A preparation method of an electrode material for electro-adsorption coupled electro-catalytic reduction of nitrate

Abstract

The application discloses a preparation method of an electrode material for electro-adsorption coupled electro-catalytic reduction of nitrate, and first prepares Mxene material Ti3C2Tx; then dissolves nickel salt, aluminum salt and palladium salt in deionized water, adjusts the pH value to 9-10, and then carries out hydrothermal reaction; after the end, cooling to room temperature, washing with water and drying, then calcining, cooling to obtain Pd / LDO; finally, Ti3C2Tx and Pd / LDO are added into deionized water, uniformly dispersed, and then filtered into a film shape, and vacuum dried to obtain the electrode material. The noble metal catalyst Pd with reduction activity is combined with the layered metal oxide loaded on the surface of MXene to prepare the electrode material, compared with the prior art, the electrode prepared from the material is particularly suitable for electro-adsorption coupled electro-catalytic reduction of nitrate, can overcome the mass transfer obstacle of nitrate, and realizes the centralized separation, concentration and electro-catalysis of nitrate in a single electrochemical device.

Description

Technical Field

[0001] This invention relates to a method for preparing an electrode material for electro-adsorption coupled with electrocatalytic reduction of nitrate, belonging to the field of environmental engineering technology. Background Technology

[0002] The rapid development of modern industry and agriculture has led to increasingly serious nitrate pollution in groundwater. Nitrate pollution causes eutrophication, and when it enters the human body through drinking water and the food chain, it poses a serious threat to human health. Therefore, removing nitrates from groundwater is an urgent task. In recent years, electrochemical technology has attracted widespread attention from researchers due to its advantages such as high efficiency, ease of operation, and environmental friendliness.

[0003] Electroadsorption technology creates an electrostatic field by applying an external voltage. Charged ions in water are adsorbed onto electrodes with opposite charges, causing the ions to accumulate and concentrate on the electrode surface, thus reducing the concentration of nitrate in the water. However, electroadsorption only transfers and concentrates pollutants, failing to completely eliminate them. Electrocatalytic reduction is a method that involves charge transfer from the cathode electrode interface to the solute interface, thereby reducing the corresponding electrolyte molecules in the solution. Compared to other methods, it is more efficient and economical, with simple reactor equipment, small footprint, high nitrogen selectivity, and no need for any chemical reagents, making it easy to industrialize. However, during the electrocatalytic reduction of nitrate, the negative charge on the electrode surface and nitrate ions exhibit electrostatic repulsion, resulting in low nitrate concentration near the electrode, low reduction rate, and long reaction time for nitrate removal. Moreover, the low concentration of nitrate in the natural environment or wastewater, regardless of the mass transfer performance of the catalyst, will significantly hinder the electrocatalytic performance.

[0004] To achieve rapid removal of nitrates from water, this invention proposes a coupling of electroadsorption and electrocatalysis technologies. First, electroadsorption is used to capture dilute nitrates, concentrating them at the electrode. Then, electrocatalysis reduces the nitrates, achieving a highly efficient and harmless conversion. However, electroadsorption requires a positive pressure, while electrocatalysis requires a negative pressure. Even though the nitrate ions adsorbed during electroadsorption are prone to desorption at the beginning of electrocatalysis, mitigating this desorption during the electrocatalysis stage remains a technical challenge. Therefore, the key to this coupling technology is the electrode design, which must achieve the synergistic coupling of nitrate capture, concentration, and conversion into nitrogen gas.

[0005] Mxene materials are novel transition metal carbides / nitrides with high specific surface area, high conductivity, and high charge-discharge electrochemical stability. Their unique two-dimensional structure provides channels and storage space for ion flow in water, and they can form films independently without the need for binders. They also exhibit good flexibility and high plasticity, making them highly promising for applications in electroadsorption. However, Mxene electrodes cannot electrocatalyze the reduction of nitrate, and therefore, there are currently no reports on the use of Mxene in electroadsorption-coupled electrocatalytic electrodes. Summary of the Invention

[0006] The purpose of this invention is to address the aforementioned problems in the prior art by providing a method for preparing electrode materials for electroadsorption coupled with electrocatalytic reduction of nitrates.

