Ligand engineering towards electrocatalytic urea synthesis on a molecular catalyst | Nature Communications

Nature Communications volume 15, Article number: 8858 (2024) Cite this article

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Electrocatalytic C-N coupling from carbon dioxide and nitrate provides a sustainable alternative to the conventional energy-intensive urea synthetic protocol, enabling wastes upgrading and value-added products synthesis. The design of efficient and stable electrocatalysts is vital to promote the development of electrocatalytic urea synthesis. In this work, copper phthalocyanine (CuPc) is adopted as a modeling catalyst toward urea synthesis owing to its accurate and adjustable active configurations. Combining experimental and theoretical studies, it can be observed that the intramolecular Cu-N coordination can be strengthened with optimization in electronic structure by amino substitution (CuPc-Amino) and the electrochemically induced demetallation is efficiently suppressed, serving as the origination of its excellent activity and stability. Compared to that of CuPc (the maximum urea yield rate of 39.9 ± 1.9 mmol h−1 g−1 with 67.4% of decay in 10 test cycles), a high rate of 103.1 ± 5.3 mmol h−1 g−1 and remarkable catalytic durability have been achieved on CuPc-Amino. Isotope-labelling operando electrochemical spectroscopy measurements are performed to disclose reaction mechanisms and validate the C-N coupling processes. This work proposes a unique scheme for the rational design of molecular electrocatalysts for urea synthesis.

Urea [CO(NH2)2] is a vital chemical in the fields of energy, agriculture, medicine, and fine chemical synthesis1,2,3. Currently, the industrial synthesis of urea is still dominated by the energy-intensive Bosch-Meiser method with ammonia (NH3) as nitrogen source, operating at high temperature and pressure (150–200 °C, 150–250 bar)2,4. The Haber-Bosch is the well-established method under harsh conditions to produce NH3 based on massive consumption of fossil fuels, and most of the produced NH3 is adopted for subsequent urea synthesis5,6,7,8. The current urea synthetic route poses a major challenge to sustainable development. In this context, the renewable-energy powered electrocatalysis is emerged as a potential solution to drive the reaction under milder conditions. Pioneer works have been devoted on electrocatalytic and electrochemical NH3 synthesis from electroreduction of nitrogenous species, including dinitrogen, nitric oxide, nitrite, and nitrate9,10,11,12,13,14. However, the separation of NH3, mainly from the aqueous phase, is a considerable challenge, and the subsequent urea synthesis still needs to be carried out under harsh conditions15.

To address the above issues, the electrocatalytic C–N coupling of carbon dioxide (CO2) and nitrogenous species serves as promising alternative to enable direct and sustainable urea production, with skipping the intermediate ammonia synthesis process16,17. Nitrate (NO3–) is a harmful water pollutant, which mainly derived from fossil fuel combustion and industrial discharge18,19. The electrocatalytic C-N coupling of CO2 and NO3− enables the green synthesis of urea and the waste upgrading driven by electricity, showing great significance in maintenance of global carbon and nitrogen balances16,20,21. In recent years, researchers have made some important advances in this field, but are currently limited by low urea synthesis activity. As the crucial component of electrocatalytic system, the design of efficient and stable electrocatalysts is the focus of promoting the development of urea synthesis.

Metal phthalocyanine (MPc) has been adopted as heterogeneous molecular catalysts owing to its accurate and adjustable active configurations22. By modulating the metal centers and tuning the ligand engineering, a series of electrocatalytic small molecule conversion reactions (e.g., oxygen reduction reaction23,24,25, nitrogen reduction reaction26, and carbon dioxide reduction reaction27,28) have been realized. Although tremendous progresses have been achieved, the application of metal phthalocyanine materials in electrocatalytic C-N coupling systems is rarely reported29. Copper based materials are generally demonstrated as potential electrocatalysts toward urea synthesis from coupling of CO2 and NO3–30,31,32,33,34. Typically, Ye et al. realized the efficient C-N coupling on copper single-atom alloy by matching kinetics35. Li and collaborators achieved the efficient urea electrosynthesis via innovatively alternating Cu-W bimetallic C-N coupling sites with the state-of-the-art efficiency of 70% at ultra-low applied potential36. Meanwhile, copper phthalocyanine (CuPc) is considered to exhibit activity in electrocatalytic CO2-to-CH4 conversion, but it still remains controversial due to metal center dissolution and structural instability during electrolysis37,38,39.

In this work, CuPc is selected as a modeling catalyst to investigate its electrochemical performances toward urea synthesis. Combining experimental and theoretical studies, it can be observed that the intramolecular Cu-N coordination can be strengthened by amino substitution (CuPc-Amino) with increased coordination number and optimized electronic structure. The Raman spectroscopy, X-ray photoelectron spectroscopy, and inductively coupled plasma-optical emission spectrometer were used to analyze the catalyst states before and after the electrolysis, and it was found that the demetallization and electrochemically induced structural collapse only occurred in the unsubstituted CuPc, serving as the origination of excellent activity and stability of CuPc-Amino. Compared to that of CuPc (the maximum urea yield rate of 39.9 ± 1.9 mmol h−1 g−1 with 67.4% of decay in 10 test cycles), a high rate of 103.1 ± 5.3 mmol h−1 g−1 and remarkable catalytic durability have been achieved on CuPc-Amino. Isotope-labelling operando electrochemical spectroscopy measurements were performed to disclose reaction mechanisms and validate the C-N coupling processes.

