Photocatalytic CO2 reduction using La-Ni bimetallic sites within a covalent organic framework

Site-Selective Zn-Metalation in Poly-Triphenyl Amine-based Porous Organic Polymer for Solid-Gas Phase CO2 Photoreduction

Harvesting solar energy to produce value-added chemicals from carbon dioxide (CO2) presents a promising route for addressing the complexities of sustainable energy systems and environmental issues. In this context, the development of metal-coordinated porous organic polymers (POPs) offers a vital avenue for improving the photocatalytic performance of organic motifs. The current study presents a metal-integrated photocatalytic system (namely, Zn@BP-POP) developed via a one-pot Friedel-Crafts (F.C.) acylation strategy, for solid-gas phase photochemical CO2 reduction to CO (CO2RR). The postsynthetic incorporation of metal (Zn) active sites on the host polymeric backbone of BP-POP significantly influences the catalytic activity. Notably, Zn@BP-POP demonstrates good photocatalytic performance in the absence of any cocatalyst and photosensitizer yielding CO while impeding the competitive hydrogen evolution reaction (HER) from water. The experimental findings collectively propose that the observed catalytic activity and selectivity arise from the synergistic interplay between the singular zinc catalytic centers and the light-harvesting capacity of the highly conjugated polymeric backbone. Further, X-ray absorption spectroscopy (XAS) analysis has significantly highlighted the prominent role played by the ZnN2O4 single sites in the polymeric framework for activating the gaseous CO2 molecules. Further, time-dependent density functional theory (DFT) analysis also reveals the thermodynamic feasibility of CO2RR over HER under optimized reaction conditions. This work cumulatively presents an effective strategy to demonstrate the importance of metal-active sites and effectively establish their structure-activity relationship during photocatalysis.

Post-synthetic Metalation on the Ionic TiO2 Surface to Enhance Metal-CO2 Interaction During Photochemical CO2 Reduction

During the photochemical CO2 reduction reaction, CO2 adsorption on the catalyst's surface is a crucial step where the binding mode of the [metal-CO2] adduct directs the product selectivity and efficiency. Herein, an ionic TiO2 nanostructure stabilized by polyoxometalates (POM), ([POM]x@TiO2), is prepared and the sodium counter ions present on the surface to balance the POMs' charge are replaced with copper(II) ions, (Cux[POM]@TiO2). The microscopic and spectroscopic studies affirm the copper exchange without altering the TiO2 core and weak coordination of copper (II) ions to the POMs' surface. Band structure analysis suggests the photo-harvesting efficiency of the TiO2 core with the conduction band edge higher than the reduction potential of CuII/I and multi-electron CO2 reduction potentials. Photochemical CO2 reduction with Cux[POM]@TiO2 results in 30 μmol gcat. -1 CO (79 %) and 8 μmol gcat -1 of CH4 (21 %). Quasi-in-situ Raman study provides evidence in support of CO2 adsorption on the Cux[POM]@TiO2 surface. 13C and D2O labeling studies affirm the {Cu-[CO2]-} adduct formation. Despite the photo-harvesting ability of Nax[POM]@TiO2 itself, the poor CO2 adsorption ability of sodium ions highlights the crucial role of copper ion CO2 photo-reduction. Characterization of the {M-[η2-CO2]-} species via surface tuning validates the CO2 activation and photochemical reduction pathway proposed earlier.

