A bimetal-loaded boron-doped carbon nitride nanosheet heterojunction, its preparation method and application

Abstract

The present invention discloses a heterojunction of bimetal-loaded boron-doped carbon nitride nanosheets. One end of the heterojunction is carbon nitride nanosheets loaded with metal Cu, and the other end is nitrogen-deficient boron-doped carbon nitride nanosheets loaded with metal Pd. The preparation process is as follows: (1) preparing Cu-CNN by photodeposition method; (2) preparing BDCNN by thermal reduction; (3) preparing Pd-BDCNN by ethanol reduction method; (4) preparing Cu-CNN / Pd-BDCNN by electrostatic self-assembly. When the heterojunction of bimetal-loaded boron-doped carbon nitride nanosheets of the present invention is used for the photothermal catalytic dry reforming reaction of methane, Cu-CNN and Pd-BDCNN synergistically improve the activity of the photothermal catalytic dry reforming reaction of methane, and have excellent photothermal catalytic effects.

Description

Technical Field

[0001] The present invention belongs to the technical fields of preparation of nanomaterials and photothermal catalysis, and particularly relates to a heterojunction of bimetal-loaded boron-doped carbon nitride nanosheets, a preparation method thereof, and an application thereof in the dry reforming reaction of methane by photothermal catalysis. Background Art

[0002] The dry reforming reaction of methane is to react methane and carbon dioxide to convert them into important value-added chemicals, such as hydrogen, carbon monoxide, ethane, ethylene, methanol, ethanol, etc. Efficiently and selectively converting methane and carbon dioxide into specific high-value-added chemicals has always been the goal of scientific research workers. Although the traditional thermal catalytic dry reforming of methane has a high conversion efficiency, it requires harsh reaction conditions such as high temperature and high pressure, and serious carbon deposition occurs, resulting in high energy consumption and uneconomical reaction; the new photocatalytic dry reforming of methane has attracted much attention as a mild reaction method, but currently its conversion efficiency is too low and its industrial application value is not high; the photothermal catalytic dry reforming of methane that combines photocatalysis and relatively low-temperature thermal catalysis can well take into account the advantages of both, and can carry out the reaction under mild conditions and have a high conversion efficiency. As a typical organic semiconductor material, graphitic carbon nitride material has excellent properties such as visible light responsiveness, low-cost synthesis, and high chemical / thermal stability. However, the easy and rapid recombination of its photo-generated electrons and holes, and the insufficient oxidation ability of methane restrict the application of carbon nitride. Summary of the Invention

[0003] In order to solve the problems existing in the prior art, the purpose of the present invention is to provide a heterojunction of bimetal-loaded boron-doped carbon nitride nanosheets, a preparation method thereof, and an application thereof in the dry reforming reaction of methane by photothermal catalysis. One end of the heterojunction is a carbon nitride nanosheet loaded with metal Cu, and the other end is a nitrogen-deficient and boron-doped carbon nitride nanosheet loaded with metal Pd. When it is used for the dry reforming reaction of methane by photothermal catalysis, the two cooperate to improve the activity of the dry reforming reaction of methane by photothermal catalysis.

[0004] In order to achieve the above technical purpose, the present invention adopts the following technical solutions:

[0005] A heterojunction of bimetal-loaded boron-doped carbon nitride nanosheets, one end of the heterojunction is a carbon nitride nanosheet loaded with metal Cu, and the other end is a nitrogen-deficient and boron-doped carbon nitride nanosheet loaded with metal Pd.

[0006] Preferably, in the heterojunction, the mass ratio of the carbon nitride nanosheet loaded with metal Cu to the nitrogen-deficient and boron-doped carbon nitride nanosheet loaded with metal Pd is 9:1 to 1:9.

[0007] Preferably, the thickness of the carbon nitride nanosheet is 10-20 nm.

[0008] Preferably, in the metal Cu-loaded carbon nitride nanosheets, the loading amount of metal Cu is 0.01-1 wt%.

