Highly crystallized carbon nitride, preparation method thereof and application of highly crystallized carbon nitride in photocatalytic production of hydrogen peroxide

Abstract

The application provides high-crystalline carbon nitride, a preparation method thereof and application thereof in photocatalytic production of hydrogen peroxide. The preparation method of the high-crystalline carbon nitride comprises the following steps: S1: calcining melamine under a nitrogen atmosphere, and cooling, washing and drying after calcination to obtain bulk-phase carbon nitride; and S2: uniformly mixing potassium chloride, lithium chloride and the bulk-phase carbon nitride, and then calcining under a nitrogen atmosphere, and cooling, washing and drying after calcination to obtain high-crystalline carbon nitride. The high-crystalline carbon nitride has high crystallinity, a large surface area and more active sites, and has excellent photocatalytic performance, which is conducive to efficient photocatalytic production of hydrogen peroxide.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of photocatalytic material preparation, in particular to a high-crystalline carbon nitride, a preparation method thereof and application thereof in photocatalytic production of hydrogen peroxide. BACKGROUND

[0002] Hydrogen peroxide (H2O2) is a multifunctional green oxidant that plays an important role in water treatment processes, and is widely used in disinfection and pollutant degradation due to its ability to transform into highly oxidizing hydroxyl radicals. Compared to the traditional anthraquinone method for synthesizing hydrogen peroxide, the generation of H2O2 by photocatalytic selective two-electron oxygen reduction has the unique advantage of environmental sustainability, so photocatalysis is considered a promising method for sustainable production of H2O2. However, the cost-effectiveness of practical solar applications is a barrier, and the abundant photocatalyst components on Earth and the efficient photonic chemical conversion process are two key factors affecting cost-effectiveness. Therefore, designing efficient and low-cost photocatalysts is a major challenge to achieve sustainable H2O2 production.

[0003] Polymeric carbon nitride (PCN) has been widely used in various solar-driven chemical conversion reactions, such as water splitting, CO2 reduction, organic chemical conversion reactions, etc. In addition, due to the selective 2e - ORR was found to have a kinetic advantage, so the PCN framework is particularly suitable for solar production of hydrogen peroxide. Although PCN has unique advantages such as abundant raw material sources, chemical stability, non-toxicity and visible light activity, the PCN framework has a low dielectric constant, and the exciton is localized on the triazine unit, and the polaron jumps through the overlapping molecular orbitals in the direction perpendicular to the triazine unit in Brownian motion, and the dissociation efficiency of the exciton is low. Therefore, a method is expected to improve the solar-chemical conversion efficiency by expanding the visible light absorption and promoting the dissociation of the exciton.

[0004] In view of this, the present application is provided. SUMMARY

[0005] The purpose of the present application is to provide a high-crystalline carbon nitride, a preparation method thereof and application thereof in photocatalytic production of hydrogen peroxide, which has high crystallinity, surface area and more abundant active sites, and excellent photocatalytic performance.

[0006] The present application provides a preparation method of a high-crystalline carbon nitride, comprising the following steps:

[0007] S1: calcining melamine under a nitrogen atmosphere, cooling, washing and drying after calcination to obtain bulk carbon nitride;

[0008] S2: uniformly mixing potassium chloride, lithium chloride and bulk-phase carbon nitride, and then calcining under a nitrogen atmosphere, cooling, washing and drying after calcination to obtain high-crystallinity carbon nitride.

[0009] In the above preparation method, the mass ratio among melamine, potassium chloride, lithium chloride and bulk-phase carbon nitride can be (7-9):(5-6):(4-5):(1-3).

[0010] In the above preparation method, the nitrogen atmosphere is a high-purity nitrogen atmosphere, and the purity of the high-purity nitrogen is ≥99.9%; in addition, the nitrogen flow rate can be controlled to be 50-250 mL / min, for example, 150-250 mL / min.

[0011] In the above preparation method, the calcination temperature can be 400-600℃, wherein the calcination temperature in step S1 can be 400-500℃, and the calcination temperature in step S2 can be 450-550℃; the heating rate during calcination can be 2-6℃ / min, for example, 4-6℃ / min; and the calcination time can be 2-4h.

[0012] In the above preparation method, the washing includes performing 2-4 times of the following steps: adding deionized water for washing, and then separating by reduced-pressure suction filtration. In addition, the drying temperature can be 50-70℃, and the drying time can be 10-15h.

