Heterogeneous electron transfer processes

One also obtains analogous findings with trace-crossing effects for the electropolymerization of thiophene and pyrrole. This cannot be explained by a simple linear reaction sequence, as presented in Scheme I, because it indicates competing homogeneous and heterogeneous electron transfer processes. Measurements carried out in a diluted solution of JV-phenylcarbazole provide a more accurate insight into the reaction mechanism (Fig. 2). 

Interfacial electron transfer is the critical process occurring in all electrochemical cells in which molecular species are oxidized or reduced. While transfer of an electron between an electrode and a solvated molecule or ion is conceptually a simple reaction, rates of heterogeneous electron transfer processes depend on a multitude of factors and can vary over many orders of magnitude. Since control of interfacial electron transfer rates is usually essential for successful operation of electrochemical devices, understanding the kinetics of these reactions has been and remains a challenging and technologically important goal. 

As mentioned in Chapter 1, Section 2.2, it is quite common that a heterogeneous electron transfer process is complicated by homogeneous chemical reactions that involve the species Ox and/or Red. In this light, the chemical complications are classified 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>. 

One of the most intriguing aspects of electrochemistry involves the homogeneous chemical reactions that often accompany heterogeneous electron-transfer processes occurring at the electrode-solution interface. The addition or removal of an electron from a molecule generates a new redox state, which can be chemically reactive. A variety of mechanisms, some of which involve complicated sequences of electrode and chemical reactions, have been characterized. Several of the more common mechanisms with examples of applicable chemical systems are described next. More examples are given in Chaps. 21 and 23. 

The electrochemical formation of a radical ion from an aromatic compound or other highly conjugated species is, generally, fast and, therefore, the kinetics of the heterogeneous electron transfer process usually do not interfere with the kinetics of the follow-up reactions to be studied. For species with only one or two double bonds, the initial electron transfer process is often slow and may even be rate determining. In such cases, the kinetics of the follow-up reactions may be studied only with some difficulty. One method is to use a so-called mediator (Med) which serves to shuttle electrons between the substrate and the electrode. Thus, the slow electron transfer between the substrate and the electrode is replaced by two fast electron transfer processes, between the mediator and the electrode, and between the oxidised or reduced mediator and the substrate. In this event, the single reaction of Equation 6.4 is replaced by the two reactions in Scheme 6.8 [32 ]. It is seen that the mediator is recycled and consequently needs be present in only small, non-stoichiometric amounts. 

The application of electrochemical methods for the study of the kinetics and mechanisms of reactions of electro chemically generated intermediates is intimately related to the thermodynamics and kinetics of the heterogeneous electron transfer process and to the mode of transport of material to and from the working electrode. For that reason, we review below some basics, including the relationship between potential and current (Section 6.5.1), the electrochemical double layer and the double layer charging current (Section 6.5.2), and the... 

Fig. 1.13 Influence of the applied potential E on the energy barrier of the heterogeneous electron transfer process ...

Figure 2.9 Schematics illustrating an adsorbate-electrode interface in aqueous solution, plus the corresponding thermal activation and electron tunneling steps associated with a heterogeneous electron transfer process...

Figure 2.10 Parabolic free energy curves for heterogeneous electron transfer processes... 

Figure 5.37 Schematic illustration of a photoswitch process in which the solution-phase species acts as the photoactive moiety. In this instance, photoisomerization alters the ability of the redox-active solution-phase moiety to recognize the monolayer and undergo a heterogeneous electron transfer process...

The time range of the electrochemical measurements has been decreased considerably by using more powerful -> potentiostats, circuitry, -> microelectrodes, etc. by pulse techniques, fast -> cyclic voltammetry, -> scanning electrochemical microscopy the 10-6-10-1° s range has become available [iv,v]. The electrochemical techniques have been combined with spectroscopic ones (see -> spectroelectrochemistry) which have successfully been applied for relaxation studies [vi]. For the study of the rate of heterogeneous -> electron transfer processes the ILIT (Indirect Laser Induced Temperature) method has been developed [vi]. It applies a small temperature perturbation, e.g., of 5 K, and the change of the open-circuit potential is followed during the relaxation period. By this method a response function of the order of 1-10 ns has been achieved. 

Ftg. 11 Reaction coordinate diagrams for simple heterogeneous electron transfer processes at an electrode held at a potential of for a range of differing values... 

