Strategies for improving the photocatalytic performance of metal-organic frameworks for CO2 reduction: A review

A Phenazine-Based Two-Dimensional Covalent Organic Framework for Photochemical CO2 Reduction with Increased Selectivity for Two-Carbon Products

The reduction of carbon dioxide (CO₂) into valuable products will contribute to sustainable carbon use. Here we report the photocatalytic reduction of CO₂ to carbon monoxide, formate, and oxalate ions using a redox-active phenazine-based 2D covalent organic framework (Phen-COF) and its phenazine monomer. Under similar irradiation conditions, Phen-COF produced 2.9 times more CO, 11 times more formate, and 13 times more oxalate compared to equimolar amounts of the monomeric phenazine, demonstrating that the COF architecture enhances catalytic performance (TOFCOF: 10-7 s-1 CO, 10-8 s-1 formate, and 10-11 s-1 oxalate). Structural analysis, including X-ray diffraction and N₂ porosimetry, confirmed the COF's long-range order and porosity. Mechanistic studies suggest a sequential formate-to-oxalate pathway, with CO and formate acting as intermediates. These results demonstrate the potential of the COF architecture to improve the performance of metal-free, redox-active aromatic systems such as phenazines to facilitate efficient and selective CO₂ conversion under mild conditions.

Metal cluster-mediated photocatalysis: synthesis, characterization and application

The escalation of global energy crises and environmental degradation has intensified the focus on photocatalytic technology, which harnesses solar energy for direct chemical reactions in a green manner. Metal clusters serve as multifunctional components in photocatalytic systems, with their unique properties (such as dimensions, composition, and surface modification) offering a plethora of regulatory mechanisms for designing innovative and efficient cluster-based photocatalysts. These improvements enhance light absorption, charge separation, and catalytic activity. This comprehensive review explores the fundamental principles and applications of photocatalytic technology, emphasizing the role of metal cluster materials in advancing this field. The synthetic methodologies, especially AI-assisted synthesis, characterization techniques, and modification strategies of metal cluster materials are introduced in detail, highlighting their significance in enhancing photocatalytic performance. The applications of metal clusters in various photocatalytic processes are also discussed, including water splitting, CO2 reduction, N2 fixation, pollutant degradation, H2O2 generation and selective organic synthesis, showcasing their potential in environmental remediation and energy transformation. Finally, the review concludes with an outlook on future research directions, emphasizing the need for innovating synthesis methods, developing advanced characterizations techniques, and optimizing catalytic performance to address existing challenges and unlock the full potential of cluster-based photocatalysts.

Photocatalytic Carbon Dioxide Reduction with Imidazolium-Based Ionic Liquids

The growing urgency of addressing climate change caused by greenhouse gas emissions and dwindling fossil fuel supplies has heightened the need for effective strategies to capture and utilize carbon dioxide. Photocatalytic CO2 conversion, inspired by natural photosynthesis, presents a viable approach for transforming CO2 into useful C1-C3 chemical intermediates for industrial purposes. However, the inherent stability of CO2 and the competing hydrogen evolution reaction (HER) introduce significant obstacles. Imidazolium-based ionic liquids can pre-activate CO2, accelerate reaction kinetics, and act as eco-friendly solvents or additives. Systems employing ionic liquids with catalysts, such as homogeneous organocatalysts and heterogeneous materials like Metal-Organic Frameworks (MOFs) and quantum dots, offer potential solutions to these challenges. This review focuses on the role of ionic liquids in both homogeneous and heterogeneous photocatalytic processes, emphasizing their use in CO2 reduction and highlighting recent mechanistic insights for imidazolium-based species.

Recent Progress of Three-Dimensional Graphene-Based Composites for Photocatalysis

Converting solar energy into fuels/chemicals through photochemical approaches holds significant promise for addressing global energy demands. Currently, semiconductor photocatalysis combined with redox techniques has been intensively researched in pollutant degradation and secondary energy generation owing to its dual advantages of oxidizability and reducibility; however, challenges remain, particularly with improving conversion efficiency. Since graphene's initial introduction in 2004, three-dimensional (3D) graphene-based photocatalysts have garnered considerable attention due to their exceptional properties, such as their large specific surface area, abundant pore structure, diverse surface chemistry, adjustable band gap, and high electrical conductivity. Herein, this review provides an in-depth analysis of the commonly used photocatalysts based on 3D graphene, outlining their construction strategies and recent applications in photocatalytic degradation of organic pollutants, H2 evolution, and CO2 reduction. Additionally, the paper explores the multifaceted roles that 3D graphene plays in enhancing photocatalytic performance. By offering a comprehensive overview, we hope to highlight the potential of 3D graphene as an environmentally beneficial material and to inspire the development of more efficient, versatile graphene-based aerogel photocatalysts for future applications.

