Photoelectrocatalytic urea synthesis: Surface engineering of the constructions, mechanism insights, and technological challenges

Summary

Traditional urea production methods, primarily the Haber-Bosch and Bosch-Meiser processes, consume substantial amounts of energy and have significant environmental impacts, calling for more sustainable alternatives. Alternatively, photoelectrocatalytic (PEC) urea synthesis utilizes sunlight and electrochemistry to facilitate C–N coupling reactions under mild, environmentally friendly conditions. This review focuses on the recent progress in electrocatalytic, photocatalytic, and photoelectrocatalytic urea synthesis, with a focus on methodology, technological constraints and innovations. Key limitations include low Faradaic efficiencies, restricted catalytic selectivity and mechanistic understanding, and suboptimal charge-transfer processes. We discuss the potential of several photo-electroactive constructions, and quantification methodologies, highlighting a need for standardized testing protocols. Economic aspects of the technology are also discussed, with focus on the industrial feasibility of green urea production. Although PEC synthesis offers notable benefits over conventional methods, it still faces challenges, including catalyst durability, high energy consumption, and limited scalability. This review provides strategies to overcome these challenges and advance PEC urea synthesis toward real-world implementation in sustainable fertilizer manufacturing.

Graphical abstract

Catalysis; Theoretical photochemistry; Analytical Electrochemistry

Introduction

Urea is classified as a significant component in the human food chain. According to reports, the global demand for urea is expected to reach 200 million metric tons by 2026. Urea, widely used as an organic nitrogen fertilizer, has a significant impact on the health-related quality of life for over 50% of the global population.¹ Urea is a primary raw material for the fabrication of a wide range of fine chemicals used in medicine, agriculture, and various industrial sectors, including plastics, resins, and adhesives.² Other fascinating applications of this compound include its utilization in sensor design for enzymatic reactions³ and fuel cell technologies.^4,5 The industrial-scale production of urea relies on the direct reaction of ammonia (NH₃) and carbon dioxide (CO₂) under extreme conditions, including high temperatures and pressures, as well as specialized reactors.⁶ The NH₃ used in urea production is primarily produced from N₂ and H₂ through various reactions, including the Bosch-Meiser and Haber-Bosch processes. The Bosch-Meiser process uses N₂ as the nitrogen source, which is energy-intensive because the considerable dissociation energy (941 kJ/mol) is required to break the N≡N bond. Therefore, this technique is considered undesirable due to its high energy consumption, dependency on fossil fuels, and negative environmental impact.^7,8

Today, the Haber-Bosch process is the most common method for NH₃ production, employing principles similar to those of the Bosch-Meiser process but with further optimizations for greater efficiency and large-scale manufacturing. Nevertheless, the Bosch-Meiser process remains essential in advancing industrial NH₃ production. Considering the mentioned drawbacks, it is suggested that the Haber-Bosch and Bosch-Meiser processes can be replaced with innovative, eco-friendly, and energy-efficient approaches for urea synthesis.⁹

Electrocatalysis, utilizing renewable electricity under milder conditions, has emerged as a promising, eco-friendly solution. It reduces the energy barrier of chemical reactions, providing substantial advantages and more efficiently addressing the practical challenges.¹⁰ Due to their cost-effectiveness and superior performance compared to conventional catalytic systems, electrochemical urea synthesis and production technologies have garnered considerable attention in scientific research and emerging markets.¹¹ Electrocatalysis has also gained significant attention for sustainable urea synthesis. The main challenge is to catalyze efficient C–N coupling between carbon and nitrogen intermediates while minimizing undesired side reactions such as C–C coupling or H₂ evolution.^12,13 In this regard, electrocatalytic C−N bond coupling for converting CO₂ and nitrogen sources (N₂, nitrate (NO₃⁻), nitrite (NO₂⁻), NH₃) into urea in ambient conditions has recently gained significant attention as a sustainable strategy for urea synthesis. However, traditional electrocatalysts have demonstrated limited performance in this area due to their poor chemical adsorption and weak surface C−N bond coupling.¹⁴

Therefore, ongoing research focuses on developing novel electroactive structures with enhanced physicochemical properties, optimized morphology, and improved electrochemical mass transfer to overcome these limitations in lab-scale projects. Various electrocatalytic structures, such as metallic alloys,¹⁵ mixed metal oxides,¹⁴ defective materials,¹⁶ organometallic structures,¹⁷ and semiconductors,¹⁸ have been widely explored to address this challenge, with a particular focus on improving selectivity as a key parameter in scaling up and industrializing the reaction. Photocatalytic C–N coupling is a promising, potentially green alternative strategy for synthesizing urea by directly coupling CO₂ and nitrogen sources under light irradiation, typically using a semiconductor catalyst.¹⁹ This approach offers a safe, controllable, and environmentally sustainable route compared to traditional urea production methods, as mentioned before.²⁰ The conventional urea synthesis process requires harsh reaction conditions (high temperature and pressure), making it a carbon-emitting, energy-intensive technology that contradicts carbon-neutrality goals. Whereas photocatalytic approaches can operate at ambient conditions, reducing both energy use and operational risk.²¹

Additionally, by utilizing sunlight as a renewable energy source and directly converting CO₂, which helps in carbon capture and reduces greenhouse gas emissions,²² photocatalytic approaches offer a pathway for carbon-neutral production by directly transforming CO₂ into valuable chemicals.²³ Photocatalytic systems may also simplify process design by combining NH₃ generation and urea formation in a single reactor.²⁴ If catalysts are properly optimized, they can further limit byproducts and reduce environmental burden.²⁵ Finally, semiconductor photocatalysts can be engineered to enhance selectivity, activity, and stability, allowing for optimized performance across a range of conditions.²⁶ Despite these advantages, photocatalysts often suffer from high charge-carrier recombination rates and significant challenges in product controllability due to competing side reactions. To address these limitations and enhance overall performance, the methods of electrocatalysis and photocatalysis can be synergistically integrated using both light and electrical inputs, motivating the development of photoelectrocatalysts.

Over the past few decades, photoelectrocatalytic (PEC) technologies have demonstrated significant potential as sustainable solutions for clean energy production, utilizing methods such as water (H₂O) splitting and CO₂ reduction.²⁷ These approaches have also been explicitly explored to enhance the yield and selectivity of organic synthesis reactions.²⁸ Recently, researchers have focused on the efficient application of PEC methods to synthesize high-value chemicals, such as urea, through C–N coupling reactions on the surface of electrodes under light, taking advantage of the negligible external bias required and the virtually unlimited availability of solar energy.²⁹ The key point in developing such an innovative tool is the strategic modification of the electrode surfaces with appropriate functionalities to catalyze the oxidation-reduction couplings, preventing the generation of undesirable byproducts. This review provides a comprehensive roadmap for advancing green urea synthesis by systematically comparing various techniques, including electrocatalytic, photocatalytic, and PEC methods, while emphasizing the synergistic potential of integrating PEC systems with electro- and photocatalysis as an underexplored strategy to enhance efficiency and sustainability in the field. Unlike previous studies, this review offers deeper mechanistic insights and a more comprehensive perspective by exploring various precursor mixtures, including CO₂ + N₂, CO₂ + NO₃⁻, and CO₂ + NO, as well as the latest advancements in catalysts, such as Cu-TiO₂, Cu₂O, and Ru-doped systems. Additionally, it examines emerging approaches, including single-atom and hybrid catalysts. By going beyond the typical focus on catalysts and addressing critical challenges, such as low Faradaic efficiency, stability issues, and energy efficiency, it tries to bridge the gap between fundamental research and industrial scalability.

It also highlights gaps in urea quantification, emphasizing the need for standardized measurement protocols by addressing the analytical techniques, such as liquid chromatography-mass spectrometry (LC-MS), ultraviolet–visible spectrophotometry (UV-Vis), and nuclear magnetic resonance (NMR), essential for accurately measuring urea in catalytic systems. Furthermore, the research investigates the industrial feasibility of green urea production, comparing energy costs with traditional methods and discussing scalability, an aspect typically neglected in laboratory-focused reviews. Finally, it identifies research gaps and suggests future directions, particularly in optimizing PEC reactor design, offering a detailed guide that bridges laboratory advancements with practical, large-scale implementation.

Electrocatalytic C–N coupling for urea synthesis

Electrocatalytic C–N coupling has emerged as a promising approach for constructing C–N bonds and producing essential chemicals, such as urea, under mild ambient conditions.¹³ In contrast to the Haber–Bosch process, which relies on hydrogen produced via energy-intensive methods such as the H₂O-gas shift reaction³⁰ and steam–methane reforming,³¹ leading to significant CO₂ emissions,³² electrosynthesis harnesses hydrogen directly from H₂O, the most abundant resource on Earth.³³ Therefore, in recent years, there has been significant interest in urea synthesis via electrochemical activation of various nitrogen precursors and CO₂ on solid surfaces under mild conditions, thereby facilitating C–N coupling.^2,34 Xiong et al. discovered a new electrocatalytic strategy for urea synthesis through the oxidative coupling of CO and NH₃ on platinum (Pt) electrodes.³⁵ Figure 1A shows that the total current density increases from approximately 1.5 to 13.7 mA cm⁻² as the applied potential is raised from 0.5 to 0.8 V. Further increasing the potential to 0.9 V results in a decrease in current density to about 5.2 mA cm⁻², indicating the onset of competing reactions or catalyst limitations. Notably, a high Faradaic efficiency for urea synthesis (∼70%) is achieved at 0.5 V, demonstrating excellent selectivity at lower potentials. As presented in Figure 1B, the electro-oxidative coupling of CO and NH₃ over a commercial 70 wt. % Pt catalyst can deliver an impressive electrocatalytic C–N bond formation rate of 100 mmol h⁻¹ g⁻¹ at 0.8 V. According to Figure 1C, urea can be synthesized using renewable electricity through a two-step process in which CO₂ is electrochemically reduced to CO at the cathode, after which the CO is electro-oxidatively coupled with NH₃ at the anode of another reactor to form the urea. In this work, an efficient, alternative pathway for forming C–N bonds is demonstrated, avoiding challenges associated with reductive electrosynthesis, including hydrogen evolution.

Figure 1.

Electrocatalytic performance of urea synthesis on 70 wt.% Pt/C

(A) Potential-dependent FE and total current density (the inserted photograph shows the post-reaction electrolytes after DAMO assays).

(B) Comparison of the optimal electrocatalytic C–N bond formation rate and FE in this work, with reported values in the literature.

(C) A schematic of the envisioned renewable electricity used for urea synthesis.³⁵

Copyright 2024@Springer Nature.

The performance metrics of various electrocatalysts for urea synthesis, based on different carbon and nitrogen feedstocks, are presented in Table 1.

Table 1.

Performance of various electrocatalysts for urea synthesis

Catalyst	Precursors (N- + C-source)	Applied potential (V vs. RHE)	Urea yield rate (μg/mg.h)	Faradaic Eff. (%)	Reference
Ru-Pd/WO3/MXene	N2 + CO2	−0.4	227	23.7	Govindan et al.36
Fe/MoS2	NO3− + CO2	−0.5	18.98	54.98	Du et al.37
AuCu nanofiber	NO2− + CO2	−0.3	215	18.2	Liu et al.38
Cu99Ni1	NO2− + CO2	−0.6	361.3	25.1	Zhang et al.39
Cu–N–C–SACs	NO3− + CO2	−0.5	120	28	Leverett et al.40
Vo-In-TiO2	NO3− + CO2	−0.65	759.8	9.06	Mao et al.41
Cu3Mo2O9	NO3− + CO2	−0.70	177	40	Zhang et al.42
Dynamic Bi nanosheets	N2 + CO2	−0.40	334.6	12.5	Wang et al.43
P–Cu/Fe2O3	NO3− + CO2	−0.68	62.74	73.81	Deng et al.44
Cuδ+/Cu0	NO3− + CO2	−1.2	386.4	25.2	Chen et al.45
FeNC-CeO2	NO3− + CO2	−0.5	20969.2	89.3	Zhang et al.46
Diatomic Alloy catalyst CuPd1Rh1	NO3− + CO2	−0.5	53.2	72.1	Chen et al.47
Cu1-MoS2	NO3− + CO2	−0.6	23.3	57.02	Du et al.48

Most of the above-mentioned systems suffer from low conversion efficiency and urea production yield, which remain substantial challenges in the field. These issues arise from the slow C–N coupling kinetics, limited CO₂ adsorption capacity on metal catalysts, low reactant concentrations (CO₂/NO₃⁻) at high current densities due to poor mass transfer,⁴⁹ and disordered adsorption/conversion processes that hinder essential intermediate interactions during urea formation.⁵⁰ To improve catalytic efficiency and urea yield, it is crucial to enhance CO₂/NO₃⁻ adsorption, speed up their conversion, and stabilize major intermediates. A promising approach is the design of nano-confinement architecture, which spatially confines reaction pathways to specific domains, stabilizing reactive intermediates and controlling the dispersion of active components. Liu et al. synthesized mesoporous carbon hollow spheres (MCHS) and solid carbon spheres (CS) as catalyst supports to explore the impact of nano-confinement on C‒N coupling, employing Cu and Ru as bimetallic catalytic sites. Taking advantage of the CuRu synergy and nano-confinement effects, CuRu/MCHS achieved an impressive urea yield of 12.51 g h⁻¹ g_cat⁻¹ at 250 mA cm⁻², maintaining 125 h of stability (Figure 2A).⁵¹ To reveal the mechanistic differences between the nano-confined and non-confined systems, density functional theory (DFT) calculations were performed (Figure 2B). Adsorption energy analyses revealed that CO₂ exhibited stronger bonding affinity with Cu (−1.58 eV) than Ru (−1.40 eV), whereas NO₃⁻ preferentially interacted with Ru (−1.65 eV) rather than Cu (−1.12 eV). Based on the DFT results, they proposed two different pathways for C–N coupling in urea electrosynthesis, including (∗OCO + ∗NO → ∗OCONO) and (∗COOH + ∗NH₂ → ∗COOHNH₂). In the ∗OCONO pathway, the coupling requires an energy barrier of 0.79 eV. While the ∗COOHNH₂ pathway exhibits a smaller energy barrier of 0.64 eV during the first C–N coupling step, it follows a much longer reaction path compared to the ∗OCONO pathway. After the initial C–N bond formation, the energy barrier for ∗CO₂ hydrogenation is reduced from 0.67 eV (∗OCO + ∗NH₂ → ∗COOH + ∗NH₂) to 0.33 eV (∗OCONH₂ → ∗COOHNH₂), suggesting that urea formation becomes easier and faster after the first C–N coupling stage.

Figure 2.

Structural characterization of CuRu/MCHS and CuRu/CS electro-catalyst

(A) Synthesis process and microstructure of carbon spheres (CS), mesoporous carbon hollow spheres (MCHS), and CuRu/MCHS, alongside a schematic representation of the design principles of confined and nonconfined systems in urea electrosynthesis.

(B) Reaction diagrams based on DFT calculations for urea production on the CuRu surface and corresponding atomic configurations. The gray, blue, red, white, pink, and turquoise spheres represent C, N, O, H, Cu, and Ru atoms, respectively.

(C) Urea synthesis on CuRu/MCHS and CuRu/CS.⁵¹

Copyright 2025@Springer Nature.

Figure 2C illustrates that the nano-confined structure of CuRu/MCHS enables early C–N coupling via the ∗OCONO pathway rather than the ∗COOHNH₂ pathway typically followed in non-confined systems. Therefore, the final step for efficient urea production is the ∗OCONO pathway, where the coupling of ∗OCO and ∗NO to ∗OCONO initiates the process. The incorporation of Ru into the CuRu catalyst significantly lowers the energy barriers for this pathway, making it the preferred route for urea electrosynthesis while minimizing side reactions, such as HCOOH formation. The ∗OCONO pathway is more kinetically favorable, and nano-confinement helps break through the initial thermodynamic energy barrier, allowing the reaction to proceed more efficiently with fewer side reactions. The diagram shows how nano-confinement stabilizes intermediates, thereby enhancing the overall efficiency of urea electrosynthesis by shifting the coupling reaction toward a faster and more effective pathway.

Despite the increasing number of reports in this field, the topic continues to face significant challenges, including an unclear mechanistic description of the processes, low production yield, and poor Faradaic efficiency, which severely hinder its extensive use in practical applications.⁵² In addition, due to the complex multi-electron pathway involving a variety of intermediates, the reaction kinetics are slowed, and more energy is required to achieve practical current densities than expected.⁵³ Considering this, achieving selective C–N coupling is essential, and understanding the underlying reaction mechanisms is crucial for designing efficient electrocatalysts capable of precise and controllable C–N bond formation.⁵⁴ Electrocatalytic C–N coupling involves the simultaneous electrochemical activation of carbon-containing reactants, such as CO₂, and nitrogen-containing compounds, such as NH₃, nitric oxide (NO), NO₂⁻, and NO₃⁻, with both reacting at the same electrode surface. Depending on the electrochemical environment, these reactions are either reductive cathodic or oxidative anodic C–N couplings.⁵⁵ In cathodic C–N coupling reactions, C–N bonds are formed through electrochemical reduction processes occurring at the cathode surface, while anodic C–N coupling involves the formation of carbon–nitrogen bonds through oxidative electron transfer processes occurring at the anode surface.⁵⁶ The selection of reactants and the characteristics of intermediate species play a crucial role in determining reaction pathways and product selectivity.⁵⁷ The wise choice of reactants is key to directing the formation of desired C–N coupling products, making it essential for precise and efficient organic synthesis.⁵⁸

Mechanism of urea synthesis from CO₂ and nitrogenous sources

Several types of nitrogen species, including NO₂⁻,⁵⁹ NO₃⁻,⁴⁰ molecular N₂,¹⁵ and NH₃⁶⁰ have been widely investigated for electrocatalytic coupling with CO₂ on electrode surfaces to produce urea. A thorough understanding of the reaction mechanisms involving these precursors is critical to optimizing efficiency under ambient conditions. Beyond theoretical studies employing established methods, such as DFT, advanced in situ spectroscopic techniques have proven instrumental in elucidating these pathways. For instance, Raman spectroscopy, synchrotron radiation Fourier transform infrared spectroscopy (SR-FTIR), and sum-frequency generation infrared spectroscopy (SFG-IR) have been successfully applied to unravel the mechanistic details of electrocatalytic urea synthesis from these nitrogen-containing compounds.^2,52

Urea formation from carbon dioxide and nitrogen

Both N₂ and CO₂ are chemically inert due to their strong bonding energies of 941 kJ mol⁻¹ for N≡N and 806 kJ mol⁻¹ for C=O, making their activation challenging,⁶¹ while electrocatalysts facilitate the activation of these molecules, enabling catalytic reactions under milder conditions.⁶² However, competing reduction processes of CO₂ to CO, formate, or methane and N₂ to NH₃ or hydrazine (N₂H₄) hinder efficient C–N coupling and often lead to low urea selectivity.^15,63 Recent advancements have focused on overcoming these limitations by strategically designing and developing tailored electrocatalysts.⁶⁴ Govindan et al. developed a series of bifunctional heterojunction electrocatalysts by anchoring Ru-Pd alloy nanoparticles onto 2D MXene and WO₃ nanosheets (Ru-Pd–WO₃/MXene) for urea synthesis via the electrochemical coupling of CO₂ and nitrogen.³⁶ According to their results, the optimum catalyst achieved a Faradaic efficiency of 23.7% and an excellent urea production yield of 227 μg_urea mg_cat⁻¹ h⁻¹. The C–N coupling mechanism for urea formation was further investigated using DFTcalculations, which proposed that the reaction is initiated by the reduction of CO₂ to adsorbed ∗CO on the electrode surface. This intermediate then reacts with nitrogen to form ∗NCON, which subsequently undergoes protonation to yield urea.

