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 (approximate to 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 (X-Fe5C2) gradually decreased, while the other (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.