JP7708396B2 - Method for producing carbon dioxide reducing photocatalyst particles - Google Patents

Description

The present invention relates to a method for producing carbon dioxide reducing photocatalyst particles.

Water splitting and carbon dioxide reduction technology using semiconductor photocatalyst particles has attracted attention as a technology that can solve energy problems and environmental problems. By supporting nanoparticles made of silver (Ag) or the like as a co-catalyst on these photocatalyst particles, it is expected that the effect of trapping electrons generated by photoexcitation to promote charge separation and the effect of selecting carbon dioxide reduction products can be expected. For example, in a normal photocatalyst, water is decomposed into hydrogen (H ₂ ) and oxygen (O ₂ ) by light irradiation. In contrast, in a photocatalyst supporting silver particles, carbon monoxide (CO) is generated together with hydrogen (H ₂ ) by reduction of carbon dioxide (CO ₂ ). As such a carbon dioxide reduction photocatalyst, gallium oxide particles supporting silver nanoparticles as a co-catalyst (gallium oxide particles supporting silver nanoparticles) are known.

Carbon monoxide (CO) is an important starting material in the chemical and industrial industries, and can be reacted with hydrogen to synthesize various fuels and chemicals. In such synthesis, a mixture 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 in a ratio of CO:H ₂ = 1:2 is used to synthesize methanol.

In producing synthesis gas (syngas), it is advantageous to use a carbon dioxide reduction photocatalyst that produces carbon monoxide and hydrogen. In order to efficiently obtain synthesis gas, it is desirable for the carbon dioxide reduction photocatalyst to be able to control the carbon monoxide production ratio, i.e., selectivity, and to produce a large amount of gas. Here, the CO selectivity is the ratio of the carbon monoxide (CO) gas production rate to the total of the hydrogen (H ₂ ) gas production rate (amount produced) and the carbon monoxide (CO) gas production rate produced by the reduction reaction, as expressed by the following formula (1).

As a method for supporting metal nanoparticles, methods such as impregnation and photoelectrodeposition (photoelectrodeposition) have been conventionally known. For example, Patent Document 1 describes a method for reducing carbon dioxide, which includes irradiating CO ₂ , H ₂ O, and a photocatalyst with light to generate CO by a reaction that reduces CO ₂ , using a catalyst in which silver is supported on gallium oxide by photoelectrodeposition or impregnation, adding gallium oxide powder to an alcoholic aqueous solution containing a silver precursor such as silver nitrate and mixing it, then irradiating light to reduce the silver precursor, and adding gallium oxide to an aqueous silver precursor solution and stirring it, removing water, drying it by heating, and then calcining it in air (claims 1, 2, and [0015] of Patent Document 1).

Although not related to carbon dioxide reduction catalysts, Patent Document 2 discloses a method for producing a precious metal nanomaterial, which includes a step of dispersing one or more types of precious metal oxide in a solvent by irradiating ultrasonic waves to obtain a precious metal oxide dispersion liquid, and a step of heating the precious metal oxide dispersion liquid. It also describes that the solvent further contains a carrier for supporting the precious metal, and that it is possible to obtain a precious metal nanomaterial supported with high dispersibility on the surface of the carrier, which can be suitably used as a catalyst for fuel cells, a catalyst for material synthesis, etc. (Claims 1, 6, and [0014] of Patent Document 2).

JP 2012-192302 A JP 2008-24968 A

However, the inventors' research revealed that the photocatalysts produced by the impregnation method and photoelectrodeposition method proposed in Patent Document 1 have a certain degree of effectiveness, but there is room for improvement. That is, in order to fully exert the effect of the promoter silver (Ag), it is desirable to increase the amount of silver supported to a certain extent. It is also desirable for the silver particle size to be small, on the order of several tens of nanometers. This is because catalytic activity is lost if the particle size is too large. In this regard, photocatalyst particles produced by the impregnation method and photoelectrodeposition method have the problem that if the silver concentration (supported amount) is increased, the silver particles aggregate and the particle size becomes large. For this reason, it is difficult to support a high amount of silver nanoparticles with a small particle size.

Patent Document 2 does not intend to use precious metal nanomaterials as carbon dioxide reduction photocatalysts, much less to reduce the particle size of nanomaterials to the order of several tens of nanometers. In fact, in the examples of cited document 2, it merely teaches that precious metal (Pt) nanoparticle-supported spherical carbon and precious metal (Pt) nanotubes are produced, and that they are suitable for use as catalysts for fuel cells, catalysts for material synthesis, medical and food additives, and conductive pastes (Patent Document 2, [0043] to [0062]).

