Single-atom cobalt-loaded carbon nanofiber cathode and preparation method and application thereof

Abstract

The application specifically relates to a single-atom cobalt-loaded carbon nanofiber cathode and a preparation method and application thereof. The preparation method of the cathode comprises the following steps: 1) preparing a spinning solution with a mass concentration of 10-30 wt% by using a solvent for electrospinning of a precursor, dissolving a cobalt precursor in the spinning solution, and then preparing nanofibers by electrospinning of the spinning solution; and 2) placing the nanofibers in an oven for pre-oxidation and in a tubular furnace for high-temperature carbonization to obtain the single-atom cobalt-loaded carbon nanofiber cathode. The prepared single-atom cobalt-loaded carbon nanofiber cathode has uniform single-atom cobalt particle size loaded on the carbon nanofiber, and in a two-electron oxygen reduction reaction for production of hydrogen peroxide, the cathode has the characteristics of high activity, high selectivity and high stability, and can be used for efficient degradation of various types of antibiotics in wastewater in an electro-Fenton system.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of environmental functional materials and electrochemical water treatment technology, and relates to a carbon nanofiber cathode for electro-Fenton, in particular to preparation and application of a single-atom cobalt-loaded carbon nanofiber cathode. BACKGROUND

[0002] With the continuous development of social economy, environmental pollution has become a problem of great concern to mankind, especially the widespread use of antibiotics. It is estimated that the global annual consumption of antibiotics will be between 1x10 5 and 2x10 5 tons from 2010 to 2030. A large amount of antibiotics and their degradation intermediates enter the water environment, and the presence of antibiotics has been detected in rivers, lakes, seawater and even drinking water. Due to the extremely low natural degradation rate of antibiotics, the food chain of organisms can be changed, which poses a serious health risk to aquatic animals and even humans. Therefore, it is urgent to develop effective treatment technology for wastewater containing antibiotics to quickly and efficiently remove antibiotics.

[0003] Electrochemical advanced oxidation processes are widely used in wastewater treatment because they can generate strong oxidizing free radicals that can effectively degrade antibiotics, and have the advantages of high energy efficiency, easy operation, environmental friendliness and strong versatility. Electro-Fenton is one of the most promising electrochemical advanced oxidation processes. In the electro-Fenton system, H2O2 can be continuously generated by driving with green electricity, and Fe 3+ is reduced to Fe 2+ , so that the generated H2O2 reacts rapidly with Fe 2+ to produce a large amount of strong oxidizing free radicals, thereby efficiently removing antibiotics. Compared with traditional Fenton technology, electro-Fenton technology does not require the addition of H2O2, which can reduce the storage and transportation costs of H2O2 drugs, and the Fe 2+ produced by electrode reduction can be used to maintain the continuous reaction, thereby reducing the amount of iron sludge and the cost of secondary pollution treatment. In addition, the electro-Fenton technology has high treatment efficiency and relatively short required residence time, which helps to improve the efficiency and treatment capacity of wastewater treatment. The properties of the cathode electrode in the electro-Fenton system determine the generation of H2O2, and the amount of H2O2 further affects the generation of strong oxidizing free radicals. Therefore, designing a cathode electrode that can efficiently generate and utilize H2O2 is crucial to improving the degradation efficiency of antibiotics in the electro-Fenton system.

[0004] Monatomic catalysts have extremely high atom utilization, especially metal monatomic catalysts anchored on nitrogen-doped carbon substrates, which can be designed and synthesized to fully expose active sites, increase the number of active sites and intrinsic activity. For example, cobalt (Co) monatomic catalysts anchored on nitrogen-doped carbon catalysts show great potential for H2O2 electro-synthesis through the dominant 2-electron oxygen reduction pathway in acidic environments, and are expected to be used as effective cathode materials for electro-Fenton degradation of antibiotics. However, monatomic atoms have very high surface energy and migration ability, and are prone to agglomeration to form nanoparticles with lower surface energy and larger size during catalysis, thereby reducing their catalytic performance. Carbon nanofibers obtained by electrospinning coupled with carbonization can be used as effective carriers for monatomic catalysts to prepare cathode materials, which not only improve the electrical conductivity, but also prevent the movement and aggregation of monatomic atoms to maintain stable performance.