Metal-organic frameworks for organic transformations by photocatalysis and photothermal catalysis

Organic transformation by light-driven catalysis, especially, photocatalysis and photothermal catalysis, denoted as photo(thermal) catalysis, is an efficient, green, and economical route to produce value-added compounds. In recent years, owing to their diverse structure types, tunable pore sizes, and abundant active sites, metal-organic framework (MOF)-based photo(thermal) catalysis has attracted broad interest in organic transformations. In this review, we provide a comprehensive and systematic overview of MOF-based photo(thermal) catalysis for organic transformations. First, the general mechanisms, unique advantages, and strategies to improve the performance of MOFs in photo(thermal) catalysis are discussed. Then, outstanding examples of organic transformations over MOF-based photo(thermal) catalysis are introduced according to the reaction type. In addition, several representative advanced characterization techniques used for revealing the charge reaction kinetics and reaction intermediates of MOF-based organic transformations by photo(thermal) catalysis are presented. Finally, the prospects and challenges in this field are proposed. This review aims to inspire the rational design and development of MOF-based materials with improved performance in organic transformations by photocatalysis and photothermal catalysis.

Plant-Based Phytochemicals for Synthesis of Z-Scheme In2O3/CdS Heterostructures: DFT Analysis and Photocatalytic CO2 Reduction to HCOOH and CO

Photocatalytic CO2 reduction shows potential for mitigating industrial emissions. Z-scheme In2O3/CdS(bio) heterostructures (25 nm, 217.0 m2 g-1 surface area) with a more negative conduction band synthesized using phytochemicals present in Aegle marmelos with short microwave irradiation inhibit CdS(bio) photocorrosion forming SO42-. In2O3/CdS(bio) increased the photocurrent density (0.82 μA cm-2) and CO2 adsorption (0.431 mmol g-1) significantly compared to CdS(bio) and In2O3(bio) NPs. Heterostructures increased decay time and reduced PL intensity by 46.28 and 61.80% over those of CdS(bio) and In2O3(bio) NPs. Density functional theory (DFT)-optimized geometry, band structure analysis, and density of states (DOS) studies indicate that the DOS of CdS is modified with In2O3 incorporation, enhancing charge separation. Optimal 0.4In2O3/CdS(bio) heterostructures exhibit remarkable CO2 conversion to HCOOH/CO production of 514.4/162 μmol g-1 h-1 (AQY 4.44/2.45%), surpassing CdS(bio) and In2O3(bio) by 9 and 6.5 times, and retain their morphological and structural stability. This study provides valuable insight for developing bio-based CdS heterostructures for photocatalytic CO2 reduction.

S-scheme heterojunction of crystalline carbon nitride nanosheets and ultrafine WO3 nanoparticles for photocatalytic CO2 reduction

Highly crystalline carbon nitride polymers have shown great opportunities in overall water photosplitting; however, their mission in light-driven CO2 conversion remains to be explored. In this work, crystalline carbon nitride (CCN) nanosheets of poly triazine imide (PTI) embedded with melon domains are fabricated by KCl/LiCl-mediated polycondensation of dicyandiamide, the surface of which is subsequently deposited with ultrafine WO3 nanoparticles to construct the CCN/WO3 heterostructure with a S-scheme interface. Systematic characterizations have been conducted to reveal the compositions and structures of the S-scheme CCN/WO3 hybrid, featuring strengthened optical capture, enhanced CO2 adsorption and activation, attractive textural properties, as well as spatial separation and directed movement of light-triggered charge carriers. Under mild conditions, the CCN/WO3 catalyst with optimized composition displays a high photocatalytic activity for reducing CO2 to CO in a rate of 23.0 µmol/hr (i.e., 2300 µmol/(hr·g)), which is about 7-fold that of pristine CCN, along with a high CO selectivity of 90.6% against H2 formation. Moreover, it also manifests high stability and fine reusability for the CO2 conversion reaction. The CO2 adsorption and conversion processes on the catalyst are monitored by in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), identifying the crucial intermediates of CO2*-, COOH* and CO*, which integrated with the results of performance evaluation proposes the possible CO2 reduction mechanism.

