CN112028038A

CN112028038A - Preparation method and application of alkalized carbon nitride nanotube

Info

Publication number: CN112028038A
Application number: CN202010902865.2A
Authority: CN
Inventors: 周易; 周亮; 周彦波; 刘勇弟; 雷菊英; 张金龙
Original assignee: East China University of Science and Technology
Current assignee: East China University of Science and Technology
Priority date: 2020-09-01
Filing date: 2020-09-01
Publication date: 2020-12-04
Anticipated expiration: 2040-09-01
Also published as: CN112028038B

Abstract

The invention provides a preparation method of an alkalized carbon nitride nanotube, which is characterized in that ammonium chloride and potassium chloride are firstly hydrothermally alkalized to synthesize an alkalized melamine nanorod, and then the alkalized carbon nitride nanotube is synthesized in a calcining mode. The method combines a hydrothermal method and a calcination method, successfully connects a large number of hydroxyl groups on the surface of carbon nitride, and simultaneously maintains the specific morphology of the hexagonal nanotube. The successful introduction of hydroxyl groups effectively inhibits the self-decomposition of hydrogen peroxide, realizes the high-efficiency photocatalytic spontaneous hydrogen peroxide generation without adding a sacrificial agent, and simultaneously reveals the influence on light through the detection of pHAnother main species for catalyzing hydrogen peroxide generation reaction is protonic hydrogen (H)⁺) The proton hydrogen is generated by the ionization reaction of the hydroxyl on the surface of the carbon nitride, and the photocatalysis synthesis of the hydrogen peroxide is promoted. The part of the work provides a new method for developing a high-efficiency nonmetal hydrogen peroxide photocatalyst in a spontaneous system.

Description

Preparation method and application of alkalized carbon nitride nanotube

Technical Field

The invention relates to a preparation method and application of an alkalized carbon nitride nanotube.

Background

H₂O₂Due to the unique property, the method is widely applied to the fields of environmental remediation, biological treatment, chemical industry and the like. At present, anthraquinone method, alcohol oxidation method, electrochemical synthesis method and other methods are generally adopted in industry to prepare H₂O₂. However, these processes consume high energy during the manufacturing process and generate a large amount of organic by-products, which have a great impact on the environment. Therefore, the method is suitable for H which is economical, efficient and environment-friendly₂O₂The development of the production method is particularly important.

Semiconductor-based photocatalytic reduction of oxygen to produce H₂O₂Is considered to be a very promising preparation method. Shiraishi et al found that mesoporous carbon nitride rich in surface defects produced H under photocatalysis compared to conventional bulk carbon nitride₂O₂Showing better activity. The photocatalytic production of H in these above mentioned semiconductors₂O₂In the system, researchers add some sacrificial agents additionally to obtain higher H₂O₂And (4) yield. Ethanol is the most used sacrificial agent, and the main mechanism is that ethanol in solution can capture photogenerated holes and generate hydrogen ions (formula 1-1) to participate in H₂O₂The synthesis reaction (formula 1-2). In addition, the ethanol consumes holes in time, so that more electrons are further generated, and the reduction of oxygen to generate H is promoted₂O₂The reaction of (1). However, the continuous addition of sacrificial agents in industrial processes not only increases the difficulty of product purification but also increases the production cost, so this is not a sustainable solution.

R-CH₂OH+2h⁺→R-CHO+2H⁺ 1-1

O₂+2H⁺+2e^-→H₂O₂ 1-2

In view of the cost of operation, scientists have been working on developing self-luminescence catalysis without adding sacrifice to produce H₂O₂And (4) preparing the system. For example: kofuji et al reported a carbon nitride-aromatic diimine-graphene nanocomposite that efficiently reacts with water and oxygen under visible light excitation to produce H₂O₂. Hirakawa et al developed a BiVO-based solution₄Au nanoparticle-loaded systems selectively enhancing two-electron reduction of oxygen to promote self-luminescent catalytic H production₂O₂. Moon et al developed a CoPi-rGO/TiO₂Can produce H efficiently by photocatalysis without adding other sacrificial agents₂O₂. However, these photocatalysts produce H spontaneously₂O₂The system adopts complex semiconductor composite materials or noble metals. These modifications may contribute to H in part by adjusting the energy band or increasing absorption of light₂O₂And (4) generating. It is worth noting that H is currently achieved₂O₂The yield still can hardly meet the actual industrial requirements. In addition, in most photocatalytic systems with or without sacrificial agents, H₂O₂There is a bottleneck in the yield of (A), and a certain amount of H is accumulated in the reaction system with the lapse of time₂O₂Moiety H₂O₂In-situ decomposition reactions (equations 1-3, 1-4, 1-5, 1-6) occur. And H₂O₂The higher the amount, the more violent the decomposition reaction proceeds, and after a certain time, H₂O₂The yield does not increase any more. At present, there are few reports focusing on solving this H₂O₂The problem of self-decomposition.

