Visible light-responsive photocatalyst

Abstract

To provide a visible light-responsive photocatalyst comprising carbon nitride and having superior photocatalytic effect.SOLUTION: The present invention relates to a visible light-responsive photocatalyst comprising carbon nitride and silver phosphate.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a visible light responsive photocatalyst. [Background technology]

[0002] Photocatalysts absorb light energy and generate electrons (e - ) and holes (h + ) pair, but this hole (h + ) converts the moisture on the catalyst surface into hydroxyl radicals, which have strong oxidizing power, and these hydroxyl radicals decompose organic matter such as pollutants, dirt, and bacteria, thereby exerting their function. The effect is proportional to the amount of light energy absorbed and is related to the band gap energy of the photocatalyst. The lower the band gap energy Eg, the greater the light energy that can be absorbed, and is related to the upper limit of the wavelength λ that can be absorbed by the following formula: Eg = Planck's constant × speed of light / λ = 1240 / λ

[0003] Titanium oxide (TiO2), which is widely used as a photocatalyst, has a bandgap energy of 3.2 eV and can utilize ultraviolet light with a wavelength of 388 nm or less, but it can only utilize just under 3% of the total energy of sunlight. Patent Document 1 discloses an efficient method for producing monoclinic bismuth vanadate (BiVO4), which has an even lower bandgap energy of 2.4 eV and can utilize the energy of visible light and ultraviolet light with a wavelength of 517 nm or less, but it can only utilize just under 19% of the total energy of sunlight.

[0004] Other known visible light responsive photocatalysts include graphite carbon nitride g-C3N4 (Non-Patent Document 1) and a calcined product of melamine C3H6N6 and BiVO4 (Patent Document 2). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2020-111476 [Patent Document 2] Japanese Patent Publication No. 2021-137789 [Non-patent literature]

[0006] [Non-Patent Document 1] "GS Alliance synthesizes and commercializes graphitic carbon nitride (g-C3N4), a visible-light-responsive photocatalyst that can also be used in artificial photosynthesis," published October 4, 2018, https: / / www.atpress.ne.jp / news / 167523 Summary of the Invention [Problem to be solved by the invention]

[0007] The visible light responsive photocatalysts disclosed in Patent Documents 1 and 2 and Non-Patent Document 1 do not have sufficient photocatalytic effects. An object of the present invention is to obtain a visible light responsive photocatalyst that contains carbon nitride and has excellent photocatalytic effects. [Means for solving the problem]

[0008] The present inventors have found that a photocatalyst containing carbon nitride and silver phosphate has a high visible light responsive photocatalytic effect, and have completed the present invention.

[0009] That is, the present invention (1) is a visible light responsive photocatalyst containing carbon nitride and silver phosphate.

[0010] The present invention (2) is a visible light responsive photocatalyst according to the present invention (1), in which carbon nitride and silver phosphate are composited.

[0011] The present invention (3) is a visible light responsive photocatalyst according to the present invention (2), in which silver phosphate is attached to the surface of carbon nitride.

[0012] The present invention (4) is the visible light responsive photocatalyst according to any one of the present inventions (1) to (3), wherein the ratio of the average particle size of the carbon nitride to the average particle size of the silver phosphate is 1 or more.

[0013] In the present invention (5), the number of silver phosphate particles attached to the surface of the carbon nitride is 100 μm 2 The visible light responsive photocatalyst according to any one of the present inventions (1) to (4) has 10 or more per unit area.

[0014] The present invention (6) is a method for producing the visible light responsive photocatalyst according to any one of the present inventions (1) to (5), which comprises a step of mixing carbon nitride, a water-soluble silver salt, a water-soluble phosphate, and water.

[0015] The present invention (7) is a method for producing a visible-light-responsive photocatalyst according to the present invention (6), further comprising a step of producing carbon nitride by calcining melamine two to four times at a temperature of 630°C or higher, and using the obtained carbon nitride in the mixing step.

[0016] The present invention (8) is a method for producing a visible light responsive photocatalyst according to the present invention (6) or (7), wherein the molar ratio of the water-soluble phosphate to the water-soluble silver salt is 1 or less.

