Electrode processes involving coupled homogeneous reactions

11 Electrode processes involving coupled homogeneous reactions 

Interesting reactions occur when the charge transfer at the electrode is associated with homogeneous reactions in solution that can precede or follow the electron transfer reaction at the electrode. A selection of possible schemes is shown in Table 6.1. Note the presence of many organic compounds the reduction or oxidation of these compounds involves, in many cases, the addition or removal of hydrogen, which 

We consider three simple schemes, shown in Fig. 6.11, and examine the effect of homogeneous coupled reactions on the current at the electrode they are CE, EC, and EC, where E represents an electrochemical step (at the electrode) and C a chemical step (in solution). The equations to calculate the rate constants from experimental measurements for the various types of electrode can be found in the specialized literature. In most studies the electrochemical step has been considered reversible—thus, in the following, the rate constant for the electrode reaction is not indicated. 

The concentration of A is less than in the absence of the chemical step, and is dependent on the value of K. Measurement of the diminution in current allows the determination of the values of kx and The position of the voltammetric curve on the potential axis is not affected by the homogeneous step (Fig. 6.11a). 

The chemical step reduces the quantity of A at the electrode surface and consequently causes a shift of the voltammetric wave to more positive 

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Cyclic voltammetry is a powerful tool for investigating electrode processes involving coupled homogeneous reactions. We exemplify with the EC mechanism ... 

Since the 1960s, cyclic voltanunetry has been the most widely used technique for studies of electrode processes with coupled chentical reactions. The theory was developed for numerous mechanisms involving different combinations of reversible, quasi-reversible, and irreversible heterogeneous ET and homogeneous steps. Because of space limitations, we will only consider two well-studied examples—(i.e., first-order reversible reaction preceding reversible ET) and E Ci (i.e., reversible ET followed by a first-order irreversible reaction)—to illnstrate general principles of the coupled kinetics measurement. A detailed discussion of other mechanisms can be found in Chapter 12 of reference (1) and references cited therein, including a seminal publication by Nicholson and Shain (19). 

More complex electrode processes than those described above involve consecutive electron transfer or coupled homogeneous reactions. The theory of these reactions is also more complicated, but they correspond to a class of real, important reactions, particularly involving organic and biological compounds. 

Many interesting processes occurring at the liquid/liquid interface involve coupled homogeneous chemical reactions. In principle, electrochemical methods used for probing complicated mechanisms at metal electrodes (61) can be employed at the ITIES. However, many of these techniques (e.g., rotating ring-disk electrode or fast-scan cyclic voltammetry) are hard to adapt to liquid/liquid measurements. Because of technical problems, few studies of multistep processes at the ITIES have been reported to date (1,62). 

Let us first consider briefly how the use of mass transport as a variable can provide a guide to the reaction mechanism and give quantitative kinetic detail. As an illustration, we consider the behaviour of CE and EC processes (where E signifies electron transfer and C represents a chemical step) at a rotating disc electrode (RDE). This hydrodynamic system has already been discussed by Albery et al. and the reader is referred to Chap. 4 for details. CE and EC processes represent the simplest conceivable electrode reactions involving coupled homogeneous kinetics mechanistic examples of both types are shown in Table 1. In the discussion which follows, the electron-transfer reaction in the two mechanisms is considered to be a cathodic process the extension to the anodic case is trivial. 

Oxidation-reduction processes involving the Fe -Fe couple have been intensively studied, and the elementary electron exchange, taking place either in solution or at an electrode interface, has served for a long time as a test case for theoretical interpretations of electron transfer kinetics. For reactions involving ground-state ions, the mechanisms of the reactions are now well understood for the homogeneous systems, for example, electron transfer can occur, as demonstrated by Taube, via either a simple outer-sphere electron... 

As will now be clear from the first Chapter, electrochemical processes can be rather complex. In addition to the electron transfer step, coupled homogeneous chemical reactions are frequently involved and surface processes such as adsorption must often be considered. Also, since electrode reactions are heterogeneous by nature, mass transport always plays an important and frequently dominant role. A complete analysis of any electrochemical process therefore requires the identification of all the individual steps and, where possible, their quantification. Such a description requires at least the determination of the standard rate constant, k, and the transfer coefficients, and ac, for the electron transfer step, or steps, the determination of the number of electrons involved and of the diffusion coefficients of the oxidised and reduced species (if they are soluble in either the solution or the electrode). It may also require the determination of the rate constants of coupled chemical reactions and of nucleation and growth processes, as well as the elucidation of adsorption isotherms. A complete description of this type is, however, only ever achieved for very simple systems, as it is generally only possible to obtain reliable quantitative data about the slowest step in the overall reaction scheme (or of two such steps if their rates are comparable). 

In the second chapter, Appleby presents a detailed discussion and review in modem terms of a central aspect of electrochemistry Electron Transfer Reactions With and Without Ion Transfer. Electron transfer is the most fundamental aspect of most processes at electrode interfaces and is also involved intimately with the homogeneous chemistry of redox reactions in solutions. The subject has experienced controversial discussions of the role of solvational interactions in the processes of electron transfer at electrodes and in solution, especially in relation to the role of Inner-sphere versus Outer-sphere activation effects in the act of electron transfer. The author distils out the essential features of electron transfer processes in a tour de force treatment of all aspects of this important field in terms of models of the solvent (continuum and molecular), and of the activation process in the kinetics of electron transfer reactions, especially with respect to the applicability of the Franck-Condon principle to the time-scales of electron transfer and solvational excitation. Sections specially devoted to hydration of the proton and its heterogeneous transfer, coupled with... 

A systematic description of all possible combinations of homogeneous chemical processes coupled to electron transfer at an electrode surface is impossible because an infinite range of theoretically possible reaction schemes can be constructed. Unfortunately, a consistent form of nomenclature for defining the possible web of reaction pathways has not yet been invented. However, the lUPAC nomenclature [89] is of assistance with respect to simple reaction schemes. In this article, the commonly employed descriptors for electron transfer (E) and chemical (C) sequences of reaction steps, e.g. ECEC, will be used for a sequence of reactions involving electron transfer-chemical process-electron transfer-chemical process. Reaction schemes involving branching of a reaction pathway will be considered later. 

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