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 degrees C, while C diffusion into metallic Fe requires much higher temperature (similar to 180 degrees C). The carburization rate increases with increasing H-2/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 epsilon'()-carbide, while chi-Fe5C2 is formed at lower H-2/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. (C) 2021 The Author(s). Published by Elsevier Inc.

Automated summary

The formation of Fe-carbide phases is crucial for Fischer-Tropsch synthesis catalysts. Carburization of Raney Fe showed that CO dissociation occurs at -150 degrees C, while C diffusion into metallic Fe requires higher temperature. The carburization rate increases with H2/CO ratio, leading to predominant formation of carbon-rich ε'-carbide.