Combined catalyst–pathway design for efficient and green urea synthesis

Abstract

Urea electrosynthesis from CO₂ + NO₃⁻ co-electrolysis (UECN) offers a promising pathway for sustainable urea synthesis, while necessitating rational catalyst/pathway design to boost CO₂/NO₃⁻ co-activation and C–N coupling. Herein, we demonstrate an atomically dispersed Ag on oxygen vacancy (OV)-rich TiO₂ (Ag₁/TiO_2−x) catalyst for efficient UECN. Theoretical calculations and in situ spectroscopic analyses reveal the synergy of OV-induced Ag_1-OV and Ti_OV dual-sites in promoting the selective C–N coupling of *CO and *NO₂ for target urea generation. Notably, with the integration of plasma-assisted air-to-NO_x⁻ conversion and CO₂ + NO_x⁻ co-electrolysis, Ag₁/TiO_2−x achieves an exceptional urea faradaic efficiency of 75.7% and a urea yield rate of 74.6 mmol h⁻¹ g⁻¹ along with excellent catalytic stability.

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Green foundation

1. We pioneer a plasma-electrocatalytic pathway for efficient and green urea synthesis using atmospheric air (N₂/O₂) and CO₂ as feedstocks, eliminating fossil fuel-derived NH₃ required in conventional processes. The Ag₁/TiO_2−x catalyst enables selective C–N coupling of *CO/*NO₂ intermediates via dual-site synergy for target urea generation.

2. Quantitatively, our strategy achieves a record-high urea faradaic efficiency of 75.7% and urea yield rate of 74.6 mmol h⁻¹ g⁻¹ along with excellent catalytic stability. Qualitatively, it bypasses the energy-intensive Haber–Bosch process and enables much lower energy consumption versus industrial urea synthesis under mild conditions.

3. Future work should optimize plasma energy efficiency using renewable electricity and scale catalyst loading and membrane-electrode assemblies for industrial deployment.

Urea (CO(NH₂)₂) plays a vital role in agricultural and industrial development.¹ The industrial urea synthesis remains tethered to the energy-intensive Bosch–Meiser process under harsh conditions, giving rise to enormous global energy consumption and huge CO₂ emissions.² Electrocatalytic C–N coupling of CO₂ and nitrogenous species (i.e., N₂/NO₃⁻) offers a promising alternative pathway for sustainable urea synthesis by repurposing waste carbon/nitrogen sources while leveraging renewable energy.^3–6 Recent studies have demonstrated the feasibility of urea electrosynthesis from CO₂ + NO₃⁻ co-electrolysis (UECN), simultaneously realizing environmental remediation and value-added urea production.⁷ Nevertheless, UECN faces sluggish reaction kinetics due to the inefficient C–N coupling and low selectivity from competing reaction pathways (NO₃⁻-to-NH₃ reduction reaction (NO₃RR) and hydrogen evolution reaction (HER)).^8–11 These challenges necessitate innovative UECN catalyst/pathway design to enhance activity, selectivity and stability for efficient and durable urea electrosynthesis.

Recent advances highlight single-atom catalysts (SACs) and oxygen vacancy (OV) engineering as promising strategies, in which SACs maximize atom utilization and tailor active sites, while OVs modulate the catalyst electronic structure to enhance intermediate stabilization and accelerate charge transfer.^12–14 For instance, integrating metal single atoms into OV-rich oxides can effectively boost CO₂ activation and concurrently leverage metal–support interactions to promote C–N coupling.^14–18 Despite these merits, simultaneously achieving high urea yield rate and faradaic efficiency (FE_urea) remains challenging due to the competitive adsorption of various C/N reactants, inherent scaling relations in multi-step reactions, and insufficient stabilization of key C/N intermediates.¹⁰ In addition, the NO₃⁻ feedstocks still indirectly depend on fossil fuels, which undermines the process sustainability, thus underscoring the need for nitrogen sources that require facile activation with sustainable production.¹⁹

Herein, we demonstrate an atomically dispersed Ag on OV-rich TiO₂ (Ag₁/TiO_2−x) catalyst for efficient UECN. By combining extensive experimental and theoretical calculations, we elucidate how Ag single atoms and OV-rich TiO_2−x synergistically promote C–N coupling while suppressing the competing side reactions towards efficient and selective urea generation. Notably, with integration of plasma-assisted air-to-NO_x⁻ conversion and CO₂ + NO_x⁻ co-electrolysis, Ag₁/TiO_2−x exhibits outstanding UECN performance, achieving an FE_urea of 75.7% and a urea yield rate of 74.6 mmol h⁻¹ g⁻¹ along with excellent catalytic stability.

