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

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-Fe2O3 to Fe3O4, accompanied simultaneously with the formation of atomic (Cα) and polymeric (Cβ) carbonaceous species on the surface of the catalyst. With time on stream the Fe3O4 formed in the near-surface regions was converted gradually to χ-Fe5C2, and the amounts of Cα and Cβ species presented an increasing trend. Subsequently, these species were combined partly together to form the graphitic-type (Cδ) carbonaceous species, and the Cδ species largely covered on the surface of iron carbides. The formation of iron carbides (especially for χ-Fe5C2), Cα and Cβ species on the surface layers promoted the catalytic activity, whereas the Cδ species formed restrained the active sites for FTS reaction.

A systematic investigation was carried on transformation mechanism of surface/bulk structures and carbonaceous species under CO atmosphere over a precipitated Fe-based catalyst, and correlation between iron phases, surface carbon species and FTS activity.Figure optionsDownload high-quality image (128 K)Download as PowerPoint slideHighlights
► The Cα and Cβ species were formed gradually with the conversion of magnetite to χ-Fe5C2 on the surface layers under CO atmosphere.
► The formation of χ-Fe5C2, Cα and Cβ species on the surface layers promoted the catalytic activity.
► The Cδ species formed restrained the active sites for FTS reaction.