Morales, Fernando; Grandjean, Didier; Mens, Ad; de Groot, Frank M. F.; Weckhuysen, Bert M.

Journal of Physical Chemistry B (2006), 110(17), 8626-8639 CODEN: JPCBFK; ISSN: 1520-6106. English.

The effects of the addition of manganese to TiO2-supported cobalt Fischer-Tropsch (FT) catalysts prepared by different methods were studied by a combination of XRD, temperature-programmed reduction (TPR), 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 as Co3-xMnxO4 solns., as revealed by XRD and XAFS. Characterization of the catalysts after H2 reduction at 350.degree. 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 solns. were initially present in the catalyst precursors. Also, the presence of manganese generally gave larger cobalt agglomerates (.apprx.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 compds. could be identified: (1) a highly dispersed Ti2MnO4-type phase located at the TiO2 surface and (2) a less dispersed MnO phase being in the proximity of the cobalt particles. Also, the MnO was also found to exist partially mixed with a CoO phase as rock-salt Mn1-xCoxO-type solid solns. The existence of the later solns. was further confirmed by scanning TEM with EELS (STEM-EELS) for a Mn-rich sample. Finally, the cobalt active site composition in the catalysts after reduction at 300.degree. and 350.degree. was linked to the catalytic performances obtained under reaction conditions of 220.degree., 1 bar, and H2/CO = 2. The catalysts with larger Co0 particles (approx. >5 nm) and lower Co reduction extents displayed a higher intrinsic hydrogenation activity and a longer catalyst lifetime. The MnO and Mn1-xCoxO species effectively promoted these larger Co0 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 Co0 particles.