Method for producing carbon dioxide reduction photocatalytic particles

Abstract

To provide a method for producing photocatalytic particles with excellent CO2 reduction photocatalytic performance, comprising TiO2-based matrix particles. [Solution] A method for producing carbon dioxide reduction photocatalytic particles, wherein the carbon dioxide reduction photocatalytic particles comprise a base material particle mainly composed of titanium dioxide (TiO2) and a co-catalyst containing silver (Ag) and cobalt (Co) supported on the surface of the base material particle, and the method comprises a step of supporting silver (Ag) and cobalt (Co) on the surface of the base material particle, wherein the silver is supported by an ultrasonic reduction method in which ultrasonic waves are irradiated onto a reaction solution containing base material particles, a silver (Ag) supply source and a reducing solution.

Description

[Technical Field]

[0001] This invention relates to a method for producing carbon dioxide reduction photocatalytic particles. [Background technology]

[0002] Carbon dioxide (CO2) reduction technology using semiconductor photocatalytic particles is attracting attention as a technology that can solve energy and environmental problems. By supporting nanoparticles made of silver (Ag) or other materials as a co-catalyst on these photocatalytic particles, it is expected that the electrons generated by photoexcitation will be trapped, promoting charge separation and selective CO2 reduction products will be produced. For example, in a normal photocatalyst, water is decomposed into hydrogen (H2) and oxygen (O2) upon light irradiation. In contrast, in a photocatalyst supported with silver (Ag) particles, carbon monoxide (CO) is produced along with hydrogen (H2) upon reduction of carbon dioxide (CO2).

[0003] As such a CO2-reducing photocatalyst, gallium oxide particles supported with silver nanoparticles as a co-catalyst (silver nanoparticle-supported gallium oxide particles) are known. For example, Patent Document 1 discloses a CO2 reduction method characterized by using a silver-supported gallium oxide photocatalyst to generate CO by irradiating CO2, H2O, and the photocatalyst with light to reduce CO2 (Claim 1 of Patent Document 1). It is also described that the photocatalyst is one in which silver is supported on gallium oxide by photoelectrodeposition or impregnation (Claim 2 of Patent Document 1).

[0004] Carbon monoxide (CO) is an important starting material in the chemical industry and other industrial sectors, and it can be reacted with hydrogen (H2) to synthesize various fuels and chemical substances. Therefore, in CO2 reduction photocatalysts, a large amount of CO is produced (CO generation rate), as well as a high CO generation ratio (CO selectivity). Here, CO selectivity is the ratio of the CO generation rate to the sum of the H2 generation rate and CO generation rate produced by the reduction reaction, as shown in equation (1) below.

[0005]

number

[0006] Incidentally, titanium dioxide (TiO2) is a well-known photocatalytic material. TiO2 has the function of decomposing substances (mainly organic substances) by triggering chemical reactions (oxidation and reduction reactions) through light absorption. Furthermore, TiO2 is an oxide of titanium (Ti) with a Clark number of 9, and has the advantage of being inexpensive and stably available because it is an abundant resource. In addition, TiO2 is chemically stable and has the characteristics of being harmless and non-toxic to the human body and the environment. For this reason, TiO2 is widely used in applications such as air purification, including the removal of malodors, VOCs (volatile organic compounds), and NOx contained in vehicle exhaust gases, as well as swimming pool water purification, sterilization, and antibacterial effects.

[0007] TiO2 exhibits n-type semiconductor properties, with a band gap of approximately 3.0-3.2 eV (equivalent wavelength of approximately 388-412 nm). Therefore, it absorbs ultraviolet light with energy greater than the band gap, causing electrons in the valence band (eV) to be lost. - This excites electrons (e) in the conduction band. - ) is generated, and holes (h + (Figure 1) is generated. The holes and electrons generated by light absorption move separately within the TiO2 crystal, causing reduction and oxidation reactions, respectively. TiO2, in particular, has strong oxidizing power, and it oxidizes and decomposes organic compounds that come into contact with it, producing carbon dioxide and water. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2012-192302 [Overview of the project] [Problems that the invention aims to solve]

[0009] Thus, titanium dioxide (TiO2) has excellent advantages such as being inexpensive, stable, harmless, and non-toxic, and is widely used as a photocatalyst for the decomposition of organic substances. However, there was a problem in that even when TiO2 was used for the reduction of carbon dioxide (CO2), it was not possible to obtain excellent CO2 reduction photocatalytic performance (CO generation rate, CO selectivity).

[0010] This will be explained using the band structure of TiO2 (Figure 1). As shown in Figure 1, protons (H + ) Reduction potential (H2 / H + Assuming the voltage is 0V, the lower edge potential of the conduction band for TiO2 is -0.19V. On the other hand, the reduction potential of CO2 (CO / CO2) is -0.11V, and the difference with the lower edge potential of the conduction band is small. Because the overpotential is small, it is thought that the photocatalytic conversion of CO2 does not proceed effectively.

[0011] In view of these conventional problems, the inventors conducted diligent research. As a result, they found that in photocatalytic particles comprising titanium dioxide (TiO2) matrix particles, by supporting a co-catalyst containing silver (Ag) and cobalt (Co) on the surface of the matrix particles, and by performing Ag support by ultrasonic reduction, photocatalytic particles with excellent CO2 reduction photocatalytic performance can be obtained.

[0012] This invention was completed based on such findings, and aims to provide a method for producing photocatalytic particles with excellent CO2 reduction photocatalytic performance, which are equipped with TiO2-based matrix particles. [Means for solving the problem]

[0013] The present invention encompasses the following embodiments (1) to (7). In this specification, the expression "~" includes the numerical values ​​at both ends. That is, "X~Y" is synonymous with "X or more and Y or less". Furthermore, in this specification, any combination of preferred embodiments can be adopted as long as technical consistency can be maintained. For example, one of the preferred numerical ranges can be arbitrarily combined with the other.

[0014] (1) A method for producing carbon dioxide reduction photocatalytic particles, The carbon dioxide reduction photocatalyst particles comprise a base material particle mainly composed of titanium dioxide (TiO2), and a co-catalyst containing silver (Ag) and cobalt (Co) supported on the surface of the base material particle. The above method includes a step of supporting silver (Ag) and cobalt (Co) on the surface of the base material particles, A method for supporting silver (Ag) by ultrasonic reduction, which involves irradiating a reaction solution containing matrix particles, a silver (Ag) supply source, and a reducing solution with ultrasound.

[0015] (2) The method of (1) above, wherein the cobalt (Co) is supported by an ultrasonic reduction method in which ultrasound is irradiated onto a reaction solution containing matrix particles, a cobalt (Co) supply source and a reducing solution.

[0016] (3) The method of (2) above, wherein the cobalt (Co) source is at least one selected from the group consisting of cobalt(III) chloride, cobalt(II) hydroxide, cobalt(II) nitrate, and hydrates thereof.

[0017] (4) The titanium oxide (TiO2) contained in the matrix particles contains both anatase-type crystalline phase and rutile-type crystalline phase, according to any of the methods (1) to (3) above.

[0018] (5) Any of the methods (1) to (4) above, wherein the amount of silver (Ag) supported on the surface of the base material particles is 0.1% by mass or more and 1.0% by mass or less relative to the base material particles.

[0019] (6) Any of the methods (1) to (5) above, wherein the molar ratio (Co / Ag ratio) of the amount of cobalt (Co) supported on the surface of the base material particles to the amount of silver (Ag) supported is 0.2 or more and 2 or less.

[0020] (7) The silver (Ag) supported on the surface of the base material particles is silver (Ag) particles with an average particle size of 0.1 nm or more and 10 nm or less, according to any of the methods (1) to (6) above. [Effects of the Invention]

[0021] According to the present invention, a method for producing photocatalytic particles having excellent CO2 reduction photocatalytic performance, comprising TiO2-based matrix particles, is provided. [Brief explanation of the drawing]

[0022] [Figure 1] This shows the band structure of TiO2 and the reduction potential of CO2. [Figure 2] This shows the crystal structure of TiO2 (anatase and rutile forms). [Figure 3] This shows the CO2 reduction mechanism of photocatalyst particles supported by a co-catalyst. [Figure 4] An example of an evaluation apparatus for CO2 reduction photocatalyst performance evaluation testing is shown. [Figure 5] This paper illustrates the mechanism of metal Ag particle loading using ultrasonic reduction. [Figure 6] The manufacturing flow of photocatalytic particles is shown (Example A1). [Figure 7] The manufacturing flow of photocatalytic particles is shown (Example A2). [Figure 8] The manufacturing flow of photocatalytic particles is shown (Example A3). [Figure 9] The manufacturing flow of photocatalytic particles is shown (Example A4). [Figure 10] The manufacturing flow of photocatalytic particles is shown (Example A5). [Figure 11] The manufacturing flow of photocatalytic particles is shown (Example A6). [Figure 12] STEM and EDX elemental mapping images of photocatalytic particles are shown (Example A4). [Figure 13] STEM images of photocatalytic particles and the particle size distribution of Ag particles are shown (Examples A1 to A4). [Figure 14] The diffuse reflectance spectra of photocatalytic particles are shown (Examples A1 to A4). [Figure 15] The diffuse reflectance spectra of photocatalytic particles are shown (Examples A2, A4-A6). [Figure 16] The CO2 reduction photocatalytic performance of photocatalytic particles is shown (Examples A1 to A4). [Figure 17]The CO2 reduction photocatalytic performance of photocatalytic particles is shown (Examples A2, A4-A6). [Figure 18] The XRD patterns of the matrix particles (TiO2) are shown (Examples B1 to B7). [Figure 19] The CO2 reduction photocatalytic performance of photocatalytic particles is shown (Examples B3 and B6). [Figure 20] The CO2 reduction photocatalytic performance of the photocatalytic particles is shown (Examples B2, B4, and B5). [Figure 21] The CO2 reduction photocatalytic performance of the photocatalytic particles is shown (Examples B1, B3, and B4). [Figure 22] The CO2 reduction photocatalytic performance of the photocatalytic particles is shown (Examples B2, B6, and B7). [Figure 23] The CO2 reduction photocatalytic performance of photocatalytic particles is shown (Examples C1 to C4). [Figure 24] The CO2 reduction photocatalytic performance of photocatalytic particles is shown (Examples D1 to D5). [Figure 25] The CO2 reduction photocatalytic performance of the photocatalytic particles is shown (Examples E1 to E7). [Figure 26] The CO2 reduction photocatalytic performance of photocatalytic particles is shown (Example A4). [Figure 27] The CO2 reduction photocatalytic performance of photocatalytic particles is shown (Examples G1 to G4). [Figure 28] The relationship between light irradiation time and the amount of photocatalytic reaction products produced is shown (Example A4). [Modes for carrying out the invention]

[0023] Specific embodiments of the present invention (hereinafter referred to as "these embodiments") are described below. However, the present invention is not limited to the following embodiments, and various modifications are possible as long as they do not alter the essence of the invention. Furthermore, in this specification, any combination of preferred embodiments can be adopted as long as technical consistency can be maintained. For example, one of the preferred numerical ranges can be arbitrarily combined with the other.

