*3.8. Long-Term Stability Test*

Generally, catalyst stability in POM is greatly influenced by deactivation resulting from sintering, metal agglomeration, carbon deposition, and the disappearance of active sites due to oxidation at reaction conditions. Usually, these deactivation effects occur simultaneously, but sometimes one of them predominates. Among the catalysts used in this study, Co-800 showed best results and so it was selected for a prolonged activity test at 800 ◦C for 24 h (Figure 9). It is worth to mention that the catalyst maintained stable activity throughout the complete run. *Processes* **2019**, *7*, 141 12 of 15

**Figure 9.** CH4 conversion, CO selectivity and H2/CO ratio over Co-800 (5%Co/Al2O3–ZrO2) catalyst calcined at 800 °C over 24 h on stream in POM at 800 °C. **Figure 9.** CH<sup>4</sup> conversion, CO selectivity and H2/CO ratio over Co-800 (5%Co/Al2O3–ZrO<sup>2</sup> ) catalyst calcined at 800 ◦C over 24 h on stream in POM at 800 ◦C.

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The stable activity may be attributed to the presence of ZrO2 that leads to coke suppression as revealed by TGA and TPO (Figure 10). The presence of ZrO2 imparts two advantages to the catalysts: (i) It renders basic character (Figure 5) to the catalysts which in turn makes it capable of activating CO2 (CO2 → CO + O\*) because it enhances the dissociative chemisorption of CO2 in metal/ZrO2 interface; (ii) it suppresses the carbon deposition as an outcome of its higher oxygen storage capacity which provides more active oxygen species by redox activity (C\* + O\* → CO). This is the reason why the catalysts showed very low coking, making them long-term stable. Similar studies were conducted using Pt/Al2O3–ZrO2 and Ni/Al2O3–ZrO2 catalysts; higher activity and stability to syngas were reported [37,38]. This behavior is due to the rise in capacity of dissociative chemisorption of CO2 over Pt-ZrO2 and Ni-ZrO2. Therefore, based on the stability analysis, it can be concluded that the catalyst operated at 800 °C was more stable than the one tested at 700 °C (Figure 8a–8d). The stable activity may be attributed to the presence of ZrO<sup>2</sup> that leads to coke suppression as revealed by TGA and TPO (Figure 10). The presence of ZrO<sup>2</sup> imparts two advantages to the catalysts: (i) It renders basic character (Figure 5) to the catalysts which in turn makes it capable of activating CO<sup>2</sup> (CO<sup>2</sup> → CO + O\*) because it enhances the dissociative chemisorption of CO<sup>2</sup> in metal/ZrO<sup>2</sup> interface; (ii) it suppresses the carbon deposition as an outcome of its higher oxygen storage capacity which provides more active oxygen species by redox activity (C\* + O\* → CO). This is the reason why the catalysts showed very low coking, making them long-term stable. Similar studies were conducted using Pt/Al2O3–ZrO<sup>2</sup> and Ni/Al2O3–ZrO<sup>2</sup> catalysts; higher activity and stability to syngas were reported [37,38]. This behavior is due to the rise in capacity of dissociative chemisorption of CO<sup>2</sup> over Pt-ZrO<sup>2</sup> and Ni-ZrO2. Therefore, based on the stability analysis, it can be concluded that the catalyst operated at 800 ◦C was more stable than the one tested at 700 ◦C (Figure 8a–d).
