Multifunctional UiO-66 MOFs in Photocatalytic and Electrocatalytic Nitrogen Reduction: A New Horizon in Green Ammonia Synthesis

Abstract

The conventional production of ammonia (NH₃) rests on the energy-intensive Haber process, driving the scientific community to search for sustainable catalysts capable of reducing the inert NN bond under ambient conditions. Among the various porous materials examined for the nitrogen reduction reaction (NRR), UiO-66, a Zr-based MOF, stands out owing to its exceptional robustness, high BET surface area, tunable porosity and strong Lewis acidity at the Zr⁴⁺ nodes, making it a highly versatile platform for both photocatalytic and electrocatalytic NH₃ production. This review aims to deliver a comprehensive overview of UiO-66-based materials for photocatalytic (P-NRR) and electrocatalytic (E-NRR) nitrogen reduction, while providing a detailed account of the key parameters governing NRR, including its thermodynamics and kinetics, the challenges of activating the inert NN bond, and competing hydrogen evolution reaction (HER). Several synthesis methods for UiO-66 are critically discussed with respect to how each approach influences material properties, including crystallinity, surface area, stability, and porosity. Targeted modification strategies, such as metal doping, linker functionalization, defect engineering, and composite formation, are systematically explored, each aimed at introducing suitable active sites and establishing a favorable environment for N₂ adsorption and activation across both photocatalytic and electrocatalytic systems. P-NRR and E-NRR performances are critically analyzed, with due consideration given to NH₃ detection methods, including colorimetric assays, ion chromatography, and ¹H NMR spectroscopy, which are essential for reliable quantification. Finally, current challenges and future directions are outlined to guide the rational design of efficient, selective, and robust UiO-66-based catalysts for sustainable NH₃ synthesis.

1. Introduction

In Earth’s atmosphere, various gases are present, among which nitrogen constitutes 78% of the atmosphere. Because of its abundance, nitrogen plays an essential role in biomolecules such as amino acids, proteins, nucleic acids, and enzymes. So, it is a necessary element for sustaining life. In the N₂ molecule, the two nitrogen atoms are connected by an N–N triple bond, which is why most of the organisms cannot metabolize it into simpler products. However, these organisms can readily metabolize the atmospheric ammonia. NH₃ is also one of the most important industrial compounds because it is used to synthesize various chemicals and pharmaceuticals. In nature, ammonia is only obtained from various geochemical cycles, like lightning and some biological pathways that use nitrogenase as the enzyme. To fulfill the ever-increasing demand for ammonia in advanced agriculture and industry, an artificial nitrogen-fixation pathway is essential.

Nowadays, the majority of NH₃ is produced by the Haber-Bosch process, which involves the reaction of N₂ and H₂ to form ammonia in the presence of a suitable catalyst. However, this process has serious limitations, such as harsh reaction conditions and the use of expensive hydrogen as a precursor. Also, this process has a substantial endowment to global CO₂ emissions and results in a very low yield (≈15%) per cycle of NH₃, which leads to the decarbonization and energization of NH₃ production pathways. Hence, there is a need to explore a sustainable, greener pathway for NH₃ synthesis.

Currently, both P-NRR and E-NRR are emerging as promising tactics for the formation of NH₃ under normal conditions. The inertness of the N₂ molecule is evidenced by the following factors: it has a BDE of 940.95 kJ mol^–1, a wide HOMO–LUMO band gap of 10.82 eV and an ionization energy (I.E.) of 15.58 eV. So, from the above factors, it is clear that the nonpolar, inert NN molecule is reluctant to donate or accept electrons. As the N₂ molecule is extremely stable in nature, it is difficult for traditional photocatalysts to activate and reduce the strong NN bond. An efficient photocatalyst must have suitable active sites that act as Lewis acidic centers with a high surface area that can energize the N₂ molecule by polarizing the inert NN bond, thereby weakening it and ultimately reducing the activation energy required for dissociation. Consequently, the activation of the nitrogen molecule represents the most critical step for the artificial nitrogen fixation reaction.

In recent years, this photocatalytic technique for the conversion of N₂ to NH₃ has been widely used, as it proceeds under mild reaction conditions, and numerous photocatalysts (both homogeneous and heterogeneous) have been reported, such as TiO₂, perovskites, g-C₃N₄, LDH, etc. However, the use of homogeneous catalysts is associated with many drawbacks, such as low yield of ammonia, high cost of the catalyst and very short lifetime of the activated catalyst. Similarly, in the case of heterogeneous catalysis, the catalytic activity is less satisfactory due to the narrow light absorption range of the photocatalyst, rapid recombination of e^–-h⁺ (exciton) pairs and a smaller number of active catalytic centers.

Moreover, E-NRR is considered a more efficient and greener route for NH₃ synthesis, as its feedstocks are derived entirely from water and atmospheric nitrogen. However, the major drawbacks of E-NRR are the high bond dissociation energy of the nitrogen molecule, poor solubility of N₂ in water and the HER, which competes with N₂ adsorption and activation, ultimately affecting E-NRR efficiency and selectivity. A highly effective NRR catalyst consisting of noble/non-noble metals, metal-free carbons, MXenes, etc., retains high Faradaic efficiency (F.E.) for the conversion of N₂ to NH₃. Emerging scientific studies have underscored the application of UiO-66-NH₂ and its heterojunction with other carbonaceous materials such as graphene and MWCNTs as tremendously active electrocatalysts. In addition to this, insights into the hydrolytic resistance of UiO-66 and the free-radical route of UiO-66-NH₂ in the degradation of the catalyst demonstrate their viability in water-based applications. ,

Conventional electrocatalysis is often restricted by scaling relations and fewer exposed active sites. These limitations are addressed by utilizing MOF-based materials. These MOFs and their derivatives tremendously lower the energy barrier for sustainable nitrogen electrochemistry by offering larger surface areas and precise active centers. In addition, incorporating structural rearrangement and compositional fine-tuning into contemporary MOF design helps synchronize intermediate adsorption while favorably overpowering the competing HER. , The optimal materials must possess superior hydrophobicity to suppress the competing HER, while preserving robust stability and high activity. Consequently, designing low-cost, high-efficiency light/electricity-driven catalysts for N₂ reduction to form NH₃ has emerged as a core research focus.

Metal–organic frameworks are highly crystalline materials obtained by the synergy between inorganic metal ions or metal clusters and multidentate ligands. ,, These MOFs have been considered among the most useful materials for NRR due to their outstanding structural features, such as high structural functionality, tunable porosity, and chemical adaptability. , The above noble properties enable MOFs to behave as well-organized supports and catalytic platforms, providing multiple active centers, a regulated electronic environment, and modifiable hydrophobicity. − Specifically, in P-NRR, MOF-based materials have a wide absorption range, and the functional group modification of ligands in the MOF framework facilitates modulation of their absorption spectrum. These properties make MOFs highly suitable candidates for P-NRR. Additionally, for the E-NRR, MOFs serve as precursors for conductive, durable nanoporous carbon materials. During the pyrolysis process, these MOFs can produce single-atom catalysts (SACs), which ultimately enhance nitrogen fixation activity. From the above discussion, it is clear that MOFs and their related materials have immense value for the photocatalytic and electrocatalytic reduction of N₂ to NH₃. ,

The Zr-based MOF, UiO-66, consist of Zr -oxo clusters, which are connected by 1,4-benzenedicarboxylic acid (BDC) as ligands. Its framework layout creates a robust, crystalline material with hierarchical pores- a core 11 A⁰ octahedral cavity and 8 Å cages- designed for various catalysis and adsorption processes. It exhibits excellent chemical and thermal stability, with a Brunauer- Emmett- Teller surface area of about 1200 m²/g. The specific surface area and porosity of UiO-66 can be increased by the use of modulators like monotopic ligands (monocarboxylic acids), defect engineering, or substitution with different functional group derivatives of UiO-66, resulting in better absorptivity. UiO-66 is therefore considered an excellent material for both P-NRR and E-NRR. The photocatalytic nitrogen fixation reaction generally proceeds through two important steps: the nitrogen reduction reaction (NRR) and the nitrogen oxidation reaction (NOR). The NRR pathways generally proceed via a dissociative mechanism, where the strong NN bond is first broken on the surface of the catalyst, subsequently forming ammonia through stepwise surface hydrogenation. Due to the exceptional surface area, tunable structure and high porosity of UiO-66, it can easily activate the strong N₂ molecule under light irradiation. In addition, UiO-66 is considered a promising material for constructing heterojunctions with other semiconducting materials, metals, and so on, facilitating enhanced charge transfer and excellent photocatalytic efficiency. ,

For E-NRR, UiO-66 also provides a favorable environment for N₂ adsorption, activation, and reduction. This is due to the high Lewis acidity of Zr⁴⁺ metal ions, which polarize the strong NN bond that ultimately activates the stable N₂ molecule, a major drawback of E-NRR. Additionally, the functionalized UiO-66 enhances electrical conductivity, improves electronic structures, and inhibits the competing HER, ultimately increasing NH₃ production. The electrocatalytic performance of UiO-66 can also be improved by incorporating SACs and nanoparticle additives. From the above discussion, it is clear that UiO-66 is one of the most efficient platforms for both photocatalytic and electrocatalytic NH₃ synthesis. −

To date, numerous reviews have been published on MOFs for various energy generation aspects. However, there is a scarcity of literature that clearly analyses both photo and electrochemical processes for ammonia synthesis. So, a systematic and precise overview of UiO-66-based materials for sustainable NH₃ production is essential. In this review article, the fundamentals of NH₃ synthesis are briefly discussed, providing insight into N₂ reduction, its thermodynamics and kinetics, and the differences between P-NRR and E-NRR processes, as well as an explanation of converting N₂ to NH₃ using various techniques, followed by the detection of NH₃ through different methods. Also, different types of parameters are mentioned to understand the yield and selectivity of product formation and synthesis of UiO-66, along with the advantages and disadvantages of each method, highlighting the novelty of UiO-66, including its stability, porosity, and surface area. Additionally, it clearly illustrates how the catalytic activities of UiO-66 are improved by various modification strategies, such as doping, functionalization, and composite formation. The advancement of UiO-66-based hybrid materials in photocatalysis is illustrated in Scheme , highlighting key breakthroughs and the development of various functional hybrid materials.

1. Timeline of the Development of UiO-66-Based MOFs for P-NRR and E-NRR.

2. Fundamentals of Ammonia Synthesis

2.1. Thermodynamics and Kinetics of NRR

To fully understand the elemental limitations of NRR and to design efficient UiO-66-based MOF catalysts, it is important to separate the thermodynamic and kinetic factors that control N₂ activation. Structurally, the hydrogenation of inert N₂ to NH₃ is a thermodynamically allowed process under ambient conditions, which is confirmed by the negative free energy of formation (ΔG _f ^° = −16.48 kJ mol^–1). Therefore, the overwhelming obstacles of NRR do not come from the macro-thermodynamics of the overall reaction, but rather from substantial kinetic barriers and poor catalytic selectivity against the competing HER. The problems of N₂ activation are mainly kinetic, governed by the extreme inertness of the N₂ molecule and competitive molecular dynamics on the catalyst surface.

Owing to the strong NN triple bond in the N₂ molecule, it is highly inert. The underactivity of the N₂ molecule cannot be explained based only on the BDE of the NN bond (941 kJ mol^–1). This is supported by the fact that acetylene (BDE = 962 kJ mol^–1) and carbon monoxide (BDE = 1070 kJ mol^–1) have higher BDE than the N₂ molecule, but both are highly reactive as compared to N₂. This is better understood from the dissociation of the first bond in the NN bond, which needs 410 kJ mol^–1 energy. 222 kJ mol^–1 energy is required for the dissociation of the first bond in acetylene (CHCH). Hence, it is clear that in the N₂ molecule, more energy is required to start the breakage of the NN bond than the CHCH bond. Due to the lower dipole moment of the N₂ molecule, its proton affinity is lower than that of methane. Also, NN has a negative electron affinity (−1.90 eV), a high ionization energy (15.84 eV), and a gap between its HOMO and LUMO of 10.82 eV. The above discussions depict that the activation of strong N₂ is extremely difficult, as protonation is challenging even in the addition of strong acids. The formation of NH₃ (ΔH _f ^° = −92.22 kJ mol^–1, ΔG _f ^° = −16.48 kJ mol^–1) from the N₂ molecule by the complete hydrogenation is thermodynamically allowed; however, complete hydrogenation never happens and does not proceed spontaneously at a detectable rate under ambient conditions. In the process of partial hydrogenation of the N₂ molecule, two intermediates are formed, i.e., diazene (N₂H₂, ΔH _f ^° = 212.9 kJ mol^–1 for (Z)-diazene) and hydrazine (N₂H₄, ΔH _f ^° = 95.35 kJ mol^–1), which have very high enthalpies for their formation. As these intermediates are associated with a high energy barrier for ammonia synthesis, homogeneous (uncatalyzed) reactions have serious limitations. In the case of heterogeneous catalyzed synthesis of NH₃, chemisorption of the N₂ molecule and the catalyst surface alter the reaction pathway by lowering the energy barriers. The process still requires some amount of energy for the reaction to occur. Therefore, it is clear that the proton affinities of N₂ are less than those of diazene (N₂H₂, 803 kJ mol^–1) and hydrazine (N₂H₄, 853.2 kJ mol^–1), offering easier protonation of these compounds than that of N₂. Also, the difference in energy between HOMO and LUMO of hydrazine­(N₂H₄) is 7.44 eV, which is less than that of N₂(10.82 eV). So, ammonia synthesis from the inert N₂ molecule involves, in the beginning, the hydrogenation of N₂ and the subsequent steps are kinetically challenging.

