Hydrogen-intercalation PdZn bimetallene for urea electro-synthesis from nitrate and carbon dioxide

Abstract

Electrochemical co-reduction of carbon dioxide and nitrate is a green technology to replace traditional energy-intensive methods for urea synthesis, and the development of high-performance catalysts is still a great challenge. Here, we propose the incorporation of nonmetallic hydrogen and oxophilic zinc into palladium metallene for the preparation of hydrogen-intercalation PdZn (H-PdZn) bimetallene, serving as an active electrocatalyst for co-reduction of carbon dioxide and nitrate to synthesize urea via the C–N coupling reaction. The H-PdZn bimetallene affords a high urea yield of 314.17 μg h⁻¹ mg⁻¹ and a Faraday efficiency of 24.39%, better than those of PdZn bimetallene (144.25 μg h⁻¹ mg⁻¹ and 16.03%). The strong electronic interaction between the Pd, Zn and H atoms can induce the downshift of the Pd d-band center of H-PdZn bimetallene, which can promote the formation of the key intermediates of *NH₂ and *CO, and lower the energy barrier for their C–N coupling to synthesize urea. This work offers a hydrogenation strategy for the construction of advanced PdH-based metallenes towards electrochemical C–N coupling to synthesize urea.

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Introduction

Urea (CO(NH₂)₂) is not only one of the important organic chemicals, but also the most widely used nitrogen fertilizer, which greatly supports global agricultural and social development.^1–3 The industrial synthesis of urea usually involves carbon dioxide and ammonia under harsh conditions (NH₃ + CO₂ → CO(NH₂)₂, 150–250 bar, and 180–200 °C),^4,5 and this process consumes about 80% global production of NH₃ obtained from the Haber–Bosch method (N₂ + H₂ → NH₃, 150–350 bar, and 350–550 °C), which requires substantial fossil fuel consumption and results in large CO₂ emissions.^6–9 To address this problem, there is an urgent need to develop a green and sustainable technology for urea synthesis under mild conditions. Recent reports have demonstrated that electrocatalytic co-reduction of N₂ and CO₂ has been a promising alternative method for urea production under mild conditions via the C–N coupling reaction.^10,11 However, the inherent chemical inertness of N₂ and ultralow solubility severely limited the urea yield and Faraday efficiency.^10–14 Nitrate has high solubility in water and a relatively low dissociation energy of the N=O bond (204 kJ mol⁻¹),^15–18 which is an ideal nitrogenous reactant with CO₂ for C–N coupling to produce urea. However, this method involves a 16-electron reaction process, usually resulting in complex product distribution and unsatisfactory activity and selectivity.^11,19,20 Moreover, the competitive hydrogen evolution reaction (HER) is another challenge that inhibits urea electro-synthesis.⁶ It is highly desired to design highly selective catalysts for urea production.

Palladium has been widely demonstrated to be a highly active catalyst for the NO₃⁻ reduction reaction (NO₃RR) and CO₂ reduction reaction (CO₂RR).^21–23 As such, Pd electrocatalysts have been utilized for co-reduction of NO₃⁻ and CO₂ to synthesize urea via C–N coupling,^24,25 but they usually suffer from unsatisfactory selectivity for the urea product. Designing the morphology and optimizing the composition are considered as two effective strategies for regulating catalytic performance.^26–28 Metallene as a novel ultrathin 2D sheet structure has abundant exposed metal active sites, high electrical conductivity and high atomic utilization, attracting intensive attention in electrocatalysis.^29,30 Moreover, the combination of Pd with other elements is a promising strategy that not only effectively modifies the electronic structure of Pd, but also reduces the use of precious metals.^31,32 Palladium hydride (PdH) has received widespread attention in electrocatalysis due to the unique physicochemical properties, such as nitrogen reduction reaction,³³ CO₂ reduction,³⁴ formic acid oxidation,³⁵ and methanol oxidation.³⁶ Moreover, it has been reported that the introduction of oxophilic Zn can facilitate the generation of *CO and *NH₂ intermediates and facilitate their C–N coupling reaction to synthesize urea because of strong electron transfer from oxophilic Zn to Cu.³⁷ Therefore, it is highly desired to develop hydrogen-intercalated bimetallene for urea electro-synthesis.

