Carbon dioxide-reducing photocatalyst particles

Abstract

To provide a carbon dioxide reduction photocatalyst particle excellent in a catalytic performance.SOLUTION: A carbon dioxide reduction photocatalyst particle includes a gallium oxide (Ga2O3) particle, and a metal gold coat metal silver nanoparticle carried on a surface of the gallium oxide particle. The metal gold coat metal silver nanoparticle includes a metal silver (Ag) nanoparticle and a gold (Au) coat disposed on a surface of the metal silver nanoparticle. The carbon dioxide reduction photocatalyst particle has a peak within a range of wavelength 350 to 550 nm in a diffuse reflectance spectrum.SELECTED DRAWING: Figure 9

Description

Technical Field

[0001] The present invention relates to carbon dioxide-reducing photocatalyst particles.

Background Art

[0002] Water splitting and carbon dioxide reduction technologies using semiconductor photocatalyst particles have attracted attention as technologies that can solve energy and environmental problems. By supporting nanoparticles composed of silver (Ag) or the like as a cocatalyst on this photocatalyst particle, it is possible to expect an effect of trapping electrons generated by photoexcitation to promote charge separation and an effect of selecting carbon dioxide reduction products. For example, in a normal photocatalyst, water is decomposed into hydrogen (H 2 ) and oxygen (O 2 ) by light irradiation. On the other hand, in a photocatalyst supporting silver particles, carbon monoxide (CO) is generated together with hydrogen (H 2 ) by the reduction of carbon dioxide (CO 2 ). As such a carbon dioxide-reducing photocatalyst, gallium oxide particles supporting silver nanoparticles (silver nanoparticle-supported gallium oxide particles) are known.

[0003] Carbon monoxide (CO) is an important starting material in the chemical industry and industry, and it is possible to synthesize various fuels and chemical substances by reacting this with hydrogen. In such a synthesis, a mixed gas of carbon monoxide (CO) and hydrogen (H 2 ) is used, and this gas is called synthesis gas (syngas). For example, synthesis gas containing carbon monoxide and hydrogen at a ratio of CO:H 2 = 1:2 is used for the synthesis of methanol.

[0004] In producing synthesis gas (syngas), it is convenient to use a carbon dioxide-reducing photocatalyst that generates carbon monoxide and hydrogen. In order to efficiently obtain synthesis gas, it is desired that the carbon dioxide-reducing photocatalyst can control the production ratio of carbon monoxide, that is, the selectivity, and has a large gas production amount. Here, the CO selectivity is, as expressed by the following formula (1), hydrogen (H 2) It is the ratio of the generation rate of carbon monoxide (CO) gas to the sum of the generation rate (generation amount) of gas and the generation rate of carbon monoxide (CO) gas.

Number

[0005] By the way, as methods for supporting metal nanoparticles, methods such as an impregnation method and a photoelectrodeposition method (photoelectrochemical deposition method) have been conventionally known. For example, in Patent Document 1 regarding a method for reducing carbon dioxide, CO 2 and H 2 O and a photocatalyst are irradiated with light to generate CO by a reaction for reducing CO 2 by using a catalyst in which silver is supported on gallium oxide by a photoelectrodeposition method or an impregnation method. In the photoelectrodeposition method, gallium oxide powder is put into an aqueous alcohol solution containing a silver precursor such as silver nitrate, mixed, and then irradiated with light to reduce the silver precursor. In the impregnation method, gallium oxide is added to an aqueous silver precursor solution, stirred, water is removed, then heated and dried, and further fired in air (Claims 1, 2, and

[0015] of Patent Document 1).

[0006] Although it is not related to a carbon dioxide reduction catalyst, Patent Document 2 discloses a method for producing a noble metal nanomaterial including a step of irradiating ultrasonic waves to disperse one or more noble metal oxides in a solvent to obtain a noble metal oxide dispersion, and a step of heating the noble metal oxide dispersion. It is described that the solvent further contains a carrier for supporting a noble metal, and since it is possible to obtain a noble metal nanomaterial supported on the surface of the carrier with high dispersibility, it can be suitably used for a catalyst for fuel cells, a catalyst for material synthesis, etc. (Claims 1, 6, and

[0014] of Patent Document 2).

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] However, as a result of investigations by the present inventors, it has been found that although the photocatalysts produced by the impregnation method and the photoelectrodeposition method proposed in Patent Document 1 have certain effects, there is still room for improvement. That is, in order to fully exert the effect of silver (Ag) as a co-catalyst, it is desirable to increase its loading amount to a certain extent. Also, it is desired that the particle size of silver is small, on the order of several tens of nm. This is because if the particle size is too large, the catalytic activity will be lost. In this regard, in the photocatalyst particles produced by the impregnation method and the photoelectrodeposition method, when the silver concentration (loading amount) is increased, there is a problem that silver particles aggregate and the particle size becomes large. Therefore, it is difficult to support silver nanoparticles with a small particle size at a high loading amount.

