Synthetic strategies for MOF-based single-atom catalysts for photo- and electro-catalytic CO2 reduction

Isomeric Cu(I) Azolate Frameworks Showing Contrasting Electrocatalytic CO2 Reduction Selectivities and Stabilities

Metal‒organic frameworks have attracted wide interest in the electrocatalytic CO2 reduction reaction (eCO2RR), but their differences of performances originated from chemical composition and stabilities are rarely concerned. Here, isomeric Cu(I) triazolate frameworks (MAF-2Fa and MAF-2Fb) with similar thermal/chemical stabilities but very different coordination modes are used for eCO2RR studies. MAF-2Fa with monotypic planar dinuclear Cu(I) coordination mode achieves high selectivity for C2H4 (53%) and C2 products (70%), with almost unchanged over a wide potential window (‒1.1 to ‒1.5 V), making it among one of the best Cu-complex electrocatalysts. In contrast, MAF-2Fb with multiple Cu(I) coordination modes (including planar/bent dinuclear, linear mononuclear, and trigonal mononuclear ones) showed low C2/C1 products without significant differences. More interestingly, MAF-2Fa can maintain its performance for at least 8 h, whereas MAF-2Fb decomposed into inorganics with inferior performance after 1.5 h. The significant differences of eCO2RR selectivities and stabilities are elucidated by computational simulations and operando electrochemical tests.

Safety Landscape of Therapeutic Nanozymes and Future Research Directions

Oxidative stress and inflammation are at the root of a multitude of diseases. Treatment of these conditions is often necessary but current standard therapies to fight excessive reactive oxygen species (ROS) and inflammation are often ineffective or complicated by substantial safety concerns. Nanozymes are emerging nanomaterials with intrinsic enzyme-like properties that hold great promise for effective cancer treatment, bacterial elimination, and anti-inflammatory/anti-oxidant therapy. While there is rapid progress in tailoring their catalytic activities as evidenced by the recent integration of single-atom catalysts (SACs) to create next-generation nanozymes with superior activity, selectivity, and stability, a better understanding and tuning of their safety profile is imperative for successful clinical translation. This review outlines the current applied safety assessment approaches and provides a comprehensive summary of the safety knowledge of therapeutic nanozymes. Overall, nanozymes so far show good in vitro and in vivo biocompatibility despite considerable differences in their composition and enzymatic activities. However, current safety investigations mostly cover a limited set of basic toxicological endpoints, which do not allow for a thorough and deep assessment. Ultimately, remaining research gaps that should be carefully addressed in future studies are highlighted, to optimize the safety profile of therapeutic nanozymes early in their pre-clinical development.

Electrochemical conversion of CO2 using metalorganic frameworks-based materials: A review on recent progresses and outlooks

Global warming has been mainly attributed to the excessive release of carbon dioxide (CO2) to the atmosphere. Several CO2 capture and conversion technologies have been developed in the past few decades with their own merits and limitations. Electrochemical conversion of CO2 is one of the most attractive techniques for combating CO2 emissions. However, the efficacy of the electrochemical reduction of CO2 hinges on the efficiency of the utilized materials (i.e., electrocatalysts). Metal organic frameworks (MOFs)-based materials have recently emerged as attractive tools for various applications, including the electrochemical conversion of CO2. Although there are some review articles on CO2 capture and conversion using different materials, reviews focusing specifically on the electrochemical conversion of CO2 using MOFs-based materials are still comparatively lacking. Additionally, the field of electrochemical conversion of CO2 into valuable chemicals is currently gaining high momentum, requiring comprehensive and recent reviews, which would provide researchers/professionals with a quick and easy access to the recent developments in this rapidly evolving research area. Accordingly, this article comprehensively reviews recent studies on the electrochemical conversion of CO2 using pristine/modified/functionalized MOFs as well as composite materials containing MOFs. Additionally, single atom catalysts (SACs) derived from MOFs and their applications for the electrochemical conversion of CO2 has also been reviewed. Furthermore, obstacles, challenges, limitations, and remaining research gaps have been identified, and future works to tackle them have been highlighted. Overall, this review article provides valuable discussion and insights into the recent advancements in the field of electrochemical conversion of CO2 into chemicals using MOFs-based materials.

Single-Atom based Metal-Organic Framework Photocatalysts for Solar-Fuel Generation

The growing demand for fossil fuels and subsequent CO2 emissions prompted a search for alternate sources of energy and a reduction in CO2. Photocatalysis driven by solar light has been found as a potential research area to tackle both these problems. In this direction, SAC@MOF (Single-atom loaded MOFs) photocatalysis is an emerging field and a promising technology. The unique properties of single-atom catalysts (SACs), such as high catalytic activity and selectivity, are leveraged in these systems. Photocatalysis, focusing on the utilization of Metal-Organic Frameworks (MOFs) as platforms for creating single-atom catalysts (SACs) characterized by metal single-atoms (SAs) as their active sites, are noted for their unparalleled atomic efficiency, precisely defined active sites, and superior photocatalytic performance. The synergy between MOFs and SAs in photocatalytic systems is meticulously examined, highlighting how they collectively enhance photocatalytic efficiency. This review examines SAC@MOF development and applications in environmental and energy sectors, focusing on synthesis and stabilization methods for SACs on MOFs and also characterization techniques vital for understanding these catalysts. The potential of SAC@MOF in CO2 Photoreduction and Photocatalytic H2 evolution is highlighted, emphasizing its role in green energy technologies and advances in materials science and Photocatalysis.