Ag₁/TiO_2−x was synthesized through a combined hydrothermal/annealing method. The XRD pattern (Fig. S1) of Ag₁/TiO_2−x reveals the characteristic TiO₂ diffraction peaks (PDF#86-1157) with no Ag-related species observed, indicating that the introduced Ag species are ultrasmall and highly dispersed on TiO_2−x. A typical nanosheet morphology is observed in the TEM images of Ag₁/TiO_2−x (Fig. 1a and Fig. S2a). The HRTEM (Fig. S2b) image of Ag₁/TiO_2−x displays clear lattice fringes with an interplanar distance of 0.352 nm, assigned to the (101) plane of Ag₁/TiO_2−x. The AC-STEM (Fig. 1b) image of Ag₁/TiO_2−x displays numerous atomically dispersed bright spots, assigned to isolated Ag atoms. The atomic-level isolation of Ag species can be further confirmed by the corresponding atomic intensity line scanning and 3D surface intensity profile (Fig. 1c). Elemental mapping images of Ag₁/TiO_2−x (Fig. 1d) reveal a homogeneous dispersion of Ag atoms on the TiO_2−x substrate. Inductively coupled plasma analysis shows that the Ag content is 3.37 wt%. The EPR spectra (Fig. S3) display much stronger g-signals of TiO_2−x and Ag₁/TiO_2−x relative to bulk TiO₂, indicating the existence of rich oxygen vacancies (OVs) on both TiO_2−x and Ag₁/TiO_2−x. Meanwhile, Ag₁/TiO_2−x exhibits a similar g-signal intensity to TiO_2−x, implying that the introduced Ag species have little impact on the OV concentration of TiO_2−x.

Fig. 1 (a–d) Characterization of Ag₁/TiO_2−x: (a) TEM image, (b) AC-STEM image and the corresponding (c) intensity line scanning profile and 3D intensity profile, and (d) elemental mapping images. (e–h) Ag K-edge XAS analyses: (e) XANES spectra, (f) EXAFS spectra, (g) EXAFS fitting curve of Ag₁/TiO_2−x and (h) wavelet transform analyses of Ag foil, Ag₂O and Ag₁/TiO_2−x.

XAS is used to study the coordination and valence environment of Ag₁/TiO_2−x. The XANES (Fig. 1e) spectra reveal that the Ag K-edge of Ag₁/TiO_2−x is located between Ag foil (0) and Ag₂O (+1), and linear fitting analysis reveals an average Ag valence state of +0.65 for Ag₁/TiO_2−x (Fig. S4). The EXAFS spectra (Fig. 1f) of Ag₁/TiO_2−x exhibit prominent Ag–O coordination with no Ag–Ag coordination (2.75 Å) observed, suggesting that the loaded Ag species are atomically dispersed on the TiO_2−x substrate,^20–22 thus ruling out the formation of Ag nanoparticles or clusters. Quantitative EXAFS fitting analysis (Fig. 1g) shows that the isolated Ag atoms are coordinated with two surrounding O atoms, forming an Ag–O₂ geometric unit (Fig. 1g, inset, Table S1). The corresponding wavelet transform contour plots (Fig. 1h) reveal a sole intensity maximum of Ag–O at 3.9 Å⁻¹ for Ag₁/TiO_2−x, further confirming that isolated Ag atoms are highly dispersed on the TiO_2−x substrate.

