A low-defect pti photocatalyst based on double-halogen regulation and a preparation method and application thereof

Abstract

This invention discloses a polytriazine imide (PTI) photocatalytic material based on dihalogen ion-regulated crystallization, its preparation method, and its applications. By constructing a synergistic molten salt system of lithium bromide and sodium chloride / potassium chloride, staged control of crystal nucleation and growth is achieved during thermal polymerization, allowing the material to undergo a crystallization path from a metastable phase to a stable phase, thereby inducing the formation of a crystalline carbon nitride material with partially ion-deficient structures. This structure can effectively reduce the defect state density and transform deep-level traps into shallow-level traps, thus significantly improving the separation and migration efficiency of photogenerated carriers. The prepared material exhibits excellent performance in the photocatalytic water splitting reaction, with an apparent quantum efficiency exceeding 30%. The method of this invention is simple, highly controllable, and has good prospects for large-scale application.

Description

Technical Field

[0001] This invention belongs to the field of heterogeneous catalysis and photocatalytic water splitting technology, specifically relating to a dihalogen-regulated polytriazineimide photocatalyst and its preparation method and application, and particularly to a crystalline carbon nitride material that improves the photocatalytic water splitting performance by regulating the crystallization path, ion intercalation state and defect structure of polytriazineimide through mixed halide molten salts. Background Technology

[0002] Photocatalytic water splitting to produce hydrogen using solar energy is an ideal way to directly convert solar energy into chemical energy, offering advantages such as a wide availability of raw materials, mild reaction conditions, and environmental friendliness. Carbon nitride materials are considered one of the most promising photocatalytic water splitting materials due to their simple composition, low cost, and good chemical stability.

[0003] Polytriazine imide (PTI) is a highly crystalline allotrope of polymeric carbon nitride, possessing a relatively regular interlayer stacking structure and a well-developed in-plane conjugated framework, showing promising application potential in photocatalytic water splitting reactions. Current PTI is typically prepared via molten salt-assisted thermal polymerization, often using salt systems such as LiCl / KCl or LiBr / KBr to promote precursor condensation and crystal formation. During this process, halide ions and alkali metal ions can intercalate into the PTI framework channels, thereby affecting its crystal structure, local electric field, electronic structure, and carrier dynamics. However, existing technologies still have the following problems: On the one hand, while the resulting PTI structure is highly stable when using a single chloride salt system, a high crystallization energy barrier needs to be overcome during nucleation, which is not conducive to reducing defect density; on the other hand, although using a single bromine salt system can lower the nucleation energy barrier, the larger Br⁻ particles easily weaken the interlayer stabilization effect, leading to a decrease in crystal structure stability, and thus introducing more structural defects and deep trapped states, inhibiting the effective migration of photogenerated carriers.

[0004] Therefore, developing a method for preparing PTI crystals that can simultaneously achieve low nucleation energy barriers, high structural stability, low defect density, and high charge separation efficiency, and applying it to efficient photocatalytic water splitting, is of significant scientific importance and practical value. Summary of the Invention

[0005] To address the aforementioned problems, this invention provides a dihalogen-regulated polytriazine imide photocatalyst, its preparation method, and its applications. This method utilizes a mixed molten salt system constructed from lithium bromide, sodium chloride, and potassium chloride. By introducing a competitive nucleation and growth mechanism during thermal polymerization, the material's crystallization process follows a phased evolution path from a metastable phase to a stable phase, thereby inducing the formation of partially ion-deficient structures while maintaining the integrity of the crystal framework. These ion-deficient structures effectively reduce the material's defect density, mitigate the adverse effects of deep-trapped states, and promote the separation, transport, and surface reactions of photogenerated carriers, thus improving the performance of polytriazine imide in the photocatalytic water splitting reaction.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A method for preparing a dihalogen-regulated polytriazine imide photocatalyst includes the following steps:

[0008] (1) The carbon nitride precursor was mixed with sodium chloride, potassium chloride and lithium bromide to obtain a precursor / molten salt mixture;

[0009] (2) Place the mixture obtained in step (1) in a container and pretreat it at 400°C for 4 to 8 hours to promote the initial polycondensation of the precursor and obtain a prepolymer intermediate;

[0010] (3) The intermediate obtained in step (2) is subjected to further heat treatment at 500-600℃ for 8-24 hours under closed conditions to cause deaming polycondensation and crystal growth reaction, resulting in a mixture of PTI crystals and molten salt.

