Electron transfer through oxide

A lively subsection in applications of quantum theory to transitions at electrodes concerns the tunneling of electrons through oxide films. This work has been led by Schmickler (1980, 1996), who has used a quantum mechanical approach known as resonance tunneling to explain the unexpected curvature of Tafel lines for electron transfer through oxide-covered electrodes (Fig. 9.21). 

Dehydrogenation, Ammoxidation, and Other Heterogeneous Catalysts. Cerium has minor uses in other commercial catalysts (41) where the element s role is probably related to Ce(III)/Ce(IV) chemistry. Styrene is made from ethylbenzene by an alkah-promoted iron oxide-based catalyst. The addition of a few percent of cerium oxide improves this catalyst s activity for styrene formation presumably because of a beneficial interaction between the Fe(II)/Fe(III) and Ce(III)/Ce(IV) redox couples. The ammoxidation of propjiene to produce acrylonitrile is carried out over catalyticaHy active complex molybdates. Cerium, a component of several patented compositions (42), functions as an oxygen and electron transfer through its redox couple. 

Damjanovic, A., Birss, V. I. and Boudreaux, D. S. (1991) Electron transfer through thin anodic oxide films during the oxygen evolution reactions at Pt... 

In the thermodynamically redox-stable resting state, CcOs all Cu ions are in the Cu state and all hemes are Fe . From this state, CcOs can be reduced by one to four electrons. One-electron reduced CcOs are aerobically stable with the electron delocalized over the Cua and heme a sites. The more reduced forms—mixed-valence (two-electron reduced), three-electron reduced, and fully (four-electron) reduced—bind O2 rapidly and reduce it to the redox level of oxide (—2 oxidation state) within <200 p-s [Wikstrom, 2004 Michel, 1999]. This rate is up to 100 times faster than the average rate of electron transfer through the mammalian respiratory chain under normal... 

An interesting way to generate telluronium dications involves electron transfer through a 71-conjugated system to a spatially remote sulfoxide sulfur atom in a domino manner. Treatment of substrate 141 with triflic anhydride results in reduction of the terminal sulfoxide group with simultaneous oxidation of the tellurium atom in the para-position and formation of a trichalcogen dicationic moiety 144143 through the intermediate sulfonium salt 142 and quinoid structure 143 (Scheme 52). 

Analysis of the above data led to the conclusion that all of the redox reactions proceed with electron transfer through the [CuL2]2+/+ redox couple, and that the change in number of ligands occurs in the Cu(I) oxidation state. This interpretation is given as pathway I in Fig. 5. 

The polypyrrole molecular interface has been electrochemically synthesized between the self-assembled protein molecules and the electrode surface for facilitating the enzyme with electron transfer to the electrode. Figure 9 illustrates the schematic procedure of the electrochemical preparation of the polypyrrole molecular interface. The electrode-bound protein monolayer is transferred in an electrolyte solution containing pyrrole. The electrode potential is controlled at a potential with a potentiostat to initiate the oxidative polymerization of pyrrole. The electrochemical polymerization should be interrupted before the protein monolayer is fully covered by the polypyrrole layer. A postulated electron transfer through the polypyrrole molecular interface is schematically presented in Fig. 10. 

Oxidative Phosphorylation Electron transfer through the cytochrome system liberating free energy which is transformed into high-energy phosphate bonds. [NIH]... 

In any cell that depends on aerobic metabolism, if the rate of ATP utilisation increases, the rate of the Krebs cycle, electron transfer and oxidative phosphorylation must also increase. The mechanism of regulation discussed here is for mammalian skeletal muscle since, to provide sufficient ATP to maintain the maximal power output, at least a 50-fold increase in flux through the cycle is required so that the mechanism is easier to study (Figure 9.22). 

Eventually, the electrons in PQBH2 pass through the cytochrome b6f complex (Fig. 19-49). The electron initially removed from P680 is replaced with an electron obtained from the oxidation of water, as described below. The binding site for plastoquinone is the point of action of many commercial herbicides that kill plants by blocking electron transfer through the cytochrome b6f complex and preventing photosynthetic ATP production. 

In general, the electron transfer reaction (Reaction 2) controls the over-all rate of reaction, and the nature of the catalyst has a profound effect on the kinetics of oxidation (23). Thus Wallace et al. (23) have compared the catalytic efficiencies of metal phthalocyanines with metal pyrophosphates, phosphates, phosphomolybdates, and phosphotungstates. The activity of metal pyrophosphates was ascribed to the ease of electron transfer through the metal coordination shell, the reaction being suggested to occur at the solid pyrophosphate-liquid interface. On the other hand, the catalytic effectiveness of a series of metals, added to solution as simple salts, has been explained in terms of their ability to form soluble complexes containing thiols (13). It was not clear whether the high rates of oxidation were caused by the solubility of metal complexes or by the peculiar nature of the thiol complexes. 

These are hemoproteins that catalyze electron transfer through the reversible change in oxidation state of the heme iron.637 They are involved in the respiratory chain, and in a wide range of other processes such as photosynthesis and the nitrogen cycle. Over 50 cytochromes have been studied, notably cytochrome c, which is one of the best studied biological molecules. Bacteria in particular produce a wide range of cytochromes which currently are attracting much attention. 

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