DFT calculations were carried out to elucidate the electronic structure of Ag₁/TiO_2−x (Fig. S5). Differential charge density maps (Fig. S6) show more enriched electron density of Ag₁/TiO_2−x compared to TiO₂, TiO_2−x and Ag₁/TiO₂, suggesting that OV and Ag₁ are both favorable for boosting the electronic interaction and this is further verified by the remarkable Ag/O/Ti orbital hybridization shown by the partial density of states (PDOS, Fig. S7). In addition, Ag₁/TiO_2−x reveals the equilibrium energy and temperature states below 698 K (Fig. S8) during the simulation, verifying the excellent thermodynamic stability of Ag₁/TiO_2−x. Furthermore, the Ag₁ introduction modulates the electronic structure of TiO_2−x to cause a decreased work function of Ag₁/TiO_2−x (Fig. S9), thereby facilitating efficient proton-coupled electron transfer to promote the electrocatalytic UECN kinetics.^23–25

The electrochemical UECN performance of Ag₁/TiO_2−x was first evaluated in an H-cell containing a CO₂-saturated electrolyte (0.1 M KNO₃ and 0.1 M KHCO₃).²⁶ Liquid and gas products were detected using colorimetric and gas chromatographic methods (Fig. S10), respectively. Linear sweep voltammetry (LSV, Fig. S11a) curves show that compared to the Ar-saturated electrolyte, the CO₂-saturated electrolyte presents a relatively higher current density (j), suggesting the pronounced catalytic UECN activity of Ag₁/TiO_2−x. A chronoamperometric test for 1 h of electrolysis at various potentials (Fig. S11b) was then performed to quantitatively determine the produced urea on Ag₁/TiO_2−x. As shown in Fig. 2a, Ag₁/TiO_2−x exhibits the highest FE_urea of 40.1% with a corresponding urea yield rate of 25.9 mmol h⁻¹ g⁻¹ at −0.7 V, which are higher than those of most reported catalysts (Table S2). The FEs of other by-products were also tested with NH₃ being the primary by-product (Fig. 2b).

Fig. 2 (a) UECN performances of Ag₁/TiO_2−x at different potentials. (b) FEs of different products on Ag₁/TiO_2−x. (c) ¹³C NMR spectra of the ¹³CO(NH₂)₂ standard sample and those fed with ¹³CO₂ after electrolysis at −0.7 V. (d) Schematic diagram of the flow cell. (e) LSV curves of Ag₁/TiO_2−x in an H-cell and flow cell. (f) UECN performances of Ag₁/TiO_2−x at different potentials in the flow cell. (g) Comparison of urea yield rates and FE_urea in the H-cell and flow cell. (h) Comparison of the catalytic performance between Ag₁/TiO_2−x and recently reported catalysts.

Several tests were carried out to verify the N/C sources of the produced urea. ¹H nuclear magnetic resonance (NMR) experiments show that using ¹⁵NO₃⁻ as the N-source results in a characteristic doublet chemical shift of CO (¹⁵NH₂)₂ (Fig. S12), while employing ¹³CO₂ leads to a typical singlet chemical shift of ¹³CO(NH₂)₂ (Fig. 2c), evidencing that urea is exclusively produced through combined CO₂ and NO_x⁻. Fig. S13 shows negligible urea detection without the CO₂ feeding gas, in an NO_x⁻-free electrolyte or at the open circuit potential (OCP), suggesting that the generated urea originates from the UECN process. We further studied the long-term electrolysis of Ag₁/TiO_2−x, which showed a rather stable current density (Fig. S14), indicating the good long-term stability of Ag₁/TiO_2−x. In addition, as shown in Fig. S15 during the eight consecutive electrolysis cycles, FE_urea and urea yield rate show no obvious changes, indicating the outstanding cycling stability of Ag₁/TiO_2−x.^27–29 After the stability test, Ag₁/TiO_2−x retains the atomic Ag dispersion and Ag content (Fig. S16), confirming the robust structure of Ag₁/TiO_2−x.^30,31

