*3.1. Zr MOFs*

In 2011, Wang et al. [75] chelated metal ions (such as Ir, Re, and Ru) with 4,4-biphenyldicarboxylic acid derivatives as organic ligands to construct MOFs, and the Zr-based MOF (UiO-67) systems with different metal doping were obtained. A similar synthesis strategy has also been adopted by Wang et al. who used ligand H2L4 for photocatalytic reduction of CO2 to CO [76]. The total conversion number (TON) of CO2 reduction can reach 10.9. The photocatalytic activity can be improved by doping a variety of photoactive metal nanoparticles inside MOFs. Subsequently, the authors observed a significant decrease in photocatalytic activity through a series of comparison experiments, which proved that the metal nanoparticles themselves are the real active sites that are involved in the photocatalytic reaction.

In 2015, Xu et al. [77] chose Zr-MOF (PCN-222) containing porphyrin as catalysts and found that it could be used as a stable photocatalyst to reduce CO2 to formate ion under visible light. It was found that PCN-222 exhibited broad-spectrum absorption properties. There existed a series of extremely long lifetime electron trap states in the material, which could inhibit the recombination of photogenerated charge carriers and improve the photoreduction efficiency of CO2. In 2016, Chen et al. [78] synthesized a new microporous stable zirconium-based metal organic skeleton (NNU-28) from 4,4'-(anthracene-9,10-bis (2,1-ethynylphenyl) dicarboxylic acid, which was used to reduce CO2 to formate while using triethanolamine as the sacrifice agent. Under visible light irradiation, the rate of catalytic conversion of CO2 to formate ion was 183.3 μmol/h. It was found that, in the catalytic reaction, the ligands produced about 27.3% formate ions, while the metal clusters

produced about 77.7% formate ions. Under light irradiation, anthracene derivative ligands not only acted as an effective light collector, but it also sensitized Zr6 oxygen clusters through the LMCT (linker-to-metal charge transfer) process. At the same time, the ligand itself can also be stimulated to form free radicals and produce photogenerated electrons. Figure 2 shows two catalytic pathways for the reduction of CO2 to formate. This strategy is helpful for the design and development of MOFs materials with efficient visible light response [78].

**Figure 2.** Two catalytic pathways for the reduction of CO2 to formate. Reprinted from ref. 78 with permission by the Royal Society of Chemistry.

In 2018, Sun et al. [79] synthesized a porous zirconium based metal-organic framework [(Zr6O4(OH)4(L)·6DMF) while using dicarboxyl ligands (H2L=2,2'-diamino -4,4'-stilbene dicarboxylic acid, DMF) with conjugated imine function. The materials showed high chemical stability and remarkable visible light absorption properties. The average rate of HCOO- formation of MOFs is about 96.2 μmol/h.

Sun et al. [80] compared the activity of NH2-UiO-66(Zr) and NH2-MIL-125(Ti) for photocatalytic reduction of CO2 under visible light. The results showed that the catalytic performance of NH2-UiO-66(Zr) was higher than that of NH2-MIL-125(Ti) under the same reaction conditions. This is ascribed to the effective transfer of photogenerated electrons from ATA to Zr-O clusters, and made Zr-O clusters efficient photocatalytic active sites. Furthermore, some ATA ligands were replaced by 2,5-diamino terephthalic acid (DTA) and the mixed ligand NH2-UiO-66(Zr) was obtained. It was found that the CO2 conversion of mixed NH2-UiO-66(Zr) was 50% higher than that of pure NH2-UiO-66(Zr). This may be because the mixed NH2-UiO-66(Zr) showed strong photoabsorption capacity and large CO2 adsorption capacity, so its photocatalytic activity is obviously improved.

