Designing covalent organic frameworks with Co-O(4) atomic sites for efficient CO(2) photoreduction

Co2+ coordination-assisted molecularly imprinted covalent organic framework for selective extraction of ochratoxin A

Covalent organic frameworks (COFs) are attractive materials for sample pretreatment due to their tunable structures and functions. However, the precise recognition of contaminant in complex environmental matrices by COFs remains challenging owing to their insufficient specific active sites. Herein, we report Co2+ coordination-assisted molecularly imprinted flexible COF (MI-COF@Co2+) for selective recognition of ochratoxin A (OTA). The MI-COF@Co2+ was prepared via one-step polymerization of 3,3-dihydroxybenzidine, 2,4,6-tris(4-formylphenoxy)- 1,3,5-triazine, Co2+ and template. The flexible units endowed COFs with the self-adaptable ability to regulate the molecular conformation and coordinate with Co2+ to locate the imprinted cavities. The coordination interaction greatly improved the adsorption capacity and selectivity of MI-COF@Co2+ for OTA. The prepared MI-COF@Co2+ was used as solid phase extraction adsorbent for high-performance liquid chromatography determination of OTA with the detection limit of 0.03 ng mL-1 and the relative standard deviation of < 2.5 %. In addition, this method permitted interference-free determination of OTA in real samples with recovery from 89.5 % to 102.8 %. This work provides a simple way to improve the selectivity of COFs for the determination of hazardous compounds in complex environments.

Asymmetric Atomic Dual-Sites for Photocatalytic CO2 Reduction

Atomically dispersed active sites in a photocatalyst offer unique advantages such as locally tuned electronic structures, quantum size effects, and maximum utilization of atomic species. Among these, asymmetric atomic dual-sites are of particular interest because their asymmetric charge distribution generates a local built-in electric potential to enhance charge separation and transfer. Moreover, the dual sites provide flexibility for tuning complex multielectron and multireaction pathways, such as CO2 reduction reactions. The coordination of dual sites opens new possibilities for engineering the structure-activity-selectivity relationship. This comprehensive overview discusses efficient and sustainable photocatalysis processes in photocatalytic CO2 reduction, focusing on strategic active-site design and future challenges. It serves as a timely reference for the design and development of photocatalytic conversion processes, specifically exploring the utilization of asymmetric atomic dual-sites for complex photocatalytic conversion pathways, here exemplified by the conversion of CO2 into valuable chemicals.

Dual-Atom P-Co-Dy Charge-Transfer Bridge on Black Phosphorus for Enhanced Photocatalytic CO2 Reduction

The synergistic effect of rare earth single-atoms and transition metal single-atoms may enable us to achieve some unprecedented performance and characteristics. Here, Co-Dy dual-atoms on black phosphorus with a P-Co-Dy charge-transfer bridge are designed and fabricated as the active center for the CO2 photoreduction reaction. The synergistic effect of Co-Dy on the performance of black phosphorus is studied by combining X-ray absorption spectroscopy, ultrafast spectral analysis, and in situ technology with DFT calculations. The results show that the Co and Dy bimetallic active site can promote charge transfer by the charge transfer bridge from P to Dy, and then to Co, thereby improving the photocatalytic activity of black phosphorus. The performance of catalysts excited at different wavelength light indicates that the 4G11/2/2I15/2/4F9/2→6H15/2 and 4F9/2→6H13/2 emissions of Dy can be absorbed by black phosphorus to improve the utilization of sunlight. The in situ DRIFTS and density functional theory (DFT) calculations are used to investigate the CO2 photoreduction pathway. This work provides an depth insight into the mechanism of dual-atom catalysts with enhanced photocatalytic performance, which helps to design novel atomic photocatalysts with excellent activity for CO2 reduction reactions.

Solar-Driven Conversion of CO2 to C2 Products by the 3d Transition Metal Intercalates of Layered Lead Iodides

Photocatalytic CO2 reduction to high-value-added C2+ products presents significant challenges, which is attributed to the slow kinetics of multi-e- CO2 photoreduction and the high thermodynamic barrier for C-C coupling. Incorporating redox-active Co2+/Ni2+ cations into lead halide photocatalysts has high potentials to improve carrier transport and introduce charge polarized bimetallic sites, addressing the kinetic and thermodynamic issues, respectively. In this study, a coordination-driven synthetic strategy is developed to introduce 3d transition metals into the interlamellar region of layered organolead iodides with atomic precision. The resultant bimetallic halide hybrids exhibit selective photoreduction of CO2 to C2H5OH using H2O vapor at the evolution rates of 24.9-31.4 µmol g-1 h-1 and high selectivity of 89.5-93.6%, while pristine layered lead iodide yields only C1 products. Band structure calculations and photoluminescence studies indicate that the interlayer Co2+/Ni2+ species greatly contribute to the frontier orbitals and enhance exciton dissociation into free carriers, facilitating carrier transport between adjacent lead iodide layers. In addition, Bader charge distribution calculations and in situ experimental spectroscopic studies reveal that the asymmetric Ni-O-Pb bimetallic catalytic sites exhibit intrinsic charge polarization, promoting C-C coupling and leading to the formation of the key *OC-CHO intermediate.

