Carbon dioxide reduction photocatalyst particles and method for producing the same

Abstract

To provide carbon dioxide reduction photocatalyst particles and a method for producing the same, which enable compatibility between CO gas evolution rate and CO selectivity at a high level.SOLUTION: Provided are carbon dioxide reduction photocatalyst particles including base particles and silver (Ag) particles carried on the surfaces of the base particles, the base particles comprising a compound expressed by the general formula K2YTa5O15.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to carbon dioxide reducing photocatalyst particles and a method for producing the same. [Background technology]

[0002] Water splitting and carbon dioxide reduction technology using semiconductor photocatalyst particles is attracting attention as a technology that can solve energy and environmental problems. By supporting particles made of silver (Ag) or other materials as co-catalysts on these photocatalyst particles, it is expected that the electrons generated by photoexcitation can be trapped to promote charge separation and that carbon dioxide reduction products can be selected. For example, with a conventional photocatalyst, water is split into hydrogen (H2) and oxygen (O2) when irradiated with light. In contrast, with a photocatalyst supported by silver (Ag) particles, carbon dioxide (CO2) is reduced to carbon monoxide (CO) along with hydrogen (H2).

[0003] Carbon monoxide (CO) is an important starting material in the chemical and industrial industries, and can be reacted with hydrogen to synthesize various fuels and chemicals. Therefore, a high carbon monoxide production rate, i.e., a high CO selectivity, is desirable for a carbon dioxide reduction photocatalyst. Here, CO selectivity is the ratio of the CO gas production rate to the sum of the hydrogen (H2) gas production (production) rate and carbon monoxide (CO) gas production rate produced by the reduction reaction, as expressed in the following equation (1).

[0004]

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[0005] Conventionally, methods for supporting metal particles such as silver (Ag) include chemical reduction, impregnation, and photoelectrodeposition (photoelectrodeposition). For example, Patent Document 1 describes a method for reducing carbon dioxide, which involves irradiating CO2, HO, and a photocatalyst with light to generate CO2 through a CO2-reducing reaction, using a catalyst in which silver is supported on gallium oxide by photoelectrodeposition or impregnation, adding gallium oxide powder to an alcoholic aqueous solution containing a silver precursor such as silver nitrate, mixing the mixture, and then irradiating the mixture with light to reduce the silver precursor, and impregnation, which involves adding gallium oxide to an aqueous silver precursor solution, stirring the mixture, removing the water, drying the mixture by heating, and then calcining the mixture in air (claims 1, 2, and

[0015] of Patent Document 1).

[0006] Although not related to carbon dioxide reduction catalysts, Patent Document 2 discloses a method for producing a noble metal nanomaterial, which comprises the steps of irradiating ultrasonic waves to disperse one or more noble metal oxides in a solvent to obtain a noble metal oxide dispersion, and heating the noble metal oxide dispersion (Claim 1 of Patent Document 2). It also describes that by further adding a carrier for supporting the noble metal to the solvent, it is possible to obtain a noble metal nanomaterial supported with high dispersibility on the surface of the carrier, which can be suitably used as a catalyst for fuel cells, a catalyst for material synthesis, etc. (Claim 6 and

[0014] of Patent Document 2). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-192302 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-024968 Summary of the Invention [Problem to be solved by the invention]

[0008] Although carbon dioxide reduction photocatalysts have been proposed for some time, there was room for improvement in conventional photocatalysts. In other words, in order to efficiently obtain carbon monoxide (CO) through a photocatalytic reaction, it is important to increase the CO gas generation rate and CO selectivity. However, conventional photocatalysts have limitations in terms of increasing CO selectivity while maintaining a high CO gas generation rate.

[0009] The present inventors have conducted extensive research in light of this problem, and as a result have discovered that carbon dioxide-reducing photocatalyst particles containing a combination of base particles and a co-catalyst with a specific composition can achieve both a high CO gas generation rate and a high CO selectivity.

[0010] The present invention was completed based on these findings, and an objective of the present invention is to provide carbon dioxide-reducing photocatalyst particles that can achieve both a high CO gas generation rate and a high CO selectivity, and a method for producing the same. [Means for solving the problem]

[0011] The present invention encompasses the following embodiments (1) to (7). In this specification, the expression "to" includes both the numerical values at both ends. In other words, "X to Y" is synonymous with "X or more and Y or less." In addition, in this specification, any combination of suitable embodiments can be adopted as long as technical consistency can be achieved. For example, one of the suitable numerical ranges can be combined with the other.

[0012] (1) A composition comprising base particles and metallic silver (Ag) particles supported on the surfaces of the base particles, The base particles have the general formula: K2YTa5O 15 Compounds represented by the formula: Carbon dioxide reduction photocatalytic particles.

[0013] (2) The carbon dioxide reducing photocatalyst particles according to (1), wherein the amount of the metallic silver (Ag) particles supported is more than 0% by mass and less than 1.0% by mass relative to the base material particles.

[0014] (3) The carbon dioxide reducing photocatalyst particles according to (1) or (2) above, wherein the metallic silver (Ag) particles have an average particle size of 5 nm or more and 30 nm or less.

