Metal–Organic Framework-Based Materials for Photocatalytic Nitrogen Fixation: History
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Contributor:
Linkun Fan
,
Qin Yu
,
Jiazhen Chen
,
Usman Khan
,
Xusheng Wang
,
Junkuo Gao
,
LinKun Fan

Metal–organic frameworks (MOFs) are coordination polymers with high porosity that are constructed from molecular engineering. Constructing MOFs as photocatalysts for the reduction of nitrogen to ammonia is a newly emerging but fast-growing field, owing to MOFs’ large pore volumes, adjustable pore sizes, controllable structures, wide light harvesting ranges, and high densities of exposed catalytic sites. They are also growing in popularity because of the pristine MOFs that can easily be transformed into advanced composites and derivatives, with enhanced catalytic performance.

  • Haber–Bosch process
  • solar energy
  • photocatalytic nitrogen fixation

1. Introduction

Nitrogen is a necessary element to sustain life. It is abundant in the Earth’s atmosphere in the form of virtually inert dinitrogen (N2) gas that most organisms cannot metabolize [1]; instead, “fixed” forms such as ammonia (NH3) can be metabolized. Before the widespread use of the Haber–Bosch process in the 1950s, fixed nitrogen was mainly derived from natural nitrogen fixation that involved geochemical processes such as lightning, and biological processes that used nitrogenase [2]. The Haber–Bosch process is a traditional and primary means for artificial nitrogen fixation, which converts N2 and H2 to NH3 in the presence of a catalyst (Equation (1)). Now, nitrogen fixation via the Haber–Bosch process has exceeded natural nitrogen fixation. Nearly half of the existing human population will not exist without nitrogen fertilizers from the Haber–Bosch process. However, the Haber–Bosch process requires the expensive raw material hydrogen, intensive energy, and harsh conditions, including high temperatures (400–500 °C) and pressures (20–50 MPa). Therefore, finding a mild and green artificial nitrogen fixation method to replace or replenish the Haber–Bosch process is urgent.
N2(g) + 3H2(g) → 2NH3 (g) 
In recent years, photocatalysis technologies have been widely used in various fields [3][4][5][6][7][8][9][10]. With the development of photocatalysts, photocatalysis technology also makes the production of ammonia in mild conditions possible [11]. Currently, several important homogeneous and heterogeneous photocatalysts for nitrogen fixation are available, such as Mo complex [12][13], Fe complex [14], TiO2 [15][16][17][18][19], ZnO [20], La2TiO5 [21], KNbO3 [22], Ti-zeolite [23], BiOBr [24][25], Fe2O3 [26], and g-C3N4 [27]. However, classical organometallic complexes (homogeneous catalysts) have not found practical application due to their low yields (around 10 equivalents of ammonia per equivalent of the catalyst), rapid catalyst deactivation, and the high costs of those catalysts. Thus, homogeneous catalysts for ammonia synthesis are currently the most useful for understanding nitrogenase. Although the stability has dramatically increased for such heterogeneous catalysts, their catalytic performance is still unsatisfactory, especially in the visible light region; most of those photocatalysts suffer from a narrow light absorption range, rapid photogenerated electron–hole combination, and a lack of rich catalytic sites.
Metal–organic frameworks (MOFs) are constructed by inorganic metal ions/metal clusters and multidentate organic ligands [28]. Since MOF-5, a highly porous crystalline material constituted by terephthalic acid (BDC) ligands and Zn4O clusters, was reported in Nature by Yaghi in 1999, MOFs began to develop rapidly [29]. The next two decades witnessed intensive efforts by numerous researchers to reveal new structures and applications of MOFs [30][31][32][33][34][35][36][37][38][39]. The naming of MOFs also has certain rules, mainly in the following four aspects: material composition (i.e., metal–organic framework, abbreviated as MOF) [29], structure (i.e., zeolitic imidazolate framework, abbreviated as ZIF) [40], function (i.e., multivariate metal–organic framework, abbreviated as MTV-MOF) [41], and the name of the laboratory or university (i.e., Materials of Institute Lavoisier, abbreviated as MIL) [33]. This naming method was subsequently adopted by most researchers [42][43][44][45][46]. Owing to their specific structural features such as large specific surface areas, high porosity, well-defined crystallinity, and increased numbers of active sites, MOFs have been widely used in gas adsorption and separation [47][48], fluorescence [49], sensing [44][45][50][51][52], ion conductivity [53], optoelectronics [54], thermal catalysis [55][56][57][58][59], electrocatalysis [60][61], photocatalysis [41][62][63][64], and so on. As photocatalysts, MOFs have many advantages: (i) the large specific surface area and highly ordered pore structure contribute to the mass transfer of reactants; (ii) the adjustable ligands make MOFs possess the ability to harvest light in a wide range; (iii) introducing defects into MOFs can expose more active sites, enhancing their nitrogen fixation activity [65][66][67][68]. Thus far, compared with photocatalytic hydrogen production and photocatalytic carbon dioxide (CO2) reduction, the study of photocatalytic nitrogen fixation by MOFs is still in its initial stages of development.
The recent achievements in state-of-the-art MOF-based materials for photocatalytic nitrogen fixation have been further summarized. Finally, the summary and perspectives of MOF-based materials for photocatalytic nitrogen fixation were presented (Figure 1).
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Figure 1. MOF-based materials for photocatalytic nitrogen fixation.

