Unveiling Electrochemical Urea Synthesis by Co-Activation of CO2 and N2 with Mott-Schottky Heterostructure Catalysts

Multifunctional Strategies of Advanced Electrocatalysts for Efficient Urea Synthesis

The electrochemical reduction of nitrogenous species (such as N2, NO, NO2 -, and NO3 -) for urea synthesis under ambient conditions has been extensively studied due to their potential to realize carbon/nitrogen neutrality and mitigate environmental pollution, as well as provide a means to store renewable electricity generated from intermittent sources such as wind and solar power. However, the sluggish reaction kinetics and the scarcity of active sites on electrocatalysts have significantly hindered the advancement of their practical applications. Multifunctional engineering of electrocatalysts has been rationally designed and investigated to adjust their electronic structures, increase the density of active sites, and optimize the binding energies to enhance electrocatalytic performance. Here, surface engineering, defect engineering, doping engineering, and heterostructure engineering strategies for efficient nitrogen electro-reduction are comprehensively summarized. The role of each element in engineered electrocatalysts is elucidated at the atomic level, revealing the intrinsic active site, and understanding the relationship between atomic structure and catalytic performance. This review highlights the state-of-the-art progress of electrocatalytic reactions of waste nitrogenous species into urea. Moreover, this review outlines the challenges and opportunities for urea synthesis and aims to facilitate further research into the development of advanced electrocatalysts for a sustainable future.

Trace Cu-Induced Low C─N Coupling Barrier on Amorphous Co Metallene Boride for Boosting Electrochemical Urea Production

The electrochemical C─N coupling of carbon dioxide (CO2) and nitrate(NO3 -) is an alternative strategy to the traditional high-energy industrial pathway for urea synthesis, which urgently requires the design of efficient catalysts to achieve high yield and Faraday efficiency (FE). Here, amorphous low-content copper-doped cobalt metallene boride (a-Cu0.1CoBx metallene) is designed for urea synthesis via electrochemical C─N coupling. The a-Cu0.1CoBx metallene can drive electrocatalytic C─N coupling of CO2 and NO3 - for urea synthesis in CO2-saturated 0.1 m KNO3 electrolyte, with 27.7% of FE and 312 µg h-1 mg-1 cat. of yield at -0.5 V, as well as superior cycling stability. The in situ Fourier transform infrared and theoretical calculations reveal that electronic effect between Cu, Co, and B causes Cu and Co as dual active sites to promote the adsorption of reactants. Furthermore, the introduced trace Cu reduces the reaction energy barrier of the C─N coupling to facilitate urea synthesis. This work provides a promising route for the optimization of Co-based metallene for the electrosynthesis of urea through C─N coupling.

Boosting Electroreduction of Nitrate and CO2 to Urea on a Tandem Fe1/MoS2 Catalyst

Urea electrosynthesis by coelectrolysis of NO3- and CO2 (UENC) holds enormous promise for sustainable urea production, while the efficient UENC process relies on the rational design of high-performance catalysts to facilitate the electrocatalytic C-N coupling efficiency and the hydrogenation reaction process. Herein, Fe single atoms supported on MoS2 (Fe1/MoS2) are developed as a highly effective and robust catalyst for UENC. Theoretical calculations and operando spectroscopic measurements reveal a tandem catalysis mechanism of the Fe1-S3 motif and MoS2-edge to jointly promote the UENC process, where the Fe1-S3 motif drives the early C-N coupling and subsequent *CO2NO2-to-*CO2NH2 step. The generated *CO2NH2 is then migrated from the Fe1-S3 motif to the nearby MoS2-edge, which facilitates the *CO2NH2 → *COOHNH2 step for urea formation. Noticeably, Fe1/MoS2 assembled in a flow cell reaches a maximum urea Faraday efficiency of 54.98% with a corresponding urea yield rate of 18.98 mmol h-1 g-1, performing at the top level among all of the UENC catalysts reported to date.

Electrocatalytic Synthesis of Urea: An In-depth Investigation from Material Modification to Mechanism Analysis

Industrial urea synthesis production uses NH3 from the Haber-Bosch method, followed by the reaction of NH3 with CO2, which is an energy-consuming technique. More thorough evaluations of the electrocatalytic C-N coupling reaction are needed for the urea synthesis development process, catalyst design, and the underlying reaction mechanisms. However, challenges of adsorption and activation of reactant and suppression of side reactions still hinder its development, making the systematic review necessary. This review meticulously outlines the progress in electrochemical urea synthesis by utilizing different nitrogen (NO3 -, N2, NO2 -, and N2O) and carbon (CO2 and CO) sources. Additionally, it delves into advanced methods in materials design, such as doping, facet engineering, alloying, and vacancy introduction. Furthermore, the existing classes of urea synthesis catalysts are clearly defined, which include 2D nanomaterials, materials with Mott-Schottky structure, materials with artificially frustrated Lewis pairs, single-atom catalysts (SACs), and heteronuclear dual-atom catalysts (HDACs). A comprehensive analysis of the benefits, drawbacks, and latest developments in modern urea detection techniques is discussed. It is aspired that this review will serve as a valuable reference for subsequent designs of highly efficient electrocatalysts and the development of strategies to enhance the performance of electrochemical urea synthesis.

State of Play of Critical Mineral-Based Catalysts for Electrochemical E-Refinery to Synthetic Fuels

The pursuit of decarbonization involves leveraging waste CO2 for the production of valuable fuels and chemicals (e.g., ethanol, ethylene, and urea) through the electrochemical CO2 reduction reactions (CO2RR). The efficacy of this process heavily depends on electrocatalyst performance, which is generally reliant on high loading of critical minerals. However, the supply of these minerals is susceptible to shortage and disruption, prompting concerns regarding their usage, particularly in electrocatalysis, requiring swift innovations to mitigate the supply risks. The reliance on critical minerals in catalyst fabrication can be reduced by implementing design strategies that improve the available active sites, thereby increasing the mass activity. This review seeks to discuss and analyze potential strategies, challenges, and opportunities for improving catalyst activity in CO2RR with a special attention to addressing the risks associated with critical mineral scarcity. By shedding light onto these aspects of critical mineral-based catalyst systems, this review aims to inspire the development of high-performance catalysts and facilitates the practical application of CO2RR technology, whilst mitigating adverse economic, environmental, and community impacts.

A Perspective on Electrochemical Point Source Utilization of CO2 and Other Flue Gas Components to Value Added Chemicals

Electrochemical CO2 reduction reaction (eCO2RR) has been explored extensively for mitigation of noxious CO2 gas generating C1 and C2+ hydrocarbons and oxygenates as value-added fuels and chemicals with remarkable selectivity. The source of CO2 being a pure CO2 feed, it does not fully satisfy the real-time digestion of industrial exhausts. Besides the detrimental effect of noxious gas mixture leading to global warming, there is a huge capital investment in purifying the flue gas mixtures from industries. The presence of other impurity gases affects the eCO2RR mechanism and its activity and selectivity toward C2+ products dwindle drastically. Impurities like NOx, SOx, O2, N2, and halide ions present in flue gas mixture reduce the conversion and selectivity of eCO2RR significantly. Instead of wiping out these impurities via separation processes, new strategies from material chemistry and electrochemistry can open new avenues for turning foes to friends! In this perspective, the co-electroreduction will vividly discussed and supporting role of different heteroatom-containing impurity gases with CO2, generating highly stable C─N, C─S, C─X bonds, and highlight the existing limitations and providing probable solutions for attaining further success in this field and translating this to industrial exhaust streams.

Orderly Coating of Bilayer Polymer to Tailor Microenvironment for Efficient C-N Coupling Toward Highly Selective Urea Electrosynthesis

Electrosynthesis of urea from co-reduction of carbon dioxide and nitrate is a promising alternative to the industrial process. However, the overwhelming existence of proton and nitrate as well as the insufficient supply of CO2 at the reaction interface usually result in complex product distributions from individual nitrate reduction or hydrogen evolution, instead of C-N coupling. In this work, we systematically optimize this microenvironment through orderly coating of bilayer polymer to specifically tackle the above challenges. Polymer of intrinsic microporosity is chosen as the upper polymer to achieve physical sieving, realizing low water diffusivity for suppressing hydrogen evolution and high gas permeability for smooth mass transfer of CO2 at the same time. Polyaniline with abundant basic amino groups is capable of triggering chemical interaction with acidic CO2 molecules, so that is used as the underlying polymer to serve as CO2 concentrator and facilitate the carbon source supply for C-N coupling. Within this tailored microenvironment, a maximum urea generation yield rate of 1671.6 μg h-1 mg-1 and a high Faradaic efficiency of 75.3 % are delivered once coupled with efficient electrocatalyst with neighboring active sites, which is among the most efficient system of urea electrosynthesis.

