Stability and Reactivity of ϵ−χ−θ Iron Carbide Catalyst Phases in Fischer−Tropsch Synthesis: Controlling μ_C

Abstract

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).

Metal organic framework-mediated synthesis of highly active and stable Fischer-Tropsch catalysts

“…This remarkable difference is related to the different carbidization degree upon exposure to syngas. Moreover, the encapsulating carbon matrix seems to avoid oxidation of the active carbide phase under reaction conditions 37 . Mössbauer spectroscopy shows that catalysts prepared by the MOFMS approach offer an intimate contact between Fe and C, favouring the formation of w À Fe 5 C 2 , which is well known to be the most active phase in the FTS process.…”

The effect of Fe–Rh alloying on CO hydrogenation to C2+ oxygenates

“…The analysis was completed by fitting the sample diffraction to an appropriate model where the lattice constants, scale factor, peak profile functions, and atomic potentials were varied to produce a simulated diffraction pattern nearly identical to the experimental XRD data. The models were chosen based upon knowledge of synthesis, reaction conditions, and phases previously identified in similar studies, that is, Rh metal, Fe metal, Fe-Rh alloys (FeRh and Fe0.7Rh0.3), FeO, Fe2O3, Fe3C, Fe2C, Fe5C2, anatase TiO2, and rutile TiO2 [20][21][22][23][24][25][26][27][28][29][30][31][32]. A complete refinement provides information about phase quantification, lattice constants, and particle size.…”

“…According to the refinement results summarized in Table 3, the Fe metal content is reduced at the expense of Fe3C formation. Carburization of Fe is not unexpected as the reaction conditions for CO hydrogenation are very similar to Fischer-Tropsch synthesis (FTS) where Fe carbides are formed regardless of the initial phase of the catalyst [27,29,30]. Fe3C is generally accepted as a spectator or deactivation phase, and therefore, its contribution in the CO conversion and product distribution is not expected to be substantial [30,34,35].…”

ε-Iron carbide as a low-temperature Fischer–Tropsch synthesis catalyst

“…Pure w-Fe 5 C 2 nanoparticles (NPs) have been synthesized and supported, and proven to be better than a reduced haematite catalyst in terms of CO conversion and product selectivity 24 . In contrast, O-carbides are favoured in high carbon potential (m c ) surroundings, that is, low temperature (o473 K) and high CO partial pressure, with m c more sensitive to temperature than to pressure 25 . However, kinetic factors (lattice deformation, carbon diffusion) are adverse to their formation at low temperature.…”

“…Attractively, a recent theoretical study illustrated that the barriers on Fe 2 C for CO dissociation and hydrogenation are both the lowest amongst Fe 2 C(011), Fe 5 C 2 (010), Fe 3 C(001) and Fe 4 C(100) (not found in FTS 25 ) surfaces with carbon vacancies 26 . According to a volcano-plot for metal-CO bond strength versus FTS activity for iron and cobalt 27 , increasing the carbon content in iron carbide will weaken the Fe-CO bonding and hence enhance the activity.…”