H₂O₂+H⁺+e^-→H₂O+OH· 1-3

H₂O₂+2H⁺+2e^-→2H₂O 1-4

H₂O₂+h⁺→H⁺+HO₂· 1-5

H₂O₂+2h⁺→O₂+2H⁺ 1-6

In contrast, the invention provides a preparation method of the alkalized carbon nitride nanotube, which comprises the steps of synthesizing the alkalized melamine nanorod by hydrothermal alkalization of ammonium chloride and potassium chloride, and synthesizing the alkalized carbon nitride nanotube by calcination. The prepared alkalized carbon nitride hexagonal nanotube has stable and unique hexagonal nanotube structure, great amount of hydroxyl radicals planted on the surface for photocatalytic H production₂O₂Plays an important role in the reaction of (1). Compared with common carbon nitride, the material has excellent photocatalytic H production without adding sacrificial agent₂O₂And (4) performance. In which H of the best performing modified catalyst₂O₂The yield is about 13 times that of common carbon nitride. More importantly, in the work, the alkalization modification and the shape regulation are proposed to improve H₂O₂The yield mechanism discusses the formation mode of various reactant species and H pair₂O₂Inhibition of self-decomposition.

Disclosure of Invention

Ammonium chloride and potassium chloride are firstly hydrothermally alkalized to synthesize alkalized melamine nanorods, and then the alkalized carbon nitride nanotubes are synthesized by a calcining mode. The method combines a hydrothermal method and a calcination method, successfully connects a large number of hydroxyl groups on the surface of carbon nitride, and simultaneously maintains the specific morphology of the hexagonal nanotube. The material is proved to have good photoelectric property by a series of characterization means. The successful introduction of hydroxyl groups can effectively inhibit the self-decomposition of hydrogen peroxide, realize the high-efficiency photocatalytic spontaneous hydrogen peroxide generation without adding a sacrificial agent, and generate 120.8 mu mol/L H within 30min in the sample of the optimum alkalization amount ACNT-5 under simulated sunlight₂O₂. Through a capture experiment and an EPR test, the main active species in the system is explored to be OH. The surface hydroxyl groups of ACNT-5 efficiently consume holes, promote electron and hole separation of the material, and generate OH suppressing H₂O₂Self-decomposition reaction of (1). At the same timeThe pH detection reveals that another main species influencing the photocatalytic hydrogen peroxide generation reaction is proton hydrogen (H)⁺) Generated by ionization reaction of hydroxyl on the surface of carbon nitride, promotes photocatalysis to synthesize the hydrogen peroxide. The part of the work provides a new method for developing a high-efficiency nonmetal hydrogen peroxide photocatalyst in a spontaneous system. The specific scheme is as follows:

a method for preparing alkalized carbon nitride nanotubes comprises the following steps: weighing melamine, potassium chloride and ammonium chloride with preset mass, dispersing into deionized water, heating and stirring under an oil bath until the melamine is completely dissolved, transferring the obtained transparent solution into a hydrothermal kettle, placing the hydrothermal kettle into an oven, heating for preset time at preset temperature, naturally cooling to room temperature, washing the obtained sample with water and alcohol for a plurality of times in sequence, and drying to obtain the alkalized melamine nanorod; and placing the alkalized melamine nanorod in a muffle furnace to be calcined for preset time, after the calcining process is finished, naturally cooling the alkalized melamine nanorod to room temperature, fully grinding the obtained yellow sample, washing the yellow sample with water for a plurality of times, and drying the yellow sample in a vacuum drying oven to obtain the alkalized carbon nitride nanotube.

Further, 1.0g of melamine, 1.0 to 10.0g of potassium chloride and 0.2g of ammonium chloride were weighed and dispersed in 70mL of deionized water.

Further, 1.0g of melamine, 5.0g of potassium chloride and 0.2g of ammonium chloride were weighed and dispersed in 70mL of deionized water.

Further, the oil bath temperature was 80 ℃.

Further, the hydrothermal kettle is heated in an oven at 200 ℃ for 10 hours.

Further, the calcination was carried out in a muffle furnace at a rate of 2.5 ℃/min up to 550 ℃ and held for 4 hours.

Further, the mixture is dried in a vacuum drying oven at 60 ℃.

Further, a method for producing hydrogen peroxide by photocatalysis is characterized in that a preset mass of the alkalized carbon nitride nanotube prepared by the preparation method is weighed and placed in a photocatalysis tube, deionized water is added, oxygen flow is used for bubbling for a preset time, the mixed solution is continuously stirred under a dark condition to achieve adsorption-desorption balance, and then a lamp is turned on for irradiation to prepare the hydrogen peroxide.

Further, 50mg of the alkalized carbon nitride nanotube prepared by the preparation method according to any one of claims 1 to 7 is weighed and placed in a photocatalytic tube, 50mL of deionized water is added, oxygen flow is used for bubbling for 20min, the mixed solution is continuously stirred for 30min under dark conditions to reach adsorption-desorption equilibrium, and then the lamp is turned on for irradiation to prepare hydrogen peroxide.

