## 3 Results and Discussion
### Catalytic Evaluation Studies  
In Figure 6, the conversion profiles of CO over temperature for bare ZnO, CeO\({}_{2}\) and CeO\({}_{2}\)/ZnO mixed oxides are shown. It is evident that all mixed oxides demonstrate superior performance as compared to bare ones. A similar performance was obtained between mixed oxides, with the CeO\({}_{2}\)/ZnO-0.4 being slightly better. To gain insight into the role of preparation method and the synergistic interaction between the different counterparts, the catalytic performance of a mechanical mixture (CeO\({}_{2}\)+ZnO-0.4) of exactly the same composition to the optimum sample (CeO\({}_{2}\)/ZnO-0.4) was explored in parallel. The bare ceria nanorods (CeO\({}_{2}\)) exhibit a catalytic profile shifted by ca. 30 \({}^{\circ}\)C to higher temperature than the optimum CeO\({}_{2}\)/ZnO-0.4 sample. ZnO is much less active with a profile located ca. 170 degrees higher. More importantly, the conversion profile of the mechanical mixture CeO\({}_{2}\)+ZnO-0.4 is shifted by ca. 100 \({}^{\circ}\)C to higher temperatures as compared to that of CeO\({}_{2}\)/ZnO-0.4, revealing the beneficial interaction between CeO\({}_{2}\) and ZnO, which is induced by the preparation method (see experimental Section 2.1). To obtain a better understanding in relation to the intrinsic activity of investigated samples, the specific activity, both in terms of catalyst mass (\(\mu\)mol g\({}^{-1}\) s\({}^{-1}\)) and surface area (\(\mu\)mol m\({}^{-2}\) s\({}^{-1}\)), was also estimated (Table 4). It is evident that the CeO\({}_{2}\)/ZnO-0.4 sample offers the best performance, in terms of both conversion and mass-normalized specific activity, revealing its superior reactivity. It should be noted, however, that in terms of area-normalized activity, both bare ceria and mixed oxides exhibited an inferior activity as compared to bare ZnO. The latter can be attributed to the low surface area of ZnO (7.05 m\({}^{2}\)/g), which is about one order of magnitude lower compared to bare CeO\({}_{2}\) and CeO\({}_{2}\)/ZnO mixed oxides. Therefore, on the basis of the present results, the enhanced catalytic performance of CeO\({}_{2}\)/ZnO mixed oxides can be attributed to a compromise between redox and surface  
Figure 6: CO conversion profiles of CeO\({}_{2}\), ZnO and CeO\({}_{2}\)/ZnO samples. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{4}{c}{**H\({}_{2}\) Uptake (mmol H\({}_{2}\)/g) and OSC (mmol O\({}_{2}\)/g)**} \\ \cline{2-6}  & **Peak A** & **Peak B** & **Peak C** & **H\({}_{2}\) uptake** & **OSC** \\ \hline CeO\({}_{2}\) & - & - & 0.59 & 0.59 & 0.29 \\ CeO\({}_{2}\)/ZnO–0.2 & 0.15 & 0.21 & 0.61 & 0.97 & 0.48 \\ CeO\({}_{2}\)/ZnO–0.4 & 0.15 & 0.23 & 0.63 & 1.01 & 0.50 \\ CeO\({}_{2}\)/ZnO–0.6 & 0.16 & 0.25 & 0.63 & 1.04 & 0.52 \\ ZnO & 0.14 & 0.17 & - & 0.31 & 0.15 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Redox features of CeO\({}_{2}\) and CeO\({}_{2}\)/ZnO samples. properties. Moreover, the optimum CeO\({}_{2}\)/ZnO-0.4 sample exhibits an apparent activation energy (E\({}_{\text{a}}\)) of 32.1 kJ/mol, much lower than that of bare ceria (44.2 kJ/mol), ZnO (42.1 kJ/mol) and CeO\({}_{2}\)+ZnO-0.4 mechanical mixture (43.2 kJ/mol). These findings unveil the lower energy barrier for CO oxidation over the hydrothermally prepared mixed oxides, as compared to single oxides and mechanical mixture, demonstrating the beneficial synergistic interactions induced by synthesis procedure. The CO oxidation process investigated here can be corroborated on the basis of a redox-type (Mars-van Krevelen) mechanism. This particular mechanism involves CO chemisorption on Ce\({}^{8+}\) active sites, followed by oxygen activation on oxygen vacancies [3; 21; 41]. CO oxidation is taking place between the Ce\({}^{8+}\)-CO and adjacent oxygen species, followed by the regeneration of active sites and the reoccupation of oxygen vacancies through gas phase oxygen. The proposed reaction sequence is described in detail in our previous work [21].