Ti3+-mediated MIL-125(Ti) by metal substitution for boosting photocatalytic N2 fixation

Photocatalysis, which uses sunlight, N2 and H2O to produce NH3, is a more sustainable approach to N2 fixation than the Haber-Bosch process. However, its efficiency is severely limited by the difficulty of activating NN bonds. This work presents metal (M = Cu, Fe, V)-substituted MIL-125(Ti) (MIL-(MTi)) for photocatalytic N2 fixation without using any sacrificial agents. Structural characterizations reveal that the active sites including oxygen vacancies (OV) and Ti3+ species are formed by the resulting crystal distortion due to the partial substitution of Ti4+ by other metal ions (Cu+, Fe2+, V3+) in MIL-125(Ti). MIL-(CuTi) possesses a larger number of OV and Ti3+ compared to MIL-(FeTi) and MIL-(VTi) due to the larger valence difference between Cu+ and Ti4+. These active sites not only promote the adsorption and activation of N2 and H2O, but also facilitate the photogenerated charge mobility. Photogenerated holes oxidize H2O to produce O2 and H+. Photogenerated electrons reduce N2 activated on Ti3+ sites by combining with H+ to form NH4+. Therefore, MIL-(CuTi) shows the highest NH4+ production rate 46.5 µmol·h-1·g-1, which is much higher than that (1.2 µmol·h-1·g-1) of the pristine MIL-125(Ti). This work provides a new insight into rational design for artificial N2 fixation systems by the construction of the active site.

Rapid Charge Transfer Endowed by Interfacial Ni-O Bonding in S-scheme Heterojunction for Efficient Photocatalytic H2 and Imine Production

Cooperative coupling of H2 evolution with oxidative organic synthesis is promising in avoiding the use of sacrificial agents and producing hydrogen energy with value-added chemicals simultaneously. Nonetheless, the photocatalytic activity is obstructed by sluggish electron-hole separation and limited redox potentials. Herein, Ni-doped Zn0.2 Cd0.8 S quantum dots are chosen after screening by DFT simulation to couple with TiO2 microspheres, forming a step-scheme heterojunction. The Ni-doped configuration tunes the highly active S site for augmented H2 evolution, and the interfacial Ni-O bonds provide fast channels at the atomic level to lower the energy barrier for charge transfer. Also, DFT calculations reveal an enhanced built-in electric field in the heterojunction for superior charge migration and separation. Kinetic analysis by femtosecond transient absorption spectra demonstrates that expedited charge migration with electrons first transfer to Ni2+ and then to S sites. Therefore, the designed catalyst delivers drastically elevated H2 yield (4.55 mmol g-1  h-1 ) and N-benzylidenebenzylamine production rate (3.35 mmol g-1  h-1 ). This work provides atomic-scale insights into the coordinated modulation of active sites and built-in electric fields in step-scheme heterojunction for ameliorative photocatalytic performance.

Construction of a direct Z-scheme CeO2/UiO-66-NH2 heterojunction for boosting photocatalytic organic pollutant degradation and H2 evolution performance

In this study, hollow CeO₂ nanospheres were grown on UIO-66-NH₂ nanosheets to form a novel CeO₂/UiO-66-NH₂ (abbreviation, CU_x) Z-scheme heterojunction photocatalyst by calcination and hydrothermal method for hydrogen production and photocatalytic degradation of organic pollutants. Under visible light, the H₂ generation rate of the CU_0.50 composite was 5662.1 μmol g^-1 h^-1, which was 22 and 7 fold than that of pure CeO₂ and pure UiO-66-NH₂, respectively. In addition, compared with CeO₂ and UiO-66-NH₂, the as-prepared CU_x composites exhibited enhanced photo-degradation efficiencies for tetracycline (TC) and 2,4-dichlorophenol (DCP) under simulated solar light irradiation. Among them, the CU_0.50 composite demonstrated the highest photocatalytic performance and reached 91.5% for TC, and 94.3% for DCP. In addition, a logical solid-state Z-type electron transfer mechanism is presented with the results of radical scavenging and ESR experiments to illustrate the intensive decomposed ability of the photocatalytic system. The enhanced photocatalytic performance of CU_x heterostructures can be attributed to the formation of a band-position-matched hollow structure heterojunction between CeO₂ and UIO-66-NH₂, which can effectively inhibit the recombination of carriers and increase the specific surface area as well as the light absorption. Moreover, the oxidation and reduction ability of the charge carriers was also increased. This work resulted in a feasible idea for removing organic pollutants and hydrogen production by traditional inorganic semiconductor/MOF-based heterostructured photocatalysts.

