Fabrication of p-n Heterostructured Photocatalysts with Triazine-Based Covalent Organic Framework and CuInS2 for High-Efficiency CO2 Reduction.

The application of covalent organic frameworks (COFs) for the photocatalytic reduction of CO2 is mostly limited by severe charge recombination and low sunlight utilization. Herein, a triazine-based COF with an electron-rich and large π-conjugated system (TCOF) was employed as a building block and integrated with CuInS2 (CIS) to construct a noble-metal-free and high-efficiency photocatalyst for CO2 reduction. The in situ growth of CIS nanosheets on TCOF creates a p-n heterojunction, named CIS@TCOF. Compared with TCOF, the CIS@TCOF heterostructure exhibits a dramatically boosted photocatalytic performance in the reduction of CO2. The produced HCOOH yield over 10 wt % CIS@TCOF can be up to 171.2 µmol g-1 h-1 under visible light irradiation with good reproducibility, which is about 3 times as high as that over TCOF. Further in-depth studies indicate that the introduction of CIS not only enhances the visible light utilization but also restrains the recombination of photogenerated electron-hole pairs efficiently and facilitates the photoinduced charge transfer via the p-n heterojunction system due to the unique structural and compositional features. This research shows the great potential of COFs as efficient photocatalytic carbon fixation materials and provides a versatile route to construct semiconductor-COF heterostructures for photocatalysis.

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.

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

Photocatalytic CO2 reduction is an appealing strategy for mitigating the environmental effects of greenhouse gases while simultaneously producing valuable carbon-neutral fuels. Numerous attempts have been made to produce effective and efficient photocatalysts for CO2 reduction. In contrast, the selection of competitive catalysts continues to be a substantial hindrance and a considerable difficulty in the development of photocatalytic CO2 reduction. It is vital to emphasize different techniques for building effective photocatalysts to improve CO2 reduction performance in order to achieve a long-term sustainability. Metal-organic frameworks (MOFs) are recently emerging as a new type of photocatalysts for CO2 reduction due to their excellent CO2 adsorption capability and unique structural characteristics. This review examines the most recent breakthroughs in various techniques for modifying MOFs in order to improve their efficiency of photocatalytic CO2 reduction. The advantages of MOFs using as photocatalysts are summarized, followed by different methods for enhancing their effectiveness for photocatalytic CO2 reduction via partial ion exchange of metal clusters, design of bimetal clusters, the modification of organic linkers, and the embedding of metal complexes. For integrating MOFs with semiconductors, metallic nanoparticles (NPs), and other materials, a number of different approaches have been also reviewed. The final section of this review discusses the existing challenges and future prospects of MOFs as photocatalysts for CO2 reduction. Hopefully, this review can stimulate intensive research on the rational design and development of more effective MOF-based photocatalysts for visible-light driven CO2 conversion.

Core-Shell Covalently Linked Graphitic Carbon Nitride-Melamine-Resorcinol-Formaldehyde Microsphere Polymers for Efficient Photocatalytic CO2 Reduction to Methanol.

Photocatalytic reduction of CO2 with light and H2O to form CH3OH is a promising route to mitigate carbon emissions and climate changes. Although semiconducting metal oxides are potential photocatalysts for this reaction, low photon efficiency and leaching of environmentally unfriendly toxic metals limit their applicability. Here, we report metal-free, core-shell photocatalysts consisting of graphitic carbon nitride (g-C3N4, CN) covalently linked to melamine-resorcinol-formaldehyde (MRF) microsphere polymers for this reaction. Covalent linkage enabled efficient separation of photo-generated carriers and photocatalysis. Using 100 mg of a photocatalyst containing 15 wt % CN, a CH3OH yield of 0.99 µmol·h-1 was achieved at a reaction temperature of 80 °C and 0.5 MPa with external quantum efficiencies ranging from 5.5% at 380 nm to 1.7% at 550 nm. The yield was about 20 and 10 times higher than that of its components CN and MRF, respectively. Characterization with X-ray photoelectron spectroscopy, transmission electron microscopy, and bulk and surface elemental analyses supported the formation of a core-shell structure and the charge transfer in the C-N bond at the CN-MRF interface between the methoxy group in the 2,4-dihydroxylmethyl-1,3-diphenol part of MRF and the terminal amino groups in CN. This enhanced ligand-to-ligand charge transfer resulted in 67% of the photo-excited internal charge transferred from CN to the hydroxymethylamino group in MRF, whose amino group was the catalytic site for the CO2 photocatalytic reduction to CH3OH. This study provides a series of new metal-free photocatalyst designs and insights into the molecular-level structure-mediated photocatalytic response.

Atomically Dispersed Vanadium Sites Anchored on N-Doped Porous Carbon for the Efficient Oxidative Coupling of Amines to Imines.

