Non-equilibrium vibrational kinetics in adlayers: outline of an alternative approach to catalytic processes

Abstract

It is suggested that catalytic processes at solid surfaces should be treated according to non-equilibrium vibrational kinetics as opposed to current Boltzmann equilibrium assumptions. Vibrational non-equilibrium in adlayers can be established as a result of competition between processes leading to disposal of vibrational energy to the solid and processes of anharmonic vibration–vibration up-pumping in the adlayer, regarded as a two-dimensional gas. The chemisorption kinetics presented leads to quasi-steady-state conditions defined by non-equilibrium Treanor-like vibrational distribution functions exhibiting an overpopulation of upper vibrational levels of surface species with respect to corresponding Boltzmann values. It is shown that the activation entropy of a catalytic isotope exchange reaction can be taken as a measure of this overpopulation. Experimental rates of heterogeneous isotope exchange reactions and connected chemisorption data are presented supporting this point of view, which provides, in particular, a straightforward interpretation of the long debated `compensation effect' in heterogeneous catalysis.