Tailoring Layer Number of 2D Porphyrin-Based MOFs Towards Photocoupled Electroreduction of CO2

Atomically Dispersed Metal Catalysts for the Conversion of CO2 into High-Value C2+ Chemicals

The conversion of carbon dioxide (CO2) into value-added chemicals with two or more carbons (C2+) is a promising strategy that cannot only mitigate anthropogenic CO2 emissions but also reduce the excessive dependence on fossil feedstocks. In recent years, atomically dispersed metal catalysts (ADCs), including single-atom catalysts (SACs), dual-atom catalysts (DACs), and single-cluster catalysts (SCCs), emerged as attractive candidates for CO2 fixation reactions due to their unique properties, such as the maximum utilization of active sites, tunable electronic structure, the efficient elucidation of catalytic mechanism, etc. This review provides an overview of significant progress in the synthesis and characterization of ADCs utilized in photocatalytic, electrocatalytic, and thermocatalytic conversion of CO2 toward high-value C2+ compounds. To provide insights for designing efficient ADCs toward the C2+ chemical synthesis originating from CO2, the key factors that influence the catalytic activity and selectivity are highlighted. Finally, the relevant challenges and opportunities are discussed to inspire new ideas for the generation of CO2-based C2+ products over ADCs.

2D Metal Porphyrin-Based MOFs and ZIF-8 Composite-Derived Carbon Materials Containing M-Nx Active Sites as Bifunctional Electrocatalysts for Zinc-Air Batteries

The main impediment to the development of zinc-air batteries is the sluggish kinetics of the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Transition metal N-doped carbon catalysts offer a promising alternative to noble metal catalysts, with metal-organic framework (MOF)-derived carbon material catalysts being particularly noteworthy. Here, we synthesized MxP-Z-C carbon catalysts by combining two-dimensional (2D) metal porphyrin-based MOFs (MxPMFs, x = Fe, Co, Ni, Mn) and three-dimensional zeolitic imidazole framework-8 (ZIF-8) through electrostatic interaction, followed by carbonization. ZIF-8 was inserted between the layers of MxPMFs to prevent its Π-Π stacking, allowing the active sites to become fully exposed. MxP-Z-C demonstrated an impressive catalytic activity for both the ORR and the OER reactions. Among them, FeP-Z-C showed the best catalytic activity. The half-wave potential for ORR was 0.92 V (vs the reversible hydrogen electrode (RHE)), while the overpotential for the OER was 290 mV. In addition, the zinc-air battery assembled by FeP-Z-C exhibited high power density (133.14 mW cm-2) and significant specific capacity (816 mAh gZn-1), indicating considerable potential as a bifunctional catalyst for electronic devices.

Two-Dimensional Nickel Porphyrinic Metal-Organic Framework-Modified Electrode for Electrochemical Sensing

Due to their highly exposed active sites and high aspect ratio caused by their substantial lateral dimension and thin thickness, two-dimensional (2D) metal-organic framework (MOF) nanosheets are currently considered a potential hybrid material for electrochemical sensing. Herein, we present a nickel-based porphyrinic MOF nanosheet as a versatile and robust platform with an enhanced electrochemical detection performance. It is important to note that the nickel porphyrin ligand reacted with Cu(NO3)2·3H2O in a solvothermal process, with polyvinylpyrrolidone (PVP) acting as the surfactant to control the anisotropic development of creating a 2D Cu-TCPP(Ni) MOF nanosheet structure. To realize the exceptional selectivity, sensitivity, and stability of the synthesized 2D Cu-TCPP(Ni) MOF nanosheet, a laser-induced graphene electrode was modified with the MOF nanosheet and employed as a sensor for the detection of p-nitrophenol (p-NP). With a detection range of 0.5-200 μM for differential pulse voltammetry (DPV) and 0.9-300 μM for cyclic voltammetry (CV), the proposed sensor demonstrated enhanced electrochemical performance, with the limit of detection (LOD) for DPV and CV as 0.1 and 0.3 μM, respectively. The outstanding outcome of the sensor is attributed to the 2D Cu-TCPP(Ni) MOF nanosheet's substantial active surface area, innate catalytic activity, and superior adsorption capacity. Furthermore, it is anticipated that the proposed electrode sensor will make it possible to create high-performance electrochemical sensors for environmental point-of-care testing since it successfully detected p-NP in real sample analysis.