Selective Synthesis of Organonitrogen Compounds via Electrochemical C-N Coupling on Atomically Dispersed Catalysts

The C-N coupling reaction demonstrates broad application in the fabrication of a wide range of high value-added organonitrogen molecules including fertilizers (e.g., urea), chemical feedstocks (e.g., amines, amides), and biomolecules (e.g., amino acids). The electrocatalytic C-N coupling pathways from waste resources like CO2, NO3-, or NO2- under mild conditions offer sustainable alternatives to the energy-intensive thermochemical processes. However, the complex multistep reaction routes and competing side reactions lead to significant challenges regarding low yield and poor selectivity toward large-scale practical production of target molecules. Among diverse catalyst systems that have been developed for electrochemical C-N coupling reactions, the atomically dispersed catalysts with well-defined active sites provide an ideal model platform for fundamental mechanism elucidation. More importantly, the intersite synergy between the active sites permits the enhanced reaction efficiency and selectivity toward target products. In this Review, we systematically assess the dominant reaction pathways of electrocatalytic C-N coupling reactions toward various products including urea, amines, amides, amino acids, and oximes. To guide the rational design of atomically dispersed catalysts, we identify four key stages in the overall reaction process and critically discuss the corresponding catalyst design principles, namely, retaining NOx/COx reactants on the catalyst surface, regulating the evolution pathway of N-/C- intermediates, promoting C-N coupling, and facilitating final hydrogenation steps. In addition, the advanced and effective theoretical simulation and characterization technologies are discussed. Finally, a series of remaining challenges and valuable future prospects are presented to advance rational catalyst design toward selective electrocatalytic synthesis of organonitrogen molecules.

Photoelectrocatalytic Synthesis of Urea from Carbon Dioxide and Nitrate over a Cu2O Photocathode

Transformation of carbon dioxide and nitrate ions into urea offers an attractive route for both nitrogen fertilizer production and environmental remediation. However, achieving this transformation under mild conditions remains challenging. Herein, we report an efficient photoelectrochemical method for urea synthesis by co-reduction of carbon dioxide and nitrate ion over a Cu2O photocathode, delivering urea formation rate of 29.71±2.20 μmol g-1 h-1 and Faradaic efficiency (FE) of 12.90±1.15 % at low external potential (-0.017 V vs. reversible hydrogen electrode). Experimental data combined with theoretical calculations suggest that the adsorbed CO* and NO2* species are the key intermediates, and associated C-N coupling is the rate-determining step. This work demonstrates that Cu2O is an efficient catalyst to drive co-reduction of CO2 and NO3 - to urea under light irradiation with low external potential, showing great opportunity of photoelectrocatalysis as a sustainable tool for value-added chemical synthesis.

Electrocatalysts for Urea Synthesis from CO2 and Nitrogenous Species: From CO2 and N2/NOx Reduction to urea synthesis

The traditional industrial synthesis of urea relies on the energy-intensive and polluting process, namely the Haber-Bosch method for ammonia production, followed by the Bosch-Meiser process for urea synthesis. In contrast, electrocatalytic C-N coupling from carbon dioxide (CO2) and nitrogenous species presents a promising alternative for direct urea synthesis under ambient conditions, bypassing the need for ammonia production. This review provides an overview of recent progress in the electrocatalytic coupling of CO2 and nitrogen sources for urea synthesis. It focuses on the role of intermediate species and active site structures in promoting urea synthesis, drawing from insights into reactants' adsorption behavior and interactions with catalysts tailored for CO2 reduction, nitrogen reduction, and nitrate reduction. Advanced electrocatalyst design strategies for urea synthesis from CO2 and nitrogenous species under ambient conditions are explored, providing insights for efficient catalyst design. Key challenges and prospective directions are presented in the conclusion. Mechanistic studies elucidating the C-N coupling reaction and future development directions are discussed. The review aims to inspire further research and development in electrocatalysts for electrochemical urea synthesis.

Sustainable Electrosynthesis of N,N-Dimethylformamide via Relay Catalysis on Synergistic Active Sites

Electrified synthesis of high-value organonitrogen chemicals from low-cost carbon- and nitrogen-based feedstocks offers an economically and environmentally appealing alternative to traditional thermocatalytic methods. However, the intricate electrochemical reactions at electrode surfaces pose significant challenges in controlling selectivity and activity, especially for producing complex substances such as N,N-dimethylformamide (DMF). Herein, we tackle this challenge by developing relay catalysis for efficient DMF production using a composite WO2-NiOOH/Ni catalyst with two distinctive active sites. Specifically, WO2 selectively promotes dimethylamine (DMA) electrooxidation to produce strongly surface-bound (CH3)2N*, while nearby NiOOH facilitates methanol electrooxidation to yield more weakly bound *CHO. The disparity in binding energetics of the key C- and N-intermediates expedites C-N coupling at the WO2-NiOOH interface. In situ infrared spectroscopy with isotope-labeling experiments, quasi-in situ electron paramagnetic resonance trapping experiments, and electrochemical operating experiments revealed the C-N coupling mechanism and enhanced DMF-synthesis selectivity and activity. In situ X-ray absorption spectroscopy (XAS) and postreaction transmission electron microscopy (TEM) studies verified the stability of WO2-NiOOH/Ni during extended electrochemical operation. A Faradaic efficiency of ∼50% and a production rate of 438 μmol cm-2 h-1 were achieved at an industrially relevant current density of 100 mA cm-2 over an 80 h DMF production period. This study introduces a new paradigm for developing electrothermo relay catalysis for the sustainable and efficient synthesis of valuable organic chemicals with industrial potential.

Synergy of Photogenerated Electrons and Holes toward Efficient Photocatalytic Urea Synthesis from CO2 and N2

Directly coupling N2 and CO2 to synthesize urea by photocatalysis paves a sustainable route for urea synthesis, but its performance is limited by the competition of photogenerated electrons between N2 and CO2, as well as the underutilized photogenerated holes. Herein, we report an efficient urea synthesis process involving photogenerated electrons and holes in respectively converting CO2 and N2 over a redox heterojunction consisting of WO3 and Ni single-atom-decorated CdS (Ni1-CdS/WO3). For the photocatalytic urea synthesis from N2 and CO2 in pure water, Ni1-CdS/WO3 attained a urea yield rate of 78 μM h-1 and an apparent quantum yield of 0.15 % at 385 nm, which ranked among the best photocatalytic urea synthesis performance reported. Mechanistic studies reveal that the N2 was converted into NO species by ⋅OH radicals generated from photogenerated holes over the WO3 component, meanwhile, the CO2 was transformed into *CO species over the Ni site by photogenerated electrons. The generated NO and *CO species were further coupled to form *OCNO intermediate, then gradually transformed into urea. This work emphasizes the importance of reasonably utilizing photogenerated holes in photocatalytic reduction reactions.

Liquid Phase Exfoliation of Few-Layer Non-Van der Waals Chromium Sulfide

Exfoliation of 2D non-Van der Waals (non-vdW) semiconductor nanoplates (NPs) from inorganic analogs presents many challenges ahead for further exploring of their advanced applications on account of the strong bonding energies. In this study, the exfoliation of ultrathin 2D non-vdW chromium sulfide (2D Cr2S3) by means of a combined facile liquid-phase exfoliation (LPE) method is successfully demonstrated. The morphology and structure of the 2D Cr2S3 material are systematically examined. Magnetic studies show an obvious temperature-dependent uncompensated antiferromagnetic behavior of 2D Cr2S3. The material is further loaded on TiO2 nanorod arrays to form an S-scheme heterojunction. Experimental measurements and density functional theory (DFT) calculations confirm that the formed TiO2@Cr2S3 S-scheme heterojunction facilitates the separation and transmission of photo-induced electron/hole pairs, resulting in a significantly enhanced photocatalytic activity in the visible region.

