From molecular metal complex to metal-organic framework: The CO2 reduction photocatalysts with clear and tunable structure

Microenvironment Regulation of {Co4IIO4} Cubane for Syngas Photosynthesis

It is a great challenging task for selectivity control of both CO₂ photoreduction and water splitting to produce syngas via precise microenvironment regulation. Herein, a series of UiO-type Eu-MOFs (Eu-bpdc, Eu-bpydc, Ru_x-Eu-bpdc, and Ru_x-Eu-bpydc) with different surrounding confined spaces were designed and synthesized. These photosensitizing Ru_x-Eu-MOFs were used as the molecular platform to encapsulate the [Co^II₄(dpy{OH}O)₄(OAc)₂(H₂O)₂]²⁺ (Co₄) cubane cluster for constructing Co₄@Ru_x-Eu-MOF (x = 0.1, 0.2, and 0.4) heterogeneous photocatalysts for efficient CO₂ photoreduction and water splitting. The H₂ and CO yields can reach 446.6 and 459.8 μmol·g^-1, respectively, in 10 h with Co₄@Ru_0.1-Eu-bpdc as the catalyst, and their total yield can be dramatically improved to 2500 μmol·g^-1 with the ratio of CO/H₂ ranging from 1:1 to 1:2 via changing the photosensitizer content in the confined space. By increasing the N content around the cubane, the photocatalytic performance drops sharply in Co₄@Ru_0.1-Eu-bpydc, but with an enhanced proportion of CO in the final products. In the homogeneous system, the Co₄ cubane was surrounding with Ru photosensitizers via week interactions, which can drive water splitting into H₂ with >99% selectivity. Comprehensive structure-function analysis highlights the important role of microenvironment regulation in the selectivity control via constructing homogeneous and heterogeneous photocatalytic systems. This work provides a new insight for engineering a catalytic microenvironment of the cubane cluster for selectivity control of CO₂ photoreduction and water splitting.

Coupling Ruthenium Bipyridyl and Cobalt Imidazolate Units in a Metal-Organic Framework for an Efficient Photosynthetic Overall Reaction in Diluted CO2

Artificial photocatalytic CO₂ reduction, using water as the reductant, is challenging mainly because it is difficult for multiple functional units to cooperate efficiently. Here, we show that the classic photosensitive and H₂O-oxidizing ruthenium bipyridyl units and CO₂-reducing cobalt imidazolate units can be incorporated into a metal-organic framework using a classic organic ligand, imidazo[4,5-f][1,10]phenanthroline. Under visible light without additional sacrificial agents and photosensitizers, the overall conversion of CO₂ and H₂O to CO and O₂ was achieved by the multifunctional photocatalyst in the CH₃CN/H₂O mixed solvent with a high CO production rate of 11.2 μmol g^-1 h^-1 and CO selectivity of ca. 100%. Thanks to its ultramicroporous structure with moderately strong CO₂ adsorption ability, the photocatalyst also exhibited high performances with CO/CH₄ production rates of 5.15/0.62 and 4.26/0.20 μmol g^-1 h^-1 in the gas phase with pure and even diluted CO₂, respectively. Photoluminescence emission spectroscopy and photoelectrochemical tests confirmed that the photosensitive and catalytic units cooperated well to give suitable photocatalytic redox potentials and fast electron-hole separation.

Deciphering Distinct Overpotential-Dependent Pathways for Electrochemical CO2 Reduction Catalyzed by an Iron-Terpyridine Complex

[Fe(tpyPY2Me)]²⁺ ([Fe]²⁺) is a homogeneous electrocatalyst for converting CO₂ into CO featuring low overpotentials of <100 mV, near-unity selectivity, and high activity with turnover frequencies faster than 100 000 s^-1. To identify the origins of its exceptional performance and inform future catalyst design, we report a combined computational and experimental study that establishes two distinct mechanistic pathways for electrochemical CO₂ reduction catalyzed by [Fe]²⁺ as a function of applied overpotential. Electrochemical data shows the formation of two catalytic regimes at low (η_TOF/2 of 160 mV) and high (η_TOF/2 of 590 mV) overpotential plateaus. We propose that at low overpotentials [Fe]²⁺ undergoes a two-electron reduction, two-proton-transfer mechanism (electrochemical-electrochemical-chemical-chemical, EECC), where turnover occurs through the dicationic iron complex, [Fe]²⁺. Computational analysis supports the importance of the singlet ground-state electronic structure for CO₂ binding and that the rate-limiting step is the second protonation in this low-overpotential regime. When more negative potentials are applied, an additional electron-transfer event occurs through either a stepwise or proton-coupled electron-transfer (PCET) pathway, enabling catalytic turnover from the monocationic iron complex ([Fe]⁺) via an electrochemical-chemical-electrochemical-chemical (ECEC) mechanism. Comparison of experimental kinetic data obtained from variable controlled potential electrolysis (CPE) experiments with direct product detection with calculated rates obtained from the energetic span model supports the PCET pathway as the most likely mechanism. Moreover, we build upon this mechanistic understanding to propose the design of an improved ligand framework that is predicted to stabilize the key transition states identified in our study and explore their electronic structures using an energy decomposition analysis. Taken together, this work highlights the value of synergistic computational/experimental approaches to decipher mechanisms of new electrocatalysts and direct the rational design of improved platforms.

