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).SELECTED DRAWING: Figure 4

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 these photocatalyst particles with co-catalyst particles made of silver (Ag) or other materials, it is expected that the effects of trapping electrons generated by photoexcitation, promoting charge separation, and selecting carbon dioxide reduction products can be achieved. 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 in carbon dioxide-reducing photocatalyst particles containing base particles of a zinc tantalate composition and metallic silver particles, adjusting the composition of the base particles to make them zinc-deficient can significantly improve the CO gas generation rate and CO selectivity, thereby achieving excellent catalytic performance.

[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 (5). 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 yCarbon dioxide-reducing photocatalytic particles having a composition represented by (where x and y satisfy 0 < x < 1.0 and 0 < y < 6.0).

[0014] (2) The carbon dioxide-reducing photocatalytic particles according to (1) above, wherein the supported amount of the metallic silver (Ag) particles is more than 0% by mass and 7.0% by mass or less with respect to the base particles.

[0015] (3) The carbon dioxide-reducing photocatalytic particles according to (1) or (2) above, wherein the CO selectivity is 50% or more in the CO2 reduction photocatalytic performance evaluation test.

[0016] (4) A method for producing carbon dioxide-reducing photocatalytic particles containing base particles and metallic silver (Ag) particles supported on the surface of the base particles, comprising the following steps; A step of adding the base particles and a silver (Ag) source to a reducing solution to prepare a reaction solution, and A step of irradiating the reaction solution with ultrasonic waves to produce base particles supporting metallic silver (Ag) particles. The base 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).

[0017] (5) The method according to (4) above, wherein the supported amount of the metallic silver (Ag) particles is more than 0% by mass and 7.0% by mass or less with respect to the base particles.

Effect of the Invention

[0018] According to the present invention, carbon dioxide-reducing photocatalytic particles having excellent catalytic performance and a method for producing the same are provided.

Brief Description of the Drawings

[0019] [Figure 1] Shows the mechanism of the photocatalytic reaction. [Figure 2] It is a cross-sectional schematic view showing an example of the evaluation apparatus. [Figure 3] Shows the mechanism of Ag loading by the ultrasonic reduction method. [Figure 4] The X-ray diffraction pattern of the base material particles is shown.

Embodiments for Carrying out the Invention

[0020] Specific embodiments 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 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 preferred embodiments can be adopted. For example, one and the other of preferred numerical ranges can be arbitrarily combined.

[0021] <<1. Carbon Dioxide Reduction Photocatalyst Particles>> The carbon dioxide reduction photocatalyst particles of the present embodiment (hereinafter may be collectively referred to simply as "photocatalyst particles") include base material particles and 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 0 < x < 1.0 and 0 < y < 6.0).

[0022] The base material particles of the present embodiment function as a main catalyst. These base material particles have a composition represented by the general formula: Zn x Ta2O y Here, x and y satisfy 0 < x < 1.0 and 0 < y < 6.0. These base material particles have a composition lacking zinc (Zn). That is, the stoichiometric composition of zinc tantalate is represented by ZnTa2O6. In contrast, in the base material particles of the present embodiment, Zn is lacking compared to the stoichiometric composition of zinc tantalate (ZnTa2O6).

[0023] By forming the base particles with zinc-deficient zinc tantalate, the catalytic performance (CO gas generation rate and CO selectivity) of the photocatalyst particles is significantly improved. Although it should not be construed as a limiting interpretation, the following mechanism is considered as the reason. In addition to zinc tantalate, a plurality of tantalum oxides coexist in zinc-deficient zinc tantalate. It is speculated that these multiple phases contact at the atomic level to form a heterostructure, and as a result, promote the charge separation that generates electrons and holes (positive holes) by light irradiation.

[0024] From the perspective of improving the catalytic performance, the amount of zinc x and the amount of oxygen y in the base 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, and even more preferably satisfy 0.75 ≦ x ≦ 0.95 and 5.75 ≦ y ≦ 5.95. Incidentally, zinc (Zn), tantalum (Ta), and oxygen (O) usually exist in the base particles in the form of divalent cations, pentavalent cations, and divalent anions, respectively, to form a compound. In this case, the amount of zinc x and the amount of oxygen 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 points may be generated, and in that case, the above-described relationship is not necessarily satisfied.

