Copper Active Sites in Metal–Organic Frameworks Advance CO₂ Adsorption and Photocatalytic Conversion

Abstract

The photocatalytic reduction of CO₂ into high-value chemicals utilizing solar energy represents a sustainable approach to mitigating greenhouse gas emissions and advancing renewable chemical production. Recently, copper-based metal–organic frameworks (Cu-MOFs) have been extensively researched for their potential in photocatalytic CO₂ reduction, due to their high affinity for capturing CO₂, the presence of unsaturated Cu sites, and their advantageous photochemical properties. In this review, we first provide an overview of Cu active sites in the secondary building units (SBUs) of MOFs, focusing on their selective adsorption of CO₂ gas and analyzing the mechanisms of the multi-electron transfer processes involved in Cu-based photocatalytic reduction of CO₂. Ultimately, this article outlines the existing obstacles and suggests potential avenues for future research.

1. Introduction

The intensification of anthropogenic activities has led to a significant increase in atmospheric CO₂ concentrations, exceeding 400 ppm, thereby accelerating global warming and climate change [1,2,3]. To mitigate these effects, research has concentrated on two primary strategies: physical sequestration of CO₂ via geological storage methods and chemical conversion of CO₂ into value-added chemicals and fuels through catalytic processes [4,5]. Chemical conversion encompasses direct electrochemical and photocatalytic reduction pathways, with the latter leveraging solar energy to drive the endothermic transformation of CO₂ into simpler carbon-based molecules [6]. Photocatalytic reduction mimics natural photosynthesis, employing semiconductor photocatalysts to absorb photons and generate electron–hole pairs that facilitate the reduction of CO₂ to C₁ and C₂ hydrocarbons and oxygenates, such as CO, CH₄, CH₃OH, C₂H₅OH, and HCHO [7,8]. Overcoming the kinetic and thermodynamic stability of CO₂, characterized by a bond enthalpy of approximately +805 kJ mol⁻¹ for the C=O bonds, remains a critical challenge [9]. Recent advances in photocatalysts focus on enhancing charge separation, expanding light absorption, and increasing the number of active sites to significantly improve the quantum efficiency of CO₂ reduction under solar illumination [10,11,12]. This approach offers a sustainable pathway for renewable energy storage and greenhouse gas mitigation by converting atmospheric CO₂ into useful chemical feedstocks and fuels with minimized energy input and environmental impact.

The photocatalytic reduction of CO₂ involves sequential steps: photon absorption, charge carrier generation and separation, and CO₂ adsorption and activation [13,14]. Upon photon absorption, electron–hole pairs are generated within the semiconductor photocatalyst. Effective separation of these charge carriers is vital to prevent recombination and improve the reduction efficiency [15]. Strategies such as forming heterojunctions or introducing electron traps enhance charge separation by creating internal electric fields or localized states that facilitate electron migration [16]. Concurrently, CO₂ molecules adsorb onto the catalyst’s active sites, where increased surface area and exposed catalytic sites enhance adsorption energy and lower activation energy barriers, thereby accelerating reduction kinetics [17,18]. Despite progress in expanding the light absorption spectrum and reducing charge recombination [19], achieving high catalytic selectivity and overall efficiency remains challenging [20], necessitating the development of photocatalysts with broad spectral absorption, high charge mobility, abundant active sites, and robust CO₂ adsorption capacity to advance sustainable solar-to-chemical energy conversion [21,22].

Metal–organic frameworks (MOFs) are crystalline materials composed of metal ions or clusters, named secondary building units (SBUs), coordinated with multi-dentate organic linkers, forming highly ordered, porous networks with exceptionally high specific surface areas, tunable pore dimensions, and abundant open metal sites [23,24,25]. These structural features confer advantages for a range of applications, including gas adsorption and separation, chemical sensing, catalysis, and CO₂ capture and conversion [26,27,28]. Notably, MOFs exhibit remarkable potential as photocatalysts due to their broad-spectrum light absorption capabilities, which can be engineered through linker functionalization and metal node selection to optimize visible-light harvesting; their ability to facilitate the efficient separation and migration of photogenerated electron–hole pairs via electron-trapping centers; and the presence of uniformly distributed, accessible catalytically active sites, which enhance reaction kinetics and our mechanistic understanding of structure–activity relationships [29,30,31]. In addition to photocatalytic conversion, MOFs are employed in catalytic cycloaddition reactions and electrochemical reduction of CO₂, leveraging their tunable pore environment, metal site density, and electronic properties to steer reaction selectivity and efficiency [32,33,34,35]. Consequently, the intrinsic structural versatility and functional tunability of MOFs make them highly promising platforms for advancing sustainable and efficient CO₂ capture, activation, and conversion processes driven by solar energy.

