Water oxidation in all oxygenic photosynthetic organisms is catalysed by the Mn(4)CaO(4) cluster of Photosystem II. This cluster has inspired the development of synthetic manganese catalysts for solar energy production. A photoelectrochemical device, made by impregnating a synthetic tetranuclear-manganese cluster into a Nafion matrix, has been shown to achieve efficient water oxidation catalysis. We report here in situ X-ray absorption spectroscopy and transmission electron microscopy studies that demonstrate that this cluster dissociates into Mn(II) compounds in the Nafion, which are then reoxidized to form dispersed nanoparticles of a disordered Mn(III/IV)-oxide phase. Cycling between the photoreduced product and this mineral-like solid is responsible for the observed photochemical water-oxidation catalysis. The original manganese cluster serves only as a precursor to the catalytically active material. The behaviour of Mn in Nafion therefore parallels its broader biogeochemistry, which is also dominated by cycles of oxidation into solid Mn(III/IV) oxides followed by photoreduction to Mn(2+).
Hocking, R., Brimblecombe, R., Chang, L-Y., Singh, A., Cheah, M. H., Glover, C., Casey, W., & Spiccia, L. (2011). Water-oxidation catalysis by manganese in a geochemical-like cycle. Nature Chemistry, 3(6), 461 - 466. https://doi.org/10.1038/NCHEM.1049