JP2022137741A - Photocatalyst particle for carbon dioxide reduction - Google Patents

Abstract

To provide a photocatalyst particle for carbon dioxide reduction having improved CO selectivity.SOLUTION: A photocatalyst particle for carbon dioxide reduction contains a gallium oxide (Ga2O3) particle, and a metal silver (Ag) nanoparticle and a silver chromate (Ag2CrO4) nanoparticle supported on the surface of the gallium oxide particle.SELECTED DRAWING: Figure 5

Description

The present invention relates to carbon dioxide reduction photocatalyst particles.

Water splitting and carbon dioxide reduction technology using semiconductor photocatalyst particles is attracting attention as a technology that can solve energy problems and environmental problems. By supporting nanoparticles made of silver (Ag) or the like on these photocatalyst particles as a co-catalyst, it is possible to expect the effect of trapping electrons generated by photoexcitation to promote charge separation and the effect of selecting carbon dioxide reduction products. . For example, an ordinary photocatalyst decomposes water into hydrogen (H ₂ ) and oxygen (O ₂ ) by light irradiation. On the other hand, in a photocatalyst carrying silver particles, carbon monoxide (CO) is produced together with hydrogen (H ₂ ) by reduction of carbon dioxide (CO ₂ ). As such a photocatalyst for carbon dioxide reduction, gallium oxide particles supporting silver nanoparticles as promoters (silver nanoparticle-supporting gallium oxide particles) are known.

Carbon monoxide (CO) is an important starting material in the chemical industry and industry, and can be reacted with hydrogen to synthesize a variety of fuels and chemicals. Therefore, it is desirable that the carbon monoxide reduction photocatalyst has a high carbon monoxide generation ratio, that is, a high CO selectivity. Here, the CO selectivity is the sum of the generation rate (amount) of hydrogen (H ₂ ) gas generated by the reduction reaction and the generation rate of carbon monoxide (CO) gas, as represented by the following formula (1). is the ratio of the rate of carbon monoxide (CO) gas generation to the

By the way, methods such as an impregnation method and a photoelectrodeposition method (photoelectrodeposition method) have been conventionally known as methods for supporting metal nanoparticles. For example, Patent Literature 1 describes a method for reducing carbon dioxide by irradiating CO ₂ , H ₂ O, and a photocatalyst with light to reduce CO ₂ , thereby generating CO. In the photoelectrodeposition method, gallium oxide powder is added to an alcoholic aqueous solution containing a silver precursor such as silver nitrate, mixed, and then irradiated with light to reduce the silver precursor.In the impregnation method, It is described that gallium oxide is added to an aqueous silver precursor solution, stirred, water is removed, dried by heating, and then fired in air (claims 1 and 2 and [0015] of Patent Document 1).

Further, although it does not relate to a carbon dioxide reduction catalyst, Patent Document 2 describes a step of irradiating ultrasonic waves to disperse one or more noble metal oxides in a solvent to obtain a noble metal oxide dispersion, and A method for producing a noble metal nanomaterial is disclosed, which comprises the step of heating the dispersion liquid, and the solvent further contains a carrier for carrying the noble metal, and the noble metal is carried on the surface of the carrier with a high degree of dispersibility. Since it is possible to obtain a precious metal nanomaterial in a state where it has been decomposed, it is described that it can be suitably used as a catalyst for fuel cells, a catalyst for material synthesis, etc. (Claim of Patent Document 2 Items 1, 6 and [0014]).

JP 2012-192302 A Japanese Unexamined Patent Application Publication No. 2008-24968

However, as a result of investigations by the present inventors, it was found that although the photocatalyst supported by silver by the impregnation method or photoelectrodeposition method proposed in Patent Document 1 has certain effects, there is still room for improvement. In other words, in order to fully exhibit the effect of silver (Ag), which is a co-catalyst, it is desirable to increase the amount of silver (Ag) supported to some extent. In addition, it is desired that the grain size of silver is small and is on the order of several tens of nanometers. This is because if the particle size is too large, catalytic activity is lost. However, the photocatalyst particles produced by the impregnation method or the photoelectrodeposition method have a problem that when the silver concentration (supported amount) is increased, the silver particles aggregate to increase the particle size. Therefore, it is difficult to support a large amount of silver nanoparticles with a small particle size.

Patent Document 2 does not intend to use the noble metal nanomaterial for the carbon dioxide reduction photocatalyst, much less to reduce the particle diameter of the nanomaterial to the order of several tens of nanometers. In fact, in Cited Document 2, in the examples, noble metal (Pt) nanoparticle-supported spherical carbon and noble metal (Pt) nanotubes are produced, and it is suitable for fuel cell catalysts, material synthesis catalysts, medical and food additives, and conductive pastes. ([0043] to [0062] of Patent Document 2).

