光合放氧复合物锰簇光组装*

在光系统II水氧化过程中, 由锰簇及其附近一个具氧化还原活性的酪氨酸Y_Z组成的光合放氧复合物通过4个连续的氧化还原反应将水裂解为质子和氧气。在光系统II的功能性组装过程中,光合放氧复合物锰簇是通过一个被称作光组装的过程形成的。结合当前的研究进展,本文详尽地介绍了光合放氧复合物锰簇光组装的动力学模型、与锰簇形成相关的蛋白及锰簇的外围配体、锰与去锰光系统II的结合特性、其它辅助因子对锰簇光组装的影响及锰簇光组装的机理。此外,结合我们的研究结果,对人工合成的锰化合物与去锰光系统II放氧复合物的光组装进行了综述和讨论。本文最后介绍了目前光合放氧复合物锰簇结构及功能研究中存在的一些问题,并对光合放氧复合物锰簇结构及功能研究的应用前景进行了展望。     

光合放氧复合物结构及其放氧机理的研究*

光合水氧化是地球上最重要的生化过程之一.光合放氧生物包括光系统Ⅰ(PSⅠ)和光系统Ⅱ(PSⅡ)两种类型反应中心,光系统Ⅱ反应中心能以水作为电子给体,利用光能氧化水产生质子和氧气.对于水如何被氧化这个难题前人已做了大量的工作,但到目前为止放氧复合物(OEC)的结构及水氧化的机理仍不清楚.本文结合当前研究结果,就光合放氧复合物的结构及光合放氧机理进行了综述,希望能有助于推进这方面的工作.     

光系统Ⅱ水氧化机制的研究进展

光系统Ⅱ(PSⅡ)的水氧化机制对于揭示光合作用的奥秘和促进人工光合的发展具有重要意义，但由于目前还不能通过实验确定处于S₄态的Mn₄CaO₅释放氧气的确切机制，因此该机制长期以来广受关注.自Kok循环模型提出后，PSⅡ的水氧化机制研究取得了巨大的进展.本文首先介绍了光合作用和PSⅡ的整体结构、PSⅡ的放氧复合物(OEC)、电子传递链及Kok循环，其次重点概述了目前几种广为人知的水氧化机制假设，包括“水亲核进攻”机制(WNA)、“自由基耦合”机制(RC)、“亲核氧-氧耦合”机制(NOOC)以及“双通道”机制，以期为后续PSⅡ水氧化机制研究和人工光合研究提供参考.     

单核第一过渡周期金属水氧化催化剂（英文）

由于传统化石能源的不可再生性,其储量日益减少.同时,传统化石能源的使用对环境产生了巨大影响,给人类社会带来了一系列问题,包括温室效应、酸雨等.因此,进入二十一世纪以后,人类面临着日益严峻的能源危机和环境问题,寻找清洁、高效的替代能源已经迫在眉睫.太阳能被认为是一种洁净的可再生能源.自然界通过光合作用将太阳能转化为化学能,在这一过程中,水被氧化产生氧气,同时释放出的电子和质子通过和二氧化碳作用生成碳水化合物.为了模拟这一过程,人工光合作用可以直接将电子和质子结合形成氢气.由此生成的氢气也被认为是洁净的可再生能源,因为在其燃烧过程中只产生水.因此,通过光致水分解析氢析氧的人工光合作用受到了越来越广泛的重视.水分解可以分为两个独立的半反应,即水的氧化析氧和水的还原析氢.水的氧化无论在热力学还是动力学方面,都存在着非常大的阻碍.在热力学上,两分子的水氧化生成一分子氧气需要提供很多能量(ΔE=1.23 V vs NHE).在动力学上,由于涉及到四个氢原子和两个氧原子的重组,并且涉及到氧氧键形成并释放出一分子氧气,因此水氧化是一个非常缓慢的过程.在自然界,水的氧化主要发生在光合作用中,在绿色植物的叶绿体中完成.通过对光合作用的研究,科学家们发现氧气的产生由光系统Ⅱ(PSII)中的释氧中心来完成.释氧中心是一个钙锰簇合物,由四个锰和一个钙组成(Mn_4CaO_x).自然界水分解产生氧气的过程给了我们很大启示,对设计和研究高效稳定的水氧化催化剂具有一定的指导意义.目前水氧化催化剂主要有两大类.第一类是基于材料的水氧化催化剂.该类催化剂的催化效率高,过电势小,但是对水氧化催化过程的机理缺乏深入研究.第二类是基于金属配合物的分子催化剂.相比基于材料的催化剂,分子催化剂具有以下特点:(1)分子催化剂的结构可以通过实验手段表征清楚;(2)可以结合光谱对水氧化的机理进行深入研究,可以对催化过程中间体进行表征;(3)催化剂的结构可以从分子水平上进行修饰,因此可以更好地研究催化效率与结构之间的关系,为设计高效、稳定的催化剂提供必要信息;(4)比较容易组装成分子器件从而应用到实际的水氧化装置中;(5)通过实验与理论的结合,对氧氧成键提出新的认识与理解.近几年来,一些单核的金属配合物逐渐被发现可以高效、稳定地催化水氧化.研究表明,一些基于钌和铱的催化剂具有良好的催化活性,但由于金属钌和铱储量少、价格昂贵等因素,限制了该类催化剂的大量使用.由于第一过渡系金属元素具有储量丰富、安全无毒、廉价易得等优势,第一过渡周期金属化合物逐渐成为科学家们研究的热点.近几年来,基于第一过渡系金属的水氧化催化剂已经有大量报道.本文主要总结了近几年来基于第一过渡系金属的单核水氧化分子催化剂.通过对催化机理进行深入的讨论,特别是对氧氧成键的总结,本文将对设计合成结构新颖、具有高催化效率和良好稳定性的水氧化分子催化剂提供理论依据.     

