Oxidation of Hägg Carbide during High-Temperature Fischer–Tropsch Synthesis: Size-Dependent Thermodynamics and In Situ Observations

Abstract

Hagg carbide (χ-Fe 5 C 2 ) is considered to be the primary active phase for high-temperature iron-based Fischer− Tropsch synthesis (HTFTS). Hagg carbide may be oxidized to magnetite during FTS depending on the chemical potential of oxygen, μ O , which may be related to the partial pressure of the oxidizing agents, H 2 O or CO 2 . 3Fe 5 C 2 + 26H 2 O → 5Fe 3 O 4 + 6CO + 26H 2 , and 3Fe 5 C 2 + 26CO 2 → 5Fe 3 O 4 + 32CO. Magnetite is believed to be active for the water−gas shift reaction but inactive for the HTFTS, and thus, its formation could subsequently contribute to the loss in the FT activity. An in situ magnetometer was used to follow the oxidation behavior of Hagg carbide by either H 2 O or CO 2 under realistic high-temperature FT process conditions. Hagg carbide is not magnetic at the high temperature used for the FT process, while magnetite is. Thus, the transformation of Hagg carbide to magnetite can be followed by tracking its magnetization and by employing in situ X-ray diffraction, at relevant conditions. The results indicated that the oxidation of Hagg carbide and the concurrent catalyst deactivation at these conditions are strongly dependent on the H 2 O levels present in the reactor. No oxidation was observed at CO 2 levels up to 8 bar, while in agreement with the thermodynamic calculations conducted in this study, H 2 O-induced oxidation was observed at 4 bar during 3 to 20 h exposure. It may be speculated that lower H 2 O levels could also contribute to Hagg carbide oxidation if the exposure times are longer. Magnetite can be transformed back to Hagg carbide upon lowering the H 2 O partial pressure or if H 2 O is removed altogether. This fast reversibility in the phase transformation has also been coupled with an activity gain. More importantly, it has been shown that magnetite may not be solely responsible for the water−gas shift activity during the FTS.

Transient behavior of CO and CO2 hydrogenation on Fe@SiO2 core-shell model catalysts – A stoichiometric analysis of experimental data

“…Especially at low temperatures and pressures the RWGS is strongly preferred, if surface oxides are present [35,54,58,59]. In the present case iron oxides are possibly formed during preceding hydrogenation of CO/CO2 mixtures and during the CO2 hydrogenation itself [33,60], which hence lead to high selectivity towards CO of more than 97 %. Carbon deposition and thus iron carbide formation during CO2 hydrogenation plays a minor role for Fe@SiO2 catalysts, as reported previously [35].…”

“…In particular, thermogravimetric analysis combined with mass spectrometry (TGA-MS) allows to analyze the carbon content in the catalysts [64]. Additionally, vibrating sample magnetometer (VSM) analysis [33] and X-ray adsorption near edge structure (XANES) spectroscopy, recently used for studying influence of the oxidic support for CO2 hydrogenation on iron based catalysts [65], can help to determine the degree of oxidation and carburization of the active iron phase.…”

“…oxide species are formed by transformation of carbidic or metallic iron species in H2O rich atmosphere within the confined core-shell structure during hydrogenation of CO2 rich syngas. This transformation was recently discussed by Claeys et al [33] for high temperature Fischer-Tropsch synthesis at elevated pressure, but can be adapted to the present reaction conditions, as well. For core-shell catalysts, in particular, we assume that the microporous SiO2 shell restricts diffusion to a certain extent.…”

“…Especially during COx hydrogenation carburization and oxidation of the Fe based catalyst plays an important role regarding catalytic activity [23,[30][31][32] and hence influences the preferred reaction pathway [29]. For instance, under Fischer-Tropsch conditions it is known that transformation of iron carbide based catalysts into iron oxides can occur dynamically via oxidation within the first few hours, induced by water formed during reaction [33]. As a consequence, this directly affects the WGS activity and thus the product distribution [34].…”

“…To overcome these limitations and expedite further investigation of structure-activity relations, nanostructured core-shell catalysts with its unique and tunable properties are applied in catalysis research [24,37,38]. Changes of size and shape of the active core nanoparticles caused during conversion can be studied for example ex situ by a transmission electron microscopy (TEM) [35] or in situ via environmental TEM [39] or vibrating-sample magnetometer [33], as the core-shell arrangement is well-defined and stable during reaction [40]. Studies on Fe based core-shell catalysts already cover a broad variety of important catalytic processes, e.g., ammonia decomposition [40], esterification [41], alcohol oxidation [42], olefin hydrogenation [43], and Fischer-Tropsch synthesis [44].…”