Co-doped carbon nitride / phosphomolybdic acid composite catalyst as well as preparation method and application thereof

Abstract

The invention relates to a co-doped carbon nitride / phosphomolybdic acid composite catalyst as well as a preparation method and application thereof. The composite catalyst is composed of co-doped carbon nitride and phosphomolybdic acid loaded on the co-doped carbon nitride; the co-doped carbon nitride is graphite phase carbon nitride co-doped with sodium, phosphorus and oxygen, and the co-doped carbon nitride is prepared by mixing a cyanamide compound and sodium hexametaphosphate and calcining at high temperature. The composite catalyst is used for photocatalytic production of hydrogen peroxide, and a method for photocatalytic production of hydrogen peroxide comprises the following steps: dispersing the co-doped carbon nitride / phosphomolybdic acid composite catalyst in an aqueous solution containing a sacrificial agent, and performing photocatalytic preparation of hydrogen peroxide under irradiation of a xenon lamp. Compared with the prior art, the preparation method provided by the invention is simple and good in repeatability, the charge separation efficiency of the prepared photocatalytic material is remarkably improved, and the photocatalytic hydrogen peroxide preparation efficiency is high.

Description

Technical Field

[0001] The present invention belongs to the technical field of catalysts, and in particular relates to a co-doped carbon nitride / phosphomolybdic acid composite catalyst, a preparation method thereof, and an application thereof. Background Art

[0002] As an environmentally friendly oxidant, hydrogen peroxide is widely used in fields such as disinfection, organic synthesis, sewage treatment, bleaching, and energy. At present, the main method for producing hydrogen peroxide industrially is the anthraquinone method, but it has many inevitable drawbacks, such as complex production processes, high energy consumption, and serious environmental pollution. Therefore, there is an urgent need for a low-carbon and environmentally friendly hydrogen peroxide production method. Photocatalytic production of hydrogen peroxide is a simple, green, and sustainable production method that converts water and oxygen into hydrogen peroxide using solar energy, which has received extensive attention from researchers. Therefore, the development of highly efficient, stable, and low-cost photocatalysts has become a research hotspot in this field.

[0003] Graphitic carbon nitride (g-C3N4) is an environmentally friendly and low-cost semiconductor material with a unique two-dimensional conjugated structure, high chemical stability, easy modification, and excellent redox properties. Therefore, g-C3N4 shows great potential in the field of photocatalysis. However, g-C3N4 has disadvantages such as insufficient visible light absorption ability, small specific surface area, and low separation efficiency of photogenerated carriers, which greatly reduces its photocatalytic production efficiency of hydrogen peroxide.

[0004] CN116726968A discloses a potassium and multi-type iodine co-doped carbon nitride, a preparation method thereof, and an application thereof, which improves the photocatalytic production rate of hydrogen peroxide by introducing potassium and multi-type iodine. However, the separate element co-doping still limits the transfer efficiency of photogenerated electrons, and the photocatalytic production efficiency of hydrogen peroxide still needs to be improved.

[0005] Therefore, aiming at the problems of existing co-doped carbon nitride in photocatalytic production of hydrogen peroxide, it is urgent to develop a new type of g-C3N4 modified catalyst to further improve the photocatalytic production efficiency of hydrogen peroxide. Summary of the Invention

[0006] The purpose of the present invention is to provide a co-doped carbon nitride / phosphomolybdic acid composite catalyst, a preparation method thereof, and an application thereof in order to further improve the photocatalytic production efficiency of co-doped carbon nitride for hydrogen peroxide.

[0007] The purpose of the present invention can be achieved by the following technical solutions:

[0008] The present invention first provides a co-doped carbon nitride / phosphomolybdic acid composite catalyst, which is composed of co-doped carbon nitride and phosphomolybdic acid supported on the co-doped carbon nitride;

[0009] The co-doped carbon nitride is sodium, phosphorus, and oxygen co-doped graphitic carbon nitride, and the co-doped carbon nitride is obtained by mixing a cyanamide compound and sodium hexametaphosphate and then calcining at a high temperature.