In this work, hydrogen-intercalation PdZn (H-PdZn) bimetallene is synthesized by the incorporation of nonmetallic hydrogen and oxophilic zinc into Pd metallene using N,N-dimethylformamide (DMF) as the hydrogen source, which serves as a high-efficiency catalyst for urea electro-synthesis via C–N coupling from CO₂ and nitrate. The H-PdZn bimetallene has an ultrathin bent and folded morphology, demonstrating an excellent urea yield (R_urea, 314.17 μg h⁻¹ mg⁻¹) and Faraday efficiency (FE, 24.39%). The ultrathin nanosheet structure and the strong electronic interaction between Pd, Zn and H can provide rich active centers and cause the decrease of the Pd d-band center, which can optimize the adsorption energy of intermediates on active sites, thus enhancing the electrocatalytic performance for urea production. This work offers an attractive approach for the construction of hydrogen-intercalated Pd-based metallene for the electrochemical C–N coupling reaction to synthesize urea.

Experimental

Chemicals and materials

Sodium chloro-palladate (Na₂PdCl₄), zinc acetylacetonate (Zn(acac)₂), potassium hydroxide (KOH), N,N-dimethylformamide (DMF), ethylene glycol (EG), potassium nitrate (KNO₃), potassium bicarbonate (KHCO₃), potassium sulfate (K₂SO₄), sodium hydroxide (NaOH), diacetyl monoxime (DAMO), ferric chloride (FeCl₃), salicylic acid (C₇H₆O₃), sodium citrate (C₆H₅Na₃O₇·2H₂O), N-(1-naphthyl) ethylenediamine dihydrochloride (C₁₂H₁₄N₂·2HCl), sulfanilamide (C₆H₈N₂O₂S), sodium hypochlorite (NaClO), Nafion solution (0.5 wt%), and phosphoric acid (H₃PO₄, ≥85%) were all purchased from Aladdin Chemicals. Diethylenetriamine (DETA), thiosemicarbazide (TSC) and sodium nitroferricyanide (C₅FeN₆Na₂O) were obtained from Macklin. All chemicals and materials were used as received without further purification.

Preparation of H-PdZn bimetallene

All chemicals and reagents were received and used without further purification. PdZn bimetallene was first synthesized via a one-step hydrothermal method according to the literature with slight modifications.²⁶ Typically, 1 g of KOH was dissolved in 6 mL of N,N-dimethylformamide (DMF) and 4 mL of ethylene glycol under ultrasonication for 30 min, followed by adding 300 μL of 0.1 M Na₂PdCl₄ DMF solution, 150 μL of 0.1 M Zn(acac)₂ DMF solution and 5 mL of diethylenetriamine (DETA) to form a homogeneous solution. The solution mixtures were then put into a Teflon-lined reactor and heated at 200 °C for 8 h. After that, PdZn bimetallene was obtained by centrifugation and washing for several cycles. The obtained PdZn bimetallene (8 mg) was dispersed in 20 mL of DMF, which was heated at 160 °C for 16 h. Finally, the H-PdZn bimetallene was collected by centrifugation and washing for several cycles.

