For electrochemically coupled reduction of CO2 and NO3 - Method for preparing catalyst for producing urea

Abstract

The present invention relates to the field of electrocatalytic energy storage, and aims to provide a method for electrochemically coupling the co-reduction of CO2 and NO3 ‑ A method for preparing a catalyst for producing urea. The method comprises: mixing the precursors of a nitrogen source and a carbon source, performing continuous gradient heating under an inert atmosphere, keeping the temperature at 207°C for 2 hours, keeping the temperature at 550°C for 6 hours, and keeping the temperature at 900°C for 3 hours; and obtaining a catalyst product after cooling. The large number of C=N‑H functional groups contained in the two-dimensional carbon layer structure of the catalyst of the present invention provide catalytic active sites, which can achieve higher atomic utilization and better stability; the optimized C=N active sites are more conducive to the CRR reaction and can promote the occurrence of the C‑N coupling process. By adopting a one-pot gradient pyrolysis method, there is no need for tedious steps such as pretreatment and post-treatment acid washing, and the reactant precursor raw materials used are abundant in reserves and low in cost.

Description

Technical Field

[0001] The present invention relates to a method for electrochemically coupling the co-reduction of CO2 and NO3 — The invention discloses a method for preparing a catalyst for producing urea, belonging to the field of electrocatalytic energy storage. Background Art

[0002] Urea is one of the most important nitrogen fertilizers, and the development of the urea industry is crucial for meeting the growing demand. However, current large-scale industrial urea synthesis still relies on traditional methods, typically using two sequential reactions: N₂ + H₂ → NH₃ and NH₃ + CO₂ → CO(NH₂)₂. These processes require high temperatures, pressures, and energy consumption, and the raw gas synthesis is accompanied by significant emissions of the greenhouse gas CO₂. Therefore, the development of efficient, clean, and energy-efficient urea synthesis technologies is crucial.

[0003] In recent years, inspired by the electrocatalytic synthesis of ammonia technology, electrocatalytic synthesis of urea is considered to be the most promising technology to replace industrial urea synthesis. In 2020, Chen et al. first proposed the electrochemical coupling of N2 and CO2, and achieved 3.36mmol g using TiO2-supported PdCu nanoparticle catalysts. -1 h -1 The synthesis rate of urea was 5.91mmolg using non-precious metal catalyst Bi-BiVO4 heterojunction. -1 h -1 Urea synthesis yield (Yuan, Menglei, et al. Angew. Chem. Int. Ed. 133. 19 (2021): 11005-11013.).

[0004] However, the current method of preparing urea by coupling CO2 reduction (CRR) and N2 reduction (ENRR) half reactions still faces the bottleneck of low efficiency and low selectivity. The main reason is that the ENRR half reaction has a low solubility of N2 (K H =6.24×10 -4 mol L -1 atm -1 ), and a strong N≡N bond (940.95 kJ mol -1 ) requires a high activation energy, resulting in a very low utilization efficiency of N2. Recently, NO3 — Reduction to NH3 (NtrRR) has become an effective alternative to ENRR and has achieved a faradaic efficiency of up to 95% for ammonia production. Therefore, coupling the CRR and NtrRR reactions will hopefully achieve the reduction of waste gas (CO2) and wastewater (NO3— ) is effectively utilized to produce high value-added urea through electrochemical reaction.

[0005] For simultaneous reduction of CO2 and NO3 — In an electrochemical reaction system, how to control the reaction process to achieve efficient CN coupling is particularly critical. Therefore, the rational design of efficient and highly selective catalysts is particularly important for the effective conversion and utilization of energy. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to overcome the deficiencies in the prior art and provide a method for electrochemically coupling the reduction of CO2 and NO3. — A method for preparing a catalyst for producing urea.

[0007] To solve the technical problem, the solution of the present invention is:

[0008] Provides electrochemical coupling for the co-reduction of CO2 and NO3 — A method for preparing a catalyst for producing urea comprises the steps of:

[0009] (1) Weigh the nitrogen source and carbon source precursors at a mass ratio of 40:1, mix them, and place them in a quartz boat;

[0010] (2) The mixed precursor was transferred to a tube furnace and heated continuously from room temperature under inert atmosphere by increasing the temperature to 207°C for 2 h, 550°C for 6 h, and 900°C for 3 h.

[0011] (3) After cooling naturally to room temperature, a black powdery solid is obtained, which is the NC catalyst.

[0012] As a preferred embodiment of the present invention, the nitrogen source is melamine.

[0013] As a preferred embodiment of the present invention, the carbon source is glucose.

