Reactivity of oxygen states at caesium surfaces towards carbon monoxide and carbon dioxide

Abstract

The development of oxygen states at caesium surfaces at 80 K has been studied by X-ray photoelectron spectroscopy. Core-level O 1s spectra have enabled three distinct oxygen states to be assigned to isolated oxygen adatoms, O^δ–, peroxo-type species, O₂^δ–, and oxide, O^2–. The O^δ– state is only observed at low coverages and is the precursor to oxide formation. The reactivity of these species towards carbon monoxide and carbon dioxide has been studied through monitoring the O 1s and C 1s spectra and quantitative analysis of the stoichiometry of the reactions. Both the anionic form of chemisorbed carbon dioxide, CO₂^δ–, and surface carbonate species can be formed through reactions of the individual oxygen species with either CO or CO₂ while the CO₂^δ– species can be transformed to carbonate species by exposure to dioxygen. All reactions are of low activation energy and occur at 80 K. A reaction scheme is proposed which illustrates the various reaction pathways available when carbon monoxide–dioxygen mixtures are exposed to a caesium surface at 80 K; for oxygen rich mixtures surface oxidation dominates while for carbon monoxide rich mixtures both chemisorbed carbon dioxide, CO2^δ–, and surface carbonate form.