The influence of surface properties on catalytic activities of tantalum carbides

The Ta2O5 or Ta (99.99%, Aldrich) was carburized using pure CH4 in a temperature-programmed manner. The BET surface areas of these materials varied with different molar hourly space velocity, suggesting that the sorption properties of these materials were strong functions of space velocity employed. The tantalum carbides proved to be active NH3 decomposition catalysts. The ammonia decomposition activities of tantalum carbides normalized by the oxygen uptake increased with increasing surface area, suggesting that NH3 decomposition over these materials is structure-sensitive. This behavior may be ascribed to variations in the surface stoichiometry with surface area as confirmed by XPS results. The ammonia decomposition activation energies for all the tantalum carbides were similar and averaged 39 kcal/mol, which is in good agreement with those reported in the literature. The activities of the tantalum carbides were much lower than those of vanadium and molybdenum carbide catalysts. This difference was probably due to the degree of electron transfer between metals and carbon in transition metal carbides, (C) 1999 Elsevier Science B.V. All rights reserved.