Ray absorption Spectroscopy of Mn/Co/TiO2 Fischer-Tropsch catalysts:: Relationships between preparation method, molecular structure, and catalyst performance

被引:117
作者
Morales, F [1 ]
Grandjean, D [1 ]
Mens, A [1 ]
de Groot, FMF [1 ]
Weckhuysen, BM [1 ]
机构
[1] Univ Utrecht, Dept Chem, NL-3584 CA Utrecht, Netherlands
关键词
D O I
10.1021/jp0565958
中图分类号
O64 [物理化学(理论化学)、化学物理学];
学科分类号
070304 ; 081704 ;
摘要
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 H-2 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 (similar to 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 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 degrees C, 1 bar, and H-2/CO = 2. The catalysts with larger Co0 particles (similar to > 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 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 Co-0 particles.
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页码:8626 / 8639
页数:14
相关论文
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