The invention has the following beneficial effects:

1. photocatalytic properties of alkalized carbon nitride nanotubes (ACNT)

The H of the material under the conditions of pure water, oxygen and simulated sunlight irradiation is measured₂O₂The yield, results are shown in FIG. 10 a. H of CNT compared with CN₂O₂The yield is 21.38 mu mol/L which is 2.3 times of CN, and the hexagonal nanotube structure is favorable for improving the photocatalytic activity of the material. Compared with a layered stacking structure of bulk-phase carbon nitride, the special structural morphology provides more active sites for photocatalytic reaction, and accelerates the reaction mass transfer process in multiphase reaction.

In addition to morphology regulation and control, the alkalization modification of the carbon nitride further improves H₂O₂The yield of (2). The alkalized series of catalysts all showed much higher photocatalytic effect than CN and CNT. Of note, ACNT-5 produced 120.8. mu. mol/L of H in 30min₂O₂. According to incomplete statistics, this yield is higher than most photocatalytic spontaneous systems reported in recent years (as shown in table 2). To compare the performance of different catalysts more intuitively, first order reaction kinetics (In (C)₀C) ═ kt) the photocatalytic product H was calculated₂O₂Reaction rate constant (FIG. 10 b). Apparently, the reaction rate k of ACNT-5 is 13 times that of CN. The chemical stability of the material was also determined, after six cycles, H₂O₂The yield of (2) still reached more than 90% of the first measurement result (FIG. 10 c). The performance of the catalyst is not obviously weakened in multiple cycles, which shows that the material has good chemical stability, and can be cleaned by a scanning electron microscope after multiple cyclesWell-preserved hexagonal nanotubular morphology was clearly observed (fig. 10 d).

Some of the optoelectronic properties of the material were characterized (fig. 11). It is generally accepted that the PL signal is generated by the recombination of photo-generated electrons and holes in some semiconductors. Compared to CN, the PL signal of ACNT-5 is severely attenuated (FIG. 11a), indicating that the separation of photogenerated electrons from holes is very efficient. In the EIS-Nyquist curve (FIG. 11b), ACNT-5 has the smallest ring, indicating the smallest impedance, which means that after light excitation, photo-generated electrons and holes do not undergo massive recombination inside ACNT-5, but effectively separate and migrate to participate in the photocatalytic H production₂O₂The reaction of (1). The fluorescence lifetime profile of the material (FIG. 11c, d) also supports this phenomenon. Compared with CN, the fluorescence lifetime of ACNT-5 is greatly shortened by 7.2 times, and the great reduction of the fluorescence lifetime also indicates that the holes are rapidly reacted with surface hydroxyl groups after being separated. The ACNT-5 has a special shape which exposes more active sites, and a large number of hydroxyl groups introduced into the surface have strong reactivity, so that photogenerated holes can be rapidly captured, and the generation of more electrons by photocatalysis is promoted. The electrons can rapidly react with water, oxygen and the like to generate H₂O₂. In general, ACNT-5 shows more excellent photoelectric property and is beneficial to producing H by photocatalysis₂O₂The reaction takes place.

It is assumed that the change in band structure may be another important factor for improving the photocatalytic activity of the material, and the forbidden bandwidth of the material was measured (fig. 12). The absorption spectrum shows that the ACNT series samples have stronger absorption in an ultraviolet region, and the photocatalytic activity is favorably improved. However, it was found by calculation that the band structure of the material was not greatly changed by the alkalifying modification compared to CN, so the change in band structure was not a main factor of the improvement in the activity of the material.

2. Production of H from alkalized carbon nitride nanotube₂O₂Photocatalytic mechanism of

To further explore H in the ACNT system₂O₂The major active species in the system were analyzed in combination with the capture experiments and EPR tests for a large increase in yield (fig. 13). Koji at CNOn the line, it was clearly observed that the signal was ascribed to DMPO-. O₂ ^-Indicating that in the CN system, H is₂O₂By reducing O mainly in a plurality of steps₂Reduction to H₂O₂. Interestingly, in the ACNT-5 system, although DMPO-. O was not found₂-but a signal of DMPO-. OH was detected (FIG. 13 a). In fact, OH does not produce H₂O₂The major active species of (c). Therefore, to confirm the results of this experiment, a series of capture experiments were designed. In the photocatalytic production of H₂O₂P-benzoquinone is added as O in the experiment₂Trapping agent, isopropanol as. OH trapping agent and triethanolamine as h⁺A capture agent. When p-benzoquinone was added to the CN system (FIG. 13b), H₂O₂The yield is reduced by over 95 percent, which is consistent with the measurement result of EPR, and the main active species in CN system is O₂-. In the ACNT-5 system, no significant decrease in yield was observed, indicating that O₂Not in production H₂O₂Plays a main role in the reaction. After the addition of isopropanol (FIG. 13c), the yield of the ACNT-5 system decreased by about 25%. The electron-hole separation efficiency of the ACNT-5 photocatalyst has been maintained at a high level, and thus, when isopropanol captures. OH, H is promoted₂O₂Self-decomposition of (see equations 1-3). And in h⁺In the trapping experiment (fig. 13d), the yield of the CN system was significantly increased, mainly due to the improved photocatalytic activity by facilitating the separation of electrons from holes. For ACNT-5, the capture of H + renders-OH unoxidizable to OH, resulting in H₂O₂Is not inhibited, so that H₂O₂The yield of (2) is reduced. From the above results, it was preliminarily judged that the surface hydroxyl group of ACNT-5 efficiently consumes holes, promotes the separation of electrons from holes of the material, and generates OH suppressing H₂O₂Self-decomposition reaction of (1). While the present invention has been described in detail with reference to the preferred embodiments, it should be understood that the above description should not be taken as limiting the invention.