In E-NRR, there is a transfer of six protons and six electrons in multistage processes. The following are the reversible potentials in acidic and basic media of the N₂ reduction and hydrogen formation reaction.

$${\mathrm{N}}_2(\mathrm{g})+6{\mathrm{H}}^{+}(\mathrm{aq})+6{\mathrm{e}}^{-}\rightleftharpoons 2{\mathrm{NH}}_3(\mathrm{aq})E^{°}=0.092\mathrm{V}\mathrm{vs}\mathrm{RHE}$$ 1

$$2{\mathrm{H}}^{+}(\mathrm{aq})+2{\mathrm{e}}^{-}\rightleftharpoons {\mathrm{H}}_2(\mathrm{g})E^{°}=0.000\mathrm{V}\mathrm{vs}\mathrm{RHE}$$ 2

$${\mathrm{N}}_2(\mathrm{g})+6{\mathrm{H}}_2\mathrm{O}(\mathrm{l})+6{\mathrm{e}}^{-}\rightleftharpoons 2{\mathrm{NH}}_3+6{\mathrm{OH}}^{-}(\mathrm{aq})E^{°}=0.092\mathrm{V}\mathrm{vs}\mathrm{RHE}$$ 3

$$2{\mathrm{H}}_2\mathrm{O}(\mathrm{l})+2{\mathrm{e}}^{-}\rightleftharpoons {\mathrm{H}}_2(\mathrm{g})+2{\mathrm{OH}}^{-}(\mathrm{aq})E^{°}=0.000\mathrm{V}\mathrm{vs}\mathrm{RHE}$$ 4

Application of negative potentials to the electrode enhances the electron density at the active site, thereby facilitating the N₂ uptake. The two equations of N₂ reduction have the same equilibrium potentials­(E°) as those of the two equations involving HERs. In HER, for the production of an H₂ molecule, two electrons are required in a single half reaction. While in the case of reduction of N₂ to NH₃, the E° can be calculated by taking an average of multiple steps because the reaction takes place in three sequential steps involving two electrons in each step. If the E° values are not specified, they can be calculated from ΔG _f° (298 K), which is determined experimentally.

Due to the rapid disproportionation of diazene (N₂H₂) into N₂ and N₂H₄, it cannot be analyzed spectroscopically in solution. Also, its ΔG _f° value is never determined experimentally. But the gas-phase study predicts a minimum limit for ΔH _f° ≥ 204.1 ± 2.2 kJ·mol^–1 for trans-N₂H₂. Using the value of ΔG _f ^° (298 K) 230.1 kJ mol^–1 from CCSD­(T)/AVTZ calculations, this method facilitates the determination of E°, creating a precise yardstick for the Δ_f H° of trans-N₂H_2(g).

From the accepted thermochemical data and ΔG _f°(298 K) for trans-N₂H₂, the E° values can be calculated easily for eqs and . The E° value of 0.092 V in eq is consistent with the cumulative potential derived from the individual steps outlined in eqs – for the 6e^– dinitrogen reduction reaction. The E° value for trans-N₂H₂ in eqs and was obtained from the theoretical value of ΔG _f°(298 K), and the E^o in eq for NH₃(aq) was obtained from accepted ΔG _f°(298 K).

$${\mathrm{N}}_2+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\rightleftharpoons {\mathrm{N}}_2{\mathrm{H}}_2(\mathrm{g})E^{°}=-1.19\mathrm{V}\mathrm{vs}\mathrm{RHE}$$ 5

$${\mathrm{N}}_2{\mathrm{H}}_2(\mathrm{g})+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\rightleftharpoons {\mathrm{N}}_2{\mathrm{H}}_4(\mathrm{aq})E^{°}=0.529\mathrm{V}\mathrm{vs}\mathrm{RHE}$$ 6

$${\mathrm{N}}_2{\mathrm{H}}_4(\mathrm{aq})+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\rightleftharpoons 2{\mathrm{NH}}_3(\mathrm{aq})E^{°}=0.939\mathrm{V}\mathrm{vs}\mathrm{RHE}$$ 7

Equation is a highly endothermic reaction, as the value of E° is negative, which is also confirmed by the lower FE for NH₃ production. The other factors that affect the electrochemical N₂ reduction are the negative potentials of one-electron intermediates, which constitute the major thermodynamic constraint. The E° for one-electron reduction of N₂ is −2.78 V, but in some reported studies, the E° value for 1e^– reduction of N₂ to N₂ ^–(aq) is ≈−4.16 eV vs RHE, and for 1e^–, H⁺ reduction to N₂H_(g) is −3.2 V vs RHE. From the above discussion, it is clear that the potential barrier for the 1e- reduction of N₂ is extremely high.

$${\mathrm{N}}_2+{\mathrm{e}}^{-}\rightleftharpoons {\mathrm{N}}_2^{-}(\mathrm{g})E^{°}=-2.78\mathrm{V}\mathrm{vs}\mathrm{RH}$$ 8

In this process, the hydrogen evolution reaction is dominated by the hexa-electron cathodic reduction of N₂ to NH₃, which is thermodynamically favored (eqs –). The problem in N₂ reduction lies in the first step, which requires high energy input, and the subsequent species can easily hydrogenate. So, there is a need to find a way to reduce N₂ through catalyst design. The difference in adsorption energies between the intermediates OH and OOH is 3.2 ± 0.2 eV, which is higher than the 2.46 eV difference for an ideal catalyst. They differ in energy because

The intermediates coordinate to the catalyst surface along an identical pathway. So, for the oxygen evolution and reduction reaction to occur, a minimum of 0.25–0.35 V overpotential is required. Also, in the case of the N₂ reduction, the B. Es of the intermediate NH₂ and N₂H are not independent, so a minimum of −0.4 V overpotential is required due to the unfavorable energetic scaling between these transient species. The scaling relationship must be avoided to develop a good electrocatalyst. Thus, an effective electrocatalyst should be able to bind intermediates via distinct pathways to circumvent the scaling relationship.

2.2. Photo- and Electrocatalytic NRR: Principles and Differences

In both P-NRR and E-NRR processes, the strong nitrogen (N₂) molecule is converted into ammonia (NH₃). In P-NRR, light energy, whether from the sun or a suitable lamp, drives the chemical reaction. The light is generally absorbed by a semiconducting material that acts as the active photocatalyst (Figure a). On the other hand, electrical energy is required for E-NRR. The electrocatalytic cell consists of two electrodes, an anode and a cathode. The cathode is the site of nitrogen reduction, while the anode is responsible for the oxidation reaction (such as oxygen evolution), completing the circuit (Figure b). ,

1.

(a) Schematic diagram showing the P-NRR. (b) E-NRR mechanism comprising (i) Dissociative pathway, (ii) Associative distal pathway, and (iii) Associative alternating pathway. Reproduced with permission from ref . Copyright 2024 Elsevier.

We know the nitrogen atoms in a nitrogen molecule­(N₂) are held together by strong NN bonds. So, for the reduction reaction to occur, the nitrogen atoms must have considerable energy to overcome the energy barrier. The extra energy must be supplied by external sources, such as light irradiation, electrocatalytic polarization etc. In P-NRR, a complete understanding of its kinetics requires an understanding of the photocatalyst’s efficiency, doping, functionalization, and composite formation, among other factors. On the other hand, the electrocatalytic synthesis of ammonia (NH₃) from N₂ and H₂ proceeds at lower temperatures and atmospheric pressure, demonstrating characteristics of green reactions. Both P-NRR and E-NRR are devoid of high-energy input, unlike the energy-intensive Haber-Bosch process. From the above discussions, it is clear that P-NRR and E-NRR are promising alternatives to the Haber-Bosch process for ammonia (NH₃) synthesis, as they proceed under milder conditions.

2.3. Comparison between P-NRR and E-NRR

Both P-NRR and E-NRR processes are environmentally friendly. But the main difference lies in the nature of the energy source that drives the redox reaction. In the case of the electrocatalytic reaction, an external electrical bias induces the chemical reaction, whereas in photocatalysis, light irradiation initiates it.

However, both approaches face limitations in ammonia yield. The low yield in both photocatalytic and electrocatalytic processes is due to many factors. High electron–hole recombination rates, inappropriate band gaps, sluggish electron transfer, and the competing HER all contribute to reduced efficiency in photo- and electro-induced nitrogen reduction. A comparative study is provided in Table , summarizing the various operating principles and key performance characteristics of PNRR & ENRR.

1. Comparative Description of P-NRR and E-NRR, Highlighting Key Features.

features	P-NRR	E-NRR
Energy Source	Solar or artificial light drives the reaction via photon absorption.	External electrical energy is applied to an electrochemical cell.
Driving force	A semiconductor absorbs photons → electron–hole pairs reduce N2.	An applied potential induces electron transfer at the cathode during N2 reduction.
Reaction Sites	Surface of photoactive semiconductor	Cathode–electrolyte interface within an electrochemical cell
Operating Conditions	Ambient T, P; no external bias; light intensity-dependent.	Ambient T, P; controlled by applied voltage and electrolyte composition.
Efficiency Range	Quantum yield ≤ 1%; dependent on light wavelength.	Faradaic efficiency ≈ 0.1–10%; depends on overpotential.
Major limitations	Short lifetime of exciton pairs, low adsorptivity of N2 on the catalyst surface	Lower stability of the catalyst, competing with the hydrogen evolution reaction (HER)
Benefits	Use of solar energy is widely accepted, so the setup is easy and convenient.	Electron supply occurs in a more controlled manner and is effective at higher current densities.

2.4. Catalytic Metrics: Faradaic Efficiency, NH₃ Yield Rate, and Selectivity

2.4.1. Faradaic Efficiency

The major side reactions in E-NRR are HER and hydrazine (N₂H₄) generation, so it is very important to determine the selectivity of NH₃ generation in an electrocatalytic system. External energy must be supplied for the production of NH₃ in E-NRR. The FE of NH₃ generation is the ratio of the quantity of electricity used to generate NH₃ (Q _NH3 ) to the total electricity transmitted through the electrode (Q _total).

Mathematically, Faradaic efficiency (FE) = 100% × (3F × C _NH3 × V)/Q _total

Where the factor 3F arises from the requirement of three electrons per NH₃ molecule, F = Faraday constant (96,500), C = Conc. Of NH₃(mol L^–1) in the electrolyte, V = Volume (L) of the electrolyte, Q _total = Total quantity of electricity passed through that electrolyte.

The Faradaic efficiency (FE) can give information about how to design and assess highly selective catalysts. Also, it gives an idea about the impact of side reactions on the evolution of NH₃. In E-NRR, the Faradaic efficiency (FE) for NH₃ selectivity is typically low.

2.4.2. NH₃ Yield Rate

In E-NRR, the NH₃ yield can be expressed in various units that represent the quantity of NH₃ formed per hour (or per second) per unit of catalyst. Commonly used units include μg h^–1 cm^–2, μg h^–1 mg^–1, μmol s^–1 cm^–2, μmol h^–1 g^–1. In the above units, cm^–2 stands for the geometric area of the catalyst on the working electrode, and mg^–1 depicts normalized catalyst quantity. Mathematically, ammonia (NH₃) can be illustrated by the equation r _NH3 = (C _NH3 × V)/(t × Δ), where V is the volume­(mL) of the electrolyte in the cathode cell, t is the time (s, min or h), Δ and is the catalyst amount (mg_cat or mg_metal). The currently attained NH₃ yield is typically at the μg level; therefore, many researchers report NH₃ yield in μg h^–1 cm^–2 and/or μg h^–1 mg^–1 as preferred units. −

2.4.3. Selectivity

The HER has a lower overpotential than N₂ reduction; therefore, hydrogen (H₂) formation is more kinetically facile than NH₃ formation, which leads to very low NH₃ selectivity and Faradaic efficiency (FE). Although the low overpotential indicates superior energy conversion, it is associated with low selectivity. To find out the cause of the low selectivity of solid electrocatalysts toward the synthesis of ammonia, Singh et al., by taking a qualitative model, proved that the rate of ammonia production is a zero-order reaction with respect to electron and proton densities. At the same time, the HER is a first-order reaction. To increase the selectivity of ammonia (NH₃), the limitation of either electrons or protons at the electrode surface is essential. This can be done by following these four paths: (1) Limit proton availability at the electrode surface by reducing the concentration of proton donors in the bulk solution; (2) Limit the proton transfer rate by protecting the electrode surface with an aprotic, hydrophobic layer; (3) Limit the electron transfer rate by adding a thin insulator between the metal electrode and bulk solution; (4) Limit the electron transfer rate by supplying electrons slowly using photo absorbers. Also, it has been observed that the limitation of proton availability is extremely difficult due to the presence of H₃O⁺ ions (in acidic medium) and H₂O molecules (in basic or neutral solution), which can act as proton donors. From the above discussion, it is clear that decreasing the transfer rate of either electron or proton alone may not be sufficient to solve the problem of low NH₃ yield. Therefore, the low selectivity of N₂ reduction toward NH₃ formation remains a major challenge and necessitates advanced catalyst and electrode design strategies. ,

2.5. Detection of Ammonia

For the detection of ammonia or ammonium content, many methods have been devised by considering the concentration range, application and scientific area. Among the various methods proposed, the most common are spectrophotometry (Colorimetric assays), ion chromatography (IC), and ¹H NMR spectroscopy.

Spectrophotometry (Colorimetric assays) is considered the most economical and accepted method for quantification of ammonia/ammonium ions. These Colorimetric assays can be performed through Nessler’s reagent (NR) assays, contingent on the reaction between NH₃ and Nessler’s reagent or the indophenol blue assay (IB), contingent on the Berthelot reaction. Both these methods have some advantages, like ease of adaptability, high sensitivity, and the requirement of minimal instrumentation. Still, they also have limitations, such as sensitivity to solution acidity/alkalinity, sacrificial agents, metal cations, CO₂ in air, and reaction time or temperature, which hamper the identification and quantification of ammonia. The use of Nessler’s reagent in the NR assay is responsible for its toxicity because it contains poisonous Hg metal. Also, under acidic conditions or at high concentrations, the indophenol blue method is less accurate.

Ion chromatography (IC) is a reliable method for the detection of ammonia as it is highly sensitive and replicable. In this process, the stationary phase consists of the cation-exchange material and the mobile phase contains the eluent. The interaction between the mobile and stationary phases is responsible for the separation of cationic species. These cations pass through the columns at different rates, depending on the interactions, and are thereby separated. The obtained cations can be identified with the help of a conductometric detector by analyzing them with the retention times of known standards. The peak areas of the sample help in quantitative analysis and can be obtained by comparing the peak areas with those of a known standard sample. But the major disadvantages of this method are the instrument cost and the requirement for technical expertise.

The NH₄ ⁺ ions and the well-known alkali and alkaline-earth metal ions can be simultaneously separated and detected in the IC method with high sensitivity and greater reliability in a single run. Still, the major disadvantage of this method is the interference from ions with similar retention times. However, the primary drawback of this method is interference from ions with similar retention times. For instance, the detection of Na⁺ and NH₄ ⁺ ions is difficult when the concentrations of these ions are significantly different because it is very tedious to get sufficient resolution. To overcome the problem several methods are adopted i.e., (a) Use of columns with higher cation exchange capacity that will give better resolution for NH₄ ⁺ ions than Na⁺ ions; (b) Column switching technique can enhance the resolution between NH₄ ⁺ and Na⁺ ions; (c) Also, the application of macrocyclic ligands either in the mobile phase or in stationary phase can eliminate the above problems because these ligands have different selectivity for cations in their cavities.

