Molecular complexes in water oxidation: Pre-catalysts or real catalysts

Artificial photosynthesis is considered a promising method to produce clean and renewable energy by sunlight. To accomplish this aim, development of efficient and robust catalysts for water oxidation and hydrogen production is extremely important. Owing to the advantages of easily modified structures and traceable catalytic processes, molecular water oxidation catalysts (WOCs) attract much attention during the past decade. However, the transformation of molecular WOCs to metal oxides/hydroxides or metal ions may occur under the harsh catalytic conditions, making the identification of true active species difficult. In this article, recent progress on molecular complexes acting as real catalysts or precursors toward water oxidation was briefly reviewed. We summarized the commonly used physical techniques and chemical methods that enable to distinguish homogeneous catalysts from heterogeneous catalysts. The factors that affect the nature of WOCs, such as reaction conditions, transition metal centers, and supporting ligands were discussed as well.     

Heteroatom‐doped MoSe2 Nanosheets with Enhanced Hydrogen Evolution Kinetics for Alkaline Water Splitting

Electrochemical water splitting for hydrogen generation is a vital part for the prospect of future energy systems, however, the practical utilization relies on the development of highly active and earth‐abundant catalysts to boost the energy conversion efficiency as well as reduce the cost. Molybdenum diselenide (MoSe₂) is a promising nonprecious metal‐based electrocatalyst for hydrogen evolution reaction (HER) in acidic media, but it exhibits inferior alkaline HER kinetics in great part due to the sluggish water adsorption/dissociation process. Herein, the alkaline HER kinetics of MoSe₂ is substantially accelerated by heteroatom doping with transition metal ions. Specifically, the Ni‐doped MoSe₂ nanosheets exhibit the most impressive catalytic activity in terms of lower overpotential and larger exchange current density. The density functional theory (DFT) calculation results reveal that Ni/Co doping plays a key role in facilitating water adsorption as well as optimizing hydrogen adsorption. The present work paves a new way to the development of low‐cost and efficient electrocatalysts towards alkaline HER.     

A Dinuclear Ruthenium‐Based Water Oxidation Catalyst: Use of Non‐Innocent Ligand Frameworks for Promoting Multi‐Electron Reactions

Insight into how H₂O is oxidized to O₂ is envisioned to facilitate the rational design of artificial water oxidation catalysts, which is a vital component in solar‐to‐fuel conversion schemes. Herein, we report on the mechanistic features associated with a dinuclear Ru‐based water oxidation catalyst. The catalytic action of the designed Ru complex was studied by the combined use of high‐resolution mass spectrometry, electrochemistry, and quantum chemical calculations. Based on the obtained results, it is suggested that the designed ligand scaffold in Ru complex 1 has a non‐innocent behavior, in which metal–ligand cooperation is an important part during the four‐electron oxidation of H₂O. This feature is vital for the observed catalytic efficiency and highlights that the preparation of catalysts housing non‐innocent molecular frameworks could be a general strategy for accessing efficient catalysts for activation of H₂O.     

