Covalent triazine framework / weak acid gallium salt compound and electrocatalytic hydrogen peroxide production catalyst

Abstract

The invention provides a covalent triazine framework / weak acid gallium salt compound and a catalyst for producing hydrogen peroxide through electro-catalysis. The preparation method comprises the following steps: successively dispersing a porous covalent triazine framework of 2, 2 '-dipyridyl, soluble gallium salt and a chelating agent in a non-aqueous polar solvent, carrying out chelation reaction at a certain temperature, and finally filtering, washing and drying to obtain a target product of chelating gallium salt in inner holes of the covalent triazine framework. The preparation process of the covalent triazine frame / gallium salt compound is simple, the pollution to the environment is small, and the covalent triazine frame / gallium salt compound has excellent performance of electrocatalytically reducing oxygen into hydrogen peroxide. And the two-electron selectivity of 85-93% and relatively high reaction activity are shown.

Description

Technical Field

[0001] The invention relates to the preparation of an electrocatalyst, and is applied to fields such as green industrial synthesis represented by electrocatalytic production of hydrogen peroxide. Background Art

[0002] Hydrogen peroxide (H2O2) is an important chemical product with the advantages of high active oxygen content, strong oxidizing property and environmental protection. It is widely used in many fields such as chemical synthesis, paper bleaching, industrial wastewater treatment, degradation of organic pollutants and fuel cells. At present, the industrial preparation of hydrogen peroxide generally adopts the anthraquinone redox reaction. This method has many disadvantages, including the need for complex reactors and separation devices, the generation of various by-products, high energy consumption and high pollution, and the need to use hydrogen as a hydrogen source, which cannot meet the requirements of green and sustainable development. In contrast, the electrocatalytic two-electron oxygen reduction reaction (2e - ORR) can use protons in water as the source of hydrogen and realize hydrogen peroxide synthesis under the drive of electrical energy, which is energy-saving and environmentally friendly and has great development potential. - ORR electrosynthesis of hydrogen peroxide has attracted extensive attention. - In the ORR reaction process, the catalyst plays a very critical role. An efficient catalyst can effectively reduce the overpotential of the reaction and improve the activity and selectivity of the reaction. Covalent triazine framework (CTFs) is a nitrogen-containing aromatic polymer framework with a triazine ring. Its highly porous characteristics give it a sufficiently large specific surface area, thereby providing a large number of adsorption sites for gallium salts. Its excellent stability gives CTFs great potential as a substrate material for ORR electrocatalysts. It is reported that gallium has high selectivity, long-term good stability and high specific activity for ORR, which can lead to an increase in the intermediate bonding energy of oxygen, thereby enhancing the catalytic activity for ORR (10.1021 / acs.nanolett.8b00028). However, in this report, Ga was doped into octahedral PtNi, and the structure used was special, the preparation process was relatively complex, and the reaction conditions were relatively harsh. In contrast, the present invention selects a solvothermal method, which is not only simple to operate, mild in preparation conditions, but also can obtain the target product in one step. Summary of the invention

[0003] In view of the above technical problems, the present invention provides a method for preparing a covalent triazine framework (CTF) / weak acid gallium salt complex for electrocatalytic production of hydrogen peroxide, comprising the following steps: The first step: uniformly disperse the covalent triazine framework CTF and the soluble gallium salt in a non-aqueous polar solvent to obtain a dispersion. The non-aqueous polar solvent is used to inhibit the gallium salt from hydrolyzing or forming a strong crystal field coordination with OH and water molecules (thereby reducing the chelate coordination concentration of gallium atoms and bidentate N in CTF). The non-aqueous polar solvent includes methanol, tetrahydrofuran, NN dimethylformamide, etc.

[0004] The mass ratio of CTF to gallium salt is 1:0.5~2.2; the dispersion of CTF in non-aqueous polar solvent is 1~2 g L -1 The CTF is obtained by mixing [2,2'-bipyridyl]-5,5'-dimethylmethanol and benzene monomer biphenyl-4,4'-dicarboxamidine or terephthalamidine, adding cesium carbonate, and washing and freeze-drying the obtained solid product after precipitation.

[0005] The soluble gallium salts include gallium chloride, gallium nitrate pentahydrate, gallium nitrate nonahydrate and gallium sulfate.

[0006] Step 2: Add the chelating agent to the above dispersion in batches for multiple times, and after ultrasonic homogenization, heat and stir to react. The heating temperature is 30~100℃ and the time is 6~24h.

[0007] The chelating agent includes one or more of malonic acid, oxalic acid, benzoic acid and phenanthrenequinone; and the molar ratio of the amount to the atomic weight of gallium metal in the soluble gallium salt is 1.10-1.58:1.