Choi et al. [81] reported the synthesis of composited catalysts by covalently binding ReI(CO)3(bpydc)Cl(as Re TC) to UiO-67 to Re*n*-MOFs (n is the density of Re TC in the pores of MOF). Subsequently, the MOF was further modified with cubic silver nanoparticles to obtain Ag-Re*n*-MOF, thus the photocatalytic activity of CO2 conversion was significantly improved (Figure 3A, [81]). The PXRD (powder X-ray diffraction) patterns showed that the single crystal Re3-MOF structure is preserved when different amount of Re TC is introduced into Re*n*-MOF (Figure 3B, [81]). By studying the process of photocatalytic conversion of CO2 by Re*n*-MOF (Figure 3C, [81]), it was found that the catalytic activity of Re3-MOF was the highest. In addition, under visible light irradiation, the activity of AgRe3-MOF was five times higher than that of Re3-MOF, and the conversion efficiency of CO2 to CO was increased by seven times. This is mainly because MOF has large porosity and CO2 adsorption capacity, which is conducive to the occurrence of catalytic reduction reaction. On the other hand, precious metals have a wide range of photo absorption and are easier to trap photogenerated electrons due to the lower Fermi levels. At the same time, their stability could be further improved due to the strong covalent bond between Re TC and MOF.

**Figure 3.** Structures of Ren-MOF and Ag Ren-MOF based catalysts (**A**), PXRD of Ren-MOFs (**B**), and the photocatalytic activity of Ren-MOF (**C**). Reprinted from ref. 81 with permission by the American Chemical Society.

Lee et al. [82] used UiO-66 (Zirconium 1,4-Carboxybenzene) as a precursor to obtain UiO-66-CAT with Cr3+ or Ga3+ sites as catalysts for photocatalytic CO2 reduction. In the presence of TEOA and BNAH, the TON (turnover number) values of UiO-66-Cr CAT and UiO-66- Ga CAT are 11.22 ±0.37 and 6.14±0.22, and the amount of HCOOH that is produced by catalytic reduction of CO2 after visible light irradiation for 6h were (51.73±2.64) and (28.78 ±2.52) μmol, respectively. The activity of UiO-66-Cr CAT is about twice higher than that of UiO-66-Ga CAT, which is mainly attributed to the fact that Cr3+ is more efficient than Ga3+ for the rapid transfer of electrons. At the same time, Cr derivatives show higher reduction efficiency than Ga derivatives due to their open shell structure.

Zhang et al. [83] reported Zr- porphyrin MOF (MOF-525-Co) as efficient catalysts for CO2 conversion. Using TEOA as a sacrificial agent, MOF-525-Co could efficiently catalyze the reduction of CO2 to CO and CH4 under visible light irradiation. When compared with Zn-MOF-525 and MOF-525, MOF-525-Co showed the highest catalytic activity and CO2 adsorption capacity. The metallized MOFs is obviously improved, and exhibited strong charge separation ability and energy conversion efficiency. The highest catalytic performance of cobalt metallized MOFs is mainly due to the fact that the introduction of monoatomic Co into MOF-525 can significantly improve the electron-hole separation efficiency in porphyrin ligands. At the same time, the photogenerated electrons rapidly migrated from the porphyrin center to the surface of the catalyst, thus the electrons with long lifetime were obtained, which effectively activated the CO2 molecules that were adsorbed on the Co center.

Su et al. [84] prepared a series of Cd0.2Zn0.8S@UiO-66-NH2 composites with different UiO-66-NH2 content by solvothermal method, which were used for photocatalytic reduction of CO2 to CH3OH. The results showed that the single UiO-66-NH2 showed no activity for photocatalytic CO2 reduction, but Cd*x*Zn1-*x*S with adjustable composition and band gap could be e fficiently excited by visible light. All of the Cd0.2Zn0.8S@UiO-66-NH2 samples showed excellent photocatalytic activity when compared with Cd0.2Zn0.8S. When the content of UiO-66-NH2 was 20% (mass fraction), the catalyst showed the best photocatalytic activity, and the formation rate of CH3OH is 3.4 times higher than that of single structure Cd0.2Zn0.8S. This is mainly due to the e ffective charge separation and transfer at the interface between Cd0.2Zn0.8S and UiO-66-NH2. Thus, the photogenerated electrons that were absorbed by Cd0.2Zn0.8S and UiO-66-NH2 can be quickly transferred to the surface for CO2 reduction. In addition, Cd0.2Zn0.8S@UiO-66-NH2 photocatalyst showed excellent stability in the process of photocatalytic reduction of CO2.