To efficiently promote the urea formation reaction, anodic oxidation of H₂O to enrich the medium with H⁺ and (e⁻) is crucial. During this process, two H₂O molecules are oxidized at the Ru-Pd anode, generating substantial amounts of hydronium ions (H₃O⁺) and electrons, as well as CO₂ and NO₃⁻ species. Equations 1, 2, 3, 4, 5, 6, and 7 describe the proposed mechanism of the urea formation reaction over the catalyst mentioned above.

Anode:

$$2{\mathrm{H}}_2\mathrm{O}\to {\mathrm{O}}_2+{4\mathrm{H}}^{+}+{4\mathrm{e}}^{-}$$ (Equation 1)

Cathode:

$${\text{CO}}_2+{}^{\ast }+{\mathrm{e}}^{-}\to {}^{\ast }{\text{CO}}_2^{-}$$ (Equation 2)

$${}^{\ast }{\text{CO}}_2^{-}+{\mathrm{M}}^{+}\to {}^{\ast }{\text{CO}}_2^{-}-{\mathrm{M}}^{+}$$ (Equation 3)

$${}^{\ast }{\text{CO}}_2^{-}+{\mathrm{M}}^{+}+{\mathrm{H}}_2\mathrm{O}\to {}^{\ast }\text{COOH}+{\mathrm{M}}^{+}+{\text{OH}}^{-}$$ (Equation 4)

$${}^{\ast }\text{COOH}+{\mathrm{e}}^{-}\to {}^{\ast }\text{COOH}+{\text{OH}}^{-}$$ (Equation 5)

$${}^{\ast }\text{CO}+{{}^{\ast }\mathrm{N}}_2\to {}^{\ast }\text{NCON}\to {}^{\ast }\text{NCONH}\to {}^{\ast }\text{NHCONH}\to {}^{\ast }\text{NHCON}{\mathrm{H}}_2\to {}^{\ast }\mathrm{N}{\mathrm{H}}_2\text{CON}{\mathrm{H}}_2$$ (Equation 6)

Overall:

$${\mathrm{N}}_2+{\text{CO}}_2+{6\mathrm{H}}^{+}+{6\mathrm{e}}^{-}\to {\mathrm{H}}_2\mathrm{O}+\mathrm{N}{\mathrm{H}}_2\text{CON}{\mathrm{H}}_2$$ (Equation 7)

DFT results for the photoelectrochemical reduction of CO₂ and N₂ to urea indicate that Ru–Pd/WO₃/MXene exhibits a stronger tendency for the chemisorption of N₂ compared to pure MXene or WO₃. The results also confirmed that incorporating Ru-Pd into the WO₃/Mxene structure can remarkably shift the density of states (DOSs) toward the Fermi level, indicating a substantial improvement in the electrical conductivity of the material (Figures 3A–3C). Based on the initial stage of the PEC process and free energy diagram of the reduction reaction (Figures 3D and 3E), an excellent urea yield could be achieved at low onset potential over the prepared bifunctional surface compared to the recently reported Pd-containing heterostructures, such as Pd–CeO₂²⁰ and Pd₁Cu₁/TiO₂-400.¹⁵ The enhanced selectivity of the catalyst results from a carefully designed multifunctional heterostructure, in which each component plays a specific role in supporting distinct steps of the C–N coupling process. Ru–Pd alloy nanoparticles activate CO₂ and convert it to ∗CO, with the bimetallic combination lowering the energy barrier for this key intermediate. WO₃ nanosheets, with a low conduction band, weaken the N≡N bond to activate ∗N₂, while MXene (Ti₃C₂T_x) provides a highly conductive surface, adsorbing and activating N₂ and preventing nanoparticle clumping. This 2D-on-2D architecture enhances charge transfer and creates a favorable electronic environment for C–N coupling between ∗CO and ∗N₂, forming ∗NCON. The design minimizes competing reactions by inhibiting hydrogen evolution, guiding ∗CO toward C–N coupling, and preventing NH₃ formation through controlled N₂ activation.

Figure 3.

A DFT study was conducted to investigate Ru–Pd/WO₃/MXene for accelerated CO₂N₂RR

DOS of Ti₃C₂Tx (A), WO₃ (B), and Ru−Pd/WO₃/MXene (C), mechanism of N₂-CO₂ reduction on the prepared catalyst (D), and free energy diagram for the reaction through an alternative and distal pathway on Ru−Pd/WO₃/MXene (E).³⁶

Copyright 2024, American Chemical Society.

Similar theoretical and experimental insights have been reported by other researchers investigating Zn-Mn diatomic catalyst structures⁶⁵ and Ti₂@C₃N₄.⁶⁶ All findings are based on the understanding that the chemical adsorption of the reactants is a key step in the reaction, driven by interactions between N₂ and/or CO₂ and the metallic active sites on the electrocatalyst components. Another DFT study on CuB₁₂ single-layer structures revealed that the CuB₁₂ surface exhibits excellent catalytic performance along with the CO₂ and OCOH pathways, with a low limiting potential of approximately 0.23 V and a low kinetic barrier of 0.54 eV for the key C–N bond-forming step (∗CO + ∗NHNH → ∗NHCONH), indicating strong intrinsic activity and high selectivity toward urea formation. The high selectivity of the CuB₁₂ monolayer arises from its electron-deficient boron sites, which strongly adsorb and activate both CO₂ and N₂ while stabilizing key intermediates such as ∗CO and ∗NHNH in close proximity. Selectivity is further enhanced by suppressing CO₂ reduction pathways leading to CH₃OH or CH₄, as further hydrogenation of ∗CO to ∗CHO is thermodynamically and kinetically less favorable than C–N coupling. In parallel, the reaction mechanism bypasses complete N₂ reduction to NH₃ by promoting coupling between CO and partially hydrogenated N₂ species, preventing NH₃ from becoming a dominant nitrogenous by-product, even though the initial N₂ activation step shares a similar energy barrier. Additionally, weak hydrogen adsorption on boron sites intrinsically suppresses the hydrogen evolution reaction, ensuring that protons are preferentially consumed in urea-forming hydrogenation steps rather than H₂ generation. Together, these features enable the CuB₁₂ catalyst to efficiently steer reaction pathways toward selective urea production.⁶⁷

Urea formation from carbon monoxide and ammonia

Direct C–N coupling between CO and NH₃ is challenging due to the inert nature of CO; however, under suitable catalytic conditions, this barrier can be overcome by activating the CO molecule, thereby facilitating the coupling reaction toward urea formation.³⁵ The electrocatalytic C–N coupling of CO and NH₃ on noble metal catalysts has recently been investigated to elucidate the reaction mechanism, employing electrochemical and computational techniques coupled with in situ spectroscopic analysis, particularly nuclear magnetic resonance (NMR).³⁵ The process demonstrated a strong performance on Pt catalysts, achieving approximately 70% selectivity and a C–N bond formation rate of 100 mmol h⁻¹ g⁻¹ at ambient conditions. The proposed reaction mechanism also involves sequential 2H⁺-coupled electron-transfer steps leading to the formation of the intermediate isocyanic acid (HNCO). The HNCO intermediate was subsequently neutralized to form cyanate, which then reacted with ammonium to yield urea. The practical significance of this method lies in its efficient utilization of toxic CO gas to synthesize high-value-added urea through coupling with nitrogen, an abundant atmospheric component.

Urea formation from carbon dioxide and nitrites

Previous studies suggested that the C–N coupling step during urea electrosynthesis from NO₂⁻ and CO₂ likely originates from ∗NH₂ and ∗CO intermediates.⁶⁸ A possible mechanism for this reaction was proposed by Wang’s research team, considering their experimental results for electrocatalytic urea generation on the surface of the AuCu self-assembled nanofiber catalysts³⁸:

$${\text{CO}}_2+{2\mathrm{H}}^{+}+{2\mathrm{e}}^{-}\to {}^{\ast }\text{CO}+{\mathrm{H}}_2\mathrm{O}$$ (Equation 8)

$${\text{NO}}_2^{-}+{6\mathrm{H}}^{+}+{5\mathrm{e}}^{-}\to {{}^{\ast }\text{NH}}_2+2{\mathrm{H}}_2\mathrm{O}$$ (Equation 9)

$${}^{\ast }\text{CO}+2{{}^{\ast }\text{NH}}_2\to {\text{NH}}_2\text{CO}{\text{NH}}_2$$ (Equation 10)

The proposed pathway involved the simultaneous formation of ∗CO and ∗NH₂ intermediates on electrode surfaces, followed by their coupling to form urea. The authors also clearly claimed that no urea formation was observed when CO₂ and NO₂⁻ were replaced with CO and NH₄⁺, respectively. Another team reported a similar mechanistic trend for the reaction over a Cu₉₉Ni₁ catalyst, supported by DFT simulations and in situ FTIR spectroscopy.⁶⁹ However, a key deviation from the standard NH₃-based pathway was observed in the work of Zhang et al., where the oxygen vacancies in ZnO play a role by providing active sites for the direct C–N coupling of CO₂ and NO₂⁻ derivatives. According to their results from in situ ATR (attenuated total reflectance)-FTIR spectroscopy, the electrochemical generation of ∗NH₂ and ∗COOH via proton-coupled electron transfer steps represents an initial key step in the urea formation pathway. This claim was verified by the detection of their characteristic peaks in ATR-FTIR spectra.

$${\text{NO}}_2^{-}+{6\mathrm{H}}^{+}+{5\mathrm{e}}^{-}\to {{}^{\ast }\text{NH}}_2+4{\mathrm{H}}_2\mathrm{O}$$ (Equation 11)

$${\text{CO}}_2+{\mathrm{e}}^{-}+{\mathrm{H}}^{+}\to {}^{\ast }\text{COOH}$$ (Equation 12)

$${}^{\ast }\text{COOH}+2{{}^{\ast }\text{NH}}_2+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\to {\text{NH}}_2\text{CO}{\text{NH}}_2+\text{CO}+{\mathrm{H}}_2\mathrm{O}$$ (Equation 13)

Urea formation from carbon dioxide and nitrates

There is a global concern about NO₃⁻ content in groundwater, primarily due to the excessive use of fertilizers and other industrial activities.⁷⁰ Given that the N=O bond (204 kJ mol⁻¹) has a substantially lower dissociation energy than the N≡N bond (941 kJ mol⁻¹), electrocatalytic reduction of NO₃⁻ has emerged as an attractive and environmentally friendly approach, particularly when coupled with CO₂ co-reduction, enabling urea synthesis via the C–N coupling process.⁷¹ Considering such advantages along with the growing interest in sustainable nitrogen and carbon utilization, recent research has increasingly focused on CO₂ and NO₃⁻ as commonly used reactants in electrocatalytic urea synthesis. This focus results from the fact that these reactants are abundant, sustainable, and allow direct carbon-nitrogen coupling under mild conditions.⁷² The efficiency and pathway of urea formation, however, strongly depend on the choice of nitrogen precursor and its oxidation state. As a deeply oxidized species, NO₃⁻ undergoes a more complex reduction process than NO₂⁻, as in many studies with NO₃⁻ and CO₂, the C–N bond forms via ∗NO₃, ∗NO_2, and ∗CO/∗CO₂.⁷³ The series of reactions outlined in Equations 14, 15, 16, 17, 18, 19, 20, and 21 describes a rational mechanism for urea formation. NO₃⁻ ions are first reduced to ∗NH₂⁻ on the catalyst surface. This reaction involves the consumption of protons (H⁺) and electrons (e⁻), leading to the formation of ∗NH₂ and H₂O.

$${\text{NO}}_3^{-}+{2\mathrm{H}}^{+}+{2\mathrm{e}}^{-}\to {}^{\ast }\mathrm{N}{\mathrm{O}}_2^{-}+{\mathrm{H}}_2\mathrm{O}$$ (Equation 14)

$${}^{\ast }{\text{NO}}_2^{-}+{2\mathrm{H}}^{+}+{2\mathrm{e}}^{-}\to {}^{\ast }\mathrm{N}{\mathrm{O}}^{-}+{\mathrm{H}}_2\mathrm{O}$$ (Equation 15)

$${}^{\ast }\mathrm{N}{\mathrm{O}}^{-}+{4\mathrm{H}}^{+}+{4\mathrm{e}}^{-}\to {}^{\ast }{\text{NH}}_2+{\mathrm{H}}_2\mathrm{O}$$ (Equation 16)

Overall, NO₃⁻ is reduced to an amine group (∗NH₂) on the catalyst surface (∗), with the release of H₂O:

$${\text{NO}}_3^{-}+{8\mathrm{H}}^{+}+{8\mathrm{e}}^{-}\to {}^{\ast }{\text{NH}}_2+3{\mathrm{H}}_2\mathrm{O}$$ (Equation 17)

DFT exploration shows that ∗NH₂, as a reactive nucleophile, is kinetically accessible and stable on transition metal surfaces.⁷⁴

$${\text{CO}}_2+{\mathrm{e}}^{-}+{\mathrm{H}}^{+}\to {}^{\ast }\text{COOH}$$ (Equation 18)

$${}^{\ast }\text{COOH}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\to {}^{\ast }\text{CO}+{\mathrm{H}}_2\mathrm{O}$$ (Equation 19)

∗COOH is a transient intermediate, while ∗CO is the stable surface species that is adsorbed strongly enough to exclude further reduction of CO₂.⁷⁵

$${}^{\ast }\text{CO}+{}^{\ast }{\text{NH}}_2+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\to {}^{\ast }\text{CO}{\text{NH}}_2+\text{CO}+{\mathrm{H}}_2\mathrm{O}$$ (Equation 20)

DFT calculations also demonstrated a lower formation energy for ∗CONH₂ on Te−Pd (111) than that of pure Pd.⁷⁶

$${}^{\ast }\text{CO}{\text{NH}}_2+{}^{\ast }{\text{NH}}_2+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\to \mathrm{N}{\mathrm{H}}_2\text{CO}{\text{NH}}_2$$ (Equation 21)

To elucidate the mechanism of C–N coupling during the co-reduction of NO₃⁻ and CO₂, the online differential electrochemical mass spectrometry (DEMS) technique was applied. Meng et al. demonstrated that ∗NH₂ and ∗CO are the primary intermediates in the reaction mechanism over Cu-Zn alloy, as indicated by the significant decrease in the signal intensity of CO and NH₂ upon the consumption of the reactants.⁷⁷ Further mechanistic insights were obtained by investigating the energy barriers associated with forming key intermediates on various metal facets, including Zn and Cu-Zn alloys. Despite these findings, many studies still suggest that ∗NO₂ and ∗CO/∗CO₂ are the dominant intermediates in NO₃⁻-to-urea conversion. Zhang et al. introduced copper molybdate (Cu₃Mo₂O₉) nanorods as an active hydrogen pump catalyst that actively generates and supplies surface-adsorbed hydrogen (∗H) at Cu sites to hydrogenate key nitrogen and carbon intermediates. In the nitrogen-converting route, ∗NO is rapidly converted to ∗HNO and then to ∗NH₂O or ∗NHOH, ultimately yielding the ∗NH₂ intermediate. In the carbon conversion pathway, CO₂ is converted to ∗COOH, and then hydrogenated to the ∗HOCOH intermediate, which is more electrophilic than ∗CO and favors nucleophilic attack by ∗NH₂. The C–N coupling occurs between ∗NH₂ and ∗HOCOH, forming ∗H₂NCOOH, which, upon dehydration, gives ∗H₂NCO and is finally hydrogenated to urea. The synergistic effect of Cu sites for ∗H generation and NO₃⁻/NO₂⁻ activation, together with Mo-O motifs that stabilize intermediates and modulate electronic structure, suppresses side reactions and enables the formation of a new, lower-energy pathway via hydrogenated intermediates.⁴² Using a combination of DFT calculations, DEMS, temperature-programmed desorption (TPD), and in situ Raman spectroscopy, Zhao et al. identified the rate-determining step and the prominent intermediates.⁷⁸ They designed an alternating Cu–W bimetallic catalyst (CuWO₄) with high-valence W⁶⁺ sites specifically tailored to stabilize ∗NO₂ intermediates from NO₃⁻ reduction, while adjacent Cu²⁺ sites favor the formation of ∗CO intermediates from CO₂ reduction at remarkably low potentials (Figure 4A). This wise arrangement brings the carbon and nitrogen intermediates close together, dramatically increasing the probability of direct C–N coupling. The prepared CuWO₄ demonstrates outstanding performance, achieving a urea Faradaic efficiency of 70.1 ± 2.4% and a production rate of 98.5 ± 3.2 μg h⁻¹ mg⁻¹ at a low potential of −0.2 V versus RHE, significantly exceeding both its single-metal counterparts (CuO and WO₃) and previous state-of-the-art catalysts.⁷⁸ A comprehensive mechanistic study, using in situ Raman spectroscopy, DEMS, and TPD, confirmed that ∗CO and ∗NO₂ are the key surface-bound intermediates. Importantly, changing the reactants in the experiments proved that only CO (or its aqueous derivatives) and NO₂ gas could replace CO₂ and NO₃⁻ to produce urea, confirming their roles as the essential coupling partners. DFT calculations provided an atomic-level understanding, showing that the rate-determining step is the C–N coupling between ∗CO and ∗NO₂ to form ∗CONO₂ (Figures 4B–4D) This coupling is thermodynamically more favorable (ΔG = −0.70 eV) than the competing hydrogenation of ∗NO₂ to NH₃ (ΔG = −0.32 eV) or ∗CO to hydrocarbons, which explains the reason for the high selectivity of the system. The electronic structure at the bimetallic interface promotes electron transfer, facilitating this key coupling step. Therefore, this study successfully highlights a general design principle by constructing catalysts with complementary, well-arranged sites that stabilize and bring key intermediates together, and effectively control complex reaction pathways. This strategy minimizes dominant side reactions, such as NH₃ or hydrogen evolution, and enables highly selective and efficient synthesis of urea from abundant feedstocks, such as CO₂ and NO₃⁻.

