## 5 Implication in Catalysis
### Co\({}_{2}\) Hydrogen
Interestingly, the Cu/CeO\({}_{2}\)-NR catalyst is totally selective to CO (rWGS reaction), reaching the equilibrium at temperatures higher than 400 \({}^{\circ}\)C. Hence, the combination of Ni with CeO\({}_{2}\)-NR favors the CO\({}_{2}\) methanation reaction, whereas the rWGS reaction is favored over Cu/CeO\({}_{2}\)-NR. Moreover, stability experiments performed over the optimum catalytic materials (rod-shaped samples), revealed a stable performance without any deterioration of activity/selectivity [33; 39]. The superior conversion and selectivity performance of Ni/CeO\({}_{2}\)-NR is additionally demonstrated by comparing it with the corresponding commercial ceria (CeO\({}_{2}\)-Com) and Ni/CeO\({}_{2}\)-Com of identical Ni/Ce atomic ratio (Ni/Ce = 0.25, corresponding to ca. 8 wt.% Ni loading; see Figure 14). Apparently, nickel-ceria nanorods are much superior as compared to the commercial ceria-supported sample (Ni/CeO\({}_{2}\)-Com), clearly revealing the major role of ceria support nanostructure. In particular, the Ni/CeO\({}_{2}\)-Com sample shows a CH\({}_{4}\) yield at 300 \({}^{\circ}\)C of ca. 7%, as compared to 92% for as-prepared Ni/CeO\({}_{2}\)-NR. These findings are of particular importance, revealing that by appropriately adjusting the composition (e.g., ceria with various TMs) and structural defects (e.g., by modulating ceria support morphology), highly active composites can be obtained. In line with the aforementioned findings, significant morphological effects were also revealed during the CO\({}_{2}\) hydrogenation to CH\({}_{3}\)OH; the rod-shaped Cu/ceria samples have shown the highest methanol yield, ascribed mainly to the synergistic interactions between copper and ceria nanorods, as compared to nanocubes or nanoparticles [76]. Similarly, Cu/CeO\({}_{2}\)-NR shows the highest rWGS activity at atmospheric pressure as compared to nanospheres, since the formation of active intermediates is facilitated on the {110} facets of nanorods [84]. Moreover, among different morphologies (rods, cubes, octahedrons and polyhedrons), Pd supported on ceria nanorods has shown the highest activity and space time yield of methanol, due to their abundance in oxygen vacancies [149]. In this point, it should be mentioned that CO\({}_{2}\) hydrogenation catalysts of high activity and selectivity can be also obtained through the development of multicomponent systems [150; 151]. For instance, highly active bimetallic composites supported on ceria nanoparticles (Cu-Ni/CeO\({}_{2}\)) of specific morphology (e.g., nanorods and nanotubes) can be obtained for the hydrogenation of carbon dioxide to methanol, due to the synergism between Cu-Ni alloy and ceria [150; 151]. Figure 13: CO\({}_{2}\) conversion (**a**) and selectivity to CH\({}_{4}\) (**b**) for bare CeO\({}_{2}\)-NR and various metal-based catalysts (Fe, Cu, Co, Ni). Reaction conditions: Weight Hourly Space Velocity (WHSV) = 30,000 mL g\({}^{-1}\) h\({}^{-1}\), H\({}_{2}\):CO\({}_{2}\) = 4:1, P = 1 atm [39]. Dotted lines represent the corresponding equilibrium for rWGS and Sabatier reactions. At this point, driven by the aforementioned superior reactivity of ceria-based nanocomposites in various processes, it should be underlined that the development of structured catalysts, such as monoliths or pellets, coated with the catalytically active nanocomposites is a prerequisite for industrial applications. The materials platform for real-life industrial applications can be obtained by bringing together the following into one structure: the (i) superior catalytic activity of ceria-based nanocomposites of specific architecture; (ii) appropriate adhesion, pore structure, and permeability of washcoat; and (iii) enhanced thermal stability, mechanical strength, and low pressure drop of ceramic or metallic substrates [152; 153; 154].