Molecular Engineering for Modulating Photocatalytic Hydrogen Evolution of Fully Conjugated 3D Covalent Organic Frameworks

Covalent organic frameworks (COFs) have recently shown great potential for photocatalytic hydrogen production. Currently almost all reports are focused on two-dimensional (2D) COFs, while the 3D counterparts are rarely explored due to their non-conjugated frameworks derived from the sp3 carbon based tetrahedral building blocks. Here, we rationally designed and synthesized a series of fully conjugated 3D COFs by using the saddle-shaped cyclooctatetrathiophene derivative as the building block. Through molecular engineering strategies, we thoroughly discussed the influences of key factors including the donor-acceptor structure, hydrophilicity, specific surface areas, as well as the conjugated/non-conjugated structures on their photocatalytic hydrogen evolution properties. The as-synthesized fully conjugated 3D COFs could generate the hydrogen up to 40.36 mmol h-1 g-1. This is the first report on intrinsic metal-free 3D COFs in photocatalytic hydrogen evolution application. Our work provides insight on the structure design of 3D COFs for highly-efficient photocatalysis, and also reveals that the semiconducting fully conjugated 3D COFs could be a useful platform in clear energy-related fields.

Efficient Photocatalytic CO2 Reduction in Ellagic Acid-Based Covalent Organic Frameworks

Employing covalent organic frameworks (COFs) for the photocatalytic CO2 reduction reaction (CDRR) to generate high-value chemical fuels and mitigate greenhouse gas emissions represents a sustainable catalytic conversion approach. However, achieving superior photocatalytic CDRR performance is hindered by the challenges of low charge separation efficiency, poor stability, and high preparation costs associated with COFs. Herein, in this work, we utilized perfluorinated metallophthalocyanine (MPcF16) and the organic biomolecule compound ellagic acid (EA) as building blocks to actualize functional covalent organic frameworks (COFs) named EPM-COF (M = Co, Ni, Cu). The designed EPCo-COF, featuring cobalt metal active sites, demonstrated an impressive CO production rate and selectivity in the photocatalytic CO2 reduction reaction (CDRR). Moreover, following alkaline treatment (EPCo-COF-AT), the COF exposed carboxylic acid anion (COO-) and hydroxyl group (OH), thereby enhancing the electron-donating capability of EA. This modification achieved a heightened CO production rate of 17.7 mmol g-1 h-1 with an outstanding CO selectivity of 97.8% in efficient photocatalytic CDRR. Theoretical calculations further illustrated that EPCo-COF-AT functionalized with COO- and OH can effectively alleviate the energy barriers involved in the CDRR process, which facilitates the proton-coupled electron transfer processes and enhances the photocatalytic performance on the cobalt active sites within EPCo-COF-AT.

Photosensitizing metal covalent organic framework with fast charge transfer dynamics for efficient CO2 photoreduction

Designing artificial photocatalysts for CO2 reduction is challenging, mainly due to the intrinsic difficulty of making multiple functional units cooperate efficiently. Herein, three-dimensional metal covalent organic frameworks (3D MCOFs) were employed as an innovative platform to integrate a strong Ru(ii) light-harvesting unit, an active Re(i) catalytic center, and an efficient charge separation configuration for photocatalysis. The photosensitive moiety was precisely stabilized into the covalent skeleton by using a rational-designed Ru(ii) complex as one of the building units, while the Re(i) center was linked via a shared bridging ligand with an Ru(ii) center, opening an effective pathway for their electronic interaction. Remarkably, the as-synthesized MCOF exhibited impressive CO2 photoreduction activity with a CO generation rate as high as 1840 μmol g-1 h-1 and 97.7% selectivity. The femtosecond transient absorption spectroscopy combined with theoretical calculations uncovered the fast charge-transfer dynamics occurring between the photoactive and catalytic centers, providing a comprehensive understanding of the photocatalytic mechanism. This work offers in-depth insight into the design of MCOF-based photocatalysts for solar energy utilization.

Bipolaronic Motifs Induced Spatially Separated Catalytic Sites for Tunable Syngas Photosynthesis From CO2

Photocatalytic reduction of CO2 into syngas is a promising way to tackle the energy and environmental challenges; however, it remains a challenge to achieve reaction decoupling of CO2 reduction and water splitting. Therefore, efficient production of syngas with a suitable CO/H2 ratio for Fischer-Tropsch synthesis can hardly be achieved. Herein, bipolaronic motifs including Co(II)-pyridine N motifs and Co(II)-imine N motifs are rationally designed into a crystalline imine-linked 1,10-phenanthroline-5,6-dione-based covalent organic framework (bp-Co-COF) with a triazine core. These featured structures with spatially separated active sites exhibit efficient photocatalytic performance toward CO2-to-syngas conversion with a suitable CO/H2 ratio (1:1-1:3). The bipolaronic motifs enable a highly separated electron-hole state, whereby the Co(II)-pyridine N motifs tend to be the active sites for CO2 activation and accelerate the hydrogenation to form *COOH intermediates; whilst, the Co(II)-imine N motifs increase surface hydrophilicity for H2 evolution. The photocatalytic reductions of CO2 and H2O thus decouple and proceed via a concerted way on the bipolaronic motifs of bp-Co-COF. The optimal bp-Co-COF photocatalyst achieves a high syngas evolution rate of 15.8 mmol g-1 h-1 with CO/H2 ratio of 1:2, outperforming previously reported COF-based photocatalysts.

