The crystal structures of the iron carbides

• Main Author: Du Plessis, Hester Esna
• Published: 2008
• Subjects: Crystal structures; Cementite; Fischer-Tropsch process
• Online Access: http://hdl.handle.net/10210/421

Iron carbides are amongst the crystalline phases formed during Fischer-Tropsch synthesis to produce hydrocarbons (Dry, 1990, Niemantsverdriet et al., 1980), using iron catalysts. The small crystallite size of the iron carbides causes peak broadening in XRD and prevented complete structure determinations in the past (Hagg, 1931; Retief, 1999; Senateur et al., 1962). Fortunately new instrumentation and techniques, such as fast powder X-ray diffractometers and software for structure determination, are now available to study crystal structures. Five different iron carbide phases are known to form during Fischer-Tropsch synthesis i.e. Hägg carbide (χ-Fe5C2), pseudo-hexagonal iron carbide (έ- Fe2.2C), hexagonal iron carbide (ε-Fe3C), Eckström-Adcock iron carbide (Fe7C3) and cementite (θ-Fe3C). Since the structure of cementite θ-Fe3C is well-known (Westgren & Phragmen, 1922) this study focused on the remainder, i.e. the determination of the crystal structures of the first four iron carbides: Hägg carbide (χ-Fe5C2), pseudo-hexagonal iron carbide (έ -Fe2.2C), hexagonal iron carbide (ε-Fe3C) and Eckström-Adcock iron carbide (Fe7C3). This study consisted of the preparation of iron carbides, structure determinations of these iron carbides, determination of reactions of the iron carbides during Fischer-Tropsch synthesis (FTS) (in situ XRD) and the stability of Hägg carbide (χ-Fe5C2) during FTS under commercial fixed bed reactor conditions. Time-temperature-transformation graphs were determined for iron catalysts with and without potassium promoter. The first step in the structure determination process was the preparation of almost pure samples. Samples of Hagg carbide (:t-FesC2), pseudo-hexagonal 8 -Fe22C iron carbide and hexagonal 1::-Fe3C iron carbide were prepared as pure as possible using the Anton Paar XRK600 reaction chamber attached to an X'Pert Pro multi-purpose diffractometer (N!PD). Eckstrom-Adcock iron carbide (Fe7C3) was available in spent catalyst from a fluidized-bed hydrocarbon synthesis plant at SASOL. These samples were characterized using room temperature and low temperature (77 K) Moss bauer absorption spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM, Philips CM200). Thermo-gravimetric analysis and conductivity measurements were done to characterize the carbonaceous species in the samples. Molecular modelling calculations were done using CASTEP (N!ilman et al., 2000; Payne et al., 1992) to determine the total lattice energies of the iron carbide structures.... === Prof. G.J. Kruger Prof. J.P.R. de Villiers