¹H NMR spectroscopy is the most powerful method for quantifying NH₄ ⁺ ion concentration. This can be done by taking ¹⁵N in place of ¹⁴N as the reactant. This isotopic-labeling experiment selectively quantifies ¹⁵NH₄ ⁺ relative to ¹⁴NH₄ ⁺ by identifying unnecessary contaminants. From this experiment, different patterns are obtained due to heteronuclear coupling between ¹⁴N–¹H and ¹⁵N–¹H, because ¹⁴N (I = 1) and ¹⁵N (I = 1/2) have different spin values, forming a triplet (¹ J = 52 Hz) and a doublet (¹ J = 73 Hz) of the NH₄ ⁺ signal at 7 ppm. In addition to quantification of ¹⁴NH₄ ⁺ and ¹⁵NH₄ ^+, this procedure also enables unambiguous differentiation between the two species. However, this method requires specialized expertise and costly machinery.

From the above-discussed method, it is clear that each technique has its own advantages and limitations and can be applicable for the identification of NH₄ ⁺ concentration. While carrying out the detection, the researcher should consider factors like pH, solvents, coexisting ions and impurities that can negatively affect the measurements. Although ion chromatography is often regarded as a highly reliable technique for quantification, the combination of different approaches leads to accurate detection of ammonia produced during NRR.

3. Synthesis of UIO-66 MOF

Nucleation and growth are key steps in the synthesis of UiO-66-based MOFs, which are widely exploited in practical applications due to their ease of fabrication, high surface-to-volume ratio, and outstanding permanence. However, because the process is sensitive to a number of variables, including reaction time, temperature, and precursor concentration, controlling the size, shape, and degree of flaws in the crystallites is still difficult. The catalytic effectiveness of UiO-66-based materials is substantially controlled by grain size, defect concentration, and porosity, which affect reactant diffusion, charge separation, and active site availability. Thus, customizing UiO-66-based MOFs for effective and sustainable green ammonia synthesis requires a thorough grasp of the synthesis–structure–property interaction. This section discusses the many benefits and drawbacks of the synthetic methods used to grow polycrystalline UiO-66-based MOFs, emphasizing how various strategies affect crystallinity, morphology, and defect concentration, which in turn affect catalytic performance in NRR processes.

3.1. Solvothermal

The solvothermal method is the widespread and most versatile technique for fabricating MOFs, which involves the dissolution of organic linkers (ligands) and a metal precursor in a suitable solvent and is subjected to elevated temperature and pressure in a sealed Teflon-lined stainless-steel autoclave. (Figure a). The solvothermal environment enables the controlled nucleation and growth of crystalline structures over a defined period. Depending on the intended product and its characteristics, solvothermal synthesis can employ a variety of solvents. The procedure is known as hydrothermal when water is employed. DMF, methanol, ethanol, ammonia, and supercritical CO₂ are examples of common solvents that are chosen based on the coordination behavior and solubility of the reactants. The choice of solvent not only affects the solubility of precursors but also plays a critical role in modulating the crystallinity, particle size, and porosity of the resulting material. ,

2.

(a) Evolution of MOF by the reaction between metal nodes and organic molecules as linkers by the solvothermal method. Reproduced with permission from ref , Copyright 2022 American Chemical Society. (b) SEM and TEM images of UiO-66 and chloro@UiO-66e (c) UV–vis DRS of UiO-66 and chloro@UiO-66e.Reproduced with permission from ref . Copyright 2024 American Chemical Society. (d) Formation of MOF by the mechanochemical method. Reproduced with permission from ref , Copyright 2024 Environmental Engineering Research. (e) Thermogram depicting the decomposition of UiO-66-NH₂ at 10 °C min^–1 in aerobic conditions, reproduced with permission from ref , Copyright 2023 American Chemical Society. (f) FTIR spectra of GO-UiO-66-NH₂ and its related materials. Reproduced with permission from ref , Copyright 2025 American Chemical Society. (g) Synthesis of MOF by microwave-assisted methods. Reproduced with permission from ref , Copyright 2022 American Chemical Society.

The solvothermal approach provides a flexible and efficient way to adjust crystal shape, particle size, and defect concentration in the production of UiO-66-based MOFs. Usually, terephthalic acid (1,4-benzenedicarboxylic acid, H₂BDC) or its functionalized derivatives (such as amino-or nitro-substituted BDC) are employed as organic linkers, and metal salts (such as ZrCl₄ or HfCl₄) are employed as metal precursors. Temperature, reaction duration, solvent polarity (usually DMF), metal-to-ligand ratio, and the kind and number of modulators, such as acetic or formic acid, all have a significant impact on the crystallization process. Together, these variables influence the kinetics of nucleation and crystal development, allowing for exact manipulation of structural characteristics and defect engineering. Because of its ability to create highly crystalline, porous frameworks and its scalability and reproducibility, the solvothermal process continues to be a mainstay for the production of UiO-66-based products. , For instance, Gurmessa et al. synthesized pristine UiO-66 using the standard solvothermal route, with ZrOCl₂·8H₂O and BDC as precursors. What actually gives this framework its stability is the strength of the Zr–carboxylate bond itself, and you can see that directly in the FTIR data: the carbonyl stretch at 1660 cm^–1 tells you BDC’s carboxylate groups are coordinating to the Zr-oxo clusters, not just sitting there unreacted. PXRD backs this up too. The sharp peaks at 7.46° and 8.12° are not just “proof of crystallinity” in the abstract sense; low-angle peaks like these specifically point to a porous, ordered cubic structure, since peak intensity at small 2θ tracks with how much pore volume the material has. And that same metal–linker bonding is really what’s behind the thermal behavior too: TGA shows the framework holding together until 450 °C, with the early ∼26.5% weight loss below 300 °C being just solvent and moisture leaving, not the framework itself breaking down. In addition to its thermal stability, porosity significantly influences the properties of UiO-66, making it an excellent material requiring a high surface area, such as for N₂ adsorption.

To improve the visible-light response, Sun et al. synthesized Chlor@UiO-66 via the standard solvothermal route followed by postsynthetic modification (PSM). PXRD confirms that the UiO-66 framework remains intact after chlorophyll loading, and that is actually a more telling result than it first appears; it means the optical and electronic changes discussed later (the band gap dropping from 3.78 eV to ∼1.7 eV) are happening without disturbing the host lattice at all. Chlorophyll is acting as a surface-anchored sensitizer rather than a dopant, so the framework’s porosity and structural integrity stay intact while its electronic behavior gets rewritten from the outside. A morphological study by SEM and TEM of Chlor@UiO-66 shows a particle size of 500–700 nm, indicating the particle is highly dispersed with a uniform octahedral structure (Figure b), which is in agreement with the pristine UiO-66. This also says something about how gentle the PSM step is just soaking UiO-66 in chlorophyll extract and stirring in the dark for 2 h does not etch or reshape the crystals at all, which is a useful design takeaway if the goal is sensitizing a MOF without sacrificing morphology-dependent properties. Successful incorporation of the biobased sensitizer chlorophyll into the Zr-MOF is additionally confirmed by EDS mapping. To elucidate the intermolecular bonding, density functional theory (DFT) techniques were conducted, revealing that chlorophyll molecules adsorb around Zr nodes of UiO-66 via Zr–N interactions, manifesting a stronger adsorption affinity of −1.29 eV compared to −0.47 eV at Zr–Mg interaction sites. This detail carries more weight than it looks like on its own: those open Zr sites only exist because the synthesis deliberately builds in ligand defects using acetic acid as a modulator. So, the defect engineering is not a side step in the protocol; it is the entire reason chlorophyll has somewhere to anchor. Without those missing-linker defects, there would be no exposed Zr–N coordination site, and the sensitization strategy simply would not work. Hence, chlorophyll-sensitized defective UiO-66 is considered a suitable contender for improved photocatalytic applications. Moreover, the UV–vis DRS study validated that Chlor@UiO-66 exhibits strong absorption in the visible region, with the absorption edge shifting toward the red end of the spectrum due to the conjugated structure of the chlorophyll molecule (Figure c), making it an outstanding contender for visible-light-induced catalysis. And this ties back to the structural-retention point, because the underlying Zr-oxo framework is not altered, that red shift can be attributed cleanly to chlorophyll’s own electronic contribution rather than any change in the MOF itself, which the authors later back up directly through the narrowed band gaps (1.67–1.72 eV) and the XPS/XANES shifts at the Zr center, pointing to real electron donation from chlorophyll into the framework.

3.2. Mechanochemical

In the mechanochemical method, percussors usually absorb mechanical energy during mechanical or grinding processes. This absorbs mechanical energy and can initiate a chemical reaction. The amount of mechanical energy absorbed depends on the impact between the ball and the reactants. Also, excessive impact may destroy the starting material, leading to an amorphous product. Generally, for optimization and to obtain a high-crystallinity product, a small amount of solvent should be added to the reactants during milling, as the applied liquid enhances molecular mobility, increases reaction rates, and produces high-crystallinity materials (Figure d). − By applying mechanochemical methods, many popular MOFs, such as UiO-66, MIL-101, ZIF-8, and HKUST-1, are synthesized with higher surface areas than those obtained by conventional methods. For instance, in 2023, Wenger et al. prepared amino-functionalized UiO-66 using a greener, sustainable approach, distinct from the high-energy solvothermal method that relies on toxic DMF as the solvent. This represents a deliberate mechanistic shift rather than a simple solvent substitution: mechanochemical synthesis drives framework assembly through molecular diffusion and direct mechanical energy input instead of solvent-mediated nucleation, which is what allows the reaction to proceed in 90 min rather than the 24 h typically required under solvothermal conditions. This sustainable pathway uses zirconium oxychloride (ZrOCl₂·8H₂O) as the metal node, 2-aminoterephthalic acid as the organic linker, and glacial acetic acid as the liquid-assisted grinding agent to facilitate crystal growth. The obtained MOF’s structural robustness and crystallinity were verified by PXRD, which matched the simulated UiO-66-NH₂ pattern via Le Bail refinement, confirming successful framework formation. The TGA further evaluated the thermal stability of this MOF (Figure e), and the resulting weight-loss profile was additionally used to estimate the linker-to-node ratio in the framework, which was found to be 6.7 linkers per Zr₆ cluster rather than the ideal value of 6. This suggests a measurable population of missing-linker defects, consistent with the comparatively fast, kinetically driven crystallization characteristic of mechanochemical synthesis relative to the slower, thermodynamically controlled growth typical of solvothermal routes.

To lessen the environmental impact, Bigdelifam et al. synthesized a GO nanocomposite via a green, biphasic mechanochemical ball-milling process, preferring a solid-state reaction over a conventional solvent-consuming process. At the outset, graphene oxide (GO) underwent a chlorination phase by treating with phosphorus­(V) chloride (PCl₅) to form GO-Cl. This intermediate was thereafter coground with 2-aminoterephthalic acid (NH₂–BDC) and zirconium­(IV) acetate in successive 40 min intervals to facilitate the formation of the MOF structure directly on the graphene framework. The choice of zirconium acetate over more common zirconium salts is itself a deliberate design decision: acetate functions as a Brønsted base within the reaction mixture, promoting deprotonation of the linker’s carboxyl groups and thereby facilitating coordination between the metal center and the organic ligand under purely mechanical, solvent-free conditions. Following synthesis, the material was washed with ethanol and DMF to remove residual unreacted precursors. Characterization techniques confirm the successful integration of the amino-functionalized MOF onto the GO surface. XRD patterns illustrate the structural integrity of the composite, as it retained the characteristic crystalline peaks of UiO-66-NH₂, while the primary GO peak at 2θ = 12.2° was no longer observed. Rather than indicating loss or degradation of the GO component, this disappearance reflects effective dispersion: the low GO content present is intercalated by UiO-66-NH₂ particles to the point that GO’s own diffraction signature is suppressed, pointing to intimate physical contact between the two phases rather than simple physical blending. FT-IR spectroscopy further confirmed the chemical modification, specifically noting the development of a C–Cl vibration at 824 cm^–1 amid chlorination and subsequent NH₂ and C–N stretching vibrations (3458–1373 cm^–1) in the final composite (Figure f). Notably, the oxygen-containing functional peaks of GO itself decreased in intensity within the composite, which is attributed to direct coordination between GO’s carboxyl groups and Zr⁴⁺ centers. This is the chemical basis for the hybrid’s stability: GO is not merely a passive scaffold for MOF deposition but is covalently/coordinatively integrated into the framework through its own oxygen-containing groups bonding with the amine groups and open metal sites of UiO-66-NH₂. A morphological study using SEM revealed that the typically wrinkled GO sheets were homogeneously decorated with cubic MOF nanocrystals, with particle sizes ranging from 80 to 105 nm.