Strain-Induced Surface Interface Dual Polarization Constructs PML-Cu/Bi12O17Br2 High-Density Active Sites for CO2 Photoreduction

The insufficient active sites and slow interfacial charge transfer of photocatalysts restrict the efficiency of CO2 photoreduction. The synchronized modulation of the above key issues is demanding and challenging. Herein, strain-induced strategy is developed to construct the Bi-O-bonded interface in Cu porphyrin-based monoatomic layer (PML-Cu) and Bi12O17Br2 (BOB), which triggers the surface interface dual polarization of PML-Cu/BOB (PBOB). In this multi-step polarization, the built-in electric field formed between the interfaces induces the electron transfer from conduction band (CB) of BOB to CB of PML-Cu and suppresses its reverse migration. Moreover, the surface polarization of PML-Cu further promotes the electron converge in Cu atoms. The introduction of PML-Cu endows a high density of dispersed Cu active sites on the surface of PBOB, significantly promoting the adsorption and activation of CO2 and CO desorption. The conversion rate of CO2 photoreduction to CO for PBOB can reach 584.3 μmol g-1, which is 7.83 times higher than BOB and 20.01 times than PML-Cu. This work offers valuable insights into multi-step polarization regulation and active site design for catalysts.

Anchoring of Metal Complexes on Au25 Nanocluster for Enhanced Photocoupled Electrocatalytic CO2 Reduction

Atomically precise Au nanoclusters (NCs) with discrete energy levels can be used as photosensitizers for CO2 reduction. However, tight ligand capping of Au NCs hinders CO2 adsorption on its active sites. Here, a new hybrid material is obtained by anchoring of thiol functionalized terpyridine metal complexes (metal=Ru, Ni, Fe, Co) on Au NCs by ligand exchange reactions (LERs). The anchoring of Ru and Ni complexes on Au25 NC (Au25 -Ru and Au25 -Ni) leads to adequate CO2 to CO conversion for photocoupled electrocatalytic CO2 reduction (PECR) in terms of high selectivity, with Faradaic efficiency of CO (FECO ) exceeding 90 % in a wide potential range, remarkable activity (CO production rate up to two times higher than that for pristine Au25 PET18 ) and extremely large turnover frequencies (TOFs, 63012 h-1 at -0.97 V for Au25 -Ru and 69989 h-1 at -1.07 V vs. RHE for Au25 -Ni). Moreover, PECR stability test indicates the excellent long-term stability of the modified NCs in contrast with pristine Au NCs. The present approach offers a novel strategy to enhance PECR activity and selectivity, as well as to improve the stability of Au NCs under light illumination, which paves the way for highly active and stable Au NCs catalysts.

Two-Dimensional Porphyrinic Metal-Organic Framework Composites as a Photocatalytic Platform for Chemoselective Hydrogenation

Chemoselective hydrogenation with high efficiency under ambient conditions remains a great challenge. Herein, an efficient photocatalyst, the 2D porphyrin metal-organic framework composite AmPy/Pd-PPF-1(Cu), featuring AmPy (1-aminopyrene) sitting axially on a paddle-wheel unit, has been rationally fabricated. The 2D AmPy/Pd-PPF-1(Cu) composite acts as a photocatalytic platform, promoting the selective hydrogenation of quinolines to tetrahydroquinolines with a yield up to 99%, in which ammonia borane serves as the hydrogen donor. The AmPy molecules coordinated on a 2D MOF not only enhance the light absorption capacity but also adjust the layer spacing without affecting the network structure of 2D Pd-PPF-1(Cu) nanosheets. Through deuterium-labeling experiments, in situ X-ray photoelectron spectroscopy, electron paramagnetic resonance studies, and density functional theory calculations, it is disclosed that Cu paddle-wheel units in 2D AmPy/Pd-PPF-1(Cu) nanosheets behave as the active site for transfer hydrogenation, and metalloporphyrin ligand and axial aminopyrene molecules can enhance the light absorption capacity and excite photogenerated electrons to Cu paddle-wheel units, assisting in photocatalysis.

