A g-C3N4-based composite catalyst, a preparation method thereof, and application in piezoelectric catalytic reforming of lignocellulose for hydrogen production

Abstract

The present invention provides a g-C3N4-based composite catalyst, a preparation method thereof, and an application thereof in piezoelectric catalytic reforming of lignocellulose to hydrogen, which relates to the technical field of composite catalyst preparation. The above-mentioned g-C3N4-based composite catalyst comprises g-C3N4 micro-rods and noble metal nanoparticles loaded on the surface of the g-C3N4 micro-rods, and the preparation method thereof is as follows: S1. Hydrothermally treat a mixed solution of melamine and concentrated nitric acid to prepare a g-C3N4 micro-rod precursor; calcine the g-C3N4 micro-rod precursor to obtain g-C3N4 micro-rods; S2. Use the photodeposition method to load noble metal nanoparticles on the surface of the g-C3N4 micro-rods prepared in step S1. The g-C3N4-based composite catalyst prepared by the preparation method proposed by the present invention has excellent piezoelectric catalytic performance, and can convert lignocellulose and the like into hydrogen under normal temperature and pressure only by means of external mechanical force.

Description

Technical Field

[0001] The present invention relates to the technical field of composite catalyst preparation, and in particular to a g-C3N4-based composite catalyst, a preparation method thereof, and an application in piezoelectric catalytic reforming of lignocellulose to produce hydrogen. Background Art

[0002] Hydrogen is regarded as an ideal energy carrier to replace fossil fuels due to its high calorific value combustion characteristics, zero carbon emission advantage and renewable attribute. At present, industrial hydrogen production mainly relies on methane steam reforming technology, which needs to operate at a high temperature of 800 °C, and has significant defects such as high energy consumption and high carbon emission intensity. In contrast, photocatalytic water splitting technology can directly convert water into hydrogen energy using solar energy at room temperature and atmospheric pressure, showing great potential for sustainable development. In recent years, researchers have proposed using lignocellulosic biomass waste (derived from photosynthesis products such as agricultural and forestry waste) as a sacrificial agent for photo-generated holes. This strategy can not only significantly improve the hydrogen production efficiency of the photocatalytic system, but also realize the high-value utilization of biomass resources and effectively alleviate the problem of dependence on fossil energy.

[0003] However, the photocatalytic reforming of lignocellulose biomass to produce hydrogen still faces multiple challenges: (1) Lignocellulose is composed of cellulose, hemicellulose and lignin through a complex cross-linked structure. Its high crystallinity and chemical bond stability lead to a high dissociation energy barrier. The band gap widths of conventional semiconductor catalysts mostly concentrate on 2.5 - 3.5 eV, and the oxidation potential of photo-generated holes is difficult to effectively break its chemical bonds; (2) Natural lignocellulose (such as straw, wood chips, etc.) mostly exists in a solid state, and its light scattering effect severely restricts the light energy utilization rate, resulting in the photocatalytic reaction can only occur on the surface layer of the system, and the overall efficiency is significantly limited.

[0004] Piezoelectrocatalysis technology provides a new idea for solving the above bottlenecks. Its mechanism is similar to photocatalysis, both relying on carriers to drive redox reactions, but the core difference is that mechanical energy replaces light energy as the driving force. By mechanically stressing, a directional polarization electric field is formed inside the piezoelectric material, thereby inducing the effective separation and migration of carriers. Compared with traditional photocatalytic systems, piezoelectrocatalysis exhibits two significant advantages: Firstly, mechanical vibration waves have excellent medium penetration, which can break through the bottleneck that photon transmission in photocatalytic systems is limited by the solution surface layer and material light transmittance, and achieve the overall effect on biomass suspension systems with low transparency; Secondly, under the action of mechanical stress, an instantaneous ultra-high potential (up to the order of hundreds of volts) can be generated on the surface of the piezoelectric material, far exceeding the oxidation ability of traditional photocatalytic systems. This super-strong electric field endows the system with stronger redox ability. It should be noted that current research on piezoelectrocatalytic hydrogen production is still in the basic research stage, and easily degradable small molecules such as glucose, methanol, and triethanolamine are generally used as sacrificial agents. If the reaction substrate can be extended to strategic resources such as lignocellulose, not only can the cost of hydrogen production raw materials be significantly reduced, but also the high-value utilization of agricultural and forestry waste can be realized, with dual environmental benefits of energy conversion and carbon fixation and emission reduction.

