| The feasibility of gas phase hydrodechlorination (HDC) of chlorobenzene (CB) and 1,3-dichlorobenzene (DCB) over the temperature range 325--573 K has been demonstrated for unsupported and supported (activated carbon (AC), graphite, graphitic nanofiber (GNF), Al2O3 and SiO2) palladium and nickel catalysts. The role of carbon structural characteristics were considered in the case of CB HDC over carbon (AC, graphite and GNF) supported Pd and the effect of carbon surface composition (presence of oxygen groups) was examined in the case of CB and DCB HDC over physical mixtures of bulk Pd+carbon. Critical carbon characteristics, i.e. structure, surface area and surface chemistry, are shown to impact on HDC activity/selectivity. The nature of the surface reactive hydrogen associated with supported Pd has been probed and can be categorized as: Pd hydride; chemisorbed hydrogen; spillover hydrogen. The contribution of hydrogen from Pd hydride to catalytic HDC is demonstrated where this source of hydrogen is fully replenished in a reactivation step. Participation of hydrogen spillover in HDC over mixtures of Al2O 3 with bulk Pd, bulk Ni, Pd/Al2O3 and Ni/Al 2O3 is illustrated where the inclusion of Al2O 3 serves to significantly (by up to an order of magnitude) raise HDC activity. Palladium is significantly more active (up to three orders of magnitude) than Ni, a result of the higher H2 diffusivity in Pd. The, performance of Al2O3 supported Pd-Ni is considered where the bimetallic delivered intermediate activity when compared with the two monometallic systems. Under conditions where HDC was operated under chemical control over Pd/Al2O3, the addition of Ni to Pd in the bimetallic samples resulted in a decrease in HDC activity that was greater than that which resulted from a dilution of Pd/Al2O3 with Ni/Al2O3 (in a physical mixture), suggesting a surface chemical interaction that inhibits HDC performance. |
