(618d) Tuning Selectivity of CO2 Methanation over MgO-Ni/SiO2

Greenhouse gas emission produced from fossil fuel burning is proven to be the primary reason for global warming. In this respect, CO₂ methanation using renewable hydrogen is particularly interesting as a carbon-neutral process because CH₄ is produced from CO₂, other than from untapped crude oil. As a catalyst, transition metal Ni with moderate performance but lower price than noble metals is a viable option. However, the mechanism is still debated due to its complexity and the unclear catalyst structure. Herein, the structure of a promoted active catalyst, MgO-Ni/SiO₂, is unambiguously resolved and a reaction mechanism for CO₂ methanation over MgO-Ni/SiO₂ is proposed.

MgO-Ni/SiO₂ catalysts with varying MgO were prepared by sequential incipient-wetness impregnation. For MgO loadings from 0 to 3 wt.%, the Ni metallic surface area decreased by half, indicating MgO covered part of the metallic Ni, which was confirmed by HRSTEM-EDX (Figure 1(a)). In situ QXAS during H₂-TPR showed that NiO in the as-prepared samples could be completely reduced into metallic Ni.

Catalytic performance tests showed that MgO addition dramatically increased the CH₄ selectivity at the same CO₂ conversion (Figure 1(b)). As MgO itself is hardly active, the selectivity increase was ascribed to the interaction of Ni and MgO. The formation of CH₄ from CO₂ might follow two pathways, direct or indirect with CO* as the key intermediate. Kinetic and in situ DRIFTS tests point towards the indirect route. Moreover, transient in situ DRIFTS proved that on the MgO-added samples the transformation CO → HCO* → CH₄ occurs faster.

This work successfully constructed a methanation catalyst, where MgO partially covered metallic Ni. The mechanism of the CO₂ methanation process over MgO-Ni/SiO₂ was detailed (Figure 1(c)), as well as the significance of the MgO/Ni interface, which is critical to develop new, highly active and selective catalysts for CO₂ utilization.