CN / CoNC nano-composite photocatalyst as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of photocatalytic materials, and particularly relates to a CN / CoNC nano-composite photocatalyst as well as a preparation method and application thereof. The CN / CoNC nano-composite photocatalyst is prepared, and the photocatalytic performance is improved by embedding Co monatomic (CoSA) derived from MOF into nitrogen carbide (NC) and integrating the Co monatomic (CoSA) with CN. The CN / CoNC nano composite material can realize effective charge transfer, enhance visible light absorption and accelerate formation of a hydrogen production active center, so that the photocatalytic activity is remarkably improved under visible light (lambda is greater than or equal to 420 nm). Besides, by optimizing distribution of CoSA in NC and combination of CoSA and a CN frame, the photocatalyst keeps high stability, and a potential solution is provided for promoting development of the field of photocatalytic hydrogen production.

Description

Technical Field

[0001] The invention belongs to the technical field of photocatalytic materials, and specifically relates to a CN / CoNC nanocomposite photocatalyst and a preparation method and application thereof. Background Art

[0002] The information disclosed in the background of the invention is only intended to enhance the understanding of the overall background of the invention and should not be necessarily regarded as an acknowledgment or any form of suggestion that the information constitutes the prior art already known to a person skilled in the art.

[0003] Energy is the foundation of sustainable development of human society. However, the rapid consumption of fossil energy and the increase in waste around the world have caused serious energy crises and environmental problems. There is an urgent need to develop clean and efficient alternative energy sources. Hydrogen energy has become a very promising solution because of its clean and pollution-free characteristics and combustion without by-products. Photocatalytic water decomposition technology for hydrogen production, as a frontier field of solar energy conversion and utilization, can directly convert solar energy into chemical energy, achieve efficient energy conversion, and provide a green and sustainable path to alleviate energy shortages and environmental degradation. The core of this technology is the development of photocatalysts with high catalytic performance. Its research is of great strategic significance for promoting the development of clean energy and responding to global energy and environmental challenges.

[0004] Polymeric graphite carbon nitride (g-C3N4, CN for short) is a new type of non-metallic photocatalyst. Graphite carbon nitride has a graphite-like layered material structure, with triazine rings as structural units, stable chemical structure, high corrosion resistance, and unique electronic and energy band structures. It is a very promising photocatalyst. Polymeric graphite carbon nitride shows potential for photocatalytic hydrogen production due to its visible light absorption and photostability. However, polymeric graphite carbon nitride has many defects in hydrogen production due to its low charge separation and migration ability, limited active centers, and poor light absorption ability. Although researchers have modified it through strategies such as element doping, heterostructure construction, and morphology regulation, there are still huge challenges in maintaining material stability while improving catalytic activity. Summary of the invention

[0005] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a CN / CoNC nanocomposite photocatalyst and its preparation method and application. Cobalt single atoms (CoSA) derived from metal organic frameworks (MOFs) are embedded in carbonized nitrogen (NC) (called CoNC), and then introduced into CN to prepare a CN / CoNC nanocomposite photocatalyst, which improves the photocatalytic activity while maintaining the high stability of the material.

[0006] In order to achieve the above object, the present invention is implemented through the following technical solutions: In a first aspect, the present invention provides a method for preparing a CN / CoNC nanocomposite photocatalyst, comprising the following steps: S1. Prepare methanol solutions of cobalt salt and 2-methylimidazole respectively; Uniformly add the methanol solution of cobalt salt into the methanol solution of 2-methylimidazole, ultrasonicate, and mix evenly. After the mixed solution undergoes a solvothermal reaction, dry it to obtain ZIF-67.

[0007] S2. Mix and grind ZIF-67 with urea, and calcine it under the protection of an inert atmosphere to obtain the CN / CoNC nanocomposite photocatalyst.

[0008] In some embodiments, in step S1, the cobalt salt is selected from any one of cobalt chloride, cobalt nitrate, and cobalt sulfate, preferably cobalt nitrate; the mass ratio of the cobalt salt to 2-methylimidazole is 1:1 - 1.3.

[0009] In some embodiments, in step S1, ultrasonicate for 20 - 40 minutes. Ultrasonic operation can ensure uniform mixing of the solution, avoid local concentration unevenness, ultrasonic can promote the dispersion of reactants, accelerate initial nucleation, and form more uniform crystal nuclei.

