Recent Advances in Photocatalytic CO₂ Utilisation Over Multifunctional Metal–Organic Frameworks

Abstract

The efficient conversion of carbon dioxide (CO₂) to high-value chemicals using renewable solar energy is a highly attractive but very challenging process that is used to address ever-growing energy demands and environmental issues. In recent years, metal–organic frameworks (MOFs) have received significant research attention owing to their tuneability in terms of their composition, structure, and multifunctional characteristics. The functionalisation of MOFs by metal nanoparticles (NPs) is a promising approach used to enhance their light absorption and photocatalytic activity. The efficient charge separation and strong CO₂ binding affinity of hybrid MOF-based photocatalysts facilitate the CO₂ conversion process. This review summarises the latest advancements involving noble metal, non-noble-metal, and miscellaneous species functionalised MOF-based hybrid photocatalysts for the reduction of CO₂ to carbon monoxide (CO) and other value-added chemicals. The novel synthetic strategies and their corresponding structure–property relationships have also been discussed for solar-to-chemical energy conversion. Furthermore, the current challenges and prospects in practical applications are also highlighted for sustainable energy production.

1. Introduction

The continuous increase in the greenhouse gas (CO₂) concentration in the atmosphere has become a global environmental concern [1,2]. To mitigate this environmental issue, significant research efforts have been directed towards CO₂ capture and storage (CCS) to decrease the level of emissions [3,4,5]. However, the high costs involved in CO₂ capture and transportation restrict the practical application of this process. Direct CO₂ conversion into value-added chemicals such as CO, methanol, and other hydrocarbons [6] is a promising route to overcome the dependency on non-renewable fossil fuels [7,8,9]. However, a significant amount of energy is required to break the highly stable molecules of CO₂, which have a bond enthalpy of 805 kJ mol⁻¹. Therefore, the use of abundant and reliable solar energy to carry out the photocatlytic CO₂ reduction is appealing and has received tremendous research attention for the sustainable conversion of solar energy to chemical energy by mimicking the natural photosynthetic process [10,11,12,13,14,15,16]. The CO₂ photoreduction product formation is dependent on the number of electrons and protons transferred in the reaction, as summarised in Equations (1)–(7) in

Table 1. Multielectron reactions of CO₂ reduction and their reduction potentials at pH 7.0 with reference to a normal hydrogen electrode (NHE) at 25 °C and 1 atm.

Equation Number	Reaction	Reduction Potential (V) vs. NHE
1	CO2 + 2$H^{+}$ + 2$e^{-} \to$ HCOOH	−0.61
2	CO2 + 4$H^{+}$ + 4$e^{-} \to$ HCHO + H2O	−0.53
3	CO2 + 2$H^{+}$ + 2$e^{-} \to$ CO + H2O	−0.48
4	CO2 + 6$H^{+}$ + 6$e^{-} \to$ CH3OH + H2O	−0.38
5	CO2 + 8$H^{+}$ + 8$e^{-} \to$ CH4 + 2H2O	−0.24
6	H2O$\to$ ½ O2 + 2$H^{+}$ + 2$e^{-}$	+0.82
7	2$H^{+}$ + 2$e^{-} \to$ H2	−0.41

, along with their reduction potentials compared to normal hydrogen electrodes (NHE) at pH 7.0 [17]. The photocatalytic CO₂ reduction process involves three main steps: (i) visible light absorption by the photocatalyst, (ii) generation of charge carriers, and (iii) efficient adsorption and activation of CO₂ on the surface of photocatalyst [18]. In the past few decades, research efforts have been focused on expanding the visible light absorption by incorporating the multifunctional properties of photocatalysts to promote the conversion efficiency [19,20,21,22,23,24,25]. However, the current development concerning the photocatalytic efficiency for CO₂ reduction is far from satisfactory because of the fast recombination of charge carriers, lower specific surface area values, and poor CO₂ adsorption capacity of photocatalysts. Therefore, it is important to design and develop photocatalysts that can address the aforementioned issues. CO₂ reduction using photothermal and thermochemical conversions, where enhanced yields using localised high temperatures generated via thermal relaxation processes have been reported [26,27,28,29,30,31], are mainly facilitated by surface reactions. However, in this review, we focus exclusively on photocatalytic reduction of CO₂, with a primary focus on metal–organic frameworks.

Metal–organic frameworks (MOFs) are a new class of porous solids prepared by linking multi-dentate organic ligands with a metal or metal clusters, leading to the development of single-site solid catalysts [32,33,34,35,36]. They have emerged as promising materials for a range of applications, including acid-based catalysis [37,38,39,40], redox reactions [41,42,43], drug delivery [44,45,46,47,48], asymmetric reactions [49,50,51,52,53,54,55], gas sorption [56,57,58,59], and photocatalysis [60,61,62,63,64], because of their large surface areas and high levels of porosity. In particular, MOFs are promising for photocatalysis due to the flexible ways in which they can be tailored toward a specific property. A number of structural parameters, such as the pore size, shape, topology, dimensionality, and chemical environment, can be altered through careful selection of the metal clusters and organic linkers that constitute their building blocks [65]. The structural arrangement assists in the integration of active sites and photosensitisers in a single material through metal nodes and organic linkers, allowing the development of multifunctional MOFs. Their hierarchical organisation allows the incorporation of both light absorption and catalytic components to achieve efficient solar utilisation in artificial photosynthesis. MOF-5 was amongst the first photoactive MOF materials studied for its charge separation efficiency under light irradiation, which led to the further development of various semiconducting MOFs for photocatalytic applications [66,67]. The functionalisation of MOFs can be carried out in situ or by post-synthetic modification (PSM). In situ functionalisation can be achieved using various ligands or metals during the synthesis of MOFs, while the post-synthetic approach involves the modification of the synthesised parent MOF by introducing organic functionalities, metal nanoparticles (NPs), oxides, carbon heterostructures, amines, and quantum dots [68].

MOFs are promising materials for photocatalytic CO₂ reduction for the following reasons: (i) their light absorption properties can be tuned, leading to improved charge separation efficiency; (ii) they possess the capability to capture CO₂; (iii) the constituent metal and organic linkers can participate in the catalytic reaction; (iv) the structural porosity allows access of guest molecules to active sites for efficient photocatalytic reduction [69,70]. Figure 1a illustrates the process of photocatalytic CO₂ reduction over MOFs under solar light irradiation. Although there are several published review papers summarising the synthesis, strategies, and applications in photocatalytic reactions, it is essential to provide a timely progress of the recent research advances in this field [17,18,71,72,73,74,75,76,77,78,79,80,81,82]. Figure 1b displays the continuous increase in the thousands of publications reported every year based on MOFs and CO₂ reduction reactions, showing the popularity of this field amongst scientists around the world. In this comprehensive review article, we present the recent advancements on the synthesis and catalytic application of functionalised MOFs. The article is divided into three different sections focusing on the visible light responsive MOFs functionalised by (i) noble metal NPs, (ii) non-noble metal NPs, and (iii) miscellaneous species for photocatalytic CO₂ reduction to CO, methane (CH₄), methanol (CH₃OH), and other value-added chemicals. The factors affecting the efficiency of CO₂ reduction, including the lifetimes of charge carriers and the CO₂ adsorption ability, are also reviewed. Finally, the current challenges and future opportunities of MOFs for photocatalytic CO₂ reduction are discussed.

2. Photocatalytic CO₂ Utilisation by MOF Hybrids

2.1. Noble-Metal-Based MOFs

Within the photocatalytic literature, the use of noble metals is ubiquitous. Owing to the ability to harness localised surface plasmonic resonance (LSPR), it is possible to tune the excitation frequencies of noble metal NPs to the visible spectral range through variations in particle size, shape, and dielectric environment [83,84,85,86,87,88]. The excited plasmons can undergo decay via radiative [89] and non-radiative processes [90]. Radiative decay involves the re-emission of photons, whereas non-radiative decay takes place via the generation of electron–hole pairs (e⁻–h⁺), known as Landau damping (LD) [83]. LD is a quantum mechanical phenomenon involving the transfer of plasmons into e⁻–h⁺ pairs within femtoseconds. Additionally, the large-field enhancement effects are also generated inside and near the metal NPs due to the collective electronic oscillations [91,92]. The field enhancement effects have been widely applied in various applications, such as in surface-enhanced Raman scattering (SERS), tip-enhanced Raman scattering (TERS), sensing, and nanophotonics [83]. Plasmonic heating is another possible non-radiative energy dissipation pathway leading to temperature increments of the catalytic system [93,94]. The photo-induced formation of electron–hole pairs makes these NPs ideal candidates for photocatalytic processes, such as the reduction of CO₂ to high-value, renewable fuels such as hydrogen (H₂), methanol, and formic acid [95]. The heterogenisation of MOFs with other catalytically active metal such as Pt and Rh has also been discussed as an effective strategy for the preparation of hybrid photocatalysts for CO₂ utilisation. Within this section, the activities of noble-metal-based photocatalysts will be examined, which are summarised in

Table 2. Noble metal species incorporated within MOFs for photocatalytic CO₂ utilisation.