Boosting Electrochemical Urea Synthesis via Constructing Ordered Pd-Zn Active Pair

Electrochemical co-reduction of nitrate (NO3-) and carbon dioxide (CO2) has been widely regarded as a promising route to produce urea under ambient conditions, however the yield rate of urea has remained limited. Here, we report an atomically ordered intermetallic pallium-zinc (PdZn) electrocatalyst comprising a high density of PdZn pairs for boosting urea electrosynthesis. It is found that Pd and Zn are responsible for the adsorption and activation of NO3- and CO2, respectively, and thus the co-adsorption and co-activation NO3- and CO2 are achieved in ordered PdZn pairs. More importantly, the ordered and well-defined PdZn pairs provide a dual-site geometric structure conducive to the key C-N coupling with a low kinetical barrier, as demonstrated on both operando measurements and theoretical calculations. Consequently, the PdZn electrocatalyst displays excellent performance for the co-reduction to generate urea with a maximum urea Faradaic efficiency of 62.78% and a urea yield rate of 1274.42 μg mg-1 h-1, and the latter is 1.5-fold larger than disordered pairs in PdZn alloys. This work paves new pathways to boost urea electrosynthesis via constructing ordered dual-metal pairs.

Urea Electrosynthesis from Nitrate and CO2 on Diatomic Alloys

Urea electrosynthesis from co-electrolysis of NO3 - and CO2 (UENC) offers a promising technology for achieving sustainable and efficient urea production. Herein, a diatomic alloy catalyst (CuPd1Rh1-DAA), with mutually isolated Pd and Rh atoms alloyed on Cu substrate, is theoretically designed and experimentally confirmed to be a highly active and selective UENC catalyst. Combining theoretical computations and operando spectroscopic characterizations reveals the synergistic effect of Pd1-Cu and Rh1-Cu active sites to promote the UENC via a tandem catalysis mechanism, where Pd1-Cu site triggers the early C-N coupling and promotes *CO2NO2-to-*CO2NH steps, while Rh1-Cu site facilitates the subsequent protonation step of *CO2NH2 to *COOHNH2 toward the urea formation. Impressively, CuPd1Rh1-DAA assembled in a flow cell presents the highest urea Faradaic efficiency of 72.1% and urea yield rate of 53.2 mmol h-1 gcat -1 at -0.5 V versus RHE, representing nearly the highest performance among all reported UENC catalysts.

Breaking the Scaling Relationship in C-N Coupling via the Doping Effects for Efficient Urea Electrosynthesis

Electrochemical C-N coupling reaction based on carbon dioxide and nitrate have been emerged as a new "green synthetic strategy" for the synthesis of urea, but the catalytic efficiency is seriously restricted by the inherent scaling relations of adsorption energies of the active sites, the improvement of catalytic activity is frequently accompanied by the decrease in selectivity. Herein, a doping engineering strategy was proposed to break the scaling relationship of intermediate binding and minimize the kinetic barrier of C-N coupling. A thus designed SrCo0.39Ru0.61O3-δ catalyst achieves a urea yield rate of 1522 μg h-1 mgcat. -1 and faradic efficiency of 34.1 % at -0.7 V versus reversible hydrogen electrode. A series of characterizations revealed that Co doping not only induces lattice distortion but also creates rich oxygen vacancies (OV) in the SrRuO3. The oxygen vacancies weaken the adsorption of *CO and *NH2 intermediates on the Co and Ru sites respectively, and the strain effects over the Co-Ru dual sites promoting the occurrence of C-N coupling of the two monomers instead of selective hydrogenating to form by-products. This work presents an insight into molecular coupling reactions towards urea synthesis via the doping engineering on SrRuO3.

Tandem Catalysts Enabling Efficient C-N Coupling toward the Electrosynthesis of Urea

The development of a methodology for synthesizing value-added urea (CO(NH2)2) via a renewable electricity-driven C-N coupling reaction under mild conditions is highly anticipated. However, the complex catalytic active sites that act on the carbon and nitrogen species make the reaction mechanism unclear, resulting in a low efficiency of C-N coupling from the co-reduction of carbon dioxide (CO2) and nitrate (NO3 -). Herein, we propose a novel tandem catalyst of Mo-PCN-222(Co), in which the Mo sites serve to facilitate nitrate reduction to the *NH2 intermediate, while the Co sites enhance CO2 reduction to carbonic oxide (CO), thus synergistically promoting C-N coupling. The synthesized Mo-PCN-222(Co) catalyst exhibited a noteworthy urea yield rate of 844.11 mg h-1 g-1, alongside a corresponding Faradaic efficiency of 33.90 % at -0.4 V vs. reversible hydrogen electrode (RHE). By combining in situ spectroscopic techniques with density functional theory calculations, we demonstrate that efficient C-N coupling is attributed to a tandem system in which the *NH2 and *CO intermediates produced by the Mo and Co active sites of Mo-PCN-222(Co) stabilize the formation of the *CONH2 intermediate. This study provides an effective avenue for the design and synthesis of tandem catalysts for electrocatalytic urea synthesis.

Efficient Electrochemical Co-Reduction of Carbon Dioxide and Nitrate to Urea with High Faradaic Efficiency on Cobalt-Based Dual-Sites

Renewable electricity-powered nitrate/carbon dioxide co-reduction reaction toward urea production paves an attractive alternative to industrial urea processes and offers a clean on-site approach to closing the global nitrogen cycle. However, its large-scale implantation is severely impeded by challenging C-N coupling and requires electrocatalysts with high activity/selectivity. Here, cobalt-nanoparticles anchored on carbon nanosheet (Co NPs@C) are proposed as a catalyst electrode to boost yield and Faradaic efficiency (FE) toward urea electrosynthesis with enhanced C-N coupling. Such Co NPs@C renders superb urea-producing activity with a high FE reaching 54.3% and a urea yield of 2217.5 µg h-1 mgcat. -1, much superior to the Co NPs and C nanosheet counterparts, and meanwhile shows strong stability. The Co NPs@C affords rich catalytically active sites, fast reactant diffusion, and sufficient catalytic surfaces-electrolyte contacts with favored charge and ion transfer efficiencies. The theoretical calculations reveal that the high-rate formation of *CO and *NH2 intermediates is crucial for facilitating urea synthesis.

Catalysts for C-N coupling in urea electrosynthesis under ambient conditions from carbon dioxide and nitrogenous species

Urea is an indispensable nitrogen-containing organic compound in modern human life. However, the current industrial synthesis of urea involves ammonia, which is produced through the Haber-Bosch process under harsh reaction conditions, causing huge energy consumption and heavy environmental pollution. Electrochemical reduction of carbon dioxide (CO2) and nitrogenous species (N2, NOx- and NO) have achieved significant progress, offering a promising approach for the electrochemical C-N coupling to produce urea under ambient conditions. Urea synthesis driven by renewable electricity represents a suitable alternative to the traditional process, contributing to the goal of carbon neutrality and nitrogen cycles. However, challenges such as low yield rate, poor selectivity and unveiled reaction mechanisms still need to be addressed. This review provides a summary of the latest catalysts utilized in urea electrosynthesis, aiming to provide guidance and prospects for the development of high-performance catalysts.

Mesoporous Electrocatalysts with p-n Heterojunctions for Efficient Electroreduction of CO2 and N2 to Urea

The electrocatalytic synthesis of high-value-added urea by activating N2 and CO2 is a green synthesis technology that has achieved carbon neutrality. However, the chemical adsorption and C-N coupling ability of N2 and CO2 on the surface of the catalyst are generally poor, greatly limiting the improvement of electrocatalytic activity and selectivity in electrocatalytic urea synthesis. Herein, novel hierarchical mesoporous CeO2/Co3O4 heterostructures are fabricated, and at an ultralow applied voltage of -0.2 V, the urea yield rate reaches 5.81 mmol g-1 h-1, with a corresponding Faraday efficiency of 30.05%. The hierarchical mesoporous material effectively reduces the mass transfer resistance of reactants and intermediates, making it easier for them to access active centers. The emerging space-charge regions at the heterointerface generate local electrophilic and nucleophilic regions, facilitating CO2 targeted adsorption in the electrophilic region and activation to produce *CO intermediates and N2 targeted adsorption in the nucleophilic region and activation to generate *N ═ N* intermediates. Then, the electrons in the σ orbitals of *N ═ N* intermediates can be easily accepted by the empty eg orbitals of Co3+ in CeO2/Co3O4, which presents a low-spin state (LS: t2g6eg0). Subsequently, *CO couples with *N ═ N* to produce the key intermediate *NCON*. Interestingly, it was discovered through in situ Raman spectroscopy that the CeO2/Co3O4 catalyst has a reversible spinel structure before and after the electrocatalytic reaction, which is due to the surface reconstruction of the catalyst during the electrocatalytic reaction process, producing amorphous active cobalt oxides, which is beneficial for improving electrocatalytic activity.

