Surface composition change of chlorine-doped catalyst Ni(Cl_x)/CeO₂ in methanation reaction

The oxide sample NiO/CeO₂ with feed atomic ratio of Ni/Ce at 40%, prepared by co-precipitation method and calcination at 500 °C for 2 h, was impregnated by aqueous solution of NH₄Cl to dope chlorine ions. After the impregnated samples were dried and calcined at 400 °C for 2 h, the calcined samples NiO(Cl_x)/CeO₂ (x=0.1–0.5) were characterized by means of X-ray diffraction (XRD) and temperature programmed reduction (TPR) techniques. It was comfirmed that the doped chlorine ions hindered reduction of Ni²⁺ ions in the calcined samples, and suppressed adsorption of CO₂ and CO on the reduced sample Ni(Cl_0.3)/CeO₂. The reduced samples Ni(Cl_x)/CeO₂ (x=0.0–0.5) were used as catalysts for selective methanation of CO in H₂-rich gas. When chlorine ions were doped at the feed atomic ratio of Cl/Ce(x) equal to 0.3–0.5, CO in the H₂-rich gas could be removed to below 10 ppm with a high selectivity more than 50% in a wide reaction temperature range of 220–280 °C. However, the selectivity of CO methanation decreased with reaction time in the durability tests over the catalyst Ni(Cl_0.3)/CeO₂ at the reaction temperature of 260 °C and even at 220 °C. The lowering of the selectivity was found to be related with the surface composition change of the catalyst in the catalytic reaction.