Ruthenium complexes with non-innocent ligands: Electron distribution and implications for catalysis

Ruthenium complexes with the non-innocent ligands (NILs) benzoquinone, iminobenzoquinone and benzoquinonediimine and their redox derivatives exhibit intriguing electronic properties. With the proper ligand set the NIL π* orbitals mix extensively with the ruthenium dπ orbitals resulting in delocalized electron distributions and non-integer oxidation states, and in most of these systems a particular ruthenium oxidation state dominates. This review critically examines the electronic structure of Ru–NIL systems from both an experimental and computational (DFT) perspective. The electron distribution within these complexes can be modulated by altering both the ancillary ligands and the NIL, and in a few cases the resultant electron distributions are exploited for catalysis. The Ru–NIL systems that perform alcohol oxidation and water oxidation catalysis are discussed in detail. The Tanaka catalyst, an anthracene-bridged dinuclear Ru complex, is an intriguing example of a Ru–NIL framework in catalysis. Unlike other known ruthenium water oxidation catalysts, the two Ru atoms remain low valent during the catalytic cycle according to DFT calculations, some experimental evidence, and predictions based on the behavior of the related mononuclear species.     

钙钛矿型水氧化电催化剂

在全球能源结构“清洁化”转型的背景下,可再生能源的开发与利用能够有效解决能源危机与环境问题,符合我国的可持续发展路线。能源转换与储存技术贯穿着循环能源技术的各个环节,是新型能源框架的核心支撑。 水氧化反应是众多能源体系（例如, 水裂解反应、 二氧化碳还原反应、 氮还原反应和金属-空气电池）的重要半反应, 但其动力学缓慢, 严重限制了设备的能源效率, 阻碍了相应技术的广泛应用。因此, 亟需开发具有低成本、 高活性、 强稳定性的水氧化电催化剂以降低反应能垒,进而推动能源转换与存储设备的工业化发展。钙钛矿型材料的晶体结构包容性强, 元素组成涵盖广泛, 具有丰富而独特的电子特性, 易于实现表面化学与电子结构的精准调控, 因此被公认为理想的催化材料设计平台。本文综述了钙钛矿型水氧化电催化剂的最新研究进展。首先介绍了钙钛矿型材料的晶体结构和电子特性,归纳了制备钙钛矿型氧化物的代表性的合成策略。通过讨论近期钙钛矿型水氧化电催化剂在酸性和碱性介质中的研究进展, 强调了钙钛矿型电催化剂结构与催化性能间的构效关系。 最后, 我们总结了钙钛矿型水氧化电催化剂在实际应用中面临的挑战与机遇, 提出了相应的建议与解决方案, 期望能使读者更清晰地认识到该领域的未来发展方向。     

The effect of humidity on the ozonolysis of unsaturated compounds in aerosol particles

Atmospheric aerosol particles are important in many atmospheric processes such as: light scattering, light absorption, and cloud formation. Oxidation reactions continuously change the chemical composition of aerosol particles, especially the organic mass component, which is often the dominant fraction. These ageing processes are poorly understood but are known to significantly affect the cloud formation potential of aerosol particles. In this study we investigate the effect of humidity and ozone on the chemical composition of two model organic aerosol systems: oleic acid and arachidonic acid. These two acids are also compared to maleic acid an aerosol system we have previously studied using the same techniques. The role of relative humidity in the oxidation scheme of the three carboxylic acids is very compound specific. Relative humidity was observed to have a major influence on the oxidation scheme of maleic acid and arachidonic acid, whereas no dependence was observed for the oxidation of oleic acid. In both, maleic acid and arachidonic acid, an evaporation of volatile oxidation products could only be observed when the particle was exposed to high relative humidities. The particle phase has a strong effect on the particle processing and the effect of water on the oxidation processes. Oleic acid is liquid under all conditions at room temperature (dry or elevated humidity, pure or oxidized particle). Thus ozone can easily diffuse into the bulk of the particle irrespective of the oxidation conditions. In addition, water does not influence the oxidation reactions of oleic acid particles, which is partly explained by the structure of oxidation intermediates. The low water solubility of oleic acid and its ozonolysis products limits the effect of water. This is very different for maleic and arachidonic acid, which change their phase from liquid to solid upon oxidation or upon changes in humidity. In a solid particle the reactions of ozone and water with the organic particle are restricted to the particle surface and hence different regimes of reactivity are dictated by particle phase. The potential relevance of these three model systems to mimic ambient atmospheric processes is discussed.     

