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 [Fe 2+ −Fe 3+ ] unit as the key active site driving the reaction. Fe 2+ promotes the cleavage of the O−O bond in H 2 O 2 to generate the OH • radical, while Fe 3+ aids in the desorption of OH • . Under photocatalytic conditions, the enhanced activity results from rapid Fe 2+ /Fe 3+ cycling driven by a photogenerated electron. In contrast, thermocatalysis relies on additional H 2 O 2 to reduce Fe 3+ to Fe 2+ . 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 Fe 2 O 3 (012) catalyst reveals that while both promote O−O bond under photoirradiation, Fe 2 O 3 (012) suffers from stronger OH binding due to its higher electron donation capacity. In addition, the V-shaped surface structure of Fe 2 O 3 (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 Fe 2 O 3 .