[0010] In some embodiments, in step S1, the solvothermal treatment is carried out at 110 - 120 °C for 3 - 5 hours. For example, the temperature can be 111 °C, 112 °C, 113 °C, 114 °C, 115 °C, 116 °C, 117 °C, 118 °C, 119 °C. If the temperature is too low, the reaction is slow; if it is too high, it may cause framework decomposition or by-product formation. If the time is insufficient, crystallization is incomplete; if the time is too long, it may cause overgrowth or aggregation of crystals.

[0011] In some embodiments, in step S1, the drying operation is carried out in a vacuum furnace at 70 - 90 °C for 20 - 24 h.

[0012] In some embodiments, in step S2, the mass ratio of ZIF-67 to urea is (1 - 9):10000. Among them, the mass ratio of ZIF-67 to urea is (1 - 9):10000, and it can be the ratio of any value between 1 - 9 to 10000, such as: 2, 3, 4, 5, 6, 7, 8. Preferably, the mass ratio of ZIF-67 to urea is 5:10000. A lower content of ZIF-67 may lead to insufficient Co loading, thereby reducing the photocatalytic activity. On the other hand, exceeding this mass ratio will cause aggregation or shielding effects, reduce the dispersion of CoSA, and thus reduce the activity.

[0013] In some embodiments, in step S2, the inert atmosphere is argon or nitrogen, and the calcination operation is to heat up to 550-600 °C at a heating rate of 4-6 °C / min, hold for 3-5 hours, and the holding time is preferably 3-4 hours. The heating rate can be 4 °C / min, 5 °C / min, 6 °C / min, and the heating rate is preferably 5 °C / min. After the above calcination, it is naturally cooled to obtain the CN / CoNC nanocomposite photocatalyst.

[0014] In the present invention, the calcination temperature is preferably 550-600 °C. If the calcination temperature is too high, the organic ligand 2-methylimidazole may be completely carbonized to form nitrogen-doped cobalt nanoparticles, and even the cobalt nanoparticles may further aggregate to form a sintered body of metallic cobalt, and the carbon layer is further graphitized, resulting in a reduction in nitrogen doping. And the above temperature range can ensure the formation of graphitic carbon nitride from urea. The slow heating rate of 4-6 °C / min helps to form a uniform CoNC structure of ZIF-67. Rapid heating will cause local collapse of the carbon matrix or the formation of macropores, and the polyhedral structure may be damaged, forming irregular porous carbon or fragmented morphology.

[0015] In a second aspect, the present invention provides a CN / CoNC nanocomposite photocatalyst prepared by the preparation method described in the first aspect of the present invention.

[0016] The described CN / CoNC nanocomposite photocatalyst can achieve efficient charge transfer, enhance visible light absorption, and accelerate the formation of hydrogen production active centers, significantly improving the photocatalytic activity. The combination between CoNC and CN maintains the high stability of the catalyst.

[0017] In a third aspect, the present invention provides an application of the CN / CoNC nanocomposite photocatalyst described in the second aspect of the present invention in photocatalytic water splitting for hydrogen production.

[0018] In a fourth aspect, the present invention provides a method for photocatalytic water splitting for hydrogen production, including the following steps: (1), Mix the CN / CoNC nanocomposite photocatalyst, sacrificial agent and water to obtain a suspension; (2), Deposit Pt on the CN / CoNC nanocomposite photocatalyst to obtain a mixed solution; (3), Apply a light source to the mixed solution and perform photocatalytic hydrolysis under visible light to produce hydrogen.

[0019] In some embodiments, in step (1), the sacrificial agent is triethanolamine (TEOA), and the mass-volume ratio of the CN / CoNC nanocomposite photocatalyst to the sacrificial agent is (4-6):1 mg / mL.

[0020] In some embodiments, the loading amount of Pt in step (2) is 1-3% of the weight of the photocatalyst. As an auxiliary agent, Pt is beneficial to reducing the overpotential during hydrogen production and aggregating the photoelectrons during the photocatalytic reaction process. There is no limitation on the Pt loading method, and it can be carried out according to the conventional operations well-known to those skilled in the art. For example, the operation can be carried out as follows: Add a certain amount of H2PtCl6 solution to the above mixed solution, stir magnetically for 5 min, and then the rotation speed is 500-700 r / min; then irradiate the suspension with visible light of a 300 W xenon lamp for 10-30 min, and then stir vigorously for 20 min.

[0021] In some embodiments, the visible light in step (3) is provided by a 300 W xenon lamp with a band-pass filter (λ ≥ 420 nm).