[0015] (4) The carbon dioxide reducing photocatalyst particles according to any one of (1) to (3) above, wherein in a particle size frequency distribution curve of the metallic silver (Ag) particles, the frequency of particles with a particle size of 20 nm is 15% by number or more.

[0016] (5) Carbon dioxide reduction photocatalyst particles according to any one of (1) to (4) above, which have a CO selectivity of 50% or more in a CO2 reduction photocatalytic performance evaluation test.

[0017] (6) A method for producing carbon dioxide-reducing photocatalyst particles comprising base particles and metallic silver (Ag) particles supported on the surfaces of the base particles, comprising: A step of adding base particles and a silver (Ag) source to a reduction solution to prepare a reaction solution; and a step of irradiating the reaction solution with ultrasonic waves to prepare base particles carrying metallic silver (Ag) particles, The base particles have the general formula: K2YTa5O 15 The method of claim 1, comprising administering to a subject a compound represented by the formula:

[0018] (7) The method according to (6), wherein the amount of the metallic silver (Ag) particles supported is more than 0% by mass and less than 1.0% by mass relative to the base material particles. [Effects of the Invention]

[0019] According to the present invention, there are provided carbon dioxide-reducing photocatalyst particles that can achieve both a high CO gas generation rate and a high CO selectivity, and a method for producing the same. [Brief explanation of the drawings]

[0020] [Figure 1] The mechanism of the photocatalytic reaction is shown. [Figure 2] FIG. 1 is a cross-sectional view illustrating an example of an evaluation device. [Figure 3] The mechanism of Ag particle loading by ultrasonic reduction method is shown. [Figure 4]This shows a TEM image of photocatalyst particles loaded with a co-catalyst using the ultrasonic reduction method (Example 1). [Figure 5] This shows a TEM image of photocatalyst particles loaded with a co-catalyst by chemical reduction (Example 2). [Figure 6] The particle size frequency distribution curves of the promoter (Ag particles) are shown (Examples 1 to 3). [Figure 7] The diffuse reflectance spectra of photocatalyst particles are shown (Examples 1 to 3). [Figure 8] The CO2 reduction photocatalytic performance of photocatalyst particles is shown (Example 1 and Example 2). DETAILED DESCRIPTION OF THE INVENTION

[0021] A specific embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described below. However, the present invention is not limited to the following embodiment, and various modifications are possible within the scope of the present invention.

[0022] <<1. Carbon dioxide reduction photocatalyst particles>> The carbon dioxide reduction photocatalyst particles of this embodiment (hereinafter sometimes referred to as "photocatalyst particles") contain base particles and metallic silver (Ag) particles supported on the surfaces of the base particles. The base particles have the general formula: K2YTa5O 15 The compound includes a compound represented by the formula:

[0023] The base particles function as the main catalyst. The base particles have the general formula: K2YTa5O 15(hereinafter, sometimes referred to as "KYTO") as the main component. Here, the main component is a component that accounts for 50% or more by mass of the base particle. By using KYTO as the main component, photocatalyst particles with excellent catalytic performance can be obtained. If the base particle itself has little water reduction activity but has oxidative activity, supporting silver, which acts as a CO2 reduction site, generates oxygen through water oxidation on the surface of the base particle, and also generates CO through CO2 reduction on the surface of the silver promoter, which is presumably improving catalytic performance. The proportion of the main component in the base particle is preferably high. The proportion of the main component may be 60% by mass or more, 70% by mass or more, 80% by mass or more, or 90% by mass or more.

[0024] Metallic Ag particles function as co-catalysts, and their particle size is on the order of nanometers or less. Supporting such fine Ag particles improves the catalytic performance of photocatalyst particles. This is because supporting fine co-catalysts increases the number of surface active sites of the base material particles. The average particle size of the metallic Ag particles is preferably 5 nm or more and 30 nm or less. The average particle size is the number-average particle size of the Ag particles. Specifically, the photocatalyst particles are observed using a TEM, the particle size of the supported metallic Ag particles is measured, and the number-average particle size is calculated from the measured particle size.

[0025] Preferably, the amount of supported metal Ag particles (Ag concentration) is more than 0 mass% and less than 1.0 mass% relative to the base material particles. Increasing the supported amount to a certain extent allows the promoter effect to be fully exerted. Specifically, when the photocatalyst is used for CO2 reduction, it becomes possible to significantly increase the CO gas generation rate and CO selectivity. On the other hand, by keeping the supported amount moderate, it is possible to suppress a decrease in the CO gas generation rate. The supported amount is more preferably 0.1 mass% or more and 0.5 mass% or less. The supported amount can be adjusted by controlling manufacturing conditions such as the amount of Ag source added during the production of photocatalyst particles.

[0026] Preferably, in the particle size frequency distribution curve of the metallic Ag particles, the frequency of particles with a particle size of 20 nm is 15% or more by number. Photocatalyst particles carrying Ag particles with such a particle size distribution have even better catalytic performance. Specifically, in a CO2 reduction photocatalytic performance evaluation test, a higher CO gas generation rate can be achieved while maintaining a high level of CO selectivity. The particle size frequency distribution curve is a curve that shows the relationship between the particle size of Ag particles and their frequency on a number basis.

