Covalent organic frameworks for direct photosynthesis of hydrogen peroxide from water, air and sunlight

Enhancing the crystallinity of covalent organic frameworks to achieve improved photocatalytic hydrogen peroxide production under ambient conditions

Photocatalytic production of hydrogen peroxide (H2O2) presents a promising strategy for environmental remediation and energy production. However, achieving clean and efficient H2O2 production under ambient conditions without organic sacrificial agents remains challenging. Enhancing the low crystallinity of covalent organic frameworks (COFs) can promote the separation and transmission of photo-generated carriers, thereby boosting their photocatalytic performance. Herein, we introduce a novel synthetic approach by substituting traditional acetic acid catalysts with organic base catalysts to enhance the crystallinity of β-ketoenamine-linked COF, TpBD-COF. Compared to TpBD-COF-A synthesized using acetic acid catalysts, TpBD-COF-B, synthesized with organic base catalysts, exhibited advancements including increased absorption intensity in the visible spectrum, reduced photoluminescence intensity, enhanced photo-generated carrier separation performance, and a 2.1-fold increase in photocatalytic H2O2 production. Under visible light irradiation, TpBD-COF-B achieved a photocatalytic H2O2 production rate of 533 µmol/h/g using only air and water, without the need for organic sacrificial agents. Furthermore, TpBD-COF-B also exhibited good performance in long-term catalytic production experiments, tests with actual water bodies, and cyclic usage experiments. This study offers a strategy for enhancing the crystallinity of COFs to improve their photocatalytic activity, with promising applications in clean energy production and environmental remediation.

Revealing the Mechanism of Exciton Spontaneous Separation at Room Temperature for Efficient Photocatalytic Hydrogen Peroxide Synthesis

The photocatalytic synthesis of hydrogen peroxide (H2O2) at room temperature has garnered significant attention as an environmentally friendly alternative to traditional anthraquinone oxidation processes. However, the low exciton dissociation efficiency at room temperature often hinders photocatalytic performance. In this study, it is demonstrated that tuning the substitution sites of electron donors in Donor-Acceptor (D-A) conjugated polymers can significantly enhance exciton dissociation by reducing exciton activation energy, which facilitates the spontaneous separation of excitons at room temperature. For comparison, materials with exciton separation energies ≈89 meV exhibit a hydrogen peroxide production rate of 2692 µmol·g-1·h-1. In contrast, the main material developed in this work, O-PTAQ, demonstrates a substantially lower exciton separation energy of 22 meV, resulting in a hydrogen peroxide production rate of 4989 µmol·g-1·h-1 under ambient conditions, outperforming most reported organic semiconductors. This enhancement is attributed to the increased electron delocalization in the electron donors, which lowers exciton activation energy to promote efficient exciton separation. The findings highlight the critical role of molecular-level structural tuning in enhancing exciton dissociation, providing a promising strategy for the development of high-efficiency photocatalysts for sustainable H2O2 production.

Synthesis of SnO₂/COF Green Nanomaterials for Effective Pesticide Decomposition and Promoting Tomato Plants Growth

In last few decades, the agriculture sector is facing various type of crops diseases originated by crop pests. Among various crops the tomato plant is greatly affected by many pests such as aphids and whiteflies, which are badly decreasing tomato plant yield and effecting its growth. In last few years, various type of pesticides such as Neonicotinoids and Pyrethroids are employed which are badly effecting eco-system and water bodies. In this research work, we prepared SnO2 nanosheets (SONS) by in-situ and green synthesis approach. Remarkably, SONS exhibit a larger surface area, tailored pore size, and higher catalytic performance than SnO2 nanoparticles (SONP). To further improve the efficiency of SONS, we coupled it with covalent organic farmwork nanosheets (COFNS) via the hydrothermal approach. The SONS@COFNS hybrid nanocatalysts exhibit improved carrier migration, enhanced porosity, multiple active sites, and exceptional light absorption capabilities. The as prepared green nanomaterials delivered improved activities for Neonicotinoids and Pyrethroids degradation. Remarkably, the most active sample 6COFNS/SONS showed the highest degradation efficiency (94 %), which is approximately 1.92 times higher than the degradation efficiency of pristine SONS (49 %). This work will ultimately contribute to developing green, ecofriendly nanomaterials for pesticides degradation and promoting tomato plants growth.

Regulating Electron Distribution in Regioisomeric Covalent Organic Frameworks for Efficient Solar-Driven Hydrogen Peroxide Production

Covalent organic frameworks (COFs) are emerging as a transformative platform for photocatalytic hydrogen peroxide (H2O2) production due to their highly ordered structures, intrinsic porosity, and molecular tunability. Despite their potential, the inefficient utilization of photogenerated charge carriers in COFs significantly restrains their photocatalytic efficiency. This study presents two regioisomeric COFs, α-TT-TDAN COF and β-TT-TDAN COF, both incorporating thieno[3,2-b]thiophene moieties, to investigate the influence of regioisomerism on the excited electron distribution and its impact on photocatalytic performance. The β-TT-TDAN COF demonstrates a remarkable solar-to-chemical conversion efficiency of 1.35%, outperforming its α-isomeric counterpart, which is merely 0.44%. Comprehensive spectroscopic and computational investigations reveal the critical role of subtle substitution change in COFs on their electronic properties. This structural adjustment intricately connects molecular structure to charge dynamics, enabling precise regulation of electron distribution, efficient charge separation and transport, and localization of excited electrons at active sites. Moreover, this finely tuned interplay significantly enhances the efficiency of the oxygen reduction reaction. These findings establish a new paradigm in COF design, offering a molecular-level strategy to advance COFs and reticular materials toward highly efficient solar-to-chemical energy conversion.

Efficient Photosynthesis of Hydrogen Peroxide from Water and Air Over Water-Dispersible Anthraquinone-Based Porous Aromatic Frameworks

Photosynthesis of hydrogen peroxide from earth-abundant water and air over organo-based semiconducting materials is a promising alternative to the traditional anthraquinone (AQ) method. However, the generally hydrophobic nature of organic semiconductors has led to their poor dispersibilities in aqueous systems, which built huge barriers for photon capture and reactant contact in water-based photocatalysis. Aiming at this issue, this study reports the facile synthesis of AQ-based porous aromatic frameworks (AQ-PAFs) by coupling AQ fragments with thiophene-derived linkers via robust carbon-carbon bonds. Remarkably, the interfacial hydrogen bonding interactions between water molecules and AQ sites on the surface improve the general hydrophilicity of AQ-PAFs, which can be well-dispersed in water-only systems with uniform particle size distributions. Moreover, the AQ moieties also function as mediators of photoinduced electrons, and the protons produced from water oxidation reaction (WOR), which would kinetically favor the charge separation and subsequent electron transfer reactions. The mono-dispersed AQ-PAF photocatalyst promotes hydrogen peroxide (H2O2) photosynthesis from water and air under visible light achieving a high productivity of 7124 µmol g-1 h-1 in the absence of any organic alcohol reagents among organic semiconductor photocatalysts. Furthermore, a continuous H2O2 photosynthesis for 190 h is also achieved in a flow system.

Three-Dimensional Covalent Organic Frameworks with jcg Topology Based on a Trinodal Strategy

The development of three-dimensional (3D) covalent organic frameworks (COFs) holds significant promise for various applications, but the conventional uninodal or binodal design strategies limit their structural diversity. In this work, we present a novel trinodal strategy for the synthesis of 3D COFs featuring both microporous and mesoporous nanochannels. Using powder X-ray diffraction (PXRD), computational simulations, and high-resolution transmission electron microscopy (HR-TEM), we demonstrate that employing an 8-c building block with reduced symmetry, which can be considered as 4- and 3-connected subunits, along with planar 4-c building blocks, results in an unprecedented [4 + 3 + 4]-c jcg net. This structure features rare saddle-shaped eight-membered rings and mirror-symmetrical chains. Furthermore, the incorporation of chromophore pyrene and redox-active triphenylamine components, coupled with structural conjugation, imparts tunable photophysical and electronic properties to these COFs, making them promising candidates for photocatalytic H2O2 production. This work highlights the potential of the trinodal strategy in creating intricate COF architectures and enhances their applicability in heterogeneous photocatalysis.

