Asymmetric Atomic Dual-Sites for Photocatalytic CO2 Reduction

Ag-Doped hollow Multi-Shelled structure TiO2 for highly selective photocatalytic CO2 reduction

The photocatalytic conversion of CO2 into valuable CH4 offers a sustainable solution to pressing environmental and energy challenges. However, this process is hindered by several factors, including the low adsorption and activation of CO2, rapid recombination of photogenerated charge carriers, and limited selectivity. Herein, hollow multi-shelled Ag-doped TiO2 (Ag/TiO2) nanospheres were successfully synthesized for highly selective photocatalytic CO2 methanation. Time-resolved photoluminescence (TRPL) analysis reveals that Ag doping extends the carrier lifetime from 1.32 ns to 49.11 ns, effectively suppressing recombination. X-ray photoelectron spectroscopy (XPS) confirms that Ag doping leads to a redistribution of electron at Ag and Ti sites, thereby optimizing the adsorption of CO2 on the catalyst. Density functional theory (DFT) calculations indicate that Ag doping strengthens CO2 adsorption (the adsorption energy from -1.54 to -2.04 eV) and affects the desorption of intermediates, thereby altering the reaction products to favor the production of CH4 instead of CO. Moreover, the hollow multi-shelled structure endows Ag/TiO2 with a large specific surface area (142.4 m2/g), which is conducive to the adsorption and activation of CO2. Consequently, the CH4 yield of Ag/TiO2 reached 89.51 μmol·g-1·h-1, which is approximately 6 times greater than that of pristine TiO2. Additionally, the selectivity for CH4 improved to 95 %. These findings highlight the potential of Ag-doped TiO2 for efficient CO2 photoreduction.

Linkages Chemistry of Covalent Organic Frameworks in Photocatalysis and Electrocatalysis

Covalent organic frameworks (COFs) have emerged as promising candidates for electrocatalysis and photocatalysis applications due to their structurally ordered architectures and tunable physicochemical properties. In COFs, organic building blocks are linked via covalent bonds, and the structural and electronic characteristics of COFs are critically governed by their linkage chemistry. These linkages influence essential material attributes including surface area, crystallinity, hydrophobicity, chemical stability, and the optoelectronic behavior (e.g., photoelectron separation efficiency, electron conductivity, and reductive activity), which collectively determine catalytic performance in energy conversion systems. A systematic understanding of linkage engineering in COFs not only advances synthetic methodologies but also provides innovative solutions to global energy and environmental challenges, thereby accelerating the development of sustainable technologies for clean energy production and environmental remediation.

A perspective on NiCo2O4-based photocatalysts: from fundamentals, modification strategies to applications

Semiconductor-mediated photocatalysis holds promise in both clean energy supply and environmental remediation, and is regarded as one of the most ideal approaches to achieving carbon neutrality in the future. Low-cost, visible-light-response, and high-efficiency semiconductor photocatalysts are key to the practical application of photocatalysis. Compared with the binary counterpart Co3O4, ternary NiCo2O4 has high cation disorder, variable electronic structure, and asymmetry dual metal sites, thus exhibiting great potential in developing high-efficiency visible-light-response photocatalysts. In this review, the fundamental advantages and drawbacks of NiCo2O4 for heterogenous photocatalysis are first introduced in terms of the crystal structure, electronic band structure, and surface atom exposure. Modification strategies for NiCo2O4-based photocatalysts are then surveyed and commented on in detail, including the aspects of morphology regulation, defect engineering, construction of heterojunctions, loading cocatalysts, and integration of multiple strategies. Furthermore, the research progress of NiCo2O4-based photocatalysts for water splitting, carbon dioxide reduction, and pollution degradation is summarized. In particular, some research gaps and problems with property modifications and practical applications are also highlighted in this context, which offers specific directions for future research. Finally, this review is concluded by outlining the challenges and opportunities of NiCo2O4-based photocatalysts for solar energy conversion.

