Carbon dioxide reduction photocatalyst particle and method for producing the same

Abstract

To provide a carbon dioxide reduction photocatalyst particle enabling balanced improvement of CO gas generation rate and CO selectivity, and a method for producing the same.SOLUTION: A carbon dioxide reduction photocatalyst particle comprising a base particle and a metallic silver (Ag) particle carried on the surface of the base particle, wherein the base particle contains strontium niobate (SrNb2O6), wherein a diffuse reflectance spectrum indicates a peak A located in a wavelength region of 200 nm or more to 300 nm or less and a peak B located in a wavelength region of 350 nm or more to 500 nm or less, and wherein a ratio (IB / IA) of an intensity (IB) of the peak B to an intensity (IA) of the peak A is 20% or less.SELECTED DRAWING: Figure 4B

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 silver (Ag) particles or other co-catalysts on these photocatalyst particles, it is expected that they will trap electrons generated by photoexcitation, promote charge separation, and select carbon dioxide reduction products. 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 is still room for improvement in conventional photocatalysts. That is, to efficiently obtain carbon monoxide (CO) through a photocatalytic reaction, it is important to increase both the CO gas generation rate and CO selectivity in a balanced manner. However, with conventional photocatalysts, it is difficult to increase both the CO gas generation rate and CO selectivity in a balanced manner, and there is a problem with low CO selectivity in particular.

[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 specific base particles and co-catalysts can improve the CO gas generation rate and CO selectivity in a balanced manner.

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

[0011] The present invention encompasses the following aspects (1) to (5). In this specification, the expression "to" includes both the numerical values ​​at both ends. That is, "X to Y" is synonymous with "X or more and Y or less." In addition, in this specification, any combination of suitable aspects 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 matrix particles include strontium niobate (SrNb2O6), In the diffuse reflectance spectrum, a peak A is located in the wavelength range of 200 nm to 300 nm, and a peak B is located in the wavelength range of 350 nm to 500 nm, and the intensity of the peak A (I A ) to the intensity of peak B (I B ) ratio (I B / I A) is 20% or less.

[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 not more than 8.0% by mass relative to the base material particles.

[0014] (3) Carbon dioxide reduction photocatalyst particles according to (1) or (2) above, which have a CO selectivity of 50% or more in a CO2 reduction photocatalyst performance evaluation test.

[0015] (4) 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, the method comprising the following steps: 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 method, wherein the host particles comprise strontium niobate (SrNb2O6).

[0016] (5) The method according to (4), wherein the amount of the metallic silver (Ag) particles supported is more than 0% by mass and 8.0% by mass or less with respect to the base material particles. [Effects of the Invention]

[0017] According to the present invention, carbon dioxide-reducing photocatalyst particles that can increase the CO gas generation rate and CO selectivity in a balanced manner, and a method for producing the same are provided. [Brief explanation of the drawings]

[0018] [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 4A] STEM image of photocatalytic particles is shown (Example 1). [Figure 4B]STEM image of photocatalytic particles is shown (Example 1). [Figure 5A] A STEM image of photocatalytic particles is shown (Example 2). [Figure 5B] A STEM image of photocatalytic particles is shown (Example 2). [Figure 6] 1 shows the diffuse reflectance spectra of photocatalytic particles (Examples 1 and 2). [Figure 7] 1 shows the CO2 reduction photocatalytic performance of comparative catalyst particles (Examples 1 and 2). DETAILED DESCRIPTION OF THE INVENTION

[0019] 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.

[0020] <<1. Carbon dioxide reduction photocatalyst particles>> The carbon dioxide reduction photocatalyst particles (hereinafter sometimes referred to as "photocatalyst particles") of this embodiment comprise base particles and metallic silver (Ag) particles supported on the surfaces of these base particles. The base particles also comprise strontium niobate (SrNb2O6). In the diffuse reflectance spectrum, the photocatalyst particles exhibit a peak A located in the wavelength range of 200 nm to 300 nm and a peak B located in the wavelength range of 350 nm to 500 nm, and the intensity of the peak A (I A ) to the intensity of peak B (I B ) ratio (I B / I A ) is 20% or less.

