Preparation method for obtaining oxygen vacancy-rich Bi2WO6 material through alkali treatment and application of oxygen vacancy-rich Bi2WO6 material in electro-catalytic nitrogen reduction

Abstract

The invention discloses a preparation method for obtaining an oxygen vacancy-rich Bi2WO6 material through alkali treatment and application of the oxygen vacancy-rich Bi2WO6 material in electro-catalytic nitrogen reduction. According to the invention, Bi2WO6 is prepared by adopting a hydrothermal method, and then a low-concentration NaOH solution is used for etching to obtain the Bi2WO6 material with rich oxygen vacancies. Rich oxygen vacancies are introduced into Bi2WO6, and the oxygen vacancies can be used as excellent charge carriers and reaction active sites, so that the charge transfer rate between a catalyst and N2 is promoted, and the adsorption and activation capability on N2 is improved, thereby improving the activity of catalyzing NRR. The alkali etching treatment improves the Bi element content in Bi2WO6 at the same time, and is beneficial to inhibition of hydrogen evolution side reaction, thereby improving the selectivity of the catalyst to NRR. The prepared Bi2WO6 material with rich oxygen vacancies has relatively high ammonia yield and Faraday efficiency when being applied to an electro-catalytic nitrogen reduction process, and shows good stability.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of electrocatalytic material preparation, and particularly relates to a preparation method of an alkali-treated abundant-oxygen-vacancy Bi2WO6 material and application of the material in electrocatalytic nitrogen reduction. BACKGROUND

[0002] Ammonia is considered as a potential carbon-free energy carrier for maintaining the long-term development of human society due to its high energy density, easy storage and transportation. In addition, ammonia is an important industrial product, which is widely used in agricultural and industrial production fields such as chemical fertilizer, nitric acid, refrigerant, resin, rubber, explosive, etc. Therefore, the global demand for ammonia is large and continues to grow. Nitrogen is the highest component in the atmosphere, and the process of converting N2 into NH3 or other nitrogen-containing compounds is called “nitrogen fixation”, which has far-reaching influence and great value. However, due to the stable chemical properties of N2, the N≡N bond has a high bond energy (941 kJ / mol), and the thermodynamic cleavage energy of the first bond is as high as 410 kJ / mol of energy, which is difficult to dissociate, and a large amount of energy needs to be input to realize this process.

[0003] At present, large-scale synthesis of ammonia is mainly completed by Haber-Bosch process, which makes the synthesis of ammonia commercialized and is the most mature artificial nitrogen fixation technology in the world. However, Haber-Bosch process needs to be carried out under high temperature and high pressure conditions, consumes a large amount of non-renewable energy and emits a large amount of greenhouse gases. The electrocatalytic nitrogen reduction reaction (ENRR) which can be carried out at room temperature and normal pressure is a feasible way to realize green synthesis of ammonia. In theory, as long as a sufficient voltage is applied, N2 can be electrocatalytically reduced to NH3. However, in practice, ENRR lacks catalysts that can produce significant ammonia yield and high faradic efficiency. Since most of the catalysts with NRR activity also have high activity for proton reduction to H2, most of the protons and electrons in the system will tend to occur in the hydrogen evolution reaction (HER) which is faster in kinetics, which leads to poor selectivity of the catalyst for nitrogen reduction reaction (NRR). At present, the strategy to solve this problem is mainly from the perspective of catalyst design, that is, by reasonably designing the catalyst, the effective adsorption and activation of N2 are realized while the HER is inhibited.

[0004] Bi2WO6 is an important layered perovskite crystal material, which is composed of [WO4] 2- layers and [Bi2O2] 2+ layers alternately stacked. This unique alternating layered structure has a large built-in electric field and asymmetric polarization effect, which can promote effective separation of carriers, thus being beneficial to the improvement of catalytic performance. However, due to the [Bi2O2] 2+The surface lacks active electrons and active sites, cannot effectively adsorb N2, and due to the insufficient "π-back donation" process of Bi atoms, the ability of dissociation and activation of N2 needs to be improved. In addition, as a wide band gap semiconductor, the electrical conductivity of Bi2WO6 is poor, which is not conducive to the charge transfer process in ENRR. SUMMARY

[0005] The purpose of the present application is to provide a preparation method of an alkali-treated Bi2WO6 material with rich oxygen vacancies and its application in electrocatalytic nitrogen reduction.