Strengthening built-in electric field and enriching active sites on cobalt-doped ZnSn(OH)6/ZnWO4 heterojunction to promote photocatalytic reduction of CO2

The photocatalytic activity of photocatalysts is often limited by rapid recombination of photo-induced electron-hole pairs, insufficient active sites and slow reaction kinetics. In this study, the Co-doped ZnSn(OH)6/ZnWO4 heterojunctions with oxygen vacancies and Lewis basic sites were synthesized for efficient photocatalytic CO2 reduction. The Co-doped ZnSn(OH)6/ZnWO4 exhibited superior photoelectrochemical properties to the ZnSn(OH)6 and ZnWO4. Results of kelvin probe force microscopy (KPFM) and electron density difference calculations demonstrated that Co doping induced lattice distortion in ZnSn(OH)6, generating a local electric field, which, in synergy with oxygen vacancies in ZnWO4, further enhanced the built-in electric field (IEF) within the Co-doped ZnSn(OH)6/ZnWO4 heterojunction, significantly accelerating carriers separation. Density functional theory (DFT) calculation also revealed that the Lewis basicity of ZnSn(OH)6 and oxygen vacancies in ZnWO4 enhanced CO2 adsorption on the Co-doped ZnSn(OH)6/ZnWO4 heterojunction, facilitating CO2 conversion. The Co-ZnSn(OH)6/ZnWO4-VO composite exhibited the highest CO production rate (90.18 μmol·g-1·h-1) during CO2 reduction, which was 25.47 and 1.28 times of those of ZnSn(OH)6 and Co-ZnSn(OH)6/ZnWO4, respectively. The main reaction intermediates were identified and CO2 reduction mechanism was proposed. This work provides reference to improve photocatalytic activity by enhancing IEF and increasing active sites in heterojunctions.

Engineering Local Coordination and Electronic Structures of Dual-Atom Catalysts

Heterogeneous dual-atom catalysts (DACs), defined by atomically precise and isolated metal pairs on solid supports, have garnered significant interest in advancing catalytic processes and technologies aimed at achieving sustainable energy and chemical production. DACs present board opportunities for atomic-level structural and property engineering to enhance catalytic performance, which can effectively address the limitations of single-atom catalysts, including restricted active sites, spatial constraints, and the typically positive charge nature of supported single metal species. Despite the rapid progress in this field, the intricate relationship between local atomic environments and the catalytic behavior of dual-metal active sites remains insufficiently understood. This review highlights recent progress and major challenges in this field. We begin by discussing the local modulation of coordination and electronic structures in DACs and its impact on catalytic performance. Through specific case studies, we demonstrate the importance of optimizing the entire catalytic ensemble to achieve efficient, selective, and stable performance in both model and industrially relevant reactions. Additionally, we also outline future research directions, emphasizing the challenges and opportunities in synthesis, characterization, and practical applications, aiming to fully unlock the potential of these advanced catalysts.

Asymmetric Triple-Atom Sites Combined with Oxygen Vacancy for Selective Photocatalytic Conversion of CO2 to Propionic Acid

Photocatalytic CO2 reduction to multicarbon products is an emerging approach for achieving carbon neutrality; however, the design of active sites that effectively promote multistep C-C coupling remains a challenge. Here, we propose a straightforward defect engineering approach to construct asymmetric triple-atom sites (Cu-Cuδ+-Wδ+) on CuWO4 with oxygen vacancies (OVs) (named CWO-OVs). The optimized CWO-OVs achieve a photochemical synthesis rate of propionic acid (C3H6O2, PA) of 86.46±2.92 μmol g-1 h-1, with an electron-based selectivity of 89.27 %, which exhibits a remarkable advantage in the field of photocatalytic CO2 reduction to C2+ products. Experimental results and density functional theory calculations corroborate the prominent role of OVs in inducing the triple-atom sites: (1) the asymmetric Cu-Cuδ+ triggers the first step of C1-C1 coupling to form *CH2CH3; (2) Cuδ+-Wδ+ facilitates subsequent C2-C1 bonding, ultimately leading to PA production. This charge-asymmetric cascade reaction system offers new insights into the design of efficient photocatalysts for the synthesis of multi-carbon products.

