Experimental\nand DFT Analysis To Anchor the Role of\nSO<sub>2</sub> in the Heterogeneous NO Reduction over a Char Surface:\nSO<sub><i>x</i></sub>‑Mediated Oxygen Rearrangement

The heterogeneous reduction of NO\nby char is important for NO source\ncontrol during the combustion of organic solid wastes. SO<sub>2</sub> manifests a significant impact of the char–NO reaction, yet\nthe inherent mechanism is still vague. In the present study, fixed-bed\nexperiments, in-situ X-ray photoelectron spectroscopy (in-situ XPS),\nand density functional theory (DFT) calculations were combined to\noffer empirical and theoretical evidence on the detailed impact mechanism\nof SO<sub>2</sub> on the char–NO reaction. Herein, we propose\nthe surface SO<sub><i>x</i></sub>-mediated oxygen rearrangement\n(SO<sub><i>x</i></sub>-MOR) mechanism for the first time.\nSO<sub>2</sub> can directly promote char–NO reduction by intervening\nin the oxygen rearrangement process. In addition, SO<sub>2</sub> and\nO<sub>2</sub> play a synergetic role in enhancing the NO reduction.\nSO<sub>2</sub> can be oxidated into SO<sub>3</sub> and adsorbed on\nthe char edge, impeding the fast oxidation of char by O<sub>2</sub>, and enhancing NO reduction. Meanwhile, NO can be reduced via the\nSO<sub>3</sub>-MOR mechanism, which possesses a thermodynamic priority\nlower than that of the SO<sub>2</sub>-MOR mechanism. In-situ XPS results\nfurther verified the surface chemical behaviors of SO<sub><i>x</i></sub> and the higher competitiveness of the SO<sub>2</sub>-MOR mechanism. The present study lays a foundation for NO source\ncontrol by SO<sub>2</sub>.