## 5 Implication in Catalysis
### CO Oxidation  
CO oxidation reaction has been widely studied in heterogeneous catalysis due to its involvement to various energy and environmental processes, such as the control of exhaust emissions from mobile and stationary sources, and fuel cell applications. Moreover, from the fundamental point of view, CO oxidation can be employed as a model reaction to disclose structure-performance relationships. Ceria-based transition metal oxides have been extensively studied for CO oxidation, due mainly to their unique redox properties [2; 3; 9; 56] and the peculiar metal-ceria synergistic interactions [16; 41; 123; 124]. However, as previously mentioned, the distinct characteristics of both the active phase and ceria carrier (e.g., particle's size and shape) can exert a major impact on the redox properties, as well as the metal-support interactions, which can be consequently reflected on the catalytic activity. In this perspective, we recently explored the impact of ceria shape (nanocubes, nanorods, and nanopolyhedra) on the CO oxidation activity of various transition metals (e.g., Fe and Cu) supported on ceria nanoparticles. Herein, we summarize and discuss the main findings, which are interpreted based on the aforementioned characterization studies. Figure 8 depicts the CO oxidation performance of the different bare ceria nanoshapes (NR, NC, and NP) in comparison to that of transition metal-based catalysts, i.e., Fe/CeO\({}_{2}\) and Cu/CeO\({}_{2}\) samples. Regarding, at first, the CO conversion of bare ceria, it is evident that significant differences are obtained between the samples, demonstrating the key role of morphology. On the basis of T\({}_{50}\), i.e., the required temperature for 50% conversion, the following order is observed for bare ceria: NR (320 \({}^{\circ}\)C) < NP (350 \({}^{\circ}\)C) < NC (385 \({}^{\circ}\)C). It is noteworthy that the conversion order of bare ceria samples totally coincides with their relative concentration in labile oxygen species (expressed by OSC) and oxygen vacancies (expressed by the ratio of I\({}_{\text{D}}\)/I\({}_{\text{F2g}}\)). The latter clearly reveals the major role of ceria shape towards determining the redox/structural properties and in consequence the catalytic behavior, as further discussed below. Most significantly, the incorporation of transition metals (Fe and Cu) to ceria drastically improves the catalytic performance, not affecting, however, the activity order obtained for single CeO\({}_{2}\) samples. This clearly implies the guided role of support morphology on the  
Figure 8: Ceria nanoparticles morphological effects (nanorods = NR, nanocubes = NC, nanopolyhedra = NP) on the CO oxidation performance of bare CeO\({}_{2}\), Cu/CeO\({}_{2}\), and Fe/CeO\({}_{2}\) samples. catalytic performance of ceria-based transition metal catalysts. Apparently, the rod-shaped samples exhibit the optimum catalytic efficiency, demonstrating complete CO conversion at 250 and 100 \({}^{\circ}\)C for Fe/CeO\({}_{2}\) and Cu/CeO\({}_{2}\) samples, respectively. It should be also mentioned that the optimum catalysts (Cu/CeO\({}_{2}\)-NR and Fe/CeO\({}_{2}\)-NR) attained a stable conversion performance in short-term stability tests (24 h). In view of this fact, BET and XRD analyses of the spent catalysts revealed no textural or structural modifications. In particular, ceria's mean crystallite size was slightly increased to 13 nm in comparison to 11.6 nm of the fresh sample, while the BET surface area remained practically unaltered (72.8 m\({}^{2}\)/g in comparison to 75.4 m\({}^{2}\)/g of the fresh sample, Table 4) [34; 38]. The superior reactivity of samples with rod-like shape was further verified by a kinetic study, under differential reaction conditions, which revealed the following order in relation to apparent activation energy (E\({}_{\text{a}}\)): CeO\({}_{2}\)-NR (44.2 kJ/mol) \(<\) CeO\({}_{2}\)-NP (46.7 kJ/mol) \(<\) CeO\({}_{2}\)-NC (49.8 kJ/mol) [38].