From Single-Atom to Dual-Atom: A Universal Principle for the Rational Design of Heterogeneous Fenton-like Catalysts

Developing efficient heterogeneous Fenton-like catalysts is the key point to accelerating the removal of organic micropollutants in the advanced oxidation process. However, a general principle guiding the reasonable design of highly efficient heterogeneous Fenton-like catalysts has not been constructed up to now. In this work, a total of 16 single-atom and 272 dual-atom transition metal/nitrogen/carbon (TM/N/C) catalysts for H2O2 dissociation were explored systematically based on high-throughput density functional theory and machine learning. It was found that H2O2 dissociation on single-atom TM/N/C exhibited a distinct volcano-type relationship between catalytic activity and •OH adsorption energy. The favorable •OH adsorption energies were in the range of -3.11 ∼ -2.20 eV. Three different descriptors, namely, energetic, electronic, and structural descriptors, were found, which can correlate the intrinsic properties of catalysts and their catalytic activity. Using adsorption energy, stability, and activation energy as the evaluation criteria, two dual-atom CoCu/N/C and CoRu/N/C catalysts were screened out from 272 candidates, which exhibited higher catalytic activity than the best single-atom TM/N/C catalyst due to the synergistic effect. This work could present a conceptually novel understanding of H2O2 dissociation on TM/N/C and inspire the structure-oriented catalyst design from the viewpoint of volcano relationship.

Asymmetric Coordination in Cobalt Single-Atom Catalysts Enables Fast Charge Dynamics and Hierarchical Active Sites for Two-Stage Kinetics in Photodegradation of Organic Pollutants

Single-atom catalysts (SACs) have attracted growing interest in solar-driven catalysis, though challenges persist due to symmetrical metal coordination, which results in limited driving force and sluggish charge dynamics. Additionally, uneven energy and mass distribution complicate reaction pathways, ultimately restricting solar energy utilization and catalytic efficiency. Herein, we synthesized cobalt single atoms decorated carbon nitride catalysts featuring a highly asymmetric Co─C2N3 coordination, tailored for photocatalytic organic pollutants removal. Advanced experimental studies and simulation results collectively revealed that the unique microenvironment surrounding Co single atoms improved charge dynamics and created reactive hot spots, facilitating the generation of reactive oxygen species during the photocatalytic degradation of organic pollutants. These enhanced charge dynamics, combined with hierarchical active sites, resulted in two-stage reaction kinetics and excellent stability for the degradation of bisphenol A in wastewater, distinctly outperforming the first-stage kinetics observed for polymeric carbon nitride. This work advances the understanding of structure-performance relationships in SAC-based photocatalytic degradation and offers valuable insights for the design of next-generation SACs in environmental catalysis.

Poly-Phosphamide Catalyzed Visible-Light-Driven CH4 and Dark-Phase-Mediated Cyclic Carbonate Productions Utilizing CO2

A poly-phosphamide (POP) with a band gap of 2.8 eV is used for the photochemical conversion of CO2 into CH4 and chemical conversion of CO2 and organo-epoxides into cyclic carbonates. The Tauc plot and Mott Schottky analyses indicate the conduction band potential at -1.49 V (vs NHE), much more negative than the multi-electron CO2 reduction potential and the lifetime of the photo-excited electron is found 2.8 ns. On photoirradiation of 420 nm light, the POP in the presence of triethanolamine or ascorbic acid can selectively convert CO2 into CH4 (≈99%) with a yield of 4.6 mmol g-1. On visible-light irradiation, the drop of charge-transfer resistance (Rct) and an enhancement of cathodic current further confirm the photon-harvesting efficiency of the POP. In situ, FTIR study identifies the CO2 adsorption to the POP and possible reaction intermediate, like *-CO, *-CH2OH. POP also behaves as a catalyst for CO2 conversion to cyclic carbonates under solvent-free conditions with more than 98% yield. After the light-phase and dark-phase reactions, POP can be successfully recycled at least five times without structural degradation. Herein, the POP acts as a bi-functional, and recyclable polymeric organic material to convert CO2 to essential feedstocks under mild reaction conditions.

