Revisiting the syngas conversion to olefins over Fe-Mn bimetallic catalysts: Insights from the proximity effects

Abstract

Fe-Mn bimetallic catalysts have shown great promises in direct conversion of syngas to olefins via Fischer-Tropsch Synthesis (FTS). However, due to the structural complexity of Fe-based catalysts and the interference of secondary promoters, the promotional effects of Mn on phase transition of Fe-based FTS catalysts and catalytic performance are still under debate. Herein, Fe-Mn bimetallic catalysts were prepared from two different methods, namely solution combustion method (SCM) and co-precipitation (CP) method. Fe4Mn1-SCM exhibited an excellent C5+ selectivity (45 %) and stability over 100 h. In contrast, the product distribution given by Fe4Mn1-CP was dominated by C1-C4 hydrocarbons (85 %). The superior chain growth ability of the Fe4Mn1-SCM was attributed to the abundant interfacial Fe-MnOx structure, which retarded the oxygen removal and carbon diffusion process, resulting in less χ-Fe5C2 phase but more ε-Fe2.2C phase that was highly active for C–C coupling. Moreover, Mn-induced promotional effects were analyzed via kinetic measurements, physiochemical characterizations, Operando Raman-mass spectroscopy and theoretical calculations. It was found that the Fe-Mn proximity played a key role in controlling the interaction of structural and electronic impacts between Mn and Fe species. These findings may provide new insights for fabrication of Fe-based FTS catalysts with tunable product distribution.