Metal-organic frameworks for CO₂ photoreduction

Abstract

Metal-organic frameworks (MOFs) have attracted much attention because of their large surface areas, tunable structures, and potential applications in many areas. In recent years, MOFs have shown much promise in CO₂ photoreduction. This review summarized recent research progresses in MOF-based photocatalysts for photocatalytic reduction of CO₂. Besides, it discussed strategies in rational design of MOF-based photocatalysts (functionalized pristine MOFs, MOF-photosensitizer, MOF-semiconductor, MOF-metal, and MOF-carbon materials composites) with enhanced performance on CO₂ reduction. Moreover, it explored challenges and outlook on using MOF-based photocatalysts for CO₂ reduction.

Keywords

metal-organic frameworks (MOFs) / photocatalysis / CO₂ photoreduction / composite

Introduction

Carbon dioxide (CO₂) released in the combustion of fossil fuels, such as oil, coal, and natural gas, makes up the majority of greenhouse gas emissions, which is the major contributor to global warming [¹,²]. Therefore, it is imperative to develop technologies to reduce CO₂ emissions. To date, the main approaches developed to reduce CO₂ emissions include ① CO₂ capture and sequestration and ② CO₂ conversion and utilization. CO₂ capture and sequestration is a set of technologies developed for reducing CO₂ emissions from fossil-fueled power plants. CO₂ capture is to separate CO₂ from gas mixtures via chemical absorption using an agent such as monoethanol amine or physical adsorption using solid adsorbents such as activated carbons and metal organic frameworks (MOFs), membrane separation and cryogenic separation at a low temperature [³]. CO₂ sequestration refers to long-term storage of CO₂ in ocean, soils, plants, and geologic formations [³,⁴]. However, these technologies are relatively energy-intensive and thus are not cost-effective. An alternative sustainable approach to mitigating CO₂ emissions is CO₂ utilization and conversion. CO₂ utilization describes the uses of CO₂ in both physical processes such as welding medium and chemical processes such as chemical synthesis. The CO₂ utilization technology has found applications in several industries such as carbonated drinks, dry ice, solvent, food preservation, refrigerant, and fire extinguisher. These direct CO₂ utilization applications, however, have a small effect on the overall CO₂ emission reduction due to the limited usage [⁵]. CO₂ utilization can also be employed indirectly in industries to promote a process such as in enhanced gas recovery, enhanced oil recovery, and enhanced geothermal systems. CO₂ conversion refers to the transformation of CO₂ into valuable products such as fuels and chemicals. CO₂ can be utilized directly as a feedstock to react with other components to form chemical products such as urea and formic acid under heat and/or pressure. CO₂ can also be utilized indirectly as a building block of a chemical product [⁶]. It is worth noting that CO₂ is a highly stable molecule. The C-O bond strength in CO₂ molecule is 364 kJ/mol and the carbon atom has the highest oxidation state, therefore, CO₂ conversion into valuable chemicals generally requires a significant input of energy and the use of a catalyst [⁷]. The main approaches to converting CO₂ into valuable products include photocatalysis [⁸,⁹], chemical fixation [¹⁰], hydrogenation [¹¹], and electrocatalysis [¹²]. Among these methods, photocatalytic reduction of CO₂ attracts much attention because it converts CO₂ into useful products by utilizing solar energy via a clean and sustainable route. Under sunlight irradiation, photocatalysts can induce CO₂ reduction and convert it into fuels and chemicals, mainly including CO, HCHO, HCOOH, CH₃OH, C₂H₅OH, and CH₄, etc., which is determined by the number of electrons and protons (e^-/H⁺) transferred in the reactions. The selectivity of product and efficiency of CO₂ reduction may have been affected by the thermodynamic reduction potentials and reaction conditions. H₂O is commonly used as a solvent for CO₂ photocatalytic reduction because it is of low cost and is natural abundance of hydrogen. The photoreduction reactions of CO₂ in aqueous solution at pH= 7 and their reduction potentials with reference to the normal hydrogen electrode (NHE) at 25°C and 1 atm are given in Table 1 [¹³,¹⁴].

Photocatalysis is a process that converts solar energy to chemical energy, where solar energy is the driving force for the excitation and transfer of holes and electrons to induce oxidation and reduction reactions. The photocatalytic CO₂ reduction process can be explained as follows: first, under light illumination with energy greater than the band gap of the photocatalysts, electrons are excited from the valence band (VB) to the conduction band (CB), generating an equal number of holes in the VB. Secondly, electron-hole pairs separate from each other and move to the surface of photocatalyst. Finally, the electrons reduce CO₂ into chemical products, while the holes oxidize a sacrificial agent or H₂O. To induce CO₂ photoreduction, it requires that reduction potential and oxidation potential of the reaction is less negative and less positive than the CB edge and the VB edge of the photocatalyst, respectively.

Since Fujishima et al. reported on the feasibility of using TiO₂ for photoelectron chemical water splitting under ultraviolet (UV) irradiation [¹⁵], many different photocatalysts have been developed, most of which are inorganic semiconductors such as TiO₂ [¹⁶], CdS [¹⁷], ZnO [¹⁸], ZnS [¹⁹], Fe₂O₃ [²⁰], g-C₃N₄ [²¹], Ag₃PO₄ [²²] and their composites. Some cocatalysts such as Pt have also been explored [²³,²⁴]. However, the wide band gap, high recombination rate of electron‐hole pairs, low adsorption capacities for CO₂, and less tunable structure of the conventional semiconductors limit their practical applications. For example, TiO₂ is mainly used for UV light photocatalysis because of its wide band gap (3–3.2 eV) [²⁵]. ZnO and CdS are not stable under irradiation in an aqueous solution, making them inactive over time [²⁶,²⁷]. Therefore, it is imperative to develop new photocatalytic materials with finely tunable energy band structures and high stability.

Metal-organic frameworks (MOFs), a new class of inorganic-organic hybrid material composed of inorganic metal clusters and organic linkers which are connected through coordination bonds, have attracted much interest due to their large surface areas, tunable structures and high porosity [²⁸–³⁴]. These excellent properties enable their wide applications in many fields, such as gas storage and separation [³⁵], catalysis [³⁶], drug delivery [³⁷], water treatments [³⁸], and sensing [³⁹]. Recently, MOFs have found applications in CO₂ photoreduction, degradation of organics, and chemical synthesis as photocatalytic materials [⁴⁰–⁴²]. In photocatalytic reaction, MOFs undergo a similar process to traditional semiconductors, but differently, the VB and CB are described as the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in MOFs, respectively [³⁶,⁴³]. In general, the HOMO and LUMO energy levels are associated with the redox potential energy levels of the organic linker and the metal-oxo cluster, respectively [⁴²]. The tunable organic linkers and/or metal clusters in MOFs can act as antennas to harvest light to generate electron-hole pairs for photocatalysis. Most of the photocatalytic reactions in MOFs reported to date are ascribed to a localized metal-to-ligand charge transfer (MLCT), a ligand-to-metalcharge transfer (LMCT), or a p–p* transition of the aromatic ligand [³⁶]. MOFs are utilized for photocatalytic reduction of CO₂ because of their attributes as follows: ① both the organic linkers and metal clusters can serve as the light harvesting sites and they can be tailored to tune the optical absorption range of MOFs [⁴⁴], ② some MOFs have a catalytic activity resulting from the catalytically active organic linkers and/or unsaturated metal sites [⁴⁵,⁴⁶], ③ MOFs have a large surface area and a high CO₂ adsorption capacity, and the high CO₂ concentration in the pores can facilitate the photocatalytic reactions, and ④ the three-dimensional porous structures and high surface areas enable MOFs to incorporate foreign photoactive species into their frame works, through which photocatalytic reactions can be enhanced by the synergistic cooperation of the metal clusters, organic linkers and the incorporated active sites [⁴⁷]. Based on these unique properties, MOFs are promising photocatalysts for CO₂ reduction.

MOF photocatalysts can be classified into two categories: ① Pristine MOF photocatalysts, which areal so called “opportunistic” photocatalysts. The photocatalytic properties of some pristine MOFs are a consequence of the catalytically active organic linkers and unsaturated metal sites [⁴⁸], and some MOFs are semiconductors [⁴⁹–⁵¹] whose photocatalytic reactions are completed through LMCT. Most pristine MOFs as photocatalysts, however, have large band gaps and barely absorb visible light, which limits their practical applications. These MOFs generally have a low photon energy utilization efficiency, and are typically employed for photocatalytic degradation of organics which have high thermodynamic driving forces and low kinetic barriers [³⁶]. ② Modified MOF photocatalysts, which refer to MOFs functionalized to increase light harvesting and decrease the recombination rate of the photo-generated charge carriers for photocatalytic activity enhancement. To date, many strategies have been developed for enhancing the photocatalytic properties of MOFs under solar illumination, including the decoration of the metal clusters and organic linkers [⁵²,⁵³], incorporation of foreign photocatalytic species such as metal particles, semiconductors [⁵⁴–⁵⁶], and photosensitizers [⁵⁷–⁵⁹].

