電催化氮集成二氧化碳還原反應(yīng)合成有機氮化合物

摘要：以化石能源如煤、石油和天然氣為主要能源的社會發(fā)展模式，不僅導(dǎo)致不可再生資源枯竭，還引發(fā)大氣中CO2濃度持續(xù)上升的問題。隨著人們對能源結(jié)構(gòu)認(rèn)識的深化和生態(tài)環(huán)境保護意識的增強，尋求有效的清潔CO2固定和轉(zhuǎn)化技術(shù)已成為研究熱點。這些技術(shù)可利用太陽能、風(fēng)能、潮汐能和地?zé)崮艿瓤稍偕茉?，促進人工碳循環(huán)、碳儲存，并緩解環(huán)境惡化。在眾多CO2固定和催化轉(zhuǎn)化技術(shù)中，常溫常壓下的CO2還原技術(shù)受到可再生能源的驅(qū)動，有助于人工碳循環(huán)、碳儲存，減輕環(huán)境退化。目前，水溶液中的電催化CO2還原研究已取得顯著進展，但在制造其他重要的有機小分子，如尿素、酰胺、胺及其衍生物，甚至氨基酸方面，仍有未開發(fā)的潛力。這些產(chǎn)品在肥料、化學(xué)品合成、醫(yī)藥化學(xué)和航空工業(yè)等領(lǐng)域有廣泛應(yīng)用，引起了廣泛研究興趣。通過氮集成的電催化CO2還原反應(yīng)制造有機氮化合物，能顯著提高CO2電還原技術(shù)的實際應(yīng)用價值，同時也為生物小分子的起源提供參考，因此具有重要意義。然而，該過程涉及CO2和含氮無機物的電化學(xué)耦合，包含多步電子和質(zhì)子轉(zhuǎn)移過程，因此面臨著緩慢的動力學(xué)和復(fù)雜的反應(yīng)機制。在本綜述中，我們詳細(xì)討論了氮集成電催化CO2還原生成不同產(chǎn)物的具體反應(yīng)路徑和合理的催化劑設(shè)計策略，這對于指導(dǎo)高效電催化劑的設(shè)計至關(guān)重要。盡管已經(jīng)通過一系列策略取得了一定的研究進展，但仍然存在一些需要解決的挑戰(zhàn)，這限制了它們在大規(guī)模實際應(yīng)用中的發(fā)展。最后，我們對該領(lǐng)域的發(fā)展限制和改進的可能方向進行了討論，希望這能有助于氮集成電催化CO2還原反應(yīng)催化劑的進一步發(fā)展。

關(guān)鍵詞：CO2還原反應(yīng)；含氮化合物；有機氮化合物；電催化；C-N偶聯(lián)

中圖分類號：O643

Abstract： The social development model relying on coal， oil， natural gas，and other fossil fuels as the primary energy sources has not only hastenedthe depletion of non-renewable resources but also led to a continuousincrease in atmospheric CO2 concentration. As human society’sunderstanding of energy structures deepens and environmentalconsciousness grows， the pursuit of effective clean CO2 capture andcatalytic conversion technologies has become a research priority. This isessential for promoting adjustments to the energy mix and achieving globalcarbon neutrality through artificial carbon cycling. Among the various CO2capture and catalytic conversion technologies， electrochemical catalyticCO2 reduction （CO2RR） at ambient temperature and pressure holdspromise for advancing artificial carbon cycling， carbon storage， and mitigating environmental degradation. This technologycan be driven by intermittent renewable energy sources such as solar energy， wind energy， tidal power， geothermal energy，etc. Furthermore， using water as a clean proton source， a wide array of chemicals can be synthesized. While recent studieshave made significant progress in CO2RR within aqueous solutions， there remains untapped potential in generating otherimportant small organic molecules like urea， amides， amines， derivatives， and even amino acids. These compounds areof great interest due to their widespread applications in fertilizers， chemical synthesis， pharmaceuticals， and the aerospaceindustry. The electrocatalytic synthesis of organonitrogen compounds through N-integrated CO2RR （NCR） is consideredcrucial for improving the practical applications and offering a reference for biological small molecules. However， NCRinvolves multi-step electron and proton transfer processes， leading to current challenges， including slow kinetics and acomplex reaction mechanism. In this review， we delve into the detailed reaction pathways and the rational design ofcatalysts for different NCR products， which are vital for developing highly efficient electrocatalysts. Although some progresshas been made through various strategies， there are still challenges to overcome， limiting their large-scale practicalapplications. The discussion concludes by addressing these existing limitations and outlining potential avenues for futureimprovements. We hope that this feature article will be instrumental in the development of novel electrocatalysts for NCR.

