Reduced State of the Graphene Oxide@Polyoxometalate Nanocatalyst Achieving High-Efficiency Nitrogen Fixation under Light Driving Conditions

[Ru(bpy)3]2+ Derivatives-Incorporated POM@MOFs with Good Photocatalytic Activity for Visible-Light-Driven Oxidative Coupling of Amines to Imines

Two novel POM@MOFs, {H3Zn2.5(H2O)10[Ru(dcbpy)3][PMo11VIMoVO40]}·2H2O (RuZn-PMo) and {H3Ni2.5(H2O)12[Ru(dcbpy)3][PMo11VIMoVO40]}·5H2O (RuNi-PMo), have been synthesized through a traditional hydrothermal method. They are composed of [Ru(bpy)3]2+-derived hexa-carboxylate and Keggin-type anion [PMo11VIMoVO40]4-. In addition, their structures were well characterized by various spectroscopic methods. Under the irradiation of visible light (λ > 400 nm), RuZn-PMo and RuNi-PMo as heterogeneous photocatalysts showed efficient photocatalytic performance in the coupling reaction of amines, with TONs of 451 and 454, respectively. Moreover, RuZn-PMo exhibited excellent reusability and stability after three continuous reaction cycles. Besides, EPR measurements were performed to elucidate the reaction mechanism.

Construction of Conjugated Organic Polymers for Efficient Photocatalytic Hydrogen Peroxide Generation with Adequate Utilization of Water Oxidation

The visible-light-driven photocatalytic production of hydrogen peroxide (H2O2) is currently an emerging approach for transforming solar energy into chemical energy. In general, the photocatalytic process for producing H2O2 includes two pathways: the water oxidation reaction (WOR) and the oxygen reduction reaction (ORR). However, the utilization efficiency of ORR surpasses that of WOR, leading to a discrepancy with the low oxygen levels in natural water and thereby impeding their practical application. Herein, we report a novel donor-bridge-acceptor (D-B-A) organic polymer conjugated by the Sonogashira-Hagihara coupling reaction with tetraphenylethene (TPE) units as the electron donors, acetylene (A) as the connectors and pyrene (P) moieties as the electron acceptors. Notably, the resulting TPE-A-P exhibits a remarkable solar-to-chemical conversion of 1.65% and a high BET-specific surface area (1132 m2·g-1). Furthermore, even under anaerobic conditions, it demonstrates an impressive H2O2 photosynthetic efficiency of 1770 μmol g-1 h-1, exceeding the vast majority of previously reported photosynthetic systems of H2O2. The outstanding performance is attributed to the effective separation of electrons and holes, along with the presence of sufficient reaction sites facilitated by the incorporation of alkynyl electronic bridges. This protocol presents a successful method for generating H2O2 via a water oxidation reaction, signifying a significant advancement towards practical applications in the natural environment.

Designing a hybrid nanomaterial based on Cr-containing polyoxometalate and graphene oxide as an electrocatalyst for the hydrogen evolution reaction

A new polyoxometalate (POM)-based hybrid nanomaterial (denoted as PMo11-Cr-mGO) was designed via covalent interaction between the Cr(acac)3 complex and [PMo11O39]7- followed by immobilization on the surface of modified graphene oxide (mGO). The prepared nanomaterial was characterized using a series of physicochemical techniques. X-ray diffraction (XRD), Raman analysis, transmission electron microscopy (TEM), and FE-SEM-EDS revealed the preservation of layered GO during the formation of the desired hybrid nanomaterial. Fourier transform infrared (FT-IR), X-ray photoelectron spectroscopy (XPS), and elemental analysis confirmed the immobilization of POM (PMo11-Cr) on the surface of mGO and the formation of PMo11-Cr-mGO. In order to evaluate the performance of PMo11-Cr-mGO in the hydrogen evolution reaction (HER), electrochemical measurements were also performed. The resulting PMo11-Cr-mGO exhibited excellent HER activities with a low overpotential of 153 mV at 10 mA cm-2 and good durability in acidic media, thus emerging as one of the most efficient POM-based electrocatalysts.

