## 2 Results and Discussion
### Catalysts' Characterization
#### 2.1.2 Spectroscopy Studies (FTIR, UV-Vis, Raman, and \({}^{29}\)Si MAS-NMR)
In addition, the oxidation of Ce\({}^{3+}\) to Ce\({}^{4+}\) is not thermodynamically favored under the conditions used in this work (i.e., non-aerated solution and pH = 11.2) [64] and [Ce(OH)\({}_{3}\)] can be regarded as the main cerium compound in solution [67]. Based on the above discussion and the well-known self-assembled systems used for synthesizing MCM-41 [59], this work proposes the following mechanism for cerium incorporation: (i) CTA\({}^{+}\) cations are counterbalanced by basic silicate anions; (ii) the latter interacts through hydrogen bonds with [Ce(OH)\({}_{3}\)] species; and (iii) the gradual condensation under basic conditions leads to silicate oligomers containing Si-O-Si and Ce-O-Si bonds. The higher disorder detected by [PERSON] for occluded CTA\({}^{+}\) indicates that cerium incorporation into silica causes different electrostatic interactions between CTA\({}^{+}\) cations and the anionic surface. Since both cerium(III) precursors are highly soluble salts with similar stability constants [68], the differences observed between Ce-MCM-Cl and Ce-MCM-NO\({}_{3}\) by the techniques used in this work must be related to the different anions. [PERSON] and co-workers [69] studied the effect of anions on alkyltrimethylammonium micelles and observed that micelle aggregation increases in the following order: OH\({}^{-}<<\)Cl\({}^{-}<\)CH\({}_{3}\)SO\({}_{4}^{-}<\)Br\({}^{-}<\)NO\({}_{3}\)\({}^{-}\). For that reason, nitrate anions induce longer rods of CTA\({}^{+}\) cations in solution than chloride [70]. In addition, the molar ratio used in the synthesis of Ce-MCM-41 materials presented a concentration of nitrate or chloride anions close to CTAB, exhibiting 1 TEOS:0.04 Ce:0.12A:0.152 CTAB:2.8 NH\({}_{4}\)OH:141.2H\({}_{2}\)O, where A = NO\({}_{3}\)\({}^{-}\) or Cl\({}^{-}\). Since NO\({}_{3}\)\({}^{-}\) ions have a higher ionic exchange constant on CTAB micelles than Cl\({}^{-}\)[71], they are expected to present a stronger binding to CTA\({}^{+}\) rods. For Ce-MCM-NO\({}_{3}\), nitrate ions bonded to CTA\({}^{+}\) could also be hydrogen bonded to [Ce(OH)\({}_{3}\)]. This layer of concentrated cerium(III) species will aggregate during calcination to form non-framework CeO\({}_{2}\) species on the surface of the MCM-41 structure. The higher disordering level observed for Ce-MCM-Cl can be associated to a higher degree of framework Ce-O-Si due to the lower binding between chloride ions and CTA\({}^{+}\) molecules. Thus, the physicochemical differences among both cerium modified samples may arise fundamentally from an anion effect in the disturbance of CTA\({}^{+}\) molecule assemblies, especially the formation of different cerium species in the calcined materials. Further effects in the formation of cerium species can be perceived in the calcined materials. Figure 4 displays the Raman spectra of the calcined materials between 650 and 150 cm\({}^{-1}\). In general, they all present signals common to the amorphous nature of the silica present in MCM-41. At the studied range, the Si-MCM-41 spectrum presents a signal at 483 cm\({}^{-1}\), typical of the stretching mode of Si-O bonds for fourfold cyclosiloxane groups [72; 73; 74]. For Ce-MCM-Cl, a weak peak appears aside the cyclosiloxane group mode at 460 cm\({}^{-1}\) and this signal becomes dominant over the 483 cm\({}^{-1}\) mode in the spectrum of Ce-MCM-NO\({}_{3}\). This feature is commonly attributed in the literature to the symmetric stretching of Ce-O bonds of the fluorite crystal structure of CeO\({}_{2}\)[75] and was assigned as the \(F_{2\mathrm{g}}\) mode of CeO\({}_{2}\). The spectrum of crystalline CeO\({}_{2}\) was obtained for means of comparison (Figure S3 in Supplementary Material) and the \(F_{2\mathrm{g}}\) mode presents itself as a strong signal centered in 463 cm\({}^{-1}\).