[0013] In the above preparation method, the mixing can be ball-milling mixing, and the frequency during ball-milling mixing can be 25-45Hz, for example, 35-45Hz; and the ball-milling mixing time can be 0.5-2h, for example, 0.5-1h.

[0014] The preparation method of the present application is performed under ionothermal conditions, and the crystallinity is improved by adjusting the structural characteristics of carbon nitride; at the same time, the mass transfer and heat transfer in the molten salt are enhanced, which is more conducive to the formation of a highly ordered carbon nitride framework structure.

[0015] The present application also provides a high-crystallinity carbon nitride prepared according to the above preparation method.

[0016] Specifically, the surface area (BET) of the high-crystallinity carbon nitride of the present application is >120m 2 / g, for example, 125-130m 2 / g.

[0017] The present application also provides the use of the above high-crystallinity carbon nitride in the photocatalytic production of hydrogen peroxide.

[0018] The implementation of the present application has at least the following advantages:

[0019] 1. The preparation method of highly crystalline carbon nitride of the present invention is simple, the raw materials are widely available and inexpensive, and the finished highly crystalline carbon nitride material is a solid powder that is non-toxic and harmless, easy to separate from water, and beneficial to practical applications.

[0020] 2. The highly crystalline carbon nitride of the present invention has higher crystallinity, surface area, and more active sites, wherein the surface area is >120m². 2 / g,K + With a mass ratio >8%, the photon conversion efficiency is improved, resulting in superior photocatalytic performance;

[0021] 3. The highly crystalline carbon nitride of the present invention can efficiently produce hydrogen peroxide, with a high quantum yield in the wavelength range of 405-420 nm and a photocatalytic hydrogen peroxide production of up to 9.63 mmol / h. It can be recycled multiple times and maintains high efficiency in hydrogen peroxide production, and has good photocatalytic stability. Attached Figure Description

[0022] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0023] Figure 1 Transmission electron microscope images of KCCN, CN, and CCN photocatalytic materials; where a represents CN, b represents CCN, and c represents KCCN.

[0024] Figure 2 This is a high-resolution TEM image of the KCCN photocatalytic material from Example 1;

[0025] Figure 3 XRD spectra of KCCN, CN, and CCN photocatalytic materials;

[0026] Figure 4 FT-IR spectra of KCCN, CN, and CCN photocatalytic materials;

[0027] Figure 5 Photocatalytic hydrogen peroxide production performance test diagrams for KCCN, CN, and CCN photocatalytic materials;

[0028] Figure 6 The apparent quantum efficiency spectrum of the KCCN photocatalytic material in Example 1 was obtained by measuring the hydrogen peroxide production performance and UV-Vis diffuse reflectance spectra at different incident light wavelengths.

[0029] Figure 7The graph shows the long-term hydrogen peroxide production performance of the KCCN photocatalytic material in Example 1.

[0030] Figure 8 The graph shows the hydrogen peroxide production cycle stability of the KCCN photocatalytic material in Example 1. Detailed Implementation

[0031] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0032] It should be noted that the terminology used herein is for the purpose of describing particular implementations only and is not intended to limit the exemplary implementations according to this application. As used herein, the singular form includes the plural form unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this description, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0033] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] Example 1

[0035] The preparation method of highly crystalline carbon nitride in this embodiment includes the following steps:

[0036] S1: Preparation of bulk carbon nitride

[0037] 8g of melamine was weighed and placed in a ceramic crucible. It was calcined in a tube furnace at 450℃ for 3h under a high-purity nitrogen atmosphere (purity ≥99.9%). The heating rate during calcination was 5℃ / min, and the nitrogen flow rate was 200mL / min. After calcination, the mixture was cooled to room temperature and a pale yellow solid was obtained. The solid was washed three times with deionized water. After each washing, the solid was collected by vacuum filtration and dried in a vacuum drying oven at 60℃ for 12h. The resulting pale yellow bulk carbon nitride (named CN) was obtained.

[0038] S2: Preparation of highly crystalline carbon nitride

[0039] Weigh 2g of the bulk carbon nitride obtained in step S1, and mix it with 5.5g of potassium chloride and 4.5g of lithium chloride in a ball mill jar. The ball milling frequency is 40Hz and the ball milling time is 0.5h. After ball milling, remove the mixture and place it in a ceramic crucible. Calcinate it in a tube furnace at 500℃ for 3h under a high-purity nitrogen atmosphere (purity ≥99.9%). The heating rate during calcination is 5℃ / min, and the nitrogen flow rate is 200mL / min. After calcination, cool to room temperature and remove to obtain a pale yellow solid. Wash it repeatedly with deionized water three times. After each washing, filter under reduced pressure to collect the solid. Place it in a vacuum drying oven at 60℃ and dry for 12h to obtain highly crystalline carbon nitride (named KCCN).

