Preparation method of coralline high-crystallinity carbon nitride

Abstract

The invention provides a preparation method of coralline high-crystallinity carbon nitride. The preparation method comprises the following steps: (1) grinding a nitrogen-rich precursor to control bulk density, and carrying out controllable calcination to obtain conical tubular carbon nitride; and (2) grinding and uniformly mixing the conical tubular carbon nitride and an alkaline metal mixed salt, carrying out controllable calcination, washing, and drying to obtain the target required coralline-shaped high-crystallization carbon nitride material. The method is simple and deep in exploration, the carbon nitride material obtained through morphology-crystal form-electron coordination control has unique morphology and excellent catalytic performance, and the problems of low specific surface area, poor carrier dynamics performance and the like of the carbon nitride material are solved. The coralline morphology of the target photocatalyst has high specific surface area, and active sites are increased; the co-doping of a triazine ring, cyano and potassium ions in the high-crystallinity molecular structure reduces the carrier recombination and increases the light utilization efficiency. The catalyst has outstanding photocatalytic reduction activity on water-soluble hexavalent chromium.

Description

Technical Field

[0001] The present invention belongs to the technical field of the preparation and application of photocatalytic materials, and particularly relates to a preparation method of coral-shaped highly crystalline carbon nitride. Background Art

[0002] Graphitic carbon nitride (g-C3N4) has been used in the field of photocatalysis due to its stable structure, adjustable band structure and good light response ability. However, due to its still having disadvantages such as low specific surface area, low charge separation and transfer efficiency, some modification treatments are still needed. Common modification methods are classified as: morphology regulation, doping engineering, defect engineering and heterostructure construction. Among them, the regulation of morphology can increase the specific surface area and porosity, which is the most direct and effective method to increase active sites and promote mass transfer. In terms of crystal structure, enhancing the crystallinity of the sample is an effective method to improve its light response ability. In addition, in the molecular structure, doping engineering is an important means to optimize carrier dynamics. Summary of the Invention

[0003] The present invention aims to enrich the morphology types and preparation methods of carbon nitride photocatalyst materials, and provides a preparation method and application of coral-shaped highly crystalline carbon nitride. By combining morphology-crystal form-electron regulation, problems such as low specific surface area and poor carrier dynamics performance are solved, and the photocatalytic degradation performance of carbon nitride materials is improved.

[0004] To achieve the above object, the present invention adopts the following technical solutions:

[0005] In the first aspect of the present invention, a preparation method of coral-shaped highly crystalline carbon nitride is provided, including the following steps:

[0006] (1) Grinding a nitrogen-rich precursor, controlling the bulk density, and obtaining tapered tubular carbon nitride after calcination;

[0007] (2) Mixing and grinding the tapered tubular carbon nitride and an alkaline metal mixed salt in a certain mass ratio, calcining, cooling to room temperature, washing multiple times to remove the excess mixed salt, filtering and drying to obtain coral-shaped highly crystalline carbon nitride.

[0008] Among them, in step (1), the calcination reaction is carried out in a tubular furnace under a nitrogen atmosphere; in step (2), the calcination reaction is carried out in a muffle furnace. Among them, in step (1), the nitrogen-rich precursor is a material rich in carbon and nitrogen elements and capable of thermal polymerization (such as melamine, urea, dicyandiamide, thiourea).

[0009] Wherein, in step (1), the calcination temperature is 450°C to 600°C (for example, 450°C, 500°C, 550°C, 600°C), the heating rate is 1 to 10°C / min (for example, 1, 2.5, 5, 8, 10°C / min), and the holding time is 0.5 to 4 hours.

[0010] Among them, in step (2), the alkaline metal mixed salt has various compositions (such as KCl-LiCl, KCl-NaCl, KCl-LiCl-NaCl).

[0011] Wherein, in step (2), the mass ratio of the alkaline metal mixed salt to the conical tubular carbon nitride is 5:1 to 20:1.

[0012] Wherein, in step (2), the calcination temperature is 450°C to 650°C (for example, 450°C, 500°C, 550°C, 600°C, 650°C), the heating rate is 5°C / min, and the holding time is 0.5 to 6 hours.

[0013] Wherein, in step (2), the mixture after calcination is ultrasonically washed with 100 mL of water for 15 minutes, filtered through a 0.45 μm water membrane, and the solid is collected and washed multiple times until the pH value of the final filtrate is 7. The washed carbon nitride powder is collected and dried in an oven at a drying temperature of 60° C. for 12 hours.