[0024] <<1. Photocatalytic particles that reduce carbon dioxide>> The carbon dioxide reduction photocatalytic particles of this embodiment (hereinafter sometimes collectively referred to as "photocatalytic particles") include base particles and a cocatalyst supported on the surface of the base particles. The base particles contain titanium dioxide (TiO2) as a main component. The cocatalyst contains silver (Ag) and cobalt (Co). The photocatalytic particles of this embodiment exhibit peak A located in the wavelength range of 200 nm or more and 400 nm or less and peak B located in the wavelength range of more than 400 nm and 800 nm or less in the diffuse reflection spectrum, and the intensity (I A ) of peak B relative to the intensity (I B ) of peak A, i.e., the ratio (I B / I A ) is 50% or less.<

[0025] The photocatalytic particles of this embodiment are used for carbon dioxide (CO2) reduction. That is, they are used for the purpose of reducing carbon dioxide (CO2) to produce carbon monoxide (CO). CO is an important starting material in the chemical industry and industry. Therefore, photocatalytic particles with excellent CO2 reduction photocatalytic performance are useful for solving energy problems and environmental problems.

[0026] <Base particles> The base particles included in the photocatalytic particles are components that function as the main catalyst and contain titanium dioxide (TiO2) as the main component. Here, the main component is a component that occupies a ratio of 50% by mass or more in the base particles. By containing TiO2 as the main component, excellent CO2 reduction photocatalytic performance is brought about in combination with the effect of an appropriate cocatalyst. From the viewpoint of enhancing the catalytic performance, it is preferable that the ratio of the main component (TiO2) in the base particles is higher. The main component ratio may be 60% by mass or more, 70% by mass or more, 80% by mass or more, or 90% by mass or more.

[0027] Three types of crystal structures are known for TiO2: anatase, rutile, and brookite. Of these, the anatase and rutile types are used in practical applications. Figure 2 shows the crystal structures of anatase and rutile TiO2. Anatase TiO2 has a low-temperature stable tetragonal crystal structure with a band gap of 3.2 eV. Rutile TiO2 has a high-temperature stable tetragonal crystal structure with a band gap of 3.0 eV.

[0028] The TiO2 contained in the matrix particles of the embodiment may contain either anatase-type crystalline phase or rutile-type crystalline phase. However, from the viewpoint of further improving the CO2 reduction photocatalytic performance, it is preferable to contain anatase-type crystalline phase, and it is particularly preferable to contain both anatase-type and rutile-type crystalline phases. The reason for the improved photocatalytic performance should not be interpreted as limiting, but it is thought that the energy difference due to the difference in band gaps between anatase-type TiO2 and rutile-type TiO2 promotes electron transfer. In addition, since anatase-type TiO2 and rutile-type TiO2 have different light absorption ranges, it is thought that including a mixture of them can broaden the usable wavelength range of light.

[0029] When the TiO2 contained in the matrix particles includes an anatase-type crystalline phase, the full width at half maximum (FWHM) of the X-ray diffraction pattern based on the (101) plane of the anatase-type crystalline phase is preferably between 0.1° and 1.0°. Limiting the FWHM within this range further improves the CO2 reduction photocatalytic performance. The reason for this, although it should not be interpreted restrictively, is speculated as follows: A narrow FWHM indicates high crystallinity. High crystallinity results in fewer defect structures, which is thought to suppress the recombination of electrons and holes generated by charge separation.

[0030] The specific surface area (SSA) of the matrix particles is not particularly limited. However, the SSA of the matrix particles is 10 m². 2 / g or more 500m 2A value of less than / g is preferable. Limiting the SSA within this range further improves the CO2 catalytic performance. The reason for this is that, although it should not be interpreted as limiting, a larger specific surface area increases the area that absorbs light, and also increases the number of reaction sites.

[0031] <Auxiliary catalyst> The photocatalytic particles of this embodiment include a co-catalyst supported on the surface of the base material particles. The co-catalyst contains silver (Ag) and cobalt (Co). In other words, the base material particles support Ag and Co as co-catalysts.

[0032] The supported Ag undergoes charge separation (electron e) of photocatalytic particles irradiated with light. - and hole h + It promotes the separation and formation of carbon dioxide. Therefore, by supporting Ag on the surface of the matrix particles, the CO2 reduction photocatalytic performance (CO generation rate, CO selectivity) is significantly increased. The reason for this should not be interpreted in a limited way, but the following mechanism is possible. That is, even if the matrix particles themselves have little water reduction activity and oxidizing activity, by supporting Ag, which is a CO2 reduction site, oxygen generation occurs through the oxidation of water on the surface of the matrix particles. At the same time, CO generation occurs through CO2 reduction on the surface of the Ag co-catalyst, thereby improving catalytic performance.

[0033] The chemical state and form of the supported Ag are not limited. However, metallic Ag particles are preferred. The average particle size of the metallic Ag particles is preferably 0.1 nm to 10 nm, more preferably 0.1 nm to 5 nm, and even more preferably 0.1 nm to 1.5 nm. Here, the average particle size is the number-average particle size of the metallic Ag particles. This is determined by observing the photocatalytic particles with a transmission electron microscope (TEM), measuring the particle size of the supported metallic Ag particles, and calculating the number-average particle size from the measured particle size. More specifically, it can be determined by the method used in the examples described later or a similar method.

[0034] The amount of silver (Ag) supported on the surface of the base material particles is preferably more than 0% by mass and 8.0% by mass or less relative to the base material particles. By increasing the amount of Ag to a certain extent, the effect of Ag as a co-catalyst can be fully exerted. On the other hand, if the amount of Ag is excessively high, the number of surface active sites on the base material particles decreases, which may impair the function of the base material particles as the main catalyst. From the viewpoint of improving the CO2 reduction photocatalytic performance, the amount of Ag supported is more preferably 0.1% by mass or more and 1.0% by mass or less relative to the base material particles, and even more preferably 0.3% by mass or more and 0.5% by mass or less. Note that the amount of Ag supported is the amount of metallic Ag supported.

[0035] The supported Co, like Ag, undergoes charge separation of photocatalytic particles upon light irradiation (electron e - and hole h + It promotes the separation and formation of [components]. It also promotes the oxidation reaction and enhances activity. Therefore, by supporting Co together with Ag on the surface of the matrix particles, the CO2 reduction photocatalytic performance (CO generation rate, CO selectivity) is significantly increased.

[0036] The chemical state of the supported Co is not limited. It may be a metal, or a compound such as an oxyhydroxide or hydroxide. Furthermore, the form of the Co is not limited. It may be particulate or not. However, it is preferable that the Co is uniformly supported. In the following, the supported Co may be referred to as the Co species.

[0037] The molar ratio (Co / Ag ratio) of the amount of cobalt (Co) supported to the amount of silver (Ag) supported on the surface of the base material particles is preferably greater than 0 and less than or equal to 20. By increasing the amount of Co supported to a certain extent, the effect of Co as a co-catalyst can be fully exerted. On the other hand, if the amount of Co supported is excessively high, the number of surface active sites on the base material particles decreases, which may impair the function of the base material particles as the main catalyst. From the viewpoint of improving the CO2 reduction photocatalytic performance, a Co / Ag ratio of 0.1 to 10 is more preferable, 0.2 to 2 is even more preferable, and 0.2 to 1 is particularly preferable. Note that the amount of Co supported refers to the amount of metallic Co supported.

[0038] The photocatalytic particles of this embodiment exhibit a peak A located in the wavelength range of 200 nm to 400 nm and a peak B located in the wavelength range of over 400 nm to 800 nm in the diffuse reflectance spectrum. Furthermore, the intensity of peak A (I A The intensity of peak B relative to (I B ) ratio (I B / I A ) is 50% or less.

[0039] Peak intensity ratio in diffuse reflectance spectrum (I B / I A By controlling the peak intensity ratio (I), the CO2 reduction catalyst performance can be further improved. Specifically, in CO2 reduction photocatalyst performance evaluation tests, it is possible to achieve a higher CO generation rate while maintaining a high level of CO selectivity. B / I A The peak intensity ratio (I) is preferably 50% or less, and more preferably 48% or less. On the other hand, in order to fully exert the function of the co-catalyst (Ag particles), B / I A ) should be reasonably high. Peak intensity ratio (I B / I A The amount of ) is preferably 0.5% or more, more preferably 1% or more, and even more preferably 2% or more.

[0040] Furthermore, it is sufficient for at least one peak to exist in each of the wavelength ranges: 200 nm to 400 nm and 400 nm to 800 nm. If multiple peaks exist in each wavelength range, the intensity of the largest peak is used. A or I B It should be defined as follows.

[0041] A diffuse reflectance spectrum is an absorption spectrum obtained by the diffuse reflectance method. When light is shone on a solid sample such as a powder, some of the light escapes the sample through a scattering process. In the scattering process, some of the irradiated light is reflected from the sample surface, and the rest penetrates the sample. Some of the incident light that penetrates is absorbed by the electronic transition state of the sample, and the rest escapes the sample. An absorption spectrum can be obtained from the intensity of the incident light (ultraviolet or visible light) and the intensity of the light after the scattering process.

[0042] In the diffuse reflectance spectrum, there is a peak in a specific wavelength range (200 nm to 400 nm, and over 400 nm to 800 nm), and the peak intensity ratio (I B / I A The detailed mechanism by which photocatalytic particles with a certain valency (Ag,Co) within a predetermined range (50% or less) exhibit superior CO2 reduction photocatalytic performance is unknown. However, it is speculated that the particle size and electronic transition state of the supported co-catalyst (Ag,Co) may be involved. In other words, the spectrum obtained by diffuse reflectance (diffuse reflectance spectrum) reflects the electronic transition state of the sample, such as the valency of the species, coordination structure, and ligand field. Furthermore, when the sample is a fine particle, it may exhibit different absorption energies depending on its particle size. For example, when metallic Ag particles are fine particles, it is known that they give an absorption band in the wavelength range corresponding to plasmon resonance. Therefore, it is thought that the photocatalytic particles of this embodiment, which have a peak in a specific wavelength range, have supported co-catalysts (Ag,Co) that are fine and have a unique electronic transition state.

[0043] On the other hand, the particle size and electronic transition state of the co-catalyst (Ag,Co) are expected to affect the catalytic performance of the photocatalyst, such as the CO selectivity. This will be explained based on the CO2 reduction mechanism of co-catalyst-supported photocatalyst particles (Figure 3). Figure 3 shows the catalytic reaction when photocatalyst particles dispersed in an aqueous solution containing isopropanol (iPrOH,C3H7OH) as a hole scavenger are irradiated with light.