Generally, the low CO₂ solubility and mass transfer limitation of an H-cell give rise to compromised UECN performance. To further improve the UECN activity of Ag₁/TiO_2−x, we employed a flow cell (Fig. 2d) that can enhance the CO₂ mass transfer during UECN electrolysis. As shown in the LSV curves (Fig. 2e), Ag₁/TiO_2−x exhibits a much larger current density in the flow cell than in the H-cell, suggesting that the enhanced UECN activity of Ag₁/TiO_2−x is enabled by the flow cell. Quantitative measurement at various potentials (Fig. 2f and Fig. S17) shows that Ag₁/TiO_2−x in the flow cell reaches the maximum FE_urea of 66.1% with the corresponding urea yield rate of 61.6 mmol h⁻¹ g⁻¹, significantly outperforming its H-cell counterpart (Fig. 2g) and most reported catalysts (Fig. 2h and Fig. S18). Moreover, the current density remains virtually constant during 36 h of continuous electrolysis (Fig. S19) and there are no significant changes in the UECN performances for eight consecutive electrolysis cycles (Fig. S20),^32,33 highlighting the excellent UECN performance of the Ag₁/TiO_2−x/flow cell toward efficient and durable urea electrosynthesis.

To elucidate the underlying reasons for the enhanced UECN performance of Ag₁/TiO_2−x, operando FTIR spectroscopy was first employed to detect the key intermediates generated during the UECN electrolysis at various potentials. As displayed in Fig. 3a and b, Ag₁/TiO_2−x exhibits a pronounced infrared band of *NO₂ at 1430 cm⁻¹.³⁴ The infrared bands probed at 2050 cm⁻¹ and 1330 cm⁻¹ are assigned to *CO and *COOH, respectively.³⁵ The infrared band at 1400 cm⁻¹ corresponds to the stretching mode of the C–N bond, while the bands at 1580 cm⁻¹ and 1690 cm⁻¹ are assigned to the generated *CONO₂ and *CONH₂ intermediates, respectively.^36,37 The pronounced bands of C–N bonds and *CONO₂ and *CONH₂ intermediates alongside the urea-related band (∼1170 cm⁻¹)²⁶ collectively confirm that Ag₁/TiO_2−x can effectively drive the UECN process and promote C–N coupling towards urea generation. Furthermore, online DEMS measurements (Fig. S21) show the distinct signals of *CONO₂ (m/z = 74), *CONH₂ (m/z = 44), *NO₂ (m/z = 46), *CO (m/z = 28) and *COOH (m/z = 45), while no signals of *NO₂H (m/z = 47) can be found. The absence of the *NO₂H intermediate and the presence of *CONO₂ and *CONH₂ intermediates disclose that Ag₁/TiO_2−x preferentially proceeds through the *CO + *NO₂ coupling pathway (*NO₃ → *NO₂ → *CONO₂ → *CONOOH → *CONO → *CONHO → *CONH → *CONH₂ → *CO NO₂NH₂ → urea) to drive the UECN process towards urea formation.

Fig. 3 Operando FTIR spectra of (a and b) Ag₁/TiO_2−x and (c) TiO_2−x during UECN electrolysis at various potentials. (d and e) Operando Raman spectra of (d) Ag₁/TiO_2−x and (e) TiO_2−x during UECN electrolysis at various potentials.

In stark contrast, pristine TiO_2−x (Fig. 3c) exhibits the dominant *CO band at 2050 cm⁻¹ but negligible C–N (1400 cm⁻¹), *CONO₂ (1580 cm⁻¹) and CONH₂ (1690 cm⁻¹) signals, highlighting its limited C–N coupling capability despite efficient CO₂ → *CO conversion. This is further supported by the operando Raman spectra, where pristine TiO_2−x displays a weak N–C–N stretching band (Fig. 3e), in contrast to the much stronger N–C–N band observed in Ag₁/TiO_2−x (Fig. 3d). These operando FTIR/Raman results underscore the synergistic roles of Ag₁ and TiO_2−x in enhancing the UECN process, where TiO_2−x facilitates the reduction of CO₂ to *CO while Ag₁ promotes the conversion of NO_x⁻ to *NO₂ and drives the C–N coupling of generated *NO₂ and *CO for urea formation.