[0022] Under visible light irradiation, photoexcited electrons migrate from the conduction band (CB) of CN to the CoSA sites in CoNC. The CoSA sites are the main active sites for electron accumulation. CoSA promotes the initial electron transfer and plays a crucial role in improving carrier separation and migration. After the electrons migrate to the CoSA sites, they will further migrate to the Pt cocatalyst, and the reduction of protons (H + ) generates H2. The NC component in CoNC can stabilize CoSA and help guide electrons from CN to CoSA, thus realizing efficient charge transfer and preventing electron recombination. At the same time, the holes generated in the valence band (VB) of CN are consumed by triethanolamine (TEOA). As a sacrificial agent, TEOA donates electrons to CN and maintains efficient electron migration. This strategy promotes effective charge separation and transfer, thereby improving the photocatalytic hydrogen production activity.

[0023] The beneficial effects of the present invention are as follows: (1) The CN / CoNC nanocomposite photocatalyst prepared by the present invention realizes the precise construction and distribution of CoSA single atoms in NC and combines it with the CN network to improve the photocatalytic hydrogen production performance while maintaining photocatalytic stability. The hydrogen production performance of the best CN / CoNC-5 (1794.1 μmol h -1 g -1 ) is much higher than that of pure CN (292.4 μmol h -1 g -1 ).

[0024] (2) The present invention optimizes the calcination temperature and time of the precursor mixture in a tubular furnace to obtain the highest density of MOF-derived CoSA embedded in NC while maintaining the structural integrity and high stability of the NC framework. In a tubular furnace with an argon gas flow, the calcination process is precisely controlled to avoid structural deformation, thereby enabling the efficient embedding of CoSA into NC and its successful combination with CN. Description of the Drawings

[0025] The accompanying drawings forming a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments and descriptions thereof of the present invention are used to explain the present invention and do not constitute an improper limitation of the present invention.

[0026] Figure 1 X-ray diffraction spectrograms of CoNC, CN, and CN / CoNC-5.

[0027] Figure 2 Fourier transform infrared spectrograms of CN, CoNC, and CN / CoNC-5 photocatalysts.

[0028] Figure 3 UV-visible diffuse reflectance spectrograms of CN and CN / CoNC-5.

[0029] Figure 4 Photoluminescence spectrograms of CN and CN / CoNC-5 photocatalysts.

[0030] Figure 5 Comparison chart of photocatalytic hydrogen production rates of CN and CN / CoNC photocatalysts prepared in the examples.

[0031] Figure 6 Cyclic stability test chart of CN / CoNC-5 nanocomposite photocatalyst. Specific Embodiments

[0032] It should be noted that the following detailed description is exemplary and is intended to provide further illustration of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention belongs.

[0033] The technical solution of the present invention will be further described below in conjunction with specific embodiments.

[0034] Example 1: Synthesis of ZIF-67: (1) Add 1.539 g of Co(NO3)2·6H2O to 45 mL of methanol, disperse evenly to obtain a methanol solution of Co(NO3)2·6H2O.

[0035] (2) 1.731 g of 2-methylimidazole was dispersed in 45 mL of methanol to obtain a methanol solution of 2-methylimidazole.

[0036] (3) Then, the methanol solution of Co(NO3)2·6H2O was uniformly added to the methanol solution of 2-methylimidazole and ultrasonically treated for 30 minutes.

[0037] (4) The mixed solution was transferred to a polytetrafluoroethylene-lined autoclave and reacted at 120° C. for 4 hours.

[0038] (5) The resulting precipitate was collected by centrifugation and washed several times with ethanol.

[0039] (6) After drying in a vacuum oven at 80 °C for 24 h, ZIF-67 was obtained.

[0040] Embodiment 2: Synthesis of CN / CoNC-1 photocatalyst: (1) 1 mg ZIF-67 and 10 g urea were placed in an agate mortar and ground for 1 h to ensure that ZIF-67 and urea were fully mixed.

[0041] (2) The mixed solid powder was placed in a ceramic dish, and heated to 600°C at a rate of 5°C / min in a tube furnace under an argon atmosphere, kept at that temperature for 4 h, and naturally cooled to room temperature to obtain the CN / CoNC-1 photocatalyst.

[0042] Embodiment 3: Synthesis of CN / CoNC-3 photocatalyst: (1) 3 mg ZIF-67 and 10 g urea were placed in an agate mortar and ground for 1 h to ensure that ZIF-67 and urea were fully mixed.

[0043] (2) The mixed solid powder was placed in a ceramic dish, and heated to 600°C at a rate of 5°C / min in a tube furnace under an argon atmosphere, kept at that temperature for 4 h, and naturally cooled to room temperature to obtain the CN / CoNC-3 photocatalyst.

