Abstract
The surface and catalytic properties of pure and Fe2O3-doped Co3O4/MgO solids were investigated. The extent of loading was maintained at 38 wt% Co3O4 and the amount of dopant varied between 1.2 wt% and 11.0 wt% Fe2O3. The pure and doped solids were prepared by calcination at 400–1000°C of pure and ferric nitrate-treated magnesium carbonate impregnated with cobalt nitrate solution. The samples thus obtained were examined by TG, DTA and XRD methods, nitrogen adsorption studies at −196°C and the catalysis of H2O2 decomposition at 30–50°C.
The results revealed that Fe2O3 treatment of the Co3O4/MgO system followed by calcination at 400°C resulted in an increase in the particle size of the Co3O4 and MgO phases, whilst the opposite effect was observed when the doped solids were calcined at 600°C. Doping of the Co3O4/MgO system followed by calcination at 400°C effected a measurable decrease in its BET surface area with the reverse effect being observed upon heating at 600°C. An increase in the calcination temperature of pure Co3O4/MgO up to 800°C brought about an abrupt decrease in the intensity of all the diffraction lines associated with the Co3O4 phase. These disappeared completely upon heating the system at 900°C or 1000°C due to dissolution of the cobalt oxide in the MgO lattice to form a solid solution.
A cobalt ferrite phase was detected in the variously doped solids calcined at 800–1000°C even when the initial solids had been treated with 2.4 wt% Fe2O3. This finding suggests that most of the ferrite phase produced was contained in the outermost surface layers of the treated samples.
Treatment of the investigated system with Fe2O3 followed by calcination at 400°C and 800°C led to a progressive decrease in the catalytic activity of the resulting solid due to an increase in the particle size (a decrease in the degree of dispersion) of the catalytically-activeconstituent, i.e. the Co3O4 phase, and due to the conversion of Co3O4 into a CoFe2O4 phase, respectively. In contrast, doping followed by calcination at 600°C led to an increase in the catalytic activity of the resulting solid due to an effective decrease in the particle size of the Co3O4 phase. Calcination of the Fe2O3-doped system at 400–800°C did not modify the mechanism of the catalyzed reaction but changed the concentration of active sites without changing their energetic nature.