Methanation of CO2 over nickel catalysts: Impacts of acidic/basic sites on formation of the reaction intermediates

Methanation of carbon oxides is an important route to produce synthetic gas for civil use. Understanding acidic/basic properties of catalyst on the reaction intermediates in methanation provides important information for developing highly active catalysts. In this study, the Ni/Al2O3, Ni/SiO2 and Ni/H beta 40 catalysts with varied distribution of acidic/basic sites were prepared and evaluated. The results indicated the importance of basic/acidic sites in determining the catalytic activities and the reaction intermediates formed. Ni/SiO2 and Ni/H beta 40 with medium to strong basic sites were far more active and selective than the Ni/Al2O3 in CO2 methanation. The in situ Diffuse Reflection Infrared Fourier Transform Spectroscopy (DRIFTS) study indicated that formate species, the important reaction intermediate for CH4 formation, was absent over Ni/H beta 40 and Ni/SiO2 while was accumulated over Ni/Al2O3, relating to presence of the positively charged Lewis acidic sites that had a strong affinity to the negatively charged formate species, deterring subsequent conversion of formate to CH4. In addition, the absorption of *CO over Ni/H beta 40 and Ni/SiO2 facilitated its activation and further conversion to methane, resulting in low CO selectivity and low tendency towards coking.