基于金属钴配合物的均相光催化还原二氧化碳研究进展

本文综述了自20世纪80年代以来基于钴配合物的均相光催化二氧化碳还原研究成果,以钴配合物催化剂的结构分类并结合时间顺序回顾了近四十年来该领域的发展轨迹,重点总结了用于光催化二氧化碳还原研究的金属钴配合物的结构、催化活性以及光催化体系的构成等特点,分析了该领域面临的挑战并展望了未来的发展方向。     

光催化还原二氧化碳全反应的研究进展

通过光催化将二氧化碳(CO₂)还原为可持续的绿色太阳能燃料是同时解决环境问题和能源危机的极具前景的方案.尽管迄今为止已经进行了广泛的研究,但实现高转化率、高选择性和高稳定性的光催化二氧化碳还原仍有许多障碍.如将水作为电子供体而非牺牲试剂,能够使反应的吉布斯自由能变ΔG>0,这对于真正实现理想化的人工光合作用至关重要,但同时也会为光催化还原CO₂体系带来更多的挑战.我们首先简要介绍了光催化还原CO₂的机理与挑战,而后根据目前光催化还原CO₂在无牺牲剂体系中出现的问题总结了对应的策略以及最新的研究进展,包括能带结构的调整、助催化剂的负载、异质结的构建、 MOFs与COFs材料的设计等方面,最后对目前仍未解决的问题以及未来实现工业化应用的阻碍进行了总结.     

Heterogeneous Molecular Systems for Photocatalytic CO2 Reduction with Water Oxidation

Artificial photosynthesis—reduction of CO₂ into chemicals and fuels with water oxidation in the presence of sunlight as the energy source—mimics natural photosynthesis in green plants, and is considered to have a significant part to play in future energy supply and protection of our environment. The high quantum efficiency and easy manipulation of heterogeneous molecular photosystems based on metal complexes enables them to act as promising platforms to achieve efficient conversion of solar energy. This Review describes recent developments in the heterogenization of such photocatalysts. The latest state‐of‐the‐art approaches to overcome the drawbacks of low durability and inconvenient practical application in homogeneous molecular systems are presented. The coupling of photocatalytic CO₂ reduction with water oxidation through molecular devices to mimic natural photosynthesis is also discussed.     

钙钛矿材料光催化还原CO2综述

利用半导体材料光催化还原CO2合成可燃物是目前解决能源危机和缓解温室效应的理想途径.本文对几种钙钛矿型材料,包括纯无机卤化物钙钛矿材料、金属有机钙钛矿材料、氧化物型钙钛矿材料和复合型钙钛矿材料在光催化还原CO2领域的应用进行了简单的归纳与总结.     

TiO2光催化还原CO2

光催化还原CO₂技术在CO₂的治理与利用方面有着潜在的应用价值和良好的开发前景。该文简要综述了近年来用于光催化还原CO₂反应的TiO₂光催化剂材料,包括纯TiO₂催化剂、负载型TiO₂催化剂、金属改性TiO₂催化剂、半导体复合TiO₂催化剂和有机光敏化TiO₂催化剂等,并介绍了各类催化剂光催化还原CO₂的反应性能。     

Improving the Photocatalytic Reduction of CO2 to CO through Immobilisation of a Molecular Re Catalyst on TiO2

The photocatalytic activity of phosphonated Re complexes, [Re(2,2′‐bipyridine‐4,4′‐bisphosphonic acid) (CO)₃(L)] (ReP; L=3‐picoline or bromide) immobilised on TiO₂ nanoparticles is reported. The heterogenised Re catalyst on the semiconductor, ReP–TiO₂ hybrid, displays an improvement in CO₂ reduction photocatalysis. A high turnover number (TON) of 48 mol_CO mol_Re^?1 is observed in DMF with the electron donor triethanolamine at λ>420 nm. ReP–TiO₂ compares favourably to previously reported homogeneous systems and is the highest TON reported to date for a CO₂‐reducing Re photocatalyst under visible light irradiation. Photocatalytic CO₂ reduction is even observed with ReP–TiO₂ at wavelengths of λ>495 nm. Infrared and X‐ray photoelectron spectroscopies confirm that an intact ReP catalyst is present on the TiO₂ surface before and during catalysis. Transient absorption spectroscopy suggests that the high activity upon heterogenisation is due to an increase in the lifetime of the immobilised anionic Re intermediate (t_{50 %}>1 s for ReP–TiO₂ compared with t_{50 %}=60 ms for ReP in solution) and immobilisation might also reduce the formation of inactive Re dimers. This study demonstrates that the activity of a homogeneous photocatalyst can be improved through immobilisation on a metal oxide surface by favourably modifying its photochemical kinetics.     

