Raman spectroscopy was employed to monitor iron-catalzyed CO₂-Fischer–Tropsch synthesis toward higher hydrocarbons with alkali metal-doped ferrous oxalate as precatalyst. The thermal transformation of MFeC₂O₄ into Fe₃O₄ as the major phase did not show a significant dependency on the presence and type of alkali metal dopant. The formation of carbon species, observable as G and D bands in the Raman spectra, was significantly more pronounced in the alkali metal-doped catalysts. The reoxidation by water vapor was hampered for the alkali metal-promoted catalytic systems.

Abstract

Knowledge about dynamic changes of the phase composition during catalytic reactions is indispensable for gaining a better understanding of catalyst structure-reactivity relationships. For this purpose, in situ Raman spectroscopic experiments were conducted to monitor the structural changes of ferrous oxalate without and with an alkali metal promoter during CO₂ hydrogenation to C₂₊-hydrocarbons under realistic reaction conditions (340 °C, 15 bar). The kind of promoter had no significant influence on the thermal transition of iron oxalate into the magnetite (Fe₃O₄) phase as the major phase under inert gas. In contrast, the promoters did have a substantial impact on the phase composition during CO₂ hydrogenation. The Raman spectroscopic results obtained, underpinned with X-ray diffraction measurements, demonstrated a much stronger development of carbon-containing species, notably iron carbides (FeC_x) and carbon deposits, when using the alkali metal-promoted catalysts. The formation of carbon deposits may indirectly indicate the occurrence of carbidization, however, no quantitative correlation was discovered. It was also found that the presence of alkali metal promoters hampered the reoxidation of FeC_x toward Fe₃O₄ with water vapor.