Computationally revisiting pH- and ligand-dependence of Fenton reaction selectivity and activity in aqueous solution

The Fe-based Fenton reaction is pivotal in generating reactive oxidative species (ROS) such as OH˙ radicals and iron-oxo Fe^IVO²⁺ to degrade wastewater pollutants, yet the selectivity origin of ROS remains debated. Using <i>ab initio</i> molecular dynamics and microkinetic modeling, we investigate the atomic-level Fenton reaction mechanism catalyzed by the Fe^III-complex [(Cl^-)₃Fe^III(H₂O)₃] in aqueous solution to quantify ROS activity and selectivity. We demonstrate that Fe^III is first reduced to Fe^II<i>via</i> H₂O₂ deprotonation and OOH˙ release, after which Fe^II enables O-O bond cleavage of a second H₂O₂, producing OH˙ and Fe^III-OH^-. The Fe^III-OH^- intermediate can either be protonated or oxidized by OH˙ to form Fe^IVO²⁺, driving a pH-dependent selectivity switch: OH˙ dominates at pH < 2.5, while Fe^IVO²⁺ prevails at pH > 2.5. Moreover, Fe-complex ligands regulate Fe^III-OH^- stability and affect ROS selectivity/activity by modulating the OH intermediate binding strength, which linearly correlates with the O-O bond cleavage barrier and OH˙ desorption kinetics. Comparing homogeneous Fe-complex catalysis to the state-of-the-art heterogeneous FeOCl, we highlight that optimized OH binding at the Fe^II⋯Fe^III dual site of FeOCl facilitates O-O bond cleavage while ensuring efficient OH˙ desorption, leading to higher activity. These findings provide atomic-level insights into pH-dependent ROS selectivity and ligand effects, advancing our understanding of both homogeneous and heterogeneous Fenton catalysis.