Design of Metal-Organic Frameworks Based Photocatalysts and Their Application in Uranium Reduction

Ying Peng; Xiaowen Zhang; Mi Li; Yu Zhang; Yilong Hua; Xiaoyan Wu

doi:10.7536/PC20251108

Progress in Chemistry ›› 2026, Vol. 38 ›› Issue (5) : 832-848 DOI: 10.7536/PC20251108

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Design of Metal-Organic Frameworks Based Photocatalysts and Their Application in Uranium Reduction

Ying Peng ¹
, Xiaowen Zhang ¹^,²^,^*
, Mi Li ¹^,²
, Yu Zhang ¹^,²
, Yilong Hua ¹^,²
, Xiaoyan Wu ¹^,²

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Abstract

The photocatalytic reduction of uranium can utilize sunlight， and convert soluble U（VI） into recyclable solid substances， thereby solving the two major problems of radioactive wastewater treatment and seawater uranium extraction. Metal-organic frameworks （MOFs）， a new type of functional porous three-dimensional （3D） crystalline materials， have large specific surface area， special metal clusters， designable organic ligands， adjustable unique morphology and pore structure， hence attracted extensive attention in the fields of photocatalysis and photochemistry. This review summarizes the design strategies for optimizing the performance of MOFs-based photocatalysts， probes into the mechanism of photocatalytic U（VI） reduction by MOFs， and elaborates on the application of MOFs in photocatalytic U（VI） reduction. Finally， the current challenges of MOFs photocatalysts in the application of U（VI） reduction are discussed， and a forward-looking perspective on the future research directions are proposed.

Contents

1 Introduction

2 Design of MOFs-based photocatalysts

2.1 Optimization of MOFs structure

2.2 Synthesis of MOFs composites

2.3 Synthesis of MOFs derivatives

3 Application of MOFs-based photocatalysts in U（Ⅵ） reduction

3.1 Mechanism of photocatalytic U（Ⅵ） reduction

3.2 Design directions of MOFs-based photocatalysts

3.3 Performances of different MOFs-based photo-catalysts for U（Ⅵ） reduction

4 Conclusion and outlook

Graphical abstract

Keywords

metal-organic frameworks / photocatalysis / uranium reduction / mechanism

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Ying Peng, Xiaowen Zhang, Mi Li, Yu Zhang, Yilong Hua, Xiaoyan Wu. Design of Metal-Organic Frameworks Based Photocatalysts and Their Application in Uranium Reduction. Progress in Chemistry, 2026, 38(5): 832-848 DOI:10.7536/PC20251108

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1 Introduction

Nuclear energy, characterized by its high energy density, small carbon footprint, and environmental friendliness, is a crucial green energy source for achieving the "dual carbon" goals and an inevitable future energy source that humanity must prioritize in its development.^[1]. With the orderly development of nuclear energy, the quantities of radioactive waste gas, wastewater, and solid waste are increasing, particularly radioactive wastewater. Uranium, as the most important element in the nuclear industry, is a major component of radioactive wastewater, typically existing in the form of hexavalent uranyl ions (UO₂²⁺). UO₂²⁺ has a long half-life (T_1/2=4.5×10⁹ years) and very high water solubility, posing serious threats to human health (such as kidney damage) and the environment through surface water and groundwater.^[2].

Currently, common methods for treating uranium-containing wastewater include chemical precipitation, evaporation concentration, adsorption, ion exchange, and membrane separation. Chemical precipitation generates large amounts of radioactive solid waste, evaporation consumption consumes significant energy, and adsorption requires substantial quantities of adsorbents. Compared with these traditional removal methods, converting soluble U(VI) into insoluble U(IV) represents a highly promising strategy for removing or recovering uranium from aqueous solutions. It has been demonstrated that during photocatalytic reduction processes, uranium(VI) in aqueous solutions can be effectively reduced and removed^[3-4], and exhibits excellent selectivity toward U(VI) even in the presence of various coexisting ions^[5]. Thus, the photocatalytic reduction method can directly utilize solar energy to drive the highly selective reduction of U(VI), producing solid uranium compounds that are convenient to recover. This provides a new technical strategy for the removal/extraction of uranium from radioactive wastewater and holds broad application prospects.

Metal-organic frameworks (MOFs) are porous crystalline three-dimensional solid materials constructed from central metal ions/clusters and multidentate organic ligands^[6-7]. Asshown in Figure 1, the metal clusters in the MOF structure act as independent semiconductor quantum dots, which can be directly excited by light or activated by organic ligands^[8-9]. Among them, MOFs containing variable-valence metal ions (such as Ti⁴⁺/Ti³⁺, Zr⁴⁺/Zr³⁺, Fe³⁺/Fe²⁺) possess relatively strong reducing capabilities, effectively driving photocatalytic reduction reactions. Secondly, the organic ligands in the MOF structure can absorb photons under solar irradiation at specific wavelengths, exciting electrons and transferring them to the positively charged metal ions bonded to them. This provides superior conditions for the development of MOF materials into semiconductor materials^[10]. Furthermore, the tunable pore structure can provide suitable channels and internal space for photocatalytic reactions, as well as an extremely large surface area^[11]. Compared with the commercialized photocatalyst TiO₂, MOF-based photocatalysts have significant advantages in specific surface area. For example, the Yaghi research group^[12] synthesized MOF-210 in 2010, whose specific surface area approached the extreme limit of solid materials, reaching up to 6240 m²/g. Therefore, by selecting metal catalytic centers with high-efficiency photocatalytic reduction activity, combining them with organic ligands exhibiting strong light absorption performance, and utilizing the porous characteristics of MOFs, various MOF catalysts with high catalytic efficiency, strong stability, high selectivity, or visible-light response can be designed and synthesized.

In recent years, MOFs have attracted significant attention due to their excellent photocatalytic properties and are widely applied in H₂ generation, CO₂ reduction, Cr(VI) reduction, and other research fields^[11,13-16]; however, MOF-based photocatalysts in the field of radioactive wastewater treatment are still in their infancy. This paper summarizes design strategies for enhancing the photocatalytic performance of MOFs through structural optimization, composite construction, and derivative synthesis. From multiple perspectives, including improving light utilization efficiency, reducing electron-hole recombination rates, and simplifying catalytic conditions, typical research achievements of MOF-based materials in the field of U(VI) photoreduction are listed. Finally, current challenges are proposed, and the development trends of MOF-based materials in U(VI) photoreduction are discussed.

2 Design of MOF-based photocatalysts

Traditional MOFs usually have some drawbacks, such as a narrow photoresponse range, few catalytic active sites, and a tendency for photogenerated carrier recombination during photocatalytic reduction.^[17]. To overcome these deficiencies, a series of MOF materials with high photocatalytic activity have been designed, including: (1) optimizing the MOF structure by refining the three elements of metal centers, organic ligands, and porosity to enhance the photoresponse of MOFs and broaden the photoresponse range for effective utilization of solar energy; (2) synthesizing MOF composites to promote the separation and migration of photogenerated carriers through synergistic catalysis and band structure adjustment; (3) generating MOF derivatives to improve the stability and catalytic activity of MOFs through morphological and structural functionalization.

2.1 Optimize MOF structure

2.1.1 Optimization of metal clusters

The optimization directions for metal nodes mainly focus on the types of metals, as well as coordination modes, bonding methods, and dimensional differences. In addition, metal doping within the nodes is also an effective approach to improve the performance of MOF photocatalysts.^[18-19].

First, the selection of the metal center significantly influences the photocatalytic capability of the designed MOF photocatalysts. The research group of Yuan Yupeng^[20]reacted different trivalent metal ions Fe³⁺, Al³⁺, and Cr³⁺with terephthalic acid to construct MOF materials MIL-53(M) (M = Fe, Al, or Cr) possessing identical framework structures. Ultraviolet-visible (UV-Vis) spectroscopy revealed that the absorption bands of MIL-53(Fe), MIL-53(Cr), and MIL-53(Al) were centered at 455, 395, and 320 nm, respectively, with correspondingE_gvalues of 2.72, 3.20, and 3.87 eV, indicating that MIL-53(M) with different metal centers exhibits varying responses to light.

