Electrocatalysis charge transfer

Electrocatalysis Again by definition, an electrocatalyst is a solid, in fact an electrode, which can accelerate a process involving a net charge transfer, such as e.g. the anodic oxidation of H2 or the cathodic reduction of 02 in solid electrolyte cells utilizing YSZ ... 

Electrocatalysis refers to acceleration of a charge transfer reaction and is thus restricted to Faradaic efficiency, A, values between -1 and 1. Electrochemical promotion (NEMCA) refers to electrocatalytically assisted acceleration of a catalytic (no net charge-transfer) reaction, so that the apparent Faradaic efficiency A is not limited between -1 and 1. 

Temperature programmed desorption, TPD detection of backspillover species, 228 of oxygen, 228 Thermodynamics of adsorption, 306 of spillover, 104, 499 Three phase boundaries charge transfer at, 114 electrocatalysis at, 115 length, measurement of, 243 normalized length, 243 Time constants ofNEMCA analysis of, 198 and backspillover, 198 prediction of, 200... 

So far, uncatalysed electrochemical processes have had to compete with catalytic organic processes. There is considerable scope for a specific catalyst to be developed for specific organic electrochemical reactions. This implies reduced overpotential and acceleration of slow chemical rather than relatively fast charge-transfer steps (Jansson, 1984). Electrocatalysis... 

In recent years, electrochemical charge transfer processes have received considerable theoretical attention at the quantum mechanical level. These quantal treatments are pivotal in understanding underlying processes of technological importance, such as electrode kinetics, electrocatalysis, corrosion, energy transduction, solar energy conversion, and electron transfer in biological systems. 

The approach to P given above is a simplification, although it does show why the effect of the change in the electrode potential on the charge-transfer rate is less titan that expected if the full potential were applied, an important realization. Another virtue of the early theory is the basis it gives to a theory of electrocatalysis. 

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—... 

Electrocatalysis is, in the majority of cases, due to the chemical catalysis of the chemical steps in an electrochemical multi-electron reaction composed of a sequence of charge transfers and chemical reactions. Two factors determine the effective catalytic activity of a technical electrocatalysts its chemical nature, which decisively determines its absorptive and fundamental catalytic properties and its morphology, which determines mainly its utilization. A third issue of practical importance is long-term stability, for which catalytic properties and utilization must occasionally be sacrificed. 

The selective facilitation of the charge transfer of the species of interest is called electrocatalysis. In such a case, the species of interest are transformed at energies substantially lower than those of the interferants. The higher selectivity therefore implies a lower applied potential at the modified working electrode, which exhibits such selective electrocatalytic properties. In such a situation, the choice of the... 

The changes in the potential profile of the interfacial region because specific adsorption do indeed affect the electrode kinetics of charge transfer processes, particularly when these have an inner sphere character [13, 26] (see Fig. 1.12). When this influence leads to an improvement of the current response of these processes, the global effect is called electrocatalysis. ... 

Tetracyanoquinodimethane (TCNQ) and many of its derivatives are easily reduced to anions of the type TCNQ-, which form salts with various cations. With many cations, e.g., tetrathiafulvalene cations (TTF+), and N-methyl phenazinium cations (NMP+), the TCNQ- anions form electronically conducting salts (- molecular metals, -> charge-transfer complexes) that can be used as electrodes, especially because of their electrocatalytic properties (- biosensors, -> electrocatalysis, -> molecular metals) [i,ii]. TCNQ undergoes insertion electrochemical reactions (-> insertion electrochemistry) leading to TCNQ salts [iii, iv]. Polymers... 

Tetrathiafulvalene electrodes — Tetrathiafulvalene (TTF) and many of its derivatives are easily oxidized to form cations of the type TTF+. With various anions, most prominent is the tetracyanoquinodimethane anion (TCNQ-), these cations form - charge-transfer complexes, i.e., salts with metal-like conductivities (- molecular metals). These salts are used in electrochemistry as electrodes or to modify the surface of electrodes, for the purpose of achieving desirable electrocatalytic properties (- electrocatalysis). Tetrathiafulvalene-substituted polystyrenes have also been synthesized and used as modified electrodes [ii]. 