MOF Hybrid Photocatalyst	Synthetic Methodology	Light Source	Photoreduction Product	Photocatalytic Performance	Reference
Ag@Co-ZIF-9	In situ reduction	200 W Xe ≥420 nm	CO	CO + H2 = 102.6 µmol h−1	[96]
Ag@MIL-101-Cr	In situ reduction	300 W Xe 400–780 nm	CO/CH4	CO = 808.2 µmol g−1h−1 CH4 = 427.5 µmol g−1h−1 H2 = 82.1 µmol g−1h−1	[97]
Ag@Co-MOF-74	Solvothermal transformation	300 W Xe 400–1000 nm	CO/H2	CO ≈ 10 µmolh−1 H2 ≈ 17 µmolh−1	[98]
Au@PPF-3	Dispersion	200 W Xe ≥400 nm	HCOOH	42.3 µmol g−1h−1	[99]
Au@TiO2	Solvothermal/pyrolitic conversion	200 W Hg/Xe 200–750 nm	CH4	10 ppm/h	[100]
Pt@MIL-125(Ti)	In situ reduction	300 W Xe 420–800 nm	HCOO−	H2 = 235 µmol HCOO− = 13 µmol	[101]
Pt@NH2-UiO-68	Solvothermal	300 W Xe 400–780 nm	CO	400 µmol g−1	[102]
Ag⊂Re3-MOF	Mixed linker Solvothermal	300 W Xe 400–700 nm	CO	0.093 h−1	[15]
Cp*Rh@UiO-67	Linker Exchange	300 W Xe ≥415 nm	H2/HCOO−	TON HCOO− = 47 TON H2 = 36 HCOO− + H2 = 1.2 µmol h−1	[103]

below.

2.1.1. Ag-Based Functionalised MOFs

The introduction of NPs onto MOF architectures can be facilitated through a variety of synthetic strategies, and largely depends on the stability of the MOF towards NP formation conditions. Consideration of the intended site for NPs is important, and timely characterisation is often reported to help ascertain the locations of NPs in terms of whether they reside within the porous frameworks or decorate the outer surfaces. Isolation of NPs within the porous framework is usually achieved through solvothermal methods, by mixing pre-synthesised NPs into the precursor solution containing the metal salts and organic linker. This method “grows” the framework around the NPs, encompassing them within its porous system. Conversely, in situ reduction of NP precursors or the direct deposition of NPs onto MOFs often isolates NPs on the surfaces. The latter of these two strategies provide a less synthetically demanding route for NPs@MOF catalysts, as the former relies heavily on inherent MOF stability towards reductive conditions using borohydrides or H₂.

Min Choi et al. attached a rhenium-based photocentre (Re^I(CO)₃Cl) to the same bipyridyl dicarboxylic acid linker (ReTC) for mounting within UiO-67 [15]. In the synthesis of the Re_n-framework, a solvothermal protocol was established, in which varying stoichiometric ratios of the bipyridyl and photoactive linker were added to the precursor solution to yield frameworks with varying Re contents (Figure 2a). Under visible light illumination using trimethylamine as a sacrificial electron donor, it was found that the framework consisting of 3 photoactive ReTC linkers per unit cell provided the highest activity for conversion of CO₂ to CO. Structural analysis of the Re₃-MOF showed that the ReTC linkers covalently bound within the octahedral cavities of the framework and adopted a fac arrangement about the Re-centre, as confirmed by Fourier difference maps, by IR and NMR imaging, and by the presence of a metal-to-ligand charge-transfer (MLCT) band at 400 nm.

To further improve the photocatalytic activity, the Re₃-MOF was coupled to the Ag nanocubes to produce a bifunctional catalyst, as shown in Figure 2b. Following the same solvothermal method used to prepare the Re₃-MOF but with the addition of pre-formed Ag nanocubes, the Re₃-MOF was crystallised around the nanocubes to produce Ag⊂Re_n-MOF, the thickness of which was controlled through the synthesis time. Figure 2c,d display the transmission electron spectroscopy (TEM) image of the Ag⊂Re₃-MOF, in which the Re₃-MOF was constructed on the surface of Ag nanocubes. The maximum absorption in the ultraviolet-visible (UV-vis) spectra of Ag⊂Re₃-MOF compared to the Re₃-MOF and Ag nanocube is also shown in Figure 2e. In a similar manner to NPs, the Ag nanocubes can spatially localise the photoactive Re centres to intensified electric fields at their plasmon resonance frequency, which can be orders of magnitudes higher than the incident electromagnetic field. Under identical photocatalytic conditions, the Ag⊂Re₃-MOF exhibited a 5-fold enhancement of activity over the Re₃-MOF. Notably, Ag nanocubes encapsulated in a thinner MOF layer (16 nm) provided a 7-fold enhancement of the photocatalytic activity (ca. TON = 0.09/hr). This observation was attributed to the exponential decay of the near field from LSPR with increasing distance from the surface of the nanocube. Control experiments using MOF layers with no ReTC (Ag⊂Re₀-MOF) showed no activity, ruling out the possibility of Ag being responsible for CO production. Furthermore, upon coating a similar loading of ReTC onto similar sized Cu NPs, which do not observe LSPR characteristics that match the absorption features of ReTC, activity enhancement was not observed.

Using a photodeposition method to form NPs, Chen et al. decorated cobalt based-zeolitic imidazolate framework (Co-ZIF-9) samples with varying quantities of Ag NPs in order to enhance the previously reported catalytic potential of bare Co-ZIF-9 [104], as well as its capability as a co-catalyst within a cadmium sulfide(CdS)–bypyridine photocatalytic system [96,105]. In the presence of a photosensitiser ((Ru(bpy)₃)Cl₂·6H₂O) (bpy = 2’,2-bipyridine) and triethanolamine (TEOA), Co-ZIF-9 provided 13.3 and 19.3 µmol of CO and H₂, respectively. Owing to the Co²⁺ centres within the framework, the deep-purple appearance of Co-ZIF-9 provides strong adsorption in the visible spectral range above 500 nm. With the addition of Ag NPs, the absorption range of the hybrid catalyst expanded to include the higher energy wavelengths in the 350–500 nm region. With 2.07 wt % of Ag NPs, an immediate change in selectivity was observed; CO generation increased to 17.2 µmol and H₂ evolution decreased to 14.8 µmol. The optimum wt % of Ag within the framework was found to be 5 wt %. To elucidate the source of the reduction product, Chen et al. [96] conducted an isotopic experiment using carbon-13-labelled CO₂ (¹³CO₂). Under the same reaction conditions, mass spectrometry reported peaks at m/z 29 and 45 only, with no signal detected at m/z 28. This confirmed that the produced CO was the result of successful reduction of the CO₂ feedstock rather than decomposition of other organics within the photocatalytic system.

Through the reduction of an AgNO₃ precursor using NaBH₄, Guo et al. decorated the external surfaces of pre-formed chromium terephthalate metal-organic framework, MIL-101(Cr) (Matérial Institut Lavoisier) nanocrystals with ~8 nm Ag NPs [97]. Displaying the characteristic deep-green colour of Cr³⁺, bare MIL-101(Cr) strongly absorbs in the visible region. The MOF decorated with Ag NPs expanded the absorption corresponding to the LSPRs of Ag in the absorption wavelength range into the near-IR region (400–1200 nm), providing a greater coverage of the solar spectrum. By efficiently engineering different sizes of MIL-101(Cr) nanocrystals through varying the MOF-precursor concentrations, Guo et al. effectively demonstrated the crystal-size dependency of the photocatalytic system in the photoreduction of CO₂ to CO, H₂, and CH₄ products. Across a sample range encompassing crystal sizes from 80 to 800 nm, it was shown that the Ag-decorated MIL-101(Cr) catalysts with the smallest crystallite size provided superior catalytic activity in TEAOH, yielding rates of 808.2, 427.5, and 82.1 µmolg⁻¹h⁻¹ for CO, H₂, and CH₄, respectively. The photoreduction rates using 800 nm crystallites were 23, 18, and 35 times lower than the 80 nm system, respectively. Without the inclusion of Ag NPs, the 80 nm catalysts provided the highest rates for bare MIL-101(Cr) but showed vastly decreased performance when compared to those decorated with Ag NPs. Furthermore, under identical conditions and NP loading, Ag NPs dispersed onto Al₂O₃ provided a very low rate of CO production (8.8 µmolg⁻¹h⁻¹).

Using a simplified model, the size-dependent activity was attributed to the densities of unit cells on the edges and corners as percentages of the total surface unit cells. For the 80 nm catalyst, 63.6% of surface unit cells within the crystal lie along edges and corners. This percentage significantly decreases with increasing crystal size, with the 800 nm particle displaying only 6.6% of unit cells along edges and corners. This gives rise to favourable photo-responsive properties for the smaller crystallites, providing superior photocurrents under illumination, and concomitantly the lowest electron transfer resistance. Together, the deposition of Ag NPs provides enhanced light adsorption, while size-engineered MIL-101(Cr) facilitates enhanced charge-separation.