Enhanced Photocatalytic Activity of Hyper-Cross-Linked Polymers Toward Amines Oxidation Coupled with H2 O2 Generation through Extending Monomer's Conjugation Degree

Visible-light-driven amines oxidation coupled with hydrogen peroxide (H₂ O₂ ) generation is a promising way to convert solar energy to chemical energy. Herein, a series of hyper-cross-linked polymers (HCPs) photocatalysts with different arenes monomers, including benzene (BE), diphenyl (DP), p-terphenyl (TP), or p-quaterphenyl (QP), were synthesized by simple Friedel-Crafts alkylation reaction. Owing to the maximum monomer's conjunction degree and excellent oxygen (O₂ ) adsorption capacity, QP-HCPs exhibited highest photocatalytic activity for benzylamine oxidation coupled with H₂ O₂ generation under the irradiation of 455 nm Blue LED lamp. More than 99 % of benzylamine could be converted to N-benzylidenebenzylamine within 60 min. In addition, nearly stoichiometric H₂ O₂ was synchronously obtained with a high production rate of 9.3 mmol g_cat ^-1 h^-1 . Our work not only demonstrated that the photocatalytic activity of HCPs photocatalysts significantly depends on monomer's conjunction degree, but also provided a new strategy for converting solar energy to chemical energy.

Oxygen vacancies assist a facet effect to modulate the microstructure of TiO2 for efficient photocatalytic O2 activation

Defect engineering is recognized as an effective route to obtaining highly active photocatalytic materials. However, the current understanding of the role of defects in photocatalysts mainly comes from their independent functional analysis, ignoring the synergy between defects and the chemical environment, especially with crystal facets. Herein, oxygen vacancy (V_O)-rich TiO₂ nanostructures with different dominant exposed facets were prepared, and the microstructural changes induced by the synergy between the V_O and facet effect and the performance difference of photocatalytic O₂ activation were explored. The results showed that the combination of high concentration V_O and the {101} facet is more conducive to improving the photocatalytic performance of TiO₂, which is significantly superior to the combination of low concentration V_O and the {101} facet as well as the combination of high concentration V_O and the {001} facet. The experimental and theoretical results clarified the dependence of each stage of photocatalysis on two factors. Specifically, V_O plays a more significant role in energy band regulation, improving the dynamic behavior of photogenerated charges and enhancing the adsorption and activation of O₂, while the facet effect made more contributions to reducing the thermodynamic energy barrier of ROS formation and conversion. The excellent ability of O₂ activation enables T101-V_O to show potential application characteristics in the removal of RhB and bacterial disinfection. This work established a link between defect and facet effects, providing new insights into understanding defect function in photocatalysts.

Ir Cluster-Anchored MOFs as Peroxidase-Mimetic Nanoreactors for Diagnosing Hydrogen Peroxide-Related Biomarkers

