Abstract
A pure Co3O4/Al2O3 sample was prepared by the wet impregnation method using finely powdered aluminium hydroxide and cobalt nitrate dissolved in the least amount of distilled water. Maintaining the amount of Co3O4 constant at 16.7 mol%, different samples were doped by impregnating with known amounts of cerium or vanadium nitrates prior to impregnation with cobalt nitrate. The pure and doped samples were precalcined at 600°C and 800°C, respectively. The dopant concentrations employed were 0.5, 1.0, 2.0 and 3.0 mol% CeO2 or V2O5. The resulting materials were studied by XRD and EDX methods, N2 adsorption at −196°C and CO oxidation by O2 at 130–220°C.
The results obtained revealed that the surface concentrations of cobalt species in pure and variously doped Co3O4/Al2O3 solids precalcined at 600°C were more than twice those present in the bulk of these solids. Heating the doped solid at 600°C and 800°C increased the degree of dispersion of Co3O4 by decreasing the crystallite size. This effect was more pronounced in the case of V2O5 doping, with the surface concentration of cobalt species being increased to a greater extent than with CeO2 doping. Similarly, heating the doped solid at 600°C resulted in a progressive but measurable increase in the lattice parameter of Co3O4 due to the formation of solid solutions.
On heating the V2O5-doped solid at 800°C, a portion of the added V2O5 inter-acted with Co3O4 to yield cobalt vanadate while the other portion contributed to the formation of a solid solution. Doping followed by heating at 600°C increased the catalytic activity of the doped solids in CO oxidation by O2. In contrast, solids precalcined at 800°C exhibited the opposite effect. The doping process changed the concentration of active sites involved in the catalytic reaction rather than changing the mechanism of the catalytic reaction.