The catalysts of CuPc and CuPc-Amino were fabricated according to the previous reports27,40,41, see “Methods” for more details. The X-ray diffraction (XRD) patterns of both CuPc and CuPc-Amino in Fig. 1a exhibit the consistent characteristic peaks with the simulated data, indicating a α-phase structure of the as-synthesized catalysts. Ultraviolet-visible (UV-vis) spectra were performed to validate the substitution of amino groups. As illustrated in Fig. 1b, the absorption peaks in the UV region (300–400 nm) and the visible region (600–750 nm) can be assigned to the B band and Q band respectively, attributing to the n–π* transition and π–π* transition on phthalocyanine. A blue shift of Q band from 670.2 nm (CuPc) to 648.1 nm (CuPc-Amino) is closely related to electron-donating ability of the substituted amino groups42,43. The formation of unsubstituted CuPc (576.05 g mol−1) and tetraamine-substituted CuPc-Amino (635.11 g mol−1) were confirmed by matrix-assisted laser desorption/ionization time-of-flight mass (MALDI-TOF MS) data (Fig. 1c)44. The high-resolution transmission electron microscopy (HR-TEM) images of CuPc and CuPc-Amino (Figure S1) show no obvious aggregation of metal sites.

a X-ray diffraction patterns of CuPc-Amino, CuPc and the simulated data. b UV-vis spectra and (c) MALDI-TOF MS spectra of CuPc-Amino and CuPc. d FT-EXAFS spectra and (e) Wavelet-transform plots for Cu element of CuPc-Amino, CuPc and Cu foil.

The atomically dispersed sites in electrocatalysts are further validated by X-ray absorption spectroscopy (XAS), and both of the CuPc and CuPc-Amino catalysts show a characteristic Cu2+ peak (1s–3d transition) at ~8985 eV in the Cu K-edge X-ray absorption near edge structure (XANES) data (Figure S2)37. Meanwhile, the Fourier transform extended X-ray absorption fine structure (FT-EXAFS) analysis for Cu in CuPc and CuPc-Amino exhibit the similar peak position at about 1.65 Å originated from the Cu-N bonds (Fig. 1d). No obvious Cu-Cu (~2.27 Å) bond was observed in these two catalysts. More importantly, the peak intensity of Cu-N bond was enhanced with the substitution of amino groups. The further data fitting results in Figure S3 suggest the increase in coordination numbers from 3.9 ± 0.2 (CuPc) to 4.2 ± 0.1 (CuPc-Amino), and the fitting parameters in Table S1 suggest that the Cu-N bond distance of the CuPc catalyst was reduced after introduction of amino groups. Wavelet-transform extended X-ray absorption fine structure (WT-EXAFS) was also conducted to identify the Cu-N and Cu-Cu paths in CuPc-Amino, CuPc and Cu foil. This conclusion is clearly illustrated in the WT-EXAFS analysis (Fig. 1e).

The electrocatalytic performances toward urea synthesis were carried out in a H-type cell. The linear sweep voltammetry curves for CuPc-Amino were firstly acquired and an obvious reduction peak appeared at about −1.0 V versus reversible hydrogen electrode (RHE) during co-electrolysis (Figure S4), which might be attributed to the interaction between N-containing and C-containing intermediate species. We next evaluated the electrocatalytic performances of the co-reduction of CO2 and NO3– in 0.1 M KHCO3 with the addition of 0.05 M KNO3 over the CuPc-Amino and CuPc cathodes, respectively. The concentrations of urea products were determined by the urease decomposition method and the yielded ammonia was quantified by the indophenol blue method (Figure S5)16. Tuning the applied potential from −1.2 V to −1.7 V (Figure S6 and S7), the yields of urea show a volcano-type tendency, and −1.6 V is optimal to give urea with a 103.1 ± 5.3 mmol h−1 g−1 yield rate over CuPc-Amino, which is approximately 2.6 times higher than that over CuPc (Fig. 2a). A higher Faradaic efficiency value of 11.9 ± 0.6% and a partial current density of 8.2 ± 0.5 mA cm−2 were also achieved over CuPc-Amino (Fig. 2b, c). The electroreduction of NO3– (NO3RR) over CuPc-Amino and CuPc was performed and higher ammonia yield rates were acquired over CuPc-Amino during the whole potential range, implying the enhancement of nitrate activation and reduction by the amino-substitution. The sharply decreased ammonia yields over CuPc-Amino in co-electrolysis suggest the transformation of NO3RR path to C-N coupling, which was not observed during co-electrolysis tests on CuPc (Figure S8). All the products in co-electrolysis over CuPc-Amino and CuPc were quantitatively analyzed by an on-line gas chromatography (Figure S9) and ultraviolet spectrophotometry (Figure S10)16, and the distributions of products were summarized in Figure S11.