[0014] As a preferred embodiment of the present invention, the inert atmosphere is achieved by continuously introducing argon gas.

[0015] As a preferred embodiment of the present invention, when performing continuous gradient heating, the heating rate is controlled to be 5°C / min.

[0016] The present invention further provides a catalyst prepared by the above method for electrochemical coupling of CO2 and NO3 — The application method in preparing urea comprises the following steps:

[0017] (1) dispersing a powdered catalyst in a dispersion to obtain a slurry; coating the slurry on a hydrophobic carbon paper and drying it naturally to obtain an NC catalytic electrode sheet;

[0018] (2) A three-electrode H-type electrolytic cell was constructed using an NC catalytic electrode sheet as the working electrode, a Pt sheet as the counter electrode, and an Ag / AgCl electrode as the reference electrode. The working electrode and the reference electrode were located in the cathode region, the counter electrode was located in the anode region, and the cells were separated by a diaphragm. The electrolyte was a mixed solution of 0.1 M KHCO3 + 0.1 M KNO3.

[0019] (3) Continuously introduce CO2 into the H-type electrolytic cell and apply power to convert CO2 and NO3 — Catalytic reduction to urea.

[0020] As a preferred embodiment of the present invention, the dispersion is prepared by mixing deionized water, ethylene glycol, and Nafion in a volume ratio of 48:1:1.

[0021] As a preferred embodiment of the present invention, the diaphragm is a proton exchange membrane.

[0022] As a preferred embodiment of the present invention, the H-type electrolytic cell is energized by a constant voltage method, and the loaded voltage range is -0.3V to -0.7V vs. RHE.

[0023] Description of the invention principle:

[0024] The innovation of this invention lies in the use of gradient temperature control to achieve melting, condensation, and carbonization of nitrogen and carbon source precursors in a one-pot process. By maintaining the reaction at 207°C, melamine is thoroughly mixed with the other reactants while still molten; maintaining the reaction at 550°C allows the carbon and nitrogen sources to condense to form a carbon trinitrogen tetrakis (C3N4) structure; and maintaining the reaction at 900°C allows some nitrogen atoms to detach from the carbon layer. In this way, an NC catalyst containing a two-dimensional carbon layer with nitrogen defects containing C=NH functional groups is prepared.

[0025] Based on this unique catalyst microstructure, the reversible adsorption and desorption of hydrogen in C=NH can be utilized to optimize the reaction pathway during the electrochemically coupled catalytic reaction, promote the occurrence of the CN coupling process, and achieve higher selectivity for catalytic synthesis of urea.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] (1) The NC catalyst prepared by the present invention has a large number of C=NH functional groups contained in its two-dimensional carbon layer structure, which provides catalytic active sites, can achieve higher atomic utilization and better stability.

[0028] (2) In the NC catalyst of the present invention, the non-metallic active site of C=NH first undergoes a dehydrogenation process during the NtrRR process. The optimized C=N active site is more conducive to the CRR reaction and can promote the occurrence of the CN coupling process.

[0029] (3) The present invention adopts a one-pot gradient pyrolysis method, which does not require tedious steps such as pretreatment and post-treatment pickling, and the reactant precursor raw materials used are abundant in reserves and low in cost.

[0030] (4) The NC catalyst prepared in the present invention can achieve 596.2 μg mg at 0.5 V vs. RHE. -1 h -1 The urea production rate and Faradaic efficiency of 65.7% are higher than those of metal-containing catalysts. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is the STEM image of NC in Example 1 of the present invention;

[0032] Figure 2 is the XPS of NC in Example 1 of the present invention;

[0033] Figure 3 Schematic diagram of the electrochemical cell for preparing urea by electrochemical CN coupling in Example 1 of the present invention;

[0034] Figure 4 The Faradaic efficiency and selectivity of urea production of NC (a) and Cu1 / NC (b) in Example 1 of the present invention are as follows;

[0035] Figure 5 is the urea yield of NC and Cu1 / NC in Example 1 of the present invention;

[0036] Figure 6 The performance of NC in Example 1 of the present invention is compared with the performance of other reported catalysts;

[0037] Figure 7 Comparison of urea production stability of NC in Example 1 of the present invention;

[0038] Figure 8 The NC in Example 1 of the present invention co-reduces CO2 and NO3 at different voltages — In situ infrared spectrum of

[0039] Figure 9 The NC alone reduces NO3 at different voltages in Example 1 of the present invention — In situ infrared spectrum of

[0040] Figure 10 The NC in Example 1 of the present invention uses different isotopes of CO2 and NO3 — Raman spectrum. DETAILED DESCRIPTION

[0041] The present invention will be described in detail below with reference to specific embodiments, but the protection scope of the present invention is not limited to these embodiments.