Figure 4.

Schematic illustration of design strategy for bioinspired alternating bimetallic sites of CuWO₄ catalyst for urea electrosynthesis and DFT calculation of urea synthesis mechanism on bimetallic CuWO₄ (111) surface

(A) CuWO₄ catalyst design, showing bioinspired alternating bimetallic sites for urea electrosynthesis.

(B) Free-energy diagram illustrating urea formation and NO₃⁻ reduction on the CuWO₄ (111) facet.

(C) Mechanism of C–N coupling between ∗CO and ∗NO₂, showing the initial, transition, and final states of ∗CONO₂ formation. Atom colors: gray = C, blue = N, red = lattice O, orange = Cu, cyan = W, yellow = adsorbate O.

(D) Charge density difference for the co-adsorption of ∗CO and ∗NO₂ (left) and for the adsorbed ∗CONO₂ (right) on the CuWO₄ (111) facet. The iso-value is 0.002 e/ų, with electron accumulation shown in yellow and depletion in cyan.⁷⁸

Copyright@2023, Springer Nature.

Moreover, by testing alternative carbon sources (CO₃²⁻, HCO₃⁻, CO, HCHO, CH₃OH) while keeping the nitrogen source constant, the authors demonstrated that only precursors that form the adsorbed ∗CO intermediate can lead to urea, confirming ∗CO as the essential carbon partner. In parallel, screening different nitrogen sources (NO₂, NO₂⁻, NO, NH₂OH, NH₃) with CO₂ revealed that only NO₂ gas could substitute for NO₃⁻, ∗NO₂ as a nitrogen intermediate. This highlights the crucial role of precursor selection, which dictates which nitrogen intermediates are generated and how effectively they couple with carbon species. Choosing an appropriate nitrogen source for urea synthesis is inherently complex because it requires balancing thermodynamic feasibility, kinetic behavior, and practical system constraints. No single option is universally superior, as the choice fundamentally shapes the overall process design. Although NO₃⁻ is thermodynamically favorable based on standard potential, it demands the highest proton consumption per urea molecule. From a thermodynamic perspective, NO is favorable when compared to the competing anodic H₂O-splitting reaction, as it shows the smallest change in Gibbs free energy per urea molecule. In terms of kinetics, NO₂⁻ and NO provide higher current densities than nitrogen gas. This is consistent with the previous electrochemical observations, in which N₂ behaves inertly compared to other nitrogen sources. Practically, N₂ and NO are gases, while NO₃⁻ and NO₂⁻ are dissolved in the electrolyte, allowing independent control over nitrogen and CO₂ transport. Regardless of the nitrogen source, a liquid phase is required in the cell, necessitating carefully designed zero-gap configurations to minimize voltage. Although NO₃⁻, NO₂⁻, and NO are more reactive, they are less abundant than N₂, a key consideration for real-world applications and the overall feasibility of the process.⁷⁹ For instance, pursuing N₂ activation aligns with long-term sustainability goals when abundant renewable energy is available, whereas short-term, practically deployable electrosynthesis is more effectively achieved with NO₃⁻/NO₂⁻ reactants, especially when paired with highly active electrocatalysts capable of transforming these pollutants into useful products.⁷⁹ Table 2 summarizes various nitrogen precursors for urea synthesis, outlining their respective advantages and challenges, thereby linking experimental evidence to strategic catalyst and reactant design.

Table 2.

Nitrogen precursors for urea synthesis: advantages and disadvantages

Nitrogen precursor	Activation difficulty	Nitrogen intermediates	Advantages	Challenges	Reference
N2 (Nitrogen gas)	very high	∗N2, ∗NH2, ∗NCON, ∗NNH	abundant, clean, sustainable	extremely difficult activation due to a strong N≡N bond; very low Faradaic efficiency; requires harsh conditions	Chen et al.15; Foster et al.80
NO3− (Nitrate ion)	moderate	∗NO3, ∗NO2, ∗NO, ∗NH2	high solubility; good selectivity when ∗NO2 stabilizes, widely available	multiple reduction steps; competing NH3 formation; requires precise control of intermediates	Yu et al.81; Zeng et al.82
NO (Nitric oxide)	low-moderate	∗NO, ∗NO2, ∗N, ∗NH2CO	easy activation; strong N–metal interaction; rapid formation of urea intermediates	toxic; unstable; undesired side reactions; handling difficulties	Cai et al.83; Long et al.84; Shi et al.85
NO2− (Nitrite ion)	low	∗NO2, ∗CO2, ∗NOOH, ∗NH2	high urea yield; low activation energy; low N=O bond dissociation energy; water-soluble	competing NH3 formation; mass-transport sensitive; selectivity depends on catalyst	Hu et al.86; Shin et al.87; Cai et al.83
NH3/NH4+ (Ammonia)	very low	∗NH2, ∗NH, ∗NCON	easy to activate; simple chemistry; can couple electrochemically with ∗CO	primarily produces H2 or reduces to N2; not ideal for direct C–N coupling; low selectivity for urea	Krebs et al.88; Krase et al.89

Mechanistic divergence and the catalyst-dependent pathway

As discussed in the previous sections, the literature reports several conflicting C–N coupling pathways, including the coupling of ∗CO and ∗N₂ to form an ∗NCON intermediate (Equation 6), the reaction of ∗COOH and ∗NH₂ intermediates (Equation 13), and the coupling of ∗CO and ∗NO₂ according to Zhao et al.’s study.⁷⁸ This review argues that such divergence does not reflect errors in analysis; rather, it represents a significant insight that the urea synthesis mechanism is highly catalyst-dependent and not universal. The nature of the active sites, whether a metal alloy, metal oxide, or single-atom catalyst (SAC), along with the specific nitrogen precursor (N₂ versus NO₃⁻), fundamentally determines which intermediates are energetically favored and, therefore, which reaction pathway predominates. For instance, in the Ru-Pd/WO₃/MXene heterostructure, the Ru sites provide strong N₂ binding that facilitates N₂ activation, while the Cu or Pd sites favor CO₂ reduction to ∗CO. This dual functionality facilitates the coupling of nitrogenous and carbonaceous intermediates via the ∗NCON pathway (Equation 29), which is energetically plausible. Therefore, cooperative interactions between different sites within a heterostructure play a critical role.

As discussed in urea formation from CO₂ and NO₃⁻, the choice of nitrogen precursor is crucial, as it fundamentally alters the reaction pathway. Table 2 demonstrates that N₂ is highly inert, requires significant energy for activation, and typically undergoes associative or dissociative reduction via ∗N₂ or ∗NH₂ intermediates. In contrast, NO₃⁻, as an oxidized and solvated species, is reduced through intermediates such as ∗NO₂, ∗NO, and ∗NH₂.

On the other hand, oxide-based catalysts with abundant oxygen vacancies, such as ZnO or Vo–In–TiO₂, readily activate H₂O to supply protons and preferentially stabilize ∗COOH (from CO₂) and ∗NH₂ (from NO₂⁻ reduction). For these systems, the ∗COOH + ∗NH₂ coupling pathway (Equation 13) is therefore the more probable route.

In the PEC systems, light and an applied electrical bias operate as distinct synergistic parameters that together drive chemical reactions, each shaping catalyst behavior and reaction mechanisms in fundamentally different ways. Light primarily generates electron–hole pairs within a semiconductor when photon energy exceeds the bandgap, creating high-energy charge carriers, altering surface potentials, and sometimes restructuring the catalyst surface. In contrast, the applied electrical bias controls the separation and utilization of these photogenerated charges by establishing the electric field, setting the Fermi level, and tuning the overpotential, thereby affecting the binding energies and reaction pathways of surface intermediates. Their interaction creates a unique operational regime in which light supplies part of the reaction energy while the bias directs charge flow and sets the thermodynamic driving force, enabling mechanistic pathways not accessible in purely electrochemical or purely photocatalytic systems. Consequently, reaction performance reflects a non-linear combination of light intensity and applied potential, implying that the selectivity and activity depend sensitively on how these two inputs are jointly tuned.

The absence of a single, universal mechanism underscores the need for future research to move beyond merely proposing mechanistic pathways and instead prioritize catalyst designs that both minimize the energy barrier for a targeted, high-selectivity route and concurrently elevate the kinetic barriers for all competing parasitic reactions.

Electrocatalytic materials for sustainable urea production

As mentioned in the previous sections, CO₂ and nitrogenous compounds, such as N₂, NO₂⁻, NO₃⁻, and NO, are the primary reactants for urea production under electrocatalysis conditions. One key challenge in this approach is achieving the controlled, simultaneous reduction of CO₂ and N-containing species within the same reaction environment. Therefore, the rational design of advanced electrocatalytic materials to control reaction selectivity and enhance urea yield is a key point in this field. Considering several crucial factors, including the mass transfer rate at the catalyst surface/interface, the number of accessible active sites, and the electronic behavior at the catalyst–electrode interface, various solid materials have been explored for efficient urea electrosynthesis over the past decades.⁹⁰ In this section, the focus is on the primary principles and essential factors in designing rational electrocatalysts for efficient urea production, as well as the remarkable challenges that restrict their large-scale use in real-world applications.

Transition metals

Due to the unique properties such as high stability, low cost, abundance in nature, desirable electronic characteristics, and environmental friendliness, transition metals such as Zn, Fe, Cu, Co, V, Mo, Ce, W, and others have received increasing attention for promoting the electrocatalytic synthesis reactions.^{14,66,91,92,93,94} Precise engineering of structural characteristics and atomic-level morphology significantly influences the electrochemical performance and catalytic activity of transition-metal-based materials. As a result, considerable effort has been devoted to designing and fabricating SACs in recent years to enhance the electrocatalysis of CO₂ and nitrogenous compound reduction, NH₃ synthesis, hydrogen evolution, and C–N coupling reactions.² The SACs, which consist of individually isolated atoms, offer high atom efficiency and well-defined active sites, making them highly effective for catalytic C–N coupling reactions. Fe-based SACs have demonstrated the capability to simultaneously activate N₂ and CO₂, thereby facilitating C–N coupling, which is essential for urea formation. Du et al. synthesized Fe single atoms supported on MoS₂ (Fe₁/MoS₂) as effective catalysts for urea electrosynthesis.³⁷ This catalyst achieves a Faradaic efficiency of 54.98% and a urea yield of 18.98 mmol h⁻¹ g⁻¹, in a flow cell, which is attributed to a well-designed tandem catalytic mechanism in which Fe₁–S₃ sites preferentially drive the initial C–N coupling while MoS₂ edge sites facilitate the subsequent hydrogenation of the ∗CO₂NH₂ intermediate to ∗COOHNH₂ and thereafter urea. Although the results are scientifically impressive, reproducing them may be challenging because the formation of the Fe₁–S₃ active sites is highly sensitive to synthesis conditions, and their verification relies on specialized techniques such as AC-STEM and synchrotron-based XAS, which are not widely accessible. In addition, scaling the process is hindered by difficulties in preserving single-atom sites at high catalyst loadings and in demonstrating catalyst stability beyond 100 h of operation.

Cu–N–C SACs have shown significant potential in the electrochemical synthesis of urea from CO₂ and NO₃⁻. A study reported a Faradaic efficiency of up to 28% for urea synthesis using Cu–N–C catalysts. This performance is attributed to the Cu–N₄ coordination structure of the catalyst, which enhances CO₂ reduction, while its modified sites facilitate NO₃⁻ reduction, thereby promoting effective C–N coupling.⁴⁰

Similarly, Ni–N–C has proven effective for the activation of CO₂ and NO₃⁻ in urea synthesis. Hua et al. developed an efficient and scalable method for synthesizing Ni–N–C catalysts using guar gum, an abundant and low-cost natural polymer, as a template. The resulting Ni–N–C catalyst features a tri-coordinated Ni–N₃ configuration, which enhances catalytic activity, and a hierarchical porous structure that facilitates mass and charge transport. This catalyst demonstrated stable operation for over 70 h in a membrane electrode assembly, achieving a total energy efficiency of 41.0%.⁹⁵

Bimetallic catalysts have shown synergistic interactions that improve the efficiency of C–N coupling in the electrochemical production of urea from CO₂ and nitrogen-containing sources. For example, Cu–Fe alloys combine the advantages of both metals, in which Cu facilitates CO₂ reduction to ∗CO intermediates, while Fe promotes the reduction of NO₃⁻ or N₂, enabling effective C–N bond formation.⁹⁶

Cu–Zn bimetallic systems improve the adsorption and conversion of CO₂ and nitrogen species, with research showing that the integration of Cu and Zn oxides can reach a Faradaic efficiency of up to 22.27% for urea synthesis at −0.8 V vs. RHE.⁹⁷

Metal oxides and hydroxides have emerged as effective catalysts for electrochemical urea synthesis, owing to their ability to stabilize reaction intermediates and facilitate the co-reduction of CO₂ and nitrogen species. Titanium dioxide (TiO₂) with oxygen vacancies has enhanced activation of both CO₂ and NO₂⁻, promoting efficient C–N coupling. Mao et al. have highlighted oxygen vacancy-rich indium-doped titanium dioxide (Vo-In-TiO₂) for efficiently synthesizing urea by co-reducing NO₃⁻ and CO₂. The catalyst achieves a Faradaic efficiency of 9.06% and a urea yield of 759.8 μg mg⁻¹ h⁻¹ at −0.65 V vs. RHE. Its high performance is due to the synergistic effect of oxygen vacancies (Vo) and In³⁺ ions: Vo sites facilitate NO₃⁻ reduction to ∗NH₂, while In³⁺ enhances CO₂ capture and promotes the coupling of ∗NH₂ and ∗CO₂ to form urea. This “relay catalysis” approach offers a promising strategy for designing efficient urea-synthesis catalysts.⁴¹ Cobalt-based oxides like Co₃O₄ have redox-active surfaces that are well-suited for NO₃⁻ reduction. Studies show that Co₃O₄ demonstrates strong electrocatalytic activity in converting NO₃⁻ to NH₃, a crucial intermediate in synthesizing urea.⁹⁸

Noble metal alloys and hybrids

Alloys and hybrid-based noble metal catalysts (Au, Pd, and Pt) also show great potential to catalyze the electrochemical urea synthesis at ambient conditions. These materials have been widely employed in various electrocatalytic applications, including CO₂ reduction, NH₃ synthesis, H₂ evolution, and NO₃⁻ reduction reactions, and have indicated excellent performances.^99,100

Owing to their unique electronic properties for controlling the formation of the desirable intermediates through C–N coupling, along with high chemical stability and excellent electrocatalytic activity, these catalysts are considered highly promising for urea electrosynthesis.^18,101 Recent advancements have demonstrated the efficacy of Pt catalysts in the electrocatalytic oxidative coupling of carbon monoxide (CO) with NH₃ to synthesize urea under mild conditions. Xiong et al. developed a method for synthesizing urea through electrocatalytic oxidative coupling between CO and NH₃ on Pt catalysts. They obtained an optimal selectivity of approximately 70% for urea, maintaining over 50% selectivity across a broad range of applied potentials, with an electrocatalytic C–N bond formation rate reaching 100 mmol h⁻¹ g⁻¹. Mechanistic studies suggest that the Pt-catalyzed reaction produces cyanate from CO and NH₃, which then undergoes the Wöhler reaction to yield urea.³⁵ Bimetallic alloys of noble metals, such as Au-Pd and Pt-Ru, are pivotal in catalysis and electrocatalysis due to their synergistic effects from tailored electronic and geometric interactions.¹⁰² For example, Au–Pd alloy demonstrates high efficiency in oxidation reactions by combining Pd reactivity with the ability of Au to stabilize intermediates.¹⁰³ These alloys exhibit tunable selectivity based on their composition and structure, and their inherent corrosion resistance is further enhanced through alloying, making them highly durable in harsh environments.¹⁰⁴ Furthermore, bimetallic alloys composed of Pd¹⁵ and Au³⁸ with copper nanoparticles were precisely engineered to incorporate structural defects, thereby enhancing the density of electroactive sites.

Earth-abundant and non-noble metals

Although noble metals such as Pt, gold (Au), and silver (Ag) exhibit excellent catalytic activity, their high cost and scarcity have driven significant interest in more abundant, cost-effective non-noble metal catalysts. These include transition metal-based catalysts (such as iron (Fe), nickel (Ni), copper (Cu), cobalt (Co), and manganese (Mn)), along with their alloy catalysts (like Cu-Fe), which offer comparable efficiency, improved durability, and beneficial synergistic interactions.¹⁰⁵ Additionally, metal-organic frameworks (MOFs) offer notable benefits over conventional catalysts due to their high surface area and tunable properties. Their porosity and surface area improve interactions between active sites and reactants, as well as facilitate efficient mass transport of ionic electrolytes, a feature lacking in traditional catalysts such as hydroxides, perovskites, and metal oxides. Moreover, the adaptable metal ions in MOFs enhance the performance and stability of electron-transfer reactions.¹⁰⁶ Carbon-based materials, such as doped graphene, promote efficient charge transfer and represent promising non-noble options. While such catalysts are cost-effective, sustainable, and customizable, they still face challenges in achieving the activity and selectivity of noble metals, maintaining stability under reaction conditions, and clarifying their operational mechanisms. Ongoing research focused on optimizing these materials and understanding their mechanisms is crucial for advancing scalable, commercially viable, sustainable urea production. This research includes rational design, surface engineering, and innovative synthesis techniques to enhance the active sites, electronic structure, and morphology of the catalysts. By fine-tuning these elements, it becomes possible to develop more efficient and stable non-noble transition-metal-based catalysts for sustainable urea production, making them suitable for large-scale industrial applications.¹⁰⁶ Geng et al. developed a high-efficiency dual-site electrocatalyst for ambient urea synthesis by synthesizing graphitic carbon encapsulated amorphous iron and iron oxide nanoparticles on carbon nanotubes (Fe(a)@CFe₃O₄/CNTs) using a liquid phase laser irradiation method.¹⁰⁷ The resulting electrocatalyst achieved a notable urea yield of 1341.3 ± 112.6 μg h⁻¹ mg_cat⁻¹ and a faradaic efficiency of 16.5 ± 6.1%, at a potential of −0.65 V (vs. RHE) in a 0.1 M KNO₃ electrolyte under ambient conditions. In situ FTIR and NMR analyses indicated that the Fe(a)@C sites selectively reduce NO₃⁻ to ∗NH₂ intermediates while the Fe₃O₄ sites reduce CO₂ to ∗CO intermediates. This dual-active-site synergy lowers the energy barrier for C–N coupling, enabling efficient urea formation. This study presents a sustainable approach to urea production by utilizing waste nitrogen and carbon sources using earth-abundant, non-noble metals as an electrocatalyst.

