Method for preparing urea by electrochemically catalyzing carbon dioxide and nitrogen

Abstract

The invention discloses a method for preparing urea through electrochemical catalysis of carbon dioxide and nitrogen, and belongs to the technical field of value-added chemicals constructed through an electrochemical method.The method comprises the steps that S1, a copper dimer is synthesized; s2, synthesizing graphite phase carbon nitride; s3, synthesizing copper diatoms loaded on the surface of the graphite phase carbon nitride; s4, taking silver / silver chloride as a reference electrode, taking a platinum net as a counter electrode, taking the copper diatomic catalyst as a working electrode, and carrying out electrochemical reaction in an H-type electrolytic tank to obtain a urea product; rich and cheap carbon dioxide and nitrogen in nature are used as reaction raw materials, water is used as a proton source, and green, clean and mild synthesis of urea under room-temperature room pressure is realized. More importantly, the catalyst shows good activity, selectivity and stability in a urea electrochemical synthesis reaction; the catalyst can become a low-cost and high-efficiency catalyst applied to urea electrosynthesis.

Description

Technical Field

[0001] This invention relates to the field of electrochemical methods for constructing value-added chemicals, and more particularly to a method for preparing urea from carbon dioxide and nitrogen through electrochemical catalysis. Background Technology

[0002] Currently, industrial urea production requires liquid ammonia and carbon dioxide to undergo a harsh reaction under high temperature and pressure conditions. This process consumes approximately 2% of the world's total energy consumption annually, accompanied by substantial carbon dioxide emissions. With the depletion of fossil fuels and the increase in greenhouse gas emissions, there is an urgent need to develop environmentally friendly and sustainable green synthesis methods for urea production. Compared to conventional thermocatalysis and other hydrogenation methods, utilizing renewable electricity to synthesize urea from abundant natural resources is a highly promising strategy. Electrocatalytic synergistic conversion of carbon dioxide and nitrogen to produce urea not only achieves rational resource utilization but also bypasses the ammonia synthesis step in the industrial route, transforming a high-energy-consuming and highly polluting industrial process into a low-energy-consuming, mild, and clean process driven by renewable energy. This has significant research value and broad development prospects.

[0003] At ambient temperature and pressure, the electrocatalytic conversion of carbon dioxide and nitrogen exhibits low activity and selectivity. This is mainly due to (1) weak chemisorption of carbon dioxide and nitrogen gases on the electrode catalyst surface; (2) the need for high overpotentials to dissociate the highly stable nitrogen-nitrogen triple bond and carbon-oxygen double bond; and (3) the complex intermediates and numerous side reactions involved in the synergistic electroreduction of carbon dioxide and nitrogen, resulting in low urea selectivity. To improve the reactivity of the electrocatalytic conversion of carbon dioxide and nitrogen to urea, researchers have focused on constructing catalysts with multiple reaction sites. These multiple sites can adsorb and activate carbon dioxide and nitrogen respectively, thereby improving the efficiency of the carbon-nitrogen coupling reaction. Currently, the electrode catalyst materials used in the electrocatalytic conversion of carbon dioxide and nitrogen to urea still have some limitations, such as low Faraday efficiency, low yield, and insufficient electrocatalytic stability of the urea product.

[0004] Diatomic catalysts are a class of atomically dispersed metal catalysts anchored to the surface of a support. They have maximized atomic utilization and adjustable coordination configurations, and are widely used in the field of catalysis. Summary of the Invention

[0005] The present invention aims to provide a method for preparing urea from carbon dioxide and nitrogen by electrochemical catalysis, so as to solve the problems mentioned in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A method for preparing urea from carbon dioxide and nitrogen by electrochemical catalysis, the method comprising the following steps:

[0008] S1. Synthesis of copper dimer:

[0009] Copper chloride dihydrate was dissolved in deionized water. A methanol solution of bipyridine and an aqueous solution of oxalic acid were added dropwise to the aqueous solution of copper chloride dihydrate. The mixture was then stirred and washed and centrifuged to obtain copper dimer.

[0010] S2, Synthesis of graphitic carbon nitride:

[0011] Urea was heated in a muffle furnace and then kept at a certain temperature. The resulting powder was cooled to room temperature, then heated to a certain temperature and kept at that temperature again to finally obtain graphitic carbon nitride.

[0012] S3. Copper diatomic synthesis supported on the surface of graphitic carbon nitride:

[0013] Copper dimer was dispersed in methanol, and then added to a methanol solution containing graphitic carbon nitride. After stirring, centrifugation, drying, and calcination, the loaded copper diatomic electrode catalyst was obtained.