钌基催化剂催化的气固相反应

催化剂被广泛应用于各种化学品的生产,从原子尺度了解整个催化反应体系有利于合理设计新型催化剂.参与气固相反应的催化剂主要有贵金属催化剂和过渡金属催化剂.近年来, Ru基催化剂由于在低温低压下表现出良好的催化活性而广泛应用于一些气固相反应.本文对 Ru的基本性质、氧化行为以及 Ru基催化剂的理论研究进行综述.介绍了钌基催化剂参与的气固相反应,包括挥发性有机物的催化氧化、一氧化碳优先氧化(PROX)、氨合成、氯化氢氧化以及甲烷部分氧化,分析了催化性能与理化性质之间的构效关系,提出了钌基催化剂在相关反应中存在的问题以及未来发展趋势. Ru具有多种氧化态,在 Ru基催化剂参与的气固相反应中,金属 Ru和/或 RuO2被认为是活性物种,通常反应温度在400oC以下. Ru (0001)晶面在 O2存在条件下,随着氧气含量的不同会从中间态过渡到氧化态,实验证明该晶面属于 RuO2.理论研究证实了在反应过程中 RuO2的存在,并提出了核壳结构,对于其它气固相反应的机理研究有一定启发.挥发性有机物(VOC)的催化氧化主要集中烷烃、烯烃、芳烃以及卤代烃的催化氧化,催化剂的理化性质包括颗粒粒径、价态和晶体结构等对催化活性有很大影响,并且 Ru基催化剂对卤代烃的催化氧化表现出良好的抗卤性,同时多卤代副产物低于其它贵金属体系. Ru基催化剂在低温条件下对 PROX具有高的活性和选择性,并且可以有效抑制 H2氧化、CO甲烷化和CO2甲烷化等副反应发生.氨合成的难点在于 N≡N具有很强的解离能,许多研究表明,氨合成使用的 Ru基催化剂的催化性能与载体性质密切相关, Ru与载体之间强的相互作用使得电子可以迅速地从载体转移到 Ru颗粒上,掺杂其它有效元素可能会提供更多的氧空位和有效防止高温焙烧导致催化剂烧结.对于 HCl氧化虽然研究较少,但是 Over等人对 HCl氧化机理进行了深入研究,并且日本住友化工设计的 Ru基催化剂已经商业化. Ru基催化剂可以有效降低甲烷部分氧化的反应温度和压力,并具有高的选择性和稳定性,避免副产物生成.现有催化系统以及新型催化剂开发仍面临诸多挑战,例如：对于单一 VOC氧化过程和多元 VOCs催化氧化的机理和动力学需要进一步研究;对于氨合成需要寻求具有高电导率的载体,从而将电子快速转移到 Ru颗粒表面,使得氨合成在更低温度下进行;为了避免副产物生成,需确保新型 Ru基催化剂上PROX和甲烷部分氧化在低温低压条件下进行; Ru基催化剂理化性质对活性的影响以及失活等问题需要进一步研究.     

Pt Nanoparticles Supported on Nitrogen‐Doped‐Carbon‐Decorated CeO2 for Base‐Free Aerobic Oxidation of 5‐Hydroxymethylfurfural

Currently, the base‐free aerobic oxidation of biomass‐derived 5‐hydroxymethylfurfural (HMF) to produce 2,5‐furandicarboxylic acid (FDCA) is attracting intense interest due to its prospects for the green, sustainable, and promising production of biomass‐based aromatic polymers. Herein, we have developed a new Pt catalyst supported on nitrogen‐doped‐carbon‐decorated CeO₂ (NC‐CeO₂) for the aerobic oxidation of HMF in water without the addition of any homogeneous base. It was demonstrated that the small‐sized Pt particles could be well dispersed on the surface of the hybrid NC‐CeO₂ support, and the activity of the supported Pt catalyst depended strongly on the surface structure and properties of the catalysts. The as‐fabricated Pt/NC‐CeO₂ catalyst, with abundant surface defects, enhanced basicity, and favorable electron‐deficient metallic Pt species, enabled an almost 100 % yield of FDCA in water with molecular oxygen (0.4 MPa) at 110 °C for 8 h without the addition of any homogeneous base, which is indicative of exceptional catalytic performance. Furthermore, this Pt/NC‐CeO₂ catalyst also showed good stability and reusability owing to strong metal–support interactions. An understanding of the role of surface structural defects and basicity of the hybrid NC‐CeO₂ support provides a basis for the rational design of high‐performance and stable supported metal catalysts with practical applications in various transformations of biomass‐derived compounds.     

Design of Single Gold Atoms on Nitrogen‐Doped Carbon for Molecular Recognition in Alkyne Semi‐Hydrogenation

Single‐atom heterogeneous catalysts with well‐defined architectures are promising for deriving structure–performance relationships, but the challenge lies in finely tuning the structural and electronic properties of the metal. To tackle this point, a new approach based on the surface diffusion of gold atoms on different cavities of N‐doped carbon is presented. By controlling the activation temperature, the coordination neighbors (Cl, O, N) and the oxidation state of the metal can be tailored. Semi‐hydrogenation of various alkynes on the single‐atom gold catalysts displays substrate‐dependent catalytic responses; structure insensitive for alkynols with γ‐OH and unfunctionalized alkynes, and sensitive for alkynols with α‐OH. Density functional theory links the sensitivity for alkynols to the strong interaction between the substrate and specific gold‐cavity ensembles, mimicking a molecular recognition pattern that allows to identify the cavity site and to enhance the catalytic activity.     