A ZnO-based Catalytic System for the Synthesis of Hydrogen Peroxide from Air

Hydrogen peroxide (H2O2) has a wide range of applications as an eco-friendly and sustainable oxidant. However, the clean, efficient and convenient synthesis of this compound remains challenging. This work demonstrates a rationally designed electron-self-supplied catalytic system capable of generating H2O2 from water and atmospheric oxygen without extra energy input. This catalytic system is made of a ZnO coating containing oxygen vacancies and a Zn substrate. The ZnO catalyst layer obtains electrons from the Zn substrate to synthesize H2O2. The H2O2 concentration produced by this catalytic system is up to 17.9 mM without any secondary processing. This remarkably high concentration is attributed to the formation of a liquid film on the hydrophilic ZnO surface that promotes the oxygen reduction reaction by accelerating the transfer of oxygen from the ambient air to the catalyst surface. By integrating with atmospheric fog collection, this system can continuously collect H2O2 directly from the air.

Building a Confluence Charge Transfer Pathway in COFs for Highly Efficient Photosynthesis of Hydrogen Peroxide from Water and Air

Sunlight-driven photosynthesis by covalent organic frameworks (COFs) from water and air without using sacrificial reagents is a promising H2O2 fabrication approach but is still restricted by the insufficient charge separation and sluggish 2e- water oxidation process. Herein, we provide a facile strategy to simultaneously improve charge separation and water oxidation in COFs via confining the charge transfer pathways from two diversion ones to a confluence one through regulating the site of nitrogen in bipyridine. Combining in-situ characterization with computational calculations, we reveal that compared to COF-BD1 containing two diversion charge transfer pathways, the charge transfer pathway in COF-BD2 is confined to a confluence due to the electron-deficiency effect of nitrogen, which greatly accelerates the intermolecular and out-of-plane charge transfer. Via effectively reducing the energy barrier of rate-determining water oxidation reaction (WOR), the subsequent water oxidation process to produce the key *OH intermediate in COF-BD2 is also greatly facilitated, boosting the yield of H2O2 (5211 µmol g-1 h-1) from water, oxygen, and light without sacrificial agents or additional energy consumption. We further demonstrate that H2O2 can be efficiently produced by COF-BD2 in a broad pH range, in real water, and in an enlarged reactor using natural sunlight for water decontamination.

Dehydration-enhanced Ion Recognition of Triazine Covalent Organic Frameworks for High-resolution Li+/Mg2+ Separation

The precise and rapid extraction of lithium from salt-lake brines is critical to meeting the global demand for lithium resources. However, it remains a major challenge to design ion-transport membranes with accurate recognition and fast transport path for the target ion. Here, we report a triazine covalent organic framework (COF) membrane with high resolution for Li+ and Mg2+ that enables fast Li+ transport while almost completely inhibiting Mg2+ permeation. The remarkably high rejection of Mg2+ by the COF membrane is achieved via imposed ion dehydration and the construction of the energy well. The proper hydrophilic environment of the COF channel promotes the dissociation of Li+ from the negatively charged functional groups, allowing Li+ for hopping transport supported by the sulfonate side-chains to shorten the diffusion path of Li+. Under high-salinity electrodialysis conditions, the COF membrane demonstrates robust Li+/Mg2+ separation performance (No Mg2+ were detected in the collected solution), achieving efficient lithium recovery and high product purity (Li2CO3: 99.3 %). This membrane design strategy enables high energy efficiency and powerful lithium extraction in the electrodialysis lithium extraction process, and can be generalized to other energy and separation related membranes.

Rational Fabrication of a Robust, Exfoliatable Catechol-Porphyrin Covalent Network for Enhanced H2O2 Photosynthesis

Covalent organic frameworks (COFs) containing dioxin-linkages are highly valued for their exceptional chemical stability, which is essential for practical use. However, research on dioxin-based COFs remains limited. Herein, a unique nonplanar 2D COF, designated as TCP-COF, constructed from catechol-porphyrin units interconnected by 1,4-dioxin bonds, exhibiting a staggered AAA stacking pattern, is presented. Remarkably, TCP-COF can undergo in situ exfoliation to produce ultrathin 2D nanosheets when it is utilized as a photocatalyst for hydrogen peroxide (H2O2) generation in water and air, without the need for additives. This exfoliation process is primarily driven by the distortion of porphyrin units and weak π-π interaction between adjacent layers in TCP-COF. The resultant ultrathin nanosheets significantly reinforce catalytic activity, achieving a photocatalytic H2O2 production rate of 3077 µmol g-1 h-1. The mechanism underlying H2O2 photosynthesis is further explored through a combination of experimental analyses and theoretical calculations. This study provides valuable insights for the development of efficient COF-based photocatalysts for H2O2 evolution.

Efficient Photocatalytic Synthesis of Hydrogen Peroxide Facilitated by Triptycene-Based 3D Covalent Organic Frameworks

Covalent organic frameworks (COFs) are widely studied for hydrogen peroxide (H₂O₂) photosynthesis, with 3D COFs standing out for their porous structures and chemical stability. However, the difficult preparation of 3D COFs and the low efficiency in separating photo-generated electrons and holes (e- and h+) limits the efficient production of H2O2. In this study, two kinds of [6+3] 3D COFs (XJU-1, XJU-2) with significant charge separation, achieving record-breaking H₂O₂ photocatalysis rates of 34 777 and 11 922 µmol g⁻¹ h⁻¹, respectively. XJU-1's superior efficiency stems from its larger pores, enhancing material transport and oxygen (O2) activation. Experimental and theoretical studies have demonstrated that triptycene monomers achieve significant charge separation toward triazine via imine bonds. Moreover, the dimer's smaller singlet-triplet energy gap (∆ES-T) and triptycene's orthogonal configuration enhance singlet oxygen (1O2) production, enabling multiple H2O2 generation pathways. Ultimately, through the oxygen reduction reaction (ORR) pathway, rapid generation of H2O2 can be achieved at multiple catalytic sites. XJU-1 mainly follows a mixed pathway involving 1e--ORR and 2e--ORR, and XJU-2 primarily follows the 2e--ORR pathway, respectively. These open the door of triptycene-based 3D COFs applied in continuous, efficient, and stable photosynthesis of H2O2.

Modulating N-Heterocyclic Microenvironment in β-Ketoenamine Covalent Organic Frameworks to Boost Overall Photosynthesis of H2O2

Covalent Organic Frameworks (COFs) have emerged as promising platforms for photocatalytic synthesis of hydrogen peroxide (H2O2) due to their tunable chemical compositions and efficient catalytic functionalities. Inspired by the role of the microenvironment in enzyme catalysis, this study introduces various N-heterocyclic species into β-ketoenamine COFs (Nx-COFs, where Nx represents the number of nitrogen atoms in the N-heterocycle) to regulate the microenvironment around catalytic sites on acceptor-donor-acceptor (A-D-A) COFs foroverall H2O2 photosynthesis in pure water. The Nx-COFs exhibit distinct H2O2 photosynthetic rates following the number of nitrogen atoms sequence of N3-COF > N2-COF > N1-COF > N0-COF, with N3-COF with triazine structure showing the highest H2O2 generation rate (4881 µmol h-1 g-1) and the decent solar-to-chemical conversion (SCC) efficiency (0.413%), surpassing many existing COF-based catalysts. In situ characterization and theoretical calculations support the experimental results, revealing that N-heterocyclic species promote the photosynthesis of H2O2 through both an indirect stepwise single-electron oxygen reduction reaction (1e- ORR) mechanism and a direct two-electron water oxidation (2e- WOR) pathway. This study advances the design paradigm of photocatalysts by modulating the microenvironment within A-D-A COFs, paving the way for the development of more efficient and robust photocatalytic systems for the overall photosynthesis of H2O2.