On the other hand, when synthesizing metal nanoparticles, there is known a method of synthesizing uniform and fine particles by using a large amount of an organic protective agent while lowering the raw material concentration. However, when photocatalyst particles are produced by such a method, there are problems in that the yield of photocatalyst particles is low and the organic protective agent adheres to the particle surfaces. Since the adhering organic protective agent lowers the catalytic activity, it is necessary to calcine the catalyst particles at a high temperature in order to decompose and remove the organic protective agent. In the catalyst particles that have undergone such calcination, the particle size of the metal nanoparticles becomes large. Therefore, it is difficult to support fine silver nanoparticles with high dispersion and high loading by conventional methods, and there is a limit in efficiently producing photocatalyst particles with excellent catalytic performance, especially CO selectivity. .

The present inventors have conducted studies in view of such conventional problems, and have found that by a simple method of irradiating gallium oxide particles and a silver supply source with ultrasonic waves, silver nanoparticles serving as promoters are highly dispersed and highly dispersed. It has been found that a photocatalyst with improved CO selectivity can be obtained by precipitating at a supporting rate and further performing chromium (Cr) treatment.

The present invention was completed based on such findings, and an object of the present invention is to provide carbon dioxide reduction photocatalyst particles having improved CO selectivity.

The present invention includes the following aspects (1) to (5). In the present specification, the expression "-" includes both numerical values. That is, "X to Y" is synonymous with "X or more and Y or less".

(1) Carbon dioxide reduction comprising gallium oxide (Ga ₂ O ₃ ) particles, metallic silver (Ag) nanoparticles and silver chromate (Ag ₂ CrO ₄ ) nanoparticles supported on the surface of the gallium oxide particles Photocatalytic particles.

(2) The carbon dioxide reduction photocatalyst particles of (1) above, wherein the average particle size of the metallic silver (Ag) nanoparticles is 10.0 to 50.0 nm.

(3) The carbon dioxide reduction photocatalyst particles of (1) or (2) above, wherein the silver chromate (Ag ₂ CrO ₄ ) nanoparticles have an average particle size of 1.0 to 30.0 nm.

(4) The above (1) to (3), wherein the supported amount of the silver (Ag) component is 0.3 to 10.0% by mass with respect to the gallium oxide (Ga ₂ O ₃ ) particles in terms of metallic silver. Any carbon dioxide reduction photocatalyst particle.

(5) The carbon dioxide reduction photocatalyst particles according to any one of (1) to (4) above, which have a CO selectivity of 30% or more in a CO ₂ reduction photocatalyst performance evaluation test.

According to the present invention, carbon dioxide reduction photocatalyst particles with improved CO selectivity are provided.

It is a cross-sectional schematic diagram which shows an example of an evaluation apparatus. It shows the mechanism of photocatalytic reaction. The mechanism of silver nanoparticle support is shown. 1 shows XRD patterns of examples and comparative samples. The STEM photograph of the example sample is shown. A STEM photograph of a comparative example sample is shown.

A specific embodiment of the present invention (hereinafter referred to as "this embodiment") will be described below. However, the present invention is not limited to the following embodiments, and various modifications are possible without changing the gist of the present invention.

<<1. Photocatalyst particles>>
The carbon dioxide reduction photocatalyst particles of the present embodiment (hereinafter sometimes collectively referred to as “photocatalyst particles”) include gallium oxide (Ga ₂ O ₃ ) particles and metallic silver (Ag) supported on the surface of the gallium oxide particles. ) nanoparticles and silver chromate (Ag ₂ CrO ₄ ) nanoparticles. In this photocatalyst particle, Ag nanoparticles and Ag ₂ CrO ₄ nanoparticles are supported in a dispersed state on the surface of Ga ₂ O ₃ particles. This makes the photocatalyst particles excellent in catalytic performance.

Gallium oxide (Ga ₂ O ₃ ) particles function as a main catalyst, and are not particularly limited as long as they are used for photocatalysts. Ga ₂ O ₃ is known to be α-type, β-type, γ-type, δ-type, and ε-type, and any of them may be used. However, the β form (β-Ga ₂ O ₃ ), which is a stable oxide, is preferred. The particle size is also not particularly limited. For example, the particles have an average particle diameter of 0.3 to 5.0 μm. Furthermore, the shape of the particles is not particularly limited. For example, it may be spherical, amorphous, or anisotropic (rod-shaped, plate-shaped, etc.). When the particles are rod-shaped, for example, particles having a long axis diameter of 1.0 to 5.0 μm and a short axis diameter of 0.3 to 1.0 μm can be used.

