Two-Dimensional Graphene-Assisted Formation of the Cylindrical Iron Carbide Nanocapsules for Fischer-Tropsch Synthesis

Iron-based Fischer-Tropsch synthesis provides a promising alternative as a direct and non-petroleum route for producing olefins from syngas (CO + H₂). In general, the promoter is required to regulate the activity and olefins selectivity, and the major challenges come from the catalyst deactivation due to the carbon deposition and sintering at drastic reaction conditions (340 ^oC, high pressure). In this study, we show a totally different strategy for solving these problems. We find that the originally large spherical Fe₂O₃ particles of around 120 nm assisted by graphene without additional promoter can gradually evolve during the FTS reaction, forming much smaller cylindrical Hägg carbide nanocapsules of about 10 nm with single (510) crystal facet that displays excellent activity of nearly 40 times of the unmodified one, olefins selectivity of 76% (84% if methane is not considered) and high stability. In contrast, the original Fe₂O₃ catalyst and carbon-containing polymer material (e.g. PAA, PVP) modified Fe₂O₃ catalysts exhibit significantly lower selectivity to olefins, higher selectivity to methane and deactivate rapidly. Different from the above mentioned peculiar evolution of iron particles, the iron particles over these spent catalysts aggregate into larger ones having multiple crystal facets, e.g. χ-Fe₅C₂ (510), and χ-Fe₅C₂ (-202), and at the same time serious carbon deposition is observed, which mainly accounts for the catalyst deactivation. In the case of activated carbon and carbon nanofibers supported iron catalysts with initially small iron particles below 10 nm, irregular morphology and multiple crystal facets of iron carbides are also observed over the spent catalysts. The DFT calculations demonstrate that χ-Fe₅C₂ (510) crystal facet is more active and favorable for the formation of olefins than χ-Fe₅C₂ (-202) crystal facet, which is well in accordance with our experimental results. The presented methodology provides a new strategy to control the morphology and crystal facet of iron carbide during the catalytic reaction for solving the toughest problems in heterogeneous catalysis.