## 3 Results and Discussion  
### FT-IR Spectroscopic Studies  
#### 3.1.1 Pyrocatech  
FTIR spectra of CeO\({}_{2}\), pure P, and samples P/CeO\({}_{2}\) are presented in Figure 1. Interpretation of the obtained results, performed on the basis of experimental data [51; 52] and the results of quantum chemical calculations of the frequencies of normal vibrations of pyrocatechol in the crystalline and gaseous states [53], are presented in Table 1. The symbol \(\
u\) is used to denote valence vibrations, \(\beta\) for out-of-plane deformation vibrations, and \(\delta\) for non-planar deformation vibrations. Figure 1: Fourier transform-infrared (FT-IR) spectra of pure CeO\({}_{2}\) (a), samples of P/CeO\({}_{2}\) with different contents of FA (0.1: b; 0.3: c; 0.6: d; 0.9: e; 1.2 mmol/g: f) and pure P (g). According to [53], most of the bands of this dihydroxybenzene have a mixed shape. Figure 1 shows significant changes in the P/CeO\({}_{2}\) spectra compared with pure P (Figure 1, Table 1), which is the result of the interaction of P molecules with oxide. The absorption bands at 721 and 937 cm\({}^{-1}\), which correspond to C-H stretching vibrations [53] of pure pyrocatechol molecules in the crystalline state, are a sign of association due to intermolecular hydrogen bonds [53]. The disappearance of these absorptions for P/CeO\({}_{2}\) indicates the destruction of this association structure of P due to interaction with the oxide surface. The obtained spectra of the studied samples contain several other features indicating the interaction of OH groups of pyrocatechol with CeO\({}_{2}\). In particular, one of these signs is a significant decrease in the intensity of the bands 756 (\(\delta\)(OH) + \(\delta\)(CH)) and 769 cm\({}^{-1}\) (\(\
u\)(CC) + \(\
u\)(CO) + \(\beta\)(CCC)) [53]. There are bands 849 and 859 cm\({}^{-1}\) (Figure 1) for pyrocatechol in the region of 800-900 cm\({}^{-1}\), which according to [48; 53], are mixed and associated with non-planar deformation vibrations of CH, deformation vibrations of CC, as well as vibrations of COH groups. For P/CeO\({}_{2}\) samples, changes in this region of the spectrum are observed due to the interaction of OH groups of P with the oxide surface. Absorption at 1041 cm\({}^{-1}\) for pyrocatechol is mainly due to the stretching vibrations of the CC, and 1095 cm\({}^{-1}\) is associated with vibrations of CC, CH, and COH groups. For P/CeO\({}_{2}\) samples, these bands are shifted toward lower frequencies, and their relative intensity decreases. In addition, for P/CeO\({}_{2}\) samples, changes were found in the range 1150-1400 cm\({}^{-1}\). This region's absorption mainly corresponds to stretching and bending vibrations of COH [51; 53]. Analysis of the IR spectra of pyrocatechol in the crystalline state and the gas phase [48] shows that the ultrathin structure of the bands in this region is associated with the formation of intermolecular and intramolecular bonds. From Figure 1, it is seen that the maximum of 1167 cm\({}^{-1}\), which is due mainly to \(\
u\)(CO) vibration [53], essentially disappears for all concentrations of P/CeO\({}_{2}\). At the same time, the intensity of the bands at 1188 and 1365 cm\({}^{-1}\), which partially corresponds to the \(\beta\)(COH) vibrations [51; 53], decreases significantly. Instead of a wide band with three peaks at 1242, 1255 (\(\beta\)(COH) [51; 53]), and 1281 cm\({}^{-1}\) (\(\
u\)(CO)) [51; 53], for lower concentrations, one maximum was detected at 1261 cm\({}^{-1}\), and only when the concentration of P increases to 0.6 mmol/g, a peak appears at 1273 cm\({}^{-1}\).