Asymmetrical Interactions between Ni Single Atomic Sites and Ni Clusters in a 3D Porous Organic Framework for Enhanced CO2 Photoreduction

3D porous organic frameworks, which possess the advantages of high surface area and abundant exposed active sites, are considered ideal platforms to accommodate single atoms (SAs) and metal nanoclusters (NCs) in high-performance catalysts; however, very little research has been conducted in this field. In the present work, a 3D porous organic framework containing Ni1 SAs and Nin NCs is prepared through the metal-assisted one-pot polycondensation of tetraaldehyde and hexaaminotriptycene. The single metal sites and metal clusters confined in the 3D space created a favorable micro-environment that facilitated the activation of chemically inert CO2 molecules, thus promoting the overall photoconversion efficiency and selectivity of CO2 reduction. The 3D-NiSAs/NiNCs-POPs, as a CO2 photoreduction catalyst, demonstrated an exceptional CO production rate of 6.24 mmol g-1 h-1, high selectivity of 98%, and excellent stability. The theoretical calculations uncovered that asymmetrical interaction between Ni1 SAs and Nin NCs not only favored the bending of CO2 molecules and reducing the CO2 reduction energy, but also regulated the electronic structure of the catalyst leading to the optimal binding strength of intermediates.

Hydrogen-Bond-Network Breakdown Boosts Selective CO2 Photoreduction by Suppressing H2 Evolution

Conventional strategies for highly efficient and selective CO2 photoreduction focus on the design of catalysts and cocatalysts. In this study, we discover that hydrogen bond network breakdown in reaction system can suppress H2 evolution, thereby improving CO2 photoreduction performance. Photosensitive poly(ionic liquid)s are designed as photocatalysts owing to their strong hydrogen bonding with solvents. The hydrogen bond strength is tuned by solvent composition, thereby effectively regulating H2 evolution (from 0 to 12.6 mmol g-1 h-1). No H2 is detected after hydrogen bond network breakdown with trichloromethane or tetrachloromethane as additives. CO production rate and selectivity increase to 35.4 mmol g-1 h-1 and 98.9 % with trichloromethane, compared with 0.6 mmol g-1 h-1 and 26.2 %, respectively, without trichloromethane. Raman spectroscopy and theoretical calculations confirm that trichloromethane broke the systemic hydrogen bond network and subsequently suppressed H2 evolution. This hydrogen bond network breakdown strategy may be extended to other catalytic reactions involving H2 evolution.

Boosting CO2 Electroreduction over a Covalent Organic Framework in the Presence of Oxygen

Herein, we propose an oxygen-containing species coordination strategy to boost CO2 electroreduction in the presence of O2. A two-dimensional (2D) conjugated metal-covalent organic framework (MCOF), denoted as NiPc-Salen(Co)2-COF that is composed of the Ni-phthalocyanine (NiPc) unit with well-defined Ni-N4-O sites and the salen(Co)2 moiety with binuclear Co-N2O2 sites, is developed and synthesized for enhancing the CO2RR under aerobic condition. In the presence of O2, one of the Co sites in the NiPc-Salen(Co)2-COF that coordinated with the intermediate of *OOH from ORR could decrease the energy barrier of the activation of CO2 molecules and stabilize the key intermediate *COOH of the CO2RR over the adjacent Co center. Besides, the oxygen species axially coordinated Ni-N4-O sites can favor in reducing the energy barrier of the intermediate *COOH formation for the CO2RR. Thus, NiPc-Salen(Co)2-COF exhibits high oxygen-tolerant CO2RR performance and achieves outstanding CO Faradaic efficiency (FECO) of 97.2 % at -1.0 V vs. the reversible hydrogen electrode (RHE) and a high CO partial current density of 40.3 mA cm-2 at -1.1 V in the presence of 0.5 % O2, which is superior to that in pure CO2 feed gas (FECO=94.8 %, jCO=19.9 mA cm-2). Notably, the NiPc-Salen(Co)2-COF achieves an industrial-level current density of 128.3 mA cm-2 in the flow-cell reactor with 0.5 % O2 at -0.8 V, which is higher than that in pure CO2 atmosphere (jCO=104.8 mA cm-2). It is worth noting that an excellent FECO of 86.8 % is still achieved in the presence of 5 % O2 at -1.0 V. This work provides an effective strategy to enable the CO2RR under O2 atmosphere by utilizing the *OOH intermediates of ORR to boost CO2 electroreduction.

Designable Photo-Responsive Micron-Scale Ultrathin Peptoid Nanobelts for Enhanced Performance on Hydrogen Evolution Reaction

The development of high-reactive single-atom catalysts (SACs) based on long-range-ordered ultrathin organic nanomaterials (UTONMs) (i.e., below 3 nm) provides a significant tactic for the advancement in hydrogen evolution reactions (HER) but remains challenging. Herein, photo-responsive ultrathin peptoid nanobelts (UTPNBs) with a thickness of ≈2.2 nm and micron-scaled length are generated using the self-assembly of azobenzene-containing amphiphilic ternary alternating peptoids. The pendants hydrophobic conjugate stacking mechanism reveals the formation of 1D ultralong UTPNBs, whose thickness is dictated by the length of side groups that are linked to peptoid backbones. The photo-responsive feature is demonstrated by a reversible morphological transformation from UTPNBs to nanospheres (21.5 nm) upon alternative irradiation with UV and visible lights. Furthermore, the electrocatalyst performance of these aggregates co-decorated with nitrogen-rich ligand of terpyridine (TE) and uniformly-distributed atomic platinum (Pt) is evaluated toward HER, with a photo-controllable electrocatalyst activity that highly depended on both the presence of Pt element and structural characteristic of substrates. The Pt-based SACs using TE-modified UTPNBs as support exhibit a favorable electrocatalytic capacity with an overpotential of ≈28 mV at a current density of 10 mA cm-2. This work presents a promising strategy to fabricate stimuli-responsive UTONMs-based catalysts with controllable HER catalytic performance.

