## 5 Implication in Catalysis
### CO Oxidation
A similar trend was acquired for TMs/ceria samples, indicating once again the pivotal role of ceria morphology towards determining the activity of ceria-based transition metal samples [38]. Moreover, the conversion performance of TMs/ceria samples prepared through the hydrothermal and consequent impregnation method surpasses, by far, the corresponding performance of mechanical mixtures (M\({}_{\text{x}}\)O\({}_{\text{y}}\) + CeO\({}_{2}\)-NR), clearly revealing the synergistic interactions between metal and support [38]. Finally, from a practical perspective, the catalytic performance of M\({}_{\text{x}}\)O\({}_{\text{y}}\)/CeO\({}_{2}\)-NR samples considerably surpasses the corresponding efficiency of typical precious metal oxidation catalysts, such as Pt/Al\({}_{2}\)O\({}_{3}\) (half-conversion temperature of ca. 230 \({}^{\circ}\)C under similar reaction conditions), revealing the effectiveness of shape engineering towards the rational design of cost-efficient noble metal-free catalysts [13]. The establishment of reliable structure-property relationships is crucial on the way to rational design and fabrication of highly-active composites. In other words, and in relation to the present findings, it is of paramount importance to disclose reliable correlations between key physicochemical parameters and the CO oxidation of M\({}_{\text{x}}\)O\({}_{\text{y}}\)/CeO\({}_{2}\) catalysts. The aforementioned results can be well interpreted on the grounds of a Mars-van Krevelen mechanism, thoroughly described in our previous works [34; 38; 75]. Briefly, CO oxidation reaction proceeds through the following steps: (i) the chemisorption of CO on metal active sites followed by its migration to the metal-support interface, (ii) the activation of molecular oxygen to oxygen vacancies, (iii) the surface reaction between CO and active oxygen; and (iv) the replenishment of oxygen vacancies by gas phase oxygen. In view of this mechanistic scheme, the pivotal role of redox properties linked to oxygen vacancies and oxygen mobility is apparent. Indeed, perfect relationships between the reaction rate (r\({}_{\text{co}}\), mmol s\({}^{-1}\) g\({}^{-1}\)) and the following activity descriptors were disclosed: (i) population of oxygen vacancies expressed by I\({}_{\text{D}}\)/I\({}_{\text{F2g}}\) ratio, and (ii) oxygen storage capacity (OSC), as shown in Figure 9. Similar relationships have been obtained for Cu- and Fe-based samples, revealing the pivotal role of support morphology [34; 38]. Apparently, the CO consumption rate follows the order NR \(>\) NP \(>\) NC, which perfectly matched with the order of OSC and oxygen vacancies (I\({}_{\text{D}}\)/I\({}_{\text{F2g}}\)). Hence, the improved OSC of nanorods with [100] and [110] exposed facets, linked with their abundance in oxygen vacancies, can be considered responsible for their enhanced CO oxidation performance. Correspondingly, both experimental and theoretical studies have revealed that the creation of anionic vacancies on ceria nanostructures is favored in the following order: [111] \(<\) [100] \(<\) [110] [22; 24; 65; 112; 125; 113; 114; 115; 116; 117; 118; 119; 120; 121; 122; 123; 124; 125; 126; 127; 128; 129; 130; 131; 132], which verifies the superiority of ceria nanorods during CO oxidation [133; 105; 134]. In a similar manner, it has been found that CO oxidation over CuO\({}_{\text{x}}\)/CeO\({}_{2}\) catalysts is sensitive to the structures of both copper and ceria, while the CuO\({}_{\text{x}}\)-ceria interaction depends on ceria's exposed facets, following the order: [110]-rods \(>\) {111}-polyhedra \(>\) {100}-cubes [135]. Similar effects have been reported for various noble metals (e.g., Au, Pd, and Pt) anchored on ceria nanorods during CO oxidation, water-gas shift (WGS) reaction, methanol steam reforming, etc. [68]; interfacial interactions in conjunction to the improved redox properties of ceria nanorods were mainly considered responsible for the enhanced catalytic efficiency [68].