A review on metal-organic frameworks for photoelectrocatalytic applications

Semiconductor-based photoelectrocatalytic processes have attracted considerable research interest for solar energy collection and storage. Photoelectrocatalysis is a heterogeneous photocatalytic process in which a bias potential is applied to a photoelectrode, and thus the photoelectrocatalytic performance is closely related to the photoelectrode prepared by semiconductors. Among various semiconductors, metal-organic frameworks (MOFs) have attracted more and more attention because of their unique properties such as optical properties and adjustable structure. Herein, a comprehensive review on different MOFs (Ti-based, Zn-based, Co-based, Fe-based, Cu-based, and mixed metal-based MOFs) for heterogeneous photoelectrocatalysis is carried out and, in particular, the application of this technique for CO₂ conversion and water splitting is discussed. In addition, the challenges and development prospects of MOFs in photoelectrocatalysis are also presented.     

Recent Advances in Metal–Organic Frameworks for Photo-/Electrocatalytic CO2 Reduction

Considerable attention has been paid to the utilization of CO₂, an abundant carbon source in nature. In this regard, porous catalysts have been eagerly explored with excellent performance for photo-/electrocatalytic reduction of CO₂ to high valued products. Metal–organic frameworks (MOFs), featuring large surface area, high porosity, tunable composition and unique structural characteristics, have been widely exploited in catalytic CO₂ reduction. This Minireview first reports the current progress of MOFs in CO₂ reduction. Then, a specific interest is focused on MOFs in photo-/electrocatalytic reduction of CO₂ by modifying their metal centers, organic linkers, and pores. Finally, the future directions of study are also highlighted to satisfy the requirement of practical applications.     

Accelerated Transfer and Spillover of Carbon Monoxide through Tandem Catalysis for Kinetics-boosted Ethylene Electrosynthesis

Cu-based catalysts have been widely applied in electroreduction of carbon dioxide (CO₂ER) to produce multicarbon (C₂₊) feedstocks (e.g., C₂H₄). However, the high energy barriers for CO₂ activation on the Cu surface is a challenge for a high catalytic efficiency and product selectivity. Herein, we developed an in situ *CO generation and spillover strategy by engineering single Ni atoms on a pyridinic N-enriched carbon support with a sodalite (SOD) topology (Ni-SOD/NC) that acted as a donor to feed adjacent Cu nanoparticles (NPs) with *CO intermediate. As a result, a high C₂H₄ selectivity of 62.5 % and an industrial-level current density of 160 mA cm⁻² at a low potential of −0.72 V were achieved. Our studies revealed that the isolated NiN₃ active sites with adjacent pyridinic N species facilitated the *CO desorption and the massive *CO intermediate released from Ni-SOD/NC then overflowed to Cu NPs surface to enrich the *CO coverage for improving the selectivity of CO₂ER to C₂H₄.     

Ag+-Doped InSe Nanosheets for Membrane Electrode Assembly Electrolyzer toward Large-Current Electroreduction of CO2 to Ethanol

It is an appealing approach to CO₂ utilization through CO₂ electroreduction (CO₂ER) to ethanol at high current density; however, the commonly used Cu-based catalysts cannot sustain large current during CO₂ER despite their capability for ethanol production. Herein, we report that Ag⁺-doped InSe nanosheets with Se vacancies can address this grand challenge in a membrane electrode assembly (MEA) electrolyzer. As revealed by our experimental characterization and theoretical calculation, the Ag⁺ doping, which can tailor the electronic structure of InSe while diversifying catalytically active sites, enables the formation of key reaction intermediates and their sequential evolution into ethanol. More importantly, such a material can well work for large-current conditions in MEA electrolyzers with In²⁺ species stabilized via electron transfer from Ag to Se. Remarkably, in an MEA electrolyzer by coupling cathodic CO₂ER with anodic oxygen evolution reaction (OER), the optimal catalyst exhibits an ethanol Faradaic efficiency of 68.7 % and a partial current density of 186.6 mA cm⁻² on the cathode with a full-cell ethanol energy efficiency of 26.1 % at 3.0 V. This work opens an avenue for large-current production of ethanol from CO₂ with high selectivity and energy efficiency by rationally designing electrocatalysts.     