Key Words： CO2 reduction reaction; Nitrogenous species; Organonitrogen compound; Electrocatalysis;C-N coupling

1 Introduction

The societal development model reliant on coal， oil， naturalgas， and other fossil fuels as its primary energy sources has notonly accelerated the depletion of non-renewable resources butalso led to a continuous surge in atmospheric CO2 concentration.This substantial CO2 emission has disrupted the carbon cyclewithin the ecosystem and caused a range of severeenvironmental issues， including the greenhouse effect， glacierand permafrost melting， and seawater acidification 1，2. As humansociety’s understanding of energy structures deepens andenvironmental awareness grows， the quest for effective andclean CO2 capture and catalytic conversion technology hasbecome a research priority. The goal is to promote adjustmentsto the energy mix and achieve global carbon neutrality throughartificial carbon cycling 3，4. Among the numerous CO2 captureand catalytic conversion technologies， electrochemical catalyticCO2 reduction （CO2RR） at ambient temperature and pressureholds promise for advancing artificial carbon cycling， carbonstorage， and mitigating environmental degradation. Thistechnology can be powered by intermittent renewable energysources such as solar energy， wind energy， tidal power，geothermal energy， etc. 5. Furthermore， water serves as a cleansource of protons， enabling the synthesis of various chemicals.In recent years， thanks to the relentless efforts of advancedscientific researchers， the development of CO2RR in aqueoussolutions has made significant strides and achieved fruitfulresults 6，7. However， the research is still in its early stages， withCO2RR primarily yielding products such as CO， HCOOH， CH4，C2H4， C2H5OH， and so on 8，9. Electrochemical CO2RR shouldn’tbe limited to just C， H， and O atoms; many other crucial smallcarbon-containing organic molecules used in the chemicalindustry can also be produced via electrochemical CO2RR， suchas urea， amides， amines， and even amino acids 10. Therefore，integrating bond-forming reactions involving N atoms intoelectrocatalytic CO2RR （NCR） to produce organonitrogencompounds is desirable. These target compounds are ofsignificant interest due to their widespread applications infertilizers， chemical synthesis， pharmaceutical chemistry， andthe aerospace industry （Fig. 1） 11，12. Thus， converting CO2 intohigher-value products to promote the industrialization of CO2RRis a critical consideration.

To synthesize organonitrogen compounds using mild andenvironmentally friendly technology， electrochemical synthesismethods have been developed. These methods utilize CO2 andN2， NO3?， NO2?， or NH3 as the carbon and nitrogen sources，respectively 13. However， this approach involves a multi-stepprocess of electron and proton transfer， posing challenges due toslow dynamics and a complex reaction mechanism. For instance，moderate *CO adsorption promotes the C-N coupling process，while less *CO adsorption leads to the desorption of gaseousbyproduct CO. Strengthening *CO adsorption increases theprotonation process for CH4 formation， and even adsorption sitepoisoning can result in H―H bond coupling formation of byproductsH2 14，15. Therefore， achieving specific product preparation with high selectivity involves optimizing theregulation of specific adsorption species. Current research in thisfield is still at a relatively early stage. In CO2RR， C-C couplingis usually accompanied by a significant reduction ofintermediates 16–18. When N-containing compounds areintroduced， the N-intermediates involved in the C-N couplingare often nucleophilic， making them prone to attack the partiallypositively charged *C ＝ O. On one hand， this makes itchallenging to retain carboxyl groups in the product structure，and on the other hand， it makes carboxyl groups that may formsusceptible to attack and conversion into amides by Ncontainingintermediates 19，20.

2 Reaction mechanisms of NCR

C-N coupling reaction was a chemical reaction in which Catom and N atom form a stable covalent bond. The process wascritically important in life as the underpinning for the productionof many essential biomolecules such as proteins and nucleicacids 21–24. Understanding and realizing C-N coupling throughcatalytic chemistry is of paramount importance. In this process，CO2 and nitrogenous species undergo a series of cascadereactions： CO2 can be reduced to C-intermediates like *CO，*COOH， *CO2?， and *CCO 25–30， while nitrogenous species can be reduced to N-intermediates like NN， *NH2， and *NO2. Theseintermediates then react to ultimately form a C―N bond 31–34.

However， the specific reaction pathways and mechanisms ofC-N coupling are highly complex， resulting in multiple potentialroutes for synthesizing urea alone. In theory， a wide range ofproducts can be generated from C-N coupling， includingimportant chemicals such as amines， urea， nitrogenousheterocyclic compounds， and even vital bioactive substanceslike amino acids and amino alcohols 35–37. This chaptersummarizes the main reaction pathways of electrocatalytic C-Ncoupling based on current research achievements （Fig. 2）.

Due to the variety of nitrogenous species involved in C-Ncoupling reactions with CO2， urea synthesis can proceed throughvarious mechanisms 38. The specific reaction mechanismremains somewhat debated， but it’s established that catalystssignificantly influence the reaction rate and selectivity. Onesuggested mechanism involves the formation of intermediates*CO2NO2 through the coupling reaction between *NO2 and*CO2 species， which is crucial for the electrocatalytic reductionof urea. After protonation of *NO2 in *CO2NO2 to form*CO2NH2， further protonation results in the *COOHNH2intermediate. The subsequent reaction of *COOHNH2spontaneously proceeds thermodynamically， yielding urea 39.Another mechanism proposes that the *OCNO intermediate isalso key to the electrocatalytic reduction of C-N coupling withCO2， with the stable nitrogenous species reduction product *NOpromoting *OCNO to enhance the selectivity of urea production 40.The *NH and *CO species produced through the reduction ofNH3 and CO2 can also couple to form a pivotal intermediate，*NHCO， for urea synthesis 41，42. Additionally， due to thematching of molecular orbitals between *CO and *N2，intermediates are formed through the reaction of *N2 and *CO2，with *CO2 reduced to *CO via proton coupling. The C-Ncoupling of *CO and *N2 can also generate the key intermediate*NCON for urea synthesis 33. Some research suggests thatduring urea synthesis， *COOH plays a critical role. The amineintermediate *NH2 and the carboxyl group intermediate *COOH undergo a coupling reaction process to generate urea 43，44.There’s also a mechanism involving the C-N coupling of theamine intermediate （*NH2）， carbonyl intermediate （*CO）， andcarboxyl intermediate （*COOH） to produce urea 34. In thispathway， the amine， carbonyl， and carboxyl intermediatesworked together to form a C―N bond， which then lead to theformation of urea. There was increasing literature coverage ofthe electrocatalytic reduction of CO2 for C-N coupling in thesynthesis of urea. The exploration of the mechanism for ureasynthesis has been become increasingly clear due to advancedcharacterization methods.