[0010] Further, the mass ratio of phosphomolybdic acid to co-doped carbon nitride is (0.1 - 0.5):1.

[0011] Further, in the co-doped carbon nitride, the doping rate of sodium atoms is 2% - 5%, the doping rate of phosphorus atoms is 8% - 12%, and the doping rate of oxygen atoms is 18% - 22%.

[0012] Further, the particle size of the co-doped carbon nitride / phosphomolybdic acid composite catalyst is 1 - 20 μm.

[0013] Further, the co-doped carbon nitride / phosphomolybdic acid composite catalyst has a layered structure and a rough surface.

[0014] The present invention also provides a method for preparing a co-doped carbon nitride / phosphomolybdic acid composite catalyst, comprising the following steps:

[0015] S1: Mix a cyanamide compound and sodium hexametaphosphate and then calcine to obtain co-doped carbon nitride;

[0016] S2: Disperse the co-doped carbon nitride obtained in S1 in water to obtain a suspension;

[0017] S3: Add an aqueous solution of phosphomolybdic acid to the suspension obtained in S2 for reaction, and dry the product to obtain the co-doped carbon nitride / phosphomolybdic acid composite catalyst.

[0018] Further, in step S1, the mass ratio of the cyanamide compound to sodium hexametaphosphate is (0.2 - 0.6):1.

[0019] Further, in step S1, the cyanamide compound includes one or more of melamine or dicyandiamide.

[0020] Further, in step S1, the calcination temperature is 500 - 550 °C, the heating rate of calcination is 2 - 5 °C / min, and the calcination time is 4 - 5 h.

[0021] Further, in step S2, the concentration of the suspension is 5 - 10 g / L.

[0022] Further, in step S3, the reaction is carried out under stirring conditions, and the reaction time is 10 - 12 h.

[0023] Further, in step S3, the drying temperature is 100 - 110 °C, and the drying time is 6 - 8 h.

[0024] The present invention also provides an application of a co-doped carbon nitride / molybdophosphoric acid composite catalyst, and the composite catalyst is used for photocatalytic production of hydrogen peroxide; the method for photocatalytic production of hydrogen peroxide is as follows: dispersing the co-doped carbon nitride / molybdophosphoric acid composite catalyst in an aqueous solution containing a sacrificial agent, and photocatalytically preparing hydrogen peroxide under the irradiation of a xenon lamp.

[0025] Further, the sacrificial agent includes one or more of ethanol, methanol, isopropanol, formic acid, and lactic acid.

[0026] Further, the addition amount of the co-doped carbon nitride / molybdophosphoric acid composite catalyst is 0.5 - 1.5 g / L.

[0027] Further, the power of the xenon lamp is 100 - 500 W, preferably 300 W.

[0028] Further, the photocatalytic time is 2 - 6 h.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] (1) For the sodium, phosphorus, and oxygen co-doped carbon nitride / molybdophosphoric acid composite catalyst prepared by the present invention, the electronic energy level structure of the catalyst can be adjusted by element doping to improve the separation efficiency of photogenerated carriers; at the same time, the loaded molybdophosphoric acid has excellent electron acceptance and donation abilities, which can further transfer photogenerated electrons, inhibit the recombination of photogenerated carriers, and further improve the photocatalytic efficiency.

[0031] (2) For the sodium, phosphorus, and oxygen co-doped carbon nitride / molybdophosphoric acid composite catalyst prepared by the present invention, a photocatalytic hydrogen peroxide production test is carried out. The results show that after 4 h of illumination, the hydrogen peroxide concentration can be > 500 μmol / L, while the hydrogen peroxide photocatalytically generated by g-C3N4 prepared under the same conditions is only 52.3 μmol / L.

[0032] (3) The preparation method of the present invention is simple and easy to implement, has strong repeatability, can be mass-produced, and has an industrial application prospect. Description of the Drawings

[0033] Figure 1 It is the scanning electron microscope image of CN in Comparative Example 1.

