**3. Reaction mechanisms**

*2.2.2.2. Nickel reducibility*

70 New Advances in Hydrogenation Processes - Fundamentals and Applications

In general, the promotion of methanation catalysts with addition of second metals would enhance the nickel reducibility [63, 64]. The improvements in the Co reducibility may occur without any effect on the Co dispersion for the Ni-Ce/USY catalysts [41]. While the effect of promotion with Ce on the Ni reducibility is particularly pronounced with the alumina-supported Ni catalysts [63]. Compared to the unpromoted Ni/Al2O3, the lower reduction temperature of NiO in Ni-CeO2/Al2O3 samples implies that addition of CeO2 decreased the reduction temperature by altering the interaction between Ni and Al2O3, and improved the catalyst reducibility [16, 63, 64]. CNTs-supported catalysts exhibited better catalytic performance than the traditional Al2O3-supported catalysts [16], which attributed to the outstanding reduction properties of the CNTs-supported catalysts, which provided much more active sites for CO2 methanation. As shown in H2-TPR analysis (**Figure 8**), the accession of Ce could effectively promote the reduction of the nickel oxides, the high reduction peak temperature, corresponding to the highly dispersed nickel oxides in intimate contact with the exterior walls of the CNTs, decreased from 480 to 460°C for the 12Ni/CNT and 12Ni4.5Ce/CNT [16], which suggested easily reducible nickel species on the surface of the 12Ni4.5Ce/CNT catalyst, which may due to

the interaction change between the metal oxides and CNTs by the addition of cerium.

**Figure 8.** H2-TPR profiles of the catalysts. (a) 12Ni/CNT, (b) 12Ni4.5Ce/CNT, (c) 12Ni/Al2O3, (d) 12Ni4.5Ce/Al2O3 [16].

Recently, a new kind of γ-Al2O3−ZrO2−TiO2−CeO2 composite oxide supported Ni-based catalysts was synthesized for CO2 methanation [65]. The optimal catalytic activity of the composite oxide supported Ni-based catalysts was achieved because of the improvements in the reducibility. According to the H2-TPR profile for all the catalysts, the high temperature peak (weakly interacted with Al2O3, or called Ni rich phase) shifts downward for the composite oxide-supported Ni-based catalysts, suggesting a weaker interaction between NiO and the composite support. Furthermore, the reduction of the Ni rich phase According to the previous research, the reaction mechanism was difficult to establish mainly because of the different opinions on the intermediate and the methane formation process. Two feasible reaction mechanisms were proposed for CO2 methanation in the past decades. The first one involves the CO2 convert to CO prior to methanation, and the subsequent reaction follows the same mechanism as CO methanation [69]. Similar to the mechanism of CO2 hydrogenation to CH3OH, someone considered CO was an intermediate [32], and the CO hydrogenation to methane also been focused [70, 71]. The other mechanism involves the direct CO2 hydrogenation to methane without forming CO as an intermediate [72]. However, the mechanism depends on different catalysis systems, which are still under investigation.

$$\text{H}\_2 + 2\text{ }^\ast \rightarrow 2\text{H}^\ast \tag{2}$$

$$\text{CO}\_2 + ^\text{\textasciicircum} \text{CO}\_2\text{\textasciicircum} \tag{3}$$

$$\text{CO}\_2^\* + ^\* \rightarrow \text{CO}^\* + \text{O}^\* \tag{4}$$

$$\text{'}\text{'}\text{O}^\* + \text{'} \rightarrow \text{''}^\* + \text{O}^\*\tag{5}$$

$$\text{C}^\* + \text{H}^\* \to \text{CH}^\* \tag{6}$$

$$\text{CH}^\* + 3\text{H}^\* \rightarrow \text{CH}\_4 + ^\* \tag{7}$$

$$\text{O}^\* + \text{H}^\* \to \text{OH}^\* \tag{8}$$

$$\text{OH}^\* + \text{H}^\* \rightarrow \text{H}\_2\text{O}^\* \rightarrow \text{H}\_2\text{O} + \text{\textasciicircum} \tag{9}$$

The atomic hydrogen dissociated from Ni sites in the MSN may facilitate the formation of methane, as shown in **Figure 9**. The oxygen vacancies will be formed when H2 react with the surface oxygen along with the water generation, which activate additional CO2 to fill the vacancies and produce CO. During the CO2 methanation reaction, CO was also suggested as the alternative product, which was an intermediate, as shown in Eqs. (2)–(9) [32]. Therefore, the higher CH4 selectivity can be explained by the enhanced supply of adsorbed hydrogen to the activated adsorbed CO intermediate, which was the rate-determining step [73]. However, some researchers considered that the main mechanism for CO2 methanation does not require CO as the reaction intermediate [28, 74], which can be explained by the importance of weak basic sites the adsorption of CO2 [28].

Density functional theory is helpful in understanding the mechanistic aspects of the reactions. Different mechanisms of CO2 methanation on Ni(111) surfaces were investigated, and the energy barrier of 237.4 kJ mol−1 is acquired for the dissociation of CO into C and O species, which support that CO2 is converted to CO, subsequently to carbon before hydrogenation [75].

