Unraveling the\nOrigin of Superior Activity of FeOCl\nfor Photocatalytic Fenton Reaction

The photoaccelerated Fenton reaction on semiconductors\nhas gained\nincreasing attention for wastewater treatment, with FeOCl showing\nsuperior OH<sup>•</sup> radical generation. However, the atomic-level\nmechanism behind this enhanced activity remains unclear. In this study,\nwe performed first-principles calculations to compare the photo-Fenton\nreaction on FeOCl(100) under photocatalytic and thermocatalytic conditions.\nOur results identify the [Fe<sup>2+</sup>–Fe<sup>3+</sup>]\nunit as the key active site driving the reaction. Fe<sup>2+</sup> promotes\nthe cleavage of the O–O bond in H<sub>2</sub>O<sub>2</sub> to\ngenerate the OH<sup>•</sup> radical, while Fe<sup>3+</sup> aids\nin the desorption of OH<sup>•</sup>. Under photocatalytic conditions,\nthe enhanced activity results from rapid Fe<sup>2+</sup>/Fe<sup>3+</sup> cycling driven by a photogenerated electron. In contrast, thermocatalysis\nrelies on additional H<sub>2</sub>O<sub>2</sub> to reduce Fe<sup>3+</sup> to Fe<sup>2+</sup>. Although the photogenerated holes can\nalso contribute by trapping OH<sup>–</sup> to form OH<sup>•</sup> radical, their effect is relatively secondary due to the lower hole-trapping\ncapacity of the FeOCl(100) surface compared to electron trapping.\nA comparison with the Fe<sub>2</sub>O<sub>3</sub>(012) catalyst reveals\nthat while both promote O–O bond under photoirradiation, Fe<sub>2</sub>O<sub>3</sub>(012) suffers from stronger OH binding due to\nits higher electron donation capacity. In addition, the V-shaped surface\nstructure of Fe<sub>2</sub>O<sub>3</sub>(012) promotes bidentate adsorption,\nintensifying the excessive adsorption of intermediates and limiting\nOH<sup>•</sup> desorption. In contrast, FeOCl’s moderate\nOH adsorption contributes to its superior catalytic efficiency. This\nstudy provides atomic-level insights into the photo-Fenton mechanism,\nhighlighting the role of light, and offers guidance for designing\nmore effective Fenton catalysts by comparing FeOCl with Fe<sub>2</sub>O<sub>3</sub>.