A method for preparing an RhCu alloy nanosphere electrocatalyst by ultrasonic-assisted hydrothermal method

Abstract

The present invention belongs to the technical field of electrocatalytic materials, specifically a method for preparing RhCu alloy nanosphere electrocatalysts by ultrasonic-assisted hydrothermal method and the performance study of electrocatalytic co-reduction of nitrate and carbon dioxide to produce urea. The raw materials are mixed evenly and placed in a closed high-temperature and high-pressure reaction kettle to prepare RhCu alloy nanospheres by ultrasonic hydrothermal method. The electrocatalytic effect of the RhCu alloy nanospheres prepared by ultrasonic hydrothermal method is significantly better than that of the RhCu alloy nanospheres prepared by traditional hydrothermal method. The advantages of the ultrasonic-assisted hydrothermal method include simple reaction process, uniform mixing and easy control of the morphology of nanomaterials. In addition, ultrasonic waves can also break up agglomerates, thereby producing more uniform product composition. The preparation process of the RhCu alloy nanospheres of the present invention is simple and the catalytic effect is excellent. It is an excellent electrocatalytic material and can be used in new energy fields such as electrocatalytic synthesis of urea.

Description

Technical Field

[0001] The present invention belongs to the technical fields of nanomaterials and electrocatalysis, and particularly relates to a method for preparing an RhCu alloy nanosphere electrocatalyst by an ultrasonic-assisted hydrothermal method. Background Art

[0002] Urea is a nitrogen fertilizer with a nitrogen content as high as 46%, and can be used in fields such as crops and feeds. The industrial production of urea requires two consecutive processes, namely N2 + 3H2 → 2NH3 and 2NH3 + CO2 → CO(NH2)2 + H2O. Among them, the Haber-Bosch reaction for converting nitrogen into ammonia needs to be carried out under the conditions of high temperature (400 - 500 °C) and high pressure (100 - 200 bar), which will lead to huge energy consumption and a large amount of greenhouse gas emissions. The electrocatalytic synthesis of urea technology has gradually become the key to solving the above energy and environmental problems. Some studies have shown that nitrogen and carbon dioxide can be electrocatalytically synthesized into urea under normal temperature and pressure conditions. However, the high dissociation energy of the N≡N bond in nitrogen molecules (941 kJ mol -1 ) and the low solubility of nitrogen in water limit the coupling reaction of nitrogen and carbon dioxide on the catalyst surface. Nitrate (NO3 - ) is a very ideal nitrogen-containing reactant. Considering the relatively low dissociation energy of the N=O bond (204 kJ mol -1 ) and the high solubility of nitrate in aqueous electrolytes, nitrate and carbon dioxide may become another effective way to drive the electrocatalytic co-reduction for the preparation of urea. Although the urea yield when nitrate is used as a reactant is higher than that when nitrogen is used as a reactant, its Faraday efficiency value is still relatively low and cannot yet meet the actual needs. Therefore, there is an urgent need to develop efficient electrocatalysts for the co-reduction reaction of nitrate ions and carbon dioxide for the efficient synthesis of urea.

[0003] Although rhodium (Rh)-based materials are often used as efficient electrocatalysts for the oxygen reduction reaction, due to the good affinity of Rh atoms for nitrate ions and other nitrogen-containing intermediates, they also show great potential in the electrocatalytic synthesis of urea. In order to alleviate the shortage of Rh scarcity and reduce the cost of preparing electrocatalysts, alloying non-precious metal copper (Cu) with Rh metal can optimize the electronic structure of Rh and inhibit the competition from the hydrogen evolution reaction, thereby improving the Faraday efficiency of the electrocatalytic synthesis of urea. At present, the synthesis methods of RhCu alloys include electro-displacement method, water bath synthesis method, polyol combined with microwave heating method and hydrothermal method. Among them, the hydrothermal method is considered to be one of the most effective methods for constructing nano-RhCu catalysts due to its simple process. In the present invention, ultrasonic treatment is added to the hydrothermal method. Ultrasonic-assisted hydrothermal synthesis is beneficial to the controllable changes in the crystallinity, morphology and structure of the nano-alloy, thereby changing the electrocatalytic performance of the RhCu alloy. The field of preparing RhCu alloys by this method is still blank. Summary of the Invention

[0004] The purpose of the present invention is to prepare RhCu alloy nanospheres by ultrasonic-assisted hydrothermal method and improve the performance of electrocatalytic synthesis of urea.