[0009] Patent Document 2 does not intend to use noble metal nanomaterials for carbon dioxide reduction photocatalysts, let alone aim to reduce the particle size of nanomaterials to the order of several tens of nm. In fact, in Cited Document 2, it only teaches in the examples that noble metal (Pt) nanoparticle-supported spherical carbon or noble metal (Pt) nanotubes are produced, and that they are suitable for catalysts for fuel cells, catalysts for material synthesis, medical and food additives, and conductive pastes (

[0043] -

[0062] of Patent Document 2).

[0010] On the other hand, when synthesizing metal nanoparticles, a method is known in which the raw material concentration is lowered and a large amount of organic protecting agent is used to synthesize uniform and fine particles. However, when photocatalyst particles are produced by such a method, there are problems that the yield of the photocatalyst particles is low and the organic protecting agent adheres to the particle surface. Since the attached organic protecting agent reduces the catalytic activity, it is necessary to calcine the catalyst particles at a high temperature to decompose and remove it. In the catalyst particles that have undergone such calcination, the particle size of the metal nanoparticles becomes large. Therefore, it has been difficult to support fine silver nanoparticles at a high dispersion and a high loading amount by conventional methods, and there has been a limit in efficiently manufacturing photocatalyst particles with excellent catalytic performance.

[0011] In view of such conventional problems, the present inventors conducted studies and found that by a simple method of irradiating gallium oxide particles and a silver source with ultrasonic waves, silver nanoparticles serving as a cocatalyst can be deposited with high dispersion and high loading rate. Furthermore, by coating the surface of the silver nanoparticles with gold, the amount of hydrogen generation can be increased while maintaining the amount of carbon monoxide generation. As a result, photocatalytic particles with excellent catalytic performance can be obtained. 2 generation amount, and as a result, it was found that photocatalytic particles with excellent catalytic performance can be obtained.

[0012] The present invention has been completed based on such findings, and an object thereof is to provide carbon dioxide reduction photocatalytic particles having excellent catalytic performance.

Means for Solving the Problems

[0013] The present invention includes the following aspects. 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".

[0014] Gallium oxide (Ga 2 O 3 ) particles and metal gold-coated metal silver nanoparticles supported on the surface of the gallium oxide particles, The metal gold-coated metal silver nanoparticles include metal silver (Ag) nanoparticles and a gold (Au) coat provided on the surface of the metal silver nanoparticles, Carbon dioxide reduction photocatalytic particles having a peak in the wavelength range of 350 to 550 nm in the diffuse reflection spectrum.

Effects of the Invention

[0015] According to the present invention, carbon dioxide reduction photocatalytic particles having excellent catalytic performance are provided.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Mode for Carrying Out the Invention

[0017] The specific embodiments of the present invention (hereinafter referred to as "the present embodiment") will be described below. However, the present invention is not limited to the following embodiments, and various modifications are possible without departing from the gist of the present invention.

[0018] <<1. Photocatalytic Particles>> The carbon dioxide reduction photocatalytic particles of the present embodiment (hereinafter may be collectively referred to as "photocatalytic particles") are gallium oxide (Ga 2 O 3 ) particles and metal gold-coated metal silver nanoparticles supported on the surface of the gallium oxide particles. These metal gold-coated metal silver nanoparticles (hereinafter may be collectively referred to as "gold-coated silver nanoparticles") have metal silver (Ag) nanoparticles and a gold (Au) coat provided on the surface thereof.

[0019] Gallium oxide (Ga 2 O 3 ) particles function as a main catalyst and are not particularly limited as long as they are used in a photocatalyst. Ga 2 O 3 has α-type, β-type, γ-type, δ-type and ε-type, and any of them may be used. However, the β-type (β-Ga 2 O3 ) is preferable. The particle size is not particularly limited either. For example, the average particle diameter of the particles is 0.3 to 5.0 μm. Furthermore, the particle shape is not particularly limited either. For example, spherical, irregular, anisotropic (rod or plate-like, etc.) shapes can be mentioned. When the particles are rod-shaped, for example, those with a major axis diameter of 1.0 to 5.0 μm and a minor axis diameter of 0.3 to 1.0 μm can be used.

[0020] The gold-coated silver nanoparticles function as a cocatalyst, and their average particle diameter is on the order of nm. By making the nanoparticles that function as a cocatalyst finer, the catalytic performance of the photocatalyst is enhanced. On the other hand, if the nanoparticles are larger than the nm order, there is a risk that the function as a cocatalyst such as catalytic activity will be lost, or the surface active sites of the gallium oxide particles will decrease.

