Sodium sulfate / crystalline carbon nitride composite material and preparation and application thereof

Abstract

This invention discloses a sodium sulfate / crystallized carbon nitride composite material, comprising a crystallized carbon nitride matrix and Na2SO4 intercalated therein, wherein the Na2SO4 is mainly intercalated on the (002) crystal plane of crystallized g-C3N4. This invention utilizes Na2SO4 to regulate the crystallization of g-C3N4 and modify it through salt intercalation. During the preparation of the composite photocatalytic material, the introduced Na2SO4 can act as a denitrification condensation reaction site during high-temperature calcination, accelerating the conversion of melamine to g-C3N4 and promoting the formation of crystallized g-C3N4 with ordered internal unit arrangement. Simultaneously, the Na2SO4 salt itself intercalates into the crystallized g-C3N4 structure, serving as an interfacial reaction site to optimize the oxygen reduction to H2O2 reaction kinetics, significantly enhancing photocatalytic activity. The preparation method of the composite material described in this invention is simple and environmentally friendly, providing a new approach for the preparation of high-performance photocatalytic carbon nitride materials.

Description

Technical Field

[0001] This invention belongs to the field of photocatalytic materials technology, specifically relating to a sodium sulfate / crystallized carbon nitride composite material, its preparation method, and its application. Background Technology

[0002] Graphitic carbon nitride (g-C3N4) is a low-cost, non-toxic, narrow-bandgap organic polymer photocatalyst with excellent physicochemical stability, good visible light response, and outstanding photocatalytic activity. However, traditional bulk g-C3N4 has many internal defects and low crystallinity, leading to severe recombination of photogenerated carriers and relatively low photocatalytic activity.

[0003] Currently, methods for improving the photocatalytic activity of g-C3N4 materials mainly include semiconductor coupling, co-catalyst modification, and crystallinity regulation. Semiconductor coupling refers to coupling another semiconductor with g-C3N4, which can not only significantly broaden the spectral response range but also improve the separation efficiency of photogenerated carriers, thereby enhancing the photocatalytic activity of g-C3N4. Co-catalyst modification of g-C3N4 can effectively improve the separation and transport efficiency of photogenerated carriers and accelerate the redox reaction rate on the surface, thereby enhancing the photocatalytic performance of g-C3N4. Crystallinity regulation improves the orderliness of the internal structural units of traditional bulk g-C3N4 and reduces internal structural defects, promoting the separation of photogenerated carriers and thus enhancing the photocatalytic performance of g-C3N4.

[0004] The three common modification methods mentioned above (semiconductor coupling, co-catalyst modification, and crystallinity control) are widely used to improve the photocatalytic activity of g-C3N4. Crystallinity control, in particular, aims to address the intrinsic amorphous structural defects of bulk g-C3N4 and is a highly efficient and important method. Currently, the main methods developed for synthesizing crystallized g-C3N4 include microwave-assisted synthesis, high-pressure synthesis, and molten salt synthesis. Microwave-assisted and high-pressure methods have high equipment requirements and limited room for improvement. The molten salt method, using LiCl / KCl eutectic salt as a solvent to control the polycondensation reaction, has been widely used to synthesize crystallized g-C3N4.

[0005] However, existing molten salt methods require the use of two or more salts with a eutectic point below 550°C. o The presence of eutectic salts (C) limits the types of eutectic salts suitable for controlling the crystallinity of g-C3N4, and these typically require the use of toxic LiCl salts. Furthermore, the eutectic salts need to be removed subsequently to obtain crystallized g-C3N4. In addition, the effect of conventionally prepared molten salt-based crystallized g-C3N4 on improving photocatalytic activity is limited, relying solely on the ordered crystallization structure to promote carrier separation (performance is unlikely to exceed 10 times that of conventional C3N4). Therefore, further exploration of environmentally friendly and efficient high-performance g-C3N4 material preparation processes is of significant research and application value. Summary of the Invention

[0006] One of the objectives of this invention is to address the problems and shortcomings of existing crystallized g-C3N4 synthesis technology by providing a sodium sulfate / crystallized carbon nitride composite material based on the Na2SO4 regulation mechanism. This material utilizes Na2SO4 to regulate the synthesis of crystallized g-C3N4 and achieves the intercalation of Na2SO4 salt in g-C3N4, thereby significantly improving photocatalytic performance.

[0007] The second objective of this invention is to provide a method for preparing the sodium sulfate / crystallized carbon nitride composite material, which involves a simple and environmentally friendly preparation process.

