Characterization of photo-enhanced water oxidation in cobalt oxide catalysts using model Co₄O₄ cubane

Hydrogen generation through water oxidation would allow for an abundant resource that could replace fossil fuels as a primary source of energy. Water oxidation catalysts still operate at high overpotentials, and the dominant mechanisms for O–O bond formation are still under debate. The cobalt oxide catalyst CoPi is a type of water oxidation catalyst that, in addition to its similarity to the oxygen evolving complex in photosystem-II, has photocurrent effects which have selective enhancement and inhibition of catalytic activity depending on the wavelength of light—600 nm and 400 nm, respectively. Understanding these photocurrent effects provides insight into favorable conditions for water oxidation, which could guide the design of future catalysts. However, due to its amorphous structure, it is difficult to characterize the dynamics of CoPi, and thus molecular models provide better resolution. Using Co₄O₄(OAc)₄(Py)₄ (Co₄O₄) as a model for the CoPi active site, the ultrafast excited state dynamics at the 400 nm excitation were probed using X-ray transient absorption at the cobalt K-edge. In combination with quantum chemical calculations, the results here explore the early-time changes in electronic and molecular structure upon excitation.