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J. Phys. Chem. B 2001, 105, 7762-7770
Electronic and Chemical Properties of Ce0.8Zr0.2O2(111) Surfaces: Photoemission, XANES, Density-Functional, and NO2 Adsorption Studies Gang Liu, Jose´ A. Rodriguez,* Jan Hrbek, and Joseph Dvorak Chemistry Department, BrookhaVen National Laboratory, Upton, New York 11973
Charles H. F. Peden EnVironmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352 ReceiVed: April 2, 2001; In Final Form: June 7, 2001
Synchrotron-based high-resolution photoemission, conventional X-ray (Mg KR) 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 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 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, and 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 examined. Ne+ ion sputtering was used to generate substantial concentrations of Ce3+, Zr2+, and Zr0 centers on the oxide surfaces. On CeO2-x(111), we observed NO3, NO2, and N 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+ f Ce4+; Zr0 f Zr2+). The NOx species desorbed from the oxides at temperatures ranging 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 compared to those of other common dopant agents (Ca, Ti, and Cu).
I. Introduction Zirconia-doped ceria (Ce1-xZrxO2) is a complex material and an important component of catalysts used in automotive exhaustgas converters.1 Today, the protection of the environment is receiving much more attention than ever before. One of the efforts for environmental cleanup is focused on controlling the emission of toxic pollutants such as nitrogen oxides NOx (NO2, NO, N2O), which are produced in automotive engines by the oxidation of nitrogen.2 In many situations, the decomposition rate of nitrogen oxides is slow without catalytic assistance. Therefore, it is necessary to employ appropriate catalysts to reduce the emission of NOx compounds (denoted as the DeNOx process) into the air.1,3 Due to its redox properties, ceria (CeO2) is a key component in three-way automotive catalysts.4,5 Ceriasupported noble-metal catalysts are capable of storing oxygen under oxidizing conditions and releasing oxygen under reducing conditions via the facile conversion between Ce4+ and Ce3+ oxidation states.6 Recently, significant attention7-13 has been directed toward correlating the electronic and geometric structures of nonstoichiometric ceria with their chemical and catalytic * Corresponding author. Fax: 631-344-5815. E-mail:
[email protected].
properties. Oxygen anion vacancies in ceria surfaces are considered to play an essential role in catalytic reactions.13-16 To enhance the redox properties and thermal stability of pure ceria, zirconia (ZrO2) is often mixed as an additive to form solid solutions of the Ce1-xZrxO2 type. Zr-doped cerium oxides have been reported to enhance the oxygen storage capacity (OSC) and the rate for the oxygen release in automotive catalysts.16-22 They also retard thermal deactivation.16-22 The focus has been on examining possible correlations between the CeO2-ZrO2 interactions and differences in the behavior of Ce1-xZrxO2 and CeO2. It has been suggested that ceria structural modifications mediated by zirconia are responsible for the enhanced OSC properties of ceria-zirconia mixed oxides.18,19,22 However, the mechanisms for such doping effects remain uncertain and are still a matter of debate. The exact role of pure and Zr-doped ceria in DeNOx processes in automobile catalytic converters is not clear.5,6 Understanding the details of NOx chemistry on Ce1-xZrxO2 surfaces has both practical and academic interests.3,5 NO2 is formed in automotive engines during the combustion of fuels under oxygen-rich conditions. In general, little is known about the chemistry of NO2 on oxide surfaces at a molecular level.3a,23-27 It has been
10.1021/jp011224m CCC: $20.00 © 2001 American Chemical Society Published on Web 07/25/2001
Properties of Ce0.8Zr0.2O2(111) Surfaces found that for fully oxidized polycrystalline CeO2 and CeO2 powder, at 300 K, nitrate is the only species that exists on the surface after adsorption of NO2.3a However, on partially reduced ceria (CeO2-x), a surface mixture of N, NO, and NO3 was observed at 300 K with simultaneous transformation of Ce3+ to Ce4+ ions.3a Adsorbed nitrate has been detected as a product of the reaction of NO2 with MgO(100),3a ZnO(0001),28a and TiO2(110)28b at high coverage and moderate temperatures (
