Unusual physical and chemical properties of Cu in Ce _1-xCu _xO ₂ oxides

The structural and electronic properties of Ce _1-xCu _xO 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 ₂O or CuO. The lattice of the Ce _1-xCu _xO 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-xCu _xO _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-xCu _xO 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-xCu _xO 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-xCu _xO ₂ nanoparticles was rather reversible, without the generation of a significant amount of CuO or Cu ₂O phases during reoxidation. This reversible process demonstrates the unusual structural and chemical properties of the Cu-doped ceria materials.