Regulating Water Adsorption Sites of Keto-Enamine COF by Base Exfoliation and Deprotonation for Enhanced Humidity Response

Covalent organic framework (COF) has received much attention owing to its unique framework structure formed by diverse organic units. However, challenges, including low conductivity, structure instability, and limited control of adsorption and desorption processes, stimulate the modification of COF in electronic sensors. Herein, inspired by the alterable structure of COF in different solvents, a facile base exfoliation and deprotonation method is proposed to regulate the water adsorption sites and improve the intrinsic conductivity of TpPa-1 COF. TpPa-1 COF powders are exfoliated to nanosheets to increase water adsorption, while the deprotonation is utilized to adjust the affinity of water molecules on TpPa-1 COF framework, contributing to water accumulation in the 1D pores. The as-fabricated TpPa-1 COF sensor exhibits a decreased recovery time from 419 to 49 s, forming a linear relation between relative humidity (RH) value and humidity response. The excellent chemical stability of the covalent bond of TpPa-1 COF contributes to the excellent stable device performance in 30 days, promoting further integration and data analysis in respiration monitoring.

Rational design of covalent organic frameworks/NaTaO3 S-scheme heterostructure for enhanced photocatalytic hydrogen evolution

As a typical perovskite material, NaTaO3 has been regarded as a potential catalyst for photocatalytic hydrogen evolution (PHE) process, due to its excellent photoelectric property and superior chemical stability. However, the photocatalytic activity of pure NaTaO3 was largely restricted by its poor visible-light absorption ability and rapid recombination of photogenerated charge carriers. Therefore, a covalently bonded TpBpy covalent organic framework (COF)/NaTaO3 (TpBpy/NaTaO3) heterostructure was designed and synthesized by the post modification strategy with (3-aminopropyl) triethoxysilane (APTES) and the in situ solvothermal process. Benefiting from the enhanced built-in electric field by the interfacial covalent bonds and the formation of S-scheme heterostructure between TpBpy and NaTaO3, which were proved by the Ar+-cluster depth profile and X-ray photoelectron spectroscopy (XPS), as well as density functional theory (DFT) calculation results, both the charge transfer efficiency and the PHE performance of the TpBpy/NaTaO3 composites were significantly improved. Additionally, the composites exhibited an excellent absorption performance in the visible region, which was also beneficial for the photocatalytic process. As expected, the optimal TpBpy/20%NaTaO3 composite achieved a remarkable hydrogen evolution rate of 17.3 mmol·g-1·h-1 (10 mg of catalyst) under simulated sunlight irradiation, which was about 173 and 2.4 times higher than that of pure NaTaO3 and TpBpy, respectively. This work provided a novel strategy for constructing highly effective and stable semiconductor/COFs heterostructures with strong interfacial interaction for photocatalytic hydrogen evolution.

TpPa-1 COFs-Enhanced Zwitterion Hydrogel for Efficient Harvesting of Atmospheric Water

Zwitterionic hydrogel, serving as carriers for hygroscopic salts, holds significant potential in atmospheric water harvesting. However, their further application is limited by structural collapse in high-concentration salt solution and poor photothermal conversion performance. Herein, the graded pore structure of poly-3-[dimethyl-[2-(2-methylprop-2-enoyloxy)ethyl]azaniumyl]propane-1-sulfonate (PDMAPS) zwitterionic hydrogel/TpPa-1 covalent organic frameworks (COFs)/LiCl composite (named as PCL composite hydrogel) is proposed, which leads to the accelerated diffusion effect for water molecules. As a result, the vapor adsorption capacity of the optimal composite hydrogel (PCL-42) reaches 2.88 g g-1 within 12 hours under conditions of 25 °C and 90 % RH. Simultaneously, the maximum temperature of PCL-42 composite could reach 53.9 °C after 9 minutes under a simulated solar intensity of 1.0 kW m-2, releasing 91 % of the adsorbed water in 3 hours, providing a promising prospect for efficient solar-driven atmospheric water harvesting. One cycle could collect 7.55 g of fresh water under outdoor conditions, and the maximum daily water production may reach 2.71 kg kg-1. The reason lies in that TpPa-1 COFs lead hydrogel to form a gradient pore structure, which may accelerate the transport of water molecules, increase the loading capacity of LiCl and enhance the photothermal property.

