CN115430454A - Porous flaky carbon nitride photocatalytic material and preparation method thereof - Google Patents

Abstract

The invention belongs to the technical field of photocatalysis, relates to a semiconductor photocatalyst, and provides a porous flaky carbon nitride photocatalytic material and a preparation method thereof; the invention firstly synthesizes porous flaky carbon nitride precursor by molecular self-assembly of melamine and cyanuric acid, and then OH released by KOH is polymerized by heat ^‑ The compound reacts with amino on the edge of carbon nitride to form nitrogen carbon-cyano, the nitrogen carbon-cyano is used as an electron acceptor to form a unique electron donor-acceptor system with nitrogen atoms on a carbon nitride heptazine ring, so that efficient transmission of photon-generated carriers in molecules is promoted, and the problem of serious recombination of photon-generated carriers in the carbon nitride is solved; meanwhile, the electron-withdrawing property of the nitrogen carbon-cyano group serves as an active site in the photocatalytic reaction, so that the photocatalytic reaction is promoted to be carried out efficiently; the porous sheet structure of the carbon nitride improves the agglomeration of the nano-sheets,the contact opportunity of reactants and active sites is increased; finally, the invention shows excellent activity of photocatalytic reduction of carbon dioxide under visible light.

Description

A kind of porous carbon nitride photocatalytic material and preparation method thereof

technical field

The invention belongs to the technical field of photocatalysis, relates to a semiconductor photocatalyst, and specifically provides a porous sheet carbon nitride photocatalytic material containing an electron donor-acceptor system with enhanced activity of visible light photocatalytic reduction of carbon dioxide and a preparation method thereof.

Background technique

The energy crisis and environmental problems caused by the massive consumption of traditional energy and excessive exhaust emissions threaten the human living environment, among which carbon dioxide is one of the main gases that cause the greenhouse effect; therefore, how to reduce the concentration of CO ₂ in the atmosphere and effectively use Carbon dioxide has become a research hotspot. Photocatalytic technology can reduce carbon dioxide to carbon monoxide and hydrocarbon fuels, which can simultaneously solve the problems of energy shortage and environmental pollution, and has attracted widespread attention.

The core of the field of photocatalysis technology is to develop highly active semiconductor photocatalysts. In recent years, carbon nitride (g~C ₃ N ₄ ) has been widely used in the field of photocatalysis due to its metal-free and moderate energy band structure; however, bulk Due to the disadvantages of easy recombination of photogenerated carriers and insufficient exposure of active sites, phase carbon nitride only shows weak photocatalytic activity. To solve this problem, designing porous sheet-like carbon nitride is one of the effective measures to improve photocatalytic activity. Among them, the electron donor-acceptor system can effectively promote intramolecular carrier migration and effectively inhibit carrier recombination, further improving photocatalytic activity. The photocatalytic activity of carbon nitride; the nitrogen atom on the heptazine ring in carbon nitride is often used as an electron donor. In order to construct an efficient electron donor-acceptor transport system, the choice of electron acceptor is very important. At present, some studies have reported using selenium and dibromoquinoline as electron acceptors, such as the literature "Ou.HH, Chen.XR, Lin.LH, Fang.YX, Wang.XC, Biomimetic donor-acceptor motifsin conjugated polymers for promoting exciton splitting and charge separation, Angew. Chem. Int. Ed., 2018, 57, 8729-8733.", "Fan.X, Zhang.L, Cheng.R, Wang.M, Li.M, Zhou. Y, Shi.J, Construction of graphitic C ₃ N ₄ -based intramolecular Donor-Acceptor conjugated copolymers for photocatalytic hydrogen evolution, ACSCatal., 2015, 5, 5008-5015.”; however, these methods are not environmentally friendly and costly Problems, not conducive to large-scale industrial production.

Contents of the invention

The purpose of the present invention is to provide a porous sheet-like carbon nitride photocatalytic material and its preparation method in view of the defects of the carbon nitride material itself and the deficiencies in the prior art; characteristics, it was proposed for the first time to use the nitrogen atom in the heptazine ring as the electron donor and the nitrogen-carbon-cyano group as the electron acceptor to construct an electron donor-acceptor system in situ in the porous carbon nitride, so that the porous sheet Carbon nitride photocatalytic materials exhibit excellent photocatalytic carbon dioxide reduction activity under visible light.

