Electron tunneling heterogeneous

Examples of electron tunneling reactions on the surface of heterogeneous catalysts have been discussed in Chap. 7. These reactions provide electron transfer between spatially separated donor and acceptor centres on the surface of heterogeneous catalysts as well as between the centres one of which is on the surface of the catalyst and the other is in the subsurface layer. Such processes are expected to be important for photocatalytic reactions, as well as for thermal catalytic reactions proceeding at low temperatures by heterolytic mechanisms. 

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... 

Figure 2.9 Schematics illustrating an adsorbate-electrode interface in aqueous solution, plus the corresponding thermal activation and electron tunneling steps associated with a heterogeneous electron transfer process...

An even more fundamental reason why protein structural heterogeneity is not selected to assist or retard electron tunneling arises from the... 

The experimental detection and quantification of surface species on in situ soil particles and other natural colloids is a difficult area of research because of the sample heterogeneity, low surface concentrations, and the necessity to investigate the solid adsorbents in the presence of water. Unambiguous information can be obtamed only with in situ surface spectroscopy, such as x-ray photoelectron (XPS), extended x-ray absorption hne structure (EXFAS), x-ray absorption near-edge structure (XANES), melastic electron tunneling (lETS), and electron energy loss (EELS) spectroscopies. Recent advances in the development of nonevasive, in situ spectroscopic scarmed-probe and microscopic techniques have been applied successfully to study mineral particles in aqueous suspensions (Hawthorne, 1988 Hochella and White, 1990). 

There are two distinguishable mechanisms of the electrical conduction in such heterogeneous mixtures. One is the percolation mechanism, and the other is the electron tunneling mechanism. 

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