[0027] Preferably, the photocatalytic particles have two peaks in the diffuse reflectance spectrum: Peak A located in the wavelength range of 200 nm or more and less than 350 nm, and Peak B located in the wavelength range of 300 nm or more and less than 600 nm. Here, the wavelength of Peak B is higher than the wavelength of Peak A. In addition, the intensity of Peak A (I A ) versus the intensity of peak B (I B ) ratio (I B / I A The peak intensity ratio (I B / I A By controlling the peak intensity ratio (I), the catalytic performance of the photocatalyst particles can be further improved. Specifically, in the CO2 reduction photocatalytic performance evaluation test, a higher CO gas generation rate can be achieved while maintaining a high level of CO selectivity. B / I A ) is preferably 40% or less, more preferably 30% or less, and even more preferably 20% or less. On the other hand, in order to fully exhibit the function of the promoter (Ag particles), the peak intensity ratio (I B / I A It is desirable that the peak intensity ratio (I B / I A ) is preferably 0.5% or more, more preferably 1% or more, and even more preferably 2% or more.

[0028] It is sufficient that at least one peak exists in each of the wavelength ranges of 200 nm or more and less than 350 nm and 300 nm or more and less than 600 nm. For example, when multiple peaks exist in the wavelength ranges of 300 nm or more and less than 600 nm, the intensity ratio (I B / I A ) should be within a specified range (50% or less).

[0029] A diffuse reflectance spectrum is an absorption spectrum obtained by the diffuse reflectance method. When light is irradiated onto a solid sample such as a powder, some of the light leaves the sample through a scattering process. In the scattering process, part of the irradiated light is reflected from the surface of the sample, and the rest penetrates into the sample. Of the incident light that penetrates, part is absorbed by the electronic transition state of the sample, and the rest leaves the sample. An absorption spectrum can be obtained from the incident light intensity of ultraviolet or visible light and the light intensity after the scattering process.

[0030] The diffuse reflectance spectrum has peaks in specific wavelength ranges (200 nm or more and less than 350 nm, 300 nm or more and less than 600 nm), and the peak intensity ratio (I B / I A Photocatalyst particles with a ratio of 1000 to 10000 within a predetermined range (50% or less) exhibit a high CO gas generation rate, but the details of this mechanism are unknown. However, it is speculated that the particle size of the metal Ag particles may be related to the electronic transition state. In other words, the spectrum obtained by diffuse reflectance (diffuse reflectance spectrum) reflects the electronic transition state of the sample, such as the species valence, coordination structure, and ligand field. Furthermore, when the sample is a fine particle, it may exhibit different absorption energy depending on the particle size. For example, it is known that fine metal Ag particles have an absorption band in the wavelength range corresponding to plasmon resonance. Therefore, the photocatalyst particles of this embodiment, which have a peak in a specific wavelength range, are thought to have fine, specific electronic transition states for the supported Ag particles.

[0031] On the other hand, it is expected that the particle size and electron transition state of the Ag particles will affect the catalytic performance of the photocatalyst, for example, the CO selectivity. This will be explained based on the mechanism of CO2 reduction of photocatalyst particles carrying metal Ag particles (Fig. 1). As shown in Fig. 1, when light with energy hν is irradiated onto the particles, electrons (e - ) and holes (h + At this time, the metallic Ag particles (promoter) undergo charge separation (electron e- and hole h + promotes the separation of holes (h + ) reacts with the surrounding water (H2O), and the reaction shown in the following formula (2) proceeds to the right, producing oxygen (O2) and protons (H + ) are generated. On the other hand, electrons (e - ) is carbon dioxide (CO2) and protons (H + As a result of this reaction, the reactions shown in the following equations (3) and (4) proceed to the right, producing carbon monoxide (CO), water (H2O), and hydrogen (H2). In principle, the reaction shown in the following equation (5) proceeds when the reactions in the following equations (2) to (4) are combined.

[0032]

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[0033] If the reactions of the above formulas (3) and (4) occur at the same rate, the CO selectivity (CO generation rate / (H generation rate + CO generation rate)) is constant, as shown in the above formula (5). However, in reality, these reactions do not necessarily occur at the same rate. When the reaction of the above formula (3) occurs preferentially, the CO selectivity increases.

[0034] It has been reported that in the reaction of formula (3) above, carbon dioxide (CO2) is adsorbed onto the catalyst surface as carbonate species, and upon irradiation with light, it is converted into a reaction intermediate, formate species, which then interacts with water molecules to form carbon monoxide (CO). This report also suggests that the Ag promoter promotes the generation of the reaction intermediate. Therefore, by enhancing the charge separation effect and reaction intermediate generation effect of metallic Ag particles (promoter), the reaction of formula (3) above occurs preferentially, and it is expected that CO selectivity will be further improved.

[0035] In this regard, it is believed that in photocatalyst particles that have a peak in a specific wavelength region of the diffuse reflectance spectrum and whose peak intensity ratio is within a specified range, the supported Ag particles are present on the surface of the base particle in a fine state with a unique electronic state. By supporting fine Ag particles with a unique electronic transition state, the number of CO2 reduction sites on the photocatalyst particles increases compared to cases where Ag particles are supported by other methods, which is thought to lead to improved catalytic activity, particularly the rate of CO gas generation.

