Heterogeneous electron exchange

Fernandez and Zon [181], when studying the heterogeneous electron exchange between N,7V,N, 7V -tetramethyl-/ -phenylendiamine and its mono-cation radical in 12 aprotic and hydrogen-bonded solvents, using a platinum electrode, have found that Eq. (40) is approximately fulfilled with a = 0.53. 

Its heterogeneous electron exchange with the electrode must be fast. 

Most electrocatalytic reactions are based on a signal generated by heterogeneous electron exchange between the dissolved analyte and the electrode which has been modified as described above. However, the use of M-N4 complexes as homogeneous electrocatalysts where both the catalyst and the analyte are in solu-... 

The dependence of surface potential and reaction rate on the NADH concentration are similar (Fig. 11), and the potential shift in the given system is proportional to the oxidation rate of NADH. The obtained result agrees well with the theoretical model of a heterogeneous catalytic reaction proposed earlier [23-25], according to which the potential shift at the interface between immiscible liquids relative to the supporting electrolyte is directly proportional to the rate of the heterogeneous electron-exchange reaction. 

Fig. 5. Plot of apparent electron self exchange rate constants kf P, derived from polymer De values for films containing the indicated metals, mixed valent states, and ligands, all in acetonitrile, using Equation 2, vs. literature heterogeneous electron transfer rate constants k° for the corresponding monomers in nitrile solvents. See Ref. 6 for details. (Reproduced from Ref. 6. Copyright 1987 American Chemical Society.)...

Dowden (19) developed a theory of heterogeneous catalysis on the basis of electron exchanges between catalysts and adsorbates [see also Boudart (19a)]. Hauffe and Engell (20), Aigrain and Dugas (21), as well as Weisz (22), tried to relate chemisorption on semiconductors to the charge distributions in the adsorbing semiconductors. 

In a direct electrolysis, the electron is exchanged between the electrode and the substrate, and the rate of the reaction depends on the electrode potential and the rate constant of the heterogeneous electron-transfer reaction. In an indirect electrolysis, the electron is primarily exchanged with a substance (a mediator) that exchanges the electron with the substrate in a chemical reaction, and the rate does not depend on the ability of the substrate to exchange an electron with the electrode. 

Thus for large amplitudes, the current is logarithmically related to overpotential as shown in Figure 2.17. Tafel plots (Fig. 2.17) are frequently employed by physical electrochemists to determine exchange currents and transfer coefficients. There are many other ways to obtain these parameters experimentally, but such numbers are rarely of interest to the analytical chemist. As we will see later, the rate of the heterogeneous electron transfer relative to other controlling factors (e.g., diffusion and coupled chemical reactions) is of critical importance to most experiments. 

The type of electron exchange, which can take place either by direct heterogeneous electron transfer between the electrode and the substrate or by using an indirect process, in which a redox catalyst mediates the exchange of redox equivalents between the electrode and the substrate... 

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... 

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