a Urea yield rates, (b) Faradaic efficiencies and (c) Partial current densities for urea synthesis of CuPc-Amino and CuPc at various applied potentials. d Electrochemical stability tests on CuPc-Amino and CuPc at −1.6 V versus RHE. Electrochemical measurements were performed without iR compensation. The error bars represent the standard deviation for three independent measurements. All measurements were performed in a H-cell at 25 °C, the electrolyte for electrochemical C−N coupling is CO2-saturated 0.1 M KHCO3 and 0.05 M KNO3 (pH = 6.8 ± 0.03), and the catalyst loading is 0.2 mg cm−2. The cathodic chamber was continuously purged with high-purity CO2 with a flow rate of 30 mL min–1 and stirred at a rotation rate of 500 r.p.m. during the electrocatalytic process.

Controlled experiments were carried out and the urea products only can be detected when the CO2 and NO3− reactants co-exist (Figure S12). The 1H nuclear magnetic resonance (NMR) spectroscopy was adopted to qualitatively determine the yielded products with isotope-labelled 15NO3− as the nitrogen source. The urea products were directly (Figure S13) and indirectly detected (Figure S14) by the NMR method. The CO(14NH2)2 and the isotope-labelling CO(15NH2)2 can be distinguished by the singlet peak at ~5.62 ppm and the doublet peak at ~5.57 ppm and ~5.68 ppm, respectively15. When 14NO3– and 15NO3– were used as nitrogen sources in co-electrolysis measurements, the corresponding urea products were obtained, indicating that the nitrogen source in urea products originates from nitrate feedstocks. Comparable urea yields were obtained via the 1H NMR method and the urease decomposition method (Figure S13b), implying the reliability of urea detection in this work. However, the yield rate of urea was found to be considerably lower when CO₂ was combined with 15NO₃– than when urease and CO₂ were combined with 14NO₃–. There may be a kinetic nitrogen isotope effect due to the difference in nitrogen atoms, which affects the reaction rate, thus suppressing the urea yield rates in our case45,46. However, the specific cause need further investigation, and in the follow-up work, we will conduct research and analysis on the kinetic nitrogen isotope effect. Urea can be detected after urease decomposition into corresponding ammonia, and the results in Figure S14 also indicate that the nitrogen atoms in the urea products were derived from electrolysis process of nitrate reactant31. The urea synthesis rate proposed in this work is higher than most of the reports under comparable evaluation conditions, as illustrated in Figure S15 and Table S2. We then evaluated the electrochemical stability of catalysts at -1.6 V versus RHE (Figure S16), the sharp decreases in urea yield rate (Fig. 2d, a 67.4% of decay in 10 test cycles) and Faradaic efficiency (Figure S17) indicate the poor stability of unsubstituted CuPc, on the contrary, no obvious decay of urea yield rates was observed over CuPc-Amino, exhibiting its remarkable electrocatalytic durability.

To disclose the origination of electrochemical activity, the electrostatic potential (ESP) of CuPc-Amino and CuPc were analyzed. The site possessing more negative ESP has the stronger ability to attract electrophiles and thus is more possibly to be the reactive site47. According to the results in Fig. 3a and Supplementary Data 1, the ESP of Cu site in CuPc-Amino was calculated to be 5.64 kcal mol−1, which is much lower than that of Cu site in CuPc (20.41 kcal mol−1), enabling the efficient adsorption and activation of electrophilic CO2 and nitrate, as well as their intermediate species. The Hirshfeld charge analysis results in Fig. 3b suggest that the Cu center in the CuPc-Amino bear fewer positive charges than that in CuPc, that is, the electron density in active center of CuPc-Amino is higher than CuPc48. This would make the Cu center in CuPc-Amino binds oxygenated species more strongly than the CuPc49. The theoretical analysis is consistent with the experimental results in Fig. 2 and Figure S8. The increase of coordination numbers in Fig. 1d and Figure S3 also might contribute to the improvement of intrinsic urea synthesis activity50,51. The amino substitution optimizes the electronic structure of CuPc, and possibly promotes the adsorption and interaction of oxygen-containing electrophilic intermediate species, thus enhancing the activity of electrocatalytic urea synthesis.

a Electrostatic potential (ESP) of CuPc-Amino and CuPc. b Hirshfeld charges of CuPc-Amino and CuPc. The orange, gray, blue and white balls represent Cu, C, N and H atoms, respectively. X-ray photoelectron spectra of as-prepared and post-electrolysis (c) CuPc and (d) CuPc-Amino. Raman spectra of as-prepared and post-electrolysis (e) CuPc and (f) CuPc-Amino.