The rapid realization of metal substitution (doping) and its structural diversity provide new insights for photocatalytic reduction based on MOF materials. Wang Siqi et al.^[21]partially substituted Fe for In to prepare bimetallic NH₂-MIL-68(In_αFe_1-_α) photocatalysts. The introduction of Fe provides an effective carrier transfer pathway for the intermetallic charge transfer process, thereby reducing the electron-hole recombination rate and making the charge separation and transport efficiency of NH₂-MIL-68(In_αFe_1-_α) catalysts significantly higher than that of the original NH₂-MIL-68(In). Wang et al.^[22]successfully prepared Ti-ion doped UiO-66 using a post-synthetic modification method. The introduction of titanium enhanced the optical properties of UiO-66 by forming oxygen-bridged heterometallic Zr-Ti clusters. Although the adsorption capacity of UiO-66(Ti) decreased due to the reduced specific surface area, compared to pure UiO-66, the photocatalytic degradation rate of methyl blue by UiO-66(Ti) increased from zero to 82.2%.

2.1.2 Optimization of organic ligands

The light absorption properties of organic ligands inherently exhibit differences; the band gaps of most MOFs are primarily determined by the ligands, and using organic ligands with narrow band gaps is one of the effective methods to obtain MOFs with excellent photoresponse.^[23]. There are three types of design strategies based on organic ligands that can be used to adjust the band gaps of MOFs, asFigure 2shows^[24]. Among them, the most effective and widely used method is modifying the ligands of existing MOFs with functional groups. For example, Shen et al.^[25]compared the effects of NH₂, NO₂, and Br groups on the photocatalytic activity of UiO-66-X. The results show that electron-donating functional groups such as —NH₂increase the electron density around the metal centers of MOFs, promoting the rapid separation and transfer of photogenerated carriers, thereby enhancing the photocatalytic reduction activity of the catalyst. Additionally, an increase in the conjugated system within the ligand also causes a red shift in the absorption peak of MOFs, thereby improving their catalytic performance under visible light.^[10]. The third common method is to graft other organic fragments (such as photosensitive organic groups) onto MOFs to replace the ligands via post-synthetic modification.^[23].

2.1.3 Regulation of crystal defects

Structural defects generate unsaturated sites around metal centers, providing additional pathways for the migration of photogenerated carriers, promoting their separation, and thereby enhancing the catalytic activity of MOFs^[26-27]. Defects in the MOF lattice can be generated by altering precursor synthesis parameters, synthesis temperature, reaction time, or by adding coordination modulators^[23-28]. Guo et al.^[29]synthesized a series of defective Zr-MOF-X (X = 160, 240, 320, 400, representing the molar ratio of acetic acid to ZrCl₄) using an acetic acid-mediated defect regulation method. Compared with the pristine defect-free Zr-MOF, Zr-MOF-X exhibited larger BET specific surface areas and pore volumes, even displaying mesoporous structures; the structural defects in Zr-MOF-X improved charge transfer, generated abundant Zr(III) catalytic active sites, and resulted in a slower decay process than that of defect-free Zr-MOF, thereby allowing long-lived Zr(III) species in defective Zr-MOF-X to be fully exposed to high concentrations of CO₂ environments, improving the photocatalytic efficiency of CO₂ reduction. Similarly, Zhao et al.^[30]effectively controlled organic linker defects by adjusting the amount of formic acid, obtaining UiO-67 with different CO₂ reduction capabilities, among which UiO-67-200 exhibited the strongest ability to reduce CO₂ to CO (12.29 μmol/(g·h)). Density functional theory and UV-Vis spectral analysis revealed that appropriate organic linker defects enhance the ligand-to-metal charge transfer process by promoting electron transfer between the highest occupied molecular orbital and the lowest unoccupied molecular orbital.

Most defect engineering studies for photocatalytic applications are based on Zr-based MOFs. However, Lázaro et al.^[31]conducted a comprehensive study on defect engineering of titanium heterometallic MOFs (MUV-10) using 5-hydroxyisophthalic acid (Iso-OH) and 5-fluoroisophthalic acid (Iso-F) as modulators, reasonably explaining the impact of material properties on their photocatalytic hydrogen production activity. The study shows that adding approximately 9% Iso-OH modulator generates about 40% defects, leading to a reduced bandgap and extended excited-state lifetime; in MOFs derived under Iso-F modulation, the higher photocatalytic activity is attributed to the influence of divergent defect compensation patterns on material photostability and the increase in surface area after introducing the Iso-F modulator.

2.2 Synthesis of MOF composites

2.2.1 Composite with precious metals

Three-dimensional MOFs possess abundant pore volume and regular, stable pore structures, facilitating the regulation of metal nanoparticle size, making them highly advantageous carriers for encapsulating metal nanoparticles. After noble metal nanoparticles such as Pt, Au, and Pb are introduced into MOF photocatalytic systems, they primarily enhance photocatalytic performance in the following three aspects: (1) utilizing the plasmonic effect of noble metals to improve the efficiency of MOF materials in harnessing visible light; (2) noble metals capturing photogenerated carriers from MOFs, thereby promoting electron-hole separation and suppressing their recombination; (3) noble metals increasing the active sites for photocatalysis.^[32].

Wu Ling Research Group^[33]Highly dispersed noble metal nanoparticles were immobilized on MOFs via a simple photodeposition process to synthesize M@MIL-100(Fe) (where M represents Au, Pd, or Pt) nanocomposites. The deposition of noble metals effectively improves the charge separation efficiency of MIL-100(Fe) under visible light irradiation, thereby enhancing the photocatalytic activity of M@MIL-100(Fe) for visible-light-driven photocatalytic redox reactions. In particular, Pt@MIL-100(Fe) with an average Pt particle size of 2 nm exhibits significantly enhanced photocatalytic activity, attributed to the combined effects of increased light absorption intensity and effective separation of photogenerated charge carriers. Further studies have found that Ag/UiO-66-NH₂ materials can also enhance the visible light absorption capability of MOFs and improve the separation efficiency of photogenerated carriers, thereby significantly improving the photocatalytic reduction capacity of the composites.^[34].

2.2.2 Forming a heterojunction

A heterojunction refers to a structure composed of two or more semiconductor materials with different band structures, lattice constants, and electronic properties. Compared to single semiconductor materials, heterojunction structures not only exhibit optimized band gaps and band configurations but also possess stronger light absorption capabilities, making them a new focus in the field of photocatalysis research. Heterojunctions can be classified into traditional heterojunctions, Z-scheme heterojunctions, the emerging S-scheme heterojunctions, and Schottky heterojunctions. Various types of heterojunctions have different advantages in photocatalysis; for example, the Type-II structure in traditional heterojunctions offers better e^--h⁺ separation efficiency and a wider light absorption range; in Z-scheme heterojunctions, oxidation and reduction sites are separated, facilitating the separation of photogenerated carriers; in the S-scheme heterojunction mode, powerful photogenerated e^- and h⁺ are retained in the conduction band of the reduction-type photocatalyst (RP) and the valence band of the oxidation-type photocatalyst (OP), respectively, while useless photogenerated carriers recombine, effectively promoting the separation of photogenerated carriers between OP and RP; the barrier of a Schottky heterojunction acts like a "city wall," ensuring unidirectional charge transfer at the metal-semiconductor interface, preventing the recombination of photogenerated carriers, and thereby promoting their spatial separation^[35].

Coupling low-cost metal oxide semiconductors with MOF frameworks is one of the common methods for synthesizing catalysts with heterogeneous structures. Rad et al.^[36]ZnO was compounded with MOF materials to prepare a ZnO@MOF-46 composite photocatalyst with a core-shell heterogeneous structure. Its excellent photocatalytic activity increased the methylene blue degradation efficiency from 12.6% (without catalyst), 41.5% (with pure ZnO), and 51% (with pure MOF-46) to 81%. TiO₂is a classic semiconductor catalyst that responds only to ultraviolet light. The research group of Liu Fuqiang^[37]added TiO₂to the MOF synthesis precursor solution and, using a simple solvothermal method, synthesized a heterogeneous structure (TMFe-x) composite with NH₂-MIL-88B(Fe), asFigure 3shows. This heterojunction enables efficient reduction of Cr(VI) driven by sunlight in neutral aqueous media; it also exhibits good stability, maintaining its photocatalytic performance after completing four reaction cycles. The S-TiO₂/UiO-66-NH₂composite synthesized by ball milling also exhibits excellent photocatalytic activity under natural sunlight, simultaneously achieving the reduction of Cr(VI) and the degradation of bisphenol A^[38].

