*3.9. Post (Long Term Test) Characterizations*

*3.9. Post (Long Term Test) Characterizations*  Temperature-programmed oxidation (TPO) was conducted to characterize the nature of coke deposit over Co-800 catalyst after the long-term POM test (Figure 10a). Zhang investigated the TPO profiles for reforming reaction and assigned three peaks as Cα (150–220 °C), Cβ (530–600 °C) and Cγ (~650 °C) whereas the peak above 700 °C might indicate the oxidation of graphitic/inactive carbon [39]. We applied this model to our catalysts. As per TPO profile, the intensity maxima of Cα was found at 293 °C corresponding to the most active carbon which is responsible for the transformation Temperature-programmed oxidation (TPO) was conducted to characterize the nature of coke deposit over Co-800 catalyst after the long-term POM test (Figure 10a). Zhang investigated the TPO profiles for reforming reaction and assigned three peaks as Cα (150–220 ◦C), Cβ (530–600 ◦C) and Cγ (~650 ◦C) whereas the peak above 700 ◦C might indicate the oxidation of graphitic/inactive carbon [39]. We applied this model to our catalysts. As per TPO profile, the intensity maxima of Cα was found at 293 ◦C corresponding to the most active carbon which is responsible for the transformation into synthesis gas. The maximum at 593 ◦C represents Cβ which may be attributed

agreement with TEM images (Figure 6c). When TGA (Figure 10b) was performed after the test over 24 h with Co-800 catalyst at 800 °C, it was found that there was insignificant (˂1%) rise in the coke amount on the catalyst surface. The low amount of carbon may be attributed to the much amount of

active and amorphous carbon type which is also registered by TPO.

into synthesis gas. The maximum at 593 °C represents Cβ which may be attributed to intermediate amorphous carbon and could be transformed into CO at high temperature. Finally, the peak at 665 to intermediate amorphous carbon and could be transformed into CO at high temperature. Finally, the peak at 665 ◦C possessing the lowest intensity may be ascribed to Cγ, an inert carbon intermediate which is transformed into filamentous or graphitic features. The intensity of the signal for the most active carbon (Cα) is higher which implies that these species are predominant. These findings are in agreement with TEM images (Figure 6c). When TGA (Figure 10b) was performed after the test over 24 h with Co-800 catalyst at 800 ◦C, it was found that there was insignificant (<1%) rise in the coke amount on the catalyst surface. The low amount of carbon may be attributed to the much amount of active and amorphous carbon type which is also registered by TPO. *Processes* **2019**, *7*, 141 13 of 15

**Figure 10.** (**a**) Temperature-programmed oxidation and (**b**) TGA for Co-800 (5%Co/Al2O3-–rO2) catalyst calcined at 800 °C after the long-term POM test at 800 °C. **Figure 10.** (**a**) Temperature-programmed oxidation and (**b**) TGA for Co-800 (5%Co/Al2O3—rO<sup>2</sup> ) catalyst calcined at 800 ◦C after the long-term POM test at 800 ◦C.

#### **4. Conclusions 4. Conclusions**

119.

