Transformation of carbonaceous species and its influence on catalytic performance for iron-based Fischer–Tropsch synthesis catalyst

Abstract

Transformation behavior of carbonaceous species over a precipitated iron-based Fischer–Tropsch synthesis (FTS) catalyst was investigated by Mössbauer effect spectroscopy (MES), X-ray photoelectron spectroscopy (XPS), CO temperature-programmed desorption (CO-TPD), temperature-programmed hydrogenation (TPH), high resolution transmission electron microscopy (HRTEM) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The catalytic activities were tested in a fixed bed reactor. It was found that during carburization the fresh catalyst was firstly reduced from α-Fe 2O 3 to Fe 3O 4, accompanied simultaneously with the formation of atomic (C α ) and polymeric (C β ) carbonaceous species on the surface of the catalyst. With time on stream the Fe 3O 4 formed in the near-surface regions was converted gradually to χ-Fe 5C 2, and the amounts of C α and C β species presented an increasing trend. Subsequently, these species were combined partly together to form the graphitic-type (C δ ) carbonaceous species, and the C δ species largely covered on the surface of iron carbides. The formation of iron carbides (especially for χ-Fe 5C 2), C α and C β species on the surface layers promoted the catalytic activity, whereas the C δ species formed restrained the active sites for FTS reaction.