Reactions with homogeneous chemical steps

Consider an electrochemical reaction of the type Red Ox + e under conditions when the chemical reaction (and equilibrium) 

When the current is anodic, component Red is consumed and the equilibrium in the electrolyte close to the surface is disturbed reaction (13.37) will start to proceed from left to right, producing additional amounts of species Red. In this case the chemical precedes the electrochemical reaction. However, when the current is cathodic, substance Red is produced and the chemical reaction (13.37), now as a subsequent reaction, will occur from right to left. When component Ox rather than component Red is involved in the chemical reaction, this reaction will be the preceding reaction for cathodic currents, but otherwise all the results to be reported below remain valid. 

Preceding and subsequent homogeneous chemical reactions that occur in the bulk solution are very common. Examples include dehydration (when only a nonhydrated form of the substance is involved in the electrochemical reaction), protonation (e.g., of the anions of organic acids), and decay of complexes (in metal deposition from solutions of complex salts). 

Equilibrium between substances A and Red will be preserved during current flow not only in the bulk solution but even near the electrode surface when the chemical reaction has a high exchange rate. Therefore, a change in surface concentration of the substance Red which occurs as a result of the electrochemical reaction will give 

When calculating the surface concentration of substance Red in Eq. (4.11), we must use the combined limiting CD of both substances. 

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In addition to the chemical steps, which are the only steps involved in stoichiometric or in homogeneous catalysis reactions, heterogeneous catalysis reactions involve also physical steps, i.e. transport (transfer) of organic molecules (and heat) from the reaction mixture to the active sites of the solid catalyst and vice versa.113-151 Another difference deals with the chemical steps, which do not occur in the fluid phase, but for part of them involve both fluid and solid phases (chemisorption and desorption), the other part occurring at the surface of the catalyst.113-151... 

This section concerns heterogeneous electron transfer reactions coupled with homogeneous chemical reactions in which either the electroactive species A or the product of the electron transfer B participate as reactants. Perturbations of electrochemical responses of different techniques evoked by these reactions enable the elucidation of the mechaism and the evaluation of the kinetic parameters of the chemical steps. Chemical reactions that are indicated in the electrochemical way occur in the thin layer reaction layer) adjacent to the electrode surface only. This is illustrated in Fig. 1 where the concentration dependence of the product B on the distance from the electrode plane (with and without follow-up chemical reaction) is plotted. It must be stressed that the kinetics and the electrode mechanism are affected not only by the nature of the electroactive as well as electroinactive species including the type of the solvent, but also by the electrode material and substances adsorbed on the electrode surface. 

Firstly, we should define the types of complexity which need to be considered when dealing with homogeneous chemical reactions coupled to electron transfer. The most common one is that the conversion of primary intermediates into final product is, in fact, a sequence of several, maybe four or five, elementary steps. In addition to defining the reaction pathway, it is necessary to decide which step is the rate determining one and also to consider the possibility that two steps have approximately the same rate, or that the r.d.s. changes, say with concentration of electroactive species. It is, however, also common in organic electrochemistry to find that the electrode reaction leads to a mixture of products and this is a clear indication of a branch mechanism where two competing reactions have comparable rates branch mechanisms can even lead to the same product. A further uncertainty arises as to the source of electrons does the second... 

A more interesting situation is found when the homogeneous redox reaction is combined with a chemical reaction between the electrocatalyst and the substrate. In this case, the catalytic process is called chemical catalysis. 3 This mechanism is depicted in Scheme 2 for reduction. The coupling of the electron transfer and the chemical reaction takes place via an inner-sphere mechanism and involves the formation of a catalyst-substrate [MC-S] complex. Here the selectivity of the mechanism is determined by the chemical step. Metal complexes are ideal candidates... 

The electrode reaction (2.46) is followed by a first-order homogeneous chemical reaction (2.47), in which the product of the electrode reaction O is converted to a final electroinactive product Y. By analogy with the CE mechanism, the chemical step can proceed as ... 

The net result of a photochemical redox reaction often gives very little information on the quantum yield of the primary electron transfer reaction since this is in many cases compensated by reverse electron transfer between the primary reaction products. This is equally so in homogeneous as well as in heterogeneous reactions. While the reverse process in homogeneous reactions can only by suppressed by consecutive irreversible chemical steps, one has a chance of preventing the reverse reaction in heterogeneous electron transfer processes by applying suitable electric fields. We shall see that this can best be done with semiconductor or insulator electrodes and that there it is possible to study photochemical primary processes with the help of such electrochemical techniques 5-G>7>. 

If, in indirect electrolyses, the homogeneous reaction step is a chemical step combined with a redox reaction (mechanism B), some more advantages are obtained ... 

For complex mechanisms such as ECE or other schemes involving at least two electron transfer steps with interposed chemical reactions, double electrodes offer a unique probe for the determination of kinetic parameters. Convection from upstream to downstream electrodes allows the study of fast homogeneous processes. The general reaction scheme for an ECE mechanism can be written... 

Two types of redox catalysts are used in indirect electrolyses [3] (1) pure, outer-sphere, or non-bonded electron transfer agents, and (2) redox reagents that undergo a homogeneous chemical reaction that is intimately combined with a redox step. This may be called inner-sphere electron transfer or bonded electron transfer mechanism. 

Sometimes the oxidized species can exist in two forms in chemical equilibrium, with one of them electro-inactive in the potential range where the electrochemical process occurs. This type of reaction pathway is known as a CE mechanism because a homogeneous chemical reaction (C) precedes the heterogeneous electrochemical process (E). If the chemical step is of first or pseudo-first order, the process can be expressed by the reaction scheme ... 

It should be noted that the detailed modelling of heterogeneous catalytic reactions faces some specific difficulties. Compared with homogeneous systems, the limits of the field wherein the law of mass action analog (the surface-action law) can be correctly applied are less distinct. Still less reliable are the elementary step constants. Nevertheless, we believe that, despite the complexity of "real kinetics , the importance of studying the models fitting the law of mass action cannot be undervalued. These models describe the chemical components of a complex catalytic process properly and, on the other hand, they are a necessary step that can be treated as a first approximation. Our study is devoted to the analysis of just these models. 

Another case of interest with LSV is the catalytic system, described above in the section on potential steps. The equations are the same except that the potential here is not constant and very negative, following (2.29) in its dimensionless form. For small and intermediate rates of the homogeneous chemical reaction (dimensionless constant K), the same procedure as mentioned above, that is, convergence simulations, must be used. For large K, however, the LSV curves become sigmoid, with a plateau equal to the current for the potential step, G = y/K. This can be used to test methods. Figure 2.7 shows LSV curves for some A -values, where this effect is seen. 

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