## 3 Results and Discussion
### Thermal Transformations of Model Lignin Compounds on the Surface of CeO\({}_{2}\)  
#### 3.2.1 Pyrocatech  
The study of thermal transformations of P/CeO\({}_{2}\) samples by the TPD MS method is presented in Figure 5. According to thermograms and the P/T-curve (Figure 5), the thermal decomposition of P/CeO\({}_{2}\) occurred in several stages at the range of 50-750 \({}^{\circ}\)C. At the same time, no desorption of pyrocatechol in molecular form (M.r. = 110 Da, \(m/z\) 110) was observed over the entire temperature range studied. This indicates that pyrocatechol binds to the surface of nanoceria, and its pyrolysis occurs due to the transformation of surface complexes. This is confirmed by the data of IR spectroscopy, according to which changes in the absorption of COH groups and the appearance of a new band at 1297 cm\({}^{-1}\) indicate chemisorption of pyrocatechol. In this case, one feature was observed: the peaks at a temperature of about 120 \({}^{\circ}\)C on the TPD curves for aliphatic series ions \(m/z\) 99, 85, 71, 57, and 43. According to [76], such a set of ions is characteristic of aliphatic compounds, namely, alkyl derivatives, alicyclic alcohols (\(m/z\) 99 (C\({}_{6}\)H\({}_{11}\)O), \(m/z\) 85 (C\({}_{6}\)H\({}_{12}\)), \(m/z\) 71 (C\({}_{5}\)H\({}_{11}\)), \(m/z\) 57 (C\({}_{4}\)H\({}_{9}\)), and \(m/z\) 43 (C\({}_{3}\)H\({}_{7}\))). The formation of aliphatic products can result from the decomposition of phenolate complexes of pyrocatechol. It is known that the Ar-OH bond is one of the strongest types of C-O bonds, for example, for guaiacol and other phenol derivatives [35]. It is believed that, before breaking, this bond must first be weakened by hydrogenating the aromatic ring [43,77,78]. According to [43], deoxygenation in the presence of the catalysts CeO\({}_{2}\) and CeO\({}_{2}\)-ZrO\({}_{2}\) occurs more easily from the saturated cycle than from the unsaturated cycle. This is due to the dissociation energy of the CO bond in alcohols, which decreases in the following order: aromatic alcohol (469 kJ/mol), secondary alcohol (385 kJ/mol), primary alcohol (383 kJ/mol), tertiary alcohol (379 kJ/mol) [43]. We do not rule out the possibility of such a process on the surface of CeO\({}_{2}\), because during the study, we recordedan intense signal of hydrogen evolution (\(m/z\) 3) (Figure 5B). The formation of aliphatic products was likely due to complex redox processes on the surface of nanoceria. However, the intensity of these processes was low. The main processes were of deep destruction of pyrocatech. This conclusion can be made by comparing the intensities of TPD peaks of the aliphatic series with TPD peaks, which characterize the processes of dehydration (\(m/z\) 18), decarboxylation (\(m/z\) 44), and desorption of CO and C\({}_{2}\)H\({}_{4}\) (\(m/z\) 28) (Figure 5C,D). The strong influence of the interactions with catalyst surface on the structure of the pyrocatech molecule was also indicated by the obtained IR spectra. Figure 1 shows that significant changes occurred not only in the absorption of COH groups, through which the interaction occurs, but also in the absorption of the aromatic ring. In particular, the absorption at 1471 cm\({}^{-1}\) shifted by 12 cm\({}^{-1}\) to the high-frequency region, and the absorption at 1603 and 1620 cm\({}^{-1}\) was almost invisible for small concentrations (0.1-0.3 mmol/g). Figure 6 and Table 5 show the results of a DTG/DTA/TG study of CeO\({}_{2}\) and P/CeO\({}_{2}\) samples. The main weight loss of the CeO\({}_{2}\) sample is recorded in the temperature range 20-200 \({}^{\circ}\)C, which was probably associated with the desorption of water from the oxide surface. The decomposition of the G/CeO\({}_{2}\) sample, according to the DTG curve (Figure 6A), proceeded in two stages: 100-150 \({}^{\circ}\)C and 150-430 \({}^{\circ}\)C. All stages were exothermic. The main weight loss occurred in the second stage.