Computationally Revisiting pH- and Ligand-Dependence of Fenton Reaction Selectivity and Activity in Aqueous Solution

Abstract

The Fe-based Fenton reaction is pivotal in generating reactive oxidative species (ROS) such as OH• radical and iron-oxo FeIVO2+ to degrade wastewater pollutants, yet the selectivity origin of ROS remains debated. Using ab initio molecular dynamics and microkinetic modeling, we investigate the atomic-level Fenton reaction mechanism catalyzed by FeIII-complex [(Cl−)3FeIII(H2O)3] in aqueous solution to quantify ROS activity and selectivity. We demonstrate that FeIII is first reduced to FeII via H2O2 deprotonation and OOH• release, after which FeII enables O−O bond cleavage of a second H2O2, producing OH• and FeIII−OH−. The FeIII−OH− intermediate can either be protonated or oxidized by OH• to form FeIVO2+, driving a pH-dependent selectivity switch: OH• dominates at pH < 2.5, while FeIVO2+ prevails at pH > 2.5. Moreover, Fe-complex ligands regulate FeIII−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 FeII…FeIII 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.