二维配位聚合物[Mn(2，4-DAA)2(4，4′-bipy)]n的合成、晶体结构及荧光性质研究

近年来,人们对锰配合物的研究引起极大兴趣,这主要归因于锰的配合物有其生物功能.生物体内的过氧氢化酶、过氧歧化酶和放氧复合物,这3种酶的作用都涉及锰(Ⅱ)离子,锰离子参与了生物体内的氧化还原反应,是绿色植物的第二类光合系统的活性中心[1].     

非均相催化臭氧氧化反应机制

臭氧催化氧化作为高级氧化技术是目前水处理领域研究的热点,其中非均相臭氧催化氧化技术因其氧化能力强、降低臭氧投加量特别是能显著提高有机物矿化率等优点而备受关注。非均相催化臭氧氧化领域不断研究新的催化剂,但是其反应过程及机制更加复杂。催化臭氧氧化的性能很大程度上取决于催化剂及其表面性质。污染物在催化剂表面形成络合物,或者臭氧在催化剂表面分解产生不同的含氧物种如表面氧原子、过氧化物和羟基自由基等。本文评述了非均相臭氧催化氧化反应中存在的多种机理,主要是自由基理论、氧空位理论、表面原子氧理论、表面络合物理论和臭氧直接氧化理论。催化剂表面的羟基基团是主要的催化活性中心,本文探讨了表面羟基基团催化反应机制,得出催化剂表面性质决定其表面活性位点的特性及含量,对诱导臭氧分解产生含氧活性物种起了关键作用;概述了催化剂改性后的结构形态、比表面积及其性能和作用机制;并讨论了非均相臭氧催化氧化反应催化剂未来的发展趋势,为催化臭氧氧化污水处理技术提供了理论参考。     

二氧化铈表面氧的活化及对氧化反应的催化作用

稀土催化材料的研究和发展为La和Ce等高丰度稀土元素的高质、高效利用提供了有效的途径．Ce基催化剂由于具有优异的储放氧性能,在氧化反应中得到广泛应用．本文介绍了采用理论模拟的方法,在研究Ce的4f轨道和结构弛豫对CeO2氧空位的形成和迁移中的作用以及作为载体时助催化作用的最新研究成果,提出了铈锆固溶体高储放氧性能的本质原因,为进一步制备高性能的氧化铈基储放氧材料和催化剂提供了理论基础．同时,对铈基催化剂在甲烷催化燃烧、CO催化氧化和卤代烃催化燃烧等反应中的应用和催化作用进行了综述,重点讨论了CeO2及其复合催化剂的氧化还原性能与其活性之间的关系．最后对在铈基催化剂研究中存在的问题和发展思路提出了思考和展望．     