[0040] Example 2

[0041] The preparation method of highly crystalline carbon nitride in this embodiment includes the following steps:

[0042] S1: Preparation of bulk carbon nitride

[0043] 7g of melamine was weighed and placed in a ceramic crucible. It was calcined in a tube furnace at 400℃ for 4h under a high-purity nitrogen atmosphere (purity ≥99.9%). The heating rate during calcination was 4℃ / min, and the nitrogen flow rate was 150mL / min. After calcination, the mixture was cooled to room temperature and a pale yellow solid was obtained. The solid was washed three times with deionized water. After each washing, the solid was collected by vacuum filtration and dried in a vacuum drying oven at 50℃ for 15h. The result was pale yellow bulk carbon nitride.

[0044] S2: Preparation of highly crystalline carbon nitride

[0045] Weigh 1g of the bulk carbon nitride obtained in step S1, and mix it with 5g of potassium chloride and 5g of lithium chloride in a ball mill jar. The ball milling frequency is 35Hz and the ball milling time is 1h. After ball milling, remove the mixture and place it in a ceramic crucible. Calcinate it in a tube furnace at 450℃ for 4h under a high-purity nitrogen atmosphere (purity ≥99.9%). The heating rate during calcination is 4℃ / min, and the nitrogen flow rate is 150mL / min. After calcination, cool to room temperature and remove to obtain a pale yellow solid. Wash it repeatedly with deionized water three times. After each washing, filter under reduced pressure to collect the solid. Place it in a vacuum drying oven at 50℃ and dry for 15h to obtain highly crystalline carbon nitride.

[0046] Example 3

[0047] The preparation method of highly crystalline carbon nitride in this embodiment includes the following steps:

[0048] S1: Preparation of bulk carbon nitride

[0049] 9g of melamine was weighed and placed in a ceramic crucible. It was calcined in a tube furnace at 500℃ for 2h under a high-purity nitrogen atmosphere (purity ≥99.9%). The heating rate during calcination was 6℃ / min and the nitrogen flow rate was 250mL / min. After calcination, the mixture was cooled to room temperature and a pale yellow solid was obtained. The solid was washed three times with deionized water. After each washing, the solid was collected by vacuum filtration and dried in a vacuum drying oven at 70℃ for 10h. The result was pale yellow bulk carbon nitride.

[0050] S2: Preparation of highly crystalline carbon nitride

[0051] Weigh 3g of the bulk carbon nitride obtained in step S1, and mix it with 6g of potassium chloride and 4g of lithium chloride in a ball mill jar. The ball milling frequency is 45Hz and the ball milling time is 0.5h. After ball milling, remove the mixture and place it in a ceramic crucible. Calcinate it in a tube furnace at 450℃ for 2h under a high-purity nitrogen atmosphere (purity ≥99.9%). The heating rate during calcination is 6℃ / min, and the nitrogen flow rate is 250mL / min. After calcination, cool to room temperature and remove the solid to obtain a pale yellow solid. Wash the solid three times with deionized water. After each washing, filter the solid under reduced pressure and dry it in a vacuum drying oven at 70℃ for 10h to obtain highly crystalline carbon nitride.

[0052] Compare with Example 1

[0053] This comparative example uses the bulk carbon nitride (named CN) prepared in Example 1 as a control.

[0054] Compare with Example 2

[0055] The preparation method of this comparative example is as follows:

[0056] Weigh 5.5g of potassium chloride, 4.5g of lithium chloride, and 2g of melamine and place them in a ball mill jar for ball milling. The ball milling frequency is 40Hz, and the milling time is 0.5h. After ball milling, remove the mixture and place it in a ceramic crucible. Calcinate it in a tube furnace at 500℃ for 3h under a high-purity nitrogen atmosphere (purity ≥99.9%). The heating rate during calcination is 5℃ / min, and the nitrogen flow rate is 200mL / min. After calcination, cool to room temperature and remove the solid to obtain a pale yellow solid. Wash the solid three times with deionized water. After each washing, filter the solid under reduced pressure and dry it in a vacuum drying oven at 60℃ for 12h to obtain crystalline carbon nitride (named CCN).

