Catalyst Deactivation in Syngas Tar Cracking: A Multianalysis Study of Coke Deposition on γ‑Al₂O₃‑Supported Iron...

Catalyst deactivation caused by coke deposition remains the main limitation to the catalytic removal of tars in hot syngas cleaning. This study investigates the characteristics of coke layers deposited over two iron-based, alumina-supported catalysts. The first, Fe/γ-Al₂O₃, just doped with a market-available iron active phase; the second, RM/γ-Al₂O₃, uses red mud obtained as a by-product of the Bayer process for aluminum extraction. Catalytic cracking tests were carried out at 800 °C, without and with steam, by using naphthalene as a model tar component. A multitechnique approach combining FT-IR, Raman spectroscopy, and SEM-EDX analyses was applied to fresh and spent catalysts to assess the role of coke formation and its features in governing the activity and stability of catalysts for tar reforming. FT-IR spectroscopy provided qualitative insights into the presence and evolution of surface functional groups. Raman spectroscopy was used to quantify the relative distribution of amorphous and graphitic coke under different operating conditions. SEM-EDX analyzed surface morphology and elemental composition, providing visual indications of carbonaceous deposits. The overall results show that Fe/γ-Al₂O₃ achieves the largest hydrogen yields due to its higher iron loading, even though, when steam was not used, it undergoes severe deactivation caused by amorphous and graphitic coke deposition. RM/γ-Al₂O₃ exhibits a lower hydrogen production but a slower deactivation, likely related to the high content of alkali and alkaline earth metals on its surface, which reduces coke deposits. The results provide useful information for designing more reliable catalysts for tar syngas reforming.