Redox properties of water on the oxidized and reduced surfaces of CeO2(1 1 1)

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 Å epitaxial CeO2(1 1 1) film grown on yttria-stabilized ZrO2(1 1 1). Oxidation with O2 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 Ce3d3/2 4f0 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 O2 at RT resulted in nearly complete conversion of Ce3+ to Ce4+. Annealing at 773 K led to a surface in which ≈40% 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 O2. 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, H2 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.