A General Strategy to Immobilize Single-Atom Catalysts in Metal-Organic Frameworks for Enhanced Photocatalysis

Promoted hydrogenation of CO2 to methanol over single-atom Cu sites with Na+-decorated microenvironment

Although single-atom Cu sites exhibit high efficiency in CO2 hydrogenation to methanol, they are prone to forming Cu nanoparticles due to reduction and aggregation under reaction conditions, especially at high temperatures. Herein, single-atom Cu sites stabilized by adjacent Na+ ions have been successfully constructed within a metal-organic framework (MOF)-based catalyst, namely MOF-808-NaCu. It is found that the electrostatic interaction between the Na+ and Hδ- species plays a pivotal role in upholding the atomic dispersion of Cu in MOF-808-NaCu during CO2 hydrogenation, even at temperatures of up to 275°C. This exceptional stabilization effect endows the catalyst with excellent activity (306 g·kgcat-1·h-1), high selectivity to methanol (93%) and long-term stability at elevated reaction temperatures, far surpassing the counterpart in the absence of Na+ (denoted as MOF-808-Cu). This work develops an effective strategy for the fabrication of stable single-atom sites for advanced catalysis by creating an alkali-decorated microenvironment in close proximity.

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.

Construction of Nickel Single Atoms by Using the Inherent Confined Space in Template-Occupied Mesoporous Silica

Due to their maximum atomic use of metal sites, single-atom catalysts (SACs) exhibit excellent catalytic activity in a variety of reactions. Although many techniques have been reported for the production of SACs, the construction of single atoms through a convenient strategy is still challenging. Here, we provide a facile method to prepare nickel SACs by utilizing the inherent confined space between the template and silica walls in template-occupied mesoporous silica KIT-6 (TOK). After the introduction of nickel-containing precursors into the inherent confined space of the TOK by solid-phase grinding, Ni SACs can be produced promptly during calcination. Single Ni atoms create a covalent Ni-O-Si structure in the TOK, as indicated by density functional theory (DFT) calculations and experimental data. This synthetic approach is easy to scale up, and 10 g of sample can be effortlessly synthesized using ball milling. The resultant Ni SACs were applied to the oxygen evolution reaction and exhibited higher catalytic activity and stability than the comparative sample synthesized in the absence of confined space.

In Situ UV-Vis-NIR Absorption Spectroscopy and Catalysis

This review highlights in situ UV-vis-NIR range absorption spectroscopy in catalysis. A variety of experimental techniques identifying reaction mechanisms, kinetics, and structural properties are discussed. Stopped flow techniques, use of laser pulses, and use of experimental perturbations are demonstrated for in situ studies of enzymatic, homogeneous, heterogeneous, and photocatalysis. They access different time scales and are applicable to different reaction systems and catalyst types. In photocatalysis, femto- and nanosecond resolved measurements through transient absorption are discussed for tracking excited states. UV-vis-NIR absorption spectroscopies for structural characterization are demonstrated especially for Cu and Fe exchanged zeolites and metalloenzymes. This requires combining different spectroscopies. Combining magnetic circular dichroism and resonance Raman spectroscopy is especially powerful. A multitude of phenomena can be tracked on transition metal catalysts on various supports, including changes in oxidation state, adsorptions, reactions, support interactions, surface plasmon resonances, and band gaps. Measurements of oxidation states, oxygen vacancies, and band gaps are shown on heterogeneous catalysts, especially for electrocatalysis. UV-vis-NIR absorption is burdened by broad absorption bands. Advanced analysis techniques enable the tracking of coking reactions on acid zeolites despite convoluted spectra. The value of UV-vis-NIR absorption spectroscopy to catalyst characterization and mechanistic investigation is clear but could be expanded.

Manipulating the Microenvironment of Single Atoms by Switching Support Crystallinity for Industrial Hydrogen Evolution

Modulating the microenvironment of single-atom catalysts (SACs) is critical to optimizing catalytic activity. Herein, we innovatively propose a strategy to improve the local reaction environment of Ru single atoms by precisely switching the crystallinity of the support from high crystalline and low crystalline, which significantly improves the hydrogen evolution reaction (HER) activity. The Ru single-atom catalyst anchored on low-crystalline nickel hydroxide (Ru-LC-Ni(OH)2 ) reconstructs the distribution balance of the interfacial ions due to the activation effect of metal dangling bonds on the support. Single-site Ru with a low oxidation state induces the aggregation of hydronium ions (H3 O+ ), leading to the formation of a local acidic microenvironment in alkaline media, breaking the pH-dependent HER activity. As a comparison, the Ru single-atom catalyst anchored on high-crystalline nickel hydroxide (Ru-HC-Ni(OH)2 ) exhibits a sluggish Volmer step and a conventional local reaction environment. As expected, Ru-LC-Ni(OH)2 requires low overpotentials of 9 and 136 mV at 10 and 1000 mA cm-2 in alkaline conditions and operates stably at 500 mA cm-2 for 500 h in an alkaline seawater anion exchange membrane (AEM) electrolyzer. This study provides a new perspective for constructing highly active single-atom electrocatalysts.

