High-crystallinity phosphorus-doped carbon nitride photocatalyst as well as preparation method and application thereof

Abstract

The invention relates to the technical field of photocatalysis, and discloses a high-crystallinity phosphorus-doped carbon nitride photocatalyst and a preparation method and application thereof, the preparation method comprises the following steps: (1) mixing melamine, phosphoric acid and water to obtain a mixed solution; (2) carrying out hydrothermal reaction on the mixed solution, and then drying to obtain an intermediate product; (3) mixing the intermediate product with mixed salt, and then calcining; wherein the mixed salt comprises potassium chloride and lithium chloride. The photocatalytic activity of g-C3N4 is greatly improved through crystallization degree optimization and phosphorus doping synergistic effect, so that the obtained phosphorus-crystalline carbon nitride photocatalyst has excellent photocatalytic activity, can be widely applied to various application scenes of photocatalysis, and particularly can be applied to photocatalytic degradation of NOx and photocatalytic degradation of organic matters.

Description

Technical Field

[0001] The present invention relates to the field of photocatalysis technology, and particularly relates to a phosphorus-doped graphitic carbon nitride photocatalyst with high crystallinity, a preparation method thereof, and an application thereof. Background Art

[0002] Graphitic carbon nitride (g-C 3 N 4 ) is a non-metallic, two-dimensional conjugated polymer semiconductor composed of tri-s-triazine structural units, with a band gap of about 2.70 eV and an optical absorption limit of 460 nm. Due to its non-metallic property, visible light responsiveness, physical and chemical stability, simple preparation method, and inexpensive and easily available raw materials, it has become a research hotspot in the field of photocatalysis in recent years and has been widely used in the fields of photocatalytic hydrogen production, photocatalytic degradation of nitrogen oxides (NOx), and photocatalytic degradation of organic matter.

[0003] g-C 3 N 4 The traditional preparation method of g-C 3 N 4 is the thermal polycondensation method, that is, precursors such as cyanamide, dicyandiamide, melamine, ammonium thiocyanate, urea, or thiourea are calcined at a temperature of 550 - 600 °C; during this process, the precursor molecules gradually condense and polymerize by removing small molecules, and finally bulk g-C 3 N 4 is formed. This preparation method is simple to operate. However, during the traditional thermal polymerization process, it is controlled by reaction kinetics, and the synthesized original g-C 3 N 4 has problems such as poor crystallinity, narrow light absorption range, and serious recombination of photo-generated carriers, resulting in poor photocatalytic performance and greatly restricting its application in the field of photocatalysis. Summary of the Invention

[0004] The purpose of the present invention is to overcome the problems existing in the original g-C 3 N 4 such as poor crystallinity, narrow light absorption range, and serious recombination of photo-generated carriers, resulting in poor photocatalytic performance, and to provide a phosphorus-doped graphitic carbon nitride photocatalyst with high crystallinity, a preparation method thereof, and an application thereof.

[0005] To achieve the above purpose, on the one hand, the present invention provides a preparation method of a phosphorus-doped graphitic carbon nitride photocatalyst, the method comprising: (1) Mixing melamine, phosphoric acid, and water to obtain a mixed solution; (2) Performing a hydrothermal reaction on the mixed solution, and then drying to obtain an intermediate product; (3) Mixing the intermediate product with a mixed salt, and then calcining; wherein, the mixed salt comprises lithium chloride and potassium chloride.

[0006] Preferably, in step (1), the dosage ratio of melamine to phosphoric acid is 4 g: 0.5 - 2 mL; Preferably, in step (1), the dosage ratio of melamine to water is 4 g: 50 - 100 mL.

[0007] Preferably, in step (1), the mixing is carried out under stirring, and the stirring time is 20 - 40 min.

[0008] Preferably, in step (2), the conditions of the hydrothermal reaction include: the temperature is 150 - 200 °C, and the time is 5 - 15 h.

[0009] Preferably, in step (2), the drying temperature is 40 - 60 °C.

[0010] Preferably, in step (3), the weight ratio of the dosage of the intermediate product to the mixed salt is 1:8 - 12.

