## 2 Results and Discussion
### UV-Vis and Photoluminescence Spectroscopy  
Figure 6A displays the UV/Vis absorbance spectra of the as-prepared CeNR, CeNR-BuOOH, and CeNR-H\({}_{2}\)O\({}_{2}\) powders. The peaks attributed to the _4f_\(\rightarrow\)5\(d\) transitions of Ce\({}^{3+}\) (220-320 nm), are observed in all spectra. The excitation spectra (Figure 7) also exhibit Ce\({}^{3+}\)_4f_\(\rightarrow\)5\(d\) transitions. A broad band (cut-off wavelength 480-550 nm) is also observed due to charge transfer transitions from O 2\(p\) to Ce 4_f_[27; 28; 29; 30]: a broadening of this band is observed especially in the case of CeNR-H\({}_{2}\)O\({}_{2}\) relative to CeNR possibly due to the increasing number of surface defects [13]. The optical energy gap, \(\varepsilon_{\mathrm{d}}\), and the indirect band gap, \(\varepsilon_{\mathrm{i}}\), were calculated for a suspension of CeNR in ethanol (Figures 6B and 6C). The values obtained (\(\varepsilon_{\mathrm{d}}=3.1\) eV and \(\varepsilon_{\mathrm{i}}=2.5\) eV) are in accord with the band gaps of ceria bulk [31; 32]. The band gap values were not estimated for Ce-BuOOH and CeNR-H\({}_{2}\)O\({}_{2}\) due their low dispersion and stability in suspension. However, the similarity of the absorption spectra measured for the CeNR and CeNR-BuOOH powders (Figure 6A) suggest that the energy gaps of both materials may be comparable, albeit may differ somewhat from that of CeNR-H\({}_{2}\)O\({}_{2}\). \begin{table}
\begin{tabular}{|c|c|c|c|} \hline
**Substrate** & **Conversion** & **Product** & **Selectivity** \\  & **(\%)** & **(\%)** & **(\%)** \\ \hline Ethylbenzene & & 1-phenyl-ethyl-_tert_-butyl-peroxide & \\  & 25 & & 100 \\ \hline Cyclohexene & & 1-(_tert_-butyl-peroxy)-2-cyclohexene & \\  & 22 & & 86\({}^{\mathrm{a}}\) \\ \hline Cyclohexanol & & Cyclohexanone & \\  & 33 & & 100 \\ \hline \end{tabular}  
* By-products are mainly 2-cyclohexen-1-one and minor amounts of 2-cyclohexen-1-ol.  
\end{table} 
Table 3: Oxidation of different substrates with t-BuOOH, in the presence of CeNR, using acetonitrile as solvent, during 24 h, at 55 \({}^{\mathrm{o}}\)C.  
The emission spectra at -262 \({}^{\circ}\)C of CeNR, CeNR-H\({}_{2}\)O\({}_{2}\) and CeNR-PhEt-BuOOH (excitation at 332 nm) display two overlapping broad bands at _ca._ 375 and 410 nm, exemplified for CeNR-H\({}_{2}\)O\({}_{2}\) by spectral deconvolution (Figure 8A). In contrast to that observed for CeNT-H\({}_{2}\)O\({}_{2}\), in the case of CeNR  
Figure 7: \(-\)11 K excitation spectra of CeNT monitoring at 370.1 nm (black), 390.2 nm (red) and 450 nm (blue). and CeNR-PhEt-BuOOH, a series of sharp peaks at _ca._ 370.1, 375.8, 380.6, 386.3 and 390.5 nm were discerned superimposed to the broad band envelope. Whereas the two broad bands are unequivocally (because their energy separation is 2270 cm-1) attributed to _Ce3+_5_sd_-2_F5/2,7/2_ transitions [33,34], the nature of the sharp peaks is still open to discussion: some authors suggest that the peaks are attributed to the hopping from localized Ce3+ states or different defect levels to the _O 2p_ band [35]. The emission spectra of CeNR and CeNR600 excited at 332 nm are displayed in Figure 8 (inset) showing the same sharp peaks (360-400 nm) associated with the hopping from localized Ce3+ states or different defect levels to the _O 2p_ band superimposed to the _5d_-2_F5/2,7/2_ broad transitions. Although the emission spectra of all the different materials clearly show the presence of Ce3+ ions, the differences between the relative intensity of the sharp peaks and the _5d_-2_F5/2,7/2_ and _4f_-2_O 2p_ broad transitions when comparing CeNR600 to the non-calcined samples suggest that the calcination process modifies the distribution of Ce3+ ions present in bulk regions and/or in surface defects [19]. These results together with the catalytic ones, suggest that the presence of Ce3+/Ce4+ in the material is not sufficient for the existence of catalytic activity and most likely catalytic activation is associated to a certain population of defect sites.