Abstract
The oxidation states and compositions of the surfaces of CuAl 2O 4 and NiO promoted CuAl 2O 4 were investigated by X-ray photoelectron spectroscopy (XPS). It is shown that an appreciable enrichment of the copper content within the probing depth of CuAl 2O 4 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 Cu 2O and Al 2O 3. Reduction in CO and H 2 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 O 2 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 Cu 2+. The low oxidation states seem to be essential for the catalytic activity of the system.
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