### *3.2. Zn MOFs*

In 2015, Wang et al. [85] reported the establishment of CO2 photoreduction system while using the CdS semiconductor and Co-ZIF-9 as catalyst and co-catalyst, respectively. Under mild reaction conditions, the reaction system of bipyridine and triethanolamine showed high catalytic activity when CO2 was deoxidized to CO under visible light irradiation. Under the irradiation of monochromatic light at a wavelength of 420 nm, the quantum e fficiency could reach 1.93%.

In 2018, Wang et al. [86] synthesized a series of ZIF-67 nanocrystals with a di fferent morphology by the solvent induction method. Taking the advantages of MOF, the capture of CO2 was controlled by controlling its morphology, and their photocatalytic performance was further improved. In the same year, Chen [87] and co-workers fabricated the Ag-Co-ZIF-9 nanocomposited materials with di fferent Ag loading by the photo deposition method to study the e ffect of Ag NPs on the reaction performance of Co-ZIF-9 in CO2 photo reduction reaction. In this study, Co-ZIF-9, with a rod structure was obtained by the reflux method, and ultra-small Ag nanoparticles (< 5 nm) were doped into Co-ZIF-9 by photodeposition. With the help of photosensitizer, the Ag@Co-ZIF-9 composite showed the catalytic performance of converting CO2 to CO under the irradiation of visible light. With the increase of Ag nanoparticles, the formation of CO obviously increased while the amount of H2 decreased. When compared with pure Co-ZIF-9, the photocatalytic activity of Ag@Co-ZIF-9 can be improved by two times (about 28.4 μmol CO), and selectivity about 20% (22.9 μmol H2). The experimental results showed that Ag NPs in Co-ZIF-9 could act as an electron trap and active site for CO2 reduction, thus the e fficiency and selectivity of MOF materials in CO2 photo reduction were improved.

Subsequently, Ye et al. [88] developed and used the ultra-thin two-dimensional Zn-MOF nanoliths to reduce CO2 to CO. They firstly tried to establish two novel non-precious metal mixed photocatalytic systems. The catalyst showed excellent photocatalytic activity and selectivity under mild reaction conditions. It was confirmed that the Zn-MOF nanoparticles show better charge transfer ability than the Zn-MOF bulk materials via electrochemical impedance and PL (photoluminescence) spectroscopy analysis, thus stronger photocatalytic e fficiency and selectivity were obtained. This provides feasibility for the application of photocatalysis in the development of various two-dimensional (2D) MOF materials.

In 2018, Zhao et al. [89] prepared Zn2GeO4/Mg-MOF-74 composites by the hydrothermal method (Figure 4). When the water was used as agent, the photocatalytic activity of Zn2GeO4/Mg-MOF-74 for CO2 reduction reaction is higher than that of pure Zn2Ge4 nanorods or the physical mixture of Zn2GeO4 and Mg-MOF-74. This is mainly due to the stronger CO2 adsorption performance of Mg-MOF-74, the lower recombination probability of photogenerated electron-hole pair and more alkali metal sites on the surface of Mg-MOF-74. In addition, the e ffect of H2O on the reaction was also studied and the results show that H2O is the reducing agen<sup>t</sup> and hydrogen source involved in the reaction. In the process of reduction, the photogenerated electrons from the conduction band reduce CO2 to CO and HCOOH, by the reaction of CO2+2e- <sup>+</sup>2H<sup>+</sup>→HCOOH and CO2+2e- <sup>+</sup>2H<sup>+</sup>→CO+H2O, in which the content of HCOOH is very small.

