Applicability and limits of the ensemble model in catalysis by metals: The kinetics of ethane hydrogenolysis over Pt/SiO2

摘要

The rate of ethane hydrogenolysis over the EUROPT 1 (Pt/SiO2) catalyst has been studied over a large range of pressure and temperature. At ethane pressures, P-E, lower than 1.3 kPa the hydrocarbon coverage is shown to be negligible. For T, P conditions which correspond to a degree of hydrogen coverage, theta(H), below 0.6 ML (monolayer), the rate r is found to be related to theta(H) and P-E, by the equation, r = k(0)P(E)e(-E0/RT)(1 - beta theta(H))(X), With k(0) approximately 4 times smaller than the number of ethane molecules colliding with the Pt surface, E-0 = 55 +/- 7 kJ mol(-1), X = 9 +/- 2, and beta = 1.3 +/- 0.1. This rate equation which models on a quantitative ground the complex variations of r with T and P might be considered as describing a two-body process between gaseous ethane and surface ensembles composed of X adjacent platinum atoms free from adsorbed hydrogen, Eo being the activation energy. The constant beta takes into consideration the fact that at saturation the surface stoichiometry H/Pt-s is larger than unity. In a T, P domain which corresponds to hydrogen coverage above 0.6 ML, a marked change in the evolution of the kinetic parameters is observed: the apparent activation energy starts to decrease and the order versus the hydrogen pressure starts to be less negative by decreasing the temperature. The above purely statistical model does not apply anymore to this kinetic domain, which could be characterised by some H adsorption ordering. This work shows that the ensemble model, successfully applied to describe catalytic reactions occurring over Rh and Ni catalysts, can be extended to the case of Pt, within a domain characterized by a moderate hydrogen coverage and a purely statistical adsorption process. (C) 1998 Academic Press. [References: 33]