Heterogeneous Catalysis for Water Oxidation by an Iridium Complex Immobilized on Bipyridine‐Periodic Mesoporous Organosilica

Heterogenization of metal‐complex catalysts for water oxidation without loss of their catalytic activity is important for the development of devices simulating photosynthesis. In this study, efficient heterogeneous iridium complexes for water oxidation were prepared using bipyridine‐bridged periodic mesoporous organosilica (BPy‐PMO) as a solid chelating ligand. The BPy‐PMO‐based iridium catalysts (Ir‐BPy‐PMO) were prepared by postsynthetic metalation of BPy‐PMO and characterized through physicochemical analyses. The Ir‐BPy‐PMOs showed high catalytic activity for water oxidation. The turnover frequency (TOF) values for Ir‐BPy‐PMOs were one order of magnitude higher than those of conventional heterogeneous iridium catalysts. The reusability and stability of Ir‐BPy‐PMO were also examined, and detailed characterization was conducted using powder X‐ray diffraction, nitrogen adsorption, ¹³C DD MAS NMR spectroscopy, TEM, and XAFS methods.     

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.     

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.     

Hydrogen Spillover Enhanced Hydroxyl Formation and Catalytic Activity Toward CO Oxidation at the Metal/Oxide Interface

H₂‐promoted catalytic activity of oxide‐supported metal catalysts in low‐temperature CO oxidation is of great interest but its origin remains unknown. Employing an FeO(111)/Pt(111) inverse model catalyst, we herewith report direct experimental evidence for the spillover of H(a) adatoms on the Pt surface formed by H₂ dissociation to the Pt?FeO interface to form hydroxyl groups that facilely oxidize CO(a) on the neighboring Pt surface to produce CO₂. Hydroxyl groups and coadsorbed water play a crucial role in the occurrence of hydrogen spillover. These results unambiguously identify the occurrence of hydrogen spillover from the metal surface to the noble metal/metal oxide interface and the resultant enhanced catalytic activity of the metal/oxide interface in low‐temperature CO oxidation, which provides a molecular‐level understanding of both H₂‐promoted catalytic activity of metal/oxide ensembles in low‐temperature CO oxidation and hydrogen spillover.     

有序多孔过渡金属氧化物及其负载贵金属催化剂对挥发性有机物氧化的催化性能

大部分的挥发性有机物(VOCs)污染环境,危害人身健康.目前,我国虽然已开展了治理 VOCs污染的工作,但还缺乏有效的、拥有自主知识产权的 VOCs治理技术,因此研发新型高效 VOCs处理技术迫在眉睫.催化氧化法是公认的最有效消除 VOCs的途径之一,而高性能催化剂的研发是实现该过程的关键.近年来,人们围绕消除 VOCs的高效且价廉的催化剂的研发开展了卓有成效的工作,许多过渡金属氧化物、混合或复合金属氧化物及其负载贵金属催化剂均被认为是有效的催化氧化材料.与体相材料相比,多孔材料具有发达的孔道结构和高的比表面积,一方面有利于反应物的扩散、吸附和脱附,因而具有更高的催化活性和选择性;另一方面有利于活性组分(如贵金属等)在多孔材料表面的高分散,抑制活性组分的烧结,因而具有更好的催化稳定性.本文简述了近年来多孔金属氧化物在环境污染物消除领域的研究进展,阐述了以有序介孔或大孔过渡金属氧化物、钙钛矿型氧化物和负载贵金属催化剂的制备及其对典型 VOCs(如苯系物、醇类、醛类及酮类等)氧化的催化性能,重点介绍了四类催化材料,包括有序介孔过渡金属氧化物或复合氧化物(Co3O4, MnO2, Fe2O3, Cr2O3和 LaFeO3等)催化剂,有序介孔金属氧化物负载贵金属(Au/Co3O4, Au/MnO2和 Pd/Co3O4等)催化剂,三维有序大孔过渡金属氧化物或复合氧化物(Fe2O3, LaMnO3, La0.6Sr0.4MnO3和 La2CuO4等)催化剂,以及三维有序大孔金属氧化物负载贵金属(Au/Co3O4, Au/LaCoO3, Au/La0.6Sr0.4MnO3和 AuPd/Co3O4等)催化剂的制备及其物化性质与对苯、甲苯、二甲苯、乙醇、丙酮、甲醛、甲烷或氯甲烷等 VOCs氧化的催化性能之间的相关性.借助二氧化硅或聚甲基丙烯酸甲酯微球等硬模板,采用纳米浇铸法可制备出二维或三维的有序单一或多级孔道结构的金属氧化物.研究表明,多孔金属氧化物的催化性能远优于其体相甚至纳米催化剂的.有序多孔材料的优异催化性能与其拥有大的比表面积、高的吸附氧物种浓度、优良的低温还原性、独特的孔道结构、活性组分的高分散以及贵金属与氧化物载体之间的强相互作用等有关.探明影响催化剂活性的因素有利于从原子水平上认识催化过程,为新型高效催化剂的设计与制备奠定基础.本文还指出了此类研究中存在的一些问题,例如利用硬模板法制备多孔材料的缺点是目标催化剂的收率低,硬模板浪费严重,大规模制备多孔催化剂势必增加制备成本,这些问题有待于妥善解决.与此同时,还展望了 VOCs消除技术的未来发展趋势,采用多种技术联用的方法有望最大程度地提高 VOCs的消除效率.     

