## 3 Results
### Drift  
To elucidate the reaction pathway, the nR and the Ni-nR samples were analyzed using the DRIFT technique. The significant species detected on the surface are listed in Table 4. The DRIFT spectra of nR are seen in the range between 4000 and 2600 cm\({}^{-1}\) (Figure 8a) and between 2100 and 1150 cm\({}^{-1}\) (Figure 8b). Two bands around 3710 and 3618 cm\({}^{-1}\) were identified in the hydroxide vibrational region due to mono-coordinated -OH (type I) and bridged -OH (type II) vibrations on Ce\({}^{8+}\) site [46; 47; 48; 49]. These bands occurred at all temperatures but were most intense at 300 \({}^{\circ}\)C. At this distinct temperature, a further signal at 3507 cm\({}^{-1}\) was also uncovered, which could be due to vibrations of triple-bridged -OH on Ce\({}^{8+}\) site or to the hydroxyl group of bicarbonate species [50; 51]. As the temperature increased, the intensity of this band gradually decreased and, at 500 \({}^{\circ}\)C, it was difficult to detect. Figure 7: HR-TEM micrograph of Ni-nR catalyst after catalytic test at low (**a**) and high magnification (**b**), STEM imaging with Ce-L, and Ni-k mapping (**c**). The arrow in panel (**a**) points to the Ni particles. The interaction of CO2 in combination with H2 over the CeO2 surface was already evident at 200 \({}^{\circ}\)C by the appearance of a weak band at 2845 cm\({}^{-1}\), due to v(CH) vibrations of formate species [52; 53]. This band increased in intensity with a temperature up to 400 \({}^{\circ}\)C, and was accompanied by another less intense band at 2931 cm\({}^{-1}\), due to the corresponding v\({}_{\text{a}}\)(CO2) + \(\delta\)(CH) vibrations [52; 53]. As the temperature continued to rise, the intensity of these bands decreased rapidly. Other bands appeared from 400 \({}^{\circ}\)C onwards due to the formation of methane (band at 3015 cm\({}^{-1}\)) and carbon monoxide (see bands at 2117 and 2174 cm\({}^{-1}\) in Figure 8b). In the region between 2200 and 1150 cm\({}^{-1}\), the observed bands are mainly due to the interaction of CO2 with the CeO2 surface. It is important to emphasize that pre-reduction up to 500 \({}^{\circ}\)C guarantees a Ce3+ containing layers (surface-subsurface), as evidenced by a  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & & & & \\ \(\text{Ge}\) & & & & \\ \(\text{Ge}\) & & &  
negative, although not very intense, peak at 2114 cm\({}^{-1}\) corresponding to the Ce\({}^{3+}\) electronic transition, a phenomenon also observed on CeO\({}_{2}\) solid solutions with aliovalent elements, such as Gd\({}^{3+}\) or Sm\({}^{3+}\) by [PERSON] et al. (2014). At 200 \({}^{\circ}\)C, intense bands appeared at 1612, 1390, and 1216 cm\({}^{-1}\) assigned to the \(\
u_{\rm a}(CO_{3})\), \(\
u_{\rm s}(CO_{3})\) and \(\delta(\rm OH)\) vibrations of bicarbonate species chemisorbed on reduced CeO\({}_{2}\)[55; 56; 12; 52]. Numerous other bands have been discovered that are not yet clearly resolved, but we can focus on the most important ones: the bands at 1562 and 1272 cm\({}^{-1}\) are due to the \(\
u_{\rm a}(CO_{3})\) and \(\
u_{\rm s}(CO_{3})\) vibrations of bidentate carbonate; whereas the bands at 1462 and 1293 cm\({}^{-1}\) should come from the \(\
u_{\rm a}(CO_{3})\) vibrations of monodentate carbonate [55; 56; 57]. In agreement with the spectra in Figure 8a, as the temperature increased, the vibrations associated with formate species became more intense, and the signal from the bands associated with bicarbonates (1612, 1390, and 1216 cm\({}^{-1}\)) gradually decreased. The formate species, probably in a bidentate configuration, showed characteristic bands at 1569 and 1352 cm\({}^{-1}\) due to the \(\
u_{\rm a}(CO_{2})\) and \(\
u_{\rm s}(CO_{2})\)+ \(\delta(\rm CH)\) vibrations, respectively. These bands were most pronounced up to the highest temperature of 500 \({}^{\circ}\)C, but the interaction of CO\({}_{2}\) on the CeO\({}_{2}\) surface remained evident through other bands at \(\sim\)1562 and \(\sim\)1272 cm\({}^{-1}\) associated with bidentate carbonate.