Iron-Complex-Based Supramolecular Framework Catalyst for Visible-Light-Driven CO2 Reduction

Development of a Ru-porphyrin-based supramolecular framework catalyst for styrene epoxidation

A new microporous supramolecular-framework Ru(II)-porphyrin catalyst containing non-covalent interactions between pyrenylphenyl moieties at the meso-position of the porphyrin ring is synthesised and structurally characterised. This recyclable catalyst expedites styrene epoxidation more efficiently than homogeneous Ru-porphyrin catalytic systems.

Electronic structure modulation via single atom interfacial engineering for selective atmospheric CO2 photoreduction

Single-atom iridium was interfacially engineered on CuFeS2 quantum dots (QDs) with highly efficient CO2 photoreduction performance. The electronic structure modulation yielded a remarkable CO yield rate of 32.5 μmol g-1 h-1 with 92.2% selectivity under atmospheric conditions. This work demonstrates the potential of single-atom interfacial engineering for advancing CO2 photoreduction.

Precisely Constructing Molecular Junctions in Hydrogen-Bonded Organic Frameworks for Efficient Artificial Photosynthetic CO2 Reduction

The development of artificial photocatalysts to convert CO2 into renewable fuels and H2O into O2 is a complex and crucial task in the field of photosynthesis research. The current challenge is to enhance photogenerated charge separation, as well as to increase the oxidation capability of materials. Herein, a molecular junction-type porphyrin-based crystalline photocatalyst (Ni-TCPP-TPyP) was successfully self-assembled by incorporating a nickel porphyrin complex as a reduction site and pyridyl porphyrin as an oxidation site via hydrogen bonding and π-π stacking interactions. The resulting material has a highly crystalline structure, and the formation of inherent molecular junctions can accelerate photogenerated charge separation and transport. Thus, Ni-TCPP-TPyP achieved an excellent CO production rate of 309.3 μmol g-1 h-1 (selectivity, ~100 %) without the use of any sacrificial agents, which is more than ten times greater than that of single-component photocatalyst (Ni-TCPP) and greater than that of the most organic photocatalysts. The structure-function relationship was investigated by femtosecond transient absorption spectroscopy and density functional theory calculations. Our work provides new insight for designing efficient artificial photocatalysts, paving the way for the development of clean and renewable fuels through the conversion of CO2 using solar energy.

Charge Transport Approaches in Photocatalytic Supramolecular Systems Composing of Semiconductor and Molecular Metal Complex for CO2 Reduction

The design of photocatalytic supramolecular systems composing of semiconductors and molecular metal complexes for CO2 reduction has attracted increasing attention. The supramolecular system combines the structural merits of semiconductors and metal complexes, where the semiconductor harvests light and undertakes the oxidative site, while the metal complex provides activity for CO2 reduction. The intermolecular charge transfer plays crucial role in ensuring photocatalytic performance. Here, we review the progress of photocatalytic supramolecular systems in reduction of CO2 and highlight the interfacial charge transfer pathways, as well as their state-of-the-art characterization methods. The remaining challenges and prospects for further design of supramolecular photocatalysts are also presented.

Pivotal Impact Factors in Photocatalytic Reduction of CO2 to Value-Added C1 and C2 Products

Over the past decades, CO2 greenhouse emission has been considerably increased, causing global warming and climate change. Indeed, converting CO2 into valuable chemicals and fuels is a desired option to resolve issues caused by its continuous emission into the atmosphere. Nevertheless, CO2 conversion has been hampered by the ultrahigh dissociation energy of C=O bonds, which makes it thermodynamically and kinetically challenging. From this prospect, photocatalytic approaches appear promising for CO2 reduction in terms of their efficiency compared to other traditional technologies. Thus, many efforts have been made in the designing of photocatalysts with asymmetric sites and oxygen vacancies, which can break the charge distribution balance of CO2 molecule, reduce hydrogenation energy barrier and accelerate CO2 conversion into chemicals and fuels. Here, we review the recent advances in CO2 hydrogenation to C1 and C2 products utilizing photocatalysis processes. We also pin down the key factors or parameters influencing the generation of C2 products during CO2 hydrogenation. In addition, the current status of CO2 reduction is summarized, projecting the future direction for CO2 conversion by photocatalysis processes.

