Covalent triazine framework / weak acid gallium salt complexes and electrocatalytic hydrogen peroxide production catalysts

Abstract

The application provides a covalent triazine framework / weak acid gallium salt composite and an electrocatalytic hydrogen peroxide catalyst. A porous covalent triazine framework of 2,2'-dipyridyl, a soluble gallium salt and a chelating agent are dispersed in a non-water polar solvent in sequence, a chelation reaction is carried out at a certain temperature, and finally the target product of the gallium salt chelated in the inner hole of the covalent triazine framework is obtained after filtration, washing and drying. The preparation process of the covalent triazine framework / gallium salt composite is simple, the pollution to the environment is small, the covalent triazine framework / gallium salt composite has excellent electrocatalytic oxygen reduction to hydrogen peroxide performance. It shows 85-93% two-electron selectivity and high reaction activity.

Description

TECHNICAL FIELD

[0001] The present application relates to the preparation of electrocatalysts, which are applied to the field of green industrial synthesis represented by electrocatalytic production of hydrogen peroxide. BACKGROUND

[0002] Hydrogen peroxide (H2O2) is an important chemical product, which has the advantages of high active oxygen content, strong oxidation and green environmental protection, and has a wide range of applications in chemical synthesis, paper bleaching, industrial wastewater treatment, organic pollutant degradation and fuel cells. At present, hydrogen peroxide is generally prepared by anthraquinone redox reaction in industry. This method has many shortcomings, including the need for complex reactors and separation devices, the generation of multiple 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 a source of hydrogen to synthesize hydrogen peroxide under the drive of electrical energy, which is energy-saving and environmentally friendly, and has great development potential. Therefore, 2e - ORR electric synthesis of hydrogen peroxide has received extensive attention. In the process of 2e - ORR reaction, the catalyst plays a very key role. Efficient catalysts can effectively reduce the overpotential of the reaction and improve the activity and selectivity of the reaction. Covalent triazine framework (CTF) is a nitrogen-containing aromatic polymer framework with a triazine ring. The highly porous nature provides a large specific surface area for gallium salt, and the excellent stability endows CTF with great potential as an ORR electrocatalyst base material. It is reported that gallium has high selectivity for ORR, long-term good stability and high specific activity, which can lead to an increase in oxygen intermediate bonding energy, thereby enhancing the catalytic activity of ORR (10.1021 / acs.nanolett.8b00028). However, the report selects Ga-doped octahedral PtNi, which has a special structure and a complex preparation process, and the reaction conditions are harsh. In contrast, the present application selects a solvothermal method, which not only has a simple preparation process, mild preparation conditions, but also can obtain the target product in one step. SUMMARY

[0003] In view of the above technical problems, the present application provides a preparation method of a covalent triazine framework (CTF) / weak acid gallium salt composite for electrocatalytic production of hydrogen peroxide, comprising the following steps:

[0004] The first step is to 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 hydrolysis of the gallium salt or the formation of a strong crystal field coordination with OH and water molecules (thus reducing the chelation coordination concentration of gallium atoms and bidentate N in the CTF). The non-aqueous polar solvent includes methanol, tetrahydrofuran, N-N dimethylformamide, etc.

[0005] The mass ratio of the CTF and the gallium salt is 1:0.5-2.2, and the dispersion degree of the CTF in the non-aqueous polar solvent is 1-2 g / L. -1 The CTF is [2,2'-dipyridyl]-5,5'-diyl methanol, and the benzene monomer biphenyl-4,4'-diimidazole or p-phenylenediamine is mixed, then cesium carbonate is added, and after precipitation, the obtained solid product is washed and freeze-dried.

[0006] The soluble gallium salt includes gallium chloride, gallium nitrate pentahydrate, gallium nitrate nonahydrate, and gallium sulfate, etc.

[0007] The second step is to add a chelating agent to the above dispersion in batches and multiple times, uniformly ultrasonic, and then perform a heating and stirring reaction. The heating temperature is 30-100 DEG C, and the time is 6-24 h.

[0008] The chelating agent includes one or more of malonic acid, oxalic acid, benzoic acid, and phenanthraquinone, and the molar ratio of the amount to the amount of gallium metal atoms in the soluble gallium salt is 1.10-1.58:1.