Utilising another member of the ZIF-family, Deng et al. utilised a solvothermal treatment of a pre-formed, Ag-NP-decorated Co-ZIF-67 framework with an appropriate linker (2,5-dihydroxyterephthalic acid, H₄DOBDC) in order to induce a transformation to an Ag-NP-functionalised, hollow Co-MOF-74 [98]. Through this novel strategy, Co-MOF-74 particles with a double-layer hollow shell structure were prepared without any remarkable change in the morphology in comparison to original ZIF-67 particles.

Various experiments were conducted by changing the molar ratios of H₄DOBDC to ZIF-67 (n(L)/n(M)) and the concentration of H₄DOBDC (c(L)) to yield hollow shells, as shown in Figure 3. Owing to the cobalt density difference between Co-MOF-74 and Co-ZIF-67 and because H₄DOBDC molecules are bigger than the ZIF-67 channel, the transformation of a ZIF-67 particle to MOF-74 occurs from outside to inside. The low number of Co atoms available to fill in the spaces of the original ZIF-67 particles facilitates the formation of MOF-74’s hollow structure. In the instance of higher n(L)/n(M) ratios but low c(L), single-layered hollow structures will be formed. Compared to traditionally synthesised Co-MOF-74 (MOF-74-C), the transformation of ZIF-67 resulted in monodisperse 300 nm particles of Co-MOF-74 (MOF-74-T) composed of ca. 30 nm crystallites. Although providing lower total N₂ adsorption capacity, MOF-74-T gave a much larger external surface area and increased pore volume, which was attributed to the hollow nature of the MOF architecture. To demonstrate the structural advantages of MOF-74-T and AgNPs@MOF-74 compared to the MOF-74-C, the efficiency levels of the co-catalysts were tested in the photocatalytic conversion of CO₂ to CO in the presence of Ru(bpy)₃Cl₂ and TEOA. It was found that all catalysts displayed H₂ generation ability, however superior performance was shown by the AgNPs@MOF-74 catalyst (CO = ca. 10 µmol/h, H₂ = ca. 17 µmol/h), which was attributed to the LSPR effect of Ag NPs. MOF-74-T provided decreased activity without decoration of NPs, however still provided better performance than the conventionally crystallised MOF by a factor of 1.8 (MOF-74-T: CO = ca. 4.4 µmol/h, H₂ = ca. 8 µmol/h). Deng et al. [98] concluded that the larger external surface area of the hollow structure leads to the exposure of more co-catalytic active sites, while the thin shell allows diffusion of the reactants and bulky Ru(bpy)₃Cl₂.

2.1.2. Au-Based Functionalised MOFs

In order to replicate the intriguing catalytic performance of ultra-thin materials, such as carbon-based sheets, metals, and metal dichalcogenides, compared to other heterogeneous morphologies, Chen et al. employed a Co²⁺-based porphyrin paddle wheel framework (PPF) to host Au NPs [99]. Figure 4a–f display the scanning electron microscopy (SEM), TEM, and atomic force microscopy (AFM) images of PPF and Au–PPF catalysts with different magnifications. To achieve thin sheets, polyvinylpyrrolidone (PVP) was employed as a structure-directing agent (SDA) to restrict growth along the c-axis, resulting in square sheets of 1 µm width and 20 nm thickness. Similar synthesis in the absence of SDA resulted in thicker nanosheets (120 nm).

To decorate the sheets with pre-synthesised Au NPs, Chen et al. [99] added citrate-modified NPs to a dispersion of PPF-3 nanosheets. In order to achieve adequate adhesion between NPs and the PPF-3 sheets, thermal treatment of the PPF-3 in DMF was required. Before washing, it was found that the negative zeta potential of the NPs (−19.4 mV) made it difficult for them to bind to the surface of the PPF-3, which also produced a negative value (−5.9 mV) arising from the surface-bound PVP. Treatment and removal of the SDA shifted the PPF-3 zeta potential to a positive value (+4.9 mV), allowing favourable electrostatic interaction and uniform dispersion across the external surfaces of the PPF-3 sheets. This observation was also partly attributed to the migration of Co²⁺ ions within the PPF-3 sheets.

Using an acetonitrile–ethanol mixture (v/v = 4:1), the photocatalysts produced formic acid (HCOOH) as the sole product. Both Au-NP-decorated PPF-3 sheets provided higher HCOOH production rates compared to their parent frameworks, with the thin-sheet hybrid providing a superior rate of 42.7 µmolg⁻¹h⁻¹, representing a 5-fold increase in activity compared to the framework without Au NPs (8.3 µmolg⁻¹h⁻¹). Identical catalysis on thicker sheets without NPs provided further inferior activity, clearly demonstrating the importance of the framework thickness to intrinsic electron transfer and mass transport properties. The substitution of ethanol for other aliphatic alcohols such as n-propanol and n-butanol decreased the rates of HCOOH production to 18.5 and 12.1 µmolg⁻¹h⁻¹, respectively.

Following electro- and photochemical analyses, Chen et al. [99] attributed the activity of the Au NP/PPF-3 system to two factors: (i) upon visible light illumination, electrons from the PPF-3 highest occupied molecular orbital (HOMO) transition to the lowest unoccupied molecular orbital (LUMO) and are transferred to the Co²⁺ centres, generating Co⁺ ions; (ii) deposition of Au NPs amplifies the local electric fields induced by LSPR, resulting in the absorption of more photons and leading to a higher concentration of Co⁺ ions.

Beyond its function as a porous host, Khaletskaya et al. [100] showed amine functionalised-MIL-125 (NH₂-MIL-125(Ti)) to be suitable as a sacrificial precursor for the synthesis of TiO₂. Motivated by the challenges associated with the controllable production of nanoscale TiO₂ particles and the discernible advantages this can bring to the field of photocatalysis, the pyrolytic conversion of MOF precursors offers routes to unusual metal oxide morphologies, as their porosity provides a host matrix for the loading of additional components [100]. Utilising a solvothermal encapsulation synthesis protocol, Khaletskaya et al. [100] placed pre-formed, monodisperse Au NPs (2.1 ± 0.8 nm) within the precursor MOF solution and heated the resulting mixture to induce crystallisation of the framework. This resulted in a hybrid material for which much larger Au NPs (3.8–20 nm) were identified as residing primarily on the outer surface. Following successful crystallisation, the hybrid MOF was heated in dry O₂ at 450 °C for 2 h to convert the decorated framework into an Au/TiO₂ hybrid. Through this synthetic pathway, 300–450 nm nanocrystals retaining the size and morphology of the host framework were produced, validating the synthetic rationale. A similar process was performed using bare NH₂-MIL-125(Ti) to produce a TiO₂ reference. Notably, analysis by powder X-ray diffraction (PXRD) and Fourier transform (FT) Raman spectroscopy revealed that the presence of Au NPs induced selective crystallisation into the rutile phase, whereas the TiO₂ reference contained traces of anatase [100].

In the photocatalytic formation of CH₄ from a CO₂/H₂O mixture (1.5 and 0.6%, respectively) diluted in He, the Au NP/TiO₂ outperformed the TiO₂ reference catalyst, producing ca. 50 ppm CH₄ across a 6 h reaction. Interestingly, to establish the stability and reusability across subsequent cycles, Khaletskaya et al. [100] observed significant increase in activity for the TiO₂ reference catalyst in the 2nd cycle. Citing similar peculiarities to the work of Strunk et al. [106] on Au/Ti/SBA-15, this observation was attributed to the availability of a residual “carbon pool” on the TiO₂ reference catalyst surface at the beginning of the 2nd cycle [106]. These surface species were generated in the 1st cycle and could not be removed through evacuation between cycles. Consistent CH₄ production was observed for the Au NP/TiO₂ hybrid, which clearly outperformed commercial P25 and AUROlite (commercial Au/TiO₂) [106].

2.1.3. Pt- and Rh-Based Functionalised MOFs

Employing a different member of the MIL family, Sun et al. compared the activity of Pt and Au NPs on NH₂-MIL-125(Ti) for the formation of hydrogen and formate (HCOO⁻) [101]. MIL-125(Ti) and its amino-based derivative, NH₂-MIL-125(Ti), have been explored extensively for their photocatalytic potential. Reported first by Férey and coworkers, MIL-125(Ti) absorbs strongly in the UV region and has been shown to provide photochromic behaviour when irradiated in the presence of alcohols [107]. To extend the light absorption into the visible region, the terephthalic acid linker was replaced with the amino derivative, 2-aminoterephthalic acid, by Li and co-workers to prepare NH₂-MIL-125(Ti) [108]. In carrying out this substitution, it was found that not only had the absorption properties of the framework been extended into the visible region (Figure 5), the CO₂ sorption capacity had also improved compared to the parent framework.

