A nano boron-copper composite catalyst, a preparation method thereof, and a method for electrocatalytic synthesis of urea

Abstract

The present invention discloses a nano boron-copper composite catalyst, a preparation method thereof, and a method for electrocatalytic synthesis of urea, which relates to the technical field of catalysts. The preparation method of the nano boron-copper composite catalyst of the present invention is to first mix a copper precursor and a boron precursor with a molten salt and then grind to obtain a powder, and then calcine the powder in an inert atmosphere at a certain temperature to obtain a boron-copper salt composite, and finally wash the boron-copper salt composite with water, wash it with alcohol, and dry it to obtain the nano boron-copper composite catalyst. The present invention realizes the electrocatalytic synthesis of urea under mild conditions and achieves the effect of high-yield synthesis of urea by carrying out boron thermal reduction in different molten salts to synthesize the nano boron-copper salt composite catalyst and electrocatalytically synthesizing urea with it.

Description

Technical Field

[0001] The present invention belongs to the technical field of catalysts, and particularly relates to a nano boron-copper composite catalyst, a preparation method thereof, and a method for electrocatalytic synthesis of urea. Background Art

[0002] As is well known, more than 40% of the global food production depends on chemical fertilizers. And urea, as an important nitrogen source, has an annual output of more than 100 million tons. Urea is not only easy to store, convenient to use, but also has little damage to the soil. It is a chemical nitrogen fertilizer with a large usage amount and the highest nitrogen content. The industrial development of urea is of great significance for meeting the growing population demand.

[0003] Currently, the Haber-Bosch method dominates the industrial production of NH 3 and most of the produced NH 3 is used as a raw material for synthesizing urea. Traditional urea synthesis consists of two consecutive processes. Nitrogen reacts with hydrogen to produce ammonia, and ammonia reacts with carbon dioxide to produce urea. Both of these processes are carried out under high temperature and high pressure (100 - 200 bar and 400 - 500 °C). The Haber-Bosch method for synthesizing ammonia consumes about 2% of the global energy every year. This method relies on various complex types of equipment and multi-cycle processes to improve the conversion efficiency. Therefore, it is particularly important to explore an efficient and economical urea synthesis route under mild conditions. Summary of the Invention

[0004] In view of this, the present invention provides a nano boron-copper composite catalyst, a preparation method thereof, and a method for electrocatalytic synthesis of urea, which solve the disadvantages of strict reaction conditions, huge energy consumption, and low efficiency in the prior art when synthesizing urea from carbon dioxide and ammonia under high temperature and high pressure.

[0005] To achieve the above object, the present invention provides the following technical solutions: A preparation method of a nano boron-copper composite catalyst, comprising the following steps:

[0006] (1) Mix a copper precursor and a boron precursor with a molten salt and then grind to obtain a powder;

[0007] (2) Calcinate the obtained powder in an inert atmosphere at a certain temperature to obtain a boron-copper salt composite;

[0008] (3) Wash, alcohol-wash, and dry the obtained boron-copper salt composite to obtain a nano boron-copper composite catalyst.

[0009] Preferably, the copper precursor in step (1) includes a copper-containing compound; the copper-containing compound includes but is not limited to copper chloride, copper oxide, and copper nitrate.

[0010] The boron precursor includes boron-containing compounds; the boron-containing compounds include, but are not limited to, sodium borohydride, boric acid, and boron oxide.

[0011] Preferably, the molar ratio of copper to boron in the copper precursor and the boron precursor is 1:(0.5 - 3).

[0012] Preferably, the molten salt in step (1) includes NaPO 4 -KPO 4 molten mixture, NaCl-KCl molten mixture, LiSO 4 -K 2 SO 4 molten mixture.

[0013] Preferably, the mass ratio of the total mass of the copper precursor and the boron precursor to the molten salt in step (1) is 1:(10 - 30).

[0014] Preferably, the calcination temperature in step (2) is 650°C - 1000°C, and the heating rate is 5°C / min - 10°C / min.

[0015] Preferably, the alcohol in step (3) includes methanol, ethanol, and propanol; the drying temperature is 60°C - 80°C, and the drying time is 12h - 24h.

[0016] The present invention also provides a nano boron-copper composite catalyst prepared by the preparation method.

[0017] The present invention also provides a method for electrocatalytic synthesis of urea using the nano boron-copper composite catalyst. Using the nano boron-copper composite catalyst as the working electrode, a graphite rod as the counter electrode, and an Ag / AgCl electrode as the reference electrode, CO 2 is introduced into the electrolyte, and an H-type electrolytic cell is used as the reaction device for electrocatalytic reaction.

