Acta Physico-Chimica Sinica ›› 2020, Vol. 36 ›› Issue (1): 1906087.doi: 10.3866/PKU.WHXB201906087

Special Issue: Special Issue in Honor of Academician Youqi Tang on the Occasion of His 100th Birthday

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

Huabo Zhao1,Ding Ma2,*()   

  1. 1 National institute of Low Carbon and Clean Energy, Beijing 102211, P. R. China
    2 College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China
  • Received:2019-06-27 Accepted:2019-08-19 Published:2019-08-22
  • Contact: Ding Ma E-mail:dma@pku.edu.cn
  • Supported by:
    the National Key Research and Development Program of China(2017YFB0602500)

Abstract:

Iron carbides, especially Hägg carbide (χ-Fe5C2), have become a topic of significant research interest due to their potential application in various fields over the past decades. For Fischer-Trö psch (F-T) synthesis, χ-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 χ-Fe5C2 has been studied since its discovery. The C2/c crystal structure of Hägg carbide was initially resolved in the 1960s. Because various iron oxides and carbides always co-exist in the synthesized χ-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 χ-Fe5C2 via traditional preparation methods owing to the metastable phase of χ-Fe5C2. Hence, tremendous efforts have been devoted to the synthesis of χ-Fe5C2. Recently, some processes to prepare single-phase and structure-controlled χ-Fe5C2 nanostructures have been reported. Many iron and carbon precursors can be used to prepare Hägg carbide. Carburization in solid-solid, solid-gas, and solid-liquid phases can be adopted to synthesize χ-Fe5C2 of various sizes and morphologies. The success of synthetic chemistry has provided novel insights into the mechanism of phase transformation in χ-Fe5C2. More details regarding the formation of the χ-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 χ-Fe5C2 in catalysis has also benefited from novel synthesis strategies. With the development of these preparation methods, tuning the activity and selectivity of χ-Fe5C2 has become possible. A heterostructure of small Co/χ-Fe5C2 with low cobalt loading showed an unexpectedly high CO conversion rate at low temperature. Beyond classical F-T synthesis, χ-Fe5C2 is a promising catalyst for the production of light olefins, long chain α-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 χ-Fe5C2 have been evaluated to some extent. Herein, the recent progress in the synthesis, structural analysis, formation mechanisms, and catalytic performance of χ-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.

Key words: χ-Fe5C2, Fischer Tröpsch synthesis, Materials synthesis, In situ characterization, Promoter

MSC2000: 

  • O643.3