Integrating Hybrid Perovskite Nanocrystals into Metal-Organic Framework as Efficient S-Scheme Heterojunction Photocatalyst for Synergistically Boosting Controlled Radical Photopolymerization under 980 nm NIR Light

S-scheme heterojunction photocatalyst MAPbI3@PCN-222 with light absorption extending to the NIR region is constructed by embedding organic-inorganic hybrid perovskite (MAPbI3) into porphyrinic Zr-MOF (PCN-222). Both in situ X-ray photoelectron spectroscopy, ultraviolet photoelectron spectral characterization, and photocatalytic polymerization experiment prove the formation of S-scheme heterojunction. MAPbI3@PCN-222 with a low dosage (90 ppm) displays an impressive photocatalytic ability for 980 nm light-mediated photoinduced electron/energy-transfer-reversible addition-fragmentation chain-transfer (PET-RAFT) polymerization in air. The well-defined controllable-molecular weight polymers including block copolymers and ultrahigh-molecular weight polymers can be achieved with narrow distributions (Mw/Mn < 1.20) via rapid photopolymerization. The industrial application potential of the photocatalyst also has been proved by scale-up synthesis of polymers with low polydispersity under NIR light-induced photopolymerization in a large-volume reaction system (200 mL) with high monomer conversion up to 99%. The penetration photopolymerization through the 5 mm polytetrafluoroethylene plate and excellent photocontrollable behavior illustrate the existence of long-term photogenerated electron transfer of heterojunction and abundant free radicals in photopolymerization. The photocatalyst still retains high catalytic activity after 10 cycles of photopolymerization in air. It is revealed for the first time that the special PET-RAFT polymerization pathway is initiated by the aldehyde-bearing α-aminoalkyl radical derived from the oxidization of triethanolamine (TEOA) by the heterojunction photocatalyst. This research offers a new insight into understanding the NIR-light-activated PET-RAFT polymerization mechanism in the presence of TEOA.

Exploring the Roles of Single Atom in Hydrogen Peroxide Photosynthesis

This comprehensive review provides a deep exploration of the unique roles of single atom catalysts (SACs) in photocatalytic hydrogen peroxide (H2O2) production. SACs offer multiple benefits over traditional catalysts such as improved efficiency, selectivity, and flexibility due to their distinct electronic structure and unique properties. The review discusses the critical elements in the design of SACs, including the choice of metal atom, host material, and coordination environment, and how these elements impact the catalytic activity. The role of single atoms in photocatalytic H2O2 production is also analysed, focusing on enhancing light absorption and charge generation, improving the migration and separation of charge carriers, and lowering the energy barrier of adsorption and activation of reactants. Despite these advantages, several challenges, including H2O2 decomposition, stability of SACs, unclear mechanism, and low selectivity, need to be overcome. Looking towards the future, the review suggests promising research directions such as direct utilization of H2O2, high-throughput synthesis and screening, the creation of dual active sites, and employing density functional theory for investigating the mechanisms of SACs in H2O2 photosynthesis. This review provides valuable insights into the potential of single atom catalysts for advancing the field of photocatalytic H2O2 production.

Self-supporting, hierarchically hollow structured NiFe-PBA electrocatalyst for efficient alkaline seawater oxidation

Seawater electrolysis, taking advantage of the huge seawater resource, holds great promise for sustainable hydrogen generation. Compared to conventional water electrolysis, seawater electrolysis is more challenging because of the more complex and corrosive electrolyte and competitive side reactions, which necessitates the development of highly efficient and stable electrocatalysts. In this study, a self-supporting, highly porous NiFe-PBA (Prussian-blue-analogue) electrocatalyst with a hierarchically hollow nanostructure is introduced, which exhibits impressive catalytic performance towards the oxygen evolution in alkaline seawater electrolytes. In NiFe-PBA, the synergistic interaction between Ni and Fe improves intrinsic conductivity for efficient electron transfer, enhances chemical stability in seawater, and boosts overall electrocatalytic activity. The direct use of self-supporting NiFe-PBA as an electrocatalyst avoids the energy-intensive and tedious pyrolysis procedure during the preparation process while making use of the tailored morphological, structural, and compositional benefits of PBA-based materials. By combining the NiFe-PBA catalyst with the NiMoN cathode, the constructed two-electrode electrolyzer achieved a high current density of 500 mA cm-2 at a low cell voltage of 1.782 V for overall electrolysis of alkaline seawater, demonstrating excellent durability for 100 hours. Our findings have important implications for the hydrogen economy and sustainable development through the development of robust and efficient PBA-based electrocatalysts for seawater electrolysis.