For further revealing the promoting mechanism of amino substitution on electrocatalytic stability, the cyclic voltammograms (CVs) from the CuPc-Amino and CuPc were obtained in 0.1 M KHCO3 (Figure S18). In the cathodic scan, the CuPc sample exhibits two reduction peaks corresponding to the reduction of Cu2+ to Cu+ and the reduction of Cu+ to Cu0. It indicates that the copper centers are electrochemically instable in CuPc, while the electrochemical reduction for the copper centers in CuPc-Amino sample was efficiently suppressed. Figure 3c shows the Cu 2p X-ray photoelectron spectra (XPS) of as-prepared CuPc and after electrolysis at −1.6 V. The peaks in as-prepared CuPc could be assigned to Cu2+52, while two peaks of about 952 eV and about 933 eV arising from Cu+ or Cu0 appeared after electrolysis53. Notably, the chemical states of copper centers in CuPc-Amino were barely affect by the cathodic reduction (Fig. 3d). Cu LMM Auger spectrum of the post-electrolysis CuPc was performed. As illustrated in Figure S19, the peak at the kinetic energy of 915.1 eV can be assigned to Cu2+ species. Meanwhile, the peaks at 910.6 eV and 917.7 eV belong to Cu+ and Cu0 species respectively, which are generated by electrochemical reduction processes54. Raman spectroscopy of as prepared CuPc exhibited the characteristic peaks at 681 and 1108 cm−1 (Fig. 3e), attributing to the in plane full symmetric Cu-N stretch and in plane diag symmetric N-Cu-N bend respectively55,56. The decrease in these peak intensities with post-electrolysis implies that the demetallation of Cu centers occurred in CuPc57. The Raman peaks shifted to 690 and 1126 cm−1 (Fig. 3f) in the as-prepared CuPc-Amino are owing to the substitution of amino groups, meanwhile, the Cu-N coordination remains unchanged after electrolysis.

The inductively coupled plasma-optical emission spectrometer (ICP-OES) were used to analyze to identify whether the metal leaching into electrolyte occurs during electrochemical process. As illustrated in Figure S20, the copper leaching was only observed with CuPc electrode. HR-TEM images were acquired after electrolysis and the metal aggregation to form Cu nanoparticles was exclusively observed for CuPc (Figure S21). The aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of CuPc-Amino before and after co-electrolysis have been supplemented as Figure S22 in the revised manuscript. The bright dots in Figure S22a indicate atomic dispersion of Cu centers in CuPc-Amino catalyst, and no obvious aggregation was observed after electrolysis, indicating its superior structural stability during electrolysis process. Through theoretical calculations58, it was found that amino substitution can strengthen the intramolecular metal ligand engineering enhance the binding energy of Cu and N atoms (Table S3), thus inhibiting the demetallation and improving the structural and electrochemical stability of the catalyst. We then evaluated the effect of substituents on the urea synthesis activity. The electron-withdrawing carboxyl and sulfonic acid substituted CuPc, and the electron-donating hydroxyl modified CuPc were synthesized, respectively. In general, the electron-donating groups (amino and hydroxyl) tend to promote the urea synthesis activity, whereas the electron-withdrawing groups (sulfonic acid and carboxyl) tend to suppress the catalytic activity of CuPc (Figure S23), and the urea synthesis activity shows a trend of enhancement with the increase of electron-donating ability59, implying a positive correlation between the increase of catalytic activity and the electron-donating ability.

To gain an in-depth understanding of the catalytic process, isotope-labelling operando synchrotron-radiation Fourier transform infrared (SR-FTIR) experiments were carried out for the CuPc-Amino to monitor the evolution of bonding structure under working conditions (Figure S24)15,20. As shown in Fig. 4a, the band at ~1639 cm−1 can be assigned to the stretching vibration of C = O60,61, suggesting the adsorption and activation of oxygen-containing species occurred on the catalyst surface. More importantly, a stretching vibration of C-N at ~1403 cm−1 emerged as the solid evidence for the occurrence of C–N coupling process62. The band at ~2159 cm−1 is attributed to the stretching vibration of OCNO species63, which derived from coupling of *CO and *NO intermediates as reported16,64. The peak intensities of C-N and OCNO are potential dependent and reached their peak values at −1.6 V, which is consistent with the electrochemical measurements. The activation of these electrophilic oxygenated species and the generation of C-N bond are highly correlated to the aforementioned electronic localization on CuPc-Amino. Isotope-labelling operando SR-FTIR measurements were conducted for the CuPc-Amino catalyst with 14NO3− or 15NO3− as the nitrogen source. As seen in Fig. 4b, the vibrations of C−15N and OC15NO are shifted toward lower wavenumbers by about 33 cm−1. These shifts are attributed to the isotope effect65, validating that the nitrogen atoms in urea products were derived from the nitrate reactants via electrocatalytic process.

a Infrared signal in the range of 1200–2400 cm−1 at various applied potentials for CuPc-Amino during co-electrolysis. OCV represents open circuit voltage. b Infrared signal at -1.6 V versus RHE for CuPc-Amino during the electrocoupling of 14NO3–/15NO3– and CO2 processes.