Metallic silver (Ag) nanoparticles function as co-catalysts and have an average particle size of nm order. By making the metallic silver nanoparticles finer, the catalytic performance of the photocatalyst is enhanced. On the other hand, if the silver nanoparticles are larger than the nanometer order, there is a possibility that the function as a co-catalyst such as catalytic activity may be lost, or the number of surface active sites of the gallium oxide particles may decrease.

The average particle size of the metallic silver (Ag) nanoparticles is preferably 10.0-50.0 nm. By setting the average particle diameter to 10.0 nm or more, it becomes possible to form silver nanoparticles without the need to lower the Ag concentration (supported amount) or add an organic surface protective agent. On the other hand, by setting the average particle size to 50.0 nm or less, it is possible to prevent problems such as loss of co-catalyst function such as catalytic activity and reduction of surface active sites of gallium oxide particles. . More preferably, the average particle size is 10.0 to 30.0 nm. The average particle size can be obtained by observing the photocatalyst particles with a transmission electron microscope (TEM). As an example, there is a method of determining the particle size distribution of silver nanoparticles by visual inspection or image analysis software, and calculating the average value from this particle size distribution.

Silver chromate (Ag ₂ CrO ₄ ) nanoparticles have a function of assisting the co-catalyst function of metallic silver nanoparticles. That is, chromium (Cr) ions originating from silver chromate react with carbon dioxide (CO ₂ ) to form carbonates when acting as a catalyst. Therefore, by providing the photocatalyst particles with silver chromate nanoparticles, carbonate and carbonate ions generated therefrom can be present in the vicinity of the silver nanoparticles serving as cocatalysts, and as a result, the reduction of carbon dioxide can be effectively promoted. becomes possible. Ag ₂ CrO ₄ nanoparticles can be produced by mixing Ga ₂ O ₃ particles supporting metallic silver with an aqueous solution of chromium (III) nitrate and then heat-treating the mixture.

Silver chromate (Ag ₂ CrO ₄ ) nanoparticles preferably have an average particle size of 1.0 to 30.0 nm. By setting the average particle size to 1.0 nm or more, there is an effect that the particles can be stably supported without agglomeration. On the other hand, by setting the average particle size to 30.0 nm or less, there is an effect that the catalytic performance can be improved by increasing the number of active sites. More preferably, the average particle size is 1.0 to 20.0 nm. The average particle size of silver chromate nanoparticles may be measured by the same method as for metallic silver nanoparticles.

The supported amount of silver (Ag) component is preferably 0.3 to 10.0% by mass in terms of metal silver with respect to gallium oxide (Ga ₂ O ₃ ) particles. Here, the supported amount of silver component is the total amount of silver (Ag) contained in each of the metallic silver nanoparticles and the silver chromate nanoparticles. If the supported amount is excessively small, it becomes difficult to sufficiently exhibit the effect of the co-catalyst. Therefore, the CO gas generation rate and CO selectivity are low when photocatalysts are used for _CO2 reduction. On the other hand, if the supported amount is excessively large, the CO gas generation rate decreases. The supported amount can be adjusted by controlling the blending amount of the silver supply source and the ultrasonic treatment conditions. The amount of silver nanoparticles supported may be 0.5% by mass or more, may be 1.0% by mass or more, may be 3.0% by mass or more, or may be 5.0% by mass or more. good. Further, the supported amount may be 7.5% by mass or less, 5.0% by mass or less, 3.0% by mass or less, or 1.0% by mass or less.

The photocatalyst particles of the present embodiment are excellent in catalytic performance, particularly in CO selectivity. For example, the CO selectivity is 30% or more in the CO ₂ reduction photocatalyst performance evaluation test. Therefore, it becomes possible to increase the ratio of the amount of CO gas generated by CO ₂ reduction. The CO selectivity may be 40% or higher, 50% or higher, 60% or higher, or 70% or higher. Although the upper limit of the CO selectivity is not particularly limited, it is typically 90% or less, more typically 80% or less.

