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

It is suggested that catalytic processes at solid surfaces should be treate d according to non-equilibrium vibrational kinetics as opposed to current B oltzmann equilibrium assumptions. Vibrational non-equilibrium in adlayers c an be established as a result of competition between processes leading to d isposal of vibrational energy to the solid and processes of anharmonic vibr ation-vibration up-pumping in the adlayer, regarded as a two-dimensional ga s. The chemisorption kinetics presented leads to quasi-steady-state conditi ons defined by non-equilibrium Treanor-like vibrational distribution functi ons exhibiting an overpopulation of upper vibrational levels of surface spe cies with respect to corresponding Boltzmann values. It is shown that the a ctivation entropy of a catalytic isotope exchange reaction can be taken as a measure of this overpopulation. Experimental rates of heterogeneous isoto pe exchange reactions and connected chemisorption data are presented suppor ting this point of view, which provides, in particular, a straightforward i nterpretation of the long debated 'compensation effect' in heterogeneous ca talysis. (C) 2000 Elsevier Science B.V. All rights reserved.