**4. Conclusions and perspectives**

(101) plane possesses a relatively lower activation energy for the CO dissociation, resulting in

Finally, the detailed mechanism was proposed for CO2 methanation over metal-based MSNs [81]. As shown in **Figure 11**, CO2 and H2 were adsorbed and dissociated on the metal active sites to form CO, O, and H, followed by the migration of these atoms to the MSN surface. Subsequently, the CO dissociated from the active sites interacted with the MSN oxide surfaces to form the carbonyl, including bridged and linear carbonyl, and the H atom in the reaction facilitated the formation of bidentate formate. And the above three species were responsible for the methane formation, among them, the main route for the methane formation was due to the bidentate formate species, and the MSN support served as the sites for carbonyl species,

the highly catalytic activity toward CO2 methanation reaction [80].

74 New Advances in Hydrogenation Processes - Fundamentals and Applications

**Figure 10.** Reaction mechanism of CO2 methanation [78].

which act as a precursor to methane formation [81].

**Figure 11.** Plausible mechanism of CO2 methanation on M/MSN [81].

CO2 has been promoted to an important carbon resource for conversion and utilization, and CO2 hydrogenation is a feasible and powerful process, especially for methanation. However, CO2 is chemically stable and thermodynamically unfavorable. To eliminate the limitations on the conversion and selectivity, various technical directions and specific research approaches on rational design of catalysts and exploration of reaction mechanisms have been presented.

Noble metal catalysts such as Ru, Rh and Pd are efficient for the formation of methane under relatively mild operating conditions, but the high cost as well as their limited availability restricts their practical applications. Therefore, researchers have paid increasing attention on the immobilization of homogenous catalysts to combine the efficient activity with the properties of separation and recyclability. Ni- and Co-based catalysts are, of course, more practical for industrial applications compared to noble metal catalysts. The catalysts with larger surface areas and higher metal dispersion can usually possess higher activity and selectivity, and longer stability in the hydrogenation of CO2. However, the Ni-based catalysts are more resistless to carbon formation compared with noble metal catalysts. Thus, one strategy has to be proposed for pursuing high-performance catalysts with abilities of low-temperature methanation and resisting carbon formation. In addition, understanding the fundamental mechanisms of CO2 methanation and explore its relationship with catalyst active site structures using both theoretical calculations (molecular/electronic level modeling) and experimental approaches to tailor new catalyst structures are considerably needed.