Reports on the synthesis of amides via electrocatalytic C-Ncoupling are relatively few， and research on its mechanism isalso limited 44. CO2 is reduced to *CO2 on the catalyst surface，forming an active carbon intermediate. Subsequently， the lonepair electrons of the nitrogen atom in NH3 attack the Cintermediate，leading to the formation of formamide. As shownin Fig. 3， the electro-synthesis process of acetamide is similar tothe formation mechanism of formamide， with CO2 being reducedto *CO2 on the catalyst surface. Two *CO2 molecules thenundergo a coupling reaction， resulting in a *C ＝ C ＝ Ointermediate， which is both the starting material of the reactionand a critical step in amide formation. The lone pair electrons ofthe N atom in NH3 then attack the median carbon in the C2intermediate， forming acetamide. It was important to note that ahighly reduced C2 intermediate was the crucial factor in theselectivity of acetamide because these of the intermediates aspart of the synthetic pathway of acetamide. However， forformamide， a highly reduced C2 intermediate didn’t participatein its synthesis pathway， and therefore the selectivity offormamide didn’t increase to the same extent. Overall， theformation of both formamide and acetamide involved thereduction of CO2 and reaction with NH3， wherein the lone pairelectrons of the N atom in NH3 play a key role 45. The discoveryof these reaction pathways provided foundational understandingto further optimize and improve the synthesis of high selectivitycatalysts.

Based on recent reports， amines were generated during theelectro-catalytic reduction of CO2 and the reduction reaction ofNO3? 46. CO2 and NO3? were respectively reduced to aldehydecompounds and hydroxylamine intermediates， whichspontaneously condensed to form oxime-aldehyde compounds.Oxime-aldehyde compounds， the key intermediates in carbonnitrogencoupling reactions， were ultimately converted into target amine compounds after a series of reduction reactions.Specifically， for the synthesis of methylamine， formaldehyde（HCHO） and hydroxylamine （NH2OH） intermediates were firstproduced via the electro-catalytic reduction of CO2. Theformaldehyde and hydroxylamine intermediates thenspontaneously condensed， forming an oxime-aldehydecompound. The oxime-aldehyde compound underwent anelectrochemical reduction reaction to produce methylamine（CH3NH2）. During this process， hydroxylamine， being the mostreduced intermediate， may serve as N-intermediate for C-Ncoupling 35. For the synthesis of ethylamine， via the integrationof CO2 reduction reaction with NO3?， acetaldehyde （CH3CHO）and hydroxylamine intermediates were produced. Similar to thesynthesis of methylamine， acetaldehyde and hydroxylamineintermediates spontaneously condensed to form an oximealdehydecompound. The oxime-aldehyde compound thenunderwent a series of reduction reactions， includingelectrochemical reduction and autoreduction reactions， toultimately yield ethylamine （CH3CH2NH2） 46. The mainmechanisms for carbon-nitrogen coupling to form aminecompounds included the generation of aldehyde compounds andhydroxylamine intermediates， the spontaneous condensation toform oxime-aldehyde compounds， and a series of reductionreactions leading to the final target amine compounds in Fig. 4.It was worth noting that the current reports on the coupledreactions of CO2 reduction and nitrogenous species reductionwere relatively scarce， and the mechanisms of many couplingswere not clearly elucidated， particularly the reaction steps andintermediates under different nitrogen source conditions， whichremain uncertain. Of course， the electrocatalytic C-N couplingreaction was still a relatively new field， and more experimentsand theoretical explorations were needed to detail themechanism of C-N coupling reactions. There was in urgent needof the development of advanced catalysts with superior C-Ncoupling selectivity and exploration of reaction mechanisms.The study of electrocatalytic C-N coupling still faced difficultiesdue to the lack of in-depth understanding， but it was also filled with new opportunities and challenges. In particular， the directelectrocatalytic synthesis of important value products such asamino acids or amines from CO2 and nitrogenous species maybring a significant challenge that could potentially triggerrevolutionary technological changes in human societydevelopment. Therefore， understanding and designing highlyselective catalysts based on reaction mechanisms was especiallyimportant.

3 Urea synthesis

Urea， as a significant fertilizer， provided nutrients for morethan half of the global population. Industrial production of NH3for urea synthesis was commonly achieved through the Haber-Bosch process， with about 80% of the ammonia being used forurea production. This process involved energy-intensivereactions under harsh conditions of high pressure andtemperature， accounting for over 2% of global energyconsumption 47–49. Consequently， exploring the electrochemicalproduction of urea from CO2， which offers relatively mildconditions， has garnered interest. In the electrocatalytic synthesisof urea， two primary factors affected the selectivity and yield ofurea： the adsorption behavior of the carbon and nitrogenousspecies on the catalyst surface， and the concurrent reduction tofacilitate the formation of intermediates necessary for ureasynthesis 50–52. Here， we summarized the material characteristicsthat influence urea synthesis， including active sites， charge，defects， and coordination structures. We believed thatcomparative analysis of these findings will enhance ourunderstanding of C-N coupling， which was crucial for furtheroptimizing the reaction system.

