Heterogenous electron transfer inner sphere

Figure 2 Schematic representation of an heterogeneous electron transfer taking place through an inner-sphere mechanism at a negatively charged electrode...

The inner-sphere X can be calculated from the measured force constants of the redox molecule in each oxidation state. It is possible to have distinctly different Aig values for each oxidation state. The outer sphere X for heterogeneous electron transfer can be estimated from the dielectric continuum theory (Eq. 20) [238] ... 

In heterogeneous redox reactions similar reaction sequences are observed usually an encounter (outer-sphere or inner-sphere) surface complex is formed to facilitate the subsequent electron transfer. 

In this picture, the electron transfer processes mediated by metallic electrodes (redox reactions in a heterogeneous phase) can also be classified to proceed according to outer-sphere or inner-sphere mechanisms (obviously, considering the electrode surface as a reagent). 

Bridge mediation mechanisms in heterogeneous outer sphere electrochemical reactions has also been theoretically treated using the pull—push and push-pull mechanistic concepts [84]. Schmidt [85] has considered theoretically homogeneous inner sphere bridge electron transfer reactions without atom or ion transfer. Bridge mediation in electron transfer reactions may also involve simultaneous atom or ion transfer. Heyrovsky [86] invoked mediation of electron transfer by formation of bridges to explain the enhancement of the rate of electroreduction of indium (III) ions in the presence of specifically adsorbed halide ions on mercury. 

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... 

Vanadyl (V02 +) is an ideal cation for the study of heterogeneous oxidations for several reasons (1) the electron-transfer behavior of VO2 + is in many aspects similar to that of Fe2+ (Rosseinsky, 1972, Wehrli et al., in press), (2) the experimental conditions can be chosen so that vanadyl is completely adsorbed at pH >4 (Fig, 2), (3) the adsorbed V(IV) species have been characterized by ENDOR spectroscopy as inner-sphere surface complexes (=MO)xVO (Motschi and Rudin, 1984). Adsorption experiments are compatible with x = 2. The oxygenation rates of V02+ adsorbed to anatase and <5-Al203 follow the empirical rate law... 

The importance of Marcus theoretical work on electron transfer reactions was recognized with a Nobel Prize in Chemistry in 1992, and its historical development is outlined in his Nobel Lecture.3 The aspects of his theoretical work most widely used by experimentalists concern outer-sphere electron transfer reactions. These are characterized by weak electronic interactions between electron donors and acceptors along the reaction coordinate and are distinct from inner-sphere electron transfer processes that proceed through the formation of chemical bonds between reacting species. Marcus theoretical work includes intermolecular (often bimolecular) reactions, intramolecular electron transfer, and heterogeneous (electrode) reactions. The background and models presented here are intended to serve as an introduction to bimolecular processes. 

Outer-sphere electron transfers can be treated in a more general way than inner-sphere processes, where specific chemistry and interactions are important. For this reason, the theory of outer-sphere electron transfer is much more highly developed, and the discussion that follows pertains to these kinds of reactions. However, in practical applications, such as in fuel cells and batteries, the more complicated inner-sphere reactions are important. A theory of these requires consideration of specific adsorption effects, as described in Chapter 13, as well as many of the factors important in heterogeneous catalytic reactions (56). 

A study of the irreversible reduction of several Co ", Rh" and Ir" complexes revealed no correlation between the polarographic Ey and several spectroscopic parameters but, interestingly, it was found that a linear correlation existed for several of the Co " complexes between the y, and In where was the rate constant for homogeneous electron transfer, when [Ru(NH3)6] was used as reductant. The theoretical foundation for this relationship is that Ey is linearly related to In (the heterogeneous rate constant for electrochemical reduction) and, from the theories of Marcus and Hush, the ratio of k for a series of compounds is the same as the ratio of the rate constants k for a constant reductant provided both pathways are outer sphere. The mechanistic implication of the relationship is not clear it may simply mean that both pathways proceed via an outer sphere mechanism as no correlation was found between y, and the values of kgx for reduction by which can undergo homogeneous electron transfer by an inner sphere mechanism. 

Electron transfer, heterogeneous or homogeneous, can be dassified as outer sphere or inner sphere, according to the extent of interaction between the electron donor and the electron acceptor (Figure 1-13). 

---