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

Abstract

The morphological, phase transformations and carbon‐layer growth for unpromoted and K‐promoted iron catalysts were investigated over time during Fischer–Tropsch synthesis. Catalysts were activated in CO for 24 h, which transformed hematite into a mixture containing 93 % iron carbide and 7 % magnetite for the unpromoted catalyst and 81 % iron carbide and 19 % magnetite for the K‐promoted catalyst. Initially, the activated catalysts had high CO conversions (≈85 %); however, the conversions decreased to approximately 30 % after approximately 280 h of synthesis time. For the unpromoted catalyst, the amount of iron carbide gradually decreased over time while the corresponding magnetite phase increased. However, for the K‐promoted one, only one iron carbide phase (χ‐Fe5C2) gradually decreased, while the other ($\acute \epsilon $‐Fe2.2C) phase steadily increased and magnetite remained unchanged. TEM analyses revealed that for the K‐promoted catalyst, carbon deposition increased over time, unlike that of the unpromoted catalyst.

Iron‐Based Fischer–Tropsch Synthesis for the Efficient Conversion of Carbon Dioxide into Isoparaffins

“…On the basis of the reaction approach, the active sites on the iron catalyst should be divided into two types. One is probably an oxidic iron phase, which appears to be active for the RWGS reaction (pathway 1) by abstraction of an O atom in CO 2 by a H radical to form CO. Another is the well‐known iron carbide phase (Fe 5 C 2 ) for the dissociation of CO to carbon–carbon propagation by FTS (pathway 3) …”

“…One is probably an oxidic iron phase, which appears to be active for the RWGS reaction (pathway 1) by abstraction of an Oa tom in CO 2 by aHr adical to form CO. Another is the well-known iron carbide phase (Fe 5 C 2 )f or the dissociation of CO to carbon-carbon propagation by FTS (pathway3). [32,33] As illustrated in Scheme1,t he hydrogenation of CO 2 over the iron catalystp roduces products or intermediates, including CO, water,a nd hydrocarbons. CO is the reactioni ntermediate formed in the first step by the RWGS reaction.…”

Fischer–Tropsch Synthesis: Deactivation as a Function of Potassium Promoter Loading for Precipitated Iron Catalyst

“…While some researchers have reported that FTS activity either increases [2,6,13] or passes through a maximum as a function of potassium loading [11,20], others have found that potassium either has no effect on the activity for FTS [21] or suppresses it [17,20]. In our previous study [22], we reported the activity/stability of an unpromoted and a 2 % potassium promoted iron catalyst during FTS, and the activities of these catalysts decreased in a similar manner with time on stream.…”

“…Mössbauer effect spectroscopy is a useful technique that can provide quantitative information on the fractions of various iron phases present in the catalyst samples [22]. The iron phase composition of unpromoted and various potassium promoter loaded iron catalysts, after carburization and during FTS, as determined by fitting the Möss-bauer spectra at 20 K measurements, are listed in Table 2.…”

“…Results of previous studies have shown that the formation of surface carbides is required before the catalyst can exhibit high activity [9,17,25]. It is well known that the distribution of iron phases in the catalyst changes with time-on-stream [22,26]. The oxidation of the metallic iron and/or the iron carbide phases is believed to be one of the factors contributing to catalyst deactivation.…”