[0005] In the prior art, a cobalt monatomic catalyst with cobalt atoms anchored on carbon nanofibers is disclosed in Chinese patent application CN 109119649 A, which has good 4-electron oxygen reduction reaction and oxygen evolution catalytic performance, and is applied to fuel cells and metal-air batteries. However, the obtained composite catalyst is mainly used in the energy field, which is significantly different from the catalyst for generating hydrogen peroxide through 2-electron oxygen reduction reaction. The latter requires optimal adsorption energy for specific oxygen-containing intermediates to improve its activity and selectivity for H2O2 generation. CN 113832574 A discloses a coordination atom-doped porous carbon fiber confined transition metal monatomic material, but this method requires the use of hydrofluoric acid etching to remove the SiO2 hard template, which is dangerous and complex. SUMMARY

[0006] The present application aims to overcome the deficiencies in the prior art and provides a single-atom cobalt-loaded carbon nanofiber cathode for degrading antibiotics based on electro-Fenton reaction, as well as a preparation method and application thereof.

[0007] The present application is achieved by the following technical solution: a single-atom cobalt-loaded carbon nanofiber cathode for degrading antibiotics based on electro-Fenton reaction, which has high H2O2 generation activity and selectivity, and can be effectively used for degrading various antibiotics through electro-Fenton reaction. The present application obtains the single-atom cobalt-loaded carbon nanofiber cathode through electrospinning coupled with carbonization process, which has a self-supporting structure and good mechanical properties. By applying an external Fe 2+ The H2O2 generated by the electrode can generate a variety of active oxygen such as hydroxyl radicals, superoxide radicals and singlet oxygen, thereby efficiently degrading various antibiotics such as tetracycline hydrochloride, levofloxacin and sulfadiazine. The preparation method of the cathode comprises the following steps:

[0008] (1) Preparation of cobalt-containing precursor

[0009] The cobalt nitrate hexahydrate and 2-methyl imidazole are respectively added into methanol, stirred at room temperature, the precipitate is separated from the solution by high-speed centrifugation, the precipitate is washed with methanol, and then freeze-dried to obtain a cobalt-containing precursor;

[0010] (2) Preparation of cobalt-containing nanofiber

[0011] The cobalt-containing precursor prepared in step (1) is dissolved in an N,N-dimethylformamide solution together with polyacrylonitrile, and electrospinning is performed after stirring to obtain a cobalt-containing nanofiber.

[0012] (3) Preparation of single-atom cobalt-loaded carbon nanofiber cathode

[0013] The cobalt-containing nanofiber prepared in step (2) is pre-oxidized in an oven at 240-250 DEG C, then loaded into a graphite box and placed in a tube furnace for calcination and carbonization at 900-1000 DEG C, and after washing and drying, a single-atom cobalt-loaded carbon nanofiber cathode (Co@N-CNFs) is obtained.

[0014] In the step (1), the mass ratio of the cobalt nitrate hexahydrate and 2-methyl imidazole is 1:4-1:8, and the volume of the methanol is 300 mL.

[0015] In the step (2), the mass ratio of the cobalt-containing precursor and polyacrylonitrile is 1:2-1:4, and the volume of the N,N-dimethylformamide solution is 30 mL.

[0016] In the step (2), the process parameters of the electrospinning process are as follows: the spinning speed is 1.2-1.8 mL / h, the distance from the collector to the needle is 13-17 cm, the spinning voltage is 18-20 kV, the temperature is 30±2 DEG C, and the humidity is 30±10 %.

[0017] A single-atom cobalt-loaded carbon nanofiber cathode (Co@N-CNFs) is prepared by the above-mentioned method.

[0018] The single-atom cobalt-loaded carbon nanofiber cathode, the preparation method and the application thereof have the following beneficial effects:

[0019] 1. The present application uses cobalt metal salt and 2-methyl imidazole as reaction precursors, and uniformly loads single-atom catalyst on carbon nanofiber through electrospinning and high-temperature calcination. The reaction precursor preparation process is simple, and a high-temperature and high-pressure reaction kettle is not needed. The reaction precursor serves as a cobalt source when carbonized, so that cobalt is uniformly loaded on carbon nanofiber in the form of single atoms, and the fully exposed active sites exhibit excellent catalytic effect.

[0020] 2、The prepared single-atom cobalt-loaded carbon nanofiber cathode is a self-supporting electrode with good mechanical properties, which can be directly used as a cathode, while most powder-type single-atom catalysts need to be made into a slurry and coated on a carrier for use.

[0021] 3、The single-atom cobalt-loaded carbon nanofiber cathode (Co@N-CNFs) is used as a cathode electrode in the electro-Fenton method to degrade and remove the common antibiotic tetracycline hydrochloride in water environment, and shows extremely high degradation efficiency, with a removal rate of more than 99% in 5 minutes, and the removal rate still being more than 99% after 10 cycles, and good reusability. In addition, the method also shows high degradation capacity for antibiotics such as levofloxacin and sulfadiazine. The method is green and safe, simple to operate, and has good application prospect. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 is a scanning electron microscope image of the single-atom cobalt-loaded carbon nanofiber cathode.