[0005] In view of this, it is necessary to design an improved g-C3N4-based composite catalyst, its preparation method, and its application in piezoelectrocatalytic reforming of lignocellulose to hydrogen to solve the above problems. Summary of the Invention

[0006] The object of the present invention is to provide a g-C3N4-based composite catalyst, its preparation method, and its application in piezoelectrocatalytic reforming of lignocellulose to hydrogen.

[0007] On the one hand, to achieve the above-mentioned invention object, the present invention provides a g-C3N4-based composite catalyst, including g-C3N4 micro-rods and noble metal nanoparticles loaded on the surface of the g-C3N4 micro-rods;

[0008] The g-C3N4 micro-rods have a length of 100 - 200 μm and a width of 10 - 40 μm; the content of the noble metal nanoparticles in the composite catalyst is 1 - 4 wt%.

[0009] Furthermore, the present invention also provides a preparation method of the g-C3N4-based composite catalyst, including the following steps:

[0010] S1. Prepare a g-C3N4 micro-rod precursor by hydrothermal treatment of a mixed solution of melamine and concentrated nitric acid; calcine the g-C3N4 micro-rod precursor to obtain g-C3N4 micro-rods;

[0011] S2. Use the photodeposition method to load noble metal nanoparticles on the surface of the g-C3N4 micro-rods prepared in step S1.

[0012] Preferably, in step S2, the loading of noble metal nanoparticles on the surface of the g-C3N4 micro-rods is carried out as follows: the g-C3N4 micro-rods, a salt or acid solution containing a noble metal element, and an alkaline solution containing lignocellulose are mixed, and under stirring conditions, irradiated with light for 1 - 4 h to obtain the g-C3N4-based composite catalyst.

[0013] Preferably, the alkaline solution is obtained by dissolving lignocellulose in an NaOH solution; the concentration of the NaOH solution is 1 - 4 mol / L, and the concentration of lignocellulose in the alkaline solution is 0 - 5 g / L.

[0014] Preferably, in step S1, the preparation method of the g-C3N4 micro-rods is as follows: concentrated nitric acid is added to an aqueous solution of melamine, and after mixing evenly, hydrothermal reaction is carried out at 140 - 160 °C for 8 - 12 h to obtain a g-C3N4 micro-rod precursor; under an inert atmosphere, the g-C3N4 micro-rod precursor is calcined to obtain the g-C3N4 micro-rods.

[0015] Preferably, the temperature of the calcination treatment is 500 - 600 °C, and the time is 4 - 6 h.

[0016] Preferably, the light source used during light irradiation is an ultraviolet lamp with ≤395 nm or a composite light source containing a wavelength of ≤395 nm.

[0017] Preferably, the lignocellulose is one or more of microcrystalline cellulose, hemicellulose, lignin, starch, branches, leaves, grass, bamboo powder, wood blocks, cotton, and cotton cloth.

[0018] On the other hand, the g-C3N4-based composite catalyst prepared by the preparation method provided by the present invention can be applied to piezoelectrocatalytic reforming of lignocellulose for hydrogen production. The specific application method is as follows: after mixing the g-C3N4-based composite catalyst with an NaOH solution in which the substrate is dissolved, under mechanical force, the g-C3N4-based composite catalyst induces the substrate to undergo a hydrogen evolution reaction; the substrate is one of agricultural and forestry biomass waste, industrial by-products, and synthetic materials.

[0019] The beneficial effects of the present invention are:

[0020] 1. The preparation method of the g-C3N4-based composite catalyst provided by the present invention is to first prepare g-C3N4 micro-rods, and then load a specific amount of noble metal nanoparticles on their surfaces. By utilizing the excellent stress response characteristics of the rod-like structure and generating a larger dipole moment due to its micro-scale size to form a high-intensity piezoelectric electric field, the loading of noble metal nanoparticles effectively improves the separation efficiency of piezoelectric electrons and the intrinsic activity of the surface hydrogen evolution reaction. Under the combined action of the above two aspects, the composite catalyst is endowed with more excellent catalytic performance in the piezoelectric catalysis process.