Structural Deterioration of Well-Faceted MOFs upon H2S Exposure and Its Effect in the Adsorption Performance

The structural deterioration of archetypical, well-faceted metal–organic frameworks (MOFs) has been evaluated upon exposure to an acidic environment (H₂S). Experimental results show that the structural damage highly depends on the nature of the hybrid network (e.g., softness of the metal ions, hydrophilic properties, among others) and the crystallographic orientation of the exposed facets. Microscopy images show that HKUST-1 with well-defined octahedral (111) facets is completely deteriorated, ZIF-8 with preferentially exposed (110) facets exhibits a large external deterioration with the development of holes or cavities in the mesoporous range, whereas UiO-66-NH₂ with (111) exposed facets, and PCN-250 with (100) facets does not reflect any sign of surface damage. Despite the selectivity in the external deterioration, X-ray diffraction and gas adsorption measurements confirm that indeed all MOFs suffer an important internal deterioration, these effects being more severe for MOFs based on softer cations (e.g., Cu-based HKUST-1 and Fe-based PCN-250). These structural changes have inevitable important effects in the final adsorption performance for CO₂ and CH₄ at low and high pressures.     

Modeling gas separation in metal-organic frameworks

The gas adsorption and CO₂ separation properties of 9 different metal-organic frameworks (MOFs) have been modelled with grand canonical Monte Carlo (GCMC) adsorption simulations. Adsorption of both pure gases and gas mixtures has been studied. MOFs are shown to have high selectivity for polar gases such as CO₂ over non-polar gases such as N₂. Selectivity of one polar gas from another can be altered by changing the polarity of the framework, pore geometry and also temperature. Often features that lead to good selectivity of CO₂ from N₂ also lead to poor selectivity of CO₂ from H₂O.     

离子液体功能化金属有机骨架材料催化CO2环加成反应研究进展

金属有机骨架(Metal-organic frameworks, MOFs)材料因具有超大比表面积、可修饰的化学结构、可调的孔隙形状和大小、开放的金属位点等独特的结构优越性而被广泛用于催化CO₂环加成反应的研究中。然而,大部分MOFs材料在此反应中往往需要在助催化剂或溶剂的存在下才能发挥其催化性能,这也导致了产物分离困难、资源浪费等问题。因此,开发能够单独催化CO₂环加成反应的MOFs材料成为当前科学家们研究的热点。在MOFs骨架上或孔腔内修饰离子液体是构筑此类催化体系的一种重要途径。本文对近年来这类MOFs的构筑策略、催化CO₂环加成反应的性能以及催化机理进行了总结,同时还对MOFs组成、形貌以及催化反应条件等因素对催化活性的影响进行了探讨。     

An Overview of Metal-Organic Framework Based Electrocatalysts: Design and Synthesis for Electrochemical Hydrogen Evolution,Oxygen Evolution,and Carbon Dioxide Reduction Reactions

Due to the increasing global energy demands, scarce fossil fuel supplies, and environmental issues, the pursued goals of energy technologies are being sustainable, more efficient, accessible, and produce near zero greenhouse gas emissions. Electrochemical water splitting is considered as a highly viable and eco-friendly energy technology. Further, electrochemical carbon dioxide (CO₂) reduction reaction (CO₂RR) is a cleaner strategy for CO₂ utilization and conversion to stable energy (fuels). One of the critical issues in these cleaner technologies is the development of efficient and economical electrocatalyst. Among various materials, metal-organic frameworks (MOFs) are becoming increasingly popular because of their structural tunability, such as pre- and post- synthetic modifications, flexibility in ligand design and its functional groups, and incorporation of different metal nodes, that allows for the design of suitable MOFs with desired quality required for each process. In this review, the design of MOF was discussed for specific process together with different synthetic methods and their effects on the MOF properties. The MOFs as electrocatalysts were highlighted with their performances from the aspects of hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and electrochemical CO₂RR. Finally, the challenges and opportunities in this field are discussed.     

Optimization of Reaction Conditions towards Multiple Types of Framework Isomers and Periodic‐Increased Porosity: Luminescence Properties and Selective CO2 Adsorption over N2

Three new metal‐organic frameworks (MOFs) were prepared by solvo(hydro)thermolysis and further characterized as framework isomers. The structural transformation from non‐porous to porous MOFs and the purity of these products can be modulated by controlling the reaction temperature. The periodic‐increased porosity observed was further confirmed by CO₂ adsorption isotherms. Owing to the presence of acylamide groups in the pore walls and the flexible nature of the skeleton of these MOFs, highly selective CO₂ adsorption over N₂ was observed, as well as structure‐dependent periodic varieties in luminescence properties.     

