Effect of reducing agents on microstructure and catalytic performance of precipitated iron-manganese catalyst for Fischer-Tropsch synthesis

Effects of reducing agents on the textural properties and bulk/surface phase compositions of a precipitated iron-manganese catalyst were investigated by N-2-physisorption, X-ray photoelectron spectroscopy (XRD), Mossbauer effect spectroscopy (MES), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy (LRS). Fischer-Tropsch synthesis (FTS) was performed in a slurry-phase continuously stirred tank reactor. The characterization results indicated that the hematite in the fresh catalyst was converted mainly to magnetite in H-2 atmosphere without the formation of intermediate metallic iron. Large amounts of Fe3O4 and small amounts of epsilon'-Fe2.2C and chi-Fe2.5C were formed after syngas pretreatment. In contrast, CO activation led to the formation of large amounts of chi-Fe2.5C and carbonaceous species on the surface of magnetite. In the FTS reaction, the CO-activated catalyst presented the highest initial activity compared to the H-2 and syngas-reduced catalysts, and remained unchanged in the activity following the transformation of iron carbides to Fe3O4. Furthermore, the FTS activity of the H-2-reduced catalyst increased gradually accompanied with the conversion of magnetite to iron carbides. All of the results suggested that the formation of iron carbides (especially for chi-Fe2.5C) on the surface layers provides probably the active sites for FTS, whereas the Fe3O4 formed plays a negligible role in the FTS activity. (C) 2011 Elsevier B.V. All rights reserved.