Nanostructurally Engineering Covalent Organic Frameworks for Boosting CO2 Photoreduction

Herein, a series of imine-linked covalent organic frameworks (COFs) are developed with advanced ordered mesoporous hollow spherical nanomorphology and ultra-large mesopores (4.6 nm in size), named OMHS-COF-M (M = H, Co, and Ni). The ordered mesoporous hollow spherical nanomorphology is revealed to be formed via an Ostwald ripening mechanism based on a one-step self-templated strategy. Encouraged by its unique structural features and outstanding photoelectrical property, the OMHS-COF-Co material is applied as the photocatalyst for CO2-to-CO reduction. Remarkably, it delivers an impressive CO production rate as high as 15 874 µmol g-1 h-1, a large selectivity of 92.4%, and a preeminent cycling stability. From in/ex situ experiments and density functional theory (DFT) calculations, the excellent CO2 photoreduction performance is ascribed to the desirable cooperation of unique ordered mesoporous hollow spherical host and abundant isolated Co active sites, enhancing CO2 activation, and improving electron transfer kinetics as well as reducing the energy barriers for intermediates *COOH generation and CO desorption.

Multisite Cocatalysis: Single atomic Pt2+/Pt0 active sites synergistically improve the simulated sunlight driven H2O-to-H2 conversion performance of Sb2S3 nanorods

Single atomic metal (SAM) cocatalysis is a potential strategy to improve the performance of photocatalytic materials. However, the cocatalytic mechanism of SAM sites in different valence states is rarely reported. Herein, single atomic Pt2+/Pt0 active sites were anchored on Sb2S3 nanorods to synergistically improve the photoactivity for hydrogen production under simulated sunlight. Experimental results and density functional theory calculations indicated that the coexistence of single atomic Pt2+/Pt0 sites synergistically improves the broadband light harvesting and promotes the Sb2S3-to-Pt electron transfer following inhibited photoexciton recombination kinetics and enhanced H proton adsorption capacity, resulting higher and more durable photoactivity for hydrogen production. Therefore, the optimal Sb2S3-Pt0.9‰ composite catalyst achieved remarkably enhanced hydrogen evolution rate of 1.37 mmol∙g-1∙h-1 (about 105-fold greater of that of Sb2S3 NRs) under faintly alkaline condition, and about 5.41 % of apparent quantum yield (AQY700 nm) was achieved, which shows obvious superiority in hydrogen production by contrasting with the reported Sb2S3-based photocatalysts and conventional semiconductor photocatalytic materials modified with noble metals. This study elucidate a well-defined mechanism of multisite cocatalysis for photoactivity improvement.

Photocatalytic CO2 reduction by topologically matched polymer-polymer heterojunction nanosheets

Conversion of solar energy into chemical fuel can be achieved through a number of routes but direct conversion, via photocatalysis, is potentially the simplest and cheapest route to the transformation of low-value substances, water and CO2, to useful chemical fuels or feedstocks such as hydrogen, formate, methanol, and syngas. 2D polymers, including carbon nitrides and COFs, have emerged as one of the most promising classes of organic photocatalysts for solar fuels production due to their energetic tunability, charge transport properties and robustness. They are, however, difficult to process and so there have been limited studies into the formation of heterojunction materials incorporating these components. In this work we use our novel templating approach to combine topologically matched imine-based donor polymers with acceptor polymers formed through Knoevenagel condensation. An efficient heterojunction interface was formed by matching the isostructural nodes and linkers that make up the D1 and A1 semiconductors and this was reflected in the increased photocatalytic activity of the heterojunction material T1. Tuning of the templating synthesis route to give heterojunctions with optimised donor : acceptor ratios, as well as the photocatalytic conditions, resulted in CO production rates that were between 1.5 and 10 times higher than those of the individual polymers. A further set of polymers A5 and D5 were developed with more optimised structures for CO2 reduction including increased overpotential for the reduction reaction and the presence of co-catalyst chelating groups. These had increased activity compared to the group 1 family and again showed higher activity for CO production by the templated heterojunction, T5, than either individual component or a physical mixture of the donor and acceptor.

