Electronic and chemical properties of Ce0.8Zr0.2O2(111) surfaces: Photoemission, XANES, density-functional, and NO2 adsorption studies

Synchrotron-based high-resolution photoemission, conventional X-ray (Mg Ka) photoemission (XPS), X-ray absorption near-edge spectroscopy (XANES), and first-principles density-functional calculations have been used to study th e electronic properties of a Ce0.8Zr0.2O2 mixed-metal oxide. The results of density-functional calculations show that the band gap in bulk Ce0.8Zr0.2O 2 is similar to0.6 eV smaller than that 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 nonequivalent Zr-O distances, at l east three different types of oxygen atoms are found in the Ce0.8Zr0.2O2 sy stem. O K-edge XANES spectra for a series of Ce1-xZrxO2 (x = 0, 0.1, 0.2, 0 .3, and 1) compounds show a distinctive line shape for the mixed-metal oxid es that cannot be attributed to a SUM Of CeO2 and ZrO2 features, supporting the idea that the 0 atoms in Ce1-xZrxO2 are in a special chemical environm ent. XPS Ce 3d core-level spectra show the presence of Ce3+ cations even af ter prolonged oxidation with oxygen gas, which may be related to the relati ve 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 examined. Ne+ ion sputtering was us ed to generate substantial concentrations of Ce3+, Zr2+, and Zr-0 centers o n the oxide surfaces. On CeO2-x(111), we observed NO3, NO2, and N upon adso rption of NO2. In contrast, only NO2 and N were detected after adsorption o f 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+; Zr- 0 --> Zr2+). The NO, species desorbed from the oxides at temperatures rangi ng from 400 to 800 K, leaving N adatoms on the surfaces. The effects of Zr on the electronic and chemical properties of ceria are discussed and compar ed to those of other common dopant agents (Ca, Ti, and Cu).