[0005] To achieve the above purpose, the technical solution adopted by the present invention is: preparing RhCu alloy nanospheres by ultrasonic-assisted hydrothermal method, including the following steps:

[0006] (1) Dispersing the copolymer into an organic solvent and stirring to obtain a copolymer solution;

[0007] (2) Adding hydrochloric acid solution, sodium hexachlororhodate aqueous solution, copper chloride aqueous solution and ascorbic acid solution to the copolymer solution obtained in step (1), mixing evenly to obtain a mixed solution;

[0008] (3) Putting the mixed solution obtained in step (2) into a reaction kettle for ultrasonic hydrothermal reaction;

[0009] (4) Centrifuging the product obtained after the reaction in step (3), washing with a solvent and drying, and the obtained powder product is RhCu alloy nanospheres.

[0010] In an embodiment of the present invention, the copolymer described in step (1) is a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymer; the organic solvent is DMF.

[0011] In an embodiment of the present invention, the mass-volume ratio of the copolymer to DMF in step (1) is 20 mg:(3-4) mL, and specifically, it can be selected as 20 mg:3.2 mL.

[0012] In one embodiment of the present invention, the concentration of the hydrochloric acid solution in step (2) is 0.8 - 1.2 mol / L, and specifically, it can be selected as 1 mol / L; the concentration of the sodium hexachlororhodate solution is 0.03 - 0.05 mol / L, and specifically, it can be selected as 0.04 mol / L; the concentration of the copper chloride solution is 0.03 - 0.05 mol / L, and specifically, it can be selected as 0.04 mol / L; the concentration of the ascorbic acid solution is 0.09 - 0.11 mol / L, and specifically, it can be selected as 0.1 mol / L.

[0013] In one embodiment of the present invention, the volume ratio of the hydrochloric acid solution to the sodium hexachlororhodate solution in step (2) is 1:(4 - 6), and specifically, it can be selected as 1:5.

[0014] In one embodiment of the present invention, the volume ratio of the hydrochloric acid solution to the copper chloride solution in step (2) is 1:(4 - 6), and specifically, it can be selected as 1:5.

[0015] In one embodiment of the present invention, the volume ratio of the hydrochloric acid solution to the ascorbic acid solution in step (2) is 1:(9 - 11), and specifically, it can be selected as 1:10.

[0016] In one embodiment of the present invention, the reaction conditions of the ultrasonic hydrothermal reaction in step (3) are that the reaction temperature is 100 - 110 °C, the reaction time is 3 - 5 h, and the ultrasonic power is 250 - 350 W. Specifically, it can be selected that the reaction temperature is 100 °C, the reaction time is 4 h, and the ultrasonic power is 300 W.

[0017] In one embodiment of the present invention, the conditions of centrifugation in step (4) are that the rotation speed is 12000 - 16000 rpm and the centrifugation time is 15 - 25 min; the cleaning with a solvent is carried out with acetone and water.

[0018] The RhCu alloy nanospheres prepared by the present invention can be used as a catalyst for the electrocatalytic co - reduction of nitrate and carbon dioxide to produce urea.

[0019] The beneficial effects achieved by the present invention compared with the prior art are as follows:

[0020] (1) The present invention has the characteristic of a simple synthesis method;

[0021] (2) The nanospheres prepared by the ultrasonic - assisted hydrothermal method of the present invention have the advantages of uniform structure and small particle size;

[0022] (3) The RhCu alloy nanospheres prepared by the present invention exhibit excellent performance in the electrocatalytic co - reduction of nitrate and carbon dioxide to produce urea. Description of the Drawings

[0023] Figure 1 It is a schematic diagram of the preparation process of the RhCu alloy nanospheres prepared in the present invention.

[0024] Figure 2 It is a SEM photograph of the RhCu alloy nanospheres prepared in the present invention.