[0021] The base material particles function as the main catalyst. The base material particles contain strontium niobate (SrNb2O6) as the main component. Here, the term "main component" refers to a component that accounts for 50% or more by mass of the base material particles. By using SrNb2O6 as the main component, photocatalyst particles with excellent catalytic performance can be obtained. While this should not be interpreted as being limited, it is believed that this is related to the fact that SrNb2O6 has a layered crystal structure, which includes deformable [NbO6] octahedra and has high-energy Nb 4d orbitals. From the perspective of improving catalytic performance, a high proportion of the main component (SrNb2O6) in the base material particles is preferable. The proportion of the main component may be 60% or more by mass, 70% or more by mass, 80% or more by mass, or 90% or more by mass.

[0022] Metallic Ag particles function as a co-catalyst. Supporting co-catalyst Ag particles improves the catalytic performance of photocatalyst particles. Even if the base material particles themselves have little water reduction ability but have oxidizing ability, supporting Ag, which acts as a CO2 reduction site, causes oxygen generation through water oxidation on the surface of the base material particles. At the same time, CO2 is reduced on the Ag co-catalyst surface to generate CO, which is presumably improving catalytic performance. 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.

[0023] Preferably, the amount of supported metal Ag particles (Ag concentration) is more than 0 mass% and not more than 8.0 mass% relative to the base material particles. By increasing the supported amount to a certain extent, the effect of the promoter can be fully exerted. Specifically, when the photocatalyst is used for CO2 reduction, it is 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 1.0 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.

[0024] The photocatalytic particles of this embodiment exhibit, in the diffuse reflectance spectrum, a peak A located in the wavelength region of 200 nm or more and 300 nm or less and a peak B located in the wavelength region of 350 nm or more and 500 nm or less. A ) to the intensity of peak B (I B ) ratio (I B / I A ) is 20% or less.

[0025] Peak intensity ratio (I B / I A By controlling the peak intensity ratio (I B / I A ) is preferably 15% or less, more preferably 10% or less. On the other hand, in order to fully utilize 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.

[0026] It is sufficient that at least one peak exists in each of the wavelength ranges of 200 nm to 300 nm and 350 nm to 500 nm. If multiple peaks exist in each wavelength range, the intensity of the largest peak is defined as I. A or I B It can be determined as follows.

[0027] 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.

[0028] The diffuse reflectance spectrum has peaks in specific wavelength ranges (200 nm to 300 nm, 350 nm to 500 nm), and the peak intensity ratio (I B / I A Photocatalyst particles with a ratio of 1000 to 10000 within a predetermined range (20% 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.

[0029] 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 + ) and 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.

[0030]

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[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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 CO selectivity; it is sufficient if it is 100% or less.

[0035] 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.

[0036] 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):

[0037]

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[0038] <<2. Photocatalytic particle manufacturing method>> The photocatalyst particles of this embodiment are not limited in their manufacturing method as long as they satisfy the above-mentioned requirements. However, they are preferably manufactured by an ultrasonic reduction method. That is, it is preferable that the photocatalyst particles are made to support Ag particles by an ultrasonic reduction method.

[0039] A suitable manufacturing 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). The base particles contain strontium niobate (SrNbO). Each step is described in detail below.

[0040] <Mixing process> In the mixing process, the base material particles and a silver (Ag) source are added to the reducing solution to prepare a reaction solution. The base material particles can be commercially available or can be ground, or they can be synthesized. For example, strontium niobate (SrNbO) particles can be synthesized by calcining a mixture of strontium chloride hexahydrate (SrCl2·6H2O) and niobium oxide (NbO5). The synthesized base material particles can be washed and / or ground.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] <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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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 not more than 8.0% by mass, more preferably 0.1% by mass or more and not more than 1.0% by mass, relative to the base material particles.