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

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

The particle size and electronic transition state of silver nanoparticles are expected to affect the catalytic performance of photocatalysts, such as CO selectivity. This will be explained based on the mechanism of CO ₂ reduction of gallium oxide particles (photocatalyst particles) supported by metallic silver nanoparticles (Fig. 2). As shown in FIG. 2, when particles are irradiated with light having energy hν, electrons (e ⁻ ) and holes (h ⁺ ) are generated in gallium oxide particles (semiconductor photocatalyst particles). At this time, the metallic silver nanoparticles (promoter) promote charge separation (separation of electrons e ⁻ and holes h ⁺ ). As a result of the reaction of the holes (h ⁺ ) with the surrounding moisture (H ₂ O), the reaction shown in the following formula (2) proceeds rightward to generate oxygen (O ₂ ) and protons (H ⁺ ). . On the other hand, electrons (e ⁻ ) react with carbon dioxide (CO ₂ ) and protons (H ⁺ ). Water (H ₂ O) and hydrogen (H ₂ ) are produced. Further, when the reactions of the following formulas (2) to (4) are combined, in principle, the reaction shown in the following formula (5) proceeds.

If the reaction of formula (3) and the reaction of formula (4) occur to the same extent, then as shown in formula (5) above, the CO selectivity (CO generation rate/(H generation rate ₊ CO generation rate)) is constant. In practice, however, these reactions do not always occur to the same extent. The preferential reaction of the formula (3) increases the CO selectivity.

In the reaction of formula (3) above, carbon dioxide (CO ₂ ) is adsorbed on the surface of the catalyst as carbonate species, and is converted to formate species, which is a reaction intermediate by irradiation with light, and then water There are reports that it interacts with molecules to become carbon monoxide (CO). This report also suggests that the silver promoter promotes the formation of reaction intermediates. Therefore, by enhancing the action of charge separation and the action of generating reaction intermediates by the metallic silver nanoparticles (promoter), the reaction of the above formula (3) occurs preferentially, and the CO selectivity is further improved. Be expected. In this regard, in the photocatalyst particles of the present embodiment, since the supported silver nanoparticles are fine and have a unique electronic transition state, these act in combination to effect charge separation and generate reaction intermediates It is speculated that the effect of

<<2. Method for producing photocatalyst particles>>
As long as the photocatalyst of the present embodiment satisfies the requirements described above, the production method is not limited. However, it is preferably obtained by irradiating gallium oxide (Ga ₂ O ₃ ) particles and a silver (Ag) ion source in a reducing solution with ultrasonic waves and further treating them with Cr. A preferred embodiment of the production method will be specifically described below.
does not mean

The method for producing photocatalyst particles preferably comprises the following steps; a step of preparing gallium oxide (Ga ₂ O ₃ ) particles and a silver (Ag) source (preparation step); reducing the gallium oxide particles and the silver source; A step of adding to the liquid to prepare a reaction solution (mixing step), a step of irradiating the reaction solution with ultrasonic waves to prepare gallium oxide particles supporting metallic silver nanoparticles (sonication step), and metallic silver A step of producing gallium oxide particles supporting metallic silver (Ag) nanoparticles and silver chromate (Ag ₂ CrO ₄ ) nanoparticles by subjecting gallium oxide particles supporting nanoparticles to Cr treatment (Cr treatment step). including. Details of each step are described below.

<Preparation process>
In the preparation step, gallium oxide (Ga ₂ O ₃ ) particles and a silver (Ag) source are provided. Gallium oxide (Ga ₂ O ₃ ) particles are not particularly limited as long as they are used for photocatalysts. Ga ₂ O ₃ is known to be α-type, β-type, γ-type, δ-type, and ε-type, and any of them may be used. However, the β form (β-Ga ₂ O ₃ ), which is a stable oxide, is preferred. The particle size is also not particularly limited. For example, the particles have an average particle size of 0.3 to 5.0 μm. Furthermore, the shape of the particles is not particularly limited. For example, it may be spherical, amorphous, or anisotropic (rod-shaped, plate-shaped, etc.). When the particles are rod-shaped, for example, particles having a long axis diameter of 1.0 to 5.0 μm and a short axis diameter of 0.3 to 1.0 μm can be used.

A silver (Ag) supply source is not limited as long as it can supply silver. Specific examples include oxides, inorganic metal salts and/or organometallic compounds. Inorganic metal salts include nitrates, chlorides and/or sulfates. The silver source may or may not be soluble in the reducing liquid. A preferred silver source comprises silver oxide. Since silver oxide is composed only of silver ions and oxygen ions, it is easy to handle and free from problems such as waste disposal. Ag ₂ O, AgO and Ag ₂ O ₃ , which have different oxidation numbers of silver, are known as silver oxides, and any of them can be used. Ag ₂ O is preferred, however, as it is more readily available. The size of the silver supply source is also not particularly limited. For example, the silver source has an average particle size of 0.3 to 3.0 μm.

