**3. Mechanisms**

Photocatalytic CO2 reduction involves three basic processes. Under light irradiation, the electron-hole pairs could be generated in semiconductor materials upon the absorption of photons with larger energy than the forbidden band gap [65]. Subsequently, the photoexcited electron-hole pairs separate and migrate to the active sites on the surface of the semiconductor. In this process, it is necessary to reduce the bulk phase and surface recombination of photogenerated electron-hole pairs. This is the major factor limiting the e fficiency of photocatalytic reduction of CO2 [66,67]. After that, oxidation and reduction reactions occur on the surface of the semiconductor. At this time, electrons with strong enough reducing ability can reduce CO2 molecules into hydrocarbons, such as CO, CH4, and CH3OH, and holes with oxidizing ability oxidize H2O molecules to release O2, O2-, and other substances [68]. The conversion e fficiency of photocatalytic CO2 reduction depends on the capacity of the light-trapping ability of the semiconductor material, the e fficiency of photo-generated carrier generation and separation, and the thermodynamic equilibrium of the surface catalytic reactions. From the kinetic point of view, the e ffective absorption of light, the e fficient separation and migration of photo-generated electron-hole pairs, and the su fficient reactive sites on the catalyst surface are an important prerequisite for the high-e fficiency photocatalytic conversion of CO2 while using semiconductor materials [69].

The detailed mechanisms for photocatalytic CO2 reduction process have not been discovered so far. However, mechanism studies in recent years provide valuable information to unravel this process [70]. At present, it is commonly accepted that photocatalytic CO2 reduction is a multi-electron reduction process, as described in the Equations (2)–(8). It can be seen that the reaction process is accompanied by some unstable substances, namely intermediates. The corresponding products are di fferent due to the specific reaction route and the number of electrons obtained during the reaction [71,72]. According to the number of electrons that were obtained by C atom, the products can be carbon monoxide, methane, formic acid, methanol, etc. [73]. In some special reaction system, some multi-carbon compounds such as ethane, acetic acid, and other compounds can also be obtained. From the perspective of Gibbs free energy, photocatalytic reduction of CO2 is an uphill reaction, that is Δ G > 0. If the reaction proceeds, a large amount of energy injection (such as incident photons) is required.

Reaction Eredox/ (V vs NHE,PH=7)

$$\text{CO}\_2 + \text{e}^- \rightarrow \text{CO}^- \qquad -1.90\tag{1}$$

$$\rm{CO\_2 + H^+ + 2e^- \rightarrow HCO\_2^-} \qquad \quad \rm{-0.49} \tag{2}$$

CO2 + 2H<sup>+</sup> + 2e−→CO + H2O −0.53 (3)

$$\rm CO\_2 + 4H^+ + 4e^- \rightarrow \rm HCHO + \rm H\_2O \qquad \quad -0.48 \tag{4}$$

$$\rm{CO\_2 + 6H^+ + 6e^- \rightarrow CH\_3OH + H\_2O} \qquad \quad -0.38 \tag{5}$$

$$\text{CO}\_2 + 8\text{H}^+ + 8\text{e}^- \rightarrow \text{CH}\_4 + 2\text{H}\_2\text{O} \qquad -0.24\tag{6}$$

$$\text{2H}^+ + \text{2e}^- \rightarrow \text{H}\_2 \qquad \quad -0.41\tag{7}$$

$$\text{H}\_2\text{O} \to 0.5\,\text{O}\_2 + 2\text{H}^+ + 2\text{e}^-\qquad 0.82\tag{8}$$

Hendon et al. [74] elucidated the electronic structure of MIL-125 with aminated linkers through a combination of synthesis and computation. They also discussed the band gap modification of MIL-125, a TiO2/1,4-benzenedicarboxylate (bdc) MOF, and the possible mechanism for the photocatalytic CO2 reduction was proposed (Figure 1).

**Figure 1.** (**a**) the valence band is composed of the bdc C 2p orbitals (shown on the right), making these favorable for linker-based band gap modifications; (**b**) the conduction band is composed ofO 2p orbitals and Ti 3d orbitals (shown on the right). (**c**) PBEsol band structures for synthetic MIL-125 (black), 10%-MIL-125-NH2 (blue), 10%-MIL-125-(NH2)2/90%-MIL-125-NH2 (orange) and the theoretical 10%-MIL-125-(NH2)2 (green). (**d**) HSE06-calculated VB and CB energies of MIL-125-NH2 models containing increasing numbers of bdc-NH2 linkers [i.e. 0 (MIL-125) to 12 (100%-MIL-125-NH2)] per unit cell. MOFs materials for photocatalytic CO2 reduction. Reprinted from ref. 74 with permission by the American Chemical Society.

Photocatalytic CO2 reduction using MOFs-based materials as catalysts has drawn dramatic research interests in recent years. It is easy to design MOFs materials with accessible metal sites, specific hetero-atoms, and the ordered structure of functionalized organic ligands. This can effectively improve the efficiency of electron-hole separation and the photocatalytic performance. Porosity can make MOFs expose more active sites and channels for reactant adsorption. This can improve the charge transfer efficiency as well as improve its utilization efficiency of solar energy while inhibiting the recombination of the photo-induced electron-hole pairs in the bulk phase. Based on the above merits, people try to use different MOFs for photocatalytic CO2 reduction. In the following text, we will introduce three typical MOFs for photocatalytic CO2 reduction and their catalytic performances. New insights for the dominating factors on activity and selectivity of product will also be discussed. Table 1 summarizes the research progress of several typical MOF materials for photocatalytic CO2 reduction in recent years.


**Table 1.** the research progress of several typical metal-organic frameworks (MOF) materials for photocatalyticCO2reduction.