Synthetic strategies for MOF-based single-atom catalysts for photo- and electro-catalytic CO2 reduction

The excessive CO₂ emission has resulted in climate changes, which has threatened human existence. Photocatalytic and electrocatalytic CO₂ reduction, driven by wind electricity and solar energy, are feasible ways of tackling carbon dioxide emission, as both energies are clean and renewable. Single-atom catalyst (SAC) is a candidate owing to excellent electrocatalytic and photocatalytic performance. Methods for preparing an SAC by using metal-organic frameworks (MOFs) as support or precursors are summarized. Also, applications in energy conversion are exhibited. However, the real challenge is to improve the selectivity of catalytic reactions to yield higher value products, which is to be discussed.

Utilization of Low-Concentration CO2 with Molecular Catalysts Assisted by CO2-Capturing Ability of Catalysts, Additives, or Reaction Media

Increasing concentration of atmospheric CO₂ is a worldwide concern and continues to trigger various environmental problems. Photo- or electrocatalytic CO₂ reduction (CO₂-Red) using solar energy, i.e., artificial photosynthesis, is a prospective technique owing to its sustainability and the usefulness of the reaction products. Concentrations of CO₂ in exhaust gases from industries are several % to 20%, and that in the atmosphere is about 400 ppm. Although condensation processes of CO₂ require high energy consumption and cost, pure CO₂ has been used in most of the reported studies for photo- and electrocatalytic CO₂-Red because the reaction between CO₂ and the catalyst could be one of the rate-limiting steps. To address these issues and provide a repository of potential techniques for other researchers, this perspective summarizes the catalytic systems reported for the reduction of low-concentration CO₂, which utilize a combination of catalytic CO₂-Red and CO₂-capturing reactions (or CO₂ adsorption). First, we describe CO₂ insertions into M-X bonds of the catalysts, which increase the rate constants and/or equilibrium constants for CO₂ binding on the catalysts, and modifications of the second coordination sphere to stabilize the CO₂-bound catalysts. Furthermore, we discuss the reaction media used for catalytic CO₂-Red that have the unique effect of increasing CO₂ concentrations around the catalysts. These reaction media include typical CO₂-capturing additives, ionic liquids, and metal-organic frameworks.

Engineering Hierarchical Architecture of Metal-Organic Frameworks for Highly Efficient Overall CO2 Photoreduction

Previous studies on syntheses of metal-organic frameworks (MOFs) for photocatalytic CO₂ reduction are mainly focused on the exquisite control over the net topology and the functionality of metal clusters/organic building blocks. This contribution demonstrates that the rational design of MOF-based photocatalyst can be further extended to the hierarchical structure at micrometer scales well beyond the conventional MOF design at the molecular level. By taking advantage of the disparity of two selective MOFs in nucleation kinetics, a hierarchical core-shell MOF@MOF structure is successfully constructed through a simple one-pot synthesis. Besides inheriting the high porosity, crystallinity, and robustness of parent MOFs, the obtained heterojunction exhibits extended photoresponse, optimized band alignment with large overpotential, and greatly enhanced photogenerated charge separation, which would be hardly realized by the merely molecular-level assembly. As a result, the challenging overall CO₂ photoreduction is achieved, which generates a record high HCOOH production (146.0 µmol/g/h) without using any sacrificial reagents. Moreover, the core-shell structure exhibits a more effective use of photogenerated electrons than the individual MOFs. This work shows that harnessing the hierarchical architecture of MOFs present a new and effective alternative to tuning the photocatalytic performance at a mesoscopic level.

