Highly efficient electrocatalytic urea synthesis from carbon dioxide and nitrogen based on electron cluster-induced precipitation technology

Abstract

The present invention discloses a method for efficiently synthesizing urea from carbon dioxide and nitrogen based on electron cluster-induced precipitation technology, which relates to the field of electrocatalysis technology and comprises the following steps: S1, catalyst preparation; including precursor preparation and crystallization-pyrolysis process; S2, electrocatalytic urea synthesis experiment; including experimental device construction, electrolyte and gas supply, and electrochemical testing; S3, reaction mechanism analysis; in the present invention, the CoN3-CoAC / NC electrocatalyst constructed by the crystallization-pyrolysis route has an electron precipitation effect generated by the strong coupling of its Co clusters with the Co-N3 sites, which promotes d orbital charge redistribution, optimizes the reactant adsorption configuration, reduces the C-N coupling barrier, improves intrinsic activity and accelerates reaction kinetics; the electron delocalization design based on the CoN3-CoAC / NC system breaks the symmetrical electronic arrangement of the reactants, promotes electron transfer, and realizes effective urea electrosynthesis; and the CoN3-CoAC / NC catalyst exhibits excellent NRR and CO2RR performance.

Description

Technical Field

[0001] The present invention relates to the field of electrocatalysis technology, and in particular to a method for efficiently synthesizing urea from carbon dioxide and N2 based on electron cluster induced precipitation technology. Background Art

[0002] Global warming and environmental problems are becoming increasingly serious due to excessive emissions of carbon dioxide (CO2).

[0003] There has been renewed interest in exploring potential solutions for achieving high-value utilization of CO2. Electrochemical CO2 reduction (ECR) technology powered by renewable electricity is considered a promising technology for producing valuable carbon derivatives (such as CO, HCOOH, CH3OH, C2H4, etc.). However, the original ECR route still faces the challenge of limited product variety. Urea is the most representative product of this reaction. It is an important nitrogen fertilizer with the highest nitrogen content and is also a basic component for the manufacture of urea-formaldehyde, urea-melamine-formaldehyde resin, and barbiturates.

[0004] In this context, compared with conventional thermal catalytic processes (150–550 °C, 150–350 bar), urea electrosynthesis eliminates the stringent reaction requirements and enables the direct conversion of inert CO2 and N2 reactants under ambient conditions. For example, PdCu alloy nanoparticles (NPs) supported on TiO2 nanosheets have achieved efficient electrocatalytic urea synthesis. In addition to noble metal palladium-based catalysts, Bi-BiVO4

[24] and BiFeO3 / BiVO4 heterostructure catalysts have been developed for the co-reduction of CO2 and N2 molecules to produce urea. Apparently, the built-in electric field at the heterostructure interface enables the adsorption and activation of active molecules. Despite encouraging progress, the activity and selectivity of urea electrosynthesis remain unsatisfactory, which may be attributed to the suboptimal adsorption and coupling of active intermediates. Therefore, the key to overcoming the above obstacles lies in the precise design of active sites to enhance the interaction between molecules and catalysts and to couple the desired intermediates in a targeted manner. In fact, increasing the number of active sites and improving the intrinsic activity of each active site are considered to be two effective strategies to promote electrocatalytic activity. More specifically, catalysts with single metal atom sites, including well-defined cobalt pyrrolidine dianhydride conducting metal-organic frameworks (Co-PMDA-2mbIM) and copper phthalocyanine nanotubes (CuPcNTs), both of which feature isolated metal active sites, have been shown to be effective for electrocatalytic urea synthesis. However, the metal atoms in single-atom catalysts are prone to migration and aggregation during long-term electrolysis due to structural instability.

[0005] In view of this, a method for efficiently synthesizing urea from carbon dioxide and N2 using electron cluster induced evolution technology is provided to overcome the above problems. Summary of the Invention

[0006] The object of the present invention is to provide a method for efficiently synthesizing urea from carbon dioxide and N2 using electron cluster induced precipitation technology to solve the problems raised in the above-mentioned background technology.

[0007] To solve the above technical problems, the present invention provides a method for efficiently synthesizing urea from carbon dioxide and N2 using electron cluster induced precipitation technology, comprising the following steps:

[0008] S1. Catalyst preparation; including precursor preparation, crystallization-pyrolysis process;

[0009] The precursor preparation includes selecting ZIF-67 with a purity of ≥99.5% as a precursor for preparing the electrocatalyst;

[0010] The crystallization-pyrolysis process involves placing the ZIF-67 precursor in a precisely temperature-controlled tube furnace for annealing;

[0011] S2. Electrocatalytic urea synthesis experiment; including experimental device construction, electrolyte and gas supply, and electrochemical testing; electrochemical testing includes CoN3-CoAC / NC catalyst performance testing, stability testing, and comparison of catalyst performance;

[0012] S3. Reaction mechanism analysis.

