Insights into the synergistic effect of active sites in FeMn catalysts for regulating the product distribution of Fischer-Tropsch synthesis

Mn has been recognized as an effective promoter for Fe-based catalysts in Fischer-Tropsch synthesis (FTS), while the surface enrichment mechanisms of Mn during FTS and their structure-activity relationships with olefin selectivity remain elusive. In this study, to clarify the Mn surface enrichment mechanism in low-Mn-doped Fe 2 O 3 catalysts and the regulatory effect of pretreatment on product selectivity in FTS, the structural evolution, dynamic changes in surface species, and structure-performance relationship were systematically investigated for alkali-free xMn-Fe catalysts, which were prepared via modulation of the pretreatment method. The results show that pretreatment significantly regulates the degree of Mn enrichment and catalytic performance. The freeze-dried 7Mn-Fe-F-D sample exhibits the optimal FTS activity, achieving a CO conversion of 81.8% and a C 2 -C 4 = selectivity of 50.9%. In contrast, the high-temperature calcined 7Mn-Fe-650C sample favors long-chain hydrocarbons, with linear α-olefins accounting for 69.8% of C 4 + hydrocarbon products. Mechanistically, freeze-drying promotes the reduction and carburization of Fe species by overcoming the reduction process of Fe 2 O 3 , while high-temperature calcination forms stable α-Fe 2 O 3 to inhibit the reduction and carburization processes. Pretreatment enables precise control of olefin chain length by regulating the degree of Mn enrichment and phase structure, establishing a clear structure-activity relationship for Fe-based catalysts in FTS. • Surface enrichment behavior of Mn in low Mn-doped iron catalysts in FTS is clarified. • A facile pretreatment strategy is developed to achieve regulation of olefin product distribution in FTS. • Structural evolution of iron species under different pretreatment is elucidated via in-situ XRD. • The dynamic reduction/carburization-product selectivity relationship of low Mn-doped iron catalysts is revealed.