Combination of Organic Dye and Iron for CO2 Reduction with Pentanuclear Fe2Na3 Purpurin Photocatalysts

Molecular photocatalysts designed with earth-abundant elements are rare and challenging in artificial photosynthesis study. Herein, we report a multimetallic Fe2Na3 purpurin (1) complex for the reduction of CO2 in DMF under visible-light irradiation. The photocatalytic system achieves 91% selectivity and 2625 ± 334 turnovers of CO in 120 h, which is among the highest reported for a noble-metal-free catalyst without an additional photosensitizer. UV-vis and electrochemical studies suggest that the mechanism involves subsequent reductions and protonations of 1 to generate [FeII2Na3((H)2PP)6]5- and [FeIII2Na3((H)2PP)6]3- as the active photocatalysts in CO2 reduction.

Highly Efficient Photocatalytic Reduction of CO2 to CO by In Situ Formation of a Hybrid Catalytic System Based on Molecular Iron Quaterpyridine Covalently Linked to Carbon Nitride

Efficient and selective photocatalytic CO₂ reduction was obtained within a hybrid system that is formed in situ via a Schiff base condensation between a molecular iron quaterpyridine complex bearing an aldehyde function and carbon nitride. Irradiation (blue LED) of an CH₃ CN solution containing 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH), triethylamine (TEA), Feqpy-BA (qpy-BA=4-([2,2':6',2'':6'',2'''-quaterpyridin]-4-yl)benzaldehyde) and C₃ N₄ resulted in CO evolution with a turnover number of 2554 and 95 % selectivity. This hybrid catalytic system unlocks covalent linkage of molecular catalysts with semiconductor photosensitizers via Schiff base reaction for high-efficiency photocatalytic reduction of CO₂ , opening a pathway for diverse photocatalysis.

Ferrocene-Functionalized Crystalline Biomimetic Catalysts for Efficient CO2 Photoreduction

Photoreducing carbon dioxide (CO2) into highly valued chemicals or energy products has been recognized as one of the most promising proposals to degrade atmospheric CO2 concentration and achieve carbon neutrality. Adenine with a photosensitive amino group and aromatic nitrogen atom can strongly interact with CO2 and has been authenticated for its catalytic activity for the CO2 photoreduction reaction (CO2RR). Herein, two adenine-constructed crystalline biomimetic photocatalysts (Co2-AW and Co2-AF) were designed and synthesized to achieve CO2RR. Between them, Co2-AF displayed higher photocatalytic activity (225.8 μmol g-1 h-1) for CO2-to-HCOOH conversion than that of Co2-AW. It was found that the superior charge transfer capacity of the functional ferrocene group in Co2-AF is the primary reason to facilitate the photocatalytic performance efficiently. Additionally, this work also demonstrated the great potential of the ferrocene group as an electron donor and mediator in improving the photocatalytic activity of crystalline coordination catalysts.

Metallated Isoindigo-Porphyrin Covalent Organic Framework Photocatalyst with a Narrow Band Gap for Efficient CO2 Conversion

Photocatalytic CO₂ reduction into formate (HCOO^-) has been widely studied with semiconductor and molecule-based systems, but it is rarely investigated with covalent organic frameworks (COFs). Herein, we report a novel donor-acceptor COF named Co-PI-COF composed of isoindigo and metallated porphyrin subunits that exhibits high catalytic efficiency (∼50 μmol formate g^-1 h^-1) at low-power visible-light irradiation and in the absence of rare metal cocatalysts. Density functional theory calculations and experimental diffuse-reflectance measurements are used to explain the origin of catalytic efficiency and the particularly low band gap (0.56 eV) in this material. The mechanism of photocatalysis is also studied experimentally and is found to involve electron transfer from the sacrificial agent to the excited Co-PI-COF. The observed high-efficiency conversion could be ascribed to the enhanced CO₂ adsorption on the coordinatively unsaturated cobalt centers, the narrow band gap, and the efficient transfer of the charge originating from the postsynthetic metallation. It is anticipated that this study will pave the way toward the design of new simple and efficient catalysts for photocatalytic CO₂ reduction into useful products.

Strategies for Breaking Molecular Scaling Relationships for the Electrochemical CO2 Reduction Reaction

The electrocatalytic CO2 reduction reaction (CO2RR) is a promising strategy for converting CO2 to fuels and value-added chemicals using renewable energy sources. Molecular electrocatalysts show promise for the selective conversion...

Heterogenization of molecular cobalt catalysts in robust metal–organic frameworks for efficient photocatalytic CO2 reduction

Efficient, selective and recyclable heterogeneous catalysts for photocatalytic CO2 reduction to CO under visible light irradiation are readily prepared by immobilization of cobalt molecular catalysts into Zr(iv)-based MOFs.

Influences of the precursor molar ratio in synthesis on the structures and visible-light driven CO2 reduction into solar fuel of CdS catalyst

Synthesizing highly efficient photocatalysts for photoreduction of CO2 into solar fuel is of great significance for solving the energy shortage and environmental pollution. Here a series of CdS photocatalysts are...

