How Carbon and Nitrogen Intermediates Decide the Fate of Electrocatalytic Urea Synthesis

Urea is one of the most consequential molecules humanity manufactures. It feeds roughly half the world’s crops as a nitrogen fertilizer and underpins the production of plastics, resins, and pharmaceuticals. Yet the industrial route to making it remains stubbornly energy-hungry. The Bosch-Meiser process reacts ammonia with carbon dioxide at 150 to 200 degrees Celsius and pressures of 15 to 25 megapascals, and the ammonia itself must first be forged in the Haber-Bosch reactor at 400 to 600 degrees Celsius and 200 to 350 atmospheres. Together, these two steps consume an estimated 1 to 2 percent of global annual energy while releasing substantial carbon dioxide. A review published in Advanced Science by researchers at the University of Auckland and Huazhong University of Science and Technology now takes stock of a radically different proposition: building the carbon-nitrogen bonds of urea directly at an electrode, at room temperature, from carbon dioxide and nitrate or nitrite ions dissolved in water.

The appeal of electrochemical co-reduction is twofold. First, it sidesteps the need to isolate ammonia as an intermediate, instead coupling carbon and nitrogen species in a single electrochemical step. Second, it can turn a pollution problem into a feedstock. Nitrate and nitrite are abundant in industrial wastewater, and their infiltration into groundwater causes methemoglobinemia, a blood disorder that impairs oxygen transport. Compared with molecular nitrogen, whose triple bond carries a bond energy of roughly 941 kilojoules per mole, the nitrogen-oxygen bonds in nitrate and nitrite are far easier to break, making them ideal nitrogen sources for coupling with carbon dioxide. The challenge is that the reaction is extraordinarily intricate: the full urea synthesis pathway involves sixteen proton-coupled electron transfer steps and two distinct carbon-nitrogen coupling steps, and the precise sequence in which those bonds form has remained stubbornly ambiguous.

The review organizes the field around five representative coupling pathways for the first, and most critical, carbon-nitrogen bond. In the earliest route, adsorbed carbon dioxide couples with surface-bound nitrogen dioxide to form a carbon dioxide-nitrogen dioxide intermediate before any deep hydrogenation occurs. Work on a sulfur-modified indium hydroxide catalyst, reported by Lv and colleagues, combined operando synchrotron-radiation infrared spectroscopy with density functional theory to show that the barrier for nitrogen dioxide to couple with carbon dioxide, 0.35 electron volts, is substantially lower than the barrier for its further hydrogenation, 0.62 electron volts. Because coupling happens early, competing ammonia formation is kinetically suppressed, and the catalyst achieved a urea Faradaic efficiency of 53.4 percent with nearly complete carbon selectivity. Similar evidence on molybdenum oxide nanoclusters and uncoordinated zinc nanosheets, the latter probed by online differential electrochemical mass spectrometry, confirmed that carbon dioxide and nitrogen dioxide can couple directly without passing through carbon monoxide.

A second family of pathways involves carbon monoxide as the carbon partner. On copper tungstate, in situ Raman spectroscopy detected adsorbed carbon monoxide and nitrogen dioxide at 1979 and 1428 wavenumbers respectively, with both signals peaking at exactly the potential where urea yield and Faradaic efficiency were highest. When downstream intermediates were substituted into the reaction, urea formation vanished, reinforcing the assignment. On a copper silicon oxide catalyst, attenuated total reflection infrared spectroscopy and mass spectrometry caught the carbon monoxide-nitrogen dioxide coupling product at 1695 wavenumbers, the earliest hydrogenation-stage intermediate observed. A third route pairs carbon monoxide with nitric oxide to form an isocyanate-like intermediate. On a copper-based porphyrin metal-organic framework, surface-enhanced infrared spectroscopy captured all three species, and the coupling step showed a negative Gibbs free energy change, making it kinetically spontaneous. On cerium dioxide enriched with oxygen vacancies, the carbon monoxide-nitric oxide coupling barrier was a mere 0.27 electron volts, far below the 1.75 electron volts needed for nitric oxide protonation, so the coupling wins outright.

The fourth pathway defers coupling to the very end of the reduction cascade, joining carbon monoxide with an amino intermediate after nitrogen has been reduced through seven electron transfer steps. Substitution experiments on nickel-cerium oxide catalysts, in which carbon dioxide, carbon monoxide, formaldehyde, and methanol were tested as carbon sources, showed that urea formed only when carbon dioxide or carbon monoxide was present, while formamide as a nitrogen source also yielded urea, pinpointing the carbamoyl intermediate as the key species. Because this route demands deep reduction of both reactants, it typically operates at more negative potentials, and its Achilles heel is that both partners can be lost to side products: carbon monoxide desorbing as gas, or the amino intermediate hydrogenating all the way to ammonia. The fifth and most surprising pathway abandons the Langmuir-Hinshelwood logic of two adsorbed species entirely. On boron-doped copper, in situ infrared spectroscopy showed that carbon dioxide alone could not be activated on the surface, yet in the presence of nitrate both the urea carbon-nitrogen signal and an adsorbed carbonate-like signal appeared. By varying nitrate concentration and carbon dioxide partial pressure, researchers demonstrated that urea production saturated with nitrate but kept rising with carbon dioxide pressure, the signature of an Eley-Rideal mechanism in which gaseous carbon dioxide directly attacks surface-bound nitrogen intermediates.