[0008] A third objective of this invention is to provide the application of the sodium sulfate / crystallized carbon nitride composite material as a photocatalyst.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A sodium sulfate (Na2SO4) / crystallized carbon nitride (g-C3N4) composite material comprising a crystallized carbon nitride matrix and Na2SO4 (Na salt) intercalated therein, wherein the Na2SO4 is mainly intercalated on the (002) crystal plane of the crystallized g-C3N4.

[0010] Furthermore, the theoretical dosage of Na2SO4 is 5-25% of melamine.

[0011] Furthermore, the light absorption range of the sodium sulfate / crystallized carbon nitride composite material (Na2SO4 / g-C3N4) is significantly broadened, enabling it to respond to a wider spectral range of visible light (380-550 nm; the visible light response range of conventional g-C3N4 material is 380-480 nm); in addition, its band gap can be reduced to 2.25 eV (the band gap of conventional bulk g-C3N4 is 2.57 eV).

[0012] The above-mentioned method for preparing a sodium sulfate / crystallized carbon nitride composite material includes the following steps: S1. Melamine powder is uniformly dispersed in water to obtain melamine dispersion (suspension); sodium sulfate (Na2SO4) powder is added to the melamine dispersion and mixed evenly to obtain precursor solution; The concentration of Na2SO4 in the precursor solution is greater than 0 and does not exceed 0.01 g / mL; The mass ratio of melamine to sodium sulfate introduced was 4~20:1; S2. The obtained precursor solution was freeze-dried under vacuum to obtain a white powder; The vacuum freeze-drying includes freezing and vacuum drying steps, wherein the freezing time is more than 4 hours and the vacuum drying time is more than 24 hours; S3. The obtained white powder is calcined and ground to obtain the sodium sulfate / crystallized carbon nitride composite material. The calcination temperature is 500~750 ℃, and the time is 1~8h.

[0013] Furthermore, the concentration of Na2SO4 in the precursor solution is preferably 0.002~0.01 g / mL; more preferably 0.005~0.007 g / mL.

[0014] According to the above scheme, in S1, the dispersion and mixing steps adopt a stirring process, with a temperature of 5~35 ℃, a stirring rate of 200~800 r / min, and a time of more than 0.5 hours.

[0015] According to the above scheme, in S2, the temperature of vacuum freeze drying is below -40℃ and the time is more than 48 hours.

[0016] Furthermore, in the vacuum freeze-drying step, the freezing time is preferably 4~12h, more preferably 5~7h; the vacuum drying time is 24~72h. Furthermore, the calcination temperature in S3 is 500~600℃, more preferably 540~560℃. o C; and / or the calcination time is 1~4h, more preferably 2~4h.

[0017] Furthermore, the heating rate during the calcination process is 2℃ / min.

[0018] According to the above scheme, the grinding time in step S3 is more than 0.25 hours.

[0019] Preferably, the grinding time is 0.25~0.5 h.

[0020] The present invention also provides an application of the above-mentioned sodium sulfate / crystallized carbon nitride composite material as a photocatalyst, which significantly improves the performance of photocatalytic oxygen reduction to hydrogen peroxide (H2O2), reaching 12.5 times the photocatalytic performance of traditional bulk g-C3N4, achieving an order-of-magnitude improvement.

[0021] The principles of this invention include: This invention proposes for the first time a crystallization regulation and intercalation modification mechanism based on a single component Na2SO4 salt. Melamine and sodium sulfate are mixed and freeze-dried to obtain a precursor, which is then calcined at high temperature to synthesize a Na2SO4 intercalated crystallized g-C3N4 composite material in one step. During the process of calcining and freeze-drying the precursor, the introduced Na2SO4 can serve as a denitrification condensation reaction site, accelerating the condensation reaction rate of melamine to g-C3N4 and promoting the formation of crystallized g-C3N4 with ordered internal unit arrangement. At the same time, Na2SO4 molecules can be intercalated into the interlayer of the π-π conjugated stacked (002) crystal plane of g-C3N4 to form a stable Na2SO4 intercalated crystallized g-C3N4 structure, thereby obtaining Na2SO4 / g-C3N4.

[0022] Compared to traditional g-C3N4, the crystallized g-C3N4 structure obtained in this invention has fewer internal defects, which can effectively promote carrier separation and migration. At the same time, Na2SO4 intercalated into the crystallized g-C3N4 structure can serve as an interfacial reaction site, optimizing the interfacial oxygen reduction to H2O2 reaction kinetics. In addition, Na2SO4 intercalation into crystallized g-C3N4 can also significantly broaden the light absorption range and narrow the band gap of g-C3N4. These advantages ensure its excellent photocatalytic activity.