3.3. Microwave

The microwave technique was discovered by P.L. Spencer in 1946 at Raytheon Corporation. This method is used for the rapid preparation of MOFs by using microwave radiation as the source. To synthesize the material, the starting materials along with the solvents are put in a Teflon vessel and placed in a microwave at a suitable temperature and time (Figure g). Microwave radiation creates thermal energy by inducing rotation in dipolar molecules, producing heat through molecular friction in the liquid mixture. As electromagnetic radiation directly interacts with the solution of the precursors, this process is not energy-intensive and comes under the category of greener synthetic methods. In microwave synthesis, the obtained product has high selectivity and good crystallinity. Kang et al. prepared UiO-66 using a green, sustainable microwave synthesis method, employing TA, Zr (OC₃H₇)₄, CH₃COOH, and DMF as precursors. These precursors were loaded into an MW-inert glass tube, which was then irradiated with microwave radiation. The obtained materials were subjected to centrifugation, followed by washing and drying to get the product. ,

In this regard, Guerrero et al. synthesized Pd@UiO-66-NH₂ via microwave synthesis using a mixture of ZrOCl₂·8H₂O and BDC-NH₂ in DMF with trifluoroacetic acid (TFA) as modulator. The choice of TFA over other common modulators is itself a deliberate design decision rather than an arbitrary one: stronger acids with low pK _a values remain bound to the Zr-cluster throughout synthesis, leaving missing-linker positions once the material is activated, whereas weaker acids with a pK _a closer to that of the linker itself (such as acetic or formic acid) are more readily displaced by the linker, yielding larger but less defective crystals. TFA was therefore selected specifically because it maximizes defect generation while preserving structural stability. To confirm the success of synthesis, PXRD depicts the retention of the characteristic UiO-66 framework, though a degree of peak broadening was observed following Pd­(II) loading and its subsequent reduction to Pd nanoparticles, indicating a slight, expected degree of matrix degradation associated with the harsh in situ reduction conditions (NaBH₄) rather than with the parent MOF synthesis itself. This structural robustness was further corroborated by FTIR, where the persistence of characteristic peaks corresponding to −NH₂ (3500–3390 cm^–1), COO^– (1573, 1385 cm^–1), and aromatic CC (1499 cm^–1) confirmed that Pd incorporation did not disrupt the organic linker environment. However, TGA revealed thermal stability of up to 300 °C, lower than that of pristine UiO-66-NH₂, suggesting that microwave irradiation introduced structural defects into the framework despite overall crystallinity being preserved. This defect generation is not an incidental side effect but a functional necessity: missing-linker defects expose additional coordinatively unsaturated Zr sites, which are essential for anchoring and stabilizing the Pd species during nanoparticle formation. In this sense, the very feature that lowers thermal stability is the same feature that enables the composite’s catalytic function. Well-dispersed Pd nanoparticles throughout UiO-66-NH₂ were confirmed by STEM. Complementing this, XPS revealed a predominant Pd⁰ signal alongside a minor Pd²⁺ fraction, confirming that the reduction step successfully generated metallic Pd nanoparticles, with only a small portion reoxidized upon exposure to air. Separately, the N 1s peak was found to shift to higher binding energy and broaden relative to pristine UiO-66-NH₂, which is the evidence the authors point to for interaction between the framework’s amine groups and the Pd species, indicating strong metal–support interactions that anchor the Pd nanoparticles within the framework.

In contrast to the postsynthetic method (PSM), Fu et al. prepared Cr-doped UiO-66 by an in situ approach, which modifies the properties during crystallization, thereby providing better sustainability. The accuracy of Cr-doped UiO-66 was confirmed by the XRD study, which gives three dominating peaks at 7.4°, 8.5°, and 25.8° that correspond to (111), (200), and (600) crystal planes, respectively (Figure a). The fact that these peak positions remained unshifted relative to undoped UiO-66 indicates that Cr incorporation did not substitute into or distort the long-range Zr₆O₄(OH)₄ cluster framework; rather, the dopant appears to be accommodated without disrupting the parent crystal lattice, consistent with the absence of any new vibrational bands in the corresponding FTIR spectra. Building on this structural preservation, BET analysis revealed that Cr doping enhanced the surface area to 1524 m²/g for the most heavily doped sample (MW-1.5Cr-UiO-66), compared to 1272 m²/g for the undoped microwave-synthesized UiO-66, suggesting that Cr incorporation introduced additional porosity. This porosity gain is corroborated by the pore size distribution data, which show new and more numerous peaks in the Cr-doped samples relative to the undoped material, pointing to a redistribution of micropore sizes rather than simple bulk enlargement of existing pores. However, this near-20% gain in surface area did not translate proportionally into water uptake: saturated water vapor adsorption rose by only 8.9% across the same series. The authors attribute this disconnect to the underlying adsorption mechanism itself, because UiO-66’s micropores (0.5–1.0 nm) sit below the critical pore diameter for capillary condensation of water (2.0 nm); water is taken up through cluster formation within the pores rather than through pore filling. Under this mechanism, narrow pore necks restrict water molecule diffusion regardless of how much additional surface area or micropore volume is generated by Cr doping, which is why the porosity and uptake gains scale so differently.

3.

(a) XRD patterns of Cr-doped UiO-66. Reproduced with permission from ref . Copyright 2021 Energy Proceedings. (b) Procedure for synthesis of HP-UiO-66 by the reflux method; (c) FESEM images of HP-UiO-66; (d) EDX study of HP-UiO-66; (e) nitrogen adsorption/desorption isotherms; (f) BJH pore size distribution for HP-UiO-66. Reproduced with permission from ref . Copyright 2024 Environmental Science and Pollution Research.

3.4. Reflux

Reflux is an extensively used conventional technique for the synthesis of nanomaterials, enabling continuous heating of the reaction mixture at a constant temperature, typically the boiling point of the solvent, without the loss of solvent due to evaporation. This is achieved by using a condenser, which cools the vapor back into liquid form and returns it to the reaction flask. In MOF synthesis, reflux plays a pivotal role by providing a stable thermal environment that enables the Lewis acid–base interaction between metallic cations and organic building blocks. The constant temperature and solvent retention enable uniform crystal growth over extended reaction times, thereby improving crystallinity and morphology. Unlike solvothermal methods that require sealed, high-pressure autoclaves, the reflux method is simpler, safer, and more scalable, operating under atmospheric pressure (Figure b). For lab-scale or modified MOF synthesis, it is a highly efficient method since it provides greater control over functionalization and defect engineering. The development of distinct and highly crystalline MOF structures is promoted by consistent heating. It lowers the possibility of dry reactions and overheating by continuously condensing and returning the solvent to the flask. This guarantees that during the synthesis, the reaction environment stays constant. , For instance, Eram et al. disclosed the synthesis of hierarchically mesoporous UiO-66 using a sacrificial Zn₄O­(BC)₆ precursor template, prepared by refluxing zinc nitrate and benzoic acid in DMF at 120 °C for 12 h. Zirconium chloride (ZrCl₄) and 1,4-benzenedicarboxylic acid (H₂BDC) were then introduced to this mixture and refluxed for a further 24 h at the same temperature to grow the Zr-BDC framework around the zinc-benzoate scaffold. This templating step is the actual source of the hierarchical mesoporosity: rather than crystallizing UiO-66 directly, as in conventional solvothermal synthesis, the Zn₄O­(BC)₆ template is destroyed and washed away with successive DMF rinses, leaving behind void spaces within the otherwise microporous Zr framework. Without this sacrificial step, the product would be the conventional microporous UiO-66 rather than the meso/macroporous hierarchical structure targeted here. The obtained suspension was then washed repeatedly with DMF until the zinc template was fully removed, followed by acetone washes to eliminate residual DMF, before drying and vacuum activation to obtain the final product. Structural, morphological, and compositional characterization via FESEM (Figure c) and EDX (Figure d) depicted a clear porous, sponge-like structure characterized by an average particle size of less than 100 nm, with most particles retaining a broadly cubic morphology despite some deviation toward more spherical shapes. This irregular, sponge-like character is itself consistent with the templating mechanism described above: the random distribution of porous cavities reflects the random spatial arrangement of the now-removed zinc-benzoate template within the growing Zr framework, rather than the regular, well-defined micropore lattice typical of conventionally synthesized UiO-66. These hierarchical structures were further supported by an extensive surface area of 1011 m²/g (Figure e) and an internal pore volume of 0.76 cm³/g (Figure f).

Advancing toward a more sustainable protocol, Gao et al. synthesized carboxyl-functionalized UiO-66-(COOH)₂ in a green, DMF-free medium by taking 1,2,4,5-benzene tetracarboxylic acid (H₄btec), zirconium tetrachloride (ZrCl₄), and H₂O as solvent, with constant reflux heating at 100 °C for 24 h. This DMF-free, water-based route is a deliberate departure from the conventional solvothermal synthesis of UiO-66 analogues. The structural and morphological characteristics of the resulting framework were confirmed using various characterization techniques, such as PXRD and SEM. PXRD plays a particularly important role beyond simply confirming successful synthesis: the diffraction peaks remained unchanged when the material was immersed in solutions across a pH range of 2 to 7, demonstrating that the framework retains its crystallinity even under the acidic conditions later required for cation adsorption, a prerequisite for any practical use of the material’s carboxyl groups as binding sites. The peak at 1700 cm^–1 in FTIR spectra indicates that the material has a −COOH group, which is responsible for the capture of metal ions. This is possible because H₄btec carries four carboxylate arms, but only two of these coordinate to the Zr₆O₄(OH)₄ nodes during framework formation; the remaining two arms stay uncoordinated and are uniformly distributed throughout the structure. This is the structural basis for the material’s selectivity, as the free carboxyl groups are accessible for binding cations without requiring any disruption to the underlying UiO-66 topology, which is what allows the framework to capture metal ions while still retaining its parent crystal structure and porosity.

The preparation of UiO-66 and its derivatives by various synthetic methods, including solvothermal, mechanochemical, microwave, and reflux methods, offers distinct advantages in crystallite size, morphology, etc., but also has limitations, such as prolonged reaction times and limited modularity. By varying parameters such as temperatures, solvents and modulators, various formation pathways of UiO-66 can be engaged, depicting its robustness as a wonderful material for manifold applications. Table below summarizes the pros and cons of the above synthesis methods, along with the effects of various parameters on them.

2. Comparative Study Depicting the Advantages and Disadvantages of Various Synthetic Methods, along with Their Effects on the Structural and Catalytic Activities of UiO-66.

synthesis method	influence on UiO-66 properties	effect on catalytic parameters	advantages	limitations
Solvothermal	High crystallinity, well-defined morphology of the octahedral type; Adjustable size, porosity, defect density; Choice of modulator and solvent determines the quality of the framework	High surface area amplifies adsorption of N2; Modulator-promoted defects introduce active sites by exposing the metal nodes; Controlled defect density improves NRR selectivity.	Highly flexible and reproducible; Defect density is well controlled; Large-scale applications; The most well-established method is the UiO-66 framework.	Operates at high temperature, high pressure; Reaction times are long; Mostly uses environmentally hazardous solvents.
Mechanochemical	Moderate crystallinity; small, relatively uniform particles; Solvent-free process; supports direct composite fabrication.	Smaller particle size improves active site exposure; Composite integration improves interfacial charge transfer.	Solvent-free and environmentally friendly; Energy consumption is low; One-step composite formation is enabled.	Excessive milling may cause framework amorphization; Limited ability to control particle size uniformity; Scalability and reproducibility validation still needed.
Microwave	Fast nucleation rate, good crystallinity and selectivity; High surface area in functionalized UiO-66 analogues; Homogeneous dispersion of incorporated moieties	Enhanced surface area boosts N2 adsorption capacity; Microwave-induced defects result in increased active sites.	Fast and energy-saving method; More sustainable than the traditional method, as it is a greener method; Uniform morphology of the particles	Can lead to defects, reducing thermal stability; Usually on a small scale; Defect type and density are difficult to control
Reflux	Uniform crystal growth under stable atmospheric conditions; Compatible with DMF-free and aqueous synthesis routes; Accessible mesoporous structure	The mesoporous structure facilitates reactant diffusion and mass transfer, enabling controlled functionalization and defect engineering.	Easier and safer, no need for an autoclave; Scalable with higher precision in functionalization; More flexibility in greener solvents	Longer reaction times; lower precision in defect engineering compared to solvothermal; Lower surface area depending on the conditions

4. In Situ Characterization Techniques

In moderate conditions, the reduction of N₂ to form NH₃ is extremely difficult due to the high bond strength of NN and the competitive hydrogen evolution reaction (HER). To determine the particular reaction pathway, a complete understanding of the catalyst surface and key intermediates under real operating conditions is required. ,

For this, different characterization techniques are performed to maintain the gap between the theoretical evaluations and real-time catalyst behavior by monitoring the electronic configuration and tracking the various reaction intermediates.

4.1. Vibrational Spectroscopies: For Intermediates

These techniques are particularly used for analyzing various reactive intermediates at the solid–liquid–gas interface.

4.1.1. In Situ FTIR

It primarily detects the catalytic surface with the help of infrared radiation. In NRR, the formation of N–N and N–H bonds can be determined with the help of stretching frequencies. Also, during the mechanistic pathways, the formation of various intermediates (*N₂H _y ) depicts whether the reaction follows an associative distal or alternating pathway. liquid-phase or surface-bound coreactants. Also, it gives information about the competing HER by auditing liquid-phase or surface-bound coreactants.

4.1.2. In Situ Raman Spectroscopy

This is a highly sensitive spectroscopic characterization technique, particularly used to determine symmetric molecular vibrations of homodiatomic molecules and metal–ligand bonds that exhibit low frequencies.

In NRR, it shows the formation of M–N bonds upon chemisorption of N₂ molecules on the surface of the catalyst. In the formation of NH₃ by UiO-66 and its derivatives, this technique stands as a diagnostic tool for monitoring localized structural collapse during negative potential polarization.

4.2. Electronic and Chemical State Analysis

This type of analysis gives a proper idea about the oxidation states of active metal centers and how these metals transfer these electrons to the adsorbed N₂ on the surface of the catalyst during NRR.

4.2.1. In Situ X-ray Photoelectron Spectroscopy (XPS)

Unlike XPS, which depends on ultrahigh vacuum, in situ electrochemical XPS, which operates at ambient pressure, helps researchers to analyze the changes in the surface in the operating environment.

In NRR, this technique helps reveal the creation of superior mixed-valence pairs accelerated by an applied potential. This movement of electrons helps in identifying whether a particular oxidation state is responsible for a negligible potential, which is considered the rate-determining step during the activation of nitrogen.

4.2.2. In Situ X-ray Absorption Spectroscopy (XAS)

In situ XAS, comprising X-ray absorption near-edge structure (XANES) and Extended X-ray absorption fine structure (EXAFS), gives information about the electronic and structural properties of a specific element.

In NRR, XANES determines the real-time changes in the oxidation state of the metal under high reducing potentials, whereas transient bond lengths and coordination number are obtained by EXAFS. Both these characterization techniques help the researchers to design modified structures where defects or amorphous active sites are formed that enhance the NRR performance.