The literature reports the substrateInfluence of the mechanism of the sub-transfer, H₂O₂The yield of (a) increases with decreasing pH. Thus, the change in pH during the reaction was monitored (as shown in fig. 14). For CN, the pH of the system slightly increased during dark adsorption, both with and without the addition of ethanol. This is due to hydrolysis of the amino groups on the surface of the carbon nitride (see equations 1-7). After the lamp was turned on, the pH continued to decrease as the reaction proceeded. H⁺Generated by oxidizing water molecules through photogenerated holes, ethanol can capture photogenerated holes to generate more H after a sacrificial agent is added⁺(see equation 1-1), therefore the production of H + is much higher than in CN systems without sacrificial agent, resulting in lower pH. In addition, the timely consumption of holes facilitates the generation of more electrons, H₂O₂The yield of (2) is 3 times that of the CN system without the sacrificial agent.

R-NH₂+H₂O→R-NH₃ ⁺+OH- 1-7

R-OH→R-O-+H⁺ 1-8

In the ACNT-5 system, reasonable catalyst design allows the catalyst itself to have the full function of the added sacrificial agent. Although the initial pH of the ACNT-5 spontaneous system is higher than that of the CN spontaneous system, the pH begins to drop during dark adsorption due to ionization reactions of a large number of hydroxyl groups on the surface (see equations 1-8). After light irradiation, the hydroxyl groups introduced on the surface have faster and more efficient reactivity than the sacrificial agent, so that the pH in the system is rapidly reduced to 4.38, which is lower than that of the CN system (5.10) added with the sacrificial agent. Shows that a large amount of H is generated in the ACNT-5 system⁺And effectively consumes the cavity and completely replaces the function of the sacrificial agent.

Drawings

FIG. 1 is a schematic diagram of the preparation mechanism of alkalized carbon nitride nanotubes (ACNT)

FIG. 2(a) SEM image of commercial melamine; (b) SEM image of melamine nanorod; (c) a field emission map of the CNTs; (d) TEM image of the basified broken carbon nitride after KOH; (e) SEM images of NH4Cl and KCl post-basified carbon nitride; (f) SEM image of alkalized melamine nanorod

FIG. 3(a, b, c) FESEM image of ACNT-5; (d) TEM image of ACNT-5

FIG. 4(a, b, c) TEM image of CN, ACNT-1 and ACNT-10; (d) SEM picture of CN; FESEM pictures of (e, f) ACNT-1 and ACNT-10

FIG. 5(a, b) N2 adsorption-desorption curves for CN and ACNT-5

FIG. 6(a) XRD patterns of CN, CNT, ACNT-1, ACNT-5 and ACNT-10; (b) FTIR patterns of alkalized g-C3N4 prepared by post-alkalization of ACNT-1, ACNT-10 and KON

FIG. 7(a) FTIR plots of CN and ACNT-5; XPS spectra of N1s, O1s and K2p of (b, c, d) CN and ACNT-5

FIG. 8(a, b, c, d) photographs of dispersibility of CN and ACNT-5 under different conditions

FIG. 9(a, b, c, d) photograph of contact angles of CN, ACNT-1, ACNT-5 and ACNT-10

FIG. 10(a) is a graph showing the effect of photocatalytic hydrogen peroxide generation by CN, CNT, ACNT-1, ACNT-5 and ACNT-10; (b) the reaction rate constant of hydrogen peroxide generated by photocatalysis of CN, CNT, ACNT-1, ACNT-5 and ACNT-10 is repeated for a plurality of times to obtain an error bar; (c) the cycle effect graph of ACNT-5; (d) FESEM image of ACNT-5 after 6 cycles of experiment

FIG. 11(a) PL spectrum of CN and ACNT-5; (b) impedance spectrogram of CN and ACNT-5; (c, d) fluorescence lifetime spectrum of CN and ACNT-5

FIG. 12(a) UV-visible diffuse reflectance spectra of CN, ACNT-1, ACNT-5, and ACNT-10; (b) CN, ACNT-1, ACNT-5 and ACNT-10 are converted by a Kubelka-Munk formula to obtain corresponding forbidden bandwidth

FIG. 13(a) EPR spectra of CN and ACNT-5 photocatalytic hydrogen peroxide production systems; (b, c, d) CN and ACNT-5 are respectively added with BQ, IPA and TEOA trapping agent to generate hydrogen peroxide effect diagram, and repeated experiments are carried out for a plurality of times to obtain error bars

FIG. 14(a) Effect of sacrificial agents on the photocatalytic activity of CN systems and pH change during the reaction; (b) a schematic mechanism diagram of the hydrogen peroxide photocatalytic generation of a CN system added with a sacrificial agent; (c) the photocatalytic activity of the ACNT-5 system and the pH change in the reaction are improved; (d) mechanism diagram of ACNT self-luminescence catalysis hydrogen peroxide generation system

Detailed Description

The present invention will be described in more detail below with reference to specific examples, but the scope of the present invention is not limited to these examples.

