Effect of thermal treatments on catalyst reducibility and activity in nickel supported on RHA–Al2O3 systems

RHA–Al2O3 composite oxide supports were prepared by impregnation of rice husk ash with an aluminum sulfate solution, and were then used to prepare Ni/RHA–Al2O3 catalysts by the ion exchange method. The supports and catalyst precursors were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The thermal decomposition of catalyst precursors was examined by a thermogravimetric analyzer (TGA). In addition, the effect of calcination temperature on the catalyst reducibility was investigated by the temperature-programmed reduction (TPR) technique. Results show that the decomposition temperature of layered nickel compound to nickel oxide started above 773 K. Reduction in NiO from the thermal decomposition of the layered nickel compound was found to be particularly difficult. The reduction temperature increased with the increase in calcination temperature. The catalytic activities of nickel catalysts were tested by CO2 hydrogenation with H2/CO2 ratios of 4/1 at 773 K. The effects of calcination temperature, calcination time, reduction temperature, reduction time and alumina content on the activity of catalysts were examined extensively. An optimal condition for calcination and reduction of catalysts has been determined from activity considerations in the hydrogenation of CO2.