[0025] Figure 3 It is an EDX spectrum of the RhCu alloy nanospheres prepared in the present invention.

[0026] Figure 4 It is a performance diagram of the electrocatalytic co-reduction of nitrate and carbon dioxide into urea by the RhCu alloy nanospheres prepared in the present invention. Specific Embodiments

[0027] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims of this application.

[0028] PEO-PPO-PEO was purchased from Sigma-Aldrich; DMF and hydrochloric acid (HCl, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China); 5% Nafion solution was purchased from Sigma-Aldrich; sodium hexachlororhodate(III) (Na3RhCl6, ≥17.1%) was purchased from Alfa Aesar; copper chloride (CuCl2, 98%) and ascorbic acid (AA, 99%) were purchased from Adamas; CO2 gas (99.999%) was purchased from Wuxi Xinyi Instrument Technology Co., Ltd.

[0029] Example 1

[0030] This example includes the following steps:

[0031] (1) Weigh 20 mg of the PEO-PPO-PEO triblock copolymer and disperse it into a DMF solution with a volume of 3.2 mL, and stir until it becomes clear to obtain a copolymer solution.

[0032] (2) Measure the volume of the hydrochloric acid solution to be 0.8 mL, the solution concentration to be 1 mol / L, the volume of the sodium hexachlororhodate(III) solution to be 4 mL, the solution concentration to be 0.04 mol / L, the volume of the copper chloride solution to be 4 mL, the solution concentration to be 0.04 mol / L, and the volume of the ascorbic acid solution to be 8 mL, the solution concentration to be 0.1 mol / L. Add them to the copolymer solution in step (1), mix evenly, and the solution becomes transparent light brown.

[0033] (3) Put the transparent light brown solution obtained in step (2) into a high-temperature and high-pressure autoclave for ultrasonic hydrothermal reaction at a temperature of 100 °C, a reaction time of 4 h, and an ultrasonic power of 300 W.

[0034] (4) Centrifuge the product obtained from the ultrasonic hydrothermal reaction in step (3) at a speed of 14,000 rpm for 20 min, remove the supernatant, wash / centrifuge the solid continuously with acetone and water 5 times to remove the residual polymer, and finally obtain the product after drying, denoted as RhCu alloy nanospheres.

[0035] The RhCu alloy nanospheres were tested for the electrocatalytic co-reduction of nitrate and carbon dioxide to urea. At -0.6 V, the urea yield was 26.81 mmol g -1 h -1 , and the Faraday efficiency was 34.82%.

[0036] Example 2

[0037] Change the mass of the PEO-PPO-PEO triblock copolymer in Example 1 to 10 mg, and the rest are the same as in Example 1. The finally obtained material is denoted as RhCu alloy nanospheres-1. The RhCu alloy nanospheres-1 were tested for the electrocatalytic co-reduction of nitrate and carbon dioxide to urea. At -0.6 V, the urea yield was 3.10 mmol g -1 h -1 , and the Faraday efficiency was 5.00%.

[0038] Example 3

[0039] Change the mass of the PEO-PPO-PEO triblock copolymer in Example 1 to 30 mg, and the rest are the same as in Example 1. The finally obtained material is denoted as RhCu alloy nanospheres-2. The RhCu alloy nanospheres-2 were tested for the electrocatalytic co-reduction of nitrate and carbon dioxide to urea. At -0.6 V, the urea yield was 17.36 mmol g -1 h -1 , and the Faraday efficiency was 15.25%.

[0040] Example 4

[0041] Change the ultrasonic hydrothermal reaction in Example 1 to a hydrothermal reaction, and the rest are the same as in Example 1. The finally obtained material is denoted as RhCu alloy nanospheres-3. The RhCu alloy nanospheres-3 were tested for the electrocatalytic co-reduction of nitrate and carbon dioxide to urea. At -0.6 V, the urea yield was 18.53 mmol g -1 h -1 , and the Faraday efficiency was 13.27%.

