Cylindrical photocatalytic cell

Abstract

To provide a cylindrical photocatalyst cell which can improve handleability, and achieves high photocatalyst activity and can enhance photoconversion efficiency, for instance, when BiVO4 is used as a photocatalyst for artificial photosynthesis.SOLUTION: A cylindrical photocatalyst cell stores a porous glass particle carrier formed of a plurality of glass particles and a photocatalyst in a cylindrical container having an inner diameter D of 0.8 cm or more and light permeability, wherein when light is emitted in a diameter direction of the cylindrical container from the side face of the cylindrical container, a ratio (transmission light amount / irradiation light amount) of a transmission light amount to an irradiation light amount is 0.05% or more and 10% or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a cylindrical photocatalyst cell with controlled transmitted light amount and a photocatalytic reaction method using the same. Although not particularly limited, it is preferably used for performing a photocatalytic reaction that photoreduces a redox substance in water and involves an oxidation reaction of water, and relates to a cylindrical photocatalyst cell with controlled transmitted light amount and a photocatalytic reaction method using the same.

Background Art

[0002] It is known that compounds such as titanium oxide exhibit a catalytic action when irradiated with light and are called photocatalysts. One of the catalytic actions is an oxidative decomposition action that oxidizes organic substances on the photocatalyst surface and decomposes them into CO2, H2O, etc. Utilizing this property, product development aiming at environmental purification such as deodorization, VOC removal, stain removal, antibacterial and sterilization is underway by oxidatively decomposing harmful substances present in the environment into harmless substances. For example, tiles having an antibacterial action by the added photocatalyst, air purifiers that decompose harmful substances in the air by the photocatalyst supported on an air filter, etc. can be mentioned.

[0003] In addition, water splitting technology utilizing the catalytic action is expected. This technology is a technology that uses light energy and a photocatalyst to decompose water to generate hydrogen and oxygen and is expected as artificial photosynthesis technology. Specifically, by irradiating light with energy above the band gap of the photocatalyst, electrons in the valence band of the photocatalyst are excited to the conduction band, electrons are generated in the conduction band, and holes are generated in the valence band. Water is oxidized by the holes to generate oxygen and protons [referred to as the "oxygen generation system". The following reaction formula (1)], while on the other hand, protons are reduced by the electrons to generate hydrogen [referred to as the "hydrogen generation system". The following reaction formula (2)]. 2H2O → O2+ 4H + + 4e - ···(1) 4H + + 4e - → 2H2···(2)

[0004] Many water splitting technologies are based on a mechanism that simultaneously promotes oxygen generation and hydrogen generation on a single photocatalyst (hereinafter referred to as the "one-step mechanism"). In the case of this one-step mechanism, as conditions for the photocatalyst, it is essential that the valence band of the photocatalyst is more positive than the oxidation potential of water (+1.23 V) and has the potential to oxidize water with holes to generate oxygen, and that the conduction band of the photocatalyst is more negative than the reduction potential of water (0 V) and has the potential to reduce water with excited electrons to generate hydrogen. However, since the conductor of the visible light-responsive photocatalyst is more positive than the reduction potential of water, it cannot reduce water. In the one-step mechanism, a photocatalyst that satisfies the above essential conditions usually does not exist, making water splitting difficult.

[0005] One method for improving the problem of visible light responsiveness of the one-step mechanism is a two-step mechanism called the "Z-scheme," in which oxygen generation and hydrogen generation are separated into two types of photocatalysts and the two are connected by an oxidant-reductant. In the Z-scheme, in the oxygen generation system, holes oxidize water to generate oxygen and at the same time reduce the oxidized form of the oxidant-reductant to generate the reduced form. On the other hand, in the hydrogen generation system, excited electrons reduce water to generate hydrogen and at the same time oxidize the reduced form of the oxidant-reductant to generate the oxidized form. It is essential that the redox potential of the oxidant-reductant is more positive than the conduction band of the oxygen generation system and more negative than the valence band of the hydrogen generation system. A visible light-responsive photocatalyst whose conduction band is more positive than the reduction potential of water can be used for the oxygen generation system if its valence band is more positive than the oxidation potential of water. However, since the combination of the two types of photocatalysts that satisfy the above essential conditions is limited, water splitting is still difficult.