[0009] Preferably, in the nitrogen-deficient boron-doped carbon nitride nanosheets loaded with metal Pd, the loading amount of metal Pd is 0.5-5 wt%, and the doping amount of boron is 0.05-0.5 wt%.

[0010] The present invention also provides a preparation method of the above-mentioned bimetal-loaded boron-doped carbon nitride nanosheet heterojunction, including the following steps:

[0011] (1) By using a photodeposition method, load metal Cu on the carbon nitride nanosheets to obtain metal Cu-loaded carbon nitride nanosheets;

[0012] (2) Mix the carbon nitride nanosheets with sodium borohydride or boric acid evenly, and perform a thermal reduction treatment under a protective atmosphere to obtain nitrogen-deficient boron-doped carbon nitride nanosheets;

[0013] (3) By using an ethanol reduction method, load metal Pd on the nitrogen-deficient boron-doped carbon nitride nanosheets to obtain metal Pd-loaded nitrogen-deficient boron-doped carbon nitride nanosheets;

[0014] (4) Assemble the metal Cu-loaded carbon nitride nanosheets and the metal Pd-loaded nitrogen-deficient boron-doped carbon nitride nanosheets by electrostatic self-assembly to obtain a bimetal-loaded boron-doped carbon nitride nanosheet heterojunction.

[0015] In the present invention, the carbon nitride nanosheets can be prepared by existing conventional methods, such as by urea heat treatment and ultrasonic exfoliation.

[0016] Preferably, in step (1), the specific process of the photodeposition method is as follows: Add copper nitrate trihydrate to water, stir and dissolve; then add the carbon nitride nanosheets and disperse them by ultrasonic waves; finally, add methanol and stir well to obtain a dispersion; irradiate and reduce Cu ions in this dispersion under ultraviolet-visible light (λ>300 nm), and obtain metal Cu-loaded carbon nitride nanosheets through filtration, washing, and drying.

[0017] Preferably, the mass ratio of copper nitrate trihydrate to the carbon nitride nanosheets is 0.006-0.6:1.

[0018] Preferably, in step (2), the mass ratio of the carbon nitride nanosheets to sodium borohydride or boric acid is 4:0.1-2; the temperature of the thermal reduction treatment is 300-500 °C.

[0019] Preferably, in step (3), the specific process of the ethanol reduction method is as follows: Palladium chloride is added to absolute ethanol and stirred until dissolved; then nitrogen-deficient boron-doped carbon nitride nanosheets are added, and Pd ions are reduced by ultrasonic stirring. After filtration, washing, and drying, metal Pd-loaded nitrogen-deficient boron-doped carbon nitride nanosheets are obtained.

[0020] Preferably, the mass ratio of palladium chloride to nitrogen-deficient boron-doped carbon nitride nanosheets is 0.01 - 0.2:1.

[0021] Preferably, in step (4), the specific process of the electrostatic self-assembly is as follows: Metal Cu-loaded carbon nitride nanosheets are added to a hydrochloric acid solution, and protonation is carried out by ultrasonic stirring. After filtration, washing, and drying, acidified Cu-loaded carbon nitride nanosheets are obtained; the acidified Cu-loaded carbon nitride nanosheets are added to water and stirred to disperse; then metal Pd-loaded nitrogen-deficient boron-doped carbon nitride nanosheets are added, and ultrasonic stirring is carried out. After filtration, washing, and drying, a bimetal-loaded boron-doped carbon nitride nanosheet heterojunction is obtained.

[0022] Preferably, the mass ratio of metal Cu-loaded carbon nitride nanosheets to metal Pd-loaded nitrogen-deficient boron-doped carbon nitride nanosheets is 9:1 - 1:9.

[0023] The present invention also provides an application of the above-mentioned bimetal-loaded boron-doped carbon nitride nanosheet heterojunction, which is used for the photothermal catalytic dry reforming reaction of methane.

