Electrocatalysis principles

First-Principles Simulation of the Active Sites and Reaction Environment in Electrocatalysis... 

Based on the results obtained in the investigation of the effects of modulation of the electron density by the nuclear vibrations, a lability principle in chemical kinetics and catalysis (electrocatalysis) has been formulated in Ref. 26. This principle is formulated as follows the greater the lability of the electron, transferable atoms or atomic groups with respect to the action of external fields, local vibrations, or fluctuations of the medium polarization, the higher, as a rule, is the transition probability, all other conditions being unchanged. Note that the concept lability is more general than... 

The aim of this overview is first to present the general principles of electrocatalysis by metal complexes, followed by a series of selected examples published over the last 20 years illustrating the major electrochemical reactions catalyzed by metal complexes and their potential applications in synthetic and biomimetic processes, and also in the development of sensory devices. The area of metal complex catalysts in electrochemical reactions was reviewed in 1990.1... 

In this paper, we will discuss the thermodynamic principles involved in fuel cells as well as the kinetic aspects of their half cell reactions. In the kinetic considerations, we will also touch, briefly, on the fundamental problem of electrocatalysis. We will then proceed to describe different types of fuel cells and finally present the status of this new electrical generation device. 

DMFCs and direct ethanol fuel cells (DEFCs) are based on the proton exchange membrane fuel cell (PEM FC), where hydrogen is replaced by the alcohol, so that both the principles of the PEMFC and the direct alcohol fuel cell (DAFC), in which the alcohol reacts directly at the fuel cell anode without any reforming process, will be discussed in this chapter. Then, because of the low operating temperatures of these fuel cells working in an acidic environment (due to the protonic membrane), the activation of the alcohol oxidation by convenient catalysts (usually containing platinum) is still a severe problem, which will be discussed in the context of electrocatalysis. One way to overcome this problem is to use an alkaline membrane (conducting, e.g., by the hydroxyl anion, OH ), in which medium the kinetics of the electrochemical reactions involved are faster than in an acidic medium, and then to develop the solid alkaline membrane fuel cell (SAMFC). 

PossibiKties of electrocatalysis of reactions at electrodes are among the powerful incentives for the electrochemical study of POMs. Interesting results were obtained both in electrocatalytic reductions and oxidations, provided the appropriate form of the POM is used. Two recent reviews devoted to the electrochemical properties of polyoxometalates as electrocatalysts are available [8, 9]. The second one focuses more specifically on electrocatalysis on modified electrodes. In the present text, attention will be drawn specially to the basic principles that could be considered to govern most of solution processes. The principles will be illustrated by several recent experimental results, even though earlier achievements will also be described briefly. 

The development of a consistent theory for a dissociative electron transfer is a recent challenge in the field of theoretical electrocatalysis. Progress in this field of electrochemistry has involved the use of an harmonic Morse curves [25] instead of harmonic approximations. Applying the principles of the theory of the activated complex to adiabatic dissociative electron transfer reactions, the work of Saveant resulted in the following expressions [24] for the Gibbs energy of activation... 

The chemistry of electrochemical reaction mechanisms is the most hampered and therefore most in need of catalytic acceleration. Therefore, we understand that electrochemical catalysis does not, in principle, differ much fundamentally and mechanistically from chemical catalysis. In addition, apart from the fact that charge-transfer rates and electrosorption equilibria do depend exponentially on electrode potential—a fact that has no comparable counterpart in chemical heterogeneous catalysis—in many cases electrocatalysis and catalysis of electrochemical and chemical oxidation or reduction processes follow very similar if not the same pathways. For instance as electrochemical hydrogen oxidation and generation is coupled to the chemical splitting of the H2 molecule or its formation from adsorbed hydrogen atoms, respectively, electrocatalysts for cathodic hydrogen evolution—... 

Sviridov, Dmitry V. he obtained his Ph.D. (1987) and D.Sc. (1999) degrees in Physical Chemistry from Belarussian State University (BSU). He currently holds an appointment of Professor of Chemistry at BSU and Principle Investigator in the Institute for Physico-Chemical Problems, BSU, Minsk, Belarus Republic. His scientific interests include photoelectrochemistry of semiconductors and molecular aggregates, electrocatalysis and environmental photocatalysis. E-mail ... 