To achieve the above object, the technical scheme adopted in the present invention is as follows:

A porous sheet-like carbon nitride photocatalytic material, characterized in that the carbon nitride has a porous sheet-like structure, wherein the porous sheet-like carbon nitride nanosheet has a thickness of 10-60 nm and a pore diameter of 0.1-0.4 μm ; The carbon nitride has a nitrogen carbon-cyano functional group, and the nitrogen carbon-cyano functional group serves as an electron acceptor.

Further, the porous sheet carbon nitride photocatalytic material photocatalytically reduces carbon dioxide into methane and carbon monoxide under visible light.

The preparation method of the above-mentioned porous sheet carbon nitride photocatalytic material is characterized in that it comprises the following steps:

Step 1, preparation of porous sheet carbon nitride precursor;

Step 1.1, dispersing melamine in ethanol to obtain an ethanol suspension of melamine; dispersing cyanuric acid in ethanol to obtain an ethanol suspension of cyanuric acid;

Step 1.2, mixing the ethanol suspension of melamine and the ethanol suspension of cyanuric acid, and stirring until a precipitate is formed to obtain a mixed suspension;

Step 1.3, centrifuging, washing, and drying the mixed suspension in sequence to obtain a porous carbon nitride precursor;

Step 2, preparation of porous sheet carbon nitride photocatalytic material;

Step 2.1, dissolving KOH in a mixed solution of ethanol and pyridine to obtain a dispersion;

Step 2.2, dispersing the porous flaky carbon nitride precursor into the dispersion liquid, then drying and evaporating to dryness to obtain a powder sample;

Step 2.3, placing the powder sample in a muffle furnace, and sintering to obtain a porous sheet carbon nitride photocatalytic material.

Further, in step 1.2, the molar ratio of melamine to cyanuric acid is: (0.1-2):(0.1-2).

Further, in step 1.3, the drying temperature is 50-70°C.

Further, in step 2.1, the mass of the KOH is 0.1-1 g, the mass concentration of the dispersion is 1×10 ⁴ to 5×10 ⁴ mg/L; the volume of pyridine and ethanol in the mixed solution of ethanol and pyridine The ratio is 1:10.

Further, in step 2.2, the mass of the porous sheet-like carbon nitride precursor is 2-10 g, and the temperature for drying and evaporating to dryness is 20-80°C.

Further, in step 2.3, the specific process of sintering is: heating from room temperature to 550° C. at a rate of 1-5° C./min, keeping the temperature at 550° C. for 2-8 hours, and then naturally cooling to room temperature.

Compared with prior art, the beneficial effect of the present invention is:

1) The porous sheet-like carbon nitride photocatalytic material containing the electron donor-system provided by the present invention can significantly enhance the photocatalytic activity from three aspects: first, nitrogen carbon-cyano group is used as electron acceptor and carbaheptazine nitride The nitrogen atoms on the ring form a unique electron donor-acceptor system, which promotes the efficient transport of photogenerated carriers in the molecule and solves the serious problem of recombination of photogenerated carriers in carbon nitride; secondly, the introduced nitrogen The carbon-cyano group has a unique electron-withdrawing property, which acts as an active site in the photocatalytic reaction and promotes the efficient photocatalytic reaction; finally, due to the electron-withdrawing effect of the nitrogen atom on the pyridine ring, the electron cloud density of the carbon atom on the ring Reduced, especially in the 2-position and 4-position, the electron cloud density is lower, so the nucleophilic substitution reaction on the ring is easy to occur, resulting in a porous sheet structure of the carbon nitride product, which improves the agglomeration of nanosheets , increasing the chance of contact between the reactant and the active site;

2) The preparation method of the porous carbon nitride photocatalytic material containing the electron donor-system provided by the present invention has the advantages of simple process, environmental protection and low cost;

3) The present invention uses molecular self-assembly combined with alkali-assisted methods to prepare porous sheet-like carbon nitride containing electron donor-systems to improve the activity of photocatalytic reduction of carbon dioxide, which can be promoted in the field of photocatalysis.