Autocatalytic Interfacial Synthesis of Self-Standing Amide-Linked Covalent Organic Framework Membranes

The synthesis of crystalline covalent organic frameworks (COFs) has in principle relied on reversible dynamic chemistry. A general method to synthesize irreversibly bonded COFs is urgently demanded for driving the COF chemistry to a new era. Here we report a universal two-step method for the straightforward synthesis of irreversibly amide-linked COF (AmCOF) membranes by autocatalytic interfacial polymerization (AIP). Highly crystalline amide and imine bilinker COF (AICOF) membranes are readily synthesized by AIP strategy which ingeniously leverages interfacial polymerization to generate amide units followed by an autocatalytic condensation that forms imine bonds. Then, the fully amide-linked AmCOF membranes with Turing structures can be prepared through irreversible linker renovation. The universality of this method has been exemplified by nine AmCOF membranes. Among them, the AmCOF-1 membrane exhibits superior performance for H2O2 photosynthesis (4353 μmol g-1 h-1) and high stability, enabling continuous production of H2O2 under sunlight for 150 h without sacrificial agents. Mechanistic investigations reveal that the greatly improved properties are attributable to the built-in robust amide knots, facilitating full separation of electrons and holes, ultra-long exciton diffusion length, and fast dissociation of excitons within the AmCOF channels.

g-C3N4 S-Scheme Homojunction through Van der Waals Interface Regulation by Intrinsic Polymerization Tailoring for Enhanced Photocatalytic H2 Evolution and CO2 Reduction

The effective S-scheme homojunction relies on the precise regulation of band structure and construction of advantaged charge migration interfaces. Here, the electronic structural properties of g-C3N4 were modulated through meticulous polymerization of self-assembled supramolecular precursors. Experimental and DFT results indicate that both the intrinsic bandgap and surface electronic characteristics were adjusted, leading to the formation of an in-situ reconstructed homojunction interface facilitated by intrinsic van der Waals forces. The homojunction catalyst, composed of g-C3N4 nanodots and ultra-thin g-C3N4 nanoflakes, exhibited a significant S-scheme carrier separation mechanism, which enhances the utilization of electrons and holes. Consequently, under AM 1.5 light irradiation (~100 mW/cm2), the g-C3N4 homojunction photocatalyst achieved a remarkable hydrogen evolution rate of 580 μmol h-1. Furthermore, a reversed CH4 selectivity in CO2 reduction was observed, yielding 80.30 μmol g-1 h-1 with a selectivity of 96.86 %, in contrast to the performance of bulk g-C3N4, which produced only 2.22 μmol g-1 h-1 with the 15.69 % CH4 selectivity. These findings not only highlight the significant potential of the g-C3N4 homojunction photocatalyst for hydrogen production and CO2 reduction but also propose a superior and effective strategy for optimizing the structural properties of g-C3N4, which are crucial for the design of photocatalytic reactions.

A Covalent Organic Framework as Photocatalyst for Smart Conversion Between Photooxidation and Photoreduction and H2O2 Production in Full pH Environment

Developing multifunctional photocatalysts with intelligent self-adjusting is of great significance in the photocatalytic process. Herein, a smart covalent organic framework (Por-HQ-COF) with a phenol-quinone conversion structure with pH changes is constructed for photooxidation, photoreduction, and H2O2 production. As a smart photocatalyst, Por-HQ-COF can convert into Por-BQ-COF intelligently with a trigger including solution pH, and vice versa. The reconstruction of phenol-quinone conversion not only significantly alters the morphologies and the specific surface areas of the COF, but also leads to an entirely change in the band energy and charge distribution to influence photoelectric properties. As a result, under acidic conditions, Por-BQ-COF converts into Por-HQ-COF automatically and can photoreduce high concentration Cr(VI) to Cr(III) efficiently. Under neutral conditions, the superoxide anions (·O2 -) initiate the Por-HQ-COF reconstruction into Por-BQ-COF to accelerate photooxidation to degrade high-concentration TC. Under alkaline conditions, Por-HQ-COF converts into Por-BQ-COF, can effectively photosynthesize H2O2 (1525 µmol h-1 g-1 at λ > 420 nm) in the absence of any sacrificial reagents, and reveal the strong alkalinity lower the energy barrier of hydrogen extraction from H2O and clarify active sites for H2O2 production. This work provides a new strategy for developing smart photocatalysts and fulfill the application across the full pH environment.

Bottom-up Synthesis of Piezoelectric Covalent Triazine-based Nanotube for Hydrogen Peroxide Production from Water and Air

Carbon nanotubes (CNTs) are nanoscale tubular materials with superior mechanical strength and electronic properties. However, the conventional CNTs are inherently non-piezoelectric, mainly due to the lack of polar structures with pure carbon elements. The direct synthesis of fully conjugated and polarized organic nanotubes with desired piezoelectric properties remains a challenge. Herein, we report the bottom-up synthesis of a new type of covalent triazine-based nanotube (CTN-1) as a novel piezoelectric material. The CTN-1 comprises of high surface area, nitrogen-rich and fully conjugated structure, which provides a series of merits for piezoelectric catalytic processes. These structural features combined with one-dimensional tubular morphology endow CTN-1 with excellent mechanical stimuli response and thus displaying prominent piezoelectric properties via pronounced nanocurvature effect. We further show that the CTN-1 enables the efficient synthesis of H2O2 from water in the air via mechanical energy conversion, with an excellent piezocatalytic H2O2 evolution rate of 4115 μmol g-1 h-1, which exceeds other reported piezoelectric materials. The piezocatalysis by the CTN-1 can be practically integrated into a self-Fenton system, which exhibits excellent pollutant degradation capability. This work demonstrates the enormous potential of a new type of piezoelectric synthetic nanotube from organic frameworks for the in situ synthesis valuable chemicals.

Vinylene-Linking of Polycyclic Aromatic Hydrocarbons to π-Extended Two-Dimensional Covalent Organic Framework Photocatalyst for H2O2 Synthesis

Polycyclic aromatic hydrocarbons (PAHs) hold the predominant role either as individual molecules or building blocks in the field of organic semiconductors or nanocarbons. Connecting PAHs via sp2-carbon bridges to form high-crystalline π-extended structures is highly desired not only for enlarging the regimes of two-dimensional materials but also for achieving exceptional properties/functions. In this work, we developed 5,10-dimethyl-4,9-diazapyrene as a key monomer, whose two methyl groups at the positions adjacent to nitrogen atoms, can helpfully increase the solubility, and serve as the active connection sites. In the presence of organic acids, this monomer enables smoothly conducting Knoevenagel condensation to form two vinylene-linked PAH-cored COFs, which show high-crystalline honeycomb structures with large surface areas up to 1238 m2 g-1. Owing to the direct connection mode of PAH building blocks with vinylene, the as-prepared COFs possess spatially extended π-conjugation and substantial semiconducting properties. Consequently, their visible-light photocatalysis with exceptional activity and durability was manifested to generate H2O2 up to 3820 μmol g-1 h-1 in pure water, and even 17080 μmol g-1 h-1 using benzyl alcohol as a hole sacrificial agent.

