Glutathione catalysis and the reaction mechanisms of glutathione-dependent enzymes.

PubMed

Deponte, Marcel

2013-05-01

Glutathione-dependent catalysis is a metabolic adaptation to chemical challenges encountered by all life forms. In the course of evolution, nature optimized numerous mechanisms to use glutathione as the most versatile nucleophile for the conversion of a plethora of sulfur-, oxygen- or carbon-containing electrophilic substances. This comprehensive review summarizes fundamental principles of glutathione catalysis and compares the structures and mechanisms of glutathione-dependent enzymes, including glutathione reductase, glutaredoxins, glutathione peroxidases, peroxiredoxins, glyoxalases 1 and 2, glutathione transferases and MAPEG. Moreover, open mechanistic questions, evolutionary aspects and the physiological relevance of glutathione catalysis are discussed for each enzyme family. It is surprising how little is known about many glutathione-dependent enzymes, how often reaction geometries and acid-base catalysts are neglected, and how many mechanistic puzzles remain unsolved despite almost a century of research. On the one hand, several enzyme families with non-related protein folds recognize the glutathione moiety of their substrates. On the other hand, the thioredoxin fold is often used for glutathione catalysis. Ancient as well as recent structural changes of this fold did not only significantly alter the reaction mechanism, but also resulted in completely different protein functions. Glutathione-dependent enzymes are excellent study objects for structure-function relationships and molecular evolution. Notably, in times of systems biology, the outcome of models on glutathione metabolism and redox regulation is more than questionable as long as fundamental enzyme properties are neither studied nor understood. Furthermore, several of the presented mechanisms could have implications for drug development. This article is part of a Special Issue entitled Cellular functions of glutathione. Copyright © 2012 Elsevier B.V. All rights reserved.

Pop-It Beads to Introduce Catalysis of Reaction Rate and Substrate Depletion Effects

ERIC Educational Resources Information Center

Gehret, Austin U.

2017-01-01

A kinesthetic classroom activity was designed to help students understand enzyme activity and catalysis of reaction rate. Students served the role of enzymes by manipulating Pop-It Beads as the catalytic event. This activity illuminates the relationship between reaction rate and reaction progress by allowing students to experience first-hand the…

Heterogeneous Molecular Catalysis of Electrochemical Reactions: Volcano Plots and Catalytic Tafel Plots.

PubMed

Costentin, Cyrille; Savéant, Jean-Michel

2017-06-14

We analyze here, in the framework of heterogeneous molecular catalysis, the reasons for the occurrence or nonoccurrence of volcanoes upon plotting the kinetics of the catalytic reaction versus the stabilization free energy of the primary intermediate of the catalytic process. As in the case of homogeneous molecular catalysis or catalysis by surface-active metallic sites, a strong motivation of such studies relates to modern energy challenges, particularly those involving small molecules, such as water, hydrogen, oxygen, proton, and carbon dioxide. This motivation is particularly pertinent for what concerns heterogeneous molecular catalysis, since it is commonly preferred to homogeneous molecular catalysis by the same molecules if only for chemical separation purposes and electrolytic cell architecture. As with the two other catalysis modes, the main drawback of the volcano plot approach is the basic assumption that the kinetic responses depend on a single descriptor, viz., the stabilization free energy of the primary intermediate. More comprehensive approaches, investigating the responses to the maximal number of experimental factors, and conveniently expressed as catalytic Tafel plots, should clearly be preferred. This is more so in the case of heterogeneous molecular catalysis in that additional transport factors in the supporting film may additionally affect the current-potential responses. This is attested by the noteworthy presence of maxima in catalytic Tafel plots as well as their dependence upon the cyclic voltammetric scan rate.

Switching on Elusive Organometallic Mechanisms with Photoredox Catalysis

PubMed Central

Terrett, Jack A.; Cuthbertson, James D.; Shurtleff, Valerie W.; MacMillan, David W. C.

2015-01-01

Transition metal-catalyzed cross-coupling reactions have become one of the most utilized carbon–carbon and carbon–heteroatom bond-forming reactions in chemical synthesis. More recently, nickel catalysis has been shown to participate in a wide variety of C–C bond forming reactions, most notably Negishi, Suzuki–Miyaura, Stille, Kumada, and Hiyama couplings1,2. Despite the tremendous advances in C–C fragment couplings, the ability to forge C–O bonds in a general fashion via nickel catalysis has been largely unsuccessful. The challenge for nickel-mediated alcohol couplings has been the mechanistic requirement for the critical C–O bond forming step (formally known as the reductive elimination step) to occur via a Ni(III) alkoxide intermediate. In this manuscript, we demonstrate that visible light-excited photoredox catalysts can modulate the preferred oxidation states of nickel alkoxides in an operative catalytic cycle, thereby providing transient access to Ni(III) species that readily participate in reductive elimination. Using this synergistic merger of photoredox and nickel catalysis, we have developed a highly efficient and general carbon–oxygen coupling reaction using abundant alcohols and aryl bromides. More significantly, we have developed a general strategy to “switch on” important yet elusive organometallic mechanisms via oxidation state modulations using only weak light and single-electron transfer (SET) catalysts. PMID:26266976

Charge-tagged ligands: useful tools for immobilising complexes and detecting reaction species during catalysis

PubMed Central

Limberger, Jones; Leal, Bárbara C.; Monteiro, Adriano L.

2015-01-01

In recent years, charge-tagged ligands (CTLs) have become valuable tools in organometallic catalysis. Insertion of an ionic side chain into the molecular skeleton of a known ligand has become a useful protocol for anchoring ligands, and consequently catalysts, in polar and ionic liquid phases. In addition, the insertion of a cationic moiety into a ligand is a powerful tool that can be used to detect reaction intermediates in organometallic catalysis through electrospray ionisation mass spectrometry (ESI-MS) experiments. The insertion of an ionic tag ensures the charge in the intermediates independently of the ESI-MS. For this reason, these ligands have been used as ionic probes in mechanistic studies for several catalytic reactions. Here, we summarise selected examples on the use of CTLs as immobilising agents in organometallic catalysis and as probes for studying mechanisms through ESI-MS. PMID:28553458

Origin of fast catalysis in allylic amination reactions catalyzed by Pd-Ti heterobimetallic complexes.

PubMed

Walker, Whitney K; Kay, Benjamin M; Michaelis, Scott A; Anderson, Diana L; Smith, Stacey J; Ess, Daniel H; Michaelis, David J

2015-06-17

Experiments and density functional calculations were used to quantify the impact of the Pd-Ti interaction in the cationic heterobimetallic Cl2Ti(N(t)BuPPh2)2Pd(η(3)-methallyl) catalyst 1 used for allylic aminations. The catalytic significance of the Pd-Ti interaction was evaluated computationally by examining the catalytic cycle for catalyst 1 with a conformation where the Pd-Ti interaction is intact versus one where the Pd-Ti interaction is severed. Studies were also performed on the relative reactivity of the cationic monometallic (CH2)2(N(t)BuPPh2)2Pd(η(3)-methallyl) catalyst 2 where the Ti from catalyst 1 was replaced by an ethylene group. These computational and experimental studies revealed that the Pd-Ti interaction lowers the activation barrier for turnover-limiting amine reductive addition and accelerates catalysis up to 10(5). The Pd-Ti distance in 1 is the result of the N(t)Bu groups enforcing a boat conformation that brings the two metals into close proximity, especially in the transition state. The turnover frequency of classic Pd π allyl complexes was compared to that of 1 to determine the impact of P-Pd-P coordination angle and ligand electronic properties on catalysis. These experiments identified that cationic (PPh3)2Pd(η(3)-CH2C(CH3)CH2) catalyst 3 performs similarly to 1 for allylic aminations with diethylamine. However, computations and experiment reveal that the apparent similarity in reactivity is due to very fast reaction kinetics. The higher reactivity of 1 versus 3 was confirmed in the reaction of methallyl chloride and 2,2,6,6-tetramethylpiperidine (TMP). Overall, experiments and calculations demonstrate that the Pd-Ti interaction induces and is responsible for significantly lower barriers and faster catalysis for allylic aminations.

Oxidase catalysis via aerobically generated hypervalent iodine intermediates

NASA Astrophysics Data System (ADS)

Maity, Asim; Hyun, Sung-Min; Powers, David C.

2018-02-01

The development of sustainable oxidation chemistry demands strategies to harness O2 as a terminal oxidant. Oxidase catalysis, in which O2 serves as a chemical oxidant without necessitating incorporation of oxygen into reaction products, would allow diverse substrate functionalization chemistry to be coupled to O2 reduction. Direct O2 utilization suffers from intrinsic challenges imposed by the triplet ground state of O2 and the disparate electron inventories of four-electron O2 reduction and two-electron substrate oxidation. Here, we generate hypervalent iodine reagents—a broadly useful class of selective two-electron oxidants—from O2. This is achieved by intercepting reactive intermediates of aldehyde autoxidation to aerobically generate hypervalent iodine reagents for a broad array of substrate oxidation reactions. The use of aryl iodides as mediators of aerobic oxidation underpins an oxidase catalysis platform that couples substrate oxidation directly to O2 reduction. We anticipate that aerobically generated hypervalent iodine reagents will expand the scope of aerobic oxidation chemistry in chemical synthesis.

Biodiesel forming reactions using heterogeneous catalysis

NASA Astrophysics Data System (ADS)

Liu, Yijun

Biodiesel synthesis from biomass provides a means for utilizing effectively renewable resources, a way to convert waste vegetable oils and animal fats to a useful product, a way to recycle carbon dioxide for a combustion fuel, and production of a fuel that is biodegradable, non-toxic, and has a lower emission profile than petroleum-diesel. Free fatty acid (FFA) esterification and triglyceride (TG) transesterification with low molecular weight alcohols constitute the synthetic routes to prepare biodiesel from lipid feedstocks. This project was aimed at developing a better understanding of important fundamental issues involved in heterogeneous catalyzed biodiesel forming reactions using mainly model compounds, representing part of on-going efforts to build up a rational base for assay, design, and performance optimization of solid acids/bases in biodiesel synthesis. As FFA esterification proceeds, water is continuously formed as a byproduct and affects reaction rates in a negative manner. Using sulfuric acid (as a catalyst) and acetic acid (as a model compound for FFA), the impact of increasing concentrations of water on acid catalysis was investigated. The order of the water effect on reaction rate was determined to be -0.83. Sulfuric acid lost up to 90% activity as the amount of water present increased. The nature of the negative effect of water on esterification was found to go beyond the scope of reverse hydrolysis and was associated with the diminished acid strength of sulfuric acid as a result of the preferential solvation by water molecules of its catalytic protons. The results indicate that as esterification progresses and byproduct water is produced, deactivation of a Bronsted acid catalyst like H2SO4 occurs. Using a solid composite acid (SAC-13) as an example of heterogeneous catalysts and sulfuric acid as a homogeneous reference, similar reaction inhibition by water was demonstrated for homogeneous and heterogeneous catalysis. This similarity together with

Kinetics and Catalysis Demonstrations.

ERIC Educational Resources Information Center

Falconer, John L.; Britten, Jerald A.

1984-01-01

Eleven videotaped kinetics and catalysis demonstrations are described. Demonstrations include the clock reaction, oscillating reaction, hydrogen oxidation in air, hydrogen-oxygen explosion, acid-base properties of solids, high- and low-temperature zeolite reactivity, copper catalysis of ammonia oxidation and sodium peroxide decomposition, ammonia…

Functionalization of multi-walled carbon nanotubes with iron phthalocyanine via a liquid chemical reaction for oxygen reduction in alkaline media

NASA Astrophysics Data System (ADS)

Yan, Xiaomei; Xu, Xiao; Liu, Qin; Guo, Jia; Kang, Longtian; Yao, Jiannian

2018-06-01

Iron single-atom catalyst in form of iron-nitrogen-carbon structure possesses the excellent catalytic activity in various chemical reactions. However, exploring a sustainable and stable single-atom metal catalyst still faces a great challenge due to low yield and complicated synthesis. Here, we report a functional multi-wall carbon nanotubes modified with iron phthalocyanine molecules via a liquid chemical reaction and realize the performance of similar single-atom catalysis for oxygen reduction reaction. A serial of characterizations strongly imply the structure change of iron phthalocyanine molecule and its close recombination with multi-wall carbon nanotubes, which are in favor of ORR catalysis. Compared to commercial platinum-carbon catalyst, composites exhibit superior activity for oxygen reduction reaction with higher half-wave potential (0.86 V), lower Tafel slope (38 mV dec-1), higher limiting current density and excellent electrochemical stability. The corresponding Zinc-air battery also presents higher maximum power density and discharge stability. Therefore, these findings provide a facile route to synthesize a highly efficient non-precious metal carbon-based catalyst.

Diversity-oriented asymmetric catalysis (DOAC): stereochemically divergent synthesis of thiochromanes using an imidazoline-aminophenol-nickel-catalyzed Michael/Henry reaction.

PubMed

Arai, Takayoshi; Yamamoto, Yushi

2014-03-21

The (S,S)-diphenylethylenediamine-derived imidazoline-aminophenol-Ni complex catalyzed tandem asymmetric Michael/Henry reaction of 2-mercaptobenzaldehydes with β-nitrostyrenes to give the corresponding (2S,3R,4R)-2-aryl-3-nitrothiochroman-4-ols in up to 99% diastereoselectivity with 95% ee was demonstrated in diversity-oriented asymmetric catalysis. Reduction of the nitro group of the chiral thiochromanes gave a new series of (2S,3R,4R)-3-amino-2-arylthiochroman-4-ols with retention of the strereoselectivity.

Orthogonal tandem catalysis

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lohr, Tracy L.; Marks, Tobin J.

2015-05-20

Tandem catalysis is a growing field that is beginning to yield important scientific and technological advances toward new and more efficient catalytic processes. 'One-pot' tandem reactions, where multiple catalysts and reagents, combined in a single reaction vessel undergo a sequence of precisely staged catalytic steps, are highly attractive from the standpoint of reducing both waste and time. Orthogonal tandem catalysis is a subset of one-pot reactions in which more than one catalyst is used to promote two or more mechanistically distinct reaction steps. This Perspective summarizes and analyses some of the recent developments and successes in orthogonal tandem catalysis, withmore » particular focus on recent strategies to address catalyst incompatibility. We also highlight the concept of thermodynamic leveraging by coupling multiple catalyst cycles to effect challenging transformations not observed in single-step processes, and to encourage application of this technique to energetically unfavourable or demanding reactions.« less

Single-molecule Force Spectroscopy Approach to Enzyme Catalysis*

PubMed Central

Alegre-Cebollada, Jorge; Perez-Jimenez, Raul; Kosuri, Pallav; Fernandez, Julio M.

2010-01-01

Enzyme catalysis has been traditionally studied using a diverse set of techniques such as bulk biochemistry, x-ray crystallography, and NMR. Recently, single-molecule force spectroscopy by atomic force microscopy has been used as a new tool to study the catalytic properties of an enzyme. In this approach, a mechanical force ranging up to hundreds of piconewtons is applied to the substrate of an enzymatic reaction, altering the conformational energy of the substrate-enzyme interactions during catalysis. From these measurements, the force dependence of an enzymatic reaction can be determined. The force dependence provides valuable new information about the dynamics of enzyme catalysis with sub-angstrom resolution, a feat unmatched by any other current technique. To date, single-molecule force spectroscopy has been applied to gain insight into the reduction of disulfide bonds by different enzymes of the thioredoxin family. This minireview aims to present a perspective on this new approach to study enzyme catalysis and to summarize the results that have already been obtained from it. Finally, the specific requirements that must be fulfilled to apply this new methodology to any other enzyme will be discussed. PMID:20382731

Single-molecule force spectroscopy approach to enzyme catalysis.

PubMed

Alegre-Cebollada, Jorge; Perez-Jimenez, Raul; Kosuri, Pallav; Fernandez, Julio M

2010-06-18

Enzyme catalysis has been traditionally studied using a diverse set of techniques such as bulk biochemistry, x-ray crystallography, and NMR. Recently, single-molecule force spectroscopy by atomic force microscopy has been used as a new tool to study the catalytic properties of an enzyme. In this approach, a mechanical force ranging up to hundreds of piconewtons is applied to the substrate of an enzymatic reaction, altering the conformational energy of the substrate-enzyme interactions during catalysis. From these measurements, the force dependence of an enzymatic reaction can be determined. The force dependence provides valuable new information about the dynamics of enzyme catalysis with sub-angstrom resolution, a feat unmatched by any other current technique. To date, single-molecule force spectroscopy has been applied to gain insight into the reduction of disulfide bonds by different enzymes of the thioredoxin family. This minireview aims to present a perspective on this new approach to study enzyme catalysis and to summarize the results that have already been obtained from it. Finally, the specific requirements that must be fulfilled to apply this new methodology to any other enzyme will be discussed.

Diversifying Homogenous Au(I)-Catalysis through New Reaction Discovery

NASA Astrophysics Data System (ADS)

Motika, Stephen

Homogenous Au(I)-catalysis has become a valuable synthetic tool to activate a host of unsaturated carbon functional groups towards nucleophilic addition. Over the course of the past two decades, many have embarked on new journeys within this field. Notably, the advancements in this field hinge on the development of new ligand systems that impart novel reactivity at the metal. Our group has focused on this area, as we have successfully demonstrated the utility of 1,2,3-triazoles as ligands for gold and a host of other transition metals and Lewis acids. With respect to gold catalysis, these ligands enhance the stability of the metal center, thus inhibiting typical reductive decomposition pathways that have plagued this field. The enhanced stability comes with a price though as higher temperatures can be required. We've addressed this challenge by discovering an interesting synergy between triazole-gold and Lewis acids, allowing us to overcome the lower reactivity of these catalysts. During my time as a graduate student, I have focused heavily on enlisting these catalytic systems for new reaction discovery. In my first experimental chapter, I was able to develop an interesting reaction cascade in which triazole-gold and secondary amine catalysts were used. I started with a well-known gold-catalyzed Claisen rearrangement of propargyl vinyl ether, yielding functionalized allenes. The identical oxidation state between these allenes and synthetically appealing dienals was an impetus to develop a new isomerization strategy. After screening various conditions, I was able to successfully execute this design. Most of the work I have been involved in over the past two years has surrounded a gold-catalyzed hydroboration to yield interesting hetercocycles containing a N-B bond. The N-B bond offers some unique properties as it is isoelectronic to a C-C double bond. Despite the simplicity in this design, it would become apparent early on in this research that mitigating the

Selective reduction of carboxylic acids to aldehydes with hydrosilane via photoredox catalysis.

PubMed

Zhang, Muliang; Li, Nan; Tao, Xingyu; Ruzi, Rehanguli; Yu, Shouyun; Zhu, Chengjian

2017-09-12

The direct reduction of carboxylic acids to aldehydes with hydrosilane was achieved through visible light photoredox catalysis. The combination of both single electron transfer and hydrogen atom transfer steps offers a novel and convenient approach to selective reduction of carboxylic acids to aldehydes. The method also features mild conditions, high yields, broad substrate scope, and good functional group tolerance, such as alkyne, ester, ketone, amide and amine groups.

Space and time-resolved probing of heterogeneous catalysis reactions using lab-on-a-chip

NASA Astrophysics Data System (ADS)

Navin, Chelliah V.; Krishna, Katla Sai; Theegala, Chandra S.; Kumar, Challa S. S. R.

2016-03-01

Probing catalytic reactions on a catalyst surface in real time is a major challenge. Herein, we demonstrate the utility of a continuous flow millifluidic chip reactor coated with a nanostructured gold catalyst as an effective platform for in situ investigation of the kinetics of catalytic reactions by taking 5-(hydroxymethyl)furfural (HMF) to 2,5-furandicarboxylic acid (FDCA) conversion as a model reaction. The idea conceptualized in this paper can not only dramatically change the ability to probe the time-resolved kinetics of heterogeneous catalysis reactions but also used for investigating other chemical and biological catalytic processes, thereby making this a broad platform for probing reactions as they occur within continuous flow reactors.Probing catalytic reactions on a catalyst surface in real time is a major challenge. Herein, we demonstrate the utility of a continuous flow millifluidic chip reactor coated with a nanostructured gold catalyst as an effective platform for in situ investigation of the kinetics of catalytic reactions by taking 5-(hydroxymethyl)furfural (HMF) to 2,5-furandicarboxylic acid (FDCA) conversion as a model reaction. The idea conceptualized in this paper can not only dramatically change the ability to probe the time-resolved kinetics of heterogeneous catalysis reactions but also used for investigating other chemical and biological catalytic processes, thereby making this a broad platform for probing reactions as they occur within continuous flow reactors. Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nr06752a

Improving the throughput of batch photochemical reactions using flow: Dual photoredox and nickel catalysis in flow for C(sp2)C(sp3) cross-coupling.

PubMed

Abdiaj, Irini; Alcázar, Jesús

2017-12-01

We report herein the transfer of dual photoredox and nickel catalysis for C(sp 2 )C(sp 3 ) cross coupling form batch to flow. This new procedure clearly improves the scalability of the previous batch reaction by the reactor's size and operating time reduction, and allows the preparation of interesting compounds for drug discovery in multigram amounts. Copyright © 2016 Elsevier Ltd. All rights reserved.

Confined catalysis under two-dimensional materials

PubMed Central

Li, Haobo; Xiao, Jianping; Bao, Xinhe

2017-01-01

Confined microenvironments formed in heterogeneous catalysts have recently been recognized as equally important as catalytically active sites. Understanding the fundamentals of confined catalysis has become an important topic in heterogeneous catalysis. Well-defined 2D space between a catalyst surface and a 2D material overlayer provides an ideal microenvironment to explore the confined catalysis experimentally and theoretically. Using density functional theory calculations, we reveal that adsorption of atoms and molecules on a Pt(111) surface always has been weakened under monolayer graphene, which is attributed to the geometric constraint and confinement field in the 2D space between the graphene overlayer and the Pt(111) surface. A similar result has been found on Pt(110) and Pt(100) surfaces covered with graphene. The microenvironment created by coating a catalyst surface with 2D material overlayer can be used to modulate surface reactivity, which has been illustrated by optimizing oxygen reduction reaction activity on Pt(111) covered by various 2D materials. We demonstrate a concept of confined catalysis under 2D cover based on a weak van der Waals interaction between 2D material overlayers and underlying catalyst surfaces. PMID:28533413

Birnessite catalysis of the Maillard Reaction: Its significance in natural humification

NASA Astrophysics Data System (ADS)

Jokic, A.; Frenkel, A. I.; Vairavamurthy, M. A.; Huang, P. M.

Although mineral colloids are known to play a significant role in transforming organic matter in soils and sediments, there still are many gaps in our understanding of the mechanisms of organic-mineral interactions. In this study, we investigated the role of a major oxide-mineral birnessite (a form of Mn(IV) oxide) in catalyzing the condensation reaction between sugars and amino acids, the Maillard reaction, for forming humic substances. The Maillard reaction is perceived to be a major pathway in natural humification. Using a suite of spectroscopic methods (including ESR, XANES, EXAFS and 13C NMR), our results show that Mn(IV) oxide markedly accelerates the Maillard reaction between glucose and glycine at ranges of temperatures and pH typical of natural environments. These results demonstrate the importance of manganese oxide catalysis in the Maillard reaction, and its significance in the natural abiotic formation of humic substances.

Linear scaling relationships and volcano plots in homogeneous catalysis - revisiting the Suzuki reaction.

PubMed

Busch, Michael; Wodrich, Matthew D; Corminboeuf, Clémence

2015-12-01

Linear free energy scaling relationships and volcano plots are common tools used to identify potential heterogeneous catalysts for myriad applications. Despite the striking simplicity and predictive power of volcano plots, they remain unknown in homogeneous catalysis. Here, we construct volcano plots to analyze a prototypical reaction from homogeneous catalysis, the Suzuki cross-coupling of olefins. Volcano plots succeed both in discriminating amongst different catalysts and reproducing experimentally known trends, which serves as validation of the model for this proof-of-principle example. These findings indicate that the combination of linear scaling relationships and volcano plots could serve as a valuable methodology for identifying homogeneous catalysts possessing a desired activity through a priori computational screening.

Fundamental insights into interfacial catalysis.

PubMed

Gong, Jinlong; Bao, Xinhe

2017-04-03

Surface and interfacial catalysis plays a vital role in chemical industries, electrochemistry and photochemical reactions. The challenges of modern chemistry are to optimize the chemical reaction processes and understand the detailed mechanism of chemical reactions. Since the early 1960s, the foundation of surface science systems has allowed the study of surface and interfacial phenomena on atomic/molecular level, and thus brought a number of significant developments to fundamental and technological processes, such as catalysis, material science and biochemistry, just to name a few. This themed issue describes the recent advances and developments in the fundamental understanding of surface and interfacial catalysis, encompassing areas of knowledge from metal to metal oxide, carbide, graphene, hexagonal boron nitride, and transition metal dichalcogenides under ultrahigh vacuum conditions, as well as under realistic reaction conditions.

Candida parapsilosis: A versatile biocatalyst for organic oxidation-reduction reactions.

PubMed

Chadha, Anju; Venkataraman, Sowmyalakshmi; Preetha, Radhakrishnan; Padhi, Santosh Kumar

2016-10-01

This review highlights the importance of the biocatalyst, Candida parapsilosis for oxidation and reduction reactions of organic compounds and establishes its versatility to generate a variety of chiral synthons. Appropriately designed reactions using C. parapsilosis effect efficient catalysis of organic transformations such as deracemization, enantioselective reduction of prochiral ketones, imines, and kinetic resolution of racemic alcohols via selective oxidation. This review includes the details of these biotransformations, catalyzed by whole cells (wild type and recombinant strains), purified enzymes (oxidoreductases) and immobilized whole cells of C. parapsilosis. The review presents a bioorganic perspective as it discusses the chemo, regio and stereoselectivity of the biocatalyst along with the structure of the substrates and optical purity of the products. Fermentation scale biocatalysis using whole cells of C. parapsilosis for several biotransformations to synthesize important chiral synthons/industrial chemicals is included. A comparison of C. parapsilosis with other whole cell biocatalysts for biocatalytic deracemization and asymmetric reduction of carbonyl and imine groups in the synthesis of a variety of enantiopure products is presented which will provide a basis for the choice of a biocatalyst for a desired organic transformation. Thus, a wholesome perspective on the present status of C. parapsilosis mediated organic transformations and design of new reactions which can be considered for large scale operations is provided. Taken together, C. parapsilosis can now be considered a 'reagent' for the organic transformations discussed here. Copyright © 2016 Elsevier Inc. All rights reserved.

Silver versus gold catalysis in tandem reactions of carbonyl functions onto alkynes: a versatile access to furoquinoline and pyranoquinoline cores.

PubMed

Godet, Thomas; Vaxelaire, Carine; Michel, Carine; Milet, Anne; Belmont, Philippe

2007-01-01

An efficient and versatile tandem process of acetalization and cycloisomerization reactions has been developed for the reactions of 1-alkynyl-2-carbonylquinoline substrates. The reaction occurs thanks to Au(I) and Ag(I) catalysis. Silver(I) catalysis has been extensively studied (11 different silver species) on a broad range of quinoline derivatives (variation of alkyne substituent, of carbonyl function and of nucleophiles), leading to a variety of furoquinoline and pyranoquinoline moieties. An insight is given for the presumed mechanism along with DFT-B3 LYP/6-31G** calculations to address the 6-endo and 5-exo regioselectivities observed.

Reaction of azides and enolisable aldehydes under the catalysis of organic bases and Cinchona based quaternary ammonium salts.

PubMed

Destro, Dario; Sanchez, Sandra; Cortigiani, Mauro; Adamo, Mauro F A

2017-06-21

Herein we report a two-step sequence for the preparation of amides starting from azides and enolisable aldehydes. The reaction proceeded via the formation of triazoline intermediates that were converted into amides via Lewis acid catalysis. Preliminary studies on the preparation of triazolines under chiral phase transfer catalysis are also presented, demonstrating that enantioenriched amides could be prepared from achiral aldehydes in moderate to low enantioselectivity.

Multi-catalysis cascade reactions based on the methoxycarbonylketene platform: diversity-oriented synthesis of functionalized non-symmetrical malonates for agrochemicals and pharmaceuticals.

PubMed

Ramachary, Dhevalapally B; Venkaiah, Chintalapudi; Reddy, Y Vijayendar; Kishor, Mamillapalli

2009-05-21

In this paper we describe new multi-catalysis cascade (MCC) reactions for the one-pot synthesis of highly functionalized non-symmetrical malonates. These metal-free reactions are either five-step (olefination/hydrogenation/alkylation/ketenization/esterification) or six-step (olefination/hydrogenation/alkylation/ketenization/esterification/alkylation), and employ aldehydes/ketones, Meldrum's acid, 1,4-dihydropyridine/o-phenylenediamine, diazomethane, alcohols and active ethylene/acetylenes, and involve iminium-, self-, self-, self- and base-catalysis, respectively. Many of the products have direct application in agricultural and pharmaceutical chemistry.

Synergistic Catalysis: A Powerful Synthetic Strategy for New Reaction Development

PubMed Central

Allen, Anna E.; MacMillan, David W. C.

2012-01-01

Synergistic catalysis is a synthetic strategy wherein both the nucleophile and the electrophile are simultaneously activated by two separate and distinct catalysts to afford a single chemical transformation. This powerful catalysis strategy leads to several benefits, specifically synergistic catalysis can (i) introduce new, previously unattainable chemical transformations, (ii) improve the efficiency of existing transformations, and (iii) create or improve catalytic enantioselectivity where stereocontrol was previously absent or challenging. This perspective aims to highlight these benefits using many of the successful examples of synergistic catalysis found in the literature. PMID:22518271

Rational design of competitive electrocatalysts for the oxygen reduction reaction in hydrogen fuel cells

NASA Astrophysics Data System (ADS)

Stolbov, Sergey; Alcántara Ortigoza, Marisol

2012-02-01

The large-scale application of one of the most promising clean and renewable sources of energy, hydrogen fuel cells, still awaits efficient and cost-effective electrocatalysts for the oxygen reduction reaction (ORR) occurring on the cathode. We demonstrate that truly rational design renders electrocatalysts possessing both qualities. By unifying the knowledge on surface morphology, composition, electronic structure and reactivity, we solve that sandwich-like structures are an excellent choice for optimization. Their constituting species couple synergistically yielding reaction-environment stability, cost-effectiveness and tunable reactivity. This cooperative-action concept enabled us to predict two advantageous ORR electrocatalysts. Density functional theory calculations of the reaction free-energy diagrams confirm that these materials are more active toward ORR than the so far best Pt-based catalysts. Our designing concept advances also a general approach for engineering materials in heterogeneous catalysis.

Development of a reaction cell for in-situ/operando studies of surface of a catalyst under a reaction condition and during catalysis.

PubMed

Nguyen, Luan; Tao, Franklin Feng

2016-06-01

Tracking surface chemistry of a catalyst during catalysis is significant for fundamental understanding of catalytic performance of the catalyst since it allows for establishing an intrinsic correlation between surface chemistry of a catalyst at its working status and its corresponding catalytic performance. Ambient pressure X-ray photoelectron spectroscopy can be used for in-situ studies of surfaces of different materials or devices in a gas. To simulate the gaseous environment of a catalyst in a fixed-bed a flowing gaseous environment of reactants around the catalyst is necessary. Here, we report the development of a new flowing reaction cell for simulating in-situ study of a catalyst surface under a reaction condition in gas of one reactant or during catalysis in a mixture of reactants of a catalytic reaction. The homemade reaction cell is installed in a high vacuum (HV) or ultrahigh vacuum (UHV) environment of a chamber. The flowing gas in the reaction cell is separated from the HV or UHV environment through well sealings at three interfaces between the reaction cell and X-ray window, sample door and aperture of front cone of an energy analyzer. Catalyst in the cell is heated through infrared laser beam introduced through a fiber optics interfaced with the reaction cell through a homemade feedthrough. The highly localized heating on the sample holder and Au-passivated internal surface of the reaction cell effectively minimizes any unwanted reactions potentially catalyzed by the reaction cell. The incorporated laser heating allows a fast heating and a high thermal stability of the sample at a high temperature. With this cell, a catalyst at 800 °C in a flowing gas can be tracked readily.

Core/shell face-centered tetragonal FePd/Pd nanoparticles as an efficient non-Pt catalyst for the oxygen reduction reaction

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhu, Huiyuan; Jiang, Guangming; Zhang, Xu

We report the synthesis of core/shell face-centered tetragonal (fct)-FePd/Pd nanoparticles (NPs) via reductive annealing of core/shell Pd/Fe 3O 4 NPs followed by temperature-controlled Fe etching in acetic acid. Among three different kinds of core/shell FePd/Pd NPs studied (FePd core at similar to 8 nm and Pd shell at 0.27, 0.65, or 0.81 nm), the fct-FePd/Pd-0.65 NPs are the most efficient catalyst for the oxygen reduction reaction (ORR) in 0.1 M HClO 4 with Pt-like activity and durability. This enhanced ORR catalysis arises from the desired Pd lattice compression in the 0.65 nm Pd shell induced by the fct-FePd core. Lastly,more » our study offers a general approach to enhance Pd catalysis in acid for ORB.« less

Core/shell face-centered tetragonal FePd/Pd nanoparticles as an efficient non-Pt catalyst for the oxygen reduction reaction

DOE PAGES

Zhu, Huiyuan; Jiang, Guangming; Zhang, Xu; ...

2015-10-04

We report the synthesis of core/shell face-centered tetragonal (fct)-FePd/Pd nanoparticles (NPs) via reductive annealing of core/shell Pd/Fe 3O 4 NPs followed by temperature-controlled Fe etching in acetic acid. Among three different kinds of core/shell FePd/Pd NPs studied (FePd core at similar to 8 nm and Pd shell at 0.27, 0.65, or 0.81 nm), the fct-FePd/Pd-0.65 NPs are the most efficient catalyst for the oxygen reduction reaction (ORR) in 0.1 M HClO 4 with Pt-like activity and durability. This enhanced ORR catalysis arises from the desired Pd lattice compression in the 0.65 nm Pd shell induced by the fct-FePd core. Lastly,more » our study offers a general approach to enhance Pd catalysis in acid for ORB.« less

Pop-it beads to introduce catalysis of reaction rate and substrate depletion effects.

PubMed

Gehret, Austin U

2017-03-04

A kinesthetic classroom activity was designed to help students understand enzyme activity and catalysis of reaction rate. Students served the role of enzymes by manipulating Pop-It Beads as the catalytic event. This activity illuminates the relationship between reaction rate and reaction progress by allowing students to experience first-hand the effect of substrate depletion on catalyzed reaction rate. Preliminary findings based on survey results and exam performance suggest the activity could prove beneficial to students in the targeted learning outcomes. Unique to previous kinesthetic approaches that model Michaelis-Menten kinetics, this activity models the effects of substrate depletion on catalyzed reaction rate. Therefore, it could prove beneficial for conveying the reasoning behind the initial rate simplification used in Michaelis-Menten kinetics. © 2016 by The International Union of Biochemistry and Molecular Biology, 45(2):179-183, 2017. © 2016 The International Union of Biochemistry and Molecular Biology.

High-Potential Electrocatalytic O2 Reduction with Nitroxyl / NOx Mediators: Implications for Fuel Cells and Aerobic Oxidation Catalysis

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gerken, James B.; Stahl, Shannon S.

2015-07-15

Efficient reduction of O2 to water is a central challenge in energy conversion and aerobic oxidation catalysis. In the present study, we investigate the electrochemical reduction of O2 with soluble organic nitroxyl and nitrogen oxide (NOx) mediators. When used alone, neither organic nitroxyls, such as TEMPO (2,2,6,6-tetramethyl-1-piperidinyl-N-oxyl), nor NOx species, such as sodium nitrite, are effective mediators of electrochemical O2 reduction. The combination of nitroxyl/NOx species, however, mediates sustained O2 reduction at electrochemical potentials of 0.19–0.33 V (vs. Fc/Fc+) in acetonitrile containing trifluoroacetic acid. Mechanistic analysis of the coupled redox reactions supports a process in which the nitrogen oxide catalystmore » drives aerobic oxidation of a nitroxyl mediator to an oxoammonium species, which then is reduced back to the nitroxyl at the cathode. The electrolysis potential is dictated by the oxoammonium/nitroxyl reduction potential. The high potentials observed with this ORR system benefit from the mechanism-based specificity for four-electron reduction of oxygen to water mediated by NOx species, together with kinetically efficient reduction of oxidized NOx species by TEMPO and other organic nitroxyls. This research was supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center, funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.« less

Direct sp(3)C-H acroleination of N-aryl-tetrahydroisoquinolines by merging photoredox catalysis with nucleophilic catalysis.

PubMed

Feng, Zhu-Jia; Xuan, Jun; Xia, Xu-Dong; Ding, Wei; Guo, Wei; Chen, Jia-Rong; Zou, You-Quan; Lu, Liang-Qiu; Xiao, Wen-Jing

2014-04-07

Sequence catalysis merging photoredox catalysis (PC) and nucleophilic catalysis (NC) has been realized for the direct sp(3) C-H acroleination of N-aryl-tetrahydroisoquinoline (THIQ). The reaction was performed under very mild conditions and afforded products in 50-91% yields. A catalytic asymmetric variant was proved to be successful with moderate enantioselectivities (up to 83 : 17 er).

Heterobimetallic Catalysis: Platinum-Gold-Catalyzed Tandem Cyclization/C-X Coupling Reaction of (Hetero)Arylallenes with Nucleophiles.

PubMed

Alonso, José Miguel; Muñoz, María Paz

2018-04-16

Heterobimetallic catalysis offers new opportunities for reactivity and selectivity but still presents challenges, and only a few metal combinations have been explored so far. Reported here is a Pt-Au heterobimetallic catalyst system for the synthesis of a family of multi-heteroaromatic structures through tandem cyclization/C-X coupling reaction. Au-catalyzed 6-endo-cyclization takes place as the first fast step. Pt-Au clusters are proposed to be responsible for the increased reactivity in the second step, that is, the intermolecular nucleophilic addition which occurs through an outer-sphere mechanism by hybrid homogeneous-heterogeneous catalysis. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

A Molecular Reaction Cycle with a Solvatochromic Merocyanine Dye: An Experiment in Photochemistry, Kinetics, and Catalysis.

ERIC Educational Resources Information Center

Abdel-Kader, M. H.; Steiner, U.

1983-01-01

Three experiments using merocyanine M suitable as an integrated laboratory experience for undergraduates are described. Experiments demonstrate: complete molecular cycle composed of photochemical, thermal, and protolytic reaction steps; kinetics of cis-trans isomerization of the dye; and mechanism of base catalysis for thermal isomerization of the…

Noninnocent Proton-Responsive Ligand Facilitates Reductive Deprotonation and Hinders CO 2 Reduction Catalysis in [Ru(tpy)(6DHBP)(NCCH 3 )] 2+ (6DHBP = 6,6'-(OH) 2 bpy)

DOE PAGES

Duan, Lele; Manbeck, Gerald F.; Kowalczyk, Marta; ...

2016-04-14

Ruthenium complexes with proton-responsive ligands [Ru(tpy)(nDHBP)(NCCH 3)](CF 3SO 3) 2 (tpy = 2,2':6',2"-terpyridine; nDHBP = n,n'-dihydroxy-2,2'-bipyridine, n = 4 or 6) were examined in this study for reductive chemistry and as catalysts for CO 2 reduction. Electrochemical reduction of [Ru(tpy)(nDHBP)(NCCH 3)] 2+ generates deprotonated species through interligand electron transfer in which the initially formed tpy radical anion reacts with a proton source to produce singly and doubly deprotonated complexes that are identical to those obtained by base titration. A third reduction (i.e., reduction of [Ru(tpy)(nDHBP–2H +)] 0) triggers catalysis of CO 2 reduction; however, the catalytic efficiency is strikingly lowermore » than that of unsubstituted [Ru(tpy)(bpy)(NCCH 3)] 2+ (bpy = 2,2'-bipyridine). Cyclic voltammetry, bulk electrolysis, and spectroelectrochemical infrared experiments suggest the reactivity of CO 2 at both the Ru center and the deprotonated quinone-type ligand. Lastly, the Ru carbonyl formed by the intermediacy of a metallocarboxylic acid is stable against reduction, and mass spectrometry analysis of this product indicates the presence of two carbonates formed by the reaction of DHBP–2H + with CO 2.« less

Supramolecular catalysis beyond enzyme mimics.

PubMed

Meeuwissen, Jurjen; Reek, Joost N H

2010-08-01

Supramolecular catalysis - the assembly of catalyst species by harnessing multiple weak intramolecular interactions - has, until recently, been dominated by enzyme-inspired approaches. Such approaches often attempt to create an enzyme-like 'active site' and have concentrated on reactions similar to those catalysed by enzymes themselves. Here, we discuss the application of supramolecular assembly to the more traditional transition metal catalysis and to small-molecule organocatalysis. The modularity of self-assembled multicomponent catalysts means that a relatively small pool of catalyst components can provide rapid access to a large number of catalysts that can be evaluated for industrially relevant reactions. In addition, we discuss how catalyst-substrate interactions can be tailored to direct substrates along particular reaction paths and selectivities.

Chemical catalysis of nitrate reduction by iron (II)

NASA Astrophysics Data System (ADS)

Ottley, C. J.; Davison, W.; Edmunds, W. M.

1997-05-01

Experiments have been conducted to investigate the chemical reduction of nitrate under conditions relevant to the often low organic carbon environment of groundwaters. At pH 8 and 20 ± 2°C, in the presence of Cu(II), NO 3- was chemically reduced by Fe(II) to NH 4+ with an average stoichiometric liberation of 8 protons. The rate of the reaction systematically increased with pH in the range pH 7-8.5. The half-life for nitrate reduction, t 1/2, was inversely related to the total molar copper concentration, [Cu T], by the equation log t 1/2 = -1.35 log [Cu T] -2.616, for all measured values of t 1/2 from 23 min to 15 days. At the Cu(II) concentrations used of 7 × 10 -6 -10 -3 M, Cu was present mainly as a solid phase, either adsorbed to the surfaces of precipitated iron oxides or as a saturated solid. It is this solid phase copper rather than CU 2+ in solution which is catalytically active. Neither magnetite, which was formed as a product of the reaction, nor freshly prepared lepidocrocite catalysed the reaction, but goethite did. Although traces of oxygen accelerated the reaction, at higher partial pressures (>0.01 atm) the reduction of nitrate was inhibited, probably due to competition between NO 3- and O 2 for Fe(II). Appreciable catalytic effects were also observed for solid phase forms of Ag(I), Cd(H), Ni(H), Hg(II), and Pb(II). Mn(II) enhanced the rate slightly, and there was evidence for slow abiotic reduction in the absence of any added metal catalysts. These results suggest that the chemical reduction of nitrate at catalytic concentrations and temperatures appropriate to groundwater conditions is feasible on a timescale of months to years.

Operando chemistry of catalyst surfaces during catalysis.

PubMed

Dou, Jian; Sun, Zaicheng; Opalade, Adedamola A; Wang, Nan; Fu, Wensheng; Tao, Franklin Feng

2017-04-03

Chemistry of a catalyst surface during catalysis is crucial for a fundamental understanding of mechanism of a catalytic reaction performed on the catalyst in the gas or liquid phase. Due to the pressure- or molecular density-dependent entropy contribution of gas or liquid phase of the reactants and the potential formation of a catalyst surface during catalysis different from that observed in an ex situ condition, the characterization of the surface of a catalyst under reaction conditions and during catalysis can be significant and even necessary for understanding the catalytic mechanism at a molecular level. Electron-based analytical techniques are challenging for studying catalyst nanoparticles in the gas or liquid phase although they are necessary techniques to employ. Instrumentation and further development of these electron-based techniques have now made in situ/operando studies of catalysts possible. New insights into the chemistry and structure of catalyst nanoparticles have been uncovered over the last decades. Herein, the origin of the differences between ex situ and in situ/operando studies of catalysts, and the technical challenges faced as well as the corresponding instrumentation and innovations utilized for characterizing catalysts under reaction conditions and during catalysis, are discussed. The restructuring of catalyst surfaces driven by the pressure of reactant(s) around a catalyst, restructuring in reactant(s) driven by reaction temperature and restructuring during catalysis are also reviewed herein. The remaining challenges and possible solutions are briefly discussed.

A paradigm shift for radical SAM reactions: The organometallic intermediate Ω is central to catalysis.

PubMed

Byer, Amanda S; Yang, Hao; McDaniel, Elizabeth C; Kathiresan, Venkatesan; Impano, Stella; Pagnier, Adrien; Watts, Hope; Denler, Carly; Vagstad, Anna; Piel, Jörn; Duschene, Kaitlin S; Shepard, Eric M; Shields, Thomas P; Scott, Lincoln G; Lilla, Edward A; Yokoyama, Kenichi; Broderick, William E; Hoffman, Brian M; Broderick, Joan B

2018-06-28

Radical S-adenosyl-L-methionine (SAM) en-zymes comprise a vast superfamily catalyzing diverse reactions essential to all life through ho-molytic SAM cleavage to liberate the highly-reactive 5-deoxyadenosyl radical (5-dAdo•). Our recent observation of a catalytically compe-tent organometallic intermediate Ω that forms dur-ing reaction of the radical SAM (RS) enzyme py-ruvate formate-lyase activating-enzyme (PFL-AE) was therefore quite surprising, and led to the question of its broad relevance in the superfamily. We now show that Ω in PFL-AE forms as an in-termediate under a variety of mixing order condi-tions, suggesting it is central to catalysis in this enzyme. We further demonstrate that Ω forms in a suite of RS enzymes chosen to span the totality of superfamily reaction types, implicating Ω as essential in catalysis across the RS superfamily. Finally, EPR and electron nuclear double reso-nance spectroscopy establish that Ω involves an Fe-C5 bond between 5-dAdo• and the [4Fe-4S] cluster. An analogous organometallic bond is found in the well-known adenosylcobalamin (co-enzyme B12) cofactor used to initiate radical reac-tions via a 5'-dAdo• intermediate. Generation of a 5'-dAdo• intermediate via homolytic metal-carbon bond cleavage thus appears to be similar for Ω and coenzyme B12. However coenzyme B12 is involved in enzymes catalyzing of only a small number (~12) of distinct reactions, while the RS superfamily has more than 100,000 distinct se-quences and over 80 reaction types character-ized to date. The appearance of Ω across the RS superfamily therefore dramatically enlarges the sphere of bio-organometallic chemistry in Nature.

Fe Stabilization by Intermetallic L1 0-FePt and Pt Catalysis Enhancement in L1 0-FePt/Pt Nanoparticles for Efficient Oxygen Reduction Reaction in Fuel Cells

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Junrui; Xi, Zheng; Pan, Yung -Tin

We report in this article a detailed study on how to stabilize a first-row transition metal (M) in an intermetallic L1 0-MPt alloy nanoparticle (NP) structure and how to surround the L1 0-MPt with an atomic layer of Pt to enhance the electrocatalysis of Pt for oxygen reduction reaction (ORR) in fuel cell operation conditions. Using 8 nm FePt NPs as an example, we demonstrate that Fe can be stabilized more efficiently in a core/shell structured L1 0-FePt/Pt with a 5 Å Pt shell. The presence of Fe in the alloy core induces the desired compression of the thin Ptmore » shell, especially the 2 atomic layers of Pt shell, further improving the ORR catalysis. This leads to much enhanced Pt catalysis for ORR in 0.1 M HClO 4 solution (both at room temperature and 60°C) and in the membrane electrode assembly (MEA) at 80°C. The L1 0-FePt/Pt catalyst has a mass activity of 0.7 A/mg Pt from the half-cell ORR test and shows no obvious mass activity loss after 30,000 potential cycles between 0.6 V and 0.95 V at 80°C in the MEA, meeting the DOE 2020 target (<40% loss in mass activity). Here, we are extending the concept and preparing other L1 0-MPt/Pt NPs, such as L1 0-CoPt/Pt NPs, with reduced NP size as a highly efficient ORR catalyst for automotive fuel cell applications.« less

Fe Stabilization by Intermetallic L1 0-FePt and Pt Catalysis Enhancement in L1 0-FePt/Pt Nanoparticles for Efficient Oxygen Reduction Reaction in Fuel Cells

DOE PAGES

Li, Junrui; Xi, Zheng; Pan, Yung -Tin; ...

2018-02-07

We report in this article a detailed study on how to stabilize a first-row transition metal (M) in an intermetallic L1 0-MPt alloy nanoparticle (NP) structure and how to surround the L1 0-MPt with an atomic layer of Pt to enhance the electrocatalysis of Pt for oxygen reduction reaction (ORR) in fuel cell operation conditions. Using 8 nm FePt NPs as an example, we demonstrate that Fe can be stabilized more efficiently in a core/shell structured L1 0-FePt/Pt with a 5 Å Pt shell. The presence of Fe in the alloy core induces the desired compression of the thin Ptmore » shell, especially the 2 atomic layers of Pt shell, further improving the ORR catalysis. This leads to much enhanced Pt catalysis for ORR in 0.1 M HClO 4 solution (both at room temperature and 60°C) and in the membrane electrode assembly (MEA) at 80°C. The L1 0-FePt/Pt catalyst has a mass activity of 0.7 A/mg Pt from the half-cell ORR test and shows no obvious mass activity loss after 30,000 potential cycles between 0.6 V and 0.95 V at 80°C in the MEA, meeting the DOE 2020 target (<40% loss in mass activity). Here, we are extending the concept and preparing other L1 0-MPt/Pt NPs, such as L1 0-CoPt/Pt NPs, with reduced NP size as a highly efficient ORR catalyst for automotive fuel cell applications.« less

Diphenylbutadienes Syntheses by Means of the Wittig Reaction: Experimental Introduction to the Use of Phase Transfer Catalysis.

ERIC Educational Resources Information Center

Gillois, J.; And Others

1980-01-01

The synthesis of 1,4-diphenylbutadiene by means of the Wittig reaction is presented as suitable for organic chemistry students at the end of a basic laboratory program to apply laboratory skills and display understanding of the use of phase transfer catalysis and its application in syntheses. (CS)

Insights into reaction mechanisms in heterogeneous catalysis revealed by in situ NMR spectroscopy.

PubMed

Blasco, Teresa

2010-12-01

This tutorial review intends to show the possibilities of in situ solid state NMR spectroscopy in the elucidation of reaction mechanisms and the nature of the active sites in heterogeneous catalysis. After a brief overview of the more usual experimental devices used for in situ solid state NMR spectroscopy measurements, some examples of applications taken from the recent literature will be presented. It will be shown that in situ NMR spectroscopy allows: (i) the identification of stable intermediates and transient species using indirect methods, (ii) to prove shape selectivity in zeolites, (iii) the study of reaction kinetics, and (iv) the determination of the nature and the role played by the active sites in a catalytic reaction. The approaches and methodology used to get this information will be illustrated here summarizing the most relevant contributions on the investigation of the mechanisms of a series of reactions of industrial interest: aromatization of alkanes on bifunctional catalysts, carbonylation reaction of methanol with carbon monoxide, ethylbenzene disproportionation, and the Beckmann rearrangement reaction. Special attention is paid to the research carried out on the role played by carbenium ions and alkoxy as intermediate species in the transformation of hydrocarbon molecules on solid acid catalysts.

Sustainable Catalysis_Energy efficient reactions and Applications

EPA Science Inventory

This book chapter discusses various catalysts for environmental remediation. Detailed information on catalysis using ferrate and ferrite oxidation, TiO2 photocatalysis, and new catalysts (i.e., graphene, perovskites and graphitic carbon nitride) is provided for the degradation of...

Catalysis-reduction strategy for sensing inorganic and organic mercury based on gold nanoparticles.

PubMed

Li, Xiaokun; Zhang, Youlin; Chang, Yulei; Xue, Bin; Kong, Xianggui; Chen, Wei

2017-06-15

In view of the high biotoxicity and trace concentration of mercury (Hg) in environmental water, developing simple, ultra-sensitive and highly selective method capable of simultaneous determination of various Hg species has attracted wide attention. Here, we present a novel catalysis-reduction strategy for sensing inorganic and organic mercury in aqueous solution through the cooperative effect of AuNP-catalyzed properties and the formation of gold amalgam. For the first time, a new AuNP-catalyzed-organic reaction has been discovered and directly used for sensing Hg 2+ , Hg 2 2+ and CH 3 Hg + according to the change of the amount of the catalytic product induced by the deposition of Hg atoms on the surface of AuNPs. The detection limit of Hg species is 5.0pM (1 ppt), which is 3 orders of magnitude lower than the U.S. Environmental Protection Agency (EPA) limit value of Hg for drinking water (2 ppb). The high selectivity can be exceptionally achieved by the specific formation of gold amalgam. Moreover, the application for detecting tap water samples further demonstrates that this AuNP-based assay can be an excellent method used for sensing mercury at very low content in the environment. Copyright © 2016 Elsevier B.V. All rights reserved.

Ruthenium nanocatalysis on redox reactions.

PubMed

Veerakumar, Pitchaimani; Ramdass, Arumugam; Rajagopal, Seenivasan

2013-07-01

Nanoparticles have generated intense interest over the past 20 years due to their high potential applications in different areas such as catalysis, sensors, nanoscale electronics, fuel and solar cells and optoelectronics. As the large fractions of metal atoms are exposed to the surface, the use of metal nanoparticles as nanocatalysts allows mild reaction conditions and high catalytic efficiency in a large number of chemical transformations. They have emerged as sustainable heterogeneous catalysts and catalyst supports alternative to conventional materials. This review focuses on the synthesis, characterization and catalytic role of ruthenium nanoparticles (RuNPs) on the redox reactions of heteroatom containing organic compounds with the green reagent H2O2, a field that has attracted immense interest among the chemical, materials and industrial communities. We intend to present a broad overview of Ru nanocatalysts for redox reactions with an emphasis on their performance, stability and reusability. The growth in the chemistry of organic sulfoxides and N-oxides during last decade was due to their importance as synthetic intermediates for the production of a wide range of chemically and biologically active molecules. Thus design of efficient methods for the synthesis of sulfoxides and N-oxides becomes important. This review concentrates on the catalysis of RuNPs on the H2O2 oxidation of organic sulfides to sulfoxides and amines to N-oxides. The deoxygenation reactions of sulfoxides to sulfides and reduction of nitro compounds to amines are fundamental reactions in both chemistry and biology. Here, we also highlight the catalysis of metal nanoparticles on the deoxygenation of sulfoxides and sulfones and reduction of nitro compounds with particular emphasis on the mechanistic aspects.

An MM and QM Study of Biomimetic Catalysis of Diels-Alder Reactions Using Cyclodextrins

PubMed Central

Chen, Wei; Sun, Lipeng; Tang, Zhiye; Ali, Zulfikhar A.; Wong, Bryan M.; Chang, Chia-en A.

2018-01-01

We performed a computational investigation of the mechanism by which cyclodextrins (CDs) catalyze Diels-Alder reactions between 9-anthracenemethanol and N-cyclohexylmaleimide. Hydrogen bonds (Hbonds) between N-cyclohexylmaleimide and the hydroxyl groups of cyclodextrins were suggested to play an important role in this catalytic process. However, our free energy calculations and molecular dynamics simulations showed that these Hbonds are not stable, and quantum mechanical calculations suggested that the reaction is not promoted by these Hbonds. The binding of 9-anthracenemethanol and N-cyclohexylmaleimide to cyclodextrins was the key to the catalytic process. Cyclodextrins act as a container to hold the two reactants in the cavity, pre-organize them for the reactions, and thus reduce the entropy penalty to the activation free energy. Dimethyl-β-CD was a better catalyst for this specific reaction than β-CD because of its stronger van der Waals interaction with the pre-organized reactants and its better performance in reducing the activation energy. This computational work sheds light on the mechanism of the catalytic reaction by cyclodextrins and introduces new perspectives of supramolecular catalysis. PMID:29938117

C3N4-H5PMo10V2O40: a dual-catalysis system for reductant-free aerobic oxidation of benzene to phenol

NASA Astrophysics Data System (ADS)

Long, Zhouyang; Zhou, Yu; Chen, Guojian; Ge, Weilin; Wang, Jun

2014-01-01

Hydroxylation of benzene is a widely studied atom economical and environmental benign reaction for producing phenol, aiming to replace the existing three-step cumene process. Aerobic oxidation of benzene with O2 is an ideal and dream process, but benzene and O2 are so inert that current systems either require expensive noble metal catalysts or wasteful sacrificial reducing agents; otherwise, phenol yields are extremely low. Here we report a dual-catalysis non-noble metal system by simultaneously using graphitic carbon nitride (C3N4) and Keggin-type polyoxometalate H5PMo10V2O40 (PMoV2) as catalysts, showing an exceptional activity for reductant-free aerobic oxidation of benzene to phenol. The dual-catalysis mechanism results in an unusual route to create phenol, in which benzene is activated on the melem unit of C3N4 and O2 by the V-O-V structure of PMoV2. This system is simple, highly efficient and thus may lead the one-step production of phenol from benzene to a more practical pathway.

C3N4-H5PMo10V2O40: a dual-catalysis system for reductant-free aerobic oxidation of benzene to phenol

PubMed Central

Long, Zhouyang; Zhou, Yu; Chen, Guojian; Ge, Weilin; Wang, Jun

2014-01-01

Hydroxylation of benzene is a widely studied atom economical and environmental benign reaction for producing phenol, aiming to replace the existing three-step cumene process. Aerobic oxidation of benzene with O2 is an ideal and dream process, but benzene and O2 are so inert that current systems either require expensive noble metal catalysts or wasteful sacrificial reducing agents; otherwise, phenol yields are extremely low. Here we report a dual-catalysis non-noble metal system by simultaneously using graphitic carbon nitride (C3N4) and Keggin-type polyoxometalate H5PMo10V2O40 (PMoV2) as catalysts, showing an exceptional activity for reductant-free aerobic oxidation of benzene to phenol. The dual-catalysis mechanism results in an unusual route to create phenol, in which benzene is activated on the melem unit of C3N4 and O2 by the V-O-V structure of PMoV2. This system is simple, highly efficient and thus may lead the one-step production of phenol from benzene to a more practical pathway. PMID:24413448

Frustrated Lewis pairs: from concept to catalysis.

PubMed

Stephan, Douglas W

2015-02-17

CONSPECTUS: Frustrated Lewis pair (FLP) chemistry has emerged in the past decade as a strategy that enables main-group compounds to activate small molecules. This concept is based on the notion that combinations of Lewis acids and bases that are sterically prevented from forming classical Lewis acid-base adducts have Lewis acidity and basicity available for interaction with a third molecule. This concept has been applied to stoichiometric reactivity and then extended to catalysis. This Account describes three examples of such developments: hydrogenation, hydroamination, and CO2 reduction. The most dramatic finding from FLP chemistry was the discovery that FLPs can activate H2, thus countering the long-existing dogma that metals are required for such activation. This finding of stoichiometric reactivity was subsequently evolved to employ simple main-group species as catalysts in hydrogenations. While the initial studies focused on imines, subsequent studies uncovered FLP catalysts for a variety of organic substrates, including enamines, silyl enol ethers, olefins, and alkynes. Moreover, FLP reductions of aromatic anilines and N-heterocycles have been developed, while very recent extensions have uncovered the utility of FLP catalysts for ketone reductions. FLPs have also been shown to undergo stoichiometric reactivity with terminal alkynes. Typically, either deprotonation or FLP addition reaction products are observed, depending largely on the basicity of the Lewis base. While a variety of acid/base combinations have been exploited to afford a variety of zwitterionic products, this reactivity can also be extended to catalysis. When secondary aryl amines are employed, hydroamination of alkynes can be performed catalytically, providing a facile, metal-free route to enamines. In a similar fashion, initial studies of FLPs with CO2 demonstrated their ability to capture this greenhouse gas. Again, modification of the constituents of the FLP led to the discovery of reaction

Heteromultimetallic catalysis for sustainable organic syntheses.

PubMed

Lorion, Mélanie M; Maindan, Karan; Kapdi, Anant R; Ackermann, Lutz

2017-12-07

Fully complementary bimetallic catalysis has been identified as an increasingly powerful tool for molecular transformations, which was largely inspired by early examples of sequential catalytic transformations. Thus, energy-efficient one-pot reactions involving different metal catalysts orchestrated in concert constitute an attractive alternative to multi-step protocols, with major recent progress through the elegant ligand design in heterobimetallic catalysis as well as sustainable photo-induced C-H transformations, among others. This review provides a critical assessment of the state of the art in heterobimetallic catalysis for sustainable organic syntheses (SOS), highlighting key advances and representative examples until summer 2017.

Catalysis of Dialanine Formation by Glycine in the Salt-Induced Peptide Formation Reaction.

NASA Astrophysics Data System (ADS)

Suwannachot, Yuttana; Rode, Bernd M.

1998-02-01

Mutual catalysis of amino acids in the salt-induced peptide formation (SIPF) reaction is demonstrated for the case of glycine/alanine. The presence of glycine enhances dialanine formation by a factor up to 50 and enables dialanine formation at much lower alanine concentrations. The actual amounts of glycine play an important role for this catalytic effect, the optimal glycine concentration is 1/8 of the alanine concentration. The mechanism appears to be based on the formation of the intermediate Gly-Ala-Ala tripeptide, connected to one coordination site of copper(II) ion, and subsequent hydrolysis to dialanine and glycine.

Catalysis of dialanine formation by glycine in the salt-induced peptide formation reaction.

PubMed

Suwannachot, Y; Rode, B M

1998-02-01

Mutual catalysis of amino acids in the salt-induced peptide formation (SIPF) reaction is demonstrated for the case of glycine/alanine. The presence of glycine enhances dialanine formation by a factor up to 50 and enables dialanine formation at much lower alanine concentrations. The actual amounts of glycine play an important role for this catalytic effect, the optimal glycine concentration is 1/8 of the alanine concentration. The mechanism appears to be based on the formation of the intermediate Gly-Ala-Ala tripeptide, connected to one coordination site of copper(II) ion, and subsequent hydrolysis to dialanine and glycine.

Frontiers, Opportunities, and Challenges in Biochemical and Chemical Catalysis of CO2

DOE Office of Scientific and Technical Information (OSTI.GOV)

Appel, Aaron M.; Bercaw, John E.; Bocarsly, Andrew B.

2013-08-14

Our central premise is that catalytic scientists can learn by studying how these important metabolic processes occur in nature. Complementarily, biochemists can learn by studying how catalytic scientists view these same chemical transformations promoted by synthetic catalysts. From these studies, hypotheses can be developed and tested through manipulation of enzyme structure and by synthesizing simple molecular catalysts to incorporate different structural features of the enzymes. It is hoped that these studies will lead to new and useful concepts in catalyst design for fuel production and utilization. This paper describes the results of a workshop held to explore these concepts inmore » regard to the development of new and more efficient catalytic processes for the conversion of CO2 to a variety of carbon-based fuels. The organization of this overview/review is as follows: 1) The first section briefly explores how interactions between the catalysis and biological communities have been fruitful in developing new catalysts for the reduction of protons to hydrogen, the simplest fuel generation reaction. 2) The second section assesses the state of the art in both biological and chemical reduction of CO2 by two electrons to form either carbon monoxide (CO) or formate (HCOO-). It also attempts to identify common principles between biological and synthetic catalysts and productive areas for future research. 3) The third section explores both biological and chemical processes that result in the reduction of CO2 beyond the level of CO and formate, again seeking to identify common principles and productive areas of future research. 4) The fourth section explores the formation of carbon-carbon bonds in biological and chemical systems in the same vein as the other sections. 5) A fifth section addresses the role of non-redox reactions of CO2 in biological systems and their role in carbon metabolism, with a parallel discussion of chemical systems. 6) In section 6, the topics

Parallel array of nanochannels grafted with polymer-brushes-stabilized Au nanoparticles for flow-through catalysis.

