BIMETALLIC CATALYSTS .5. KINETICS OF THE HYDROGENOLYSIS OF BUTANE OVER PT-MO/SIO2 CATALYSTS

The kinetics of the hydrogenolysis of butane on various Pt-Mo catalysts deposited on silica with the same metal content but with various Mo/(Mo + Pt) ratios has been studied. The hydrogenolysis of the two different kinds of CC bonds has been distinguished. It is shown that the addition of Mo to Pt drastically changes the selectivity of the reaction, but that this selectivity is approximately the same for all the bimetallic Pt-Mo catalysts. Using a rate equation that had already been used for the hydrogenolysis of alkanes (Maurel, R., Leclercq, G., and Leclercq, L., J. Catal. 44, 68 (1975)), calculation has been made of the values of the equilibrium adsorption constants (λ) and those of the rate constants of the CC bond splitting steps (k) for the hydrogenolysis of the terminal CC bonds (λ1 and k1) and of the middle CC bond (λ2 and k2). In this way it has been shown that on Pt alone, two different intermediaries with high λ values are formed in which the CC bonds will be broken further with different selectivities but with low rate constants. On the contrary, on all Pt-Mo catalysts, butane is adsorbed as a single intermediate adsorbed species with a much lower adsorption equilibrium constant than on Pt alone, but this adsorbed species is much more reactive (k1 and k2).The adsorption equilibrium constants are the same for all Pt-Mo taking into account the margin of error, which shows that, on all Pt-Mo. butane is adsorbed on the same mixed Pt-Mo site of constant composition whatever the catalyst composition. The number of these active sites (proportional to k1 and k2) increases as Mo/(Mo + Pt) increases up to 25-30%. then it decreases. © 1993 Academic Press, Inc.