To clarify the active sites of the CuPc-Amino and catalytic reaction mechanism, we evaluated evaluated nitrate reduction and CO2 reduction activities over catalysts (CuPc-Amino, CuPc, Pc-Amino and Pc), and the results (Figure S25) indicate that pure Pc without Cu center possesses considerable nitrate activation and reduction ability, which is related to its azacyclic structure66, and the introduction of Cu center significantly promotes the CO2 reduction process. Combining urea synthesis activity and operando spectroscopic analysis, we proposed the following urea synthesis mechanism: nitrate and CO2 were firstly adsorbed and activated at Pc and Cu sites respectively, and then converted into *NO and *CO intermediates through electrochemical reduction, and urea was formed finally through two-step C–N couplings and multi-step hydrogenation processes. The detailed reaction mechanism has been illustrated in Figure S25d.

Electrocatalytic urea synthesis via C-N coupling of carbon dioxide and nitrate shows great significance for the sustainable development of urea industry. This work aims at promoting the activity and stability for urea synthesis on a modeling molecular catalyst. The substitution of amino groups into copper phthalocyanine induces electronic localization and the higher electron density favors to adsorb electrophilic oxygenated species, thus enhancing the intrinsic activity for C-N coupling and urea synthesis. At the same time, the amino-substitution strengthens the intramolecular metal coordination to suppress the electrochemically induced metal demetallation and structural collapse, promoting the structural and electrochemical stability. Compared to that of unsubstituted CuPc, a high urea yield rate of 103.1 ± 5.3 mmol h−1 g−1 and remarkable durability have been realized over CuPc-Amino. This work provides a unique scheme for rational design of urea synthesis electrocatalyst, paving the way for application of sustainable urea synthesis in the future.

Carbon papers (HCP020N, thickness: 0.19 ± 0.01 mm) were purchased from Hesen company (Shanghai, China). These cathodic substrates were annealed in a Muffle furnace at 400 °C for 16 h before use. Glass carbon electrode holder (PEEK) purchased from GaossUnion Company (Wuhan, China). Copper (II) acetate monohydrate was purchased from Ourchem. KHCO3 ( ≥ 99.99% metals basis, 99.7-100.5% dry basis), trimellitic anhydride, 4-nitrophtalonitrile, 1,2-dicyanobenzene, was bought from Aladdin Biochemical Technology Co., Ltd. Potassium nitrate (KNO3, > 99%), potassium chloride (KCl, AR), sodium hydroxide (NaOH, AR), salicylic acid (C7H6O3), trisodium citrate dihydrate (C6H5Na3O7·2H2O, AR), ethylenediaminetetraacetic acid disodium salt (C10H14N2Na2O8·2H2O) Potassium hydroxide (KOH, AR), hydrogen peroxide 30% aqueous solution (H2O2), ammonium chloride (NH4Cl), Copper chloride (CuCl2·2H2O, > 99%), methanol (CH3OH), isopropanol (C3H8O), N,N-dimethylformamide (DMF), and ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O AR) were received from Sinopharm Chemical Reagent Co., Ltd. Sodium hypochlorite (NaClO),11-15% available chlorine was bought from Thermo scientific. Sodium nitroprusside dihydrate (Na2[Fe(CN)5NO]·2H2O) was purchased from Energy Chemical. K15NO3 (Isotopic abundance: 99 atom%) was obtained from Aladdin Biochemical Technology Co., Ltd. Urease (Urease activity: U1500−20KU, Batch number: Lot#SLCJ5647) from Canavalia ensiformis (Jack bean) was purchased from Sigma. Sodium sulfide nine-hydrate is purchased in Macklin reagent. All chemicals were purchased and used as received without further purification unless otherwise stated. CO2 (99.99%) and Ar (99.999%) were purchased from Airgas.

The synthesis of CuPc is based on some modified procedures reported in the literature29. 0.738 g 1,2-Dicyanobenzene (5.76 mmol), 0.287 g CuCl2 · 2H2O (1.44 mmol), and 30 mg (NH4)6Mo7O24 · 4H2O catalyst were placed in ethylene glycol solvent. After stirring at 60 °C for 30 min, the solution was placed into a teflon-lined 100 mL autoclave, which was kept at 180 °C for 20 h. The formed precipitate was naturally cooled to room temperature and then washed three times each with 1.0 N hydrochloric acid, hot water, and ethanol to remove residual reagents. Finally, the obtained CuPc powder was dried in a vacuum oven at 65 °C for 24 h.

The synthesis of CuPc-Amino is based on some modified procedures reported in the literature27. 1.73 g 4-Nitrophthalitrile (10 mmol), 0.426 g CuCl2 · 2H2O (2.5 mmol), 4.80 g urea (80 mmol) and catalytic capacity (NH4)6Mo7O24 · 4H2O. The mixture is first ground evenly in the agate mortar. The solid mixture is then reacted in a tubular furnace at 170 °C for 5 h at a rate of 2 °C min−1. The product was mixed in hydrochloric acid (1 M, 200 mL) and stirred at 90 °C for 1 h. Strain the solids, then stir in sodium hydroxide (1 M, 200 mL) for 1 h. The crude product was filtered, washed with water for 3 times, and then purified with methanol in Soxhlet extraction. The resulting solids are then dissolved in DMF and filtered. After vacuum drying for 24 h, the obtained dark green solid was CuPc-NO2. The previously synthesized 0.75 g CuPc-NO2 (1 mmol), 4.8 gNa2S·9H2O (20 mmol), 1 mL deionized water and 25 mL DMF were mixed in a three-neck round bottomed bottle and stirred at 60 °C under nitrogen atmosphere for 12 h. Then, washed with water for 3 times and vacuum dried. The obtained solids were added into 100 mL (5 wt%) sodium hydroxide solution and boiled for 4 h. The sediment was then washed and filtered with water. Pour the resulting solids into 250 mL pure water and add 1 mol L−1 hydrochloric acid to adjust the pH (~5.0). The filtered precipitates were poured into 250 mL distilled water, 1 mol L−1 potassium hydroxide solution was added to adjust the pH (~8.0), and then boiled for 4 h. The precipitation was filtered and collected to obtain the dark green compound (CuPc-Amino).