[0017] The present invention (9) is a method for producing a visible-light responsive photocatalyst according to any one of the present inventions (6) to (8), wherein the water-soluble silver salt is silver nitrate.

[0018] The present invention (10) is a method for producing a visible light responsive photocatalyst according to any one of the present inventions (6) to (9), wherein the water-soluble phosphate is trisodium phosphate.

[0019] The present invention (11) is a method for producing a visible-light responsive photocatalyst according to any one of the present inventions (6) to (10), wherein the stirring time is 2 hours or more. [Effects of the Invention]

[0020] The visible light-responsive photocatalyst of the present invention has a high photocatalytic effect. In addition, by using carbon nitride, which is a cheap raw material, it is possible to reduce costs, and it is expected to have effects such as water purification by decomposing organic environmental pollutants under visible light, hydrogen generation by water splitting, sterilization, and deodorization. [Brief explanation of the drawings]

[0021] [Figure 1] 1 shows XRD charts of visible light responsive photocatalysts containing carbon nitride and silver phosphate prepared in Examples 1 to 3. [Figure 2] 1 shows the photocatalytic effect of the visible light responsive photocatalysts containing carbon nitride and silver phosphate prepared in Examples 1 to 3. [Figure 3] 1 shows the appearance of the visible light responsive photocatalysts containing carbon nitride and silver phosphate prepared in Examples 1 to 3 during transmittance measurement. [Figure 4] 1 shows SEM images of carbon nitride and visible light responsive photocatalysts containing carbon nitride and silver phosphate prepared in Examples 1 to 3. [Figure 5] 1 shows XRD charts of visible light responsive photocatalysts containing carbon nitride and silver phosphate prepared in Examples 4 to 8. [Figure 6] 1 shows the photocatalytic effect of the visible light responsive photocatalysts containing carbon nitride and silver phosphate prepared in Examples 4 to 8 after irradiation for 1 hour. [Figure 7] 1 shows the irradiation time dependence of the photocatalytic effect of the visible light responsive photocatalyst containing carbon nitride and silver phosphate prepared in Example 5. [Figure 8] (a) SEM images of carbon nitride and (b) SEM images of the visible light-responsive photocatalyst containing carbon nitride and silver phosphate prepared in Example 5. [Figure 9] The photocatalytic effects of the BiVO4 powder and g-C3N4 powder prepared in Comparative Examples 1 and 2 are shown. [Figure 10] 1 shows XRD charts of visible light responsive photocatalysts containing carbon nitride and bismuth vanadate produced in Comparative Examples 3 to 7. [Figure 11] 1 shows the photocatalytic effects of the visible light responsive photocatalysts containing carbon nitride and bismuth vanadate prepared in Comparative Examples 3 to 7. DETAILED DESCRIPTION OF THE INVENTION

[0022] <<Visible light responsive photocatalyst>> The visible light responsive photocatalyst of the present invention is characterized by containing carbon nitride and silver phosphate.

[0023] The visible light responsive photocatalyst is not particularly limited in its form as long as it contains carbon nitride and silver phosphate, but it is preferable that carbon nitride and silver phosphate are composited to have a portion made of carbon nitride and a portion made of silver phosphate. In particular, it is more preferable that silver phosphate is attached to the surface of carbon nitride to make it a photocatalyst interparticle electron transfer type.

[0024] Carbon nitride, which constitutes visible light-responsive photocatalysts, consists of nitrogen and carbon atoms. Among carbon nitrides, C3N4 is preferred due to its energy stability. The crystalline forms of carbon nitride include low-dimensional crystals such as graphitic carbon nitride (g-C3N4) and polymeric carbon nitride (PCN); cubic carbon nitride (c-C3N4) and hexagonal carbon nitride (β-C3N4); and soft carbon nitride (a-CN x ), hard carbon nitride (ta-CN x Among these, low-dimensional crystals are preferred, graphite-like carbon nitride (g-C3N4) is more preferred, and graphite-like carbon nitride (tg-C3N4) with a triazine ring skeleton is even more preferred.

[0025] It is preferable to use carbon nitride obtained by calcining melamine (C3N3(NH2)3). The calcination temperature is preferably 500°C or higher, more preferably 600°C or higher, and even more preferably 630°C or higher. There is no particular upper limit, but it is generally set to 800°C or lower. The calcination time is preferably 1 to 6 hours, more preferably 2 to 6 hours.

