Catalytic and structural properties of ruthenium bimetallic catalysts: Hydrogenolysis of propane and n-butane on Ru/Al2O3 catalysts modified by a Group 14 element

The addition of Ge as Ge(n)Bu(4) to H-covered Ru/Al2O3 (RuEC1) leads after in situ reduction at high temperature (HTR1; 758 K) to decreases in H-2 chemisorption capacity and even more marked falls in activity for hydrogenolysis of propane and of n-butane. Changes in product selectivities are slight, and all Ge-containing samples exhibit similar characteristics to the parent RuEC1, which is very highly dispersed. Although loss of activity is chiefly due to encapsulation of the Ru-o particles by amorphous GeOx species, these affect neighbouring active sites by decreasing the equilibrium constant for alkane dehydrogenation to the reactive species. This conclusion follows from determinations of the rate-dependence on H-2 pressure and mathematical modelling of the results. Similar behaviour is shown by the less well-dispersed RuEC3, where activity loss and selectivity changes are partly due to encapsulation by GeOx of smaller particles: some evidence for selective blocking of Ru atoms in low coordination number sites by Sn species formed analogously is however obtained. H-2 pressure-dependence of the rate of n-butane hydrogenolysis was determined at four different temperatures with a GeOx-modified RuEC3, whence a true activation energy (50 kJ mol(-1)) and an enthalpy change for the alkane dehydrogenation step were obtained. Oxidation and low-temperature reduction (O/LTR) of catalysts previously subjected to HTR1 led to very large increases in rate, and to changes in product selectivites, that suggested the creation of large, essentially pure, Ru particles; the modifier could not be wholly re-united with them by a second HTR. Ru/Al2O3 catalysts of the NI series, prepared from inorganic precursors, showed greater dispersion in the presence of the modifier, the catalytic behaviour being in harmony with this observation.