[0013] Furthermore, during the crystallization-pyrolysis process, the temperature control accuracy of the tubular furnace was ±0.5°C, the heating rate was set to 5°C / min, the cooling rate was 3°C / min, the atmosphere was high-purity nitrogen, and the gas flow rate was 50 mL / min.

[0014] Furthermore, during the crystallization-pyrolysis process, CoN4-CoAC / NC was formed when the heat treatment time was set to 10 min.

[0015] Furthermore, during the crystallization-pyrolysis process, as the pyrolysis time increases, some Co-N bonds break to form CoN3-CoAC / NC.

[0016] Furthermore, during the crystallization-pyrolysis process, the calcination time was extended to obtain CoNP / NC containing nanoparticles.

[0017] Furthermore, the performance test of the CoN3-CoAC / NC catalyst was carried out 5 times under the voltage condition of -0.4 V versus RHE.

[0018] Furthermore, in the stability test, when the CoN3-CoAC / NC electrode was continuously electrolyzed for 10 h, the current density was recorded every 30 minutes and the current density was stable at 0.5 A / cm 2The fluctuation range is less than 5%.

[0019] Furthermore, in comparison of the catalyst performance, CoNP / NC and CoN4-CoAC / NC were evaluated under the same conditions.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] The urea yield of CoN3-CoAC / NC catalyst reached 20.83 mmol / h at -0.4 V vs. RHE. -1 g -1 , the Faraday efficiency is 23.73%, setting a record for urea electrosynthesis.

[0022] The electron evolution effect promotes the redistribution of d orbital charge, optimizes the adsorption configuration of N2 and CO2, generates ideal intermediates, reduces the CN coupling barrier, improves the intrinsic activity of urea electrosynthesis, and accelerates the reaction kinetics.

[0023] The current density of the CoN3-CoAC / NC catalyst was stable during continuous electrolysis for 10 h. After five cyclic stability tests, the electrochemical performance, crystal structure, morphology and chemical state remained good, showing good structural rigidity.

[0024] The catalyst also exhibits excellent performance in the nitrogen reduction reaction (NRR) and carbon dioxide reduction reaction (CO2RR), ensuring the full co-reduction of inert N2 and CO2 to urea. Its performance surpasses that of comparable catalysts, such as CoNP / NC and CoN4-CoAC / NC, highlighting the important contribution of electron delocalization in enhancing the intrinsic activity of the catalyst. This provides important insights into the design of efficient urea electrosynthesis catalysts and helps to reveal the underlying activation mechanism of the electron evolution effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a first schematic diagram of the method for efficiently electrocatalyzing urea synthesis of carbon dioxide and N2 based on electron cluster induced precipitation technology of the present invention;

[0026] Figure 2 This is a second schematic diagram of the method for efficiently electrocatalyzing urea synthesis of carbon dioxide and N2 based on electron cluster induced precipitation technology of the present invention;

[0027] Figure 3 This is a third schematic diagram of the method for efficiently electrocatalyzing urea synthesis from carbon dioxide and N2 based on electron cluster induced precipitation technology of the present invention;

[0028] Figure 4 This is a fourth schematic diagram of the method for efficiently electrocatalyzing urea synthesis from carbon dioxide and N2 based on electron cluster induced precipitation technology of the present invention;

[0029] Figure 5 This is a fifth schematic diagram of the method for efficiently electrocatalyzing urea synthesis from carbon dioxide and N2 based on electron cluster induced precipitation technology of the present invention;

[0030] Figure 6 This is the sixth principle diagram of the method for efficiently electrocatalyzing urea synthesis from carbon dioxide and N2 based on electron cluster induced precipitation technology of the present invention. DETAILED DESCRIPTION

[0031] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0032] See also Figures 1-6 , the present invention provides a technical solution:

[0033] It is necessary to add that:

[0034] Figure 1 :(a) Schematic diagram of the electron localization effect on the CoN4-CoAC / NC structure; (b) Charge density distribution of Co-N4 / NC and (c) CoN4-CoAC / NC with an isopar value of 0.005eBohr-3; (d) The difference charge density of N2+CO2 molecules adsorbed on CoN4-CoAC / NC with an isopar value of 0.005eBohr-3, where yellow and cyan represent charge accumulation and charge depletion, respectively. (e) Schematic diagram of the electron release effect on the CoN3-CoAC / NC structure; (f) Charge density distribution of Co-N3 / NC and (g) CoN3-CoAC / NC with an isopar value of 0.005eBohr-3; (h) The difference charge density of N2+CO2 molecules adsorbed on CoN3-CoAC / NC with an isopar value of 0.005eBohr-3, where yellow and cyan represent charge accumulation and charge depletion, respectively.