[0057] Experimental Example 1

[0058] The morphology, structure, light absorption properties, and photocatalytic activity of the highly crystalline carbon nitride KCCN of Example 1, the bulk carbon nitride CN of Control Example 1, and the crystalline carbon nitride CCN of Control Example 2 were studied using TEM, XRD, XPS, and DRS. The photocatalytic hydrogen peroxide production performance was tested using the following methods:

[0059] 10 mg of KCCN, CN, and CCN photocatalysts were dispersed in 50 mL of 5 vol.% ethanol solution, respectively. The solutions were ultrasonicated for 15 min, stirred for 15 min to reach adsorption-desorption equilibrium, and then saturated with oxygen for 30 min. Finally, light was applied at an intensity of 100 mW / cm². 2 The reaction was carried out under xenon lamp irradiation at room temperature, with the gas pressure of the reaction system at 1 standard atmosphere. Samples were taken at specified time intervals, and the supernatant was used for hydrogen peroxide detection. The concentration of hydrogen peroxide produced was determined by ultraviolet-visible spectrophotometry, using N,N-diethylphenylenediamine sulfate (DPD) and peroxidase (POD) as the indicator, with a maximum absorption wavelength of 551 nm, and a 0.5 M phosphate solution as the buffer solution.

[0060] Figure 1 Transmission electron microscope (TEM) images of KCCN, CN, and CCN photocatalytic materials. Figure 1 The results show that CN exhibits a typical irregular layered structure. Figure 1 a), while CCN exhibits a nanorod-like structure with a size of approximately 150 nm. Figure 1 b). KCCN is synthesized by secondary polymerization of CN in LiCl-KCl eutectic salt. Its edges are felt-like, significantly increasing the surface area. In particular, the felt-like structure can provide a long light path for light collection, expose active sites, and enhance the photocatalytic reaction. Figure 1 c).

[0061] Table 1 shows the BET surface area results of KCCN, CN, and CCN photocatalytic materials determined by nitrogen physical adsorption method.

[0062] Table 1. BET surface area of ​​KCCN, CN, and CCN photocatalytic materials

[0063] Photocatalytic material CN CCN KCCN BET surface area (m 2 / g) 1.2 73.9 127.5

[0064] The results in Table 1 further confirm that the specific surface area of ​​the KCCN photocatalytic material prepared in Example 1 is significantly increased.

[0065] Figure 2 This is a high-resolution TEM image of the KCCN photocatalytic material from Example 1; Figure 2 The clear lattice fringes confirm the high crystallinity of the KCCN photocatalytic material, and the lattice spacing of 1.06 nm was measured by fast Fourier transform, corresponding to the (100) crystal plane of KCCN.

[0066] Figure 3 The XRD spectra of KCCN, CN, and CCN photocatalytic materials are shown. Figure 3The results show that CN has two typical X-ray diffraction peaks at 13.1° and 27.54°, representing the (100) and (002) crystal planes of carbon nitride, respectively, which are attributed to the in-plane heptaazine unit structure and interlayer stacking structure. Compared with CN, CCN shows two new peaks at lower 8.19° and 12.05°, representing the (100) and (110) crystal planes, respectively. Compared with CCN, the peak intensity of KCCN attributable to the (100) crystal plane increases and moves from 8.19° to 8.35°, indicating a decrease in interplane spacing; the main peak of the (002) crystal plane moves from 27.0° to 27.46°, and the full width at half maximum (FWHM) decreases significantly, indicating a decrease in interlayer spacing and a significant improvement in crystallinity.

[0067] Figure 4 The images show the FT-IR spectra of KCCN, CN, and CCN photocatalytic materials. Figure 4 The results show that the overall patterns of KCCN and CCN are similar to those of CN, indicating that the application of the ionothermal method did not alter the core structure of the carbon nitride framework. At 802 cm⁻¹ -1 The absorption peak at 1100–1700 cm⁻¹ is attributed to the out-of-plane bending vibration of the heptaazine units in the framework; -1 The typical absorption peak at 2179 cm⁻¹ is due to the skeletal stretching vibration of the aromatic heterocycle. -1 The peak value at 3000-3500 cm⁻¹ indicates the presence of cyano groups in CCN and KCCN. -1 The strong and broad absorption peak at 993 cm⁻¹ is a result of the NH stretching vibration. In addition, there is a peak at 993 cm⁻¹. -1 The absorption peaks appearing at this point are attributed to the CN-K group, and the peak value of KCCN is significantly increased compared to CCN.