第一过渡系列金属元素单核水氧化催化剂的最新进展

水的氧化是光合作用的重要步骤,其提供用于二氧化碳固定的电子和质子,以及生物圈所必需的氧气.在将太阳能转换为化学能的人工光合作用中,设计合成高效稳定的水氧化催化剂是研究的关键.目前的催化体系主要是基于钌和铱等贵金属的金属氧化物纳米颗粒和多核金属配合物.基于钌和铱的单核催化体系近年来也得到了广泛的发展.最近几年,第一过渡系列金属元素单核水氧化催化剂快速得到重视.作为配合物中心原子,它们不仅具有丰富的氧化态,而且因相对充足的蕴藏和较低的开采冶炼成本,其具有钌和铱等贵金属不可比拟的重大优势和广阔的应用前景.本文总结了近几年第一过渡系列金属元素单核水氧化催化剂的进展,并在此基础上,简单讨论了氧—氧键的生成,为进一步设计新颖、具有高催化效率和高稳定性的单核水氧化催化剂提供了理论依据.     

Mechanisms of antioxidant action. The behaviour of sulphur dioxide in autoxidizing systems

Sulphur dioxide shows both chain-breaking and peroxide decomposing functions in autoxidizing systems. In a hydroperoxide initiated oxidation, it also shows a pro-oxidant effect at molar ratios (hydroperoxide to sulphur dioxide) of ?1. This reaction results in the formation of sulphur trioxide which is shown to be the true catalyst for peroxide decomposition. Sulphur trioxide is hydrated by water formed by dehydration of oxidation products and becomes less active as a catalyst as a result of phase separation.     

Mn掺杂的铁电半导体BiFeO3本征光催化氧化水活性的研究

开发稳定高效的可见光吸收的氧化物半导体光催化剂是太阳能光催化分解水的一个重要研究方向.最近我们提出(J.Mater.Chem.A,2020,8,6863?6873),具有室温铁电性质的BiFeO3(BFO)薄膜体系表现出低光电流密度响应是由于铁电畴壁/界面处的电荷复合,而该作用在纳米粒子催化剂体系中应该会大大减少.为了...     

Supramolecular Water Oxidation with Ru–bda‐Based Catalysts

Extremely slow and extremely fast new water oxidation catalysts based on the Ru–bda (bda=2,2′‐bipyridine‐6,6′‐dicarboxylate) systems are reported with turnover frequencies in the range of 1 and 900 cycles s^?1, respectively. Detailed analyses of the main factors involved in the water oxidation reaction have been carried out and are based on a combination of reactivity tests, electrochemical experiments, and DFT calculations. These analyses give a convergent interpretation that generates a solid understanding of the main factors involved in the water oxidation reaction, which in turn allows the design of catalysts with very low energy barriers in all the steps involved in the water oxidation catalytic cycle. We show that for this type of system π‐stacking interactions are the key factors that influence reactivity and by adequately controlling them we can generate exceptionally fast water oxidation catalysts.     

Design of Mononuclear Ruthenium Catalysts for Low‐Overpotential Water Oxidation

Water oxidation is a key reaction in natural photosynthesis and in many schemes for artificial photosynthesis. Inspired by energy challenges and the emerging understanding of photosystem II, the development of artificial molecular catalysts for water oxidation has become a highly active area of research in recent years. In this Focus Review, we describe recent achievements in the development of single‐site ruthenium catalysts for water oxidation with a particular focus on the overpotential of water oxidation. First, we introduce the general scheme to access the high‐valent ruthenium–oxo species, the key species of the water‐oxidation reaction. Next, the mechanisms of the O? O bond formation from the active ruthenium–oxo species are described. We then discuss strategies to decrease the onset potentials of the water‐oxidation reaction. We hope this Focus Review will contribute to the further development of efficient catalysts toward sustainable energy‐conversion systems.     