[0021] The silver cocatalyst in the gold-coated silver nanoparticles has the function of selectively generating carbon monoxide (CO) in the photoreduction of carbon dioxide (CO 2 ). On the other hand, gold promotes the generation of hydrogen by the reduction of hydrogen ions (H 2 ) rather than the reduction of CO + ). When gold-coated silver nanoparticles are used as a cocatalyst as in this embodiment, since gold has a larger work function than silver, the charge separation of electrons and holes generated by photoexcitation is further promoted. Therefore, the photocatalytic reaction is likely to occur, and as a result, an increase in the amount of generated gas is expected.

[0022] The photocatalytic particles of this embodiment preferably have a peak in the wavelength range of 350 to 550 nm in the diffuse reflection spectrum. Thereby, the photocatalytic particles have excellent catalytic performance. The diffuse reflection spectrum is an absorption spectrum obtained by the diffuse reflection method. When light is irradiated on a solid sample such as powder, a part of it goes out of the sample through the scattering process. In the scattering process, a part of the irradiated light is reflected on the sample surface, and the rest enters the sample. The incident light that has entered is partially absorbed depending on the electron transition state of the sample, and the rest goes out of the sample. The absorption spectrum can be obtained from the incident light intensity of ultraviolet light or visible light and the light intensity after the scattering process.

[0023] In the photocatalytic particles of this embodiment, the average particle diameter of the gold-coated silver nanoparticles is preferably 10.0 to 50.0 nm. By setting the average particle diameter to 10.0 nm or more, it becomes possible to form silver nanoparticles without the need to lower the Ag concentration (loading amount) or add an organic surface protecting agent. On the other hand, by setting the average particle diameter to 50.0 nm or less, it is possible to prevent problems such as loss of the function as a cocatalyst such as catalytic activity or a decrease in the surface active sites of the gallium oxide particles. The average particle diameter is more preferably 10.0 to 30.0 nm. The average particle diameter can be determined by observing the photocatalytic particles using a transmission electron microscope (TEM). As an example, there is a method of obtaining the particle size distribution of the gold-coated silver nanoparticles by visual observation or image analysis software and calculating the number average value from this particle size distribution.

[0024] In the photocatalytic particles of this embodiment, the loading amount of the gold-coated silver nanoparticles is preferably 0.3 to 10.0% by mass based on the gallium oxide particles. If the loading amount is excessively small, it becomes difficult to sufficiently exhibit the effect of the cocatalyst. Therefore, when using the photocatalyst for CO 2 reduction, the CO gas generation rate and the CO selectivity become low. On the other hand, if the loading amount is excessively large, the CO gas generation rate decreases. The loading amount can be adjusted by controlling the blending amounts of the gold and silver sources and the ultrasonic treatment conditions during the production of the photocatalytic particles. The loading amount of the gold-coated silver nanoparticles may be 0.5% by mass or more, 1.0% by mass or more, 3.0% by mass or more, or 5.0% by mass or more. Also, the loading amount may be 7.5% by mass or less, 5.0% by mass or less, 3.0% by mass or less, or 1.0% by mass or less.

[0025] In the photocatalytic particles of this embodiment, the loading amount (coating amount) of metallic gold (Au) in the gold-coated silver nanoparticles is preferably 0.1 to 20.0% by mass based on metallic silver (Ag). If the gold coating amount is excessively small, it becomes difficult to sufficiently exhibit the effect of the gold coating. On the other hand, if the gold coating amount is excessively large, the effect of the gold cocatalyst becomes large and CO is not generated and H 2Only the generation will occur. The amount of gold coating may be 0.1% by mass or more, may be 0.3% by mass or more, may be 0.5% by mass or more, or may be 1.0% by mass or more. Also, the amount of gold coating may be 20.0% by mass or less, may be 10.0% by mass or less, may be 5.0% by mass or less, or may be 3.0% by mass or less.

[0026] The photocatalytic particles of this embodiment are CO 2 In the CO reduction photocatalytic performance evaluation test, it is preferable that the CO selectivity of the photocatalytic particles is 20% or more. Thereby, it becomes possible to increase the ratio of the amount of CO gas generated by the CO 2 reduction. The CO selectivity may be 30% or more. The upper limit of the CO selectivity is not particularly limited, but is typically 90% or less, more typically 80% or less.

[0027] CO 2 The CO reduction photocatalytic performance evaluation test may be performed using a known evaluation apparatus. An example of the evaluation apparatus is shown in FIG. 1. The evaluation apparatus (2) is composed of a tank (4) and a high-pressure mercury (Hg) lamp (6) provided inside this tank (4). An evaluation solution (22) is placed inside the tank (4). The tank (4) is also provided with a gas introduction pipe (8), a gas discharge pipe (10), a pH meter (12), a rubber stopper (14), and a stirrer (16). A bubbling filter (18) is provided at the tip of the gas introduction pipe (8). The mercury lamp (6) is cooled by the cooling water (20) flowing around it.

