Carbon dioxide reduction photocatalyst particle and method for producing the same

Abstract

To provide a carbon dioxide reduction photocatalyst particle having superior catalytic performance 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 has a composition represented by the general formula: ZnxTa2Oy (where x and y satisfy 0<x≤1.0 and 0<y≤6.0), and a film containing a chromium compound is present on at least part of the surface of the metallic silver (Ag) particle.SELECTED DRAWING: Figure 6B

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 has attracted attention as a technology that can solve energy and environmental problems. By supporting these photocatalyst particles with co-catalyst particles made of silver (Ag) or other materials, it is expected that the photocatalyst particles can trap electrons generated by photoexcitation, promoting charge separation and selecting 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, it is desirable for a carbon dioxide reduction photocatalyst to have a high carbon monoxide generation rate (CO gas generation rate) as well as a high carbon monoxide production ratio (CO selectivity). Here, CO selectivity is the ratio of the CO gas generation rate to the sum of the hydrogen (H2) gas generation rate and carbon monoxide (CO) gas generation 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 reduce CO2 and generate CO2 through a reaction; the photoelectrodeposition or impregnation method uses a catalyst in which silver is supported on gallium oxide; the photoelectrodeposition method involves 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 the impregnation method involves adding gallium oxide to an aqueous silver precursor solution, stirring the mixture, removing the water, drying 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 including a carrier for supporting the noble metal in 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] However, the inventors' investigations have revealed that conventional carbon dioxide reduction photocatalysts have room for improvement in terms of catalytic performance. For example, the catalyst proposed in Patent Document 1, in which silver is supported on gallium oxide by photoelectrodeposition or impregnation, has insufficient catalytic performance. Furthermore, Patent Document 2 does not aim to produce a carbon dioxide reduction photocatalyst, and the catalytic performance of the resulting nanomaterial is unknown.

[0009] The present inventors conducted research in light of these problems. As a result, they discovered that carbon dioxide reduction photocatalyst particles with high catalytic performance can be obtained by supporting metallic silver (Ag) on ​​base particles containing zinc tantalate (ZnTa2O6). Based on this discovery, they filed patent applications (Japanese Patent Application Nos. 2023-125816 and 2023-210982).

[0010] On the other hand, further improvement in the catalytic performance of carbon dioxide-reducing photocatalysts is desired. As a result of further investigations, the present inventors have discovered that by further providing a film containing a chromium (Cr) compound on carbon dioxide-reducing photocatalyst particles containing base particles of a zinc tantalate composition and metallic silver (Ag) particles, the CO gas generation rate can be significantly improved and excellent catalytic performance can be obtained.

[0011] The present invention was completed based on these findings, and an object of the present invention is to provide carbon dioxide reducing photocatalyst particles having excellent catalytic performance and a method for producing the same. [Means for solving the problem]

[0012] The present invention encompasses the following aspects (1) to (10). In this specification, the expression "to" includes the numerical values ​​on both ends. In other words, "X to Y" is synonymous with "at least X and at most Y."

[0013] (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: Zn x Ta2O yA composition represented by (where x and y satisfy 0 < x ≤ 1.0 and 0 < y ≤ 6.0), and A carbon dioxide reduction photocatalyst particle in which a film containing a chromium (Cr) compound is present on at least a part of the surface of the metallic silver (Ag) particles.

[0014] (2) The carbon dioxide reduction photocatalyst particle according to (1) above, wherein the supported amount of the metallic silver (Ag) particles is more than 0% by mass and 8.0% by mass or less with respect to the base particles.

[0015] (3) The carbon dioxide reduction photocatalyst particle according to (1) or (2) above, wherein the chromium (Cr) compound is at least one selected from the group consisting of chromium (III) hydroxide and chromium (III) oxide.

[0016] (4) A compositional localization part exists on the particle surface of the base particles, the Zn / Ta molar ratio (surface Zn / Ta ratio) of the compositional localization part is smaller than the Zn / Ta molar ratio (bulk Zn / Ta ratio) of the whole particles, and the difference (ΔZn / Ta ratio) between the bulk Zn / Ta ratio and the surface Zn / Ta ratio is within the range of 0.01 or more and 0.30 or less. The carbon dioxide reduction photocatalyst particle according to any one of (1) to (3) above.

[0017] (5) The carbon dioxide reduction photocatalyst particle according to any one of (1) to (4) above, wherein the supported amount of the film containing the chromium (Cr) compound is 0.01% by mass or more and 0.30% by mass or less in terms of chromium (Cr).

[0018] (6) The carbon dioxide reduction photocatalyst particle according to any one of (1) to (5) above, wherein the CO selectivity is 50% or more in a CO2 reduction photocatalyst performance evaluation test.

[0019] (7) A method for producing a carbon dioxide reduction photocatalyst particle containing base particles and metallic silver (Ag) particles supported on the surface of the base particles, A step of adding base particles and a silver (Ag) source to a reducing solution to prepare a reaction solution, A step of irradiating the reaction solution with ultrasonic waves to prepare base particles supporting metallic silver (Ag) particles, and A step of providing a film containing a chromium (Cr) compound on at least a part of the surface of the silver (Ag) particles supported on the surface of the base material particles, is included. The base material particles have a general formula: Zn x Ta2O y And has a composition represented by (where x and y satisfy 0 < x ≤ 1.0 and 0 < y ≤ 6.0), method.

[0020] (8) In the step of providing the film containing the chromium (Cr) compound, base material particles supporting the silver (Ag) particles are dispersed in an aqueous solution in which a chromium (Cr) raw material is dissolved in water to prepare a dispersion liquid, and after irradiating the dispersion liquid with ultraviolet light (UV light), it is dried, the method of (7) above.

[0021] (9) The method of (7) or (8) above, wherein the loading amount of the silver (Ag) particles is more than 0% by mass and 8.0% by mass or less with respect to the base material particles.

