Exploring the molecular mechanisms of reactions at surfaces (original) (raw)
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Transformations of Organic Molecules over Metal Surfaces: Insights from Computational Catalysis
Chemical Record, 2016
Much-needed progress in catalytic science, in particular regarding heterogeneous catalysis, is associated with the transition from largely empirical research to rational design of new and improved catalysts and catalytic processes. To achieve this goal, fundamental atomic-scale understanding of catalytic processes is required, which can be achieved with the help of theoretical modeling, in particular, using methods based on quantum chemical calculations. In this review we illustrate the current progress by discussing examples from the authors' work in which complex reaction networks involving organic molecules on transition-metal surfaces have been studied using density functional theory. We review some of the success stories where theory helped to interpret experimental observations and provided atomistic insights into the mechanisms, which were not definitively known before. In other cases, partial disagreement between theoretical results and existing experimental evidence calls for further reconciliation studies.
2007
We could naively think that the best catalyst is the one that makes the reaction as fast as possible but this is not the case. The best catalyst is the one that works the way we expect, at the speed we need: an explosive reaction can be less interesting than a slow one. Also, a catalyst that accelerates unwanted reactions that may spoil (poison) the catalytic device is certainly noxious. Schematically, a reaction catalyzed by a solid surface can be described by three elementary processes: adsorption of the reactants, formation of the products on the catalyst's surface, and desorption of the products. Reactants form an activated complex, called the transition state, that decomposes into products. Transition state corresponds to a maximum in the energy profile plot from initial to final state. The height of the energy gap between transition state and initial state is called energy barrier and is equal High Resolution Scanning Electron Microscopy, Field Ion Microscopy (FIM), Scanning Tunnelling Microscopy (STM). In figure I.2 we can see a practical example of STM utility in understanding microscopic details of a reacting surface. These different types of experiments have provided us with a wealth of detailed information about surface structures, adsorption geometries, bond strengths and elementary reactions steps. A realistic heterogeneous catalyst consists in a disordered surface with many facets, defects, terraces with different behaviors among the various Theoretical modeling of surfaces At the present time theoretical study of surface properties is achieved mainly through computational methods that practically consist in large-scale computer simulations of a set of atoms interacting and positioned in a defined space at our will. The choice of the parameters and conditions determin-Water on transition metals Water is perhaps the most important and most pervasive chemical of our planet. The influence of water permeates virtually all areas of biochemical, chemical and physical importance, and is especially evident in phenomena occurring at the interfaces of solid surfaces. The progress in understanding the properties of water on solid surfaces is evident both in areas for which surface science methodology has traditionally been strong (catalysis and electronic materials) and also in new areas not traditionally studied by surface In this thesis we will present two different studies involving water reactions on transition metal surfaces. The first one involves study of water dissociation on flat and stepped platinum surfaces through the estimate of energy barriers for different reaction pathways. Data at the present time published
2016
Author(s): Avanesian, Talin | Advisor(s): Christopher, Phillip | Abstract: In this work we utilized quantum chemical calculations coupled with numerical and analytical models to predict macroscopic observables associated with catalytic and photocatalytic processes on transition metal surfaces. All the predicted macroscopic observables were validated based on experimental measurements. The theoretical models developed here provide atomic scale insights into the mechanisms of catalytic and photocatalytic processes and suggest areas for future research in the design of novel catalysts.In the first part of this dissertation, we focused on understanding characteristics that control performance of late transition metals for the catalytic reduction of CO2 by H2. By coupling Density Functional Theory (DFT) calculations with mean-field microkinetic models we found that on Ru-based catalysts CHO* dissociation to CH* and O* is the rate determining step (RDS) for CH4 formation, while CO* desorp...
Microchimica Acta, 2006
Heterogeneous catalysis is one of the fields of modern technology, in which a characterization of structural and chemical properties of solid surfaces at the microscopic level is of enormous importance. For a long time, such insights have been precluded by the complexity of most catalytically active materials. Recently, substantial progress has been made, however, toward a microscopic-level understanding of complex catalyst surfaces. We discuss the driving factors for these advancements, which are based on the development of new well-defined model systems as well as on advances in experimental technology and theory. Scrutinizing the example of planar model catalysts, we identify the process of linking structural and chemical information to microscopic reaction kinetics as a particular challenging aspect of today's work. We review the kinetic effects which may have an influence on the reaction kinetics on complex surfaces. As an example how structural and kinetic information can be correlated at the microscopic level we discuss the case of surface oxidation and oxygen induced restructuring of Pd nanoparticles as studied by molecular beam methods.