5. Features of UIO-66: Framework, Stability and Porosity

UiO-66­(Zr) is a promising UiO (University of Oslo) family, prepared by taking ZrCl₄ salts (Zr⁴⁺) as the metal center and Terephthalic acid (TPA) as the linker. It consists of Zr₆O₄(OH)₄ units connected by 1,4-benzenedicarboxylic acid (BDC) linkers, leading to a Face-centered cubic (FCC) structure, and each Zr ion is coordinated by 8 oxygen atoms. The opening of UiO-66 consists of four octahedral enclosures­(1.1 nm) and eight tetrahedral cages (0.8 nm). Access to these voids is facilitated by triangular windows with a diameter of 0.6 nm. , There is a strong coordinate bond between the oxyphilic Zr⁴⁺ cluster and the carboxylate groups of the BDC linkers, resulting in high thermal, chemical, and mechanical stability of UiO-66, making it a robust candidate for various catalytic applications. Moreover, the stability of MOFs depends on the nature of the metal clusters and organic linkers, as explained by HSAB theory. In UiO-66, the metal center, Zr⁴⁺ (a hard metal), and the linkers’ carboxylate ions (O, a hard base) confer exceptional stability compared to other combinations, such as Zn²⁺ (a soft metal) in place of Zr. Therefore, the Zr-based UiO-66 is considered a suitable candidate for various research applications due to its robust framework and versatility. Kim et al. demonstrated that UiO-66 exhibits exceptional stability in water and acidic conditions but is relatively unstable in strongly basic conditions. Under basic conditions, the hydroxide ions (OH^–) can easily attack Zr–carboxylate coordination bonds, leading to the rupture of the framework. The structural and morphological integrity of UiO-66 MOF toward H₂O and other solvents can be analyzed by different spectroscopic techniques like SEM, High-performance liquid chromatography (HPLC), Brunauer–Emmett–Teller (BET) surface area and XRD patterns, etc. The XRD patterns show that UiO-66 exhibits excellent structural and mechanical stability against solvents like water, acetone, benzene and DMF, etc. , Thus, it is assumed that the rigid Zr₆O₄(OH)₄ cluster plays a pivotal role in regulating the structural stability of UiO-66. In the structure of UiO-66, each Zr metal center is 8-coordinated to the BDC ligand. This high coordination number of Zr is among the highest reported for metal nodes in MOFs, contributing significantly to the framework’s robustness. ,

The structural derivatives of UiO-66, such as UiO-66-NO₂, UiO-66-NH₂, UiO-66-(CH₃)₂, and UiO-66-Br, have been investigated for their resistance to air and water. These compounds are stable for a month under air and remain stable in water for up to 10 days. This indicates that there is no significant change in the crystallinity of the mentioned UiO-66 derivatives within this duration, as confirmed by XRD analysis. From this study, it is observed that UiO-66-(CH₃)₂ remains stable under strongly basic (pH 14) and strongly acidic (pH 1) conditions. However, the pristine UiO-66 MOF exhibits stability up to PH 12. It is concluded that UiO-66 exhibits remarkable stability in acidic and neutral conditions, with moderate resistance to basic environments, depending on functionalization. , Porosity is one of the most important features of UiO-66-based MOFs, and it ultimately depends on the synthesis method. This porosity is due to the different arrangements of metal nodes and linkers in the backbone of the MOF structure. It can be varied through well-known postsynthetic modification strategies by altering the functionality of linkers and by introducing defects. , Porosity and surface area are complementary; i.e., high porosity is always associated with high surface area. For example, the pristine UiO-66 has a BET surface area of approximately 1200 m²/g and a pore volume of 0.49 cm³/g. Likewise, the modulated synthesis of defective UiO-66 using terminal carboxylic acids exhibits a significantly higher surface area of 1890 m²/g and a pore volume of 0.88 cm³/g. ,

As a result, the modulated UiO-66 exhibits an outstanding adsorption capacity of 366 mg/g, which is higher than that of pristine UiO-66 (39 mg/g). He et al. synthesized defective UiO-66-HAc, which creates linker defects in the MOF structure, exposing more Zr sites as active sites. This enhancement increases the electrochemical surface area and facilitates N₂ adsorption, ultimately improving the conversion of N₂ to NH₃. Similarly, Shi et al. synthesized phosphomolybdic acid-mediated defective UiO-66–2HPMo, which exhibits excellent nitrogen reduction reaction efficiency because the introduction of defects increases the specific surface area and the number of catalytic centers, leading to improved R _NH3 .

6. Modification Strategies

Doping, functionalization, and composite formation are among the most effective strategies for tuning the surface, electronic, and structural properties of metal–organic frameworks, thereby enhancing their performance in energy storage and environmental applications. In a doping strategy, the intrinsic electronic structure of a semiconducting material changes without largely altering the host crystal framework. By introducing dopants or defects, it creates band gap shortening, expands the light-absorption spectrum and suppresses fast electron–hole recombination due to the formation of localized charge-trapping centers. −

Functionalization of MOFs involves the incorporation of specific structural groups, such as −NH₂, −SH, −SO₃H, and −NO₂, onto organic linkers, or partial ligand substitution, which can regulate light absorption, electronic distribution, adsorption selectivity, and surface polarity. To overcome intrinsic limitations of MOFs, particularly low electrical conductivity and limited mechanical robustness, composite formation with other conductive or semiconducting materials, such as metal oxides, CNTs, graphene, or metal nanoparticles, is widely utilized. Together, these unifying design principles construct a tailored interface, which generates synergistic interfacial interactions that drastically improve charge transfer, structural stability, and overall catalytic efficiency (Scheme ). ,

2. Modification of UiO-66 and Its Derivatives for NRR.

6.1. Doping

Metal-ion doping is a useful strategy for adjusting the electronic properties of UiO-66 without damaging the framework. The pristine UiO-66 exhibits low visible-light absorption and a moderate electron density at the Zr active sites. This limits efficient π-backdonation to the adsorbed N₂ molecules. The introduction of low-electronegativity metal ions can redistribute electron density at the catalytic sites, thereby improving N₂ activation.

For instance, Tan et al. doped UiO-66-NH₂ with Ca²⁺. Because of its lower electronegativity compared to Zr⁴⁺, Ca²⁺ acts as an electronic modulator rather than an active site. XPS results showed a lowering in the binding energy of the Zr 3d peak, suggesting an electron enrichment at the Zr sites. DFT calculations also supported the increased electron back-donation to the π* orbital of N₂, making it easier to ignite the NN bond. In addition, Ca²⁺ doping enhanced the separation efficiency of electron–hole pairs, as indicated by decreased PL intensity, increased carrier lifetime, and reduced interfacial charge-transfer resistance. The addition of diamino ligands further reduced the bandgap energy (from 2.8 to 2.15 eV), thereby enhancing visible-light absorption and electron availability. The optimized UN­(Zr-0.30Ca)-4 photocatalyst exhibited an NH₃ evolution rate of 759.3 μ mol g^–1 h^–1, which was about seven times higher. Hence, the synergistic effect of Ca²⁺ incorporation and diamino ligand modification led to a reduction in the bandgap and enhanced the efficiency of carrier separation, proving that metal doping can control both structural and conductivity behavior of UiO-66.

Based on the Ca²⁺ doping approach mentioned above, Tan et al. in 2021 further proved that postsynthetic Ti⁴⁺ exchange into UiO-66-NH₂ achieves a more integrated framework modification at the metal nodes, which directly affects the E-NRR. In this case, Ti⁴⁺ partially replaces Zr⁴⁺ in the Zr-oxo clusters rather than being deposited on the surface, as evidenced by the XRD pattern retaining the framework structure with some peak shifts to higher 2θ values due to the contraction of the lattice parameters caused by the smaller ionic radius of Ti⁴⁺. No extra diffraction peaks appeared for TiO₂, which means that the metal node exchange was successful without the formation of secondary phases. Nevertheless, from the structural analysis, it was found that the increased Ti⁴⁺ substitution resulted in a depletion in BET surface area and micropore volume, and TEM images also indicated a morphology change from well-defined octahedral crystals to more rounded particles at longer durations of metal node exchange, which implies that excessive substitution could destroy the ordered framework. Despite the observed decrease in porosity, moderate Ti doping has been found to improve the NRR performance. The UiO–Zr-Ti-5d sample with the optimal doping level shows an NH₃ formation rate of 1.16 × 10^–10 mol cm^–2 s^–1 with a FE of 80.36% at −0.3 V vs RHE, which is superior to that of the pristine UiO-66-NH₂ material. Electrochemical impedance spectroscopy indicates a decrease in electron transfer resistance after Ti doping, suggesting an efficient electron-transfer process. At the same time, the C _dl results verify an enhanced electrochemically active surface area for the optimally doped sample. The enhancement is attributed to the role of Ti species as extra-active centers for N₂ activation, enhanced charge-transfer kinetics, and the inhibition of the challenging HER through the synergistic coexistence of Zr and Ti sites. However, an overdose of Ti substitution causes degradation in structure and performance, thus underlining the importance of controlled dopant concentration to achieve a balance between active site density, electronic conductivity, and framework integrity.

For the enhancement of NRR performance, researchers form structural defects by dispersing transition metals into the structure of UiO-66. These single metal atoms attach to the coordinates of missing Zr nodes or linkers form lower-coordination states with excellent orbital overlaps, thereby reducing the energy barrier for the flow of electrons into the π* ABMO of N₂.

6.2. Functionalization

Besides metal doping, UiO-66 functionalization by linker modification or postsynthetic anchoring has been extensively investigated to control its electronic properties and catalytic performance. Compared with UiO-66, UiO-66-NH₂ shows enhanced N₂ adsorption capacity and catalytic activity. The addition of the −NH₂ functional group reduces the band gap and increases light absorption, thereby improving charge separation efficiency and catalytic performance for NH₃ production, with high stability and recyclability.

Chlorophyll-sensitized defective UiO-66 (Chlor@UiO-66) was synthesized by postsynthetic modification, as reported by Sun et al. The UiO-66 framework was found to be intact after chlorophyll modification, as evidenced by PXRD, SEM, and TEM analysis. FTIR, XPS, XANES, and EXAFS studies confirmed the coordination interaction between chlorophyll and the exposed Zr atoms, which formed Zr–N bonds. The XPS spectrum showed that the Zr 3d peaks were negatively shifted, suggesting that the electron density around the Zr centers increased because of the electron-donating property of the chlorophyll porphyrin ring. The XANES spectrum also confirmed the increased electron density around the Zr atoms. UV–vis DRS analysis revealed a large red shift and a reduction in the band gap energy from 3.78 eV (UiO-66) to about 1.7 eV after chlorophyll sensitization. Photoluminescence analysis indicated suppressed recombination of photoexcited carriers. Mott–Schottky analysis revealed a more negative conduction band edge for Chlor@UiO-66 than for UiO-66, which facilitated electron transfer for N₂ reduction. DFT calculations supported interfacial electron transfer from chlorophyll to Zr-oxo clusters (Δq = 0.14e) and found that the E _a for the rate-limiting step (*N₂ → *NNH) was lessened following functionalization. Moreover, the calculated N₂ adsorption energy became more negative than that of UiO-66, suggesting improved N₂ activation.

Functionalization of UiO-66 thus alters the electronic environment of the Zr nodes, extends the visible absorption band, and facilitates N₂ adsorption and activation without compromising structural integrity.

Introducing certain functional groups, such as –NH₂, into the organic linker alters the local electronic environment. This –NH₂ group creates hydrogen-bonding networks by altering the framework’s hydrophilicity. Although the amino functionalized ligands do less polarize the strong N₂, it influences the local electron density and electrostatic interaction of polar reaction intermediates or protons, acting as a cocatalyst that enhances proton-coupled electron transfer (PCET).

6.3. Composite Formation

Besides doping and functionalization, the synthesis of composite materials has also been proven to be an efficient method to overcome the inherent disadvantages of bare UiO-66, especially its low electrical conductivity and fast charge recombination rate. By combining UiO-66 with conductive or photoactive components, the charge-transfer process at the interface, particle dispersion, and adsorption capacity can be greatly optimized. −

Ren et al. prepared a UiO-66/PDA/CNNS composite via bioinspired adhesion using polydopamine (PDA) as an interfacial bridge. Structural characterization (SEM and TEM) revealed that CNNS strongly interacted with the octahedral UiO-66 surface, presenting an armor-like structure, in contrast to the aggregation observed in the physical mixture. XRD and FTIR results confirmed that the UiO-66 crystal structure was preserved after composite preparation. The addition of PDA facilitated the creation of a four-step electron transfer process (CNNS → PDA → ligand → Zr metal node), which increased the average fluorescence lifetime from 6.9 ns (UiO-66) and 3.3 ns (CNNS) to 8.1 ns for the UiO-66/PDA/CNNS composite. The intensity of photoluminescence was reduced substantially, suggesting that the recombination of the photoexcited carriers was suppressed. UV–vis DRS spectra indicated that the absorption edge broadened into the visible region, and band alignment analysis indicated that the energy was favorably aligned for the transfer of electrons from CNNS to UiO-66. In addition, the results of DFT and GCMC calculations indicated that the N₂ adsorption–desorption was increased and the activation energy barrier of the rate-determining step was decreased. Consequently, the optimized UiO-66/PDA/CNNS hybrid showed an NH₃ evolution rate of 147.8 μ mol h^–1 g^–1 under full-spectrum irradiation, which was much superior than that of UiO-66 and the physically mixed composite.

For electrocatalytic NRR, Chen et al. obtained UiO-66-NH₂/CNT combinations via a hydrothermal method. The XRD results established that the UiO-66-NH₂ framework structure was preserved after the introduction of CNTs. The TEM images revealed that UiO-66-NH₂ nanocrystals were evenly distributed within the CNT network, which suppressed the aggregation of UiO-66-NH₂ nanoparticles and slightly reduced the average particle size. The CNT network ensured a continuous conductive channel for electron transfer in the NRR process. XPS results showed that the chemical state of Zr was not affected, confirming that CNT mainly served as a conductive support. Electrochemical tests revealed that NH₃ yield increased after CNT addition, due to enhanced charge-transfer efficiency. Nevertheless, as CNT concentration increased, HER activity also increased, leading to a gradual reduction in Faradaic efficiency. The UiO-66-NH₂/CNT-0.15 catalyst showed the highest NH₃ yield of 1.85 × 10^–10 mol cm^–2 s^–1 with a FE of 37.78% at −0.3 V vs RHE. These works have shown that the formation of composites in UiO-66-based systems mainly improves catalytic activity by modifying charge transfer at interfaces, dispersing active sites, broadening light absorption ranges (in photocatalysis), and creating conductive electron channels (in electrocatalysis), while preserving the inherent structural stability of the MOF structure.