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.

Energy-efficient extraction of linear alkanes from various isomers using structured metal-organic framework membrane

Extraction of low concentration linear alkanes (C5-C7) from various isomers is critical for the petrochemical industry. At present, the separation of alkane isomers is mainly accomplished by distillation, which results in substantial energy expenditure. Metal-organic frameworks (MOFs) with well-tailored nanopores have been demonstrated to be capable of realizing molecule-level separation. In this study, oriented HKUST-1 membranes are formulated according to the morphology-biased principle and finally realized with a low dose synthesis method for terminating undesired crystal nucleation and growth. The fully exposed triangular sieving pore array of the membrane induces configuration entropic diffusion to split linear alkanes from mono-branched and di-branched isomers as well as their cyclical counterparts. Typically, the current separation technique consumes 91% less energy than vacuum distillation. Furthermore, our membranes can realize one-step extraction of normal-pentane, normal-hexane and normal-heptane from a ten-component alkane isomer solution that mimics light naphtha.

Bioinspired microenvironment modulation of metal-organic framework-based catalysts for selective methane oxidation

Inspiration from natural enzymes enabling creationary catalyst design is appealing yet remains extremely challenging for selective methane (CH4) oxidation. This study presents the construction of a biomimetic catalyst platform for CH4 oxidation, which is constructed by incorporating Fe-porphyrin into a robust metal-organic framework, UiO-66, furnished with saturated monocarboxylic fatty acid bearing different long alkyl chains. The catalysts demonstrate the high efficiency in the CH4 to methanol (CH3OH) conversion at 50 °C. Moreover, the selectivity to CH3OH can be effectively regulated and promoted through a fine-tuned microenvironment by hydrophobic modification around the Fe-porphyrin. The long-chain fatty acids anchored on the Zr-oxo cluster of UiO-66 can not only tune the electronic state of the Fe sites to improve CH4 adsorption, but also restrict the amount of H2O2 around the Fe sites to reduce the overoxidation. This behavior resembles the microenvironment regulation in methane monooxygenase, resulting in high CH3OH selectivity.

Induced Production of Atypical Naturally Nonpreferred Metal-Organic Frameworks and Their Detachment via Provoking Post-Mismatching

The structures of metal-organic frameworks (MOFs) are typically determined by the building blocks that compose them and the conditions under which they are formed. MOFs tend to adopt a thermodynamically and/or kinetically stable structure (naturally preferred form). Thus, constructing MOFs with naturally nonpreferred structures is a challenging task, as it requires avoiding the easier pathway toward a naturally preferred MOF. Herein, an approach to construct naturally nonpreferred dicarboxylate-linked MOFs employing reaction templates is reported. This strategy relies on the registry between the surface of the template and the cell lattice of a target MOF, which reduces the effort required to form naturally nonpreferred MOFs. Reactions of p-block trivalent metal ions (Ga3+ and In3+ ) with dicarboxylic acids typically produce preferred MIL-53 or MIL-68. However, the surface of UiO-67 (and UiO-66) template exhibits the well-defined hexagonal lattice, which induce the selective formation of a naturally nonpreferred MIL-88 structure. Inductively grown MIL-88s are purely isolated from the template via provoking a post-mismatch in their lattices and weakening the interfacial interaction between product and template. It is also discovered that an appropriate template for effective induced production of naturally nonpreferred MOFs shall be properly selected based on the cell lattice of a target MOF.

Imaging the dynamic influence of functional groups on metal-organic frameworks

Metal-organic frameworks (MOFs) with different functional groups have wide applications, while the understanding of functionalization influences remains insufficient. Previous researches focused on the static changes in electronic structure or chemical environment, while it is unclear in the aspect of dynamic influence, especially in the direct imaging of dynamic changes after functionalization. Here we use integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM) to directly 'see' the rotation properties of benzene rings in the linkers of UiO-66, and observe the high correlation between local rigidity and the functional groups on the organic linkers. The rigidity is then correlated to the macroscopic properties of CO2 uptake, indicating that functionalization can change the capability through not only static electronic effects, but also dynamic rotation properties. To the best of our knowledge this is the first example of a technique to directly image the rotation properties of linkers in MOFs, which provides an approach to study the local flexibility and paves the way for potential applications in capturing, separation and molecular machine.

