Morales Fernando, Grandjean Didier, Mens Ad, de Groot Frank M F, Weckhuysen Bert M
Inorganic Chemistry and Catalysis, Department of Chemistry, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands.
J Phys Chem B. 2006 May 4;110(17):8626-39. doi: 10.1021/jp0565958.
The effects of the addition of manganese to a series of TiO(2)-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 Co(3)O(4) clusters in the range 15-35 nm and a MnO(2)-type phase, which existed either dispersed on the TiO(2) surface or covering the Co(3)O(4) particles. Manganese was also found to coexist with the Co(3)O(4) in the form of Co(3-x)Mn(x)O(4) solutions, as revealed by XRD and XAFS. Characterization of the catalysts after H(2) reduction at 350 degrees 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 Co(3)O(4) to Co(0). The presence of manganese was found to hamper the cobalt reducibility, with this effect being more severe when Co(3-x)Mn(x)O(4) solutions were initially present in the catalyst precursors. Moreover, the presence of manganese generally led to the formation of larger cobalt agglomerates ( approximately 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(2)MnO(4)-type phase located at the TiO(2) 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 Mn(1-x)Co(x)O-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 degrees 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 Co(0) particles ( approximately >5 nm) and lower Co reduction extents displayed a higher intrinsic hydrogenation activity and a longer catalyst lifetime. Interestingly, the MnO and Mn(1-x)Co(x)O species effectively promoted these larger Co(0) particles by increasing the C(5+) selectivity and decreasing the CH(4) production, while they did not significantly influence the selectivity of the catalysts containing very small Co(0) particles.
通过结合X射线衍射（XRD）、程序升温还原（TPR）、透射电子显微镜（TEM）以及钴和锰K边的原位X射线吸收精细结构（XAFS）光谱，研究了向一系列通过不同方法制备的TiO₂负载钴费托（FT）催化剂中添加锰的影响。煅烧后，催化剂通常由15 - 35 nm范围内的大Co₃O₄簇和MnO₂型相组成，该MnO₂型相要么分散在TiO₂表面，要么覆盖Co₃O₄颗粒。XRD和XAFS表明，锰还以Co₃₋ₓMnₓO₄固溶体的形式与Co₃O₄共存。通过XAFS和TEM对350℃下H₂还原后的催化剂进行表征，结果显示主要形成了非常小的Co(0)颗粒（约2 - 6 nm），这表明钴相在从Co₃O₄还原为Co(0)的过程中倾向于重新分散。发现锰的存在会阻碍钴的还原，当催化剂前驱体中最初存在Co₃₋ₓMnₓO₄固溶体时，这种影响更为严重。此外，锰的存在通常会导致还原后形成更大的钴团聚体（约8 - 15 nm），这可能是钴还原度降低的结果。XAFS结果表明，所有还原后的催化剂中的锰完全处于Mn²⁺状态，并且可以鉴定出两种明显不同的化合物：（1）位于TiO₂表面的高度分散的Ti₂MnO₄型相；（2）在钴颗粒附近分散性较差的MnO相。此外，还发现MnO部分与CoO相以岩盐Mn₁₋ₓCoₓO型固溶体的形式混合。对于富锰样品，通过扫描透射电子显微镜与电子能量损失谱（STEM - EELS）进一步证实了后一种固溶体的存在。最后，将300℃和350℃下还原后的催化剂中的钴活性位点组成与在220℃、1 bar和H₂/CO = 2的反应条件下获得的催化性能联系起来。具有较大Co(0)颗粒（约>5 nm）和较低钴还原程度的催化剂表现出较高的固有氢化活性和较长的催化剂寿命。有趣的是，MnO和Mn₁₋ₓCoₓO物种通过提高C₅⁺选择性和降低CH₄生成量有效地促进了这些较大的Co(0)颗粒，而它们对含有非常小的Co(0)颗粒的催化剂的选择性没有显著影响。