PubMed

Liu, Jianxi; Ma, Shuanhong; Wei, Qiangbing; Jia, Lei; Yu, Bo; Wang, Daoai; Zhou, Feng

2013-12-07

Smart systems on the nanometer scale for continuous flow-through reaction present fascinating advantages in heterogeneous catalysis, in which a parallel array of straight nanochannels offers a platform with high surface area for assembling and stabilizing metallic nanoparticles working as catalysts. Herein we demonstrate a method for finely modifying the nanoporous anodic aluminum oxide (AAO), and further integration of nanoreactors. By using atomic transfer radical polymerization (ATRP), polymer brushes were successfully grafted on the inner wall of the nanochannels of the AAO membrane, followed by exchanging counter ions with a precursor for nanoparticles (NPs), and used as the template for deposition of well-defined Au NPs. The membrane was used as a functional nanochannel for novel flow-through catalysis. High catalytic performance and instantaneous separation of products from the reaction system was achieved in reduction of 4-nitrophenol.

Parallel array of nanochannels grafted with polymer-brushes-stabilized Au nanoparticles for flow-through catalysis

NASA Astrophysics Data System (ADS)

Liu, Jianxi; Ma, Shuanhong; Wei, Qiangbing; Jia, Lei; Yu, Bo; Wang, Daoai; Zhou, Feng

2013-11-01

Smart systems on the nanometer scale for continuous flow-through reaction present fascinating advantages in heterogeneous catalysis, in which a parallel array of straight nanochannels offers a platform with high surface area for assembling and stabilizing metallic nanoparticles working as catalysts. Herein we demonstrate a method for finely modifying the nanoporous anodic aluminum oxide (AAO), and further integration of nanoreactors. By using atomic transfer radical polymerization (ATRP), polymer brushes were successfully grafted on the inner wall of the nanochannels of the AAO membrane, followed by exchanging counter ions with a precursor for nanoparticles (NPs), and used as the template for deposition of well-defined Au NPs. The membrane was used as a functional nanochannel for novel flow-through catalysis. High catalytic performance and instantaneous separation of products from the reaction system was achieved in reduction of 4-nitrophenol.

Coupling Solar Energy into Reactions: Materials Design for Surface Plasmon-Mediated Catalysis.

PubMed

Long, Ran; Li, Yu; Song, Li; Xiong, Yujie

2015-08-26

Enabled by surface plasmons, noble metal nanostructures can interact with and harvest incident light. As such, they may serve as unique media to generate heat, supply energetic electrons, and provide strong local electromagnetic fields for chemical reactions through different mechanisms. This solar-to-chemical pathway provides a new approach to solar energy utilization, alternative to conventional semiconductor-based photocatalysis. To provide readers with a clear picture of this newly recognized process, this review presents coupling solar energy into chemical reactions through plasmonic nanostructures. It starts with a brief introduction of surface plasmons in metallic nanostructures, followed by a demonstration of tuning plasmonic features by tailoring their physical parameters. Owing to their tunable plasmonic properties, metallic materials offer a platform to trigger and drive chemical reactions at the nanoscale, as systematically overviewed in this article. The design rules for plasmonic materials for catalytic applications are further outlined based on existing examples. At the end of this article, the challenges and opportunities for further development of plasmonic-mediated catalysis toward energy and environmental applications are discussed. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Exploring reaction pathways for O-GlcNAc transferase catalysis. A string method study.

PubMed

Kumari, Manju; Kozmon, Stanislav; Kulhánek, Petr; Štepán, Jakub; Tvaroška, Igor; Koča, Jaroslav

2015-03-26

The inverting O-GlcNAc glycosyltransferase (OGT) is an important post-translation enzyme, which catalyzes the transfer of N-acetylglucosamine from UDP-N-acetylglucosamine (UDP-GlcNAc) to the hydroxyl group of the Ser/Thr of cytoplasmic, nuclear, and mitochondrial proteins. In the past, three different catalytic bases were proposed for the reaction: His498, α-phosphate, and Asp554. In this study, we used hybrid quantum mechanics/molecular mechanics (QM/MM) Car-Parrinello molecular dynamics to investigate reaction paths using α-phosphate and Asp554 as the catalytic bases. The string method was used to calculate the free-energy reaction profiles of the tested mechanisms. During the investigations, an additional mechanism was observed. In this mechanism, a proton is transferred to α-phosphate via a water molecule. Our calculations show that the mechanism with α-phosphate acting as the base is favorable. This reaction has a rate-limiting free-energy barrier of 23.5 kcal/mol, whereas reactions utilizing Asp554 and water-assisted α-phosphate have barriers of 41.7 and 40.9 kcal/mol, respectively. Our simulations provide a new insight into the catalysis of OGT and may thus guide rational drug design of transition-state analogue inhibitors with potential therapeutic use.

Chiral poly-rare earth metal complexes in asymmetric catalysis

PubMed Central

Shibasaki, Masakatsu

2006-01-01

Asymmetric catalysis is a powerful component of modern synthetic organic chemistry. To further broaden the scope and utility of asymmetric catalysis, new basic concepts for the design of asymmetric catalysts are crucial. Because most chemical reactions involve bond-formation between two substrates or moieties, high enantioselectivity and catalyst activity should be realized if an asymmetric catalyst can activate two reacting substrates simultaneously at defined positions. Thus, we proposed the concept of bifunctional asymmetric catalysis, which led us to the design of new asymmetric catalysts containing two functionalities (e.g. a Lewis acid and a Brønsted base or a Lewis acid and a Lewis base). These catalysts demonstrated broad reaction applicability with excellent substrate generality. Using our catalytic asymmetric reactions as keys steps, efficient total syntheses of pharmaceuticals and their biologically active lead natural products were achieved. PMID:25792774

The Development of Visible-Light Photoredox Catalysis in Flow.

PubMed

Garlets, Zachary J; Nguyen, John D; Stephenson, Corey R J

2014-04-01

Visible-light photoredox catalysis has recently emerged as a viable alternative for radical reactions otherwise carried out with tin and boron reagents. It has been recognized that by merging photoredox catalysis with flow chemistry, slow reaction times, lower yields, and safety concerns may be obviated. While flow reactors have been successfully applied to reactions carried out with UV light, only recent developments have demonstrated the same potential of flow reactors for the improvement of visible-light-mediated reactions. This review examines the initial and continuing development of visible-light-mediated photoredox flow chemistry by exemplifying the benefits of flow chemistry compared with conventional batch techniques.

The Development of Visible-Light Photoredox Catalysis in Flow

PubMed Central

Garlets, Zachary J.; Nguyen, John D.

2014-01-01

Visible-light photoredox catalysis has recently emerged as a viable alternative for radical reactions otherwise carried out with tin and boron reagents. It has been recognized that by merging photoredox catalysis with flow chemistry, slow reaction times, lower yields, and safety concerns may be obviated. While flow reactors have been successfully applied to reactions carried out with UV light, only recent developments have demonstrated the same potential of flow reactors for the improvement of visible-light-mediated reactions. This review examines the initial and continuing development of visible-light-mediated photoredox flow chemistry by exemplifying the benefits of flow chemistry compared with conventional batch techniques. PMID:25484447

Enzyme catalysis with small ionic liquid quantities.

PubMed

Fischer, Fabian; Mutschler, Julien; Zufferey, Daniel

2011-04-01

Enzyme catalysis with minimal ionic liquid quantities improves reaction rates, stereoselectivity and enables solvent-free processing. In particular the widely used lipases combine well with many ionic liquids. Demonstrated applications are racemate separation, esterification and glycerolysis. Minimal solvent processing is also an alternative to sluggish solvent-free catalysis. The method allows simplified down-stream processing, as only traces of ionic liquids have to be removed.

Enzyme catalysis in microgravity: steady-state kinetic analysis of the isocitrate lyase reaction.

PubMed

Ranaldi, Francesco; Vanni, Paolo; Giachetti, Eugenio

2003-01-21

Two decades of research in microgravity have shown that certain biochemical processes can be altered by weightlessness. Approximately 10 years ago, our team, supported by the European Space Agency (ESA) and the Agenzia Spaziale Italiana, started the Effect of Microgravity on Enzyme Catalysis project to test the possibility that the microgravity effect observed at cellular level could be mediated by enzyme reactions. An experiment to study the cleavage reaction catalyzed by isocitrate lyase was flown on the sounding rocket MASER 7, and we found that the kinetic parameters were not altered by microgravity. During the 28th ESA parabolic flight campaign, we had the opportunity to replicate the MASER 7 experiment and to perform a complete steady-state analysis of the isocitrate lyase reaction. This study showed that both in microgravity and in standard g controls the enzyme reaction obeyed the same kinetic mechanism and none of the kinetic parameters, nor the equilibrium constant of the overall reaction were altered. Our results contrast with those of a similar experiment, which was performed during the same parabolic flight campaign, and showed that microgravity increased the affinity of lipoxygenase-1 for linoleic acid. The hypotheses suggested to explain this change effect of the latter were here tested by computer simulation, and appeared to be inconsistent with the experimental outcome.

Recent Advances in Nickel Catalysis

PubMed Central

Tasker, Sarah Z.; Standley, Eric A.; Jamison, Timothy F.

2015-01-01

Preface The field of nickel catalysis has made tremendous advances in the past decade. There are several key properties of nickel that have allowed for a broad range of innovative reaction development, such as facile oxidative addition and ready access to multiple oxidation states. In recent years, these properties have been increasingly understood and leveraged to perform transformations long considered exceptionally challenging. Herein, we discuss some of the most recent and significant developments in homogeneous nickel catalysis with an emphasis on both synthetic outcome and mechanism. PMID:24828188

From containers to catalysts: supramolecular catalysis within cucurbiturils.

PubMed

Pemberton, Barry C; Raghunathan, Ramya; Volla, Sabine; Sivaguru, Jayaraman

2012-09-24

Cucurbiturils are a family of molecular container compounds with superior molecular recognition properties. The use of cucurbiturils for supramolecular catalysis is highlighted in this concept. Both photochemical reactions as well as thermal transformations are reviewed with an eye towards tailoring substrates for supramolecular catalysis mediated by cucurbiturils. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Solar fuels generation and molecular systems: is it homogeneous or heterogeneous catalysis?

PubMed

Artero, Vincent; Fontecave, Marc

2013-03-21

Catalysis is a key enabling technology for solar fuel generation. A number of catalytic systems, either molecular/homogeneous or solid/heterogeneous, have been developed during the last few decades for both the reductive and oxidative multi-electron reactions required for fuel production from water or CO(2) as renewable raw materials. While allowing for a fine tuning of the catalytic properties through ligand design, molecular approaches are frequently criticized because of the inherent fragility of the resulting catalysts, when exposed to extreme redox potentials. In a number of cases, it has been clearly established that the true catalytic species is heterogeneous in nature, arising from the transformation of the initial molecular species, which should rather be considered as a pre-catalyst. Whether such a situation is general or not is a matter of debate in the community. In this review, covering water oxidation and reduction catalysts, involving noble and non-noble metal ions, we limit our discussion to the cases in which this issue has been directly and properly addressed as well as those requiring more confirmation. The methodologies proposed for discriminating homogeneous and heterogeneous catalysis are inspired in part by those previously discussed by Finke in the case of homogeneous hydrogenation reaction in organometallic chemistry [J. A. Widegren and R. G. Finke, J. Mol. Catal. A, 2003, 198, 317-341].

Homogeneous Catalysis by Transition Metal Compounds.

ERIC Educational Resources Information Center

Mawby, Roger

1988-01-01

Examines four processes involving homogeneous catalysis which highlight the contrast between the simplicity of the overall reaction and the complexity of the catalytic cycle. Describes how catalysts provide circuitous routes in which all energy barriers are relatively low rather than lowering the activation energy for a single step reaction.…

TePtFe Nanotubes as High-Performing Bifunctional Electrocatalysts for the Oxygen Reduction Reaction and Hydrogen Evolution Reaction.

PubMed

Li, Wenqiang; Amiinu, Ibrahim Saana; Ye, Bei; Wang, Zhe; Zhu, Jiawei; Kou, Zongkui; Mu, Shichun

2018-04-25

Currently, a multicomponent platinum-based alloy has been applied as a promising electrocatalyst to improve catalysis and lower the usage of the noble metal platinum. Herein, a tellurium nanowire (NW)-derived ternary TePtFe nanotube (NT) electrocatalyst has been prepared by the Kirkendall effect. The TePtFe NT formed consists of small single-crystal nanoparticles and voids with an open-end and hollow structure. The TePtFe NT electrocatalyst presents an impressive catalytic activity and stability for both the oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). Its ORR specific activity and mass activity are 8.5 and 2.4 times, respectively, improved relative to those of commercial platinum catalysts. It is also impressive that, for the HER, a very low overpotential of 28.1 mV at 10 mA cm -2 can be achieved; this is lower than that of platinum (51.8 mV) catalysts in 0.1 m HClO 4 , and the activity is improved, even after 5000 cycles. This work reveals that TePtFe NTs can be employed as nanocatalysts with an impressive catalytic activity and stability for application in fuel cells and hydrogen production. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Electric Fields and Enzyme Catalysis

PubMed Central

Fried, Stephen D.; Boxer, Steven G.

2017-01-01

What happens inside an enzyme’s active site to allow slow and difficult chemical reactions to occur so rapidly? This question has occupied biochemists’ attention for a long time. Computer models of increasing sophistication have predicted an important role for electrostatic interactions in enzymatic reactions, yet this hypothesis has proved vexingly difficult to test experimentally. Recent experiments utilizing the vibrational Stark effect make it possible to measure the electric field a substrate molecule experiences when bound inside its enzyme’s active site. These experiments have provided compelling evidence supporting a major electrostatic contribution to enzymatic catalysis. Here, we review these results and develop a simple model for electrostatic catalysis that enables us to incorporate disparate concepts introduced by many investigators to describe how enzymes work into a more unified framework stressing the importance of electric fields at the active site. PMID:28375745

Seventh BES (Basic Energy Sciences) catalysis and surface chemistry research conference

DOE Office of Scientific and Technical Information (OSTI.GOV)

Not Available

1990-03-01

Research programs on catalysis and surface chemistry are presented. A total of fifty-seven topics are included. Areas of research include heterogeneous catalysis; catalysis in hydrogenation, desulfurization, gasification, and redox reactions; studies of surface properties and surface active sites; catalyst supports; chemical activation, deactivation; selectivity, chemical preparation; molecular structure studies; sorption and dissociation. Individual projects are processed separately for the data bases. (CBS)

First principles investigation of heterogeneous catalysis on metal oxide surfaces

NASA Astrophysics Data System (ADS)

Ghoussoub, Mireille

Metal oxides possess unique electronic and structural properties that render them highly favourable for applications in heterogeneous catalysis. In this study, computational atomistic modelling based on Density Functional Theory was used to investigate the reduction of carbon dioxide over hydroxylated indium oxide nanoparticles, as well at the activation of methane over oxygen-covered bimetallic surfaces. The first study employed metadynamics-biased ab initio molecular dynamics to obtain the free energy surface of the various reaction steps at finite temperature. In the second study, the nudged elastic band method was used to probe the C-H activation mechanisms for different surface configurations. In both cases, activation energies, reaction energies, transition state structures, and charge analysis results are used to explain the underlying mechanistic pathways.

Heterogeneous Diels–Alder catalysis for biomass-derived aromatic compounds

DOE PAGES

Settle, Amy E.; Berstis, Laura; Rorrer, Nicholas A.; ...

2017-05-17

In this tutorial review, we provide an overview of heterogeneous Diels–Alder catalysis for the production of lignocellulosic biomass-derived aromatic compounds. Diels–Alder reactions afford an extremely selective and efficient route for carbon–carbon cycloadditions to produce intermediates that can readily undergo subsequent dehydration or dehydrogenation reactions for aromatization. As a result, catalysis of Diels–Alder reactions with biomass-derived dienes and dienophiles has seen a growth of interest in recent years; however, significant opportunities remain to (i) tailor heterogeneous catalyst materials for tandem Diels–Alder and aromatization reactions, and (ii) utilize biomass-derived dienes and dienophiles to access both conventional and novel aromatic monomers. As such,more » this review discusses the mechanistic aspects of Diels–Alder reactions from both an experimental and computational perspective, as well as the synergy of Brønsted–Lewis acid catalysts to facilitate tandem Diels–Alder and aromatization reactions. Heterogeneous catalyst design strategies for Diels–Alder reactions are reviewed for two exemplary solid acid catalysts, zeolites and polyoxometalates, and recent efforts for targeting direct replacement aromatic monomers from biomass are summarized. In conclusion, we point out important research directions for progressing Diels–Alder catalysis to target novel, aromatic monomers with chemical functionality that enables new properties compared to monomers that are readily accessible from petroleum.« less

Tandem catalysis: a new approach to polymers.

PubMed

Robert, Carine; Thomas, Christophe M

2013-12-21

The creation of polymers by tandem catalysis represents an exciting frontier in materials science. Tandem catalysis is one of the strategies used by Nature for building macromolecules. Living organisms generally synthesize macromolecules by in vivo enzyme-catalyzed chain growth polymerization reactions using activated monomers that have been formed within cells during complex metabolic processes. However, these biological processes rely on highly complex biocatalysts, thus limiting their industrial applications. In order to obtain polymers by tandem catalysis, homogeneous and enzyme catalysts have played a leading role in the last two decades. In the following feature article, we will describe selected published efforts to achieve these research goals.

Catalysis in prebiotic chemistry RNA synthesis

NASA Astrophysics Data System (ADS)

Ferris, J.; Joshi, P.; Wang, K.; Huang, W.; Miyakawa, S.

It is proposed that catalysis by minerals and metal ions had a central role in the steps that led to the origins of life. In particular, the formation of biopolymers in the presence of water requires catalysis to compete with hydrolytic processes. Catalysis is required to limit the number of isomers generated so that the longer polymers necessary for the origins of life formed. Montmorillonite clay catalyzes the formation of 6 14 mers of RNA from activated monomers of A, G, U and C in- aqueous solution. Daily addition of activated monomers to a 10 mer primer results in the formation of 40-50 mers of adenylic acid and 30 mers of uridylic acid. The sequence selectivity and regioselectivity in phosphodiester bond formation results from the montmorillonite catalysis. Reaction of D, L-activated monomers of A and U leads to the preferential formation of homochiral dimers (eg. D, D and L, L-- pApA). These data and any more recent developments will be discussed.

Photoredox Catalysis in Organic Chemistry

PubMed Central

2016-01-01

In recent years, photoredox catalysis has come to the forefront in organic chemistry as a powerful strategy for the activation of small molecules. In a general sense, these approaches rely on the ability of metal complexes and organic dyes to convert visible light into chemical energy by engaging in single-electron transfer with organic substrates, thereby generating reactive intermediates. In this Perspective, we highlight the unique ability of photoredox catalysis to expedite the development of completely new reaction mechanisms, with particular emphasis placed on multicatalytic strategies that enable the construction of challenging carbon–carbon and carbon–heteroatom bonds. PMID:27477076

Carbene-catalysed reductive coupling of nitrobenzyl bromides and activated ketones or imines via single-electron-transfer process

PubMed Central

Li, Bao-Sheng; Wang, Yuhuang; Proctor, Rupert S. J.; Zhang, Yuexia; Webster, Richard D.; Yang, Song; Song, Baoan; Chi, Yonggui Robin

2016-01-01

Benzyl bromides and related molecules are among the most common substrates in organic synthesis. They are typically used as electrophiles in nucleophilic substitution reactions. These molecules can also be activated via single-electron-transfer (SET) process for radical reactions. Representative recent progress includes α-carbon benzylation of ketones and aldehydes via photoredox catalysis. Here we disclose the generation of (nitro)benzyl radicals via N-heterocyclic carbene (NHC) catalysis under reductive conditions. The radical intermediates generated via NHC catalysis undergo formal 1,2-addition with ketones to eventually afford tertiary alcohol products. The overall process constitutes a formal polarity-inversion of benzyl bromide, allowing a direct coupling of two initially electrophilic carbons. Our study provides a new carbene-catalysed reaction mode that should enable unconventional transformation of (nitro)benzyl bromides under mild organocatalytic conditions. PMID:27671606

Carbene-catalysed reductive coupling of nitrobenzyl bromides and activated ketones or imines via single-electron-transfer process

NASA Astrophysics Data System (ADS)

Li, Bao-Sheng; Wang, Yuhuang; Proctor, Rupert S. J.; Zhang, Yuexia; Webster, Richard D.; Yang, Song; Song, Baoan; Chi, Yonggui Robin

2016-09-01

Benzyl bromides and related molecules are among the most common substrates in organic synthesis. They are typically used as electrophiles in nucleophilic substitution reactions. These molecules can also be activated via single-electron-transfer (SET) process for radical reactions. Representative recent progress includes α-carbon benzylation of ketones and aldehydes via photoredox catalysis. Here we disclose the generation of (nitro)benzyl radicals via N-heterocyclic carbene (NHC) catalysis under reductive conditions. The radical intermediates generated via NHC catalysis undergo formal 1,2-addition with ketones to eventually afford tertiary alcohol products. The overall process constitutes a formal polarity-inversion of benzyl bromide, allowing a direct coupling of two initially electrophilic carbons. Our study provides a new carbene-catalysed reaction mode that should enable unconventional transformation of (nitro)benzyl bromides under mild organocatalytic conditions.

Green Chemistry by Nano-Catalysis

EPA Science Inventory

The approach of using MW technique with nano-catalysis and benign aqueous reaction medium can offer an extraordinary synergistic effect with greater potential than these three individual components in isolation. To illustrate the ‘‘proof-of-concept’’ of this “Green and Sustainabl...

Reduction of chemical reaction models

NASA Technical Reports Server (NTRS)

Frenklach, Michael

1991-01-01

An attempt is made to reconcile the different terminologies pertaining to reduction of chemical reaction models. The approaches considered include global modeling, response modeling, detailed reduction, chemical lumping, and statistical lumping. The advantages and drawbacks of each of these methods are pointed out.

Lewis acid-Lewis acid heterobimetallic cooperative catalysis: mechanistic studies and application in enantioselective aza-Michael reaction.

PubMed

Yamagiwa, Noriyuki; Qin, Hongbo; Matsunaga, Shigeki; Shibasaki, Masakatsu

2005-09-28

The full details of a catalytic asymmetric aza-Michael reaction of methoxylamine promoted by rare earth-alkali metal heterobimetallic complexes are described, demonstrating the effectiveness of Lewis acid-Lewis acid cooperative catalysis. First, enones were used as substrates, and the 1,4-adducts were obtained in good yield (57-98%) and high ee (81-96%). Catalyst loading was successfully reduced to 0.3-3 mol % with enones. To broaden the substrate scope of the reaction to carboxylic acid derivatives, alpha,beta-unsaturated N-acylpyrroles were used as monodentate, carboxylic acid derivatives. With beta-alkyl-substituted N-acylpyrroles, the reaction proceeded smoothly and the products were obtained in high yield and good ee. Transformation of the 1,4-adducts from enones and alpha,beta-unsaturated N-acylpyrroles afforded corresponding chiral aziridines and beta-amino acids. Detailed mechanistic studies, including kinetics, NMR analysis, nonlinear effects, and rare earth metal effects, are also described. The Lewis acid-Lewis acid cooperative mechanism, including the substrate coordination mode, is discussed in detail.

Recent advances in heterobimetallic catalysis across a "transition metal-tin" motif.

PubMed

Das, Debjit; Mohapatra, Swapna Sarita; Roy, Sujit

2015-06-07

Heterobimetallic catalysts, bearing a metal-metal bond between a transition metal (TM) and a tin atom, are very promising due to their ability in mediating a wide variety of organic transformations. Indeed the utilization of such catalysts is a challenging and evolving area in the field of homogeneous catalysis. Catalysis across a 'TM-Sn' motif is an emerging area in the broader domain of multimetallic catalysis. The present review apprises the chemists' community of the past, present and future scope of this versatile catalytic motif. The TM-Sn catalyzed reactions presented include, among others, Friedel-Crafts alkylation, carbonylation, polymerization, cyclization, olefin metathesis, Heck coupling, hydroarylation Michael addition and tandem coupling. The mechanistic aspects of the reactions have been highlighted as well.

Flexibility, Diversity, and Cooperativity: Pillars of Enzyme Catalysis

PubMed Central

Hammes, Gordon G.; Benkovic, Stephen J.; Hammes-Schiffer, Sharon

2011-01-01

This brief review discusses our current understanding of the molecular basis of enzyme catalysis. A historical development is presented, beginning with steady state kinetics and progressing through modern fast reaction methods, NMR, and single molecule fluorescence techniques. Experimental results are summarized for ribonuclease, aspartate aminotransferase, and especially dihydrofolate reductase (DHFR). Multiple intermediates, multiple conformations, and cooperative conformational changes are shown to be an essential part of virtually all enzyme mechanisms. In the case of DHFR, theoretical investigations have provided detailed information about the movement of atoms within the enzyme-substrate complex as the reaction proceeds along the collective reaction coordinate for hydride transfer. A general mechanism is presented for enzyme catalysis that includes multiple intermediates and a complex, multidimensional standard free energy surface. Protein flexibility, diverse protein conformations, and cooperative conformational changes are important features of this model. PMID:22029278

Polymer-silica hybrids for separation of CO2 and catalysis of organic reactions

NASA Astrophysics Data System (ADS)

Silva Mojica, Ernesto

Porous materials comprising polymeric and inorganic segments have attracted interest from the scientific community due to their unique properties and functionalities. The physical and chemical characteristics of these materials can be effectively exploited for adsorption applications. This dissertation covers the experimental techniques for fabrication of poly(vinyl alcohol) (PVA) and silica (SiO2) porous supports, and their functionalization with polyamines for developing adsorbents with potential applications in separation of CO2 and catalysis of organic reactions. The supports were synthesized by processes involving (i) covalent cross-linking of PVA, (ii) hydrolysis and poly-condensation of silica precursors (i,e,. sol-gel synthesis), and formation of porous structures via (iii) direct templating and (iv) phase inversion techniques. Their physical structure was controlled by the proper combination of the preparation procedures, which resulted in micro-structured porous materials in the form of micro-particles, membranes, and pellets. Their adsorption characteristics were tailored by functionalization with polyethyleneimine (PEI), and their physicochemical properties were characterized by vibrational spectroscopy (FTIR, UV-vis), microscopy (SEM), calorimetry (TGA, DSC), and adsorption techniques (BET, step-switch adsorption). Spectroscopic investigations of the interfacial cross-linking reactions of PEI and PVA with glutaraldehyde (GA) revealed that PEI catalyzes the cross-linking reactions of PVA in absence of external acid catalysts. In-situ IR spectroscopy coupled with a focal plane array (FPA) image detector allowed the characterization of a gradient interface on a PEI/PVA composite membrane and the investigation of the cross-linking reactions as a function of time and position. The results served as a basis to postulate possible intermediates, and propose the reaction mechanisms. The formulation of amine-functionalized CO2 capture sorbents was based on the

Recent progress in asymmetric bifunctional catalysis using multimetallic systems.

PubMed

Shibasaki, Masakatsu; Kanai, Motomu; Matsunaga, Shigeki; Kumagai, Naoya

2009-08-18

The concept of bifunctional catalysis, wherein both partners of a bimolecular reaction are simultaneously activated, is very powerful for designing efficient asymmetric catalysts. Catalytic asymmetric processes are indispensable for producing enantiomerically enriched compounds in modern organic synthesis, providing more economical and environmentally benign results than methods requiring stoichiometric amounts of chiral reagents. Extensive efforts in this field have produced many asymmetric catalysts, and now a number of reactions can be rendered asymmetric. We have focused on the development of asymmetric catalysts that exhibit high activity, selectivity, and broad substrate generality under mild reaction conditions. Asymmetric catalysts based on the concept of bifunctional catalysis have emerged as a particularly effective class, enabling simultaneous activation of multiple reaction components. Compared with conventional catalysts, bifunctional catalysts generally exhibit enhanced catalytic activity and higher levels of stereodifferentiation under milder reaction conditions, attracting much attention as next-generation catalysts for prospective practical applications. In this Account, we describe recent advances in enantioselective catalysis with bifunctional catalysts. Since our identification of heterobimetallic rare earth-alkali metal-BINOL (REMB) complexes, we have developed various types of bifunctional multimetallic catalysts. The REMB catalytic system is effective for catalytic asymmetric Corey-Chaykovsky epoxidation and cyclopropanation. A dinucleating Schiff base has emerged as a suitable multidentate ligand for bimetallic catalysts, promoting catalytic syn-selective nitro-Mannich, anti-selective nitroaldol, and Mannich-type reactions. The sugar-based ligand GluCAPO provides a suitable platform for polymetallic catalysts; structural elucidation revealed that their higher order polymetallic structures are a determining factor for their function in the

Supramolecular Approaches To Control Activity and Selectivity in Hydroformylation Catalysis

PubMed Central

2018-01-01

The hydroformylation reaction is one of the most intensively explored reactions in the field of homogeneous transition metal catalysis, and many industrial applications are known. However, this atom economical reaction has not been used to its full potential, as many selectivity issues have not been solved. Traditionally, the selectivity is controlled by the ligand that is coordinated to the active metal center. Recently, supramolecular strategies have been demonstrated to provide powerful complementary tools to control activity and selectivity in hydroformylation reactions. In this review, we will highlight these supramolecular strategies. We have organized this paper in sections in which we describe the use of supramolecular bidentate ligands, substrate preorganization by interactions between the substrate and functional groups of the ligands, and hydroformylation catalysis in molecular cages. PMID:29657887

Metallic nanocatalysts for electrochemical CO2 reduction in aqueous solutions.

PubMed

Wang, Yuanxing; Niu, Cailing; Wang, Dunwei

2018-05-16

How to effectively and efficiently reduce carbon dioxide (CO 2 ) to value-added chemicals represent a frontier in catalysis research. Due to the high activation energy needs and the endothermic nature of CO 2 reduction, the reactions are difficult to carry out. When H 2 O is present, hydrogen evolution reactions (HER) often compete favorably with CO 2 reduction reactions. For these reactions, catalysts are of critical importance to CO 2 reduction. In this article, we review the various metal nanocatalysts for electrochemical CO 2 reduction (ECR) reactions. In recognition of the importance of H 2 O to CO 2 reduction, we focus our discussions on systems in aqueous solutions. Nanostructured metal catalysts are chosen for the discussions because they represent the most effective catalysts for ECR. After a brief introduction of the fundamental principles of ECR, we devote the rest of the article on the discussions of various types of nanostructured metallic catalysts, which are categorized by their compositions and working mechanisms. Lastly, strategies for improving reaction efficiency and selectivity are discussed. Copyright © 2018 Elsevier Inc. All rights reserved.

RNA catalysis and the origins of life

NASA Technical Reports Server (NTRS)

Orgel, Leslie E.

1986-01-01

The role of RNA catalysis in the origins of life is considered in connection with the discovery of riboszymes, which are RNA molecules that catalyze sequence-specific hydrolysis and transesterification reactions of RNA substrates. Due to this discovery, theories positing protein-free replication as preceding the appearance of the genetic code are more plausible. The scope of RNA catalysis in biology and chemistry is discussed, and it is noted that the development of methods to select (or predict) RNA sequences with preassigned catalytic functions would be a major contribution to the study of life's origins.

Tandem catalysis of ring-closing metathesis/atom transfer radical reactions with homobimetallic ruthenium–arene complexes

PubMed Central

Borguet, Yannick; Sauvage, Xavier; Zaragoza, Guillermo; Demonceau, Albert

2010-01-01

Summary The tandem catalysis of ring-closing metathesis/atom transfer radical reactions was investigated with the homobimetallic ruthenium–indenylidene complex [(p-cymene)Ru(μ-Cl)3RuCl(3-phenyl-1-indenylidene)(PCy3)] (1) to generate active species in situ. The two catalytic processes were first carried out independently in a case study before the whole sequence was optimized and applied to the synthesis of several polyhalogenated bicyclic γ-lactams and lactones from α,ω-diene substrates bearing trihaloacetamide or trichloroacetate functionalities. The individual steps were carefully monitored by 1H and 31P NMR spectroscopies in order to understand the intimate details of the catalytic cycles. Polyhalogenated substrates and the ethylene released upon metathesis induced the clean transformation of catalyst precursor 1 into the Ru(II)–Ru(III) mixed-valence compound [(p-cymene)Ru(μ-Cl)3RuCl2(PCy3)], which was found to be an efficient promoter for atom transfer radical reactions under the adopted experimental conditions. PMID:21160564

Enzyme catalysis-electrophoresis titration for multiplex enzymatic assay via moving reaction boundary chip.

PubMed

Zhong, Ran; Xie, Haiyang; Kong, Fanzhi; Zhang, Qiang; Jahan, Sharmin; Xiao, Hua; Fan, Liuyin; Cao, Chengxi

2016-09-21

In this work, we developed the concept of enzyme catalysis-electrophoresis titration (EC-ET) under ideal conditions, the theory of EC-ET for multiplex enzymatic assay (MEA), and a related method based on a moving reaction boundary (MRB) chip with a collateral channel and cell phone imaging. As a proof of principle, the model enzymes horseradish peroxidase (HRP), laccase and myeloperoxidase (MPO) were chosen for the tests of the EC-ET model. The experiments revealed that the EC-ET model could be achieved via coupling EC with ET within a MRB chip; particularly the MEA analyses of catalysis rate, maximum rate, activity, Km and Kcat could be conducted via a single run of the EC-ET chip, systemically demonstrating the validity of the EC-ET theory. Moreover, the developed method had these merits: (i) two orders of magnitude higher sensitivity than a fluorescence microplate reader, (ii) simplicity and low cost, and (iii) fairly rapid (30 min incubation, 20 s imaging) analysis, fair stability (<5.0% RSD) and accuracy, thus validating the EC-ET method. Finally, the developed EC-ET method was used for the clinical assay of MPO activity in blood samples; the values of MPO activity detected via the EC-ET chip were in agreement with those obtained by a traditional fluorescence microplate reader, indicating the applicability of the EC-ET method. The work opens a window for the development of enzymatic research, enzyme assay, immunoassay, and point-of-care testing as well as titration, one of the oldest methods of analysis, based on a simple chip.

Bite angle effects of diphosphines in C-C and C-X bond forming cross coupling reactions.

PubMed

Birkholz, Mandy-Nicole; Freixa, Zoraida; van Leeuwen, Piet W N M

2009-04-01

Catalytic reactions of C-C and C-X bond formation are discussed in this critical review with particular emphasis on cross coupling reactions catalyzed by palladium and wide bite angle bidentate diphosphine ligands. Especially those studies have been collected that allow comparison of the ligand bite angles for the selected ligands: dppp, BINAP, dppf, DPEphos and Xantphos. Similarities with hydrocyanation and CO/ethene/MeOH reactions have been highlighted, while rhodium hydroformylation has been mentioned as a contrasting example, in which predictability is high and steric and electronic effects follow smooth trends. In palladium catalysis wide bite angles and bulkiness of the ligands facilitate generally the reductive elimination thus giving more efficient cross coupling catalysis (174 references).

Acid–base catalysis over perovskites: a review

DOE Office of Scientific and Technical Information (OSTI.GOV)

Polo-Garzon, Felipe; Wu, Zili

We present that perovskite catalysts have been extensively studied for reduction–oxidation (redox) reactions; however, their acid–base catalytic properties are still under-explored. This review collects work aiming to study the acid–base catalytic properties of perovskites. Reports regarding combined acid–base/redox catalysis over perovskites lie beyond the scope of the present review. For the characterization of acid–base properties, researchers have studied the interaction of probe molecules with perovskite surfaces by means of multiple techniques that provide information about the density, strength and type of adsorption sites. The top-surface composition of perovskites, which relates to the abundance of the acid–base sites, has been studiedmore » by means of low energy ion scattering (LEIS), and, the less surface sensitive, conventional X-ray photoelectron spectroscopy (XPS). Probe reactions, with the conversion of 2-propanol as the common choice, have also been employed for characterizing the acid–base catalytic properties of perovskites. The complex nature of perovskite surfaces, which explains the still absent fundamental relations between the structure of the catalyst and reaction rates/selectivity, encounters a great challenge due to the surface reconstruction of these materials. In this review, we devote a special section to highlight recent publications that report the impact of surface reconstruction and particle shape on acid–base catalysis over perovskites. In addition, we review promising catalytic performances of perovskite catalysts for other reactions of interest. Challenges in acid–base catalysis over perovskites focus on the development of time-resolved monolayer-sensitive characterization of surfaces under operando conditions and the discernment of combined acid–base/redox reaction mechanisms. Finally, opportunities lay on tuning the acid–base characteristics of perovskites with computation-based catalytic descriptors to

Acid–base catalysis over perovskites: a review

DOE PAGES

Polo-Garzon, Felipe; Wu, Zili

2018-01-15

We present that perovskite catalysts have been extensively studied for reduction–oxidation (redox) reactions; however, their acid–base catalytic properties are still under-explored. This review collects work aiming to study the acid–base catalytic properties of perovskites. Reports regarding combined acid–base/redox catalysis over perovskites lie beyond the scope of the present review. For the characterization of acid–base properties, researchers have studied the interaction of probe molecules with perovskite surfaces by means of multiple techniques that provide information about the density, strength and type of adsorption sites. The top-surface composition of perovskites, which relates to the abundance of the acid–base sites, has been studiedmore » by means of low energy ion scattering (LEIS), and, the less surface sensitive, conventional X-ray photoelectron spectroscopy (XPS). Probe reactions, with the conversion of 2-propanol as the common choice, have also been employed for characterizing the acid–base catalytic properties of perovskites. The complex nature of perovskite surfaces, which explains the still absent fundamental relations between the structure of the catalyst and reaction rates/selectivity, encounters a great challenge due to the surface reconstruction of these materials. In this review, we devote a special section to highlight recent publications that report the impact of surface reconstruction and particle shape on acid–base catalysis over perovskites. In addition, we review promising catalytic performances of perovskite catalysts for other reactions of interest. Challenges in acid–base catalysis over perovskites focus on the development of time-resolved monolayer-sensitive characterization of surfaces under operando conditions and the discernment of combined acid–base/redox reaction mechanisms. Finally, opportunities lay on tuning the acid–base characteristics of perovskites with computation-based catalytic descriptors to

Anion-π Catalysis of Enolate Chemistry: Rigidified Leonard Turns as a General Motif to Run Reactions on Aromatic Surfaces.

PubMed

Cotelle, Yoann; Benz, Sebastian; Avestro, Alyssa-Jennifer; Ward, Thomas R; Sakai, Naomi; Matile, Stefan

2016-03-18

To integrate anion-π, cation-π, and ion pair-π interactions in catalysis, the fundamental challenge is to run reactions reliably on aromatic surfaces. Addressing a specific question concerning enolate addition to nitroolefins, this study elaborates on Leonard turns to tackle this problem in a general manner. Increasingly refined turns are constructed to position malonate half thioesters as close as possible on π-acidic surfaces. The resulting preorganization of reactive intermediates is shown to support the disfavored addition to enolate acceptors to an absolutely unexpected extent. This decisive impact on anion-π catalysis increases with the rigidity of the turns. The new, rigidified Leonard turns are most effective with weak anion-π interactions, whereas stronger interactions do not require such ideal substrate positioning to operate well. The stunning simplicity of the motif and its surprisingly strong relevance for function should render the introduced approach generally useful. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Supporting palladium metal on gold nanoparticles improves its catalysis for nitrite reduction

NASA Astrophysics Data System (ADS)

Qian, Huifeng; Zhao, Zhun; Velazquez, Juan C.; Pretzer, Lori A.; Heck, Kimberly N.; Wong, Michael S.

2013-12-01

Nitrate (NO3-) and nitrite (NO2-) anions are often found in groundwater and surface water as contaminants globally, especially in agricultural areas due to nitrate-rich fertilizer use. One popular approach to studying the removal of nitrite/nitrate from water has been their degradation to dinitrogen via Pd-based reduction catalysis. However, little progress has been made towards understanding how the catalyst structure can improve activity. Focusing on the catalytic reduction of nitrite in this study, we report that Au NPs supporting Pd metal ("Pd-on-Au NPs") show catalytic activity that varies with volcano-shape dependence on Pd surface coverage. At room temperature, in CO2-buffered water, and under H2 headspace, the NPs were maximally active at a Pd surface coverage of 80%, with a first-order rate constant (kcat = 576 L gPd-1 min-1) that was 15x and 7.5x higher than monometallic Pd NPs (~4 nm; 40 L gPd-1 min-1) and Pd/Al2O3 (1 wt% Pd; 76 L gPd-1 min-1), respectively. Accounting only for surface Pd atoms, these NPs (576 L gsurface-Pd-1 min-1) were 3.6x and 1.6x higher than monometallic Pd NPs (160 L gsurface-Pd-1 min-1) and Pd/Al2O3 (361 L gsurface-Pd-1 min-1). These NPs retained ~98% of catalytic activity at a chloride concentration of 1 mM, whereas Pd/Al2O3 lost ~50%. The Pd-on-Au nanostructure is a promising approach to improve the catalytic reduction process for nitrite and, with further development, also for nitrate anions.Nitrate (NO3-) and nitrite (NO2-) anions are often found in groundwater and surface water as contaminants globally, especially in agricultural areas due to nitrate-rich fertilizer use. One popular approach to studying the removal of nitrite/nitrate from water has been their degradation to dinitrogen via Pd-based reduction catalysis. However, little progress has been made towards understanding how the catalyst structure can improve activity. Focusing on the catalytic reduction of nitrite in this study, we report that Au NPs supporting Pd

Electromagnetic field redistribution induced selective plasmon driven surface catalysis in metal nanowire-film systems

PubMed Central

Pan, Liang; Huang, Yingzhou; Yang, Yanna; Xiong, Wen; Chen, Guo; Su, Xun; Wei, Hua; Wang, Shuxia; Wen, Weijia

2015-01-01

For the novel interpretation of Raman spectrum from molecule at metal surface, the plasmon driven surface catalysis (PDSC) reactions have become an interesting topic in the research field of surface enhanced Raman scattering (SERS). In this work, the selective PDSC reactions of p,p’-dimercaptoazobenzene (DMAB) produced from para-aminothiophenol (PATP) or 4-nitrobenzenethiol (4NBT) were demonstrated in the Ag nanowires dimer-Au film systems. The different SERS spectra collected at individual part and adjacent part of the same nanowire-film system pointed out the importance of the electromagnetic field redistribution induced by image charge on film in this selective surface catalysis, which was confirmed by the simulated electromagnetic simulated electro- magnetic field distributions. Our result indicated this electromagnetic field redistribution induced selective surface catalysis was largely affected by the polarization and wavelength of incident light but slightly by the difference in diameters between two nanowires. Our work provides a further understanding of PDSC reaction in metal nanostructure and could be a deep support for the researches on surface catalysis and surface analysis. PMID:26601698

Transition-State Ensembles Navigate the Pathways of Enzyme Catalysis.

PubMed

Mickert, Matthias J; Gorris, Hans H

2018-06-07

Transition-state theory (TST) provides an important framework for analyzing and explaining the reaction rates of enzymes. TST, however, needs to account for protein dynamic effects and heterogeneities in enzyme catalysis. We have analyzed the reaction rates of β-galactosidase and β-glucuronidase at the single molecule level by using large arrays of femtoliter-sized chambers. Heterogeneities in individual reaction rates yield information on the intrinsic distribution of the free energy of activation (Δ G ‡ ) in an enzyme ensemble. The broader distribution of Δ G ‡ in β-galactosidase compared to β-glucuronidase is attributed to β-galactosidase's multiple catalytic functions as a hydrolase and a transglycosylase. Based on the catalytic mechanism of β-galactosidase, we show that transition-state ensembles do not only contribute to enzyme catalysis but can also channel the catalytic pathway to the formation of different products. We conclude that β-galactosidase is an example of natural evolution, where a new catalytic pathway branches off from an established enzyme function. The functional division of work between enzymatic substates explains why the conformational space represented by the enzyme ensemble is larger than the conformational space that can be sampled by any given enzyme molecule during catalysis.

Activity of N-coordinated multi-metal-atom active site structures for Pt-free oxygen reduction reaction catalysis: Role of *OH ligands

DOE PAGES

Holby, Edward F.; Taylor, Christopher D.

2015-03-19

We report calculated oxygen reduction reaction energy pathways on multi-metal-atom structures that have previously been shown to be thermodynamically favorable. We predict that such sites have the ability to spontaneously cleave the O₂ bond and then will proceed to over-bind reaction intermediates. In particular, the *OH bound state has lower energy than the final 2 H₂O state at positive potentials. Contrary to traditional surface catalysts, this *OH binding does not poison the multi-metal-atom site but acts as a modifying ligand that will spontaneously form in aqueous environments leading to new active sites that have higher catalytic activities. These *OH boundmore » structures have the highest calculated activity to date.« less

Reduction Potentials of [FeFe]-Hydrogenase Accessory Iron-Sulfur Clusters Provide Insights into the Energetics of Proton Reduction Catalysis.

PubMed

Artz, Jacob H; Mulder, David W; Ratzloff, Michael W; Lubner, Carolyn E; Zadvornyy, Oleg A; LeVan, Axl X; Williams, S Garrett; Adams, Michael W W; Jones, Anne K; King, Paul W; Peters, John W

2017-07-19

An [FeFe]-hydrogenase from Clostridium pasteurianum, CpI, is a model system for biological H 2 activation. In addition to the catalytic H-cluster, CpI contains four accessory iron-sulfur [FeS] clusters in a branched series that transfer electrons to and from the active site. In this work, potentiometric titrations have been employed in combination with electron paramagnetic resonance (EPR) spectroscopy at defined electrochemical potentials to gain insights into the role of the accessory clusters in catalysis. EPR spectra collected over a range of potentials were deconvoluted into individual components attributable to the accessory [FeS] clusters and the active site H-cluster, and reduction potentials for each cluster were determined. The data suggest a large degree of magnetic coupling between the clusters. The distal [4Fe-4S] cluster is shown to have a lower reduction potential (∼ < -450 mV) than the other clusters, and molecular docking experiments indicate that the physiological electron donor, ferredoxin (Fd), most favorably interacts with this cluster. The low reduction potential of the distal [4Fe-4S] cluster thermodynamically restricts the Fd ox /Fd red ratio at which CpI can operate, consistent with the role of CpI in recycling Fd red that accumulates during fermentation. Subsequent electron transfer through the additional accessory [FeS] clusters to the H-cluster is thermodynamically favorable.

Reduction Potentials of [FeFe]-Hydrogenase Accessory Iron–Sulfur Clusters Provide Insights into the Energetics of Proton Reduction Catalysis

DOE Office of Scientific and Technical Information (OSTI.GOV)

Artz, Jacob H.; Mulder, David W.; Ratzloff, Michael W.

An [FeFe]-hydrogenase from Clostridium pasteurianum, CpI, is a model system for biological H 2 activation. In addition to the catalytic H-cluster, CpI contains four accessory iron-sulfur [FeS] clusters in a branched series that transfer electrons to and from the active site. In this work, potentiometric titrations have been employed in combination with electron paramagnetic resonance (EPR) spectroscopy at defined electrochemical potentials to gain insights into the role of the accessory clusters in catalysis. EPR spectra collected over a range of potentials were deconvoluted into individual components attributable to the accessory [FeS] clusters and the active site H-cluster, and reduction potentialsmore » for each cluster were determined. The data suggest a large degree of magnetic coupling between the clusters. The distal [4Fe-4S] cluster is shown to have a lower reduction potential (~ < -450 mV) than the other clusters, and molecular docking experiments indicate that the physiological electron donor, ferredoxin (Fd), most favorably interacts with this cluster. The low reduction potential of the distal [4Fe-4S] cluster thermodynamically restricts the Fd ox/Fd red ratio at which CpI can operate, consistent with the role of CpI in recycling Fd redthat accumulates during fermentation. In conclusion, subsequent electron transfer through the additional accessory [FeS] clusters to the H-cluster is thermodynamically favorable.« less

Reduction Potentials of [FeFe]-Hydrogenase Accessory Iron–Sulfur Clusters Provide Insights into the Energetics of Proton Reduction Catalysis

DOE PAGES

Artz, Jacob H.; Mulder, David W.; Ratzloff, Michael W.; ...

2017-06-21

An [FeFe]-hydrogenase from Clostridium pasteurianum, CpI, is a model system for biological H 2 activation. In addition to the catalytic H-cluster, CpI contains four accessory iron-sulfur [FeS] clusters in a branched series that transfer electrons to and from the active site. In this work, potentiometric titrations have been employed in combination with electron paramagnetic resonance (EPR) spectroscopy at defined electrochemical potentials to gain insights into the role of the accessory clusters in catalysis. EPR spectra collected over a range of potentials were deconvoluted into individual components attributable to the accessory [FeS] clusters and the active site H-cluster, and reduction potentialsmore » for each cluster were determined. The data suggest a large degree of magnetic coupling between the clusters. The distal [4Fe-4S] cluster is shown to have a lower reduction potential (~ < -450 mV) than the other clusters, and molecular docking experiments indicate that the physiological electron donor, ferredoxin (Fd), most favorably interacts with this cluster. The low reduction potential of the distal [4Fe-4S] cluster thermodynamically restricts the Fd ox/Fd red ratio at which CpI can operate, consistent with the role of CpI in recycling Fd redthat accumulates during fermentation. In conclusion, subsequent electron transfer through the additional accessory [FeS] clusters to the H-cluster is thermodynamically favorable.« less

Enantio-Relay Catalysis Constructs Chiral Biaryl Alcohols over Cascade Suzuki Cross-Coupling-Asymmetric Transfer Hydrogenation

NASA Astrophysics Data System (ADS)

Zhang, Dacheng; Gao, Xiaoshuang; Cheng, Tanyu; Liu, Guohua

2014-05-01

The construction of chiral biaryl alcohols using enantio-relay catalysis is a particularly attractive synthetic method in organic synthesis. However, overcoming the intrinsic incompatibility of distinct organometallic complexes and the reaction conditions used are significant challenges in asymmetric catalysis. To overcome these barriers, we have taken advantage of an enantio-relay catalysis strategy and a combined dual-immobilization approach. We report the use of an imidazolium-based organopalladium-functionalized organic-inorganic hybrid silica and ethylene-coated chiral organoruthenium-functionalized magnetic nanoparticles to catalyze a cascade Suzuki cross-coupling-asymmetric transfer hydrogenation reaction to prepare chiral biaryl alcohols in a two-step, one-pot process. As expected, the site-isolated active species, salient imidazolium phase-transfer character and high ethylene-coated hydrophobicity can synergistically boost the catalytic performance. Furthermore, enantio-relay catalysis has the potential to efficiently prepare a variety of chiral biaryl alcohols. Our synthetic strategy is a general method that shows the potential of developing enantio-relay catalysis towards environmentally benign and sustainable organic synthesis.

Quantifying ligand effects in high-oxidation-state metal catalysis

NASA Astrophysics Data System (ADS)

Billow, Brennan S.; McDaniel, Tanner J.; Odom, Aaron L.

2017-09-01

Catalysis by high-valent metals such as titanium(IV) impacts our lives daily through reactions like olefin polymerization. In any catalysis, optimization involves a careful choice of not just the metal but also the ancillary ligands. Because these choices dramatically impact the electronic structure of the system and, in turn, catalyst performance, new tools for catalyst development are needed. Understanding ancillary ligand effects is arguably one of the most critical aspects of catalyst optimization and, while parameters for phosphines have been used for decades with low-valent systems, a comparable system does not exist for high-valent metals. A new electronic parameter for ligand donation, derived from experiments on a high-valent chromium species, is now available. Here, we show that the new parameters enable quantitative determination of ancillary ligand effects on catalysis rate and, in some cases, even provide mechanistic information. Analysing reactions in this way can be used to design better catalyst architectures and paves the way for the use of such parameters in a host of high-valent processes.

Industrial Catalysis: A Practical Guide

NASA Astrophysics Data System (ADS)

Farrauto, Robert J.

Every student of chemistry, material science, and chemical engineering should be schooled in catalysis and catalytic reactions. The reason is quite simple; most products produced in the chemical and petroleum industry utilize catalysts to enhance the rate of reaction and selectivity to desired products. Catalysts are also extensively used to minimize harmful byproduct pollutants in environmental applications. Enhanced reaction rates translate to higher production volumes at lower temperatures with smaller and less exotic materials of construction necessary. When a highly selective catalyst is used, large volumes of desired products are produced with virtually no undesirable byproducts. Gasoline, diesel, home heating oil, and aviation fuels owe their performance quality to catalytic processing used to upgrade crude oil.

Enantioselective Bronsted Acid Catalysis with Chiral Pentacarboxycyclopentadienes

NASA Astrophysics Data System (ADS)

Gheewala, Chirag

This thesis details the design and development of pentacarboxycyclopentadienes (PCCPs) as a new platform for enantioselective Bronsted acid catalysis. Prior to this research, enantioselective Bronsted acid catalysis was limited to the BINOL (and variations thereof) framework. While this catalyst platform has paved the way for a myriad of novel asymmetric chemical transformations, the utility of this catalyst scaffold has suffered from its lengthy and expensive preparations. As an alternative, starting from readily available 1,2,3,4,5-pentacarbomethoxycyclopentadiene and various chiral alcohols and amines, the synthesis of a library of strongly acidic chiral catalysts is described. The utility of these novel acid catalysts is explored in various transformations. As a prelude to the heart of this work, Chapter 1 focuses on the advancements made in asymmetric Bronsted acid catalysis through BINOL-phosphate derived catalysts, focusing on the major accomplishments made by researchers since 2004. The provided review highlights the utility of these chiral acid catalysts but also reveals the need for a new scaffold that is more affordable and accessible. Chapter 2 discusses the background of PCCPs, including its initial discovery and subsequent applications. Our work in developing novel transesterified and amidated derivatives is discussed with accompanying crystal structures of achiral and chiral PCCPs. pKa measurements demonstrate the capacity of PCCPs to be used as strong Bronsted acid catalysts and are compared to literature values of known Bronsted acid catalysts. Chapter 3 focuses on the utility of PCCPs as enantioselective Bronsted acid catalysts in a variety of chemical transformations including the Mukaiyama-Mannich reaction, transfer hydrogenation, Pictet-Spengler reaction, diaryl alcohol substitution, Mukayaiama oxocarbenium aldol reaction, and [4+2]-cycloaddition. Catalyst loadings down to 0.01 mol% and reaction scale up to 25 grams in the Mukaiyama

Oxygen evolution reaction catalysis

DOEpatents

Haber, Joel A.; Jin, Jian; Xiang, Chengxiang; Gregoire, John M.; Jones, Ryan J.; Guevarra, Dan W.; Shinde, Aniketa A.

2016-09-06

An Oxygen Evolution Reaction (OER) catalyst includes a metal oxide that includes oxygen, cerium, and one or more second metals. In some instances, the cerium is 10 to 80 molar % of the metals in the metal oxide and/or the catalyst includes two or more second metals. The OER catalyst can be included in or on an electrode. The electrode can be arranged in an oxygen evolution system such that the Oxygen Evolution Reaction occurs at the electrode.

Out-of-equilibrium catalysis of chemical reactions by electronic tunnel currents.

PubMed

Dzhioev, Alan A; Kosov, Daniel S; von Oppen, Felix

2013-04-07

We present an escape rate theory for current-induced chemical reactions. We use Keldysh nonequilibrium Green's functions to derive a Langevin equation for the reaction coordinate. Due to the out of equilibrium electronic degrees of freedom, the friction, noise, and effective temperature in the Langevin equation depend locally on the reaction coordinate. As an example, we consider the dissociation of diatomic molecules induced by the electronic current from a scanning tunnelling microscope tip. In the resonant tunnelling regime, the molecular dissociation involves two processes which are intricately interconnected: a modification of the potential energy barrier and heating of the molecule. The decrease of the molecular barrier (i.e., the current induced catalytic reduction of the barrier) accompanied by the appearance of the effective, reaction-coordinate-dependent temperature is an alternative mechanism for current-induced chemical reactions, which is distinctly different from the usual paradigm of pumping vibrational degrees of freedom.

Catalysis for Fluorination and Trifluoromethylation

PubMed Central

Furuya, Takeru; Kamlet, Adam S.; Ritter, Tobias

2011-01-01

Preface Recent advances in catalysis have made the incorporation of fluorine into complex organic molecules easier than ever before, but selective, general, and practical fluorination reactions remain sought after. Fluorination of molecules often imparts desirable properties such as metabolic and thermal stability, and fluorinated molecules are therefore frequently used as pharmaceuticals or materials. Even with the latest advances in chemistry, carbon–fluorine bond formation in complex molecules is still a significant challenge. Within the last few years, new reactions to make organofluorides have emerged and exemplify how to overcome some of the intricate challenges associated with fluorination. PMID:21614074

Mechanistic Studies at the Interface Between Organometallic Chemistry and Homogeneous Catalysis

DOE Office of Scientific and Technical Information (OSTI.GOV)

Casey, Charles P

Mechanistic Studies at the Interface Between Organometallic Chemistry and Homogeneous Catalysis Charles P. Casey, Principal Investigator Department of Chemistry, University of Wisconsin - Madison, Madison, Wisconsin 53706 Phone 608-262-0584 FAX: 608-262-7144 Email: casey@chem.wisc.edu http://www.chem.wisc.edu/main/people/faculty/casey.html Executive Summary. Our goal was to learn the intimate mechanistic details of reactions involved in homogeneous catalysis and to use the insight we gain to develop new and improved catalysts. Our work centered on the hydrogenation of polar functional groups such as aldehydes and ketones and on hydroformylation. Specifically, we concentrated on catalysts capable of simultaneously transferring hydride from a metal center and a proton frommore » an acidic oxygen or nitrogen center to an aldehyde or ketone. An economical iron based catalyst was developed and patented. Better understanding of fundamental organometallic reactions and catalytic processes enabled design of energy and material efficient chemical processes. Our work contributed to the development of catalysts for the selective and mild hydrogenation of ketones and aldehydes; this will provide a modern green alternative to reductions by LiAlH4 and NaBH4, which require extensive work-up procedures and produce waste streams. (C5R4OH)Ru(CO)2H Hydrogenation Catalysts. Youval Shvo described a remarkable catalytic system in which the key intermediate (C5R4OH)Ru(CO)2H (1) has an electronically coupled acidic OH unit and a hydridic RuH unit. Our efforts centered on understanding and improving upon this important catalyst for reduction of aldehydes and ketones. Our mechanistic studies established that the reduction of aldehydes by 1 to produce alcohols and a diruthenium bridging hydride species occurs much more rapidly than regeneration of the ruthenium hydride from the diruthenium bridging hydride species. Our mechanistic studies require simultaneous transfer of hydride from

Anion-π Catalysis on Fullerenes.

PubMed

López-Andarias, Javier; Frontera, Antonio; Matile, Stefan

2017-09-27

Anion-π interactions on fullerenes are about as poorly explored as the use of fullerenes in catalysis. However, strong exchange-correlation contributions and the localized π holes on their surface promise unique selectivities. To elaborate on this promise, tertiary amines are attached nearby. Dependent on their positioning, the resulting stabilization of anionic transition states on fullerenes is shown to accelerate disfavored enolate addition and exo Diels-Alder reactions enantioselectively. The found selectivities are consistent with computational simulations, particularly concerning the discrimination of differently planarized and charge-delocalized enolate tautomers by anion-π interactions. Enolate-π interactions on fullerenes are much shorter than standard π-π interactions and anion-π interactions on planar surfaces, and alternative cation-π interactions are not observed. These findings open new perspectives with regard to anion-π interactions in general and the use of carbon allotropes in catalysis.

Toward benchmarking in catalysis science: Best practices, challenges, and opportunities

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bligaard, Thomas; Bullock, R. Morris; Campbell, Charles T.

Benchmarking is a community-based and (preferably) community-driven activity involving consensus-based decisions on how to make reproducible, fair, and relevant assessments. In catalysis science, important catalyst performance metrics include activity, selectivity, and the deactivation profile, which enable comparisons between new and standard catalysts. Benchmarking also requires careful documentation, archiving, and sharing of methods and measurements, to ensure that the full value of research data can be realized. Beyond these goals, benchmarking presents unique opportunities to advance and accelerate understanding of complex reaction systems by combining and comparing experimental information from multiple, in situ and operando techniques with theoretical insights derived frommore » calculations characterizing model systems. This Perspective describes the origins and uses of benchmarking and its applications in computational catalysis, heterogeneous catalysis, molecular catalysis, and electrocatalysis. As a result, it also discusses opportunities and challenges for future developments in these fields.« less

Toward benchmarking in catalysis science: Best practices, challenges, and opportunities

DOE PAGES

Bligaard, Thomas; Bullock, R. Morris; Campbell, Charles T.; ...

2016-03-07

Benchmarking is a community-based and (preferably) community-driven activity involving consensus-based decisions on how to make reproducible, fair, and relevant assessments. In catalysis science, important catalyst performance metrics include activity, selectivity, and the deactivation profile, which enable comparisons between new and standard catalysts. Benchmarking also requires careful documentation, archiving, and sharing of methods and measurements, to ensure that the full value of research data can be realized. Beyond these goals, benchmarking presents unique opportunities to advance and accelerate understanding of complex reaction systems by combining and comparing experimental information from multiple, in situ and operando techniques with theoretical insights derived frommore » calculations characterizing model systems. This Perspective describes the origins and uses of benchmarking and its applications in computational catalysis, heterogeneous catalysis, molecular catalysis, and electrocatalysis. As a result, it also discusses opportunities and challenges for future developments in these fields.« less

Water Oxidation Catalysis for NiOOH by a Metropolis Monte Carlo Algorithm.

PubMed

Hareli, Chen; Caspary Toroker, Maytal

2018-05-08

Understanding catalytic mechanisms is important for discovering better catalysts, particularly for water splitting reactions that are of great interest to the renewable energy field. One of the best performing catalysts for water oxidation is nickel oxyhydroxide (NiOOH). However, only one mechanism has been adopted so far for modeling catalysis of the active plane: β-NiOOH(01̅5). In order to understand how a second reaction mechanism affects catalysis, we perform Density Functional Theory + U (DFT+U) calculations of a second mechanism for water oxidation reaction of NiOOH. Then, we use a Metropolis Monte Carlo algorithm to calculate how many catalytic cycles are completed when two reaction mechanisms are competing. We find that within the Metropolis algorithm, the second mechanism has a higher overpotential and is therefore not active even for large applied biases.

Catalysis by clusters with precise numbers of atoms

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tyo, Eric C.; Vajda, Stefan

2015-07-03

Clusters that contain only a small number of atoms can exhibit unique and often unexpected properties. The clusters are of particular interest in catalysis because they can act as individual active sites, and minor changes in size and composition – such as the addition or removal of a single atom – can have a substantial influence on the activity and selectivity of a reaction. Here we review recent progress in the synthesis, characterization and catalysis of well-defined sub-nanometre clusters. We examine work on size-selected supported clusters in ultra-high vacuum environments and under realistic reaction conditions, and explore the use ofmore » computational methods to provide a mechanistic understanding of their catalytic properties. We also highlight the potential of size-selected clusters to provide insights into important catalytic processes and their use in the development of novel catalytic systems.« less

Advances in copper-catalyzed C-C coupling reactions and related domino reactions based on active methylene compounds.