[0044] As shown in Figure 3, when light with energy hν is irradiated onto the particles, electrons (e) are released into the photocatalytic particles, which are semiconductors. - ) and holes (h + ) occurs. At this time, the co-catalyst (Ag,Co) performs charge separation (electron e - and hole h + It promotes the separation of (h + As a result of the reaction between the Co species and the surrounding isopropanol (iPrOH,C3H7OH), the reaction shown in equation (2) below proceeds to the right, resulting in the formation of acetone (CH3COCH3) and protons (H + ) is generated. Furthermore, electrons (e - ) carbon dioxide (CO2) and protons (H) on Ag particles + As a result of the reaction with (), the reactions shown in equations (3) and (4) below proceed to the right, producing carbon monoxide (CO), water (H2O), and hydrogen (H2). Furthermore, if the reactions in equations (2) to (4) below are combined, in principle the reaction shown in equation (5) below will proceed.

[0045]

number

number

number

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[0046] If the reactions in equation (3) and (4) occur to the same extent, the CO selectivity (CO evolution rate / (H2 evolution rate + CO evolution rate)) will be constant, as shown in equation (5) above. However, in reality, these reactions do not necessarily occur to the same extent. The reaction in equation (3) occurs preferentially, resulting in a higher CO selectivity.

[0047] In the reaction of equation (3) above, it has been reported that carbon dioxide (CO2) is adsorbed on the catalyst surface as a carbonate species, and upon light irradiation, it is transformed into a formate species, which is a reaction intermediate, and then interacts with water molecules to become carbon monoxide (CO). This report also suggests that the Ag co-catalyst promotes the formation of the reaction intermediate. Therefore, it is expected that by enhancing the charge separation effect and the reaction intermediate formation effect of metallic Ag particles (co-catalyst), the reaction of equation (3) above will occur preferentially, and the CO selectivity will be further improved.

[0048] In this regard, in photocatalytic particles that have a peak in a specific wavelength range of the diffuse reflectance spectrum and whose peak intensity ratio is within a predetermined range, it is thought that the supported Ag particles exist on the surface of the matrix particles in a fine state with a unique electronic state. It is thought that the presence of fine Ag particles with a unique electronic transition state increases the number of CO2 reduction sites on the photocatalytic particles compared to when they are supported by other methods, and that this leads to improved catalytic activity, especially the CO generation rate. Furthermore, it is thought that the presence of Co species in a highly dispersed state on the surface of the matrix particles containing TiO2 increases the number of oxidation sites, which contributes to the promotion of charge separation and improved activity.

[0049] The photocatalytic particles of this embodiment exhibit excellent catalytic performance, particularly in terms of CO selectivity. For example, in CO2 reduction photocatalytic performance evaluation tests, the CO selectivity is 30% or higher. This makes it possible to increase the proportion of CO gas generated by CO2 reduction. The CO selectivity may be 40% or higher, 50% or higher, 60% or higher, 70% or higher, 80% or higher, or even 90% or higher. There is no particular upper limit to the CO selectivity; it may be 100% or less.

[0050] Furthermore, the photocatalytic particles of this embodiment have a high CO generation rate. For example, in a CO2 reduction photocatalyst performance evaluation test, when 0.2 g of photocatalytic particles were dispersed in 200 mL of aqueous solution, the CO generation rate was 1.0 μmolh. -1 It is extremely high. The CO generation rate is 2.0 μmolh-1 The above is sufficient, and 3.0 μmolh -1 The above is sufficient, and 4.0 μmolh -1 That's fine too.

[0051] The CO2 reduction photocatalyst performance evaluation test can be performed using a known evaluation apparatus. An example of an evaluation apparatus is shown in Figure 4. The evaluation apparatus consists of a tank and a xenon (Xe) lamp installed outside the tank. The evaluation solution is placed inside the tank. The tank is also equipped with a gas inlet pipe, a gas outlet pipe, a pH meter, a rubber stopper, and a stirrer. A bubbling filter is provided at the end of the gas inlet pipe.

[0052] The evaluation test should be carried out as follows: Prepare an evaluation solution by mixing pure water, sodium bicarbonate (NaHCO3), isopropanol, and the sample (photocatalytic particles). Place this evaluation solution in the tank of the evaluation apparatus and stir with a stirrer. Blow carbon dioxide (CO2) gas through the gas inlet tube, and at the same time irradiate the evaluation solution with light from a xenon (Xe) lamp. After irradiation for a predetermined time, introduce the generated gas through the gas exhaust tube into a gas chromatograph and analyze it there. Determine the generation rate of hydrogen (H2) gas and carbon monoxide (CO) gas from this analysis. Using the obtained generation rates, calculate the CO selectivity based on equation (1) below.

[0053]

number

[0054] The photocatalytic particles of this embodiment, as described above, possess titanium dioxide (TiO2)-based matrix particles that have the excellent advantages of being inexpensive, stable, harmless, and non-toxic, while exhibiting excellent CO2 reduction photocatalytic performance (CO generation rate, CO selectivity). Such photocatalytic particles are extremely useful in solving energy and environmental problems.

[0055] Furthermore, the manufacturing method of the photocatalytic particles in this embodiment is not limited as long as the above requirements are satisfied. However, it is preferable that the photocatalytic particles are obtained by a manufacturing method that includes a step of supporting silver (Ag) and cobalt (Co) on the surface of a base material particle (co-catalyst support step), and the silver (Ag) support is performed by an ultrasonic reduction method in which ultrasound is irradiated onto a reaction solution containing the base material particle, a silver (Ag) supply source, and a reducing solution. It is even more preferable that the photocatalytic particles are obtained by a manufacturing method in which the silver (Ag) support is performed by an ultrasonic reduction method in which ultrasound is irradiated onto a reaction solution containing the base material particle, a silver (Ag) supply source, and a reducing solution, and the Co support is further performed by an ultrasonic reduction method in which ultrasound is irradiated onto a reaction solution containing the base material particle, a cobalt (Co) supply source, and a reducing solution.

[0056] <<2. Method for producing carbon dioxide reduction photocatalytic particles>> This embodiment also covers a method for producing carbon dioxide reduction photocatalytic particles. These carbon dioxide reduction photocatalytic particles comprise a base material particle mainly composed of titanium dioxide (TiO2) and a co-catalyst containing silver (Ag) and cobalt (Co) supported on the surface of the base material particle. The production method of this embodiment includes a step of supporting silver (Ag) and cobalt (Co) on the surface of the base material particle (co-catalyst supporting step). Furthermore, the silver (Ag) is supported by an ultrasonic reduction method in which ultrasound is irradiated onto a reaction solution containing the base material particle, a silver (Ag) supply source, and a reducing solution.

[0057] <Catalyst support process> In the co-catalyst loading step, silver (Ag) and cobalt (Co) are loaded onto the surface of the base material particles. Through this step, photocatalytic particles of this embodiment are obtained, comprising base material particles and a co-catalyst containing Ag and cobalt (Co). The Ag loading and Co loading in the co-catalyst loading step may be performed simultaneously or separately. Furthermore, if performed separately, the order of Ag loading and Co loading is not limited.

[0058] (Ag-carrying) The manufacturing method of this embodiment is characterized by performing Ag loading by ultrasonic reduction, which involves irradiating a reaction solution containing matrix particles, an Ag supply source, and a reducing solution with ultrasound. This makes it possible to produce photocatalytic particles with excellent CO2 reduction photocatalytic performance (CO generation rate, CO selectivity).

[0059] For Ag loading using ultrasonic reduction, a reaction solution containing matrix particles, an Ag supply source, and a reducing solution is first prepared. Details of the matrix particles used for co-catalyst loading are as previously described. Specifically, the matrix particles mainly contain titanium dioxide (TiO2). The proportion of the main component (TiO2) in the matrix particles may be 60% by mass or more, 70% by mass or more, 80% by mass or more, or 90% by mass or more. The TiO2 contained in the matrix particles may contain either anatase-type crystalline phase or rutile-type crystalline phase. However, it is preferable that it contains anatase-type crystalline phase, and it is particularly preferable that it contains both anatase-type and rutile-type crystalline phases. When the TiO2 contained in the matrix particles contains anatase-type crystalline phase, the full width at half maximum (FWHM) of the X-ray diffraction pattern based on the (101) plane of the anatase-type crystalline phase is preferably 0.1° to 1.0°. The specific surface area (SSA) of the matrix particles is 10 m². 2 / g or more 500m 2 Preferably less than / g

[0060] The source of Ag is not limited as long as it can supply Ag. Specifically, it can be at least one selected from the group consisting of oxides, inorganic metal salts, and organometallic compounds. Examples of inorganic metal salts include nitrates, chlorides, and / or sulfates. The source of Ag may or may not dissolve in the reducing solution. A preferred source of Ag is silver oxide. Since silver oxide consists only of silver ions and oxygen ions, it is easy to handle and does not pose problems such as waste disposal. Silver oxide is known in the form of Ag2O, AgO, and Ag2O3, which have different oxidation states of silver, and all can be used. However, Ag2O is preferred because it is more readily available. The size of the source of Ag is also not particularly limited. For example, the average particle size of the source of Ag is 0.3 to 3.0 μm.

[0061] The reducing solution is not limited as long as it is a liquid with reducing properties. It may be a liquid that has reducing properties itself, or it may be a liquid in which a reducing agent has been dissolved. However, it is preferable that it be a liquid that has reducing properties itself. It is also not necessary to contain a separate reducing agent. As such a reducing solution, alcohols such as ethanol and propanol are preferred because they have low toxicity and are readily available. A mixture of alcohols and water can also be used. However, when using a mixture, if the water content is excessively high, it will be difficult to exert a sufficient reducing effect. Therefore, the water content in the reducing solution is preferably 50% by volume or less, and more preferably 25% by volume or less. The lower limit of the water content is not particularly limited and may be 0% by volume.

[0062] The reaction solution can be prepared by adding the base material particles and the Ag supply source to the reducing solution. The proportions of base material particles and Ag supply source should be determined based on the desired Ag load of the photocatalytic particles. In other words, increasing the proportion of Ag supply source relative to the amount of base material particles will yield photocatalytic particles with a high Ag load, while decreasing the proportion of Ag supply source relative to the amount of base material particles will yield photocatalytic particles with a low Ag load.

[0063] Next, the resulting reaction solution is irradiated with ultrasound. When irradiated with ultrasound, the surface of the Ag supply source is reduced, generating metallic Ag particles, which are then supported on the surface of the base material particles. Thus, base material particles with supported metallic Ag particles are obtained.

