Nanoparticle‐Size Effects on Iron Reduction and Carbidization in Silica‐Supported Catalysts Under H 2 and CO

ABSTRACT Iron‐based catalysts are frequently employed in heterogeneous catalysis reactions, particularly in the Fischer–Tropsch (FT) synthesis. The understanding of phase transformations of iron‐based catalysts under reductive atmosphere remains incomplete due to the high sensitivity to activation and reaction conditions. The quantitative use of in situ advanced characterization techniques is a crucial requirement for identification under realistic conditions of the structure of the catalyst prior to, and after activation, and of the intermediate phases formed during activation and leading to the active phase. In this study, we monitored the phenomena that occur during the reduction under 5%‐H 2 and carbidization under 5%‐CO of the iron oxide nanoparticles supported on silica (Fe/SiO 2 ), by using in situ XAS and TEM. The multimodal analysis of the reduction under hydrogen revealed the consecutive transformation of the hematite into magnetite and wüstite, the latter being transformed before full reduction, into a core‐shell type species FeO x ‐Fe never evidenced before, characterized by a decreasing oxygen gradient from the periphery to the center of the particle. We also demonstrate that, during the reduction of the iron‐based catalyst under CO, the formation of the FT active species, the iron carbide, is a surface process governed by the morphology and size of the particles.