Artificial photosynthesis: from molecular catalysts for light-driven water splitting to photoelectrochemical cells

Photosynthesis has been for many years a fascinating source of inspiration for the development of model systems able to achieve efficient light-to-chemical energetic transduction. This field of research, called "artificial photosynthesis," is currently the subject of intense interest, driven by the aim of converting solar energy into the carbon-free fuel hydrogen through the light-driven water splitting. In this review, we highlight the recent achievements on light-driven water oxidation and hydrogen production by molecular catalysts and we shed light on the perspectives in terms of implementation into water splitting technological devices.     

Source of Selectivity in Oxidative Cross‐Coupling of Aryls by Solvent Effect of 1,1,1,3,3,3‐Hexafluoropropan‐2‐ol

Solvents such as 1,1,1,3,3,3‐hexafluoroisopropanol (HFIP) with a high capacity for donating hydrogen bonds generate solvates that enter into selective cross‐coupling reactions of aryls upon oxidation. When electric current is employed for oxidation, reagent effects can be excluded and a decoupling of nucleophilicity from oxidation potential can be achieved. The addition of water or methanol to the electrolyte allows a shift of oxidation potentials in a specific range, creating suitable systems for selective anodic cross‐coupling reactions. The shift in the redox potentials depends on the substitution pattern of the substrate employed. The concept has been expanded from arene–phenol to phenol–phenol as well as phenol–aniline cross‐coupling. This driving force for selectivity in oxidative coupling might also explain previous findings using HFIP and hypervalent iodine reagents.     

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.     

Boosting Electrochemical Water Oxidation with Metal Hydroxide Carbonate Templated Prussian Blue Analogues

The development of efficient and stable catalyst systems with low‐cost, abundant, and non‐toxic materials is the primary demand for electrochemical water oxidation. A unique method is reported for the syntheses of metal hydroxide carbonate templated Prussian blue analogues (PBAs) on carbon cloth and their outstanding water oxidation activities in alkaline medium. The best water oxidation activity is obtained with cobalt hydroxide carbonate templated t‐Co^II‐Co^III with an overpotential as low as 240 mV to reach a current density of 10 mA cm^?2. It produces constant current over 50 h in chronoamperometric measurements. Moreover, the catalysts outperform the activities of the PBAs prepared without any template and even the noble metal catalyst RuO₂. Spectroscopic and microscopic studies show that the PBAs are transformed into layered hydroxide–oxyhydroxide structures during electrochemical process and provide the active sites for the water oxidation.     

