Group VI transition metal carbides as alternatives in the hydrodechlorination of chlorofluorocarbons

The synthesis of transition metal carbides of tungsten and molybdenum has b een carried out via temperature programmed reactions (TPRs) of metal oxides or passivated nitrides. Their specific chloropentafluoroethane conversion rates were at best one order of magnitude less than that of a reference Pd based catalyst. The intrinsic rates range from 4.7 to 14.7 nmol m(-2) s(-1) and decrease as follows: Mo2C>WC>W2C approximate to WC1-x>MoC1-x. The grou p VI carbide samples catalyse hydrodehalogenation and dehydrofluorination. WC appears to be as selective towards pentafluoroethane (HFC-125) as the Pd based catalyst. Then the selectivity decreases in the following sequence: W2C>Mo2C>WC1-xMoC1-x. All the carbide catalysts deactivate at the early sta ges of the reaction. Based on the XPS results and the product distribution of the reaction, the deactivation has been mainly attributed to a site bloc king phenomenon due to a strong deposit of polymeric carbon and of hydroflu orocarbon polymers. Polymerisation of detected unsaturated compounds take p lace on acidic sites probably generated by fluoride and/or chloride in the course of the reaction. (C) 2000 Elsevier Science B.V. All rights reserved.