The obtained results show that the ZrO2–Al2O3-supported Ni and/or Co catalysts for syngas production via partial oxidation exhibit a high surface area. Co/Al2O3–ZrO2 catalysts demonstrated superior catalytic performance, giving high methane conversion and selectivity to CO and H2 at 700 °C and reached up to 100% selectivity to H2 and 84% methane conversion at 800 °C. Increasing the calcination temperature from 550 °C to 800 °C resulted in strong metal-support interaction which endowed resistance against sintering. The presence of ZrO2 in the binary oxide enhanced the surface area and number of basic sites in the catalysts. Several factors can assist to obtain stable and active catalysts as the presence of basic sites by addition of ZrO2-facilitated CO2 dissociation, generation of oxygen intermediates, and removal of deposited carbon over the catalyst surface. Furthermore, the effect of calcination at a higher temperature of 800 °C stabilizes high dispersion of Ni and/or Co on the support, thereby avoiding metal agglomeration which in turn improved coke resistance. Eventually, monometallic Co-based catalyst calcined at 800 °C was found to have the highest activity but not Ni-based catalyst, which is unexpected. On the other hand, bimetallic Ni–Co-550 showed highest activity at low calcination temperature. Finally, increasing the calcination and reaction temperatures led to higher activity but posed no adverse effects on stability. It is worth mentioning that Co-800 catalyst used at 800 °C was found to have excellent stability over 24 h on stream. Recently, Dedov and co-workers utilized neodymium-calcium cobaltate-based catalysts for syngas production via partial oxidation of methane by using a fixed-bed flow reactor [17]. They reportedly attained 85% methane conversion and selectivity of CO and H2 close to 100% at very high temperature (925 °C). Another study used Ni(Co)–Gd0.1Ti0.1Zr0.1Ce0.7O2 catalyst at 900 °C for the production of syngas via partial oxidation of methane [14]. They obtained 80–90% methane conversion, 85–95% selectivity for CO and 79% selectivity for H2. Methane conversion was somewhat higher but the selectivity to CO The obtained results show that the ZrO2–Al2O3-supported Ni and/or Co catalysts for syngas production via partial oxidation exhibit a high surface area. Co/Al2O3–ZrO<sup>2</sup> catalysts demonstrated superior catalytic performance, giving high methane conversion and selectivity to CO and H<sup>2</sup> at 700 ◦C and reached up to 100% selectivity to H<sup>2</sup> and 84% methane conversion at 800 ◦C. Increasing the calcination temperature from 550 ◦C to 800 ◦C resulted in strong metal-support interaction which endowed resistance against sintering. The presence of ZrO<sup>2</sup> in the binary oxide enhanced the surface area and number of basic sites in the catalysts. Several factors can assist to obtain stable and active catalysts as the presence of basic sites by addition of ZrO2-facilitated CO<sup>2</sup> dissociation, generation of oxygen intermediates, and removal of deposited carbon over the catalyst surface. Furthermore, the effect of calcination at a higher temperature of 800 ◦C stabilizes high dispersion of Ni and/or Co on the support, thereby avoiding metal agglomeration which in turn improved coke resistance. Eventually, monometallic Co-based catalyst calcined at 800 ◦C was found to have the highest activity but not Ni-based catalyst, which is unexpected. On the other hand, bimetallic Ni–Co-550 showed highest activity at low calcination temperature. Finally, increasing the calcination and reaction temperatures led to higher activity but posed no adverse effects on stability. It is worth mentioning that Co-800 catalyst used at 800 ◦C was found to have excellent stability over 24 h on stream. Recently, Dedov and co-workers utilized neodymium-calcium cobaltate-based catalysts for syngas production via partial oxidation of methane by using a fixed-bed flow reactor [17]. They reportedly attained 85% methane conversion and selectivity of CO and H<sup>2</sup> close to 100% at very high temperature (925 ◦C). Another study used Ni(Co)–Gd0.1Ti0.1Zr0.1Ce0.7O<sup>2</sup> catalyst at 900 ◦C for the production of syngas via partial oxidation of methane [14]. They obtained 80–90% methane conversion, 85–95% selectivity for CO and 79% selectivity for H2. Methane conversion was somewhat higher but the selectivity to CO and H<sup>2</sup>

**Funding:** The research is funded by Deanship of Scientific Research at King Saud University project No. RGP-

and H2 was still lower even at a higher temperature. Based on the activity of catalysts reported in

previous studies, our catalysts showed higher activity and selectivity at lower temperature.

shared data analysis. H.S. and A.A.I. contributed in writing the paper and edited it.

was still lower even at a higher temperature. Based on the activity of catalysts reported in previous studies, our catalysts showed higher activity and selectivity at lower temperature.

**Author Contributions:** A.S.A.-F., A.H.F. and Y.A. carried out all experiments and characterization tests as well as shared in the analysis of the data and writing of the manuscript. U.A., H.A. and A.E.A. wrote the paper and shared data analysis. H.S. and A.A.I. contributed in writing the paper and edited it.

**Funding:** The research is funded by Deanship of Scientific Research at King Saud University project No. RGP-119.

**Acknowledgments:** The authors would like to extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for funding this research group No. RGP-119.

**Conflicts of Interest:** The authors declare no conflict of interest.