Exploring multifaceted and highly sensitive biosensors is a major challenge in biotechnology and medical diagnosis. Here, we create a new iridium (Ir) cluster-anchored metal-organic framework (MOF, namely, Ir_NCs@Ti-MOF via a coordination-assisted strategy) as a peroxidase (POD)-mimetic nanoreactor for colorimetrically diagnosing hydrogen peroxide-related biomarkers. Owing to the Ir_NCs-N/O coordination of Ti-MOF and unique enzymatic properties of Ir clusters, the Ir_NCs@Ti-MOF exhibits exceptional and exclusive POD-mimetic activities (K_m = 3.94 mM, V_max = 1.70 μM s^-1, and turnover number = 39.64 × 10^-3 s^-1 for H₂O₂), thus demonstrating excellent POD-mimetic detecting activity and also super substrate selectivity, which is considerably more efficient than recently reported POD mimetics. Colorimetric studies disclose that this Ir_NCs@Ti-MOF-based nanoreactor shows multifaceted and efficient diagnosing activities and substrate selectivity, such as a limit of detection (LOD): 14.12 μM for H₂O₂ at a range of 0-900 μM, LOD: 3.41 μM for l-cysteine at a range of 0-50 μM, and LOD: 20.0 μM for glucose at a range of 0-600 μM, which enables an ultrasensitive and visual determination of abundant H₂O₂-related biomarkers. The proposed design will not only provide highly sensitive and cheap colorimetric biosensors in medical resource-limited areas but also offer a new path to engineering customizable enzyme-mimetic nanoreactors as a powerful tool for accurate and rapid diagnosis.

Charge Separation by Creating Band Bending in Metal-Organic Frameworks for Improved Photocatalytic Hydrogen Evolution

Metal-organic frameworks (MOFs) have been intensively studied as a class of semiconductor-like materials in photocatalysis. However, band bending, which plays a crucial role in semiconductor photocatalysis, has not yet been demonstrated in MOF photocatalysts. Herein, a representative MOF, MIL-125-NH₂ , is integrated with the metal oxides (MoO₃ and V₂ O₅ ) that feature appropriate work functions and energy levels to afford the corresponding MOF composites. Surface photovoltage results demonstrate band bending in the MOF composites, which gives rise to the built-in electric field of MIL-125-NH₂ , boosting the charge separation. As a result, the MOF composites present 56 and 42 times higher activities, respectively, compared to the pristine MOF for photocatalytic H₂ production. Upon depositing Pt onto the MOF, ∼6 times higher activity is achieved. This work illustrates band bending of MOFs for the first time, supporting their semiconductor-like nature, which would greatly promote MOF photocatalysis.

Engineering Catalytic Interfaces in Cuδ+/CeO2-TiO2 Photocatalysts for Synergistically Boosting CO2 Reduction to Ethylene

Photocatalytic CO₂ conversion into a high-value-added C₂ product is a highly challenging task because of insufficient electron deliverability and sluggish C-C coupling kinetics. Engineering catalytic interfaces in photocatalysts provides a promising approach to manipulate photoinduced charge carriers and create multiple catalytic sites for boosting the generation of C₂ product from CO₂ reduction. Herein, a Cu^δ+/CeO₂-TiO₂ photocatalyst that contains atomically dispersed Cu^δ+ sites anchored on the CeO₂-TiO₂ heterostructures consisting of highly dispersed CeO₂ nanoparticles on porous TiO₂ is designedly constructed by the pyrolytic transformation of a Cu²⁺-Ce³⁺/MIL-125-NH₂ precursor. In the designed photocatalyst, TiO₂ acts as a light-harvesting material for generating electron-hole pairs that are efficiently separated by CeO₂-TiO₂ interfaces, and the Cu-Ce dual active sites synergistically facilitate the generation and dimerization of *CO intermediates, thus lowering the energy barrier of C-C coupling. As a consequence, the Cu^δ+/CeO₂-TiO₂ photocatalyst exhibits a production rate of 4.51 μmol^-1·g_cat^-1·h^-1 and 73.9% selectivity in terms of electron utilization for CO₂ to C₂H₄ conversion under simulated sunlight, with H₂O as hydrogen source and hole scavenger. The photocatalytic mechanism is revealed by operando spectroscopic methods as well as theoretical calculations. This study displays the rational construction of heterogeneous photocatalysts for boosting CO₂ conversion and emphasizes the synergistic effect of multiple active sites in enhancing the selectivity of C₂ product.

Metal–organic-framework-based photocatalysts for microorganism inactivation: a review

A metal–organic framework (MOF) is a porous coordination material composed of multidentate organic ligands and metal ions or metal clusters.

