JP7637372B2 - Carbon dioxide reduction photocatalyst particles - Google Patents

Description

The present invention relates to carbon dioxide reduction 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. Therefore, in a carbon dioxide reduction photocatalyst, it is desirable to have a high carbon monoxide production rate, i.e., a high CO selectivity. Here, the CO selectivity is the ratio of the carbon monoxide (CO) gas generation rate to the total of the hydrogen (H ₂ ) gas generation rate (generation amount) and the carbon monoxide (CO) gas generation rate generated by the reduction reaction, as expressed by the following formula (1).

Meanwhile, methods such as impregnation and photoelectrodeposition (photoelectrodeposition) have been known as methods for supporting metal nanoparticles. 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 the mixture, and then irradiating light to reduce the silver precursor, and impregnation adding gallium oxide to an aqueous silver precursor solution, stirring the mixture, removing water, and then heating and drying the mixture, and then calcining the mixture 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 the steps of dispersing one or more types of precious metal oxide in a solvent by irradiating the solvent with ultrasonic waves to obtain a precious metal oxide dispersion, and heating the precious metal oxide dispersion. 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 on the surface of the carrier with high dispersibility, 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. However, with photocatalyst particles produced by the impregnation method or photoelectrodeposition method, there is a 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, Patent Document 2 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 calcined at high temperatures to decompose and remove it. In catalyst particles that have been calcined in this way, the particle size of the metal nanoparticles becomes large. Therefore, using conventional methods, it is difficult to support fine silver nanoparticles in a high dispersion and loading amount, and there are limitations in efficiently producing photocatalyst particles with excellent catalytic performance, especially CO selectivity.

The inventors have conducted research in light of these conventional problems and have discovered that by using a simple method of irradiating gallium oxide particles and a silver source with ultrasound, it is possible to precipitate silver nanoparticles as promoters with high dispersion and high loading, thereby obtaining a photocatalyst with improved CO selectivity.

The present invention was completed based on these findings, and aims to provide carbon dioxide reduction photocatalyst particles with improved CO selectivity.

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."

The present invention relates to a gallium oxide (Ga ₂ O ₃ ) particle and a metallic silver (Ag) nanoparticle supported on the surface of the gallium oxide particle,
Carbon dioxide reducing photocatalyst particles having a diffuse reflectance spectrum peak within the wavelength range of 350 to 550 nm.

The present invention provides carbon dioxide reduction photocatalyst particles with improved CO selectivity.

FIG. 2 is a schematic cross-sectional view showing an example of an evaluation device. The mechanism of photocatalytic reaction is shown. 1 shows the mechanism of silver nanoparticle loading. 1 shows an optical photograph of an example sample. 1 shows XRD patterns of the example and comparative sample. 1 shows XRD patterns (enlarged views) of the example and comparative sample. 1 shows XRD patterns (enlarged views) of the example and comparative sample. 1 shows an SEM photograph of an example sample (Example 3). 1 shows an SEM photograph of a comparative sample (Comparative Example 2). 1 shows an STEM photograph of an example sample (Example 3). 1 shows an STEM photograph of an example sample (Example 4). 1 shows an STEM photograph of a comparative sample (Comparative Example 1). 1 shows an STEM photograph of a comparative sample (Comparative Example 2). 1 shows the particle size distribution of silver nanoparticles in example samples. 1 shows the particle size distribution of silver nanoparticles in the samples of the examples and comparative examples. 1 shows the diffuse reflectance spectra of the example and comparative example samples. 1 shows the relationship between the amount of silver carried and the CO selectivity. 4 shows SEM photographs of the example sample before and after heat treatment. TG curves of the example sample before and after heat treatment are shown. 4 shows DTA curves of the example sample before and after heat treatment. 4 shows XRD patterns of the example sample before and after heat treatment.

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 that does not change the gist of the present invention.

1. Photocatalyst particles The carbon dioxide reduction photocatalyst particles of this embodiment (hereinafter, may be collectively referred to as "photocatalyst particles") include gallium oxide (Ga ₂ O ₃ ) particles and metallic silver (Ag) nanoparticles supported on the surface of the gallium oxide particles. The 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, anisotropic (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.