钯基氧还原和乙醇氧化反应电催化剂:关于结构和机理研究的一些近期见解

质子交换膜燃料电池和直接乙醇燃料电池已成为可持续性清洁能源研究的一个聚焦点。在燃料电池中,氧还原反应和乙醇氧化反应是两个重要的反应,其相关高活性、高稳定性并且廉价的催化剂的研发仍然存在很多问题,极大地制约了燃料电池的大规模商业化应用。其中的挑战主要来自于对纳米催化剂结构和反应机理的有限认识。由于实验表征理论计算的结合,对钯基合金纳米材料电催化剂的研究得到了很大的进展。本文从实验和理论计算两个方面出发,重点讨论了应用于氧还原反应和乙醇氧化反应的钯和钯基电催化剂的结构和反应机理方面的近期研究的一些见解。这些见解对未来催化剂的设计与优化有一定的启发意义。     

钯基氧还原和乙醇氧化反应电催化剂：关于结构和机理研究的一些近期见解（英文）

质子交换膜燃料电池和直接乙醇燃料电池已成为可持续性清洁能源研究的一个聚焦点。在燃料电池中,氧还原反应和乙醇氧化反应是两个重要的反应,其相关高活性、高稳定性并且廉价的催化剂的研发仍然存在很多问题,极大地制约了燃料电池的大规模商业化应用。其中的挑战主要来自于对纳米催化剂结构和反应机理的有限认识。由于实验表征理论计算的结合,对钯基合金纳米材料电催化剂的研究得到了很大的进展。本文从实验和理论计算两个方面出发,重点讨论了应用于氧还原反应和乙醇氧化反应的钯和钯基电催化剂的结构和反应机理方面的近期研究的一些见解。这些见解对未来催化剂的设计与优化有一定的启发意义。     

Pt/HM和Pd/HM催化剂表面氧的恢复与供出活化能测定

催化氧化是最重要的工业催化反应之一。七十年代以来,关于氧化催化剂的基础研究取得了长足的进步,采用各种手段对氧化催化剂的表面及体相氧种进行了考察。本文作者根据研究实用氧化催化剂的经验,结合控制论方法,亦提出了多相催化氧化反应的集团结构适应性规律。近年来,作者用TPD和吸附平衡法研究了V_2O_5/SiO_2催化剂表面氧的种类、数目和供出活性,较好地关联了催化氧化的一些事实,使以上规律更趋于半定量化。在多相氧化反应中,催化剂表面活性氧不断处于供出和恢复的循环之中,催化剂表面氧中心的供     

The possible role of proton-coupled electron transfer (PCET) in water oxidation by photosystem II

All higher life forms use oxygen and respiration as their primary energy source. The oxygen comes from water by solar-energy conversion in photosynthetic membranes. In green plants, light absorption in photosystem II (PSII) drives electron-transfer activation of the oxygen-evolving complex (OEC). The mechanism of water oxidation by the OEC has long been a subject of great interest to biologists and chemists. With the availability of new molecular-level protein structures from X-ray crystallography and EXAFS, as well as the accumulated results from numerous experiments and theoretical studies, it is possible to suggest how water may be oxidized at the OEC. An integrated sequence of light-driven reactions that exploit coupled electron-proton transfer (EPT) could be the key to water oxidation. When these reactions are combined with long-range proton transfer (by sequential local proton transfers), it may be possible to view the OEC as an intricate structure that is "wired for protons".     

What spectroscopy reveals concerning the Mn oxidation levels in the oxygen evolving complex of photosystem II: X-ray to near infra-red