[0052] <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.

[0053] 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.

[0054] In this way, photocatalyst particles consisting of base material particles carrying metal Ag particles can be obtained.

[0055] 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.

[0056] 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.

[0057] 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]

[0058] The present invention will be described in more detail with reference to the following examples and comparative examples, but the present invention is not limited to these examples.

[0059] (1) Preparation of photocatalytic particles [Example 1 (Example)] In Example 1, strontium niobate (SrNb2O6) particles were synthesized as base particles. Next, Ag particles were reduced and generated by ultrasonic reduction (USR), and the generated Ag particles were supported on the surface of the SrNb2O6 particles as a co-catalyst. The amount of co-catalyst (Ag particles) supported (Ag concentration) relative to the base particles was 0.5 mass%. The specific production procedure was as follows.

[0060] <Base material particle synthesis process> First, we synthesized the SrNb2O6 particles, which served as the base material. Specifically, 6.00 g of strontium chloride hexahydrate (SrCl2·6H2O) and 2.00 g of niobium oxide (Nb2O5) were mixed in an alumina mortar for 10 minutes. The resulting mixture was then calcined in air at 900°C for 2 hours. The calcined 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 overnight at 80°C. The dried powder was then sieved through a 100-mesh sieve to obtain SrNb2O6 particles.

[0061] <Co-catalyst supporting step> Silver oxide (AgO, Fujifilm Wako Pure Chemical Industries, Ltd.) particles were prepared. Next, 1.2 g of SrNbO 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.

[0062] 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 oscillator (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 Ag2O 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 SrNb2O6 particles as photocatalyst particles.

[0063] [Example 2 (Comparative Example)] In Example 2, the base material particles (SrNb2O6) synthesized in Example 1 were used to reduce and support the promoter (Ag particles) by chemical reduction (CR). The amount of promoter (Ag particles) supported (Ag concentration) relative to the base material particles was 0.5 mass%. The specific production procedure was as follows.

[0064] <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.

[0065] 0.75 g of the SrNb2O6 particles synthesized in Example 1 were 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 1.5 hours to induce a chemical reduction reaction. The liquid (slurry) after chemical reduction was filtered, and the resulting powder was collected. The collected powder was then dried at room temperature to obtain photocatalyst particles.

[0066] [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.

[0067] <Co-catalyst supporting step> Gallium oxide particles (Ga2O3, High Purity Chemical Laboratory Co., Ltd.) and silver oxide (Ag2O, Fujifilm Wako Pure Chemical Industries, Ltd.) 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 (Fujifilm Wako Pure Chemical Industries, Ltd.) was used as the reducing solution. This produced a reaction solution.

[0068] The obtained 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 with 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 to silver (Ag). Next, the product generated by the treatment was filtered, then washed with ethanol (10 mL), and dried at 60 °C in the atmosphere for 0.5 hours to obtain metal silver nanoparticle-supported gallium oxide particles as photocatalyst particles.

[0069] (2) Evaluation of photocatalyst particles Regarding the photocatalyst particle samples obtained in Examples 1 to 3, evaluations of various properties were carried out as follows.

[0070] <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 with a transmission electron image under the condition of an acceleration voltage of 200 kV. Also, the particle diameters of 100 or more cocatalysts (Ag particles) were measured from the obtained STEM image, and the number average value was determined as the average particle diameter.

[0071] <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 carried out under the conditions of a bandwidth: 1.0 nm, a measurement range: 800 - 200 nm, a data acquisition interval: 0.1 nm, and a scanning speed: 200 nm / min -1 of.

[0072] <CO2 reduction photocatalytic performance> The CO₂ reduction photocatalytic performance of the samples was evaluated using the evaluation apparatus shown in Fig. 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. The gas generated after 1 hour of irradiation 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).