[0011] Preferably, in the mixed salt of step (3), the weight ratio of the dosage of lithium chloride to potassium chloride is 55 - 65:41.

[0012] Preferably, in step (3), the conditions of the calcination include: the temperature is 500 - 600 °C, and the time is 3 - 5 h; Preferably, step (3) further includes: washing and drying the product obtained by calcination.

[0013] In the second aspect of the present invention, a phosphorus-doped graphitic carbon nitride photocatalyst is provided, and the phosphorus-doped graphitic carbon nitride photocatalyst is prepared by the method as described above.

[0014] In the third aspect of the present invention, an application of the phosphorus-doped graphitic carbon nitride photocatalyst as described above in photocatalytic degradation of NOx is provided.

[0015] In the technical solution provided by the present invention, melamine is selected as the precursor and phosphoric acid is used as the doping agent for phosphorus element. Based on the induction of phosphoric acid under hydrothermal conditions, part of melamine is hydrolyzed in-situ into cyanuric acid, and then melamine, cyanuric acid and phosphoric acid achieve self-assembly of the supramolecular precursor through hydrogen bond interaction to obtain an intermediate product. The intermediate product is optimized in crystallinity by the molten salt method to obtain a phosphorus-doped g-C 3 N 4 photocatalyst. Through the synergistic effect of crystallinity optimization and phosphorus doping, the present invention greatly improves the photocatalytic activity of g-C 3 N 4 , so that the obtained phosphorus-doped graphitic carbon nitride photocatalyst has excellent photocatalytic activity and can be widely applied to various application scenarios of photocatalysis, especially in photocatalytic degradation of NOx and photocatalytic degradation of organic substances. Description of the Drawings

[0016] Figure 1 XRD patterns of the phosphorus-doped carbon nitride photocatalyst prepared in Examples 1-3 of the present invention and the pristine carbon nitride prepared in Comparative Example 1; Figure 2 Photocatalytic NOx degradation performance of the products prepared in Example 1 of the present invention and Comparative Examples 1-3; Figure 3 Photocatalytic organic matter degradation performance of the products prepared in Examples 1-2 of the present invention and Comparative Example 1. Detailed Description of the Invention

[0017] The following provides a detailed description of the specific embodiments of the present invention with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the present invention.

[0018] In the ranges disclosed herein, the endpoints and any value are not limited to the exact range or value, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of each range, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be considered to be specifically disclosed herein.

[0019] The present invention provides a method for preparing a highly crystalline phosphorus-doped carbon nitride photocatalyst, which comprises the following steps: (1) Mixing melamine, phosphoric acid and water to obtain a mixed solution; (2) Subjecting the mixed solution to a hydrothermal reaction and then drying to obtain an intermediate product; (3) Mixing the intermediate product with a mixed salt and then calcining; wherein the mixed salt comprises lithium chloride and potassium chloride.

[0020] In the method of the present invention, the highly crystalline g-C 3 N 4 prepared by the molten salt method has fewer bulk phase defects, a more ordered structure, a higher degree of polymerization and a more extended conjugated plane, which is beneficial to promoting the transfer of photo-generated carriers, reducing the recombination of electron-hole pairs, and ultimately enhancing the photocatalytic activity of highly crystalline g-C 3 N 4 ; At the same time, by doping phosphorus into g-C 3 N 4 , the visible light spectral response range of g-C 3 N 4 can be broadened. The present invention simultaneously solves the problems of the original g-C 3 N 4Numerous drawbacks such as poor crystallinity, narrow visible light absorption range, severe carrier recombination, and small specific surface area significantly limit the photocatalytic activity of g-C 3 N 4 .

[0021] In a preferred embodiment, in step (1), the dosage ratio of melamine to phosphoric acid is 4 g: 0.5 - 2 mL. Specifically, for example, it can be 4 g: 0.5 mL, 4 g: 0.6 mL, 4 g: 0.7 mL, 4 g: 1 mL, 4 g: 1.2 mL, 4 g: 1.5 mL, or 4 g: 2 mL. At the above doping levels, the prepared photocatalyst has better photocatalytic activity.

[0022] In the method of the present invention, the water used in step (1) is distilled water.

