XPS investigation on Co-Cu mixed oxide catalysts

Metastable and isomorphous cobalt-copper hydroxysalts with Co:Cu = 100:0, 70:30 and 50:50 atomic ratios have been prepared as precursors by coprecipitation at constant pH from solutions of cobalt and copper nitrates added to a solution of NaHCO3. Mixed oxides were obtained by decomposition of the precursors at 723 K for 24 h in air and in N2. The surface of both precursors and mixed oxides was investigated by XPS. Only Co2+ and Cu2+ species are revealed for the precursors. The mixed oxides present CO2+, CO3+, Cu+ and Cu2+ species depending on the composition and on the calcination atmosphere. Quantitative analysis has evidenced a cobalt enrichment at the surface for the Co : Cu = 70:30 and 50:50 precursors. With respect to mixed oxides, the samples calcined in N2 are characterized by a homogeneous distribution of cobalt and copper both in the bulk and on the surface; in contrast, for the corresponding samples calcined in air the original cobalt enrichment of the precursors is preserved.