[0006] The preparation method of the alkali-treated Bi2WO6 material with rich oxygen vacancies is: Step S1: Bi(NO3)3 and Na2WO4 are dissolved in ethylene glycol respectively, under stirring, the Na2WO4 solution is added dropwise into the Bi(NO3)3 solution, then the mixed solution is transferred to a high-pressure reaction kettle, reacted at 140-200℃ for 5-20h, cooled to room temperature, the precipitate is washed with deionized water and ethanol respectively, and dried to obtain Bi2WO6; Step S2: the Bi2WO6 prepared in step S1 is dispersed in a 0.1-1 mol / L NaOH solution, stirred for no more than 3h, then the precipitate is washed with deionized water and ethanol respectively after filtration, and dried to obtain a Bi2WO6 material with rich oxygen vacancies.

[0007] The Bi2WO6 material with rich oxygen vacancies prepared above is applied in electrocatalytic nitrogen reduction reaction.

[0008] The specific operation of the electrocatalytic nitrogen reduction reaction is: a three-electrode system is used, the Bi2WO6 material with rich oxygen vacancies is used as the working electrode, the Ag / AgCl electrode is used as the reference electrode, the platinum mesh is used as the counter electrode, and the Li2SO4 solution is used as the electrolyte, nitrogen gas is introduced into the system, and N2 is converted into NH3 by electrocatalysis.

[0009] The application adopts a hydrothermal method to prepare Bi2WO6, and then uses a low-concentration NaOH solution to etch to obtain a Bi2WO6 material with rich oxygen vacancies. The rich oxygen vacancies are introduced into the Bi2WO6, which can serve as excellent charge carriers and reaction active sites, promote the charge transfer rate between the catalyst and N2, and improve the adsorption and activation capacity of N2, so as to improve the activity of catalytic NRR. The alkali etching treatment simultaneously improves the content of Bi element in the Bi2WO6, which is beneficial to inhibit the hydrogen evolution side reaction, so as to improve the selectivity of the catalyst to NRR. The prepared Bi2WO6 material with rich oxygen vacancies is applied to the process of electrocatalytic nitrogen reduction, and exhibits excellent catalytic performance, and the ammonia yield can reach 6 times of that of the Bi2WO6 catalyst without alkali treatment under the same conditions, has a high ammonia yield and Faraday efficiency, and exhibits good stability. BRIEF DESCRIPTION OF DRAWINGS

[0010] Figure 1 The SEM image of the product obtained in Example 2 of the application.

[0011] Figure 2 The SEM image of the product obtained in Comparative Example 1 of the application.

[0012] Figure 3 The XRD image of the product obtained in Examples 1-5 and Comparative Example 1 of the application.

[0013] Figure 4 The EPR image of the product obtained in Example 2 and Comparative Example 1 of the application.

[0014] Figure 5 The content image of Bi and W elements of the product obtained in Example 2 and Comparative Example 1 of the application.

[0015] Figure 6 The UV-vis image (a) of the obtained NH4Cl standard solution measured by indigo blue colorimetry; the linear curve image (b) of the NH4Cl standard solution and absorbance.

[0016] Figure 7 The UV-vis image (a) of the obtained hydrazine standard solution measured by Watt-Chrisp method; the linear curve image (b) of the hydrazine standard solution and absorbance.

[0017] Figure 8 The nitrogen reduction test of the product obtained in Examples 1-5 and Comparative Example 1 of the application at a potential of-0.4 V vs. RHE, and the UV-vis curve (a) of the electrolyte after coloration; the corresponding ammonia yield and Faraday efficiency (b).

[0018] Figure 9The UV-vis curve of the electrolyte after color development by Watt-Chrisp method after nitrogen reduction test of the product obtained in Example 2 of the present application at different potentials.

[0019] Figure 10 The chronoamperogram (a) of the product obtained in Example 2 of the present application at-0.4 V vs. RHE for 12 hours; the UV-vis curve of the electrolyte after 5 times of electrochemical test (b). DETAILED DESCRIPTION

[0020] In order to make the present application clearer, the above technical solutions will be described in detail below in conjunction with the description and specific embodiments, but the protection scope of the present application is not limited by this. Example 1

[0021] Step S1: 2.425 g of Bi(NO3)3·5H2O and 0.825 g of Na2WO4·2H2O were weighed and dissolved in 15 mL of ethylene glycol respectively, and completely dissolved to form a transparent solution after magnetic stirring for 0.5 h. Under magnetic stirring, the Na2WO4 solution was slowly added to the Bi(NO3)3 solution, and stirring was continued for 0.5 h, then the mixed solution was transferred to a 50 mL reaction kettle, and after reaction at 160 ℃ for 12 h, it was cooled to room temperature. The precipitate was washed with deionized water and ethanol for 3 times respectively, and dried in a vacuum drying oven at 60 ℃ for 12 h to obtain Bi2WO6.

