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 Mossbauer 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 alpha-Fe2O3 to Fe3O4, accompanied simultaneously with the formation of atomic (C-alpha) and polymeric (C-beta) carbonaceous species on the surface of the catalyst. With time on stream the Fe3O4 formed in the near-surface regions was converted gradually to chi-Fe5C2, and the amounts of C-alpha and C-beta species presented an increasing trend. Subsequently, these species were combined partly together to form the graphitic-type (C-delta) carbonaceous species, and the C-delta species largely covered on the surface of iron carbides. The formation of iron carbides (especially for chi-Fe5C2), C-alpha and C-beta species on the surface layers promoted the catalytic activity, whereas the C-delta species formed restrained the active sites for FTS reaction. (C) 2011 Elsevier B.V. All rights reserved.