A Novel Copper Metal-Organic Framework Catalyst for the Highly Efficient Conversion of CO2 with Propargylic Amines

The rapid increase in atmospheric carbon dioxide has resulted in the greenhouse effect. Hence, carbon dioxide capture and further fixation into valuable chemical products are particularly important for reducing atmospheric...

Feeding Carbonylation with CO2 via the Synergy of Single-Site/Nanocluster Catalysts in a Photosensitizing MOF

Solar-driven carbonylation with CO₂ replacing toxic CO as a C1 source is of considerable interest; however it remains a great challenge due to the inert CO₂ molecule. Herein, we integrate cobalt single-site and ultrafine CuPd nanocluster catalysts into a porphyrin-based metal-organic framework to construct composite photocatalysts (Cu₁Pd₂)_z@PCN-222(Co) (z = 1.3, 2.0, and 3.0 nm). Upon visible light irradiation, excited porphyrin can concurrently transfer electrons to Co single sites and CuPd nanoclusters, providing the possibility for coupling CO₂ photoreduction and Suzuki/Sonogashira reactions. This multicomponent synergy in (Cu₁Pd₂)_1.3@PCN-222(Co) can not only replace dangerous CO gas but also dramatically promote the photosynthesis of benzophenone in CO₂ with over 90% yield and 97% selectivity under mild condition. Systematic investigations clearly decipher the function and collaboration among different components in these composite catalysts, highlighting a new insight into developing a sustainable protocol for carbonylation reactions by employing greenhouse gas CO₂ as a C1 source.

Templating Synthesis of Metal-Organic Framework Nanofiber Aerogels and Their Derived Hollow Porous Carbon Nanofibers for Energy Storage and Conversion

A kind of metal-organic framework (MOF) aerogels are synthesized by the self-assembly of uniform and monodisperse MOF nanofibers. Such MOF nanofiber aerogels as carbon precursors can effectively avoid the aggregation of nanofibers during calcination, resulting in the formation of well-dispersed hollow porous carbon nanofibers (HPCNs). Moreover, HPCNs with well-dispersion are investigated as sulfur host materials for Li-S batteries and electrocatalysts for cathode oxygen reduction reaction (ORR). On the one hand, HPCNs act as hosts for the encapsulation of sulfur into their hierarchical micro- and mesopores as well as hollow nanostructures. The obtained sulfur cathode exhibits excellent electrochemical features, good cycling stability and high coulombic efficiency. On the other hand, HPCNs exhibit better electrocatalytic activity than aggregated counterparts for ORR. Furthermore, a highly active single atom electrocatalyst can be prepared by the carbonization of bimetallic MOF nanofiber aerogels. The results indicate that well-dispersed HPCNs show enhanced electrochemical properties in contrast to their aggregated counterparts, suggesting that the dispersion situation of nanomaterials significantly influence their final performance. The present concept of employing MOF nanofiber aerogels as precursors will provide a new strategy to the design of MOF-derived nanomaterials with well-dispersion for their applications in energy storage and conversion.

Effect of Micropores of a Porous Coordination Polymer on the Product Selectivity in RuII Complex-catalyzed CO2 Reduction

To develop an efficient CO2 reduction catalyst, hybridizing a molecular catalyst and a porous coordination polymer (PCP) is a promising strategy because it can combine both advantages of the precise reactivity control of the former and the CO2 adsorption property of the latter. Although several PCP hybrid catalysts have been reported to date, the CO2 sorption behavior and the CO2 reduction reactivity have been investigated separately, and the CO2 enrichment during the catalysis is still unclear. We report CO2 photoreduction under different temperatures and pressures using a PCP-RuII complex hybrid catalyst. The product selectivity (CO or HCOOH) varied depending on the reaction conditions. The altered selectivity could be interpreted in terms of the CO2 capture in the micropores of a PCP.

Electrostatic Attraction-Driven Assembly of a Metal-Organic Framework with a Photosensitizer Boosts Photocatalytic CO2 Reduction to CO

Reducing CO₂ into fuels via photochemical reactions relies on highly efficient photocatalytic systems. Herein, we report a new and efficient photocatalytic system for CO₂ reduction. Driven by electrostatic attraction, an anionic metal-organic framework Cu-HHTP (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) as host and a cationic photosensitizer [Ru(phen)₃]²⁺ (phen = 1,10-phenanthroline) as guest were self-assembled into a photocatalytic system Ru@Cu-HHTP, which showed high activity for photocatalytic CO₂ reduction under laboratory light source (CO production rate of 130(5) mmol g^-1 h^-1, selectivity of 92.9%) or natural sunlight (CO production rate of 69.5 mmol g^-1 h^-1, selectivity of 91.3%), representing the remarkable photocatalytic CO₂ reduction performance. More importantly, the photosensitizer [Ru(phen)₃]²⁺ in Ru@Cu-HHTP is only about 1/500 in quantity reported in the literature. Theoretical calculations and control experiments suggested that the assembly of the catalysts and photosensitizers via electrostatic attraction interactions can provide a better charge transfer efficiency, resulting in high performance for photocatalytic CO₂ reduction.