Highly Productive C3H4/C3H6 Trace Separation by a Packing Polymorph of a Layered Hybrid Ultramicroporous Material

Ultramicroporous materials can be highly effective at trace gas separations when they offer a high density of selective binding sites. Herein, we report that sql-NbOFFIVE-bpe-Cu, a new variant of a previously reported ultramicroporous square lattice, sql, topology material, sql-SIFSIX-bpe-Zn, can exist in two polymorphs. These polymorphs, sql-NbOFFIVE-bpe-Cu-AA (AA) and sql-NbOFFIVE-bpe-Cu-AB (AB), exhibit AAAA and ABAB packing of the sql layers, respectively. Whereas NbOFFIVE-bpe-Cu-AA (AA) is isostructural with sql-SIFSIX-bpe-Zn, each exhibiting intrinsic 1D channels, sql-NbOFFIVE-bpe-Cu-AB (AB) has two types of channels, the intrinsic channels and extrinsic channels between the sql networks. Gas and temperature induced transformations of the two polymorphs of sql-NbOFFIVE-bpe-Cu were investigated by pure gas sorption, single-crystal X-ray diffraction (SCXRD), variable temperature powder X-ray diffraction (VT-PXRD), and synchrotron PXRD. We observed that the extrinsic pore structure of AB resulted in properties with potential for selective C₃H₄/C₃H₆ separation. Subsequent dynamic gas breakthrough measurements revealed exceptional experimental C₃H₄/C₃H₆ selectivity (270) and a new benchmark for productivity (118 mmol g^-1) of polymer grade C₃H₆ (purity >99.99%) from a 1:99 C₃H₄/C₃H₆ mixture. Structural analysis, gas sorption studies, and gas adsorption kinetics enabled us to determine that a binding "sweet spot" for C₃H₄ in the extrinsic pores is behind the benchmark separation performance. Density-functional theory (DFT) calculations and Canonical Monte Carlo (CMC) simulations provided further insight into the binding sites of C₃H₄ and C₃H₆ molecules within these two hybrid ultramicroporous materials, HUMs. These results highlight, to our knowledge for the first time, how pore engineering through the study of packing polymorphism in layered materials can dramatically change the separation performance of a physisorbent.

Single-Atom Nano-Islands (SANIs): A Robust Atomic-Nano System for Versatile Heterogeneous Catalysis Applications

Academician Tao Zhang from China and co-workers designed the first Pt₁ /FeO_x single-atom catalysts (SACs) in 2011, and they proposed the concept of "single-atom catalysis" in the field of heterogeneous catalysis. Generally, it is easy for active metal single-atom sites on a carrier to migrate and aggregate, which results in poor performance; or the chemical bond between the metal atom and carrier is too strong (immovable), which results in passivation of the active site. Recently, "nano-island" type SACs were designed, in which the active metal atoms are isolated on the "islands", and can move within the respective "island", but the migration across the "island" is blocked, to achieve a dynamic confinement design of single atoms (that is, a "moving but not aggregating" design philosophy). Herein, a new concept of "single-atom nano-islands (SANIs)" is proposed to describe these congeneric "atomic-nano" systems in heterogeneous catalysis fields. Particularly, the SANIs are divided into three categories: "one-island-one-atom", "one-island-multi-atoms", and "island-sea synergism" architectures. The scientific significance and application principles of SANIs in versatile heterogeneous catalysis fields (i.e., thermocatalysis, electrocatalysis, and photocatalysis) are summarized. The challenges and proposals of SANIs are also provided.

Photothermal-enabled single-atom catalysts for high-efficiency hydrogen peroxide photosynthesis from natural seawater

Hydrogen peroxide (H₂O₂) is a powerful industrial oxidant and potential carbon-neutral liquid energy carrier. Sunlight-driven synthesis of H₂O₂ from the most earth-abundant O₂ and seawater is highly desirable. However, the solar-to-chemical efficiency of H₂O₂ synthesis in particulate photocatalysis systems is low. Here, we present a cooperative sunlight-driven photothermal-photocatalytic system based on cobalt single-atom supported on sulfur doped graphitic carbon nitride/reduced graphene oxide heterostructure (Co-CN@G) to boost H₂O₂ photosynthesis from natural seawater. By virtue of the photothermal effect and synergy between Co single atoms and the heterostructure, Co-CN@G enables a solar-to-chemical efficiency of more than 0.7% under simulated sunlight irradiation. Theoretical calculations verify that the single atoms combined with heterostructure significantly promote the charge separation, facilitate O₂ absorption and reduce the energy barriers for O₂ reduction and water oxidation, eventually boosting H₂O₂ photoproduction. The single-atom photothermal-photocatalytic materials may provide possibility of large-scale H₂O₂ production from inexhaustible seawater in a sustainable way.