[0026] In particular, continuous calcination of melamine at 630°C or higher for a long period of time tends to impair the catalytic activity of the resulting carbon nitride. Therefore, when calcining at 630°C or higher, it is preferable to repeat the calcination multiple times rather than performing a single long-term calcination. For example, a method can be used in which calcination is repeated at 630°C or higher for 0.5 to 6 hours, with the number of repetitions being preferably 2 to 4, and more preferably 2 to 3. After each calcination, it is preferable to cool the reactant to room temperature before performing the next calcination. When short calcinations are repeated multiple times, the total time for calcination at 630°C or higher is preferably 4 to 6 hours.

[0027] The silver phosphate that constitutes the visible light responsive photocatalyst can be synthesized, for example, by (1) a solid-phase reaction between Ag2O and P2O5 (see JP 2023-158860 A), (2) a solid-phase reaction between AgCl and P2O5 (see JP 2022-141332 A), or (3) a liquid-phase reaction between a water-soluble silver salt and a water-soluble phosphate salt, which are used in the method of the present invention described below.

[0028] The silver phosphate prepared by method (1) may contain unreacted Ag2O or P2O5. If Ag2O or P2O5 is contained, it is preferable that a portion of Ag3PO4 is present on the surface of the visible light responsive photocatalyst (in contact with the atmosphere).

[0029] When the visible light responsive photocatalyst contains Ag2O, the content is preferably 0.3 moles or more, more preferably 0.7 moles or more, per mole of Ag3PO4. If the content is less than 0.3 moles, sufficient photocatalytic activity may not be obtained. The upper limit of the Ag2O content is not particularly limited, but it can be less than 1 mole per mole of Ag3PO4.

[0030] When the visible light responsive photocatalyst contains P2O5, the content is preferably 0.7 mol or less, more preferably 0.3 mol or less, per 1 mol of Ag3PO4. If the content exceeds 0.7 mol, sufficient photocatalytic activity may not be obtained.

[0031] The silver phosphate prepared by method (2) may contain unreacted AgCl or P2O5. If Ag2O or P2O5 is contained, it is preferable that a portion of Ag3PO4 is present on the surface of the visible light responsive photocatalyst (in contact with the atmosphere).

[0032] When Ag2O is contained in the visible light responsive photocatalyst, its content is preferably 10 to 90 mass%, more preferably 30 to 80 mass%. If it is less than 10 mass%, the amount of Ag supplied from AgCl will be small, resulting in a small amount of Ag3PO4 produced. On the other hand, if it exceeds 90 mass%, there will be an excess of AgCl (very little PO5), which tends to make it impossible to produce a satisfactory photocatalytic material.

[0033] Of the total surface area of ​​the visible light-responsive photocatalyst, the proportion of carbon nitride is preferably 55 to 95%, and the proportion of silver phosphate is preferably 5 to 45%. When the proportion of carbon nitride is less than 55% or more than 95%, the effect of electron transfer between photocatalyst particles tends not to be obtained. The surface area proportion can be determined by image analysis based on SEM images.

[0034] The average particle size of the visible light responsive photocatalyst is preferably 1 to 20 μm, and more preferably 5 to 10 μm. If it is less than 1 μm, the area irradiated with visible light or ultraviolet light becomes small, and the catalyst tends to be less activated. If it exceeds 20 μm, the particle size of bismuth vanadate also becomes large, and the surface area becomes large, which tends to reduce the activity of the visible light responsive photocatalyst. The average particle size can be determined by image analysis based on SEM images.

[0035] As mentioned above, when carbon nitride and silver phosphate are composited, the ratio of the average particle size of the carbon nitride particles to the average particle size of the silver phosphate particles in the composite is preferably 1 or more, more preferably 2 or more, and even more preferably 10 or more. When the ratio of the average particle size of the carbon nitride particles to the average particle size of the silver phosphate particles is 1 or more, visible light-responsive photocatalytic activity tends to be easily obtained. The average particle sizes of the carbon nitride particles and the silver phosphate particles can be determined by image analysis based on SEM images.

