Combined In Situ X-ray Powder Diffractometry/Raman Spectroscopy of Iron Carbide and Carbon Species Evolution in Fe(-Na-S)/α-Al2O3 Catalysts during Fischer-Tropsch Synthesis

A Na-S promoted Fe-based Fischer-Tropsch synthesis (FTS) catalyst converts a H-2/CO gas mixture into hydrocarbons with enriched C-2-C-4 olefin content. Above 300 degrees C, the carbon-depositing Boudouard reaction competes with the FTS reaction for CO as reactant. By making use of a combined in situ X-ray powder diffractometry (XRPD)/Raman spectroscopy setup, the simultaneous evolution of the FexOy/alpha-Fe/FexC phases and various formed carbon species has been monitored at 340 degrees C and 10 bar. CO carburized, Na-S promoted and unpromoted Fe(-Na-S)/alpha-Al2O3 catalysts were investigated. The various Fe phases present were quantified with Rietveld quantitative phase analysis (R-QPA) from the in situ collected XRPD patterns. The observed D- and G-bands in the in situ Raman spectra were analyzed for their relative intensities, band widths, and positions and compared to reference carbon materials. It was found that amorphous carbon with C sp(3) and C sp(2) in chain-like ordering evolved toward carbon nanofiber-like structures during FTS. Na-S promotion and initial CO carburization at temperatures >= 340 degrees C led to an increased amount of cyclic sixfold C sp(2) species. Preliminary carbon deposits present in the catalysts decreased the initial fast increase of the Raman band intensities, while Na-S promotion increased Raman band intensity growth after the initial fast increase period. The carbon species evolution was unaffected by the presence of specific Fe carbides or by carbide-to-carbide transitions. Na-S promotion aided in the reduction of Fe3O4 by (H-2:)CO to carbon-depositing Fe carbides. The results obtained add to our further understanding on the role of Fe and carbon species during a high-temperature FTS reaction.