Integration of zirconium-based metal-organic framework with CdS for enhanced photocatalytic conversion of CO2 to CO

It is a promising strategy to prepare composite photocatalysts based on MOFs and semiconductors for enhancing photocatalytic reduction of carbon dioxide (CO₂). A family of binary composite photocatalysts (CdS@UiO-66-NH₂) with different CdS contents have been designed and synthesized, which have been explored for photocatalytic reduction of CO₂. CdS@UiO-66-NH₂ can efficiently convert CO₂ into CO under visible light irradiation via the solid-gas mode in the absence of sacrificial agents and photosensitizers. The generation rate of CO can reach up to 280.5 μmol g^-1 h^-1, which is 2.13-fold and 2.9-fold improvements over the pristine CdS and UiO-66-NH₂, respectively, and the selectivity for CO is very high. Furthermore, this kind of photocatalysts can still maintain great photocatalytic activity in CO₂/N₂ mixed atmosphere with different CO₂ concentrations. The outstanding performances of CdS@UiO-66-NH₂ may be attributed to the existence of the direct Z-scheme heterojunction, which possesses the enhanced separation and migration of photo-generated charge carriers between UiO-66-NH₂ and CdS, available specific surface areas and improved visible light absorption ability as well as abundant reaction active sites. This case reveals that MOF-based composite photocatalysts exhibit promising potential applications in the field of CO₂ conversion.

Charge Transfer from Donor to Acceptor in Conjugated Microporous Polymer for Enhanced Photosensitization

Photosensitization associated with light absorption and energy/electron-transfer represents the central processes for photosynthesis. However, it's still a challenge to develop a heavy-atom-free (HAF) strategy to improve the sensitizing ability of polymeric photosensitizers. Herein, we propose a new protocol to significantly improve the photosensitization by decorating mother conjugated microporous polymer (CMP-1) with polycyclic aromatic hydrocarbons (PAHs), resulting in a series of CMPs (CMP-2-4). Systematic study reveals that covalent modification with PAHs can transfer charge to Bodipy in CMP to further facilitate both intersystem crossing and electron-hole separation, which can dramatically boost energy-/electron-transfer reactions. Remarkably, CMP-2 as a representative CMP can efficiently drive the photosynthesis of methyl phenyl sulfoxide with 92 % yield, substantially higher than that of CMP-1 (32 %). Experiments and theory calculations demonstrate the structure-property-activity relationship of these CMPs, opening a new horizon for developing HAF heterogeneous photosensitizers with highly efficient sensitizing activity by rational structure regulation at a molecular level.

High loading of Mn(ii)-metalated porphyrin in a MOF for photocatalytic CO2 reduction in gas-solid conditions

A high loading of Mn(ii)-metalated porphyrin was achievable in a 2D porphyrin-based Mn-MOF induced by an ionic liquid. The excellent stability, sufficient redox potential, atomically dispersed porphyrin Mn(ii) sites, desired CO₂ affinity, high visible light-harvesting and efficient charge separation, endow this MOF with the overall photocatalytic conversion of CO₂ to CH₄ in gas-solid conditions.

Electrochemical conversion of CO2 into value-added carbon with desirable structures via molten carbonates electrolysis

Direct conversion of CO₂ to high value-added carbon products based on molten salt electrochemistry has been proven to be a feasible approach to solve the climate problem and achieve carbon neutrality. In this work, carbon nanotubes (CNTs), carbon spheres (CSs) and honeycomb carbon are synthesized by electrolysis of a single or multiple alkali metal carbonate electrolyte. The elemental composition, morphology and structure, crystallinity and graphitization degree of carbon products are characterized by electron dispersive spectroscopy (EDS), scanning electron microscopy (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD) and Raman microspectroscopy (RAM). The results demonstrate that a high yield of CNTs is obtained in Li₂CO₃ electrolyte by regulating the electrolysis temperature and current density. Compared to pure Li₂CO₃, Li-Na carbonate electrolyte with 1 wt% stannic oxide/cerium oxide (SnO₂/GeO₂) favors CS formation rather than CNT formation. Additionally, honeycomb carbon products are generated in Li-Na-K electrolyte, when the electrolysis temperature is lower than 600 °C. Overall, this work provides a novel carbon neutral strategy where high value-added carbon products are synthesized using CO₂ as a carbon source.

