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 H-2 at 350 degrees C produces Co-0 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 phase at the TiO2 surface, (2) a less dispersed MnO phase close to the cobalt particles that coexists with (3) a rock salt-type Mn1-x.CoxO solid solution. Similarly, large amount of spinet solid solution in the calcined state favors the formation of Mn1-xCoxO-type solid solution during reduction showing that one of the main roles of the Mn promoter is to limit Co reducibility.