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

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Iron-based Fischer-Tropsch (FT) catalysts with a mol-based formula of (100 - x)Fe/xMn/5Cu/17SiO (x ≤ 20), were prepared using co-precipitation methods. The calcined catalysts were first activated in H for 12 h, then reacted in flowing syngas at 1.8 atm, 280 °C, and a 2:1 ratio of H :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 Fe C concentration and increased Fe O concentration. Principal Component Analysis (PCA) of Fe indicates that the Fe O , Fe O , θ-Fe C 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 Mn O , 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 (Fe Mn ) O . FEFF calculations have shown relatively good agreement for Mn-substitution of octahedral Fe-sites in Fe O (28664-ICSD), specifically in the pre-edge region; corresponding to the composition (Fe Mn ) O . Fe-based FT catalysts deactivate when carbon deposition occurs on larger iron carbide clusters. This study has shown less carbon deposition, Fe C formation, and higher CO hydrogenation activity with the Mn-promoted catalysts. This indicates that (Fe Mn ) O was responsible for the formation of smaller clusters of Fe C, which were more active for CO hydrogenation and were less prone to deactivation through carbon deposition. © 2009 Elsevier B.V. 2 2 2 x 3 4 2 3 3 4 3 2 3 1-y y 3 4 3 4 1-y y 3 4 x 1-y y 3 4 x

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Applied Catalysis A: General

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