## 2 Results and Discussion
### Catalysts' Characterization
#### 2.1.2 Spectroscopy Studies (FTIR, UV-Vis, Raman, and \({}^{29}\)Si MAS-NMR)  
To investigate the origin of non-framework CeO\({}_{2}\) species, as well as the consequences of using different cerium precursors, Raman spectroscopy was performed in both as-synthesized and calcined MCM-41 materials. The Raman spectra of as-synthesized materials can give information on how an assembly of surfactants is affected by other ionic species in the synthetic medium. Raman signals of organic molecules are dominant in this situation and the occluded CTA\({}^{+}\) cations can be in the same arrangement from the synthetic medium, counter-balancing the negative charges from deprotonated silanol groups. Also, the use of CTA\({}^{+}\) as a probe to access information towards microstructural characteristics from the framework of as-synthesized MCM-41 has been little explored in the literature [56; 57; 51], encouraging the use of conformational-sensible spectroscopic techniques, such as Raman spectroscopy, in the unraveling of microstructural changes. Figure 3 displays the spectra of the as-synthesized materials and, for means of comparison, the spectra of template molecules with two different molecular orderings were acquired: A 23 wt.% aqueous solution of cetyltrimethylammonium bromide (CTAB) and solid CTAB (Figure S2 in Supplementary Material). The concentration of 23 wt.% was chosen because it is in the range of the concentration where surfactant molecules form an hexagonal liquid-crystal phase in solution [58]. The comparison between the spectra of the as-synthesized materials and the two template standards clearly shows that CTA\({}^{+}\) cations are organized as cylindrical micelles inside the pores of as-synthesized materials, as it is expected based on the mechanism for MCM-41 crystallization [59]. The assignment of each vibrational mode observable in the spectra of the as-synthesized materials and template standards is summarized in Table S1 (see Supplementary Material). For the solid CTAB, two of the most important Raman bands that contain information towards molecular configuration are those in 1061 and 1126 cm\({}^{-1}\), attributed to C-C symmetric and asymmetric stretching, respectively [57; 60; 61]. These are characteristic of extended hydrocarbon chains with all-_trans_ ordered structures in which alternate carbon atoms move in the opposite direction. Other important vibrational modes at 2848/2880 and 2941/2958 cm\({}^{-1}\) were assigned to the C-H symmetric/asymmetric stretching of  
Figure 3: Raman spectra of the as-synthesized materials between 400 and 1700 cm\({}^{-1}\) (**A**) and 2800 and 3000 cm\({}^{-1}\) (**B**). The signals used for the determination of the disorder parameters are indicated. CH\({}_{2}\) and CH\({}_{3}\), respectively. These bands are the strongest in the Raman spectra and highly sensitive to molecular conformation [57; 62]. In comparison with solid CTAB, the Raman spectra of CTAB solution and occluded CTA\({}^{+}\) in as-synthesized materials showed less and broader signals, as well as other relevant differences. First, a new signal appeared at 1076 cm\({}^{-1}\) for both as-synthesized materials and CTAB solution. This signal, which was not observed for solid CTAB, was assigned to the C-C stretching mode of hydrocarbon chains in the gauche configuration and is expected in more disordered systems [57; 60]. Second, common signals found for solid CTAB were absent or became overlapped for CTA\({}^{+}\) molecules in as-synthesized materials or in solution, especially those at 1175 and 1464 cm\({}^{-1}\), associated to the dynamic motion of molecules in supramolecular assemblies [57]. This result indicates that occluded CTA\({}^{+}\) molecules in as-synthesized materials act as it were in a liquid-like state [57]. Finally, intensity and shift changes in conformational-sensitive peaks between 2850 and 3000 cm\({}^{-1}\) were also observed for CTA\({}^{+}\) occluded and in solution.