COMPARED ACTIVITIES OF PLATINUM AND HIGH SPECIFIC SURFACE-AREA MO2C AND WC CATALYSTS FOR REFORMING REACTIONS .1. CATALYST ACTIVATION AND STABILIZATION - REACTION OF NORMAL-HEXANE

The catalytic properties of high specific surface area (>150 m2/g) molybdenum and tungsten carbides are studied in hydrocarbon-reforming reactions and compared to conventional platinum supported on alumina. The isomerization of n-hexane is used as a test reaction. TPR and XPS analyses show that the raw materials are contaminated by oxygen and need an activation process to become reactive. These analyses also show that Mo2C is decomposed by pure hydrogen at 800°C to form methane and metallic molybdenum. Different methods of reductive activation are tested: high-temperature reduction (800°C) by hydrogen leads to metallic Mo and W on the surface (catalytically unreactive for reforming); coreduction by a mixture of pentane and hydrogen (700°C) gives active catalysts but less so than conventional platinum, probably because of the presence of carboneous residues formed by decomposition of the n-pentane. Trace amounts of different group VIII transition metals (≤500 ppm) can catalyze the activation process, probably by preventing the formation of the carboneous residues. Mo2C activated by 500 ppm of Pt, Ir, or Ru can reach total specific activities 6 to 7 times higher than the conventional Pt catalyst. However in terms of yield, the best carbide, activated by Ir, only doubles the performance of conventional platinum, with a high amount of cracked molecules formed in parallel. Clean surfaces of Mo2C or WC can be much more reactive than conventional Pt catalysts in terms of specific activity, isomerization, plus cracking; however, the best selectivity in isomers never exceeds 30% while selectivity on Pt is usually in the range 75 to 85%. © 1992.