EXAMPLES preparation of alkalized carbon nitride nanotubes

Example 1

1.0g of melamine, 1.0g of potassium chloride and 0.2g of ammonium chloride were weighed into 70mL of deionized water and heated with stirring at 80 ℃ in an oil bath until the melamine was completely dissolved. The obtained transparent solution is transferred to a hydrothermal kettle, and the hydrothermal kettle is heated in an oven at 200 ℃ for 10 hours. And after naturally cooling to room temperature, washing the obtained sample with water and alcohol for a plurality of times, and drying to obtain the alkalized melamine nanorod. The alkalinized melamine nanorods were placed in a porcelain ark, raised to 550 ℃ in a muffle furnace at a rate of 2.5 ℃/min and incubated for 4 h. And after the calcining process is finished, naturally cooling to room temperature, fully grinding the obtained yellow sample, washing for a plurality of times, and drying in a vacuum drying oven at 60 ℃. The final sample was labeled ACNT-1.

Example 2

1.0g of melamine, 5.0g of potassium chloride and 0.2g of ammonium chloride were weighed into 70mL of deionized water and heated with stirring at 80 ℃ in an oil bath until the melamine was completely dissolved. The obtained transparent solution is transferred to a hydrothermal kettle, and the hydrothermal kettle is heated in an oven at 200 ℃ for 10 hours. And after naturally cooling to room temperature, washing the obtained sample with water and alcohol for a plurality of times, and drying to obtain the alkalized melamine nanorod. The alkalinized melamine nanorods were placed in a porcelain ark, raised to 550 ℃ in a muffle furnace at a rate of 2.5 ℃/min and incubated for 4 h. And after the calcining process is finished, naturally cooling to room temperature, fully grinding the obtained yellow sample, washing for a plurality of times, and drying in a vacuum drying oven at 60 ℃. The final sample was labeled ACNT-5.

Example 3

1.0g of melamine, 10.0g of potassium chloride and 0.2g of ammonium chloride were weighed into 70mL of deionized water and heated with stirring at 80 ℃ in an oil bath until the melamine was completely dissolved. The obtained transparent solution is transferred to a hydrothermal kettle, and the hydrothermal kettle is heated in an oven at 200 ℃ for 10 hours. And after naturally cooling to room temperature, washing the obtained sample with water and alcohol for a plurality of times, and drying to obtain the alkalized melamine nanorod. The alkalinized melamine nanorods were placed in a porcelain ark, raised to 550 ℃ in a muffle furnace at a rate of 2.5 ℃/min and incubated for 4 h. And after the calcining process is finished, naturally cooling to room temperature, fully grinding the obtained yellow sample, washing for a plurality of times, and drying in a vacuum drying oven at 60 ℃. The final sample was labeled ACNT-10.

Comparative example 1

Preparation of bulk phase carbon nitride

10g of melamine were weighed into a porcelain ark, raised to 550 ℃ in a muffle furnace at a rate of 2.5 ℃/min and held for 4 h. And after the calcining process is finished, naturally cooling to room temperature, fully grinding the obtained yellow sample, washing for a plurality of times, and drying in a vacuum drying oven at 60 ℃. The final sample was labeled CN.

Comparative example 2

Preparation of carbon nitride nanotubes

1.0g of melamine was weighed into 70mL of deionized water and heated with stirring at 80 ℃ in an oil bath until the melamine was completely dissolved. The obtained transparent solution is transferred to a hydrothermal kettle, and the hydrothermal kettle is heated in an oven at 200 ℃ for 10 hours. And after naturally cooling to room temperature, washing the obtained sample with water and alcohol for a plurality of times, and drying to obtain the melamine nanorod. The melamine nanorods were placed in a porcelain ark, raised to 550 ℃ in a muffle furnace at a rate of 2.5 ℃/min and incubated for 4 h. And after the calcining process is finished, naturally cooling to room temperature, fully grinding the obtained yellow sample, washing for a plurality of times, and drying in a vacuum drying oven at 60 ℃. The final sample was labeled as CNT.