[0035] Figure 2 : (a) HAADF-STEM image of CoN3-CoAC / NC. (b) Aberration-corrected scanning transmission electron microscopy (STEM) image of CoN3-CoAC / NC; (c) Enlarged aberration-corrected STEM image of the white dashed area of ​​CoN3-CoAC / NC. (d) XRD patterns of CoN3-CoAC / NC, CoN4-CoAC / NC, and CoNP / NC samples.

[0036] (e) Energy dispersive X-ray spectroscopy (EDX) elemental mapping of CoN3-CoAC / NC. (f) XPS survey spectra of CoN3-CoAC / NC, CoN4-CoAC / NC, and CoNP / NC catalysts.

[0037] Figure 3 : (d) Schematic diagram of the preparation of CoN3-CoAC / NC, CoN4-CoAC / NC and CoNP / NC samples.

[0038] See Figures 1-6 As shown, an embodiment of a method for efficiently electrocatalyzing urea synthesis from carbon dioxide and N2 based on electron cluster induced precipitation technology:

[0039] S1. Catalyst preparation;

[0040] Precursor preparation;

[0041] High-purity (purity ≥ 99.5%) ZIF-67 is selected as a precursor for preparing the electrocatalyst, and its purity is accurately measured and verified by elemental analysis and spectral analysis.

[0042] crystallization-pyrolysis process;

[0043] The ZIF-67 precursor was annealed in a precisely controlled tube furnace with a temperature accuracy of ±0.5°C, a heating rate of 5°C / min, and a cooling rate of 3°C / min. The atmosphere was high-purity nitrogen (≥99.999%) at a flow rate of 50 mL / min.

[0044] Preparation of CoN4-CoAC / NC;

[0045] When the heat treatment time is set to 10 minutes, CoN4-CoAC / NC is formed. X-ray diffraction (XRD) analysis ( Figure 2 (d)), characteristic diffraction peaks appear at 2θ=10.2°, 12.4°, 22.3°, 32.1°, and 34.5°. Energy dispersive X-ray spectroscopy (EDS) element distribution diagram ( Figure 2 (e) shows that Co, N, and C are evenly distributed, with atomic percentages of Co approximately 12%, N approximately 28%, and C approximately 60%. X-ray photoelectron spectroscopy (XPS) survey spectrum ( Figure 2 In (f), the Co2p3 / 2 peak is located at 780.2 eV, indicating a specific chemical environment.

[0046] Preparation of CoN3-CoAC / NC;

[0047] As the pyrolysis time increases, some Co-N bonds break to form CoN3-CoAC / NC. EDS element distribution map ( Figure 2 (e)) shows that the atomic percentages of Co, N, and C are about 10% Co, 25% N, and 65% C. XPS survey spectrum ( Figure 2 The Co2p3 / 2 peak in (f) shifts to 781.5 eV, indicating a change in the chemical environment.

[0048] Preparation of CoNP / NC;

[0049] Further extending the calcination time yielded CoNP / NC nanoparticles. Transmission electron microscopy (TEM) revealed spherical nanoparticles with an average particle size of approximately 20-30 nm. XRD, EDS, and XPS results were significantly different from those of CoN3-CoAC / NC and CoN4-CoAC / NC, with significant changes in the XPS peak shape and binding energy of Co.

[0050] S2, electrocatalytic urea synthesis experiment;

[0051] Setting up the experimental device;

[0052] A standard three-electrode electrochemical cell was used. Working electrode preparation: the catalyst (CoN3-CoAC / NC, CoNP / NC or CoN4-CoAC / NC) was mixed with Nafion solution (mass fraction 5%) at a mass ratio of 1:1 to prepare a slurry, which was evenly coated on an area of ​​1 cm 2 The catalyst loading on the carbon paper electrode was 0.5 mg / cm 2 The counter electrode is a platinum electrode with a purity of 99.9% and an area of ​​2 cm 2 The reference electrode is a reversible hydrogen electrode (RHE).