Covalent organic frameworks

Covalent organic frameworks (COFs) are a novel category of porous, crystalline materials composed of small organic building blocks connected through strong covalent bonds involving light elements such as H, B, C, N, and O.¹⁰⁸ The unique characteristics of COFs, including high porosity, well-ordered crystalline networks, lightweight nature, excellent thermal stability, and high surface area, make them highly suitable candidates for C–N coupling reactions.¹⁰⁹

Due to their high surface area, tunable porosity, and low density, COFs have emerged as promising materials for facilitating C–N coupling reactions, including urea synthesis.¹¹⁰ These properties enable COFs to serve as efficient catalysts or catalyst supports, thereby enhancing the accessibility of active sites and promoting effective molecular interactions.¹¹¹ In urea synthesis, COFs can stabilize metal nanoparticles or active catalytic sites, which are crucial for activating nitrogenous and carbonyl compounds. Additionally, the thermal and chemical stability of COFs ensures robust performance under the harsh reaction conditions typically required for C–N bond formation, making them excellent platforms for sustainable, recyclable catalytic systems in urea production.¹¹⁰ Li et al. developed three-dimensional COFs (3D COFs) functionalized with benzene, pyrazine, and tetrazine cores to modulate catalytic performance through varying nitrogen content. The obtained results demonstrate a progressive improvement in electrical conductivity, light absorption, charge separation efficiency, and co-adsorption capacity for NH₃ and CO₂ from benzene-to pyrazine-to tetrazine-containing COFs. Among them, the tetrazine-based COF shows the most efficient photocatalytic performance for urea production, reaching a yield of 523 μmol g⁻¹ h⁻¹, 40 and 4 times greater than the benzene- and pyrazine-based COFs, respectively. This enhanced activity, attributed to the nitrogen-rich heterocyclic environment, as validated by in situ spectroscopic studies and DFT calculations, offers a promising route to sustainable urea synthesis via photocatalysis.¹¹⁰

P-block element catalysts

Catalysts containing p-block elements, such as B, In, O, and N, have recently shown promising performance for sustainable electrocatalytic urea production due to their intriguing physicochemical properties and intrinsic poor hydrogen adsorption.¹¹² These elements are critical in stabilizing active sites, modulating electronic environments, and enabling novel reaction pathways. For example, boron (B) based catalysts such as boron nanosheets,¹¹³ boron carbides,¹¹⁴ boron nitride nanosheets,¹¹⁵ and boron phosphide nanoparticles¹¹⁶ possess inherent empty p orbitals and therefore demonstrate efficient N₂ activation or protonation; thus, they could serve as a useful dopant to introduce active sites on the surface of other materials.¹¹² P-block elements are often integrated into transition metal-based catalysts to enhance synergistic interactions. The strong interaction between P-block elements and N 2p orbitals results in excellent adsorption and activation of nitrogen sources. Chen et al. incorporated the p-block element bismuth (Bi) into the 1T-MoS₂ phase to synthesize a Bi₂S₃/MoS₂ composite. Combining the excellent conductivity of 1T-MoS₂ and the strong N₂ adsorption and activation capabilities of Bi, the catalyst achieved an NH₃ yield rate of 54.64 μg h⁻¹ mg⁻¹ and a high Faradaic efficiency of 58.56%. This study highlights the synergistic interaction between p-block elements and transition-metal-based catalysts in promoting efficient NH₃ synthesis via nitrogen reduction.¹¹⁷ Carbon-based materials, such as graphene and nitrogen-doped carbon nanosheets, are p-block elements that act as sustainable supports for Pd or Cu nanoparticles, promoting better dispersion and reusability. These p-block-derived supports enhance catalytic performance in reactions such as the Suzuki-Miyaura coupling and urea electrosynthesis.¹¹⁸

In addition, the integration of defect-engineered p-block-element-based catalysts represents a cutting-edge strategy to enhance catalytic performance in C–N coupling reactions.¹¹⁹ Oxygen-deficient InOOH structures are an example of defect engineering in metal oxide catalysts, where creating oxygen vacancies significantly boosts catalytic efficiency. These vacancies act as electron-rich sites that enhance the adsorption of reactants, such as CO₂ and nitrogen compounds, thereby stabilizing crucial intermediates, such as ∗CONH₂, which are necessary for C–N bond formation. Furthermore, oxygen vacancies modulate the local electronic environment, thereby lowering the energy requirements for vital steps such as CO₂ activation and N–H bond coupling. Such defects also enhance charge transfer, a crucial process for electrochemical reactions, including urea synthesis and cross-coupling reactions. Notably, the oxygen-deficient surfaces of InOOH reflect a design approach also found in hybrid catalysts, such as Cu-In oxides. In both cases, oxygen vacancies play a crucial role by enabling the simultaneous activation of CO₂ and NO₃⁻, a critical step in the electrochemical synthesis of urea.¹²⁰

Grain boundary-rich Bi₂Se₃ nanosheets are an exciting example of how structural defects can enhance catalyst performance. The defective edges and grain boundaries possess a high density of unsaturated active sites, facilitating the adsorption of reactants such as aryl halides and amines. Due to its two-dimensional architecture, these defects enhance electron mobility, thereby accelerating the redox cycles essential for electrocatalytic C–N coupling. Moreover, these defects modulate the local environment of Bi and Se atoms, enabling tunable selectivity and facilitating suitable reaction pathways toward desired products. Bi₂Se₃ is an excellent example of how p-block elements utilize their flexible oxidation states and structural adaptability to enable efficient catalysis, offering a sustainable alternative to noble metal-based systems.¹²¹

As an innovative strategy, researchers have focused on the rational combination of the above-mentioned electrocatalyst candidates to suppress the formation of unwanted intermediates and block associated mechanistic pathways. This approach also aims to control the electronic behavior at the electrode/electrolyte interface, facilitating C–N bond activation and increasing the electrocatalysis efficiency.⁶⁸ More importantly, identifying the optimized ratios of transition metals, noble metals, and p-block elements components, particularly when single-atom engineering of the constructions is involved, is essential to conceptually designing an effective catalyst. Accordingly, systematic efforts should focus on developing cost-effective and scalable methods to rapidly produce highly efficient electrocatalysts from these material families, enabling large-scale urea production for industrial applications.

Challenges and opportunities in electrocatalyst design and sustainable applications

Multiple electrochemical charge-transfer processes involving H⁺ and e^– species make C–N coupling reactions highly competitive and complex. This competition increases the energy barrier for the overall process, ultimately lowering the yield of urea formation and reducing the Faradaic efficiency. The key challenge in this area is to achieve simultaneous reductions of carbon sources and nitrogenous compounds, thereby forming favorable intermediates that lead to the selective formation of urea.

To overcome these challenges and achieve optimal urea production efficiency, an effective electrocatalyst should meet several key criteria, including facilitating the strong adsorption and efficient activation of the reactants on its surface, enhancing the C–N coupling reaction between the desired intermediates and active sites, and lowering the energy barrier of the favorable reaction steps while suppressing competing side reactions.

On one hand, researchers should consider the intrinsic characteristics of the reactants, especially nitrogen-containing compounds, to reduce the energy costs and promote the reaction more effectively. For instance, the solubility of N₂ in an electrolyte medium is lower than that of NO₂⁻ and NO₃⁻. Moreover, its activation is much more difficult due to the strong chemical bond between the two nitrogen atoms.

On the other hand, key technical parameters, such as overpotential, Faradaic efficiency, current density, electrode stability, and C/N selectivity, should be carefully monitored and optimized to accurately evaluate and improve the performance of urea electrosynthesis.²

Conventional electrocatalytic technologies utilize applied electric current and voltage to the electrode surface, precisely controlling the reaction kinetics and directing the selectivity of specific reactions at the electrode surface. These capabilities are often difficult to achieve through traditional thermal or homogeneous catalytic methods.¹²² This electrochemical control enables the fine-tuning of reaction pathways under milder conditions, activating inert molecules and offering greater specificity in product formation, particularly in complex multi-electron processes such as urea electrosynthesis.⁷⁹

Additionally, these systems often exhibit reaction rates that favor the formation of higher-valued product distributions, which is a significant advantage when considering scale-up and practical applications.¹²³ Utilizing electrical energy to power modern electrocatalytic systems addresses the key challenges associated with traditional reaction conditions.¹²⁴ This strategy eliminates the need for high-temperature and pressure conditions to drive the reactions effectively while preventing the emission of hazardous gases such as CO and CO₂, which can have an adverse impact on the environment.¹²⁵ Beyond this, in processes such as urea electrosynthesis, gases such as CO and CO₂, as well as nitrogenous species such as NO and NO₂⁻, serve as key reactants in the electrochemical reactions.⁵⁷

Therefore, large-scale urea electrosynthesis could significantly contribute to reducing the effects of global warming by utilizing atmospheric CO₂, thereby helping to mitigate climate change.¹²⁶ At the same time, it can contribute to air purification in urban areas by converting toxic gaseous pollutants, such as CO and NO, commonly emitted by industrial activities and vehicles, into valuable chemical products.¹²⁷

Conventional catalytic methods are limited by steady reaction conditions and energy-intensive processes, whereas electrosynthesis offers a high degree of tunability, enabling precise control over product purity, sustainability, and selectivity.¹²⁸ This adaptability makes electrosynthesis approaches particularly well-suited to meet the goals of green chemistry and sustainability, offering cleaner, more efficient, and environmentally friendly alternatives to traditional processes.^129,130 However, a significant challenge in electrocatalysis lies in designing and constructing highly stable electroactive surfaces that can function effectively as catalysts to ensure long-term stability under reaction conditions while maintaining high activity and selectivity in practical electrosynthesis applications.¹³¹ Utilizing renewable materials to achieve carbon neutrality and promote long-term sustainability has emerged as a highly promising strategy, garnering significant attention over the past few years.¹³² However, the high energy requirements in manufacturing single-atom engineered catalysts¹³³ and the lack of a precise mechanistic illustration of the interfacial phenomena¹³⁴ limit their practical applications. Within these concepts, the scientific community should significantly improve the fundamentals of electrosynthesis by establishing robust standards, conducting detailed product analyses, evaluating the long-term stability of electrode architectures, and developing large-scale electrochemical systems.¹³⁵ Another significant challenge in industrial electrocatalysis is the demand for high-concentration electrolytes and high-temperature electrolysis conditions to achieve efficient operation.¹³²

However, elevated temperatures often compromise the stability of most electroactive materials, particularly at the laboratory scale. This issue is further worsened by using high-concentration electrolytes, which create highly corrosive environments for testing. Overcoming these challenges is essential to establishing synthetic electrocatalysis as a robust and scalable tool that facilitates its transition from laboratory research to real-world applications.

Photocatalytic synthesis of urea

Due to their notable safety, eco-friendly nature, and simplicity of operation, solar-driven photocatalytic urea synthesis systems have recently gained considerable attention as a sustainable and adaptable alternative to the Bosch–Meiser process.^136,137 Photocatalysis involves the generation of multiple electron-hole pairs upon absorbing light photons on the surface of solid materials.¹³⁸ The inherent potential of photocatalytic materials can be improved by creating electron-rich sites in their crystal lattice. These sites are created through strategies such as defect, interfacial, facet, and junction engineering, as well as heteroatom doping and precise control over structural morphology.¹³⁹ Achieving efficient electron transfer to drive the feasible multi-electron reduction of nitrogenous reactants (NO, N₂, NO₃⁻, NO₂⁻, NH₃) and CO₂ during C–N coupling reactions is crucial but challenging for urea photosynthesis.

The earliest report on urea photocatalytic synthesis was published in 1998 by Kuwabata et al.¹⁴⁰ Simultaneous reduction of CO₂ and NO₃⁻ ions occurred on the surface of nano-sized TiO₂ supported on a polyvinylpyrrolidone gel film (TiO₂/PVP). This group achieved a urea yield of 1.9 mM after 5 h of reaction time under irradiation from a 500 W high–pressure mercury arc lamp. Interestingly, the systematic investigation of the reaction pathway revealed that the reduction of NO₃⁻ ions is the rate-limiting step in urea production over the photocatalysts. Subsequently, a variety of titanium oxide-based photocatalysts, such as Cu-containing TiO₂ particles,¹⁴¹ polymeric TiO₂ composites,¹⁴² and zeolite-based Fe-TiO₂ structures,¹³⁶ have been successfully developed for urea synthesis from different reactants. Over the last decade, significant research efforts have been focused on designing advanced heterojunction architectures and supporting photocatalyst systems to enhance urea production yields and understand surface reaction mechanisms.

For instance, Zhang et al. utilized a Ni₁-CdS/WO₃ heterojunction as an efficient photocatalytic approach for urea synthesis from CO₂ and N₂.¹⁴³ The catalyst achieved a urea yield rate of 78 μMh⁻¹, an apparent quantum yield of 0.15% at 385 nm, and a solar-to-urea conversion efficiency of 0.024%. Mechanistic studies reveal that this catalyst effectively uses photogenerated electrons and holes to convert CO₂ and N₂ to urea. The CO₂ adsorbed on the Ni site was reduced into ∗CO by photogenerated electrons. While the photogenerated holes reacted with H₂O to generate ^·OH radicals, further converting the N₂ adsorbed on the WO₃ surface into NO species. As shown in Figure 5A, the scanning electron microscopy (SEM) image of WO₃ nanorods displayed an average diameter of ∼100 nm with smooth surfaces. The high-resolution transmission electron microscopy (HRTEM) image of Ni₁-CdS/WO₃ showed two distinct sets of fringes with interplanar spacings of 3.34 and 3.91 Å, which are attributed to the (111) crystal plane of CdS and the (001) crystal plane of WO₃, respectively. (Figure 5B). Additionally, the elemental mapping images of Ni₁-CdS/WO₃ reveal a uniform distribution of Ni, Cd, S, W, and O elements (Figure 5C). Ni K-edge X-ray absorption fine structure (XAFS) was used to further examine the chemical states and coordination structures of the Ni species. The Ni K-edge spectra in R space, shown in Figure 5D, reveal a prominent peak at approximately 1.8 Å, attributed to the Ni-S coordination. No distinct peaks for Ni–O or Ni–Ni bonds were detected, confirming the atomic dispersion of Ni species on the CdS surface in Ni₁-CdS/WO₃. Additionally, in situ X-ray photoelectron spectroscopy (XPS) measurements were performed to investigate the direction of photogenerated carrier flow in the heterojunctions. As demonstrated in Figure 5E, under illumination, the CdS/WO₃ showed a 0.2 eV positive shift in the W 4f spectra and a 0.4 eV negative shift in the Cd 3d spectra compared to the dark conditions, which indicates that the photogenerated electrons and holes are primarily concentrated in the CdS and WO₃ components, respectively. A similar trend was observed for Ni₁-CdS/WO₃, except that the photogenerated electrons, which initially accumulated at CdS, were further transferred to the Ni single atoms. The in situ Ni L-edge X-ray absorption spectra of Ni₁-CdS/WO₃ under different conditions are shown in Figure 5F, confirming electron transfer to the Ni sites. Compared to the dark condition, the Ni L-edge peak intensity decreased significantly under illumination, attributed to photogenerated electrons occupying the unoccupied Ni 3d orbital. The schematic illustration of the flow direction of photogenerated carriers in Ni₁-CdS/WO₃ is shown in Figure 5G. The differences in work functions between CdS and WO₃ triggered electron transfer when they contacted each other, generating an internal electric field at their interface. This field caused electrons to move from CdS to WO₃, leading to band bending in both materials. As a result, the photogenerated electrons in the conduction band (CB) of WO₃ were driven toward CdS, where they recombined with the photogenerated holes in the valence band (VB) of CdS. At the same time, electrons in the CB of CdS and holes in the VB of WO₃ remained separated, promoting the reduction and oxidation reactions. The introduction of Ni single atoms further facilitated the transfer of photogenerated electrons from CdS to the Ni sites, improving the separation of the photogenerated carriers in Ni₁-CdS/WO₃. Figure 5H presents the schematic illustration of the photocatalytic urea synthesis mechanism over the Ni₁-CdS/WO₃ catalyst. When illuminated, photogenerated electrons and holes accumulate on the Ni₁-CdS and WO₃ components, respectively. CO₂ is first adsorbed on the Ni site, where it is reduced to ∗CO by photogenerated electrons. At the same time, photogenerated holes react with H₂O to generate ^·OH radicals, which then convert N₂ adsorbed on the WO₃ surface into NO species. These NO species move to the Ni site, where they react with ∗CO in a series of steps to form ∗OCNO and ∗ONCONO intermediates. Finally, ONCONO undergoes hydrogenation via a multi-step proton-electron-coupling process, yielding urea.

Figure 5.

The WO₃ and Ni₁-CdS/WO₃ nanostructures were synthesized, and their structural characterization and the mechanism of photogenerated carrier in them are demonstrated

(A) SEM image of WO₃ nanorods (scale bars, 500 nm).

(B) HRTEM image of Ni₁-CdS/WO₃ (scale bars, 5 nm).

(C) Elemental mapping images of Ni₁-CdS/WO₃ (scale bars, 50 nm).

(D) The Ni K-edge spectra of Ni1-CdS/WO₃ in R space, (inset: corresponding geometric configuration for Ni₁-CdS/WO₃, in which the green = Ni atom, mauve = Cd atom, yellow = S atom, red = O atom, and indigo = W atom.

(E) In situ XPS tests in W 4f spectra over Ni₁-CdS/WO₃.

(F) In situ Ni L-edge tests of Ni₁-CdS/WO₃.

(G) Flow direction of photogenerated carriers in Ni₁-CdS/WO₃.

(H) Photocatalytic urea synthesis mechanism over Ni₁-CdS/WO₃ catalyst.¹⁴³

Copyright@2025, Wiley Online Library.

Two heterojunction composites, S-defect engineered NiCoP/ZnIn₂S_4−x¹⁴⁴ and D-CdS@3D-BiOBr,¹⁴⁵ have been successfully examined for visible-light-driven photocatalytic urea synthesis. The interfacial charge transfer at the semiconducting junctions effectively maintained a high concentration of photogenerated electrons, thereby enhancing catalytic performance. Based on mechanistic understanding, the photocatalytic activity of these structures is further examined in the following sections.

