Ammonia decomposition over vanadium carbide catalysts

Vanadium carbides were synthesized by the temperature-programmed carburization of vanadium oxide precursors (V2O5) with pure CH4 or a mixture of 49.9% CH4 in H-2. It was found that the structural properties of these materials were strong functions of the heating rate and space velocity employed, The vanadium carbides with high surface areas could be prepared using high space velocities or low heating rates. The vanadium carbides proved to be active NH3 decomposition catalysts. Since the activity varied with changes in the surface area and particle size, ammonia decomposition over the vanadium carbides appeared to be structure-sensitive. This response was considered to be due to variations in the surface stoichiometry with particle size. In general, the activities of the vanadium carbides were about 2 times less than those of Mo2C catalyst while vanadium carbides had higher activities by a factor of 1 or 2 than those of Pt/C catalyst. The difference in activities of the vanadium and molybdenum carbides might be related to the degree of electron transfer between metals and carbon. A similarity between the catalytic behaviors of vanadium carbides and platinum metal appeared to be due to the similar electronic and magnetic properties of these materials, (C) 1999 Academic Press.