Surface structure, role electrocatalysis

The intrinsic exchange current density,/ , is not a mere materials constant, but it depends on size distributions of catalyst nanoparticles, their surface structure, as well as surface composition in the case of alloy catalysts like PtRu. In this section, we discuss modeling approaches that highlight particle size effects and the role of surface heterogeneity in fuel cell electrocatalysis. 

During the past two decades, a great deal of work in surface electrochemistry has been aimed at elucidating the role of the local symmetry of surface atoms in electrocatalysis, particularly for the kinetics of the ORR on platinum single-crystal surfaces. It is now well established that the kinetics of the ORR on Pt(hkl) surfaces are sensitive to the surface structure [88, 89] and arise because of structure-sensitive adsorption of spectator species, such as H plimited scope of this report, it will not be possible to review aU... 

The use of CeOs-based materials in catalysis has attracted considerable attention in recent years, particularly in applications like environmental catalysis, where ceria has shown great potential. This book critically reviews the most recent advances in the field, with the focus on both fundamental and applied issues. The first few chapters cover structural and chemical properties of ceria and related materials, i.e. phase stability, reduction behaviour, synthesis, interaction with probe molecules (CO. O2, NO), and metal-support interaction — all presented from the viewpoint of catalytic applications. The use of computational techniques and ceria surfaces and films for model catalytic studies are also reviewed. The second part of the book provides a critical evaluation of the role of ceria in the most important catalytic processes three-way catalysis, catalytic wet oxidation and fluid catalytic cracking. Other topics include oxidation-combustion catalysts, electrocatalysis and the use of cerium catalysts/additives in diesel soot abatement technology. 

The next section gives a brief overview of the main computational techniques currently applied to catalytic problems. These techniques include ab initio electronic structure calculations, (ab initio) molecular dynamics, and Monte Carlo methods. The next three sections are devoted to particular applications of these techniques to catalytic and electrocatalytic issues. We focus on the interaction of CO and hydrogen with metal and alloy surfaces, both from quantum-chemical and statistical-mechanical points of view, as these processes play an important role in fuel-cell catalysis. We also demonstrate the role of the solvent in electrocatalytic bondbreaking reactions, using molecular dynamics simulations as well as extensive electronic structure and ab initio molecular dynamics calculations. Monte Carlo simulations illustrate the importance of lateral interactions, mixing, and surface diffusion in obtaining a correct kinetic description of catalytic processes. Finally, we summarize the main conclusions and give an outlook of the role of computational chemistry in catalysis and electrocatalysis. 

Interfacial structure The role of electrochemical phenomena at interfaces between ionic, electronic, photonic, and dielectric materials is reviewed. Also reviewed are opportunities for research concerning microstructure of solid surfaces, the influence of the electric field on electrochemical processes, surface films including corrosion passivity, electrocatalysis and adsorption, the evolution of surface shape, and self-assembly in supramolecular domains. 

The electrode surface serves the role of a catalyst for the charge transfer process and often also for coupled preceding or following chemical processes. Unfortunately, electrocatalytic processes for the most part are not well understood. Of critical importance is the structure of the electrochemical interface, particularly with adsorption of various species. The limited structural information concerning such interfaces is a serious deterrent of the development of electrocatalysis as a precise science (see the later section in this chapter on "Surface Reactions"). 

Moreover, adsorption has other important roles in electrochemical technology. In electrocatalysis, adsorption of intermediates is a key step since their presence on the surface provides alternative lower energy pathways (see Chapter 7). Also, adsorption of species not directly involved in the electron transfer process is used to modify electrode reactions, to change the product, to modify the structure of metal deposits, and to slow down electron transfer reactions (as in corrosion inhibition). 

In recent years, much attention was given to the role of neutral species and specifically adsorbed anions on the structure of the electrochemical interface, the electric field distribution in their vicinity and their role in electrocatalysis. EC STM became a key experimental technique in providing insight into the structure of the adsorbed neutral species or the specifically adsorbed anions copresent with underpotential deposited metallic layers 52-54) and in supporting data on the anion surface coverage based on chronocoulometry experiments (55-57). These structural results derived fi om various experimental approaches have led to a contemporary model of the electrified solid-liquid interface which is presented in Figure 4. 

In addition to studies focusing exclusively on the catalyst surface, the catalyst support (when employed) can play a major role in enhacing the activity/selectivity via morphologic, electronic, and physico-chemical effects. These factors have been extensively explored in the case of thermochemical heterogeneous reactions where a variety of compounds and structures have been successfully used on an industrial scale as catalyst supports (e.g., oxides, sulfides, meso- and microporous materials (molecular sieves), polymers, carbons [251-256]). In electrocatalysis, on the other hand, the practical choice of support in gas diffusion electrodes has been largely limited thus far to carbon black particles. The high electronic conductivity requirement, combined wifli electrochemical stability and cost effectivness, imposes serious restrictions on the type of materials that could be employed as supports in electrocatalysis. 

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