Molybdenum carbide catalysts 3. Turnover rates for the hydrogenolysis of n-butane

Abstract

The hydrogenolysis of n-butane was studied on Mo and M02C supported on A12O3 as well as on bulk carbides Mo2C and α-MoC1-x. Well-reduced molybdenum clusters supported on Al2O3 were prepared according to the method of Burwell and co-workers. The clusters were transformed in CH4H2 mixtures into M02C as shown by X-ray absorption spectroscopy. Turnover rates, vt, referred to the number of surface metal atoms titrated by irreversible chemisorption of CO were very high both on supported and unsupported catalysts. The value of vt for supported Mo2C clusters was slightly less than that for Ru and Os, the most active metals known for the hydrogenolysis of alkanes. The reaction was found to be structure-sensitive since vt, values changed with particle size and crystal structure of catalysts. The presence of polymeric carbon on the surface, or possibly oxygen, and the structure sensitivity of the reaction seem responsible for the low vt of unsupported Mo2C reported previously. Molybdenum surfaces that were initially metallic transformed to carbides under reaction conditions, indicating that the active catalytic phase of metallic Mo was also Mo2C.