[0018] Preferably, the electrolyte is a mixed solution of bicarbonate and nitrate.

[0019] The present invention has the following beneficial effects:

[0020] 1. The present invention selects copper precursors and boron precursors with low cost, reduces the production cost, and the obtained catalyst has higher use value and execution effect.

[0021] 2. The method for synthesizing the nano boron-copper composite catalyst of the present invention has the characteristics of simple synthesis process, low cost, less energy consumption, and mild conditions.

[0022] 3. The nano boron - copper composite catalyst obtained in the present invention has excellent performance in electrocatalytic synthesis of urea, realizing electrocatalytic synthesis of urea under mild conditions and achieving a high - yield synthesis of urea. The urea yield can reach 15 mmol / h / g, and the maximum Faraday efficiency reaches 22%. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is the transmission electron microscopy image of the boron - copper composite catalyst in Example 1;

[0024] Figure 2 It is the transmission electron microscopy image of the boron - copper composite catalyst in Example 2;

[0025] Figure 3 It is the transmission electron microscopy image of the boron - copper composite catalyst in Example 3;

[0026] Figure 4 It is the transmission electron microscopy image of the boron - copper composite catalyst in Example 4;

[0027] Figure 5 It is the graph of the yield and Faraday efficiency of electrocatalytic synthesis of urea in Example 5;

[0028] Figure 6 It is the graph of the yield and Faraday efficiency of electrocatalytic synthesis of urea in Example 6;

[0029] Figure 7 It is the graph of the yield and Faraday efficiency of electrocatalytic synthesis of urea in Example 7;

[0030] Figure 8 It is the graph of the yield and Faraday efficiency of electrocatalytic synthesis of urea in Comparative Example 4;

[0031] Figure 9 It is the graph of the yield and Faraday efficiency of electrocatalytic synthesis of urea in Comparative Example 5;

[0032] Figure 10 It is the UV curve of UV - visible ammonia color development;

[0033] Figure 11 It is the standard curve of ammonia color development. DETAILED DESCRIPTION OF THE INVENTION

[0034] Now, various exemplary embodiments of the present invention will be described in detail. This detailed description should not be considered as a limitation of the present invention, but should be understood as a more detailed description of certain aspects, characteristics, and implementation schemes of the present invention.

[0035] The present invention provides a preparation method of a nano boron - copper composite catalyst, which is prepared by the following preparation steps:

[0036] (1) Mix the copper precursor and the boron precursor with the molten salt and grind thoroughly until the powder particles are fine and evenly mixed to obtain a powder.

[0037] In the present invention, the copper precursor includes a copper-containing compound; the copper-containing compound includes but is not limited to copper chloride, copper oxide, and copper nitrate; the boron precursor includes a boron-containing compound; the boron-containing compound includes but is not limited to sodium borohydride, boric acid, and boron oxide. The boron precursor also includes boron powder; the molar ratio of copper to boron in the copper precursor and the boron precursor is preferably 1:(0.5 - 3), more preferably 1:(0.8 - 2.1).

[0038] In the present invention, the molten salt includes but is not limited to NaPO 4 -KPO 4 molten mixture, NaCl-KCl molten mixture, LiSO 4 -K 2 SO 4 molten mixture, preferably the NaCl-KCl molten mixture; the molar ratio of NaCl to KCl in the NaCl-KCl molten mixture is preferably (1 - 3):(1 - 3), more preferably 1:1. In the present invention, the mass ratio of the total mass of the copper precursor and the boron precursor to the mass of the molten salt is 1:(10 - 30), preferably 1:(10 - 20).

[0039] (2) Calcinate the obtained powder in an inert atmosphere at a certain temperature to obtain a boron copper salt complex.

[0040] In the present invention, calcining the obtained powder in an inert atmosphere at a certain temperature specifically means: transferring the obtained powder to a glass tube for sealing or transferring it to an alumina crucible, and calcining it in an inert atmosphere at a certain temperature; the calcination temperature is preferably 400°C - 1000°C, more preferably 650°C - 1000°C, the heating rate is preferably 5°C / min - 10°C / min, the calcination time is preferably 1h - 3h, more preferably 1.5 - 2.5h; the calcination device is preferably one of a muffle furnace and a tube furnace.

[0041] (3) Wash the obtained boron copper salt complex with water, wash it with alcohol, and dry it to obtain a nano boron copper complex catalyst.