Under visible light irradiation, NH₂-MIL-125(Ti) was shown to be active in the photoreduction of CO₂ to provide HCOO⁻, whereas the non-substituted framework provided no activity, signifying the importance of the amino group in enhancing the adsorption range of the framework [109]. In the formation of NPs via in situ reduction with H₂, Sun et al. demonstrated that Pt- and Au-NP-deposited NH₂-MIL-125(Ti) samples provide 235 and 40 µmol of H₂, respectively, as the major products over 8 h, whereas no detectable H₂ was observed with NH₂-MIL-125(Ti). This was attributed to the ability of the noble metal NPs to trap excited electrons from the parent framework and provide redox reaction sites for hydrogen evolution. A similar system was demonstrated by Horiuchi et al. using Pt NPs, providing 33 µmol of H₂ after 9 h [110]. Despite this activity, with reference to the previous work by Li and coworkers [108], Sun et al. anticipated a reduction in HCOO⁻ production upon deposition of noble metal NPs [101]. In the presence of sacrificial electron donor, TEOA, the organic linker within NH₂-MIL-125(Ti), acts as an antenna, transitioning into a long-lived excited charge separation state upon visible light absorption into the ligand-to-metal charge transfer (LMCT) band. This is followed by the transfer of an electron from the organic linker to the metal node. Within these nodes, octomeric Ti-O units (Ti₈O₈(OH)₄) are considered as mimics of isolated titanium oxide quantum dots and undergo reduction to the active Ti³⁺ photocatalytic species, producing HCOO⁻. It was suggested that the noble metal NPs would reduce the possibility of electron transfer to the metal nodes, diminishing the generation of the active Ti³⁺ species. As predicted, under identical photocatalytic conditions, the framework with Au NPs provided a lower HCOO⁻ (9.06 µmol) yield than the bare NH₂-MIL-125(Ti) (10.75 µmol). The same was not observed for the framework with Pt NPs, however, which provided a 21% increase in activity (12.96 µmol). The use of electron spin resonance (ESR) spectroscopy and DFT calculations revealed that the disparity in activity was attributed to H₂ spillover from the Pt NPs, resulting in the migration of H to corner-sharing oxygen atoms between Ti octahedra, forming Ti³⁺ species. The thermodynamic barrier for this process was calculated to be lower in the case of Pt compared to Au (0.53 vs. 0.94 eV), providing the observed activity [101].

Using an amino derivative of UiO-68(Zr), NH₂-UiO-68, Guo et al. demonstrated the dependence of CO₂ photoreduction performance on the locations of NPs and the lengths of linkers within the MOF hybrids [102]. Contrasting step-wise and one-pot synthesis strategies, Guo et al. deposited Pt NPs (2 wt %) onto the surface (Pt(2)/NH₂-UiO-68) by adding the pre-synthesised MOF into a Pt NP suspension or embedded them within the MOF (Pt(2)@NH₂-UiO-68) via a solvothermal method. Using surface-sensitive, high-resolution X-ray photoelectron spectroscopy (XPS), contrasting Pt peak areas were observed, providing NP locations for catalysts containing equal Pt wt % values, with increased areas observed for surface NPs. The effect of the NP location on the ability of the catalysts to trap electrons was clearly evident during photo and electrochemical measurements, with Pt(2)@NH₂-UiO-68 displaying a 3-fold increase in transient photocurrent response, a significantly smaller capacitance, and an almost fully quenched photoluminescence emission intensity compared to Pt(2)/NH₂-UiO-68. Such observations imply that the position of the Pt NPs has a great influence on the charge separation efficiency in the charge utilisation processes.

In the photoreduction of CO₂ under visible light in the presence of TEOA, CO was produced as the sole product using these catalyst systems. Compared to the bare framework, all Pt NPs containing catalysts provided increased CO production rates. In particular, Pt(2)@NH₂-UiO-68 provided the best performance, displaying a 3-fold enhancement in activity (400.2 µmolg⁻¹) compared to the Pt(2)/NH₂-UiO-68 catalyst with external NPs. This 2 wt % catalyst further outperformed identically synthesised catalysts with loadings of 1 and 4 wt %, as well as a physical mixture of 2 wt % Pt NPs and NH₂-UiO-68 (37.8 µmolg⁻¹), signifying the effect of the NP location on the photocatalytic performance. From these observations and associated Mott–Schottky measurements, Guo et al. [102] suggested that the n-type semiconductor behaviour provided by the NH₂-UiO-68 framework with a 2.87 eV band gap facilitated electron transfer from the framework to the Pt NPs, effectively promoting rapid charge transfer, as illustrated in Figure 6. The location of LUMO of the MOF framework at −0.60 V and the energy level of Pt at 1.15 V facilitates the efficient electron transfer under visible light irradiation. The electron-rich Pt species exhibit much higher photocatalytic activity in the CO₂ reduction reaction.

To differentiate the effect of the linker length, an identical catalyst with embedded NPs was synthesised using NH₂-UiO-66 (analogous framework by varying the length of the linker) and tested under identical photo-reductive conditions, providing a reduced yield of CO (102.6 µmolg⁻¹). This observation was attributed to the ability of the NH₂-UiO-68-based framework to accommodate more CO₂ guest molecules.

Aside from the decoration of NPs, another synthetic route for imparting noble-metal-based photocatalytic properties upon a MOF is to mount photoactive moieties onto the organic linker, most commonly performed via PSM procedures. Using UiO-67 as the host framework, Chambers et al. demonstrated this synthetic strategy using a rhodium-based linker modelled upon the biphenyl dicarboxylic acid linker of the original MOF, as illustrated in Scheme 1 [103]. Using a bipyridyl dicarboxylic acid analogue, a photoactive Cp*RhCl₂ group (Cp* = pentamethylcyclopendadiene) was bound to the pyridyl nitrogen atoms (CpRh(bpydc)). Owing to the large accommodating pores and inherent stability of the crystallised UiO-67 framework towards moisture and temperature, the framework could undergo post-synthetic ligand exchange, a common synthetic protocol used to insert linkers that would not otherwise crystallise into the framework through traditional solvothermal methods [111]. This process exchanged bound biphenyl linkers with the Cp*Rh linker, heterogenising the photoactive rhodium centre and yielding a series of functionalised, isoreticular MOFs, namely, 5%, 10%, 20%, and 35%-Cp*Rh@UiO-67. Notably, confirming the successful linker exchange, the NMR analysis showed peaks of the displaced biphenyl linker in equivalent stoichiometric quantities. An identical process using a bipyridyl analogue, a linker without dicarboxylic acid groups (CpRh(byr)), provided no such peaks. Although uptake of the CpRh(byr) was observed, it was concluded that it resided within the pores of the framework and could easily be washed out through sequential washing.

In establishing the photocatalytic behaviour of the Cp*Rh@UiO-67 in the presence of TEOA and the photosensitiser, Ru(bpy)₃Cl₂, the rates of HCOO⁻ and H₂ generation were compared to that of the homogeneous CpRh(bpydc) linker, as well as the bipyridyl analogue (CpRh(byr)). Other than formate, no other carbon-containing products were detected. In the homogenous phase, CpRh(byr) outperformed CpRh(bpydc) in both HCOO⁻ and H₂ production (TON[HCOO⁻] = 125 vs. 42, TON[H₂] = 55 vs. 38, respectively), which was attributed to the electron-withdrawing ancillary carboxylate groups of CpRh(bpydc). Under identical conditions, using a Cp*Rh@UiO-67 framework of near-equivalent Rh loading, the heterogenised catalyst provided comparable performance compared to the homogeneous CpRh(bpydc) (TON[HCOO⁻] = 47 vs. 42, TON[H₂] = 36 vs. 38, MOF vs. homogeneous). Both systems showed comparable stability over several hours, with a decrease in performance attributed to the breakdown of the photosensitiser rather than the photocatalyst. In particular, the longevity and reusability of the Cp*Rh@UiO-67 was confirmed over 6 cycles, and a cumulative photoactivity in excess of 4 days with only a 20% loss in activity was observed, which was ascribed to continual handling of the catalyst (1.4 mg) and perspective mass losses. Studying the effect of Rh % on catalytic activity, it was found that 10% loading was optimal in the heterogeneous system, beyond which formate production decreased rapidly with concomitant increase in H₂ formation. Above 10% loading in the homogenous system, all activity significantly decreased, suggesting that the heterogenisation procedure provided a stabilisation effect, reducing the possibility of bimetallic deactivation pathways.

Table 3. Catalytic descriptors summarising noble metal species incorporated within MOFs for photocatalytic CO₂ utilisation.

Descriptor	MOF Hybrid Photocatalyst	Photocatalytic Trend	Ref
Crystallite size	Ag@MIL-101-Cr	The smallest MIL-101 crystallite decorated with Ag provided superior activity	[97]
Organic linker	Pt@MIL-125(Ti)	The use of 2-aminoterephthalic acid instead of terephthalic acid organic linker enhanced the visible light absorption and CO2 adsorption capacity	[101]
Surface area and shell thickness	Ag@Co-MOF-74	Large surface area and thin shell allowed exposure to active sites and easy diffusion of substrates, respectively	[98]
NP location	Pt@NH2-UiO-68	Pt NPs embedded within the MOF displayed 3-fold enhanced activity compared with NPs present on surface	[102]
Length of organic linker	Pt@NH2-UiO-68	NH2-UiO-68 gave higher product yields than NH2-UiO-66 due to its ability to accommodate more CO2 molecules	[102]
Thickness of framework	Au@PPF-3	Au decorated thin PPF-3 sheets provided superior activity than with thicker sheets	[99]
Bifunctionality	Ag⊂Re3-MOF	The combination of Re3-MOF with Ag nanocubes enhanced light absorption and catalytic activity (five-fold) compared with bare Re3-MOF.	[15]

summarises the MOF hybrid photocatalysts discussed in this section by corelating the specific catalytic descriptors with the observed photocatalytic activity trend, including the crystallite size, nature of the organic linker, surface area, thickness of the MOF framework, location of the NPs, length of the organic linker, and effect of introducing bifunctionality within the MOF framework.