Cobalt-based metal-organic frameworks for the photocatalytic reduction of carbon dioxide

Metal-organic frameworks (MOFs) are porous materials composed of metal centers and organic connectors. They are formed by complexation reactions and exhibit characteristics of both polymers and coordination compounds. They exhibit numerous advantageous features, including a large specific surface area, adjustable pore size/shape, and modifiable pore wall functional groups. Consequently, MOFs have been extensively applied in the photocatalytic reduction of carbon dioxide (CO2). Despite considerable research on cobalt-based MOFs, the photocatalytic reduction of CO2 in the presence of these materials remains challenging. The present review summarizes the current studies concerning the utilization of cobalt-based MOFs in the photocatalytic reduction of CO2. Additionally, approaches used to enhance the catalytic reduction performance are evaluated and the challenges associated with Co-based MOFs are discussed.

Transition-Metal-Modified Vanadoborate Clusters as Stable and Efficient Photocatalysts for CO2 Reduction

Photocatalytic carbon dioxide reduction (CO₂RR) is considered to be a promising sustainable and clean approach to solve environmental issues. Polyoxometalates (POMs), with advantages in fast, reversible, and stepwise multiple-electron transfer without changing their structures, have been promising catalysts in various redox reactions. However, their performance is often restricted by poor thermal or chemical stability. In this work, two transition-metal-modified vanadoborate clusters, [Co(en)₂]₆[V₁₂B₁₈O₅₄(OH)₆]·17H₂O (V₁₂B₁₈-Co) and [Ni(en)₂]₆[V₁₂B₁₈O₅₄(OH)₆]·17H₂O (V₁₂B₁₈-Ni), are reported for photocatalytic CO₂ reduction. V₁₂B₁₈-Co and V₁₂B₁₈-Ni can preserve their structures to 200 and 250 °C, respectively, and remain stable in polar organic solvents and a wide range of pH solutions. Under visible-light irradiation, CO₂ can be converted into syngas and HCOO^- with V₁₂B₁₈-Co or V₁₂B₁₈-Ni as catalysts. The total amount of gaseous products and liquid products for V₁₂B₁₈-Co is up to 9.5 and 0.168 mmol g^-1 h^-1. Comparing with V₁₂B₁₈-Co, the yield of CO for V₁₂B₁₈-Ni declines by 1.8-fold, while that of HCOO^- increases by 35%. The AQY of V₁₂B₁₈-Co and V₁₂B₁₈-Ni is 1.1% and 0.93%, respectively. These values are higher than most of the reported POM materials under similar conditions. The density functional theory (DFT) calculations illuminate the active site of CO₂RR and the reduction mechanism. This work provides new insights into the design of stable, high-performance, and low-cost photocatalysts for CO₂ reduction.

Coordination-driven discrete metallo-supramolecular assembly for rapid and selective photochemical CO2 reduction in aqueous solution

A discrete metallo-supramolecular assembly composed of six iron(ii) cations and twelve redox-active terpyridine fragments has been developed for the highly efficient visible-light-driven reduction of CO2 to CO with a TON of 14 956 and 99.6% selectivity in the presence of an organic thermally activated delayed fluorescence (TADF) photosensitizer 4CzIPN in aqueous solution. The photochemical system proceeds rapidly with a turnover frequency (TOF) of 276 min-1. It is demonstrated that the redox-active terpyridine fragments in the assembly are reduced by the photosensitizer which could further act as an electron reservoir for CO2 reduction, resulting in the highly efficient reduction of CO2. This work shows that discrete metallo-supramolecular assemblies could be used for robust photochemical CO2 reduction.

Water-Assisted Highly Efficient Photocatalytic Reduction of CO2 to CO with Noble Metal-Free Bis(terpyridine)iron(II) Complexes and an Organic Photosensitizer

Photocatalytic CO₂ reduction reaction is believed to be a promising approach for CO₂ utilization. In this work, a noble metal-free photocatalytic system, composed of bis(terpyridine)iron(II) complexes and an organic thermally activated delayed fluorescence compound, has been developed for selective reduction of CO₂ to CO with a maximum turnover number up to 6320, 99.4% selectivity, and turnover frequency of 127 min^-1 under visible-light irradiation in dimethylformamide/H₂O solution. More than 0.3 mmol CO was generated using 0.05 μmol catalyst after 2 h of light irradiation. The apparent quantum yield was found to be 9.5% at 440 nm (180 mW cm^-2). Control experiments and UV-vis-NIR spectroscopy studies further demonstrated that water strongly promoted the photocatalytic cycle and terpyridine ligands rather than Fe(II) were initially reduced during the photocatalytic process.

