## 3 Results and Discussion
### Redox Properties (H\({}_{2}\)-TPR)  
By means of H\({}_{2}\)-TPR experiments, the quality and quantity of the active oxygen sites of the catalysts can be determined. Such information is important for reactions following a redox-type mechanism, such as CO oxidation. Figure 5 shows the reduction profiles of bare and mixed oxides in the temperature range of 100-800 \({}^{\circ}\)C. The CeO\({}_{2}\) sample shows a peak at 500-700 \({}^{\circ}\)C (peak C),  
Figure 4: SEM–EDS analysis of the CeO\({}_{2}\)/ZnO–0.2 sample.  
Figure 3: TEM images of (**a**) ZnO, (**b**) CeO\({}_{2}\), (**c**) CeO\({}_{2}\)/ZnO–0.2, (**d**) CeO\({}_{2}\)/ZnO–0.4 and (**e**) CeO\({}_{2}\)/ZnO–0.6.  
which is ascribed to the loosely bound surface species, contrary to bulk oxygen, which is reduced at temperatures higher than 700 \({}^{\circ}\)C [1; 36; 37; 38]. For the pure ZnO sample, two broad peaks are found located at 150-300 \({}^{\circ}\)C (peak A) and 410-530 \({}^{\circ}\)C (peak B) regions, which can be attributed to the reduction of the hydroxyl species related to ZnO and the surface oxygen reduction from ZnO, respectively [39]. Additionally, in the pristine ZnO, the Zn\({}^{2+}\) reduction to Zn\({}^{0}\) has been reported at 465 \({}^{\circ}\)C [40]. The TPR profiles of CeO\({}_{2}\)/ZnO samples consist of the distinct peaks of both CeO\({}_{2}\) and ZnO phases. It is worth noticing that ZnO addition to CeO\({}_{2}\) does not lead to a shift of TPR peaks, as recently found for CeO\({}_{2}\)/TiO\({}_{2}\)[35] and Fe\({}_{2}\)O\({}_{3}\)/CeO\({}_{2}\)[2] mixed oxides. This implies the structurally independent nature of ZnO and CeO\({}_{2}\) in the mixed oxides. The latter is in agreement with XRD and TEM results showing distinct ZnO nanoparticles in the vicinity of CeO\({}_{2}\) nanorods without incorporation of Zn in the nanostructure of CeO\({}_{2}\) nanorods.  
To gain greater insight into the influence of ZnO on the reducibility of CeO\({}_{2}\)/ZnO oxides, the H\({}_{2}\) uptake in the temperature range of 50-700 \({}^{\circ}\)C was estimated by the quantification of the TPR peaks (Table 3). Interestingly, ZnO addition results in an increase of H\({}_{2}\) uptake in the CeO\({}_{2}\)/ZnO samples, implying a facilitation of surface oxygen species detachment. It is also of worth noticing the increase of H\({}_{2}\) uptake related to ceria reduction (peak C), despite the progressive decrease of ceria content. The latter denotes the facile reduction of ceria capping oxygen upon the increase of ZnO content. In particular, all mixed oxides exhibit a total H\({}_{2}\) consumption of ca. 1.0 mmol H\({}_{2}\)/g, as compared to 0.59 and 0.31 mmol H\({}_{2}\)/g of bare CeO\({}_{2}\) and ZnO, respectively. In terms of oxygen storage capacity (OSC), the following trend is obtained: CeO\({}_{2}\)/ZnO-0.6 (0.52 mmol O\({}_{2}\)/g) \(>\) CeO\({}_{2}\)/ZnO-0.4 (0.50 mmol O\({}_{2}\)/g) \(>\) CeO\({}_{2}\)/ZnO-0.2 (0.48 mmol O\({}_{2}\)/g) \(>\) CeO\({}_{2}\) (0.29 mmol O\({}_{2}\)/g) \(>\) ZnO (0.15 mmol O\({}_{2}\)/g). This order coincides relatively well with the catalytic activity (see below), revealing the key role of reducibility.  
Figure 5: H\({}_{2}\)-TPR profiles of CeO\({}_{2}\) and the CeO\({}_{2}\)/ZnO samples.  
The abundance of reducible oxygen species is expected to affect the oxygen mobility and in turn the CO oxidation process, as will be further discussed below.