Active site identification and CO oxidation in UiO-66-XX thin films

Metal-organic frameworks (MOFs) offer an intrinsically porous and chemically tunable platform for gas adsorption, separation, and catalysis. We investigate thin film derivatives of the well-studied Zr-O based MOF powders to understand their adsorption properties and reactivity with their adaption to thin films, involving diverse functionality with the incorporation of different linker groups and the inclusion of embedded metal nanoparticles: UiO-66, UiO-66-NH₂, and Pt@UiO-66-NH₂. Using transflectance IR spectroscopy, we determine the active sites in each film upon consideration of the acid-base properties of the adsorption sites and guest species, and perform metal-based catalysis with CO oxidation of a Pt@UiO-66-NH₂film. Our study shows how surface science characterization techniques can be used to characterize the reactivity and the chemical and electronic structure of MOFs.

Zr- and Ti-based metal-organic frameworks: synthesis, structures and catalytic applications

Recently, Zr- and Ti-based metal-organic frameworks (MOFs) have gathered increasing interest in the field of chemistry and materials science, not only for their ordered porous structure, large surface area, and high thermal and chemical stability, but also for their various potential applications. Particularly, the unique features of Zr- and Ti-based MOFs enable them to be a highly versatile platform for catalysis. Although much effort has been devoted to developing Zr- and Ti-based MOF materials, they still suffer from difficulties in targeted synthesis, especially for Ti-based MOFs. In this Feature Article, we discuss the evolution of Zr- and Ti-based MOFs, giving a brief overview of their synthesis and structures. Furthermore, the catalytic uses of Zr- and Ti-based MOF materials in the previous 3-5 years have been highlighted. Finally, perspectives on the Zr- and Ti-based MOF materials are also proposed. This work provides in-depth insight into the advances in Zr- and Ti-based MOFs and boosts their catalytic applications.

Photo-Induced Switching of CO2 Hydrogenation Pathway towards CH3 OH Production over Pt@UiO-66-NH2 (Co)

It is highly desired to achieve controllable product selectivity in CO₂ hydrogenation. Herein, we report light-induced switching of reaction pathways of CO₂ hydrogenation towards CH₃ OH production over actomically dispersed Co decorated Pt@UiO-66-NH₂ . CO, being the main product in the reverse water gas shift (RWGS) pathway under thermocatalysis condition, is switched to CH₃ OH via the formate pathway with the assistance of light irradiation. Impressively, the space-time yield of CH₃ OH in photo-assisted thermocatalysis (1916.3 μmol g_cat ^-1  h^-1 ) is about 7.8 times higher than that without light at 240 °C and 1.5 MPa. Mechanism investigation indicates that upon light irradiation, excited UiO-66-NH₂ can transfer electrons to Pt nanoparticles and Co sites, which can efficiently catalyze the critical elementary steps (i.e., CO₂ -to-*HCOO conversion), thus suppressing the RWGS pathway to achieve a high CH₃ OH selectivity.

Silver nanoparticle enhanced metal-organic matrix with interface-engineering for efficient photocatalytic hydrogen evolution

Integrating plasmonic nanoparticles into the photoactive metal-organic matrix is highly desirable due to the plasmonic near field enhancement, complementary light absorption, and accelerated separation of photogenerated charge carriers at the junction interface. The construction of a well-defined, intimate interface is vital for efficient charge carrier separation, however, it remains a challenge in synthesis. Here we synthesize a junction bearing intimate interface, composed of plasmonic Ag nanoparticles and matrix with silver node via a facile one-step approach. The plasmonic effect of Ag nanoparticles on the matrix is visualized through electron energy loss mapping. Moreover, charge carrier transfer from the plasmonic nanoparticles to the matrix is verified through ultrafast transient absorption spectroscopy and in-situ photoelectron spectroscopy. The system delivers highly efficient visible-light photocatalytic H₂ generation, surpassing most reported metal-organic framework-based photocatalytic systems. This work sheds light on effective electronic and energy bridging between plasmonic nanoparticles and organic semiconductors.

Metal-Organic Framework-Based Photocatalysis for Solar Fuel Production

Metal-organic frameworks (MOFs) represent a novel class of crystalline inorganic-organic hybrid materials with tunable semiconducting behavior. MOFs have potential for application in photocatalysis to produce sustainable solar fuels, owing to their unique structural advantages (such as clarity and modifiability) that can facilitate a deeper understanding of the structure-activity relationship in photocatalysis. This review takes the photocatalytic active sites as a particular perspective, summarizing the progress of MOF-based photocatalysis for solar fuel production; mainly including three categories of solar-chemical conversions, photocatalytic water splitting to hydrogen fuel, photocatalytic carbon dioxide reduction to hydrocarbon fuels, and photocatalytic nitrogen fixation to high-energy fuel carriers such as ammonia. This review focuses on the types of active sites in MOF-based photocatalysts and discusses their enhanced activity based on the well-defined structure of MOFs, offering deep insights into MOF-based photocatalysis.