[0073] [Number]

[0074] (3) Evaluation Results [STEM Observation] The STEM images of the photocatalytic particles prepared in Example 1 and Example 2 are shown in Figs. 4A and 4B (Example 1), and Figs. 5A and 5B (Example 2). Note that Figs. 4B and 5B are enlarged images of Figs. 4A and 5A. In any of the samples, Ag particles with a particle size of about 10 to 12 nm were observed.

[0075] [Diffuse Reflection Spectrum] The diffuse reflection spectra of the photocatalytic particle samples of Example 1 and Example 2 are each shown in Fig. 6. Note that Fig. 6 also shows the diffuse reflection spectrum of the base material (SrNb₂O₆) particles. Also, the spectral intensity shown on the vertical axis of Fig. 6 is normalized so that the intensity of peak A existing in the wavelength range of 200 nm or more and 300 nm or less is 100% for each sample.

[0076] The diffuse reflectance spectra of the photocatalyst particle samples (Examples 1 and 2) loaded with Ag particles show peaks (Peak A and Peak B) in two wavelength ranges: 200 nm to 300 nm and 350 nm to 500 nm. In contrast, the diffuse reflectance spectrum of the base material (SrNb2O6) particles alone shows a peak in the wavelength range of 200 nm to 300 nm, but no peak in the wavelength range of 350 nm to 500 nm. From this, it is believed that the peak in the wavelength range of 200 nm to 300 nm (Peak A) is caused by light absorption by the SrNb2O6 particles, and the peak in the wavelength range of 350 nm to 500 nm (Peak B) is caused by the plasmons of the loaded Ag nanoparticles.

[0077] In Example 1, where Ag particles were supported by the ultrasonic reduction (USR) method, the peak intensity ratio (I B / I A ) was 9.2%, whereas in Example 2 where loading was performed by chemical reduction (CR), the peak intensity ratio was 37.9%.

[0078] The CO2 reduction photocatalytic performance (gas generation rate and CO selectivity) is shown in Table 1 below and Figure 7. Table 1 below also shows the average particle size of Ag particles determined from STEM images.

[0079] In Examples 1 and 2, which used SrNb2O6 particles as the base material, the CO selectivity was high, at 98% or more. In addition, Example 1, in which Ag particles were supported by the ultrasonic reduction method, had a higher CO gas production rate than Example 2, in which the co-catalyst was supported by the chemical reduction method.

[0080] In contrast, in Example 3, in which Ga2O3 particles were used as the base material, the CO gas generation rate was high, but the CO selectivity was low at approximately 30%.

[0081] [Table 1]

[0082] From the above results, it can be seen that the carbon dioxide-reducing photocatalyst of this embodiment can increase the CO gas generation rate and CO selectivity in a balanced manner. [Explanation of symbols]

[0083] 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. The carrier includes base particles and metallic silver (Ag) particles supported on the surfaces of the base particles, The base particles are made of strontium niobate (SrNb ２ O ６ ), In the diffuse reflectance spectrum, a peak A is located in a wavelength range of 200 nm or more and 300 nm or less, and a peak B is located in a wavelength range of 350 nm or more and 500 nm or less, and the intensity of the peak A (I Ａ ) to the intensity of peak B (I Ｂ ) ratio (I Ｂ / I Ａ ) is 20% or less.

2. 2. The carbon dioxide reducing photocatalyst particles according to claim 1, wherein the amount of the metallic silver (Ag) particles supported is more than 0% by mass and 8.0% by mass or less with respect to the base material particles.

3. CO ２ 3. The carbon dioxide reducing photocatalyst particles according to claim 1, which have a CO selectivity of 50% or more in a reduction photocatalyst performance evaluation test.

4. A method for producing carbon dioxide reducing photocatalyst particles comprising base material particles and metallic silver (Ag) particles supported on the surfaces of the base material particles, the method comprising the following steps: 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 are made of strontium niobate (SrNb ２ O ６ ) a method comprising:

5. The method according to claim 4, wherein the amount of the metallic silver (Ag) particles supported is greater than 0 mass % and not more than 8.0 mass % relative to the base material particles.

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