Recent progress of metal-organic framework-based nanozymes with oxidoreductase-like activity

Nanozymes, a class of synthetic nanomaterials possessing enzymatic catalytic properties, exhibit distinct advantages such as exceptional stability and cost-effectiveness. Among them, metal-organic framework (MOF)-based nanozymes have garnered significant attention due to their large specific surface area, tunable pore size and uniform structure. MOFs are porous crystalline materials bridged by inorganic metal ions/clusters and organic ligands, which hold immense potential in the fields of catalysis, sensors and drug carriers. The combination of MOFs with diverse nanomaterials gives rise to various types of MOF-based nanozyme, encompassing original MOFs, MOF-based nanozymes with chemical modifications, MOF-based composites and MOF derivatives. It is worth mentioning that the metal ions and organic ligands in MOFs are perfectly suited for designing oxidoreductase-like nanozymes. In this review, we intend to provide an overview of recent trends and progress in MOF-based nanozymes with oxidoreductase-like activity. Furthermore, the current obstacles and prospective outlook of MOF-based nanozymes are proposed and briefly discussed. This comprehensive analysis aims to facilitate progress in the development of novel MOF-based nanozymes with oxidoreductase-like activity while serving as a valuable reference for scientists engaged in related disciplines.

Solar-Driven Ammonia Production through Engineering of the Electronic Structure of a Zr-Based MOF

As a hydrogen carrier and a vital component in fertilizer production, ammonia (NH3) is set to play a crucial role in the planet's future. While its industrial production feeds half of the global population, it uses fossil fuels and emits greenhouse gases. To tackle this issue, photocatalytic nitrogen fixation using visible light is emerging as an effective alternative method. This strategy avoids carbon dioxide (CO2) emissions and harnesses the largest share of sunlight. In this work, we successfully incorporated a 5-nitro isophthalic acid linker into MOF-808 to introduce structural defects and open metal sites. This has allowed modulation of the electronic structure of the MOF and effectively reduced the band gap energy from 3.8 to 2.6 eV. Combination with g-C3N4 enhanced further NH3 production, as these two materials possess similar band gap energies, and g-C3N4 has shown excellent performance for this reaction. The nitro groups serve as acceptors, and their integration into the MOF structure allowed effective interaction with the free electron pairs on N-(C)3 in the g-C3N4 network nodes. Based on DFT calculations, it was concluded that the adsorption of N2 molecules on open metal sites caused a decrease in their triple bond energy. The modified MOF-808 showed superior performance compared with the other MOFs studied in terms of N2 photoreduction under visible light. This design concept offers valuable information about how to engineer band gap energy in MOF structures and their combination with appropriate semiconductors for solar-powered photocatalytic reactions, such as N2 or CO2 photoreduction.

Series of Cadmium-Organic Frameworks Based on Mixed Flexible and Rigid Ligands: Single-Crystal-to-Single-Crystal Transformations, Sorption, and Luminescence Properties

Here, we present a series of Cd(II) coordination polymers containing two types of ligands: sterically rigid terephthalate derivatives (bdc-NO22- and bdc-Br2-) and flexible bis(2-methylimidazolyl)propane (bmip). The combination of two types of ligands is used to obtain and characterize compounds by single crystal and powder X-ray diffraction, FT-IR, elemental analysis, and TGA. Guest exchange results in structural transformations. 2-fold interpenetrated 1·DMF and 2·DMF rapidly undergo to 4-fold interpenetrated 1·Et2O, 1·EtOH, and 1·H2O, or 2·Et2O, respectively. Also, changes in the coordinating numbers and length of the N,N'-donor bmip ligand were observed according to single crystal X-ray analysis. Activated guest-free compounds [Cd(bdc-NO2)(bmip)] (1) and [Cd(bdc-Br)(bmip)] (2) are shown to be porous with a BET surface area of 103 and 283 m2·g-1, respectively. Moreover, both compounds demonstrate gate-opening behavior of ethylene adsorption isotherms at low pressures (<1 bar) and highly selective adsorption of benzene over cyclohexane or lower alcohols. Also, both compounds demonstrate a strong dependence of the maximum of the photoluminescence emission on an excitation wavelength. As a result, the photoluminescence color changes from white to red and from blue to red through green and yellow for compounds 1 and 2, respectively, with excitation wavelength changing from 360 to 540 nm.