3.1 Regulation of active sites

To achieve efficient urea production， limitations of selectiveadsorption and activation of carbon or nitrogen reactants can beovercome by introducing single atom metals into the catalyst，enhancing reaction sites for C-N coupling and its inhibitoryeffects on side reactions 42，53–55. Leverett et al. presented apioneering application of single-atom catalysts in the field ofelectro-synthesis of urea 56. They successfully immobilized Cusingle atoms onto a graphene support through a solutionimpregnation-annealing approach， resulting in a Cu-N-Ccoordination configuration that facilitates efficient reduction ofCO2 and NO3?. The coordination structure of the single-atomcatalyst transformed from Cu-N4 to Cu-N4?x-C with an increasein pyrolysis temperature. The former configuration preferredCO2 reduction， while achieved higher yields. The Cu-N4?x-Csingle-atom catalyst achieved a remarkable FE of 28% for ureaproduction with the yield of 1800 μg?h?1?mg?1 at ?0.9 V vs.reversible hydrogen electrode （RHE）. The observation ofpreferential formation of the *COOH intermediate at the Cu-N4site during the electrocatalytic process underlines its crucial roleas the rate-determining step for CO2 reduction and ureasynthesis. Zhang et al. fabricated N-coordinated transition metalatomic sites anchoring on a porous carbon framework via pyrolysis of coordination polymer， and subsequentlysuccessfully synthesized a diatomic catalyst with Fe-Ni pairs atan atomic distance of 0.25 nm （Fig. 5a，b） 57. The separated Fe-N4 and Ni-N4 sites worked synergistically to enhance theadsorption and activation of NO3? and CO2， triggering a plethoraof activated C- and N-species and increasing the possibility ofthese intermediates encountering and coupling to produce thecrucial C ― N bond. In situ synchrotron radiation Fouriertransform infrared （SR-FTIR） tests observed infrared bands at～1978 and ～2170 cm?1， corresponding to the stretching vibrationsof N＝O and C＝O 58，59， respectively， demonstrating the coactivationbinding of nitrate ions and carbon dioxide on thecatalyst. Furthermore， the prominent infrared band at ～1694cm?1 belongs to *NHCO， which is closely related to ureageneration （Fig. 5c）. Subsequently， *NHCO and *NO rapidlycombined to form the key intermediate *NHCONO （Fig. 5d）.The urea synthesis performance reached 20.2 mmol?h?1?g?1， witha corresponding Faraday efficiency （FE） of 17.8% under 0.1mol?L?1 KHCO3 electrolyte. Kong et al. loaded different metalsingle atoms onto porous boron nitride materials （M/p-BN），allowing the metal single atoms to form asymmetric active siteswith two adjacent boron atoms 60. Density functional theory（DFT） calculations showed that both Fe/p-BN and Co/p-BNmaterials exhibit excellent catalytic performance. They utilizedthe strong and weak electronic polarization effects to fullyactivate nitrogen molecules， reducing the kinetic barrier for C-Ncoupling and exhibiting good selectivity for the NCON*intermediate.

The incorporation of Fe-based nanoparticles into the catalyst，resulting in dual active sites， provides an unexpected boost to theproduction of urea. Geng et al. successfully modified crystallineFe3O4 and carbon-coated amorphous Fe （Fe（a）@C） onto carbonnanotube （CNT） carriers through a liquid-phase laser irradiationstrategy 61. By utilizing these two iron-based active componentsas dual active sites， they were able to enhance the adsorption andactivation of NO3? and CO2 in a synergistic manner. The in situFTIR measurements detected the stretching vibrations of the CNbond， confirming the formation of Fe2+-urea complexes throughthe O atom in C＝O and the N atom in N―H. Furthermore， DFTcalculations revealed that Fe3O4 promotes the reduction of CO2to the *CO intermediate， while Fe（a）@C was conducive to theformation of the *NH2 intermediate. In 0.1 mol?L?1 KNO3electrolyte， the urea yield of Fe（a）@C-Fe3O4/CNTs was 1341.3± 112.6 μg?h?1?mgcat?1 with a FE of 16.5% ± 6.1% at ?0.65 V.

Non-metallic electrocatalysts have also attracted considerableattention in the field of electrocatalytic synthesis of urea due totheir high structural adjustability 62. Liu et al. fabricated fluorinedopedcarbon nanotubes （F-CNT） as an electrocatalyst for ureasynthesis （Fig. 5e） 63. The incorporation of F atom， a highlyelectronegative element， introduced positive charges into thecarbon framework. These F-doped materials exhibited inhibitionof the hydrogen evolution reaction and improvedelectroreduction of CO2 and NO?3. DFT calculations indicatedthat the fluorine-doped graphite external layer provided plentiful“C-F2” active sites， which facilitated the formation of reactionintermediates such as *CO and *NH2， thereby promoting C―Ncoupling. The calculated energy of formation for *CONH2 on FCNTand undoped CNT were ?1.32 and ?0.07 eV， respectively（Fig. 5f）. At an applied potential of ?0.65 V， the F-CNTelectrocatalyst demonstrated a urea productivity of 6.36mmol?h?1?gcat?1， accompanied by a FE of 18%. The potential useof silicon-infused graphene-analogous carbon nitride （SiC6N6）as an electrocatalyst for urea synthesis was investigated by Royet al. using first-principles estimations 64. The incorporation oftwo Si atoms into the C6N6 framework was found to enhanceCO2 adsorption and its subsequent reduction to CO under acidicconditions. The formed CO species then reacted with activatedN ― N bonds， resulting in the formation of *N（CO）N*.Furthermore， the application of an optimal confining potentialfacilitated the conversion of activated *N2 ― COOH to*N（CO）N*. Urea synthesis exhibited a significantly lowerelectrochemical initiation potential compared to ammoniasynthesis or hydrogen evolution reactions， with a theoretical FEapproaching 100%.