PubMed

Liu, Yunyun; Wan, Jie-Ping

2012-06-01

Active methylene compounds are a major class of reaction partners for C-C bond formation with sp(2) C-X (X = halide) fragments. As one of the most-classical versions of the Ullmann-type coupling reaction, activated-methylene-based C-C coupling reactions have been efficiently employed in a large number of syntheses. Although this type of reaction has long relied on noble-metal catalysis, the renaissance of copper catalysis at the end of last century has led to dramatic developments in Ullmann C-C coupling reactions. Owing to its low cost, abundance, as well as excellent catalytic activity, the exceptional atom economy of copper catalysis is gaining widespread attention in various organic synthesis. This review summarizes the advances in copper-catalyzed intermolecular and intramolecular C-C coupling reactions that use activated methylene species as well as in tandem reactions that are initiated by this transformation. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Nanoreactors for green catalysis

PubMed Central

De Martino, M Teresa; Abdelmohsen, Loai K E A; Rutjes, Floris P J T

2018-01-01

Sustainable and environmentally benign production are key drivers for developments in the chemical industrial sector, as protecting our planet has become a significant element that should be considered for every industrial breakthrough or technological advancement. As a result, the concept of green chemistry has been recently defined to guide chemists towards minimizing any harmful outcome of chemical processes in either industry or research. Towards greener reactions, scientists have developed various approaches in order to decrease environmental risks while attaining chemical sustainability and elegancy. Utilizing catalytic nanoreactors for greener reactions, for facilitating multistep synthetic pathways in one-pot procedures, is imperative with far-reaching implications in the field. This review is focused on the applications of some of the most used nanoreactors in catalysis, namely: (polymer) vesicles, micelles, dendrimers and nanogels. The ability and efficiency of catalytic nanoreactors to carry out organic reactions in water, to perform cascade reaction and their ability to be recycled will be discussed. PMID:29719570

Mechanistic understanding of surface plasmon assisted catalysis on a single particle: cyclic redox of 4-aminothiophenol

DOE PAGES

Xu, Ping; Kang, Leilei; Mack, Nathan H.; ...

2013-10-21

We investigate surface plasmon assisted catalysis (SPAC) reactions of 4-aminothiophenol (4ATP) to and back from 4,4'-dimercaptoazobenzene (DMAB) by single particle surface enhanced Raman spectroscopy, using a self-designed gas flow cell to control the reductive/oxidative environment over the reactions. Conversion of 4ATP into DMAB is induced by energy transfer (plasmonic heating) from surface plasmon resonance to 4ATP, where O 2 (as an electron acceptor) is essential and H 2O (as a base) can accelerate the reaction. In contrast, hot electron (from surface plasmon decay) induction drives the reverse reaction of DMAB to 4ATP, where H 2O (or H 2) acts asmore » the hydrogen source. More interestingly, the cyclic redox between 4ATP and DMAB by SPAC approach has been demonstrated. Finally, this SPAC methodology presents a unique platform for studying chemical reactions that are not possible under standard synthetic conditions.« less

Shape-selective synthesis of Sn(MoO4)2 nanomaterials for catalysis and supercapacitor applications.

PubMed

Sakthikumar, K; Ede, Sivasankara Rao; Mishra, Soumyaranjan; Kundu, Subrata

2016-06-07

Size and shape-selective Sn(MoO4)2 nanomaterials have been synthesized for the first time using a simple hydrothermal route by the reaction of Sn(ii) chloride salt with sodium molybdate in CTAB micellar media under stirring at 60 °C temperature for about three hours. Needle-like and flake-like Sn(MoO4)2 nanomaterials were synthesized by optimizing the CTAB to metal salt molar ratio and by controlling other reaction parameters. The eventual diameter and length of the nanoneedles are ∼100 ± 10 nm and ∼850 ± 100 nm respectively. The average diameter of the flakes is ∼250 ± 50 nm. The synthesized Sn(MoO4)2 nanomaterials can be used in two potential applications, namely, catalytic reduction of nitroarenes and as an anodic material in electrochemical supercapacitors. From the catalysis study, it was observed that the Sn(MoO4)2 nanomaterials could act as a potential catalyst for the successful photochemical reduction of nitroarenes into their respective aminoarenes within a short reaction time. From the supercapacitor study, it was observed that the Sn(MoO4)2 nanomaterials of different shapes show different specific capacitance (Cs) values and the highest Cs value was observed for Sn(MoO4)2 nanomaterials having a flake-like morphology. The highest Cs value observed was 109 F g(-1) at a scan rate of 5 mV s(-1) for the flake-like Sn(MoO4)2 nanomaterials. The capacitor shows an excellent long cycle life along with 70% retention of the Cs value, even after 4000 consecutive cycles at a current density of 8 mA cm(-2). Other than the applications in catalysis and supercapacitors, the synthesized nanomaterials can find further applications in photoluminescence, sensor and other energy-related devices.

Porous Carbon-Hosted Atomically Dispersed Iron-Nitrogen Moiety as Enhanced Electrocatalysts for Oxygen Reduction Reaction in a Wide Range of pH

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fu, Shaofang; Zhu, Chengzhou; Su, Dong

As one of the alternatives to replace precious metal catalysts, transition-metal–nitrogen–carbon (M–N–C) electrocatalysts have attracted great research interest due to their low cost and good catalytic activities. Despite nanostructured M–N–C catalysts can achieve good electrochemical perfor-mances, they are vulnerable to aggregation and insufficient catalytic sites upon continuous catalytic reaction. In this work, metal–organic frameworks derived porous single-atom electrocatalysts (SAEs) were successfully prepared by simple pyrolysis procedure without any further posttreatment. Combining the X-ray absorption near-edge spectroscopy and electrochemical measure-ments, the SAEs have been identified with superior oxygen reduction reaction (ORR) activity and stability compared with Pt/C catalysts in alkaline condition.more » More impressively, the SAEs also show excellent ORR electrocatalytic perfor-mance in both acid and neutral media. This study of nonprecious catalysts provides new insights on nanoengineering catalytically active sites and porous structures for nonprecious metal ORR catalysis in a wide range of pH.« less

Porous Carbon-Hosted Atomically Dispersed Iron–Nitrogen Moiety as Enhanced Electrocatalysts for Oxygen Reduction Reaction in a Wide Range of pH

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fu, Shaofang; Zhu, Chengzhou; Su, Dong

2018-02-01

As one of the alternatives to replace precious metal catalysts, transition-metal–nitrogen–carbon (M–N–C) electrocatalysts have attracted great research interest due to their low cost and good catalytic activities. Despite nanostructured M–N–C catalysts can achieve good electrochemical perfor-mances, they are vulnerable to aggregation and insufficient catalytic sites upon continuous catalytic reaction. In this work, metal–organic frameworks derived porous single-atom electrocatalysts (SAEs) were successfully prepared by simple pyrolysis procedure without any further posttreatment. Combining the X-ray absorption near-edge spectroscopy and electrochemical measure-ments, the SAEs have been identified with superior oxygen reduction reaction (ORR) activity and stability compared with Pt/C catalysts in alkaline condition.more » More impressively, the SAEs also show excellent ORR electrocatalytic perfor-mance in both acid and neutral media. This study of nonprecious catalysts provides new insights on nanoengineering catalytically active sites and porous structures for nonprecious metal ORR catalysis in a wide range of pH.« less

Highly Regioselective Indoline Synthesis under Nickel/Photoredox Dual Catalysis.

PubMed

Tasker, Sarah Z; Jamison, Timothy F

2015-08-05

Nickel/photoredox catalysis is used to synthesize indolines in one step from iodoacetanilides and alkenes. Very high regioselectivity for 3-substituted indoline products is obtained for both aliphatic and styrenyl olefins. Mechanistic investigations indicate that oxidation to Ni(III) is necessary to perform the difficult C-N bond-forming reductive elimination, producing a Ni(I) complex, which in turn is reduced to Ni(0). This process serves to further demonstrate the utility of photoredox catalysts as controlled single electron transfer agents in multioxidation state nickel catalysis.

Catalysis by dihydrofolate reductase and other enzymes arises from electrostatic preorganization, not conformational motions

PubMed Central

Adamczyk, Andrew J.; Cao, Jie; Kamerlin, Shina C. L.; Warshel, Arieh

2011-01-01

The proposal that enzymatic catalysis is due to conformational fluctuations has been previously promoted by means of indirect considerations. However, recent works have focused on cases where the relevant motions have components toward distinct conformational regions, whose population could be manipulated by mutations. In particular, a recent work has claimed to provide direct experimental evidence for a dynamical contribution to catalysis in dihydrofolate reductase, where blocking a relevant conformational coordinate was related to the suppression of the motion toward the occluded conformation. The present work utilizes computer simulations to elucidate the true molecular basis for the experimentally observed effect. We start by reproducing the trend in the measured change in catalysis upon mutations (which was assumed to arise as a result of a “dynamical knockout” caused by the mutations). This analysis is performed by calculating the change in the corresponding activation barriers without the need to invoke dynamical effects. We then generate the catalytic landscape of the enzyme and demonstrate that motions in the conformational space do not help drive catalysis. We also discuss the role of flexibility and conformational dynamics in catalysis, once again demonstrating that their role is negligible and that the largest contribution to catalysis arises from electrostatic preorganization. Finally, we point out that the changes in the reaction potential surface modify the reorganization free energy (which includes entropic effects), and such changes in the surface also alter the corresponding motion. However, this motion is never the reason for catalysis, but rather simply a reflection of the shape of the reaction potential surface. PMID:21831831

Nanobubbles: An Effective Way to Study Gas-Generating Catalysis on a Single Nanoparticle.

PubMed

Li, Shuping; Du, Ying; He, Ting; Shen, Yangbin; Bai, Chuang; Ning, Fandi; Hu, Xin; Wang, Wenhui; Xi, Shaobo; Zhou, Xiaochun

2017-10-11

Gas-generating catalysis is important to many energy-related research fields, such as photocatalytic water splitting, water electrolysis, etc. The technique of single-nanoparticle catalysis is an effective way to search for highly active nanocatalysts and elucidate the reaction mechanism. However, gas-generating catalysis remains difficult to investigate at the single-nanoparticle level because product gases, such as H 2 and O 2 , are difficult to detect on an individual nanoparticle. Here, we successfully find that nanobubbles can be used to study the gas-generating catalysis, i.e., H 2 generation from formic acid dehydrogenation on a single Pd-Ag nanoplate, with a high time resolution (50 ms) via dark-field microscopy. The research reveals that the nanobubble evolution process includes nucleation time and lifetime. The nucleation rate of nanobubbles is proportional to the catalytic activity of a single nanocatalyst. The relationship between the catalytic activity and the nucleation rate is quantitatively described by a mathematical model, which shows that an onset reaction rate (r onset ) exists for the generation of nanobubbles on a single Pd-Ag nanoplate. The research also reveals that a Pd-Ag nanoplate with larger size usually has a higher activity. However, some large-sized ones still have low activities, indicating the size of the Pd-Ag nanoplate is not the only key factor for the activity. Notablely, further research shows that Pd content is the key factor for the activity of single Pd-Ag nanoplates with similar size. The methodology and knowledge acquired from this research are also applicable to other important gas-generating catalysis reactions at the single-nanoparticle level.

Mechanical catalysis on the centimetre scale

PubMed Central

Miyashita, Shuhei; Audretsch, Christof; Nagy, Zoltán; Füchslin, Rudolf M.; Pfeifer, Rolf

2015-01-01

Enzymes play important roles in catalysing biochemical transaction paths, acting as logical machines through the morphology of the processes. A key challenge in elucidating the nature of these systems, and for engineering manufacturing methods inspired by biochemical reactions, is to attain a comprehensive understanding of the stereochemical ground rules of enzymatic reactions. Here, we present a model of catalysis that can be performed magnetically by centimetre-sized passive floating units. The designed system, which is equipped with permanent magnets only, passively obeys the local causalities imposed by magnetic interactions, albeit it shows a spatial behaviour and an energy profile analogous to those of biochemical enzymes. In this process, the enzyme units trigger physical conformation changes of the target by levelling out the magnetic potential barrier (activation potential) to a funnel type and, thus, induce cascading conformation changes of the targeted substrate units reacting in parallel. The inhibitor units, conversely, suppress such changes by increasing the potential. Because the model is purely mechanical and established on a physics basis in the absence of turbulence, each performance can be explained by the morphology of the unit, extending the definition of catalysis to systems of alternative scales. PMID:25652461

Mechanical catalysis on the centimetre scale.

PubMed

Miyashita, Shuhei; Audretsch, Christof; Nagy, Zoltán; Füchslin, Rudolf M; Pfeifer, Rolf

2015-03-06

Enzymes play important roles in catalysing biochemical transaction paths, acting as logical machines through the morphology of the processes. A key challenge in elucidating the nature of these systems, and for engineering manufacturing methods inspired by biochemical reactions, is to attain a comprehensive understanding of the stereochemical ground rules of enzymatic reactions. Here, we present a model of catalysis that can be performed magnetically by centimetre-sized passive floating units. The designed system, which is equipped with permanent magnets only, passively obeys the local causalities imposed by magnetic interactions, albeit it shows a spatial behaviour and an energy profile analogous to those of biochemical enzymes. In this process, the enzyme units trigger physical conformation changes of the target by levelling out the magnetic potential barrier (activation potential) to a funnel type and, thus, induce cascading conformation changes of the targeted substrate units reacting in parallel. The inhibitor units, conversely, suppress such changes by increasing the potential. Because the model is purely mechanical and established on a physics basis in the absence of turbulence, each performance can be explained by the morphology of the unit, extending the definition of catalysis to systems of alternative scales.

Protein dynamics and enzyme catalysis: insights from simulations.

PubMed

McGeagh, John D; Ranaghan, Kara E; Mulholland, Adrian J

2011-08-01

The role of protein dynamics in enzyme catalysis is one of the most active and controversial areas in enzymology today. Some researchers claim that protein dynamics are at the heart of enzyme catalytic efficiency, while others state that dynamics make no significant contribution to catalysis. What is the biochemist - or student - to make of the ferocious arguments in this area? Protein dynamics are complex and fascinating, as molecular dynamics simulations and experiments have shown. The essential question is: do these complex motions have functional significance? In particular, how do they affect or relate to chemical reactions within enzymes, and how are chemical and conformational changes coupled together? Biomolecular simulations can analyse enzyme reactions and dynamics in atomic detail, beyond that achievable in experiments: accurate atomistic modelling has an essential part to play in clarifying these issues. This article is part of a Special Issue entitled: Protein Dynamics: Experimental and Computational Approaches. Copyright © 2010 Elsevier B.V. All rights reserved.

The energy landscape of adenylate kinase during catalysis

DOE PAGES

Kerns, S. Jordan; Agafonov, Roman V.; Cho, Young-Jin; ...

2015-01-12

Kinases perform phosphoryl-transfer reactions in milliseconds; without enzymes, these reactions would take about 8,000 years under physiological conditions. Despite extensive studies, a comprehensive understanding of kinase energy landscapes, including both chemical and conformational steps, is lacking. In this paper, we scrutinize the microscopic steps in the catalytic cycle of adenylate kinase, through a combination of NMR measurements during catalysis, pre-steady-state kinetics, molecular-dynamics simulations and crystallography of active complexes. We find that the Mg 2+ cofactor activates two distinct molecular events: phosphoryl transfer (>10 5-fold) and lid opening (10 3-fold). In contrast, mutation of an essential active site arginine decelerates phosphorylmore » transfer 10 3-fold without substantially affecting lid opening. Finally, our results highlight the importance of the entire energy landscape in catalysis and suggest that adenylate kinases have evolved to activate key processes simultaneously by precise placement of a single, charged and very abundant cofactor in a preorganized active site.« less

The energy landscape of adenylate kinase during catalysis

PubMed Central

Kerns, S. Jordan; Agafonov, Roman V.; Cho, Young-Jin; Pontiggia, Francesco; Otten, Renee; Pachov, Dimitar V.; Kutter, Steffen; Phung, Lien A.; Murphy, Padraig N.; Thai, Vu; Alber, Tom; Hagan, Michael F.; Kern, Dorothee

2014-01-01

Kinases perform phosphoryl-transfer reactions in milliseconds; without enzymes, these reactions would take about 8000 years under physiological conditions. Despite extensive studies, a comprehensive understanding of kinase energy landscapes, including both chemical and conformational steps, is lacking. Here we scrutinize the microscopic steps in the catalytic cycle of adenylate kinase, through a combination of NMR measurements during catalysis, pre-steady-state kinetics, MD simulations, and crystallography of active complexes. We find that the Mg2+ cofactor activates two distinct molecular events, phosphoryl transfer (>105-fold) and lid-opening (103-fold). In contrast, mutation of an essential active-site arginine decelerates phosphoryl transfer 103-fold without substantially affecting lid-opening. Our results highlight the importance of the entire energy landscape in catalysis and suggest that adenylate kinases have evolved to activate key processes simultaneously by precise placement of a single, charged and very abundant cofactor in a pre-organized active site. PMID:25580578

Mechanistic studies on the phosphoramidite coupling reaction in oligonucleotide synthesis. I. Evidence for nucleophilic catalysis by tetrazole and rate variations with the phosphorus substituents.

PubMed Central

Dahl, B H; Nielsen, J; Dahl, O

1987-01-01

Tetrazole catalyzed reactions of a series of phosphoramidites, 5'-O-DMTdT-3'-O-P(OR1)NR2(2) (1a-h), with 3'-O-SiButPh2-6-N-benzoyl-dA (2a) in acetonitrile solution have been studied. It is found that the coupling rate depends very much on whether tetrazole is added before or after 2a, and that dialkylammonium tetrazolide salts are inhibitors. These and other facts are evidence that the reactions are subjected to nucleophilic catalysis by tetrazole, in addition to acid catalysis. The rate variations with phosphorus substituents of 1a-h are NEt2 greater than NPri2 greater than N(CH2CH2)O greater than NMePh, and OMe greater than OCH2CH2CN greater than OCHMeCH2CN greater than OCMe2CH2CN much greater than OC6H4Cl. The inhibitor properties of dialkylammonium tetrazolides have practical consequences for the efficiency of DNA syntheses, when in situ prepared phosphoramidites are used; the same would apply for segmented, simultaneous syntheses or syntheses where recycling is performed. PMID:3822837

Oxygen reduction reaction: A framework for success

DOE PAGES

Allendorf, Mark D.

2016-05-06

Oxygen reduction at the cathode of fuel cells typically requires a platinum-based material to catalyse the reaction, but lower-cost, more stable catalysts are sought. Here, an intrinsically conductive metal–organic framework based on cheaper elements is shown to be a durable, structurally well-defined catalyst for this reaction.

Sequential rhodium/palladium catalysis: enantioselective formation of dihydroquinolinones in the presence of achiral and chiral ligands.

PubMed

Zhang, Lei; Qureshi, Zafar; Sonaglia, Lorenzo; Lautens, Mark

2014-12-08

Compatible combinations of achiral and chiral ligands can be used in rhodium/palladium catalysis to achieve highly enantioselective domino reactions. The difference in rates of catalysis and minimal effects of ligand interference confer control in the domino sequence. The "all-in-one" 1,4-conjugate arylation and C-N cross-coupling through sequential Rh/Pd catalysis provides access to enantioenriched dihydroquinolinone building blocks. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Nanostructured Membranes for Enzyme Catalysis and Green Synthesis of Nanoparticles

EPA Science Inventory

Macroporous membranes functionalized with ionizable macromolecules provide promising applications in toxic metal capture at high capacity, nanoparticle synthesis, and catalysis. Our low-pressure membrane approach is marked by reaction and separation selectivity and their tunabil...

Nanostructured Membranes for Green Synthesis of Nanoparticles and Enzyme Catalysis

EPA Science Inventory

Macroporous membranes functionalized with ionizable macromolecules provide promising applications in toxic metal capture at high capacity, nanoparticle synthesis, and catalysis. Our low‐pressure membrane approach is marked by reaction and separation selectivity and their tunabili...

Single-layer graphdiyne-covered Pt(111) surface: improved catalysis confined under two-dimensional overlayer

NASA Astrophysics Data System (ADS)

Chen, Xi; Lin, Zheng-Zhe

2018-05-01

In recent years, two-dimensional confined catalysis, i.e., the enhanced catalytic reactions in confined space between metal surface and two-dimensional overlayer, makes a hit and opens up a new way to enhance the performance of catalysts. In this work, graphdiyne overlayer was proposed as a more excellent material than graphene or hexagonal boron nitride for two-dimensional confined catalysis on Pt(111) surface. Density functional theory calculations revealed the superiority of graphdiyne overlayer originates from the steric hindrance effect which increases the catalytic ability and lowers the reaction barriers. Moreover, with the big triangle holes as natural gas tunnels, graphdiyne possesses higher efficiency for the transit of gaseous reactants and products than graphene or hexagonal boron nitride. The results in this work would benefit future development of two-dimensional confined catalysis. [Figure not available: see fulltext.

Heterogeneous Catalysis.

ERIC Educational Resources Information Center

Vannice, M. A.

1979-01-01

Described is a graduate course in catalysis offered at Penn State University. A detailed course outline with 30 lecture topics is presented. A list of 42 references on catalysis used in place of a textbook is provided. (BT)

Synergistic Anion-(π) n-π Catalysis on π-Stacked Foldamers.

PubMed

Bornhof, Anna-Bea; Bauzá, Antonio; Aster, Alexander; Pupier, Marion; Frontera, Antonio; Vauthey, Eric; Sakai, Naomi; Matile, Stefan

2018-04-11

In this report, we demonstrate that synergistic effects between π-π stacking and anion-π interactions in π-stacked foldamers provide access to unprecedented catalytic activity. To elaborate on anion-(π) n -π catalysis, we have designed, synthesized and evaluated a series of novel covalent oligomers with up to four face-to-face stacked naphthalenediimides (NDIs). NMR analysis including DOSY confirms folding into π stacks, cyclic voltammetry, steady-state and transient absorption spectroscopy the electronic communication within the π stacks. Catalytic activity, assessed by chemoselective catalysis of the intrinsically disfavored but biologically relevant addition reaction of malonate half thioesters to enolate acceptors, increases linearly with the length of the stacks to reach values that are otherwise beyond reach. This linear increase violates the sublinear power laws of oligomer chemistry. The comparison of catalytic activity with ratiometric changes in absorption and decreasing energy of the LUMO thus results in superlinearity, that is synergistic amplification of anion-π catalysis by remote control over the entire stack. In computational models, increasing length of the π-stacked foldamers correlates sublinearly with changes in surface potentials, chloride binding energies, and the distances between chloride and π surface and within the π stack. Computational evidence is presented that the selective acceleration of disfavored but relevant enolate chemistry by anion-π catalysis indeed originates from the discrimination of planar and bent tautomers with delocalized and localized charges, respectively, on π-acidic surfaces. Computed binding energies of keto and enol intermediates of the addition reaction as well as their difference increase with increasing length of the π stack and thus reflect experimental trends correctly. These results demonstrate that anion-(π) n -π interactions exist and matter, ready for use as a unique new tool in catalysis

Engaging unactivated alkyl, alkenyl and aryl iodides in visible-light-mediated free radical reactions

NASA Astrophysics Data System (ADS)

Nguyen, John D.; D'Amato, Erica M.; Narayanam, Jagan M. R.; Stephenson, Corey R. J.

2012-10-01

Radical reactions are a powerful class of chemical transformations. However, the formation of radical species to initiate these reactions has often required the use of stoichiometric amounts of toxic reagents, such as tributyltin hydride. Recently, the use of visible-light-mediated photoredox catalysis to generate radical species has become popular, but the scope of these radical precursors has been limited. Here, we describe the identification of reaction conditions under which photocatalysts such as fac-Ir(ppy)3 can be utilized to form radicals from unactivated alkyl, alkenyl and aryl iodides. The generated radicals undergo reduction via hydrogen atom abstraction or reductive cyclization. The reaction protocol utilizes only inexpensive reagents, occurs under mild reaction conditions, and shows exceptional functional group tolerance. Reaction efficiency is maintained upon scale-up and decreased catalyst loading, and the reaction time can be significantly shortened when the reaction is performed in a flow reactor.

Evolutionarily Conserved Linkage between Enzyme Fold, Flexibility, and Catalysis

PubMed Central

Ramanathan, Arvind; Agarwal, Pratul K.

2011-01-01

Proteins are intrinsically flexible molecules. The role of internal motions in a protein's designated function is widely debated. The role of protein structure in enzyme catalysis is well established, and conservation of structural features provides vital clues to their role in function. Recently, it has been proposed that the protein function may involve multiple conformations: the observed deviations are not random thermodynamic fluctuations; rather, flexibility may be closely linked to protein function, including enzyme catalysis. We hypothesize that the argument of conservation of important structural features can also be extended to identification of protein flexibility in interconnection with enzyme function. Three classes of enzymes (prolyl-peptidyl isomerase, oxidoreductase, and nuclease) that catalyze diverse chemical reactions have been examined using detailed computational modeling. For each class, the identification and characterization of the internal protein motions coupled to the chemical step in enzyme mechanisms in multiple species show identical enzyme conformational fluctuations. In addition to the active-site residues, motions of protein surface loop regions (>10 Å away) are observed to be identical across species, and networks of conserved interactions/residues connect these highly flexible surface regions to the active-site residues that make direct contact with substrates. More interestingly, examination of reaction-coupled motions in non-homologous enzyme systems (with no structural or sequence similarity) that catalyze the same biochemical reaction shows motions that induce remarkably similar changes in the enzyme–substrate interactions during catalysis. The results indicate that the reaction-coupled flexibility is a conserved aspect of the enzyme molecular architecture. Protein motions in distal areas of homologous and non-homologous enzyme systems mediate similar changes in the active-site enzyme–substrate interactions, thereby

Evolutionarily conserved linkage between enzyme fold, flexibility, and catalysis.

PubMed

Ramanathan, Arvind; Agarwal, Pratul K

2011-11-01

Proteins are intrinsically flexible molecules. The role of internal motions in a protein's designated function is widely debated. The role of protein structure in enzyme catalysis is well established, and conservation of structural features provides vital clues to their role in function. Recently, it has been proposed that the protein function may involve multiple conformations: the observed deviations are not random thermodynamic fluctuations; rather, flexibility may be closely linked to protein function, including enzyme catalysis. We hypothesize that the argument of conservation of important structural features can also be extended to identification of protein flexibility in interconnection with enzyme function. Three classes of enzymes (prolyl-peptidyl isomerase, oxidoreductase, and nuclease) that catalyze diverse chemical reactions have been examined using detailed computational modeling. For each class, the identification and characterization of the internal protein motions coupled to the chemical step in enzyme mechanisms in multiple species show identical enzyme conformational fluctuations. In addition to the active-site residues, motions of protein surface loop regions (>10 Å away) are observed to be identical across species, and networks of conserved interactions/residues connect these highly flexible surface regions to the active-site residues that make direct contact with substrates. More interestingly, examination of reaction-coupled motions in non-homologous enzyme systems (with no structural or sequence similarity) that catalyze the same biochemical reaction shows motions that induce remarkably similar changes in the enzyme-substrate interactions during catalysis. The results indicate that the reaction-coupled flexibility is a conserved aspect of the enzyme molecular architecture. Protein motions in distal areas of homologous and non-homologous enzyme systems mediate similar changes in the active-site enzyme-substrate interactions, thereby impacting

Nanocrystal assembly for tandem catalysis

DOEpatents

Yang, Peidong; Somorjai, Gabor; Yamada, Yusuke; Tsung, Chia-Kuang; Huang, Wenyu

2014-10-14

The present invention provides a nanocrystal tandem catalyst comprising at least two metal-metal oxide interfaces for the catalysis of sequential reactions. One embodiment utilizes a nanocrystal bilayer structure formed by assembling sub-10 nm platinum and cerium oxide nanocube monolayers on a silica substrate. The two distinct metal-metal oxide interfaces, CeO.sub.2--Pt and Pt--SiO.sub.2, can be used to catalyze two distinct sequential reactions. The CeO.sub.2--Pt interface catalyzed methanol decomposition to produce CO and H.sub.2, which were then subsequently used for ethylene hydroformylation catalyzed by the nearby Pt--SiO.sub.2 interface. Consequently, propanal was selectively produced on this nanocrystal bilayer tandem catalyst.

Simulations of chemical catalysis

NASA Astrophysics Data System (ADS)

Smith, Gregory K.

This dissertation contains simulations of chemical catalysis in both biological and heterogeneous contexts. A mixture of classical, quantum, and hybrid techniques are applied to explore the energy profiles and compare possible chemical mechanisms both within the context of human and bacterial enzymes, as well as exploring surface reactions on a metal catalyst. A brief summary of each project follows. Project 1 - Bacterial Enzyme SpvC The newly discovered SpvC effector protein from Salmonella typhimurium interferes with the host immune response by dephosphorylating mitogen-activated protein kinases (MAPKs) with a beta-elimination mechanism. The dynamics of the enzyme substrate complex of the SpvC effector is investigated with a 3.2 ns molecular dynamics simulation, which reveals that the phosphorylated peptide substrate is tightly held in the active site by a hydrogen bond network and the lysine general base is positioned for the abstraction of the alpha hydrogen. The catalysis is further modeled with density functional theory (DFT) in a truncated active-site model at the B3LYP/6-31 G(d,p) level of theory. The truncated model suggested the reaction proceeds via a single transition state. After including the enzyme environment in ab initio QM/MM studies, it was found to proceed via an E1cB-like pathway, in which the carbanion intermediate is stabilized by an enzyme oxyanion hole provided by Lys104 and Tyr158 of SpvC. Project 2 - Human Enzyme CDK2 Phosphorylation reactions catalyzed by kinases and phosphatases play an indispensable role in cellular signaling, and their malfunctioning is implicated in many diseases. Ab initio quantum mechanical/molecular mechanical studies are reported for the phosphoryl transfer reaction catalyzed by a cyclin-dependent kinase, CDK2. Our results suggest that an active-site Asp residue, rather than ATP as previously proposed, serves as the general base to activate the Ser nucleophile. The corresponding transition state features a

Catalysis of Nuclear Reactions by Electrons

NASA Astrophysics Data System (ADS)

Lipoglavšek, Matej

2018-01-01

Electron screening enhances nuclear reaction cross sections at low energies. We studied the nuclear reaction 1H(19F,αγ)16O in inverse kinematics in different solid hydrogen targets. Measured resonance strengths differed by up to a factor of 10 in different targets. We also studied the 2H(p,γ)3He fusion reaction and observed electrons emitted as reaction products instead of γ rays. In this case electron screening greatly enhances internal conversion probability.

Surface catalysis of uranium(VI) reduction by iron(II)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liger, E.; Charlet, L.; Van Cappellen, P.

1999-10-01

Colloidal hematite ({alpha}-Fe{sub 2}O{sub 3}) is used as model solid to investigate the kinetic effect of specific adsorption interactions on the chemical reduction of uranyl (U{sup VI}O{sub 2}{sup 2+}) by ferrous iron. Acid-base titrations and Fe(II) and uranyl adsorption experiments are performed on hematite suspensions, under O{sub 2}- and CO{sub 2}-free conditions. The results are explained in terms of a constant capacitance surface complexation model of the hematite-aqueous solution interface. Two distinct Fe(II) surface complexes are required to reproduce the data: ({equivalent{underscore}to}Fe{sup III}OFe{sup II}){sup +} (or {equivalent{underscore}to}Fe{sup III}OFe{sup II}(OH{sub 2}){sub n}{sup +}) and {equivalent{underscore}to}Fe{sup III}OFe{sup II}OH{sup 0} (or {equivalent{underscore}to}Fe{sup III}OFe{supmore » II}(OH{sub 2}){sub n{minus}1}OH{sup 0}). The latter complex represents a significant fraction of total adsorbed Fe(II) at pH {gt} 6.5. Uranyl binding to the hematite particles is characterized by a sharp adsorption edge between pH 4 and pH 5.5. Because of the absence of competing aqueous carbonate complexes, uranyl remains completely adsorbed at pH {gt} 7. A single mononuclear surface complex accounts for the adsorption of uranyl over the entire range of experimental conditions. Although thermodynamically feasible, no reaction between uranyl and Fe(II) is observed in homogeneous solution at pH 7.5, for periods of up to three days. In hematite suspensions, however, surface-bound uranyl reacts on a time scale of hours. Based on Fourier Transformed Infrared spectra, chemical reduction of U(VI) is inferred to be the mechanism responsible for the disappearance of uranyl. The kinetics of uranyl reduction are quantified by measuring the decrease with time of the concentration of U(VI) extractable from the hematite particles by NaHCO{sub 3}. In the presence of excess Fe(II), the initial rate of U(VI) reduction exhibits a first-order dependence on the

Asymmetric photoredox transition-metal catalysis activated by visible light.

PubMed

Huo, Haohua; Shen, Xiaodong; Wang, Chuanyong; Zhang, Lilu; Röse, Philipp; Chen, Liang-An; Harms, Klaus; Marsch, Michael; Hilt, Gerhard; Meggers, Eric

2014-11-06

Asymmetric catalysis is seen as one of the most economical strategies to satisfy the growing demand for enantiomerically pure small molecules in the fine chemical and pharmaceutical industries. And visible light has been recognized as an environmentally friendly and sustainable form of energy for triggering chemical transformations and catalytic chemical processes. For these reasons, visible-light-driven catalytic asymmetric chemistry is a subject of enormous current interest. Photoredox catalysis provides the opportunity to generate highly reactive radical ion intermediates with often unusual or unconventional reactivities under surprisingly mild reaction conditions. In such systems, photoactivated sensitizers initiate a single electron transfer from (or to) a closed-shell organic molecule to produce radical cations or radical anions whose reactivities are then exploited for interesting or unusual chemical transformations. However, the high reactivity of photoexcited substrates, intermediate radical ions or radicals, and the low activation barriers for follow-up reactions provide significant hurdles for the development of efficient catalytic photochemical processes that work under stereochemical control and provide chiral molecules in an asymmetric fashion. Here we report a highly efficient asymmetric catalyst that uses visible light for the necessary molecular activation, thereby combining asymmetric catalysis and photocatalysis. We show that a chiral iridium complex can serve as a sensitizer for photoredox catalysis and at the same time provide very effective asymmetric induction for the enantioselective alkylation of 2-acyl imidazoles. This new asymmetric photoredox catalyst, in which the metal centre simultaneously serves as the exclusive source of chirality, the catalytically active Lewis acid centre, and the photoredox centre, offers new opportunities for the 'green' synthesis of non-racemic chiral molecules.

Asymmetric photoredox transition-metal catalysis activated by visible light

NASA Astrophysics Data System (ADS)

Huo, Haohua; Shen, Xiaodong; Wang, Chuanyong; Zhang, Lilu; Röse, Philipp; Chen, Liang-An; Harms, Klaus; Marsch, Michael; Hilt, Gerhard; Meggers, Eric

2014-11-01

Asymmetric catalysis is seen as one of the most economical strategies to satisfy the growing demand for enantiomerically pure small molecules in the fine chemical and pharmaceutical industries. And visible light has been recognized as an environmentally friendly and sustainable form of energy for triggering chemical transformations and catalytic chemical processes. For these reasons, visible-light-driven catalytic asymmetric chemistry is a subject of enormous current interest. Photoredox catalysis provides the opportunity to generate highly reactive radical ion intermediates with often unusual or unconventional reactivities under surprisingly mild reaction conditions. In such systems, photoactivated sensitizers initiate a single electron transfer from (or to) a closed-shell organic molecule to produce radical cations or radical anions whose reactivities are then exploited for interesting or unusual chemical transformations. However, the high reactivity of photoexcited substrates, intermediate radical ions or radicals, and the low activation barriers for follow-up reactions provide significant hurdles for the development of efficient catalytic photochemical processes that work under stereochemical control and provide chiral molecules in an asymmetric fashion. Here we report a highly efficient asymmetric catalyst that uses visible light for the necessary molecular activation, thereby combining asymmetric catalysis and photocatalysis. We show that a chiral iridium complex can serve as a sensitizer for photoredox catalysis and at the same time provide very effective asymmetric induction for the enantioselective alkylation of 2-acyl imidazoles. This new asymmetric photoredox catalyst, in which the metal centre simultaneously serves as the exclusive source of chirality, the catalytically active Lewis acid centre, and the photoredox centre, offers new opportunities for the `green' synthesis of non-racemic chiral molecules.

Serial crystallography captures enzyme catalysis in copper nitrite reductase at atomic resolution from one crystal.

PubMed

Horrell, Sam; Antonyuk, Svetlana V; Eady, Robert R; Hasnain, S Samar; Hough, Michael A; Strange, Richard W

2016-07-01

Relating individual protein crystal structures to an enzyme mechanism remains a major and challenging goal for structural biology. Serial crystallography using multiple crystals has recently been reported in both synchrotron-radiation and X-ray free-electron laser experiments. In this work, serial crystallography was used to obtain multiple structures serially from one crystal (MSOX) to study in crystallo enzyme catalysis. Rapid, shutterless X-ray detector technology on a synchrotron MX beamline was exploited to perform low-dose serial crystallography on a single copper nitrite reductase crystal, which survived long enough for 45 consecutive 100 K X-ray structures to be collected at 1.07-1.62 Å resolution, all sampled from the same crystal volume. This serial crystallography approach revealed the gradual conversion of the substrate bound at the catalytic type 2 Cu centre from nitrite to nitric oxide, following reduction of the type 1 Cu electron-transfer centre by X-ray-generated solvated electrons. Significant, well defined structural rearrangements in the active site are evident in the series as the enzyme moves through its catalytic cycle, namely nitrite reduction, which is a vital step in the global denitrification process. It is proposed that such a serial crystallography approach is widely applicable for studying any redox or electron-driven enzyme reactions from a single protein crystal. It can provide a 'catalytic reaction movie' highlighting the structural changes that occur during enzyme catalysis. The anticipated developments in the automation of data analysis and modelling are likely to allow seamless and near-real-time analysis of such data on-site at some of the powerful synchrotron crystallographic beamlines.

Evidence for single metal two electron oxidative addition and reductive elimination at uranium.

PubMed

Gardner, Benedict M; Kefalidis, Christos E; Lu, Erli; Patel, Dipti; McInnes, Eric J L; Tuna, Floriana; Wooles, Ashley J; Maron, Laurent; Liddle, Stephen T

2017-12-01

Reversible single-metal two-electron oxidative addition and reductive elimination are common fundamental reactions for transition metals that underpin major catalytic transformations. However, these reactions have never been observed together in the f-block because these metals exhibit irreversible one- or multi-electron oxidation or reduction reactions. Here we report that azobenzene oxidises sterically and electronically unsaturated uranium(III) complexes to afford a uranium(V)-imido complex in a reaction that satisfies all criteria of a single-metal two-electron oxidative addition. Thermolysis of this complex promotes extrusion of azobenzene, where H-/D-isotopic labelling finds no isotopomer cross-over and the non-reactivity of a nitrene-trap suggests that nitrenes are not generated and thus a reductive elimination has occurred. Though not optimally balanced in this case, this work presents evidence that classical d-block redox chemistry can be performed reversibly by f-block metals, and that uranium can thus mimic elementary transition metal reactivity, which may lead to the discovery of new f-block catalysis.

Cobalt-manganese-based spinels as multifunctional materials that unify catalytic water oxidation and oxygen reduction reactions.

PubMed

Menezes, Prashanth W; Indra, Arindam; Sahraie, Nastaran Ranjbar; Bergmann, Arno; Strasser, Peter; Driess, Matthias

2015-01-01

Recently, there has been much interest in the design and development of affordable and highly efficient oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) catalysts that can resolve the pivotal issues that concern solar fuels, fuel cells, and rechargeable metal-air batteries. Here we present the synthesis and application of porous CoMn2 O4 and MnCo2 O4 spinel microspheres as highly efficient multifunctional catalysts that unify the electrochemical OER with oxidant-driven and photocatalytic water oxidation as well as the ORR. The porous materials were prepared by the thermal degradation of the respective carbonate precursors at 400 °C. The as-prepared spinels display excellent performances in electrochemical OER for the cubic MnCo2 O4 phase in comparison to the tetragonal CoMn2 O4 material in an alkaline medium. Moreover, the oxidant-driven and photocatalytic water oxidations were performed and they exhibited a similar trend in activity to that of the electrochemical OER. Remarkably, the situation is reversed in ORR catalysis, that is, the oxygen reduction activity and stability of the tetragonal CoMn2 O4 catalyst outperformed that of cubic MnCo2 O4 and rivals that of benchmark Pt catalysts. The superior catalytic performance and the remarkable stability of the unifying materials are attributed to their unique porous and robust microspherical morphology and the intrinsic structural features of the spinels. Moreover, the facile access to these high-performance materials enables a reliable and cost-effective production on a large scale for industrial applications. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Photoredox Catalysis for the Generation of Carbon Centered Radicals.

PubMed

Goddard, Jean-Philippe; Ollivier, Cyril; Fensterbank, Louis

2016-09-20

Radical chemistry has witnessed over the last decades important advances that have positioned it as a methodology of choice in synthetic chemistry. A number of great attributes such as specific reactivities, the knowledge of the kinetics of most elementary processes, the functional group tolerance, and the possibility to operate cascade sequences are clearly responsible for this craze. Nevertheless, at the end of the last century, radical chemistry appeared plagued by several hurdles to overcome such as the use of environmentally problematic mediators or the impossibility of scale up. While the concept of photocatalysis was firmly established in the coordination chemistry community, its diffusion in organic synthetic chemistry remained sporadic for decades until the end of the 2000s with the breakthrough merging of organocatalysis and photocatalysis by the MacMillan group and contemporary reports by the groups of Yoon and Stephenson. Since then, photoredox catalysis has enjoyed particularly active and intense developments. It is now the topic of a still increasing number of publications featuring various applications from asymmetric synthesis, total synthesis of natural products, and polymerization to process (flow) chemistry. In this Account, we survey our own efforts in this domain, focusing on the elaboration of new photocatalytic pathways that could lead to the efficient generation of C-centered functionalized alkyl and aryl radicals. Both reductive and oxidative manifolds are accessible through photoredox catalysis, which has guided us along these lines in our projects. Thus, we studied the photocatalytic reduction of onium salts such as sulfoniums and iodoniums for the production of the elusive aryl radical intermediates. Progressing to more relevant chemistry for synthesis, we examined the cleavage of C-O and the C-Br bonds for the generation of alkyl C-centered radicals. Activated epoxides could serve as valuable substrates of a photocatalyzed variant of

The mechanism of montmorillonite catalysis in RNA synthesis

NASA Astrophysics Data System (ADS)

Joshi, Prakash

The formation of complex prebiotic molecules on the early Earth is likely to have involved a component of mineral catalysis. Amongst the variety of clay minerals that have been investigated by us for their ability to catalyze the formation of RNA oligomers is montmorillonite. These are 2:1 layer silicates that have a wide range of chemical compositions [(Na,Ca)0.33(Al,Fe,Mg)2(Si,Al)4O10(OH)2.nH2O]. They are commonly produced by the weathering of silicic volcanic ashes to form Bentonite. Once formed, montmorillonites gradually transform to Illites at a modest pressure and temperature. Of the many samples of montmorillonite that we have experimentally examined, a selected subset has been observed to be catalytic for RNA synthesis (Joshi et. al., 2009; Aldersley et al., 2011). Those that have been observed to be excellent catalysts come from a restricted range of elemental compositions. The recent identification of phyllosilicates including montmorillonite on Mars (Bishop et al., 2008) raises the possibility that such processes may have taken place there too. The extent of catalysis depended not only upon the magnitude of the negative charge on the montmorillonite lattice and the number of cations associated with it, but also on the pH at which the reaction is promoted. The isotherm and catalysis studies were extended to provide binding information and catalytic outcomes over a wide pH range. When cations in raw montmorillonite are completely replaced by sodium ions, the resulting Na+-montmorillonite does not catalyze oligomer formation because the ions saturate the interlayer between the platelets of montmorillonite, which blocks the binding of the activated monomers. Acid washed montmorillonite titrated to pH 6-8 with alkali metal ions, serves as the model catalyst for this RNA synthesis (Aldersley et. al., 2011). The optimal binding occurred in the region of maximal oligomer formation. X-ray diffraction studies revealed changes in layer separations of

A Course in Heterogeneous Catalysis Involving Video-Based Seminars.

ERIC Educational Resources Information Center

White, Mark G.

1984-01-01

A video-based format was used during a graduate seminar course designed to educate students on the nature of catalysis, to help transfer information among students working on similar problems, and to improve communication skills. The mechanics of and student reaction to this seminar course are discussed. (JN)

Surface- and Tip-Enhanced Raman Spectroscopy in Catalysis

PubMed Central

2016-01-01

Surface- and tip-enhanced Raman spectroscopy (SERS and TERS) techniques exhibit highly localized chemical sensitivity, making them ideal for studying chemical reactions, including processes at catalytic surfaces. Catalyst structures, adsorbates, and reaction intermediates can be observed in low quantities at hot spots where electromagnetic fields are the strongest, providing ample opportunities to elucidate reaction mechanisms. Moreover, under ideal measurement conditions, it can even be used to trigger chemical reactions. However, factors such as substrate instability and insufficient signal enhancement still limit the applicability of SERS and TERS in the field of catalysis. By the use of sophisticated colloidal synthesis methods and advanced techniques, such as shell-isolated nanoparticle-enhanced Raman spectroscopy, these challenges could be overcome. PMID:27075515

Selective Catalytic Reduction over Cu/SSZ-13: Linking Homo- and Heterogeneous Catalysis

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gao, Feng; Mei, Donghai; Wang, Yilin

Active centers in Cu/SSZ-13 selective catalytic reduction (SCR) catalysts have been recently identified as isolated Cu2+ and [CuII(OH)]+ ions. A redox reaction mechanism has also been established, where Cu-ions cycle between CuI and CuII oxidation states during SCR reaction. While the mechanism for the reduction half-cycle (CuII CuI) is reasonably well understood, that for the oxidation half-cycle (CuI CuII) remains an unsettled debate. Herein we report detailed reaction kinetics on low-temperature standard NH3-SCR, supplemented by DFT calculations, as strong evidence that the low-temperature oxidation half-cycle occurs with the participation of two isolated CuI ions, via formation of a transient [CuI(NH3)2]+-O2-[CuI(NH3)2]+more » intermediate. The feasibility of this reaction mechanism is confirmed from DFT calculations, and the simulated energy barrier and rate constants are consistent with experimental findings. Significantly, the low-temperature standard SCR mechanism proposed here provides full consistency with low-temperature SCR kinetics. The authors gratefully acknowledge the US Department of Energy (DOE), Energy Efficiency and Renewable Energy, Vehicle Technologies Office for the support of this work. The research described in this paper was performed in the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is operated for the US DOE by Battelle.« less

Heterogeneous Catalysis of Polyoxometalate Based Organic–Inorganic Hybrids

PubMed Central

Ren, Yuanhang; Wang, Meiyin; Chen, Xueying; Yue, Bin; He, Heyong

2015-01-01

Organic–inorganic hybrid polyoxometalate (POM) compounds are a subset of materials with unique structures and physical/chemical properties. The combination of metal-organic coordination complexes with classical POMs not only provides a powerful way to gain multifarious new compounds but also affords a new method to modify and functionalize POMs. In parallel with the many reports on the synthesis and structure of new hybrid POM compounds, the application of these compounds for heterogeneous catalysis has also attracted considerable attention. The hybrid POM compounds show noteworthy catalytic performance in acid, oxidation, and even in asymmetric catalytic reactions. This review summarizes the design and synthesis of organic–inorganic hybrid POM compounds and particularly highlights their recent progress in heterogeneous catalysis. PMID:28788017

Reduction potentials of protein disulfides and catalysis of glutathionylation and deglutathionylation by glutaredoxin enzymes.

PubMed

Ukuwela, Ashwinie A; Bush, Ashley I; Wedd, Anthony G; Xiao, Zhiguang

2017-11-09

Glutaredoxins (Grxs) are a class of GSH (glutathione)-dependent thiol-disulfide oxidoreductase enzymes. They use the cellular redox buffer GSSG (glutathione disulfide)/GSH directly to catalyze these exchange reactions. Grxs feature dithiol active sites and can shuttle rapidly between three oxidation states, namely dithiol Grx(SH) 2 , mixed disulfide Grx(SH)(SSG) and oxidized disulfide Grx(SS). Each is characterized by a distinct standard reduction potential [Formula: see text] The [Formula: see text] values for the redox couple Grx(SS)/Grx(SH) 2 are available, but a recent estimate differs by over 100 mV from the literature values. No estimates are available for [Formula: see text] for the mixed disulfide couple Grx(SH)(SSG)/(Grx(SH) 2  + GSH). This work determined both [Formula: see text] and [Formula: see text] for two representative Grx enzymes, Homo sapiens HsGrx1 and Escherichia coli EcGrx1. The empirical approaches were verified rigorously to overcome the sensitivity of these redox-labile enzymes to experimental conditions. The classic method of acid 'quenching' was demonstrated to shift the thiol-disulfide redox equilibria. Both enzymes exhibit an [Formula: see text] (vs. SHE) at a pH of 7.0. Their [Formula: see text] values (-213 and -230 mV for EcGrx1 and HsGrx1, respectively) are slightly less negative than that ([Formula: see text]) of the redox buffer GSSG/2GSH. Both [Formula: see text] and [Formula: see text] vary with log [GSH], but the former more sensitively by a factor of 2. This confers dual catalytic functions to a Grx enzyme as either an oxidase at low [GSH] or as a reductase at high [GSH]. Consequently, these enzymes can participate efficiently in either glutathionylation or deglutathionylation. The catalysis is demonstrated to proceed via a monothiol ping-pong mechanism relying on a single Cys residue only in the dithiol active site. © 2017 The Author(s). Published by Portland Press Limited on behalf of the Biochemical Society.

Hydrolytic Glycosidic Bond Cleavage in RNA Nucleosides: Effects of the 2'-Hydroxy Group and Acid-Base Catalysis.

PubMed

Lenz, Stefan A P; Kohout, Johnathan D; Wetmore, Stacey D

2016-12-22

Despite the inherent stability of glycosidic linkages in nucleic acids that connect the nucleobases to sugar-phosphate backbones, cleavage of these bonds is often essential for organism survival. The current study uses DFT (B3LYP) to provide a fundamental understanding of the hydrolytic deglycosylation of the natural RNA nucleosides (A, C, G, and U), offers a comparison to DNA hydrolysis, and examines the effects of acid, base, or simultaneous acid-base catalysis on RNA deglycosylation. By initially examining HCOO - ···H 2 O mediated deglycosylation, the barriers for RNA hydrolysis were determined to be 30-38 kJ mol -1 higher than the corresponding DNA barriers, indicating that the 2'-OH group stabilizes the glycosidic bond. Although the presence of HCOO - as the base (i.e., to activate the water nucleophile) reduces the barrier for uncatalyzed RNA hydrolysis (i.e., unactivated H 2 O nucleophile) by ∼15-20 kJ mol -1 , the extreme of base catalysis as modeled using a fully deprotonated water molecule (i.e., OH - nucleophile) decreases the uncatalyzed barriers by up to 65 kJ mol -1 . Acid catalysis was subsequently examined by selectively protonating the hydrogen-bond acceptor sites of the RNA nucleobases, which results in an up to ∼80 kJ mol -1 barrier reduction relative to the corresponding uncatalyzed pathway. Interestingly, the nucleobase proton acceptor sites that result in the greatest barrier reductions match sites typically targeted in enzyme-catalyzed reactions. Nevertheless, simultaneous acid and base catalysis is the most beneficial way to enhance the reactivity of the glycosidic bonds in RNA, with the individual effects of each catalytic approach being weakened, additive, or synergistic depending on the strength of the base (i.e., degree of water nucleophile activation), the nucleobase, and the hydrogen-bonding acceptor site on the nucleobase. Together, the current contribution provides a greater understanding of the reactivity of the glycosidic

Probing the Origin of the Compromised Catalysis of E. coli Alkaline Phosphatase in its Promiscuous Sulfatase Reaction

PubMed Central

Catrina, Irina; O'Brien, Patrick J.; Purcell, Jamie; Nikolic-Hughes, Ivana; Zalatan, Jesse G.; Hengge, Alvan C.; Herschlag, Daniel

2008-01-01

The catalytic promiscuity of E. coli alkaline phosphatase (AP) and many other enzymes provides a unique opportunity to dissect the origin of enzymatic rate enhancements via a comparative approach. Here we use kinetic isotope effects (KIEs) to explore the origin of the 109-fold greater catalytic proficiency by AP for phosphate monoester hydrolysis relative to sulfate monoester hydrolysis. The primary 18O KIEs for the leaving group oxygen atoms in the AP-catalyzed hydrolysis of p-nitrophenyl phosphate (pNPP) and p-nitrophenylsulfate (pNPS) decrease relative to the values observed for nonenzymatic hydrolysis reactions. Prior linear free energy relationship results suggest that the transition states for AP-catalyzed reactions of phosphate and sulfate esters are ‘loose’ and indistinguishable from that in solution, suggesting that the decreased primary KIEs do not reflect a change in the nature of the transition state but rather a strong interaction of the leaving group oxygen atom with an active site Zn2+ ion. Furthermore, the KIEs for the two reactions are identical within error, suggesting that the differential catalysis of these reactions cannot be attributed to differential stabilization of the leaving group. In contrast, AP perturbs the KIE for the nonbridging oxygen atoms in the reaction of pNPP but not pNPS, suggesting a differential interaction with the transferred group in the transition state. These and prior results are consistent with a strong electrostatic interaction between the active site bimetallo Zn2+ cluster and one of the nonbridging oxygen atoms on the transferred group. We suggest that the lower charge density of this oxygen atom on a transferred sulfuryl group accounts for a large fraction of the decreased stabilization of the transition state for its reaction relative to phosphoryl transfer. PMID:17411045

Reactivity, characterization, equilibrium thermodynamics and hetero-bimetallic studies of tridentate and tetradentate complexes relevant to syngas catalysis

NASA Astrophysics Data System (ADS)

Imler, Gregory H.

The unifying objective of this work is the study of model systems that will assist in the development of new metal catalysts capable of converting carbon monoxide and hydrogen (syngas) into organic oxygenates at mild temperature and pressure. The selective catalytic transformation of carbon monoxide and hydrogen to liquid fuels and chemical feedstocks represents a major "Grand Challenge" of catalysis science. A core objective is the study of a macrocycle that is related to a porphyrin ligand in order to circumvent some of the disadvantages of utilizing porphyrins in catalysis. The rhodium complex of the macrocycle dibenzotetramethylaza[14]annulene ([(tmtaa)Rh]2]) was reacted with a series of small molecules relevant to CO reduction and hydrogenation. Several complexes were formed that demonstrated the ability to partially reduce and hydrogenate carbon monoxide, including the dirhodium ketone (Rh-C(O)-Rh) and an example of a thermodynamically favorable metal formyl complex ((tmtaa)Rh-C(O)H). An important feature of this work is the measurement of thermodynamic data to provide experimental benchmarks for obtaining key species in CO reduction and hydrogenation. A thorough study of the (tmtaa)Rh system will help in identifying structural features that assist or hinder CO hydrogenation. All reactions are monitored by 1H NMR which permits determination of solution thermodynamics from equilibrium constants obtained by NMR peak integrations. DFT calculations have supplemented experimental results by providing estimates to compare with the experimentally determined thermodynamic data. These computations also provide insight into the structures and thermodynamics of species that cannot be observed directly such as short lived intermediates and thermodynamically unfavorable products. Heterobimetallic complexes of (tmtaa)Rh• with a second metal radical have been utilized to attempt to convert CO to organic products. Reactivity with CO and H2 has been accomplished, with most

Linking protein motion to enzyme catalysis.

PubMed

Singh, Priyanka; Abeysinghe, Thelma; Kohen, Amnon

2015-01-13

Enzyme motions on a broad range of time scales can play an important role in various intra- and intermolecular events, including substrate binding, catalysis of the chemical conversion, and product release. The relationship between protein motions and catalytic activity is of contemporary interest in enzymology. To understand the factors influencing the rates of enzyme-catalyzed reactions, the dynamics of the protein-solvent-ligand complex must be considered. The current review presents two case studies of enzymes-dihydrofolate reductase (DHFR) and thymidylate synthase (TSase)-and discusses the role of protein motions in their catalyzed reactions. Specifically, we will discuss the utility of kinetic isotope effects (KIEs) and their temperature dependence as tools in probing such phenomena.

Improving gold catalysis of nitroarene reduction with surface Pd

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pretzer, Lori A.; Heck, Kimberly N.; Kim, Sean S.

2016-04-01

Nitroarene reduction reactions are commercialized catalytic processes that play a key role in the synthesisof many products including medicines, rubbers, dyes, and herbicides. Whereas bimetallic compositionshave been studied, a better understanding of the bimetallic structure effects may lead to improved indus-trial catalysts. In this work, the influence of surface palladium atoms supported on 3-nm Au nanoparticles(Pd-on-Au NPs) on catalytic activity for 4-nitrophenol reduction is explored. Batch reactor studies indi-cate Pd-on-Au NPs exhibit maximum catalytic activity at a Pd surface coverage of 150 sc%, with aninitial turnover frequency of ~3.7 mol-nitrophenol/mol-metalsurface/s, which was ~5.5× and ~13× moreactive than pure Au NPsmore » and Pd NPs, respectively. Pd NPs, Au NPs, and Pd-on-Au NPs below 175 sc%show compensation behavior. Three-dimensional Pd surface ensembles (with ~4–5 atoms) previouslyidentified through X-ray adsorption spectroscopy provide the active sites responsible for the catalyticmaximum. These results demonstrate the ability to adjust systematically a structural feature (i.e., Pdsurface coverage) to yield a more active material.« less

Regulating the surface of nanoceria and its applications in heterogeneous catalysis

NASA Astrophysics Data System (ADS)

Ma, Yuanyuan; Gao, Wei; Zhang, Zhiyun; Zhang, Sai; Tian, Zhimin; Liu, Yuxuan; Ho, Johnny C.; Qu, Yongquan

2018-03-01

Ceria (CeO2) as a support, additive, and active component for heterogeneous catalysis has been demonstrated to have great catalytic performance, which includes excellent thermal structural stability, catalytic efficiency, and chemoselectivity. Understanding the surface properties of CeO2 and the chemical reactions occurred on the corresponding interfaces is of great importance in the rational design of heterogeneous catalysts for various reactions. In general, the reversible Ce3+/Ce4+ redox pair and the surface acid-base properties contribute to the superior intrinsic catalytic capability of CeO2, and hence yield enhanced catalytic phenomenon in many reactions. Particularly, nanostructured CeO2 is characterized by a large number of surface-bound defects, which are primarily oxygen vacancies, as the surface active catalytic sites. Many efforts have therefore been made to control the surface defects and properties of CeO2 by various synthetic strategies and post-treatments. The present review provides a comprehensive overview of recent progress in regulating the surface structure and composition of CeO2 and its applications in catalysis.

Electrostatic transition state stabilization rather than reactant destabilization provides the chemical basis for efficient chorismate mutase catalysis.

PubMed

Burschowsky, Daniel; van Eerde, André; Ökvist, Mats; Kienhöfer, Alexander; Kast, Peter; Hilvert, Donald; Krengel, Ute

2014-12-09

For more than half a century, transition state theory has provided a useful framework for understanding the origins of enzyme catalysis. As proposed by Pauling, enzymes accelerate chemical reactions by binding transition states tighter than substrates, thereby lowering the activation energy compared with that of the corresponding uncatalyzed process. This paradigm has been challenged for chorismate mutase (CM), a well-characterized metabolic enzyme that catalyzes the rearrangement of chorismate to prephenate. Calculations have predicted the decisive factor in CM catalysis to be ground state destabilization rather than transition state stabilization. Using X-ray crystallography, we show, in contrast, that a sluggish variant of Bacillus subtilis CM, in which a cationic active-site arginine was replaced by a neutral citrulline, is a poor catalyst even though it effectively preorganizes chorismate for the reaction. A series of high-resolution molecular snapshots of the reaction coordinate, including the apo enzyme, and complexes with substrate, transition state analog and product, demonstrate that an active site, which is only complementary in shape to a reactive substrate conformer, is insufficient for effective catalysis. Instead, as with other enzymes, electrostatic stabilization of the CM transition state appears to be crucial for achieving high reaction rates.

Advances in Gold Catalysis and Understanding the Catalytic Mechanism.

PubMed

Ishida, Tamao; Koga, Hiroaki; Okumura, Mitsutaka; Haruta, Masatake

2016-10-01

When gold is deposited as nanoparticles (NPs) with mean diameters of 2-5 nm or clusters with mean diameters below 2 nm onto a variety of supports such as metal oxides, carbons, polymers, etc., the supported Au NPs exhibit unique catalytic properties, while bulk Au is almost inert as a catalyst. A lot of research works indicate that the key factors of the catalysis by supported Au NPs are the selection of the supports, the control of the Au NP size, the shape of the Au NPs, and the strong junction between Au NPs and the supports, because the perimeter zone around Au NPs acts as the active site for many reactions. In order to elucidate the origin of catalysis by supported Au NPs, the interplay between physicochemical analysis, computational studies, and rational experiments for catalysis by supported Au NPs is becoming more and more important. This article summarizes our experiences and progress in such interplay. © 2016 The Chemical Society of Japan & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Gold Redox Catalysis through Base-Initiated Diazonium Decomposition toward Alkene, Alkyne, and Allene Activation.

PubMed

Dong, Boliang; Peng, Haihui; Motika, Stephen E; Shi, Xiaodong

2017-08-16

The discovery of photoassisted diazonium activation toward gold(I) oxidation greatly extended the scope of gold redox catalysis by avoiding the use of a strong oxidant. Some practical issues that limit the application of this new type of chemistry are the relative low efficiency (long reaction time and low conversion) and the strict reaction condition control that is necessary (degassing and inert reaction environment). Herein, an alternative photofree condition has been developed through Lewis base induced diazonium activation. With this method, an unreactive Au I catalyst was used in combination with Na 2 CO 3 and diazonium salts to produce a Au III intermediate. The efficient activation of various substrates, including alkyne, alkene and allene was achieved, followed by rapid Au III reductive elimination, which yielded the C-C coupling products with good to excellent yields. Relative to the previously reported photoactivation method, our approach offered greater efficiency and versatility through faster reaction rates and broader reaction scope. Challenging substrates such as electron rich/neutral allenes, which could not be activated under the photoinitiation conditions (<5 % yield), could be activated to subsequently yield the desired coupling products in good to excellent yield. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Recent advances of lanthanum-based perovskite oxides for catalysis

DOE PAGES

Zhu, Huiyuan; Zhang, Pengfei; Dai, Sheng

2015-09-21

There is a need to reduce the use of noble metal elements especially in the field of catalysis, where noble metals are ubiquitously applied. To this end, perovskite oxides, an important class of mixed oxide, have been attracting increasing attention for decades as potential replacements. Benefiting from the extraordinary tunability of their compositions and structures, perovskite oxides can be rationally tailored and equipped with targeted physical and chemical properties e.g. redox behavior, oxygen mobility, and ion conductivity for enhanced catalysis. Recently, the development of highly efficient perovskite oxide catalysts has been extensively studied. This review article summarizes the recent developmentmore » of lanthanum-based perovskite oxides as advanced catalysts for both energy conversion applications and traditional heterogeneous reactions.« less

Catalysis by Metallic Nanoparticles in Solution: Thermosensitive Microgels as Nanoreactors

NASA Astrophysics Data System (ADS)

Roa, Rafael; Angioletti-Uberti, Stefano; Lu, Yan; Dzubiella, Joachim; Piazza, Francesco; Ballauff, Matthias

2018-05-01

Metallic nanoparticles have been used as catalysts for various reactions, and the huge literature on the subject is hard to overlook. In many applications, the nanoparticles must be affixed to a colloidal carrier for easy handling during catalysis. These "passive carriers" (e.g. dendrimers) serve for a controlled synthesis of the nanoparticles and prevent coagulation during catalysis. Recently, hybrids from nanoparticles and polymers have been developed that allow us to change the catalytic activity of the nanoparticles by external triggers. In particular, single nanoparticles embedded in a thermosensitive network made from poly(N-isopropylacrylamide) (PNIPAM) have become the most-studied examples of such hybrids: immersed in cold water, the PNIPAM network is hydrophilic and fully swollen. In this state, hydrophilic substrates can diffuse easily through the network, and react at the surface of the nanoparticles. Above the volume transition located at 32°C, the network becomes hydrophobic and shrinks. Now hydrophobic substrates will preferably diffuse through the network and react with other substrates in the reaction catalyzed by the enclosed nanoparticle. Such "active carriers", may thus be viewed as true nanoreactors that open new ways for the use of nanoparticles in catalysis. In this review, we give a survey on recent work done on these hybrids and their application in catalysis. The aim of this review is threefold: we first review hybrid systems composed of nanoparticles and thermosensitive networks and compare these "active carriers" to other colloidal and polymeric carriers (e.g. dendrimers). In a second step we discuss the model reactions used to obtain precise kinetic data on the catalytic activity of nanoparticles in various carriers and environments. These kinetic data allow us to present a fully quantitative comparison of different nanoreactors. In a final section we shall present the salient points of recent efforts in the theoretical modeling of

Molecular Catalysis of the Electrochemical and Photochemical Reduction of CO2 with Earth-Abundant Metal Complexes. Selective Production of CO vs HCOOH by Switching of the Metal Center.