[0064] The mechanism of metal Ag particle loading is explained with reference to Figure 5. When ultrasound is irradiated, compression waves are generated in the reaction solution, and these compression waves generate repeated positive and negative pressure. During the negative pressure cycle, countless tiny bubbles are generated in the reaction solution due to evaporation. During the positive pressure cycle, these bubbles collapse, exerting a strong impact force on the surroundings. This phenomenon is called ultrasonic cavitation. Cavitation uniformly disperses the matrix particles and Ag source in the reaction solution and cleans their surfaces. Cavitation also generates tiny, high-temperature, high-pressure hot spots. These hot spots decompose and reduce the Ag source and act on the reaction solution to generate radicals, which further promote the decomposition and reduction of the Ag source. In this way, metal Ag particles are generated from the Ag source.

[0065] For example, when solid silver oxide (Ag2O) is used as the Ag source, hot spots and radicals act on the Ag2O, causing it to decompose and be reduced on the surface, resulting in the precipitation of Ag particles. These Ag particles gradually grow, and once they reach a certain size, the interfacial stress between the Ag2O and the Ag particles becomes critical, causing them to desorb. Alternatively, ultrasonic waves can generate intermediate products from the Ag2O, and hot spots and radicals act on these intermediate products to generate Ag particles in the reaction solution. The desorbed or generated Ag particles move to the surface of the matrix particles due to the physical action of the ultrasonic waves and are adsorbed there. In this way, matrix particles supporting Ag particles are obtained. The Ag particles produced by ultrasonic reduction are fine. Furthermore, since organic protective agents and high-temperature calcination are unnecessary, it is possible to produce matrix particles (photocatalytic particles) supporting Ag particles in a fine state.

[0066] If photocatalytic particles contain Ag sources other than Ag particles, it can be difficult to fully utilize the co-catalyst effect. In this regard, sonication makes it possible to obtain photocatalytic particles that contain almost no Ag sources (such as silver oxide). For example, it is possible to obtain photocatalytic particles in which no peak of silver oxide (Ag2O) is observed in the X-ray diffraction pattern.

[0067] Special equipment is not necessary for ultrasonic treatment, and an apparatus equipped with a normal ultrasonic oscillator may be used. For example, a commercially available ultrasonic cleaner may be used. The treatment may also be carried out under normal conditions. For example, the frequency of the ultrasonic waves may be 20 to 100 kHz. The ultrasonic treatment may be continuously performed at the same frequency, or the frequency may be switched during the treatment using a frequency oscillation switching mode. The number of times of frequency switching may be once or a plurality of times. By treating at a low frequency (for example, 26 kHz), it becomes possible to further improve the dispersibility of the base material particles in the liquid. Also, the output of the ultrasonic waves may be 10 to 500 W, or may be 50 to 200 W. Furthermore, the treatment time may be 1 to 10 hours. By increasing 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 the cocatalyst (Ag particles) supported. The treatment time is preferably 2 hours or more and 10 hours or less.

[0068] The product obtained by ultrasonic treatment (base material particles supporting Ag particles) exists in a dispersed or precipitated state in the reaction solution. Therefore, the product may be recovered from the reaction solution and dried. Known separation means such as filtration or centrifugation may be used for the recovery. Also, the drying may be carried out under conditions where the Ag particles do not undergo excessive grain growth, for example, at 100°C or lower.

[0069] The amount of Ag supported in the finally obtained photocatalyst particles can be adjusted by controlling the blending amount of the Ag source and the conditions of the Ag supporting method. The amount of Ag supported on the surface of the base material particles is preferably more than 0 mass% and 8.0 mass% or less, more preferably 0.1 mass% or more and 1.0 mass% or less, and even more preferably 0.3 mass% or more and 0.5 mass% or less with respect to the base material particles.

[0070] <Co loading> The method of Co loading is not particularly limited. It may be carried out by ultrasonic reduction, similar to Ag loading, or by other methods such as photoelectrodeposition, chemical reduction, or impregnation. However, from the viewpoint of obtaining photocatalytic particles with even better CO2 reduction photocatalytic performance, it is preferable to use ultrasonic reduction. That is, it is preferable to carry out Co loading by ultrasonic reduction, in which ultrasound is irradiated onto a reaction solution containing matrix particles, a cobalt (Co) supply source, and a reducing solution.

[0071] When performing Co-supporting using ultrasonic reduction, a reaction solution containing matrix particles, a Co supply source, and a reducing solution is first prepared. Details of the matrix particles and reducing solution are as described above. Specifically, the matrix particles used in preparing the reducing solution mainly contain titanium dioxide (TiO2). The reducing solution is not limited as long as it is a liquid with reducing properties, but alcohols such as ethanol or propanol, which have low toxicity and are readily available, are preferred. The matrix particles used for Ag-supporting and Co-supporting are the same. On the other hand, the reducing solutions may be the same or different. However, when performing Ag-supporting and Co-supporting simultaneously, the same reducing solution is naturally used.

[0072] The Co source is not limited as long as it can supply Co. Specifically, it can be at least one selected from the group consisting of oxides, inorganic metal salts, and organometallic compounds. More specifically, it can be cobalt(III) chloride (CoCl3), cobalt(II) hydroxide (Co(OH)2), cobalt(II) nitrate (Co(NO3)2), or cobalt(II,III) oxide (CoO, Co2O3, Co3O4, etc.). However, from the viewpoint of obtaining photocatalytic particles with excellent photocatalytic performance, it is preferable that the Co source is at least one selected from the group consisting of cobalt(III) chloride, cobalt(II) hydroxide, cobalt(II) nitrate, and their hydrates.

[0073] Next, the resulting reaction solution is irradiated with ultrasound. The ultrasound irradiation conditions are the same as those for Ag support. When ultrasound is irradiated, Co moves from the Co supply source and is supported on the surface of the matrix particles as Co species. However, the mechanism of Co support by ultrasound irradiation is not necessarily the same as the mechanism of Ag support. Also, the chemical state and form of the supported Co species are not necessarily the same as those of Ag. The supported Co species may be a metal, or a compound such as an oxyhydroxide or hydroxide. Furthermore, the Co species may be particulate or not.

[0074] As mentioned above, in the manufacturing method of this embodiment, Ag loading and Co loading may be performed simultaneously or separately. For example, Ag loading and Co loading may be performed simultaneously by ultrasonic reduction, in which case ultrasound is applied to a reaction solution containing the base material particles, Ag supply source, Co supply source, and reducing solution. When Ag loading and Co loading are performed separately, a method may be used in which ultrasound is applied to a reaction solution containing the base material particles, Ag supply source, and reducing solution, a Co supply source is added to the reaction solution after ultrasound application, and ultrasound is applied again, or conversely, a method may be used in which ultrasound is applied to a reaction solution containing the base material particles, Co supply source, and reducing solution, an Ag supply source is added to the reaction solution after ultrasound application, and ultrasound is applied again.

[0075] Although the method of Co-supporting using ultrasonic reduction has been described above, the manufacturing method of this embodiment is not limited to the method of Co-supporting using ultrasonic reduction. Co-supporting may also be performed by other methods such as photoelectrodeposition, chemical reduction, or impregnation.

[0076] When performing Co-supporting by photodeposition (PD), known methods can be employed. For example, a mixture containing water, matrix particles (TiO2 particles or Ag-supported TiO2 particles, etc.) and a water-soluble Co salt can be prepared, and the resulting mixture can be irradiated with UV light. Upon UV irradiation, Co ions in the mixture precipitate and are supported on the surface of the matrix particles. Examples of water-soluble Co salts include nitrate Co(Co(NO3)2).

[0077] When performing Co-supporting by chemical deposition (CR), known methods can be employed. For example, a mixture containing water, matrix particles (TiO2 particles or Ag-supported TiO2 particles, etc.), a water-soluble Co salt, and a reducing agent can be prepared. A chemical reduction reaction can then be carried out in the resulting mixture to form a product, which can then be recovered from the resulting slurry and dried. This yields Co-supported matrix particles. Examples of water-soluble Co salts include Co nitrate. Examples of reducing agents include sodium phosphinate (NaH2PO2·H2O).

[0078] When performing Co-supporting by the impregnation (IMP) method, known techniques can be employed. For example, a mixture containing water, an aqueous solution of Co salt, and a base material particle (TiO2 particle or Ag-supported TiO2 particle, etc.) can be prepared, and the resulting mixture can be dried and calcined. This yields Co-supported base material particles. Examples of water-soluble Co salts include Co nitrate.

[0079] The amount of Co supported in the final photocatalytic particles can be adjusted by controlling the amount of Co source used and the conditions of the Co support method. The molar ratio of cobalt (Co / Ag) to silver (Ag) supported on the surface of the matrix particles (Co / Ag ratio) is preferably greater than 0 and 20 or less, more preferably between 0.1 and 10, even more preferably between 0.2 and 2, and particularly preferably between 0.2 and 1.

[0080] <Heat treatment process> If necessary, the Ag-supported matrix particles, the Co-supported matrix particles, or the Ag and Co-supported matrix particles may be heat-treated. The heat treatment is preferably carried out at 100°C or higher in an oxygen-containing atmosphere. No special equipment is required for the heat treatment; a general electric furnace capable of firing in air can be used. However, the photocatalytic particles of this embodiment are not limited to those that have been heat-treated. Photocatalytic particles with excellent catalytic activity can be obtained without heat treatment.

[0081] In this way, photocatalytic particles consisting of Ag and Co-supported matrix particles can be obtained. By employing this manufacturing method, it is possible to easily obtain photocatalytic particles with excellent CO2 reduction photocatalytic performance (CO generation rate, CO selectivity). The detailed reason for this is unknown, but it is presumed that the supported Ag particles produced by ultrasonic reduction are fine and possess a unique electronic transition state.

[0082] In other words, it is thought that the majority of the silver (Ag) produced by ultrasonic reduction treatment is in an ultrafine state. Furthermore, it is thought that the high temperature and high pressure hot spots and radicals generated during ultrasonic treatment create a unique electronic transition state. In fact, there are reports that the hot spots generated by ultrasound reach temperatures of nearly 5000°C, and it is easy to predict that even a momentary action by such high-temperature hot spots will change the electronic transition state. It is hypothesized that the combined action of this fine particle size and unique electronic transition state leads to an improvement in the CO2 reduction photocatalyst performance.

[0083] Furthermore, this manufacturing method allows, though not limited to, the use of compounds such as silver oxide that do not dissolve in the reaction solution as an Ag supply source in a solid state. When compounds that do not dissolve in the reaction solution are used, the reaction solution does not contain harmful substances such as anions, and wastewater treatment is easy. However, the manufacturing method of this embodiment does not exclude the use of compounds that dissolve in the reaction solution. Even in such cases, the effect of precipitating metallic Ag particles with high dispersion and high support rate can be obtained. [Examples]

[0084] The present invention will be described in more detail using the following examples and comparative examples. However, the present invention is not limited to the following examples.