[0006] Another form of the two-step mechanism is the photocatalyst / electrolysis hybrid type. Although electrolysis is easy to generate hydrogen from water, it requires a large voltage. The photocatalyst / electrolysis hybrid type can generate hydrogen from water at the minimum required voltage by supplementing the hydrogen generation system with electrical energy. In the photocatalyst / electrolysis hybrid type, the photocatalyst electrode and the hydrogen generation electrode are electrically connected in series via an external power source. In the photocatalyst, which is an oxygen generation system, holes oxidize water to generate oxygen and electrons, and a voltage is applied by an external power source to reach a potential more negative than the reduction potential of water. In the hydrogen generation electrode, which is a hydrogen generation system, water is reduced to generate hydrogen.

[0007] In another form of the photocatalyst / electrolysis hybrid type, Fe 3+ / Fe 2+ is used as a redox agent. In the first step, light is irradiated on the photocatalyst in an aqueous solution of Fe 3+ , water is oxidized by holes to generate oxygen and protons, and Fe 3+ is reduced by electrons to generate Fe 2+ . In the second step, an aqueous solution of Fe 2+ is electrolyzed to generate Fe 3+ and hydrogen (see Patent Documents 1 and 2).

[0008] Among these, examples of photocatalysts include titanium oxide (TiO2), tungsten oxide (WO3), iron oxide (Fe2O3), tin oxide (SnO2), zinc oxide (ZnO), strontium titanate (SrTiO3), cadmium sulfide (CdS), zirconium oxide (ZrO2), etc. Among them, mainly titanium oxide has been put into practical use.

[0009] On the other hand, in the decomposition of water using Fe 3+ / Fe 2+ as a redox agent, one of the main problems related to its conversion efficiency is that the conversion efficiency of light energy in the visible region is low. Solar light, which is the main light energy, has only about 3% of light energy in the ultraviolet region, and the utilization of visible light is very important. Therefore, the development of visible light-responsive photocatalysts has been carried out. The above-mentioned Fe 3+ / Fe 2+As a photocatalyst used in a photocatalyst / electrolytic hybrid type with oxidation-reduction reaction, it has high visible light responsiveness and contains oxygen and Fe 2+ Due to its high production capacity, bismuth vanadate (BiVO4) is expected to be a photocatalyst for artificial photosynthesis (see, for example, Patent Document 3).

[0010] When BiVO4 is used industrially, it is desirable from the standpoint of ease of handling to immobilize it on a solid transparent support rather than as a powder. For example, a bismuth vanadate laminate has been disclosed in which a substrate is placed in a precursor solution containing a vanadium salt and a bismuth salt, and a bismuth vanadate layer is formed on the substrate by microwave-assisted chemical bath deposition (see Patent Document 4). This laminate is expected to be applied to photoelectrodes. However, this method is limited to substrates that can be heated by microwave, such as glass substrates with an FTO film, and it is difficult to apply supports with higher transparency. Also, because it is a flat support, the amount of photocatalyst supported is not sufficient. A yellow pigment having protruding bismuth vanadate crystals on the surface of a flaky material such as glass flakes coated with titanium dioxide is known (see Patent Document 5). This is obtained by adding a flaky material to an acid solution containing bismuth and vanadium to form a suspension, and then adding an alkaline aqueous solution and aging it. However, the purpose of this is to ensure the saturation as a pigment, and there is no disclosure in terms of improving photocatalytic performance or ease of handling. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] Japanese Patent Application Publication No. 11-157801 [Patent Document 2] Japanese Patent Publication No. 2001-233602 [Patent Document 3] Japanese Patent Publication No. 2017-100057 [Patent Document 4] International Publication WO2014 / 136783 [Patent Document 5] Japanese Patent Publication No. 2004-155876 [Overview of the project] [Problems that the invention aims to solve]

[0012] In other words, when using BiVO4 and other photocatalysts for artificial photosynthesis, it is important to achieve both ease of handling and high photocatalytic activity, but conventional technologies are still insufficient. Therefore, after diligent research, the inventors discovered that the above problem can be solved by supporting a photocatalyst on a porous glass particle carrier formed from multiple glass particles to create a photocatalytic composite, housing this composite in a cylindrical container to form a cylindrical photocatalytic cell, and controlling the ratio of transmitted light to irradiated light (transmitted light amount / irradiated light amount) to a predetermined range when light is irradiated, thereby completing the present invention. [Means for solving the problem]

[0013] In other words, the present invention relates to a cylindrical photocatalytic cell characterized in that a porous glass particle carrier formed of a plurality of glass particles and a photocatalyst are housed in a light-transmitting cylindrical container having an inner diameter D of 0.8 cm or more, and when light is irradiated from the side of the cylindrical container in the diametrical direction of the cylindrical container, the ratio of transmitted light to irradiated light (transmitted light amount / irradiated light amount) is 0.05% or more and 10% or less. Sunlight is preferably used as the light (light source) in such a cylindrical photocatalytic cell.