2. MOF-Based Materials for Photocatalytic Nitrogen Fixation

Due to their large pore volumes, adjustable pore sizes, wide light harvesting range, and high densities of exposed catalytic sites, MOFs have been explored for photocatalytic nitrogen fixation. However, it is still in its preliminary stages of development. In addition to pure MOFs, advanced MOF composites and MOF derivatives have also been studied for photocatalytic nitrogen fixation (Table 1).
Table 1. Photocatalytic nitrogen fixation by MOF-based materials.
Photocatalyst Light Source Sacrificial Agents NH3 Yield AQY/% Ref.
Pure MOFs
NH2-MIL-125 (Ti) Xe Lamp (300 W, L40) None 12.25 μmolg−1 h−1 0.26 (400 nm) [69]
OH-MIL-125 (Ti) Xe Lamp (300 W, L40) None 5.04 μmolg−1 h−1 / [69]
CH3-MIL-125 (Ti) Xe Lamp (300 W, L40) None 1.39 μmolg−1 h−1 / [69]
UiO-66-UV-Vis Xe Lamp (300 W, UV-vis) None 256.60 μmolg−1 h−1 / [70]
UiO-66(SH)2-200 Xe Lamp (300 W, L40) None 32.40 μmolg−1 h−1 0.45 (420 nm) [71]
MIL-101(Fe) Xe Lamp (300 W, full-spectrum) None 50.36 μmolg−1 h−1 / [72]
MIL-100(Fe) Xe Lamp (300 W, full-spectrum) None 46.53 μmolg−1 h−1 / [72]
MIL-88(Fe) Xe Lamp (300 W, full-spectrum) None 40.04 μmolg−1 h−1 / [72]
MIL-53(FeII/FeIII)-0.1 Xe Lamp (300 W, L42) K2SO3 306.00 μmolg−1 h−1 0.12 (420 nm) [73]
Al–PMOF(Fe) Xe Lamp (100 mWcm−2, L42) CH3OH 7.06 μmolg−1 h−1 / [74]
MOF-76(Ce) Xe Lamp (300 W, full-spectrum) None 34.20 μmolg−1 h−1 / [75]
Gd-IHEP-8 Xe Lamp (300 W, AM 1.5 G filter) None 220.00 μmolg−1 h−1 2.25 (365 nm) [76]
Gd-IHEP-7 Xe Lamp (300 W, AM 1.5 G filter) None 128.00 μmolg−1 h−1 1.72 (365 nm) [76]
U(0.5Hf) Xe Lamp (300 W, full-spectrum) K2SO3 351.80 μmolg−1 h−1 0.1 (420 nm) [77]
U(0.5Hf)-2SH Xe Lamp (300 W, L42) K2SO3 116.10 μmolg−1 h−1 0.55 (420 nm) [77]
NU6(Ce–Hf) Xe Lamp (300 W, full-spectrum) K2SO3 158.4 μmolg−1 h−1 0.65 (380 nm) [78]
MOF composites
ZIF-67@PMO12 Xe Lamp (300 W, full-spectrum) C2H5OH 39.40 μmolg−1 h−1 / [79]
ZIF-67@PMO11V Xe Lamp (300 W, full-spectrum) C2H5OH 70.00 μmolg−1 h−1 / [79]
ZIF-67@PMO10V2 Xe Lamp (300 W, full-spectrum) C2H5OH 74.80 μmolg−1 h−1 / [79]
ZIF-67@PMO9V3 Xe Lamp (300 W, full-spectrum) C2H5OH 134.60 μmolg−1 h−1 / [79]
ZIF-67@PMO4V8 Xe Lamp (300 W, full-spectrum) C2H5OH 149.00 μmolg−1 h−1 / [79]
Au@UiO-66 Xe Lamp (300 W, L42) None 18.90 μmolg−1 h−1 1.54 (520 nm) [80]
MOF-74@C3N4 Xe Lamp (300 W, L40) CH3OH 330.00 μmolg−1 h−1 / [81]
MIL-125@TiO2 Xe Lamp (300 W, 200 mWcm−2) None 102.70 μmolg−1 h−1 / [82]
GSCe (Graphene@Ce-UiO-66) LED (6 W, 365 nm) None 110.24 μmolg−1 h−1 9.25 (365 nm) [83]
9MX-MOF Xe Lamp (300 W, full-spectrum) Na2SO3 88.79 μmolg−1 h−1 / [84]
MOF derivatives
Ru–In2O3 HPNs Xe Lamp (300 W, AM 1.5 G filter) CH3OH 44.50 μmolg−1 h−1 / [85]

