Manganese (Mn) oxides are some of the most redox reactive minerals in natural waters, with a strong tendency to grab electrons from organic molecules, thereby breaking down natural organic matter into carbon dioxide. Working at the Molecular Foundry and the ALS, researchers used the ultrafast spectroscopy facilities to develop a new mechanism for the photoreduction of birnessite that considered electronic and structural transitions that occurred on the picosecond, nanosecond, and much longer timescales. The result is one of the most detailed depictions of a mineral redox reaction ever established experimentally.
In surface waters, including oceans, rivers and lakes, sunlight is well known to drive manganese oxide redox chemistry. It had long been assumed that electron-rich organic molecules were required to serve as electron donors for Mn reduction driven by light. A recent theoretical prediction, however, suggested that photoexcitation of birnessite, a common Mn(IV) oxide that forms a lamellar nanosheet structure, could lead to net reduction in water in the absence of any organic molecule. This work has confirmed the prediction, and additional insights into the mechanisms of the redox reaction, have now been revealed.