Hydrodechlorinationo of Dichloridifluoromethane (CFC-12) on Silica-Supported Palladium and Palladium-Gold Catalysts

Silica-supported palladium and palladium-gold catalysts were screened in the reaction of dichlorodifluoromethane with dihydrogen at 180_C. The Au-containing sample appeared superior as far as the selectivity towards difluoromethane (desired reaction product) is concerned (95% vs. ~30% for Pd/SiO2). A comparative X-ray diffraction study of the catalysts, before and after reaction, confirmed our earlier findings that during the hydrodechlorination massive amounts of carbon originated from CCl2F2 dissolve in palladium. However, a similar conclusion cannot be drawn from XRD studies of the Pd-Au/SiO2 catalyst, because upon entering Pd bulk both gold and carbon may produce analogous shifts of the XRD reflections. For that reason, temperature programmed hydrogenation of carbon deposited in the Pd and a Pd-Au catalyst was used. In contrast to the behaviour of monometallic Pd/SiO2, only insignificant quantity of carbon was found in the bimetallic Pd-Au/SiO2 sample subjected to hydrodechlorination. This result supports the idea that a complete hydrodehalogenation of CCl2F2 to methane would occur via bare carbon adspecies. Thus, one can associate a superior catalytic behaviour of Pd-Au alloy catalysts (selectivity to CH2F2 ~95%) with a low abundance of C1 adspecies.