A series of iron oxide catalysts prepared from ferric/ferrous mixed salts were studied to explore the structure–performance relationship for CO2 hydrogenation. The high selectivity to C5+ hydrocarbons (47 %) at a high CO2 conversion (34 %) could be maintained when the initial Fe3+/Fe2+ ratio in preparation varies from 2/1 to 2.5/0.5, while both the CO2 conversion and selectivity to C5+ hydrocarbons show an obvious decline as the ratio deviates from the range. Combined with various characterization results, the volcano-like evolution of CO2 conversion and selectivity to C5+ hydrocarbons is related to the structure and phase transition of the iron oxide catalysts. With the increase of Fe3+/Fe2+ ratio, the catalyst composition gradually changes from Fe3O4 nanoparticles to β-FeOOH nanorods, meanwhile, the size of Fe3O4 nanoparticles decreases. The smaller Fe3O4 nanoparticles could provide higher active surface area for improved reducibility and CO2 adsorption capacity, whereas the presence of β-FeOOH nanorods is not favorable for the reduction and carburization of catalysts. Consequently, the variation of initial phase composition and structure would affect the formation of Hägg iron carbide (χ-Fe5C2) phase during reaction, which has decisive effect on higher hydrocarbons synthesis from CO2 hydrogenation.
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