Dynamics of Surface Reactions

Abstract

Catalysis concerns the rate of a chemical reaction which is usually expressed as a function of the concentrations of the species involved in the reaction and a set of rate constants k i which depend on temperature T. Within the framework of transition state theory (TST) [1], which forms the basis of chemical kinetics, the latter are given bywhereby E* is the activation energy (= the height of the energy barrier along the 'reaction coordinate', i.e. the path with minimum energy with respect to all other degrees of freedom for nuclear motion). The term ΔS *The basic idea of TST consists in the assumption that at all stages along the reaction coordinate thermal equilibrium is established, so that the temperature (T) is the only essential (macroscopic) parameter. This requires that energy exchange between the various degrees of freedom of the particle interacting with the surface and the heat bath of the solid occur much faster than the process of chemical transformation. This chapter will discuss effects, for which this condition is not fulfilled and hence the observed phenomena are governed by dynamics rather than by kinetics. Energy exchange between adsorbate and solidAn estimate of the typical energy relaxation time of a chemisorbed particle, t relax , can be obtained from ob-

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