[0023] Figure 2 is a high-angle annular dark field scanning transmission electron microscope image of the single-atom cobalt-loaded carbon nanofiber cathode.

[0024] Figure 3 is a H2O2 production performance graph of the single-atom cobalt-loaded carbon nanofiber cathode.

[0025] Figure 4 is a tetracycline hydrochloride degradation effect graph of the single-atom cobalt-loaded carbon nanofiber cathode.

[0026] Figure 5 is a 10-time repeated tetracycline hydrochloride degradation effect graph of the single-atom cobalt-loaded carbon nanofiber cathode.

[0027] Figure 6 is a degradation of different types of antibiotics effect graph of the single-atom cobalt-loaded carbon nanofiber cathode. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical scheme and advantages of the embodiments of the present application more clear and explicit, the technical scheme in the embodiments of the present application will be described clearly, completely and meticulously below, but the technical content of the present application is not limited to the range described. When the specific technology or condition is not specified in the embodiments, the technology or condition described in the literature or according to the product manual is used. When the manufacturer of all reagents or instruments is not specified, it is a conventional product that can be obtained from the market.

[0029] The single-atom cobalt-loaded carbon nanofiber cathode (Co@N-CNFs) and its preparation method and application will be described in detail below.

[0030] Example 1, Preparation of single-atom cobalt-loaded carbon nanofiber cathode (Co@N-CNFs)

[0031] The preparation method of the single-atom cobalt-loaded carbon nanofiber cathode (Co@N-CNFs) provided by the embodiment of the present application comprises:

[0032] (1) Preparation of cobalt-containing precursor

[0033] 4g of cobalt nitrate hexahydrate and 25g of 2-methylimidazole were added to 300mL of methanol respectively, and stirred at room temperature. The precipitate was separated from the solution by high-speed centrifugation, and the precipitate was washed with methanol and freeze-dried to obtain the cobalt-containing precursor.

[0034] (2) Preparation of cobalt-containing nanofiber

[0035] 1g of the cobalt-containing precursor prepared in step (1) and 3g of polyacrylonitrile were dissolved in 30mL of N,N-dimethylformamide solution, and then electrospun after stirring. The spinning speed was 1.6mL / h, the distance between the collector and the needle was 15cm, the spinning voltage was 19kV, the temperature was 30℃, and the humidity was 30%, to obtain the cobalt-containing nanofiber.

[0036] (3) Preparation of single-atom cobalt-loaded carbon nanofiber cathode

[0037] The cobalt-containing nanofiber prepared in step (2) was pre-oxidized in an oven at 250℃, and then loaded into a graphite box and calcined at 1000℃ in a tube furnace. After cleaning and drying, the single-atom cobalt-loaded carbon nanofiber cathode (Co@N-CNFs) was obtained.

[0038] The single-atom cobalt-loaded carbon nanofiber cathode (Co@N-CNFs) was observed by scanning electron microscopy, as shown in Figure 1 The surface morphology of Co@N-CNFs is shown in the figure. The surface of Co@N-CNFs is relatively smooth, and there are some granular structures, which are formed by carbonization of cobalt-containing precursor particles. Figure 2 The morphology of Co@N-CNFs under high-angle annular dark field scanning transmission electron microscopy is shown in the figure. There are bright white spots (highlighted with gray circles), which indicate that atomic-level dispersed cobalt metal is successfully formed on Co@N-CNFs.

[0039] Example 2, H2O2 production performance of single-atom cobalt-loaded carbon nanofiber cathode (Co@N-CNFs)

[0040] The above obtained Co@N-CNFs prepared in Example 1 were used as the cathode, a platinum sheet electrode was used as the anode, and a mercury sulfate electrode was used as the reference electrode. A hydrogen peroxide production capacity determination experiment was performed in an H-type electrolytic cell with Na2SO4(0.1 mol / L, pH = 3) as the supporting electrolyte. The concentration of H2O2 was determined by the potassium titanium oxalate spectrophotometric method.

[0041] The efficiency of electro-Fenton is significantly affected by the amount of H2O2 generated by electricity, as shown in Figure 3 , with the gradual increase of the voltage applied to the cathode, the H2O2 yield increased from 39.3 mg·L -1 ·h -1 to 807.3 mg·L -1 ·h -1 , while the current efficiency decreased slightly from 82.0% to 68.5%. This shows that the Co@N-CNFs prepared by the present application have good H2O2 production capacity and good selectivity for 2-electron oxygen reduction reaction.