[0023] Compared with the prior art, the beneficial effects of the present invention include: 1) This invention is the first to propose using Na2SO4 to regulate the crystallization and modify g-C3N4 through salt intercalation. In the preparation of the composite photocatalytic material, the Na2SO4 (Na salt) introduced in the precursor can act as a denitrification condensation reaction site during high-temperature calcination to accelerate the conversion of melamine to g-C3N4, promoting the formation of crystallized g-C3N4 with ordered internal unit arrangement. At the same time, Na2SO4 salt can be intercalated into the crystallized g-C3N4 structure and can act as an interfacial reaction site to optimize the oxygen reduction to H2O2 reaction kinetics. The synergistic effect of the two results in a significant improvement in photocatalytic activity, providing a new approach for the preparation of high photocatalytic performance carbon nitride materials. 2) This invention can effectively broaden the light absorption range of g-C3N4, narrow the band gap, promote the effective separation of photogenerated carriers in the bulk phase, and improve the kinetics of the reaction between interfacial oxygen reduction and H2O2 production. 3) This invention employs a one-step calcination method, which involves a simple preparation method with low equipment requirements. It does not use toxic raw materials, does not produce toxic substances during the preparation process, and the product is non-toxic, making it suitable for large-scale synthesis. It also has the advantages of being simple, green, and efficient, and has good economic and environmental benefits. Attached Figure Description

[0024] Figure 1XRD patterns of conventional bulk g-C3N4 and Na2SO4 / g-C3N4 prepared in Example 1 before and after hydrothermal treatment; wherein, A is the XRD pattern of conventional bulk g-C3N4 and Na2SO4 / g-C3N4 prepared in Example 1; B is the XRD pattern of conventional bulk g-C3N4 and Na2SO4 / g-C3N4 prepared in Example 1 after hydrothermal treatment (180°C). o XRD pattern after removing Na2SO4 (C / 1 h); Figure 2 TEM images of conventional bulk g-C3N4 and Na2SO4 / g-C3N4 prepared in Example 1; wherein, A and B are TEM images of conventional bulk g-C3N4 and C and D are TEM images of Na2SO4 / g-C3N4 prepared in Example 1; Figure 3 XPS spectra of conventional bulk g-C3N4 and Na2SO4 / g-C3N4 prepared in Example 1; wherein, A is the full XPS spectrum of conventional bulk g-C3N4 and Na2SO4 / g-C3N4, and B, C, and D are the high-resolution XPS spectra of Na 1s, O 1s, and S 2p of conventional bulk g-C3N4 and Na2SO4 / g-C3N4 prepared in Example 1, respectively; Figure 4 The spectroscopic analysis results are for conventional bulk g-C3N4 and Na2SO4 / g-C3N4 prepared in Example 1; where A is the UV-vis spectrum and B is the Tauc plot. Figure 5 The photocatalytic performance analysis results of traditional bulk g-C3N4, Na2SO4 / g-C3N4 prepared in Example 1, and crystallized g-C3N4 photocatalysts for H2O2 production are shown. Among them, A is the H2O2 production rate change over time, B is the H2O2 generation rate of different samples, and C is the cycle performance test graph. Figure 6 The electrochemical and fluorescence performance analysis results are shown for traditional bulk g-C3N4 and Na2SO4 / g-C3N4 prepared in Example 1; where A is the photocurrent response curve, B is the electrochemical impedance change curve, C is the steady-state spectrum and D is the transient fluorescence spectrum. Detailed Implementation

[0025] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the specification and accompanying drawings.

[0026] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0027] In the following examples, the photocatalytic H2O2 production performance of Na2SO4 / g-C3N4 was evaluated by reducing oxygen under visible light. The specific steps are as follows: First, 50 mg of photocatalyst powder was weighed and placed in a 100 mL quartz reactor (the reactor is equipped with a circulating water cooling system). Then, 50 mL of 10 vol% ethanol aqueous solution was added (ethanol acts as a sacrificial agent to capture holes in this photocatalytic system). The reaction was maintained with uniform magnetic stirring throughout, and the reaction was carried out at a rate of 200 mL / min. -1 High-purity O2 was continuously introduced into the reaction system at a constant flow rate. Before illumination, the reaction system was continuously stirred in the dark for 30 minutes to ensure dissolved oxygen reached saturation. Subsequently, a 420 nm LED light source (light intensity: 80 mW·cm²) was used. -2 The photocatalyst reaction was carried out. During the reaction, 2 mL of suspension was taken out every 30 min, centrifuged to remove the photocatalyst particles, and the resulting filtrate was mixed with an equal volume of 0.2 mol·L⁻¹ water. - 1 After mixing the C4K2O9Ti·2H2O colorimetric reagent and developing the color for 2 min, the absorbance was measured at a wavelength of 385 nm using a UV-Vis spectrophotometer, and the H2O2 concentration was calculated using the standard curve method.