PubMed

Chen, Lingjing; Guo, Zhenguo; Wei, Xi-Guang; Gallenkamp, Charlotte; Bonin, Julien; Anxolabéhère-Mallart, Elodie; Lau, Kai-Chung; Lau, Tai-Chu; Robert, Marc

2015-09-02

Molecular catalysis of carbon dioxide reduction using earth-abundant metal complexes as catalysts is a key challenge related to the production of useful products--the "solar fuels"--in which solar energy would be stored. A direct approach using sunlight energy as well as an indirect approach where sunlight is first converted into electricity could be used. A Co(II) complex and a Fe(III) complex, both bearing the same pentadentate N5 ligand (2,13-dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene), were synthesized, and their catalytic activity toward CO2 reduction was investigated. Carbon monoxide was formed with the cobalt complex, while formic acid was obtained with the iron-based catalyst, thus showing that the catalysis product can be switched by changing the metal center. Selective CO2 reduction occurs under electrochemical conditions as well as photochemical conditions when using a photosensitizer under visible light excitation (λ > 460 nm, solvent acetonitrile) with the Co catalyst. In the case of the Fe catalyst, selective HCOOH production occurs at low overpotential. Sustained catalytic activity over long periods of time and high turnover numbers were observed in both cases. A catalytic mechanism is suggested on the basis of experimental results and preliminary quantum chemistry calculations.

Importance of Loop L1 Dynamics for Substrate Capture and Catalysis in Pseudomonas aeruginosa d-Arginine Dehydrogenase.

PubMed

Ouedraogo, Daniel; Souffrant, Michael; Vasquez, Sheena; Hamelberg, Donald; Gadda, Giovanni

2017-05-16

Mobile loops located at the active site entrance in enzymes often participate in conformational changes required to shield the reaction from bulk solvent, to control the access of the substrate to the active site, and to position residues for substrate binding and catalysis. In d-arginine dehydrogenase from Pseudomonas aeruginosa (PaDADH), previous crystallographic data suggested that residues 45-47 in the FAD-binding domain and residues 50-56 in the substrate-binding domain in loop L1 could adopt two distinct conformations. In this study, we have used molecular dynamics, kinetics, and fluorescence spectroscopy on the S45A and A46G enzyme variants of PaDADH to investigate the impact of mutations in loop L1 on the catalytic function of the enzyme. Molecular dynamics showed that the mutant enzymes have probabilities of being in open conformations that are higher than that of wild-type PaDADH of loop L1, yielding an increased level of solvent exposure of the active site. In agreement, the flavin fluorescence intensity was ∼2-fold higher in the S45A and A46G enzymes than in wild-type PaDADH, with a 9 nm bathochromic shift of the emission band. In the variant enzymes, the k cat /K m values with d-arginine were ∼13-fold lower than in wild-type PaDADH. Moreover, the pH profiles for the k cat value with d-arginine showed a hollow, consistent with restricted proton movements in catalysis, and no saturation was achieved with the alternate substrate d-leucine in the reductive half-reaction of the variant enzymes. Taken together, the computational and experimental data are consistent with the dynamics of loop L1 being important for substrate capture and catalysis in PaDADH.

The hydrolysis of epoxides catalyzed by inorganic ammonium salts in water: kinetic evidence for hydrogen bond catalysis.

PubMed

Nozière, B; Fache, F; Maxut, A; Fenet, B; Baudouin, A; Fine, L; Ferronato, C

2018-01-17

Naturally-occurring inorganic ammonium ions have been recently reported as efficient catalysts for some organic reactions in water, which contributes to the understanding of the chemistry in some natural environments (soils, seawater, atmospheric aerosols, …) and biological systems, and is also potentially interesting for green chemistry as many of their salts are cheap and non-toxic. In this work, the effect of NH 4 + ions on the hydrolysis of small epoxides in water was studied kinetically. The presence of NH 4 + increased the hydrolysis rate by a factor of 6 to 25 compared to pure water and these catalytic effects were shown not to result from other ions, counter-ions or from acid or base catalysis, general or specific. The small amounts of amino alcohols produced in the reactions were identified as the actual catalysts by obtaining a strong acceleration of the reactions when adding these compounds directly to the epoxides in water. Replacing the amino alcohols by other strong hydrogen-bond donors, such as trifluoroethanol (TFE) or hexafluoroisopropanol (HFIP) gave the same results, demonstrating that the kinetics of these reactions was driven by hydrogen-bond catalysis. Because of the presence of many hydrogen-bond donors in natural environments (for instance amines and hydroxy-containing compounds), hydrogen-bond catalysis is likely to contribute to many reaction rates in these environments.

Visible-Light-Promoted Trifluoromethylthiolation of Styrenes by Dual Photoredox/Halide Catalysis.

PubMed

Honeker, Roman; Garza-Sanchez, R Aleyda; Hopkinson, Matthew N; Glorius, Frank

2016-03-18

Herein, we report a new visible-light-promoted strategy to access radical trifluoromethylthiolation reactions by combining halide and photoredox catalysis. This approach allows for the synthesis of vinyl-SCF3 compounds of relevance in pharmaceutical chemistry directly from alkenes under mild conditions with irradiation from household light sources. Furthermore, alkyl-SCF3-containing cyclic ketone and oxindole derivatives can be accessed by radical-polar crossover semi-pinacol and cyclization processes. Inexpensive halide salts play a crucial role in activating the trifluoromethylthiolating reagent towards photoredox catalysis and aid the formation of the SCF3 radical. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Combining silver catalysis and organocatalysis: a sequential Michael addition/hydroalkoxylation one-pot approach to annulated coumarins.

PubMed

Hack, Daniel; Chauhan, Pankaj; Deckers, Kristina; Hermann, Gary N; Mertens, Lucas; Raabe, Gerhard; Enders, Dieter

2014-10-03

A highly stereoselective one-pot procedure for the synthesis of five-membered annulated hydroxycoumarins has been developed. By merging primary amine catalysis with silver catalysis, a series of functionalized coumarin derivatives were obtained in good yields (up to 91%) and good to excellent enantioselectivities (up to 99% ee) via a Michael addition/hydroalkoxylation reaction. Depending on the substituents on the enynone, the synthesis of annulated six-membered rings is also feasible.

UV Catalysis, Cyanotype Photography, and Sunscreens

NASA Astrophysics Data System (ADS)

Lawrence, Glen D.; Fishelson, Stuart

1999-09-01

This laboratory experiment is intended for a chemistry course for non-science majors. The experiment utilizes one of the earliest photographic processes, the cyanotype process, to demonstrate UV catalysis of chemical reactions. In addition to making photographic prints from negatives, the process can be used to test the effectiveness of sunscreens and the relative efficacy of the SPF (sun protection factor) rating of sunscreens. This is an inexpensive process, requiring solutions of ammonium ferric citrate and potassium ferricyanide, with options to use hydrogen peroxide and ammonium hydroxide solutions. Students can prepare their own UV-sensitized paper with the indicated chemicals and watch the photographic image appear as it is exposed to sunlight or fluorescent UV lamps in a light box designed for use in this experiment. The laboratory experiment should stimulate discussion of UV catalysis, photographic processes and photochemistry, sunscreens, and UV damage to biological organisms. The chemicals used are relatively nontoxic, and the procedure is simple enough to be used by groups of diverse ages and abilities.

Predictive Modeling in Actinide Chemistry and Catalysis

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yang, Ping

2016-05-16

These are slides from a presentation on predictive modeling in actinide chemistry and catalysis. The following topics are covered in these slides: Structures, bonding, and reactivity (bonding can be quantified by optical probes and theory, and electronic structures and reaction mechanisms of actinide complexes); Magnetic resonance properties (transition metal catalysts with multi-nuclear centers, and NMR/EPR parameters); Moving to more complex systems (surface chemistry of nanomaterials, and interactions of ligands with nanoparticles); Path forward and conclusions.

Understanding titanium-catalysed radical-radical reactions: a DFT study unravels the complex kinetics of ketone-nitrile couplings.

PubMed

Streuff, Jan; Himmel, Daniel; Younas, Sara L

2018-04-03

The computational investigation of a titanium-catalysed reductive radical-radical coupling is reported. The results match the conclusions from an earlier experimental study and enable a further interpretation of the previously observed complex reaction kinetics. Furthermore, the interplay between neutral and cationic reaction pathways in titanium(iii)-catalysed reactions is investigated for the first time. The results show that hydrochloride additives and reaction byproducts play an important role in the respective equilibria. A full reaction profile is assembled and the computed activation barrier is found to be in reasonable agreement with the experiment. The conclusions are of fundamental importance to the field of low-valent titanium catalysis and the understanding of related catalytic radical-radical coupling reactions.

Perspective: Defining and quantifying the role of dynamics in enzyme catalysis

PubMed Central

Warshel, Arieh; Bora, Ram Prasad

2016-01-01

Enzymes control chemical reactions that are key to life processes, and allow them to take place on the time scale needed for synchronization between the relevant reaction cycles. In addition to general interest in their biological roles, these proteins present a fundamental scientific puzzle, since the origin of their tremendous catalytic power is still unclear. While many different hypotheses have been put forward to rationalize this, one of the proposals that has become particularly popular in recent years is the idea that dynamical effects contribute to catalysis. Here, we present a critical review of the dynamical idea, considering all reasonable definitions of what does and does not qualify as a dynamical effect. We demonstrate that no dynamical effect (according to these definitions) has ever been experimentally shown to contribute to catalysis. Furthermore, the existence of non-negligible dynamical contributions to catalysis is not supported by consistent theoretical studies. Our review is aimed, in part, at readers with a background in chemical physics and biophysics, and illustrates that despite a substantial body of experimental effort, there has not yet been any study that consistently established a connection between an enzyme’s conformational dynamics and a significant increase in the catalytic contribution of the chemical step. We also make the point that the dynamical proposal is not a semantic issue but a well-defined scientific hypothesis with well-defined conclusions. PMID:27179464

Perspective: Defining and quantifying the role of dynamics in enzyme catalysis.

PubMed

Warshel, Arieh; Bora, Ram Prasad

2016-05-14

Enzymes control chemical reactions that are key to life processes, and allow them to take place on the time scale needed for synchronization between the relevant reaction cycles. In addition to general interest in their biological roles, these proteins present a fundamental scientific puzzle, since the origin of their tremendous catalytic power is still unclear. While many different hypotheses have been put forward to rationalize this, one of the proposals that has become particularly popular in recent years is the idea that dynamical effects contribute to catalysis. Here, we present a critical review of the dynamical idea, considering all reasonable definitions of what does and does not qualify as a dynamical effect. We demonstrate that no dynamical effect (according to these definitions) has ever been experimentally shown to contribute to catalysis. Furthermore, the existence of non-negligible dynamical contributions to catalysis is not supported by consistent theoretical studies. Our review is aimed, in part, at readers with a background in chemical physics and biophysics, and illustrates that despite a substantial body of experimental effort, there has not yet been any study that consistently established a connection between an enzyme's conformational dynamics and a significant increase in the catalytic contribution of the chemical step. We also make the point that the dynamical proposal is not a semantic issue but a well-defined scientific hypothesis with well-defined conclusions.

Metal-Free Carbon-Based Materials: Promising Electrocatalysts for Oxygen Reduction Reaction in Microbial Fuel Cells

PubMed Central

Sawant, Sandesh Y.; Han, Thi Hiep; Cho, Moo Hwan

2016-01-01

Microbial fuel cells (MFCs) are a promising green approach for wastewater treatment with the simultaneous advantage of energy production. Among the various limiting factors, the cathodic limitation, with respect to performance and cost, is one of the main obstacles to the practical applications of MFCs. Despite the high performance of platinum and other metal-based cathodes, their practical use is limited by their high cost, low stability, and environmental toxicity. Oxygen is the most favorable electron acceptor in the case of MFCs, which reduces to water through a complicated oxygen reduction reaction (ORR). Carbon-based ORR catalysts possessing high surface area and good electrical conductivity improve the ORR kinetics by lowering the cathodic overpotential. Recently, a range of carbon-based materials have attracted attention for their exceptional ORR catalytic activity and high stability. Doping the carbon texture with a heteroatom improved their ORR activity remarkably through the favorable adsorption of oxygen and weaker molecular bonding. This review provides better insight into ORR catalysis for MFCs and the properties, performance, and applicability of various metal-free carbon-based electrocatalysts in MFCs to find the most appropriate cathodic catalyst for the practical applications. The approaches for improvement, key challenges, and future opportunities in this field are also explored. PMID:28029116

Bioinspired Electrocatalysis of Oxygen Reduction Reaction in Fuel Cells Using Molecular Catalysts.

PubMed

Zion, Noam; Friedman, Ariel; Levy, Naomi; Elbaz, Lior

2018-04-23

One of the most important chemical reactions for renewable energy technologies such as fuel cells and metal-air batteries today is oxygen reduction. Due to the relatively sluggish reaction kinetics, catalysts are necessary to generate high power output. The most common catalyst for this reaction is platinum, but its scarcity and derived high price have raised the search for abundant nonprecious metal catalysts. Inspired from enzymatic processes which are known to catalyze oxygen reduction reaction efficiently, employing transition metal complexes as their catalytic centers, many are working on the development of bioinspired and biomimetic catalysts of this class. This research news article gives a glimpse of the recent progress on the development of bioinspired molecular catalyst for oxygen reduction, highlighting the importance of the molecular structure of the catalysts, from advancements in porphyrins and phthalocyanines to the most recent work on corroles, and 3D networks such as metal-organic frameworks and polymeric networks, all with nonpyrolyzed, well-defined molecular catalysts for oxygen reduction reaction. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Dynamically Achieved Active Site Precision in Enzyme Catalysis

PubMed Central

2015-01-01

Conspectus The grand challenge in enzymology is to define and understand all of the parameters that contribute to enzymes’ enormous rate accelerations. The property of hydrogen tunneling in enzyme reactions has moved the focus of research away from an exclusive focus on transition state stabilization toward the importance of the motions of the heavy atoms of the protein, a role for reduced barrier width in catalysis, and the sampling of a protein conformational landscape to achieve a family of protein substates that optimize enzyme–substrate interactions and beyond. This Account focuses on a thermophilic alcohol dehydrogenase for which the chemical step of hydride transfer is rate determining across a wide range of experimental conditions. The properties of the chemical coordinate have been probed using kinetic isotope effects, indicating a transition in behavior below 30 °C that distinguishes nonoptimal from optimal C–H activation. Further, the introduction of single site mutants has the impact of either enhancing or eliminating the temperature dependent transition in catalysis. Biophysical probes, which include time dependent hydrogen/deuterium exchange and fluorescent lifetimes and Stokes shifts, have also been pursued. These studies allow the correlation of spatially resolved transitions in protein motions with catalysis. It is now possible to define a long-range network of protein motions in ht-ADH that extends from a dimer interface to the substrate binding domain across to the cofactor binding domain, over a distance of ca. 30 Å. The ongoing challenge to obtaining spatial and temporal resolution of catalysis-linked protein motions is discussed. PMID:25539048

Dynamically achieved active site precision in enzyme catalysis.

PubMed

Klinman, Judith P

2015-02-17

CONSPECTUS: The grand challenge in enzymology is to define and understand all of the parameters that contribute to enzymes' enormous rate accelerations. The property of hydrogen tunneling in enzyme reactions has moved the focus of research away from an exclusive focus on transition state stabilization toward the importance of the motions of the heavy atoms of the protein, a role for reduced barrier width in catalysis, and the sampling of a protein conformational landscape to achieve a family of protein substates that optimize enzyme-substrate interactions and beyond. This Account focuses on a thermophilic alcohol dehydrogenase for which the chemical step of hydride transfer is rate determining across a wide range of experimental conditions. The properties of the chemical coordinate have been probed using kinetic isotope effects, indicating a transition in behavior below 30 °C that distinguishes nonoptimal from optimal C-H activation. Further, the introduction of single site mutants has the impact of either enhancing or eliminating the temperature dependent transition in catalysis. Biophysical probes, which include time dependent hydrogen/deuterium exchange and fluorescent lifetimes and Stokes shifts, have also been pursued. These studies allow the correlation of spatially resolved transitions in protein motions with catalysis. It is now possible to define a long-range network of protein motions in ht-ADH that extends from a dimer interface to the substrate binding domain across to the cofactor binding domain, over a distance of ca. 30 Å. The ongoing challenge to obtaining spatial and temporal resolution of catalysis-linked protein motions is discussed.

Characterizing Chain Processes in Visible Light Photoredox Catalysis

PubMed Central

Cismesia, Megan A.

2015-01-01

The recognition that Ru(bpy)32+ andsimilar visible light absorbing transition metal complexes can be photocatalysts for a variety of synthetically useful organic reactions has resulted in a recent resurgence of interest in photoredox catalysis. However, many of the critical mechanistic aspects of this class of reactions remain poorly understood. In particular, the degree to which visible light photoredox reactions involve radical chain processes has been a point of some disagreement that has not been subjected to systematic analysis. We have now performed quantum yield measurements to demonstrate that threerepresentative, mechanistically distinct photoredox processes involve product-forming chain reactions. Moreover, we show that the combination of quantum yield and luminescence quenching experiments provides a rapid method to estimate the length of these chains. Together, these measurements constitute a robust, operationally facile strategy for characterizing chain processes in a wide range of visible light photoredox reactions. PMID:26668708

Catalysis by Dust Grains in the Solar Nebula

NASA Technical Reports Server (NTRS)

Kress, Monika E.; Tielens, Alexander G. G. M.

1996-01-01

In order to determine whether grain-catalyzed reactions played an important role in the chemistry of the solar nebula, we have applied our time-dependent model of methane formation via Fischer-Tropsch catalysis to pressures from 10(exp -5) to 1 bar and temperatures from 450 to 650 K. Under these physical conditions, the reaction 3H2 + CO yields CH4 + H2O is readily catalyzed by an iron or nickel surface, whereas the same reaction is kinetically inhibited in the gas phase. Our model results indicate that under certain nebular conditions, conversion of CO to methane could be extremely efficient in the presence of iron-nickel dust grains over timescales very short compared to the lifetime of the solar nebula.

High-Potential Electrocatalytic O2 Reduction with Nitroxyl/NOx Mediators: Implications for Fuel Cells and Aerobic Oxidation Catalysis

PubMed Central

2015-01-01

Efficient reduction of O2 to water is a central challenge in energy conversion and many aerobic oxidation reactions. Here, we show that the electrochemical oxygen reduction reaction (ORR) can be achieved at high potentials by using soluble organic nitroxyl and nitrogen oxide (NOx) mediators. When used alone, neither organic nitroxyls, such as 2,2,6,6-tetramethyl-1-piperidinyl-N-oxyl (TEMPO), nor NOx species, such as sodium nitrite, are effective ORR mediators. The combination of nitroxyl/NOx species, however, mediates sustained O2 reduction with overpotentials as low as 300 mV in acetonitrile containing trifluoroacetic acid. Mechanistic analysis of the coupled redox reactions supports a process in which the nitrogen oxide catalyst drives aerobic oxidation of a nitroxyl mediator to an oxoammonium species, which then is reduced back to the nitroxyl at the cathode. The electrolysis potential is dictated by the oxoammonium/nitroxyl reduction potential. The overpotentials accessible with this ORR system are significantly lower than widely studied molecular metal-macrocycle ORR catalysts and benefit from the mechanism-based specificity for four-electron reduction of oxygen to water mediated by NOx species, together with kinetically efficient reduction of oxidized NOx species by TEMPO and other organic nitroxyls. PMID:27162977

Bridging the Gap Between Theory and Experiment to Derive a Detailed Understanding of Hammerhead Ribozyme Catalysis

PubMed Central

Lee, Tai-Sung; Wong, Kin-Yiu; Giambasu, George M.; York, Darrin M.

2016-01-01

Herein we summarize our progress toward the understanding of hammerhead ribozyme (HHR) catalysis through a multiscale simulation strategy. Simulation results collectively paint a picture of HHR catalysis: HHR first folds to form an electronegative active site pocket to recruit a threshold occupation of cationic charges, either a Mg2+ ion or multiple monovalent cations. Catalytically active conformations that have good in-line fitness are supported by specific metal ion coordination patterns that involve either a bridging Mg2+ ion or multiple Na+ ions, one of which is also in a bridging coordination pattern. In the case of a single Mg2+ ion bound in the active site, the Mg2+ ion undergoes a migration that is coupled with deprotonation of the nucleophile (C17:O2′). As the reaction proceeds, the Mg2+ ion stabilizes the accumulating charge of the leaving group and significantly increases the general acid ability of G8:O2′. Further computational mutagenesis simulations suggest that the disruptions due to mutations may severely impact HHR catalysis at different stages of the reaction. Catalytic mechanisms supported by the simulation results are consistent with available structural and biochemical experiments, and together they advance our understanding of HHR catalysis. PMID:24156941

Modeling microbial reaction rates in a submarine hydrothermal vent chimney wall

NASA Astrophysics Data System (ADS)

LaRowe, Douglas E.; Dale, Andrew W.; Aguilera, David R.; L'Heureux, Ivan; Amend, Jan P.; Regnier, Pierre

2014-01-01

The fluids emanating from active submarine hydrothermal vent chimneys provide a window into subseafloor processes and, through mixing with seawater, are responsible for steep thermal and compositional gradients that provide the energetic basis for diverse biological communities. Although several models have been developed to better understand the dynamic interplay of seawater, hydrothermal fluid, minerals and microorganisms inside chimney walls, none provide a fully integrated approach to quantifying the biogeochemistry of these hydrothermal systems. In an effort to remedy this, a fully coupled biogeochemical reaction-transport model of a hydrothermal vent chimney has been developed that explicitly quantifies the rates of microbial catalysis while taking into account geochemical processes such as fluid flow, solute transport and oxidation-reduction reactions associated with fluid mixing as a function of temperature. The metabolisms included in the reaction network are methanogenesis, aerobic oxidation of hydrogen, sulfide and methane and sulfate reduction by hydrogen and methane. Model results indicate that microbial catalysis is generally fastest in the hottest habitable portion of the vent chimney (77-102 °C), and methane and sulfide oxidation peak near the seawater-side of the chimney. The fastest metabolisms are aerobic oxidation of H2 and sulfide and reduction of sulfate by H2 with maximum rates of 140, 900 and 800 pmol cm-3 d-1, respectively. The maximum rate of hydrogenotrophic methanogenesis is just under 0.03 pmol cm-3 d-1, the slowest of the metabolisms considered. Due to thermodynamic inhibition, there is no anaerobic oxidation of methane by sulfate (AOM). These simulations are consistent with vent chimney metabolic activity inferred from phylogenetic data reported in the literature. The model developed here provides a quantitative approach to describing the rates of biogeochemical transformations in hydrothermal systems and can be used to constrain the

Are there dynamical effects in enzyme catalysis? Some thoughts concerning the enzymatic chemical step.

PubMed

Tuñón, Iñaki; Laage, Damien; Hynes, James T

2015-09-15

We offer some thoughts on the much debated issue of dynamical effects in enzyme catalysis, and more specifically on their potential role in the acceleration of the chemical step. Since the term 'dynamics' has been used with different meanings, we find it useful to first return to the Transition State Theory rate constant, its assumptions and the choices it involves, and detail the various sources of deviations from it due to dynamics (or not). We suggest that much can be learned about the key current questions for enzyme catalysis from prior extensive studies of dynamical and other effects in the case of reactions in solution. We analyze dynamical effects both in the neighborhood of the transition state and far from it, together with the situation when quantum nuclear motion is central to the reaction, and we illustrate our discussion with various examples of enzymatic reactions. Copyright © 2015 The Authors. Published by Elsevier Inc. All rights reserved.

CO- and HCl-free synthesis of acid chlorides from unsaturated hydrocarbons via shuttle catalysis

NASA Astrophysics Data System (ADS)

Fang, Xianjie; Cacherat, Bastien; Morandi, Bill

2017-11-01

The synthesis of carboxylic acid derivatives from unsaturated hydrocarbons is an important process for the preparation of polymers, pharmaceuticals, cosmetics and agrochemicals. Despite its industrial relevance, the traditional Reppe-type carbonylation reaction using pressurized CO is of limited applicability to laboratory-scale synthesis because of: (1) the safety hazards associated with the use of CO, (2) the need for special equipment to handle pressurized gas, (3) the low reactivity of several relevant nucleophiles and (4) the necessity to employ different, often tailor-made, catalytic systems for each nucleophile. Herein we demonstrate that a shuttle-catalysis approach enables a CO- and HCl-free transfer process between an inexpensive reagent, butyryl chloride, and a wide range of unsaturated substrates to access the corresponding acid chlorides in good yields. This new transformation provides access to a broad range of carbonyl-containing products through the in situ transformation of the reactive acid chloride intermediate. In a broader context, this work demonstrates that isodesmic shuttle-catalysis reactions can unlock elusive catalytic reactions.

Molecular-Level Design of Heterogeneous Chiral Catalysis

DOE Office of Scientific and Technical Information (OSTI.GOV)

Francisco Zaera

2012-03-21

The following is a proposal to continue our multi-institutional research on heterogeneous chiral catalysis. Our team combines the use of surface-sensitive analytical techniques for the characterization of model systems with quantum and statistical mechanical calculations to interpret experimental data and guide the design of future research. Our investigation focuses on the interrelation among the three main mechanisms by which enantioselectivity can be bestowed to heterogeneous catalysts, namely: (1) by templating chirality via the adsorption of chiral supramolecular assemblies, (2) by using chiral modifiers capable of forming chiral complexes with the reactant and force enantioselective surface reactions, and (3) by formingmore » naturally chiral surfaces using imprinting chiral agents. Individually, the members of our team are leaders in these various aspects of chiral catalysis, but the present program provides the vehicle to generate and exploit the synergies necessary to address the problem in a comprehensive manner. Our initial work has advanced the methodology needed for these studies, including an enantioselective titration procedure to identify surface chiral sites, infrared spectroscopy in situ at the interface between gases or liquids and solids to mimic realistic catalytic conditions, and DFT and Monte Carlo algorithms to simulate and understand chirality on surfaces. The next step, to be funded by the monies requested in this proposal, is to apply those methods to specific problems in chiral catalysis, including the identification of the requirements for the formation of supramolecular surface structures with enantioselective behavior, the search for better molecules to probe the chiral nature of the modified surfaces, the exploration of the transition from supramolecular to one-to-one chiral modification, the correlation of the adsorption characteristics of one-to-one chiral modifiers with their physical properties, in particular with their

Effect of Intermolecular Distance on Surface-Plasmon-Assisted Catalysis.

PubMed

Wu, Shiwei; Liu, Yu; Ma, Caiqing; Wang, Jing; Zhang, Yao; Song, Peng; Xia, Lixin

2018-06-26

4-Aminothiophenol (PATP) and 4-aminophenyl disulfide (APDS) in contact with silver will form H 2 N-C 6 H 4 -S-Ag (PATP-Ag), and under the conditions of surface-enhanced Raman spectroscopy (SERS), a coupling reaction will generate 4,4-dimercaptoazobenzene (DMAB). DMAB is strongly Raman-active, showing strong peaks at ν ≈ 1140, 1390, and 1432 cm -1 , and is widely used in surface-plasmon-assisted catalysis. Using APDS, PATP, p-nitrothiophenol (PNTP), and p-nitrodiphenyl disulfide (NPDS) as probe molecules, Raman spectroscopy and imaging techniques have been used to study the effect of intermolecular distance on surface-plasmon-assisted catalysis. Theoretically, PATP-Ag formed from APDS will be bound at proximal Ag atoms on the Ag surface due to S-S bond cleavage. The results show that APDS is more prone to surface-plasmon-assisted catalytic coupling due to the smaller distance between surface PATP-Ag moieties than those derived from PATP. Therefore, APDS has a higher reaction efficiency, better Raman activity, and better Raman imaging than does PATP. Analogous experiments with PNTP and NPDS gave similar results. Thus, this technique has great application prospects in the fields of surface chemistry and materials chemistry.

High performance platinum single atom electrocatalyst for oxygen reduction reaction

NASA Astrophysics Data System (ADS)

Liu, Jing; Jiao, Menggai; Lu, Lanlu; Barkholtz, Heather M.; Li, Yuping; Wang, Ying; Jiang, Luhua; Wu, Zhijian; Liu, Di-Jia; Zhuang, Lin; Ma, Chao; Zeng, Jie; Zhang, Bingsen; Su, Dangsheng; Song, Ping; Xing, Wei; Xu, Weilin; Wang, Ying; Jiang, Zheng; Sun, Gongquan

2017-07-01

For the large-scale sustainable implementation of polymer electrolyte membrane fuel cells in vehicles, high-performance electrocatalysts with low platinum consumption are desirable for use as cathode material during the oxygen reduction reaction in fuel cells. Here we report a carbon black-supported cost-effective, efficient and durable platinum single-atom electrocatalyst with carbon monoxide/methanol tolerance for the cathodic oxygen reduction reaction. The acidic single-cell with such a catalyst as cathode delivers high performance, with power density up to 680 mW cm-2 at 80 °C with a low platinum loading of 0.09 mgPt cm-2, corresponding to a platinum utilization of 0.13 gPt kW-1 in the fuel cell. Good fuel cell durability is also observed. Theoretical calculations reveal that the main effective sites on such platinum single-atom electrocatalysts are single-pyridinic-nitrogen-atom-anchored single-platinum-atom centres, which are tolerant to carbon monoxide/methanol, but highly active for the oxygen reduction reaction.

One-Pot Synthesis of Fused Pyrroles via a Key Gold Catalysis-Triggered Cascade

PubMed Central

Zheng, Zhitong; Tu, Huangfei

2014-01-01

A two-step, one-pot synthesis of fused pyrroles is realized by firstly condensing N-alkynylhydroxammonium salt with readily enolizable ketone under mild basic condition and then subjecting the reaction mixture to a gold catalyst, which triggers a cascade reaction featured by a facile initial 3.3-sigmatropic rearrangement of the gold catalysis product, i.e., an N,O-dialkenylhydroxamine. The reaction provides a facile access to polycyclic pyrroles in moderate to good yields. PMID:24482098

Hydrogen tunneling links protein dynamics to enzyme catalysis.

PubMed

Klinman, Judith P; Kohen, Amnon

2013-01-01

The relationship between protein dynamics and function is a subject of considerable contemporary interest. Although protein motions are frequently observed during ligand binding and release steps, the contribution of protein motions to the catalysis of bond making/breaking processes is more difficult to probe and verify. Here, we show how the quantum mechanical hydrogen tunneling associated with enzymatic C-H bond cleavage provides a unique window into the necessity of protein dynamics for achieving optimal catalysis. Experimental findings support a hierarchy of thermodynamically equilibrated motions that control the H-donor and -acceptor distance and active-site electrostatics, creating an ensemble of conformations suitable for H-tunneling. A possible extension of this view to methyl transfer and other catalyzed reactions is also presented. The impact of understanding these dynamics on the conceptual framework for enzyme activity, inhibitor/drug design, and biomimetic catalyst design is likely to be substantial.

Hydrogen Tunneling Links Protein Dynamics to Enzyme Catalysis

PubMed Central

Klinman, Judith P.; Kohen, Amnon

2014-01-01

The relationship between protein dynamics and function is a subject of considerable contemporary interest. Although protein motions are frequently observed during ligand binding and release steps, the contribution of protein motions to the catalysis of bond making/breaking processes is more difficult to probe and verify. Here, we show how the quantum mechanical hydrogen tunneling associated with enzymatic C–H bond cleavage provides a unique window into the necessity of protein dynamics for achieving optimal catalysis. Experimental findings support a hierarchy of thermodynamically equilibrated motions that control the H-donor and -acceptor distance and active-site electrostatics, creating an ensemble of conformations suitable for H-tunneling. A possible extension of this view to methyl transfer and other catalyzed reactions is also presented. The impact of understanding these dynamics on the conceptual framework for enzyme activity, inhibitor/drug design, and biomimetic catalyst design is likely to be substantial. PMID:23746260

Aligned carbon nanotube with electro-catalytic activity for oxygen reduction reaction

DOEpatents

Liu, Di-Jia; Yang, Junbing; Wang, Xiaoping

2010-08-03

A catalyst for an electro-chemical oxygen reduction reaction (ORR) of a bundle of longitudinally aligned carbon nanotubes having a catalytically active transition metal incorporated longitudinally in said nanotubes. A method of making an electro-chemical catalyst for an oxygen reduction reaction (ORR) having a bundle of longitudinally aligned carbon nanotubes with a catalytically active transition metal incorporated throughout the nanotubes, where a substrate is in a first reaction zone, and a combination selected from one or more of a hydrocarbon and an organometallic compound containing an catalytically active transition metal and a nitrogen containing compound and an inert gas and a reducing gas is introduced into the first reaction zone which is maintained at a first reaction temperature for a time sufficient to vaporize material therein. The vaporized material is then introduced to a second reaction zone maintained at a second reaction temperature for a time sufficient to grow longitudinally aligned carbon nanotubes over the substrate with a catalytically active transition metal incorporated throughout the nanotubes.

Beyond relationships between homogeneous and heterogeneous catalysis

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dixon, David A.; Katz, Alexander; Arslan, Ilke

2014-08-13

Scientists who regard catalysis as a coherent field have been striving for decades to articulate the fundamental unifying principles. But because these principles seem to be broader than chemistry, chemical engineering, and materials science combined, catalytic scientists commonly interact within the sub-domains of homogeneous, heterogeneous, and bio-catalysis, and increasingly within even narrower domains such as organocatalysis, phase-transfer catalysis, acid-base catalysis, zeolite catalysis, etc. Attempts to unify catalysis have motivated researchers to find relationships between homogeneous and heterogeneous catalysis and to mimic enzymes. These themes have inspired vibrant international meetings and workshops, and we have benefited from the idea exchanges andmore » have some thoughts about a path forward.« less

Cathodic current enhancement via manganese and oxygen related reactions in marine biofilms

NASA Astrophysics Data System (ADS)

Strom, Matthew James

Corrosion is a threat that has economic, and environmental impacts worldwide. Many types of corrosive attack are the subject of ongoing research. One of these areas of research is microbiologically influenced corrosion, which is the enhancement and/or initiation of corrosion events caused by microorganisms. It is well known that colonies of microorganisms can enhance cathodic currents through biofilm formation. The aim of the present work was to elucidate the role of manganese in enhancing cathodic currents in the presence of biofilms. Repeated polarizations conducted in Delaware Bay waters, on biofilm coated Cr identified potentially sustainable reduction reactions. The reduction of MnO2 and the enhancement of the oxygen reduction reaction (ORR) were proven to be factors that influence cathodic current enhancement. The removal of ambient oxygen during polarizations resulted in a shutdown of cathodic current enhancement. These field data led to an exploration of the synergistic relationship between MnO2 and the ORR. Laboratory studies of the catalysis of peroxide disproportionation by MnO2 were monitored using a hanging mercury drop electrode. Experiments were run at an ambient sweater pH of 8 and pH 9, which simulated the near-surface conditions typical of cathodes immersed in seawater. Rapid reoxidation at the more basic pH was shown to allow manganese to behave as a persistent catalyst under the typical electrochemical surface conditions of a cathode. As a result a mechanism for ORR enhancement by manganese was proposed as a unique mechanism for cathodic current enhancement in biofilms. A separate field study of Delaware biofilms on stainless steel coupled to a sacrificial Al anode was carried out to identify the ORR enhancement mechanism and sustainable redox reactions at the cathode. Chemical treatments of glutaraldehyde and formaldoxime were applied to cathodes with biofilms to distinguish between enzymatic and MnO2 related ORR enhancement. The results ruled

Appreciating Formal Similarities in the Kinetics of Homogeneous, Heterogeneous, and Enzyme Catalysis

ERIC Educational Resources Information Center

Ashby, Michael T.

2007-01-01

Because interest in catalysts is widespread, the kinetics of catalytic reactions have been investigated by widely diverse groups of individuals, including chemists, engineers, and biologists. This has lead to redundancy in theories, particularly with regard to the topics of homogeneous, heterogeneous, and enzyme catalysis. From a pedagogical…

High performance platinum single atom electrocatalyst for oxygen reduction reaction

PubMed Central

Liu, Jing; Jiao, Menggai; Lu, Lanlu; Barkholtz, Heather M.; Li, Yuping; Wang, Ying; Jiang, Luhua; Wu, Zhijian; Liu, Di-jia; Zhuang, Lin; Ma, Chao; Zeng, Jie; Zhang, Bingsen; Su, Dangsheng; Song, Ping; Xing, Wei; Xu, Weilin; Wang, Ying; Jiang, Zheng; Sun, Gongquan

2017-01-01

For the large-scale sustainable implementation of polymer electrolyte membrane fuel cells in vehicles, high-performance electrocatalysts with low platinum consumption are desirable for use as cathode material during the oxygen reduction reaction in fuel cells. Here we report a carbon black-supported cost-effective, efficient and durable platinum single-atom electrocatalyst with carbon monoxide/methanol tolerance for the cathodic oxygen reduction reaction. The acidic single-cell with such a catalyst as cathode delivers high performance, with power density up to 680 mW cm−2 at 80 °C with a low platinum loading of 0.09 mgPt cm−2, corresponding to a platinum utilization of 0.13 gPt kW−1 in the fuel cell. Good fuel cell durability is also observed. Theoretical calculations reveal that the main effective sites on such platinum single-atom electrocatalysts are single-pyridinic-nitrogen-atom-anchored single-platinum-atom centres, which are tolerant to carbon monoxide/methanol, but highly active for the oxygen reduction reaction. PMID:28737170

High performance platinum single atom electrocatalyst for oxygen reduction reaction

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liu, Jing; Jiao, Menggai; Lu, Lanlu

For the large-scale sustainable implementation of polymer electrolyte membrane fuel cells in vehicles, high-performance electrocatalysts with low platinum consumption are desirable for use as cathode material during the oxygen reduction reaction in fuel cells. Here we report a carbon black-supported cost-effective, efficient and durable platinum single-atom electrocatalyst with carbon monoxide/methanol tolerance for the cathodic oxygen reduction reaction. The acidic single-cell with such a catalyst as cathode delivers high performance, with power density up to 680 mW cm –2 at 80 °C with a low platinum loading of 0.09 mgPt cm –2, corresponding to a platinum utilization of 0.13 gPt kWmore » –1 in the fuel cell. Good fuel cell durability is also observed. As a result, theoretical calculations reveal that the main effective sites on such platinum single-atom electrocatalysts are single-pyridinic-nitrogen-atom-anchored single-platinum-atom centres, which are tolerant to carbon monoxide/methanol, but highly active for the oxygen reduction reaction.« less

High performance platinum single atom electrocatalyst for oxygen reduction reaction

DOE PAGES

Liu, Jing; Jiao, Menggai; Lu, Lanlu; ...

2017-07-24

For the large-scale sustainable implementation of polymer electrolyte membrane fuel cells in vehicles, high-performance electrocatalysts with low platinum consumption are desirable for use as cathode material during the oxygen reduction reaction in fuel cells. Here we report a carbon black-supported cost-effective, efficient and durable platinum single-atom electrocatalyst with carbon monoxide/methanol tolerance for the cathodic oxygen reduction reaction. The acidic single-cell with such a catalyst as cathode delivers high performance, with power density up to 680 mW cm –2 at 80 °C with a low platinum loading of 0.09 mgPt cm –2, corresponding to a platinum utilization of 0.13 gPt kWmore » –1 in the fuel cell. Good fuel cell durability is also observed. As a result, theoretical calculations reveal that the main effective sites on such platinum single-atom electrocatalysts are single-pyridinic-nitrogen-atom-anchored single-platinum-atom centres, which are tolerant to carbon monoxide/methanol, but highly active for the oxygen reduction reaction.« less

Engaging Alkenyl Halides with Alkylsilicates via Photoredox Dual Catalysis.

PubMed

Patel, Niki R; Kelly, Christopher B; Jouffroy, Matthieu; Molander, Gary A

2016-02-19

Single-electron transmetalation via photoredox/nickel dual catalysis provides the opportunity for the construction of Csp(3)-Csp(2) bonds through the transfer of alkyl radicals under very mild reaction conditions. A general procedure for the cross-coupling of primary and secondary (bis-catecholato)alkylsilicates with alkenyl halides is presented. The developed method allows not only alkenyl bromides and iodides but also previously underexplored alkenyl chlorides to be employed.

Elucidating the mechanism for the reduction of nitrite by copper nitrite reductase--a contribution from quantum chemical studies.

PubMed

De Marothy, S A; Blomberg, M R A; Siegbahn, P E M

2007-01-30

Density functional methods have been applied to investigate the properties of the active site of copper-containing nitrite reductases and possible reaction mechanisms for the enzyme catalysis. The results for a model of the active site indicate that a hydroxyl intermediate is not formed during the catalytic cycle, but rather a state with a protonated nitrite bound to the reduced copper. Electron affinity calculations indicate that reduction of the T2 copper site does not occur immediately after nitrite binding. Proton affinity calculations are indicative of substantial pK(a) differences between different states of the T2 site. The calculations further suggest that the reaction does not proceed until uptake of a second proton from the bulk solution. They also indicate that Asp-92 may play both a key role as a proton donor to the substrate, and a structural role in promoting catalysis. In the D92N mutant another base, presumably a nearby histidine (His-249) may take the role as the proton donor. On the basis of these model calculations and available experimental evidence, an ordered reaction mechanism for the reduction of nitrite is suggested. An investigation of the binding modes of the nitric oxide product and the nitrite substrate to the model site has also been made, indicating that nitric oxide prefers to bind in an end-on fashion to the reduced T2 site.

Homogeneous, Heterogeneous, and Enzymatic Catalysis.

ERIC Educational Resources Information Center

Oyama, S. Ted; Somorjai, Gabor A.

1988-01-01

Discusses three areas of catalysis: homegeneous, heterogeneous, and enzymatic. Explains fundamentals and economic impact of catalysis. Lists and discusses common industrial catalysts. Provides a list of 107 references. (MVL)

Synergistic Effect of Nitrogen in Cobalt Nitride and Nitrogen-Doped Hollow Carbon Spheres for Oxygen Reduction Reaction

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhong, Xing; Liu, Lin; Jiang, Yu

The need for inexpensive and high-activity oxygen reduction reaction (ORR) electrocatalysts has attracted considerable research interest over the past years. Here we report a novel hybrid that contains cobalt nitride/nitrogen-rich hollow carbon spheres (CoxN/NHCS) as a high-performance catalyst for ORR. The CoxN nanoparticles were uniformly dispersed and confined in the hollow NHCS shell. The performance of the resulting CoxN/NHCS hybrid was comparable with that of a commercial Pt/C at the same catalyst loading toward ORR, but the mass activity of the former was 5.7 times better than that of the latter. The nitrogen in both CoxN and NHCS, especially CoxN,more » could weaken the adsorption of reaction intermediates (O and OOH), which follows the favourable reaction pathway on CoxN/NHCS according to the DFT-calculated Gibbs free energy diagrams. Our results demonstrated a new strategy for designing and developing inexpensive, non-precious metal electrocatalysts for next-generation fuels. The authors acknowledge the financial support from the National Basic Research Program (973 program, No. 2013CB733501) and the National Natural Science Foundation of China (No. 21306169, 21101137, 21136001, 21176221 and 91334013). Dr. D. Mei is supported by the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle. Computing time was granted by the grand challenge of computational catalysis of the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL). EMSL is a national scientific user facility located at Pacific Northwest National Laboratory (PNNL) and sponsored by DOE’s Office of Biological and Environmental Research.« less

Surface Modification of Boron-Doped Diamond with Microcrystalline Copper Phthalocyanine: Oxygen Reduction Catalysis.

PubMed

Gan, Patrick; Foord, John S; Compton, Richard G

2015-10-01

Surface modification of boron-doped diamond (BDD) with copper phthalocyanine was achieved using a simple and convenient dropcast deposition, giving rise to a microcrystalline structure. Both unmodified and modified BDD electrodes of different surface terminations (namely hydrogen and oxygen) were compared via the electrochemical reduction of oxygen in aqueous solution. A significant lowering of the cathodic overpotential by about 500 mV was observed after modification of hydrogen-terminated (hydrophobic) diamond, while no voltammetric peak was seen on modified oxidised (hydrophilic) diamond, signifying greater interaction between copper phthalocyanine and the hydrogen-terminated BDD. Oxygen reduction was found to undergo a two-electron process on the modified hydrogen-terminated diamond, which was shown to be also active for the reduction of hydrogen peroxide. The lack of a further conversion of the peroxide was attributed to its rapid diffusion away from the triple phase boundary at which the reaction is expected to exclusively occur.

Surface Modification of Boron-Doped Diamond with Microcrystalline Copper Phthalocyanine: Oxygen Reduction Catalysis

PubMed Central

Gan, Patrick; Foord, John S; Compton, Richard G

2015-01-01

Surface modification of boron-doped diamond (BDD) with copper phthalocyanine was achieved using a simple and convenient dropcast deposition, giving rise to a microcrystalline structure. Both unmodified and modified BDD electrodes of different surface terminations (namely hydrogen and oxygen) were compared via the electrochemical reduction of oxygen in aqueous solution. A significant lowering of the cathodic overpotential by about 500 mV was observed after modification of hydrogen-terminated (hydrophobic) diamond, while no voltammetric peak was seen on modified oxidised (hydrophilic) diamond, signifying greater interaction between copper phthalocyanine and the hydrogen-terminated BDD. Oxygen reduction was found to undergo a two-electron process on the modified hydrogen-terminated diamond, which was shown to be also active for the reduction of hydrogen peroxide. The lack of a further conversion of the peroxide was attributed to its rapid diffusion away from the triple phase boundary at which the reaction is expected to exclusively occur. PMID:26491640

Will water act as a photocatalyst for cluster phase chemical reactions? Vibrational overtone-induced dehydration reaction of methanediol

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kramer, Zeb C.; Takahashi, Kaito; Skodje, Rex T.

2012-04-28

The possibility of water catalysis in the vibrational overtone-induced dehydration reaction of methanediol is investigated using ab initio dynamical simulations of small methanediol-water clusters. Quantum chemistry calculations employing clusters with one or two water molecules reveal that the barrier to dehydration is lowered by over 20 kcal/mol because of hydrogen-bonding at the transition state. Nevertheless, the simulations of the reaction dynamics following OH-stretch excitation show little catalytic effect of water and, in some cases, even show an anticatalytic effect. The quantum yield for the dehydration reaction exhibits a delayed threshold effect where reaction does not occur until the photon energymore » is far above the barrier energy. Unlike thermally induced reactions, it is argued that competition between reaction and the irreversible dissipation of photon energy may be expected to raise the dynamical threshold for the reaction above the transition state energy. It is concluded that quantum chemistry calculations showing barrier lowering are not sufficient to infer water catalysis in photochemical reactions, which instead require dynamical modeling.« less

The balance of electric field and interfacial catalysis in promoting water dissociation in bipolar membranes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yan, Zhifei; Zhu, Liang; Li, Yuguang C.

Bipolar membranes maintain a steady pH in electrolytic cells through water autodissociation at the interface between their cation- and anion-exchange layers. We analyze the balance of electric field and catalysis in accelerating this reaction.

The balance of electric field and interfacial catalysis in promoting water dissociation in bipolar membranes

DOE PAGES

Yan, Zhifei; Zhu, Liang; Li, Yuguang C.; ...

2018-01-01

Bipolar membranes maintain a steady pH in electrolytic cells through water autodissociation at the interface between their cation- and anion-exchange layers. We analyze the balance of electric field and catalysis in accelerating this reaction.

Studying Chemical Reactions, One Bond at a Time, with Single Molecule AFM Techniques

NASA Astrophysics Data System (ADS)

Fernandez, Julio M.

2008-03-01

The mechanisms by which mechanical forces regulate the kinetics of a chemical reaction are unknown. In my lecture I will demonstrate how we use single molecule force-clamp spectroscopy and protein engineering to study the effect of force on the kinetics of thiol/disulfide exchange. Reduction of disulfide bond via the thiol/disulfide exchange chemical reaction is crucial in regulating protein function and is of common occurrence in mechanically stressed proteins. While reduction is thought to proceed through a substitution nucleophilic bimolecular (SN2) reaction, the role of a mechanical force in modulating this chemical reaction is unknown. We apply a constant stretching force to single engineered disulfide bonds and measure their rate of reduction by dithiothreitol (DTT). We find that while the reduction rate is linearly dependent on the concentration of DTT, it is exponentially dependent on the applied force, increasing 10-fold over a 300 pN range. This result predicts that the disulfide bond lengthens by 0.34 å at the transition state of the thiol/disulfide exchange reaction. In addition to DTT, we also study the reduction of the engineered disulfide bond by the E. coli enzyme thioredoxin (Trx). Thioredoxins are enzymes that catalyze disulfide bond reduction in all organisms. As before, we apply a mechanical force in the range of 25-450 pN to the engineered disulfide bond substrate and monitor the reduction of these bonds by individual enzymes. In sharp contrast with the data obtained with DTT, we now observe two alternative forms of the catalytic reaction, the first requiring a reorientation of the substrate disulfide bond, causing a shortening of the substrate polypeptide by 0.76±0.07 å, and the second elongating the substrate disulfide bond by 0.21±0.01 å. These results support the view that the Trx active site regulates the geometry of the participating sulfur atoms, with sub-ångström precision, in order to achieve efficient catalysis. Single molecule

Reactivating Catalytic Surface: Insights into the Role of Hot Holes in Plasmonic Catalysis.

PubMed

Peng, Tianhuan; Miao, Junjian; Gao, Zhaoshuai; Zhang, Linjuan; Gao, Yi; Fan, Chunhai; Li, Di

2018-03-01

Surface plasmon resonance of coinage metal nanoparticles is extensively exploited to promote catalytic reactions via harvesting solar energy. Previous efforts on elucidating the mechanisms of enhanced catalysis are devoted to hot electron-induced photothermal conversion and direct charge transfer to the adsorbed reactants. However, little attention is paid to roles of hot holes that are generated concomitantly with hot electrons. In this work, 13 nm spherical Au nanoparticles with small absorption cross-section are employed to catalyze a well-studied glucose oxidation reaction. Density functional theory calculation and X-ray absorption spectrum analysis reveal that hot holes energetically favor transferring catalytic intermediates to product molecules and then desorbing from the surface of plasmonic catalysts, resulting in the recovery of their catalytic activities. The studies shed new light on the use of the synergy of hot holes and hot electrons for plasmon-promoted catalysis. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Catalytic formal [2+2+1] synthesis of pyrroles from alkynes and diazenes via Ti(II)/Ti(IV) redox catalysis.

PubMed

Gilbert, Zachary W; Hue, Ryan J; Tonks, Ian A

2016-01-01

Pyrroles are structurally important heterocycles. However, the synthesis of polysubstituted pyrroles is often challenging. Here, we report a multicomponent, Ti-catalysed formal [2+2+1] reaction of alkynes and diazenes for the oxidative synthesis of penta- and trisubstituted pyrroles: a nitrenoid analogue to classical Pauson-Khand-type syntheses of cyclopentenones. Given the scarcity of early transition-metal redox catalysis, preliminary mechanistic studies are presented. Initial stoichiometric and kinetic studies indicate that the mechanism of this reaction proceeds through a formally Ti(II)/Ti(IV) redox catalytic cycle, in which an azatitanacyclobutene intermediate, resulting from [2+2] alkyne + Ti imido coupling, undergoes a second alkyne insertion followed by reductive elimination to yield pyrrole and a Ti(II) species. The key component for catalytic turnover is the reoxidation of the Ti(II) species to a Ti(IV) imido via the disproportionation of an η(2)-diazene-Ti(II) complex.

Catalytic formal [2+2+1] synthesis of pyrroles from alkynes and diazenes via TiII/TiIV redox catalysis

NASA Astrophysics Data System (ADS)

Gilbert, Zachary W.; Hue, Ryan J.; Tonks, Ian A.

2016-01-01

Pyrroles are structurally important heterocycles. However, the synthesis of polysubstituted pyrroles is often challenging. Here, we report a multicomponent, Ti-catalysed formal [2+2+1] reaction of alkynes and diazenes for the oxidative synthesis of penta- and trisubstituted pyrroles: a nitrenoid analogue to classical Pauson-Khand-type syntheses of cyclopentenones. Given the scarcity of early transition-metal redox catalysis, preliminary mechanistic studies are presented. Initial stoichiometric and kinetic studies indicate that the mechanism of this reaction proceeds through a formally TiII/TiIV redox catalytic cycle, in which an azatitanacyclobutene intermediate, resulting from [2+2] alkyne + Ti imido coupling, undergoes a second alkyne insertion followed by reductive elimination to yield pyrrole and a TiII species. The key component for catalytic turnover is the reoxidation of the TiII species to a TiIV imido via the disproportionation of an η2-diazene-TiII complex.

Asymmetric Catalysis with Organic Azides and Diazo Compounds Initiated by Photoinduced Electron Transfer.

PubMed

Huang, Xiaoqiang; Webster, Richard D; Harms, Klaus; Meggers, Eric

2016-09-28

Electron-acceptor-substituted aryl azides and α-diazo carboxylic esters are used as substrates for visible-light-activated asymmetric α-amination and α-alkylation, respectively, of 2-acyl imidazoles catalyzed by a chiral-at-metal rhodium-based Lewis acid in combination with a photoredox sensitizer. This novel proton- and redox-neutral method provides yields of up to 99% and excellent enantioselectivities of up to >99% ee with broad functional group compatibility. Mechanistic investigations suggest that an intermediate rhodium enolate complex acts as a reductive quencher to initiate a radical process with the aryl azides and α-diazo carboxylic esters serving as precursors for nitrogen and carbon-centered radicals, respectively. This is the first report on using aryl azides and α-diazo carboxylic esters as substrates for asymmetric catalysis under photoredox conditions. These reagents have the advantage that molecular nitrogen is the leaving group and sole byproduct in this reaction.

Modeling of the oxygen reduction reaction for dense LSM thin films

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yang, Tao; Liu, Jian; Yu, Yang

In this study, the oxygen reduction reaction mechanism is investigated using numerical methods on a dense thin (La 1-xSr x) yMnO 3±δ film deposited on a YSZ substrate. This 1-D continuum model consists of defect chemistry and elementary oxygen reduction reaction steps coupled via reaction rates. The defect chemistry model contains eight species including cation vacancies on the A- and B-sites. The oxygen vacancy is calculated by solving species transportation equations in multiphysics simulations. Due to the simple geometry of a dense thin film, the oxygen reduction reaction was reduced to three elementary steps: surface adsorption and dissociation, incorporation onmore » the surface, and charge transfer across the LSM/YSZ interface. The numerical simulations allow for calculation of the temperature- and oxygen partial pressure-dependent properties of LSM. The parameters of the model are calibrated with experimental impedance data for various oxygen partial pressures at different temperatures. The results indicate that surface adsorption and dissociation is the rate-determining step in the ORR of LSM thin films. With the fine-tuned parameters, further quantitative analysis is performed. The activation energy of the oxygen exchange reaction and the dependence of oxygen non-stoichiometry on oxygen partial pressure are also calculated and verified using the literature results.« less

Modeling of the oxygen reduction reaction for dense LSM thin films

DOE PAGES

Yang, Tao; Liu, Jian; Yu, Yang; ...

2017-10-17

In this study, the oxygen reduction reaction mechanism is investigated using numerical methods on a dense thin (La 1-xSr x) yMnO 3±δ film deposited on a YSZ substrate. This 1-D continuum model consists of defect chemistry and elementary oxygen reduction reaction steps coupled via reaction rates. The defect chemistry model contains eight species including cation vacancies on the A- and B-sites. The oxygen vacancy is calculated by solving species transportation equations in multiphysics simulations. Due to the simple geometry of a dense thin film, the oxygen reduction reaction was reduced to three elementary steps: surface adsorption and dissociation, incorporation onmore » the surface, and charge transfer across the LSM/YSZ interface. The numerical simulations allow for calculation of the temperature- and oxygen partial pressure-dependent properties of LSM. The parameters of the model are calibrated with experimental impedance data for various oxygen partial pressures at different temperatures. The results indicate that surface adsorption and dissociation is the rate-determining step in the ORR of LSM thin films. With the fine-tuned parameters, further quantitative analysis is performed. The activation energy of the oxygen exchange reaction and the dependence of oxygen non-stoichiometry on oxygen partial pressure are also calculated and verified using the literature results.« less

State-of-the-art Nanofabrication in Catalysis.

PubMed

Karim, Waiz; Tschupp, Simon A; Herranz, Juan; Schmidt, Thomas J; Ekinci, Yasin; van Bokhovenac, Jeroen A

2017-04-26

We present recent developments in top-down nanofabrication that have found application in catalysis research. To unravel the complexity of catalytic systems, the design and use of models with control of size, morphology, shape and inter-particle distances is a necessity. The study of well-defined and ordered nanoparticles on a support contributes to the understanding of complex phenomena that govern reactions in heterogeneous and electro-catalysis. We review the strengths and limitations of different nanolithography methods such as electron beam lithography (EBL), photolithography, extreme ultraviolet (EUV) lithography and colloidal lithography for the creation of such highly tunable catalytic model systems and their applications in catalysis. Innovative strategies have enabled particle sizes reaching dimensions below 10 nm. It is now possible to create pairs of particles with distance controlled with an extremely high precision in the order of one nanometer. We discuss our approach to study these model systems at the single-particle level using X-ray absorption spectroscopy and show new ways to fabricate arrays of single nanoparticles or nanoparticles in pairs over a large area using EBL and EUV-achromatic Talbot lithography. These advancements have provided new insights into the active sites in metal catalysts and enhanced the understanding of the role of inter-particle interactions and catalyst supports, such as in the phenomenon of hydrogen spillover. We present a perspective on future directions for employing top-down nanofabrication in heterogeneous and electrocatalysis. The rapid development in nanofabrication and characterization methods will continue to have an impact on understanding of complex catalytic processes.

Catalysis and biocatalysis program

NASA Technical Reports Server (NTRS)

1991-01-01

The annual report presents the fiscal year (FY) 1990 research activities and accomplishments for the Catalysis and Biocatalysis Program of the Advanced Industrial Concepts Division (AICD), Office of Industrial Technologies of the Department of Energy (DOE). The mission of the AICD is to create a balanced program of high risk, long term, directed interdisciplinary research and development that will improve energy efficiency and enhance fuel flexibility in the industrial sector. The Catalysis and Biocatalysis Program's technical activities were organized into five work elements: the Molecular Modeling and Catalysis by Design element; the Applied Microbiology and Genetics element; the Bioprocess Engineering element; the Separations and Novel Chemical Processes element; and the Process Design and Analysis element.

Proton Mediated Chemistry and Catalysis in a Self-Assembled Supramolecular Host

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pluth, Michael; Bergman, Robert; Raymond, Kenneth

2009-04-10

Synthetic supramolecular host assemblies can impart unique reactivity to encapsulated guest molecules. Synthetic host molecules have been developed to carry out complex reactions within their cavities, despite the fact that they lack the type of specifically tailored functional groups normally located in the analogous active sites of enzymes. Over the past decade, the Raymond group has developed a series of self-assembled supramolecules and the Bergman group has developed and studied a number of catalytic transformations. In this Account, we detail recent collaborative work between these two groups, focusing on chemical catalysis stemming from the encapsulation of protonated guests and expandingmore » to acid catalysis in basic solution. We initially investigated the ability of a water-soluble, self-assembled supramolecular host molecule to encapsulate protonated guests in its hydrophobic core. Our study of encapsulated protonated amines revealed rich host-guest chemistry. We established that self-exchange (that is, in-out guest movement) rates of protonated amines were dependent on the steric bulk of the amine rather than its basicity. The host molecule has purely rotational tetrahedral (T) symmetry, so guests with geminal N-methyl groups (and their attendant mirror plane) were effectively desymmetrized; this allowed for the observation and quantification of the barriers for nitrogen inversion followed by bond rotation. Furthermore, small nitrogen heterocycles, such as N-alkylaziridines, N-alkylazetidines, and N-alkylpyrrolidines, were found to be encapsulated as proton-bound homodimers or homotrimers. We further investigated the thermodynamic stabilization of protonated amines, showing that encapsulation makes the amines more basic in the cavity. Encapsulation raises the effective basicity of protonated amines by up to 4.5 pK{sub a} units, a difference almost as large as that between the moderate and strong bases carbonate and hydroxide. The thermodynamic

Catalysis Science Initiative: Catalyst Design by Discovery Informatics

DOE Office of Scientific and Technical Information (OSTI.GOV)

Delgass, William Nicholas; Abu-Omar, Mahdi; Caruthers, James

Catalysts selectively enhance the rates of chemical reactions toward desired products. Such reactions provide great benefit to society in major commercial sectors such as energy production, protecting the environment, and polymer products and thereby contribute heavily to the country’s gross national product. Our premise is that the level of fundamental understanding of catalytic events at the atomic and molecular scale has reached the point that more predictive methods can be developed to shorten the cycle time to new processes. The field of catalysis can be divided into two regimes: heterogeneous and homogeneous. For the heterogeneous catalysis regime, we have usedmore » the water-gas shift (WGS) reaction (CO + H2O + CO2 + H2O) over supported metals as a test bed. Detailed analysis and strong coupling of theory with experiment have led to the following conclusions: • The sequence of elementary steps goes through a COOH intermediate • The CO binding energy is a strong function of coverage of CO adsorbed on the surface in many systems • In the case of Au catalysts, the CO adsorption is generally too weak on surface with close atomic packing, but the enhanced binding at corner atoms (which are missing bonding partners) of cubo-octahedral nanoparticles increases the energy to a near optimal value and produces very active catalysts. • Reaction on the metal alone cannot account for the experimental results. The reaction is dual functional with water activation occurring at the metal-support interface. It is clear from our work that the theory component is essential, not only for prediction of new systems, but also for reconciling data and testing hypotheses regarding potential descriptors. Particularly important is the finding that the interface between nano-sized metal particles and the oxides that are used to support them represent a new state of matter in the sense that the interfacial bonding perturbs the chemical state of both metals atoms and the

Computational Design of Clusters for Catalysis

NASA Astrophysics Data System (ADS)

Jimenez-Izal, Elisa; Alexandrova, Anastassia N.

