Method for improving crystallinity and catalytic water decomposition performance of polyheptazine imino carbon nitride photocatalyst

Abstract

The invention discloses a method for improving the crystallinity and catalytic water decomposition performance of a polyheptazine imino carbon nitride photocatalyst, and belongs to the technical field of photocatalytic material preparation. The preparation method comprises the following steps: firstly, by taking melamine as a precursor, carrying out thermal polymerization to obtain an oligomer melt consisting of heptazine units; then carrying out high-temperature fused salt calcination treatment in a nitrogen atmosphere to obtain carbon nitride with a certain crystallinity; and adding sodium chloride as a structure regulator, fully grinding, calcining in vacuum, washing with water, and drying to obtain the crystalline-phase carbon nitride photocatalyst with high crystallinity. The method is simple, the cost is low, the actual production requirement is met, and the synthesized photocatalyst has good polymerization degree and light absorption performance, has excellent activity of fully decomposing water to produce hydrogen and oxygen under the condition of no sacrificial agent, and has the potential of realizing photocatalytic full-decomposition water diversion and the prospect of industrial application.

Description

Technical Field

[0001] The invention belongs to the technical field of photocatalytic material preparation, and specifically relates to a method for improving the crystallinity and catalytic water decomposition performance of a polyheptazine imine-based carbon nitride photocatalyst. Background Art

[0002] Rapid economic and social development cannot be separated from the massive consumption of energy, but fossil energy still dominates the current energy structure. Fossil energy takes tens of millions of years to form under harsh conditions, and it is difficult to recover in a short period of time after consumption. In addition, the rapid consumption of fossil energy has brought about serious environmental problems. As an important form of energy on the earth, how to efficiently convert solar energy into high-quality, usable energy is one of the main challenges currently faced. Photocatalytic technology is an emerging technology that uses semiconductor photocatalysts as a medium to convert solar energy into chemical energy or to drive chemical reaction processes that require energy consumption, such as photocatalytic carbon dioxide reduction, photolysis of water to produce hydrogen, and photocatalytic degradation of pollutants. It has broad application prospects.

[0003] At present, the widely developed and utilized photocatalysts are inorganic semiconductor materials. Most of these inorganic semiconductor materials have wide band gaps and can only absorb ultraviolet light, and the utilization rate of visible light is not high. Some metal sulfides are prone to photocorrosion under light conditions and have poor stability. Graphite phase carbon nitride is a polymer semiconductor material composed of non-metallic elements. It has the advantages of simple preparation method, reasonable band gap structure, visible light response, and stable chemical structure. However, the preparation of carbon nitride by direct thermal condensation often leads to insufficient specific surface area and active sites, ineffective regulation of microstructure, and serious recombination of photogenerated charge carriers. These problems greatly restrict the performance improvement and application scope of carbon nitride-based photocatalysts. In addition, due to the insufficient driving force for oxidation of the valence band of carbon nitride and the slow water oxidation rate, hole sacrificial agents such as methanol and triethanolamine are usually added to consume photogenerated holes. The addition of sacrificial agents is not conducive to the separation and purification of products, and also reduces economic benefits. Summary of the invention

[0004] The purpose of the present invention is to provide a method and application for improving the crystallinity and catalytic water decomposition performance of poly(heptazine imine) carbon nitride photocatalyst. The prepared photocatalyst has a high crystallinity, greatly reduces the structural defects of the catalyst, promotes the separation of photogenerated electrons and holes, and has a high solar light utilization rate without sacrificial agents.

[0005] In order to achieve the above object, the present invention adopts the following technical solution: A method for improving the crystallinity and catalytic water decomposition performance of a poly(heptazine imine) carbon nitride photocatalyst comprises the following steps: (1) Place the precursor in a crucible, heat it to the target temperature at a certain heating rate, hold the temperature for a certain period of time, and then cool it naturally to obtain an oligomer melon composed of heptazine units; (2) Mix and grind the oligomer melon obtained in step (1) with molten salt evenly, then heat it to the target temperature at a certain heating rate under an inert atmosphere, hold the temperature for a period of time, and then cool it naturally. Wash it with deionized water multiple times to remove the excess salt to obtain PHI; (3) Grind the PHI obtained in step (2) with a structure regulator sodium chloride, then heat it to the target temperature at a certain heating rate under vacuum, hold the temperature for a period of time, wash it with water and dry it to obtain a crystalline phase carbon nitride photocatalyst, named PHI-re.