基于催化氧化技术去除甲醛的研究进展

甲醛（HCHO）作为挥发性有机物（Volatile Organic Compounds,VOCs）,其催化氧化技术具有起燃温度低、设备简单、净化效率高等优点,被广泛采用,催化氧化催化剂主要为贵金属以及过渡金属氧化物.我们综述了近年来催化氧化甲醛的最新研究进展,尤其是甲醛催化氧化机理和提高催化活性的策略.最后,对催化氧化技术在甲醛催化氧化反应中的未来发展方向和趋势进行了展望.     

Nanocarbons for Catalytic Desulfurization

Nanocarbon catalysts are green and sustainable alternatives to metal‐based catalysts for numerous catalytic transformations. The application of nanocarbons for environmental catalysis is an emerging research discipline and has undergone rapid development in recent years. In this focus review, we provide a critical analysis of state‐of‐the‐art nanocarbon catalysts for three different catalytic desulfurization processes. In particular, we focus on the advantages and limitations as well as the reaction mechanisms of the nanocarbon catalysts at the molecular level.     

Transition metal-based electrocatalysts for overall water splitting

Electrochemical overall water splitting is attracting a broad focus as a promising strategy for converting the electrical output of renewable resources into chemical fuels, specifically oxygen and hydrogen. However, the urgent challenge in water electrolysis is to search for low-cost, high-efficiency catalysts based on earth-abundant elements as an alternative to the high-cost but effective noble metal-based catalysts. The transition metal-based catalysts are more appealing than the noble metal catalysts because of its low cost, high performance and long stability. Some recent advances for the development in overall water splitting are reviewed in terms of transition metal-based oxides, carbides, phosphides, sulfides, and hybrids of their mixtures as hybrid bifunctional electrocatalysts. Concentrating on different catalytic mechanisms, recent advances in their structural design, controllable synthesis, mechanistic insight, and performance-enhancing strategies are proposed. The challenges and prospects for the future development of transition metal-based bifunctional electrocatalysts are also addressed.     

An Update on Formic Acid Dehydrogenation by Homogeneous Catalysis

Formic acid (FA) has been extensively studied as one of the most promising hydrogen energy carriers today. The catalytic decarboxylation of FA ideally leads to the formation of CO₂ and H₂ that can be applied in fuel cells. A large number of transition‐metal based homogeneous catalysts with high activity and selectivity have been reported for the selective FA dehydrogentaion. In this review, we discussed the recent development of C,N/N,N‐ligand and pincer ligand‐based homogeneous catalysts for the FA dehydrogenation reaction. Some representative catalysts are further evaluated by the CON/COF assessment (catalyst on‐cost number)/(catalyst on‐cost frequency). Conclusive remarks are provided with future challenges and opportunities.     

非均相Fenton催化剂的组成结构设计与性能优化

非均相Fenton催化技术解决了均相Fenton反应存在的问题,具有pH适用范围广以及催化剂易于回收利用等优点,因而成为水处理领域的研究热点。本文首先介绍了非均相Fenton反应用于降解有机污染物的发展、反应机理以及机理的研究方法。总结了非均相Fenton催化剂的种类,主要包括铁氧化物、其它金属氧化物、金属有机框架材料。重点讨论了提高非均相Fenton催化剂活性及稳定性的方法,包括通过调控催化剂的形貌、尺寸、孔结构使催化剂具有更高的比表面积,将活性组分负载在具有高比表面积的载体上,通过与其它金属复合以及引入光、超声、微波等外场。最后,对非均相Fenton催化技术的发展进行了展望。     

Highly Dense Isolated Metal Atom Catalytic Sites: Dynamic Formation and In Situ Observations