[0011] (4) After cooling the mixture obtained in step (3), take it out, wash it thoroughly with pure water to remove residual molten salt, and then dry it to obtain the dihalogen-regulated PTI photocatalyst.

[0012] Furthermore, the carbon nitride precursor mentioned in step (1) is one of urea, melamine, and melamine.

[0013] Further, the mass ratio of the carbon nitride precursor, sodium chloride, potassium chloride and lithium bromide in step (1) is 1:5:4:(1-5).

[0014] Furthermore, the pretreatment temperature in step (1) is preferably 400℃, and the pretreatment time is preferably 4-12 h.

[0015] Furthermore, the heating rate of the heat treatment in step (2) is 60-120℃ / h.

[0016] Furthermore, the drying process described in step (4) is to dry at 60-80°C overnight.

[0017] A dihalogen-regulated polytriazine imide photocatalyst prepared by the above-described method. The photocatalyst exhibits high crystallinity, low defect density, and partial ion vacancy characteristics. Its crystal structure involves a crystallization regulation process with both chloride and bromide ions, forming an electronic structure conducive to carrier separation and migration.

[0018] Furthermore, the aforementioned dihalogen-regulated polytriazine imide photocatalyst is applied to photocatalytic water splitting. Preferably, the photocatalyst is dispersed in pure water, and the water splitting reaction is carried out under light irradiation. Simultaneous hydrogen and oxygen production is achieved by in-situ photodeposition of a co-catalyst on the catalyst surface. Specifically, CoO2 is used in this paper. x As a co-catalyst for oxygen production, PtCrO x It was used as a hydrogen production co-catalyst and tested under 300 W xenon lamp irradiation.

[0019] Compared with the prior art, the present invention has the following advantages:

[0020] (1) The present invention adopts a dual-halogen mixed molten salt system, uses lithium bromide to reduce the crystallization energy barrier in the nucleation stage, and uses sodium chloride / potassium chloride to guide more stable crystal growth in the growth stage, thereby realizing the staged control of the PTI crystallization path, taking into account both the lower nucleation energy barrier and the higher structural stability.

[0021] (2) The competitive crystallization mechanism established in this invention can induce the formation of partially ion-deficient structures, reduce the intercalation ion content without destroying the PTI main framework, reduce structural defects and deep trap states, and improve charge separation and migration efficiency.

[0022] (3) The material obtained in this invention exhibits excellent activity in the photocatalytic water splitting reaction, with the optimized sample showing H2 and O2 evolution rates of up to 915 μmol·h. -1 and 458 μmol·h -1 The apparent quantum efficiency under 365 nm monochromatic illumination can reach 39%, which is more than 300 times higher than that of the control sample 1 and about 1.5 times higher than that of the control sample 2.

[0023] (3) The material obtained by the present invention has good photochemical stability and can maintain a stable stoichiometric ratio for hydrogen and oxygen production after 12 consecutive cycles of testing, indicating that it has good application prospects. Attached Figure Description

[0024] Figure 1 Schematic diagram of the PTI crystallization pathway regulated by dihalogens.

[0025] Figure 2 X-ray diffraction (XRD) patterns of samples from Example 1, Comparative Example 1, and Comparative Example 2.

[0026] Figure 3 Example 1: iDPC-STEM image and ion vacancy distribution map of the sample.

[0027] Figure 4 UV-Vis absorption spectra and defect structure characterization diagrams of the samples from Example 1, Comparative Example 1, and Comparative Example 2.

