Catalyst for electro-catalysis nitrate reduction and preparation method thereof

Abstract

The invention discloses a catalyst for electro-catalysis nitrate reduction and a preparation method. The catalyst is a nickel-molybdenum / foamed nickel composite catalyst. The catalyst has a macroporous frame structure; in the catalyst, the molar ratio of nickel to molybdenum is (2: 1)-(6: 1). According to the catalyst, foamed nickel is used as a substrate, nickel nitrate and sodium molybdate are used as raw materials, and the nickel molybdenum / foamed nickel composite catalyst grows on the foamed nickel through hydrothermal reduction and hydrogen reduction in sequence. The catalyst provided by the invention can realize 97% of ammonia production selectivity at a relatively low overpotential, and can keep the activity without obvious decline in a stability experiment of 10 cycles. Under the synergistic effect of nickel and molybdenum in the catalyst, nickel can effectively promote activation of water under the alkaline condition, so that protons are provided for reduction of nitrate radicals for reaction. Molybdenum enables the conversion process of nitrate to be more convenient and efficient, and is especially beneficial to conversion of nitrate to nitrite.

Description

Technical Field

[0001] The present application relates to a catalyst for electrocatalytic nitrate reduction and a preparation method thereof, belonging to the field of electrocatalysis. Background Art

[0002] Ammonia is an important chemical raw material and has important applications in agriculture, textiles, plastics and pharmaceuticals. It is a mild, hydrogen-rich energy storage medium with high energy density (4.3kWh kg -1 Ammonia is a highly bioavailable fuel, easy to transport, and holds great promise as a next-generation energy source and hydrogen carrier. Currently, ammonia is primarily produced through the Haber-Bosch industrial ammonia synthesis process, which requires high temperatures (300-500°C) and high pressures (15-35 MPa), and produces large amounts of carbon dioxide greenhouse gases.

[0003] NO 3- It is a common pollutant that is discharged into surface and groundwater in large quantities, posing a serious threat to people's health. At the same time, the increase in nitrate concentration in water bodies will cause eutrophication of water bodies and cause damage to the environment. 3- The NO bond energy is low (205 kJ mol -1 ), which easily breaks down and converts into ammonia, making it a potential nitrogen source for ammonia production. Currently, electrocatalytic nitrate reduction still faces challenges, including suppressing the competing reaction for hydrogen production, improving catalytic activity and selectivity, and effectively recovering the product ammonia.

[0004] Electrocatalytic nitrate reduction is a complex multi-electron, multi-proton transfer process, and the catalyst significantly affects its conversion rate, product selectivity, and catalytic stability. Currently, several different catalysts have been reported in the literature, including some containing precious metals such as Au (para-Au / C, Precision Chemistry, 2024), Pt (PtRu, Journal of Catalysis, 2021), and Ru (Ru / RuO2, J.Am.Chem.Soc., 2020). However, due to the high price of precious metals, it is difficult to achieve large-scale application in industry. Some catalysts based on transition metals such as Cu (BiCo@Cu, Advanced Materials, 2023), Co (CoP-CNS, Nature Communications, 2022), Fe (Cu-Fe3O4, Nano Letters, 2023), and Ni (Ni-NSA-VNi, Journal of Materials Chemistry A, 2021) also show high selectivity, but the catalyst stability is poor and the current density is low. Summary of the Invention

[0005] The object of the present invention is to provide a nickel-molybdenum / nickel foam catalyst with excellent ammonia production activity, selectivity and cyclic stability and a preparation method thereof, which is used for the efficient electrocatalytic reduction of nitrate to ammonia.

[0006] According to one aspect of the present application, a catalyst for electrocatalytic nitrate reduction is provided;

[0007] The catalyst is a nickel-molybdenum / foam nickel composite catalyst;

[0008] The catalyst is based on nickel foam and covered with nanosheets composed of nickel and molybdenum;

[0009] The catalyst has a macroporous framework structure;

[0010] In the catalyst, the molar ratio of nickel to molybdenum is 2:1 to 6:1.