Polyoxometalate-Structured Materials: Molecular Fundamentals and Electrocatalytic Roles in Energy Conversion

Polyoxometalates (POMs), a kind of molecular metal oxide cluster with unique physical-chemical properties, have made essential contributions to creating efficient and robust electrocatalysts in renewable energy systems. Due to the fundamental advantages of POMs, such as the diversity of molecular structures and large numbers of redox active sites, numerous efforts have been devoted to extending their application areas. Up to now, various strategies of assembling POM molecules into superstructures, supporting POMs on heterogeneous substrates, and POMs-derived metal compounds have been developed for synthesizing electrocatalysts. From a multidisciplinary perspective, the latest advances in creating POM-structured materials with a unique focus on their molecular fundamentals, electrocatalytic roles, and the recent breakthroughs of POMs and POM-derived electrocatalysts, are systematically summarized. Notably, this paper focuses on exposing the current states, essences, and mechanisms of how POM-structured materials influence their electrocatalytic activities and discloses the critical requirements for future developments. The future challenges, objectives, comparisons, and perspectives for creating POM-structured materials are also systematically discussed. It is anticipated that this review will offer a substantial impact on stimulating interdisciplinary efforts for the prosperities and widespread utilizations of POM-structured materials in electrocatalysis.

Aryl selenonium vs. aryl sulfonium counterions in polyoxometalate chemistry: the impact of Se+ cationic centers on the photocatalytic reduction of dichromate

A selenonium organic counter ion has been used in polyoxometalate chemistry to develop a new aryl selenonium polyoxometalate (POM) hybrid, and its photocatalytic properties have been explored in comparison with an aryl sulfonium POM-hybrid counterpart for the first time. The chalcogenonium counterions, namely, methyldiphenylsulfonium trifluoromethane sulfonate (MDPST) and methyldiphenylselenonium trifluoromethane sulfonate (MDPSeT), and their octamolybdate ([Mo8O26]4-) hybrids, 1 and 2, with the general formula (C13H13X)4[Mo8O26] (where X = S for 1 and Se for 2) were synthesized and characterized. Hybrids 1 and 2 vary in their chalcogenonium cationic center (S+vs. Se+), which enabled a direct comparison of their photocatalytic properties as a function of the cationic center. The photocatalytic activities of hybrids 1 and 2 were tested using the reduction of dichromate (Cr2O72-) as a model reaction under UV irradiation. A 99% photocatalytic reduction of Cr2O72- with a rate constant of 0.0305 min-1 was achieved with hybrid 2, while only a 67% reduction with a rate constant of 0.0062 min-1 was observed with hybrid 1 in 180 minutes. The better catalytic performance of hybrid 2 may be correlated to the larger atomic radii of Se than S, which helps in better stabilizing the photogenerated electron-hole (e--h+) pair on the POM cluster by polarizing its lone pair more efficiently compared to S. The catalytic recyclability was tested for up to 4 cycles using hybrid 2, and up to 98% reduction was obtained even after the 4th cycle. Recyclability tests and control experiments also indicated the generation of some elemental Se through possible cleavage of some C-Se bonds of MDPSe under prolonged UV exposure during catalysis, and the Se thus generated was found to contribute to the catalytic reduction of dichromate. This study, therefore, opens new avenues for aryl selenonium moieties and their POM hybrids for potential catalytic applications.

Solar-to-H2 O2 Energy Conversion by the Photothermal Effect of a Polymeric Photocatalyst via a Two-Channel Pathway

With a view to using solar energy, the exploitation of near-infrared (NIR) light, which constitutes about 50 % of solar energy, in photocatalytic H₂ O₂ synthesis remains challenging. In this study, resorcinol-formaldehyde (RF), which has a relatively low bandgap and high conductivity, is introduced for photothermal catalytic generation of H₂ O₂ under ambient conditions. Owing to the promoted surface charge transfer rate under high temperature, the photosynthetic yield reaches roughly 2000 μm within 40 min under 400 mW cm^-2 irradiation with a solar-to-chemical conversion (SCC) efficiency of up to 0.19 % at 338 K under ambient conditions, exceeding the rate of photocatalysis with a cooling system by a factor of about 2.5. Notably, the H₂ O₂ produced by RF during photothermal process was formed via a two-channel pathway, leading to the overall promotion of H₂ O₂ formation. The resultant H₂ O₂ can be applied in situ for pollutant removal. This work offers a sustainable and economical route for the efficient formation of H₂ O₂ .

