Abstract
The O-O coupling process in water oxidation on the gamma FeOOH hydroxide
catalyst is simulated by means of density functional theory using model
iron cubane cluster
Fe4O4(OH)4. A key
reactive intermediate is proposed to be the HO-FeIV-O•
oxyl unit with terminal oxo radical formed from vertex
HO-FeIV-OH moiety by withdrawal of proton-electron
pair. The O-O coupling goes via water nucleophilic attack on the oxyl
oxygen to form the O-O bond with a remarkably low barrier of 11
kcal/mol. This process is far more effective than alternative scenario
based on direct interaction of two ferryl FeIV=O sites
(with estimated barrier of 36 kcal/mol) and is comparable with the
coupling between terminal oxo center and three-coordinated lattice oxo
center (12 kcal/mol barrier). The process of hydroxylation of terminal
oxygen inhibits the O-O coupling. Nevertheless, being more effective for
ferryl oxygen, the hydroxylation in fact enhances selectivity of the O-O
coupling initiated by the oxyl oxygen.