Incorporating Transition-Metal Phosphides Into Metal-Organic Frameworks for Enhanced Photocatalysis

Metal-organic frameworks (MOFs) have been shown to be an excellent platform in photocatalysis. However, to suppress electron-hole recombination, a Pt cocatalyst is usually inevitable, especially in photocatalytic H₂ production, which greatly limits practical application. Herein, for the first time, monodisperse, small-size, and noble-metal-free transitional-metal phosphides (TMPs; for example, Ni₂ P, Ni₁₂ P₅ ), are incorporated into a representative MOF, UiO-66-NH₂ , for photocatalytic H₂ production. Compared with the parent MOF and their physical mixture, both TMPs@MOF composites display significantly improved H₂ production rates. Thermodynamic and kinetic studies reveal that TMPs, behaving similar ability to Pt, greatly accelerate the linker-to-cluster charge transfer, promote charge separation, and reduce the activation energy of H₂ production. Significantly, the results indicate that Pt is thermodynamically favorable, yet Ni₂ P is kinetically preferred for H₂ production, accounting for the higher activity of Ni₂ P@UiO-66-NH₂ than Pt@UiO-66-NH₂ .

Development of MOF-based heterostructures for photocatalytic hydrogen evolution

The construction of metal-organic framework (MOF) based heterostructures are a promising strategy to improve the photocatalytic hydrogen evolution activities of MOFs. For the intrinsic porosity, inorganic-organic hybrid nature and structural tunability of MOFs, a wide variety of MOF-based heterostructured photocatalysts with improved hydrogen evolution activities have been fabricated. In this frontier article, we present the latest advances in MOF-based heterostructures for photocatalytic hydrogen evolution. The opportunities and challenges related to MOF-based heterostructured photocatalysts are also presented.

A Ti-MOF Decorated With a Pt Nanoparticle Cocatalyst for Efficient Photocatalytic H2 Evolution: A Theoretical Study

Pt nanoparticles (NPs) are often used as cocatalysts to enhance the photocatalytic hydrogen production catalyzed by the metal organic framework (MOF) materials. The catalytic efficiency of many Pt/MOF systems can be greatly improved when Pt NPs are used as cocatalysts. In this work, the Pt/20%-MIL-125-(SCH₃)₂ was chosen as the template material to understand the functional role of a Pt metal cocatalyst in the catalytic process. Experimentally, the catalytic activity of Pt/20%-MIL-125-(SCH₃)₂ is more than 100 times that of the system without the help of Pt NPs. Firstly, we proposed a searching algorithm, which is based on the combined Monte Carlo (MC) method and principal component analysis (PCA) algorithm, to find that the most probable adsorption site of the Pt₁₃ nanocluster loaded on the (001) surface of 20%-MIL-125-(SCH₃)₂. Next, by using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, we revealed that the accumulation of some positive charges on the Pt₁₃ cluster and proton adsorbed on the Pt₁₃ cluster, which can promote the separation of photogenerated electrons and holes, thus improving the photocatalytic efficiency. This work not only provides a method to obtain the adsorption configuration of metal clusters on various MOFs but also provides a new insight into increasing photocatalytic efficiency for H₂ production in Pt/MOF systems.

Localized surface plasmon resonance enhanced visible-light-driven CO2 photoreduction in Cu nanoparticle loaded ZnInS solid solutions

Visible-light-driven photocatalysts have shown tremendous prospects in solving the energy crisis and environmental problems, thanks to their wide spectral response and high quantum efficiency. Several strategies including the expansion of visible light response and the improvement of solar energy utilization and photocatalytic quantum efficiency via more effective separation of photogenerated carriers are the current focuses of research that direct the design and fabrication of viable photocatalysts. Herein, a series of composite photocatalysts assembled from plasmonic Cu nanoparticles (NPs) and Zn3In2S6 (ZIS) solid solutions were synthesized by means of a simple solvothermal method. In comparison with the pristine ZIS semiconductor, Cu NP loaded ZIS solid solutions showed greatly enhanced photocatalytic activity, selectivity and stability towards CO2 reduction under visible irradiation. Of note was that the optimized ZIS-Cu2 exhibited an enhanced CH4 production rate of ca. 292 μL g-1 h-1 and a selectivity of ca. 71.1%, which were among the highest numbers reported hitherto. The localized surface plasmon resonance (LSPR) effect, shown by surface Cu NPs, was believed to play a critical role in the enhanced CO2 photoreduction efficiency. More importantly, the introduction of plasmonic Cu NPs could restrain the recombination of photogenerated electron-hole pairs and promote the migration of photogenerated electrons to better participate in the photocatalytic CO2 reduction in the presence of water vapor. This work thus provides a facile means to design robust and flexible composite photocatalysts for visible-light-driven CO2 photoreduction with high efficiency.

