Stable Water Oxidation Inside A Cage

Solar fuels can be made by splitting water into H₂ and O₂ with the help of sunlight. Most photosynthetic organisms use a manganese-centered structure known as photosystem II for the O₂ evolution portion of that process. But the most active synthetic mimics of that system break down quickly under catalytic conditions as the catalyst molecules react with one another to form a variety of inert species. Siddhartha Das and Binod Nepal of Utah State University have now found a way to keep the catalyst molecules apart by assembling them inside the pores of a metal-organic framework (MOF), like a ship in a bottle (Angew. Chem. Int. Ed. 2013, DOI: 10.1002/anie.201301327). The team prepared the system by floating the reactants into the MOF sequentially. Their assembled Mn unit is too large to diffuse out the way its components came in, or to diffuse between neighboring chambers. The resulting MOF-catalyst system, constructed entirely from Earth-abundant elements, sustains water oxidation for more than seven hours with no signs of slowing. The same catalyst without the MOF cage produces O₂ twice as fast but lasts less than seven minutes. The group now hopes to develop a photoactive MOF to make the reaction self-contained and solar-driven.