Covalent Organic Frameworks for Boosting H2O2 Photosynthesis via the Synergy of Multiple Charge Transfer Channels and Polarized Field

Covalent organic frameworks (COFs) serve as one of the most promising candidates for hydrogen peroxide (H2O2) photosynthesis, while attaining high-performance COFs remains a formidable challenge due to the insufficient separation of photogenerated charges. Here, through the rational design of bicarbazole-based COFs (Cz-COFs), we showcase the first achievement in piezo-photocatalytic synthesis of H2O2 using COFs. Noteworthily, the ethenyl group-modified Cz-COFs (COF-DH-Eth) demonstrates a record-high yield of H2O2 (9212 μmol g-1 h-1) from air and pure water through piezo-photocatalysis, which is ca. 2.5 times higher than that of pristine Cz-COFs without ethenyl groups (COF-DH-H) under identical condition and COF-DH-Eth without ultrasonic treatment. The H2O2 production rate originates from the synergistic effect between an ultrasonication-induced polarized electric field and the spatially separated multiple charge transfer channels, which significantly promote the utilization of photogenerated electrons by directional transfer from bicarbazole groups to the ethenyl group-modified benzene rings. Several Cz-COFs and bifluorenylidene-based COFs (COF-BFTB-H) with similar twisted monomers exhibit obvious piezoelectric performance for promoting H2O2 generation, signifying that organic ligands with a twistable structure play a crucial role in creating broken symmetry structures, thereby establishing piezoelectric properties in COFs.

Induced Charge-Compensation Effect for Boosting Photocatalytic Water Splitting in Covalent Organic Frameworks

Imine-based covalent organic frameworks (COFs) are promising for photocatalytic water splitting, but their performance is often constrained by inefficient charge separation due to the high electron localization nature of polar imine bonds. In this study, we have optimized the electron delocalization across the imine linkage within a COF by implementing a charge compensation effect. This effect is achieved when a strong electron-donating thieno[3,2-b]thiophene linker is directly attached to the iminic carbon of a zinc-porphyrinic COF. This modification significantly reduces the electron binding effect within the imine bonds of the COF, facilitating both in-plane charge separation and out-plane charge transfer to the catalytic site. Conversely, the use of strong electron-withdrawing pyrizine linker aggravates the electron localization at the imine linkage in the ZnP-Pz variant. Consequently, ZnP-Tt shows a substantially improved photocatalytic water-splitting activity under visible light irradiation, with a hydrogen evolution of 44288±2280 μmol g-1 in 4 h, which exceeds the ZnP-Pz counterpart by a factor of 10. These results offer fresh perspectives for the design of imine-based COFs to overcome their limitations in charge separation efficiency.

The Engineering Screen of Photoactive Nanozymes Based on N-Containing Covalent Organic Frameworks for Antibacterial Application

Covalent organic frameworks (COFs) are a class of highly efficient photocatalytic organic semiconductor materials, which have been developed for the design of photoactive nanozymes. Nitrogen (N)-heterocycles could effectively improve their photocatalytic activity of COFs. However, the systematic exploration of photoactive nanozymes based on N-containing COFs is still lacking. In this work, a series of N-containing Schiff-base linkages of COFs are designed and synthesized to explore high-performance photoactive nanozymes. In addition, Fe ions are introduced through post-modification of COFs, which can not only effectively extend the band-edge absorption of COFs to the red-light region and thereby broaden its biological applications, but also introduce single site of Fe to enrich the types of free radicals in the catalytic products. The activities of photoactive nanozymes based on N-containing COFs are systematically studied, and their catalytic mechanisms are uncovered. Interestingly, it is not as commonly recognized that the more content of N, the better for photocatalysis or the construction of photoactive nanozyme. Furthermore, the selected photoactive nanozymes are used for antibacterial applications, which showed good activity against Escherichia coli.

Polymeric Carbon Nitride Edged with Spatially Isolated Donor and Acceptor for Sunlight-Driven H2O2 Synthesis and In-Situ Utilization

On-site H2O2 activation attracts much attention in energy conversion and environment remediation et al., yet remains challenging in its highly efficient and sustainable synthesis. Herein, we grafted a pair of spatially isolated donor (methoxyphenyl unit) and acceptor (anthraquinone unit) in polymeric carbon nitride edges, which induce directional electron-hole transfer to the two spatially separated dual active centers. Specifically, photogenerated electrons in the anthraquinone unit facilitate the 2e- ORR, while the methoxyphenyl unit, which gathers photogenerated holes, enables rapid 4e- WOR. More impressively, the anthraquinone unit also exhibits strong proton extraction capabilities to boost the generation of *OOH intermediates and H2O2. Consequently, the synthesized donor-polymeric carbon nitride-acceptor (DPA) catalyst shows a remarkable H2O2 yield of 6497.1 μM h-1 g-1 in pure water, surpassing traditional DP and PA catalysts. Because of its high efficiency, the H2O2 product can efficiently degrade and mineralize various organic contaminants in a continuous-flow self-Fenton reactor under sunlight irradiation. Our work presents an unprecedented approach to designing photocatalysts with efficient H2O2 synthesis and practical application from a molecular engineering perspective.

Modulating Core Polarity in Metal-free Covalent Organic Frameworks for Selective Electrocatalytic Hydrogen Peroxide Production

Tuning the charge density at the active site to balance the adsorption ability and reactivity of oxygen is extremely significant for driving a two-electron oxygen reduction reaction (ORR) to produce hydrogen peroxide (H2O2). Herein, we have highlighted the influence of intermolecular polarity in covalent organic frameworks (COFs) on the efficiency and selectivity of electrochemical H2O2 production. Different C3 symmetric building blocks have been utilized to regulate the charge density at the active sites. The benzene-cored COF, which exhibits reduced polarity than the triazine-cored COF, displayed enhanced performance in H2O2 production, achieving 93.1 % selectivity for H2O2 at 0.4 V with almost two-electron transfer and a faradaic efficiency of 90.5 %. In-situ electrochemical Raman spectroscopy and scanning electrochemical microscopy (SECM) were employed to confirm H2O2 generation and analyze spatial reactivity patterns. These techniques provided detailed insights into localized catalytic behavior, emphasizing the influence of core polarity.

Linkage Microenvironment Modulation in Triazine-Based Covalent Organic Frameworks for Enhanced Photocatalytic Hydrogen Peroxide Production

Covalent organic frameworks (COFs), known for the precise tunability of molecular structures, hold significant promise for photocatalytic hydrogen peroxide (H2O2) production. Herein, by systematically altering the quinoline (QN) linkages in triazine (TA)-based COFs via the multi-component reactions, six R-QN-TA-COFs are synthesized with identical skeletons but different substituents. The fine-tuning of the optoelectronic properties and local microenvironment of COFs is allowed, thereby optimizing charge separation and improving interactions with dissolved oxygen. Consequently, MeO-QN-TA-COF is customized to achieve an impressive rate of H2O2 production up to 7384 µmol g⁻1 h⁻1 under an air atmosphere in water without any sacrificial agents, surpassing most of the reported COF photocatalysts. Its high stability is demonstrated through five consecutive recycling experiments and the characterization of the recovered COF. The reaction mechanism for the H2O2 production is further investigated using a suite of quenching experiments, in situ spectroscopic analysis, and theoretical calculations. The enhanced photocatalytic H2O2 production over MeO-QN-TA-COF is through 2e⁻ oxygen reduction reaction and water oxidation reaction pathways. Overall, the crucial role of linkage microenvironment modulation in the design of COFs for solar-driven effective photocatalytic H2O2 production.