2.1. Pristine MOFs

2.1.1. Transition Metal-Based MOFs

The unoccupied and occupied d-orbitals in some transition metals-based MOFs have appropriate energy and symmetry, which makes these MOFs effectively adsorb and activate N2. The empty d orbital of open metal sites in MOFs can accept the electrons from the occupied σ orbital of N2. At the same time, the occupied d orbitals of open metal sites donate electrons to the empty π* orbital of N2. The back donated bonds not only weaken N≡N, but also strengthen the metal-nitrogen bond [86], making transition metals-based MOFs for photocatalytic nitrogen fixation possible.

Cerium-Based MOFs

Cerium, with its electron configuration of [Xe]4f26s2, exhibits flexible valence transformation behavior between Ce3+ and Ce4+, with occupied 4f1 and unoccupied 4f0 orbitals, respectively. On this basis, the cerium with empty orbits and filled orbitals can mimic π back donation for further catalysis [87].
Recently, Zhang et al. applied MOF-76(Ce) to effectively transform N2 into NH3, mimicking π back donation [75]. MOF-76(Ce) nanorods with Ce coordinate unsaturated sites (Ce-CUS) were prepared using the solvothermal method (Figure 2a). Under full-spectrum light source irradiation, the photogenerated electrons were first transferred to Ce-CUS sites, then to the π antibonding orbital of adsorbed N2 molecules on it. The electrons entering the π antibonding orbital would weaken the N≡N bond, significantly improving photocatalytic nitrogen fixation efficiency. The photocatalytic nitrogen fixation stability of MOF-76(Ce) is comparable to CeO2 (Figure 2b). Moreover, due to the synergy between Ce-CUS and N2, the photocatalytic nitrogen fixation activity of MOF-76(Ce) is higher than CeO2, with an activity of 34.2 μmolg−1 h−1. It should be noted that this is the first kind of study that involved using MOFs for nitrogen fixation.
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Figure 2. (a) Synthesis diagram of MOFs-76(Ce). (b) MOFs-76(Ce) stability test of photocatalytic nitrogen fixation. Reproduced with permission [75]. Copyright 2019, American Chemical Society.

