Unusual Physical and Chemical Properties of Cu in Ce_1‐xCu_xO₂ Oxides.

Abstract

Oxides. -Structural and electronic properties of nanoparticles of the title solid solutions prepared by a reverse microemulsion method are characterized by XRD, XAFS, Raman spectroscopy, and DFT calculations. The Cu atoms embedded in ceria have an oxidation state higher than those of the cations in Cu2O or CuO. The lattices of the title systems still adopt a fluorite-type structure but are highly distorted. The doping of CeO 2 with Cu introduces a large strain into the oxide lattice and favors the formation of O vacancies. The chemical activities of the prepared nanoparticles are characterized by the reactions with H2 and O2. The reduction of the solid solutions is completely reversible without generation of significant amounts of CuO or Cu2O phases during reoxidation. -(WANG, X.; RODRIGUEZ*, J. A.; HANSON, J. C.; GAMARRA, D.; MARTINEZ-ARIAS, A.; FERNANDEZ-GARCIA, M.; J.

Characterization of Active Sites/Entities and Redox/Catalytic Correlations in Copper-Ceria-Based Catalysts for Preferential Oxidation of CO in H2-Rich Streams

“…5-8 nm nanocrystals [46,47]) onto which copper oxide entities have been dispersed. The chemical state of copper has been shown to be similar for all samples and, in turn, similar to that observed for a CuO reference, with small differences being attributed to interactions with the ceria-related support [18,37], as evidenced by XANES (Figure 1). The existence of a fully oxidized state, Cu(II), of copper in the initial calcined catalysts is also in agreement with XPS investigation, as shown in Figure 2.…”

“…In contrast, the surface area appreciably increases with the copper amount for the samples prepared by microemulsion-coprecipitation, in correlation with the aforementioned introduction of copper into the ceria lattice, which basically induces a decrease in primary particle size (Table 1). On the other hand, concerning the electronic state of copper, XANES (as also shown below) and XPS analyses revealed that the starting sample calcined under dilute O 2 at 500 °C presents copper in a fully oxidized Cu 2+ chemical state with relatively few differences (except for the crystalline CuO detected in 5CuO/CeO 2 ) between the samples concerning copper electronic characteristics [37,40].…”

“…As shown in Table 1, lattice parameters estimated from XRD analysis of the fluorite peaks are close to those expected for pure ceria for all the samples. It must be taken into account, however, that copper introduction into the ceria fluorite lattice is not expected to induce significant changes in this parameter [29,37]. Indeed, on the basis of analysis of lattice microstrain (determined from analysis of XRD results) and Raman results, significant differences between samples prepared by impregnation and microemulsion-coprecipitation were revealed [37,40].…”

“…It must be taken into account, however, that copper introduction into the ceria fluorite lattice is not expected to induce significant changes in this parameter [29,37]. Indeed, on the basis of analysis of lattice microstrain (determined from analysis of XRD results) and Raman results, significant differences between samples prepared by impregnation and microemulsion-coprecipitation were revealed [37,40]. These have been related to the fact that, as expected, all the copper remains essentially at the sample surface in the samples prepared by copper impregnation, while at least a part of the copper appears to be incorporated into the ceria fluorite lattice for those prepared by coprecipitation within microemulsion.…”

“…In this sense, generally speaking, the properties of copper oxide entities for CO oxidation promotion depend strongly on their dispersion degree and/or related degree of interaction with ceria [18,25,26,36,37]. Nevertheless, although reaction models for CO oxidation (competing or not with H 2 oxidation, as it occurs under CO-PROX conditions) have been proposed on the basis of indirect analysis of redox or catalytic properties for this type of catalyst [38,39]; direct evidence on redox changes taking place in the catalyst under the reactant atmosphere, which could provide details on the nature of active sites for the processes taking place under CO-PROX conditions in this type of catalyst, has, to the best of our knowledge, only recently been reported in a series of contributions from our laboratory [18,40,41].…”