Recently, copper-based metal–organic frameworks (Cu-MOFs) have shown remarkable efficacy in the photocatalytic reduction of CO₂ [36,37,38,39]. The coordinatively unsaturated copper centers act as active catalytic sites, effectively adsorbing and activating CO₂ molecules by lowering the activation energy required for reduction [40,41]. Their high surface area and accessible open copper sites further enhance CO₂ adsorption, increasing the local concentration of reactants at active sites and thus boosting reduction rates [42]. Moreover, the organic linkers and metal centers work synergistically to expand the light absorption spectrum, enabling more efficient capture of solar energy and its conversion into chemical energy [43]. The combination of efficient light harvesting, effective charge separation, strong CO₂ adsorption, and active copper sites positions Cu-MOFs as highly promising photocatalysts for CO₂ reduction, paving the way for the conversion of solar energy into valuable chemical fuels.

To enable the strategic development of advanced Cu-MOFs for solar-driven CO₂ reduction, it is important to deliver a thorough and up-to-date overview of the most recent scientific progress in this dynamic field. While several prior reviews have addressed aspects of MOF photocatalysis, there remains a need for an updated perspective that highlights new developments, offers novel insights, and discusses unresolved challenges [44,45,46,47]. In this review, we initially elucidate the fundamental mechanisms governing photocatalytic CO₂ reduction, providing a comprehensive mechanistic framework. Building on this foundation, we highlight the critical importance of active site sensitivity and CO₂ adsorption affinity in dictating catalytic performance. Subsequently, we systematically analyze the role of various copper (Cu) active sites in MOFs, including Cu (I), Cu (II), and single-atom Cu catalytic centers, in modulating CO₂ adsorption characteristics. Based on these insights, we present a detailed mechanistic investigation of the multi-electron transfer pathways in Cu-MOF-catalyzed photocatalytic CO₂ reduction. This includes an in-depth discussion of the two-electron, six-electron, eight-electron, and twelve-/fourteen-electron reduction processes, along with the identification and characterization of key reaction intermediates and their associated energetics. Finally, we critically assess the current limitations and unresolved challenges in MOF-based photocatalytic systems for CO₂ reduction, and we discuss potential avenues for future research to enhance their performance and facilitate sustainable CO₂ utilization.

2. Principles of MOF Photocatalysts for CO₂ Conversion

Leveraging MOF-based photocatalysts to harness solar energy represents a promising approach for converting CO₂ into valuable fuels and chemical commodities [48,49,50,51,52]. The process fundamentally involves the photoexcitation of electrons from the HOMO, primarily contributed by the organic linker, to the LUMO, mainly derived from the metal clusters, upon photon absorption [40]. This excitation generates separated charge carriers, with electrons in the LUMO and holes in the HOMO that must be effectively separated and transported to catalytic sites [53]. The photogenerated electrons in the LUMO diffuse to active metal centers within the MOFs, where they participate in multi-electron transfer reactions with adsorbed CO₂ molecules, initiating reduction pathways (Figure 1). For efficient reduction, the LUMO must be thermodynamically aligned above the redox potentials of the desired reduction products; for example, the reduction of CO₂ to CO requires a LUMO energy exceedingly approximately –0.104 V, while reductions to formic acid (~–0.171 V), methanol (~0.016 V), and methane (~0.169 V) demand correspondingly higher LUMO energies [54,55,56]. The tunability of MOFs, achieved by engineering organic linkers and metal nodes, allows precise modulation of their electronic structures, particularly their HOMO and LUMO, thereby optimizing light absorption, charge separation efficiency, and redox potentials [57]. For oxidation reactions occurring at the HOMO, the photogenerated holes typically oxidize water molecules or sacrificial agents (SAs), producing oxygen or other oxidation products.

Rational design strategies that emphasize ligand functionalization and the strategic selection of metal nodes can effectively enhance the photocatalytic performance of MOFs. Furthermore, MOFs inherently possess several intrinsic features that promote efficient CO₂ reduction: (1) both organic linkers and unsaturated metal centers can serve as active catalytic sites; (2) their high surface area and porosity facilitate substantial CO₂ adsorption, increasing the local CO₂ concentration at active sites and lowering activation energy barriers; (3) the highly ordered porous architecture enables the incorporation of guest photoactive species, further augmenting photocatalytic activity [59,60,61,62,63,64]. Collectively, these properties underscore the immense potential of MOFs as versatile and highly tunable platforms for solar-driven CO₂ conversion.