[0021] 2. The g-C3N4-based composite catalyst provided by the present invention can convert lignocellulose and the like into hydrogen at normal temperature and pressure only by means of external mechanical force (such as ultrasonic vibration, etc.), and still exhibit good catalytic performance after multiple cycles. Brief Description of the Drawings

[0022] Figure 1 SEM image of the g-C3N4 micro-rods prepared in Example 1 of the present invention;

[0023] Figure 2 TEM image of the g-C3N4-based composite catalyst prepared in Example 1 of the present invention;

[0024] Figure 3 Cyclic stability result diagram of the g-C3N4-based composite catalyst prepared in Example 1 of the present invention for piezoelectric catalytic cellulose reforming to produce hydrogen;

[0025] Figure 4 Hydrogen production performance comparison diagram of the g-C3N4-based composite catalyst prepared in Example 1 of the present invention for different lignocellulose substrates;

[0026] Figure 5 Hydrogen production rate of the g-C3N4-based composite catalysts prepared in Examples 1 to 4 of the present invention;

[0027] Figure 6 Hydrogen production rate of the composite catalysts prepared in Example 1 and Examples 5 to 8 of the present invention;

[0028] Figure 7 Hydrogen production rate of the composite catalysts prepared in Example 1 and Examples 9 to 12 of the present invention;

[0029] Figure 8 Hydrogen production rate of the composite catalysts prepared in Example 1 and Comparative Examples 1-2 of the present invention. Detailed Embodiments

[0030] In order to make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0031] Here, it should also be noted that in order to avoid obscuring the present invention with unnecessary details, only the structures and / or processing steps closely related to the solution of the present invention are shown in the drawings, while other details less relevant to the present invention are omitted.

[0032] In addition, it should also be noted that the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements not only includes those elements, but also includes other elements not explicitly listed, or further includes elements inherent to such process, method, article or device.

[0033] On the one hand, the present invention provides a g-C3N4-based composite catalyst, comprising:

[0034] g-C3N4 micro-rods, used as a carrier, with a length of 100 - 200 μm and a width of 10 - 40 μm;

[0035] Noble metal nanoparticles, loaded on the surface of g-C3N4, and the content of noble metal nanoparticles in the composite catalyst is 1 - 4 wt%; wherein, the noble metal is one of Ag, Au, Pt, Pd, Ru, preferably Ag, and when the loaded particles are Ag nanoparticles, their particle size is 10 - 20 nm.

[0036] Furthermore, the present invention also provides a preparation method of the above-mentioned g-C3N4-based composite catalyst, comprising the following steps:

[0037] S1. Prepare g-C3N4 micro-rods;

[0038] S2. Use the photodeposition method to load noble metal nanoparticles on the surface of the g-C3N4 micro-rods prepared in step S1 to obtain the g-C3N4-based composite catalyst.

[0039] As an implementation manner of the present invention, in step S1, the preparation method of the g-C3N4 micro-rods is as follows: Add concentrated nitric acid to an aqueous solution of melamine, mix evenly, and then carry out a hydrothermal reaction at 140 - 160 °C for 8 - 12 h to obtain a g-C3N4 micro-rod precursor; under an inert atmosphere, perform a calcination treatment on the g-C3N4 micro-rod precursor to obtain the g-C3N4 micro-rods.

[0040] Specifically, the temperature of the calcination treatment is 500 - 600 °C, the time is 4 - 6 h, and the heating rate from room temperature to the target temperature during the calcination process is 5 °C / min; the inert atmosphere is one of nitrogen, helium, argon, etc. The concentration of melamine dispersed in water is 33.3 g / L, and its ratio to concentrated nitric acid (concentration of 68%) is: 0.667 g of melamine / 1 mL of concentrated nitric acid.

[0041] As an embodiment of the present invention, in step S2, the loading of noble metal nanoparticles on the surface of g-C3N4 micro-rods is carried out as follows: Mix the g-C3N4 micro-rods and a salt or acid solution containing a noble metal element with an alkaline solution in which lignocellulose is dissolved. Under stirring conditions, irradiate with light for 1 - 4 h to obtain the g-C3N4-based composite catalyst. Among them, the light source for the light irradiation process is an ultraviolet lamp with a wavelength ≤ 395 nm or a composite light source containing a wavelength ≤ 395 nm; the alkaline solution is obtained by dissolving lignocellulose in a NaOH solution until it reaches a saturated state, and the concentration of the NaOH solution is 1 - 4 mol / L, preferably 3 mol / L; lignocellulose includes but is not limited to one or several of microcrystalline cellulose, hemicellulose, lignin, starch, branches, leaves, grass, bamboo powder, wood blocks, cotton, and cotton cloth. In the above process, cellulose serves as a hole scavenger. If the concentration is too low, it will directly affect the deposition of silver and the hydrogen production reaction rate, while if the concentration is too high, it will increase the solution viscosity and affect the ultrasonic cavitation effect.

