Many studies have reported that CO₂ methanation follows the formate route on different nickel catalysts such as Ni/MgO [198], Ni-Mn/Al@Al₂O₃ [199], Ni/Y₂O₃ [200], Ni/ZrO₂ [196,201], Ni/ultra-stable Y (USY) zeolite [139], and Ni@C [102]. For example, Xu and coworkers [196] discussed the formation and evolution of CO₂ adsorbed species on Ni/c-ZrO₂ by in situ FTIR and DFT calculations. CO₂ methanation on Ni/c-ZrO₂ was dominated by the formate pathway as follows: CO₂*→ HCOO* → H₂COO* → H₂COOH* → H₂CO* → CH₂*→ CH₄*, which is the same as that shown in Figure 4. CO was a by-product instead of a reaction intermediate, which could not further form CH₄, and the DFT calculations also confirmed the formate pathway, which was highly consistent with the in situ FTIR results. Solis-Garcia et al. [201] also found that CO₂ methanation follows the formate pathway over Ni/ZrO₂ and no CO species were observed during the reaction. The possible reaction pathway of the CO₂ methanation over Ni@C was also investigated by CO₂-TPD measurements and in situ FTIR characterization. All results demonstrated that CO₂ methanation over Ni@C catalyst proceeded via the formate route without involving CO as an intermediate [102]. Aldana et al. [41] also found that the main CO₂ methanation mechanism on Ni-CZ_sol–gel was the formate pathway, which does not require CO as reaction intermediate. They also found that H₂ was dissociated on Ni⁰ sites while CO₂ was activated on the ceria–zirconia support to form carbonates and then further into CH₄, suggesting that a stable metal–support interface is beneficial for the adsorption of CO₂.

In another study, Pan et al. [202] found that the reaction pathway on Ni/γ-Al₂O₃ and Ni/Ce_0.5Zr_0.5O₂ all followed the formate pathway, only differing in reactive basic sites. On the Ni/Ce_0.5Zr_0.5O₂ catalyst, CO₂ adsorption on medium basic sites formed bidentate formate, whereas CO₂ adsorption on surface oxygen resulted in the monodentate formate. Due to the faster hydrogenation of monodentate formate, it was assumed to be the main reaction route on the Ni/Ce_0.5Zr_0.5O₂ catalyst. For CO₂ methanation on Ni/γ-Al₂O₃, hydrogenation of bidentate formate was the main reaction route as bidentate formate was the main adsorption and intermediate species and CO₂ adsorbed on strong basic sites of Ni/γ-Al₂O₃ will not participate in the CO₂ methanation reaction. It was assumed that medium basic sites are responsible for promoting the formation of monodentate formate species, thus enhancing CO₂ methanation activity. CO₂ methanation reaction pathways on Ni/Ce_0.5Zr_0.5O₂ and Ni/γ-Al₂O₃ are shown in Figure 5.

The CO pathway involves the dissociation of CO₂ to CO prior to methanation, and in the subsequent reaction, CO is converted to CH₄ by reacting with H₂ [203]. Karelovic et al. showed the direct dissociation of CO₂. The reactions below summarize the reduction process (Equations (3) and (4)). The excess amount of CO generated in the first reaction deposits on the catalyst, which produces coking effects. To avoid this problem, the methanation of CO must proceed much faster than the CO production, and the CO₂ methanation reaction must take place at low temperatures. Therefore, the direct dissociation of CO₂ to CO_ads and O_ads often occur over a variety of noble metal-based catalysts at low temperature [49,204,205]. In addition, the formation of nickel carbonyls Ni(CO)₄ would cause the deactivation of Ni-based catalysts [162].

Therefore, CO₂ methanation occurred via the CO pathway only over some Ni-based catalysts including Ni/CeO₂ [103], Ni/F-SBA-15 [113], and Ni-sepiolite [206]. The CO pathway over Ni/CeO₂ could be proven by in situ FTIR. The FTIR adsorption bands at 2017 cm⁻¹ were assigned to the CO adsorption state, and the bands at 2120 and 2170 cm⁻¹ were ascribed to gas phase CO, which indicated that CO₂ molecules can be converted to CO molecules on the surface of the Ni/CeO₂ catalyst. Characterization results indicated that CO species generated from the reduction of CO₂ molecules by nickel active sites and surface oxygen vacancies promoted CO₂ methanation [103]. Bukhari et al. found that Ni metals on Ni/F-SBA-15 (Fibrous type SBA-15) contributed to the CO₂ dissociation into CO and O species as well as the dissociation of H₂ into atomic hydrogen species. The linear carbonyl group came from the dissociation of CO₂, which was an intermediate during CO₂ methanation and could be seen at 2055 cm⁻¹. Then, the adsorbed CO species interacted with surface oxygen, producing bidentate and unidentate carbonate groups, thus CH₄ [113]. Cerdá-Moreno et al. [206] also found linearly and bridged bonded CO as intermediates during CO₂ methanation over a Ni-sepiolite catalyst.

There are also many factors influencing the CO₂ methanation mechanism. The addition of promoters affects the formation of intermediates. Mg or Ca modified Ni/Al₂O₃ catalysts promote the formation of the carbonate species due to the increased basicity, while Sr or Ba modified catalysts promoted *CO and H₂CO* formation [177]. The nature of nickel active sites also influence the CO₂ methanation mechanism. Zhou et al. [158] found that CO₂ methanation took the pathway of CO over the Ni/TiO₂ catalyst with Ni (111) as the principal exposing facet, while the catalyst with multi-facets followed the formate route, with which nickel was only functional for hydrogen dissociation. The location of nickel active sites also affects the CO₂ methanation reaction pathways [153]. Controlling nickel being on either the interior or the exterior of adjacent siloxene nanosheets is achieved by employing different solvents in the preparation process, which determines the reaction intermediates and pathways for CO₂ methanation, as shown in Figure 6. CO₂ methanation occurred through the formate pathway over Ni@SiXNS-EtOH with nickel active sites being on the interior of adjacent siloxene nanosheets while CO₂ methanation followed the CO pathway when nickel was at the exterior of adjacent siloxene nanosheets on Ni@SiXNS-H₂O.

The different preparation methods can also influence the reaction pathway of CO₂ methanation. Jia et al. [135] used the operando DRIFT analyses to demonstrate the CO₂ methanation pathway on Ni/ZrO₂ obtained via different preparation methods. CO₂ methanation over the plasma decomposed catalyst follows the Co-hydrogenation route. The exposed high-coordinate Ni (111) facets of the plasma decomposed catalyst facilitate the decomposition of CO₂ and formates into adsorbed CO. The subsequent hydrogenation of adsorbed CO leads to the production of methane. However, the thermally decomposed catalyst with a complex Ni crystal structure and more defects mainly takes the pathway of direct formate hydrogenation.