Stepwise construction of multi-component metal-organic frameworks

Multi-component metal-organic frameworks (MC-MOFs) are crystalline porous materials containing multiple organic ligands or mixed metals, which manifest new properties beyond the linear combination of the single component. However, the traditional one-pot synthesis method for MOFs is not always applicable for synthesizing MC-MOFs due to the competitive coordination of multiple ligands and metals. Therefore, the stepwise construction of MC-MOFs has been explored, which enables more precise control of the heterogeneity within the ordered MC-MOFs. This review provides a summary of the synthesis strategies, namely, ligand exchange, coordinative modification, covalent modification, ligand metalation, cluster metalation, and use of mixed-metal precursors, for the stepwise construction of MC-MOFs. Furthermore, we discuss the applications of MC-MOFs with ordered arrangements of multiple functionalities, focusing on gas adsorption and separation, water remediation, heterogeneous catalysis, luminescence, and chemical sensing.

Visible-light-induced photocatalytic CO2 reduction over zirconium metal organic frameworks modified with different functional groups

The reduction of CO₂ into high value-added chemicals and fuels by a photocatalytic technology can relieve energy shortages and the environmental problems caused by greenhouse effects. In the current work, an amino-functionalized zirconium metal organic framework (Zr-MOF) was covalently modified with different functional groups via the condensation of Zr-MOF with 2-pyridinecarboxaldehyde (PA), salicylaldehyde (SA), benzaldehyde (BA), and trifluoroacetic acid (TA), named Zr-MOF-X (X = PA, SA, BA, and TA), respectively, through the post-synthesis modification. Compared with Zr-MOF and Zr-MOF-TA, the introduction of PA, SA, or BA into the framework of Zr-MOF can not only enhance the visible-light harvesting and CO₂ capture, but also accelerate the photogenerated charge separation and transfer, thereby improving the photocatalytic ability of Zr-MOF for CO₂ reduction. These results indicate that the modification of Zr-MOF with electron-donating groups can promote the photocatalytic CO₂ reduction. Therefore, the current work provides an instructive approach to improve the photocatalytic efficiency of CO₂ reduction through the covalent modification of MOFs.

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 A2 BX6 , A2 B(I)B(III)X6 , and A3 B2 X9 -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.

Bi2MoO6 Embedded in 3D Porous N,O-Doped Carbon Nanosheets for Photocatalytic CO2 Reduction

Artificial photosynthesis is promising to convert solar energy and CO₂ into valuable chemicals, and to alleviate the problems of the greenhouse effect and the climate change crisis. Here, we fabricated a novel photocatalyst by directly growing Bi₂MoO₆ nanosheets on three-dimensional (3D) N,O-doped carbon (NO-C). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show that the designed photocatalyst ensured the close contact between Bi₂MoO₆ and NO-C, and reduced the stacking of the NO-C layers to provide abundant channels for the diffusion of CO₂, while NO-C can allow for fast electron transfer. The charge transfer in this composite was determined to follow a step-scheme mechanism, which not only facilitates the separation of charge carriers but also retains a strong redox capability. Benefiting from this unique 3D structure and the synergistic effect, BMO/NO-C showed excellent performance in photocatalytic CO₂ reductions. The yields of the best BMO/NO-C catalysts for CH₄ and CO were 9.14 and 14.49 μmol g^-1 h^-1, respectively. This work provides new insights into constructing step-scheme photocatalytic systems with the 3D nanostructures.

Photoreactive Carbon Dioxide Capture by a Zirconium-Nanographene Metal-Organic Framework

The mechanism of photochemical CO₂ reduction to formate by PCN-136, a Zr-based metal-organic framework (MOF) that incorporates light-harvesting nanographene ligands, has been investigated using steady-state and time-resolved spectroscopy and density functional theory (DFT) calculations. The catalysis was found to proceed via a "photoreactive capture" mechanism, where Zr-based nodes serve to capture CO₂ in the form of Zr-bicarbonates, while the nanographene ligands have a dual role of absorbing light and storing one-electron equivalents for catalysis. We also find that the process occurs via a "two-for-one" route, where a single photon initiates a cascade of electron/hydrogen atom transfers from the sacrificial donor to the CO₂-bound MOF. The mechanistic findings obtained here illustrate several advantages of MOF-based architectures in molecular photocatalyst engineering and provide insights on ways to achieve high formate selectivity.