[0022] (10) The method according to any one of (7) to (9) above, wherein the loading amount of the film containing the chromium (Cr) compound is 0.01% by mass or more and 0.30% by mass or less in terms of chromium (Cr). [Advantages of the Invention]

[0023] According to the present invention, carbon dioxide reduction photocatalyst particles having excellent catalytic performance and a method for producing the same are provided. [Brief Description of the Drawings]

[0024] [Figure 1] Shows the mechanism of the photocatalytic reaction. [Figure 2] It is a cross-sectional schematic view showing an example of an evaluation apparatus. [Figure 3] Shows the mechanism of Ag loading by the ultrasonic reduction method. [Figure 4] Shows the X-ray diffraction pattern of the base material particles. [Figure 5] Shows the X-ray absorption spectrum of the photocatalyst particles. [Figure 6A] Shows the TEM image of the photocatalyst particles (before Cr treatment). [Figure 6B] The TEM image of the photocatalyst particles (after Cr treatment) is shown. [Figure 7] The relationship between the amount of Cr charged and the amount of Cr supported during the production of the photocatalyst particles is shown. [Figure 8] The relationship between the amount of Cr charged and the CO2 reduction photocatalytic performance is shown. [Figure 9] The change over time in the CO2 photoreduction activity of the photocatalyst particles is shown. [Figure 10] The SEM images of the photocatalyst particles before and after light irradiation are shown. [Figure 11] The SEM-EDX analysis results of the photocatalyst particles after the catalytic reaction are shown.

Mode for Carrying Out the Invention

[0025] Specific embodiments of the present invention (hereinafter referred to as "the present embodiments") will be described below. However, the present invention is not limited to the following embodiments, and various modifications are possible without changing the gist of the present invention. Also, in this specification, as long as technical consistency can be achieved, any combination of suitable aspects can be adopted. For example, one and the other of suitable numerical ranges can be arbitrarily combined.

[0026] <<1. Carbon Dioxide Reduction Photocatalyst Particles>> The carbon dioxide reduction photocatalyst particles of the present embodiment (hereinafter may be simply collectively referred to as "photocatalyst particles") include base material particles and metallic silver (Ag) particles supported on the surface of the base material particles. The base material particles have a composition represented by the general formula: Zn x Ta2O y (where x and y satisfy ❶<x≦❶.❶ and ❶<y≦❻.❶). Also, a film containing a chromium (Cr) compound exists on at least a part of the surface of the metallic silver (Ag) particles.

[0027] The base material particles of the present embodiment function as the main catalyst. These base material particles have the general formula: Zn x Ta2O yIt has a composition represented by. Here, x and y satisfy 0 < x ≤ 1.0 and 0 < y ≤ 6.0. When x = 1.0 and y = 6.0, this base material particle has a stoichiometric composition of zinc tantalate (ZnTa₂O₆).

[0028] On the other hand, when x < 1.0 and y < 6.0, the base material particle has a composition lacking zinc (Zn). By forming it with a zinc tantalate composition moderately lacking in zinc, the CO gas generation rate of the photocatalyst particles increases. Although it should not be construed restrictively, in addition to zinc tantalate, a plurality of tantalum oxides coexist in the zinc-deficient zinc tantalate. It is speculated that these multiple phases contact at the atomic level to form a heterostructure, and as a result, it may promote charge separation that generates electrons and holes (positive holes) upon light irradiation.

[0029] From the perspective of improving the CO gas generation rate, the zinc amount x and oxygen amount y in the base material particle composition preferably satisfy 0 < x ≤ 1.0 and 5.0 < y ≤ 6.0, more preferably satisfy 0.5 ≤ x ≤ 1.0 and 5.5 ≤ y ≤ 6.0, still more preferably satisfy 0.7 ≤ x ≤ 1.0 and 5.7 ≤ y ≤ 6.0, and particularly preferably satisfy 0.8 ≤ x ≤ 0.9 and 5.8 ≤ y ≤ 5.9. Incidentally, zinc (Zn), tantalum (Ta), and oxygen (O) usually exist in the base material particle in the form of a compound formed by a divalent cation, a pentavalent cation, and a divalent anion, respectively. In this case, the zinc amount x and oxygen amount y satisfy the relationship y = 5 + x, thereby satisfying the charge neutrality condition. However, Zn, Ta, and O may be ions having valences other than those described above. Also, crystal defects such as vacant lattice sites may be generated, and in that case, the above-described relationship is not necessarily satisfied.

[0030] The base particles may be single-phase or may be composed of a mixed phase of multiple crystalline phases. For example, they may be composed of a single phase of zinc tantalate (ZnTa2O6), or a mixed phase of zinc tantalate (ZnTa2O6) and tantalum oxide (Ta2O5 and / or TaO2). Furthermore, the base particles may contain other component elements as long as they contain tantalum (Ta), zinc (Zn), and oxygen (O) so as to satisfy the above-mentioned composition. However, from the viewpoint of utilizing excellent catalytic performance, it is desirable that the proportion of component elements other than Ta, Zn, and O be low. Specifically, the proportion of other component elements is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 1% by mass or less.

[0031] Metallic Ag particles function as a co-catalyst. By supporting metallic Ag particles, the catalytic performance of the photocatalyst particles, especially the CO selectivity, is improved. This will be explained based on the mechanism of CO2 reduction of metallic Ag-supported photocatalyst 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.

[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 the photocatalyst particles of this embodiment, the amount (Ag concentration) of metal Ag particles (promoter) supported relative to the base material particles is preferably more than 0% by mass and not more than 8.0% by mass. By increasing the supported amount to a certain extent, the effect of the promoter can be fully exerted. Specifically, CO selectivity increases. On the other hand, if the promoter supported amount is excessively large, the area of ​​the base material particle surface that receives ultraviolet light decreases. Furthermore, the Ag particles function as a recombination center for electrons and holes, which may result in a decrease in the gas generation rate. From the viewpoint of achieving both a high level of gas generation rate and CO selectivity, the promoter supported amount is more preferably 0.1% by mass or more and 4.0% by mass or less, and even more preferably 0.3% by mass or more and 3.0% by mass or less.

