Preparation method and application of platinum-molybdenum dual-doped carbon nitride catalyst

Abstract

The present invention relates to a preparation method of a platinum-molybdenum dual-doped carbon nitride catalyst. Specifically, the method comprises the following steps: heat-treating urea in a sealed container at 500-600°C, placing the heat-treated product in an air atmosphere and continuously thermally oxidizing it at 500-600°C to obtain graphite-phase carbon nitride; placing the graphite-phase carbon nitride in a molybdenum source solution, fully impregnating it, and then calcining it at 300-400°C to obtain molybdenum-doped carbon nitride; ultrasonically mixing the molybdenum-doped carbon nitride and a platinum source in anhydrous ethanol; irradiating the mixture under light conditions for 4-6 hours, separating the solid from the liquid, and washing and drying the solid to obtain the platinum-molybdenum dual-doped carbon nitride; and using the catalyst to photocatalytically reduce carbon dioxide to prepare methane, achieving high-efficiency and high-selectivity reduction of carbon dioxide to methane.

Description

Technical Field

[0001] The present invention relates to the technical field of photocatalytic materials, and in particular to a preparation method and application of a platinum-molybdenum dual-doped carbon nitride photocatalytic material. Background Art

[0002] As one of the products of carbon dioxide reduction, methane is an efficient hydrocarbon energy source. However, the reaction route for converting carbon dioxide to methane is long, the number of transferred electrons is large, and the energy barrier is high, resulting in low conversion efficiency. In the traditional method, a large amount of platinum is directly loaded on carbon nitride to improve the selectivity of carbon nitride materials for the photocatalytic reduction of carbon dioxide to methane. However, this method uses a large amount of precious metal platinum materials, which undoubtedly increases the cost of material preparation. In addition, platinum often agglomerates during the traditional photodeposition process, resulting in a larger deposit size and a smaller specific surface area of ​​the active sites, which limits its photocatalytic effect and makes it difficult to further improve the methane selectivity. Summary of the Invention

[0003] The present invention provides a method for preparing a platinum-molybdenum dual-doped carbon nitride catalyst. The platinum doped catalyst prepared by the method has a smaller nanometer size and a more uniform elemental valence state. The dual-metal doping effectively reduces the amount of precious metal platinum used, thereby having higher cost-effectiveness.

[0004] The preparation method of the platinum-molybdenum dual-doped carbon nitride catalyst of the present invention is as follows:

[0005] 1. Place urea in an alumina crucible, seal it, and calcine it at 500-600℃ for 1-3h. Then place the calcined product in an air atmosphere and continue calcining at 500-600℃ for 1-3h. Grind it to obtain graphite phase carbon nitride.

[0006] 2. Place graphite phase carbon nitride in a molybdenum source solution and fully immerse it. Calcine the graphite phase carbon nitride that has fully absorbed the solution at 300-400°C for 1-3 hours and grind it to obtain molybdenum-doped carbon nitride, wherein the amount of molybdenum source added is 5-10% of the mass of the graphite phase carbon nitride.

[0007] 3. After ultrasonically mixing the molybdenum-doped carbon nitride and the platinum source in anhydrous ethanol, the mixture is irradiated under light conditions for 4-6 hours to deposit platinum ions on the molybdenum-doped carbon nitride. The solid-liquid separation is carried out, and the solid is washed and dried to obtain platinum-molybdenum dual-doped carbon nitride, wherein the amount of platinum source added is 5-10% of the mass of the molybdenum-doped carbon nitride.

[0008] The urea of ​​the present invention was purchased from Tianjin Fengchuan Chemical Reagent Technology Co., Ltd.

[0009] The molybdenum source includes sodium molybdate dihydrate and ammonium molybdate heptahydrate.

[0010] Another object of the present invention is to apply the platinum-molybdenum dual-doped carbon nitride catalyst prepared by the above method to the photocatalytic reduction of carbon dioxide to produce methane, and to evaluate the catalytic performance of the catalyst using a photocatalytic carbon dioxide reduction system equipped with a gas cylinder and a gas chromatograph.