X-ray diffraction (XRD) was performed on D8 Advance X-ray diffractometer using Cu Kα1 (Brucker). Aberration-corrected high angle annular dark field (HAADF)-scanning transmission electron microscope (STEM) images were acquired on a Titan ETEM G2 80-300 electron microscope. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) analysis were performed on an AXIS SUPRA system (Shimadzu, Kratos Axis Supra Japan) using monochromatic Al Kα radiation. Matrix Assisted Laser Desorption Ionization Time of Flight Mass Spectrometry (MALDI-TOF-MS) was performed using Shimadzu AXIMA-Confidence test. UV-vis spectra were collected on a PerkinElmer (Lambda 365) visible UV spectrophotometer. ICP-OES was tested using the Agilent ICP-OES 5800 instrument. Fourier infrared spectroscopy (FTIR) was tested using a Thermo Scientific iN10 model instrument (USA). Raman spectra were acquired on a photocurrent Raman imaging atlas, Witec Alpha300R.

The electrolytes for electrocatalytic C-N coupling were prepared as follows: 10.012 g of KHCO3 and 5.055 g of KNO3 were dissolved into the deionized water (Conductivity: 18.25 MΩ cm−1). The total volume of the solution is then determined to one liter (1 L), thus the concentrations of KHCO3 and KNO3 are 0.1 M and 0.05 M respectively. The same volume of electrolytes (40 mL for each) was taken out and added into the cathodic and anodic chambers of the electrolytic cell respectively, and the cathodic electrolyte was purged with high-purity CO2 gas before electrochemical tests. The preparation of CO2 reduction electrolyte is similar to the above procedures, except that nitrate is not added. The sole nitrate reduction electrolyte was consistent with the C-N coupling electrolyte except that it was saturated with Ar gas before the test. Other than storage at room temperature, no additional storage conditions are required for the above electrolytes.

A Nafion 117 membrane (Dupont) with a geometric area of 2 × 2 cm2 (thickness: 183 um) was used to separate the cathodic and the anodic chambers. Nafion membranes were pre-treated as follows: Firstly, Nafion 117 membranes are boiled in a 5% aqueous solution of H2O2 at 80 °C for 1 hto remove organic impurities from the membrane. After that, the membrane is repeatedly rinsed with deionized water and immersed in deionized water at 80 °C for 1 h to completely remove the residual H2O2. And the membrane was preserved in deionized water at room temperature before use. Electrocatalysis was conducted in a H-type cell with a three-electrode configuration on a CHI760E electrochemical station (CH Instruments Ins., China). For the acquirement of I-t curves, it is carried out by setting applied potential (versus RHE) and electrolysis time using chronoamperometric test procedure. For the acquirement of LSV curves, the linear voltammetry scan test was applied with a negative scanning mode over the potential range from 0.0 V to −2.0 V (versus RHE).

The electrolyte utilized for coupling reactions was CO2-saturated 0.1 M KHCO3 with 0.05 M KNO3. Ag/AgCl electrode filled with saturated KCl solution and a carbon rod were used as reference electrode and counter electrode, respectively. All potentials in this study were measured against the Ag/AgCl reference electrode and converted to the RHE reference scale by ERHE = EAg/AgCl + 0.0591 × pH + 0.197 V, where pH is the average pH value of electrolyte. The pH of the electrolyte was measured on a FE28-CN pH-meter (Mettler Toledo). The pH value of a CO2-saturated electrolyte is 6.8 ± 0.03, and it is 8.3 ± 0.06 for Ar-saturated electrolyte. Before the electrochemical test, the cathode electrolyte (40 mL) was continuously fed with CO2 gas with a flow rate of 50 mL min−1 for 20 min to achieve saturation. Then, the flow rate was set to 30 mL min−1 during the electrocatalytic process. Electrocatalytic of urea synthesis was conducted in CO2-saturated 0.1 M KHCO3 containing 0.05 M KNO3 and the single electrolysis time is 20 min for each cycle. Isotope-labelling experiments were conducted except that the N-source was replaced by the isotope-labelled K15NO3. The LSV and CV curves were recorded at a scan rate of 10 mV s−1 by loading CuPc-Amino or CuPc on glassy carbon electrode.