A comprehensive review on effective parameters on microalgae productivity and carbon capture rate

Rising carbon emissions caused by population growth and industrialization is a significant environmental challenge in various countries. To combat this issue, Renewable Energy (RE) and Carbon Capture and Storage (CCS) technologies should be commercialized to reduce Greenhouse Gas (GHG) emissions and generate carbon-free energy. One such technology is the use of microalgae, which can directly capture CO2 from the air through photosynthesis and potentially produce biofuels due to their high energy content. However, the carbon capture rate of microalgae varies globally due to numerous parameters and variables affecting microalgae productivity. Additionally, microalgae productivity and carbon capture formulas yield different results worldwide, especially in outdoor industrial-scale cultivation. This research aims to comprehensively review the effective variables and parameters in carbon capture by microalgae based on microalgae productivity and carbon capture formulas. The research also ranked countries based on CO2 production in four different categories to determine whether the biggest carbon producer countries could exhibit suitable weather conditions for microalgae cultivation. Findings reveal optimal ranges of critical variables in the microalgae growth formula, including temperature, solar radiation intensity, Photon Flux Density (PFD), and sunlight duration. The study also analyzes microalgae cultivation, carbon capture, and oxygen production formula in different systems such as Open Ponds (OP), Tubular Photobioreactors (TPBR), and Flat Plate Photobioreactors (FPPBR), while discussing other influential parameters. In conclusion, emphasizing the adjustment and utilization of optimal values of effective parameters in microalgae cultivation not only holds promise for future carbon capture by microalgae but also pushes human beings toward sustainable development goals.

Engineering 0D/2D Architecture of Ni(OH)2 Nanoparticles on Covalent Organic Framework Nanosheets for Selective Visible-Light-Driven CO2 Reduction

Low-dimensional materials serving as photocatalysts favor providing abundant unsaturated active sites and shortening the charge transport distance, but the high surface energy readily causes the aggregation that limits their application. Herein, it is demonstrated that 2D covalent organic framework (COF) TpBD nanosheets are effective in the dispersion and stabilization of 0D Ni(OH)2 . The COF precursor TpBD is synthesized from the Schiff base condensation of 1,3,5-triformylphloroglucinol (Tp) and benzidine (BD) and exfoliated into 2D nanosheets named BDNs via ultrasonication. The formation of highly dispersive 0D Ni(OH)2 on BDNs is reached under a mild weak basic condition, enabling robust active sites for CO2 adsorption/activation and rapid interface cascaded electron transport channels for the accumulation of long-lived photo-generated charges. The champion catalyst 30%Ni-BDNs effectively catalyze the CO2 to CO conversion under visible-light irradiation, offering a high CO evolution rate of 158.4 mmol g-1 h-1 and turnover frequency of 51 h-1 . By contrast, the counterpart photocatalyst, the bulk TpBD stabilized Ni(OH)2 , affords a much lower CO evolution rate and selectivity. This work demonstrates a new avenue to simultaneously construct efficient active sites and electron transport channels by coupling 0D metal hydroxides and 2D COF nanosheets for CO2 photoreduction.

Pyrolysis Free Out-of-Plane Co-Single Atomic Sites in Porous Organic Photopolymer Stimulates Solar-Powered CO2 Fixation

Herein, a facile strategy is illustrated to develop pyrolysis-free out-of-plane coordinated single atomic sites-based M-POP via a one-pot Friedel Craft acylation route followed by a post-synthetic metalation. The optimized geometry of the Co@BiPy-POP clearly reveals the presence of out-of-plane Co-single atomic sites in the porous backbone. This novel photopolymer Co@BiPy-POP shows extensive π-conjugations followed by impressive light harvesting ability and is utilized for photochemical CO2 fixation to value-added chemicals. A remarkable conversion of styrene epoxide (STE) to styrene carbonate (STC) (≈98%) is obtained under optimized photocatalytic conditions in the existence of promoter tert-butyl ammonium bromide (TBAB). Synchrotron-based X-ray adsorption spectroscopy (XAS) analysis reveals the single atom coordination sites along with the metal (Co) oxidation number of +2.16 in the porous network. Moreover, in situ diffuse reflectance spectroscopy (DRIFTS) and electron paramagnetic resonance (EPR) investigations provide valuable information on the evolution of key reaction intermediates. Comprehensivecomputational analysis also helps to understand the overall mechanistic pathway along with the interaction between the photocatalyst and reactants. Overall, this study presents a new concept of fabricating porous photopolymers based on a pyrolysis-free out-of-plane-coordination strategy and further explores the role of single atomic sites in carrying out feasible CO2 fixation reactions.

Single-Atom Catalysts on Covalent Organic Frameworks for Energy Applications

Single-atom catalysts (SACs) have been investigated and applied to energy conversion devices. However, issues of metal agglomeration, low metal loading, and substrate stability have hindered realization of the SACs' full potential. Recently, covalent organic framework (COF)-based SACs have emerged as promising materials to enable highly efficient catalytic reactions. Here, we summarize the representative COF-based SACs and their wide application in clean energy devices and conversion reactions, such as hydrogen evolution reaction, carbon dioxide reduction reaction, nitrogen reduction reaction, oxygen reduction reaction, and oxygen evolution reaction. Based on their catalysis conditions, these reactions are categorized into photocatalyzed and electrocatalyzed reactions. We also summarize their design strategies, including heteroatom inclusion, donor-acceptor pairs, pore engineering, interface engineering, etc. Although COF-based SACs are promising, more efforts, such as linkage engineering, functional groups, ionization, multifunctional sites for cocatalyzed systems, etc., could improve them to be the ideal SAC materials. At the end, we provide our perspectives on where the field will proceed in the next 5 years.