二氰胺钴、镍调控钒酸铋界面载流子传输及光催化产氧研究（英文）

光催化水分解反应是解决当前世界范围严峻的能源与环境问题的一种有效途径.光催化分解水过程可以分为产氢和产氧两个半反应.产氧反应过程复杂,动力学缓慢,是光催化分解水的限速步骤,因此需要探索性能优异的水氧化催化剂(WOCs)来提高产氧半反应的效率.钒酸铋近年来被广泛研究并应用于光催化产氧领域.钒酸铋拥有合适的带宽(2.4 eV)以及较好的稳定性,但是其应用受到其严重的电子空穴复合率、较低的电荷传输能力以及较差的反应动力学的限制.以往研究表明,通过构建复合光催化体系可以有效促进光生电荷的分离与传输,提高材料的光催化性能.因此,我们提出构建新型的BiVO_4/M(dca)_2(M=Co,Ni)复合体系,其中,BiVO_4作为光敏化剂,M(dca)_2作为水氧化催化剂.红外测试和紫外可见测试的结果表明,M(dca)_2通过物理吸附的方式附着在BiVO_4表面,形成BiVO_4/M(dca)_2复合光催化剂体系.复合体系的产氧活性相较于纯BiVO_4有明显的提升.光催化产氧测试结果表明,BiVO_4/Co(dca)2和BiVO_4/Ni(dca)_2复合体系的产氧活性分别可达508.1和297.7μmol/(h·g),而纯BiVO_4的产氧活性只有252.2μmol/(h·g).进一步的稳定性测试结果表明,BiVO_4/Co(dca)2复合体系在30 h的测试过程中能够保持稳定的活性.ICP-MS和XPS的表征结果证明了催化过程中分子催化剂良好的稳定性,排除了反应过程中生成氧化物进而促进产氧活性的可能.对该复合体系的一系列电化学表征证明,M(dca)_2有效改善了BiVO_4/电解液界面的电荷传输性能,从而促进了光催化产氧性能.其中,莫特-肖特基测试表明,M(dca)_2的加入增大了能带弯曲,提高了空穴传递的驱动力,阻抗谱的测试证明了复合体系具有较低的界面电阻,有利于载流子的迁移.通过对复合体系光生载流子分离和注入效率的表征,可以证明,在BiVO_4/M(dca)_2复合体系中,光生空穴能够有效地从BiVO_4迁移到M(dca)_2,进而参与光催化产氧反应并且光催化活性有明显的提升.其中,由于Co(dca)2能够更加有效地改善BiVO_4/电解质的水氧化反应动力学过程,其活性显著优于BiVO_4/Ni(dca)_2体系和纯BiVO_4.此外,基于实验结果和各项表征,我们进一步提出了BiVO_4/Co(dca)2光催化产氧反应的反应机理:光照条件下,BiVO_4中电子跃迁至导带,进而被牺牲剂消耗,而价带上的空穴则传递至分子催化剂进行化学反应,其中,分子催化的反应机理遵循水亲核攻击的模型.     

Electrocatalytic selective oxidation of ethylene glycol: A concise review of catalyst development and reaction mechanism with comparison to thermocatalytic oxidation process

As an important water and seawater degradable plastic monomer, glycolic acid can be synthesized by selective oxidation of ethylene glycol. This review recapitulates recent advances in electrocatalytic ethylene glycol oxidation reaction (EGOR) from the aspects of catalytic performance and reaction mechanism. For catalytic performance evaluation, target product yield and space-time yield are correlated and analyzed for electrocatalytic and thermocatalytic EGOR systems. To elucidate the rationale behind the electrocatalytic selective oxidation of ethylene glycol, previous works using in situ Fourier transform infrared spectroscopy, online differential electrochemical mass spectrometry, ion chromatography, and theoretical calculations to investigate EGOR are systematically reviewed. Finally, the advantages of electrocatalytic EGOR are summarized by comparing electrocatalytic and thermocatalytic processes.     

Oxidation of Water to Molecular Oxygen by One-Electron Oxidants on Transition Metal Hydroxides

Surveyed in this review are the most important achievements in the research and development of catalysts based on Mn, Fe, Co, and Cu hydroxides for the oxidation of water to molecular oxygen by chemical oxidizing agents obtained, for the most part, at Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences. An analysis of the results of kinetic studies on water oxidation in the presence of the above-menthioned catalysts together with data obtained by quantum chemistry methods allowed us to make a conclusion on the general nature and process mechanism both in the presence of artificial catalytic systems based on metal hydroxides and the natural enzyme photosystem II of green plants. The most important properties of hydroxo compounds responsible for catalytic activity in the oxidation of water by one-electron oxidants are discussed, and a possible reaction mechanism is considered.     

Self-assembly of active IrO2 colloid catalyst on an ITO electrode for efficient electrochemical water oxidation

Active catalysts for water oxidation to evolve O(2) are required for the construction of artificial photosynthetic devices that are expected to be promising energy-providing systems in the future. The citrate-stabilized IrO(2) colloid was self-assembled onto an indium tin oxide (ITO) electrode to form a monolayer of the colloidal IrO(2) particles when it was dipped in the colloid solution. The self-assembly could be achieved by a chemical interaction between carboxylate groups on the citrate stabilizer and hydroxyl groups on the ITO surface to form ester bonds. Efficient electrocatalysis for water oxidation was demonstrated using the electrode modified by the self-assembled IrO(2) colloid to yield the highest turnover frequency ((2.3-2.5) x 10(4) h(-1)) of IrO(2) in the hitherto-reported catalysts for electrochemical water oxidation.     

