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

The H-2 pressure dependence of rates of hydrogenolysis of ethane, prop ane and n-butane on Ru/Al2O3 catalysts differing in dispersion and typ e of pretreatment has been measured at a number of temperatures, and t he results interpreted in terms of a mechanism involving adsorbed part ially dehydrogenated intermediates CnHx, and modelled by the derived r ate expression. The rate-limiting step is taken as the reaction of CnH x 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 equilib rium 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 consta nts that describe them, change markedly with dispersion, and with pret reatment: 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 hi gh temperature (753 K). What is most striking is that differences in a ctivity seem to be determined much more by the constants K-A and K-H t han by the rate constant k(1), which when expressed per Ru surface ato m varies at most three-fold. In particular K-H is much larger after th e first high-temperature reduction (HTR1) than after O/LTR. The true a ctivation energy derived from the temperature-dependence of k(1) is ab out the same for each alkane (approximate to 60 kJ mol(-1)), the entha lpy changes for H-2 chemisorption are small and for alkane dehydrogena tion they lie between 50 and 130 kJ mol(-1). The manner in which produ ct selectivities vary with H-2 pressure also depends on dispersion and pretreatment, the dominant factor being the strength of H-2 chemisorp tion. Thus on a very highly dispersed catalyst for which K-H is large, intermediate product selectivities are high because the high concentr ation of H atoms facilitates desorption of adsorbed species, and for t his reason also selectivities scarcely respond to changes in H-2 press ure. With the same catalyst after O/LTR, however, when K-H is much dec reased, 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 th e result of variations in chemisorption energetics, and their conseque ntial effects on surface coverage, than of kinetic effects; this conce pt also accounts for dispersion-dependent differences in the temperatu re-dependence of product selectivities previously reported.