A novel Z-scheme heterojunction, SrTiO₃-FeS-CoWO₄, has been recently examined for photocatalytic urea generation from CO₂ and N₂.¹⁴⁶ An excellent urea production yield of 8054.2 μg ⋅ g_cat⁻¹ ⋅ h⁻¹ indicates that the performance of the fabricated system is significantly higher than the current state-of-the-art technologies. The enhanced charge transfer facilitated by the FeS semiconducting bridge during construction helps preserve the photogenerated electrons, enabling the efficient co-reduction of the precursors and promoting the progress of C–O coupling. Furthermore, the engineered photocatalysts benefit from three distinct types of metal active sites, each selectively promoting the formation of desired products while minimizing impurity concentrations. Diffuse reflectance spectroscopy (DRS) results revealed that the ternary system exhibited significantly enhanced visible-light absorption compared to its pure and dual components (Figure 6A), partly due to surface plasmon resonance effects of iron sulfide. Mott–Schottky plots were employed to determine the flat band potentials and Fermi levels, which subsequently enabled the calculation of the conduction band (CB) and valence band (VB) positions for constructing the energy band diagram of SrTiO₃-FeS-CoWO₄ (Figure 6B). Furthermore, ultrafast spectroscopic analysis (Figures 6C–6E) confirmed the Z-scheme charge-transfer mechanism, as evidenced by both photoluminescence (PL) and transient absorption spectroscopy (TAS), demonstrating highly efficient interfacial charge transfer at the SrTiO₃-FeS-CoWO₄ junctions, thereby significantly enhancing photocatalytic performance. Transient absorbance spectroscopy (Figures 6F and 6G) revealed that the synergistic integration of the three semiconducting components significantly extends the average lifetime of the electron-hole pairs through efficient charge separation. This improvement is attributed to accelerated charge migration and suppressed recombination, enabling electrons accumulated in the system to simultaneously reduce CO₂ and N₂ effectively.

Figure 6.

Optical characterization

(A) The results of DRS.

(B) Heterojunction’s energy band diagram of the construction.

(C) PL spectra.

(D and E) Transient absorbance spectra (on nanosecond timescale) of the ternary photocatalyst, and the synchronized kinetic trace plots at 266 nm.

(F and G) The results of transient absorbance spectroscopy (on a microsecond timescale) of the final construction, and the analogous kinetic trace plots at 520 nm, the results of EPR for detecting: (H) hydroxyl radicals and (I) superoxide ion-radicals.

Adapted with permission from ref.¹⁴⁶ Copyright 2024, Wiley Online Library.

Meanwhile, the photogenerated holes facilitate H₂O oxidation to produce H⁺, thereby supporting the reduction reactions and preventing the oxidation of N₂. Additionally, electron paramagnetic resonance (EPR) analysis was employed further to elucidate the charge-transfer mechanism within the constructed system. The results indicated a substantial generation of hydroxyl radicals and a relatively weak signal for superoxide ion radicals in the system. This observation can be readily explained by the band edge potentials of the individual semiconducting components (Figures 6H and 6I).

Reaction pathways in photocatalytic urea production

In general, heterogeneous photocatalysis involves illuminating a solid semiconductor, resulting in the rapid generation of electron-hole pairs that produce reactive radicals that drive oxidation-reduction reactions.¹⁴⁷ Using instrumental techniques such as electron spin resonance (ESR) spectroscopy¹⁴⁸ and analyzing band electrochemical potentials, as well as experimental methods such as scavenging tests,¹⁴⁹ researchers were able to describe the charge migration trends and identify the primary reactive species involved in photocatalysis. Subsequently, benefiting from the in situ identification of reaction intermediates and the previous findings, a well-supported mechanistic illustration of the process could be presented. Due to the extensive research on charge transfer mechanisms at the photocatalyst interface, this section focuses on the reaction mechanism of urea formation, with an emphasis on intermediate identification.

According to Kuwabata et al.,¹⁴⁰ photocatalytic urea formation involves simultaneous reduction reactions of the reactants, in this case, NO₃⁻ and CO₂. Accordingly, the overall process requires 16 electrons to be transferred through a multi-electron reaction as follows:

$${2\text{NO}}_3^{-}+{\text{CO}}_2+{18\mathrm{H}}^{+}+{16\mathrm{e}}^{-}\to {\text{NH}}_2\text{CO}{\text{NH}}_2+7{\mathrm{H}}_2\mathrm{O}$$ (Equation 22)

The formation of urea under photocatalytic conditions may proceed via additional reaction pathways (Equations 23, 24, 25, and 26), depending on the nature of the precursors involved¹³⁶:

$${2\text{NO}}_3^{-}+\text{CO}+{16\mathrm{H}}^{+}+{14\mathrm{e}}^{-}\to {\text{NH}}_2\text{CO}{\text{NH}}_2+6{\mathrm{H}}_2\mathrm{O}$$ (Equation 23)

$${2\text{NO}}_3^{-}+\text{HCOOH}+{16\mathrm{H}}^{+}+{14\mathrm{e}}^{-}\to {\text{NH}}_2\text{CO}{\text{NH}}_2+7{\mathrm{H}}_2\mathrm{O}$$ (Equation 24)

$$2{\text{NH}}_2\text{OH}+{\text{CO}}_2+{4\mathrm{H}}^{+}+{4\mathrm{e}}^{-}\to {\text{NH}}_2\text{CO}{\text{NH}}_2+3{\mathrm{H}}_2\mathrm{O}$$ (Equation 25)

$$2\text{NO}+{\text{CO}}_2+{10\mathrm{H}}^{+}+{10\mathrm{e}}^{-}\to {\text{NH}}_2\text{CO}{\text{NH}}_2+3{\mathrm{H}}_2\mathrm{O}$$ (Equation 26)

All the above-mentioned reactions proceed through multi-electron steps where the required electrons can be supplied by photocatalysis activation and electron-rich sites within the semiconductor crystal structures. For instance, Mohamed et al. suggested a multi-step reaction mechanism (Equations 27, 28, 29, and 30) describing the reduction of NO₃⁻ ions on TiO₂ nanoparticles¹⁵⁰:

$${\text{NO}}_3^{-}+{2\mathrm{H}}^{+}+{2\mathrm{e}}^{-}\to {\text{NO}}_2^{-}+{\mathrm{H}}_2\mathrm{O}$$ (Equation 27)

$${\text{NO}}_2^{-}+{2\mathrm{H}}^{+}+{2\mathrm{e}}^{-}\to {\text{NO}}^{-}+{\mathrm{H}}_2\mathrm{O}$$ (Equation 28)

$${\text{NO}}^{-}+{3\mathrm{H}}^{+}+{2\mathrm{e}}^{-}\to {\text{NH}}_2\text{OH}$$ (Equation 29)

$${\text{NH}}_2\text{OH}+{3\mathrm{H}}^{+}+{2\mathrm{e}}^{-}\to {\mathrm{H}}_2\mathrm{O}+{\text{NH}}_4^{+}$$ (Equation 30)

According to Srinivas et al., the photogenerated NH₃ can react with CO₂ on the surface of the supported TiO₂ photocatalysts via another multi-electron mechanism to produce urea¹⁵¹:

$${\text{NH}}_4^{+}+{\text{CO}}_2+{\mathrm{e}}^{-}\to {\text{NH}}_2\text{COO}{\text{NH}}_4$$ (Equation 31)

$${\text{NH}}_2\text{COO}{\text{NH}}_4\to {\mathrm{H}}_2\mathrm{O}+{\text{NH}}_2\text{CO}{\text{NH}}_2$$ (Equation 32)

Based on the mechanisms presented in the preceding equations, heteroatom-metal active sites could play a distinctive role in directing the selective formation of urea on the surface of MOF-based photocatalysts, as reported by El-Shahat and Abdelhameed.¹³⁷ They provided a detailed description of the electron-hole migration pathways across the junctions of the Mo(MnO₄)₅@Ce-BTC photocatalyst, demonstrating its ability to generate a sufficient population of photogenerated electrons to drive the simultaneous reduction reactions of CO₂ and NH₃. The mechanism of urea generation, involving the utilization of photogenerated electrons and active sites of Mo, Mn, and Ce on the catalyst surface, is proposed as shown in Figure 7. Initially, CO₂ and NH₃ molecules are adsorbed onto the active sites of the catalyst, including Ce sites in the Ce-BTC framework and Mo–Mn oxide sites from the doped nanoparticles. Upon exposure to visible light, electrons are excited from the VB to the CB of two metal compounds, leaving behind holes in the VB. Photogenerated electrons then migrate from the CB of Ce-BTC to the CB of Mo–Mn, while the holes move from the VB of Ce-BTC to the VB of Mo–Mn. The heterojunction between Ce-BTC and Mo(MnO₄)₅ promotes efficient separation of photogenerated electrons and holes, reducing recombination. During the reaction, electrons take part in the coupling mechanism. It is suggested that the photocatalytic mechanism for urea generation involves the coupling of NH₃ with adsorbed CO₂ on the Mo(MnO₄)₅@Ce-BTC composite, forming the intermediate H₂NCOOH. This intermediate then reacts with NH₃ to produce ammonium carbamate (H₂NCO₂NH₄), which undergoes a one-step hydrogenation to form urea through a multi-step process, as outlined in the reaction pathways of Equations 31 and 32.

Figure 7.

Mechanistic illustration of urea formation on the surface of Mo(MnO₄)₅@Ce-BTC photocatalyst, considering the metal active sites’ tendency to better promote the multi-electron reaction pathway

Adapted with permission from ref.¹³⁷ Copyright 2024, Nature.

Liang et al. investigated urea synthesis using noble-metal-free NiCoP/ZnIn₂S_4−x photocatalysts under ambient conditions and proposed that the reduction of CO₂ to CO is the first step in the mechanism. Photo-excited ∗N=N∗ species reacted with CO to form ∗NCON∗, the primary intermediate in urea formation, which is then hydrogenated to produce urea.¹⁴⁴ Spin-polarized electrons generated by the bi-functional photocatalyst were demonstrated as the primary driving force for urea production in this system. In another study, Maimaiti et al. investigated the synergetic interaction between non-metal defects, specifically oxygen vacancies and metal (Ti) active sites on the surface of Ti³⁺-doped TiO₂ nanoparticles supported on the carbon nanotubes with iron cores (Ti³⁺-TiO₂/Fe-CNTs) as a photocatalyst.¹⁵² According to their report, the dual catalytic active sites, oxygen vacancies, and Ti³⁺ jointly facilitate the reduction of CO₂ and N₂ for urea synthesis through an optimal spatial arrangement. Ti³⁺ species specifically enhance the adsorption of N₂, which is subsequently activated by the photogenerated electrons to form intermediates such as –NH–NH–, NH₂–, and –N=N–. Simultaneously, CO₂ molecules are adsorbed at the oxygen vacancy sites located near Ti³⁺ centers, where they undergo a structural transformation from the linear to the geniculate configuration. Subsequently, the C–O bonds are broken, facilitating C–N coupling and ultimately leading to the formation of urea.

In a related study, Xia et al. investigated the photocatalytic activity of a series of Pd-decorated molecular sieve LTA-3A (referred to as Pd-LTA-3A photocatalysts) for solar-driven urea synthesis.¹⁵³ Acting as the main photothermal driving force of the urea formation reaction, Pd nanoparticles catalyzed the NH₃ dissociation on the catalyst surface. Meanwhile, the zeolite support played a complementary role by adsorbing the H₂O byproduct, thereby shifting the reaction equilibrium toward increased urea production. A mechanistic illustration of the reaction, supported by the diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and DFT calculations, revealed that Pd–NH₂ and Pd–H are the most likely nitrogen-containing intermediates formed immediately after the NH₃ dissociation at the surface-active sites. These intermediates can rapidly form carbamate intermediate species through nucleophilic addition to CO₂. The subsequent intermediates, NHCO and NHCONH_2, are generated via an oxidation step followed by a second nucleophilic addition reaction involving Pd–NH₂, with Pd–H contributing. These intermediates then react with Pd–H, ultimately leading to the formation of urea.

Irfan et al.¹⁴⁶ proposed a direct reduction pathway of N₂ and CO₂ leading to urea formation (Equation 33), based on the emergence of the prominent peaks attributing to the stretching vibrations of NH₂ (3623 cm⁻¹) and NH (3702 cm⁻¹) intermediates, along with the characteristic peak at 1440 cm⁻¹ corresponding to C–N bonding.

$${\mathrm{N}}_2+{\text{CO}}_2+{6\mathrm{H}}^{+}+{6\mathrm{e}}^{-}\to {\text{NH}}_2\text{CO}{\text{NH}}_2+{\mathrm{H}}_2\mathrm{O}$$ (Equation 33)

To further investigate the urea formation pathway on the SrTiO₃-FeS-CoWO₄ photocatalyst surface, a systematic DFT calculation was conducted. Based on the results, the following energetically favorable route was proposed for urea generation under the photocatalytic condition (Equations 34, 35, 36, 37, 38, and 39)¹⁴⁶:

$${\mathrm{N}}_2+{\text{CO}}_2\to {}^{\ast }{\mathrm{N}}_2+{}^{\ast }{\text{CO}}_2$$ (Equation 34)

$${}^{\ast }{\mathrm{N}}_2+{}^{\ast }{\text{CO}}_2\to {}^{\ast }{\mathrm{N}}_2+{}^{\ast }\text{COOH}$$ (Equation 35)

$${}^{\ast }{\mathrm{N}}_2+{}^{\ast }\text{COOH}\to {}^{\ast }{\mathrm{N}}_2+{}^{\ast }\text{CO}$$ (Equation 36)

$${}^{\ast }{\mathrm{N}}_2+{}^{\ast }\text{CO}\to {}^{\ast }\text{NCON}$$ (Equation 37)

$${}^{\ast }\text{NCON}\to {}^{\ast }\text{NCHONH}$$ (Equation 38)

$${}^{\ast }\text{NHCONH}\to {\text{NH}}_2\text{CO}{\text{NH}}_2$$ (Equation 39)

The C–N coupling was identified as the rate-determining step; however, due to the presence of dual metallic active sites that facilitate the adsorption and activation of N₂ and CO₂, the SrTiO₃-FeS-CoWO₄ photocatalyst demonstrated a significantly enhanced ability to lower the energy barriers associated with this crucial step, compared to the single and dual semiconducting components.

Based on the above statements, the development of environmentally friendly approaches, such as photocatalysis, for the clean and efficient production of high-value-added urea is gaining increasing attention among researchers. Due to rapid advances in emerging instrumental techniques for mechanistic elucidation, further advancements in the rational design and scaling-up of reactors and photoreaction systems are expected in the near future.

Challenges in photocatalytic urea synthesis from a catalyst perspective: Side reactions, selectivity, and yield

Urea synthesis requires simultaneous reduction of both carbon and nitrogen precursors. Therefore, photogenerated electrons are the vital driving force for the reduction steps in light-driven urea production systems.¹⁵⁴ However, positively charged holes with strong oxidizing capability threaten the efficiency and selectivity of the system by adversely interfering with the simultaneous reduction reactions of the carbonous and nitrogenous sources.¹⁵⁵ When NO₃⁻ is used as the nitrogen precursor in aqueous photocatalytic urea synthesis, side reactions significantly impact selectivity and overall yield. In photocatalytic NO₃⁻ reduction systems, NO₃⁻ rarely forms a single product; instead, it produces multiple competing products, including NO₂⁻, NH₄⁺, and N₂, with a selectivity strongly dependent on the catalyst type and reaction conditions. In the presence of H₂O, photogenerated holes can oxidize H₂O molecules and generate reactive ^·OH radicals, which could facilitate the cyclic reductant reformation of NO₃^-.¹⁵⁶ These side products form because photogenerated holes and ^·OH radicals not only consume charge carriers that would otherwise accelerate reductive C–N coupling but also re-oxidize reduced nitrogen intermediates back toward NO₃⁻ or other less reduced states, decreasing the effectiveness of NO₃⁻ utilization. As a result, urea selectivity in NO₃⁻-based systems is fundamentally limited by competing oxidative pathways that divert electrons and holes toward byproduct formation, highlighting the need for catalyst designs that suppress these side reactions while promoting selective C–N bond formation.¹⁵⁶ Conversely, ^·OH non-selectively oxidizes organic intermediates, essential for urea synthesis,¹³⁶ and can degrade the urea produced. Park et al. demonstrated that a large amount of photogenerated holes and highly reactive ^·OH radicals are produced from H₂O oxidation on the TiO₂ surface under UV irradiation, which could readily drive non-selective oxidation reactions.¹⁵⁷ Nonselective reactions become problematic for urea synthesis because the same holes and ^·OH radicals compete with the desired reductive C–N coupling, consuming charge carriers and oxidizing key intermediate, effectively lowering the net selectivity toward urea and diverting reactive electrons and holes into oxidative pathways. Together, these findings illustrate that photogenerated holes and ^·OH species act as major sinks for reactive electrons and holes, reducing selectivity and effective urea yield unless their formation and reactivity are carefully controlled through catalyst design or operating conditions. Experimental explorations of the photocatalytic NO₃⁻/CO₂ co-reduction and photoelectrochemical urea synthesis indicated that under ambient conditions, some undesired byproducts such as NO₂⁻, NH₄⁺, and even H₂O₂ could be formed alongside urea, reflecting the poor selectivity toward strategic goals. For example, photoelectrochemical reduction of NO₃⁻ with CO₂ over a Cu₂O photocathode produced urea at a rate of about 29.7 μmol g⁻¹ h⁻¹ with a relatively low Faradaic efficiency of ∼12.9%, indicating that nearly 90% of the electrons are consumed by side reactions rather than contributing to urea formation.¹⁵⁸ Another harmful consequence of ^·OH activity is the dimerization of N₂O to toxic N₂O₄, exposure to 100 ppm of which can cause pulmonary edema or even be fatal.^159,160 Under the applied irradiation conditions, particularly with UV light, direct photolysis of the reactants may occur as all possess unsaturated chemical bonds in their molecular structures. The work by Wang et al. shows that photocatalytic urea synthesis using CO₂, N₂, and H₂O faces the typical selectivity challenges inherent to aqueous photocatalytic C–N coupling.¹⁶¹ In the visible-light-driven photocatalytic system employing CdS@Bi₂WO₆ for the co-reduction of CO₂ and N₂ in H₂O, the reported reaction yield of urea is modest but notable, with the optimized 20 wt % CdS@Bi₂WO₆ catalyst achieving a production rate of approximately 10.0 μmol g⁻¹ h⁻¹ under visible illumination. The existence of competing side reactions, such as the hydrogen evolution reaction (HER), the partial reduction of CO₂ to carbonaceous intermediates such as CO or HCOO⁻, and the N₂ reduction to NH₃ or other nitrogenous byproducts, is common in systems of this type. These side pathways divert charge carriers away from the targeted C–N coupling and decrease the efficiency of urea formation, highlighting the ongoing need for catalyst designs that enhance charge separation, suppress non-selective reduction and oxidation reactions, and steer electrons specifically toward urea production. Other research has reported that NO₃⁻ photolysis under ultraviolet irradiation in the 270–330 nm wavelength range generates ^·OH radicals, consumes some reactants, and thereby shifts the equilibrium of photocatalytic reactions toward a lower photocatalytic yield.¹⁶² The findings from this research on NO₃⁻ photodissociation, particularly via spin-forbidden singlet–triplet transitions, could be applied to broader photochemical processes, such as urea synthesis, particularly in the design of productive systems for light-driven reactions and catalysis. Specifically, understanding how excited triplet states influence reactions could be important for optimizing light-driven urea-forming processes in artificial photosynthesis or solar-based chemical synthesis. Similar reactions could be expected for NO, NO₂⁻, and N₂ precursors. Recent theoretical studies have indicated that direct photolytic dissociation of CO₂ into CO + O through singlet and triplet pathways can occur in the range of 150–210 nm and provides molecular-level photodissociation dynamics and cross-sections.¹⁶³ This suggests that if such dissociation occurs during CO₂ reduction processes, it may interfere with the desired C-O coupling reactions, potentially leading to less efficient or undesirable product formation. Another challenge in the field is the generation of superoxide radical ions (^·O₂⁻) through the consumption of photogenerated electrons by molecular oxygen.¹⁶⁴ Although ^·O₂⁻ has sufficient potential to produce environmentally friendly O₂ and regenerate nitrogenous precursors such as NO₂⁻ by reacting with other photoexcited species in the reaction medium,¹⁶⁵ a significant concern is the strong affinity of urea to attach this species,¹⁶⁶ as demonstrated by both theoretical and experimental studies. As mentioned earlier, photooxidation and other undesired photoreduction pathways are unavoidable in photocatalytic systems. These side reactions can significantly influence reaction selectivity and pose challenges for scaling up systems for real-world applications. Like other photocatalysis-based technologies, there are considerable scalability challenges, among which the stability of photocatalysts and their anti-corrosion properties, mass transfer issues, and reactor design complexities need to be substantially addressed.^167,168 As a technical point, the adsorption capacity of reactants on the surface of photocatalysts, closely linked to surface texture engineering, is critical for reactant activation and overall efficiency and is considered a key stage in photocatalyst design and fabrication.¹⁶⁹ For instance, N₂, as a nitrogenous source of the urea photocatalytic reaction, is almost inert and should be sufficiently activated to participate in the C–O coupling photoreactions.¹⁷⁰ Single-atom engineering at the surface and interface of materials leverages the optoelectronic properties of unique heteroatoms to enhance the adsorption and activation of carbon and nitrogen precursors, providing a rational strategy to overcome these challenges.^171,172 Adsorption/activation challenges indeed affect the scalability of the other two catalytic urea synthesis approaches, including electrocatalysis¹⁷³ and photoelectrocatalysis,¹⁷⁴ as well as those not limited to photocatalysis. Optimizing the large-scale design of photoreactors and the physicochemical properties of catalysts poses significant economic challenges for the development of photocatalysis technologies.¹⁷⁵