[0036] The photocatalyst particles of this embodiment have excellent catalytic performance, particularly CO selectivity. For example, in a CO2 reduction photocatalytic performance evaluation test, the CO selectivity is 30% or more. This makes it possible to increase the proportion of CO gas generated by CO2 reduction. The CO selectivity may be 40% or more, 50% or more, 60% or more, 70% or more, 80% or more, or even 90% or more. There is no particular upper limit to the CO2 selectivity, but it is typically 98% or less, more typically 95% or less.

[0037] The CO2 reduction photocatalytic performance evaluation test may be performed using a known evaluation device. An example of the evaluation device is shown in Figure 2. The evaluation device (2) is composed of a tank (4) and a high-pressure mercury (Hg) lamp (6) installed inside the tank (4). The tank (4) contains an evaluation solution (22). The tank (4) also includes a gas inlet pipe (8), a gas outlet pipe (10), a pH meter (12), a rubber stopper (14), and a stirrer (16). A bubbling filter (18) is installed at the tip of the gas inlet pipe (8). The mercury lamp (6) is cooled by cooling water (20) flowing around it.

[0038] The evaluation test can be performed as follows. Pure water, sodium bicarbonate (NaHCO3), and a sample (photocatalyst particles) are mixed to prepare an evaluation solution (22). This evaluation solution (22) is placed in the tank (4) of the evaluation device (2) and stirred with a stirrer (16). Carbon dioxide (CO2) gas (30) is blown in through the gas inlet pipe (8), and simultaneously UV light from a high-pressure mercury lamp (6) is irradiated onto the evaluation solution (22). After irradiation for a predetermined time, the generated gas (32) is introduced into a gas chromatograph (34) through the gas outlet pipe (10) and analyzed there. This analysis determines the generation (production) rates of hydrogen (H2), oxygen (O2), and carbon monoxide (CO) gases. The obtained generation rates are used to calculate the CO selectivity according to the following equation (1):

[0039]

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[0040] <<2. Photocatalytic particle manufacturing method>> This embodiment also relates to a method for producing carbon dioxide-reducing photocatalyst particles. The photocatalyst particles include base particles and metallic silver (Ag) particles supported on the surfaces of the base particles. The photocatalyst particles may be produced by any method as long as they satisfy the above-mentioned requirements. They may be produced by techniques such as ultrasonic reduction, chemical reduction, impregnation, and photoelectrodeposition. However, production by ultrasonic reduction is preferred. In other words, photocatalyst particles on which Ag particles are supported by ultrasonic reduction are preferred. This makes it possible to produce photocatalyst particles with a higher CO gas generation rate while maintaining a high level of CO selectivity.

[0041] A suitable production method using ultrasonic reduction includes the following steps: a step of adding base particles and a silver (Ag) source to a reduction solution to prepare a reaction solution (mixing step), and a step of irradiating the reaction solution with ultrasonic waves to prepare base particles carrying metallic silver (Ag) particles (ultrasonic treatment step). 15 The details of each step are explained below.

[0042] <Mixing process> In the mixing process, base material particles and a silver (Ag) source are added to the reducing solution to prepare a reaction solution. The base material particles may be commercially available products or may be crushed and used. Alternatively, they may be synthesized. For example, base material (KYTO) particles can be synthesized by the flux method. Specifically, a mixture of yttrium oxide (YO), tantalum oxide (TaO), and potassium chloride (KCl) is fired, and the resulting fired product is washed with water to remove excess flux, thereby obtaining base material particles. The synthesized base material particles may also be crushed.

[0043] The size of the base material particles is not particularly limited. For example, the average particle size of the particles is 0.3 to 5.0 μm. Furthermore, the shape of the particles is not particularly limited. Examples include spherical, irregular, and anisotropic shapes (rod or plate-like). When the particles are rod-shaped, for example, particles with a major axis diameter of 1.0 to 5.0 μm and a minor axis diameter of 0.3 to 1.0 μm can be used.

[0044] The Ag supply source is not limited as long as it can supply Ag. Specific examples include oxides, inorganic metal salts, and / or organic metal compounds. Examples of inorganic metal salts include nitrates, chlorides, and / or sulfates. The Ag supply source may or may not be soluble in the reducing solution. A suitable Ag supply source includes silver oxide. Silver oxide is composed only of silver ions and oxygen ions, making it easy to handle and eliminating waste disposal issues. Known silver oxides include Ag2O, AgO, and Ag2O3, which have different oxidation numbers of silver, and all of them can be used. However, Ag2O, which is more readily available, is preferred. The size of the Ag supply source is also not particularly limited. For example, the average particle size of the Ag supply source is 0.3 to 3.0 μm.

[0045] The blending ratio of the base material particles and the Ag supply source can be adjusted so that the amount of co-catalyst (Ag particles) loaded in the final photocatalyst particles is the desired value. If the co-catalyst loading is too low, it becomes difficult to fully utilize the co-catalyst's effect. As a result, when photocatalyst particles are used for CO2 reduction, the CO gas generation rate and CO selectivity will be low. On the other hand, if the co-catalyst loading is too high, the CO gas generation rate will decrease.