Metallic silver (Ag) nanoparticles function as a co-catalyst and have an average particle size on the order of nm. Refining metallic silver nanoparticles improves the catalytic performance of the photocatalyst. On the other hand, if silver nanoparticles are larger than the order of nm, there is a risk that they will lose their co-catalyst function, such as catalytic activity, or that the surface active sites of the gallium oxide particles will decrease.

The photocatalyst particles of this embodiment have a peak in the wavelength range of 350 to 550 nm in the diffuse reflectance spectrum. This gives the photocatalyst particles 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 metallic 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, and a decrease in the number of 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 silver nanoparticles is determined visually or by image analysis software, and the average value is calculated from this particle size distribution.

In the photocatalyst particles of this embodiment, the amount of metallic 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 cocatalyst. 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 blending amount of the silver source and the ultrasonic treatment conditions. The amount of 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.

The photocatalyst particles of this embodiment have excellent catalytic performance, particularly CO selectivity. For example, in a _CO2 reduction photocatalyst performance evaluation test, the CO selectivity is 30% or more. Therefore, it is possible to increase the proportion of CO gas generated by _CO2 reduction. The CO selectivity may be 40% or more, 50% or more, 60% or more, or 70% 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. 2. 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 the photocatalyst particles of this embodiment, which have a peak in a specific wavelength range (350 to 550 nm) in the diffuse reflectance spectrum, show high CO selectivity are unknown. However, it is speculated that the particle size of the metallic silver nanoparticles and the electronic transition state are 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, when the metallic silver 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 photocatalyst particles of this embodiment, which have a peak in a specific wavelength range (350 to 550 nm), have fine and unique electronic transition states of the supported silver nanoparticles.

On the other hand, the particle size and electronic transition state of 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 metallic silver nanoparticles (Figure 2). As shown in Figure 2, when light having energy hν is irradiated to 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 action of charge separation and the action of reaction intermediate generation by the metallic silver nanoparticles (promoter), in this regard. In this regard, it is speculated that the photocatalyst particles of this embodiment have a complex action of the supported silver nanoparticles, which are fine and have a unique electronic transition state, and therefore the action of charge separation and the action of reaction intermediate generation are enhanced.

2. Method for producing photocatalyst particles The photocatalyst of this embodiment is not limited in its production method. However, it is preferable that the photocatalyst particles (carbon dioxide reduction photocatalyst particles) are composite particles (metallic silver-supported gallium oxide particles) obtained by irradiating gallium oxide (Ga ₂ O ₃ ) particles and a silver (Ag) ion source with ultrasonic waves in a reduction solution. It is also preferable that the photocatalyst particles (carbon dioxide reduction photocatalyst particles) are heated composite particles (metallic silver-supported gallium oxide particles) obtained by further heat-treating the composite particles obtained by irradiating gallium oxide (Ga ₂ O ₃ ) particles and a silver (Ag) ion source with ultrasonic waves in a reduction solution at a temperature of 100° C. or higher in an oxygen-containing atmosphere. A preferred embodiment of the production method will be specifically described below.
This is not the case.

The method for producing photocatalyst particles preferably 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 the gallium oxide particles and the silver supply source to a reduction solution to prepare a reaction solution (mixing step), and a step of irradiating the reaction solution with ultrasonic waves to prepare gallium oxide particles carrying metallic silver nanoparticles (ultrasonic treatment step). Each step will be described in detail 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 step>
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 low. 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 gallium oxide particles carrying metallic silver nanoparticles. At this time, the surface of the silver source in the reaction solution is ultrasonically reduced to become metallic silver (Ag) nanoparticles, which are carried on the surface of the gallium oxide particles. The gallium oxide particles carrying metallic silver nanoparticles become photocatalyst particles.

The mechanism of metallic silver nanoparticle support is explained using Figure 3. When ultrasonic waves are irradiated, compressional 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 give a strong impact force to 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 carrying silver nanoparticles are obtained. The silver nanoparticles generated by ultrasonic reduction are minute. In addition, since no organic protective agent or high-temperature firing is required, it is possible to prepare gallium oxide particles (photocatalyst particles) carrying fine silver nanoparticles.

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. 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.

