Deciphering Evolution Pathway of Supported NO₃^•Enabled via Radical Transfer from ^•oH to Surface NO₃^-Functionality for Oxidative Degradation of Aqueous Contaminants

NO₃^•can compete with omnipotent ^•OH/SO₄^•-in decomposing aqueous pollutants because of its lengthy lifespan and significant tolerance to background scavengers present in H₂O matrices, albeit with moderate oxidizing power. The generation of NO₃^•, however, is of grand demand due to the need of NO₂^•/O₃, radioactive element, or NaNO₃/HNO₃in the presence of highly energized electron/light. This study has pioneered a singular pathway used to radicalize surface NO₃^-functionalities anchored on polymorphic α-/γ-MnO₂surfaces (α-/γ-MnO₂-N), in which Lewis acidic Mn^2+/3+and NO₃^-served to form ^•OH via H₂O₂dissection and NO₃^•via radical transfer from ^•OH to NO₃^-(^•OH → NO₃^•), respectively. The elementary steps proposed for the ^•OH → NO₃^•route could be energetically favorable and marginal except for two stages such as endothermic ^•OH desorption and exothermic ^•OH-mediated NO₃^-radicalization, as verified by EPR spectroscopy experiments and DFT calculations. The Lewis acidic strength of the Mn^2+/3+species innate to α-MnO₂-N was the smallest among those inherent to α-/β-/γ-MnO₂and α-/γ-MnO₂-N. Hence, α-MnO₂-N prompted the rate-determining stage of the ^•OH → NO₃^•route (^•OH desorption) in the most efficient manner, as also evidenced by the analysis on the energy barrier required to proceed with the ^•OH → NO₃^•route. Meanwhile, XANES and in situ DRIFT spectroscopy experiments corroborated that α-MnO₂-N provided a larger concentration of surface NO₃^-species with bi-dentate binding arrays than γ-MnO₂-N. Hence, α-MnO₂-N could outperform γ-MnO₂-N in improving the collision frequency between ^•OH and NO₃^-species and in facilitating the exothermic transition of NO₃^-functionalities to surface NO₃^•analogues per unit time. These were corroborated by a greater efficiency of α-MnO₂-N in decomposing phenol, in addition to scavenging/filtration control runs and DFT calculations. Importantly, supported NO₃^•species provided 5-7-fold greater efficiency in degrading textile wastewater than conventional ^•OH and supported SO₄^•-analogues we discovered previously.