氧空位增强光催化还原CO2性能方面的研究进展

利用太阳能和半导体光催化剂,将CO₂光催化还原转变成碳氢燃料,是缓解温室效应、全球变暖、环境污染和能源危机等一系列问题的理想途径。 本文对氧空位增强的光催化还原CO₂反应机理进行归纳,并分别针对还原产物为C1和C2组分的光催化体系进行概括总结。 作为CO₂光催化还原过程的第一步,CO₂捕获光催化剂导带上的电子生成CO2·-是反应的速控步骤。 氧空位的引入及其带来的金属配位不饱和点,利于CO₂捕获电子生成CO2·-,进而促进CO₂光催化还原过程。 最后,提出当前氧空位增强光催化还原CO₂过程仍然存在的问题,且对发展前景进行展望。     

Photocatalytic Reduction of CO2 on TiO2 and Other Semiconductors

Rising atmospheric levels of carbon dioxide and the depletion of fossil fuel reserves raise serious concerns about the ensuing effects on the global climate and future energy supply. Utilizing the abundant solar energy to convert CO₂ into fuels such as methane or methanol could address both problems simultaneously as well as provide a convenient means of energy storage. In this Review, current approaches for the heterogeneous photocatalytic reduction of CO₂ on TiO₂ and other metal oxide, oxynitride, sulfide, and phosphide semiconductors are presented. Research in this field is focused primarily on the development of novel nanostructured photocatalytic materials and on the investigation of the mechanism of the process, from light absorption through charge separation and transport to CO₂ reduction pathways. The measures used to quantify the efficiency of the process are also discussed in detail.     

Strategies for Designing Nanoparticles for Electro‐ and Photocatalytic CO2 Reduction

Conversion of carbon dioxide (CO₂) into value‐added chemicals has attracted much attention because it can not only resolve global warming issues by reducing CO₂ accumulation in the atmosphere, but also produce renewable hydrocarbon fuels that are important feedstocks for the chemical industry. Among the diverse approaches reported, CO₂ reduction via electro‐ and photocatalytic methods is at the center of topics due to potential engineering of reaction performance through rational design of catalyst features. In this Minireview, we highlight recent strategies for designing nanoparticles to maximize the reaction efficiency and selectivity; from a materials viewpoint, these strategies can provide critical information to guide future research directions.     

Pivotal Role of Holes in Photocatalytic CO2 Reduction on TiO2

Evidence is provided that in a gas-solid photocatalytic reaction the removal of photogenerated holes from a titania (TiO₂) photocatalyst is always detrimental for photocatalytic CO₂ reduction. The coupling of the reaction to a sacrificial oxidation reaction hinders or entirely prohibits the formation of CH₄ as a reduction product. This agrees with earlier work in which the detrimental effect of oxygen-evolving cocatalysts was demonstrated. Photocatalytic alcohol oxidation or even overall water splitting proceeds in these reaction systems, but carbon-containing products from CO₂ reduction are no longer observed. H₂ addition is also detrimental, either because it scavenges holes or because it is not an efficient proton donor on TiO₂. The results are discussed in light of previously suggested reaction mechanisms for photocatalytic CO₂ reduction. The formation of CH₄ from CO₂ is likely not a linear sequence of reduction steps but includes oxidative elementary steps. Furthermore, new hypotheses on the origin of the required protons are suggested.     

