Revealing the dominant reactive oxygen species in aqueous amine solutions for carbon capture

Abstract

Reactive oxygen species (ROS) play a critical role in the oxidative degradation of amine solvents for carbon dioxide (CO₂) capture, resulting in solvent loss and the formation of harmful byproducts. While hydroxyl radicals (˙OH) or direct oxidation by metal cations have been proposed as potential initiators of the oxidative degradation, the specific ROS involved remains unclear. In this study, we propose that superoxide (O₂⁻) may be the dominant ROS under alkaline conditions during CO₂ capture, based on the thermodynamic analysis of reduction reactions. Using quantum mechanical (QM) calculations, we further demonstrate that ferrous iron (Fe²⁺) complexes, when coordinated to electron-donating ligands such as carbamates and bicarbonates, exhibit significantly lower reduction potentials, making them effective reductants for O₂. The large concentration of these ligands in CO₂-loaded amine solution may allow for the production of such reductive Fe complexes and in turn facilitate O₂ reduction to O₂⁻. Finally, we propose a reaction mechanism, supported by molecular dynamics simulations, where O₂⁻ initiates the decomposition of protonated monoethanolamine. These findings offer new insights into the primary ROS involved in the oxidative degradation of aqueous amines and suggest strategies to mitigate solvent degradation in CO₂ capture processes.