[0014] S4. Using silver / silver chloride as the reference electrode, a platinum mesh as the counter electrode, and the copper diatomic catalyst as the working electrode, an electrochemical reaction is carried out in an H-type electrolytic cell to obtain urea product.

[0015] Furthermore, in the synthesis of copper dimers, the molar concentration of copper chloride dihydrate is 0.05-0.10 M, the molar concentration of bipyridine is 0.05-0.20 M, and the molar concentration of oxalic acid is 0.05-0.10 M.

[0016] Furthermore, in the synthesis of graphitic carbon nitride, the first heating is carried out at 500-600 degrees Celsius for 1-2 hours, and the second heating is carried out at 500-600 degrees Celsius for 3-4 hours.

[0017] Furthermore, in the process of synthesizing copper diatomic molecules supported on the surface of graphitic carbon nitride, the mass of copper dimer added is 0.1-1.0 g, the volume of methanol is 40-60 mL, the mass of graphitic carbon nitride added is 40-50 mg, and the volume of methanol is 3-6 mL; the calcination temperature is 200-300 degrees Celsius, and the duration is 8-15 hours.

[0018] Furthermore, in step S4, both the cathode and anolyte are potassium bicarbonate solutions.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] This invention utilizes a calcination method to prepare graphitic carbon nitride modified with copper diatoms. This catalyst contains abundant copper diatomic catalytic sites, which are beneficial for binding carbon dioxide and nitrogen reactants. The copper diatoms loaded on the graphitic carbon nitride induce a gradient change in the charge density at the interface. The two copper atoms serve as catalytic sites for carbon-containing and nitrogen-containing species, respectively, exhibiting decoupled adsorption and activation characteristics. The uniformly dispersed atomic-level reaction sites on the surface of the graphitic carbon nitride effectively promote the contact between reactants and catalyst, thereby significantly improving the Faraday efficiency and yield of electrochemical synthesis of urea.

[0021] Secondly, graphitic carbon nitride has good stability and conductivity, which can maintain the stability and reusability of the catalyst during the catalytic process, and realize the preparation of urea with high activity, high selectivity and high stability. Attached Figure Description

[0022] Figure 1 This is an aberration-corrected transmission electron microscope image of the copper diatomic electrode catalyst in this invention.

[0023] Figure 2 This is a scanning electron microscope image of the copper diatomic electrode catalyst in this invention.

[0024] Figure 3 This is the X-ray diffraction pattern of the copper diatomic electrode catalyst in this invention.

[0025] Figure 4 The graph shows the electrochemical synthesis yield of urea using copper diatomic electrode catalysts with loadings of 0.73%, 1.37%, 2.29%, and 3.00% in this invention.

[0026] Figure 5 The diagram shows the Faradaic efficiency of the electrochemical synthesis of urea using copper diatomic electrode catalysts with loadings of 0.73%, 1.37%, 2.29%, and 3.00% in this invention.

[0027] Figure 6 This is a graph showing 10 cycles of testing of the copper diatomic electrode catalyst in this invention;

[0028] Figure 7 This is a flowchart of a method for preparing urea from carbon dioxide and nitrogen through electrochemical catalysis. Detailed Implementation

[0029] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments:

[0030] This invention provides a copper diatomic catalyst for the electrochemical synthesis of urea using carbon dioxide and nitrogen as raw materials. The copper diatomic catalyst is uniformly dispersed on the surface of graphitic carbon nitride prepared by calcination and exhibits high activity and selectivity in the electrocatalytic synthesis of urea.

[0031] like Figure 7 As shown, the preparation method of the copper diatomic electrode catalyst includes the following steps:

[0032] First, the synthesis of copper dimers:

[0033] Copper chloride dihydrate was dissolved in deionized water. A methanol solution of bipyridine and an aqueous solution of oxalic acid were added dropwise to the aqueous solution of copper chloride dihydrate. The mixture was then stirred and washed and centrifuged to obtain copper dimer.

[0034] Subsequently, the synthesis of graphitic carbon nitride was carried out:

[0035] Urea is heated in a muffle furnace and then kept at a certain temperature. The resulting powder is cooled to room temperature, then heated to a certain temperature and kept at that temperature again to finally obtain graphitic carbon nitride.

[0036] Finally, copper diatomic synthesis on the surface of graphitic carbon nitride:

[0037] Copper dimers were dispersed in methanol, and then added to a methanol solution containing graphitic carbon nitride. After stirring, centrifugation, drying, and calcination, the loaded copper diatomic catalyst was obtained.