Construction of S-Scheme Cs2AgBiBr6/BiVO4 Heterojunctions with Fast Charge Transfer Kinetics Toward Promoted Photocatalytic Conversion of CO2

Lead-based halide perovskites (LHPs) have been widely explored by researchers in the field of photocatalysis. However, the poor stability and toxicity of LHPs limit their large-scale applications. Here, lead-free Cs2AgBiBr6/BiVO4 (CABB/BVO)-X% (X = 30, 50, 100) S-scheme heterojunction composites are prepared by electrostatic assembly, and their catalytic activity for photoreduction of CO2 is evaluated. After 3 h of simulated solar irradiation, the prepared CABB/BVO-50% composites show the highest CO yield and electron consumption rate of 143.59 and 352.22 µmol g-1, which are 9.2 and 7.8 times higher than that of CABB alone, respectively. In addition, the prepared CABB/BVO-50% photocatalysts exhibit 81.5% high selectivity for CO. The generation of an internal electric field (IEF) between the two materials and the generation of S-scheme heterojunctions are powerfully confirmed by employing various characterization techniques and DFT calculations. The low carrier recombination rate, bandgap-matched heterointerfaces, and exceptional S-scheme charge transfer mechanism are primarily responsible for the outstanding performance. This work provides new insights into the design of efficient lead-free perovskites-based photocatalytic materials.

Asymmetric Atomic Pt-B Dual-Site Catalyst for Efficient Photoreforming of Waste Polylactic Acid Plastics in Seawater

Waste plastic has imposed significant burdens on marine ecosystems. Converting plastic into high-value products via photocatalysis is an emerging and promising approach, but its low activity and product selectivity pose great challenges. Herein, we report a carbon nitride-anchored atomically dispersed Pt-B dual-site catalyst (Pt SA/BCN100) for the photoreforming of polylactic acid (PLA) into high-value chemicals and H2 in seawater. Experiments and DFT calculations reveal that significantly enhanced charge transfer occurs between the Pt site and the B site, and the hole-rich B site can selectively trigger the activation and cleavage of the C-H and C-C bonds of PLA to form acetic acid (AA), while the electron-rich Pt site drives the reduction of H protons to H2. As a result, Pt SA/BCN100 exhibits a high H2 evolution rate of 993 μmol gcatal-1 h-1 and an AA production rate of 300 μmol gcatal-1 h-1 with a selectivity of over 98%. We also demonstrate the direct photoreforming of g-scale real-world PLA wastes and low concentrations of PLA microplastics in natural seawater. Techno-economic analysis and environmental assessment show that this catalytic system can significantly reduce carbon emissions and has potential commercial value.

One-Step Molten Salt Constructing Double S-Scheme K0.2WO3/NiO/NiWO4 Heterojunction for Photocatalytic CO2 Reduction

Rapid charge separation and transfer is the key scientific problem in photocatalysis. The construction of S-scheme heterojunction is one of the effective strategies to promote charge separation and maintain the strong redox properties. Herein, the NiO, K0.2WO3, and NiWO4 ternary double S-scheme K0.2WO3/NiO/NiWO4 heterojunction (W/NiO) was created by a one-step molten salt method. Ultraviolet-visible (UV-Vis) diffuse reflectance spectra, photoluminescence (PL) spectra, photoelectrochemistry tests, and other analyses revealed that the double S-scheme heterostructure broadened the spectral response range of NiO and promoted its separation of photocarriers. Compared with pristine NiO, the modified double S-scheme heterojunction enhanced the surface adsorption of water molecules and the accumulation of intermediate product of HCOO-, and optimized the CO2 reduction system, realizing the improved CO yield of 373 μmol·g-1·h-1 in Ru(byp)32+/ethanolamine of CO2 reduction system. This study indicates that double S-scheme heterojunction could facilitate efficient photogenerated charge transfer and separation, thereby achieving high activity and selectivity for CO2 photoreduction. Our work provides a reference for the one-step construction of double S-scheme heterojunction.

