English

• The extensive utilization of urea (nitrogen fertilizers, medical products, pesticides) has significantly propelled the growth of the urea industry [1,2]. Presently, industrial synthesis of urea involves two key thermochemical stages: (i) The conversion of nitrogen (N₂) and hydrogen (H₂) to ammonia (NH₃) at temperatures ranging from 450 ℃ to 600 ℃ and pressures between 100 bar and 250 bar; and (ii) the subsequent reaction of NH₃ with carbon dioxide (CO₂) to form urea at temperatures of 180–230 ℃ and pressures of 150–250 bar [3,4]. Despite being a prevalent technique for decades, the future of this technology faces challenges due to its stringent reaction conditions and the substantial requirement for liquid ammonia NH₃ [5]. In contrast, the emerging method of urea electrosynthesis, which involves coupling CO₂ with a nitrogen source under ambient conditions, offers an innovative pathway [6–11]. This pathway not only significantly reduces energy consumption but also plays a crucial role in addressing the energy crisis and mitigating global warming caused by excessive CO₂ emissions.

  In the realm of urea electrosynthesis, the use of N₂ as a primary source for urea production has recently captured considerable attention [7,12-16]. The limited solubility of N₂ in water at ambient conditions and the high energy required to break its N≡N bond (941 kJ/mol), nevertheless, typically results in suboptimal performance [17–22]. Conversely, nitrate (NO₃^–), being water-soluble, is a more favorable nitrogen source for its relatively lower dissociation energy of the N=O bond (204 kJ/mol), which facilitates enhanced reaction kinetics for urea electrosynthesis [6,10,23-28]. In addition, the electrochemical reduction of NO₃^– is considered as an environmentally friendly method, particularly for its ability to remove the abundant yet harmful NO₃^– in groundwater [29,30]. At the mechanistic level, introducing NO₃^– as a nitrogen source involves the electrocatalytic reduction of CO₂ to *CO and NO₃^– to *NH₂ on the catalyst's active surface, which followed by a non-electrocatalytic coupling that forms C—N bond for urea production [31]. Nonetheless, significant advancements in this field are currently hindered by the challenge of identifying efficient catalysts that can effectively facilitate the C—N bonding process.

  Current research primarily center on indium (In)- [25,32-34], ruthenium (Ru)- [35,36], and copper (Cu)-based catalysts [9,26,27,37-40] for the electrosynthesis of urea. However, their on-scale applications are hindered by the high costs of In and Ru, and the susceptibility of Cu to corrosion during NO₃^– reduction [41–44]. In natural processes, the reduction of NO₃^– is accomplished via enzymatic cascades involving NO₃^– reductase with a molybdenum (Mo) cofactor, and Mo sites are also known to catalyze CO₂ reduction [45–47]. Recent study has also demonstrated the effectiveness of molybdenum oxide nanoclusters in urea electrolysis through the conversion of CO₂ and NO₃^– [48]. In comparison, 2D molybdenum carbide (Mo₂C) is emerging as a highly appealing material due to its higher conductivity and fully exposed Mo active sites [49–51]. Mo₂C has been reported to show activity in both NO₃^– and CO₂ reduction reaction [51–53]. It is thus anticipated that Mo₂C can significantly enhance the C—N coupling process required for efficient urea electrolysis.

  Herein, we propose for the efficient urea electrosynthesis via the co-reduction of NO₃^– and CO₂ on Mo₂C nanosheets-decorated carbon sheets (Mo₂C/C). The as-prepared Mo₂C/C affords a high urea yield rate of 579.13 µg h^–1 mg^–1 and a high Faradaic efficiency (FE) of 44.80% at –0.5 V versus reversible hydrogen electrode (RHE). Furthermore, it demonstrates outstanding electrochemical stability, consistently performing well over a duration of 12 h and through 8 successive cycles. Additionally, we employed density functional theory (DFT) calculations to delve deeper into the underlying mechanisms of the urea electrosynthesis process.

