Journal of Catalysis 223 (2004) 98–105 www.elsevier.com/locate/jcat Chlorofluorocarbon dechlorination on Pd(111): effect of chlorine stoichiometry Boonchuan Immaraporn, a PingPing Ye, a and Andrew J. Gellman b,∗ a Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213, USA b Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA Received 17 July 2003; revised 22 December 2003; accepted 12 January 2004 Abstract 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: CF 3 CCl 3 , CF 3 CFCl 2 , CF 3 CF 2 Cl, and CF 3 CHFCl. The desorption energies, E des , of the CFCs were found to increase with increasing number of chlorine atoms in the following order: CF 3 CF 2 Cl < CF 3 CHFCl < CF 3 CFCl 2 < CF 3 CCl 3 . The rate constants and apparent barriers to C–Cl cleavage, E app , on the Pd(111) surface were measured using CF 3 CFCl 2 , CF 3 CF 2 Cl, and CF 3 CHFCl. The trend in E app was found to be opposite that for E des : CF 3 CFCl 2 < CF 3 CHFCl < CF 3 CF 2 Cl. For CF 3 CFCl 2 and CF 3 CHFCl the intrinsic activation energy for C–Cl cleavage on the Pd(111) surface, E C–Cl , was determined by adding the desorption energies and the apparent activation energies for dissociative adsorption, E C–Cl = E app +E app . In contrast with the other compounds, CF 3 CCl 3 adsorbed on the Pd(111) surface dechlorinates rather than desorbing during heating, thus allowing direct measurement of E C–Cl . Comparison of E C–Cl and E des among the CFCs suggests that the variations in E des have a greater influence on the dechlorination reactivity than the variations in E C–Cl .  2004 Elsevier Inc. All rights reserved. Keywords: Dechlorination; Hydrodechlorination; Chlorofluorocarbons; Palladium 1. Introduction Chlorofluorocarbons (CFCs) are believed to harm the en- vironment by depleting the stratospheric ozone layer. As a result they are no longer produced for commercial use and alternative compounds have been developed with similar physicochemical properties but less potential for destruc- tion of the ozone layer. One such class of compounds are the hydrofluorocarbons (HFCs), which can be produced by hydrodechlorination of CFCs [1–3]. Currently a number of HFCs such as CF 3 CFH 2 are used as replacements for their counterpart CFCs such as CF 3 CFCl 2 . One route to obtain HFCs is the hydrodechlorination of CFCs over supported Pd catalysts [1]. CF 3 CFCl 2 + 2H 2 Pd → CF 3 CFH 2 + 2HCl. Several mechanisms and rate-determining steps have been proposed for the catalytic hydrodechlorination reaction * Corresponding author. E-mail address: ag4b@andrew.cmu.edu (A.J. Gellman). shown above. A recent study suggested that the rate- determining step involves C–Cl bond cleavage possibly via oxidative addition [4,5]. Although there is not complete agreement on the rate-determining step, previous studies have shown that rates of CFC dechlorination on the Pd(111) surface correlate quite well with rates of hydrodechlori- nation on Pd catalysts [6,7]. The agreement in the trend between rates of dechlorination and rates of catalytic hy- drodechlorination implies that C–Cl cleavage must be kinet- ically significant in the hydrodechlorination reaction. Previous studies of the hydrodechlorination of CFCs over several catalysts have revealed that as the number of chlorine atoms attached to a carbon atom increases, the rate of hydrodechlorination also increases [4,5,7,8]. For in- stance, on Pt/Al 2 O 3 catalysts the rate of hydrodechlorina- tion of CCl 4 is 100 times faster than that of CH 3 Cl un- der identical conditions [8]. On supported Pd catalysts, a systematic study of the hydrodechlorination of four dif- ferent CFCs reported by Thompson et al. revealed that the ratio of the turnover rates for hydrodechlorination of CF 3 CCl 3 :CF 3 CFCl 2 :CF 3 CHFCl:CF 3 CF 2 Cl is 10 7 :10 3 :3:1 0021-9517/$ – see front matter  2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2004.01.013