Reinventing primary reactive oxygen species evolved from H₂O₂ heterolysis on FeOCl via SiO₂ encapsulation

Fe^δ+/3+ defects on FeOCl surface interact with H₂O₂ to produce diverse reactive oxygen species (ROS) including ^•OH, ^•OOH, O₂^•-, Fe⁴⁺=O, and ¹O₂, whose relative contributions to aqueous pollutant degradation have been debatable and only partially clarified. Herein, SiO₂ with O^2- acting as an electron donor served to encapsulate FeOCl to form FeOCl-SiO₂, whose interface bore Fe^δ+/3+ distinct from those of FeOCl surface under H₂O₂-containing aqueous phases in terms of composition and electron affinity. The FeOCl-SiO₂ interface bore plentiful Fe^δ+ and minute Fe³⁺, from which Fe⁴⁺=O was generated yet remained barely accessible to bulky contaminants. Conversely, the FeOCl surface afforded plentiful Fe³⁺ and a non-negligible amount of Fe^δ+, from which copious O₂^•- and a moderate amount of Fe⁴⁺=O were produced, respectively, with high accessibility to bulky pollutants. Albeit with the production of ^•OH on FeOCl and FeOCl-SiO₂, plots of their initial contaminant decomposition rates versus contaminant ionization potentials subjected to the correction for contaminant adsorption or Fe^δ+/3+ leaching along with scavenging/recycle runs corroborated that Fe⁴⁺=O and/or ¹O₂ function as the major ROS in fragmenting aqueous wastes upon exposure of the FeOCl-containing catalysts to H₂O₂-rich conditions. This was unanticipated when considering that the lifetimes and redox potentials of Fe⁴⁺=O and ¹O₂ are smaller than the corresponding values of ^•OH and that the evolution of ^•OH, Fe⁴⁺=O, and ¹O₂ on FeOCl was energetically favorable, as demonstrated by density functional theory calculations.