Effect of silver on the catalytic hydrodechlorination of tetrachloromethane over silica-supported palladium-silver catalysts

Two silica-supported palladium based catalysts characterized by overall metal loadings 10 wt.%, and atomic Pd : Ag ratios of 100 : 0 and 70 : 30, were prepared by incipient wetness impregnation, subjected to reduction in hydrogen and characterized by XRD. It was found that incipient wetness co-impregnation of silica resulted in the formation of a catalyst characterized by relatively small metal crystallites (~11 nm) and significant extent of Pd-Ag alloying. The catalytic performance was investigated in the hydrodechlorination of tetrachloromethane in the gas phase. Silver addition to palladium in the co-impregnated catalysts had very pronounced effect, vastly decreasing the overall activity (conversion) and also the selectivity towards hydrocarbons, propensity characteristic for the monometallic palladium catalysts working in CCl4 hydrodechlorination carried out in the gas phase. Instead, large amounts of C2HxCly dimeric products were formed. These dimers are considered as possible coke precursors. Post-reaction samples of both catalysts contained large amount of carbon which entered the Pd and Pd-Ag phases. This carbon can be easily removed by treatment with hydrogen at 450ºC.