The product obtained by ultrasonic treatment (gallium oxide particles carrying metallic 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 silver nanoparticles, for example, at 100°C or less. As mentioned above, the amount of metallic silver nanoparticles carried is preferably 0.3 to 10.0% by mass relative to the gallium oxide particles. The amount carried can be adjusted by controlling the amount of silver source mixed and the ultrasonic treatment conditions.

<Heat treatment process>
If necessary, the gallium oxide particles carrying metallic silver nanoparticles may be heat-treated at a temperature of 100°C or higher in an oxygen-containing atmosphere. By subjecting the silver-loaded gallium oxide particles produced by the ultrasonic loading method to heat treatment, the organic matter derived from ethanol remaining on the particle surface can be removed, and the particle size of the silver (Ag) nanoparticles can be reduced. Due to the action of this heat treatment, CO can be more selectively generated when used as a _CO2 reduction photocatalyst.

The heat treatment is preferably carried out at 100°C or higher in an oxygen-containing atmosphere. There is no need to use special equipment for the heat treatment; a general electric furnace capable of firing in air can be used. At temperatures below 100°C, the effect on the decomposition of organic matter and on the particle size of silver (Ag) nanoparticles is small, and no effect can be expected.

In this way, photocatalyst particles consisting of gallium oxide particles carrying metallic silver nanoparticles can be obtained.

By adopting this manufacturing method, it is possible to easily obtain catalyst particles with excellent catalytic performance, especially CO selectivity. Although the detailed reason is unknown, it is speculated that this is because the supported 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 hot spots generated by ultrasonic treatment 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 combined action of this fine particle size and the unique electronic transition state results in excellent CO selectivity.

In addition, this manufacturing method makes it possible to deposit metallic silver nanoparticles with high dispersion and high loading rate. 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 the amount is more than the optimal range, the particle diameter of the silver nanoparticles becomes large, losing effects such as catalytic activity, and reducing the surface active sites of gallium oxide ([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 this manufacturing method, although not limited thereto, it is possible to use a compound such as silver oxide that does not dissolve in the reaction solution as a silver supply 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. 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 dissolves 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.

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.

[Experimental Example A]
(1) Preparation of photocatalyst particles Example 1
Photocatalyst particles (metallic silver nanoparticle-supported gallium oxide particles) were produced by reducing silver nanoparticles using ultrasonic treatment. The silver concentration (supported amount) relative to gallium oxide was 0.5 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 step>
The prepared gallium oxide particles (1 g) and silver oxide (5 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 condition of a two-frequency switching oscillation of 28 kHz and 45 kHz and an output of 100 W. The treatment time was changed between 0 and 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). Next, the product generated by the treatment was filtered, washed with ethanol (10 mL), and dried in the air at 60°C for 0.5 hours to obtain gallium oxide particles carrying metallic silver nanoparticles as photocatalyst particles.

Example 2
Except for changing the amount of silver oxide added in the mixing step to 11 mg, photocatalyst particles were produced in the same manner as in Example 1. The silver concentration relative to gallium oxide was 1.0 mass%.

Example 3
Except for changing the amount of silver oxide added in the mixing step to 32 mg, photocatalyst particles were produced in the same manner as in Example 1. The silver concentration relative to gallium oxide was 3.0 mass%.

Example 4
Except for changing the amount of silver oxide added in the mixing step to 54 mg, photocatalyst particles were produced in the same manner as in Example 1. The silver concentration relative to gallium oxide was 5.0 mass%.

Comparative Example 1
Photocatalyst particles were produced by reducing silver nanoparticles using the conventional impregnation method. First, ultrapure water (20 mL), gallium oxide (1 g), and silver nitrate aqueous solution (0.1 M, 2.8 mL) were mixed, and the resulting mixed solution was heated at 80 ° C. for 1 hour using a water bath. Next, the heated mixed solution was dried in air at 80 ° C. for 3 hours, and the resulting dried product was baked at 450 ° C. for 2 hours to obtain photocatalyst particles. The silver concentration relative to gallium oxide was 3.0 mass%.

