χ-Fe5C2: Structure, Synthesis, and Tuning of Catalytic Properties

Iron carbides, especially Hagg carbide (chi-Fe5C2), have become a topic of significant research interest due to their potential application in various fields over the past decades. For Fischer-Tropsch (F-T) synthesis, chi-Fe5C2 has been confirmed as an active phase. In addition, this well-known catalytic material is a candidate for potential application in electrochemistry, magnetic imaging, and various therapies. The physical chemistry, including structure, stability, and catalytic properties of chi-Fe5C2 has been studied since its discovery. The C2/c crystal structure of Hagg carbide was initially resolved in the 1960s. Because various iron oxides and carbides always co-exist in the synthesized chi-Fe5C2 samples, the structure model still faces challenges. The crystal structure is being revised with high-purity samples using modern characterization techniques and theoretical methods. However, it is very difficult to obtain the pure phase of chi-Fe5C2 via traditional preparation methods owing to the metastable phase of chi-Fe5C2. Hence, tremendous efforts have been devoted to the synthesis of chi-Fe5C2. Recently, some processes to prepare single-phase and structure-controlled chi-Fe5C2 nanostructures have been reported. Many iron and carbon precursors can be used to prepare Hagg carbide. Carburization in solid-solid, solid-gas, and solid-liquid phases can be adopted to synthesize chi-Fe5C2 of various sizes and morphologies. The success of synthetic chemistry has provided novel insights into the mechanism of phase transformation in chi-Fe5C2. More details regarding the formation of the chi-Fe5C2 structure in the solid-gas and solid-liquid phases have been revealed via in situ characterization methods. The formation and crystallization of an Fe-C amorphous composite is likely the key step. The application of chi-Fe5C2 in catalysis has also benefited from novel synthesis strategies. With the development of these preparation methods, tuning the activity and selectivity of chi-Fe5C2 has become possible. A heterostructure of small Co/chi-Fe5C2 with low cobalt loading showed an unexpectedly high CO conversion rate at low temperature. Beyond classical F-T synthesis, chi-Fe5C2 is a promising catalyst for the production of light olefins, long chain alpha-olefins, aromatics, and alcohol synthesis by modification with other elements. Combining density functional theory (DFT) calculations and kinetic analysis, the roles of promoters and interaction with chi-Fe5C2 have been evaluated to some extent. Herein, the recent progress in the synthesis, structural analysis, formation mechanisms, and catalytic performance of chi-Fe5C2 is summarized. A collection of synthesis methods is presented, and novel methods for regulating catalytic properties are reviewed. We believe that advanced synthesis methods are key to a deeper understanding and better utilization of this material.