Chapter 3, by Rolando Guidelli, deals with another aspect of major fundamental interest, the process of electrosorption at electrodes, a topic central to electrochemical surface science Electrosorption Valency and Partial Charge Transfer. Thermodynamic examination of electrochemical adsorption of anions and atomic species, e.g. as in underpotential deposition of H and metal adatoms at noble metals, enables details of the state of polarity of electrosorbed species at metal interfaces to be deduced. The bases and results of studies in this field are treated in depth in this chapter and important relations to surface -potential changes at metals, studied in the gas-phase under high-vacuum conditions, will be recognized. Results obtained in this field of research have significant relevance to behavior of species involved in electrocatalysis, e.g. in fuel-cells, as treated in chapter 4, and in electrodeposition of metals. 

If the heterogeneous charge transfer for the generation of the reactive intermediates is complicated by some surface character of the process it may be possible to circumvent the problem by using electrocatalysis. During electrocatalysis (Savdant, 1980) charge transfer at the electrode involves a catalyst redox couple, O/R, in a thermodynamically reversible reaction (4). 

When an electrocatalytic reaction involves a primary step of molecular dissociative chemisorption, for example, a c,e mechanism, then the electrocatalysis arises more directly, in the same way as for many regular catalytic processes that involve such a step of dissociative chemisorption. In this type of electrocatalytic reaction, the dissociated adsorbed fragments, for example, adsorbed H in H2 oxidation, become electrochemically ionized or oxidized in one or more charge-transfer steps following the initial dissociation. The rate... 

The term electrocatalysis was coined by Kobosev and Monblanova at the beginning of the 1930s [68]. However, it has been only in the last 30 years or so that this terminology has been commonly employed to describe the study of electrode processes where charge-transfer reactions have a strong dependence on the nature of the electrode material [69, 70]. Virtually every electrochemical reaction where chemical bonds are broken or formed is electrocatalytic, and the kinetics can vary by orders of magnitude for different electrode materials [13]. This is true even for... 

The term electrocatalysis was first introduced by Grubb in 1963 [1] in connection with the anodic and cathodic charge-transfer reactions in fuel cells. However, the first systematic experimental investigations of various electrocatalysts had been carried out in the 1920s by Bowden and Rideal [2] and the concept and first interpretation of electrocatalysis had been introduced in the 1930s by Horiuti and Polanyi [3]. Their theory and its extensions and improvements have been analyzed lucidly by Bockris [4-6]. Recent reviews of progress in electrocatalysis can be found in more general [6-8] or specialized books [9-11]. 

The total cell activation overpotential is the sum of the activation overpotentials at the anode and cathode, as shown in Fig. 13 for the case of H2O electrolysis using Pt electrodes in alkaline solutions. The two overpotentials can be separated by the use of a reference electrode. Thus, the use of reference electrodes is essential for the study of electrocatalysis, since in this case one can individually study the dependence of each electrode overpotential on the current and thus assess the elec-trocatalytic performance of each electrode. The best electrocatalyst, for each charge-transfer reaction is, obviously, the one that minimizes the activation overpotential. 

Charge transfer is not unique to electrocatalysis as even a cursory survey of the catalytic literature can show. Indeed, oxidation (18-21), desulfurization (21), and reduction (22) mechanisms have been proposed, involving electron transfer between catalyst and reactant, to explain activity and selectivity effects. Electronic interactions between adsorbate bonds and d-band electrons of the catalyst are also used commonly to explain strength of adsorption (21,23,24). This electron exchange or transfer in conventional catalysis and electrocatalysis, and steps such as adsorption, surface reaction, and desorption, point toward expected similarities between the two catalytic... 

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