As mentioned, the CO2 adsorption is a crucial step for methanation. Indeed, CO2 dissociation is the rate-limiting step. CO2 dissociation over Rh-based catalysts is influenced by the CO coverage on the surface and the strength of the bond Rh−CO, and the hydrogen adsorption at the surface is competed with CO2 adsorption. Due to the preferential adsorption of CO2 and the accumulation of CO on the surface, hydrogen coverage on the rhodium catalyst is very small [76]. However, CO2 adsorption on the medium basic sites of Ni/Ce0.5Zr0.5O2 results in monodentate carbonates, and monodentate formate derived from monodentate carbonate on medium basic sites, which could be hydrogenated more quickly than bidentate formate derived from hydrogen carbonate. The medium basic sites were proposed to promote the formation of monodentate formate species, thus to enhance the activity [77].

\* \* H 2 2H <sup>2</sup> + ® (2)

\* \* CO CO 2 2 + ® (3)

\*\* \* \* CO CO O <sup>2</sup> +® + (4)

\*\* \* \* CO C O +® + (5)

\*\* \* C H CH + ® (6)

\*\* \* CH 3H CH 4 +® + (7)

\*\* \* O H OH + ® (8)

\*\* \* \* OH H H O H O 2 2 +® ® + (9)

The atomic hydrogen dissociated from Ni sites in the MSN may facilitate the formation of methane, as shown in **Figure 9**. The oxygen vacancies will be formed when H2 react with the surface oxygen along with the water generation, which activate additional CO2 to fill the vacancies and produce CO. During the CO2 methanation reaction, CO was also suggested as the alternative product, which was an intermediate, as shown in Eqs. (2)–(9) [32]. Therefore, the higher CH4 selectivity can be explained by the enhanced supply of adsorbed hydrogen to the activated adsorbed CO intermediate, which was the rate-determining step [73]. However, some researchers considered that the main mechanism for CO2 methanation does not require CO as the reaction intermediate [28, 74], which can be explained by the importance of weak

Density functional theory is helpful in understanding the mechanistic aspects of the reactions. Different mechanisms of CO2 methanation on Ni(111) surfaces were investigated, and the energy barrier of 237.4 kJ mol−1 is acquired for the dissociation of CO into C and O species, which support that CO2 is converted to CO, subsequently to carbon before hydrogenation [75].

As mentioned, the CO2 adsorption is a crucial step for methanation. Indeed, CO2 dissociation is the rate-limiting step. CO2 dissociation over Rh-based catalysts is influenced by the CO coverage on the surface and the strength of the bond Rh−CO, and the hydrogen adsorption at the surface is competed with CO2 adsorption. Due to the preferential adsorption of CO2 and the accumulation of CO on the surface, hydrogen coverage on the rhodium catalyst is very

basic sites the adsorption of CO2 [28].

72 New Advances in Hydrogenation Processes - Fundamentals and Applications

**Figure 9.** A probable mechanism for Ni/MSN whereby spillover of atomic hydrogen from Ni interacts with C(a) species and sequentially hydrogenates carbon until the product methane desorbs [32].

In addition, at 383 K, a reaction mechanism was proposed for the carbon dioxide methanation reaction on 2% Ru/TiO2, which investigate the precursor existence for the adsorbed CO and reaction intermediate, and the side-product, formate was also found adsorbing on the support [78], which suggested the surface intermediate corresponding to the adsorbed formate on the metal-support interface, and the measured formate infrared bands are corresponding to the diffused formate species from the interface to the support. A pathway involving hydrogen carbonate is also presented for the formation of the interfacial formate, because the species is formed on the support during the reaction, and the transient response is consistent with the response of a CO precursor. The reaction mechanism that could account for all of these observations is presented in **Figure 10** [78].

To make a better understanding of the adsorption of possible intermediates, the reaction mechanism and factors determining the product selectivity, DFT calculations were considered to be a suitable method to investigate the hydrogenation process of CO2 and CO on the Ru(0001) surface [79]. For CO2 hydrogenation, the HCOO intermediate are firstly formed from the adsorbed CO2 hydrogenation, and subsequently produces an adsorbed CHO and O species. The active C and CH species then undergo stepwise hydrogenation to CH2, CH3 and CH4, or the CHx species, and further transforms to longer carbon chains. From the calculation results, CH3 hydrogenation is considered to be the rate determining step in the sequence of C hydrogenation on the Ru(0001) surface, and the lowest barrier channel of C–C coupling occurs via the CH + CH reaction [79]. In addition, the study based on DFT calculations on a Ru nanoparticle supported on the TiO2 catalyst further confirms the stronger electron transfer from the Ru cluster to the TiO2(101) facet than to TiO2(001); the Ru species supported on the (101) plane possesses a relatively lower activation energy for the CO dissociation, resulting in the highly catalytic activity toward CO2 methanation reaction [80].

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

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

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