[0085] [Experimental Example A] In Experimental Example A, co-catalysts (Ag,Co) were supported on TiO2-based matrix particles using different methods, and the effect of the support method on the CO2 reduction photocatalytic performance was investigated.

[0086] (1) Production of photocatalytic particles [Example A1 (Comparative Example)] In Example A1, the co-catalyst (Ag,Co) was supported by photoelectrolysis (PD method). The manufacturing flow for Example A1 is shown in Figure 6.

[0087] First, titanium dioxide (TiO2) particles were prepared as the matrix particles. JRC-TIO-4(2) (manufactured by Nippon Aerosil Co., Ltd.), a reference catalyst of the Catalysis Society of Japan, was used as the TiO2 particles. In addition, 0.1 M aqueous solutions of silver nitrate (AgNO3 aqueous solution, Fujifilm Wako Pure Chemical Industries, Ltd.) and cobalt nitrate (Co(NO3)2 aqueous solution, Fujifilm Wako Pure Chemical Industries, Ltd.) were prepared as Ag and Co raw materials, respectively.

[0088] A mixture was prepared by adding 1.2 g of prepared TiO2 particles, 0.56 mL of AgNO3 aqueous solution, and 0.56 mL of Co(NO3)2 aqueous solution to 1 L of ultrapure water. Next, the mixture was purged with argon (Ar) gas, and then the mixture was irradiated with UV light from a 400 W high-pressure Hg lamp for 3 hours while blowing Ar gas at a flow rate of 30 mL / min to reduce the Ag ions. The mixture after light irradiation was filtered to recover the powder, and the recovered powder was dried at room temperature to obtain photocatalytic particles consisting of AgCo / TiO2 particles (TiO2 particles supported with Ag and Co). In the obtained photocatalytic particles, the amount of Ag supported was 0.5 mass% relative to the TiO2 particles. The molar ratio of the amount of Co supported to the amount of Ag supported (Co / Ag ratio) was 1.

[0089] [Example A2 (comparative example)] In Example A2, the co-catalyst (Ag,Co) was supported by a chemical reduction method (CR method). The manufacturing flow for Example A2 is shown in Figure 7.

[0090] First, TiO2 particles (JRC-TIO-4(2)) similar to those in Example A1 were prepared as the matrix particles. In addition, 0.1 M aqueous solutions of silver nitrate (Fujifilm Wako Pure Chemical Industries, Ltd., AgNO3 aqueous solution) and 0.1 M aqueous solution of cobalt nitrate (Fujifilm Wako Pure Chemical Industries, Ltd., Co(NO3)2 aqueous solution) were prepared as Ag and Co raw materials, respectively. Furthermore, sodium phosphinate monohydrate (Fujifilm Wako Pure Chemical Industries, Ltd., NaH2PO2·H2O) was prepared as a reducing agent, and this was mixed with ultrapure water to prepare a 0.4 M aqueous solution of sodium phosphinate (NaH2PO2 aqueous solution).

[0091] A mixture was prepared by adding 0.75 g of prepared TiO2 particles to 50 mL of ultrapure water, and the resulting mixture was maintained at 80°C in a water bath. Next, 0.35 mL of AgNO3 aqueous solution (0.1 M), 0.35 mL of Co(NO3)2 aqueous solution (0.1 M), and 0.75 mL of NaH2PO2 aqueous solution (0.4 M) were added to the mixture, and the mixture was stirred at 80°C for 1.5 hours to induce a chemical reduction reaction. The liquid (slurry) after chemical reduction was filtered to recover the resulting powder, and the recovered powder was dried at room temperature to obtain photocatalytic particles consisting of AgCo / TiO2 particles. In the obtained photocatalytic particles, the amount of Ag supported was 0.5 mass% relative to the TiO2 particles. The molar ratio of the amount of Co supported to the amount of Ag supported (Co / Ag ratio) was 1.

[0092] [Example A3 (Comparative Example)] In Example A3, the co-catalyst (Ag,Co) was supported using the impregnation method (IMP method). The manufacturing flow for Example A3 is shown in Figure 8.

[0093] First, TiO2 particles (JRC-TIO-4(2)) similar to those in Example A1 were prepared as the matrix particles. In addition, 0.1 M aqueous solutions of silver nitrate (Fujifilm Wako Pure Chemical Industries, Ltd., AgNO3 aqueous solution) and 0.1 M aqueous solution of cobalt nitrate (Fujifilm Wako Pure Chemical Industries, Ltd., Co(NO3)2 aqueous solution) were prepared as the Ag and Co raw materials, respectively.

[0094] 1.2 g of prepared TiO2 particles and 20 mL of ultrapure water were mixed, and the resulting mixture was maintained at 80°C in a water bath. Next, 0.56 mL of AgNO3 aqueous solution and 0.56 mL of Co(NO3)2 aqueous solution were added to the mixture, and the mixture was stirred at 80°C for 1 hour. The heated and stirred mixture was dried in air at 80°C for 3 hours, and the resulting dried material was calcined at 450°C for 2 hours to obtain photocatalytic particles consisting of AgCo / TiO2 particles. In the obtained photocatalytic particles, the amount of Ag supported was 0.5% by mass relative to the total amount of photocatalytic particles. The molar ratio of the amount of Co supported to the amount of Ag supported (Co / Ag ratio) was 1.

[0095] [Example A4] In Example A4, the co-catalyst (Ag,Co) was supported using the ultrasonic reduction method (USR method). The manufacturing flow for Example A4 is shown in Figure 9.

[0096] First, TiO2 particles (JRC-TIO-4(2)) similar to those in Example A1 were prepared as the matrix particles. In addition, silver oxide particles (Fujifilm Wako Pure Chemical Industries, Ltd., Ag2O particles) and cobalt hydroxide particles (Fujifilm Wako Pure Chemical Industries, Ltd., Co(OH)2 particles) were prepared as Ag raw materials (Ag source) and Co raw materials (Co source). Furthermore, isopropanol (Fujifilm Wako Pure Chemical Industries, Ltd., i-PrOH) was prepared as the reducing solution.

[0097] A reaction solution was prepared by adding 1.2 g of TiO2 particles, 6.5 mg of Ag2O particles, and 5.2 mg of Co(OH)2 particles to 50 mL of i-PrOH. The resulting reaction solution was then subjected to ultrasonic treatment using an ultrasonic device (QUAVA mini, Kaijo Co., Ltd.). The ultrasonic treatment was performed at a frequency of 26 kHz and an output of 100 W for 2 hours. The temperature of the reaction solution was maintained at 40°C during the ultrasonic treatment. This treatment reduced the Ag2O in the reaction solution to metallic Ag. The product generated by the treatment was recovered by filtration, washed with i-PrOH, and dried in air at 70°C for 1 hour to obtain photocatalytic particles consisting of AgCo / TiO2 particles. In the obtained photocatalytic particles, the amount of Ag supported was 0.5 mass% relative to the TiO2 particles. The molar ratio of the amount of Co supported to the amount of Ag supported (Co / Ag ratio) was 1.

[0098] [Example A5 (Comparative Example)] In Example A5, the base material particles (TiO2 particles) were ultrasonically treated, and the resulting treated TiO2 particles were then supported with a co-catalyst (Ag,Co) using a chemical reduction method (CR method). The manufacturing flow for Example A5 is shown in Figure 10.

[0099] First, TiO2 particles (JRC-TIO-4(2)) similar to those in Example A1 were prepared as the base material particles, and isopropanol (Fujifilm Wako Pure Chemical Industries, Ltd., i-PrOH) was prepared as the reducing solution. In addition, 0.1 M aqueous solutions of silver nitrate (Fujifilm Wako Pure Chemical Industries, Ltd., AgNO3 aqueous solution) and 0.1 M aqueous solution of cobalt nitrate (Fujifilm Wako Pure Chemical Industries, Ltd., Co(NO3)2 aqueous solution) were prepared as the Ag and Co raw materials, respectively. Furthermore, sodium phosphinate monohydrate (Fujifilm Wako Pure Chemical Industries, Ltd., NaH2PO2·H2O) was prepared as a reducing agent, and this was mixed with ultrapure water to prepare a 0.4 M aqueous solution of sodium phosphinate (NaH2PO2 aqueous solution).

[0100] A reaction solution was prepared by adding 1.2 g of prepared TiO2 particles to 50 mL of i-PrOH. The resulting reaction solution was then subjected to ultrasonic treatment using an ultrasonic device (QUAVA mini, Kaijo Co., Ltd.). The ultrasonic treatment was performed at a frequency of 26 kHz and an output of 100 W for 3 hours. The temperature of the reaction solution was maintained at 40°C during ultrasonic treatment. After ultrasonic treatment, the TiO2 particles were recovered from the reaction solution by filtration, washed with i-PrOH, and dried in air at 70°C for 1 hour.

[0101] A mixture was prepared by adding 0.75 g of sonicated and dried TiO2 particles to 50 mL of ultrapure water, and the resulting mixture was maintained at 80°C in a water bath. Next, 0.35 mL of AgNO3 aqueous solution (0.1 M), 0.35 mL of Co(NO3)2 aqueous solution (0.1 M), and 0.75 mL of NaH2PO2 aqueous solution (0.4 M) were added to the mixture, and the mixture was stirred at 80°C for 1.5 hours to induce a chemical reduction reaction. The liquid (slurry) after chemical reduction was filtered to recover the resulting powder, and the recovered powder was dried at room temperature to obtain photocatalytic particles consisting of AgCo / TiO2 particles. In the obtained photocatalytic particles, the amount of Ag supported was 0.5 mass% relative to the TiO2 particles. The molar ratio of the amount of Co supported to the amount of Ag supported (Co / Ag ratio) was 1.

[0102] [Example A6] In Example A6, Ag was loaded using the ultrasonic reduction method (USR method), followed by Co loading using the chemical reduction method. The manufacturing flow for Example A6 is shown in Figure 11.

[0103] First, TiO2 particles (JRC-TIO-4(2)) similar to those in Example A1 were prepared as the base material particles, and isopropanol (Fujifilm Wako Pure Chemical Industries, Ltd., i-PrOH) was prepared as the reducing solution. In addition, silver oxide particles (Fujifilm Wako Pure Chemical Industries, Ltd., Ag2O particles) and a 0.1 M cobalt nitrate aqueous solution (Fujifilm Wako Pure Chemical Industries, Ltd., Co(NO3)2 aqueous solution) were prepared as the Ag raw material (Ag supply source) and Co raw material, respectively. Furthermore, sodium phosphinate monohydrate (Fujifilm Wako Pure Chemical Industries, Ltd., NaH2PO2·H2O) was prepared as a reducing agent, and this was mixed with ultrapure water to prepare a 0.4 M sodium phosphinate aqueous solution (NaH2PO2 aqueous solution).