Unprecedented Photocatalytic Hydrogen Peroxide Production via Covalent Triazine Frameworks Constructed from Fused Building Blocks

Covalent organic frameworks (COFs) have garnered attention for their potential in photocatalytic hydrogen peroxide (H2O2) production. However, their photocatalytic efficiency is impeded by insufficient exciton dissociation and charge carrier transport. Constructing COFs with superior planarity is an effective way to enhance the π-conjugation degree and facilitate electron-hole separation. Nonetheless, the conventional linear linkers of COFs inevitably introduce torsional strain that disrupts coplanarity. Herein, we address this issue by introducing inherently coplanar triazine rings as linkers and fused building blocks as monomers to create covalent triazine frameworks (CTFs) with superior coplanarity. Both experimental and theoretical calculations confirm that CTFs constructed from fused building blocks significantly enhance the electron-hole separation efficiency and improve the photocatalytic performance, compared to the CTFs constructed with non-fused building blocks. The frontier molecular orbitals and electrostatic potentials (ESP) revealed that the oxygen reduction reaction (ORR) is preferentially facilitated by the triazine rings, with the water oxidation reaction (WOR) likely occurring at the thiophene-containing moiety. Remarkably, CTF-BTT achieved an exceptional H2O2 production rate of 74956 μmol g-1 h-1 when employing 10 % benzyl alcohol (V/V) as a sacrificial agent in an O2-saturated atmosphere, surpassing existing photocatalysts by nearly an order of magnitude. Our findings provide valuable insights for designing highly coplanar polymer-based photocatalysts that enhance the solar-to-chemical energy conversion process.

Isomerization of Covalent Organic Frameworks for Efficiently Activating Molecular Oxygen and Promoting Hydrogen Peroxide Photosynthesis

Constitutional-isomerized covalent organic frameworks (COFs), constructed by swapping monomers around imine bonds, have attracted attention for their distinct optoelectronic properties, which significantly impact photocatalytic performance. However, limited research has delved into the inherent relationship between isomerization and the enhancement of H2O2 photosynthesis. Herein, a pair of isomeric COFs linked by imine bonds (PB-PT-COF and PT-PB-COF) is synthesized, and it is proved that isomeric COFs exhibit different rate-determining steps in the generation process of H2O2, resulting in a twofold increase in photocatalytic efficiency. Specifically, PT-PB-COF demonstrates effective adsorption and activation of molecular oxygen (O2 + e- → •O2 - + e- → H2O2), leading to a significant improvement in H2O2 photocatalytic efficiency. In contrast, PB-PT-COF exhibits robust interaction with H2O, enabling direct oxidation of H2O (H2O + h+ → H2O2). This study provides a thorough understanding of the intrinsic mechanism underlying the constitutional-isomerized COFs in the photocatalytic H2O2 generation, offering insights for further optimizing building units.

Unlocking Topological Effects in Covalent Organic Frameworks for High-Performance Photosynthesis of Hydrogen Peroxide

Covalent organic frameworks (COFs) offer a compelling platform for the efficient photosynthesis of hydrogen peroxide (H2O2). Constructed with diverse topologies from various molecular building units, COFs can exhibit unique photocatalytic properties. In this study, three π-conjugated 2D sp2 carbon-linked COFs with distinctly different topologies (hcb, sql, and hxl) are designed to investigate the topological effect on the overall photosynthesis of H2O2 from water and oxygen. Despite their similar chemical and band structures, the QP-HPTP-COF with hxl topology outperformed other COFs in the photosynthesis of H2O2, demonstrating a remarkable solar-to-chemical conversion efficiency of 1.41%. Comprehensive characterizations confirmed that the hxl topology can substantially improve charge separation and transfer, thereby significantly enhancing photocatalytic performance. This study not only unravels the topology-directed charge carrier dynamics in COFs but also establishes a molecular engineering framework for developing high-performance photocatalysts for sustainable H2O2 production.

Covalent Organic Frameworks for Photocatalytic Hydrogen Peroxide Evolution

Covalent organic frameworks (COFs) are considered advanced class materials due to their exotic structural and optical properties. The abundance of starting monomers with variable linkage motifs may give rise to multiple conformations in either 2D or 3D fashion. Tailoring of the abovementioned properties has facilitated the application of COFs in a wide range of applications, which are strongly correlated with energy conversion schemes. Having a crystalline porous character and a large set of donor-acceptor combinations, COFs are expected to make huge impact in photocatalytic processes. In this Review, we present the recent advances in the development of semiconducting COF-based systems towards the photocatalytic hydrogen peroxide evolution. An overview is given about the effect of various parameters on the photocatalytic performance, such as charge transfer tuning, wettability by chemical functionalization, topology, porosity and crystallinity. Various challenges are discussed, and constructive insights are given for the development of highly functional COF-based photocatalysts for H2O2 evolution.

Photocatalytic applications of covalent organic frameworks: synthesis, characterization, and utility

Photocatalysis has emerged as an energy efficient and safe method to perform organic transformations, and many semiconductors have been studied for use as photocatalysts. Covalent organic frameworks (COFs) are an established class of crystalline, porous materials constructed from organic units that are easily tunable. COFs importantly display semiconductor properties and respectable photoelectric behaviour, making them a strong prospect as photocatalysts. In this review, we summarize the design, synthetic methods, and characterization techniques for COFs. Strategies to boost photocatalytic performance are also discussed. Then the applications of COFs as photocatalysts in a variety of reactions are detailed. Finally, a summary, challenges, and future opportunities for the development of COFs as efficient photocatalysts are entailed.

Celebrating Ten Years of Covalent Organic Frameworks for Solar Energy Conversion: Past, Present and Future

Accelerated anthropogenic emission of greenhouse gases due to increasing energy demands has created a negative impact on our planet. Therefore, the replacement of fossil by renewable energy resources has become of paramount interest, both societally and scientifically. It is within this setting that organic photocatalysts have emerged as a new generation of earth-abundant catalysts for the conversion of solar radiation into chemical energy. In 2014, the first example of a covalent organic framework (COF) photocatalyst for the hydrogen evolution reaction was reported by our group, which has not only marked the beginning of COF photocatalysis for solar fuel production but also helped to accelerate research into "soft photocatalysis" based on porous polymers in general. In the last decade, significant progress has been made toward developing COFs as robust, molecularly precise platforms emulating artificial photosynthesis. This mini-review commemorates the 10th anniversary of COF photocatalysis and gives a brief historical overview of the milestones in the field since its inception in 2014. We review milestones in the development of COFs for solar fuel production and related photocatalytic transformations, including hydrogen evolution, oxygen evolution, overall water splitting, CO2 reduction, N2 fixation, oxygen reduction, and alcohol oxidation. We discuss lessons learned for the design of structure-property-function relationships in COF photocatalysts, and future perspectives and challenges for the field of "soft photocatalysis" are given.

Synergistic Tri-efficiency Enhancement Utilizing Functionalized Covalent Organic Frameworks for Photocatalytic H2O2 Production

The overall maximization of photocatalytic H2O2 production efficiency urgently requires the comprehensive optimization of each step in multiplex photocatalysis. Despite numerous endeavors, isolated researches focusing on single efficiencies hinder further advancements in overall catalytic activity. In this work, a series of imine-linked COFs (TT-COF-X), incorporating electronically tunable functional groups (X = ─H, ─OMe, ─OH, ─Br), are rationally fabricated for visible-light-driven H2O2 production via a dual-channel pathway involving 2e- water oxidation and 2e- oxygen reduction. Combined simulations and characterizations reveal that the synergistic modification of functional groups for electronic conjugation and locally intramolecular polarity collectively enhanced light absorption, charge separation and transfer, and interface water-oxygen affinity efficiency. Notably, femtosecond time-resolved transient absorption (fs-TA) reveals that the polarity-induced built-in electric field play a crucial role in facilitating exciton dissociation by reacting BIEF-mediated shallow trapping state. The simultaneously optimal tri-efficiency ultimately results in the highest H2O2 production rate of 3406.25 µmol h-1 g-1 and apparent quantum yields of 8.1% of TT-COF-OH. This study offers an emerging strategy to rational design of photocatalysts from the comprehensive tri-efficiency-oriented perspective.

