The role of H₂ in Fe carburization by CO in Fischer-Tropsch catalysts

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.