[0042] Melamine, glucose, and copper nitrate used in the following examples were purchased from Sigma-Aldrich Co., Ltd. and were of analytical grade.

[0043] Part I Preparation and Application of Catalysts

[0044] Example 1 Preparation of NC catalyst

[0045] In this embodiment, it is used for electrochemical coupling of CO2 and NO3 — The catalyst for producing urea, the preparation method thereof comprises the following steps:

[0046] (1) Weigh 16 g of dicyandiamide (DADC) and 400 mg of glucose (GC), mix well, and place in a quartz boat.

[0047] (2) Place the quartz boat in a tube furnace and continuously introduce argon gas to maintain an inert atmosphere;

[0048] (3) Set up a gradient temperature program and perform continuous gradient heating at a heating rate of 5°C / min: heat from room temperature to 207°C and then hold for 2 hours to allow melamine to be fully mixed with glucose in a molten state. Heat from 207°C to 550°C and then hold for 6 hours to allow the carbon source and nitrogen source to condense to form a carbon trinitrogen tetrakis (C3N4) structure. Heat from 550°C to 900°C and then hold for 3 hours to allow some nitrogen atoms to detach from the carbon layer, forming an atomic-level dispersed structure.

[0049] (4) After cooling naturally to room temperature, a black powdery solid is obtained, which is the NC catalyst.

[0050] Application method examples:

[0051] The NC catalyst prepared in Example 1 was used for electrochemical coupling of CO2 and NO3 — The preparation of urea comprises the following steps:

[0052] (1) Take 100 μL of Nafion and add 100 μL of isopropanol and 4.8 mL of deionized water. Ultrasonicate for 5 min to prepare 5 mL of 1 wt‰ Nafion solution.

[0053] (2) 2 mg of powdered catalyst NC was added to 1 mL of 1 wt‰ Nafion solution and ultrasonicated for 2 h to obtain a uniformly dispersed 2 mg / mL catalyst slurry;

[0054] (3) Cut a 1.5 cm × 1 cm hydrophobic carbon paper and apply 100 μL of catalyst slurry to the hydrophobic carbon paper 1 cm away. 2 The surface was dried naturally to obtain a carbon paper electrode with a coating amount of NC catalyst of 0.2 mg / cm 2 ;

[0055] (4) Build a three-electrode system in an H-type electrolytic cell (such as Figure 3 As shown in the figure): A carbon paper electrode coated with NC catalyst is used as the working electrode for the electrocatalytic reaction of urea synthesis, the counter electrode is a Pt sheet, the reference electrode is an Ag / AgCl electrode, and the electrolyte is 0.1M KHCO3+0.1M KNO3; wherein, the working electrode and the reference electrode are located in the cathode region, and the counter electrode is located in the anode region, separated by a proton exchange membrane.

[0056] (5) CO2 is continuously introduced into the H-type electrolytic cell, and the H-type electrolytic cell is energized by a constant voltage method (with a 0.1V interval and a loaded voltage range of -0.3V to -0.7V vs. RHE), and CO2 and NO3 — Catalytic reduction to urea. Figure 3 The arrow on the upper left side represents the excretion of excess CO2.

[0057] Part II Performance Testing and Analysis of Catalysts

[0058] 1. Preparation of Metal Catalyst in Comparative Example 1

[0059] This comparative example adds the use of metal ions on the basis of Example 1, specifically including:

[0060] 16g of dicyandiamide (DADC), 400mg of glucose (GC), and 10mg of copper nitrate (Cu(NO3)2) were weighed, mixed, and placed in a quartz boat. A continuous gradient heating treatment was then performed according to the same procedures as steps (2)-(4) in Example 1 to obtain a Cu1 / NC catalyst.

[0061] 2. Performance test method:

[0062] (1) Take 2 mg of NC catalyst and 2 mg of Cu1 / NC catalyst respectively, disperse the powder in 1 mL of dispersion solution, and mix well to prepare 2 mg mL -1 The two slurries are as follows: the ratio of the dispersion is: deionized water: ethylene glycol: Nafion = 480uL: 10uL: 10uL.

[0063] (2) The above two slurries were coated on 1×1.5 cm hydrophobic carbon paper to obtain 0.2 mg cm -1 catalytic cathode.