  The X-ray diffraction (XRD) pattern of Mo₂C/C, as shown in Fig. 1a, exhibits distinct diffraction peaks at approximately 34.53°, 38.05°, 35.59°, 52.31°, 61.87°, 69.77°, 72.83°, 74.99° and 75.99°. These peaks closely align with those of standard Mo₂C (JCPDS No. 03–065–8766) [52,54]. Additionally, a broadened diffraction peak at around 21.6° is attributed to carbon [55], originating from the pyrolysis of glucose. Scanning electron microscopy (SEM) image (Fig. 1b) reveals interconnected, relatively smooth carbon sheets. Transmission electron microscopy (TEM) images (Figs. 1c and d) depict densely packed nanosheets on the surfaces of carbon sheet. In Fig. 1e, the high-resolution TEM (HRTEM) image of Mo₂C/C is presented. The selected area electron diffraction (SAED) patterns and line scan of the HRTEM image, indicated by red boxes 1 and 2 in Fig. 1f and Fig. S1 (Supporting information) from Fig. 1e, confirm the good crystallinity of Mo₂C, featuring a lattice spacing of 0.256 nm, which corresponds to the (100) plane of Mo₂C. Elemental distribution analysis using high-angle annular. High-angle annular dark field scanning TEM (HAADF-STEM) and corresponding elemental imaging in Fig. 1g reveal a homogeneous distribution of Mo and C atoms across the structure, indicating that the sheet-like nanoparticles are composed of Mo₂C uniformly distributed within the carbon sheet. Subsequent X-ray photoelectron spectroscopy (XPS) analysis of Mo₂C/C provide insights into the surface electronic states. The Mo 3d spectra (Fig. 1h) show peaks at 228.9 and 231.98 eV, corresponding to the binding energies of Mo²⁺, indicative of Mo₂C. Other doublets observed at 229.5/232.7 and 232.6/235.8 eV can be attributed to oxidized molybdenum phases Mo⁴⁺ and Mo⁶⁺, respectively, likely due to surface oxidation [56,57]. The C 1s spectra (Fig. 1i) consist of three subpeaks at 284.8, 285.6, and 288.8 eV, which are assigned to Mo-C, C—C, and O=C—O bonds, respectively [55,58,59].

  Figure 1

  

  Figure 1.  (a) XRD pattern of Mo₂C/C. (b) SEM image of Mo₂C/C. (c) Low and (d) high magnification TEM images of Mo₂C/C. (e) High-resolution TEM image of Mo₂C/C. (f) SAED pattern and line scan and of the HRTEM image indicated by the red box 1 in (e). (g) HAADF-STEM and its corresponding mapping images of Mo₂C/C. XPS spectra of Mo₂C/C in the (h) Mo 3d, (i) C 1s regions.

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  The intrinsic CO₂ reduction reaction (CO₂RR) and NO₃^– reduction reaction (NO₃RR) performance of Mo₂C/C were initially evaluated in an H-type electrochemical cell. The linear sweep voltammetry (LSV) curve in CO₂-saturated 0.2 mol/L Na₂SO₄ (Fig. 2a) exhibited a higher current density compared to that in pure 0.2 mol/L Na₂SO₄, indicating significant CO₂RR activity. Similarly, an obviously increase in current density was observed in 0.2 mol/L Na₂SO₄ containing 0.05 mol/L NO₃^–, demonstrating effective NO₃RR (Fig. 2b). Based on these LSV curves, the potential window from –0.5 V to –0.9 V was selected for subsequent CO₂RR and NO₃RR experiments, respectively. Mo₂C/C displayed commendable CO₂RR capacity for CO generation (Fig. 2c, Figs. S2 and S3 in Supporting information). The production of NH₃ was quantified using the indophenol blue method, with the corresponding calibration curve presented in Fig. S4 (Supporting information). Mo₂C/C also showed notable NO₃RR efficiency in generating NH₃ (Fig. 2d, Figs. S5 and S6 in Supporting information). In essence, the promising electrocatalytic performance of Mo₂C/C in both CO₂RR and NO₃RR underscores its potential for efficient electrocatalytic urea synthesis.

  Figure 2

  

  Figure 2.  (a) LSV curves of Mo₂C/C in 0.2 mol/L Na₂SO₄ with and without saturated CO₂, respectively. (b) LSV curves of Mo₂C/C in 0.2 mol/L Na₂SO₄ with and without 0.05 mol/L NO₃⁻, respectively. (c) FEs of CO in CO₂RR process at various potentials on Mo₂C/C. (d) NH₃ yields and FEs on Mo₂C/C for NO₃RR in 0.2 mol/L Na₂SO₄ + 0.05 mol/L NO₃⁻ with various applied potentials.