On the other hand, when synthesizing metal nanoparticles, a method is known in which the raw material concentration is reduced and a large amount of organic protective agent is used to synthesize uniform and fine particles. However, when photocatalyst particles are produced using this method, there are problems in that the yield of photocatalyst particles is low and the organic protective agent adheres to the particle surface. Since the attached organic protective agent reduces the catalytic activity, the catalyst particles must be fired at high temperatures to decompose and remove it. In catalyst particles that have been fired in this way, the particle size of the metal nanoparticles becomes large. Therefore, using conventional methods, it is difficult to highly disperse and support fine silver nanoparticles in high amounts, and there are limitations to efficiently producing photocatalyst particles with excellent catalytic performance.

The present inventors have conducted studies in light of such conventional problems, and have discovered that by a simple method of irradiating gallium oxide particles and a silver supply source with ultrasound, it is possible to precipitate silver nanoparticles as promoters with high dispersion and high loading, and further, by gold-coating the surfaces of the silver nanoparticles, it is possible to increase the amount of _H2 generated while maintaining the amount of CO generated, and as a result, it is possible to obtain photocatalyst particles with excellent catalytic performance.

The present invention was completed based on these findings, and its objective is to provide a method for producing carbon dioxide reduction photocatalyst particles with excellent catalytic performance.

The present invention encompasses the following aspects. In this specification, the expression "-" includes both ends of the expression. In other words, "X-Y" is synonymous with "X or more and Y or less."

Providing gallium oxide ( _Ga2O3 ) particles and _a silver (Ag) source;
adding the gallium oxide particles and the silver source to a reduction solution to prepare a reaction solution;
A method for producing carbon dioxide reducing photocatalyst particles, comprising: a step of irradiating the reaction solution with ultrasound to produce gallium oxide particles carrying metallic silver nanoparticles; and a step of supplying a gold (Au) source to the reaction solution to produce photocatalyst particles consisting of gallium oxide particles carrying metallic gold-coated metallic silver nanoparticles.

The present invention provides a method for producing carbon dioxide reducing photocatalyst particles with excellent catalytic performance.

FIG. 2 is a schematic cross-sectional view showing an example of an evaluation device. 1 shows the mechanism of silver nanoparticle loading. 1 shows an STEM photograph of photocatalyst particles (Example 1). 1 shows an STEM photograph of photocatalyst particles (Example 2). 1 shows an STEM photograph of photocatalyst particles (Example 3). 1 shows the EDX line analysis results of nanoparticles (Example 1). 1 shows the EDX line analysis results of nanoparticles (Example 3). 1 shows the diffuse reflectance spectra of photocatalyst particles (Examples 1 to 3). The CO2 _reduction photocatalytic performance of photocatalyst particles (Examples 1 to 3) is shown.

A specific embodiment of the present invention (hereinafter referred to as "the present embodiment") is described below. However, the present invention is not limited to the following embodiment, and various modifications are possible within the scope of the present invention.

<<1. Photocatalyst particles >>
The carbon dioxide reduction photocatalyst particles of this embodiment (hereinafter sometimes referred to as "photocatalyst particles") include gallium oxide ( _Ga2O3 ) particles and metallic _gold -coated metallic silver nanoparticles supported on the surfaces of the gallium oxide particles. The metallic gold-coated metallic silver nanoparticles (hereinafter sometimes referred to as "gold-coated silver nanoparticles") have metallic silver (Ag) nanoparticles and a gold (Au) coating provided on the surfaces thereof.

Gallium oxide (Ga ₂ O ₃ ) particles function as a main catalyst, and are not particularly limited as long as they are used for photocatalysis. Ga ₂ O ₃ is known to be of α-type, β-type, γ-type, δ-type, and ε-type, and any of them may be used. However, β-type (β-Ga ₂ O ₃ ), which is a stable oxide, is preferable. The size of the particles is also not particularly limited. For example, the average particle diameter of the particles is 0.3 to 5.0 μm. Furthermore, the shape of the particles is also not particularly limited. For example, spherical, irregular, and anisotropic shapes (rod or plate-like, etc.) can be mentioned. When the particles are rod-like, 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.

