Te-Doped Pd Nanocrystal for Electrochemical Urea Production by Efficiently Coupling Carbon Dioxide Reduction with Nitrite Reduction

Recent progress in electrochemical C-N coupling: metal catalyst strategies and applications

Electrochemical C-N coupling reactions hold significant importance in the fields of organic chemistry and green chemistry. Conventional methods for constructing C-N bonds typically rely on high temperatures, high pressures, and other conditions that are energy-intensive and prone to generating environmental pollutants. In contrast, the electrochemical approaches employ electrical energy as the driving force to achieve C-N bond formation under ambient conditions, representing a more environment-friendly and sustainable alternative. The notable advantages of electrochemical C-N coupling include high efficiency, good selectivity, and mild reaction conditions. Through rational design of corresponding electrocatalysts, it is possible to achieve efficient C-N bond coupling at low potentials. Moreover, the electrochemical methods allow for precise control over reaction conditions, thereby avoiding side reactions and by-products that are common for conventional methods, improving both selectivity and product purity. Despite the extensive research efforts devoted to exploring the potential of electrochemical C-N coupling, the design of efficient and stable metal catalysts remains a significant challenge. In this review, we summarize and evaluate the latest strategies developed for designing metal catalysts, and their application prospects for different nitrogen sources such as N2 and NOx. We delineate how the control over nanoscale structures, morphologies, and electronic properties of metal catalysts can optimize their performance in C-N coupling reactions, and discuss the performances and advantages of single-metal catalysts, bimetallic catalysts, and single-atom catalysts under various reaction conditions. By summarizing the latest research achievements, particularly in the development of high-efficiency catalysts, the application of novel catalyst materials, and the in-depth study of reaction mechanisms, this review aims to provide insights for future research in the field of electrochemical C-N coupling, and demonstrates that rationally designed metal catalysts could not only enhance the efficiency and selectivity of electrochemical C-N coupling reactions, but also offer conceptual frameworks for other electrochemical reactions.

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.

Efficient Electrosynthesis of Urea over Single-Atom Alloy with Electronic Metal Support Interaction

Urea electrosynthesis from carbon dioxide (CO2) and nitrate (NO3 -) is an alternative approach to traditional energy-intensive urea synthesis technology. Herein, we report a CuAu single-atom alloy (SAA) with electronic metal support interaction (EMSI), achieving a high urea yield rate of 813.6 μg h-1 mgcat -1 at -0.94 V versus reversible hydrogen electrode (vs. RHE) and a Faradaic efficiency (FE) of 45.2 % at -0.74 V vs. RHE. In situ experiments and theoretical calculations demonstrated that single-atom Cu sites modulate the adsorption behavior of intermediate species. Bimetallic sites synergistically accelerate C-N bond formation through spontaneous coupling of *CO and *NO to form *ONCO as key intermediates. More importantly, electronic metal support interaction between CuAu SAA and CeO2 carrier further modulates electron structure and interfacial microenvironment, endowing electrocatalysts with superior activity and durability. This work constructs SAA electrocatalysts with EMSI effect to tailor C-N coupling at the atomic level, which can provide guidance for the development of C-N coupling systems.

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.

FeCu bimetallic clusters for efficient urea production via coupling reduction of carbon dioxide and nitrate

Urea electrosynthesis has appeared to meet the nitrogen cycle and carbon neutrality with energy-saving features. Copper can co-electrocatalyze among CO2 and nitrogen species to generate urea, however developing effective electrocatalysts is still an obstacle. Here, we developed a nitrogen-doped porous carbon loaded with FeCu clusters that convert CO2 and NO3- into urea, with the highest Faradaic efficiency of 39.8 % and yield rate of 1024.6 μg h-1 mgcat.-1, under optimized ambient conditions, exceeding that at the Fe or Cu homogeneous sites. Furthermore, a favorable CN coupling pathway originates from *NHCO and *NHCONO two intermediates with lower free energy barriers on FeCu dual active sites are verified through in-situ Fourier transform infrared spectroscopy and theoretical calculations. This research might provide deep insights into coupling mechanisms and investigation of efficient catalysts for green urea production.