[0104] A reaction solution was prepared by adding 1.2 g of TiO2 particles and 6.5 mg of Ag2O particles to 50 mL of i-PrOH. The resulting reaction solution was then subjected to ultrasonic treatment using an ultrasonic device (QUAVA mini, Kaijo Co., Ltd.). The ultrasonic treatment was performed at a frequency of 26 kHz and an output of 100 W for 2 hours. The temperature of the reaction solution was maintained at 40°C during the ultrasonic treatment. This treatment reduced the Ag2O in the reaction solution to metallic Ag. The product generated by the treatment was recovered by filtration, washed with i-PrOH, and dried in air at 70°C for 1 hour to obtain Ag / TiO2 particles.

[0105] Next, 0.75 g of the obtained Ag / TiO2 particles were added to 50 mL of ultrapure water to prepare a mixture, and the resulting mixture was maintained at 80°C in a water bath. Then, 0.35 mL of Co(NO3)2 aqueous solution (0.1 M) and 1.5 mL of NaH2PO2 aqueous solution (0.4 M) were added to the mixture, and the mixture was stirred at 80°C for 1.5 hours to induce a chemical reduction reaction. The liquid (slurry) after chemical reduction was filtered to recover the resulting powder, and the recovered powder was dried at room temperature to obtain photocatalytic particles consisting of AgCo / TiO2 particles. In the obtained photocatalytic particles, the amount of Ag supported was 0.5 mass% relative to the TiO2 particles. The molar ratio of the amount of Co supported to the amount of Ag supported (Co / Ag ratio) was 1.

[0106] (2) Evaluation For the samples (photocatalyst particles) obtained in Examples A1 to A6, various properties were evaluated as follows.

[0107] <STEM Observation> The samples were observed with a scanning transmission electron microscope (STEM). Also, the elemental distribution was examined by energy dispersive X-ray spectroscopy (EDX) using an analyzer attached to the microscope. STEM observation and EDX analysis were performed at a magnification of 5 million times. Further, from the TEM images obtained by the observation, the particle size of the cocatalyst Ag particles supported on the base material particles was determined. Specifically, the TEM images were imported into image analysis software (imageJ). Then, focusing on one Ag particle in the image, its average diameter (particle size) was determined. The same operation was performed for 200 particles, and the distribution and average value (average particle size) of the particle size based on the number were determined.

[0108] <Diffuse Reflection Spectrum> The diffuse reflection spectrum of the sample in the solid state was measured using an ultraviolet-visible near-infrared spectrophotometer (JASCO, V-650). The measurement was performed under the conditions of a bandwidth: 1.0 nm, a measurement range: 800 - 200 nm, a data acquisition interval: 0.1 nm, and a scanning speed: 200 nm / min.

[0109] <CO2 Reduction Photocatalytic Performance> The CO2 reduction photocatalytic performance of the sample was evaluated using the evaluation apparatus shown in Fig. 4. First, a mixed solution (200 mL) of isopropanol (iPrOH) and ultrapure water prepared to 0.1 M, NaHCO3 (0.1 M), and photocatalyst particles (0.2 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, it was irradiated with light using a 300 W xenon (Xe) 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 and CO. Then, the CO selectivity was calculated based on the following formula (1). Also, the filtrate of the reaction solution was analyzed using capillary GC.

[0110]

Equation

[0111] (3) Evaluation results <STEM Observation> The STEM image and EDX elemental mapping image of the photocatalytic particles of Example A4 are shown in Fig. 12. Ag particles were observed on the surface of the base particles (TiO2). On the other hand, the Co species were uniformly distributed.

[0112] The STEM images of the photocatalytic particles of Examples A1 to A4 are shown in Fig. 13. The particle size distribution of the Ag particles determined from the STEM images is also shown.

[0113] In Examples A1 to A3 in which the cocatalyst was supported by the photoelectrodialysis method (PD method), the chemical reduction method (CR method), or the impregnation method (IMP method), the average particle diameter of the supported Ag particles was 2.0 to 2.4 nm. In contrast, in Example A4 in which the cocatalyst was supported by the ultrasonic reduction method (USR method), the average particle diameter of the supported Ag particles was as fine as 1.2 nm. Also, in Examples A3 and A4, it was found that the particle size distribution of the Ag particles was sharp and they were uniformly supported on the surface of the base particles (TiO2 particles).

[0114] <Diffuse reflection spectrum> The diffuse reflection spectra obtained for the photocatalytic particles of Examples A1 to A6 are shown in Figs. 14 and 15. In Figs. 14 and 15, the diffuse reflection spectra of the TiO2 particles as the base particles are also shown.

[0115] In the diffuse reflection spectra of photocatalyst particle samples (Examples A1 to A6) supporting cocatalysts (Ag, Co), peaks (Peak A and Peak B) are observed in two wavelength ranges: a wavelength range of 200 nm or more and 400 nm or less, and a wavelength range of more than 400 nm and 800 nm or less. In contrast, in the diffuse reflection spectrum of only the base material (TiO2) particles, although a peak is observed in the wavelength range of 200 nm or more and 400 nm or less, no peak is observed in the wavelength range of more than 400 nm and 800 nm or less. From this, the peak (Peak A) in the wavelength range of 200 nm or more and 400 nm or less is considered to be caused by the light absorption of TiO2 particles, and the peak (Peak B) in the wavelength range of more than 400 nm and 800 nm or less is considered to be caused by the plasmon of the supported Ag nanoparticles or the light absorption (d-d transition) of Co species.

[0116]

[0119] In Example A4 in which the cocatalyst (Ag, Co) was supported by the ultrasonic reduction method (USR method), the peak intensity ratio (I B / I A ) was 3.62%, and in Example A3 in which the cocatalyst was supported by the impregnation method (IMP method), the peak intensity ratio (I B / I A ) was 3.76%. In contrast, in Example A2 in which the cocatalyst was supported by the chemical reduction method (CR method), the peak intensity ratio was 52.3%, and in Example A1 in which the catalyst was supported by the photoelectrodeposition method (PD method), the peak intensity ratio was 76.4%.

[0117] <CO2 Reduction Photocatalyst Performance> The CO2 reduction photocatalyst performance (H2 and CO generation rates, CO selectivity) obtained for the photocatalyst particles of Examples A1 to A4 is shown in FIG. 16.

[0118] In Examples A1 to A with the cocatalyst (Ag, Co) supported by the photoelectrodeposition method, chemical reduction method, or impregnation method, the CO generation rate was relatively small at 1.5 μmolh -1 . Among these, the samples of Examples A1 and A2 had a CO selectivity of less than 7%. In contrast, in Example A4 with the cocatalyst supported by the ultrasonic reduction method, the CO generation rate was relatively large at 2.5 μmolh -1 . Also, the CO selectivity was very high at 95% or more.

[0119] Figure 17 shows the CO2 reduction photocatalytic performance (H2 and CO generation rate, CO selectivity) obtained for the photocatalytic particles of Example A5 and Example A6, along with the results obtained for Example A2 and Example A4.

[0120] In example A5, where a co-catalyst (Ag,Co) was supported on ultrasonically treated base material particles (TiO2 particles) by chemical reduction, the CO generation rate was 1.5 μmolh -1 The CO selectivity was relatively small. Furthermore, the CO selectivity was less than 70%. On the other hand, in example A6, where Ag was supported by ultrasonic reduction followed by chemical reduction, the CO generation rate was 2.5 μmolh -1 The CO selectivity was also relatively high, and over 90%.

[0121] From the above results, it was found that when co-catalysts (Ag,Co) are supported by methods other than ultrasonic reduction (photoelectrolysis, chemical reduction, impregnation), it is difficult to obtain photocatalytic particles with excellent CO2 reduction photocatalytic performance (CO generation rate, CO selectivity). However, by performing at least Ag support using ultrasonic reduction, it is possible to obtain photocatalytic particles with excellent photocatalytic performance. Furthermore, it was found that by performing not only Ag but also Co support using ultrasonic reduction, photocatalytic particles exhibiting even better photocatalytic performance can be obtained.

[0122] [Experimental Example B] In Experimental Example B, a co-catalyst (Ag,Co) was supported on TiO2-based matrix particles using ultrasonic reduction. Various matrix particles (TiO2 particles) with different crystalline states and specific surface areas were used, and their effects were investigated.

[0123] (1) Production of photocatalytic particles [Example B1] In Example B1, JRC-TIO-2 (manufactured by Fuji Titanium Industries Co., Ltd.), a reference catalyst of the Catalysis Society of Japan, was used as the TiO2 particles. Otherwise, photocatalytic particles consisting of AgCo / TiO2 particles were obtained using the same procedure as in Example A4.

[0124] [Example B2] In Example B2, JRC-TIO-4(2) (manufactured by Nippon Aerosil Co., Ltd.), a reference catalyst of the Catalysis Society of Japan, was used as the TiO2 particles. Otherwise, photocatalytic particles consisting of AgCo / TiO2 particles were obtained using the same procedure as in Example A4.

[0125] [Example B3] In Example B3, JRC-TIO-9 (manufactured by Furukawa Machinery & Metal Co., Ltd.), a reference catalyst of the Catalysis Society of Japan, was used as the TiO2 particles. Otherwise, photocatalytic particles consisting of AgCo / TiO2 particles were obtained using the same procedure as in Example A4.

[0126] [Example B4] In Example B4, JRC-TIO-13 (manufactured by Teika Co., Ltd.), a reference catalyst of the Catalysis Society of Japan, was used as the TiO2 particles. Otherwise, photocatalytic particles consisting of AgCo / TiO2 particles were obtained using the same procedure as in Example A4.

[0127] [Example B5] In Example B5, JRC-TIO-16 (manufactured by Sakai Chemical Industry Co., Ltd.), a reference catalyst of the Catalysis Society of Japan, was used as the TiO2 particles. Otherwise, photocatalytic particles consisting of AgCo / TiO2 particles were obtained using the same procedure as in Example A4.

[0128] [Example B6] In Example B6, ST-01 (manufactured by Ishihara Sangyo Co., Ltd.) was used as the TiO2 particles. Otherwise, photocatalytic particles consisting of AgCo / TiO2 particles were obtained using the same procedure as in Example A4.

[0129] [Example B7] In Example B7, TiO2 synthesized by the HyCOM (Hydrothermal Crystallization in Organic Media) method (hereinafter referred to as "HyCOM") was used as the TiO2 particles. Otherwise, photocatalytic particles consisting of AgCo / TiO2 particles were obtained using the same procedure as in Example A4.

