Turnover rate, reaction order, and elementary steps for the hydrodechlorination of chlorofluorocarbon compounds on palladium catalysts

The rates of hydrodechlorination catalyzed by Pd supported on carbon for fo ur chlorofluorocarbons spanned a range of 7 orders of magnitude. The rates scaled up to the bond strength of the carbon-chlorine bond for the gas-phas e reactant. This finding demonstrates that the rate-determining step involv es the scission of the C-Cl bond and suggests, through Polanyi and linear f ree-energy relationships, that rates for other compounds can be estimated i f the C-Cl bond strength is known. The reaction orders for the most abundan t products are approximately first-order for the chlorine-containing compou nd, half-order in H-2, and inverse first-order in HCl. The reaction steps c onsistent with these orders include a rate-determining step involving the a dsorption of the chlorofluorocarbon to a single site (which could be a sing le surface palladium atom) and equilibrated steps between gas-phase H2, gas -phase HCl, and adsorbed hydrogen and chlorine atoms. The rates on the supp orted catalysts are comparable to the ones reported before on a Pd foil, in dicating that the support does not play a role in the reaction. The product distribution is independent of conversion, implying that the various produ cts are formed from a single visit of the reactant on the surface and not f rom readsorption of gas-phase products. The four compounds studied were chl oropentafluoroethane (C-3-CF2Cl), 2-chloro-1,1,1,2-tetrafluoroethane (CF3-C FClH), 1,1-dichlorotetrafluoroethane (CF3-CFCl2), and 1, 1, 1-trichloro-2,2 ,2-trifluoroethane (CF3-CCl3).