Constructing Photocatalytic Covalent Organic Frameworks with Aliphatic Linkers

Photocatalytic covalent organic frameworks (COFs) are typically constructed with rigid aromatic linkers for crystallinity and extended π-conjugation. However, the essential hydrophobicity of the aromatic backbone can limit their performances in water-based photocatalytic reactions. Here, we for the first time report the synthesis of hydrophilic COFs with aliphatic linkers [tartaric acid dihydrazide (TAH) and butanedioic acid dihydrazide] that can function as efficient photocatalysts for H2O2 and H2 evolution. In these hydrophilic aliphatic linkers, the specific multiple hydrogen bonding networks not only enhance crystallization but also ensure an ideal compatibility of crystallinity, hydrophilicity, and light harvesting. The resulting aliphatic linker COFs adopt an unusual ABC stacking, giving rise to approximately 0.6 nm nanopores with an improved interaction with water guests. Remarkably, both aliphatic linker-based COFs show strong visible light absorption, along with a narrow optical band gap of ∼1.9 eV. The H2O2 evolution rate for TAH-COF reaches up to 6003 μmol h-1 g-1, in the absence of sacrificial agents, surpassing the performance of all previously reported COF-based photocatalysts. Theoretical calculations reveal that the TAH linker can enhance the indirect two-electron oxygen reduction reaction for H2O2 production by improving the O2 adsorption and stabilizing the *OOH intermediate. This study opens a new avenue for constructing semiconducting COFs using nonaromatic linkers.

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.

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.

Synchrotron Radiation Based X-ray Absorption Spectroscopy: Fundamentals and Applications in Photocatalysis

Photocatalysis is one of the most promising green technologies to utilize solar energy for clean energy achievement and environmental governance. There is a knotty problem to rational designing high-performance photocatalyst, which largely depends on an in-depth insight into their structure-activity relationships and complex photocatalytic reaction mechanisms. Synchrotron radiation based X-ray absorption spectroscopy (XAS) is an important characterization method for photocatlayst to offer the element-specific key geometric and electronic structural information at the atomic level, on this basis, time-resolved XAS technique has a huge impact on mechanistic understanding of photochemical reaction owing to their powerful ability to probe, in real-time, the electronic and geometric structures evolution within photocatalysis reactions. This review will focus on the fundamentals of XAS and their applications in photocatalysis. The detailed applications obtained from XAS is described through the following aspects: 1) identifying local structure of photocatalyst; 2) uncovering in situ structure and chemical state evolution during photocatalysis; 3) revealing the photoexcited process. We will provide an in depth understanding on how the XAS method can guide the rational design of highly efficient photocatalyst. Finally, a systematic summary of XAS and related significance is made and the research perspectives are suggested.

Passivation of 2D Cs2PbI2Cl2 Nanosheets for Efficient and Stable CsPbI3 Quantum Dot Solar Cells

Inorganic CsPbI3 perovskite quantum dots (PQDs) possess remarkable optical properties, making them highly promising for photovoltaic applications. However, the inadequate stability resulting from internal structural instability and the complex external surface chemical environment of CsPbI3 PQDs has hindered the development of CsPbI3 PQD solar cells (PQDSCs). In this work, the capping layer composed of inorganic two-dimensional (2D) Ruddlesden-Popper (RP) phase Cs2PbI2Cl2 nanosheets (NSs) is introduced, which may be effectively treated to improve the surface properties of the CsPbI3 PQD film. This modification serves to passivate defects by filling cesium and iodine vacancies while optimizing the energy band arrangement and preventing humidity intrusion, leading to the meliorative stability and photovoltaic performance. The optimized CsPbI3 PQDSCs achieve an enhanced power conversion efficiency (PCE) of 14.73%, with the superb stability of only a 16% efficiency loss after being exposed to ambient conditions (30 ± 5% RH) for 432 h.

