The nature and reactivity of chemisorbed oxygen and oxide overlayers at metal surfaces as revealed by photoelectron spectroscopy

Abstract

X-ray photoelectron spectroscopy has shown that chemisorbed oxygen and oxide overlayers at both nickel and titanium surfaces exhibit both extensive non-stoichiometry and variable oxidation (redox) states. Assignment of core-level features has been possible through studies of defective bulk oxides and their thermally induced conversion to ‘more perfect’ oxides. Localized oxidation states. Ti2+ and Ti3+ are also shown to form when carbon monoxide (Ti2+) and nitric oxide (Ti2+, Ti3+) are chemisorbed at titanium surfaces. This is the first spectroscopic evidence for localized and discrete oxidation states in the chemisorption of these diatomic molecules at metal surfaces. Transient surface oxygen species O−(s) and O−2(s) are shown to be intermediates in the formation of stable chemisorbed oxygen O2-(a) at Mg(0001) and Zn(0001) surfaces. These reactive transients play an important role in the chemistry observed when dioxygen is coadsorbed with ammonia and pyridine at Zn(0001) and Mg(0001) surfaces, ammonia and pyridine acting as specific chemical probes for the short-lived oxygen surface intermediates.