2018-04-01

When small clusters are studied in chemical physics or physical chemistry, one perhaps thinks of the fundamental aspects of cluster electronic structure, or precision spectroscopy in ultracold molecular beams. However, small clusters are also of interest in catalysis, where the cold ground state or an isolated cluster may not even be the right starting point. Instead, the big question is: What happens to cluster-based catalysts under real conditions of catalysis, such as high temperature and coverage with reagents? Myriads of metastable cluster states become accessible, the entire system is dynamic, and catalysis may be driven by rare sites present only under those conditions. Activity, selectivity, and stability are highly dependent on size, composition, shape, support, and environment. To probe and master cluster catalysis, sophisticated tools are being developed for precision synthesis, operando measurements, and multiscale modeling. This review intends to tell the messy story of clusters in catalysis.

Thermoswitchable catalysis controlled by reversible dispersion/aggregation change of nanoreactors in the presence of α-CD polymers

NASA Astrophysics Data System (ADS)

Li, Yinfeng; Hu, Jie; Niu, Chengrong; Leng, Jinghang; Li, Songjun

2018-06-01

The present work was aimed at preparing a thermosensitive nanoreactor system which could adjust its dispersion/aggregation status according to external temperature change to achieve the switchable catalysis. The mesoporous silica nanoparticle (MSNP) was selected as the framework material of the nanoreactor, and Ag nanoparticles were encapsulated in the mesoporous silica by an in situ reaction. Dodecyl groups were introduced onto MSNP surface, which could transform reversibly between complexation and disassociation with α-cyclodextrin (CD) cavity upon temperature change. It was found that the nanoreactors aggregated and the catalysis was effectively switched ‘off’ in the presence of CD polymers at low temperature (20 °C). However, when the temperature increased to 50 °C, the nanoreactors redispersed and catalysis successfully switched ‘on’.

Thermoswitchable catalysis controlled by reversible dispersion/aggregation change of nanoreactors in the presence of α-CD polymers.

PubMed

Li, Yinfeng; Hu, Jie; Niu, Chengrong; Leng, Jinghang; Li, Songjun

2018-06-01

The present work was aimed at preparing a thermosensitive nanoreactor system which could adjust its dispersion/aggregation status according to external temperature change to achieve the switchable catalysis. The mesoporous silica nanoparticle (MSNP) was selected as the framework material of the nanoreactor, and Ag nanoparticles were encapsulated in the mesoporous silica by an in situ reaction. Dodecyl groups were introduced onto MSNP surface, which could transform reversibly between complexation and disassociation with α-cyclodextrin (CD) cavity upon temperature change. It was found that the nanoreactors aggregated and the catalysis was effectively switched 'off' in the presence of CD polymers at low temperature (20 °C). However, when the temperature increased to 50 °C, the nanoreactors redispersed and catalysis successfully switched 'on'.

Metal-catalyzed Decarboxylative Fluoroalkylation Reactions.

PubMed

Ambler, Brett R; Yang, Ming-Hsiu; Altman, Ryan A

2016-12-01

Metal-catalyzed decarboxylative fluoroalkylation reactions enable the conversion of simple O-based substrates into biologically relevant fluorinated analogs. Herein, we present decarboxylative methods that facilitate the synthesis of trifluoromethyl- and difluoroketone-containing products. We highlight key mechanistic aspects that are critical for efficient catalysis, and that inspired our thinking while developing the reactions.

High-Valent Organometallic Copper and Palladium in Catalysis

PubMed Central

Hickman, Amanda J.; Sanford, Melanie S.

2015-01-01

Preface Copper and palladium catalysts are critically important for numerous commercial chemical processes. Improvements in the activity, selectivity, and scope of these catalysts have the potential to dramatically reduce the environmental impact and increase the sustainability of chemical reactions. One rapidly emerging strategy to achieve these goals is to exploit “high-valent” copper and palladium intermediates in catalysis. This review describes exciting recent advances involving both the fundamental chemistry and the applications of these high-valent metal complexes in numerous synthetically useful catalytic transformations. PMID:22498623

A Continuum of Progress: Applications of N-Hetereocyclic Carbene Catalysis in Total Synthesis

PubMed Central

Izquierdo, Javier; Hutson, Gerri E.; Cohen, Daniel T.; Scheidt, Karl A.

2013-01-01

N-Heterocyclic carbene (NHC) catalyzed transformations have emerged as powerful tactics for the construction of complex molecules. Since Stetter’s report in 1975 of the total synthesis of cis-jasmon and dihydrojasmon by using carbene catalysis, the use of NHCs in total synthesis has grown rapidly, particularly over the last decade. This renaissance is undoubtedly due to the recent developments in NHC-catalyzed reactions, including new benzoin, Stetter, homoenolate, and aroylation processes. These transformations employ typical as well as Umpolung types of bond disconnections and have served as the key step in several new total syntheses. This Minireview highlights these reports and captures the excitement and emerging synthetic utility of carbene catalysis in total synthesis. PMID:23074146

Single Platinum Atoms Electrocatalysts: Oxygen Reduction and Hydrogen Oxidation Reactions

DOE PAGES

Vukmirovic, Miomir B.; Teeluck, Krishani M.; Liu, Ping; ...

2017-08-08

We prepared atomically dispersed catalyst consisting of Pt atoms arranged in a c(2 × 2) array on RuO2(110) substrate. A large interatomic distance of Pt atoms in a c(2 × 2) phase precludes the reactants to interact with more than one Pt atoms. A strong bond of Pt atoms with RuO2 prevents agglomeration of Pt atoms to form 2D-islands or 3D-clusters. The activities of single Pt atom catalyst for the oxygen reduction and hydrogen oxidation reactions were determined and compared with those of bulk Pt. It has lower catalytic activity for the oxygen reduction reaction and similar activity for hydrogenmore » oxidation reaction compared to Pt(111). This was explained by a large calculated up-shift of the dband center of Pt atoms and larger Pt-Pt interatomic distance than that of Pt(111). Our information is of considerable interest for further development of electrocatalysis.« less

Old Yellow Enzyme: Stepwise reduction of nitro-olefins and catalysis of aci-nitro tautomerization

PubMed Central

Meah, Younus; Massey, Vincent

2000-01-01

The Old Yellow Enzyme has been shown to catalyze efficiently the NADPH-linked reduction of nitro-olefins. The reduction of the nitro-olefin proceeds in a stepwise fashion, with formation of a nitronate intermediate that is freely dissociable from the enzyme. The first step involves hydride transfer from the enzyme-reduced flavin to carbon 2 of the nitro-olefin. The protonation of the nitronate at carbon 1 to form the final nitroalkane product also is catalyzed by the enzyme and involves Tyr-196 as an active site acid/base. This residue also is involved in aci-nitro tautomerization of nitroalkanes, the first example of a nonredox reaction catalyzed by the enzyme. PMID:10995477

Iron-Catalyzed C-O Bond Activation: Opportunity for Sustainable Catalysis.

PubMed

Bisz, Elwira; Szostak, Michal

2017-10-23

Oxygen-based electrophiles have emerged as some of the most valuable cross-coupling partners in organic synthesis due to several major strategic and environmental benefits, such as abundance and potential to avoid toxic halide waste. In this context, iron-catalyzed C-O activation/cross-coupling holds particular promise to achieve sustainable catalytic protocols due to its natural abundance, inherent low toxicity, and excellent economic and ecological profile. Recently, tremendous progress has been achieved in the development of new methods for functional-group-tolerant iron-catalyzed cross-coupling reactions by selective C-O cleavage. These methods establish highly attractive alternatives to traditional cross-coupling reactions by using halides as electrophilic partners. In particular, new easily accessible oxygen-based electrophiles have emerged as substrates in iron-catalyzed cross-coupling reactions, which significantly broaden the scope of this catalysis platform. New mechanistic manifolds involving iron catalysis have been established; thus opening up vistas for the development of a wide range of unprecedented reactions. The synthetic potential of this sustainable mode of reactivity has been highlighted by the development of new strategies in the construction of complex motifs, including in target synthesis. The most recent advances in sustainable iron-catalyzed cross-coupling of C-O-based electrophiles are reviewed, with a focus on both mechanistic aspects and synthetic utility. It should be noted that this catalytic manifold provides access to motifs that are often not easily available by other methods, such as the assembly of stereodefined dienes or C(sp 2 )-C(sp 3 ) cross-couplings, thus emphasizing the synthetic importance of this mode of reactivity. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Some reflections on the understanding of the oxygen reduction reaction at Pt(111)

PubMed Central

Gómez-Marín, Ana M; Rizo, Ruben

2013-01-01

Summary The oxygen reduction reaction (ORR) is a pivotal process in electrochemistry. Unfortunately, after decades of intensive research, a fundamental knowledge about its reaction mechanism is still lacking. In this paper, a global and critical view on the most important experimental and theoretical results regarding the ORR on Pt(111) and its vicinal surfaces, in both acidic and alkaline media, is taken. Phenomena such as the ORR surface structure sensitivity and the lack of a reduction current at high potentials are discussed in the light of the surface oxidation and disordering processes and the possible relevance of the hydrogen peroxide reduction and oxidation reactions in the ORR mechanism. The necessity to build precise and realistic reaction models, which are deducted from reliable experimental results that need to be carefully taken under strict working conditions is shown. Therefore, progress in the understanding of this important reaction on a molecular level, and the choice of the right approach for the design of the electrocatalysts for fuel-cell cathodes is only possible through a cooperative approach between theory and experiments. PMID:24455454

Some reflections on the understanding of the oxygen reduction reaction at Pt(111).

PubMed

Gómez-Marín, Ana M; Rizo, Ruben; Feliu, Juan M

2013-12-27

The oxygen reduction reaction (ORR) is a pivotal process in electrochemistry. Unfortunately, after decades of intensive research, a fundamental knowledge about its reaction mechanism is still lacking. In this paper, a global and critical view on the most important experimental and theoretical results regarding the ORR on Pt(111) and its vicinal surfaces, in both acidic and alkaline media, is taken. Phenomena such as the ORR surface structure sensitivity and the lack of a reduction current at high potentials are discussed in the light of the surface oxidation and disordering processes and the possible relevance of the hydrogen peroxide reduction and oxidation reactions in the ORR mechanism. The necessity to build precise and realistic reaction models, which are deducted from reliable experimental results that need to be carefully taken under strict working conditions is shown. Therefore, progress in the understanding of this important reaction on a molecular level, and the choice of the right approach for the design of the electrocatalysts for fuel-cell cathodes is only possible through a cooperative approach between theory and experiments.

Cosmic strings and baryon decay catalysis

NASA Technical Reports Server (NTRS)

Gregory, Ruth; Perkins, W. B.; Davis, A.-C.; Brandenberger, R. H.

1989-01-01

Cosmic strings, like monopoles, can catalyze proton decay. For integer charged fermions, the cross section for catalysis is not amplified, unlike in the case of monopoles. The catalysis processes are reviewed both in the free quark and skyrmion pictures and the implications for baryogenesis are discussed. A computation of the cross section for monopole catalyzed skyrmion decay is presented using classical physics. Also discussed are some effects which can screen catalysis processes.

Design of a new reactor-like high temperature near ambient pressure scanning tunneling microscope for catalysis studies.

PubMed

Tao, Franklin Feng; Nguyen, Luan; Zhang, Shiran

2013-03-01

Here, we present the design of a new reactor-like high-temperature near ambient pressure scanning tunneling microscope (HT-NAP-STM) for catalysis studies. This HT-NAP-STM was designed for exploration of structures of catalyst surfaces at atomic scale during catalysis or under reaction conditions. In this HT-NAP-STM, the minimized reactor with a volume of reactant gases of ∼10 ml is thermally isolated from the STM room through a shielding dome installed between the reactor and STM room. An aperture on the dome was made to allow tip to approach to or retract from a catalyst surface in the reactor. This dome minimizes thermal diffusion from hot gas of the reactor to the STM room and thus remains STM head at a constant temperature near to room temperature, allowing observation of surface structures at atomic scale under reaction conditions or during catalysis with minimized thermal drift. The integrated quadrupole mass spectrometer can simultaneously measure products during visualization of surface structure of a catalyst. This synergy allows building an intrinsic correlation between surface structure and its catalytic performance. This correlation offers important insights for understanding of catalysis. Tests were done on graphite in ambient environment, Pt(111) in CO, graphene on Ru(0001) in UHV at high temperature and gaseous environment at high temperature. Atom-resolved surface structure of graphene on Ru(0001) at 500 K in a gaseous environment of 25 Torr was identified.

Probing eudesmane cation-π interactions in catalysis by aristolochene synthase with non-canonical amino acids.

PubMed

Faraldos, Juan A; Antonczak, Alicja K; González, Verónica; Fullerton, Rebecca; Tippmann, Eric M; Allemann, Rudolf K

2011-09-07

Stabilization of the reaction intermediate eudesmane cation (3) through interaction with Trp 334 during catalysis by aristolochene synthase from Penicillium roqueforti was investigated by site-directed incorporation of proteinogenic and non-canonical aromatic amino acids. The amount of germacrene A (2) generated by the mutant enzymes served as a measure of the stabilization of 3. 2 is a neutral intermediate, from which 3 is formed during PR-AS catalysis by protonation of the C6,C7 double bond. The replacement of Trp 334 with para-substituted phenylalanines of increasing electron-withdrawing properties led to a progressive accumulation of 2 that showed a good correlation with the interaction energies of simple cations such as Na(+) with substituted benzenes. These results provide compelling evidence for the stabilizing role played by Trp 334 in aristolochene synthase catalysis for the energetically demanding transformation of 2 to 3.

Steering Asymmetric Lewis Acid Catalysis Exclusively with Octahedral Metal-Centered Chirality.

PubMed

Zhang, Lilu; Meggers, Eric

2017-02-21

Catalysts for asymmetric synthesis must be chiral. Metal-based asymmetric catalysts are typically constructed by assembling chiral ligands around a central metal. In this Account, a new class of effective chiral Lewis acid catalysts is introduced in which the octahedral metal center constitutes the exclusive source of chirality. Specifically, the here discussed class of catalysts are composed of configurationally stable, chiral-at-metal Λ-configured (left-handed propeller) or Δ-configured (right-handed propeller) iridium(III) or rhodium(III) complexes containing two bidentate cyclometalating 5-tert-butyl-2-phenylbenzoxazole (dubbed IrO and RhO) or 5-tert-butyl-2-phenylbenzothiazole (dubbed IrS and RhS) ligands in addition to two exchange-labile acetonitriles. They are synthetically accessible in an enantiomerically pure fashion through a convenient auxiliary-mediated synthesis. Such catalysts are of interest due to their intrinsic structural simplicity (only achiral ligands) and the prospect of an especially effective asymmetric induction due to the intimate contact between the chiral metal center and the metal-coordinated substrates or reagents. With respect to chiral Lewis acid catalysis, the bis-cyclometalated iridium and rhodium complexes provide excellent catalytic activities and asymmetric inductions for a variety of reactions including Michael additions, Friedel-Crafts reactions, cycloadditions, α-aminations, α-fluorinations, Mannich reactions, and a cross-dehydrogenative coupling. Mechanistically, substrates such as 2-acyl imidazoles are usually activated by two-point binding. Exceptions exist as for example for an efficient iridium-catalyzed enantioselective transfer hydrogenation of arylketones with ammonium formate, which putatively proceeds through an iridium-hydride intermediate. The bis-cyclometalated iridium complexes catalyze visible-light-induced asymmetric reactions by intertwining asymmetric catalysis and photoredox catalysis in a unique

Catalysis and prebiotic RNA synthesis

NASA Technical Reports Server (NTRS)

Ferris, James P.

1993-01-01

The essential role of catalysis for the origins of life is discussed. The status of the prebiotic synthesis of 2',5'- and 3'5'-linked oligomers of RNA is reviewed. Examples of the role of metal ion and mineral catalysis in RNA oligomer formation are discussed.

A novel differential electrochemical mass spectrometry method to determine the product distribution from parasitic Methanol oxidation reaction on oxygen reduction reaction catalysts

NASA Astrophysics Data System (ADS)

Jurzinsky, Tilman; Kurzhals, Philipp; Cremers, Carsten

2018-06-01

The oxygen reduction reaction is in research focus since several decades due to its importance for the overall fuel cell performance. In direct methanol fuel cells, the crossover of methanol and its subsequent parasitic oxidation are main issues when it comes to preventing fuel cell performance losses. In this work, we present a novel differential electrochemical mass spectrometry method to evaluate oxygen reduction reaction catalysts on their tolerance to methanol being present at the cathode. Besides this, the setup allows to measure under more realistic fuel cell conditions than typical rotating disc electrode measurements, because the oxygen reduction reaction is evaluated in gaseous phase and a gas diffusion electrode is used as working electrode. Due to the new method, it was possible to investigate the oxygen reduction reaction on two commonly used catalysts (Pt/C and Pt3Co/C) in absence and presence of methanol. It was found, that Pt3Co/C is less prone to parasitic current losses due to methanol oxidation reaction. By connecting a mass spectrometer to the electrochemical cell, the new method allows to determine the products formed on the catalysts due to parasitic methanol electrooxidation.

High-performance oxygen reduction and evolution carbon catalysis: From mechanistic studies to device integration

DOE PAGES

To, John W. F.; Ng, Jia Wei Desmond; Siahrostami, Samira; ...

2016-11-30

The development of high-performance and low-cost oxygen reduction and evolution catalysts that can be easily integrated into existing devices is crucial for the wide deployment of energy storage systems that utilize O 2-H 2O chemistries, such as regenerative fuel cells and metal-air batteries. Herein, we report an NH 3-activated N-doped hierarchical carbon (NHC) catalyst synthesized via a scalable route, and demonstrate its device integration. The NHC catalyst exhibited good performance for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), as demonstrated by means of electrochemical studies and evaluation when integrated into the oxygen electrode of amore » regenerative fuel cell. The activities observed for both the ORR and the OER were comparable to those achieved by state-of-the-art Pt and Ir catalysts in alkaline environments. We have further identified the critical role of carbon defects as active sites for electrochemical activity through density functional theory calculations and high-resolution TEM visualization. As a result, this work highlights the potential of NHC to replace commercial precious metals in regenerative fuel cells and possibly metal-air batteries for cost-effective storage of intermittent renewable energy.« less

High-performance oxygen reduction and evolution carbon catalysis: From mechanistic studies to device integration

DOE Office of Scientific and Technical Information (OSTI.GOV)

To, John W. F.; Ng, Jia Wei Desmond; Siahrostami, Samira

The development of high-performance and low-cost oxygen reduction and evolution catalysts that can be easily integrated into existing devices is crucial for the wide deployment of energy storage systems that utilize O 2-H 2O chemistries, such as regenerative fuel cells and metal-air batteries. Herein, we report an NH 3-activated N-doped hierarchical carbon (NHC) catalyst synthesized via a scalable route, and demonstrate its device integration. The NHC catalyst exhibited good performance for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), as demonstrated by means of electrochemical studies and evaluation when integrated into the oxygen electrode of amore » regenerative fuel cell. The activities observed for both the ORR and the OER were comparable to those achieved by state-of-the-art Pt and Ir catalysts in alkaline environments. We have further identified the critical role of carbon defects as active sites for electrochemical activity through density functional theory calculations and high-resolution TEM visualization. As a result, this work highlights the potential of NHC to replace commercial precious metals in regenerative fuel cells and possibly metal-air batteries for cost-effective storage of intermittent renewable energy.« less

Role of dynamics in enzyme catalysis: substantial versus semantic controversies.

PubMed

Kohen, Amnon

2015-02-17

CONSPECTUS: The role of the enzyme's dynamic motions in catalysis is at the center of heated contemporary debates among both theoreticians and experimentalists. Resolving these apparent disputes is of both intellectual and practical importance: incorporation of enzyme dynamics could be critical for any calculation of enzymatic function and may have profound implications for structure-based drug design and the design of biomimetic catalysts. Analysis of the literature suggests that while part of the dispute may reflect substantial differences between theoretical approaches, much of the debate is semantic. For example, the term "protein dynamics" is often used by some researchers when addressing motions that are in thermal equilibrium with their environment, while other researchers only use this term for nonequilibrium events. The last cases are those in which thermal energy is "stored" in a specific protein mode and "used" for catalysis before it can dissipate to its environment (i.e., "nonstatistical dynamics"). This terminology issue aside, a debate has arisen among theoreticians around the roles of nonstatistical vs statistical dynamics in catalysis. However, the author knows of no experimental findings available today that examined this question in enzyme catalyzed reactions. Another source of perhaps nonsubstantial argument might stem from the varying time scales of enzymatic motions, which range from seconds to femtoseconds. Motions at different time scales play different roles in the many events along the catalytic cascade (reactant binding, reprotonation of reactants, structural rearrangement toward the transition state, product release, etc.). In several cases, when various experimental tools have been used to probe catalytic events at differing time scales, illusory contradictions seem to have emerged. In this Account, recent attempts to sort the merits of those questions are discussed along with possible future directions. A possible summary of current

Autoinduced catalysis and inverse equilibrium isotope effect in the frustrated Lewis pair catalyzed hydrogenation of imines.

PubMed

Tussing, Sebastian; Greb, Lutz; Tamke, Sergej; Schirmer, Birgitta; Muhle-Goll, Claudia; Luy, Burkhard; Paradies, Jan

2015-05-26

The frustrated Lewis pair (FLP)-catalyzed hydrogenation and deuteration of N-benzylidene-tert-butylamine (2) was kinetically investigated by using the three boranes B(C6F5)3 (1), B(2,4,6-F3-C6H2)3 (4), and B(2,6-F2-C6H3)3 (5) and the free activation energies for the H2 activation by FLP were determined. Reactions catalyzed by the weaker Lewis acids 4 and 5 displayed autoinductive catalysis arising from a higher free activation energy (2 kcal mol(-1)) for the H2 activation by the imine compared to the amine. Surprisingly, the imine reduction using D2 proceeded with higher rates. This phenomenon is unprecedented for FLP and resulted from a primary inverse equilibrium isotope effect. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Catalysis of aptamer-modified AuPd nanoalloy probe and its application to resonance scattering detection of trace UO(2)2+.

PubMed

Liang, Aihui; Zhang, Yi; Fan, Yanyan; Chen, Chunqiang; Wen, Guiqing; Liu, Qingye; Kang, Caiyan; Jiang, Zhiliang

2011-08-01

AuPd nanoalloy and nanopalladium with a diameter of 5 nm were prepared, using sodium citrate as the stabilizing agent and NaBH(4) as the reductant. The nanocatalyst containing palladium on the surface exhibited a strong catalytic effect on the slow NiP particle reaction between NiCl(2) and NaH(2)PO(2), and the NiP particle system showed a resonance scattering (RS) peak at 508 nm. The RS results showed that the Pd atom on AuPd nanoalloy surface is the catalytic center. Combining the aptamer cracking reaction of double-stranded DNA (dsDNA)-UO(2)(2+), AuPd nanoalloy aggregation, and AuPd nanoalloy catalysis, both AuPd nanoalloy RS probe and AuPd nanoalloy catalytic RS assays were developed for the determination of 40-250 pmol L(-1) UO(2)(2+) and 5.0-50 pmol L(-1) UO(2)(2+), respectively. This journal is © The Royal Society of Chemistry 2011

Surface-enhanced Raman scattering studies of the reduction of p-nitroaniline catalyzed by a nanonized Ag porous-glass hybrid composite.

PubMed

Huang, Genin Gary; Sou, Nga-Lai; Hung, Mei-Jou

2016-09-05

Nanonized noble metal composites have been known for their excellent catalytic properties. However, the mechanism and intermediates formed on the surfaces of nanocatalysts during catalysis are speculated with mostly insufficient evidence. In this study, to obtain further understanding of the roles of noble metal nanocatalysts in a catalytic reaction, surface-enhanced Raman scattering (SERS) was used to monitor the surfaces of silver (Ag) nanocatalysts. Furthermore, UV-Vis spectrometry was used to trace the concentration variations of reactants and products in bulk solutions, thereby correlating the variations of the Ag nanocatalyst surfaces with those in the bulk solutions. Nanonized Ag porous-glass hybrid composites were prepared by reducing naked Ag nanoparticles on porous-glass filter plates and were used as catalysts for nitroanilines reduction. The complete process was monitored using SERS and UV-Vis spectrometry simultaneously. The results indicated that the reactant and product molecules adsorbed on the Ag nanocatalysts can reach equilibrium, and the equilibrium is affected by the reaction conditions, including reducing agent concentration, pH of the reaction system, and temperature. In addition, the reduction of reactants in the bulk solutions is also related to the behavior of Ag nanocatalyst surfaces. Furthermore, Ag nanocatalysts can act as electron relays even if their surfaces are occupied by reactants and products. Analyzing the collected SERS and UV-Vis spectra can provide a new insight into Ag nanoparticle catalysis, and the role of Ag nanocatalysts can be further comprehended. Copyright © 2016 Elsevier B.V. All rights reserved.

Organic radicals for the enhancement of oxygen reduction reaction in Li-O2 batteries.

PubMed

Tesio, A Y; Blasi, D; Olivares-Marín, M; Ratera, I; Tonti, D; Veciana, J

2015-12-25

We examine for the first time the ability of inert carbon free-radicals as soluble redox mediators to catalyze and enhance the oxygen reduction reaction in a (TEGDME)-based electrolyte. We demonstrate that the tris(2,4,6-trichlorophenyl)methyl (TTM) radical is capable of chemically favoring the oxygen reduction reaction improving significantly the Li-O2 battery performance.

Mesoporous Phosphate Heterostructures: Synthesis and Application on Adsorption and Catalysis

NASA Astrophysics Data System (ADS)

Moreno-Tost, Ramón; Jiménez-Jiménez, José; Infantes-Molina, Antonia; Cavalcante, Celio L.; Azevedo, Diana C. S.; Soriano, María Dolores; López Nieto, José Manuel; Jiménez-López, Antonio; Rodríguez-Castellón, Enrique

Porous phosphate heterostructures (PPHs) are solids formed by a layered metal(IV) phosphate expanded with silica galleries obtained by combining the two main strategies for obtaining mesoporous materials [pillared layered structures (PLS') and MCM-41]. The different synthetic pathways for obtaining mesoporous phosphate structures with silica galleries with Zr- or Ti-doped silica, the study of their structural, textural and acid properties, its functionalisation with different organic substances such as propionitrile, 3-aminopropyl triethoxysilane, (3-mercaptopropyl)trimethoxysilane, vinyltrimethoxysilane, phenyltriethoxysilane and 3-(triethoxysilyl)propionitrile are discussed. The preparation of metal-supported catalysts and their application in gas separation, adsorption and catalysis are reviewed. Specifically, the use of Cu- and Fe-exchanged PPH for the adsorption of benzothiophene and the separation of propane/propene is the main application as adsorbent. The hydrotreating of aromatic hydrocarbons using ruthenium-impregnated catalysts via hydrogenation and hydrogenolysis/hydrocracking for the production of clean diesel fuels, the selective catalytic reduction of NO from stationary and mobile sources by using Cu-PPH with 1, 3 and 7 wt% of Cu and the selective oxidation of hydrogen sulphide to sulphur with vanadium-containing PPH are the three catalytic reactions of environmental interest studied.

Noncontact Atomic Force Microscopy: An Emerging Tool for Fundamental Catalysis Research.

PubMed

Altman, Eric I; Baykara, Mehmet Z; Schwarz, Udo D

2015-09-15

Although atomic force microscopy (AFM) was rapidly adopted as a routine surface imaging apparatus after its introduction in 1986, it has not been widely used in catalysis research. The reason is that common AFM operating modes do not provide the atomic resolution required to follow catalytic processes; rather the more complex noncontact (NC) mode is needed. Thus, scanning tunneling microscopy has been the principal tool for atomic scale catalysis research. In this Account, recent developments in NC-AFM will be presented that offer significant advantages for gaining a complete atomic level view of catalysis. The main advantage of NC-AFM is that the image contrast is due to the very short-range chemical forces that are of interest in catalysis. This motivated our development of 3D-AFM, a method that yields quantitative atomic resolution images of the potential energy surfaces that govern how molecules approach, stick, diffuse, and rebound from surfaces. A variation of 3D-AFM allows the determination of forces required to push atoms and molecules on surfaces, from which diffusion barriers and variations in adsorption strength may be obtained. Pushing molecules towards each other provides access to intermolecular interaction between reaction partners. Following reaction, NC-AFM with CO-terminated tips yields textbook images of intramolecular structure that can be used to identify reaction intermediates and products. Because NC-AFM and STM contrast mechanisms are distinct, combining the two methods can produce unique insight. It is demonstrated for surface-oxidized Cu(100) that simultaneous 3D-AFM/STM yields resolution of both the Cu and O atoms. Moreover, atomic defects in the Cu sublattice lead to variations in the reactivity of the neighboring O atoms. It is shown that NC-AFM also allows a straightforward imaging of work function variations which has been used to identify defect charge states on catalytic surfaces and to map charge transfer within an individual

Catalysis of heat-to-work conversion in quantum machines

PubMed Central

Ghosh, A.; Latune, C. L.; Davidovich, L.; Kurizki, G.

2017-01-01

We propose a hitherto-unexplored concept in quantum thermodynamics: catalysis of heat-to-work conversion by quantum nonlinear pumping of the piston mode which extracts work from the machine. This concept is analogous to chemical reaction catalysis: Small energy investment by the catalyst (pump) may yield a large increase in heat-to-work conversion. Since it is powered by thermal baths, the catalyzed machine adheres to the Carnot bound, but may strongly enhance its efficiency and power compared with its noncatalyzed counterparts. This enhancement stems from the increased ability of the squeezed piston to store work. Remarkably, the fraction of piston energy that is convertible into work may then approach unity. The present machine and its counterparts powered by squeezed baths share a common feature: Neither is a genuine heat engine. However, a squeezed pump that catalyzes heat-to-work conversion by small investment of work is much more advantageous than a squeezed bath that simply transduces part of the work invested in its squeezing into work performed by the machine. PMID:29087326

Catalysis of heat-to-work conversion in quantum machines

NASA Astrophysics Data System (ADS)

Ghosh, A.; Latune, C. L.; Davidovich, L.; Kurizki, G.

2017-11-01

We propose a hitherto-unexplored concept in quantum thermodynamics: catalysis of heat-to-work conversion by quantum nonlinear pumping of the piston mode which extracts work from the machine. This concept is analogous to chemical reaction catalysis: Small energy investment by the catalyst (pump) may yield a large increase in heat-to-work conversion. Since it is powered by thermal baths, the catalyzed machine adheres to the Carnot bound, but may strongly enhance its efficiency and power compared with its noncatalyzed counterparts. This enhancement stems from the increased ability of the squeezed piston to store work. Remarkably, the fraction of piston energy that is convertible into work may then approach unity. The present machine and its counterparts powered by squeezed baths share a common feature: Neither is a genuine heat engine. However, a squeezed pump that catalyzes heat-to-work conversion by small investment of work is much more advantageous than a squeezed bath that simply transduces part of the work invested in its squeezing into work performed by the machine.

Catalysis of heat-to-work conversion in quantum machines.

PubMed

Ghosh, A; Latune, C L; Davidovich, L; Kurizki, G

2017-11-14

We propose a hitherto-unexplored concept in quantum thermodynamics: catalysis of heat-to-work conversion by quantum nonlinear pumping of the piston mode which extracts work from the machine. This concept is analogous to chemical reaction catalysis: Small energy investment by the catalyst (pump) may yield a large increase in heat-to-work conversion. Since it is powered by thermal baths, the catalyzed machine adheres to the Carnot bound, but may strongly enhance its efficiency and power compared with its noncatalyzed counterparts. This enhancement stems from the increased ability of the squeezed piston to store work. Remarkably, the fraction of piston energy that is convertible into work may then approach unity. The present machine and its counterparts powered by squeezed baths share a common feature: Neither is a genuine heat engine. However, a squeezed pump that catalyzes heat-to-work conversion by small investment of work is much more advantageous than a squeezed bath that simply transduces part of the work invested in its squeezing into work performed by the machine.

Dual gold catalysis: σ,π-propyne acetylide and hydroxyl-bridged digold complexes as easy-to-prepare and easy-to-handle precatalysts.

PubMed

Hashmi, A Stephen K; Lauterbach, Tobias; Nösel, Pascal; Vilhelmsen, Mie Højer; Rudolph, Matthias; Rominger, Frank

2013-01-14

A series of dinuclear gold σ,π-propyne acetylide complexes were prepared and tested for their catalytic ability in dual gold catalysis that was based on the reaction of an electrophilic π-complex of gold with a gold acetylide. The air-stable and storable catalysts can be isolated as silver-free catalysts in their activated form. These dual catalysts allow a fast initiation phase for the dual catalytic cycles without the need for additional additives for acetylide formation. Because propyne serves as a throw-away ligand, no traces of the precatalyst are generated. Based on the fast initiation process, side products are minimized and reaction rates are higher for these catalysts. A series of test reactions were used to demonstrate the general applicability of these catalysts. Lower catalyst loadings, faster reaction rates, and better selectivity, combined with the practicability of these catalysts, make them ideal catalysts for dual gold catalysis. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Visible-Light Photoredox Catalysis: Selective Reduction of Carbon Dioxide to Carbon Monoxide by a Nickel N-Heterocyclic Carbene-Isoquinoline Complex

DOE Office of Scientific and Technical Information (OSTI.GOV)

Thoi, VanSara; Kornienko, Nick; Margarit, C

2013-06-07

The solar-driven reduction of carbon dioxide to value-added chemical fuels is a longstanding challenge in the fields of catalysis, energy science, and green chemistry. In order to develop effective CO2 fixation, several key considerations must be balanced, including (1) catalyst selectivity for promoting CO2 reduction over competing hydrogen generation from proton reduction, (2) visible-light harvesting that matches the solar spectrum, and (3) the use of cheap and earth-abundant catalytic components. In this report, we present the synthesis and characterization of a new family of earth-abundant nickel complexes supported by N-heterocyclic carbene amine ligands that exhibit high selectivity and activity formore » the electrocatalytic and photocatalytic conversion of CO2 to CO. Systematic changes in the carbene and amine donors of the ligand have been surveyed, and [Ni(Prbimiq1)]2+ (1c, where Prbimiq1 = bis(3-(imidazolyl)isoquinolinyl)propane) emerges as a catalyst for electrochemical reduction of CO2 with the lowest cathodic onset potential (Ecat = 1.2 V vs SCE). Using this earth-abundant catalyst with Ir(ppy)3 (where ppy = 2-phenylpyridine) and an electron donor, we have developed a visible-light photoredox system for the catalytic conversion of CO2 to CO that proceeds with high selectivity and activity and achieves turnover numbers and turnover frequencies reaching 98,000 and 3.9 s1, respectively. Further studies reveal that the overall efficiency of this solar-to-fuel cycle may be limited by the formation of the active Ni catalyst and/or the chemical reduction of CO2 to CO at the reduced nickel center and provide a starting point for improved photoredox systems for sustainable carbon-neutral energy conversion.« less

Uncovering the Roles of Oxygen in Cr(III) Photoredox Catalysis.

PubMed

Higgins, Robert F; Fatur, Steven M; Shepard, Samuel G; Stevenson, Susan M; Boston, David J; Ferreira, Eric M; Damrauer, Niels H; Rappé, Anthony K; Shores, Matthew P

2016-04-27

A combined experimental and theoretical investigation aims to elucidate the necessary roles of oxygen in photoredox catalysis of radical cation based Diels-Alder cycloadditions mediated by the first-row transition metal complex [Cr(Ph2phen)3](3+), where Ph2phen = bathophenanthroline. We employ a diverse array of techniques, including catalysis screening, electrochemistry, time-resolved spectroscopy, and computational analyses of reaction thermodynamics. Our key finding is that oxygen acts as a renewable energy and electron shuttle following photoexcitation of the Cr(III) catalyst. First, oxygen quenches the excited Cr(3+)* complex; this energy transfer process protects the catalyst from decomposition while preserving a synthetically useful 13 μs excited state and produces singlet oxygen. Second, singlet oxygen returns the reduced catalyst to the Cr(III) ground state, forming superoxide. Third, the superoxide species reduces the Diels-Alder cycloadduct radical cation to the final product and reforms oxygen. We compare the results of these studies with those from cycloadditions mediated by related Ru(II)-containing complexes and find that the distinct reaction pathways are likely part of a unified mechanistic framework where the photophysical and photochemical properties of the catalyst species lead to oxygen-mediated photocatalysis for the Cr-containing complex but radical chain initiation for the Ru congener. These results provide insight into how oxygen can participate as a sustainable reagent in photocatalysis.

Push or Pull? Proton Responsive Ligand Effects in Rhenium Tricarbonyl CO 2 Reduction Catalysts

DOE PAGES

Manbeck, Gerald F.; Muckerman, James T.; Szalda, David J.; ...

2015-02-19

Proton responsive ligands offer control of catalytic reactions through modulation of pH-dependent properties, second coordination sphere stabilization of transition states, or by providing a local proton source for multi-proton, multi-electron reactions. Two fac-[ReI(α-diimine)(CO)₃Cl] complexes with α-diimine = 4,4'- (or 6,6'-) dihydroxy-2,2'-bipyridine (4DHBP and 6DHBP) have been prepared and analyzed as electrocatalysts for reduction of carbon dioxide. Consecutive electrochemical reduction of these complexes yields species identical to those obtained by chemical deprotonation. An energetically feasible mechanism for reductive deprotonation is proposed in which the bpy anion is protonated followed by loss of H₂ and 2H⁺. Cyclic voltammetry reveals a two-electron, three-wavemore » system owing to competing EEC and ECE pathways. The chemical step of the ECE pathway might be attributed to the reductive deprotonation. but cannot be distinguished from chloride dissociation. The rate obtained by digital simulation is approximately 8 s⁻¹. Under CO₂, these competing reactions generate a two-slope catalytic waveform with onset potential of –1.65 V vs Ag/AgCl. Reduction of CO₂ to CO by the [ReI (4DHBP–2H⁺)(CO)₃]⁻ suggests the interaction of CO₂ with the deprotonated species or a third reduction followed by catalysis. Conversely, the reduced form of [Re(6DHBP)(CO)₃Cl] converts CO₂ to CO with a single turnover.« less

Heterobimetallic transition metal/rare earth metal bifunctional catalysis: a Cu/Sm/Schiff base complex for syn-selective catalytic asymmetric nitro-Mannich reaction.

PubMed

Handa, Shinya; Gnanadesikan, Vijay; Matsunaga, Shigeki; Shibasaki, Masakatsu

2010-04-07

The full details of a catalytic asymmetric syn-selective nitro-Mannich reaction promoted by heterobimetallic Cu/Sm/dinucleating Schiff base complexes are described, demonstrating the effectiveness of the heterobimetallic transition metal/rare earth metal bifunctional catalysis. The first-generation system prepared from Cu(OAc)(2)/Sm(O-iPr)(3)/Schiff base 1a = 1:1:1 with an achiral phenol additive was partially successful for achieving the syn-selective catalytic asymmetric nitro-Mannich reaction. The substrate scope and limitations of the first-generation system remained problematic. After mechanistic studies on the catalyst prepared from Sm(O-iPr)(3), we reoptimized the catalyst preparation method, and a catalyst derived from Sm(5)O(O-iPr)(13) showed broader substrate generality as well as higher reactivity and stereoselectivity compared to Sm(O-iPr)(3). The optimal system with Sm(5)O(O-iPr)(13) was applicable to various aromatic, heteroaromatic, and isomerizable aliphatic N-Boc imines, giving products in 66-99% ee and syn/anti = >20:1-13:1. Catalytic asymmetric synthesis of nemonapride is also demonstrated using the catalyst derived from Sm(5)O(O-iPr)(13).

Low NO Concentration Dependence of Reductive Nitrosylation Reaction of Hemoglobin*

PubMed Central

Tejero, Jesús; Basu, Swati; Helms, Christine; Hogg, Neil; King, S. Bruce; Kim-Shapiro, Daniel B.; Gladwin, Mark T.

2012-01-01

The reductive nitrosylation of ferric (met)hemoglobin is of considerable interest and remains incompletely explained. We have previously observed that at low NO concentrations the reaction with tetrameric hemoglobin occurs with an observed rate constant that is at least 5 times faster than that observed at higher concentrations. This was ascribed to a faster reaction of NO with a methemoglobin-nitrite complex. We now report detailed studies of this reaction of low NO with methemoglobin. Nitric oxide paradoxically reacts with ferric hemoglobin with faster observed rate constants at the lower NO concentration in a manner that is not affected by changes in nitrite concentration, suggesting that it is not a competition between NO and nitrite, as we previously hypothesized. By evaluation of the fast reaction in the presence of allosteric effectors and isolated β- and α-chains of hemoglobin, it appears that NO reacts with a subpopulation of β-subunit ferric hemes whose population is influenced by quaternary state, redox potential, and hemoglobin dimerization. To further characterize the role of nitrite, we developed a system that oxidizes nitrite to nitrate to eliminate nitrite contamination. Removal of nitrite does not alter reaction kinetics, but modulates reaction products, with a decrease in the formation of S-nitrosothiols. These results are consistent with the formation of NO2/N2O3 in the presence of nitrite. The observed fast reductive nitrosylation observed at low NO concentrations may function to preserve NO bioactivity via primary oxidation of NO to form nitrite or in the presence of nitrite to form N2O3 and S-nitrosothiols. PMID:22493289

Cobamide-mediated enzymatic reductive dehalogenation via long-range electron transfer

PubMed Central

Kunze, Cindy; Bommer, Martin; Hagen, Wilfred R.; Uksa, Marie; Dobbek, Holger; Schubert, Torsten; Diekert, Gabriele

2017-01-01

The capacity of metal-containing porphyrinoids to mediate reductive dehalogenation is implemented in cobamide-containing reductive dehalogenases (RDases), which serve as terminal reductases in organohalide-respiring microbes. RDases allow for the exploitation of halogenated compounds as electron acceptors. Their reaction mechanism is under debate. Here we report on substrate–enzyme interactions in a tetrachloroethene RDase (PceA) that also converts aryl halides. The shape of PceA’s highly apolar active site directs binding of bromophenols at some distance from the cobalt and with the hydroxyl substituent towards the metal. A close cobalt–substrate interaction is not observed by electron paramagnetic resonance spectroscopy. Nonetheless, a halogen substituent para to the hydroxyl group is reductively eliminated and the path of the leaving halide is traced in the structure. Based on these findings, an enzymatic mechanism relying on a long-range electron transfer is concluded, which is without parallel in vitamin B12-dependent biochemistry and represents an effective mode of RDase catalysis. PMID:28671181

Cobamide-mediated enzymatic reductive dehalogenation via long-range electron transfer.

PubMed

Kunze, Cindy; Bommer, Martin; Hagen, Wilfred R; Uksa, Marie; Dobbek, Holger; Schubert, Torsten; Diekert, Gabriele

2017-07-03

The capacity of metal-containing porphyrinoids to mediate reductive dehalogenation is implemented in cobamide-containing reductive dehalogenases (RDases), which serve as terminal reductases in organohalide-respiring microbes. RDases allow for the exploitation of halogenated compounds as electron acceptors. Their reaction mechanism is under debate. Here we report on substrate-enzyme interactions in a tetrachloroethene RDase (PceA) that also converts aryl halides. The shape of PceA's highly apolar active site directs binding of bromophenols at some distance from the cobalt and with the hydroxyl substituent towards the metal. A close cobalt-substrate interaction is not observed by electron paramagnetic resonance spectroscopy. Nonetheless, a halogen substituent para to the hydroxyl group is reductively eliminated and the path of the leaving halide is traced in the structure. Based on these findings, an enzymatic mechanism relying on a long-range electron transfer is concluded, which is without parallel in vitamin B 12 -dependent biochemistry and represents an effective mode of RDase catalysis.

Testing Electrostatic Complementarity in Enzyme Catalysis: Hydrogen Bonding in the Ketosteroid Isomerase Oxyanion Hole

PubMed Central

Kraut, Daniel A; Sigala, Paul A; Pybus, Brandon; Liu, Corey W; Ringe, Dagmar; Petsko, Gregory A

2006-01-01

A longstanding proposal in enzymology is that enzymes are electrostatically and geometrically complementary to the transition states of the reactions they catalyze and that this complementarity contributes to catalysis. Experimental evaluation of this contribution, however, has been difficult. We have systematically dissected the potential contribution to catalysis from electrostatic complementarity in ketosteroid isomerase. Phenolates, analogs of the transition state and reaction intermediate, bind and accept two hydrogen bonds in an active site oxyanion hole. The binding of substituted phenolates of constant molecular shape but increasing p K a models the charge accumulation in the oxyanion hole during the enzymatic reaction. As charge localization increases, the NMR chemical shifts of protons involved in oxyanion hole hydrogen bonds increase by 0.50–0.76 ppm/p K a unit, suggesting a bond shortening of ˜0.02 Å/p K a unit. Nevertheless, there is little change in binding affinity across a series of substituted phenolates (ΔΔG = −0.2 kcal/mol/p K a unit). The small effect of increased charge localization on affinity occurs despite the shortening of the hydrogen bonds and a large favorable change in binding enthalpy (ΔΔH = −2.0 kcal/mol/p K a unit). This shallow dependence of binding affinity suggests that electrostatic complementarity in the oxyanion hole makes at most a modest contribution to catalysis of ˜300-fold. We propose that geometrical complementarity between the oxyanion hole hydrogen-bond donors and the transition state oxyanion provides a significant catalytic contribution, and suggest that KSI, like other enzymes, achieves its catalytic prowess through a combination of modest contributions from several mechanisms rather than from a single dominant contribution. PMID:16602823

A new type of power energy for accelerating chemical reactions: the nature of a microwave-driving force for accelerating chemical reactions.

PubMed

Zhou, Jicheng; Xu, Wentao; You, Zhimin; Wang, Zhe; Luo, Yushang; Gao, Lingfei; Yin, Cheng; Peng, Renjie; Lan, Lixin

2016-04-27

The use of microwave (MW) irradiation to increase the rate of chemical reactions has attracted much attention recently in nearly all fields of chemistry due to substantial enhancements in reaction rates. However, the intrinsic nature of the effects of MW irradiation on chemical reactions remains unclear. Herein, the highly effective conversion of NO and decomposition of H2S via MW catalysis were investigated. The temperature was decreased by several hundred degrees centigrade. Moreover, the apparent activation energy (Ea') decreased substantially under MW irradiation. Importantly, for the first time, a model of the interactions between microwave electromagnetic waves and molecules is proposed to elucidate the intrinsic reason for the reduction in the Ea' under MW irradiation, and a formula for the quantitative estimation of the decrease in the Ea' was determined. MW irradiation energy was partially transformed to reduce the Ea', and MW irradiation is a new type of power energy for speeding up chemical reactions. The effect of MW irradiation on chemical reactions was determined. Our findings challenge both the classical view of MW irradiation as only a heating method and the controversial MW non-thermal effect and open a promising avenue for the development of novel MW catalytic reaction technology.

A new type of power energy for accelerating chemical reactions: the nature of a microwave-driving force for accelerating chemical reactions

PubMed Central

Zhou, Jicheng; Xu, Wentao; You, Zhimin; Wang, Zhe; Luo, Yushang; Gao, Lingfei; Yin, Cheng; Peng, Renjie; Lan, Lixin

2016-01-01

The use of microwave (MW) irradiation to increase the rate of chemical reactions has attracted much attention recently in nearly all fields of chemistry due to substantial enhancements in reaction rates. However, the intrinsic nature of the effects of MW irradiation on chemical reactions remains unclear. Herein, the highly effective conversion of NO and decomposition of H2S via MW catalysis were investigated. The temperature was decreased by several hundred degrees centigrade. Moreover, the apparent activation energy (Ea’) decreased substantially under MW irradiation. Importantly, for the first time, a model of the interactions between microwave electromagnetic waves and molecules is proposed to elucidate the intrinsic reason for the reduction in the Ea’ under MW irradiation, and a formula for the quantitative estimation of the decrease in the Ea’ was determined. MW irradiation energy was partially transformed to reduce the Ea’, and MW irradiation is a new type of power energy for speeding up chemical reactions. The effect of MW irradiation on chemical reactions was determined. Our findings challenge both the classical view of MW irradiation as only a heating method and the controversial MW non-thermal effect and open a promising avenue for the development of novel MW catalytic reaction technology. PMID:27118640

Palladium/N-heterocyclic carbene catalysed regio and diastereoselective reaction of ketones with allyl reagents via inner-sphere mechanism

PubMed Central

Bai, Da-Chang; Yu, Fei-Le; Wang, Wan-Ying; Chen, Di; Li, Hao; Liu, Qing-Rong; Ding, Chang-Hua; Chen, Bo; Hou, Xue-Long

2016-01-01

The palladium-catalysed allylic substitution reaction is one of the most important reactions in transition-metal catalysis and has been well-studied in the past decades. Most of the reactions proceed through an outer-sphere mechanism, affording linear products when monosubstituted allyl reagents are used. Here, we report an efficient Palladium-catalysed protocol for reactions of β-substituted ketones with monosubstituted allyl substrates, simply by using N-heterocyclic carbene as ligand, leading to branched products with up to three contiguous stereocentres in a (syn, anti)-mode with excellent regio and diastereoselectivities. The scope of the protocol in organic synthesis has been examined preliminarily. Mechanistic studies by both experiments and density functional theory (DFT) calculations reveal that the reaction proceeds via an inner-sphere mechanism—nucleophilic attack of enolate oxygen on Palladium followed by C–C bond-forming [3,3']-reductive elimination. PMID:27283477

Reduction Reaction Activity on Pt-Monolayer-Shell PdIr/Ni-core Catalysts

DOE Office of Scientific and Technical Information (OSTI.GOV)

Song, Liang; Vukmirovic, Miomir B.; Adzic, Radoslav R.

Platinum monolayer oxygen reduction reaction catalysts present promising way of reducing the Pt content without scarifying its fuel cell performance. We present a facile way of preparing Pt monolayer shell PdIr-based core catalysts, which showed much higher activity for oxygen reduction reaction than that of TKK 46.6% Pt/C catalyst. Among tested samples, PtMLPd2Ir/Ni/C performs the best with Pt and Platinum Group Metal mass activity around 9 and 0.25 times higher of that of TKK 46.6% Pt/C. In addition, accelerated aging test indicates its excellent durability.

Reduction Reaction Activity on Pt-Monolayer-Shell PdIr/Ni-core Catalysts

DOE PAGES

Song, Liang; Vukmirovic, Miomir B.; Adzic, Radoslav R.

2018-05-14

Platinum monolayer oxygen reduction reaction catalysts present promising way of reducing the Pt content without scarifying its fuel cell performance. We present a facile way of preparing Pt monolayer shell PdIr-based core catalysts, which showed much higher activity for oxygen reduction reaction than that of TKK 46.6% Pt/C catalyst. Among tested samples, PtMLPd2Ir/Ni/C performs the best with Pt and Platinum Group Metal mass activity around 9 and 0.25 times higher of that of TKK 46.6% Pt/C. In addition, accelerated aging test indicates its excellent durability.

Ultrasensitive and selective signal-on electrochemical DNA detection via exonuclease III catalysis and hybridization chain reaction amplification.

PubMed

Ren, Wang; Gao, Zhong Feng; Li, Nian Bing; Luo, Hong Qun

2015-01-15

This work reported a novel, ultrasensitive, and selective platform for electrochemical detection of DNA, employing an integration of exonuclease III (Exo-III) assisted target recycling and hybridization chain reaction (HCR) for the dual signal amplification strategy. The hairpin capture probe DNA (C-DNA) with an Exo-III 3' overhang end was self-assembled on a gold electrode. In the presence of target DNA (T-DNA), C-DNA hybridized with the T-DNA to form a duplex region, exposing its 5' complementary sequence (initiator). Exo-III was applied to selectively digest duplex region from its 3-hydroxyl termini until the duplex was fully consumed, leaving the remnant initiator. The intact T-DNA spontaneously dissociated from the structure and then initiated the next hybridization process as a result of catalysis of the Exo-III. HCR event was triggered by the initiator and two hairpin helper signal probes labeled with methylene blue, facilitating the polymerization of oligonucleotides into a long nicked dsDNA molecule. The numerous exposed remnant initiators can trigger more HCR events. Because of integration of dual signal amplification and the specific HCR process reaction, the resultant sensor showed a high sensitivity for the detection of the target DNA in a linear range from 1.0 fM to 1.0 nM, and a detection limit as low as 0.2 fM. The proposed dual signal amplification strategy provides a powerful tool for detecting different sequences of target DNA by changing the sequence of capture probe and signal probes, holding a great potential for early diagnosis in gene-related diseases. Copyright © 2014 Elsevier B.V. All rights reserved.

Direct Acylation of C(sp(3))-H Bonds Enabled by Nickel and Photoredox Catalysis.

PubMed

Joe, Candice L; Doyle, Abigail G

2016-03-14

Using nickel and photoredox catalysis, the direct functionalization of C(sp(3))-H bonds of N-aryl amines by acyl electrophiles is described. The method affords a diverse range of α-amino ketones at room temperature and is amenable to late-stage coupling of complex and biologically relevant groups. C(sp(3))-H activation occurs by photoredox-mediated oxidation to generate α-amino radicals which are intercepted by nickel in catalytic C(sp(3))-C coupling. The merger of these two modes of catalysis leverages nickel's unique properties in alkyl cross-coupling while avoiding limitations commonly associated with transition-metal-mediated C(sp(3))-H activation, including requirements for chelating directing groups and high reaction temperatures. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Synthesis, characterization and heterogeneous base catalysis of amino functionalized lanthanide metal-organic frameworks

NASA Astrophysics Data System (ADS)

Huang, Jinping; Li, Chunmei; Tao, Lingling; Zhu, Huilin; Hu, Gang

2017-10-01

Lanthanide metal-organic frameworks (Ln-MOFs) are featured by their tolerance to water and dense structure. In this work, an amine-functionalized Ln-MOF was facilely synthesized by coordination of terbium with 2-aminoterephthalic acid under the condition of microwave irradiation. The crystal structure was characterized by single crystal X-ray diffraction, FT-IR, Raman, TG-DTA and XPS analysis. The basic catalytic activity of the NH2-Tb-MOF was evaluated for Knoevenagel condensation and Henry reactions. Apart from the high activity and 100% selectivity to the condensation product, the NH2-Tb-MOF catalyst could be easily recycled and reused owing to the high stability of the MOF framework formed by coordination of Tb3+ with carboxylic groups. Remarkably, the NH2-Tb-MOF exhibited size-selective catalysis to substrates. For the small-sized reactants, it displayed comparable activity to the homogeneous catalyst of aniline owing to the high dispersion of NH2- active sites and the low diffusion limits. However, in the same reaction system, extremely poor activity in Knoevenagel condensation and Henry reaction for the bulky substrate 4-(tert-butyl) benzaldehyde was observed due to the both effects of substitute and inhibition of diffusion into the micropores. Crystal structure analysis provided a mechanistic evidence that the heterogeneous base catalysis arose from the amino groups densely distributed inside the micropores.

Synergism between microwave irradiation and enzyme catalysis in transesterification of ethyl-3-phenylpropanoate with n-butanol.

PubMed

Yadav, Ganapati D; Pawar, Sandip V

2012-04-01

Lipase catalyzed transesterification was investigated to study the synergistic effect of microwave irradiation and enzyme catalysis. Transesterification of ethyl-3-phenylpropanoate with n-butanol was chosen as the model reaction using immobilized enzymes such as Novozyme 435, Lipozyme RMIM and Lipozyme TL IM with microwave irradiation. Novozyme 435 was the best catalyst. The effect of various parameters affecting the conversion and initial rates of transesterification were studied to establish kinetics and mechanism. There is synergism between enzyme catalysis and microwave irradiation. The analysis of initial rate data and progress curve data showed that the reaction obeys the Ping-Pong bi-bi mechanism with inhibition by n-butanol. The theoretical predictions and experimental data match very well. These studies were also extended to other alcohols such as 2-phenyl-1-propanol, n-octanol, benzyl alcohol, iso-amyl alcohol, 2-hexanol and 2-pentanol under otherwise similar conditions. Copyright © 2012. Published by Elsevier Ltd.

Molecular complexity from polyunsaturated substrates: the gold catalysis approach.

PubMed

Fensterbank, Louis; Malacria, Max

2014-03-18

Over the last two decades, electrophilic catalysis relying on platinum(II), gold(I), and gold(III) salts has emerged as a remarkable synthetic methodology. Chemists have discovered a large variety of organic transformations that convert a great assortment of highly functionalized precursors into valuable final products. In many cases, these methodologies offer unique features, allowing access to unprecedented molecular architectures. Due to the mild reaction conditions and high function compatibility, scientists have successfully developed applications in total synthesis of natural products, as well as in asymmetric catalysis. In addition, all these developments have been accompanied by the invention of well-tailored catalysts, so that a palette of different electrophilic agents is now commercially available or readily synthesized at the bench. In some respects, researchers' interests in developing homogeneous gold catalysis can be compared with the Californian gold rush of the 19th century. It has attracted into its fervor thousands of scientists, providing a huge number of versatile and important reports. More notably, it is clear that the contribution to the art of organic synthesis is very valuable, though the quest is not over yet. Because they rely on the intervention of previously unknown types of intermediates, new retrosynthetic disconnections are now possible. In this Account, we discuss our efforts on the use of readily available polyunsaturated precursors, such as enynes, dienynes, allenynes, and allenenes to give access to highly original polycyclic structures in a single operation. These transformations transit via previously undescribed intermediates A, B, D, F, and H that will be encountered later on. All these intermediates have been determined by both ourselves and others by DFT calculations and in some cases have been confirmed on the basis of experimental data. In addition, dual gold activation can be at work in some of these transformations

Heterogeneous Catalysis.

ERIC Educational Resources Information Center

Miranda, R.

1989-01-01

Described is a heterogeneous catalysis course which has elements of materials processing embedded in the classical format of catalytic mechanisms and surface chemistry. A course outline and list of examples of recent review papers written by students are provided. (MVL)

Role of Dynamics in Enzyme Catalysis: Substantial versus Semantic Controversies

PubMed Central

2015-01-01

Conspectus The role of the enzyme’s dynamic motions in catalysis is at the center of heated contemporary debates among both theoreticians and experimentalists. Resolving these apparent disputes is of both intellectual and practical importance: incorporation of enzyme dynamics could be critical for any calculation of enzymatic function and may have profound implications for structure-based drug design and the design of biomimetic catalysts. Analysis of the literature suggests that while part of the dispute may reflect substantial differences between theoretical approaches, much of the debate is semantic. For example, the term “protein dynamics” is often used by some researchers when addressing motions that are in thermal equilibrium with their environment, while other researchers only use this term for nonequilibrium events. The last cases are those in which thermal energy is “stored” in a specific protein mode and “used” for catalysis before it can dissipate to its environment (i.e., “nonstatistical dynamics”). This terminology issue aside, a debate has arisen among theoreticians around the roles of nonstatistical vs statistical dynamics in catalysis. However, the author knows of no experimental findings available today that examined this question in enzyme catalyzed reactions. Another source of perhaps nonsubstantial argument might stem from the varying time scales of enzymatic motions, which range from seconds to femtoseconds. Motions at different time scales play different roles in the many events along the catalytic cascade (reactant binding, reprotonation of reactants, structural rearrangement toward the transition state, product release, etc.). In several cases, when various experimental tools have been used to probe catalytic events at differing time scales, illusory contradictions seem to have emerged. In this Account, recent attempts to sort the merits of those questions are discussed along with possible future directions. A possible

Steric and electronic ligand perturbations in catalysis: asymmetric allylic substitution reactions using C2-symmetrical phosphorus-chiral (bi)ferrocenyl donors.

PubMed

Nettekoven, U; Widhalm, M; Kalchhauser, H; Kamer, P C; van Leeuwen, P W; Lutz, M; Spek, A L

2001-02-09

Three series of P-chiral diphosphines based on ferrocene (1a-f, 2a-c) and biferrocenyl skeletons (3a-c), including novel ligands 1f and 3c, were employed in palladium-catalyzed allylic substitution reactions. Steric effects imposed by the phosphine residues were studied using C2-symmetrical donors 1 (1 = 1,1'-bis(arylphenylphosphino)ferrocene with aryl groups a = 1-naphthyl, b = 2-naphthyl, c = 2-anisyl, d = 2-biphenylyl, e = 9-phenanthryl, and f = ferrocenyl), whereas para-methoxy- and/or para-trifluoromethyl substitution of the phenyl moieties in 1a enabled investigation of ligand electronic effects applying ferrocenyl diphosphines 2a-c. Ligands 3 (3 = 2,2'-bis- (arylphenylphosphino)-1,1'-biferrocenyls with aryl substituents a,c = 1-naphthyl (diastereomers) and b = 2-biphenylyl) allowed for comparison of backbone structure effects (bite angle variation) in catalysis. Linear and cyclic allylic acetates served as substrates in typical test reactions; upon attack of soft carbon and nitrogen nucleophiles on (E)-1,3-diphenylprop-2-ene-1-yl acetate the respective malonate, amine, or imide products were obtained in enantioselectivities of up to 99% ee. A crystal structure analysis of a palladium 1,3-diphenyl-eta 3-allyl complex incorporating ligand (S,S)-1a revealed a marked distortion of the allyl fragment, herewith defining the regioselectivity of nucleophile addition.

Computational catalyst screening: Scaling, bond-order and catalysis

DOE PAGES

Abild-Pedersen, Frank

2015-10-01

Here, the design of new and better heterogeneous catalysts needed to accommodate the growing demand for energy from renewable sources is an important challenge for coming generations. Most surface catalyzed processes involve a large number of complex reaction networks and the energetics ultimately defines the turn-over-frequency and the selectivity of the process. In order not to get lost in the large quantities of data, simplification schemes that still contain the key elements of the reaction are required. Adsorption and transition state scaling relations constitutes such a scheme that not only maps the reaction relevant information in terms of few parametersmore » but also provides an efficient way of screening for new materials in a continuous multi-dimensional energy space. As with all relations they impose certain restrictions on what can be achieved and in this paper, I show why these limitations exist and how we can change the behavior through an energy-resolved approach that still maintains the screening capabilities needed in computational catalysis.« less

The Pauson-Khand reaction: the catalytic age is here!

PubMed

Gibson, Susan E; Stevenazzi, Andrea

2003-04-25

As a consequence of growing environmental awareness, it is now inappropriate to design a synthetic metal-mediated transformation that involves a noncatalytic use of toxic and expensive transition-metal species. One of the earliest examples of such a metal-mediated transformation is the Pauson-Khand reaction, a [2+2+1] cyclocarbonylation that generates a cyclopentenone. Despite the early descriptions by Pauson and co-workers of catalytic versions of the reaction with octacarbonyldicobalt(0), applications of the Pauson-Khand reaction have to date almost exclusively used approaches that involve stoichiometric quantities of cobalt-carbonyl complexes. In the last decade, and, most markedly, in the last two to three years, however, there have been many exciting and novel developments in the catalytic Pauson-Khand reaction. Furthermore, asymmetric catalysis of the Pauson-Khand reaction has been shown to be a viable process. In view of the impressive developments in Pauson-Khand catalysis in the last two to three years, we present a comprehensive and critical coverage of the catalytic Pauson-Khand reaction that is designed to facilitate its application and to point to exciting future developments.

Direct one-pot reductive amination of aldehydes with nitroarenes in a domino fashion: catalysis by gum-acacia-stabilized palladium nanoparticles.