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  In the process of urea electrosynthesis, CO₂ was continuously bubbled into a solution of 0.2 mol/L Na₂SO₄ containing 0.05 mol/L NO₃^–. Initially, the potential performance was assessed using LSV curves (Fig. 3a). The presence of CO₂ elicited a current response, suggesting the potential occurrence of electrocatalytic C–N bonding crucial for urea synthesis. Subsequent chronoamperometry experiments were conducted to further confirm urea production (Fig. S7 in Supporting information). Urea detection was performed using the urease decomposition method, with corresponding UV–vis absorption spectra shown in Fig. S8 (Supporting information). And calculated urea yields and FEs at various potentials are presented in Fig. 3b. The UV–vis spectrum indicates a prominent absorbance at 655 nm at –0.9 V, corresponding to a urea yield of 3004.70 µg h^–1 mg^–1. A high FE of 44.80% was recorded at –0.5 V, surpassing previously most of the reported urea electrosynthesis catalysts (Table S1 in Supporting information). Comprehensive evaluation of urea electrosynthesis also involved quantitative detection of byproducts. Gas products (N₂, H₂, and CO) were monitored using online gas chromatography. The concentration of produced NO₂^– was determined using the Griess method (Figs. S9 and S10 in Supporting information). As depicted in Fig. 3c, combined with Fig. 3b, at potentials below –0.5 V, the excess release of NH₃ and H₂ from intensified hydrogen evolution blocks the adsorption sites of CO₂ and NO₃^–. This impacts the urea electrosynthesis performance, leading to a diversified distribution of products, with NO₂^–, N₂, and CO also being detected. Enhanced hydrogen evolution significantly hinders the formation of C–N bonds by coupling *CO with *NH₂. The recyclability and stability of the electrocatalyst are crucial for industrial applications. Therefore, an 8-cycle electrosynthesis of urea was conducted, with results shown in Fig. S11 (Supporting information), Figs. 3d and e. The consistent current response, urea FEs, and yields indicate the durable stability of Mo₂C/C. Furthermore, a 12-h long-term electrolysis test at –0.5 V in CO₂-saturated 0.2 mol/L Na₂SO₄ containing 0.05 mol/L NO₃^– showed a steady current density, underlining the catalyst's potential for practical applications. The stability of Mo₂C/C during NO₃RR is further confirmed by a steady current density over 12 h of prolonged electrolysis (Fig. S12 in Supporting information). Additionally, post-electrolysis analysis shows nearly identical LSV curves (Fig. S13 in Supporting information), well-preserved morphology (Fig. S14 in Supporting information), and consistent XRD pattern (Fig. S15 in Supporting information), collectively demonstrating the high durability and stability of Mo₂C/C in this process.

  Figure 3

  

  Figure 3.  (a) LSV curves of Mo₂C/C in Ar- and CO₂-saturated 0.2 mol/L Na₂SO₄ + 0.05 mol/L NO₃⁻. (b) Urea yields and FEs of Mo₂C/C at different potentials. (c) FEs of NH₃, CO, N₂, H₂, NO₂⁻ and urea on Mo₂C/C at different potentials during the electrosynthesis of urea. (d) Urea FEs at –0.5 V during consecutive recycling test. (e) Urea yields and chronoamperometry curves during consecutive recycling test.

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  To further reveal the underlying mechanism of urea electrosynthesis on 2D Mo₂C nanosheets from NO₃^– and CO₂, DFT computations were carried out. The atomic structure of Mo₂C is depicted in Fig. S16 (Supporting information). These calculations explored the entire free energy landscape of urea formation. The NO₃^– reduction follows a multi-step pathway: NO₃^– → NO₃* → NO₂* + OH* → NO₂* → NO* + OH* → NO* → HNO*→ H₂NO* → NH₂*+ OH* → NH₂* [31]. Here, the formation of NO* represents the potential rate-limiting step, exhibiting a ΔG value of 0.36 eV (Fig. 4a). The presence of multiple Mo active sites significantly enhances the dissociation and hydrogenation of NO₃*, NO₂*, and H₂NO* intermediates. The CO₂ reduction process follows the pathway CO₂ → COOH* → CO*, easily proceeding to *CO (Fig. S17 in Supporting information). Two mechanisms, Eley–Raideal and Langmuir-Hinshelwood, were then considered for the N—C-N coupling process. As depicted in Fig. 4b, the energy barrier for inserting CO between two independently adsorbed NH₂* intermediates, leading to urea formation, is only 0.78 eV. This step is also thermodynamically favorable, with a ΔG value of –0.19 eV, indicating that CO and NH₂* coupling is energetically advantageous both kinetically and thermodynamically. To assess Mo₂C nanosheet's selectivity for urea production, potential competitive reactions, such as NO₃^– reduction to NH₃, CO* hydrogenation, and the hydrogen evolution reaction (HER), were examined. The results revealed higher ΔG values for NH₂* reduction to NH₃ and CO* hydrogenation (1.04 eV and 0.03 eV, respectively) compared to N—C-N coupling (–0.19 eV) (Fig. 4c), suggesting the thermodynamic unfeasibility of these reactions. For HER, its selectivity was evaluated by comparing the free energy change of H* adsorption with NO₃^– adsorption on Mo active sites. As shown in Fig. S18 (Supporting information), ΔG_H* is –0.70 eV, more positive than ΔG_NO, thus indicating a preference for NO₃^– reduction over HER. Consequently, the side reactions of NO₃^– reduction, CO* hydrogenation, and HER are substantially less favored compared to urea production, affirming the high selectivity of Mo₂C towards urea formation. Ab initio molecular dynamic (AIMD) simulations confirm that at 500 K, Mo₂C maintains its structure during 10 ps of annealing, with minimal fluctuations in total energy and temperature (Fig. S19 in Supporting information), further demonstrating Mo₂C's robust stability in urea formation.