COF-Topological Quantum Material Nano-heterostructure for CO2 to Syngas Production under Visible Light

Efficient solar-driven syngas production (CO+H2 mixture) from CO2 and H2O with a suitable photocatalyst and fundamental understanding of the reaction mechanism are the desired approach towards the carbon recycling process. Herein, we report the design and development of an unique COF-topological quantum material nano-heterostructure, COF@TI with a newly synthesized donor-acceptor based COF and two dimensional (2D) nanosheets of strong topological insulator (TI), PbBi2Te4. The intrinsic robust metallic surfaces of the TI act as electron reservoir, minimising the fast electron-hole recombination process, and the presence of 6s2 lone pairs in Pb2+ and Bi3+ in the TI helps for efficient CO2 binding, which are responsible for boosting overall catalytic activity. In variable ratio of acetonitrile-water (MeCN : H2O) solvent mixture COF@TI produces syngas with different ratios of CO and H2. COF@TI nano-heterostructure enables to produce higher amount of syngas with more controllable ratios of CO and H2 compared to pristine COF. The electron transfer route from COF to TI was realized from Kelvin probe force microscopy (KPFM) analysis, charge density difference calculation, excited state lifetime and photoelectrochemical measurements. Finally, a probable mechanistic pathway has been established after identifying the catalytic sites and reaction intermediates by in situ DRIFTS study and DFT calculation.

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.

Boosting exciton dissociation and charge transfer in CsPbBr3 QDs via ferrocene derivative ligation for CO2 photoreduction

Photo-catalytic CO2 reduction with perovskite quantum dots (QDs) shows potential for solar energy storage, but it encounters challenges due to the intricate multi-electron photoreduction processes and thermodynamic and kinetic obstacles associated with them. This study aimed to improve photo-catalytic performance by addressing surface barriers and utilizing multiple-exciton generation in perovskite QDs. A facile surface engineering method was employed, involving the grafting of ferrocene carboxylic acid (FCA) onto CsPbBr3 (CPB) QDs, to overcome limitations arising from restricted multiple-exciton dissociation and inefficient charge transfer dynamics. Kelvin Probe Force Microscopy and XPS spectral confirmed successfully creating an FCA-modulated microelectric field through the Cs active site, thus facilitating electron transfer, disrupting surface barrier energy, and promoting multi-exciton dissociations. Transient absorption spectroscopy showed enhanced charge transfer and reduced energy barriers, resulting in an impressive CO2-to-CO conversion rate of 132.8 μmol g-1 h-1 with 96.5% selectivity. The CPB-FCA catalyst exhibited four-cycle reusability and 72 h of long-term stability, marking a significant nine-fold improvement compared to pristine CPB (14.4 μmol g-1 h-1). These results provide insights into the influential role of FCA in regulating intramolecular charge transfer, enhancing multi-exciton dissociation, and improving CO2 photoreduction on CPB QDs. Furthermore, these findings offer valuable knowledge for controlling quantum-confined exciton dissociation to enhance CO2 photocatalysis.

Recent progress of heterogeneous catalysts for transfer hydrogenation under the background of carbon neutrality

Under the background of carbon neutrality, the direct conversion of greenhouse CO2 to high value added fuels and chemicals is becoming an important and promising technology. Among them, the generation of liquid C1 products (formic acid and methanol) has made great progress; nevertheless, it encounters the problem of how to use it efficiently to solve the overcapacity issue. In this review, we suggest that the catalytic transfer hydrogenation using formic acid and methanol as the hydrogen sources is a critical and potential route for the substitution for the fossil fuel-derived H2 to generate essential bulk and fine chemicals. We mainly focus on summarizing the recent progress of heterogeneous catalysts in such reactions, including thermal- and photo-catalytic processes. Finally, we also propose some challenges and opportunities for this development.