Electrocatalytic Hydrogenation and Oxidation in Aqueous Conditions†

Water molecule contains one oxygen and two hydrogen atoms, making it a potential oxygen and hydrogen source. Electrocatalytic organic reduction and oxidation using water as oxygen and/or hydrogen donors provide an environmentally friendly and sustainable strategy to replace traditional chemical‐driven stoichiometric reactions that use sacrificial reagents. Furthermore, the development of electrochemical synthesis provides a potential application for low tension photoelectricity, which is not cost‐effective during boosted voltage and application. In the last decade, electrocatalytic redox reactions of organic molecules in aqueous media had shown progress owing to the development of electrode materials and water‐splitting technology. This paper highlights several electrocatalytic systems and corresponding mechanisms for both hydrogenation and oxidative transformation of representative compounds. The activation process of protons and water on the working electrode surface has received special focus. Furthermore, paired electrolysis using water as the oxygen and hydrogen source has been demonstrated. This paired system combines hydrogenation and oxidation half‐reactions in one cell using water as the hydrogen and oxygen source, resulting in high atomic and electron utilization rates.     

分子催化剂催化水氧化过程及其O–O成键机理

随着化石燃料的不断消耗和生存环境的日益恶化,可再生、清洁且环境友好的新能源逐渐受到广泛关注与利用.太阳能作为一种洁净的可再生能源,在自然界中,植物可以通过光合作用将太阳能转换成化学能.在该过程中,水分子在光系统II中被氧化而释放出氧气,伴随生成的质子和电子进一步将二氧化碳转化为蕴含生物质能的碳水化合物.在光系统II中,叶绿素P680被光照激发生成阳离子自由基P680^·+,其具有很强的氧化能力,可以从附近的析氧中心中夺取电子.析氧中心通过这一过程失去4个电子,可以将两分子水氧化生成一分子氧气和4个质子.作为水裂解的半反应之一,水氧化在热力学方面需要很多能量来断裂4个O-H键(ΔE=1.23 V vs.NHE),在动力学方面涉及4个氢原子与2个氧原子的重组以及氧气的释放,因而水氧化析氧是一个非常缓慢的过程,如何高效稳定地催化水氧化一直是人们研究的热点和难点.研究发现,自然界中存在的析氧中心为Mn₄CaO(x)的钙锰簇合物,在水氧化过程中生成的Mn=O物种可以被游离的水分子亲核进攻形成O-O键,也可以与桥连μ-O(H)反应生成O-O键.通过对析氧中心持续的研究,在过去几十年中设计合成了一系列具有水氧化催化活性的基于金属配合物的分子催化剂.分子催化剂催化水氧化一般主要分为金属-氧物种的演化过程以及O-O成键过程.通常,金属-氧物种可以通过失电子或质子耦合的失电子过程逐步生成高价态的金属-氧物种,其引发的O-O成键过程通常是水氧化催化循环的决速步骤.基于之前的研究成果,目前主要报道了五种不同的O-O成键机理:(1)水亲核进攻金属-氧物种的WNA机理,(2)金属-氧自由基耦合的I2M机理,(3)金属-羟基自由基耦合的HC机理,(4)分子内进攻桥连氧的IOC机理以及(5)氧化还原异构的RI机理.本文综述了过去几十年水氧化分子催化剂的发展,总结了贵金属钌和铱配合物到第一过渡金属锰、铁、钴、镍和铜配合物催化水氧化过程中金属-氧物种的生成与演化,重点阐述了引发O-O成键过程的高价态金属-氧物种的种类及其不同的O-O成键机理.重点总结了O-O成键中WNA机理与I2M机理的异同,并阐述了催化剂设计对WNA与I2M机理选择性的影响.通过对金属-氧物种种类和O-O成键机理的总结,将有助于进一步设计合成高效稳定的水氧化分子催化剂.