[0042] As an embodiment of the present invention, in step S2, the salt or acid solution containing a noble metal element is specifically one of AgNO3, H2PtCl6, HAuCl4, K2PdCl6, and RuCl3 solutions. The solution concentration can be selected and adjusted according to actual application requirements, and this is not limited herein.

[0043] As an embodiment of the present invention, in step S2, the loading amount of noble metal nanoparticles in the g-C3N4-based composite catalyst is 1 - 4 wt%. To achieve the above purpose, an appropriate amount of g-C3N4 micro-rods can be added during the preparation process according to the content of the noble metal element in the salt or acid solution containing the noble metal element.

[0044] On the other hand, the present invention also provides an application method of the g-C3N4-based composite catalyst in piezo-catalytic lignocellulose reforming for hydrogen production, which specifically includes the following steps: After mixing the g-C3N4-based composite catalyst with a NaOH solution in which the substrate is dissolved, under the action of mechanical force (such as ultrasonic vibration, etc.), the g-C3N4-based composite catalyst generates piezo-charges to induce the substrate to undergo a hydrogen evolution reaction. Specifically, the substrate includes agricultural and forestry biomass wastes such as cellulose / hemicellulose / lignin, bamboo powder, nut shells, straws, leaves, and herbaceous plants, industrial by-products such as waste paper, cotton textile scraps, and wood processing scraps, and synthetic materials such as polylactic acid plastics.

[0045] The following further illustrates the g-C3N4-based composite catalyst, its preparation method, and the application of piezo-catalytic cellulose reforming for hydrogen production provided by the present invention in combination with specific examples:

[0046] Example 1

[0047] In this embodiment, a g-C3N4-based composite catalyst was prepared. The specific preparation method includes the following steps:

[0048] Disperse 5 g of melamine in 150 mL of deionized water. After stirring for 15 min, add 7.5 mL of concentrated nitric acid, let it stand for 30 min, and then carry out a hydrothermal reaction at 150 °C for 10 h. After the reaction is completed, wash the obtained solid product with deionized water 3 - 5 times, and then wash it once with absolute ethanol. After the washing is completed, dry it at 60 °C for 3 h to obtain the g-C3N4 micro-rod precursor. Place the g-C3N4 micro-rod precursor in a quartz boat, and in a tubular furnace with N2 flowing, heat it from room temperature to 550 °C at a heating rate of 5 °C / min and calcine it for 4 h to obtain g-C3N4 micro-rods;

[0049] Disperse 50 mg of g-C3N4 micro-rods in 200 mL of an alkaline solution, and then add a certain amount of AgNO3 solution to the above solution to ensure that the loading amount of Ag is 2 wt%. Place the mixed solution obtained in the above step in a reactor, seal the reactor with a silica gel plug, and irradiate it with a 395 nm ultraviolet lamp under magnetic stirring for 1 h. After the irradiation is completed, centrifuge to collect the solid sample, and wash the solid sample with deionized water until the supernatant is neutral. Then dry the obtained product in a vacuum drying oven at 60 °C to prepare the g-C3N4-based composite catalyst. Among them, the alkaline solution is the supernatant obtained by dissolving microcrystalline cellulose (Merida, CAS: 9004-34-6) in a NaOH solution at 25 °C until saturation. The concentration of the NaOH solution is 3 mol / L, and the concentration of the dissolved microcrystalline cellulose is about 3 - 3.75 g / L. In addition, before the ultraviolet light irradiation, the inside of the reactor needs to be purged with argon for 10 min.

