New information about the active sites for the water gas shift (WGS) reaction over Cu-CeO 2 systems was obtained using in-situ, time-resolved X-ray diffraction (TR-XRD), X-ray absorption spectroscopy (TR-XAS, Cu K and Ce L 3 edges), and infrared spectroscopy (DRIFTS). Cu-CeO 2 nanoparticles prepared by a novel reversed microemulsion method (doped Ce 1-x Cu x O 2 sample) and an impregnation method (impregnated CuO x / CeO 2 sample) were studied. The results from all of the samples indicate that both metallic copper and oxygen vacancies in ceria were involved in the generation of active sites for the WGS reaction. Evidence was found for a synergistic Cu-O vacancy interaction. This interaction enhances the chemical activity of Cu, and the presence of Cu facilitates the formation of O vacancies in ceria under reaction conditions. Water dissociation occurred on the O vacancy sites or the Cu-O vacancy interface. No significant amounts of formate were formed on the catalysts during the WGS reaction. The presence of strongly bound carbonates is an important factor for the deactivation of the catalysts at high temperatures. This work identifies for the first time the active sites for the WGS reaction on Cu-CeO 2 catalysts and illustrates the importance of in situ structural studies for heterogeneous catalytic reactions.

Two series of nanostructured oxidized copper-cerium catalysts with varying copper loadings, and prepared, respectively, by impregnation of ceria and by coprecipitation of the two components within reverse microemulsions, have been characterized in detail at structural and electronic levels by X-ray diffraction (XRD), Raman spectroscopy, high-resolution electron microscopy (HREM), X-ray energy dispersive spectroscopy (XEDS), X-ray photoelectron spectroscopy (XPS) (including Ar + -sputtering), and X-ray absorption fine structure (XAFS). These results have been correlated with analysis of their catalytic properties for preferential oxidation of CO in a H 2 -rich stream (CO-PROX), complemented by Operando-DRIFTS. A relevant difference between the two series of catalysts concerns the nature of the support for the surface-dispersed copper oxide entities, which is essentially ceria for the samples prepared by impregnation and a Ce-Cu mixed oxide for those prepared by microemulsion-coprecipitation. The existence of copper segregation in the form of copper oxide or copper-enriched Cu-Ce mixed oxides for the latter type of samples is uniquely revealed by nanoprobe XEDS and XPS Ar + -sputtering experiments. The CO oxidation activity under CO-PROX conditions is correlated to the degree of support-promoted reduction achieved by the dispersed copper oxide particles under reaction conditions. Nevertheless, catalysts which display higher CO oxidation activity are generally more efficient also for the undesired H 2 oxidation reaction. The balance between both reactions results in differences in the CO-PROX activity between the two series of catalysts which are examined on the basis of the structural differences found.

Catalysts based on combinations between copper and cerium oxides are analyzed with respect to their catalytic properties for preferential oxidation of CO in a H2-rich stream (CO-PROX) by means of DRIFTS, XANES, and Raman under Operando conditions. The results allow analyzing entities/species responsible for the CO and H2 oxidation reactions taking place during the CO-PROX process. While CO oxidation takes place at copper oxide support interfacial sites and its activity correlates with the degree of reduction achieved on the dispersed copper oxide particles, H2 oxidation apparently proceeds when a massive copper oxide reduction occurs. This opens the possibility to modulate the catalytic behavior of these types of catalysts by acting, respectively, on the interfacial redox properties and on the dispersed copper oxide redox properties.