Robust Covalent Organic Frameworks for Photosynthesis of H2O2: Advancements, Challenges and Strategies.

Since 2020, covalent organic frameworks (COFs) are emerging as robust catalysts for the photosynthesis of hydrogen peroxide (H2O2), benefiting from their distinct advantages. However, the current efficiency of H2O2 production and solar-to-chemical energy conversion efficiency (SCC) remain suboptimal due to various constraints in the reaction mechanism. Therefore, there is an imperative to propose efficiency improvement strategies to accelerate the development of this reaction system. This comprehensive review delineates recent advances, challenges, and strategies in utilizing COFs for photocatalytic H2O2 production. It explores the fundamentals and challenges (e.g., oxygen (O2) mass transfer rate, O2 adsorption capacity, response to sunlight, electron-hole separation efficiency, charge transfer efficiency, selectivity, and H2O2 desorption) associated with this process, as well as the advantages, applications, classification, and preparation strategies of COFs for this purpose. Various strategies to enhance the performance of COFs in H2O2 production are highlighted. The review aims to stimulate further advancements in utilizing COFs for photocatalytic H2O2 production and discusses potential prospects, challenges, and application areas in this field.

Recent Advances and Challenges in Efficient Selective Photocatalytic CO2 Methanation.

Solar-driven carbon dioxide (CO2 ) methanation holds significant research value in the context of carbon emission reduction and energy crisis. However, this eight-electron catalytic reaction presents substantial challenges in catalytic activity and selectivity. In this regard, researchers have conducted extensive exploration and achieved significant developments. This review provides an overview of the recent advances and challenges in efficient selective photocatalytic CO2 methanation. It begins by discussing the fundamental principles and challenges in detail, analyzing strategies for improving the efficiency of photocatalytic CO2 conversion to CH4 comprehensively. Subsequently, it outlines the recent applications and advanced characterization methods for photocatalytic CO2 methanation. Finally, this review highlights the prospects and opportunities in this area, aiming to inspire CO2 conversion into high-value CH4 and shed light on the research of catalytic mechanisms.

Electronic Structure Modulation of Oxygen-Enriched Defective CdS for Efficient Photocatalytic H2 O2 Production.

Artificial photosynthesis for hydrogen peroxide (H2 O2 ) presents a sustainable and environmentally friendly approach to generate clean fuel and chemicals. However, the catalytic activity is hindered by challenges such as severe charge recombination, insufficient active sites, and poor selectivity. Here, a robust strategy is proposed to regulate the electronic structure of catalyst by the collaborative effect of defect engineering and dopant. The well designed oxygen-doped CdS nanorods with S2- defects and Cd2+ 4d10 electron configuration (CdS-O,Sv ) is successfully synthesized, and the Cd2+ active sites around S defects or oxygen atoms exhibit rapid charge separation, suppressed carrier recombination, and enhanced charge utilization. Consequently, a remarkable H2 O2 production rate of 1.62 mmol g-1  h-1 under air conditions is acquired, with an apparent quantum yield (AQY) of 9.96% at a single wavelength of 450 nm. This work provides valuable insights into the synergistic effect between defect and doping on catalytic activity.

Alternating 3rd- to 2nd-Order Charge Reaction Kinetics on Bismuth Vanadate Photoanodes with Ultrathin Bismuth Metal-Organic-Frameworks.

The most challenging obstacle for photocatalysts to efficiently harvest solar energy is the sluggish surface redox reaction (e. g., oxygen evolution reaction, OER) kinetics, which is believed to originate from interface catalysis rather than the semiconductor photophysics. In this work, we developed a light-modulated transient photocurrent (LMTPC) method for investigating surface charge accumulation and reaction on the W-doped bismuth vanadate (W : BiVO4) photoanodes during photoelectrochemical water oxidation. Under illuminating conditions, the steady photocurrent corresponds to the charge transfer rate/kinetics, while the integration of photocurrent (I~t) spikes during the dark period is regarded as the charge density under illumination. Quantitative analysis of the surface hole densities and photocurrents at 0.6 V vs. reversible hydrogen electrode results in an interesting rate-law kinetics switch: a 3rd-order charge reaction behavior appeared on W : BiVO4, but a 2nd-order charge reaction occurred on W : BiVO4 surface modified with ultrathin Bi metal-organic-framework (Bi-MOF). Consequently, the photocurrent for water oxidation on W : BiVO4/Bi-MOF displayed a 50 % increment. The reaction kinetics alternation with new interface reconstruction is proposed for new mechanism understanding and/or high-performance photocatalytic applications.