[0008] The chelating agent added in the present invention is some negatively charged organic weak acid or alcohol, and CTF is also negatively charged, so the whole environment is negatively charged, and thus can be electrostatically bonded with gallium cations. More importantly, the organic weak acid or alcohol used in the present invention contains bidentate / multidentate oxygen, and CTF contains a large number of bidentate N binding sites, so it can firmly chelate metal ions. Taking malonic acid as the chelating agent, the structure of the chelate is shown as follows:

[0009] Step 3: Obtain the catalyst through filtering, washing and drying.

[0010] In some embodiments, one or more non-aqueous polar solvents such as methanol, tetrahydrofuran, NN-dimethylformamide (DMF) are selected and washed repeatedly for at least three times. After filtering, the sample is placed in a vacuum drying oven and dried for 6 to 24 hours to obtain a catalyst.

[0011] The covalent triazine framework / gallium salt complex prepared by the preparation method of the present invention is used as a catalyst for electrocatalytic production of hydrogen peroxide.

[0012] A catalyst for electrocatalytically producing hydrogen peroxide is a covalent triazine framework / gallium salt complex prepared by the preparation method.

[0013] The preparation method is simple and has little pollution to the environment. The covalent triazine framework / gallium salt complex has excellent electrocatalytic performance in reducing oxygen to hydrogen peroxide, showing 85-93% two-electron selectivity and high reaction activity. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 a is the ORR diagram of the sample in Example 1, b is the hydrogen peroxide selectivity of the sample in Example 1, and c is the number of transferred electrons of the sample in Example 1.

[0015] Figure 2 a is the ORR diagram of the sample in Example 2, b is the hydrogen peroxide selectivity of the sample in Example 2, and c is the number of transferred electrons of the sample in Example 2.

[0016] Figure 3 a is the ORR diagram of the sample in Example 3, b is the hydrogen peroxide selectivity of the sample in Example 3, and c is the number of transferred electrons of the sample in Example 3.

[0017] Figure 4 a is the ORR diagram of the sample in Example 4, b is the hydrogen peroxide selectivity of the sample in Example 4, and c is the number of transferred electrons of the sample in Example 4.

[0018] Figure 5 a is the ORR diagram of the sample in Example 5, b is the hydrogen peroxide selectivity of the sample in Example 5, and c is the number of transferred electrons of the sample in Example 5.

[0019] Figure 6 The XRD diagrams of Examples 1 and 3 are shown.

[0020] Figure 7 The scanning electron microscope image and element distribution mapping diagram of Example 1.

[0021] Figure 8 This is the EDS diagram of the element content of Example 1.

[0022] Fig. 9 The scanning electron microscope image and element distribution mapping diagram of Example 3.

[0023] Fig.10 This is the EDS diagram of the element content of Example 3. DETAILED DESCRIPTION

[0024] Characterization conditions: The ORR test method in the embodiment of the present invention is as follows: 980 μL of deionized water, 980 μL of ethanol and 40 μL of Nafion are mixed evenly, and then 1 mg of the sample is added to the solution and ultrasonicated for 2 h to make it evenly dispersed. The obtained slurry is then dropped onto the rotating ring disk electrode with a loading of 0.02 mg cm -2 , and then a three-electrode system using a reference electrode (mercuric oxide), a counter electrode (carbon rod), and a working electrode was tested in an oxygen-saturated 0.1 M KOH solution.

[0025] Preparation of covalent triazine framework (CTF): In a round-bottom flask (100 mL), [2,2'-bipyridyl]-5,5'-dimethylmethanol (86 mg, 0.4 mmol), benzene monomer biphenyl-4,4'-dicarboxamidine or terephthalamidine (190 mg, 0.8 mmol) and cesium carbonate (318 mg, 1.0 mmol) were added to a dimethyl sulfoxide solution (25.0 mL). The precipitate in the yellow suspension was washed thoroughly with dilute hydrochloric acid (1M, 50 mL) to remove the residual cesium carbonate, and then washed several times with water, ethanol and tetrahydrofuran. The filtered solid was freeze-dried for 24 hours to obtain yellow CTF.

[0026] Embodiment 1: In a round-bottom flask (25 mL), 5 mg CTF was dissolved in 2 ml DMF. After ultrasonic homogenization, 1.2 ml of the prepared GaCl3 solution (148.6 mg GaCl3 dissolved in 100 ml DMF) was added. After ultrasonic homogenization, the above solution was heated in a water bath at 338K (65°C) and reacted with magnetic stirring for 2 h. Then, 30 μl of the prepared malonic acid solution (176 mg dissolved in 10 ml DMF) was added in batches at an interval of 30 min for a total of 5 times. After continuing the reaction for 10 h, the precipitate was filtered out and washed with DMF several times to remove the residual GaCl3 in the precipitate. Finally, it was placed in a vacuum drying oven and dried for 12 h to obtain a covalent triazine framework / gallium complex.

