Catalysis Controlled by the Constitution of Active Sites

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Publisher Summary This chapter describes catalysis controlled by the constitution of active sites. A catalytic reaction is composed of a series of elementary processes, and the surface atoms are necessarily involved in making intermediates or activated complexes, as well as in the adsorption of reactant and product molecules. The participation of surface atoms is indispensable in all of the elementary processes accomplishing the catalysis, but the kinetics is mainly influenced by the manner of the participation of surface atoms in the rate-determining steps. The orbital symmetry rule in chemical reactions suggests that the highest occupied molecular orbital (HOMO) of one reaction partner and the lowest unoccupied molecular orbital (LUMO) of the other should meet the symmetry requirements. The catalytic isomerization reaction of olefins is caused by either an associative or a dissociative mechanism. The associative mechanism involves either σ-alkyl or alkyl ion intermediates, and the dissociative mechanism involves allylic intermediates. Physical processes—such as internal and external rotation of intermediates—are indispensable elementary processes in some catalytic reactions.