Highly Selective and Efficient Solar-Light-Driven CO2 Conversion with an Ambient-Stable 2D/2D Co2 P@BP/g-C3 N4 Heterojunction.

Renewable solar-driven carbon dioxide (CO2 ) conversion to highly valuable fuels is an economical and prospective strategy for both the energy crisis and ecological environment disorder. However, the selectivity and activity of current photocatalysts have great room for improvement due to the diversification and complexity of products. Here, an ambient-stable 2D/2D Co2 P@BP/g-C3 N4 heterojunction is designed for highly selective and efficient photocatalytic CO2 reduction reaction. The resulting Co2 P@BP/g-C3 N4 material has a remarkable conversion of CO2 to carbon monoxide (CO) with an ≈96% selectivity, coupled with a dramatically increased CO generation rate of 16.21 µmol g-1 h-1 , which is 5.4 times higher than pristine graphitic carbon nitride (g-C3 N4 ). In addition, this photocatalyst exhibits good ambient stability of black phosphorus (BP) without oxidation even over 180 days. The excellent photocatalytic selectivity and activity of Co2 P@BP/g-C3 N4 heterojunction are attributed to its lower energy barriers of *COOH, *CO, and *+CO in the process of CO2 reduction, coupled with rapid charge transfer at the heterointerfaces of BP/g-C3 N4 and Co2 P/BP. This is solidly verified by both density functional theory calculation and mechanism experiments. Therefore, this work holds great promise for an ambient-stable efficient and high selectivity photocatalyst in solar-driven CO2 conversion.

3D Gold-Modified Cerium and Cobalt Oxide Catalyst on a Graphene Aerogel for Highly Efficient Catalytic Formaldehyde Oxidation.

A hard template method is used to prepare porous gold-doped cerium and cobalt oxide (Au-Cex Coy ) materials. A series of 3D Au-Ce x Coy /graphene aerogel (GA) composites is then fabricated by a facile heating method. The obtained catalysts possess a well-defined structure of ordered arrays of nanotubes and good performance in formaldehyde (HCHO) oxidation. The composition and surface elemental valence states of the catalysts are modulated by the Ce/Co molar ratio. The Au-Cex Coy catalyst and graphene oxide sheets are well compounded within 60 s through a diamine cross-linker to form 3D Au-Cex Coy /GA composites. In addition, the resulting catalyst of 3 wt% Au-Ce3 Co/GA achieves ≈55% conversion at room temperature and 100% conversion when the reaction temperature is raised at 60 °C. The synergistic effect between CeO2 and Co3 O4 promotes the migration of oxygen species and the activation of Au, which facilitates HCHO oxidation. The method used to prepare the 3D catalyst could be used to produce other catalytic materials with good replication of the template. In addition, these findings provide a simple method for rapid fabrication of catalyst/GA composites. The superior activity and stability of the 3D Au-Ce3 Co/GA catalyst make it potentially applicable in HCHO removal.

Eye-Readable Detection and Oxidation of CO with a Platinum-Based Catalyst and a Binuclear Rhodium Complex.

Toxic gases that are colorless and odorless, such as CO, are a major environmental concern and require early detection to prevent serious toxicological effects. In this study, a unique system (Pt/HMSs-BRC) was fabricated by combining a catalyst (Pt/hollow mesoporous silica spheres, Pt/HMSs) with a silica gel containing an adsorbed chromogenic probe (binuclear rhodium complex, BRC). The process is a simple method to prepare well-dispersed and uniform Pt nanoparticles. The Pt/HMSs-BRC materials demonstrated early CO detection and excellent catalytic performance for CO oxidation. The probe exhibited remarkable color modulation from gray-violet to light-yellow when exposed to CO concentration levels above 50 ppm, and the color of the chromogenic probe was fully recoverable. By a kinetics-assisted discrimination method and DFT calculations, it was found that the corner Pt sites are the dominant active sites for CO oxidation.

Engineering 3D Ru/Graphene Aerogel Using Metal-Organic Frameworks: Capture and Highly Efficient Catalytic CO Oxidation at Room Temperature.