[0042] In the present invention, the number of water washing times is preferably 3 times to remove inorganic salts and by-product BxOy; the alcohol includes methanol, ethanol, and propanol, preferably ethanol; the drying is preferably carried out in a vacuum oven; the drying temperature is preferably 60°C - 80°C, and the drying time is preferably 12h - 24h.

[0043] The present invention also provides a nano boron copper complex catalyst prepared by the above preparation method.

[0044] The present invention also provides a method for electrocatalytic synthesis of urea using the nano boron - copper composite catalyst. Using the nano boron - copper composite catalyst as the working electrode, a graphite rod as the counter electrode, and an Ag / AgCl electrode as the reference electrode, CO is introduced into the electrolyte 2 , and an H - type electrolytic cell is used as the reaction device for electrocatalytic reaction.

[0045] In the present invention, the electrolyte is preferably a mixed solution of bicarbonate and nitrate; the concentration of the bicarbonate is preferably 0.1M - 0.5M; the bicarbonate is preferably one of potassium bicarbonate and sodium bicarbonate; the concentration of the nitrate is preferably 0.01M - 0.05M; the nitrate is preferably one of potassium nitrate and sodium nitrate.

[0046] Compared with the energy - consuming industrial synthesis process, electrochemical synthesis has the advantages of low cost and less emissions, and is a new green synthesis strategy. The traditional electrocatalytic coupling of nitrogen and carbon dioxide provides a feasible solution for the direct production of synthetic urea under environmental conditions. However, due to the energy barrier of the N≡N bond (941 kJ / mol), it is not easy to activate. The present invention couples nitrate with carbon dioxide to drive the electrocatalytic reaction to synthesize urea, which can effectively achieve C - N coupling and inhibit the influence of side reactions such as hydrogen evolution reaction, thereby increasing the urea yield and Faraday efficiency.

[0047] In the present invention, the electrolyte is preferably a mixed solution of 0.1M KHCO 3 and 0.05M KNO 3 . The method for electrocatalytic synthesis of urea using the nano boron - copper composite catalyst of the present invention achieves a maximum urea yield of 11.8 mmol / h / g and a maximum Faraday efficiency of 5.8% at - 0.65V (V vs. RHE).

[0048] Example 1

[0049] Preparation of nano boron - copper composite catalyst

[0050] 1) Take 135 mg of copper chloride and 28 mg of boron powder and place them in a mortar and grind them with 1.7 g of sodium chloride and 1.7 g of potassium chloride for 15 minutes until the powder particles are evenly mixed.

[0051] 2) Transfer the ground powder to a glass tube for sealing, place it in a tubular furnace filled with argon, heat and melt the powder at a heating rate of 10℃ / min until the temperature rises to 900℃, and maintain it for 1 h to obtain a boron - copper salt composite.

[0052] 3) After the obtained boron-copper salt complex is naturally cooled, it is washed three times with warm water at 80°C, then washed with ethanol and dried in a vacuum drying oven at 80°C for 12 h.

[0053] The transmission electron microscope of the boron-copper complex obtained in this example is as Figure 1 shown. The boron-copper complex is granular, with a nanostructured distribution. After the catalyst is formulated into ink, it is evenly distributed on the carbon cloth with a diffusion layer.

[0054] Example 2

[0055] Preparation of nano boron-copper complex catalyst

[0056] 1) Take 100 mg of copper oxide and 50 mg of sodium borohydride and place them in a mortar and grind them thoroughly with 2 g of sodium phosphate and 2 g of potassium phosphate for 15 minutes until the powder particles are evenly mixed.

[0057] 2) Transfer the ground powder to an alumina crucible, place it in a tubular furnace filled with argon, heat and melt the powder at a heating rate of 10°C / min until the temperature rises to 810°C, and maintain it for 1 h to obtain a boron-copper salt complex.

[0058] 3) After the obtained boron-copper salt complex is naturally cooled, it is washed three times with warm water at 80°C, then washed with ethanol and dried in a vacuum drying oven at 80°C for 12 h.

[0059] The transmission electron microscope of the boron-copper complex obtained in this example is as Figure 2 shown. The boron-copper complex is in the form of particle accumulation, with a nanostructured distribution. After the catalyst is formulated into ink, it is evenly distributed on the carbon cloth with a diffusion layer.

[0060] Example 3

[0061] Preparation of nano boron-copper complex catalyst

[0062] 1) Take 135 mg of copper chloride and 50 mg of sodium borohydride and place them in a mortar and grind them thoroughly with 1.8 g of lithium sulfate and 1.8 g of potassium sulfate for 15 minutes until the powder particles are evenly mixed.

