Non-equilibrium vibrational steady state in chemisorption and catalysis

Abstract

The possibility for obtaining non-equilibrium vibrational steady state of the adlayer in chemisorption and catalysis, that is non-Boltzmann distribution of the vibrational ladder, has been studied by kinetic approach. The processes of vibrational quantum transfer between adatoms and either the solid or the gas lattice have been modeled by rate equations in terms of relaxation times for quantum exchange. It is shown that, in general, chemisorption occurs under non-equilibrium steady state which exhibits a displacement from the Boltzmann distribution depending upon adsorption flux and relaxation times. In case of large adsorption enthalpies it is found that the overpopulation of the upper bound vibrational level, with respect to the Boltzmann population, can be much greater than unity. The consequences of this effect on the overall rate of chemisorption are analysed and discussed. The kinetic approach is also employed for studying the steady state distributions in the model case of a simple exoergic reaction. The computation indicates that the system is bistable, namely it admits two steady state solutions which are the Boltzmann distribution and the non-equilibrium distribution derived by Treanor et al. [J. Chem. Phys. 48 (1968) 1798]. The conditions that ought to be verified to ensure the settlement of the non-equilibrium state are established in terms of the anharmonicity of the potential, recombination probability and relaxation times.