Chemisorptive replacement of surface oxygen by hydrogen halides (HCl and HBr) at Pb(110) surfaces. Photoelectron spectroscopic and kinetic evidence for a metastable chloride overlayer

Abstract

Photoelectron spectroscopy has established that although the hydrogen halides (HC1 and HBr) are not chemisorbed at a Pb(110) surface, chemisorptive replacement of preadsorbed oxygen leads to the formation of a halide overlayer in the temperature range 100–295 K. Surface oxygen plays a dual role, activating the hydrogen halide and also influencing the stability of the halide overlayer.The chloride and bromide overlayers show distinct differences in their reactivities to the hydrogen halides. Although the bromide overlayer is unreactive, the chloride formed via oxygen replacement is metastable and removed (desorbed) on exposure to HCl(g) at low temperature. This reaction is critically dependent on the oxygen coverage and is observed only when the latter is <0.1. Chloride overlayers generated by the dissociative chemisorption of dichlorine are unreactive to HCl(g), while mixed halide overlayers exhibit specific reactivities indicative of highly localised interactions. Angular dependent X.p.s. studies of the overlayers suggest a correlation between surface geometry and stability.The kinetics of chloride desorption in the presence of both HCl(g) and HBr(g) suggest that weakening of the metal–overlayer (PbCl2) bonding arises from the formation of a surface complex of the type Cl–—HCl. The magnitude of this energy of interaction is influenced by the presence of sub-surface oxygen, which determines the charge on the chlorine adatom through a localised ligand effect. The activation energy of chloride desorption is 10.8 kJ mol–1 and its rate exhibits a first-order dependence on both HCl(g) and HBr(g).