Atomically dispersed noble‐metal catalysts with highly dense active sites are promising materials with which to maximise metal efficiency and to enhance catalytic performance; however, their fabrication remains challenging because metal atoms are prone to sintering, especially at a high metal loading. A dynamic process of formation of isolated metal atom catalytic sites on the surface of the support, which was achieved starting from silver nanoparticles by using a thermal surface‐mediated diffusion method, was observed directly by using in situ electron microscopy and in situ synchrotron X‐ray diffraction. A combination of electron microscopy images with X‐ray absorption spectra demonstrated that the silver atoms were anchored on five‐fold oxygen‐terminated cavities on the surface of the support to form highly dense isolated metal active sites, leading to excellent reactivity in catalytic oxidation at low temperature. This work provides a general strategy for designing atomically dispersed noble‐metal catalysts with highly dense active sites.     

基于立方烷结构的分子催化剂在光催化水氧化中的研究进展

随着化石燃料大量使用带来的气候变化和环境污染问题日趋严重,寻找清洁高效的可再生能源用做传统化石燃料的替代品,已经成为当前的研究热点。光驱动的水分解反应被认为是太阳能制氢的可行途径。水的全分解包括两个半反应-水的氧化和质子还原。其中水的氧化反应是一个涉及四个电子和四个质子转移的复杂过程,需要很高的活化能,被认为是全分解水反应的瓶颈步骤。因此,开发高效、稳定、廉价丰产的水氧化催化剂是人工光合作用突破的关键因素。立方烷具有类似自然界光合作用酶光系统II(PSII)活性中心Mn_4CaO_5簇的结构,世界各国的科学家受自然界光合作用的启发,开发出了许多基于过渡金属的立方烷结构的催化剂,常见的有锰、钴和铜等立方烷催化剂。本文简要地综述了近年来立方烷分子催化剂在光催化水氧化中的研究进展。首先介绍了立方烷基光催化水氧化反应历程,继而详细介绍了基于有机配体的立方烷配合物和全无机的多金属氧酸盐立方烷水氧化催化剂,其次是半导体(BiVO4或聚合的氮化碳(PCN))为捕光材料复合立方烷分子催化剂的水氧化体系最新研究进展。最后总结并展望了该领域所面临的挑战及其前景。     

基于非贵金属催化剂常温常压电化学合成氨

氨是一种重要的化工原料和能量载体,“哈伯反应”是工业上合成氨最主要的方法,但是该方法存在着能耗高,大量排放温室气体CO₂以及转化率低等问题。近年来,常温常压下基于多相催化剂的电化学还原N₂反应(NRR)来制备氨因其原料(N₂ + H₂O)易得,不依赖传统化石能源以及条件温和等原因而表现出巨大的应用潜能,并受到了科学家的广泛关注。然而目前NRR仍存在着如催化剂以贵金属材料为主,催化效率低和催化机理未明确等问题亟待解决。本综述主要总结了电催化NRR的最新研究成果,首先介绍了电催化NRR热力学和催化机理,接着重点列举了基于非贵金属催化剂的研究进展,包括过渡金属氧化物、氮化物、硫化物、非金属催化剂及单原子催化剂等,然后讨论了几种NRR电催化剂的改性方法,以及常见的产物氨的定性定量方法,最后,就目前该研究方向中仍待解决的问题进行了总结,并对下一步的研究进行了展望。     

In Situ Generation and Stabilization of Accessible Cu/Cu2O Heterojunctions inside Organic Frameworks for Highly Efficient Catalysis

Heterostructural metal/metal oxides are the very promising substituents of noble‐metal catalysts; however, generation and further stabilization of accessible metal/metal oxide heterojunctions are very difficult. A strategy to encapsulate and stabilize Cu/Cu₂O nanojunctions in porous organic frameworks in situ is developed by tuning the acrylate contents in copper‐based metal–organic frameworks (Cu‐MOFs) and the pyrolytic conditions. The acrylate groups play important roles on improving the polymerization degree of organic frameworks and generating and stabilizing highly dispersed and accessible Cu/Cu₂O heteronanojunctions. As a result, pyrolysis of the MOF ZJU‐199, consisting of three acrylates per ligand, generates abundant heterostructural Cu/Cu₂O discrete domains inside porous organic matrices at 350 °C, demonstrating excellent catalytic properties in liquid‐phase hydrogenation of furfural into furfuryl alcohol, which are much superior to the non‐noble metal‐based catalysts.