Effect of Al2O3 and MoO3 doping of Co3O4 solid on its surface and catalytic properties

Pure and doped Co3O4 samples were prepared by thermal decomposition at 500-1000 degrees C of pure basic cobalt carbonate and of the basic carbonate treated with aluminium nitrate and ammonium molybdate. The amounts of dopants added were varied within the range 0.75-6 mol% Al2O3 and 0.025-6 mol% MoO3. The influence of this treatment on the specific surface areas and catalytic activities of the Co3O4 solids was investigated using nitrogen absorption at -196 degrees C and studies of the decomposition of H2O2 at 30-50 degrees C. The results obtained revealed that doping of cobaltic oxide solids with either Al2O3 or MoO3, followed by calcination at 500 degrees C and 700 degrees C. resulted in a progressive increase in the BET surface areas. This increase was, however, more pronounced in the case of MoO3 doping. Calcination of the doped solids at 900 degrees C led to an increase in the BET surface areas of the Al2O3-treated solids and to a small decrease in the specific surface areas of the MoO3-doped samples. Calcination of the variously doped solids at 500-900 degrees C brought about a decrease in their catalytic activity to an extent proportional to the amount of dopant added. Thus, treatment of Co3O4 solids with 6 mol% Al2O3 followed by calcination at 500 degrees C, 700 degrees C and 900 degrees C effected a decrease of 36.9, 42.8 and 67.5% in their activities (expressed as reaction rate constant per unit area) measured at 30 degrees C. The decrease in the catalytic activity of Co3O4 solids due to MoO3 doping was greater than that effected by Al2O3 doping. The doping process did not change the mechanism of the catalytic reaction but effectively decreased the concentration of CO3+-CO2+ ion pairs acting as the active sites involved in the catalytic process.