[0034] Figure 2 It is the scanning electron microscope image of CN-NaPO-PMA-30 in Example 3.

[0035] Figure 3 It is the full X-ray photoelectron spectroscopy spectrum of CN-NaPO-PMA-30 in Example 3.

[0036] Figure 4X-ray photoelectron spectroscopy high-resolution spectrum of CN-NaPO-PMA-30 in Example 3.

[0037] Figure 5 Infrared spectra of CN, CN-NaPO, and CN-NaPO-PMA-30 in Comparative Example 1, Comparative Example 2, and Example 3.

[0038] Figure 6 UV-visible spectra of CN, CN-NaPO, and CN-NaPO-PMA-30 in Comparative Example 1, Comparative Example 2, and Example 3.

[0039] Figure 7 Band gaps of CN, CN-NaPO, and CN-NaPO-PMA-30 in Comparative Example 1, Comparative Example 2, and Example 3.

[0040] Figure 8 Performance graphs of photocatalytic preparation of hydrogen peroxide by the catalyst materials prepared in the examples and comparative examples.

[0041] Figure 9 Hydrogen peroxide content prepared by photocatalyzing CN-NaPO-PMA-30 in Example 3 for 4 h under different atmospheres.

[0042] Figure 10 Cyclic stability of photocatalytic preparation of hydrogen peroxide by CN-NaPO-PMA-30 in Example 3. Detailed implementation manners

[0043] The present invention will be described in detail below with reference to the accompanying drawings and specific examples. This example is implemented on the premise of the technical solution of the present invention, and the detailed implementation manners and specific operation processes are given, but the protection scope of the present invention is not limited to the following examples.

[0044] Unless otherwise specified, the reagents, methods, instruments, and equipment used in the present invention are conventional reagents, methods, instruments, and equipment in the art. Unless otherwise specified, the reagents and materials used in the following examples are all commercially available. Among them, dicyandiamide (analytical pure) is purchased from Sinopharm Chemical Reagent Co., Ltd.; sodium hexametaphosphate (analytical pure) is purchased from Sinopharm Chemical Reagent Co., Ltd.; phosphomolybdic acid hydrate (analytical pure) is purchased from Shanghai Titan Scientific Co., Ltd.

[0045] Example 1:

[0046] This example provides a co-doped carbon nitride / phosphomolybdic acid composite catalyst, and the specific preparation method is as follows:

[0047] (1) Weigh 2 g of dicyandiamide and 0.6 g of sodium hexametaphosphate, grind and mix them evenly in a mortar for 30 min, transfer them to a crucible, and calcine them in a muffle furnace at 550 °C for 4 h with a heating rate of 2 °C / min to obtain sodium, phosphorus, and oxygen co-doped carbon nitride (CN-NaPO).

[0048] (2) Weigh 200 mg of CN-NaPO, add 20 mL of deionized water, and ultrasonically disperse it for 30 min to obtain a CN-NaPO suspension.

[0049] (3) Dissolve 20 mg of phosphomolybdic acid (PMA) in 10 mL of deionized water, slowly add it to the CN-NaPO suspension, stir at room temperature for 12 h, then place it in an oven at 100 °C for drying. Wash the obtained solid three times with deionized water, and then dry it in an oven at 60 °C to obtain a composite catalyst material loaded with phosphomolybdic acid, denoted as CN-NaPO-PMA-10. Here, 10 represents that the mass ratio of the added phosphomolybdic acid to CN-NaPO is 10%.

[0050] Example 2:

[0051] This example provides a co-doped carbon nitride / phosphomolybdic acid composite catalyst. The difference from Example 1 is that in this example, 40 mg of phosphomolybdic acid is added to the CN-NaPO suspension, and the finally obtained composite catalyst material is denoted as CN-NaPO-PMA-20.

[0052] Example 3:

[0053] This example provides a co-doped carbon nitride / phosphomolybdic acid composite catalyst. The difference from Example 1 is that in this example, 60 mg of phosphomolybdic acid is added to the CN-NaPO suspension, and the finally obtained composite catalyst material is denoted as CN-NaPO-PMA-30.

