The Electronic Factor in Heterogeneous Catalysis

Publisher Summary This chapter describes the electronic factor in heterogeneous catalysis. In quantum mechanical theory, the molecular orbital treatment of a crystalline solid considers the outer electrons as belonging to the crystal as a whole. At temperatures above absolute zero, there are two possible processes that may occur: either electrons may be thermally excited from the impurity levels to the empty band or electrons from the filled band may be excited into the impurity levels. In both cases, partly occupied bands are produced and the crystal becomes conducting. Chemisorption of one or more of the reactants is essential in heterogeneous catalytic reactions, and a consideration of the nature of the bond between surface and substrate is, therefore, of decisive importance. The effectiveness of atoms or molecules as catalyst poisons will depend on their size and strength of bonding to the catalyst and will, therefore, depend on geometric and electronic considerations. The chemical bond by means of which the poison is linked to the metallic surface resembles the ordinary dative bond in which the poison is donor.