<Mixing process>
In the mixing step, the prepared gallium oxide particles and silver supply source are added to the reducing liquid to prepare a reaction liquid. The mixing ratio of the gallium oxide particles and the silver source may be adjusted so that the amount of supported silver in the finally obtained photocatalyst is a desired value. If the amount of supported silver is too small, it becomes difficult to sufficiently exhibit the effect of the co-catalyst. Therefore, the CO gas generation rate and CO selectivity are low when photocatalysts are used for _CO2 reduction. On the other hand, if the amount of silver carried is excessively large, the CO gas generation rate decreases.

The reducing liquid is not limited as long as it is a reducing liquid. It may be a reducing liquid itself, or a non-reducing liquid in which a reducing agent is dissolved. However, it is preferably a liquid which itself has reducing properties. It may also contain no separate reducing agent. Alcohols such as ethanol and propanol, which have low toxicity and are readily available, are preferable as such a reducing liquid. Mixtures of alcohols and water can also be used. However, when a mixed solution is used, if the water content is excessively high, it becomes difficult to exhibit a sufficient reducing action. Therefore, the content of water in the reducing liquid is preferably 50% by volume or less, more preferably 25% by volume or less. The lower limit of water content is not particularly limited, and may be 0% by volume.

<Ultrasonic treatment process>
In the ultrasonic treatment step, the obtained reaction solution is irradiated with ultrasonic waves to produce gallium oxide particles supporting metallic silver nanoparticles. At this time, the surface portion of the silver supply source in the reaction liquid is ultrasonically reduced to form metallic silver (Ag) nanoparticles, which are supported on the surfaces of the gallium oxide particles. Gallium oxide particles supporting metallic silver nanoparticles become photocatalyst particles.

The mechanism of supporting metallic silver nanoparticles will be described with reference to FIG. When ultrasonic waves are applied, compressional waves are generated in the reaction solution, and these compressional waves generate repeated positive and negative pressures. During the negative pressure cycle, evaporation causes countless fine air bubbles in the reaction solution. During a positive pressure cycle, this bubble collapses and exerts a strong impact force on the surroundings. This phenomenon is called ultrasonic cavitation. The cavitation uniformly disperses the gallium oxide particles and the silver supply source in the reaction liquid and cleans the surface thereof. In addition, cavitation generates minute, high-temperature and high-pressure hot spots. The generated hot spots decompose and reduce the silver source, and act on the reaction solution to generate radicals, which accelerate the decomposition and reduction of the silver source. Thus, metallic silver nanoparticles are produced 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 deposit silver nanoparticles. The silver nanoparticles grow gradually, and when they grow to a certain extent, the interfacial stress between the silver oxide and the silver nanoparticles reaches a limit and they are detached. Alternatively, an intermediate product is produced from silver oxide by the action of ultrasonic waves, and hot spots and radicals act on this intermediate product to produce silver nanoparticles in the reaction solution. The desorbed or generated silver nanoparticles move to the gallium oxide particle surface and are adsorbed there by the physical action of ultrasonic waves. Thus, gallium oxide particles supporting silver nanoparticles are obtained. The silver nanoparticles produced by ultrasonic reduction are very small. In addition, since an organic protective agent and high-temperature baking are not required, it is possible to produce gallium oxide particles (photocatalyst particles) carrying silver nanoparticles in a fine state.

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

It is not necessary to use a special device for the ultrasonic treatment, and an ordinary device equipped with an ultrasonic wave source may be used. For example, a commercially available ultrasonic cleaner may be used. The treatment may also be performed under normal conditions. For example, the frequency of ultrasound may be 20-100 kHz, and may be 28-45 kHz. Ultrasonic treatment may be performed continuously at the same frequency, or the frequency may be switched during treatment using a frequency oscillation switching mode. The number of frequency switching may be one or more. Processing in a frequency oscillation switching mode (for example, a dual frequency switching oscillation mode of 28 kHz/45 kHz) makes it possible to further improve the dispersibility of the gallium oxide particles in the liquid. The output of the ultrasonic wave may be 10-500W, or 50-200W. Furthermore, the treatment time may be from 1 to 10 hours. By lengthening the treatment time, it is possible to convert all of the silver supply 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 supported amount of silver nanoparticles.

The product (gallium oxide particles supporting metallic silver nanoparticles) obtained by the ultrasonic treatment exists in a dispersed or precipitated state in the reaction solution. Therefore, the product may be recovered from the reaction solution and dried. For recovery, known separation means such as filtration and centrifugation may be used. Drying may be performed under conditions that do not cause excessive grain growth of the silver nanoparticles, for example, 100° C. or less. As described above, the amount of metal silver nanoparticles supported is preferably 0.3 to 10.0% by mass with respect to the gallium oxide particles. The supported amount can be adjusted by controlling the blending amount of the silver source and the ultrasonic treatment conditions.