Description of drawings

Fig. 1 is the synthesizing mechanism diagram of porous sheet carbon nitride photocatalyst material in embodiment 1.

Fig. 2 is the Fourier transform infrared absorption spectrum (FTIR) of the materials prepared in Example 1 and Comparative Examples 1-2.

3 is a scanning electron microscope image (SEM) of the materials prepared in Example 1 and Comparative Examples 1-2; wherein, (a) is Example 1, (b) is Comparative Example 1, and (c) is Comparative Example 2.

Fig. 4 is the electrochemical impedance diagram (EIS) and transient photocurrent response diagram of the material prepared in embodiment 1 and comparative example 1～2; Wherein, (a) is the transient photocurrent response diagram, (b) electrochemical Impedance diagram.

Figure 5 is a photocatalytic carbon dioxide reduction performance diagram of the materials prepared in Example 1 and Comparative Examples 1 to 2 under visible light radiation; wherein, (a) is a carbon monoxide performance diagram produced by photocatalytic reduction of carbon dioxide, and (b) is a photocatalytic The performance diagram of methane produced by reducing carbon dioxide, (c) is the yield diagram of methane and carbon monoxide produced by photocatalytic reduction of carbon dioxide.

detailed description

The purpose of the present invention, technical solutions and beneficial effects will be further described in detail below in conjunction with the embodiments and accompanying drawings. Limitation of Scope of Protection.

Example 1

This embodiment provides a porous sheet-like carbon nitride photocatalytic material, the carbon nitride is a porous sheet-like structure, the thickness of the porous sheet-like carbon nitride nanosheet is 10-60 nm, and the pore diameter is 0.1-0.4 μm; The carbon nitride has nitrogen-carbon-cyano functional groups, and the nitrogen-carbon-cyano functional groups are used as electron acceptors; the porous sheet carbon nitride photocatalytic material is obtained by molecular self-assembly and alkali-assisted strategies, and its porous sheet structure The formation mechanism of carbon-nitrogen-cyano functional groups is shown in Figure 1. First, a porous sheet-like carbon nitride precursor is synthesized by molecular self-assembly from melamine and cyanuric acid, and then the added KOH melts during thermal polymerization, and the released OH ^- reacts with the amino group at the edge of the carbon nitride to form an electron-withdrawing group (nitrogen-carbon-cyano).

More specifically, the preparation process of the above-mentioned porous sheet carbon nitride photocatalytic material includes the following steps:

Step 1, preparation of porous sheet carbon nitride precursor;

Step 1.1, dissolve 5 g of melamine in 200 mL of ethanol, stir for 4 hours until the melamine is evenly dispersed, and obtain an ethanol suspension of melamine; disperse 5.1 g of cyanuric acid in 200 mL of ethanol, stir for 4 h until the cyanuric acid is evenly dispersed, and obtain a cyanuric acid suspension ethanol suspension;

Step 1.2, mix the ethanol suspension of melamine prepared in step 1.1 with the ethanol suspension of cyanuric acid, stir for 15min to form a white precipitate, and obtain a mixed suspension; wherein the molar ratio of melamine to cyanuric acid is 1:1;

Step 1.3. Centrifuge the mixed suspension obtained in step 1.2, wash it several times with deionized water and ethanol, and then dry it in an oven at 50°C to obtain a white product, which is a porous sheet carbon nitride precursor (cyanuric acid-melamine , abbreviated as CM);

Step 2, preparation of porous sheet carbon nitride containing electron donor-system:

Step 2.1, adding 0.5g KOH to 20ml of ethanol and pyridine mixed solution, wherein the volume ratio of pyridine and ethanol is 1:10; after dissolving, add 8g of the porous sheet carbon nitride precursor (CM) obtained in step 1, Stir evenly to obtain a dispersion;

Step 2.2, drying the dispersion obtained in step 2.1 in an oven at 50°C to obtain a white powder sample;

Step 2.3, put the white powder sample obtained in step 2.2 in a muffle furnace, raise the temperature from room temperature to 550 °C at a rate of 2.5 °C/min, and keep it at 550 °C for 4 hours; after completion, naturally cool to room temperature, A yellow powder is obtained, which is a porous carbon nitride sheet containing an electron donor-acceptor system (abbreviated as MCN-0.5).