[0022] Step S2: 0.2 g of the Bi2WO6 sample obtained in step S1 was weighed and dispersed into 40 mL of 0.5 mol / L NaOH solution, and stirred at room temperature for 0.25 h. After filtration, the obtained solid was washed with deionized water and ethanol for 3 times respectively, and then dried in a vacuum drying oven at 60 ℃ for 12 hours to obtain a Bi2WO6 material with rich oxygen vacancies. Example 2

[0023] Step S1: 2.425 g of Bi(NO3)3·5H2O and 0.825 g of Na2WO4·2H2O were weighed and dissolved in 15 mL of ethylene glycol respectively, and completely dissolved to form a transparent solution after magnetic stirring for 0.5 h. Under magnetic stirring, the Na2WO4 solution was slowly added to the Bi(NO3)3 solution, and stirring was continued for 0.5 h, then the mixed solution was transferred to a 50 mL reaction kettle, and after reaction at 160 ℃ for 12 h, it was cooled to room temperature. The precipitate was washed with deionized water and ethanol for 3 times respectively, and dried in a vacuum drying oven at 60 ℃ for 12 h to obtain Bi2WO6.

[0024] Step S2: Weigh 0.2 g of the Bi2WO6 sample obtained in Step S1, disperse it in 40 mL of 0.5 mol / L NaOH solution, and stir at room temperature for 0.5 h. After filtration, wash the obtained solid three times with deionized water and ethanol respectively, and then dry it in a vacuum drying oven at 60 ℃ for 12 hours to obtain Bi2WO6 material with abundant oxygen vacancies. Example 3

[0025] Step S1: Weigh 2.425 g Bi(NO3)3·5H2O and 0.825 g Na2WO4·2H2O, and dissolve them separately in 15 mL of ethylene glycol. After magnetic stirring for 0.5 h, the solutions are completely dissolved to form a transparent solution. Under magnetic stirring, the Na2WO4 solution is slowly added dropwise to the Bi(NO3)3 solution, and stirring is continued for 0.5 h. The mixed solution is then transferred to a 50 mL reaction vessel and reacted at 160 °C for 12 h, followed by cooling to room temperature. The precipitate is washed three times with deionized water and ethanol, respectively, and dried under vacuum at 60 °C for 12 h to obtain Bi2WO6.

[0026] Step S2: Weigh 0.2 g of the Bi2WO6 sample obtained in Step S1, disperse it in 40 mL of 0.5 mol / L NaOH solution, and stir at room temperature for 0.75 h. After filtration, wash the obtained solid three times with deionized water and ethanol respectively, and then dry it in a vacuum drying oven at 60 ℃ for 12 hours to obtain Bi2WO6 material with abundant oxygen vacancies. Example 4

[0027] Step S1: Weigh 2.425 g Bi(NO3)3·5H2O and 0.825 g Na2WO4·2H2O, and dissolve them separately in 15 mL of ethylene glycol. After magnetic stirring for 0.5 h, the solutions are completely dissolved to form a transparent solution. Under magnetic stirring, the Na2WO4 solution is slowly added dropwise to the Bi(NO3)3 solution, and stirring is continued for 0.5 h. The mixed solution is then transferred to a 50 mL reaction vessel and reacted at 160 °C for 12 h, followed by cooling to room temperature. The precipitate is washed three times with deionized water and ethanol, respectively, and dried under vacuum at 60 °C for 12 h to obtain Bi2WO6.

[0028] Step S2: Weigh 0.2 g of the Bi2WO6 sample obtained in Step S1, disperse it in 40 mL of 0.5 mol / L NaOH solution, and stir at room temperature for 1 h. After filtration, wash the obtained solid three times with deionized water and ethanol respectively, and then dry it in a vacuum drying oven at 60 °C for 12 h to obtain Bi2WO6 material with abundant oxygen vacancies. Example 5

[0029] Step S1: Weigh 2.425 g Bi(NO3)3·5H2O and 0.825 g Na2WO4·2H2O, and dissolve them separately in 15 mL of ethylene glycol. After magnetic stirring for 0.5 h, the solutions are completely dissolved to form a transparent solution. Under magnetic stirring, the Na2WO4 solution is slowly added dropwise to the Bi(NO3)3 solution, and stirring is continued for 0.5 h. The mixed solution is then transferred to a 50 mL reaction vessel and reacted at 160 °C for 12 h, followed by cooling to room temperature. The precipitate is washed three times with deionized water and ethanol, respectively, and dried under vacuum at 60 °C for 12 h to obtain Bi2WO6.