[0011] The electrocatalytic reduction of nitrate is carried out in an H-type dual-chamber reaction cell, where the cathode chamber and the anode chamber are separated by an anion exchange membrane. A nickel-molybdenum / foam nickel catalyst is used as the working electrode. A constant voltage is applied to allow the nitrate in the cathode chamber solution to synthesize ammonia through an electrochemical reduction reaction.

[0012] According to a second aspect of the present application, a method for preparing a nickel-molybdenum / nickel foam composite catalyst is provided, comprising at least the following steps:

[0013] Step I: placing a mixed solution containing nickel salt and molybdenum salt, and nickel foam in a sealed container, and performing reaction I to obtain a nickel foam sample;

[0014] Step II: reducing the nickel foam sample obtained in step I in a mixed atmosphere of argon and hydrogen to obtain the catalyst.

[0015] Optionally, in step I, the nickel salt is selected from at least one of nickel nitrate, nickel sulfate, and nickel chloride;

[0016] The molybdenum salt is selected from at least one of sodium molybdate, potassium molybdate and ammonium molybdate.

[0017] Optionally, the molar ratio of the nickel salt to the molybdenum salt is 1:8 to 8:1.

[0018] Optionally, the molar ratio of the nickel salt to the molybdenum salt is 1:4 to 4:1.

[0019] Optionally, the conditions of reaction 1 are as follows:

[0020] The temperature of the reaction I is 120-180°C;

[0021] The reaction time is 5 to 10 hours.

[0022] Optionally, in step II, the mixed atmosphere includes argon and hydrogen.

[0023] Optionally, in step II, the ventilation rate of the argon gas is 100 to 300 sccm.

[0024] Optionally, in step II, the hydrogen gas flow rate is 10 to 50 sccm.

[0025] Optionally, in step II, the reduction conditions are as follows:

[0026] The reduction heating rate is 3-5°C / min;

[0027] The reduction temperature is 25 to 550° C.

[0028] The reduction time is 30 minutes to 2 hours.

[0029] Alternatively, a method for preparing a nickel-molybdenum / nickel foam catalyst for electrocatalytic reduction of nitrate to ammonia comprises the following steps:

[0030] 1) The nickel foam substrate was ultrasonically cleaned in acetone, isopropyl alcohol, ethanol, and ultrapure water for 30 min, followed by drying;

[0031] 2) preparing 30 mL of a mixed solution of nickel nitrate hexahydrate and sodium molybdate dihydrate, wherein the molar ratio of nickel to molybdenum is in the range of 1:4 to 4:1;

[0032] 3) pouring the mixed solution into a reaction kettle, placing the nickel foam therein, and hydroheating at 150° C. for 6 h. After hydroheating, ultrasonically treating the obtained nickel foam sample and vacuum drying;

[0033] 4) The vacuum-dried sample was heated from room temperature to 550° C. at a rate of 3° C. / min in a mixed atmosphere of argon and hydrogen (200 sccm of argon and 30 sccm of hydrogen) and subjected to hydrogen reduction at this temperature for 1 h to obtain the nickel-molybdenum / foamed nickel composite catalyst.

[0034] The beneficial effects of this application include:

[0035] 1) The catalyst prepared in this application is a novel non-precious metal catalyst that uses clean electrical energy for electrocatalytic nitrate reduction at room temperature and pressure, achieving high Faradaic efficiency and ammonia yield. Compared with previously reported precious metal catalysts, this approach reduces production costs.

[0036] 2) The catalyst described in the present application can achieve an ammonia production selectivity of up to 97% at a relatively low overpotential, and can maintain its activity without significant degradation during a 10-cycle stability experiment.

[0037] 3) The synergistic effect of nickel and molybdenum in the catalyst described herein: nickel effectively promotes water activation under alkaline conditions, thereby providing protons for nitrate reduction. Molybdenum makes the nitrate conversion process more convenient and efficient, especially facilitating the conversion of nitrate to nitrite. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 This is an SEM image of nickel-molybdenum / nickel foam in Example 1 of the present application;

[0039] Figure 2 This is the standard curve diagram in Example 2 of the present application;

[0040] Figure 3 This is a graph showing the selectivity and activity of the catalyst for ammonia production at different voltages in Example 3 of the present application;

[0041] Figure 4 This is a graph showing the selectivity and activity of nitrate reduction of nickel-molybdenum catalysts with different ratios in Example 5 of the present application;

[0042] Figure 5 This is a graph showing the stability test of the electrocatalytic ammonia production cycle of the catalyst in Example 6 of the present application. DETAILED DESCRIPTION

[0043] The present application is described in detail below with reference to embodiments, but the present application is not limited to these embodiments.