[0023] In some embodiments, in step (1), the dosage ratio of melamine to water is 4 g: 50 - 100 mL, preferably 50 - 60 mL.

[0024] In some embodiments, in step (1), the mixing is carried out under stirring, and the stirring time is 20 - 40 min. Specifically, step (1) includes: adding melamine and water to the reactor, and then adding phosphoric acid, and stirring for 20 - 40 min.

[0025] In order to make the degree of self-assembly of the supramolecular precursor induced by the hydrothermal reaction appropriate, in a preferred embodiment, in step (2), the conditions of the hydrothermal reaction include: the temperature is 150 - 200 °C, and the time is 5 - 15 h. Specifically, the temperature of the hydrothermal reaction can be 150 °C, 160 °C, 170 °C, 180 °C, 190 °C, or 200 °C, preferably 170 - 190 °C; the time of the hydrothermal reaction can be 5 h, 6 h, 7 h, 85 h, 10 h, 12 h, 13 h, or 15 h, preferably 8 - 12 h.

[0026] For the drying temperature in step (2), the present invention does not make a special limitation. In a specific embodiment, in step (2), the drying temperature is 40 - 60 °C. The present invention also does not limit the specific drying time, as long as the water is evaporated completely. Specifically, the drying time is 24 - 36 h.

[0027] The molten salt method refers to preparing a reaction mixture by mixing a mixed salt and raw materials in a certain proportion. After mixing evenly, the salt is heated to melt, and the reactants undergo a chemical reaction in the molten salt. In the method described in the present invention, the mixed salt package is lithium chloride and potassium chloride. In a preferred embodiment, in step (3), in the mixed salt, the weight ratio of the amounts of lithium chloride and potassium chloride is 55-65:41. By selecting the mixed salt with the above-mentioned component ratio, the prepared phosphorus-doped carbon nitride photocatalyst has higher crystallinity.

[0028] In the most preferred embodiment, in step (3), in the mixed salt, the weight ratio of the amounts of lithium chloride and potassium chloride is 59:41. In this way, the prepared phosphorus-doped carbon nitride photocatalyst has the highest crystallinity.

[0029] In a preferred embodiment, in step (3), the weight ratio of the amount of the intermediate product to the mixed salt is 1:8-12. Specifically, for example, it can be 1:8, 1:9, 1:10, 1:11 or 1:12, and the most preferred is 1:10.

[0030] In the method described in the present invention, in step (3), the calcination temperature can be 500-600 °C. Specifically, for example, it can be 500 °C, 510 °C, 520 °C, 530 °C, 540 °C, 550 °C, 570 °C, 580 °C or 600 °C, and the preferred is 540-560 °C.

[0031] In the method described in the present invention, in step (3), the calcination time can be 3-5 h, and the preferred is 3.5-4.5 h.

[0032] In some embodiments, step (3) further includes: washing and drying the product obtained by calcination.

[0033] The present invention also provides a phosphorus-doped carbon nitride photocatalyst, and the phosphorus-doped carbon nitride photocatalyst is prepared by the method as described above. The preparation method of the phosphorus-doped carbon nitride photocatalyst adopts a strategy of synergistic modification of crystallinity optimization and non-metal doping. Therefore, the phosphorus-doped carbon nitride photocatalyst prepared by this method has excellent photocatalytic activity.

[0034] Since the phosphorus-doped carbon nitride photocatalyst provided by the present invention has excellent photocatalytic activity, it can be widely used in the field of photocatalysis. For example, photocatalytic hydrogen production, photocatalytic degradation of NOx and organic substances, etc. Based on this, the present invention also provides the application of the phosphorus-doped carbon nitride photocatalyst as described above in photocatalytic degradation of NOx.

[0035] In addition, the present invention also provides the application of the phosphorus-doped carbon nitride photocatalyst as described above in photocatalytic degradation of organic substances.

[0036] The present invention will be described in detail below through embodiments, but the protection scope of the present invention is not limited thereto. The experimental methods in the following embodiments are all conventional methods in the art unless otherwise specified. The experimental materials used in the following embodiments are all commercially available products unless otherwise specified.

[0037] Example 1 This example is used to illustrate the phosphorus-doped carbon nitride photocatalyst and its preparation method according to the present invention.