The gold-coated silver nanoparticles function as co-catalysts, and their average particle size is on the order of nanometers. By making the nanoparticles that function as co-catalysts finer, the catalytic performance of the photocatalyst is improved. On the other hand, if the nanoparticles are larger than the order of nanometers, there is a risk that they will lose their co-catalyst functions, such as catalytic activity, or that the surface active sites of the gallium oxide particles will decrease.

The silver co-catalyst in the gold-coated silver nanoparticles selectively produces carbon monoxide (CO) in the photoreduction of carbon dioxide (CO ₂ ). On the other hand, gold promotes hydrogen production by reduction of hydrogen ions (H ⁺ ) rather than reduction of CO ₂ . When gold-coated silver nanoparticles are used as a co-catalyst as in this embodiment, gold has a larger work function than silver, and therefore promotes charge separation of electrons and holes generated by photoexcitation. This makes it easier for the photocatalytic reaction to occur, and as a result, an increase in the amount of gas produced is expected.

The photocatalyst particles of this embodiment preferably have a peak in the wavelength range of 350 to 550 nm in the diffuse reflectance spectrum. This provides the photocatalyst particles with excellent catalytic performance. The diffuse reflectance spectrum is an absorption spectrum obtained by the diffuse reflectance method. When light is irradiated onto a solid sample such as a powder, a portion of the light leaves the sample after a scattering process. In the scattering process, a portion of the irradiated light is reflected by the surface of the sample, and the remainder penetrates into the sample. A portion of the penetrating incident light is absorbed by the electronic transition state of the sample, and the remainder leaves the sample. The absorption spectrum can be obtained from the incident light intensity of ultraviolet or visible light and the light intensity after the scattering process.

In the photocatalyst 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 is possible to form silver nanoparticles without the need to lower the Ag concentration (loading amount) or add an organic surface protective 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 function as a cocatalyst, such as catalytic activity, or a decrease in the surface active sites of 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 photocatalyst particles using a transmission electron microscope (TEM). As an example, a method is given in which the particle size distribution of the gold-coated silver nanoparticles is determined visually or by image analysis software, and the number average value is calculated from this particle size distribution.

In the photocatalyst particles of this embodiment, the amount of gold-coated silver nanoparticles supported is preferably 0.3 to 10.0% by mass relative to the gallium oxide particles. If the amount supported is too small, it becomes difficult to fully exert the effect of the co-catalyst. Therefore, when the photocatalyst is used for _CO2 reduction, the CO gas generation rate and CO selectivity become low. On the other hand, if the amount supported is too large, the CO gas generation rate decreases. The amount supported can be adjusted by controlling the amount of gold and silver source blended and the ultrasonic treatment conditions during the production of the photocatalyst particles. The amount of gold-coated silver nanoparticles supported 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. The amount supported 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.

In the photocatalyst particles of this embodiment, the amount of metallic gold (Au) carried (coated amount) in the gold-coated silver nanoparticles is preferably 0.1 to 20.0 mass% relative to metallic silver (Ag). If the amount of gold coating is too small, it becomes difficult to fully exert the effect of the gold coating. On the other hand, if the amount of gold coating is too large, the effect of the gold promoter becomes large, and only H ₂ is generated without generating CO. The amount of gold coating may be 0.1 mass% or more, 0.3 mass% or more, 0.5 mass% or more, or 1.0 mass% or more. The amount of gold coating may be 20.0 mass% or less, 10.0 mass% or less, 5.0 mass% or less, or 3.0 mass% or less.

In the photocatalyst particles of this embodiment, the CO selectivity of the photocatalyst particles is preferably 20% or more in a _CO2 reduction photocatalyst performance evaluation test. This makes it possible to increase the proportion of the amount of CO gas generated by _CO2 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.

The _CO2 reduction photocatalytic performance evaluation test may be performed using a known evaluation device. An example of the evaluation device is shown in FIG. 1. The evaluation device (2) is composed of a tank (4) and a high-pressure mercury (Hg) lamp (6) installed inside the tank (4). The tank (4) is filled with an evaluation solution (22). The tank (4) is also equipped with a gas inlet pipe (8), a gas outlet pipe (10), a pH meter (12), a rubber stopper (14), and a stirrer (16). A bubbling filter (18) is installed at the tip of the gas inlet pipe (8). The mercury lamp (6) is cooled by cooling water (20) flowing around it.