[0009] In the present application, some negatively charged weak organic acids or alcohols are added, and the CTF is also negatively charged, so the whole environment is negatively charged, and thus can be combined with gallium ions by electrostatic force. More importantly, the weak organic acids or alcohols used in the present application contain bidentate / multidentate oxygen, and the CTF contains a large number of bidentate N binding sites, so the metal ions can be firmly chelated. When malonic acid is used as the chelating agent, the structure of the chelate is as follows:

[0010]

[0011] The third step is to filter, wash, and dry to obtain the catalyst.

[0012] In some embodiments, one or more non-aqueous polar solvents such as methanol, tetrahydrofuran, N-N-dimethylformamide (DMF), etc. are repeatedly washed at least three times, and after filtration, the sample is placed in a vacuum drying oven and dried for 6-24 h to obtain the catalyst.

[0013] The covalent triazine framework / gallium salt complex prepared by the preparation method described in the present application is used as a catalyst for electrocatalytic production of hydrogen peroxide.

[0014] The application discloses an electrocatalytic hydrogen peroxide catalyst, and a covalent triazine frame / gallium salt composite prepared by the preparation method.

[0015] The preparation method is simple in process and small in environmental pollution, and the covalent triazine frame / gallium salt composite has excellent electrocatalytic oxygen reduction to hydrogen peroxide performance, exhibits 85-93% two-electron selectivity and high reaction activity. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 a is an ORR graph of the sample of Example 1, b is hydrogen peroxide selectivity of the sample of Example 1, and c is the number of transferred electrons of the sample of Example 1.

[0017] Figure 2 a is an ORR graph of the sample of Example 2, b is hydrogen peroxide selectivity of the sample of Example 2, and c is the number of transferred electrons of the sample of Example 2.

[0018] Figure 3 a is an ORR graph of the sample of Example 3, b is hydrogen peroxide selectivity of the sample of Example 3, and c is the number of transferred electrons of the sample of Example 3.

[0019] Figure 4 a is an ORR graph of the sample of Example 4, b is hydrogen peroxide selectivity of the sample of Example 4, and c is the number of transferred electrons of the sample of Example 4.

[0020] Figure 5 a is an ORR graph of the sample of Example 5, b is hydrogen peroxide selectivity of the sample of Example 5, and c is the number of transferred electrons of the sample of Example 5.

[0021] Figure 6 a is an XRD graph of Examples 1 and 3.

[0022] Figure 7 a is a scanning electron microscope graph and element distribution Mapping graph of Example 1.

[0023] Figure 8 a is an element content EDS graph of Example 1.

[0024] Figure 9 a is a scanning electron microscope graph and element distribution Mapping graph of Example 3.

[0025] Figure 10 a is an element content EDS graph of Example 3. DETAILED DESCRIPTION

[0026] Characterization conditions:

[0027] The ORR test method in the embodiment of the present application: take 980 μL of deionized water, 980 μL of ethanol and 40 μL of Nafion, mix uniformly, then take 1 mg of the sample, add it to the solution, ultrasonic for 2 h to make it uniformly dispersed. Then drop the obtained slurry on a rotating ring-disk electrode, and the loading is 0.02 mg cm -2 Then use a reference electrode (mercury oxide), a counter electrode (carbon rod), and a working electrode three-electrode system to test in a 0.1 M KOH solution saturated with oxygen.

[0028] Preparation of covalent triazine framework (CTF):

[0029] In a round-bottom flask (100 mL), add [2,2'-bipyridyl]-5,5'-diylmethanol (86 mg, 0.4 mmol) to a dimethyl sulfoxide solution (25.0 mL), a yellow suspension of precipitate in benzene monomer biphenyl-4,4'-diimidamide or p-phenylenediamine (190 mg, 0.8 mmol) and cesium carbonate (318 mg, 1.0 mmol), wash the precipitate with dilute hydrochloric acid (1M, 50 mL) to remove residual cesium carbonate, then wash with water, ethanol and tetrahydrofuran several times. After filtration, the solid is freeze-dried for 24 h to obtain a yellow CTF.