[0036] The photocatalyst particles of this embodiment have a film containing a chromium (Cr) compound (hereinafter, sometimes referred to as a "Cr compound film") on at least a portion of the surface of metallic silver (Ag) particles. By providing the Cr compound film, the catalytic activity of the photocatalyst particles, particularly the CO gas generation rate, is significantly improved. The reason for this, although not to be construed as limiting, is presumed to be that covering the Ag particles with a Cr compound film promotes the supply of CO to the reduction active sites on the Ag particle surface. It is also believed that hydrogen generation and reverse reactions are suppressed, thereby improving catalytic activity.

[0037] The type of Cr compound is not limited as long as it is a compound containing Cr. However, at least one selected from the group consisting of silver chromate (AgCrO), chromium (III) hydroxide (Cr(OH)), and chromium (III) oxide (CrO) is preferred, at least one selected from the group consisting of chromium (III) hydroxide and chromium (III) oxide is more preferred, and chromium (III) hydroxide is particularly preferred.

[0038] The amount of the Cr compound film is not limited. However, the amount of chromium (Cr) equivalent (Cr loading amount) is preferably 0.01% by mass or more and 0.30% by mass or less. The higher the Cr loading amount, the higher the photocatalytic performance of the photocatalyst particles, particularly the gas generation rate. Furthermore, by appropriately suppressing the Cr loading amount, the photocatalyst particles can be easily manufactured. By setting the Cr compound film loading amount within the above-mentioned range, it is possible to easily obtain photocatalyst particles with excellent photocatalytic performance. Note that the Cr compound only needs to be provided so as to cover at least a portion of the Ag particle surface. It may also be present in a location other than the Ag particle surface, for example, on the surface of the base material particle. Furthermore, the Cr loading amount is the amount of Cr (Cr concentration) in the entire photocatalyst particle, and can be analyzed by a method such as ICP emission spectrometry. More specifically, it is analyzed by the method described in the examples below or a method equivalent thereto.

[0039] The photocatalyst particles of this embodiment may include a co-catalyst other than metal Ag particles or Cr compounds. For example, particles containing zinc (Zn) and / or copper (Cu) may be supported on the surface of the base particle. Furthermore, a Cr compound film may be present on at least a portion of the surface of the particles containing zinc (Zn) and / or copper (Cu).

[0040] Preferably, a localized composition region exists on the surface of the base particle. The Zn / Ta molar ratio (surface Zn / Ta ratio) of this localized composition region is preferably smaller than the Zn / Ta molar ratio (bulk Zn / Ta ratio) of the entire particle, and it is particularly preferable that the difference between the bulk Zn / Ta ratio and the surface Zn / Ta ratio (ΔZn / Ta ratio) be in the range of 0.01 to 0.30. Here, the localized composition region is a region present on the surface of the base particle that contains an excess of Ta compared to the composition of the entire base particle. In other words, when comparing the Zn / Ta molar ratio (bulk Zn / Ta ratio) of the entire base particle with the Zn / Ta molar ratio (surface Zn / Ta ratio) of the localized composition region, the region where the surface Zn / Ta ratio is smaller than the bulk Zn / Ta ratio is the localized composition region. Providing a localized composition region on the surface of the base particle can further improve the catalytic performance of the photocatalyst particle. This is believed to be because the localized composition region with an excess Ta composition acts as an oxidation site. In other words, it is believed that the presence of localized Ta-excess regions in the composition increases the number of oxidation sites, resulting in higher activity of the photocatalyst particles.

[0041] The bulk Zn / Ta ratio is determined by analyzing the photocatalyst particles or base material particles using inductively coupled plasma (ICP) emission spectroscopy. The surface Zn / Ta ratio is determined by analyzing the particles using X-ray photoelectron spectroscopy (XPS). The ICP analysis and XPS analysis may be performed under the conditions described in the examples below or similar conditions.

[0042] The photocatalyst particles of this embodiment have excellent catalytic performance, particularly CO selectivity. This makes it possible to increase the proportion of CO gas generated by CO reduction. For example, in a CO reduction photocatalytic performance evaluation test, the CO selectivity is 50% or more. The CO selectivity may be 60% or more, 70% or more, 80% or more, or even 90% or more. There is no upper limit to the CO selectivity; it is typically 100% or less.

[0043] 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) comprises 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.

[0044] 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 rates of hydrogen (H2), oxygen (O2), and carbon monoxide (CO). The obtained generation rates are used to calculate the CO selectivity according to the following equation (1):

[0045]

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[0046] <<2. Photocatalytic particle manufacturing method>> As long as the photocatalytic particles of the present embodiment satisfy the above-described requirements, the manufacturing method thereof is not limited. However, it is preferably manufactured by the following method.

[0047] A preferred manufacturing method of the photocatalytic particles includes the following steps: a step of preparing a reaction solution by adding base material particles and a silver (Ag) source to a reducing solution (mixing step), a step of irradiating the reaction solution with ultrasonic waves to prepare base material particles carrying silver (Ag) particles (ultrasonic treatment step), and a step of providing a film containing a chromium (Cr) compound on at least a part of the surface of the silver (Ag) particles carried on the surface of the base material particles (Cr treatment step). Further, the base material particles have a composition represented by the general formula: Zn x Ta2O y (where x and y satisfy 0 < x ≤ 1.0 and 0 < y ≤ 6.0). Details of each step will be described below.

[0048] <Mixing step> In the mixing step, base material particles and a silver (Ag) source are added to a reducing solution to prepare a reaction solution. Here, as the base material particles, those having a composition represented by the general formula: Zn x Ta2O y (where x and y satisfy 0 < x ≤ 1.0 and 0 < y ≤ 6.0) are used.