[0028] In the following embodiments, the methods for characterizing the microstructure and structure of Na2SO4 / g-C3N4 include: The microstructure of the samples was characterized by transmission electron microscopy (TEM, model: JEM-2100F). The crystal structure was tested and analyzed using an X-ray diffractometer (XRD, model: Rigaku III); Its elemental composition and microstructure were analyzed using X-ray photoelectron spectroscopy (XPS); The ultraviolet-visible diffuse reflectance spectrum (UV-vis DRS) was measured using a U-3900H spectrophotometer; In addition, steady-state fluorescence spectra and transient fluorescence spectra were monitored using an F-4700 fluorescence spectrophotometer (Hitachi, Japan) and an FLS920 fluorescence spectrophotometer (Edinburgh Instruments, UK), respectively.

[0029] Example 1 A sodium sulfate / crystallized carbon nitride composite material, the preparation method of which includes the following steps: S1. Disperse 2.0 g of melamine powder in 50 mL of deionized water and stir until uniform to obtain a melamine suspension; add 0.3 g of sodium sulfate (Na2SO4) powder to the melamine suspension and stir until uniform to obtain a precursor solution; the concentration of Na2SO4 in the precursor solution is 0.006 g / mL. S2. The obtained precursor solution is heated at -40°C. o C. Vacuum freeze drying is performed at a pressure below 0.5 MPa, with a freezing time of 6 hours and a vacuuming time of 48 hours (to complete the removal of moisture), resulting in a white powder with melamine and Na2SO4 mixed evenly. S3. The obtained white powder is placed in a muffle furnace and calcined. Specific steps include: using 2... o The temperature was programmed to rise at a rate of C / min to 550. o C, keep warm and calcine for 2 hours to obtain a yellow solid; then grind the obtained yellow solid for 0.25 hours to obtain a uniform powder, which is sodium sulfate / crystallized carbon nitride composite material.

[0030] The XRD pattern of the sodium sulfate / crystallized carbon nitride composite material (Na2SO4 / g-C3N4) obtained in this embodiment is shown below. Figure 1 A. Traditional bulk phase g-C3N4 (melamine precursor directly calcined at high temperature in a muffle furnace using traditional preparation process, the calcination procedure is the same as S3) at 13.0 o and 27.3 o Characteristic diffraction peaks corresponding to the g-C3N4 (100) and (002) crystal planes appeared at the locations (see...). Figure 1 (A) Compared with bulk g-C3N4, the Na2SO4 / g-C3N4 obtained in this embodiment not only showed characteristic diffraction peaks corresponding to the (100) and (002) crystal planes of g-C3N4, but also exhibited characteristic diffraction peaks of Na2SO4, indicating that the Na2SO4 / g-C3N4 composite photocatalyst was successfully synthesized. In addition, compared with bulk g-C3N4, the intensity of characteristic diffraction peaks of the (100) and (002) crystal planes of g-C3N4 in the obtained Na2SO4 / g-C3N4 was reduced, which can be attributed to the intercalation of Na2SO4 in the interlayer (002) and in-plane (100), which interfered with the diffraction of the g-C3N4 crystal plane. In order to eliminate the interference of intercalated Na2SO4, the bulk g-C3N4 and the Na2SO4 / g-C3N4 composite photocatalyst were simultaneously subjected to hydrothermal operation (180°C). o(C, 1 h) to remove Na2SO4, the resulting products were named hydrothermal g-C3N4 and crystallized g-C3N4, respectively. Compared with g-C3N4, the (002) diffraction peak of the Na2SO4 / g-C3N4 composite material was significantly enhanced after Na2SO4 removal, which is attributed to its increased crystallinity. Simultaneously, it can be observed that the (002) diffraction peak shifted to a higher angle by 0.2. o (27.5) o Offset to 27.7 o This is due to the more ordered interlayer stacking and reduced interlayer spacing of the resulting crystallized g-C3N4. The above XRD results prove the successful synthesis of Na2SO4 / g-C3N4.