Comparative Example 2
Photocatalyst particles were produced by reducing and generating silver nanoparticles using the conventional photoelectrodeposition method. First, ultrapure water (1 L), gallium oxide (1 g), and silver nitrate aqueous solution (0.1 M, 2.8 mL) were mixed, and argon (Ar) gas was blown into the resulting mixed solution to replace the gas. Next, while blowing argon gas at a flow rate of 30 mL/min, UV light was irradiated from a 400 W high-pressure Hg lamp for 5 hours to reduce silver (Ag) ions. The solution after light irradiation was filtered, and the powder was collected. The collected powder was dried at room temperature to obtain photocatalyst particles. The silver concentration relative to gallium oxide was 3.0% by mass.

Comparative Example 3
Photocatalyst particles were produced in the same manner as in Example 2, except that pure water (50 mL) was used instead of the reducing solution (ethanol) in the mixing process. However, even after 3 hours of ultrasonic treatment, no change in the color of the particles was observed. It is believed that water alone has a low reducing power and the reduction reaction from silver oxide to silver does not proceed.

Comparative Example 4
Without supporting silver nanoparticles, gallium oxide particles alone were used as photocatalyst particles.

(2) Evaluation of photocatalyst particles The samples obtained in Examples 1 to 4 and Comparative Examples 1, 2, and 4 were evaluated for various properties as follows. In Examples 1 to 4, the samples that had been subjected to ultrasonic treatment for 3 hours were evaluated in a manner other than visual observation. In addition, the sample in Comparative Example 3 was not evaluated because the reduction reaction from silver oxide to silver had not progressed.

<Visual Observation>
The samples were visually inspected for color.

<X-ray diffraction (XRD)>
The sample was analyzed by X-ray diffraction to examine its crystal phase. The analysis conditions were as follows:

- X-ray diffraction equipment: Spectris Corporation, X'Pert PRO MRD
- Radiation source: CuKα radiation - Tube voltage: 45 kV
-Tube current: 40mA
- Scan speed: 5.5°/min - Scan range (2θ): 20 to 60°

<SEM Observation>
The sample was observed using a scanning electron microscope (SEM; Carl ZEISS, ULTRA55) at an accelerating voltage of 1 kV.

<STEM observation>
The sample was observed using a scanning transmission electron microscope (STEM; Hitachi High-Technologies Corporation, HD2700) to determine the average particle size of the silver nanoparticles. Observation was performed using a transmission electron image at an acceleration voltage of 200 kV. The average particle size was determined as follows. Particle sizes at 70 or more points were measured from the STEM image, and the average value was taken as the average particle size.

<Diffuse reflectance spectrum>
The diffuse reflectance spectrum of the solid state sample was measured in the wavelength range of 200 to 800 nm using an ultraviolet-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 2. First, ultrapure water (1 L), _NaHCO3 (0.1 M) and photocatalyst particles (0.5 g) were mixed to prepare an evaluation solution. Next, this evaluation solution was placed in the tank of the evaluation 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 <Visual observation>
Fig. 4 shows optical photographs of samples (Examples 1 to 4 and Comparative Example 4) prepared by changing the ultrasonic irradiation time and the amount of supported silver. As the ultrasonic irradiation time increased, the color changed from white to brown. Also, the color changed to a darker color as the amount of supported silver increased.

<X-ray diffraction (XRD)>
The X-ray diffraction (XRD) patterns obtained for Examples 1 to 4 and Comparative Example 4 are shown in Figures 5 to 7 together with the patterns of silver (Ag) and silver oxide (Ag ₂ O). Here, Figures 6 and 7 are enlarged views of Figure 5. In the samples (Examples 1 to 4) prepared by the ultrasonic reduction method, no pattern consistent with silver oxide (Ag ₂ O) was observed. In particular, as shown in the enlarged view of Figure 6, even in the sample (Example 4) in which the amount of silver carried was increased to 5 mass%, no peak specific to silver oxide (Ag ₂ O) was observed near 2θ = 32.7 °. From this, it was found that Examples 1 to 4 do not contain silver oxide. On the other hand, in these samples (Examples 1 to 4), a pattern consistent with silver (Ag) was observed. In particular, as shown in the enlarged view of Figure 7, in the samples (Examples 3 and 4) in which the amount of silver carried was 3 mass% or 5 mass%, a peak specific to silver (Ag) was observed near 2θ = 44.5 °, and the intensity was higher as the amount carried was larger. This indicates that in the samples prepared by the ultrasonic reduction method, the raw silver source (silver oxide) was entirely reduced to silver.