Metal sulfides are a class of reduced semiconductor photocatalysts with narrow band gaps and negative conduction band potentials, capable of absorbing visible and infrared light, exhibiting excellent charge separation, strong photoreduction ability, and low redox energy barriers. However, single metal sulfides usually suffer from low quantum efficiency and photocorrosion problems^[39]. Wang et al.^[40]successfully synthesized In₂S₃@MIL-125(Ti) core-shell composite materials (abbreviated as MLS) using a solvothermal method and applied them to the removal of tetracycline from wastewater. The results showed that the photocatalytic degradation ability of MLS was significantly higher than that of In₂S₃and MIL-125(Ti) monomers. This is mainly because MIL-125(Ti) can provide high surface area and more open mesoporous channels, and the heterojunction composed of S—Ti—O bonds facilitates the transfer of photogenerated carriers. Composite materials CdS/MIL-125(Ti), Ag₂S/MIL-125(Ti), and CuS/MIL-125(Ti) synthesized by the photodeposition method showed that the introduction of sulfide quantum dots not only does not change the original structure and morphological characteristics of MIL-125(Ti), but also extends its light absorption range from the ultraviolet region to the visible region, improving the utilization rate of the solar spectrum^[41].

Two-dimensional materials possess an ultra-thin layered structure. Their unique electronic structure and physical properties endow them with excellent electron mobility and a wide band gap, enabling them to modulate the band gap of MOFs, thereby enhancing the catalytic performance of MOFs. Zhang et al.^[42]prepared a visible-light-responsive composite photocatalyst (GR/MIL-53(Fe)) by hybridizing MIL-53(Fe) with two-dimensional graphene (GR) via a solvothermal process. Under visible light irradiation, the GR/MIL-53(Fe)-H₂O₂system exhibited significantly higher photocatalytic degradation activity towards Rhodamine B (RhB) than the MIL-53(Fe)-H₂O₂system, completely decomposing RhB within 60 min with a removal rate reaching 100%. The band gap of g-C₃N₄is approximately 2.7 eV, allowing it to absorb blue-violet light with wavelengths shorter than 475 nm in the solar spectrum. Compositing it with MOFs not only significantly improves the photocatalytic performance of MOFs but also constructs new composite MOFs with high visible-light responsiveness. Guo et al.^[43]prepared a novel visible-light-driven catalyst, g-C₃N₄/MIL-53(Al), using a room-temperature stirring method. The study found that the g-C₃N₄(20% (mass fraction))/MIL-53(Al) photocatalyst exhibited optimal photocatalytic degradation efficiency, approximately 5 times higher than that of pure g-C₃N₄, which is mainly attributed to the efficient separation of electron-hole pairs and low charge transfer resistance. Meanwhile, it also demonstrated excellent stability and reusability. Similarly, g-C₃N₄/MIL-53(Fe), g-C₃N₄/MIL-125(Ti), and g-C₃N₄/UiO-66 heterojunction composites also exhibit excellent photocatalytic performance under visible light^[44-46]. MXenes are a series of two-dimensional transition metal carbides and nitrides with graphene-like structures, possessing excellent conductivity and catalytic performance^[47]. The research group of Li Xiaoming^[48]synthesized Ti₃C₂-modulated MIL-125-NH₂-based nano-hybrids with double heterojunctions via a one-step solvothermal strategy, as shown in Figure 4. Due to the electrostatic attraction between Ti⁴⁺ ions and the oxygen-containing groups on Ti₃C₂, Ti₃C₂formed a tight bond with NH₂-MIL-125(Ti); when the content of Ti₃C₂increased in the precursor solution, TiO₂formed on the surface of NH₂-MIL-125(Ti), and the formed TiO₂transformed from two-dimensional nanosheets into three-dimensional nanoparticles. The optimized NH₂-MIL-125(Ti)(TiO₂)/Ti₃C₂nano-hybrids showed a 1.65-fold increase in Hλ > 420 nm) and an 11.5-fold increase in the degradation efficiency of tetracycline hydrochloride (TC-HCl). Similar MOFs/MXene heterostructures, such as MXOF, UNiMOF/Ti₂O₂production under visible light irradiation (₃C₂, ZIF-8/Ti₃C₂T_x, UiO-66/MXene, Sn-Bi-MOF/Ti₃C₂, and MIL-88A(Fe)/Ti₃C₂, have all played a good photocatalytic role and been successfully applied to pollutant degradation^[49-54].

Covalent organic frameworks (COFs) are emerging porous crystalline materials with tunable composition, structure, and function, formed by polydentate organic building blocks connected via covalent bonds, offering high surface area. The unique π-conjugated structures and π-π stacked layers of COFs endow them with a broad visible-light absorption range and high charge separation efficiency; thus, heterostructures composed with MOFs exhibit enhanced photocatalytic capabilities. Xue et al.^[55]employed an in situ growth method to incorporate a two-dimensional conductive MOF ((Cu₃(HHTP)₂)) as a noble-metal-free cocatalyst into the covalent organic framework COF (Tp-Pa-1). Based on the coordination between Cu ions and the carbonyl oxygen and enamine nitrogen groups in Tp-Pa-1, the prepared MOF/COF composite catalyst exhibited strong interactions and excellent stability. The optimized Cu₃(HHTP)₂: Tp-Pa-1 (with a ratio of 1:15) photocatalyst achieved the highest hydrogen evolution rate under visible light irradiation, approximately 93 times that of pure Tp-Pa-1, and even slightly higher than that of the Pt (3%) (by mass)/Tp-Pa-1 composite catalyst. This indicates that integrating conductive Cu₃(HHTP)₂can replace noble-metal cocatalysts as an excellent electron extractor, significantly promoting charge transfer and suppressing the recombination of photogenerated electron-hole pairs. Zheng et al.^[56]utilized the benzoic acid groups on the surface of COFs (B-CTF) as nucleation sites for MOFs to construct a B-CTF-Ti-MOF crystalline hybrid material via an in situ self-assembly process. The photocatalytic hydrogen production rate was 11.8 times that of pure CTF‐1, demonstrating superior activity and stability.

2.2.3 Polyoxometalate-based Metal-Organic Frameworks (POMOFs)

Polyoxometalates, also known as polyacids (POMs), are nanoscale metal-oxygen cluster compounds formed by high oxidation state pre-transition metal ions (such as V, Mo, W, etc.) and oxygen.^[57]. POMs possess excellent redox properties, can combine with various functional materials to achieve synergistic effects between them, such as synthesizing POMOFs with MOFs. Zhang et al.^[58]synthesized the POMOF [Ni₃(bpp)₃·(H₂O)₁₁(PW₉^VIW₃^VO₄₀)]·2H₂O (bpp = 3,5′-bis(pyridin-4-yl)pyridine) via a hydrothermal method by introducing rigid pyridyl-containing ligands into the POM system. In the POMOF, Ni ions coordinate simultaneously with the N-donor ligand bpp and the O-donor ligand POMs, forming a three-dimensional framework with a doubly interpenetrated structure featuring large stacking channels. This POMOF exhibits excellent degradation performance towards Rhodamine B (RhB) under UV irradiation, achieving a 90% degradation rate within 120 min, with no significant decrease in degradation efficiency after three cycles of photocatalysis. To explore the influence of V-shaped organic ligands on complex helical or meso-helical structures in POMOFs, Jia et al.^[59]successfully isolated under hydrothermal conditions an unprecedented helical structure generated from two types of metal-organic subunits A and B, fused together in an A-B-A-B pattern in (H[K₂Ag₉(DPT)₇(μ-2-O)₂(H₂O)₄][SiW₁₂O₄₀]₂). Under UV irradiation, the photocatalytic decolorization rate of RhB within 300 min was 75.9%, which is 1.82 times that of pure (NBu₄)₄[SiW₁₂O₄₀].

2.2.4 Fiber/MOFs Composites

MOF powders face challenges such as difficult recovery and potential secondary water pollution. Loading MOFs onto easily separable supports to prepare composite materials has become an important approach to solving these problems. Yu Fan et al.^[60]modified UiO-66-NH₂ using isocyanate-terminated polyurethane oligomers to enhance its adsorption performance for Cr(VI), and then loaded the PU/UiO-66-NH₂ onto polypropylene (PP) nonwoven fabric to prepare a PU/UiO-66-NH₂/PP nonwoven fabric composite material. The PU/UiO-66-NH₂and PP nonwoven fabric exhibit strong bonding strength; after 30 minutes of ultrasonic washing with water, the maximum mass loss rate was only 4.5%. Under visible light conditions, the rate of decrease in the mass concentration of Cr₂O₇^2-was significantly faster than in the absence of light; after 180 minutes of reaction, the mass concentration of Cr₂O₇^2-in the solution approached zero, indicating that the composite material exerts a visible-light photocatalytic effect on Cr₂O₇^2-.