So far, a number of good literature reviews have summarized the photocatalytic applications of MOFs [⁶⁰–⁶⁹], most of which have discussed the role of MOFs and their performances in water splitting [⁷⁰–⁷³], degradation of organic pollutants [⁷⁴–⁷⁷], hydrogen evolution [³⁶,⁷⁸–⁸⁰], and CO₂ conversion [³⁶,⁴³,⁶⁶,⁷⁸,⁸⁰–⁸⁵]. Therefore, this review is mainly focused on reviewing the recent progress of MOF-based photocatalysts in CO₂ photoreduction. Besides, it discusses modification strategies of MOFs and their photocatalytic activities in CO₂ reduction. Moreover, it explores the challenges and future perspectives of MOFs-based photocatalysts in CO₂ photoreduction.

Modification of pristine MOFs as photocatalysts

Compared to traditional inorganic semiconductors, it is more convenient to tune the optical properties and consequently the photocatalytic properties of MOFs by modification of the metal clusters or organic linkers. It is reported that the linker decoration can change the energy band gap of MOFs by shifting the photo absorption edge from the UV to visible light region [⁸⁶]. Many strategies have been developed to functionalize organic linkers and metal clusters [⁴⁴,⁸⁷–⁹²], such as amine and photosensitizer functionalization of organic linkers and incorporation of foreign metal cations into organic linkers and metal sites (e.g., Ti–O, Fe–O, and Zr–O clusters). The performances of recent photocatalytic MOFs for CO₂ photoreduction are summarized in Table 2.

Metal cluster nodes

MOFs have three-dimensional structures constructed from the interconnection of metal cluster nodes with organic linkers. Besides photoactive ligands, metal cluster nodes in MOFs can also drive photocatalytic CO₂ reduction. To date, Ti-, Zr-, and Fe-based MOFs are among the most investigated photocatalytic MOFs whose photoactive metal nodes in metal-oxo clusters can initiate photocatalytic reaction by trapping photo-excited electrons and decreasing recombination rate of electron-hole pairs [⁸³]. These MOFs share the similarity that they all contain metal ions with variable valence states (e.g., Ti⁴⁺/Ti³⁺ , Zr⁴⁺/Zr³⁺ , and Fe³⁺/Fe²⁺ ), which enables their effectiveness on photocatalytic reduction.

Of the Ti-based MOFs, MIL-125(Ti) (Ti₈O₈(OH)₄(O₂C-C₆H₄-CO₂)₆) is the most studied MOF, which is constructed from the Ti₈O₈(OH)₄ secondary building units (SBUs) and 1,4-benzenediacarboxylate (BDC) ligands. Fu et al. investigated the CO₂ photoreduction over MIL-125(Ti) [⁵³]. 2.41 mmol HCOO^- was detected in the acetonitrile (MeCN) solvent with TEOA under 365 nm UV light irradiation for 10 h. A number of strategies have been developed to functionalize MIL-125(Ti) to enhance the photocatalytic reduction of CO₂, such as organic linker decoration and incorporation of foreign photocatalytic components into MOFs to form MOF composites. These strategies will be discussed in Section 2.2 and Section 3.

Zr-based MOFs, which have robust structures and excellent thermal and chemical stabilities [¹¹²,¹¹³], are another popular class of photocatalytic MOFs since their synthesis in 2008 [¹¹⁴]. The representative examples are UiO-66(Zr) and UiO-67(Zr), which are constructed by integrating Zr₆O₄(OH)₄ SBUs with BDC ligands and 4,4′-biphenyldicarboxylic acid (BPDC) ligands, respectively [¹¹⁴,¹¹⁵]. Compared to Ti-based MOFs, Zr-based MOFs have more negative redox potential (Ti⁴⁺/Ti³⁺ (–0.1 V), Zr⁴⁺/Zr³⁺ (–1.06 V)) [¹¹⁶,¹¹⁷]. However, UiO-66 exhibits no absorption under visible light irradiation owing to the higher redox potential energy level of the Zr₆O₄(OH)₄ SBUs in UiO-66 than the LUMO of the BDC linkers, leading to the low efficiency in LMCT and consequently low efficacy in CO₂ photoreduction [⁹⁴]. Partial substitution of metal cations in MOFs can introduce metal-to-metal charge transfer, which can promote photocatalytic performance especially under visible light irradiation [⁹⁴,⁹⁶]. A bimetallic UiO-based MOF, NH₂-UiO-66(Zr/Ti), was prepared by Cohen and coworkers by partially substituting Zr in NH₂-UiO-66(Zr) with Ti [⁹⁴]. NH₂-UiO-66(Zr/Ti) had a better performance in photocatalytic CO₂ reduction under visible light irradiation compared to NH₂-UiO-66(Zr). The reason for this is that Ti ions incorporated enable the SBUs to accept the electrons generated via light absorption by the organic linkers. No HCOO^- was produced over the parent UiO-66(Zr)-NH₂, demonstrating that Ti was critical for photocatalysis.

Sun et al. also prepared Ti-substituted NH₂-UiO-66(Zr/Ti) MOFs (NH₂-UiO-66(Zr/Ti)-120-16 and NH₂-UiO-66(Zr/Ti)-100-4) doped with different amounts of Ti by a post-synthetic exchange method and examined their photocatalytic performance on CO₂ reduction under visible light irradiation (Fig. 1(a)) [⁹⁶]. NH₂-UiO-66(Zr/Ti)-120-16 had an enhanced photocatalytic activity toward CO₂ conversion with a yield of 5.8 mmol/mol of HCOO^- after 10 h visible light irradiation in MeCN solvent with TEOA as a sacrificial agent, which was 1.7 times of that observed over NH₂-UiO-66(Zr) (3.4 mmol/mol) under similar conditions. In contrast, NH₂-UiO-66(Zr/Ti)-100-4 produced 4.2 mmol mol^-1 of HCOO^-, which was less than NH₂-UiO-66(Zr/Ti)-120-16, but still higher than that over the pristine NH₂-UiO-66(Zr). The enhancement in photocatalytic performance over Ti-substituted NH₂-UiO-66(Zr/Ti) MOFs is associated with the increase in CO₂ adsorption capacity and photocatalytic sites, both of which result from Ti doped into Zr-O clusters of NH₂-UiO-66(Zr). Based on the experimental observations and theoretical studies, the mechanism for enhanced photocatalytic reactions over NH₂-UiO-66(Zr/Ti) is proposed (see Fig. 1(b)). When Zr⁴⁺ centers in Zr₆O₄(OH)₄ are partially substituted by Ti⁴⁺ to form (Ti/Zr)₆O₄(OH)₄, the excited NH₂-BDC upon visible light irradiation can transfer electrons to either Zr⁴⁺ or Ti⁴⁺ centers. The theoretical calculations show that there is a higher probability for electrons to be transferred to Ti⁴⁺ than that to Zr⁴⁺ centers, leading to the formation of (Ti³⁺/Zr⁴⁺)₆O₄(OH)₄ SBUs. The Ti³⁺ in the excited (Ti³⁺/Zr⁴⁺ )₆O₄(OH)₄ SBUs can play a role of electron donor to donate electrons to Zr⁴⁺ , leading to Ti⁴⁺–O–Zr³⁺ formation. As a result, the substituted Ti center in NH₂-UiO-66(Zr/Ti) facilitates the interfacial charge transfer from the excited NH₂-BDC to Zr–O clusters, which boosts the enhanced photocatalytic reactions over NH₂-UiO-66(Zr/Ti).

