Chemisorption of oxygen at Ag(110) surfaces and its role in adsorbate activation

Abstract

Oxygen has been shown by X-ray photoelectron spectroscopy (X.p.s.) and ultraviolet photoelectron spectroscopy (u.p.s.) studies to be adsorbed both molecularly and dissociatively at an Ag(110) surface at 80 K. Chemisorbed chlorine inhibits the formation of the dissociative state. The interaction of water vapour with Ag(110) is influenced strongly, even at 80 K, by the presence of O^2–(a). The formation of surface hydroxyls and hydrogen-bonded water clusters is discussed. Surface oxygen undergoes a chemisorptive replacement reaction with HCl(g) below 175 K. At 473 K there is strong evidence from both X.p.s. and u.p.s. for three states of oxygen; one of these is a strongly chemisorbed dioxygen species, while the other two involve dissociated oxygen one of which is sub-surface. [See also ref. (1).] The relative proportions of the dioxygen and dissociated oxygen species can be controlled by the presence of surface chlorine. Comparisons are made between the core and valence-level spectra observed with the Ag(110)–Cl + O₂ system and those obtained with an industrial silver catalyst known to be active in ethylene epoxidation.