PubMed

Sreedhar, B; Reddy, P Surendra; Devi, D Keerthi

2009-11-20

This note describes the direct reductive amination of carbonyl compounds with nitroarenes using gum acacia-palladium nanoparticles, employing molecular hydrogen as the reductant. This methodology is found to be applicable to both aliphatic and aromatic aldehydes and a wide range of nitroarenes. The operational simplicity and the mild reaction conditions add to the value of this method as a practical alternative to the reductive amination of carbonyl compounds.

Method of reduction of nitroaromatics by enzymatic reaction with redox enzymes

DOEpatents

Shah, Manish M.

2000-01-01

A method for the controlled reduction of nitroaromatic compounds such as nitrobenzene and 2,4,6-trinitrotoluene by enzymatic reaction with redox enzymes, such as Oxyrase (Trademark of Oxyrase, Inc., Mansfield, Ohio).

Catalysis in human hypoxanthine-guanine phosphoribosyltransferase: Asp 137 acts as a general acid/base.

PubMed

Xu, Y; Grubmeyer, C

1998-03-24

Hypoxanthine-guanine phosphoribosyltransferase (HGPRTase) catalyzes the reversible formation of IMP and GMP from their respective bases hypoxanthine (Hx) and guanine (Gua) and the phosphoribosyl donor 5-phosphoribosyl-1-pyrophosphate (PRPP). The net formation and cleavage of the nucleosidic bond requires removal/addition of a proton at the purine moiety, allowing enzymic catalysis to reduce the energy barrier associated with the reaction. The pH profile of kcat for IMP pyrophosphorolysis revealed an essential acidic group with pKa of 7.9 whereas those for IMP or GMP formation indicated involvement of essential basic groups. Based on the crystal structure of human HGPRTase, protonation/deprotonation is likely to occur at N7 of the purine ring, and Lys 165 or Asp 137 are each candidates for the general base/acid. We have constructed, purified, and kinetically characterized two mutant HGPRTases to test this hypothesis. D137N displayed an 18-fold decrease in kcat for nucleotide formation with Hx as substrate, a 275-fold decrease in kcat with Gua, and a 500-fold decrease in kcat for IMP pyrophosphorolysis. D137N also showed lower KD values for nucleotides and PRPP. The pH profiles of kcat for D137N were severely altered. In contrast to D137N, the kcat for K165Q was decreased only 2-fold in the forward reaction and was slightly increased in the reverse reaction. The Km and KD values showed that K165Q interacts with substrates more weakly than does the wild-type enzyme. Pre-steady-state experiments with K165Q indicated that the phosphoribosyl transfer step was fast in the forward reaction, as observed with the wild type. In contrast, D137N showed slower phosphoribosyl transfer chemistry, although guanine (3000-fold reduction) was affected much more than hypoxanthine (32-fold reduction). In conclusion, Asp137 acts as a general catalytic acid/base for HGPRTase and Lys165 makes ground-state interactions with substrates.

Nitrogen-doped graphdiyne as a metal-free catalyst for high-performance oxygen reduction reactions

NASA Astrophysics Data System (ADS)

Liu, Rongji; Liu, Huibiao; Li, Yuliang; Yi, Yuanping; Shang, Xinke; Zhang, Shuangshuang; Yu, Xuelian; Zhang, Suojiang; Cao, Hongbin; Zhang, Guangjin

2014-09-01

Fuel cells and metal-air batteries will only become widely available in everyday life when the expensive platinum-based electrocatalysts used for the oxygen reduction reactions are replaced by other efficient, low-cost and stable catalysts. We report here the use of nitrogen-doped graphdiyne as a metal-free electrode with a comparable electrocatalytic activity to commercial Pt/C catalysts for the oxygen reduction reaction in alkaline fuel cells. Nitrogen-doped graphdiyne has a better stability and increased tolerance to the cross-over effect than conventional Pt/C catalysts.Fuel cells and metal-air batteries will only become widely available in everyday life when the expensive platinum-based electrocatalysts used for the oxygen reduction reactions are replaced by other efficient, low-cost and stable catalysts. We report here the use of nitrogen-doped graphdiyne as a metal-free electrode with a comparable electrocatalytic activity to commercial Pt/C catalysts for the oxygen reduction reaction in alkaline fuel cells. Nitrogen-doped graphdiyne has a better stability and increased tolerance to the cross-over effect than conventional Pt/C catalysts. Electronic supplementary information (ESI) available: Detailed RDE and RRDE experiments, additional tables and figures. See DOI: 10.1039/c4nr03185g

Cp2 TiX Complexes for Sustainable Catalysis in Single-Electron Steps.

PubMed

Richrath, Ruben B; Olyschläger, Theresa; Hildebrandt, Sven; Enny, Daniel G; Fianu, Godfred D; Flowers, Robert A; Gansäuer, Andreas

2018-04-25

We present a combined electrochemical, kinetic, and synthetic study with a novel and easily accessible class of titanocene catalysts for catalysis in single-electron steps. The tailoring of the electronic properties of our Cp 2 TiX-catalysts that are prepared in situ from readily available Cp 2 TiX 2 is achieved by varying the anionic ligand X. Of the complexes investigated, Cp 2 TiOMs proved to be either equal or substantially superior to the best catalysts developed earlier. The kinetic and thermodynamic properties pertinent to catalysis have been determined. They allow a mechanistic understanding of the subtle interplay of properties required for an efficient oxidative addition and reduction. Therefore, our study highlights that efficient catalysts do not require the elaborate covalent modification of the cyclopentadienyl ligands. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Material-Efficient Microfluidic Platform for Exploratory Studies of Visible-Light Photoredox Catalysis.

PubMed

Coley, Connor W; Abolhasani, Milad; Lin, Hongkun; Jensen, Klavs F

2017-08-07

We present an automated microfluidic platform for in-flow studies of visible-light photoredox catalysis in liquid or gas-liquid reactions at the 15 μL scale. An oscillatory flow strategy enables a flexible residence time while preserving the mixing and heat transfer advantages of flow systems. The adjustable photon flux made possible with the platform is characterized using actinometry. Case studies of oxidative hydroxylation of phenylboronic acids and dimerization of thiophenol demonstrate the capabilities and advantages of the system. Reaction conditions identified through droplet screening translate directly to continuous synthesis with minor platform modifications. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Cobalt nanoparticles on charcoal: a versatile catalyst in the Pauson-Khand reaction, hydrogenation, and the reductive Pauson-Khand reaction.

PubMed

Son, Seung Uk; Park, Kang Hyun; Chung, Young Keun

2002-10-31

[formula: see text] Dispersions of nanometer-sized cobalt particles with very high stability were prepared in charcoal and analyzed by electron microscopy and X-ray analysis. The resulting cobalt nanoparticles on charcoal (CNC) were successfully used as a catalyst for the carbonylative cycloaddition of alkyne, alkene, and carbon monoxide (Pauson-Khand reaction), hydrogenation, and the reductive Pauson-Khand reaction.

High-Level Spectroscopy, Quantum Chemistry, and Catalysis: Not just a Passing Fad.

PubMed

Neese, Frank

2017-09-04

Quantum chemistry can be used as a powerful link between theory and experiment for studying reactions in all areas of catalysis. The key feature of this approach is the combination of quantum chemistry with a range of high-level spectroscopic methods. This allows for conclusions to be reached that neither theory nor experiment would have been able to obtain in isolation. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Oligomerization reactions of ribonucleotides - The reaction of the 5'-phosphorimidazolide of nucleosides on montmorillonite and other minerals

NASA Technical Reports Server (NTRS)

Ferris, James P.; Ertem, Gozen

1992-01-01

The reaction of ImpA in the presence of Na(+)-montmorillonite 22A or Na(+)-Volclay in aqueous, pH 8 solution gives a 50-60 percent yield of dimers and trimers (pA)2 and (pA)3. The ratio of 3',5'-phosphodiester bond formation is twice as great as 2',5'-bond formation. The reaction requires the presence of Mg2+ and is inhibited by 0.4 M imidazole. N-methylimidazole enhances the rate of the reaction but does not cause major changes in yield or product composition. Higher yields were obtained when Li+- or Ca2+-montmorillonites were used in place of Na+-montmorillonite. Little or no phosphodiester bond formation was observed with Mg2+- or Al3+-montmorillonite. Montmorillonites other than 22A and Volclay exhibited litle or no catalysis. In addittion, little or no catalysis was exhibited in ferrugenous smectite, nontronite, allophane, imogolite or sepiolite. Oligomers were also formed by the reaction of ImpG, 2-methylImpG, ImpC and ImpU in the presence of Na+-montmorillonite. The pyrimidine nucleotides gave significantly lower yields of oligomers.

Abiotic CO2 reduction during geologic carbon sequestration facilitated by Fe(II)-bearing minerals

NASA Astrophysics Data System (ADS)

Nielsen, L. C.; Maher, K.; Bird, D. K.; Brown, G. E.; Thomas, B.; Johnson, N. C.; Rosenbauer, R. J.

2012-12-01

Redox reactions involving subsurface minerals and fluids and can lead to the abiotic generation of hydrocarbons from CO2 under certain conditions. Depleted oil reservoirs and saline aquifers targeted for geologic carbon sequestration (GCS) can contain significant quantities of minerals such as ferrous chlorite, which could facilitate the abiotic reduction of carbon dioxide to n-carboxylic acids, hydrocarbons, and amorphous carbon (C0). If such reactions occur, the injection of supercritical CO2 (scCO2) could significantly alter the oxidation state of the reservoir and cause extensive reorganization of the stable mineral assemblage via dissolution and reprecipitation reactions. Naturally occurring iron oxide minerals such as magnetite are known to catalyze CO2 reduction, resulting in the synthesis of organic compounds. Magnetite is thermodynamically stable in Fe(II) chlorite-bearing mineral assemblages typical of some reservoir formations. Thermodynamic calculations demonstrate that GCS reservoirs buffered by the chlorite-kaolinite-carbonate(siderite/magnesite)-quartz assemblage favor the reduction of CO2 to n-carboxylic acids, hydrocarbons, and C0, although the extent of abiotic CO2 reduction may be kinetically limited. To investigate the rates of abiotic CO2 reduction in the presence of magnetite, we performed batch abiotic CO2 reduction experiments using a Dickson-type rocking hydrothermal apparatus at temperatures (373 K) and pressures (100 bar) within the range of conditions relevant to GCS. Blank experiments containing CO2 and H2 were used to rule out the possibility of catalytic activity of the experimental apparatus. Reaction of brine-suspended magnetite nanoparticles with scCO2 at H2 partial pressures typical of reservoir rocks - up to 100 and 0.1 bars respectively - was used to investigate the kinetics of magnetite-catalyzed abiotic CO2 reduction. Later experiments introducing ferrous chlorite (ripidolite) were carried out to determine the potential for

Palladium/N-heterocyclic carbene catalysed regio and diastereoselective reaction of ketones with allyl reagents via inner-sphere mechanism

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bai, Da -Chang; Yu, Fei -Le; Wang, Wan -Ying

The palladium-catalysed allylic substitution reaction is one of the most important reactions in transition-metal catalysis and has been well-studied in the past decades. Most of the reactions proceed through an outer-sphere mechanism, affording linear products when monosubstituted allyl reagents are used. Here, we report an efficient Palladium-catalysed protocol for reactions of beta-substituted ketones with monosubstituted allyl substrates, simply by using N-heterocyclic carbene as ligand, leading to branched products with up to three contiguous stereocentres in a ( syn, anti)-mode with excellent regio and diastereoselectivities. The scope of the protocol in organic synthesis has been examined preliminarily. As a result, mechanisticmore » studies by both experiments and density functional theory ( DFT) calculations reveal that the reaction proceeds via an inner-sphere mechanism-nucleophilic attack of enolate oxygen on Palladium followed by C-C bond-forming [3,3']-reductive elimination.« less

Palladium/N-heterocyclic carbene catalysed regio and diastereoselective reaction of ketones with allyl reagents via inner-sphere mechanism

DOE PAGES

Bai, Da -Chang; Yu, Fei -Le; Wang, Wan -Ying; ...

2016-06-10

The palladium-catalysed allylic substitution reaction is one of the most important reactions in transition-metal catalysis and has been well-studied in the past decades. Most of the reactions proceed through an outer-sphere mechanism, affording linear products when monosubstituted allyl reagents are used. Here, we report an efficient Palladium-catalysed protocol for reactions of beta-substituted ketones with monosubstituted allyl substrates, simply by using N-heterocyclic carbene as ligand, leading to branched products with up to three contiguous stereocentres in a ( syn, anti)-mode with excellent regio and diastereoselectivities. The scope of the protocol in organic synthesis has been examined preliminarily. As a result, mechanisticmore » studies by both experiments and density functional theory ( DFT) calculations reveal that the reaction proceeds via an inner-sphere mechanism-nucleophilic attack of enolate oxygen on Palladium followed by C-C bond-forming [3,3']-reductive elimination.« less

Catalysis-dependent selenium incorporation and migration in the nitrogenase active site iron-molybdenum cofactor.

PubMed

Spatzal, Thomas; Perez, Kathryn A; Howard, James B; Rees, Douglas C

2015-12-16

Dinitrogen reduction in the biological nitrogen cycle is catalyzed by nitrogenase, a two-component metalloenzyme. Understanding of the transformation of the inert resting state of the active site FeMo-cofactor into an activated state capable of reducing dinitrogen remains elusive. Here we report the catalysis dependent, site-selective incorporation of selenium into the FeMo-cofactor from selenocyanate as a newly identified substrate and inhibitor. The 1.60 Å resolution structure reveals selenium occupying the S2B site of FeMo-cofactor in the Azotobacter vinelandii MoFe-protein, a position that was recently identified as the CO-binding site. The Se2B-labeled enzyme retains substrate reduction activity and marks the starting point for a crystallographic pulse-chase experiment of the active site during turnover. Through a series of crystal structures obtained at resolutions of 1.32-1.66 Å, including the CO-inhibited form of Av1-Se2B, the exchangeability of all three belt-sulfur sites is demonstrated, providing direct insights into unforeseen rearrangements of the metal center during catalysis.

Heterogeneous Organo-Catalysis: Sustainable Pathways to ...

EPA Pesticide Factsheets

Glucose and fructose are among the most abundant plant-derived materials1 and have been converted into useful building units often used in the drug discovery and polymer architecture.2 Unfortunately, most of these conversions require mineral acids and complex heterogeneous catalysis systems which suffer from the diminished activity and recyclability issues.3 Herein, we report a highly reactive and inexpensive heterogeneous sulfonated graphitic carbon nitride (Sg-CN), endowed with strong acidity that readily transforms carbohydrates to furanics. The ready availability and benign nature of the material and its stability over the several reaction cycles renders this catalyst very useful in organic synthesis, polymer industry and in the preparation of drug precursors. Poster presentation at the 253rd American Chemical Society (ACS) National meeting in San Francisco, CA

Enzymatic catalysis of formation of Z-aspartame in ionic liquid - An alternative to enzymatic catalysis in organic solvents.

PubMed

Erbeldinger, M; Mesiano, A J; Russell, A J

2000-01-01

We present the first report of enzymatic catalysis in an ionic liquid. The virtually nonexistent vapor pressure makes ionic liquids an exciting new alternative for enzyme-catalyzed syntheses in environmentally friendly environments. Z-aspartame was synthesized in a thermolysin-catalyzed reaction of carbobenzoxy-L-aspartate and L-phenylalanine methyl ester hydrochloride in 1-butyl-3-methylimidazolium hexafluorophosphate (BP6). Ionic liquids such as BP6 are thermally stable and have a remarkable range of temperatures over which they remain liquid (300 degrees C). With an initial rate of 1.2 +/- 0.1 nmol min(-)(1) mg(-)(1), we observed a competitive rate in comparison to that of enzymatic synthesis in organic solvent. Additionally, the enzyme exhibits outstanding stability, which would normally require immobilization.

Merging Visible Light Photoredox Catalysis with Metal Catalyzed C–H Activations: On the Role of Oxygen and Superoxide Ions as Oxidants

PubMed Central

2016-01-01

Conspectus The development of efficient catalytic systems for direct aromatic C–H bond functionalization is a long-desired goal of chemists, because these protocols provide environmental friendly and waste-reducing alternatives to classical methodologies for C–C and C–heteroatom bond formation. A key challenge for these transformations is the reoxidation of the in situ generated metal hydride or low-valent metal complexes of the primary catalytic bond forming cycle. To complete the catalytic cycle and to regenerate the C–H activation catalyst, (super)stoichiometric amounts of Cu(II) or Ag(I) salts have often been applied. Recently, “greener” approaches have been developed by applying molecular oxygen in combination with Cu(II) salts, internal oxidants that are cleaved during the reaction, or solvents or additives enabling the metal hydride reoxidation. All these approaches improved the environmental friendliness but have not overcome the obstacles associated with the overall limited functional group and substrate tolerance. Hence, catalytic processes that do not feature the unfavorable aspects described above and provide products in a streamlined as well as economically and ecologically advantageous manner would be desirable. In this context, we decided to examine visible light photoredox catalysis as a new alternative to conventionally applied regeneration/oxidation procedures. This Account summarizes our recent advances in this expanding area and will highlight the new concept of merging distinct redox catalytic processes for C–H functionalizations through the application of visible light photoredox catalysis. Photoredox catalysis can be considered as catalytic electron-donating or -accepting processes, making use of visible-light absorbing homogeneous and heterogeneous metal-based catalysts, as well as organic dye sensitizers or polymers. As a consequence, photoredox catalysis is, in principle, an ideal tool for the recycling of any given metal

Inorganic dendrimers: recent advances for catalysis, nanomaterials, and nanomedicine.

PubMed

Caminade, Anne-Marie

2016-10-07

Dendrimers are hyperbranched polymers having a perfectly defined structure because they are synthesized step-by-step in an iterative fashion, and not by polymerization reactions. Some dendrimers are considered as inorganic, as they possess inorganic atoms at each branching point. Among numerous examples, two families of inorganic dendrimers have emerged as particularly promising: silicon-containing dendrimers, particularly carbosilanes, and phosphorus-containing dendrimers, particularly phosphorhydrazones. This tutorial review will display the main properties of both families of dendrimers in the fields of catalysis, materials and biology/nanomedicine. Emphasis will be put on the most recent and promising examples.

Systematic Doping of Cobalt into Layered Manganese Oxide Sheets Substantially Enhances Water Oxidation Catalysis.

PubMed

McKendry, Ian G; Thenuwara, Akila C; Shumlas, Samantha L; Peng, Haowei; Aulin, Yaroslav V; Chinnam, Parameswara Rao; Borguet, Eric; Strongin, Daniel R; Zdilla, Michael J

2018-01-16

The effect on the electrocatalytic oxygen evolution reaction (OER) of cobalt incorporation into the metal oxide sheets of the layered manganese oxide birnessite was investigated. Birnessite and cobalt-doped birnessite were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and conductivity measurements. A cobalt:manganese ratio of 1:2 resulted in the most active catalyst for the OER. In particular, the overpotential (η) for the OER was 420 mV, significantly lower than the η = 780 mV associated with birnessite in the absence of Co. Furthermore, the Tafel slope for Co/birnessite was 81 mV/dec, in comparison to a Tafel slope of greater than 200 mV/dec for birnessite. For chemical water oxidation catalysis, an 8-fold turnover number (TON) was achieved (h = 70 mmol of O 2 /mol of metal). Density functional theory (DFT) calculations predict that cobalt modification of birnessite resulted in a raising of the valence band edge and occupation of that edge by holes with enhanced mobility during catalysis. Inclusion of extra cobalt beyond the ideal 1:2 ratio was detrimental to catalysis due to disruption of the layered structure of the birnessite phase.

Investigation of the electrocatalytic oxygen reduction and evolution reactions in lithium–oxygen batteries

DOE PAGES

Zheng, Dong; Zhang, Xuran; Qu, Deyu; ...

2015-04-21

Oxygen reduction and oxygen evolution reactions were examined on graphite electrodes with different crystal orientations. The kinetics for the redox couple O 2/O 2 •- are very fast, therefore no catalyst seems necessary to assist the charge transfer process. Apparently, the main source of the overpotential for the O 2 reduction reaction is from mass diffusion. Li 2O 2 becomes soluble in non-aqueous electrolytes in the presence of the tetraethylammonium tetrafluoroborate additive. The soluble B-O 2 2- ions can be oxidized electro-catalytically. The edge orientation of graphite demonstrates superior catalytic activity for the oxidation over basal orientation. The findings revealmore » an opportunity for recharging Li-air batteries efficiently and a new strategy of developing the catalyst for oxygen evolution reaction.« less

Direct synthesis of bimetallic PtCo mesoporous nanospheres as efficient bifunctional electrocatalysts for both oxygen reduction reaction and methanol oxidation reaction

NASA Astrophysics Data System (ADS)

Wang, Hongjing; Yu, Hongjie; Li, Yinghao; Yin, Shuli; Xue, Hairong; Li, Xiaonian; Xu, You; Wang, Liang

2018-04-01

The engineering of electrocatalysts with high performance for cathodic and/or anodic catalytic reactions is of great urgency for the development of direct methanol fuel cells. Pt-based bimetallic alloys have recently received considerable attention in the field of fuel cells because of their superior catalytic performance towards both fuel molecule electro-oxidation and oxygen reduction. In this work, bimetallic PtCo mesoporous nanospheres (PtCo MNs) with uniform size and morphology have been prepared by a one-step method with a high yield. The as-made PtCo MNs show superior catalytic activities for both oxygen reduction reaction and methanol oxidation reaction relative to Pt MNs and commercial Pt/C catalyst, attributed to their mesoporous structure and bimetallic composition.

Ketene reactions with tertiary amines.

PubMed

Allen, Annette D; Andraos, John; Tidwell, Thomas T; Vukovic, Sinisa

2014-01-17

Tertiary amines react rapidly and reversibly with arylketenes in acetonitrile forming observable zwitterions, and these undergo amine catalyzed dealkylation forming N,N-disubstituted amides. Reactions of N-methyldialkylamines show a strong preference for methyl group loss by displacement, as predicted by computational studies. Loss of ethyl groups in reactions with triethylamine also occur by displacement, but preferential loss of isopropyl groups in the phenylketene reaction with diisopropylethylamine evidently involves elimination. Quinuclidine rapidly forms long-lived zwitterions with arylketenes, providing a model for catalysis by cinchona and related alkaloids in stereoselective additions to ketenes.

Catalysis of Methyl Group Transfers Involving Tetrahydrofolate and B12

PubMed Central

Ragsdale, Stephen W.

2011-01-01

This review focuses on the reaction mechanism of enzymes that use B12 and tetrahydrofolate (THF) to catalyze methyl group transfers. It also covers the related reactions that use B12 and tetrahydromethanopterin (THMPT), which is a THF analog used by archaea. In the past decade, our understanding of the mechanisms of these enzymes has increased greatly because the crystal structures for three classes of B12-dependent methyltransferases have become available and because biophysical and kinetic studies have elucidated the intermediates involved in catalysis. These steps include binding of the cofactors and substrates, activation of the methyl donors and acceptors, the methyl transfer reaction itself, and product dissociation. Activation of the methyl donor in one class of methyltransferases is achieved by an unexpected proton transfer mechanism. The cobalt (Co) ion within the B12 macrocycle must be in the Co(I) oxidation state to serve as a nucleophile in the methyl transfer reaction. Recent studies have uncovered important principles that control how this highly reducing active state of B12 is generated and maintained. PMID:18804699

The Nernst equation applied to oxidation-reduction reactions in myoglobin and hemoglobin. Evaluation of the parameters.

PubMed

Saroff, Harry A

Analyses of the binding of oxygen to monomers such as myoglobin employ the Mass Action equation. The Mass Action equation, as such, is not directly applicable for the analysis of the binding of oxygen to oligomers such as hemoglobin. When the binding of oxygen to hemoglobin is analyzed, models incorporating extensions of mass action are employed. Oxidation-reduction reactions of the heme group in myoglobin and hemoglobin involve the binding and dissociation of electrons. This reaction is described with the Nernst equation. The Nernst equation is applicable only to a monomeric species even if the number of electrons involved is greater than unity. To analyze the oxidation-reduction reaction in a molecule such as hemoglobin a model is required which incorporates extensions of the Nernst equation. This communication develops models employing the Nernst equation for oxidation-reduction reactions analogous to those employed for hemoglobin in the analysis of the oxygenation (binding of oxygen) reaction.

Heterogeneous catalysis in complex, condensed reaction media

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cantu, David C.; Wang, Yang-Gang; Yoon, Yeohoon

Many reactions required for the upgrading of biomass into fuels and chemicals—hydrogenation, hydrodeoxygenation, hydrocracking—are ostensibly similar to those practiced in the upgrading of petroleum into fuels. But, repurposing hydroprocessing catalysts from refinery operations to treat bio-oil has proved to be unsatisfactory. New catalysts are needed because the composition of the biogenic reactants differs from that of petroleum-derived feedstocks (e.g. the low concentration of sulfur in cellulose-derived biomass precludes use of metal sulfide catalysts unless sulfur is added to the reaction stream). New processes are needed because bio-oils oligomerize rapidly, forming intractable coke and “gunk”, at temperatures so low that themore » desired upgrading reactions are impractically slow, and so low that the bio-oil upgrading must be handled as a condensed fluid. Ideally, the new catalysts and processes would exploit the properties of the multiple phases present in condensed bio-oil, notably the polarizability and structure of the fluid near a catalyst’s surface in the cybotactic region. The results of preliminary modeling of the cybotactic region of different catalyst surfaces in the hydrogenation of phenol suggest that Pd catalysts supported on hydrophilic surfaces are more active than catalysts based on lipophilic supports because the former serve to enhance the concentration of the phenol in the vicinity of the Pd. The effect stems from thermodynamics, not the rate of mass transport. This work was supported by the US Department of Energy, Office of Energy Efficiency and Renewable Energy, Bioenergy Technologies Office. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle.« less

Accelerated gas-liquid visible light photoredox catalysis with continuous-flow photochemical microreactors.

PubMed

Straathof, Natan J W; Su, Yuanhai; Hessel, Volker; Noël, Timothy

2016-01-01

In this protocol, we describe the construction and use of an operationally simple photochemical microreactor for gas-liquid photoredox catalysis using visible light. The general procedure includes details on how to set up the microreactor appropriately with inlets for gaseous reagents and organic starting materials, and it includes examples of how to use it to achieve continuous-flow preparation of disulfides or trifluoromethylated heterocycles and thiols. The reported photomicroreactors are modular, inexpensive and can be prepared rapidly from commercially available parts within 1 h even by nonspecialists. Interestingly, typical reaction times of gas-liquid visible light photocatalytic reactions performed in microflow are lower (in the minute range) than comparable reactions performed as a batch process (in the hour range). This can be attributed to the improved irradiation efficiency of the reaction mixture and the enhanced gas-liquid mass transfer in the segmented gas-liquid flow regime.

Carbon-doped BN nanosheets for metal-free photoredox catalysis

PubMed Central

Huang, Caijin; Chen, Cheng; Zhang, Mingwen; Lin, Lihua; Ye, Xinxin; Lin, Sen; Antonietti, Markus; Wang, Xinchen

2015-01-01

The generation of sustainable and stable semiconductors for solar energy conversion by photoredox catalysis, for example, light-induced water splitting and carbon dioxide reduction, is a key challenge of modern materials chemistry. Here we present a simple synthesis of a ternary semiconductor, boron carbon nitride, and show that it can catalyse hydrogen or oxygen evolution from water as well as carbon dioxide reduction under visible light illumination. The ternary B–C–N alloy features a delocalized two-dimensional electron system with sp2 carbon incorporated in the h-BN lattice where the bandgap can be adjusted by the amount of incorporated carbon to produce unique functions. Such sustainable photocatalysts made of lightweight elements facilitate the innovative construction of photoredox cascades to utilize solar energy for chemical conversion. PMID:26159752

Enzyme catalysis captured using multiple structures from one crystal at varying temperatures.

PubMed

Horrell, Sam; Kekilli, Demet; Sen, Kakali; Owen, Robin L; Dworkowski, Florian S N; Antonyuk, Svetlana V; Keal, Thomas W; Yong, Chin W; Eady, Robert R; Hasnain, S Samar; Strange, Richard W; Hough, Michael A

2018-05-01

High-resolution crystal structures of enzymes in relevant redox states have transformed our understanding of enzyme catalysis. Recent developments have demonstrated that X-rays can be used, via the generation of solvated electrons, to drive reactions in crystals at cryogenic temperatures (100 K) to generate 'structural movies' of enzyme reactions. However, a serious limitation at these temperatures is that protein conformational motion can be significantly supressed. Here, the recently developed MSOX (multiple serial structures from one crystal) approach has been applied to nitrite-bound copper nitrite reductase at room temperature and at 190 K, close to the glass transition. During both series of multiple structures, nitrite was initially observed in a 'top-hat' geometry, which was rapidly transformed to a 'side-on' configuration before conversion to side-on NO, followed by dissociation of NO and substitution by water to reform the resting state. Density functional theory calculations indicate that the top-hat orientation corresponds to the oxidized type 2 copper site, while the side-on orientation is consistent with the reduced state. It is demonstrated that substrate-to-product conversion within the crystal occurs at a lower radiation dose at 190 K, allowing more of the enzyme catalytic cycle to be captured at high resolution than in the previous 100 K experiment. At room temperature the reaction was very rapid, but it remained possible to generate and characterize several structural states. These experiments open up the possibility of obtaining MSOX structural movies at multiple temperatures (MSOX-VT), providing an unparallelled level of structural information during catalysis for redox enzymes.

Photoactivity of N-doped ZnO nanoparticles in oxidative and reductive reactions

NASA Astrophysics Data System (ADS)

Oliveira, Jéssica A.; Nogueira, André E.; Gonçalves, Maria C. P.; Paris, Elaine C.; Ribeiro, Caue; Poirier, Gael Y.; Giraldi, Tania R.

2018-03-01

N-doped ZnO is a prospective material for photocatalytic reactions. However, only oxidative paths are well investigated in the literature. This paper describes a comparative study about ZnO and ZnO:N potential for oxidative and reductive reactions, probed by rhodamine B dye photodegradation and CO2 photoreduction. The materials were prepared by the polymeric precursor method, using urea as a nitrogen source, and different heat treatments were used to observe their effects on surface decontamination, crystallinity, particle sizes and shapes, and photocatalytic performance. ZnO and ZnO:N presented a wurtzite crystalline structure and nanometric-scale particles. Samples submitted to higher temperatures showed lower specific surface areas, but higher crystallinity and lower contents of species adsorbed on their surfaces. On the other hand, the photocatalysts annealed in shorter times presented smaller crystallite sizes and lower crystallinity. These factors influenced the photoactivity in both conditions, i.e., oxidation and reduction reactions, under the ultraviolet and visible light, indicating that structural factors influenced the adequate charge separation and consequent photocatalytic activity since the as-synthesized samples were versatile photocatalysts in both redox reactions.

Bio-inspired nanocatalysts for the oxygen reduction reaction.

PubMed

Grumelli, Doris; Wurster, Benjamin; Stepanow, Sebastian; Kern, Klaus

2013-01-01

Electrochemical conversions at fuel cell electrodes are complex processes. In particular, the oxygen reduction reaction has substantial overpotential limiting the electrical power output efficiency. Effective and inexpensive catalytic interfaces are therefore essential for increased performance. Taking inspiration from enzymes, earth-abundant metal centres embedded in organic environments present remarkable catalytic active sites. Here we show that these enzyme-inspired centres can be effectively mimicked in two-dimensional metal-organic coordination networks self-assembled on electrode surfaces. Networks consisting of trimesic acid and bis-pyridyl-bispyrimidine coordinating to single iron and manganese atoms on Au(111) effectively catalyse the oxygen reduction and reveal distinctive catalytic activity in alkaline media. These results demonstrate the potential of surface-engineered metal-organic networks for electrocatalytic conversions. Specifically designed coordination complexes at surfaces inspired by enzyme cofactors represent a new class of nanocatalysts with promising applications in electrocatalysis.

Phosphoryl Transfer Reaction in RNA in Alkaline Conditions.

PubMed

Bertran, Joan; Oliva, Antoni; Branchadell, Vicenç; Acosta-Silva, Carles

2018-06-25

In this work we have studied the phosphoryl transfer reaction in RNA in alkaline conditions by theoretically exploring the influence of several solvents. The calculations have been carried out using the M06-2X functional while the solvents are taken as a continuum using the SMD method. The main results are that the O2'-P-O5' angle in the reactants, the free activation energies and the reaction mechanism are clearly dependent on the dielectric constant of the environment, thus showing that the electrostatic term is determining for this chemical system with two negative charges. Our study seems to indicate that water, the solvent with the greatest dielectric constant, would be the one that mostly increases the reaction rate. As this is not the case in enzymatic catalysis, one has to conclude that, in the case of proteins as well as in the case of ribozymes, the enzymatic catalysis is not mainly due to the solvent reaction field, but to local electrical fields due to the enzyme preorganization. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Unraveling the role of protein dynamics in dihydrofolate reductase catalysis

PubMed Central

Luk, Louis Y. P.; Javier Ruiz-Pernía, J.; Dawson, William M.; Roca, Maite; Loveridge, E. Joel; Glowacki, David R.; Harvey, Jeremy N.; Mulholland, Adrian J.; Tuñón, Iñaki; Moliner, Vicent; Allemann, Rudolf K.

2013-01-01

Protein dynamics have controversially been proposed to be at the heart of enzyme catalysis, but identification and analysis of dynamical effects in enzyme-catalyzed reactions have proved very challenging. Here, we tackle this question by comparing an enzyme with its heavy (15N, 13C, 2H substituted) counterpart, providing a subtle probe of dynamics. The crucial hydride transfer step of the reaction (the chemical step) occurs more slowly in the heavy enzyme. A combination of experimental results, quantum mechanics/molecular mechanics simulations, and theoretical analyses identify the origins of the observed differences in reactivity. The generally slightly slower reaction in the heavy enzyme reflects differences in environmental coupling to the hydride transfer step. Importantly, the barrier and contribution of quantum tunneling are not affected, indicating no significant role for “promoting motions” in driving tunneling or modulating the barrier. The chemical step is slower in the heavy enzyme because protein motions coupled to the reaction coordinate are slower. The fact that the heavy enzyme is only slightly less active than its light counterpart shows that protein dynamics have a small, but measurable, effect on the chemical reaction rate. PMID:24065822

Merging photoredox and nickel catalysis: decarboxylative cross-coupling of carboxylic acids with vinyl halides.

PubMed

Noble, Adam; McCarver, Stefan J; MacMillan, David W C

2015-01-21

Decarboxylative cross-coupling of alkyl carboxylic acids with vinyl halides has been accomplished through the synergistic merger of photoredox and nickel catalysis. This new methodology has been successfully applied to a variety of α-oxy and α-amino acids, as well as simple hydrocarbon-substituted acids. Diverse vinyl iodides and bromides give rise to vinylation products in high efficiency under mild, operationally simple reaction conditions.

Merging Photoredox and Nickel Catalysis: Decarboxylative Cross-Coupling of Carboxylic Acids with Vinyl Halides

PubMed Central

2015-01-01

Decarboxylative cross-coupling of alkyl carboxylic acids with vinyl halides has been accomplished through the synergistic merger of photoredox and nickel catalysis. This new methodology has been successfully applied to a variety of α-oxy and α-amino acids, as well as simple hydrocarbon-substituted acids. Diverse vinyl iodides and bromides give rise to vinylation products in high efficiency under mild, operationally simple reaction conditions. PMID:25521443

Stabilization of AuNPs by monofunctional triazole linked to ferrocene, ferricenium, or coumarin and applications to synthesis, sensing, and catalysis.

PubMed

Li, Na; Zhao, Pengxiang; Igartua, María E; Rapakousiou, Amalia; Salmon, Lionel; Moya, Sergio; Ruiz, Jaime; Astruc, Didier

2014-11-03

Monofunctional triazoles linked to ferrocene, ferricenium, or coumarin (Cou), easily synthesized by copper-catalyzed azide alkyne (CuAAC) "click" reactions between the corresponding functional azides and (trimethylsilyl)acetylene followed by silyl group deprotection, provide a variety of convenient neutral ligands for the stabilization of functional gold nanoparticles (AuNPs) in polar organic solvents. These triazole (trz)-AuNPs are very useful toward a variety of applications to synthesis, sensing, and catalysis. Both ferrocenyl (Fc) and isostructural ferricenium linked triazoles give rise to AuNP stabilization, although by different synthetic routes. Indeed, the first direct synthesis and stabilization of AuNPs by ferricenium are obtained by the reduction of HAuCl4 upon reaction with a ferrocene derivative, AuNP stabilization resulting from a synergy between electrostatic and coordination effects. The ferricenium/ferrocene trz-AuNP redox couple is fully reversible, as shown by cyclic voltammograms that were recorded with both redox forms. These trz-AuNPs are stable for weeks in various polar solvents, but at the same time, the advantage of trz-AuNPs is the easy substitution of neutral trz ligands by thiols and other ligands, giving rise to applications. Indeed, this ligand substitution of trz at the AuNP surface yields a stable Fc-terminated nanogold-cored dendrimer upon reaction with a Fc-terminated thiol dendron, substitution of Cou-linked trz with cysteine, homocysteine, and glutathione provides remarkably efficient biothiol sensing, and a ferricenium-linked trz-AuNP catalyst is effective for NaBH4 reduction of 4-nitrophenol to 4-aminophenol. In this catalytic example, the additional electrostatic AuNP stabilization modulates the reaction rate and induction time.

Exact model reduction of combinatorial reaction networks

PubMed Central

Conzelmann, Holger; Fey, Dirk; Gilles, Ernst D

2008-01-01

Background Receptors and scaffold proteins usually possess a high number of distinct binding domains inducing the formation of large multiprotein signaling complexes. Due to combinatorial reasons the number of distinguishable species grows exponentially with the number of binding domains and can easily reach several millions. Even by including only a limited number of components and binding domains the resulting models are very large and hardly manageable. A novel model reduction technique allows the significant reduction and modularization of these models. Results We introduce methods that extend and complete the already introduced approach. For instance, we provide techniques to handle the formation of multi-scaffold complexes as well as receptor dimerization. Furthermore, we discuss a new modeling approach that allows the direct generation of exactly reduced model structures. The developed methods are used to reduce a model of EGF and insulin receptor crosstalk comprising 5,182 ordinary differential equations (ODEs) to a model with 87 ODEs. Conclusion The methods, presented in this contribution, significantly enhance the available methods to exactly reduce models of combinatorial reaction networks. PMID:18755034

Thiourea-catalyzed Diels–Alder reaction of a naphthoquinone monoketal dienophile

PubMed Central

Kramer, Carsten S

2013-01-01

Summary A variety of organocatalysts were screened for the catalysis of the naphthoquinone monoketal Diels–Alder reaction. In this study we found that Schreiner's thiourea catalyst 10 and Jacobson's thiourea catalyst 12 facilitate the cycloaddition of the sterically hindered naphthoquinone monoketal dienophile 3 with diene 4. The use of thiourea catalysis allowed for the first time the highly selective synthesis of the exo-product 2a in up to 63% yield. In this reaction a new quaternary center was built. The so formed cycloaddition product 2a represents the ABC tricycle of beticolin 0 (1) and is also a valuable model substrate for the total synthesis of related natural products. PMID:23946836

Robust Catalysis on 2D Materials Encapsulating Metals: Concept, Application, and Perspective.

PubMed

Deng, Jiao; Deng, Dehui; Bao, Xinhe

2017-11-01

Great endeavors are undertaken to search for low-cost, rich-reserve, and highly efficient alternatives to replace precious-metal catalysts, in order to cut costs and improve the efficiency of catalysts in industry. However, one major problem in metal catalysts, especially nonprecious-metal catalysts, is their poor stability in real catalytic processes. Recently, a novel and promising strategy to construct 2D materials encapsulating nonprecious-metal catalysts has exhibited inimitable advantages toward catalysis, especially under harsh conditions (e.g., strong acidity or alkalinity, high temperature, and high overpotential). The concept, which originates from unique electron penetration through the 2D crystal layer from the encapsulated metals to promote a catalytic reaction on the outermost surface of the 2D crystal, has been widely applied in a variety of reactions under harsh conditions. It has been vividly described as "chainmail for catalyst." Herein, recent progress concerning this chainmail catalyst is reviewed, particularly focusing on the structural design and control with the associated electronic properties of such heterostructure catalysts, and also on their extensive applications in fuel cells, water splitting, CO 2 conversion, solar cells, metal-air batteries, and heterogeneous catalysis. In addition, the current challenges that are faced in fundamental research and industrial application, and future opportunities for these fantastic catalytic materials are discussed. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Role of the pyridine nitrogen in pyridoxal 5'-phosphate catalysis: activity of three classes of PLP enzymes reconstituted with deazapyridoxal 5'-phosphate.

PubMed

Griswold, Wait R; Toney, Michael D

2011-09-21

Pyridoxal 5'-phosphate (PLP; vitamin B(6))-catalyzed reactions have been well studied, both on enzymes and in solution, due to the variety of important reactions this cofactor catalyzes in nitrogen metabolism. Three functional groups are central to PLP catalysis: the C4' aldehyde, the O3' phenol, and the N1 pyridine nitrogen. In the literature, the pyridine nitrogen has traditionally been assumed to be protonated in enzyme active sites, with the protonated pyridine ring providing resonance stabilization of carbanionic intermediates. This assumption is certainly correct for some PLP enzymes, but the structures of other active sites are incompatible with protonation of N1, and, consequently, these enzymes are expected to use PLP in the N1-unprotonated form. For example, aspartate aminotransferase protonates the pyridine nitrogen for catalysis of transamination, while both alanine racemase and O-acetylserine sulfhydrylase are expected to maintain N1 in the unprotonated, formally neutral state for catalysis of racemization and β-elimination. Herein, kinetic results for these three enzymes reconstituted with 1-deazapyridoxal 5'-phosphate, an isosteric analogue of PLP lacking the pyridine nitrogen, are compared to those for the PLP enzyme forms. They demonstrate that the pyridine nitrogen is vital to the 1,3-prototropic shift central to transamination, but not to reactions catalyzed by alanine racemase or O-acetylserine sulfhydrylase. Not all PLP enzymes require the electrophilicity of a protonated pyridine ring to enable formation of carbanionic intermediates. It is proposed that modulation of cofactor electrophilicity plays a central role in controlling reaction specificity in PLP enzymes.

Catalysis of concerted reactions by antibodies: the Claisen rearrangement.

PubMed Central

Hilvert, D; Carpenter, S H; Nared, K D; Auditor, M T

1988-01-01

Monoclonal antibodies were prepared against a transition state analog inhibitor of chorismate mutase (EC 5.4.99.5). One of the antibodies catalyzes the rearrangement of chorismate to prephenate with rate accelerations of more than 2 orders of magnitude compared to the uncatalyzed reaction. Saturation kinetics were observed, and at 25 degrees C the values of kcat and Km were 1.2 X 10(-3) s-1 and 5.1 X 10(-5) M respectively. The transition state analog was shown to be a competitive inhibitor of the reaction with Ki equal to 0.6 microM. These results demonstrate the feasibility of using rationally designed immunogens to generate antibodies that catalyze concerted reactions. PMID:3393525

Synergistic Diazo-OH Insertion/Conia-Ene Cascade Catalysis for the Stereoselective Synthesis of γ-Butyrolactones and Tetrahydrofurans.

PubMed

Hunter, Arianne C; Schlitzer, Steven C; Sharma, Indrajeet

2016-11-02

A novel and highly efficient diazo-OH insertion/Conia-ene cascade reaction of readily available homopropargylic acids and alcohols with diazo carbonyl compounds is described. The cascade reaction involves a synergistic Rh/Ag/Au catalyst cocktail and proceeds instantly with a variety of substituted diazo compounds and acids/alcohols to provide functionalized γ-butyrolactones and tetrahydrofurans with complete regio- and stereoselectivity. The unprecedented rate-enhancement, complete stereoselectivity, and the enabling of new Conia-ene cyclizations suggest a concerted [4+1]-cycloaddition reaction pathway under synergistic (Rh/Ag/Au)-catalysis conditions. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Heterogeneous Catalysis: Deuterium Exchange Reactions of Hydrogen and Methane

ERIC Educational Resources Information Center

Mirich, Anne; Miller, Trisha Hoette; Klotz, Elsbeth; Mattson, Bruce

2015-01-01

Two gas phase deuterium/hydrogen exchange reactions are described utilizing a simple inexpensive glass catalyst tube containing 0.5% Pd on alumina through which gas mixtures can be passed and products collected for analysis. The first of these exchange reactions involves H[subscript 2] + D[subscript 2], which proceeds at temperatures as low as 77…

Silylative Reductive Amination of α,β-Unsaturated Aldehydes: A Convenient Synthetic Route to β-Silylated Secondary Amines.

PubMed

Kim, Eunae; Park, Sehoon; Chang, Sukbok

2018-04-17

Described here is a reductive amination/hydrosilylation cascade of α,β-unsaturated aldehydes mediated by a Lewis acidic borane catalyst. The present reaction system provides an one-pot synthetic route towards β-silylated secondary amines that have not been accessible by other previous catalysis. Comparative 1 H NMR studies on the silylative reduction of enimines revealed that steric bulkiness of primary amine reactants strongly affects both catalytic efficiency and regioselectivity. This strategy was applicable to a broad range of substrates and amenable to one-pot gram-scale synthesis. Moreover, a diastereoselective introduction of the β-silyl group was also found to be feasible (d.r. up to 71:29). © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Optimizing Metalloporphyrin-Catalyzed Reduction Reactions for In Situ Remediation of DOE Contaminants

DOE Office of Scientific and Technical Information (OSTI.GOV)

Schlautman, Mark A.

2013-07-14

Past activities have resulted in a legacy of contaminated soil and groundwater at Department of Energy facilities nationwide. Uranium and chromium are among the most frequently encountered and highest-priority metal and radionuclide contaminants at DOE installations. Abiotic chemical reduction of uranium and chromium at contaminated DOE sites can be beneficial because the reduced metal species are less soluble in water, less mobile in the environment, and less toxic to humans and ecosystems. Although direct biological reduction has been reported for U(VI) and Cr(VI) in laboratory studies and at some field sites, the reactions can sometimes be slow or even inhibitedmore » due to unfavorable environmental conditions. One promising approach for the in-situ remediation of DOE contaminants is to develop electron shuttle catalysts that can be delivered precisely to the specific subsurface locations where contaminants reside. Previous research has shown that reduction of oxidized organic and inorganic contaminants often can be catalyzed by electron shuttle systems. Metalloporphyrins and their derivatives are well known electron shuttles for many biogeochemical systems, and thus were selected to study their catalytic capabilities for the reduction of chromium and uranium in the presence of reducing agents. Zero valent iron (ZVI) was chosen as the primary electron donor in most experimental systems. Research proceeded in three phases and the key findings of each phase are reported here. Phase I examined Cr(VI) reduction and utilized micro- and nano-sized ZVI as the electron donors. Electron shuttle catalysts tested were cobalt- and iron-containing metalloporphyrins and Vitamin B12. To aid in the recycle and reuse of the nano-sized ZVI and soluble catalysts, sol-gels and calcium-alginate gel beads were tested as immobilization/support matrices. Although the nano-sized ZVI could be incorporated within the alginate gel beads, preliminary attempts to trap it in sol-gels were

Magnetic catalysis and inverse magnetic catalysis in (2 +1 )-dimensional gauge theories from holographic models

NASA Astrophysics Data System (ADS)

Rodrigues, Diego M.; Capossoli, Eduardo Folco; Boschi-Filho, Henrique

2018-06-01

We study the deconfinement phase transition in (2 +1 )-dimensional holographic S U (N ) gauge theories in the presence of an external magnetic field from the holographic hard and soft wall models. We obtain exact solutions for the critical temperature of the deconfinement transition for any range of magnetic field. As a consequence, we find a critical magnetic field (Bc), in which the critical temperature (Tc) vanishes; for B catalysis and for B >Bc we have a magnetic catalysis.

The role of phosphate in a multistep enzymatic reaction: reactions of the substrate and intermediate in pieces.

PubMed

Kholodar, Svetlana A; Allen, C Leigh; Gulick, Andrew M; Murkin, Andrew S

2015-02-25

Several mechanistically unrelated enzymes utilize the binding energy of their substrate's nonreacting phosphoryl group to accelerate catalysis. Evidence for the involvement of the phosphodianion in transition state formation has come from reactions of the substrate in pieces, in which reaction of a truncated substrate lacking its phosphorylmethyl group is activated by inorganic phosphite. What has remained unknown until now is how the phosphodianion group influences the reaction energetics at different points along the reaction coordinate. 1-Deoxy-D-xylulose-5-phosphate (DXP) reductoisomerase (DXR), which catalyzes the isomerization of DXP to 2-C-methyl-D-erythrose 4-phosphate (MEsP) and subsequent NADPH-dependent reduction, presents a unique opportunity to address this concern. Previously, we have reported the effect of covalently linked phosphate on the energetics of DXP turnover. Through the use of chemically synthesized MEsP and its phosphate-truncated analogue, 2-C-methyl-D-glyceraldehyde, the current study revealed a loss of 6.1 kcal/mol of kinetic barrier stabilization upon truncation, of which 4.4 kcal/mol was regained in the presence of phosphite dianion. The activating effect of phosphite was accompanied by apparent tightening of its interactions within the active site at the intermediate stage of the reaction, suggesting a role of the phosphodianion in disfavoring intermediate release and in modulation of the on-enzyme isomerization equilibrium. The results of kinetic isotope effect and structural studies indicate rate limitation by physical steps when the covalent linkage is severed. These striking differences in the energetics of the natural reaction and the reactions in pieces provide a deeper insight into the contribution of enzyme-phosphodianion interactions to the reaction coordinate.

The CoFactor database: organic cofactors in enzyme catalysis.

PubMed

Fischer, Julia D; Holliday, Gemma L; Thornton, Janet M

2010-10-01

Organic enzyme cofactors are involved in many enzyme reactions. Therefore, the analysis of cofactors is crucial to gain a better understanding of enzyme catalysis. To aid this, we have created the CoFactor database. CoFactor provides a web interface to access hand-curated data extracted from the literature on organic enzyme cofactors in biocatalysis, as well as automatically collected information. CoFactor includes information on the conformational and solvent accessibility variation of the enzyme-bound cofactors, as well as mechanistic and structural information about the hosting enzymes. The database is publicly available and can be accessed at http://www.ebi.ac.uk/thornton-srv/databases/CoFactor.

Charge Transfer and Catalysis at the Metal Support Interface

DOE Office of Scientific and Technical Information (OSTI.GOV)

Baker, Lawrence Robert

Kinetic, electronic, and spectroscopic characterization of model Pt–support systems are used to demonstrate the relationship between charge transfer and catalytic activity and selectivity. The results show that charge flow controls the activity and selectivity of supported metal catalysts. This dissertation builds on extensive existing knowledge of metal–support interactions in heterogeneous catalysis. The results show the prominent role of charge transfer at catalytic interfaces to determine catalytic activity and selectivity. Further, this research demonstrates the possibility of selectively driving catalytic chemistry by controlling charge flow and presents solid-state devices and doped supports as novel methods for obtaining electronic control over catalyticmore » reaction kinetics.« less

Preparation of the vulcan XC-72R-supported Pt nanoparticles for the hydrogen evolution reaction in PEM water electrolysers

NASA Astrophysics Data System (ADS)

Du Nguyen, Huy; Thuy Luyen Nguyen, T.; Nguyen, Khac Manh; Ha, Thuc Huy; Hien Nguyen, Quoc

2015-01-01

Pt nanoparticles on vulcan XC-72R support (Pt/vulcan XC-72R) were prepared by the impregnation-reduction method. The Pt content, the morphological properties and the electrochemical catalysis of the Pt/vulcan XC 72R materials have been investigated by ICP-OES analysis, FESEM, TEM, and cyclic voltammetry. These materials were then used as catalyst for hydrogen evolution reaction at the cathode of proton exchange membrane (PEM) water electrolysers. The best catalyst was Pt/vulcan XC-72R prepared by the impregnation-reduction method which is conducted in two reducing steps with the reductants of sodium borohydride and ethylene glycol, respectively. The current density of PEM water electrolysers reached 1.0 A cm-2 when applying a voltage of 2.0 V at 25 °C.

Recent advances in N-heterocyclic carbene (NHC)-catalysed benzoin reactions

PubMed Central

Menon, Rajeev S; Biju, Akkattu T

2016-01-01

Summary N-Heterocyclic carbenes (NHCs) have emerged as a powerful class of organocatalysts that mediate a variety of organic transformations. The Benzoin reaction constitutes one of the earliest known carbon–carbon bond-forming reactions catalysed by NHCs. The rapid growth of NHC catalysis in general has resulted in the development of a variety of benzoin and benzoin-type reactions. An overview of such NHC-catalysed benzoin reactions is presented. PMID:27340440

Recent advances in N-heterocyclic carbene (NHC)-catalysed benzoin reactions.

PubMed

Menon, Rajeev S; Biju, Akkattu T; Nair, Vijay

2016-01-01

N-Heterocyclic carbenes (NHCs) have emerged as a powerful class of organocatalysts that mediate a variety of organic transformations. The Benzoin reaction constitutes one of the earliest known carbon-carbon bond-forming reactions catalysed by NHCs. The rapid growth of NHC catalysis in general has resulted in the development of a variety of benzoin and benzoin-type reactions. An overview of such NHC-catalysed benzoin reactions is presented.

Multi-Scale Computational Enzymology: Enhancing Our Understanding of Enzymatic Catalysis

PubMed Central

Gherib, Rami; Dokainish, Hisham M.; Gauld, James W.

2014-01-01

Elucidating the origin of enzymatic catalysis stands as one the great challenges of contemporary biochemistry and biophysics. The recent emergence of computational enzymology has enhanced our atomistic-level description of biocatalysis as well the kinetic and thermodynamic properties of their mechanisms. There exists a diversity of computational methods allowing the investigation of specific enzymatic properties. Small or large density functional theory models allow the comparison of a plethora of mechanistic reactive species and divergent catalytic pathways. Molecular docking can model different substrate conformations embedded within enzyme active sites and determine those with optimal binding affinities. Molecular dynamics simulations provide insights into the dynamics and roles of active site components as well as the interactions between substrate and enzymes. Hybrid quantum mechanical/molecular mechanical (QM/MM) can model reactions in active sites while considering steric and electrostatic contributions provided by the surrounding environment. Using previous studies done within our group, on OvoA, EgtB, ThrRS, LuxS and MsrA enzymatic systems, we will review how these methods can be used either independently or cooperatively to get insights into enzymatic catalysis. PMID:24384841

Aromatic Chlorosulfonylation by Photoredox Catalysis.

PubMed

Májek, Michal; Neumeier, Michael; Jacobi von Wangelin, Axel

2017-01-10

Visible-light photoredox catalysis enables the efficient synthesis of arenesulfonyl chlorides from anilines. The new protocol involves the convenient in situ preparation of arenediazonium salts (from anilines) and the reactive gases SO 2 and HCl (from aqueous SOCl 2 ). The photocatalytic chlorosulfonylation operates at mild conditions (room temperature, acetonitrile/water) with low catalyst loading. Various functional groups are tolerated (e.g., halides, azides, nitro groups, CF 3 , SF 5 , esters, heteroarenes). Theoretical and experimental studies support a photoredox-catalysis mechanism. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Molecular catalysis science: Perspective on unifying the fields of catalysis.

PubMed

Ye, Rong; Hurlburt, Tyler J; Sabyrov, Kairat; Alayoglu, Selim; Somorjai, Gabor A

2016-05-10

Colloidal chemistry is used to control the size, shape, morphology, and composition of metal nanoparticles. Model catalysts as such are applied to catalytic transformations in the three types of catalysts: heterogeneous, homogeneous, and enzymatic. Real-time dynamics of oxidation state, coordination, and bonding of nanoparticle catalysts are put under the microscope using surface techniques such as sum-frequency generation vibrational spectroscopy and ambient pressure X-ray photoelectron spectroscopy under catalytically relevant conditions. It was demonstrated that catalytic behavior and trends are strongly tied to oxidation state, the coordination number and crystallographic orientation of metal sites, and bonding and orientation of surface adsorbates. It was also found that catalytic performance can be tuned by carefully designing and fabricating catalysts from the bottom up. Homogeneous and heterogeneous catalysts, and likely enzymes, behave similarly at the molecular level. Unifying the fields of catalysis is the key to achieving the goal of 100% selectivity in catalysis.

Molecular catalysis science: Perspective on unifying the fields of catalysis

PubMed Central

Ye, Rong; Hurlburt, Tyler J.; Sabyrov, Kairat; Alayoglu, Selim; Somorjai, Gabor A.

2016-01-01

Colloidal chemistry is used to control the size, shape, morphology, and composition of metal nanoparticles. Model catalysts as such are applied to catalytic transformations in the three types of catalysts: heterogeneous, homogeneous, and enzymatic. Real-time dynamics of oxidation state, coordination, and bonding of nanoparticle catalysts are put under the microscope using surface techniques such as sum-frequency generation vibrational spectroscopy and ambient pressure X-ray photoelectron spectroscopy under catalytically relevant conditions. It was demonstrated that catalytic behavior and trends are strongly tied to oxidation state, the coordination number and crystallographic orientation of metal sites, and bonding and orientation of surface adsorbates. It was also found that catalytic performance can be tuned by carefully designing and fabricating catalysts from the bottom up. Homogeneous and heterogeneous catalysts, and likely enzymes, behave similarly at the molecular level. Unifying the fields of catalysis is the key to achieving the goal of 100% selectivity in catalysis. PMID:27114536

Molecular catalysis science: Perspective on unifying the fields of catalysis

DOE PAGES

Ye, Rong; Hurlburt, Tyler J.; Sabyrov, Kairat; ...

2016-04-25

Colloidal chemistry is used to control the size, shape, morphology, and composition of metal nanoparticles. Model catalysts as such are applied to catalytic transformations in the three types of catalysts: heterogeneous, homogeneous, and enzymatic. Real-time dynamics of oxidation state, coordination, and bonding of nanoparticle catalysts are put under the microscope using surface techniques such as sumfrequency generation vibrational spectroscopy and ambient pressure X-ray photoelectron spectroscopy under catalytically relevant conditions. It was demonstrated that catalytic behavior and trends are strongly tied to oxidation state, the coordination number and crystallographic orientation of metal sites, and bonding and orientation of surface adsorbates. Itmore » was also found that catalytic performance can be tuned by carefully designing and fabricating catalysts from the bottom up. Homogeneous and h eterogeneous catalysts, and likely enzymes, behave similarly at the molecular level. Finally, unifying the fields of catalysis is the key to achieving the goal of 100% selectivity in catalysis.« less

Direct vs. indirect pathway for nitrobenzene reduction reaction on a Ni catalyst surface: a density functional study.

PubMed

Mahata, Arup; Rai, Rohit K; Choudhuri, Indrani; Singh, Sanjay K; Pathak, Biswarup

2014-12-21

Density functional theory (DFT) calculations are performed to understand and address the previous experimental results that showed the reduction of nitrobenzene to aniline prefers direct over indirect reaction pathways irrespective of the catalyst surface. Nitrobenzene to aniline conversion occurs via the hydroxyl amine intermediate (direct pathway) or via the azoxybenzene intermediate (indirect pathway). Through our computational study we calculated the spin polarized and dispersion corrected reaction energies and activation barriers corresponding to various reaction pathways for the reduction of nitrobenzene to aniline over a Ni catalyst surface. The adsorption behaviour of the substrate, nitrobenzene, on the catalyst surface was also considered and the energetically most preferable structural orientation was elucidated. Our study indicates that the parallel adsorption behaviour of the molecules over a catalyst surface is preferable over vertical adsorption behaviour. Based on the reaction energies and activation barrier of the various elementary steps involved in direct or indirect reaction pathways, we find that the direct reduction pathway of nitrobenzene over the Ni(111) catalyst surface is more favourable than the indirect reaction pathway.

Illustrating Catalysis with Interlocking Building Blocks: Correlation between Structure of a Metallocene Catalyst and the Stereoregularity of Polypropylene

ERIC Educational Resources Information Center

Horikoshi, Ryo; Kobayashi, Yoji; Kageyama, Hiroshi

2013-01-01

Catalysis with transition-metal complexes is a part of the inorganic chemistry curriculum and a challenging topic for upper-level undergraduate and graduate students. A hands-on teaching aid has been developed for use during conventional lectures to help students understand these catalytic reactions. A unique method of illustrating the…

General acid-base catalysis mediated by nucleobases in the hairpin ribozyme

PubMed Central

Kath-Schorr, Stephanie; Wilson, Timothy J.; Li, Nan-Sheng; Lu, Jun; Piccirilli, Joseph A.; Lilley, David M. J.

2012-01-01

The catalytic mechanism by which the hairpin ribozyme accelerates cleavage or ligation of the phosphodiester backbone of RNA has been incompletely understood. There is experimental evidence for an important role for an adenine (A38) and a guanine (G8), and it has been proposed that these act in general acid-base catalysis. In this work we show that a large reduction in cleavage rate on substitution of A38 by purine (A38P) can be reversed by replacement of the 5′-oxygen atom at the scissile phosphate by sulfur (5′-PS), which is a much better leaving group. This is consistent with A38 acting as the general acid in the unmodified ribozyme. The rate of cleavage of the 5′-PS substrate by the A38P ribozyme increases with pH log-linearly, indicative of a requirement for a deprotonated base with a relatively high pKa. On substitution of G8 by diaminopurine, the 5′-PS substrate cleavage rate at first increases with pH and then remains at a plateau, exhibiting an apparent pKa consistent with this nucleotide acting in general base catalysis. Alternative explanations for the pH dependence of hairpin ribozyme reactivity are discussed, from which we conclude that general acid-base catalysis by A38 and G8 is the simplest and most probable explanation consistent with all the experimental data. PMID:22958171

Conformational Plasticity of an Enzyme during Catalysis: Intricate Coupling between Cyclophilin A Dynamics and Substrate Turnover

PubMed Central

McGowan, Lauren C.; Hamelberg, Donald

2013-01-01

Enzyme catalysis is central to almost all biochemical processes, speeding up rates of reactions to biological relevant timescales. Enzymes make use of a large ensemble of conformations in recognizing their substrates and stabilizing the transition states, due to the inherent dynamical nature of biomolecules. The exact role of these diverse enzyme conformations and the interplay between enzyme conformational dynamics and catalysis is, according to the literature, not well understood. Here, we use molecular dynamics simulations to study human cyclophilin A (CypA), in order to understand the role of enzyme motions in the catalytic mechanism and recognition. Cyclophilin A is a tractable model system to study using classical simulation methods, because catalysis does not involve bond formation or breakage. We show that the conformational dynamics of active site residues of substrate-bound CypA is inherent in the substrate-free enzyme. CypA interacts with its substrate via conformational selection as the configurations of the substrate changes during catalysis. We also show that, in addition to tight intermolecular hydrophobic interactions between CypA and the substrate, an intricate enzyme-substrate intermolecular hydrogen-bonding network is extremely sensitive to the configuration of the substrate. These enzyme-substrate intermolecular interactions are loosely formed when the substrate is in the reactant and product states and become well formed and reluctant to break when the substrate is in the transition state. Our results clearly suggest coupling among enzyme-substrate intermolecular interactions, the dynamics of the enzyme, and the chemical step. This study provides further insights into the mechanism of peptidyl-prolyl cis/trans isomerases and the general interplay between enzyme conformational dynamics and catalysis. PMID:23332074

Catalysis and biocatalysis program

NASA Technical Reports Server (NTRS)

Ingham, J. D.