Titanium Metal-Based MOFs

Ti-based MOFs have been widely explored in photocatalytic hydrogen production [88][89][90], photocatalytic CO2 reduction [91][92], and environmental protection; however, photocatalytic nitrogen fixation is still in its preliminary stages of development.
Recently, the group initially synthesized three functional group-decorated isostructural MOFs (NH2-MIL-125 (Ti), OH-MIL-125 (Ti), and CH3-MIL-125 (Ti)) for visible-light-driven photocatalytic nitrogen fixation [69]. The introduced functional groups effectively enlarged the light-harvesting range of MIL-125 (Ti) from the ultraviolet (UV) region to the visible light region. After being exposed to visible light (400–800 nm) for about 15 h, the produced ammonia of NH2-MIL-125 (Ti), CH3-MIL-125 (Ti), and OH-MIL-125 (Ti) reached 183.76 μmolg−1, 20.88 μmolg−1, and 75.48 μmolg−1, respectively. It should be noted that no sacrificial agent participated in the photocatalytic nitrogen fixation process for those MOFs. The reason for the highest nitrogen fixation rate of NH2-MIL-125 (Ti) can be explained by the introduction of NH2, which significantly increased light absorption to 550 nm, and by the exposed Ti coordinational unsaturated sites induced by a linker defect. The photocatalysis mechanism can be illustrated as follows: under visible light irradiation, photogenerated electrons in organic ligands are transferred to exposed Ti coordinational unsaturated sites and reduce Ti4+ to Ti3+; then, the electron in Ti3+ is further transferred to the π anti-bond of N2 to weaken the strong N≡N bond, eventually reducing the N2 to NH3.

Zirconium-Based MOFs

Defective UiO-66(Zr) series MOFs reflect excellent application prospects in photocatalytic hydrogen production, pollutant degradation, and absorption, which result from their large surface areas and suitable pore structures [93][94][95].
Recently, UiO-66-UV-vis with linker defects induced by light was published by Gao et al. [70] to improve photocatalytic nitrogen fixation activity. In order to improve the increase in photocatalytic nitrogen fixation activity that was attributed to linker defect but not cluster defect, three different types of UiO-66, UiO-66-fresh, UiO-66-UV-vis, and UiO-66-PSE, were studied for photocatalytic nitrogen fixation. UiO-66-fresh, UiO-66-UV-vis, and UiO-66-PSE stand for fresh prepared UiO-66, UV-vis light treated UiO-66, and defect repaired UiO-66 through a post-synthetic ligand exchange process (PSE), respectively. Compared with UiO-66-fresh, the coordinated formic acid and acetic acid were removed in UiO-66-UV-vis after exposure to ultraviolet light. The exposed coordination of unsaturated metal sites greatly enhanced the activity of UiO-66-UV-vis up to 256.6 μmolg−1 h−1. The coordination unsaturated Zr node on UiO-66 can inject photogenerated electrons into the antibonding π-orbitals of N2 to promote the activation and dissociation of N2. In contrast, the exposed coordination unsaturated metal sites were recovered by the terephthalic acid linker in UiO-66-PSE, which resulted in lower photocatalytic activity, even lower than that of UiO-66-fresh.
Guo et al. also introduced defect Zr-based MOFs created by thermal treatment, using UiO-66(SH)2-200 as the photocatalyst for the reduction of N2 [71]. The Zr clusters were dehydrated by thermal treatment, thus providing accessible [Zr6O6] sites for N2 adsorption and activation. The optical temperature was 200 °C. SH groups were also introduced into UiO-66 to improve the absorption edge of the light to visible light. UiO-66(SH)2-200 shows a photocatalytic nitrogen fixation activity up to 32.4 μmolg−1 h−1 under visible light. In-situ DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) revealed that the N2 molecule was gradually reduced to an NxHy intermediate and to NH3, finally [96]. DFT (density functional theory) calculations further revealed that the photoelectron initiates the reduction of the N2, immediately followed by the protonation of the activated N2.

