Single Atom and Multiatom Sites

These terms are simply convenient labels for two broad classes of catalytic reactions and should not be considered an absolute characterization. While changes in areal TOF with variation in catalyst dispersion can be understood, the rationale presented to explain structure insensitivity is rather complex.24 This is especially true in that a growing number of such reactions have been shown to have areal TOFs that vary with changing catalyst dispersion when they are run imder appropriate reaction conditions.25-27 u would seem, then, that a criterion 

Similar conclusions have been drawn from a study of the hydrogenolysis and isomerization of cyclopropane over catalysts made up of supported rhenium clusters of differing nuclearity.29 The hydrogenolysis of cyclopropane was found to take place only over those catalysts having at least a Re3 nuclearity while the hydrogenation of propene, an isomerization product of cyclopropane, took place on Re I species (Eqn. 3.1). 

These results indicate that, perhaps, a better definition of structure sensitive reactions would be those that occur over ensembles of surface atoms while structure insensitive reactions are those that are promoted by single atom active sites. 

It is necessary, however, to define more clearly the nature of the specific active sites, be they ensembles or single atoms, that are responsible for promoting specific reactions. A number of approaches to this problem have been made over the years with varying degrees of success. 

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