[0054] Example 4:

[0055] This example provides a co-doped carbon nitride / phosphomolybdic acid composite catalyst. The difference from Example 1 is that in this example, 80 mg of phosphomolybdic acid is added to the CN-NaPO suspension, and the finally obtained composite catalyst material is denoted as CN-NaPO-PMA-40.

[0056] Example 5:

[0057] This example provides a co-doped carbon nitride / phosphomolybdic acid composite catalyst. The difference from Example 1 is that in this example, 100 mg of phosphomolybdic acid is added to the CN-NaPO suspension, and the finally obtained composite catalyst material is denoted as CN-NaPO-PMA-50.

[0058] Comparative Example 1:

[0059] This comparative example is only graphitic carbon nitride CN (i.e., g-C3N4), and the preparation method is as follows: Weigh 2 g of dicyandiamide, transfer it to a crucible, place it in a muffle furnace and calcine it at 550 °C for 4 h with a heating rate of 2 °C / min to obtain graphitic carbon nitride, denoted as CN.

[0060] Comparative Example 2:

[0061] This comparative example is only sodium, phosphorus, and oxygen co-doped carbon nitride CN-NaPO, that is, phosphomolybdic acid is not added. The preparation method is as follows: Weigh 2 g of dicyandiamide and 0.6 g of sodium hexametaphosphate, grind and mix them evenly in a mortar for 30 min, transfer them to a crucible, place it in a muffle furnace and calcine it at 550 °C for 4 h with a heating rate of 2 °C / min to obtain sodium, phosphorus, and oxygen co-doped carbon nitride (CN-NaPO).

[0062] The morphologies of CN prepared in Comparative Example 1 of the present invention and CN-NaPO-PMA-30 prepared in Example 3 were characterized by scanning electron microscopy, and the results are shown in Figure 1 and Figure 2 respectively. It can be seen from the figure that CN has a layered structure and its surface is relatively smooth and flat. CN-NaPO-PMA-30 also has a layered structure, but its surface becomes rough and many particles are attached.

[0063] X-ray photoelectron spectroscopy (XPS) was used to test CN-NaPO-PMA-30 prepared in Example 3 of the present invention to characterize the elemental composition and chemical states of each element of CN-NaPO-PMA-30. The full XPS spectrum is shown in Figure 3 and the high-resolution spectrum is shown in Figure 4 respectively. According to the XPS data, CN-NaPO-PMA-30 is composed of C, N, O, P, Na, and Mo, and the element ratios are 29.23%, 35.52%, 20.87%, 9.44%, 3.69%, and 1.25% respectively.

[0064] Infrared spectroscopy was used to test CN-NaPO-PMA-30 prepared in Example 3 of the present invention and the catalyst materials prepared in Comparative Example 1 and Comparative Example 2. The results are shown in Figure 5 respectively. The peaks near 3000 - 3300 cm -1 correspond to the stretching vibrations of -OH and N-H, the peak at 2173 cm -1 is attributed to the asymmetric stretching vibration of C≡N, and the absorption peaks in the range of 1200 - 1700 cm -1 are attributed to the stretching vibrations of C-N and C=N. Compared with CN and CN-NaPO, the peak of CN-NaPO-PMA-30 at 890 cm -1 is enhanced, which is caused by Mo-O in the loaded phosphomolybdic acid. The peak at 810 cm -1The peak at [location] corresponds to the stretching vibration of the tri-s-triazine ring. After loading phosphomolybdic acid, the peak intensity weakens because the combination of phosphomolybdic acid and carbon nitride causes partial changes in the triazine ring structure.