Experiments and data

1. Photocatalytic production of hydrogen peroxide

The photocatalytic performance of each catalyst was determined by producing H under simulated sunlight conditions₂O₂The amount of (c) is evaluated. Weighing 50mg of catalyst, placing the catalyst in a photocatalytic tube, adding 50mL of deionized water, carrying out bubbling with oxygen flow for 20min, and continuously stirring the mixed solution for 30min under a dark condition to achieve adsorption-desorption balance. In some of the experiments with sacrificial agent, 40mL of water was measured and 10mL of ethanol was added. The lamp was then turned on and 1mL of the liquid was taken at regular intervals in a centrifuge tube and centrifuged. Weighing 0.5mL of supernatant and 2mL of potassium iodide solution with the concentration of 0.1mol/LMixing 0.05mL of 0.01mol/L ammonium molybdate aqueous solution, standing for 10min, measuring absorbance of the solution at 350nm, and calculating by formula 1-7 to obtain H₂O₂And (4) concentration C. Wherein the value of absorbance A is effective within 0-1, and the measurement is carried out after dilution treatment if the value exceeds 1. The concentration C is expressed in. mu. mol/L. The working current of the Xe lamp is 15A, the power is 300W, the lamp is 4cm away from the liquid level, and the filter used for simulating sunlight is AM 1.5.

Physicochemical Properties of ACNT

In the process of preparing the series of ACNT, a traditional post-alkalization method is tried, wherein the post-alkalization method is to directly prepare a NaOH or KOH solution with a certain concentration, disperse a sample into the solution, continuously stir, heat the solution to 120 ℃ by using an oil bath until the water in the solution is completely evaporated, then centrifuge and wash the sample for a plurality of times, and dry the sample.

As shown in the upper half of fig. 1. First, commercial melamine takes on a blocky structure (fig. 2 a). After the hydrothermal process, melamine is partially hydrolyzed to cyanuric acid. The hexagonal nanorod structure is formed by self-assembly of melamine and cyanuric acid generated in situ (fig. 2 b). After calcination, a hollow carbon nitride nanotube-like structure was observed (fig. 2 c). The resulting CNTs were then dispersed in aqueous NaOH or KOH solution, then continuously stirred at 120 ℃ and the water was evaporated off. However, electron microscopy characterization found that the post-basification protocol completely destroyed the morphology of the material, which was dispersed into multiple small chunks (fig. 2 d). Subsequently, KCl and NH were used₄The Cl mixed solution is used for replacing the strong alkali solution to carry out the post-alkalization operation again, the shape can still not be perfectly maintained, most of the tubular structures are still damaged (figure 2e), and the alkalization degree of the material is low. Therefore, KCl and NH are adopted₄Cl is used as a raw material, the alkalized melamine hexagonal nanorod (shown in figure 2f) is synthesized by a hydrothermal method, and the hexagonal nanotube structure of the material is successfully maintained and the alkalization degree of the material is controllable by calcining and thermal polycondensation.

As shown in fig. 3, ACNT-5 exhibits a perfect hexagonal nanotube-like structure. The tube length can reach dozens of microns and the diameter is between 1 and 3 microns, which shows that the preparationThe method is continuous and reliable. The interior of the tube is of a hollow structure with larger volume and thinner tube wall, and the structure is beneficial to the mass transfer process in the liquid phase. And for the bulk phase carbon nitride CN, it is a layered structure with uneven surface (fig. 4). Meanwhile, for the ACNT-1 and the ACNT-10, although the alkalization degrees are different, the hexagonal hollow nanotube structure is still maintained, and the damage of the alkalization modification means to the material morphology is effectively prevented through a reasonable designed experimental scheme. The specific surface area of ACNT-5 was as high as 63.53m by the BET test (FIG. 5)²G, much higher than 8.13m of CN²(ii) in terms of/g. The hexagonal nanotube-shaped structure of the ACNT series samples not only exposes more active sites, but also has larger specific surface area, thereby being beneficial to the mass transfer process.

In the XRD pattern (fig. 6a), all samples had two characteristic diffraction peaks at 27.5 ° and 13.0 °, corresponding to (002) and (100) planes of carbon nitride, respectively, indicating that the bulk chemical structure was not greatly changed in the alkalization modification. In addition, the ACNT series samples shifted peak positions toward large angles and the peak intensity was greatly reduced due to the special hexagonal morphology resulting in a reduction in the interlayer spacing of the (002) planes. In the FTIR spectrum of the material (FIG. 7a), at 880cm^-1And 1200 ion 1680cm^-1The peak ascribed to the triazine unit and the aromatic carbon nitrogen heterocycle can be clearly observed. Compared with CN, ACNT-5 has two additional peaks at 999cm^-1And 2140cm^-1Respectively, corresponding to hydroxyl groups introduced on the surface of the carbon nitride. In the alkalization process, K⁺Plays an important role in introducing hydroxyl groups. In addition, ACNT-5 is at 2177cm^-1The peak was ascribed to some functional bonds formed by C and N (e.g., C.ident.N, etc.), and similar peaks were observed in ACNT-1, ACNT-10 and carbon nitride materials treated with KOH (FIG. 6 b).