3.2 Regulation of charge

The electron transfer facilitated the formation of criticalintermediates， reducing the energy barrier for C-N coupling andenhancing the catalytic performance of urea electro-synthesis.Meng et al. developed a Cu@Zn core-shell nanowire structureby oxidizing a copper mesh anode followed by calcination toobtain cuprous oxide （Fig. 6a） 65. A hydrothermal method wasemployed to grow zinc oxide nanorods on the surface of thecuprous oxide， which was then electrochemically reduced toform the Cu@Zn core-shell structure. By virtue of Cu’s higherwork function （4.63 eV） compared to Zn （4.30 eV）， the transferof electrons from zinc to copper reduced the electronconcentration around Zn， thus facilitating the reduction process.Differential electrochemical mass spectrometry， attenuatedFTIR and DFT calculations corroborated the transfer ofelectrons from Zn to Cu， promoting the formation of *NH2 and*CO intermediates （Fig. 6b，c）. At ?1.02 V， the Cu@Zn catalyst exhibited urea productivity and FE of 7.29 μmol?cm?2?h?1 and9.28%， respectively. Xiong et al. conducted a study using DFTto explore the catalytic performance of α-borane catalystsmodified with various single metal atoms， such as Ti， Cr， Nb，Mo， and Ta. Their findings revealed a strong correlation betweenthe catalytic activity and the d-band center and charge density ofthe active center atom 66. The generation of *NCON on Nb@α-B was observed as the highest limiting potential reaction step（?1.45 V）， involving the formation of *CO and *N2 andsubsequent C ― N bond coupling. On the other hand， theformation of *CO*NH2NH2 with the lowest limiting potential（?0.16 V） occurred through the elementary reaction stepinvolving *CO and *NHNH2 along the *CO pathway. Zhang etal. successfully achieved in situ growth of Co-Ni bimetallicoxides （Co-NiOx@GDY） on the surface of graphdiyne （GDY），harnessing the interface and molecular interactions betweenGDY and metal oxides to facilitate charge transfer 67. Themolecular signals on the catalyst surface were investigated atdifferent potentials using an in situ SR-FTIR， including thecharacteristic stretching vibration of C ≡ C （associated withGDY）， CO2 adsorption peak， N―H bonds， C＝O stretchingsignal， and O―H， the signals of bending and wagging of N―Hbonds provided evidence for the formation of *NH2intermediates （Fig. 6d）. The incorporation of GDY can markedlyenhance the charge transfer within the sample， resulting in asubstantial improvement in conductivity and an increase in thenumber of active sites. The multi-step process involved thecoupling of protons along with the corresponding electrontransfer， leading to the formation of the crucial *NH2intermediate. Co-NiOx@GDY exhibits a urea yield of 913.2μg?h?1?mgcat?1 and 64.3% of FE at ?0.7 V. Yuan et al.successfully synthesized a highly conductive Co-PMDA-2-mbIM （PMDA = pyromellitic dianhydride; 2-mbIM = 2-methylbenzimidazole） MOF catalyst （Fig. 6e）， which exhibits a ureayield of 14.47 mmol?h?1?g?1 with a FE of 48.97% at 0.5 V 68. Thecatalyst exhibits host-guest molecular interactions， leading to theconversion of a fraction of high-spin Co3+ ions within the CoO6octahedra to intermediate-spin Co4+ states. Additionally，localized electrophilic and nucleophilic regions are formed.Specifically， N2， rich in electrons， adsorbed onto electrophilic Cosites within the CoO6 octahedra， whereas electron-deficient CO2molecules adsorb onto nucleophilic N sites within the Co-PMDA-2-mbIM framework. This adsorption process generated*N＝N* and *CO intermediates. Notably， electrons present inthe σ orbital of *N＝N* species can efficiently occupy the egorbitals of high-spin Co3+ ions， effectively facilitating the C-Ncoupling reaction and resulting in the formation of *NCON*urea precursors （Fig. 6f）. Yuan et al. successfully synthesized aMott-Schottky heterojunction structure of Bi-BiVO4， whichexhibits a urea yield of 5.91 mmol?h?1?g?1 with a FE of 12.55%at ?0.4 V 34. This structure demonstrated self-driven chargetransfer and the formation of a space charge region. Remarkably，it facilitated the adsorption and activation of CO2 and N2molecules at specific nucleophilic and electrophilic sites 69，while effectively preventing CO poisoning and the formation of*NNH intermediates. Consequently， this mechanism ensured theexothermic coupling between *N＝N* intermediates and CO，leading to the formation of *NCON* urea precursors. Liu et al.focused on the Cu（100） crystal surface in a neutral electrolyteand employed molecular dynamics simulations to investigate thefundamental reasons behind C-N coupling 70. The simulationstook into account both the electrode potential and the dynamicnature of the solvent. It was revealed that *NH and *CO arecrucial precursor species for the formation of C―N bonds at lowoverpotentials. However， at high overpotentials， the competingreduction of CO2 narrows the potential range for urea synthesis.Under these conditions， C-N coupling is achieved through theadsorption of *NH and solvated CO species. Yang et al.investigated the theoretical aspects of urea synthesis via C-Ncoupling on the Cu （111） crystal surface 71. The first C―N bondformation during the catalytic process was achieved directlythrough the coupling of CO2 molecules in the gas phase， withoutinvolving any intermediates resulting from CO2 reduction （suchas *COOH and *CO）. The reaction followed the Eley-Ridealmechanism and only required a single active site. The relativelysmall deformation energies of CO2 and N1 （the surface-boundnitrogen intermediate） resulted in faster C-N coupling dynamics，and the interaction between the two species was attributed tocharge transfer from N1 to CO2.