Engineering Single-Atom Sites into Pore-Confined Nanospaces of Porphyrinic Metal-Organic Frameworks for the Highly Efficient Photocatalytic Hydrogen Evolution Reaction

As a type of heterogeneous catalyst expected for the maximum atom efficiency, a series of single-atom catalysts (SACs) containing spatially isolated metal single atoms (M-SAs) have been successfully prepared by confining M-SAs in the pore-nanospaces of porphyrinic metal-organic frameworks (MOFs). The prepared MOF composites of M-SAs@Pd-PCN-222-NH₂ (M = Pt, Ir, Au, and Ru) display exceptionally high and persistent efficiency in the photocatalytic hydrogen evolution reaction with a turnover number (TON) of up to 21713 in 32 h and a beginning/lasting turnover frequency (TOF) larger than 1200/600 h^-1 based on M-SAs under visible light irradiation (λ ≥ 420 nm). The photo-/electrochemical property studies and density functional theory calculations disclose that the close proximity of the catalytically active Pt-SAs to the Pd-porphyrin photosensitizers with the confinement and stabilization effect by chemical binding could accelerate electron-hole separation and charge transfer in pore-nanospaces, thus promoting the catalytic H₂ evolution reaction with lasting effectiveness.

Metal-organic framework (MOF)-5/CuO@ZnIn2S4 core-shell Z-scheme tandem heterojunctions for improved charge separation and enhanced photocatalytic performance

Interface engineering is regarded as an effective strategy for charge separation. Metal-organic framework (MOF)-5/CuO@ZnIn₂S₄ core-shell Z-scheme tandem heterojunctions with a three-dimensional floral spherical shape are prepared by a two-step solvothermal and oxidative method. The flower spherical core-shell structure enhances multiple reflections and refractions of light and thus improves light utilization efficiently. In addition, this core-shell structure can supply sufficient active sites for photocatalytic reactions. Meanwhile, the composition of Z-scheme tandem heterojunctions and the photothermal effect contributed to the spatial charge separation and accelerated the photocatalytic process. The photocatalytic hydrogen production rate of MOF-5/CuO@ZnIn₂S₄ (1938.3 μmol g^-1 h^-1) is 18 times higher than that of pristine MOF-5, and the photocatalytic degradation efficiency of 2,4-dichlorophenol and phenol can reach up to 98.7% and 97.3%, respectively. In addition, multiple cycle experiments demonstrate high stability, which is favorable for practical applications.

Electronegativity Enhanced Strong Metal-Support Interaction in Ru@F-Ni3N for Enhanced Alkaline Hydrogen Evolution

Precious metals (Pt, Ir, Ru, and so on) and related compounds usually demonstrate superb catalytic activity for electrochemical hydrogen production. However, scarcity and stability are still challenges for hydrogen evolution reaction, even for single-atomic-site electrocatalysts. Herein, a fluorine (F) doping strategy is proposed to enhance the strong metal-support interaction between the F-doped Ni₃N support and the loaded ruthenium (Ru) species. Via synergistically modulating both the Ru loading amount and F doping concentration, outstanding HER activity was achieved in Ru@F-Ni₃N with an overpotential (η) of 115 mV at 100 mA cm^-2, superior to the benchmark Pt/C (η = 201 mV). Density functional theory simulation in combination with X-ray photoelectron spectra and X-ray absorption spectroscopy characterizations convincingly demonstrate that, with the strongest electronegativity, F doping could effectively stabilize Ru atoms doped in the F-Ni₃N substrate and simultaneously reduce the H bonding strength, which accelerated the desorption of H₂. These findings provide a facile strategy to modulate both catalytic activities and stabilities of heteroatom-loaded catalytic materials.

Effect of sub-minimum inhibitory concentration of ceftriaxone on the expression of outer membrane proteins in Salmonella enterica serovar Typhi

