Types of Chemisorption Bonds

Every heterogeneous catalytic process begins with the act of adsorption. Therefore, the theory of heterogeneous catalysis should proceed from the 

A system of adsorbed particles is often treated as a two-dimensional gas covering the adsorbent surface. Such an approach is quite justified and fruitful, as long as we are dealing with physical adsorption when the influence of the adsorbent on the adsorbate can be regarded as a weak perturbation. In case of chemical adsorption (the most frequent in catalysis), the concept of a two-dimensional gas becomes untenable. In this case the adsorbed particles and the lattice of the adsorbent form a single quantum-mechanical system and must be regarded as a whole. In such a treatment the electrons of the crystal lattice are direct participants of the chemical processes on the surface of the crystal in some cases they even regulate these processes. 

We shall proceed from a concept which in a certain sense is contrary to that of the two-dimensional gas. We shall treat the chemisorbed particles as impurities of the crystal surface, in other words, as structural defects disturbing the strictly periodic structure of the surface. In such an approach, which we first developed in 1948 (I), the chemisorbed particles and the lattice of the adsorbent are treated as a single quantum-mechanical system, and the chemisorbed particles are automatically included in the electronic system of the lattice. We observe that this by no means denotes that the adsorbed particles are rigidly localized they retain to a greater or lesser degree the ability to move ( creep ) over the surface. 

In such an approach there is no fundamental difference between the chemisorbed particles and the biographical structural defects which are always present on any real surface. The only difference is that the chemisorbed particles are free to leave the surface for the gaseous phase and to return to the surface from this phase, whereas the biographical defects must be considered as rigidly bound to the surface and incapable of interchange with the gaseous phase. 

In a number of theoretical papers it has been shown that a chemisorbed particle considered as a structural defect of the surface is a center of localization for the free electrons of the lattice, serving as a trap for them and thus playing the part of an acceptor for the free electrons. Or (depending on the nature of the particle) it may serve as a center of localization of a free hole, thus playing the part of a donor. 

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Different types of chemisorption sites may be observed, each with a characteristic A value. Several adsorbed states appear to exist for CO chemisorbed on tungsten, as noted. These states of chemisorption probably have to do with different types of chemisorption bonding, maybe involving different types of surface sites. Much of the evidence has come initially from desorption studies, discussed immediately following. 

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