Electrochemical measurements

All electrochemical measurements were carried out using a CHI660E electrochemical workstation in an H-type cell using a three-electrode system. Catalyst-coated carbon paper (1 × 1 cm²), a Pt sheet and an Ag/AgCl (saturated KCl) electrode were employed as the working electrode, counter electrode and reference electrode, respectively. The obtained electrocatalyst (5 mg) was dispersed in 950 μL of ethanol and 50 μL of Nafion solution to form a catalyst ink under ultrasonication. Then, 100 μL of ink was added dropwise on a piece of carbon paper (1 × 1 cm²) and dried under ambient conditions to form the working electrode with a loading of about 0.5 mg cm⁻². Prior to electrochemical experiments, CO₂ was continuously bubbled into the cathode chamber to obtain CO₂-saturated 0.1 M KNO₃. Then, CO₂ was continuously flowed into the cathode chamber with stirring at a flow rate of 300 rpm for urea electro-synthesis. For comparison, electrochemical NO₃RR tests were conducted in 0.1 M KNO₃ without CO₂ supply, and electrochemical CO₂RR tests were performed in CO₂-saturated 0.1 M KHCO₃ solution with a constant flow of CO₂. All measured potentials were referenced to the reversible hydrogen electrode (RHE). The electrochemical measurements and product detection are described in the ESI file† in detail.

Results and discussion

The preparation of H-PdZn bimetallene via a two-step method is depicted in Fig. 1. During the preparation of PdZn bimetallene by a solvothermal reaction, DETA can combine with metal precursors to form chelates, thus resulting in a decreased reduction rate of metal precursors,³⁸ which can allow the selective growth of nanosheets. Furthermore, the DMF is easily decomposed into DMA and CO at elevated temperatures in the presence of KOH,³⁹ which can facilitate facet regulation and induce the formation of ultra-thin nanosheets. After that, active hydrogen atoms can be generated from the decomposition of DMF, which can be easily adsorbed on the surface of the PdZn bimetallene. Subsequently, hydrogen atoms gradually penetrate into the lattices to form H-PdZn bimetallene due to the strong affinity between H and Pd,^40,41 resulting in lattice expansion.

Fig. 1 Graphic synthesis process of H-PdZn bimetallene.

The morphology of H-PdZn bimetallene is first investigated. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and transmission electron microscopy (TEM) images show that H-PdZn bimetallene has a folded ultrathin nanosheet structure (Fig. 2a and b), which is similar to that of PdZn bimetallene (Fig. S1a and b†). Meanwhile, the transmission electron microscopy (TEM) image and corresponding selected area electron diffraction (SAED) pattern reveal four distinct concentric rings that match well with the (111), (200), (220), and (311) facets of H-PdZn bimetallene (Fig. S2†). The atomic force microscopy (AFM) image of H-PdZn bimetallene exhibits an average thickness of about 1.2 nm (Fig. 2c), which can expose sufficient under-coordinated metal atoms that are favorable for the adsorption and activation of substances in the electrocatalytic process. As shown in Fig. 2d, H-PdZn bimetallene has a clear lattice stripe with a spacing of 0.230 nm, which is larger than that of the (111) plane of PdZn bimetallene (0.222 nm, Fig. S1c†), indicating lattice expansion due to the incorporation of hydrogen atoms into the lattice gap. In addition, element mapping images obviously show the uniform distribution of Pd and Zn in the samples (Fig. 2e), with the similar Pd/Zn atomic ratios of the samples from energy dispersive X-ray spectroscopy (EDS) (Fig. S1d and S3†).

Fig. 2 The characterization of H-PdZn bimetallene. (a) HAADF-STEM and (b) TEM images. (c) AFM image and corresponding height profile. (d) HRTEM image and corresponding Fourier-filtered lattice fringes and atomic absorption intensity profile. (e) HAADF-STEM image and corresponding element mapping images.