[0053] electrolyte and gas supply;

[0054] Electrolyte preparation: Dissolve 0.1 mol / L KHCO₃ in deionized water and adjust the pH to 7.0 with dilute hydrochloric acid. Introduce a mixture of CO₂ (at a flow rate of 20 mL / min, purity ≥99.99%) and N₂ (at a flow rate of 10 mL / min, purity ≥99.99%) into the electrolyte.

[0055] electrochemical testing;

[0056] Linear sweep voltammetry (LSV) tests were performed in the potential range of -1.0 V to 0 V (vs RHE) at a scan rate of 5 mV / s and a data acquisition frequency of 1 Hz.

[0057] CoN3-CoAC / NC catalyst performance test;

[0058] At a voltage of -0.4 V to RHE, the experiment was repeated five times, and the urea yield of the CoN3-CoAC / NC catalyst reached 20.83 mmol / h -1 g -1 , the Faradaic efficiency (FE) is 23.73% ( Figure 4 (f)), the relative standard deviation (RSD) of urea yield was 2.5%, and the RSD of FE was 3.0%. Compared with the reported catalysts, such as catalyst A (urea yield 15.2 mmolh -1 g -1 , FE is 18.5%), catalyst B (urea yield 18.3 mmolh - 1g -1 , FE is 20.2%), etc., which has obvious advantages.

[0059] Stability testing;

[0060] When the CoN3-CoAC / NC electrode was electrolyzed continuously for 10 h, the current density was recorded every 30 min and was stable at 0.5 A / cm 2 After five cycles of stability testing, XRD analysis showed that the main diffraction peak angle (2θ) and intensity changed by less than 2%, TEM images showed that the average particle size changed by less than 2nm, and the Co binding energy in the XPS spectrum changed by less than 0.3eV.

[0061] Comparison of catalyst performance;

[0062] CoNP / NC and CoN4-CoAC / NC were evaluated under the same conditions. The urea yield of CoNP / NC was 0.80 mmol / h -1 g -1 , FE was 1.22%, urea yield RSD was about 5%, and FE RSD was about 6%; the urea yield of CoN4-CoAC / NC was 5.57 mmolh -1 g -1 , FE was 6.92%, the RSD of urea yield was about 4%, and the RSD of FE was about 5%. Compared with CoN3-CoAC / NC, the activity and selectivity were significantly lower.

[0063] S3, reaction mechanism analysis;

[0064] Electron delocalization effect and reactant activation;

[0065] Through the analysis of surface electrostatic potential, it is known that the charge distribution of N2 and CO2 molecules is symmetrical, and the surface charge density is approximately For the Co-N4 / NC and Co-N3 / NC systems, the density of electronic states near the Fermi surface calculated by density functional theory (DFT) is: Co-N4 / NC is 10 states / eV, and Co-N3 / NC is 15 states / eV. The energy of the empty orbital of the Co-N3 single atom site is at -2.5eV. When the CoN4-CoAC / NC model adsorbs N2 and CO2, the number of electron transfers is less than 0.01e, while the charge density change of the CoN3-CoAC / NC system reaches Break the symmetrical electron arrangement and promote electron transfer.

[0066] Orbital electron rearrangement and formation of reaction intermediates;

[0067] X-ray absorption fine structure (XAFS) spectroscopy analysis shows that the interaction between Co clusters and Co-N3 domains causes the d orbital charge to shift from 3d 7 4s 1 Become 3D 8 4s 0 The absorption edge shifts 2 eV toward higher energy. Molecular dynamics simulations show that the N adsorption energy decreases from -0.5 eV in the Co-N-CoAC / NC system to -0.8 eV in the CoN-CoAC / NC system, and the CO adsorption energy decreases from -0.3 eV to -0.6 eV, generating N=N and *CO intermediates. DFT calculations indicate that the interaction between the bonding and anti-bonding orbitals of the gas molecules and the catalyst reduces the CN coupling barrier by approximately 15 kJ / mol.

[0068] The synergistic advantages of the CoN3-CoAC / NC catalyst in multiple properties;

[0069] Nitrogen reduction reaction (NRR) performance of CoN3-CoAC / NC catalyst: the onset potential is -0.2 V, the half-wave potential is -0.4 V, and the current density reaches 1.2 A / cm at -0.6 V. 2 Carbon dioxide reduction reaction (CO2RR) performance: The onset potential is -0.3V, and the selectivity for generating CO reaches 80%. These properties synergistically ensure that N2 and CO2 are fully reduced to urea.

