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

Abstract

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 °C, and a 2:1 ratio of H 2: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 x C concentration and increased Fe 3O 4 concentration. Principal Component Analysis (PCA) of Fe indicates that the Fe 2O 3, Fe 3O 4, θ-Fe 3C 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 2O 3, 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 1− y Mn y ) 3O 4. FEFF calculations have shown relatively good agreement for Mn-substitution of octahedral Fe-sites in Fe 3O 4 (28664-ICSD), specifically in the pre-edge region; corresponding to the composition (Fe 1− y Mn y ) 3O 4. Fe-based FT catalysts deactivate when carbon deposition occurs on larger iron carbide clusters. This study has shown less carbon deposition, Fe x C formation, and higher CO hydrogenation activity with the Mn-promoted catalysts. This indicates that (Fe 1− y Mn y) 3O 4 was responsible for the formation of smaller clusters of Fe x C, which were more active for CO hydrogenation and were less prone to deactivation through carbon deposition.