Achieving Almost 100% Selectivity in Photocatalytic CO2 Reduction to Methane via In-Situ Atmosphere Regulation Strategy

Artificial photosynthesis, harnessing solar energy to convert CO2 into hydrocarbons, presents a promising solution for climate change and energy scarcity. However, photocatalytic CO2 reduction often terminates at the CO stage due to limited electron transfer capacity, hindering the formation of higher-energy hydrocarbons such as CH4. This study introduces, for the first time, an in-situ atmosphere regulation strategy, refined from molecular imprinting methodologies, using dynamically reacting molecules to precisely engineer photocatalytic surface sites for selective *CO adsorption and hydrogenation in CO2-to-CH4 conversion. Specifically, the single-atom Cu catalyst (Cu-SA-CO) is prepared by anchoring single-atom Cu onto defective TiO2 substrates (Cu-SA-CO) under a CO reduction atmosphere. Under illumination, the catalyst exhibited outstanding CH4 selectivity (almost 100%) and productivity (58.5 µmol g-1 h-1). Mechanistic investigations reveal that the coordination environment of the Cu single atoms is significantly affected by dynamically reacting molecules (CO and *CHxO) during synthesis, leading to a Ti-Cu-O structure. The structure, with the synergistic interaction between Cu single atoms and oxygen defects, significantly enhances *CO adsorption and hydrogenation, thereby promoting the formation of methane. This work pioneers the use of dynamically reactive molecules as imprinted templates to tune photocatalytic CO2 reduction selectivity, providing a novel avenue for designing efficient photocatalysts.

Mechanical Bond Induced Enhancement and Purification of Pyrene Emission in the Solid State

To address key concerns on solid-state pyrene-based luminescent materials, we propose a novel and efficient mechanical bond strategy. This strategy results in a transformation from ACQ to AIE effect and a remarkable enhancement of pyrene emission in the solid state. Moreover, an unusual purification of emission is also achieved. Through computational calculation and experimental characterisation, finally determined by X-ray diffraction analysis, we prove that the excellent emissions result from mechanical bond induced refinement of molecular arrangements, including reduced π-π stacking, well-ordered packing and enhanced structural stability. This work demonstrates the potential of mechanical bond in the field of organic luminescent molecules, providing a new avenue for developing high-performance organic luminescent materials.

A robust Fe-based heterogeneous photocatalyst for the visible-light-mediated selective reduction of an impure CO2 stream

The transformation of CO2 into value-added products from an impure CO2 stream, such as flue gas or exhaust gas, directly contributes to the principle of carbon capture and utilization (CCU). Thus, we have developed a robust iron-based heterogeneous photocatalyst that can convert the exhaust gas from the car into CO with an exceptional production rate of 145 μmol g-1 h-1. We characterized this photocatalyst by PXRD, XPS, ssNMR, EXAFS, XANES, HR-TEM, and further provided mechanistic experiments, and multi-scale/level computational studies. We have reached a clear understanding of its properties and performance that indicates that this highly robust photocatalyst could be used to design an efficient visible-light-mediated reduction strategy for the transformation of impure CO2 streams into value-added products.

Controlled crystallisation of porous crystals of luminescent platinum(II) complexes by electronic tuning of ancillary ligands

Porous molecular crystals (PMCs) have gained significant importance as next-generation functional porous materials. However, the selective crystallisation of the PMC phase remains a challenge. Herein, we have systematically controlled the stability of the luminescent PMC phase prepared using the luminescent Pt(II) complex [Pt(pbim)(N^O)] (pbim = 2-phenylbenzimidazolate, N^O = N-heteroaryl carboxylate) with Pt⋯Pt electronic interactions. The PMC phase formation varied significantly among the complexes depending on the heteroaryl group of the ancillary N^O ligand; the oxazolyl-bearing complex did not form a PMC phase, whereas the pyrazyl- and 5-fluoropyridyl-bearing complexes spontaneously formed a porous structure. This difference was rationalised by the π-stacking capability of the heteroaryl group of the ancillary ligand. Furthermore, owing to the presence of the one-dimensional Pt⋯Pt chains in this PMC phase, the photophysical properties of PMCs resulting from the Pt⋯Pt interactions were also significantly changed by the ancillary ligands.

Fluorinated chlorin chromophores for red-light-driven CO2 reduction

The utilization of low-energy photons in light-driven reactions is an effective strategy for improving the efficiency of solar energy conversion. In nature, photosynthetic organisms use chlorophylls to harvest the red portion of sunlight, which ultimately drives the reduction of CO2. However, a molecular system that mimics such function is extremely rare in non-noble-metal catalysis. Here we report a series of synthetic fluorinated chlorins as biomimetic chromophores for CO2 reduction, which catalytically produces CO under both 630 nm and 730 nm light irradiation, with turnover numbers of 1790 and 510, respectively. Under appropriate conditions, the system lasts over 240 h and stays active under 1% concentration of CO2. Mechanistic studies reveal that chlorin and chlorinphlorin are two key intermediates in red-light-driven CO2 reduction, while corresponding porphyrin and bacteriochlorin are much less active forms of chromophores.