Modulating Charge Separation of Oxygen-Doped Boron Nitride with Isolated Co Atoms for Enhancing CO2 -to-CO Photoreduction

To alleviate the greenhouse effect and address the related energy crisis, solar-driven reduction of carbon dioxide (CO2 ) to value-added products is considered as a sustainable strategy. However, the insufficient separation and rapid recombination of photogenerated charge carriers during photocatalysis greatly limit their reduction efficiency and practical application potential. Here, isolated Cobalt (Co) atoms are successfully decorated into oxygen-doped boron nitride (BN) via an in situ pyrolysis method, achieving greatly improved catalytic activity and selectivity to the carbon monoxide (CO) product. X-ray absorption fine spectroscopy demonstrates that the isolated Co atoms are stabilized by the O and N atoms with an unsaturated CoO2 N1 configuration. Further experimental investigation and theoretical simulations confirm that the decorated Co atoms not only work as the real active center during the CO2 reduction process, but also perform as the electron pump to promote the electron/hole separation and transfer, resulting in greatly accelerated reaction kinetics and improved activity. In addition, the CoO2 N1 coordination geometry is favorable to the conversion from *CO2 to *COOH, which shall be considered as a selectivity-determining step for the evolution of the CO products. The surface modulation strategy at the atomic level opens a new avenue for regulating the reaction kinetics for photocatalytic CO2 reduction.

An emerging direction for nanozyme design: from single-atom to dual-atomic-site catalysts

Nanozymes, a new class of functional nanomaterials with enzyme-like characteristics, have recently made great achievements and have become potential substitutes for natural enzymes. In particular, single-atomic nanozymes (Sazymes) have received intense research focus on account of their versatile enzyme-like performances and well-defined spatial configurations of single-atomic sites. More recently, dual-atomic-site catalysts (DACs) containing two neighboring single-atomic sites have been explored as next-generation nanozymes, thanks to the flexibility in tuning active sites by various combinations of two single-atomic sites. This minireview outlines the research progress of DACs in their synthetic approaches and the latest characterization techniques highlighting a series of representative examples of DAC-based nanozymes. In the final remarks, we provide current challenges and perspectives for developing DAC-based nanozymes as a guide for researchers who would be interested in this exciting field.

Photoredox Coupling of CO2 Reduction with Benzyl Alcohol Oxidation over Ternary Metal Chalcogenides (ZnmIn2S3+m, m = 1-5) with Regulable Products Selectivity

Integrating photocatalytic CO2 reduction with selective benzyl alcohol (BA) oxidation in one photoredox reaction system is a promising way for the simultaneous utilization of photogenerated electrons and holes. Herein, ZnmIn2S3+m (m = 1-5) semiconductors (ZnIn2S4, Zn2In2S5, Zn3In2S6, Zn4In2S7, and Zn5In2S8) with various composition faults were synthesized via a simple hydrothermal method and used for effective selective dehydrocoupling of benzyl alcohol into high-value C-C coupling products and reduction of CO2 into syngas under visible light. The absorption edge of ZnmIn2S3+m samples shifted to shorter wavelengths as the atomic ratio of Zn/In was increased. The conduction band and valence band position can be adjusted by changing the Zn/In ratio, resulting in controllable photoredox ability for selective BA oxidation and CO2 reduction. For example, the selectivity of benzaldehyde (BAD) product was reduced from 76% (ZnIn2S4, ZIS1) to 27% (Zn4In2S7, ZIS4), while the selectivity of hydrobenzoin (HB) was increased from 22% to 56%. Additionally, the H2 formation rate on ZIS1 (1.6 mmol/g/h) was 1.6 times higher than that of ZIS4 (1.0 mmol/g/h), and the CO formation rate on ZIS4 (0.32 mmol/g/h) was three times higher than that of ZIS1 (0.13 mmol/g/h), demonstrating that syngas with different H2/CO ratios can be obtained by controlling the Zn/In ratio in ZnmIn2S3+m. This study provides new insights into unveiling the relationship of structure-property of ZnmIn2S3+m layered crystals, which are valuable for implementation in a wide range of environment and energy applications.