At the same time, this example also provides Comparative Example 1 and Comparative Example 2; wherein, the difference between Comparative Example 1 and Example 1 is that the precursor prepared in Step 1 is directly raised from room temperature to 550 °C at a heating rate of 2.5 °C/min. ℃, and kept at 550 ℃ for 4h to make a porous sheet carbon nitride material; comparative example 2 is bulk carbon nitride, and its preparation process is: 5g melamine is ground in a mortar for 10 minutes, put into a crucible, Calcined at 550°C for 4h, the heating rate was set at 5°C/min, and the light yellow powder obtained after sintering was bulk carbon nitride.

The photocatalytic carbon dioxide reduction activity experiment was carried out on the materials obtained in the above-mentioned Example 1 and Comparative Examples 1-2. The specific steps were as follows: first, take 30 mg of the sample and place it in the crucible lid, add 3 ml of ethanol and ultrasonically disperse it for 5 minutes;

Then, the sample after ultrasonic dispersion was dried to form a film, and 0.5ml deionized water was added dropwise;

Finally, check the airtightness of the reaction and evacuate the reactor, feed carbon dioxide and keep the system pressure at about 70-80Kpa, turn on the water circulation device to ensure that the reactor temperature is maintained at room temperature, and set up a photocatalytic online analysis system (Perfect Light Labsolar 6A ) of the reaction time and cycle, turn on the 300W xenon lamp to start the photocatalytic carbon dioxide reduction experiment, and detect the carbon dioxide photoreduction product by gas chromatography.

As shown in Figure 2, it is the Fourier transform infrared absorption spectrum (FTIR) of the prepared material of embodiment 1 and comparative examples 1～2; As can be seen from the figure, embodiment 1 and comparative examples 1～2 all show and carbon nitride similar Chemical structure, but the characteristic peak of nitrogen carbon-cyano group appeared in Example 1 at 2173cm ^-1 , indicating that cyano group was successfully introduced into carbon nitride.

As shown in Figure 3, it is the scanning electron microscope figure (SEM) of the prepared material of embodiment 1 and comparative examples 1～2; As can be seen from the figure, the morphology of the bulk phase carbon nitride prepared in comparative example 2 is agglomerated nanosheets, and Through the preparation method of molecular self-assembly, using the electron-absorbing effect of the nitrogen atom on the pyridine ring, the electron cloud density of the carbon atom on the ring is reduced, especially at the 2-position and 4-position. The nucleophilic substitution reaction is easy to occur, resulting in the carbon nitride product having a porous sheet-like structure, that is, the porous sheet-like carbon nitride prepared in Example 1 and Comparative Example 1 is obtained, which greatly facilitates the exposure of active sites.

As shown in Figure 4, it is the electrochemical impedance diagram (EIS) and the transient photocurrent response diagram of the material prepared in embodiment 1 and comparative examples 1～2; Wherein, as can be seen from figure (a), the material prepared in embodiment 1 shows The highest photocurrent density is obtained, indicating that the electron donor-acceptor system inhibits the recombination of carriers, and the electron donor-acceptor system promotes the migration of carriers in the molecule, which increases the effective utilization of photogenerated carriers; from Figure (b) shows that the equivalent impedance of the material prepared in Example 1 is the smallest, indicating that due to the introduction of nitrogen carbon-cyano group as the electron acceptor and the formation of an electron donor-acceptor between the nitrogen atom in the carboheptazine ring system, which effectively promotes carrier transfer.

