Grasping pros and cons of SiO₂ proximal to Lewis acidic metal cations in activating H₂O₂ heterolysis under electric environments

Lewis acidic metals (M^δ+) cleave H₂O₂ to evolve ^•OH used for fragmenting aqueous wastes. M^δ+ species are situated near SiO₂ and generate interfacial M^δ+-SiO₂ domains, where M^δ+ species can avoid aggregation, retain ^•OH productivity with minimal leaching via chelation to the –OH of SiO₂, elevate the collisions between M^δ+ and H₂O₂ or between ^•OH and aqueous pollutants via confinement, and destabilize contaminants via electron (e^-) transfer. The merits of SiO₂ stated earlier, however, remain abstract and only partially clarified. Herein, interfacial Ni^δ+-SiO₂ domains were created on SiO₂-modified NiS/Ni₉S₈ (NiS/Ni₉S₈-SiO₂) to elaborate the aforementioned advantages of SiO₂ under electric conditions, where it was hypothesized that e^- transfer can occur from O^2–/di-sulfide (S₂^2-)/poly-sulfide (S_N^2-) to Ni^δ+ species. SiO₂ was verified to increase the amount of Ni^δ+ speices for NiS/Ni₉S₈-SiO₂ and reduce their Lewis acidic strengths, which was conducive to lowering the energy barrier required for H₂O₂ dissection. SiO₂ was demonstrated to hardly promote the collisions of Ni^δ+ with H₂O₂ because of the hydrogen bonding of H₂O₂ with SiO₂, reduce Ni^δ+ leaching via e^- acceptance from O^2–/S₂^2-/S_N^2- rather than via –OH chelation, while making e^- migration from Ni^δ+ to aqueous organics scarcely compelling.