A single-atom Ce-modified α-Fe₂O₃ catalyst (Fe_0.93Ce_0.07O_x catalyst with 7% atomic percentage of Ce) was synthesized by a citric acid-assisted sol-gel method, which exhibited excellent performance for selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) over a wide operating temperature window. Remarkably, it maintained ∼93% NO conversion efficiency for 168 h in the presence of 200 ppm SO₂ and 5 vol % H₂O at 250 °C. The structural characterizations suggested that the introduction of Ce leads to the generation of local Fe-O-Ce sites in the FeO_x matrix. Furthermore, it is critical to maintain the atomic dispersion of the Ce species to maximize the amounts of Fe-O-Ce sites in the Ce-doped FeO_x catalyst. The formation of CeO₂ nanoparticles due to a high doping amount of Ce species leads to a decline in catalytic performance, indicating a size-dependent catalytic behavior. Density functional theory (DFT) calculation results indicate that the formation of oxygen vacancies in the Fe-O-Ce sites is more favorable than that in the Fe-O-Fe sites in the Ce-free α-Fe₂O₃ catalyst. The Fe-O-Ce sites can promote the oxidation of NO to NO₂ on the Fe_0.93Ce_0.07O_x catalyst and further facilitate the reduction of NO_x by NH₃. In addition, the decomposition of NH₄HSO₄ can occur at lower temperatures on the Fe_0.93Ce_0.07O_x catalyst containing atomically dispersed Ce species than on the α-Fe₂O₃ reference catalyst, resulting in the good SO₂/H₂O resistance ability in the NH₃-SCR reaction.

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