<SEM Observation>
SEM photographs obtained for the samples of Example 3 and Comparative Example 2 are shown in Figures 8 and 9, respectively. The bright (white) particles present in the figures are silver nanoparticles. In both samples, silver nanoparticles were supported on the surfaces of gallium oxide particles. However, the particle size of the silver nanoparticles was small in the sample prepared by the ultrasonic reduction method (Example 3), whereas the particle size was large in the sample prepared by the photoelectrodeposition method (Comparative Example 2). Although not shown in the figures, the silver nanoparticles were uniformly generated in the samples prepared by the ultrasonic reduction method (Examples 1 to 4), and almost no aggregated particles were observed. In addition, when the amount of silver nanoparticles supported was increased, the number of silver nanoparticles increased, but almost no change was observed in the particle size.

<STEM observation>
STEM photographs obtained for the samples of Example 3, Example 4, Comparative Example 1, and Comparative Example 2 are shown in Figures 10 to 13, respectively. The black particles present in the figures are silver nanoparticles. As in the results of the SEM photographs, silver nanoparticles were supported on the surfaces of gallium oxide particles in all samples. Furthermore, the silver nanoparticles had small particle sizes in the samples produced by the ultrasonic reduction method (Examples 3 and 4), whereas the particle sizes were large in the samples produced by the impregnation method or photoelectrodeposition method (Comparative Examples 1 and 2).

Figures 14 and 15 show the particle size distribution of silver nanoparticles determined based on STEM photographs. Table 1 shows the average particle size calculated from the particle size distribution. The samples (Examples 1 to 4) produced by the ultrasonic reduction method all had an average particle size within the range of 20.0 ± 2.0 nm, and the average particle size hardly changed even when the amount of silver carried was changed. The particle size distribution was also relatively sharp. In contrast, the samples (Comparative Examples 1 and 2) produced by the impregnation method and photoelectrodeposition method had a large average particle size of 241 nm or 276 nm, and the particle size distribution was broad.

<Diffuse reflectance spectrum>
The diffuse reflectance spectra obtained for Examples 1 to 4, Comparative Example 1, Comparative Example 2, and Comparative Example 4 are shown in FIG. 16. In the samples (Examples 1 to 4) prepared by the ultrasonic reduction method, a peak was observed at wavelengths of 350 to 550 nm. Furthermore, the intensity of this peak increased as the amount of supported silver increased. In contrast, no clear peak was observed at wavelengths of 350 to 550 nm in the samples (Comparative Examples 1 and 2) prepared by the impregnation method or the photoelectrodeposition method and the sample without supported silver (Comparative Example 4). From this, it was found that the peak observed at wavelengths of 350 nm to 550 nm was due to silver nanoparticles generated by ultrasonic reduction.

< _CO2 reduction photocatalytic performance>
The _CO2 reduction photocatalytic performance (gas generation rate and CO selectivity) obtained for Examples 1 to 4, Comparative Example 1, Comparative Example 2, and Comparative Example 4 is shown in Table 2 and FIG. 17. The samples (Examples 1 to 4) in which silver nanoparticles were supported by the ultrasonic reduction method had a significantly higher CO selectivity than the sample without silver support (Comparative Example 4). In addition, the higher the amount of silver supported, the higher the CO selectivity. It is presumed that charge separation was promoted by supporting silver nanoparticles. In particular, the sample with a support amount of 0.5 mass% (Example 1) also maintained a high CO gas generation rate. On the other hand, the samples (Comparative Examples 1 and 2) prepared by the impregnation method and the photoelectrodeposition method had inferior CO selectivity compared to the sample with the same support amount prepared by the ultrasonic reduction method (Example 3).

[Experimental Example B]
In Experimental Example B, the silver nanoparticles were reduced and generated by ultrasonic treatment to obtain gallium oxide particles carrying metallic silver nanoparticles, which were then subjected to a heat treatment to produce photocatalyst particles.

(1) Preparation of photocatalyst particles

Example 5
The gallium oxide particles carrying metallic silver nanoparticles obtained in Example 3 were heat-treated in air at 100° C. for 5 minutes to prepare photocatalyst particles.

Example 6
The gallium oxide particles carrying metallic silver nanoparticles obtained in Example 3 were heat-treated in air at 200° C. for 5 minutes to prepare photocatalyst particles.