Noble metals (Au, Pt, and Ru) loaded into carbon supports show excellent performance for CO oxidation. Herein, a tunable metal-organic framework (MOF) coating is applied to a macroscopic 3D Ru/graphene aerogel (Ru/GA) composite, using a facial step-by-step method. The open macroporous structure of the Ru/GA provides pathways for the access and diffusion of reactant and product molecules. The resulting HK (HKUST-1)-containing MOF composite exhibits good performance for CO adsorption. It can simultaneously adsorb and oxidize CO, which improves the reaction rate. In this work, the catalytic efficiency of the resulting catalyst is higher than that (≈48.4%) of the Ru/GA. These findings provide a simple method for increasing the instantaneous concentration of reactants around the catalyst, which in turn increases the reaction rate. The catalytic performances of composites subjected to different pretreatment conditions are also investigated. Hopefully, this finding may provide a feasible direction for the effective management of the diverse environment issues.

Recent advances in ambient-stable black phosphorus materials for artificial catalytic nitrogen cycle in environment and energy.

Nitrogen cycle is crucial for the Earth's ecosystem and human-nature coexistence. However, excessive fertilizer use and industrial contamination disrupt this balance. Semiconductor-based artificial nitrogen cycle strategies are being actively researched to address this issue. Black phosphorus (BP) exhibits remarkable performance and significant potential in this area due to its unique physical and chemical properties. Nevertheless, its practical application is hindered by ambient instability. This review covers the synthesis methods of BP materials, analyzes their instability factors under environmental conditions, discusses stability improvement strategies, and provides an overview of the applications of ambient-stable BP materials in nitrogen cycle, including N2 fixation, NO3- reduction, NOx removal and nitrides sensing. The review concludes by summarizing the challenges and prospects of BP materials in the nitrogen cycle, offering valuable guidance to researchers.

Cobalt Phosphide-Supported Single-Atom Pt Catalysts for Efficient and Stable Hydrogen Generation from Ammonia Borane Hydrolysis.

Ammonia borane (AB) has emerged as a promising chemical hydrogen storage material. The development of efficient, stable, and cost-effective catalysts for AB hydrolysis is the key to achieving hydrogen energy economy. Here, cobalt phosphide (CoP) is used to anchor single-atom Pt species, acting as robust catalysts for hydrogen generation from AB hydrolysis. Thanks to the high Pt utilization and the synergy between CoP and Pt species, the optimized Pt/CoP-100 catalyst exhibits an unprecedented hydrogen generation rate, giving a record turnover frequency (TOF) value of 39911 mo l H 2 mo l Pt - 1 mi n - 1 ${\mathrm{mo}}{{{\mathrm{l}}}_{{{{\mathrm{H}}}_{\mathrm{2}}}}}{\mathrm{\ mo}}{{{\mathrm{l}}}_{{\mathrm{Pt}}}}^{{\mathrm{ - 1}}}{\mathrm{\ mi}}{{{\mathrm{n}}}^{{\mathrm{ - 1}}}}$ and turnover number of 2926829 mo l H 2 mo l Pt - 1 ${\mathrm{mo}}{{{\mathrm{l}}}_{{{{\mathrm{H}}}_{\mathrm{2}}}}}{\mathrm{\ mo}}{{{\mathrm{l}}}_{{\mathrm{Pt}}}}^{{\mathrm{ - 1}}}$ at room temperature. These metrics surpass those of all existing state-of-the-art supported metal catalysts by an order of magnitude. Density functional theory calculations reveal that the integration of single-atom Pt onto the CoP substrate significantly enhances adsorption and dissociation processes for both water and AB molecules, thereby facilitating hydrogen production from AB hydrolysis. Interestingly, the TOF value is further elevated to 54878 mo l H 2 mo l Pt - 1 mi n - 1 ${\mathrm{mo}}{{{\mathrm{l}}}_{{{{\mathrm{H}}}_{\mathrm{2}}}}}{\mathrm{\ mo}}{{{\mathrm{l}}}_{{\mathrm{Pt}}}}^{{\mathrm{ - 1}}}{\mathrm{\ mi}}{{{\mathrm{n}}}^{{\mathrm{ - 1}}}}$ under UV-vis light irradiation, which can be attributed to the efficient separation and mobility of photogenerated carriers at the Pt-CoP interface. The findings underscore the effectiveness of CoP as a support for single-atom metals in hydrogen production, offering insights for designing high-performance catalysts for chemical hydrogen storage.