Multi-drug resistance (MDR) in Salmonella is one of the major reasons for foodborne outbreaks worldwide. Decreased susceptibility of Salmonella Typhi to first-line drugs such as ceftriaxone, ciprofloxacin, and azithromycin has raised concern. Reduced outer membrane proteins (OMPs) permeability and increased efflux pump transportation are considered to be the main reasons for the emergence of antibiotic resistance in Salmonella. The present study aimed to assess the expression of OMPs at sub-lethal concentrations of ceftriaxone in S. Typhi (Sl5037/BC, and Sl05). The S. Typhi strains were exposed to sub-MIC and half of the sub-MIC concentrations of ceftriaxone at three different time intervals (0 min, 40 min, and 180 min) and analyzed for differential expression of OMPs. Further, the expression variation of OMP encoding genes (yaeT, ompX, lamb, ompA, and ybfM) in response to ceftriaxone was evaluated using real-time PCR. The genes like lamB, ompX, and yaeT showed significant downregulation (p < 0.05) compared to the control without antibiotic exposure, whereas ybfM and ompA showed a moderate downregulation. The expression of omp genes such as lamB, ompA, ompX, ybfM, and yaeT were found to be low in the presence of ceftriaxone, followed by time and dose-dependent. The study provides insights into the possible involvement of OMPs in drug resistance of S. Typhi, which could help develop a therapeutic strategy to combat MDR isolates of S. Typhi.

Active Sites In Situ Implanted Hybrid Zeolitic Imidazolate Frameworks for a Water Oxidation Catalyst

Metal-organic frameworks (MOFs) have been a focus of research because of their unique porous structure, but they are usually not directly for electrocatalysis. Herein, we prepared a special class of Fe/Zn/Mo-based trimetallic hybrid zeolitic imidazolate frameworks by in situ solvothermal synthesis that have the potential to act directly as highly efficient oxygen evolution reaction electrocatalysts. This work provides a foundation for the preparation of multimetal MOFs and expands the investigation of electrocatalysts.

Ni2P NPs loaded on methylthio-functionalized UiO-66 for boosting visible-light-driven photocatalytic H2 production

The UiO-66 family shows promising photocatalytic prospects in water splitting for hydrogen evolution under visible light irradiation due to its suitable band gap and adequate active sites. In this work, novel Ni₂P/UiO-66-(SCH₃)₂ composites were prepared by a simple solvothermal method. These as-synthesized samples were fully characterized by XRD, SEM, TEM, HRTEM, EDS, and XPS methods. The effectiveness of visible light driven photocatalytic water-splitting to produce hydrogen was investigated in the presence of sacrificial agents. The results showed that the optimal hydrogen yield of 5 wt% Ni₂P/UiO-66-(SCH₃)₂ is 3724.22 μmol g^-1 h^-1, reaching almost 187 times that of pristine UiO-66-(SCH₃)₂ (19.93 μmol g^-1 h^-1). Meanwhile, long term cycling stability tests also showed that Ni₂P/UiO-66-(SCH₃)₂ composites present an excellent photocatalytic H₂ production stability. Photoelectrochemical performance analysis revealed that the high catalytic activity of the composite materials could be associated with the synergistic effect of UiO-66-(SCH₃)₂ and Ni₂P. Light stimulates UiO-66-(SCH₃)₂ to generate electrons and holes, and Ni₂P as a cocatalyst could effectively transmit electrons and boost photogenerated charge separation. This work provides a reference for exploring UiO-66 family catalysts with good catalytic activity.

Photocatalytic Biocidal Coatings Featuring Zr6Ti4-Based Metal-Organic Frameworks

The world is currently suffering socially, economically, and politically from the recent pandemic outbreak due to the coronavirus disease 2019 (COVID-19), and those in hospitals, schools, and elderly nursing homes face enhanced threats. Healthcare textiles, such as masks and medical staff gowns, are susceptible to contamination of various pathogenic microorganisms, including bacteria and viruses. Metal-organic frameworks (MOFs) can potentially address these challenges due to their tunable reactivity and ability to be incorporated as porous coatings on textile materials. Here, we report how incorporating titanium into the zirconium-pyrene-based MOF NU-1000, denoted as NU-1012, generates a highly reactive biocidal photocatalyst. This MOF features a rare ligand migration phenomenon, and both the Ti/Zr center and the pyrene linker act synergistically as dual active centers and widen the absorption band for this material, which results in enhanced reactive oxygen species generation upon visible light irradiation. Additionally, we found that the ligand migration process is generally applicable to other csq topology Zr-MOFs. Importantly, NU-1012 can be easily incorporated onto cotton textile cloths as a coating, and the resulting composite material demonstrates fast and potent biocidal activity against Gram-negative bacteria (Escherichia coli), Gram-positive bacteria (Staphylococcus epidermidis), and T7 bacteriophage virus with up to a 7-log(99.99999%) reduction within 1 h under simulated daylight.