Iron-Based MOFs

Fe is an essential transition metal in nitrogenase, which plays a vital role in photocatalytic nitrogen fixation. There have been many types of research on reducing N2 by utilizing the Fe active site [97][98][99][100]. When the Fe metal site is exposed for adsorbing N2, the unpaired electrons of the d orbital in Fe will transfer to the π antibonding orbital of the N2 molecule to form a strong Fe–N bond that weakens the N≡N bond.
Recently, in order to prove the importance of the transition metal Fe for photocatalytic nitrogen fixation [72], Li et al. studied the nitrogen fixation performance of MIL-101(Fe), MIL-100(Fe), MIL-88(Fe), and MIL-101(Cr). Notably, MIL-101(Fe) exhibited the highest photocatalytic nitrogen activity (50.36 μmolg−1 h−1), whereas isostructural MIL-101(Cr) had almost no activity. DFT calculations revealed that MIL-101(Fe) showed more electronic supply capacity, higher adsorption energy of N2, and a lower reaction barrier than MIL-101(Cr), confirming the important role Fe has during photocatalytic nitrogen fixation .
Owing to its unique multi-iron metallocluster (Fe2+3Fe3+4M3+, M = Mo, V, Fe), nitrogenase exhibits excellent nitrogen fixation activity. Recently, Zhao et al. synthesized a MIL-53(Fe2+/Fe3+) containing both Fe2+ and Fe3+ for photocatalytic nitrogen fixation [73]. In this MOF, Fe2+ and Fe3+ simulated Fe2+ activity sites and high valence metal ions (M = Mo, V, Fe) in nitrogenase. The Fe3+ in MIL-53(Fe2+/Fe3+) can be partly reduced into Fe2+ by ethylene glycol (EG), and the Fe2+/Fe3+ ratio can be regulated from 0.18:1 to 1.21:1 by changing the EG content. Notably, when the ratio of Fe2+/Fe3+ was 1.06:1, photocatalytic nitrogen fixation activity reached its highest value of 306 μmolh−1 g−1.
The proposed mechanism for photocatalytic nitrogen fixation by MIL-53(Fe2+/Fe3+) can be divided into six steps. (i) N2 adsorbs onto Fe2+ active sites of MOF, in which the electrons transfer to N2 to form the Fe3+-azo intermediate. (ii) Then, the hydrogen transfer quickly occurs in the Fe3+-azo intermediate. (iii) In the photo-excitation stage, holes transfer to the Fe–O− radical, and Fe3+ reduces to Fe2+ by electrons. (iv) The sacrificial agent K2SO3 eliminates the holes, and Fe2+-hydrazo transforms into Fe3+-hydrazo, with H2N–NH2 species appearing. (v) Fe3+-hydrazo transfers to Fe3+-amine with further hydrogenation. (vi) Finally, the NH3 releases gradually, benefitting from continuous hydrogenation.

Transitional Bimetallic MOFs

Similarly to MIL-53(Fe2+/Fe3+), An et al. developed bimetallic MOFs containing Zr and Hf for photocatalytic nitrogen fixation. Zr simulates the Fe2+ active site in nitrogenase, acting as the active site through the π antibonding mechanism. Hf imitates the high valence metal in nitrogenase to promote electron transfer and utilization. An SH group was also introduced to extend the absorption edge to the visible light region [77]. As the SH group expands the absorption spectrum to 502 nm and the synergistic effect of Zr and Hf, U(0.5Hf)-2SH with 50% Hf exhibits extremely superior photocatalytic nitrogen fixation activity (116.1 μmolh−1 g−1) under visible light. The constructed ligand-to-metal-to-metal electron transfer (LMMET) pathway in MOFs promotes the transferring and utilization of photogenerated electrons. Under visible light irradiation, the electrons move from the highest occupied molecular orbit (HOMO) to the lowest occupied molecular orbit (LOMO). At the same time, the holes are consumed by the sacrificial agent K2SO3, the electrons are transferred to the multi-metal clusters, and finally the N2 is reduced to NH3. Except for zirconium- and hafnium-based MOFs, An et al. further synthesized a stable amino-functionalized UiO-66 with bimetallic Ce-Hf nodes for photocatalytic nitrogen fixation [78]. In this MOF, the introduced NH2 group expands the absorption edge, the Ce species acts as an electron buffer tank to enhance electron transfer, and the Hf species plays the part of active catalytic sites to improve the selectivity of the nitrogen fixation reaction. When the molar ratio of Ce-Hf is 1:1, the nitrogen fixation activity was the highest (158.4 μmolh−1 g−1) under visible light, with K2SO3 as the sacrificial agent.

2.1.2. Post-Transition Metal-Based MOFs

Compared with transition metal-based MOFs, the post-transition metal-based variations are rarely reported to reduce nitrogen to ammonia as photocatalysts.

Gadolinium-Based MOFs

Hu et al. developed two viologen-based radical-containing metal–organic frameworks, Gd-IHEP-7 and Gd-IHEP-8 [76]. A single-crystal-to-single-crystal (SCSC) transformation occurred from two-dimensional (2D) Gd-IHEP-7 to three-dimensional (3D) Gd-IHEP-8 when heating the Gd-IHEP-7 in the air at 120 °C. With a rearrangement of the Gd3+ coordination environment, enhanced photocatalytic nitrogen fixation activity emerged with the SCSC transformation. Chemisorption of N2 onto the catalytic sites is a pre-condition for photocatalytic nitrogen fixation. Both the adsorbed N2 on active Gd metal sites for Gd-IHEP-7 and Gd-IHEP-8 were activated, evidenced by the elongated N–N bond length and the shortened Ga–N bond length. Compared with the distal (D) route, the alternative (A) route is more favorable for both Gd-IHEP-7 and Gd-IHEP-8. While even theoretical calculations indicate that similar photocatalytic nitrogen fixation pathways exist for both RMOF (Gd-IHEP-7, Gd-IHEP-8), the intermediates for Gd-IHEP-8 showed better stability, resulting in a better nitrogen fixation activity of 220 µmolh−1 g−1.