[0030] Step S2: Weigh 0.2 g of the Bi2WO6 sample obtained in Step S1, disperse it in 40 mL of 0.5 mol / L NaOH solution, and stir at room temperature for 3 h. After filtration, wash the obtained solid three times with deionized water and ethanol respectively, and then dry it in a vacuum drying oven at 60 ℃ for 12 hours to obtain Bi2WO6 material with abundant oxygen vacancies. Comparative Example 1

[0031] Procedure: Weigh 2.425 g Bi(NO3)3·5H2O and 0.825 g Na2WO4·2H2O, and dissolve them separately in 15 mL of ethylene glycol. After magnetic stirring for 0.5 h, the solutions are completely dissolved to form a transparent solution. Under magnetic stirring, the Na2WO4 solution is slowly added dropwise to the Bi(NO3)3 solution, and stirring is continued for 0.5 h. The mixed solution is then transferred to a 50 mL reaction vessel and reacted at 160 °C for 12 h, followed by cooling to room temperature. The precipitate is washed three times with deionized water and ethanol, respectively, and dried under vacuum at 60 °C for 12 h to obtain Bi2WO6.

[0032] Figure 1 and Figure 2 SEM images of the products obtained in Example 2 and Comparative Example 1 are shown. Comparing the two images, it can be seen that the morphology of the products obtained before and after alkali treatment is similar; both are three-dimensional spherical structures assembled from two-dimensional nanosheets, with a diameter of 500-700 nm. This indicates that alkali treatment did not cause any change in the basic morphology, and this special structure, with its high specific surface area, excellent electron transport performance, good structural stability, and optimized mass transfer efficiency, can exhibit excellent catalytic performance.

[0033] Figure 3These are the XRD patterns of the samples obtained in Examples 1 to 5 and Comparative Example 1. As can be seen from the figures, the XRD patterns of the six samples are similar, indicating that they have the same phase composition. The diffraction peaks at 28.3°, 32.9°, 47.1°, 55.8°, 58.5°, 68.8°, 76.1°, and 78.5° correspond to the (131), (002), (202), (331), (262), (400), (333), and (204) crystal planes of the orthorhombic Bi2WO6 (JCPDS No. 39-0256), respectively. No impurity peaks were observed, indicating that all synthesized samples have high purity.

[0034] EPR testing further demonstrated the effect of alkali treatment on increasing the oxygen vacancy concentration in Bi2WO6 samples. Figure 4 As shown, compared to Comparative Example 1, Example 2 exhibits a pair of symmetrically distributed peaks with greater intensity at a g value of 2.004, confirming that the alkali treatment in this invention successfully increased the oxygen vacancy concentration on the Bi2WO6 surface. These oxygen vacancies are expected to enhance the catalyst's ability to adsorb and activate N2, thereby improving its ENRR performance.

[0035] The content of Bi and W elements on the surface of the products obtained in Comparative Example 2 and Comparative Example 1 ( Figure 5 As can be seen, the content of W in the Bi₂WO₆ sample decreased after alkali treatment, while the content of Bi increased. This is because [WO₄]₂... 2- The layer is soluble in NaOH solution, while [Bi₂O₂] 2+ The layer is insoluble, therefore alkaline etching removed part of the [WO4] content. 2- The layers then undergo charge redistribution, resulting in changes in elemental composition and chemical environment as seen in Example 2. It has been reported that a high Bi content helps provide more active sites for NRR, thereby improving nitrogen fixation performance.

[0036] Next, the catalytic performance of the Bi2WO6 material with abundant oxygen vacancies prepared in this invention in the field of electrocatalytic NRR will be evaluated. The specific methods are as follows: The electrocatalytic nitrogen reduction reaction is carried out through a three-electrode system (with a 1 cm electrode). 2A carbon paper coated with 0.25 mg of catalyst was used as the working electrode, a Pt mesh as the counter electrode, and an Ag / AgCl electrode (saturated KCl solution) as the reference electrode. The electrolyte was 0.1 M Li₂SO₄ solution. Electrochemical tests were performed in an H-type electrolytic cell containing a Nafion membrane, and a CHI760 electrochemical workstation was used. The working electrode was prepared by dispersing 5 mg of catalyst (the catalyst being the product obtained in Examples 1-5 and Comparative Example 1) in a mixed solution of 400 μL isopropanol, 600 μL deionized water, and 30 μL 5.0 wt.% Nafion solution, followed by sonication for at least 1 h to obtain a stable suspension. 50 μL of this suspension was then coated onto a 1×1 cm² plate. 2 Apply to carbon paper and allow to dry naturally at room temperature before use.

