Thermal stability of cobalt oxides doped with MgO and Al 2O 3

Abstract

The influence of MgO and Al 2O 3 doping on the thermal decomposition of cobaltic oxide to cobaltous oxide and the re-oxidation of CoO to Co 3O 4 was studied using DTA, with controlled rate of heating and cooling, and infrared and X-ray diffraction techniques. X-ray diffraction and infrared absorption spectra were obtained for different solids subjected to both sudden and slow cooling. The DTA investigation revealed that MgO slightly decreased both the thermal stability of Co 3O 4 and the susceptibility of CoO to oxidation by O 2 to form Co 3O 4. By contrast, Al 2O 3 considerably increased the thermal stability of cobaltic oxide to an extent proportional to the amount of dopant oxide, attaining a maximum stability of 58% for the 4.5% Al 2O 3-doped solid. Furthermore, Al 2O 3 effectively enhanced the reactivity of CoO for oxidation to Co 3O 4. The marked thermal stability of Co 3O 4, induced by doping with Al 2O 3, was confirmed by both X-ray diffraction and infrared absorption spectrometry investigations. The sudden cooling of MgO and Al 2O 3-doped cobalt oxide specimens from 1000°C to room temperature, was accompanied by the appearance of a strong absorption band at 1380 cm −1. The intensity of this band, which was more pronounced in the case of Al 2O 3-doped solids, decreased considerably with slow cooling of the doped oxide specimens. The possible mechanisms of MgO and Al 2O 3-doping of cobalt oxides were discussed.