Aluminium-Based MOFs

The post-transition metals can not only function as active sites as in Gd-IHEP-8, but they also act as metal nodes that impart high framework stability to MOFs. Recently, Shang et al. developed two porphyrin-based metal–organic frameworks for photocatalytic nitrogen fixation, named Al-PMOF and Al-PMOF(Fe). Compared with Al-PMOF, Al-PMOF(Fe) not only has Al as the metal node to stabilize the framework of the MOF, but also has Fe as the active center to adsorb and reduce the N2 to NH3 [74]. The structure of Al-PMOF(Fe) is shown in Figure 3a. The atomically isolated Fe in Al-PMOF(Fe) was proven through X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). Photocatalytic nitrogen fixation experiments showed that Al-PMOF(Fe) had better activity than Al-PMOF under visible light, and that the produced NH3 originated from N2. It was confirmed that the addition of Fe as the active center effectively increased the adsorption of N2 and further enhanced the photocatalytic performance (Figure 3b,c). DFT calculations were further applied to establish the photocatalytic nitrogen fixation reaction pathway (Figure 3d–f). The first hydrogenation from N2* to N2H* showed no obvious difference between alternating and distal pathways, but further hydrogenation showed that the alternating pathway seemed more likely to occur. Notably, the release of 1NH3 in the distal process requires significant energy, making the reaction difficult.
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Figure 3. (a) The structure of Al-PMOF(Fe). (b) N2 adsorption isotherms. (c) Photocatalytic nitrogen fixation activity of Al-PMOF(Fe) and Al-PMOF. (d) Charge different maps for Al-PMOF(Fe) adsorbing N2. (e) Alternating and (f) distal pathways in Al-PMOF(Fe). Reproduced with permission [74]. Copyright 2021, American Chemical Society.
MOFs can also function as photosensitizers and transfer the photogenerated electrons to active sites in other functional materials that are in contact with them. For example, Ding et al. combined N-defect thin film g-C3N4 (DF-C3N4) with nano MOF to form Nano MOF-74@DF-C3N4 composite for enhanced photocatalytic nitrogen reduction. A Z-type heterojunction was formed between Nano-MOF-74 and DF-C3N4, which clearly improved the separation efficiency of photogenerated electrons and holes. The Z-type heterojunction photocatalyst structure first proposed by Bard et al. in 1979, can provide the entire system with a stronger redox ability to promote photocatalytic activity. It should be noted that another carrier transfer process of type II is competitive with the Z-type process, and the fluorescence lifetime results indicated that the Z-type is the main process. In this Z-type carrier transfer process, the photogenerated electrons in the conduction band of MOFs are firstly transferred to the valence band of DF-C3N4, then become re-excited to the conduction band of DF-C3N4, and finally anticipate the reduction of N2 adsorbed on N defect sites of DF-C3N4.
Another similar example is the MOFs composite MIL-125@TiO2 with core-shell structure synthesized by Wang et al., using the post thermal solvent method (Figure 10a) [95]. The thickness of TiO2 nanosheets can be controlled by the reaction time with thioacetamide. Under the synergistic effect of MIL-125 and TiO2, MIL-125@TiO2-2 h exhibited an activity up to 89.5 µmolg−1 h−1 under visible light, which was much higher than the pristine MIL-125 (Figure 10b). In MIL-125@TiO2, photogenerated electrons can be transferred to both the coordination unsaturated Ti sites of defect MIL-125 and TiO2, and further reduce the adsorbed N2 to NH3.
