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

Fe ^δ+/3+ defects on FeOCl surface interact with H ₂O ₂ to produce diverse reactive oxygen species (ROS) including ^•OH, ^•OOH, O ₂ ^•-, Fe ⁴⁺=O, and ¹O ₂, whose relative contributions to aqueous pollutant degradation have been debatable and only partially clarified. Herein, SiO ₂ with O ^2- acting as an electron donor served to encapsulate FeOCl to form FeOCl-SiO ₂, whose interface bore Fe ^δ+/3+ distinct from those of FeOCl surface under H ₂O ₂-containing aqueous phases in terms of composition and electron affinity. The FeOCl-SiO ₂ interface bore plentiful Fe ^δ+ and minute Fe ³⁺, from which Fe ⁴⁺=O was generated yet remained barely accessible to bulky contaminants. Conversely, the FeOCl surface afforded plentiful Fe ³⁺ and a non-negligible amount of Fe ^δ+, from which copious O ₂ ^•- and a moderate amount of Fe ⁴⁺=O were produced, respectively, with high accessibility to bulky pollutants. Albeit with the production of ^•OH on FeOCl and FeOCl-SiO ₂, 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 Fe ⁴⁺=O and/or ¹O ₂ function as the major ROS in fragmenting aqueous wastes upon exposure of the FeOCl-containing catalysts to H ₂O ₂-rich conditions. This was unanticipated when considering that the lifetimes and redox potentials of Fe ⁴⁺=O and ¹O ₂ are smaller than the corresponding values of ^•OH and that the evolution of ^•OH, Fe ⁴⁺=O, and ¹O ₂ on FeOCl was energetically favorable, as demonstrated by density functional theory calculations.