Non-metal boron atoms on a CuB12 monolayer as efficient catalytic sites for urea production

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.

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.

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.

Electrocatalytic CO2 Reduction to Ethylene: From Advanced Catalyst Design to Industrial Applications

The value-added chemicals, monoxide, methane, ethylene, ethanol, ethane, and so on, can be efficiently generated through the electrochemical CO2 reduction reaction (eCO2 RR) when equipped with suitable catalysts. Among them, ethylene is particularly important as a chemical feedstock for petrochemical manufacture. However, despite its high Faradaic efficiency achievable at relatively low current densities, the substantial enhancement of ethylene selectivity and stability at industrial current densities poses a formidable challenge. To facilitate the industrial implementation of eCO2 RR for ethylene production, it is imperative to identify key strategies and potential solutions through comprehending the recent advancements, remaining challenges, and future directions. Herein, the latest and innovative catalyst design strategies of eCO2 RR to ethylene are summarized and discussed, starting with the properties of catalysts such as morphology, crystalline, oxidation state, defect, composition, and surface engineering. The review subsequently outlines the related important state-of-the-art technologies that are essential in driving forward eCO2 RR to ethylene into practical applications, such as CO2 capture, product separation, and downstream reactions. Finally, a greenhouse model that integrates CO2 capture, conversion, storage, and utilization is proposed to present an ideal perspective direction of eCO2 RR to ethylene.

Implications of the Pore Size of Graphitic Carbon Nitride Monolayers on the Selectivity of Dual-Boron Atom Catalysts for the Reduction of N2 to Urea and Ammonia: A Computational Investigation

The formation of urea by electrocatalytic means remains a great challenge due to the lack of a suitable catalyst that is capable of not only activating inert N2 and CO2 molecules but also circumventing the complexity associated with subsequent reaction steps leading to urea formation. Herein, by means of comprehensive density functional theory simulations, we investigate the catalytic activity of highly stable transition-metal-free dual-boron atom-doped graphitic carbon-nitride monolayers with different pore sizes toward urea production under ambient conditions. As per the results, dual boron atoms impregnated in g-C2N and g-C6N6 monolayers with large pore diameters can successfully activate the N2 molecule and lead to the spontaneous formation of the *NCO*N intermediate, which is the most crucial step for urea formation via direct coupling of N2 and CO2. Interestingly, the B2@g-C2N and B2@g-C6N6 favor urea production with low limiting potentials of -1.11 and -1.18 V compared to very high limiting potentials of -1.71 and -1.88 V, respectively, for ammonia synthesis, leading to an almost 100% Faradaic efficiency for urea formation over ammonia. The dual-boron doping in g-C3N4 with a smaller pore size depicts comparatively weaker N2 adsorption than g-C2N and g-C6N6 counterparts. Further, B2@g-C3N4 prefers ammonia formation at a very low limiting potential of -0.40 V compared to a very high limiting potential of -2.11 V for urea formation. Thus, our findings clearly highlight the critical role played by the pore size of carbon-nitride monolayers in tuning the reactivity and catalytic activity of dual-boron atom catalysts toward urea formation in a selective manner, thereby providing valuable guidance in exploring other highly efficient urea catalysts.

Revealing electrocatalytic C N coupling for urea synthesis with metal–free electrocatalyst

Urea is ubiquitous in agriculture and industry, but its production consumes a lot of energy. The conversion of nitrogen (N₂) and carbon dioxide (CO₂) into urea via an electrocatalytic CN coupling reaction under ambient conditions would be a major boon to sustainable development. However, designing a metal - free catalyst with high activity and selectivity for urea remains a major challenge. Herein, by means of density functional theory (DFT) and ab - initio molecular dynamics (AIMD) computations, the B₁₂ cluster doped on nitrogenated graphene (C₂N) substrate catalyst (B₁₂@C₂N) with superior stability was designed for electrocatalytic urea synthesis starting from the CO₂ and N₂ through four reaction mechanisms. The nature of the co-adsorption activation of CO₂ and N₂ on the B₁₂@C₂N catalyst was investigated, the electrochemical proton - electron transfer steps and the CN thermochemical coupling led to the synthesis of urea. The study showed that the B₁₂@C₂N catalyst exhibited high catalytic activity for urea synthesis with the lowest limiting potential of - 1.01 V following the *HNNH mechanism compared with other mechanisms. The potential - determining step (PDS) is the formation of the *CO+*NH₂NH₂ species. However, the two - step CN coupling barriers of *NCON species are 0.13 eV and 0.60 eV using AIMD and a "slow - growth" sampling approach in an explicit water molecules model. Calculations also showed that the byproducts of carbon monoxide (CO), methane (CH₄), methanol (CH₃OH), ammonia (NH₃), and hydrogen (H₂) can be inhibited on the B₁₂@C₂N catalyst. Therefore, the metal - free catalyst not only has a good performance for the hydrogenation of CO₂ and N₂ promoting the electrochemical reaction, but also facilitates CN thermochemical coupling for urea synthesis. This work provides new insights into the synthesis of urea via the CN coupling reaction on a metal - free electrocatalyst, a process that could contribute to greenhouse gas mitigation to help meet carbon neutrality targets.

