## 3 Results
### Drift
From the chemical species that can act as the main reaction intermediates on the nR and Ni-nR systems, the following reaction steps during the CO\({}_{2}\) methanation can be hypothesized: (i) H\({}_{2}\) favors the formation of oxygen vacancies and Ce- OH species; (ii) CO\({}_{2}\) is chemisorbed on the \"basic\" CeO\({}_{2-\delta}\) surface forming carbonates (mainly bidentate); (iii) the conversion of carbonates to formates, is obtained at high temperatures on the CeO\({}_{2-\delta}\) support; (iv) Ni\({}^{0}\) plays a crucial role in the activation of the H\({}_{2}\) molecule through its dissociation into reactive H\({}^{\star}\), and thus its spillover on the CeO\({}_{5}\) surface; (v) the hydrogen spillover by Ni\({}^{0}\) favors the formation of formates at low temperature and their hydrogenation to CH\({}_{4}\). To better clarify the methanation reaction pathway, we also performed an in operando DRIFT during CO hydrogenation. The characterization was carried out up to the maximum temperature of 400 \({}^{\circ}\)C. Figure 10a shows a magnification between 4000-2600 cm\({}^{-1}\) and Figure 10b in the interval 2150 and 1150 cm\({}^{-1}\). At the temperature of 200 \({}^{\circ}\)C, very labile bands assigned to Ce-OH were observed, whereas monodentate formate was already evident with the \(\
u\)(CH) and \(\
u_{\rm a}\)(CO\({}_{2}\)) vibrations at 2831 and 1602 cm\({}^{-1}\), respectively. In addition to the bands associated with gaseous CO, there was an intense peak at 2059 cm\({}^{-1}\) assigned to \(\
u\)(CO-Ni) of linear nickel carbonyl and other unresolved bands between \(\sim\)1960 and \(\sim\)1680 cm\({}^{-1}\), probably due to bridged nickel carbonyl vibrations. Increasing the temperature to 300 \({}^{\circ}\)C favored a significant methane formation, with characteristic bands at 3016 and 1305 cm\({}^{-1}\). Moreover, the intensity of the formate bands, mainly the bidentate type at 2830 and 1596 cm\({}^{-1}\), increased alongside a significant decrease in the CO and Ni-CO bands. At the maximum temperature of 400 \({}^{\circ}\)C, there was a marked decrease in all formate bands, the absence of Ni-CO species, and the prominent bands were only those of CH\({}_{4}\) gas. In general, the presence of carbonates was not observed at any temperature. The CO was never oxidized by an oxygen storage process through the CeO\({}_{2}\), as no band of CO\({}_{2}\) was observed, clearly indicating that the surface was always maintained in a CeO\({}_{5}\) reduced state. Figure 9: DRIFT spectra of Ni-nR catalyst exposed to reaction mixture CO\({}_{2}\):H\({}_{2}\):He = 5:20:25 cm\({}^{3}\) min\({}^{-1}\) from 200–400 \({}^{\circ}\)C between 4000–2600 cm\({}^{-1}\) (**a**), and 2150 and 1150 cm\({}^{-1}\) (**b**). Before the experiment, the sample was reduced in situ at 500 \({}^{\circ}\)C. Symbol definition: * hydroxide species, \(\blacklozenge\) formate, + CH\({}_{4}\), \(\blacktriangledown\) bidentate carbonate, \(\bigstar\) carboxylate. It has been shown that the hydrogenation of CO can follow a mechanism involving formates [60]. In this case, they were also formed by exploiting the hydroxyl and the oxygen vacancy formed upon Ce\({}^{4+}\)\(\rightarrow\)Ce\({}^{3+}\) reduction by H\({}_{2}\). Indeed, the low intensity of the Ce-OH species observed in Figure 10 could be a confirmation of this. The formates were then converted to CH\({}_{4}\) thanks to the spillover effect of H\({}^{*}\) generated by the dissociation of H\({}_{2}\) chemisorbed on the Ni\({}^{0}\) surface, as stated before. The CO chemisorbed on Ni could act as a spectator and not be directly hydrogenated to methane. Thus, to summarize the results obtained with the DRIFT technique, it is possible to define Ni-nR as a bifunctional system, in which Ni plays a fundamental role in the activation of H\({}_{2}\), while CeO\({}_{2-\delta}\) acts on CO\({}_{2}\). Obviously, in our catalyst with a low Ni loading, the exposed surface is mainly that of CeO\({}_{5}\).