Potassium as a Structural Promoter for an Iron/Activated Carbon Catalyst: Unusual Effect of Component Deposition Order on Magnetite Particle Size and Catalytic Behavior in Fischer-Tropsch Synthesis

The structural and catalytic properties of nanosized Fe3O4 particles deposited on activated charcoal (AC) are drastically affected by the sequence of component deposition order. Preliminary alkalization of AC followed by Fe deposition leads to formation of small magnetite particles (modal size of 4 nm) with uniform distribution. This catalyst demonstrates much higher iron-time yield in FT synthesis than counterparts prepared by co-impregnation and sequential impregnation with a reverse order of component deposition (Fe, then K). The catalysts were characterized by nitrogen adsorption, X-ray diffraction, transmission electron microscopy, temperature-programmed reduction in Ar/H-2, and CO/H-2 flow and in situ magnetic measurements. After calcinations in argon, catalysts contain iron predominantly in the form of magnetite with minor amount of hematite in some cases. Activation of the catalyst in the CO/H-2 stream prior catalytic test leads to the formation of surface Hegg carbide Fe5C2. Spent catalysts contain considerable amount of magnetite, which is presumably due to partial oxidation of carbide by water in the course of the FT reaction.