<Heat treatment process>
If necessary, the gallium oxide particles supporting the metallic silver nanoparticles may be heat-treated at a temperature of 100° C. or higher in an oxygen-containing atmosphere. When the silver-supported gallium oxide particles prepared by the ultrasonic support method are heat-treated, ethanol-derived organic substances remaining on the particle surface can be removed, and the particle size of the silver (Ag) nanoparticles can be reduced. Due to the effect of this heat treatment, CO can be produced more selectively when used as a CO ₂ reduction photocatalyst.

The heat treatment is preferably performed at 100° C. or higher in an atmosphere containing oxygen. It is not necessary to use a special apparatus for heat treatment, and a general electric furnace capable of firing in the atmosphere can be used. At a temperature of less than 100° C., the decomposition of organic substances and the effect on the particle size of silver (Ag) nanoparticles are small, and the effect cannot be expected.

<Cr treatment step>
In the Cr treatment step, silver chromate (Ag ₂ CrO ₄ ) is produced on the gallium oxide particles supporting the metallic silver nanoparticles. Thus, gallium oxide particles supporting metallic silver (Ag) nanoparticles and silver chromate (Ag ₂ CrO ₄ ) nanoparticles can be obtained.

The method of Cr treatment is not limited as long as the metallic silver nanoparticles and the silver chromate nanoparticles can be supported. However, it is preferable to follow the procedure below. That is, the gallium oxide particles supporting metallic silver are immersed in an aqueous solution in which a chromium (Cr) compound is dissolved and then dried to precipitate the chromium (Cr) compound on the surface of the gallium oxide particles supporting metallic silver. As the chromium compound, water-soluble compounds such as chromium (III) nitrate, chromium sulfate hydrate, and chromium chloride hydrate may be used. After that, the gallium oxide particles supporting metallic silver on which the chromium (Cr) compound is deposited are subjected to heat treatment. Silver (Ag) and chromium (Cr) react with each other to form silver chromate (Ag ₂ CrO ₄ ). The heat treatment may be performed, for example, at a temperature of 400-700° C. for 1-10 hours.

By adopting such a production method, it is possible to easily obtain catalyst particles excellent in catalytic performance, particularly in CO selectivity. Although the detailed reason is unknown, it is speculated that the supported silver nanoparticles produced by ultrasonic reduction have fine and unique electronic transition states. That is, the silver nanoparticles (promoter) produced by the ultrasonic reduction treatment have a small particle size. In addition, it is considered that the unique electronic transition state is caused by the action of high-temperature and high-pressure hot spots and radicals generated during the ultrasonic treatment. In fact, it has been reported that hot spots generated by ultrasonic waves have a high temperature of nearly 5000 ° C. It is easy to change the electronic transition state by the instantaneous action of such a high temperature hot spot. is expected. It is speculated that this fine particle size and the unique electronic transition state act in combination to bring about excellent CO selectivity.

In addition, according to such a production method, it is possible to precipitate metallic silver nanoparticles with high dispersion and high loading, and silver chromate (Ag ₂ CrO ₄ ) nanoparticles can be supported on the surface of the gallium oxide particles. This allows carbonate ions to exist in the vicinity of the silver nanoparticles serving as promoters, and as a result, it is possible to effectively promote the reduction of carbon dioxide. Furthermore, according to such a production method, it is possible to use a compound such as silver oxide, which is not dissolved in the reaction solution, as a silver supply source in a solid state, although it is not limited thereto. When a compound that does not dissolve in the reaction liquid is used, the reaction liquid does not contain harmful substances such as anions, and waste liquid treatment is easy.

On the other hand, it is difficult to support metallic silver nanoparticles with high dispersion and high loading rate by the conventionally proposed impregnation method and photo-electrodeposition method. For example, in Patent Document 1, regarding the amount of silver supported, it is 0.2 to 2% by mass with respect to gallium oxide when using the photoelectrodeposition method, and 0.05 to 2% when using the impregnation method. It is described that if the amount is more than the range, the particle size of the silver nanoparticles becomes large and the effect such as catalytic activity is lost, or the surface active sites of gallium oxide are reduced ([0014] of Patent Document 1 and [0015]).

Further, in the impregnation method and the photoelectrodeposition method proposed in Patent Document 1, a precursor solution such as an aqueous solution in which silver nitrate is dissolved is used. Anions such as nitrate ions contained in such a precursor solution are harmful substances that cause air pollution. Therefore, the impregnation method and the photoelectrodeposition method require waste liquid treatment to detoxify harmful substances. However, the production method of this embodiment does not exclude the use of a compound that dissolves in the reaction solution. Even in such a case, it is possible to obtain the effect of depositing metallic silver nanoparticles with high dispersion and high loading.