[0070] In summary, the CoN3-CoAC / NC catalyst prepared via a crystallization-pyrolysis strategy in this example exhibits excellent performance at -0.4 V vs. RHE, providing a basis for the design of efficient urea electrosynthesis catalysts and overcoming the limitations of existing technologies. The advantages of the CoN3-CoAC / NC catalyst are highlighted through mechanisms such as electron delocalization and orbital electron rearrangement, as well as comparative experiments.

[0071] Further explanation is needed:

[0072] exist Figure 1(a) Schematic diagram of the electron localization effect on the CoN4-CoAC / NC structure. Electrons are confined to specific regions, forming relatively stable electron clouds around certain atoms. This makes it difficult for N2 and CO2 molecules adsorbed on the surface to acquire electrons and change their electronic structure. For example, in this confined electronic environment, the outer electrons of the N2 molecule cannot effectively interact with the catalyst, hindering effective adsorption and activation.

[0073] and Figure 1 (e) Schematic diagram of the electron release effect on the CoN3-CoAC / NC structure shows that electrons are clearly released from the region where the cobalt atom cluster (CoAC) and the Co-N3 single atom site interact synergistically. These released electrons can move freely within the system and participate in interactions with N2 and CO2 molecules when they approach. For example, electrons can be transferred to the antibonding orbital of the N2 molecule, weakening the N-N bond and creating favorable conditions for subsequent adsorption, activation, and reaction, ultimately triggering an efficient urea electrosynthesis process.

[0074] for Figure 1 The isobaric values ​​of Co-N4 / NC and Co-N3 / NC in (b) and (f) are 0.005 eBohr -3 The charge density distribution of Co-N4 / NC, visualized and analyzed using the professional VESTA software (version 3.5.8), shows that the charge density distribution of Co-N4 / NC is relatively uniform and symmetrical, with electron clouds arranged regularly around the atoms, consistent with its relatively symmetrical structure. In this structure, the electron clouds form a nearly circular distribution around the Co and N atoms, and the electron density does not vary much in all directions.

[0075] The charge density distribution of Co-N3 / NC is clearly asymmetric, especially near the Co-N3 single-atom site. Around this site, the electron cloud density varies significantly, with a relatively high density near the Co atom and a low density on the other side. This asymmetry stems from the presence of empty orbitals that can accommodate excess electrons, giving it a unique electronic structure and laying the foundation for the subsequent electron delocalization effect.

[0076] Further observation Figure 1 The isosurface value of adsorption on CoN4-CoAC / NC and CoN3-CoAC / NC in (d) and (h) is 0.005 eBohr -3 The difference charge density diagram of N2+CO2 molecules is obtained by using a high-precision KPFM-1000 charge density measuring instrument (with a measurement accuracy of up to ) to measure and draw. Figure 1In (d), the yellow (charge accumulation) and cyan (charge depletion) regions are relatively dispersed, with minimal color variation. This suggests that when CoN4-CoAC / NC adsorbs N2 and CO2, electron transfer between the molecule and the catalyst is minimal, resulting in low levels of charge accumulation and depletion. For example, in the N2 molecule adsorption region, the charge density change is almost negligible, further confirming the role of electron localization in hindering reactant activation.

[0077] And in Figure 1 In (h), the CoN3-CoAC / NC system after adsorption of N2+CO2 molecules has clear yellow and cyan regions and significant color changes, with a charge density change of up to It can be seen that there is an obvious charge accumulation area around the N2 molecule, indicating that electrons have been transferred from the catalyst to the N2 molecule. A similar situation is also seen near the CO2 molecule, which intuitively shows that electrons have been significantly transferred from the catalyst to the reactant molecules and redistributed within the system. This strongly proves that the electron precipitation effect promotes the electron transfer between the adsorbate and the catalyst, breaking the symmetrical electronic arrangement of the inert reactants, thereby enabling the system to trigger an effective urea electrosynthesis process.

[0078] Figure 2 (a) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of CoN3-CoAC / NC is obtained by using a high-brightness electron gun (the electron gun brightness reaches 5×106A / cm 2 The image was acquired using a JEM-2100F transmission electron microscope (accelerating voltage 200 kV) equipped with a 1.5-μm CMOS process. In this image, atoms appear as bright spots, with the brightness of the bright spots proportional to the square of the atomic number.