[0024] Compared with the prior art, the advantages of the present invention are as follows:

[0025] For the bimetal-loaded boron-doped carbon nitride nanosheet heterojunction of the present invention, one end is based on unmodified carbon nitride, and the other end is based on boron-doped modified carbon nitride. The heterojunction interface formed in this way is closer, and the interface can better transport electrons; boron doping changes the energy band structure of carbon nitride. After forming the heterojunction, the overall light absorption performance is improved and the separation of photo-generated carriers is promoted; the introduction of nitrogen vacancies can, on the one hand, enhance the conductivity of carbon nitride, and on the other hand, nitrogen vacancies can promote gas adsorption; then, the unmodified carbon nitride loaded with metal Cu is used as one end of the heterojunction, which has better electron transport and reduction activity for carbon dioxide; while the nitrogen-deficient boron-doped modified carbon nitride nanosheets loaded with metal Pd are used as the other end of the heterojunction, which has stronger oxidation performance and dehydrogenation performance for methane; the two cooperate to improve the activity of the photothermal catalytic dry reforming reaction of methane. Description of the Drawings

[0026] Figure 1 It is an XRD characterization diagram of carbon nitride materials with different modification and doping;

[0027] Figure 2For the optical absorption characterization and fluorescence characterization of differently modified and functionalized carbon nitride materials;

[0028] Figure 3 AFM characterization diagram of carbon nitride nanosheets;

[0029] Figure 4 SEM and TEM characterization diagrams of differently modified and functionalized carbon nitride materials, SEM: (a) CNN; TEM: (b) Cu-CNN, (c) Pd-BDCNN, (d) Cu-CNN / Pd-BDCNN.

[0030] Explanation of symbols in the figure: CNN: Carbon nitride nanosheets; BDCNN: Nitrogen-deficient and boron-doped carbon nitride nanosheets; Cu-CNN: Cu-loaded carbon nitride nanosheets; Pd-BDCNN: Pd-loaded nitrogen-deficient and boron-doped carbon nitride nanosheets; Cu-CNN / Pd-BDCNN: Heterojunction of Cu-Pd-loaded boron-doped carbon nitride nanosheets. Specific implementation mode

[0031] The following further elaborates on the present invention with reference to embodiments, but the protection scope of the present invention is not limited to these embodiments.

[0032] In the present invention, the specific preparation process of carbon nitride nanosheets is as follows:

[0033] 30 g of urea is added to a 70 mL crucible, the crucible lid is covered, and it is placed in a muffle furnace. It is heated to 550 °C at a rate of 5 °C / min and maintained for 2 h to obtain a yellow solid; the yellow solid is ground into powder in a mortar, poured into a 250 mL beaker, 200 mL of deionized water is added, stirred for 30 min, and then ultrasonicated for 2 h; the suspension obtained by ultrasonication is filtered, washed twice with deionized water, and dried in an 80 °C oven for 2 h to obtain a pale yellow powder; the pale yellow powder is evenly spread on a porcelain boat, the porcelain boat lid is covered, and it is placed in a muffle furnace. It is heated to 550 °C at a rate of 5 °C / min and maintained for 2 h to obtain carbon nitride nanosheets CNN. The scanning electron microscope photograph of CNN is shown in Figure 4 (a), and it can be seen that the carbon nitride is porous and sheet-like; the atomic force microscope characterization of CNN is as shown in Figure 3 shown, and the thickness of the nanosheets is about 15 nm.

[0034] The present invention evaluates the activity of the catalyst through a photothermal catalytic closed reactor. The reaction conditions are: 140 °C, 1 mg of catalyst, methane:carbon dioxide:argon = 1:1:8, 300 W xenon lamp (ultraviolet + visible), and the reaction lasts for 4 h. The gases hydrogen and carbon monoxide produced by the reaction are quantitatively sampled online by gas chromatography (Fuli 9790).