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.

Bridging molecular photosensitizer and catalyst on carbon nanotubes toward enhanced selectivity and durability for CO2 photoreduction

Homogenous molecular photocatalysts for CO2 reduction, especially metal complex-based photosensitizer‒catalyst assemblages, have been attracting extensive research interests due to their efficiency and customizability. However, their low durability and recyclability limit practical applications. In this work, we immobilized the catalysts of metal terpyridyl complexes and the photosensitizer of [Ru(bpy)3]Cl2 onto the surface of carbon nanotubes through covalent bonds and electrostatic interactions, respectively, transforming the homogeneous system into a heterogeneous one. Our characterizations prove that these metal complexes are well dispersed on CNTs with a high loading (ca. 12 wt.%). Photocatalytic measurements reveal that catalytic activity is remarkably enhanced when the molecular catalysts are anchored, which is three times higher than that of homogeneous molecular catalysts. Moreover, when the photosensitizer of [Ru(bpy)3]Cl2 is immobilized, the side reaction of hydrogen evolution is completely suppressed and the selectivity for CO production reaches 100%, with its durability also significantly improved. This work provides an effective pathway for constructing heterogeneous photocatalysts based on rational assembly of efficient molecular photosensitizers and catalysts.

Ligand Defect-Induced Active Sites in Ni-MOF-74 for Efficient Photocatalytic CO2 Reduction to CO

The conversion of CO2 into valuable carbon-based products using clean and renewable solar energy has been a significant challenge in photocatalysis. It is of paramount importance to develop efficient photocatalysts for the catalytic conversion of CO2 using visible light. In this study, the Ni-MOF-74 material is successfully modified to achieve a highly porous structure (Ni-74-Am) through temperature and solvent modulation. Compared to the original Ni-MOF-74, Ni-74-Am contains more unsaturated Ni active sites resulting from defects, thereby enhancing the performance of CO2 photocatalytic conversion. Remarkably, Ni-74-Am exhibits outstanding photocatalytic performance, with a CO generation rate of 1380 µmol g-1 h-1 and 94% CO selectivity under visible light, significantly surpassing the majority of MOF-based photocatalysts reported to date. Furthermore, experimental characterizations reveal that Ni-74-Am has significantly higher efficiency of photogenerated electron-hole separation and faster carrier migration rate for photocatalytic CO2 reduction. This work enriches the design and application of defective MOFs and provides new insights into the design of MOF-based photocatalysts for renewable energy and environmental sustainability. The findings of this study hold significant promise for developing efficient photocatalysts for CO2 reduction under visible-light conditions.

π-Sticked Metal‒Organic Monolayers for Single-Metal-Site Dependent CO2 Photoreduction and Hydrogen Evolution Reaction

Hierarchical self-assembly of 2D metal‒organic layers (MOLs) for the construction of advanced functional materials have witnessed considerable interest, due to the increasing atomic utilizations and well-defined atom‒property relationship. However, the construction of atomically precise MOLs with mono-/few-layered thickness through hierarchical self-assembly process remains a challenge, mostly because the elaborate long-range order is difficult to control via conventional noncovalent interaction. Herein, a quadruple π-sticked metal‒organic layer (πMOL) is reported with checkerboard-like lattice in ≈1.0 nanometre thickness, on which the catalytic selectivity can be manipulated for highly efficient CO2 reduction reaction (CO2RR) and hydrogen evolution reaction (HER) over a single metal site. In saturated CO2 aqueous acetonitrile, Fe-πMOL achieves a highly effective CO2RR with the yield of ≈3.98 mmol g‒1 h‒1 and 91.7% selectivity. In contrast, the isostructural Co-πMOL as well as mixed metallic FeCo-πMOL exhibits a high activity toward HER under similar conditions. DFT calculations reveal that single metal site exhibits the significant difference in CO2 adsorption energy and activation barrier, which triggers highly selective CO2RR for Fe site and HER for Co site, respectively. This work highlights the potential of supramolecular π…π interaction for constructing monolayer MOL materials to uniformly distribute the single metal sites for artificial photosynthesis.