[0007] This invention uses Mxene as a substrate, grows layered metal oxides (LDO) on its surface, and further loads Pd onto the Mxene / LDO surface to obtain a Pd / Mxene / LDO composite material, which is used as an electrode for electro-adsorption coupled electrocatalytic reduction of nitrate. The specific technical solution is as follows:

[0008] A method for preparing an electrode material for electroadsorption coupled with electrocatalytic reduction of nitrate includes the following steps:

[0009] (1) Preparation of Mxene material Ti3C2Tx;

[0010] (2) Dissolve nickel salt, aluminum salt and palladium salt in deionized water to obtain a mixed solution. Adjust the pH value to 9-10, and then transfer the mixed solution to a hydrothermal reactor for hydrothermal reaction. After the reaction is completed, cool to room temperature, wash the product with water and dry it, and then place it in a tube furnace. Heat the product to 500-700℃ in an air atmosphere and keep it at that temperature for 2-5 hours. Then cool it down to 200-300℃ and introduce H2 / Ar mixed gas. Keep it at that temperature for 1-4 hours. After cooling, Pd / LDO is obtained.

[0011] (3) The Mxene material Ti3C2Tx prepared in step (1) and the Pd / LDO prepared in step (2) are added to deionized water, dispersed evenly, filtered into a thin film, and dried under vacuum to obtain an electrode material for electro-adsorption coupled electrocatalytic reduction of nitrate.

[0012] Further, in step (1), the preparation method of the Mxene material Ti3C2Tx is as follows: LiF is added to hydrochloric acid solution, stirred evenly, and then Ti3AlC2 powder is added. The mass ratio of LiF to Ti3AlC2 powder is 1:1. The temperature is raised to 50℃, and the reaction is stirred for 30-40h. After centrifugation and washing several times with deionized water, the mixture is finally freeze-dried under vacuum to obtain the Mxene material Ti3C2Tx.

[0013] Furthermore, in step (2), the molar ratio of nickel salt: aluminum salt: palladium salt is 3:1:(0.1-0.8). Too low a palladium content will lead to insufficient reduction of nitrate, while too high a content will lead to waste of precious metals.

[0014] Furthermore, in step (2), the nickel salt is one or a combination of two or more of nickel chloride hexahydrate, nickel nitrate hexahydrate, nickel sulfate hexahydrate, or nickel carbonate; the aluminum salt is one or a combination of two or more of aluminum nitrate nonahydrate, aluminum chloride, aluminum sulfate, or aluminum carbonate; and the palladium salt is palladium chloride, palladium nitrate, or a combination of both.

[0015] Furthermore, in step (2), the hydrothermal reaction temperature is 120-180℃, and the reaction time is 8-24h. If the hydrothermal time is too short and the temperature is too low, the metal ion precipitation rate will be too slow and the crystal growth will be incomplete; if the hydrothermal time is too long and the temperature is too high, it will be detrimental to the preservation of the Pd / LDO structure.

[0016] Furthermore, in step (2), the volume content of H2 in the H2 / Ar mixture is 10%.

[0017] Furthermore, in step (3), the mass ratio of the Mxene material Ti3C2Tx to Pd / LDO is 1:(1-3). If this ratio is exceeded, too little Mxene will result in poor electrode conductivity, which is not conducive to charge transfer, and too little Pd / LDO will not be conducive to improving electroadsorption capacity and electrocatalytic efficiency.

[0018] Furthermore, in step (3), the vacuum drying temperature is 60°C and the time is 8-12 hours.

[0019] The application of the electrode material prepared by the above method in the electro-adsorption coupled electrocatalytic reduction of nitrate.

[0020] The beneficial effects of this invention are:

[0021] 1) This invention provides an electrode material for electro-adsorption coupled with electrocatalytic reduction of nitrate. It combines a noble metal catalyst Pd with reduction activity with a layered metal oxide supported on the surface of MXene as a bifunctional electroadsorbent and electrocatalyst in a nanostructure. Compared with the prior art, this electrode allows for the centralized separation, concentration and electrocatalysis of nitrate in a single electrochemical device, which can overcome the mass transfer barrier of nitrate and achieve its efficient and harmless conversion.

[0022] 2) The electrode material of the present invention contains Mxene material, which is a novel transition metal carbide / nitride with high specific surface area, high conductivity and high charge-discharge electrochemical stability; its unique two-dimensional structure can provide channels and storage space for the flow of ions in water, and can form a film independently without the need for binders, with good flexibility and strong plasticity.

[0023] 3) The electrode material of the present invention contains LDO, which has a positive charge on its surface and can effectively adsorb nitrate ions in water. It also has a "memory effect", which can prevent nitrate ions from being rapidly desorbed even under negative voltage conditions and can enhance the mass transfer barrier of nitrate ions during negative voltage reduction.