Recently, Fe-based MOFs as photocatalysts for CO₂ reduction have attracted much interest owing to the fact that the Fe–O clusters can be directly photoexcited to induce electron transfer from O^2- to Fe³⁺ to form Fe²⁺ under visible light irradiation, which drives the photocatalytic reaction [⁹⁵,¹¹⁸]. Since the Fe sites play a role of photocatalytic centers, Fe-based MOFs are capable of photocatalytically reducing CO₂ under visible light irradiation in the absence of LMCT. Wang et al. reported a series of Fe-based MOFs, MIL-101(Fe), MIL-53(Fe), MIL-88B(Fe), all of which had a photocatalytic activity for CO₂ reduction to produce HCOO^- under visible light irradiation [⁹⁵]. It showed that the photocatalytic activity of MIL-101(Fe), MIL-53(Fe) and MIL-88B(Fe) toward CO₂ conversion into HCOO^- in MeCN solvent with TEOA as a sacrificial reactant was 59.0, 29.7, and 9.0 mmol, respectively, after visible light irradiation for 8 h. Of the three investigated Fe-based MOFs, MIL-101(Fe) had the best photocatalytic performance attributing to the existence of the unsaturated Fe sites in its structure that were absent in MIL-53(Fe) and MIL-88B(Fe). MIL-53(Fe) had a better activity than MIL-88B(Fe) attributing to its higher CO₂ adsorption capacity (13.5 g/cm³) than MIL-88B(Fe) (10.4 g/cm³) which might be ascribed to its one dimensional framework structure which possessed a better electro-conductivity. The photocatalytic activities of the three Fe-based MOFs were enhanced by amine functionalization. The HCOO^- yield of 178, 46.5, and 30 mmol was produced over NH₂-MIL-101(Fe), NH₂-MIL-53(Fe), and NH₂-MIL-88B(Fe), respectively, under the same photocatalytic conditions, which were higher in comparison to the parent Fe-MOFs. This photocatalytic activity enhancement is caused by the existence of the dual excitation pathways: excitation of NH₂-functionalized organic ligands to transfer electrons to the Fe center in addition to the direct excitation of Fe-O clusters (see Fig. 2).

Modification of organic linkers

Functionalization of organic linkers has been considered as an effective approach to increase the absorption of light and decrease the recombination rate of the photo-generated charge carriers and consequently to improve the photocatalytic reduction of CO₂. To date, several different organic linker functionalization strategies have been developed for enhancing the photocatalytic performance of MOFs toward CO₂ reduction under light illumination, including amine functionalization, utilization of porphyrin-based organic linkers, and photosensitizer functionalization.

Amine functionalization

Studies have shown that amine groups incorporated into the organic linkers in MOFs contribute to the CO₂ adsorption capacity enhancement, adsorption region broadening, and CO₂ photoreduction performance boost. The introduction of amine groups into aromatic polycarboxylates in MOFs can increase the interactions between CO₂ molecules and the modified linkers [¹¹⁹]. The CO₂ photoreduction improvement is attributed to the lone pair of electrons in amine groups, which can interact with the p*-orbitals of the benzene ring, leading to the donation of electrons to the anti-bonding orbitals. This interaction enables the formation of a new higher energy HOMO level, which leads to a broader optical absorption region and consequently enhanced CO₂ photoreduction [⁶⁹].

In 2012, Li and coworkers [⁵³], for the first time, prepared an amino-functionalized MOF, NH₂-MIL-125(Ti), and examined CO₂ adsorption and photoreduction under visible light irradiation. NH₂-MIL-125(Ti) had a higher CO₂ adsorption capacity (132.2 cm³/g) in comparison to that of MIL-125(Ti) (98.6 cm³/g) due to the increased interaction between CO₂ molecules and the amine-modified linkers. Moreover, amine functionalization led to a broader adsorption range of 550 nm for NH₂-MIL-125(Ti) compared to MIL-125(Ti) with an adsorption range of 350 nm (see Fig. 3(a)). Photoreduction of CO₂ over NH₂-MIL-125(Ti) was performed in MeCN solvent with TEOA as an electron donor. After visible light irradiation for 10 h, 8.14 mmol of HCOO^– was obtained, which was much higher than that of MIL-125(Ti) with no yield. The photocatalytic mechanism is proposed as follows (see Fig. 3(b)): under visible light irradiation, electrons are transferred from organic linkers to Ti⁴⁺ cations, and Ti⁴⁺ cations are reduced to Ti³⁺ by TEOA acting as an electron donor, leading to the formation of a long-lived excited charge separation. Ti³⁺ cations subsequently reduce CO₂ to HCOO^-. In 2013, the same research group (Li and coworkers) [⁹³] developed another amino-functionalized MOF, NH₂-UiO-66(Zr), and found that it had a higher activity for CO₂ photoreduction than previously reported NH₂-MIL-125(Ti) in the presence of TEOA as a sacrificial agent. Similar to previous reports, substitution of the organic linker in UiO-66(Zr) by NH₂-BDC led to a broader optical absorption range of NH₂-UiO-66(Zr) due to the increased interaction between the NH₂-BDC linker and the Zr-O clusters. CO₂ uptake of NH₂-UiO-66(Zr) (68 cm³/g) was also improved in comparison to the parent UiO-66(Zr) (53 cm³/g) owing to the enhanced interactions of the NH₂ functional groups with the CO₂ molecules [¹¹⁹,¹²⁰]. After visible light irradiation over NH₂-UiO-66(Zr) for 10 h, 13.2 mmol of HCOO^– was produced. Partial substitution of the organic linker NH₂-BDC by (NH₂)₂-BDC in NH₂-UiO-66(Zr) further improved its CO₂ photoreduction activity, which gave 20.7 mmol of HCOO^- under the same conditions as over NH₂-UiO-66(Zr) (Fig. 4). The improvement in CO₂ photocatalytic reduction over the mixed NH₂-UiO-66(Zr) is attributed to enhanced light absorption in the visible region and increased CO₂ adsorption (71 cm³/g). Similar results were reported by Cohen and coworkers [⁹⁴] who investigated the CO₂ photocatalytic reduction performance over (NH₂)₂-UiO-66(Zr) and (NH₂)₂-UiO-66(Zr/Ti) prepared by the introduction of diamine-substituted ligands into (NH₂)-UiO-66(Zr) and (NH₂)-UiO-66(Zr/Ti), respectively. It was found that an increase of the amino group number introduced new energy levels for additional light absorption and charge transfer, leading to enhancement in photocatalytic activities.

Utilization of porphyrin-based organic linkers

Porphyrins have complex cyclic structures consisting of four pyrrole rings linked to each other by methine groups. Due to their strong interactions with CO₂, high light adsorption efficiency and catalytic performance, porphyrin-based organic linkers have been incorporated into MOFs for CO₂ photoreduction.

Su and coworkers [⁹⁷] prepared a rhodium(III)-porphyrin zirconium MOF (Rh-PMOF-1(Zr)) using a Rh-based metalloporphyrin tetracarboxylic ligand Rh(TCPP)Cl (TCPP= tetrakis(4-carboxyphenyl) porphyrin) and ZrCl₄. The CO₂ adsorption and photoreduction over Rh-PMOF-1(Zr) were examined. Rh-PMOF-1(Zr) had a high CO₂ adsorption capacity of 53 cm³/gat 298 K. After visible-light irradiation for 18 h, the yield of HCOO^– reached 6.1 mmol/mmol_cat. This showed that Rh-PMOF-1(Zr) had a long-lived excited-state under vacuum at 298 K with the lifetime of 207 ms, which contributed to the improvement in the photocatalytic activity of Rh-PMOF-1(Zr). Two catalytic reactions contribute to the CO₂ photocatalytic reduction to the formation of HCOO^– over Rh-PMOF-1(Zr): ① metalloporphyrin ligand plays a role of an antenna to harvest light, generate electrons, and transfer electrons to the zirconium oxo clusters, reducing CO₂ to HCOO^–, and ② the rhodium-porphyrin ligands serve as photocatalytic centers toward CO₂ photoreduction.

Sharifnia and coworkers [⁹⁸] used TCPP as ligand and Zn(NO₃)₂·6H₂O to prepare a porphyrin-based MOF (Zn/PMOF) and performed photocatalytic reduction of CO₂ over Zn/PMOF in the presence of H₂O vapor under UV-visible light. After 4 h irradiation, Zn/PMOF had a CO₂ photoreduction activity with CH₄ formation of 10.43mmol. Only a small amount of CH₄ (10.77 mmol) was produced by extending the irradiation time to 24 h, which may have been caused by the fact that the intermediate product and/or byproduct formed after 4 h have saturated the active sites.

Huang et al. [⁹⁹] synthesized two types of porphyrin-based MOFs, Al/PMOF and Cu-Al/PMOF, and compared their CO₂ adsorption capacities and photoreduction of CO₂ toward methanol. Al/PMOF was synthesized using TCPP as organic linker and AlCl₃·6H₂O while Cu-Al/PMOF was produced by doping Cu²⁺ into Al/PMOF. It showed that Cu²⁺ in Cu-Al/PMOF contributed to the enhanced photocatalytic reduction of CO₂. Cu-Al/PMOF had a higher CO₂ adsorption capacity (277.4 mg/g) than that of Al/PMOF (153.1 mg/g). Moreover, the methanol formation rate over Cu-Al/PMOF (262.6 ppm/(g·h)) was 7 times as high as that of Al/PMOF (37.5 ppm/(g·h)). As demonstrated by in situ Fourier transform infrared (FT-IR) spectra, CO₂ could be chemically adsorbed on the Cu site in Cu-Al/PMOF, where the linear CO₂ molecules would bend, thus lowering the reaction barrier and improving the photocatalytic efficiency.