3.3 Regulation of defect

Defects played a pivotal role in catalysts design， exertingsignificant influences on their catalytic performance andstability. These defects had the ability to modulate the selectivityof catalysts， enabling the selective generation of desired productsunder specific reaction conditions. Additionally， defects caninduce alterations in the diffusion properties of catalysts， leadingto enhanced rates of adsorption and diffusion of reactants on thecatalyst surface， thereby bolstering the overall reaction rate.Consequently， when designing and synthesizing catalysts， it wasimperative to comprehensively consider the impact of defects inorder to achieve catalytic performance that is both moreefficient， stable， and selective.

In a recent investigation， Liu et al. utilized a one-step chemicalreduction method to facilitate the self-assembly of exposed （111）facet AuCu alloy nanowires into nanofibers in the presence of a4-aminopyridine solution （Fig. 7a） 72. The resulting nanofiberswere then employed for the electroreduction synthesis of ureafrom NO2? and CO2， ultimately yielding a productivity of 3.889mg?h?1?mgcat?1. Remarkably， the nanofibers demonstrated a FEof 24.7% at ?1.55 V， showcasing their ability to promote desiredchemical transformations （Fig. 7b，c）. The distinctive onedimensionalstructure exhibited anisotropic characteristics andpossessed a large specific surface area， thereby facilitatingimproved utilization of bimetallic atoms， expediting chargetransfer rates， and preventing the undesirable occurrences ofcatalyst aggregation and dissolution. Furthermore， it wasdetermined that the AuCu nanofibers harbored structural defectssuch as twin boundaries and stacking faults， which serve ascatalytically active sites. Huang et al. conducted a comparativeanalysis of the electrocatalytic rates for urea production using 10different metals 73. Zn emerged as the most efficient catalystamong the tested metals. Subsequently， ZnO nanosheets weresubjected to electrochemical reduction， leading to the formationof Zn nanobelts. These Zn nanobelts were employed as catalystsfor the electrochemical synthesis of urea from NO and CO2. Theelectrochemical reduction process resulted in the disappearanceof the reduction peak in the polarization curve （Fig. 7d）. Inaddition， in situ X-ray diffraction （XRD） spectra indicated aprogressive decline in the ZnO diffraction peaks over time， whilethe intensity of the Zn diffraction peaks steadily increased （Fig.7e）. Notably， during the electrocatalytic reaction， NO and CO2were separately reduced to *NH2 and *CO intermediates， whichsubsequently underwent coupling reactions to form ureamolecules. The achieved optimum urea production rate wasdetermined to be 15.13 mmol?h?1?g?1， accompanied by FE of11.26%. Krzywda et al. utilized surface-enhanced Ramanspectroscopy and mass spectrometry to reveal the formation ofsoluble active species similar to Cu―C≡N on the Cu electrodesurface during the presence of NO3? and CO2 74. The appearanceof this active species was found to be a key contributing factorto the surface restructuring of Cu. Additionally， Cu-Bi bimetalliccatalysts were designed by Wu et al. to investigate the impact ofdefects on the performance of urea electro synthesis 39，75.

