Structure-activity relationship in nanostructured copper-ceria-based preferential CO oxidation catalysts

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.