Ionic liquid post-modified carboxylate-rich MOFs for efficient catalytic CO2 cycloaddition under solvent-free conditions

The synthesis of cyclic carbonates through cycloaddition reactions between epoxides and carbon dioxide (CO2) is an important industrial process. Metal-Organic Frameworks (MOFs) have functional and ordered pore structures, making them attractive catalysts for converting gas molecules into valuable products. One approach to enhance the catalytic activity of MOFs in CO2 cycloaddition reactions is to create open metal sites within MOFs. In this study, the amino-functionalized rare earth Gd-MOF (Gd-TPTC-NH2) and its ionic liquid composite catalysts (Gd-TPTC-NH-[BMIM]Br) were synthesized using 2'-amino-[1,1':4',1''-terphenyl]-3,3'',5,5''-tetracarboxylic acid (H4TPTC-NH2) as the ligand. The catalytic performance of these two catalysts was observed in the cycloaddition reaction of CO2 and epoxides. Under the optimized reaction conditions, Gd-TPTC-NH-[BMIM]Br can effectively catalyze the cycloaddition reaction of a variety of epoxide substrates with good to excellent yields of cyclic carbonate products. Comparatively, epichlorohydrin and epibromohydrin, which possess halogen substituents, promote higher yields of cyclic carbonates due to the electron-withdrawing nature of Cl and Br substituents. Additionally, the Gd-TPTC-NH-[BMIM]Br catalyst demonstrated good recyclability and reproducibility, maintaining its catalytic activity without any changes in its structure or properties after five reuse cycles.

Recent Advances in Hierarchical Porous Engineering of MOFs and Their Derived Materials for Catalytic and Battery: Methods and Application

Hierarchical porous materials have attracted the attention of researchers due to their enormous specific surface area, maximized active site utilization efficiency, and unique structure and properties. In this context, metal-organic frameworks (MOFs) offer a unique mix of properties that make them particularly appealing as tunable porous substrates containing highly active sites. This review focuses on recent advances in the types and synthetic strategies of hierarchical porous MOFs and their derived materials. Furthermore, it highlights the relationship between the mass diffusion and transport of hierarchical porous structures and the pore size with examples and simulations, while identifying their potential and limitations. On this basis, how the synthesis conditions affect the structure and electrochemical properties of MOFs based hierarchical porous materials with different structures is discussed, highlighting the prospects and challenges for the synthetization, as well as further scientific research and practical applications. Finally, some insights into current research and future design ideas for advanced MOFs based hierarchical porous materials are presented.

Atomic Distance Engineering in Metal Catalysts to Regulate Catalytic Performance

It is very important to understand the structure-performance relationship of metal catalysts by adjusting the microstructure of catalysts at the atomic scale. The atomic distance has an essential influence on the composition of the environment of active metal atom, which is a key factor for the design of targeted catalysts with desired function. In this review, we discuss and summarize strategies for changing the atomic distance from three aspects and relate their effects on the reactivity of catalysts. First, the effects of regulating bond length between metal and coordination atom at one single-atom site on the catalytic performance are introduced. The bond lengths are affected by the strain effect of the support and high-shell doping and can evolve during the reaction. Next, the influence of the distance between single-atom sites on the catalytic performance is discussed. Due to the space matching of adsorption and electron transport, the catalytic performance can be adjusted with the shortening of site distance. In addition, the effect of the arrangement spacing of the surface metal active atoms on the catalytic performance of metal nanocatalysts is studied. Finally, a comprehensive summary and outlook of the relationship between atomic distance and catalytic performance is given.

Progress on Single-Atom Photocatalysts for H2 Generation: Material Design, Catalytic Mechanism, and Perspectives

Solar energy utilization is of great significance to current challenges of the energy crisis and environmental pollution, which benefit the development of the global community to achieve carbon neutrality goals. Hydrogen energy is also treated as a good candidate for future energy supply since its combustion not only supplies high-density energy but also shows no pollution gas. In particular, photocatalytic water splitting has attracted increasing research as a promising method for H2 production. Recently, single-atom (SA) photocatalysts have been proposed as a potential solution to improve catalytic efficiency and lower the costs of photocatalytic water splitting for H2 generation. Owing to the maximized atom utilization rate, abundant surface active sites, and tunable coordination environment, SA photocatalysts have achieved significant progress. This review reviews developments of advanced SA photocatalysts for H2 generation regarding the different support materials. The recent progress of titanium dioxide, metal-organic frameworks, two-dimensional carbon materials, and red phosphorus supported SA photocatalysts are carefully discussed. In particular, the material designs, reaction mechanisms, modulation strategies, and perspectives are highlighted for realizing improved solar-to-energy efficiency and H2 generation rate. This work will supply significant references for future design and synthesis of advanced SA photocatalysts.