Recent advances in visible-light-driven carbon dioxide reduction by metal-organic frameworks

Metal-organic frameworks (MOFs) have emerged as promising materials and have attracted researchers due to their unique chemical and physical properties-design flexibility, tuneable pore channels, a high surface-to-volume ratio that allow their distinct application in diverse research fields-gas storage, gas separation, catalysis, adsorption, drug delivery, ion exchange, sensing, etc. The rapidly growing CO₂ in the atmosphere is a global concern due to the excessive use of fossil fuels in the current era. CO₂ is the prime cause of global warming and should be ameliorated either through adsorption or conversion into value-added products to protect the environment and mankind. Nowadays, MOFs are exploited as a photocatalyst for applications of CO₂ reduction. Since the use of semiconductors limits the use of visible light for photocatalytic reduction of CO₂, MOFs are promising options. The current review describes recent development in the application of MOFs as host, composites, and their derivatives in photocatalytic reduction of CO₂ to CO and different organic chemicals (HCOOH, CH₃OH, CH₄). Efficient charge separation and visible light absorption by incorporation of active sites for efficient photocatalysis have been discussed. The selection of material for high CO₂ uptake and potential strategies for the rational design and development of high-performance catalysts are outlined. Major challenges and future perspectives have also been discussed at the last of the review.

Influence of the Physical Properties on the Antibacterial and Photocatalytic Behavior of Ag-Doped Indium Sulfide Film Deposited by Spray Pyrolysis

Spray pyrolysis was used to deposit indium sulfide (In2S3) films, with or without silver doping. The films are polycrystalline, and the inclusion of Ag in the In2S3 structure leads to the formation of a solid solution, with the crystallite size of the order of tens of nanometers. In2S3 films exhibit a semiconductive behavior, and the incorporation of Ag leads to an increase of the charge carrier concentration, enhancing the electrical conductivity of the films. The small polaron hopping mechanism, deduced by the fittings according to the double Jonscher variation, explains the evolution of the direct current (dc) conductivity at high temperature of the Ag-doped indium sulfide. From impedance spectroscopy, it was found that the doped film presents dielectric relaxation, and Nyquist diagrams indicate the importance of the grain and the grain boundaries’ contributions to the transport phenomena. The physical characteristics of the films have an influence on the photocatalytic performance, achieving photodegradation efficiency above 80% (85.5% in the case of Ag doping), and on the antibacterial activity. The obtained results indicate that indium sulfide films are good candidates for environmental and biological applications, confirming a multifunctional nature.

Promoting photocatalytic CO2 reduction with a molecular copper purpurin chromophore

CO₂ reduction through artificial photosynthesis represents a prominent strategy toward the conversion of solar energy into fuels or useful chemical feedstocks. In such configuration, designing highly efficient chromophores comprising earth-abundant elements is essential for both light harvesting and electron transfer. Herein, we report that a copper purpurin complex bearing an additional redox-active center in natural organic chromophores is capable to shift the reduction potential 540 mV more negative than its organic dye component. When this copper photosensitizer is employed with an iron porphyrin as the catalyst and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole as the sacrificial reductant, the system achieves over 16100 turnover number of CO from CO₂ with a 95% selectivity (CO vs H₂) under visible-light irradiation, which is among the highest reported for a homogeneous noble metal-free system. This work may open up an effective approach for the rational design of highly efficient chromophores in artificial photosynthesis.

Self-assembly of TiO2/ZIF-8 nanocomposites for varied photocatalytic CO2 reduction with H2O vapor induced by different synthetic methods

Photoreduction of carbon dioxide (CO₂) provides an effective perspective for solving the energy crisis and environmental problems. Herein, two types of composite photocatalysts (TiO₂/ZIF-8) based on ZIF-8 and TiO₂ have been designed and synthesized with the help of the grinding method and the solid-synthesis method. Both composite photocatalysts are employed for the photocatalytic reduction of CO₂. In composite photocatalysts prepared by the grinding method, ZIF-8 particles are distributed on the surface of TiO₂, and provide extra available spaces for storing CO₂, which is beneficial for improving their photoreduction performances. As a result, an enhanced CO formation rate of 21.74 μmol g^-1 h^-1 with a high selectivity of 99% is obtained for this family of composite photocatalysts via the solid-gas mode without photosensitizers and sacrificial agents. For comparison, the other family of composite photocatalysts synthesized via the solid-synthesis method possesses structures similar to ZIF-8, where TiO₂ is encapsulated inside the framework of ZIF-8. This structural feature obstructs the contact between the active sites of TiO₂ and CO₂, and leads to lower activities. The best CO formation rate of this family is only 10.67 μmol g^-1 h^-1 with 90% selectivity. Both the structural features of the two families of photocatalysts are described to explain their differences in photoreduction performances. The experimental finding reveals that different synthetic approaches indeed result in diversified structures and varied photocatalytic performances. This work affords a new scalable and efficient approach for the rational design of efficient photocatalysts in the area of artificial photosynthesis.