Additionally, employing reliable and durable illumination sources for large-scale photocatalytic systems increases energy costs, making them less competitive with conventional urea synthesis methods.¹⁷⁶ Integrating photocatalytic and electrocatalytic approaches offers a promising solution to many of the technical challenges associated with each technique, aiming to develop practical constructions for the large-scale production of high-quality urea. This idea will be comprehensively discussed in detail in the following section.

Based on the statements discussed above, photocatalysis has attracted growing attention in recent years as a sustainable approach.²⁶ Although the large-scale implementation of this approach at the pilot plant level raises cost concerns, limiting its economic viability, photocatalysis offers distinct environmental advantages and benefits from significant advances in material engineering at the atomic and molecular levels. The coexistence of photo-oxidation pathways, such as H₂O oxidation to ^·OH radicals and reduction reactions (CO₂/N₂ activation), in photocatalytic urea synthesis systems poses a fundamental challenge, as both processes compete for photogenerated electrons and holes. This competition directly reduces urea production yield and selectivity by diverting charge carriers toward side reactions instead of C–N coupling.^146,177

Moreover, studies have reported on the direct photooxidation of urea through radical-mediated reactions under irradiation.^157,178 Accordingly, appropriate strategies should be implemented to minimize the undesirable impacts of photogenerated holes in these systems. The addition of external hole scavenging agents, often used to reduce hole concentration in photocatalysis, may not be an appropriate approach as these substances are not easily separable and can, hence, interfere with final product purity. Despite the limited number of studies on the photocatalytic synthesis of urea in recent years (Table 3), the existing drawbacks of photocatalytic synthesis systems should be prioritized rather than solely advancing the design of new photocatalytic materials.

Table 3.

A brief overview of the recent photocatalytic systems developed for urea production over the past few years

Photocatalyst	Reactants	Electrolyte composition	Irradiation source, light intensity	Reaction rate	Reference
NiCoP/ZnIn2S4−x	N2 + CO2	H2O	visible light (>420 nm), 100 mW/cm2	19.6 μmol g−1 h−1	Liang et al.144
Pd/LTA-3A	NH3 + CO2	gas phase	solar energy	87 μmol g−1 h−1	Xia et al.153
Ti3+-TiO2/Fe-CNTs	N2 + CO2	Na2SO4, 0.5 M	300 W high-pressure Hg lamp	177.5 μmol L−1 g−1 h−1	Maimaiti et al.152
Mo(MnO4)5@Ce-BTC	NH3 + CO2	gas phase	visible light, 300 mW/cm2	70 μmol L−1 h−1	El-Shahat and Abdelhameed137
2D-CdS@3D-BiOBr	NO3− + CO2	Na2SO4, 0.2 M	300 W xenon lamp,0.2 W cm−2	15 μmol L−1 g−1 h−1	Wang et al.145
Cu SA-TiO2	N2 + CO2	H2O	365 nm LED light, 433 mW cm−2	432.13 μg g−1	Li et al.139
Fe-TiO2/HZSM-5	NO3− + CO2	KNO3, 1.6 Mm	high-pressure Hg lamp (250 W)	6.25 μg L−1 g−1	Srinivas et al.151
CdS-Bi2WO6	N2 + CO2	H2O	visible light (>365 nm)	10 μmol g−1 h−1	Wang et al.161

Generally speaking, in C–N coupling electrocatalytic systems, competing reduction processes (e.g., CO₂ to CO, formate, or methane and N₂ to NH₃ or N₂H₄) hinder efficient C–N coupling and often lead to low urea selectivity. In such systems, chemical adsorption of the reactants is a key step to prompt the ultimate reaction, driven by effective interactions between the precursors and electrocatalytic active sites on the surface. Achieving the controlled, simultaneous reduction of the precursors within the same reaction environment is the key challenge of this approach. To overcome these obstacles and reach optimal urea production efficiency, an effective electrocatalyst should meet several significant criteria, including facilitating the strong adsorption and efficient activation of the reactants on its surface, enhancing the C–N coupling reaction between the desired intermediates and active sites, and lowering the energy barrier of the favorable reaction steps while suppressing competing side reactions. Furthermore, technical parameters, such as overpotential, Faradaic efficiency, current density, electrode stability, and C/N selectivity, should be precisely monitored and optimized to maximize the performance of urea electrosynthesis. The capability of directly controlling the reaction kinetics and selectivity in electrochemical systems is often difficult to achieve through traditional thermal or homogeneous catalytic methods. This enables the fine-tuning of the reaction pathways under moderate conditions, activating inert molecules and offering a better specificity in product generation, especially in complex multi-electron processes such as urea electrosynthesis. Also, these systems mostly exhibit reaction rates that favor the formation of higher-valued product distribution, which is a significant priority for scale-up and practical applications. This strategy eliminates the need for high-pressure and temperature circumstances to drive the reactions effectively while suppressing the emission of hazardous gases such as CO to the atmosphere. Hence, large-scale urea electrosynthesis could significantly contribute to reducing the effects of global warming by utilizing atmospheric CO₂, thereby helping to mitigate climate change. It can also contribute to air purification in urban areas by converting toxic gaseous pollutants, such as CO and NO, frequently emitted by industrial activities and vehicles, into valuable chemical products. However, a significant challenge in electrocatalysis lies in designing and constructing highly stable electroactive surfaces that can act effectively as catalysts to assure long-term stability during the process. Another notable challenge in industrial electrocatalysis is the need for high-concentration electrolytes and high temperatures to ensure maximum yield, while the elevated temperatures compromise the stability of electroactive materials, specifically at the laboratory scale. This issue is further worsened by using high-concentration electrolytes, which provide a highly corrosive medium for conducting.

Regarding the photocatalytic approach, its eco-friendly nature, remarkable safety, and simplicity of operation make this strategy a more sustainable and adaptable alternative for urea production compared to the Bosch–Meiser process. In this context, the successful and efficient electron transfer to facilitate the multi-electron reduction of nitrogen and carbon source reactants toward C–N coupling occurrence is still crucial, but challenging. Substantially, the photogenerated electrons are the vital driving force to prompt the reduction steps in light-driven urea production systems. In a contrary manner, positively charged holes with strong oxidizing characters restrict the efficiency and selectivity of systems by adversely interfering with the simultaneous reduction processes. The catalyst construction plays a pivotal role here. For instance, with NO₃⁻ precursor-contained systems, NO₃⁻ rarely forms a single product; it produces a range of products, including NO₂⁻, NH₄⁺, and N₂, with a selectivity strongly dependent on the catalyst type and reaction conditions. In the presence of H₂O, photogenerated holes can oxidize H₂O molecules and generate reactive ^·OH radicals, which could promote some unfavorable side reactions such as cyclic reductant reformation of NO₃⁻, which not only consume charge carriers that would otherwise accelerate reductive C–N coupling, but also re-oxidize reduced nitrogen intermediates back toward NO₃⁻ or other less reduced states, decreasing the effectiveness of NO₃⁻ utilization. As a result, urea selectivity in NO₃⁻-based systems is fundamentally limited by competing oxidative pathways that divert electrons and holes toward useless byproducts, highlighting the need for rational catalyst design that suppresses these side occurrences while steering the selective C–N bond formation. Also, ^·OH radicals could non-selectively oxidize some key organic intermediates, essential for urea synthesis. Another harmful consequence of ^·OH activity is the dimerization of N₂O to toxic N₂O₄, 100 ppm of which can cause pulmonary edema or even be fatal to humans. Therefore, photogenerated holes and ^·OH species act as major sinks for reactive electrons, reducing selectivity and effective urea yield unless their formation and reactivity are carefully controlled through catalyst design or operating conditions. As another reactive species from photocatalysis, ^·O₂⁻ indicates a strong affinity for urea, as repeatedly supported by theoretical and experimental studies. These are among the undesired photooxidation and photoreduction pathways that are unavoidable in photocatalytic systems. These concerns can significantly influence scaling up photocatalytic urea synthesis systems for real-world applications. Additionally, employing robust and lasting illumination sources for large-scale utilization increases energy costs, making such systems less competitive with urea electrosynthesis. However, integrating both approaches offers a promising solution to overcome many of the technical challenges associated with each one toward developing practical systems for the large-scale production of high-quality urea.

Photoelectrocatalytic synthesis of urea

The integration of photocatalytic and electrochemical strategies in PEC synthesis creates a powerful hybrid approach that utilizes the advantages of both methods. In contrast to photocatalysis, PEC systems rely on either an applied voltage or built-in electric fields to drive the separation of charge carriers.^29,179 This reduces recombination-related energy losses and improves overall reaction efficiency. Compared to purely electrocatalytic systems, PEC configurations can directly utilize solar energy to lower the external electrical input required to drive thermodynamically demanding reactions such as C–N coupling. This dual capability creates a unique opportunity to modulate reaction pathways more precisely by adjusting both photonic and electrochemical parameters, including light intensity, applied voltage, and interfacial energetics.⁸ Although PEC technology is still in its early stages, this integrated approach offers clear advantages over using either method alone and shows strong potential as a more energy-efficient and flexible pathway for sustainable urea production. By adjusting parameters such as light intensity, applied voltage, and surface energetics, researchers can more precisely investigate reaction pathways. This level of control is particularly important for complex, multi-electron processes like producing urea from CO₂ and nitrogen sources, where managing intermediates and selectivity is essential.

Photoelectrocatalysis: from fundamental principles to urea synthesis applications

Photoelectrocatalysts combine the synergistic effects of photocatalysis and electrocatalysis by integrating the charge-transfer control of electrochemistry with the photoexcitation capabilities of photocatalysis.¹⁸⁰ These catalytic systems, which utilize light and an applied electrical bias, can be constructed by immobilizing photoactive particles onto suitable conductive substrates that function as working electrodes. In addition to a working electrode, a reference electrode, and an auxiliary electrode are also essential components, enabling precise potential control and accurate monitoring of the electron transfer occurrences. Under the irradiation source, electron-hole charge carriers are generated on the surface of semiconductor-based electrodes. An applied electrical bias prevents the recombination of charge carriers; therefore, the separated charges can drive redox processes such as CO₂ and NO₃⁻ reduction (Figure 8A).¹⁸² The urea photoelectrosynthesis process (Figure 8B) focuses explicitly on accumulating photogenerated electrons and facilitating their kinetically and thermodynamically favorable transfer, aiming to reduce nitrogen and carbon sources simultaneously. Notably, a key advantage of the photoelectrocatalysis system is that the catalytic photoreduction and photooxidation half-reactions occur separately at the auxiliary and working electrodes. This spatial separation provides a versatile quantitative investigation of the desired reactions within an isolated medium.^183,184 Photoelectrocatalysis, which combines the significant benefits of photocatalysis with the speed, simplicity, accuracy, and sensitivity inherent in electrocatalysis, is now a leading green technology for addressing environmental, energy, and chemical synthesis concerns.¹⁸⁵

Figure 8.

Urea synthesis routes

(A) a simple photoelectrocatalytic system.¹⁸¹ Copyright 2024, espublisher.

(B) Photoelectrosynthesis pathway of urea.

Adapted with permission from ref.²⁹ Copyright 2024, Wiley Online Library.

The efficiency of PEC systems depends on how effectively light is absorbed and how efficiently charge carriers (electrons and holes) are generated. The interaction of light with materials directly influences the generation of these carriers; therefore, photon management is crucial for overcoming thermodynamic limitations and improving the efficiency of activation reactions. Plasmonic materials, especially those with surface plasmon resonance (SPR), can generate energetic electrons and holes, so-called hot carriers. These hot carriers can then drive catalytic reactions, enhancing light-matter interactions and lowering activation barriers, thereby improving reaction rates and product selectivity. However, the efficient capture of these hot carriers, particularly holes, which have shorter mean-free paths than electrons, is the main challenge. Moreover, the effective separation and utilization of both hot electrons and holes is a key point to improving the performance of PEC systems. The catalyst design and its morphology also play a vital role in enhancing the PEC productivity. Materials such as Cu, Bi, and noble metals could be particularly prominent for activating CO₂ and nitrogen precursors due to their unique optical properties and catalytic behavior at the nanoscale level. A precise optical and structural design is needed to maximize light absorption while minimizing shadow loss. Moreover, achieving strong light-matter coupling between plasmonic modes and dielectric resonances further enhances carrier generation and reaction dynamics, thereby improving selectivity and energy demand for processes such as urea synthesis.¹⁸⁶

Recent advancements in photoelectrocatalytic urea synthesis

Although urea conversion via photoelectrocatalysis has been frequently reported,^{187,188,189,190,191,192} the literature specifically addressing PEC urea synthesis remains limited. Zheng et al.¹⁹³ developed a series of bi-metallic Cu-Ti dual active sites on TiO₂ to enhance the photoelectrochemical C–N coupling reaction between CO₂ and NO₃⁻, aiming at urea synthesis. Compared to pure TiO₂, the Cu-Ti/TiO₂ catalyst showed a 2.66-fold enhancement in urea production yield (52.45 μmol L⁻¹ h⁻¹) with a Faradic efficiency of 68% under ambient conditions and an applied voltage of −0.5 V. The free energy formation of various intermediates on the materials was theoretically calculated using DFT, with experimental insights obtained from in situ DRIFTS measurements. The high performance arises from proximity-enabled C–N coupling, in which Cu sites strongly adsorb and reduce CO₂ to ∗CO while adjacent Ti sites selectively generate ∗NH₂ from NO₃⁻, allowing direct ∗CO–∗NH₂ coupling with a lower energy barrier than competing reactions. The PEC system provides a unique advantage by using photogenerated holes to dynamically suppress side reactions such as CO desorption and NH₃ formation, thereby steering selectivity toward urea. This preferential binding suppressed competing reactions while enabling selective C–N coupling for urea formation. Although this work presents an insightful and convincing strategy for PEC urea synthesis, both reproducibility and scalability remain significant challenges. Reproducing the results requires precise control over single-atom Cu placement and Cu–Ti spacing, which are highly sensitive to synthesis conditions and difficult to achieve without advanced characterization tools. Scaling up further demands the fabrication of large, stable photoelectrodes that preserve single-atom integrity, provide uniform light exposure, and operate reliably over long periods. Additionally, the complexity and cost of PEC reactors, uncertainties around long-term durability, reliance on NO₃⁻ feedstocks, and the difficulty of separating urea at low concentrations represent significant barriers to practical implementation.

In another work, Li et al.²⁹ developed an efficient Cu₂O-based photocathode with cubic morphology for the photoelectrochemical urea synthesis from NO₃⁻ and CO₂. The urea production yield on this photoelectrode reached 29.71 μmol g⁻¹ h⁻¹ with a Faradaic efficiency of 12.9% at a minimal external bias of −0.017 V vs. RHE. A comparative investigation of the catalytic activity of pure semiconductors highlighted the strong potential of Cu₂O for urea formation under photoelectrosynthesis conditions (Figure 9A). The phase purity and well-defined cubic morphology of the Cu₂O photocatalyst were also confirmed by XRD analysis (Figure 9B) and microscopy images (Figures 9C and 9D), demonstrating the successful synthesis of pure phase without CuO impurities and precise morphological control of Cu₂O. Figure 9E reveals a strong potential-dependent behavior, where applied bias modulated urea production rates across an order of magnitude (5–29 μmol g⁻¹ h⁻¹) while simultaneously affecting Faradaic efficiency. The observed voltage sensitivity indicates that competing reduction pathways become dominant at higher overpotentials, suggesting an optimal potential range for selective urea formation. Therefore, precise control of the applied potential is critical to enhance both urea yield and electron utilization. This fact shows the importance of optimizing the rational correlation between the illumination source intensity, external potential, and resulting urea formation rate. The observed decline in catalytic activity during the initial recycling runs (Figure 9F) was primarily caused by the photochemical reduction of Cu₂O to metallic copper (Cu⁰) under operational conditions, emphasizing the need for further improved strategies to stabilize the photoelectrode surface composites. Figure 9G presents the incident photon-to-current efficiency (IPCE) results, which illustrate how effectively photon energy contributes to electric current generation. As shown in the plots, the higher IPCE values correspond to improved PEC performance. This effect was particularly pronounced in the 350–500 nm wavelength range, highlighting the excellent light-harvesting capability of Cu₂O and its efficient photogenerated charge migration under illumination. The key innovation of this research is the use of solar energy to generate an internal photovoltage, thereby significantly reducing electrical energy input. In fact, exposure to light boosts urea production by nearly 9 times compared to conventional dark electrocatalysis. In addition, the experimental methodology in this study is reproducible primarily because it employs commercially available Cu₂O and rigorously validated analytical techniques. However, achieving consistent results faces challenges, including photocorrosion, partial reduction of Cu₂O to metallic Cu during operation, and strong sensitivity to light-source characteristics and PEC cell configuration. Scaling up the process presents even greater difficulties, including inherently low production rates that would require very large photoreactor areas, the need to develop stable and conductive protective coatings to prevent the degradation of the Cu₂O photocathode, complex reactor designs to ensure uniform illumination and efficient mass transport, and the necessity of balancing overall energy efficiency with capital and operational costs.