[0064] (3) Using the NC catalytic electrode as the working electrode, the Pt electrode as the counter electrode, and the Ag / AgCl electrode as the reference electrode, a three-electrode H-type electrolytic cell (such as Figure 3As shown); wherein, the working electrode and the reference electrode are located in the cathode region, the counter electrode is located in the anode region, separated by a proton exchange membrane in the middle, and the electrolyte is a mixed solution of 0.1M KHCO3+0.1M KNO3;

[0065] (4) Before testing, pass Ar for 15 minutes to remove oxygen from the electrolyte.

[0066] (5) During the test, CO2 gas was continuously introduced, and a voltage was applied at a constant voltage of -0.3 V to -0.7 V vs. RHE for 30 min at 0.1 V intervals. That is, voltages of -0.3 V, -0.4 V, -0.5 V, -0.6 V, and -0.7 V were applied during the loading process. The catholyte after the reaction was collected and replaced with a new catholyte before repeating the test.

[0067] (6) Extract the electrolyte after testing and quantitatively analyze the urea, NH3, and NO3 in the electrolyte by UV spectrophotometry — and N2H4 content.

[0068] The formula for calculating the Faradaic efficiency of urea is as follows:

[0069]

[0070] Where n: number of transferred electrons; F: Faraday constant, 96485C mol -1 ; C: urea concentration; V: volume of electrolyte; M: relative molecular weight of urea is 60; Q: total electrolytic charge.

[0071] The formula for calculating the yield of urea is as follows:

[0072]

[0073] Where, C: urea concentration; V: electrolyte volume; T: reaction time; m: catalyst mass.

[0074] The N-selectivity of nitrogen-containing products is calculated as follows:

[0075]

[0076] M(N): molar mass of a nitrogen-containing product; M total (N): molar mass of total nitrogen-containing products.

[0077] 3. Analysis and Conclusion

[0078] Figure 1 The atomic structure morphology of the NC catalyst was characterized, and the dark field scanning transmission electron microscopy (STEM) image showed a two-dimensional nitrogen-doped carbon layer with some defective structures.

[0079] Figure 2 The full XPS spectrum analysis of the NC catalyst shows that in addition to nitrogen oxide, graphitic nitrogen, pyrrolic nitrogen, and pyridinic nitrogen, there is also an obvious C=NH peak.

[0080] Figure 4 The NC catalysts ( Figure 4 a) and Cu1 / NC( Figure 4 b) Faradaic efficiency and selectivity of the catalysts for urea synthesis at different potentials. The NC catalyst exhibits significantly higher catalytic activity and selectivity than the Cu1 / NC catalyst. As the voltage increases, the reaction favors the production of NH3, resulting in a gradual decrease in the urea Faradaic efficiency. The NC catalyst achieves a maximum Faradaic efficiency of 65.7% and a nitrogen selectivity of ~38% at -0.5 V vs. RHE. Due to the high NtrRR selectivity of the Cu1 / NC catalyst, the two competing reactions favor the production of NH3, resulting in a Faradaic efficiency of less than 20% for urea. As the voltage increases, the NH3 product becomes the dominant product.

[0081] Figure 5 The yields of urea synthesized by Cu1 / NC catalyst and NC catalyst at different potentials were statistically analyzed. It can be seen that the urea yield of NC catalyst is much higher than that of Cu1 / NC catalyst, and can reach 596.2 μg mg at -0.5 V vs. RHE. -1 h -1 urea production rate.

[0082] Figure 6 By comparing the electrochemical performance of NC catalysts with other reported catalysts, it can be found that NC catalysts have advantages over those currently reported in terms of both Faradaic efficiency and yield, and their catalytic performance is much higher than that of some reported precious metal catalysts.

[0083] At a given -0.5V vs. RHE, the urea Faraday efficiency of the catalyst was tested five times using the same NC catalyst electrode. Figure 7 It can be seen that the average Faradaic efficiency of 5 times is about 60%, showing considerable cycling stability.

[0084] The applicant further used in-situ Raman and in-situ infrared to reveal the active sites of the catalytic reaction at the atomic and molecular scales and to explore the reaction mechanism of CN coupling. The specific experimental methods are described as follows:

[0085] Electrochemical in situ infrared (ATR-SEIRAS) experiments were performed using an INVENIOR FTIR spectrometer (Bruker) equipped with a mercury-cadmium-telluride (MCT) detector. Au thin films were chemically deposited on the reflective surface of a faceted Si crystal, acting as a conductive substrate to achieve surface enhancement effects.