2.3 Generate MOF derivatives

Porous MOF derivatives obtained through high-temperature calcination not only inherit the high specific surface area, controllable structure, and regular pore distribution of MOFs but also exhibit higher stability; therefore, MOFs serve as excellent sacrificial templates for synthesizing porous photocatalysts. The preparation of MOF derivatives is typically divided into three steps: precursor design, calcination process, and post-treatment.^[61]. Precursor design can directly regulate the particle size and morphology of MOFs, making it a crucial step in designing MOF-derived photocatalysts; meanwhile, the calcination temperature and atmosphere during the calcination process determine the morphology, structure, and porous architecture of the derived catalysts. Additionally, after calcination, methods such as acid washing and etching can be employed to remove metals or metal compounds occupying space within the MOF derivatives, or to modify the defects and active sites of the derivatives.^[62].

2.3.1 Metal-free porous carbon

Designing metal-free porous carbons derived from MOFs primarily uses MOF precursors containing low-boiling-point metals, such as Zn-based MOFs (e.g., ZIF-8, MAF-5, MAF-6)^[63]. Bai et al.^[64]directly carbonized ZIF-8 within a temperature range of 600–1000 °C to obtain metal-free carbon materials. Characterization results showed that the specific surface area and pore volume of MOF-derived porous carbons increased with rising carbonization temperatures, accompanied by an increase in ultramicropore content. Abbasi et al.^[65]and Ahmed et al.^[66]also found that materials obtained by carbonizing ZIF-8 at 1000 °C exhibited the largest specific surface area, pore volume, and porosity. However, excessively high carbonization temperatures lead to shrinkage and deformation of the carbon material, increased content of disordered carbon atoms, collapse of the MOF structure, and ultimately prevent the acquisition of ordered porous carbons^[67-68]. The Zn/C ratio in MOFs and the pyrolysis duration also significantly affect the surface area and pore structure of the prepared carbon-based materials^[64,69].

Acid washing can also be used for the post-calcination treatment of MOFs to generate metal-free porous carbon. Zhang et al.^[70]heated activated MAF-5, MAF-6, and MAF-32 under a nitrogen atmosphere, followed by hydrochloric acid cleaning to obtain nitrogen-doped porous carbon materials CMAF-5, CMAF-6, and CMAF-32; their pore volumes and surface areas were higher than those of the original MOFs. Similarly, Bhadra et al.^[69,71]also obtained metal-free carbon materials with highly porous characteristics by washing MOF carbides with hydrochloric acid solution. In addition to HCl acid washing, HF can also be used for acid washing in post-calcination treatment. Li et al.^[72]carbonized MOF-253 (an Al-based MOF) in an argon atmosphere at 700, 800, 900, and 1000 °C to synthesize various MOF-derived carbon-based nanomaterials. To remove Al₂O₃, the materials were treated with 20% HF and then washed with deionized water. The treated carbon-based materials had a high N content (3.7%~9.4% by mass), and the BET surface area reached 630~1490 m²/g.

2.3.2 Metal-doped carbon materials

Direct carbonization of MOFs containing metals (Co, Ni, Fe, etc.) yields corresponding metal/metal oxide carbon-based materials. During the carbonization process, the reduction potential of the metal atoms plays a crucial role in the resulting metal-doped carbon products.^[73]. Typically, when the reduction potential of the metal in the MOFs is 0.27 V or higher, the pyrolysis product consists of metal nanoparticles; however, when the reduction potential of the metal is less than 0.27 V, metal oxides are obtained.^[63].

Torad et al.^[74]Under a nitrogen atmosphere, ZIF-67 powder was directly carbonized. High-resolution elemental imaging results indicated that the content of pure cobalt in the carbon matrix was approximately 42% (by mass), and also confirmed the presence of a small amount of cobalt oxides (6%~7% by mass) formed by the slow oxidation of deposited cobalt nanoparticles in air. When the carbonization temperature was increased to 900 °C, the sample was found to be fully graphitized with highly crystalline cobalt particles. Wang Shaobin's research group^[75]A series of in-situ carbon-doped titanium dioxide (Cx/TiO₂) composites with porous and crystalline structures were successfully synthesized via low-temperature calcination of MIL-125(Ti). Compared with the benchmark titanium dioxide photocatalyst (P25), Cx/TiO₂exhibits high specific surface area, a narrower band gap, and a favorable photogenerated electron-hole ratio, demonstrating excellent activity and stability for the photocatalytic degradation of BPA. During the BPA degradation process, holes and superoxide radicals are the primary oxidizing species. MOFs of other transition metals can also serve as precursors to generate metal-doped carbon materials, such as MIL-101(Fe)^[76], UiO-66(Zr)^[77], Ni-MOF-74^[78], Mg-MOF-74^[79], Cu-MOF^[80], etc.

Direct pyrolysis of single-metal MOFs to prepare metal-carbon composites may result in limited active sites, which inevitably affects the material's performance. Therefore, MOF precursors can be designed to synthesize metal-carbon composites with different functions^[81], such as introducing other metal sources (Co, Fe, Ni, Mn, etc.). Liu et al.^[82]dissolved Zn(NO₃)₂·6H₂O and FeSO₄·7H₂O with benzimidazole in DMF for solvothermal reaction to generate a bimetallic MOF (Zn/Fe-ZIF-X); then Zn/Fe-ZIF-X was maintained at 1000 °C in a nitrogen flow for 1 h, washed with sulfuric acid, and an Fe-N-C mixture was synthesized. The resulting FeNC-20-1000 hybrid electrocatalyst exhibited excellent oxygen reduction reaction (ORR) activity and good stability in both acidic and alkaline solutions.

2.3.3 Non-metal heteroatom-doped carbon materials

Other heteroatoms (such as N, P, or S) can be doped by immersing MOFs in organic solutions containing these elements, ultimately yielding heteroatom-doped carbon materials after pyrolysis. Gao et al.^[83]N)₃N₄nanosheet photocatalysts (DCNNS) by directly calcining a mixture of urea and MOFs. The optimized DCNNS exhibited a maximum H₂generation rate of approximately 484.09 mmol/h at 420 nm, with a quantum efficiency of about 3.43%, and its photocatalytic U(VI) reduction activity was enhanced by approximately 6.09 times.

Introducing functional groups into MOFs followed by carbonization is another popular method for synthesizing MOF-derived materials. Liu et al.^[84]introduced amino organic functional groups into MOF-5 and synthesized amino-functionalized MOF-5 derived carbon materials (TEPA-C-MOF-5) via short-term high-temperature treatment. It exhibits strong uranium adsorption capacity (550 mg/g), and the surface carbon layer enhances its stability in water or humid environments.

2.3.4 Single-atom catalysts (SACs)

The preparation of MOF-derived SACs typically employs methods such as in-situ node evolution, encapsulation confinement, and ligand anchoring.^[85]In-situ node evolution utilizes the metal nodes of MOFs themselves as the source of surface metal atoms required for synthesizing single-atom catalysts, representing a highly common strategy. For instance, using ZIF-8 as a template, a catalyst composed of atomically dispersed monovalent zinc species tightly integrated with nitrogen-doped carbon nanosheets (Zn/NC NSs) was successfully synthesized.^[86]MOFs possess the characteristics of adsorbing and confining guest substances, effectively preventing the detachment and aggregation of surface single atoms; therefore, the encapsulation confinement method can be employed to selectively synthesize efficient single-atom catalysts. For example, using ZIF-67 as a template, guest metal salts were adsorbed onto the inner and outer surfaces via solvothermal reactions, thereby obtaining iron-doped Co₃O₄ layered hollow nanosheets (named Fe-Co₃O₄ HHNPs) catalyst.^[87]The ligand anchoring method provides possibilities for adjusting metal-organic framework materials to achieve designs with ordered stacking structures possessing high catalytic activity. For example, using ZIF-8 as a support material and ferrocene as a guest material, an iron sulfide catalyst was prepared.^[88].