[0031] Figure 2 In the diagram, A and B are the TEM and HRTEM images of the traditional bulk g-C3N4, respectively, and C and D are the TEM and HRTEM images of the Na2SO4 / g-C3N4 obtained in Example 1, respectively. It can be seen that the traditional bulk g-C3N4 exhibits an aggregated, bulk, disordered amorphous structure, consistent with the widely reported characteristics of bulk g-C3N4; however, after using Na2SO4 for regulation, the macroscopic structure of the resulting Na2SO4 / g-C3N4 did not change significantly. Figure 2 C), but high-resolution TEM images ( Figure 2 D) Two clear and orderly lattice stripes appeared. After measurement, their lattice spacing was 0.32 nm and 0.62 nm, which correspond to the (002) crystal plane of g-C3N4 and the characteristic lattice of Na2SO4, respectively, further indicating that Na2SO4 / g-C3N4 was successfully synthesized.

[0032] Figure 3 XPS spectra of conventional bulk g-C3N4 and Na2SO4 / g-C3N4 prepared in Example 1. Figure 3 As shown in Figure A, the traditional bulk g-C3N4 exhibits distinct C 1s, N 1s, and O 1s XPS peaks. The C and N elements originate from its intrinsic structure, while the O element mainly originates from water or oxygen adsorbed on the sample surface. The obtained Na2SO4 / g-C3N4, in addition to the intrinsic XPS peaks of g-C3N4, also shows new Na 1s, Na KLL Auger peaks, S 2p XPS peaks, and an enhanced O 1s XPS peak (its O 1s peak is significantly enhanced compared to unmodified C3N4), which is mainly attributed to the intercalated Na2SO4. To further verify the above hypothesis, the high-resolution XPS spectra of Na 1s, O 1s, and S 2p are shown below. Figure 3 B to Figure 3 As shown in D. High-resolution XPS spectra of Na 1s and S 2p ( Figure 3 B and Figure 3 (D) indicates that a large amount of Na and S are present in the obtained composite material, and S mainly exists in the +6 valence form. Furthermore, the O 1s high-resolution XPS spectrum ( Figure 3 C) A new peak appears at 532.5 eV, whose binding energy corresponds to sulfate (SO4) ions. 2- Therefore, the high-resolution XPS spectra above confirm the presence of Na2SO4. In summary, the XRD, TEM, and XPS results fully confirm the successful synthesis of Na2SO4 / g-C3N4.

[0033] Figure 4 In the figure, A represents the UV-vis spectra of traditional bulk g-C3N4 and Na2SO4 / g-C3N4 prepared in Example 1. Compared to the light absorption curve of traditional bulk g-C3N4, the absorption edge of Na2SO4 / g-C3N4 shows a significant red shift, indicating that it can absorb a wider range of visible light. The UV-vis spectra are converted into Tauc plots, as shown below. Figure 4 As shown in B, by drawing tangents, the band gap of the conventional bulk g-C3N4 is 2.57 eV, while the band gap of the Na2SO4 / g-C3N4 obtained in this invention is 2.25 eV. The reduced band gap makes it easier to be excited by visible light, which is beneficial for improving the photocatalytic activity for H2O2 production.

[0034] Figure 5 A shows the curves of the amount of H2O2 produced by photocatalytic oxygen reduction in conventional bulk g-C3N4 and Na2SO4 / g-C3N4 prepared in Example 1, as a function of time. The photocatalytic H2O2 production rate of conventional bulk g-C3N4 can be calculated to be 48.6 μmol·L⁻¹. -1 ·h -1 The photocatalytic H2O2 production rate of Na2SO4 / g-C3N4 after Na2SO4 intercalation was significantly improved, reaching 607.2 μmol·L⁻¹. -1 ·h -1 The H2O2 production activity is 12.5 times that of traditional bulk g-C3N4. This significantly enhanced photocatalytic activity can be attributed to the synergistic effect of Na2SO4 intercalation and crystallized g-C3N4. To verify this mechanism, the Na2SO4 intercalated in crystallized g-C3N4 was removed to obtain crystallized g-C3N4, and its photocatalytic H2O2 production activity was tested. The results are as follows: Figure 5 As shown in B. Clearly, after removing the intercalated Na₂SO₄, the photocatalytic activity of the resulting crystallized g-C₃N₄ for H₂O₂ production is 189.1 μmol·L⁻¹. -1 ·h -1 Compared to the photocatalytic activity of Na2SO4 / g-C3N4 (607.2 μmol·L⁻¹), -1 ·h -1The rate of H2O2 production decreased significantly, but it was still lower than that of H2O2 production by bulk g-C3N4 (48.6 μmol·L⁻¹). -1 ·h -1 The superior photocatalytic activity of the obtained Na2SO4 / g-C3N4 is mainly attributed to the synergistic effect of Na2SO4 intercalation and crystallized g-C3N4.