3. Cu Sites in MOFs for CO₂ Adsorption

In photocatalytic CO₂ reduction, the efficiency critically hinges on the catalyst’s ability to adsorb and activate CO₂ molecules at active sites, which, in Cu-MOFs, is achieved through the presence of coordinatively unsaturated Cu centers acting as Lewis acid sites that form stable coordinate covalent bonds with the quadrupolar CO₂, stabilizing it via electrostatic interactions between the electron-deficient metal centers and the partial negative charges on the oxygen atoms of CO₂ [65,66,67,68,69,70]. The highly porous, high-surface-area framework provides an extensive internal volume, enabling physical adsorption and facilitating the diffusion of CO₂ molecules; during synthesis, pore sizes can be precisely tuned to match the molecular dimensions of CO₂ (~3.3 Å), promoting size-selective adsorption and minimizing the competitive binding of undesired gases [71]. Functional groups such as amino or hydroxyl groups incorporated into organic linkers introduce additional hydrogen-bonding and dipolar interactions with CO₂, further increasing the binding energy and residence time—factors that are critical for activation and subsequent reduction [72]. The organic linkers extend light absorption into the visible spectrum, enhancing photon harvesting and charge generation, while electronic coupling between the metal centers and organic linkers promotes rapid charge transfer, minimizing electron–hole recombination and efficiently delivering electrons to adsorbed CO₂ molecules [73]. The stable coordination environment maintains structural integrity under operational conditions, preventing framework degradation, while the electrostatic quadrupole moment of CO₂ interacts favorably with the local electrostatic potential created by the open metal sites and functional groups, thus improving selectivity. Therefore, features such as accessible open metal sites, tailored pore structures, specific chemical functionalities, and extended light absorption synergistically enhance CO₂ adsorption, activation, and reduction, underpinning the high catalytic performance of Cu-MOFs in solar-driven CO₂ conversion.

3.1. Cu (I) Sites in MOFs

To address the challenges involved in developing an MOF-based integrated carbon capture and utilization material suitable for point sources in the cement industry, optimal MOFs should possess open metal sites (OMSs) for catalytic CO₂ conversion, exhibit high stability under elevated temperatures and humid conditions, and selectively adsorb CO₂ over other flue gas components. Cu(I)-MOFs are particularly promising due to their well-established catalytic capabilities for CO₂ conversion, making them a strong candidate for this application. Sengupta et al. developed the Cu (I)-MOF NU-2100 (Cu₂(BBTA), where H₂BBTA = 1,5-Dihydrobenzo[1,2-d:4,5-d’]bis([1,2,3]triazole)) for integrated CO₂ capture and conversion (Figure 2a). Under mild reaction conditions (50 °C and H₂:CO₂ = 3:1), NU-2100 demonstrated effective CO₂ adsorption and catalytic conversion to formic acid with 100% selectivity (Figure 2b) [74]. Unlike in the as-synthesized MOF with acetonitrile molecules, the CO₂ molecules in the activated form are preferentially localized at the vertices of the zigzag channels. As shown in Figure 2, this spatial distribution promotes weak C−H···OCO hydrogen-bonding interactions between the electron-deficient pyrazine and pyridine rings and the CO₂ molecules, facilitating selective physisorption within the MOF’s confined environment [75]. This development represents a significant advancement toward the design of next-generation MOFs engineered for integrated carbon capture and conversion processes under industrial conditions.

3.2. Cu (II) Sites in MOFs

Although Cu (I)-based MOFs exhibit exceptional catalytic efficiency for CO₂ conversion, their operational stability is frequently compromised by their susceptibility to oxidative degradation and hydrolysis under ambient air and moisture conditions. Hence, MOFs featuring coordinatively unsaturated Cu (II) centers demonstrate superior CO₂ adsorption performance, driven by the availability of accessible open metal sites that enable strong, localized Lewis acid–base interactions and facilitate selective and reversible host–guest binding. A hexacarboxylate linker was developed and used to construct a Cu(II)-based MOF, known as MFM-160 (Figure 3a). The ligand structure, featuring a triazine backbone with a triangular configuration of isophthalic acid groups, was engineered to enhance CO₂ uptake and enable the selective isolation of C2 hydrocarbons from methane (Figure 3b) [76]. These coordinating moieties facilitate the formation of an rht topology framework through integration with Cu(II) paddlewheel nodes. The rht topology has been extensively exploited in the development of MOFs with exceptional gas storage and separation performance, owing to its robust and accessible pore architecture. Furthermore, an amide-functionalized MOF, designated MFM-136 (Figure 3c), demonstrated a high CO₂ adsorption capacity of 12.6 mmol g⁻¹ at 20 bar and 298 K (Figure 3d). Notably, MFM-136 represents the first example of an acylamide pyrimidyl isophthalate MOF devoid of open metal sites, providing a unique system to elucidate guest–host interactions involving amide functionalities [77]. Neutron diffraction coupled with inelastic neutron scattering confirms that pendant amide functionalities do not establish direct coordination or hydrogen-bonding interactions with guest molecules such as CO₂ or CH₄. These findings highlight that functional group presence alone is insufficient for targeted guest affinity; instead, adsorption efficiency is predominantly governed by the spatial arrangement of the pore channels and the overall framework topology, which modulate host–guest interactions at the molecular level. Thus, gas adsorption performance results from complex interplay among the pore size, framework topology, and functional group chemistry. This underscores the importance of pore architecture and the chemical environment in collectively modulating guest affinity and selectivity in porous materials.

