Theory of Heterogeneous Electron-Transfer Reactions

The previous sections dealt with a generalized theory of heterogeneous electron-transfer kinetics based on macroscopic concepts, in which the rate of the reaction was expressed in terms of the phenomenological parameters, and a. While useful in helping to organize the results of experimental studies and in providing information about reaction mechanisms, such an approach cannot be employed to predict how the kinetics are affected by such factors as the nature and structure of the reacting species, the solvent, the electrode material, and adsorbed layers on the electrode. To obtain such information, one needs a microscopic theory that describes how molecular structure and environment affect the electron-transfer process. 

The Marcus Theory can also be applied for heterogeneous electron transfer reaction at electrode surfaces [24 and references therein]. The electronic coupling between the protein and the electrode can be varied using different self-assembled monolayers controlling the orientation of the redox active protein on the surface and the distance between the redox active site of the protein and the electrode. The driving force is related to the appHed potential and the redox potential of the protein. In many cases the rate of electron transfer across the protein-electrode interface is limited by conformational reorganization. This has focussed the efforts of many groups on tailored interaction between proteins and enzymes and electrode surfaces. 

Clegg AD, Rees NV, Klymenko OV, Coles BA, Compton RG (2004) Marcus theory of outer-sphere heterogeneous electron transfer reactions dependence of the standard electrochemical rate constant on the hydrodynamic radius from high precision measurements of the oxidation of anthracene and its derivatives in nonaqueous solvents using the high-speed channel electrode. J Am Chem Soc 126(19) 6185-6192... 

This series covers recent advances in electrocatalysis and electrochemistry and depicts prospects for their contribution into the present and future of the industrial world. It illustrates the transition of electrochemical sciences from a solid chapter of physical electrochemistry (covering mainly electron transfer reactions, concepts of electrode potentials and stmcture of the electrical double layer) to the field in which electrochemical reactivity is shown as a unique chapter of heterogeneous catalysis, is supported by high-level theory, connects to other areas of science, and includes focus on electrode surface structure, reaction environment, and interfacial spectroscopy. 

A further example of the use of this technique to introduce a ferrocene redox centre to a platinum surface is given in equation (32). A comparative survey was made of the rates of heterogeneous charge transfer between the platinum electrode and ferrocene both in solution and immobilized on the surface. Both processes show an Arrhenius temperature dependence but AGact(soIii) / A( ACT(surface bound). Absolute rate theory was unsatisfactory for the surface reaction and the need to involve electron tunnelling and a specific model for the conformation of the surface was indicated.66... 

The Marcus theory is the most widely applied theory used to describe electron transfer reactions and is equally applicable to photoinduced, interfacial and thermally driven electron transfers. The fundamental difference between these processes lies primarily in the nature of the driving force. In the case of heterogeneous electron... 

The Marcus theory, outlined above in Section 2.2.1 for homogeneous reactions, directly addresses these issues and is widely accepted as the most powerful and complete description of both heterogeneous and homogeneous electron transfer reactions. Its application to heterogeneous processes will be described in the following section. 

Marcus theory — Figure. Standard Gibbs energy, Ge, as a function of reaction coordinate (q, horizontal axis) for an electron transfer reaction, for homogeneous or heterogeneous charge transfer reactions... 

Using the Marcus theory, the a value (see -> charge-transfer coefficient) can be predicted, and its dependence on the potential applied. For low - over potentials, and when neither Ox nor Red are specifically adsorbed on the electrode surface, a should be approximately equal to 0.5. Further, the theory describes the relation between homogeneous and heterogeneous rate constants characteristic of the same redox system. An interesting prediction from Marcus theory is the existence of a so-called inverted region for the homogeneous electron transfer reactions, of importance to the phenomenon of... 

---