**Figure 4.** Schematic illustration of the synthesis of the Zn2GeO4/Mg-MOF-74 composites. Reprinted from ref. 89 with permission by the Royal Society of Chemistry.

In 2018, Cardoso et al. [90] modified TiO2 nanotubes and formed a core-shell structure by layer growth of ZIF-8 nanoparticles on the surfaces. The FT-IR spectra show that the host-guest interaction depends on the pore structure and chemical properties of MOF connectors. Under UV irradiation at room temperature, CO2 can be photocatalyzed to methanol and ethanol fuel on the electrode of composited materials. Zinc-based MOF not only provided the adsorption/activation of CO2, but also acted as a light absorber to transfer excited electrons for photocatalytic reduction.

Sadeghi et al. [91] synthesized zinc-based porphyrin (Zn/PMOF), which could catalytically reduce CO2 to CH4 under light irradiation. The results showed that the yield of CH4 was 10.43 μmol when Zn/PMOF was used as photocatalyst. After 4h irradiation, Zn/PMOF was much higher than that of CH4 when ZnO was used as photocatalyst. At the same time, Zn/PMOF as photocatalyst showed high selectivity for CO2reduction, and it has better stability and repeatability when comparing to ZnO.

Yan et al. [92] loaded different amounts of TiO2 on Co-ZIF-9 to construct Co-ZIF-9/TiO2 nanostructure composites (ZIFx/T, x is the mass ratio of Co-ZIF-9 in the composites, T is TiO2). The results showed that ZIF0.03/ T showed the best catalytic conversion efficiency of CO2, and the yield of Ti/T is 2.1 times higher than that of pure TiO2 catalyst after irradiation for 10h. Linear sweep voltammetry in CO2 saturated solution further reveals that Co-ZIF-9 can effectively activate CO2 and reduce the CO2 reduction initiation potential of ZIFx/T (x ≤ 0.10). In addition, the photoluminescence spectra show that the ZIFx/T composites that were prepared by in-situ synthesis showed higher charge separation efficiency. Therefore, better CO2 adsorption capacity and charge separation rate are beneficial to the high activity of ZIFx/T nanostructures in photocatalytic transformation.

Maina et al. [93] designed a catalytic system based on membrane reactor. The controllable encapsulation of TiO2 and Cu2+ doped TiO2 nanoparticles (Cu-TiO2) in ZIF-8 film was realized by the rapid thermal deposition (RTD) method (Figure 5A, [93]). Under ultraviolet irradiation, the Cu-TiO2/ ZIF-8 hybrid film showed high photocatalytic activity. The results show that, when compared with the amount produced by the original ZIF-8 film alone, the yields of CO and CH3OH increased by 188% and 50%, respectively (Figure 5B, [93]). Further studies showed that the yields of photocatalytic reduction of CO2 to CH3OH and CO depend on the content of Cu-TiO2 nanoparticles that are loaded on MOF films (Figure 5C, [93]). When the loading of Cu-TiO2 nanoparticles is 7 μg, Cu-TiO2/ZIF-8 exhibited the best catalytic efficiency. When compared with the original ZIF-8 film, the yields of CO and CH3OH increased by 23.3% and 70%, respectively. The sharp increase of product originated from the synergistic effect between the ability of semiconductor nanoparticles to produce photoexcited electrons under light irradiation and the high CO2 adsorption capacity of MOF.

**Figure 5.** Fabrication of Cu-TiO2/ ZIF-8 membranes (**A**), effect of membrane composition (**B**) and Cu-TiO2nanoparticles loading on the product yields (**C**). Reprinted from ref. 93 with permission by the American Chemical Society.