[0049] The base material particles may be used as they are or after being pulverized as commercially available products. Alternatively, they may be synthesized. For example, they can be synthesized by firing a mixture of zinc oxide (ZnO) and tantalum pentoxide (Ta2O5). The firing may be performed, for example, under the condition of holding the mixture at a temperature of 900°C or higher and 1100°C or lower in an air atmosphere for 1 hour or longer. Further, the fired product obtained by firing may be subjected to particle size adjustment treatments such as pulverization and classification. The base material particles may be single-phase or may be composed of a mixed phase of a plurality of crystal phases.

[0050] The size of the particles is not particularly limited. For example, the average particle diameter of the particles is 0.3 to 5.0 μm. Furthermore, the shape of the particles is not particularly limited. For example, spherical, irregular, or anisotropic (rod or plate-like, etc.) shapes can be mentioned. 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.

[0051] The silver (Ag) supply source is not limited as long as it can supply silver (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. Since silver oxide is composed only of silver ions and oxygen ions, it is easy to handle and does not pose problems such as waste disposal. 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.

[0052] 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, it is preferable that the liquid be a liquid with reducing properties itself. It may also 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 preferred. 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.

[0053] <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.

[0054] The mechanism of metallic Ag loading 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 hot spots with high temperature and pressure. 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.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] 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.

[0059] The loading amount of the cocatalyst (Ag particles) in the finally obtained photocatalytic particles can be adjusted by controlling the blending amount of the Ag source and the ultrasonic treatment conditions. The loading amount of the metallic Ag particles is preferably more than 0% by mass and 8.0% by mass or less, more preferably 0.1% by mass or more and 4% by mass or less, and even more preferably 0.3% by mass or more and 3% by mass or less, based on the base material particles.

[0060] <Heat treatment step> If necessary, the base material particles supporting the metallic Ag particles may be heat-treated. When the Ag-supported base material particles prepared by the ultrasonic loading method are heat-treated, organic substances remaining on the particle surface, for example, organic substances derived from ethanol, can be removed, and furthermore, the particle size of the Ag particles can be reduced. By the action of this heat treatment, CO can be more selectively generated when used as a CO2 reduction photocatalyst.

[0061] The heat treatment is preferably carried out at 100°C or higher in an atmosphere containing oxygen. There is no need to use a special apparatus for the heat treatment, and a general electric furnace capable of firing in the atmosphere can be used. At a temperature lower than 100°C, the decomposition of organic substances and the influence on the particle size of the Ag particles are small, and the effect cannot be expected. However, the photocatalytic particles of the present embodiment are not limited to those subjected to heat treatment. Even without heat treatment, photocatalytic particles with sufficiently high catalytic activity can be obtained.

[0062] <Cr treatment step> In the Cr treatment step, a film containing a chromium (Cr) compound (Cr compound film) is provided on at least a part of the surface of the metallic silver (Ag) particles supported on the surface of the base material particles. As long as the Cr compound film can be provided, the method of Cr treatment is not limited. However, it is preferably carried out by the following method. That is, base material particles supporting metallic silver (Ag) particles are dispersed in an aqueous solution in which a chromium (Cr) raw material is dissolved in water to prepare a dispersion liquid, and after irradiating the obtained dispersion liquid with ultraviolet light (UV light), it is dried. Thereby, a chromium (Cr) compound is deposited on the surface of the Ag particles.

[0063] The Cr raw material is not particularly limited as long as it is water-soluble. Examples include sodium chromate tetrahydrate (Na2CrO4·4H2O), potassium chromate (K2CrO4), chromium(III) nitrate (Cr(NO3)3), chromium sulfate hydrate (Cr2(SO4)3·nH2O), and / or chromium chloride hydrate (CrCl3·nH2O). To enhance the catalytic performance of the photocatalyst particles, it is preferable to use a relatively large amount of the Cr raw material (Cr feed amount). Specifically, it is preferable to adjust the Cr feed amount so that the Cr / Ag ratio is 0.1 or greater, and more preferably so that the Cr / Ag ratio is 1 or greater. Here, the Cr / Ag ratio is the molar ratio of the Cr feed amount in Cr terms to the amount of supported metal Ag particles. The coating amount can be adjusted by controlling the amount of Cr feed in the aqueous solution.

[0064] In this way, the photocatalyst particles of this embodiment can be obtained. By employing this manufacturing method, catalyst particles with excellent catalytic performance, particularly in terms of CO gas generation rate and CO selectivity, can be easily obtained. Furthermore, although not limited to, compounds such as silver oxide that are insoluble in the reaction solution can be used in their solid state as an Ag supply source. When a compound that is insoluble in the reaction solution is used, the reaction solution does not contain harmful substances such as anions, making waste liquid disposal easy. However, the manufacturing method of this embodiment does not exclude the use of compounds that are soluble in the reaction solution. Even in such cases, the effect of precipitating metal Ag particles with high dispersion and high loading can be obtained. [Example]

[0065] 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.

[0066] [Experimental Example A] In Experimental Example A, several photocatalyst particle samples with different base particle compositions were prepared to investigate the effect of the base particle composition on photocatalytic performance. In addition, samples with and without a Cr compound film were prepared to investigate the effect of the presence or absence of a Cr compound film.

[0067] (1) Preparation of photocatalyst particles [Example 1] In Example 1, Ag particles as a cocatalyst were supported on the base material particles, and a Cr compound film was further provided on the surface of the Ag particles to prepare photocatalyst particles. The composition of the base material particles was Zn x Ta2O 5+x (where x = 1.00). The support of Ag particles was carried out by ultrasonic reduction method, and the supported amount of Ag particles was 2.0% by mass based on the base material particles. Also, the charged amount of the Cr raw material (Cr charged amount) during the Cr treatment was adjusted so that the molar ratio of Ag:Cr = 1:1 (Cr / Ag ratio = 1).

