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 by mixing solutions of the nitrates and calcining at 1223 K, followed by treatment with washing solutions of either ammonium carbonate or nitric acid in order to extract uncombined copper oxide. Ammonium carbonate was only effective for removal of dispersed CuO, whereas nitric acid 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 are copper species derived from CuAl₂O₄ and CuCr₂O₄ spinels rather than from CuO. The CuCr₂O₄ catalyst exhibited higher activity, suggesting that Cu species in tetrahedral coordination lead to higher activity. Also, prereduction with H₂ produced higher activity than prereduction with CO, which can be attributed to a higher surface concentration of active species or their precursors after the former treatment. Activity results, together with TPR and XPS characterization point out that both Cu⁰ and Cu⁺ species are involved in the mechanism of CO oxidation.