X-ray Absorption Spectroscopy of Mn/Co/TiO2 Fischer-Tropsch Catalysts: Relationships between Preparation Method, Molecular Structure, and Catalyst Performance

The effects of the addition of manganese to a series of TiO2-supported cobalt Fischer-Tropsch (FT) catalysts prepared by different methods were studied by a combination of X-ray diffraction (XRD), temperature-programmed reduction (TPR), transmission electron microscopy (TEM), and in situ X-ray absorption fine structure (XAFS) spectroscopy at the Co and Mn K-edges. After calcination, the catalysts were generally composed of large Co3O4 clusters in the range 15-35 nm and a MnO2-type phase, which existed either dispersed on the TiO2 surface or covering the Co3O4 particles. Manganese was also found to coexist with the Co3O4 in the form of Co3-xMnxO4 solutions, as revealed by XRD and XAFS. Characterization of the catalysts after H2 reduction at 350 °C by XAFS and TEM showed mostly the formation of very small Co0 particles (around 2-6 nm), indicating that the cobalt phase tends to redisperse during the reduction process from Co3O4 to Co0. The presence of manganese was found to hamper the cobalt reducibility, with this effect being more severe when Co3-xMnxO4 solutions were initially present in the catalyst precursors. Moreover, the presence of manganese generally led to the formation of larger cobalt agglomerates (8-15 nm) upon reduction, probably as a consequence of the decrease in cobalt reducibility. The XAFS results revealed that all reduced catalysts contained manganese entirely in a Mn2+ state, and two well-distinguished compounds could be identified: (1) a highly dispersed Ti2MnO4-type phase located at the TiO2 surface and (2) a less disperse