3.3. Cu Single-Atom Sites in MOFs

Single-atom Cu leverages maximal atomic utilization and distinctive electronic configurations to achieve enhanced catalytic activity and exceptional selectivity, thereby enabling the precise modulation of reaction pathways and facilitating the design of highly efficient, atomically precise catalytic systems. Mageed et al. engineered a single-atom copper catalyst supported on UiO-66, achieved by covalently anchoring individual copper atoms to defect sites in the zirconium-based framework [78]. Computational studies supported by experimental findings revealed that copper interacts with the MOF through hydroxyl and coordinated water ligands at defect sites on zirconium oxide clusters, contributing to the material’s stability and catalytic performance. The development of highly efficient single-atom-site electrocatalysts for the electrochemical reduction of carbon dioxide to methane has garnered significant interest for its potential use in intermittent renewable energy storage. However, realizing practical and selective catalytic systems remains a considerable challenge. Chen et al. developed N-heterocyclic carbene (NHC)-ligated copper single-atom sites (Cu SASs) embedded in a metal–organic framework (2Bn-Cu@UiO-67), shown in Figure 4c, which achieved an outstanding Faradaic efficiency (FE) of 81% for CO₂ reduction to CH₄ at @1.5 V vs. RHE with a current density of 420 mA cm⁻². [79] The material’s porosity enhanced mass transport, promoting efficient diffusion of CO₂ molecules to the active copper sites and thereby boosting catalytic accessibility. As Figure 4d shows, NU-1000 was functionalized with primary amino groups (–NH₂), introduced via ligand modification, prior to MOF synthesis to enhance CO₂ affinity, while post-synthetic solvent-assisted ligand exchange was employed to incorporate thiol (–SH) functionalities within the framework, providing affinity for copper species. This altered catalyst demonstrated complete selectivity for methanol synthesis at temperatures reaching 280 °C, with an approximate production rate of 100 milligrams of methanol per gram of catalyst per hour (Figure 4e) [80]. Additionally, the precisely characterized single-atom active sites of this catalyst will facilitate comprehensive mechanistic studies.

3.4. Pros and Cons of Cu (I, II, and Single-Atom)-Based MOFs

Cu(I)-based MOFs exhibit notable electronic properties, including high electrical conductivity, strong light absorption, and redox activity, making them promising candidates for photocatalysis, electrocatalysis, and electronic applications; however, their stability is often compromised under oxidative and humid conditions due to their susceptibility to oxidation to Cu(II), and their synthesis demands a strictly controlled inert atmosphere, which can limit scalability [74,75]. Cu(II)-based MOFs are distinguished by their exceptional thermal and chemical stability, diverse coordination geometries, and facile synthesis, enabling their widespread use in gas storage, separation, and heterogeneous catalysis; nevertheless, they generally possess lower electrical conductivity and limited redox flexibility, restricting their utility in applications requiring dynamic electron transfer [76,77]. Single-atom Cu MOFs leverage atomic dispersion to maximize catalytic site efficiency and selectivity, offering unique quantum confinement effects and electronic properties that enhance catalytic performance; however, the synthesis of stable, atomically dispersed Cu sites is technically challenging, often requiring sophisticated techniques to prevent the migration or aggregation of single atoms under operational conditions, which also complicates large-scale production and precise characterization [78,79,80].

4. Photocatalytic CO₂ Reduction on Cu-MOFs

Previous studies have shown that copper-based metal–organic frameworks demonstrate exceptional sensitivity and affinity toward CO₂ adsorption, owing to the presence of coordinatively unsaturated copper sites and tunable pore environments. However, the conversion of CO₂ through this pathway requires several coordinated proton and electron transfer steps, leading to the formation of different intermediate species and making it challenging to steer the reaction toward a specific product [81]. In this section, we explore the photochemically initiated reactions that occur during both the two-electron and complex multi-electron reduction pathways of CO₂.

