Axial coordination engineering for single-atom catalysts in bifunctional oxidation of NO and mercury

Coal-fired power plants are major emitters of nitrogen oxides (NO) and elemental mercury (Hg⁰), both of which pose significant environmental and health risks. While wet flue gas desulfurization (WFGD) and electrostatic precipitators (ESP) are effective in removing oxidized mercury (Hg²⁺) and particulate-bound mercury (Hg^p), capturing volatile Hg⁰ remains a significant challenge. Catalytic oxidation is a promising approach to convert NO and Hg⁰ into their more easily captured oxidized forms (NO₂ and Hg²⁺), highlighting the need for highly efficient catalysts. In this study, graphene-supported iron single-atom catalysts (Fe SACs) with various axial ligands were systematically investigated using density functional theory (DFT). Adsorption energies of O₂ and NO, along with energy barriers for key oxidation steps, were calculated to evaluate catalytic performance. Among the ten Fe₁N₄-X catalysts examined, Fe₁N₄-Br exhibited the lowest reaction energy barriers, while Fe₁N₄-H₂O showed the highest turnover frequency (TOF) for both NO and Hg⁰ oxidation under simulated flue gas conditions. These results demonstrate the importance of axial ligand coordination in tuning catalytic activity. This work offers theoretical insights for the rational design of high-performance SACs for pollutant control in coal-fired flue gas treatment systems.