Heterogeneous electron-transfer processes are important, they have been extensively studied and they span a wide range of phenomena. These include studies of electron transfer from (or to) molecular substrates within (or across) micelles and vesicles, porous solid media, layered materials, and to (or from) semiconductors and electrodes. 

When the heterogeneous electron-transfer process at the electrode becomes slow and irreversible, the use of the direct OTTLE/Nernst experiment is inconvenient because of the uncertainties associated with a slow equilibration process. A mediated OTTLE/Nernst experiment should rather be considered, where a redox mediator Mox/Mred characterized by a high heterogeneous rate constant is added to the cell (Eq. 111). The concentration ratio of the mediator couple will be adjusted quickly to the applied electrode potential E and, furthermore, it will be in a redox equilibrium (Eq. 112) with the redox pair O/R in the bulk solution, according to Eq. 113. 

This chapter is meant to serve both as a guide for the beginner and as an overview for the nonelectrochemist with a need to know the methods available. Approximately half of the chapter is concerned with various aspects of linear sweep and cyclic voltammetry in view of the importance and widespread use of these techniques. Some general aspects of the heterogeneous electron transfer process, and the chemical reactions associated with it, are introduced in this part. Electrochemical reactions in which the electroactive substrate is formed in a chemical reaction in solution prior to the electron transfer [1-5] and catalysis of chemical reactions by electron transfer [6] are not included in this chapter. The reader interested in the details of such reactions should consult the presentations referred to. The reader is encouraged also to consult Chapter 1, where a number of basic electrochemical concepts are discussed in detail. 

Common to all the methods discussed earlier is that B is generated at the electrode surface, that is, by a direct electron exchange between the electrode and the substrate A. This approach is, however, sometimes hampered by the limitations imposed by the heterogeneous nature of the electron transfer reaction. For instance, studies of the kinetics of fast follow-up reactions may be difficult or even impossible owing to interference from the rate of the heterogeneous electron transfer process. In such cases, the kinetics of the follow-up reactions may be studied instead by an indirect method, generally known as redox catalysis [5,124-126]. Another application of redox... 

Detailed studies of ligand and solvent effects on electrochemical reductions of Cr111 amino-polycarboxylates have been performed.741,742 The observed differences in electrochemical behavior between cis- and trans-N204 Cr111 complexes have been explained by different Jahn-Teller distortions (JTD) of their Cr11 analogs (Section 4.6.6.8).741 Electrochemical reduction of Crm-edta complexes has been used as a model reaction in studies of heterogeneous electron-transfer processes.743... 

Although the formalism provides a unified description of homogeneous and heterogenous electron-transfer processes, there are many aspects of electron-transfer reactions that are not readily accommodated within this framework. These include atom-transfer reactions, specific-bridging and ion-pairing effects, oxidation and reduction of coordinated ligands, and the unique chemistry associated with many electron-transfer processes. These aspects as well as the successes and failures of the electron-transfer models are elaborated in the following sections. 

There are several examples of catenanes where ring movements can be induced by external stimulations like simple chemical reactions or homogeneous or heterogeneous electron transfer processes [91-93], but only very few cases are reported in which the stimulus employed is light. It has been shown that in azobenzene-containing [2]catenanes like 31 + (Fig. 29) it is possible to control the rate of thermally activated rotation of the macrocyclic components by photoisomerization of the azobenzene moiety [119, 120]. Such systems can be viewed as molecular-level brakes operated by light. 

In this equation, and represent the surface concentrations of the oxidized and reduced forms of the electroactive species, respectively k° is the standard rate constant for the heterogeneous electron transfer process at the standard potential (cm/sec) and oc is the symmetry factor, a parameter characterizing the symmetry of the energy barrier that has to be surpassed during charge transfer. In Equation (1.2), E represents the applied potential and E° is the formal electrode potential, usually close to the standard electrode potential. The difference E-E° represents the overvoltage, a measure of the extra energy imparted to the electrode beyond the equilibrium potential for the reaction. Note that the Butler-Volmer equation reduces to the Nernst equation when the current is equal to zero (i.e., under equilibrium conditions) and when the reaction is very fast (i.e., when k° tends to approach oo). The latter is the condition of reversibility (Oldham and Myland, 1994 Rolison, 1995). 

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