The FeIVO• oxyl unit as a key intermediate in water oxidation on the FeIIIhydroxide: DFT predictions

• Published in: International journal of quantum chemistry 2021-05, Vol.121 (10), p.n/a
• Main Authors: Shubin, Aleksandr A., Kovalskii, Viktor Yu, Ruzankin, Sergey Ph, Zilberberg, Igor L., Parmon, Valentin N., Tomilin, Felix N., Avramov, Pavel V.
• Format: Article
• Language: English
• Subjects: Chemistry; Coupling; Cubane; Density functional theory; Hydroxylation; negative spin density; Oxidation; Oxygen; oxyl oxygen; Physical chemistry; Quantum physics; Selectivity; the FeOOH hydroxide; the OO coupling; Water chemistry; water oxidation
• ISSN: 0020-7608; 1097-461X
• DOI: 10.1002/qua.26610

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. The “initial” vertex FeIII(OH) moiety forms this intermediate at the calculated overpotential of 0.93 V by adding one water molecule and withdrawing two proton–electron pairs. 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. Photoelectrochemical water splitting is a promising process in the solar energy conversion to renewable fuels. The OO coupling reaction limits the performance of this process. Although the best catalyst is known to be that based on (Ni,Fe)OOH, a consensus on the mechanism has yet to be obtained. On base of DFT, the present work suggests that the HOFeIVO• oxyl site constitutes an origin of the catalyst activity due to the spin‐polarization of radical center.