Electrochemical Synthesis of Urea: Co-Reduction of Nitrite and Carbon Dioxide on Binuclear Cobalt Phthalocyanine

Exploration of molecular catalysts with the atomic-level tunability of molecular structures offers promising avenues for developing high-performance catalysts for the electrochemical co-reduction reaction of carbon dioxide (CO2) and nitrite (NO2 -) into value-added urea. In this work, a binuclear cobalt phthalocyanine (biCoPc) catalyst is prepared through chemical synthesis and applied as a C─N coupling catalyst toward urea. Achieving a remarkable Faradaic efficiency of 47.4% for urea production at -0.5 V versus reversible hydrogen electrode (RHE), this biCoPc outperforms many known molecular catalysts in this specific application. Its unique planar macromolecular structure and the increased valence state of cobalt promote the adsorption of nitrogenous and carbonaceous species, a critical factor in facilitating the multi-electron C─N coupling. Combining highly sensitive in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) with density functional theory (DFT) calculations, the linear adsorbed CO (COL) and bridge adsorbed CO (COB) is captured on biCoPc catalyst during the co-reduction reaction. COB, a pivotal intermediate in the co-reduction from CO2 and nitrite to urea, is evidenced to be labile and may be attacked by nitrite, promoting urea production. This work demonstrates the importance of designing molecular catalysts for efficient co-reduction of CO2 and nitrite to urea.

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.

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.

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.

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.

Molecular tuning boosts asymmetric C-C coupling for CO conversion to acetate

Electrochemical carbon dioxide/carbon monoxide reduction reaction offers a promising route to synthesize fuels and value-added chemicals, unfortunately their activities and selectivities remain unsatisfactory. Here, we present a general surface molecular tuning strategy by modifying Cu2O with a molecular pyridine-derivative. The surface modified Cu2O nanocubes by 4-mercaptopyridine display a high Faradaic efficiency of greater than 60% in electrochemical carbon monoxide reduction reaction to acetate with a current density as large as 380 mA/cm2 in a liquid electrolyte flow cell. In-situ attenuated total reflectance surface-enhanced infrared absorption spectroscopy reveals stronger *CO signal with bridge configuration and stronger *OCCHO signal over modified Cu2O nanocubes by 4-mercaptopyridine than unmodified Cu2O nanocubes during electrochemical CO reduction. Density function theory calculations disclose that local molecular tuning can effectively regulate the electronic structure of copper catalyst, enhancing *CO and *CHO intermediates adsorption by the stabilization effect through hydrogen bonding, which can greatly promote asymmetric *CO-*CHO coupling in electrochemical carbon monoxide reduction reaction.

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.

FeOOH with Low Spin State Iron as Electron Acceptors for High Yield Rate Electrosynthesis of Urea from Nitrate and Carbon Dioxide

Co electroreduction of carbon dioxide and nitrate to synthesize urea provides an alternative strategy to high energy-consumption traditional methods. However, the complexity of the reaction mechanism and the high energy barrier of nitrate reduction result in a diminished production of urea. Herein, a convenient electrodeposition technique to prepare the FeOOH with low spin state iron that increases the yield rate of urea efficiently is employed. According to soft X-ray Absorption Spectroscopy and theoretical calculations, the unique configuration of low spin state iron as electron acceptors can effectively induce electron pair transfer from the occupied σ orbitals of intermediate * NO to empty d orbitals of iron. This σ→d donation mechanism leads to a reduction in the energy barrier associated with the rate-determining step (* NOOH→* NO + * OH), hence augmenting the urea generation. The low spin state iron presents a high urea yield rate of 512 µg h-1  cm-2 , representing approximately two times compared to the medium spin state iron. The key intermediates (* NH2 and * CO) in the formation of C─N bond are detected with in situ Fourier transform infrared spectroscopy. The coupling of * NH2 and * CO contributes to the formation of * CONH2 , which subsequently endures multi-step proton-coupled electron transfer to generate urea.

