Selectivity Of Metal Catalysts Research Articles

Constructing Highly Efficient ZnO Nanocatalysts with Exposed Extraordinary (110) Facet for CO2 Electroreduction.

Electrochemical reduction of CO2 to highly valuable products is a promising way to reduce CO2 emissions. The shape and facets of metal nanocatalysts are the key parameters in determining the catalytic performance. However, the exposed crystal facets of ZnO with different morphologies and which facets achieve a high performance for CO2 reduction are still controversial. Here, we systematically investigate the effect of the facet-dependent reactivity of reduction of CO2 to CO on ZnO (nanowire, nanosheet, and flower-like). The ZnO nanosheet with exposed (110) facet exhibited prominent catalytic performance with a Faradaic efficiency of CO up to 84% and a current density of -10 mA cm-2 at -1.2 V versus RHE, far outperforming the ZnO nanowire (101) and ZnO nanoflower (103). Based on detailed characterizations and kinetic analysis, the ZnO nanosheet (110) with porous architecture increased the exposure of active sites. Further studies revealed that the high CO selectivity originated from the enhancement of CO2 adsorption and activation on the ZnO (110) facet, which promoted the conversion of CO2 toward CO. This study provides a new way to tailor the activity and selectivity of metal catalysts by engineering exposed specific facets.

Oxidative cross-coupling of thiols for S-X (X = S, N, O, P, and C) bond formation: mechanistic aspects.

This review focuses on the reactive intermediates (disulfides, sulfenyl halides, thiyl radicals, sulfenium cations, and metal-organosulfur species) and the mechanisms of the recently reported oxidative couplings of thiols. These intermediates are generated by chemical oxidants, transition metal catalysts, electrochemistry, and photochemistry. Chemical oxidant-mediated reactions involve radical, halogenated, or cationic intermediates, or disulfides. Transition metal-catalyzed mechanisms proposed various metal-organosulfur intermediates to elucidate the reactivity and selectivity of metal catalysts. In electro- and photooxidation, direct oxidation/reduction mechanisms of reactants at the electrode or indirect oxidation/reduction of reactants in the presence of redox catalysts have been reported. The following sections are based on the products, thiosulfonates (S-S bond), sulfenamides, sulfinamides, and sulfonamides (S-N bond), sulfinates (S-O bond), thiophosphine oxides and thiophosphates (S-P bond), and sulfides, sulfoxides, and sulfones (S-C bond) and discuss the reaction mechanisms and the above-mentioned key intermediates for product formation. The contents of this review will provide helpful information, guiding the choice of oxidative coupling conditions for the synthesis of various organosulfur compounds with high yields and selectivity.

Rational design of selective metal catalysts for alcohol amination with ammonia

The lack of selectivity for the direct amination of alcohols with ammonia (a modern and clean route for the synthesis of primary amines) is an unsolved problem. Here, we combine first-principles calculations, scaling relations, kinetic simulations and catalysis experiments to determine the key factors that govern the activity and selectivity of metal catalysts for this reaction. We show that the loss of selectivity towards primary amines is linked to a surface-mediated C–N bond coupling between two N-containing intermediates: CH3NH and CH2NH. The barrier for this step is low enough to compete with the main surface hydrogenation reactions and it can be used as a descriptor for selectivity. The activity and selectivity maps (using the C and O adsorption energies as descriptors) were combined for the computational screening of 348 dilute bimetallic catalysts. Among the best theoretical candidates, Co98.5Ag1.5 and Co98.5Ru1.5 (5 wt% Co) were identified experimentally to be the most promising catalysts.