Single-atom catalysts effectively integrate the respective advantages of homogeneous and heterogeneous catalysts and are a pioneering research frontier in catalysis by virtue of their maximized utility of metal atoms and distinct atomic configuration. However, development of such catalysts is still in the early stages. Herein, atomically dispersed vanadium (V) sites that are coordinated by N atoms and inlaid within N-incorporated porous carbon networks were prepared through a top-down strategy by annealing a V-containing metal-organic framework, NH2-MIL-101(V), followed by acid etching. The resulting V-N-C-600 catalyst exhibits unexpected catalytic reactivity, selectivity, and robust stability for the direct aerobic oxidation of benzylamine to generate N-benzylidene benzylamine with molecular oxygen under mild conditions. The turnover frequency reaches 53.9 h-1, which is much superior to those achieved over the commercial V2O5 and state-of-the-art non-noble metal heterogeneous catalysts reported in the literature. Kinetic analysis reveals a low activation energy barrier (37 kJ mol-1) for the benzylamine oxidation and indicates that a carbocationic intermediate is involved in the reaction mechanism. The synergistic effect between the isolated V single-atomic sites and N-doped hierarchically porous carbon network boosts the performance of V-N-C-600. Moreover, V-N-C-600 exhibits a wide generality for the efficient synthesis of a set of symmetrical imines, unsymmetrical imines, and imine derivatives.

Comparative study on different strategies for synthesizing all-silica DD3R zeolite crystals with a uniform morphology and size.

In the last three decades, the all-silica deca-dodecasil 3R (DD3R) zeolite has been extensively studied as a significant potential class of porous materials in adsorptive separations. However, the use of most existing synthesis methods is unable to produce pure DD3R crystals with a uniform morphology and size. The present research, is therefore intended to provide a facile protocol to synthesize pure DD3R crystals with a controllable morphology and size and with a high reproducibility and productivity. Special attention was focused on investigating the effects of the type of seeds and the mineralizing reagent on the phase-purity, morphology, and crystal size of the resultant DD3R crystals. Various techniques, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), N2 adsorption-desorption at 77 K, and thermogravimetric analysis (TGA) were then used to characterize the synthesized samples. The results show that by adding a small amount of "amorphous" DD3R or "amorphous" ZSM-58 seeds, the pure DD3R crystals with a uniform morphology and size can be synthesized using 1-adamantanamine (1-ADA) as a structure-directing agent (SDA), KF was used as a mineralizing reagent, and LUDOX AS-30 as a silicon source at 443 K for 1 d. In addition, the pure, large and uniform hexahedron DD3R crystals can be prepared using fumed silica as seeds, although the crystallization time takes a longer period of 3 d. The present work could stimulate fundamental research and industrial applications of the all-silica DD3R zeolite.

Synergistic Catalysis of Ruthenium Nanoparticles and Polyoxometalate Integrated Within Single UiO-66 Microcrystals for Boosting the Efficiency of Methyl Levulinate to γ-Valerolactone.

The synthesis of heterogeneous cooperative catalysts in which two or more catalytically active components are spatially separated within a single material has generated considerable research efforts. The multiple functionalities of catalysts can significantly improve the efficiency of existing organic chemical transformations. Herein, we introduce ruthenium (Ru) nanoparticles (NPs) on the surfaces of a metal-organic framework pre-encapsulated with polyoxometalate silicotungstic acid (SiW) UiO-66 (University of Oslo [UiO]) and prepared a 2.0% Ru/11.7% SiW@UiO-66 porous hybrid using the impregnation method. The close synergistic effect of metal Ru NPs, SiW, and UiO-66 endow 2.0% Ru/11.7% SiW@UiO-66 with increased activity and stability for complete methyl levulinate (ML) conversion and exclusive γ-valerolactone (GVL) selectivity at mild conditions of 80°C and at a H2 pressure of 0.5 MPa. Effectively, this serves as a model reaction for the upgrading of biomass and outperforms the performances of the constituent parts and that of the physical mixture (SiW + Ru/UiO-66). The highly dispersed Ru NPs act as active centers for hydrogenation, while the SiW molecules possess Brønsted acidic sites that cooperatively promote the subsequent lactonization of MHV to generate GVL, and the UiO-66 crystal accelerates the mass transportation facilitated by its own porous structure with a large surface area.

Temperature modulation of defects in NH2-UiO-66(Zr) for photocatalytic CO2 reduction.

Defect engineering can be a promising approach to improve the photocatalytic performance of metal-organic frameworks (MOFs). Herein, a series of defective NH2-UiO-66(Zr) materials were synthesized via simply controlling the synthesis temperature, with concentrated HCl as the modulator and then these as-prepared samples were used to systematically investigate the effects of their structural defects on photocatalytic CO2 reduction. Remarkably, these MOFs with defects exhibit significantly enhanced activities in photocatalytic CO2 reduction, compared with the material without defects. The defect engineering creates active binding sites and more open frameworks in the MOF, and thus facilitates the photo-induced charge transfer and restrains the recombination of photo-generated charges efficiently. The current work provides an instructive approach to improve the photocatalytic efficiency by taking advantage of the structural defects in MOFs, and could also inspire more work on the design of advanced defective MOFs.