Tuning Intermediates Adsorption and C─N Coupling for Efficient Urea Electrosynthesis Via Doping Ni into Cu

Simultaneous electrochemical reduction of nitrite and carbon dioxide (CO2 ) under mild reaction conditions offers a new sustainable and low-cost approach for urea synthesis. However, the development of urea electrosynthesis thus far still suffers from low selectivity due to the high energy barrier of * CO formation and the subsequent C─N coupling. In this work, a highly active dendritic Cu99 Ni1 catalyst is developed to enable the highly selective electrosynthesis of urea from co-reduction of nitrite and CO2 , reaching a urea Faradaic efficiency (FE) and production rate of 39.8% and 655.4 µg h-1  cm-2 , respectively, at -0.7 V versus reversible hydrogen electrode (RHE). In situ Fourier-transform infrared spectroscopy (FT-IR) measurements together with density functional theory (DFT) calculations demonstrate that Ni doping into Cu can significantly enhance the adsorption energetics of the key reaction intermediates and facilitate the C─N coupling. This work not only provides a new strategy to design efficient electrocatalysts for urea synthesis but also offers deep insights into the mechanism of C─N coupling during the co-reduction of nitrite and CO2 .

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.

Exploration of a novel electrochemical CN coupling process: Urea synthesis from direct air carbon capture with nitrate wastewater

Direct air capture (DAC) can be used to decrease the CO2 concentration in the atmosphere, but this requires substantial energy consumption. If residual waste carbon (in the form of bicarbonate solution) from DAC can be directly reused, it might present a novel method for overcoming the aforementioned challenges. Electrochemical CN coupling methods for synthesizing urea have garnered considerable attention for waste carbon utilization, but the carbon source is high-purity CO2. No research has been conducted regarding the application of bicarbonate solution as the carbon source. This study proposes a proof-of-concept electrochemical CN coupling process for synthesizing urea using bicarbonate solution from DAC as the carbon source and nitrate from wastewater as the nitrogen source. These results confirmed the feasibility of synthesizing urea using a three-electrode system employing TF and CuInS2/TF as the working electrodes via potentiostatic electrolysis. Under the optimal conditions (initial pH 5.0 and applied potential of -1.3 V vs. Ag/AgCl), the urea yield after 2 h of electrolysis reached 3017.2 μg h-1 mgcat.-1 and an average Faradaic efficiency of 19.6 %. The in-situ attenuated total reflection surface-enhanced infrared absorption spectroscopy indicated a gradual increase in the intensity of the -CONH bond signal on the surface of the CuInS2/TF electrode as the reaction progressed. This implied that this bond may be a key chemical group in this process. The density functional theory calculations demonstrated that *CONH was a pivotal intermediate during CN coupling, and a two-step CN coupling reaction path was proposed. *NH + *CO primarily transformed into *CONH, followed by the conversion reaction of *CONH + *NO to *NOCONH2. This study offers a groundbreaking approach for waste carbon utilization from DAC and holds the potential to furnish technical underpinnings for advancing electrochemical CN coupling methods.

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.

Critical Review of Pd-Catalyzed Reduction Process for Treatment of Waterborne Pollutants

Catalyzed reduction processes have been recognized as important and supplementary technologies for water treatment, with the specific aims of resource recovery, enhancement of bio/chemical-treatability of persistent organic pollutants, and safe handling of oxygenate ions. Palladium (Pd) has been widely used as a catalyst/electrocatalyst in these reduction processes. However, due to the limited reserves and high cost of Pd, it is essential to gain a better understanding of the Pd-catalyzed decontamination process to design affordable and sustainable Pd catalysts. This review provides a systematic summary of recent advances in understanding Pd-catalyzed reductive decontamination processes and designing Pd-based nanocatalysts for the reductive treatment of water-borne pollutants, with special focus on the interactions and transformation mechanisms of pollutant molecules on Pd catalysts at the atomic scale. The discussion begins by examining the adsorption of pollutants onto Pd sites from a thermodynamic viewpoint. This is followed by an explanation of the molecular-level reaction mechanism, demonstrating how electron-donors participate in the reductive transformation of pollutants. Next, the influence of the Pd reactive site structure on catalytic performance is explored. Additionally, the process of Pd-catalyzed reduction in facilitating the oxidation of pollutants is briefly discussed. The longevity of Pd catalysts, a crucial factor in determining their practicality, is also examined. Finally, we argue for increased attention to mechanism study, as well as precise construction of Pd sites under batch synthesis conditions, and the use of Pd-based catalysts/electrocatalysts in the treatment of concentrated pollutants to facilitate resource recovery.