[0042] Example 3, Performance of Single-Atom Cobalt Carbon Nanofiber Cathode (Co@N-CNFs) in Degradation of Tetracycline Hydrochloride

[0043] The above obtained Co@N-CNFs prepared in Example 1 were used as the cathode, a platinum sheet electrode was used as the anode, and a mercury sulfate electrode was used as the reference electrode. A hydrogen peroxide production capacity determination experiment was performed in an H-type electrolytic cell with Na2SO4(0.1 mol / L, pH = 3) as the supporting electrolyte, and the concentration of tetracycline hydrochloride was 30 mg / L. The concentration of tetracycline hydrochloride was determined by high performance liquid chromatography, and 10 repeated tetracycline hydrochloride degradation experiments were performed.

[0044] As shown in Figure 4 , with the gradual negative shift of the electrode potential applied to the cathode, the removal efficiency of Co@N-CNFs for tetracycline hydrochloride increased significantly, and within 30 minutes of reaction time, the removal rate of Co@N-CNFs for tetracycline hydrochloride increased from 78.0% to 100%. At the same time, Co@N-CNFs effectively removed more than 99% of tetracycline hydrochloride within 5 minutes at a rate of 1.063 min -1 under an electrode potential of -2V, showing its high efficiency in removing antibiotics. As shown in Figure 5 , 10 repeated tetracycline hydrochloride degradation experiments were performed, and the removal rate was greater than 99% in 10 min, and the removal rate was still greater than 99% after 10 cycles, indicating that Co@N-CNFs had good reusability.

[0045] Example 4, Performance of Single-Atom Cobalt Carbon Nanofiber Cathode (Co@N-CNFs) in Degradation of Different Types of Antibiotics

[0046] The above obtained Co@N-CNFs prepared in Example 1 were used as cathode, platinum sheet electrode as anode, and mercury sulfate electrode as reference electrode, and the degradation capacity determination experiment was carried out in multiple electrolytic cells with Na2SO4(0.1 mol / L, pH = 3) as supporting electrolyte, and the concentrations of tetracycline hydrochloride, levofloxacin and sulfadiazine were 30 mg / L respectively, and the concentration of antibiotics was determined by high performance liquid chromatography.

[0047] As shown in Figure 5 , Co@N-CNFs also have good performance in degrading levofloxacin and sulfadiazine. Levofloxacin and sulfadiazine were completely removed after 15 min of reaction, indicating that Co@N-CNFs have satisfactory catalytic degradation activity for various types of antibiotics.

Claims

1. A method for preparing a cathode loaded with single-atom cobalt carbon nanofibers, characterized in that: The cathode contains cobalt metal, nitrogen, and carbon elements, and the contact angle of the material exceeds 140°. The cathode, with its supported single-atom cobalt-carbon nanofibers, is a self-supporting structure. The nitrogen content is 1%–5%, and the cobalt element is atomically dispersed, with a loading of 0.01%–5%. The method for preparing the cathode is characterized by the following steps: (1) Preparation of cobalt-containing precursors: Cobalt nitrate hexahydrate and 2-methylimidazole were added to methanol and stirred thoroughly at room temperature. The precipitate was separated from the solution by high-speed centrifugation. The precipitate was then washed with methanol and freeze-dried to obtain the cobalt-containing precursor. (2) Preparation of cobalt-containing nanofibers: The cobalt-containing precursor obtained in step (1) and polyacrylonitrile were dissolved in N,N-dimethylformamide solution, stirred and then electrospun to obtain cobalt-containing nanofibers. (3) Preparation of cathodes supported on single-atom cobalt carbon nanofibers: The cobalt-containing nanofibers obtained in step (2) were pre-oxidized in an oven at 240-250°C, then placed in a graphite box and calcined in a tube furnace at 900-1000°C for carbonization. After cleaning and drying, a cathode loaded with single-atom cobalt carbon nanofibers (Co@N-CNFs) was obtained. The cathode is used as the working electrode in the electro-Fenton reaction system.

2. The preparation method according to claim 1, characterized in that, In step (1), the mass ratio of cobalt nitrate hexahydrate to 2-methylimidazole is in the range of 1:4 to 1:

8.

3. The preparation method according to claim 1, characterized in that, In step (2), the mass ratio of the cobalt-containing precursor to polyacrylonitrile is in the range of 1:2 to 1:

4.

4. The preparation method according to claim 1, characterized in that, In step (2), the process parameters for electrospinning are: spinning speed of 1.2 to 1.8 mL / h, distance from collector to needle of 13 to 17 cm, spinning voltage of 18 to 20 kV, temperature of 30 ± 2 ℃, and humidity of 30 ± 10%.

5. The cathode loaded with single-atom cobalt carbon nanofibers prepared by any of the preparation methods described in claims 1-4.

6. The use of a cathode supported on single-atom cobalt carbon nanofibers as described in claim 5, characterized in that, The cathode is used as the working electrode in an electro-Fenton reaction system to degrade antibiotic pollutants in water.

Images