[0038] In this preparation method:

[0039] When synthesizing copper dimers, the molar concentration of copper chloride dihydrate is 0.05-0.10 M, the molar concentration of bipyridine is 0.05-0.20 M, and the molar concentration of oxalic acid is 0.05-0.10 M. The purpose of stirring is to promote the formation of copper dimers, and washing and centrifugation are to obtain copper dimers.

[0040] In the synthesis of graphitic carbon nitride, the first heating is carried out at 500-600 degrees Celsius for 1-2 hours, and the second heating is carried out at 500-600 degrees Celsius for 3-4 hours, in order to form graphitic carbon nitride.

[0041] In the synthesis of copper diatoms supported on the surface of graphitic carbon nitride, the mass of copper dimer added is 0.1-1.0 g, the volume of methanol is 40-60 mL, and the mass of graphitic carbon nitride added is 40-50 mg, the volume of methanol is 3-6 mL. This step also includes stirring, centrifugation, and drying to ensure that the copper diatoms form and are uniformly dispersed on the surface of the graphitic carbon nitride. The calcination temperature is 200-300 degrees Celsius, and the duration is 8-15 hours.

[0042] Furthermore, a method for electrocatalytic synthesis of urea from carbon dioxide and nitrogen is provided, comprising the following steps:

[0043] Using silver / silver chloride as the reference electrode, a platinum mesh as the counter electrode, and the copper diatomic catalyst as the working electrode, an electrochemical reaction is carried out in an H-type electrolytic cell to obtain urea product.

[0044] Both the cathode and anode electrolytes are potassium bicarbonate solutions.

[0045] During the electrochemical reaction, carbon dioxide gas is continuously introduced.

[0046] During the electrochemical reaction, the applied voltage is set to -0.50 volts to -0.70 volts, preferably -0.55 volts.

[0047] This invention employs a calcination method to prepare copper diatomic catalysts, which is simple, low-cost, and highly reproducible. The invention utilizes graphitic carbon nitride-supported copper diatomic catalysts to electrocatalyze the conversion of carbon dioxide and nitrogen into urea, exhibiting excellent urea selectivity and yield, and providing a new approach for customizing electrochemical urea synthesis catalysts.

[0048] The specific implementation process is as follows:

[0049] Preparation of a copper diatomic catalyst with a loading of 0.73%

[0050] (1) Dissolve 1.6 mmol bipyridine and 0.8 mmol oxalic acid in 15 mL methanol and 15 mL deionized water respectively using an ultrasonic instrument.

[0051] (2) Dissolve 1.6 mmol of copper chloride dihydrate in 25 mL of deionized water using an ultrasonic instrument, and add the solution obtained in step (1) dropwise to the copper chloride dihydrate aqueous solution. Stir continuously for 1 hour, wash with water and methanol respectively, centrifuge three times, and obtain a light blue solid by vacuum drying.

[0052] (3) Place 20 grams of urea into a crucible and place it in a muffle furnace. Heat it to 550 degrees Celsius at a rate of 20 degrees Celsius per minute and keep it for 2 hours. After cooling to room temperature, reheat it to 550 degrees Celsius at a rate of 5 degrees Celsius per minute and keep it for 3 hours to obtain a light yellow solid powder.

[0053] (4) Weigh 42 mg of the copper dimer obtained in step (2) and disperse it in 10 mL of methanol. Weigh 0.5 g of the graphite phase carbon nitride obtained in step (3) and disperse it in 30 mL of methanol. Add the copper dimer methanol dispersion dropwise to the graphite phase carbon nitride methanol dispersion. Stir at room temperature for 30 hours, centrifuge to obtain solid, and dry to obtain solid powder.

[0054] (5) The solid obtained in step (4) is placed in a crucible and placed in a muffle furnace and heated to 250 degrees Celsius at a rate of 2 degrees Celsius per minute. The temperature is then maintained for 10 hours to obtain a supported copper diatomic catalyst.

[0055] from Figure 1 It can be seen that the distance between the two copper atoms in the final copper diatomic structure is about 0.24 nanometers.

[0056] from Figure 2 It can be seen that its main morphology is a layered structure of graphitic carbon nitride, indicating that the support structure is intact.

[0057] from Figure 3 It can be seen that its main crystal planes belong to the graphitic carbon nitride phase, indicating that the copper element is dispersed in atomic form.

[0058] Example 2: Preparation of a copper diatomic catalyst with a loading of 1.37%

[0059] Replace the amount of copper chloride dihydrate in Example 1 with 2.4 mmol.

[0060] Example 3: Preparation of a copper diatomic catalyst with a loading of 2.29%

[0061] Replace the amount of copper chloride dihydrate in Example 1 with 3.2 mmol.