The study examined two distinct reaction mechanisms termeddistal， wherein protonation occurs at the terminal nitrogen atom，and alternating， where protonation alternates between two Natoms76. It was found that the determining steps varied forcatalysts without defects and those with metal defects in Cu-Bi：in the former case， the determining steps for both distal andalternating mechanisms were identified as *NN + *CO →*NCON* with a barrier energy of 2.63 eV. Conversely， for thelatter case， the determining step for the distal mechanisminvolved *NCON* → NHHCON （barrier energy： 0.78 eV）， andfor the alternating mechanism， it involved *NH + *CO →*NCON* （barrier energy： 0.76 eV） （Fig. 7f）. In comparison toseveral control samples， the Cu-Bi alloy with defects exhibited amaximum urea concentration of 0.45 ± 0.06 mg?L?1 at ?0.4 V vs.RHE， accompanied by a FE of 8.7% ± 1.7%. Lv et al. havepresented their investigation on the application of hydrogenatedindium hydroxide （In（OH）3） electrocatalyst with exposed {100}crystal facets in the electro synthesis of urea 39. The adsorptionof CO2 onto the catalyst induces an N-type to P-typesemiconductor transition， leading to the formation of a holeenrichedlayer on the catalyst surface， which effectivelysuppresses the hydrogen evolution reaction. The {100} crystalfacets exhibit the lowest energy barrier， thus promoting the C-Ncoupling reaction of *NO2 and *CO2 intermediates. Remarkably，the yield of urea reached 533.1 μg?h?1?mg?1 at ?0.6 V vs. RHE，accompanied by high FE （53.4%）. Meanwhile， the impact ofoxygen vacancies on the electro synthesis of urea usinghydroxyindium oxide （InOOH） catalyst was investigated by thesame group 77. Employing a combination of in situ FTIRspectroscopy and theoretical calculations， it was revealed thatthe protonation of *CO2NH2 represents the rate-determining stepin urea synthesis. Furthermore， the presence of O vacancies wasobserved to reconstruct the electronic structure of the activesurface sites， resulting in a reduction of the energy barrier for the*CO2NH2 → *COOHNH2 conversion. Notably， the InOOHcatalyst with O vacancies demonstrated a commendable ureaproduction rate of 592.5 μg?h?1?mgcat?1， accompanied by FE of51%. Wei et al. have successfully improved the catalyticefficiency of cerium dioxide （CeO2） by introducing a significantnumber of oxygen vacancies （VO） on its surface 78， resulting inan impressive urea production rate of 943.6 mg?h?1?g?1.Intermediate *NO can undergo adsorption on oxygen vacanciesand subsequently couple with *CO， forming *OCNOintermediates that effectively avoid the protonation reaction of*NO. In situ and vibrational spectroscopic analyses wereperformed to compare the evolution of intermediates duringelectrocatalytic reactions. The presence of *OCNOintermediates observed on the VO-CeO2 catalyst confirmed thatoxygen vacancy-mediated changes to the reaction pathways canoccur. Conversely， the selectivity for C-N coupling was less inthe absence of oxygen vacancies， and the proton-coupledelectron transfer of *NO continued as the primary reactionmechanism.

4 Amides synthesis

Benefiting from the extensive research and application of Cucatalysts for CO2 electroreduction with excellent C2 productselectivity， coupling intermediates of C2 products withintermediates of nitrogen reduction could greatly broaden therange of C-N coupling products， of which amide was greatproduct of high economic value. Li and his co-worker usedcommercially available Cu or CuO particles as electrocatalysts（Fig. 8a，b）， using CO2 as the carbon source to generateformamide and acetamide as the main C―N products， with themaximum FE of formamide and acetamide being 0.4% and 10%respectively （at ?0.58 V， corresponding to partial currentdensities of 0.2 and 2.2 mA?cm?2， respectively） 45. Throughinfrared spectroscopy analysis， new vibrational bands related tothe reaction of CO2 and NH3 were observed. Under thecoexistence of CO2 and NH3， positive and negative bands relatedto carbonates and ammonium salts appeared. These spectralfeatures are consistent with the features of *COO and *COOHintermediates observed in previous studies， supporting theformation pathway of formamide and acetamide （Fig. 8c，d）.Preliminary mechanistic studies suggested that theelectrochemical synthesis pathways of formamide and formateshare the same initial *CO2 intermediate， which forms productscontaining C―N bonds through nucleophilic attack by NH3，while the authors also studied C-N coupling with NO3? or NO2? asthe nitrogen source instead of NH3 （Fig. 8e，f）. Althoughacetamide and formamide were produced， the catalyticselectivity and activity were significantly reduced. Although thiswork did not innovate in materials， it updated the understandingof the utilization of CO2 resources and provided a mechanisticexplanation. The key steps of the reaction were the activation ofthe *CO2 intermediate and the nucleophilic attack by NH3. Bydeveloping catalysts with higher selectivity through thismechanism， synthesis of products with different selectivity maybe achieved， offering innovative avenues for the sustainable useand development of CO2 resources.

5 Amines synthesis

Molecular catalysts possessed well-defined active sites andtunable fine structures， allowing performance optimizationbased on the reaction mechanism， but their application inCO2RR was extremely inadequate. Among them， cobaltphthalocyanine （CoPc） was often used as an efficient catalyst forreducing CO2 to CO 79，80.

However， Wang et al. found that the immobilization of CoPcon carbon nanotubes could firstly convert CO2 to CO through atwo-electron process， and CO was continuously reduced toMeOH through a four-electron-four-proton process， following adomino process （Fig. 9a，b） 81. At present， CoPc was the onlycatalyst other than Cu-based catalysts that achieved synthesis ofMeOH through electrochemical CO2RR with an appreciablecurrent density （Fig. 9c）.

On this basis， their group introduced the nitrogenous species（NO3?） into the CO2RR system and successfully realized thesynthesis of methylamine in aqueous medium in Fig. 9d 35. As shown in Fig. 9e， the average FE （methylamine） was ～12%， andthe catalytic activity could be maintained for more than 16 h at?0.92 V. The whole reaction process involved the 14-electronsand 15-protons transformation process. And the formationprocess of methylamine molecules was an 8-step catalyticcascade process， which was realized by the coupling behavior ofadjacent C-intermediates and N-intermediates on the catalystsurface. The key step in forming C-N coupling is the overflowof hydroxylamine from the NO3? reduction reaction， and thespontaneous and rapid reaction with the formaldehyde from theCO2RR to form formaldoxime， which is further reduced tomethylamine. The condensation reaction had fast kinetics andgreat thermodynamics， as a suitable step for the coupling of twointermediates under ambient conditions. This work provided asuccessful example of sustainable synthesis of alkylamines frominorganic carbon and nitrogen species. Then they exploited thiskey pathway of aldehyde-aldoxime-amine in C-N couplingprocess to synthesize more complex amines.Oxide-derived Cu catalysts were obtained from theelectroreduction of CuO nanoparticle， and they firstly achievedelectrochemical conversion of CO2 and NO3? to ethylamine 46， a20-electron and 21-proton reduction cascade process （Fig. 9f）.Although the current FE and yield were far from satisfactory，this provided an excellent reaction pathway to synthesize aminesby NCR， which had great guiding significance for mechanismresearch and product scope expansion.

