Carbon-chlorine bond dissociation on the Pd(111) surface

Dissociation of the C-Cl bonds in straight chain alkyl chlorides (CH3(CH2)( n)Cl, n = 0-5) adsorbed on the Pd(111) surface has been investigated using temperature programmed desorption (TPD) and X-ray photoemission spectroscop y (XPS). Short chain alkyl chlorides adsorbed on the Pd(111) surface at low temperatures desorb during heating because the intrinsic activation energy for C-Cl bond cleavage (Delta EC-Cl) is greater than the desorption energy (Delta E-des). Systematically increasing the alkyl chain length increases Delta-E-des until it is greater than Delta EC-Cl. The value of Delta EC-Cl was estimated by determining Delta E-des of the smallest alkyl chloride to dissociate during heating, i.e., the smallest alkyl chloride with Delta EC- Cl less than or equal to Delta E-des. The TPD and XPS studies showed that w hile adsorbed CH3(CH2)(2)Cl desorbs from the Pd(111) surface during heating , CH3(CH2)(3)Cl is the shortest alkyl chloride to dissociate. This implies that Delta EC-Cl > Delta E-des for CH3(CH2)(2)Cl and Delta EC-Cl < Delta E- des for CH3(CH2)(3)Cl. The estimated range for the value of Delta EC-Cl is 60-64 kJ/mol and is consistent with the value of Delta EC-Cl estimated from previous studies of the dechlorination of fluorinated 1,1-dichloroethanes on the Pd(111) surface. (C) 2000 Academic Press.