In neat UiO-66, the genuine active centers for NRR are particularly exposed at coordinately unsaturated Zr sites formed by missing linkers or at dual metal centers (Zr/Ti synergism). Upon creating composites with various conductive materials such as MXenes, graphene oxide, etc., these active sites form strong chemical bonds with the parent material, which suppress the competing HER, enhance the interfacial charge transfer and stabilize the N₂ intermediates to propel high FE. Table comprehensively summarizes the various modification techniques employed for UiO-66-based materials and their effects on the underlying mechanisms governing P-NRR and E-NRR performance. Moreover, A detailed account of the structural engineering strategies and mechanistic pathways of UiO-66-based catalysts for both P-NRR and E-NRR is further presented in Table .

3. Summary of Modification Strategies and Underlying Mechanisms in UiO-66 for Photocatalytic and Electrocatalytic Nitrogen Reduction Reactions.

modification strategy	P-NRR primary mechanism	E-NRR primary mechanism
Defect Engineering (Missing linkers/Missing clusters)	The lifetime of the exciton increases as the electron trap in the midgap states that is formed due to the formation of a defect, and this extended lifetime of the photogenerated electron is responsible for the reduction of N2.	Exposes a large no of unsaturated Zr sites, which selectively enhances the adsorption of N2 while kinetically poising the H2 adsorption.
Loading of Noble Metal	It acts as an electron sink. The internal electric field facilitates the transfer of photogenerated electrons from the CB of UiO-66 to the sink, thereby separating them from holes.	Creates d-orbital backdonation via σ or π-bond for elongating the strong NN bond and also regulates the overpotential for NRR.
Ligand Functionalization	These functional groups act as chromophores, which shortens the band gap of neat UiO-66, shifting the absorption band to the visible region.	Changes the electron density and hydrophilic environments inside the pores that ultimately reduce H+ adsorption for HER but enhance the affinity for N2.
Heterojunction Formation	Establishes a z-scheme composite that retains electrons in the catalyst for reduction and possesses holes for water oxidation that supply protons. Also enhances the interfacial charge migration that lessens the chance of recombination.	Increases the electrical conductivity of the framework, which enhances utilization of electrons and supplies interfacial active centers that are responsible for lowering the activation energy for NRR.

4. Structural Engineering and Mechanistic Pathway of UiO-66 Based Catalysts for P-NRR and E-NRR.

catalytic mechanism	guiding architecture	structure–property correlation and surface-active site	reaction pathway and dynamics
P-NRR	Band Gap tailoring: Introducing electron-donating groups like –NH2 or creating composites with materials like g-C3N4, Mxene, etc., to transition the absorption from UV to the visible region. Charge Carrier dynamics: Create a Z-scheme or type-II heterojunction to diminish electron–hole recombination. Photosensitization: Embed light-absorbing large conjugated molecules, such as porphyrins or phthalocyanines	Functional groups such as –NH2, –OCH3, etc., introduce localized electron states. The adsorption and activation of N2 are facilitated by open Zr6 clusters as active sites.	Photoactivation: Creates an abundance of electron–hole pairs by enhancing absorption of visible-light absorption. Carrier Lifetime extension: Heterojunctions or photosensitization are responsible for the spatial charge distribution that acts as a driving force for the transfer of highly energetic photogenerated electrons directly to the active sites where N2 molecules are adsorbed.
E-NRR	Defect Engineering: The missing linker creates defects in the SBUs [Zr6O4(OH)4]. Integration of cocatalyst: Lowering of NRR potential occurs due to the loading of SACs or MNPs onto the structure of UiO-66. Surface functional groups: Introduce polar surface functional groups to tailor the microenvironment.	The coordinatively unsaturated Zr-metal sites formed due to missing linker defects provide binding sites for N2 adsorption and stabilize intermediate species (*N2H x )″. Polar functional groups enhance the gas affinity for adsorbing N2 on the active sites.	Interfacial electronic transfer: The open Zr sites exhibit Lewis acidity, accepting electron density from the N2 molecule through σ-interaction, which in turn weakens and elongates the inert NN bond via metal-to-ligand π-backdonation. Suppression of HER: Also, the exposed unsaturated Zr sites enhance hydrogenation of N2, suppressing the competing HER.

7. UIO-66-Based Photocatalysts for NH₃ Production

7.1. Doped MOF

Doping is a high-energy pathway for enhancing the properties of MOFs, and it can be achieved using Various organic, inorganic, or hydride precursors within the structure of MOFs. The doped species can alter the coordination environments of both the linkers and metal nodes by competing with other metal cations for ligands. The introduction of dopants increases the surface area and provides suitable active sites for adsorption.

Zhao et al. in 2023 prepared Au nanoparticle-loaded UiO-66 for photocatalytic N₂ fixation. The introduction of metal NPs into the MOF structure enhances the photocatalytic activity in two ways: i.e., Au NPs concentration enhances the N₂ adsorptivity around the MOF surface by providing suitable active sites, and the metal–carrier interactions between the MOFs and Au NPs enhance the charge transfer by providing the required environment for photocatalytic applications. Mechanistically, Au’s vacant orbitals enable π-back-donation that weakens the NN bond, while the Au–MOF work-function difference drives directional electron flow that limits recombination, substrate activation and charge separation handled by the same metal. Photocatalytic performance analysis confirmed that the NH₄ ⁺ produced was ∼4.7 times (66.28 μmol g^–1h^–1) that of pure UiO-66 (14.14 μmol g^–1 h^–1), when the loading concentration was 1.5 wt %. Above that, the ammonium ion generation decreases due to the aggregation of Au NPs, showing that loading is a trade-off: beyond optimal dispersion, Au buries active sites and shortens recombination pathways instead of helping. From the UV–vis DRS study, it was obtained that the introduction of Au decreases the band gap to 3.94 eV from 4.05 eV (pristine UiO-66), which was further supported by the PL spectra. The shift is modest and, with XRD showing no phase change, points to interfacial Schottky-junction states rather than true bandgap engineering, consistent with the XPS binding-energy shift indicating electron transfer from framework to metal. It was observed that loading of Au into the pure UiO-66 suppresses the electron–hole recombination by facilitating Au/UiO-66 to absorb visible radiation, reinforced by weaker PL, higher photocurrent, and a smaller EIS arc, together pointing to Au acting as an electron sink rather than the bandgap shift driving the gain alone. This suggests a transferable design rule: a metal with both favorable work-function mismatch and accessible d-orbitals can address charge separation and substrate activation in one step, though the yield, while sacrificial-agent-free, still trails ternary systems like UiO-66/PDA/CNNS (147.8 μmol g^–1 h^–1) that rely on a sacrificial donor to get there.

Applying the concept of electronic modulation, Tan et al. synthesized a low-electronegativity Ca^2+‑doped UiO-66-NH₂, where the diamino ligand was introduced into the structure of the MOF to reduce the band gap that ultimately expands the light absorption range, thereby overcoming the energy barriers for photocatalytic N2 fixation. Ca²⁺ was selected over other alkaline earth metals (Mg^2+, Sr^2+, Ba²⁺⁾ specifically for its size match with Zr⁴⁺ and its low electronegativity, which makes it a poor electron acceptor, so charge pushed onto Ca²⁺ does not stay trapped there but continues toward Zr. The introduction of Ca²⁺ into Zr₆ nodes generates an environment Zr–O–Ca that facilitates N₂ adsorption and activation due to the electron releasing nature of Ca²⁺ which was confirmed from grand canonical monte Carlo (GCMC) and DFT calculation and this environment under light irradiation facilitates charge transfer occurs via LMCT toward the Zr metal and the localized electron redistribution occurs due to the instability of Ca⁺ ions making the Zr nodes rich in electrons which donates its excess d-electrons into the antibonding molecular orbitals of N₂, thereby weakening the strong NN bond. Among all the samples, UN (Zr-0.30 Ca) exhibits an NH₃ yield of 222.2 μmol g^–1 h^–1, which increases to 759.3 μmol g^–1 h^–1 upon addition of the 2,5-diamino ligand, though it is worth noting this top-yield sample, UN­(Zr-0.30Ca)-4, sustains only two reaction cycles before degrading; the authors’ stability and mechanistic work is instead anchored on the next-best UN­(Zr-0.30Ca)-3 (546.7 μmol g^–1 h^–1), which holds over four cycles, a yield-versus-durability trade-off that matters when comparing this number against other systems. corroborated by optical studies that reveal that increasing doping decreases the band gap of the material by introducing midgap states and facilitating exciton movement. More precisely, the band gap narrows from 2.80 eV down to a minimum of 2.74 eV at this 0.30 doping ratio before widening again at higher Ca²⁺ content, a nonmonotonic trend that mirrors the activity curve, and DFT attributes it to growing Zr 4d orbital contribution near the conduction band rather than to a permanent structural change. Upon incorporation of Ca²⁺ and ligand modification, the photocatalyst’s impedance radius improves charge-carrier dynamics. This synergistic effect, which eases interfacial charge transfer, confirms that UN (Zr-0.30Ca)-4 has a high charge-transfer efficiency (Figure a).

4.

(a) Scheme for photocatalytic N2 fixation mechanism. Reproduced with permission from ref . Copyright 2023 Elsevier. (b) Probability distribution plots of N2 molecules in the adsorption of (1) UiO-66, (2) UC and (3) UPC models at 298 K simulated by the GCMC methods (c) Suggested pathways for N2 reduction over UPC, NH3 formation rates of (d) -UiO-66, UPC, UPCdS, UPT and (e) MIL-53, MPC, MPCdS, MPT under the full spectrum. Reproduced with permission from ref . Copyright 2022 RSC.

Doping of UiO-66 with heteroatoms or functionalized ligands decreases the bandgap, enabling effective visible-light absorption and the formation of defect-rich Lewis-active sites that facilitate N₂ adsorption and activation. Though charge-carrier density is enhanced by doping, the quantum efficiency is hampered by reduced availability of d-electrons for π-backdonation and by reduced selectivity against competing HER.

7.2. Surface-Functionalized MOF

Functionalization has a very important role in precisely tuning the physicochemical properties of metal organic frameworks like UiO-66, which ultimately enhances the efficiency of the materials for various catalytic applications. Sun et al. in 2024 synthesized chlorophyll-functionalized FC@UiO-66e for pNRR, achieving a higher ammonia yield of 73.1 μmol g^–1 h^–1, which is 11 times that of neat UiO-66 (6.3 μmol g^–1 h^–1) upon loading 1.6 wt % of chlorophyll from F. japonica. DFT shows chlorophyll anchors via its pyrrole nitrogen to exposed Zr sites (Zr–N, −1.29 eV), a much stronger interaction than the alternative Zr–Mg contact (−0.47 eV). Structural and morphological integrity was validated by PXRD and SEM analysis. The 5-cycle reusability itself comes from a separate NH₃-yield cycling test (Figure c); PXRD, SEM, and XPS afterward confirmed the crystal structure, morphology, and electronic state stayed intact, explaining why that stability holds. Electrochemical analysis, such as transient photocurrent (TPC) and electrochemical impedance spectroscopy (EIS), revealed that functionalization with chlorophyll decreases charge-transfer resistance, thereby enhancing charge-carrier separation, consistent with the PL quenching also observed for FC@UiO-66e. These findings were further supported by a DFT study that evaluated differential charge density and Bader charge analysis (Δq = 0.14 e), which showed a charge flow from the conjugated chlorophyll to the Zr-oxo clusters, making them electron-sufficient; chlorophyll essentially acting as an electron reservoir feeding Zr’s empty d-orbitals, which then back-donate into adsorbed N₂. In addition, the integration of chlorophyll reinforces the π-conjugated system, narrowing the band gap (via UV–vis DRS/Tauc, E _g falls from 3.78 to ∼1.7 eV) and lowering the conduction band (via Mott–Schottky); XANES/TDOS instead support the electron enrichment at Zr behind that shift. Eventually, relative to pristine UiO-66 (−0.08 eV), inclusion of chlorophyll facilitates better N₂ adsorption (−0.14 eV) and a reduced activation energy barrier of 1.70 eV, with in situ FTIR and DFT together pointing to an alternating hydrogenation pathway where the first protonation step (N₂ → NNH) is rate-limiting and specifically what chlorophyll eases.

Surface functionalization of UiO-66 with various groups such as –NH₂, −OH, –SH, –NO₂, −SO₃H or defects, etc., critically increases the NRR by decreasing the conduction/valence Band edges for visible-light absorption and forming a localized environment that facilitates N₂ chemisorption. However, while these strategies enhance charge separation and provide the required active centers, they can decrease surface area by blocking pores or decrease framework stability under harsh aqueous conditions. So, the present research should focus on tuning these functional groups to suppress HER, thereby optimizing FE.

7.3. Composite MOF

MOFs’ noble properties, like high surface area, tunable architecture and high porosity, can be optimized through the formation of composites with functional materials. Polymers, metal oxides and carbon nanostructures are some of the functional materials that can be combined with MOFs, resulting in improved mechanical strength, enhanced stability and tailored surface properties. ,

Ren et al. in 2022 synthesized a composite UiO-66/PDA/graphitic carbon nitride nanosheet (UiO-66/PDA/CNNS) for photocatalytic NH₃ generation, where the composite manifested enhanced performance by facilitating multistep charge transfer with an impressive NH₃ yield of 147.8 μ mol h^–1 g^–1. From UV–vis diffuse reflectance spectra, the composite exhibited a higher wavelength absorption range of 487 nm, higher than UiO-66 (308 nm) and CNNS (427 nm), indicating that the formation of the nanocomposite expands the absorption range of UiO-66 from the UV to the visible region. This enhanced light harvesting, however, is only meaningful if the photogenerated carriers are effectively separated, which was confirmed by PL spectroscopy. While UiO-66, PDA, and CNNS individually exhibited intense emission peaks around 470 nm, UiO-66/PDA/CNNS showed a much lower PL intensity, indicating suppressed recombination of photogenerated electron–hole pairs in both UiO-66 and CNNS. This suppression is attributed to the introduction of conductive PDA, which improves the lifetime of photogenerated electrons and holes by enabling a multistep charge-transfer pathway (Figure b).