The working electrode for chronoamperometry tests was based on a catalyst-modified carbon paper configuration. The carbon paper (HCP020N, Hesen Company) was firstly annealed at 400 °C for 16 h to enhance its hydrophilicity. Then, the carbon substrate was cut into rectangular pieces with a geometric area of 1 × 0.5 cm2. 2 mg of catalyst and 20 μL of Nafion solution (Dupont, 5 wt%) were dispersed in 480 uL of dispersant (isopropyl alcohol: water = 2:1) with ultrasonication for 40 min to form a homogeneous catalyst ink. Subsequently, 25 uL of the catalyst ink was drop-casted onto the above carbon substrate and dried naturally. Thus, the catalyst loading is 0.2 mg cm−2. The electrode for linear voltammetry scan measurements was based on a circular glassy carbon configuration with a diameter of 0.5 cm. The preparation of catalyst inks and catalyst loading (0.2 mg cm−2) are consistent with the above tests.

The gaseous products were detected by an online gas chromatograph (SP-7890), thermal conductivity detector (TCD), and flame ionization detector (FID) was used for H2, CO and CH4 quantification. Ultrahigh purity Ar (99.999%) was used as the carrier gas. The volume ratio of gaseous products was calibrated by standard curves from standard gas (Figure S9).

Nitrate concentrations were determined by using N-(-1-naphthyl)-ethylenediamine dihydrochloride spectrophotometric method16. The color developer was prepared as follows: 0.8 g p-aminobenzene sulfonamide was added to a mixed solution of 10 mL of water and 2 mL of phosphoric acid, and then 0.04 g of N-(-1-naphthyl)-ethylenediamine dihydrochloride was dissolved in the above solution. 5 mL distilled water and 50 μL of electrolyte was mixed with 100 μL chromogenic agent. Placing in a dark place and react for 20 min. The absorbances were recorded at 540 nm (Figure S10).

Ammonia concentrations were measured by the indophenol blue method with the absorbances recorded on an ultraviolet-visible spectroscopy (UV-vis)31. The indophenol blue developing agents were as follows: (A) 1 M NaOH solution containing 5 wt% sodium citrate and 5 wt % salicylic acid, (B) 0.05 M NaClO solution, (C) 1 wt% sodium nitroferricyanide solution. Then 1.8 mL of electrolyte was obtained from the catholic chamber, 200 μL sodium salt solution EDTA and 2 mL (A), 1 mL (B), 200 μL (C) mixed for 2 h in the dark. Because the concentration of ammonium ions after dark reaction is more than 8 ppm, it is difficult to accurately quantify the concentration of ammonium ions by ultraviolet spectrophotometer. Therefore, the method of simultaneous dilution is used to detect the concentration of ammonium ions, and the dilution factor is 2.66 times. The details are as follows: 2.5 mL of the blank electrolyte was added to a 5 mL small centrifuge tube, and 1.5 mL of the dark reaction solution was added, and the absorbance was measured after mixing evenly. Absorbance measurements were performed at λ = 662 nm (Figure S5).

Urea products were determined by the urease decomposition method. The details of urease decomposition are as follows: A mixture of 1.8 mL urea electrolyte and 0.2 mL urease solution with a concentration of 5 mg mL−1 was formed and fully reacted on a constant temperature shaking bed at 37 °C for 40 min. Since one urea molecule can be decomposed into two ammonia molecules. After the decomposition, the moles of urea were calculated by the equation:

The mammonia and murease represent the moles of ammonia before and after urease decomposition process, respectively.

For gas products, the equation for calcualtion of Faradaic efficiency was presented as follows:

where n is the number of electron transfer for the generation of corresponding one molecule. It is 2 for carbon monoxide and hydrogen, and 8 for methane. P is the atmospheric pressure (1.013 × 105 Pa), V is the gas flow rate determined by a flowmeter, v (vol%) is the volume ratio of gas products in the gas chromatography sampling loop, F is the Faradaic constant (96485 C mol−1), R is the ideal gas constant (8.314 m3 Pa mol−1 K−1), T is the reaction temperature (298 K), Q is the electric quantity in product collection process.

For liquid products, the equation for calcualtion of Faradaic efficiency was presented as follows:

where n is the number of electron transfer for the generation of corresponding one molecule. It is 2 for nitrite, 8 for ammonia, and 16 for urea. F is the Faradaic constant (96485 C mol−1), C is the concentration of generated products (ppm, mg L−1), V is the volume of the cathodic electrolyte (0.04 L), M is the relative molecular mass (62 for nitrite, 17 for ammonia, and 60 for urea), Q is the electric quantity in product collection process.

The NMR method was utilized to detect indirectly and directly urea products. Based on the determination of produced ammonia after urease decomposition, the way of indirect detection of urea was generated. As a typical NMR test procedure, 500 μL of electrolyte and 100 μL of d6-DMSO were mixed at a ratio of 5:1 as the deuterated solvent. Before the NMR measurements, the pH of the above solution was adjusted to ~3 by addition of an appropriate amount of 1 M HCl. The measurements process before urease decomposition was consistent of the procedure after urease decomposition, and the recorded data were the accumulated results of 1024 scans on a Bruker at 600 MHz (Figure S14). The direct NMR method was exploited to determine the urea on a Bruker at 800 MHz, which equipped with an ultra-low temperature probe (liquid helium), and the recorded data were the accumulated results of 2048 scans (Figure S13).