[0044] Embodiment 4: Synthesis of CN / CoNC-5 photocatalyst: (1) 5 mg ZIF-67 and 10 g urea were placed in an agate mortar and ground for 1 h to ensure that ZIF-67 and urea were fully mixed.

[0045] (2) The mixed solid powder was placed in a ceramic dish, and heated to 600°C at a rate of 5°C / min in a tube furnace under an argon atmosphere, kept at that temperature for 4 h, and naturally cooled to room temperature to obtain the CN / CoNC-5 photocatalyst.

[0046] Example 5: Synthesis of CN / CoNC-7 photocatalyst: (1) Put 7 mg of ZIF-67 and 10 g of urea into an agate mortar and grind for 1 hour to ensure sufficient mixing of ZIF-67 and urea.

[0047] (2) Place the mixed solid powder in a ceramic dish, and in a tubular furnace, under an argon atmosphere, heat it to 600 °C at a heating rate of 5 °C / min, hold for 4 h, and then cool it naturally to room temperature to obtain the CN / CoNC-7 photocatalyst.

[0048] Example 6: Synthesis of CN / CoNC-9 photocatalyst: (1) Put 9 mg of ZIF-67 and 10 g of urea into an agate mortar and grind for 1 hour to ensure sufficient mixing of ZIF-67 and urea.

[0049] (2) Place the mixed solid powder in a ceramic dish, and in a tubular furnace, under an argon atmosphere, heat it to 600 °C at a heating rate of 5 °C / min, hold for 4 h, and then cool it naturally to room temperature to obtain the CN / CoNC-5 photocatalyst.

[0050] Comparative Example 1: Synthesis of CN photocatalyst: (1) Put 10 g of urea into an agate mortar and grind for 1 hour.

[0051] (2) Then place the white solid powder in a ceramic dish, and in a tubular furnace, under an argon atmosphere, heat it to 600 °C at a heating rate of 5 °C / min, hold for 4 h, and then cool it naturally to room temperature to obtain the CN photocatalyst.

[0052] Experimental Example 1 The photocatalytic H2 production rates of CN and CN / CoNC-5 photocatalysts were measured and studied.

[0053] Photocatalytic H2 evolution measurement device: The photocatalytic water splitting experiment was carried out in a 500 mL Pyrex top-irradiation reactor equipped with a closed gas circulation and vacuum pumping device (Labsolar-6A, Perfectlight, Beijing, China).

[0054] According to the conventional method, 50 mg of the photocatalyst was loaded into 100 mL of an aqueous solution containing 10% (volume fraction) of TEAO. Then, 1 wt% of Pt was loaded onto the photocatalyst by photodeposition of H2PtCl6·6H2O. Before irradiation with a 300 W xenon lamp (λ ≥ 420 nm), the reaction mixture was evacuated several times to remove air. During the reaction, the solution was continuously stirred and maintained at 6 °C using circulating cooling water. The generated H2 was measured by gas chromatography (GC 1120, SHP Shanghai), which was connected to a thermal conductivity detector and a 5A molecular sieve column, with N2 as the carrier gas.

[0055] Performance analysis Figure 1 are the X-ray diffraction (XRD) spectra of CoNC, CN, and CN / CoNC-5. As Figure 1 shown, CoNC shows broad peaks at 44.3°, 51.5°, and 76.0°, corresponding to the (111), (200), and (220) planes of metallic Co, respectively, demonstrating the presence of cobalt nanoparticles in NC. Pure CN shows two characteristic diffraction peaks at 13.1° and 27.7°, which are attributed to the (100) and (002) crystal planes of graphitic carbon nitride, respectively. Among them, the diffraction peak at 13.1° originates from the tri-s-triazine groups in the plane, while the strong peak at 27.7° corresponds to the interlayer stacking between adjacent carbon-nitrogen layers. For the CN / CoNC-5 sample, both peaks of CN are retained, while the peak intensities slightly decrease. This result indicates the successful combination of CoNC and CN while maintaining the CN framework.

[0056] Figure 2 are the Fourier transform infrared (FTIR) spectra of CN, CoNC, and CN / CoNC-5 photocatalysts. As Figure 2 shown, the FTIR spectrum of pure CN shows representative bands at 814 cm -1 , 1220 - 1644 cm -1 and 3000 - 3600 cm -1 , which are attributed to triazine ring vibration, CN stretching vibration, and -NH / -OH groups, respectively. The CN / CoNC-5 nanocomposite retains these characteristic peaks, but the transmittance decreases and the peak positions shift slightly. This indicates the successful combination of CoNC and CN.