Thermo-enhanced photocatalytic oxidation of amines to imines over MIL-125-NH2@Ag@COF hybrids under visible light

Thermo-enhanced photocatalysis combines the advantages of thermocatalysis and photocatalysis and provides a very promising approach for the selective oxidation of organic compounds to value-added chemicals. In this work, the amino group in MIL-125-NH₂ first reacts with formaldehyde to form the reducing group (-NH-CH₂OH), which can in situ auto reduce the introduced Ag⁺ ions to Ag clusters/nanoparticles in the cavities. Then the formed MIL-125-NH-CH₂OH@Ag was further coated with a covalent organic framework (COF) through imine bonds to form a series of MIL-125-NH-CH₂OH@Ag@COF hybrids. Oxidative coupling of amines was selected to evaluate the photocatalytic performance of these materials under visible light at set temperatures (20-60 °C). With an optimized composition, MIL-125-NH-CH₂OH@Ag-0.5@COF-2 not only improves the optical properties, but also exhibits the highest conversion (almost 100%) of benzylamine under visible light at 60 °C and good stability for at least three cycles. Free radical capture experiments and electron spin resonance detection demonstrated that holes (h⁺), hydroxyl (˙OH) and superoxide radicals (O₂˙^-) were the active species. The results prove that the MIL-125-NH-CH₂OH@Ag@COF hybrid possessed higher photocatalytic performance than individual MIL-125-NH₂, Ag and COF on account of the efficient separation and transfer of photoinduced electrons and holes. Moreover, the promotion of the reaction temperature on the photocatalytic oxidation of amines has been reported, revealing that the conversion of benzylamine over MIL-125-NH-CH₂OH@Ag-0.5@COF-2 at 60 °C is nearly twice as high as that at 20 °C under visible light irradiation. Therefore, the thermo-enhanced photocatalytic oxidation performance of the MOF@Ag@COF hybrid demonstrates the great potential of thermal energy for further improving the photocatalytic selective oxidation performance.

Bi2S3@NH2-UiO-66-S composites modulated by covalent interfacial reactions boost photodegradation and the oxidative coupling of primary amines

The Bi₂S₃@NH₂-UiO-66-S heterostructures have been synthesized via covalent interfacial reactions, exhibiting excellent performance in the photodegradation of methylene and the oxidative coupling of primary amines compared to reported photocatalysts.

Au/TiO2 nanobelts: thermal enhancement vs. plasmon enhancement for visible-light-driven photocatalytic selective oxidation of amines into imines

Au NPs improve the photocatalytic activity of TiO2 only in a low temperature range. Excessive Au NPs loaded on TiO2 inhibit the photocatalytic amine conversion due to the decreased oxygen vacancies and poor amine adsorption ability.

Synergistic Pd Single Atoms, Clusters, and Oxygen Vacancies on TiO2 for Photocatalytic Hydrogen Evolution Coupled with Selective Organic Oxidation

Developing efficient photocatalysts for synchronously producing H₂ and high value-added chemicals holds great promise to enhance solar energy conversion. Herein, a facile strategy of simultaneously engineering Pd cocatalyst and oxygen vacancies (V_O s) on TiO₂ to promote H₂ production coupled with selective oxidation of benzylamine is demonstrated. The optimized Pd_SA+C /TiO₂ -V_O photocatalyst containing Pd single atoms (SAs), clusters (C), and V_O s exhibits much superior performance to those of TiO₂ -V_O and Pd_SA /TiO₂ -V_O counterparts. The production rates of H₂ and N-benzylidenebenzylamine over Pd_SA+C /TiO₂ -V_O are 52.7 and 1.5 times those over TiO₂ -V_O , respectively. Both experimental and theoretical studies have elucidated the synergistic effect of Pd SAs, clusters, and V_O s on TiO₂ in boosting the photocatalytic reaction. The presence of Pd SAs facilitates the generation and stabilization of abundant V_O s by the formation of PdOTi³⁺ atomic interface, while Pd clusters promote the photogenerated charge separation and afford the optimum active sites for H₂ evolution. Surface V_O s of TiO₂ guarantee the efficient adsorption and dissociation/activation of reactant molecules. This study reveals the effect of active-site engineering on the photocatalysis and is expected to shed substantial light on future structure design and modulation of semiconductor photocatalysts.