[0086] The sample was drop-cast onto an Au-deposited Si crystal and used to prepare the working electrode of a custom-made spectroelectrochemical cell mounted on an ATR accessory. Figure 8 The left figure shows the results of in-situ infrared observations in the range of 1000-2000cm -1 Data within the wavelength range, the right figure is 2600-3800cm -1 Data within the wavelength range. It can be seen that CO2 and NO3 - In the co-reduction system, within the voltage range of 0.0V to -0.7V, the increase of voltage is accompanied by the increase of CN (1228cm -1 )、NCN(1461cm -1 )、C=O(1677cm -1 )、NH(1625cm -1 ) and other typical urea chemical bonds, indicating the occurrence of CN coupling process and the formation of urea. - During the reaction, it was observed that the growth range of the low voltage (0.0V to -0.15V) was Figure 9 ), a reduction in NH bonds was observed, indicating that the defective CNH plays the role of a proton pump in the catalytic process, and the proton on C=NH participates in the reaction as a reactant.

[0087] In situ Raman experiments were performed using an improved electrochemical Raman cell (a three-electrode polyvinylidene fluoride cell with a 0.5 mm thick sapphire window) and a confocal Raman spectrometer (Renishaw inVia). A 532 nm excitation laser (10%) with a power of 50 mW was used as the light source, the grating was 1800, and it was focused on the sample surface through a 50X telephoto objective lens. During the Raman measurement, the distance between the sapphire window and the working electrode surface was less than 0.1 mm, which made the solution layer on the working electrode surface very thin, so the attenuation effect of the solution layer on the Raman signal could be minimized. CN (~1415 cm -1 ) and NH(~1170cm -1 ) further supports the formation of C-N bonds and urea. In this catalytic system, the catalyst itself has a carbon and nitrogen source, so it is necessary to verify the sources of carbon and nitrogen in urea. Figure 10 The observed structure of in-situ Ramanian isotopes, with the left image being 1150 cm -1 ~1250cm -1 Data within the range, the right picture is 1300cm -1 ~1450cm -1 The data in the range. 12 CO2 or 14 NO3 - Replace with13 CO2 or 15 NO3 - After that, the positions of CN and NH peaks shifted, indicating that the C and N in urea came from the introduced CO2 and NO3 in the electrolyte. - In situ experiments combined with isotope observations reveal the active sites of catalytic reactions and the electrocatalytic reaction mechanism at the molecular and atomic levels.

[0088] The nitrogen-doped two-dimensional carbon catalyst used in the present invention has simple synthesis steps and low cost of synthetic raw materials, which greatly reduces the manufacturing cost of the catalyst. - At the same time, waste acid NO3 - treatment of greenhouse gas CO2 and synthesis of high-value urea.

[0089] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. An electrochemical coupling of CO2 and NO3 - A method for preparing urea, characterized in that The following steps are involved: (1) dispersing a powdered catalyst in a dispersion to obtain a slurry; coating the slurry on a hydrophobic carbon paper and drying it naturally to obtain an NC catalytic electrode sheet; (2) A three-electrode H-type electrolytic cell was constructed using an NC catalytic electrode sheet as the working electrode, a Pt sheet as the counter electrode, and an Ag / AgCl electrode as the reference electrode. The working electrode and the reference electrode were located in the cathode region, the counter electrode was located in the anode region, and the cells were separated by a diaphragm. The electrolyte was a mixed solution of 0.1 M KHCO3 + 0.1 M KNO3. (3) Continuously introduce CO2 into the H-type electrolytic cell and apply power to convert CO2 and NO3 - Catalytic reduction to urea; The catalyst is prepared by the following method: (a) Weigh nitrogen source and carbon source precursors at a mass ratio of 40:1, mix them, and place them in a quartz boat; (b) The mixed precursor was transferred to a tube furnace and heated continuously under an inert atmosphere starting from room temperature at 207°C for 2 h, 550°C for 6 h, and 900°C for 3 h. (c) After cooling naturally to room temperature, a black powdery solid is obtained, which is the NC catalyst.

2. The method according to claim 1, characterized in that The dispersion is prepared by mixing deionized water, ethylene glycol and Nafion in a volume ratio of 48:1:

1.

3. The method according to claim 1, characterized in that The membrane is a proton exchange membrane.

4. The method according to claim 1, wherein The H-type electrolytic cell was powered using a constant voltage method, and the loaded voltage range was -0.3 V to -0.7 V vs. RHE.

5. The method according to claim 1, wherein The nitrogen source is melamine.

6. The method according to claim 1, characterized in that The carbon source is glucose.

7. The method according to claim 1, characterized in that The inert atmosphere was achieved by continuously flowing argon gas.

8. The method according to claim 1, characterized in that When performing continuous gradient heating, the heating rate was controlled at 5°C / min.

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