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Effect of potassium promoter on phase transformation during the isothermal and temperature programmed reduction of the α-Fe₂O₃ catalyst were systematically investigated by in situ XRD. The reduction pathway of Fe₂O₃ based catalysts follows the same sequence of α-Fe₂O₃ → Fe₃O₄ → α-Fe during the isothermal reduction, in contrast to that of α-Fe₂O₃ → Fe₃O₄ → FeO/ α-Fe→ α-Fe during the temperature programmed reduction. The potassium promoter notably inhibits the reduction from α-Fe₂O₃ to Fe₃O₄ due to the enhanced strength of Fe-O bonds, whereas it either accelerates or has no effect on the formation rate of α-Fe crystallites during reduction. The potassium promoter increases the CO conversion during FTS, while enhances C₅+, olefins, CO₂ selectivity, and decreases CH₄ selectivity.