Photocatalytic Reduction of Carbon Dioxide: Designing the Active Sites and Tracking the Pathways

Photocatalytic reduction of carbon dioxide (CO2) realizes the recycling of carbon emissions and storage of solar energy into the bonding of organics at the same time, and thus attract great interest in the field of energy and environment. However, the current photocatalytic performance of CO2 reduction cannot match the industrial application. The design of highly efficient photocatalysts with precise selectivity and reliable long-term stability is still a big challenge, partially because the mechanism of photocatalytic CO2 reduction to guide the design and fabrication, is not completely clear yet. The reduction can involve at most eight electrons for each CO2 molecule, during which several pathways might be opened up at the active sites to consume photocarriers to influence the selectivity and stability. The reduction pathways are dependent on the electronic structure and property of active sites, and the photocatalytic performance can be optimized if those pathways are thermodynamically or kinetically compatible for the target production. This review will summarize the strategy for designing the active sites on the surface of photocatalysts and the investigation on the relation between the active sites and pathways for photocatalytic CO2 reduction, looking ahead at the future development of the photocatalysts and devices for CO2 reduction.

Enhanced Charge Transfer in Poly(Heptazine Imide) Synergistically Induced by Donor-Acceptor Motifs and Ohmic Junctions for Efficient Photocatalytic CO2 Reduction

Poly(heptazine imide) (PHI) has received widely interest in the photocatalytic CO2 reduction due to its good crystallinity and complete in-plane structure. However, its poor photo-induced carrier separation and migration efficiency and insufficient active sites results in undesirable photocatalytic CO2 reduction performance. Herein, we designed and constructed a novel ohmic junction photocatalyst by integrating melamine edge-modified PHI (mel-PHI) with extended π-conjugated system with TiN (TiN/mel-PHI) for enhancing the photocatalytic CO2 reduction activity. Strikingly, the photocatalytic CO2 reduction yield of the optimal TiN/mel-PHI is 62.64 μmol g-1 h-1, which is 5.6 and 2.8 times higher than PHI (11.26 μmol g-1 h-1) and mel-PHI (22.32 μmol g-1 h-1), respectively. The superior photocatalytic CO2 reduction activity is attributed not only to the formation of D-A structure by the introduction of melamine, which extends the π-conjugation system, alters the electronic structure of PHI, and accelerates the charge separation and migration, but also to the induced internal electric field by ohmic junction further enhances the charge separation and migration efficiency. Meanwhile, the synergistic effect of mel-PHI and TiN enriched the electron number of TiN, reducing the CO2 reduction potential. This work highlights the synergistic enhancement of charge transfer between D-A motifs and ohmic junctions, confirming their potential in optimizing photocatalysts.

Asymmetric Charge Distribution in Atomically Precise Metal Nanoclusters for Boosted CO2 Reduction Catalysis

Recently, atomically precise metal nanoclusters (NCs) have been widely applied in CO2 reduction reaction (CO2RR), achieving exciting activity and selectivity and revealing structure-performance correlation. However, at present, the efficiency of CO2RR is still unsatisfactory and cannot meet the requirements of practical applications. One of the main reasons is the difficulty in CO2 activation due to the chemical inertness of CO2. Constructing symmetry-breaking active sites is regarded as an effective strategy to promote CO2 activation by modulating electronic and geometric structure of CO2 molecule. In addition, in the subsequent CO2RR process, asymmetric charge distributed sites can break the charge balance in adjacent adsorbed C1 intermediates and suppress electrostatic repulsion between dipoles, benefiting for C-C coupling to generate C2+ products. Although compared to single atoms, metal nanoparticles, and inorganic materials the research on the construction of asymmetric catalytic sites in metal NCs is in a newly-developing stage, the precision, adjustability and diversity of metal NCs structure provide many possibilities to build asymmetric sites. This review summarizes several strategies of construction asymmetric charge distribution in metal NCs for boosting CO2RR, concludes the mechanism investigation paradigm of NCs-based catalysts, and proposes the challenges and opportunities of NCs catalysis.