Recent advances of pure/independent covalent organic framework membrane materials: preparation, properties and separation applications.

Covalent organic frameworks (COF) are porous crystalline polymers connected by covalent bonds. Due to their inherent high specific surface area, tunable pore size, and good stability, they have attracted extensive attention from researchers. In recent years, COF membrane materials developed rapidly, and a large amount of research work has been presented on the preparation methods, properties, and applications of COF membranes. This review focuses on the research on independent/pure continuous COF membranes. First, based on the membrane formation mechanism, COF membrane preparation methods are categorized into two main groups: bottom-up and top-down. Four methods are presented, namely, solvothermal, interfacial polymerization, steam-assisted conversion, and layer by layer. Then, the aperture, hydrophilicity/hydrophobicity and surface charge properties of COF membranes are summarized and outlined. According to the application directions of gas separation, water treatment, organic solvent nanofiltration, pervaporation and energy, the latest research results of COF membranes are presented. Finally, the challenges and future directions of COF membranes are summarized and an outlook provided. It is hoped that this work will inspire and motivate researchers in related fields.

Biomimetic cellulose-nanocrystalline-based composite membrane with high flux for efficient purification of oil-in-water emulsions.

The massive discharge of oily wastewater and oil spills are causing serious pollution to water resources. It is urgent to require clean and efficient method of purifying oily emulsions. Although the separation membranes with selective wettability have been widely used in the efficient purification of oil/water emulsions. It is still very challenging to develop functional films that are environmentally friendly, fouling resistant, inexpensive, easy to prepare, easy to scale, and highly efficient. Cellulose nanocrystalline-based composite membranes (CCM) were prepared by surface-initiated atom transfer radical polymerization (SATRP) and vacuum self-assembly. The prepared CCM is superhydrophilic and superoleophobic underwater due to the hydrophilic nature of the modified cellulose-nanocrystalline and the micro-nano surface structure. The CCM shows high separation efficiency (> 99.9 %), high flux (16,692 L-1·m-2·h-1·bar-1) for surfactant-stabilized oil-in-water emulsions, good cycle stability and anti-fouling performance. This biomass-derived membrane is green, cheap, easy to manufacture, scalable, super-wettability, and durability, it promises to be an alternative to separation membranes in today's market.

A polydimethylsiloxane-based sponge for water purification and interfacial solar steam generation.

A better knowledge for the design and synthesis of low-cost, novel porous materials is highly desirable in various fields such as recyclable solar desalination and liquid recycling. Herein, a polydimethylsiloxane-based sponge with a web-like three-dimensional (3D) interconnected porous structure was developed for effective recovery of liquids and the continuous interfacial solar steam generation (ISSG). The sponge is capable of conducting directional transport of oil or organic solvents at temperatures above 32 °C while automatically controlling the desorption of the organic phase below 28 °C. The synergistic combination between high light absorption (above 95 %) and light-to-heat conversion efficiency (99.87 %) resulted in a considerably high seawater evaporation rate (1.66 Kg m-2h-1) under 1 sun. The self-regeneration of the evaporator is facilitated by the salt barrier function of the large channels of the smart sponge with high hydraulic conductivity. This sponge can maintain a maximum evaporation rate up to the 5 consecutive days operation with the co-benefit of real-time regeneration and the reversible switching of the wettability. The reusable smart sponge evaporators are highly efficient in generating clean water from seawater with satisfactory ion rejection rates (above 99.6 %). As such, the prepared sponge shows great potential in environmental restoration, metal recovery, and water regeneration.

A 3D smart wood membrane with high flux and efficiency for separation of stabilized oil/water emulsions.

