Catalytic hydrodechlorination of chlorinated aromatics on the platinum(111) surface.

Abstract

The reaction of chlorobenzene and the dechlorination of the isomers of chlorotoluene were characterized on the Pt(111) surface using temperature programmed reaction spectroscopy (TPRS) and fluorescence yield near edge spectroscopy (FYNES). Studies of disproportionation, hydrogenation, and oxidation of these two chlorinated aromatics have provided new molecular understanding of the surface reactions involved. For both molecules, hydrogenolysis results in facile dechlorination below room temperature. Oxidation inhibits dechlorination by removing hydrogen from the reaction system. For chlorotoluene isomers, methyl substituents proximate to chloro groups enhance dechlorination substantially. The position of the methyl group decreases the dechlorination temperature by 70 K for ortho-chlorotoluene relative to para-chlorotoluene. These results and the nature of the surface intermediates found add substantially to our basic understanding of these important surface reactions. During hydrogenation and disproportionation of chlorobenzene on the Pt(111) surface thermal hydrodechlorination results in the formation of two HCl desorption peaks, one at 270 K, and the other at 420 K. Comparison of the integrated peak areas for these two reaction pathways indicates a 3:2 yield ratio (270 K: 420 K). Temperature programmed-FYNES show that a cyclohexadiene intermediate is formed above 250 K that is coadsorbed in a 3:2 ratio with molecular chlorobenzene. The remaining molecular chlorobenzene is dechlorinated in the 420 K range to form additional adsorbed cyclohexadiene. For chlorotoluene, substituent effects dominate reaction of the C-Cl bond at low temperatures. The enhanced reactivity from methyl decreases the reaction limited dechlorination temperature from 420 K for para-chlorotoluene to 350 K for ortho chlorotoluene. While the electron donating methyl substituent enhances low temperature dechlorination and HCl formation, the chloro group decreases the reactivity of the proximate methyl substituent towards dehydrogenation. Additionally coadsorbed atomic oxygen inhibits low temperature HCl formation. The effects of atomic oxygen on the dechlorination reactions show that oxidation inhibits low temperature dechlorination substantially for both chlorobenzene and the isomers of chlorotoluene. At low temperature, both chlorobenzene and chlorotoluene undergo a stepwise oxydehydrogenation process. The low temperature HCl formation pathway is inhibited by atomic oxygen which abstracts the hydrogen to form water. HCl is only formed after surface oxygen has been depleted.