Interfacial Assembly Immobilization of Tripodal Iron Terpyridyl Coordination Oligomers on Carbon Nitride for Efficient Photocatalytic CO2 Reduction

Surface modification of cheap light-absorbing materials by noble-metal-free molecular catalysts to construct high efficiency photocatalytic systems has recently attracted great research interest but remains a great challenge. Here, we constructed carbon nitride-based composite photocatalysts through interfacial covalently assembling tripodal Fe(II)-terpyridyl (Fe-TerPyTa)n coordination oligomers on oxidized graphitic carbon nitride (O-C3N4) via a surface-initiated alternative reaction between TerPyTa and Fe(BF4)2. The obtained O-C3N4@(Fe-TerPyTa)n possesses visible-light-absorbing C3N4 and active Fe-TerPyTa with superior capability in photocatalytic CO2 reduction. Due to the well-defined structure and integrated functionality, the O-C3N4@(Fe-TerPyTa)1 hybrid catalyst displays significantly enhanced catalytic performance in CO2 reduction with outstanding CO selectivity (almost 100%) and a maximum CO evolution rate of 91.1 μmol g-1 h-1, which is nearly 100-fold higher than that of pristine O-C3N4. The fluorescence emission, time-resolved fluorescence, and electrochemical impedance spectrum studies indicate that the improved catalytic efficiency is mainly attributed to the direct covalent and coordinative interactions between the oligomer and the O-C3N4. Such a configuration can enhance the charge transfer from the O-C3N4 to the redox center of Fe-TerPyTa, which further passes the electrons to the CO2 molecules to produce CO. Moreover, due to the kinetical preference for electrons to reduce H2O-to-H2, the reducing products can be regulated from pure CO to its CO/H2 mixture (analogue of syngas) with tunable ratios by changing the water content in the reaction solution.

Crystal-facet modulated pathway of CO2 photoreduction on Bi4NbO8Cl nanosheets boosting production of value-added solar fuels

Two nanosheets of Bi4NbO8Cl were successfully synthesized for photocatalytic conversion of CO2 into solar fuel, featuring differently exposed (001) and (201) facets. The exposure of these specific facets facilitates C-C coupling to generate ethanol, and (201) facet typically accelerates this process.

Covalent Organic Frameworks for Photocatalytic Hydrogen Peroxide Evolution

Covalent organic frameworks (COFs) are considered advanced class materials due to their exotic structural and optical properties. The abundance of starting monomers with variable linkage motifs may give rise to multiple conformations in either 2D or 3D fashion. Tailoring of the abovementioned properties has facilitated the application of COFs in a wide range of applications, which are strongly correlated with energy conversion schemes. Having a crystalline porous character and a large set of donor-acceptor combinations, COFs are expected to make huge impact in photocatalytic processes. In this Review, we present the recent advances in the development of semiconducting COF-based systems towards the photocatalytic hydrogen peroxide evolution. An overview is given about the effect of various parameters on the photocatalytic performance, such as charge transfer tuning, wettability by chemical functionalization, topology, porosity and crystallinity. Various challenges are discussed, and constructive insights are given for the development of highly functional COF-based photocatalysts for H2O2 evolution.

Application of porous crystalline framework materials towards direct flue gas conversion

The photocatalytic direct conversion of carbon dioxide (CO2) from flue gas into high-value products is regarded as one of the most promising approaches to achieving carbon neutrality. Nevertheless, this direct conversion process encounters significant challenges, primarily due to practical limitations such as low CO2 concentrations and the presence of interfering substances. Porous crystalline framework materials exhibit considerable potential in flue gas conversion, attributed to their robust CO2 capture capabilities, well-defined and tunable structures, high specific surface areas, and plentiful catalytic sites. This review highlights strategies to improve the capture and activation of low-concentration CO2 by porous crystalline materials including functionalization of organic ligands, creation of open metal sites (OMSs) and Lewis basic sites (LBSs), as well as strategies to improve the catalytic activity of flue gas reforming, which encompasses anchoring of catalytic sites to the skeleton, fabricating composites, and preparing derived materials. The review aims to provide insights and guidance for the design and development of efficient catalysts specifically tailored for flue gas reforming.