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

The details of the mechanism by which photocatalyst particles with a peak in a specific wavelength range (350-550 nm) in the diffuse reflectance spectrum show high CO selectivity are unknown. However, it is speculated that the particle size of the gold-coated silver nanoparticles and the electronic transition state may be related. In other words, the spectrum obtained by the diffuse reflectance method (diffuse reflectance spectrum) reflects the electronic transition state of the sample, such as the valence of the species, the coordination structure, and the ligand field. In addition, when the sample is a fine particle, it may show different absorption energy depending on the particle size. For example, it is known that fine particles give an absorption band in the wavelength range corresponding to plasmon resonance. Therefore, in the photocatalyst particles of this embodiment with a peak in a specific wavelength range (350-550 nm), the gold-coated silver nanoparticles are considered to have a fine and unique electronic transition state.

On the other hand, the particle size and electronic transition state of the gold-coated silver nanoparticles are expected to affect the catalytic performance of the photocatalyst, for example, the CO selectivity. This will be explained based on the mechanism of _CO2 reduction of gallium oxide particles (photocatalyst particles) supported by gold-coated silver nanoparticles. When light with energy hν is irradiated onto the particles, electrons ( ^e- ) and holes (h ⁺ ) are generated in the gallium oxide particles (semiconductor photocatalyst particles). At this time, the metallic silver nanoparticles (co-catalyst) promote charge separation (separation of electrons ^e- and holes h ⁺ ). As a result of the reaction of the holes (h ⁺ ) with the surrounding moisture ( _H2O ), the reaction shown in the following formula (2) proceeds to the right, and oxygen ( _O2 ) and protons (H ⁺ ) are generated. On the other hand, as a result of electrons (e ^- ) reacting with carbon dioxide (CO ₂ ) and protons (H ⁺ ), the reactions shown in the following formulas (3) and (4) proceed to the right, producing carbon monoxide (CO), water (H ₂ O), and hydrogen (H ₂ ). Furthermore, by combining the reactions in the following formulas (2) to (4), the reaction shown in the following formula (5) proceeds in principle.

When the reactions of the above formulas (3) and (4) occur to the same extent, the CO selectivity (CO generation rate/( _H2 generation rate+CO generation rate)) is constant as shown in the above formula (5). However, in reality, these reactions do not necessarily occur to the same extent. When the reaction of the above formula (3) occurs preferentially, the CO selectivity increases.

In the reaction of the above formula (3), it has been reported that carbon dioxide (CO ₂ ) is adsorbed on the catalyst surface as a carbonate species, and is converted to a formate species, which is a reaction intermediate, by irradiation with light, and then interacts with water molecules to become carbon monoxide (CO). This report also suggests that the silver promoter promotes the generation of reaction intermediates. Therefore, it is expected that the reaction of the above formula (3) will occur preferentially and the CO selectivity will be further improved by enhancing the charge separation action and reaction intermediate generation action of the promoter, which is a result of the photocatalyst particles of this embodiment having fine gold-coated silver nanoparticles with a unique electronic transition state. In this regard, it is speculated that the action of charge separation and the action of reaction intermediate generation are enhanced by the combined action of these.

<<2. Method for producing photocatalyst particles>>
The method for producing carbon dioxide reduction photocatalyst particles of this embodiment includes the following steps: a step of preparing gallium oxide (Ga ₂ O ₃ ) particles and a silver (Ag) supply source (preparation step), a step of adding gallium oxide particles and a silver supply source to a reduction solution to prepare a reaction solution (mixing step), a step of irradiating the reaction solution with ultrasonic waves to prepare gallium oxide particles carrying metallic silver nanoparticles (ultrasonic treatment step), and a step of supplying a gold (Au) supply source to the reaction solution to prepare photocatalyst particles consisting of gallium oxide particles carrying metallic gold-coated metallic silver nanoparticles (galvanic substitution reaction step). Details of each step are described below.

<Preparation process>
In the preparation step, gallium oxide (Ga ₂ O ₃ ) particles and a silver (Ag) supply source are prepared. The gallium oxide (Ga ₂ O ₃ ) particles are not particularly limited as long as they are used in photocatalysis. Ga ₂ O ₃ is known to be of α-type, β-type, γ-type, δ-type, and ε-type, and any of them may be used. However, β-type (β-Ga ₂ O ₃ ), which is a stable oxide, is preferable. The size of the particles is also not particularly limited. For example, the average particle diameter of the particles is 0.3 to 5.0 μm. Furthermore, the shape of the particles is also not particularly limited. For example, spherical, irregular, and anisotropic shapes (rod or plate-like, etc.) can be mentioned. When the particles are rod-like, 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.