[0028] The evaluation test may be performed as follows. Pure water, sodium hydrogen carbonate (NaHCO 3 ) and a sample (photocatalytic particles) are mixed to prepare an evaluation solution (22). This evaluation solution (22) is placed in the tank (4) of the evaluation apparatus (2) and stirred with a stirrer (16). Carbon dioxide (CO 2)Gas (30) is blown in from the gas introduction pipe (8), and at the same time, UV light is irradiated from the high-pressure Hg lamp (6) onto the evaluation solution (22). After irradiation for a predetermined time, the generated gas (32) is introduced through the gas discharge pipe (10) into gas chromatography (34) and analyzed there. By this analysis, the generation rates (generation amounts) of hydrogen (H 2 ), oxygen (O 2 ) and carbon monoxide (CO) are determined. Using the obtained generation rates, the CO selectivity is calculated based on the following formula (1). [Number]

[0029] Photocatalytic particles having a peak in a specific wavelength range (350 to 550 nm) in the diffuse reflection spectrum exhibit a high CO selectivity, and the details of the mechanism are unknown. However, it is speculated that the particle size of the gold-coated silver nanoparticles and the electron transfer state may be related. That is, the spectrum obtained by the diffuse reflection method (diffuse reflection spectrum) reflects the electron transfer state of the sample such as the species valence, coordination structure, and ligand field. When the sample is fine particles, it may exhibit different absorption energies depending on the particle size. For example, when it is fine particles, it is known to give an absorption band in the wavelength range corresponding to surface plasmon resonance. Therefore, in the photocatalytic particles of this embodiment having a peak in the specific wavelength range (350 to 550 nm), it is considered that the gold-coated silver nanoparticles have a fine and unique electron transfer state.

[0030] On the other hand, it is expected that the particle size and electron transfer state of the gold-coated silver nanoparticles affect the catalytic performance of the photocatalyst, for example, the CO 2 reduction mechanism will be explained. When light with energy hν irradiates the particles, electrons (e - ) and holes (h + ) are generated in the gallium oxide particles (semiconductor photocatalytic particles). At this time, the metallic silver nanoparticles (co-catalyst) perform charge separation (electrons e - ) and holes h+ promote the separation). When a hole (h + ) reacts with surrounding moisture (H 2 O), the reaction shown in the following formula (2) proceeds to the right, and oxygen (O 2 ) and a proton (H + ) are generated. On the other hand, when an electron (e - ) reacts with carbon dioxide (CO 2 ) and a proton (H + ), the reactions shown in the following formulas (3) and (4) proceed to the right, and carbon monoxide (CO) and water (H 2 O) and hydrogen (H 2 ) are generated. Also, when the reactions of the following formulas (2) to (4) are combined, the reaction shown in the following formula (5) proceeds in principle.

Equation

Equation

Equation

Equation

[0031] When the reactions of the above formula (3) and the reaction of the formula (4) occur to the same extent, as shown in the above formula (5), the CO selectivity (CO generation rate / (H 2 generation rate + CO generation rate)) is constant. However, in reality, these reactions do not always occur to the same extent. When the reaction of the above formula (3) occurs preferentially, the CO selectivity increases.

[0032] In the reaction of the above formula (3), carbon dioxide (CO 2) is adsorbed on the catalyst surface as a carbonate species, and after being changed to a formate species which is a reaction intermediate by light irradiation, it interacts with water molecules to become carbon monoxide (CO). There is also a report suggesting that a silver co-catalyst promotes the formation of reaction intermediates. Therefore, by enhancing the charge separation effect and the reaction intermediate formation effect by the co-catalyst, the reaction of the above formula (3) is expected to occur preferentially, and the CO selectivity is further improved. In this regard, it is speculated that the photocatalyst particles of the present embodiment have fine gold-coated silver nanoparticles with a specific electron transfer state, and thus these act in a complex manner to enhance the charge separation effect and the reaction intermediate formation effect.

[0033] <<2. Method for manufacturing photocatalyst particles>> The manufacturing method of the photocatalyst of the present embodiment is not limited. However, gallium oxide (Ga 2 O 3 ) particles and a silver (Ag) ion source are irradiated with ultrasonic waves in a reducing solution to deposit silver nanoparticles, and then a gold (Au) ion source is added to the solution to coat the surface of the silver nanoparticles with gold, which is preferably obtained. Specific embodiments of the preferred manufacturing method will be described in detail below.

