The Geochemical Origins of Water-Oxidation Catalysis

技术描述。前寒武纪从非生物到生物氧气生产的转变仍然是地球科学的一个永恒的谜团。三种产生o2的mn酶促反应中有两种涉及矿物样结构，包括光系统II中mn -过氧化氢酶（一种二聚体结构）对H2O2的歧化和4 - mn -ca氧化物簇（水氧化中心，或WOC）对水的氧化。长期以来，WOC一直被与含有锰氧化物类立方烷部分的矿物进行比较，这导致了这些反应的化学家模型，包括MnIV，IV，IV，III -立方烷氧化物，它们不仅类似于光系统ii的camn4 -氧化物核心，而且类似于birnite - MnO2薄片的核心。我们发现，在光催化电池中，这些分子在负载下确实会转化为一种类似氢氧化镁的结构——形成二氧化锰纳米片，正是这些类似矿物质的纳米片，而不是精细的分子，催化水氧化成O2。此外，该反应与海洋中锰的生物地球化学循环大致相似——水中Mn（II）的初始氧化导致Mn(III)/Mn（IV）固体氧化物，它吸收可见光光子，光激发O(2p)=Mn（3d）配体到金属的电荷转移，并且与释放Mn2+(aq)和活性氧不相称。在本研究项目中，我们详细介绍了这些矿物样二氧化锰片中产生氧气的途径。更广泛的意义和重要性。这个项目对于帮助人类找到真正可持续的能源资源具有重要意义。人类社会需要找到一种廉价的、地球上储量丰富的催化剂，来帮助捕获可见光中的能量。地球上丰富的材料，如锰氧化物，是必不可少的，因为催化剂必须为100亿人工作——由稀有元素制成的催化剂是不可能昂贵的。使用矿物来捕获光能也是有利的，因为它们比更精细的催化剂持续时间更长。2)该项目对理解光合作用的起源，以及地球上矿物成分与生物系统之间的关系也有意义。我们想了解一种像泥土一样常见的氧化锰矿物是如何成为光合作用的核心酶的。土壤矿物质和酶之间的转变是如何发生的？在大气中有氧气之前，是什么能量驱动了反应？3)这项工作将有助于加强美国与澳大利亚的科学合作，以及美国学生的国际培训。NSF国际科学与工程办公室支持该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.