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

Collapse

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

Collapse

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

Collapse

An In Situ Fabricated Hydrogel Polymer - Palladium Nanocomposite Electrocatalyst for the HER: Critical Role of the Polymer in Realizing High Efficiency and Stability

Development of general and simple designs of catalytic electrodes for the hydrogen evolution reaction (HER) is critical. The present work demonstrates the multiple roles played by a hydrogel polymer in the fabrication and activity enhancement of the nanoelectrocatalyst. A nanocomposite thin film of Pd with the insulating hydrogel, poly(2-hydroxyethyl methacrylate) (PHEMA), is fabricated through a facile in situ process, the polymer itself functioning as the reducing/stabilizing agent in the formation of Pd nanoparticles. Pd-PHEMA on Ni foam enables efficient HER in alkaline medium with a low overpotential; the polymer enables the electrocatalysis by its swelling and confinement of the electrolyte. Most significantly, when the electrode is subjected to an optimized cycling protocol, the overpotential decreases steadily, reaching an impressively low value of 36 mV (@10 mA cm-2 ). A low Tafel slope (68 mV dec-1 ), high exchange current density, Faradaic efficiency and TOF (3.27 mA cm-2 , 99 %, 122.7 h-1 ), and extended stability are achieved. Detailed investigations reveal the active role of the polymer in the evolution of the nanocatalyst, itself undergoing favorable morphological changes. The study illustrates the widened scope for developing efficient and stable catalytic electrodes with hydrogel polymers and unique features that promote the generation of green hydrogen.

Electrochemical C-N Bond Formation within Boron Imidazolate Cages Featuring Single Copper Sites

Electrocatalysis expands the ability to generate industrially relevant chemicals locally and on-demand with intermittent renewable energy, thereby improving grid resiliency and reducing supply logistics. Herein, we report the feasibility of using molecular copper boron-imidazolate cages, BIF-29(Cu), to enable coupling between the electroreduction reaction of CO2 (CO2RR) with NO3- reduction (NO3RR) to produce urea with high selectivity of 68.5% and activity of 424 μA cm-2. Remarkably, BIF-29(Cu) is among the most selective systems for this multistep C-N coupling to-date, despite possessing isolated single-metal sites. The mechanism for C-N bond formation was probed with a combination of electrochemical analysis, in situ spectroscopy, and atomic-scale simulations. We found that NO3RR and CO2RR occur in tandem at separate copper sites with the most favorable C-N coupling pathway following the condensation between *CO and NH2OH to produce urea. This work highlights the utility of supramolecular metal-organic cages with atomically discrete active sites to enable highly efficient coupling reactions.

Self-supported P-doped NiFe2O4 micro-sheet arrays for the efficient conversion of nitrite to ammonia

The nitrite reduction reaction (NO2-RR) is an important process for eliminating toxic nitrites from water while simultaneously producing high-value ammonia under ambient conditions. For the aim to improve the NO2-RR efficiency, we designed a new synthetic strategy to prepare a phosphorus-doped three-dimensional NiFe2O4 catalyst loaded onto a nickel foam in-situ and evaluated its performance for the reduction of NO2- to NH3. The catalyst achieved a high Faradaic efficiency (FE) of 95.39%, and an ammonia (NH3) yield rate of 34788.51 µg h-1 cm-2 at - 0.45 V vs. RHE. A high NH3 yield rate and FE were maintained after 16 cycles at - 0.35 V vs. RHE in an alkaline electrolyte. This study provides a new direction for the rational design of highly stable electrocatalysts for the conversion of NO2- to NH3.

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.

