Extremely Low CO/H2 Ratio Activation of a Precipitated Iron Catalyst for Enhanced Fischer–Tropsch Performance

Fischer–Tropsch synthesis (FTS) is a non-petroleum and effective route to produce fuels and key chemicals from syngas. Precipitated iron catalyst, as an industrial catalyst for FTS has attracted much attention. The catalytic performance of iron catalysts strongly depends upon the activation progress, which is of significance to industrial application. Herein, we regulated the CO/H2 ratio of pretreatment gas and investigated its influence on a precipitated iron catalyst. The textural structure, phase compositions, and carbon deposit of the catalysts were analyzed systematically by X-ray diffraction, transmission electron microscopy, Mössbauer effect spectroscopy, Raman, X-ray photoelectron spectroscopy, and temperature-programmed reaction. It was found that activation with an extremely low CO/H2 ratio (3:100) achieves a balance of CO dissociation, permeation, and diffusion and facilitates the formation of χ-Fe5C2 species coated with a thin carbon layer. Such a catalyst exhibits high CO conversion of 68%, selectivity to C5+ hydrocarbons of 93.6%, and excellent stability under typical medium-temperature Fischer–Tropsch conditions (270 °C and 2.0 MPa). In contrast, pure H2 reduction is favorable to form ε′-Fe2.2C. With the increase of the CO/H2 ratio, the carbon deposition becomes more severe. By calculation of the intrinsic activity (TOF) of different iron carbides (ε′-Fe2.2C and χ-Fe5C2), we established the influence of the activation atmosphere on the FTS catalytic performance.