[0034] The method for manufacturing photocatalyst particles preferably includes the following steps: a step of preparing gallium oxide (Ga 2 O 3 ) particles and a silver (Ag) source (preparation step), a step of adding gallium oxide particles and a silver source to a reducing solution to prepare a reaction solution (mixing step), a step of irradiating the reaction solution with ultrasonic waves to prepare gallium oxide particles supporting metallic silver nanoparticles (ultrasonic treatment step), and a step of supplying a gold (Au) source to the reaction solution to prepare photocatalyst particles composed of gallium oxide particles supporting metallic gold-coated metallic silver nanoparticles (galvanic substitution reaction step). Details of each step will be described below.

[0035] <Preparation step> In the preparation step, gallium oxide (Ga 2 O 3) Prepare a particle and a silver (Ag) source. Gallium oxide (Ga 2 O 3 ) particles are not particularly limited as long as they are used for a photocatalyst. Ga 2 O 3 has α-type, β-type, γ-type, δ-type and ε-type, and any of them may be used. However, the β-type (β-Ga 2 O 3 ), which is a stable oxide, is preferred. The size of the particles is not particularly limited either. For example, the average particle diameter of the particles is 0.3 to 5.0 μm. Furthermore, the shape of the particles is not particularly limited. For example, spherical, irregular, and anisotropic shapes (such as rod or plate shapes) can be mentioned. When the particles are rod-shaped, for example, those with a major axis diameter of 1.0 to 5.0 μm and a minor axis diameter of 0.3 to 1.0 μm can be used.

[0036] The silver (Ag) source is not limited as long as it can supply silver. Specifically, oxides, inorganic metal salts and / or organometallic compounds can be mentioned. Examples of inorganic metal salts include nitrates, chlorides and / or sulfates. The silver source may or may not dissolve in the reducing solution. Preferably, the silver source contains silver oxide. Since silver oxide is composed of only silver ions and oxygen ions, it is easy to handle and there are no problems such as waste treatment. Silver oxide includes Ag 2 O, AgO and Ag 2 O 3 which are known, and any of them can be used. However, Ag 2 O which is more easily available is preferred. The size of the silver source is not particularly limited either. For example, the average particle diameter of the silver source is 0.3 to 3.0 μm.

[0037] <Mixing step> In the mixing step, the prepared gallium oxide particles and the silver source are added to the reducing solution to prepare a reaction solution. The mixing ratio of the gallium oxide particles and the silver source may be adjusted so that the silver loading amount in the finally obtained photocatalyst becomes a desired value. If the silver loading amount is excessively small, it becomes difficult to sufficiently exhibit the effect of the cocatalyst. Therefore, CO 2When a photocatalyst is used for reduction, the CO gas generation rate and the CO selectivity decrease. On the other hand, when the silver loading is excessively high, the CO gas generation rate decreases.

[0038] The reducing liquid is not limited as long as it is a liquid having reducibility. It may itself be a liquid having reducibility, or may be a liquid in which a reducing agent is dissolved in a liquid having no reducibility. However, it is preferably a liquid having reducibility itself. Also, it may not contain a separate reducing agent. As such a reducing liquid, alcohols such as ethanol and propanol, which are low in toxicity and easily available, are preferable. A mixed solution of alcohols and water can also be used. However, when using a mixed solution, if the water content is excessively high, it becomes difficult to exhibit a sufficient reducing action. Therefore, the water content in the reducing liquid is preferably 50% by volume or less, more preferably 25% by volume or less. The lower limit value of the water content is not particularly limited and may be 0% by volume.

[0039] <Ultrasonic treatment step> In the ultrasonic treatment step, the obtained reaction solution is irradiated with ultrasonic waves to produce photocatalyst particles composed of gallium oxide particles supporting metal silver nanoparticles. At this time, the surface portion of the silver source in the reaction solution is ultrasonically reduced to become metal silver (Ag) nanoparticles, which are supported on the surface of the gallium oxide particles.

[0040] The mechanism of supporting metal silver nanoparticles will be described with reference to Fig. 2. When ultrasonic waves are irradiated, a rough wave is generated in the reaction solution, and positive and negative repetitive pressures are generated by this rough wave. During the negative pressure cycle, innumerable fine bubbles are generated in the reaction solution by evaporation. During the positive pressure cycle, these bubbles collapse and give a strong impact force to the surroundings. This phenomenon is called ultrasonic cavitation. By cavitation, gallium oxide particles and a silver source in the reaction solution are uniformly dispersed and their surfaces are cleaned. In addition, cavitation generates minute hot spots with high temperature and high pressure. The generated hot spots decompose and reduce the silver source, act on the reaction solution to generate radicals, and these radicals promote the decomposition and reduction of the silver source. In this way, metal silver nanoparticles are generated from the silver source.

