Unraveling the Structure of Mn‐Promoted Co/TiO2 Fischer‐Tropsch Catalysts by In Situ X‐Ray Absorption Spectroscopy

Combination of in situ X‐ray absorption spectroscopy (XAFS) at the Co and Mn K‐edges with electron microscopy (STEM‐EELS) has allowed to unravel the complex structure of a series of unpromoted and Mn promoted TiO2‐supported cobalt Fischer‐Tropsch catalysts prepared by homogeneous deposition precipitation (HDP), both in their calcined and reduced states. After calcination the catalysts are generally composed of large Co3O4 aggregates (13–20 nm) and a MnO2‐type phase that is either dispersed on the TiO2 surface or, for the major part, covering the Co3O4 particles. Additionally Mn is also forming a spinel‐type Co3−xMnxO4 solid solution at the surface of the Co3O4 particles. In pure Co or when small amount of this spinel‐type phase are formed during calcination, reduction in H2 at 350 °C produces Co0 particles of variable sizes (3.5–15 nm) otherwise Co reduction is limited to the Co2+ state. Manganese that exists entirely in a Mn2+ state in the reduced catalysts is forming (1) a highly dispersed Ti2MnO4‐type ...