4.1. Two-Electron Reduction Process

In photocatalytic CO₂ reduction pathways involving two-electron transfer processes, the predominant reaction products are carbon monoxide (CO) and formic acid (HCOOH), resulting from the selective reduction of CO₂ through controlled electron and proton transfer mechanisms. For CO₂ reduction to CO, Cu-MOFs have been synthesized for photocatalytic reactions. As shown in Figure 5a, coordinated water molecules bound to the metal centers are readily displaced, exposing active metal sites where CO₂ molecules can adsorb. Due to the lower LUMO energy level of MOF-Cu compared to [Ru(bpy)₃]²⁺, photogenerated electrons in the LUMO of [Ru(bpy)₃]²⁺ can be transferred to the surface of MOF-Cu. Subsequently, the adsorbed CO₂ molecule accepts an electron at the active site, forming a radical CO₂ intermediate. Through a proton-assisted two-electron transfer mechanism, this intermediate is ultimately reduced to carbon monoxide (CO). Finally, the excited state of the photosensitizer undergoes reductive quenching by the sacrificial electron donor TIPA, leading to the release of CO from the catalyst surface [82]. Feng et al. reported the design of an MOF called mPT-Cu/Re, which incorporates cuprous photosensitizers (Cu/Re-PSs) specifically engineered for CO₂ reduction reactions (Figure 5b) [83]. Under 350−700 nm light irradiation, the mPT-Cu/Re catalyst facilitated continuous CO₂ reduction to CO over a 48-hour period in a mixed solvent of N, N-dimethylacetamide (DMA) and water (v/v = 0.96:0.04), using BIH as the sacrificial electron donor. The gas chromatography analysis results, shown in Figure 5c, revealed a total turnover number (TON) of 1328 for CO production, with a CO to H₂ selectivity ratio of 9:1.

A series of amine-functionalized Ti-based MOFs, including NH₂-MIL-125(Ti), were synthesized using metalation, incorporation, and photodeposition techniques to introduce Cu and Ag into the framework. Among these, the post-synthetically metalated MOF with Cu²⁺ (MOF_met_0.5%Cu) demonstrated superior catalytic activity for CO₂ reduction to HCOOH [84]. Additionally, single-site Co and Cu catalytically active centers were further encapsulated within the UiO-67 framework to facilitate directed and efficient electron transfer, thereby enhancing the overall efficiency of CO₂ photoreduction (Figure 6a). The improved activity of UiO67-Ir-Cou 6/Co and UiO67-Ir-Cou 6/Cu is due to efficient electron flow from BIH to the excited MOF and fast intra-framework transfer to metal active sites, enhancing the CO₂ reduction efficiency (Figure 6b) [85]. Cu incorporation enhances the photocatalytic performance of MOFs for CO₂ reduction by increasing their CO₂ adsorption capacity and decreasing their LUMO overpotentials, thereby facilitating more efficient charge transfer and activation of CO₂ molecules.

4.2. Six-Electron Reduction Process

The six-electron conversion of CO₂ to CH₃OH demands a higher energy input and involves more complex challenges than the two-electron pathway. Goyal et al. demonstrated that the addition of metals to a material can modify its surface properties and enhance its overall efficiency [86]. Incorporated metal acts as a charge-carrier trap, boosting reaction rates and extending the lifespan of separated electron–hole pairs. During photocatalysis, electrons transition from lower to higher energy levels, forming a Schottky barrier at the metal–semiconductor interface. This barrier traps electrons, preventing their recombination with holes by blocking their migration back to the valence band. Visible light irradiation is predicted to excite electrons into the conduction band (CB) of Cu/ZIF-8 catalysts, facilitating charge separation. In contrast, pure ZIF-8 does not produce methanol, likely because CO₂ reduction to methanol involves proton–electron transfer with a standard potential of E (CO₂/CH₃OH) = –0.38 V vs. NHE (Figure 7). Semiconductors are characterized by two key features: (1) the generation of a charge-separated state when the photon energy exceeds the bandgap, and (2) the mobility of charge carriers, which migrate away from their generation sites to minimize recombination and enable surface reactions.