Review on strategies for improving the added value and expanding the scope of CO2 electroreduction products

The electrochemical reduction of CO2 into value-added chemicals has been explored as a promising solution to realize carbon neutrality and inhibit global warming. This involves utilizing the electrochemical CO2 reduction reaction (CO2RR) to produce a variety of single-carbon (C1) and multi-carbon (C2+) products. Additionally, the electrolyte solution in the CO2RR system can be enriched with nitrogen sources (such as NO3-, NO2-, N2, or NO) to enable the synthesis of organonitrogen compounds via C-N coupling reactions. However, the electrochemical conversion of CO2 into valuable chemicals still faces challenges in terms of low product yield, poor faradaic efficiency (FE), and unclear understanding of the reaction mechanism. This review summarizes the promising strategies aimed at achieving selective production of diverse carbon-containing products, including CO, formate, hydrocarbons, alcohols, and organonitrogen compounds. These approaches involve the rational design of electrocatalysts and the construction of coupled electrocatalytic reaction systems. Moreover, this review presents the underlying reaction mechanisms, identifies the existing challenges, and highlights the prospects of the electrosynthesis processes. The aim is to offer valuable insights and guidance for future research on the electrocatalytic conversion of CO2 into carbon-containing products of enhanced value-added potential.

Amorphous Bismuth-Tin Oxide Nanosheets with Optimized C-N Coupling for Efficient Urea Synthesis

Closing the carbon and nitrogen cycles by electrochemical methods using renewable energy to convert abundant or harmful feedstocks into high-value C- or N-containing chemicals has the potential to transform the global energy landscape. However, efficient conversion avenues have to date been mostly realized for the independent reduction of CO2 or NO3-. The synthesis of more complex C-N compounds still suffers from low conversion efficiency due to the inability to find effective catalysts. To this end, here we present amorphous bismuth-tin oxide nanosheets, which effectively reduce the energy barrier of the catalytic reaction, facilitating efficient and highly selective urea production. With enhanced CO2 adsorption and activation on the catalyst, a C-N coupling pathway based on *CO2 rather than traditional *CO is realized. The optimized orbital symmetry of the C- (*CO2) and N-containing (*NO2) intermediates promotes a significant increase in the Faraday efficiency of urea production to an outstanding value of 78.36% at -0.4 V vs RHE. In parallel, the nitrogen and carbon selectivity for urea formation is also enhanced to 90.41% and 95.39%, respectively. The present results and insights provide a valuable reference for the further development of new catalysts for efficient synthesis of high-value C-N compounds from CO2.

Theoretical study of transition metal-doped β12 borophene as a new single-atom catalyst for carbon dioxide electroreduction

The electrocatalytic carbon dioxide reduction reaction (CO2RR) presents a viable and cost-effective approach for the elimination of CO2 by transforming it into valuable products. Nevertheless, this process is impeded by the absence of exceptionally active and stable catalysts. Herein, a new type of electrocatalyst of transition metal (TM)-doped β12-borophene (TM@β12-BM) is investigated via density functional theory (DFT) calculations. Through comprehensive screening, two promising single-atom catalysts (SACs), Sc@β12-BM and Y@β12-BM, are successfully identified, exhibiting high stability, catalytic activity and selectivity for the CO2RR. The C1 products methane (CH4) and methanol (CH3OH) are synthesized with limiting potentials (UL) of -0.78 V and -0.56 V on Sc@β12-BM and Y@β12-BM, respectively. Meanwhile, CO2 is more favourable for reduction into the C2 product ethanol (CH3CH2OH) compared to ethylene (C2H4) via C-C coupling on these two SACs. More importantly, the dynamic barriers of the key C-C coupling step are 0.53 eV and 0.73 eV for the "slow-growth" sampling approach in the explicit water molecule model. Furthermore, Sc@β12-BM and Y@β12-BM exhibit higher selectivity for producing C1 compounds (CH4 and CH3OH) than C2 (CH3CH2OH) in the CO2RR. Compared with Sc@β12-BM, Y@β12-BM demonstrates superior inhibition of the competitive hydrogen evolution reaction (HER) in the liquid phase. These results not only demonstrate the great potential of SACs for direct reduction of CO2 to C1 and C2, but also help in rationally designing high-performance SACs.

Advances in Catalysts for Urea Electrosynthesis Utilizing CO2 and Nitrogenous Materials: A Mechanistic Perspective

Electrocatalytic urea synthesis from CO2 and nitrogenous substances represents an essential advance for the chemical industry, enabling the efficient utilization of resources and promoting sustainable development. However, the development of electrocatalytic urea synthesis has been severely limited by weak chemisorption, poor activation and difficulties in C-N coupling reactions. In this review, catalysts and corresponding reaction mechanisms in the emerging fields of bimetallic catalysts, MXenes, frustrated Lewis acid-base pairs and heterostructures are summarized in terms of the two central mechanisms of molecule-catalyst interactions as well as chemical bond cleavage and directional coupling, which provide new perspectives for improving the efficiency of electrocatalytic synthesis of urea. This review provides valuable insights to elucidate potential electrocatalytic mechanisms.

Selective Electrocatalytic Conversion of Nitric Oxide to High Value-Added Chemicals

The artificial disturbance in the nitrogen cycle has necessitated an urgent need for nitric oxide (NO) removal. Electrochemical technologies for NO conversion have gained increasing attention in recent years. This comprehensive review presents the recent advancements in selective electrocatalytic conversion of NO to high value-added chemicals, with specific emphasis on catalyst design, electrolyte composition, mass diffusion, and adsorption energies of key intermediate species. Furthermore, the review explores the synergistic electrochemical co-electrolysis of NO with specific carbon source molecules, enabling the synthesis of a range of valuable chemicals with C─N bonds. It also provides in-depth insights into the intricate reaction pathways and underlying mechanisms, offering valuable perspectives on the challenges and prospects of selective NO electrolysis. By advancing comprehension and fostering awareness of nitrogen cycle balance, this review contributes to the development of efficient and sustainable electrocatalytic systems for the selective synthesis of valuable chemicals from NO.

Activation of Ga Liquid Catalyst with Continuously Exposed Active Sites for Electrocatalytic C-N Coupling

Environmentally friendly electrocatalytic coupling of CO2 and N2 for urea synthesis is a promising strategy. However, it is still facing problems such as low yield as well as low stability. Here, a new carbon-coated liquid alloy catalyst, Ga79Cu11Mo10@C is designed for efficient electrochemical urea synthesis by activating Ga active sites. During the N2 and CO2 co-reduction process, the yield of urea reaches 28.25 mmol h-1 g-1, which is the highest yield reported so far under the same conditions, the Faraday efficiency (FE) is also as high as 60.6 % at -0.4 V vs. RHE. In addition, the catalyst shows excellent stability under 100 h of testing. Comprehensive analyses showed that sequential exposure of a high density of active sites promoted the adsorption and activation of N2 and CO2 for efficient coupling reactions. This coupling reaction occurs through a thermodynamic spontaneous reaction between *N=N* and CO to form a C-N bond. The deformability of the liquid state facilitates catalyst recovery and enhances stability and resistance to poisoning. Moreover, the introduction of Cu and Mo stimulates the Ga active sites, which successfully synthesises the *NCON* intermediate. The reaction energy barrier of the third proton-coupled electron transfer process rate-determining step (RDS) *NHCONH→*NHCONH2 was lowered, ensuring the efficient synthesis of urea.

