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), temperatureprogrammed 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 Co 0 particles (around 2-6 nm), indicating that the cobalt phase tends to redisperse during the reduction process from Co3O4 to Co 0 . 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 Mn 2+ state, and two well-distinguished compounds could be identified: (1) a highly dispersed Ti 2MnO4-type phase located at the TiO2 surface and (2) a less dispersed MnO phase being in the proximity of the cobalt particles. Furthermore, the MnO was also found to exist partially mixed with a CoO phase in the form of rock-salt Mn1-xCoxO-type solid solutions. The existence of the later solutions was further confirmed by scanning transmission electron microscopy with electron energy loss spectroscopy (STEM-EELS) for a Mn-rich sample. Finally, the cobalt active site composition in the catalysts after reduction at 300 and 350 °C was linked to the catalytic performances obtained under reaction conditions of 220 °C, 1 bar, and H2/CO ) 2. The catalysts with larger Co 0 particles ( >5 nm) and lower Co reduction extents displayed a higher intrinsic hydrogenation activity and a longer catalyst lifetime. Interestingly, the MnO and Mn1-xCoxO species effectively promoted these larger Co 0 particles by increasing the C5+ selectivity and decreasing the CH4 production, while they did not significantly influence the selectivity of the catalysts containing very small Co 0 particles.