Surface reactivity as revealed by photoelectron spectroscopy

Abstract

The chemical specificity induced by chemisorbed oxygen at atomically clean surfaces is shown to have diverse implications for the fundamental mechanisms of surface processes. Photoelectron spectroscopy, used in a dynamic mode, has enabled the synergistic chemistry associated with the chemistry of coadsorbed molecules (nitric oxide with water and ammonia) at Zn(0001) and Mg(0001) surfaces to be attributed to the activation of the O-H and N -H bonds. The step-wise reduction of chemisorbed nitrogen to NH (a) and NH 2 (a) has been established; with Mg(0001) dehydrogenation of the surface amide rather than complete hydrogenation to NH 3 (g) favoured. The reaction pathway is critically dependent on temperature, the concentrations of O 2- (a) and OH(a) and the strength of the zinc-nitrogen and magnesium-nitrogen bonds. At Mg(100) surfaces preadsorbed oxygen, through an electrostatic (ligand) effect, controls the two chemisorption states of acetic acid, one involving sp 3 and the other sp 2 hybridization. Analogous ligand effects are involved in the correlation established between the reactivity of chloride overlayers at pB(110) surfaces and the presence of surface oxygen. In the reaction of halogen hydrides with Pb(110), surface oxygen plays two distinct roles: activation of the halogen hydride and stabilization of the chloride overlayer. The surface oxygen is a prerequisite for both the generation of the overlayer and the reactivity of the overlayer. Two distinct types of oxygen are invoked, one of which is subsurface. Oxygen interaction with nickel and titanium has been shown through analysis of Ni(2p) and Ti(2p) spectra to involve variable oxidation states. Ti 2+ and Ti 3+ are formed preferentially at the titanium —overlayer interface while Ti 4+ is predom inantly at the oxide—gas interface. A localized bonding model for the oxygen overlayer is suggested, which also has analogies with observations m ade of m ixed-halide (chloride and brom ide) overlayers at Pb(110) surfaces.