The crystalline structure of the samples is then characterized by X-ray diffraction (XRD). As shown in Fig. 3a, the XRD diffraction peaks of PdZn bimetallene shift to higher angles compared to those of standard face centered cubic (fcc) Pd (JCPDS card 46-1043), which reveals the lattice contraction of the samples because of the introduction of smaller Zn atoms into the Pd lattice, demonstrating the successful formation of PdZn bimetallene. Interestingly, the diffraction peaks of H-PdZn bimetallene are shifted to lower angles compared to those of PdZn bimetallene, suggesting lattice expansion caused by hydrogen incorporation into the Pd lattice gap.^33,42 Furthermore, by analyzing the correlation between lattice parameters and the composition of palladium hydride according to Vegard's law, a more precise estimation of the H : (Pd + Zn) ratio can be derived, approximating 0.32 (Fig. S4, see the ESI† for details).⁴³ X-ray photoelectron spectroscopy (XPS) is further applied to determine the chemical state of the samples. The existence of Pd and Zn elements in H-PdZn bimetallene is confirmed by the XPS survey spectrum (Fig. 3b). In the high-resolution Pd 3d XPS spectra (Fig. 3c), H-PdZn bimetallene shows two obvious high peaks at 340.96 and 335.69 eV, which are attributed to Pd⁰ 3d_3/2 and Pd⁰ 3d_5/2, while the other two smaller peaks at 342.37 and 336.53 eV are attributed to Pd²⁺ due to oxidation in air.^44,45Fig. 3d shows the Zn 2p XPS spectra of the samples, in which a pair of peaks for H-PdZn bimetallene are visible at 1021.1 and 1044.3 eV, belonging to the Zn²⁺ spin–orbit double peaks of Zn 2p_3/2 and Zn 2p_1/2 due to the high oxophilicity of Zn. Compared to PdZn bimetallene, the Pd 3d and Zn 2p XPS peaks in H-PdZn bimetallene are shifted positively by 0.3 and 0.2 eV, respectively, suggesting stronger electronic interaction between Pd and H than that between Zn and H.⁴⁶ Furthermore, the XPS valence band spectrum is recorded to analyze the electronic structure of the samples. As shown in Fig. 3e, the peak width at half height of H-PdZn bimetallene (4.5 eV) is smaller than that of PdZn bimetallene (4.7 eV), indicating an increase in the Pd–Pd distance in H-PdZn bimetallene. The shrinkage bandwidth of the valence band spectrum reveals that the incorporation of hydrogen atoms into the metal lattice leads to the formation of metal hydrides.⁴⁷ Furthermore, the d-band center of Pd is determined from the valence band spectrum (see the ESI Experimental method for details†),^48,49 which is calculated to be −3.30 eV for PdZn bimetallene and −3.62 eV for H-PdZn bimetallene (Fig. 3f). Compared to PdZn bimetallene, the d-band center of H-PdZn bimetallene is further away from the Fermi level, which is due to the fact that the state density of H-PdZn bimetallene decreases after the hydrogen intercalation into the PdZn bimetallene. Moreover, the negative shift of the d-band center of H-PdZn bimetallene indicates that hydrogen incorporation can optimize the adsorption energy of intermediates on active sites.⁵⁰

Fig. 3 (a) XRD patterns of the samples. (b) XPS survey spectrum of H-PdZn bimetallene. XPS spectra of (c) Pd 3d and (d) Zn 2p for H-PdZn bimetallene. (e) Valence band structures of the samples. (f) The d-band center of the samples calculated from XPS valence band spectra.