[0006] Further, the precursor is one or more of melamine, urea, dicyandiamide, and thiourea.

[0007] Further, in step (1), the heating rate is 1 °C / min to 15 °C / min, the target temperature is 450 °C to 575 °C, and the holding time is 0.5 h - 10 h.

[0008] Further, in step (2), the molten salt is one or more of lithium chloride, sodium chloride, potassium chloride, rubidium chloride, cesium chloride, magnesium chloride, calcium chloride, and strontium chloride.

[0009] Further, in step (2), the heating rate is 1 °C / min to 15 °C / min, the target temperature is 450 °C to 625 °C, and the holding time is 0 h to 48 h.

[0010] Further, in step (2), the mass ratio of the oligomer melon to the molten salt is 1:(1 - 20).

[0011] Further, in step (3), the heating rate is 1 °C / min - 15 °C / min, the target temperature is 300 °C - 600 °C, and the holding time is 0 h - 48 h.

[0012] Further, in step (3), the mass ratio of PHI to the structure regulator sodium chloride is 1:(1 - 50).

[0013] Further, in step (3), PHI-re is heptazine imino carbon nitride, and the microscopic morphology is nanospheres. Apply the prepared highly crystalline phase heptazine imino carbon nitride photocatalyst to one-step photoexcitation photocatalytic overall water splitting for hydrogen production, specifically: while using the highly crystalline phase carbon nitride photocatalyst, use Pt and CoOx as the hydrogen production and oxygen production co-catalysts respectively, and carry out water splitting for hydrogen production and oxygen production under full light illumination.

[0014] The present invention has the following beneficial effects: (1) The carbon nitride photocatalyst PHI-re prepared by the present invention is a highly crystalline carbon nitride obtained by further polymerizing PHI, further reducing the structural defects in PHI prepared by the one-step molten salt method, reducing cyano groups, and then forming highly crystalline heptazine-based carbon nitride under the action of a molten salt template and vacuum calcination.

[0015] (2) Compared with the heptazine crystalline carbon nitride prepared by the one-step molten salt method, it has higher crystallinity, greatly reducing the structural defects on the surface of heptazine carbon nitride, which is beneficial to the separation of photo-generated electrons and holes, and realizing efficient photocatalytic overall water splitting for hydrogen production without a sacrificial agent.

[0016] (3) The entire preparation process of the present invention is simple and easy to control, with low energy consumption and cost, meeting the actual production needs and being conducive to large-scale promotion. Description of the Drawings

[0017] Figure 1 Scanning electron micrographs of the heptazine carbon nitride photocatalysts obtained in Example 2 and Comparative Example 1: (a) Comparative Example 1 and (b) Example 2.

[0018] Figure 2 Comparison chart of powder X-ray diffraction spectra of the heptazine carbon nitride photocatalysts obtained in Example 2 and Comparative Example 1.

[0019] Figure 3 UV-visible diffuse reflectance spectra of the heptazine carbon nitride photocatalysts obtained in Example 2 and Comparative Example 1 Figure 4 Fourier transform infrared spectra of the heptazine carbon nitride photocatalysts obtained in Example 2 and Comparative Example 1.

[0020] Figure 5 High-resolution transmission electron micrograph of the heptazine carbon nitride photocatalyst obtained in Example 2.

[0021] Figure 6 Comparison chart of powder X-ray diffraction spectra of the heptazine carbon nitride photocatalysts obtained in Examples 1-5.

[0022] Figure 7 Comparison chart of the photocatalytic overall water splitting activities of the heptazine carbon nitride photocatalysts obtained in Examples 1-5.