[0014] The coral-like highly crystalline carbon nitride prepared by the present invention has a tubular morphology with nano-thorns all over the surface, which is similar to a hollow coral structure.

[0015] The coral-like highly crystalline carbon nitride prepared by the present invention has significantly improved crystallinity, and at the same time, triazine rings, cyanide groups and potassium ions are doped, so the recombination of photogenerated electron-hole pairs is effectively suppressed. It has a strong reduction driving force during photocatalytic reaction, and can therefore be used for photocatalytic reduction of hexavalent chromium.

[0016] The second aspect of the present invention provides the use of carbon nitride prepared by any of the above methods in the photocatalytic reduction of hexavalent chromium.

[0017] Preferably, the application conditions are: the reaction temperature is 25° C., the concentration of hexavalent chromium is 100 mg / L, the concentration of coral-like highly crystalline carbon nitride is 0.4 g / L, and the solution is subjected to photocatalytic degradation for 20 minutes.

[0018] The preparation mechanism of the coral-like highly crystalline carbon nitride of the present invention is:

[0019] Cyanamide was used as a self-sacrificing gas template for calcination. Utilizing the characteristic that gas clusters accumulate and diffuse outward to form an air current during the high-temperature pyrolysis process, conical tubular carbon nitride was induced to be prepared; then, through the polymerization-promoting and exfoliating effects of the liquid environment in molten salt calcination on carbon nitride, coral-like highly crystalline carbon nitride was obtained.

[0020] Compared with the prior art, the advantages of the present invention are as follows:

[0021] (1) The present invention prepared carbon nitride with a unique coral-like morphology, and the specific surface area was increased by an order of magnitude, which is much higher than the bulk carbon nitride obtained by the one-step calcination method in other literatures (the specific surface area is less than 10 m 2 / g), significantly increasing the reactive active sites.

[0022] (2) The present invention utilizes the polymerization-promoting effect of molten salt calcination on carbon nitride to improve its crystallinity and effectively increase its response to visible light.

[0023] (3) The present invention utilizes the exfoliating effect of alkaline metal molten salt on carbon nitride to construct the co-doping of triazine rings, cyano groups and potassium ions, effectively improving the electronic structure of carbon nitride and promoting the separation and transfer of photo-generated carriers. Description of the Drawings

[0024] Figure 1 It is the scanning electron microscope image of the conical tubular carbon nitride prepared in Example 1 of the present invention and the coral-like highly crystalline carbon nitride prepared in Example 2;

[0025] Figure 2 It is the nitrogen adsorption-desorption curve of the conical tubular carbon nitride prepared in Example 1 of the present invention and the coral-like highly crystalline carbon nitride prepared in Example 2;

[0026] Figure 3 It is the X-ray diffraction spectrum of the conical tubular carbon nitride prepared in Example 1 of the present invention and the coral-like highly crystalline carbon nitride prepared in Example 2;

[0027] Figure 4 It is the Fourier transform infrared spectrum of the conical tubular carbon nitride prepared in Example 1 of the present invention and the coral-like highly crystalline carbon nitride prepared in Example 2;

[0028] Figure 5 It is the result diagram of the visible light catalytic degradation of hexavalent chromium by the conical tubular carbon nitride prepared in Example 1 of the present invention and the coral-like highly crystalline carbon nitride prepared in Example 2. Detailed Embodiments

[0029] To make the content of the present invention easier to understand, the technical solutions of the present invention will be further described below in conjunction with specific drawings and embodiments, but the present invention is not limited thereto.

[0030] In a first aspect of the present invention, a method for preparing coral-like highly crystalline carbon nitride is provided, comprising the following steps:

[0031] (1) Grind the nitrogen-rich precursor, control the bulk density, and obtain conical tubular carbon nitride after calcination.

[0032] Among them, the nitrogen-rich precursor is a material rich in carbon and nitrogen elements and capable of thermal polymerization (such as melamine, urea, dicyandiamide, thiourea). The calcination reaction is carried out in a tubular furnace under a nitrogen atmosphere, the calcination temperature is 450°C to 600°C (such as 450°C, 500°C, 550°C, 600°C), the heating rate is 1 to 10°C / minute (such as 1, 2.5, 5, 8, 10°C / minute), and the heat preservation time is 0.5 to 4 hours.