Al-Sehemi A, Alargarsamy S, Gajendran P, Ramachandran R. Development of Different Kinds of Electrocatalyst for the Electrochemical Reduction of Carbon Dioxide Reactions: An Overview

Significant advancements have been made in the development of CO2 reduction processes for applications such as electrosynthesis, energy storage, and environmental remediation. Several materials have demonstrated great potential in achieving high activity and selectivity for the desired reduction products. Nevertheless, these advancements have primarily been limited to small-scale laboratory settings, and the considerable technical obstacles associated with large-scale CO2 reduction have not received sufficient attention. Many of the researchers have been faced with persistent challenges in the catalytic process, primarily stemming from the low Faraday efficiency, high overpotential, and low limiting current density observed in the production of the desired target product. The highlighted materials possess the capability to transform CO2 into various oxygenates, including ethanol, methanol, and formates, as well as hydrocarbons such as methane and ethane. A comprehensive summary of the recent research progress on these discussed types of electrocatalysts is provided, highlighting the detailed examination of their electrocatalytic activity enhancement strategies. This serves as a valuable reference for the development of highly efficient electrocatalysts with different orientations. This review encompasses the latest developments in catalyst materials and cell designs, presenting the leading materials utilized for the conversion of CO2 into various valuable products. Corresponding designs of cells and reactors are also included to provide a comprehensive overview of the advancements in this field.

MOF-derived single-atom catalysts for oxygen electrocatalysis in metal-air batteries

Electrocatalysts play a critical role in oxygen electrocatalysis, enabling great improvements for the future development and application of metal-air batteries. Single-atom catalysts (SACs) derived from metal-organic frameworks (MOFs) are promising catalysts for oxygen electrocatalysis since they are endowed with the merits of a distinctive electronic structure, a low-coordination environment, quantum size effect, and strong metal-support interaction. In addition, MOFs afford a desirable molecular platform for ensuring the synthesis of well-dispersed SACs, endowing them with remarkably high catalytic activity and durability. In this review, we focus on the current status of MOF-derived SACs used as catalysts for oxygen electrocatalysis, with special attention to MOF-derived strategies for the fabrication of SACs and their application in various metal-air batteries. Finally, to facilitate the future deployment of high-performing SACs, some technical challenges and the corresponding research directions are also proposed.

Interaction between Single Metal Atoms and UiO-66 Framework Revealed by Low-Dose Imaging

Atomically dispersed metals encapsulated in metal-organic frameworks (MOFs) have attracted extensive attention in catalysis and energy fields. Amino groups were considered conducive to the formation of single atom catalysts (SACs) due to the strong metal-linker interactions. Here, atomic details of Pt₁@UiO-66 and Pd₁@UiO-66-NH₂ are revealed using low-dose integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM). Single Pt atoms locate on the benzene ring of p-benzenedicarboxylic acid (BDC) linkers in Pt@UiO-66, while single Pd atoms are adsorbed by the amino groups in Pd@UiO-66-NH₂. However, Pt@UiO-66-NH₂ and Pd@UiO-66 show obvious clusters. Therefore, amino groups do not always favor the formation of SACs, and density functional theory (DFT) calculations indicate that a moderate binding strength between metals and MOFs is preferred. These results directly reveal the adsorption sites of single metal atoms in UiO-66 family, paving the way for understanding the interaction between single metal atoms and the MOFs.

Superstructures of Zeolitic Imidazolate Frameworks to Single- and Multiatom Sites for Electrochemical Energy Conversion

The exploration of electrocatalysts with high catalytic activity and long-term stability for electrochemical energy conversion is significant yet remains challenging. Zeolitic imidazolate framework (ZIF)-derived superstructures are a source of atomic-site-containing electrocatalysts. These atomic sites anchor the guest encapsulation and self-assembly of aspheric polyhedral particles produced using microreactor fabrication. This review provides an overview of ZIF-derived superstructures by highlighting some of the key structural types, such as open carbon cages, 1D superstructures, hollow structures, and the interconversion of superstructures. The fundamentals and representative structures are outlined to demonstrate the role of superstructures in the construction of materials with atomic sites, such as single- and dual-atom materials. Then, the roles of ZIF-derived single-atom sites for the electroreduction of CO₂ and electrochemical synthesis of H₂ O₂ are discussed, and their electrochemical performance for energy conversion is outlined. Finally, the perspective on advancing single- and dual-atom electrode-based electrochemical processes with enhanced redox activity and a low-impedance charge-transfer pathway for cathodes is provided. The challenges associated with ZIF-derived superstructures for electrochemical energy conversion are discussed.