[0036] The content of silver phosphate in the visible light responsive photocatalyst is preferably 30 to 80 mass %, more preferably 40 to 70 mass %.

[0037] The number of silver phosphate particles attached to the surface of carbon nitride is not particularly limited, but it is preferable that the number of silver phosphate particles attached to the surface of carbon nitride be 100 μm or more. 2 The number of particles per unit area is preferably 10 or more, and more preferably 20 or more. There is no particular upper limit, but the number is preferably 500 or less, and more preferably 300 or less.

[0038] The photocatalytic activity of the visible light-responsive photocatalyst can be evaluated, for example, by a decolorization test using a methylene blue solution. After mixing a 0.5 mM methylene blue solution with the visible light-responsive photocatalyst, the mixture is irradiated with visible light for 1 hour. The visible light-responsive photocatalyst is then removed by centrifugation, and the light transmittance at a wavelength of 662 nm of the resulting solution is measured. The recovery rate calculated using the following formula is preferably 80% or more, more preferably 85% or more, and even more preferably 90% or more. Recovery rate [%] = (solution transmittance / water transmittance) x 100

[0039] The visible light responsive photocatalyst of the present invention can be suitably used for deodorization, sterilization, and antifouling applications. It can also be applied to systems to be purified using visible light, and is particularly suitable for decomposing endocrine disruptors such as nonylphenol, bisphenol A, and natural estrogens present in water systems to be purified.

[0040] <<Method for producing visible light responsive photocatalyst>> The method for producing a visible-light-responsive photocatalyst of the present invention is characterized by comprising a step of mixing carbon nitride, a water-soluble silver salt, a water-soluble phosphate, and water. Fine silver phosphate that is poorly soluble in water is precipitated on the carbon nitride from the water-soluble silver salt and the water-soluble phosphate dissolved in water, thereby producing a visible-light-responsive photocatalyst in which carbon nitride and silver phosphate are combined. Simply mixing poorly water-soluble silver phosphate with carbon nitride makes it difficult to combine the two.

[0041] The amount of water is preferably 100 to 100,000 parts by mass, more preferably 1,000 to 10,000 parts by mass, per 100 parts by mass of carbon nitride.

[0042] The water-soluble silver salt may be any silver salt that is soluble in water, such as silver nitrate.

[0043] The water-soluble phosphate may be any phosphate that is soluble in water, such as trisodium phosphate.

[0044] Since silver phosphate is produced from a water-soluble silver salt and a water-soluble phosphate, it is most preferable to react the water-soluble silver salt and the water-soluble phosphate in a 1:1 molar ratio, but they do not necessarily have to be reacted in equimolar ratios. In order to make effective use of the expensive water-soluble silver salt, the reaction can also be carried out with an excess of water-soluble phosphate, i.e., 1 to 5 moles of water-soluble phosphate per mole of water-soluble silver salt.

[0045] The amount of silver phosphate added is preferably 0.5 to 3 mol, more preferably 0.8 to 2 mol, per 1 mol of carbon nitride.

[0046] The method for mixing carbon nitride, water-soluble silver salt, water-soluble phosphate, and water is not particularly limited. Water may be added to a container containing these solid raw materials, the solid raw materials may be added to a container containing water, or an aqueous solution or dispersion containing each raw material may be added to the container. From the viewpoint of optimal chemical reaction, a preferred method is to first mix carbon nitride and water-soluble phosphate, add water to homogenize, and then finally add an aqueous solution in which a water-soluble silver salt has been dissolved.

[0047] The mixing time is not particularly limited, but is preferably 2 hours or more, more preferably 2 to 12 hours, more preferably 2 to 10 hours, even more preferably 2 to 6 hours, and even more preferably 2 to 3 hours. The reaction solution may be stirred during mixing. The stirring speed is preferably 100 to 800 rpm, more preferably 300 to 600 rpm.

[0048] The precipitate can be collected by filtration, centrifugation, etc. The collected precipitate may be washed with a washing liquid such as water, ethanol, methanol, or acetone, as needed. [Example]

[0049] Examples of the present invention will be described below, but the present invention is not limited to the following examples.