Density functional theory in surface chemistry and catalysis
Proceedings of the National Academy of Sciences, 2011
Recent advances in the understanding of reactivity trends for chemistry at transition-metal surfaces have enabled in silico design of heterogeneous catalysts in a few cases. The current status of the field is discussed with an emphasis on the role of coupling theory and experiment and future challenges.
Quantum Monte Carlo Calculations on a Benchmark Molecule–Metal Surface Reaction: H2 + Cu(111)
Journal of Chemical Theory and Computation, 2017
Accurate modeling of heterogeneous catalysis requires the availability of highly accurate potential energy surfaces. Within density functional theory, these can unfortunatelydepend heavily on the exchange-correlation functional. High-level ab initio calculations, on the other hand, are challenging due to the system size and the metallic character of the metal slab. Here, we present a quantum Monte Carlo (QMC) study for the benchmark system H 2 + Cu(111), focusing on the dissociative chemisorption barrier height. These computationally extremely challenging ab initio calculations agree to within 1.6 ± 1.0 kcal/mol with a chemically accurate semiempirical value. Remaining errors, such as time-step errors and locality errors, are analyzed in detail in order to assess the reliability of the results. The benchmark studies presented here are at the cutting edge of what is computationally feasible at the present time. Illustrating not only the achievable accuracy but also the challenges arising within QMC in such a calculation, our study presents a clear picture of where we stand at the moment and which approaches might allow for even more accurate results in the future.
The Journal of Physical Chemistry B, 2006
The mechanism that controls bond breaking at transition metal surfaces has been studied with sum frequency generation (SFG), scanning tunneling microscopy (STM), and catalytic nanodiodes operating under the highpressure conditions. The combination of these techniques permits us to understand the role of surface defects, surface diffusion, and hot electrons in dynamics of surface catalyzed reactions. Sum frequency generation vibrational spectroscopy and kinetic measurements were performed under 1.5 Torr of cyclohexene hydrogenation/dehydrogenation in the presence and absence of H 2 and over the temperature range 300-500 K on the Pt(100) and Pt(111) surfaces. The structure specificity of the Pt(100) and Pt(111) surfaces is exhibited by the surface species present during reaction. On Pt(100), π-allyl c-C 6 H 9 , cyclohexyl (C 6 H 11 ), and 1,4cyclohexadiene are identified adsorbates, while on the Pt(111) surface, π-allyl c-C 6 H 9 , 1,4-cyclohexadiene, and 1,3-cyclohexadiene are present. A scanning tunneling microscope that can be operated at high pressures and temperatures was used to study the Pt(111) surface during the catalytic hydrogenation/dehydrogenation of cyclohexene and its poisoning with CO. It was found that catalytically active surfaces were always disordered, while ordered surface were always catalytically deactivated. Only in the case of the CO poisoning at 350 K was a surface with a mobile adsorbed monolayer not catalytically active. From these results, a CO-dominated mobile overlayer that prevents reactant adsorption was proposed. By using the catalytic nanodiode, we detected the continuous flow of hot electron currents that is induced by the exothermic catalytic reaction. During the platinum-catalyzed oxidation of carbon monoxide, we monitored the flow of hot electrons over several hours using a metal-semiconductor Schottky diode composed of Pt and TiO 2 . The thickness of the Pt film used as the catalyst was 5 nm, less than the electron mean free path, resulting in the ballistic transport of hot electrons through the metal. The electron flow was detected as a chemicurrent if the excess electron kinetic energy generated by the exothermic reaction was larger than the effective Schottky barrier formed at the metalsemiconductor interface. The measurement of continuous chemicurrent indicated that chemical energy of exothermic catalytic reaction was directly converted into hot electron flux in the catalytic nanodiode. We found the chemicurrent was well-correlated with the turnover rate of CO oxidation separately measured by gas chromatography. † Part of the special issue "Charles B. Harris Festschrift".