[0068] <Synthesis of base material particles> Zinc oxide (ZnO): 0.814x [g] (where x = 1.00), tantalum pentoxide (Ta2O5): 4.42 [g], and ultrapure water: 5 [mL] were put into an alumina mortar and mixed for 10 minutes. The obtained mixture was filtered to recover the solid content, and the recovered solid content was dried overnight at a temperature of 80 °C and further calcined at 1000 °C for 50 hours in an air atmosphere. The product obtained by calcination was sized with a 100-mesh sieve to obtain Zn x Ta2O 5+x particles (where x = 1.00).

[0069] <Support of Ag particles> The synthesized Zn x Ta2O 5+x particles: 1.2 [g] and silver oxide (Fuji Film Wako Pure Chemical Industries, Ltd., Ag2O) particles: 6.5 [mg] were added to the reducing solution: 50 [mL] to prepare a reaction solution. Ethanol (Fuji Film Wako Pure Chemical Industries, Ltd.) was used as the reducing solution. The charged amount of the Ag source (Ag2O) was 2.0% by mass in terms of Ag based on the base material particles (Zn x Ta2O 5+x particles).

[0070] The obtained reaction solution was placed in an ultrasonic device (Honda Electronics Co., Ltd., WT-100-M) for 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. Also, the treatment time was 3 hours. At this time, the temperature of the reaction solution was maintained at 40 °C. By this treatment, Ag2O in the reaction solution was reduced to change into metallic Ag. After filtering and collecting the product generated by the treatment, it was washed with ethanol: 10 [mL], and dried at 60 °C in the air for 1 hour to obtain Zn x Ta2O 5+x particles loaded with Ag. Also, it was confirmed by analysis that the Ag loading amount and the Ag charged amount were the same (2.0 mass% in terms of Ag with respect to the base particles).

[0071] <Cr treatment> The obtained Zn loaded with metallic Ag x Ta2O 5+x particles: 0.75 [g], sodium chromate tetrahydrate (Na2CrO4·4H2O): 8.1 [mg], and ultrapure water: 1 [L] were mixed, and argon (Ar) gas was blown into the obtained mixed solution for gas replacement. At this time, the charged amount of the Cr raw material (Na2CrO4·4H2O) was adjusted so that the molar ratio of Ag:Cr = 1:1. Next, while blowing argon gas at a flow rate of 30 [mL / min], UV light was irradiated with a 400 W high-pressure Hg lamp for ½ hour to reduce Cr ions. The solution after light irradiation was filtered to collect the powder, and the collected powder was dried at room temperature to obtain photocatalyst particles.

[0072] [Example 2] In Example 2, the base particle composition was changed to Zn x Ta2O 5+x (where x = 0.95). Otherwise, photocatalyst particles were prepared in the same manner as in Example 1.

[0073] [Example 3] In Example 3, the base particle composition was changed to Zn x Ta2O 5+x (where x = 0.85). Otherwise, photocatalyst particles were prepared in the same manner as in Example 1.

[0074] [Example 4] In Example 4, the base material particle composition is Zn x Ta2O 5+x (However, x was changed to 0.75). Photocatalyst particles were produced in the same manner as in Example 1 except for this.

[0075] [Example 5] In Example 5, the base particle composition is Zn x Ta2O 5+x (However, x was changed to 0.50). Photocatalyst particles were produced in the same manner as in Example 1 except for this.

[0076] [Example 6] In Example 6, the base particle composition is Zn x Ta2O 5+x (However, x was changed to 0.25). Photocatalyst particles were produced in the same manner as in Example 1 except for this.

[0077] [Example 7] In Example 7, the base material particle composition was Zn x Ta2O 5+x (However, x=0.85). In addition, the amount of Ag particles carried was changed to 0.5 mass % relative to the base material particles. Other than that, photocatalyst particles were produced in the same manner as in Example 1.

[0078] [Example 8] In Example 8, the base material particle composition was Zn x Ta2O 5+x (However, x=0.85). In addition, the amount of Ag particles carried was changed to 5.0 mass % relative to the base material particles. Other than that, photocatalyst particles were produced in the same manner as in Example 1.

[0079] [Example 9 (Comparative Example)] In Example 9, no Cr treatment was performed. Other than that, photocatalyst particles were produced in the same manner as in Example 1.

[0080] (2) Evaluation of photocatalytic particles The base material particles and photocatalyst particles obtained in Examples 1 to 9 were used as samples, and various properties were evaluated as follows.

[0081] <xrd> The base material particles were analyzed by X-ray diffraction method (XRD). The analysis was carried out under the following conditions.

[0082] - X-ray diffractometer: Rigaku Corporation, Ultima IV - X-ray source: Cu Kα ray (λ = 0.154 nm) - Measurement range (2θ): 10 to 70° - Sampling width: 0.02° - Scan speed: 0.02° / min - Tube voltage: 40 kV - Tube current: 40 mA - Divergence slit: 1 / 8° - Divergence vertical limit slit: 10 mm - Receiving slit: Open

[0083] <X-ray absorption spectroscopy> Spectral analysis was performed by X-ray absorption spectroscopy using the large-scale synchrotron radiation facility Spring-8. Specifically, spectra in the Cr-K shell absorption band were measured by the transmission method and the fluorescence method at the beamline BL14B02 of Spring-8.

[0084] <TEM observation> The sample was observed using a field emission transmission electron microscope (JEOL Ltd., JEM-2100F; FE-TEM), and a TEM image was taken.

[0085] <SEM observation> The sample was observed using a field emission scanning electron microscope (Hitachi High-Technologies Corporation, SU-8220), and an SEM image was taken.

