Degradation of Multiply-Chlorinated Hydrocarbons on Cu(100)

Abstract

Dechlorination and hydrodechlorination of multiply-chlorinated ethanes and propanes on a clean Cu(100) surface have been studied by Auger electron spectroscopy, temperature-programmed desorption, and chemical displacement measurements. The rate-limiting step in degradation is dissociation of the first C−Cl bond in the molecule, and this process is more facile in CCl2 groups than in CCl groups. The activation energies for C−Cl bond scission on Cu(100) are 12−20% of the gas phase bond dissociation energies, and the extent of dissociation by the physisorbed molecules is a sensitive function of the relative rates of C−Cl bond scission and molecular desorption. The chlorinated hydrocarbon fragments generated on the surface by C−Cl bond cleavage undergo facile α- or β-chlorine elimination while all C−H bonds remain intact. β-Chlorine elimination dominates in cases where chlorine is present at both the α- and β-positions, and as a result of β-chlorine elimination, alkenes are generated and evolved to the gas phase. In cases where no β-Cl is present, α-chlorine elimination yields surface carbene intermediates, which readily couple to form longer chain alkenes. Surface hydrogen atoms readily scavenge these carbene intermediates to form alkyl groups. Upon thermal activation, these alkyls are converted to alkenes via β-hydride elimination or to alkanes via coupling with surface hydrogen. All processes subsequent to the initial dissociative adsorption of the chlorinated hydrocarbon occur with 100% selectivity, and all hydrocarbon products are evolved from the surface below 300 K. No carbon is detected on the surface after reaction, but Cl remains adsorbed up to 750 K where it is evolved as CuCl.