4.3. Eight-Electron Reduction Process

The process of converting CO₂ into CH₄ and CH₃COOH under solar illumination is an intricate multi-electron reaction pathway that includes numerous intermediate steps and yields different secondary products. Wu et al. engineered a surfactant-free method to encapsulate Cu₂O nanowires within Cu₃(BTC)₂ metal–organic frameworks (BTC = 1,3,5-benzene tricarboxylate) [87]. The slow dissolution of Cu²⁺ from Cu₂O, combined with the rapid nucleation of Cu₃(BTC)₂, resulted in the encapsulation of Cu₂O nanowires within the Cu₃(BTC)₂ framework (Figure 8a). This process was visually indicated by color changes in CM, from reddish-brown to red and eventually to green, signaling the sequential growth of Cu₂O and Cu₃(BTC)₂. Cu₃(BTC)₂ demonstrated an outstanding ability to catalytically reduce carbon dioxide into methane (Figure 8b). Jincai Zhao’s group developed a type II heterojunction photocatalyst by assembling two-dimensional materials through π–π interactions, combining graphitic carbon nitride with a copper-porphyrin-containing metal–organic framework. [88] The 2D-MOF component incorporated tetrakis(4-carboxyphenyl)porphyrin (TCPP) as the organic linker and was designated Cu-ZnTCPP. In this nomenclature, “Cu” refers to the copper node (Cu₂(COO)₄) in a paddle-wheel structure coordinated by carboxylate groups, while “Zn” denotes the central Zn ion (ZnN₄) within the porphyrin ring (Figure 8c). As shown in Figure 8d, computational studies demonstrated that the Cu-ZnTCPP/g-C₃N₄ composite effectively stabilizes key intermediates in the stepwise reduction pathway, thereby promoting the selective conversion of CO₂ to CO.

4.4. Twelve-/Fourteen-Electron Reduction Process

The photocatalytic reduction of CO₂ to C₂ hydrocarbons (C₂H₅OH and C₂H₄) is highly desirable due to its greater value. However, it remains a significant challenge owing to the complex multi-electron transfer process and the slow kinetics associated with C-C bond formation, which necessitate the coordinated action of multiple active sites working synergistically. Xu et al. co-immobilized Cu single-atom catalysts and Cu-doped Au nanoparticles on a mesoporous, photoactive MOF (ACN), creating a confined environment that combines multiple catalytic functionalities (Figure 9a) [89]. The detailed mechanism was elucidated through in situ DRIFTS analysis (Figure 9b). The increased absorption peaks at 1372 cm⁻¹ and 1430 cm⁻¹ were attributed to CO₃²⁻ and HCO₃⁻ species, respectively, indicating CO₂ adsorption on the surface of ACN. The characteristic peaks at 1646 cm⁻¹ and 1737/1710 cm⁻¹, corresponding to the key intermediates *COOH and *CHO, progressively intensified, providing direct evidence for the formation of CO and CH₄. For C₂ hydrocarbon formation, signals corresponding to intermediates *OCCO, *OCCHO, and *OCCHOH were observed at 1549/1531 cm⁻¹, 1468 cm⁻¹, and 1310 cm⁻¹, respectively, confirming the occurrence of C–C coupling steps. In summary, the identification of these key C₂ intermediates offers crucial mechanistic insights into the CO₂ conversion pathway leading to C₂ hydrocarbons.

4.5. Electronic and Geometric Factors of CO₂ Conversion Mechanism

Product selectivity in photocatalytic CO₂ reduction over Cu-MOFs is governed by a complex interplay of electronic and geometric factors at the active sites [90]. Electronically, the d-electron configuration of copper centers and their associated surface electronic states modulate the adsorption energies and stabilization of key reaction intermediates such as *COOH, *CO, and *CHO [91]. These intermediates’ binding affinities influence the reaction pathways, favoring the formation of specific products like CO, methane, or methanol. Geometrically, the local coordination environment—encompassing the surface facet orientation, atomic arrangement, defect sites, and pore architecture—dictates the adsorption modes and spatial orientation of reactants and intermediates [92]. Variations in active site geometry alter the transition state energies and facilitate or hinder particular reaction pathways, thereby steering selectivity toward different reduction products. Ultimately, the synergistic effects of the electronic structure and geometric configuration of Cu sites within the MOF matrix underpin mechanistic control over the product distribution in photocatalytic CO₂ reduction.

The mechanistic pathways of photocatalytic CO₂ reduction in Cu-MOFs display both commonalities and distinct differences when compared to other transition-metal-based MOFs, primarily arising from variations in their electronic structure, redox flexibility, and intermediate stabilization. In Cu-MOFs, the partially filled d-orbitals of copper facilitate efficient electron transfer and stabilize key reaction intermediates such as *COOH, *CO, and *CHO, thereby enabling multiple pathways that lead to a broad product spectrum including CO, methane, and methanol [93,94]. In contrast, transition metals like Co, Ni, and Fe possess different electronic configurations and redox potentials, which influence the adsorption energies and transition state barriers of intermediates, thereby biasing the reaction toward specific products—such as Fe-MOFs promoting multi-electron transfer pathways toward formate or other C1 products [95]. These differences in electronic properties and active site geometries modulate the stabilization of reaction intermediates and transition states, ultimately dictating the dominant mechanistic routes and product selectivity. A comprehensive understanding of these pathway distinctions provides critical insights for the rational design of MOFs with tailored active sites to optimize selectivity and efficiency in photocatalytic CO₂ reduction.