[0050] The SEM image of the g-C3N4 micro-rods prepared in this embodiment is as Figure 1 shown. It can be seen from the figure that g-C3N4 is a solid rod-like structure, 100 - 200 μm in length and 10 - 40 μm in width. The partial TEM image of the g-C3N4-based composite catalyst prepared in this embodiment is as Figure 2As shown in the figure, it can be seen that Ag nanoparticles are loaded on the surface of g-C3N4, and their size is 10 nm. The g-C3N4-based composite catalyst prepared in this example was applied to piezo-catalytic cellulose reforming for hydrogen production. The specific test steps are as follows: 5 mg of the composite catalyst was dispersed in 20 mL of an alkaline solution of microcrystalline cellulose. The alkaline solution of microcrystalline cellulose was the supernatant obtained by dissolving microcrystalline cellulose (Merida, CAS: 9004-34-6) in a NaOH solution at 25 °C until saturated. The concentration of the NaOH solution was 3 mol / L. It was sonicated for 3 h at the central position (i.e., the position with the strongest vibration) of an ultrasonic cleaner with a power of 160 W and a frequency of 40 kHz. After each hour, the gaseous product was collected with a syringe and injected into a gas chromatograph (s▪sun GC-9860) to detect the H2 production, and the hydrogen production rate was calculated. After the test, the catalyst was separated from the substrate, the supernatant was removed, the same volume of freshly prepared NaOH solution of microcrystalline cellulose was added, and after purging with argon, the next test was carried out. A total of three cycles were tested, and each cycle was tested for 12 h. The results are as Figure 3 As shown in the figure, it can be seen that the catalytic activity of the composite catalyst was basically maintained unchanged during the three cycles.

[0051] Furthermore, the present invention explored the catalytic activity of the above-mentioned g-C3N4-based composite catalyst on different substrates such as hemicellulose, lignin, plastics, paper, cotton cloth, wood powder, bamboo powder, grass, and leaves. 5 mg of the composite catalyst was dispersed in 20 mL of alkaline solutions of different substrates for testing. The alkaline solution was the supernatant obtained by dissolving different substrates in a NaOH solution at 25 °C until saturated. The concentration of the NaOH solution was 3 mol / L. The hydrogen production rate results under different substrates are as Figure 4 shown. The results show that the hydrogen production efficiency of different substrates is different. Among them, the hydrogen production efficiency of PLA (polylactic acid) can reach 1759 µmol / g / h; among natural biomasses, the hydrogen production efficiency of biomasses such as iris leaves, foxtail grass leaves, and sapindus leaves can reach about 1300 µmol / g / h.

[0052] Example 2

[0053] The difference between Example 2 and Example 1 is only that: when preparing the g-C3N4-based composite catalyst, the addition amount of silver nitrate is different from that in Example 1, and the Ag loading amount is 1 wt%, and other experimental parameters and experimental conditions are the same as those in Example 1, which will not be elaborated here.

[0054] Example 3

[0055] Example 3 is only different from Example 1 in that when preparing the g-C3N4-based composite catalyst, the addition amount of silver nitrate is different from that in Example 1, and the loading amount of Ag is 3 wt%, and other experimental parameters and experimental conditions are the same as those in Example 1, which will not be elaborated here.

[0056] Example 4

[0057] Example 4 is only different from Example 1 in that when preparing the g-C3N4-based composite catalyst, the addition amount of silver nitrate is different from that in Example 1, and the loading amount of Ag is 4 wt%, and other experimental parameters and experimental conditions are the same as those in Example 1, which will not be elaborated here.

[0058] To compare the performance of the g-C3N4-based composite catalysts prepared in Examples 1 to 4 in piezoelectric catalytic hydrogen production from cellulose reforming, they were applied to piezoelectric catalytic hydrogen production from cellulose reforming. The specific steps are as follows: Disperse 5 mg of the composite catalyst in 20 mL of the alkaline solution of microcrystalline cellulose. The alkaline solution of microcrystalline cellulose is the supernatant obtained by dissolving microcrystalline cellulose (Merida, CAS: 9004-34-6) in a NaOH solution at 25 °C until saturated, and the concentration of the NaOH solution is 3 mol / L. Ultrasonic for 3 h at the central position (i.e., the position with the strongest vibration) of an ultrasonic cleaner with a power of 160 W and a frequency of 40 kHz. Every 1 hour, use a syringe to collect the gaseous product and inject it into a gas chromatograph (s▪sun GC-9860) to detect the H2 production amount, and calculate the hydrogen production rate. The results are shown in Table 1. It can be seen from the data in the table that the Ag loading amount will affect the catalytic performance of the composite catalyst, and the catalytic activity of the composite catalyst is the highest when the loading amount is 2 wt%.

