An activity and XANES study of Mn-promoted, Fe-based Fischer-Tropsch catalysts

被引:65
作者
Campos, Andrew [1 ]
Lohitharn, Nattaporn [2 ]
Roy, Amitava [3 ]
Lotero, Edgar [4 ]
Goodwin, James G., Jr. [4 ]
Spivey, James J. [1 ]
机构
[1] Louisiana State Univ, Cain Dept Chem Engn, Baton Rouge, LA 70803 USA
[2] Logos Technol Inc, Ashburn, VA 20147 USA
[3] Louisiana State Univ, Ctr Adv Microstruct & Devices, Baton Rouge, LA 70806 USA
[4] Clemson Univ, Dept Chem & Biomol Engn, Clemson, SC 29634 USA
关键词
Fischer-Tropsch; Iron; Manganese; XANES; MANGANESE OXIDE CATALYSTS; IRON-BASED CATALYST; MOSSBAUER-SPECTROSCOPY; SURFACE CHARACTERIZATION; CATION DISTRIBUTION; REDUCTION BEHAVIOR; CARBON-MONOXIDE; SUPPORTED FE; HYDROGENATION; CARBURIZATION;
D O I
10.1016/j.apcata.2009.11.015
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
Iron-based Fischer-Tropsch (FT) catalysts with a mol-based formula of (100 - x)Fe/xMn/5Cu/17SiO(2) (x <= 20), were prepared using co-precipitation methods. The calcined catalysts were first activated in H-2 for 12 h, then reacted in flowing syngas at 1.8 atm, 280 degrees C, and a 2:1 ratio of H2:CO. The fresh and reacted catalysts were characterized using X-ray absorption near-edge structure (XANES) to determine changes in the oxidation state and the atomic-level environment of the Fe and Mn atoms. XANES spectra of the fresh calcined and reacted catalyst were taken using the K edges of Fe (7.112 KeV) and Mn (6.540 KeV) for various Mn-metal loadings (x = 0, 5, 20). The FT activity significantly increased with Mn promotion, indicating significant Fe-Mn interactions. The least squares fitting of the reacted catalyst shows that higher Mn loadings lead to decreased FexC concentration and increased Fe3O4 concentration. Principal Component Analysis (PCA) of Fe indicates that the Fe2O3, Fe3O4, theta-Fe3C phases were present in either the calcined or reacted catalyst. One additional Fe-containing phase was present in the catalyst but was not identified using the Fe standards. The PCA of Mn showed the presence of Mn2O3, as well as one additional Mn-containing phase. The Mn XANES of the reacted 95Fe5Mn and 80Fe20Mn catalysts show that Mn was a mixture of the 2+ and 3+ oxidation states. The average oxidation state of Mn in the reacted 95Fe5Mn catalyst was 2.24 +/- 0.07, consistent with the formation of an additional phase, identified as (Fe1-yMny)(3)O-4. FEFF calculations have shown relatively good agreement for Mn-substitution of octahedral Fe-sites in Fe3O4 (28664-ICSD), specifically in the pre-edge region; corresponding to the composition (Fe1-yMny)(3)O-4. Fe-based FT catalysts deactivate when carbon deposition occurs on larger iron carbide clusters. This study has shown less carbon deposition, FexC formation, and higher CO hydrogenation activity with the Mn-promoted catalysts. This indicates that (Fe1-yMny)(3)O-4 was responsible for the formation of smaller clusters of FexC, which were more active for CO hydrogenation and were less prone to deactivation through carbon deposition. (C) 2009 Elsevier B.V. All rights reserved.
引用
收藏
页码:12 / 16
页数:5
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