5. Advanced Characterization

To achieve highly efficient photocatalysts, detailed characterization techniques are essential for unveiling critical processes such as light absorption, charge separation, and surface redox reactions. Numerous reviews have summarized the methodologies employed in photocatalysis research [96]. MOFs, with their atomically precise and tunable structures, offer a versatile platform for gaining deeper insights into photocatalytic mechanisms. In this context, advanced analytical methods become particularly valuable. This section first discusses single-crystal X-ray diffraction (SCXRD), which offers high-resolution, atomic-level insights into MOFs, enabling accurate determinations of connectivity, pore topology, and functionalization critical for elucidating structure–property relationships. Subsequently, X-ray absorption spectroscopy (XAS) provides element-specific, in situ insights into the electronic and local structural changes occurring in photocatalysts during operation, aiding in understanding their mechanisms and optimizing their performance. Finally, in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) facilitates the identification of reaction intermediates and provides mechanistic insights into catalytic processes.

Understanding and visualizing gas–adsorbent interactions is essential for developing improved materials for adsorption applications. As shown in Figure 10, structural analyses of N₂, Ar, CO₂, and C₃H₆ adsorption in Cu-MOF-1 under air-free conditions confirmed that gas exposure restored the larger pore phase, consistent with SPXRD results [97]. All gases except Ar interact with Cu(II) sites, revealing secondary (N₂, CO₂) and tertiary (N₂) adsorption sites. Based on the SCXRD results, the Cu–gas distances indicate predominantly strong (N₂, CO₂, C₃H₆) and weaker (Ar, N₂, CO₂, C₃H₆) interactions. The gas loadings from SCXRD match the adsorption measurements, except that for Ar due to its higher disorder. Notably, the first single-crystal structure of a Cu–C₃H₆ adduct was obtained at 7.5 bar/290 K, with Cu–C distances of 2.33 Å, the shortest among the studied gases, confirming strong host–guest interactions. C₃H₆ preferentially forms single C₃H₆-Cu (II) complexes, interacting mainly with the framework rather than being centrally confined.

6. Summary

In summary, we initially delineated the fundamental mechanistic principles governing photocatalytic CO₂ reduction, establishing a comprehensive theoretical framework that encompasses charge transfer dynamics, intermediate formation, and product selectivity. Building upon this foundation, we underscored the critical importance of active site sensitivity and CO₂ adsorption affinity in modulating catalytic activity and selectivity. Subsequently, we systematically analyzed the role of various copper (Cu) active centers within MOFs—Cu (I), Cu (II), and single-atom Cu sites—influencing CO₂ adsorption energies and activation barriers. Leveraging these insights, we presented an in-depth mechanistic investigation of the multi-electron transfer pathways involved in Cu-MOF-catalyzed photocatalytic CO₂ reduction, encompassing two-electron, six-electron, eight-electron, and twelve-/fourteen-electron reduction processes. This analysis included a detailed characterization of key reaction intermediates, transition states, and their associated energetics, elucidating the thermodynamic and kinetic factors governing product formation. We discussed strategic directions for future research aimed at optimizing active site engineering, enhancing charge separation, and integrating these materials into scalable, sustainable CO₂ conversion technologies.

Copper-based MOFs exhibit significant advantages in both their CO₂ adsorption capacity (key performance indicators: uptake of CO₂, adsorption isotherms) and photocatalytic reduction efficiency (key performance indicators: conversion rate, product selectivity) owing to their highly tunable pore structures and the presence of coordinatively unsaturated copper sites that promote strong CO₂ affinity. The redox versatility of copper ions (Cu(I)/Cu(II)) facilitates efficient multi-electron transfer processes and the stabilization of reaction intermediates, thereby enhancing catalytic activity under visible light irradiation. Furthermore, the electronic structure of copper enables precise modulation of the MOF’s bandgap and light absorption properties, optimizing the charge separation and transfer essential for photocatalysis. As shown in

Table 1. Adsorption and photocatalytic CO₂ reduction capacity of MOF-based systems. TEOA: triethanolamine; BIH: 1,3 dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole.