Kinetically matched C-N coupling toward efficient urea electrosynthesis enabled on copper single-atom alloy

Chemical C-N coupling from CO2 and NO3-, driven by renewable electricity, toward urea synthesis is an appealing alternative for Bosch-Meiser urea production. However, the unmatched kinetics in CO2 and NO3- reduction reactions and the complexity of C- and N-species involved in the co-reduction render the challenge of C-N coupling, leading to the low urea yield rate and Faradaic efficiency. Here, we report a single-atom copper-alloyed Pd catalyst (Pd4Cu1) that can achieve highly efficient C-N coupling toward urea electrosynthesis. The reduction kinetics of CO2 and NO3- is regulated and matched by steering Cu doping level and Pd4Cu1/FeNi(OH)2 interface. Charge-polarized Pdδ--Cuδ+ dual-sites stabilize the key *CO and *NH2 intermediates to promote C-N coupling. The synthesized Pd4Cu1-FeNi(OH)2 composite catalyst achieves a urea yield rate of 436.9 mmol gcat.-1 h-1 and Faradaic efficiency of 66.4%, as well as a long cycling stability of 1000 h. In-situ spectroscopic results and theoretical calculation reveal that atomically dispersed Cu in Pd lattice promotes the deep reduction of NO3- to *NH2, and the Pd-Cu dual-sites lower the energy barrier of the pivotal C-N coupling between *NH2 and *CO.

Photocatalytic Co-Reduction of N2 and CO2 with CeO2 Catalyst for Urea Synthesis

The effective conversion of carbon dioxide (CO2 ) and nitrogen (N2 ) into urea by photocatalytic reaction under mild conditions is considered to be a more environmentally friendly and promising alternative strategies. However, the weak adsorption and activation ability of inert gas on photocatalysts has become the main challenge that hinder the advancement of this technique. Herein, we have successfully established mesoporous CeO2-x nanorods with adjustable oxygen vacancy concentration by heat treatment in Ar/H2 (90 % : 10 %) atmosphere, enhancing the targeted adsorption and activation of N2 and CO2 by introducing oxygen vacancies. Particularly, CeO2 -500 (CeO2 nanorods heated treatment at 500 °C) revealed high photocatalytic activity toward the C-N coupling reaction for urea synthesis with a remarkable urea yield rate of 15.5 μg/h. Besides, both aberration corrected transmission electron microscopy (AC-TEM) and Fourier transform infrared (FT-IR) spectroscopy were used to research the atomic surface structure of CeO2 -500 at high resolution and to monitor the key intermediate precursors generated. The reaction mechanism of photocatalytic C-N coupling was studied in detail by combining Density Functional Theory (DFT) with specific experiments. We hope this work provides important inspiration and guiding significance towards highly efficient photocatalytic synthesis of urea.

Recent advances of H-intercalated Pd-based nanocatalysts for electrocatalytic reactions

The intercalation of H into Pd-based nanocatalysts plays a crucial role in optimizing the catalytic performance by tailoring the structural and electronic properties. We herein present a comprehensive review about the recent progress of interstitial hydrogen atom modified Pd-based nanocatalysts for various energy-related electrocatalytic reactions. Before systematically manifesting the great potential of Pd-based hydrides for electrocatalytic applications, we have briefly illustrated the synthesis strategies and corresponding mechanisms for the Pd-based hydrides. This is followed by a comprehensive discussion about the fundamentals and functions of H intercalation in tailoring their physicochemical and electrochemical properties. Subsequently, we focus on the widespread application of Pd-based hydrides for electrocatalytic reactions, with the emphasis on the role of H intercalation played in determining electrocatalytic performance. Finally, the future direction and perspectives regarding the development of more efficient Pd-based hydrides are also manifested.

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.

