**4. Conclusions**

In this work, an alumina-supported cobalt–iron sample (15Co–30Fe/Al2O3) was prepared and used in the catalytic methane decomposition and regeneration process. In situ regeneration of spent catalysts was performed with different oxidizing forced periodic cycling at 180, 360, and 720 min. The X-ray powder diffraction results showed the presence of γ-Al2O3 phase, Fe3O4 magnetite and CoFe2O4 spinel in fresh catalyst, whereas SP-90 min spent catalyst provided only graphite, Fe<sup>0</sup> and CoC2 reflections. The X-ray powder diffraction profile of SP-720 min spent/regenerated catalyst exhibited a new allotropic phase identified as θ-Al2O3. Transmission electron microscopy of spent and spent/regenerated samples indicated the formation of filamentous multi-walled carbon nanotubes; these nanotubes were formed by large amounts of encapsulating carbon on the surface which played an important role in the explanation of the reaction mechanism. Furthermore, different carbon bonds, detected by laser Raman spectroscopy, revealed the presence of graphite carbon structure on the molecular level. The reduction peaks were ascribed to the reduction of Co2+ species to nanoparticles of Co<sup>0</sup> (below 450 ◦C) and Fe3+ → Fe2+ → Fe<sup>0</sup> (between 450 and 900 ◦C). The catalytic activities of spent/regenerated catalysts exhibited lower activity at a higher cycle number. Hydrogen yield was

affected by the oxidizing regeneration number (same trend as methane conversion); on the other hand, carbon deposit (calculated by atomic absorption spectroscopy) followed the reverse trend. Indeed, the formation of carbon encapsulating on the metallic surface could be limited by the Cads + H2 → CH4 reverse reaction. This observation has been confirmed by the hydrogasification of carbon deposit (in H2-TPR toward 700 ◦C) by releasing methane (C(s) + 2H2 → CH4). The authors believe that the main factors responsible for the catalyst deactivation are coke deposition and weak sintering of the metallic active phase (Co–Fe) which occurred during the catalytic methane decomposition reaction and regeneration process.

**Author Contributions:** A.S.A.-F., A.H.F., A.A.I., and A.E.A. carried out all experiments and characterization tests as well as shared in the analysis of the data and shared in the writing of the manuscript. Both S.B. and A.B. wrote the paper and shared data analysis. R.-L.A.-O. performed TGA measurements as well as sharing in the writing of the manuscript.

**Funding:** This research was funded by the Deanship of Scientific Research at King Saud University, Project No. RG-1435-078.

**Acknowledgments:** The authors would like to express their sincere appreciation to the Deanship of Scientific Research at King Saud University for its funding for this research group Project No. RG-1435-078.

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