[0035] Furthermore, the photocatalytic activity of the obtained Na2SO4 / g-C3N4 for H2O2 production remained at its initial high level after four cycles of testing. Figure 5 (C), which fully demonstrates the stability of the photocatalytic H2O2 production of Na2SO4 / g-C3N4.

[0036] Figure 6 Electrochemical and fluorescence performance analysis results for conventional bulk g-C3N4 and Na2SO4 / g-C3N4 prepared in Example 1; where A is the photocurrent response curve, B is the electrochemical impedance spectroscopy curve, C is the steady-state spectrum, and D is the transient fluorescence spectrum. Figure 6 As shown in Figure A, the obtained Na₂SO₄ / g-C₃N₄ exhibits a significantly enhanced photocurrent response compared to the traditional bulk g-C₃N₄, indicating a significant improvement in its photogenerated carrier separation efficiency. Figure 6 As shown in Figure B, the impedance radius of Na2SO4 / g-C3N4 is smaller than that of the conventional bulk g-C3N4, indicating that its interfacial resistance is reduced and its photogenerated carrier transport is more efficient. Figure 6 As shown in Figure C, the traditional bulk g-C3N4 exhibits a strong emission peak, while the emission peak intensity of Na2SO4 / g-C3N4 is significantly reduced, confirming its lower photogenerated carrier recombination rate. Figure 6 As shown in Figure D, the average lifetime of photogenerated carriers in Na2SO4 / g-C3N4 is 14.26 ns, which is longer than that of the conventional bulk g-C3N4 photocatalyst (10.23 ns), indicating that more photogenerated carriers can participate in the interfacial redox reaction. These characterizations fully demonstrate that, compared to conventional bulk g-C3N4, Na2SO4 / g-C3N4 significantly improves the separation and migration efficiency of photogenerated carriers, thus exhibiting excellent photocatalytic activity for H2O2 production.

[0037] Example 2 A sodium sulfate / crystallized carbon nitride composite material is prepared in a manner largely the same as in Example 1, except that the amount of Na2SO4 used is set to 0 g / mL, 0.002 g / mL, 0.004 g / mL, 0.006 g / mL, 0.008 g / mL, and 0.01 g / mL, respectively.

[0038] The results showed that when the amount of Na2SO4 added was 0.3 g (0.006 g / mL), the photocatalytic activity of Na2SO4 / g-C3N4 for H2O2 production was optimal, reaching 607.2 μmol·L⁻¹. -1 ·h -1 It is 12.5 times (48.6 μmol·L⁻¹) more potent than traditional bulk g-C₃N₄ photocatalysts. -1 ·h -1 This represents an order-of-magnitude increase.

[0039] When the amount of Na2SO4 added is less than 0.3 g (<0.006 g / mL), the photocatalytic activity of Na2SO4 / g-C3N4 for H2O2 production increases with increasing Na2SO4 content. When the amount of Na2SO4 added exceeds 0.3 g (>0.006 g / mL), the photocatalytic activity of Na2SO4 / g-C3N4 for H2O2 production decreases with increasing Na2SO4 content, but remains higher than the photocatalytic activity of traditional bulk g-C3N4 photocatalysts.

[0040] When the amount of Na2SO4 added exceeds 0.5 g (>0.01 g / mL), no product is obtained after calcination when mixed with melamine. This is because excessive Na2SO4 hinders the condensation reaction of melamine to convert to g-C3N4, causing the raw materials and intermediates to sublimate and be lost during calcination, thus failing to obtain the g-C3N4 product. Therefore, in the preparation of Na2SO4 / g-C3N4, the optimal amount of Na2SO4 added is 0.1-0.5 g, corresponding to a concentration of 0.002-0.01 g / mL (the amount of water as dispersant is 50 mL), and a mass fraction relative to melamine (2.0 g) of 5%-25%.