1993-01-01

This final report presents a summary of research activities and accomplishments for the Catalysis and Biocatalysis Program, which was renamed the Biological and Chemical Technologies Research (BCTR) Program, currently of the Advanced Industrial Concepts Division (AICD), Office of Industrial Technologies of the Department of Energy (DOE). The Program was formerly under the Division of Energy Conversion and Utilization Technologies (ECUT) until the DOE reorganization in April, 1990. The goals of the BCTR Program are consistent with the initial ECUT goals, but represent an increased effort toward advances in chemical and biological technology transfer. In addition, the transition reflects a need for the BCTR Program to assume a greater R&D role in chemical catalysis as well as a need to position itself for a more encompassing involvement in a broader range of biological and chemical technology research. The mission of the AICD is to create a balanced Program of high risk, long-term, directed interdisciplinary research and development that will improve energy efficiency and enhance fuel flexibility in the industrial sector. Under AICD, the DOE Catalysis and Biocatalysis Program sponsors research and development in furthering industrial biotechnology applications and promotes the integrated participation of universities, industrial companies, and government research laboratories.

Multimetallic Catalysis Enabled Cross-Coupling of Aryl Bromides with Aryl Triflates

PubMed Central

Ackerman, Laura K.G.; Lovell, Matthew M.

2015-01-01

Transition metal-catalyzed strategies for the formation of new C-C bonds have revolutionized the field of organic chemistry, enabling the efficient synthesis of ligands, materials, and biologically active molecules.1–3 In cases where a single metal fails to promote a selective or efficient transformation, the synergistic cooperation4 of two distinct catalysts – multimetallic catalysis – can be employed instead. Many important reactions rely on multimetallic catalysis,5 including the Wacker oxidation of olefins6–8 and the Sonogashira coupling of alkynes with aryl halides.9–10 However, the application of this strategy, even in recently developed methods11, has largely been limited to the use of metals with distinct reactivities, with only one metal catalyst undergoing an oxidative addition.12 In this manuscript, we demonstrate that cooperativity between two d10 metal catalysts, (bipyridine)nickel and (1,3-bis(diphenylphosphino)propane)palladium, enables a general cross-Ullman reaction.13–15 Our method couples aryl bromides with aryl triflates directly, eliminating the use of arylmetal reagents and avoiding the challenge of differentiating between multiple C–H bonds that is required for many C–H activation methods.16–17 The selectivity does not require an excess of either substrate and originates from the orthogonal activity of the two catalysts and the relative stability of the two arylmetal intermediates. While (dppp)Pd reacts preferentially with aryl triflates to afford a persistent intermediate, (bpy)Ni reacts preferentially with aryl bromides to form a transient, reactive intermediate. Although each catalyst forms less than 5% cross product in isolation, together they are able to achieve up to 94% yield. Our results reveal a new, general method for the synthesis of biaryls, heteroaryls, and dienes, as well as a new mechanism for selective transmetalation between two catalysts. We anticipate that this reaction will simplify the synthesis of

Facet‐Controlled Synthetic Strategy of Cu2O‐Based Crystals for Catalysis and Sensing

PubMed Central

Shang, Yang

2015-01-01

Shape‐dependent catalysis and sensing behaviours are primarily focused on nanocrystals enclosed by low‐index facets, especially the three basic facets ({100}, {111}, and {110}). Several novel strategies have recently exploded by tailoring the original nanocrystals to greatly improve the catalysis and sensing performances. In this Review, we firstly introduce the synthesis of a variety of Cu2O nanocrystals, including the three basic Cu2O nanocrystals (cubes, octahedra and rhombic dodecahedra, enclosed by the {100}, {111}, and {110} facets, respectively), and Cu2O nanocrystals enclosed by high‐index planes. We then discuss in detail the three main facet‐controlled synthetic strategies (deposition, etching and templating) to fabricate Cu2O‐based nanocrystals with heterogeneous, etched, or hollow structures, including a number of important concepts involved in those facet‐controlled routes, such as the selective adsorption of capping agents for protecting special facets, and the impacts of surface energy and active sites on reaction activity trends. Finally, we highlight the facet‐dependent properties of the Cu2O and Cu2O‐based nanocrystals for applications in photocatalysis, gas catalysis, organocatalysis and sensing, as well as the relationship between their structures and properties. We also summarize and comment upon future facet‐related directions. PMID:27980909

Facet-Controlled Synthetic Strategy of Cu2O-Based Crystals for Catalysis and Sensing.

PubMed

Shang, Yang; Guo, Lin

2015-10-01

Shape-dependent catalysis and sensing behaviours are primarily focused on nanocrystals enclosed by low-index facets, especially the three basic facets ({100}, {111}, and {110}). Several novel strategies have recently exploded by tailoring the original nanocrystals to greatly improve the catalysis and sensing performances. In this Review, we firstly introduce the synthesis of a variety of Cu 2 O nanocrystals, including the three basic Cu 2 O nanocrystals (cubes, octahedra and rhombic dodecahedra, enclosed by the {100}, {111}, and {110} facets, respectively), and Cu 2 O nanocrystals enclosed by high-index planes. We then discuss in detail the three main facet-controlled synthetic strategies (deposition, etching and templating) to fabricate Cu 2 O-based nanocrystals with heterogeneous, etched, or hollow structures, including a number of important concepts involved in those facet-controlled routes, such as the selective adsorption of capping agents for protecting special facets, and the impacts of surface energy and active sites on reaction activity trends. Finally, we highlight the facet-dependent properties of the Cu 2 O and Cu 2 O-based nanocrystals for applications in photocatalysis, gas catalysis, organocatalysis and sensing, as well as the relationship between their structures and properties. We also summarize and comment upon future facet-related directions.

Photothermal-enhanced catalysis in core-shell plasmonic hierarchical Cu7S4 microsphere@zeolitic imidazole framework-8.

PubMed

Wang, Feifan; Huang, Yanjie; Chai, Zhigang; Zeng, Min; Li, Qi; Wang, Yuan; Xu, Dongsheng

2016-12-01

Conventional semiconductor photocatalysis based on band-edge absorption remains inefficient due to the limited harvesting of solar irradiation and the complicated surface/interface chemistry. Herein, novel photothermal-enhanced catalysis was achieved in a core-shell hierarchical Cu 7 S 4 nano-heater@ZIF-8 heterostructures via near-infrared localized surface plasmon resonance. Our results demonstrated that both the high surface temperature of the photothermal Cu 7 S 4 core and the close-adjacency of catalytic ZIF-8 shell contributed to the extremely enhanced catalytic activity. Under laser irradiation (1450 nm, 500 mW), the cyclocondensation reaction rate increased 4.5-5.4 fold compared to that of the process at room temperature, in which the 1.6-1.8 fold enhancement was due to the localized heating effect. The simulated sunlight experiments showed a photothermal activation efficiency (PTAE) of 0.07%, further indicating the validity of photothermal catalysis based on the plasmonic semiconductor nanomaterials. More generally, this approach provides a platform to improve reaction activity with efficient utilization of solar energy, which can be readily extended to other green-chemistry processes.

Asymmetric catalytic cascade reactions for constructing diverse scaffolds and complex molecules.

PubMed

Wang, Yao; Lu, Hong; Xu, Peng-Fei

2015-07-21

With the increasing concerns about chemical pollution and sustainability of resources, among the significant challenges facing synthetic chemists are the development and application of elegant and efficient methods that enable the concise synthesis of natural products, drugs, and related compounds in a step-, atom- and redox-economic manner. One of the most effective ways to reach this goal is to implement reaction cascades that allow multiple bond-forming events to occur in a single vessel. This Account documents our progress on the rational design and strategic application of asymmetric catalytic cascade reactions in constructing diverse scaffolds and synthesizing complex chiral molecules. Our research is aimed at developing robust cascade reactions for the systematic synthesis of a range of interesting molecules that contain structural motifs prevalent in natural products, pharmaceuticals, and biological probes. The strategies employed to achieve this goal can be classified into three categories: bifunctional base/Brønsted acid catalysis, covalent aminocatalysis/N-heterocyclic carbene catalysis, and asymmetric organocatalytic relay cascades. By the use of rationally designed substrates with properly reactive sites, chiral oxindole, chroman, tetrahydroquinoline, tetrahydrothiophene, and cyclohexane scaffolds were successfully assembled under bifunctional base/Brønsted acid catalysis from simple and readily available substances such as imines and nitroolefins. We found that some of these reactions are highly efficient since catalyst loadings as low as 1 mol % can promote the multistep sequences affording complex architectures with high stereoselectivities and yields. Furthermore, one of the bifunctional base/Brønsted acid-catalyzed cascade reactions for the synthesis of chiral cyclohexanes has been used as a key step in the construction of the tetracyclic core of lycorine-type alkaloids and the formal synthesis of α-lycorane. Guided by the principles of

Three Rate-Constant Kinetic Model for Permanganate Reactions Autocatalyzed by Colloidal Manganese Dioxide: The Oxidation of L-Phenylalanine.

PubMed

Perez-Benito, Joaquin F; Ferrando, Jordi

2014-12-26

The reduction of permanganate ion to MnO(2)-Mn(2)O(3) soluble colloidal mixed oxide by l-phenylalanine in aqueous phosphate-buffered neutral solutions has been followed by a spectrophotometric method, monitoring the decay of permanganate ion at 525 nm and the formation of the colloidal oxide at 420 nm. The reaction is autocatalyzed by the manganese product, and three rate constants have been required to fit the experimental absorbance-time kinetic data. The reaction shows base catalysis, and the values of the activation parameters at different pHs have been determined. A mechanism including both the nonautocatalytic and the autocatalytic reaction pathways, and in agreement with the available experimental data, has been proposed. Some key features of this mechanism are the following: (i) of the two predominant forms of the amino acid, the anionic form exhibits a stronger reducing power than the zwitterionic form; (ii) the nonautocatalytic reaction pathway starts with the transfer of the hydrogen atom in the α position of the amino acid to permanganate ion; and (iii) the autocatalytic reaction pathway involves the reduction of Mn(IV) to Mn(II) by the amino acid and the posterior reoxidation of Mn(II) to Mn(IV) by permanganate ion.

Oxidative and photoxidative polymerization of humic suprastructures by heterogeneous biomimetic catalysis.

PubMed

Nuzzo, Assunta; Piccolo, Alessandro

2013-05-13

The meso-tetra(2,6-dichloro-3-sulfonatophenyl)porphyrinate of manganese(III) chloride [Mn-(TDCPPS)Cl] biomimetic catalyst immobilized on spacer-functionalized kaolinite clay mineral was employed in the oxidative coupling reaction of a dissolved humic acid (HA) suprastructure with either chemical (H2O2) or UV-light oxidation. The changes in molecular size of humic matter subjected to catalyzed oxidative reaction were followed by high-performance size exclusion chromatography (HPSEC) with UV-vis and refractive index (RI) detectors in series, and by thermogravimetric (TGA) analysis. Both the enhanced molecular size shown by differences between HPSEC chromatograms of humic reaction mixtures at either pH 6 or 3.5 and the increase of thermogravimetric stability suggest that the heterogeneous biomimetic catalysis promoted the stabilization of humic conformations by new intermolecular covalent bonds during oxidative coupling. The similarity between chemical and light-induced oxidation results suggests potential multiple applications of the kaolinite-supported heterogeneous catalyst in controlling the reactivity of natural organic matter within biogeochemical cycles and environmental reactions.

A full understanding of oxygen reduction reaction mechanism on Au(1 1 1) surface

NASA Astrophysics Data System (ADS)

Yang, Yang; Dai, Changqing; Fisher, Adrian; Shen, Yanchun; Cheng, Daojian

2017-09-01

Oxygen reduction and hydrogen peroxide reduction are technologically important reactions in energy-conversion devices. In this work, a full understanding of oxygen reduction reaction (ORR) mechanism on Au(1 1 1) surface is investigated by density functional theory (DFT) calculations, including the reaction mechanisms of O2 dissociation, OOH dissociation, and H2O2 dissociation. Among these ORR mechanisms on Au(1 1 1), the activation energy of \\text{O}2* hydrogenation reaction is much lower than that of \\text{O}2* dissociation, indicating that \\text{O}2* hydrogenation reaction is more appropriate at the first step than \\text{O}2* dissociation. In the following, H2O2 can be formed with the lower activation energy compared with the OOH dissociation reaction, and finally H2O2 could be generated as a detectable product due to the high activation energy of H2O2 dissociation reaction. Furthermore, the potential dependent free energy study suggests that the H2O2 formation is thermodynamically favorable up to 0.4 V on Au(1 1 1), reducing the overpotential for 2e - ORR process. And the elementary step of first H2O formation becomes non-spontaneous at 0.4 V, indicating the difficulty of 4e - reduction pathway. Our DFT calculations show that H2O2 can be generated on Au(1 1 1) and the first electron transfer is the rate determining step. Our results show that gold surface could be used as a good catalyst for small-scale manufacture and on-site production of H2O2.

Water Oxidation by a Cytochrome P450: Mechanism and Function of the Reaction

PubMed Central

Prasad, Brinda; Mah, Derrick J.; Lewis, Andrew R.; Plettner, Erika

2013-01-01

P450cam (CYP101A1) is a bacterial monooxygenase that is known to catalyze the oxidation of camphor, the first committed step in camphor degradation, with simultaneous reduction of oxygen (O2). We report that P450cam catalysis is controlled by oxygen levels: at high O2 concentration, P450cam catalyzes the known oxidation reaction, whereas at low O2 concentration the enzyme catalyzes the reduction of camphor to borneol. We confirmed, using 17O and 2H NMR, that the hydrogen atom added to camphor comes from water, which is oxidized to hydrogen peroxide (H2O2). This is the first time a cytochrome P450 has been observed to catalyze oxidation of water to H2O2, a difficult reaction to catalyze due to its high barrier. The reduction of camphor and simultaneous oxidation of water are likely catalyzed by the iron-oxo intermediate of P450cam, and we present a plausible mechanism that accounts for the 1∶1 borneol:H2O2 stoichiometry we observed. This reaction has an adaptive value to bacteria that express this camphor catabolism pathway, which requires O2, for two reasons: 1) the borneol and H2O2 mixture generated is toxic to other bacteria and 2) borneol down-regulates the expression of P450cam and its electron transfer partners. Since the reaction described here only occurs under low O2 conditions, the down-regulation only occurs when O2 is scarce. PMID:23634216

Tuning electronic properties by oxidation-reduction reactions at graphene-ruthenium interface

DOE PAGES

Kandyba, Viktor; Al-Mahboob, Abdullah; Giampietri, Alessio; ...

2018-06-06

Mass production of graphene is associated with the growth on catalysts used also in other chemical reactions. In this study, we exploit the oxidation-reduction to tailor the properties of single layer graphene domains with incorporated bi-layer patches on ruthenium. Using photoelectron spectromicroscopy techniques, we find that oxygen, intercalating under single layer and making it p-doped by the formation of Ru-O x, does not intercalate under the bilayer patches with n-doped upper layer, but decorates them under single layer surrounding creating lateral p-n junctions with chemical potential difference of 1.2 eV. O-reduction by thermal treatment in vacuum results in C-vacancy defectsmore » enhancing electronic coupling of remained graphene to Ru, whereas in H 2, vacancy formation is suppressed. Also, for the domains below 15–25 μm size, after O-reduction in H 2, graphene/Ru coupling is restored, while wrinkle pattern produced by O-intercalation is irreversible and can trap reaction products between the wrinkles and Ru surface step edges. In fact, in certain regions of bigger domains, the products, containing H 2O and/or its fragments, remain at the interface, making graphene decoupled and undoped.« less

Absorbing states in a catalysis model with anti-Arrhenius behavior.

PubMed

de Andrade, M F; Figueiredo, W

2012-04-28

We study a model of heterogeneous catalysis with competitive reactions between two monomers A and B. We assume that reactions are dependent on temperature and follow an anti-Arrhenius mechanism. In this model, a monomer A can react with a nearest neighbor monomer A or B, however, reactions between monomers of type B are not allowed. We assume attractive interactions between nearest neighbor monomers as well as between monomers and the catalyst. Through mean-field calculations, at the level of site and pair approximations, and extensive Monte Carlo simulations, we determine the phase diagram of the model in the plane y(A) versus temperature, where y(A) is the probability that a monomer A reaches the catalyst. The model exhibits absorbing and active phases separated by lines of continuous phase transitions. We calculate the static, dynamic, and spreading exponents of the model, and despite the absorbing state be represented by many different microscopic configurations, the model belongs to the directed percolation universality class in two dimensions. Both reaction mechanisms, Arrhenius and anti-Arrhenius, give the same set of critical exponents and do not change the nature of the universality class of the catalytic models.

Origin of fatty acid synthesis - Thermodynamics and kinetics of reaction pathways

NASA Technical Reports Server (NTRS)

Weber, Arthur L.

1991-01-01

The primitiveness of contemporary fatty acid biosynthesis was evaluated by using the thermodynamics and kinetics of its component reactions to estimate the extent of its dependence on powerful and selective catalysis by enzymes. Since this analysis indicated that the modern pathway is not primitive because it requires sophisticated enzymatic catalysis, an alternative pathway of primitive fatty acid synthesis is proposed that uses glycolaldehyde as a substrate. In contrast to the modern pathway, this primitive pathway is not dependent on an exogenous source of phosphoanhydride energy. Furthermore, the chemical spontaneity of its reactions suggests that it could have been readily catalyzed by the rudimentary biocatalysts available at an early stage in the origin of life.

Biaxially strained PtPb/Pt core/shell nanoplate boosts oxygen reduction catalysis

DOE PAGES

Bu, Lingzheng; Zhang, Nan; Guo, Shaojun; ...

2016-12-16

Compressive surface strains have been necessary to boost oxygen reduction reaction (ORR) activity in core/shell M/Pt catalysts (where M can be Ni, Co, Fe). We report a class of PtPb/Pt core/shell nanoplate catalysts that exhibit large biaxial tensile strains. The stable Pt (110) facets of the nanoplates have high ORR specific and mass activities that reach 7.8 milliampere per centimeter square and 4.3 ampere per milligram of platinum at 0.9 volts versus the reversible hydrogen electrode (RHE), respectively. Density functional theory calculations revealed that the edge-­Pt and top (bottom)-Pt (110) facets undergo large tensile strains that help optimize the Pt-­Omore » bond strength. The intermetallic core and uniform 4 layers of Pt shell of the PtPb/Pt nanoplates appear to underlie the high endurance of these catalysts, which can undergo 50,000 voltage cycles with negligible activity decay and no apparent structure and composition changes.« less

Biaxially strained PtPb/Pt core/shell nanoplate boosts oxygen reduction catalysis

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bu, Lingzheng; Zhang, Nan; Guo, Shaojun

Compressive surface strains have been necessary to boost oxygen reduction reaction (ORR) activity in core/shell M/Pt catalysts (where M can be Ni, Co, Fe). We report a class of PtPb/Pt core/shell nanoplate catalysts that exhibit large biaxial tensile strains. The stable Pt (110) facets of the nanoplates have high ORR specific and mass activities that reach 7.8 milliampere per centimeter square and 4.3 ampere per milligram of platinum at 0.9 volts versus the reversible hydrogen electrode (RHE), respectively. Density functional theory calculations revealed that the edge-­Pt and top (bottom)-Pt (110) facets undergo large tensile strains that help optimize the Pt-­Omore » bond strength. The intermetallic core and uniform 4 layers of Pt shell of the PtPb/Pt nanoplates appear to underlie the high endurance of these catalysts, which can undergo 50,000 voltage cycles with negligible activity decay and no apparent structure and composition changes.« less

Rhenium(VII) Catalysis of Prins Cyclization Reactions

PubMed Central

Tadpetch, Kwanruthai; Rychnovsky, Scott D.

2009-01-01

The rhenium(VII) complex O3ReOSiPh3 are particularly effective catalyst for Prins cyclizations using aromatic and α,β-unsaturated aldehydes. The reaction conditions are mild and the highly substituted 4-hydroxy tetrahydropyran products are formed stereoselectively. Rhenium(VII) complexes appear to spontaneously form esters with alcohols and to directly activate electron rich alcohols for solvolysis. Re2O7 and perrhenic acid were equally effective in catalyzing these cyclizations. PMID:18816133

Beyond alkyl transfer: Synthesis of main group metal (Mg, Ca, Zn) silyl and tris(oxazolinyl)borato complexes and their stoichiometric and catalytic reactions with borane Lewis acids and carbonyls

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lampland, Nicole Lynn

2015-05-09

Recently, the fundamental knowledge of main group metal chemistry has grown. This progress is crucial for the further development of main group metal compounds in silicon chemistry and catalysis and for advancing their applications as green alternatives to many rare earth and transition metal compounds. This thesis focuses on reactivity beyond the welldocumented alkyl-transfer applications for main group metals, and it highlights examples of reactions with Lewis acids and the reduction of carbonyls.

Amorphous nickel boride membrane on a platinum-nickel alloy surface for enhanced oxygen reduction reaction.

PubMed

He, Daping; Zhang, Libo; He, Dongsheng; Zhou, Gang; Lin, Yue; Deng, Zhaoxiang; Hong, Xun; Wu, Yuen; Chen, Chen; Li, Yadong

2016-08-09

The low activity of the oxygen reduction reaction in polymer electrolyte membrane fuel cells is a major barrier for electrocatalysis, and hence needs to be optimized. Tuning the surface electronic structure of platinum-based bimetallic alloys, a promising oxygen reduction reaction catalyst, plays a key role in controlling its interaction with reactants, and thus affects the efficiency. Here we report that a dealloying process can be utilized to experimentally fabricate the interface between dealloyed platinum-nickel alloy and amorphous nickel boride membrane. The coating membrane works as an electron acceptor to tune the surface electronic structure of the platinum-nickel catalyst, and this composite catalyst composed of crystalline platinum-nickel covered by amorphous nickel boride achieves a 27-times enhancement in mass activity relative to commercial platinum/carbon at 0.9 V for the oxygen reduction reaction performance. Moreover, this interactional effect between a crystalline surface and amorphous membrane can be readily generalized to facilitate the 3-times higher catalytic activity of commercial platinum/carbon.

Recent Developments of Electrochemical Promotion of Catalysis in the Techniques of DeNOx

PubMed Central

Tang, Xiaolong; Yi, Honghong; Chen, Chen; Wang, Chuan

2013-01-01

Electrochemical promotion of catalysis reactions (EPOC) is one of the most significant discoveries in the field of catalytic and environmental protection. The work presented in this paper focuses on the aspects of reaction mechanism, influencing factors, and recent positive results. It has been shown with more than 80 different catalytic systems that the catalytic activity and selectivity of conductive catalysts deposited on solid electrolytes can be altered in the last 30 years. The active ingredient of catalyst can be activated by applying constant voltage or constant current to the catalysts/electrolyte interface. The effect of EPOC can improve greatly the conversion rate of NOx. And it can also improve the lifetime of catalyst by inhibiting its poisoning. PMID:23970835

Hydrogen-Bonding Catalysis and Inhibition by Simple Solvents in the Stereoselective Kinetic Epoxide-Opening Spirocyclization of Glycal Epoxides to Form Spiroketals

PubMed Central

Wurst, Jacqueline M.; Liu, Guodong; Tan, Derek S.

2011-01-01

Mechanistic investigations of a MeOH-induced kinetic epoxide-opening spirocyclization of glycal epoxides have revealed dramatic, specific roles for simple solvents in hydrogen-bonding catalysis of this reaction to form spiroketal products stereoselectively with inversion of configuration at the anomeric carbon. A series of electronically-tuned C1-aryl glycal epoxides was used to study the mechanism of this reaction based on differential reaction rates and inherent preferences for SN2 versus SN1 reaction manifolds. Hammett analysis of reaction kinetics with these substrates is consistent with an SN2 or SN2-like mechanism (ρ = −1.3 vs. ρ = −5.1 for corresponding SN1 reactions of these substrates). Notably, the spirocyclization reaction is second-order dependent on MeOH and the glycal ring oxygen is required for second-order MeOH catalysis. However, acetone cosolvent is a first-order inhibitor of the reaction. A transition state consistent with the experimental data is proposed in which one equivalent of MeOH activates the epoxide electrophile via a hydrogen bond while a second equivalent of MeOH chelates the sidechain nucleophile and glycal ring oxygen. A paradoxical previous observation that decreased MeOH concentration leads to increased competing intermolecular methyl glycoside formation is resolved by the finding that this side reaction is only first-order dependent on MeOH. This study highlights the unusual abilities of simple solvents to act as hydrogen-bonding catalysts and inhibitors in epoxide-opening reactions, providing both stereoselectivity and discrimination between competing reaction manifolds. This spirocyclization reaction provides efficient, stereocontrolled access to spiroketals that are key structural motifs in natural products. PMID:21539313

Chemical Synthesis of Complex Molecules Using Nanoparticle Catalysis

PubMed Central

Cong, Huan; Porco, John A.

2011-01-01

Nanoparticle catalysis has emerged as an active topic in organic synthesis. Of particular interest is the development of enabling methodologies to efficiently assemble complex molecules using nanoparticle catalysis. This Viewpoint highlights recent developments and discusses future perspectives in this emerging field. PMID:22347681

Iridium catalysis: application of asymmetric reductive hydrogenation.

PubMed

Cadu, Alban; Andersson, Pher G

2013-10-28

Iridium, despite being one of the least abundant transition metals, has found several uses. N,P-ligated iridium catalysts are used to perform many highly selective reactions. These methodologies have been developed extensively over the past 15 years. More recently, the application of iridium N,P catalysts in asymmetric hydrogenation has been a focus of research to find novel applications and to expand on their current synthetic utility. The aim of this perspective is to highlight the advances made by the Andersson group.

2013 Chemical reactions at surfaces. Surfaces in Energy and the Environment. Gordon Research Conference and Gordon Research Seminar (April 28 - May 3, 2013 - Les Diablerets, Switzerland)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Stair, Peter C.

presentations on chemistry at solid and liquid surfaces of relevance to catalysis, synthesis, photochemistry, environmental science, and tribology. Topics include: Fundamental Surface Chemistry; Catalysis; Solid Liquid and Aerosol Interfaces; Surface Photochemistry; Synthesis of Surfaces; Environmental Interfaces; Hot Topics in Surface Chemical Reactions; Tribology; Gas-Surface Scattering and Reactions; Novel Materials and Environments.

Controlling energy flow in multimetallic nanostructures for plasmonic catalysis

NASA Astrophysics Data System (ADS)

Aslam, Umar; Chavez, Steven; Linic, Suljo

2017-10-01

It has been shown that photoexcitation of plasmonic metal nanoparticles (Ag, Au and Cu) can induce direct photochemical reactions. However, the widespread application of this technology in catalysis has been limited by the relatively poor chemical reactivity of noble metal surfaces. Despite efforts to combine plasmonic and catalytic metals, the physical mechanisms that govern energy transfer from plasmonic metals to catalytic metals remain unclear. Here we show that hybrid core-shell nanostructures in which a core plasmonic metal harvests visible-light photons can selectively channel that energy into catalytically active centres on the nanostructure shell. To accomplish this, we developed a synthetic protocol to deposit a few monolayers of Pt onto Ag nanocubes. This model system allows us to conclusively separate the optical and catalytic functions of the hybrid nanomaterial and determine that the flow of energy is strongly biased towards the excitation of energetic charge carriers in the Pt shell. We demonstrate the utility of these nanostructures for photocatalytic chemical reactions in the preferential oxidation of CO in excess H2. Our data demonstrate that the reaction occurs exclusively on the Pt surface.

Reaction engineering for materials processing in space: Reduction of ilmenite by hydrogen and carbon monoxide

NASA Technical Reports Server (NTRS)

Zhao, Y.; Shadman, F.

1991-01-01

Oxygen is a consumable material which needs to be produced continuously in most space missions. Its use for propulsion as well as life support makes oxygen one of the largest volume chemicals to be produced in space. Production of oxygen from lunar materials is of particular interest and is very attractive possibility. The kinetics and mechanism of reduction of ilmenite by carbon monoxide and hydrogen at 800 to 1100 C were investigated. The temporal profiles of conversion for carbon monoxide have a sigmoidal shape and indicate the presence of three different stages (induction, acceleration, and deceleration) during the reduction reaction. The apparent activation energy decreases from 18 kcal/mole at 10 percent conversion to 10 kcal/mole at 50 percent conversion. The reaction is first order with respect to carbon monoxide under the experimental conditions studied. Both SEM and EDX analysis show that the diffusion of Fe product away from the reaction front and through the TiO2 phase, followed by the nucleation and growth of a separate Fe phase are important steps affecting the process kinetics. The results from hydrogen reduction show that the mechanism of ilmenite reduction by hydrogen is similar to that by carbon monoxide. However, the titanium dioxide can be further reduced by hydrogen at 800 to 1000 C. The detailed comparison and theoretical modeling of both reduction processes is presented.

Recent advances in the design of tailored nanomaterials for efficient oxygen reduction reaction

DOE PAGES

Lv, Haifeng; Li, Dongguo; Strmcnik, Dusan; ...

2016-04-11

In the past decade, polymer electrolyte membrane fuels (PEMFCs) have been evaluated for both automotive and stationary applications. One of the main obstacles for large scale commercialization of this technology is related to the sluggish oxygen reduction reaction that takes place on the cathode side of fuel cell. Consequently, ongoing research efforts are focused on the design of cathode materials that could improve the kinetics and durability. Majority of these efforts rely on novel synthetic approaches that provide control over the structure, size, shape and composition of catalytically active materials. This article highlights the most recent advances that have beenmore » made to tailor critical parameters of the nanoscale materials in order to achieve more efficient performance of the oxygen reduction reaction (ORR).« less

Interface confined hydrogen evolution reaction in zero valent metal nanoparticles-intercalated molybdenum disulfide

PubMed Central

Chen, Zhongxin; Leng, Kai; Zhao, Xiaoxu; Malkhandi, Souradip; Tang, Wei; Tian, Bingbing; Dong, Lei; Zheng, Lirong; Lin, Ming; Yeo, Boon Siang; Loh, Kian Ping

2017-01-01

Interface confined reactions, which can modulate the bonding of reactants with catalytic centres and influence the rate of the mass transport from bulk solution, have emerged as a viable strategy for achieving highly stable and selective catalysis. Here we demonstrate that 1T′-enriched lithiated molybdenum disulfide is a highly powerful reducing agent, which can be exploited for the in-situ reduction of metal ions within the inner planes of lithiated molybdenum disulfide to form a zero valent metal-intercalated molybdenum disulfide. The confinement of platinum nanoparticles within the molybdenum disulfide layered structure leads to enhanced hydrogen evolution reaction activity and stability compared to catalysts dispersed on carbon support. In particular, the inner platinum surface is accessible to charged species like proton and metal ions, while blocking poisoning by larger sized pollutants or neutral molecules. This points a way forward for using bulk intercalated compounds for energy related applications. PMID:28230105

Time-resolved chromatographic analysis and mechanisms in adsorption and catalysis.

PubMed

Roubani-Kalantzopoulou, Fani

2009-03-06

The main object of this review is the study of fundamentals of adsorption and heterogeneous catalysis, a benefit for the understanding of adsorptive and catalytic properties. This work aims to define and record, with the utmost accuracy, the phenomena and the possible reactions. A new methodology for the study of the adsorption is presented, which is a version of the well-known inverse gas chromatography. This reversed-flow inverse gas chromatography (RF-IGC) is technically very simple, and it is combined with a mathematical analysis that gives the possibility for the estimation of various physicochemical parameters related to adsorbent or catalyst characterization, under conditions compatible with the operation of real adsorbents and catalysts. On this base, this methodology has been successfully applied to the study of the impact of air pollutants, volatile organic and/or inorganic, on many solids such as marbles, ceramics, oxide-pigments of works of art, building materials, authentic statues of the Greek Archaeological Museums. Moreover, this methodology proved to be a powerful tool for studying the topography of active sites of heterogeneous surfaces in the nano-scale domain. Thus, some very important local quantities for the surface chemistry have been determined experimentally for many solids including thin films. These physicochemical local quantities (among which adsorption energy and entropy, surface diffusion coefficient, probability density function) have been determined from the experimental pairs of height of extra chromatographic peaks and time by a nonlinear least-squares method, through personal computer programs written in GW BASIC and lately in FORTRAN. Through the time-resolved analysis the surface characterization of the examined materials took place. In addition, the kinetic constants responsible for adsorption/desorption and surface chemical reactions have also been calculated. Thus, important answers have been provided to the following

Photoredox Catalysis Unlocks Single-Electron Elementary Steps in Transition Metal Catalyzed Cross-Coupling

PubMed Central

2016-01-01

Since initial reports, cross-coupling technologies employing photoredox catalysts to access novel reactivity have developed with increasing pace. In this Outlook, prominent examples from the recent literature are organized on the basis of the elementary transformation enabled by photoredox catalysis and are discussed in the context of relevant historical precedent in stoichiometric organometallic chemistry. This treatment allows mechanistic similarities inherent to odd-electron transition metal reactivity to be generalized to a set of lessons for future reaction development. PMID:27280163

Hydrothermal Synthesis of Platinum-Group-Metal-Free Catalysts: Structural Elucidation and Oxygen Reduction Catalysis

DOE PAGES

Gokhale, Rohan; Tsui, Lok-Kun; Roach, Kristin; ...

2017-12-07

In this paper, a hydrothermal approach to generate a platinum-group-metal-free (PGM-free) Fe-N-C catalyst for the oxygen reduction reaction (ORR) is introduced. The process involves partial carbonization by hydrothermal means followed by thermal treatment to obtain the final catalysts. Detailed X-ray scattering analysis of the glucose-imidazole catalysts (termed as GLU-IMID-C catalysts), obtained for the first time with the use of CarbonXS GUI program, reveals the presence of face-centered cubic (FCC) iron nanoparticles embedded in partially graphitic carbon in all catalyst variations. We also report the physical characterization of these catalysts by using X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller surface area analysis, and transmissionmore » electron microscopy. The electrocatalytic behavior of the catalysts towards oxygen reduction is studied separately in acidic and alkaline electrolytes by rotating ring disk electrode measurements. The catalysts exhibit high ORR activity in acidic (0.5 M H 2SO 4) and alkaline (0.1 M KOH) electrolytes. Lastly, a precursor structure-performance relationship of these catalysts and their performance trends in both electrolytes has been discussed in this work.« less

Hydrothermal Synthesis of Platinum-Group-Metal-Free Catalysts: Structural Elucidation and Oxygen Reduction Catalysis

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gokhale, Rohan; Tsui, Lok-Kun; Roach, Kristin

In this paper, a hydrothermal approach to generate a platinum-group-metal-free (PGM-free) Fe-N-C catalyst for the oxygen reduction reaction (ORR) is introduced. The process involves partial carbonization by hydrothermal means followed by thermal treatment to obtain the final catalysts. Detailed X-ray scattering analysis of the glucose-imidazole catalysts (termed as GLU-IMID-C catalysts), obtained for the first time with the use of CarbonXS GUI program, reveals the presence of face-centered cubic (FCC) iron nanoparticles embedded in partially graphitic carbon in all catalyst variations. We also report the physical characterization of these catalysts by using X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller surface area analysis, and transmissionmore » electron microscopy. The electrocatalytic behavior of the catalysts towards oxygen reduction is studied separately in acidic and alkaline electrolytes by rotating ring disk electrode measurements. The catalysts exhibit high ORR activity in acidic (0.5 M H 2SO 4) and alkaline (0.1 M KOH) electrolytes. Lastly, a precursor structure-performance relationship of these catalysts and their performance trends in both electrolytes has been discussed in this work.« less

Controlling Gas-Phase Reactions for Efficient Charge Reduction Electrospray Mass Spectrometry of Intact Proteins

PubMed Central

Frey, Brian L.; Lin, Yuan; Westphall, Michael S.; Smith, Lloyd M.

2006-01-01

Charge reduction electrospray mass spectrometry (CREMS) reduces the charge states of electrospray-generated ions, which concentrates the ions from a protein into fewer peaks spread over a larger m/z range, thereby increasing peak separation and decreasing spectral congestion. An optimized design for a CREMS source is described that provides an order-of-magnitude increase in sensitivity compared to previous designs and provides control over the extent of charge reduction. Either a corona discharge or an α-particle source was employed to generate anions that abstract protons from electrosprayed protein cations. These desired ion/ion proton transfer reactions predominated, but some oxidation and ion-attachment reactions also occurred leading to new peaks or mass-shifted broader peaks while decreasing signal intensity. The species producing these deleterious side-reactions were identified, and conditions were found that prevented their formation. Spectrometer m/z biases were examined because of their effect upon the signal intensity of higher m/z charge-reduced protein ions. The utility of this atmospheric pressure CREMS was demonstrated using a cell lysate fraction from E. coli. The spectral simplification afforded by CREMS reveals more proteins than are observed without charge reduction. PMID:16198118

Asymmetric Fluorination of α-Branched Cyclohexanones Enabled by a Combination of Chiral Anion Phase-Transfer Catalysis and Enamine Catalysis using Protected Amino Acids

PubMed Central

2015-01-01

We report a study involving the successful merger of two separate chiral catalytic cycles: a chiral anion phase-transfer catalysis cycle to activate Selectfluor and an enamine activation cycle, using a protected amino acid as organocatalyst. We have demonstrated the viability of this approach with the direct asymmetric fluorination of α-substituted cyclohexanones to generate quaternary fluorine-containing stereocenters. With these two chiral catalytic cycles operating together in a matched sense, high enantioselectivites can be achieved, and we envisage that this dual catalysis method has the potential to be more broadly applicable, given the breadth of enamine catalysis. It also represents a rare example of chiral enamine catalysis operating successfully on α-branched ketones, substrates commonly inert to this activation mode. PMID:24684209

Conformational plasticity of an enzyme during catalysis: intricate coupling between cyclophilin A dynamics and substrate turnover.

PubMed

McGowan, Lauren C; Hamelberg, Donald

2013-01-08

Enzyme catalysis is central to almost all biochemical processes, speeding up rates of reactions to biological relevant timescales. Enzymes make use of a large ensemble of conformations in recognizing their substrates and stabilizing the transition states, due to the inherent dynamical nature of biomolecules. The exact role of these diverse enzyme conformations and the interplay between enzyme conformational dynamics and catalysis is, according to the literature, not well understood. Here, we use molecular dynamics simulations to study human cyclophilin A (CypA), in order to understand the role of enzyme motions in the catalytic mechanism and recognition. Cyclophilin A is a tractable model system to study using classical simulation methods, because catalysis does not involve bond formation or breakage. We show that the conformational dynamics of active site residues of substrate-bound CypA is inherent in the substrate-free enzyme. CypA interacts with its substrate via conformational selection as the configurations of the substrate changes during catalysis. We also show that, in addition to tight intermolecular hydrophobic interactions between CypA and the substrate, an intricate enzyme-substrate intermolecular hydrogen-bonding network is extremely sensitive to the configuration of the substrate. These enzyme-substrate intermolecular interactions are loosely formed when the substrate is in the reactant and product states and become well formed and reluctant to break when the substrate is in the transition state. Our results clearly suggest coupling among enzyme-substrate intermolecular interactions, the dynamics of the enzyme, and the chemical step. This study provides further insights into the mechanism of peptidyl-prolyl cis/trans isomerases and the general interplay between enzyme conformational dynamics and catalysis. Copyright © 2013 Biophysical Society. Published by Elsevier Inc. All rights reserved.

Electrophilic activation of alkynes for enyne cycloisomerization reactions with in situ generated early/late heterobimetallic Pt-Ti catalysts.

PubMed

Talley, Michael R; Stokes, Ryjul W; Walker, Whitney K; Michaelis, David J

2016-06-14

In situ formation of heterobimetallic Pt-Ti catalysts enables rapid room temperature catalysis in enyne cycloisomerization reactions. The Lewis acidic titanium atom in the ligand framework is shown to be essential for fast catalysis. A range of enyne substrates are efficiently cyclized to carbocycles and heterocycles in high yield.

Heterogeneous catalysis with lasers

NASA Astrophysics Data System (ADS)

George, T. F.

1986-06-01

Theoretical techniques have been developed to describe a variety of laser-induced molecular rate processes occurring at solid surfaces which are involved in heterogeneous catalysis. Such processes include adsorption, migration, chemical reactions and desorption. The role of surface phonons in laser-selective processes and laser heating has been analyzed. The importance of electronic degrees of freedom has been considered for semiconductor and metal substrates, with special emphasis on the laser excitation of surface states. Surface-modified photochemistry has also been investigated, where the effect of a metal surface on the resonance fluorescence spectrum of a laser-driven atom/molecule has been assessed by means of surface-dressed optical Bloch equations. It is seen that the spectrum can be significantly different from the gas-phase case. Two related gas-surface collision processes have also been studied. First, the feasibility of the formation of the electron-hole pairs in a semiconductor by vibrationally excited molecules has been explored. Second, charge transfer in ion-surface collisions has been examined for both one-electron and two-electron transfer processes. Work has been initiated on microstructures and rough structures, including clusters and surface gratings.

Electric-field enhanced performance in catalysis and solid-state devices involving gases

DOEpatents

Blackburn, Bryan M.; Wachsman, Eric D.; Van Assche, IV, Frederick Martin

2015-05-19

Electrode configurations for electric-field enhanced performance in catalysis and solid-state devices involving gases are provided. According to an embodiment, electric-field electrodes can be incorporated in devices such as gas sensors and fuel cells to shape an electric field provided with respect to sensing electrodes for the gas sensors and surfaces of the fuel cells. The shaped electric fields can alter surface dynamics, system thermodynamics, reaction kinetics, and adsorption/desorption processes. In one embodiment, ring-shaped electric-field electrodes can be provided around sensing electrodes of a planar gas sensor.

Synthesis of novel chiral phosphinocyrhetrenyloxazoline ligands and their application in asymmetric catalysis.

PubMed

Bolm, Carsten; Xiao, Li; Kesselgruber, Martin

2003-01-07

Several novel planar chiral phosphinocyrhetrenyloxazolines have been synthesized, and their catalytic activities have been evaluated in a variety of asymmetric catalytic reactions. Preferable effects as compared to their ferrocenyl analogues have been observed in asymmetric allylic amination and asymmetric hydrosilylation, and up to 97% ee and 72% ee were reached, respectively. The Lewis basicity of the phosphorus on the ferrocene and the cyrhetrene, which contributes to their different behavior in catalysis, has been deduced by 31P NMR spectroscopy analysis, as indicated by 1J(77Se-31P) in the corresponding phosphine selenides.

Bimetallic Effect of Single Nanocatalysts Visualized by Super-Resolution Catalysis Imaging

DOE PAGES

Chen, Guanqun; Zou, Ningmu; Chen, Bo; ...

2017-11-01

Compared with their monometallic counterparts, bimetallic nanoparticles often show enhanced catalytic activity associated with the bimetallic interface. Direct quantitation of catalytic activity at the bimetallic interface is important for understanding the enhancement mechanism, but challenging experimentally. Here using single-molecule super-resolution catalysis imaging in correlation with electron microscopy, we report the first quantitative visualization of enhanced bimetallic activity within single bimetallic nanoparticles. We focus on heteronuclear bimetallic PdAu nanoparticles that present a well-defined Pd–Au bimetallic interface in catalyzing a photodriven fluorogenic disproportionation reaction. Our approach also enables a direct comparison between the bimetallic and monometallic regions within the same nanoparticle. Theoreticalmore » calculations further provide insights into the electronic nature of N–O bond activation of the reactant (resazurin) adsorbed on bimetallic sites. Subparticle activity correlation between bimetallic enhancement and monometallic activity suggests that the favorable locations to construct bimetallic sites are those monometallic sites with higher activity, leading to a strategy for making effective bimetallic nanocatalysts. Furthermore, the results highlight the power of super-resolution catalysis imaging in gaining insights that could help improve nanocatalysts.« less

Simultaneous reduction and nitrogen functionalization of graphene oxide using lemon for metal-free oxygen reduction reaction

NASA Astrophysics Data System (ADS)

Begum, Halima; Ahmed, Mohammad Shamsuddin; Cho, Sung; Jeon, Seungwon

2017-12-01

Inspire by the vision of finding a simple and green method for simultaneous reduction and nitrogen (N)-functionalization of graphene oxide (GO), a N-rich reduced graphene oxide (rGO) has been synthesized through a facile and ecofriendly hydrothermal strategy while most of the existing methods are involving with multiple steps and highly toxic reducing agents that are harmful to human health and environment. In this paper, the simultaneous reduction and N-functionalization of GO using as available lemon juice (denoted as Lem-rGO) for metal-free electrocatalysis towards oxygen reduction reaction (ORR) is described. The proposed method is based on the reduction of GO using of the reducing and the N-precursor capability of ascorbic acid and citric acid as well as the nitrogenous compounds, respectively, that containing in lemon juice. The resultant Lem-rGO has higher reduction degree, higher specific surface area and better crystalline nature with N-incorporation than that of well investigated ascorbic acid and citric acid treated rGO. As a result, it shows better ORR electrocatalytic activity in respect to the improved onset potential, electron transfer rate and kinetics than those typical rGO catalysts. Moreover, it shows a significant tolerance to the anodic fuels and durability than the Pt/C during ORR.

Positron studies in catalysis research

NASA Astrophysics Data System (ADS)

During the past eight months, the authors have made progress in several areas relevant to the eventual use of positron techniques in catalysis research. They have come closer to the completion of their positron microscope, and at the same time have performed several studies in their non-microscopic positron spectrometer which should ultimately be applicable to catalysis. The current status of the efforts in each of these areas is summarized in the following sections: Construction of the positron microscope (optical element construction, data collection software, and electronic sub-assemblies); Doppler broadening spectroscopy of metal silicide; Positron lifetime spectroscopy of glassy polymers; and Positron lifetime measurements of pore-sizes in zeolites.

Catalytic reduction of CO 2 by H 2 for synthesis of CO, methanol and hydrocarbons: challenges and opportunities

DOE PAGES

Porosoff, Marc D.; Yan, Binhang; Chen, Jingguang G.

2015-10-22

Ocean acidification and climate change are expected to be two of the most difficult scientific challenges of the 21st century. Converting CO 2 into valuable chemicals and fuels is one of the most practical routes for reducing CO 2 emissions while fossil fuels continue to dominate the energy sector. Reducing CO 2 by H 2 using heterogeneous catalysis has been studied extensively, but there are still significant challenges in developing active, selective and stable catalysts suitable for large-scale commercialization. We study the catalytic reduction of CO 2 by H 2 can lead to the formation of three types of products:more » CO through the reverse water–gas shift (RWGS) reaction, methanol via selective hydrogenation, and hydrocarbons through combination of CO 2 reduction with Fischer–Tropsch (FT) reactions. In addition, investigations into these routes reveal that the stabilization of key reaction intermediates is critically important for controlling catalytic selectivity. Furthermore, viability of these processes is contingent on the development of a CO 2-free H 2 source on a large enough scale to significantly reduce CO 2 emissions.« less

Wavelet-based surrogate time series for multiscale simulation of heterogeneous catalysis

DOE Office of Scientific and Technical Information (OSTI.GOV)

Savara, Aditya Ashi; Daw, C. Stuart; Xiong, Qingang

We propose a wavelet-based scheme that encodes the essential dynamics of discrete microscale surface reactions in a form that can be coupled with continuum macroscale flow simulations with high computational efficiency. This makes it possible to simulate the dynamic behavior of reactor-scale heterogeneous catalysis without requiring detailed concurrent simulations at both the surface and continuum scales using different models. Our scheme is based on the application of wavelet-based surrogate time series that encodes the essential temporal and/or spatial fine-scale dynamics at the catalyst surface. The encoded dynamics are then used to generate statistically equivalent, randomized surrogate time series, which canmore » be linked to the continuum scale simulation. As a result, we illustrate an application of this approach using two different kinetic Monte Carlo simulations with different characteristic behaviors typical for heterogeneous chemical reactions.« less

Wavelet-based surrogate time series for multiscale simulation of heterogeneous catalysis

DOE PAGES

Savara, Aditya Ashi; Daw, C. Stuart; Xiong, Qingang; ...

2016-01-28

We propose a wavelet-based scheme that encodes the essential dynamics of discrete microscale surface reactions in a form that can be coupled with continuum macroscale flow simulations with high computational efficiency. This makes it possible to simulate the dynamic behavior of reactor-scale heterogeneous catalysis without requiring detailed concurrent simulations at both the surface and continuum scales using different models. Our scheme is based on the application of wavelet-based surrogate time series that encodes the essential temporal and/or spatial fine-scale dynamics at the catalyst surface. The encoded dynamics are then used to generate statistically equivalent, randomized surrogate time series, which canmore » be linked to the continuum scale simulation. As a result, we illustrate an application of this approach using two different kinetic Monte Carlo simulations with different characteristic behaviors typical for heterogeneous chemical reactions.« less

Mechanistic Details and Reactivity Descriptors in Oxidation and Acid Catalysis of Methanol

DOE Office of Scientific and Technical Information (OSTI.GOV)

Deshlahra, Prashant; Carr, Robert T.; Chai, Song-Hai

2015-02-06

Acid and redox reaction rates of CH₃OH-O₂ mixtures on polyoxometalate (POM) clusters, together with isotopic, spectroscopic, and theoretical assessments of catalyst properties and reaction pathways, were used to define rigorous descriptors of reactivity and to probe the compositional effects for oxidative dehydrogenation (ODH) and dehydration reactions. ³¹P-MAS NMR, transmission electron microscopy and titrations of protons with di-tert-butylpyridine during catalysis showed that POM clusters retained their Keggin structure upon dispersion on SiO₂ and after use in CH₃OH reactions. The effects of CH₃OH and O₂ pressures and of D-substitution on ODH rates show that C-H activation in molecularly adsorbed CH₃OH is themore » sole kinetically relevant step and leads to reduced centers as intermediates present at low coverages; their concentrations, measured from UV-vis spectra obtained during catalysis, are consistent with the effects of CH₃OH/O₂ ratios predicted from the elementary steps proposed. First-order ODH rate constants depend strongly on the addenda atoms (Mo vs W) but weakly on the central atom (P vs Si) in POM clusters, because C-H activation steps inject electrons into the lowest unoccupied molecular orbitals (LUMO) of the clusters, which are the d-orbitals at Mo⁶⁺ and W⁶⁺ centers. H-atom addition energies (HAE) at O-atoms in POM clusters represent the relevant theoretical probe of the LUMO energies and of ODH reactivity. The calculated energies of ODH transition states at each O-atom depend linearly on their HAE values with slopes near unity, as predicted for late transition states in which electron transfer and C-H cleavage are essentially complete. HAE values averaged over all accessible O-atoms in POM clusters provide the appropriate reactivity descriptor for oxides whose known structures allow accurate HAE calculations. CH₃OH dehydration proceeds via parallel pathways mediated by late carbenium-ion transition states

Spatially resolved observation of crystal-face-dependent catalysis by single turnover counting

NASA Astrophysics Data System (ADS)

Roeffaers, Maarten B. J.; Sels, Bert F.; Uji-I, Hiroshi; de Schryver, Frans C.; Jacobs, Pierre A.; de Vos, Dirk E.; Hofkens, Johan

2006-02-01

Catalytic processes on surfaces have long been studied by probing model reactions on single-crystal metal surfaces under high vacuum conditions. Yet the vast majority of industrial heterogeneous catalysis occurs at ambient or elevated pressures using complex materials with crystal faces, edges and defects differing in their catalytic activity. Clearly, if new or improved catalysts are to be rationally designed, we require quantitative correlations between surface features and catalytic activity-ideally obtained under realistic reaction conditions. Transmission electron microscopy and scanning tunnelling microscopy have allowed in situ characterization of catalyst surfaces with atomic resolution, but are limited by the need for low-pressure conditions and conductive surfaces, respectively. Sum frequency generation spectroscopy can identify vibrations of adsorbed reactants and products in both gaseous and condensed phases, but so far lacks sensitivity down to the single molecule level. Here we adapt real-time monitoring of the chemical transformation of individual organic molecules by fluorescence microscopy to monitor reactions catalysed by crystals of a layered double hydroxide immersed in reagent solution. By using a wide field microscope, we are able to map the spatial distribution of catalytic activity over the entire crystal by counting single turnover events. We find that ester hydrolysis proceeds on the lateral {1010} crystal faces, while transesterification occurs on the entire outer crystal surface. Because the method operates at ambient temperature and pressure and in a condensed phase, it can be applied to the growing number of liquid-phase industrial organic transformations to localize catalytic activity on and in inorganic solids. An exciting opportunity is the use of probe molecules with different size and functionality, which should provide insight into shape-selective or structure-sensitive catalysis and thus help with the rational design of new or

Multistep Reduction Kinetics of Fine Iron Ore with Carbon Monoxide in a Micro Fluidized Bed Reaction Analyzer

NASA Astrophysics Data System (ADS)

Chen, Hongsheng; Zheng, Zhong; Chen, Zhiwei; Yu, Wenzhou; Yue, Junrong

2017-04-01

The reduction kinetics of Brazilian hematite by CO is investigated in a Micro Fluidized Bed Reaction Analyzer (MFBRA) using an analyzing method based on Johnson-Mehl-Avrami (JMA) model at temperatures of 973 K (700 °C), 1023 K (750 °C), 1073 K (800 °C), and 1123 K (850 °C). The solid products at different reduction stages are evaluated by SEM/EDS and XRD technologies. Results indicate that the reduction process is better to be discussed in terms of a parallel reaction model that consists of the reactions of hematite to wüstite and wüstite to iron, rather than a stepwise route. Meanwhile, the controlling mechanism of the reduction process is found to vary with temperature and the degree of conversion. The overall process is controlled by the gas-solid reaction occurring at the iron/wüstite interface in the initial stages, and then is limited by the nucleation of wüstite, and finally shifts to diffusion control. Moreover, the reactions of hematite to wüstite and wüstite to iron take place simultaneously but with different time dependences, and the apparent activation energies of hematite to wüstite and wüstite to iron are determined as 83.61 and 80.40 KJ/mol, respectively.

Stable platinum nanoclusters on genomic DNA–graphene oxide with a high oxygen reduction reaction activity

PubMed Central

Tiwari, Jitendra N.; Nath, Krishna; Kumar, Susheel; Tiwari, Rajanish N.; Kemp, K. Christian; Le, Nhien H.; Youn, Duck Hyun; Lee, Jae Sung; Kim, Kwang S.

2013-01-01

Nanosize platinum clusters with small diameters of 2–4 nm are known to be excellent catalysts for the oxygen reduction reaction. The inherent catalytic activity of smaller platinum clusters has not yet been reported due to a lack of preparation methods to control their size (<2 nm). Here we report the synthesis of platinum clusters (diameter ≤1.4 nm) deposited on genomic double-stranded DNA–graphene oxide composites, and their high-performance electrocatalysis of the oxygen reduction reaction. The electrochemical behaviour, characterized by oxygen reduction reaction onset potential, half-wave potential, specific activity, mass activity, accelerated durability test (10,000 cycles) and cyclic voltammetry stability (10,000 cycles) is attributed to the strong interaction between the nanosize platinum clusters and the DNA–graphene oxide composite, which induces modulation in the electronic structure of the platinum clusters. Furthermore, we show that the platinum cluster/DNA–graphene oxide composite possesses notable environmental durability and stability, vital for high-performance fuel cells and batteries. PMID:23900456

Conjugated Microporous Polymers for Heterogeneous Catalysis.

PubMed

Zhou, Yun-Bing; Zhan, Zhuang-Ping

2018-01-04

Conjugated microporous polymers (CMPs) are a class of crosslinked polymers that combine permanent micropores with π-conjugated skeletons and possess three-dimensional (3D) networks. Compared with conventional materials such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), CMPs usually have superior chemical and thermal stability. CMPs have made significant progress in heterogeneous catalysis in the past seven years. With a bottom-up strategy, catalytic moieties can be directly introduced into in the framework to produce heterogeneous CMP catalysts. Higher activity, stability, and selectivity can be obtained with heterogeneous CMP catalysts in comparison with their homogeneous analogs. In addition, CMP catalysts can be easily isolated and recycled. In this review, we focus on CMPs as an intriguing platform for developing various highly efficient and recyclable heterogeneous catalysts in organic reactions. The design, synthesis, and structure of these CMP catalysts are also discussed in this focus review. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Don't Forget Product Recovery in Catalysis Research-Check the Distillation Resistance.

PubMed

Lange, Jean-Paul

2017-01-10

When investigating a new catalytic reaction, scientists may wonder if the crude reaction product can be easily separated and purified. We present here a new concept-distillation resistance-to assess the potential of distillation as a purification technique and to guide catalyst formulation or operation at the very early stage of the research. Distillation resistance, which has been developed from the analysis of 15 industrial distillation trains, can be quickly calculated with knowledge of only the product composition and atmospheric boiling points of the components. It can be directly converted into a preliminary distillation cost that considers investment and energy cost. Its application and its potential guidance in catalysis research are illustrated through a few cases studies derived from biorefinery processes. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Novel nanowire-structured polypyrrole-cobalt composite as efficient catalyst for oxygen reduction reaction

PubMed Central

Yuan, Xianxia; Li, Lin; Ma, Zhong; Yu, Xuebin; Wen, Xiufang; Ma, Zi-Feng; Zhang, Lei; Wilkinson, David P.; Zhang, Jiujun

2016-01-01

A novel nanowire-structured polypyrrole-cobalt composite, PPy-CTAB-Co, is successfully synthesized with a surfactant of cetyltrimethylammounium bromide (CTAB). As an electro-catalyst towards oxygen reduction reaction (ORR) in alkaline media, this PPy-CTAB-Co demonstrates a superior ORR performance when compared to that of granular PPy-Co catalyst and also a much better durability than the commercial 20 wt% Pt/C catalyst. Physiochemical characterization indicates that the enhanced ORR performance of the nanowire PPy-CTAB-Co can be attributed to the high quantity of Co-pyridinic-N groups as ORR active sites and its large specific surface area which allows to expose more active sites for facilitating oxygen reduction reaction. It is expected this PPy-CTAB-Co would be a good candidate for alkaline fuel cell cathode catalyst. PMID:26860889

Detection of Reaction Intermediates in Mg2+-Dependent DNA Synthesis and RNA Degradation by Time-Resolved X-Ray Crystallography.

PubMed

Samara, Nadine L; Gao, Yang; Wu, Jinjun; Yang, Wei

2017-01-01

Structures of enzyme-substrate/product complexes have been studied for over four decades but have been limited to either before or after a chemical reaction. Recently using in crystallo catalysis combined with X-ray diffraction, we have discovered that many enzymatic reactions in nucleic acid metabolism require additional metal ion cofactors that are not present in the substrate or product state. By controlling metal ions essential for catalysis, the in crystallo approach has revealed unprecedented details of reaction intermediates. Here we present protocols used for successful studies of Mg 2+ -dependent DNA polymerases and ribonucleases that are applicable to analyses of a variety of metal ion-dependent reactions. © 2017 Elsevier Inc. All rights reserved.

On the mechanism of the palladium catalyzed intramolecular Pauson-Khand-type reaction.

PubMed

Lan, Yu; Deng, Lujiang; Liu, Jing; Wang, Can; Wiest, Olaf; Yang, Zhen; Wu, Yun-Dong

2009-07-17

Density functional theory calculations and experimental studies have been carried out on the intramolecular Pauson-Khand-Type reaction mediated by a PdCl(2)-thiourea catalyst, which proceeds under mild reaction conditions and provides a useful alternative to traditional Pauson-Khand reactions. The classical mechanism of the Pauson-Khand reaction involving the alkyne/alkene C-C bond formation as the key step has been found to be energetically unfavorable and is not in line with the experimental observations. A novel reaction mechanism has been proposed for the reaction. The first step involves the cis-halometalation of the alkyne, followed by sequential alkene and carbonyl insertion. The rate-determining fourth step is an intramolecular C-Cl oxidative addition, leading to a Pd(IV) intermediate. A C-C bond formation by reductive elimination completes the reaction. The mechanism is in agreement with the key experimental observations including (1) the need of a chloride for catalytic activity and the absence of catalysis with Pd(OAc)(2) alone; (2) the rate acceleration by the addition of LiCl; both with PdCl(2) and Pd(OAc)(2) catalysts; and (3) the preferred formation of the trans diastereomer in substituted cases. The cis halometalation and the formation and stability of the Pd(IV) intermediate is studied in detail and provides general insights into these novel steps.

Evidence of enzymatic catalysis of oxygen reduction on stainless steels under marine biofilm.

PubMed

Faimali, Marco; Benedetti, Alessandro; Pavanello, Giovanni; Chelossi, Elisabetta; Wrubl, Federico; Mollica, Alfonso

2011-04-01

Cathodic current trends on stainless steel samples with different surface percentages covered by biofilm and potentiostatically polarized in natural seawater were studied under oxygen concentration changes, temperature increases, and additions of enzymic inhibitors to the solution. The results showed that on each surface fraction covered by biofilm the oxygen reduction kinetics resembled a reaction catalyzed by an immobilised enzyme with high oxygen affinity (apparent Michaelis-Menten dissociation constant close to K(O(2))(M)  ≈ 10 μM) and low activation energy (W ≈ 20 KJ mole(-1)). The proposed enzyme rapidly degraded when the temperature was increased above the ambient (half-life time of ∼1 day at 25°C, and of a few minutes at 50°C). Furthermore, when reversible enzymic inhibitors (eg sodium azide and cyanide) were added, the cathodic current induced by biofilm growth was inhibited.

Catalysis in the 3rd Dimension: How Organic Molecules May be Formed

NASA Technical Reports Server (NTRS)

Freund, Friedemann; DeVincenzi, D. (Technical Monitor)

2001-01-01

Catalysis is often little more than a word to phenomenologically describe the fact that a reaction follows a pat1 that leads to products of an unexpected kind or of unexpected yield. Low activation energy barriers for intermediates are recognized as the most likely cause why a system deviates from the thermodynamic pull towards minimizing its free energy and ends up in a metastable state. Seldom is the mechanism known. This i: particularly true for heterogeneous catalysis under hydrothermal conditions with minerals as catalysts. It is commonly assumed that catalytic action takes place across solid-fluid interfaces and that, on the atomic level, interfaces are just 2-dimensional contacts. This makes it difficult to understand, for instance, the assembly of long-chain carboxylic (fatty) acids. 3y studying single crystals that grew from a melt in the presence of H2O and CO2, we can show: (1) that numerals take up the fluid components into solid solution, (2) that some-thing happens converting them to -educedH and C, (3) that C atoms segregate into dislocations and tie C-C bonds. The products are medium-to-long chain Cn protomolecules, with some C-H attached, pre-assembled in the dislocations. Upon solvent extraction, these proto-molecules turn into carboxylic and dicarboxylic acids. This observation suggests that, in a very elementary step, catalysis under hydrothermal conditions leading to fatty acids involves the pre-assembly of Cn entities in the interface that is not 2-D but extends into the 3rd dimension, with dislocations as synthesis sites.

Transient Interaction Between Reduction and Slagging Reactions of Wustite in Simulated Cohesive Zone of Blast Furnace

NASA Astrophysics Data System (ADS)

Ma, Kaihui; Xu, Jian; Deng, Junyi; Wang, Dongdong; Xu, Yang; Liao, Zhehan; Sun, Chengfeng; Zhang, Shengfu; Wen, Liangying

2018-06-01

The blast furnace cohesive zone plays an important role in the gas flow distribution and heat-transfer efficiency. Previous work mainly employed temperature-based indices to evaluate and predict the shape and thickness of the cohesive zone, whereas the internal reactions and related effects on the softening and melting properties of a complex burden are ignored. In this study, an innovative index, namely, shrinkage rate (SR), is first proposed to directly estimate the shrinkage behavior of wustite (FeO)-packed bed inside a simulated cohesive zone. The index is applied as the temperature increases to elucidate the transient interaction between reduction and slagging reactions. Results show that the thermally induced slagging reaction causes the packed bed to shrink at lower temperature, and the SR doubles when compounds with low melting temperature are generated by adding a reasonable concentration of CaO or SiO2. The reduction reaction becomes the driving force during the shrinkage of the packed bed between 1173 K and 1273 K when CO is introduced in the mixture gas. Then, the dominating factors for further shrinkage include slagging, reduction, or both factors. These factors vary with respect to the added compounds or temperature.