[0035] Example 1

[0036] (1) Preparation of Cu-loaded carbon nitride nanosheets by photo-deposition method:

[0037] In 90 mL of deionized water, 11.54 mg of copper nitrate trihydrate was added and stirred until dissolved. Then, 100 mg of the as-prepared carbon nitride nanosheets CNN was added, and the mixture was sonicated for 10 min. Subsequently, 10 mL of methanol was added and stirred for 10 min. The dispersion was irradiated under ultraviolet-visible light (λ > 300 nm) for 2 h to fully reduce the Cu ions. The product was obtained by filtration, washed three times with deionized water, and dried in vacuum at 60 °C for 2 h to obtain metal Cu-loaded carbon nitride nanosheets Cu-CNN, where the loading amount of metal Cu was 0.05 wt%. The transmission electron microscopy image of Cu-CNN is shown in Figure 4 (b). No obvious metal Cu was observed, but it can be seen from Table 2 that the mass percentage of Cu element in Cu-CNN / Pd-BDCNN was 0.024%, indicating that metal Cu was successfully loaded, although the content was relatively low and the metal particles were relatively small.

[0038] (2) Preparation of nitrogen-deficient boron-doped carbon nitride nanosheets by thermal reduction method:

[0039] 400 mg of carbon nitride nanosheet powder and 160 mg of sodium borohydride powder were ground in a mortar and mixed for 10 min. The mixed powder was evenly spread on a porcelain boat, covered with the boat lid, and placed in a tube furnace. Under a nitrogen atmosphere, it was heated to 450 °C at a rate of 5 °C / min and heat-treated for 1 h. After the obtained powder was cooled to room temperature, it was dispersed in 50 mL of ethanol, sonicated for 10 min, stirred for 10 min, separated by suction filtration, and washed twice with deionized water and ethanol respectively. Finally, it was dried in a vacuum drying oven at 60 °C for 2 h to obtain nitrogen-deficient boron-doped carbon nitride nanosheets BDCNN, where the doping amount of B was 0.153 wt%.

[0040] (3) Preparation of Pd-loaded nitrogen-deficient boron-doped carbon nitride nanosheets by ethanol reduction method:

[0041] 8.5 mg of palladium chloride was dissolved in 100 mL of absolute ethanol, and then 100 mg of nitrogen-deficient boron-doped carbon nitride nanosheets BDCNN was added. The mixture was sonicated for 1 h and stirred for 2 h. After stirring, it was filtered through a sintered glass funnel, washed twice with absolute ethanol and acetonitrile respectively, and then placed in a vacuum drying oven at 60 °C and dried overnight to obtain metal Pd-loaded nitrogen-deficient boron-doped carbon nitride nanosheets Pd-BDCNN, where the loading amount of metal Pd was 2 wt%. The transmission electron microscopy image of Pd-BDCNN is shown in Figure 4 (c), and the particle size of metal Pd was 5 nm. It can be seen from the elemental content analysis in Table 2 that the mass percentage of Pd in Cu-CNN / Pd-BDCNN was 1.795%.

[0042] (4) Preparation of Cu-Pd loaded boron-doped carbon nitride nanosheet heterojunction by electrostatic self-assembly:

[0043] Prepare 200 mL of a hydrochloric acid solution with a concentration of 1.5 mol / L, add 80 mg of metal Cu-loaded carbon nitride nanosheets, stir for 10 min, sonicate for 1 h, and then vigorously stir the suspension at room temperature for another 2 h for protonation. The obtained acid suspension is filtered and washed with deionized water to remove excess hydrochloric acid until the pH value is close to 7. It is placed in an oven at 80 °C and dried for 2 h to obtain acidified Cu-loaded carbon nitride nanosheets;

[0044] Pour 100 mL of deionized water into a 150 mL beaker, add the above-mentioned acidified Cu-loaded carbon nitride nanosheets, stir for 10 min, then add 80 mg of metal Pd-loaded nitrogen-deficient boron-doped carbon nitride nanosheets Pd-BDCNN, stir for 10 min, sonicate for 30 min, stir for 2 h, and finally filter and wash three times with deionized water. It is placed in a vacuum drying oven at 60 °C and dried overnight to obtain a Cu-Pd loaded boron-doped carbon nitride nanosheet heterojunction Cu-CNN / Pd-BDCNN, where the mass ratio of Cu-CNN to Pd-BDCNN is 1:1. The transmission electron microscope image of Cu-CNN / Pd-BDCNN is shown in Figure 4 (d), and it can be clearly seen that there are two parts of carbon nitride nanosheets with and without metal Pd loading.