[0027] Figure 1 a is the ORR diagram of the sample in Example 1, with an initial potential of 0.79 V vs RHE and a limiting current density of 1.05 mAcm -2 , the half-wave potential is 0.52V vs RHE, and Figure b is the hydrogen peroxide selectivity of the sample in Example 1, which can be concluded that the hydrogen peroxide selectivity is 93%. Figure 1 c is the number of transferred electrons of the sample in Example 1, and it can be concluded that the number of transferred electrons is 2.13.

[0028] Figure 6This is the XRD diagram of the sample in Example 1. Combined with the standard card, it can be concluded that gallium does not form compounds such as oxide salts, indicating that gallium is chelated with CTF and small molecule chelating agents in the form of single atoms, thereby roughly maintaining the crystal structure of CTF, and also indicating the successful preparation of the covalent triazine framework / gallium salt complex. Figure 7 This is the element mapping diagram of the sample in Example 1. Figure 8 This is the XPS graph of the sample in Example 1, in which the Ga element content is 5.88%.

[0029] Embodiment 2: In a round-bottom flask (25 mL), 5 mg CTF was added and dissolved in 2 ml DMF. After ultrasonic homogenization, 1.5 ml of the prepared GaCl3 solution (148.6 mg GaCl3 dissolved in 100 ml DMF) was added. After ultrasonic homogenization, the above solution was heated in a water bath at 338 K and magnetically stirred for 2 h. Then, 37.5 μl of the prepared malonic acid solution (176 mg dissolved in 10 ml DMF) was added in batches for 5 times with an interval of 30 min. After continuing the reaction for 10 h, the precipitate was filtered out and washed with DMF several times to remove the residual GaCl3 in the precipitate. Finally, it was placed in a vacuum drying oven and dried for 12 hours to obtain a covalent triazine framework / gallium complex.

[0030] Figure 2 a is the ORR diagram of the sample in Example 2, with an initial potential of 0.78 V vs RHE and a limiting current density of 0.99 mA cm -2 , the half-wave potential is 0.48 V vs RHE, Figure 2 b is the hydrogen peroxide selectivity of the sample in Example 2, and it can be concluded that the hydrogen peroxide selectivity is 89%. Figure 2 c is the number of transferred electrons of the sample in Example 2, and it can be concluded that the number of transferred electrons is 2.25.

[0031] Embodiment 3: In a round-bottom flask (25 mL), 5 mg CTF was added and dissolved in 2 ml methanol. After ultrasonic homogenization, 2 ml of the prepared GaCl3 solution (148.6 mg GaCl3 dissolved in 100 ml DMF) was added. After ultrasonic homogenization, the above solution was heated in a water bath at 338 K and magnetically stirred for 2 h. Then, 40 μl of the prepared oxalic acid solution (152 mg dissolved in 10 ml DMF) was added in batches at an interval of 30 min for a total of 5 times. After continuing the reaction for 10 h, the precipitate was filtered out and washed with methanol and DMF several times to remove the residual GaCl3 in the precipitate. Finally, it was placed in a vacuum drying oven and dried for 12 hours to obtain a covalent triazine framework / gallium complex.

[0032] Figure 3a is the ORR diagram of the sample in Example 3, with an initial potential of 0.79 V vs RHE and a limiting current density of 1.15 mA cm -2 , the half-wave potential is 0.59 V vs RHE, Figure 3 b is the hydrogen peroxide selectivity of the sample in Example 3, and it can be concluded that the hydrogen peroxide selectivity is 86%. Figure 3 c is the number of transferred electrons of the sample in Example 3, and it can be concluded that the number of transferred electrons is 2.26.