Accelerated Synthesis of TS-2 Zeolite via 2D Heterogeneous Nucleation for Efficient H2O2 Production

MEL type c is crucial for addressing energy and environmental crises, yet efficient synthesis remains a challenge due to thermodynamic and kinetic limitations. In this work, TS-2 as typical zeolite is successfully synthesized with high efficiency (12 h with 92% yield) by introducing titanate acid (TA) 2D nanosheet into a hydrothermal synthesis system. A newly defined TA/TS-2 heterostructure is precisely identified as being incorporated into the zeolite framework via a heterogeneous nucleation mechanism. Ab initio molecular dynamics simulations deeply revealed the nucleation and growth mechanisms of the TA/TS-2 heterostructure. The formation energy barrier of Ti─O─Si structural units (88 kJ mol-1) is much lower than that of Si─O─Si units (119 kJ mol-1), leading to more efficient growth of the Ti─O─Si structure. The polarized electronic properties of Ti─O─Si (negative LUMO orbital and larger polarization) further enhanced the reaction probability and stability of Ti─Si bonding. This obtained TA/TS-2 heterostructure also demonstrated superior activity for photocatalytic production of hydrogen peroxide, which can be attributed to the abundant conductive band holes and narrow bandgap. This research provides an effective strategy for using 2D nanosheets to accelerate zeolite production, as well as an in-depth molecular-level insight into the nucleation and growth processes.

Al-N3 Bridge Site Enabling Interlayer Charge Transfer Boosts the Direct Photosynthesis of Hydrogen Peroxide from Water and Air

Manipulating the electronic environment of the reactive center to lower the energy barrier of the rate-determining water oxidation step for boosting the direct generation of H2O2 from water, air, and sunlight is fascinating yet remains a grand challenge. Driven by a first-principles screening across a series of metal single atoms in carbon nitride, we report a class of an Al-N3 bridge site enabling interlayer charge transfer in carbon nitride nanotubes (CNNT-Al) for the highly efficient photosynthesis of H2O2 directly from water, oxygen, and sunlight. We demonstrate that the interlayered Al-N3 bridge site in CNNT-Al is able to activate the neighboring surface N atom for promoting the rate-determining step of the two-electron water oxidation to H2O2. It is also able to act as a bridge for enhancing the vertical interlaminar charge transfer due to the hybridization between the 3s and 3p states of the interstitial Al atom and the conduction band of two adjacent carbon nitride layers. Collectively, these factors lead to a highest photocatalytic mass activity of 1410.2 μmol g-1 h-1 (with a photocatalyst concentration of 1 g L-1) for direct photosynthesis of H2O2 out of all CN-based photocatalysts and a 7-fold higher solar-to-chemical conversion efficiency (0.73%) compared to that of the natural photosynthesis of typical plants (∼0.1%). Most importantly, the CNNT-Al-based flow reactor can steadily produce H2O2 for 200 h and be directly used for the on-site degradation of organic dye in water. The CNNT-Al-based flow reactor can also kill a 10 times higher concentration of bacteria in deionized water than that in natural water with 100% efficiency, which makes our design economically appealing for practical water treatment.

Nitrogen-Site Engineering in Covalent Organic Frameworks for H2O2 Photogeneration via Dual Channels of Indirect Two-Electron O2 Reduction

Photocatalytic H2O2 production is a green and sustainable route, but far from meeting the increasing demands of industrialization due to the rapid recombination of the photogenerated charge carriers and the sluggish reaction kinetics. Effective strategies for precisely regulating the photogenerated carrier behavior and catalytic activity to construct high-performance photocatalysts are urgently needed. Herein, a nitrogen-site engineering strategy, implying elaborately tuning the species and densities of nitrogen atoms, is applied for H2O2 photogeneration performance regulation. Different nitrogen heterocycles, such as pyridine, pyrimidine, and triazine units, are polymerized with trithiophene units, and five covalent organic frameworks (COFs) with distinct nitrogen species and densities on the skeletons are obtained. Fascinatingly, they photocatalyzed H2O2 production via dominated two-electron O2 reduction processes, including O2-O2 •‒-H2O2 and O2-O2 •‒-O2 1-H2O2 dual pathways. Just in the air and pure water, the multicomponent TTA-TF-COF with the maximum nitrogen densities triazine nitrogen densities exhibited the highest H2O2 production rate of 3343 µmol g-1 h-1, higher than most of other reported COFs. The theoretical calculation revealed the higher activity is due to the easy formation of O2 •‒ and O2 1 in different catalytic process. This study gives a new insight into designing photocatalysis at atomic level.

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.

A Dual-Active Covalent Triazine Framework Film for Efficient Visible-Light-Driven Hydrogen Peroxide Production

Photocatalytic hydrogen peroxide production from water and oxygen offers a clean and sustainable alternative to the conventional energy-intensive anthraquinone oxidation method. Compared to powdered covalent triazine frameworks (CTFs), the film morphology of CTFs provides better connectivity in 2D, yielding several advantages: more efficient connections between active sites, reduced electron-hole pair recombination, increased resistance to superoxide radical induced corrosion, and decreased light scattering. Leveraging these benefits, it has incorporated dual active sites for both the oxygen reduction reaction (ORR) and the water oxidation reaction (WOR) into a CTF film system. This dual-active CTF film demonstrated an exceptional hydrogen peroxide production rate of 19 460 µmol h⁻¹ m⁻2 after 1 h and 17 830 µmol h⁻¹ m⁻2 after 5 h under visible light irradiation (≥420 nm) without the need for sacrificial agents.

Constructing Electron-Rich Ru Clusters on Non-Stoichiometric Copper Hydroxide for Superior Biocatalytic ROS Scavenging to Treat Inflammatory Spinal Cord Injury

Traumatic spinal cord injury (SCI) represents a complex neuropathological challenge that significantly impacts the well-being of affected individuals. The quest for efficacious antioxidant and anti-inflammatory therapies is both a compelling necessity and a formidable challenge. Here, in this work, the innovative synthesis of electron-rich Ru clusters on non-stoichiometric copper hydroxide that contain oxygen vacancy defects (Ru/def-Cu(OH)2), which can function as a biocatalytic reactive oxygen species (ROS) scavenger for efficiently suppressing the inflammatory cascade reactions and modulating the endogenous microenvironments in SCI, is introduced. The studies reveal that the unique oxygen vacancies promote electron redistribution and amplify electron accumulation at Ru clusters, thus enhancing the catalytic activity of Ru/def-Cu(OH)2 in multielectron reactions involving oxygen-containing intermediates. These advancements endow the Ru/def-Cu(OH)2 with the capacity to mitigate ROS-mediated neuronal death and to foster a reparative microenvironment by dampening inflammatory macrophage responses, meanwhile concurrently stimulating the activity of neural stem cells, anti-inflammatory macrophages, and oligodendrocytes. Consequently, this results in a robust reparative effect on traumatic SCI. It is posited that the synthesized Ru/def-Cu(OH)2 exhibits unprecedented biocatalytic properties, offering a promising strategy to develop ROS-scavenging and anti-inflammatory materials for the management of traumatic SCI and a spectrum of other diseases associated with oxidative stress.

Structurally Locked High-Crystalline Covalent Triazine Frameworks Enable Remarkable Overall Photosynthesis of Hydrogen Peroxide

The development of green and efficient hydrogen peroxide (H2O2) production is of great interest but remains challenging. Herein, we develop a new and simple strategy via locking the coplanarity in highly crystalline covalent triazine frameworks (CTFs) to remarkably boost direct photosynthesis of H2O2 from oxygen and water. The exfoliated ultrathin 2D-CTF nanosheets exhibit excellent photocatalytic H2O2 evolution with an ultrahigh solar-to-chemical efficiency of 0.91% and a superb apparent quantum yield of 16.8% at 420 nm, surpassing all previous CTFs and most of the metal-free photocatalysts ever reported. Our detailed experimental and theoretical studies reveal that the spatially locked structure in the crystalline CTF photocatalyst can not only greatly enhance the separation and transfer of photoexcited charge-carriers for promoting H2O2 photogeneration but also alter the local electronic structures that unexpectedly turn water oxidation from a four-electron route to a two-electron pathway, resulting in a 100% atom utilization efficiency. This work provides valuable insights into the designed synthesis of highly efficient metal-free photocatalysts and precise control over photocatalytic reaction pathways in organic materials.