[0062] Example 4: Preparation of a copper diatomic catalyst with a loading of 3.00%

[0063] Replace the amount of copper chloride dihydrate in Example 1 with 4.0 mmol.

[0064] Example 5: The performance of the copper diatomic catalyst in this invention for the electrocatalytic conversion of carbon dioxide and nitrogen into urea was tested using a three-electrode system in an H-type electrolytic cell.

[0065] The reference electrode was a silver / silver chloride electrode, the counter electrode was a platinum mesh electrode, and the working electrolysis was the copper diatomic catalyst prepared in Examples 1-4.

[0066] Both the cathode and anode electrolytes are 1 mol / L potassium bicarbonate solutions. The voltages set for the electrocatalytic process are -0.50 V, -0.55 V, -0.60 V, -0.65 V, and -0.70 V. The electrochemical reaction time is 2 hours, and carbon dioxide gas is continuously introduced into the cathode electrolyte during the reaction.

[0067] The performance tests of electrochemical synthesis of urea with different copper diatomic loadings in this invention are shown in the figure. Figure 4 .from Figure 4It can be seen that the urea yield changes with voltage. When the voltage is -0.55 volts, the urea yield can reach 441.9 mg / hour per gram of catalyst.

[0068] The Faraday efficiency of different copper diatomic loadings in the electrochemical synthesis of urea is shown in the figure. Figure 5 .from Figure 5 It can be seen that adjusting the loading of copper diatoms can effectively control the electrochemical synthesis efficiency of urea. When the loading is 2.29%, the urea Faraday efficiency reaches the highest of 52.8%.

[0069] Example 6: The stability of a copper diatom catalyst with a loading of 2.29% was tested using the electrochemical urea synthesis apparatus described in Example 5. The catalyst was cycled 10 times at -0.55 V, with each cycle lasting 2 hours, for a total of 20 hours. Figure 6 It can be seen that the yield of urea on the catalyst surface and the Faraday efficiency were not reduced.

[0070] The above descriptions are merely embodiments of the present invention, and common knowledge such as specific technical solutions and / or characteristics are not described in detail here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the technical solutions of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the implementation of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A method for preparing urea from carbon dioxide and nitrogen by electrochemical catalysis, characterized in that, The preparation method includes the following steps: S1. Synthesis of copper dimer: Copper chloride dihydrate was dissolved in deionized water. A methanol solution of bipyridine and an aqueous solution of oxalic acid were added dropwise to the aqueous solution of copper chloride dihydrate. The mixture was then stirred and washed and centrifuged to obtain copper dimer. S2, Synthesis of graphitic carbon nitride: Urea was heated in a muffle furnace and then kept at a certain temperature. The resulting powder was cooled to room temperature, then heated to a certain temperature and kept at that temperature again to finally obtain graphitic carbon nitride. S3. Copper diatomic synthesis supported on the surface of graphitic carbon nitride: Copper dimer was dispersed in methanol, and then added to a methanol solution containing graphitic carbon nitride. After stirring, centrifugation, drying, and calcination, the loaded copper diatomic electrode catalyst was obtained. S4. Using silver / silver chloride as the reference electrode, a platinum mesh as the counter electrode, and the copper diatomic catalyst as the working electrode, an electrochemical reaction is carried out in an H-type electrolytic cell to obtain urea product.

2. The method for preparing urea from carbon dioxide and nitrogen by electrochemical catalysis according to claim 1, characterized in that, When synthesizing copper dimers, the molar concentration of copper chloride dihydrate is 0.05-0.10 M, the molar concentration of bipyridine is 0.05-0.20 M, and the molar concentration of oxalic acid is 0.05-0.10 M.

3. The method for preparing urea from carbon dioxide and nitrogen by electrochemical catalysis according to claim 1, characterized in that, In the synthesis of graphitic carbon nitride, the first heating is carried out at 500-600 degrees Celsius for 1-2 hours, and the second heating is carried out at 500-600 degrees Celsius for 3-4 hours.

4. The method for preparing urea from carbon dioxide and nitrogen by electrochemical catalysis according to claim 1, characterized in that, In the process of synthesizing copper diatomic molecules supported on the surface of graphitic carbon nitride, the mass of copper dimer added is 0.1-1.0 g, the volume of methanol is 40-60 mL, the mass of graphitic carbon nitride added is 40-50 mg, and the volume of methanol is 3-6 mL; the calcination temperature is 200-300 degrees Celsius, and the duration is 8-15 hours.

5. The method for preparing urea from carbon dioxide and nitrogen by electrochemical catalysis according to claim 1, characterized in that, In step S4, both the cathode and anode electrolytes are potassium bicarbonate solutions.

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