Carbon-hydrogen bond activation over tungsten carbide catalysts

Abstract

Tungsten carbides with surface areas ranging from 4 to 39 m2/g were prepared by the temperature-programmed carburization of tungsten oxide and nitride powders with pure CH4 or a 48.9% CH4 in H2 mixture. Oxygen uptakes on the carbides were low most likely because of the presence of graphitic carbon at the surface. Nevertheless, the carbides were active and selective for the dehydrogenation of butane at temperatures between 623 and 723 K and atmospheric pressure with and without H2 in the reactant feed. With H2 in the feed, the higher surface area carbides ([ges] 36 m2/g) were as active as a Pt-Sn/[gamma]-Al2O3 catalyst, but their selectivities were different. Without H2 in the feed, the selectivities of the carbides were similar to those of the Pt-Sn/[gamma]-Al2O3 catalyst, but their reaction rates were 1-3 orders of magnitude lower. The high surface area materials were also active for the hydrogenolysis of butane. Because the catalytic properties of the carbides varied with the average particle size, we concluded that butane dehydrogenation was structure-sensitive over these materials, and suspected that this behavior was due to variations in the surface stoichiometry as well as the particle faceting.