[0079] It can be clearly seen that the cobalt atomic clusters (CoAC) present a certain agglomeration morphology, but are relatively dispersed on the carbon matrix. In the image, the bright spots formed by Co atoms are relatively bright, which is because Co has a larger atomic number. The brightness difference can be used to distinguish Co atoms from atoms of other elements (such as N and C atoms), and the bright spots formed by N and C atoms are darker. At the same time, the Co-N3 single atom sites are relatively evenly distributed around in the form of relatively dark but clearly distinguishable single bright spots. Through statistical analysis of 15 different fields of view in different areas of the image, it was found that this distribution state has good consistency, and the overall state of good dispersion and synergistic existence of the two is shown. This structural feature provides a structural basis for the subsequent electron cluster induced precipitation effect and efficient electrocatalytic urea synthesis.

[0080] Figure 2(b) The aberration-corrected scanning transmission electron microscope (STEM) image was obtained using an advanced aberration corrector (with aberration correction accuracy of ) was obtained using a Titan Themis G2 STEM instrument (accelerating voltage of 300kV), which can further improve the resolution and clearly distinguish atomic-level details.

[0081] In this image, we can see the specific spatial relationship between the Co-N3 single atom site and the cobalt atom cluster. For example, the average distance between them can be measured by image measurement and statistical analysis (using professional image analysis software ImageJ, the measurement accuracy can reach ) is approximately Such a distance facilitates interaction between the two and induces electron precipitation effect.

[0082] and Figure 2 The aberration-corrected STEM image of the white dashed area in (c) focuses on a more detailed local structure. By magnifying this area 5 million times, the coordination environment of the Co-N3 single-atom site can be clearly seen. For example, the number of N atoms coordinating to it (the statistical average is 3) and the connection between it and the cobalt atom cluster (the presence of covalent chemical bonds is determined by observing the relative positions of the atoms and the electron cloud distribution), providing an intuitive and precise basis for a deeper understanding of the relationship between catalyst structure and performance.

[0083] Figure 2 (d) X-ray diffraction (XRD) patterns of CoN3-CoAC / NC, CoN4-CoAC / NC, and CoNP / NC samples were obtained using a D8Advance X-ray diffractometer (using a CuKα radiation source with a wavelength of The detection was performed with a tube voltage of 40 kV and a tube current of 40 mA. The scanning range was set to 5°-80°, the scanning step was 0.02°, and the scanning speed was 2° / min. These parameter settings ensured the acquisition of high-resolution and accurate diffraction peak information.

[0084] For CoN3-CoAC / NC, the characteristic diffraction peaks appear at specific 2θ = 10.2°, 12.4°, 22.3°, 32.1°, and 34.5°. Their peaks are sharp and their intensities are relatively stable, reflecting the unique crystal structure of the sample. By comparing with the ICDD database, it can be seen that the diffraction peak at 10.2° corresponds to the (100) crystal plane, and the lattice spacing is calculated by the Bragg equation as follows: 12.4° corresponds to the (010) crystal plane, and the lattice spacing is 22.3° corresponds to the (110) crystal plane, and the lattice spacing is 32.1° corresponds to the (200) crystal plane, and the lattice spacing is 34.5° corresponds to the (210) crystal plane, and the lattice spacing is There are obvious differences in peak shape, peak intensity, and crystal plane and lattice spacing corresponding to the diffraction peaks compared with CoN4-CoAC / NC, reflecting the different crystal structure characteristics caused by structural changes such as partial Co-N bond breaking.

[0085] The XRD peaks of CoNP / NC show a greater difference from the previous two. In addition to the appearance of some characteristic diffraction peaks related to nanoparticles (such as the new diffraction peak at 2θ = 44.2°, which corresponds to the crystal structure characteristics of the formed Co nanoparticles, the XRD peaks of CoNP / NC are analyzed to be face-centered cubic crystal structure with a lattice constant of

[0086] ) and a relatively broadened overall peak shape. According to the Scherrer equation, this broadening of the diffraction peak occurs when the average nanoparticle size is approximately 25 nm. These differences can accurately identify samples with different structures and explain their different performance in electrocatalytic urea synthesis from a structural perspective.

[0087] Figure 2 (e) The energy dispersive X-ray spectroscopy (EDX) elemental map of CoN3-CoAC / NC was detected and drawn using a high-resolution EDX detector (energy resolution up to 129 eV, element detection range covering multiple elements from boron (B) to uranium (U)) installed on a SU8010 scanning electron microscope (accelerating voltage of 15 kV, beam current of 10 nA).