The silver (Ag) supply source is not limited as long as it can supply silver. Specific examples include oxides, inorganic metal salts, and/or organic metal compounds. Examples of inorganic metal salts include nitrates, chlorides, and/or sulfates. The silver supply source may be soluble in the reducing solution, or may not be soluble in the reducing solution. Preferably, the silver supply source includes silver oxide. Since silver oxide is composed only of silver ions and oxygen ions, it is easy to handle and there is no problem with waste disposal. As silver oxide, Ag ₂ O, AgO, and Ag ₂ O _3, which have different oxidation numbers of silver, are known, and any of them can be used. However, Ag ₂ O, which is more easily available, is preferable. The size of the silver supply source is also not particularly limited. For example, the average particle size of the silver supply source is 0.3 to 3.0 μm.

<Mixing process>
In the mixing step, the prepared gallium oxide particles and silver source are added to the reduction solution to prepare a reaction solution. The mixing ratio of the gallium oxide particles and the silver source may be adjusted so that the amount of silver carried in the final photocatalyst is the desired value. If the amount of silver carried is too small, it becomes difficult to fully exert the effect of the co-catalyst. Therefore, when the photocatalyst is used for _CO2 reduction, the CO gas generation rate and CO selectivity are reduced. On the other hand, if the amount of silver carried is too large, the CO gas generation rate is reduced.

The reducing liquid is not limited as long as it is a liquid having reducing properties. It may be a liquid having reducing properties by itself, or may be a liquid having no reducing properties in which a reducing agent is dissolved. However, it is preferable that the liquid has reducing properties by itself. It is also not necessary to contain a separate reducing agent. As such a reducing liquid, alcohols such as ethanol and propanol, which are less toxic and easily available, are preferable. A mixture of alcohols and water can also be used. However, when using a mixed liquid, if the water content is excessively high, it becomes difficult to exert a sufficient reducing effect. Therefore, the water content in the reducing liquid 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.

<Ultrasonic treatment process>
In the ultrasonic treatment step, the reaction solution is irradiated with ultrasonic waves to produce photocatalyst particles consisting of gallium oxide particles supported by metallic silver nanoparticles. During this process, the surface of the silver source in the reaction solution is ultrasonically reduced to become metallic silver (Ag) nanoparticles, which are supported on the surface of the gallium oxide particles.

The mechanism of metallic silver nanoparticle support is explained using Figure 2. When ultrasonic waves are irradiated, compression waves are generated in the reaction solution, which generates repeated positive and negative pressures. During negative pressure cycles, countless fine bubbles are generated in the reaction solution due to evaporation. During positive pressure cycles, these bubbles collapse and exert a strong impact force on the surroundings. This phenomenon is called ultrasonic cavitation. Cavitation uniformly disperses the gallium oxide particles and silver source in the reaction solution and cleans its surface. Cavitation also generates tiny hot spots of high temperature and pressure. The generated hot spots decompose and reduce the silver source, and also act on the reaction solution to generate radicals, which promote the decomposition and reduction of the silver source. In this way, metallic silver nanoparticles are generated from the silver source.

For example, when solid silver oxide (Ag ₂ O) is used as a silver supply 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. The silver nanoparticles grow gradually, and when they grow to a certain extent, the interface stress between the silver oxide and the silver nanoparticles becomes a limit and they are detached. Alternatively, an intermediate product is generated from the silver oxide by the action of ultrasound, and hot spots and radicals act on this intermediate product to generate silver nanoparticles in the reaction solution. The detached or generated silver nanoparticles move to the surface of the gallium oxide particles by the physical action of ultrasound and are adsorbed there. In this way, gallium oxide particles supported by silver nanoparticles are obtained. The silver nanoparticles generated by ultrasonic reduction are fine. In addition, since no organic protective agent or high-temperature firing is required, it is possible to prepare gallium oxide particles (photocatalyst particles) supported with silver nanoparticles in a fine state.

If the photocatalyst particles contain a silver source other than silver nanoparticles, it may be difficult to fully exert the effect of the co-catalyst. In this regard, by ultrasonic treatment, it is possible to obtain photocatalyst particles that contain almost no silver source (silver oxide, etc.). For example, it is possible to obtain photocatalyst particles in which no peak of silver oxide (Ag ₂ O) is observed in the X-ray diffraction pattern.

