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Abstract: A model kinetics is presented for describing adatom recombination ruled by the ‘hot atom’ mechanism. Hot atoms are identified with adatoms populating the excited vibrational states of the adsorption potential well. The disposal of vibrational quanta to the solid and the transfer of quanta among adatoms are taken into account by the kinetics. In the model case of three-levels the kinetics is solved analytically. It is shown that the competition between recombination and quantum transfer to the solid may drive the system far from equilibrium, leading to an enhancement of the reaction rate. The equilibrium non-equilibrium transition is studied in terms of control parameters, such as the gas flux and the vibrational relaxation time, and it is further characterized through the behaviour of the “apparent” activation energy of the reaction which is found to depend, strongly, on the vibrational state of the adlayer. [Copyright &y& Elsevier]

Abstract: In the framework of the ‘hot atom’ reaction mechanism we report on the rate of adatom recombination at catalytic surfaces. The modeling deals with a flat surface where hot atoms are identified with the adatoms populating the excited states of the vibrational ladder of the adsorption potential well. The dynamics of the energy transfer between the adlayer and the solid, that is the exchange of vibrational quanta between the adatoms and the solid, has been explicitly taken into account in the master equations together with the adsorption and recombination processes. At steady state the analytical solution of the kinetics is found to be in good agreement with the numerical solutions for a four-levels system. It is shown that the recombination rate at steady state is strongly dependent upon the ratio between the probabilities (per unit time) of quantum dissipation to the solid and of adatom recombination. This ratio is a key quantity of the kinetic model since it rules a continuous transition towards higher rates which is ascribed to a progressive displacement of the vibrational state of the adlayer, from the equilibrium condition. [Copyright &y& Elsevier]

It is shown that a vibrational non-equilibrium model of the chemisorption process can be employed for interpreting experimental data on adsorption-stimulated desorption. Further analysis of chemisorption rates, on a variety of substrates, also leads to the conclusion that desorption can be stimulated by the adsorption process, which determines an enhancement of the population of the upper bound vibrational level of the adspecies. The presence of this stimulation is revealed by a significant modification of the adsorption kinetics with respect to those expected under Boltzmann equilibrium conditions. An inverse correlation between rates of desorption and rates of vibrational relaxation of the adspecies stems from this analysis. A decrease of the relaxation rates with increasing surface coverage is also obtained for all the systems investigated. [Copyright &y& Elsevier]

Non-equilibrium vibrational kinetics, as recently developed for adlayers, is applied to the catalytic oxidation of CO by either molecular or atomic oxygen. This model allows one to fit experimental distribution functions of the product CO2(g) molecules leaving the surface with hyperthermal energies. An LH* mechanism involving non-equilibrium vibrational distribution functions (vdfs) of the relevant adspecies is presented showing the basic consequences of non-equilibrium at the catalytic surface. In this picture `hot atoms'' are seen as mobile, vibrationally excited adspecies with their vibrational ladders populated according to non-equilibrium Treanor distribution functions. [Copyright &y& Elsevier]