CO2 methanation vs reverse WGS activity on Co/gamma-Al2O3 catalysts at atmospheric pressure: effect of cobalt loading and silica addition on selectivity and stability

Catalysts containing 5 wt% and 13.6 wt% cobalt on pure alumina and 5 wt% silica-containing alumina were prepared by conventional impregnation procedure and calcined at 1023 K for 5 h. The samples were charac-terized both as prepared, pre-reduced and after reaction by means of X-ray Diffraction, FTIR and DR-UV-Vis-NIR spectroscopies and by X-ray Pholotoelectron Spectroscopy. After pre-treatment in H2/N2 atmosphere, they were tested in CO2 hydrogenation at atmospheric pressure in the temperature range 523-773 K. The material with 5 wt% Co on pure gamma-Al2O3 was found predominantly active as a reverse Water Gas Shift (rWGS) catalyst producing mainly CO and approaching rWGS forecasted thermodynamic equilibrium at the highest temperatures. The addition of silica decreases selectivity to CO, with an enhanced methanation activity. Instead, the materials with 13.6 wt% Co on pure gamma-Al2O3 was found to act mainly as a methanation catalyst, although with the coproduction of limited amounts of CO. The rWGS catalytic activity is apparently stable while slight deactivation for methanation was found, attributed to deposits of stick-like carbon residues. Investigated 13.6 wt% Co/gamma-Al2O3 is less selective for methanation than comparable Ni/gamma-Al2O3 catalysts while but more stable than unsupported cobalt and Co/SiO2 catalysts. Silica addition to 13.6 wt% Co does not modify significantly the catalytic activity while it slightly decreases stability. The effect of silica is a combination of two partially contrasting phenomena, i.e. the increase of the total surface area of the support and the decrease of the available surface area fraction for cobalt dispersion.