Redox properties of water on the oxidized and reduced surfaces of CeO2(111)

We present X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD) results probing the surface chemistry of water on the oxidized and reduced surfaces of a 500 Angstrom epitaxial CeO2(1 1 1) film grown on yttria-stabilized ZrO2(1 1 1). Oxidation with O-2 at 773 K under UHV conditions was sufficient to generate XPS spectra reflective of fully oxidized CeO2(1 1 1). Surface reduction was carried out by annealing in UHV between 773 and 973 K, and the level of reduction was quantified using changes in the Ce3d(3/2) 4f(0) photoemission peak at 917 eV which results primarily from Ce4+ sites. As expected, the level of surface reduction (generation of Ce3+ sites) increased with increasing temperature. These Ce3+ sites were primarily in the first layer based on the fact that exposure of the film to O-2 at RT resulted in nearly complete conversion of Ce3+ to Ce4+. Annealing at 773 K led to a surface in which approximate to40% of the surface Ce4+ sites were reduced to Ce3+, whereas annealing at higher temperatures led to more substantial reduction of the first layer along with some subsurface reduction that was not reoxidized by RT exposure to O-2. Comparisons with results in the literature for reduction of single crystal CeO2(1 1 1) surfaces suggest that the volume-to-surface ratio of ceria samples influences, in part, the reduction conditions that result in detectable levels of surface Ce3+ sites. In other words, the annealing temperatures required to achieve a specific extent of surface reduction likely depends on the thickness of the sample. Water TPD studies on the oxidized CeO2(1 1 1) surface reveal strong coverage dependence that destabilizes high coverages of water relative to low coverages. The presence of surface reduction (on the order 30% oxygen vacancy sites) removes much of the coverage dependent behavior. TPD uptake measurements, H-2 TPD spectra and XPS spectra in the Ce3d core level and Ce4f valence band (VB) regions all indicate that little or no irreversible water decomposition or Ce3+ oxidation was observed for water on this reduced surface. In contrast, exposure of water at 650 K resulted in additional surface reduction above that observed from annealing at 650 K in the absence of water. This is attributed to a redistribution of oxygen vacancies from the bulk to the surface as a result of high temperature water treatment. Because water oxidation of Ce3+ surface sites has been observed for reduced ceria powders, but was not observed on the reduced CeO2(1 1 1) surfaces studied here, we propose that the reduced (1 1 1) surface is more resistant than non-(1 1 1) terminations to being oxidized by water. (C) 2002 Published by Elsevier Science B.V.