[0041] For example, when silver oxide (Ag 2 O), which is solid, is used as the silver source, hot spots and radicals act on the silver oxide, and the silver oxide is decomposed and reduced on the surface to precipitate silver nanoparticles. These silver nanoparticles gradually grow, and when they grow to a certain extent, the interfacial stress between the silver oxide and the silver nanoparticles reaches the limit and they desorb. Alternatively, an intermediate product is generated from the silver oxide by the action of ultrasonic waves, and hot spots and radicals act on this intermediate product to generate silver nanoparticles in the reaction solution. The desorbed or generated silver nanoparticles move to the surface of the gallium oxide particles by the physical action of ultrasonic waves and adsorb there. In this way, gallium oxide particles supporting silver nanoparticles are obtained. The silver nanoparticles generated by ultrasonic reduction are minute. In addition, since an organic protective agent and high-temperature baking are not required, it is possible to produce gallium oxide particles (photocatalyst particles) supporting silver nanoparticles in a fine state.

[0042] When the photocatalyst particles contain a silver source other than silver nanoparticles, it may be difficult to fully exhibit the effect of the cocatalyst. In this regard, according to ultrasonic treatment, it is possible to obtain photocatalyst particles that hardly contain a silver source (such as silver oxide). For example, in the X-ray diffraction pattern, photocatalyst particles in which the peak of silver oxide (Ag 2 O) is not observed can be obtained.

[0043] 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, or may be 28 to 45 kHz. The ultrasonic treatment may be continuously carried out 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 may be a plurality of times. By carrying out the treatment in a frequency oscillation switching mode (for example, a two-frequency switching oscillation mode of 28 kHz / 45 kHz), it becomes possible to further improve the dispersibility of the gallium oxide particles in the liquid. Also, the output of the ultrasonic waves may be 10 to 500 W, or may be 50 to 200 W. Further, the treatment time may be 1 to 10 hours. By lengthening the treatment time, it is possible to convert all of the silver source into silver nanoparticles and support them on the gallium oxide particles. On the other hand, by shortening the treatment time, it is possible to adjust the amount of silver nanoparticles supported.

[0044] <Galvanic replacement reaction step> In the galvanic replacement reaction step, a gold source is supplied to the reaction solution after the reduced metallic silver is generated. Thereby, photocatalytic particles composed of gallium oxide particles supporting metal gold-coated metal silver nanoparticles are produced. By the galvanic replacement reaction between gold ions and metallic silver, the metallic silver on the surface of the metal silver nanoparticles becomes silver ions and dissolves. Then, instead, metallic gold is deposited, and metal gold-coated metal silver nanoparticles are formed.

[0045] The gold source is not particularly limited as long as it can be dissolved in the solvent (such as alcohol or water) of the reaction solution. For example, tetrachloride gold(III) acid (HAuCl 4 ) and / or its hydrate can be mentioned. The supply of the gold source may be carried out by adding and mixing the gold source to the reaction solution. Ultrasonic irradiation treatment may be carried out during or after the addition of the gold source. The liquid temperature of the reaction solution is preferably maintained at 20 to 60 °C, more preferably 30 to 50 °C, during or after the addition of the gold source.

[0046] The product obtained by the galvanic replacement reaction (gallium oxide particles supporting gold-coated silver nanoparticles) exists in a dispersed or precipitated state in the reaction solution. Therefore, the product may be recovered from the reaction solution and dried. For the recovery, known separation means such as filtration or centrifugation may be used. Also, the drying may be performed under conditions where the gold-coated silver nanoparticles do not undergo excessive grain growth, for example, at 100 °C or lower. In this way, photocatalytic particles composed of metal nanoparticle-supported gallium oxide particles can be obtained. As described above, it is preferable that the loading amount of the gold-coated silver nanoparticles is 0.3 to 10.0% by mass based on the gallium oxide particles. The loading amount can be adjusted by controlling the blending amounts of the gold and silver sources and the ultrasonic treatment conditions.

[0047] According to the production method of the present embodiment, particularly by performing ultrasonic treatment, it becomes possible to deposit silver nanoparticles with high dispersion and a high loading rate. In fact, it has been confirmed that silver nanoparticles can be supported at a high concentration of 5% by mass, thereby obtaining photocatalytic particles excellent in CO selectivity. On the other hand, in the conventionally proposed impregnation method and photoelectrodeposition method, it is difficult to support metal silver nanoparticles with high dispersion and a high loading rate. For example, Patent Document 1 states that regarding the silver loading amount, in the case of the photoelectrodeposition method, it is 0.2 to 2% by mass based on gallium oxide, and in the case of the impregnation method, it is 0.05 to 2%. When it is more than the optimum range, the particle size of the silver nanoparticles becomes large and the effects such as catalytic activity are lost, or the surface active sites of gallium oxide are reduced (

[0014] and

[0015] of Patent Document 1).