Co3O4@CdS Hollow Spheres Derived from ZIF-67 with a High Phenol and Dye Photodegradation Activity

The Co₃O₄@CdS double-layered hollow spheres were first prepared by the template-removal method with the assistance of the ZIF-67 material; the structure has been proved by transmission electron microscopy (TEM). The Co₃O₄@CdS hollow spheres calcinated at 400 °C exhibited the highest photodegradation activity. Nearly 90% phenol was degraded after 2 h of visible-light irradiation. More than 80% rhodamine-B (RhB) was degraded within the first 30 min and nearly eliminated after 1 h of irradiation. The mechanism of the photodegradation reaction was investigated. Based on the analysis of electron spin resonance (ESR) spectra and radical trapping test, it was found that superoxide radicals are the major oxidative species for dye degradation and holes and hydroxyl radicals are the major oxidative species for phenol degradation. These results may be used in industrial wastewater treatment. The reaction obeys first-order reaction kinetics, and the rate constant of the Co₃O₄@CdS hollow sphere in dye degradation is 0.05 min^-1 and that in phenol degradation is 0.02 min^-1, which is three times higher than that of CdS nanoparticles. These results indicated the high oxidizing ability of the samples.

Nanocrystalline Ag3PO4 for Sunlight- and Ambient Air-Driven Oxidation of Amines: High Photocatalytic Efficiency and a Facile Catalyst Regeneration Strategy

Selective oxidation of amines to imines using sunlight as clean and renewable energy source is an important but challenging chemical transformation because of high reactivity of the generated imines and lack of visible light-responsive materials with high conversion rates. In addition, oxygen gas has to be purged in the reaction mixture in order to increase the reaction efficiency which, in itself, is an energy-consuming process. Herein, we report, for the first time, the use of Ag₃PO₄ as an excellent photocatalyst for the oxidative coupling of benzyl amines induced by ambient air in the absence of any external source of molecular oxygen at room temperature. The conversion efficiency for the selective oxidation of benzyl amine was found to be greater than 95% with a selectivity of >99% after 40 min of light irradiation indicating an exceptionally high conversion efficiency with a rate constant of 0.002 min^-1, a turnover frequency of 57 h^-1, and a quantum yield of 19%, considering all of the absorbed photons. Ag₃PO₄, however, is known for its poor photostability owing to a positive conduction band position and a favorable reduction potential to metallic silver. Therefore, we further employed a simple catalyst regeneration strategy and showed that the catalyst can be recycled with negligible loss of activity and selectivity.

Multimetal Incorporation into 2D Conductive Metal-Organic Framework Nanowires Enabling Excellent Electrocatalytic Oxidation of Benzylamine to Benzonitrile

