Pd-Au/Sibunit carbon catalysts: Characterization and catalytic activity in hydrodechlorination of dichlorodifluoromethane (CFC-12)

A series of Sibunit carbon-supported palladium-gold catalysts prepared by different methods (wet and incipient wetness impregnation and direct redox reaction) were characterized by various techniques and investigated in the reaction of dichlorodifluoromethane with dihydrogen. The direct redox method resulted in a higher degree of Pd-Au alloying than the impregnation procedures. A temperature-programmed hydride decomposition (TPHD) study appears to be a very promising technique for characterization of Pd-Au bimetal. A clear correlation between TPHD spectra and X-ray diffraction data for Pd-Au catalysts allows proposal of the former technique for diagnosing the quality of Pd-Au alloying. The selectivity toward difluoromethane (desired reaction product) was increased upon introducing gold to palladium; however, this enhancement depended very much on the degree of Pd-Au alloying. For the catalysts prepared by the direct redox reaction method the selectivity for difluoromethane increased from <70 (for Pd) to nearly 90% for bimetallic samples at the highest temperature of screening, 180°C. Such a high selectivity enhancement was not observed for Pd-Au/C catalysts prepared by impregnation methods, which showed a lesser degree of Pd-Au alloying. During the reaction of dichlorodifluoromethane hydrodechlorination substantial amounts of carbon dissolve in the bulk of the palladium (or Pd-rich alloy) phase. Independent studies with a model Pd/SiO2 catalyst showed that this carbon originates from the CFC molecule, not from the carbon support. It is interesting that a massive carbiding of palladium takes place at a very early stage of the reaction, when also the most important changes in catalytic behavior occur. Thus, it may be speculated that the surface state of the working catalyst must be correlated with the extent of the bulk carbiding. An easy removal of carbon from palladium by hydrogen at the reaction temperature confirms our earlier idea that methane formation from dichlorodifluoromethane may occur via hydrogenation of C, surface species. (C) 2002 Elsevier Science (USA).