Low-Dimensional High-Entropy Alloys for Advanced Electrocatalytic Reactions

Low-dimensional high-entropy alloy (HEA) nanomaterials are widely employed as electrocatalysts for energy conversion reactions, due to their inherent advantages, including high electron mobility, rich catalytically active site, optimal electronic structure. Moreover, the high-entropy, lattice distortion, and sluggish diffusion effects also enable them to be promising electrocatalysts. A thorough understanding on the structure-activity relationships of low-dimensional HEA catalyst play a huge role in the future pursuit of more efficient electrocatalysts. In this review, we summarize the recent progress of low-dimensional HEA nanomaterials for efficient catalytic energy conversion. By systematically discussing the fundamentals of HEA and properties of low-dimensional nanostructures, we highlight the advantages of low-dimensional HEAs. Subsequently, we also present many low-dimensional HEA catalysts for electrocatalytic reactions, aiming to gain a better understanding on the structure-activity relationship. Finally, a series of upcoming challenges and issues are also thoroughly proposed as well as their future directions.

Electrochemical Wastewater Refining: A Vision for Circular Chemical Manufacturing

Wastewater is an underleveraged resource; it contains pollutants that can be transformed into valuable high-purity products. Innovations in chemistry and chemical engineering will play critical roles in valorizing wastewater to remediate environmental pollution, provide equitable access to chemical resources and services, and secure critical materials from diminishing feedstock availability. This perspective envisions electrochemical wastewater refining─the use of electrochemical processes to tune and recover specific products from wastewaters─as the necessary framework to accelerate wastewater-based electrochemistry to widespread practice. We define and prescribe a use-informed approach that simultaneously serves specific wastewater-pollutant-product triads and uncovers a mechanistic understanding generalizable to broad use cases. We use this approach to evaluate research needs in specific case studies of electrocatalysis, stoichiometric electrochemical conversions, and electrochemical separations. Finally, we provide rationale and guidance for intentionally expanding the electrochemical wastewater refining product portfolio. Wastewater refining will require a coordinated effort from multiple expertise areas to meet the urgent need of extracting maximal value from complex, variable, diverse, and abundant wastewater resources.

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%.

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.

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.

Dynamic Reconstitution Between Copper Single Atoms and Clusters for Electrocatalytic Urea Synthesis

Electrocatalytic CN coupling between carbon dioxide and nitrate has emerged to meet the comprehensive demands of carbon footprint closing, valorization of waste, and sustainable manufacture of urea. However, the identification of catalytic active sites and the design of efficient electrocatalysts remain a challenge. Herein, the synthesis of urea catalyzed by copper single atoms decorated on a CeO₂ support (denoted as Cu₁ -CeO₂ ) is reported. The catalyst exhibits an average urea yield rate of 52.84 mmol h^-1 g_cat. ^-1 at -1.6 V versus reversible hydrogen electrode. Operando X-ray absorption spectra demonstrate the reconstitution of copper single atoms (Cu₁ ) to clusters (Cu₄ ) during electrolysis. These electrochemically reconstituted Cu₄ clusters are real active sites for electrocatalytic urea synthesis. Favorable CN coupling reactions and urea formation on Cu₄ are validated using operando synchrotron-radiation Fourier transform infrared spectroscopy and theoretical calculations. Dynamic and reversible transformations of clusters to single-atom configurations occur when the applied potential is switched to an open-circuit potential, endowing the catalyst with superior structural and electrochemical stabilities.

Metal functionalization of two-dimensional nanomaterials for electrochemical carbon dioxide reduction

With the mechanical exfoliation of graphene in 2004, researchers around the world have devoted significant efforts to the study of two-dimensional (2D) nanomaterials. Nowadays, 2D nanomaterials are being developed into a large family with varieties of structures and derivatives. Due to their fascinating electronic, chemical, and physical properties, 2D nanomaterials are becoming an important type of catalyst for the electrochemical carbon dioxide reduction reaction (CO₂RR). Here, we review the recent progress in electrochemical CO₂RR using 2D nanomaterial-based catalysts. First, we briefly describe the reaction mechanism of electrochemical CO₂ reduction to single-carbon (C₁) and multi-carbon (C₂₊) products. Then, we discuss the strategies and principles for applying metal materials to functionalize 2D nanomaterials, such as graphene-based materials, metal-organic frameworks (MOFs), and transition metal dichalcogenides (TMDs), as well as applications of resultant materials in the electrocatalytic CO₂RR. Finally, we summarize the present research advances and highlight the current challenges and future opportunities of using metal-functionalized 2D nanomaterials in the electrochemical CO₂RR.