CuOx/TiO2光催化水蒸气还原CO2反应研究

以商品TiO2-P25为原料,通过浸渍法负载一定量过渡金属Cu,得到一系列不同含量的CuOx/TiO2光催化剂.利用X射线衍射(XRD),X-射线光电子能谱(XPS),BET,高分辨率透射镜(HRTEM),X射线荧光光谱(XRF)和光致发光光谱(PL)等方法对催化剂进行了详细表征,在自建的光催化反应器中评价了气态水光催化还原CO2反应的活性和CH4收率.结果表明负载CuOx后的TiO2纳米材料光催化性能显著提高,其中1%CuOx/TiO2样品紫外光照72 h后,CH4生成量达到了24.86μmol.gTi-1.同时,CuOx负载量、反应温度、反应时间等因素对CH4收率均有显著影响.     

Molecular Porous Photosystems Tailored for Long‐Term Photocatalytic CO2 Reduction

The molecular‐level structuration of two full photosystems into conjugated porous organic polymers is reported. The strategy of heterogenization gives rise to photosystems which are still fully active after 4 days of continuous illumination. Those materials catalyze the carbon dioxide photoreduction driven by visible light to produce up to three grams of formate per gram of catalyst. The covalent tethering of the two active sites into a single framework is shown to play a key role in the visible light activation of the catalyst. The unprecedented long‐term efficiency arises from an optimal photoinduced electron transfer from the light harvesting moiety to the catalytic site as anticipated by quantum mechanical calculations and evidenced by in situ ultrafast time‐resolved spectroscopy.     

均相体系可见光催化还原CO2研究进展和面临挑战

利用太阳能将CO₂还原成可以利用的燃料或者有机物,是洁净新能源的重要研究方向,具有很大的挑战性。均相体系作为光催化还原CO₂最早研究的体系,对于CO₂还原的分子机理研究、新型催化剂设计以及体系优化方面具有重要指导意义。近五年来,均相体系光催化还原CO₂的研究取得了显著进展。本综述将对近五年来均相体系中光催化还原CO₂取得的重要研究进展进行综述,并对均相光催化体系发展面临的挑战进行了总结。     

Recent Advances and Future Perspectives of Lead-Free Halide Perovskites for Photocatalytic CO2 Reduction

It is still challenging to design and develop the state-of-the-art photocatalysts toward CO₂ photoreduction. Enormous researchers have focused on the halide perovskites in the photocatalytic field for CO₂ photoreduction, due to their excellent optical and physical properties. The toxicity of lead-based halide perovskites prevents their large-scale applications in photocatalytic fields. In consequence, lead-free halide perovskites (LFHPs) without the toxicity become the promising alternatives in the photocatalytic application for CO₂ photoreduction. In recent years, the rapid advances of LFHPs have offer new chances for the photocatalytic CO₂ reduction of LFHPs. In this review, we summarize not only the structures and properties of A₂BX₆, A₂B(I)B(III)X₆, and A₃B₂X₉-type LFHPs but also their recent progresses on the photocatalytic CO₂ reduction. Furthermore, we also point out the opportunities and perspectives to research LFHPs photocatalysts for CO₂ photoreduction in the future.     

Robust Binding between Carbon Nitride Nanosheets and a Binuclear Ruthenium(II) Complex Enabling Durable,Selective CO2 Reduction under Visible Light in Aqueous Solution

Carbon nitride nanosheets (NS‐C₃N₄) were found to undergo robust binding with a binuclear ruthenium(II) complex ( RuRu′ ) even in basic aqueous solution. A hybrid material consisting of NS‐C₃N₄ (further modified with nanoparticulate Ag) and RuRu ′ promoted the photocatalytic reduction of CO₂ to formate in aqueous media, in conjunction with high selectivity (approximately 98 %) and a good turnover number (>2000 with respect to the loaded Ru complex). These represent the highest values yet reported for a powder‐based photocatalytic system during CO₂ reduction under visible light in an aqueous environment. We also assessed the desorption of RuRu ′ from the Ag/C₃N₄ surface, a factor that can contribute to a loss of activity. It was determined that desorption is not induced by salt additives, pH changes, or photoirradiation, which partly explains the high photocatalytic performance of this material.     

