CATALYTIC AND STRUCTURAL-PROPERTIES OF RUTHENIUM BIMETALLIC CATALYSTS - KINETICS OF HYDROGENOLYSIS OF LOWER ALKANES ON VARIOUSLY PRETREATED RU/AL2O3 CATALYSTS

The H-2 pressure dependence of rates of hydrogenolysis of ethane, propane and n-butane on Ru/Al2O3 catalysts differing in dispersion and type of pretreatment has been measured at a number of temperatures, and the results interpreted in terms of a mechanism involving adsorbed partially dehydrogenated intermediates CnHx, and modelled by the derived rate expression. The rate-limiting step is taken as the reaction of CnHx with an adsorbed H atom. We thus obtain best-fit values of the rate constant k(1), the H-2 adsorption equilibrium constant K-H, an equilibrium constant for the dehydrogenation of the alkane K-A, and of x, for each set of results. The shapes of the kinetic curves, and the constants that describe them, change markedly with dispersion, and with pretreatment: oxidation and low-temperature reduction (O/LTR), as well as causing some loss of dispersion, gives rise to other effects, ascribed to 'morphological' factors, not seen when catalysts are reduced at high temperature (753 K). What is most striking is that differences in activity seem to be determined much more by the constants K-A and K-H than by the rate constant k(1), which when expressed per Ru surface atom varies at most three-fold. In particular K-H is much larger after the first high-temperature reduction (HTR1) than after O/LTR. The true activation energy derived from the temperature-dependence of k(1) is about the same for each alkane (approximate to 60 kJ mol(-1)), the enthalpy changes for H-2 chemisorption are small and for alkane dehydrogenation they lie between 50 and 130 kJ mol(-1). The manner in which product selectivities vary with H-2 pressure also depends on dispersion and pretreatment, the dominant factor being the strength of H-2 chemisorption. Thus on a very highly dispersed catalyst for which K-H is large, intermediate product selectivities are high because the high concentration of H atoms facilitates desorption of adsorbed species, and for this reason also selectivities scarcely respond to changes in H-2 pressure. With the same catalyst after O/LTR, however, when K-H is much decreased, selectivities respond sensitively to H-2 pressure because the adsorption is weaker. It is then deduced that approximately two more H atoms are required to effect desorption of intermediates as a product alkane than to cause further C-C bond breaking. Our results strongly suggest that structure-sensitivity in alkane hydrogenolysis is more the result of variations in chemisorption energetics, and their consequential effects on surface coverage, than of kinetic effects; this concept also accounts for dispersion-dependent differences in the temperature-dependence of product selectivities previously reported.