Ye and coworkers [¹⁰⁰] prepared MOF-525 (Zr₆O₄(OH)₄(TCPP-H₂)₃) by integrating Zr₆ clusters with porphyrin-based organic linkers. MOF-525-Co and MOF–525-Zn were also developed by the introduction of coordinatively unsaturated Co sites and Zn sites into the porphyrin units of MOF-525, respectively. As shown in Fig. 5, both MOF-525-Co (33.6 cm³/g) and MOF-525-Zn (28.1 cm³/g) had a higher CO₂ adsorption capacity than that of pristine MOF-525 (25.3 cm³/g) due to the enhanced interaction between CO₂ molecules and the introduced open Co and Zn metal sites. CO₂ photoreduction over MOF-525, MOF-525-Co and MOF-525-Zn was performed in the presence of MeCN solvent and TEOA as an electron donor under visible light irradiation for 6 h, and two products, CO and CH₄ were produced. MOF-525-Co had the highest CO evolution rate of 200.6 mmol/(g·h) (yield: 2.42 mmol), and a CH₄ evolution rate of 36.76 mmol/(g·h) (yield: 0.42 mmol), followed by MOF-525-Zn (CO, 111.7 mmol/(g·h); CH₄, 11.635 mmol/(g·h)) and MOF-525 (CO, 64.02  mmol/(g·h); CH₄, 6.2 mmol/(g·h)). The photocatalytic activity enhancement for MOF-525-Co and MOF–525-Zn is partially ascribed to the difference in their charge separation efficiencies. MOF-525-Co and MOF-525-Zn had an enhanced efficiency in charge separation because of the lower energy at Co or Zn site, to which the electrons can be transferred from porphyrin units with a higher easiness. Furthermore, MOF-525-Co had a higher electron transfer efficiency (62.1 %) than that of MOF-525-Zn (24.9 %). These partially account for the better catalytic activity of MOF-525-Co than that of MOF-525-Zn. In addition, MOF-525-Co had a good stability and a good reproducible photocatalytic activity after three cycles (Fig. 5).

Xu et al. [¹⁰¹] prepared a zirconium-porphyrin MOF, PCN-222 (also named as MOF-545 or MMPF-6), by employing zirconium (IV) chloride and TCPP as ligand. PCN-222 had a CO₂ uptake of 35 cm³/gat 298 K and 1 atm. Photocatalytic reduction of CO₂ over PCN-222 was performed in MeCN as solvent and TEOA as a sacrificial agent. After visible light irradiation for 10 h, HCOO^- was produced with a yield of 30 mmol, which was much higher than that observed over the TCPP ligand alone (2.4 mmol), as shown in Fig. 6. It is proposed that upon irradiation, the TCPP in PCN-222 acts as an antenna to harvest visible light, generate electron–hole pairs, and transfer electrons to the Zr-oxo clusters toward CO₂ reduction to HCOO^- in the presence of TEOA as the electron donor. Two factors contribute to the enhancement in photocatalytic performance of PCN-222: ① the high CO₂ adsorption capacity of PCN-222 might enable higher interaction with CO₂ in MeCN, thereby promoting the photocatalytic reaction, and ② the ultrafast transient absorption and photoluminescence spectroscopy reveals that the emergence of an extremely long-lived electron trap state in PCN-222 significantly suppresses the electron-hole recombination, thus enhancing the CO₂ photoreduction efficiency.

Photosensitizer functionalization

Photosensitizers are able to harvest light to generate electron-hole pairs and act as catalyst sites for CO₂ photoreduction. Photoactive metal complexes, such as Ru, Re, and Ir-based polypyridine units, have been widely used to functionalize MOFs to enhance their CO₂ photocatalytic reaction activities by introducing catalytic active centers and photosensitive sites for visible light harvesting [¹²¹–¹²³].

Yan et al. [¹⁰²] produced an Eu-Ru(phen)₃-MOF (phen= phenanthroline) by integrating the triangular Ru(phen)₃-derived tricarboxylate ligand as photosensitizer into Eu-MOF with Eu(III)₂(m₂-H₂O) SBUs. Transient absorption results and theoretical calculations showed that photo-excitation of the Ru metalloligands in Eu-Ru(phen)₃-MOF initiated electron transfer into the nodes to generate dinuclear [Eu(II)]₂ active sites, which selectively converted CO₂ to HCOO^- with a yield of 47 mmol in 10 h in the presence of MeCN and TEOA under visible light irradiation. This was higher than that observed over the Ru(phen)₃-derived tricarboxylate acid metalloligandalone under the same condition.

Lin and coworkers [¹⁰³] incorporated Re^I(CO)₃(5,5′-dcbpy)Cl (at 4 wt% doping level, 5,5′‐dcbpy= 2,2’-bipyridine-5,5′-dicarboxylic acid) into the UiO-67 framework built from Zr6(µ₃-O)₄(µ₃-OH)₄ SBUs and BPDC ligands, and tested the photocatalytic reduction of CO₂ in MeCN solvent and trimethylamine (TEA) as a sacrificial agent under visible light. The total turnover number (TON) of CO and H₂ over the as-doped UiO-67 reached 5.0 and 0.5, respectively, under visible light irradiation of 6 h. After 20 h, CO-TON and H₂-TON reached 10.9 and 2.5, respectively, which were higher than that observed for the bare Re^I(CO)₃(5,5′-dcbpy)Cl (CO-TON= 7.0, H₂-TON= 1.0) under the same condition because of the decomposition of Re^I(CO)3(5,5′-dcbpy)Cl under irradiation. This enhanced TON is proposed to be associated with the stabilization effect of the active-site isolation of the immobilized Re-based catalysts in the framework. Lately, the same group produced another photosensitizer-functionalized MOF, Zr₆(O)₄(OH)₄[Re(CO)₃Cl(bpydb)]₆(MOF-1), by integrating the elongated linear (bpy)Re(CO)₃Cl^-containing dicarboxylate ligand (bpydb= 4,4’-(2,2’-bipyridine-5,5′-diyl)dibenzoate; bpy= 2,2’-bipyridine) with Zr₆(µ₃-O)₄(µ₃-OH)₄ SBUs (Fig. 7) [¹⁰⁴]. Under the same experimental condition as described above, CO-TON and H₂-TON reached 6.44 and 0.4 in 6 h, respectively, which were higher than those of the homogeneous counterpart (CO-TON= 1.12 and H₂-TON 0.18). Ryu et al. [⁵²] embedded Re^I(CO)₃(5,5′-dcbpy)Cl and amine (-NH₂) functional group within UiO-67 (dented as Re-MOF-NH₂) and varied the ratio of the-NH₂ functional groups from 0 to 80 mol%. Photocatalytic CO₂ conversion was performed in the presence of TEA under visible light. The results showed that Re-MOF-NH₂ incorporated with 33 mol% of-NH₂ functional groups had the highest photocatalytic CO₂ conversion rate (1.5 mmol/(g·h)) to CO, which was three times as that observed for Re-MOF without-NH₂ incorporation (0.5 mmol/(g·h)). The enhancement in photocatalytic activity is caused by the induced different bond lengths for Re-CO in Re^I(CO)₃(5,5′-dcbpy)Cl by the incorporation of-NH₂ functional groups, which endows the intermolecular stabilization of carbamate with CO₂, thus boosting the photocatalytic activity. Fontecave and coworkers [¹⁰⁵] functionalized UiO-67 MOF by replacing 5%–35% of BPDC linkers with Cp*Rh(5,5′-bpydb)Cl₂ (Cp* = pentamethylcyclopentadiene) (named as Cp*Rh@UiO-67). Under visible light irradiation for 10 h in the presence of acetonitrile (ACN) and TEOA, HCOO^--TON and H₂-TON of 10%-Cp*Rh@UiO-67 reached 47 and 36, respectively, with [Ru(bpy)₃]Cl₂ as a photosensitizer.