[0088] In the spectrum, by accurately detecting and analyzing the characteristic X-ray energy peaks corresponding to different elements (Co, N, and C), not only can the presence of these three elements in the catalyst be intuitively confirmed, but their relative content can also be accurately calculated based on the peak intensity through a quantitative analysis algorithm (using the ZAF correction algorithm with a correction accuracy of up to ±5%), which corroborates the atomic percentage data mentioned above (Co approximately 10%, N approximately 25%, and C approximately 65%). At the same time, it can be observed from the spectrum that the elements are uniformly distributed throughout the detection area. The accuracy of the element distribution uniformity judgment is ensured by taking the average of multiple measurements of 10 seconds each at different locations (at least 20 different and representative locations are selected). This uniform distribution shows that these elements are evenly dispersed throughout the CoN3-CoAC / NC structure, which is crucial for ensuring that all parts of the catalyst can effectively participate in the electrocatalytic reaction and exert a synergistic effect.

[0089] Figure 2(f) X-ray photoelectron spectroscopy (XPS) survey spectra of CoN3-CoAC / NC, CoN4-CoAC / NC, and CoNP / NC catalysts were obtained in an ultrahigh vacuum environment (vacuum better than 1×10 -8 The measurements were performed using an Escalab 250Xi X-ray photoelectron spectrometer equipped with an AlKα ray source (power 300 W, energy 1486.6 eV) at 400 Pa.

[0090] Prior to measurement, the instrument was rigorously calibrated using standard samples (e.g., the Au4f7 / 2 peak was calibrated at 84.0 eV and the Ag3d5 / 2 peak was calibrated at 368.2 eV) to ensure measurement accuracy. The Co 2p3 / 2 peak in CoN3-CoAC / NC is located at 781.5 eV, shifting from 780.2 eV in CoN4-CoAC / NC. This shift, revealed by high-precision peak fitting of the spectra (using a Gaussian-Lorentzian hybrid function with an error within ±0.1 eV), indicates a shift in the chemical coordination environment of Co in CoN3-CoAC / NC, due to the partial breakage of Co-N bonds and interactions with Co-N3 single-atom sites and cobalt clusters.

[0091] At the same time, the photoelectron spectral peaks of other elements (such as N and C) were analyzed. The N1s peak in CoN3-CoAC / NC showed two obvious peaks at 398.5eV and 400.2eV, corresponding to pyridinic nitrogen and pyrrolic nitrogen. Peak fitting analysis showed that the relative content ratio of pyridinic nitrogen and pyrrolic nitrogen was approximately 3:2. The C1s peak showed three peaks at 284.5eV, 285.2eV, and 286.7eV, corresponding to graphitic carbon, defective carbon, and carbon bound to nitrogen, respectively. The characteristics of these photoelectron spectral peaks of each element collectively reflect the chemical composition and chemical environment of the catalyst, which is one of the key characterization bases for understanding the unique performance of the catalyst in the present invention.

[0092] Figure 3 (d) Schematic diagrams illustrating the preparation of CoN3-CoAC / NC, CoN4-CoAC / NC, and CoNP / NC samples from a ZIF-67 precursor by varying the pyrolysis time. Throughout the preparation process, the TF-1200 tube furnace (capable of reaching a maximum heating temperature of 1200°C with a temperature uniformity of ±1°C) was used. Its precise temperature control and stable heating performance provide a reliable environment for annealing.

[0093] As can be seen from the schematic, after the ZIF-67 precursor is placed in a tube furnace, the Co-N bond within the ZIF-67 gradually changes with annealing time. During the initial short period (10 minutes), due to the temperature and time constraints, the Co coordination environment remains relatively intact, forming CoN4-CoAC / NC. At this point, the ZIF-67 structural framework is largely intact, and the Co atoms remain in their original coordination positions. This structure was verified using various characterization methods, including XRD, EDS, and XPS (the specific parameters and analysis of each method have been detailed previously).

[0094] Subsequently, as the pyrolysis time increased to about 30 minutes, some Co-N bonds broke under the action of heat, forming the desired CoN3-CoAC / NC sample. During this process, the chemical environment of the Co atoms changed, and some Co atoms began to aggregate to form atomic clusters (AC), which together with the remaining Co-N structure formed a unique CoN3-CoAC / NC structure. The structural changes can be clearly seen through corresponding structural characterization (such as the distribution of atomic clusters and single-atom sites observed in STEM images).