Which pathway dominates, the review argues, is not a matter of chance but of thermodynamics and kinetics on each specific surface. The early carbon dioxide-nitrogen dioxide route thrives where carbon dioxide hydrogenation is sluggish and nitrogen dioxide hydrogenation carries a high barrier, so nitrogen dioxide accumulates and waits for a coupling partner. The carbon monoxide routes require catalysts that convert carbon dioxide to carbon monoxide at low overpotential while simultaneously resisting the over-reduction of nitrogen species, because the two half-reactions otherwise operate in mismatched potential windows and their intermediates never coexist. The amino route succeeds only where hydrogenation of the amino intermediate is deliberately slowed. And the gaseous carbon dioxide route can even function on catalysts with no carbon dioxide reduction activity at all, though coupling an unactivated linear carbon dioxide molecule carries its own energetic price. No single mechanism is universal; the dominant route is a property of the catalyst-electrolyte-potential combination.

From this mechanistic picture, the authors distill a unified framework of intermediate regulation. On the carbon side, strategies include tuning adsorption geometry, promoting carbon monoxide generation, balancing its binding strength, and managing its spatial delivery. A striking example is a dual-metal hetero-single-atom catalyst in which iron and copper-boron coordination sites force carbon dioxide into a single-oxygen, O-monodentate adsorption mode. Isotope labeling with carbon-13 confirmed that carbon dioxide, not carbon monoxide, was the carbon source, and the calculated coupling barrier dropped from 1.02 to 0.19 electron volts, delivering a 71.9 percent Faradaic efficiency and a yield of over 2000 micrograms per hour per milligram of catalyst. Geometrically, the O-linked configuration leaves the carbon atom protruding and exposed, while electronically it sharpens the carbon atom’s electrophilicity toward nucleophilic nitrogen partners. Elsewhere, spillover engineering shuttles carbon monoxide from palladium-gold alloy sites across an interface to ruthenium dioxide where nitrogen intermediates reside, and diatomic iron-nickel catalysts weld the two half-reactions together at the atomic scale.

On the nitrogen side, the central battle is against desorption and over-hydrogenation. Ionic-liquid modification of a copper zeolitic framework strengthened adsorption of both carbon monoxide and nitrogen dioxide, lowering their adsorption energies and pushing urea yield to 140 micromoles per hour per square centimeter with 55.3 percent Faradaic efficiency. Oxygen vacancies in cerium dioxide raise the hydrogenation barriers of nitrogen intermediates so that coupling with carbon monoxide becomes the energetically preferred step. Comparative experiments on iron phosphosulfide showed that switching from nitrate to the more easily hydrogenated nitrite slashed urea efficiency while boosting ammonia, a direct demonstration that preserving coupling-active nitrogen species is the decisive lever. The review also surveys the in situ toolkit, including infrared and Raman spectroscopies and online mass spectrometry, that underpins these assignments, while candidly noting their limits: detecting a downstream intermediate does not prove where the bond actually formed, and many coupling intermediates are too short-lived, low-coverage, or nonvolatile to observe directly.

The path forward, the authors contend, lies in sharper instruments and better theory working in concert. Time-resolved and surface-enhanced spectroscopies could catch transient early-stage intermediates, operando techniques that replicate true electrode polarization and local pH would replace misleading simulated conditions, and isotope labeling with carbon-13 and nitrogen-15 can strip away background interference. On the computational side, models must graduate from idealized surfaces to realistic electrochemical environments that include electric fields and solvent structure, and kinetic analysis must supplement thermodynamic energy landscapes, since an intermediate with a low coupling barrier but a vanishingly short lifetime may never actually couple. If experiment and computation can be woven into a self-constraining framework built on experimentally testable descriptors, the field will move beyond cataloging possible mechanisms to identifying the dominant one on any given catalyst. That, in turn, would transform electrocatalytic urea synthesis from a laboratory curiosity into a genuine technology: a process that cleans nitrate from wastewater, recycles carbon dioxide, and manufactures the world’s most important fertilizer with a fraction of today’s energy budget.

Subject of Research: Electrocatalytic C–N coupling mechanisms for urea synthesis from CO2 and nitrate/nitrite

Article Title: Understanding and Regulating C–N Coupling Pathways for Electrocatalytic Urea Synthesis From CO2 and NO3−/NO2−

Article References: Chen, X., Li, Z., Cai, L., Kilmartin, P., Yu Xia, B., & Wang, Z. (2026). Understanding and Regulating C–N Coupling Pathways for Electrocatalytic Urea Synthesis From CO 2 and NO 3 − /NO 2 −. Advanced Science, 13(56), Article e77499. https://doi.org/10.1002/advs.77499

Image Credits: AI Generated

DOI: 10.1002/advs.77499

Keywords: electrocatalysis, urea synthesis, CO2 reduction, nitrate reduction, C–N coupling, reaction intermediates, in situ spectroscopy, density functional theory, Eley–Rideal mechanism, oxygen vacancies, single-atom catalysts, wastewater remediation