[0059] The hydrogen production rates of the g-C3N4-based composite catalysts prepared in Examples 1 to 4 are as Figure 5 shown. Comparing them with g-C3N4 micro-rods (i.e., without loading silver nanoparticles), the results show that the catalytic activity is significantly improved after loading silver nanoparticles, and the catalytic activity of the composite catalyst obtained when the loading amount of silver nanoparticles is 2 wt% is the highest. This is because when the loading amount of silver nanoparticles is too low, the piezoelectric charge separation efficiency is insufficient, and at the same time, the hydrogen production active sites are also insufficient, inhibiting the catalytic activity; but when the loading amount is too high, silver nanoparticles are prone to agglomeration on the surface of g-C3N4 micro-rods, reducing the active area of silver exposure, resulting in a decrease in catalytic performance and a decrease in H2 yield.

[0060] Table 1 H2 yields of the g-C3N4-based composite catalysts prepared in Examples 1 to 4

[0061] Project <![CDATA[H2 production rate (μmol / g / h) <!-- 5 -->]]> Example 1 834 Example 2 686 Example 3 381 Example 4 301

[0062] Examples 5 to 8

[0063] Examples 5 to 8 are only different from Example 1 in that when loading metals on the surface of g-C3N4 micro-rods, instead of using AgNO3 solution to load metals, K2PdCl6, HAuCl4, H2PtCl6, and RuCl3 solutions are used to achieve the loading of Pt, Au, Pd, and Ru respectively, and the loading amount of each metal is 2 wt%. Other experimental parameters and conditions are the same as those in Example 1 and will not be elaborated here. The hydrogen production results of the composite catalysts prepared in Example 1 and Examples 5 to 8 when used for piezoelectric catalytic reforming of cellulose to hydrogen are shown in Table 2 and Figure 6 as follows. It can be seen from the data in the table that when the loaded metal is silver, the catalyst activity is the highest.

[0064] Table 2 Hydrogen production results of the composite catalysts prepared in Example 1 and Examples 5 to 8 when used for piezoelectric catalytic reforming of cellulose to hydrogen

[0065] Project Metal solution <![CDATA[H2 production rate (μmol / g / h)]]> Example 1 <![CDATA[AgNO3]]> 834 Example 5 <![CDATA[K2PdCl6]]> 371 Example 6 <![CDATA[HAuCl4]]> 355 Example 7 <![CDATA[H2PtCl6]]> 265 Example 8 <![CDATA[RuCl3]]> 475

[0066] Example 9

[0067] Example 9 is only different from Example 1 in that when loading Ag on the surface of g-C3N4 micro-rods, the concentration of the NaOH solution used is 1 mol / L, and when the composite catalyst is mixed with the NaOH solution in which the substrate is dissolved during the application process, the concentration of the NaOH solution used is 3 mol / L. Other experimental parameters and conditions are the same as those in Example 1 and will not be elaborated here.

[0068] Examples 10 to 12

[0069] Examples 10 to 12 are only different from Example 9 in that when the composite catalyst is mixed with the NaOH solution in which the substrate is dissolved during the application process, the concentration of the NaOH solution used is different from that in Example 9. Other experimental parameters and conditions are the same as those in Example 9 and will not be elaborated here.

[0070] The hydrogen production rates of the composite catalysts prepared in Example 1 and Example 9 are as Figure 7 shown in and Table 3. The results show that using a 3 mol / L NaOH solution has a better effect when loading silver nanoparticles. This is because the high concentration of NaOH increases the solubility of microcrystalline cellulose, thereby improving the deposition efficiency of Ag and changing its size distribution.

[0071] The measured hydrogen production rates of Examples 9 to 12 are as Figure 7 shown in and Table 3. The results show that the hydrogen production performance of the same catalyst is greatly affected by the NaOH concentration, and the hydrogen production performance is the best when the concentration is 3 mol / L. This is because too low a NaOH concentration will inhibit the dissolution of cellulose, while too high a NaOH concentration will increase the solution viscosity and is not conducive to ultrasonic cavitation.

