Electrode processes electron transfer

In Section 1.4 it was assumed that the rate equation for the h.e.r. involved a parameter, namely the transfer coefficient a, which was taken as approximately 0-5. However, in the previous consideration of the rate of a simple one-step electron-transfer process the concept of the symmetry factor /3 was introduced, and was used in place of a, and it was assumed that the energy barrier was almost symmetrical and that /3 0-5. Since this may lead to some confusion, an attempt will be made to clarify the situation, although an adequate treatment of this complex aspect of electrode kinetics is clearly impossible in a book of this nature and the reader is recommended to study the comprehensive work by Bockris and Reddy. ... 

As in chemical systems, however, the requirement that the reaction is thermodynamically favourable is not sufficient to ensure that it occurs at an appreciable rate. In consequence, since the electrode reactions of most organic compounds are irreversible, i.e. slow at the reversible potential, it is necessary to supply an overpotential, >] = E — E, in order to make the reaction proceed at a conveniently high rate. Thus, secondly, the potential of the working electrode determines the kinetics of the electron transfer process. 

The major effect of an increase in temperature on the actual electron transfer process is to increase A , and hence to enhance the reversibility of the electrode process. The reversible potential is, however, itself temperature dependent, and... 

C19-0123. A cell is set up using two zinc wires and two solutions, one containing 0.250 M ZnCl2 solution and the other containing 1.25 M Zn (N03)2 solution, (a) What electrochemical reaction occurs at each electrode (b) Draw a molecular picture showing spontaneous electron transfer processes at the two zinc electrodes, (c) Compute the potential of this cell. 

Let us consider a cathode electron transfer process at metal electrode. The role of the electron donor is played here by the metal electrode. The specific feature of this donor consists of the fact that its electron energy spectrum is practically continuous... 

Lantz, J. M. and Corn, R. M. (1994) Electrostatic field measurements and hand fiattening during electron-transfer processes at single-crystal Ti02 electrodes by electric field-induced optical second harmonic generation. J. Phys. Chem., 98, 4899-4905. 

A solid-liquid interface will have three aspects to its structure the atomic 1.1 structure of the solid electrode, the structure of any adsorbed layer and the Structure structure of the liquid layer above the electrode. All three of these are of fundamental importance in the understanding of the electron transfer processes at the core of electrochemistry and we must consider all three if we are to arrive at a fundamental understanding of the subject. 

Even for the Cl oxidation process on the Ru02/Ti02 electrode, for which the individual electron transfer processes are sufficiently fast for thermodynamic equilibrium to be maintained, at least close to E°, it is highly unlikely that two electrons are transferred simultaneously from two CP ions that are exactly the right distance apart for Cl2 bond formation to take place. It is far more likely that some kind of radical intermediate is involved which is stabilised by complexation on the surface. 

Similar to those observed with the cysteine-modified electrode in Cu, Zn-SOD solution [98], CVs obtained at the MPA-modified Au electrode in phosphate buffer containing Fe-SOD or Mn-SOD at different potential scan rates (v) clearly show that the peak currents obtained for each SOD are linear with v (not v 1/2) over the potential scan range from 10 to 1000 mVs-1. This observation reveals that the electron transfer of the SODs is a surface-confined process and not a diffusion-controlled one. The previously observed cysteine-promoted surface-confined electron transfer process of Cu, Zn-SOD has been primarily elucidated based on the formation of a cysteine-bridged SOD-electrode complex oriented at an electrode-solution interface, which is expected to sufficiently facilitate a direct electron transfer between the metal active site in SOD and Au electrodes. Such a model appears to be also suitable for the SODs (i.e. Cu, Zn-SOD, Fe-SOD, and Mn-SOD) with MPA promoter. The so-called... 

FIGURE 12.2 Schematic depicting a direct electron transfer process between an enzyme and the electrode, acting as an anode in this case. 