A universal synthesis of ultrathin Pd-based nanorings for efficient ethanol electrooxidation

Metallic nanorings (NRs) with open hollow structures are of particular interest in energy-related catalysis due to their unique features, which include the high utilization of active sites and facile accessibility for reactants. However, there is still a lack of general methods for synthesizing Pd-based multimetallic NRs with a high catalytic performance. Herein, we develop a template-directed strategy for the synthesis of ultrathin PdM (M = Bi, Sb, Pb, BiPb) NRs with a tunable size. Specifically, ultrathin Pd nanosheets (NSs) are used as a template to steer the deposition of M atoms and the interatomic diffusion between Pd and M, subsequently resulting in the hollow structured NRs. Taking the ethanol oxidation reaction (EOR) as a proof-of-concept application, the PdBi NRs deliver a substantially improved activity relative to the Pd NSs and commercial Pd/C catalysts, simultaneously showing outstanding stability and CO tolerance. Mechanistically, density functional theory (DFT) calculations disclose that the incorporation of Bi reduces the energy barrier of the rate-determining step in the EOR C₂-path, which, together with the high ratio of exposed active sites, endows the PdBi NRs with an excellent EOR activity. We believe that our work can illuminate the general synthesis of multimetallic NRs and the rational design of advanced electrocatalysts.

Ultrasonic-assisted hydrothermal synthesis of RhCu alloy nanospheres for electrocatalytic urea production

Herein, the electronic structure of RhCu nanospheres was optimized and the size of the nanoparticles was reduced by an ultrasonic-assisted hydrothermal method. The performance of electrocatalytic urea synthesis was improved with an enhanced faradaic efficiency and urea yield rate of 34.82 ± 2.47% and 26.81 ± 0.62 mmol g^-1 h^-1, respectively. This work opens a novel insight into synthesizing an electrocatalyst by ultrasonic treatment for urea production.

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.

Fascinating Electrocatalysts with Dispersed Di-Metals in MN3 -M'N4 Moiety as Two Active Sites Separately for N2 and CO2 Reduction Reactions and Jointly for CN Coupling and Urea Production

The idealized urea electrocatalyst is crucial to boost the CN coupling reaction and simultaneously suppress their isolated reduction process after adsorbing N₂ and CO₂ molecules. Therefore, the dispersed MN₃ -M'N₄ moiety is investigated systematically, including 26 homonuclear and 650 heteronuclear di-metal systems. After, 205 stable systems are selected using lowest-energy principle and ab initio molecular dynamics simulations. According to three possible pathways, NCON, CO, and OCOH to produce urea, a five-step high-throughput screening method for excellent catalytic activity and a five-aspect high-throughput screening strategy for outstanding catalytic selectivity are proposed, respectively. The potential determined steps and the limiting potential through three pathways are identified. The data indicates both CO pathway and OCOH pathway are more competitive at lower Gibbs free energy. Significantly, the most favorite RuN₃ -CoN₄ combination possesses an extremely low limiting potential of -0.80 V for urea production, meanwhile it exists a strong foundation for experimental preparation. This work not only broadens electrocatalytic potentiality of developing di-metals as two active sites, but also provides a feasible high-throughput screening recipe for urea production.

Stable and Ordered Body-Centered Cubic PdCu Phase for Highly Selective Hydrogenation

Phase engineering of nanomaterials plays a crucial role for regulating the catalytic performance. Nevertheless, great challenges still remain for elucidating the structure-selectivity correlation. Herein, this study demonstrates that the body-centered cubic phase of PdCu (bcc-PdCu) can serve as a highly active and selective catalyst for 3-nitrostyrene (NS) hydrogenation under mild conditions. In particular, bcc-PdCu displays a 3-nitro-ethylbenzene (NE) selectivity of 93.8% with a turnover frequency (TOF) value of 4573 h^-1 at 30 °C in the presence of H₂ . With the assistance of NH₃ ∙BH₃ , the selectivity of 3-amino-styrene (AS) reaches 94.5% with a TOF value of 13 719 h^-1 . Detailed experimental and theoretical calculations reveal that improved NE selectivity is ascribed to the selective adsorption of the CC bond and desorption of NE on bcc-PdCu. Moreover, the presence of NH₃ ∙BH₃ facilitates the selective hydrogenation of NO₂ due to their strong interaction and thus leads to the formation of AS. This work provides an efficient selective catalyst for NS hydrogenation under mild conditions, which may attract immediate interests in the fields of materials, chemistry, and catalysis.