[0038] (1) Add 4 g of melamine and 60 ml of water to a reactor, then add 0.5 ml of phosphoric acid, and stir for 30 min to obtain a mixed solution; (2) Hydrothermally react the mixed solution at 180 °C for 10 h, and then dry it at 50 °C for 24 h to obtain an intermediate product; (3) Mix the intermediate product obtained by drying in step (2) with a mixed salt (LiCl:KCl = 59:41) in a mass ratio of 1:10, calcine it in a muffle furnace at 550 °C for 4 h, and then wash and dry the obtained product to obtain a phosphorus-doped carbon nitride photocatalyst.

[0039] Example 2 This example is used to illustrate the phosphorus-doped carbon nitride photocatalyst and its preparation method according to the present invention.

[0040] (1) Add 4 g of melamine and 60 ml of water to a reactor, then add 1 ml of phosphoric acid, and stir for 30 min to obtain a mixed solution; (2) Hydrothermally react the mixed solution at 180 °C for 10 h, and then dry it at 50 °C for 24 h to obtain an intermediate product; (3) Mix the intermediate product obtained by drying in step (2) with a mixed salt (LiCl:KCl = 59:41) in a mass ratio of 1:10, calcine it in a muffle furnace at 550 °C for 4 h, and then wash and dry the obtained product to obtain a phosphorus-doped carbon nitride photocatalyst.

[0041] Example 3 This example is used to illustrate the phosphorus-doped carbon nitride photocatalyst and its preparation method according to the present invention.

[0042] (1) Add 4 g of melamine and 60 ml of water to a reactor, then add 1.5 ml of phosphoric acid, and stir for 30 min to obtain a mixed solution; (2) Hydrothermally react the mixed solution at 180 °C for 10 h, and then dry it at 50 °C for 24 h to obtain an intermediate product; (3) Mix the intermediate product obtained by drying in step (2) with the mixed salt (LiCl:KCl = 59:41) at a mass ratio of 1:10, calcine at 550 °C in a muffle furnace for 4 h, then wash and dry the obtained product to obtain the phosphorus-doped carbon nitride photocatalyst.

[0043] Comparative Example 1 Raw carbon nitride: Melamine is calcined at 550 °C in a muffle furnace for 4 h to obtain the raw carbon nitride product.

[0044] Comparative Example 2 (1) Add 4 g of melamine and 60 ml of water to the reactor, stir for 30 min to obtain a mixed solution; (2) Hydrothermally react the mixed solution at 180 °C for 10 h, then dry at 50 °C for 24 h to obtain an intermediate product; (3) Mix the intermediate product obtained by drying in step (2) with the mixed salt (LiCl:KCl = 59:41) at a mass ratio of 1:10, calcine at 550 °C in a muffle furnace for 4 h, then wash and dry the obtained product to obtain a product, denoted as "raw carbon nitride + molten salt".

[0045] Comparative Example 3 (1) Add 4 g of melamine and 60 ml of water to the reactor, then add 0.5 ml of phosphoric acid, stir for 30 min to obtain a mixed solution; (2) Hydrothermally react the mixed solution at 180 °C for 10 h, then dry at 50 °C for 24 h to obtain an intermediate product; (3) Calcinate the intermediate product obtained by drying in step (2) at 550 °C in a muffle furnace for 4 h, then wash and dry the obtained product to obtain a product, denoted as "phosphorus doping + calcination".

[0046] Test Example 1 Perform X-ray diffraction (XRD) on the phosphorus-doped carbon nitride photocatalysts prepared in Examples 1-3 and the raw carbon nitride prepared in Comparative Example 1, and the results are as Figure 1 shown.

[0047] It can be seen from Figure 1 that the phosphorus-doped carbon nitride products with high crystallinity were successfully prepared in the examples of the present invention.