[0086] <ICP analysis> The sample for ICP analysis was prepared by heating with hydrofluoric acid aqueous solution and nitric acid aqueous solution. The bulk Zn / Ta ratio was measured using an ICP emission spectroscopic analyzer (manufactured by Agilent Technologies, 5900). The element concentrations contained in the prepared solution were identified from the calibration curves prepared using ICP standard samples of each element, and the composition ratio in the photocatalyst particles was calculated from those ratios.

[0087] <XPS Analysis> For the surface analysis of photocatalyst particles, Versa Probe II (manufactured by ULVAC-PHI, Inc.) (X-ray source: Al Kα) was used. A slide glass was placed on the sample holder attached to the apparatus and fixed with insulating double-sided tape, and a conductive double-sided tape with the sample fixed thereon was fixed to the insulating double-sided tape to prepare a measurement sample.

[0088] <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 photocatalyst 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. Then, the gas generated after irradiation for a predetermined time was analyzed using gas chromatography (Shimadzu Corporation, GC-8A) to determine the generation rates of H2, O2, and CO, and the CO selectivity was calculated based on the following formula (1).

[0089]

Equation

[0090] For some samples, long-term light irradiation was performed until the irradiation time reached 15 hours. However, during the light irradiation, the irradiation was interrupted once after 5.5 hours and 10.5 hours of irradiation, and then the irradiation was resumed. And the relationship between the light irradiation time and the catalyst performance was investigated.

[0091] (3) Evaluation Results <xrd> The base particles synthesized in Examples 1 to 9 (Zn x Ta2O 5+x particles; x = 0.25 to 1.00) are shown in Fig. 4 in terms of X-ray diffraction (XRD) patterns. In Fig. 4, the standard XRD patterns of ZnTa2O6, Ta2O5, TaO2, and ZnO obtained from the Inorganic Crystal Structure Database (ICSD) are also shown.

[0092] In the XRD patterns of the base particles, patterns attributable to zinc tantalate (ZnTa2O6) and patterns attributable to tantalum oxides (Ta2O5, TaO2) were confirmed. The pattern attributable to TaO2 was shifted to the lower angle side from the standard XRD pattern. This is considered to be due to an increase in the lattice constant.

[0093] <X-ray absorption spectroscopy> The X-ray absorption spectrum at the Cr-K edge obtained for the sample (photocatalyst particles) of Example 1 is shown in Fig. 5. In Fig. 5, the spectra of chromium foil (Cr foil), chromium(III) oxide (Cr2O3), chromium(III) hydroxide (Cr(OH)3), silver chromate (Ag2CrO4), and sodium chromate tetrahydrate (Na2CrO4·4H2O) are also shown.

[0094] The spectral shape of Example 1 is similar to the spectral shapes of chromium hydroxide (Cr(OH)3) and chromium(III) oxide (Cr2O3). As a result of linear combination fitting, the amounts of Cr(OH)3 and Cr2O3 were 87% and 13% respectively. Therefore, it was found that the photocatalyst particles of Example 1 mainly contain Cr(OH)3 and Cr2O3 as Cr compounds.

[0095] <TEM observation> The TEM images of the sample (photocatalyst particles) of Example 1 are shown in Figs. 6A and 6B. Fig. 6A shows the sample before Cr treatment, and Fig. 6B shows the sample after Cr treatment. It was observed that the periphery of the co-catalyst Ag particles was coated with a Cr compound.

[0096] <CO2 reduction photocatalytic performance> The CO2 reduction photocatalytic performance (gas generation rate and CO selectivity) obtained for the samples (photocatalyst particles) of Examples 1 to 9 is shown in Table 1 below. Table 1 below shows the catalytic activity (gas generation rate, CO selectivity) at the time (irradiation time) when the CO gas generation rate is at its maximum. Table 1 below also shows the amount of Ag charged (Ag concentration), and it has been confirmed by analysis that the amount of Ag supported and the amount of Ag charged are the same.

[0097] Looking at the effect of the presence or absence of a Cr compound film, the photocatalyst particle sample with a Cr compound film (Example 1) had a higher gas generation rate and CO selectivity than a photocatalyst particle sample with the same base particle composition (x = 1.00) but without a Cr compound film (Example 9). In particular, the CO gas generation rate of the former (Example 1) (411.06 μmol / hour) was significantly higher, more than 28 times, than the CO gas generation rate of the latter (Example 9) (14.49 μmol / hour).

[0098] Looking at the effect of the base material particle composition, the CO gas generation rate increased as x decreased from 1.00 (zinc deficiency increased), reaching a maximum at x = 0.85. After that, as x decreased further, the CO gas generation rate decreased.

[0099] Looking at the effect of the amount of Ag particles supported (Ag concentration), the photocatalyst particle samples with Ag concentrations of 0.5 or 2.0 mass% (Examples 3 and 7) had higher gas generation rates and CO selectivity than the photocatalyst particle sample with the same base particle composition (x = 0.85) but an Ag concentration of 5.0 mass% (Example 8). In particular, the CO gas generation rates of the former (Examples 3 and 7) (756.76 or 902.26 μmol / hour) were significantly higher than the CO gas generation rate of the latter (Example 8) (36.46 μmol / hour).

[0100] [Table 1]

[0101] [Experimental Example B] In Experimental Example B, a number of photocatalyst particles were produced by varying the amount of Cr raw material charged, and the effect of the amount of Cr carried (Cr concentration) on the photocatalytic performance was investigated.

[0102] (1) Preparation of photocatalytic particles [Example 10] In Example 10, the amount of Ag particles supported was changed to 0.5 mass% relative to the base material particles. The amount of Cr added was adjusted so that the molar ratio of Ag:Cr was 1:0 (Cr / Ag ratio = 0). In other words, no Cr compound film was provided. Photocatalyst particles were prepared in the same manner as in Example 1.