Recent progress of photocatalysts based on tungsten and related metals for nitrogen reduction to ammonia

Photocatalytic nitrogen reduction reaction (NRR) to ammonia holds a great promise for substituting the traditional energy-intensive Haber–Bosch process, which entails sunlight as an inexhaustible resource and water as a hydrogen source under mild conditions. Remarkable progress has been achieved regarding the activation and solar conversion of N₂ to NH₃ with the rapid development of emerging photocatalysts, but it still suffers from low efficiency. A comprehensive review on photocatalysts covering tungsten and related metals as well as their broad ranges of alloys and compounds is lacking. This article aims to summarize recent advances in this regard, focusing on the strategies to enhance the photocatalytic performance of tungsten and related metal semiconductors for the NRR. The fundamentals of solar-to-NH₃ photocatalysis, reaction pathways, and NH₃ quantification methods are presented, and the concomitant challenges are also revealed. Finally, we cast insights into the future development of sustainable NH₃ production, and highlight some potential directions for further research in this vibrant field.

Spontaneous exciton dissociation in organic photocatalyst under ambient conditions for highly efficient synthesis of hydrogen peroxide

SignificanceHydrogen peroxide is a highly competitive ready-to-use product for solar energy transformation. Nevertheless, the contemporary photosynthetic systems are not efficient enough, due to severe charge recombination caused by high activation energy and binding energy of the exciton. Herein, we achieve spontaneous exciton dissociation at room temperature. Moreover, the photosynthesis of H2O2 reaches between 9,366 and 12,324 µmol·g-1 from 9 AM to 4 PM in ambient conditions, that is, sunlight irradiation, real water including fresh water and seawater, room temperature, and open air. The ultrahigh photocatalytic efficiency in ambient conditions allows the solar-to-chemical conversion in a real cost-effective and sustainable way, which represents an important step toward real applications.

Keggin-type SiW12 encapsulated in MIL-101(Cr) as efficient heterogeneous photocatalysts for nitrogen fixation reaction

The incorporation of polyoxometalates (POMs) in metal-organic frameworks (MOFs) with host-guest structure have proven to be effective strategy to rational design of heterogeneous catalysis. In this study, the Keggin-type POM@MIL-101(Cr) composite catalysts (PMo₁₂, PW₁₂ and SiW₁₂) are synthesized for nitrogen fixation reaction without sacrificial agents at room temperature in the first time. The SiW₁₂ molecules are encapsulated in smaller cavities of MIL-101(Cr) by solvothermal method and in larger cavities by impregnation method, respectively. Solvothermal synthesized catalyst has a performance of 75.56 μmol·h^-1·g^-1_cat and TOF value of 1.95 h^-1, which are about 10 and 88 times than that of Na₄SiW₁₂O₄₀. The excellent performance is ascribed to the synergistic effect of SiW₁₂ and MIL-101(Cr). The MIL-101(Cr) adsorbs a large amount of N₂ and generates sufficiently photogenerated electrons under sunlight irradiation, and electrons quickly transfer to the SiW₁₂ through hydrogen bonds. Moreover, the agglomeration effect of the homogeneous catalyst SiW₁₂ is weakened due to encapsulation with more exposed active sites. This work provides a feasible route to design and synthesize nanocomposite materials with exceptional performance for photocatalytic nitrogen fixation.

Amorphization and defect engineering in constructing ternary composite Ag/PW10V2/am-TiO2−x for enhanced photocatalytic nitrogen fixation

Ag/PW10V2/am-TiO2−x was designed by decorating OVs-enriched am-TiO2−x with Ag NPs and PW10V2. The formed Z-scheme heterojunction, Ag–am-TiO2−x interface and plentiful surface OVs account for its high photocatalytic efficiency in nitrogen fixation.