[0065] UV-visible spectroscopy tests were carried out on the CN-NaPO-PMA-30 prepared in Example 3 of the present invention and the catalyst materials prepared in Comparative Example 1 and Comparative Example 2. The results are as Figure 6 shown. After carbon nitride is doped with sodium, phosphorus, and oxygen, the UV absorption increases and there is a slight red shift. After loading phosphomolybdic acid, the UV absorption further increases and the red shift is more obvious, which is beneficial to the improvement of photocatalytic activity. The band gaps of CN, CN-NaPO, and CN-NaPO-PMA-30 are as Figure 7 shown, which are 2.66 eV, 2.59 eV, and 2.54 eV respectively. The decrease in the band gap is beneficial to the electron excitation transition and improves the photoelectric conversion efficiency.

[0066] The catalyst materials prepared in the above examples and comparative examples were used for photocatalytic preparation of hydrogen peroxide. The specific steps are as follows: Add 30 mg of the catalyst and 30 mL of 10% isopropyl alcohol aqueous solution into a quartz beaker, ultrasonicate for 15 min under dark conditions, then stir for 15 min, irradiate with a 300 W xenon lamp, continuously introduce O2 into the reaction system, take 1 mL of the solution at certain intervals, filter it with a 0.22 μm microporous filter membrane, and determine the concentration of hydrogen peroxide by the iodometric method.

[0067] The results are as Figure 8 shown. After 4 h of illumination, the concentrations of hydrogen peroxide prepared by photocatalysis in Examples 1-5 are significantly higher than those in Comparative Example 1 and Comparative Example 2. Among them, the concentrations of hydrogen peroxide prepared by photocatalysis of CN, CN-NaPO, and CN-NaPO-PMA-30 are 52.3 μmol / L, 195.8 μmol / L, and 557.6 μmol / L respectively. Compared with CN, the yield of hydrogen peroxide prepared by photocatalysis of CN-NaPO increases significantly, indicating that the doping of sodium, phosphorus, and oxygen improves the catalytic activity of carbon nitride. After loading phosphomolybdic acid, the yield of hydrogen peroxide prepared by photocatalysis of CN-NaPO-PMA-30 further increases significantly, indicating that the phosphomolybdic acid loaded on carbon nitride improves the transfer efficiency of photogenerated electrons, inhibits the recombination of photogenerated carriers, and improves the catalytic efficiency.

[0068] Under the condition that other test conditions remain unchanged, the performance of CN-NaPO-PMA-30 in photocatalytic preparation of hydrogen peroxide under different atmospheres was investigated. The results are as Figure 9 shown. The amount of hydrogen peroxide prepared by photocatalysis is the largest under oxygen conditions, and only 61.7 μmol / L under nitrogen conditions, indicating that hydrogen peroxide is mainly generated by oxygen reduction.

[0069] In addition, the catalytic cycle stability of CN-NaPO-PMA-30 was also investigated. After the photocatalytic preparation of hydrogen peroxide using CN-NaPO-PMA-30, the catalyst was recovered by centrifugation, washed with deionized water, dried, and then tested for the photocatalytic preparation of hydrogen peroxide again. This process was repeated 4 times to evaluate the catalytic cycle stability of CN-NaPO-PMA-30. The results are as Figure 10 shown. After 4 catalytic cycles, the concentration of hydrogen peroxide prepared by photocatalysis using CN-NaPO-PMA-30 was 512.9 μmol / L, the hydrogen peroxide production remained at 92%, and the catalytic activity did not decrease significantly, indicating that CN-NaPO-PMA-30 has good catalytic cycle stability.

[0070] In summary, for the sodium, phosphorus, and oxygen co-doped carbon nitride / phosphomolybdic acid composite catalyst prepared in the present invention, element doping can adjust the electronic energy level structure of the catalyst, improve the separation efficiency of photogenerated carriers. At the same time, the supported phosphomolybdic acid has excellent electron accepting and donating abilities, which can further transfer photogenerated electrons, inhibit the recombination of photogenerated carriers, and improve the photocatalytic efficiency. In the test of photocatalytic preparation of hydrogen peroxide, the results showed that after 4 hours of illumination, the concentration of hydrogen peroxide reached 557.6 μmol / L, while the hydrogen peroxide photocatalytically produced by g-C3N4 prepared under the same conditions was only 52.3 μmol / L, indicating that the co-doped carbon nitride / phosphomolybdic acid composite catalyst prepared in the present invention has high photocatalytic activity.