Ethylene adsorption on Ag(111), Rh(111) and Ir(111) by (meta)-GGA based density functional theory calculations

Accurate description of the adsorption process of reactants on metal surfaces from theory is crucial for mechanistic understanding of activity and selectivity of metal catalysts, but it remains challengeable for the nowadays first-principles theory due to the lack of proper exchange-correlation functional describing the distinct interactions involved. We studied here the potential energy surfaces of ethylene adsorption on Ag(111), Rh(111) and Ir(111) using density functional theory calculations and (meta)-GGA functional including PBE, BEEF-vdW, SCAN, and SCAN+rVV10. For ethylene adsorption on noble metal Ag(111), it is found that BEEF-vdW, SCAN and SCAN+rVV10 predict the presence of the physisorption states only. For Rh(111), both SCAN and SCAN+rVV10 find that there is a precursor physisorption state before the chemisorption state. In contrast, there is no precursor state found based on potential energy surfaces from BEEF-vdW and PBE. Whereas for Ir(111), BEEF-vdW predicts the existence of a rather shallow precursor physisorption state, in addition to the chemisorption state. Irrespective to the transition metals considered, we find that SCAN+rVV10 gives the strongest binding strength, followed by SCAN, and PBE/BEEF-vdW, accordingly. The present work highlights great dependence of potential energy surface of ethylene adsorption on transition metal surfaces and exchange-correlation functionals.

Arylruthenium(III) Porphyrin-Catalyzed C-H Oxidation and Epoxidation at Room Temperature and [RuV(Por)(O)(Ph)] Intermediate by Spectroscopic Analysis and Density Functional Theory Calculations.

The development of highly active and selective metal catalysts for efficient oxidation of hydrocarbons and identification of the reactive intermediates in the oxidation catalysis are long-standing challenges. In the rapid hydrocarbon oxidation catalyzed by ruthenium(IV) and -(III) porphyrins, the putative Ru(V)-oxo intermediates remain elusive. Herein we report that arylruthenium(III) porphyrins are highly active catalysts for hydrocarbon oxidation. Using catalyst [RuIII(TDCPP)(Ph)(OEt2)] (H2TDCPP = 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin), the oxidation of C-H bonds of various hydrocarbons with oxidant m-CPBA at room temperature gave alcohols/ketones in up to 99% yield within 1 h; use of [ nBu4N]IO4 as a mild alternative oxidant avoided formation of lactone from cyclic ketone in C-H oxidation, and the catalytic epoxidation with up to 99% yield and high selectivity (no aldehydes as side product) was accomplished within 5 min. UV-vis, electrospray ionization-mass spectrometry, resonance Raman, electron paramagnetic resonance, and kinetic measurements and density functional theory calculations lend evidence for the formation of Ru(V)-oxo intermediate [RuV(TDCPP)(O)(Ph)].

Synthesis and Stabilization of Supported Metal Catalysts by Atomic Layer Deposition

Supported metal nanoparticles are among the most important catalysts for many practical reactions, including petroleum refining, automobile exhaust treatment, and Fischer-Tropsch synthesis. The catalytic performance strongly depends on the size, composition, and structure of the metal nanoparticles, as well as the underlying support. Scientists have used conventional synthesis methods including impregnation, ion exchange, and deposition-precipitation to control and tune these factors, to establish structure-performance relationships, and to develop better catalysts. Meanwhile, chemists have improved the stability of metal nanoparticles against sintering by the application of protective layers, such as polymers and oxides that encapsulate the metal particle. This often leads to decreased catalytic activity due to a lack of precise control over the thickness of the protective layer. A promising method of catalyst synthesis is atomic layer deposition (ALD). ALD is a variation on chemical vapor deposition in which metals, oxides, and other materials are deposited on surfaces by a sequence of self-limiting reactions. The self-limiting character of these reactions makes it possible to achieve uniform deposits on high-surface-area porous solids. Therefore, design and synthesis of advanced catalysts on the nanoscale becomes possible through precise control over the structure and composition of the underlying support, the catalytic active sites, and the protective layer. In this Account, we describe our advances in the synthesis and stabilization of supported metal catalysts by ALD. After a short introduction to the technique of ALD, we show several strategies for metal catalyst synthesis by ALD that take advantage of its self-limiting feature. Monometallic and bimetallic catalysts with precise control over the metal particle size, composition, and structure were achieved by combining ALD sequences, surface treatments, and deposition temperature control. Next, we describe ALD oxide overcoats applied with atomically precise thickness control that stabilize metal catalysts while preserving their catalytic function. We also discuss strategies for generation and control over the porosity of the overcoats that allow the embedded metal particles to remain accessible by reactants, and the details for ALD alumina overcoats on metal catalysts. Moreover, using methanol decomposition and oxidative dehydrogenation of ethane as probe reactions, we demonstrate that selectively blocking low coordination metal sites by oxide overcoats can provide another strategy to enhance both the durability and selectivity of metal catalysts.