[0023] Figure 8 Comparison chart of the photocatalytic overall water splitting activities of the heptazine carbon nitride photocatalysts obtained in Example 2 and Comparative Examples 1 and 2. Detailed Embodiments

[0024] To make the content of the present invention easier to understand, the technical solutions of the present invention will be further described below in conjunction with specific embodiments, but the present invention is not limited thereto.

[0025] Based on a highly crystalline heptazine imino carbon nitride photocatalyst for visible light overall water splitting, it is prepared by the following method: Example 1 (1) Put melamine in a crucible, heat it to 550 °C at a heating rate of 2 °C / min, keep it warm for 2 hours, and naturally cool it to obtain a low polymer melon composed of heptazine units.

[0026] (2) After uniformly grinding the low polymer melon, sodium chloride, and lithium chloride according to a mass ratio of 1:3.5:6.5, heat it to 550 °C under nitrogen and keep it warm for 4 hours, then wash to remove the excess salt to obtain carbon nitride with a certain degree of crystallinity, named PHI.

[0027] (3) After fully grinding PHI and sodium chloride according to a mass ratio of 1:10, place it in an ampoule for vacuum pumping and sealing, and then calcine it at 475 °C for 6 hours. After washing and drying, a highly crystalline heptazine-based carbon nitride photocatalyst is obtained, named PHI-re.

[0028] Example 2 (1) Put melamine in a crucible, heat it to 550 °C at a heating rate of 2 °C / min, keep it warm for 2 hours, and naturally cool it to obtain a low polymer melon composed of heptazine units.

[0029] (2) After uniformly grinding the low polymer melon, sodium chloride, and lithium chloride according to a mass ratio of 1:3.5:6.5, heat it to 550 °C under nitrogen and keep it warm for 4 hours, then wash to remove the excess salt to obtain carbon nitride with a certain degree of crystallinity, named PHI.

[0030] (3) After fully grinding PHI and sodium chloride according to a mass ratio of 1:10, place it in an ampoule for vacuum pumping and sealing, and then calcine it at 500 °C for 6 hours. After washing and drying, a highly crystalline heptazine-based carbon nitride photocatalyst is obtained, named PHI-re.

[0031] Example 3 (1) Put melamine in a crucible, heat it to 550 °C at a heating rate of 2 °C / min, keep it warm for 2 hours, and naturally cool it to obtain a low polymer melon composed of heptazine units.

[0032] (2) After uniformly grinding the low polymer melon, sodium chloride, and lithium chloride according to a mass ratio of 1:3.5:6.5, heat it to 550 °C under nitrogen and keep it warm for 4 hours, then wash to remove the excess salt. Carbon nitride with a certain degree of crystallinity is obtained, named PHI.

[0033] (3) PHI was further ground with sodium chloride in a mass ratio of 1:10, placed in an ampoule for vacuum pumping and sealing, and then calcined at 525 °C for 6 hours. After washing and drying, a highly crystalline heptazine-based carbon nitride photocatalyst was obtained, named PHI-re.

[0034] Example 4 (1) Melamine was placed in a crucible and heated to 550 °C at a heating rate of 2 °C / min, held for 2 hours, and then naturally cooled to obtain a low polymer melon composed of heptazine units.

[0035] (2) The low polymer melon was ground evenly with sodium chloride and lithium chloride in a mass ratio of 1:3.5:6.5, and then held at 550 °C for 4 hours under nitrogen, and the excess salt was removed by washing. A carbon nitride with a certain degree of crystallinity was obtained, named PHI.

[0036] (3) PHI was further ground with sodium chloride in a mass ratio of 1:10, placed in an ampoule for vacuum pumping and sealing, and then calcined at 550 °C for 6 hours. After washing and drying, a highly crystalline heptazine-based carbon nitride photocatalyst was obtained, named PHI-re.

[0037] Example 5 (1) Melamine was placed in a crucible and heated to 550 °C at a heating rate of 2 °C / min, held for 2 hours, and then naturally cooled to obtain a low polymer melon composed of heptazine units.