Photosystem II (PS II), found in oxygenic photosynthetic organisms, catalyses the most energetically demanding reaction in nature, the oxidation of water to molecular oxygen and protons. The water oxidase in PS II contains a Mn(4)Ca cluster (oxygen evolving complex, OEC), whose catalytic mechanism has been extensively investigated but is still unresolved. In particular the precise Mn oxidation levels through which the cluster cycles during functional turnover are still contentious. In this, the first of several planned parts, we examine a broad range of published data relating to this question, while considering the recent atomic resolution PS II crystal structure of Umena et al. (Nature, 2011, 473, 55). Results from X-ray, UV-Vis and NIR spectroscopies are considered, using an approach that is mainly empirical, by comparison with published data from known model systems, but with some reliance on computational or other theoretical considerations. The intention is to survey the extent to which these data yield a consistent picture of the Mn oxidation states in functional PS II - in particular, to test their consistency with two current proposals for the mean redox levels of the OEC during turnover; the so called 'high' and 'low' oxidation state paradigms. These systematically differ by two oxidation equivalents throughout the redox accumulating catalytic S state cycle (states S(0)S(3)). In summary, we find that the data, in total, substantially favor the low oxidation proposal, particularly as a result of the new analyses we present. The low oxidation state scheme is able to resolve a number of previously 'anomalous' results in the observed UV-Visible S state turnover spectral differences and in the resonant inelastic X-ray spectroscopy (RIXS) of the Mn pre-edge region of the S(1) and S(2) states. Further, the low oxidation paradigm is able to provide a 'natural' explanation for the known sensitivity of the OEC Mn cluster to cryogenic near infra-red (NIR) induced turnover to alternative spin/redox states in S(2) and S(3).     

Water oxidation catalysed by manganese compounds: from complexes to 'biomimetic rocks'

One of the most fundamental processes of the natural photosynthetic reaction sequence is the light-driven oxidation of water to molecular oxygen. In vivo, this reaction takes place in the large protein ensemble Photosystem II, where a μ-oxido-Mn(4)Ca- cluster, the oxygen-evolving-complex (OEC), has been identified as the catalytic site for the four-electron/four-proton redox reaction of water oxidation. This Perspective presents recent progress for three strategies which have been followed to prepare functional synthetic analogues of the OEC: (1) the synthesis of dinuclear manganese complexes designed to act as water-oxidation catalysts in homogeneous solution, (2) heterogeneous catalysts in the form of clay hybrids of such Mn(2)-complexes and (3) the preparation of manganese oxide particles of different compositions and morphologies. We discuss the key observations from the studies of such synthetic manganese systems in order to shed light upon the catalytic mechanism of natural water oxidation. Additionally, it is shown how research in this field has recently been motivated more and more by the prospect of finding efficient, robust and affordable catalysts for light-driven water oxidation, a key reaction of artificial photosynthesis. As manganese is an abundant and non-toxic element, manganese compounds are very promising candidates for the extraction of reduction equivalents from water. These electrons could consecutively be fed into the synthesis of "solar fuels" such as hydrogen or methanol.     

Mechanisms for proton release during water oxidation in the S2 to S3 and S3 to S4 transitions in photosystem II

The new high-resolution X-ray structure of photosystem II has allowed more detailed studies than before of water oxidation at the oxygen evolving complex (OEC). In the present study the two final S-transitions of water oxidation are studied. The electron coupled proton transfers are followed from the center of the OEC to Asp61, which is considered as the start of the transfer chain through the protein to the lumenal side. It is found that the proton transfers occur in multiple steps. Structures of intermediates and energy diagrams are derived and compared to experimental observations. Since the new experimental structure of the OEC is very similar to the one suggested earlier by density functional calculations, the O-O bond formation step remains essentially the same as the one suggested five years ago. An interesting new result is that the barrier for proton transfer within the OEC actually competes with the O-O bond formation step of being rate-limiting.     

Theoretical clues to the mechanism of dioxygen formation at the oxygen-evolving complex of photosystem II

The mechanism of the generation of dioxygen at the oxygen-evolving complex (OEC) of photosystem II (PSII), a crucial step in photosynthesis, is still under debate. The simplest unit present in the OEC that can produce O2 is a dinuclear oxo-bridge manganese complex within the tetranuclear Mn4 cluster. In this paper we report a theoretical study of the model complexes [Mn2(mu-O)2(NH3)6(H2O)2]n+ (n = 2-5), for which density functional calculations have been carried out for several electronic configurations. The molecular orbital picture deduced from the calculations indicates that one-electron oxidation of the Mn2IV,IV/(O2-)2 complex (n = 4) mostly affects the oxygen atoms, thus ruling out the existence of a MnV oxidation state in this context, while the incipient formation of an O-O bond in the O2(3-) transient species evolves exothermally toward the dissociation of dioxygen and a Mn2II,III couple. These results identify the electronic features that could be needed to enable an intramolecular mechanism of oxygen-oxygen bond formation to exist at the OEC during photosynthesis.     