Theoretical computation was utilized to clarify the UECN mechanism of Ag₁/TiO_2−x. According to the previous XAS (Fig. 1e–h) and DFT results (Fig. S5–S7), four active sites on Ag₁/TiO_2−x are considered: the pristine Ti site, the Ag single-atom site (Ag₁), the OV-adjacent Ti site (Ti_OV), and the OV-adjacent Ag single-atom site (Ag_1-OV). Fig. 4a shows that among all four sites, NO₃⁻ is mostly preferentially adsorbed on the Ag_1-OV site while *CO₂ is mostly preferentially adsorbed on the Ti_OV site. Accordingly, in the UECN process, the Ag_1-OV site mainly drives NO₃⁻ reduction while the Ti_OV site drives *CO₂ reduction. The formed C-intermediates on the Ti_OV site and N-intermediates on the Ag_1-OV site would undergo C–N coupling towards urea formation.

Fig. 4 (a) Adsorption free energies of *CO₂ and *NO₂ on various active sites (Ti site, Ag₁ site, Ti_OV site and Ag_1-OV site) on Ag₁/TiO_2−x. (b and c) Free energy barriers of (b) coupling of *NO₂ with various C-intermediates and (c) coupling of *CO with various N-intermediates. (d) Free energy diagrams of CO₂ → *CO pathways. (e and f) Calculated TS energy barriers for the migration of (e) *CO from the Ti_OV site to the Ag_1-OV site and (f) migration of *NO₂ from the Ag_1-OV site to the Ti_OV site. (g) Free energy diagrams for the electrocatalytic C–N coupling of the CO₂ + NO₃⁻ pathway on Ag₁/TiO_2−x.

Previous operando FTIR results (Fig. 3a–c) indicate that *NO₂ is the critical N-intermediate in the UECN process. After analyzing the free energy barriers for the C–N coupling of *NO₂ with various C-intermediates (Fig. 4b and Fig. S22, S23), it is found that *NO₂ coupling with *CO shows the lowest energy barrier, suggesting that *CO/*NO₂ coupling is mostly energetically favored for urea formation. Likewise, for the C–N coupling of *CO with various N-intermediates, *CO coupling with *NO₂ shows the lowest energy barrier (Fig. 4c and Fig. S24). Thus, the UECN process on Ag₁/TiO_2−x involves two key steps, namely, the initial CO₂ → *CO and *NO₃ → *NO₂ and the subsequent C–N coupling of *NO₂ and *CO to generate urea (*NO₂ + *CO → *CONO₂ → *CONOOH → *CONO → *CONHO → *CONH → *CONH₂ → *CO NO₂NH₂ → urea). For the CO₂ → *CO process, compared to the Ti site (0.73 eV), the Ti_OV site shows a much lower rate-determining step (RDS) energy barrier of 0.50 eV (Fig. 4d and Fig. S25–S27), indicating that Ti_OV serves as the primary site for CO₂ → *CO reduction, thus ensuring a steady supply of the *CO intermediate necessary for the subsequent C–N coupling with *NO₂ on the Ag₁ site. For the NO₃⁻ → *NO₂ process (Fig. S28), the Ag_1-OV site (0.55 eV) exhibits a larger downhill energy change than the Ag₁ site (0.83 eV), suggesting that the Ag_1-OV site is more energetically effective in facilitating NO₃⁻ activation for *NO₂ formation. These results demonstrate that OVs play a crucial role in enhancing the capabilities of both Ag₁ and Ti sites for CO₂/NO₂⁻ activation and reduction.

