XPS study of copper aluminate catalysts

The oxidation states and compositions of the surfaces of CuAl2O4 and NiO promoted CuAl2O4 were investigated by X-ray photoelectron spectroscopy (XPS). It is shown that an appreciable enrichment of the copper content within the probing depth of CuAl2O4 occurs as compared to the bulk concentration. This trend is still enhanced by the addition of small amounts of NiO. The aluminates are being reduced in vacuo at temperatures ⩾ 200°C to form Cu2O and Al2O3. Reduction in CO and H2 at 500°C leads to the formation of metallic Cu within the probing depth, while the nickel appears to be only partially reduced. Prereduced catalysts can be reoxidized by NO and O2 at temperatures ⩾ 200°C. The original spinel structure can be reformed under fairly mild conditions (approximately 450°C), which is explained by the assumption of very small and highly reactive particles being formed during reduction-reoxidation cycles. Treatment of the catalysts in NO + CO reaction mixtures produces Cu species in low oxidation states and Cu2+. The low oxidation states seem to be essential for the catalytic activity of the system.