Boosting free radical type photocatalysis over Pd/Fe-MOFs by coordination structure engineering

The development of novel heterogeneous photocatalytic systems, along with a deep understanding of the relationship between the catalytic center chemical environment and the catalytic performance, is of great significance.

Synthesis of EDTA-bridged CdS/g-C3N4 heterostructure photocatalyst with enhanced performance for photoredox reactions

Photocatalytic reactions represent a kind of green and sustainable chemical processes for organic transformations, but the efficiency is limited by the severe recombination and/or inadequate redox potentials of photoinduced charge carriers in photocatalysts. To address these issues, herein, the CdS-EDTA/g-C₃N₄ heterostructures were designed according to Z-scheme photocatalytic mechanism and synthesized by the hydrothermal growth of CdS on g-C₃N₄ nanoflakes with assistance of EDTA chelating agent. EDTA played multiple roles in the formation of CdS-EDTA/g-C₃N₄ heterostructure photocatalysts, such as controlling the morphology of CdS nanostructures, linking CdS and g-C₃N₄ together, and boosting the charge transfer between two semiconductors. The optimized CdS-EDTA/g-C₃N₄(10%) photocatalyst exhibited much higher activities toward the selective reduction of nitrophenol and the selective oxidation of benzyl alcohol, than those of CdS/g-C₃N₄ heterostructures without EDTA. The enhanced photocatalysis of CdS-EDTA/g-C₃N₄ can be ascribed to the efficient separation and suitable photoredox potentials of photoexcited charge carriers in the EDTA-bridged Z-scheme system. This work provides the inspiration for exploring inexpensive organic electron mediators for constructing all-solid-state Z-scheme photocatalysts and demonstrates the enhanced performance of Z-scheme photocatalysts for photoredox reactions of organic transformations.

Interfacial engineering by creating Cu-based ternary heterostructures on C3N4 tubes towards enhanced photocatalytic oxidative coupling of benzylamines

Benzylamine coupling is a very important reaction for the synthesis of imine but still faces many challenges. Herein, we present a highly effective strategy towards the coupling reaction by using environmentally friendly catalysts. These catalysts are composed of Cu/Cu2O/Cu3N heterostructures supported by C3N4 tubes and the composites were synthesized by one-step hydrothermal treatment followed by calcination. Cu2O, Cu3N, and C3N4 all are responsive to visible light and the heterojunction formed can greatly enhance the charge separation. When used as photocatalysts for oxidative self-coupling of benzylamine at a low temperature of 323 K in air, Cu/Cu2O/Cu3N/C3N4 was able to give conversion and selectivity values of up to 99% and 98%, respectively. The high efficiency of the catalysts is attributable to their ability to generate large quantities of free radicals (such as ·OH and ·O2 -) under visible-light irradiation.

Metal–Organic Frameworks (MOFs) and Covalent Organic Frameworks (COFs) Applied to Photocatalytic Organic Transformations

Among the different alternatives for catalysis using metal–organic frameworks (MOFs) or covalent organic frameworks (COFs), photocatalysis has remarkably evolved during the last decade. Photocatalytic reticular materials allowed recyclability and easy separation of catalyst from the product, also reaching the activity and selectivity commonly observed for molecular systems. Recently, photocatalytic MOFs and COFs have been applied to synthetic applications in order to obtain organic molecules of different complexity. However, although a good number of works have been devoted to this issue, an updated comprehensive revision on this field is still needed. The aim of this review was to fill this gap covering the following three general aspects: (1) common strategies on the design of reticular photocatalytic materials, (2) a comprehensive discussion of the photocatalytic organic reactions achieved by the use of COFs and MOFs, and (3) some critical considerations highlighting directions that should be considered in order to make advances in the study of photocatalytic COFs and MOFs.

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.