Review on Electrocatalytic Coreduction of Carbon Dioxide and Nitrogenous Species for Urea Synthesis

The electrochemical coreduction of carbon dioxide (CO₂) and nitrogenous species (such as NO₃^-, NO₂^-, N₂, and NO) for urea synthesis under ambient conditions provides a promising solution to realize carbon/nitrogen neutrality and mitigate environmental pollution. Although an increasing number of studies have made some breakthroughs in electrochemical urea synthesis, the unsatisfactory Faradaic efficiency, low urea yield rate, and ambiguous C-N coupling reaction mechanisms remain the major obstacles to its large-scale applications. In this review, we present the recent progress on electrochemical urea synthesis based on CO₂ and nitrogenous species in aqueous solutions under ambient conditions, providing useful guidance and discussion on the rational design of metal nanocatalyst, the understanding of the C-N coupling reaction mechanism, and existing challenges and prospects for electrochemical urea synthesis. We hope that this review can stimulate more insights and inspiration toward the development of electrocatalytic urea synthesis technology.

Multi-heterointerfaces for selective and efficient urea production

A major impediment to industrial urea synthesis is the lack of catalysts with high selectivity and activity, which inhibits the efficient industrial production of urea. Here, we report a new catalyst system suitable for the highly selective synthesis of industrial urea by in situ growth of graphdiyne on the surface of cobalt-nickel mixed oxides. Such a catalyst is a multi-heterojunction interfacial structure resulting in the obvious incomplete charge-transfer phenomenon between a graphdiyne and metal oxide interface and multiple intermolecular interactions. These intrinsic characteristics are the origin of the high performance of the catalyst. Studies on the mechanism reveal that the catalyst could effectively optimize the adsorption/desorption capacities of the intermediate and promote direct C-N coupling by significantly suppressing by-product reactions toward the formation of H2, CO, N2 and NH3. The catalyst can selectively synthesize urea directly from nitrite and carbon dioxide in water at room temperature and pressure, and exhibits a record-high Faradaic efficiency of 64.3%, nitrogen selectivity (Nurea-selectivity) of 86.0%, carbon selectivity (Curea-selectivity) of ∼100%, as well as urea yield rates of 913.2 μg h-1 mgcat -1 and remarkable long-term stability.

Ambient Electrosynthesis of Urea with Nitrate and Carbon Dioxide over Iron-Based Dual-Sites

The development of efficient electrocatalysts to generate key *NH₂ and *CO intermediates is crucial for ambient urea electrosynthesis with nitrate (NO₃ ^- ) and carbon dioxide (CO₂ ). Here we report a liquid-phase laser irradiation method to fabricate symbiotic graphitic carbon encapsulated amorphous iron and iron oxide nanoparticles on carbon nanotubes (Fe(a)@C-Fe₃ O₄ /CNTs). Fe(a)@C-Fe₃ O₄ /CNTs exhibits superior electrocatalytic activity toward urea synthesis using NO₃ ^- and CO₂ , affording a urea yield of 1341.3±112.6 μg h^-1  mg_cat ^-1 and a faradic efficiency of 16.5±6.1 % at ambient conditions. Both experimental and theoretical results indicate that the formed Fe(a)@C and Fe₃ O₄ on CNTs provide dual active sites for the adsorption and activation of NO₃ ^- and CO₂ , thus generating key *NH₂ and *CO intermediates with lower energy barriers for urea formation. This work would be helpful for design and development of high-efficiency dual-site electrocatalysts for ambient urea synthesis.