Kong et al. [94] prepared CsPbBr3@ ZIFs composites by in-situ synthesis used as CO2 reduction photocatalyst with reinforcing activity (Figure 6A, [94]). The electron consumption rates of CsPbBr3@ZIF-8 and CsPbBr3@ZIF-67 are 15.498 and 29.630 <sup>μ</sup>mol·g-1·h-1, which is 1.39 and 2.66 times higher than that of pure CsPbBr3, respectively. The comparison of photocatalytic CO2 reduction performance using CsPbBr3 and CsPbBr3@ZIFs showed that the ZIF coating greatly improved the catalytic activity of CsPbBr3 (Figure 6B, [94]). In addition, six cycle experiments have been carried out on CsPbBr3@ZIF, and it was found that the electron consumption rate suffered from negligible decrease. This indicates that it possessed good stability (Figure 6C, [94]). The synergistic effect of CsPbBr3 and ZIF coating improved the stability of CsPbBr3 to water molecules and enhanced the CO2 capture ability and the charge separation efficiency. All of these lead to a higher conversion efficiency. Moreover, the catalytic active center Co in ZIF-67 could further accelerate the process of charge separation, activate CO2 molecules, and improve the catalytic activity of CO2 reduction.

**Figure 6.** Schematic illustration of the fabrication process and CO2 photoreduction process of CsPbBr3/ ZIFs (**A**) and photocatalytic CO2 reduction performances of CsPbBr3 and CsPbBr3@ZIFs (**B**,**C**). Reprinted from ref. 94 with permission by the American Chemical Society.

### *3.3. Ti MOFs*

In 2012, Fu et al. [95] reported a photosensitive MOF Ti8O8(OH)4(bdc)6(MIL-125(Ti)) for photocatalytic CO2 reduction. The photocatalytic activity evaluation indicated that Ti-MOF could efficiently reduce CO2 to HCOO- under 365 nm UV irradiation. When comparing to other MOFs, MIL-125(Ti) showed slightly higher activity. The photocatalytic results of NH2-MIL-125 showed that the concentration of HCOO- increased in the reaction system with the extension of irradiation time, and the formation of HCOO- reached 8.14 μmol within 10 hours. On one hand, the introduction of NH2 can promote the rapid transfer of electrons from O to Ti, in TiO5(OH) metal cluster. On the other hand, NH2 can significantly improve the adsorption capacity of NH2-MIL-125 (Ti) to CO2, which is beneficial for the adsorption and activation of CO2 in the process of photocatalytic reaction. In 2018, He [96] designed an MOF-based ternal-composite photocatalyst (TiO2/Cu2O/Cu3(BTC)2) to increase the density of charge carrier and promote the activation of CO2 molecules to improve the photoreduction capacity of CO2. The experimental results showed that the addition of Cu2O and Cu3(BTC)2 not only significantly improved the light conversion e fficiency of CO2, but also facilitated the formation of CH4. The increase of charge carrier density improved the overall performance of the catalyst. The PL, XPS, and DRIFT analysis verified that the coordination of unsaturated metal sites were helpful in activating CO2. This study provides a new way to solve the problems of low charge density and e fficiency CO2 activation, and it also provides a reasonable design for in-depth understanding of CO2 photoreduction and other applications of mixed nanomaterials based on MOF.

#### **4. Prospect of Photocatalytic CO2 Reduction**

The advantages of MOFs-based photocatalytic materials are obvious when comparing to conventional semiconductor materials. Thus, they have attracted more and more research attentions in photocatalysis. However, the low e fficiency of this technology still hinders its wide applications in industry. The following problems should be addressed in the future. Firstly, researchers need to put forward e ffective strategies to improve the light absorption properties and charge separation performances. Secondly, most MOFs are not as metal oxide for semiconductor photocatalysts, especially in water or under ultraviolet light, which ultimately leads to the decreased catalyst life; hence, how to enhance their robustness is another important topic. Thirdly, there are few studies on the mechanism of photocatalytic CO2 reduction in MOFs, especially the current understanding of the catalytic reaction path is still blurred. In addition, most of the reported photocatalytic CO2 reduction reactions are carried out in organic solvents, requiring additional sacrificial agents. The future materials for catalytic reduction of CO2 should be economical and environmentally friendly. Therefore, it is urgen<sup>t</sup> to solve the above problems of MOFs materials for photocatalytic CO2 reduction.
