**2.3. Cobalt-based catalysts for low-temperature methanation of CO2**

Generally, the Co-based Fischer-Tropsch catalysts exhibit a superior catalytic performance with respect to low-temperature CO2 methanation [17, 67, 68]. A higher CH4 selectivity was observed in the Fischer-Tropsch synthesis when the Co catalysts were not completely reduced or when the catalysts contain smaller Co3O4 particles [67]. When taking the coke oven gas as feed gas and using a nanosized Co3O4 catalyst, CO was easily adsorbed onto the smaller nanosized Co3O4 surface and react with H2, and the temperature at which CO completely converted to CH4 was much lower than that using nanosized Co3O4 with large particles [67].

In addition, the Ru-doped Co3O4 catalyst with a relatively rough surface shows a lower light-off temperature than that of a Co catalyst [68]. The relatively rough surface morphology of Ru-doped Co3O4 probably results from the larger ionic radius of Ru3+, which affects the dissolution-recrystallization process. Therefore, the final surface morphology of nanorods was disrupted with the addition crystalline defects. The correlation between the surface chemistry and the catalytic performances suggests that doping a noble metal to an oxide of an earth-abundant metal followed by reduction could create a chemically stable, cost-effective catalyst with a bimetallic surface, which has an equivalent or much better catalytic performance [68]. Usually, the catalytic activity affected by the catalyst composition and structure, e.g., when used the mesoporous Co/KIT-6 and Co/meso-SiO2 in CO2 methanation, the highly ordered bicontinuous mesoporous structure of the Co/KIT-6 catalyst exhibits higher methane selectivity than the Co/meso-SiO2 catalyst, and the CO2 conversion exceeds 48.9%, and the methane selectivity can be retained at 100% at 280°C [17].