Symmetrical Localized Built-in Electric Field by Induced Polarization Effect in Ionic Covalent Organic Frameworks for Selective Imaging and Killing Bacteria

Photocatalytic materials are some of the most promising substitutes for antibiotics. However, the antibacterial efficiency is still inhibited by the rapid recombination of the photogenerated carriers. Herein, we design a cationic covalent organic framework (COF), which has a symmetrical localized built-in electric field due to the induced polarization effect caused by the electron-transfer reaction between the Zn-porphyrin unit and the guanidinium unit. Density functional theory calculations indicate that there is a symmetrical electrophilic/nucleophilic region in the COF structure, which results from increased electron density around the Zn-porphyrin unit. The formed local electric field can further inhibit the recombination of photogenerated carriers by driving rapid electron transfer from Zn-porphyrin to guanidinium under light irradiation, which greatly increases the yield of reactive oxygen species. This COF wrapped by DSPE-PEG2000 can selectively target the lipoteichoic acid of Gram-positive bacteria by electrostatic interaction, which can be used for selective discrimination and imaging of bacteria. Furthermore, this nanoparticle can rapidly kill Gram-positive bacteria including 99.75% of Staphylococcus aureus and 99.77% of Enterococcus faecalis at an abnormally low concentration (2.00 ppm) under light irradiation for 20 min. This work will provide insight into designing photoresponsive COFs through engineering charge behavior.

Ultra-fast supercritically solvothermal polymerization for large single-crystalline covalent organic frameworks

Crystalline polymer materials, e.g., hyper-crosslinked polystyrene, conjugate microporous polymers and covalent organic frameworks, are used as catalyst carriers, organic electronic devices and molecular sieves. Their properties and applications are highly dependent on their crystallinity. An efficient polymerization strategy for the rapid preparation of highly or single-crystalline materials is beneficial not only to structure-property studies but also to practical applications. However, polymerization usually leads to the formation of amorphous or poorly crystalline products with small grain sizes. It has been a challenging task to efficiently and precisely assemble organic molecules into a single crystal through polymerization. To address this issue, we developed a supercritically solvothermal method that uses supercritical carbon dioxide (sc-CO2) as the reaction medium for polymerization. Sc-CO2 accelerates crystal growth due to its high diffusivity and low viscosity compared with traditional organic solvents. Six covalent organic frameworks with different topologies, linkages and crystal structures are synthesized by this method. The as-synthesized products feature polarized photoluminescence and second-harmonic generation, indicating their high-quality single-crystal nature. This method holds advantages such as rapid growth rate, high productivity, easy accessibility, industrial compatibility and environmental friendliness. In this protocol, we provide a step-by-step procedure including preparation of monomer dispersion, polymerization in sc-CO2, purification and characterization of the single crystals. By following this protocol, it takes 1-5 min to grow sub-mm-sized single crystals by polymerization. The procedure takes ~4 h from preparation of monomer dispersion and polymerization in sc-CO2 to purification and drying of the product.

Rational design of bimetallic sites in covalent organic frameworks for efficient photocatalytic oxidative coupling of amines

The conversion of organic compounds by photocatalysis under mild conditions is an environment-friendly alternative for organic transformations. In this work, the bimetallic covalent organic framework coordinated by Sr2+ and Fe2+ in the porphyrin centers with molar ratio of 2:1 (COF-Sr2Fe1) was synthesized through a two-step reaction. Under the synergistic regulation of Sr2+ and Fe2+, the separation of photogenerated charges and visible light absorption for COF-Sr2Fe1 were significantly promoted, and thus COF-Sr2Fe1 exhibited efficient photocatalytic performance towards benzylamine oxidative coupling reaction with a yield of 97 %, much higher than that of the nonmetallic covalent organic framework COF-366. Moreover, it was found that the Fe site displayed higher dehydrogenation ability and the Sr site displayed higher CN coupling ability through the density functional theory (DFT) calculations, thereby making the dehydrogenation and CN coupling steps more controllable for benzylamine oxidative coupling reaction by COF-Sr2Fe1. This work provides a strategy for designing efficient covalent organic frameworks photocatalysts, and helps to understand the oxidative coupling of amines more deeply.

Construction and characterization of a magnetic nanoparticle-supported Cu complex: a stable and active nanocatalyst for synthesis of heteroaryl-aryl and di-heteroaryl sulfides

Diaryl and di-heteroaryl sulfides exist in the structure of many drugs and important biological compounds, also these compounds are well-known in medicinal chemistry due to important biological and pharmaceutical activities. Therefore, the development of novel, ecofriendly and efficient catalytic systems for the preparation of diaryl and di-heteroaryl sulfides is a very attractive and important challenge in organic synthesis. In this attractive methodology, we wish to introduce Fe3O4-supported 3-amino-4-mercaptobenzoic acid copper complex (Fe3O4@AMBA-CuI) nanomaterials as a novel and efficient magnetically recoverable catalyst for the preparation of heteroaryl-aryl and di-heteroaryl sulfides with high yields through reaction of heteroaryl halides with aryl or heteroaryl boronic acids and S8 as the sulfur source under ecofriendly conditions. This catalytic system was very efficient and practical for a diverse range of heteroaryl substrates including benzothiazole, benzoxazole, benzimidazole, oxadiazole, benzofuran, and imidazo[1,2-a]pyridine, because the desired diaryl and di-heteroaryl sulfides were prepared with high yields. The reusability-experiments revealed that the Fe3O4@AMBA-CuI nanocatalyst can be magnetically separated and reused at least six times without a significant decrease in its catalytic activity. VSM and ICP-OES analyses confirmed that despite using the Fe3O4@AMBA-CuI nanocatalyst 6 times, the magnetic properties and stability of the catalyst were still maintained. Although all the obtained heteroaryl-aryl and di-heteroaryl sulfide products are known and previously reported, the synthesis of this number of heteroaryl-aryl and di-heteroaryl sulfides has never been reported by any previouse methods.