[0044] Unless otherwise specified, the raw materials in the examples of this application were purchased through commercial channels;

[0045] The analysis method in Example 1 of the present application is as follows:

[0046] The morphology of the catalysts was analyzed using Quanta 200F and HT7700 instruments.

[0047] The analysis of the product ammonia in the examples of this application is as follows:

[0048] The product ammonia was detected by UV-visible spectrophotometry.

[0049] The calculation formula of the product ammonia in the embodiment of this application is as follows:

[0050] Faradaic efficiency of ammonia production = nFc(NH3)V / it;

[0051] Ammonia production rate = Vc(NH3) / tS.

[0052] Where n is the number of electrons transferred when nitric acid is reduced to ammonia, which is 8; F is the Faraday constant, which is 96485 C mol -1c(NH3) is the concentration of ammonia in the solution; V is the volume of the cathode solution, which is 10 mL; i is the average current of the nitrate reduction reaction to produce ammonia; t is the time for nitrate reduction to produce ammonia, which is 40 min; S is the projected area of the catalyst on the electrode, which is 0.36 cm 2 .

[0053] Example 1 Preparation of nickel-molybdenum / nickel foam working electrode

[0054] The nickel-molybdenum / nickel foam working electrode for the electrocatalytic reduction of nitrate to ammonia was prepared using the following steps:

[0055] 1) The nickel foam was ultrasonically cleaned with different cleaning solutions for 30 min and then dried;

[0056] 2) preparing a mixed solution of nickel salt and molybdenum salt: mixing 20 mL of 0.06 mol / L nickel nitrate hexahydrate and 10 mL of 0.08 mol / L sodium molybdate dihydrate to form a mixed solution having a molar ratio of nickel to molybdenum of 1.2:0.8;

[0057] 3) pouring the mixed solution into a reaction kettle, placing the nickel foam therein, and hydroheating at 150° C. for 6 h. After hydroheating, ultrasonically treating the obtained nickel foam sample and vacuum drying;

[0058] 4) The vacuum-dried nickel foam sample was heated from room temperature to 550°C at a rate of 3°C / min in a mixed atmosphere of argon and hydrogen and then subjected to hydrogen reduction at this temperature for 1 hour to obtain the nickel-molybdenum / nickel foam composite catalyst, i.e., the electrocatalytic nitrate reduction catalyst. Its surface morphology is as follows Figure 1 The figure shows that nickel and molybdenum are densely distributed on the surface of the base nickel foam to form a nano-sheet structure.

[0059] Example 2 Detection of product ammonia

[0060] The product ammonia was detected by UV-visible spectrophotometry. This method is Nessler's method, which is as follows: dilute the reaction solution to 2 mL and neutralize it with 2 mL of 0.5 M sulfuric acid solution. After the reaction is complete, add 200 μL of Nessler reagent, let it stand for 20 minutes, and then record its absorbance at 450 nm and compare it with the standard curve. Figure 2 Compare the amount of ammonia generated by nitrate reduction.

[0061] Example 3 Nitrate reduction activity test of nickel-molybdenum / nickel foam

[0062] The nickel-molybdenum / nickel foam catalyst prepared in Example 1 was encapsulated with 706 glue to an exposed area of 0.6*0.6 cm 2, as the working electrode, platinum sheet as the counter electrode, potassium chloride saturated Ag / AgCl as the reference electrode, in a standard three-electrode system double-chamber electrolytic cell, before the start of the experiment, using 1M potassium hydroxide solution as the electrolyte, pretreated at -1.52V vs. Ag / AgCl voltage for 30min, then using 0.5M potassium nitrate and 1M potassium hydroxide mixture as the electrolyte, 30℃ water bath at normal pressure, at -0.1V RHE The reaction was carried out under voltage for 40 minutes. The ammonia concentration in the electrolyte was determined by spectrophotometry. The Faradaic efficiency of nitrate production was 97%, and the ammonia production rate was 6.08 mmol cm -2 h -1 , Figure 3 is the ammonia production rate and Faradaic efficiency of the catalyst at different voltages.