[0041] Example 3 A sodium sulfate / crystallized carbon nitride composite material is prepared in a manner largely the same as in Example 1, except that the amount of melamine used is set to 0.5 g, 1.0 g, 2.0 g, 4.0 g, 6.0 g, and 8.0 g, respectively.

[0042] The results showed that when the amount of melamine was 0.5 g, no yellow g-C3N4 product was obtained, only white residual Na2SO4, and the product had no photocatalytic activity. This was because the amount of melamine was too small, while the concentration of Na2SO4 (0.006 g / mL) was relatively excessive, hindering the condensation reaction of melamine to convert to g-C3N4. This caused the raw materials and intermediates to sublimate and be lost during calcination, resulting in no g-C3N4 product. When the amount of melamine was between 1.0 and 4.0 g, Na2SO4 / g-C3N4 formed by intercalation and crystallization of g-C3N4 with Na2SO4 could be obtained, and the photocatalytic activity was improved compared with the traditional bulk g-C3N4 photocatalyst. The product with the best photocatalytic activity was obtained when the amount of melamine was around 2.0 g. When the amount of melamine exceeds 4.0 g, i.e., when the amount of melamine reaches 6.0 g and 8.0 g, the photocatalytic activity of the product for producing H2O2 decreases slightly with the increase of melamine amount. This is because the excess melamine agglomerates during high-temperature calcination, reducing the specific surface area and the number of exposed effective reaction sites. However, the product is still Na2SO4 / g-C3N4 formed by intercalation and crystallization of g-C3N4 with Na2SO4. Its photocatalytic activity decreases with the increase of melamine amount, but its photocatalytic activity is still significantly better than that of traditional g-C3N4. This is mainly attributed to the intercalation effect of Na2SO4.

[0043] Example 4 A sodium sulfate / crystallized carbon nitride composite material is prepared in a manner similar to that of Example 1, except that the freezing time in step S2 is 1 h, 3 h, 4 h, 6 h, 9 h, and 12 h, respectively; and the vacuum drying time is 12 h, 24 h, 48 h, and 72 h, respectively.

[0044] Experimental results show that when the freezing time reaches 6 hours, 50 mL of the mixture can be completely frozen into ice blocks. When the freezing time is 1-3 hours, 50 mL of the mixture cannot be completely frozen, and some remains in a solution state, which is not conducive to subsequent vacuum drying. A freezing time of 4 hours can achieve complete freezing. Furthermore, when the freezing time exceeds 6 hours, reaching 9-12 hours, although completely frozen ice blocks are obtained, it is time-consuming. Therefore, the optimal freezing time is 6 hours. When the freezing time is 6 hours and the vacuum drying time reaches 48 hours, the water in the frozen ice blocks can be completely sublimated, resulting in a dry melamine / Na2SO4 mixture. When the vacuum drying time is less than 24 hours, some water in the ice blocks does not sublimate; when the vacuum drying time exceeds 48 hours, reaching 72 hours, a dry mixture is still obtained, but it is time-consuming. Therefore, in the preparation process of Na2SO4 / g-C3N4, the optimal freezing time and vacuum drying time during vacuum freeze-drying are 6 hours and 48 hours, respectively.

[0045] Example 5 A sodium sulfate / crystallized carbon nitride composite material is prepared using a method largely the same as in Example 1, except that the calcination temperature in step S3 is 300°C. o C, 400 o C, 500 o C, 550 o C, 700 o C, 800 o C.

[0046] Experimental results show that when the calcination temperature is 300... o At temperature C, the melamine precursor did not undergo a condensation reaction and could not be converted into g-C3N4. When the calcination temperature is 400°C... o At temperature C, the melamine precursor undergoes a condensation polymerization reaction to form the melem intermediate (heptaazine ring structure), and still cannot be converted into g-C3N4. When the calcination temperature is 500-600°C... o At temperature C, the melamine precursor undergoes condensation polymerization to form g-C3N4. When the calcination temperature reaches 700°C... o At C, at 500-600 o At temperature C, the already formed g-C3N4 structure undergoes partial decomposition, resulting in a certain reduction in product mass. This reduction is primarily attributed to the decomposed g-C3N4. However, the resulting product is still Na2SO4 / g-C3N4 formed by intercalation and crystallization of Na2SO4 g-C3N4, exhibiting significantly superior photocatalytic activity compared to traditional g-C3N4. When the temperature reaches 800°C... oAt temperature C, the g-C3N4 structure undergoes complete decomposition, yielding a white solid as residual Na2SO4. Therefore, in the preparation of Na2SO4 / g-C3N4, the preferred high-temperature calcination temperature is 500-600 °C. o C, the optimal temperature is 540-560°C. o C.