Metal-Organic Frameworks for Photocatalytic Water Splitting and CO2 Reduction

Photocatalytic water splitting and carbon dioxide (CO₂) reduction provide promising solutions to global energy and environmental issues. In recent years, metal-organic frameworks (MOFs), a class of crystalline porous solids featuring well-defined and tailorable structures as well as high surface areas, have captured great interest toward photocatalytic water splitting and CO₂ reduction. In this review, the semiconductor-like behavior of MOFs is first discussed. We then summarize the recent advances in photocatalytic water splitting and CO₂ reduction over MOF-based materials and focus on the unique advantage of MOFs for clarifying the structure-property relationship in photocatalysis. In addition, some representative characterization techniques have been presented to unveil the photocatalytic kinetics and reaction intermediates in MOF-based systems. Finally, the challenges, and perspectives for future directions are proposed.     

Organic molecules involved in Cu-based electrocatalysts for selective CO2 reduction to C2+ products

Electrocatalytic carbon dioxide (CO₂) reduction reaction (CO₂RR) is a promising process to mitigate the environmental issues caused by CO₂, as well as to produce valuable multicarbon (C₂₊) products. Significant progresses have been made to explore highly efficient Cu-based electrocatalysts for CO₂RR in recent years. Adding organic molecules into electrocatalytic systems can tune the CO₂ interaction with the electrocatalysts for CO₂RR, therefore, the final C₂₊ products, which are not solely achieved by inorganic modification. In this review, we will summarize the recent progress of the organic molecules participation in CO₂ electroreduction to C₂₊ products on Cu-based electrocatalysts. The applied organic molecules are reviewed based on the heteroatoms (N and S), with the emphasis on their roles in activity and selectivity toward C₂₊ products. A perspective on the application of organic molecules for efficient and selective CO₂RR has been provided.     

Magnetically Induced Continuous CO2 Hydrogenation Using Composite Iron Carbide Nanoparticles of Exceptionally High Heating Power

The use of magnetic nanoparticles to convert electromagnetic energy into heat is known to be a key strategy for numerous biomedical applications but is also an approach of growing interest in the field of catalysis. The heating efficiency of magnetic nanoparticles is limited by the poor magnetic properties of most of them. Here we show that the new generation of iron carbide nanoparticles of controlled size and with over 80 % crystalline Fe_2.2C leads to exceptional heating properties, which are much better than the heating properties of currently available nanoparticles. Associated to catalytic metals (Ni, Ru), iron carbide nanoparticles submitted to magnetic excitation very efficiently catalyze CO₂ hydrogenation in a dedicated continuous‐flow reactor. Hence, we demonstrate that the concept of magnetically induced heterogeneous catalysis can be successfully applied to methanation of CO₂ and represents an approach of strategic interest in the context of intermittent energy storage and CO₂ recovery.     

Elucidation of the Roles of Ionic Liquid in CO2 Electrochemical Reduction to Value-Added Chemicals and Fuels

The electrochemical reduction of carbon dioxide (CO₂ER) is amongst one the most promising technologies to reduce greenhouse gas emissions since carbon dioxide (CO₂) can be converted to value-added products. Moreover, the possibility of using a renewable source of energy makes this process environmentally compelling. CO₂ER in ionic liquids (ILs) has recently attracted attention due to its unique properties in reducing overpotential and raising faradaic efficiency. The current literature on CO₂ER mainly reports on the effect of structures, physical and chemical interactions, acidity, and the electrode–electrolyte interface region on the reaction mechanism. However, in this work, new insights are presented for the CO₂ER reaction mechanism that are based on the molecular interactions of the ILs and their physicochemical properties. This new insight will open possibilities for the utilization of new types of ionic liquids. Additionally, the roles of anions, cations, and the electrodes in the CO₂ER reactions are also reviewed.     