[0050] Manufacturing Examples 1-3 Nitrogen carbonate g-C3N4 of Production Examples 1 to 3 was prepared by calcining melamine (C3N3(NH2)3) under the following conditions. (Condition 1) Firing at 650°C for 6 hours (Condition 2) Firing at 650°C for 6 hours, then cooling to room temperature and firing at 650°C for 6 hours (Condition 3) After firing under (Condition 2), cool to room temperature again and fire at 650°C for 6 hours.

[0051] Examples 1 to 3 First, g-CN4, AgNO3, and Na3PO4·12H2O prepared in Preparation Examples 1 to 3 were mixed in a beaker in a molar ratio of 1:1:1. Then, ultrapure water (150 ml) was added, followed by the addition of an aqueous AgNO3 solution. The mixture was stirred at room temperature (24°C) at 400 rpm for 6 hours. The precipitate was then collected, and the solution was evaporated in a drying oven to obtain the g-CN4 / Ag3PO4 powder for each example. In Examples 1 to 3, the resulting silver phosphate was adjusted to 1 g.

[0052] <Crystallinity (XRD)> The XRD measurement results for the catalysts prepared in Examples 1 to 3 are shown in Figure 1. For comparison, the XRD measurement results for g-C3N4 powder and Ag3PO4 powder are also shown in Figure 1. In the mixed sample, both peaks of g-C3N4 (002) and Ag3PO4 (210, 211, etc.) were observed.

[0053] <Photocatalytic effect> 3 mL of methylene blue (MB) solution and 0.1 g of the photocatalyst prepared in Examples 1 to 3 were placed in a plastic container (MB solution concentration 0.5 mM) and irradiated with visible light for 1 hour. The solution was then placed in a quartz cell and the transmittance was measured using a spectrophotometer. The results are shown in Figure 2. The photocatalytic effect was greater than that of Ag3PO4 alone, and the transmittance was highest when the g-C3N4 powder was fired three times.

[0054] <sem> Figure 4 shows SEM photographs of the photocatalysts prepared in Examples 1 to 3. Ag3PO4 adheres to the surface of g-C3N4, forming a composite. Note that the number of Ag3PO4 particles adhered did not change depending on the number of times g-C3N4 was fired.

[0055] Examples 4 to 8 First, g-CN4, AgNO3, and Na3PO4·12H2O prepared in Preparation Example 3 (Condition 3) were mixed in a beaker to obtain molar ratios of 0.5:1:1, 0.75:1:1, 1:1:1, 1.25:1:1, and 2:1:1. Ultrapure water (150 ml) was then added, followed by an aqueous AgNO3 solution. The mixture was stirred at room temperature (24°C) at 400 rpm for 6 hours. The precipitate was then collected, and the solution was evaporated in a drying oven to obtain the g-CN4 / Ag3PO4 powder for each example. In Examples 1 to 3, the resulting silver phosphate was 1 g.

[0056] <Crystallinity (XRD)> The XRD measurement results for the catalysts prepared in Examples 4 to 8 are shown in Figure 5. For comparison, the XRD measurement results for g-C3N4 powder and Ag3PO4 powder are also shown in Figure 5. In the mixed sample, peaks for both g-C3N4 and Ag3PO4 were observed.

[0057] <Photocatalytic effect> 3 mL of methylene blue (MB) solution and 0.1 g of the catalyst prepared in Examples 4 to 8 were placed in a plastic container and irradiated with visible light for 1 hour. The solution was then placed in a quartz cell and the transmittance was measured using a spectrophotometer. Figure 6 shows the photocatalytic effect of a 0.1 mM MB solution. The highest transmittance was obtained when the ratio of g-CN:AgNO:NaPO·12H O was 0.75:1:1, demonstrating a photocatalytic effect greater than that of AgPO alone. Furthermore, as shown in Figure 7, the transmittance improved with increasing irradiation time.

[0058] <sem> SEM photographs of the catalysts prepared in Examples 4 to 8 are shown in Figure 8. Ag3PO4 was attached to the surface of g-C3N4, forming a composite. In Examples 4 to 8, the number of silver phosphate particles attached to the surface of carbon nitride was 100 μm 2 The numbers were 42, 71, 121, 135, and 207 per person, respectively.

