Chemisorption and active sites

Heterogeneous catalysis occurs on the surfaces of solid materials. These surfaces are almost invariably covered by various species—water, carbon dioxide, and other materials (including ions) from the preparative mixture or atmosphere. The majority of these species are very weakly adsorbed, or physisorbed, via van der Waals forces. However, for a chemical reaction involving an otherwise stable molecule to be initiated, a significant electronic disturbance in that molecule, or even bond scission, must occur at the catalytic surface. 

The adsorption process that brings about such a chemical modification, which may occur in several steps, is usually referred to as chemisorption. Furthermore, when two or more molecules are involved in a reaction on a catalytic surface, as in hydrogenation or oxidation processes, we usually find evidence that the major reactions occur between chemisorbed or surface species derived from each of the reactant molecules. 

It was recognized early in the history of heterogeneous catalysis that, in many instances, only a relatively small proportion of the surface was catalytically active. The pre-exponential rate factor was seen to be very small in relation to likely collision frequencies of molecules adsorbed on the surface, taking into account steric requirements, while poisoning (inhibition of the catalysed reaction) could result from surprisingly low levels of specific impurities (see below). Hence the term active sites was coined to describe those localities on the surface which would induce the desired chemical reaction. 

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