[0024] 4) The electrode material of the present invention contains Pd, which has good catalytic reduction activity and can effectively reduce nitrate to nitrogen. Detailed Implementation

[0025] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0026] Example 1

[0027] A method for preparing an electrode material for electroadsorption coupled with electrocatalytic reduction of nitrate includes the following steps:

[0028] (1) Preparation of Mxene material Ti3C2Tx: 3.2g LiF was added to 40mL of 9M hydrochloric acid solution and stirred for 30min. Then, 3.2g Ti3AlC2 powder was added, the temperature was raised to 50℃, the rotation speed was adjusted to 400rpm, and the reaction was carried out for 36h. The dispersion obtained from the reaction was washed several times by centrifugation with deionized water until the supernatant turned dark green. Then, it was freeze-dried to obtain Mxene material Ti3C2Tx.

[0029] (2) Take 0.009 mol of NiCl2·6H2O, 0.003 mol of Al(NO3)3·9H2O and 0.0003 mol of PdCl2 and add them to 100 mL of deionized water. After dissolving completely, a mixed solution is obtained. The pH value is adjusted to 10 using sodium hydroxide solution and sodium carbonate solution. Then the mixed solution is transferred to a hydrothermal reactor, heated to 120℃, reacted for 12 h, cooled to room temperature, washed with water and dried at 60℃ for 8 h, and then placed in a tube furnace. The temperature is raised to 500℃ at a rate of 2℃ / min under air atmosphere and held for 4 h. Then the temperature is lowered to 250℃ and H2 / Ar mixed gas (H2 volume content is 10%) is introduced and held for 3 h. After cooling, Pd / LDO is obtained.

[0030] (3) The Mxene material Ti3C2Tx prepared in step (1) and the Pd / LDO prepared in step (2) are mixed in a mass ratio of 1:1 and added to deionized water. After being dispersed evenly, the mixture is filtered into a thin film and dried under vacuum at 60°C for 10 hours to obtain the Mxene-based electrode material for electro-adsorption coupled electrocatalytic reduction of nitrate.

[0031] Example 2

[0032] A method for preparing an Mxene-based electrode material for electroadsorption coupled with electrocatalytic reduction of nitrate includes the following steps:

[0033] (1) Preparation of Mxene material Ti3C2Tx: 3.2g LiF was added to 40mL of 9M hydrochloric acid solution and stirred for 30min. Then, 3.2g Ti3AlC2 powder was added, the temperature was raised to 50℃, the rotation speed was adjusted to 400rpm, and the reaction was carried out for 36h. The dispersion obtained from the reaction was washed several times by centrifugation with deionized water until the supernatant turned dark green. Then, it was freeze-dried to obtain Mxene material Ti3C2Tx.

[0034] (2) Take 0.009 mol of NiCl2·6H2O, 0.003 mol of Al(NO3)3·9H2O and 0.0009 mol of PdCl2 and add them to 100 mL of deionized water. After dissolving completely, a mixed solution is obtained. The pH value is adjusted to 10 using sodium hydroxide solution and sodium carbonate solution. Then the mixed solution is transferred to a hydrothermal reactor, heated to 180℃, reacted for 10 h, cooled to room temperature, the product is washed with water and dried at 60℃ for 8 h, and then placed in a tube furnace. The temperature is raised to 700℃ at a rate of 2℃ / min under air atmosphere and held for 4 h. Then the temperature is lowered to 250℃ and H2 / Ar mixed gas (H2 volume content is 10%) is introduced and held for 3 h. After cooling, Pd / LDO is obtained.

[0035] (3) The Mxene material Ti3C2Tx prepared in step (1) and the Pd / LDO prepared in step (2) are mixed at a mass ratio of 1:3 and added to deionized water. After being dispersed evenly, the mixture is filtered to form a thin film. After being vacuum dried at 60°C for 10 hours, the Mxene-based electrode material for electro-adsorption coupled electrocatalytic reduction of nitrate is obtained.

