Unusual Physical and Chemical Properties of Cu in Ce 1-x Cu x O 2 Oxides Xianqin Wang, ² Jose ´ A. Rodriguez,* ,² Jonathan C. Hanson, ² Daniel Gamarra, ‡ Arturo Martı ´nez-Arias,* ,‡ and Marcos Ferna ´ ndez-Garcı ´a* ,‡ Chemistry Department, BrookhaVen National Laboratory, Upton, New York 11973, and Instituto de Cata ´ lisis y Petroleoquı ´mica, CSIC, Campus Cantoblanco, 28049 Madrid, Spain ReceiVed: April 15, 2005; In Final Form: August 9, 2005 The structural and electronic properties of Ce 1-x Cu x O 2 nano systems prepared by a reverse microemulsion method were characterized with synchrotron-based X-ray diffraction, X-ray absorption spectroscopy, Raman spectroscopy, and density functional calculations. The Cu atoms embedded in ceria had an oxidation state higher than those of the cations in Cu 2 O or CuO. The lattice of the Ce 1-x Cu x O 2 systems still adopted a fluorite- type structure, but it was highly distorted with multiple cation-oxygen distances with respect to the single cation-oxygen bond distance seen in pure ceria. The doping of CeO 2 with copper introduced a large strain into the oxide lattice and favored the formation of O vacancies, leading to a Ce 1-x Cu x O 2-y stoichiometry for our materials. Cu approached the planar geometry characteristic of Cu(II) oxides, but with a strongly perturbed local order. The chemical activities of the Ce 1-x Cu x O 2 nanoparticles were tested using the reactions with H 2 and O 2 as probes. During the reduction in hydrogen, an induction time was observed and became shorter after raising the reaction temperature. The fraction of copper that could be reduced in the Ce 1-x Cu x O 2 oxides also depended strongly on the reaction temperature. A comparison with data for the reduction of pure copper oxides indicated that the copper embedded in ceria was much more difficult to reduce. The reduction of the Ce 1-x Cu x O 2 nanoparticles was rather reversible, without the generation of a significant amount of CuO or Cu 2 O phases during reoxidation. This reversible process demonstrates the unusual structural and chemical properties of the Cu-doped ceria materials. Introduction CuO-CeO 2 mixed-metal oxides have important applications as electrolytes in fuel cells, 1,2 gas sensors, 3 and efficient catalysts for various reactions such as the combustion of CO and methane, 4-7 the water-gas shift reaction, 7-9 the reduction of SO 2 by CO, 10 methanol synthesis, 11 and the wet oxidation of phenol. 12 A recent study has even reported that Cu/CeO 2 catalysts exhibited superior activity when compared to the industrial Cu/ZnO catalysts for the steam-reforming methanol reaction. 13,14 Furthermore, copper oxide-based catalysts have been considered as suitable substitutes for noble metal catalysts in emission control applications for novel, clean fuels due to their high activities and lower cost. 13-15 It is anticipated that, with proper development, metal-promoted ceria catalysts should realize much higher CO conversions than commercial Cu/ZnO catalysts. 9,13-15 In this study, we investigate the behavior of nanoparticles of Ce 1-x Cu x O 2 solid solutions prepared following a novel proce- dure. 5,16,17 Since the dopant is incorporated into a fluorite-type crystal structure that is very different from the structures of CuO or Cu 2 O (see Figure 1), it is not clear what to expect for the physical and chemical properties of Ce 1-x Cu x O 2 . In principle, a Cu cation embedded in a fluorite lattice can have up to eight oxygen neighbors versus four in CuO and two in Cu 2 O. Moreover, Cu does not adopt formal oxidation states of “+4” and “+3” like Ce does. These differences open the possibility for substantial structural perturbations (stress, point, and line defects, O vacancies) 18 within the ceria lattice. Previous work indicates that Cu-doped ceria has better redox properties and oxygen storage capacity than those of pure ceria. 7 The combination of the redox properties and oxygen storage capacity of ceria as well as the interaction between ceria and Cu are usually claimed for the significant activities of CuO- CeO 2 catalysts. 7c,9,19,20,21 To address these issues, a series of Ce 1-x Cu x O 2 and CuO x /CeO 2 samples were prepared in this work. Their structural and electronic properties were investigated employing X-ray diffraction (XRD), X-ray absorption fine structure (XAFS), Raman spectroscopy, and density functional (DF) calculations. Since CuO-CeO 2 catalysts interact with hydrogen in many reaction processes, 7-15 we studied the reduction in H 2 of the Ce 1-x Cu x O 2 and CuO x /CeO 2 systems using synchrotron-based in-situ time-resolved XRD and XAFS. Recently, these techniques were successfully used to study the kinetics and mechanism for the reduction of CuO and Cu 2 O. 22,23 They showed a direct transformation pathway for CuO reduction (CuO f Cu) when there was a large supply of H 2 , while a sequential reduction pathway involving one intermediate (CuO f Cu 2 O f Cu) was observed when there was a limited supply * Corresponding authors. ² Brookhaven National Laboratory. ‡ Instituto de Cata ´lisis y Petroleoquı ´mica, CSIC. Figure 1. Crystal structures of CuO, Cu2O, and CeO2. 19595 J. Phys. Chem. B 2005, 109, 19595-19603 10.1021/jp051970h CCC: $30.25 © 2005 American Chemical Society Published on Web 09/23/2005