[0042] Example 5 Testing and Characterization

[0043] In the electrochemical test, an H-type electrolytic cell and a three-electrode test system were used. The prepared composite electrocatalytic material was used as the working electrode, the Ag / AgCl electrode was used as the reference electrode, and the graphite rod was used as the counter electrode. The electrolyte used was 30 mL of 0.1 M KNO₃ solution. Before the test, the electrolyte was purged with carbon dioxide for 30 min to remove the dissolved air therein, and the carbon dioxide flow rate was 15 mL min -1 , and the catalyst loading was ~0.02 mg cm -2 . The test was carried out in a fully enclosed state to exclude interference from other factors such as air. Conventional electrochemical test methods were used to study the electrocatalytic performance of the composite electrocatalytic material prepared in the present invention for the co-reduction of nitrate and carbon dioxide into urea.

[0044] A scanning electron microscope (SEM), X-ray energy dispersive spectroscopy (EDX), and an electrochemical workstation were used to characterize the structural morphology, element distribution, and catalytic performance of the RhCu alloy nanospheres obtained in the present invention for the co-reduction of nitrate and carbon dioxide to produce urea. The results are as follows:

[0045] (1) The SEM test results showed that the prepared RhCu alloy nanospheres exhibited a uniform distribution and the nanospheres had a uniform size, indicating that the RhCu alloy nanospheres could be prepared by an ultrasonic-assisted hydrothermal method. See the appendix Figure 2 .

[0046] (2) The EDX test results showed that the ratio of Rh and Cu elements in the prepared RhCu alloy nanospheres was 1.88:1. See the appendix Figure 3 .

[0047] (3) The electrochemical test results showed that the prepared RhCu alloy nanospheres had excellent electrocatalytic performance: at a voltage of -0.6 V, it exhibited the most excellent electrocatalytic reduction performance. Specifically, the maximum urea yield reached 26.81 mmol g -1 h -1 , and the highest Faraday efficiency was 34.82%. See the appendix Figure 4 .

[0048] Although the present invention has been disclosed above with preferred embodiments, it is not intended to limit the present invention. Anyone familiar with this technology can make various changes and modifications without departing from the technology and scope of the present invention. Therefore, the protection scope of the present invention should be defined by the claims.

Claims

1. A method for preparing RhCu alloy nanospheres, characterized in that, It includes the following steps: (1) Disperse the copolymer into an organic solvent and stir to obtain a copolymer solution; the copolymer is a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer; the mass of the copolymer is 20 mg, and the organic solvent is DMF with a volume of 3.2 mL; (2) Add hydrochloric acid solution, sodium hexachlororhodate solution, copper chloride solution and ascorbic acid solution to the copolymer solution obtained in step (1), mix evenly to obtain a mixed solution; The concentration of the hydrochloric acid solution is 0.8 - 1.2 mol / L; the concentration of the sodium hexachlororhodate solution is 0.03 - 0.05 mol / L; the concentration of the copper chloride solution is 0.03 - 0.05 mol / L; the concentration of the ascorbic acid solution is 0.09 - 0.11 mol / L; The volume of the hydrochloric acid solution is 0.8 mL; the volume of the sodium hexachlororhodate solution is 4 mL; the volume of the copper chloride solution is 4 mL; the volume of the ascorbic acid solution is 8 mL; (3) Put the mixed solution obtained in step (2) into a reaction kettle for ultrasonic hydrothermal reaction; (4) Centrifuge the product obtained after the reaction in step (3), wash it with a solvent, and dry it to obtain a powder product of RhCu alloy nanospheres.

2. According to the preparation method described in claim 1, characterized in that, The conditions of the ultrasonic hydrothermal reaction in step (3) are a temperature of 100 - 110 °C, a time of 3 - 5 h, and an ultrasonic power of 250 - 350 W.

3. According to the preparation method described in claim 1, characterized in that, The conditions of the centrifugation in step (4) are a rotation speed of 12000 - 16000 rpm and a centrifugation time of 15 - 25 min; the washing with a solvent is carried out with acetone and water.

4. Application of the RhCu alloy nanospheres prepared by the preparation method according to any one of claims 1 - 3 as a catalyst in the electrocatalytic co - reduction of nitrate and carbon dioxide to produce urea.

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