[0103] [Example 11] In Example 11, the amount of Ag particles supported was changed to 0.5 mass% relative to the base material particles. The amount of Cr added was adjusted to a molar ratio of Ag:Cr = 1:0.1 (Cr / Ag ratio = 0.1). Otherwise, photocatalyst particles were prepared in the same manner as in Example 1.

[0104] [Example 12] In Example 12, the amount of Ag particles supported was changed to 0.5 mass% relative to the base material particles. The amount of Cr added was also adjusted to a molar ratio of Ag:Cr = 1:0.5 (Cr / Ag ratio = 0.5). Otherwise, photocatalyst particles were prepared in the same manner as in Example 1.

[0105] [Example 13] In Example 13, the amount of Ag particles supported was changed to 0.5 mass% relative to the base material particles. The amount of Cr charged was also adjusted to a molar ratio of Ag:Cr = 1:1 (Cr / Ag ratio = 1). Otherwise, photocatalyst particles were produced in the same manner as in Example 1.

[0106] [Example 14] In Example 14, the amount of Ag particles supported was changed to 0.5 mass% relative to the base material particles. The amount of Cr charged was also adjusted to a molar ratio of Ag:Cr = 1:5 (Cr / Ag ratio = 5). Otherwise, photocatalyst particles were produced in the same manner as in Example 1.

[0107] [Example 15] In Example 15, the amount of Ag particles supported was changed to 0.5 mass% relative to the base material particles. The amount of Cr charged was also adjusted to a molar ratio of Ag:Cr = 1:10 (Cr / Ag ratio = 10). Otherwise, photocatalyst particles were produced in the same manner as in Example 1.

[0108] (2) Evaluation of photocatalytic particles Using the photocatalyst particles obtained in Examples 10 to 15 as samples, ICP analysis and evaluation of photocatalytic performance for CO2 reduction were carried out in the same manner as in Experimental Example A.

[0109] (3) Evaluation results The Cr concentrations (loading amounts) of the samples (photocatalytic particles) of Examples 10 to 15 are shown in Table 2 below and FIG. 7. The Cr concentrations shown here are values ​​for the samples before the CO2 reduction photocatalytic performance evaluation test was conducted. In the region where the Cr loading amount (Cr / Ag ratio) during Cr treatment was 0 to 1, the Cr concentration of the photocatalytic particles increased as the loading amount increased. In contrast, in the region where the Cr loading amount (Cr / Ag ratio) was greater than 1, the Cr concentration was almost constant.

[0110] The CO2 reduction photocatalytic performance (gas generation rate and CO selectivity) obtained for the samples (photocatalyst particles) of Examples 10 to 15 is shown in Table 2 below and Figure 8. In the region where the amount of Cr raw material charged during Cr treatment (Cr / Ag molar ratio) was 0 to 1, the gas generation rate increased as the charged amount increased. In contrast, in the region where the Cr charged amount (Cr / Ag ratio) was greater than 1, the gas generation rate remained almost constant. Furthermore, the Cr charged amount had almost no effect on the CO gas selectivity. From these results, it was found that the higher the Cr concentration (loading amount), the higher the gas generation rate, although the effect on CO selectivity was small.

[0111] [Table 2]

[0112] For the sample (Example 13) prepared with a Cr loading amount (Cr / Ag molar ratio) of 1, the Cr concentration (loading amount) after the CO2 reduction photocatalytic performance evaluation test was determined by ICP analysis to be 0.02 mass%. Furthermore, when the ultraviolet-visible spectrum of the solution after the test was analyzed, a Cr peak was observed. It is believed that the Cr contained in the Cr compound film was eluted into the solution during the evaluation test.

[0113] [Experimental Example C] In Experimental Example C, the effect of varying the light irradiation time was investigated when evaluating the CO2 reduction photocatalytic performance. Specifically, for the sample (photocatalytic particles) of Example 3 in Experimental Example A, the light irradiation time was varied from 0 to 15 hours, and the gas generation rate and CO2 selectivity were investigated at each time. In this case, the light irradiation was stopped at 5.5 and 10.5 hours, suspended overnight, and then resumed. In addition, the appearance of the sample before and after photocatalysis was observed using an SEM, and the composition of the sample after photocatalysis was analyzed using SEM-EDX.

[0114] Figure 9 shows the change over time in the CO2 photoreduction activity of photocatalytic particles. The gas generation rate was small at the beginning of irradiation, and then increased rapidly up to 1.5 hours of irradiation. This indicates that there is an induction period in photocatalytic activity. Note that the gas generation rate values ​​shown in Table 1 for Experimental Example A are the values ​​at the time when the maximum CO2 generation rate was achieved during the photocatalytic evaluation of each sample. This does not mean that these samples exhibit high activity from the early stages of photocatalysis.

[0115] As shown in Figure 9, no significant change in the gas generation rate was observed after the initial rapid increase in the rate of light irradiation. In particular, although light irradiation was interrupted at 5.5 and 10.5 hours, the gas generation rate recovered to a high level immediately after the interruption, indicating that the interruption of light irradiation had little effect.

[0116] SEM images of the sample (Example 3) before and after light irradiation are shown in Figure 10(a) to (f), where Figure 10(a) to (f) show the appearance of the sample before light irradiation (Figure 10(a)), after 0.5 hours of light irradiation (Figure 10(b)), after 1.0 hours of light irradiation (Figure 10(c)), after 1.5 hours of light irradiation (Figure 10(d)), after 2.0 hours of light irradiation (Figure 10(e)), and after 5.0 hours of light irradiation (Figure 10(f)), respectively.

[0117] In the sample before light irradiation, spherical Ag nanoparticles were uniformly distributed on the surface of the base particle (Figure 10(a)). In contrast, after 0.5 hours of light irradiation, while there were areas where Ag was uniformly loaded, Ag segregation was observed in some areas (Figure 10(b)). Furthermore, when the irradiation time was increased to 1.0 and 1.5 hours, Ag segregation became even more pronounced, and a similar trend was observed after that (2.0 and 5.0 hours) (Figures 10(c)-(f)). This suggests that Ag nanoparticles segregate during the induction period.

