Electron charge-transfer process

From the derivations in Appendix B, it is evident that the present faradaic rectification formulations for multiple-electron charge transfer not only enable the determination of kinetic parameters for each step of three-electron charge transfer processes but may also be extended to charge transfer processes involving a higher number of electrons. However, the calculations become highly involved and complicated. 

This section presents the solution corresponding to a surface two-electron charge transfer process (EE mechanism) when a sequence of potential pulses H, E2,..., Ep is applied to the reaction scheme (6.II) by assuming that all the species (Oi, 02, and O3) correspond to the oxidation states of a surface-confined molecule O. Under these conditions, Eq. (6.107) has to be replaced by... 

Recently, Kisza et al. [203] found that the total electrode reaction can be interpreted by a two-step two-electron charge transfer process with an intermediate adsorption ... 

For a Pt(lll) surface, with a surface density of 1.5 x 1015 atoms cm-2, the current density corresponding to a TOF of Is-1 is 0.18 mA cm-2 for a one-electron charge-transfer process. Such exchange current densities based on the real electrocatalyst surface area are quite typical for a decent electrocatalyst for H2 evolution or oxidation. Thus, one may conclude that the order of magnitude of the TOFs of catalytic and electrocatalytic reactions are quite similar. In the latter... 

Standard driving force for each couple is nominally the same (Hamann et al., 2005a). These collective measurements demonstrate that simple one-electron charge-transfer processes at semiconductor/liquid junctions are experimentally in accord with the Marcus-Gerischer model of interfacial charge transfer. 

Finally, here it should be noted formally that a is, of course, identical with in cases where the initial step in a reaction sequence is a one-electron charge transfer process and is itself the rate-controlling process. Also b can be related to the total number of electrons passed in the overall reaction or in the rate-controlling step through a and the stoichiometric number v. This matter has, however, been treated in various earlier works by, e.g., Horiuti and Ikusima, Bockris, and Gileadi, and so need not be examined again here since no involvement of the temperature variable arises apart from that in b itself. 

Equation 6 is referred to as the Butler-Volmer equation. Normally, for significant overpotentials, either one or the other of the two terms is dominant, so that the current-density exponentially increases with r, i.e. In i is proportional to 3tiF/RT in the case, for example, of appreciable positive t] values. Here the significance of Tafel s b coefficient (Equation 1) is seen b = dn/d In i = RT/3F for a simple, single-electron charge-transfer process. 

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