In Situ Hydroxylation of a Single-Atom Iron Catalyst for Preferential 1O2 Production from H2O2

In Fenton or Fenton-like reactions, ·OH obtained by the reductive activation of H2O2 is regarded as the major reactive oxygen species (ROS); however, the generation of other ROS like O2·– and 1O2 cannot be negligible. Since the degradation capability of a certain ROS would be varied to distinctive pollutants, the regulation of ROS production from H2O2 activation should be applicable for a more targeted pollutant degradation. Herein, a series of carbon nitride (C3N4)-supported Fe catalysts with the state of Fe ranging among single-atom, oxide-cluster, and nanoparticle catalysts were fabricated and their activities in photo-Fenton reactions were evaluated. It was uncovered that the single-atom catalyst favored the generation of O2·– and 1O2 via the oxidative activation of H2O2 and the selectivity of 1O2 toward ·OH dynamically increased with the proceeding of H2O2 activation, while the catalysts with an abundance of oxide clusters exhibited a significantly higher conversion of H2O2 to ·OH. In situ FT-IR studies demonstrated that during the H2O2 activation, the single-atom catalyst underwent a more significant surface hydroxylation than the oxide-cluster catalyst. Such a result was further consolidated by the theoretical calculation that the adsorption energy of surface hydroxyl was remarkably higher on the single-atom catalyst. Thereafter, distinctive from the easy desorption of hydroxyl during the reduction of H2O2 to ·OH in oxide-cluster catalysts, the in situ surface hydroxylation on the single-atom catalyst alters the adsorption mode of H2O2 to a H-bonded structure, which steers the selectivity of ROS generation to a more favored oxidative transformation of H2O2 to O2·–/1O2. This work uncovers the decisive role of surface hydroxyl in regulating ROS generation in a heterogeneous Fenton reaction.