[0059] Comparative Example 1 BiVO4 powder was prepared by the method described in the examples of JP 2021-104486 A.

[0060] Comparative Example 2 Melamine C3H6N6 was fired in an electric furnace at 650 °C for 3 hours and then crushed in a mortar to produce g-C3N4 powder.

[0061] <Photocatalytic effect> Three mL of methylene blue (MB) solution and 0.1 g of the catalyst prepared in Comparative Examples 1 and 2 were placed in a plastic container and irradiated with visible light for 1 hour. The solution was then placed in a quartz cell and the transmittance was measured using a spectrophotometer. Figure 9 shows the photocatalytic effect of MB solutions with concentrations of 0.1 mM and 0.5 mM. As shown in Figure 9, when the MB solution concentration was low, both BiVO4 and g-CN4 exhibited photocatalytic effects. However, when the MB solution concentration was high, the photocatalytic effects of both BiVO4 and g-CN4 were insufficient.

[0062] Comparative Examples 3 to 7 Photocatalysts were prepared using the method described in the examples of JP 2021-137789 A. Specifically, melamine C3H6N6 and BiVO4 were mixed at ratios of 1:1, 2:1, 3:1, 4:1, and 5:1, fired in an electric furnace at 650°C for 3 hours, and then pulverized in a mortar to prepare C3H6N6 / BiVO4 powders for Comparative Examples 3 to 7.

[0063] <Crystallinity (XRD)> The XRD measurement results for the photocatalysts prepared in Comparative Examples 3 to 7 are shown in Figure 10. For comparison, the XRD measurement results for g-C3N4 powder and Ag3PO4 powder are also shown in Figure 10. A peak for BiVO4 was observed, but the peak for g-C3N4, which is produced by calcining C3H6N6, was not observed.

[0064] <Photocatalytic effect> The photocatalytic effect was measured in the same manner as in Examples 1 to 3. As a result, as shown in Figure 11, the photocatalytic effect using the MB solution was observed, but even when the MB solution concentration was as low as 0.1 mM, a sufficient photocatalytic effect was not obtained. [Industrial Applicability]

[0065] The present invention provides a visible-light-responsive photocatalyst containing carbon nitride and having excellent photocatalytic activity. This visible-light-responsive photocatalyst can purify environmental pollutants, for example, by using visible light to decompose endocrine disruptors such as nonylphenol, bisphenol A, and natural estrogens contained in water systems to be purified.< / sem> < / sem>

Claims

1. Visible light-responsive photocatalyst containing carbon nitride and silver phosphate.

2. 2. The visible light responsive photocatalyst according to claim 1, wherein carbon nitride and silver phosphate are composited.

3. 3. The visible light responsive photocatalyst according to claim 2, wherein silver phosphate particles are attached to the surface of the carbon nitride.

4. 3. The visible light responsive photocatalyst according to claim 1, wherein the ratio of the average particle size of the carbon nitride to the average particle size of the silver phosphate is 1 or more.

5. The number of silver phosphate particles attached to the surface of carbon nitride is 100 μm ２ 3. The visible light responsive photocatalyst according to claim 1, wherein the number of the nuclei per nucleus is 10 or more.

6. 3. A method for producing the visible light responsive photocatalyst according to claim 1, comprising the step of mixing carbon nitride, a water-soluble silver salt, a water-soluble phosphate, and water.

7. The method for producing a visible light responsive photocatalyst according to claim 6, further comprising a step of producing carbon nitride by baking melamine at a temperature of 630°C or higher two to four times, and using the obtained carbon nitride in the mixing step.

8. 7. The method for producing a visible light responsive photocatalyst according to claim 6, wherein the molar ratio of the water-soluble phosphate to the water-soluble silver salt is 1 or less.

9. 7. The method for producing a visible light responsive photocatalyst according to claim 6, wherein the water-soluble silver salt is silver nitrate.

10. 7. The method for producing a visible light responsive photocatalyst according to claim 6, wherein the water-soluble phosphate is trisodium phosphate.

11. The method for producing a visible light responsive photocatalyst according to claim 6, wherein the stirring time is 2 hours or more.

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