2.2. Non-Noble-Metal-Based MOFs

The use of less expensive non-noble-metals [112,113,114] such as Co, Ni, Fe, and Cu in MOF photocatalysts with high catalytic activity and selectivity is still a challenge when developing low-cost materials [15,96,97,98]. It is highly desired to develop cost-effective materials with high performance and durability [115,116,117], and research efforts have focussed on developing non-noble-metal MOF analogues that rival the catalytic performance of the noble-metal-based counterparts. Much attention has been paid to Fe-containing MOFs, since iron is an earth-abundant element and iron-containing complexes are commonly used in photocatalysis. Further, Fe-based MOF materials are reported to be directly excited by visible light due to the existence of iron-oxo clusters. In particular, terephthalate- and porphyrinic-based Fe-MOFs have seen considerable attention in recent years [118,119,120,121].

Table 4. Non-noble-metal species incorporated within MOFs for photocatalytic CO₂ utilisation.

MOF Hybrid Photocatalyst	Synthetic Methodology	Light Source	Photoreduction Product	Photocatalytic Performance	Ref
NH2-Fe MOFs	Solvothermal	300 W Xe 420–800 nm	HCOO−	178 µmol (8 h)	[118]
Solvent-free NH2-Fe MOFs	Solvothermal	300 W Xe 420–780 nm	CO	87.6 μmol g−1	[119]
LHP QDs@PCN-221(Fe)	Solvothermal/PSM	300 W Xe ≥400 nm	CO/CH4	1559 µmolg−1 (80 h) CO (34%)/CH4 (66%)	[121]
In-FexTCPP-MOF	Solvothermal	300 W Xe ≥400 nm	CO	3469 μmolg−1 (24 h)	[120]

summarises the case studies of Fe-based MOF hybrid photocatalysts discussed in this section.

Acknowledging the difficulties in establishing the photocatalytic processes within NH₂-MIL-125(Ti), Wang et al. reported the photocatalytic performance over three typical Fe-based MOFs and their amino-functionalised derivatives; MIL-101(Fe), MIL-53(Fe), and MIL-88B(Fe) [118]. All three Fe-based MOFs are based upon the terephthalic acid linker bound to Fe-octahedra and the selected MIL frameworks vary only in their topology. MIL-53 is composed of infinite M-OH chains of corner-sharing octahedra bound to four neighbouring chains and is most readily recognised for its associated “breathing” ability. MIL-88B and MIL-101 both share the same trimeric Fe₃O node, within which three metal octahedra are bound by a single bridging µ₃ oxygen molecule and are connected to a further 6 nodes through the terephthalic linker. Similar to MIL-53, MIL-88B shares similar breathing properties and arranges in staggered layers, forming expandable channels upon host incorporation. Conversely, MIL-101 forms a rigid structure with two differently sizes pores composed of hexagonal and pentagonal windows. Through functionalisation with the amino linker derivative, Wang et al. [118] enhanced the absorption of the MOFs within the visible region beyond 700 nm, as well as the adsorption capacity towards CO₂ compared to the unfunctionalised MOFs.

In the presence of TEOA, all functionalised and non-functionalised MOFs reduced CO₂ to HCOO⁻, with superior activity provided by NH₂-MIL-101(Fe), producing 178 µmol in 8 h. Across the MOF series, Wang et al. [118] demonstrated a linear trend between formate production and CO₂ adsorption, with all amino-functionalised MOFs providing enhanced production compared with their parent framework. Further, using the MIL-101-based frameworks, the reduction process was shown to be photocatalytic in nature, as when a cut-off filter was employed to restrict irradiation wavelengths to above those adsorbed by the non-functionalised framework, HCOO⁻ was produced, whereas 5.6 μmol of HCOO⁻ was produced over the amino- derivative. From these observations, Wang et al. concluded that the amino derivative provides an additional excitation pathway for unfunctionalised MOFs, while visible light irradiation results in an excited charge separation state by transferring an electron from the O²⁻ to Fe³⁺ within the metal nodes, generating Fe²⁺, which is capable of reducing CO₂. For the amino-functionalised derivatives, excitation of the NH₂ functionality followed by an electron transfer from the excited organic linker to the metal centre also generates Fe²⁺, as shown in Scheme 2. This dual-excitation pathway is similar to that observed for the NH₂-MIL-125(Ti) and NH₂-Uio-66(Zr) frameworks described above [108,122].

Most recently, Dao et al. demonstrated the tuning of selectivity using the same amino-functionalised Fe-MOFs to produce CO by designing a solvent-free system, removing the organic media that are traditionally utilised in photocatalytic reductions, as depicted in Scheme 3 [119]. Within this system, the MOF photocatalyst and TEOA were uniformly dispersed into a glass fibre film. CO₂ gas was pumped into the system and circulated around a pipeline to increase the contact between the catalyst and CO₂, generating a gas−solid interface. Compared with a conventional gas−liquid−solid reaction route, the solvent-free gas−solid interface removes the organic medium, providing a more environmentally friendly catalysis route and increasing CO₂ adsorption.

Under photocatalytic conditions, all MOFs provided non-linear CO production curves, which were attributed to the high boiling point of TEOA. This facet resulted in slow volatilisation of the TEOA, and therefore little CO generation at the beginning of the cycle. For all experiments, no H₂ or CH₄ gas was evolved and no HCOOH was observed as a liquid product, as determined through NMR. Of the MOFs, NH₂-MIL-101(Fe) provided the best catalytic performance over 5 h, with a CO formation rate of 87.6 μmolg⁻¹, 5.6 times higher than that of the worst performing NH₂-MIL-53(Fe) (15.7 μmolg⁻¹). As previous identified by Wang et al. [118], the superior performance of the MIL-101 framework was attributed to the enhanced CO₂ adsorption capability, as well as coordinatively unsaturated Fe sites within the metal node of the MOF, which become available after removal of pendant H₂O ligands through thermal treatment.

Photoluminescence spectra of the Fe MOFs displayed remarkably decreased fluorescence compared to the amino-functionalised linker, which exhibits an emission at 485 nm. This quenching is attributed to the effective LMCT upon the incorporation of the amino group, and the degree of quenching matches the catalytic trend. Further, using photocurrent measurements, Dao et al. [119] showed that NH₂-MIL-101(Fe) possesses the highest photocurrent, and concomitantly the smallest resistance amongst the tested MOFs, suggesting that the higher catalytic capacity of NH₂-MIL-101(Fe) is attributed to its more efficient charge transfer ability.

Within porphyrinic-based MOF, PCN-221(Fe), Wu et al. encapsulated lead halide perovskite (LHP) quantum dots (QDs) to construct a series of composite photocatalysts of MAPbI₃@PCN-221(Fe_x) (x = 0 – 1), as shown in Figure 7 [121]. Based upon 4,4′,4”,4”’-(porphyrin-5,10,15,20-tetrayl) tetrabenzoate] (TCPP) linkers connected by zirconium nodes, the PCN-221 framework provides a unique binding site at the centre of the porphyrin linker to localise a single metal ion. By altering the ratio of bare linkers (H₂TCPP) to metal-containing linkers (TCPP(Fe)), a series of catalysts with varying Fe contents can be synthesised easily. Driven by the improvement in catalytic performance of LHP QDs for converting CO₂ into CO and CH₄ when loaded onto graphene oxide or g-C₃N₄, the strategy employed by Wu et al. [121] was founded upon three considerations: (i) QDs with ultra-small size and homogeneous distribution can be generated in the pores of the MOF; (ii) the MOF framework can provide improved stability for the LHP QDs, as well as increased catalytic performance; (iii) localisation of the LHP QDs close to the MOFs catalytic centre. The metal ion residing in the TCPP linker shortens the charge transfer distance, enhancing the charge separation efficiency of LHP QDs under visible light irradiation and improving the catalytic activity of the MOFs.

To encapsulate QDs into the pores of the MOF, as-prepared PCN-221(Fe_x) was immersed in a PbI₂ solution and then rinsed to remove residual surface PbI₂. The resulting PbI₂@PCN-221(Fe_x) was then immersed in a methyl ammonium iodide (MAI) solution to generate MAPbI₃@PCN-221(Fe_x) composite photocatalysts, with QDs of ca. 1.8 nm within the 2 nm pores.