[0063] 2) Transfer the ground powder to an alumina crucible, place it in a tubular furnace filled with argon, heat and melt the powder at a heating rate of 10°C / min until the temperature rises to 800°C, and maintain it for 1 h to obtain a boron-copper salt complex.

[0064] 3) After the obtained boron-copper salt complex is naturally cooled, it is washed three times with warm water at 80°C, then washed with ethanol and dried in a vacuum drying oven at 80°C for 12 h.

[0065] The transmission electron microscope of the boron-copper complex obtained in this example is as Figure 3As shown, the boron-copper complex is granular and has a nanostructured distribution. After preparing the catalyst into ink, it is evenly distributed on the carbon cloth with a diffusion layer.

[0066] Example 4

[0067] Preparation of Nano Boron-Copper Composite Catalyst

[0068] 1) Take 135 mg of copper chloride and 28 mg of boron powder and place them in a mortar. Grind them thoroughly with 1.7 g of sodium chloride and 1.7 g of potassium chloride for 15 minutes until the powder particles are evenly mixed.

[0069] 2) Transfer the ground powder to a glass tube for sealing. Place it in a tube furnace filled with argon and heat it at a heating rate of 10 °C / min to melt the powder until the temperature reaches 900 °C and maintain it for 1 h to obtain the boron-copper salt complex.

[0070] 3) After the obtained boron-copper salt complex is naturally cooled, wash it three times with warm water at 80 °C, then wash it with ethanol and dry it in a vacuum drying oven at 80 °C for 12 h.

[0071] The transmission electron microscope of the boron-copper complex obtained in this example is as Figure 4 shown. It can be seen from the figure that the boron-copper complex is in a lamellar stacking shape and has a nanostructured distribution. After preparing the catalyst into ink, it is evenly distributed on the carbon cloth with a diffusion layer.

[0072] Comparative Example 1

[0073] Commercially available copper powder was selected as the comparative example, and its catalyst preparation is as follows:

[0074] Take 5 mg of copper powder and disperse it in a mixed solution of ethanol and water (V:V = 3:2). Ultrasonic it in a water bath for half an hour to form a uniform ink, and then evenly distribute it on the carbon cloth with a diffusion layer for electrocatalytic testing.

[0075] Comparative Example 2

[0076] Commercially available boron powder was selected as the comparative example, and the catalyst preparation is as follows:

[0077] Take 5 mg of boron powder and disperse it in a mixed solution of ethanol and water (V:V = 3:2). Ultrasonic it in a water bath for half an hour to form a uniform ink, and then evenly distribute it on the carbon cloth with a diffusion layer for electrocatalytic testing.

[0078] Example 5

[0079] Electrocatalytic Synthesis of Urea with Nano Boron-Copper Composite Catalyst

[0080] A three - electrode system was adopted. The electrode clip clamped the carbon paper with the catalyst prepared in Example 1. The Ag / AgCl electrode was used as the reference electrode, and the platinum sheet was used as the counter electrode. A mixed solution of 0.1M KHCO 3 and 0.05M KNO 3 was used as the electrolyte. CO 2 was introduced into the electrolyte, and the applied voltage was - 0.5V to - 0.7V. The reaction was carried out for 1 h using an H - type electrolytic cell as the reaction device.

[0081] Example 6

[0082] Same as Example 5, except that the catalyst prepared in Example 1 was replaced with the catalyst prepared in Example 2.

[0083] Example 7

[0084] Same as Example 5, except that the catalyst prepared in Example 1 was replaced with the catalyst prepared in Example 3.

[0085] Comparative Example 3

[0086] A three - electrode system was adopted. The electrode clip clamped the carbon paper with the catalyst prepared in Example 4. The Ag / AgCl electrode was used as the reference electrode, and the platinum sheet was used as the counter electrode. 0.1M KHCO 3 was used as the electrolyte. N 2 was introduced into the electrolyte, and the reaction was carried out for 1 h using an H - type electrolytic cell as the reaction device under the condition of constant potential.

[0087] Comparative Example 4

[0088] Same as Example 5, except that the catalyst prepared in Example 1 was replaced with the catalyst prepared in Comparative Example 1.

[0089] Comparative Example 5

[0090] Same as Example 5, except that the catalyst prepared in Example 1 was replaced with the catalyst prepared in Comparative Example 2.

[0091] Test Example

[0092] The urea obtained in Examples 5 - 7 and Comparative Examples 3 - 5 was tested.

