The promotion role of different crystal phase MnO2 in iron-based Fischer-Tropsch catalysts

Abstract

Manganese oxide additive is an effective promoter for modulating the catalytic performance of iron-based Fischer-Tropsch synthesis (FTS) catalysts and studied extensively, however, the promotion effects of MnO2 crystal structures on iron-based FTS catalysts have never been reported. In this work, four types of MnO2 with crystal phases corresponding to α-, β-, γ- and δ-MnO2 were prepared, and their promotion functions on iron-based FTS catalysts were studied. These four types of MnO2 exhibit dramatically distinct promotion effects on the structure evolution and catalytic performances. The ferrite MnFe2O4 only forms in the α-MnO2 modified catalysts, while the β-, γ- and δ-MnO2 also present different existing forms in the catalysts. The presence of abundant oxygen vacancies on the surface of α- and δ-MnO2 notably facilitates the cleavage of Fe-O bonds and the dissociation of CO, leading to the enhancing formation of iron carbides in the activated and spent catalysts. Meanwhile, the strong Mn-Fe electronical interactions formed in α- and δ-MnO2 modified catalysts lead to high density of surface carbon species with a long residence time, dramatically improving the selectivity of linear high carbon α-olefins and heavy hydrocarbons. In contrast, the β- and γ-MnO2 modifying dramatically increases the catalytic activity, but suppresses the formation of olefins and long chain hydrocarbons, due to their relatively few amounts of surface oxygen vacancies and moderate Mn-Fe electronical interactions. This work provides new insights and in-depth understanding of the promotion roles of Mn additive, benefiting for the rational design of high performance iron-based FTS catalysts.