Carbon–Chlorine Bond Dissociation on the Pd(111) Surface

Ge Zhou,Andrew J Gellman

doi:10.1006/jcat.2000.2948

Abstract

Dissociation of the C–Cl bonds in straight chain alkyl chlorides (CH3(CH2)nCl, n=0–5) adsorbed on the Pd(111) surface has been investigated using temperature programmed desorption (TPD) and X-ray photoemission spectroscopy (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 (ΔEC–Cl) is greater than the desorption energy (ΔEdes). Systematically increasing the alkyl chain length increases ΔEdes until it is greater than ΔEC–Cl. The value of ΔEC–Cl was estimated by determining ΔEdes of the smallest alkyl chloride to dissociate during heating, i.e., the smallest alkyl chloride with ΔEC–Cl≤ΔEdes. The TPD and XPS studies showed that while adsorbed CH3(CH2)2Cl desorbs from the Pd(111) surface during heating, CH3(CH2)3Cl is the shortest alkyl chloride to dissociate. This implies that ΔEC–Cl>ΔEdes for CH3(CH2)2Cl and ΔEC–Cl<ΔEdes for CH3(CH2)3Cl. The estimated range for the value of ΔEC–Cl is 60–64 kJ/mol and is consistent with the value of ΔEC–Cl estimated from previous studies of the dechlorination of fluorinated 1,1-dichloroethanes on the Pd(111) surface.

Highlights

Chlorofluorocarbons (CFCs) are an important contributor to stratospheric ozone depletion (1, 2)
The temperature programmed desorption (TPD) and X-ray photoemission spectroscopy (XPS) studies presented in this paper suggest that CH3(CH2)3Cl is the smallest alkyl chloride that dissociates with Edes = 49–64 kJ/mol
The TPD results for increasing coverages of CH3(CH2)2Cl on the Pd(111) surface are shown in Fig. 3 and reveal that the peak desorption temperature shifts from 250 K at low coverage to about 173 K at monolayer coverage

Summary

Introduction

Chlorofluorocarbons (CFCs) are an important contributor to stratospheric ozone depletion (1, 2). CFCs are chemically inert, and once released into the atmosphere diffuse upward into the stratosphere. High energy UV radiation causes photodissociation of the CFCs to produce chlorine atoms that participate in a chain reaction in which ozone is destroyed. CFCs have had widespread commercial applications as refrigerants, foam-blowing agents, aerosol propellants, industrial solvents, and cleaning agents (3–5). Finding safe ways to dispose of CFCs and replace them is an urgent technological and ecological priority. Suitable replacements for CFCs must have physical properties that closely match those of the CFCs. One class of alternatives to CFCs are hydrofluorocarbons (HFCs), which do not contain chlorine and have low ozone depleting potential (6). CF3CFH2 is currently being used to replace CF2Cl2 in some automobile air-conditioning applications

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Conclusion