Dynamic Reconstruction of Two-Dimensional Defective Bi Nanosheets for Efficient Electrocatalytic Urea Synthesis

Catalyst surface dynamics drive the generation of active species for electrocatalytic reactions. Yet, the understanding of dominant site formation and reaction mechanisms is limited. In this study, we thoroughly investigate the dynamic reconstruction of two-dimensional defective Bi nanosheets from exfoliated Bi2Se3 nanosheets under electrochemical CO2 and nitrate (NO3 -) reduction conditions. The ultrathin Bi2Se3 nanosheets obtained by NaBH4-assisted cryo-mediated liquid-phase exfoliation are more easily reduced and reconstructed to Bi nanosheets with high-density grain boundaries (GBs; GB-rich Bi). The reconstructed GB-rich Bi catalyst affords a remarkable yield rate of 4.6 mmol h-1 mgcat. -1 and Faradaic efficiency of 32 % for urea production at -0.40 V vs. RHE. Notably, this yield rate is 2 and 8.2 times higher than those of the low-GB Bi and bulk Bi catalysts, respectively. Theoretical analysis demonstrates that the GB sites significantly reduce the *CO and *NH2 intermediate formation energy and C-N coupling energy barrier, enabling selective urea electrosynthesis on the GB-rich Bi catalyst. This work will trigger further research into the structure-activity interplay in dynamic processes using in situ techniques.

Built-in Electric Field Promotes Interfacial Adsorption and Activation of CO2 for C1 Products over a Wide Potential Window

The unsatisfactory adsorption and activation of CO2 suppress electrochemical reduction over a wide potential window. Herein, the built-in electric field (BIEF) at the CeO2/In2O3 n-n heterostructure realizes the C1 (CO and HCOO-) selectivity over 90.0% in a broad range of potentials from -0.7 to -1.1 V with a maximum value of 98.7 ± 0.3% at -0.8 V. In addition, the C1 current density (-1.1 V) of the CeO2/In2O3 heterostructure with a BIEF is about 2.0- and 3.2-fold that of In2O3 and a physically mixed sample, respectively. The experimental and theoretical calculation results indicate that the introduction of CeO2 triggered the charge redistribution and formed the BIEF at the interfaces, which enhanced the interfacial adsorption and activation of CO2 at low overpotentials. Furthermore, the promoting effect was also extended to CeO2/In2S3. This work gives a deep understanding of BIEF engineering for highly efficient CO2 electroreduction over a wide potential window.

NH3 Electrosynthesis from N2 Molecules: Progresses, Challenges, and Future Perspectives

Green ammonia (NH3), made by using renewable electricity to split nearly limitless nitrogen (N2) molecules, is a vital platform molecule and an ideal fuel to drive the sustainable development of human society without carbon dioxide emission. The NH3 electrosynthesis field currently faces the dilemma of low yield rate and efficiency; however, decoupling the overlapping issues of this area and providing guidelines for its development directions are not trivial because it involves complex reaction process and multidisciplinary entries (for example, electrochemistry, catalysis, interfaces, processes, etc.). In this Perspective, we introduce a classification scheme for NH3 electrosynthesis based on the reaction process, namely, direct (N2 reduction reaction) and indirect electrosynthesis (Li-mediated/plasma-enabled NH3 electrosynthesis). This categorization allows us to finely decouple the complicated reaction pathways and identify the specific rate-determining steps/bottleneck issues for each synthesis approach such as N2 activation, H2 evolution side reaction, solid-electrolyte interphase engineering, plasma process, etc. We then present a detailed overview of the latest progresses on solving these core issues in terms of the whole electrochemical system covering the electrocatalysts, electrodes, electrolytes, electrolyzers, etc. Finally, we discuss the research focuses and the promising strategies for the development of NH3 electrosynthesis in the future with a multiscale perspective of atomistic mechanisms, nanoscale electrocatalysts, microscale electrodes/interfaces, and macroscale electrolyzers/processes. It is expected that this Perspective will provide the readers with an in-depth understanding of the bottleneck issues and insightful guidance on designing the efficient NH3 electrosynthesis systems.

Efficient C-N coupling for urea electrosynthesis on defective Co3O4 with dual-functional sites

Urea electrosynthesis under ambient conditions is emerging as a promising alternative to conventional synthetic protocols. However, the weak binding of reactants/intermediates on the catalyst surface induces multiple competing pathways, hindering efficient urea production. Herein, we report the synthesis of defective Co3O4 catalysts that integrate dual-functional sites for urea production from CO2 and nitrite. Regulating the reactant adsorption capacity on defective Co3O4 catalysts can efficiently control the competing reaction pathways. The urea yield rate of 3361 mg h-1 gcat-1 was achieved with a corresponding faradaic efficiency (FE) of 26.3% and 100% carbon selectivity at a potential of -0.7 V vs. the reversible hydrogen electrode. Both experimental and theoretical investigations reveal that the introduction of oxygen vacancies efficiently triggers the formation of well-matched adsorption/activation sites, optimizing the adsorption of reactants/intermediates while decreasing the C-N coupling reaction energy. This work offers new insights into the development of dual-functional catalysts based on non-noble transition metal oxides with oxygen vacancies, enabling the efficient electrosynthesis of essential C-N fine chemicals.

Recent research progress on building C-N bonds via electrochemical NOx reduction

The release of NOx species (such as nitrate, nitrite and nitric oxide) into water and the atmosphere due to human being's agricultural and industrial activities has caused a series of environmental problems, including accumulation of toxic pollutants that are dangerous to humans and animals, acid rain, the greenhouse effect and disturbance of the global nitrogen cycle balance. Electrosynthesis of organonitrogen compounds with NOx species as the nitrogen source offers a sustainable strategy to upgrade the waste NOx into value-added organic products under ambient conditions. The electrochemical reduction of NOx species can generate surface-adsorbed intermediates such as hydroxylamine, which are usually strong nucleophiles and can undergo nucleophilic attack to carbonyl groups to build C-N bonds and generate organonitrogen compounds such as amine, oxime, amide and amino acid. This mini-review summarizes the most recent progress in building C-N bonds via the in situ generation of nucleophilic intermediates from electrochemical NOx reduction, and highlights some important strategies in facilitating the reaction rates and selectivities towards the C-N coupling products. In particular, the preparation of high-performance electrocatalysts (e.g., nano-/atomic-scale catalysts, single-atom catalysts, alloy catalysts), selection of nucleophilic intermediates, novel design of reactors and understanding the surface adsorption process are highlighted. A few key challenges and knowledge gaps are discussed, and some promising research directions are also proposed for future advances in electrochemical C-N coupling.

Screening of transition metal and boron atoms co-doped graphdiyne catalysts for electrocatalytic urea synthesis

Electrocatalytic CN coupling using nitrogen (N2) and carbon dioxide (CO2) as precursors offers a promising alternative for urea production under mild conditions, compared to traditional synthesis approaches. However, the design and screening of extremely efficient electrocatalysts remains a significant challenge in this field. Hence, we propose a systematic approach to screen efficient double-atom catalysts (DACs) with both metal and boron active sites, employing density functional theory (DFT). A comprehensive evaluation of 27 potential catalysts were performed, taking into account their stability, co-adsorption of N2 and CO2, as well as the potential-determining step (PDS) involved urea formation. The calculated results show that co-doped graphdiyne with CrB and MnB double atoms (CrB@GDY and MnB@GDY) emerge as potential electrocatalysts for urea production, displaying thermodynamic energy barriers of 0.41 eV and 0.66 eV, respectively. More importantly, these two DACs can significantly suppress the ammonia (NH3) and C1 products formation. Furthermore, a catalytic activity relationship between the d-band centers of the DACs and urea production performance were established. This study not only forecasts two promising DACs for subsequent experimental work but also establishes a theoretical framework for the evaluation of DACs in electrocatalytic urea synthesis.

Effect of Interlayer Spaces and Interfacial Structures on High-Performance MXene/Ionic Liquid Supercapacitors: A Molecular Dynamics Simulation

The combination of high-capacitance MXenes and wide-electrochemical-window ionic liquids (ILs) has exhibited bright prospects in supercapacitors. Several strategies, such as surficial functionalization and interlayer spacing tuning, have been used to enhance the electrochemical performance of supercapacitors. However, the lack of theoretical guidance on these strategies, including the effects of the microenvironment in the interlayer of confined ILs, hindered the further exploration of such devices. Herein, we performed molecular dynamics simulations to comprehensively investigate the effects of the interlayer space and surface terminations of MXene electrodes on capacity. The results show that the electrical double layer (EDL) structure was found to form on the interface between the MXene electrode and ILs electrolyte by analyzing the ion number density and charge density in the nanometer confined spaces. Under the same potential, the -OH terminations significantly impact the ion orientation in the EDL, particularly near the electrode surface, where cations tend to align vertically, allowing the retention of more cations at the electrode surfaces. Interestingly, such an orientation distribution was decisively from the hydrogen bonds expressed by O-H···O between the -OH termination of MXene and -OH groups of ILs. The differential capacitances of the supercapacitors were calculated by the surficial electron density, and it showed that the capacitance is a nearly one-quarter increase in the 14 Å interlayer spacing compared with that of 10 Å under an applied potential of 2 V. At the same time, the Ti3C2(OH)2 electrode had a higher differential capacitance than the Ti3C2O2 electrode, which possibly originates from the stronger hydrogen bonds to contribute to the vertical aggregation of the cations. Our results highlighted the roles of the interlayer spacing distance and surface terminations of the MXene on the performance of the type of supercapacitor.