[0048] Also, in the production method proposed in Patent Document 2, the ultrasonic treatment is only performed to disperse the noble metal oxide, and the reduction of the noble metal oxide is attempted in the heating step after the ultrasonic treatment (

[0038] of Patent Document 2). Therefore, it is clearly different from the method of the present embodiment in which the silver source (such as silver oxide) is reduced by ultrasonic treatment. In the production method of the present embodiment, it is possible to support sufficiently reduced silver nanoparticles without performing heating after the ultrasonic treatment.

[0049] Furthermore, according to the manufacturing method of the present embodiment, although not limited, it is possible to use a compound such as silver oxide that does not dissolve in the reaction solution as a silver source in a solid state. When a compound that does not dissolve in the reaction solution is used, the reaction solution does not contain harmful substances such as anions, and waste liquid treatment is easy. On the other hand, in the impregnation method and the photoelectrodeposition method proposed in Patent Document 1, a precursor solution such as an aqueous solution in which silver nitrate is dissolved is used. Anions such as nitrate ions contained in such a precursor solution are harmful substances that cause air pollution. Therefore, in the impregnation method and the photoelectrodeposition method, waste liquid treatment for detoxifying harmful substances is necessary. However, the manufacturing method of the present embodiment does not exclude the use of a compound that dissolves in the reaction solution. Even in such a case, the effect of depositing metal silver nanoparticles with high dispersion and high loading rate can be obtained.

[0050] Moreover, by adopting the manufacturing method of the present embodiment, it is possible to easily obtain catalyst particles excellent in catalyst performance, particularly CO selectivity and gas production amount. Although the detailed reason is unknown, it is presumed that this is because the silver nanoparticles generated by ultrasonic reduction are fine and have a specific electron transfer state. That is, the silver nanoparticles (promoters) generated by the ultrasonic reduction treatment have a small particle size. It is also considered that they have a specific electron transfer state due to the action of high-temperature and high-pressure hot spots and radicals generated during the ultrasonic treatment. In fact, there is a report that the hot spots generated by ultrasonic waves are at a temperature close to 5000 °C. It is easily expected that the electron transfer state will change even if such a high-temperature hot spot acts instantaneously. It is presumed that the combined action of this fine particle size and the specific electron transfer state brings about excellent catalyst performance.

Example

[0051] The present embodiment will be further specifically described by the following examples. However, the present invention is not limited to the following examples.

[0052] (1) Preparation of photocatalyst particles [Example 1] Gold-coated silver nanoparticles were generated by ultrasonic treatment and galvanic replacement reaction to produce photocatalytic particles (gallium oxide particles supported with gold-coated silver nanoparticles). The silver concentration with respect to gallium oxide was 2.97% by mass, and the gold concentration was 0.03% by mass. Specifically, samples were prepared as follows.

[0053] <Preparation step> Gallium oxide particles (Kohjundo Chemical Laboratory Co., Ltd., Ga 2 O 3 ) and silver oxide (FUJIFILM Wako Pure Chemical Corporation, Ag 2 O) were prepared. The gallium oxide particles had a purity of 99.99% and an average particle size of approximately 3 μm in major axis and approximately 1 μm in minor axis. The silver oxide had a purity of 99% and was an aggregate with a primary particle size of approximately 2 μm.

[0054] <Mixing step> The prepared gallium oxide particles (1 g) and silver oxide (32 mg) were added to a reducing solution (50 mL). Ethanol (FUJIFILM Wako Pure Chemical Corporation) was used as the reducing solution. Thereby, a reaction solution was prepared.

[0055] <Ultrasonic treatment step> The obtained reaction solution was placed in an ultrasonic device (Honda Electronics Co., Ltd., WT-100-M) and subjected to ultrasonic treatment. The ultrasonic treatment was performed with two-frequency switching oscillation of 28 kHz and 45 kHz under the condition of an output of 100 W. The treatment time was 3 hours. At this time, the temperature of the reaction solution was maintained at 40°C. By this treatment, silver oxide (Ag 2 O) in the reaction solution was reduced to silver (Ag).

[0056] <Galvanic replacement reaction step> HAuCl 4An aqueous solution (0.24 M, 6.3 μL, FUJIFILM Wako Pure Chemical Corporation) and 3 mL of ultrapure water were mixed to prepare a mixed solution. Then, with ultrasonic irradiation continuing, the mixed solution was added to the reaction solution, and ultrasonic irradiation was performed for 30 minutes. Next, after filtering the product generated by the treatment, silver chloride generated was removed by ultrasonic cleaning with 50 mL of 2.8% aqueous ammonia. Next, it was washed with ultrapure water (50 mL) and dried under the conditions of 60 °C for 1.5 hours in the air to obtain gold-coated silver nanoparticle-supported gallium oxide particles as photocatalyst particles. At this time, the silver concentration with respect to gallium oxide was 2.97% by mass, and the gold concentration was 0.03% by mass.