As an important organic intermediate, benzonitrile (BN) is widely involved in organic synthetic chemistry and pharmaceutical and dyestuff industries. However, the exploration of a more efficient and controllable synthesis technique and the corresponding greener catalysts for the synthesis of BN still poses a great challenge. Herein, with multimetallic two-dimensional conductive metal-organic frameworks (2D cMOF) as anodic electrocatalysts, we develop a green, convenient, and highly efficient electrochemical synthesis strategy for BN. Thanks to the intrinsic 2D electrically conductive structure and the optimized the multimetallic coupling catalytic effect, the resulting multimetallic 2D cMOFs exhibit excellent benzylamine (BA) electrooxidation performance. Especially, the trimetallic 2D cMOF (NiCoFe-CAT) requires an ultralow potential of 1.29 V vs reversible hydrogen electrode (RHE) to achieve a 10 mA·cm^-2 current density, which indicates the fastest reaction and the most favorable thermodynamic condition. A very high yield (0.058 mmol·mg^-1·h^-1) and faradic efficiency (∼87%) of benzonitrile are both achieved during the BA electrooxidation reaction at 1.45 V. The reaction mechanism investigations indicated that the various redox mediators of M^II/M^III (Ni, Co, Fe) may be regarded as multimetal active species to promote BA conversion. Also, the excellent cycling durability of multimetallic 2D cMOFs further promotes their potential practical applications. These electrocatalytic performances are considered excellent and nearly surpass all other reported Ni-based inorganics or MOF-based electrocatalysts for the electrocatalytic oxidation of benzylamine.

Alkene-Linked Covalent Organic Frameworks Boosting Photocatalytic Hydrogen Evolution by Efficient Charge Separation and Transfer in the Presence of Sacrificial Electron Donors

Covalent organic frameworks (COFs) are potential photocatalysts for artificial photosynthesis but they are much less explored for photocatalytic hydrogen evolution (PHE). COFs, while intriguing due to crystallinity, tunability, and porosity, tend to have low apparent quantum efficiency (AQE) and little is explored on atomistic structure-performance correlation. Here, adopting triphenylbenzene knots and phenyl linkers as a proof of concept, three structurally related COFs with different linkages are constructed to achieve a tunable COF platform and probe the effect of the linkage chemistry on PHE. Cyano-substituted alkene-linked COF (COF-alkene) yields a stable 2330 µmol h-1 g-1 PHE rate, much superior to imine- and imide-linked counterparts (<40 µmol h-1 g-1) under visible light irradiation. Impressively, COF-alkene achieves an AQE of 6.7% at 420 nm. Combined femtosecond transient absorption spectroscopy and theoretical calculation disclose the critical role of cyano-substituted alkene linkages toward high efficiency of charge separation and transfer in the presence of sacrificial electron donors-the decisive key to the superior PHE performance. Such alkene linkages can also be extended to design a series of high-performance polymeric photocatalysts, highlighting a general design idea for efficient PHE.

Discrete Supertetrahedral T5 Selenide Clusters and Their Se/S Solid Solutions: Ionic-Liquid-Assisted Precursor Route Syntheses and Photocatalytic Properties

Although supertetrahedral Tn sulfide clusters (n=2-6) have been extensively explored, the synthesis of Tn selenide clusters with n>4 has not been achieved thus far. Reported here are ionic-liquid (IL)-assisted precursor route syntheses, characterizations, and the photocatalytic properties of six new M-In-Q (M=Cu or Cd; Q=Se or Se/S) chalcogenide compounds, namely [Bmmim]₁₂ Cu₅ In₃₀ Q₅₂ Cl₃ (Im) (Q=Se (T5-1), Se_48.5 S_3.5 (T5-2); Bmmim=1-butyl-2,3-dimethylimidazolium, Im=imidazole), [Bmmim]₁₁ Cd₆ In₂₈ Q₅₂ Cl₃ (MIm) (Q=Se (T5-3), Se_28.5 S_23.5 (T5-4), Se₁₆ S₃₆ (T5-5); MIm=1-methylimidazole), and [Bmmim]₉ Cd₆ In₂₈ Se₈ S₄₄ Cl(MIm)₃ (T5-6). The cluster compounds T5-1 and T5-3 represent the largest molecular supertetrahedral Tn selenide clusters to date. Under visible-light illumination, the Cu-In-Q compounds showed photocatalytic activity towards the decomposition of crystal violet, whereas the Cd-In-Q compounds exhibited good photocatalytic H₂ evolution activity. Interestingly, the experimental results show that the photocatalytic performances of the selenide/sulfide solid solutions were significantly better than those of their selenide analogues, for example, the degradation time of the organic dye with T5-2 was much shorter than that with T5-1, whereas the photocatalytic H₂ evolution efficiencies with T5-3-T5-6 improved significantly with increasing sulfur content. This work highlights the significance of IL-assisted precursor route synthesis and the tuning of photocatalytic properties through the formation of solid solutions.