Li and coworkers [¹⁰⁶] incorporated Ru(CO)₂Cl₂ into MOF-253 (Al(OH)(5,5′‐dcbpy)), named as MOF-253-Ru(CO)₂Cl₂, and enhanced the CO₂ photoreduction by producing 0.67 mmol of HCOO^–, 1.86 mmol of CO as well as 0.09 mmol H₂ after irradiation under visible light for 8 h in MeCN and TEOA, whereas no products were detected over pristine MOF-253 under the same condition. The performance of MOF-253-Ru(CO)₂Cl₂ was further improved via photosensitizer (Ru(bpy)₂Cl₂) functionalization, by enhancing the light absorption in the visible light region. The HCOO^–, CO, and H₂ produced in 8 h over sensitized MOF-253-Ru(CO)₂Cl₂ (Ru(bpy)₂Cl₂/Ru-complex was 1: 2) was 4.84, 1.85, and 0.72 mmol, respectively, which was much higher than those observed over the non-sensitized MOF-253-Ru(CO)₂Cl₂ under the same condition. In the sensitized MOF-253-Ru(CO)₂Cl₂, Ru(CO)₂Cl₂ reacted with the surface N,N-chelated sites to form MOF-253-supported Ru(bpy)₂(X₂bpy)²⁺, which extended the light absorption edge to 630 nm, wider than that of MOF-253-Ru(CO)₂Cl₂ (470 nm), thus promoting the photocatalytic CO₂ reduction. However, a decrease in the photo-reactivity of the sensitized MOF-253-Ru(CO)₂Cl₂ was observed with increasing the amount of the photosensitizer Ru(bpy)₂Cl₂, which might be attributed to the pore blocking of MOF-253 by the Ru(bpy)₂Cl₂.

Luo and coworkers [¹⁰⁷] developed a photocatalytic MOF (Y[Ir(ppy)₂(4,4’-dcbpy)]₂[OH]) (Ir-CP, ppy: 2-phenylpyridine, 4,4’-dcbpy: 2,2’-bipyridine-4,4’-dicarboxylate) using Y(NO₃)₃ and Ir(ppy)₂(Hdcbpy). Photoreduction of CO₂ over Ir-CP was performed in MeCN and TEOA under visible light irradiation. It showed that 38.0 mmol HCOO^– was produced in 6 h, with a product formation rate of 118.8 mmol /(g·h). The photocatalytic mechanism is proposed as follows: the [Ir(ppy)₂(dcbpy)]^– unit in Ir-CP is excited under visible light and is reductively quenched by TEOA, and the CO₂ molecules are reduced to HCOO^– by getting electrons from [Ir(ppy)₂(dcbpy)]^2– units. Lately, the same group [¹⁰⁸,¹⁰⁹] incorporated Ru-polypyridine complexes into MOF structures as metalloligands and produced a series of photocatalytic MOFs. Ru-MOF, with a chemical formula of [Cd₂[Ru(4,4’-dcbpy)₃]·12H₂O]_n, was constructed from [Cd₂(CO₂)₆] SBUs and [Ru(4,4’-dcbpy)₃]^4–metalloligands [¹⁰⁸]. The morphologies of Ru-MOF can be tuned to form nanoflowers, microflakes, and bulk crystals structures by controlling the reactants concentration, as shown in Fig. 8. In the mixture of MeCN/TEOA, Ru-MOF nanoflowers had the highest HCOO^– formation rate of 77.2 mmol/(g·h) under visible light irradiation for 8 h, followed by microflakes (52.7 mmol/(g·h)) and bulk crystals (30.6 mmol/(g·h)). The Ru-MOF nanoflowers had a better photocatalytic activity than their bulk counterparts because of their high visible light harvesting and long-lasting excited-state originated from their large surface area (8.08 m²/g; microflake: 1.33 m²/g) and high energy transfer efficiency. After this work, Luo and coworkers [¹⁰⁹] synthesized two Ru-polypyridine-functionalized MOFs, [Cd₃[Ru(5,5′-dcbpy)₃]₂·2(Me₂NH₂)]_n and [Cd[Ru(bpy)(4,4′-dcbpy)₂]·3H₂O]_n, with non-interpenetrated and interpenetrated structures, respectively.CO₂ photoreduction over the two MOFs was conducted in the presence of MeCN and TEOA under visible light irradiation. It showed that the HCOO^- production over [Cd₃[Ru(5,5′-dcbpy)₃]₂·2(Me₂NH₂)]_n and [Cd[Ru(bpy)(4,4′-dcbpy)₂]·3H₂O]_n reached 16.1 and 17.2 mmol in 6 h (with a production rate of 67.5 and 71.7 mmol/(g·h)), respectively. The difference in photocatalytic performance of the two MOFs may have been associated with their structural stabilities. [Cd₃[Ru(5,5′udcbpy)₃]₂·2(Me₂NH₂)]_n has a non-interpenetrated structure in which the porous framework is supported by the coordination between metal ions and organic linkers as well as the interactions between the framework and the guest molecules in the pores. In the photocatalytic process, the exchange of guest molecules with the solvent molecules may cause the collapse of structure, leading to the decrease in photocatalytic activity. In contrast, [Cd[Ru(bpy)(4,4′-dcbpy)₂]·3H₂O]_n with an interpenetrated structure has a higher structural stability, which endows its better photocatalytic activity than [Cd₃[Ru(5,5′-dcbpy)₃]₂·2(Me₂NH₂)]_n.

In addition to the ligand functionalization strategies discussed as above, catechol- [¹¹⁰] and anthracene-based [¹¹¹] organic linkers were also utilized to functionalize MOF structures, which all had high photocatalytic performances on CO₂ reduction.

MOF composite photocatalysts

MOFs have shown much potential on CO₂ photoreduction thanks to their large surface areas, high CO₂ uptake, as well as tunable structures and optical properties. To further improve the photocatalytic performance of MOFs, photosensitizers, semiconductors, metals and carbon materials have been incorporated into MOF structures to promote photo-generated electrons transfer and charge separation processes. The performances of MOF composites as photocatalysts for CO₂ reduction are summarized in Table 3.

MOF composites incorporated with photosensitizers

As discussed in Section 2.2.3, metal-complex photosensitizers have been used to functionalize MOFs to enhance the photocatalytic activity. Photosensitizers can also be incorporated into MOFs to produce MOF composites for CO₂ photoreduction.

Ru-based photosensitizer has been incorporated into several MOFs to enhance CO₂ photocatalytic reduction [⁵⁷–⁵⁹]. Wang and coworkers [⁵⁷] investigated the performance of a series of MOFs (Co-ZIF-9, Co-MOF-74, Mn-MOF-74, Zn-ZIF-8, Zr-UiO-66-NH₂) in conjunction with [Ru(bpy)₃]Cl₂·6H₂O toward CO₂ photocatalytic reduction, where the MOF acts as a co-catalyst and [Ru(bpy)₃]Cl₂·6H₂O acts as a photosensitizer. Photoreduction of CO₂ was performed in MeCN and H₂O solvent with TEOA as a sacrificial agent. After visible light irradiation for 0.5 h, 41.8 mmol of CO and 29.9 mmol of H₂ were obtained over Co-ZIF-9 incorporated with [Ru(bpy)₃]Cl₂·6H₂O, which outperformed the counterparts with a lower yield of CO and H₂ under the same condition (as shown in Table 3). Co-ZIF-9 is a microporous cobalt-containing benzimidazolate MOF. The superior performance of Co-ZIF-9 with regard to CO₂ photocatalytic reduction is a result of the synergetic effect of the imidazolate-based ligand which has a strong interaction with CO₂ molecules for CO₂ adsorption and cobalt with electron-mediating functions. Lately, the same group incorporated [Ru(bpy)₃]Cl₂·6H₂O as a photosensitizer into Co-ZIF-67 acting as a co-catalyst [⁵⁹]. Under the similar reaction condition, 37 mmol of CO and 13 mmol of H₂ were produced over Co-ZIF-67/[Ru(bpy)₃]Cl₂·6H₂O in 0.5 h under visible light irradiation. This was higher than that observed over Zn-ZIF-8/[Ru(bpy)₃]Cl₂·6H₂O (1.8 mmol of CO and 2.0 mmol of H₂ production in 0.5 h under the same condition), where the co-catalyst Zn-ZIF-8 has the same organic linker (2-methylimidazole) as Co-ZIF-67 but different metal site (Zn-based). This further confirmed the effect ofcobalton the photocatalytic reaction.

Cohen and coworkers [⁵⁸] incorporated a photosensitizer [Ru(dmb)₃]²⁺ (dmb= 4,4′-dimethyl-2,2′-bipyridine) into UiO-67 which was functionalized with a Mn⁺bipyridine complex, Mn(5,5′‐5,5′‐dcbpy)-(CO)₃Br for CO₂ photocatalytic reduction. In the mixture of N, N-dimethylformamide (DMF), TEOA and BNAH, HCOO^- was produced with the TON of 110 under visible light irradiation for 18 h, which outperformed the UiO-67-Mn(5,5′‐dcbpy)-(CO)₃Br without a photosensitizer [Ru(dmb)₃]²⁺ and the homogeneous reference systems (Fig. 9). The superior photocatalytic performance is attributed to the isolated active sites in MOF framework, which endows the high stability of the catalyst and impedes the dimerization of the singly reduced Mn complex.