[0037] In the nitrogen reduction electrolysis experiment, N2 was first introduced into the electrolyte at a constant rate for 30 min to reach saturation, and then the electrolyte was saturated at -0.4 V. vs. A 2-hour chronoamperometry test was conducted at the potential of RHE, with N2 continuously flowing through the solution during the test. The ammonia content in the solution after electrolysis was then determined using the indophenol blue method to further determine the ammonia yield and Faraday efficiency. The applied potential was determined according to the Nernst equation (E... RHE =E Ag / AgCl +0.059 pH +0.197 V) are all converted to potentials based on the reversible hydrogen electrode.

[0038] Electrolysis product testing: Ammonia content was tested using the indophenol blue method; hydrazine content was tested using the Watt-Chrisp method; after determining the content, ammonia yield and Faraday efficiency were calculated.

[0039] The formula for calculating ammonia yield is: ; in, C NH3 It refers to the concentration of NH3 produced. V (mL) is the volume of the electrolyte, and t (h) is the reaction time. m (mg) is the mass of the catalyst.

[0040] The formula for calculating Faraday efficiency is FE%=(3 F × C NH3 × V )×100 / (17× Q ); Where F is the Faraday constant (96485.3 C mol) -1 ), Q It is the amount of electric charge.

[0041] Before performing electrocatalytic nitrogen reduction, the ammonia standard curve was first determined using the indophenol blue method with the ammonia standard solution. The results are as follows: Figure 6 The Watt-Chrisp method was used to determine the possible byproduct hydrazine using a standard curve. The results are as follows: Figure 7 .

[0042] 30 mL of 0.1 M Li₂SO₄ solution was placed in each of the two chambers of an H-type electrolytic cell. Using Examples 1-5 and Comparative Example 1 as working electrodes, constant potential nitrogen reduction tests were performed in a three-electrode system with the potential set to -0.4 V. vs. RHE, runtime 7200 s. Test results for Examples 1-5 and Comparative Example 1 are as follows: Figure 8 As shown in the results, all catalysts treated with alkali (Examples 1-5) exhibited higher ammonia yields and Faradaic efficiencies than the untreated Bi₂WO₆ catalyst. This indicates that increasing the oxygen vacancy concentration through alkali treatment effectively enhances the nitrogen reduction activity of the catalyst. Next, the content of the byproduct hydrazine in the electrolyte after nitrogen reduction tests at different potentials in Example 2 was measured. Figure 9 As shown in Example 2, almost no hydrazine was detected in the electrolyte after electrocatalytic nitrogen reduction, indicating that the catalyst of the present invention has good selectivity for nitrogen reduction to ammonia synthesis.

[0043] To investigate the catalytic stability of the prepared material, we used Example 2 as the catalyst at -0.4 V. vs. Electrocatalytic NRR was tested for 12 h at the voltage of RHE. For example... Figure 10 As shown in (a), the catalyst did not show a significant decrease in current density during the test, and further cycling experiments ( Figure 10 (b) shows that after five repeated tests, the ammonia synthesis rate and the corresponding Faraday efficiency were relatively stable, proving that the catalyst has good stability.

Claims

1. A method for preparing Bi2WO6 material with abundant oxygen vacancies through alkali treatment, characterized in that, The specific steps of the preparation method are as follows: Step S1: Dissolve Bi(NO3)3 and Na2WO4 in ethylene glycol respectively. While stirring, add the Na2WO4 solution dropwise to the Bi(NO3)3 solution. Then transfer the mixed solution to a high-pressure reactor and react at 140-200℃ for 5-20 hours. Cool to room temperature, wash the precipitate with deionized water and ethanol respectively, and dry to obtain Bi2WO6. Step S2: Disperse the Bi2WO6 obtained in step S1 into a 0.1-1 mol / L NaOH solution, stir the reaction for no more than 3 hours, filter, wash the precipitate with deionized water and ethanol respectively, and dry to obtain Bi2WO6 material with abundant oxygen vacancies.

2. The application of the Bi2WO6 material with abundant oxygen vacancies prepared by the method according to claim 1 in the electrocatalytic nitrogen reduction reaction.

3. The application according to claim 2, characterized in that, The specific operation of the electrocatalytic nitrogen reduction reaction is as follows: a three-electrode system is used, with Bi2WO6 material with abundant oxygen vacancies as the working electrode, Ag / AgCl electrode as the reference electrode, platinum mesh as the counter electrode, and Li2SO4 solution as the electrolyte. Nitrogen gas is introduced into the system to electrocatalyze the conversion of N2 to NH3.

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