Chemical basis of metal catalyst promotion

A survey is presented of the different modes of action of metal catalyst promotors. Broadly two main categories can be distinguished: structural and bifunctional promotors. In structural promotion the promoting atoms do not participate themselves in the elementary steps of a catalytic reaction. Bifunctional promotors participate themselves chemically in the catalytic reaction. Five different kinds of structural promotion can be recognized: ensemble-size regulating; valence-state promotion; polarizing-structural promotion; surface-phase and cluster complex promotion; particle size stabilizing. Each subcategory is discussed shortly and when relevant illustrated with a practical example. Three different kinds of bifunctional promotors can be distinguished: Polarizing-bifunctional promotion; direct-bifunctional promotion; chemical-bifunctional promotion. Whereas conceptually different, subcategories are not always easily experimentally distinguishable. Of most of the different kinds of promotion theoretical treatments exists. Quantum-chemical studies are referred to in the text