Coralloid W18O49@covalent organic frameworks S-scheme heterojunction for high efficiency photocatalytic aerobic oxidation

Reasonably designing and constructing efficient artificial S-mechanism photocatalysts, expanding their application in the field of photocatalytic organic synthesis, have become a hot and challenging topic in the photocatalysis. Herein, a series of coral-like W18O49@TpPa-H (TpPa-H represents COFs generated by the reaction of 1,3,5-triformylphloroglucinol (Tp) and p-phenylenediamine (Pa-H)) composites were successfully prepared by using a simple in-situ encapsulation strategy. Given the internal electric field at the S-scheme interface, W18O49 acts as an oxidative photocatalyst with sufficient positive valence band (VB) position and TpPa-H as a reductive one with enough negative conduction band (CB) position for the efficient amines oxidative coupling to imines. The resulting W18O49@TpPa-H-0.1 hybrid material shows both optimal benzylamine to imine conversion and selectivity exceeding 99 % within 4 h under 10 W 420 nm LED light irradiation, which is 9.9 and 2.8 fold greater than that of W18O49 and TpPa-H, respectively. The photocatalytic activity is even extended to 740 nm. Furthermore, the photocatalytic mechanism research confirmed that a high efficiency S-scheme heterojunction was formed between W18O49 and TpPa-H, and multiple active species, such as ·O2-, 1O2, and h+, synergistically participated in the reaction, imparting its excellent photocatalytic performance. This work may open new avenues for the development of high-efficiency COFs-based S-scheme heterojunction for organic photosynthesis.

Electrocatalytic Lignin Valorization into Aromatic Products via Oxidative Cleavage of Cα-Cβ Bonds

Lignin is the most promising candidate for producing aromatic compounds from biomass. However, the challenge lies in the cleavage of C-C bonds between lignin monomers under mild conditions, as these bonds have high dissociation energy. Electrochemical oxidation, which allows for mild cleavage of C-C bonds, is considered an attractive solution. To achieve low-energy consumption in the valorization of lignin, the use of highly efficient electrocatalysts is essential. In this study, a meticulously designed catalyst consisting of cobalt-doped nickel (oxy)hydroxide on molybdenum disulfide heterojunction was developed. The presence of molybdenum in a high valence state promoted the adsorption of tert-butyl hydroperoxide, leading to the formation of critical radical intermediates. In addition, the incorporation of cobalt doping regulated the electronic structure of nickel, resulting in a lower energy barrier. As a result, the heterojunction catalyst demonstrated a selectivity of 85.36% for cleaving the Cα-Cβ bond in lignin model compound, achieving a substrate conversion of 93.69% under ambient conditions. In addition, the electrocatalyst depolymerized 49.82 wt% of soluble fractions from organosolv lignin (OL), resulting in a yield of up to 13 wt% of aromatic monomers. Significantly, the effectiveness of the prepared electrocatalyst was also demonstrated using industrial Kraft lignin (KL). Therefore, this research offers a practical approach for implementing electrocatalytic oxidation in lignin refining.

Understanding Bridging Sites and Accelerating Quantum Efficiency for Photocatalytic CO2 Reduction

We report a novel double-shelled nanoboxes photocatalyst architecture with tailored interfaces that accelerate quantum efficiency for photocatalytic CO2 reduction reaction (CO2RR) via Mo-S bridging bonds sites in Sv-In2S3@2H-MoTe2. The X-ray absorption near-edge structure shows that the formation of Sv-In2S3@2H-MoTe2 adjusts the coordination environment via interface engineering and forms Mo-S polarized sites at the interface. The interfacial dynamics and catalytic behavior are clearly revealed by ultrafast femtosecond transient absorption, time-resolved, and in situ diffuse reflectance-Infrared Fourier transform spectroscopy. A tunable electronic structure through steric interaction of Mo-S bridging bonds induces a 1.7-fold enhancement in Sv-In2S3@2H-MoTe2(5) photogenerated carrier concentration relative to pristine Sv-In2S3. Benefiting from lower carrier transport activation energy, an internal quantum efficiency of 94.01% at 380 nm was used for photocatalytic CO2RR. This study proposes a new strategy to design photocatalyst through bridging sites to adjust the selectivity of photocatalytic CO2RR.

Pulsed-Laser-Triggered Piezoelectric Photocatalytic CO2 Reduction over Tetragonal BaTiO3 Nanocubes

The recombination of photoinduced carriers in photocatalysts is considered one of the biggest barriers to the increase of photocatalytic efficiency. Piezoelectric photocatalysts open a new route to realize rapid carrier separation by mechanically distorting the lattice of piezoelectric nanocrystals to form a piezoelectric potential within the nanocrystals, generally requiring external force (e.g., ultrasonic radiation, mechanical stirring, and ball milling). In this study, a low-power UV pulsed laser (PL) (3 W, 355 nm) as a UV light source can trigger piezoelectric photocatalytic CO2 reduction of tetragonal BaTiO3 (BTO-T) in the absence of an applied force. The tremendous transient light pressure (5.7 × 107  Pa, 2.7 W) of 355 nm PL not only bends the energy band of BTO-T, thus allowing reactions that cannot theoretically occur to take place, but also induces a pulsed built-in electric field to determine an efficient photoinduced carrier separation. On that basis, the PL-triggered piezoelectric photocatalytic CO2 reduction realizes the highest reported performance, reaching a millimole level CO yield of 52.9 mmol g-1 h-1 and achieving efficient photocatalytic CO2 reduction in the continuous catalytic system. The method in this study is promising to contribute to the design of efficient piezoelectric photocatalytic reactions.