[0047] Example 6 A sodium sulfate / crystallized carbon nitride composite material was prepared using a method largely the same as in Example 1, except that the calcination times in step S3 were 0.5 h, 1 h, 2 h, 4 h, and 6 h, respectively. Experimental results showed that when the calcination time was 1 h, the resulting g-C3N4 photocatalyst exhibited weak XRD characteristic diffraction peaks and low photocatalytic H2O2 production activity. This was because the short calcination time meant that some precursors or intermediates did not fully participate in the condensation polymerization to form the g-C3N4 structure; however, the resulting product was mainly Na2SO4 / g-C3N4, exhibiting superior photocatalytic activity compared to traditional g-C3N4. When the calcination time was 2-4 h, the resulting g-C3N4 photocatalyst possessed strong XRD characteristic diffraction peaks and the best photocatalytic H2O2 production activity. When the calcination time reached 6 h and 8 h, the XRD characteristic diffraction peaks of the obtained g-C3N4 photocatalyst were not significantly different from those calcined for 2-4 h, and the yield of g-C3N4 and the photocatalytic activity for H2O2 production were slightly reduced. Therefore, in the preparation process of Na2SO4 / g-C3N4, the preferred high-temperature calcination time is 2-4 h, with the optimal time being 2 h.

[0048] Comparative Example 1 A sodium sulfate / crystallized carbon nitride composite material is prepared using a method largely the same as in Example 1, except that the precursor solution obtained in step S2 is dried at 70°C. o In a C-type forced-air drying oven, the moisture in the precursor solution is dried by keeping it at a temperature of 12 hours. After grinding, a dry white powder is obtained.

[0049] The composite photocatalyst obtained in Comparative Example 1 was subjected to the same hydrothermal operation as in Example 1 (removing the Na2SO4 component). The XRD characteristic diffraction peaks of the (002) crystal plane of the resulting g-C3N4 photocatalyst were not enhanced (compared to conventional g-C3N4). This indicates that calcining the precursor obtained by the forced-air drying process cannot produce the g-C3N4 photocatalyst with improved crystallinity as described in this invention.

[0050] Further testing results showed that the photocatalytic activity for H2O2 production of the obtained composite photocatalyst was approximately 98.5 μmol·L⁻¹. -1 ·h -1 Its photocatalytic activity is relatively higher than that of traditional bulk g-C3N4 (48.6 μmol·L⁻¹).-1 ·h -1 No significant improvement was achieved.

[0051] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for producing a sodium sulfate / crystalline cubic nitride composite material, characterized by, Includes the following steps: S1. Melamine powder is evenly dispersed in water to obtain a melamine suspension; then sodium sulfate powder is added and mixed evenly to obtain a precursor solution. The concentration of Na2SO4 in the precursor solution is greater than 0 and does not exceed 0.01 g / mL; The mass ratio of melamine to sodium sulfate introduced was 4~20:1; S2. The obtained precursor solution was freeze-dried under vacuum to obtain a white powder; The vacuum freeze-drying includes freezing and vacuum drying steps, wherein the freezing time is more than 4 hours and the vacuum drying time is more than 24 hours; S3. The obtained white powder is calcined and ground to obtain the sodium sulfate / crystallized carbon nitride composite material. The calcination temperature is 500~750 ℃, and the time is 1~8h.

2. The production method according to claim 1, characterized by, The concentration of Na2SO4 in the precursor solution is 0.002~0.01 g / mL.

3. The preparation method according to claim 1, characterized in that, In S1, the dispersion and mixing steps employ a stirring process with a stirring temperature of 5~35 ℃.

4. The preparation method according to claim 1, characterized in that, In S2, the vacuum freeze-drying temperature is below -40℃ and the time is above 48 hours.

5. The preparation method according to claim 1, characterized in that, In the vacuum freeze-drying step, the freezing time is 4~12h; the vacuum drying time is 24~72h.

6. The preparation method according to claim 1, characterized in that, In S3, the calcination temperature is 540~560 o C.

7. The sodium sulfate / crystallized carbon nitride composite material prepared by the preparation method according to any one of claims 1 to 6, characterized in that, It comprises a crystallized carbon nitride matrix and intercalated Na2SO4 therein.

8. The application of the sodium sulfate / crystallized carbon nitride composite material of claim 7 as a photocatalyst.

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