[0028] Figure 5 Comparison of photocatalytic water splitting performance between Example 1 and Comparative Examples 1 and 2.

[0029] Figure 6 Example 1: Stability test and apparent quantum efficiency curve of the sample. Detailed Implementation

[0030] To make the contents of this invention clearer and easier to understand, the technical solution of this invention will be further described below with reference to specific embodiments, but this invention is not limited to the following embodiments. Figure 1 This is a schematic diagram of the dihalogen-regulated PTI crystallization pathway.

[0031] Example 1

[0032] 1.0 g of melamine, 5.0 g of sodium chloride, 4.0 g of potassium chloride, and 1.0 g of lithium bromide were thoroughly ground and mixed in an agate mortar to obtain a precursor / molten salt mixture. The mixture was placed in an ampoule and pretreated at 400°C for 6 h. After cooling, the ampoule was evacuated and sealed, and then heat-treated at 550°C for another 12 h to induce further deamination polycondensation and crystal growth. After the reaction, the mixture was allowed to cool naturally to room temperature. The solid product was then removed, pulverized, and thoroughly washed with deionized water to remove residual molten salt. It was then dried overnight at 60°C to obtain a white powder product, which is the dihalogen-regulated polytriazine imide photocatalyst, denoted as PTI-ClBr.

[0033] Figure 2 The XRD results shown indicate that the sample obtained in Example 1 has obvious PTI phase diffraction characteristic peaks, indicating that the obtained material has high crystallinity.

[0034] Figure 3 The electron microscopy analysis further shows that the sample exhibits a regular hexagonal crystal morphology and shows local ion vacancies at the atomic scale, while the main carbon-nitrogen framework structure remains intact.

[0035] Example 2

[0036] 1.0 g of melamine, 5.0 g of sodium chloride, 4.0 g of potassium chloride, and 2.0 g of lithium bromide were thoroughly ground and mixed in an agate mortar to obtain a homogeneous mixture. The mixture was transferred to a long-necked ampoule and heated at 400°C for 6 hours. The ampoule was then vacuum-sealed and heated at 550°C for another 12 hours. After the reaction was complete, the mixture was allowed to cool naturally to room temperature. The product was then removed, dispersed in deionized water, filtered, and washed with deionized water until no obvious ion residue remained in the washings. The product was then dried overnight at 60°C to obtain a white powder product, PTI-ClBr2.

[0037] Example 3

[0038] 1.0 g of melamine, 5.0 g of sodium chloride, 4.0 g of potassium chloride, and 3.0 g of lithium bromide were thoroughly ground and mixed in an agate mortar to obtain a homogeneous mixture. The mixture was transferred to a long-necked ampoule and heated at 400°C for 6 hours. The ampoule was then vacuum-sealed and heated at 550°C for another 12 hours. After the reaction was complete, the mixture was allowed to cool naturally to room temperature. The product was then removed, dispersed in deionized water, filtered, and washed with deionized water until no obvious ion residue remained in the washings. The product was dried overnight at 60°C to obtain a white powder, PTI-ClBr3.

[0039] Comparative Example 1

[0040] 1.0 g of melamine, 5.0 g of sodium bromide, 4.0 g of potassium bromide, and 1.0 g of lithium bromide were thoroughly ground and mixed in an agate mortar to obtain a homogeneous mixture. The mixture was transferred to a long-necked ampoule and heated at 400°C for 6 hours. The ampoule was then vacuum-sealed and heated at 550°C for another 12 hours. After the reaction was complete, the mixture was allowed to cool naturally to room temperature. The product was then removed and dispersed in deionized water, filtered, and washed with deionized water until no obvious ion residue remained in the washings. The product was then dried overnight at 60°C to obtain a white powder, which was PTI-Br.

[0041] Figure 4 The UV-Vis absorption spectra of the samples from Example 1 and Comparative Example 1 are shown.