[0011] Advantages and technical effects of the present invention:

[0012] The method of the present invention regulates the deposition of more platinum elements in a zero-valence state on the carbon nitride surface by pre-incorporating molybdenum elements, and uses cheaper molybdenum metal to replace part of the precious metal platinum, which not only reduces the cost of the catalyst, but also makes the prepared platinum catalyst smaller in size and more uniform in elemental valence. The catalyst is used to reduce carbon dioxide to produce methane and can have 100% selectivity for the photocatalytic reduction of carbon dioxide to methane. The method of the present invention is simple to prepare, requires mild conditions, and is suitable for industrial production and market promotion applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 is a SEM image of the platinum-molybdenum dual-doped carbon nitride catalyst of Example 1;

[0014] Figure 2 TEM images and platinum nanoparticle size distribution diagrams of the catalysts of Example 1 and Comparative Example 1, wherein Figure a is the TEM image of the catalyst of Example 1, and Figure b is the platinum nanoparticle size distribution diagram of the catalyst of Example 1; Figure c is the TEM image of the catalyst of Comparative Example 1, and Figure d is the platinum nanoparticle size distribution diagram of the catalyst of Comparative Example 1;

[0015] Figure 3 1 is the XPS graph of the catalysts of Example 1, Example 3 and Comparative Example 1;

[0016] Figure 4 1 is a diagram showing the catalytic effect of the platinum-molybdenum dual-doped carbon nitride catalyst of Examples 1 and 2;

[0017] Figure 5 Figure 1 is a comparison of the XPS spectra of the metal elements platinum and molybdenum before and after the catalytic reaction of the catalyst in Example 1, wherein Figure a is the XPS spectrum of Pt4f and Figure b is the XPS spectrum of Mo 3d;

[0018] Figure 6 It is a diagram of the catalytic effect under different catalytic conditions;

[0019] Figure 7 The stability test results of the catalyst in Example 1 after 12 hours of reaction;

[0020] Figure 8 This is the result of recycling the catalyst in Example 1;

[0021] Figure 9 The catalytic effects of the catalysts of Examples 1-3 and Comparative Examples 1-4 and carbon nitride (CN) for photocatalytic reduction of carbon dioxide to produce methane are shown. DETAILED DESCRIPTION

[0022] The following is a clear and complete description of the technical solutions in the embodiments of the present invention in conjunction with the embodiments of the present invention and the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0023] In the following embodiments, a photocatalytic carbon dioxide reduction system with a matching gas cylinder and gas chromatography was used to evaluate the catalytic performance of the catalyst. Specifically, 10 mg of catalyst, 45 mL of ultrapure water, and 5 mL of triethanolamine were added to a cylindrical quartz reactor with a quartz cover, and ultrasonic treatment was performed for 10 minutes to uniformly disperse the photocatalyst in the solution. The reactor was then connected to the gas line, and the vacuum pump was turned on to evacuate the entire reaction device. After evacuation, the vacuum pump was turned off, and the pipeline connected to the carbon dioxide cylinder was opened. Carbon dioxide gas was slowly filled in until the vacuum gauge displayed -30 kPa to ensure that the carbon dioxide concentration could be fully diffused into the reaction solution. The reaction solution was allowed to stand for 30 minutes to wait for the gas in the pipeline to diffuse and the solution to absorb carbon dioxide gas to saturation before the photocatalytic performance test was started. A 300 nm spectroscopy with an AM1.5 filter was used. A W xenon lamp was used as the light source, irradiating simulated sunlight from the quartz cover of the quartz reactor; the initial current intensity of the light source was set to 17A; to prevent the reaction solvent from evaporating into the chromatogram during the photocatalytic process, 278K condensed water was used to insulate the photocatalytic system; the entire reaction system was always under high-speed magnetic stirring to avoid the precipitation of photocatalyst nanoparticles and maximize light absorption; the reaction time for each set of experiments was 3h, and the system was sampled into the gas chromatograph every 30min to detect the peaks of gas products CO, CH4 and raw material CO2. Example 1