In a CO₂-saturated ethyl acetate solution containing a small amount of water as the sacrificial agent, both QD-decorated and parent framework photocatalysts were tested for the formation of CO and CH₄. Without QDs, the frameworks displayed selectivity for CH₄, whereas CO was the primary product when QDs were encapsulated or frameworks did not include any Fe. No H₂ or liquid products such as HCOOH and CH₃OH were detected for any photocatalytic systems. Without QDs, the yields of CO and CH₄ were 13 and 38 µmolg⁻¹, respectively, after 25 h of irradiation, after which the frameworks were observed to decompose. Under the same conditions, a framework containing QDs but no Fe displayed little activity. With both QDs and Fe (x < 0) incorporated into the catalysts, all exhibited significant increases in CO₂ photoreduction activity. Most notably, the photocatalyst containing 20% TCPP(Fe) linker exhibited the highest photocatalytic activity, achieving 104 µmolg⁻¹ of CO and 325 µmolg⁻¹ of CH₄. Furthermore, Wu et al. demonstrated that the encapsulation of LHP QDs provides greater stability to the pristine PCN-221(Fe_x) photocatalyst series, resulting in superior stability of MAPbI₃ QDs compared to those reported on other substrates such a ZIFs [16,114,116]. As such, the MAPbI₃@PCN-221(Fe_x) photocatalysts display significantly enhanced yields for CO₂ reduction over extended time periods (1559 µmolg⁻¹ over 80 h, CO (34%) and CH₄ (66%)) that are 25–38 times higher than the parent PCN-221(Fe_x) frameworks.

During ¹³CO₂/H₂¹⁸O isotope trace experiments, mass spectrum signals for ¹³CO (m/z = 29) and ¹³CH₄ (m/z = 17) were observed, indicating that both CO and CH₄ originated from the reduction of CO₂ rather than photo-oxidation of ethyl acetate. Similarly, the evolution of ¹⁸O₂ (m/z = 36) was observed, demonstrating that the water was the reducing agent rather than ethyl acetate. This observation was replicated upon substitution of the ethyl acetate with acetonitrile, for which photocatalytic cycles produced similar amounts of CO, CH₄, and O₂. On the basis of these experiments, Wu et al. highlighted the role this catalytic system can play in the practical application of artificial photosynthesis using water as an electron source.

Most recently, Wang et al. explored a similar methodology to achieve a 2-fold interpenetrated MOF framework containing 4 connected indium nodes and metal-doped porphyrin linkers [120]. The identity of the metal at the centre of the porphyrin linkers, as well as the overall metal content within the linkers, was varied to achieve a series of photocatalysts containing indium (In-InTCPP-MOF), cobalt (In-Co_1.71TCPP-MOF), or iron (In-Fe_xTCPP-MOF, x = 0.75/1.91/3.56). Utilising XPS measurements, the oxidation states of the Fe and Co ions within the porphyrins were probed. Through identification of the characteristic Fe 2p_3/2 and Fe 2p_1/2 peaks at binding energies of 711.0 and 724.4 eV, respectively, as well as satellite peaks at 719.1 and 728.7 eV, it was concluded that Fe-containing catalysts possessed Fe residing in a Fe(III) oxidation state. For the Co-containing MOF, binding energies of Co 2p_3/2 and Co 2p_1/2 appearing at 780.3 and 795.8 eV, respectively, indicated a Co(II) species in the porphyrin centres. Across all frameworks, In 3d peaks also confirmed the presence of In(III) species.

Within a CO₂-saturated ethyl acetate solution using L-ascorbgyl palmitate (L-AP) as the electron sacrificial agent, the Fe-containing frameworks were found to provide the highest catalytic activity, providing CO as the main product, with only trace amounts of CH₄ (ca. 0.5%) being evolved. After 24 h, no H₂ was detected. From the Fe series, In-Fe_1.91TCPP-MOF was shown to display the highest catalytic efficiency, with a total CO yield of 3469 μmolg⁻¹ in 24 h. The obtained rate of CO formation indicates the superior activity amongst most noble-metal-free MOF photocatalytic systems. Despite being similar in metal content, the In-Co_1.71TCPP-MOF provided far less CO, yet outperformed the indium-based framework. The dependence on metal identity was further illustrated by time-resolved PL spectroscopy, as the lifetime was shown to decrease from 0.65 ns for the indium-based framework to 0.19 and 0.38 ns for the Fe- and Co-substituted frameworks, respectively, suggesting rapid electron consumption in the Fe-based system.

Within this system, Wang et al. proposed a mechanistic pathway describing electron transfer from the photoexcited porphyrin linker to the single Fe³⁺ centres, achieving CO₂ reduction. To complete the photocatalytic cycle, the resulting positively charged electron holes oxidize L-AP. This mechanism was supported by calculation of the conduction band (CB, −0.51 V vs. reversible hydrogen electrode (RHE)) potential, which was found to be more negative than that of the standard reduction potential for CO₂ to CO (−0.12 V vs. RHE), thus facilitating CO₂ reduction under visible light. Further, DFT calculations of the CO₂ reduction steps using Fe− and Co−porphyrin analogues showed that the formation of [Co^II(Por²⁻)(COOH⁻)]⁻ from [Co⁰(Por)]²⁻ is approximately thermoneutral (∆G = −0.25 kcal mol⁻¹), whereas the same process within Fe analogues was calculated to be more favourable (∆G = −4.10 kcal mol⁻¹), likely due to the higher stability of [Fe^II(Por²⁻)(COOH⁻)]⁻ compared to the corresponding Co analogues.

A summary of catalytic descriptors, including the topology, excitation pathway, presence of unsaturated Fe sites, oxidation state, and proximity of QDs to MOF, has been provided for non-noble-metal functionalised MOFs in

Table 5. Catalytic descriptors enlisting non-noble-metal species incorporated within MOFs for photocatalytic CO₂ utilisation.

Descriptor	MOF Hybrid Photocatalyst	Photocatalytic Trend	Ref
Topology	NH2-Fe MOFs	Three different Fe-based MOFs, namely MIL-101, MIL-53, and MIL-88B, with different topologies and different photocatalytic performances	[118]
Excitation pathway	NH2-Fe MOFs	Difference in the excitation pathway for amino functionalised and unfunctionalised MOFs under visible light irradiation	[118]
Unsaturated Fe sites	Solvent-free NH2-Fe MOFs	Thermal treatment removes H2O ligands and exposes unsaturated Fe sites for enhanced activity	[119]
Proximity (QD to MOF)	LHP QDs@PCN-221(Fe)	Localisation of LHP QDs in close proximity to MOF imparts stability and superior photoreduction ability	[121]
Oxidation state	In-FexTCPP-MOF	The existence of Fe(III) oxidation state in MOF results in the superior photocatalytic reduction	[120]

by linking them with the photocatalytic response observed in the CO₂ utilisation.

2.3. MOFs Incorporated with Miscellaneous Species

The incorporation of transition metals within the MOF architectures has been found to be effective in terms of decreasing the rate of charge recombination and enhancing the charge separation efficiency. However, the high cost and scarcity of some important metals such as Au, Ag, and Ru limits their industrial application. This section covers the miscellaneous species involving carbon-based heterostructures, such as graphene oxide, quantum dots, and carbon nitride, used to develop non-toxic catalysts with enhanced light-harvesting ability.

Table 6. Miscellaneous species incorporated within MOFs for photocatalytic CO₂ utilisation.

MOF Hybrid Photocatalyst	Synthetic Methodology	Light Source	Photoreduction Product	Photocatalytic Performance	Ref
rGO- Ni3HITP2	Ultrasonic dispersion	100 W LED 420 nm	CO	TOF = 0.46 min−1; selectivity = 91%	[123]
CD@NH2-UiO-66	Solvothermal and embedding	Xe Lamp ≥420 nm	CO	16.6 µmol gcat −1h−1	[124]
g-C3N4/ZIF-8	In situ deposition	300 W Xe Full spectrum	CO2/H2O to CH3OH	0.75 µmol h−1 g−1	[125]
Morphology-controlled ZIF-67	Solvent-induced method	300 W Xe ≥420 nm	CO	3.89 µmol mg−1 h−1	[126]
Single atom/Co-MOF	Wet impregnation	300 W Xe 400–800 nm	CO and CH4	CO = 200.6 µmol g−1 h−1 CH4 = 36.67 µmol g−1 h−1	[80]
Co3O4 hierarchical nanosheets	Oil bath method and calcination	300 W Xe ≥400 nm	CO and H2	39.7 µmol h−1	[127]
MOF derived ZnO/NiO porous spheres	Thermal treatment from Zn-Ni MOFs	300 W Xe Full spectrum	CH3OH	1.57 µmol h−1 g−1	[128]

enlists the unique catalytic systems that have been developed recently to achieve excellent photocatalytic performances.

The rational design of multicomponent MOF catalysts by using a carbon-based matrix for enhanced charge transfer was investigated for the first time by Peng et al. [123]. The integration of reduced graphene oxide (rGO) along with a two-dimensional (2D)-MOF (Ni₃HITP₂, HITP = 2,3,6,7,10,11-hexaiminotriphenylene) heterostructure was synthesised via Coulombic and π–π interactions for the photocatalytic CO₂ reduction [123]. The photocatalyst was prepared by dispersing and ultrasonicating the Ni₃HITP₂ MOF in distilled water followed by the addition of rGO under continuous stirring. After vacuum filtration, the composite catalyst, Ni₃HITP₂/rGO, was dried and labelled as NHPG-x, where x = 0.5, 2, and 4, representing the specific amounts of MOF used during synthesis. The fabrication of the heterostructure with rGO assisted in the following ways: (i) the high surface area and conductivity of rGO assisted in the improved charge harvesting; (ii) rGO bridged the efficient charge transfer between catalytic active and light absorption components; (iii) the generation of electronic states enhanced CO₂ activation. The change in the electronic states was studied using XPS and X-ray absorption near edge structure (XANES) spectroscopic analysis. The presence of a shoulder peak at 855.6 eV in the Ni L-edge XANES spectrum for NHPG confirmed the intimate contact between the two components.

