Impacts of nickel loading on properties, catalytic behaviors of Ni/γ–Al2O3 catalysts and the reaction intermediates formed in methanation of CO2

In this study, methanation of CO2 over Ni/Al2O3 with varied nickel loading (from 0 to 50 wt%) was evaluated, striving to explore the effects of nickel loading on catalytic behaviors and the reaction intermediates formed. The results showed that agglomeration of nickel particles were closely related to interaction between nickel and alumina. Increasing nickel loading resulted in the increased proportion of nickel having medium strong interaction with alumina, the reduced reduction degree of NiO, the increase of medium to strong basic sites, the enhanced activity for methanation and the competition between reverse water gas shift (RWGS) reaction and methanation. Lower nickel loading promoted RWGS reaction while methanation of CO2 dominated at higher nickel loading. The catalyst with a nickel loading around 25% achieved the best activity for methanation. The in–situ DRIFTS studies of methanation of CO2 showed that CO2 could be absorbed on surface of metallic Ni, NiO or alumina. More metallic nickel species on alumina suppressed formation of carbonate species while promoted further conversion of HCOO∗ species and ∗CH3 species, achieving a higher catalytic efficiency. Moreover, more metallic nickel species was crucial for gasifying the carbonaceous intermediates, prevented aggregation of the intermediates to coke and achieving a higher catalytic stability.