The Geochemical Origins of Water-Oxidation Catalysis

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Technical description.The transition from abiotic to biotic O2 production in the Precambrian remains one of the abiding mysteries of Earth Science. Two of the three O2-producing Mn-enzymatic reactions involve mineral-like structures --- these include disproportionation of H2O2 by Mn-catalase (a dimer-like structure) and oxidation of water by a four-Mn-Ca-oxide cluster (the water-oxidation-center, or WOC) in Photosystem II. The WOC has long been compared to minerals that contain Mn-oxide cubane-like moieties, leading to chemist's models for these reactions, including such things as a MnIV,IV,IV,III -cubane oxide that resembles not only the CaMn4-oxide core of Photosystem-II but also the core of a birnessite-like MnO2 sheet. We discovered that these molecules actually do convert to a birnessite-like structure under load in the photocatalytic cell---nanosheets of MnO2 form and it is these mineral-like sheets, not the delicate molecules, that perform the catalysis of water oxidation to O2. The reaction is furthermore broadly similar to the biogeochemical cycling of manganese in the oceans---initial oxidation of aqueous Mn(II) leads to a Mn(III)/Mn(IV) solid oxide, which absorbs visible photons, photoexcites a O(2p)=Mn(3d) ligand-to-metal-charge transfer and disproportionate to release Mn2+(aq) and reactive oxygen species. In this research project, we detail the pathways for the O2 production in these mineral-like MnO2 sheets.Broader significance and importance. 1) This project has strong implications for helping humans to find truly sustainable energy resources. Human society needs to find a cheap, Earth-abundant catalyst that can help trap energy from visible light. Earth-abundant materials, such as manganese oxides, are essential because the catalysts must work for 10 billion humans---catalysts made from rare elements are impossibly expensive. Using minerals to trap light energy can also be advantageous, as they can last longer than more delicate catalysts. 2) This project also has implications for understanding the origins of photosynthesis, and the relationship between the mineral components and biological systems of this planet. We want to understand how a manganese-oxide mineral that was literally as common as dirt became the central enzyme for photosynthesis. How did the transition occur between the soil mineral and the enzyme? What energy source drove the reaction before there was O2 in the atmosphere? 3) This work will help to strengthen a US-Australia scientific collaboration, as well as the international training of a US student. The NSF Office of International Science and Engineering is supporting the travel of the student to the collaborating laboratory at Monash University in Australia.