There is no need to use a special device for the ultrasonic treatment, and a device equipped with a normal ultrasonic oscillation source may be used. For example, a commercially available ultrasonic cleaner may be used. The treatment may be performed under normal conditions. For example, the frequency of the ultrasonic waves may be 20 to 100 kHz, or 28 to 45 kHz. The ultrasonic treatment may be performed continuously at the same frequency, or the frequency may be switched during the treatment using a frequency oscillation switching mode. The frequency may be switched once or multiple times. By performing the treatment in a frequency oscillation switching mode (for example, a two-frequency switching oscillation mode of 28 kHz/45 kHz), it is possible to further improve the dispersibility of the gallium oxide particles in the liquid. In addition, the output of the ultrasonic waves may be 10 to 500 W, or 50 to 200 W. Furthermore, the treatment time may be 1 to 10 hours. By extending 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.

<Galvanic substitution reaction process>
In the galvanic substitution reaction process, a gold source is supplied to the reaction solution after the reduced metallic silver is produced. This produces photocatalyst particles consisting of gallium oxide particles supported by metallic gold-coated metallic silver nanoparticles. The galvanic substitution reaction between the gold ions and metallic silver causes the metallic silver on the surface of the metallic silver nanoparticles to become silver ions and dissolve. Then, metallic gold is precipitated in its place, producing metallic gold-coated metallic silver nanoparticles.

The gold source is not particularly limited as long as it dissolves in the solvent of the reaction solution (alcohol, water, etc.). For example, tetrachloroauric (III) acid (HAuCl ₄ ) and/or its hydrate can be mentioned. The gold source can be supplied by adding and mixing the gold source to the reaction solution. Ultrasonic irradiation treatment may be performed during or after the addition of the gold source. 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.

The product obtained by the galvanic substitution reaction (gallium oxide particles supported by gold-coated silver nanoparticles) exists in a dispersed or precipitated state in the reaction solution. Therefore, the product can be recovered from the reaction solution and dried. For recovery, known separation means such as filtration or centrifugation can be used. Drying can be performed under conditions that do not cause excessive particle growth of the gold-coated silver nanoparticles, for example, at 100°C or less. In this way, photocatalyst particles consisting of metal nanoparticle-supported gallium oxide particles can be obtained. As mentioned above, the amount of gold-coated silver nanoparticles supported is preferably 0.3 to 10.0% by mass relative to the gallium oxide particles. The amount supported can be adjusted by controlling the amounts of gold and silver sources mixed and the ultrasonic treatment conditions.

According to the manufacturing method of this embodiment, it is possible to precipitate silver nanoparticles with high dispersion and high loading rate, especially by performing ultrasonic treatment. In fact, it has been confirmed that silver nanoparticles can be loaded at a high concentration of 5 mass%, thereby obtaining photocatalyst particles with excellent CO selectivity. In contrast, it is difficult to load metallic silver nanoparticles with high dispersion and high loading rate using the impregnation method and photoelectrodeposition method proposed in the past. For example, Patent Document 1 describes that the amount of silver loaded is 0.2 to 2 mass% of gallium oxide in the case of photoelectrodeposition method and 0.05 to 2% in the case of impregnation method, and that if it is more than the optimal range, the particle diameter of the silver nanoparticles becomes large, the effect of catalytic activity is lost, and the surface active sites of gallium oxide are reduced ([0014] and [0015] of Patent Document 1).

In addition, in the manufacturing method proposed in Patent Document 2, ultrasonic treatment is performed only to disperse the precious metal oxide, and the precious metal oxide is reduced in a heating step after ultrasonic treatment ([0038] of Patent Document 2). Therefore, it is clearly different from the method of the present embodiment in which the silver source (silver oxide, etc.) is reduced by ultrasonic treatment. In the manufacturing method of the present embodiment, it is possible to support sufficiently reduced silver nanoparticles without heating after ultrasonic treatment.

