Electrocatalysis description

This chapter presents the design and application of a two-stage combinatorial and high-throughput screening electrochemical workflow for the development of new fuel cell electrocatalysts. First, a brief description of combinatorial methodologies in electrocatalysis is presented. Then, the primary and secondary electrochemical workflows are described in detail. Finally, a case study on ternary methanol oxidation catalysts for DMFC anodes illustrates the application of the workflow to fuel cell research. 

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. 

Since platinum in its pure state, and either alloyed or in mixtures with other metals/metal oxides, (which act as promoters), is among the most active materials for methanol oxidation, much attention has been devoted to the nature of, and mechanism involved in, the methanol oxidation reaction on platinum. As such, platinum has served as a useful model system illustrating the general features of metal electro-oxidation in an aqueous environment. There are many postulated mechanisms for the oxidation of methanol, and detailed descriptions of the same can be found in the literature [55-59] and will not be discussed in the present work, except from the point of view of contributions of IR and STM toward the understanding of the overall picture of electrocatalysis at model electrodes. 

Description of electrocatalytic processes in such modified electrodes can be derived from the intersection between the theory of Andrieux and Saveant (1980, 1988) for mediated electrocatalysis in redox polymers and those for metal oxide electrocatalysis (Lyons et al., 1992,1994 Attard, 2001 Pleus and Schulte, 2001) and the recent models for the voltammetry of microparticles given by Lovric and Scholz (1997, 1999) and Oldham (1998) and combined by Schroder et al. (2000). 

Of course, this is a crude sketch, a bare description of electrocatalysis in simple cases, but it does stress that to know which step is rate determining is more than half way to determining how to catalyze the overall reaction. ... 

The aim of this edition is to provide an up-to-date account of these recent advances. The first chapter describes a fascinating application of the X-ray diffraction technique to the study of the structure-reactivity relationship in electrocatalysis. The next two chapters illustrate the power of UV-visible spectroscopy and epifluorescence microscopy to explore electric field-driven transformations of thin organic films. Two chapters are devoted to non-linear spectroscopies at the liquid-liquid and liquid-solid interfaces, demonstrating the uniqueness of these techniques for revealing the structural details of these buried interfaces. Four chapters give a comprehensive description of applications of infrared spectroscopy to in-situ studies of electrified semiconductor-solution and metal-solution interfaces. The volume is concluded by a chapter that describes the emerging new technique of STM tip-induced surface-enhanced Raman spectroscopy. 

The theoretical description of electrocatalysis that takes into account electron and ion transfer and the transport process, the permeations of the substrates, and their combined involvement in the control over the overall kinetics has been elaborated by Albeiy and Hillman [312,313,373] and by Andrieux and Saveant [315], and a good summary can be found in [314]. Practically all of the possible cases have been considered, including Michaelis-Menten kinetics for enzyme catalysis. Inhibition, saturation, complex mediation, etc., have also been treated. The different situations have also been represented in diagrams. Based on the theoretical models, the respective forms of the Koutecky-Levich eqrration have been obtained, which make analyzing the resirlts of voltarrrmetry on stationary artd rotating disc electrodes a straightforward task. 

In Chapter 2 we discuss the use of electroactive polymer films in the important area of electrocatalysis. The material presented is also relevant for the quantitative description of the operation of amperometric chemical and biological sensors. In the latter context, the efficient operation of the amperometric sensor depends largely on how readily the polymer layer enhances the rate of substrate oxidation or reduction. This of course is related to electrocatalytic properties of the polymer film. 

The unique three-dimensional aluminosilicate crystalline lattice of zeolites gives rise to three intriguing characteristics (104). These characteristics are high cation-exchange capacity, sensitive molecular recognition (size and shape selectivity), and good catalytic activity. These properties give rise to the use of zeolite modified electrodes in sensor development, and electrocatalysis (106). These and other applications are outlined in Table 8.6. More detailed descriptions can be found in recent reviews (7, 96, 102-106), with extensive lists compiled by Rolison (Table 11 in (102)) and Walcarius (Tables 1 and 3 in (105) and Table 1 in (106)). 

In this chapter, we will consider electrocatalytic processes. It is not the intention to provide a complete description of surface electrocatalysis but rather to show how catalytic processes at electrode surfaces can be understood in much the same terms as other surface-catalyzed chemical processes. In particular, we will show that free energy diagrams, scaling relations, and activity maps are tools that are just as useful to analyze trends in electrocatalytic processes as for other heterogeneous catalytic processes. 

Before getting to the description of trends in electrocatalytic activity, we will discuss the features of surface electrocatalysis that are different from ordinary gas-phase heterogeneous catalysis. 

As a first step toward a quantitative description of electrocatalysis in micelles, we developed models for diffusion of micelle-bound electroactive molecules. Measurement of apparent diffusion coefficients (D ) of electroactive solutes in micellar solutions by voltammetry or chrono-coulometry has been used to estimate the fraction of total electroactive solute "probe" bound to micelles " , as well as micellar size If a single size distribution of micelles (i.e., a monodisperse system) exists, D can be expressed by a two-state model ... 

S. Schnur and A. Gross, Challenges in the first-principles description of reactions in electrocatalysis, Catal. Today, 2011, 165(1), 129. 

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