The kinetics of the transformation of metallic Fe to the active Fe carbide phase at the start of the Fischer-Tropsch (FT) reaction were studied. The diffusion rates of C atoms going in or out of the lattice were determined using 13C-labeled synthesis gas in combination with measurements of the transient 12C and 13C contents in the carbide by temperature-programmed hydrogenation. In the initial 20 min, C diffuses rapidly into the lattice occupying thermodynamically very stable interstitial sites. The FT reaction starts already during these early stages of carburization. When reaching steady state, the diffusion rates of C in and out of the lattice converge and the FT reaction continues via two parallel reaction mechanisms. It appears that the two outer layers of the Fe carbide are involved in hydrocarbon formation via a slow Mars-Van Krevelen-like reaction contributing to ∼10% of the total activity, while the remainder of the activity stems from a fast Langmuir-Hinshelwood reaction occurring over a minor part of the catalyst surface.

The formation of Fe-carbide phases is relevant to the synthesis of Fischer-Tropsch synthesis catalysts. We investigated the carburization of Raney Fe as a model catalyst using spectroscopic and temperature-programmed techniques. IR spectroscopy shows that CO dissociation already occurs at −150 °C, while C diffusion into metallic Fe requires much higher temperature (~180 °C). The carburization rate increases with increasing H₂/CO ratio, which can be attributed to the lower overall barrier for O removal as H₂O as compared to CO₂. O removal frees vacancies that are needed for CO dissociation. The resulting higher C coverage increases the driving force for Fe-carbide formation. A higher driving force leads to predominant formation of the more carbon-rich ε(́)-carbide, while χ-Fe₅C₂ is formed at lower H₂/CO ratio. The removal of surface O appears to be the rate-limiting step under all conditions. Initially, most of deposited C is used for Fe-carbide formation with a small contribution to hydrocarbons formation.

Using model catalysts with well-defined particle sizes and morphologies to elucidate questions regarding catalytic activity and stability has gained more interest, particularly utilizing colloidally prepared metal(oxide) particles. Here, colloidally synthesized iron oxide nanoparticles (FexOy-NPs, size ∼7 nm) on either a titania (FexOy/TiO2) or a silica (FexOy/SiO2) support were studied. These model catalyst systems showed excellent activity in the Fischer-Tropsch to olefin (FTO) reaction at high pressure. However, the FexOy/TiO2 catalyst deactivated more than the FexOy/SiO2 catalyst. After analyzing the used catalysts, it was evident that the FexOy-NP on titania had grown to 48 nm, while the FexOy-NP on silica was still 7 nm in size. STEM-EDX revealed that the growth of FexOy/TiO2 originated mainly from the hydrogen reduction step and only to a limited extent from catalysis. Quantitative STEM-EDX measurements indicated that at a reduction temperature of 350 °C, 80% of the initial iron had dispersed over and into the titania as iron species below imaging resolution. The Fe/Ti surface atomic ratios from XPS measurements indicated that the iron particles first spread over the support after a reduction temperature of 300 °C followed by iron oxide particle growth at 350 °C. Mössbauer spectroscopy showed that 70% of iron was present as Fe2+, specifically as amorphous iron titanates (FeTiO3), after reduction at 350 °C. The growth of iron nanoparticles on titania is hypothesized as an Ostwald ripening process where Fe2+ species diffuse over and through the titania support. Presynthesized nanoparticles on SiO2 displayed structural stability, as only ∼10% iron silicates were formed and particles kept the same size during in situ reduction, carburization, and FTO catalysis.

The Fe-catalyzed Fischer-Tropsch (FT) synthesis reaction is the core of the coal-to-liquids (CTL) process, which is an efficient route to convert coal into liquid fuels via synthesis gas (a mixture of CO and H₂). Conventional Fe-based FT catalysts convert typically 30% of the feed CO to undesirable CO₂during the FT step. A decrease of CO₂production in FT units can reduce the profitability of the overall CTL process. In this context, we synthesized phase-pure ϵ-Fe-carbide FT catalysts by careful control of the pretreatment and carburization conditions starting from Raney Fe. The resulting phase-pure Fe-carbide exhibits a low CO₂selectivity during the Fischer-Tropsch reaction. The developed preparation method can be applied to supported Fe catalysts and does not require expensive starting chemicals. The purity and durability of as-synthesized phase-pure ϵ-Fe-carbide catalyst under FT conditions were characterized in detail by in situ characterization techniques. A bulk phase-pure ϵ-Fe-carbide catalyst operated at 23 bar, 235 °C and a H₂/CO ratio of 1.5 shows stable performance with no deactivation during a 150 h pilot test. The catalyst is free of primary CO₂formation and presents reduced secondary CO₂formation compared to conventional Fe-based FT catalysts. These findings contribute to the development of new Fe-based FT catalysts for CTL processes.