In addition, in the production method proposed in Patent Document 2, the ultrasonic treatment is performed only to disperse the noble metal oxide, and the noble metal oxide is reduced in the heating step after the ultrasonic treatment (Patent Document 2). of [0038]). Therefore, it is clearly different from the method of the present embodiment in which the silver source (such as silver oxide) is reduced by ultrasonic treatment. In the production method of the present embodiment, sufficiently reduced silver nanoparticles can be supported without heating after ultrasonic treatment.

The present embodiment is further specifically described by the following examples. However, the invention is not limited to the following examples.

(1) Production of photocatalyst particles [Example 1]
Photocatalyst particles (metallic silver nanoparticles-supported gallium oxide particles) were produced by reducing silver nanoparticles by ultrasonic treatment. The silver concentration (supported amount) with respect to gallium oxide was set to 3.0% by mass. Specifically, samples were produced as follows.

<Preparation process>
Gallium oxide particles (Kojundo Chemical Laboratory Co., Ltd., Ga ₂ O ₃ ) and silver oxide (Fujifilm Wako Pure Chemical Industries, Ltd., Ag ₂ O) were prepared. The gallium oxide particles had a purity of 99.99% and an average particle size of about 3 μm in major axis and about 1 μm in minor axis. The silver oxide was an aggregate having a purity of 99% and 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 the reducing solution (50 mL). Ethanol (Fuji Film Wako Pure Chemical Industries, Ltd.) was used as the reducing liquid. A reaction solution was thus prepared.

<Ultrasonic treatment process>
The obtained reaction solution was placed in an ultrasonic device (WT-100-M, Honda Electronics Co., Ltd.) and subjected to ultrasonic treatment. The ultrasonic treatment was performed under the conditions of 28 kHz and 45 kHz switching oscillation and an output of 100 W. Also, the treatment time was varied 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 produced by the treatment was filtered, washed with ethanol (10 mL), and dried in the atmosphere at 60° C. for 0.5 hours to obtain gallium oxide particles supporting metallic silver nanoparticles.

<Heat treatment process>
The obtained metal silver nanoparticle-supported gallium oxide particles were heat-treated in air at 200° C. for 5 minutes.

<Cr treatment step>
Photocatalyst particles were produced by reacting Cr and silver by an impregnation method to produce Ag ₂ CrO ₄ . First, heat-treated gallium oxide supporting metallic silver nanoparticles (1.03 g) was mixed with ultrapure water (20 mL) and chromium (III) nitrate aqueous solution (0.14 M, 1.98 mL). At this time, the charging amount of Cr was adjusted so that Ag:Cr=1:1 (molar ratio). The resulting mixed solution was then heated at 80° C. for 1 hour using a water bath. The heated mixed solution was dried in air at 80° C. for 3 hours, and the resulting dried product was calcined at 450° C. for 2 hours. Photocatalyst particles were thus obtained.

[Comparative Example 1]
Photocatalyst particles were produced by performing Cr treatment by photoelectrodeposition (photoelectrodeposition). First, heat-treated gallium oxide supporting metallic silver nanoparticles (1.03 g) was mixed with ultrapure water (1 L) and chromium (III) nitrate aqueous solution (0.14 M, 1.98 mL). At this time, the charging amount of Cr was adjusted so that Ag:Cr=1:1 (molar ratio). Next, argon (Ar) gas was blown into the resulting mixed solution to replace the gas. After that, while blowing argon gas at a flow rate of 30 mL/min, the mixed solution was irradiated with UV light from a 400 W high-pressure Hg lamp for 1.5 hours to deposit chromium hydroxide on the gallium oxide supporting metallic silver nanoparticles. The powder was recovered by filtering the solution after light irradiation. The recovered powder was dried at room temperature to obtain photocatalyst particles.

[Comparative Example 2]
No Cr treatment was performed. Photocatalyst particles were obtained in the same manner as in Example 1 except for the above.

(2) Evaluation of Photocatalyst Particles Various characteristics of the samples obtained in Example 1 and Comparative Examples 1 and 2 were evaluated as follows.

<X-ray diffraction (XRD)>
A sample was analyzed by X-ray diffraction to determine its crystalline phase. The analysis conditions were as follows.

