## 3 Ceria Shape Effects on the Structural Defects and Redox Properties  
In this part, the impact of distinct morphology on the physicochemical characteristics of bare ceria nanoparticles is initially discussed. Figure 3 shows indicative TEM images of ceria nanorods, nanopolyhedra, and nanocubes. The CeO\({}_{2}\)-NR sample (Figure 3a,d) shows a rod-like morphology with a length from 25 to 200 nm. In the case of CeO\({}_{2}\)-NC (Figure 3c,f), a well-defined cube-like morphology is obtained (of about 20-30 nm), whereas nanopolyhedra of irregular size/shape are observed for CeO\({}_{2}\)-NP (Figure 3b,e). For the sake of clarity, the main textural and structural characteristics of ceria nanoparticles are summarized in Table 2. The following order, in relation to BET area, was disclosed: NC (40 m\({}^{2}\)/g) \(<\) NR (92 m\({}^{2}\)/g) \(<\) NP (109 m\({}^{2}\)/g). The average crystallite diameter of CeO\({}_{2}\) phase, calculated by applying Scherrer equation on the XRD diffractions, follows the reverse order, i.e., NP (9.5 nm) \(<\) NR (13.2 nm) \(<\) NC (19.2 nm). Among the different ceria nanostructures, ceria nanocubes exhibit the lowest BET surface area, along with the highest mean particle size. Figure 2: Schematic illustration of synthetic procedure of ceria nanoparticles of different shape (nanocubes = NC, nanorods = NR, and nanopolyhedra = NP). H\({}_{2}\)-TPR was performed to explore the impact of ceria shape on the redox properties (Figure 4). The main TPR peaks at ca. 550 and 800 \({}^{\circ}\)C are attributed to ceria's surface oxygen (O\({}_{\text{s}}\)) and bulk oxygen (O\({}_{\text{b}}\)) reduction, respectively. Interestingly, significant quantitative and qualitative differences between the reduction profiles are observed, implying the pivotal role of morphology on the redox features of ceria nanoparticles. In particular, the surface-to-bulk (O\({}_{\text{s}}\)/O\({}_{\text{b}}\)) ratio follows the order NR (1.13) > NP (0.94) > NC (0.71), which coincides with the oxygen storage capacity (OSC): NC (0.21 mmol/g) < NP (0.24 mmol/g) < NR (0.29 mmol/g). These results clearly showed that the rod-shaped ceria nanoparticles possess the highest population of weakly bound labile oxygen species. Moreover, ceria nanorods exhibit a low temperature reduction peak at 550 \({}^{\circ}\)C with a shoulder at ca. 450 \({}^{\circ}\)C, in opposition to other morphologies, which exhibit one main peak in the low temperature range. The latter is in agreement with the more facile reduction of {110} and {100} facets prevailing on ceria nanorods, as compared to {100} planes [76; 112; 114]. Hence, on the basis of the aforementioned results, it can be argued that, although ceria nanorods do not display the optimum textural properties (e.g., surface area), they possess the optimum redox characteristics in terms of population and facile reduction of loosely bound oxygen species, which is of paramount importance in several reaction processes obeying a redox mechanism, as further discussed below. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**BET Analysis**} & \multicolumn{2}{c}{**XRD Analysis**} & \multicolumn{1}{c}{**TPR Analysis**} \\ \cline{2-5}
**Sample** & \begin{tabular}{c} **BET Surface Area** \\ **(m\({}^{2}\)/g)** \\ \end{tabular} & \begin{tabular}{c} **Pore Volume** \\ **(cm\({}^{3}\)/g)** \\ \end{tabular} & \
\begin{tabular}{c} **Average Crystallite** \\ **Diameter**, **D\({}_{\text{XRD (nm)}}\)** \\ \end{tabular} &
\begin{tabular}{c} **OSC** \\ **(mmol O\({}_{2}\)/g)** \\ \end{tabular} 2 \\ \hline CeO\({}_{2}\)-NC & 40 & 0.12 & 19.2 & 0.21 \\ CeO\({}_{2}\)-NR & 92 & 0.71 & 13.2 & 0.29 \\ CeO\({}_{2}\)-NP & 109 & 1.04 & 9.5 & 0.24 \\ \hline \hline \end{tabular}  
* Calculated applying Scherrer equation. 2 Calculated by integration of the temperature programmed reduction (TPR) peaks in the low-temperature range (surface oxygen reduction). OSC, oxygen storage capacity.  
\end{table} 
Table 2: Textural, structural and redox properties of bare ceria nanoparticles.