PNNL

Abstract

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 (111) film grown on yttria-stabilized ZrO2 (111). Oxidation with O2 at 773 K under UHV conditions was sufficient to generate XPS spectra reflective of fully oxidized CeO2 (111). 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 approximately 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.