Especially at oxide surfaces, though in principle also at metals, the chemisorption of intermediates can very usefully be considered in terms of changes in the coordination chemistry of metal ions or atoms. This approach forms the modern basis for understanding electrocatalysis of the anodic Oj or Cl 2 evolution reactions at oxides or oxidized metal surfaces. 

The Sabatier principle of catalysis also finds extensive application in the area of electrocatalysis reactants should be moderately adsorbed on the catalyst/electro-catalyst surface. Very weak or very strong adsorption leads to low electrocatalytic activity. This has been demonstrated repeatedly in the literature by the use of volcano plots (Figs 23-25). In these plots, the electrocatalytic activity is plotted as a function of the adsorption energy of the key reactant or some other parameter related to it in a linear or near-linear fashion, such as the work function of the metal [5], or the metal—H bond strength when discussing the H2 evolution reaction (Fig. 24) [54] or the enthalpy of the lower-to-higher oxide transition when examining the O2 evolution reaction (Fig. 25) [55]. 

In addition to a - compared with a diaphragm - better conductivity of the membrane, advanced electrocatalysis is applied for cell voltage reduction electrocatalyst particles (for instance platinum or Ni-Zn alloy) are deposited in the boundary layer adjacent to a porous electrode material [55]. Figure 23 shows the principle. 

Surface modification of electrodes to facilitate electrocatalysis parallels in many respects the chemistry discussed above for immobilization of catalysts on organic and inorganic materials. However, the objectives are somewhat different in electrode modification. Specifically, the principle objectives in electrode modification are usually to alter electrode stability, to alter the kinetics of reactions at electrode surfaces, or to alter an electrodes electrochemical properties. Electrode modification may involve covalent attachment of electroactive compounds or coating of the electrode surface with a polymeric phase. 

Some of these principles have been the object of extensive investigation in electrocatalysis and conventional heterogeneous catalysis and they have... 

The discussion of a number of topics in electrocatalysis, including adsorption phenomena, surface reaction mechanisms and investigation techniques, electrocatalytic activity and selectivity concepts, and reaction engineering factors, may seem at first too diverse. We believe, however, that fundamental principles cannot be divorced from their natural counterpart, praxis. Here, we attempt to establish ties between basic and applied electrocatalysis and with their conventional similes, catalysis, surface physics (and spectroscopy) and reaction engineering. By taking a vitae parallelae perspective, we hope that a synthetic analysis of the present state of the art emerges. 

Electrochemical data recorded under no steady-state conditions can also be used for studying electrocatalytic processes involving porous materials. In cases where the catalytic system can be approached by homogeneous electrocatalysis in solution phase, variation of cyclic voltammetric profiles with potential scan rate (Nicholson and Shain, 1964) and/or, for instance, square-wave voltammetric responses with square-wave frequency (O Dea et al., 1981 O Dea and Osteryoung, 1993 Lovric, 2002) can be used. This situation can, in principle, be taken for highly porous materials where substrate transport, as well as charge-balancing ion transport, is allowed. On first examination, the catalytic process can be approached in the same manner... 

The treatment of such problems is more complicated than those involving only dissolved species, because one must choose an adsorption isotherm, which involves the introduction of additional parameters and, in general, nonlinear equations. In addition, the treatment must include assumptions about (a) the degree to which adsorption equilibrium is attained before the start of the electrochemical experiment (i.e., how long after the formation of a fresh electrode surface the experiment is initiated) and (b) the relative rate of electron transfer to the adsorbed species compared to that for the dissolved species. These effects complicate the evaluation of the voltammetric data and make the extraction of desired mechanistic and other information more difficult. Thus adsorption is often considered a nuisance to be avoided, when possible, by changing the solvent or changing concentrations. However, adsorption of a species is sometimes a prerequisite for rapid electron transfer (as in forms of electrocatalysis), and can be of major importance in processes of practical interest (e.g., the reduction of O2, the oxidation of aliphatic hydrocarbons, or the reduction of proteins). Our discussion here will deal with the basic principles and several important cases. 

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