Unusual Physical and Chemical Properties of Cu in Ce_1-xCu_xO₂ Oxides

Abstract

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.

Cu-doped ceria: Oxygen vacancy formation made easy

“…Moreover, in their experiments they also found that the Cu dopants always gave rise to a corresponding number of O vacancies, i.e. that vacancy formation is very much facilitated by Cu doping and the oxygen storage capacity remarkably enhanced [10]. Here, we will explore exactly that property by means of quantum-mechanical calculations.…”

“…The remainder (1.59 eV) must be attributed to the changes in electronic structure induced by the Cu 2+ dopant. Wang et al [10] ascribe the easy formation of O vacancies in Cu-doped bulk ceria to a large strain in the ceria lattice introduced by the Cu dopant.…”

“…Electrical conductivity measurements have shown that a Cu x Ce 1-x O 2-δ (0 ≤ x ≤ 0.10) solid solution exhibits good oxide-ion conductivity [8]. As for structure, Bera et al [9] presented a comprehensive experimental study of Cu-doped ceria samples using a range of methods; their EXAFS analyses for Cu x Ce 1-x O 2-δ samples with x equal to 0.03, 0.05 and 0.10 suggested that Cu has three oxygen atoms in its first coordination shell, located at a distance of 1.96 Å. Wang et al [10] performed synchrotron-based X-ray diffraction (XRD) and EXAFS measurements for x = 0.05, 0.1 and 0.2 (supported by quantum-mechanical calculations). Their EXAFS fits indicated a Cu-O coordination number of 5−6 (while their quantum-mechanical calculations suggested that the Cu atom in bulk ceria has four oxygen atom neighbors at a distance of 1.92-1.95 Å).…”

“…To the best of our knowledge all these studies explored unreduced samples, i.e. [10], while Shapovalov and Metiu [11] reported data for the Cu-O coordination structure for Cu-doped CeO 2 (111) (as part of a larger study of CO oxidation on Au-doped ceria surface). In Ref.…”

Techniques for the Study of the Structural Properties

“…63 A similar approach has been employed to explain qualitatively the shifts observed in the main band of Ce-Tb or Ce-Cu mixed oxide nanoparticles as a function of the Tb or Cu content. 76,77 Another effect of the presence of oxygen vacancies in the CeO 2 nanoparticles is the appearance of a new broad peak at ca. 600 cm -1 that can be related to a second-order phonon or a local mode centered on the vacancy positions.…”