N-doping TiO2 hollow microspheres with abundant oxygen vacancies for highly photocatalytic nitrogen fixation

Photocatalytic fixation of nitrogen to ammonia (NH₃) is a green but low-efficiency technology due to the high recombination of photo-generated carriers and poor light absorption of photocatalysts. Generally, the adsorption capacity for N₂ and the band position of TiO₂ are responsible for bandgap, light-adsorption, and the separation of photocarriers. Therefore, they play crucial roles to improve catalytic activity. Herein, N-doping TiO₂ hollow microspheres (NTO-0.5) with oxygen vacancies were synthesized via a hydrothermal method using phenolic resin microsphere as a template. The obtained NTO-0.5 achieves an impressive ammonia yield of 80.09 μmol g_cat^-1h^-1. Oxygen vacancies of NTO-0.5 were confirmed by ESR, Raman, XPS, Zeta potential, and H₂O₂ treatment for reducing oxygen vacancies. The ammonia yield of NTO-0.5 decreases to 34.78 μmol g_cat^-1h^-1 after reducing oxygen vacancies by H₂O₂ treatment, which demonstrates the importance of oxygen vacancies. The oxygen vacancies narrow the bandgap from 3.18 eV to 2.83 eV and impede the recombination of photo-generated carriers. The hollow microspheres structure is conducive to light absorption and utilization. Therefore, the synergistic effect between the oxygen vacancies and the hollow microspheres structure boosts the efficiency of photocatalytic nitrogen fixation. After four cycles, the ammonia production yield still maintains at 76.52 μmol g_cat^-1h^-1, meaning high stability. This work provides a new insight into the construction of catalysts with oxygen vacancies to enhance photocatalytic nitrogen fixation performance.

A solar-to-chemical conversion efficiency up to 0.26% achieved in ambient conditions

Artificial photosynthesis in ambient conditions is much less efficient than the solar-to-biomass conversion (SBC) processes in nature. Here, we successfully mimic the NADP-mediated photosynthetic processes in green plants by introducing redox moieties as the electron acceptors in the present conjugated polymeric photocatalyst. The current artificial process substantially promotes the charge carrier separation efficiency and the oxygen reduction efficiency, achieving a photosynthesis rate for converting Earth-abundant water and oxygen in air into hydrogen peroxide as high as 909 μmol⋅g^-1⋅h^-1 and a solar-to-chemical conversion (SCC) efficiency up to 0.26%. The SCC efficiency is more than two times higher than the average SBC efficiency in nature (0.1%) and the highest value under ambient conditions. This study presents a strategy for efficient SCC in the future.

Reduced polyoxomolybdate immobilized on reduced graphene oxide for rapid catalytic decontamination of a sulfur mustard simulant

Keggin-type polyoxometalates (POMs) were immobilized on poly(diallyldimethylammonium chloride) (PDDA) functionalized reduced graphene oxide (rGO) by a facile and broad-spectrum hydrothermal method. The prepared POMs@PDDA-rGO composites (POM = H3PMo12O40, H3PW12O40, H5PMo10V2O40) have been thoroughly characterized using a series of techniques. The three composites can catalyze the oxidative decontamination of a sulfur mustard simulant, 2-chloroethyl ethyl sulfide (CEES) in the order of PMo12@PDDA-rGO > PMo10V2@PDDA-rGO > PW12@PDDA-rGO. Notably, under ambient conditions PMo12@PDDA-rGO can convert 99% of CEES within 30 min in the presence of nearly stoichiometric aqueous H2O2 (3 wt%) and its catalytic activity is significantly higher than that of homogeneous H3PMo12O40. XPS spectral analysis and control experiments indicate that the Mo center of POM is reduced from +6 to +5 during the hydrothermal process, and the excellent catalytic performance is related to the reduction of Mo. Moreover, the PMo12@PDDA-rGO composite is stable during the decontamination process and it can be used for at least five cycles without loss of activity.