-X-ray diffractometer: Bruker Japan, D8 DISCOVER Vario-1
- Source: CuKα1 monochromated
- Tube voltage: 45kV
- Tube current: 40mA
- Scanning speed: 0.3°/min - Scanning range (2θ): 20-60°

<TEM/EDS analysis>
The samples were observed using a transmission electron microscope (TEM; HITACHI, HF-2200). Observation was performed with a transmission electron image under the condition of an acceleration voltage of 200 kV.

< _CO2 reduction photocatalyst performance>
The _CO2 reduction photocatalytic performance of the samples was evaluated using the evaluation apparatus shown in FIG. First, ultrapure water (1 L), NaHCO ₃ (0.1 M) and photocatalyst particles (0.5 g) were mixed to prepare a solution for evaluation. Next, this solution for evaluation was placed in a tank of an evaluation apparatus, and UV light was irradiated with a 400 W high-pressure Hg lamp while blowing carbon dioxide (CO ₂ ) gas at a flow rate of 30 mL/min. The gas generated after 1 hour of irradiation was analyzed using gas chromatography (Shimadzu Corporation, GC-8A) to determine the generation rate of H ₂ , O ₂ and CO. Then, the CO selectivity was calculated based on the following formula (1).

(3) Evaluation results <X-ray diffraction (XRD)>
The X-ray diffraction (XRD) patterns obtained for Example 1, Comparative Examples 1 and ₂ , together with the patterns of silver (Ag) and silver chromate (Ag2CrO4), are shown in Figures ₄ (a) and (b). shown in Ga ₂ O ₃ was seen as the main peak in all samples, but a difference was seen when analyzed in detail. Specifically, in Example 1, an Ag ₂ CrO ₄ peak was observed, while in Comparative Examples 1 and 2, no Ag ₂ CrO ₄ peak was detected. Although it is difficult to make an accurate judgment due to overlap of peaks, in Comparative Examples 1 and 2, Ag peaks were detected, whereas in Example 1, the Ag peak intensity was weak.

<TEM/EDS Observation>
STEM photographs obtained for the samples of Example 1 and Comparative Example 1 are shown in FIGS. 5 and 6, respectively. From the TEM image of Example 1, Ga ₂ O ₃ columnar particles and particles with a size of several nm were observed. Also, Ag, Cr and O were detected in particles with a size of several nanometers by EDS analysis. From this, it was found that the particles with a size of several nm were Ag ₂ CrO ₄ . In contrast, in the TEM image of Comparative Example 1, particles with a size of several nm were not observed. Ag nanoparticles as small as 20 nm were detected, and Cr appeared to reside on the Ag.

< _CO2 reduction photocatalyst performance>
The CO ₂ reduction photocatalytic performance (gas generation rate and CO selectivity) obtained for Example 1 and Comparative Examples 1 and 2 are shown in Table 1. When comparing the performance of the samples prepared by the impregnation method (Example 1 and Comparative Example 2), the Cr-treated sample (Example 1) has almost the same CO selectivity as the Cr-untreated sample (Comparative Example 2). Despite being the same, the CO production rate was about 2.5 times higher. On the other hand, the sample (Comparative Example 1) prepared by the photo-electrodeposition method showed an increased rate of CO production, but also an increased rate of H ₂ production compared to the sample (Comparative Example 2) not treated with Cr. Therefore, the CO selectivity deteriorated significantly.

2 evaluation device 4 tank 6 mercury (Hg) lamp 8 gas introduction tube 10 gas discharge tube 12 pH meter 14 rubber plug 16 stirrer 18 bubbling filter 20 cooling water 22 evaluation solution 30 CO ₂ gas 32 generated gas 34 gas chromatography

Claims (5)

A carbon dioxide reduction photocatalyst particle comprising gallium oxide (Ga ₂ O ₃ ) particles, and metallic silver (Ag) nanoparticles and silver chromate (Ag ₂ CrO ₄ ) nanoparticles supported on the surface of the gallium oxide particles. 2. The carbon dioxide reduction photocatalyst particles according to claim 1, wherein the metallic silver (Ag) nanoparticles have an average particle size of 10.0 to 50.0 nm. 3. The carbon dioxide reduction photocatalyst particles according to claim 1, wherein the silver chromate (Ag ₂ CrO ₄ ) nanoparticles have an average particle size of 1.0 to 30.0 nm. 4. The method according to any one of claims 1 to 3, wherein the supported amount of the silver (Ag) component is 0.3 to 10.0% by mass in terms of metal silver with respect to the gallium oxide (Ga ₂ O ₃ ) particles. carbon dioxide reduction photocatalyst particles. The carbon dioxide reduction photocatalyst particles according to any one of claims 1 to 4, which have a CO selectivity of 30% or more in a CO ₂ reduction photocatalyst performance evaluation test.

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