Bimetallic zeolite-imidazole framework-based heterostructure with enhanced photocatalytic hydrogen production activity

Bimetallic zeolite-imidazole frameworks with controllable flat band position, band gap and hydrogen evolution reaction characteristics were adopted as a photocatalytic hydrogen production catalyst. Furthermore, the g-C3N4-MoS2 2D-2D surface heterostructure was introduced to the ZnM-ZIF to facilitate the separation as well as utilization efficiency of the photo-exited charge carriers in the ZnM-ZIFs. On the other hand, the ZnM-ZIFs not only inhibited the aggregation of the g-C3N4-MoS2 heterostructure, but also improved the separation and transport efficiency of charge carriers in g-C3N4-MoS2. Consequently, the optimal g-C3N4-MoS2-ZnNi-ZIF exhibited an extraordinary photocatalytic hydrogen evolution activity 214.4, 37.5, and 3.7 times larger than that of the pristine g-C3N4, g-C3N4-ZnNi-ZIF and g-C3N4-MoS2, respectively, and exhibited a H2-evolution performance of 77.8 μmol h-1 g-1 under UV-Vis light irradiation coupled with oxidation of H2O into H2O2. This work will furnish a new MOF candidate for photocatalysis and provide insight into better utilization of porous MOF-based heterostructures for hydrogen production from pure water.

Critical Aspects of Metal-Organic Framework-Based Materials for Solar-Driven CO2 Reduction into Valuable Fuels

Photoreduction of CO2 into value-added fuels is one of the most promising strategies for tackling the energy crisis and mitigating the "greenhouse effect." Recently, metal-organic frameworks (MOFs) have been widely investigated in the field of CO2 photoreduction owing to their high CO2 uptake and adjustable functional groups. The fundamental factors and state-of-the-art advancements in MOFs for photocatalytic CO2 reduction are summarized from the critical perspectives of light absorption, carrier dynamics, adsorption/activation, and reaction on the surface of photocatalysts, which are the three main critical aspects for CO2 photoreduction and determine the overall photocatalytic efficiency. In view of the merits of porous materials, recent progress of three other types of porous materials are also briefly summarized, namely zeolite-based, covalent-organic frameworks based (COFs-based), and porous semiconductor or organic polymer based photocatalysts. The remarkable performance of these porous materials for solar-driven CO2 reduction systems is highlighted. Finally, challenges and opportunities of porous materials for photocatalytic CO2 reduction are presented, aiming to provide a new viewpoint for improving the overall photocatalytic CO2 reduction efficiency with porous materials.

Rapid Exciton Migration and Amplified Funneling Effects of Multi-Porphyrin Arrays in a Re(I)/Porphyrinic MOF Hybrid for Photocatalytic CO2 Reduction

A porphyrinic metal-organic framework (PMOF) known as PCN-222(Zn) was chemically doped with a molecular Re(I) catalyst-bearing carboxylate anchoring group to form a new type of metal-organic framework (MOF)-Re(I) hybrid photocatalyst. The porphyrinic MOF-sensitized hybrid (PMOF/Re) was prepared with an archetypical CO₂ reduction catalyst, (L)Re^I(CO)₃Cl (Re(I); L = 4,4'-dicarboxylic-2,2'-bipyridine), in the presence of 3 vol % water produced CO with no leveling-off tendency for 59 h to give a turnover number of ≥1893 [1070 ± 80 μmol h^-1 (g MOF)^-1]. The high catalytic activity arises mainly from efficient exciton migration and funneling from photoexcited porphyrin linkers to the peripheral Re(I) catalytic sites, which is in accordance with the observed fast exciton (energy) migration (≈1 ps) in highly ordered porphyrin photoreceptors and the effective funneling into Re(I) catalytic centers in the Re(I)-doped PMOF sample. Enhanced catalytic performance is convincingly supported by serial photophysical measurements including decisive Stern-Volmer interpretation.