For detailed analysis of N₂ adsorption and activation, GCMC and DFT studies are carried out, which show that, after composite formation, adsorption is driven by Zr₆ nodes rather than linkers, as in pristine UiO-66, due to the electron-donating and steric crowding effects of the PDA and CNNS layers. The changes in the structure from neat material to composites considerably decrease the binding energy to 115.8 kcal mol^–1 and lengthen the NN bond to 1.260 Å, which is very nearly equal to that of intermediate diazene. (Figure c). This remarkable enhancement in activation is due to the four-step electron-transfer pathways CNNS → PDA•Ligand → Metal. When the required amount of electromagnetic radiation is applied to this synthesized UPC, the excited electron transfers from the conduction band of CNNS (ECB = −0.92 V) to the conduction band of pristine UiO-66 (ECB = −0.55 V) via the permeable PDA film, and due to this, the lifetime of the electron increases to 8.1s. These transferred electrons accumulate at Zr–O clusters, as evidenced by the XPS binding-energy curve of Zr, and then adsorb N₂ via the well-known π-back-bonding mechanism. So, here, ammonia is generated by electrons supplied by the Zr–O clusters, protons from H₂O, and N₂ supplied. (Figure d,e).

In parallel, Liu et al. synthesized a Ce-based MOF embedded on graphene (GSCe) for P-NRR. The aim of integrating graphene is to modulate the activation of Ce-UiO-66, thereby increasing the lifetime of exciton pairs and facilitating superior electron transfer, as further supported by the high apparent quantum yield of 9.25% upon light irradiation.

Pristine UiO-66 undergo composite formation with a conducive semiconductor to improve the efficiency of photocatalytic N₂ reduction by providing a high surface area and narrowing its band gap (E _g), thereby extending light absorption into the visible region. This synergistic approach enhances electronic conductivity and the number of specific active centers, ultimately increasing the efficiency and selectivity of N₂ reduction to NH₃.

7.4. Defect-Induced Single-Atom Anchoring MOF

Defect engineering is considered the most noble strategy for constructing MOFs with larger pore sizes and higher volumes, enabling multiple functionalities for various applications. This hierarchical pore facilitates the diffusion of guest molecules and mass transfer within MOFs, making it a suitable adsorbent. Due to the high stability of Zr-based MOFs, their pore environments can be tailored by modifying linkers or nodes. In addition to these defects, the deposition of single atoms (SAs) on the MOF can increase photocatalytic activity via Electron-metal–support interactions (EMSI).

For instance, Ren et al. in 2024 synthesized a single-atom Ru-deposited defective UiO-66­(Zr) for improved P-NRR. They confirmed the exceptional activity is due to Robust electron-metal–support interactions (EMSI) between Ru SAs and UiO-66 via covalent bonding. In the mechanistic path, the Ru atom can donate the d-electrons to the π* antibonding orbital of N₂ by decreasing the bond order of the N–N bond. The introduction of both defects and Ru single-atom loading achieved a higher ammonia yield of 53.28 μ mol g^–1 h^–1 due to the synergistic interaction, and it is concentration-dependent, supported by the NH₃ yield increases when Ru content varies from 0.05% to 0.20%, and further loading diminished the activity due to agglomeration of Ru nanoparticles. In addition to this, the process is thermodynamically spontaneous, indicated by the shift in conduction band (CB) to −2.27 eV (Ru₁/d-UiO-66) from −1.55 eV (pristine UiO-66). The enhanced performance of this catalyst is further supported by the nonappearance of unwanted products like hydrogen­(H₂) or hydrazine­(N₂H₄) and an AQE of 0.34% at 300 nm. Further, the catalyst’s exceptional stability, as confirmed by TEM and EDS mapping, ensures that the Ru atom remains in a single-atom state.

In photocatalysis, defect-induced single-atom anchoring in UiO-66 creates missing-linker defects that stabilize the isolated metal centers, preventing agglomeration and enabling high atomic dispersion. The creation of these vacancies exposes more active sites that facilitate the transfer of electrons into the antibonding π* orbital of N₂, which ultimately lowers the activation energy of the nitrogen reduction reaction while needing precise coordination to inhibit the competing HER. Table systematically classifies the defect categories in UiO-66 MOFs, detailing the associated structural and coordination changes, diagnostic characterization techniques, and their specific influence on N₂ activation and NRR performance.

5. Defect Engineering in UiO-66 MOFs: Diagnostic Techniques and Enhanced N₂ Activation Pathways.

defect category	structural and coordination changes	main characterization techniques	influence on NRR and activation of N2
Missing-linker defect	Involves the removal of a single organic linker	TGA: the mass or weight loss of a material	Lowers the coordination no of Zr sites, exposing highly unsaturated, Lewis acidic centres that polarize the inert NN bond and facilitate σ-donation from N2.
	Lowering the co-ordination no of UiO-66 from 12 to 11 or 10-fold	FTIR: Terminal Zr-OH groups capping open defect sites give a distinct peak at ≈3730–3750 cm–1	Gives a suitable channel for the diffusion of N2 without disrupting the framework.
	Formation of open metal sites which get capped by transient species like H2O/OH–, etc.	Acid-base titration: Modulated defect sites are quantified.
Missing-cluster defect	Missing of a SBU [Zr6O4(OH)4]12+	HRTEM: Depicts the missing regular nodes	Newly formed unsaturated Zr sites effectively polarize the inert NN bond due to their high Lewis acidity.
	Formation of large mesopores occurs
	Largely changes the local coordination environment of surrounding clusters, leaving Zr highly unsaturated.	BET/PXRD: Alteration in the distribution of pore volume and reflection intensity observed at a lower angle.	Also, the drastic change in the electronic structure decreases the N2 binding affinity as compared to that of missing-linker defects, resulting in a decrease in selectivity towards HER.
Node hydroxylation defect	Thermal activation or structural changes alter the cluster core.	In situ DRIFTS/FTIR: Monitoring the peak at ≈3674 cm–1 due to μ3-OH stretching and formation of μ3-O.	Highly reactive Zr centre generated due to dehydroxylation that significantly changes N2 vs H+ affinity.
	Convert hydroxylated, 8-coordinated Zr [Zr6O4(OH)4] to 7-coordinated [Zr6O6] by the loss of 2H2O molecules	XAS(XANES/EXAFS): Track the alternation in bond length and coordination number of Zr-O.	The NRR intermediates like (*N2Hx) are stabilised by the hydrogen-bonding networks (μ3-OH) furnished by the hydroxylated nodes; however, the dehydroxylated nodes change the electron density, facilitating easy N2 capture.

7.5. Bimetallic MOF

It has been observed that the incorporation of a second metal ion into the nodes of a framework increases the catalytic, electronic and luminescent properties of MOFs. So, the partial substitution of the second metal into the secondary building blocks (SBUs) of the MOF constitutes a bimetallic system and due to the presence of two metal atoms, it shows a synergistic effect. Depending on requirements, varying the proportions of metal ions, the physicochemical properties of the MOF can be adjusted.

An et al. in 2021 synthesized the bimetallic U­(Zr–Hf)-X MOF, consisting of both Zr and Hf metals, for photocatalytic N₂ fixation, where Zr acts as the active site and Hf acts as a buffer to maximize electron utilization and diminish charge recombination. This mimics nitrogenase’s bimetallic cofactor: Hf’s more negative conduction-band potential (−2.70 eV vs Zr’s −2.44 eV) lets it relay electrons toward Zr via a ligand-to-metal-to-metal transfer pathway rather than let them recombine first. This synergistic effect between Zr and Hf is observed in materials’ textural properties; N₂ adsorption–desorption study indicates that U­(0.5Hf) exhibits an adsorption volume of 491.5 cm³ g^–1, which is more than that of pristine U­(Zr) 295.1 cm³ g^–1 and U­(Hf) 266.9 cm³ g^–1, this enhancement in gas adsorption is further supported by a higher specific surface area of 1123 m² g^–1. The introduction of specific functional groups increases the framework’s performance, though not uniformly. In U­(0.5Hf)-2SH, the mesopore diameter is 3.915 nm, which is larger than U­(0.5Hf)–OH or U­(0.5Hf). This larger pore aids diffusion, but agglomeration from the bulky −2SH groups actually collapses BET area from 1123 to 441.5 m²/g, lowering overall N₂ uptake (281.7 vs 491.5 cm³/g). The real gain from –SH is electronic: GCMC shows N₂ clustering at the metal node because of the thiols’ electron-donating character, not extra pore space. The groups like –SH, –Cl and −OH act as light-absorbing substituents, creating bathochromic shifts, shifting the absorption range from UV to the visible region, with U­(0.5Hf)-2SH absorbing at 502 nm and exhibiting a band gap of 2.65 eV with an extended average fluorescent lifetime of 4.89 ns, depicting. So, U­(0.5Hf)-2SH produces a maximum yield of 116.1 μmol g^–1h^–1 under visible light, whereas the unmodified samples barely absorb visible photons.

The photocatalytic activity of UiO-66 for the reduction of inert N₂ to NH₃ via doping and surface functionalization extends its light absorption into the visible region, and the formation of heterojunctions facilitates interfacial charge transfer and suppresses exciton recombination. The cutting-edge modification techniques, such as defect-stabilized single-atom catalysts and the synthesis of heterometallic nodes, provide suitable unsaturated coordination environments and a synergistic pathway to weaken the NN bond. In short, all these engineering pathways convert the robust UiO-66 framework into a biomimetic catalyst that accelerates the formation of green ammonia via productive electron backdonation. Table presents the photocatalytic efficiency of UiO-66 and its derivatives for the conversion of N₂ to NH₃.

6. Summary of UiO-66 and Its Nanocomposites for Photocatalytic Reduction of N₂ to NH₃ .

Sl.No	photocatalyst	metal	linker	light source	synthesis	band gap	photocatalytic efficiency (μmol g–1 h–1)	refs
1	Au/UiO-66	ZrCl4	H2BDC	300 W Xe lamp (irradiated for 1h)	Solvothermal (120 °C, 12 h)	3.94 eV	66.2
3	UN(Zr-0.30Ca)	ZrCl4	NH2-TPA	300 W Xe lamp	Solvothermal (120 °C, 24 h)	2.74 eV	759.3
3	Chlor@UiO-66	ZrCl4	H2BDC	300 W Xe lamp	Solvothermal (120 °C, 24 h)	--	73.1
4	UiO 66/PDA/CNNS	ZrCl4	TPA	300 W Xe lamp	Solvothermal (120 °C, 24 h)	--	147.8
5	Ru1/d-UiO-66	Zr, Ru	H2BDC	300 W Xe lamp	Solvothermal (120 °C, 24 h)	--	53.28
6	U(Zr–Hf)-SH	Zr, Hf	TPA	300 W Xe lamp	Solvothermal (120 °C, 24 h)	2.65 eV	∼116.1

8. UIO-66-Based Electrocatalysts for NH₃ Production

8.1. Doped MOF

The doping of UiO-66 using various techniques like ion substitution, modifying its surface and core–shell strategies enhances its properties by changing its structural, electronic and functional properties. For instance, Tan et al. synthesized bimetallic UiO–Zr-Ti electrocatalyst for nitrogen reduction reaction (NRR) by the postsynthetic exchange method. In its structural framework, Ti⁴⁺ substitutes into the same Zr–O clusters that already carry their own NRR activity (unmodified UiO-Zr alone yields 6.07 × 10–11 mol cm^–2s^–1 at FE 59.15%); what Ti specifically adds is extra active sites, its d-orbitals overlap favorably with N₂’s π orbital, while HER suppression is a joint Zr+Ti effect, thereby increasing catalytic efficiency. XRD techniques for structural characterization indicate that the framework remains intact even after the incorporation of Ti into UiO-66-NH₂, supported by the absence of impurity peaks of TiO₂ and similar diffraction patterns. However, with an increase in incubation time, there is a decrease in crystallinity as indicated by the reduction in peak intensity, clearly corroborated by the surface area of UiO–Zr-Ti-1d (488.12 m²g^–1), which is less than that of neat UiO-66 (555.24 m²g^–1). Although there is a decrease in surface area and porosity, catalytic performance is significantly enhanced due to an increase in electrochemically active surface area (higher double-layer capacitance) and charge-transfer efficiency (lower EIS resistance) and active sites provided by Ti. The electrochemical activity of UiO–Zr-Ti-5d for NRR was confirmed by LSV in 0.1 M Na₂SO₄, where current densities in N₂-saturated electrolyte are more than that of Ar-saturated environments between the potentials of −0.1 V and −0.7 V. By varying the Zr-to-Ti ratio, UiO–Zr-Ti-5d exhibits the utmost ammonia yield of 1.16 ± 0.058 × 10^–10 mol cm^–2s^–1 with a Faradaic efficiency of 80.36 ± 4.23%. Furthermore, yield and FE actually follow a volcano shape, rising as potential moves from −0.1 V to a peak at −0.3 V, then falling only beyond that point, once HER starts outcompeting N₂ adsorption. Also, the exceptional selectivity of the catalyst is supported by the fact that there is a very negligible 5.8 × 10^–13 mol cm^–2 s^–1 of hydrazine formed as an intermediate, as confirmed by the Watt-Chrisp method. Chronoamperometry was performed at −0.3 V over five cycles, with the chronoamperometric curves themselves showing no significant drop in current density across cycles, while UV–vis analysis separately confirms NH₃ yield and FE also hold steady, demonstrating UiO–Zr-Ti-5d’s own durability over this test.

Intentional doping of transition metal ions, rare earth elements or alkali metals into the structure of UiO-66 converts the insulating Zr₆ nodes into active sites, creating a suitable environment for π-backdonation into the antibonding orbitals of N₂ and increasing the framework conductivity. Although this modification strategy decreases the activation energy for the formation of intermediates like N₂H₄ and N₂H₂, the main problem lies in competing HER and low mass transfer within the microporous channels of UiO-66, which leads to low FE. To create a more efficient electrocatalyst for NRR, an accurate balance of dopant density is required, preventing framework decomposition and maintaining stability under reductive potentials.