Recent advances in photocatalytic nitrogen fixation: from active sites to ammonia quantification methods

Photocatalytic nitrogen fixation has become a hot topic in recent years due to its mild and sustainable advantages. While modifying the photocatalyst to enhance its electron separation, light absorption and nitrogen reduction abilities, the role of the active sites in the catalytic reaction cannot be ignored because the N[triple bond, length as m-dash]N nitrogen bond is too strong to activate. This review summarizes the recent research on nitrogen fixation, focusing on the active sites for N2 on the catalyst surface, classifying common active sites, explaining the main role and additional role of the active sites in catalytic reactions, and discussing the methods to increase the number of active sites and their activation ability. Finally, the outlook for future research is presented. It is hoped this review could help researchers understand more about the activation of the nitrogen molecules and lead more efforts into research on nitrogen fixation photocatalysts.

A PW12/Ag functionalized mesoporous silica-coated magnetic Fe3O4 core-shell composite as an efficient and recyclable photocatalyst

The novel composite, Fe₃O₄@SiO₂@mSiO₂-PW₁₂/Ag, was successfully prepared by in situ loading Ag nanoparticles (Ag NPs) on the surface of grafted phosphotungstate (denoted as PW₁₂) Fe₃O₄@SiO₂@mSiO₂via a photoreduction deposition method. PW₁₂ not only acts as a reducing agent and stabilizer for Ag NPs but also as a bridge to link Ag NPs and the SiO₂ shell in the loading process. Its activity toward the photodegradation of methyl orange (MO) and photoreduction of Cr₂O₇^2- anions was evaluated. Experimental results showed that Fe₃O₄@SiO₂@mSiO₂-PW₁₂/Ag with 5.3 wt% Ag loading and 18.65 wt% of PW₁₂ exhibits the highest photocatalytic efficacy, and complete degradation of MO and 91.2% photoreduction of Cr(vi) were realized under simulated sunlight for 75 min, respectively. The enhanced catalytic activities of the composite are due to its high specific surface area, the synergistic effect among the components and the formation of a heterojunction of PW₁₂/Ag. The possible enhanced photocatalytic mechanism is proposed. The catalyst is durable and can be easily recovered using a magnet for recycling without a significant loss of catalytic activity.

2D g-C3N4 as a bifunctional photocatalyst for co-catalyst and sacrificial agent-free photocatalytic N2 fixation and dye photodegradation

Photocatalytic N₂ fixation is an ecofriendly technology to produce ammonia.

Polyoxometalate modified transparent metal selenide counter electrodes for high-efficiency bifacial dye-sensitized solar cells

We present a facile one-step hydrothermal approach for the growth of PW₁₁Co/Co_0.85Se on a conductive glass substrate, which could be used as transparent CE in bifacial DSSCs with enhanced front and back efficiencies of 7.56% and 5.82%, respectively.

Hybrid catalysts of molybdovanadophosphoric acid and g-C3N4 with tunable bandgaps

The integration of semiconductors and polyoxometalates provides promising benefits for the rational tuning of hybrid materials' electronic band structures; however, the intrinsic influence of certain hybridization approaches on the resulting bandgaps of their complexes has seldom been noted. Herein, graphitic carbon nitride and a series of phosphovanadomolybdates (H3+xPMo12-xVxO40, x = 0-3) have been complexed through electrostatic charge attraction, and their optical and electronic properties are fully explored to investigate the effect of minor variations of the polyoxometalate structures on the hybrid bandgaps and electronic structures. The conduction band edge of the hybrids increases along with the expansion of the number of vanadium centers in the phosphovanadomolybdate, providing potential guidance for the design of hybrid catalysts.