MOF composites incorporated with semiconductors

Coupling MOFs with semiconductors has been shown to be another approach to reducing the recombination rate of the photo-generated charge carries and consequently enhancing the photocatalytic performance. Several types of semiconductors such as TiO₂, CdS and graphitic C₃N₄ have been embedded into MOFs to form heterogeneous structures which retain the properties of both the semiconductors and MOFs that are beneficial to CO₂ photocatalytic reactions.

MOF-TiO₂ composites

Several studies reported the development of ZIF-8/TiO₂ composites for CO₂ photoreduction. Zhang and coworkers [¹²⁴] integrated ZIF-8 into the TiO₂ film grid and conducted CO₂ photocatalytic reactions in H₂O as an electron donor without the use of a sacrificial agent under UV irradiation for 5 h. The amount of ZIF-8 in ZIF-8/TiO₂ composite was varied by repeating the ZIF-8 growth step on the TiO₂ film from one to three times, which are denoted as TiMOF-1, TiMOF-2, and TiMOF-3, respectively. TiMOF-3 with the highest amount of ZIF-8 had the highest CO₂ adsorption uptake, followed by TiMOF-2, TiMOF-1, and TiO₂. TiMOF-2 had the best performance with a CO yield of 0.53 mmol/(g·h) and a CH₄ yield of 0.18 mmol/(g·h), which was 38% and 157% higher than that observed over pure TiO₂ film under the same condition. TiMOF-2 outperformed TiMOF-3 which had the highest CO₂ adsorption capacity due to the fact that the large amount of ZIF-8 in TiMOF-3 covered the TiO₂ film, impeding the photo excitation process of TiO₂. It is proposed that both TiO₂ and ZIF-8 can be activated to generate photo-induced charge carries under UV irradiation, while TiO₂ is more effective in the photoexcitation process than ZIF-8 because of its higher photoactivity and narrower band gap (-0.45 eV vs. -0.5 eV for ZIF-8). Electrons can also transfer from TiO₂ to ZIF-8 and be involved in the photoreduction process over ZIF-8.

Cardoso and coworkers [¹²⁵] developed a MOF-based Ti/TiO₂ composite photocatalyst by growing ZIF-8 thin films on Ti/TiO₂ nanotube (NT) electrodes using a layer-by-layer process. Spectroscopic and voltammetric assays revealed that the CO₂ adsorbed on ZIF-8 formed stable carbamates. The photoelectrocatalytic reduction of CO₂ over Ti/TiO₂NT-ZIF-8 electrodes was performed in Na₂SO₄ (pH 4.5) saturated with CO₂ at a constant potential of+0.1 V under UV-visible light irradiation. 10 mmol/L of ethanol and 0.7 mmol/L of methanol were produced in 3 h, which increased around 20 and 430 times, respectively, compared to the values observed over Ti/TiO₂NT due to the low CO₂ adsorption capacity of Ti⁴⁺ species in the absence of ZIF-8.

Co-ZIF-9/TiO₂ composites with different mass ratios of Co-ZIF-9 (named as ZIF_x/T; where x represents the mass ratios of Co-ZIF-9 in the composite, which equals 0.01, 0.03, 0.10, 0.20, 0.30, 0.40, and 0.60) were synthesized via an in situ synthetic method by Ye and coworkers for CO₂ photoreduction [⁵⁴]. Of the ZIF_x/T composites, ZIF_0.03/T had the best photocatalytic performance with a CO yield of 8.79 mmol, CH₄ of 0.99 mmol and H₂ of 1.30 mmol under UV-visible light irradiation for 10 h, which was higher than that of the pure TiO₂ (3.58 mmol of CO, 0.60 mmol of CH₄ and 0.63 mmol of H₂) and Co-ZIF-9 (no CO, CH₄ or H₂ was detected). ZIF_0.03/T also had a better photocatalytic performance than the physical mixture of TiO₂ and Co-ZIF-9 at the same mass ratio of 0.03:0.97 (where 3.86 mmol of CO, 0.42 mmol of CH₄, and 0.56 mmol of H₂ were produced) due to the better charge separation. It was found that when the mass ratios of Co-ZIF-9 was higher than 0.1, the photocatalytic activity decreased with increasing the Co-ZIF-9 content in ZIF_x/T, which might be attributed to the heavier charge recombination.

Pipelzadeh et al. [¹⁴⁰] reported the CO₂ photoreduction to CH₄ and CO over a ZIF-8/TiO₂ composite with core-shell structure in a photoreactor equipped with simulated sunlight at a constant pressure (CP, 5 bar) and an intentionally controlled pressure swing (PS) at 50 mL/min (PS-50) and 100 mL/min (PS-100). A high CO yield was achieved in the PS mode at a production rate of 13.2 mmol/(g·h) (PS-50, 80% increase than CP-50 operation) and 15.6 mmol/(g·h) (PS-100, 30% increase than CP-100). The PS mode had a better promotion effect on CO production than CH₄. Continuous alteration of the reactants and product adsorption/desorption over the photocatalyst in the PS mode was beneficial to the regeneration of Ti³⁺ active sites, which contributed to the enhancement of photoreduction activity [¹⁷]. In addition, a higher gas flow rate facilitated CO removal to avoid catalyst poisoning. The calcination of ZIF-8/TiO₂ composite at 300°C further increased the yield of CO with 45.16 mmol/(g·h) under PS-100 condition, which was higher than that observed in the CP-100 mode (33.46 mmol/(g·h)) because of the higher structural stability of the ZIF-8/TiO₂ composite.

Composite photocatalysts composed of Cu-BTC (also named as HKUST-1, BTC= benzene-1,3,5-tricarboxylate) and TiO₂ were also developed for CO₂ photocatalytic reduction. Ye and coworkers [¹²⁶] synthesized a Cu-BTC/TiO₂ composite with a core-shell structure (see morphologies in Fig. 10) and evaluated the CO₂ photocatalytic performance. Under UV irradiation for 4 h, the production rate of CH₄ and H₂ from CO₂ over the bare TiO₂ reached 0.52 and 2.29 mmol/(g_TiO2·h), respectively, whereas the formation rate of CH₄ over the Cu-BTC/TiO₂ composite reached 2.64 mmol/(g_TiO2·h) with no H₂ detected. No product was produced over Cu-BTC, because its conjugated structure did not favor charge separation. This indicates that the CH₄ yield and the selectivity of CH₄ to H₂ over Cu-BTC/TiO₂ composite are significantly improved compared to the bare TiO₂ and Cu-BTC in photocatalytic reduction. Under UV irradiation, the TiO₂ in Cu-BTC/TiO₂ composite is photoexcited to generate charge pairs and the electrons can be effectively transferred to Cu-BTC, as demonstrated by the ultrafast spectroscopy. This facilitates the charge separation in TiO₂ and supplies active electrons to CO₂ molecules adsorbed on Cu-BTC, leading to an enhancement in photocatalytic activity of Cu-BTC/TiO₂ composite. Theoretical simulations demonstrate that the activation-energy barrier for CO₂ on the Cu sites in Cu-BTC will be lowered upon receiving the photo-excited electrons from TiO₂, enabling the CO₂ reduction occurring on the Cu sites of Cu-BTC and the enhanced selectivity of CH₄ to H₂ production. Lately, Wang and coworkers [¹²⁷] synthesized Cu-BTC/TiO₂ composites in microdroplets via an aerosol route. Similarly, Cu-BTC/TiO₂ composites had a higher photocatalytic performance than the pure TiO₂ and Cu-BTC. The yield of CO from CO₂ conversion over Cu-BTC/TiO₂ composites increased as a function of the molar ratio of Cu-BTC to TiO₂ ranging from 0 (i.e., bare TiO₂) to 3.33 (see Fig. 11), where the production rate of CO over 3.33Cu-BTC/TiO₂ increased to 256.35 mmol/(g_TiO2·h) as compared to the bare TiO₂ with a CO formation rate of 11.48 mmol/(g_TiO2·h). As demonstrated by the in situ diffuse reflectance infrared Fourier transform spectrometer (DRIFTS) analysis, the enhancement in the photoreduction performance of the Cu-BTC/TiO₂ composites may have been caused by the improved adsorption of reactants on the catalyst.