Molecular Co-Catalyst Confined within a Metallacage for Enhanced Photocatalytic CO2 Reduction

The construction of structurally well-defined supramolecular hosts to accommodate catalytically active species within a cavity is a promising way to address catalyst deactivation. The resulting supramolecular catalysts can significantly improve the utilization of catalytic sites, thereby achieving a highly efficient chemical conversion. In this study, the Co-metalated phthalocyanine (Pc-Co) was successfully confined within a tetragonal prismatic metallacage, leading to the formation of a distinctive type of supramolecular photocatalyst (Pc-Co@Cage). The host-guest architecture of Pc-Co@Cage was unambiguously elucidated by single-crystal X-ray diffraction (SCXRD), NMR, and ESI-TOF-MS, revealing that the single cobalt active site can be thoroughly isolated within the space-restricted microenvironment. In addition, we found that Pc-Co@Cage can serve as a homogeneous supramolecular photocatalyst that displays high CO2 to CO conversion in aqueous media under visible light irradiation. This supramolecular photocatalyst exhibits an obvious improvement in activity (TONCO = 4175) and selectivity (SelCO = 92%) relative to the nonconfined Pc-Co catalyst (TONCO = 500, SelCO = 54%). The present strategy provided a rare example for the construction of a highly active, selective, and stable photocatalyst for CO2 reduction through a cavity-confined molecular catalyst within a discrete metallacage.

Asymmetric Cu-N-La Species Enabling Atomic-Level Donor-Acceptor Structure and Favored Reaction Thermodynamics for Selective CO2 Photoreduction to CH4

Photocatalytic CO2 reduction into ideal hydrocarbon fuels, such as CH4 , is a sluggish kinetic process involving adsorption of multiple intermediates and multi-electron steps. Achieving high CH4 activity and selectivity therefore remains a great challenge, which largely depends on the efficiency of photogenerated charge separation and transfer as well as the intermediate energy levels in CO2 reduction. Herein, we construct La and Cu dual-atom anchored carbon nitride (LaCu/CN), with La-N4 and Cu-N3 coordination bonds connected by Cu-N-La bridges. The asymmetric Cu-N-La species enables the establishment of an atomic-level donor-acceptor structure, which allows the migration of electrons from La atoms to the reactive Cu atom sites. Simultaneously, intermediates during CO2 reduction on LaCu/CN demonstrate thermodynamically more favorable process for CH4 formation based on theoretical calculations. Eventually, LaCu/CN exhibits a high selectivity (91.6 %) for CH4 formation with a yield of 125.8 μmol g-1 , over ten times of that for pristine CN. This work presents a strategy for designing multi-functional dual-atom based photocatalysts.

Fluorescent and Catalytic Properties of a 2D Lamellar Zn Metal-Organic Framework with sql Network Structure

A two-dimensional (2D) lamellar Zn metal-organic framework (Zn-MOF, 1) with a fluorescent 1,6-di(pyridin-3-yl)pyrene (3-DPPy) and 1,4-benzenedicarboxylate (BDC2-) bridging linkers was prepared and structurally characterized. The chemical formula of 1 is [Zn(μ-3-DPPy)(μ-BDC)]n. The mononuclear Zn(II) ion, acting as a node, is tetrahedrally coordinated with two 3-DPPy and two BDC linkers. The coordination environment of Zn(II) is a distorted tetrahedral geometry. The Zn-MOF is the sql network structure based on topology analysis. The undulated 2D sheets of 1 tightly pack together to form a lamellar structure. The pyrene moieties are parallelly oriented to each other. The Zn-MOF is not porous, possibly because the mononuclear Zn(II) node did not form cluster-based secondary building units due to the less symmetric 3-DPPy. The steady-state fluorescence measurements indicate that the fluorescence signal of the 1 is slightly blue-shifted compared to the free 3-DPPy in the solid state. The excimer emission band at 463 nm for crystalline 3-DPPy is shifted to 447 nm for 1. The value of 447 nm is also a blue-shift value compared to nonsubstituted pyrene crystals (470 nm). Despite its nonporosity, the surface Lewis acidic sites of 1 could catalyze the transesterification of esters. Surface defect sites are responsible for this catalytic activity.

The Facile Synthesis of a Re-Complex Heterogeneous Catalysis System for Enhancing CO2 Photoreduction Activity

fac-Re(2,2'-bipyridine)(CO)₃Cl] (denoted as ReCC) is an efficient molecule-catalyst with high selectivity in the photoreduction of CO₂ to CO in a homogeneous system. However, the two major drawbacks of Re(I) complexes in the homogeneous system, easy degradation and difficult separation, seriously hinder its development in the field of industrial applications. In this paper, we designed and prepared two different Re-complex fixation systems (denoted as ReCC@TiO₂-5 wt% and ReCC-TiO₂-5 wt%) based on TiO₂ gel via the sensitization method and sol-gel method, respectively. Compared with a pure ReCC complex, both of them exhibited excellent photocatalytic reduction activity. In particular, the sol-gel hybrid system (ReCC-TiO₂-5 wt%) displayed outstanding positive synergistic effects on the photocatalytic activity and the long durability of the photocatalytic process. A series of characterizations were carried out to explore the probable photocatalytic reduction process mechanism, which provides the theoretical basis and technical support for the Re complex fixation method.