[0025] The base particles may be composed of a mixed phase of a plurality of crystal phases. For example, it may be composed of a mixed phase of zinc tantalate (ZnTa2O6) and tantalum oxides (Ta2O5 and / or TaO2). Also, 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-described composition. However, from the perspective of making the most of the excellent catalytic performance, it is desirable that the proportion of component elements other than Ta, Zn, and O is small. 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.

[0026] 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.

[0027]

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number

number

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[0028] 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.

[0029] It has been reported that in the reaction of formula (3) above, carbon dioxide (CO2) is adsorbed onto the catalyst surface as carbonate species, which are converted to formate species, a reaction intermediate, by light irradiation, and then interact 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 CO selectivity is expected to be further improved.

[0030] 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 7.0% 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 becomes small. 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% by mass or less, and even more preferably 0.3% by mass or more and 3% by mass or less.

[0031] The photocatalyst particles of this embodiment may include co-catalyst particles other than metal Ag particles. For example, particles containing zinc (Zn) and / or copper (Cu) may be supported on the surface of the base particle.

[0032] 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.

[0033] 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.

[0034] 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):

[0035]

number

[0036] <<2. Photocatalytic particle manufacturing method>> The photocatalyst particles of this embodiment are not limited in the method of production as long as they satisfy the above-mentioned requirements. However, it is preferable that they are produced by the following method.

[0037] A preferred method for producing photocatalyst 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), and a step of irradiating the reaction solution with ultrasonic waves to produce base material particles supporting metallic silver (Ag) particles (ultrasonic 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.

[0038] <Mixing step> In the mixing step, a reaction solution is prepared by adding base material particles and a silver (Ag) source to a reducing solution. Here, as the base material particles, those having a zinc-deficient zinc tantalate composition, specifically, 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. Thereby, it becomes possible to produce photocatalyst particles with significantly improved catalytic performance (CO gas generation rate and CO selectivity).

[0039] The base material particles may be used as they are or after being pulverized, either as commercially available products or 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, a particle size adjustment treatment such as pulverization or classification may be performed on the fired product obtained by firing.

[0040] The size of the particles is not particularly limited either. 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, anisotropic shapes (such as rod or plate shapes) can be mentioned. When the particles are rod-shaped, for example, those having 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.

[0041] 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.

[0042] 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.

[0043] <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.

[0044] 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, high-temperature, and 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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 metal Ag particles supported is preferably more than 0% by mass and not more than 7.0% by mass, more preferably 0.1% by mass to 4% by mass, and even more preferably 0.3% by mass to 3% by mass, based on the base material particles.

[0050] <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.

[0051] 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.

[0052] In this way, photocatalyst particles composed of metallic Ag-supported base material particles 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 metallic Ag particles with high dispersion and high loading can be obtained. [Example]

[0053] 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.

[0054] (1) Preparation of photocatalytic particles [Example 1] In Example 1, the base particles of zinc-deficient zinc tantalate composition (Zn 0.85 Ta2O5.85 Ag particles were supported on the (particles) in an amount of 0.5% by mass to prepare carbon dioxide-reducing photocatalyst particles.

[0055] <Synthesis of base particles> As the base particles, Zn 0.85 Ta2O 5.85 particles were synthesized. Specifically, zinc oxide (ZnO): 0.692 g, tantalum pentoxide (Ta2O5): 4.42 g, and ultrapure water: 5 mL were placed in 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 at a temperature of 80 °C overnight, and then 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 0.85 Ta2O 5.85 particles.

[0056] <Loading of Ag particles> Ag particles were reductively generated by ultrasonic reduction method and supported on the surface of the base particles. The loading amount of Ag particles was 0.5% by mass based on the base particles. Specifically, silver oxide (Fuji Film Wako Pure Chemical Corporation, Ag2O) particles were prepared. Next, the synthesized Zn 0.85 Ta2O 5.85 particles: 1.2 g and Ag2O particles: 6.5 mg were added to the reducing solution: 50 mL to prepare a reaction solution. Ethanol (Fuji Film Wako Pure Chemical Corporation) was used as the reducing solution.

[0057] 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 under the conditions of two-frequency switching oscillation of 28 kHz and 45 kHz and an output of 100 W. 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 metallic Ag. After the product generated by the treatment was filtered and recovered, it was washed with ethanol (10 mL) and dried at 60 °C in the air for 1 hour to obtain metal Ag-supported Zn 0.85 Ta₂O 5.85 particles as photocatalyst particles.

[0058] [Example 2] The loading amount of Ag particles was changed to 2.0 mass%. Photocatalyst particles were prepared in the same manner as in Example 1 except for this change.