[0014] Preferably, this glass particle carrier is a sintered glass particle body in which glass particles are sintered together to form a single unit, or a glass particle aggregate in which multiple glass particles are housed in a cylindrical container and the glass particles are adjacent to each other. Furthermore, the ratio (D / M) of the inner diameter D (cm) of the cylindrical container to the total mass M (g) of the porous glass particle carrier and photocatalyst per 1 cm of the length of the cylindrical container is preferably 0.23 or more and 1.32 or less.

[0015] Furthermore, the specific surface area of ​​the glass particle carrier is 12-240 cm².2 It is preferable that the value be / g.

[0016] Furthermore, the glass particles forming the glass particle carrier preferably have an average particle size of 0.1 to 2 mm. It is also preferable that the photocatalyst is supported on the surface of the glass particle carrier, and that the photocatalyst is a bismuth vanadate semiconductor.

[0017] Furthermore, it is preferable that the photocatalyst exists as a photocatalytic dispersion with water, and that the glass particle carrier is immersed in the photocatalytic dispersion in a cylindrical container to form a photocatalytic composite. In this case, at least a portion of the photocatalyst exists in the voids of the porous glass particle carrier.

[0018] Furthermore, the present invention is a photocatalytic reaction method characterized by irradiating a photocatalytic reaction apparatus, in which a porous glass particle carrier formed of a plurality of glass particles is immersed in a photocatalytic reaction solution containing a photocatalyst, a redox agent, and water, with light to reduce the redox agent in the photocatalytic reaction solution and to carry out a photocatalytic reaction accompanied by the oxidation reaction of water. [Effects of the Invention]

[0019] By utilizing the cylindrical photocatalytic cell of the present invention, it is possible to improve the handling of photocatalysts, for example, when using them for artificial photosynthesis, and to achieve high photocatalytic activity. Therefore, it becomes possible to carry out photocatalytic reactions while increasing the efficiency of light conversion from solar energy.

[0020] In particular, the cylindrical photocatalytic cell of the present invention allows for Fe in water. 3+ / Fe 2+ In the photoreduction reaction of redox products, it offers excellent handling and high efficiency in reducing oxygen and Fe 2+This allows us to obtain [the desired result]. Furthermore, it can be suitably used not only in such photoreduction reactions, but also in artificial photosynthesis technologies such as carbon dioxide reduction and hydrogen production through water splitting. Moreover, it can be applied to technologies such as air purification, water purification, antibacterial / sterilization, and antifouling / anti-fogging, and can be applied in industrial fields such as residential exteriors, residential interiors, electrical products, vehicles, roads, agriculture, water treatment, soil treatment, air treatment, and medical care. [Brief explanation of the drawing]

[0021] [Figure 1] Figure 1 is a schematic diagram illustrating the experimental setup used to evaluate the cylindrical photocatalytic cell in the example. [Figure 2] Figure 2 is a cross-sectional view of the cylindrical photocatalytic cell in the experimental setup shown in Figure 1, taken along the line X-X'. [Figure 3] Figure 3 is a schematic diagram illustrating how the Fe2+ generation rate per unit area of ​​the photocatalytic composite irradiated with light is determined using the experimental apparatus shown in Figure 1. [Modes for carrying out the invention]

[0022] The present invention will be described in detail below.

[0023] The cylindrical photocatalytic cell of the present invention comprises a photocatalyst and a porous glass particle carrier formed from multiple glass particles, housed in a light-transmitting cylindrical container with an inner diameter D of 0.8 cm or more, wherein the photocatalyst exists together with the porous glass particle carrier. In this invention, when light is irradiated from the side of the cylindrical container in the diametrical direction of the cylindrical container, the ratio of transmitted light to irradiated light (transmitted light amount / irradiated light amount) is controlled to be between 0.05% and 10%. If the ratio of transmitted light to irradiated light amount is less than 0.05%, there is the disadvantage that light does not penetrate to the depths of the porous glass carrier, and if it exceeds 10%, there is the disadvantage that the photocatalytic activity is not exhibited to the extent of the transmitted light.

[0024] Of these, known photocatalysts applicable as photocatalysts for artificial photosynthesis, etc., can be used. Examples include TiO2 (titanium dioxide), WO3 (tungsten oxide), and BiVO4 (bismuth vanadate). Of these, Fe 3+ / Fe 2+ As a photocatalyst using the oxidation-reduction reaction of as follows, it has high visible light responsiveness and is composed of oxygen and Fe 2+ BiVO4 is preferred because of its high generation capacity.