Noble metals can have different effects on photocatalysis over metal-organic frameworks (MOFs): a case study on M/NH2-MIL-125(Ti) (M=Pt and Au).

M-doped NH2-MIL-125(Ti) (M=Pt and Au) were prepared by using the wetness impregnation method followed by a treatment with H2 flow. The resultant samples were characterized by powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure (XAFS) analyses, N2-sorption BET surface area, and UV/Vis diffuse reflectance spectroscopy (DRS). The photocatalytic reaction carried out in saturated CO2 with triethanolamine (TEOA) as sacrificial agent under visible-light irradiations showed that the noble metal-doping on NH2-MIL-125(Ti) promoted the photocatalytic hydrogen evolution. Unlike that over pure NH2-MIL-125(Ti), in which only formate was produced, both hydrogen and formate were formed over Pt- and Au-loaded NH2-MIL-125(Ti). However, Pt and Au have different effects on the photocatalytic performance for formate production. Compared with pure NH2-MIL-125(Ti), Pt/NH2-MIL-125(Ti) showed an enhanced activity for photocatalytic formate formation, whereas Au has a negative effect on this reaction. To elucidate the origin of the different photocatalytic performance, electron spin resonance (ESR) analyses and density functional theory (DFT) calculations were carried out over M/NH2-MIL-125(Ti).The photocatalytic mechanisms over M/NH2-MIL-125(Ti) (M=Pt and Au) were proposed. For the first time, the hydrogen spillover from the noble metal Pt to the framework of NH2-MIL-125(Ti) and its promoting effect on the photocatalytic CO2 reduction is revealed. The elucidation of the mechanism on the photocatalysis over M/NH2-MIL-125(Ti) can provide some guidance in the development of new photocatalysts based on MOF materials. This study also demonstrates the potential of using noble metal-doped MOFs in photocatalytic reactions involving hydrogen as a reactant, like hydrogenation reactions.

Studies on photocatalytic CO(2) reduction over NH2 -Uio-66(Zr) and its derivatives: towards a better understanding of photocatalysis on metal-organic frameworks.

Metal-organic framework (MOF) NH2 -Uio-66(Zr) exhibits photocatalytic activity for CO2 reduction in the presence of triethanolamine as sacrificial agent under visible-light irradiation. Photoinduced electron transfer from the excited 2-aminoterephthalate (ATA) to Zr oxo clusters in NH2 -Uio-66(Zr) was for the first time revealed by photoluminescence studies. Generation of Zr(III) and its involvement in photocatalytic CO2 reduction was confirmed by ESR analysis. Moreover, NH2 -Uio-66(Zr) with mixed ATA and 2,5-diaminoterephthalate (DTA) ligands was prepared and shown to exhibit higher performance for photocatalytic CO2 reduction due to its enhanced light adsorption and increased adsorption of CO2 . This study provides a better understanding of photocatalytic CO2 reduction over MOF-based photocatalysts and also demonstrates the great potential of using MOFs as highly stable, molecularly tunable, and recyclable photocatalysts in CO2 reduction.

An amine-functionalized titanium metal-organic framework photocatalyst with visible-light-induced activity for CO2 reduction.

Let your light shine: the photocatalytic reduction of carbon dioxide to the formate anion under visible light irradiation is for the first time realized over a photoactive Ti-containing metal-organic framework, NH(2)-MIL-125(Ti), which is fabricated by a facile substitution of ligands in the UV-responsive MIL-125(Ti) material.

A facile hydrothermal method to BiSbO4 nanoplates with superior photocatalytic performance for benzene and 4-chlorophenol degradations.

BiSbO(4) nanoplates with a large BET specific area has been prepared successfully via a facile hydrothermal reaction from Sb(2)O(3) and Bi(NO(3))(3). The effects of reaction conditions and the precursors on the final products were investigated. It is proposed that the redox reaction between Sb(2)O(3) and Bi(NO(3))(3) plays a pivotal role in the formation of nanocrystalline BiSbO(4). The hydrothermally prepared nanocrystalline BiSbO(4) was characterized by X-ray diffraction (XRD), N(2)-sorption BET surface area, UV-vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The DRS result clarifies that BiSbO(4), originally believed to be a visible light responsive photocatalyst, is indeed UV light responsive with a band gap of 3.5 eV. The existence of Bi containing an impurity may be responsible for the visible light response of BiSbO(4) prepared via a conventional solid state reaction. BiSbO(4) nanoplates prepared via the hydrothermal method showed superior photocatalytic performance for the degradation of benzene and 4-chlorophenol (4-CP) as compared to BiSbO(4) prepared via a solid state reaction and Degussa P25. BiSbO(4(Hy)) nanoplates can be a promising photocatalyst in the treatment of environmental pollution.