Fig. 21 (a) Rectification in the AR sense electron transfers occur first between electrode MD and the HOMO of the D-o-A molecule (1 ), and between the LUMO and MA (1), thus establishing the zwitterionic excited state D+-a-A , which then (2) relaxes back into the neutral state, (b) Rectification in the anti-AR sense if auto-ionization occurs first (1), forming the excited state D+-a-A by an interaction of the intense electric field and the molecule, followed by transfers to and from the electrodes (2) and (2 ), resulting in the electron passing from MD to MA. The molecular energy levels, which in reality must perforce shift dramatically during the electron transfer process, are drawn here for simplicity as if the molecule were still isolated in the gas phase ... 

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. 

In the following sections the effect of pressure on different types of electron-transfer processes is discussed systematically. Some of our work in this area was reviewed as part of a special symposium devoted to the complementarity of various experimental techniques in the study of electron-transfer reactions (124). Swaddle and Tregloan recently reviewed electrode reactions of metal complexes in solution at high pressure (125). The main emphasis in this section is on some of the most recent work that we have been involved in, dealing with long-distance electron-transfer processes involving cytochrome c. However, by way of introduction, a short discussion on the effect of pressure on self-exchange (symmetrical) and nonsymmetrical electron-transfer reactions between transition metal complexes that have been reported in the literature, is presented. 

More complicated reactions that combine competition between first- and second-order reactions with ECE-DISP processes are treated in detail in Section 6.2.8. The results of these theoretical treatments are used to analyze the mechanism of carbon dioxide reduction (Section 2.5.4) and the question of Fl-atom transfer vs. electron + proton transfer (Section 2.5.5). A treatment very similar to the latter case has also been used to treat the preparative-scale results in electrochemically triggered SrnI substitution reactions (Section 2.5.6). From this large range of treated reaction schemes and experimental illustrations, one may address with little adaptation any type of reaction scheme that associates electrode electron transfers and homogeneous reactions. 

Sometimes a metal electrode may be directly responsible to the concentration of an anion which either gives rise to a complex or a precipitate with the respective cations of the metal. Therefore, they are termed as second-order electrodes as they respond to an ion not directly involved in the electron transfer process. The silver-silver chloride electrode, as already described in Section 16.3.1.1.3, is a typical example of a second-order electrode. In this particular instance, the coated Ag wire when dipped in a solution, sufficient AgCl dissolves to saturate the layer of solution just in contact with the respective electrode surface. Thus, the Ag+ ion concentration in the said layer of solution may be determined by the status of the solubility product (Kvfa equilibrium ... 

One of the advantages of electrochemical methods over more conventional chemical methods is the fact that the actual electron transfer process can be carried out at an electrode with a far greater degree of control than with a solution reactant. By careful application of the appropriate electrochemical techniques, it is possible to define the sequence of chemical and electron transfer steps in a given electrochemical process with... 

The theory of homogeneous electron transfer processes, as well as of the closely-related electron exchanges with metallic electrodes, has been the subject of considerable study. The proposal by Hush and by Marcus that these processes are, for simple systems, either usually electronically adiabatic or... 

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). 

The so-called diffusion layer is still a region dominated by an unequal charge distribution (i.e. in such a zone the principle of electro-neutrality is not valid) due to the electron transfer processes occurring at the electrode surface. In fact, the electrode acts as an electrostatic pump for species of... 

In the sequence of reactions (2)-(3)-(4) it was assumed that electron exchange takes place without the interaction of the species Ox and Red with the electrode surface. However, it is possible that the exchange of electrons does not occur unless the reagent Ox, or the product Red, is weakly or strongly adsorbed on the electrode surface. It is also possible that the adsorption of the species Ox or Red might cause poisoning of the electrode surface, thus preventing any electron transfer process. 

Linear Diffusion at a Planar Electrode. Commonly, in order to deduce the mathematical relationships that govern the diffusion of an electroactive species towards the electrode, one considers an electron transfer process taking place at a planar electrode, in an unstirred solution, so to make active only the diffusive motion of the redox-active species in a direction perpendicular to the electrode surface, Figure 18. 

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