[0042] Comparative Example 2

[0043] 1.0 g of melamine, 5.0 g of sodium chloride, 4.0 g of potassium chloride, and 1.0 g of lithium chloride were thoroughly ground and mixed in an agate mortar to obtain a homogeneous mixture. The mixture was transferred to a long-necked ampoule and heated at 400°C for 6 hours. The ampoule was then vacuum-sealed and heated at 550°C for another 12 hours. After the reaction was complete, the mixture was allowed to cool naturally to room temperature. The product was then removed and dispersed in deionized water, filtered, and washed with deionized water until no obvious ion residue remained in the washings. The product was then dried overnight at 60°C to obtain a white powder, which was PTI-Cl.

[0044] Application Example 1: Fully Activated Water Splitting Experiment

[0045] The hydrogen content of the prepared polymer after a catalytic experiment was analyzed using gas chromatography. The specific procedure was as follows: 100 mg of catalyst was weighed and added to 100 mL of aqueous solution, ultrasonically mixed, and then poured into the reactor. After thoroughly purging the system of air using a vacuum system, the reaction temperature was maintained at 15 °C, and a 300 W xenon lamp was turned on for the reaction. After 1 hour of reaction, the amount of hydrogen and oxygen generated was analyzed by gas chromatography (Shimadzu GC-8A).

[0046] Figure 4 The optical absorption and electron paramagnetic resonance spectra shown further indicate that PTI-ClBr has the lowest paramagnetic signal intensity and the lowest intrinsic absorption, indicating that it has the lowest defect content.

[0047] Figure 5 The graph shows the photocatalytic activity of PTI-ClBr, PTI-Cl, and PTI-Br prepared in Example 1 for total water splitting. As can be seen from the graph, compared to the PTI-Cl and PTI-Br carbon nitrides prepared under conventional conditions, the PTI-ClBr prepared in Example 1, which has a partially ion-deficient structure, exhibits significantly enhanced activity.

[0048] Figure 6 The image shows the catalytic performance of PTI-ClBr with a partially ion-deficient structure prepared in Example 1 after 12 cycles, which exhibits high catalytic stability.

[0049] The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made within the scope of the claims of the present invention should be included in the scope of the present invention.

Claims

1. A method for preparing a low-defect PTI photocatalyst based on dihalogen regulation, characterized in that: Includes the following steps: (1) The carbon nitride precursor was mixed with sodium chloride, potassium chloride and lithium bromide to obtain a mixture; (2) Place the mixture in a container and pretreat it at 400-600°C; (3) The pretreated system is heat-treated at 500-600℃ for 8-24 h under closed conditions to induce deamination polycondensation reaction and crystal growth. (4) After cooling, the product is taken out, washed with water to remove the molten salt and dried to obtain polytriazine imide crystal material.

2. The preparation method according to claim 1, characterized in that: The carbon nitride precursor mentioned in step (1) includes any one of urea, melamine, and melamine.

3. According to the preparation method of claim 1, the mass ratio of the salt system in step (1) is: precursor: sodium chloride: potassium chloride: lithium bromide = 1: 5: 4: (1–5).

4. According to the preparation method of claim 1, the pretreatment temperature in step (2) is 550℃ and the time is 12 h.

5. The preparation method according to claim 1, characterized in that: The heating rate of the heat treatment in step (3) is 60-120℃ / h.

6. A polytriazine imide crystal prepared by any one of the preparation methods according to claims 1-5, characterized in that: The polytriazine imide crystal has a partially ion-deficient structure, an intercalation system containing both Cl⁻ and Br⁻, a reduced defect state density, and a modulated trap energy level distribution.

7. The application of the polytriazine imide crystal as described in claim 6 in photocatalytic water splitting for hydrogen production, characterized in that, The photocatalyst is dispersed in pure water and subjected to a complete water splitting reaction under light irradiation. After in-situ photodeposition of a co-catalyst on the catalyst surface, simultaneous hydrogen and oxygen production is achieved.

8. The application according to claim 7, characterized in that, The inclusion of oxygen-generating co-catalyst CoO x and the hydrogen production co-catalyst PtCrO x .

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