APPLICABILITY AND LIMITS OF THE ENSEMBLE MODEL IN CATALYSIS BY METALS- THE KINETICS OF ETHANE HYDROGENOLYSIS OVER PT SIO2/

The rate of ethane hydrogenolysis over the EUROPT 1 (Pt/SiO2) catalyst has been studied over a large range of pressure and temperature. At e thane pressures, P-E, lower than 1.3 kPa the hydrocarbon coverage is s hown to be negligible. For T, P conditions which correspond to a degre e 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 s maller than the number of ethane molecules colliding with the Pt surfa ce, E-0 = 55 +/- 7 kJ mol(-1), X = 9 +/- 2, and beta = 1.3 +/- 0.1. Th is rate equation which models on a quantitative ground the complex var iations of r with T and P might be considered as describing a two-body process between gaseous ethane and surface ensembles composed of X ad jacent platinum atoms free from adsorbed hydrogen, Eo being the activa tion energy. The constant beta takes into consideration the fact that at saturation the surface stoichiometry H/Pt-s is larger than unity. I n 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 versu s the hydrogen pressure starts to be less negative by decreasing the t emperature. The above purely statistical model does not apply anymore to this kinetic domain, which could be characterised by some H adsorpt ion ordering. This work shows that the ensemble model, successfully ap plied to describe catalytic reactions occurring over Rh and Ni catalys ts, can be extended to the case of Pt, within a domain characterized b y a moderate hydrogen coverage and a purely statistical adsorption pro cess. (C) 1998 Academic Press.