The Geochemical Origins of Water-Oxidation Catalysis

在前寒武纪，从非生物产氧到生物产氧的转变仍然是地球科学长期未解的谜团之一。在三个产生O2的锰酶促反应中，有两个涉及类矿物结构-包括光系统II中的四个锰钙氧化物簇(水氧化中心，或WOC)对H_2O_2的歧化反应(二聚体结构)和水的氧化。长期以来，WOC一直被比作含有锰氧化物立方烷部分的矿物，导致了化学家对这些反应的模型，包括像MnIV，IV，IV，III-立方烷氧化物，不仅类似于Photossystem-II的CaMn4-氧化物核心，而且也类似于水钠锰矿状MnO2薄片的核心。我们发现，在光催化池中，这些分子确实在负载下转化为水钠铝石状结构-纳米MnO2片状，正是这些矿物状片状，而不是微妙的分子，执行水氧化为O2的催化。此外，该反应与海洋中锰的生物地球化学循环大体相似-水中锰(II)的初始氧化导致Mn(III)/Mn(IV)固体氧化物，该固体氧化物吸收可见光子，光激发O(2p)=Mn(3d)配体-金属电荷转移，并不成比例地释放Mn2+(Aq)和活性氧物种。在本研究项目中，我们详细介绍了在这些类矿物MnO2片材中产生O2的途径。1)该项目对帮助人类找到真正可持续的能源具有重要意义。人类社会需要找到一种廉价的、地球上丰富的催化剂，帮助从可见光中捕获能量。富含地球的材料，如锰氧化物，是必不可少的，因为催化剂必须为100亿人工作-稀有元素制成的催化剂贵得令人难以置信。使用矿物来捕捉光能也是有利的，因为它们可以比更精致的催化剂持续更长的时间。2)该项目还对理解光合作用的起源，以及地球上矿物成分和生物系统之间的关系具有重要意义。我们想要了解一种和泥土一样常见的氧化锰矿物是如何成为光合作用的中心酶的。土壤矿物质和酶之间的转换是如何发生的？在大气中有氧气之前，是什么能源驱动了这一反应？3)这项工作将有助于加强美国和澳大利亚的科学合作，以及对一名美国学生的国际培训。NSF国际科学与工程办公室正在支持这名学生前往澳大利亚莫纳什大学的合作实验室。

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.