Materials	CO2 Capacity	Conditions for Adsorption	Photocatalytic Conversion Product (μmol/g/h)	Conditions for Photocatalysis	Ref.
Cu/An (1:2)@NZU67	71.6 cm3 g−1	298 K 1 bar	EtOH: 624.17 Selectivity: 100%	Light: 450 nm, TEOA (0.3 M), CH3CN/H2O (4:1 v/v)	[98]
Cu/I2-Zr-BPDC/BPyDC	48.6 cm3 g−1	298 K 1 bar	MeOH: 9.4	Light: 450 nm, Ru(Bipy)3Cl2 (4 mg), TEOA (0.3 M)	[99]
Cu-Zr-BPyDC	31.1 cm3 g−1		MeOH: 1.88
Zr-I2-BPDC	28.7 cm3 g−1		/
Zr-BPyDC	27.3 cm3 g−1		/
1% ZIF-67CoCu(1:1)/ Bi4O5Br2	4.22 cm3 g−1	273 K 750 mmHg	CO: 6469.88 Selectivity: 97%	UV−visible light, TEOA (0.3 M), BIH (10 mg), CH3CN (4 mL) H2O (1 mL)	[100]
Mg0.4Cu0.6-MOF-74	4.58 mmol·g−1	298 K 1 bar	CO: 6.18	Visible light, H2O (2 mL)	[101]
Fe-N-TiO2/CPO-Cu	1.07 mmol·g−1	298 K 1 bar	CH4: 47.52 MeOH: 2.17	Light: 350–600 nm, H2O	[102]
CuCN	7.4 cm3 g−1	298 K 1 bar	CO: 246 Selectivity: 88%	Visible light, TEOA (1 mL), bipyridine (100 μL), CH3CN (4 mL), CoCl2 (2 μmol), H2O (1 mL)	[103]
1-K-Al-PMOF	3.46 mmol·g−1	273.15 K 1 bar	CO: 587.2	Visible light, CH3CN/H2O/TEOA (8:2:1 v/v)	[104]
Zr/Ce29%-UiO-67(NH 2, I)	78.4 cm3 g−1	298 K 1 bar	MeOH: 44.7 Selectivity: 100%	Light: 450 nm, TEOA (0.3 M), CH3CN/H2O (4:1 v/v)	[105]
20Cs2AgBiBr6/Ce-UiO-66-H	58.8 cm3 g−1	298 K 1 bar	CO: 309.01	300 W Xe lamp, H2O (50 mL)	[106]
NH2-UiO-66	1.3 mmol·g−1	298 K 1 bar	CO: 1.5	UV−visible light, gas/solid reactor	[107]
1-TiMOF	0.32 mmol·g−1	298 K 1 bar	CO: 3.74
2-TiMOF	0.36 mmol·g−1	298 K 1 bar	CO: 4.24
3-TiMOF	0.47 mmol·g−1	298 K 1 bar	CO: 3.37
4-TiMOF	0.56 mmol·g−1	298 K 1 bar	CO: 2.85

, compared to other MOFs, such as Zr-, Co-, Ce-, Fe-, and Al-based frameworks, copper MOFs often demonstrate superior adsorption capacities and catalytic efficiencies, attributed to their adaptable coordination environments, high surface areas, and cost-effective synthesis. These combined attributes make copper-based MOFs particularly promising for scalable applications in CO₂ capture and selective reduction, leveraging their structural and electronic tunability for enhanced performance.

Photocatalytic CO₂ reduction using MOFs faces a multitude of interconnected challenges that hinder its potential for sustainable energy conversion. Many photocatalysts suffer from limited quantum efficiency and suboptimal light absorption spectra, restricting the generation and utilization of photogenerated charge carriers. Rapid charge recombination further diminishes the availability of electrons for reduction reactions, thereby impairing overall catalytic performance. Achieving high selectivity toward the desired products remains difficult due to competing reaction pathways and uncontrolled product distributions. The stability of MOF-based catalysts is often compromised by photocorrosion, structural degradation, and leaching under operational conditions, undermining their long-term durability. Additionally, the scarcity of catalytically active sites with sufficient affinity for CO₂ adsorption and activation limits reaction rates, while the thermodynamic and kinetic barriers associated with multi-electron and multi-proton transfer steps result in high activation energies and sluggish kinetics. Mass transport limitations within porous structures hinder the efficient diffusion of CO₂ to active sites and impede the removal of reduction products, further reducing efficiency. When considering practical deployment, the scalability of laboratory systems to industrial-scale applications requires thorough examination, alongside challenges related to reactor design, system integration, and maintaining catalyst stability under continuous solar irradiation. Long-term operational stability under real-world solar conditions, coupled with techno-economic factors such as material costs, energy input, and process optimization, is critical for commercial viability. Addressing these issues necessitates the development of advanced MOF materials with tailored electronic structures, enhanced light-harvesting capabilities, robust stability, and improved mass transport properties, complemented by innovative engineering solutions. Cu-MOFs, in particular, offer valuable mechanistic insights and serve as a foundational platform for the rational design of highly selective, durable, and economically feasible catalysts capable of advancing photocatalytic CO₂ reduction toward practical, large-scale implementation.