Local and long range order in promoted iron-based Fischer–Tropsch catalysts: a combined in situ X-ray absorption spectroscopy/wide angle X-ray scattering study

The structural properties of three Fe-based Fischer–Tropsch synthesis (FTS) catalysts containing different amounts of Cu, K and SiO2 additives were investigated during pretreatment and FTS in a fixed bed-like reactor using combined in situ X-ray absorption fine structure (XAFS)/wide angle X-ray scattering (WAXS) techniques. This combination enabled acquisition of complementary information regarding the local environment of iron atoms from XAFS and crystalline phases from WAXS during H2 and CO/H2 pretreatment and FTS at 1 bar. The presence of the SiO2 support and promoter elements significantly influenced the structural properties of the catalysts after pretreatment. After FTS, H2 pretreated catalysts mainly consisted of amorphous θ-Fe3C clusters (unsupported catalysts) or amorphous iron (II) silicate (supported catalyst), while the CO/H2 pretreated catalysts all consisted mainly of γ-Fe and χ-Fe5C2. Catalysts activated in CO/H2 showed superior stability and activity, while H2 pretreated unsupported catalysts deactivated rapidly during the first 15 h of FTS.