# Exploring CeO\({}_{2}\)-Doped Co/SBA-15 Catalysts for Acetic Acid Oxidative Steam Reforming
## 3 Results and Discussion
### Characterization of Used Catalysts After Stability Tests  
Coke formation is widely recognized as a main deactivation factor in reforming catalysts, along with sintering and/or oxidation [66]. Thus, the used catalysts, after 50 h of time-on-stream, were characterized through XRD, TEM, Raman spectroscopy, and TGA techniques. Figure 9 displays the XRD patterns of the two spent catalysts. The presence of a new diffraction feature at 20 = 26.31\({}^{\circ}\) is noticeable, as compared with the calcined and reduced catalysts, attributed to graphitic carbon (JCPDS 01-074-2329), in both catalysts. The Co/20 CeO\({}_{2}\)-SBA-15 catalyst displayed a more intense diffraction peak, suggesting that a higher coke deposition was taking place during the reaction. Furthermore, despite the oxidizing atmosphere during the oxidative steam reforming reaction, no Co\({}_{3}\)O\({}_{4}\) peaks were observed in either sample, indicating that the active Co\({}^{0}\) species did not oxidize after 50 h of reaction. Similar to the calcined and reduced catalysts, the mean crystallite sizes of metallic Co species were calculated using the Scherrer equation, with mean crystallite sizes of 5.0 for Co/10 CeO\({}_{2}\)-SBA-15 and 4.2 nm for Co/20 CeO\({}_{2}\)-SBA-15 obtained. Taking into account the crystallite sizes of the reduced catalysts (Table 1), a small increase in the mean crystallite size can be inferred, suggesting a moderate sintering effect; according to [PERSON] et al. [66], the sintering route is significantly affected by the size of the crystal. The mass loss profiles together with the thermogravimetric derivative curves of each spent catalyst are displayed in Figure 10. In both samples, the mass percentage started decreasing when the temperature reached around 450 \({}^{\circ}\)C, with most of the weight loss in the temperature range 450-600 \({}^{\circ}\)C. The coke deposition rate was calculated as 7.2 mg\({}_{\text{Coke}}\)-\(\text{\text{\text{\  
literature, these peaks correspond to the oxidation of deposited carbon nanofilaments with differing ordering degrees. At low temperatures (460-480 \({}^{\circ}\)C), filamentous carbon species oxidation took place, which was related to the coke deposition on the metal surface. At higher temperatures (550-570 \({}^{\circ}\)C), carbon species were described as aged filamentous carbon, which would be deposited on the catalyst support [51; 67]. Coke deposited on the metallic sites would block the active sites directly, affecting the catalytic activity. In contrast, coke deposited on the support would mainly affect the reactants' diffusion. On both catalysts, the coke deposited on the support (higher temperature peak) was predominant. A higher intensity for Co/20 CeO\({}_{2}\)-SBA-15 was observed, indicating a high carbon total accumulation. The spent samples were also characterized by TEM, as shown in Figure 11. The SBA-15 porous mesostructure was preserved after carrying out the catalytic activity tests. Well-dispersed dark rounded particles can be appreciated, which correspond to Co\({}^{0}\) and CeO\({}_{2}\). Concerning coke deposition, nanofilamentous coke can be appreciated for both samples, in agreement with DTG results. According to the literature [66], this coke type is generally ascribed to transition metals, such as Co, Ni, or Fe. The particle size distribution histograms were obtained from TEM micrographs (Figure 11). The histograms show a shift in the particle size distribution to higher particle sizes in comparison with the reduced catalysts. Particularly, the Co/20 CeO\({}_{2}\)-SBA-15 catalyst presents a higher frequency of particles above 16 nm in size, deviating with respect to the Gaussian distribution profile. These higher particle sizes are related to sintering phenomena caused by the coke filament formation. These filaments would cause the detachment of the Co\({}^{0}\) crystals from the mesoporous structure, favoring their agglomeration [21; 66]. Finally, Figure 12 displays the Raman spectra of the spent catalysts.