Nickel-decorated g-C3N4 hollow spheres as an efficient photocatalyst for hydrogen evolution and oxidation of amines to imines

Photocatalysts composed of earth-abundant elements are highly desired for photocatalytic hydrogen evolution as well as oxidation of amines to imines without the requirement of precious metals.

Semiconductor nanocrystals for small molecule activation via artificial photosynthesis

The protocol of artificial photosynthesis using semiconductor nanocrystals shines light on green, facile and low-cost small molecule activation to produce solar fuels and value-added chemicals.

Electron transfer in the confined environments of metal–organic coordination supramolecular systems

In this review, we overview regulatory factors and diverse applications of electron transfer in confined environments of supramolecular host–guest systems.

A visible-light responsive metal–organic framework as an eco-friendly photocatalyst under ambient air at room temperature

A 2′,7′-dichlorofluorescein-based MOF as a reusable photocatalyst for the synthesis of 1,3-oxathiolane-2-imines from styrenes and NH₄SCN by utilizing visible light and air as the eco-sustainable and cheapest reagents.

Coordination polymers with salicylaldehyde ligands: structural diversity, selective sorption and luminescence sensing properties

Five new coordination polymers with salicylaldehyde ligands were obtained. They show diverse structures, selective dye adsorption and fluorescence sensing properties.

Visible-light-driven self-coupling and oxidative dehydrogenation of amines to imines via a Mn(ii)-based coordination polymer

The direct synthesis of various imines through visible-light-driven photocatalytic self-coupling and dehydrogenation of amines was achieved using a novel coordination polymer.

Coordination polymers with a pyridyl–salen ligand for photocatalytic carbon dioxide reduction

Fe(iii) and Mn(iii) coordination polymers with a pyridyl–salen ligand were constructed and have shown photocatalytic activity for CO₂reduction under visible-light irradiation.

g-C3N4/Uio-66-NH2 nanocomposites with enhanced visible light photocatalytic activity for hydrogen evolution and oxidation of amines to imines

g-C₃N₄/Uio-66-NH₂ (CNUIO) nanocomposites were prepared by growing an NH₂-mediated zirconium-based metal–organic framework (Uio-66-NH₂) in the presence of g-C₃N₄ nanotubes.

Encapsulating low-coordinated Pt clusters within a metal–organic framework induces spatial charge separation boosting photocatalytic hydrogen evolution

A strategy is developed to synthesize MOF photocatalyst encapsulating low-coordinate Pt clusters with high photocatalytic hydrogen-evolution performance under visible light.

Synergistic Effect over Sub-nm Pt Nanocluster@MOFs Significantly Boosts Photo-oxidation of N-alkyl(iso)quinolinium Salts

Quinolones and isoquinolones are of interest to pharmaceutical industry owing to their potent biological activities. Herein, we first encapsulated sub-nm Pt nanoclusters into Zr-porphyrin frameworks to afford an efficient photocatalyst Pt_0.9@PCN-221. This catalyst can dramatically promote electron-hole separation and ¹O₂ generation to achieve synergistic effect first in the metal-organic framework (MOF) system, leading to the highest activity in photosynthesis of (iso)quinolones in >90.0% yields without any electronic sacrificial agents. Impressively, Pt_0.9@PCN-221 was reused 10 times without loss of activity and can catalyze gram-scale synthesis of 1-methyl-5-nitroisoquinolinone at an activity of 175.8 g·g_cat⁻¹, 22 times higher than that of PCN-221. Systematic investigations reveal the contribution of synergistic effect of photogenerated electron, photogenerated hole, and ¹O₂ generation for efficient photo-oxidation, thus highlighting a new strategy to integrate multiple functional components into MOFs to synergistically catalyze complex photoreactions for exploring biologically active heterocyclic molecules.