After the formation of *CO on the Ti_OV site and *NO₂ on the Ag_1-OV site, we further investigated the electrocatalytic C–N coupling process required for urea synthesis. Before C–N coupling, *CO must migrate from Ti_OV to Ag_1-OV or *NO₂ migration from Ag_1-OV to Ti_OV must occur (Fig. S29 and S30). Transition state (TS) calculations reveal a lower energy barrier for *CO migration (0.63 eV, Fig. 4e) compared to *NO₂ migration (1.72 eV, Fig. 4f), indicating a kinetically favored *CO migration from Ti_OV to Ag_1-OV.^38,39 This preferential *CO migration facilitates effective C–N coupling with *NO₂ on the Ag_1-OV site to form *CONO₂. After the migration of *CO from the Ti_OV site to the Ag_1-OV site, the first C–N coupling of *CO and *NO₂ (*CO + *NO₂ → *CONO₂) and the second C–N coupling of *CONH₂ and *NO₂ (*CONH₂ + *NO₂ → *CONH₂NO₂) are energetically more favorable on the Ag_1-OV site (0.37 eV and 0.21 eV) over the Ag₁ site (0.68 eV and 0.53 eV) (Fig. S31–S33). This indicates that C–N coupling for urea formation is more favorable on the Ag_1-OV site. All these findings demonstrate the synergistic role of the Ag_1-OV and Ti_OV sites in promoting urea synthesis, where the Ti_OV site drives CO₂ → *CO and the Ag_1-OV site drives NO₃⁻ → *NO₂. The migration of *CO from the Ti_OV site to the Ag_1-OV site promotes selective C–N coupling with *NO₂ for urea formation.

In the UECN process, competing side reactions such as the NO₃RR and HER can impede selective urea synthesis. For the NO₃RR, as shown in Fig. S22, the free energy required for *NO₂ coupling with *CO to form *CONO₂ is significantly lower than that required for *NO₂ hydrogenation to NH₃, thereby favoring selective C–N coupling over NH₃ formation. Regarding the HER, the free energy calculations (Fig. S34) indicate that Ag_1-OV and Ti_OV sites both preferentially bind NO₃⁻/CO₂ over H. This selective NO₃⁻/CO₂ adsorption over H is further supported by molecular dynamics (MD) simulations (Fig. S35), which show stronger catalyst–NO₃⁻ and catalyst–CO₂ interactions compared to the catalyst–H interaction.⁴⁰ These results demonstrate that Ag₁/TiO_2−x can well hamper the competitive NO₃RR and HER to result in high UECN selectivity for urea generation.

For CO₂ + NO₃⁻ co-electrolysis in the UECN process, NO₃⁻ feedstock is known to be derived mainly from the Haber–Bosch-derived NH₃ that would compromise the sustainability and decarbonization objectives of electrocatalytic processes.¹⁹ These challenges underscore the need for nitrogen sources that require facile activation with sustainable production. A breakthrough lies in plasma-assisted nitrogen oxidation, which converts atmospheric air (N₂/O₂) into nitrogen oxides (NO_x) via non-thermal plasma discharge.⁴¹ Absorption of NO_x in alkaline solution generates NO_x⁻ ions (NO₂⁻/NO₃⁻).^41,42 Furthermore, plasma-derived NO_x⁻ leverages air as the sole nitrogen source, bypassing the fossil fuel-dependent NH₃ required for industrial urea synthesis and CO₂ + NO₃⁻ co-electrolysis.⁴³ Accordingly, by integrating plasma-assisted air-to-NO_x⁻ conversion and CO₂ + NO_x⁻ co-electrolysis (Fig. 5a), we propose a tandem plasma-electrochemical pathway that enables a truly sustainable urea synthesis, offering a viable pathway to mitigate global energy consumption and CO₂ emissions associated with the traditional industrial/electrochemical urea production.

Fig. 5 (a) Schematic of the plasma-electrocatalytic process for urea synthesis from air and CO₂. (b) LSV curves of Ag₁/TiO_2−x in CO₂ + NO₃⁻ and CO₂ + NO_x⁻ electrolytes. (c) UECN performances of Ag₁/TiO_2−x at different potentials. (d) Comparison of urea yield rates and FE_urea after 1 h of electrolysis in different electrolytes at optimal potentials. (e) Long-term test of Ag₁/TiO_2−x at −0.7 V. (f) Urea yield over time.