Amino-Functionalized Titanium Based Metal-Organic Framework for Photocatalytic Hydrogen Production

Photocatalytic hydrogen production using stable metal-organic frameworks (MOFs), especially the titanium-based MOFs (Ti-MOFs) as photocatalysts is one of the most promising solutions to solve the energy crisis. However, due to the high reactivity and harsh synthetic conditions, only a limited number of Ti-MOFs have been reported so far. Herein, we synthesized a new amino-functionalized Ti-MOFs, named NH₂-ZSTU-2 (ZSTU stands for Zhejiang Sci-Tech University), for photocatalytic hydrogen production under visible light irradiation. The NH₂-ZSTU-2 was synthesized by a facile solvothermal method, composed of 2,4,6-tri(4-carboxyphenylphenyl)-aniline (NH₂-BTB) triangular linker and infinite Ti-oxo chains. The structure and photoelectrochemical properties of NH₂-ZSTU-2 were fully studied by powder X-ray diffraction, scanning electron microscope, nitro sorption isotherms, solid-state diffuse reflectance absorption spectra, and Mott–Schottky measurements, etc., which conclude that NH₂-ZSTU-2 was favorable for photocatalytic hydrogen production. Benefitting from those structural features, NH₂-ZSTU-2 showed steady hydrogen production rate under visible light irradiation with average photocatalytic H₂ yields of 431.45 μmol·g⁻¹·h⁻¹ with triethanolamine and Pt as sacrificial agent and cocatalyst, respectively, which is almost 2.5 times higher than that of its counterpart ZSTU-2. The stability and proposed photocatalysis mechanism were also discussed. This work paves the way to design Ti-MOFs for photocatalysis.

Controlled Modification of Axial Coordination for Transition-Metal Single-Atom Electrocatalyst

Single-atom catalysts (SACs) have emerged as a new frontier in areas such as electrocatalysis, photocatalysis, and enzymatic catalysis. Aided by recent advances in the synthetic methodologies of nanomaterials, atomic characterization technologies, and theoretical calculation modeling, various SACs have been prepared for a variety of catalytic reactions. To meet the requirements of SACs with distinctive performance and appreciable selectivity, much research has been carried out to adjust the coordination configuration and electronic properties of SACs. This concept summarizes the latest advances in the experimental and computational efforts aimed at tuning the axial coordination of SACs. Series of atoms, functional groups or even macrocycles are oriented into the atomic metal center, and how this affects the electrocatalytic performance is also reviewed. Finally, this concept presents perspectives for the further precise design, preparation and in-situ detection of axially coordinated SACs.

Considering single-atom catalysts as photocatalysts from synthesis to application

With the ever-increased greenhouse effect and energy crisis, developing novel photocatalysts to realize high-efficient solar-driven chemicals/fuel production is of great scientific and practical significance. Recently, single-atom photocatalysts (SAPs) are promising catalysts with maximized metal dispersion and tuneable coordination environments. SAPs exhibit boosted photocatalytic performance by enhancing optical response, facilitating charge carrier transfer behaviors or directly manipulating surface reaction processes. In this regard, this article systematically reviews the state-of-the-art progress in the development and application of SAPs, especially the mechanism and performance of SAPs on various reaction processes. Some future challenges and potential research directions over SAPs are outlined at the final stage.

Engineering single-atom catalysts toward biomedical applications

Due to inherent structural defects, common nanocatalysts always display limited catalytic activity and selectivity, making it practically difficult for them to replace natural enzymes in a broad scope of biologically important applications. By decreasing the size of the nanocatalysts, their catalytic activity and selectivity will be substantially improved. Guided by this concept, the advances of nanocatalysts now enter an era of atomic-level precise control. Single-atom catalysts (denoted as SACs), characterized by atomically dispersed active sites, strikingly show utmost atomic utilization, precisely located metal centers, unique metal-support interactions and identical coordination environments. Such advantages of SACs drastically boost the specific activity per metal atom, and thus provide great potential for achieving superior catalytic activity and selectivity to functionally mimic or even outperform natural enzymes of interest. Although the size of the catalysts does matter, it is not clear whether the guideline of "the smaller, the better" is still correct for developing catalysts at the single-atom scale. Thus, it is clearly a new, urgent issue to address before further extending SACs into biomedical applications, representing an important branch of nanomedicine. This review begins by providing an overview of recent advances of synthesis strategies of SACs, which serve as a basis for the discussion of emerging achievements in improving the enzyme-like catalytic properties at an atomic level. Then, we carefully compare the structures and functions of catalysts at various scales from nanoparticles, nanoclusters, and few-atom clusters to single atoms. Contrary to conventional wisdom, SACs are not the most catalytically active catalysts in specific reactions, especially those requiring multi-site auxiliary activities. After that, we highlight the unique roles of SACs toward biomedical applications. To appreciate these advances, the challenges and prospects in rapidly growing studies of SACs-related catalytic nanomedicine are also discussed in this review.

Atomically dispersed Pt inside MOFs for highly efficient photocatalytic hydrogen evolution

Pt is carried into the channels of UiO-66 with supercritical carbon dioxide to achieve high activity for hydrogen production by photolysis.