In addition to ZIF and Cu-based MOFs, TiO₂ was also incorporated into other MOF structures to form MOF-TiO₂ composites for CO₂ photoreduction. Li and coworkers [¹²⁸] combined TiO₂ with CPO-27-Mg (also named as Mg₂(DOBDC), DOBDC= 2,5-dioxido-1,4-benzenedicarboxylate) to produce a CPO-27-Mg/TiO₂ composite via a hydrothermal self-assembly method. There exists a high concentration of open alkaline metal sites (Mg²⁺) in CPO-27-Mg structure, endowing the high CO₂ adsorption capacity of CPO-27-Mg. Under UV irradiation for 10 h, 40.9 mmol/g of CO and 23.5 mmol/g of CH₄ were produced over CPO-27-Mg/TiO₂, which were higher than those observed over pure TiO₂ (22.5 mmol/g of CO and 13.7 mmol/g of CH₄). The enhanced performance of CPO-27-Mg/TiO₂ composite on CO₂ photoreduction is attributed to its high CO₂ adsorption capacity and the existence of open Mg²⁺ metal sites. The CO₂ photoreduction over a physical mixture of TiO₂ and CPO-27-Mg (The ratio of TiO₂ to CPO-27-Mg was 6:4.) was also performed under the similar condition, and 8.5 mmol/g of H₂, 18.9 mmol/g of CO, and 7.1 mmol/g of CH₄ were produced, which were lower than those produced over the CPO-27-Mg/TiO₂ composite. This demonstrates the indispensable effect of the strong interaction between CPO-27-Mg and TiO₂ in CPO-27-Mg/TiO₂ composite for the CO₂ photoreduction. TiO₂ nanosheets were coupled with NH₂-UiO-66 using an in situ growth strategy by Petit and coworkers [¹²⁹]. The content of NH₂-UiO-66 in the composite was varied from 19 wt% to 37 wt%. NH₂-UiO-66/TiO₂ had a better performance in photo reducing CO₂ to CO than their single moiety. This improvement is a result of the enhanced abundance of long-lived charge carriers and high CO₂ adsorption capacity in NH₂-UiO-66/TiO₂ composites.

MOF-CdS composites

CdS semiconductor has been widely used for CO₂ photoreduction as a photocatalyst [¹⁴¹–¹⁴³]. Wang et al. [¹³⁰] incorporated CdS into Co-ZIF-9 and the as-obtained Co-ZIF-9/CdS composite had a good photocatalytic activity toward CO₂ conversion to CO under visible light irradiation. CO₂ photoreduction was performed in MeCN and H₂O solvent with TEOA as a sacrificial agent and bipyridine (bpy) as an assistant for electron transfer. After visible light irradiation for 1 h, 50.4 mmol of CO and 11.1 mmol of H₂ were produced over Co-ZIF-9/CdS composite, which outperformed its counterparts with lower yields of CO and H₂ and pure CdS semiconductor (0.5 mmol of CO and 1.6 mmol of H₂) under the same condition(as shown in Table 3). The photocatalytic mechanism was proposed as follows: under visible light irradiation, the CdS semiconductor was excited and charge carriers were generated. The photo-generated electrons transferred to Co-ZIF-9 and reduced the CO₂ molecules adsorbed on Co-ZIF-9 to CO. Meanwhile the protons existed in the reaction system were also reduced to H₂ by the excited electrons. Lately, Su et al. [⁵⁵] prepared a series of UiO-66-NH₂/Cd_0.2Zn_0.8S composites with different UiO-66-NH₂ contents using a solvothermal method. CO₂ photoreduction was performed over UiO-66-NH₂/Cd_0.2Zn_0.8S composites under visible light irradiation, which all had an enhanced photocatalytic activity in comparison to their single components. The UiO-66-NH₂/Cd_0.2Zn_0.8S composite with a UiO-66-NH₂ content of 20 wt% had the best photocatalytic performance with a H₂ production rate of 5846.5 mmol/(g·h) and a CH₃OH production rate of 6.8 mmol/(g·h). The efficient charge separation and transfer between Cd_0.2Zn_0.8S and UiO-66-NH₂ contributed to the enhanced photocatalytic activity of UiO-66-NH₂/Cd_0.2Zn_0.8S composites.

MOF-graphitic C₃N₄ composites

Graphitic carbon nitrides (g-C₃N₄) with different morphologies and structures have been integrated with MOFs to improve the CO₂ photoreduction activity. Wang and coworkers [¹³¹] coupled mesoporous g-C₃N₄ with Co-ZIF-9, which acted as a light harvester and co-catalyst, respectively, to fabricate a Co-ZIF-9/g-C₃N₄ composite. The Co-ZIF-9/g-C₃N₄ composite efficiently catalyzed CO₂ to CO and H₂ under visible light irradiation. 20.8 mmol of CO and 3.3 mmol of H₂ were obtained over the Co-ZIF-9/g-C₃N₄ composite in 2 h, whereas no product was detected over the pristine Co-ZIF-9 and g-C₃N₄. Liu and coworkers [⁵⁶] developed a series of ZIF-8/g-C₃N₄ composites by growing different contents of ZIF-8 nanoclusters on the surface of g-C₃N₄ nanotubes. The ZIF-8/g-C₃N₄ composites had an increased CO₂ adsorption capacity than g-C₃N₄ nanotubes without sacrificing the light absorption capacity owing to the incorporation of ZIF-8 nanoclusters. Because of the high CO₂ capture capacity of ZIF-8 and the promoted charge separation efficiency from the g-C₃N₄ nanotubes, the ZIF-8/g-C₃N₄ composites had an enhanced photocatalytic performance on CO₂ reduction, where the highest production rate of methanol reached 0.75 mmol/(g·h) over the ZIF-8/g-C₃N₄ composite in which the mass ratio of g-C₃N₄ nanotubes to ZIF-8 was 8 under light irradiation for 1 h. Under similar conditions, g-C₃N₄ nanotubes and bulk g-C₃N₄ had a production rate of methanol of 0.49 and 0.24 mmol/(g·h), respectively, whereas no methanol was produced over the pure ZIF-8 nanocrystals. In addition, g-C₃N₄ nanosheets were combined with UiO-66 [¹³²] and BIF-20 (a zeolite-like porous boron imidazolate framework) [¹³³] MOFs to form MOF/g-C₃N₄composites, which all had an enhanced performance on CO₂ photocatalytic reduction.

In addition to TiO₂, CdS and C₃N₄, another type of semiconductor was also incorporated into MOFs to generate a composite photocatalyst for CO₂ photocatalytic reduction. Wang and coworkers [¹³⁴] developed a ZIF-8/Zn₂GeO₄ composite by growing ZIF-8 nanoparticles on Zn₂GeO₄ nanorods. The ZIF-8/Zn₂GeO₄ composite inherited both the high CO₂ adsorption capacity of ZIF-8 nanoparticles and the high crystallinity of Zn₂GeO₄ nanorods. The ZIF-8/Zn₂GeO₄ composite with 25 wt% ZIF-8 had a CO₂ adsorption capacity of 15.5 cm³/g, which was higher than the pure Zn₂GeO₄ nanorods (4.9 cm³/g) due to the high CO₂ adsorption ability of ZIF-8. After 11 h of light irradiation in Na₂SO₃, the production of methanol at a rate of 0.22 mmol/(g·h) over the ZIF-8(25wt%)/Zn₂GeO₄ composite was observed. The yield of methanol over the ZIF-8(25 wt%)/Zn₂GeO₄ composite had a 62% increase in comparison to the pure Zn₂GeO₄ nanorods under light irradiation for 10 h. The enhanced CO₂ photoreduction performance of ZIF-8/Zn₂GeO₄ composite may have been resulted from the high CO₂ adsorption capacity of ZIF-8 and the higher light response.