[0130] (2) Evaluation The following evaluations were performed on the various properties of the samples (matrix particles, photocatalytic particles) from Examples B1 to B7.

[0131] <XRD(Base Material Particles)> The base material particles (TiO2 particles) were analyzed by X-ray diffraction (XRD) method to obtain an XRD pattern. The analysis was carried out under the following conditions.

[0132] - 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

[0133] Next, based on the obtained XRD pattern, the crystal phase contained in the base material particles was determined. When the TiO2 particles contained anatase-type crystal phase, the full width at half maximum (FWHM) of the diffraction peak (near 2θ = 25°) based on the anatase-type crystal phase was read. When the TiO2 particles contained rutile-type crystal phase, the full width at half maximum (FWHM) of the diffraction peak (near 2θ = 27.5°) based on the rutile-type crystal phase was read.

[0134] <Diffuse Reflection Spectrum (Photocatalyst Particles)> The diffuse reflection spectrum of the photocatalyst particles was measured to obtain the ratio (I A ) of the intensity of peak B (I B ) to the intensity of peak A (I B / I A ). The measurement was carried out in the same procedure as in Experimental Example A.

[0135] <CO2 Reduction Photocatalytic Performance (Photocatalyst Particles)> The CO2 reduction photocatalytic performance (CO generation rate, CO selectivity) of the photocatalyst particles was evaluated. The evaluation was carried out in the same procedure as in Experimental Example A.

[0136] (3) Evaluation Results <Base Material Particles> Figure 18 shows the X-ray diffraction patterns of the matrix particles (TiO2 particles) used in Examples B1 to B7. The ● marks in the figure indicate diffraction peaks based on the anatase-type crystalline phase of TiO2. The ▲ marks indicate diffraction peaks based on the rutile-type crystalline phase of TiO2.

[0137] The TiO2 particles used in Examples B1, B3, B4, B6, and B7 (JRC-TIO-2, JRC-TIO-9, JRC-TIO-13, ST-01, HyCOM) consisted of an anatase-type crystalline phase. The TiO2 particles used in Example B5 (JRC-TIO-16) consisted of a rutile-type crystalline phase. The TiO2 particles used in Example B2 (and Examples A1 to A6) (JRC-TIO-4(2)) consisted of a mixed phase of anatase-type and rutile-type crystalline phases.

[0138] Table 1 below shows the full width at half maximum (FWHM) and specific surface area (SSA) of the XRD diffraction peaks of TiO2 particles. The specific surface area (SSA) values ​​provided by the TiO2 particle supplier are shown.

[0139] [Table 1]

[0140] <Photocatalyst particles> Peak intensity ratio in the diffuse reflectance spectrum of Example B1 (I B / I A ) is 1.31%, example B2 I B / I A It is 1.45%, for example B3 I B / I A 1.87%, Example B4 I B / I A 0.61%, Example B5 I B / I A is 0%, I in example B6 B / I A 1.69%, Example B7 B / I A The figure was 4.28%.

[0141] Figure 19 shows the CO2 reduction photocatalytic performance (H2 and CO generation rates, CO selectivity) obtained for the photocatalytic particles of Example B3 and Example B6. Example B3 and Example B6 are samples with the same or similar crystalline phase and specific surface area, although they differ in the full width at half maximum (FWHM) of the XRD diffraction peaks of the base material particles (TiO2 particles) (FWHM of Example B3 is 0.79°, and FWHM of Example B6 is 1.0°).

[0142] As shown in Figure 19, the CO evolution rate in Example B3, which had a relatively small FWHM (high crystallinity), was slightly higher than that of Example B6, which had a relatively large FWHM (low crystallinity). On the other hand, the CO selectivity of Example B3 was slightly lower than that of Example B6.

[0143] Figure 20 shows the CO2 reduction photocatalytic performance obtained for the photocatalytic particles of Examples B2, B4, and B5. Example B2 is a sample using TiO2 particles composed of a mixed phase of anatase-type crystalline phase and rutile-type crystalline phase as the matrix particles. Example B4 is a sample using anatase-type TiO2 particles as the matrix particles, and Example B5 is a sample using rutile-type TiO2 particles as the matrix particles.

[0144] As shown in Figure 20, the CO generation rate of Example B2, which uses TiO2 particles composed of a mixed phase of anatase-type and rutile-type crystalline phases as the matrix particles, was higher than that of Examples B4 and B5, which use matrix particles composed of either anatase-type or rutile-type TiO2 particles alone. Furthermore, the CO selectivity of Example B2 was almost 100%.

[0145] Figure 21 shows the CO2 reduction photocatalytic performance obtained for the photocatalytic particles in Examples B1, B3, and B4. Here, the specific surface area (SSA) of the matrix particles (TiO2 particles) increases in the order of Examples B1, B4, and B3.

[0146] Looking at Figure 21, the specific surface area is moderate (59 m²). 2 In example B4, where TiO2 particles of 18 / g are used as the base material particles, the CO generation rate is lower because the specific surface area of ​​the TiO2 particles is small (18m²). 2 ( / g) For example, B1 and TiO2 particles have a large specific surface area (290-310 m²). 2The value (g) was larger than that of example B3. Furthermore, the CO selectivity for example B4 was over 90%.

[0147] Figure 22 shows the CO2 reduction photocatalytic performance obtained for the photocatalytic particles of Examples B2, B6, and B7. Comparing JRC-TIO-4(2) (Example B2) and ST-01 (Example B6), JRC-TIO-4(2) shows higher activity. From this, it can be concluded that a mixed phase of anatase and rutile types is superior to anatase type alone. On the other hand, HyCOM (Example B7) appears to produce a large amount of CO. HyCOM is known as a photocatalyst with strong oxidizing power, and it is possible that it is detecting not only CO2 reduction but also CO production due to isopropanol oxidation.

[0148] [Experimental Example C] In Experimental Example C, a co-catalyst (Ag,Co) was supported onto TiO2-based matrix particles using ultrasonic reduction. Various different compounds were used as the Co raw material, and their effects were investigated.

[0149] (1) Production of photocatalytic particles [Example C1] In Example C1, cobalt chloride hexahydrate (CoCl3·6H2O) was used as the Co raw material. Otherwise, photocatalytic particles consisting of AgCo / TiO2 particles were obtained using the same procedure as in Example A4.

[0150] [Example C2] In Example C2, cobalt hydroxide (Co(OH)2) was used as the Co raw material. Otherwise, photocatalytic particles consisting of AgCo / TiO2 particles were obtained using the same procedure as in Example A4.

[0151] [Example C3] In Example C3, cobalt nitrate hexahydrate (Co(NO3)2·6H2O) was used as the Co raw material. In other words, photocatalytic particles consisting of AgCo / TiO2 particles were obtained using the same procedure as in Example A4.

[0152] [Example C4] In Example C4, cobalt trioxide (Co3O4) was used as the Co raw material. Otherwise, photocatalytic particles consisting of AgCo / TiO2 particles were obtained using the same procedure as in Example A4.

[0153] (2) Evaluation For the samples (photocatalytic particles) of Examples C1 to C4, the diffuse reflectance spectrum and CO2 reduction photocatalytic performance (CO generation rate, CO selectivity) were evaluated. The evaluation was performed using the same procedure as in Experimental Example A.

[0154] (3) Evaluation results Peak intensity ratio in the diffuse reflectance spectrum of Example C1 (I B / I A ) is 7.19%, example C3 I B / I A 3.60%, example C4 I B / I A The percentage was 0%.

[0155] Figure 23 shows the CO2 reduction photocatalytic performance of the photocatalytic particles (H2 and CO generation rate, CO selectivity).

[0156] Example C2, which used Co(OH)2 as the CO raw material, showed the highest CO evolution rate (approximately 4.5 μmol / g). Example C1, which used CoCl3·6H2O, and Example C3, which used Co(NO3)2·6H2O, had the next highest CO evolution rates (2.6-2.9 μmol / g). Examples C1-C3 also showed high CO selectivity (over 90%). In contrast, Example C4, which used Co3O4, showed relatively low CO evolution rate and CO selectivity (approximately 1.1 μmol / g, approximately 80%).

[0157] [Experimental Example D] In Experimental Example D, a co-catalyst (Ag,Co) was supported onto TiO2-based matrix particles using ultrasonic reduction. The effect of varying the amount of Ag supported was investigated.

[0158] (1) Production of photocatalytic particles [Example D1] In Example D1, the Ag loading amount was 0% by mass with respect to the TiO2 particles. That is, Ag was not loaded. Otherwise, photocatalytic particles composed of AgCo / TiO2 particles were obtained in the same procedure as in Example A4.

[0159] [Example D2] In Example D2, the Ag loading amount was 0.1% by mass with respect to the TiO2 particles. Otherwise, photocatalytic particles composed of AgCo / TiO2 particles were obtained in the same procedure as in Example A4.

[0160] [Example D3] In Example D3, the Ag loading amount was 0.3% by mass with respect to the TiO2 particles. Otherwise, photocatalytic particles composed of AgCo / TiO2 particles were obtained in the same procedure as in Example A4.

[0161] [Example D4] In Example D4, the Ag loading amount was 0.5% by mass with respect to the TiO2 particles. That is, photocatalytic particles composed of AgCo / TiO2 particles were obtained in the same procedure as in Example A4.

[0162] [Example D5] In Example D5, the Ag loading amount was 1% by mass with respect to the TiO2 particles. Otherwise, photocatalytic particles composed of AgCo / TiO2 particles were obtained in the same procedure as in Example A4.

[0163] (2) Evaluation For the samples (photocatalytic particles) of Examples D1 to D5, the diffuse reflection spectrum and the CO2 reduction photocatalytic performance (CO generation rate, CO selectivity) were evaluated. The evaluation was carried out in the same procedure as in Experimental Example A.

[0164] (3) Evaluation Results The peak intensity ratio (I B / I A ) in the diffuse reflection spectrum of Example D1 was 0%, the I B / I A of Example D2 was 1.09%, the I B / I A of Example D3 was 2.19%, the I B / I A of Example D4 was 1.45%, and the I B / I AThe figure was 6.37%.

[0165] Figure 24 shows the CO2 reduction photocatalytic performance of the photocatalytic particles (H2 and CO generation rate, CO selectivity).

[0166] In example D1, where the Ag loading amount was 0 mass%, no CO gas generation was observed. In examples D2 to D5, where the Ag loading amount was 0.1 to 1 mass%, CO gas generation was observed, and the generation rate (CO generation rate) and CO gas selectivity were relatively large (2.2 μmol / g or more, 90% or more). In particular, the CO generation rate and CO selectivity were large in example D4, where the Ag loading amount was 0.5 mass% (approximately 4.4 μmol / g or more, 90% or more).

