(605d) Fabrication of Fe2c Embedded in Hollow Carbon Spheres: A High-Performance and Stable Catalyst for Fischer-Tropsch Synthesis

Fischer–Tropsch synthesis (FTS) is a flexible route for production of fuels and key chemicals (e.g. lower olefins) from syngas, which is derived from non-petroleum sources, such as natural gas, coal or biomass. Although trigonal prismatic carbides (TP carbides, e.g. Fe₅C₂ and Fe₃C) have been intensively investigated as active species, few researches have focused on exploitation of Fe₂C (a kind of octahedral iron carbides) in FTS, owing to its thermodynamic instability at high temperature. Herein, we proposed a protocol to fabricate Fe₂C embedded in hollow carbon spheres (HCS) catalysts and investigate them at high-temperature FTS conditions. The catalysts were obtained by pyrolyzing the coated polymer and Fe(NO₃)₃ on silica spheres and then etching the hard template. XRD, XPS, TEM and N₂ physical adsorption etc. were employed to characterize the evolution and properties of as-prepared catalysts. The results showed that the pyrolysis temperature significantly influenced the nature of Fe species, including phase, particle size and location. Interestingly, we found that the Fe₂C nano-crystalline as the dominant Fe species formed during the reaction and existed stably without transformation to TP carbides under high-temperature FTS conditions, owing to the specific chemical surroundings by carbon matrix. Moreover, the confinement effects of carbon matrix provided a good dispersion of Fe species and prevented them from migration and agglomeration. Therefore, the obtained catalysts exhibit good dispersion, robustness of geometric construction, and resistance to sintering. These peculiarities of Fe/HCS contributed to the good and stable activity in CO hydrogenation. Furthermore, the product selectivity was tuned by changing inner diameter of the HCS catalysts. As the curvature increased, the selectivity of CH₄ declined with increased O/P ratio, C₂~C₄ olefins and C₅₊ hydrocarbons, because of an improved CO/H₂ ratio inside the carbon hollow structure. This work inspires new strategy to adjust and stabilize the phase and particle size of iron carbides and provides a new idea to fabricate carbon nanoreactor with confinement effect that influence not only activity but also product distribution in FTS.