[0030] Example 1:

[0031] In a round-bottom flask (25 mL), add 5 mg of CTF to 2 ml of DMF, ultrasonically mix, then add the prepared 1.2 ml of GaCl3 solution (148.6 mg of GaCl3 dissolved in 100 ml of DMF), ultrasonically mix the above solution, then heat in a water bath at 338K (65°C) and magnetically stir for 2 h, then add the prepared 30 μl of malonic acid solution (176 mg dissolved in 10 ml of DMF) in batches with an interval of 30 min for a total of 5 times, continue to react for 10 h, then filter to obtain a precipitate, wash the precipitate with DMF several times to remove residual GaCl3 in the precipitate, and finally dry in a vacuum drying oven for 12 h to obtain a covalent triazine framework / gallium composite.

[0032] Figure 1 a is the ORR graph of the sample of example 1, the starting potential is 0.79V vs RHE, the limiting current density is 1.05 mA cm -2 , the half-wave potential is 0.52V vs RHE, b is the hydrogen peroxide selectivity of the sample of example 1, and the hydrogen peroxide selectivity is 93%, Figure 1 c is the number of transferred electrons of the sample of example 1, and the number of transferred electrons is 2.13.

[0033] Figure 6The XRD pattern of the sample of Example 1, combined with the standard card, can obtain that gallium does not form compounds such as oxide salt, which indicates that gallium is chelated with CTF and small molecule chelating agent in the form of single atom and thus the crystal structure of CTF is basically maintained, and also indicates the successful preparation of the covalent triazine framework / gallium salt complex. Figure 7 The elemental Mapping of the sample of Example 1 is as follows: Figure 8 The XPS pattern of the sample of Example 1 is as follows, in which the content of Ga element is 5.88%.

[0034] Example 2:

[0035] In a round-bottom flask (25 mL), 5 mg of CTF was dissolved in 2 ml of DMF, and after being uniformly ultrasonicated, 1.5 ml of prepared GaCl3 solution (148.6 mg of GaCl3 was dissolved in 100 ml of DMF) was added. After the above solution was uniformly ultrasonicated, it was heated in a water bath at 338 K and magnetically stirred for 2 h, and then 37.5 μl of prepared malonic acid solution (176 mg was dissolved in 10 ml of DMF) was added in batches at intervals of 30 min for a total of 5 times, and the reaction was continued for 10 h. After filtration, the precipitate was washed several times with DMF to remove residual GaCl3 in the precipitate, and finally dried in a vacuum drying box for 12 hours to obtain the covalent triazine framework / gallium complex.

[0036] Figure 2 a is the ORR pattern of the sample of Example 2, the starting potential is 0.78 V vs RHE, the limiting current density is 0.99 mA cm -2 , and the half-wave potential is 0.48 V vs RHE, Figure 2 b is the hydrogen peroxide selectivity of the sample of Example 2, which can obtain that the hydrogen peroxide selectivity is 89%, Figure 2 c is the number of transferred electrons of the sample of Example 2, which can obtain that the number of transferred electrons is 2.25.

[0037] Example 3:

[0038] In a round-bottom flask (25 mL), 5 mg of CTF was dissolved in 2 ml of methanol, and after being uniformly ultrasonicated, 2 ml of prepared GaCl3 solution (148.6 mg of GaCl3 was dissolved in 100 ml of DMF) was added. After the above solution was uniformly ultrasonicated, it was heated in a water bath at 338 K and magnetically stirred for 2 h, and then 40 μl of prepared oxalic acid solution (152 mg was dissolved in 10 ml of DMF) was added in batches at intervals of 30 min for a total of 5 times, and the reaction was continued for 10 h. After filtration, the precipitate was washed several times with methanol and DMF to remove residual GaCl3 in the precipitate, and finally dried in a vacuum drying box for 12 hours to obtain the covalent triazine framework / gallium complex.

[0039] Figure 3a is the ORR graph of the sample of Example 3, the onset potential is 0.79 V vs RHE, the limiting current density is 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 of Example 3, it can be concluded that the hydrogen peroxide selectivity is 86%, Figure 3 c is the number of transferred electrons of the sample of Example 3, it can be concluded that the number of transferred electrons is 2.26.