Porphyrin-based Bi-MOFs with Enriched Surface Bi Active Sites for Boosting Photocatalytic CO2 Reduction

The inherent challenges in using metal-organic frameworks (MOFs) for photocatalytic CO2 reduction are the combination of wide-range light harvesting, efficient charge separation and transfer as well as highly exposed catalytic active sites for CO2 activation and reduction. We present here a promising solution to satisfy these requirements together by modulating the crystal facet and surface atomic structure of a porphyrin-based bismuth-MOF (Bi-PMOF). The series of structural and photo-electronic characterizations together with photocatalytic CO2 reduction experiment collectively establish that the enriched Bi active sites on the (010) surface prefer to promote efficient charge separation and transfer as well as the activation and reduction of CO2 . Specifically, the Bi-PMOFs-120-F with enriched surface Bi active sites exhibits optimal photocatalytic CO2 reduction performance to CO (28.61 μmol h-1 g-1 ) and CH4 (8.81 μmol h-1 g-1 ). This work provides new insights to synthesize highly efficient main group p-block metal Bi-MOF photocatalysts for CO2 reduction through a facet-regulation strategy and sheds light on the surface structure-activity relationships of the MOFs.

Revealing the construction of CuOCe interfacial sites via increased support utilization for enhanced CO2 electroreduction and Li-CO2 batteries

Leveraging designed electronic oxide-metal interactions (EOMI), cerium-supported copper demonstrates remarkable competitiveness in the carbon dioxide reduction reaction (CO2RR). Nevertheless, the limited utilization efficiency of conventional cerium oxide (CeO2) support hampers the EOMI effect. Furthermore, a comprehensive understanding of the influence of distinct crystalline surfaces of CeO2 on the loaded active copper (Cu) species remains elusive. Herein, oxide carriers with diverse crystal facets are acquire for loading to load Cu species through the incorporation of cerium-based metal organic frameworks (MOFs) precursors. Simultaneously, owing to the elevated specific surface area conferred by MOF precursors, Cu/CeO2 hosts ample catalytically active sites for carbon dioxide (CO2) electrocatalytic reactions and as catalytic cathodes for lithium-CO2 (Li-CO2) batteries. Furthermore, the carbon converted from organic ligands in MOFs precursors not only proficiently immobilizes and disperses the active sites, but also enhances the inherent conductive stability of the oxide while augmenting energy utilization efficiency. Leveraging these advantages, the electrocatalyst derived from MOFs achieves a peak CO2-to-methane Faradaic efficiency of 57.9 %, whereas the assembled Li-CO2 batteries exhibit notable activity and durability, boasting a substantial full-discharge capacity of 8907 mAh/g, a discharge voltage of 2.65 V, and an extended cycle life exceeding 1000 h. Mechanistic investigations were conducted using density functional theory (DFT) calculations to thoroughly explore the impact of CeO2 carrier crystal facets, specifically (111), (100), and (110), on the loaded copper species. Notably, (110) was identified as the optimal facet due to its favorable contributions to electronic structure optimization and stability enhancement.

Exfoliation of 2D Metal-Organic Frameworks: toward Advanced Scalable Materials for Optical Sensing

Two-dimensional metal-organic frameworks (MOFs) occupy a special place among the large family of functional 2D materials. Even at a monolayer level, 2D MOFs exhibit unique sensing, separation, catalytic, electronic, and conductive properties due to the combination of porosity and organo-inorganic nature. However, lab-to-fab transfer for 2D MOF layers faces the challenge of their scalability, limited by weak interactions between the organic and inorganic building blocks. Here, comparing three top-down approaches to fabricate 2D MOF layers (sonication, freeze-thaw, and mechanical exfoliation), The technological criteria have established for creation of the layers of the thickness up to 1 nm with a record aspect ratio up to 2*10^4:1. The freezing-thaw and mechanical exfoliation are the most optimal approaches; wherein the rate and manufacturability of the mechanical exfoliation rivaling the greatest scalability of 2D MOF layers obtained by freezing-thaw (21300:1 vs 1330:1 aspect ratio), leaving the sonication approach behind (with a record 900:1 aspect ratio) have discovered. The high quality 2D MOF layers with a record aspect ratio demonstrate unique optical sensitivity to solvents of a varied polarity, which opens the way to fabricate scalable and freestanding 2D MOF-based atomically thin chemo-optical sensors by industry-oriented approach.

