Catalytic properties of copper–manganese mixed oxides prepared by coprecipitation using tetramethylammonium hydroxide

Abstract

Copper–manganese (Cu–Mn) mixed oxide catalysts were prepared by a coprecipitation technique from metal nitrates in aqueous solution using tetramethylammonium hydroxide (TMAH) as a pH regulator. The structures and properties of the mixed oxide catalysts were investigated by X-ray diffraction, extended X-ray absorption fine structure, X-ray photoelectron spectroscopy and H₂-TPR studies. Spinel-type mixed oxides were mainly formed in the Cu–Mn mixed oxides in which Cu and Mn species occupied both tetrahedrally and octahedrally coordinated sites. The occupancy of Cu and Mn in these sites depended on the Cu/Mn ratio. The average oxidation state of Cu was evaluated to be 2+ and was almost independent of the Cu/Mn ratio; reduced Cu species were also formed on the surface sites. By incorporation of Cu into Mn oxides, the average oxidation state of Mn increased both in the bulk sites and on the surface sites. The concentration of Mn on the surface sites was higher than that in the bulk sites. The Cu–Mn mixed catalysts exhibited higher activity than corresponding single-metal oxides at the reaction temperature range of 343–403 K. Formation of Cu–Mn spinel-type oxides gave rise to the increase in the catalyst surface area and the rate for CO oxidation normalized by surface area, and the Cu–Mn mixed oxides with the Cu/Mn molar ratio of 1 exhibited the highest activity for CO oxidation. CO-TPR studies revealed that in the temperature range of 343–403 K, the lattice oxygen at the first layer of the mixed oxides reacted with CO to form CO₂.