[0038] (2) The low polymer melon was ground evenly with sodium chloride and lithium chloride in a mass ratio of 1:3.5:6.5, and then held at 550 °C for 4 hours under nitrogen, and the excess salt was removed by washing. A carbon nitride with a certain degree of crystallinity was obtained, named PHI.

[0039] (3) PHI was further ground with sodium chloride in a mass ratio of 1:10, placed in an ampoule for vacuum pumping and sealing, and then calcined at 575 °C for 6 hours. After washing and drying, a highly crystalline heptazine-based carbon nitride photocatalyst was obtained, named PHI-re.

[0040] Comparative Example 1 (1) Melamine was placed in a crucible and heated to 550 °C at a heating rate of 2 °C / min, held for 2 hours, and then naturally cooled to obtain a low polymer melon composed of heptazine units.

[0041] (2) The low polymer melon was ground evenly with sodium chloride and lithium chloride in a mass ratio of 1:3.5:6.5, and then held at 550 °C for 4 hours under nitrogen, and the excess salt was removed by washing. A carbon nitride with a certain degree of crystallinity was obtained, named PHI.

[0042] Comparative Example 2 (1) Melamine was placed in a crucible and heated to 550 °C at a heating rate of 2 °C / min, held for 2 hours, and then naturally cooled to obtain a low polymer melon composed of heptazine units.

[0043] (2) The low polymer melon was ground evenly with sodium chloride and lithium chloride in a mass ratio of 1:3.5:6.5, and then held at 550 °C for 12 hours under nitrogen, and the excess salt was removed by washing. Carbon nitride with a certain degree of crystallinity was obtained and named PHI.

[0044] (3) PHI was further ground with sodium chloride in a mass ratio of 1:10, placed in an ampoule, evacuated and sealed, and then calcined at 500 °C for 12 hours. After washing and drying, a heptazine-based carbon nitride photocatalyst with high crystallinity was obtained and named PHI-re.

[0045] Figure 1 are the scanning electron microscope images of the carbon nitride photocatalysts obtained in (a) Comparative Example 1 and (b) Example 2. It can be observed from the figure that Example 2 has larger size and planar conjugation after further grinding and calcination with the sodium chloride structural agent.

[0046] Figure 2 are the comparative powder X-ray diffraction spectra of the carbon nitride photocatalysts obtained in Example 2 and Comparative Example 1. It can be found from the figure that the carbon nitride photocatalyst of Example 2 after treatment has the narrowest XRD spectral half-peak width, indicating that the crystallinity of Example 2 is better than that of Comparative Example 1.

[0047] Figure 3 are the ultraviolet-visible diffuse reflectance spectra of the carbon nitride photocatalysts obtained in Example 2 and Comparative Example 1. It can be seen from the figure that both photocatalysts have visible light absorption.

[0048] Figure 4 are the Fourier transform infrared spectra of the carbon nitride photocatalysts obtained in Example 2 and Comparative Example 1. It can be found from the figure that the prepared photocatalyst is a carbon nitride-based photocatalyst. Among them, 3000~3400, 1000~1700 and 810 nm are the characteristic infrared absorption peaks of the heptazine carbon nitride material.

[0049] Figure 5 is the high-resolution transmission electron microscope image of the carbon nitride photocatalyst obtained in Example 2. This figure further shows that the photocatalyst has a high degree of crystallinity. Among them, the lattice spacings of about 0.98 and 0.32 nm represent the planar expansion and interlayer stacking of the heptazine-based crystalline phase carbon nitride.

[0050] Figure 6Powder X-ray diffraction spectrum comparison diagram of the carbon nitride photocatalysts obtained in Examples 1-5. It can be found from the figure that the prepared carbon nitride photocatalysts all have a narrow half-peak width of the spectrum, showing a high degree of crystallinity.

