## 3 Results and Discussion
### Catalytic Evaluation Studies
In view of the above mechanistic aspects, the key role of the catalyst's redox properties is clearly manifested. In detail, the high oxygen storage capacity (Table 3), linked to enhanced reducibility and oxygen exchange kinetics, can be accounted for the improved oxidation performance. Any structural or compositional modification in ceria that can affect the ceria-oxygen or ceria-oxygen-metal bond can eventually facilitate O\({}_{2}\) activation and in turn CO oxidation. In this regard, we recently showed that the enhanced reducibility of CeO\({}_{2}\) nanorods, linked to their abundance in oxygen vacancies, is the pivotal factor for their superior catalytic performance [21]. Furthermore, we demonstrated that when a transition metal element is incorporated in the structure of ceria nanorods, even if catalytically non-active (i.e., Ti), it can affect oxygen species coordination environment and in turn the OSC and the catalytic performance [35]. These arguments in relation to the crucial role of reducibility are clearly supported by the direct relationship between the redox properties (OSC, mmol g\({}^{-1}\)) and the normalized reaction rate (umol g\({}^{-1}\) s\({}^{-1}\)), as shown in Figure 7. Interestingly, the key role of OSC as an activity descriptor is further demonstrated in the present work by including in Figure 7 relevant literature data, previously obtained with bare CeO\({}_{2}\) and CeO\({}_{2}\)/TiO\({}_{2}\) oxides [2; 35]. The latter is of particular importance towards the development of cost-effective and highly active metal oxides by appropriately adjusting their redox features. Thus, regarding the present findings, although bare ZnO is catalytically far less active compared to CeO\({}_{2}\) (Figure 6), their combination results in a synergistic effect towards the formation of CeO\({}_{2}\)/ZnO mixed oxides of improved activity. Similar synergistic interactions were previously revealed over TiO\({}_{2}\)-doped CeO\({}_{2}\)[35], where, however, the doping element was incorporated in the nanostructure, resulting in the formation of Ce-O-Ti active sites. Notably, in the present work, ZnO is not incorporated in the CeO\({}_{2}\) nanorod structure. As clearly shown by TEM images (Figure 3), ZnO nanoparticles are in close interaction with nanorods but clearly as separate particles. This is also verified by the XRD results (Figure 2), which indicate distinct oxide phases without the formation of solid solution. Hence, the improved catalytic activity of CeO\({}_{2}\)/ZnO mixed oxides, which is related to the population of active oxygen species and to the facile reduction of surface oxygen, can be attributed to the facilitation \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multirow{2}{*}{**CO Conversion (\%)**} & \multicolumn{2}{c}{**Specific Rate**} \\ \cline{3-4}  & & **r** & **r (\(\mathbf{\gamma mol\ g^{-1}\ s^{-1}}\))** & **(\(\mathbf{\mu mol\ m^{-2}\ s^{-1}}\))** \\ \hline CeO\({}_{2}\) & 5.1 & 0.056 & 0.070 \\ CeO\({}_{2}\)/ZnO-0.2 & 5.4 & 0.059 & 0.077 \\ CeO\({}_{2}\)/ZnO-0.4 & 6.9 & 0.075 & 0.121 \\ CeO\({}_{2}\)/ZnO-0.6 & 6.2 & 0.068 & 0.121 \\ ZnO & 1.3 & 0.014 & 0.201 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Conversion of CO and specific rates of CeO\({}_{2}\) and CeO\({}_{2}\)/ZnO samples at 200 \({}^{\circ}\)C. Reaction conditions: 0.2 vol.% CO and 1 vol.% O\({}_{2}\) in He. of ceria capping oxygen reduction by adjacent ZnO nanoparticles, which in turn leads to higher OSC (Figure 5, Table 3). Thus, although ZnO is not incorporated in the ceria nanorods structure, it notably contributes to surface oxygen reduction, most likely through the interfacial ZnO-CeO\({}_{2}\) sites. The latter in combination with the abundance and lower cost of zinc compared to cerium oxide is a very interesting aspect in terms of catalyst design.