[0072] Table 3 Hydrogen production results of the NaOH solution concentrations in Examples 1 and 9 to 12 and the composite catalysts prepared under the corresponding conditions when used for piezoelectric catalytic reforming of cellulose to hydrogen

[0073] Project NaOH concentration when loading Ag (mol / L) NaOH solution concentration during piezocatalysis (mol / L) <![CDATA[H2 production rate (μmol / g / h)]]> Example 1 3 3 834 Example 9 1 3 617 Example 10 1 1 358 Example 11 1 2 429 Example 12 1 4 273

[0074] Comparative Example 1

[0075] Comparative Example 1 provides a method for preparing a g-C3N4-based composite catalyst, comprising the following steps:

[0076] After spreading 10 g of urea in a covered crucible, place it in a muffle furnace, and then increase the temperature in the muffle furnace from 25 °C to 550 °C at a heating rate of 5 °C / min and hold for 4 h; after the heating is completed, grind the obtained powder sample into a finer powder to obtain g-C3N4 nanosheets with a size of 1-3 μm and a thickness of 2-4 nm;

[0077] Disperse 50 mg of the g-C3N4 nanosheets prepared in the above step in 200 mL of an alkaline solution, and add a certain amount of AgNO3 solution to ensure that the Ag loading is 2 wt%; place the mixed solution obtained in the above step in a reactor, seal the reactor with a silica gel plug, irradiate it with a 395 nm ultraviolet lamp under magnetic stirring for 1 h, after the irradiation is completed, centrifuge to collect the solid sample, wash the solid sample with deionized water until the supernatant is neutral, and then dry the obtained product in a vacuum drying oven at 60 °C to obtain the g-C3N4-based composite catalyst. Among them, the alkaline solution is obtained by dissolving microcrystalline cellulose in a NaOH solution to a saturated state, the concentration of the NaOH solution is 3 mol / L, and the concentration of the dissolved microcrystalline cellulose is 3-3.75 g / L; in addition, before putting the mixed solution into the reactor, the inside of the reactor needs to be purged with argon for 10 min.

[0078] Comparative Example 2

[0079] Comparative Example 2 provides a method for preparing a g-C3N4-based composite catalyst, comprising the following steps:

[0080] After spreading 10 g of melamine in a covered crucible, place it in a muffle furnace, and then increase the temperature in the muffle furnace from 25 °C to 550 °C at a heating rate of 5 °C / min and hold for 4 h; after the heating is completed, grind the obtained powder sample into a finer powder to obtain g-C3N4 bulk with a size of 5-10 μm;

[0081] Disperse 50 mg of the g-C3N4 bulk obtained in the above step in 200 mL of an alkaline solution, and add a certain amount of AgNO3 solution to ensure that the loading amount of Ag is 2 wt%. Place the mixed solution obtained in the above step in a reactor, seal the reactor with a silica gel plug, irradiate it with a 395 nm ultraviolet lamp under magnetic stirring for 1 h. After the irradiation, centrifuge to collect the solid sample, and wash the solid sample with deionized water until the supernatant is neutral. Then dry the obtained product in a vacuum drying oven at 60 °C to obtain the g-C3N4-based composite catalyst. Among them, the alkaline solution is obtained by dissolving microcrystalline cellulose in a NaOH solution until it reaches a saturated state. The concentration of the NaOH solution is 3 mol / L, and the concentration of the dissolved microcrystalline cellulose is 3 - 3.75 g / L. In addition, before putting the mixed solution into the reactor, the inside of the reactor needs to be purged with argon for 10 min.

[0082] To compare the performance of the g-C3N4-based composite catalysts prepared in Comparative Example 1 and Comparative Examples 1-2 in the piezo-catalytic reforming of cellulose to produce hydrogen, apply them to the piezo-catalytic reforming of cellulose to produce hydrogen. The specific steps are as follows: Disperse 5 mg of the composite catalyst in 20 mL of an alkaline solution of microcrystalline cellulose. The alkaline solution of microcrystalline cellulose is the supernatant obtained by dissolving microcrystalline cellulose (Merida, CAS: 9004-34-6) in a NaOH solution at 25 °C until it reaches a saturated state. The concentration of the NaOH solution is 3 mol / L. Ultrasonic for 3 h at the central position (i.e., the position with the strongest vibration) of an ultrasonic cleaner with a power of 160 W and a frequency of 40 kHz. Every 1 hour, use a syringe to collect the gaseous product and inject it into a gas chromatograph (s▪sun GC-9860) to detect the H2 production, and calculate the hydrogen production rate. The results are shown in Table 4. From the data in the table, it can be seen that compared with the comparative examples, the catalytic performance of the composite catalyst prepared in Example 1 is the best. This is because: it forms a regular micro-rod structure and is prone to form an oriented internal electric field under the action of ultrasonic mechanical energy. At the same time, its larger size further increases the intensity of the internal electric field.

