Open Ni site coupled with SO₄^2- functionality to prompt the radical interconversion of ^[rad]OH ↔ SO₄^[rad]− exploited to decompose refractory pollutants

Exposing SO₄²⁻ functionalities on Fe₂O₃ to an electric environment can produce supported SO₄^[rad]− species via radical interconversion of ^[rad]OH ↔ SO₄^[rad]−, which can degrade aqueous contaminants (phenol). This methodology is unique, has substantial potential, yet, is underexplored regarding how to mechanistically account for or to further accelerate radical transfer of ^[rad]OH → SO₄^[rad]−. Herein, NiO serves as a novel platform to accommodate SO₄²⁻ functionalities, whereas the resulting SO₄²⁻-modified NiO (NiO (S)) was kinetically evaluated or subjected to density functional theory (DFT) calculations to clarify the benefits of surface Ni^δ+ (δ ≤ 2), SO₄²⁻, or ^[rad]OH → SO₄^[rad]−. NiO (S) was verified to contain abundant surface-labile O, facilitate electron transfer, and thus could increase H₂O₂ productivity. The Lewis acidity of Ni^δ+ active to cleave H₂O₂ was also improved by SO₄²⁻, as evidenced by in situ diffuse reflectance Infrared Fourier transform spectra and Bader charge calculations. This helped enhance ^[rad]OH productivity of NiO (S) over NiO, whose energetics were computed and validated that ^[rad]OH production was directed by ^[rad]OH desorption from Ni^δ+. Furthermore, electron paramagnetic resonance spectroscopy and DFT calculations also substantiated NiO (S) was energetically favorable toward radical transfer from ^[rad]OH to SO₄^[rad]−, in which Ni bound to SO₄²⁻ was identified to spur the formation of supported SO₄^[rad]− via electron exchange with SO₄²⁻/^[rad]OH. In conjunction with mechanistic elucidation using theoretical calculations, kinetic assessment of H₂O₂ scission and phenol decomposition runs under various electric potentials corroborated ^[rad]OH desorption was the rate-determining step of overall ^[rad]OH → SO₄^[rad]− pathway. Importantly, NiO (S) synthesized at 400 °C could enhance the efficiency, reusability, and stability in decomposing phenol over NiO/Fe₂O₃ functionalized with SO₄²⁻ at 300, 500, or 600 °C.