Nanocrystalline Ferrihydrite-Based Catalysts for Fischer-Tropsch Synthesis: Part I. Reduction and Carburization Behavior

Temperature-programmed reduction using H₂ (H₂-TPR) and CO (CO-TPR) was carried out to investigate the reduction and carburization behavior of nanocrystalline ferrihydrite-based Fe/Cu/K/SiO₂ catalysts for use in Fischer-Tropsch synthesis (FTS). Unlike pure ferrihydrite, the ferrihydrite-based catalysts did not pass through the intermediate decomposition step of ferrihydrite (Fe₉O₂(OH)₂₃) into hematite (α-Fe₂O₃) as they were reduced into magnetite (Fe₃O₄). This is attributed to the enhanced thermal stability induced by SiO₂. For the ferrihydrite-based catalysts, the reduction of ferrihydrite into magnetite occurred in two stages because the reduction promoter, Cu, is not homogeneously distributed on the catalyst surfaces. The Cu-rich sites are likely to be reduced in the first stage, and the Cu-lean sites may be reduced in the second stage. After the ferrihydrite is reduced to magnetite, the reduction process of magnetite was similar to that for conventional hematite-based FTS catalysts: ‘magnetite→metallic iron’ and ‘magnetite→wüstite (FeO) or fayalite (Fe₂SiO₄)→metallic iron’ in the H₂ atmosphere; ‘magnetite → iron carbides’ in the CO atmosphere.