## 4 Physicochemical Properties of Ceria-Based Transition Metal (Fe, Co, Ni, and Cu) Catalysts  
Having already established the pivotal role of ceria shape on the surface chemistry and redox properties of bare ceria nanoparticles, in this section, we discuss the combined effect of metal entity (Fe, Co, Ni, and Cu), ceria morphology and interfacial interactions on the physicochemical properties of TMs/CeO\({}_{2}\) oxides.  
Figure 7 depicts representative TEM images of TMs (Fe, Co, Ni, and Cu)/CeO\({}_{2}\) samples. It is evident in all cases that ceria's morphology is not affected by the incorporation of transition metal to ceria, implying no morphological changes upon the addition of  
Figure 6: Ce 3d (**a**) and O 1s (**b**) XPS spectra of ceria nanoparticles.  
TMs into the ceria carrier. The main textural, structural, and redox characteristics of all TMs/CeO\({}_{2}\) samples are summarized in Table 4.  
Figure 7: Indicative TEM images of TM/ceria (TM: Cu, Co, Fe, and Ni) nanoparticles: (**a–c**) Cu/CeO\({}_{2}\), (**d–f**) Co/CeO\({}_{2}\), (**g–i**) Fe/CeO\({}_{2}\), and (**j–l**) Ni/CeO\({}_{2}\) samples of (**a**,**d**,**g**,**i**) rod-like, (**b**,**e**,**h**,**k**) polyhedral and (**c**,**f**,**i**,**l**) cubic morphology.  
Concerning the BET area of bare ceria nanoparticles, the following order is obtained, as has been already discussed: NP > NR > NC (Table 2). The incorporation of transition metals into ceria support results in all cases into a slight decrease in surface area. In relation to the ceria crystallite size, it follows, generally, the reverse order of the BET area, i.e., the lower the surface area the larger the ceria crystallite size. In general, and on the basis of the present textural characteristics, it could be stated that the surface area and ceria crystallite size is not significantly affected by the transition metal's nature. The textural/structural properties of TMs/ceria catalysts are mainly determined by the support characteristics, as further corroborated by TEM analysis (Figure 7).  
However, in complete contrast to textural/structural properties, the redox properties of TMs/ceria composites are notably affected upon metal incorporation to ceria carrier. In all cases, metal addition facilitates the surface oxygen reduction shifting the O\({}_{\text{s}}\) peak (Figure 4) to lower temperatures (not shown for brevity). In other words, the reduction of TMs/ceria composites is taking place at considerably lower temperatures in comparison to single metal and ceria oxides, indicating the beneficial impact of metal incorporation on surface oxygen reduction. In view of this fact, the metal-support interactions could be accounted for the enhanced reducibility and oxygen exchange kinetics of mixed oxides, as extensively discussed in previous studies [14; 25; 27; 41; 59; 61]. The latter is further demonstrated by the estimation of OSC of TMs/CeO\({}_{2}\) catalysts in the low temperature region, in which the reduction of both the metal oxide (M\({}_{\text{x}}\)O\({}_{\text{y}}\)) and CeO\({}_{2}\) surface oxygen is taking place. The obtained OSC values (Table 4) always surpasses the theoretical values corresponding to nominal metal oxide (M\({}_{\text{x}}\)O\({}_{\text{y}}\)) content, revealing the metal-induced acceleration of ceria's capping oxygen reduction. More importantly, regardless of the metal entity (Cu, Co, Fe, and Ni), the OSC generally follows the order NR > NP > NC (Table 4), clearly demonstrating the crucial role of ceria nanostructure toward determining the redox properties of TMs/CeO\({}_{2}\) catalysts.