## 3 Results
### Catalytic Activity
To further highlight the differences among the Ni-nR and Ni-nC, the kinetic rate expressed by a power-low model \(-r_{\text{CO}_{2}}=k\cdot P_{H_{2}}^{x}\cdot P_{CO_{2}}^{y}\) was determined at 350 \({}^{\circ}\)C at different reactant partial pressures with a WHSV of 240,000 cm\({}^{3}\)g\({}^{-1}\)h\({}^{-1}\), ensuring reactant conversions of less than 15%. The logarithmic dependence of the kinetic rate on the partial pressure of the reactants is shown in Figure 6. Although the different reactive steps involved in the kinetics were not highlighted, this was an essential method to compare the two catalysts. The Ni-nR catalyst showed a clear dependence of the CO\({}_{2}\) conversion rate on the increase of the H\({}_{2}\) partial pressure, resulting in an apparent reaction order of \(x_{H_{2}}=0.68\), while the effect of the CO\({}_{2}\) partial pressure was negligible and the apparent reaction order was \(y_{\text{CO}_{2}}=0.06\). Therefore, the rate equation could be approximated as a near first-order dependence on H\({}_{2}\) pressure: \(-r_{\text{CO}_{2}}=k\cdot P_{H_{2}}^{0.68}\cdot P_{CO_{2}}^{0.06}\approx k\cdot P _{H_{2}}^{0.68}\). For the Ni-nC catalyst, the influence of the H\({}_{2}\) partial pressure on the CO\({}_{2}\) conversion rate was much less pronounced with a value \(x_{H_{2}}=0.22\), and the contribution of CO\({}_{2}\) pressure was even smaller, showing an apparent reaction order \(y_{\text{CO}_{2}}=0.10\), which was only slightly higher than that observed for the Ni-nR catalyst. For Ni-nC, the following rate equation was expressed: \(-r_{\text{CO}_{2}}=k\cdot P_{H_{2}}^{0.22}\cdot P_{CO_{2}}^{0.10}\). The negligible dependence of the reaction rate on the partial pressure of CO\({}_{2}\) suggests that CO\({}_{2}\) is primarily activated by the exposed surface of the CeO\({}_{2}\) supports, having proper surface basicity [38]. Indeed, in previous work [44], a negative dependence of the CO\({}_{2}\) conversion rate on Ni/YSZ catalyst was observed, indicating that both reactants should be activated on the Ni catalytically active surface. After the catalytic test, which had a total duration of 10 h, the best performing Ni-nR catalyst was characterized by HR-TEM, as shown in Figure 7. The CeO\({}_{2}\) particles remained  
Figure 6: Effect of reactant partial pressure on CO\({}_{2}\) conversion rate for Ni-nR (symbols in red color) and Ni-nC (symbols in blue color): (**a**) H\({}_{2}\) partial pressure in the 10.1–50.7 kPa range at constant CO\({}_{2}\) partial pressure of 10.1 kPa; (**b**) CO\({}_{2}\) partial pressure in the 10.1–45.6 kPa range at constant H\({}_{2}\) partial pressure of 40.6 kPa. with a rod-like morphology without sintering or morphological changes, and the average dimensions were 13 \(\pm\) 3 nm for the diameter and 236 \(\pm\) 43 nm for the length (Figure 7a). The Ni\({}^{0}\) showed particles with sizes of 8.6 \(\pm\) 1.5 nm, thus in good agreement with the dimensions of the catalytically active Ni sites (5 nm) determined by the TPD of H\({}_{2}\). Higher magnification (Figure 7b) revealed that the actual distance between CeO\({}_{2}\) (111) planes was 3.5 A, consistent with the value of calcined catalysts. The STEM analysis (Figure 7c) showed that the Ni particles were evenly distributed, although some aggregation could be seen in some parts, indicating their partial sintering. Carbon deposits were not observed, since, feeding an H\({}_{2}\)/CO\({}_{2}\) ratio of 4, they were thermodynamically unfavorable [45].