CO2 methanation: deciphering the role of dopants (Mn, Co, and Cu) in Ni/SiO2 catalysts prepared by sol-gel chemistry

CO2 methanation is effectively catalyzed by Ni-based catalysts, and reactivity can be further tuned by the addition of promoters. Deciphering the relationship between the promoter in Ni-based catalysts and the corresponding catalytic performance in CO2 methanation mechanism is of great meaning for the development of highly active catalysts. Herein, a series of model bimetallic catalysts were prepared by sol-gel chemistry to address this fundamental challenge. Compared to Ni/SiO2 catalyst, the Mn-doped and Co-doped catalysts showed a higher methanation activity, with the former showing better performance below 250 °C and the latter showing better performance over 300 °C. On the contrary, the Cu-promoted catalyst showed a lower CO2 conversion with a lower CH4 selectivity in the whole temperature range. A comprehensive characterization study (TEM, XRD, XPS, H2-TPR, CO2-TPD, in situ DRIFTS, and TPSR analyses) suggests that the effect of promoters is not directly related to improvement of dispersion, reducibility, or basicity. Instead, we show that the promoters orient the reaction mechanism and favor the conversion of key intermediates. Mn addition has the highest promoting effect on the hydrogenation of formaldehyde intermediate (*OCH2) to methoxy intermediate (*OCH3), i.e. the rate determining step of the “RWGS+CO hydrogenation” pathway which is shown to predominate at low reaction temperature. Co addition facilitates the formation of formate species, i.e. the rate determining step of the formate pathway which is also active at high reaction temperature. Cu addition has a negative effect on the rate determining step of those two pathways, resulting a lower performance of Ni-Cu/SiO2.