Ettore Molinari

Vibrational Non-Equilibrium at Catalytic Surfaces

Inclusion in a surface reaction scheme of fast V-V up-pumping processes, bound to the anharmonicity of the vibrational levels of mobile surface species, can lead to quasi-steady states characterized by non-equilibrium Treanor vibrational distribution functions, with ν > 0 levels over-populated with respect to Boltzmann equilibrium. Non-equilibrium distributions are shown to depend on the intensity of the chemisorption processes active under the working conditions of the catalyst. The shape of these distributions determines the surface reaction rate, and reactions with widely different “true” activation energies can proceed at comparable speeds at finite surface temperatures. This provides an interpretation of the isokinetic relationship in heterogeneous catalysis based on a non-Boltzmann picture of the catalytic surface.

Evidence for Vibrational Excitation of the Adlayer in Exoergic Processes at Metal Surfaces: H-atom Abstraction and Recombination and Adsorption-stimulated Desorption of CO

Abstract The theoretical model presented in the previous paper predicts the possibility of vibrational excitation of the adlayer in exoergic process at metal surfaces to an extent determined by the interplay of reaction rates and energy dissipation into the metal. In the present paper this model will be employed for studying the following systems: a)The abstraction of Ds adspecies by Hgas and the accompanying H-atom recombination and b) the adsorption-stimulated desorption of COs in the presence of adsorbing COgas. Proper reduction of literature data provides the evidence for the existence of vibrational excitation of the H-Me and of the CO-Me adlayers, witnessed by desorption rates that are orders of magnitude larger than those expected for systems in Boltzmann equilibrium. Application of the model to Ds+Hgas relates the vibrational excitation of the adlayer and the corresponding non-Boltzmann desorption rates to the parameter Z/K and to the flux of adsorbing species. Rate coefficients K for vibrational relaxation of the H-Me bondare in the range 1013-1012 s−1 and decrease with increasing surface coverage σ. The analysis of COs desorption in the presence of adsorbing COgas confirms the dependence of desorption rates on Z/K, the coverage dependence of the rate coefficients K for energy relaxation of the CO-Me bond and brings out the predicted influence of gas pressure on the over-population of the vibrational levels of adsorbed CO. The decrease of K observed in both systems is discussed in terms of energy relaxation processes involving electron-hole pair excitation at the metal surface and it should be linked to the decrease of the surface electron density caused by the adsorbates.

Kinetic Analysis of High-Rate and of Low-Rate Regimes in CO Oxidation on Pt Group Metals: Evidence for Vibrational Excitation of the O-Rich Adlayer and for Thermal Equilibrium of the CO-Rich Phase

Abstract Proper reduction of available literature data on the heterogeneous CO+O2 reaction on Pt group metals allows one to describe the parallel process of CO desorption, in extended intervals of temperature and gas composition, both in the High-Rate regime of an oxygen rich surface and in the Low-Rate regime of a CO rich surface. The outcome of this analysis is the determination of “apparent” as well as of “true” activation energies for both CO oxidation and desorption and their dependence on surface coverage by O or by CO-adspecies. The conclusion is reached that non-equilibrium conditions of vibrational excitation of the adlayers prevail in the HR regions, while Boltzmann equilibrium characterizes LR regimes in extended temperature and pressure intervals. These results have been discussed in the framework of a theoretical model that predicts hyperthermal vibrational states of the adlayer, which depend on the efficiency of energy dissipation into the metal.