Photochemical water oxidation by uranyl nitrate

Uranyl coordination chemistry enables a wide range of chemical transformations which have been recently reviewed by Arnold et al. [Chem. Rev. 2019, 10595]. Especially the photochemical properties of uranyl complexes are promising.
Among all the photo-assisted transformations reported in the literature a recent paper targets the simultaneous water oxidation (OER) and hydrogen evolution reaction (HER) published by Burn et al [J. Am. Chem. Soc. 2014, 4797].
While the importance of molecular hydrogen as fuel is broadly acknowledged, the revalorization of depleted uranium compounds as potential HER photocatalysts would give them new extra value.

The recent density functional theory study unravels the mechanism for the photochemical water oxidation reaction promoted by uranyl nitrate upon sunlight radiation. In absence of light and in basic media, uranyl nitrate condensates to yield dimeric hydroxo-bridged compounds. A photoinduced spin crossing to the triplet manifold, leads to the formation of uranyl peroxide and hydrogen gas. To provide a detailed picture of the mechanism, characterization of minima, transition states, minimal energy crossing points and adiabatic energies were performed. Although the reported reaction is stoichiometric, further research will explore the viability achieving a photocatalytic hydrogen evolution reaction by reusing the uranyl initial monomer.

Petrus, E.; Segado, M.; Bandeira, N. A. G.; Bo, C. Unveiling a Photoinduced Hydrogen Evolution Reaction Mechanism via the Concerted Formation of Uranyl Peroxide, Inorg. Chem. 59, 8353–8360 (2020).

Key concepts