Fischer-Tropsch Synthesis: Morphology, Phase Transformation, and Carbon-Layer Growth of Iron-Based Catalysts

Iron-based Fischer-Tropsch catalysts, which are applied in the conversion of CO and H2 into longer hydrocarbon chains, are historically amongst the most intensively studied systems in heterogeneous catalysis. Despite this, fundamental understanding of the complex and dynamic chemistry of the iron-carbon-oxygen system and its implications for the rapid deactivation of the iron-based catalysts is still a developing field. Fischer-Tropsch catalysis is characterized by its multidisciplinary nature and therefore deals with a wide variety of fundamental chemical and physical problems. This critical review will summarize the current state of knowledge of the underlying mechanisms for the activation and eventual deactivation of iron-based Fischer-Tropsch catalysts and suggest systematic approaches for relating chemical identity to performance in next generation iron-based catalyst systems (210 references).

The stability and reactivity of ϵ, χ, and θ iron carbide phases in Fischer-Tropsch synthesis (FTS) catalysts as a function of relevant reaction conditions was investigated by a synergistic combination of experimental and theoretical methods. Combined in situ X-ray Absorption Fine Structure Spectroscopy/X-ray Diffraction/Raman Spectroscopy was applied to study Fe-based catalysts during pretreatment and, for the first time, at relevant high pressure Fischer-Tropsch synthesis conditions, while Density Functional Theory calculations formed a fundamental basis for understanding the influence of pretreatment and FTS conditions on the formation of bulk iron carbide phases. By combining theory and experiment, it was found that the formation of θ-Fe(3)C, χ-Fe(5)C(2), and ϵ-carbides can be explained by their relative thermodynamic stability as imposed by gas phase composition and temperature. Furthermore, it was shown that a significant part of the Fe phases was present as amorphous carbide phases during high pressure FTS, sometimes in an equivalent amount to the crystalline iron carbide fraction. A catalyst containing mainly crystalline χ-Fe(5)C(2) was highly susceptible to oxidation during FTS conditions, while a catalyst containing θ-Fe(3)C and amorphous carbide phases showed a lower activity and selectivity, mainly due to the buildup of carbonaceous deposits on the catalyst surface, suggesting that amorphous phases and the resulting textural properties play an important role in determining final catalyst performance. The findings further uncovered the thermodynamic and kinetic factors inducing the ϵ-χ-θ carbide transformation as a function of the carbon chemical potential μ(C).