Porosity as a Design Element for Developing Catalytic Molecular Materials for Electrochemical and Photochemical Carbon Dioxide Reduction

The catalytic reduction of carbon dioxide (CO2 ) using sustainable energy inputs is a promising strategy for upcycling of atmospheric carbon into value-added chemical products. This goal has inspired the development of catalysts for selective and efficient CO2 conversion using electrochemical and photochemical methods. Among the diverse array of catalyst systems designed for this purpose, 2D and 3D platforms that feature porosity offer the potential to combine carbon capture and conversion. Included are covalent organic frameworks (COFs), metal-organic frameworks (MOFs), porous molecular cages, and other hybrid molecular materials developed to increase active site exposure, stability, and water compatibility while maintaining precise molecular tunability. This mini-review showcases catalysts for the CO2 reduction reaction (CO2 RR) that incorporate well-defined molecular elements integrated into porous materials structures. Selected examples provide insights into how different approaches to this overall design strategy can augment their electrocatalytic and/or photocatalytic CO2 reduction activity.

Adsorption, activation, and conversion of carbon dioxide on small copper-tin nanoclusters

Carbon dioxide (CO₂) conversion to value-added chemicals is an attractive solution to reduce globally accelerating CO₂ emissions. Among the non-precious and abundant metals tested so far, copper (Cu) is one of the best electrocatalysts to convert CO₂ into more than thirty different hydrocarbons and alcohols. However, the selectivity for desired products is often too low. We present a computational investigation of the effects of nanostructuring, doping, and support on the activity and selectivity of Cu-Sn catalysts. Density functional theory calculations were conducted to explore the possibility of using small Cu-Sn clusters, Cu_4-nSn_n (n = 0-4), isolated or supported on graphene and γ-Al₂O₃, to activate CO₂ and convert it to carbon monoxide (CO) and formic acid (HCOOH). First, a detailed analysis of the structure, stability, and electronic properties of Cu_4-nSn_n clusters and their ability to absorb and activate CO₂ was considered. Then, the kinetics of the gas phase CO₂ direct dissociation on Cu_4-nSn_n to generate CO was determined. Finally, the mechanism of electrocatalytic CO₂ reduction to CO and HCOOH on Cu_4-nSn_n, Cu_4-nSn_n/graphene and Cu_4-nSn_n/γ-Al₂O₃ was computed. The selectivity towards the competitive electrochemical hydrogen evolution reaction on these catalysts was also considered. The Cu₂Sn₂ cluster suppresses the hydrogen evolution reaction and is highly selective towards CO, if unsupported, or HCOOH if supported on graphene. This study demonstrates that the Cu₂Sn₂ cluster is a potential candidate for the electrocatalytic conversion of the CO₂ molecule. Moreover, it identifies insightful structure-property relationships in Cu-based nanocatalysts, highlighting the influence of composition and catalyst support on CO₂ activation.

Dislocated Bilayer MOF Enables High-Selectivity Photocatalytic Reduction of CO2 to CO

The highly selective photoreduction of CO₂ into valuable small-molecule chemical feedstocks such as CO is an effective strategy for addressing the energy crisis and environmental problems. However, it remains a challenge because the complex CO₂ photoreduction process usually generates multiple possible products and requires a subsequent separation step. In this paper, 2D monolayer and bilayer porphyrin-based metal-organic frameworks (MOFs) are successfully constructed by adjusting the reaction temperature and solvent polarity with 5,10,15,20-tetrakis(4-pyridyl)porphyrin as the light-harvesting ligand. The bilayer MOF is a low-dimensional MOF with a special structure in which the upper and lower layers are arranged in dislocation and are bridged by halogen ions. This bilayer MOF exhibits 100% ultra-high selectivity for the reduction of CO₂ to CO under simulated sunlight without any cocatalyst or photosensitizer and can be recycled at least three times. The intrinsic mechanism of this photocatalytic CO₂ reduction process is explored through experimental characterization and density functional theory (DFT) calculations. This work shows that the rational design of the number of layers in 2D MOF structures can tune the stability of these structures and opens a new avenue for the design of highly selective MOF photocatalysts.

