SOURCE OF THE ACTIVATION-ENERGY IN HETEROGENEOUSLY CATALYZED-REACTIONS

Problems inherent in measuring accurately the rates of catalysed reactions, and in defining catalytic activity, are reviewed in the context of reactions of alkanes on supported metals. It is shown how compensation effects can arise by use of apparent activation energies, generated either by improper treatment of results or by variation of reactant concentrations. The source of true activation energy in catalysed reactions is thought to be lattice vibrational modes (phonons) which form a 'heat bath' from which adsorbed entities are activated to the transition state. This concept provides a logical basis for understanding why reactions of low structure-sensitivity manage with a small active centre, show small activation energies, and proceed at low temperatures, while the converse is true for reactions of high structure-sensitivity. This model also offers a satisfactory explanation for the compensation effect, associated with variation of true activation energy.