[0045] CNN, Cu-CNN, Pd-BDCNN, and Cu-CNN / Pd-BDCNN were respectively used for the photocatalytic dry reforming of methane reaction, and the performance evaluation results are shown in Table 1.

[0046] Figure 1 Figure (a) is the XRD characterization diagram of different catalysts. From the XRD characterization results in the left figure, after modification, no new diffraction peaks are added, but there are changes in the intensity and position of the diffraction peaks. Compared with the initial matrix material CNN, after boron doping and nitrogen deficiency modification, the diffraction peak intensity of the (100) crystal plane weakens. As can be seen from the enlarged view in the right figure, the position of the (002) crystal plane shifts and the peak intensity also weakens, which also proves that the modified material has been successfully obtained.

[0047] Figure 2(a) shows the optical absorption characterization of different catalysts. From top to bottom, they are Pd-BDCNN, Cu-CNN / Pd-BDCNN, and Cu-CNN. (b) shows the fluorescence characterization of different catalysts. From top to bottom, they are Cu-CNN, CNN, Pd-BDCNN, and Cu-CNN / Pd-BDCNN. From the results of optical absorption characterization, it can be seen that in the ultraviolet light region below 420 nm, the ultraviolet light absorption of Cu-CNN / Pd-BDCNN is between Cu-CNN and Pd-BDCNN. In the visible and infrared light regions above 420 nm, the light absorption performance of Cu-CNN / Pd-BDCNN is better than that of Cu-CNN and Pd-BDCNN. From the results of fluorescence characterization, it can be seen that after loading Cu on CNN, the fluorescence intensity increases to a certain extent, while after loading Pd on boron-doped nitrogen-deficient carbon nitride, the fluorescence intensity decreases significantly. The composite material Cu-CNN / Pd-BDCNN obtained by electrostatic self-assembly of Cu-CNN and Pd-BDCNN has the lowest fluorescence intensity, and the construction of the heterojunction promotes the separation of photo-generated carriers, which also proves that the composite material has been successfully obtained.

[0048] Figure 3 Figure shows the AFM characterization of carbon nitride nanosheets, and the thickness of the carbon nitride nanosheets is about 15 nm.

[0049] Figure 4 Figures show the SEM and TEM characterization of different catalysts. SEM: (a) CNN; TEM: (b) Cu-CNN, (c) Pd-BDCNN, (d) Cu-CNN / Pd-BDCNN. It can be seen from Figure (a) that carbon nitride is porous and sheet-like. No obvious metal Cu can be seen in Figure (b), but from the mass percentage content of Cu element in Table 2, it is 0.024%, which proves that metal Cu is successfully loaded, but the content is relatively small and the metal particles are relatively small. It can be seen from Figure (c) that the particle size of metal Pd is 5 nm; from the elemental content analysis in Table 2, the mass percentage content of Pd is 1.795%. It can be clearly seen from Figure (d) that there are two parts of carbon nitride nanosheets with metal Pd loaded and without metal Pd.

[0050] Table 1 Evaluation results of the catalytic performance of different samples

[0051]

[0052]

[0053] [1] Bulk carbon nitride: 10 g of melamine is placed in a 50 mL covered crucible and put into a muffle furnace. It is heated to 550 °C at a rate of 5 °C / min and calcined for 2 h to obtain a yellow block, which is ground to obtain bulk carbon nitride.

[0054] Table 2 Mass percentage of each element in the catalyst [2]

[0055]

[0056] [2] The mass percentage of each element was obtained by testing with an inductively coupled plasma optical emission spectrometer (ICP-OES).