In XPS characterization of the material, different chemical bonds were analyzed for the ACNT-5 and CN samples. The high resolution N1s spectrum (FIG. 7b) of CN showed three different nitrogen-containing chemical bonds, with the peak at 398.7eV being attributed to C-N-C bonds, the peak at 399.78eV to N- (C)3, and the peak at 401.01eV to C-N-H. In ACNT-5, however, observation is madeThe peak due to C-N-H shifts toward lower energies (400.71 eV). This change can be explained by the fact that N is more electronegative than C and that the alkalization process causes part of the C-N-H to be reduced and the C-O-H bonds to increase. Another possible cause is bond cleavage between C and N, K⁺The introduction of (2) breaks down pi electron delocalization, and in a conjugated system, N- (C)3 bonds are moved to a high-energy region. In the high resolution O1s spectrum (FIG. 7c), the peak at 532.19eV represents a bound water molecule, whereas the peak at 530.68eV is higher in ACNT-5 and is attributed to the OH bond. The results here are in agreement with the previous FTIR measurements, and a portion of the hydroxyl groups were successfully attached to the carbon nitride after basification. Also, overall, all bonds in the spectra of C1s and N1s of ACNT are shifted toward lower energy than CN, indicating a strong bond between K and N, presumably K being doped into the internal structure of carbon nitride. In the high resolution spectrum of K2p (FIG. 7d), the peak at 292.79eV is KN due to attractive interaction₃The N-K bond in (1). Meanwhile, the peak at 295.55eV represents a C-K bond. Due to the electron donor properties of K, these chemical bonds affect the electronic state of the carbon nitride surface, mainly due to the increase of oxygen species on the material surface, such as OH bonds determined by the previous measurements. From the above experimental results, it can be known that K is positively charged⁺And C, N, resulting in the cleavage of a portion of the CN bonds. Then bonding with unsaturated C, separating OH from water and generating some new chemical bonds (such as C ≡ N, (N)₂C-OH). In addition, in the photocatalytic experiment, different dispersion characteristics between materials were observed (shown in fig. 8), and the contact angle of the material was tested (shown in fig. 9). The contact angles of the ACNT series samples are all smaller than that of CN, and the improvement of the hydrophilicity also indicates the successful introduction of hydroxyl on the surface of the carbon nitride.

Advantageous effects

1. Photocatalytic properties of alkalized carbon nitride nanotubes (ACNT)

In addition to morphology regulation and control, the alkalization modification of the carbon nitride further improves H₂O₂The yield of (2). The alkalized series of catalysts all showed much higher photocatalytic effect than CN and CNT. Of note, ACNT-5 produced 120.8. mu. mol/L of H in 30min₂O₂. According to incomplete statistics, this yield is higher than most photocatalytic spontaneous systems reported in recent years (as shown in table 2). To compare the performance of different catalysts more intuitively, first order reaction kinetics (In (C)₀C) ═ kt) the photocatalytic product H was calculated₂O₂Reaction rate constant (FIG. 10 b). Apparently, the reaction rate k of ACNT-5 is 13 times that of CN. The chemical stability of the material was also determined, after six cycles, H₂O₂The yield of (2) still reached more than 90% of the first measurement result (FIG. 10 c). The performance of the catalyst was not significantly reduced over multiple cycles, indicating that the material had good chemical stability, while maintaining a well-retained hexagonal nanotubular morphology was clearly observed by scanning electron microscopy after multiple cycles (fig. 10 d).

Some of the optoelectronic properties of the material were characterized (fig. 11). It is generally accepted that the PL signal is generated by the recombination of photo-generated electrons and holes in some semiconductors. Compared to CN, the PL signal of ACNT-5 is severely attenuated (FIG. 11a), indicating that the separation of photogenerated electrons from holes is very efficient. In the EIS-Nyquist curve (FIG. 11b), ACNT-5 has the smallest ring, indicating the smallest impedance, which means that after light excitation, photo-generated electrons and holes do not undergo massive recombination inside ACNT-5, but effectively separate and migrate to participate in the photocatalytic H production₂O₂The reaction of (1). The fluorescence lifetime profile of the material (FIG. 11c, d) also supports this phenomenon. Compared with CN, the fluorescence lifetime of ACNT-5 is greatly shortened, and the reduction factor is 7.This large reduction in fluorescence lifetime by a factor of 2 also indicates that the holes rapidly react with surface hydroxyls after hole separation. The ACNT-5 has a special shape which exposes more active sites, and a large number of hydroxyl groups introduced into the surface have strong reactivity, so that photogenerated holes can be rapidly captured, and the generation of more electrons by photocatalysis is promoted. The electrons can rapidly react with water, oxygen and the like to generate H₂O₂. In general, ACNT-5 shows more excellent photoelectric property and is beneficial to producing H by photocatalysis₂O₂The reaction takes place.

