Experimental and DFT Analysis To Anchor the Role of SO₂ in the Heterogeneous NO Reduction over a Char Surface: SO_<i>x</i>-Mediated Oxygen Rearrangement

The heterogeneous reduction of NO by char is important for NO source control during the combustion of organic solid wastes. SO2 manifests a significant impact of the char–NO reaction, yet the inherent mechanism is still vague. In the present study, fixed-bed experiments, in-situ X-ray photoelectron spectroscopy (in-situ XPS), and density functional theory (DFT) calculations were combined to offer empirical and theoretical evidence on the detailed impact mechanism of SO2 on the char–NO reaction. Herein, we propose the surface SOx-mediated oxygen rearrangement (SOx-MOR) mechanism for the first time. SO2 can directly promote char–NO reduction by intervening in the oxygen rearrangement process. In addition, SO2 and O2 play a synergetic role in enhancing the NO reduction. SO2 can be oxidated into SO3 and adsorbed on the char edge, impeding the fast oxidation of char by O2, and enhancing NO reduction. Meanwhile, NO can be reduced via the SO3-MOR mechanism, which possesses a thermodynamic priority lower than that of the SO2-MOR mechanism. In-situ XPS results further verified the surface chemical behaviors of SOx and the higher competitiveness of the SO2-MOR mechanism. The present study lays a foundation for NO source control by SO2.