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

Abstract

Feδ+/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 O2- acting as an electron donor served to encapsulate FeOCl to form FeOCl-SiO2, whose interface bore Feδ+/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 Feδ+ 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 Feδ+, 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 Feδ+/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.