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.

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.

Designing a Built-In Electric Field for Efficient Energy Electrocatalysis

To utilize intermittent renewable energy as well as achieve the goals of peak carbon dioxide emissions and carbon neutrality, various electrocatalytic devices have been developed. However, the electrocatalytic reactions, e.g., hydrogen evolution reaction/oxygen evolution reaction in overall water splitting, polysulfide conversion in lithium-sulfur batteries, formation/decomposition of lithium peroxide in lithium-oxygen batteries, and nitrate reduction reaction to degrade sewage, suffer from sluggish kinetics caused by multielectron transfer processes. Owing to the merits of accelerated charge transport, optimized adsorption/desorption of intermediates, raised conductivity, regulation of the reaction microenvironment, as well as ease to combine with geometric characteristics, the built-in electric field (BIEF) is expected to overcome the above problems. Here, we give a Review about the very recent progress of BIEF for efficient energy electrocatalysis. First, the construction strategies and the characterization methods (qualitative and quantitative analysis) of BIEF are summarized. Then, the up-to-date overviews of BIEF engineering in electrocatalysis, with attention on the electron structure optimization and reaction microenvironment modulation, are analyzed and discussed in detail. In the end, the challenges and perspectives of BIEF engineering are proposed. This Review gives a deep understanding on the design of electrocatalysts with BIEF for next-generation energy storage and electrocatalytic devices.

Turning Waste into Wealth: Sustainable Production of High-Value-Added Chemicals from Catalytic Coupling of Carbon Dioxide and Nitrogenous Small Molecules

Carbon neutrality is one of the central topics of not only the scientific community but also the majority of human society. The development of highly efficient carbon dioxide (CO₂) capture and utilization (CCU) techniques is expected to stimulate routes and concepts to go beyond fossil fuels and provide more economic benefits for a carbon-neutral economy. While various single-carbon (C₁) and multi-carbon (C₂₊) products have been selectively produced to date, the scope of CCU can be further expanded to more valuable chemicals beyond simple carbon species by integration of nitrogenous reactants into CO₂ reduction. In this Review, research progress toward sustainable production of high-value-added chemicals (urea, methylamine, ethylamine, formamide, acetamide, and glycine) from catalytic coupling of CO₂ and nitrogenous small molecules (NH₃, N₂, NO₃^-, and NO₂^-) is highlighted. C-N bond formation is a key mechanistic step in N-integrated CO₂ reduction, so we focus on the possible pathways of C-N coupling starting from the CO₂ reduction and nitrogenous small molecules reduction processes as well as the catalytic attributes that enable the C-N coupling. We also propose research directions and prospects in the field, aiming to inspire future investigations and achieve comprehensive improvement of the performance and product scope of C-N coupling systems.

A Reliable and Precise Protocol for Urea Quantification in Photo/Electrocatalysis

To comply with the trend toward green and sustainable development of the fine chemical industry, multitudinous promising technologies (e.g., photocatalysis and electrocatalysis) are beginning to dabble in the green synthesis of fine chemicals, particularly urea synthesis. Whilst numerous advances are made in mechanistic understanding, the low yield reported so far also imposes more stringent requirements on the reliability and anti-interference of the detection method. Herein, the applicability of frequently used methods for urea quantification is methodically compared. In terms of the experimental results, a precise and methodical protocol for urea quantification or evaluation in photo/electrocatalysis is explored and established, with emphasis on screening quantitative methods under specific conditions and indispensable isotopic tracing experiments. The budding urea photo/electrosynthesis urgently demands a rigorous protocol, including the rapid isotopic identification and evaluation criteria, capable of promoting healthy development in the future.