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

Abstract

Lewis acidic metals (Mδ+) cleave H2O2 to evolve •OH used for fragmenting aqueous wastes. Mδ+ species are situated near SiO2 and generate interfacial Mδ+-SiO2 domains, where Mδ+ species can avoid aggregation, retain •OH productivity with minimal leaching via chelation to the –OH of SiO2, elevate the collisions between Mδ+ 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 Niδ+-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 O2–/di-sulfide (S22-)/poly-sulfide (SN2-) to Niδ+ species. SiO2 was verified to increase the amount of Niδ+ 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 Niδ+ with H2O2 because of the hydrogen bonding of H2O2 with SiO2, reduce Niδ+ leaching via e- acceptance from O2–/S22-/SN2- rather than via –OH chelation, while making e- migration from Niδ+ to aqueous organics scarcely compelling.