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 H2/CO gas mixture into hydrocarbons with enriched C2–C4 olefin content. Above 300 °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 Fe x O y /α-Fe/Fe x C phases and various formed carbon species has been monitored at 340 °C and 10 bar. CO carburized, Na–S promoted and unpromoted Fe­(−Na–S)/α-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 sp3 and C sp2 in chain-like ordering evolved toward carbon nanofiber-like structures during FTS. Na–S promotion and initial CO carburization at temperatures ≥340 °C led to an increased amount of cyclic sixfold C sp2 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 (H2:)­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.