Furthermore, according to the manufacturing method of this embodiment, although not limited thereto, it is possible to use a compound such as silver oxide that is not soluble in the reaction solution as a silver supply source in a solid state. When a compound that is not soluble in the reaction solution is used, the reaction solution does not contain harmful substances such as anions, and waste liquid treatment is easy. In contrast, the impregnation method and photoelectrodeposition method proposed in Patent Document 1 use a precursor solution such as an aqueous solution in which silver nitrate is dissolved. Anions such as nitrate ions contained in such precursor solutions are harmful substances that cause air pollution. Therefore, the impregnation method and photoelectrodeposition method require waste liquid treatment to detoxify the harmful substances. However, the manufacturing method of this embodiment does not exclude the use of a compound that is soluble in the reaction solution. Even in such a case, the effect of precipitating metallic silver nanoparticles with high dispersion and high loading rate can be obtained.

Furthermore, by adopting the manufacturing method of this embodiment, it is possible to easily obtain catalyst particles with excellent catalytic performance, especially CO selectivity and gas production. Although the detailed reason is unknown, it is speculated that this is because the silver nanoparticles generated by ultrasonic reduction are fine and have a unique electronic transition state. In other words, the silver nanoparticles (co-catalyst) generated by ultrasonic reduction treatment have a small particle size. It is also thought that the unique electronic transition state is due to the action of high-temperature and high-pressure hot spots and radicals generated during ultrasonic treatment. In fact, it has been reported that the hot spots generated by ultrasonic waves are as high as nearly 5000°C, and it is easy to predict that the electronic transition state will change if such a high-temperature hot spot acts even momentarily. It is speculated that the fine particle size and the unique electronic transition state act in combination to bring about excellent catalytic performance.

This embodiment will be described in more detail with reference to the following examples. However, the present invention is not limited to the following examples.

(1) Preparation of photocatalyst particles [Example 1]
Photocatalyst particles (gold-coated silver nanoparticle-supported gallium oxide particles) were produced by generating gold-coated silver nanoparticles through ultrasonic treatment and galvanic substitution reaction. The silver concentration relative to gallium oxide was 2.97% by mass, and the gold concentration was 0.03% by mass. Specifically, the sample was produced as follows.

<Preparation process>
Gallium oxide particles ( _Ga2O3 , _Kojundo Chemical Laboratory Co., Ltd.) and silver oxide ( _Ag2O , Fujifilm Wako Pure Chemical Industries, Ltd.) were prepared. The gallium oxide particles had a purity of 99.99% and an average particle size of about 3 μm in the long axis and about 1 μm in the short axis. The silver oxide particles had a purity of 99% and were aggregates with a primary particle size of about 2 μm.

<Mixing process>
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 Industries, Ltd.) was used as the reducing solution. This produced a reaction solution.

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

<Galvanic substitution reaction process>
A mixed solution was prepared by mixing _an aqueous solution of HAuCl4 (0.24 M, 6.3 μL, Fujifilm Wako Pure Chemical Industries, Ltd.) with 3 mL of ultrapure water. The mixed solution was added to the reaction solution while continuing the ultrasonic irradiation, and ultrasonic irradiation was performed for 30 minutes. The product generated by the treatment was then filtered, and the silver chloride generated was removed by ultrasonic cleaning with 50 mL of 2.8% ammonia water. The product was then washed with ultrapure water (50 mL) and dried in air at 60 ° C. for 1.5 hours to obtain gold-coated silver nanoparticle-supported gallium oxide particles as photocatalyst particles. The silver concentration relative to gallium oxide at this time was 2.97% by mass, and the gold concentration was 0.03% by mass.

[Example 2 (Reference Example)]
The Au coating process was not performed by the galvanic substitution reaction process, and the particles were collected by filtration after the ultrasonic treatment process and dried to obtain particles.Other than that, photocatalyst particles were produced in the same manner as in Example 1. The silver concentration relative to gallium oxide was 3.0 mass%.

[Example 3]
The amount of the _HAuCl4 aqueous solution (0.24 M) added was 190 μL. Except for this, photocatalyst particles were prepared in the same manner as in Example 1. The silver concentration relative to gallium oxide was 2.1% by mass, and the gold concentration was 0.9% by mass.

(2) Evaluation of Photocatalyst Particles For each of the samples obtained in Examples 1 to 3, various properties were evaluated as follows.

<STEM observation>
The sample was observed using a scanning transmission electron microscope (STEM; Hitachi High-Technologies Corporation, HD2700). The observation was performed using a transmission electron image at an acceleration voltage of 200 kV. In addition, elemental analysis of the nanoparticles was performed using an EDX device attached to the microscope to examine the distribution of the constituent elements.

