Effect of Alumina Modification on the Reducibility of Co3O4 Crystallites Studied on Inverse-Model Catalysts

Abstract

In order to establish the effect of alumina modification and calcination temperature on the reducibility of cobalt oxide, alumina-modified cobalt oxide crystallites containing less than 2.5 wt% Al were prepared via incipient wetness impregnation, and calcined at 300 °C or 500 °C. The catalysts were characterised using X-ray diffraction, scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy, temperature programmed reduction (TPR) and X-ray absorption near edge spectroscopy. The alumina modification was found to prevent sintering during calcination and decrease the reducibility of the catalysts. With increasing alumina loading, and increasing calcination temperature, reduction peaks shift to higher temperatures and peaks above 400 °C appear in the TPR. The kinetic evaluation shows that the decreased reducibility is due to a decrease in the pre-exponential factor, which suggests that the alumina modification hinders hydrogen activation and the nucleation of reduced cobalt phases. The catalysts are completely reduced below 800 °C, and no CoAl2O4 phase formation was observed. TPR peaks between 400 and 650 °C were assigned to the formation of a non-stoichiometric cobalt–alumina phase with cobalt ions in octahedral coordination, while peaks between 650 and 800 °C correspond to cobalt ions in tetrahedral coordination. The results show that introduction of small amounts of alumina to cobalt oxide can have drastic effects on the rate of reduction and sintering.