[0057] [Example 2 (Reference Example)] The Au coating treatment by the galvanic replacement reaction step was not performed, and the particles were collected by filtration after the ultrasonic treatment step and dried to obtain particles. Otherwise, photocatalyst particles were prepared in the same manner as in Example 1. The silver concentration with respect to gallium oxide was 3.0% by mass.

[0058] [Example 3] HAuCl 4 The addition amount of the aqueous solution (0.24 M) was 190 μL. Otherwise, photocatalyst particles were prepared in the same manner as in Example 1. The silver concentration with respect to gallium oxide was 2.1% by mass, and the gold concentration was 0.9% by mass.

[0059] (2) Evaluation of photocatalyst particles For each sample obtained in Examples 1 to 3, the evaluation of various properties was performed as follows.

[0060] <STEM Observation> The samples were observed using a scanning transmission electron microscope (STEM; Hitachi High-Technologies Corporation, HD2700). The observation was performed under the conditions of a transmission electron image with an acceleration voltage of 200 kV. Also, elemental analysis of the nanoparticles was performed using the EDX device attached to the microscope to examine the distribution of the constituent elements.

[0061] <Diffuse Reflection Spectrum> The diffuse reflection spectrum of the sample in the solid state was measured in the wavelength range of 200 to 800 nm using an ultraviolet-visible spectrophotometer (JASCO Corporation, V-650).

[0062] <CO 2 Reductive photocatalytic performance CO of the sample 2 The reductive photocatalytic performance of the sample was evaluated using the evaluation apparatus shown in Fig. 1. First, ultrapure water (1 L), NaHCO 3 (0.1 M) and photocatalytic particles (0.5 g) were mixed to prepare an evaluation solution. Next, this evaluation solution was placed in the tank of the evaluation apparatus, and while blowing carbon dioxide (CO 2 ) gas at a flow rate of 30 mL / min, UV light was irradiated with a 400 W high-pressure Hg lamp. The gas generated after 1 hour of irradiation was analyzed using gas chromatography (Shimadzu Corporation, GC-8A) to determine the generation rates of H 2 , O 2 and CO. Then, the CO selectivity was calculated based on the following formula (1).

Equation

[0063] (3) Evaluation results <STEM observation> The STEM photographs obtained for the samples of Examples 1 to 3 are shown in Figs. 3 to 5, respectively. The black particles present in the figures are silver nanoparticles. In all samples, nanoparticles were supported on the surface of gallium oxide particles.

[0064] For the samples of Example 1 and Example 3, the EDX line analysis results of the nanoparticles are shown in Figs. 6 and 7. In the sample of Example 1, it was found that the surface of the silver nanoparticles was replaced by gold (Fig. 6). On the other hand, in the sample of Example 3, gold was present not only on the surface but also inside the silver nanoparticles (Fig. 7).

[0065] <Diffuse reflection spectrum> The diffuse reflection spectra obtained for the samples of Examples 1 to 3 are shown in Fig. 8. In all the samples, peaks presumably due to silver nanoparticles were observed in the vicinity of wavelengths of 350 nm to 550 nm.

[0066] <CO 2 Reductive photocatalytic performance> The CO 2 reductive photocatalytic performance (gas generation rate and CO selectivity) obtained for Examples 1 to 3 are shown in Table 1 below and Fig. 9. In Example 1 (Au0.03%-Ag2.97%), compared with Example 2 (Ag only), the generation amounts of all gases of CO, H 2 and O 2 increased. Although the CO selectivity decreased, its value was 30.8%, which was at a level where sufficient utilization as synthesis gas could be expected. In Example 3 (Au0.9%-Ag2.1%), H 2 generation was prioritized and the CO selectivity decreased to 14.7%.

[0067] [Table 1]

Claims

**Claim 1** Gallium oxide (Ga ２ O ３ ) particles and metal gold-coated metal silver nanoparticles supported on the surface of the gallium oxide particles, and The metal gold-coated metal silver nanoparticles include silver (Ag) nanoparticles and a gold (Au) coat provided on the surface of the silver nanoparticles, which are carbon dioxide reduction photocatalyst particles having a peak in the wavelength range of 350 to 550 nm in the diffuse reflection spectrum. **Claim 2** The carbon dioxide reduction photocatalyst particles according to claim 1, wherein the average particle diameter of the metal gold-coated metal silver nanoparticles is 10.0 to 50.0 nm. **Claim 3** The carbon dioxide reduction photocatalyst particles according to claim 1 or 2, wherein the loading amount of the metal gold-coated metal silver nanoparticles is 0.3 to 10.0% by mass based on the gallium oxide particles. **Claim 4** The carbon dioxide reduction photocatalyst particles according to any one of claims 1 to 3, wherein the coating amount of gold in the metal gold-coated metal silver nanoparticles is 0.1 to 20.0% by mass based on silver.

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