Although SACs synthesized from MOFs are mainly applied in the field of electrocatalysis, their applications in photocatalysis are also being gradually explored^[89]. Zhang et al.^[90]for the first time pyrolyzed ultrathin cobalt metal-organic framework (MOF) nanosheets during the formation of g-C₃N₄, anchoring single Co(II) sites on graphitic carbon nitride (g-C₃N₄), thereby preparing various g-C₃N₄-MOLs catalysts. The photocatalytic performance of g-C₃N₄-MOLs-30 is far superior to many g-C-based₃N₄photocatalysts, benefiting from the fixation of Co(II) active sites on g-C₃N₄, which improves its optical properties and promotes the transfer of photogenerated charges, enabling adsorbed CO₂molecules to gain electrons and be reduced to CO. Shang et al.^[91]designed an Al-PMOF(Fe) single-atom catalyst derived from MOFs using porphyrin ligands and applied it to the photocatalytic nitrogen reduction reaction (NRR). The ammonia yield and formation rate of Al-PMOF(Fe) were 82% and 50% higher, respectively, than those of the iron-free single-atom Al-PMOF catalyst, because the Fe-N active sites can effectively promote the absorption of N₂by Al-PMOF(Fe) and delay electron-hole recombination, thereby enabling this single-atom catalyst to exhibit excellent catalytic performance.

SACs utilize single atoms as electron acceptors and MOFs with catalytic active centers as photosensitizers and stabilizers, significantly enhancing photocatalytic reduction reactions. Tang Junwang's research group^[92]pre-designed Ti-based MIL-125 with abundant Ti vacancies (Ti_V), leveraging the large specific surface area and abundant Ti vacancies of MIL-125(Ti_V) to adsorb Cu ions, followed by heat treatment to obtain TiO₂ photocatalysts loaded with Cu single atoms on the surface (CuSA-TiO₂). As shown in Figure 5a, the hydrogen evolution rate of CuSA-TiO₂ reached 101.7 mmol/(g·h), which is not only higher than that of pristine TiO₂, but even exceeds the hydrogen evolution rate of PtSA-TiO₂. Furthermore, the mass fraction of copper attached to TiO₂ was optimized, achieving the highest hydrogen generation activity on CuSA-TiO₂ at approximately 1.5%, realizing a quantum efficiency of 56% at 365 nm, as illustrated in Figure 5b. Six consecutive photocatalytic water splitting experiments under simulated solar irradiation confirmed the long-term stability and reproducibility of CuSA-TiO₂ (Figure 5c). The significant enhancement in photocatalytic hydrogen generation activity of CuSA-TiO₂ can be attributed to the following reasons: its synthesis strategy provided suitable sites (Ti vacancies) and a high specific surface area for copper atoms, thereby stabilizing them (Figure 5e). Secondly, abundant Cu²⁺ can effectively capture photogenerated electrons, significantly reducing the recombination of photogenerated charges (Step 1 in Figure 5d), while Cu⁺ possesses a positive potential for reducing water to generate hydrogen (Step 2 in Figure 5d), after which Cu⁺ is oxidized to Cu²⁺. This interesting reversible process or in-situ self-repair enables CuSA-TiO₂ to achieve higher photocatalytic activity than conventional Pt/TiO₂. The bonding of copper single atoms (Cu-SAs) with surface oxygen atoms also accelerates the transfer of photogenerated charge carriers from TiO₂ to Cu-SAs, ultimately increasing the hydrogen generation rate^[93].

3 Application of MOF Photocatalysts in Uranium Reduction

3.1 Mechanism of photocatalytic reduction of U(VI)

The conversion mechanism of U(VI) in photocatalytic reactions is of great significance for developing suitable photocatalysts and improving the reduction performance of U(VI). The photocatalytic mechanism of MOFs is roughly similar to that of traditional semiconductor materials (such as TiO₂, ZnO). After absorbing photons, electrons in the valence band (VB) of the metal clusters gain energy and transition to the conduction band (CB), forming holes in the VB; after the separation of photogenerated electrons and holes, U(VI) is reduced by the photogenerated electrons on the catalyst's CB or by reductive radical intermediates generated in the presence of electron sacrificial agents.

Unlike semiconductor materials, when MOFs act as photocatalysts, their metal centers are typically equated to quantum dots, while organic ligands serve as light-absorbing centers. Therefore, MOFs can achieve photocatalysis through two photoexcitation pathways: one involves direct excitation of the metal centers; the other involves excitation of the ligands to generate electron-hole pairs, where photogenerated electrons transfer from the ligands to the metal nodes, forming ligand-to-metal charge transfer (LMCT), thereby enabling the reduction of U(VI) after the separation of photogenerated carriers.^[94].

3.1.1 Electron transfer pathways for photocatalytic reduction of U(VI)

The pathways of electron transfer during the photocatalytic reduction of U(VI) are asshown in Table 1. In the single-electron reduction process, the intermediate U(V) is prone to disproportionation, producing U(VI) and U(IV); under acidic conditions, protons in the solution can participate in the two-electron reduction process to achieve the reduction of U(VI); in some aerobic systems, O₂ is reduced by electrons to generate •O₂^- radicals, which subsequently reduce U(VI); some electron-donor organic substances can also participate in the reduction reaction of U(VI), extending the lifetime of electrons and thereby promoting the reduction of U(VI)^[95].

3.1.2 Speculation on the Mechanism of Photocatalytic Reduction of U(VI) by MOFs

Currently, research on the mechanism of photocatalytic reduction of U(VI) by MOFs is mainly carried out through three approaches: characterization of the catalyst before the reaction, migration and transformation of free radicals during the reaction, and analysis of the products after the reaction.

Characterization of photocatalysts, especially their optoelectronic properties, can reveal their light absorption characteristics, estimate the transition potentials of the conduction and valence bands, reflect charge transfer rates, determine the type of photocatalyst, and infer possible carrier generation and transfer pathways. Asshown in Figure 6a^[99], in the UV-Vis diffuse reflectance absorption spectrum (UV-Vis DRS), due to the presence of MOF525, the absorption edge of MOF525@BDMTp shifts to 700 nm, and the optical absorption range is significantly broadened. The band gap widths (E_g) calculated via the Tauc plot method for BDMTp, MOF525, and BDMTp@MOF525 are 2.30, 1.80, and 1.82 eV, respectively. The flat-band potentials of BDMTp and MOF525 calculated using the Mott-Schottky method are -0.78 and -0.87 eV (vs. NHE), respectively; the positive slope of the curves indicates that both are n-type semiconductors (Figure 6b). Since the conduction band potential (E_CB) of an n-type semiconductor is approximately equal to the flat-band potential, the E_CB of BDMTp and MOF525 are estimated to be -0.78 and -0.87 eV, respectively. Based on the values of E_g and E_CB, the valence band potentials (E_VB) of BDMTp and MOF525 are calculated to be 1.52 and 0.93 eV (vs. NHE), respectively (Figure 6c). According to the electrochemical impedance spectroscopy (EIS), MOF525@BDMTp exhibits the smallest charge transfer resistance, indicating that the combination of these two materials promotes charge transfer (Figure 6d). Furthermore, as seen from the transient photocurrent response plot, MOF525@BDMTp shows the strongest photocurrent response, suggesting that this composite has a higher current density (Figure 6e). Calculated from the fluorescence decay curves, the average lifetime of MOF525@BDMTp is the shortest, at 0.41 ns (Figure 6f), indicating rapid carrier transfer between MOF525 and BDMTp in the composite.

Radical scavenging experiments can investigate the roles of different radicals in the photocatalytic reaction process, thereby inferring the catalytic mechanism. Using KIO₃, N₂, ascorbic acid (AA), methanol (MeOH), and isopropanol (IPA) as e^-, O₂, •O₂^-, h⁺, and •OH scavengers^[100]. After adding KIO₃ to capture e^-, the removal efficiency of U(VI) decreased from 57% to 24%, highlighting the critical role of e^- in the U(VI) reduction process (Figure 7). Furthermore, the addition of MeOH and IPA significantly improved the removal efficiency, indicating that scavenging h⁺ and •OH improved the separation of charges and holes, thereby enhancing electron utilization.