Metal-Organic Framework-Based Photocatalysis for Solar Fuel Production

Metal-organic frameworks (MOFs) represent a novel class of crystalline inorganic-organic hybrid materials with tunable semiconducting behavior. MOFs have potential for application in photocatalysis to produce sustainable solar fuels, owing to their unique structural advantages (such as clarity and modifiability) that can facilitate a deeper understanding of the structure-activity relationship in photocatalysis. This review takes the photocatalytic active sites as a particular perspective, summarizing the progress of MOF-based photocatalysis for solar fuel production; mainly including three categories of solar-chemical conversions, photocatalytic water splitting to hydrogen fuel, photocatalytic carbon dioxide reduction to hydrocarbon fuels, and photocatalytic nitrogen fixation to high-energy fuel carriers such as ammonia. This review focuses on the types of active sites in MOF-based photocatalysts and discusses their enhanced activity based on the well-defined structure of MOFs, offering deep insights into MOF-based photocatalysis.

Direct growth of SnS2 nanowall photoanode for high responsivity self-powered photodetectors

Tin sulfide (SnS₂) has attracted growing attention due to its environmental friendliness, tunable band gap and potential applications for high-sensitivity photodetectors. However, the low responsivity and slow response speed severely hinder its further applications. In this work, SnS₂ nanowalls have been successfully fabricated on FTO substrates by a facile hydrothermal approach. The prepared SnS₂ nanowalls were used as a photoanode material for photoelectrochemical (PEC)-type photodetectors. The SnS₂ based PEC-type photodetectors exhibit excellent photocurrent density (39.06 μA cm^-2), responsivity (1460 μA W^-1), long-term cycling stability and self-powered behavior. The responsivity of the detector is higher than that of most reported SnS₂ based PEC-type photodetectors and even some SnS₂ based photoconductive photodetectors. The high responsivity and self-powered behavior enable the extended potential applications of SnS₂ in PEC-type photodetectors.

Surface, Interface and Structure Optimization of Metal-Organic Frameworks: Towards Efficient Resourceful Conversion of Industrial Waste Gases

Industrial waste gas emissions from fossil fuel over-exploitation have aroused great attention in modern society. Recently, metal-organic frameworks (MOFs) have been developed in the capture and catalytic conversion of industrial exhaust gases such as SO₂ , H₂ S, NO_x , CO₂ , CO, etc. Based on these resourceful conversion applications, in this review, we summarize the crucial role of the surface, interface, and structure optimization of MOFs for performance enhancement. The main points include (1) adsorption enhancement of target molecules by surface functional modification, (2) promotion of catalytic reaction kinetics through enhanced coupling in interfaces, and (3) adaptive matching of guest molecules by structural and pore size modulation. We expect that this review will provide valuable references and illumination for the design and development of MOF and related materials with excellent exhaust gas treatment performance.

C2 product formation in the CO2 electroreduction on boron-doped graphene anchored copper clusters

A possible remedy for the increasing atmospheric CO₂ concentration is capturing and reducing it into valuable chemicals like methane, methanol, ethylene, and ethanol. However, a suitable catalyst for this process is still under extensive research. Small sized copper clusters have gained attention in recent years due to their catalytic activity in the CO₂ reduction reaction. Although C₂₊ products have a higher economic value, the formation of C₁ products was investigated most thoroughly. Graphene is a promising support for small copper clusters in the electrochemical reduction of CO₂. It exhibits good mechanical and electrical properties, but the weak interaction between copper and graphene is an issue. Our DFT computations reveal that small Cu clusters on the boron-doped graphene (BDG) support are promising catalysts for the electrochemical reduction of CO₂. We found facile reaction pathways towards various C₁ (carbon-monoxide, formic acid, formaldehyde, methanol or methane) and C₂ (ethanol or ethylene) products on Cu₄ and Cu₇ clusters on BDG. The reactivity is cluster-size tunable with Cu₄ being the more reactive agent, while Cu₇ shows a higher selectivity towards C₂ products.

Two-dimensional metal-organic layers constructed from Hf6/Hf12-oxo clusters and a trigonal pyramidal phosphine oxide ligand

Metal-organic layers (MOLs), a category of two-dimensional materials, have attracted wide interest due to their molecular tunability and the ease of surface modification. Herein, we reported the synthesis and structural determination of a free-standing MOL, {[Hf₆O₈H₄(HCOO)₂(H₂O·OH)₄]₃[Hf₁₂O₁₆H₈(HCOO)_6.8(H₂O·OH)_11.2](TPO)₈}_n, constructed from Hf₆-oxo and Hf₁₂-oxo clusters as secondary building units (SBUs) and the tris(4-carboxylphenyl)phosphine oxide (TPO) ligand. We establish a structure model of this new MOL based on the combined information from different characterization methods.