Positive hole quasi-free

Fig. 2.10 Schematic illustration of the mutual exclusion zone or exchange-correlation hole about a given electron within a free-electron gas. The hole has a radius, r8, corresponding to exactly one electron being excluded, thereby revealing one positive charge of underlying jellium background. The electron plus its positive hole move together through the gas of other electrons as though they are a neutral entity or quasi-particle.

A very clear-cut example for the influence of the electronic factor in supported catalysts, again involving a thin-layer metal type, is represented in Fig. 2. Here the carriers are commercially available samples of doped carborundum (SiC) which by itself is catalytically entirely inactive. In the abscissa of Fig. 2 we have arranged these samples in the order of their conductivity as stated by their manufacturers. The concentration of positive holes increases towards the right and that of the quasi-free electrons towards the left. Grains of these supports approximately 1 mm in size were covered with a thin layer of silver by the usual mirror produc-... 

A series of examples has become known recently, and more are reported in this volume, of catalytic reactions on oxide surfaces, involving electron transfer from reactant molecules to the catalyst, or vice versa. The general electronic concept of catalytic activation, first established for metals and alloys, has thus been extended to semiconductors. It appears certain that mobile quasi-free electrons or positive holes can migrate to the surface and can there bind reactant molecules in a charged or polarized state. This presupposes the presence of electrons in the conduction band (or of holes in the valence band), which in normal oxide semiconductors contains appreciable concentration of electrons only at elevated temperatures. Hence, the examples mentioned refer to high-temperature catalysis (N2O decomposition, CO oxidation). At ordinary temperatures, only those substances capable of releasing electrons from surface atoms or surface bonds, i.e., solid Lewis bases, are suitable as catalysts. This has been shown (I) to be true for the decomposition of ozone by various metal oxides. 

In n-type metal oxides, electrical conductivity arises by means of quasi-free electrons that exist because of an excess of electrons present in the lattice. N-type metal oxides are generally not active oxidation catalysts, although vanadium pentoxide (V Oj) is a notable exception. P-type metal oxides are electron-deficient in the lattice and conduct electrons by means of positive holes. These oxides are generally active oxidation catalysts. Insulators have very low electrical conduaivities because of the strictly stoichiometric metal-oxygen ratio in the lattice and very low electron (or positive hole ) mobility and are generally not active catalysts. However, insulators are often used as catalyst supports. 

At the anode neutralization of negative ions and quasi-free electrons acquired from the liquid and the possible injection of positive holes or ions need to be considered. 

The Perez model comes from an approach in which the source of mobility is the existence of quasi-punctual defects characterized by positive or negative fluctuations of packing density, whereas classical free volume theories take into account only the domains of low packing density, e.g., the holes. The model leads to the following equation for the complex modulus ... 

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