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A state-of-the-art photocatalyst for preparation of bioactive (iso)quinolones

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Synergistic catalysis of photogenerated e⁻/h⁺ and ¹O₂

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Sub-nm Pt_0.9@PCN-221 with a high efficiency of e⁻-h⁺ separation and ¹O₂ generation

An unsymmetrical covalent organic polymer for catalytic amide synthesis

Herein, we present the first report on the Covalent Organic Polymer (COP) directed non-classical synthesis of an amide bond. An economical route has been chosen for the synthesis of APC-COP using p-aminophenol and cyanuric chloride. APC-COP acts as a smart, valuable and sustainable catalyst for efficient access to the amide bond under mild conditions at room temperature in 30 min. APC-COP exhibits selectivity towards carboxylic acids over esters. The key features of this protocol involve the variety of parameters, viz. wider substrate scope, no use of additive and recyclability, which makes this approach highly desirable in gramscale synthesis. Moreover, we have shown the practical utility of the present method in the catalytic synthesis of paracetamol.

Turning on Visible-Light Photocatalytic C-H Oxidation over Metal-Organic Frameworks by Introducing Metal-to-Cluster Charge Transfer

The tailorable structure and electronic structure of metal-organic frameworks (MOFs) greatly facilitate their modulated light harvesting, redox power, and consequently photocatalysis. Herein, a representative MOF, UiO-66, was furnished by installing Fe³⁺ onto the Zr-oxo clusters, to give Fe-UiO-66, which features extended visible light harvesting, based on metal-to-cluster charge transfer (MCCT). The Fe-UiO-66 with unique electronic structure and strong oxidizing power exhibits visible light-driven water oxidation, which is impossible for pristine UiO-66. More strikingly, under visible irradiation, the generated holes over Fe-UiO-66 are able to exclusively convert H₂O to hydroxide radicals, initiating and driving the activation of stubborn C-H bond, such as toluene oxidation. The electrons reduce O₂ to O₂^•- radicals that further promote the oxidation reaction. The related catalytic mechanism and the structure-activity relationship have been investigated in detail. As far as we know, this is not only an unprecedented report on activating "inert" MOFs for photocatalytic C-H activation but also the first work on extended light harvesting and enhanced photocatalysis for MOFs by introducing an MCCT process.

Triple-Shelled Co-VSex Hollow Nanocages as Superior Bifunctional Electrode Materials for Efficient Pt-Free Dye-Sensitized Solar Cells and Hydrogen Evolution Reactions

Complex nanostructures with distinct spatial architectures and more active sites hold broad prospects in new energy conversion fields. Herein, a facile strategy was carried out to construct triple-shelled Co-VSe_x nanocages, starting with an ion-exchange process between Co-based zeolitic imidazolate framework-67 (ZIF-67) nanopolyhedrons and VO₃^- followed by the formation of triple-shelled Co-VSe_x hollow nanocages during the process of increasing the solvothermal temperature under the assistance of SeO₃^2-. Meanwhile, triple-shelled Co-VS_x and yolk-double shell Co-VO_x nanocages were fabricated as references by a similar process. Benefiting from the larger surface areas and more electrolyte adsorption sites, the triple-shelled Co-VSe_x nanocages exhibited excellent electrocatalytic performances when applied as the electrochemical catalysts for dye-sensitized solar cells (DSSC) and hydrogen evolution reactions (HER). More concretely, the DSSC based on the Co-VSe_x counter electrode showed outstanding power conversion efficiency of 9.68% when its Pt counterpart was 8.46%. Moreover, the Co-VSe_x electrocatalyst exhibited prominent HER performance with a low onset overpotential of 40 mV and a small Tafel slope of 39.1 mV dec^-1 in an acidic solution.

β-Cyclodextrin-based hollow nanoparticles with excellent adsorption performance towards organic and inorganic pollutants

In this work, β-cyclodextrin (β-CD) based hollow nanoparticles (denoted as β-CDHN) with abundant active sites and high specific surface area were first fabricated via a facile one-step method. The β-CDHN presented a maximum adsorption capacity of 2080.35, 427.35 and 120.48 mg g^-1 towards the cationic dye methylene blue (MB), heavy metal ions (Pb²⁺) and bisphenol A (BPA), respectively, much higher than those of many other adsorbents. Furthermore, β-CDHN also exhibited fast adsorption kinetics towards these pollutants with adsorption rate constants 6 to 200 times higher than those of activated carbon and other β-CD-based adsorbents, meaning the former can remove these pollutants at a much faster adsorption rate than the latter adsorbents. More importantly, the removal efficiency of these pollutants on β-CDHN almost remained stable after 10 regeneration cycles with favorable recyclability. The prepared β-CDHN show great potential in practical applications due to their low costs and high efficiency in the treatment of organic and inorganic pollutants from wastewater.

