Unraveling the Origin of Superior Activity of FeOCl for Photocatalytic Fenton Reaction

The photoaccelerated Fenton reaction on semiconductors has gained increasing attention for wastewater treatment, with FeOCl showing superior OH• radical generation. However, the atomic-level mechanism behind this enhanced activity remains unclear. In this study, we performed first-principles calculations to compare the photo-Fenton reaction on FeOCl(100) under photocatalytic and thermocatalytic conditions. Our results identify the [Fe2+–Fe3+] unit as the key active site driving the reaction. Fe2+ promotes the cleavage of the O–O bond in H2O2 to generate the OH• radical, while Fe3+ aids in the desorption of OH•. Under photocatalytic conditions, the enhanced activity results from rapid Fe2+/Fe3+ cycling driven by a photogenerated electron. In contrast, thermocatalysis relies on additional H2O2 to reduce Fe3+ to Fe2+. Although the photogenerated holes can also contribute by trapping OH– to form OH• radical, their effect is relatively secondary due to the lower hole-trapping capacity of the FeOCl(100) surface compared to electron trapping. A comparison with the Fe2O3(012) catalyst reveals that while both promote O–O bond under photoirradiation, Fe2O3(012) suffers from stronger OH binding due to its higher electron donation capacity. In addition, the V-shaped surface structure of Fe2O3(012) promotes bidentate adsorption, intensifying the excessive adsorption of intermediates and limiting OH• desorption. In contrast, FeOCl's moderate OH adsorption contributes to its superior catalytic efficiency. This study provides atomic-level insights into the photo-Fenton mechanism, highlighting the role of light, and offers guidance for designing more effective Fenton catalysts by comparing FeOCl with Fe2O3.