To further explore H in the ACNT system₂O₂The major active species in the system were analyzed in combination with the capture experiments and EPR tests for a large increase in yield (fig. 13). In the curve of CN, it was clearly observed that the result was ascribed to DMPO-. O₂ ^-Indicating that in the CN system, H is₂O₂By reducing O mainly in a plurality of steps₂Reduction to H₂O₂. Interestingly, in the ACNT-5 system, although DMPO-. O was not found₂-but a signal of DMPO-. OH was detected (FIG. 13 a). In fact, OH does not produce H₂O₂The major active species of (c). Therefore, to confirm the results of this experiment, a series of capture experiments were designed. In the photocatalytic production of H₂O₂P-benzoquinone is added as O in the experiment₂Trapping agent, isopropanol as. OH trapping agent and triethanolamine as h⁺A capture agent. When p-benzoquinone was added to the CN system (FIG. 13b), H₂O₂The yield is reduced by over 95 percent, which is consistent with the measurement result of EPR, and the main active species in CN system is O₂-. In the ACNT-5 system, no significant decrease in yield was observed, indicating that O₂Not in production H₂O₂Plays a main role in the reaction. After the addition of isopropanol (FIG. 13c), the yield of the ACNT-5 system decreased by about 25%. The electron-hole separation efficiency of the ACNT-5 photocatalyst has been maintained at a high level, and thus, when isopropanol captures. OH, H is promoted₂O₂Self-decomposition of (see equations 1-3). And in h⁺In the trapping experiment (fig. 13d), the yield of the CN system was significantly increased, mainly due to the improved photocatalytic activity by facilitating the separation of electrons from holes. For ACNT-5, the capture of H + renders-OH unoxidizable to OH, resulting in H₂O₂Is not inhibited, so that H₂O₂The yield of (2) is reduced. From the above results, it was preliminarily judged that the surface hydroxyl group of ACNT-5 efficiently consumes holes, promotes the separation of electrons from holes of the material, and generates OH suppressing H₂O₂Self-decomposition reaction of (1). While the present invention has been described in detail with reference to the preferred embodiments, it should be understood that the above description should not be taken as limiting the invention.

The literature reports that H is affected by the proton transfer mechanism₂O₂The yield of (a) increases with decreasing pH. Thus, the change in pH during the reaction was monitored (as shown in fig. 14). For CN, the pH of the system slightly increased during dark adsorption, both with and without the addition of ethanol. This is due to hydrolysis of the amino groups on the surface of the carbon nitride (see equations 1-7). After the lamp was turned on, the pH continued to decrease as the reaction proceeded. H⁺Generated by oxidizing water molecules through photogenerated holes, ethanol can capture photogenerated holes to generate more H after a sacrificial agent is added⁺(see equation 1-1), therefore the production of H + is much higher than in CN systems without sacrificial agent, resulting in lower pH. In addition, the timely consumption of holes facilitates the generation of more electrons, H₂O₂The yield of (2) is 3 times that of the CN system without the sacrificial agent.

R-NH₂+H₂O→R-NH₃ ⁺+OH^- 1-7

R-OH→R-O^-+H⁺ 1-8

While the present invention has been described in detail with reference to the preferred embodiments, it should be understood that the above description should not be taken as limiting the invention.

Claims

1. A method for preparing alkalized carbon nitride nanotubes comprises the following steps:

1) weighing melamine, potassium chloride and ammonium chloride with preset mass, dispersing into deionized water, and heating and stirring under an oil bath until the melamine is completely dissolved;

2) transferring the obtained transparent solution into a hydrothermal kettle, placing the hydrothermal kettle in an oven, heating for a preset time at a preset temperature, naturally cooling to room temperature, washing the obtained sample with water and alcohol for a plurality of times in sequence, and drying to obtain the alkalized melamine nanorod;

3) placing the alkalized melamine nanorods into a muffle furnace to be calcined for preset time, and naturally cooling the alkalized melamine nanorods to room temperature after the calcination process is finished;

4) and fully grinding the obtained yellow sample, washing with water for several times, and drying in a vacuum drying oven to obtain the alkalized carbon nitride nanotube.

2. The preparation process as claimed in the preceding claim, wherein 1.0g of melamine, 1.0-10.0g of potassium chloride and 0.2g of ammonium chloride are weighed out and dispersed in 70mL of deionized water.

3. The preparation process as claimed in the preceding claim, wherein 1.0g of melamine, 5.0g of potassium chloride and 0.2g of ammonium chloride are weighed out and dispersed in 70mL of deionized water.

4. The process according to the preceding claim, wherein the oil bath temperature is 80 ℃.

5. The preparation method as claimed in the preceding claim, wherein the hydrothermal kettle is heated in an oven at 200 ℃ for 10 h.

6. The method of claim, wherein the calcining is carried out in a muffle furnace at a rate of 2.5 ℃/min up to 550 ℃ and held for 4 hours.

7. The method of claim, further comprising drying the dried product in a vacuum oven at 60 ℃.

8. A method for producing hydrogen peroxide by photocatalysis, which is characterized in that a preset mass of the alkalized carbon nitride nanotube prepared by the preparation method of any one of claims 1 to 7 is weighed and placed in a photocatalysis tube, deionized water is added, oxygen flow is used for bubbling for a preset time, the mixed solution is continuously stirred under dark conditions to achieve adsorption-desorption balance, and then the lamp is turned on for irradiation to prepare the hydrogen peroxide.

9. The method of claim 8, wherein 50mg of the alkalized carbon nitride nanotube prepared by the preparation method of any one of claims 1 to 7 is weighed and placed in a photocatalytic tube, 50mL of deionized water is added, oxygen flow is used for bubbling for 20min, the mixed solution is continuously stirred for 30min under dark conditions to reach adsorption-desorption equilibrium, and then the hydrogen peroxide is prepared by turning on a lamp for irradiation.