Keggin-Type Polyoxometalate-Based ZIF-67 for Enhanced Photocatalytic Nitrogen Fixation

The photocatalytic reduction of N₂ to NH₃ is considered a promising strategy to alleviate human need for accessible nitrogen and environmental pollution, for which developing a photocatalyst is an effective method to complete the transformation of this process. We firstly design a series of highly efficient and stable polyoxometalates (POMs)-based zeolitic imidazolate framework-67 (ZIF-67) photocatalysts for N₂ reduction. ZIF-67 can effectively fix N₂ owing to its porosity. Integration of POMs cluster contributes enormous advantages in terms of broadening the absorption spectrum to improve sunlight utilization, enhance the stability of the materials, effectively inhibit the recombination of photo-generated electron-hole pairs, and reduce charge-transfer impedance. POMs can absorb light to convert into reduced POMs, which have stronger reducing ability to provide ample electrons to reduce N₂ . The reduced POMs can recover their oxidation state through contact with an oxidant, which forms a self-recoverable and recyclable photocatalytic fixing N₂ system. The photocatalytic activity enhances with the increasing number V substitutions in the POMs. Satisfactorily, ZIF-67@K₁₁ [PMo₄ V₈ O₄₀ ] (PMo₄ V₈ ) displays the most significant photocatalytic N₂ activity with a NH₃ yield of 149.0 μmol L^-1  h^-1 , which is improved by 83.5 % (ZIF-67) and 78.9 % (PMo₄ V₈ ). The introduction of POMs provides new insights for the design of high-performance photocatalyst nanomaterials to reduce N₂ .

Macroscale Superlubricity Achieved on the Hydrophobic Graphene Coating with Glycerol

Introduction of graphene-family nanoflakes in liquid results in a reduction in friction and enhanced wear resistance. However, the high demand for dispersity and stability of the nanoflakes in liquid largely restricted the choice of graphene-family nanoflakes thus far. This study proposed a new strategy to overcome this limitation, involving the formation of a graphene coating with deposited graphene-family nanoflakes, followed by the lubrication of the coating with glycerol solution. Pristine graphene (PG), fluorinated graphene (FG), and graphene oxide (GO) nanoflakes were chosen to be deposited on the respective SiO₂ substrates to form graphene coatings, and then an aqueous solution of glycerol was used as lubricant. The coefficient of friction (COF) and wear rate were reduced for all deposited coatings. However, the PG coating exhibited better lubrication and antiwear performance than FG and GO coatings. A robust superlubricity with COF of approximately 0.004 can be achieved by combining glycerol with the PG coating. The superlubricity mechanism was attributed to the formation of a tribofilm, mainly composed of graphene nanoflakes in the contact zone. The extremely low friction achieved on the hydrophobic graphene coating with liquid can aid in the development of a high-performing new lubrication system for industrial applications.

Photocatalytic Dinitrogen Fixation with Water on Bismuth Oxychloride in Chloride Solutions for Solar-to-Chemical Energy Conversion

Ammonia is an indispensable chemical. Photocatalytic NH₃ production via dinitrogen fixation using water by sunlight illumination under ambient conditions is a promising strategy, although previously reported catalysts show insufficient activity. Herein, we showed that ultraviolet light irradiation of a semiconductor, bismuth oxychloride with surface oxygen vacancies (BiOCl-OVs), in water containing chloride anions (Cl^-) under N₂ flow efficiently produces NH₃. The surface OVs behave as the N₂ reduction sites by the photoformed conduction band electrons. The valence band holes are consumed by self-oxidation of interlayer Cl^- on the catalyst. The hypochloric acid (HClO) formed absorbs ultraviolet light and undergoes photodecomposition into O₂ and Cl^-. These consecutive photoreactions produce NH₃ with water as the electron donor. The Cl^- in solution compensates for the removed interlayer Cl^- and inhibits catalyst deactivation. Simulated sunlight illumination of the catalyst in seawater stably generates NH₃ with 0.05% solar-to-chemical conversion efficiency, thus exhibiting significant potential of the seawater system for artificial photosynthesis.

Keggin-type polyoxometalate/thiospinel octahedron heterostructures for photoelectronic devices

Keggin-type polyoxometalate CoW₁₂/CoIn₂S₄ thiospinel heterostructures promote the further development in photoelectronic devices due to highly efficient electrocatalytic triiodide reduction, low charge-transfer resistance, and the high amount of exposed active site.