[0046] The reducing liquid is not limited as long as it is a liquid with reducing properties. It may be a liquid with reducing properties itself, or a liquid without reducing properties in which a reducing agent is dissolved. However, a liquid with reducing properties itself is preferable. It may not contain a separate reducing agent. As such a reducing liquid, alcohols such as ethanol and propanol, which are low in toxicity and easily available, are preferable. A mixture of alcohol and water can also be used. However, when using a mixture, if the water content is excessively high, it becomes difficult to exert a sufficient reduction effect. Therefore, the water content in the reducing liquid is preferably 50% by volume or less, more preferably 25% by volume or less. The lower limit of the water content is not particularly limited and may be 0% by volume.

[0047] <Ultrasonic treatment process> In the ultrasonic treatment step, ultrasonic waves are applied to the reaction solution to produce base particles carrying metallic Ag particles. During this process, the surface of the Ag supply source in the reaction solution is ultrasonically reduced to form metallic Ag particles, which are then carried on the surface of the base particles. The base particles carrying metallic Ag particles can be used as photocatalyst particles.

[0048] The mechanism of metallic Ag particle support is explained using Figure 3. When ultrasonic waves are applied, compressional waves are generated in the reaction solution, which generates repeated positive and negative pressures. During negative pressure cycles, evaporation generates countless tiny bubbles in the reaction solution. During positive pressure cycles, these bubbles collapse, exerting a powerful impact force on the surrounding area. This phenomenon is called ultrasonic cavitation. Cavitation uniformly disperses the base material particles and Ag source in the reaction solution and cleans their surfaces. Cavitation also generates tiny, high-temperature, high-pressure hot spots. The generated hot spots decompose and reduce the Ag source and act on the reaction solution to generate radicals, which promote the decomposition and reduction of the Ag source. In this way, metallic Ag particles are produced from the Ag source.

[0049] For example, when solid silver oxide (AgO) is used as the Ag source, hot spots and radicals act on the AgO, causing it to decompose and reduce on the surface, resulting in the precipitation of Ag particles. These Ag particles gradually grow, and when they reach a certain size, the interfacial stress between the AgO and the Ag particles becomes a limiting factor, causing them to detach. Alternatively, an intermediate product is generated from the AgO by the action of ultrasound, and hot spots and radicals act on this intermediate product, generating Ag particles in the reaction solution. The detached or generated Ag particles migrate to the surface of the base material particles due to the physical action of ultrasound, where they are adsorbed. In this way, base material particles carrying Ag particles are obtained. The Ag particles generated by ultrasonic reduction are very fine. Furthermore, because no organic protective agents or high-temperature calcination are required, it is possible to produce base material particles (photocatalyst particles) carrying Ag particles in a very fine state.

[0050] If photocatalyst particles contain an Ag source other than Ag particles, it may be difficult to fully exert the effect of the co-catalyst. In this regard, ultrasonic treatment makes it possible to obtain photocatalyst particles that contain almost no Ag source (silver oxide, etc.). For example, it is possible to obtain photocatalyst particles in which no silver oxide (AgO) peak is observed in the X-ray diffraction pattern.

[0051] Special equipment is not required for ultrasonic treatment; equipment equipped with a standard ultrasonic oscillation source can be used. For example, a commercially available ultrasonic cleaner can be used. Treatment can be performed under standard conditions. For example, the ultrasonic frequency can be 20 to 100 kHz or 28 to 45 kHz. Ultrasonic treatment can be performed continuously at the same frequency, or the frequency can be switched during treatment using a frequency oscillation switching mode. The frequency can be switched once or multiple times. Treatment using a frequency oscillation switching mode (e.g., a dual-frequency switching oscillation mode of 28 kHz / 45 kHz) can further improve the dispersibility of the base material particles in the liquid. The ultrasonic output can be 10 to 500 W or 50 to 200 W. The treatment time can be 1 to 10 hours. By extending the treatment time, it is possible to convert all of the Ag source into Ag particles and support them on the base material particles. On the other hand, by shortening the treatment time, it is possible to adjust the amount of co-catalyst (Ag particles) supported. The treatment time is preferably 2 to 10 hours.

[0052] The product obtained by ultrasonic treatment (base material particles carrying Ag particles) exists in a dispersed or precipitated state in the reaction solution. Therefore, the product can be recovered from the reaction solution and dried. The recovery can be performed using known separation methods such as filtration or centrifugation. Drying can be performed under conditions that do not cause excessive particle growth of the Ag particles, for example, at 100°C or below.

[0053] The amount of co-catalyst (Ag particles) supported in the final photocatalyst particles can be adjusted by controlling the amount of Ag source and the ultrasonic treatment conditions. The amount of metallic Ag particles supported is preferably more than 0% by mass and less than 1.0% by mass, more preferably 0.1% by mass or more and 0.5% by mass or less, relative to the base material particles.

[0054] <Heat treatment process> If necessary, the base material particles carrying metallic Ag particles may be heat-treated. Heat treatment of Ag-loaded base material particles prepared by ultrasonic loading can remove organic matter, such as ethanol-derived organic matter, remaining on the particle surface and further reduce the particle size of the Ag particles. This heat treatment allows for more selective CO generation when used as a CO2-reducing photocatalyst.