[0036] Example 3

[0037] A method for preparing an Mxene-based electrode material for electroadsorption coupled with electrocatalytic reduction of nitrate includes the following steps:

[0038] (1) Preparation of Mxene material Ti3C2Tx: 3.2g LiF was added to 40mL of 9M hydrochloric acid solution and stirred for 30min. Then, 3.2g Ti3AlC2 powder was added, the temperature was raised to 50℃, the rotation speed was adjusted to 400rpm, and the reaction was carried out for 36h. The dispersion obtained from the reaction was washed several times by centrifugation with deionized water until the supernatant turned dark green. Then, it was freeze-dried to obtain Mxene material Ti3C2Tx.

[0039] (2) Take 0.009 mol of NiCl2·6H2O, 0.003 mol of Al(NO3)3·9H2O and 0.0024 mol of PdCl2 and add them to 100 mL of deionized water. After dissolving completely, a mixed solution is obtained. The pH value is adjusted to 10 using sodium hydroxide solution and sodium carbonate solution. Then the mixed solution is transferred to a hydrothermal reactor, heated to 140℃, reacted for 10 h, cooled to room temperature, the product is washed with water and dried at 60℃ for 8 h, and then placed in a tube furnace. The temperature is raised to 600℃ at a rate of 2℃ / min under air atmosphere and held for 4 h. Then the temperature is lowered to 250℃ and H2 / Ar mixed gas (H2 volume content is 10%) is introduced and held for 3 h. After cooling, Pd / LDO is obtained.

[0040] (3) The Mxene material Ti3C2Tx prepared in step (1) and the Pd / LDO prepared in step (2) are mixed at a mass ratio of 1:2 and added to deionized water. After being dispersed evenly, the mixture is filtered to form a thin film and dried under vacuum at 60°C for 10 hours to obtain the Mxene-based electrode material for electro-adsorption coupled electrocatalytic reduction of nitrate.

[0041] Comparative Example 1

[0042] Preparation of LDO / Mxene electrode materials:

[0043] (1) 3.2g LiF was added to 40mL of 9M hydrochloric acid solution and stirred for 30min. Then, 3.2g Ti3AlC2 powder was added, the temperature was raised to 50℃, the rotation speed was adjusted to 400rpm, and the reaction was carried out for 36h. The dispersion obtained from the reaction was washed several times by centrifugation with deionized water until the supernatant turned dark green. Then, it was freeze-dried to obtain Mxene material Ti3C2Tx.

[0044] (2) Take 0.009 mol of NiCl2·6H2O and 0.003 mol of Al(NO3)3·9H2O and add them to 100 mL of deionized water. After dissolving completely, a mixed solution is obtained. The pH value is adjusted to 10 using sodium hydroxide solution and sodium carbonate solution. Then, the mixed solution is transferred to a hydrothermal reactor, heated to 140℃, reacted for 10 h, cooled to room temperature, washed with water and dried at 60℃ for 8 h, and then placed in a tube furnace. The temperature is raised to 600℃ at a rate of 2℃ / min under air atmosphere and held for 4 h. Then, the temperature is lowered to 250℃ and H2 / Ar mixed gas (H2 volume content is 10%) is introduced and held for 3 h. After cooling, LDO is obtained.

[0045] (3) The Mxene material Ti3C2Tx prepared in step (1) and the LDO prepared in step (2) are added to deionized water at a mass ratio of 1:2. After being dispersed evenly, the mixture is filtered into a thin film and dried under vacuum at 60°C for 10 hours to obtain the LDO / Mxene electrode material.

[0046] Comparative Example 2

[0047] Preparation of Mxene material Ti3C2Tx: 3.2g LiF was added to 40mL of 9M hydrochloric acid solution and stirred for 30min. Then, 3.2g Ti3AlC2 powder was added, the temperature was raised to 50℃, the rotation speed was adjusted to 400rpm, and the reaction was carried out for 36h. The resulting dispersion was centrifuged and washed several times with deionized water until the supernatant turned dark green. Then, it was freeze-dried to obtain Mxene material Ti3C2Tx.

[0048] The electrode materials prepared in Examples 1-3 were used to prepare electrodes and employed as working electrodes for the reduction of nitrate in water. A two-electrode system was used to test their nitrate removal performance. Electrodes prepared using LDO / Mxene material from Comparative Example 1 and Mxene material Ti3C2Tx from Comparative Example 2 were compared. The specific process is as follows: A 4*3cm electrode was prepared as the positive electrode, and an activated carbon electrode was used as the negative electrode, with a 2cm distance between them. A 100mg / L sodium nitrate solution was placed in the reactor. During the electroadsorption stage, a voltage of 1.0V was applied until adsorption equilibrium was reached. Then, the electrocatalytic reduction stage was initiated, with a constant voltage of -1.0V applied. The reaction was allowed to proceed for 2 hours. Samples were taken to analyze the concentration, and the nitrate removal rate and nitrogen selectivity were calculated using the following formulas:

[0049]

[0050]

[0051] The test results are shown in Table 1:

[0052] Table 1

[0053] Removal rate (%) Nitrogen selectivity (%) Example 1 90.8 88.5 Example 2 86.5 84.3 Example 3 83.3 80.8 Comparative Example 1 82％ 20.1％ Comparative Example 2 65％ 18.5％

[0054] As can be seen from the test results in Table 1, in the process of electroadsorption coupled with electrocatalytic reduction of nitrate, the electrode materials prepared in Examples 1-3 have higher nitrate removal rates and nitrogen selectivity compared to the electrode materials in Comparative Examples 1 and 2. Meanwhile, it can be seen that the addition of LDO significantly improves the nitrate removal rate, but the nitrogen selectivity remains poor; the addition of Pd allows nitrate to be efficiently converted into nitrogen.

Claims

1. A method for preparing an electrode material for electroadsorption coupled with electrocatalytic reduction of nitrate, characterized in that, Includes the following steps: (1) Preparation of Mxene material Ti3C2Tx; (2) Dissolve nickel salt, aluminum salt and palladium salt in deionized water to obtain a mixed solution. Adjust the pH value to 9-10, and then transfer the mixed solution to a hydrothermal reactor for hydrothermal reaction. After the reaction, cool to room temperature, wash the product with water and dry it, and then place it in a tube furnace. Heat it to 500-700℃ in an air atmosphere and keep it at that temperature for 2-5 hours. Then cool it down to 200-300℃ and introduce H2 / Ar mixed gas. Keep it at that temperature for 1-4 hours. After cooling, Pd / LDO is obtained. (3) Add the Mxene material Ti3C2Tx prepared in step (1) and the Pd / LDO obtained in step (2) to deionized water, disperse evenly, filter to thin film, and vacuum dry to obtain electrode material for electro-adsorption coupled electrocatalytic reduction of nitrate. In step (3), the mass ratio of the Mxene material Ti3C2Tx to Pd / LDO is 1:(1-3).

2. The method for preparing the electrode material for electroadsorption coupled with electrocatalytic reduction of nitrate as described in claim 1, characterized in that, In step (1), the preparation method of the Mxene material Ti3C2Tx is as follows: LiF is added to hydrochloric acid solution and stirred evenly. Then Ti3AlC2 powder is added. The mass ratio of LiF to Ti3AlC2 powder is 1:

1. The temperature is raised to 50℃ and stirred for 30-40h. After stirring and reacting, the mixture is washed several times by centrifugation with deionized water and finally freeze-dried to obtain the Mxene material Ti3C2Tx.

3. The method for preparing the electrode material for electroadsorption coupled with electrocatalytic reduction of nitrate as described in claim 1, characterized in that, In step (2), the molar ratio of nickel salt: aluminum salt: palladium salt is 3:1:(0.1-0.8).

4. The method for preparing the electrode material for electroadsorption coupled with electrocatalytic reduction of nitrate as described in claim 1, characterized in that, In step (2), the nickel salt is one or a combination of two or more of nickel chloride hexahydrate, nickel nitrate hexahydrate, nickel sulfate hexahydrate, or nickel carbonate; the aluminum salt is one or a combination of two or more of aluminum nitrate nonahydrate, aluminum chloride, aluminum sulfate, or aluminum carbonate; and the palladium salt is palladium chloride, palladium nitrate, or a combination of both.

5. The method for preparing the electrode material for electroadsorption coupled with electrocatalytic reduction of nitrate as described in claim 1, characterized in that, In step (2), the hydrothermal reaction temperature is 120-180℃ and the reaction time is 8-24 h.

6. The method for preparing the electrode material for electroadsorption coupled with electrocatalytic reduction of nitrate as described in claim 1, characterized in that, In step (2), the volume content of H2 in the H2 / Ar mixture is 10%.

7. The method for preparing the electrode material for electroadsorption coupled with electrocatalytic reduction of nitrate as described in any one of claims 1 to 6, characterized in that, In step (3), the vacuum drying temperature is 60°C and the time is 8-12 hours.

8. The application of the electrode material prepared by the method according to any one of claims 1 to 7 in the electro-adsorption coupled electrocatalytic reduction of nitrate.

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