Electrocatalytic Urea Synthesis via C-N Coupling from CO2 and Nitrogenous Species

ConspectusIndustrial urea synthesis consists of the Haber-Bosch process to produce ammonia and the subsequent Bosch-Meiser process to produce urea. Compared to the conventional energy-intensive urea synthetic protocol, electrocatalytic C-N coupling from CO2 and nitrogenous species emerges as a promising alternative to construct a C-N bond under ambient conditions and to realize the direct synthesis of high-value urea products via skipping the intermediate step of ammonia production. The main challenges for electrocatalytic C-N coupling lie in the intrinsic inertness of molecules and the competition with parallel side reactions. In this Account, we give an overview of our recent progress toward electrocatalytic C-N coupling from CO2 and nitrogenous species toward urea synthesis.To begin, we present the direct transformation of dinitrogen (N2) to the C-N bond by coelectrolysis, verifying the feasibility of direct urea synthesis from N2 and CO2 under ambient conditions. In contrast to the highly endothermic step of proton coupling in conventional N2 reduction, the N2 activation and construction of the C-N bond arise from a thermodynamic spontaneous reaction between CO (derived from CO2 reduction) and *N═N* (the asterisks represent the adsorption sites), and the crucial *NCON* species mediates the interconversion of N2, CO2, and urea. Based on theoretical guidance, the effect of N2 adsorption configurations on C-N coupling is investigated on the model catalysts with defined active site structure, revealing that the side-on adsorption rather than the end-on one favors C-N coupling and urea synthesis.Electrocatalytic C-N coupling of CO2 and nitrate (NO3-) is also an effective pathway to achieve direct urea synthesis. We summarize our progress in the C-N coupling of CO2 and NO3-, from the aspects of modulating intermediate species adsorption and reaction paths, monitoring irreversible and reversible reconstruction of active sites, and precisely constructing active sites to match activities and to boost the electrocatalytic urea synthesis. In each case, in situ electrochemical technologies and density functional theory (DFT) calculations are carried out to unveil the microscopic mechanisms for the promotion of C-N coupling and the enhancement of urea synthesis activity. In the last section, we put forward the limitations, challenges, and perspectives in these two coupling systems for further development of electrocatalytic urea synthesis.

A Universal Approach for Sustainable Urea Synthesis via Intermediate Assembly at the Electrode/Electrolyte Interface

Electrocatalytic C-N coupling process is indeed a sustainable alternative for direct urea synthesis and co-upgrading of carbon dioxide and nitrate wastes. However, the main challenge lies in the unactivated C-N coupling process. Here, we proposed a strategy of intermediate assembly with alkali metal cations to activate C-N coupling at the electrode/electrolyte interface. Urea synthesis activity follows the trend of Li+ Collapse

Key Words

• C−N Coupling
• Electrode/Electrolyte Interface
• Intermediate Assembly
• Urea Synthesis

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MESH Headings Collapse

Grants

• 2020YFA0710000 National Key R&D Program of China
• 22102054 National Natural Science Foundation of China
• 22173048 National Natural Science Foundation of China
• 22250006 National Natural Science Foundation of China
• 22261160640 National Natural Science Foundation of China
• 22202065 National Natural Science Foundation of China
• 2023JJ10002 Hunan Provincial Science Fund for Distinguished Young Scholars
• BX20200116 China Postdoctoral Science Foundation
• 2020M682540 China Postdoctoral Science Foundation
• ZR2020QB120 Natural Science Foundation of Shandong Province
• 0090/2022/AFJ Joint Scientific Research Project Funding by the National Natural Science Foundation of China and the Macao Science and Technology Development Fund
• MYRG2022-00105-IAPME Multi-Year Research Grant (MYRG) from University of Macau
• 0026/APD/2021 FDCT Funding Scheme for Postdoctoral Researchers
• 21825201 National Natural Science Foundation of China

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Affiliation(s)

Xiaojin Tu State Key Laboratory of Chem/Bio-Sensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan, China
Xiaorong Zhu School of Chemistry and Chemical Engineering, Nantong University, Nantong, Jiangsu, P. R. China
Shuowen Bo National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui, P. R. China
Xiaoran Zhang State Key Laboratory of Chem/Bio-Sensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan, China
Ruping Miao State Key Laboratory of Chem/Bio-Sensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan, China College of Material Science and Engineering, Qingdao University of Science and Technology, Qingdao, Shandong, P. R. China
Guobin Wen State Key Laboratory of Chem/Bio-Sensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan, China
Chen Chen State Key Laboratory of Chem/Bio-Sensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan, China
Jing Li Guangdong-Hong Kong-Macau Joint Laboratory for Photonic-Thermal-Electrical Energy Materials and Devices, Institute of Applied Physics and Materials Engineering, University of Macau, Avenida da Universidade, Taipa, Macau SAR, 999078, P. R. China
Yangyang Zhou State Key Laboratory of Chem/Bio-Sensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan, China
Qinghua Liu National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui, P. R. China
Dawei Chen State Key Laboratory of Chem/Bio-Sensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan, China College of Material Science and Engineering, Qingdao University of Science and Technology, Qingdao, Shandong, P. R. China
Huaiyu Shao Guangdong-Hong Kong-Macau Joint Laboratory for Photonic-Thermal-Electrical Energy Materials and Devices, Institute of Applied Physics and Materials Engineering, University of Macau, Avenida da Universidade, Taipa, Macau SAR, 999078, P. R. China
Dafeng Yan College of Chemistry and Chemical Engineering, Hubei University, Wuhan, Hubei, P. R. China
Yafei Li College of Chemistry and Materials Science, Nanjing Normal University, Nanjing, Jiangsu, P. R. China
Jianfeng Jia Key Laboratory of Magnetic Molecules and Magnetic Information Materials (Ministry of Education), School of Chemistry and Material Science, Shanxi Normal University, Taiyuan, Shanxi, P. R. China
Shuangyin Wang State Key Laboratory of Chem/Bio-Sensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan, China

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Design of Boron and Transition Metal Embedded Two-Dimensional Porous Carbon Nitride for Electrocatalytic Synthesis of Urea

Electrocatalytic coupling of CO and N2 to synthesize urea under ambient conditions is considered a promising strategy to replace traditional industrial technology. It is crucial to find efficient electrocatalysts that can adsorb and activate N2 and promote the C-N coupling reaction. Herein, a new two-dimensional porous carbon nitride material with multiactive sites is designed, in which boron and transition metal are embedded. Through a series of screening, B2Cr2, B2Mn2, and B2Os2 are predicted to be potential electrocatalysts for urea synthesis. Mechanistic studies are performed on bidentate metal-metal and metal-boron sites, and both NCON and CO mechanisms are explored. The electronic structure analysis shows that there is a strong N2 chemical adsorption within the bidentate site and that the N≡N bond is significantly activated. A new mechanism where free CO is inserted for C-N coupling within the two-dimensional porous structure is proposed.