[0068] Table 2 shows the Kc values ​​of CCN and KCCN photocatalysts measured by inductively coupled plasma optical emission spectrometry (ICP). + Mass ratio.

[0069] Table 2 KCCN and KCCN photocatalyst materials + mass ratio

[0070] Photocatalytic material CCN KCCN K + mass ratio 5.83％ 8.83％

[0071] Table 2 shows that the KCCN photocatalytic material undergoes secondary polymerization of carbon nitride in a LiCl-KCl eutectic salt, increasing the potassium ion loading within the carbon nitride framework. The incorporation of K into the carbon nitride framework... + This can increase electrical conductivity, which will facilitate carrier transfer and thus improve the photocatalytic activity of KCCN photocatalytic materials.

[0072] Figure 5The image shows the photocatalytic hydrogen peroxide production performance test results of KCCN, CN, and CCN photocatalytic materials. The photocatalytic activities of KCCN, CN, and CCN photocatalytic materials were compared by testing their hydrogen peroxide production performance over one hour. Ethanol was used as a sacrificial agent, and a xenon lamp with a light intensity of 100 mW / cm² was used as the light source to catalyze the reduction of oxygen to hydrogen peroxide. Figure 5 As shown, in one hour of hydrogen peroxide production, CN produced hydrogen peroxide at a relatively low rate of 0.32 mmol / h, while CCN showed relatively higher activity, with a hydrogen peroxide photogeneration rate of 4.78 mmol / h. KCCN, however, achieved a photocatalytic hydrogen peroxide yield of 9.63 mmol / h, approximately 26.8 times that of CN and 2.0 times that of CCN, indicating that increased crystallinity and potassium ion loading within the carbon nitride framework significantly improved photocatalytic efficiency.

[0073] Figure 6 The hydrogen peroxide production performance of the KCCN photocatalyst material prepared in Example 1 was measured at different incident light wavelengths (405 nm, 420 nm, 450 nm, 475 nm, and 500 nm), and the apparent quantum efficiency (AQY) was obtained from the UV-Vis diffuse reflectance (DRS) spectrum of KCCN. At 405 nm, the AQY reached 51.8%, and decreased with further increases in wavelength. The dependence of AQY on the incident light wavelength was consistent with the DRS spectrum, indicating that hydrogen peroxide production is based on a photocatalytic mechanism.

[0074] Figure 7 The graph shows the long-term hydrogen peroxide production performance of the KCCN photocatalytic material prepared in Example 1. Figure 7 The results showed that the concentration of hydrogen peroxide reached 28.8 mM when the reaction time was 6 h.

[0075] Figure 8 The graph shows the hydrogen peroxide production cycle stability of the KCCN photocatalytic material prepared in Example 1. Figure 8 The results showed that the concentration of hydrogen peroxide produced by KCCN in the fourth cycle was 95.5% of that in the first cycle, indicating that the KCCN photocatalytic material has good photocatalytic stability.

[0076] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An application of highly crystalline carbon nitride in the photocatalytic production of hydrogen peroxide, wherein the method for preparing highly crystalline carbon nitride comprises the following steps: S1: Melamine is calcined under a nitrogen atmosphere, and after calcination, it is cooled, washed, and dried to obtain bulk carbon nitride; S2: Potassium chloride, lithium chloride and bulk carbon nitride are mixed evenly and then calcined under a nitrogen atmosphere. After calcination, the mixture is cooled, washed and dried to obtain highly crystalline carbon nitride. The nitrogen atmosphere is a high-purity nitrogen atmosphere; the nitrogen flow rate is controlled at 200 mL / min; The mass ratio of melamine, potassium chloride, lithium chloride, and bulk carbon nitride is 8:5.5:4.5:2; In step S1, the calcination temperature is 450 ℃, the heating rate during calcination is 5 ℃ / min, and the calcination time is 3h; in step S2, the calcination temperature is 500 ℃, the heating rate during calcination is 5 ℃ / min, and the calcination time is 3h. The mixing was performed by ball milling at a frequency of 40 Hz for 0.5 h.

2. The application according to claim 1, characterized in that, The washing process includes 2-4 steps as follows: washing with deionized water followed by vacuum filtration for separation.

3. The application according to claim 1, characterized in that, The drying temperature is 50-70 ℃, and the drying time is 10-15 h.

4. The application according to claim 1, characterized in that, The surface area of ​​this highly crystalline carbon nitride is >120 m². 2 / g.

Images