The photocatalytic CO₂ reduction reaction was carried out using [Ru(bpy)₃]²⁺, TEOA, and NHPG as the photosensitiser, electron donor, and heterogeneous catalyst, respectively. A superior CO yield of 10.5 µmolh⁻¹ (TOF = 0.46 min⁻¹) over NHPG in comparison to 1.0 µmolh⁻¹ for pure rGO was obtained, along with a high selectivity value of 91.7%. Several control experiments in the absence of light photosensitiser were carried out and CO and H₂ formation was not detected. Moreover, the ¹³C isotopic studies revealed the formation of ¹³CO, confirming that the source of CO was from CO₂ and not from other carbon sources, such as TEOA and CH₃CN. The photocatalytic mechanistic pathway for the enhanced CO₂ reduction to CO using NHPG composite films can be summarised in three simple steps: (i) photosensitiser excitation under light irradiation; (ii) photoelectron transfer from rGO to metal active site in the MOF; (iii) CO₂ reduction to CO at Ni-Ni₄ sites. This work is an example of promoting the CO₂ conversion synergistically via improved electron transport and light harvesting through the integration of MOF with the rGO heterostructure.

A similar approach of encapsulating carbon dots (CDs) with the MOF was explored by Li et al. CDs are special nanomaterials with sizes below 10 nm, which also contain various polyaromatic functional groups. They can exhibit strong UV-Vis absorption due to π–π* and n–π* transitions. The NH₂-UiO-66 MOF was integrated with CDs in order to improve the efficiency of the photocatalytic CO₂ reduction [124]. The authors have compared the influence of the encapsulation of CDs within the pores (CD@NH₂-UiO-66) and on the surface of MOFs (CD/NH₂-UiO-66), as illustrated in Scheme 4. The presence of CDs leads to improved charge transfer due to the generation of heterojunctions within the MOFs. This strategy to develop novel MOF hybrid photocatalysts was investigated in the photocatalytic reduction of CO₂ to CO under visible light irradiation conditions. The rate of CO generation with pristine NH₂-UiO-66 was 3.5 µmol g_cat ⁻¹h⁻¹ and for surface-decorated CD/NH₂-UiO-66 was 4.0 µmol g_cat ⁻¹h⁻¹. A significantly superior catalytic yield of 16.6 µmol g_cat ⁻¹h⁻¹ was obtained with embedded CDs in the MOF, which was 4.5 times higher than the pristine NH₂-UiO-66, highlighting the effect of the CD location on MOF photocatalysts. The photocurrent measurements indicated the highest photocurrent value for the embedded samples, highlighting the efficient charge carrier separation in the MOF. The migration of photogenerated electrons from Zr-oxo clusters of MOFs to CDs is faster in the CD@NH₂-UiO-66 catalyst in comparison to the movement of electrons on the surface of the MOF to reach decorated CDs in CD/NH₂-UiO-66. The photocatalyst could be reused ten times without any loss of activity and while maintaining its morphology, as confirmed by SEM and XRD measurements. The incorporation of CDs as electron receptors as well as photosenitisers is an effective strategy for designing and developing photocatalysts for CO₂ utilisation.

Graphitic carbon nitride (g-C₃N₄) is one of the most studied photocatalysts for CO₂ reduction under visible light irradiation, but its limited CO₂ adsorption capacity and inferior charge separation ability restrict its use in the design of active photocatalysts. Recently, Liu et al. [125] have explored the cooperative effects of integrating semiconductors with ZIF-8 MOFs to enhance the photocatalytic redox ability, charge separation efficiency, and CO₂ adsorption ability. The decoration of tubular g-C₃N₄ (TCN) with a zeolitic imidazolate framework-8 (ZIF-8) was explored in the CO₂/H₂O vapor to CH₃OH conversion process [125]. The fabrication of photocatalyst was carried out using an in situ deposition method, in which ZIF-8 nanoclusters were grown on the g-C₃N₄ at room temperature. The negatively charged g-C₃N₄ nanotubes have a great affinity for zinc ions, and hence assist in the growth of ZIF-8 nanoclusters without any surface functionalisation. The TEM image of TCN before and after ZIF-8 deposition is shown in Figure 8a,b. To understand the kinetics of the CO₂ reduction reaction, it is essential to study the CO₂ adsorption ability on the surface of the photocatalyst. The linear relationship between the adsorption capacity and relative pressure evidences the interaction between CO₂ and g-C₃N₄/ZIF-8. The photocatalytic CH₃OH production takes place via formation of carbonate, formate, formaldehyde, and methoxy groups, as evidenced by in situ FTIR spectroscopy. Hence, the mechanism for CH₃OH generation was proposed by 2e⁻ and 2H⁺ pathways:

$$C{O}_2\to HCOOH \to HCHO \to C{H}_{3}OH$$

Figure 8c displays the obtained catalytic performances for methanol production using various catalysts, including bulk C₃N₄ (BCN), tubular C₃N₄ (TCN), and varied amounts of ZIF-8 on C₃N₄ (TCNZ). The level of methanol generation using the optimal deposition of ZIF-8 on C₃N₄ was three times higher than with the BCN photocatalyst. However, it can be seen that the ZIF-8 had bilateral effects on the CH₃OH generation by enhancing the CO₂ adsorption ability, but the charge transfer was also inhibited due to the poor electrical conductivity of ZIF-8. Hence, it has been demonstrated that reasonable combination and surface functionalisation of TCN photocatalysts can lead to synergetic tuning of photocatalytic processes in the improved conversion of CO₂ to CH₃OH.

Sun et al. studied the effects of different morphologies of ZIF-67 synthesised using the solvent-induced method with different volume ratios of methanol and water in the photocatalytic reduction of CO₂ to CO [126]. The three different morphologies, namely rhombic dodecahedral, pitaya-like, and 2D leaf-like, were labelled as ZIF-67_1, ZIF-67_2, and ZIF-67_3, respectively. The optical responses of the MOF materials were measured together with the photosensitiser [Ru(bpy)₃]Cl₂·6H₂O to understand the electronic properties. The absorption within the range of 400–700 nm was associated with the LMCT, depicting the light harvesting ability from blue to red regions of the spectrum. The CO₂ adsorption capability was also investigated for the morphology-controlled ZIF-67 materials. ZIF-67_3 had the smallest surface area and exhibited the maximum CO₂ adsorption value of 1.16 mmol g⁻¹. This was attributed to the (i) presence of flexible cavities between the 2D layers and (ii) strong interactions between the CO₂ and the linker. The photocatalytic CO₂ reduction to CO was performed in the presence of the photosensitiser and with TEOA as the electron donor. Again, ZIF-67_3, having a leaf-like morphology, displayed superior catalytic performance of 3.89 µmol mg⁻¹ h⁻¹ under visible light irradiation when compared to the values of 3.75 µmol mg⁻¹ h⁻¹ for ZIF-67_1 and 3.06 µmol mg⁻¹ h⁻¹ for ZIF-67_2.

A plausible reaction mechanism was proposed, as shown in Figure 9, which was further supported by various operating techniques, including photoluminescence and femtosecond transient absorption spectroscopy. Under visible light irradiation, the excited photosensitiser generates electron–hole pairs at the interface of ZIF-67_3 and [Ru(bpy)₃]²⁺. The electrons are then subsequently transferred to the ZIF-67_3 and TEOA to form [ZIF-67_3]⁻¹ and TEOA⁺, respectively. The electrons stored in [ZIF-67_3]⁻¹ further migrate to CO₂ molecules and assist in the formation of CO.

The incorporation of single atoms within the framework is still a challenge because of their tendency to aggregate under reaction conditions. Ye et al. explored the incorporation of single Co atoms implanted within the MOF network and investigated the charge separation efficiency in the porphyrin unit of MOF-525 [80]. In this report, MOF-525, elaborated as Zr₆O₄(OH)₄(TCPP-H₂)₃, was selected as the framework to incorporate Co single-atom species in the porphyrin unit to form MOF-525-Co. The existence of mononuclear centres of Co was confirmed by the extended X-ray absorption fine structure (EXAFS) spectroscopy. The photocatalytic performance was evaluated under visible light irradiation in the presence of TEOA as an electron donor for CO₂ reduction to CO and CH₄. The superior catalytic performance of MOF-525-Co with CO and CH₄ evolution rates of 200.6 and 36.76 µmol g⁻¹ h⁻¹, respectively, was compared with MOF-525-Zn, bare MOF-525, and the H₆TCPP ligand. Approximately 3- and 6-fold enhancements of the CO and CH₄ evolution values were obtained for MOF-525-Co compared to MOF-525. This was attributed to a higher CO₂ uptake capacity because of an increase in the number of affinity sites after metallisation of MOF-525. It was also demonstrated by experimental investigation and theoretical simulations that the photogenerated electrons could be easily transferred to the active site in MOF-525-Co, leading to the improved charge separation efficiency. Furthermore, Co-incorporated MOF-525 displayed excellent stability at 3 catalytic cycles without any significant loss of activity.

