## 5 Implication in Catalysis
### Co\({}_{2}\) Hydrogen  
The unprecedented increase of CO\({}_{2}\) levels in the atmosphere due to anthropogenic activities related mainly to fossil fuels combustion has led to serious environmental consequences. Among the different mitigation approaches, carbon dioxide conversion to value-added products via its reaction with \"green\" hydrogen, has gained particular attention [138; 139; 140; 141]. A plethora of chemical compounds, such as methane, carbon monoxide, methanol, and various hydrocarbons and oxygenates can be obtained through CO\({}_{2}\) hydrogenation [142; 143; 138]. This particular route can concurrently utilize CO\({}_{2}\) emissions  
Figure 11: Relationship between the deN\({}_{2}\)O performance and the OSC (mmol O\({}_{2}\) g\({}^{-1}\)) of bare CeO\({}_{2}\) and Co/CeO\({}_{2}\) with nanorod (NR), nanopolyhedra (NP) and nanocubes (NC) morphology. Figure 12: N\({}_{2}\)O decomposition rate of CuO\({}_{x}\)/CeO\({}_{2}\) samples of different morphology at 375 \({}^{\circ}\)C. Adapted from Reference [112]. Copyright © 2015, American Chemical Society. and the \"green\" hydrogen derived by the surplus power from non-intermittent Renewable Energy Sources (RES), providing, among others, CH\({}_{4}\) or CO through the reverse water-gas shift (rWGS) reaction (Equation (1)) and Sabatier reaction (Equation (2)), respectively:  
\[\text{CO}_{2}+4\text{H}_{2}\rightarrow\text{CH}_{4}+2\text{H}_{2}\text{O}, \text{AH}_{298\text{K}}=-164.7\text{ kJ}/\text{mol} \tag{1}\]  
\[\text{CO}_{2}+\text{H}_{2}\rightarrow\text{CO}+\text{H}_{2}\text{O},\text{ \Delta}\text{H}_{298\text{K}}=+41.3\text{ kJ}/\text{mol} \tag{2}\]  
Although noble metal catalytic materials, such as Rh and Ru, show satisfactory hydrogenation activity, their scarcity and high cost hinders widespread applications [144; 145; 146; 147]. In this regard, our recent efforts have concentrated on the rational design of NMs-free catalyst formulations. Particular emphasis was given to middle-late 3d transition metals (e.g., Cu, Ni, Co, and Fe), due to their ability to chemisorb and consequently activate the CO\({}_{2}\) molecule [33; 39]. Moreover, cerium oxide was selected as a supporting carrier due to its excellent redox/basic characteristics in conjunction to the synergistic interactions that can be developed between TMs and CeO\({}_{2}\). In line with the aforementioned fine-tuning aspects, ceria nanoparticles size/shape engineering at nanoscale could be of paramount importance. In view of this fact, and bearing in mind the crucial role of redox properties on CO\({}_{2}\) hydrogenation process [148], ceria nanoparticles of rod-like shape, offering the optimal redox properties, were selected as supporting carriers in order to obtain TMs/CeO\({}_{2}\)-NR composites. Figure 13a,b demonstrates the conversion of CO\({}_{2}\) and the selectivity to methane, respectively, for bare CeO\({}_{2}\)-NR samples as well as for TMs (Fe, Co, Ni, and Cu)/CeO\({}_{2}\)-NR. It is obvious that there are significant differences between the samples, mostly depending on metal's nature. Regarding CO\({}_{2}\) conversion, the following order is obtained: CeO\({}_{2}\) < Fe/CeO\({}_{2}\) < Cu/CeO\({}_{2}\) < Co/CeO\({}_{2}\) < Ni/CeO\({}_{2}\), signifying the pivotal role of metal-support combination. Ni/CeO\({}_{2}\)-NR exhibits by far the optimum performance, offering ca. 92% conversion at 300 \({}^{\circ}\)C. In other words, by combining Ni with ceria nanorods extremely active CO\({}_{2}\) methanation catalysts can be obtained, which practically reach the equilibrium at very low temperatures of ca. 300 \({}^{\circ}\)C. More importantly, the high conversion performance of Ni catalysts is followed by 100% selectivity to CH\({}_{4}\) (Figure 13b) in the entire temperature range studied.