Integrating Multipolar Structures and Carboxyl Groups in sp2-Carbon Conjugated Covalent Organic Frameworks for Overall Photocatalytic Hydrogen Peroxide Production

The direct production of hydrogen peroxide (H2O2) through photocatalytic reaction via H2O and O2 is considered as an ideal approach. However, the efficiency of H2O2 generation is generally limited by insufficient charge and mass transfer. Covalent organic framework (COFs) offer a promising platform as metal-free photocatalyst for H2O2 production due to their potential for rational design at the molecular level. Herein, we integrated the multipolar structures and carboxyl groups into COFs to enhance the efficiency of photocatalytic H2O2 production in pure water without any sacrificial agents. The introduction of octupolar and quadrupolar structures, along with an increase of molecular planarity, created efficient oxygen reduction reaction (ORR) sites. Meanwhile, carboxyl groups could not only boost O2 and H2O2 movement via enhancement of pore hydrophilicity, but also promote proton conduction, enabling the conversion to H2O2 from ⋅O2 -, which is the crucial intermediate product in H2O2 photocatalysis. Overall, we demonstrate that TACOF-1-COOH, consisting of optimal octupolar and quadrupolar structures, along with enrichment sites (carboxyl groups), exhibited a H2O2 yield rate of 3542 μmol h- 1 g-1 and a solar-to-chemical (SCC) efficiency of 0.55 %. This work provides valuable insights for designing metal-free photocatalysts for efficient H2O2 production.

Variation of Chemical Microenvironment of Pores in Hydrazone-Linked Covalent Organic Frameworks for Photosynthesis of H2O2

Photocatalytic synthesis of H2O2 is an advantageous and ecologically sustainable alternative to the conventional anthraquinone process. However, achieving high conversion efficiency without sacrificial agents remains a challenge. In this study, two covalent organic frameworks (COF-O and COF-C) were prepared with identical skeletal structures but with their pore walls anchored to different alkyl chains. They were used to investigate the effect of the chemical microenvironment of pores on photocatalytic H2O2 production. Experimental results reveal a change of hydrophilicity in COF-O, leading to suppressed charge recombination, diminished charge transfer resistance, and accelerated interfacial electron transfer. An apparent quantum yield as high as 10.3 % (λ=420 nm) can be achieved with H2O and O2 through oxygen reduction reaction. This is among the highest ever reported for polymer photocatalysts. This study may provide a novel avenue for optimizing photocatalytic activity and selectivity in H2O2 generation.

Molecular Engineering of Methylated Sulfone-Based Covalent Organic Frameworks for Back-Reaction Inhibited Photocatalytic Overall Water Splitting

Solar-to-hydrogen (H2) and oxygen (O2) conversion via photocatalytic overall water splitting (OWS) holds great promise for a sustainable fuel economy, but has been challenged by the backward O2 reduction reaction (ORR) with favored proton-coupled electron transfer (PCET) dynamics. Here, we report that molecular engineering by methylation inhibits the backward ORR of molecular photocatalysts and enables efficient OWS process. As demonstrated by a benchmark sulfone-based covalent organic framework (COF) photocatalyst, the precise methylation of its O2 adsorption sites effectively blocks electron transfer and increases the barrier for hydrogen intermediate desorption that cooperatively obstructs the PCET process of ORR. Methylation also repels electrons to the neighboring photocatalytic sulfone group that promotes the forward H2 evolution. The resultant DS-COF achieves an impressive inhibition of about 70 % of the backward reaction and a three-fold enhancement of the OWS performance with a H2 evolution rate of 124.7 μmol h-1 g-1, ranking among the highest reported for organic-based photocatalysts. This work provides insights for engineering photocatalysts at the molecular level for efficient solar-to-fuel conversion.

Rigid covalent organic frameworks with thiazole linkage to boost oxygen activation for photocatalytic water purification

Owing to their capability to produce reactive oxygen species (ROS) under solar irradiation, covalent organic frameworks (COFs) with pre-designable structure and unique architectures show great potentials for water purification. However, the sluggish charge separation, inefficient oxygen activation and poor structure stability in COFs restrict their practical applications to decontaminate water. Herein, via a facile one-pot synthetic strategy, we show the direct conversion of reversible imine linkage into rigid thiazole linkage can adjust the π-conjugation and local charge polarization of skeleton to boost the exciton dissociation on COFs. The rigid linkage can also improve the robustness of skeleton and the stability of COFs during the consecutive utilization process. More importantly, the thiazole linkage in COFs with optimal C 2p states (COF-S) effectively increases the activities of neighboring benzene unit to directly modulate the O2-adsorption energy barrier and improve the ROS production efficiency, resulting in the excellent photocatalytic degradation efficiency of seven toxic emerging contaminants (e.g. degrading ~99% of 5 mg L-1 paracetamol in only 7 min) and effective bacterial/algal inactivation performance. Besides, COF-S can be immobilized in continuous-flow reactor and in enlarged reactor to efficiently eliminate pollutants under natural sunlight irradiation, demonstrating the feasibility for practical application.

Multi-channel electron transfer induced by polyvanadate in metal-organic framework for boosted peroxymonosulfate activation

Catalytic peroxymonosulfate (PMS) activation processes don't solely rely on electron transfer from dominant metal centers due to the complicated composition and interface environment of catalysts. Herein the synthesis of a cobalt based metal-organic framework containing polyvanadate [V4O12]4- cluster, Co2(V4O12)(bpy)2 (bpy = 4,4'-bipyridine), is presented. The catalyst demonstrates superior degradation activity toward various micropollutants, with higher highest occupied molecular orbital (HOMO), via nonradical attack. The X-ray absorption spectroscopy and density functional theory (DFT) calculations demonstrate that Co sites act as both PMS trapper and electron donor. In situ spectral characterizations and DFT calculations reveal that the terminal oxygen atoms in the [V4O12]4- electron sponge could interact with the terminal hydrogen atoms in PMS to form hydrogen bonds, promoting the generation of SO5* intermediate via both dynamic pull and direct electron transfer process. Further, Co2(V4O12)(bpy)2 exhibits long-term water purification ability, up to 40 h, towards actual wastewater discharged from an ofloxacin production factory. This work not only presents an efficient catalyst with an electron sponge for water environmental remediation via nonradical pathway, but also provides fundamental insights into the Fenton-like reaction mechanism.

Covalent-Organic Frameworks for Selective and Sensitive Detection of Antibiotics from Water

As the consumption of antibiotics rises, they have generated some negative impacts on organisms and the environment because they are often unable to be effectively degraded, and seeking effective detection methods is currently a challenge. Covalent-organic frameworks (COFs) are new types of crystalline porous crystals created based on the strong covalent interactions between blocked monomers, and COFs demonstrate great potential in the detection of antibiotics from aqueous solutions because of their large surface area, adjustable porosity, recyclability, and predictable structure. This review aims to present state-of-the-art insights into COFs (properties, classification, synthesis methods, and functionalization). The key mechanisms for the detection of antibiotics and the application performance of COFs in the detection of antibiotics from water are also discussed, followed by the challenges and opportunities for COFs in future research.