[0051] Overall water splitting activity experiment: A reaction system composed of 100 mg of photocatalyst, 100 ml of aqueous solution, 1% mass fraction of Pt (67 μl of H2PtCl6 solution with a Pt content of 15 mg / ml, irradiated under a xenon lamp for one hour), and 0.5% mass fraction of CoOx (100 μl of Co(NO3)2 solution with a Co content of 5 mg / ml, irradiated under a xenon lamp for one hour) as co-catalysts Figure 7 Comparison diagram of the photocatalytic overall water splitting activity of the carbon nitride photocatalysts obtained in Examples 1-5. It can be found from the figure that the performance of the carbon nitride photocatalyst prepared when the calcination temperature in step (3) is 500 °C (Example 2) is significantly higher than that of the carbon nitride photocatalysts prepared at other temperatures.

[0052] Figure 8 Comparison diagram of the photocatalytic overall water splitting activity of the carbon nitride photocatalysts obtained in Example 2, Comparative Example 1, and Comparative Example 2. It can be found from the figure that the performance of the carbon nitride photocatalyst prepared at 500 °C for 6 h is significantly higher than that of the carbon nitride photocatalyst prepared at other times. Among them, the overall water splitting activity of the photocatalyst prepared in Example 2 is 370 μmol / h for hydrogen and 185 μmol / h for oxygen under full light irradiation, which is significantly higher than that of the photocatalyst prepared in Comparative Example 1.

[0053] The above are only the preferred embodiments of the present invention. All equivalent changes and modifications made according to the scope of the patent application of the present invention shall fall within the scope covered by the present invention.

Claims

1. A method for improving the crystallinity and catalytic water decomposition performance of poly(heptazine imine) carbon nitride photocatalyst, characterized in that: The following steps are involved: (1) placing the precursor in a crucible, heating it to the target temperature at a certain heating rate, and keeping it at that temperature for a certain period of time, and then naturally cooling to obtain the oligomer melon composed of heptazine units; (2) mixing and grinding the oligomer melon obtained in step (1) and the molten salt uniformly, heating to a target temperature at a certain heating rate under an inert atmosphere, keeping the temperature for a period of time, cooling naturally, and washing with deionized water for multiple times to remove excess salt, thereby obtaining PHI; (3) After grinding the PHI obtained in step (2) and the structure regulator sodium chloride, the temperature is raised to the target temperature at a certain rate under vacuum, and after keeping the temperature for a period of time, the crystalline carbon nitride photocatalyst is obtained after washing and drying, which is named PHI-re.

2. The method according to claim 1, characterized in that: The precursor is one or more of melamine, urea, dicyandiamide and thiourea.

3. The method according to claim 1, characterized in that: The heating rate in step (1) is 1°C / min to 15°C / min, the target temperature is 450°C to 575°C, and the holding time is 0.5 h to 10 h.

4. The method according to claim 1, characterized in that: The molten salt in step (2) is one or more of lithium chloride, sodium chloride, potassium chloride, rubidium chloride, cesium chloride, magnesium chloride, calcium chloride and strontium chloride.

5. The method according to claim 1, characterized in that: The heating rate in step (2) is 1°C / min to 15°C / min, the target temperature is 450°C to 625°C, and the insulation time is 0h to 48h.

6. The method according to claim 1, characterized in that: The mass ratio of the oligomer melon to the molten salt in step (2) is 1:(1-20).

7. The method according to claim 1, characterized in that: The heating rate in step (3) is 1°C / min-15°C / min, the target temperature is 300°C-600°C, and the insulation time is 0h - 48h.

8. The method according to claim 1, characterized in that: The mass ratio of PHI to the structure regulator sodium chloride in step (3) is 1:(1-50).

9. The method according to claim 1, characterized in that: The PHI-re described in step (3) is heptazine imine carbon nitride, and its microscopic morphology is nanospheres.

10. Use of the heptazine imine carbon nitride prepared by the method according to claims 1 to 9 in photocatalytic overall water decomposition to produce hydrogen, characterized in that: Heptazine-imino carbon nitride can photocatalytically split water to produce hydrogen and oxygen without sacrificial agent.

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