Selective alcohol oxidation to aldehydes and ketones over base-promoted gold–palladium clusters as recyclable quasihomogeneous and heterogeneous metal catalysts

Bimetallic gold–palladium clusters, with an average size of 1.9 nm and composed of 80 mol% gold, proved to be highly active and selective metal catalysts for the organic phase oxidation with O 2 of aliphatic, allylic and benzylic alcohols to the corresponding carbonyl products. Polyvinylpyrrolidone stabilized gold–palladium clusters dispersed in N, N-dimethylformamide emerged as promising quasihomogeneous metal catalysts for the oxidation of benzyl alcohol to benzaldehyde with full selectivity; they could be efficiently recycled with unaffected catalytic performance by solvent-resistant nanofiltration. Highly active and durable heterogeneous catalysts for the amide phase or solvent-free alcohol oxidation were prepared by the quantitative immobilization of the optimized gold–palladium clusters on the high surface area basic BaAl 2O 4 spinel support with preservation of the bimetallic clusters’ nanodispersion.

ChemInform Abstract: Highly Selective Metal Catalysts for Intermolecular Carbenoid Insertion into Primary C—H Bonds and Enantioselective C—C Bond Formation.

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Highly Selective Metal Catalysts for Intermolecular Carbenoid Insertion into Primary CH Bonds and Enantioselective CC Bond Formation

Primäre C-H-Aktivierung! Der Rhodiumkomplex I eines Doppeltaschenporphyrins katalysiert die Carbenoidinsertion in die C-H-Bindungen von n-Alkanen mit primär/sekundär-Selektivitäten bis 11.4:1 (pro C-H-Bindung, siehe Bild). Die Funktionalisierung sekundärer C-H-Bindungen mit einem Rhodiumkomplex von Haltermans chiralem Porphyrin als Katalysator ergab maximal 93 % ee. Bei diesen Reaktionen wurden auch nach fünffacher Katalysatorrückgewinnung noch Umsatzzahlen von 6477 erreicht.

Inside Cover: Highly Selective Metal Catalysts for Intermolecular Carbenoid Insertion into Primary CH Bonds and Enantioselective CC Bond Formation (Angew. Chem. Int. Ed. 50/2008)

Inside Cover: Highly Selective Metal Catalysts for Intermolecular Carbenoid Insertion into Primary CH Bonds and Enantioselective CC Bond Formation (Angew. Chem. Int. Ed. 50/2008)

Highly Selective Metal Catalysts for Intermolecular Carbenoid Insertion into Primary CH Bonds and Enantioselective CC Bond Formation

Primary CH bond activation! A Rh complex of bis-pocket porphyrin I catalyzes carbenoid insertion into the CH bonds of n-alkanes with a primary/secondary selectivity (per CH bond) of up to 11.4:1 (see picture). Enantioselective secondary CH bond functionalization catalyzed by a Rh complex of Halterman's chiral porphyrin features up to 93 % ee. These reactions exhibit up to 6477 turnovers after the catalyst was recycled five times. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Structure and selectivity of metal catalysts: revisiting bimetallic zeolite systems

The structures of Pt–Co/NaY and Ru–Co/NaY have been investigated by X-ray photoelectron spectroscopy (XPS), Extended X-ray absorption fine spectra (EXAFS) and temperature programmed spectroscopy. Formation of the bimetallic particles inside the zeolite framework, the effect of pre-treatments using O 2/H 2, He/H 2 and He/H 2/O 2/H 2 on the structure of the small bimetallic particles have also been studied. The structure of the samples was related to the activity and selectivity in CO hydrogenation. Under mild oxygen treatment the Pt–Co bimetallic particles are separated leaving the platinum particles in the supercage and transferring the Co 2+ ions into the sodalite cage or into the hexagonal prisms. Direct oxygen treatment on Ru/NaY results in the migration of the ruthenium particles to the external surface of the NaY, while the presence of cobalt in Ru–Co/NaY stabilizes the ruthenium inside the supercage. Conversely, EXAFS and XPS measurements indicate the reduction of Co 2+ ions facilitated by ruthenium and platinum, but the catalyst structure can be described by a ‘cherry-type’ model containing cobalt as its kernel. As a result the major difference between mono- and bimetallic samples is mainly controlled by the location of the metal particles and the extent of reducibility of Co 2+ ions. The olefin/paraffin ratio changes depending on the location of the metal particles, and the chain growth depends on the residence time of the species inside the zeolite. The major parameters governing the general behavior of zeolite-supported bimetallic particles, are discussed.