[0040] Figure 6 is the XRD graph of the sample of Example 3, combined with standard cards, it can be concluded that gallium does not form oxide salt compounds, indicating that gallium is chelated with CTF and small molecule chelating agent in the form of single atom, thus maintaining the crystal structure of CTF, and also indicating the successful preparation of covalent triazine framework / gallium salt complex. Figure 9 is the element Mapping graph of the sample of Example 3, Figure 10 is the XPS graph of the sample of Example 3, in which the percentage of Ga atoms is 2.68%

[0041] Example 4:

[0042] In a round-bottom flask (25 mL), 5 mg of CTF was dissolved in 2 ml of methanol, and after ultrasonic homogenization, 1.2 ml of prepared GaCl3 solution (148.6 mg of GaCl3 dissolved in 100 ml of DMF) was added. After ultrasonic homogenization of the above solution, water bath heating at 338 K and magnetic stirring reaction for 2 h, 60 μl of prepared benzoic acid solution (53.3 g dissolved in 10 ml of DMF) was added in batches with an interval of 30 min for a total of 5 times, and the reaction was continued for 10 h. After filtration, the precipitate was washed with methanol and DMF several times to remove residual GaCl3 in the precipitate. Finally, it was placed in a vacuum drying oven for 12 hours to obtain a covalent triazine framework / gallium complex.

[0043] Figure 4 a is the ORR graph of the sample of Example 4, the onset potential is 0.81 V vs RHE, the limiting current density is 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 of Example 4, it can be concluded that the hydrogen peroxide selectivity is 82%, Figure 4 c is the number of transferred electrons of the sample of Example 4, it can be concluded that the number of transferred electrons is 2.38.

[0044] Example 5:

[0045] In a round bottom flask (25 mL) was added 5 mg of CTF dissolved in 2 ml of methanol, after ultrasonic homogenization, the prepared 1.2 ml of GaCl3 solution (148.6 mg of GaCl3 dissolved in 100 ml of DMF) was added, the above solution was ultrasonically homogenized, then water bath heating at 338 K, magnetic stirring reaction for 2 h, then the prepared 60 μl of phenanthrenequinone solution (83.3 g dissolved in 10 ml of DMF) was added in batches with an interval of 30 min for 5 times, the reaction was continued for 10 h, then the precipitate was obtained by filtration, the precipitate was washed with methanol and DMF for several times to remove the residual GaCl3 in the precipitate, finally it was put into a vacuum drying box for drying for 12 hours to obtain a covalent triazine framework / gallium composite. Figure 5 a is the ORR graph of the sample of Example 5, the starting potential is 0.79 V vs RHE, and the limiting current density is 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 of Example 5, it can be concluded that the hydrogen peroxide selectivity is 75%, Figure 5 c is the number of transferred electrons of the sample of Example 5, it can be concluded that the number of transferred electrons is 2.5.

Claims

1. A method for the preparation of covalent triazine framework / weak acid gallium salt complexes, characterized by, The preparation method comprises the following steps: S1, uniformly dispersing a covalent triazine framework (CTF) and a soluble gallium salt in a non-aqueous polar solvent to obtain a dispersion liquid; S2, adding a chelating agent to the dispersion liquid in S1 in batches for multiple times, uniformly ultrasonicating, and then performing a heating and stirring reaction; S3, filtering, washing, and drying to obtain a catalyst, wherein the preparation method of the CTF is adding a benzene monomer biphenyl-4,4'-diformamide or p-phenylenediamine and cesium carbonate to [2,2'-bipyridyl]-5,5'-diylmethanol, precipitating, washing, and freeze-drying the obtained solid product to obtain the CTF; and the chelating agent is selected from one or more of malonic acid, oxalic acid, benzoic acid, and phenanthrenequinone.

2. The production method according to claim 1, characterized by, The dispersibility of the CTF described in S1 in a non-aqueous polar solvent is 1-2 g / L -1 .

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

4. The method of claim 1, wherein, The non-aqueous polar solvent comprises methanol, tetrahydrofuran, or N-N dimethylformamide.

5. 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.

6. A covalent triazine framework / gallium salt complex prepared by the preparation method in any one of claims 1-5.

7. An electrocatalytic hydrogen peroxide production catalyst characterized by, The catalyst is a covalent triazine framework / gallium salt complex prepared by the preparation method in any one of claims 1-5.

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