Protonation of an Imine-linked Covalent Organic Framework for Efficient H2O2 Photosynthesis under Visible Light up to 700 nm

Covalent organic frameworks (COFs) are promising photocatalysts for H2O2 production from water via oxygen reduction reaction (ORR). The design of COFs for efficient H2O2 production indubitably hinges on an in-depth understanding of their ORR mechanisms. In this work, taking an imine-linked COF as an example, we demonstrate that protonation of the functional units such as imine, amine, and triazine, is a highly efficient strategy to upgrade the activity levels for H2O2 synthesis. The protonation not only extends the light absorption of the COF but also provides proton sources that directly participate in H2O2 generation. Notably, the protonation simplifies the reaction pathways of ORR to H2O2, i.e. from an indirect superoxide radical (O 2 • - ${{O}_{2}^{\bullet -}}$ ) mediated route to a direct one-step two-electron route. Theoretical calculations confirm that the protonation favors H2O2 synthesis due to easy access of protons near the reaction sites that removes the energy barrier for generating *OOH intermediate. These findings not only extend the mechanistic insight into H2O2 photosynthesis but also provide a rational guideline for the design and upgradation of efficient COFs.

Stepwise Protonation of Three-Dimensional Covalent Organic Frameworks for Enhancing Hydrogen Peroxide Photosynthesis

Three-dimensional covalent organic frameworks (3D COFs), recognized for their tailorable structures and accessible active sites, offer a promising platform for developing advanced photocatalysts. However, the difficulty in the synthesis and functionalization of 3D COFs hinders their further development. In this study, we present a series of 3D-bcu-COFs with 8 connected porphyrin units linked by linear linkers through imine bonds as a versatile platform for photocatalyst design. The photoresponse of 3D-bcu-COFs was initially modulated by functionalizing linear linkers with benzo-thiadiazole or benzo-selenadiazole groups. Furthermore, taking advantage of the well-exposed porphyrin and imine sites in 3D-bcu-COFs, their photocatalytic activity was optimized by stepwise protonation of imine bonds and porphyrin centers. The dual protonated COF with benzo-selenadiazole groups exhibited enhanced charge separation, leading to an increased photocatalytic H2O2 production under visible light. This enhancement demonstrates the combined benefits of linker functionalization and stepwise protonation on photocatalytic efficiency.

Hydroxyl-Bonded Co Single Atom Site on Boroncarbonitride Surface Realizes Nonsacrificial H2O2 Synthesis in the Near-Infrared Region

Photocatalytic synthesis of hydrogen peroxide (H2O2) from O2 and H2O under near-infrared light is a sustainable renewable energy production strategy, but challenging reaction. The bottleneck of this reaction lies in the regulation of O2 reduction path by photocatalyst. Herein, the center of the one-step two-electron reduction (OSR) pathway of O2 for H2O2 evolution via the formation of the hydroxyl-bonded Co single-atom sites on boroncarbonitride surface (BCN-OH2/Co1) is constructed. The experimental and theoretical prediction results confirm that the hydroxyl group on the surface and the electronic band structure of BCN-OH2/Co1 are the key factor in regulating the O2 reduction pathway. In addition, the hydroxyl-bonded Co single-atom sites can further enrich O2 molecules with more electrons, which can avoid the one-electron reduction of O2 to •O2 -, thus promoting the direct two-electron activation hydrogenation of O2. Consequently, BCN-OH2/Co1 exhibits a high H2O2 evolution apparent quantum efficiency of 0.8% at 850 nm, better than most of the previously reported photocatalysts. This study reveals an important reaction pathway for the generation of H2O2, emphasizing that precise control of the active site structure of the photocatalyst is essential for achieving efficient conversion of solar-to-chemical.

Oxygen-Centered Organic Radicals-Involved Unified Heterogeneous Self-Fenton Process for Stable Mineralization of Micropollutants in Water

Removing organic micropollutants from water through photocatalysis is hindered by catalyst instability and substantial residuals from incomplete mineralization. Here, a novel water treatment paradigm, the unified heterogeneous self-Fenton process (UHSFP), which achieved an impressive 32% photon utilization efficiency at 470 nm, and a significant 94% mineralization of organic micropollutants-all without the continual addition of oxidants and iron ions is presented. In UHSFP, the active species differs fundamentally from traditional photocatalytic processes. One electron acceptor unit of photocatalyst acquires only one photogenerated electron to convert into oxygen-centered organic radical (OCOR), then spontaneously completing subsequent processes, including pollutant degradation, hydrogen peroxide generation, activation, and mineralization of organic micropollutants. By bolstering electron-transfer capabilities and diminishing catalyst affinity for oxygen in the photocatalytic process, the generation of superoxide radicals is effectively suppressed, preventing detrimental attacks on the catalyst. This study introduces an innovative and cost-effective strategy for the efficient and stable mineralization of organic micropollutants, eliminating the necessity for continuous chemical inputs, providing a new perspective on water treatment technologies.

Pyridine-Based Covalent Organic Frameworks with Pyridyl-Imine Structures for Boosting Photocatalytic H2O2 Production via One-Step 2e- Oxygen Reduction

Bipyridine-based covalent organic frameworks (COFs) have emerged as promising contenders for the photocatalytic generation of hydrogen peroxide (H2O2). However, the presence of imine nitrogen alters the mode of H2O2 generation from an efficient one-step two-electron (2e-) route to a two-step 2e- oxygen reduction pathway. In this work, we introduce 3,3'-bipyridine units into imine-based COF skeletons, creating a pyridyl-imine structure with two adjacent nitrogen atoms between the pyridine ring and imine linkage. This unique bipyridine-like architecture can effectively suppress the two-step 2e- ORR process at the single imine-nitrogen site, facilitating a more efficient one-step 2e- pathway. Consequently, the optimized pyridyl-imine COF (PyIm-COF) exhibits a remarkable H2O2 production rate of up to 5850 μmol h-1 g-1, nearly double that of pristine bipyridine COFs. This work provides valuable insight into the rational design of functionalized COFs for enhanced H2O2 production in photocatalysis.

Robust Covalent Organic Framework Photocatalysts for H2O2 Production: Linkage Position Matters

Covalent organic frameworks (COFs) are promising photocatalysts for hydrogen peroxide (H2O2) synthesis. However, the nature of organic polymers makes the balance between high activity and stability challenging. We demonstrate that the linkage position matters in the design of robust COF photocatalysts with durable high activity without sacrificial reagents. COFs with ortho- and para-linkages (o-COFs and p-COFs) were constructed by 1,3,5-triformylphloroglucinol with benzene-, pyridine-, pyrazine-orthodiamines and paradiamines. The pyrzaine-containing o-COFs with two pyridinic nitrogen atoms exhibited a H2O2 production rate of 4396 μmol g-1 h-1 together with long-time continuous H2O2 photosynthesis performance in pure water (48 h), superior to the corresponding p-COFs. A four-step reaction mechanism is proposed by density function calculations. Moreover, the active sites and origin of stability enhancement for o-COFs are clarified. This work provides a simple and effective molecular design strategy in the design of robust COF photocatalysts for artificial H2O2 photosynthesis.

Recent developments in photocatalytic production of hydrogen peroxide

Hydrogen peroxide (H2O2), an environmentally friendly strong oxidant and energy carrier, has attracted widespread attention in photocatalysis. Artificial photosynthesis of H2O2 using water and oxygen as raw materials, solar energy as an energy source, and semiconductor materials as catalysts is considered a promising technology. In the past few decades, encouraging progress has been made in the photocatalytic production of H2O2. Therefore, we summarize the research achievements in this field in recent years. This review first briefly introduces the reaction pathway, detection techniques and evaluation metrics. Then, the recent advances in photocatalysts are highlighted. Furthermore, the existing challenges and possible solutions in this field are presented. At last, we look forward to the future development direction of this field. This review provides valuable insights and guidance for efficient photocatalytic H2O2 production.