[0059] [Example 3] The loading amount of Ag particles was changed to 5.0 mass%. Photocatalyst particles were prepared in the same manner as in Example 1 except for this change.

[0060] [Example 4 (Comparative Example)] In Example 4, silver particles were supported on zinc tantalate base particles (ZnTa2O6 particles) with a stoichiometric composition in an amount of 5.0 mass% to prepare carbon dioxide reduction photocatalyst particles.

[0061] [Synthesis of Base Particles] ZnTa2O6 particles were synthesized as the base particles. Specifically, zinc oxide (ZnO): 0.814 g, tantalum pentoxide (Ta2O5): 4.42 g, and ultrapure water: 5 mL were placed in 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 then 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 ZnTa2O6 particles.

[0062] [Support of Ag Particles] Silver particles were reductively generated by the ultrasonic reduction method and supported on the surface of the base particles to prepare photocatalyst particles. The loading amount of Ag particles was 5.0 mass% based on the base particles. The reductive generation and support of Ag particles were carried out in the same manner as in Example 4 except that ZnTa2O6 particles were used as the base particles.

[0063] (2) Evaluation of Photocatalyst Particles Using the photocatalyst particles obtained in Examples 1 to 4 as samples, evaluations of various properties were carried out as follows.

[0064] <xrd> The powder sample of photocatalyst particles was analyzed by X-ray diffraction method (XRD). The analysis was carried out under the following conditions.

[0065] - X-ray diffractometer: Rigaku Corporation, Ultima IV - X-ray source: Cu Kα ray (λ = 0.154 nm) - Measurement range (2θ): 10~70° - 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

[0066] <CO2 reduction photocatalytic performance> The CO2 reduction photocatalytic performance of the sample 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. The gas generated after 1 hour of irradiation was analyzed using gas chromatography (Shimadzu Corporation, GC-8A) to determine the generation rates of H2, O2, and CO. Then, the CO selectivity was calculated based on the following formula (1).

[0067]

Number

[0068] (3) Evaluation results <xrd> The X-ray diffraction (XRD) patterns of the base material particles (Zn 0.85 Ta2O 5.85 particles, ZnTa2O6 particles) synthesized in Example 1 and Example 4 are shown in FIG. 4. In FIG. 4, the standard XRD patterns of ZnTa2O6, Ta2O5, TaO2, and ZnO obtained from the Inorganic Crystal Structure Database (ICSD) are also shown.

[0069] As shown in FIG. 4, diffraction patterns (peaks) attributable to ZnTa2O6 were observed in the XRD pattern of the stoichiometric composition zinc tantalate (ZnTa2O6) particles synthesized in Example 4. On the other hand, in the diffraction pattern of the zinc-deficient zinc tantalate (Zn 0.85 Ta2O 5.85 ) particles synthesized in Example 1, diffraction patterns attributable to Ta2O5 and TaO2 were observed in addition to the diffraction pattern attributable to ZnTa2O6.

[0070] <CO2 reduction photocatalytic performance> The CO2 reduction photocatalytic performance (gas generation rate and CO selectivity) obtained for the samples of Examples 1 to 4 is shown in Table 1 below.

[0071] In the example samples (Examples 1 to 3) in which the base material particles had a zinc-deficient zinc tantalate (Zn 0.85 Ta2O 5.85 ) composition, the CO selectivity was as high as 90.54% or more. Also, the CO gas generation rate was relatively large at 55.40 μmol / hour or more. In particular, in Examples 1 and 2 where the cocatalyst loading amount (Ag concentration) was 0.5 or 2.0% by mass, the CO gas generation rate was large at 102.48 μmol / hour or more.

[0072] In contrast, in the comparative example sample (Example 4) in which the base material particles were stoichiometric composition zinc tantalate (ZnTa2O6), the CO selectivity was low at 47.30%. Also, the CO gas generation rate was small at 3.90 μmol / hour.

[0073]

Table 1

[0074] 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]

[0075] 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).

2. 2. The carbon dioxide reducing photocatalyst particles according to claim 1, wherein the amount of the metallic silver (Ag) particles supported is greater than 0% by mass and not more than 7.0% by mass relative 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 have the general formula: Zn ｘ Ta ２ O ｙ wherein x and y satisfy 0<x<1.0 and 0<y<6.

0.

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 7.0 mass % relative to the base material particles.

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