Synthesis, structure, and performance of carbide phases in Fischer-Tropsch synthesis: A critical review

Iron carbides are of burgeoning interest in both fundamental scientific research and fuel technology due to their electronic, magnetic, and catalytic properties. They are arguably connected with the active phase in iron-based Fischer-Tropsch Synthesis (FTS). In FTS, different phases of iron carbide have been identified, including Hägg carbide (χ-Fe5C2), pseudo-hexagonal iron carbide (ἐ-Fe2.2C), hexagonal iron carbide (ɛ-Fe3C), cementite (θ-Fe3C), and Eckstrom-Adcock iron carbide (Fe7C3). However, despite decades of existence of FTS technology, the exact roles played by carbide phases have been the subject of intense debate due to the complex and dynamic nature of carbide phases. This drawback usually frustrates the establishment of a clear-cut structure-perfomance relationship. In light of these concerns, it is very imperative to critically review the challenges and various approaches aimed at demystifying their roles and improving their performance in FTS for sustainable production of clean fuels. First, the structures and essential properties of iron carbides were evaluated and recent research advances in the synthesis of iron carbides were discussed and compared. Second, the structural elucidations of carbide phases using in situ studies, their performances and thermodynamic stability in FT reaction were discussed. Finally, outlooks on future research directions are outlined. This review provides insights into understanding the evolution and performance of carbide phases in FTS, and motivating researchers to further contribute towards designing iron-based FTS catalysts with highly active and selective carbide phase as the predominant phase.