[0024] 1. Weigh 5g of urea and put it into an alumina crucible, wrap it with three layers of tin foil and calcine it at 550℃ for 2h. Remove the three layers of tin foil wrapped around the crucible, place the material in an air atmosphere, continue to calcine at 550℃ for 2h, and grind it to obtain graphite phase carbon nitride (CN); dissolve 12mg of sodium molybdate dihydrate in 1mL of ultrapure water, place 100mg of graphite phase carbon nitride in the sodium molybdate solution and fully immerse it. Place the graphite phase carbon nitride material that has fully absorbed the solution in a Calcinate at 350 ° C for 2h; 50 mg of calcined molybdenum-doped carbon nitride and 675 μL of 3.70 mg / mL chloroplatinic acid were placed in anhydrous ethanol and ultrasonically mixed for 0.5h. The mixed solution was placed under a 300W xenon lamp and irradiated for 5h. The filter residue was washed with anhydrous ethanol to remove the unloaded chloroplatinic acid and dried in an oven at 70 ° C for 15h to obtain a platinum-molybdenum dual-doped carbon nitride catalyst 5Pt5MoCN; the SEM image of the catalyst of this embodiment is shown in Figure 1 As can be seen from the figure: 5Pt5MoCN is a nanosheet stacking structure, and its TEM image and particle size distribution are shown in Figure 2 From the figure, it can be seen that the platinum nanoparticles of the platinum-molybdenum dual-doped carbon nitride catalyst have a particle size of >1.8nm and are significantly less in number than the catalyst doped with platinum only (Comparative Example 1), and the average nanoparticle size is also smaller; its XPS diagram is shown in Figure 3 As can be seen from the figure, in the catalyst PtCN of Comparative Example 1, platinum presents a valence of mainly +2, with valences of 0, +2, and +4 coexisting. However, when preparing the catalyst of this embodiment, molybdenum is first doped into carbon nitride, and the deposited platinum valence state is mainly 0. In addition, the increase in the molybdenum doping content also reduces the content of 0-valence platinum.

[0025] 2. Application of the catalyst of this embodiment in the photocatalytic reduction of carbon dioxide to produce methane

[0026] See the results Figure 4 As can be seen from the figure, after 3h of catalytic reaction, the methane selectivity of catalyst 5Pt5MoCN is 100%, and the methane yield is 34.9 μmol·g -1 ·h -1 ; Figure 5 a The XPS results of platinum show that the valence state of platinum is not greatly affected before and after the reaction. The XPS spectrum of Mo 3d Figure 5 b shows that the content of molybdenum decreases, indicating that molybdenum plays the role of sacrificial agent in the reaction and effectively maintains the activity of zero-valent platinum;

[0027] At the same time, a catalytic comparison experiment was conducted without adding catalyst 5Pt5MoCN, in dark conditions, and without the introduction of carbon dioxide. The other conditions were the same as above, and the results were shown in Table 2. Figure 6 As can be seen from the figure, no methane product was generated in the comparative experiment without adding catalyst 5Pt5MoCN, under dark conditions and without introducing carbon dioxide, indicating that the output of methane was produced by the photocatalytic reduction of carbon dioxide by catalyst 5Pt5MoCN in a carbon dioxide atmosphere.

[0028] 3. Stability test

[0029] The experimental method is the same as above, except that the catalyst catalytic reaction is extended from 3h to 12h. The results of the catalyst catalytic reaction for 12h are shown in Figure 7 From the figure, it can be seen that the reaction yield of the catalyst decreases slowly after 12 hours, indicating that the catalyst 5Pt5MoCN has long-term photocatalytic stability.

