## 2 Results and Discussion
### Catalysts' Characterization
#### 2.1.2 Spectroscopy Studies (FTIR, UV-Vis, Raman, and \({}^{29}\)Si MAS-NMR)
The CeO\({}_{2}\) spectrum presented more intense bands at 380 and 250 nm, owing to Ce\({}^{4+}\) ions in octahedral/polymeric and tetrahedral coordination environments, respectively. The octahedral/polymeric environment is the main coordination type for cerium in the CeO\({}_{2}\) fluorite crystal structure. Its non-stoichiometric lattice [80] gives the dioxide a high oxygen mobility and absorptions near or above 380 nm is strong evidence of non-framework species. The spectra of cerium-incorporated materials in Figure 6A showed that UV-absorption finishes around 310 nm for Ce-MCM-Cl with a steep absorption edge until 450 nm, while for Ce-MCM-NO\({}_{3}\), it ends at 355 nm with a less steep edge until 500 nm. These results are evidence that the heteroatoms in Ce-MCM-Cl have more tetrahedral coordination sites than Ce-MCM-NO\({}_{3}\), which absorbs more radiation in the visible range of the spectrum due to non-framework species [37; 51; 79]. The different positions found for the absorption edge in Ce-MCM-41 samples can be used to obtain important information towards the heteroatom by determining the optical band gap. The latter was calculated by Equation (3) (see Materials and Methods) using n = \(\frac{1}{2}\), due to direct allowed transitions, and Davis-Mott plots with suitable linear ranges were obtained (R\({}^{2}\) > 0.99). CeO\({}_{2}\) has an optical band gap of 3.02 eV, which is near to the values described in the literature for the bulk material [81]. Ce-MCM-Cl and Ce-MCM-NO\({}_{3}\) showed band gap values of 3.50 and 3.33 eV, respectively, which were lower than Si-MCM-41 (4.97 eV). As studied by [PERSON] and co-workers [73], higher band gap values of metal-incorporated MCM-41 than the metal oxide counterpart indicates high dispersion of the heteroatom in the siliceous framework. Also, the lower band gap value observed for Ce-MCM-NO  
Figure 6: Absorption spectra (**A**) and Davis-Mott plots (**B**) for the calcined MCM-41 materials. could be associated with the higher content of non-framework species identified by XRD and Raman, resulting in less dispersed CeO\({}_{2}\) particles. \({}^{29}\)Si MAS-NMR spectra of the calcined materials are available in Figure 7. As can be seen, the signals obtained for all samples were broad and overlapped in the range between \(-80\) and \(-120\) ppm. This interval is typical for the Q\({}^{n}\)-groups with a general formula of Si(OSi)\({}_{n}\)(OH)\({}_{4-n}\)[82]: (i) Geminal silanols or Q\({}^{2}\) groups (resonating around \(-90\) ppm); (ii) terminal silanols or Q\({}^{3}\) groups (resonating around \(-100\) ppm); and (iii) siloxane or Q\({}^{4}\) groups (resonating around \(-110\) ppm). The overlapping effect is typical in systems based on amorphous silica due to the wide distribution of Si-O-Si bond angles and Si-O bond lengths [82]. It was possible to perceive that, in the case of Ce-MCM-Cl, there was a slight diminishment in the signal of Q\({}^{3}\) groups, indicating a decrease in its content. This type of change was even more noticeable for Ce-MCM-NO\({}_{3}\), where all defect-related environments (Q\({}^{2}\) and Q\({}^{3}\)) were significantly reduced when compared to Si-MCM-41. Such qualitative observations indicate that the cerium incorporation is modifying the condensation of Si-O-Si bonds in the structure of these materials. To access more information towards the latter, deconvolution methods were applied and are denoted in Figure 7.  
No additional peaks to those mentioned earlier were found after applying the deconvolution methods. Therefore, the traditional parameters associated to the condensation of Si-O-Si bonds were applied to further analyze the changes perceived for cerium modified materials. Among them, the percentage of each siliceous chemical environment was calculated based on the area of each peak and they were used to determine \(Q^{4}\)/(\(Q^{2}\) + \(Q^{3}\)) ratios and the molar percentage of silanol groups (Si molar%) using Equation (4) (see Materials and Methods).