[0167] [Experimental Example E] In Experimental Example E, a co-catalyst (Ag,Co) was supported onto TiO2-based matrix particles using ultrasonic reduction. The effect of varying the molar ratio of Co to Ag (Co / Ag ratio) was investigated.

[0168] (1) Production of photocatalytic particles [Example E1] In Example E1, the molar ratio of the amount of Co supported to the amount of Ag supported (Co / Ag ratio) was set to 0. In other words, no Co was supported. Otherwise, photocatalytic particles consisting of AgCo / TiO2 particles were obtained using the same procedure as in Example A4.

[0169] [Example E2] In Example E2, the molar ratio of the Co-supported material to the Ag-supported material (Co / Ag ratio) was set to 0.1. Otherwise, photocatalytic particles consisting of AgCo / TiO2 particles were obtained using the same procedure as in Example A4.

[0170] [Example E3] In Example E3, the molar ratio of the Co-supported material to the Ag-supported material (Co / Ag ratio) was set to 0.2. Otherwise, photocatalytic particles consisting of AgCo / TiO2 particles were obtained using the same procedure as in Example A4.

[0171] [Example E4] In Example E4, the molar ratio of the Co loading amount to the Ag loading amount (Co / Ag ratio) was set to 0.5. That is, photocatalytic particles composed of AgCo / TiO2 particles were obtained in the same procedure as in Example A4.

[0172] [Example E5] In Example E5, the molar ratio of the Co loading amount to the Ag loading amount (Co / Ag ratio) was set to 1. Otherwise, photocatalytic particles composed of AgCo / TiO2 particles were obtained in the same procedure as in Example A4.

[0173] [Example E6] In Example E6, the molar ratio of the Co loading amount to the Ag loading amount (Co / Ag ratio) was set to 2. Otherwise, photocatalytic particles composed of AgCo / TiO2 particles were obtained in the same procedure as in Example A4.

[0174] [Example E7] In Example E7, the molar ratio of the Co loading amount to the Ag loading amount (Co / Ag ratio) was set to 10. Otherwise, photocatalytic particles composed of AgCo / TiO2 particles were obtained in the same procedure as in Example A4.

[0175] (2) Evaluation For the samples (photocatalytic particles) of Examples E1 to E7, the diffuse reflection spectrum and the CO2 reduction photocatalytic performance (CO generation rate, CO selectivity) were evaluated. The evaluation was performed in the same procedure as in Experimental Example A.

[0176] (3) Evaluation Results The peak intensity ratio (I B / I A ) in the diffuse reflection spectrum of Example E1 was 0%, the I B / I A of Example E2 was 1.57%, the I B / I A of Example E3 was 1.73%, the I B / I A of Example E4 was 1.53%, the I B / I A of Example E5 was 1.45%, the I B / I A of Example E6 was 3.41%, and the I B / I A of Example E7 was 6.57%.

[0177] Figure 25 shows the CO2 reduction photocatalytic performance of the photocatalytic particles (H2 and CO generation rate, CO selectivity).

[0178] In example E1, where no Co was supported (Co / Ag ratio of 0), the CO evolution rate and CO selectivity were low (approximately 0.8 μmol / g, approximately 50%). Similarly, in example E7, where a large amount of Co was supported (Co / Ag ratio of 10), the CO evolution rate was also low (approximately 0.8 μmol / g). In contrast, in examples E3 to E6, where the Co / Ag ratio was 0.2 to 2, the CO evolution rate and CO selectivity were high (approximately 3.5 μmol / g or more, 90% or more). In examples E3 to E5, where the Co / Ag ratio was 0.2 to 1, the CO evolution rate was particularly high (approximately 4.2 μmol / g or more). .

[0179] [Experimental Example F] In Experimental Example F, the influence of evaluation conditions on the CO2 reduction photocatalytic performance evaluation of photocatalytic particles supported with co-catalysts (Ag,Co) by ultrasonic reduction was investigated. Specifically, using the photocatalytic particles from Example A4, measurements were performed by changing the concentration of the hole-capturing agent (isopropanol (iPrOH)) in the evaluation solution to 0, 0.01, 0.1, 0.5, and 1 (mol / L).

[0180] Figure 26 shows the CO2 reduction photocatalytic performance (H2 and CO generation rate, CO selectivity) evaluated by varying the iPrOH concentration in the evaluation solution.

[0181] When the evaluation solution did not contain iPrOH (iPrOH concentration of 0 mol / L), the CO evolution rate was low (approximately 0.9 μmol / g). As the iPrOH concentration increased, the CO evolution rate increased, reaching its maximum at an iPrOH concentration of 0.5 mol / L (approximately 4.9 μmol / g). However, further increases in iPrOH concentration did not result in any improvement in the CO evolution rate.

[0182] [Experimental Example G] In Experimental Example G, four samples were prepared using ultrasonic reduction: a sample without a co-catalyst, a sample with only Co supported, a sample with only Ag supported, and a sample with both Ag and Co supported. The effect of the co-catalyst was then investigated.

[0183] (1) Production of photocatalytic particles [Example G1] In Example G1, no co-catalyst (Ag,Co) was supported. That is, the base material particles were used alone as photocatalytic particles for evaluation. The TiO2 particles (JRC-TIO-4(2)) used in Example A4 were used as the base material particles.

[0184] [Example G2] In Example G2, only Co was supported as a co-catalyst. Specifically, photocatalytic particles were prepared using the same procedure as in Example A4, except that Ag raw material (Ag2O particles) was not used. The amount of Co supported was 0.27 mass% relative to the TiO2 particles.

[0185] [Example G3] In Example G3, only Ag was supported as a co-catalyst. Specifically, photocatalytic particles were prepared using the same procedure as in Example A4, except that the Co raw material (Co(NO3)2·6H2O) was not used. The amount of Ag supported was 0.5 mass% relative to the TiO2 particles.

[0186] [Example G4] In Example G4, both Ag and Co were supported as co-catalysts. Specifically, photocatalytic particles were prepared using the same procedure as in Example A4. The amount of Ag supported was 0.5 mass% relative to the TiO2 particles. The molar ratio of the amount of Co supported to the amount of Ag supported (Co / Ag) was 1.

[0187] (2) Evaluation The CO2 reduction photocatalytic performance (CO generation rate, CO selectivity) was evaluated for the samples (photocatalytic particles) of Examples G1 to G4. The evaluation was performed using the same procedure as in Experimental Example A.

[0188] (3) Evaluation results Figure 27 shows the CO2 reduction photocatalytic performance of the photocatalytic particles (H2 and CO generation rate, CO selectivity).

[0189] In example G1, where no co-catalyst (Ag,Co) was supported, no CO gas generation was observed. On the other hand, in examples G2 or G3, where only Ag or Co was supported, CO gas was generated, but the generation rate (CO generation rate) was low (0.3-0.8 μmol / g). In contrast, the CO generation rate in example G4, where both Ag and Co were supported, was very high (approximately 4.5 μmol / g). The CO selectivity was also high (over 90%).

[0190] [Experimental Example H] In Experimental Example H, the relationship between light irradiation time and the amount of photocatalytic reaction products produced was investigated for photocatalytic particles supported with co-catalysts (Ag,Co) using ultrasonic reduction. Specifically, using the photocatalytic particles from Example A4, the amount of each photocatalytic reaction product (CO, H2, and CH3COCH3 (acetone)) produced was investigated when the light irradiation time was varied from 0 to 5 hours.

[0191] The amount of acetone produced was determined by filtering the evaluation solution used in the CO2 reduction photocatalyst performance evaluation to remove solid components, and then analyzing the resulting filtrate using a gas chromatograph to calculate the amount of acetone in the filtrate.

[0192] Figure 28 shows the amount of products (CO, H2, and CH3COCH3 (acetone)) produced at each light irradiation time. Figure 28 also shows the electron-hole balance (e) calculated from the CO and acetone production amounts. - / h + ) is also shown.

[0193] The amount of CO and CH3COCH3 (acetone) produced increased linearly in proportion to the light irradiation time. Furthermore, the electron-hole balance (e) calculated from the amount of CO and acetone produced was also calculated. - / h + The value was approximately 1 regardless of the light irradiation time.

[0194] As shown in Figure 3, during the photocatalytic reaction, light irradiation excites electrons e- from the valence band to the conduction band in the photocatalytic particles, and holes h- are also generated in the valence band. + A photocatalytic reaction is formed. It is thought that CO2 accepts electrons on the surface of the photocatalytic particles and is reduced to CO, while iPrOh (isopropanol) in the solution accepts holes and is oxidized to CH3COCH3 (acetone). Since the same number of electrons and holes are generated in the photocatalytic reaction, it is thought that the amount of CO produced and the amount of acetone produced will be almost the same. In fact, the electron-hole balance obtained from the measured amount of CO produced and acetone produced in the CO2 reduction photocatalyst performance evaluation was almost 1.

[0195] From the above results, it is understood that this embodiment provides a method for producing photocatalytic particles with excellent CO2 reduction photocatalytic performance, comprising TiO2-based matrix particles.

Claims

1. A method for producing carbon dioxide reduction photocatalytic particles, The carbon dioxide reduction photocatalyst particles are titanium dioxide (TiO ２ The device comprises a base material particle containing (as the main component) and a co-catalyst containing silver (Ag) and cobalt (Co) supported on the surface of the base material particle, The above method includes a step of supporting silver (Ag) and cobalt (Co) on the surface of the base material particles, A method for supporting silver (Ag) by an ultrasonic reduction method, in which ultrasound is irradiated onto a reaction solution containing matrix particles, a silver (Ag) supply source, and a reducing solution.

2. The method according to claim 1, wherein the loading of cobalt (Co) is carried out by an ultrasonic reduction method in which ultrasound is irradiated onto a reaction solution containing matrix particles, a cobalt (Co) supply source, and a reducing solution.

3. The method according to claim 2, wherein the cobalt (Co) source is at least one selected from the group consisting of cobalt(III) chloride, cobalt(II) hydroxide, cobalt(II) nitrate, and hydrates thereof.

4. Titanium oxide (TiO) contained in the aforementioned base material particles ２ The method according to claim 1 or 2, wherein the method comprises both anatase-type crystalline phase and rutile-type crystalline phase.

5. The method according to claim 1 or 2, wherein the amount of silver (Ag) supported on the surface of the base material particles is 0.1% by mass or more and 1.0% by mass or less relative to the base material particles.

6. The method according to claim 1 or 2, wherein the molar ratio (Co / Ag ratio) of the amount of cobalt (Co) supported on the surface of the base material particles to the amount of silver (Ag) supported is 0.2 or more and 2 or less.

7. The method according to claim 1 or 2, wherein the silver (Ag) supported on the surface of the matrix particles is silver (Ag) particles with an average particle size of 0.1 nm or more and 10 nm or less.

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