Promoting Photocatalytic Carbon Dioxide Reduction by Tuning the Properties of Cocatalysts

Suppressing the amount of carbon dioxide in the atmosphere is an essential measure toward addressing global warming. Specifically, the photocatalytic CO2 reduction reaction (CRR) is an effective strategy because it affords the conversion of CO2 into useful carbon feedstocks by using sunlight and water. However, the practical application of photocatalyst-promoting CRR (CRR photocatalysts) requires significant improvement of their conversion efficiency. Accordingly, extensive research is being conducted toward improving semiconductor photocatalysts, as well as cocatalysts that are loaded as active sites on the photocatalysts. In this review, we summarize recent research and development trends in the improvement of cocatalysts, which have a significant impact on the catalytic activity and selectivity of photocatalytic CRR. We expect that the advanced knowledge provided on the improvement of cocatalysts for CRR in this review will serve as a general guideline to accelerate the development of highly efficient CRR photocatalysts.

A Comprehensive Review on Graphitic Carbon Nitride for Carbon Dioxide Photoreduction

Inspired by natural photosynthesis, harnessing the wide range of natural solar energy and utilizing appropriate semiconductor-based catalysts to convert carbon dioxide into beneficial energy species, for example, CO, CH₄ , HCOOH, and CH₃ COH have been shown to be a sustainable and more environmentally friendly approach. Graphitic carbon nitride (g-C₃ N₄ ) has been regarded as a highly effective photocatalyst for the CO₂ reduction reaction, owing to its cost-effectiveness, high thermal and chemical stability, visible light absorption capability, and low toxicity. However, weaker electrical conductivity, fast recombination rate, smaller visible light absorption window, and reduced surface area make this catalytic material unsuitable for commercial photocatalytic applications. Therefore, certain procedures, including elemental doping, structural modulation, functional group adjustment of g-C₃ N₄ , the addition of metal complex motif, and others, may be used to improve its photocatalytic activity towards effective CO₂ reduction. This review has investigated the scientific community's perspectives on synthetic pathways and material optimization approaches used to increase the selectivity and efficiency of the g-C₃ N₄ -based hybrid structures, as well as their benefits and drawbacks on photocatalytic CO₂ reduction. Finally, the review concludes a comparative discussion and presents a promising picture of the future scope of the improvements.

Photocatalytic reduction of low-concentration CO2 by metal-organic frameworks

Direct conversion of diluted CO₂ to value-added chemical stocks and fuels with solar energy is an energy-saving approach to relieve global warming and realize a carbon-neutral cycle. The exploration of catalysts with both efficient CO₂ adsorption and reduction ability is significant to achieving this goal. Metal-organic frameworks (MOFs) are emerging in the field of low-concentration CO₂ reduction due to their highly tunable structure, high porosity, abundant active sites and excellent CO₂ adsorption capacity. This highlight outlines the advantages of MOFs for low-pressure CO₂ adsorption and the strategies to improve the photocatalytic performance of MOF materials at low CO₂ concentrations, including the functionalization of organic ligands, regulation of metal nodes and preparation of MOF composites or derivatives. This paper aims to provide possible avenues for the rational design and development of catalysts with the ability to reduce low-concentration CO₂ efficiently for practical applications.

Peroxymonosulphate Activation by Basolite® F-300 for Escherichia coli Disinfection and Antipyrine Degradation

In this study, the removal of persistent emerging and dangerous pollutants (pharmaceuticals and pathogens) in synthetic wastewater was evaluated by the application of heterogeneous Advanced Oxidation Processes. To do that, a Metal-Organic Framework (MOF), Basolite® F-300 was selected as a catalyst and combined with peroxymonosulfate (PMS) as oxidants in order to generate sulphate radicals. Several key parameters such as the PMS and Basolite® F-300 concentration were evaluated and optimized using a Central Composite Experimental Design for response surface methodology for the inactivation of Escherichia coli. The assessment of the degradation of an analgesic and antipyretic pharmaceutical, antipyrine, revealed that is necessary to increase the concentration of PMS and amount of Basolite® F-300, in order to diminish the treatment time. Finally, the PMS-Basolite® F-300 system can be used for at least four cycles without a reduction in its ability to disinfect and degrade persistent emerging and dangerous pollutants such as pharmaceuticals and pathogens.