Analysis of the products after the reaction, and comparison with the original photocatalyst or products under dark conditions, is a crucial step in deducing the mechanism of photocatalytic reduction of U(VI) by MOFs. There are many analytical methods, such as XRD, XPS, SEM, and elemental mapping tests. The interaction mode between U(VI) and MOF525@BDMTp was analyzed using X-ray photoelectron spectroscopy (XPS), as shown in Figure 8. The samples reacted with U(VI) in the dark and under light irradiation were named MOF525@BDMTp-U_ads and MOF525@BDMTp-U_pho(Figure 8a). U 4f peaks appeared in the XPS spectra under both dark and light conditions, and the intensity of the U 4f peak in MOF525@BDMTp-U_pho was greater than that in MOF525@BDMTp-U_ads, indicating that light irradiation promoted the adsorption and reduction of U(VI). Comparing MOF525@BDMTp with MOF525@BDMTp-U_ads, in the N 1s XPS spectrum, the binding energies of pyridinic-N, C

$\stackrel{\mathrm{ }\mathrm{ }\mathrm{ }\mathrm{ }}{̿}$

N, pyrrolic-N, and C—N all shifted towards higher binding energies, indicating an interaction between nitrogen sites and U(VI). In MOF525@BDMTp-U_pho, a U—N peak also appeared, indicating that light irradiation enhanced the activity of nitrogen sites and promoted the coordination between nitrogen sites and U(VI) (Figure 8b). According to the O 1s XPS characterization, the binding energies of Zr—O, C_ads, MOF525@BDMTp-U_pho for Zr—O, C

$\stackrel{\mathrm{ }\mathrm{ }\mathrm{ }\mathrm{ }}{̿}$

O, C—O, and —OH all increased compared to MOF525@BDMTp, indicating the existence of coordination interactions between oxygen sites and U(VI). The appearance of the U—O bond indicates that under light irradiation, the affinity between O and U(VI) increases, and the catalytic reduction performance of the material is thereby enhanced, which is related to the increased electron density around O under light irradiation (Figure 8c). During the adsorption process, uranium mainly exists in the form of U(VI), while after light irradiation, U(VI) and U(IV) coexist. This is because light irradiation enhances the affinity between active sites and U(VI), and simultaneously achieves the reduction process of U(VI) on the catalyst (Figure 8d).

In addition to various instrumental characterizations, theoretical calculations can also be used to elucidate the charge transfer process of MOF photocatalysts under light irradiation, thereby inferring the reaction mechanism of MOF photocatalytic reduction of U(VI). The density of states (DOS) and projected density of states (PDOS) of Mn-NDI-MOF were calculated using density functional theory (DFT)^[101], asFigure 9shows. In the ligand-to-metal charge transfer (LMCT) process of Mn-NDI-MOF, no energy is consumed during the transfer of electrons from the excited state of the pyridine-nitrogen-doped mesoporous material (PyNDI) to the unoccupied orbitals of manganese (Figure 9a). This indicates that the material possesses high charge transfer efficiency and low carrier recombination. In the projected density of states plot, electrons are primarily photo-excited from the N 2p and O 2p orbitals of the linker and then transferred to the 3d orbitals of manganese (Figure 9b). In the charge density difference, it can be observed that photogenerated electrons concentrate at the central metal manganese and then transfer to U(VI) (Figure 9c). From the perspective of Bader charge analysis, the amount of charge transferred from Mn-NDI-MOF to U(VI) is 1.39 electrons; U(VI) gains electrons from the Mn metal center and is reduced (Figure 9d). Under light irradiation, Mn-NDI-MOF generates photogenerated electrons, which are transferred through the ligand linkers to the metal centers and ultimately accumulate within the metal; at the catalytic active sites, U(VI) is reduced to U(IV).

3.2 Design directions for MOF photocatalysts

3.2.1 Improve sunlight utilization efficiency

Visible light accounts for approximately 43% of sunlight, while ultraviolet light accounts for only 10%. Therefore, to enhance the absorption of sunlight by catalytic materials, developing efficient visible-light-active photocatalysts is of great significance. Pristine MIL-53(Fe) can catalytically reduce U(VI) in water under visible light, achieving a removal rate of 80%.^[98]. However, very few pristine MOFs possess visible-light response capabilities.

Introducing photosensitive functional groups into MOFs or compounding them with materials possessing excellent optical properties is one of the methods to improve solar energy utilization. Yang et al.^[101]introduced naphthalene diimide (NDI) with excellent optoelectronic properties into MOFs. NDI derivatives are important regions for harvesting sunlight; under low standard reduction potentials, NDI radical anions (NDI•^-) can be easily generated via photocatalytic reduction, and the excited state NDI•^-is an effective electron donor. Without using sacrificial agents, Mn-NDI-MOF exhibits good photocatalytic reduction and removal capability for uranium, with a q_m value as high as 1142.20 mg/g. Zhao et al.^[102]synthesized a novel uranyl organic framework (UOF) with a 4-fold interpenetrated structure, HNU-81, and successfully inserted the photosensitizer Ru(bpy)32+ into suitable channels along its _a^{-axis. UV-Vis absorption spectroscopy results showed that the addition of Ru(bpy)}3₂₊^{broadened the light absorption range of HNU-81@Ru(bpy)₃, extending the wavelength range to 600 nm. In the photoelectrocatalytic U(VI) reduction reaction, the removal rate of the HNU-81@Ru(bpy)₃ photoanode was 81.2%, more than double that of HNU-81 (36.2%) under the same conditions.}

3.2.2 Improve carrier separation and transfer performance

Porphyrin (TCPP) is a class of highly π-conjugated photosensitive groups; when introduced into MOF organic ligands, it can enhance the photoresponse capability of MOFs. More importantly, it facilitates effective electron-hole separation and transfer, thereby improving photocatalytic activity.^[103-104].

Wang Ning Research Group^[105]A facile one-pot synthesis method was employed to integrate photoresponsive Ni(II)-centered porphyrin (TCPP(Ni)) ligands into UiO-66-NH₂, synthesizing TCPP(Ni)⊂MOF-X (where X represents different molar ratio combinations of TCPP(Ni) and 2-aminoterephthalic acid (ATA)) while simultaneously maintaining its crystal structure and ultra-high chemical stability. The incorporation of TCPP(Ni) ligands not only enhances visible light harvesting but also effectively promotes charge transfer from excited ATA and TCPP(Ni) ligands to Zr-oxo clusters. Through a simple surfactant-assisted strategy, two-dimensional (2D) porphyrin-based nickel metal-organic frameworks (Ni-TCPP) were successfully prepared.^[106]. Studies have found that TCPP ligands and Ni₂(COO)₄ clusters can successfully adsorb UO₂²⁺ through coordination interactions; in addition to the traditional charge transfer from excited TCPP to Ni₂(COO)₄, an additional electron transfer pathway emerges after the UO₂²⁺ adsorbed at the porphyrin center undergoes metallization. Benefiting from the dual electron transfer pathways, dual binding sites, and high light-harvesting capability of the photosensitive TCPP, the photosensitive 2D Ni-TCPP nanosheets exhibit excellent performance in photoreduction-assisted uranium recovery. Gao et al.^[107] prepared a π-conjugated MOF (Cu@Th-TCPP) and utilized it for the adsorption and photoreduction of U(VI). The highly conjugated framework of Cu@Th-TCPP is favorable for structural stability, charge transfer, and separation, while the incorporation of Cu²⁺ sites further accelerates charge carrier dynamics, thereby generating bifunctional properties.

3.2.3 Reduce the electron-hole recombination rate

Compared to single semiconductor materials, heterojunction structures can effectively separate oxidation and reduction sites, reduce the electron-hole recombination rate, and improve the utilization efficiency of photogenerated electrons and holes. Jian Ding Qiu's research group^[99]A bifunctional photocatalyst, MOF525@BDMTp, was prepared using an in situ covalent bridging method. MOF525@BDMTp exhibits high carrier mobility and conforms to a Type-II heterojunction. Under light irradiation, due to the effect of the built-in electric field, electrons transfer from MOF525 to BDMTp, increasing the electron density of BDMTp, thereby activating U(VI) binding sites. Meanwhile, BDMTp interacts with the S and N atoms in CP (chlorpyrifos, an organophosphate), promoting its hydrolysis and oxidation by h⁺ and •O₂^-. Therefore, MOF525@BDMTp ensures the simultaneous reduction of U(VI) on BDMTp and the simultaneous oxidation of CP on MOF525. A Ti-MOF@DATp core-shell bifunctional photocatalyst was synthesized by using a metal-organic framework (Ti-MOF) as the core and growing a covalent organic framework (DATp) shell on its surface in situ, achieving simultaneous photocatalytic reduction of U(VI) and oxidation of tetracycline^[100]. The staggered energy levels between Ti-MOF and DATp form a Z-Scheme heterojunction through covalent bonds; Ti-MOF serves as the site for tetracycline photo-oxidation, while DATp acts as the site for U(VI) photoreduction. The oxidation and reduction processes in Ti-MOF@DATp are effectively separated, promoting charge and hole transfer and enhancing photocatalytic performance. Experiments demonstrate that Ti-MOF@DATp can effectively achieve simultaneous photocatalytic removal of 96% U(VI) and 90% tetracycline from their mixture.