  Figure 4

  

  Figure 4.  (a) Free energy diagram and optimized geometrical structures of the involved reaction intermediates for NO₃^– reduction on Mo₂C. (b) The minimum energy pathway from (NH₂* + NH₂*+ CO) to Urea* and the corresponding energy barrier. (c) The reaction energies in co-adsorbed *NH₂–NH₂* and CO further reaction.

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  In summary, we have successfully developed Mo₂C/C as a high-active catalyst for urea synthesis through the electrocatalytic coupling of NO₃^– and CO₂. The synthesized Mo₂C/C demonstrates exceptional catalytic efficiency, achieving a high urea production rate of 579.13 µg h^–1 mg^–1 and a FE of 44.80% at −0.5 V. Additionally, it exhibits remarkable electrochemical, maintaining its performance throughout 12 h of continuous electrolysis. Theoretical insights into the mechanism indicate that the formation of *NH₂ and the retention of *CO on the Mo₂C's multiple active sites is crucial for facilitating C—N bond formation during urea electrosynthesis. This research not only showcases the potential of Mo₂C/C as an electrocatalyst for the ambient conversion of NO₃^– and CO₂ but also paves the way for future experimental and theoretical studies in developing 2D electrocatalysts with multiple active sites for sustainable production of organonitrogen compounds through C—N coupling.

  Declaration of competing 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.

  CRediT authorship contribution statement

  Yue Zhang: Data curation, Writing – original draft. Xiaoya Fan: Data curation. Xun He: Data curation. Tingyu Yan: Formal analysis. Yongchao Yao: Formal analysis. Dongdong Zheng: Investigation. Jingxiang Zhao: Conceptualization, Writing – review & editing. Qinghai Cai: Investigation. Qian Liu: Validation. Luming Li: Validation. Wei Chu: Validation. Shengjun Sun: Validation. Xuping Sun: Conceptualization, Writing – review & editing.

  Acknowledgment

  The authors acknowledge support from the Natural Science Funds for Distinguished Young Scholar of Heilongjiang Province (No. JC2018004).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.109806.

  
  
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• 

  Figure 1  (a) XRD pattern of Mo₂C/C. (b) SEM image of Mo₂C/C. (c) Low and (d) high magnification TEM images of Mo₂C/C. (e) High-resolution TEM image of Mo₂C/C. (f) SAED pattern and line scan and of the HRTEM image indicated by the red box 1 in (e). (g) HAADF-STEM and its corresponding mapping images of Mo₂C/C. XPS spectra of Mo₂C/C in the (h) Mo 3d, (i) C 1s regions.

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  Figure 2  (a) LSV curves of Mo₂C/C in 0.2 mol/L Na₂SO₄ with and without saturated CO₂, respectively. (b) LSV curves of Mo₂C/C in 0.2 mol/L Na₂SO₄ with and without 0.05 mol/L NO₃⁻, respectively. (c) FEs of CO in CO₂RR process at various potentials on Mo₂C/C. (d) NH₃ yields and FEs on Mo₂C/C for NO₃RR in 0.2 mol/L Na₂SO₄ + 0.05 mol/L NO₃⁻ with various applied potentials.

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  Figure 3  (a) LSV curves of Mo₂C/C in Ar- and CO₂-saturated 0.2 mol/L Na₂SO₄ + 0.05 mol/L NO₃⁻. (b) Urea yields and FEs of Mo₂C/C at different potentials. (c) FEs of NH₃, CO, N₂, H₂, NO₂⁻ and urea on Mo₂C/C at different potentials during the electrosynthesis of urea. (d) Urea FEs at –0.5 V during consecutive recycling test. (e) Urea yields and chronoamperometry curves during consecutive recycling test.

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  Figure 4  (a) Free energy diagram and optimized geometrical structures of the involved reaction intermediates for NO₃^– reduction on Mo₂C. (b) The minimum energy pathway from (NH₂* + NH₂*+ CO) to Urea* and the corresponding energy barrier. (c) The reaction energies in co-adsorbed *NH₂–NH₂* and CO further reaction.

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