Carbide formation during activation of iron Fisher-Tropsch catalysts

Abstract

The morphological transformations during activation of a commercial, precipitated and spray-dried, Fe₂O₃-CuO-K₂O Fischer-Tropsch Catal.yst are studied. The Catal.yst is activated in a flowing (H₂/CO = 0.7/1.0) mixture at 523 K and 543 K and subsequent reaction is carried out at 523 K. The activation temperature causes these Catal.ysts to exhibit significant differences in their Catal.ytic activity for CO hydrogenation. The solid-state phase transformations from the oxide phase (hematite) to the iron carbide are studied by high resolution transmission electron microscopy (HRTEM), X-ray diffraction and elemental analysis. The differing extents of formation of the carbide phase correlate well with the Catal.ytic activity of iron. HRTEM shows that as magnetite transforms into carbide, the crystals of magnetite break down into smaller crystallites of the carbide phase. Deposition of carbon on the surface causes the carbide crystallites to further separate from each other. Morphological transformations such as these determine the attrition of the Catal.yst and help provide clues to understanding the deactivation of iron Fischer-Tropsch Catalysts.