MXene is a new type of two-dimensional material with a graphene-like structure. It has the advantages of a large specific surface area, excellent conductivity, and abundant surface groups. Qin et al. synthesized MXene/MOF composites (Ti3C2-QD/Ni-MOF, formed as type II heterojunction; QD stands for quantum dot) by combining MXene QDs and Ni-MOF through self-assembly, which improved the photocatalytic nitrogen fixation efficiency to 88.79 μmol g−1 h−1 [97]. Under light irrigation, the electrons and holes in Ti3C2-QD were separated. Then, the photo-excited electrons were transferred to the conduction band (CB) of Ni-MOF to participate in the photocatalytic nitrogen fixation reaction. In this system, the heterojunction formed by Ti3C2-QD and Ni-MOF effectively promotes the separation of electrons and holes. Ni effectively adsorbed N2 as an active site, and promoted the nitrogen fixation process.
MOFs can also function as porous scaffolds to support functional units, and enhance the mass transfer of N2 to produce NH3. For example, Chen et al. used the UiO-66 membrane as a nanoreactor to support gold nanoparticles (AuNPs) [93] and realized a direct plasma photocatalytic nitrogen reduction reaction at room temperature and ordinary pressure. Notably, in the Au@UiO-66, UiO-66 not only effectively restricts the highly dispersed AuNPs but also ensures effective contact between AuNPs and N2 molecules in the aqueous solvent. In the gas film solution reaction interface, the N2 molecule can proceed straight into the Au@UiO-66 membrane. Each AuNP in UiO-66 could not only generate electrons through photo-excitation, but also promote the reduction of N2 as a co-catalyst. (Figure 4a). The mass loading of gold was the critical factor that affected photocatalytic nitrogen fixation. When the loading amount of Au is 1.9 wt %, the conversion of nitrogen to ammonia reaches its highest level of 0.14 mmol g−1 h−1 (Figure 4b).
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Figure 4. (a) Schematic diagram of direct plasma photocatalytic nitrogen reduction on AuNP encapsulated by UiO-66 matrix. (b) Direct plasma photocatalytic nitrogen reduction performances in the PiS system. Reproduced with permission [93]. Copyright 2021, American Chemical Society.
The introduced functional materials into MOFs can increase the photocatalytic nitrogen fixation activity and enhance the stability of composite materials. For example. Pan et al. reported on a graphene-embedded Ce-based UiO-66 photocatalyst (GSCe) [96]. With ultraviolet light irradiation, a breakage of benzene-C bonds inside Ce-UiO-66 formed active sites, which was evidenced by C K-edge X-ray absorption near its edge structure (reduced of C 1s → σ* at 292.0 eV). Even if the introduced graphene did not have any active sites, it could help control the activation process and hence stabilize the entire photocatalyst structure. With a graphene ratio of 0.35, GSCe reached its best performance, with a rate and apparent quantum efficiency (AQE) of 110.24 μmolL−1 h−1 and 9.25%, respectively. The stability of GSCe even reached 7 × 24 h.