For the initial plasma-assisted air oxidation, the plasma discharge across the needle electrode arrays generates a bright plasma zone (Fig. S36a) where high-energy electrons powerfully activate N₂/O₂ molecules into reactive NO_x (NO/NO₂) gases, as evidenced by the gas stream transition from colorless to reddish-brown after 10 min of plasma discharge (Fig. S36b).⁴² These generated NO_x gases are then absorbed into an alkaline KOH solution, resulting in the generation of NO_x⁻ (NO₂⁻/NO₃⁻) ions. Quantitative UV-vis analyses (Fig. S37 and S38) reveal that a prolonged plasma discharge time linearly enhances NO_x⁻ concentration, with NO₂⁻ as the dominant species (Fig. S39). After 60 min of continuous discharge, the total NO_x⁻ concentration reaches 118.8 mM, comprising 107.3 mM NO₂⁻ and 11.5 mM NO₃⁻ (Fig. S40, S41 and Table S3). Moreover, our plasma system presents remarkable stability, maintaining >100 mM NO_x⁻ yield across eight consecutive discharge cycles (Fig. S42), validating its robustness for stable production of NO_x⁻ feedstocks required for the downstream electrochemical co-reduction of CO₂ + NO_x⁻ to urea.

We then examined the UECN performance of Ag₁/TiO_2−x in a flow cell using a CO₂⁻saturated electrolyte containing plasma-derived NO_x⁻ (diluted to 0.1 M) and 0.1 M KHCO₃. As shown in Fig. 5b, the LSV curves present a marked enhancement in current density for CO₂ + NO_x⁻ (90.2% NO₂⁻) co-electrolysis compared to CO₂ + NO₃⁻ co-electrolysis (Fig. 5b). Such a performance enhancement is attributed to the more efficient 12-electron transfer process of the CO₂ + NO₂⁻ pathway than the 16-electron pathway required for CO₂ + NO₃⁻ co-electrolysis. As shown in Fig. 5c, the Ag₁/TiO_2−x catalyst exhibits outstanding UECN performance, achieving an FE_urea of 75.7% and a urea yield of 74.6 mmol h⁻¹ g⁻¹ at −0.7 V, significantly superior to those obtained from CO₂ + NO₃⁻ co-electrolysis (Fig. 5d). Electrocatalytic stability tests indicate the stable current density and consistent performance of Ag₁/TiO_2−x over eight electrolysis cycles (Fig. 5e), highlighting the excellent catalytic stability. To produce high-purity urea on a large scale, the electrode area is expanded to 5 × 5 cm² with the catalyst loading increased to 20 mg. After 8 h of continuous electrolysis (Fig. 5f), 0.93 g of pure urea is obtained through electrolyte evaporation, sediment dissolution in benzene and benzene evaporation. This outcome highlights the technical viability of Ag₁/TiO_2−x catalysts for potential scalable urea production via a tandem plasma-electrocatalytic pathway.

In summary, Ag₁/TiO_2−x has been validated to be a highly efficient and durable UECN catalyst. Theoretical calculations and in situ spectroscopic analyses unveil the synergistic roles of Ag_1-OV and Ti_OV sites on Ag₁/TiO_2−x in promoting urea synthesis, where the Ti_OV site drives CO₂ → *CO and the Ag_1-OV site drives NO₃⁻ → *NO₂. The migration of *CO from a Ti_OV site to an Ag_1-OV site promotes selective C–N coupling of *CO with *NO₂ for urea formation. Strikingly, by integrating plasma-assisted air-to-NO_x⁻ conversion and CO₂ + NO_x⁻ co-electrolysis, Ag₁/TiO_2−x presents outstanding UECN performance with an FE_urea of 75.7% and a urea yield of 74.6 mmol h⁻¹ g⁻¹. The present work opens up new avenues for effective and eco-friendly urea production, highlighting the transformative potential of coupled plasma-electrocatalysis technologies in advancing green chemical synthesis.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5gc04601g.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (52561042), the Gansu Province Joint Research Fund (24JRRA859) and the Industrial Support Plan Project of the Gansu Provincial Education Department (2024CYZC-22).

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