[0033] Figure 6 This is the XRD diagram of the sample in Example 3. Combined with the standard card, it can be concluded that gallium does not form compounds such as oxide salts, indicating that gallium is chelated with CTF and small molecule chelating agents in the form of single atoms, thereby roughly maintaining the crystal structure of CTF, and also indicating the successful preparation of the covalent triazine framework / gallium salt complex. Fig. 9 This is the element mapping diagram of the sample in Example 3. Fig.10 This is the XPS graph of the sample in Example 3, where the atomic percentage of Ga is 2.68% Embodiment 4: In a round-bottom flask (25 mL), 5 mg CTF was added and dissolved in 2 ml methanol. After ultrasonic homogenization, 1.2 ml of the prepared GaCl3 solution (148.6 mg GaCl3 dissolved in 100 ml DMF) was added. After ultrasonic homogenization, the above solution was heated in a water bath at 338 K and magnetically stirred for 2 h. Then, 60 μl of the prepared benzoic acid solution (53.3 g dissolved in 10 ml DMF) was added in batches at an interval of 30 min for a total of 5 times. After continuing the reaction for 10 h, the precipitate was filtered out and washed with methanol and DMF several times to remove the residual GaCl3 in the precipitate. Finally, it was placed in a vacuum drying oven and dried for 12 hours to obtain a covalent triazine framework / gallium complex.

[0034] Figure 4 a is the ORR diagram of the sample in Example 4, with an onset potential of 0.81 V vs RHE and a limiting current density of 0.98 mA cm -2 , the half-wave potential is 0.51 V vs RHE, Figure 4 b is the hydrogen peroxide selectivity of the sample in Example 4, and it can be concluded that the hydrogen peroxide selectivity is 82%. Figure 4 c is the number of transferred electrons of the sample in Example 4, and it can be concluded that the number of transferred electrons is 2.38.

[0035] Embodiment 5: In a round-bottom flask (25 mL), 5 mg CTF was added and dissolved in 2 ml methanol. After ultrasonic homogenization, 1.2 ml of the prepared GaCl3 solution (148.6 mg GaCl3 dissolved in 100 ml DMF) was added. After ultrasonic homogenization, the above solution was heated in a water bath at 338 K and magnetically stirred for 2 h. Then, 60 μl of the prepared phenanthrenequinone solution (83.3 g dissolved in 10 ml DMF) was added in batches at an interval of 30 min for a total of 5 times. After continuing the reaction for 10 h, the precipitate was filtered out and washed with methanol and DMF several times to remove the residual GaCl3 in the precipitate. Finally, it was placed in a vacuum drying oven and dried for 12 hours to obtain a covalent triazine framework / gallium complex. Figure 5 a is the ORR diagram of the sample in Example 5, with an onset potential of 0.79 V vs RHE and a limiting current density of 1.20 mA cm -2 , the half-wave potential is 0.53 V vs RHE, Figure 5 b is the hydrogen peroxide selectivity of the sample in Example 5, and it can be concluded that the hydrogen peroxide selectivity is 75%, Figure 5 c is the number of transferred electrons of the sample in Example 5, and it can be concluded that the number of transferred electrons is 2.5.

Claims

1. A method for preparing a covalent triazine framework / weak acid gallium salt complex, characterized in that: The method comprises the following preparation steps: S1, uniformly dispersing the covalent triazine framework CTF and the soluble gallium salt in a non-aqueous polar solvent to obtain a dispersion; S2, adding the chelating agent to the dispersion in S1 in batches for multiple times, and after ultrasonic homogenization, heating and stirring for reaction; S3, filtering, washing and drying to obtain a catalyst.

2. The preparation method according to claim 1, characterized in that: The mass ratio of CTF to soluble gallium salt described in S1 is 1:1.78-2.80, and the dispersion degree of CTF in non-aqueous polar solvent is 1-2 g L -1 .

3. The preparation method according to claim 2, characterized in that: The CTF is prepared by mixing [2,2'-bipyridyl]-5,5'-dimethylmethanol and benzene monomer biphenyl-4,4'-dicarboxamidine or terephthalamidine, adding cesium carbonate, and washing and freeze-drying the obtained solid product after precipitation.

4. The preparation method according to claim 1, characterized in that: The soluble gallium salt includes any one of gallium chloride, gallium nitrate pentahydrate and gallium sulfate.

5. The preparation method according to claim 1, characterized in that: The non-aqueous polar solvent includes methanol, tetrahydrofuran, or N,N-dimethylformamide.

6. The preparation method according to claim 1, characterized in that: In S2, the heating temperature is 30~100 ℃ and the time is 6~24 h.

7. The preparation method according to claim 1, characterized in that: The chelating agent comprises one or more of malonic acid, oxalic acid, benzoic acid and phenanthrenequinone; and the molar ratio of the amount thereof to the atomic weight of gallium metal in the soluble gallium salt is 1.10-1.58:

1.

8. The covalent triazine framework / gallium salt complex prepared according to the preparation method according to any one of claims 1 to 7.

9. An electrocatalytic hydrogen peroxide production catalyst, characterized in that: A covalent triazine framework / gallium salt complex prepared by the preparation method described in any one of claims 1 to 7.

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