## 3 Results and Discussion
### FT-IR Spectroscopic Studies
#### 3.1.3 Vanillic Acid  
Figure 3 shows the spectra of pure VA immobilized on CeO\({}_{2}\). The presence of different COH groups in the VA molecule complicates the interpretation of the obtained spectra. Literature data [15; 59; 64; 65] and our FT-IR spectroscopic studies of P, G, and carboxylic acids [22; 45; 50] were used to analyze the IR spectra of pure VA and VA/CeO\({}_{2}\). From the obtained spectra (Figure 3, Table 3), it was found that both the carboxyl group and the active groups of the aromatic ring of VA are involved in the interaction with CeO\({}_{2}\). In particular, for VA/CeO\({}_{2}\) samples, a decrease in the intensity of bands at 1030 cm\({}^{-1}\) (\(\
u_{\rm s}\)(COCH\({}_{3}\))), 1240 cm\({}^{-1}\) (\(\
u_{\rm ox}\)(COCH\({}_{3}\))) [15; 64] and at 1113, 1188, 1456, and 1473 cm\({}^{-1}\) (\(\beta\)(CH\({}_{3}\))) [15; 65] was observed. This indicates the participation of the methoxyl group in the interaction with the oxide surface. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Assignments**} & \multicolumn{2}{c}{**Frequency (cm\({}^{-1}\))**} & \multirow{2}{*}{**Ref.**} \\ \cline{2-2} \cline{4-4}  & **G** & & **G/CeO\({}_{2}\)** \\ \hline \(\
u_{\rm s}\)(COCH\({}_{3}\)) & 1024 & 1018 & [63] \\ \(\
u_{\rm as}\)(COCH\({}_{3}\)) & 1225 & 1217 & [63] \\ \(\
u\)(COH) & 1261 & 1261 & [63] \\ \(\
u\)(CO)\({}^{\cdot}\) & – & 1288 & [63] \\ \(\
u\)(CO)\({}^{\cdot}\) & – & 1323 & [63] \\ \(\beta\)(COH) & 1363 & – & [63] \\ \(\beta\)(CH\({}_{3}\)) & 1444 & – & [63] \\ \(\beta\)(CH\({}_{3}\)) & 1469 & – & [63] \\ \(\
u\)(CC) + \(\beta\)(CH\({}_{3}\)) & 1458 & 1456 & [63] \\ \(\
u\)(CO) & 1508 & 1498 & [63] \\ \(\
u\)(CC) & 1597 & 1595 & [63] \\ \(\
u\)(CC) & 1617 & – & [63] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Assignments of characteristic infrared bands of pure G and of G/CeO\({}_{2}\) (0.6 mmol/g). A wide intense band, which in the spectra of pure acid has two maxima at 1284 cm\({}^{-1}\) and 1299 cm\({}^{-1}\), occurs mainly as a result of vibrations of carboxyl and aromatic COH groups ((COH)\({}_{\alpha\beta}\)) of acid [15; 65]. In the spectra of the VA/CeO\({}_{2}\) samples, it undergoes significant transformations; instead of two maxima, a peak appears at 1288 cm\({}^{-1}\) and a shoulder in the 1275 cm\({}^{-1}\) region. From this, we can conclude that both the carboxyl and phenolic groups can be involved in binding to the surface of CeO\({}_{2}\). According to studies of complexes of ferulic and caffe acids with the metal ions (Cu\({}^{2+}\), Al\({}^{3+}\), Na\({}^{+}\)) CuCl\({}_{2}\), AlCl\({}_{3}\), and Na [66], new absorption bands associated with the formation of bonds between aromatic ligands of these acids and metal ions can appear in the FT-IR spectra in the region from 1090 to 1300 cm\({}^{-1}\). Their position depends on the type of metal and the reagent ratio [66]. The high reactivity of phenolic OH groups of a number of carboxylic aromatic acids (vanillic, gallic, and caffe) in interaction with cerium oxide was also recorded in [67] by UV-Vis spectroscopy. A number of signs indicating the formation of carboxylate complexes were detected in the VA/CeO\({}_{2}\) spectra. In particular, the absorption at 1686 cm\({}^{-1}\) (v(C = O)) and 920 cm\({}^{-1}\) (8(COH)) [15]) disappeared for concentrations of 0.1-0.3 mmol/g, and for higher concentrations the intensity of these bands were smaller compared to pure VA. At the same time, the broad bands appeared at \(\sim\)1410 cm\({}^{-1}\) (v\({}_{s}\)(COO\({}^{-}\))) and 1539 cm\({}^{-1}\) (v\({}_{as}\)(COO\({}^{-}\)). The presence in this part of the spectrum of absorption bands v(CC) (at 1523 cm\({}^{-1}\)--for  
Figure 3: Fourier transform-infrared (FT-IR) spectra of pure CeO\({}_{2}\) (a), samples of VA/CeO\({}_{2}\) with different contents of VA (0.1: b; 0.3: c; 0.6: d; 0.9: e; 1.2 mmol/g: f) and pure VA (g).