6 Conclusion and outlook

Catalyzing the C-N coupling reaction between the greenhousegas CO2 and nitrogenous species （such as N2， NO3?， NO2?， or NH3）to produce high-value organonitroge compounds （Table 1）through electrochemical processes has expanded the range ofreactants and products within the CO2RR domain. Thisadvancement contributes significantly to the realization ofcarbon and nitrogen recycling， thereby mitigating environmentalpollution 38，82. In this featured article， a comprehensiveexploration of the NCR （Nitrogen-Carbon Reaction） mechanismis undertaken， a critical aspect in the development of efficientelectrocatalysts. Furthermore， the article compiles rationaldesign strategies for catalysts tailored to different productoutcomes， encompassing synthesis and optimizationapproaches. While considerable research progress has beenachieved through the aforementioned strategies， certainchallenges remain， constraining their viability for large-scalepractical applications. The article delves into the currentlimitations and explores potential directions for future enhancements.

6.1 Robust competing reactions

Electrocatalytic NCR has primarily been conducted inaqueous solutions， which are susceptible to the competinghydrogen evolution reaction （HER）. This leads to lowerutilization of active hydrogen， resulting in reduced FaradaicEfficiency （FE） for the desired product. Additionally， theactivity of C-N coupling has not proven to be as effective whencompared to the self-reduction of C-intermediates and Nintermediates83. Currently， the electroreduction of nitrogenousspecies is not as well-developed as the CO2RR. Many catalystsexhibit low reaction rates and require high overpotentials.Therefore， to enhance the efficiency of the C-N coupling step， itis crucial to match the generation rates of active N-intermediatespecies and C-intermediate species. To address this challenge forvarious reactants with distinct chemical compositions， anintuitive strategy involves optimizing independent active sites toaccommodate diverse reactants 84–86. One site tailored forCO2RR can increase the binding strength and retention rate ofCO2-reducing intermediates on the surface， allowing theseintermediates to remain in place for a sufficient duration toundergo coupling， rather than desorbing as CO or otherbyproducts. The other active site designed for nitrogenousspecies reduction should exhibit ideal adsorption properties forN-intermediate species and guide their nucleophilic addition to*CO. Consequently， the development of dual active-sitecatalysts， with each site independently optimized for CO2 andnitrogenous species reduction， while also facilitating C-Ncoupling by positioning these two active sites in close proximity，has emerged as the most effective strategy.

6.2 Indistinct NCR mechanism

Mechanism research plays a pivotal role in guiding futurecatalyst design and shaping the trajectory of reactiondevelopment. It is particularly valuable for kinetic analysis of theC-N coupling step and the self-reduction reaction of the couplingintermediate. The key focus here is on optimizing the bindingstrength between the coupling intermediates and the catalysts，thereby facilitating the chemical coupling required for NCR.This approach offers significant guidance for enhancingcoupling efficiency 34，87. However， the NCR process is intricate，encompassing a multi-step electron-proton transfer process，making it challenging to elucidate a clear reaction mechanism.Hence， the application of advanced in situ characterizationtechniques is necessary to dynamically monitor the actualcatalytic reaction pathways. Techniques such as in situ Ramanspectroscopy and infrared spectroscopy are employed to trackthe transformation of adsorbed species on the catalyst’s surfaceduring NCR. Additionally， in situ electron microscopy and Xrayabsorption spectroscopy （XAS） are utilized to explorechanges in the structure and composition of the catalysts. Bycombining these experimental observations with theoreticalcalculations to simulate the catalytic NCR process， it becomespossible to verify the adsorption and desorption behaviors ofcritical intermediates （such as *COOH and *CO） at the catalyst’sinterface. Moreover， capturing the key intermediates involved inC-N coupling enhances our understanding of the NCR mechanism at a deeper level.

6.3 Limited product scope

The approach showcased in this article has successfullyenabled the production of organonitrogen compounds， primarilyencompassing urea， amides， amines， and their derivatives，through the electrocatalytic C-N coupling process in CO2RR.Notably， a small quantity of serine was obtained through theelectroreduction of CO2 and NH3 on the chiral Cu surface 88. Thisdiscovery highlighted the significance of carbonyl structures，such as H2CO-CO*， in the reaction mechanism. The synthesisof amino acids has significantly augmented the practicalapplication potential of CO2RR technology and offered insightsinto the origins of small biological molecules. Consequently， theregulation and equilibrium of adsorption and coupling behaviorsbetween carbon species and nitrogen species on the catalyst’ssurface， as well as the identification of coupling driving forcesand active sites， have proven beneficial for generating a morediverse range of organonitrogen compounds. This expansion ofthe application scope of electrocatalytic CO2RR holds promisefor various applications.

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