[0033] (2) Grind and mix the conical tubular carbon nitride and the alkaline metal mixed salt in a certain mass ratio, cool to room temperature after controlled calcination, wash repeatedly to remove the excess mixed salt, filter and dry to obtain coral-like highly crystalline carbon nitride. The calcination reaction is carried out in a muffle furnace. There are various compositions of the alkaline metal mixed salt (such as KCl-LiCl, KCl-NaCl, KCl-LiCl-NaCl). The mass ratio of the alkaline metal mixed salt to the conical tubular carbon nitride is 5:1 to 20:1. The calcination temperature is 450°C to 650°C (such as 450°C, 500°C, 550°C, 600°C, 650°C), the heating rate is 5°C / minute, and the heat preservation time is 0.5 to 6 hours. After calcination, the mixture is ultrasonically washed with 100 mL of water for 15 minutes, filtered through a 0.45 μm water membrane, and the solid is collected and washed repeatedly until the pH value of the last filtrate is 7. The washed carbon nitride powder is dried in an oven at 60°C for 12 hours, and finally the coral-like highly crystalline carbon nitride material is collected.

[0034] The technical solutions of the present invention will be further clarified below in conjunction with specific examples.

[0035] Example 1

[0036] Weigh 4 g of melamine, grind it to control the bulk density, place it in a porcelain boat, and calcine it at 500°C for 2 hours in a tubular furnace under nitrogen protection, with a heating rate of 5°C / minute. After cooling, grind it for 10 minutes to obtain conical tubular carbon nitride.

[0037] Take 20 mg of the prepared catalyst and place it in a beaker containing 50 mL of a 100 mg / L potassium dichromate solution. Sonicate for 20 minutes, stir in the dark for 20 minutes, and irradiate with a 300 W xenon lamp at a distance of 30 cm from the liquid surface for 100 minutes while stirring. Take samples every 10 minutes to test the content of hexavalent chromium.

[0038] Example 2

[0039] Weigh 4 g of melamine, grind it to control the bulk density, place it in a porcelain boat, and calcine it in a tube furnace under nitrogen protection at 500 °C for 2 hours with a heating rate of 5 °C / min. After cooling, grind it for 10 minutes to obtain conical tubular carbon nitride.

[0040] Weigh 400 mg of conical tubular carbon nitride, 2 g of potassium chloride, and 2 g of lithium chloride, and mix and grind them for 15 minutes. Transfer them to a 20 mL covered crucible, place it in a muffle furnace, and calcine it at 550 °C for 4 hours with a heating rate of 5 °C / min. After cooling, transfer the mixture to a beaker, add 100 mL of deionized water, sonicate for 15 minutes to fully dissolve the mixed salt, filter it with a 0.45 μm water membrane, collect the solid, and wash it repeatedly until the pH value of the last filtrate is about 7. Collect the washed solid powder and dry it at 60 °C for 12 hours to finally obtain a coral-like highly crystalline carbon nitride material.

[0041] Take 20 mg of the prepared catalyst and place it in a beaker containing 50 mL of a 100 mg / L potassium dichromate solution. Sonicate for 5 minutes, stir in the dark for 20 minutes, and irradiate with a 300 W xenon lamp at a distance of 30 cm from the liquid surface for 40 minutes while stirring. Take samples every 5 minutes to test the content of hexavalent chromium.

[0042] Figure 1 These are the scanning electron microscope images of the photocatalysts prepared in Examples 1 and 2 of the present invention. Example 1 has a conical tubular structure, and Example 2 has a coral-like structure with nanospines covering the tube surface.

[0043] Figure 2 These are the nitrogen adsorption-desorption isotherm curves of the photocatalysts prepared in Examples 1 and 2 of the present invention. By calculation, the specific surface area of Example 1 is 10.1 m 2 / g, and the specific surface area of Example 2 is 86.7 m 2 / g, achieving an order of magnitude improvement compared to traditional bulk carbon nitride (specific surface area less than 10 m 2 / g).

[0044] Figure 3This is the X-ray diffraction pattern of the photocatalysts prepared in Examples 1 and 2 of the present invention. In both Examples 1 and 2, there are diffraction peaks at 2θ≈27°, which belong to the (002) crystal plane of the g-C3N4 interlayer stacking. The (002) peak of Example 2 is slightly shifted to the right and significantly sharpened, indicating that the crystallinity of Example 2 is significantly enhanced and the interlayer spacing is reduced. Example 1 has a diffraction peak at 2θ≈13°, which belongs to the (100) crystal plane of the in-plane heptazine unit stacking of g-C3N4. The (100) characteristic peak of Example 2 is shifted to 2θ≈8°, indicating an increase in the unit spacing. Moreover, new peaks appear at 2θ≈20.9° and 32.1° in Example 2, and these all belong to the characteristic peaks of the triazine unit, indicating the presence of triazine ring doping in Example 2.