Non-faradaic electrochemical modification of catalytic activity in solid electrolyte cells

The catalytic activity and selectivity of metal catalysts used as electrodes in high temperature solid electrolyte cells can be altered dramatically and in a reversible manner. This is accomplished by electrochemically supplying oxygen anions onto catalytic surfaces via polarized metal-solid electrolyte interfaces. Oxygen anions, forced electrochemically to adsorb on the metal catalyst surface, alter the catalyst work function in a predictable way and lead to reaction rate increases as high as 4000%. Changes in catalytic rates typically exceed the rate of O2− transport to or from the catalyst surface by 102-3 · 105. Significant changes in product selectivity have been also observed. The case of several catalytic reactions in which this new phenomenon has been observed is presented and the origin of the phenomenon is discussed.

Non-Faradaic electrochemical modification of catalytic activity

It was found that catalytic activity and selectivity of metal catalysts can be altered dramatically and in a reversible manner. This is accomplished by electrochemically supplying oxygen anions onto catalytic surfaces via polarized solid electrolyte cells. Oxygen anions, forced electrochemically to adsorb on the catalyst surface, alter the catalyst work function in a predictable way and lead to reaction rate increases of the order of 1500%. Changes in catalytic reaction rates typically exceed the rate of O/sup 2 -/ transport to or from the catalyst surface by 10/sup 2/-10/sup 4/. The case of several catalytic reactions in which this new phenomenon has been observed is discussed.

Catalytic and electrocatalytic reactions in solid oxide fuel cells

Abstract Solid oxide electrolyte cells operating on H2 and CO as the fuel have been studied extensively in recent years and may soon provide a viable option for power generation. It has been shown recently that the same type of cells, with appropriate catalytic electrodes, can be used to carry out a number of industrially important complete or partial oxidation reactions. In this “chemical cogeneration” mode of operation valuable chemicals and electrical power are produced simultaneously. Using similar solid oxide cells operating in the oxygen pump mode, it has been found recently that the rate and product selectivity of several catalytic reactions can be altered significantly. This reversible electrochemical modification of the activity and selectivity of metal catalysts is of significant theoretical and practical interest. Progress in the areas of the use of solid electrolyte cells for chemical cogeneration and for catalytic activity and selectivity enhancement is reviewed, including some very recent unpublished results. The use of solid oxide cells for mechanistic studies of catalytic phenomena is also briefly surveyed.

Synthesis of oxygenates from CO and hydrogen over supported metal clusters

Bimetallic catalysts play an important role in many industrial catalytic processes and represent an area of intense research interest. Bimetallic systems give an opportunity to investigate geometrical versus electronic or ligand effects in catalysis. Studies of these effects can help to elucidate the key factors controlling the activity and more importantly, the selectivity of metal catalysts. For the designer of catalysts, adding a second metal component allows to systematically alter the size or electronic structure of catalytic surface ensembles. One could envision modifications of the adsorption characteristics, of the surface coverage with reactive intermediates, and consequently of the selectivity. The presence of a second metal component can make its influence felt in terms of changes in reducibility of the catalyst. Furthermore, the second metal component can in certain cases alter the deactivation behavior, a fact that has proven to be beneficial in bimetallic reforming catalysts. This chapter focuses on the modification of surface sites by addition of a second metal component and on the direct conversion of synthesis gas over organometallic clusters or cluster-derived catalysts that are not entirely subject to the Anderson-Schultz-Flory polymerization kinetics. The chapter reviews the methanation process, Fischer-Tropsch catalysts modified by addition of a second metal component, and bimetallic catalysts derived from organometallic clusters. It seeks to present an overview of supported bimetallic CO activation catalysts and to highlight major trends and fundamental principles.