[0095] Extending the calcination time (to over 60 minutes) further develops CoNP / NC containing Co nanoparticles. At this stage, the atomic clusters continue to grow and merge, ultimately forming nanoparticles. TEM observations of the nanoparticles reveal their structural changes, including their size and morphology (e.g., an average particle size of approximately 20-30 nm and a spherical shape). This schematic diagram vividly illustrates the key steps of the entire preparation process and the mechanisms by which different structures form. It is of great significance for accurately understanding and reproducing the catalyst preparation method of this invention and also demonstrates the ingenuity of achieving precise control of catalyst structure through the crystallization-pyrolysis strategy.

[0096] Summarize:

[0097] Novel catalyst design: Guided by theoretical predictions, an electrocatalyst composed of metal clusters and planar asymmetric M-N3 single-atom sites (CoN3-CoAC / NC) was prepared via a crystallization-pyrolysis route. This catalyst exploits the synergistic effect between Co clusters and Co-N3 single-atom sites.

[0098] Innovation of the CoN3-CoAC / NC system: Based on the concept of the electronic structure of the reactants and electron delocalization, the cobalt atomic clusters are coupled with the Co-N3 sites to construct a system, breaking the symmetrical electronic arrangement of the reactants and promoting electron transfer.

[0099] Crystallization-pyrolysis strategy regulation: Using ZIF-67 precursor after annealing treatment, the degree of Co-N bond breakage was controlled by controlling the calcination time to prepare samples with different structures, including CoN3-CoAC / NC, CoN4-CoAC / NC, and CoNP / NC, achieving precise control of the catalyst structure.

[0100] Highly efficient catalytic urea synthesis: CoN3-CoAC / NC catalyst achieves a urea yield of 20.83 mmol / h at -0.4 V vs. RHE. -1 g -1 , the Faraday efficiency is 23.73%, setting a record for urea electrosynthesis.

[0101] Optimize the reaction process: The electron evolution effect promotes the redistribution of d orbital charge, optimizes the N2 and CO2 adsorption configuration, generates ideal intermediates, reduces the CN coupling barrier, improves the intrinsic activity of urea electrosynthesis, and accelerates the reaction kinetics.

[0102] Improved stability: The CoN3-CoAC / NC catalyst maintained a stable current density during continuous electrolysis for 10 hours. After five cycles of stability testing, its electrochemical performance, crystal structure, morphology, and chemical state remained good, demonstrating good structural rigidity.

[0103] Enhanced Overall Performance: This catalyst also exhibits excellent performance in the nitrogen reduction reaction (NRR) and carbon dioxide reduction reaction (CO2RR), ensuring the full co-reduction of inert N2 and CO2 to urea. Its performance surpasses that of comparable catalysts, including CoNP / NC and CoN4-CoAC / NC, highlighting the important contribution of electron delocalization in enhancing the intrinsic activity of the catalyst. This also provides important insights into the design of efficient urea electrosynthesis catalysts and helps to reveal the underlying activation mechanism of the electron evolution effect.

Claims

1. A method for efficiently synthesizing urea from carbon dioxide and N2 using electron cluster induced precipitation technology, characterized in that: The following steps are involved: S1. Catalyst preparation; It includes precursor preparation, crystallization-pyrolysis process; The precursor preparation includes selecting ZIF-67 with a purity of ≥99.5% as a precursor for preparing the electrocatalyst; The crystallization-pyrolysis process involves annealing the ZIF-67 precursor in a precisely controlled tube furnace with a temperature accuracy of ±0.5°C. The heating rate is set at 5°C / min and the cooling rate is 3°C / min. The atmosphere is high-purity nitrogen at a flow rate of 50 mL / min. During this process, some Co-N bonds are broken to form CoN3-CoAC / NC. S2. Electrocatalytic urea synthesis experiment; including experimental device construction, electrolyte and gas supply, and electrochemical testing; electrochemical testing includes CoN3-CoAC / NC catalyst performance testing, stability testing, and comparison of catalyst performance; S3. Reaction mechanism analysis.

2. The method for efficiently synthesizing urea from carbon dioxide and N2 based on electron cluster induced precipitation technology according to claim 1, characterized in that: The performance test of CoN3-CoAC / NC catalyst was carried out 5 times under the voltage condition of -0.4V to RHE.

3. The method for efficiently synthesizing urea from carbon dioxide and N2 based on electron cluster induced precipitation technology according to claim 1, characterized in that: In the stability test, when the CoN3-CoAC / NC electrode was subjected to continuous electrolysis for 10 hours, the current density was recorded every 30 minutes and was stable at 0.5A / cm² with a fluctuation range of less than 5%.

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