Chlorofluorocarbon Dechlorination on Pd(111): Effect of Chlorine Stoichiometry

The kinetics of C---Cl bond cleavage on the Pd(111) surface have been measured using four chlorofluorocarbons (CFCs) containing different numbers of chlorine atoms on the primary carbon: CF3CCl3, CF3CFCl2, CF3CF2Cl, and CF3CHFCl. The desorption energies, ΔEdes, of the CFCs were found to increase with increasing number of chlorine atoms in the following order: CF3CF2Cl < CF3CHFCl < CF3CFCl2 < CF3CCl3. The rate constants and apparent barriers to C---Cl cleavage, ΔEapp, on the Pd(111) surface were measured using CF3CFCl2, CF3CF2Cl, and CF3CHFCl. The trend in ΔEapp was found to be opposite that for ΔEdes: CF3CFCl2 < CF3CHFCl < CF3CF2Cl. For CF3CFCl2 and CF3CHFCl the intrinsic activation energy for C---Cl cleavage on the Pd(111) surface, ΔEC---Cl, was determined by adding the desorption energies and the apparent activation energies for dissociative adsorption, ΔEC---Cl =ΔEapp +ΔEapp. In contrast with the other compounds, CF3CCl3 adsorbed on the Pd(111) surface dechlorinates rather than desorbing during heating, thus allowing direct measurement of ΔEC---Cl. Comparison of ΔEC---Cl and ΔEdes among the CFCs suggests that the variations in ΔEdes have a greater influence on the dechlorination reactivity than the variations in ΔEC---Cl.