n-butane dehydrogenation over vanadium carbides: Correlating catalytic andelectronic 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-buta ne. Thermogravimetric analysis coupled with X-ray diffraction indicated tha t the solid-state reaction proceeded by the following sequential reaction: V2O5 --> V2O3 --> V8C7. The space velocity and heating rates had insignific ant 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 st rongly bound to the substoichiometric vanadium carbide than to the stoichio metric material. The results were also consistent with the presence of an o xycarbide near surfaces of the passivated vanadium carbides. The passivated vanadium carbides were sufficiently activated by reduction in H-2 at 500 d egrees C for 3 h. Oxygen chemisorptive uptakes on the reduced vanadium carb ides corresponded to an O/V ratio of 0.28. This oxygen-to-metal ratio is ha lf that measured for the vanadium nitrides, suggesting that some excess car bon may have been present on surfaces of the carbides. The butane dehydroge nation 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 nit rogen. This degree of ionic bonding distinguishes the vanadium compounds fr om other carbides and nitrides and could partly explain their high dehydrog enation selectivities. Similarities between catalytic properties of the van adium carbides and nitrides were likely a consequence of their similar elec tronic structures. The p-projected density of unoccupied states near the ca rbon and nitrogen K-edges nearly identical. (C) 2000 Academic Press.