[0083] The hydrogen production rates of the composite catalysts prepared in Example 1 and Comparative Examples 1-2 are as Figure 8 shown. Compare with the blank group without a catalyst (under the condition of no catalyst, make lignocellulose undergo hydrogen evolution reaction under ultrasonic conditions). The results show that the hydrogen production efficiency with a catalyst is significantly higher than that without a catalyst, confirming that hydrogen mainly comes from piezo-catalysis rather than water splitting caused by ultrasound. Among the three catalyst samples, the composite catalyst prepared by the preparation method proposed in Example 1 of the present invention has the strongest catalytic performance for lignocellulose. This is because: the regular micro-rod structure formed in the composite catalyst is prone to form an oriented internal electric field under the action of ultrasonic mechanical energy. At the same time, its larger size further increases the intensity of the internal electric field.

[0084] Table 4 H2 production rate of the g-C3N4-based composite catalysts prepared in Example 1 and Comparative Examples 1-2

[0085] Project <![CDATA[H2 production rate (μmol / g / h)]]> Example 1 834 Comparative Example 1 337 Comparative Example 2 361

[0086] The above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A preparation method of a g-C3N4-based composite catalyst, characterized in that, The composite catalyst includes g-C3N4 micro-rods and noble metal nanoparticles supported on the surface of the g-C3N4 micro-rods; The g-C3N4 micro-rods have a length of 100-200 μm and a width of 10-40 μm; the content of the noble metal nanoparticles in the composite catalyst is 1-4 wt%; The preparation method includes the following steps: S1. Prepare a g-C3N4 micro-rod precursor by hydrothermal treatment of a mixed solution of melamine and concentrated nitric acid; calcine the g-C3N4 micro-rod precursor to obtain g-C3N4 micro-rods; The preparation method of the g-C3N4 micro-rods is as follows: Add concentrated nitric acid to an aqueous melamine solution, mix evenly, and carry out hydrothermal reaction at 140-160 °C for 8-12 h to obtain a g-C3N4 micro-rod precursor; under an inert atmosphere, calcine the g-C3N4 micro-rod precursor to obtain the g-C3N4 micro-rods; the temperature of the calcination treatment is 500-600 °C, and the time is 4-6 h; S2. Use the photodeposition method to load noble metal nanoparticles on the surface of the g-C3N4 micro-rods prepared in step S1; the loading of the noble metal nanoparticles on the surface of the g-C3N4 micro-rods is carried out as follows: Mix the g-C3N4 micro-rods and a salt or acid solution containing a noble metal element with an alkaline solution containing lignocellulose, and under stirring conditions, irradiate with light for 1-4 h to obtain the g-C3N4-based composite catalyst; the alkaline solution is obtained by dissolving lignocellulose in a NaOH solution; the concentration of the NaOH solution is 3 mol / L, the concentration of lignocellulose in the alkaline solution is 0-5 g / L, and the lignocellulose is microcrystalline cellulose.

2. The preparation method according to claim 1, characterized in that, The light source used during light irradiation is an ultraviolet lamp with a wavelength ≤ 395 nm or a composite light source containing a wavelength ≤ 395 nm.

3. Application of the g-C3N4-based composite catalyst prepared by the preparation method according to any one of claims 1-2 in piezoelectrocatalytic reforming of lignocellulose to produce hydrogen.

4. Use of the g-C3N4-based composite catalyst prepared by the preparation method according to any one of claims 1-2 in piezoelectric catalysis for the reforming of lignocellulose to produce hydrogen, characterized in that, It includes the following steps: After mixing the g-C3N4-based composite catalyst with a NaOH solution in which the substrate is dissolved, under the action of mechanical force, the g-C3N4-based composite catalyst induces the substrate to undergo a hydrogen evolution reaction; the substrate is lignocellulose.

Images