<Diffuse reflectance spectrum>
The diffuse reflectance spectrum of the solid-state sample was measured in the wavelength range of 200 to 800 nm using a UV-visible spectrophotometer (JASCO Corporation, V-650).

< _CO2 reduction photocatalytic performance>
The _CO2 reduction photocatalytic performance of the sample was evaluated using the evaluation device shown in Figure 1. First, ultrapure water (1 L), _NaHCO3 (0.1 M) and photocatalyst particles (0.5 g) were mixed to prepare an evaluation solution. Next, this evaluation solution was placed in the tank of the evaluation device, and while blowing in carbon dioxide ( _CO2 ) gas at a flow rate of 30 mL/min, UV light was irradiated from 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 _H2 , _O2 and CO generation rates. Then, the CO selectivity was calculated based on the following formula (1).

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

The results of EDX line analysis of nanoparticles for the samples of Example 1 and Example 3 are shown in Figures 6 and 7. In the sample of Example 1, it was found that the surface of the silver nanoparticles was replaced with gold (Figure 6). On the other hand, in the sample of Example 3, gold was present not only on the surface of the silver nanoparticles but also inside them (Figure 7).

<Diffuse reflectance spectrum>
8 shows the diffuse reflectance spectra obtained for the samples of Examples 1 to 3. In all samples, peaks presumably due to silver nanoparticles were observed in the wavelength range of about 350 nm to 550 nm.

< _CO2 reduction photocatalytic performance>
The _CO2 reduction photocatalytic performance (gas generation rate and CO selectivity) obtained for Examples 1 to 3 is shown in Table 1 and Figure 9 below. In Example 1 (Au 0.03% - Ag 2.97%), the amount of CO, _H2 , and _O2 gases produced increased compared to Example 2 (Ag only). Although the CO selectivity decreased, the value was 30.8%, which was a level that could be fully expected to be used as a synthetic gas. In Example 3 (Au 0.9% - Ag 2.1%), _H2 generation was prioritized, and the CO selectivity decreased to 14.7%.

Claims (10)

Providing gallium oxide ( _Ga2O3 ) particles and _a silver (Ag) source;
adding the gallium oxide particles and the silver source to a reduction solution to prepare a reaction solution;
A method for producing carbon dioxide reducing photocatalyst particles, comprising: a step of irradiating the reaction solution with ultrasound to produce gallium oxide particles carrying metallic silver nanoparticles; and a step of supplying a gold (Au) source to the reaction solution to produce photocatalyst particles consisting of gallium oxide particles carrying metallic gold-coated metallic silver nanoparticles. The method of claim 1 , wherein the silver (Ag) source comprises silver oxide (Ag ₂ O). The method according to claim 1 or 2, wherein the reducing solution contains an alcohol. The method according to any one of claims 1 to 3, wherein the frequency of the ultrasound is 28 to 45 kHz. The method according to any one of claims 1 to 4, wherein the ultrasonic irradiation is performed for 1 to 10 hours. The method according to any one of claims 1 to 5, wherein the average particle size of the metallic gold-coated metallic silver nanoparticles is 10.0 to 50.0 nm. The method according to any one of claims 1 to 6, wherein the amount of the metallic gold-coated metallic silver nanoparticles supported is 0.3 to 10.0 mass% relative to the gallium oxide particles. The method according to any one of claims 1 to 7, wherein the amount of metallic gold coated in the metallic gold-coated metallic silver nanoparticles is 0.1 to 20.0 mass% relative to the amount of metallic silver. The method according to any one of claims 1 to 8, wherein the CO selectivity of the photocatalyst particles is 20% or more in a _CO2 reduction photocatalyst performance evaluation test.
However, the CO2 _reduction photocatalyst performance evaluation test is performed under conditions in which an evaluation solution prepared by mixing ultrapure water (1 L), NaHCO3 (0.1 M) and the carbon dioxide reduction photocatalyst particles (0.5 g) is placed in a tank of an evaluation device, carbon dioxide (CO2) gas is then _{blown in} at a flow rate of 30 mL/min while UV light is irradiated from a 400 W high-pressure Hg lamp, and the gas generated after 1 hour of irradiation is analyzed using gas chromatography to determine the H2 _, O2 _and CO gas generation rates, and the CO selectivity is calculated based on the following formula (1).
The method according to any one of claims 1 to 9, wherein the photocatalyst particles have a peak in the wavelength range of 350 to 550 nm in the diffuse reflectance spectrum.