Insights into Hägg Iron-Carbide-Catalyzed Fischer–Tropsch Synthesis: Suppression of CH 4 Formation and Enhancement of C–C Coupling on χ-Fe 5 C 2 (510)

Probing the product selectivity of Fischer–Tropsch catalysts is of prime scientific and industrial importance—with the aim to upgrade products and meet various end-use applications. In this work, the mechanisms for CH 4 formation and C 1 –C 1 coupling on a thermodynamically stable, terraced-like χ-Fe 5 C 2 (510) surface were studied by DFT calculations. It was found that this surface exhibits high effective barriers of CH 4 formation for the three cases (i.e., 3.66, 2.81, and 2.39 eV), indicating the unfavorable occurrence of CH 4 formation under FTS conditions. The C + CH and CH + CH are the most likely coupling pathways, which follow the carbide mechanism. Subsequently, the effective barrier difference between CH 4 formation and C 1 –C 1 coupling was used as a descriptor to quantify FTS selectivity. A comparison of the selectivity between this surface and the reported FTS catalysts’ surfaces was discussed in detail. More interestingly, this surface shows unexpectedly high C 2+ selectivity. This indicates that manipulating the crystal facet of χ-Fe 5 C 2 catalyst can effectively tune the FTS selectivity, which will open a new avenue for highly selective Fe-based FTS catalysts.