Electrocatalytic Urea Synthesis via N2 Dimerization and Universal Descriptor

Electrocatalytic urea synthesis through N2 + CO2 coreduction and C-N coupling is a promising and sustainable alternative to harsh industrial processes. Despite considerable efforts, limited progress has been made due to the challenges of breaking inert N≡N bonds for C-N coupling, competing side reactions, and the absence of theoretical principles guiding catalyst design. In this study, we propose a mechanism for highly electrocatalytic urea synthesis using two adsorbed N2 molecules and CO as nitrogen and carbon sources, respectively. This mechanism circumvents the challenging step of N≡N bond breaking and selective CO2 to CO reduction, as the free CO molecule inserts into dimerized *N2 and binds concurrently with two N atoms, forming a specific urea precursor *NNCONN* with both thermodynamic and kinetic feasibility. Through the proposed mechanism, Ti2@C4N3 and V2@C4N3 are identified as highly active catalysts for electrocatalytic urea formation, exhibiting low onset potentials of -0.741 and -0.738 V, respectively. Importantly, taking transition metal atoms anchored on porous graphite-like carbonitride (TM2@C4N3) as prototypes, we introduce a simple descriptor, namely, effective d electron number (Φ), to quantitatively describe the structure-activity relationships for urea formation. This descriptor incorporates inherent atomic properties of the catalyst, such as the number of d electrons, the electronegativity of the metal atoms, and the generalized electronegativity of the substrate atoms, making it potentially applicable to other urea catalysts. Our work advances the comprehension of mechanisms and provides a universal guiding principle for catalyst design in urea electrochemical synthesis.

Steering competitive N2 and CO adsorption toward efficient urea production with a confined dual site

Electrocatalytic urea synthesis under mild conditions via the nitrogen (N2) and carbon monoxide (CO) coupling represents an ideal and green alternative to the energy-intensive traditional synthetic protocol. However, this process is challenging due to the more favorable CO adsorption than N2 at the catalytic site, making the formation of the key urea precursor (*NCON) extremely difficult. Herein, we theoretically construct a spatially isolated dual-site (DS) catalyst with the confinement effect to manipulate the competitive CO and N2 adsorption, which successfully guarantees the dominant horizontal N2 adsorption and subsequent efficient *NCON formation via C-N coupling and achieves efficient urea synthesis. Among all the computationally evaluated candidates, the catalyst with dual V sites anchored on 4N-doped graphene (DS-VN4) stands out and shows a moderate energy barrier for C-N coupling and a low theoretical limiting potential of -0.50 V for urea production, which simultaneously suppresses the ammonia production and hydrogen evolution. The confined dual-site introduced in this computational work has the potential to not only properly address part of the challenges toward efficient urea electrosynthesis from CO and N2 but also provide an elegant theoretical strategy for fine-tuning the strength of chemical bonds to achieve a rational catalyst design.

Progress in Photochemical and Electrochemical C-N Bond Formation for Urea Synthesis

ConspectusHere, we discuss recent advances and pressing challenges in achieving sustainable urea synthesis. Urea stands out as the most prevalent nitrogen-based fertilizer used across the globe, making up over 50% of all manufactured fertilizers. Historically, the Bosch-Meiser process has been the go-to chemical manufacturing method for urea production. This procedure, characterized by its high-temperature and high-pressure conditions, reacts ammonia with carbon dioxide to form ammonium carbamate. Subsequently, this ammonium carbamate undergoes dehydration, facilitated by heat, producing solid urea. A concerning aspect of this method is its dependency on fossil fuels, as nearly all the process heat comes from nonrenewable sources. Consequently, the Bosch-Meiser process leaves behind a considerable carbon footprint. Current estimates predict that unchecked, carbon emissions from urea production alone might skyrocket, reaching a staggering 286 MtCO2,eq/yr by 2050. Such projections paint a clear picture regarding the necessity for more eco-friendly, sustainable urea production methods. Recently, the scientific community has shown growing interest in forming C-N bonds using alternative methods. Shifting toward photochemical or electrochemical processes, as opposed to traditional thermal-based processes, promises the potential for complete electrification of urea synthesis. This shift toward process electrification is not just an incremental change; it represents a groundbreaking advancement, the first of many steps, toward achieving deep decarbonization in the chemical manufacturing sector. Since the turn of 2020, there has been a surge in research focusing on photochemical and electrochemical urea synthesis. These methods capitalize on co-reduction of carbon dioxide with nitrogenous reactants like NOx and N2. Despite the progress, there are significant challenges that hinder these processes from reaching their full potential. In this comprehensive review, we shed light on the advances made in electrified C-N bond formation. More importantly, we focus on the invaluable insights gathered over the years, especially concerning catalytic reaction mechanisms. We have dedicated a section to underline key focal areas for up-and-coming research, emphasizing catalyst, electrolyte, and reactor design. It is undeniable that catalyst design remains at the heart of the matter, as managing the co-reduction of two distinct reactants (CO2 and nitrogenous species) is complex. This process results in a myriad of intermediates, which must be adeptly managed to both maintain catalyst activity and avoid catalyst deactivation. Moreover, the electrolytes play a pivotal role, essentially dictating the creation of optimal microenvironments that drive reaction selectivity. Finally, reactor engineering stands out as crucial to ensure optimal mass transport for all involved reactants and subsequent products. We touch upon the broader environmental ramifications of urea production and bring to light potential obstacles for alternative synthesis routes. A notable mention is the urgency of accelerating the uptake and large-scale implementation of renewable energy sources.

Emerging Electrocatalysts in Urea Production

Urea synthesis from abundant CO2 and N-feedstocks via renewable electricity has attracted increasing interests, offering a promising alternative to the industrial-applied Haber-Meiser process. However, the studies toward electrochemical urea production remain scarce and appeal for more research. Herein, in this perspective, an up-to-date overview on the urea electrosynthesis is highlighted and summarized. Firstly, the reaction pathways of urea formation through various feedstocks are comprehensively discussed. Then, we focus on the strategies of materials design to improve C-N coupling efficiency by identifying the descriptor and understanding the reaction mechanism. Finally, the current challenges and disadvantages in this field are reviewed and some future development directions of electrocatalytic urea synthesis are also prospected. This Minireview aims to promote future investigations of the electrochemical urea synthesis.

Efficient C-N coupling in the direct synthesis of urea from CO2 and N2 by amorphous SbxBi1-xOy clusters

Although direct generation of high-value complex molecules and feedstock by coupling of ubiquitous small molecules such as CO2 and N2 holds great appeal as a potential alternative to current fossil-fuel technologies, suitable scalable and efficient catalysts to this end are not currently available as yet to be designed and developed. To this end, here we prepare and characterize SbxBi1-xOy clusters for direct urea synthesis from CO2 and N2 via C-N coupling. The introduction of Sb in the amorphous BiOx clusters changes the adsorption geometry of CO2 on the catalyst from O-connected to C-connected, creating the possibility for the formation of complex products such as urea. The modulated Bi(II) sites can effectively inject electrons into N2, promoting C-N coupling by advantageous modification of the symmetry for the frontier orbitals of CO2 and N2 involved in the rate-determining catalytic step. Compared with BiOx, SbxBi1-xOy clusters result in a lower reaction potential of only -0.3 V vs. RHE, an increased production yield of 307.97 μg h-1 mg-1cat, and a higher Faraday efficiency (10.9%), pointing to the present system as one of the best catalysts for urea synthesis in aqueous systems among those reported so far. Beyond the urea synthesis, the present results introduce and demonstrate unique strategies to modulate the electronic states of main group p-metals toward their use as effective catalysts for multistep electroreduction reactions requiring C-N coupling.

Modified Diacetylmonoxime-Thiosemicarbazide Detection Protocol for Accurate Quantification of Urea

Renewable photo-/electrocatalytic coreduction of CO2 and nitrate to urea is a promising method for high-value utilization of CO2 . However, because of the low yields of the urea synthesis by photo-/electrocatalysis process, the accurate quantification of low concentration urea is challenging. The traditional diacetylmonoxime-thiosemicarbazide (DAMO-TSC) method for urea detection has a high limit of quantification and accuracy, but it is easily affected by NO2 - in the solution, which limits its application scope. Thus, the DAMO-TSC method urgently requires a more rigorous design to eliminate the effects of NO2 - and accurately quantify urea in nitrate systems. Herein, a modified DAMO-TSC method is reported, which consumes NO2 - in solution through a nitrogen release reaction; hence, the remaining products do not affect the accuracy of urea detection. The results of detecting urea solutions with different NO2 - concentrations (within 30 ppm) show that the improved method can effectively control the error of urea detection within 3%.