Claims

1. A bimetallic-loaded boron-doped carbon nitride nanosheet heterojunction, characterized by: One end of the heterojunction is a metal Cu-loaded carbon nitride nanosheet, and the other end is a metal Pd-loaded nitrogen-deficient boron-doped carbon nitride nanosheet; In the heterojunction, the mass ratio of the metal Cu-supported carbon nitride nanosheets to the metal Pd-supported nitrogen-deficient boron-doped carbon nitride nanosheets is 9:1 to 1:9; In the metal Cu-loaded carbon nitride nanosheets, the loading amount of metal Cu is 0.01 to 1 wt%; In the metal Pd-loaded nitrogen-deficient boron-doped carbon nitride nanosheets, the loading amount of metal Pd is 0.5-5 wt %, and the doping amount of boron is 0.05-0.5 wt %.

2. The bimetallic-loaded boron-doped carbon nitride nanosheet heterojunction according to claim 1, characterized in that: The thickness of the carbon nitride nanosheet is 10 to 20 nm.

3. The method for preparing a bimetallic-loaded boron-doped carbon nitride nanosheet heterojunction according to any one of claims 1 to 2, characterized in that: The steps include: (1) Using the photodeposition method, metal Cu is loaded on carbon nitride nanosheets to obtain metal Cu-loaded carbon nitride nanosheets; (2) uniformly mixing carbon nitride nanosheets with sodium borohydride or boric acid, and performing thermal reduction treatment under a protective atmosphere to obtain nitrogen-deficient and boron-doped carbon nitride nanosheets; the mass ratio of carbon nitride nanosheets to sodium borohydride or boric acid is 4:0.1-2; the temperature of the thermal reduction treatment is 300-500°C; (3) using an ethanol reduction method to load metal Pd onto nitrogen-deficient and boron-doped carbon nitride nanosheets to obtain metal Pd-loaded nitrogen-deficient and boron-doped carbon nitride nanosheets; (4) Metal Cu-loaded carbon nitride nanosheets and metal Pd-loaded nitrogen-deficient boron-doped carbon nitride nanosheets are electrostatically self-assembled to obtain a bimetallic boron-doped carbon nitride nanosheet heterojunction; the specific process of the electrostatic self-assembly is as follows: the metal Cu-loaded carbon nitride nanosheets are added to a hydrochloric acid solution, ultrasonically stirred for protonation, filtered, washed, and dried to obtain acidified Cu-loaded carbon nitride nanosheets; the acidified Cu-loaded carbon nitride nanosheets are added to water, stirred and dispersed; then, metal Pd-loaded nitrogen-deficient boron-doped carbon nitride nanosheets are added, ultrasonically stirred, filtered, washed, and dried to obtain a bimetallic boron-doped carbon nitride nanosheet heterojunction; the mass ratio of the metal Cu-loaded carbon nitride nanosheets to the metal Pd-loaded nitrogen-deficient boron-doped carbon nitride nanosheets is 9:1 to 1:

9.

4. The preparation method according to claim 3, wherein: In step (1), the specific process of the photodeposition method is as follows: adding copper nitrate trihydrate to water and stirring to dissolve; then adding carbon nitride nanosheets and ultrasonically dispersing; finally adding methanol and stirring thoroughly to obtain a dispersion; irradiating the dispersion under ultraviolet-visible light to reduce Cu ions, filtering, washing, and drying to obtain metal Cu-loaded carbon nitride nanosheets; The mass ratio of the copper nitrate trihydrate to the carbon nitride nanosheets is 0.006-0.6:

1.

5. The preparation method according to claim 3, wherein: In step (3), the specific process of the ethanol reduction method is as follows: palladium chloride is added to anhydrous ethanol and stirred to dissolve; then nitrogen-deficient and boron-doped carbon nitride nanosheets are added, and Pd ions are reduced by ultrasonic stirring, and the metal Pd-loaded nitrogen-deficient and boron-doped carbon nitride nanosheets are obtained by filtering, washing, and drying; The mass ratio of the palladium chloride to the nitrogen-deficient and boron-doped carbon nitride nanosheets is 0.01-0.2:

1.

6. Use of the bimetallic-loaded boron-doped carbon nitride nanosheet heterojunction according to any one of claims 1 to 2 or the bimetallic-loaded boron-doped carbon nitride nanosheet heterojunction prepared by the preparation method according to any one of claims 3 to 5, characterized in that: It was used for photothermal catalytic methane dry reforming reaction.

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