CO2 methanation of Ni catalysts supported on tetragonal ZrO2 doped with Ca2+ and Ni2+ ions

• Ni/tetragonal-ZrO2 catalysts doped with Ni2+ and Ca2+ show high activity for CO2 methanation.
• There is an optimized Ca2+ content to enhance the activity.
• The oxygen vacancies in ZrO2 may play an important role in enhancing the catalytic activity.

This paper reports highly active CO2 methanation catalysts of Ni supported on tetragonal ZrO2 for supply of renewable energy in the form of CH4 via electrolytic H2 generation. The Ni/tetragonal ZrO2 catalysts with the nominal cationic compositions of 50 at.% Ni-(Zr + Ca) (Ca/(Zr + Ca) molar ratios of 0.125–0.333), were prepared by calcination of a ZrO2 sol mixed with nickel and calcium nitrates at 650 and 800 °C and subsequent H2 reduction of NiO. All the catalysts prepared exhibited almost 100% CH4 selectivity, and the activity increased with the calcium content up to a Ca/(Zr + Ca) molar ratio between 0.167 and 0.200, above which the activity tended to decrease. At the optimized calcium content, a tetragonal ZrO2 phase with cation vacancies introduced by Ca2+ and Ni2+ doping was predominant, in addition to fcc Ni. A monoclinic ZrO2 phase was also present below the optimized calcium content, while a Ca-rich amorphous or CaZrO3 phase was formed with tetragonal ZrO2 above the optimized calcium content. The high catalytic activity of the catalyst with the optimized composition could be correlated with the presence of tetragonal ZrO2 with oxygen vacancies.