The electrochemical tests of co-reduction of NO₃⁻ and CO₂ for urea synthesis are carried out in 0.1 M KNO₃ electrolyte bubbling with CO₂ under ambient conditions (Fig. S5†). The linear sweep voltammetry (LSV) curves of H-PdZn bimetallene in CO₂- and Ar-saturated 0.1 M KNO₃ electrolytes are displayed in Fig. 4a. In the Ar-saturated electrolyte, the cathodic current density obtained at the given applied potentials could be attributed to the NO₃RR and/or HER. Obviously, when CO₂ is introduced into the system, H-PdZn bimetallene clearly shows higher current densities at the given potentials from −0.3 to −0.7 V than those in the Ar-saturated electrolyte, which indicates that the catalyst is responsive to CO₂ and has the potential for synthesizing urea. Moreover, H-PdZn bimetallene exhibits a lower current density for the CO₂ reduction reaction in CO₂-saturated 0.1 M KHCO₃ solution without KNO₃. The increase of current density in the presence of CO₂ and NO₃⁻ reveals the occurrence of C–N coupling during CO₂RR and NO₃RR processes for urea synthesis. Besides, we also observe from the LSV curves that the current density of H-PdZn bimetallene is higher than that of PdZn bimetallene (Fig. S6†), revealing its better performance for C–N coupling to synthesize urea. The produced urea is detected by the diacetyl monoxime method according to the standard curve (see the ESI for details, Fig. S7†). Fig. 4b shows the dependence of R_urea and FE on the applied potential in CO₂-saturated 0.1 M KNO₃ electrolyte for two-hour electrolysis. The H-PdZn bimetallene exhibits the highest R_urea and FE of 314.17 μg h⁻¹ mg⁻¹ and 24.39% at −0.5 V, respectively, which are comparable with those in the literature (Tables S1 and S2†). To further highlight the improvement by inserting H into PdZn bimetallene, the electrochemical performance of PdZn bimetallene was also evaluated under the same conditions (Fig. S8†). In sharp contrast to H-PdZn bimetallene, PdZn bimetallene exhibits poor electrocatalytic performance, with an R_urea and FE of 144.25 μg h⁻¹ mg⁻¹ and 16.03% at −0.5 V, respectively. The enhanced performance of H-PdZn bimetallene can be attributed to the incorporation of interstitial H, which can downshift the Pd d-band center and thus optimize adsorption energy for intermediates. The durability of the H-PdZn bimetallene towards urea electro-synthesis is then investigated. During long-term and cycling tests, H-PdZn bimetallene maintains stable current density and shows slight changes in R_urea and FE, indicating that the H-PdZn bimetallene has good stability in the electrochemical co-reduction of CO₂ and NO₃⁻ to synthesize urea (Fig. 4c and d). In order to better illustrate the stability of the material, SEM (Fig. S9†) and XRD (Fig. S10†) of H-PdZn bimetallene after electrochemical tests were further performed. The results show that the morphological and structural characteristics of H-PdZn bimetallene essentially remain unchanged after the test, which further emphasizes the good stability of the as-synthesized material.

Fig. 4 (a) LSV curves of H-PdZn bimetallene in 0.1 M KNO₃ electrolyte with Ar or CO₂ gas feed and CO₂-saturated 0.1 M KHCO₃ solution without KNO₃. (b) R_urea and FE of H-PdZn bimetallene at different potentials. (c) R_urea and FE values of H-PdZn bimetallene at −0.5 V for six cycles. (d) Stability test of the H-PdZn bimetallene at −0.5 V.

Several comparative experiments are conducted to confirm that the detected urea originated from the electrochemical co-reduction of CO₂ and NO₃⁻. Control experiments are conducted in 0.1 M K₂SO₄ with CO₂ and 0.1 M KNO₃ without CO₂, and a negligible amount of urea is detected for both cases (Fig. 5a and b). The previous results imply that the urea indeed originates from C–N coupling of the CO₂RR and NO₃RR on the H-PdZn bimetallene. Furthermore, an isotopic labelling experiment is performed using ¹⁵NO₃⁻ as the nitrogen source. As shown in the ¹H NMR spectra (Fig. 5c and d), there is only a single peak of electrolysis solution derived from ¹⁴NO₃⁻, while there are double peaks of electrolysis solution derived from ¹⁵NO₃⁻, which are located at positions similar to those of the standards ¹⁴NH₂CO¹⁴NH₂ and ¹⁵NH₂CO¹⁵NH₂, respectively. This result further confirms the formation of urea from the electrochemical co-reduction of CO₂ and NO₃⁻ by H-PdZn bimetallene. Besides the targeted product of urea, the by-products of NH₃, NO₂⁻, CO and H₂ are inevitably produced during electrocatalysis. Therefore, the FE of each electrocatalytic product is calculated at different potentials (Fig. S11 and S12†). The NH₃ and NO₂⁻ concentrations are detected by UV-vis spectroscopy according to the standard curves (see the ESI for details, Fig. S13†) and the gas phase products (CO and H₂) are identified by on-line gas chromatography. In addition, the FE of the product is calculated when CO₂ is saturated in 0.1 M KHCO₃ and in 0.1 M KNO₃ in the absence of CO₂ supply, indicating that H-PdZn bimetallene shows high performance for the CO₂RR and NO₃RR (Fig. S14 and S15†). It is clear that there is a significant difference in FE between CO and NH₃ in both the single and coupled systems (Fig. S16†), which may be attributed to the electro-synthesis of urea.³⁷