[0071] The above description of the embodiments is provided to enable those of ordinary skill in the art to understand and use the invention. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative efforts. Therefore, the present invention is not limited to the above embodiments, and the improvements and modifications made by those skilled in the art without departing from the scope of the present invention should be within the protection scope of the present invention.

Claims

1. A co-doped carbon nitride / phosphomolybdic acid composite catalyst, characterized in that: The composite catalyst is composed of co-doped carbon nitride and phosphomolybdic acid supported on the co-doped carbon nitride; The co-doped carbon nitride is a graphite phase carbon nitride co-doped with sodium, phosphorus and oxygen. The co-doped carbon nitride is prepared by mixing a cyanamide compound and sodium hexametaphosphate and then calcining at a high temperature.

2. A co-doped carbon nitride / phosphomolybdic acid composite catalyst according to claim 1, characterized in that: The mass ratio of the phosphomolybdic acid to the co-doped carbon nitride is (0.1-0.5):

1.

3. The co-doped carbon nitride / phosphomolybdic acid composite catalyst according to claim 1, characterized in that: In the co-doped carbon nitride, the doping rate of sodium atoms is 2%-5%, the doping rate of phosphorus atoms is 8%-12%, and the doping rate of oxygen atoms is 18%-22%.

4. The co-doped carbon nitride / phosphomolybdic acid composite catalyst according to claim 1, characterized in that: The particle size of the co-doped carbon nitride / phosphomolybdic acid composite catalyst is 1-20 μm.

5. A method for preparing the co-doped carbon nitride / phosphomolybdic acid composite catalyst according to any one of claims 1 to 4, characterized in that: The following steps are involved: S1: mixing the cyanamide compound and sodium hexametaphosphate and calcining them to obtain co-doped carbon nitride; S2: dispersing the co-doped carbon nitride prepared in S1 in water to obtain a suspension; S3: adding an aqueous solution of phosphomolybdic acid to the suspension obtained in S2 to carry out a reaction, and drying the product to obtain the co-doped carbon nitride / phosphomolybdic acid composite catalyst.

6. The method for preparing a co-doped carbon nitride / phosphomolybdic acid composite catalyst according to claim 5, characterized in that: In step S1, the mass ratio of the cyanamide compound to sodium hexametaphosphate is (0.2-0.6):1, and the cyanamide compound includes one or more of melamine or dicyandiamide; The calcination temperature is 500-550°C, the calcination heating rate is 2-5°C / min, and the calcination time is 4-5h.

7. The method for preparing a co-doped carbon nitride / phosphomolybdic acid composite catalyst according to claim 5, characterized in that: In step S2, the concentration of the suspension is 5-10 g / L.

8. The method for preparing a co-doped carbon nitride / phosphomolybdic acid composite catalyst according to claim 5, characterized in that: In step S3, the reaction is carried out under stirring conditions for 10-12 hours; The drying temperature is 100-110° C., and the drying time is 6-8 hours.

9. An application of the co-doped carbon nitride / phosphomolybdic acid composite catalyst according to any one of claims 1 to 4, characterized in that: The composite catalyst is used for photocatalytic production of hydrogen peroxide; The method for producing hydrogen peroxide by photocatalysis is: The co-doped carbon nitride / phosphomolybdic acid composite catalyst is dispersed in an aqueous solution containing a sacrificial agent, and hydrogen peroxide is prepared by photocatalysis under the irradiation of a xenon lamp.

10. The use of a co-doped carbon nitride / phosphomolybdic acid composite catalyst according to claim 9, characterized in that: The sacrificial agent includes one or more of ethanol, methanol, isopropanol, formic acid and lactic acid; The addition amount of the co-doped carbon nitride / phosphomolybdic acid composite catalyst is 0.5-1.5 g / L; The power of the xenon lamp is 100-500W, and the photocatalytic time is 2-6h.

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