Oily sewage discharged from indiscriminate industrial and frequent oil spills have become a serious global problem. There is an urgent need to separate stable oil/water emulsions by efficient and environmentally friendly methods. Membrane separation technology has the advantages of low energy consumption and low cost, thus is an effective solution to the problems of oily wastewater. However, the manufacture of multifunctional membranes with high efficiency, high flux and self-cleaning using renewable materials remains a challenge. Herein, three-dimensional (3D) smart membranes with switchable superhydrophobic-hydrophilic surfaces were prepared by grafting photo-responsive poly-spiropyran (PSP) on wood-based substrates via surface atom transfer radical polymerization. This novel membrane can efficiently separate stabilized water-in-oil and oil-in-water emulsions due to reversible hydrophilic-hydrophobic transition by switching UV and visible light irradiation. Remarkably, after immobilization, the PSP grafted on the wood substrate exhibited a faster photo response effect than the free spiropyran (SP). More importantly, the prepared 3D smart membranes showed exceptional high flux (4392 L•m-2•h-1) and efficiency (above 99.99 %), good cycle stability (99.99 % after 12 times) and durability (available for at least 60 days) for the separation of surfactant-stabilized water-in-oil emulsions. This work opens a new avenue for the design of functional biomass-derived membranes for efficient and sustainable oily wastewater treatment with high flux, easy scale-up, and green regeneration.

Two-dimensional nanomaterials: synthesis and applications in photothermal catalysis.

Photothermal catalysis, as one of the emerging technologies with synergistic effects of photochemistry and thermochemistry, is highly attractive in the fields of environment and energy. Two-dimensional (2D) nanomaterials have received extensive attention toward photothermal catalysis because of their ultrathin layer structures, superior physical and optical properties, and high surface areas. These merits are beneficial for shortening the transfer distance of charge carriers, improving the efficiency of solar to thermal, and providing a great opportunity for the development of photothermal chemistry. In this review, we have summarized the state-of-art advances in various 2D nanomaterials with emphasis on the driving force and relevant mechanism of photothermal catalysis, including the involved three types, namely, localized surface plasmonic resonance (LSPR), nonradiative relaxation, and thermal vibrations of molecules. Moreover, the synthesis strategies of 2D materials and their photothermal applications in carbon dioxide (CO2) conversion, hydrogen (H2) production, volatile organic compounds (VOCs) degradation, and water (H2O) purification have been discussed in detail. Ultimately, the existing challenges and prospects of future development in the field are proposed. It is believed that this review will afford a great reference for the exploration of the high-efficiency 2D nanomaterials and their structure-activity relationship.

Multifunctional Solar Evaporator with Adjustable Island Structure Improves Performance and Salt Discharge Capacity of Desalination.

Interfacial solar steam generation (ISSG) is the main method to get fresh water from seawater or wastewater. The balance between evaporation rate and salt resistance is still a major challenge for ISSG. Herein, a wood aerogel island solar evaporator (WAISE) with tunable surface structure and wettability by synthesizing poly(n-isopropylacrylamide)-modified multi-walled carbon nanotube photothermal layers. Compared to dense surface structure evaporators, interfacial moisture transport, thermal localization, and surface water vapor diffusion of WAISE are greatly promoted, and the evaporation rate of WAISE increased by 87.64%. WAISE allows for record performance of 200 h continuous operation in 20% NaCl solution without salt accumulation. In addition, the photo-thermal-electric device is developed based on WAISE with continuous water purification, power generation, and irrigation functions. This work provides a new direction for the development of multifunctional water purification systems.

Construction of BiVO4/NiCo2O4 nanosheet Z-scheme heterojunction for highly boost solar water oxidation.

The sluggish water oxidation process is a severe obstacle for solar-driven water splitting. Therefore, it is imperative to develop a suitable photocatalyst with reduced energy barrier for strong oxidation. In this study, a Z-scheme BiVO4/NiCo2O4 (BVO/NCO) heterojunction system was designed by decorating ultrathin nickel-cobalt (NiCo2O4) spinel nanosheets on BiVO4 as an efficient photocatalyst for water oxidation. The unique structure of the system significantly reduced the energy barrier and improved the oxidation ability of BiVO4 to efficiently enhance the separation and transfer of the photogenerated carriers. Thus, the photocatalyst delivered an excellent O2 evolution performance of 1640.9 µmol∙g-1∙h-1 and showed 124% improved efficiency as compared to pristine BiVO4 and a quantum efficiency of 5.39% at 400 nm for O2 evolution. Additionally, the theoretical calculations revealed that the formation of *OOH was the rate-determining step for water oxidation. The decoration with NiCo2O4 significantly reduced the energy barrier between *O and *OOH, which eventually improved the photocatalytic performance of BVO/NCO. The results hold great promise for the potential application of spinel-based materials in efficient photocatalytic O2 evolution and offer fundamental insights into the design of efficient water oxidation heterojunctions.