Fig. 5 (a) R_urea values of H-PdZn bimetallene in CO₂-saturated 0.1 M KNO₃, Ar-saturated 0.1 M KNO₃, and CO₂-saturated 0.1 M K₂SO₄ at −0.5 V. (b) UV-vis absorbance spectra of electrolysis solutions at −0.5 V. (c and d) ¹H NMR spectra of standard ¹⁴NH₂CO¹⁴NH₂ and ¹⁵NH₂CO¹⁵NH₂ solutions and electrolysis solutions in CO₂-saturated 0.1 M ¹⁴KNO₃ and ¹⁵KNO₃ electrolytes.

The enhanced electrocatalytic performance of H-PdZn bimetallene for urea synthesis is highly related to the morphology and component. Firstly, CO stripping experiments are carried out to verify the adsorption ability of CO on active sites. As shown in Fig. S17,† the peak potential (1.08 V) of H-PdZn bimetallene is much lower than that of PdZn bimetallene (1.2 V), confirming the low adsorption energy of CO, which is beneficial for the C–N coupling reaction. On the other hand, the bent and ultrathin structure can provide abundant active sites and facilitate channels for the adsorption and activation of intermediates. The electrochemically active surface area (ECSA) serves as an indicator of the quantity of active sites present, which is directly proportional to the value of the capacitance of the electrode.^51,52 The ECSA of the catalyst is determined from the double layer capacitance (C_dl), which is derived from the cyclic voltammetry curves at different scan rates (Fig. S18a and b†). The C_dl value of H-PdZn bimetallene (1.2 mF cm⁻²) is higher than that of PdZn bimetallene (0.7 mF cm⁻²) (Fig. S18c†), indicating more active sites of H-PdZn bimetallene for urea synthesis due to the strong electronic effect after hydrogen intercalation. Furthermore, the electron transfer kinetics during urea synthesis have been investigated by electrochemical impedance spectroscopy (EIS) using the Nyquist plot. All samples show similar system ohmic resistance (R_Ω), while the semicircle radius for H-PdZn bimetallene is smaller than that for PdZn bimetallene (Fig. S18d†), proving the faster charge transfer of H-PdZn bimetallene due to the strong electron transfer among the Pd, Zn and H atoms, which can promote the kinetics of urea synthesis.

In order to further understand the C–N coupling mechanism of urea electrosynthesis for H-PdZn bimetallene, density functional theory (DFT) calculations are carried out. Firstly, we calculated the projected density of states (PDOS) of the optimized structural models of standard Pd, H-PdZn bimetallene and PdZn bimetallene with an exposed (111) plane (Fig. 6a–c). It can be seen that the main peak of the Zn 3d orbital is located at a more negative position, indicating a relatively electron-rich feature, which can serve as an electron reservoir to supply electrons to Pd and hydrogen.⁵³ Bader charge analysis exhibits that the Zn atoms transfer 6.28 e to Pd (4.98 e) and H (1.30 e) atoms in the H-PdZn bimetallene (Fig. S19†). This result confirms an apparent electron transfer among the Pd, Zn and H atoms of H-PdZn bimetallene, agreeing well with the XPS analysis. After the incorporation of Zn and H atoms, the d orbitals of Pd gradually become wide due to the orbital interaction among Pd, Zn and H, indicating the downshift of the d band center. The Pd d-band centers of Pd (111), PdZn (111), and H-PdZn (111) models are located at about −1.826, −1.910 and −1.987 eV, respectively (Fig. 6d), consistent with the analysis of valence band spectra. This may be attributed to the fact that the electron distribution caused by charge transfer regulates the position of the d-band center, which is conducive to tailoring the adsorption and activation of intermediates.