[0048] Test Example 2 (1) The photocatalytic NOx degradation performance of the product obtained in Example 1 (phosphorus-doped carbon nitride photocatalyst) and the products obtained in Comparative Examples 1-3 was tested. The test method was as follows: The prepared catalyst was sprayed onto a clean frosted glass with dimensions of 70 mm × 40 mm, and then the sample was dried at 60 °C. The steps of spraying and drying were repeated and weighed. Finally, the mass of the photocatalyst coated on the glass slide was controlled to be 14 mg. The flow rates of standard NO gas (concentration 50 ppm, balance gas N 2 ), and high-purity O 2 were controlled by a dynamic gas mixer so that the NO concentration in the gas flowing into the reactor was 1000 ppb, and the gas flow rate was stabilized at 500 mL / min. First, the reactor was placed in a dark environment, and the NO concentration at the reactor outlet was monitored in real time by a NO analyzer. When the NO concentration was stable, it indicated that the TiO 2 material on the sample surface reached the "adsorption-desorption" equilibrium. At this time, the xenon lamp was turned on, and the current and distance were adjusted so that the light intensity on the sample surface was 30 mw / cm 2 . Then, the NO concentration at the outlet was monitored by a NO analyzer. The test results are as Figure 2 shown. In the figure, c is the NO concentration at the outlet, and c o is the NO concentration at the inlet.

[0049] It can be seen from Figure 2 that the phosphorus-doped carbon nitride photocatalyst prepared in Example 1 of the present invention has a better effect on photocatalytic removal of NOx, indicating that the phosphorus-doped carbon nitride photocatalyst prepared by the method provided by the present invention has higher catalytic activity.

[0050] (2) The photocatalytic organic matter degradation performance of the products obtained in Examples 1-2 (phosphorus-doped carbon nitride photocatalysts) and the product obtained in Comparative Example 1 was tested. The test method was as follows: 30 mg of photocatalyst was added to 400 ml of 20 mg / L rhodamine B solution. After dark treatment, it was degraded under a xenon lamp. The degradation effect was measured with a UV-visible spectrophotometer every 30 min. The test results are as Figure 3 shown. In the figure, c is the concentration of the rhodamine B solution after degradation, and c o is the initial concentration of the rhodamine B solution.

[0051] It can be seen from Figure 3 that the phosphorus-doped carbon nitride photocatalyst prepared in the examples of the present invention has a better effect on catalytic degradation of organic matter, indicating that the phosphorus-doped carbon nitride photocatalyst prepared by the method provided by the present invention has higher catalytic activity.

[0052] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including the combination of each technical feature in any other suitable manner. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A method for preparing a highly crystalline phosphorus-doped carbon nitride photocatalyst, characterized in that: The method includes: (1) mixing melamine, phosphoric acid and water to obtain a mixed solution; (2) subjecting the mixed solution to a hydrothermal reaction, and then drying to obtain an intermediate product; (3) mixing the intermediate product with a mixed salt and then calcining; Wherein, the mixed salt contains lithium chloride and potassium chloride.

2. The method according to claim 1, characterized in that In step (1), the ratio of melamine to phosphoric acid is 4 g: 0.5-2 mL; and / or, In step (1), the usage ratio of melamine to water is 4 g:50-100 mL.

3. The method according to claim 1 or 2, characterized in that: In step (1), the mixing is carried out under stirring, and the stirring time is 20 to 40 minutes.

4. The method according to any one of claims 1 to 3, characterized in that: In step (2), the conditions of the hydrothermal reaction include: temperature of 150-200° C. and time of 5-15 h.

5. The method according to claim 1, characterized in that In step (2), the drying temperature is 40-60°C.

6. The method according to any one of claims 1 to 5, characterized in that: In step (3), the weight ratio of the intermediate product to the mixed salt is 1:8-12.

7. The method according to claim 1 or 7, characterized in that: In step (3), in the mixed salt, the weight ratio of the lithium chloride to the potassium chloride is 55-65:

41.

8. The method according to any one of claims 1 to 7, characterized in that: In step (3), the calcination conditions include: temperature of 500-600°C and time of 3-5h; and / or, Step (3) also includes: washing and drying the calcined product.

9. A phosphorus-doped carbon nitride photocatalyst, characterized in that: The phosphorus-doped carbon nitride photocatalyst is prepared by the method according to any one of claims 1 to 8.

10. Use of the phosphorus-doped carbon nitride photocatalyst according to claim 9 in photocatalytic degradation of NOx.

Images