8.2. Composite MOF

To eradicate the limitations of pristine MOFs, such as limited chemical/structural stability, especially in aqueous/harsh electrolytes, and poor electrical conductivity hindering electron transfer, composite formation via heterojunctions and Schottky junctions plays a very important role. Chen et al. synthesize a chain of UiO-66-NH₂/CNT-X by a simple hydrothermal method; here, the loading of CNT on the catalytic surface improves its conductivity. Rather than introducing distinct catalytic centers of its own, the CNT network functions primarily as a conductive scaffold for UiO-66-NH₂ crystallization, which suppresses particle agglomeration and yields a marginally finer, better-dispersed particle population (mean size decreasing from 125.11 to 121.85 nm). This is corroborated by electrochemical impedance spectroscopy, where the charge-transfer resistance decreases from 6.96 Ω for unmodified UiO-66-NH₂ to 2.03 Ω at the optimal CNT loading, accompanied by a corresponding increase in double-layer capacitance, indicative of a larger electrochemically active surface area. The synergistic effect between UiO-66-NH₂ and CNT therefore manifests primarily as enhanced exposure and accessibility of the existing UiO-66-NH₂ active sites, facilitated by improved charge-transfer kinetics, rather than the introduction of supplementary active centers by CNT itself. Among these fabricated composites, UiO-66-NH₂/CNT-0.15 exhibits the maximum NH₃ yield of 1.85 × 10^–10 mol cm^–2 s^–1 (22.644 μg h^–1 mg_cat ^–1) at −0.3 V (vs RHE) with FE of 37.78% (Figure a). It is noteworthy that this enhancement in yield is accompanied by a reduction in Faradaic efficiency relative to unmodified UiO-66-NH₂, which attains a markedly higher FE of 59.15% at its own optimal potential (−0.4 V). This inverse relationship is attributed to the same improvement in conductivity that benefits the nitrogen reduction reaction, also accelerating the competing hydrogen evolution reaction, such that the gain in NH₃ yield is offset by a decline in selectivity. This enhanced activity was validated by the LSV curve, where a remarkable rise in current density was observed in N₂-saturated electrolyte in comparison to that of Ar-saturated conditions, depicting the participation of nitrogen in the reduction process at potentials below −0.1 V (Figure b). In addition, the UV–vis absorption spectra from Indophenol blue spectrophotometry give a peak at 700 nm; the intensity of this peak rises as the cathodic potential shifts from −0.1 V to −0.3 V, and a following decrease in absorbance is observed at more negative potentials, beginning the challenging HER, which develops on the catalytic surface, thereby limiting the adsorption and activation of N₂ (Figure c).

5.

(a) The NH₃ yield rate and FE at different potentials for UiO-66-NH₂/CNT-0.15 (b) LSV curves of UiO-66-NH₂/CNT-0.15 obtained in Ar-saturated and N₂-saturated 0.1 mol L^–1Na₂SO₄ solution (c) UV–vis absorption spectra of the electrolyte after chronoamperometric tests for UiO-66-NH₂/CNT-0.15. Reproduced with permission from ref , Copyright 2023 Chemistry Select. (d) FE of 5 cycles at −0.3 V for UiO-66-HAc (e) Long-time i-t curve at – 0.3 V for UiO-66-HAc (f) r NH₃ and FE before and after PSE. Reproduced with permission from ref , Copyright 2022 Elsevier.

Compositing pristine UiO-66 with various semiconducting materials, such as MXenes and carbon nanotubes, addresses its insulating nature by creating a synergistic effect that decreases the energy of activation for N₂ uptake. The emergence of these composites forms a hydrophobic environment that diminishes the opposing HER by blocking the accessibility of water to the catalytic centers. However, achieving higher FE remains difficult due to restricted mass transport within the hidden micropores.

8.3. Defective MOF

Tailoring the properties of MOFs by creating defects is an excellent method. The introduction of these structural defects has several advantages for electrocatalytic N₂ reduction by increasing activity and selectivity through different mechanisms, such as the creation of active sites, modulation of the electronic structure, optimization of intermediate adsorption energy, suppression of the competitive HER, and enhanced mass and charge transfer.

He et al. in 2021 synthesized defective UiO-66-HAc for E-NRR. The introduction of defects by acetic acid exposed extra Zr₆ nodes as active sites, enhancing the electrolytic surface area, which ultimately increases the adsorption intensity of N₂ and the short-lived species formed throughout the conversion of N₂ to NH₃ through E-NRR. The current densities of N₂-saturated electrolytes are higher than those of Ar-saturated electrolytes when the potential is more negative than −0.1 V, as found from the LSV curve. So, it was clear that in N₂-saturated electrolytes, NRR may occur and the catalyst UiO-66-HAc is capable of carrying out the E-NRR. From the Watt-Chrisp method, it was confirmed that the generated N₂H₄ is much less than r _NH3. Good longevity is also required for a catalyst in addition to good catalytic activity. From the study, it was obtained that the optimum concentration of UiO-66-HAc on the working electrode could be reused for up to 5 cycles, which was run at a potential of −0.3 V for 2 h in a single cycle. After that, both the ammonia yield­(r _NH3 ) and FE remained constant (Figure d). To evaluate enduring stability, an I-t test was carried out for 96 h, and at −0.3 V, current densities remained persistent throughout the NRR (Figure e).

After 96h running, the ammonia yield r _NH3 ≈ 30.55 μ g h^–1 mg_cat ^–1 (vs 31.81 μ g h^–1 mg_cat ^–1 for 2 h of running) and ≈45.77% (vs ≈ 48.06% for 2 h of running). So, it was clear that UiO-66-HAc has satisfactory longevity for NRR to form NH₃ (Figure f). Similarly, Shi et al. in 2024 synthesized UiO-66-xHPMo as a defective MOF using phosphomolybdic acid (HPMo) as the regulator. Here, the creation of defects exposes more Zr₆ as the active sites and gives many active sites for the catalytic reduction of N₂ to form NH₃. An LSV curve was conducted for UiO-66-xHPMo and UiO-66 to find out the NRR activity. It has been observed that at −0.7 ≈ 0.1 V, the N2-saturated electrolyte has a higher current density than the Ar-saturated electrolyte, indicating the specific activity of UiO-66–2HPMo for E-NRR. For the identification of NH₃ production rate by different types of catalysts, the obtained UV–vis spectra underwent indophenol blue spectrophotometry. The absorbance curves show an initial increase followed by a decrease around 550–850 nm. Varying the applied electric potential from −0.1 V to −0.5 V, i.e., in the negative direction, the R _NH3 and FEs exhibit a volcano-shaped change law. The R _NH3 of UiO-66-xHPMo first rises and then decreases in yield was observed, and the highest ammonia yield was obtained at −0.3 V. Further increase of applied potential decreases both the R _NH3 and FEs because at lower potential, HER dominates over N₂ adsorption and subsequent reduction. The UiO-66-xHPMo exhibits the highest ammonia output as compared to unmodified UiO-66, indicating that the introduction of HPMo increases the R _NH3 yield. Among the obtained materials, UiO-66–2HPMo exhibits the highest ammonia formation of 36.61 μg h^–1 mg^–1 and a higher FE of 31.09%.

Likewise, He et al. synthesized modulated UiO-66-NH₂ functionalized with a quite stable superoxide radical­(O₂ ^·) for E-NRR. In this method, the ammonia yield (R _NH3 ) was ≈ 52.81%, with an exceptionally high FE ≈ of 85.21%.

Preparation of UiO-66 via doping or by forming a composite with various scaffolds, such as MXene, QDs, and carbon nanotubes, creates synergistic effects that enhance π-backdonation for N₂ activation, and the hydrophobic environment formed suppresses the challenging HER. Both strategies decrease the activation energy for the formation of intermediates such as diazene and hydrazine; however, achieving higher FE remains challenging due to restricted mass diffusion and entombed active sites within the micropores. So, the current research should focus on creating defect-rich centers that maximize the exposure of active sites and pore accessibility, thereby facilitating high NH₃ yields and robust stability under electrode potentials. Table summarizes the E-NRR efficacy of UiO-66 and its modified structures for the formation of NH₃ from N₂.

7. Synopsis of E-NRR Efficiency of UiO-66 and Analogues for the Production of NH₃ from Inert N₂ .

Sl.No	MOF	metal	linker	synthesis	applied potential (V vs RHE)& electrolyte	electrocatalytic efficiency	refs
1	UiO–Zr-Ti	Zr, Ti	NH2–BDC	Solvothermal (120 °C, 24 h)	–0.3 & Na2SO4	1.16 × 10–10 cm2 S–1 FE: 80.36%
2	UiO-66-NH2/CNT-0.15	Zr	NH2–BDC	Solvothermal (120 °C, 24 h)	–0.3 & Na2SO4	1.85 × 10–10 mol cm–2 s–1 22.644 μgh–1 mgcat –1 FE: 37.78%
3	UiO-66-HAc	Zr	H2BDC	Solvothermal (120 °C, 24 h)	–0.3 and 0.1 M Na2SO4	∼31.81 μgh–1 FE: ∼48.06%
4	UiO-66 2HPMo	Zr	H2BDC	Solvothermal (120 °C, 24 h)	–0.3 and 0.1 mol L–1 Na2SO4	∼36.61 μgh–1 FE: 31.09%
5	UiO-66-NH2	Zr	NH2–BDC	Solvothermal (120 °C, 24 h)	0.1 M Na2SO4	FE: ∼ 85.21%

9. Conclusion and Future Perspectives

Transitioning toward P-NRR and E-NRR from the energy-intensive Haber-Bosch process is considered a highly promising approach for sustainable ammonia production. MOFs have displayed promising substrate turnover for N₂-to-NH₃ under ambient conditions because of the high density of active centers, large surface area, and modulated electronic structure and microenvironment. Due to the high thermal and chemical stability of UiO-66 provided by Zr nodes and BDC ligands, it resists structural collapse, making it an excellent material for prolonged catalytic applications. The materials’ physicochemical properties, like crystallinity, porosity and active site density, depend on the synthetic methods such as solvothermal, mechanochemical, microwave, and reflux techniques. Furthermore, in NRR, the main difficulties are associated with activating the inert NN bond and the ever-persistent competing HER. To mitigate these hurdles, future research must move beyond neat frameworks and focus largely on deliberate structural modifications such as doping, functionalization, defect chemistry and composite formation that boost NH₃ yield and FE for both P-NRR and E-NRR systems.

Although the encouraging advancements discussed in this review show great promise, key challenges and gaps in synthesis, modification, and application remain to be addressed for UiO-66-based NRR catalysts to successfully transition from the laboratory to practical applications.

• I.Ensuring accurate E-NRR activity measurements is a persistent challenge, as NH₃ yields typically fall at the microgram level and background ammonia contamination, arising from the experimental setup, reagents, and even the operator, demands stringent protocols. Electrochemical devices, feed gases, and catalysts must all be carefully pretreated to eliminate NH₃ contamination from sources such as electrolytic cells, electrolytes, electrodes, polymer electrolyte membranes, and organic amines. The use of ¹⁵N-labeled N₂ is strongly recommended, particularly for atmospheric-pressure studies, though its high cost may limit its applicability at elevated pressures. Researchers should report all control-experiment details, including both positive and negative results, along with NH₃ yields measured by two or more independent detection methods, enabling the broader community to draw reliable conclusions and identify truly promising directions in catalyst and cell design.
• II.As highlighted throughout this review, NRR activities of UiO-66-based MOFs are frequently evaluated under varying experimental conditions, making direct performance comparisons unreliable. Standardizing key parameters, including catalyst loading, electrolyte type and concentration, counter electrode, applied voltage, and NH₃ detection method, is therefore essential for meaningful benchmarking and proper evaluation of these materials. This is compounded by inconsistent reporting of NH₃ yield itself (mol cm^–2 s^–1, μg h^–1 mg^–1, or μg h^–1 alone), which prevents direct comparison of catalytic activity across studies. Since both yield and Faradaic efficiency typically follow a volcano-shaped dependence on applied potential, a catalyst reported only at its own optimal potential cannot be fairly compared to one evaluated elsewhere on its response curve. Field-wide adoption of a common reporting format, specifying yield normalized to both electrode area and catalyst mass alongside applied potential and detection method, would improve the objectivity of cross-study comparison.
• III.To increase the photocatalytic performance of UiO-66 and its related materials for NRR, a multifaceted approach is essential, increasing the hydrophobicity of the framework and employing gas–solid reaction systems to suppress the competing HER, while mitigating nitrogen’s low water solubility through precision engineering of active centers on the surface. Developing multivariate structures, dual-metal centers, and complex vacancy pairs can further maximize the kinetics of charge transfer and substrate activation.
• IV.Research should focus on developing High-Entropy MOFs by incorporating five or more distinct metal atoms or ions into a single UiO-66 framework, thereby creating a suitable electronic environment that lowers the activation energy for N₂ adsorption and activation through lattice distortion and synergistic interactions among the various metals.
• V.Although traditional solvents such as DMF and THF provide higher solubility for N₂, their toxicity and flammability pose a major challenge, necessitating greener alternatives. Furthermore, due to NH₃’s high solubility in aqueous systems, its recovery remains difficult. Future research should therefore prioritize the use of green solvents or gas-phase NRR systems, thereby facilitating NH₃ separation.
• VI.Theoretical tools such as heat of adsorption calculations, fundamental kinetic modeling, volcano plots, and d-orbital electron distribution analysis are invaluable for predicting electrocatalyst behavior and elucidating NRR mechanisms. Expanding such investigations to cover associative and dissociative pathways, reaction intermediates, and electronic structures of UiO-66-based systems is strongly encouraged. Crucially, theoretical efforts should be coupled with advanced operando characterization techniques, including in situ XPS, TEM, and synchrotron radiation, to bridge computational predictions with experimental reality. Notably, theoretical studies on NRR photo/electrocatalysts remain virtually absent, and applying these tools to predict band structures, adsorption energetics, and surface kinetics of UiO-66-based photo/electrocatalysts represents a largely untapped and promising direction.
• VII.For large-scale industrial applications, a complete understanding of MOF performance when integrated with Gas Diffusion Electrodes (GDEs) and its behavior under extreme mechanical stress is required. Future research should also focus on developing thin-film or membrane-based UiO-66 systems compatible with continuous-flow reactors rather than conventional batch reactors
• VIII.Crucially, this transition requires moving beyond laboratory-scale catalyst design to evaluate the practical feasibility of UiO-66-based NRR systems. This demands a diligent assessment of their overall energy consumption and long-term durability of UiO-66 catalysts under continuous flow conditions, requiring milligram-to-kilogram synthesis scale-up and extensive technoeconomic analysis to verify competitiveness against the conventional Haber-Bosch process (Scheme ).

3. Prospects for Advancing the P-NRR and E-NRR Performance of UiO-66-Based Nanomaterials.

The data underlying this study are available in the published article.

A.A.D.: Conceptualization, visualization, writingoriginal draft, review, and editing. Arup Anurup Dash: Conceptualization, visualization, writing – original draft, review, and editing. Priyanka Priyadarshini: Conceptualization, visualization, writing – original draft, review, and editing. Diptirani Behera: Writing - review, and editing. Kulamani Parida: Conceptualization, visualization, and supervision.

The authors declare no competing financial interest.