PNNL

Abstract

Synchrotron-based high-resolution photoemission, conventional X-ray (Mg K?) photoemission (XPS), X-ray absorption near-edge spectroscopy (XANES), and first-principles density functional calculations have been used to study the electronic properties of a Ce0.8Zr0.2O2 mixed-metal oxide. The results of density-functional calculations show that the band gap in bulk Ce0.8Zr0.2O2 is ~ 0.6 eV smaller than in bulk CeO2, with the Zr atoms in the mixed-metal oxide showing smaller positive charges than the cations in ZrO2 or CeO2. When present in a lattice of CeO2, the Zr atoms are forced to adopt larger metal-O distances than in ZrO2, leading to a reduction in the oxidation state of this element. Due to non-equivalent Zr-O distances, at least three different types of oxygen atoms are found in the Ce0.8Zr0.2O2 system. O K-edge XANES spectra for a series of Ce1-xZrxO2 (x=0, 0.1, 0.2, 0.3, 1) compounds show a distinctive line shape for the mixed-metal oxides that cannot be attributed to a sum of CeO2 and ZrO2 features, supporting the idea that the O atoms in Ce1-xZrxO2 are in a special chemical environment. XPS Ce 3d core level spectra show the presence of Ce3+ cations even after prolonged oxidation with oxygen gas, which may be related to the relative stability of oxygen vacancy defects upon incorporation of zirconia into ceria. The interaction of NO2 gas with Ce0.8Zr0.2O2-x(111), CeO2-x(111), and Zr(Y)O2-x(111) reduced surfaces was addressed. Ne+ ion sputtering was used to generate substantial concentrations of Ce3+, Zr2+ and Zr0 centers on the oxide surfaces. On CeO2-x(111), NO3, NO2 and N were seen upon adsorption of NO2. In contrast, only NO2 and N were detected after adsorption of NO2 on Ce0.8Zr0.2O2-x(111) and Zr(Y)O2-x(111). Adsorption of NO2 induced an increase in the oxidation state of the metal cations (Ce3+-->Ce4+; Zr0-->Zr2+).