## 2 Results and Discussion
### Catalysts' Characterization
#### 2.1.2 Spectroscopy Studies (FTIR, UV-Vis, Raman, and \({}^{29}\)Si MAS-NMR)
In the case of the modified materials, the presence of this extra signal in both samples indicates the existence of non-framework cerium and that Ce-MCM-NO\({}_{3}\) possess a higher content of these species due to a much stronger intensity for this vibrational mode, which is in agreement with the XRD data presented earlier. A similar observation in the \(F_{2\mathrm{g}}\) mode appearance was made by [PERSON] and co-workers [48] for Ce-MCM-48 materials, where the mode increased in intensity with the increase of cerium content. However, such a difference between two cerium-modified MCM-41 materials with similar Si/Ce ratios as it is presented in this work has not been observed in the literature so far. Correlating with the information extracted for the as-synthesized materials, it is possible to ascertain the mechanism of crystallization proposed earlier. Thus, the disordering observed in the assembly of CTA\({}^{+}\) molecules of Ce-MCM-Cl was driven by isomorphic substitution. As for Ce-MCM-NO\({}_{3}\), part of the cerium added in the synthetic medium does not react with silicate oligomers and precipitates to form non-framework [Ce(OH)\({}_{3}\)], which converts completely to CeO\({}_{2}\) after calcination. Such a hypothesis is scarcely pointed out to explain the difference in the reactivity of cerium precursors in the synthesis of oxides, but there is an extensive literature regarding the differences in the use of precursors that leads to singular microstructural properties of these materials [52; 76; 77]. These works clearly suggest that the anions in cerium precursors have major roles in defining the properties of the final cerium-based materials more than just the mere electrostatic counterparts in the synthetic medium. Figure 5 displays the Fourier transform infrared spectroscopy (FTIR) spectra for all calcined MCM-41 materials. The vibrational modes between 1400 and 400 cm\({}^{-1}\) are typical of Si-O-Si bonds [78]. In the case of Si-MCM-41: (i) Asymmetric stretching of Si-O-Si bonds at 1232 and 1084 cm\({}^{-1}\); (ii) symmetric stretching of Si-OH bonds at 964 cm\({}^{-1}\); (iii) symmetric stretching of Si-O-Si at 802 cm\({}^{-1}\); and (iv) the Si-O-Si bending mode at 459 cm\({}^{-1}\). Changes in the wavenumber of these modes are frequently associated to isomorphic substitution of heteroelements in the framework of silica-based materials, because the average length of T-O bonds (T corresponds to any heteroatom) in the walls of the MCM-41  
Figure 4: Raman spectra of the calcined materials between 150 and 650 cm\({}^{-1}\). Figure 5: FTIR spectra of the calcined MCM-41 materials. is increased with the insertion of a larger atom than silicon [37; 49; 51]. However, the shifts observed for the cerium-incorporated materials were too small to make any assumption of isomorphic substitution. In addition, no vibrational modes attributed to Ce-O bonds, between 1600 and 900 cm\({}^{-1}\) and below 700 cm\({}^{-1}\), were evidenced. These observations (i.e., small shifts and absence of Ce-O bonds) could be respectively attributed to the low concentration of framework Ce\({}^{4+}\) ions and non-framework CeO\({}_{2}\) when compared to the amount of silica in MCM-41. Diffuse reflectance UV-Vis (DRS UV-Vis) spectra for the calcined materials are displayed in Figure 6A. When compared to Si-MCM-41, cerium-incorporated materials presented stronger absorption in the ultraviolet range and a red shift of the absorption edge due to O\({}^{2-}\)\(\rightarrow\)Ce\({}^{4+}\) charge transfer [32]. These experimental observations are proof of cerium incorporation in the materials, as already described by [PERSON] and co-workers [32]. Moreover, the greater intensity of the Ce-MCM-Cl spectrum can be due to its slightly higher cerium content (see Table 1). When compared to a standard sample of CeO\({}_{2}\) (Figure S4 in Supplementary Material), it is possible to gain information towards the coordination environment of the heteroatom, since the charge transfer transition is symmetry-sensible [37; 51; 79].