Co‐Catalyst‐Free Chemical Fixation of CO2 into Cyclic Carbonates by using Metal‐Organic Frameworks as Efficient Heterogeneous Catalysts

The concentration of carbon dioxide (CO₂) in the atmosphere is increasing at an alarming rate resulting in undesirable environmental issues. To mitigate this growing concentration of CO₂, selective carbon capture and storage/sequestration (CCS) are being investigated intensively. However, CCS technology is considered as an expensive and energy‐intensive process. In this context, selective carbon capture and utilization (CCU) as a C1 feedstock to synthesize value‐added chemicals and fuels is a promising step towards lowering the concentration of the atmospheric CO₂ and for the production of high‐value chemicals. Towards this direction, several strategies have been developed to convert CO₂, a Greenhouse gas (GHG) into useful chemicals by forming C?N, C?O, C?C, and C?H bonds. Among the various CO₂ functionalization processes known, the cycloaddition of CO₂ to epoxides has gained considerable interest owing to its 100% atom‐economic nature producing cyclic carbonates or polycarbonates in high yield and selectivity. Among the various classes of catalysts studied for cycloaddition of CO₂ to cyclic carbonates, porous metal‐organic frameworks (MOFs) have gained a special interest due to their modular nature facilitating the introduction of a high density of Lewis acidic (LA) and CO₂‐philic Lewis basic (LB) functionalities. However, most of the MOF‐based catalysts reported for cycloaddition of CO₂ to respective cyclic carbonates in high yields require additional co‐catalyst, say tetra‐n‐butylammonium bromide (TBAB). On the contrary, the co‐catalyst‐free conversion of CO₂ using rationally designed MOFs composed of both LA and LB sites is relatively less studied. In this review, we provide a comprehensive account of the research progress in the design of MOF based catalysts for environment‐friendly, co‐catalyst‐free fixation of CO₂ into cyclic carbonates.     

Water‐stable Adenine‐based MOFs with Polar Pores for Selective CO2 Capture

Here, we report two novel water‐stable amine‐functionalized MOFs, namely IISERP‐MOF26 ([NH₂(CH₃)₂][Cu₂O(Ad)(BDC)]?(H₂O)₂(DMA), 1 ) and IISERP‐MOF27 ([NH₂(CH₃)₂]_1/2[Zn₄O(Ad)₃(BDC)₂]?(H₂O)₂(DMF)_1/2, 2 ), which show selective CO₂ capture capabilities. They are made by combining inexpensive and readily available terephthalic acid and N‐rich adenine with Cu and Zn, respectively. They possess 1D channels decorated by the free amine group from the adenine and the polarizing oxygen atoms from the terephthalate units. Even more, there are dimethyl ammonium (DMA⁺) cations in the pore rendering an electrostatic environment within the channels. The activated Cu‐ and Zn‐MOFs physisorb about 2.7 and 2.2 mmol g^?1 of CO₂, respectively, with high CO₂/N₂ and moderate CO₂/CH₄ selectivity. The calculated heat of adsorption (HOA=21–23 kJ mol^?1) for the CO₂ in both MOFs suggest optimal physical interactions which corroborate well with their facile on‐off cycling of CO₂. Notably, both MOFs retain their crystallinity and porosity even after soaking in water for 24 hours as well as upon exposure to steam over 24 hours. The exceptional thermal and chemical stability, favorable CO₂ uptakes and selectivity and low HOA make these MOFs promising sorbents for selective CO₂ capture applications. However, the MOF′s low heat of adsorption despite having a highly CO₂‐loving groups lined walls is quite intriguing.     

Expanded Organic Building Units for the Construction of Highly Porous Metal–Organic Frameworks