Operando SR-FTIR measurements were conducted at the infrared beamline BL01B in the National Synchrotron Radiation Laboratory (NSRL, China). The top-plate reflection infrared device was constructed self-produced and the infrared transmission window (cut-off energy of ~625 cm−1) composed of a ZnSe crystal were employed to achieve measurements. This end station was equipped with three main components: one was an FTIR spectrometer (Bruker 66 v/s) with a KBr beam splitter, another was various detectors (herein mercury cadmium telluride detector equipped a liquid nitrogen cooled was adapt), the last was an infrared microscope (Bruker Hyperion 3000) with a ×16 objective and a high spectral resolution of 0.25 cm−1. In order to reduce the loss of infrared light, the catalyst electrode was tightly pressed against the ZnSe crystal window with a micrometre-scale gap. The spectrum of an open-circuit voltage was acquired to eliminate the impact of the background spectrum before each systemic measurement, and the measured potential ranges of the electrocoupling reaction were −1.2 to −1.7 V with an interval of 0.1 V.

All structures were optimized in the gas phase at PBE0 BSI level67, where BSI represents a basis set combination of SDD68 for Cu and 6-31 G(d,p)69 for main group atoms. Harmonic frequency analysis calculations were subsequently performed to verify the optimized geometries to be minima (no imaginary frequency) or transition states (TSs, with unique one imaginary frequency). The transition states were checked through intrinsic reaction coordinate (IRC)calculations. Improved energies were computed at PBE0/def2-TZVP70 single-point calculations including solvation effects with the SMD71 continuum solvation model. All calculations were carried out using Gaussian 16 program72. All other wavefunction analyses were finished by Multiwfn73 The color mapped isosurface graphs of electrostatic potential (ESP) and Hirshfeld charge were rendered by VMD 1.9 program74. The binding energy (Ebin) for metal decorated phthalocyanine is defined as Ebin = EMe+Pc − EPc – EMe, Where EPc, and EMe+Pc represent the total energies of pristine and metal-doped Pc, respectively. EMe is the energy of the corresponding volume of an isolated metal atom (EMe(bulk))58.

All data generated or analysed during this study are included in this Article (and Supplementary Information). Data for Figs. 1–4 are available as source data with this paper. Source data are provided with this paper.

The computational codes used in the current work are available from the corresponding author on reasonable request.

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Wang acknowledges financial support from the National Key R&D Program of China (2020YFA0710000). Chen acknowledges financial support from the National Natural Science Foundation of China (Nos. 22425021, 22250006, 22261160640, 22202065), the Hunan Provincial Science Fund for Distinguished Young Scholars (2023JJ10002), the China Postdoctoral Science Foundation (Nos. BX20200116, 2020M682540). Chen acknowledges financial support from the Natural Science Foundation of Shandong Province (ZR2020QB120). Shao acknowledges financial support from the Joint Scientific Research Project Funding by the National Natural Science Foundation of China and the Macao Science and Technology Development Fund (0090/2022/AFJ), the Multi-Year Research Grant (MYRG) from University of Macau (MYRG2022-00105-IAPME).

These authors contributed equally: Han Li, Leitao Xu, Shuowen Bo, Yujie Wang.

State Key Laboratory of Chemo/Bio-Sensing and Chemometrics, College of Chemistry and Chemical Engineering, Advanced Catalytic Engineering Research Center of the Ministry of Education, Hunan University, Changsha, P. R. China

Han Li, Leitao Xu, Yujie Wang, Han Xu, Chen Chen, Ruping Miao, Dawei Chen, Kefan Zhang & Shuangyin Wang

National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui, P. R. China

Shuowen Bo & Qinghua Liu

Institute of Applied Physics and Materials Engineering, University of Macau, Macau SAR, P. R. China

Jingjun Shen & Huaiyu Shao

Key Laboratory of Magnetic Molecules and Magnetic Information Materials (Ministry of Education), School of Chemistry and Material Science, Shanxi Normal University, Taiyuan, Shanxi, P. R. China

Jianfeng Jia

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S.W., C.C., and D.C. conceived the project. H.L. and C.C. carried out most of the materials fabrication and characterization and prepared the manuscript. L.X. performed the theoretical calculations. S.B. and Q.L. conducted the operando SR-FTIR measurements. Y.W., H.X., R.M., and K.Z. conducted part of the electrochemical measurements. J.S., H.S., and J.J. performed part of the physical characterization of the catalysts. All authors discussed the results and commented on the manuscript.

Correspondence to Chen Chen, Dawei Chen or Shuangyin Wang.

The authors declare no competing interests.

Nature Communications thanks Uttam Kumar Ghorai, Wei Ye and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. A peer review file is available.

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Li, H., Xu, L., Bo, S. et al. Ligand engineering towards electrocatalytic urea synthesis on a molecular catalyst. Nat Commun 15, 8858 (2024). https://doi.org/10.1038/s41467-024-52832-2

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