Radical atomic adsorption

These systems represent bonding to surfaces where the adsorbate atoms have unpaired electrons available for covalent interaction with unsaturated electronic states on the metal surface. We denote this bonding mechanism as radical adsorption where the open-shell electrons on the adsorbate atom can form electron pairs with the metal atoms at the surface. These radical atoms have in most cases been obtained through the dissociation of molecules on the surface. Let us make a simple picture of the electronic structure when a simple atomic adsorbate interacts with a transition metal, denoted the d-band model [31,32]. A similar description can also be found in Chapter 4. 

Adsorbate Electronic Structure and Bonding on Metal Surfaces 

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The subject of reactive SiOa has been under development for about ten years. By reactive Si02 is meant an ordinary high surface area silica, methoxylated and then pyrolysed at high temperatures. Chemisorptive centres are formed, thought to be two Si radicals associated with two anomalously reactive atoms. Adsorption of O2 gives rise to a new, 0-contain-... 

The same Chapter contains results of studies of effects of adsorption of atom particles as well as simplest free radicals on electric conductivity of semiconductor zinc oxide films. 

So far, we have focused our attention on adsorption of donor particles on semiconductor oxides. As for the effect of adsorption of acceptor particles on electrophysical characteristics, in concurrence with conclusions made none of adsorption phenomenon involving such characteristic acceptor particles as molecular and atom oxygen on -semiconductor, atoms of nitrogen and simplest alkyl and amine radicals brought about a non-monotonous change in characteristics of adsorbents, despite the fact that experiments had been conducted at various conditions. 

It has been proven by experiment that there are donor acceptor atoms and molecules of absorbate and their classification as belonging to one or another type is controlled not only by their chemical nature but by the nature of adsorbent as well (see, for instance [18, 21, 203-205]). From the standpoint of the electron theory of chemisorption it became possible to explain the effect of electron adsorption [206] as well as phenomenon of luminescence of radical recombination during chemisorption [207]. The experimental proof was given to the capability of changing of one form of chemisorption into another during change in the value of the Fermi level in adsorbent [208]. 

The results mentioned together with data outlined in Section 1.11 indicate that adsorption induced change in electric conductivity of sintered and partially reduced oxide is mostly dependent on adsorption related change in concentration of stoichiometric metal atoms which are responsible for dope electric conductivity rather than by charging of the surface of adsorbent due to transformation of radicals of O2 and O". 

These expressions describe the kinetics and the value of response of electric conductivity in oxide adsorbent during adsorption of atomic hydrogen. In general case they are applicable for adsorption of any radical particles of the donor type. However, in each specific case any of the processes of schematics (2.114) controlled by specificity of each system [129] may be a dominant process controlling both the kinetics and the stationary value of electric conductivity. 

Thus, we have considered in detail various theoretical models of effect of adsorption of molecular, atom and radical particles on electric conductivity of semiconductor adsorbents of various crystalline types. Special attention has been paid to sintered and partially reduced oxide adsorbents characterized by the bridge type of intercrystalline contacts with the dominant content of bridges of open type because of wide domain of application of this very type of adsorbents as sensitive elements used in our physical and chemical studies. 

N.N. Savvin, Mechanism of Adsorption of Atoms, Radicals and Some Simple Molecules on Metal Oxides According to the Data on Electroconductivity and IR-spectroscopy, PhD (Chemistry) Thesis, Moscow, 1980... 

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