Surface and catalytic properties of Co, Ni and Cu binary oxide systems

NiO/CuO, CuO/NiO, Co3O4/NiO, NiO/Co3O4, CuO/Co3O4 and Co3O4/CuO systems having equimolar ratios of transition metal oxides were prepared. These solids were obtained by wet impregnation method using the corresponding nitrates and carbonate salts followed by calcination at 300-750 degreesC. The crystalline phases produced at different calcination temperatures were characterized by XRD investigation. The surface and catalytic properties of the calcined products of various solids were determined by nitrogen adsorption at - 196 degreesC and CO oxidation by O-2 at 35-250 degreesC using a static method. The results obtained reveled that the specific surface areas of individual oxides were greater than those of different binary systems. The binary oxides containing Co3O4 precalcined at 300-500 degreesC constituted of Ni and Cu cobaltites. These cobaltites decomposed by heating at 750 degreesC. A portion of CuO dissolved in NiO lattice in the binary oxides system by heating at 400 degreesC and the other portion remained as a separate phase. The dissolution process resulted in the formation of NiCuO2 compound which starts to decompose by heating at >500 degreesC. Co3O4/CuO solids showed a catalytic activity, expressed as reaction rate constant per unit surface area, much greater than that Of CuO/Co3O4 and about four-fold those of the free oxides. Similarly, Co3O4/NiO was devoted with a catalytic activity much greater than that of NiO/Co3O4 and showed an activity close to those of free oxides. Finally, NiO/CuO exhibited a catalytic activity much greater than that of CuO/NiO and about three-fold those of the free oxides. The increase in precalcination temperature of various binary oxide systems from 300 to 400 degreesC brought about a measurable increase in their catalytic activities. The activation energy of the catalytic reaction was determined for various investigated systems and was found to be dependent on the nature of the system and its calcination temperature. (C) 2004 Elsevier B.V. All rights reserved.