[0055] The heat treatment is preferably carried out at 100°C or higher in an oxygen-containing atmosphere. There is no need to use special equipment for the heat treatment; a general electric furnace capable of firing in the atmosphere can be used. At temperatures below 100°C, the decomposition of organic matter and the effect on the particle size of the Ag particles are small, and no effect can be expected. However, the photocatalyst particles of this embodiment are not limited to those that have been subjected to heat treatment. Photocatalyst particles with sufficiently high catalytic activity can be obtained without heat treatment.

[0056] In this way, photocatalyst particles consisting of base material particles carrying metal Ag particles can be obtained.

[0057] By adopting this production method, it is possible to easily obtain catalyst particles with excellent catalytic performance, particularly in terms of CO gas generation rate and CO selectivity. Although the detailed reasons for this are unclear, we speculate that the supported Ag particles produced by ultrasonic reduction are fine and have a unique electronic transition state. In other words, it is believed that the majority of the silver (Ag) produced by ultrasonic reduction is in an ultrafine state. Furthermore, it is believed that the unique electronic transition state is due to the high-temperature and high-pressure hot spots and radical action generated during ultrasonic treatment. In fact, it has been reported that hot spots generated by ultrasonic treatment are as high as 5000°C. It is easy to predict that the electronic transition state will change even if such high-temperature hot spots act momentarily. We speculate that the combined action of these fine particle sizes and the unique electronic transition state results in improved catalytic performance, such as CO gas generation rate and CO selectivity.

[0058] Furthermore, this production method allows the use of a compound insoluble in the reaction solution, such as silver oxide, in its solid state as an Ag supply source, although this is not a limitation. When a compound insoluble in the reaction solution is used, the reaction solution does not contain harmful substances such as anions, making waste disposal easy.

[0059] However, the manufacturing method of this embodiment does not exclude the use of a compound that dissolves in the reaction solution, and even in such a case, the effect of precipitating metal Ag particles with high dispersion and high loading can be obtained. [Example]

[0060] The present invention will be described in more detail using the following examples and comparative examples. The present invention is not limited to the following examples.

[0061] (1) Preparation of photocatalytic particles [Example 1 (Example)] In Example 1, K2YTa5O 15(KYTO) particles were synthesized as base particles. Next, Ag particles were reduced using ultrasonic reduction (USR), and the Ag particles were supported on the surface of the KYTO particles as a co-catalyst to produce photocatalyst particles. The amount of co-catalyst supported (Ag concentration) relative to the base particles was 0.5 mass%. Specifically, the photocatalyst was produced using the following procedure.

[0062] <Base material particle synthesis process> First, KYTO particles were synthesized as the base material particles. Specifically, 0.316 g of yttrium oxide (YO), 3.09 g of tantalum oxide (TaO), and 3.41 g of potassium chloride (KCl) were mixed in an alumina mortar for 5 minutes. The resulting mixture was then calcined in air at 1145°C for 6 hours. The resulting powder was washed by stirring in 500 mL of ultrapure water at 80°C for 30 minutes. The resulting liquid (slurry) was left to stand for 10 minutes, the supernatant was removed, and 500 mL of ultrapure water was added and washed again. This washing process was repeated three times. The particles were then recovered from the washed slurry by filtration and air-dried at room temperature overnight. The dried powder was then sieved through a 100-mesh sieve to obtain KYTO particles.

[0063] <Co-catalyst supporting step> Silver oxide (AgO, Fujifilm Wako Pure Chemical Industries, Ltd.) particles were prepared. Next, 1.2 g of KYTO particles and 6.5 mg of AgO particles were added to 50 mL of reducing solution to prepare a reaction solution. Ethanol (Fujifilm Wako Pure Chemical Industries, Ltd.) was used as the reducing solution.

[0064] The resulting reaction solution was placed in an ultrasonic device (Honda Electronics Co., Ltd., WT-100-M) and subjected to ultrasonic treatment. The ultrasonic treatment was performed using a dual-frequency switching system (28 kHz and 45 kHz) at an output of 100 W. The treatment time was 3 hours. The temperature of the reaction solution was maintained at 40°C. This treatment reduced the AgO in the reaction solution to metallic Ag. The product produced by the treatment was filtered, washed with ethanol (10 mL), and dried in air at 60°C for 1 hour to obtain metallic Ag particle-supported KYTO particles as photocatalyst particles.

[0065] [Example 2 (Example)] In Example 2, the base particles (KYTO particles) synthesized in Example 1 were used to produce photocatalyst particles by reducing and supporting the co-catalyst (Ag particles) using a chemical reduction (CR) method. The amount of co-catalyst supported (Ag concentration) relative to the base particles was 0.5 mass%. Specifically, the production was carried out using the following procedure.

[0066] <Co-catalyst supporting step> A 0.1 M silver nitrate solution (Fujifilm Wako Pure Chemical Industries, Ltd., 0.1 M AgNO3aq) and sodium phosphinate monohydrate (Fujifilm Wako Pure Chemical Industries, Ltd., NaPH2O2·H2O) were prepared. Sodium phosphinate monohydrate was mixed with ultrapure water to prepare a 0.4 M sodium phosphinate solution.