Study of Excited States and Electron Transfer of Semiconductor-Metal-Complex Hybrid Photocatalysts for CO2 Reduction by Using Picosecond Time-Resolved Spectroscopies

A semiconductor-metal-complex hybrid photocatalyst was previously reported for CO₂ reduction; this photocatalyst is composed of nitrogen-doped Ta₂O₅ as a semiconductor photosensitizer and a Ru complex as a CO₂ reduction catalyst, operating under visible light (>400 nm), with high selectivity for HCOOH formation of more than 75 %. The electron transfer from a photoactive semiconductor to the metal-complex catalyst is a key process for photocatalytic CO₂ reduction with hybrid photocatalysts. Herein, the excited-state dynamics of several hybrid photocatalysts are described by using time-resolved emission and infrared absorption spectroscopies to understand the mechanism of electron transfer from a semiconductor to the metal-complex catalyst. The results show that electron transfer from the semiconductor to the metal-complex catalyst does not occur directly upon photoexcitation, but that the photoexcited electron transfers to a new excited state. On the basis of the present results and previous reports, it is suggested that the excited state is a charge-transfer state located between shallow defects of the semiconductor and the metal-complex catalyst.     

Undoped Layered Perovskite Oxynitride Li2LaTa2O6N for Photocatalytic CO2 Reduction with Visible Light

Oxynitrides are promising visible‐light‐responsive photocatalysts, but their structures are almost confined with three‐dimensional (3D) structures such as perovskites. A phase‐pure Li₂LaTa₂O₆N with a layered perovskite structure was successfully prepared by thermal ammonolysis of a lithium‐rich oxide precursor. Li₂LaTa₂O₆N exhibited high crystallinity and visible‐light absorption up to 500 nm. As opposed to well‐known 3D oxynitride perovskites, Li₂LaTa₂O₆N supported by a binuclear Ru^II complex was capable of stably and selectively converting CO₂ into formate under visible light (λ>400 nm). Transient absorption spectroscopy indicated that, as compared to 3D oxynitrides, Li₂LaTa₂O₆N possesses a lower density of mid‐gap states that work as recombination centers of photogenerated electron/hole pairs, but a higher density of reactive electrons, which is responsible for the higher photocatalytic performance of this layered oxynitride.     

Formation of Hierarchical FeCoS2–CoS2 Double‐Shelled Nanotubes with Enhanced Performance for Photocatalytic Reduction of CO2

Hierarchical FeCoS₂–CoS₂ double‐shelled nanotubes have been rationally designed and constructed for efficient photocatalytic CO₂ reduction under visible light. The synthetic strategy, engaging the two‐step cation‐exchange reactions, precisely integrates two metal sulfides into a double‐shelled tubular heterostructure with both of the shells assembled from ultrathin two‐dimensional (2D) nanosheets. Benefiting from the distinctive structure and composition, the FeCoS₂–CoS₂ hybrid can reduce bulk‐to‐surface diffusion length of photoexcited charge carriers to facilitate their separation. Furthermore, this hybrid structure can expose abundant active sites for enhancing CO₂ adsorption and surface‐dependent redox reactions, and harvest incident solar irradiation more efficiently by light scattering in the complex interior. As a result, these hierarchical FeCoS₂–CoS₂ double‐shelled nanotubes exhibit superior activity and high stability for photosensitized deoxygenative CO₂ reduction, affording a high CO‐generating rate of 28.1 μmol h^?1 (per 0.5 mg of catalyst).     

Rational Design of Carbon-Based 2D Nanostructures for Enhanced Photocatalytic CO2 Reduction: A Dimensionality Perspective

Photocatalytic CO₂ reduction is a revolutionary approach to solve imminent energy and environmental issues by replicating the ingenuity of nature. The past decade has witnessed an impetus in the rise of two-dimensional (2D) structure materials as advanced nanomaterials to boost photocatalytic activities. In particular, the use of 2D carbon-based materials is deemed as highly favorable, not only as a green material choice, but also due to their exceptional physicochemical and electrical properties. This Review article presents a diverse range of alterations and compositions derived from 2D carbon-based nanomaterials, mainly graphene and graphitic carbon nitride (g-C₃N₄), which have remarkably ameliorated the photocatalytic CO₂ performance. Herein, the rational design of the photocatalyst systems with consideration of the aspect of dimensionality and the resultant heterostructures at the interface are systematically analyzed to elucidate an insightful perspective on this pacey subject. Finally, a conclusion and outlook on the limitations and prospects of the cutting-edge research field are highlighted.