[0030] 4. Cycle experiment

[0031] The experimental method is the same as above, except that after the catalyst is catalytically reacted for 3 hours, it is separated from the solution, dried, and then put into the photocatalytic experiment. The above catalytic reaction is repeated 3 times. The results are shown in Figure 8 ,It can be seen from the figure that the catalytic effect is still maintained at 86% in the 4th cycle experiment, indicating that the catalyst has the value of multiple recycling and reuse. Example 2

[0032] The method of this embodiment is the same as that of embodiment 1, except that the molybdenum source is tetrahydrated ammonium molybdate to prepare a platinum-molybdenum dual-doped carbon nitride catalyst 5Pt5MoCN(Am). The results of the photocatalytic reduction of carbon dioxide to methane by the catalyst 5Pt5MoCN(Am) of this embodiment are shown in FIG. Figure 4 From the figure, it can be seen that after 3 hours of reaction, the selectivity of methane is 94.8% and the methane yield is 27.6 μmol·g -1 ·h -1 , a small amount of CO is produced. Example 3

[0033] Weigh 5g of urea and put it into an alumina crucible, wrap it with three layers of tin foil and calcine it at 550℃ for 2h, remove the three layers of tin foil wrapped around the crucible, place the material in an air atmosphere, continue to calcine at 550℃ for 2h, and grind it to obtain graphite phase carbon nitride; dissolve 24mg of sodium molybdate dihydrate in 1mL of ultrapure water, place 100mg of graphite phase carbon nitride in the sodium molybdate solution and fully immerse it, and place the graphite phase carbon nitride material that has fully absorbed the solution in a jar. Calcinate at 350 ° C for 2h; 50 mg of calcined molybdenum-doped carbon nitride and 675 μL of 3.70 mg / mL chloroplatinic acid are placed in anhydrous ethanol and ultrasonically mixed for 0.5h. The mixed solution is placed under a 300W xenon lamp and irradiated for 5h. The filter residue is washed with anhydrous ethanol to remove the unloaded chloroplatinic acid and dried in an oven at 70 ° C for 15h to obtain platinum-molybdenum dual-doped carbon nitride catalyst 5Pt10MoCN. Its XPS diagram is shown in FIG. Figure 3 ,It can be seen from the figure that after molybdenum is first doped into carbon nitride (CN), the valence state of platinum deposition is mainly 0 valence state, and the increase of molybdenum doping content also reduces the content of 0 valence platinum.

[0034] Comparative Example 1:

[0035] 5 g of urea was weighed and placed in an alumina crucible, wrapped with three layers of tin foil and calcined at 550 ° C for 2 h. The three layers of tin foil wrapped around the crucible were removed, and the material was placed in an air atmosphere. After continuing to calcine at 550 ° C for 2 h, it was ground to obtain graphite phase carbon nitride; 50 mg of graphite phase carbon nitride and 675 μL of 3.70 mg / mL chloroplatinic acid were placed in anhydrous ethanol and ultrasonically mixed for 0.5 h. The mixed solution was placed under a 300 W xenon lamp and irradiated for 5 h. The solution was filtered and the filter residue was washed with anhydrous ethanol to remove the unloaded chloroplatinic acid. It was dried in an oven at 70 ° C for 15 h to obtain a platinum-doped carbon nitride catalyst PtCN. The TEM image and particle size distribution of the catalyst are shown in FIG. Figure 2 , its XPS diagram is shown in Figure 3 ;

[0036] Comparative Example 2:

[0037] 5 g of urea was weighed and placed in an alumina crucible, wrapped with three layers of tin foil and calcined at 550°C for 2 h. The three layers of tin foil wrapped around the crucible were removed, and the material was placed in an air atmosphere and calcined at 550°C for another 2 h. Graphite-phase carbon nitride was then ground. 50 mg of graphite-phase carbon nitride and 1.35 mL of 3.70 mg / mL chloroplatinic acid were placed in anhydrous ethanol and ultrasonically mixed for 0.5 h. The mixed solution was placed under a 300 W xenon lamp and irradiated for 5 h. The solution was filtered and the filter residue was washed with anhydrous ethanol to remove unloaded chloroplatinic acid. The residue was dried in an oven at 70°C for 15 h to obtain a platinum-doped carbon nitride catalyst 10PtCN.