Figure 9.

Photoelectrocatalytic activity and physicochemical properties of cubic Cu₂O nanoparticles for urea synthesis

(A) Urea production yield with various semiconducting materials as the photoelectrode (band positions are pointed out in the inset). Reaction medium: KNO₃ solution (0.1 M) with a constant CO₂ flow rate of 20 mL/min and AM 1.5 G illumination source.

(B) XRD results.

(C) SEM images with scale bars of 5 μm.

(D) TEM images with scale bars of 2 nm.

(E) Urea production yield besides the FE values with changing the external potentials on Cu₂O.

(F) Recyclability experiments, and (G) IPCE curves for Cu₂O in the presence and absence of carbon dioxide.

Adapted with permission from ref.²⁹ Copyright 2024, Wiley Online Library.

TiO₂-based PEC materials incorporating Ru, Cu, and Zn have been recently synthesized by a combined hydrothermal-photodeposition approach and successfully employed for the synthesis of urea from CO₂ and NO₂, under illumination by a 300 W xenon lamp.¹⁹⁴ The results indicated that the optimal 5% of Ru in Ru-TiO₂ photocatalyst led to 50.58 μmol g⁻¹ h⁻¹ with the Faradaic efficiency of 16.66% (1.65 times higher than the Faradaic efficiency over pure TiO₂) at an external bias of −0.1 V vs. RHE. In addition to improving the light-harvesting capabilities of pure TiO₂, Ru atoms could also facilitate the efficient transfer of photogenerated electrons, thereby promoting C–N coupling and urea formation.

Despite the limited number of publications currently available on the topic, the simultaneous utilization of the major global warming contributors like CO₂ and nitrogen-containing pollutants such as NO₃⁻ and NO₂⁻ remains a substantial contaminant in the drinking H₂O resources worldwide, alongside the unique versatility aspects of the photoelectrochemical systems, which increase the potential for increased research activity in this field in the near future.

Analytical strategies for urea yield assessment in photoelectrocatalytic systems

Li et al. were the first to propose a systematic protocol for quantifying urea and other products generated from the PEC synthesis under ambient conditions.¹⁹⁵ Their work systematically evaluated urea detection approaches and ultimately developed an accurate analytical method specifically tailored for PEC systems (Figure 10). A key requirement highlighted in this protocol is the strict elimination of background contamination from carbon- and nitrogen-containing species, which is essential to avoid false-positive urea signals. Beyond contamination control, the authors emphasized the necessity of carefully designed control experiments, including catalyst-free, reactant-free, dark, light-only, and bias-only conditions, to unequivocally confirm the true origin of the detected urea. If the target product (urea) and/or a similarly effective alternative cannot be produced, the PEC system loses its practical value for urea synthesis. Furthermore, isotopic labeling experiments with ¹³CO₂ and/or ¹⁵NO₃⁻ were recognized as essential techniques for confirming C–N coupling pathways and excluding contributions from unintended sources or solution-phase reactions. Ultimately, the use of standardized performance benchmarks, normalized to catalyst mass or area, reaction time, and photon input, is essential for enabling meaningful comparisons among different PEC systems and for accurately evaluating their practical viability. It should be noted that discrepancies among urea quantification methods can substantially affect the comparability of reported yields across different studies. In particular, colorimetric UV–Vis-based assays, although simple and widely adopted, are susceptible to chemical interferences from nitrogen-containing intermediates, electrolyte components, and side products, which may lead to systematic overestimation of urea yields. As a result, urea production values derived solely from colorimetric methods are not always directly comparable to those obtained using more selective analytical techniques. In contrast, LC-MS provides higher analytical specificity by separating urea from coexisting species before detection, thereby minimizing false-positive signals and improving yield reliability, albeit at the expense of higher cost and experimental complexity. NMR spectroscopy, especially when combined with isotopic labeling (¹³CO₂ and/or ¹⁵NO₃⁻), serves as a powerful confirmatory tool that enables unambiguous identification of urea and validation of C–N coupling pathways, though its quantitative sensitivity is generally lower than that of LC-MS. Therefore, meaningful benchmarking of PEC urea synthesis requires transparent reporting of analytical methodologies, calibration procedures, and detection limits, as well as complementary validation using at least two independent techniques whenever possible.

Figure 10.

The suggested protocol for urea determination from photoelectrocatalytic synthesis routes

Adapted with permission from ref.¹⁹⁵ Copyright 2025, Wiley Online Library.

The PEC performance of a system is commonly evaluated using Faradaic efficiency and urea production yield. However, these metrics alone are insufficient without accurate and reproducible analytical quantification. Therefore, precise, highly analytical techniques with minimal bias and maximum reproducibility are required. In PEC systems, urea yield alone is insufficient to evaluate performance and must be assessed by metrics such as product selectivity, carbon and nitrogen mass balance, catalyst stability, and time-dependent yield behavior. Additionally, photon-related parameters, including light intensity, wavelength dependence, and photon-to-urea efficiency, are necessary to confirm actual photo-driven activity.

Liquid chromatography-mass spectroscopy (LC-MS) is a reliable, though costly, technique for urea detection in photoelectrosynthesis methods. Due to the distinct mass differences between urea and byproducts generated from photoelectrochemical or electrochemical side reactions, the LC-MS technique effectively isolates and quantifies urea while minimizing interferences from the byproducts based on this criterion.¹⁹⁶ As with other chromatography techniques, selecting suitable mobile phases, flow rates, and calibration standards is critical for achieving accurate analysis. In LC-MS, the individual components are first separated and then ionized for mass spectrometric detection. The separated ions are subsequently sorted by their mass-to-charge (m/z) ratio. The signal intensity at each m/z value corresponds to the relative abundance of specific species in the product mixture. Several researchers have developed reliable analytical methodologies utilizing this robust technique for the simultaneous determination of urea and other C–N-containing compounds in the complex reaction mixture.^197,198

Urea can be analyzed simply by UV-Vis spectroscopy after pretreatment with either of two common chromogenic reagents, urease-indophenol blue or diacetyl-monoxime (DAMO).^199,200 Urea (in the earlier method) and its hydrolysis product, NH₃ (in the subsequent approach), are each used directly to form colored complex solutions, which are then analyzed by UV-Vis spectrophotometry to determine their concentrations. The resulting data can be easily correlated with the urea yield in the reaction mixture. Both protocols offer a straightforward, user-friendly approach for the rapid detection of urea in photoelectrochemical reaction media; however, chemical interferences during the complexation steps remain unavoidable.¹⁹⁵

To ensure the accuracy of urea yield data obtained by UV-Vis spectroscopy, NMR spectroscopy can be used as a robust analytical technique. In this technique, DMSO-d₆ and D₂O are the most commonly used reagents, enabling rapid urea detection by ¹H-NMR, which benefits from hydrogen-deuterium exchange.²⁰¹ Since photoelectrosynthesis systems contain carbon-based compounds, ¹³C NMR is an effective characterization tool, offering strong capabilities for analyzing reaction mixtures. Herein, researchers employ ¹³CO₂ as the primary carbon source to produce ¹³C-urea through PEC synthesis.¹⁹⁵ Table 4 compares urea quantification methods, outlining their key advantages and disadvantages.

Table 4.

Advantages and disadvantages of urea quantification methods⁷⁹

Analytical method	Sensitivity and selectivity	Key advantages	Main limitations	Recommended role
HPLC/HPLC-MS	very high	excellent separation of urea from byproducts; suitable for complex media; enables multi-product analysis	high cost; limited accessibility; requires careful calibration	primary quantitative method; selectivity and mechanistic studies
UV–Vis (Indophenol/DAMO)	moderate	simple, rapid, low cost; suitable for routine screening, few influencing by-products	susceptible to chemical interferences and false positives; possible overestimation	preliminary quantification; must be validated by other techniques.
1H NMR	high	direct urea identification; minimal interference; good reproducibility, isotope labeling	lower sensitivity than LC-MS; requires deuterated solvents, and has quantification difficulties	quantitative validation and cross-checking
13C NMR (with13CO2)	high (mechanistic)	direct evidence of carbon incorporation confirms proper C–N coupling	expensive isotopes; longer acquisition time	mechanistic confirmation and isotope tracing

Mechanistic insights into photoelectrocatalytic urea synthesis via in situ FTIR analysis

The three main sequential stages of a PEC reaction include photon absorption by the photoactive component, leading to electron-hole generation within and/or on the surface, charge carriers migration through the photo/electro-active regions, and adsorption and reaction of reactants and byproducts on the photoelectrode surface to form the selective compounds on the surface of the photoelectrode, which are then desorbed into the medium for instrumental analysis.²⁰² Research on photocatalysis has focused extensively on understanding charge-transport mechanisms and on enhancing photocatalytic efficiency through morphological and structural modifications.²⁰³ To effectively elucidate product selectivity and reaction pathways, in situ analytical techniques, particularly FTIR spectroscopy, are crucial. This technique provides molecular-level insights into surface reactions, intermediate formation, and degradation pathways during photocatalytic processes.²⁰⁴ Zheng et al. used in situ FTIR to reveal that the NO₃⁻ species (characterized by bands at 1055,1240, and 1620 cm⁻¹) initially adsorb on TiO₂ active sites while CO₂-related species (1640 cm⁻¹) are anchored to Cu single-atom sites in a TiO₂/Cu-SAC system during NO reduction (Figures 11A and 11B).¹⁹³ The FTIR peaks at 1275, 1419, and 1738 cm⁻¹ are assigned to the ∗NH₂ intermediate, C–N bonding formation, and carbonamide intermediate (∗H₂NCOOH), respectively, implying coupling between ∗NH₂ and CO₂ forms ∗H₂NCOOH intermediate, which subsequently undergoes multi-step conversion to urea. Despite spectroscopic evidence suggesting a ∗NH₂/CO₂ coupling pathway indicated by peaks at 1275 cm⁻¹ [∗NH₂], 1419 cm⁻¹ [C–N], and 1738 cm⁻¹ [∗H₂NCOOH]), the authors reported the detection of NH₃ and adsorbed carbon monoxide (∗CO) peaks in their results (Figures 11C–11E), which highlights unresolved selectivity challenges in photocatalytic urea synthesis. Therefore, even though Cu/Ti sites significantly lower the barrier for urea formation, these side reactions still consume a portion of the critical ∗CO and ∗NH₂ intermediates. Suppressing these pathways is key to further increasing urea Faradaic Efficiency (FE). A targeted defect-engineering strategy can be employed to suppress CO and NH₃ side reactions and promote urea formation by simultaneously tuning the adsorption and reactivity of key intermediates on Cu–TiO₂ catalysts. By introducing oxygen vacancies into the TiO₂ support near isolated Cu atoms, the electronic environment of the Cu site can be adjusted; therefore, the key ∗CO intermediate binds more strongly and is less likely to desorb as CO gas. These vacancies donate electron density to the Cu, improving back-donation into the antibonding orbitals of CO and keeping ∗CO on the surface long enough to participate in C–N coupling, without site poisoning, thereby retaining ∗CO for C–N coupling. At the same time, oxygen-vacancy-related Ti³⁺ sites on TiO₂ can be used to inhibit hydrogenation of ∗NH₂ on Ti sites to NH₃, thereby stabilizing ∗NH₂ in a configuration less favorable for further hydrogenation by increasing the energy barrier to proton attack and extending its surface lifetime. A more integrated strategy is to create a defect-rich nano-environment in which oxygen vacancies are arranged to bring ∗CO on Cu and ∗NH₂ on neighboring Ti sites into closer electronic and geometric contact, facilitating electron transfer, improving orbital overlap, and stabilizing the transition state for C–N coupling. Collectively, these defect-induced effects are expected to lower the activation barrier of the rate-determining C–N coupling step (CONH₂ + NH₂ → urea), kinetically favoring urea formation over desorption or hydrogenation pathways.

Figure 11.

FT-IR detection of adsorbed CO₂ and nitrate on different structures

FTIR analysis of the adsorption of nitrate and CO₂ on TiO₂ (A) and single-atom doped SAC Cu-TiO₂ (B).

The results of 3D in situ FTIR for urea photoelectrosynthesis on the doped photocatalyst (C).

In situ FTIR investigation of the reaction intermediates through urea electrosynthesis (D) and photoelectrosynthesis (E).

Adapted with permission from ref.¹⁹³ Copyright 2024, Elsevier.

Hence, the PEC pathway for urea generation in this system was proposed as follows, which was in good agreement with DFT calculations.

$${}^{\ast }{\text{CO}}_2+{}^{\ast }{\text{NH}}_2\to {}^{\ast }{\text{NH}}_2+{}^{\ast }\text{COOH}\to {}^{\ast }{\text{NH}}_2+{}^{\ast }\text{CO}\to {}^{\ast }\text{CO}{\text{NH}}_2$$ (Equation 40)

In another study, in situ EPR was employed to identify the reaction mechanism of urea photoelectrosynthesis over a Cu₂O photocathode, considering the potential involvement of radicals.²⁹ No radicals were detected during the reaction, suggesting that the process may not have obeyed a radical-mediated pathway. However, when persulfate was introduced as an electron scavenger, the urea formation rate was significantly suppressed, indicating that photogenerated electrons play a crucial role in driving the reaction. Moreover, photo-assisted online DEMS results revealed that NO₂ serves as the key intermediate derived from the nitrogenous source. In addition, in situ FTIR analysis detected the characteristic peaks corresponding to CO (2073 cm⁻¹), C–N bonding (1415 and 1452 cm⁻¹), and –NH₂ (3440 cm⁻¹), providing strong evidence for the proposed urea formation mechanism outlined in the following equations:

$${\text{NO}}_3^{-}+{2\mathrm{H}}^{+}+{\mathrm{e}}^{-}\to {}^{\ast }{\text{NO}}_2+{\mathrm{H}}_2\mathrm{O}$$ (Equation 41)

$${\text{NO}}_3^{-}+hv\to {}^{\ast }{\text{NO}}_2+{\mathrm{O}}^{·-}$$ (Equation 42)

$${\text{CO}}_2+{2\mathrm{H}}^{+}+{2\mathrm{e}}^{-}\to {}^{\ast }\text{CO}+{\mathrm{H}}_2\mathrm{O}$$ (Equation 43)

$${}^{\ast }\text{CO}+{}^{\ast }{\text{NO}}_2\to {}^{\ast }\text{CO}{\text{NO}}_2$$ (Equation 44)

The final intermediate, ∗CONO₂, undergoes sequential reduction (hydrogenation) to form urea, as supported by the DFT calculations and Gibbs free energy diagrams (Figure 12). Their findings revealed that the first coupling between ∗CO and ∗NO_2, followed by the second C–N coupling between the ∗CO–NH and ∗NO_2, were recognized as the rate-determining steps in the reaction mechanism.

Figure 12.

Theoretical evidence on the mechanism of urea formation through simultaneous reduction of nitrate and carbon dioxide over a Cu₂O photoelectrode

The external cycle illustrates the suggested pathway. The elementary steps of the process are marked with blue values. The optimized geometries are exhibited with the color code indicated below, and the inset is the analogous Gibbs-free energy diagrams alongside the activation energy barriers.

Adapted with permission from ref.²⁹ Copyright 2025, Elsevier.

Economic prospects and industrial challenges in urea photoelectrosynthesis

PEC urea synthesis integrates the photocatalytic and electrochemical processes to convert carbon and nitrogen sources, thereby improving charge separation and reaction efficiency under an applied potential. PEC systems offer environmental benefits such as preventing secondary pollution, lowering operational costs, and reducing treatment duration, which are significant advantages in various photoelectrocatalysis applications.²⁰⁵ Although PEC provides a promising route for sustainable urea production, its industrial feasibility depends on overcoming significant economic and engineering challenges.²⁰⁶ Unlike the Haber–Bosch process, which benefits from over a century of large-scale deployment, PEC systems are currently limited by technical and operational obstacles. To assess the economic feasibility of PEC urea synthesis, a Techno-Economic Analysis (TEA) is necessary, focusing on key factors, including capital and operational cost, efficiency, energy consumption, and scalability.²⁰⁷ Considering capital costs, PEC systems face substantial expenses due to the use of expensive noble metal electrode materials and complex reactor designs, while the Haber-Bosch process benefits from economies of scale and optimized infrastructure, which lower its capital costs. In terms of operational costs, PEC systems face varying costs depending on renewable energy availability, long-term stability and replacement of the catalyst, and system maintenance, while the Haber-Bosch process is energy-intensive and relies heavily on natural gas but benefits from stable and highly optimized infrastructure.²⁰⁸

Regarding efficiency and energy consumption, PEC systems utilize solar energy to drive reactions at ambient temperatures and pressures, reducing external energy needs; however, current PEC systems are less efficient than the Haber-Bosch process. Although PEC consumes less energy, inefficiencies in catalyst design and light absorption limit its performance.²⁰⁹

Considering scalability, PEC systems suffer from significant scalability challenges, such as mass-transfer limitations, uniform light distribution, and maintaining homogeneous potential fields across large electrode areas. In addition, designing a mechanically robust, weather-resistant reactor, challenges of scaling up photoelectrode production, and ensuring system durability are also costly. In contrast, the Haber-Bosch process is highly optimized for large-scale NH₃ production, benefiting from industrial-scale infrastructure.

To further support the comparison, data from research and pilot-scale projects on H₂ and NH₃ production were examined alongside those from the Haber-Bosch process. A relevant case study is the European Commission-funded project on solar-driven NH₃ production, led by companies Repsol and Enagás.²¹⁰ The project aims to produce 100 kg of clean hydrogen per day using solar energy, with plans to scale up to 200 tonnes per year by connecting to the electricity grid. As another case study, Tsang’s research team developed an innovative renewable power-to- NH₃ pilot project in collaboration with Siemens, designed to produce 11 tonnes of green NH₃ annually using a 13 kW wind-powered proton-exchange membrane (PEM) H₂O electrolyzer.²¹¹ This project highlights the feasibility of integrating renewable energy sources with electrochemical processes for sustainable chemical production. It demonstrates the challenges of scaling such systems, particularly in terms of efficiency and cost, while emphasizing the importance of designing systems that are robust, scalable, and capable of continuous operation.