[0045] Figure 4 This is the Fourier transform infrared spectrum of the photocatalysts prepared in Examples 1 and 2 of the present invention. In both Examples 1 and 2, there are g-C3N4 characteristic peaks at 810 cm -1 , 1200 - 1720 cm -1 and 3000 - 3500 cm -1 . However, in Example 2, three new peaks appear at 1000 cm -1 , 1153 cm -1 and 2167 cm -1 , which belong to the asymmetric stretching vibrations of the N-K bond, the K-NC2 bond, and the cyano group respectively, indicating the presence of cyano group and potassium ion doping in Example 2.

[0046] Figure 5 This is the concentration change diagram of photocatalytic reduction of hexavalent chromium by the photocatalysts prepared in Examples 1 and 2 of the present invention. In Example 1, the complete degradation of hexavalent chromium was achieved in 90 minutes, and the calculated reaction rate constant k was 19.2×10 -3 / min. In Example 2, the complete degradation of hexavalent chromium was achieved in 15 minutes, and the calculated reaction rate constant k was 211.9×10 -3 / min. This is because the morphology-crystal form-electron synergistic regulation preparation of carbon nitride solves problems such as low specific surface area and poor carrier dynamics performance, and realizes high catalytic activity.

Claims

1. A coral-like highly crystalline carbon nitride material, characterized in that, It is a coral-like highly crystalline carbon nitride structure with nanospines covering the tube surface.

2. A preparation method of coral-like highly crystalline carbon nitride, characterized in that, It includes the following steps: (1) Grind the nitrogen-rich precursor, control the bulk density, and after calcination, obtain conical tubular carbon nitride. (2) Grind and mix the conical tubular carbon nitride obtained in step (1) with an alkaline metal mixed salt in a certain mass ratio, after controllable calcination, cool to room temperature, wash repeatedly to remove the excess mixed salt, filter and dry to obtain coral-like highly crystalline carbon nitride.

3. The preparation method of the coral-shaped highly crystalline carbon nitride according to claim 2, wherein, The calcination reaction in step (1) is carried out in a tubular furnace under a nitrogen atmosphere, and the calcination reaction in step (2) is carried out in a muffle furnace.

4. The preparation method of the coral-shaped highly crystalline carbon nitride according to claim 2, wherein, In step (1), the nitrogen-rich precursor is a material rich in carbon and nitrogen elements and capable of thermal polymerization (such as melamine, urea, dicyandiamide, thiourea).

5. The preparation method of the coral-shaped highly crystalline carbon nitride according to claim 2, wherein, In step (1), control the bulk density, the calcination temperature is 450 °C to 600 °C (such as 450 °C, 500 °C, 550 °C, 600 °C), the heating rate is 1 to 10 °C / minute (such as 1, 2.5, 5, 8, 10 °C / minute), and the holding time is 0.5 to 4 hours.

6. The preparation method of the coral-like highly crystalline carbon nitride according to claim 2, characterized in that, In step (2), the composition of the alkaline metal mixed salt has various types (such as KCl-LiCl, KCl-NaCl, KCl-LiCl-NaCl).

7. The preparation method of the coral-like highly crystalline carbon nitride according to claim 2, characterized in that In step (2), the mass ratio of the mixed salt to the conical tubular carbon nitride is 5:1 to 20:

1.

8. The preparation method of the coral-like highly crystalline carbon nitride according to claim 2, characterized in that, In step (2), the calcination temperature is 450 °C to 650 °C (such as 450 °C, 500 °C, 550 °C, 600 °C, 650 °C), the heating rate is 5 °C / minute, and the holding time is 0.5 to 6 hours.

9. The preparation method of the coral-like highly crystalline carbon nitride according to claim 2, wherein In step (2), add 100 mL of water to the calcined mixture and ultrasonically wash for 15 minutes, filter through a 0.45 μm water membrane, collect the solid and wash repeatedly until the pH value of the last filtrate is about 7. The washed carbon nitride powder is collected and dried in an oven at a drying temperature of 60 °C and a drying time of 12 hours.

10. A coral-like highly crystalline carbon nitride prepared by the preparation method according to any one of claims 1 to 9 is applied to photocatalytic degradation of hexavalent chromium in sewage, and the reaction conditions are: the reaction temperature is 25 °C, the concentration of hexavalent chromium is 100 mg / L, the concentration of a coral-like highly crystalline carbon nitride added is 0.4 g / L, and the above solution is photocatalytically degraded under light for 40 minutes.

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