NATURE OF COPPER ACTIVE-SITES IN THE CARBON-MONOXIDE OXIDATION ON CUAL2O4 AND CUCR2O4 SPINEL-TYPE CATALYSTS

Unsupported copper-aluminum and copper-chromium oxides were prepared b y mixing solutions of the nitrates and calcining at 1223 K, followed b y treatment with washing solutions of either ammonium carbonate or nit ric acid in order to extract uncombined copper oxide. Ammonium carbona te was only effective for removal of dispersed CuO, whereas nitric aci d removed both dispersed and crystalline CuO. Catalyst activity for CO oxidation increased with the extraction treatments, particularly with the nitric acid washing solution, indicating that the active sites ar e copper species derived from CuAl2O4 and CUCr2O4 spinels rather than from CuO. The CuCr2O4 catalyst exhibited higher activity, suggesting t hat Cu species in tetrahedral coordination lead to higher activity. Al so, prereduction with H-2 produced higher activity than prereduction w ith CO, which can be attributed to a higher surface concentration of a ctive species or their precursors after the former treatment. Activity results, together with TPR and XPS characterization point out that bo th Cu-0 and Cu+ species are involved in the mechanism of CO oxidation. (C) 1998 Academic Press.