As shown in Figure 5, it is the photocatalytic carbon dioxide reduction performance diagram of the materials prepared in Example 1 and Comparative Examples 1 to 2 under visible light radiation; As can be seen from the figure, compared with the bulk phase carbon nitride prepared in Comparative Example 2, the 1 The prepared porous sheet-like carbon nitride containing electron donor-acceptor system exhibited significantly enhanced photocatalytic carbon dioxide reduction activity, and the yield of methane and carbon monoxide was the highest among all samples; the enhanced photocatalytic activity indicated that the nitrogen-carbon The electron donor-acceptor system formed between the -cyano group as the electron acceptor and the nitrogen atom on the carboheptazine ring promotes the efficient transport of photogenerated carriers in the molecule, and solves the problem of photogenerated carriers in carbon nitride At the same time, the porous sheet structure improves the aggregation of nanosheets and increases the contact opportunities between reactants and active sites.

The above is only a specific embodiment of the present invention. Any feature disclosed in this specification, unless specifically stated, can be replaced by other equivalent or alternative features with similar purposes; all the disclosed features, or All method or process steps may be combined in any way, except for mutually exclusive features and/or steps.

Claims (8)

1. A porous sheet-like carbon nitride photocatalytic material, characterized in that, the carbon nitride is a porous sheet-like structure, wherein the thickness of the porous sheet-like carbon nitride nanosheets is 10 to 60 nm, and the aperture is 0.1 to 60 nm. 0.4 μm; the carbon nitride has a nitrogen carbon-cyano functional group, and the nitrogen carbon-cyano functional group acts as an electron acceptor. 2. The porous sheet-like carbon nitride photocatalytic material according to claim 1, wherein the porous sheet-like carbon nitride photocatalytic material photocatalytically reduces carbon dioxide to methane and carbon monoxide under visible light. 3. by the preparation method of the described porous sheet carbon nitride photocatalytic material of claim 1, it is characterized in that, comprises the following steps: Step 1, preparation of porous sheet carbon nitride precursor; Step 1.1, dispersing melamine in ethanol to obtain an ethanol suspension of melamine; dispersing cyanuric acid in ethanol to obtain an ethanol suspension of cyanuric acid; Step 1.2, mixing the ethanol suspension of melamine and the ethanol suspension of cyanuric acid, and stirring until a precipitate is formed to obtain a mixed suspension; Step 1.3, centrifuging, washing, and drying the mixed suspension in sequence to obtain a porous carbon nitride precursor; Step 2, preparation of porous sheet carbon nitride photocatalytic material; Step 2.1, dissolving KOH in a mixed solution of ethanol and pyridine to obtain a dispersion; Step 2.2, dispersing the porous flaky carbon nitride precursor into the dispersion liquid, then drying and evaporating to dryness to obtain a powder sample; Step 2.3, placing the powder sample in a muffle furnace, and sintering to obtain a porous sheet carbon nitride photocatalytic material. 4. according to the preparation method of the described porous sheet carbon nitride photocatalytic material of claim 3, it is characterized in that, in step 1.2, the mol ratio of described melamine and cyanuric acid is: (0.1～2): (0.1～2 ). 5. The preparation method of the porous sheet carbon nitride photocatalytic material according to claim 3, characterized in that, in step 1.3, the drying temperature is 50-70°C. 6. The method for preparing the porous sheet-like carbon nitride photocatalytic material according to claim 3, characterized in that, in step 2.1, the mass of the KOH is 0.1-1 g, and the mass concentration of the dispersion is 1×10 ⁴ - 5×10 ⁴ mg/L; the volume ratio of pyridine to ethanol in the mixed solution of ethanol and pyridine is 1:10. 7. according to the preparation method of the described porous sheet-like carbon nitride photocatalytic material of claim 3, it is characterized in that, in step 2.2, the quality of described porous sheet-like carbon nitride precursor is 2～10g, dry evaporated The temperature is 20-80°C. 8. According to the preparation method of the porous sheet-like carbon nitride photocatalytic material according to claim 3, it is characterized in that, in step 2.3, the specific process of sintering is: the temperature rise rate is raised from room temperature to 550°C, and keep warm at 550°C for 2-8 hours, then cool down to room temperature naturally.

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