Recent advances in high-crystalline conjugated organic polymeric materials for photocatalytic CO2 conversion.

The photocatalytic conversion of carbon dioxide (CO2) to high-value-added fuels is a meaningful strategy to achieve carbon neutrality and alleviate the energy crisis. However, the low efficiency, poor selectivity, and insufficient product variety greatly limit its practical applications. In this regard, conjugated organic polymeric materials including carbon nitride (g-C3N4), covalent organic frameworks (COFs), and covalent triazine frameworks (CTFs) exhibit enormous potential owing to their structural diversity and functional tunability. Nevertheless, their catalytic activities are largely suppressed by the traditional amorphous or weakly crystalline structures. Therefore, constructing relevant high-crystalline materials to ameliorate their inherent drawbacks is an efficient strategy to enhance the photocatalytic performance of conjugated organic polymeric materials. In this review, the advantages of high-crystalline organic polymeric materials including reducing the concentration of defects, enhancing the built-in electric field, reducing the interlayer hydrogen bonding, and crystal plane regulation are highlighted. Furthermore, the strategies for their synthesis such as molten-salt, solid salt template, and microwave-assisted methods are comprehensively summarized, while the modification strategies including defect engineering, element doping, surface loading, and heterojunction construction are elaborated for enhancing their photocatalytic activities. Ultimately, the challenges and opportunities of high-crystalline conjugated organic polymeric materials in photocatalytic CO2 conversion are prospected to give some inspiration and guidance for researchers.

Reaction Kinetics of Photoelectrochemical CO2 Reduction on a CuBi2O4-Based Photocathode.

The CO2 reduction reaction (CO2RR) is an essential step in natural photosynthesis and artificial photosynthesis to provide carbohydrate foods and hydrocarbon energy in the carbon-neutral cycle. However, the current solar conversion efficiencies and/or product selectivity of the CO2RR are very sluggish due to its complicated multiple-step charge transfer reactions. Here, we systematically investigate the charge transfer reaction rate during CO2 reduction on CuBi2O4 photocathodes, where the surface is modified with 3-aminopropyltriethoxysilane (APTES). We discover that the surface amine group increases the charge separation rate, significantly enhancing the surface charge transfer reaction rate. However, the surface acidity has less influence on the first-order reaction, indicating that a rate-determining step (RDS) exists in the early stage of the photoelectrochemical cell (PEC) processes. Moreover, the intensity-modulated photocurrent spectroscopy (IMPS) confirms that both surface charge transfer and the recombination rate on APTES-coated CuBi2O4 are larger than bare CuBi2O4 while possessing comparable charge transfer efficiencies. Overall, the surface charge transfer reactions under the PEC condition require designing more effective nanostructured photoelectrodes and powerful characterization methods to intrinsically increase the charge separation and transfer rate while reducing the recombination rate.

Ambient-Stable Black Phosphorus-Based 2D/2D S-Scheme Heterojunction for Efficient Photocatalytic CO2 Reduction to Syngas.

Black phosphorus (BP), an emerging remarkable photocatalytic semiconductor, is arousing strong interests in this field of solar-driven CO2 reduction, but its stability and activity are still facing huge challenges. Here, an ambient-stable and effective 2D/2D heterostructure of BP/bismuth tungstate (Bi2WO6) with oxygen vacancy is innovatively designed for syngas production via photocatalytic CO2 reduction. This work, not only resolves the stability problem of BP nanosheets by anchoring ultrasmall platinum (Pt) nanoparticles (∼2 nm) but also greatly improves the charge transfer efficiency by constructing S-scheme 2D/2D heterostructure with coupled oxygen defects. As a result, the generation rates of carbon monoxide (CO) and hydrogen (H2) remarkably reach 20.5 and 16.8 µmol g-1 h-1, respectively, which are much higher than that of reported BP-based materials, and the accomplished CO/H2 ratios (1:1-2:1) are exactly the most desirable syngas for industrial applications. Thus, this work constructs an efficient and ambient-stable BP-based photocatalyst for syngas production by CO2 reduction at mild conditions.