[0057] Figure 3 are the ultraviolet-visible diffuse reflectance spectra (UV-vis DRS) of CN and CN / CoNC-5. As Figure 3As shown, the UV-vis DRS of the CN and CN / CoNC-5 photocatalysts indicates that the optimal CN / CoNC-5 exhibits better light absorption performance than pure CN, elucidating the important role of CoNC loading in promoting visible light absorption and facilitating the formation of more photoexcited electron-hole pairs, thereby enhancing the photocatalytic H2 production performance.

[0058] Figure 4 Figure shows the photoluminescence spectra (PL) of the CN and CN / CoNC-5 photocatalysts. As Figure 4 shown, the photoluminescence intensity of CN / CoNC-5 is lower than that of bare CN, indicating a reduction in the recombination of photoexcited electron-hole pairs. This improvement in carrier separation is attributed to the synergistic effect of Co single atoms, which provide effective electron trapping sites, while the NC support enhances charge migration. In summary, the combination of CoNC and CN promotes more efficient charge separation and transport.

[0059] Figure 5 Figure shows a comparison of the photocatalytic hydrogen production rates of the CN and CN / CoNC photocatalysts prepared in the examples. As Figure 5 shown, the hydrogen production rates of all CN / CoNC samples are significantly higher than those of pure CN. The maximum activity of the CN / CoNC photocatalyst is attributed to the combined action of Co single atoms and the NC support, which serve as active sites for proton reduction and promoting charge migration, respectively. This strategy promotes more efficient separation and utilization, thereby enhancing the photocatalytic hydrogen production activity.

[0060] Figure 6 Figure shows the cyclic stability test of the CN / CoNC-5 nanocomposite photocatalyst. As Figure 6 shown, the CN / CoNC-5 nanocomposite photocatalyst maintains excellent hydrogen production stability in four consecutive reaction cycles. This result indicates that the combination of CoNC and CN not only improves the photocatalytic performance but also endows it with high stability and recyclability due to the strong interaction between CoNC and the CN network, avoiding the decomposition of the photocatalyst during repeated use.

[0061] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, various modifications and changes can be made to the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A preparation method of a CN / CoNC nanocomposite photocatalyst, characterized in that, It includes the following steps: S1. Prepare methanol solutions of cobalt salt and 2-methylimidazole respectively; Add the methanol solution of cobalt salt evenly into the methanol solution of 2-methylimidazole, carry out ultrasonic treatment, mix evenly, after the mixed solution undergoes solvothermal reaction, dry it to obtain ZIF-67; S2. Mix and grind ZIF-67 and urea in a set ratio, and calcine it under the protection of an inert atmosphere to obtain a CN / CoNC nanocomposite photocatalyst; In the step S2, the mass ratio of ZIF-67 to urea is 1-9:10000; In the step S2, the calcination operation is to heat up to 550-600 °C at a rate of 4-6 °C / min and keep the temperature for 3-5 hours.

2. The preparation method according to claim 1, characterized in that, In the step S1, the cobalt salt is selected from any one of cobalt nitrate, cobalt chloride, and cobalt sulfate, and the mass ratio of the cobalt salt to 2-methylimidazole is 1∶1-1.

3.

3. The preparation method according to claim 1, characterized in that In the step S1, the solvothermal treatment is to keep at 110-120 °C for 3-5 hours.

4. The preparation method according to claim 1, characterized in that, In the step S1, the drying operation is to dry in a vacuum furnace at 70-90 °C for 20-24 hours.

5. The preparation method according to claim 1, characterized in that, In the step S1, the ultrasonic treatment is carried out for 20-40 minutes.

6. The preparation method according to claim 1, characterized in that, In the step S2, the inert atmosphere is argon or nitrogen.

7. A CN / CoNC nanocomposite photocatalyst prepared by the preparation method according to any one of claims 1-6.

8. An application of the CN / CoNC nanocomposite photocatalyst according to claim 7 in photocatalytic water splitting for hydrogen production.

9. A method for photocatalytic water splitting to produce hydrogen, characterized in that, It includes the following steps: (1) Mix the CN / CoNC nanocomposite photocatalyst, sacrificial agent and water to obtain a suspension; (2) Deposit Pt on the CN / CoNC nanocomposite photocatalyst to obtain a mixed solution; (3) Apply a light source to the mixed solution and carry out photocatalytic hydrolysis under visible light to generate hydrogen.

10. The method according to claim 9, characterized in that In the step (1), the sacrificial agent is triethanolamine, and the mass-volume ratio of the CN / CoNC nanocomposite photocatalyst to the sacrificial agent is 4-6:1 mL / mg; in the step (2), the loading amount of Pt is 1-3% of the weight of the photocatalyst.

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