To further improve the photocatalytic performance of MOFs for uranium, researchers began working on ternary composite catalysts. The group of Zhu Wenkun^[96]synthesized MnO_x/UiO-66/Ti₃C₂T_x) with dual catalytic sites (namely MnO_x/UiO-66/Ti₃C₂T_xphotocatalyst. MnOx nanoparticles facilitate h⁺capture, while Ti₃C₂T_xnanosheets can effectively capture electrons; this spatially separated dual co-catalyst improves the separation efficiency of photogenerated electron-hole pairs. Radical scavenging experiments and EPR spectra indicate that •O₂^-radicals are generated during the U(VI) reduction process, suggesting that photogenerated electrons transfer from UiO-66 to Ti₃C₂T_x, thereby reducing U(VI) and generating •O₂^-, which further forms the stable crystalline phase of (UO₂)O₂·2H₂O.

3.2.4 Improve adsorption performance

In photocatalytic experiments, adsorption and photocatalysis often complement each other to jointly remove pollutants from water. For instance, the removal mechanism of U(VI) by S-NZVI/UiO-66 prepared via liquid-phase reduction mainly involves physical adsorption, electrostatic attraction, and the complexation of UiO-66-NO₂, as well as the reduction and complexation by S-NZVI^[108]. Therefore, enhancing the adsorption performance of MOFs is beneficial for their photocatalytic reduction of U(VI).

Li et al.^[109]Adopting a post-synthetic functionalization strategy, phosphorus-based and amino groups were modified onto PCN-222, which possesses high stability and high photoactivity, to capture U(VI) from solution via ligand complexation. Under visible light irradiation, photo-induced electrons from the MOFs can effectively reduce the U(VI) pre-enriched by the MOFs, generating neutral uranium species that detach from the MOFs, thereby regenerating active sites for capturing more U(VI). This automatic recovery process exhibits ultra-high uranium extraction capacity, is not limited by the number of adsorption sites, and can exclude competition from non-redox-active metal cations, endowing the phosphorus-based and amino-modified PCN-222 with strong uranium selectivity applicable across an extremely wide range of uranium concentrations and pH levels.

3.2.5 Simplified reduction conditions

In the process of photocatalytic uranium reduction, organic molecules are typically used as sacrificial agents to capture photogenerated holes. However, the introduction of sacrificial agents not only increases the cost of uranium removal but also causes secondary chemical pollution. Jiang et al.^[110]successfully developed a highly active and stable photocatalyst for U(VI) reduction (Ag/ZIF-8) without any sacrificial agents. Under xenon lamp irradiation without adding any sacrificial agents, Ag/ZIF-8 achieved a U(VI) removal rate of 92.8%, and even under natural light, it could enrich more than 85.8% of U(VI). Mechanistic studies indicate that the nitrogen-containing groups in ZIF-8 are highly active sites for efficient U(VI) adsorption. Furthermore, hot electrons generated on Ag nanoparticles are transferred to ZIF-8 via localized surface plasmon resonance (LSPR), directly promoting the photocatalytic reduction performance of ZIF-8.

Under air atmosphere, the activity of most photocatalysts is suppressed, and newly reduced uranium dioxide is extremely easily oxidized back into uranyl ions, redissolving into the water body. Therefore, photocatalytic uranium reduction typically requires anaerobic conditions, which hinders the practical application of photocatalysis-assisted uranium extraction technology. In recent years, researchers have developed photocatalytic uranium extraction methods capable of operating under air atmosphere, proposing new reaction mechanisms and design strategies for uranium extraction materials.^[97]. Zhang et al.^[111]synthesized MOF-modified C₃N₄composite materials, achieving a uranium removal capacity of 1355 mg/g under air atmosphere. The introduction of MOFs enhanced the band structure and optoelectronic properties of C₃N₄, enabling efficient generation and separation of electrons and holes; electrons reduce O₂to H₂O₂, while holes oxidize H₂O to O₂. The generated H₂O₂can react with UO₂²⁺to form (UO₂)O₂·2H₂O, achieving uranium immobilization under air atmosphere. Liu Yunhai's research group^[112]designed and synthesized a novel CdS/UiO-66-NH heterojunction photocatalytic material by uniformly loading CdS nanoparticles onto the surface of amino-functionalized UiO-66.₂Under air atmosphere and without adding sacrificial agents, after 15 min of visible light irradiation, CdS/UiO-66-NH₂achieved a 100% removal rate for U(VI), with a photocatalytic removal rate as high as 3.78 mmol/(g·h). The product was amorphous (UO₂)O₂·2H₂O. Notably, o-tolidine colorimetry showed no H₂O₂presence during the photocatalytic process in this reaction system, indicating that U(VI) was converted to (UO₂^-by photogenerated electrons and •O₂)O₂·2H₂O.

3.2.6 Comprehensive improvement from multiple angles

More designs do not aim to improve a single property of the catalyst, but rather modify its band structure to comprehensively enhance its catalytic performance from multiple perspectives. Inspired by ice photochemical synthesis, Liu et al.^[113]developed a ligand-assisted frozen photoreduction method to anchor atomically dispersed Cu onto amino-functionalized Zr-MOFs, synthesizing Cu SA@UiO-66-NH₂ photocatalyst. Cu single atoms (Cu SAs) act as electron acceptors, enabling electron transfer from the 2-aminoterephthalic acid (ATA) linker to the Cu SAs and Zr-oxo clusters, thereby promoting the separation and transfer of photogenerated charges. Compared to aggregated active sites (Cu Clu@UiO-66-NH₂), isolated and dispersed Cu sites help improve copper atom utilization and photocatalytic activity, resulting in Cu SA@UiO-66-NH₂ exhibiting a uranium extraction capacity of 9.16 mg/g in natural seawater.

Zhu Wenkun Research Group^[114]Utilizing the gelling properties of konjac glucomannan (KGM) and the unique structure of ZIF-8, a three-dimensional porous konjac glucomannan carbon-encapsulated carbon-doped ZnO carbon aerogel (KC@C-ZnO-X) was constructed for the adsorption and photoreduction of U(VI). KC serves as the "skeleton" encapsulating C-ZnO, which not only improves the photostability of ZnO, enabling the catalyst to be recycled, but also the synergistic effect between C-ZnO and KC promotes the separation of photogenerated charges and the generation of •O₂^-, further forming a stable (UO₃·NH₃·H₂O) crystal phase.

Liu et al.^[115]synthesized a MOF photocatalytic adsorbent (Zr/Ti-MOF-25) through defect engineering. The absence of organic linkers increased the pore size, forming a more open structure that facilitates UO₂²⁺ions to rapidly diffuse into the internal pores, thereby improving the utilization of hidden adsorption binding sites. Furthermore, the absence of organic linkers and the substitution of metal centers modulated the electronic structure of the MOFs, reducing the energy required for electron transfer from excited organic linkers to the unoccupied d-orbitals of the metal centers, thus enhancing their photocatalytic reduction capability for U(VI).

Some MOF catalysts can generate biologically toxic reactive oxygen species (ROS), possessing anti-biofouling properties, which enhances their application in real water bodies. Zhao et al.^[116]successfully prepared a heterojunction system of porphyrin zirconium metal-organic frameworks doped with nano zero-valent iron (NZVI@PCN-224). Due to the Schottky junction effect at the NZVI/MOF interface, NZVI nanomaterials immobilized on PCN-224 can improve interfacial electron transfer and separation efficiency, including photocatalytic reduction and chemical reduction; the photocatalytic effect also promotes NZVI@PCN-224 to generate more biologically toxic reactive oxygen species (ROS), thereby exhibiting high antimicrobial and anti-algal activity. This synergistic strategy combining adsorption, reduction, and anti-biofouling through chelation, chemical reduction, and photocatalysis provides new insights for the efficient treatment of liquid radioactive waste. Chen et al.^[117]prepared a three-dimensional interconnected porous network aerogel (BP@CNF-MOF) for extracting U(VI) from natural seawater. The construction of a heterojunction between black phosphorus quantum dots (BPQDs) and UiO-66-NH₂exhibited excellent photocatalytic activity, thereby promoting the reduction of U(VI) to solid U(IV), and leaving more active affinity sites on UiO-66-NH₂for further coordination with U(VI). Furthermore, this material can generate a large amount of reactive oxygen species (ROS) during photocatalytic reactions, effectively killing marine bacteria.

