ACTIVATION STUDIES WITH A PRECIPITATED IRON CATALYST FOR FISCHER-TROPSCH SYNTHESIS .1. CHARACTERIZATION STUDIES

A commercial promoted precipitated iron catalyst (100 Fe/5 Cu/4.2 K/25 SiO2 by weight) was characterized after different pretreatment condit ions and after Fischer-Tropsch (FT) synthesis in a fixed bed reactor. The BET-N-2 surface area and pore volume of the catalyst decreased aft er pretreatments in hydrogen, carbon monoxide, or syngas, Isothermal a nd temperature-programmed reduction profiles indicate that iron reduct ion occurs in two steps: facile reduction of Fe2O3 to Fe3O4, followed by slow reduction of Fe3O4 to either metallic iron (H-2 reduction) or an iron carbide (CO pretreatment), Calcined catalyst is in the form of poorly crystalline alpha-Fe2O3, which is partially converted to eithe r magnetite (Fe3O4) or a mixture of alpha-Fe and Fe3O4 after H-2 reduc tions, During FT synthesis the alpha-Fe and a portion of iron oxides a re carburized to a pseudohexagonal epsilon'-Fe-2.2,C. After CO or syng as pretreatments, the hematite is partly converted to a monoclinic chi -Fe5C2 carbide, During FT synthesis this carbide is partially converte d to magnetite, The degree of surface iron reduction, determined by X- ray photoelectron spectroscopy, was greater after the CO pretreatment at 300 degrees C for 4 h than that obtained after the H-2 reduction un der the same conditions. However, in both cases a fraction of the surf ace iron remained in the form of unreduced Fe2+/Fe3+ species, Also, th e surface carbon deposits were formed during the CO pretreatment. (C) 1995 Academic Press, Inc.