[0093] The detection method of urea is as follows:

[0094] The detection method of urea is the urease method. The principle is that one mole of urea is decomposed into two moles of ammonia. The specific operation is to take 1.8 mL of the electrolyte after the reaction, add 0.2 mL of urease solution (5 mg / mL), decompose it at 40 °C for 1 h, and the generated ammonia is determined by the indophenol blue method. By comparing with the absorbance of the electrolyte without adding urease solution, the ammonia concentration can be calculated through the ultraviolet-visible spectrum, and the urea yield can be obtained through the conversion of the standard curve ( Figure 11 ) and the formula as follows.

[0095] The molar weights of ammonia before and after the decomposition experiment are quantified by the above method and are represented as m b and m a respectively. Since 1 mole of urea can be decomposed into 1 mole of CO 2 and 2 moles of NH 3 , the molar weight of the generated urea (m urea ) can be calculated as follows:

[0096] Urea yield formula:

[0097] Murea=(m a -m b ) / 2

[0098] The Faraday efficiency of electrocatalytic urea synthesis is obtained through the following equation:

[0099] FE(%)=nFCV / 60.06×Q×100%

[0100] where F is the Faraday constant, Q is the electric charge, C is the concentration of the generated urea, V is the volume of the electrolyte, and n is the number of electron transfers in the electrochemical reaction.

[0101] The urea yield results are shown in Table 1

[0102] Table 1 Urea yield results

[0103]

[0104]

[0105] It can be seen from Figures 5 to 7 that urea is produced in Examples 5 to 7, indicating that the catalysts prepared in Examples 1 to 3 exhibit certain catalytic activities, and among them, the urea synthesis yield and Faraday effect in Example 5 are the most outstanding.

[0106] Comparative Example 3 has no catalytic activity and the yield is almost zero, indicating that the catalyst does not have the ability to activate and break the nitrogen-nitrogen triple bond and has no electrocatalytic activity for urea.

[0107] In Comparative Examples 4 to 5, Comparative Example 4 did not show catalytic activity, and the catalytic activity of Comparative Example 4 was inferior to that exhibited by Comparative Example 5.

[0108] The above-described embodiments merely represent several implementation manners of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the patent for the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all fall within the protection scope of the present invention. Therefore, the protection scope of the patent for the present invention shall be subject to the appended claims.

Claims

1. A method for electrocatalytic synthesis of urea using a nano boron - copper composite catalyst, characterized in that, Using a nano-boron copper composite catalyst as the working electrode, a graphite rod as the counter electrode, and an Ag / AgCl electrode as the reference electrode, CO is introduced into the electrolyte 2 , and an H-type electrolytic cell is used as the reaction device to carry out an electrocatalytic reaction; the electrolyte is a mixed solution of bicarbonate and nitrate; The preparation method of the nano boron - copper composite catalyst includes the following steps: (1) Mix a copper precursor and a boron precursor with a molten salt and then grind to obtain a powder; (2) Calcinate the obtained powder in an inert atmosphere at a certain temperature to obtain a boron - copper salt composite; (3) Wash the obtained boron - copper salt composite with water, alcohol, and then dry to obtain the nano boron - copper composite catalyst; In step (1), the copper precursor is selected from copper - containing compounds; the copper - containing compounds are selected from copper chloride, copper oxide, and copper nitrate; The boron precursor is selected from boron - containing compounds; the boron - containing compounds are selected from boron powder, sodium borohydride, boric acid, and boron oxide; The molten salt described in step (1) is selected from NaPO 4 -KPO 4 molten mixture, NaCl-KCl molten mixture, LiSO 4 -K 2 SO 4 molten mixture.

2. The method for electrocatalytic synthesis of urea using the nano boron - copper composite catalyst according to claim 1, characterized in that, the molar ratio of copper to boron in the copper precursor and boron precursor is 1:(0.5 - 3).

3. The method for electrocatalytic synthesis of urea using the nano boron - copper composite catalyst according to claim 1, characterized in that, the mass ratio of the total mass of the copper precursor and boron precursor to the molten salt in step (1) is 1:(10 - 30).

4. The method for electrocatalytic synthesis of urea using the nano boron - copper composite catalyst according to claim 1, characterized in that, the calcination temperature in step (2) is 650 °C - 1000 °C, and the heating rate is 5 °C / min - 10 °C / min.

5. The method for electrocatalytic synthesis of urea using the nano boron - copper composite catalyst according to claim 1, characterized in that, the alcohol in step (3) includes methanol, ethanol, and propanol; the drying temperature is 60 °C - 80 °C, and the drying time is 12 h - 24 h.

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