Efficient urea electrosynthesis from carbon dioxide and nitrate via alternating Cu-W bimetallic C-N coupling sites

Electrocatalytic urea synthesis is an emerging alternative technology to the traditional energy-intensive industrial urea synthesis protocol. Novel strategies are urgently needed to promote the electrocatalytic C-N coupling process and inhibit the side reactions. Here, we report a CuWO₄ catalyst with native bimetallic sites that achieves a high urea production rate (98.5 ± 3.2 μg h^-1 mg^-1_cat) for the co-reduction of CO₂ and NO₃^- with a high Faradaic efficiency (70.1 ± 2.4%) at -0.2 V versus the reversible hydrogen electrode. Mechanistic studies demonstrated that the combination of stable intermediates of *NO₂ and *CO increases the probability of C-N coupling and reduces the potential barrier, resulting in high Faradaic efficiency and low overpotential. This study provides a new perspective on achieving efficient urea electrosynthesis by stabilizing the key reaction intermediates, which may guide the design of other electrochemical systems for high-value C-N bond-containing chemicals.

Synergistic treatment of carbon dioxide and nitrogen-containing wastewater by electrochemical C-N coupling

Electrocatalytic CO2 reduction technology has been considered a promising approach to alleviate the severe environmental and energy issues caused by the anthropogenic over-emission of CO2. Coupling CO2 reduction with nitrogen (N)-pollutants reduction from wastewater to produce higher valued products (e.g., urea, amide, amine, etc.) could significantly extend the application scenarios and product categories of CO2 reduction technologies. This paper investigates the available CO2 and N-pollutants sources and summarizes the recent progress of electrocatalytic C-N coupling reactions. Based on the fundamental research, technical concerns for scale-up applications of C-N coupling electrocatalysis are thoroughly discussed. Finally, we prospect the opportunities and challenges with an in-depth understanding of the underlying dominant factors in applying C-N coupling electrocatalysis. Further development in recycling CO2 and N pollutants via the electrocatalytic C-N coupling process is also discussed.

Light-driven biohybrid system utilizes N2 for photochemical CO2 reduction

Attempting to couple photochemical CO₂ reduction with N₂ fixation is usually difficult, because the reaction conditions for these two processes are typically incompatible. Here, we report that a light-driven biohybrid system can utilize abundant, atmospheric N₂ to produce electron donors via biological nitrogen fixation, to achieve effective photochemical CO₂ reduction. This biohybrid system is constructed by incorporating molecular cobalt-based photocatalysts into N₂-fixing bacteria. It is found that N₂-fixing bacteria can convert N₂ into reductive organic nitrogen and create a localized anaerobic environment, which allows the incorporated photocatalysts to continuously perform photocatalytic CO₂ reduction under aerobic conditions. Specifically, the light-driven biohybrid system displays a high formic acid production rate of over 1.41 × 10^-14 mol h^-1 cell^-1 under visible light irradiation, and the organic nitrogen content undergoes an over-3-fold increase within 48 hours. This work offers a useful strategy for coupling CO₂ conversion with N₂ fixation under mild and environmentally benign conditions.

Engineering Spin States of Isolated Copper Species in a Metal-Organic Framework Improves Urea Electrosynthesis

The catalytic activities are generally believed to be relevant to the electronic states of their active center, but understanding this relationship is usually difficult. Here, we design two types of catalysts for electrocatalytic urea via a coordination strategy in a metal-organic frameworks: CuIII-HHTP and CuII-HHTP. CuIII-HHTP exhibits an improved urea production rate of 7.78 mmol h-1 g-1 and an enhanced Faradaic efficiency of 23.09% at - 0.6 V vs. reversible hydrogen electrode, in sharp contrast to CuII-HHTP. Isolated CuIII species with S = 0 spin ground state are demonstrated as the active center in CuIII-HHTP, different from CuII with S = 1/2 in CuII-HHTP. We further demonstrate that isolated CuIII with an empty [Formula: see text] orbital in CuIII-HHTP experiences a single-electron migration path with a lower energy barrier in the C-N coupling process, while CuII with a single-spin state ([Formula: see text]) in CuII-HHTP undergoes a two-electron migration pathway.

Stabilization of transition metal heterojunctions inside porous materials for high-performance catalysis

Transition metal-based heterostructural materials are a class of very promising substitutes for noble metal-based catalysts for high-performance catalysis, due to their inherent internal electric field at the interface in the heterojunctions, which could induce electron relocalization and facilitate charge carrier migration between different metal sites at heterostructural boundaries. However, redox-active metal species suffer from reduction, oxidation, migration, aggregation, leaching and poisoning in catalysis, which results in heavy deterioration of the catalytic properties of transition metal-based heterojunctions and frustrates their practical applications. To improve the stability of transition metal-based heterojunctions and sufficiently expose redox-active sites at the heterosurfaces, many kinds of porous materials have been used as porous hosts for the stabilization of non-precious metal heterojunctions. This review article will discuss recently developed strategies for encapsulation and stabilization of transition metal heterojunctions inside porous materials, and highlight their improved stability and catalytic performance through the spatial confinement effect and synergistic interaction between the heterojunctions and the host matrices.

Co engineered CoP catalyst for photochemical CO2 reduction with accelerated electron transfer endowed by the space-charge region

Photocatalytic CO₂ reduction has been regarded as an ideal method to simulate photosynthesis for achieving carbon neutralization. However, poor charge transfer efficiency limits its development. Herein, an efficient Co/CoP@C catalyst was prepared with compact contact of Co and CoP layer by using MOF as precursor. At the interface of Co/CoP, the difference in functionality between the two phases may result in uneven distribution of electrons, thus forming a self-driven space-chare region. In this region, spontaneous electron transfer is guaranteed, thus facilitating the effective separation of photogenerated carriers as well as boosting the utilization of solar energy. Furthermore, the electron density of active site Co in CoP is increased and more active sites are exposed, which promotes the adsorption and activation of CO₂ molecules. Together with suitable redox potential, low energy barrier for *COOH formation and easy desorption of CO, the reduction rate of CO₂ catalyzed by Co/CoP@C is 4 times higher than that of CoP@C.

Electrochemical Co-reduction of N2 and CO2 to Urea Using Bi2S3 Nanorods Anchored to N-Doped Reduced Graphene Oxide

Producing "green urea" using renewable energy, N₂, and CO₂ is a long-considered challenge. Herein, an electrocatalyst, Bi₂S₃/N-reduced graphene oxide (RGO), was synthesized by loading the Bi₂S₃ nanorods onto the N-RGO via a hydrothermal method. The Bi₂S₃/N-RGO composites exhibit the highest yield of urea (4.4 mmol g^-1 h^-1), which is 12.6 and 3.1 times higher than that of Bi₂S₃ (0.35 mmol g^-1 h^-1) and that of N-RGO (1.4 mmol g^-1 h^-1), respectively. N-RGO, because of its porous and open-layer structure, improves the mass transfer efficiency and stability, while the basic groups (-OH and -NH₂) promote the adsorption and activation of CO₂. Bi₂S₃ promotes the absorption and activation of inert N₂. Finally, the defect sites and the synergistic effect on the Bi₂S₃/N-RGO composites work simultaneously to form urea from N₂ and CO₂. This study provides new insights into urea synthesis under ambient conditions and a strategy for the design and development of a new material for green urea synthesis.

Electrochemical C-N coupling of CO2 and nitrogenous small molecules for the electrosynthesis of organonitrogen compounds

Electrochemical C-N coupling reactions based on abundant small molecules (such as CO₂ and N₂) have attracted increasing attention as a new "green synthetic strategy" for the synthesis of organonitrogen compounds, which have been widely used in organic synthesis, materials chemistry, and biochemistry. The traditional technology employed for the synthesis of organonitrogen compounds containing C-N bonds often requires the addition of metal reagents or oxidants under harsh conditions with high energy consumption and environmental concerns. By contrast, electrosynthesis avoids the use of other reducing agents or oxidants by utilizing "electrons", which are the cleanest "reagent" and can reduce the generation of by-products, consistent with the atomic economy and green chemistry. In this study, we present a comprehensive review on the electrosynthesis of high value-added organonitrogens from the abundant CO₂ and nitrogenous small molecules (N₂, NO, NO₂^-, NO₃^-, NH₃, etc.) via the C-N coupling reaction. The associated fundamental concepts, theoretical models, emerging electrocatalysts, and value-added target products, together with the current challenges and future opportunities are discussed. This critical review will greatly increase the understanding of electrochemical C-N coupling reactions, and thus attract research interest in the fixation of carbon and nitrogen.