## 3 Results and Discussion
### Thermal Transformations of Model Lignin Compounds on the Surface of CeO\({}_{2}\)
#### 3.2.2 Guaiacol  
Pyrolysis of guaiacol on the surface of nanoceria proceeded similarly to pyrolysis of pyrocatechol (Figure 7). Namely, there was no desorption of guaiacol in molecular form (M.r. = 124 Da, \(m/z\) 124); the formation of aliphatic products (\(m/z\) 99, 85, 71, 57, 43) occurred in the same temperature range. Consequently, their formation was probably the result of transformations of similar surface complexes P and G on the nanoceria surface. It could also be phenolate complexes, which were confirmed above using IR spectroscopy.  
The presence of peaks on the TPD curves for ions with \(m/z\) 31, 32 (CH\({}_{3}\)OH) and \(m/z\) 94 (PhOH) indicates the presence of an additional stage of pyrolysis, probably as a result of thermal transformations of G-complexes bound to the surface through the methoxyl group, the formation of which was evidenced by a number of changes recorded in the absorption of the methoxyl group during the study of these samples by the FT-IR spectroscopy (Figure 2 and Table 2).  
A peak at \(\sim\)358 \({}^{\circ}\)C on the TPD curve for the ion with \(m/z\) 78 (benzene) was recorded as well as a minor release of the product with \(m/z\) 94 (hydroxybenzene). The intensity of the latter was low. These peaks could be associated with the decomposition of G-complexes, which were formed due to the interaction of both active groups of the aromatic ring with two active centers of the cerium oxide surface. The existence of bidentate structures on the oxide surface was evidenced by the disappearance of absorption at 1363 cm\({}^{-1}\) (\(\beta\)(COH)) and the appearance of new bands at 1288 and 1323 cm\({}^{-1}\) in the IR spectra of the G/CeO\({}_{2}\) samples (Figure 2). The work [35] confirms the possibility of forming such G-complexes on the CeO\({}_{2}\) surface. It was found that the hydrodeoxygation of guaiacol over CeO\({}_{2}\)-ZrO catalysts required two oxygen vacancies [35].  
Intense peaks on the TPD curves of ions with \(m/z\) 28 (C\({}_{2}\)H\({}_{4}\)), \(m/z\) 14 (CH\({}_{2}\)), \(m/z\) 12 (C), and \(m/z\) 3 (H) at 225 \({}^{\circ}\)C were observed during thermal decomposition of the G/CeO\({}_{2}\) samples (Figure 7B). The presence of these peaks indicates intense desorption of ethylene  
Figure 7: Thermal decomposition of G/CeO\({}_{2}\) (0.6 mmol/g); mass spectra at 120 \({}^{\circ}\)C and vapor pressure measured as a function of temperature for the G/CeO\({}_{2}\) sample (**A**); TPD curves for ions with \(m/z\) 3, 18, 28, 31, and 44 (**B**); for ions with \(m/z\) 43, 57, 71, 85, and 99 (**C**); for ions with \(m/z\) 3, 12, 15, 31, 50, 78, and 94 (**D**).  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Sample** & **Stage** & **T\({}_{\text{max}}\)** (\({}^{\circ}\)**C**) & **Volatiles (\%)** & **Char (\%)** \\ \hline \multirow{3}{*}{P/CeO\({}_{2}\)} & I & 85 & 8.4 & \\  & II & 224 & 72.3 & 19.3 \\ \cline{1-1}  & \(\Sigma\)(I + II) & & 80.7 & \\ \hline \end{tabular}
\end{table}
Table 5: Pyrolysis yields for TGA pyrolysis of P/CeO2 (0.6 mmol/g).  
(C\({}_{2}\)H\({}_{4}\), M.r. = 28, \(m/z\) 28, 14, 12), that can serve as confirmation of the alkylation processes of the nanoceria surface, which occurs during the pyrolysis of G. The data presented in [35] confirm the possibility of alkylation of the oxide surface during pyrolysis of G.  
The DTG/DTA/TG study results of the G/CeO\({}_{2}\) pyrolysis are presented in Figure 8. The thermal decomposition of the sample occurred up to 400 \({}^{\circ}\)C. On the DTG curve, we can distinguish two stages of weight loss for the G/CeO\({}_{2}\) sample. The first stage corresponded to T\({}_{\text{max}}\)\(\sim\)100 \({}^{\circ}\)C, the second to T\({}_{\text{max}}\)\(\sim\)180 \({}^{\circ}\)C. All stages were exothermic. The maximum weight loss occurred in the second stage at \(\sim\)180 \({}^{\circ}\)C (Table 6).  
Analysis of TPD MS and DTG data of the G/CeO\({}_{2}\) samples (Table 6 and Figures 7 and 8) may indicate that the greatest weight loss occurs due to the decomposition of the G-complexes formed through the methoxyl group.