Regulating the Lithium Ions' Local Coordination Environment through Designing a COF with Single Atomic Co Site to Achieve Dendrite-Free Lithium-Metal Batteries

The detrimental growth of lithium dendrites and unstable solid electrolyte interphase (SEI) inhibit the practical application of lithium-metal batteries. Herein, atomically dispersed cobalt coordinate conjugated bipyridine-rich covalent organic framework (sp2 c-COF) is explored as an artificial SEI on the surface of the Li-metal anode to resolve these issues. The single Co atoms confined in the structure of COF enhance the number of active sites and promote electron transfer to the COF. The synergistic effects of the Co─N coordination and strong electron-withdrawing cyano-group can adsorb the electron from the donor (Co) at a maximum and create an electron-rich environment, hence further regulating the Li+ local coordination environment and achieving uniform Li-nucleation behavior. Furthermore, in situ technology and density functional theory calculations reveal the mechanism of the sp2 c-COF-Co inducing Li uniform deposition and promoting Li+ rapid migration. Based on these advantages, the sp2 c-COF-Co modified Li anode exhibits a low Li-nucleation barrier of 8 mV, and excellent cycling stability of 6000 h.

Performance Regulation of Single-Atom Catalyst by Modulating the Microenvironment of Metal Sites

Metal-based catalysts, encompassing both homogeneous and heterogeneous types, play a vital role in the modern chemical industry. Heterogeneous metal-based catalysts usually possess more varied catalytically active centers than homogeneous catalysts, making it challenging to regulate their catalytic performance. In contrast, homogeneous catalysts have defined active-site structures, and their performance can be easily adjusted by modifying the ligand. These characteristics lead to remarkable conceptual and technical differences between homogeneous and heterogeneous catalysts. As a recently emerging class of catalytic material, single-atom catalysts (SACs) have become one of the most active new frontiers in the catalysis field and show great potential to bridge homogeneous and heterogeneous catalytic processes. This review documents a brief introduction to SACs and their role in a range of reactions involving single-atom catalysis. To fully understand process-structure-property relationships of single-atom catalysis in chemical reactions, active sites or coordination structure and performance regulation strategies (e.g., tuning chemical and physical environment of single atoms) of SACs are comprehensively summarized. Furthermore, we discuss the application limitations, development trends and future challenges of single-atom catalysis and present a perspective on further constructing a highly efficient (e.g., activity, selectivity and stability), single-atom catalytic system for a broader scope of reactions.

High-Crystallinity BiOCl Nanosheets as Efficient Photocatalysts for Norfloxacin Antibiotic Degradation

Semiconductor photocatalysts are essential materials in the field of environmental remediation. Various photocatalysts have been developed to solve the contamination problem of norfloxacin in water pollution. Among them, a crucial ternary photocatalyst, BiOCl, has attracted extensive attention due to its unique layered structure. In this work, high-crystallinity BiOCl nanosheets were prepared using a one-step hydrothermal method. The obtained BiOCl nanosheets showed good photocatalytic degradation performance, and the degradation rate of highly toxic norfloxacin using BiOCl reached 84% within 180 min. The internal structure and surface chemical state of BiOCl were analyzed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman, Fourier transform infrared spectroscopy (FTIR), UV-visible diffuse reflectance (UV-vis), Brunauer-Emmett-Teller (BET), X-ray photoelectron spectra (XPS), and photoelectric techniques. The higher crystallinity of BiOCl closely aligned molecules with each other, which improved the separation efficiency of photogenerated charges and showed high degradation efficiency for norfloxacin antibiotics. Furthermore, the obtained BiOCl nanosheets possess decent photocatalytic stability and recyclability.

Ingenious Artificial Leaf Based on Covalent Organic Framework Membranes for Boosting CO2 Photoreduction

Covalent organic frameworks (COFs) hold the potential in converting CO₂ with water into value-added fuels and O₂ to save the deteriorating ecological environment. However, reaching high yield and selectivity is a grand challenge under metal-, photosensitizer-, or sacrificial reagent-free conditions. Here, inspired by microstructures of natural leaves, we designed triazine-based COF membranes with the integration of steady light-harvesting sites, efficient catalytic center, and fast charge/mass transfer configuration to fabricate a novel artificial leaf for the first time. Significantly, a record high CO yield of 1240 μmol g^-1 in a 4 h reaction, approximately 100% selectivity, and a long lifespan (at least 16 cycles) were achieved under gas-solid conditions without using any metal, photosensitizer, or sacrificial reagent. Unlike the existing knowledge, the chemical structural unit of triazine-imide-triazine and the unique physical form of the COF membrane are predominant for such a remarkable photocatalysis. This work opens a new pathway to simulating photosynthesis in leaves and may motivate relevant research in the future.