Activation of Precipitated Iron Fischer-Tropsch Catalysts

The Fischer-Tropsch Synthesis (FTS) is an important step in the manufacture of synthetic fuels from coal. The use of iron catalysts for FTS, though desirable from the standpoint of process economy, is hampered by problems relating to catalyst attrition and carbon buildup. In addition, from a research perspective, there has been a long-standing controversy in the literature with regard to the nature of the active phase. The effect of activation and reaction treatments on the resulting phase transformations in a commercial, precipitated and spray-dried, Fe2O3 - CuO - K2O Fischer-Tropsch catalyst have been studied. Different catalyst batches were activated in flowing H2, CO and a H2/CO = 0.7 mixture. The pretreatment method caused these catalyst samples to exhibit significant differences in their catalytic activity for FTS. The microstructure and phase composition of these catalysts were studied by electron microscopy and other bulk and surface characterization techniques in order to understand the phenomena responsible for these differences. It is observed that magnetite transforms into carbide as the catalyst is activated causing the crystals of magnetite to break down into smaller crystallites of the carbide phase. Deposition of carbon on the carbide surface causes the carbide crystallites to further separate from each other. These solid-state transformations in the catalyst during activation and reaction lead to eventual attrition of the catalyst. The results suggest that magnetite has negligible catalytic activity for FT synthesis whereas carbide formation is necessary before the catalyst becomes active. The extent of transformation into carbide correlates well with catalyst activity during the activation step