A zirconium-based metal-organic framework sensitized by thioflavin-T for sensitive photoelectrochemical detection of C-reactive protein

Herein, a novel photoelectrochemical (PEC) assay was developed for the sensitive detection of C-reactive protein (CRP) based on a zirconium-based metal-organic framework (PCN-777) as the photoelectric material and thioflavin-T (Th-T) as the effective signal sensitizer coupled with rolling circle amplification (RCA).

Visible-light harvesting pyrene-based MOFs as efficient ROS generators

The utilization of reactive oxygen species (ROS) in organic transformations is of great interest due to their superior oxidative abilities under mild conditions. Recently, metal-organic frameworks (MOFs) have been developed as photosensitizers to transfer molecular oxygen to ROS for photochemical synthesis. However, visible-light responsive MOFs for oxygen activation remains scarce. Now we design and synthesize two porous MOFs, namely, PCN-822(M) (M = Zr, Hf), which are constructed by a 4,5,9,10-(K-region) substituted pyrene-based ligand, 4,4',4'',4'''-((2,7-di-tert-butylpyrene-4,5,9,10-tetrayl)tetrakis(ethyne-2,1-diyl))-tetrabenzoate (BPETB4-). With the extended π-conjugated pyrene moieties isolated on the struts, the derived MOFs are highly responsive to visible light, possessing a broad-band adsorption from 225-650 nm. As a result, the MOFs can be applied as efficient ROS generators under visible-light irradiation, and the hafnium-based MOF, PCN-822(Hf), can promote the oxidation of amines to imines by activating molecular oxygen via synergistic photo-induced energy and charge transfer.

Carbon Nitride Co-catalyst Activation Using N-Doped Carbon with Enhanced Photocatalytic H2 Evolution

Photocatalytic water splitting holds huge potential to meet the current challenges of energy and environments. Among them, polymeric carbon nitride (CN) has drawn much attention as a promising metal-free photocatalyst. As it is known, a number of promising co-catalysts have been developed to improve catalytic reactions, Pt nanoparticles is still among the best co-catalysts for CN in photocatalytic H₂ evolution, due to the suitable Fermi level to transfer excited electrons and the low overpotential for H₂ reduction. Herein, we report the interface engineering of urea-derived bulk CN and Pt co-catalyst by using a small portion of N-doped carbon (N-C) as a transition layer with a boosted photocatalytic activity up to 7 times. It was revealed that the activation energy of the Pt co-catalyst for water reduction was lowered in the presence of N-C, and the intimate interaction between CN and N-C, ascribing to the similar elemental composition and crystal structure, promoted the efficient separation and migration of charge carriers. This study may open a new avenue to develop CN-based photocatalysts for solar fuel conversion with even higher activity by photocatalyst/co-catalyst interface engineering.

Polyoxometalate-Supported Aminocatalyst for the Photocatalytic Direct Synthesis of Imines from Alkenes and Amines

Developing efficient photocatalysts for direct oxidative coupling of alkenes and amines with O₂ under mild conditions is very significant. Herein, ZnW-PYI is well-designed by assembling a [PZnW₁₁O₃₉(H₂O)]^5- photooxidation catalyst and chiral aminocatalyst pyrrolidine-2-ylimidazole (PYI) via a coordination model. ZnW-PYI efficiently catalyzed the synthesis of imines from alkenes and amines using O₂ as the oxidant through nucleophilic catalysis by employing pyrrolidine as an organocatalyst. Combining a polyoxometalate and PYI within one single framework is an effective approach not only for stabilization and heterogenization of the redox-active catalyst and aminocatalyst but also for realization of compatibility between the reaction intermediates and synergy of multiple catalytic cycles.