## 3 Results
### Catalytic Activity  
The catalytic activity performances were evaluated in the temperature range of 200-500 \({}^{\circ}\)C, as shown in Figure 5a. Although CO\({}_{2}\) methanation is strongly exothermic and thermodynamically favored at low temperatures, the conversion threshold for both catalysts began above 275 \({}^{\circ}\)C. The CO\({}_{2}\) conversion light-off curve of the Ni-nR catalyst increased rapidly to a maximum of about 55% at 400 \({}^{\circ}\)C, and then remained constant until 500 \({}^{\circ}\)C. The Ni-nC catalyst showed a much lower conversion. For example, at 325 \({}^{\circ}\)C, the conversion was 12%, while it reached a value of 40% for the Ni-nR catalyst. However, due to the progressively increasing trend, a conversion of about 48% was obtained at 500 \({}^{\circ}\)C, which is not different from that obtained with the nanorod morphology. It is essential to mention that the CeO\({}_{2}\) supports did not exhibit any catalytic activity under the assumed  
Figure 4: TEM images of Ni-nR (**a**) and Ni-nC (**b**) catalysts calcined at 550 \({}^{\circ}\)C. The arrows in the figure (**a**,**b**) points to the NiO particles. conditions. Only the nanorod form conversion of about 2%, mainly to CO, was observed at the highest temperature of 500 \({}^{\circ}\)C.  
The selectivity to methane is plotted in Figure 5b. It was significantly higher for the Ni-nR, starting from 90% at 250 \({}^{\circ}\)C, and gradually decreasing to 70% at the highest temperature of 500 \({}^{\circ}\)C, while a particular trend was observed for the Ni-nC, where the selectivity started from 62% at 250 \({}^{\circ}\)C, and reached a maximum of 70% at 370 \({}^{\circ}\)C, and then decreased again to 53% at 500 \({}^{\circ}\)C. The yields of methane and carbon monoxide are reported in Figure 5c. The CH\({}_{4}\) yield had an increasing trend, reaching a maximum of 51% at 400 \({}^{\circ}\)C for the Ni-nR and 32% at 450 \({}^{\circ}\)C for the Ni-nC, and decreased again at higher temperatures. On the other hand, the yield of CO increased exponentially for both catalysts, but it was significantly lower for the Ni-nR, reaching 17% at 500 \({}^{\circ}\)C in contrast to a value of 25% observed for the Ni-nC one. From the Arrhenius plot of TOF for CO\({}_{2}\) reduction rate shown in Figure 5d, the apparent activation energies were 90.3 and 106.1 kJ mol\({}^{-1}\) for the Ni-nC and Ni-nR catalysts, respectively. The slight difference in the activation energy can be attributed to the exposed  
Figure 5: The catalytic activity of Ni-nR (symbols in red color) and Ni-nC (symbols in blue color) catalysts: (**a**) CO\({}_{2}\) conversion; (**b**) CH\({}_{4}\) selectivity; (**c**) CH\({}_{4}\) (circle symbols) and CO yields (square symbols); (**d**) Arrhenius plot of CO\({}_{2}\) conversion TOFs. crystallographic plane of ceria, involved in CO\({}_{2}\) activation, and the Ni particle size's effect on the interaction with H\({}_{2}\) and CO\({}_{2}\). In particular, the surface of CeO\({}_{2}\), with the nanocube morphology, exposes only the (100) faces, while the nanorod morphology exposes both the (110) and (100) faces [34; 41]. The effect of such exposed faces on the formation of oxygen vacancies is not fully defined [42]. The role of Ni particle sizes on the reaction mechanism, affecting the selectivity to CH\({}_{4}\), may also significantly alter the apparent activation energy [43]. Moreover, the TOF increased for smaller metal size, e.g., the TOF of the Ni-nR sample with a Ni\({}^{0}\) particle size of 5 nm was 0.23 s\({}^{-1}\) at 300 \({}^{\circ}\)C, while the TOF of the Ni-nC sample with a Ni\({}^{0}\) size of 7 nm decreased significantly to 0.10 s\({}^{-1}\). This confirms a positive effect of the nanorod morphology on the catalytic activity, also due to the larger dispersion of the active Ni sites.