[0067] 0.75 g of the KYTO particles synthesized in Example 1 was added to 50 mL of ultrapure water, and the resulting mixture was maintained at 80°C using a water bath. Next, 0.35 mL of a 0.1 M silver nitrate solution and 0.75 mL of a 0.4 M sodium phosphinate solution (reducing agent) were added to the mixture, and the mixture was stirred at 80°C for 1 hour and 30 minutes to induce a chemical reduction reaction. The liquid (slurry) after chemical reduction was filtered, and the resulting powder was collected. The collected powder was dried at room temperature to obtain photocatalyst particles.

[0068] [Example 3 (Comparative Example)] In Example 3, photocatalyst particles were produced using gallium oxide (Ga2O3) particles as base particles. Specifically, Ag particles were reduced and generated using an ultrasonic reduction method, and the generated Ag particles were supported on the surface of the Ga2O3 particles as a co-catalyst to produce photocatalyst particles. The amount of co-catalyst supported (Ag concentration) relative to the base particles was 0.5 mass%. Specifically, the production was carried out using the following procedure.

[0069] <Co-catalyst supporting step> Gallium oxide particles (High-Purity Chemical Research Institute Co., Ltd., Ga2O3) and silver oxide (Fuji Film Wako Pure Chemical Corporation, Ag2O) were prepared. The gallium oxide particles had a purity of 99.99%, and the silver oxide had a purity of 99%. Next, the prepared gallium oxide particles (1 g) and silver oxide (5 mg) were added to a reducing solution (50 mL). Ethanol (Fuji Film Wako Pure Chemical Corporation) was used as the reducing solution. Thereby, a reaction solution was prepared.

[0070] The obtained reaction solution was put into an ultrasonic device (Honda Electronics Co., Ltd., WT-100-M) and subjected to ultrasonic treatment. The ultrasonic treatment was performed as two-frequency switching oscillation of 28 kHz and 45 kHz under the condition of an output of 100 W. The treatment time was set to 3 hours. At this time, the temperature of the reaction solution was maintained at 40 °C. By this treatment, silver oxide (Ag2O) in the reaction solution was reduced and changed to silver (Ag). Next, the product generated by the treatment was filtered, washed with ethanol (10 mL), and dried under the condition of 60 °C in the air for 0.5 hours to obtain metal silver nanoparticle-supported gallium oxide particles as photocatalyst particles.

[0071] (2) Evaluation of photocatalyst particles Regarding the photocatalyst particle samples obtained in Examples 1 to 3, evaluations of various characteristics were performed as follows.

[0072] <STEM observation> The sample was observed using a scanning transmission electron microscope (STEM; Hitachi High-Technologies Corporation, HD2700) to obtain a STEM image. The observation was performed under the condition of a transmission electron image with an acceleration voltage of 200 kV. Also, the particle sizes of 150 cocatalysts (Ag particles) were measured from the obtained STEM image, and the particle size frequency distribution based on the number was determined. Then, the average particle size of the Ag particles was calculated from the particle size frequency distribution curve.

[0073] <Diffuse reflection spectrum> The diffuse reflection spectrum in the solid state was measured using an ultraviolet-visible near-infrared spectrophotometer (JASCO, V-650). The measurement was performed with a bandwidth of 1.0 nm, a measurement range of 800 to 200 nm, a data acquisition interval of 0.1 nm, and a scanning speed of 200 nm / min -1It was carried out under the conditions of.

[0074] <CO2 reduction photocatalytic performance> The CO2 reduction photocatalytic performance of the samples was evaluated using the evaluation apparatus shown in Figure 2. First, ultrapure water (1 L), NaHCO3 (0.1 M), and photocatalytic particles (0.5 g) were mixed to prepare an evaluation solution. Next, this evaluation solution was placed in the tank of the evaluation apparatus, and while blowing carbon dioxide (CO2) gas at a flow rate of 30 mL / min, UV light was irradiated with a 400 W high-pressure Hg lamp. After irradiation for 1 hour, the gas generated was analyzed using gas chromatography (Shimadzu Corporation, GC-8A) to determine the generation (production) rates of H2, O2, and CO gases. Then, the CO selectivity was calculated based on the following formula (1).

[0075]

Equation

[0076] (3) Evaluation results <STEM observation> The STEM images of the photocatalytic particles prepared in Example 1 and Example 2 are shown in Figure 4 (Example 1) and Figure 5 (Example 2), respectively. Although there was no difference in the appearance of the particles, it was observed that in Example 2, the cocatalyst (Ag particles) was more densely supported on the base material particles.

[0077] For the photocatalytic particles of Examples 1 to 3, the particle size frequency distribution curves of the cocatalyst (Ag particles) obtained from the STEM images are shown in Figure 6. Also, the average particle size of the Ag particles obtained from the particle size frequency distribution curve is shown in Table 1 below.

[0078] In Example 1 where the cocatalyst was supported by the ultrasonic reduction (USR) method, the average particle size of the Ag particles was 15.2 nm, which was slightly larger than the average particle size (13.0 nm) of Example 2 where the support was carried out by the chemical reduction (CR) method. Also, in Example 1, the frequency at a particle size of 20 - 25 nm in the particle size frequency distribution curve of the Ag particles was 16%, which was higher than the frequency (8.9%) of Example 2.