3.3 Application effects of MOF photocatalytic materials in reducing U(VI)

The types of MOF materials, synthesis strategies, and catalytic reduction conditions may all affect the reduction efficiency of U(VI) (Table 2). ZIF, MIL, and UiO series are common MOFs used for preparing photocatalysts, primarily due to their excellent water stability, making them suitable for radioactive wastewater treatment or uranium extraction from seawater^[7], and they are even more suitable for treating acidic radioactive wastewater. For example, under sunlight irradiation, CdS/UiO-66-NH₂ can remove 85.62% of U(VI) from mine wastewater. Only TCPP(Ni)⊂MOF, Cu SA@UiO-66-NH₂, and BP@CNF-MOF have been applied to uranium extraction from seawater, which may be related to the weak alkalinity, ionic complexity, and biofouling of seawater.

Based on the advantages of photocatalysis, MOF-based photocatalysts exhibit excellent selectivity for non-redox active metal cations, showing outstanding uranium selectivity under competition from K⁺, Na⁺, Mg²⁺, Ca²⁺, Cu²⁺, Eu³⁺, etc. However, Fe³⁺may cause interference in the U(VI) reduction reaction. For instance, under simulated sunlight irradiation in natural seawater, BP@CNF-MOF achieved a removal efficiency for Fe³⁺that reached 55.3% of the uranium ion removal rate^[117]. In radioactive wastewater, the U(VI)-F complexes formed by the combination of F^-and UO₂²⁺possess extremely high bond energies, posing challenges for the photocatalytic reduction of U(VI). However, MOF-derived Ni@C-700 not only exhibits excellent stability but also achieves a U(VI) removal efficiency exceeding 93% in a solution with a high F^-concentration of 30 g/L.

After design and synthesis, the removal rate of U(VI) by MOF-based photocatalysts basically exceeded 90%, all higher than that of the original MIL-53(Fe). Among them, IHEP-24 achieved a U(VI) removal rate approaching 100%. It can be concluded that optimizing the structure, synthesizing composites, and generating derivatives can enhance the catalytic activity of MOF-based photocatalysts and their efficiency in reducing U(VI).

4 Conclusion and Outlook

MOF porous crystalline materials, integrating the photocatalytic functionality of metal centers, the photosensitivity of conjugated organic linkers, and the abundance of active sites, have garnered attention in the field of photocatalysis. Through optimized design in areas such as framework structures, composite materials, and derived materials, some progress has been made in the application of MOFs for the photoreduction of U(VI); however, this field remains in its infancy and faces numerous challenges, primarily manifested in the following aspects.

(1) Stability issues of MOF-based photocatalysts. The stability of each MOF is limited; for example, while UiO-66 maintains its crystal structure integrity in strong HCl acid solutions at pH=1, its crystal structure disintegrates in aqueous environments with pH>7.^[7]. Although MOF composites can enhance the catalytic activity of catalysts and their removal capacity for U(VI), they increase the risk of material instability or separation and precipitation. Research on the stability of MOF photocatalysts under radiation, high salinity, biofouling, and long-term usage conditions remains scarce and requires further investigation.

(2) Issues regarding the reaction conditions for U(VI) reduction. Since the newly reduced UO₂ is easily oxidized to UO₂²⁺ and redissolves into the water body, most U(VI) reduction processes require an oxygen-free environment and must be conducted under an inert atmosphere protection. Continuous purging not only increases costs but also adds difficulties to experimental design, greatly limiting its application in actual radioactive wastewater treatment or uranium extraction from seawater.

(3) Currently, most MOF fabrication methods are costly, time-consuming, rely on organic solvents harmful to the environment, and consume significant energy. The powdery nature of the materials makes separation and regeneration for recycling difficult, leading to catalyst loss. Although complex and diverse optimization strategies endow the materials with greater potential, they increase the difficulty of cost control. These characteristics limit the quantitative production of MOF-based catalysts and hinder their large-scale application in practical scenarios.

(4) The reaction mechanism of MOF-mediated photoreduction of U(VI) is mostly inferred based on the photocatalytic mechanisms of traditional semiconductor materials (such as TiO₂, ZnO), or deduced from experimental phenomena, the aqueous chemical properties of uranium ions, and product analysis, lacking sufficient direct evidence and in-depth mechanistic studies.

In the face of challenges, to further promote the development of MOF materials in the field of uranium reduction and separation, future research efforts can advance from the following directions.

(1) Enhance the stability of materials in complex environments and improve the reproducibility of MOF-based photocatalysts. Single-atom catalysts derived from MOFs not only improve the stability of MOFs but also inherit their excellent properties, such as high specific surface area. Combined with their characteristics of maximized atom utilization and isolated active sites, they can exhibit superior performance in the photocatalytic reduction of uranium. Developing MOF/support composite systems (such as ceramics, carbon fibers, polymer materials, etc.) to construct core-shell structures is another synthesis strategy to block erosion by acidic, oxidative, or alkaline media.

(2) Develop simpler, more efficient, and cost-effective synthesis methods to enhance the processability and mass production capability of materials. MOFs possess modular structural characteristics. We can model their chemical composition and structural parameters, and based on digital models, 3D print new porous materials from numerous high-resolution complex shapes and geometric macro-frameworks, followed by targeted modification and functionalization.^[124-125].

(3) In-depth study of the mechanism of MOF catalysts in the photocatalytic reduction of U(VI), including characterization of ground and excited state intermediates and capture and identification of transient intermediate products. Utilizing the paramagnetism of radical intermediates, detect them via Electron Paramagnetic Resonance (EPR) spectroscopy to provide information on the properties and chemical environment of specific radicals. Employ in-situ characterization techniques such as in-situ HAADF-STEM, in-situ XAS, and in-situ IR spectroscopy to gain a deep understanding of reaction intermediates, actual active sites, and reaction pathways, investigate the reaction mechanisms occurring on heterogeneous photocatalysts, and establish a robust theoretical framework.

It is believed that with the continuous deepening of research, MOF materials will play an increasingly important role in the field of uranium reduction and separation, making greater contributions to energy security and ecological environmental protection.

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Funding

National Natural Science Foundation of China(52374173)

Hunan Provincial Natural Science Foundation of China(2025JJ50255)

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1 Introduction

2 Design of MOF-based photocatalysts

2.1 Optimize MOF structure

2.1.1 Optimization of metal clusters

2.1.2 Optimization of organic ligands

2.1.3 Regulation of crystal defects

2.2 Synthesis of MOF composites

2.2.1 Composite with precious metals

2.2.2 Forming a heterojunction

2.2.3 Polyoxometalate-based Metal-Organic Frameworks (POMOFs)

2.2.4 Fiber/MOFs Composites

2.3 Generate MOF derivatives

2.3.1 Metal-free porous carbon

2.3.2 Metal-doped carbon materials

2.3.3 Non-metal heteroatom-doped carbon materials

2.3.4 Single-atom catalysts (SACs)

3 Application of MOF Photocatalysts in Uranium Reduction

3.1 Mechanism of photocatalytic reduction of U(VI)

3.1.1 Electron transfer pathways for photocatalytic reduction of U(VI)

3.1.2 Speculation on the Mechanism of Photocatalytic Reduction of U(VI) by MOFs

3.2 Design directions for MOF photocatalysts

3.2.1 Improve sunlight utilization efficiency

3.2.2 Improve carrier separation and transfer performance

3.2.3 Reduce the electron-hole recombination rate

3.2.4 Improve adsorption performance

3.2.5 Simplified reduction conditions

3.2.6 Comprehensive improvement from multiple angles

3.3 Application effects of MOF photocatalytic materials in reducing U(VI)

4 Conclusion and Outlook

References

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