Two new organic building units that contain dicarboxylate sites for their self‐assembly with paddlewheel [Cu₂(CO₂)₄] units have been successfully developed to construct two isoreticular porous metal–organic frameworks (MOFs), ZJU‐35 and ZJU‐36, which have the same tbo topologies (Reticular Chemistry Structure Resource (RCSR) symbol) as HKUST‐1. Because the organic linkers in ZJU‐35 and ZJU‐36 are systematically enlarged, the pores in these two new porous MOFs vary from 10.8 Å in HKUST‐1 to 14.4 Å in ZJU‐35 and 16.5 Å in ZJU‐36, thus leading to their higher porosities with Brunauer–Emmett–Teller (BET) surface areas of 2899 and 4014 m² g^?1 for ZJU‐35 and ZJU‐36, respectively. High‐pressure gas‐sorption isotherms indicate that both ZJU‐35 and ZJU‐36 can take up large amounts of CH₄ and CO₂, and are among the few porous MOFs with the highest volumetric storage of CH₄ under 60 bar and CO₂ under 30 bar at room temperature. Their potential for high‐pressure swing adsorption (PSA) hydrogen purification was also preliminarily examined and compared with several reported MOFs, thus indicating the potential of ZJU‐35 and ZJU‐36 for this important application. Studies show that most of the highly porous MOFs that can volumetrically take up the greatest amount of CH₄ under 60 bar and CO₂ under 30 bar at room temperature are those self‐assembled from organic tetra‐ and hexacarboxylates that contain m‐benzenedicarboxylate units with the [Cu₂(CO₂)₄] units, because this series of MOFs can have balanced porosities, suitable pores, and framework densities to optimize their volumetric gas storage. The realization of the two new organic building units for their construction of highly porous MOFs through their self‐assembly with [Cu₂(CO₂)₄] units has provided great promise for the exploration of a large number of new tetra‐ and hexacarboxylate organic linkers based on these new organic building units in which different aromatic backbones can be readily incorporated into the frameworks to tune their porosities, pore structures, and framework densities, thus targeting some even better performing MOFs for very high gas storage and efficient gas separation under high pressure and at room temperature in the near future.     

功能化金属有机框架材料催化二氧化碳转化研究进展

作为主要温室气体,二氧化碳(CO2)导致了全球变暖与海洋酸化,同时CO2也是重要的C1资源.在温和条件下,利用催化剂将CO2高效、高选择性地转化为具有高附加值的化学品,对缓解CO2给气候变化带来的负面影响和减少对化石能源的依赖具有重要意义.作为一类新兴的多孔晶态材料,金属-有机框架(metal-organic fram...     

Post-modification of metal-organic framework for improved CO2 photoreduction efficiency

Utilizing metal-organic frameworks (MOFs) to design photocatalysts for CO₂ reduction catalysts is an excellent idea but currently restricted by the relatively low activity. Enhancing CO₂ affinity and tuning the oxidation state of metal clusters in MOFs might be a solution to improve the catalytic performance. Herein, the Cl-bridge atoms in the metal clusters of a cobalt MOF were easily exchanged with OH^?, which simultaneously oxidized a portion of Co(II) to Co(III) and resulted in a much enhanced photocatalytic activity for CO₂ reduction. In contrast, the original framework does not exhibit such superior activity. Comprehensive characterizations on their physicochemical properties revealed that the introduction of hydroxyl group not only greatly increases the CO₂ affinity but also alters the oxidation state of metal clusters, resulting in significantly improved photocatalytic activities for CO₂ reduction. This work provides important insight into the design of efficient photocatalysts.     

Metal-Organic Frameworks for Efficient Electrochemical Reduction of Carbon Dioxide

The large concentration of carbon dioxide (CO₂) in the atmosphere can be utilized in industrial production using effective electrocatalysts such as metal-organic frameworks (MOFs). Due to good properties such as high surface area, designable functionality, and uniform constitution, MOFs are regarded as promising electrocatalysts for the carbon dioxide electrochemical reduction reaction (eCO₂RR). This review covers the importance, challenges, and mechanism of eCO₂RR, and simply discusses the progress in the synthesis methods and characterization of MOFs. The review also thoroughly discusses the advances of single metal-based MOFs, mixed metal-based MOFs, and MOF derivatives as electrocatalysts for efficient eCO₂RR.     

Two highly porous single-crystalline zirconium-based metal-organic frameworks

Herein we report two highly porous Zr-based metal-organic frameworks(MOFs, 1 and 2) constructed by the truncated octahedral secondary building unit(SBU) of Zr_6O_4(OH)_4(CO_2)_(12) and the organic linear ligand of 4,4.-stilbenedicarboxylic acid(H_2sbdc) or4,4′-azobenezenedicarboxylic acid(H_2abdc). Both Zr-based MOFs are obtained as single crystals of suitable size for single-crystal X-ray diffraction analysis. Furthermore, these two Zr-based MOFs have been fully characterized by powder X-ray diffraction(PXRD) studies, thermogravimetric analysis(TGA), infrared spectroscopy(IR) and gas adsorption analysis. In particular, their CO_2 gas adsorption behaviors have been investigated and discussed.