Catalytic Activity at Surfaces

1 diffusion of adsorptive reactants to the active site on the solid surface  

2 adsorption of one or more reactants (adsorbates) onto the surface if molecule A is chemically adsorbed onto one of the active sites, a surface complex (S-A) is formed  

3 surface reaction A reacts with B forming molecule (A + B)  

4 desorption of products from the surface (A + B) escapes the site, thus regenerating site 

The above scheme shows the importance of both adsorption and desorption processes. Adsorption of at least one of the reactant molecules is required for catalysis to occur. If the accelerated rate of reaction is simply due to the concentration of molecules at the surface, catalysis may result from physisorption of the reactants. On the other hand, chemisorption can be used primarily to quantitatively evaluate the number of surface active sites, which are likely to promote (catalyze) chemical reactions. Chemisorption analyses are applied to physically characterize a catalyst material, to determine a catalyst s relative efficiency in promoting a particular reaction, to study catalyst poisoning, and in monitoring the degradation of catalytic activity over time of use. 

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Chemisorption denotes the situation in which an actual chemical bond is formed between the molecules and the surface atoms. A molecule undergoing chemisorption may lose its identity as the atoms are rearranged, forming new compounds that better satisfy the valences of the surface atoms. The enthalpy of chemisorption is much greater than that of physical adsorption. The basis of much catalytic activity at surfaces is that chemisorption may organize molecules into forms that can readily undergo reactions. It often is difficult to distinguish between chemisorption and physical sorption, because a chemisorbed layer may have a physically sorbed layer deposited above it. 

Catalytic activity at surfaces adsorption and catalysis, P. W. Atkins, Physical Chemistry, 4th ed., 1990, 895. Oxford Oxford University Press. 

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