4.3. MOF Derivatives

MOFs have been proposed as a potential precursor for synthesizing nanostructured materials with porosity [73,114]. After pyrolysis in different atmospheres, MOFs can be transformed into derivatives, including metal oxides, metal carbides, metal nitrides, carbon nanostructures, etc. Those MOF derivatives can be used for electrocatalysis [115], sensing [116,117], electrode materials [118], photocatalysis [119], etc.
As an example of photocatalytic nitrogen fixation, Vu et al. synthesized an In-based hollow peanut-like photocatalyst (Ru-In2O3 HPNs) through pyrolysis of MIL-68-In(Ru) precursors in the air [98]. Under solar simulator (AM 1.5 G filter) irradiation, the nitrogen fixation activity of Ru-In2O3 HPNs reached 44.5 μmolg−1 h−1. This was the result of the synergistic effects of hollow structure and Ru. Under the light, electrons and holes were separated. The electrons in the conduction band were immediately captured by Ru and oxygen vacancies after migrating to In2O3, which activated and weakened N≡N, and finally reduced N2 to NH3.

5. Summary and Future Perspective

The photocatalytic fixation of nitrogen to ammonia is a significant challenge, but it is a potential technology for green ammonia production. MOF-based materials, including pristine MOFs, MOF composites, and MOF derivatives, are excellent candidates for photocatalytic nitrogen fixation. This comprehensive review highlights recent advances in applying MOF-based materials toward photocatalytic nitrogen fixation. First, this review introduced the determination method of produced ammonia. Then, the synthetic methods of MOF-based materials were summarized. Finally, the recent achievements in MOF-based materials for photocatalytic nitrogen fixation were systematically introduced. Despite considerable progress in MOF-based materials toward photocatalysis with nitrogen fixation being achieved, many challenges remain.
Improving Reliability of NH3 Production. Before or during photocatalytic nitrogen fixation, the equipment, nitrogen sources, catalysts, solvents, etc., may be polluted. Therefore, in order to avoid unreliable conclusions, the following control experiments should be conducted: using inert Ar as a gas source, conducting the experiments under dark conditions, replacing protonic solvents (such as water) with aprotic solvents (such as DMF, DMSO), and conducting 15N2 isotope labeling experiments. Furthermore, different ammonia detection methods have their advantages and limitations; therefore, suitable detection methods should be applied in different reaction systems, and several methods should be used to detect ammonia accurately.
Reporting data in photocatalytic systems accurately. Typically, catalytic rates are expressed in terms of the amount of product divided by the amount of catalyst per unit of time. Nevertheless, this parameter can be easily manipulated to the authors’ convenience, especially in multicomponent systems where only a fraction of the catalyst represents the genuine active site. For this reason, other figures of merit such as apparent quantum yield (AQY) are also useful when comparing activities between different photocatalysts. The combination of rates plus AQY seems to be a more robust way to report catalytic performances and establish fair comparisons between different materials [120].
Developing efficient photocatalysts. The coordination of unsaturated sites in MOFs can effectively improve N2 adsorption/activation, which can be created with the defects generated. Therefore, creating abundant defects in MOFs while maintaining their highly porous structure are required for highly efficient photocatalytic nitrogen fixation. Moreover, broadening the light-harvesting range of MOFs can also increase the photocatalytic nitrogen fixation efficiency. Introducing light-absorbing functional groups into linkers such as amino groups, sulfhydryl groups, etc., or selecting strong photoactive ligands as linkers, such as porphyrin derivatives, phthalocyanine derivatives, naphthalene derivatives, etc., can greatly broaden the spectral responses of MOFs, thereby enhancing their light absorption efficiency. Constructing reasonable structures of MOFs to enhance the photogenerated electron and hole separation and avoid recombination can also dramatically enhance their photocatalytic nitrogen fixation efficiency. Finally, a rational combination with other functional materials (graphene, g-C3N4, MXene, etc.) to form advanced MOF composites, or as precursors to prepare MOF-derived materials ranging from carbon-based materials to metal-based materials, is another option to enhance their photocatalytic nitrogen fixation efficiency.
Discovering photocatalytic mechanisms using in-situ characterization techniques. Traditional characterization techniques can only reflect the state of the catalyst before and after the reaction. However, the catalytic reaction requires both a steady state and a transient state. Therefore, in-situ characterization techniques have become indispensable in catalytic reactions. Compared with traditional characterization techniques, in-situ techniques are much easier to use in capturing the complex intermediates with short lifetimes to reveal the real active sites. In-situ characterization techniques, such as in-situ extended X-ray absorption fine structure (EXAFS), infrared absorption spectra, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), thermal gravimetric analysis (TGA), XPS, Raman spectroscopy, etc., can greatly deepen our understanding of the reaction mechanisms, and further guide us to design and synthesize efficient photocatalysts. Although part of those in-situ techniques had been applied to study the photocatalytic nitrogen fixation by MOF-based materials, the research is still in its early stages.
Delving into the photocatalytic mechanism by theoretical calculation. Theoretical calculations and experiments always coexist and promote each other. MOFs with predicated structures are ideal platforms for constructing models used for theoretical calculations. With the help of theoretical calculations, important information including the electronic structures of materials, densities of states, adsorption energies of N2, reaction pathways of N2 and H2O, changes in bond lengths and angles, etc. can be obtained to enhance our understanding of the specific relationships between the structures of photocatalysts and their corresponding performances, guiding highly efficient photocatalyst design and promoting development of the photocatalytic nitrogen fixation field.
Although MOF-based materials for photocatalytic nitrogen fixation are still in their initial stages of development, it is a rapidly growing field owing to their remarkable advantages. We hope this comprehensive review will contribute to further developments in MOF-based materials for nitrogen fixation that benefit researchers who are interested or involved in this field.

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This entry is adapted from the peer-reviewed paper 10.3390/catal12091005

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