Boron- and Nitrogen-Substituted Graphene Nanoribbons as Efficient Catalysts for Oxygen Reduction Reaction

DOE PAGES

Gong, Yongji; Fei, Huilong; Zou, Xiaolong; ...

2015-02-02

Here, we show that nanoribbons of boron- and nitrogen-substituted graphene can be used as efficient electrocatalysts for the oxygen reduction reaction (ORR). Optimally doped graphene nanoribbons made into three-dimensional porous constructs exhibit the highest onset and half-wave potentials among the reported metal-free catalysts for this reaction and show superior performance compared to commercial Pt/C catalyst. Moreover, this catalyst possesses high kinetic current density and four-electron transfer pathway with low hydrogen peroxide yield during the reaction. Finally, first-principles calculations suggest that such excellent electrocatalytic properties originate from the abundant edges of boron- and nitrogen-codoped graphene nanoribbons, which significantly reduce the energymore » barriers of the rate-determining steps of the ORR reaction.« less

Investigation of hydrogen peroxide reduction reaction on graphene and nitrogen doped graphene nanoflakes in neutral solution

NASA Astrophysics Data System (ADS)

Amirfakhri, Seyed Javad; Binny, Dustin; Meunier, Jean-Luc; Berk, Dimitrios

2014-07-01

H2O2 reduction reaction (HPRR) is studied on both graphene (GNF) and nitrogen doped graphene nanoflakes in 0.1 M Na2SO4 solution by rotating disk electrode. The XPS results indicate that N-doped graphene nanoflakes with high nitrogen content, 32 at%N (N-GNF32), are synthesised successfully by an inductively-coupled thermal plasma (ICP) reactor. Pyridinic, pyrrolic and graphitic N species contribute up to 67% of the total nitrogen. Kinetic parameters such as Tafel slope and stoichiometric number suggest that HPRR occurs by the same mechanism on both GNF and N-GNF32. Although nitrogen does not change the mechanism of HPRR, the results indicate that the reaction rate of H2O2 reduction is enhanced on N-GNF32. The exchange current density of H2O2 reduction based on the active surface area of N-GNF32 is (8.3 ± 0.3) × 10-9 A cm-2, which is 6 times higher than the value determined for GNF. The apparent number of electrons involved in the process suggests that H2O2 decomposition competes with H2O2 reduction on both catalysts. Evaluation of the apparent heterogeneous reaction rate constant and the Tafel slope indicate that simultaneous reduction of O2 and H2O2 is negligible on the N-GNF32. On the other hand, the reduction of O2 and H2O2 occurs simultaneously on the GNF surface.

Rh(II)-catalyzed reactions of differentially substituted bis(diazo) functionalities.

PubMed

Bonderoff, Sara A; Padwa, Albert

2013-08-16

The chemoselective reaction of donor/acceptor (D/A) and acceptor/acceptor (A/A) diazo moieties in the same molecule was examined using 3-diazo-1-(ethyl 2-diazomalonyl)indolin-2-one under rhodium(II) catalysis. The D/A diazo group undergoes selective cyclopropanation as well as XH-insertion, leaving behind the second diazo group for a further intramolecular dipolar cycloaddition reaction.

Catalysis in high-temperature fuel cells.

PubMed

Föger, K; Ahmed, K

2005-02-17

Catalysis plays a critical role in solid oxide fuel cell systems. The electrochemical reactions within the cell--oxygen dissociation on the cathode and electrochemical fuel combustion on the anode--are catalytic reactions. The fuels used in high-temperature fuel cells, for example, natural gas, propane, or liquid hydrocarbons, need to be preprocessed to a form suitable for conversion on the anode-sulfur removal and pre-reforming. The unconverted fuel (economic fuel utilization around 85%) is commonly combusted using a catalytic burner. Ceramic Fuel Cells Ltd. has developed anodes that in addition to having electrochemical activity also are reactive for internal steam reforming of methane. This can simplify fuel preprocessing, but its main advantage is thermal management of the fuel cell stack by endothermic heat removal. Using this approach, the objective of fuel preprocessing is to produce a methane-rich fuel stream but with all higher hydrocarbons removed. Sulfur removal can be achieved by absorption or hydro-desulfurization (HDS). Depending on the system configuration, hydrogen is also required for start-up and shutdown. Reactor operating parameters are strongly tied to fuel cell operational regimes, thus often limiting optimization of the catalytic reactors. In this paper we discuss operation of an authothermal reforming reactor for hydrogen generation for HDS and start-up/shutdown, and development of a pre-reformer for converting propane to a methane-rich fuel stream.

New Possibilities for Magnetic Control of Chemical and Biochemical Reactions.

PubMed

Buchachenko, Anatoly; Lawler, Ronald G

2017-04-18

Chemistry is controlled by Coulomb energy; magnetic energy is lower by many orders of magnitude and may be confidently ignored in the energy balance of chemical reactions. The situation becomes less clear, however, when reaction rates are considered. In this case, magnetic perturbations of nearly degenerate energy surface crossings may produce observable, and sometimes even dramatic, effects on reactions rates, product yields, and spectroscopic transitions. A case in point that has been studied for nearly five decades is electron spin-selective chemistry via the intermediacy of radical pairs. Magnetic fields, external (permanent or oscillating) and the internal magnetic fields of magnetic nuclei, have been shown to overcome electron spin selection rules for pairs of reactive paramagnetic intermediates, catalyzing or inhibiting chemical reaction pathways. The accelerating effects of magnetic stimulation may therefore be considered to be magnetic catalysis. This type of catalysis is most commonly observed for reactions of a relatively long-lived radical pair containing two weakly interacting electron spins formed by dissociation of molecules or by electron transfer. The pair may exist in singlet (total electron spin is zero) or triplet (total spin is unity) spin states. In virtually all cases, only the singlet state yields stable reaction products. Magnetic interactions with nuclear spins or applied fields may therefore affect the reactivity of radical pairs by changing the angular momentum of the pairs. Magnetic catalysis, first detected via its effect on spin state populations in nuclear and electron spin resonance, has been shown to function in a great variety of well-characterized reactions of organic free radicals. Considerably less well studied are examples suggesting that the basic mechanism may also explain magnetic effects that stimulate ATP synthesis, eliminating ATP deficiency in cardiac diseases, control cell proliferation, killing cancer cells, and

Development of Catalysts and Ligands for Enantioselective Gold Catalysis

PubMed Central

Wang, Yi-Ming; Lackner, Aaron D.; Toste, F. Dean

2014-01-01

CONSPECTUS The use of Au(I) complexes for the catalytic activation of C-C π-bonds has been the subject of intense investigation in the last decade or so. The facile formation of carbon-carbon and carbon-heteroatom bonds facilitated by gold naturally led to efforts to render these transformations enantioselective. Early examples of enantioselective gold-catalyzed transformations have focused on bis(phosphinegold) complexes derived from axially chiral scaffolds. Although these complexes were highly successful in some reactions like cyclopropanation, careful choice of the weakly coordinating ligand (or counterion) was needed to obtain high levels of enantioselectivity for the case of allene hydroamination. These counterion effects led us to use the anion itself as a source of chirality, which was successful in the case of allene hydroalkoxylation. In order to expand the scope of reactions amenable to enantioselective gold catalysis to cycloadditions and other carbocyclization processes, a new class of mononuclear phosphite and phosphoramidite ligands was developed to supplement the previously widely utilized phosphines. Finally carbene ligands, in particular, the acyclic diaminocarbenes, have also been successfully applied to enantioselective transformations. PMID:24228794

Method of controlled reduction of nitroaromatics by enzymatic reaction with oxygen sensitive nitroreductase enzymes

DOEpatents

Shah, Manish M.; Campbell, James A.

1998-01-01

A method for the controlled reduction of nitroaromatic compounds such as nitrobenzene and 2,4,6-trinitrotoluene by enzymatic reaction with oxygen sensitive nitroreductase enzymes, such as ferredoxin NADP oxidoreductase.

The Influence of Marine Microfouling on the Corrosion Behaviour of Passive Materials and Copper Alloys

DTIC Science & Technology

2008-01-02

to organometallic catalysis, acidification of the electrode surface, the combined effects of elevated H20 2 and decreased pH and the production of...Ennoblement in marine waters has been ascribed to depolarization of the oxygen reduction reaction due to organometallic catalysis, acidification of the...organometallic catalysis, acidification of the electrode surface, the combined effects of elevated hydrogen peroxide (H202) and decreased pH and the production

Chemical reactions occurring during direct solar reduction of CO2.

PubMed

Lyma, J L; Jensen, R J

2001-09-28

At high temperatures carbon dioxide may absorb solar radiation and react to form carbon monoxide and molecular oxygen. The CO, so produced, may be converted by well-established means to a combustible fuel, such as methanol. We intend to make a future demonstration of the solar reduction of CO2 based on these processes. This paper, however, addresses only the problem of preserving, or even enhancing, the initial photolytic CO by quenching the hot gas with colder H2O or CO2. We present model calculations with a reaction mechanism used extensively in other calculations. If a CO2 gas stream is heated and photolyzed by intense solar radiation and then allowed to cool slowly, it will react back to the initial CO2 by a series of elementary chemical reactions. The back reaction to CO2 can be terminated with the rapid addition of CO2, water, or a mixture. Calculations show that a three-fold quench with pure CO2 will stop the reactions and preserve over 90% of the initial photolytic CO. We find that water has one of two effects. It can either increase the CO level, or it can catalyze the recombination of O and CO to CO2. The gas temperature is the determining factor. If the quench gas is not sufficient to keep the temperature below approximately 1100 K, a chain-branching reaction dominates and the reaction to CO2 occurs. If the temperature stays below that level a chain terminating reaction dominates and the CO is increased. The former case occurs below approximately a fourfold quench with a water/CO2 mixture. The later case occurs when the quench is greater than fourfold. We conclude that CO2, H2O, or a mixture may quench the hot gas stream photolyzed by solar radiation and preserve the photolytic CO.

Is It Homogeneous or Heterogeneous Catalysis Derived from [RhCp*Cl2]2? In Operando-XAFS, Kinetic and Crucial Kinetic Poisoning Evidence for Subnanometer Rh4 Cluster-Based Benzene Hydrogenation Catalysis

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bayram, Ercan; Linehan, John C.; Fulton, John L.

2011-11-23

Determining the true, kinetically dominant catalytically active species, in the classic benzene hydrogenation system pioneered by Maitlis and co-workers 34 years ago starting with [RhCp*Cl2]2 (Cp* = [{eta}5-C5(CH3)5]), has proven to be one of the most challenging case studies in the quest to distinguish single-metal-based 'homogeneous' from polymetallic, 'heterogeneous' catalysis. The reason, this study will show, is the previous failure to use the proper combination of (i) operando spectroscopy to determine the dominant form(s) of the precatalyst's mass under catalysis (i.e., operating) conditions, plus then and crucially also (ii) the previous lack of the necessary kinetic studies, catalysis being amore » 'wholly kinetic phenomenon' as J. Halpern long ago noted. An important contribution from this study will be to reveal the power of quantitiative kinetic poisoning experiments for distinguishing single-metal, or in this case subnanometer Rh4 cluster-based catalysis from larger, polymetallic Rh(0)n nanoparticle catalysis, at least under favorable conditions. The combined operando-XAFS (X-ray absorption fine structure) spectroscopy and kinetic evidences provide a compelling case for Rh4-based, with average stoichiometry 'Rh4Cp*2.4Cl4Hc', benzene hydrogenation catalysis in 2-propanol with added Et3N and at 100 C and 50 atm initial H2 pressure. The results also reveal, however, that if even ca. 1.4% of the total soluble Rh(0)n had formed nanoparticles, then those Rh(0)n nanoparticles would have been able to account for all the observed benzene hydrogenation catalytic rate (using commercial, ca. 2 nm, polyethyleneglycol-dodecylether hydrosol stabilized Rh(0)n nanoparticles as a model system). The results 'especially the poisoning methodology developed and employed' are of significant, broader interest since determining the nature of the true catalyst continues to be a central, often vexing issue in any and all catalytic reactions. The results are also of fundamental

Synthesis and x-ray characterization of cobalt phosphide (Co₂P) nanorods for the oxygen reduction reaction

DOE PAGES

Doan-Nguyen, Vicky V.T.; Su, Dong; Zhang, Sen; ...

2015-07-14

Low temperature fuel cells are clean, effective alternative fuel conversion technology. Oxygen reduction reaction (ORR) at the fuel cell cathode has required Pt as the electrocatalyst for high activity and selectivity of the four-electron reaction pathway. Targeting a less expensive, earth abundant alternative, we have developed the synthesis of cobalt phosphide (Co₂P) nanorods for ORR. Characterization techniques that include total X-ray scattering and extended X-ray absorption fine structure revealed a deviation of the nanorods from bulk crystal structure with a contraction along the b orthorhombic lattice parameter. The carbon supported nanorods have comparable activity but are remarkably more stable thanmore » conventional Pt catalysts for the oxygen reduction reaction in alkaline environments.« less

Interplay of catalysis, fidelity, threading, and processivity in the exo- and endonucleolytic reactions of human exonuclease I

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shi, Yuqian; Hellinga, Homme W.; Beese, Lorena S.

Human exonuclease 1 (hExo1) is a member of the RAD2/XPG structure-specific 5'-nuclease superfamily. Its dominant, processive 5'–3' exonuclease and secondary 5'-flap endonuclease activities participate in various DNA repair, recombination, and replication processes. A single active site processes both recessed ends and 5'-flap substrates. By initiating enzyme reactions in crystals, we have trapped hExo1 reaction intermediates that reveal structures of these substrates before and after their exo- and endonucleolytic cleavage, as well as structures of uncleaved, unthreaded, and partially threaded 5' flaps. Their distinctive 5' ends are accommodated by a small, mobile arch in the active site that binds recessed endsmore » at its base and threads 5' flaps through a narrow aperture within its interior. A sequence of successive, interlocking conformational changes guides the two substrate types into a shared reaction mechanism that catalyzes their cleavage by an elaborated variant of the two-metal, in-line hydrolysis mechanism. Coupling of substrate-dependent arch motions to transition-state stabilization suppresses inappropriate or premature cleavage, enhancing processing fidelity. The striking reduction in flap conformational entropy is catalyzed, in part, by arch motions and transient binding interactions between the flap and unprocessed DNA strand. At the end of the observed reaction sequence, hExo1 resets without relinquishing DNA binding, suggesting a structural basis for its processivity.« less

Interplay of catalysis, fidelity, threading, and processivity in the exo- and endonucleolytic reactions of human exonuclease I.

PubMed

Shi, Yuqian; Hellinga, Homme W; Beese, Lorena S

2017-06-06

Human exonuclease 1 (hExo1) is a member of the RAD2/XPG structure-specific 5'-nuclease superfamily. Its dominant, processive 5'-3' exonuclease and secondary 5'-flap endonuclease activities participate in various DNA repair, recombination, and replication processes. A single active site processes both recessed ends and 5'-flap substrates. By initiating enzyme reactions in crystals, we have trapped hExo1 reaction intermediates that reveal structures of these substrates before and after their exo- and endonucleolytic cleavage, as well as structures of uncleaved, unthreaded, and partially threaded 5' flaps. Their distinctive 5' ends are accommodated by a small, mobile arch in the active site that binds recessed ends at its base and threads 5' flaps through a narrow aperture within its interior. A sequence of successive, interlocking conformational changes guides the two substrate types into a shared reaction mechanism that catalyzes their cleavage by an elaborated variant of the two-metal, in-line hydrolysis mechanism. Coupling of substrate-dependent arch motions to transition-state stabilization suppresses inappropriate or premature cleavage, enhancing processing fidelity. The striking reduction in flap conformational entropy is catalyzed, in part, by arch motions and transient binding interactions between the flap and unprocessed DNA strand. At the end of the observed reaction sequence, hExo1 resets without relinquishing DNA binding, suggesting a structural basis for its processivity.

Analysis of Thermal and Reaction Times for Hydrogen Reduction of Lunar Regolith

NASA Technical Reports Server (NTRS)

Hegde, U.; Balasubramaniam, R.; Gokoglu, S.

2008-01-01

System analysis of oxygen production by hydrogen reduction of lunar regolith has shown the importance of the relative time scales for regolith heating and chemical reaction to overall performance. These values determine the sizing and power requirements of the system and also impact the number and operational phasing of reaction chambers. In this paper, a Nusselt number correlation analysis is performed to determine the heat transfer rates and regolith heat up times in a fluidized bed reactor heated by a central heating element (e.g., a resistively heated rod, or a solar concentrator heat pipe). A coupled chemical and transport model has also been developed for the chemical reduction of regolith by a continuous flow of hydrogen. The regolith conversion occurs on the surfaces of and within the regolith particles. Several important quantities are identified as a result of the above analyses. Reactor scale parameters include the void fraction (i.e., the fraction of the reactor volume not occupied by the regolith particles) and the residence time of hydrogen in the reactor. Particle scale quantities include the particle Reynolds number, the Archimedes number, and the time needed for hydrogen to diffuse into the pores of the regolith particles. The analysis is used to determine the heat up and reaction times and its application to NASA s oxygen production system modeling tool is noted.

Analysis of Thermal and Reaction Times for Hydrogen Reduction of Lunar Regolith

NASA Technical Reports Server (NTRS)

Hegde, U.; Balasubramaniam, R.; Gokoglu, S.

2009-01-01

System analysis of oxygen production by hydrogen reduction of lunar regolith has shown the importance of the relative time scales for regolith heating and chemical reaction to overall performance. These values determine the sizing and power requirements of the system and also impact the number and operational phasing of reaction chambers. In this paper, a Nusselt number correlation analysis is performed to determine the heat transfer rates and regolith heat up times in a fluidized bed reactor heated by a central heating element (e.g., a resistively heated rod, or a solar concentrator heat pipe). A coupled chemical and transport model has also been developed for the chemical reduction of regolith by a continuous flow of hydrogen. The regolith conversion occurs on the surfaces of and within the regolith particles. Several important quantities are identified as a result of the above analyses. Reactor scale parameters include the void fraction (i.e., the fraction of the reactor volume not occupied by the regolith particles) and the residence time of hydrogen in the reactor. Particle scale quantities include the particle Reynolds number, the Archimedes number, and the time needed for hydrogen to diffuse into the pores of the regolith particles. The analysis is used to determine the heat up and reaction times and its application to NASA s oxygen production system modeling tool is noted.

Oxygen Reduction Reaction Affected by Sulfate-Reducing Bacteria: Different Roles of Bacterial Cells and Metabolites.

PubMed

Wu, Jiajia; Liu, Huaiqun; Wang, Peng; Zhang, Dun; Sun, Yan; Li, Ee

2017-09-01

Sulfate-reducing bacteria (SRB) were found to be capable of tolerating a certain amount of oxygen (O 2 ), but how they affect oxygen reduction reaction (ORR) has not been clear. The present work investigated the impact of SRB on ORR in 3.5 wt% sodium chloride solution with the cyclic voltammetry method. The addition of SRB culture solution hampered both the reduction of O 2 to superoxide (O 2 ·- ) and hydrogen peroxide (H 2 O 2 ) to water (H 2 O), and the influence of SRB metabolites was much larger than that of bacterial cells. Sulfide and extracellular polymeric substances (EPS), typical inorganic and organic metabolic products, had great impact on ORR. Sulfide played an important role in the decrease of cathodic current for H 2 O 2 reduction due to its hydrolysis and chemical reaction activity with H 2 O 2 . EPS were sticky, easy to adsorb on the electrode surface and abundant in functional groups, which hindered the transformation of O 2 into O 2 ·- and favored the reduction of H 2 O 2 to H 2 O.

Direct observation of ultrafast-electron-transfer reactions unravels high effectiveness of reductive DNA damage

PubMed Central

Nguyen, Jenny; Ma, Yuhan; Luo, Ting; Bristow, Robert G.; Jaffray, David A.; Lu, Qing-Bin

2011-01-01

Both water and electron-transfer reactions play important roles in chemistry, physics, biology, and the environment. Oxidative DNA damage is a well-known mechanism, whereas the relative role of reductive DNA damage is unknown. The prehydrated electron (), a novel species of electrons in water, is a fascinating species due to its fundamental importance in chemistry, biology, and the environment. is an ideal agent to observe reductive DNA damage. Here, we report both the first in situ femtosecond time-resolved laser spectroscopy measurements of ultrafast-electron-transfer (UET) reactions of with various scavengers (KNO3, isopropanol, and dimethyl sulfoxide) and the first gel electrophoresis measurements of DNA strand breaks induced by and OH• radicals co-produced by two-UV-photon photolysis of water. We strikingly found that the yield of reductive DNA strand breaks induced by each is twice the yield of oxidative DNA strand breaks induced by each OH• radical. Our results not only unravel the long-standing mystery about the relative role of radicals in inducing DNA damage under ionizing radiation, but also challenge the conventional notion that oxidative damage is the main pathway for DNA damage. The results also show the potential of femtomedicine as a new transdisciplinary frontier and the broad significance of UET reactions of in many processes in chemistry, physics, biology, and the environment. PMID:21730183

Diphosphine-Protected Au 22 Nanoclusters on Oxide Supports Are Active for Gas-Phase Catalysis without Ligand Removal

DOE PAGES

Wu, Zili; Hu, Guoxiang; Jiang, De-en; ...

2016-09-29

Investigation of monodispersed and atomically-precise Au nanoclusters provides a route to understand the roles of coordination, size, and ligand effects in Au catalysis. We have explored the catalytic behavior of a newly-synthesized Au 22(L 8) 6 nanocluster (L = 1,8-bis(diphenylphosphino) octane) with in situ uncoordinated Au sites supported on TiO 2, CeO 2 and Al 2O 3. Stability of the supported Au 22 nanoclusters was probed structurally by EXAFS and HAADF-STEM, and their adsorption and reactivity for CO oxidation were investigated by IR absorption spectroscopy and temperature programed flow reaction. Low temperature CO oxidation activity was observed for the supportedmore » pristine Au 22(L 8) 6 nanoclusters without ligand removal. Isotopically labeled O 2 was used to demonstrate that the reaction pathway occurs through a redox mechanism, consistent with the observed support-dependent activity trend: CeO 2 > TiO 2 > Al 2O 3. Substantiated by density functional theory (DFT) calculations, we conclude that the uncoordinated Au sites in the intact Au 22(L 8) 6 nanoclusters are capable of adsorbing CO, activating O2 and promoting CO oxidation reaction. Thanks to the presence of the in situ coordination unsaturated Au atoms, this work is the first clear demonstration of a ligand-protected Au nanocluster that are active for gas phase catalysis without the need of ligand removal.« less

Atomically precise cluster catalysis towards quantum controlled catalysts

PubMed Central

Watanabe, Yoshihide

2014-01-01

Catalysis of atomically precise clusters supported on a substrate is reviewed in relation to the type of reactions. The catalytic activity of supported clusters has generally been discussed in terms of electronic structure. Several lines of evidence have indicated that the electronic structure of clusters and the geometry of clusters on a support, including the accompanying cluster-support interaction, are strongly correlated with catalytic activity. The electronic states of small clusters would be easily affected by cluster–support interactions. Several studies have suggested that it is possible to tune the electronic structure through atomic control of the cluster size. It is promising to tune not only the number of cluster atoms, but also the hybridization between the electronic states of the adsorbed reactant molecules and clusters in order to realize a quantum-controlled catalyst. PMID:27877723

Modeling the reactions catalyzed by coenzyme B12-dependent enzymes.

PubMed

Sandala, Gregory M; Smith, David M; Radom, Leo

2010-05-18

Enzymes accelerate chemical reactions with an exceptional selectivity that makes life itself possible. Understanding the factors responsible for this efficient catalysis is of utmost importance in our quest to harness the tremendous power of enzymes. Computational chemistry has emerged as an important adjunct to experimental chemistry and biochemistry in this regard, because it provides detailed insights into the relationship between structure and function in a systematic and straightforward manner. In this Account, we highlight our recent high-level theoretical investigations toward this end in studying the radical-based reactions catalyzed by enzymes dependent on coenzyme B(12) (or adenosylcobalamin, AdoCbl). In addition to their fundamental position in biology, the AdoCbl-dependent enzymes represent a valuable framework within which to understand Nature's method of efficiently handling high-energy species to execute very specific reactions. The AdoCbl-mediated reactions are characterized by the interchange of a hydrogen atom and a functional group on adjacent carbon atoms. Our calculations are consistent with the conclusion that the main role of AdoCbl is to provide a source of radicals, thus moving the 1,2-rearrangements onto the radical potential energy surface. Our studies also show that the radical rearrangement step is facilitated by partial proton transfer involving the substrate. Specifically, we observe that the energy requirements for radical rearrangement are reduced dramatically with appropriate partial protonation or partial deprotonation or sometimes (synergistically) both. Such interactions are particularly relevant to enzyme catalysis, because it is likely that the local amino acid environment in the active site of an enzyme can function in this capacity through hydrogen bonding. Finally, our calculations indicate that the intervention of a very stable radical along the reaction pathway may inactivate the enzyme, demonstrating that sustained

Method of controlled reduction of nitroaromatics by enzymatic reaction with oxygen sensitive nitroreductase enzymes

DOEpatents

Shah, M.M.; Campbell, J.A.

1998-07-07

A method is described for the controlled reduction of nitroaromatic compounds such as nitrobenzene and 2,4,6-trinitrotoluene by enzymatic reaction with oxygen sensitive nitroreductase enzymes, such as ferredoxin NADP oxidoreductase. 6 figs.

Photon catalysis acting as noiseless linear amplification and its application in coherence enhancement

NASA Astrophysics Data System (ADS)

Zhang, Shengli; Zhang, Xiangdong

2018-04-01

Photon catalysis is an intriguing quantum mechanical operation during which no photon is added to or subtracted from the relevant optical system. However, we prove that photon catalysis is in essence equivalent to the simpler but more efficient noiseless linear amplifier. This provides a simple and zero-energy-input method for enhancing quantum coherence. We show that the coherence enhancement holds both for a coherent state and a two-mode squeezed vacuum (TMSV) state. For the TMSV state, biside photon catalysis is shown to be equivalent to two times the single-side photon catalysis, and two times the photon catalysis does not provide a substantial enhancement of quantum coherence compared with single-side catalysis. We further extend our investigation to the performance of coherence enhancement with a more realistic photon catalysis scheme where a heralded approximated single-photon state and an on-off detector are exploited. Moreover, we investigate the influence of an imperfect photon detector and the result shows that the amplification effect of photon catalysis is insensitive to the detector inefficiency. Finally, we apply the coherence measure to quantum illumination and see the same trend of performance improvement as coherence enhancement is identified in practical quantum target detection.

Noncovalent Organocatalysis Based on Hydrogen Bonding: Elucidation of Reaction Paths by Computational Methods

NASA Astrophysics Data System (ADS)

Etzenbach-Effers, Kerstin; Berkessel, Albrecht

In this article, the functions of hydrogen bonds in organocatalytic reactions are discussed on atomic level by presenting DFT studies of selected examples. Theoretical investigation provides a detailed insight in the mechanism of substrate activation and orientation, and the stabilization of transition states and intermediates by hydrogen bonding (e.g. oxyanion hole). The examples selected comprise stereoselective catalysis by bifunctional thioureas, solvent catalysis by fluorinated alcohols in epoxidation by hydrogen peroxide, and intramolecular cooperative hydrogen bonding in TADDOL-type catalysts.

Kokes Awards for the 22nd North American Catalysis Society Meeting, June 5-10, 2011

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ribeiro, Fabio H.

2011-06-05

The biennial North American Catalysis Society (NACS) Meetings are the premiere conferences in the area of catalysis, surface science, and reaction engineering. The 22nd meeting will be held the week of June 5-10, 2011 in Detroit, Michigan. The objective of the Meetings is to bring together leading researchers for intensive scientific exchange and interactions. Financial support that offsets some of the associated costs (specifically, registration fee, airline tickets, and hotel accommodations) would encourage graduate students, and for the first time undergraduate students, to attend and participate meaningfully in this conference. The funds sought in this proposal will help support themore » Richard J. Kokes Travel Award program. Graduate students eligible for these merit-based Awards are those who study at a North American university and who will present at the Meeting. We have currently 209 applications and we expect to be able to fund about half of them. The NACS has traditionally sought to encourage graduate student, and this year for the first time undergraduate studies, participation at the National Meetings and providing financial support is the most effective means to do so. Their attendance would contribute significantly to their scientific training and communication and presentation skills. They would be exposed to the leading researchers from the US and abroad; they would meet their peers from other universities; they would learn about cutting-edge results that could benefit their research projects; and they may become interested in becoming active participants in the catalysis community. These young investigators represent the next generation of scientists and engineers, and their proper training will lead to future scientific breakthroughs and technological innovations that benefit the US economy. Advances in catalysis can come in the form of more energy-efficient and environmentally-friendly chemical processes, improved fuel cell performance

Exploration of two-enzyme coupled catalysis system using scanning electrochemical microscopy.

PubMed

Wu, Zeng-Qiang; Jia, Wen-Zhi; Wang, Kang; Xu, Jing-Juan; Chen, Hong-Yuan; Xia, Xing-Hua

2012-12-18

In biological metabolism, a given metabolic process usually occurs via a group of enzymes working together in sequential pathways. To explore the metabolism mechanism requires the understanding of the multienzyme coupled catalysis systems. In this paper, an approach has been proposed to study the kinetics of a two-enzyme coupled reaction using SECM combining numerical simulations. Acetylcholine esterase and choline oxidase are immobilized on cysteamine self-assembled monolayers on tip and substrate gold electrodes of SECM via electrostatic interactions, respectively. The reaction kinetics of this two-enzyme coupled system upon various separation distance precisely regulated by SECM are measured. An overall apparent Michaelis-Menten constant of this enzyme cascade is thus measured as 2.97 mM at an optimal tip-substrate gap distance of 18 μm. Then, a kinetic model of this enzyme cascade is established for evaluating the kinetic parameters of individual enzyme by using the finite element method. The simulated results demonstrate the choline oxidase catalytic reaction is the rate determining step of this enzyme cascade. The Michaelis-Menten constant of acetylcholine esterase is evaluated as 1.8 mM. This study offers a promising approach to exploring mechanism of other two-enzyme coupled reactions in biological system and would promote the development of biosensors and enzyme-based logic systems.

Heterogeneous Catalysis with Renewed Attention: Principles, Theories, and Concepts

ERIC Educational Resources Information Center

Dumeignil, Franck; Paul, Jean-Francois; Paul, Sebastien

2017-01-01

With the development of a strong bioeconomy sector related to the creation of next-generation biorefineries, heterogeneous catalysis is receiving renewed attention. Indeed, catalysis is at the core of biorefinery design, and many new catalysts and catalytic processes are being developed. On the one hand, they are based on knowledge acquired during…

Novel Catalysis by Gold: A Modern Alchemy

NASA Astrophysics Data System (ADS)

Haruta, Masatake

Gold has long been neglected as a catalyst because of its chemical inertness. However, when gold is deposited as nanoparticles on carbon and polymer materials as well as on base metal oxides and hydroxides, it exhibits unique catalytic properties for many reactions such as CO oxidation at a temperature as low as 200 K, gas phase direct epoxidation of propylene, and aerobic oxidation of glucose to gluconic acid. The structure-catalytic activity correlations are discussed with emphasis on the contact structure, support selection, and the size control of gold particles. Gold clusters with diameters smaller than 2 nm are expected to exhibit novel properties in catalysis, optics, and electronics depending on the size (number of atoms), shape, and the electronic and chemical interaction with the support materials. The above achievements and attempts can be regarded as a modern alchemy that creates valuables by means of the noblest element with little practical use.

Plasmon-driven sequential chemical reactions in an aqueous environment.

PubMed

Zhang, Xin; Wang, Peijie; Zhang, Zhenglong; Fang, Yurui; Sun, Mengtao

2014-06-24

Plasmon-driven sequential chemical reactions were successfully realized in an aqueous environment. In an electrochemical environment, sequential chemical reactions were driven by an applied potential and laser irradiation. Furthermore, the rate of the chemical reaction was controlled via pH, which provides indirect evidence that the hot electrons generated from plasmon decay play an important role in plasmon-driven chemical reactions. In acidic conditions, the hot electrons were captured by the abundant H(+) in the aqueous environment, which prevented the chemical reaction. The developed plasmon-driven chemical reactions in an aqueous environment will significantly expand the applications of plasmon chemistry and may provide a promising avenue for green chemistry using plasmon catalysis in aqueous environments under irradiation by sunlight.

Interplay of catalysis, fidelity, threading, and processivity in the exo- and endonucleolytic reactions of human exonuclease I

PubMed Central

Shi, Yuqian; Hellinga, Homme W.; Beese, Lorena S.

2017-01-01

Human exonuclease 1 (hExo1) is a member of the RAD2/XPG structure-specific 5′-nuclease superfamily. Its dominant, processive 5′–3′ exonuclease and secondary 5′-flap endonuclease activities participate in various DNA repair, recombination, and replication processes. A single active site processes both recessed ends and 5′-flap substrates. By initiating enzyme reactions in crystals, we have trapped hExo1 reaction intermediates that reveal structures of these substrates before and after their exo- and endonucleolytic cleavage, as well as structures of uncleaved, unthreaded, and partially threaded 5′ flaps. Their distinctive 5′ ends are accommodated by a small, mobile arch in the active site that binds recessed ends at its base and threads 5′ flaps through a narrow aperture within its interior. A sequence of successive, interlocking conformational changes guides the two substrate types into a shared reaction mechanism that catalyzes their cleavage by an elaborated variant of the two-metal, in-line hydrolysis mechanism. Coupling of substrate-dependent arch motions to transition-state stabilization suppresses inappropriate or premature cleavage, enhancing processing fidelity. The striking reduction in flap conformational entropy is catalyzed, in part, by arch motions and transient binding interactions between the flap and unprocessed DNA strand. At the end of the observed reaction sequence, hExo1 resets without relinquishing DNA binding, suggesting a structural basis for its processivity. PMID:28533382

Enzyme Catalysis To Power Micro/Nanomachines

PubMed Central

2016-01-01

Enzymes play a crucial role in many biological processes which require harnessing and converting free chemical energy into kinetic forces in order to accomplish tasks. Enzymes are considered to be molecular machines, not only because of their capability of energy conversion in biological systems but also because enzymatic catalysis can result in enhanced diffusion of enzymes at a molecular level. Enlightened by nature’s design of biological machinery, researchers have investigated various types of synthetic micro/nanomachines by using enzymatic reactions to achieve self-propulsion of micro/nanoarchitectures. Yet, the mechanism of motion is still under debate in current literature. Versatile proof-of-concept applications of these enzyme-powered micro/nanodevices have been recently demonstrated. In this review, we focus on discussing enzymes not only as stochastic swimmers but also as nanoengines to power self-propelled synthetic motors. We present an overview on different enzyme-powered micro/nanomachines, the current debate on their motion mechanism, methods to provide motion and speed control, and an outlook of the future potentials of this multidisciplinary field. PMID:27666121

Enzyme Catalysis To Power Micro/Nanomachines.

PubMed

Ma, Xing; Hortelão, Ana C; Patiño, Tania; Sánchez, Samuel

2016-10-25

Enzymes play a crucial role in many biological processes which require harnessing and converting free chemical energy into kinetic forces in order to accomplish tasks. Enzymes are considered to be molecular machines, not only because of their capability of energy conversion in biological systems but also because enzymatic catalysis can result in enhanced diffusion of enzymes at a molecular level. Enlightened by nature's design of biological machinery, researchers have investigated various types of synthetic micro/nanomachines by using enzymatic reactions to achieve self-propulsion of micro/nanoarchitectures. Yet, the mechanism of motion is still under debate in current literature. Versatile proof-of-concept applications of these enzyme-powered micro/nanodevices have been recently demonstrated. In this review, we focus on discussing enzymes not only as stochastic swimmers but also as nanoengines to power self-propelled synthetic motors. We present an overview on different enzyme-powered micro/nanomachines, the current debate on their motion mechanism, methods to provide motion and speed control, and an outlook of the future potentials of this multidisciplinary field.

Aurophilicity in gold(I) catalysis: for better or worse?

PubMed

Weber, Dieter; Gagné, Michel R

2015-01-01

This book chapter discusses the effects of aurophilicity on gold catalysis. First, a brief historic account of aurophilicity in organogold chemistry is given, focusing on the pioneering results which set the stage for its association with catalytic intermediates (gold vinyl and gold aryl complexes); this is followed by an introduction to cationic gold(I) as an electrophilic catalyst, and the first isolation of organogold intermediates from catalysis. In the main section, the growing number of reports observing aurophilic interactions in catalysis or illustrative model systems is reviewed in a non-comprehensive tutorial way. The effects of aurophilicity are discussed in the following structures: (1) the geminal diauration of C(sp²)-atoms; (2) geminal diauration of other atoms; (3) σ-π-diauration of terminal alkynes. It is apparent that in most cases efficient catalysis is hindered by aurophilic effects as less active aggregates tend to be formed from more active species [LAu]⁺, but there are a growing number of reports using aurophilicity as a driving force to access new reactivity and selectivity.

Elucidation of reaction mechanism for m -cresol hydrodeoxygenation over Fe based catalysts: A kinetic study

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hong, Yongchun; Wang, Yong

Fe based catalysts are promising for hydrodeoxygenation (HDO) of lignin derived phenolics due to their high selectivity for aromatics. In this work, the reaction mechanism of m-cresol HDO on Fe catalysts and the kinetic consequence with Pd addition were elucidated by examining the effect of H2, H2O and m-cresol pressures on toluene formation rate on Fe and PdFe catalysts. A direct CO bond cleavage mechanism is proposed for HDO catalysis on both Fe and PdFe catalysts, while Pd provides a facilitated reaction pathway at the PdFe interface and therefore promotes the catalysis on Fe without changing the high selectivity towardsmore » aromatics.« less

Understanding the effects of cationic dopants on α-MnO 2 oxygen reduction reaction electrocatalysis

DOE PAGES

Lambert, Timothy N.; Vigil, Julian A.; White, Suzanne E.; ...

2017-01-09

Nickel-doped α-MnO 2 nanowires (Ni–α-MnO 2) were prepared with 3.4% or 4.9% Ni using a hydrothermal method. A comparison of the electrocatalytic data for the oxygen reduction reaction (ORR) in alkaline electrolyte versus that obtained with α-MnO 2 or Cu–α-MnO 2 is provided. In general, Ni-α-MnO 2 (e.g., Ni-4.9%) had higher n values (n = 3.6), faster kinetics (k = 0.015 cm s –1), and lower charge transfer resistance (R CT = 2264 Ω at half-wave) values than MnO 2 (n = 3.0, k = 0.006 cm s –1, R CT = 6104 Ω at half-wave) or Cu–α-MnO 2 (Cu-2.9%,more » n = 3.5, k = 0.015 cm s –1, R CT = 3412 Ω at half-wave), and the overall activity for Ni–α-MnO 2 trended with increasing Ni content, i.e., Ni-4.9% > Ni-3.4%. As observed for Cu–α-MnO 2, the increase in ORR activity correlates with the amount of Mn 3+ at the surface of the Ni–α-MnO 2 nanowire. Examining the activity for both Ni–α-MnO 2 and Cu–α-MnO 2 materials indicates that the Mn 3+ at the surface of the electrocatalysts dictates the activity trends within the overall series. Single nanowire resistance measurements conducted on 47 nanowire devices (15 of α-MnO 2, 16 of Cu–α-MnO 2-2.9%, and 16 of Ni–α-MnO 2-4.9%) demonstrated that Cu-doping leads to a slightly lower resistance value than Ni-doping, although both were considerably improved relative to the undoped α-MnO 2. As a result, the data also suggest that the ORR charge transfer resistance value, as determined by electrochemical impedance spectroscopy, is a better indicator of the cation-doping effect on ORR catalysis than the electrical resistance of the nanowire.« less

Mono and bimetallic nanoparticles of gold, silver and palladium-catalyzed NADH oxidation-coupled reduction of Eosin-Y

NASA Astrophysics Data System (ADS)

Santhanalakshmi, J.; Venkatesan, P.

2011-02-01

Mono metallic (Au, Ag, Pd) and bimetallic (Au-Ag, Ag-Pd, Au-Pd) with 1:1 mol stoichiometry, nanoparticles are synthesized using one-pot, temperature controlled chemical method using cetyltrimethylammonium bromide (CTAB) as the capping agent. The particle sizes (Au = 5.6, Ag = 5.0, Pd = 6.0, Au-Ag = 9.2, Ag-Pd = 9.6, Au-Pd = 9.4 nm) are characterized by UV-Vis, HRTEM, and XRD measurements, respectively. CTAB bindings onto mono and bimetallic nanoparticles are analyzed by FTIR spectra. The catalytic activities of mono and bimetallic nanoparticles are tested on the reaction between NADH oxidation and Eosin-Y reduction. The effects of base, pH, ionic strength, nature of mono and bimetallic catalysts are studied and the reaction conditions are optimized. Bimetallic nanoparticles exhibited better catalysis than the mono metallic nanoparticles, which may be due to the electronic effects of the core to shell metal atoms.

Multifunctional cellulase catalysis targeted by fusion to different carbohydrate-binding modules

DOE PAGES

Walker, Johnnie A.; Takasuka, Taichi E.; Deng, Kai; ...

2015-12-21

Carbohydrate binding modules (CBMs) bind polysaccharides and help target glycoside hydrolases catalytic domains to their appropriate carbohydrate substrates. To better understand how CBMs can improve cellulolytic enzyme reactivity, representatives from each of the 18 families of CBM found in Ruminoclostridium thermocellum were fused to the multifunctional GH5 catalytic domain of CelE (Cthe_0797, CelEcc), which can hydrolyze numerous types of polysaccharides including cellulose, mannan, and xylan. Since CelE is a cellulosomal enzyme, none of these fusions to a CBM previously existed. CelEcc_CBM fusions were assayed for their ability to hydrolyze cellulose, lichenan, xylan, and mannan. Several CelEcc_CBM fusions showed enhanced hydrolyticmore » activity with different substrates relative to the fusion to CBM3a from the cellulosome scaffoldin, which has high affinity for binding to crystalline cellulose. Additional binding studies and quantitative catalysis studies using nanostructure-initiator mass spectrometry (NIMS) were carried out with the CBM3a, CBM6, CBM30, and CBM44 fusion enzymes. In general, and consistent with observations of others, enhanced enzyme reactivity was correlated with moderate binding affinity of the CBM. Numerical analysis of reaction time courses showed that CelEcc_CBM44, a combination of a multifunctional enzyme domain with a CBM having broad binding specificity, gave the fastest rates for hydrolysis of both the hexose and pentose fractions of ionic-liquid pretreated switchgrass. In conclusion, we have shown that fusions of different CBMs to a single multifunctional GH5 catalytic domain can increase its rate of reaction with different pure polysaccharides and with pretreated biomass. This fusion approach, incorporating domains with broad specificity for binding and catalysis, provides a new avenue to improve reactivity of simple combinations of enzymes within the complexity of plant biomass.« less

The mechanisms of oxygen reduction and evolution reactions in nonaqueous lithium-oxygen batteries.

PubMed

Cao, Ruiguo; Walter, Eric D; Xu, Wu; Nasybulin, Eduard N; Bhattacharya, Priyanka; Bowden, Mark E; Engelhard, Mark H; Zhang, Ji-Guang

2014-09-01

A fundamental understanding of the mechanisms of both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) in nonaqueous lithium-oxygen (Li-O2) batteries is essential for the further development of these batteries. In this work, we systematically investigate the mechanisms of the ORR/OER reactions in nonaqueous Li-O2 batteries by using electron paramagnetic resonance (EPR) spectroscopy, using 5,5-dimethyl-pyrroline N-oxide as a spin trap. The study provides direct verification of the formation of the superoxide radical anion (O2(˙-)) as an intermediate in the ORR during the discharge process, while no O2(˙-) was detected in the OER during the charge process. These findings provide insight into, and an understanding of, the fundamental reaction mechanisms involving oxygen and guide the further development of this field. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Generating carbyne equivalents with photoredox catalysis

NASA Astrophysics Data System (ADS)

Wang, Zhaofeng; Herraiz, Ana G.; Del Hoyo, Ana M.; Suero, Marcos G.

2018-02-01

Carbon has the unique ability to bind four atoms and form stable tetravalent structures that are prevalent in nature. The lack of one or two valences leads to a set of species—carbocations, carbanions, radicals and carbenes—that is fundamental to our understanding of chemical reactivity. In contrast, the carbyne—a monovalent carbon with three non-bonded electrons—is a relatively unexplored reactive intermediate; the design of reactions involving a carbyne is limited by challenges associated with controlling its extreme reactivity and the lack of efficient sources. Given the innate ability of carbynes to form three new covalent bonds sequentially, we anticipated that a catalytic method of generating carbynes or related stabilized species would allow what we term an ‘assembly point’ disconnection approach for the construction of chiral centres. Here we describe a catalytic strategy that generates diazomethyl radicals as direct equivalents of carbyne species using visible-light photoredox catalysis. The ability of these carbyne equivalents to induce site-selective carbon-hydrogen bond cleavage in aromatic rings enables a useful diazomethylation reaction, which underpins sequencing control for the late-stage assembly-point functionalization of medically relevant agents. Our strategy provides an efficient route to libraries of potentially bioactive molecules through the installation of tailored chiral centres at carbon-hydrogen bonds, while complementing current translational late-stage functionalization processes. Furthermore, we exploit the dual radical and carbene character of the generated carbyne equivalent in the direct transformation of abundant chemical feedstocks into valuable chiral molecules.

The crystal structure of xanthine oxidoreductase during catalysis: Implications for reaction mechanism and enzyme inhibition

PubMed Central

Okamoto, Ken; Matsumoto, Koji; Hille, Russ; Eger, Bryan T.; Pai, Emil F.; Nishino, Takeshi

2004-01-01

Molybdenum is widely distributed in biology and is usually found as a mononuclear metal center in the active sites of many enzymes catalyzing oxygen atom transfer. The molybdenum hydroxylases are distinct from other biological systems catalyzing hydroxylation reactions in that the oxygen atom incorporated into the product is derived from water rather than molecular oxygen. Here, we present the crystal structure of the key intermediate in the hydroxylation reaction of xanthine oxidoreductase with a slow substrate, in which the carbon–oxygen bond of the product is formed, yet the product remains complexed to the molybdenum. This intermediate displays a stable broad charge–transfer band at ≈640 nm. The crystal structure of the complex indicates that the catalytically labile Mo—OH oxygen has formed a bond with a carbon atom of the substrate. In addition, the Mo⋕S group of the oxidized enzyme has become protonated to afford Mo—SH on reduction of the molybdenum center. In contrast to previous assignments, we find this last ligand at an equatorial position in the square-pyramidal metal coordination sphere, not the apical position. A water molecule usually seen in the active site of the enzyme is absent in the present structure, which probably accounts for the stability of this intermediate toward ligand displacement by hydroxide. PMID:15148401

Oxygen Reduction Reaction on PtCo Nanocatalyst: (Bi)sulfate Anion Poisoning

NASA Astrophysics Data System (ADS)

Liu, Jie; Huang, Yan

2018-05-01

Pt alloy electrocatalysts are susceptible to anion adsorption in the working environment of fuel cells. In this work, the unavoidable bisulfate and sulfate ((bi)sulfate) poisoning of the oxygen reduction reaction (ORR) on a common PtCo nanocatalyst was studied by the rotating disk electrode (RDE) technique, for the first time to the best of our knowledge. The specific activity decreases linearly with the logarithm of (bi)sulfate concentration under various high potentials. This demonstrates that the (bi)sulfate adsorption does not affect the free energy of ORR activation at a given potential. Moreover, it is speculated that these two conditions, the adsorption of one O2 molecule onto two Pt sites and this adsorption as a rate-determining step of ORR reaction, are unlikely to exist simultaneously.

Role of metal ions in catalysis by HIV integrase analyzed using a quantitative PCR disintegration assay.

PubMed

Diamond, Tracy L; Bushman, Frederic D

2006-01-01

Paired metal ions have been proposed to be central to the catalytic mechanisms of RNase H nucleases, bacterial transposases, Holliday junction resolvases, retroviral integrases and many other enzymes. Here we present a sensitive assay for DNA transesterification in which catalysis by human immunodeficiency virus-type 1 (HIV-1) integrase (IN) connects two DNA strands (disintegration reaction), allowing detection using quantitative PCR (qPCR). We present evidence suggesting that the three acidic residues of the IN active site function through metal binding using metal rescue. In this method, the catalytic acidic residues were each substituted with cysteines. Mn2+ binds tightly to the sulfur atoms of the cysteine residues, but Mg2+ does not. We found that Mn2+, but not Mg2+, could rescue catalysis of each cysteine-substituted enzyme, providing evidence for functionally important metal binding by all three residues. We also used the PCR-boosted assay to show that HIV-1 IN could carry out transesterification reactions involving DNA 5' hydroxyl groups as well as 3' hydroxyls as nucleophiles. Lastly, we show that Mn2+ by itself (i.e. without enzyme) can catalyze formation of a low level of PCR-amplifiable product under extreme conditions, allowing us to estimate the rate enhancement due to the IN-protein scaffold as at least 60 million-fold.

Role of metal ions in catalysis by HIV integrase analyzed using a quantitative PCR disintegration assay

PubMed Central

Diamond, Tracy L.; Bushman, Frederic D.

2006-01-01

Paired metal ions have been proposed to be central to the catalytic mechanisms of RNase H nucleases, bacterial transposases, Holliday junction resolvases, retroviral integrases and many other enzymes. Here we present a sensitive assay for DNA transesterification in which catalysis by human immunodeficiency virus-type 1 (HIV-1) integrase (IN) connects two DNA strands (disintegration reaction), allowing detection using quantitative PCR (qPCR). We present evidence suggesting that the three acidic residues of the IN active site function through metal binding using metal rescue. In this method, the catalytic acidic residues were each substituted with cysteines. Mn2+ binds tightly to the sulfur atoms of the cysteine residues, but Mg2+ does not. We found that Mn2+, but not Mg2+, could rescue catalysis of each cysteine-substituted enzyme, providing evidence for functionally important metal binding by all three residues. We also used the PCR-boosted assay to show that HIV-1 IN could carry out transesterification reactions involving DNA 5′ hydroxyl groups as well as 3′ hydroxyls as nucleophiles. Lastly, we show that Mn2+ by itself (i.e. without enzyme) can catalyze formation of a low level of PCR-amplifiable product under extreme conditions, allowing us to estimate the rate enhancement due to the IN-protein scaffold as at least 60 million-fold. PMID:17085478

Taking Ockham's razor to enzyme dynamics and catalysis.

PubMed

Glowacki, David R; Harvey, Jeremy N; Mulholland, Adrian J

2012-01-29

The role of protein dynamics in enzyme catalysis is a matter of intense current debate. Enzyme-catalysed reactions that involve significant quantum tunnelling can give rise to experimental kinetic isotope effects with complex temperature dependences, and it has been suggested that standard statistical rate theories, such as transition-state theory, are inadequate for their explanation. Here we introduce aspects of transition-state theory relevant to the study of enzyme reactivity, taking cues from chemical kinetics and dynamics studies of small molecules in the gas phase and in solution--where breakdowns of statistical theories have received significant attention and their origins are relatively better understood. We discuss recent theoretical approaches to understanding enzyme activity and then show how experimental observations for a number of enzymes may be reproduced using a transition-state-theory framework with physically reasonable parameters. Essential to this simple model is the inclusion of multiple conformations with different reactivity.

Catalysis by metal-organic frameworks: fundamentals and opportunities.

PubMed

Ranocchiari, Marco; van Bokhoven, Jeroen Anton

2011-04-14

Crystalline porous materials are extremely important for developing catalytic systems with high scientific and industrial impact. Metal-organic frameworks (MOFs) show unique potential that still has to be fully exploited. This perspective summarizes the properties of MOFs with the aim to understand what are possible approaches to catalysis with these materials. We categorize three classes of MOF catalysts: (1) those with active site on the framework, (2) those with encapsulated active species, and (3) those with active sites attached through post-synthetic modification. We identify the tunable porosity, the ability to fine tune the structure of the active site and its environment, the presence of multiple active sites, and the opportunity to synthesize structures in which key-lock bonding of substrates occurs as the characteristics that distinguish MOFs from other materials. We experience a unique opportunity to imagine and design heterogeneous catalysts, which might catalyze reactions previously thought impossible.

Consequences of acid strength for isomerization and elimination catalysis on solid acids.

PubMed

Macht, Josef; Carr, Robert T; Iglesia, Enrique

2009-05-13

We address here the manner in which acid catalysis senses the strength of solid acids. Acid strengths for Keggin polyoxometalate (POM) clusters and zeolites, chosen because of their accurately known structures, are described rigorously by their deprotonation energies (DPE). Mechanistic interpretations of the measured dynamics of alkane isomerization and alkanol dehydration are used to obtain rate and equilibrium constants and energies for intermediates and transition states and to relate them to acid strength. n-Hexane isomerization rates were limited by isomerization of alkoxide intermediates on bifunctional metal-acid mixtures designed to maintain alkane-alkene equilibrium. Isomerization rate constants were normalized by the number of accessible protons, measured by titration with 2,6-di-tert-butylpyridine during catalysis. Equilibrium constants for alkoxides formed by protonation of n-hexene increased slightly with deprotonation energies (DPE), while isomerization rate constants decreased and activation barriers increased with increasing DPE, as also shown for alkanol dehydration reactions. These trends are consistent with thermochemical analyses of the transition states involved in isomerization and elimination steps. For all reactions, barriers increased by less than the concomitant increase in DPE upon changes in composition, because electrostatic stabilization of ion-pairs at the relevant transition states becomes more effective for weaker acids, as a result of their higher charge density at the anionic conjugate base. Alkoxide isomerization barriers were more sensitive to DPE than for elimination from H-bonded alkanols, the step that limits 2-butanol and 1-butanol dehydration rates; the latter two reactions showed similar DPE sensitivities, despite significant differences in their rates and activation barriers, indicating that slower reactions are not necessarily more sensitive to acid strength, but instead reflect the involvement of more unstable organic

REDUCTION OF NITROSOBENZENES AND N-HYDROXYLANILINES BY FE (II) SPECIES: ELUCIDATION OF REACTION MECHANISM

EPA Science Inventory

There has been a substantial effort toward understanding the reduction of nitroaromatics in Fe(II)-treated ferric oxide systems, little has been done to gain insight into the factors controlling the transformation of their reaction intermediates, nitrosobenzenes and N-hydroxylani...

Asymmetric Aldol Additions: A Guided-Inquiry Laboratory Activity on Catalysis

ERIC Educational Resources Information Center

King, Jorge H. Torres; Wang, Hong; Yezierski, Ellen J.

2018-01-01

Despite the importance of asymmetric catalysis in both the pharmaceutical and commodity chemicals industries, asymmetric catalysis is under-represented in undergraduate chemistry laboratory curricula. A novel guided-inquiry experiment based on the asymmetric aldol addition was developed. Students conduct lab work to compare the effectiveness of…

Robustness of the Rotary Catalysis Mechanism of F1-ATPase*

PubMed Central

Watanabe, Rikiya; Matsukage, Yuki; Yukawa, Ayako; Tabata, Kazuhito V.; Noji, Hiroyuki

2014-01-01

F1-ATPase (F1) is the rotary motor protein fueled by ATP hydrolysis. Previous studies have suggested that three charged residues are indispensable for catalysis of F1 as follows: the P-loop lysine in the phosphate-binding loop, GXXXXGK(T/S); a glutamic acid that activates water molecules for nucleophilic attack on the γ-phosphate of ATP (general base); and an arginine directly contacting the γ-phosphate (arginine finger). These residues are well conserved among P-loop NTPases. In this study, we investigated the role of these charged residues in catalysis and torque generation by analyzing alanine-substituted mutants in the single-molecule rotation assay. Surprisingly, all mutants continuously drove rotary motion, even though the rotational velocity was at least 100,000 times slower than that of wild type. Thus, although these charged residues contribute to highly efficient catalysis, they are not indispensable to chemo-mechanical energy coupling, and the rotary catalysis mechanism of F1 is far more robust than previously thought. PMID:24876384

Radicalization and Radical Catalysis of Biomass Sugars: Insights from First-principles Studies

PubMed Central

Yang, Gang; Zhu, Chang; Zou, Xianli; Zhou, Lijun

2016-01-01

Ab initio and density functional calculations are conducted to investigate the radicalization processes and radical catalysis of biomass sugars. Structural alterations due to radicalization generally focus on the radicalized sites, and radicalization affects H-bonds in D-fructofuranose more than in D-glucopyranose, potentially with outcome of new H-bonds. Performances of different functionals and basis sets are evaluated for all radicalization processes, and enthalpy changes and Gibbs free energies for these processes are presented with high accuracy, which can be referenced for subsequent experimental and theoretical studies. It shows that radicalization can be utilized for direct transformation of biomass sugars, and for each sugar, C rather than O sites are always preferred for radicalization, thus suggesting the possibility to activate C-H bonds of biomass sugars. Radical catalysis is further combined with Brønsted acids, and it clearly states that functionalization fundamentally regulates the catalytic effects of biomass sugars. In presence of explicit water molecules, functionalization significantly affects the activation barriers and reaction energies of protonation rather than dehydration steps. Tertiary butyl and phenyl groups with large steric hindrances or hydroxyl and amino groups resulting in high stabilities for protonation products drive the protonation steps to occur facilely at ambient conditions. PMID:27405843

Radicalization and Radical Catalysis of Biomass Sugars: Insights from First-principles Studies.

PubMed

Yang, Gang; Zhu, Chang; Zou, Xianli; Zhou, Lijun

2016-07-13

Ab initio and density functional calculations are conducted to investigate the radicalization processes and radical catalysis of biomass sugars. Structural alterations due to radicalization generally focus on the radicalized sites, and radicalization affects H-bonds in D-fructofuranose more than in D-glucopyranose, potentially with outcome of new H-bonds. Performances of different functionals and basis sets are evaluated for all radicalization processes, and enthalpy changes and Gibbs free energies for these processes are presented with high accuracy, which can be referenced for subsequent experimental and theoretical studies. It shows that radicalization can be utilized for direct transformation of biomass sugars, and for each sugar, C rather than O sites are always preferred for radicalization, thus suggesting the possibility to activate C-H bonds of biomass sugars. Radical catalysis is further combined with Brønsted acids, and it clearly states that functionalization fundamentally regulates the catalytic effects of biomass sugars. In presence of explicit water molecules, functionalization significantly affects the activation barriers and reaction energies of protonation rather than dehydration steps. Tertiary butyl and phenyl groups with large steric hindrances or hydroxyl and amino groups resulting in high stabilities for protonation products drive the protonation steps to occur facilely at ambient conditions.

Oxygen Reduction Reaction Activity of Platinum Thin Films with Different Densities

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ergul, Busra; Begum, Mahbuba; Kariuki, Nancy

Platinum thin films with different densities were grown on glassy carbon electrodes by high pressure sputtering deposition and evaluated as oxygen reduction reaction catalysts for polymer electrolyte fuel cells using cyclic voltammetry and rotating disk electrode techniques in aqueous perchloric acid electrolyte. The electrochemically active surface area, ORR mass activity (MA) and specific activity (SA) of the thin film electrodes were obtained. MA and SA were found to be higher for low-density films than for high-density film.

Unlocking the potential of supported liquid phase catalysts with supercritical fluids: low temperature continuous flow catalysis with integrated product separation

PubMed Central

Franciò, Giancarlo; Hintermair, Ulrich; Leitner, Walter

2015-01-01

Solution-phase catalysis using molecular transition metal complexes is an extremely powerful tool for chemical synthesis and a key technology for sustainable manufacturing. However, as the reaction complexity and thermal sensitivity of the catalytic system increase, engineering challenges associated with product separation and catalyst recovery can override the value of the product. This persistent downstream issue often renders industrial exploitation of homogeneous catalysis uneconomical despite impressive batch performance of the catalyst. In this regard, continuous-flow systems that allow steady-state homogeneous turnover in a stationary liquid phase while at the same time effecting integrated product separation at mild process temperatures represent a particularly attractive scenario. While continuous-flow processing is a standard procedure for large volume manufacturing, capitalizing on its potential in the realm of the molecular complexity of organic synthesis is still an emerging area that requires innovative solutions. Here we highlight some recent developments which have succeeded in realizing such systems by the combination of near- and supercritical fluids with homogeneous catalysts in supported liquid phases. The cases discussed exemplify how all three levels of continuous-flow homogeneous catalysis (catalyst system, separation strategy, process scheme) must be matched to locate viable process conditions. PMID:26574523

2-Diazo-1-phenyl-2-((trifluoromethyl)sulfonyl)ethan-1-one: Another Utility for Electrophilic Trifluoromethylthiolation Reactions.

PubMed

Huang, Zhongyan; Okuyama, Kenta; Wang, Chen; Tokunaga, Etsuko; Li, Xiaorui; Shibata, Norio

2016-06-01

2-Diazo-1-phenyl-2-((trifluoromethyl)sulfonyl)ethan-1-one (diazo-triflone) ( 2 ) is not only a building block but also a reagent. In this study, diazo-triflone, which was originally used for the synthesis of β-lactam triflones as a trifluoromethanesulfonyl (SO 2 CF 3 ) building block under catalyst-free thermal conditions, is redisclosed as an effective electrophilic trifluoromethylthiolation reagent under copper catalysis. A broad set of enamines, indoles, β-keto esters, pyrroles, and anilines were nicely transformed into corresponding trifluoromethylthio (SCF 3 ) compounds in good to high yields by diazo-triflone under copper catalysis via an electrophilic-type reaction. A coupling-type trifluoromethylthiolation reaction of aryl iodides was also realized by diazo-triflone in acceptable yields.

At the Dawn of the 21st Century: Is Dynamics the Missing Link for Understanding Enzyme Catalysis?

PubMed Central

Kamerlin, Shina C. L.; Warshel, Arieh

2009-01-01

Enzymes play a key role in almost all biological processes, accelerating a variety of metabolic reactions as well as controlling energy transduction, transcription and translation of genetic information, and signaling. They possess the remarkable capacity to accelerate reactions by many orders of magnitude compared to their uncatalyzed counterparts, making feasible crucial processes that would otherwise not occur on biologically relevant timescales. Thus, there is broad interest in understanding the catalytic power of enzymes on a molecular level. Several proposals have been put forward to try to explain this phenomenon, and one that has rapidly gained momentum in recent years is the idea that enzyme dynamics somehow contributes to catalysis. This review examines the dynamical proposal in a critical way considering basically all reasonable definitions, including (but not limited to) such proposed effects as “coupling between conformational and chemical motions”, “landscape searches” and “entropy funnels”. It is shown that none of these proposed effects have been experimentally demonstrated to contribute to catalysis, nor are they supported by consistent theoretical studies. On the other hand, it is clarified that careful simulation studies have excluded most (if not all) dynamical proposals. This review places significant emphasis on clarifying the role of logical definitions of different catalytic proposals, and on the need for a clear formulation in terms of the assumed potential surface and reaction coordinate. Finally, it is pointed out that electrostatic preorganization actually accounts for the observed catalytic effects of enzymes, through the corresponding changes in the activation free energies. PMID:20099310