Fig. 6 The PDOS of (a) a standard Pd crystal, (b) PdZn bimetallene and (c) H-PdZn bimetallene with the (111) plane. (d) The d-band center calculated based on the PDOS of Pd, PdZn bimetallene and H-PdZn bimetallene.

The free energy change (ΔG) values for the NO₃RR, CO₂RR and C–N coupling to urea processes were further calculated, and the optimized structural models of reaction steps are displayed in Fig. S20 and S21.† As shown in Fig. 7a, H-PdZn bimetallene has more favorable thermodynamics than PdZn bimetallene for the NO₃RR, revealing the significant effect of the incorporation of Zn and H. Similarly, for CO₂ reduction to CO, it can be seen that the formation of *CO₂ to *CO on both H-PdZn bimetallene and PdZn bimetallene is downhill. The potential-determining step (PDS) of the CO₂RR on the samples is the formation of *CO₂, with a lower energy barrier of H-PdZn bimetallene (0.23 eV) than that of PdZn bimetallene (0.33 eV), indicating the significantly improved CO₂RR performance of H-PdZn bimetallene (Fig. 7b). Therefore, the insertion of H into H-PdZn bimetallene can enhance the activity of the co-reduction of nitrate and CO₂ to *NH₂ and *CO, respectively. Finally, we calculated ΔG for C–N coupling of *NH₂ and *CO to form urea (2*NH₂ + *CO → *CO(NH₂)) on both H-PdZn bimetallene and PdZn bimetallene. The profiles exhibit the smaller uphill process of the reaction step (*NH₂ + *CO → *CONH₂) of H-PdZn bimetallene compared to that of PdZn bimetallene, with a lower energy barrier on the H-PdZn bimetallene surface compared to PdZn bimetallene (Fig. 7c), indicating promoted C–N coupling to form urea.

Fig. 7 Free energy profiles of (a) *NO₃ reduction to *NH₂ on H-PdZn bimetallene and PdZn bimetallene, (b) CO₂ reduction to *CO and (c) C–N coupling for the formation of *CONH₂ on H-PdZn bimetallene and PdZn bimetallene.

Therefore, it can be concluded that the incorporation of Zn and H atoms can promote the formation of key intermediates *CO and *NH₂ and reduce the energy barrier for C–N coupling of *CO and *NH₂ to form urea.

Conclusions

In conclusion, bent and ultrathin H-PdZn bimetallene is synthesized by inserting H into the as-synthesized PdZn bimetallene, which serves as an active catalyst for electrochemical co-reduction of CO₂ and NO₃⁻ to urea via C–N coupling. The H-PdZn bimetallene has a superior FE and R_urea of 24.39% and 314.17 μg h⁻¹ mg⁻¹ at −0.5 V, respectively, as well as excellent stability. The enhanced performance of H-PdZn bimetallene is mainly due to its unique 2D morphology and hydrogen insertion, which can provide sufficient active sites and rapid mass/charge transfer and is conducive to the C–N coupling of key *NH₂ and *CO intermediates. This study demonstrates the importance of hydrogen intercalation in the enhancement of catalytic performance for electrochemical urea synthesis.

Data availability

The data are available from the corresponding author on reasonable request.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This research was supported by the Zhejiang Provincial Natural Science Foundation of China (No. LR24B060001, LQ22B030012 and LQ23B030010) and National Natural Science Foundation of China (No. 22308330, 22372148, 22278369, 21972126, 21978264 and 21905250).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta04802d

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