Changes in Physico-Chemical Properties of Iron-Based Fischer–Tropsch Catalyst Induced by SiO2 Addition

Abstract

The effect of adding SiO₂ to a precipitated Fe-based Fischer–Tropsch catalyst was investigated. Silica was added to the catalyst either during or after precipitation. The iron-based Fischer–Tropsch catalysts were studied using Mössbauer spectroscopy, BET surface area, XRD and SEM characterization methods. Adding SiO₂ to the catalyst during precipitation or immediately after precipitation (i.e., precipitated SiO₂) results in the formation of Fe crystallites with an average diameter less than 3 nm, which have high surface areas and exhibit a strong interaction with the SiO₂ matrix. Consequently, these crystallites are resistant to reduction and carburisation. When SiO₂ was added to the catalyst after heat treatment (i.e., binder SiO₂), the resulting catalyst was observed to consist of segregated SiO₂-rich and Fe-rich phases. The distribution of K₂O in both these phases indicates that the amount of effective K₂O, i.e., that associated with Fe, is less when SiO₂ is added as a binder. The low extent of reduction and carburisation observed with catalysts that contain precipitated SiO₂ results in catalysts with low % CO conversion. A positive correlation between the amount of iron carbides present in the catalyst and the % CO conversion was observed in these studies.