MOF composites incorporated with metals

Because of their high Fermi energy levels, noble metal nanoparticles can effectively separate photo-generated charge pairs of photocatalysts [¹⁴⁴,¹⁴⁵]. Therefore, another approach for promoting the CO₂ photocatalytic reduction is to dope noble metal nanoparticles into MOFs structures to decrease the recombination rate of the photo-generated electrons and holes. Li and coworkers [¹³⁵] synthesized M-doped NH₂-MIL-125(Ti) (M= Pt and Au) and conducted CO₂ photocatalytic reaction in saturated CO₂ with TEOA as a sacrificial agent under visible light irradiation. Both H₂ and HCOO^- were produced over M/NH₂-MIL-125(Ti), while no H₂ but only HCOO^- was produced over bare NH₂-MIL-125(Ti). The reason for this is that the electron-trapping effect of noble metals promotes the hydrogen evolution. In addition, it is noted that Pt and Au had different photocatalytic performances on the production of HCOO^-. Compared to bare NH₂-MIL-125(Ti) (yield of HCOO^-: 10.75mmol), Pt/NH₂-MIL-125(Ti) had a higher production yield of HCOO^- (12.96 mmol), while Au/NH₂-MIL-125(Ti) had a lower production of HCOO^- (9.06 mmol) under visible light irradiation for 8 h. As demonstrated by ESR studies and DFT calculations, hydrogen spillover from Pt to the bridging oxygen linked to Ti atoms occurred in Pt/NH₂-MIL-125(Ti), leading to the Ti³⁺ formation and boosting the hydrogen-assisted CO₂ reduction to HCOO^-. In contrast, it was difficult to achieve the hydrogen spillover from Au to the NH₂-MIL-125(Ti) framework in Au/NH₂-MIL-125(Ti), and thus resulting in a lower HCOO^- formation over Au/NH₂-MIL-125(Ti). Fu et al. [¹³⁶] doped different contents of Co (from 1 to 3 wt%) into NH₂-MIL-125(Ti) and produced Co/NH₂-MIL-125(Ti) composites for CO₂ photoreduction under visible light irradiation. 1 wt% Co/NH₂-MIL-125(Ti) had the best performance on CO₂ photoreduction with an HCOO^- formation of 384.2 mmol in 10 h, which was 2-fold higher than that observed over pure NH₂-MIL-125(Ti) (162.8 mmol) due to the enhanced visible-light harvesting and electron transfer stemmed from the addition of Co.

Yaghi and coworkers [¹³⁷] functionalized UiO-67 with Re complexes, Re^I(CO)₃(5,5′‐dcbpy)Cl photosensitizer, of various densities (Re_n-MOF, n = 0, 1, 2, 3, 5, 11, 16, and 24 complexes per unit cell) and found that the photocatalytic activity of the MOF system can be controlled by controlling the density of Re complexes in the framework, in which Re₃-MOF had the best performance on CO₂-to-CO conversion. The photocatalytic activity was further enhanced by coating a 16 nm layer of Re₃-MOF onto Ag nanocubes (Ag⊂Re₃-MOF, see Fig. 12), where a 7-fold improvement of CO₂-to-CO reduction compared to pure Re₃-MOF under visible light irradiation was achieved. Both Re complexes and Ag nanocubes contributed to the CO₂ photocatalytic activity enhancement of Ag⊂Re₃-MOF. Because the quadrupolar localized surface plasmon resonance (LSPR) scattering peak (l_max~ 480 nm) of Ag nanocube overlapped with the absorption range of Re^I(CO)₃(5,5′‐dcbpy)Cl (400 nm<l<550 nm) in the visible region [¹⁴⁶,¹⁴⁷] and Ag⊂Re₃-MOF structure inherited the LSPR features of Ag cores, the photoactive Re metal sites within MOF shell were spatially localized into a strong electromagnetic field induced by LSPR of the Ag nanocubes for photocatalytic enhancement.

MOF composites incorporated with carbon materials

Still, another important strategy to develop active photocatalysts for CO₂ photoreduction is the incorporation of carbon materials such as graphene and its derivatives into MOFs. Graphene can act as a photosensitizer to extend the light adsorption region from the UV to the visible light region. Moreover, the excellent conductivity of graphene can facilitate the rapid transfer of the photo-generated electrons and suppress the electron-hole recombination, and subsequently boost the photocatalytic activity.

Li and coworkers [¹³⁸] synthesized UiO-66-NH₂/graphene composites via microwave-assisted in situ growth of different amounts (1–3 wt%) of UiO-66-NH₂ nanocrystals onto graphene. As compared to the pure UiO-66-NH₂ (3.1 mmol of HCOO^-, 0.11 mmol of CH₄, and 16.9 mmol of H₂ were produced.) and the UiO-66-NH₂/graphene synthesized via a traditional hydrothermal route (16.1 mmol of HCOO^- and 20.4mmol of H₂ were produced.), the as-obtained UiO-66-NH₂/graphene composites had a better performance on CO₂ photoreduction activity and selectivity under visible light irradiation. Of the UiO-66-NH₂/graphene composites as prepared, 2 wt% UiO-66-NH₂/graphene composite had the best performance, where 33.5mol of HCOO^-, 0.9 mmol of CH₄ and 13.2 mmol of H₂ were produced in 4 h. The performance enhancement is attributed to both the fine particle size and high dispersion of UiO-66-NH₂ nanocrystals, which enables more light trapping to generate charge pairs and shortens the electron transfer pathway to facilitate electron transfer. Additionally, the strong UiO-66-NH₂/ graphene interaction can also effectively accelerate electrons transfer efficiency to enhance photocatalytic activity for CO₂ reduction.

Do and coworkers [¹³⁹] incorporated different contents of amine-functionalized reduced graphene oxide (NH₂-rGO) (5–25 wt%) into a TCPP-based MOF (Al/PMOF) to form Al-PMOF/NH₂-rGO composites as photocatalysts for CO₂ reduction. Al-PMOF/NH₂-rGO composites had an enhanced photocatalytic activity for CO₂ reduction, where the HCOO^- formation rate reached 685.6 mmol /(g·h) over Al-PMOF/5 wt% NH₂-rGO under visible light irradiation in 6 h, which was much higher than the HCOO^- production rate observed over pure Al-PMOF (165.3 mmol /(g·h)). The photocatalytic mechanism is proposed as follows: upon visible light irradiation, TCPP is responsible for light harvesting and is excited to produce photo-generated electron-hole pairs, and the electrons are transferred from TCPP to graphene which acts as an electron acceptor. The electrons transferred from graphene reduce the adsorbed CO₂ to HCOO^- in the presence of TEOA that serves as a hydrogen source.

Most recently, MOF with a special microstructure in CO₂ photoreduction was reported. Lin and coworkers [¹⁴⁸] prepared Ni-based MOF (Ni₂(OH)₂BDC) monolayers (Ni MOLs) and examined the performance for photoreduction of CO₂ under visible light irradiation with [Ru(bpy)₃]Cl₂⋅6H₂O as a photosensitizer and TEOA as an electron donor. After 2 h reaction in pure CO₂, Ni MOLs had a CO production rate of 12.5 mmol/h and a H₂ production rate of 0.28 mmol/h, whereas bulk Ni MOFs had a lower CO production rate of 7.23 mmol/h. In diluted CO₂, Ni MOLs had a CO selectivity of 96.8 %, which outperformed most of the reported systems in diluted CO₂. It was proposed that the strong affinity of Ni MOLs to CO₂ molecules enabled their high CO₂ adsorption ability and stabilized the initial Ni‐CO₂ adducts, thus promoting CO₂-to-CO conversion. In addition, weak affinity of Ni MOLs to H₂O impeded the transfer of protons, thereby reducing the H₂ formation.

Conclusions and outlook

In recent years, MOFs have attracted great attention and showed much potential as photocatalysts for CO₂ reduction because of their super-high surface areas, tunable structures, and high CO₂ adsorption capacity. This review summarizes the recent research progress in the development of MOFs and MOF-based composite photocatalysts for the photocatalytic reduction of CO₂. Several strategies in improving light harvesting, CO₂ adsorption and charge separation have been discussed, which provides guidelines for rational design of MOF-based photocatalysts with enhanced performance on CO₂ reduction under visible light irradiation. Although great progress has been made in the development of MOF-based photocatalysts for CO₂ reduction, there still exist some challenges and large potential need to be explored. For instance, ① to date several thousands of MOFs with different structures have been developed, however, only several types of MOFs such as ZIFs, UiO- and MIL-based MOFs are under exploration as photocatalysts for CO₂ reduction. More efforts need to be made in investigation of many other MOF systems with active catalytic centers, which may be promising for photocatalysis. ② Most MOFs have a poor stability in aqueous solution and suffer structural collapse, which limit their practical applications in catalysis processes where water is involved. Therefore, it is imperative to develop novel MOF photocatalysts which have an excellent chemical stability in aqueous solution and meanwhile retain the photocatalytic functionality. ③ The current research achievements in photocatalytic reduction of CO₂ over MOF-based photocatalysts have not yet met the requirement for large-scale industrial applications, therefore, exploration in further improvement of CO₂ photoreduction is required. So far, many novel materials such as two-dimensional (2D) nanomaterials like graphene and 2D semiconductors have been developed; integrating these novel nanomaterials into MOF structures may be considered as a promising strategy to enhance the photocatalytic performance of MOFs. It is believed that with the rapid development progress of MOF materials and other novel photocatalytic materials, MOF-based photocatalysts will have even greater potential to fulfill the requirements for practical applications in heterogeneous photocatalysis in the future.

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