[0063] Example 4 Synthesis and Characterization of Nickel-Molybdenum / Nickel Foam Catalysts in Different Ratios

[0064] Nickel-molybdenum / nickel foam catalysts with different ratios were obtained by varying the molar ratio of nickel and molybdenum according to the synthesis method in Example 1. These catalysts were characterized to obtain the atomic ratios of nickel and molybdenum in the catalysts under different feed ratios. The results are shown in Table 1. In NiMo-xy, x represents x mmol of nickel nitrate added during the synthesis process, and y represents y mmol of sodium molybdate added during the synthesis process.

[0065] Example 5 Comparison of Nitrate Reduction Activity of Nickel-Molybdenum / Nickel Foam at Different Ratios

[0066] The nickel-molybdenum / nickel foam catalysts of different ratios in Example 4 were used for nitrate reduction tests, and the experimental method was the same as that in Example 3. Figure 4 As shown, at -0.1V RHE Under the conditions, the feed ratio is 1.2:0.8 and the Ni 2.44 Mo has the best activity and selectivity.

[0067] Example 6 Nitrate reduction stability test of nickel-molybdenum / nickel foam

[0068] The material in Example 1 was tested for its electrocatalytic reduction of NO3 using the test method of Example 3. - To test the cycling stability of the nickel-molybdenum / nickel foam catalyst, a constant current test was conducted continuously at a potential of -1.72V vs.Ag / AgCl. Figure 5 As shown, the test conditions remained the same as before, and 15 cycles were performed continuously, each cycle was 40 minutes, and the ammonia production efficiency was almost unchanged.

[0069] Table 1

[0070]

[0071] The above descriptions are merely a few embodiments of the present application and do not constitute any form of limitation to the present application. Although the present application discloses the preferred embodiments as above, they are not intended to limit the present application. Any technical personnel familiar with the present profession, without departing from the scope of the technical solution of the present application, using the technical content disclosed above to make slight changes or modifications are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A catalyst for electrocatalytic nitrate reduction, characterized in that The catalyst is a nickel-molybdenum / foam nickel composite catalyst; The catalyst is based on nickel foam and covered with nanosheets composed of nickel and molybdenum; The catalyst has a macroporous framework structure; In the catalyst, the molar ratio of nickel to molybdenum is 2:1 to 6:

1.

2. A method for preparing a nickel-molybdenum / foam nickel composite catalyst, characterized in that: At least the following steps are included: Step I: placing a mixed solution containing nickel salt and molybdenum salt, and nickel foam in a sealed container, and performing reaction I to obtain a nickel foam sample; Step II: reducing the nickel foam sample obtained in step I in a mixed atmosphere of argon and hydrogen to obtain the catalyst.

3. The preparation method according to claim 2, characterized in that In step 1, the nickel salt is selected from at least one of nickel nitrate, nickel sulfate, and nickel chloride; The molybdenum salt is selected from at least one of sodium molybdate, potassium molybdate and ammonium molybdate.

4. The preparation method according to claim 2, characterized in that In step I, the molar ratio of the nickel salt to the molybdenum salt is 1:8 to 8:

1.

5. The preparation method according to claim 4, characterized in that In step I, the molar ratio of the nickel salt to the molybdenum salt is 1:4 to 4:

1.

6. The preparation method according to claim 2, characterized in that In step 1, the conditions of reaction 1 are as follows: The temperature of the reaction I is 120-180°C; The reaction time is 5 to 10 hours.

7. The preparation method according to claim 2, characterized in that In step II, The mixed atmosphere includes argon and hydrogen.

8. The preparation method according to claim 2, characterized in that In step II, the ventilation rate of the argon gas is 100-300 sccm.

9. The preparation method according to claim 2, characterized in that In step II, the hydrogen gas flow rate is 10 to 50 sccm.

10. The preparation method according to claim 2, characterized in that In step II, the reduction conditions are as follows: The reduction heating rate is 3-5°C / min; The reduction temperature is 25 to 550° C. The reduction time is 30 minutes to 2 hours.

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