Activation Studies with a Precipitated Iron Catalyst for Fischer-Tropsch Synthesis: I. Characterization Studies

Abstract

A commercial promoted precipitated iron catalyst (100 Fe/5 Cu/4.2 K/25 SiO2 by weight) was characterized after different pretreatment conditions and after Fischer-Tropsch (FT) synthesis in a fixed bed reactor. The BET-N2 surface area and pore volume of the catalyst decreased after pretreatments in hydrogen, carbon monoxide, or syngas. Isothermal and temperature-programmed reduction profiles indicate that iron reduction occurs in two steps: facile reduction of Fe2O3 to Fe3O4, followed by slow reduction of Fe3O4 to either metallic iron (H2 reduction) or an iron carbide (CO pretreatment). Calcined catalyst is in the form of poorly crystalline α-Fe2O3, which is partially converted to either magnetite (Fe3O4) or a mixture of α-Fe and Fe3O4 after H2 reductions. During FT synthesis the α-Fe and a portion of iron oxides are carburized to a pseudohexagonal ϵ′-Fe2.2C. After CO or syngas pretreatments, the hematite is partly converted to a monoclinic χ-Fe5C2 carbide. During FT synthesis this carbide is partially converted to magnetite. The degree of surface iron reduction, determined by X-ray photoelectron spectroscopy, was greater after the CO pretreatment at 300°C for 4 h than that obtained after the H2 reduction under the same conditions. However, in both cases a fraction of the surface iron remained in the form of unreduced Fe2+/Fe3+ species. Also, the surface carbon deposits were formed during the CO pretreatment.