Visible‐Light‐Driven Water Oxidation by a Molecular Manganese Vanadium Oxide Cluster

Photosynthetic water oxidation in plants occurs at an inorganic calcium manganese oxo cluster, which is known as the oxygen evolving complex (OEC), in photosystem II. Herein, we report a synthetic OEC model based on a molecular manganese vanadium oxide cluster, [Mn₄V₄O₁₇(OAc)₃]^3?. The compound is based on a [Mn₄O₄]⁶⁺ cubane core, which catalyzes the homogeneous, visible‐light‐driven oxidation of water to molecular oxygen and is stabilized by a tripodal [V₄O₁₃]^6? polyoxovanadate and three acetate ligands. When combined with the photosensitizer [Ru(bpy)₃]²⁺ and the oxidant persulfate, visible‐light‐driven water oxidation with turnover numbers of approximately 1150 and turnover frequencies of about 1.75 s^?1 is observed. Electrochemical, mass‐spectrometric, and spectroscopic studies provide insight into the cluster stability and reactivity. This compound could serve as a model for the molecular structure and reactivity of the OEC and for heterogeneous metal oxide water‐oxidation catalysts.     

Towards Long‐Term Photostability of Nickel Hydroxide/BiVO4 Photoanodes for Oxygen Evolution Catalysts via In Situ Catalyst Tuning

Increasing long‐term photostability of BiVO₄ photoelectrode is an important issue for solar water splitting. The NiOOH oxygen evolution catalyst (OEC) has fast water oxidation kinetics compared to the FeOOH OEC. However, it generally shows a lower photoresponse and poor stability because of the more substantial interface recombination at the NiOOH/BiVO₄ junction. Herein, we utilize a plasma etching approach to reduce both interface/surface recombination at NiOOH/BiVO₄ and NiOOH/electrolyte junctions. Further, adding Fe²⁺ into the borate buffer electrolyte alleviates the active but unstable character of etched‐NiOOH/BiVO₄, leading to an outstanding oxygen evolution over 200 h. The improved charge transfer and photostability can be attributed to the active defects and a mixture of NiOOH/NiO/Ni in OEC induced by plasma etching. Metallic Ni acts as the ion source for the in situ generation of the NiFe OEC over long‐term durability.     

A model of the oxygen-evolving center of photosystem II predicted by structural refinement based on EXAFS simulations

A refined computational structural model of the oxygen-evolving complex (OEC) of photosystem II (PSII) is introduced. The model shows that the cuboidal core Mn3CaO4 with a "dangler" Mn ligated to a corner mu4-oxide ion is maximally consistent with the positioning of the amino acids around the metal cluster as characterized by XRD models and high-resolution spectroscopic data, including polarized EXAFS of oriented single crystals and isotropic EXAFS. It is, therefore, natural to expect that the proposed structural model should be particularly useful to establish the structure of the OEC, consistently with high-resolution spectroscopic data, and for elucidating the mechanism of water-splitting in PSII as described by the intermediate oxidation states of the EC along the catalytic cycle.     

Towards Long-Term Photostability of Nickel Hydroxide/BiVO4 Photoanodes for Oxygen Evolution Catalysts via In Situ Catalyst Tuning

Increasing long-term photostability of BiVO₄ photoelectrode is an important issue for solar water splitting. The NiOOH oxygen evolution catalyst (OEC) has fast water oxidation kinetics compared to the FeOOH OEC. However, it generally shows a lower photoresponse and poor stability because of the more substantial interface recombination at the NiOOH/BiVO₄ junction. Herein, we utilize a plasma etching approach to reduce both interface/surface recombination at NiOOH/BiVO₄ and NiOOH/electrolyte junctions. Further, adding Fe²⁺ into the borate buffer electrolyte alleviates the active but unstable character of etched-NiOOH/BiVO₄, leading to an outstanding oxygen evolution over 200 h. The improved charge transfer and photostability can be attributed to the active defects and a mixture of NiOOH/NiO/Ni in OEC induced by plasma etching. Metallic Ni acts as the ion source for the in situ generation of the NiFe OEC over long-term durability.