Catalytic properties of early transition metal nitrides and carbides: n-butane hydrogenolysis, dehydrogenation and isomerization

Abstract

Phase-pure early transition metal nitrides and carbides were prepared via the temperature programmed reaction of metal oxides with NH 3 or a CH 4/H 2 mixture. The nitrides and carbides were mostly mesoporous with surface areas up to 81 m 2/g. Their gravimetric butane conversion rates were generally higher than those for a Pt–Sn/Al 2O 3 catalyst. Activities for the nitrides and carbides ranged from 0.4×10 12 to 10×10 12 molecules/cm 2 s at 723 K and decreased as follows: γ-Mo 2N>W 2C≈WC>β-W 2N≈WC 1− x >β-Mo 2C>VN≈V 8C 7≫NbC≈Nb 4N 3.92. The metal atom type had the most significant effect on the activity and selectivity. The Group VI metal nitrides and carbides were much more active than the Group V metal compounds. In general, the Group VI metal compounds catalyzed butane hydrogenolysis and dehydrogenation with similar selectivities while the vanadium compounds had dehydrogenation selectivities in excess of 98%. The β-W 2N catalyst also catalyzed butane isomerization possibly as a consequence of the presence of oxygen on the surface. The effect of lattice structure was significant and obvious for the tungsten carbides where WC (hex) was almost twice as active as WC 1− x (fcc) despite having similar C/W ratios. Nitrides and carbides of the same metal and lattice structure had similar activities suggesting that the effect of the non-metal atom type was small. We believe variations in the catalytic properties of the nitrides and carbides were the result of differences between their electronic structures.