The rates of hydrodechlorination catalyzed by Pd supported on carbon for four 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-phase reactant. This finding demonstrates that the rate-determining step involves the scission of the C−Cl bond and suggests, through Polanyi and linear free-energy relationships, that rates for other compounds can be estimated if the C−Cl bond strength is known. The reaction orders for the most abundant products are approximately first-order for the chlorine-containing compound, half-order in H2, and inverse first-order in HCl. The reaction steps consistent with these orders include a rate-determining step involving the adsorption of the chlorofluorocarbon to a single site (which could be a single surface palladium atom) and equilibrated steps between gas-phase H2, gas-phase HCl, and adsorbed hydrogen and chlorine atoms. The rates on the supported catalysts are comparable to the ones reported before on a Pd foil, indicating that the support does not play a role in the reaction. The product distribution is independent of conversion, implying that the various products are formed from a single visit of the reactant on the surface and not from readsorption of gas-phase products. The four compounds studied were chloropentafluoroethane (CF3−CF2Cl), 2-chloro-1,1,1,2-tetrafluoroethane (CF3−CFClH), 1,1-dichlorotetrafluoroethane (CF3−CFCl2), and 1,1,1-trichloro-2,2,2-trifluoroethane (CF3−CCl3).

The role of particle size during the hydrogenation/dehydrogenation of cyclohexene (10 Torr C 6 H 10 , 200-600 Torr H 2 , and 273 -650 K) was studied over a series of monodisperse Pt/SBA-15 catalysts. The conversion of cyclohexene in the presence of excess H 2 (H 2 :C 6 H 10 ratio = 20-60) is characterized by three regimes: hydrogenation of cyclohexene to cyclohexane at low temperature (< 423 K), an intermediate temperature range in which both hydrogenation and dehydrogenation occur; and a high temperature regime in which the dehydrogenation of cyclohexene dominates (> 573 K). The rate of both reactions demonstrated maxima with temperature, regardless of Pt particle size. For the hydrogenation of cyclohexene, a nonArrhenius temperature dependence (apparent negative activation energy) was observed.Hydrogenation is structure insensitive at low temperatures, and apparently structure sensitive in the non-Arrhenius regime; the origin of the particle-size dependent reactivity with temperature is attributed to a change in the coverage of reactive hydrogen. Small particles were more active for dehydrogenation and had lower apparent activation energies than large particles. The selectivity can be controlled by changing the particle size, which is attributed to the structure sensitivity of both reactions in the temperature regime where hydrogenation and dehydrogenation are catalyzed simultaneously.

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