These case studies reveal both the potential and challenges of PEC systems, particularly in terms of cost and efficiency. Although PEC systems offer environmental benefits, including the potential for carbon-neutral urea production when powered by renewable energy, its high initial costs and limited scalability prevent them from competing with the Haber-Bosch process at present. However, with advancements in catalyst development and reactor design, PEC could become a competitive method in the future, especially if material costs are reduced and efficiency is improved.²¹²

To compare PEC and Haber-Bosch systems, specific benchmarks for efficiency, cost, and scalability are necessary. PEC systems must achieve 10–20% solar-to-urea efficiency, compete with Haber-Bosch, which operates at 60–70% energy efficiency on a lower heating value basis, reaching up to ∼80% under optimized conditions,²¹³ though it relies on natural gas and has significant environmental drawbacks.²¹⁴

The current PEC systems typically exhibit very low efficiencies (≪1%),^215,216 and improvements are expected with advancements in catalysts and reactor designs. A key challenge of PEC systems is electrode lifetime; stability targets reported for solar fuel and electrolysis systems span from >10³ h in laboratory demonstrations to >10⁴–10⁵ h for practical devices, corresponding to approximately year-scale operation (∼1–2 years) for early commercial viability,^217,218 whereas the Haber-Bosch process has highly stable reactors designed for long-term continuous operation. Reducing material costs by a factor of 5–10 through eliminating noble metals and utilizing more earth-abundant materials, such as Cu, Fe, or Ni catalysts, would be a critical goal. In comparison, Haber-Bosch benefits from lower material costs due to its reliance on natural gas and inexpensive iron-based catalysts. PEC urea production must reduce costs to below $500 per ton and approach current market prices for industrial urea (∼$300–500 t⁻¹) to remain competitive with the Haber–Bosch process, whose costs range from $200 to $500 per ton depending on natural gas prices.^214,219,220

However, PEC systems offer significant environmental benefits, such as the potential for carbon-neutral urea production when powered by renewable energy like solar, making it a cleaner alternative to fossil fuel-based methods. In contrast, the Haber-Bosch process has a high environmental impact due to its reliance on fossil fuels, leading to substantial CO₂ emissions and energy consumption. Despite its environmental impact, the Haber-Bosch process remains dominant due to its cost, efficiency, and established infrastructure. However, advancements in PEC catalysts and reactor design could make it a competitive method in the future, especially if it can reduce material costs and improve efficiency.²²¹

For the PEC urea production to become economically feasible, research must go beyond just improving catalytic efficiency and tackle the practical challenges associated with scaling the technology. This requires a focused effort on developing durable, earth-abundant catalysts, creating innovative reactor designs to enhance mass transport and light utilization, and conducting Techno-Economic Analysis (TEA) using data from continuous, long-term prototype operations instead of short laboratory tests.²²²

Integrating PEC technologies with circular economy principles can significantly enhance sustainability by utilizing wastewater containing NO₃⁻ as a nitrogen source for urea synthesis. This approach not only helps address NO₃⁻ pollution but also supports agricultural needs while minimizing environmental impact. When powered by renewable energy, such as sunlight, the process becomes even more sustainable, reducing carbon emissions.²²³ This approach enhances the overall feasibility and impact of PEC technologies in real-world applications while contributing to the global shift toward more sustainable, circular industrial practices. Efficient urea production from NO₃⁻ containing wastewater requires coordinated optimization of reactant selection, catalyst design, reaction conditions, and practical implementation. A key factor in this approach is the design of electrocatalysts, which must selectively facilitate C–N coupling while preventing undesirable reactions. Achieving selectivity in this process often requires dual- or multifunctional active sites that can activate both CO₂ and NO₃⁻. This can be accomplished through atomic-level strategies, including single- and dual-atom catalysts (DACs), defect engineering to modify the electronic structure and stabilize intermediates, and the use of bimetallic or compound catalysts. Additionally, structural control techniques such as facet engineering, heterostructures, and porous frameworks are essential.²²⁴

Beyond the catalyst itself, the characteristics of wastewater, including pH, NO₃⁻ and bicarbonate levels, and alkali metal cations, significantly affect reaction kinetics and selectivity. Finally, ensuring stable, long-term operation and robust performance in real wastewater environments rather than idealized electrolyte conditions remains a major challenge.²²⁵

Perspectives on the photoelectrocatalytic urea synthesis: future research direction

The development of catalysts that are highly selective, active, nontoxic, and cost-effective is crucial for PEC urea synthesis.⁵² The integrated design emphasizing novel structures, molecular modeling, advanced characterization techniques, and novel synthesis methods will help regulate the selectivity and efficiency of the catalysts. As previously noted, a typical PEC system primarily consists of a photoelectrode comprising a semiconductor and cocatalysts. The photoelectrode converts solar energy into charge carriers, generating photovoltage to drive redox reactions, with electrons and holes directing reduction and oxidation processes, respectively.²²⁶ Recent advances in photoelectrode design have focused on enhancing light absorption, promoting efficient charge separation, and optimizing cocatalyst integration. Designing catalysts for PEC urea synthesis should facilitate cooperative activation mechanisms, promote the reactions to target specific reaction pathways, maintain high selectivity by suppressing side reactions such as H₂ evolution and the complete conversion of precursors to urea, overcome mass-transfer limitations that hinder the efficient delivery of gaseous reactants like CO₂ and N₂ to the solid–liquid–gas reaction interface, and stabilize crucial intermediates. Employing advanced light absorbers offers a straightforward and impactful approach to improving solar-to-chemical conversion in PEC systems. However, effectively integrating light absorbers, cocatalysts, and counter electrodes remains a major engineering challenge. Significant efforts are required in photoelectrode design to optimize light harvesting and charge utilization for target reactions, as well as in overall system engineering to ensure that power output approaches the maximum achievable under real operating conditions. Understanding reaction mechanisms, especially in systems that produce more complex products, is another challenge.²²⁷ This mechanistic understanding is the vital bridge that connects the design of the catalyst to its ultimate performance. Although urea selectivity in many PEC systems remains below 50%, combining advanced real-time characterization techniques with thorough kinetic studies and control experiments provides a diagnostic framework for tracing reaction pathways and identifying the fundamental steps that determine selectivity and efficiency. Fortunately, various in situ and semi-in situ characterization methods have been applied in PEC research, offering valuable mechanistic insights.²²⁸ Moreover, the long-term stability of PEC systems under continuous light exposure and applied voltage remains limited, and substantial work is required to scale these technologies and demonstrate their performance under real-world operating conditions.²²⁹

In addition to catalyst issues, the most significant engineering barriers to scaling up PEC urea synthesis lie in moving from controlled laboratory setups to industrial flow-reactor systems. Key challenges include developing gas-diffusion electrodes²³⁰ that can efficiently deliver CO₂ to the reaction sites and creating durable membrane/electrode assemblies that can withstand long-term photoelectrochemical operation.²³¹ These obstacles remain crucial to making the technology practical on an industrial scale. Most PEC urea studies are conducted in batch reactors or small-scale cells, where factors such as reactant diffusion, light exposure, and catalyst/electrolyte contact can be easily controlled. However, scaling up to industrial continuous flow reactors introduces several complex challenges. Efficient mass transport, requiring constant delivery of CO₂ and nitrogen precursors and effective removal of products to avoid local concentration gradients, becomes critical. Achieving uniform light distribution across large electrode surfaces is also difficult, as uneven irradiation can reduce charge-carrier generation, lowering urea production efficiency.

Additionally, industrial reactors must manage heat accumulation resulting from light absorption to ensure stable operation and prevent catalyst degradation. Finally, optimizing flow dynamics such as turbulence, pressure drops, and residence time is essential to maintain reaction kinetics while minimizing side reactions.²³² Gas-diffusion electrodes (GDEs) are crucial to PEC urea synthesis because they help deliver CO₂ directly to the catalyst, overcoming its low solubility in H₂O and boosting reaction rates. However, designing effective GDEs is challenging because they require a balance of hydrophobic and hydrophilic properties to allow gas flow without flooding, maintain good ionic contact, and remain structurally stable during long-term operation. Scaling up adds another layer of difficulty, as producing large, uniform, and mechanically durable electrodes is technically demanding.²³³ Durable membrane/electrode assemblies are key components in PEC reactors, combining catalysts, electrodes, and ion-conducting membranes to enable efficient charge transfer. Designing a robust membrane electrode involves several engineering challenges, including mechanical stability under repeated thermal cycles, light irradiation, and electrolyte flow; chemical durability against oxidative degradation; and resistance to fouling by byproducts. Additionally, the membranes must balance ion conductivity with selectivity to maintain efficient ion transport. Integrating the membrane/electrode with gas-diffusion electrodes further increases complexity, requiring both components to be maintained in close contact and to maintain structural integrity during operation.²³⁴ At the level of the industrial system, energy efficiency requires carefully balancing light input, applied voltage, and chemical output to maintain economic feasibility.²³⁵ Industrial systems may also need integration with CO₂ capture, gas compression, and urea separation units to operate effectively.²²⁹ Furthermore, long-term reliability requires materials and designs that resist fouling, degradation, and catalyst loss, thereby reducing maintenance and operational downtime.²³⁶

Machine learning for electrocatalytic urea synthesis

Machine learning (ML) has become a powerful tool for screening catalysts in PEC urea synthesis. This is driven by advances in algorithms and their application, addressing the inherent complexity of PEC reactions, the wide variety of potential catalyst compositions and structures, and the urgent need to develop efficient, scalable systems quickly.²³⁷ The data collection, descriptor generation and selection, algorithm selection, and result evaluation are the key steps in constructing an ML model.²³⁸

Machine learning contributes to the development of photoelectrochemical catalysts in two key ways. First, it enables the construction of robust low-dimensional descriptors that effectively describe catalytic activity and selectivity, while interpretable models provide insight into the underlying reaction mechanisms.²³⁹ Second, by learning from large experimental and computational datasets, ML models can accurately predict key catalyst properties, such as band gap, charge-carrier behavior, surface area, and electronic structure, facilitating rational identification of high-activity materials and heterojunctions. This creates a cycle in which mechanistic insight improves the models, and the models, in turn, accelerate the development of better catalysts. Therefore, a scalable method for property prediction and a guiding analytical framework for high-throughput research can be developed.

Descriptors are quantitative features that represent the essential physical and chemical characteristics of a system in numerical formats that machine learning (ML) models can readily process. In electrocatalysis, effective descriptors must reflect both the specific binding configurations of adsorbates and the underlying structure–property relationships that govern catalytic behavior.²⁴⁰ Effective descriptor selection should align with reaction mechanisms, ensuring that descriptors capture the key steps in nitrogen and carbon activation during electrochemical urea synthesis while avoiding strong correlations that can lead to overfitting and reduce interpretability. Once the appropriate descriptor is chosen, various machine learning algorithms, such as random forests, artificial neural networks, and support vector machines, can be employed to model and predict the behavior of electrochemical catalysts. Ultimately, the choice of algorithm should consider the dataset size, the task complexity, and the level of interpretability required for effective catalyst discovery. The research by Zhu et al. integrates DFT with ML to screen and optimize dual-atom catalysts (DACs) for electrocatalytic urea synthesis, focusing on adsorption energies, reaction barriers, electronic structure descriptors, and overall catalytic activity.²⁴¹ The challenge in activating N₂- and CO₂-derived intermediates on bimetallic catalytic sites, especially in DACs, lies in the high energy barrier to N≡N bond cleavage and in competition from side reactions such as HER and nitrogen reduction reaction (NRR). Identifying effective catalysts for N₂ activation and selective C─N coupling requires exploring a large design space, making DFT-based methods computationally expensive. To address this, the authors integrate two machine learning models with DFT. The first model uses a random forest regressor to predict a thermodynamic descriptor (E_diff) related to overpotential, based on features such as N₂ adsorption bond length and the d-band center. The second model predicts the N₂ bond length based on DAC properties such as d-electron count and metal identity. SHAP analysis reveals that the d-electron count is crucial for N₂ activation and influences catalyst performance. By combining DFT and machine learning, the authors quickly identify TaRu–Pc as a top-performing catalyst with low overpotential and high selectivity. This approach accelerates material discovery and provides valuable insights into how electronic structure affects catalytic performance in urea electrosynthesis. Zhang et al. address the complexity of studying reaction mechanisms for electrochemical urea synthesis by proposing an active learning framework that combines ML algorithms and graph theory with traditional DFT workflows.²⁴² This approach efficiently explores and ranks potential reaction pathways on nitrogen-doped carbon catalysts, overcoming the computational cost of standard DFT methods. The framework uses a cascaded ML model with a gradient boosting classifier to predict the stability of candidate intermediates and a ridge regression model to estimate their formation energies. These models are iteratively applied in an active learning loop to select the most relevant configurations for further DFT analysis. This method reduces computational costs while maintaining accuracy, enabling the reconstruction of a complete reaction network with over 900 intermediates and 1,734 adsorption configurations. The framework demonstrates how machine learning can accelerate complex reaction studies and remain adaptable to other electrocatalytic systems.

Conclusion

The PEC synthesis of urea is gaining significant attention due to its several advantages. Firstly, it utilizes abundant sunlight and readily available atmospheric pollutants such as CO₂ and NO₃⁻ as key reactants, with engineered catalysts to produce urea. Secondly, the process is environmentally friendly, using H₂O as the solvent, generating minimal byproducts with no secondary pollution, and being driven by solar energy. Thirdly, it provides substantial energy savings by operating under mild conditions, eliminating the extreme heat and pressure required by conventional industrial methods. Finally, selectivity and reaction kinetics can be precisely optimized by adjusting the photoelectrode design, light irradiation geometry, and reactant selection, thereby offering enhanced control.

A key advantage of this system is the direct utilization of the photogenerated charge carriers to form the selective products, making the synthesis pathway inherently more straightforward and thermodynamically more favorable than conventional and/or advanced catalytic processes. While charge-transfer and separation mechanisms are well documented for the as-related photocatalytic, electrocatalytic, and PEC systems in the literature, this work focuses explicitly on clarifying the selective urea formation pathways at the surface. Beyond providing a general background for evaluating catalytic systems, the study summarizes recent progress by reviewing robust quantification protocols, advanced in situ analytical tools for tracking the urea formation yield, theoretical evidence supporting experimental findings, and demonstrating the process’s selectivity, kinetics, and thermodynamics.

The reports indicated that engineered surface defects and multiple active sites play a crucial role in enhancing reactant adsorption and facilitating C–N coupling,^120,243,244 as discussed in detail. These features are of great importance for the initial activation of inert and abundant reactants, such as atmospheric nitrogen, at the photoelectrode surface, a substantial challenge, and a significant limitation of the field. To address this, systematic and efficient functionalization of the substrates with adjustable optoelectronic properties should be prioritized.

While PEC C–H and C–C activation is well-documented,^245,246,247 controllable C–N activation for urea formation is still in its early stages, with a limited understanding of interfacial reaction mechanisms. Also, gathering atmospheric CO₂ (as one of the main reactants) from the surrounding environment is not easy and demands huge costs, restricting the system’s scalability for industrial and real-world applications. Nevertheless, this emerging technology offers key advantages, including lower energy consumption, easier product separation, and high selectivity, positioning it as a promising alternative to conventional thermochemical production strategies.

Researchers have proposed an alternative approach to C–C and C–H bond activation using the combination of light and electroactive substrates, known as electrophotocatalysis, where homogenous photocatalysis integrates with electrochemical forces to drive selective reactions. Although conceptually distinct from photoelectrocatalysis, this strategy suggests practical potential for future research aimed at favorably controlling the C–N coupling trends toward higher selectivity’s. However, a key drawback is the challenge of separating the product from the homogeneous reaction environment.

The last key factor making urea photoelectrosynthesis economically viable is the development of diverse photoelectrodes suitable for large-scale applications; however, the long-term stability of PEC systems under continuous light exposure and applied voltage as operating conditions still remains unresolved, and huge efforts should be made to overcome such limitations. Overall, utilizing cost-effective raw materials and stable precursors, along with advanced surface engineering and environmentally friendly processes, can enhance the appeal of this method as a sustainable alternative to the energy-intensive conventional approaches used in the chemical industry.

Limitations of the study

Despite the significant advances in electrocatalytic, photocatalytic, and PEC urea synthesis, several important challenges still limit their practical application and large-scale commercialization of these technologies. A major challenge is the poor selectivity of C–N coupling, as competing reactions, e.g., the HER and CO₂-to-CO reduction, frequently decrease both urea yield and Faradaic efficiency. Also, some primary reactants of the processes, including N₂ and CO₂, are chemically inert due to their strong bonding energies, and this makes the activation stages on the surface challenging. Furthermore, the urea formation reaction mechanisms and C–N coupling pathways remain inadequately understood due to the limited number of potent in situ characterization techniques available. Catalyst stability also remains a remarkable concern, as plenty of photoelectroactive materials undergo photocorrosion, structural degradation, and loss of active sites during long-term operation. Although several studies reported promising Faradaic efficiencies, the overall urea production rates and current densities seem to be unreliable for industrial scalability, primarily because of sluggish reaction kinetics and mass-transfer limitations. Reactor engineering, therefore, remains a critical bottleneck, requiring the development of advanced flow-cell systems, gas-diffusion electrodes, and optimized membrane-based reactor architectures. Precise urea quantification is another critical challenge, since contamination from electrolytes, membranes, and nitrogen-based species may lead to overestimated results, emphasizing the necessity for standardized analytical protocols and verification approaches. Hence, future progress in this field will require standardized testing protocols, advanced catalyst engineering, deeper mechanistic investigations, reactor optimization, and long-term stability explorations to achieve efficient urea yield with the focus on sustainability and scalability.

Acknowledgments

This work is supported by INSF (grant no. 40401927). Also, the authors acknowledge the partial funding from Ajman University under the grant no. 2025-IRG-CHS-4.

Author contributions

M.P.: writing – original draft, supervision, writing – review and editing. E.L.: conceptualization, writing – review and editing. S.G.: visualization, writing – review and editing, methodology. A.A.: software, visualization. A.L.: investigation, writing – review and editing. A.H.-Y.: writing – review and editing. E.A.D.: writing – review and editing, funding acquisition. M.S.: writing – review and editing. L.P.: conceptualization, writing – review and editing. C.W.: conceptualization, writing – review and editing.

Declaration of interests

The authors declare no competing interests.

Declaration of generative AI and AI-assisted technologies in the writing process

Grammarly, a free AI writing tool, was used to check the grammar of the manuscript during the preparation steps.