Example 7
The gallium oxide particles carrying metallic silver nanoparticles obtained in Example 3 were heat-treated in air at 300° C. for 5 minutes to prepare photocatalyst particles.

Example 8
The gallium oxide particles carrying metallic silver nanoparticles obtained in Example 3 were heat-treated in air at 500° C. for 5 minutes to produce photocatalyst particles.

(2) Evaluation of Photocatalyst Particles The obtained photocatalyst particles were evaluated for various properties as follows.

<SEM Observation>
The sample was observed with an SEM in the same manner as in Experimental Example A.

<TG-MS analysis>
Thermogravimetry-mass (TG-MS) analysis of the samples was performed using a TG-DTA (Bruker-AXS, 2020SA/MS9600) in a N ₂ -21% O ₂ atmosphere at a heating rate of 10°C/min.

<X-ray diffraction (XRD)>
The samples were analyzed using an X-ray diffraction apparatus (Bruker, D8 DISCOVER Vario-1).

< _CO2 reduction photocatalytic performance>
The _CO2 reduction photocatalytic performance of the sample was carried out in the same manner as in Experimental Example A.

(3) Evaluation results <SEM images after heat treatment>
SEM images of photocatalyst particles are shown in Figure 18. There was no significant change in the particle size of Ag nanoparticles in samples heat-treated at 200°C or less. On the other hand, in samples heat-treated at 300°C or more, the number of particles with slightly smaller particle sizes increased.

<Changes in residual organic matter due to heat treatment>
The TG curve and DTA curve of the photocatalyst particles are shown in Figures 19 and 20, respectively. In the sample before heat treatment (heating), _CO2 was generated at around 180°C. This suggests that decomposition of organic matter occurs at this temperature. On the other hand, in the sample heat-treated (heated) at a temperature of 200°C or higher, no sudden _CO2 generation was observed at around 180°C. It is thought that the organic matter was decomposed by the heat treatment.

<Changes in Ag compounds due to heat treatment>
The X-ray diffraction pattern of the photocatalyst particles is shown in Figure 21. The generation of _AgGaO2 was confirmed in the samples heat-treated at temperatures above 200°C. As mentioned above, SEM observation showed that the particle size of Ag nanoparticles was smaller in the samples heat-treated at temperatures above 300°C. This is thought to be due to the change of some of the Ag nanoparticles to _AgGaO2 .

<Changes in catalytic activity due to heat treatment>
The _CO2 reduction photocatalytic performance (gas generation rate and CO selectivity) of the photocatalyst particles is shown in Table 3. It was confirmed that the CO selectivity was improved by heat treatment.

2 Evaluation device 4 Tank 6 Mercury (Hg) lamp 8 Gas inlet pipe 10 Gas outlet pipe 12 pH meter 14 Rubber plug 16 Stirrer 18 Bubbling filter 20 Cooling water 22 Evaluation solution 30 _CO2 gas 32 Generated gas 34 Gas chromatography

Claims (4)

The present invention relates to a gallium oxide (Ga ₂ O ₃ ) particle and a metallic silver (Ag) nanoparticle supported on the surface of the gallium oxide particle,
A diffuse reflectance spectrum having a peak in the wavelength range of 350 to 550 nm,
The average particle size of the metallic silver nanoparticles is 10.0 to 30.0 nm;
The carbon dioxide reduction photocatalyst particles, in which the amount of the metallic silver nanoparticles supported is 3.0 to 10.0 mass% relative to the gallium oxide particles. The carbon dioxide reducing photocatalyst particle according to claim 1, which has a CO selectivity of 60% 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 one 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).

2. The carbon dioxide reducing photocatalyst particles according to claim 1 , which are composite particles obtained by irradiating gallium oxide (Ga ₂ O ₃ ) particles and a silver (Ag) ion source with ultrasonic waves in a reducing solution. The carbon dioxide reducing photocatalyst particles according to claim 1, which are composite particles obtained by irradiating gallium oxide (Ga ₂ O ₃ ) particles and a silver (Ag) ion source with ultrasonic waves in a reducing solution, and then heat-treating the composite particles at a temperature of 100° C. or higher in an oxygen-containing atmosphere, to obtain heated composite particles.

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