n-butane dehydrogenation over vanadium carbides:: Correlating catalytic and electronic properties

Vanadium carbides were prepared via the temperature-programmed reaction of V2O5 with a CH4/H-2 mixture and evaluated for the dehydrogenation of n-butane. Thermogravimetric analysis coupled with X-ray diffraction indicated that the solid-state reaction proceeded by the following sequential reaction: V2O5 --> V2O3 --> V8C7. The space velocity and heating rates had insignificant effects on the surface areas; however, the use of a high-temperature H-2 post-treatment caused a reduction in the surface area and carbon content. Temperature-programmed reduction results indicated that oxygen was more strongly bound to the substoichiometric vanadium carbide than to the stoichiometric material. The results were also consistent with the presence of an oxycarbide near surfaces of the passivated vanadium carbides. The passivated vanadium carbides were sufficiently activated by reduction in H-2 at 500 degrees C for 3 h. Oxygen chemisorptive uptakes on the reduced vanadium carbides corresponded to an O/V ratio of 0.28. This oxygen-to-metal ratio is half that measured for the vanadium nitrides, suggesting that some excess carbon may have been present on surfaces of the carbides. The butane dehydrogenation turnover frequency for the vanadium carbide catalyst was 10(-3) s(-1) at 450 degrees C. The corresponding turnover frequency for a Pt-Sn/Al2O3 catalyst was 6.3 x 10(-2) s(-1). Near-edge X-ray absorption fine structure spectroscopy indicated that the vanadium carbide and nitride catalysts were partially ionic, with charge transfer being from vanadium to carbon or nitrogen. This degree of ionic bonding distinguishes the vanadium compounds from other carbides and nitrides and could partly explain their high dehydrogenation selectivities. Similarities between catalytic properties of the vanadium carbides and nitrides were likely a consequence of their similar electronic structures. The p-projected density of unoccupied states near the carbon and nitrogen K-edges nearly identical. (C) 2000 Academic Press.