Apart from implanting various species within the MOFs [129,130], many research groups have developed novel materials derived from several monometallic and bimetallic metal–organic frameworks [122,131]. For example, Zhang et al. synthesised cobalt tetraoxide (Co₃O₄) hierarchical nanosheets (HNSs) via two-step synthesis, involving the preparation of Co-MOF using 1,4-naphthalenedicarboxylic acid as the organic linker followed by calcination (Scheme 5) [127]. Interestingly, when the isophthalic acid (H₂IPA) was used as the organic linker, nanoballs of Co₃O₄ were formed, which ultimately formed NPs after thermal treatment. The 2D morphology of HNSs was confirmed by SEM images, as shown in Figure 10a,b. The TEM images revealed the existence of smaller particles sized 10–20 nm, as shown in Figure 10c–e.

The photocatalytic performance and selectivity of CO generation using Co₃O₄ HNSs was compared with Co₃O₄ NPs and the commercial Co₃O₄, as summarised in Figure 10f. The most frequently used components, [Ru(bpy)₃]Cl₂·6H₂O and TEOA, were employed as the photosenitiser and the sacrificial agent for the reaction, respectively. As shown, the Co₃O₄ HNSs displayed superior activity and selectivity of 39.7 µmol h⁻¹ and 77%, respectively, under visible light irradiation. The apparent quantum efficiency (AQE) for CO generation was found to be 0.42% at light irradiation value of 450 nm. Several control experiments were carried out, including ¹³C isotopic studies to confirm the origin of CO as being from CO₂ and not from other carbon sources. No CO or H₂ formation was observed in the absence of a photosensitiser, TEOA, or visible light irradiation, suggesting their significance in obtaining superior photocatalytic performance. One of the major drawbacks of MOFs is their lack of reusability in practical applications. Herein, the Co₃O₄ HNSs derived from Co-MOF was found to be stable without any loss of catalytic activity, even after 4 runs. The authors explained the contribution of HNSs to the excellent catalytic performance as being due to two aspects: (i) the formation of HNSs derived from an MOF with higher surface area values, a mesoporous structure, and better CO₂ adsorption capacity; (ii) the formation of small NPs with narrow size distribution and fewer oxygen vacancies, leading to improved charge separation and migration efficiency under visible light irradiation.

Similarly, Chen and co-workers developed an MOF-derived photocatalyst for improved photocatalytic performance for CO₂ conversion to CH₃OH. They synthesised ZnO/NiO porous hollow spheres through the thermal treatment of Ni-Zn bimetallic organic frameworks (MOFs) [128]. It has been reported that hollow structures can enhance the light absorption ability because of the multiscattering effect and that the porous structure can lead to a high surface area, increased CO₂ adsorption capacity, and improved photocatalytic performance. The decomposition of bimetallic MOFs leads to the homogeneous mixing of n-type ZnO and p-type NiO, leading to the formation of p-n heterojunctions within the hybrid material. The obtained hollow spheres were labeled as ZN-10, ZN-30, and ZN-50 based on the differences in the amounts of precursor solution used in the synthesis. The XPS studies revealed the transfer of electrons from ZnO to NiO in the composite, further assisting in improving the charge separation efficiency. The photocatalytic CO₂ reduction to CH₃OH over ZN-X hollow spheres was compared with pristine ZnO and NiO metal oxides. The relatively low amounts of CH₃OH generation on ZnO (0.6 µmol h⁻¹ g⁻¹) and NiO (negligible) suggested that pure oxides are not active under the given experimental conditions. However, all ZnO/NiO composites were found be active and a superior catalytic performance was shown by ZN-30 (1.57 µmol h⁻¹ g⁻¹), which was three times greater than results obtained over ZnO. The excess amounts of NiO in the catalyst can restrict the light absorption ability and may also have acted as the charge recombination centre that led to the decrease in the photocatalytic activity of ZN-50.

Table 7. Various catalytic descriptors summarising the photocatalytic trends in the miscellaneous species incorporated into MOFs for CO₂ utilisation.

Descriptor	Photocatalyst	Photocatalytic Trends	Ref
Surface area	rGO- Ni3HITP2	rGO heterostructure creating high surface area and enhanced conductivity	[123]
Location of CDs (carbon dots)	CD@NH2-UiO-66	An approximately 4 times higher catalytic yield with embedded CDs than surface decoration	[124]
Affinity sites	g-C3N4/ZIF-8	Negatively charged g-C3N4 provided affinity to Zn ions for the growth of ZIF-8 nanoclusters	[125]
	Single atom/Co-MOF	Co metallisation increasing the number of affinity sites in MOF-525 for CO2 adsorption	[80]
Synergistic interaction	g-C3N4/ZIF-8	Improved CO2 conversion due to synergistic tuning between g-C3N4 and ZIF-8	[125]
Morphology vs. surface area	Morphology-controlled ZIF-67	Smallest surface area for 2D leaf-like ZIF-67 with maximum CO2 adsorption because of flexible cavities absent in rhombic and pitaya-like morphologies	[126]
Organic linker vs. morphology	Co3O4 hierarchical nanosheets (HNSs)	The use of 1,4-naphthalenedicarboxylic acid as organic linker formed Co3O4 HNSs and isophthalic acid formed Co3O4 NPs	[127]
NPs size vs. distribution	Co3O4 hierarchical nanosheets (HNSs)	Small NPs of HNSs with narrow size distribution enhanced migration efficiency	[127]
Monometallic vs. bimetallic	MOF derived ZnO/NiO porous spheres	Bimetallic ZnO/NiO MOFs exhibited three times greater yield than ZnO or NiO oxides	[128]

lists information on the catalytic descriptors discussed in this section, including the surface area, number of affinity sites, organic linker, effect of morphology, and location of CDs for miscellaneous species incorporated into MOFs in photocatalytic CO₂ utilisation.

3. Summary and Outlook

In summary, we have compared and contrasted the photocatalytic CO₂ reduction of three different types of MOF-based hybrid materials functionalised by (i) noble metal NPs (Ag, Au, Pd, Ru), (ii) non-noble-metal NPs (Fe, Co), and (iii) miscellaneous species, including carbon dots, r-graphene oxide, and g-C₃N₄. These MOF-based hybrid systems, along with their photocatalytic performances, have been listed in Table 2, Table 4 and Table 6. Moreover, we have linked specific catalytic descriptors to the observed photocatalytic trends at the end of each section (Table 3, Table 5 and Table 7). For example, the size of MOF crystallites can significantly impact the photocatalytic performance. The choice and length of an appropriate organic linker, the location of NPs, the surface area, and the oxidation state have been discussed, corelating the trends observed in the photocatalytic CO₂ reduction. The higher surface area of MOFs and their ability to tune the porosity, metal centres, and organic ligands are the reasons for their unique flexibility in various applications, such as in drug delivery, gas sorption, CO₂ capture, and sensing. These versatile structural properties enable photocatalytic applications in hydrogen generation, CO₂ reduction, pollutant degradation, and several organic transformation reactions. The functionalisation of MOFs by metal substitution and the integration of photo-sensitive components or semiconductors assist in improving the visible light harvesting and CO₂ conversion efficiency. We also discussed the preparative techniques and factors affecting the introduction of multifunctionalities within the MOF framework, such as carbon dots, single atoms, quantum dots, and g-C₃N₄ semiconductors, in enhancing the overall conversion efficiency. MOFs are emerging heterogeneous photocatalysts that are used for CO₂ reduction, especially because of their excellent CO₂ capture capacities.

Despite great progress being made in the design and development of MOF-based photocatalysts for CO₂ reduction, there are still some challenges and limitations in achieving artificial photosynthesis in practical applications. Owing to the organic nature of the linkers, MOFs have much lower stability toward temperature, pressure, moisture, and pH than other zeo-type materials. Many MOFs breakdown at temperatures above 350 °C, dramatically reducing the number of applications. Due to this, MOFs must be considered for more niche, lower temperature syntheses, such as fine chemical synthesis, which can utilise their shape-specific topologies and high metal contents. MOFs have various limitations, including their fast recombination of charge carriers, limited absorption in the visible light region, and complex photocatalytic mechanisms involved in the reduction reaction. The main concern regarding the use of MOFs in the photocatalytic reduction of CO₂ is the small amounts of products formed, which restricts their industrial application. The photocatalytic stability is another important issue faced by materials scientists in maintaining the number of recycling test runs. Many research reports have demonstrated the superior photocatalytic performance of MOFs when compared with inorganic semiconductors; however, continuous decreases in activity during recycling tests have hindered their large-scale use. The reports summarised here include the use of sacrificial reagents to carry out photocatalytic reduction, which are expensive and detrimental to the environment. Research efforts are required to overcome the use of such additional sacrificial reagents. The investigation of photocatalytic CO₂ reduction is still at an early stage and we predict that this article will further assist in providing timely progress and guidelines for the design of photoactive MOF-based catalysts for CO₂ reduction. We look forward to major advancements in this field for practical applications.