Tailoring Lewis acidic metals and SO₄²⁻ functionalities on bimetallic Mn-Fe oxo-spinels to exploit supported SO₄^[rad]− in aqueous pollutant fragmentation

Generation of SO₄^[rad]− anchored on metal oxides via radical transfer from ^•OH to surface SO₄²⁻ functionality (^•OH → SO₄^[rad]−) is singular, unraveled recently, and promising to decompose aqueous refractory contaminants. The core in furthering supported SO₄^[rad]− production is to reduce the energy required to accelerate the rate-determining step of the ^•OH → SO₄^[rad]− (^•OH desorption), while increasing the collision frequency between the ^•OH precursors (H₂O₂) and H₂O₂ activators (Lewis acidic metals) or between SO₄²⁻-attacking radicals (^•OH) and supported SO₄^[rad]− precursors (SO₄²⁻). Herein, Mn-substituted Fe₃O₄ oxo-spinels (Mn_XFe_3-XO₄; Mn_X) served as reservoirs to accommodate the Lewis acidic Fe/Mn and SO₄²⁻, whose properties were tailored by altering the metal compositions (X). The production of supported SO₄^[rad]− via the ^•OH → SO₄^[rad]− was of high tangibility, as confirmed by their electron paramagnetic resonance spectra coupled with those simulated. A concave trend was observed in the plot of the Lewis acidic strength of Fe/Mn versus X of Mn_X with the minimum at X ~ 1.5. Hence, Mn_1.5 could expedite ^•OH liberation from the surface most proficiently and therefore exhibited the greatest initial H₂O₂ scission rate, as corroborated by its lowest energy barrier needed for activating the ^•OH → SO₄^[rad]−. Meanwhile, a volcano-shaped trend was found in the plot of SO₄²⁻ concentration versus X of Mn_X (other than Mn₃). This could tentatively increase the collision frequency between ^•OH and SO₄²⁻ on the surface of Mn_1.5, as partially substantiated by its second largest pre-factor among the catalysts. Therefore, Mn_1.5 exhibited the highest phenol consumption rate (-r_{PHENOL, 0}) among the catalysts, which was ~ 20-fold larger than those for SO₄²⁻-modified Fe₂O₃ and NiO, which we reported previously. Mn_1.5 also outperformed other catalysts in recycling phenol degradation, fragmenting another pollutant (aniline), and mineralizing phenol/aniline.