[0038] Comparative Example 3:

[0039] 5 g of urea was weighed and placed in an alumina crucible, wrapped with three layers of tin foil and calcined at 550°C for 2 h. The three layers of tin foil wrapped around the crucible were removed, and the material was placed in an air atmosphere. After continuing to calcine at 550°C for 2 h, it was ground to obtain graphite phase carbon nitride; 12 mg of sodium molybdate dihydrate was dissolved in 1 mL of ultrapure water, 100 mg of graphite phase carbon nitride was placed in the sodium molybdate solution for full immersion, and the graphite phase carbon nitride material that had fully absorbed the solution was calcined at 350°C for 2 h; the calcined molybdenum-doped carbon nitride was placed in anhydrous ethanol and ultrasonically mixed for 0.5 h. The mixed solution was placed under a 300 W xenon lamp, irradiated for 5 h, filtered, the filter residue was collected, and dried in an oven at 70°C for 15 h to obtain a molybdenum-doped carbon nitride catalyst MoCN.

[0040] Comparative Example 4:

[0041] 5 g of urea was weighed and placed in an alumina crucible, wrapped with three layers of tin foil and calcined at 550°C for 2 h. The three layers of tin foil wrapped around the crucible were removed, and the material was placed in an air atmosphere. After continuing to calcine at 550°C for 2 h, it was ground to obtain graphite phase carbon nitride; 24 mg of sodium molybdate dihydrate was dissolved in 1 mL of ultrapure water, 100 mg of graphite phase carbon nitride was placed in the sodium molybdate solution for full immersion, and the graphite phase carbon nitride material that had fully absorbed the solution was calcined at 350°C for 2 h; the calcined molybdenum-doped carbon nitride was placed in anhydrous ethanol and ultrasonically mixed for 0.5 h. The mixed solution was placed under a 300 W xenon lamp, irradiated for 5 h, filtered, and the filter residue was collected. It was dried in an oven at 70°C for 15 h to obtain a molybdenum dual-doped carbon nitride catalyst 10MoCN.

[0042] Example 4: The catalysts of Examples 1-3 and Comparative Examples 1-4 and graphite carbon nitride (CN) were used in the photocatalytic reduction of carbon dioxide to produce methane. The results are shown in FIG. Figure 9 From the figure, it can be seen that the performance of the dual-doped catalyst in catalyzing methane is significantly higher than that of any single doped material, indicating that dual-doping can replace part of the amount of precious metal platinum, and the catalytic effect is better under the premise of higher economic benefits.

Claims

1. Use of a platinum-molybdenum dual-doped carbon nitride catalyst in the photocatalytic reduction of carbon dioxide to produce methane, characterized by: The platinum-molybdenum dual-doped carbon nitride catalyst is prepared by heat-treating urea in a sealed environment at 500-600°C, placing the heat-treated product in an air atmosphere at 500-600°C for further thermal oxidation to obtain graphite-phase carbon nitride; placing the graphite-phase carbon nitride in a molybdenum source solution for full impregnation, and then calcining it at 300-400°C to obtain molybdenum-doped carbon nitride; placing the molybdenum-doped carbon nitride and the platinum source in anhydrous ethanol for ultrasonic mixing, irradiating the mixture under light conditions for 4-6 hours, separating the solid and liquid, and washing and drying the solid.

2. The use according to claim 1, characterized in that: The amount of molybdenum source added is 12-24% of the mass of graphite phase carbon nitride, and the amount of platinum source added is 5-10% of the mass of molybdenum-doped carbon nitride.

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