Interpretation of very large differences in catalytic activity

3 Interpretation of very large differences in catalytic activity 

Such large differences are not readily accessible to experimental verification for sites on a given surface. But they are found when certain reactions are studied on different catalysts. How can they be explained  

In the point of view of Boudart, if the reactant involved in the rate determining step requires several adjacent surface atoms, i.e., a multiple sites (ensemble, cluster), a broad spectrum of activities is expected, and the larger the size of the cluster, the broader distribution the is. This is easily seen with a simple two-step sequence  

The overall reaction is A = B with the species B being the most abundant reaction intermediate, and A requires n adjacent surface atoms in the rate-determining adsorption step. The rate is then  

The Bronsted relation with a = 0.5 relates the equilibrium constant to the rate constant  

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