[0118] It is believed that the segregation of Ag nanoparticles led to the separation of reaction sites, specifically, CO2 reduction occurring at sites where Ag is present, while water oxidation occurs where Ag is not present. It is speculated that the activity improved with increasing light irradiation time because the reaction site separation promoted charge separation. Furthermore, once the Ag nanoparticles segregated, this state was maintained, so no induction period was observed even after the photocatalytic evaluation was interrupted and then resumed, and high activity was demonstrated from the initial stage of light irradiation.

[0119] Assuming that there is a difference in the composition (Zn / Ta ratio) of the base material particle between the areas where Ag is present and the areas where it is not, a point analysis of the composition was performed using SEM-EDX on the sample after light irradiation (Example 3).

[0120] The results are shown in Figure 11. In areas where Ag nanoparticles were present (areas marked "2" in the figure), the Zn / Ta ratio was 0.60. In contrast, in areas where Ag nanoparticles were not present (areas marked "1" in the figure), the Zn / Ta ratio was 0.43 to 0.50. The Zn / Ta ratio of zinc tantalate (ZnTa2O6) is 0.50 in a stoichiometric composition. Therefore, it was found that the composition in areas where Ag nanoparticles were present was a Zn-excess composition, and the composition in areas where Ag nanoparticles were not present was either a stoichiometric composition or a Ta-excess composition.

[0121] The Zn-rich regions where Ag nanoparticles segregate act as reduction sites, while the other stoichiometric or Ta-rich regions act as oxidation sites, and it is believed that the difference in Zn / Ta ratio between these regions led to the segregation of Ag nanoparticles. This segregation promotes the separation of reaction sites, resulting in highly efficient CO2 reduction and high activity.

[0122] [Experimental Example D] In Experimental Example D, for photocatalyst particle samples having base material particles with various Zn / Ta ratios, the Zn / Ta ratio of the entire base material particle (Zn / Ta ratio [bulk]) was determined by ICP analysis, and the Zn / Ta ratio on the surface of the base material particle (Zn / Ta ratio [surface]) was determined by XPS analysis.

[0123] The results obtained are shown in the following Table 3. The Zn / Ta ratio (theoretical value) shown in the following Table 3 is the Zn / Ta ratio (molar ratio) determined from the blending composition during the production of the base particles.

[0124] In all samples, the bulk Zn / Ta ratio determined by ICP analysis was almost the same as the theoretical value. 0.95 Ta2O 5.95 , Zn 0.85 Ta2O 5.85 and Zn 0.75 Ta2O 5.75 In the samples with theoretical Zn / Ta ratios of 0.48, 0.43, and 0.38, the surface Zn / Ta ratios were smaller than the bulk Zn / Ta ratios and the theoretical values, suggesting that the Ta fraction was larger on the surface of these samples.

[0125] From these results, it is speculated that by making the Zn / Ta ratio of the base material particles smaller than the stoichiometric composition, the number of oxidation sites, which are Ta-excess regions, increases, thereby increasing the activity of the photocatalyst particles.

[0126] [Table 3]

[0127] From the above results, it can be seen that this embodiment provides carbon dioxide reducing photocatalyst particles with excellent catalytic performance and a method for producing the same. [Explanation of symbols]

[0128] 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< / xrd> < / xrd>

Claims

1. The carrier includes base particles and metallic silver (Ag) particles supported on the surfaces of the base particles, The base particles have the general formula: Zn ｘ Ta ２ O ｙ (where x and y satisfy 0<x≦1.0 and 0<y≦6.0), Carbon dioxide reducing photocatalyst particles, wherein a film containing a chromium (Cr) compound is present on at least a portion of the surface of the metallic silver (Ag) particles.

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. 3. The carbon dioxide-reducing photocatalyst particles according to claim 1, wherein the chromium (Cr) compound is at least one selected from the group consisting of chromium (III) hydroxide and chromium (III) oxide.

4. 3. The carbon dioxide reducing photocatalyst particle according to claim 1, wherein a composition localized portion is present on the particle surface of the base particle, the Zn / Ta molar ratio of the composition localized portion (surface Zn / Ta ratio) is smaller than the Zn / Ta molar ratio of the entire particle (bulk Zn / Ta ratio), and the difference between the bulk Zn / Ta ratio and the surface Zn / Ta ratio (ΔZn / Ta ratio) is in the range of 0.01 or more and 0.30 or less.

5. 3. The carbon dioxide reducing photocatalyst particles according to claim 1, wherein the amount of the film containing the chromium (Cr) compound supported is 0.01% by mass or more and 0.30% by mass or less in terms of chromium (Cr).

6. 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.

7. 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; a step of irradiating the reaction solution with ultrasonic waves to prepare base particles carrying metallic silver (Ag) particles; and providing a film containing a chromium (Cr) compound on at least a portion of the surface of the metallic silver (Ag) particles supported on the surface of the base material particles, The base particles have the general formula: Zn ｘ Ta ２ O ｙ (where x and y satisfy 0<x≦1.0 and 0<y≦6.0).

8. 8. The method according to claim 7, wherein in the step of providing the film containing the chromium (Cr) compound, base material particles carrying the metallic silver (Ag) particles are dispersed in an aqueous solution in which a chromium (Cr) raw material is dissolved in water to prepare a dispersion liquid, and the dispersion liquid is irradiated with ultraviolet light (UV light) and then dried.

9. The method according to claim 7 or 8, wherein the amount of the metallic silver (Ag) particles supported is more than 0 mass % and 8.0 mass % or less relative to the base material particles.

10. 9. The method according to claim 7, wherein the amount of the film containing the chromium (Cr) compound is 0.01% by mass or more and 0.30% by mass or less in terms of chromium (Cr).

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