[0079] <Diffuse reflection spectrum> The diffuse reflectance spectra of the photocatalyst particle samples of Examples 1 to 3 are shown together with the diffuse reflectance spectrum of the base material particles (KYTO) particles in Figure 7. The spectral intensity shown on the vertical axis of Figure 7 has been normalized so that the intensity of Peak A, which exists in the wavelength range of 200 nm or more and less than 350 nm, for each sample is 100%.

[0080] The diffuse reflectance spectra of the photocatalyst particle samples (Examples 1 and 2) show peaks (Peak A and Peak B) in two wavelength ranges: from 200 nm to less than 350 nm, and from 300 nm to less than 600 nm. In contrast, the diffuse reflectance spectrum of the base material (KYTO) particles alone shows a peak in the wavelength range of from 200 nm to less than 350 nm, but no peak in the wavelength range of from 300 nm to less than 600 nm. From this, it is believed that the peak in the wavelength range of from 200 nm to less than 350 nm (Peak A) seen in the photocatalyst particle samples (Examples 1 and 2) is caused by light absorption by the KYTO particles, and the peak in the wavelength range of from 300 nm to less than 600 nm (Peak B) is caused by plasmons of the supported Ag nanoparticles.

[0081] In Example 1, where the co-catalyst was loaded by the ultrasonic reduction (USR) method, the peak intensity ratio (I B / I A ) was 10%, whereas in Example 2 where loading was performed by chemical reduction (CR), the peak intensity ratio was 63%.

[0082] The CO2 reduction photocatalytic performance (gas generation rate and CO selectivity) is shown in Table 1 below and FIG.

[0083] Looking at the influence of the base material, the sample with a KYTO base material (Example 1) had a selectivity roughly three times higher than the sample with a Ga2O3 base material (Example 3), despite having the same CO gas generation (production) rate. Ga2O3 generates a lot of hydrogen, which reduces CO selectivity, whereas KYTO suppresses hydrogen generation and increases CO selectivity.

[0084] Looking at the influence of the loading method, the sample loaded by ultrasonic reduction (Example 1) had a CO gas generation rate that was about 1.5 times higher than the sample prepared by chemical reduction (Example 2). The average particle size and particle size distribution of the Ag particles revealed that Ag nanoparticles loaded by ultrasonic reduction were larger particles loaded onto the KYTO compared to those loaded by chemical reduction. Furthermore, the diffuse reflectance spectrum suggested that light was blocked by the loaded Ag nanoparticles when chemical reduction was used.

[0085] From these findings, it is believed that by using the ultrasonic reduction method, it is possible to support Ag particles while ensuring the light-receiving range of the photocatalyst, and as a result, the CO gas generation rate is higher than when the chemical reduction method is used.

[0086] [Table 1]

[0087] From the above results, it can be seen that this embodiment provides carbon dioxide-reducing photocatalyst particles that can achieve both a high CO gas generation rate and a high CO selectivity, and a method for producing the same. [Explanation of symbols]

[0088] 2 Evaluation equipment 4 tanks 6. Mercury (Hg) lamp 8 Gas inlet pipe 10 Gas exhaust pipe 12 pH meter 14 Rubber stopper 16 Stirrer 18 Bubbling filter 20 Cooling water 22 Evaluation solution 30 CO2 gas 32 Evolved gas 34 Gas Chromatography

Claims

1. It comprises a base material particle and metallic silver (Ag) particles supported on the surface of the base material particle, The aforementioned base material particles are of the general formula: K ２ YTa ５ O １５ A compound represented by Carbon dioxide reduction photocatalytic particles.

2. The carbon dioxide reduction photocatalytic particle according to claim 1, wherein the amount of supported metallic silver (Ag) particles is greater than 0% by mass and less than 1.0% by mass relative to the base material particles.

3. The carbon dioxide reduction photocatalytic particles according to claim 1 or 2, wherein the average particle size of the metallic silver (Ag) particles is 5 nm or more and 30 nm or less.

4. The carbon dioxide reduction photocatalytic particle according to claim 1 or 2, wherein the frequency of particles with a particle size of 20 nm is 15% or more in the particle size frequency distribution curve of the metallic silver (Ag) particles.

5. CO ２ The carbon dioxide reduction photocatalyst particles according to claim 1 or 2, wherein the CO selectivity in the reduction photocatalyst performance evaluation test is 50% or more.

6. A method for producing carbon dioxide reduction photocatalytic particles comprising a base material particle and metallic silver (Ag) particles supported on the surface of the base material particle, A step of preparing a reaction solution by adding base material particles and a silver (Ag) supply source to a reducing solution, and The process includes irradiating the reaction solution with ultrasound to produce matrix particles supporting metallic silver (Ag) particles, The aforementioned base material particles are of the general formula: K ２ YTa ５ O １５ A method comprising a compound represented by the formula.

7. The method according to claim 6, wherein the amount of supported metallic silver (Ag) particles is greater than 0% by mass and less than 1.0% by mass relative to the base material particles.

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