The distributions of alkaline earth metal oxides and their promotional effects on Ni/CeO2 for CO2 methanation

To understand the role of alkaline sites played for CO2 methanation and develop better catalysts, the promoted Ni/M0.1Ce0.9Ox catalysts, in which the CeO2 are modified by different alkaline earth metal oxides with a M/Ce (M = Mg, Ca, Sr, Ba) molar ratio of 1/9, have been prepared. With the sol − gel method, different alkaline earth metal oxides present varied distribution states in the modified CeO2 support. MgO and CaO are predominantly dissolved into the CeO2 support lattice to form a pure solid solution structure, while SrO and BaO are dispersed as the carbonates on the CeO2 support surface. The modification of all the alkaline earth metal oxides on CeO2 can improve the intrinsic activity of the obtained supported Ni catalysts, whose activity follows the order of Ni/Ca0.1Ce0.9Ox > Ni/Sr0.1Ce0.9Ox > Ni/Mg0.1Ce0.9Ox > Ni/Ba0.1Ce0.9Ox > Ni/CeO2. H2-TPR, H2 adsorption–desorption and XPS results have revealed that the modification of alkaline earth metal oxides can improve Ni dispersion, induce more surface oxygen vacancies and increase both the amount and the strength of the moderate alkaline sites, which are the major factors determining the activity of the Ni/M0.1Ce0.9Ox catalysts for CO2 methanation. Ni/Ca0.1Ce0.9Ox, possessing the best Ni dispersion and the largest amount of the moderate alkaline sites, displays the highest activity among all the catalysts.