Water oxidation versus hydroxylation by the terminal oxo center of the Fe(III)-hydroxide: DFT predictions

TL;DR

This study uses DFT to compare water oxidation and hydroxylation processes at terminal oxo centers in Fe(III)-hydroxide, revealing that water oxidation via O-O coupling is energetically favorable and more efficient than other pathways.

Contribution

It provides new insights into the mechanisms of water oxidation at terminal oxo centers, highlighting the role of the HO-Fe(IV)-O• group in facilitating O-O coupling.

Findings

O-O coupling via the HO-Fe(IV)-O• group has a low energy barrier.

Water oxidation is more effective than oxo-oxo coupling on neighboring sites.

Hydroxylation of oxyl oxygen occurs with a similar barrier, indicating competitive pathways.

Abstract

The O-O coupling process is considered on terminal monocoordinated oxo centers in the gamma FeOOH hydroxide modeled by iron tetramer cubane cluster with the Fe4O4 core. The density functional theory predicts that reactive HO-Fe(IV)-O$\bullet$ group formed from hydroxide by second withdrawal of proton-electron pair is capable to couple the OH moiety of water molecule with a low barrier. This process is far more effective than direct coupling of oxo centers on neighboring metal sites and is comparable with the coupling between terminal oxo center and three-coordinated lattice oxo center. The competing process of hydroxylation of oxyl oxygen to form the hydroxo group is equally probable having similar barrier.