[0025] Furthermore, regarding the glass particle carrier, preferably, its specific surface area is 12 to 240 cm². 2 It is preferable that the value is / g. A more preferable lower limit for specific surface area is 16 cm². 2 / g, and more preferably 20cm 2 The value is / g. On the other hand, a more preferable upper limit is 120cm 2 / g, and more preferably 80cm 2 The specific surface area is given by / g. This specific surface area can be measured by image analysis, both in the case of glass particle sintered bodies and glass particle aggregates, as described later. Specifically, for example, if an SEM image is taken under the conditions described later, and the average particle size obtained from the image is D and the density of the glass particle material is ρ, then the volume V of one glass particle is V = (4 / 3)π(D / 2). 3 It is expressed as follows, and the surface area S of one glass particle is S = 4π(D / 2) 2 Since it can be expressed as, the specific surface area (surface area per unit mass) S m is S m = S / (ρV)

[0026] Furthermore, the shape of each individual glass particle forming the glass particle carrier is not particularly limited; for example, they can be spherical, elliptical, columnar, angular, plate-shaped, needle-shaped, etc. In particular, from the viewpoint that the glass particle carrier can easily form a porous shape, that a large number of pores can be formed within the glass particle carrier, and that light irradiated from the outside can be transmitted to its depths, it is preferable that the shape of the individual glass particles be spherical or elliptical.

[0027] Furthermore, the material of the glass particles should be light-transmitting, considering that a photocatalytic reaction is carried out by irradiating the photocatalyst attached to the surface of the glass particle carrier with light. In this case, considering sunlight as a suitable light source, it is preferable that the material be light-transmitting not only for ultraviolet light but also for visible light, so it is preferable that the material has a transmittance of 50% or more for light with wavelengths of at least 400 nm to 800 nm. Specifically, examples include, but are not limited to, inorganic materials such as quartz glass, borosilicate glass, soda-lime glass, and alkali-free glass, or light-transmitting resin materials such as acrylonitrile styrene, polyethylene terephthalate, and polyethylene. As will be described later, when a glass particle sintered body is used as the glass particle carrier, it is preferable to use soda-lime glass or borosilicate glass because it allows for a lower sintering temperature during the production of the sintered body. Transmittance is the ratio of the intensity of light incident at a certain wavelength to the intensity of light after it has passed through a light-transmitting material. Furthermore, in this invention, the photocatalytic reaction may be carried out using light other than sunlight, such as an external light source like a xenon lamp, a metal halide lamp, or a high-pressure mercury lamp.

[0028] Furthermore, the average particle size of the glass particles is preferably 0.1 to 2 mm, from the viewpoint of further improving the reaction efficiency in photocatalytic reactions using cylindrical photocatalytic cells. Of these, the more preferable lower limit of the average particle size is 0.2 mm, and even more preferable is 0.3 mm. If the average particle size of the glass particles is less than 0.1 mm, there is a disadvantage in that light is scattered at the surface without penetrating deep into the porous glass support. Also, if the lower limit of the average particle size is within the above range, it is possible to form pores of an appropriate size within the glass particle support and have an appropriate surface area, and a sufficient amount of photocatalyst can be present on the surface (including inside the pores) of the glass particle support. Furthermore, it is also possible to ensure the durability of the resulting glass particle support. On the other hand, the more preferable upper limit of the average particle size is 1.5 mm, and even more preferable is 1.2 mm. If the average particle size of the glass particles exceeds 2.0 mm, there is a disadvantage in that the photocatalyst aggregates inside the pores of the porous glass support, and the photocatalytic activity is not exhibited. Furthermore, this average particle size refers to the equivalent diameter of the glass particles, not only when the glass particles are spherical or elliptical, but also when they have other shapes. In the following explanation, while spherical or elliptical glass particles are preferred examples, other shapes should also be interpreted as being based on the equivalent diameter.

[0029] The average particle size of glass particles can be determined, for example, by selecting any 200 glass particle images from SEM (scanning electron microscope, magnification = 50 to 200x) images of glass particles, considering the circumscribed circle of each glass particle as its diameter, and calculating the number-average diameter (= sum of the diameters of each particle ÷ 200).

[0030] On the other hand, there are no particular limitations on the glass particle carrier, but preferably it is a sintered glass particle body in which glass particles are sintered together to form a single unit, or a glass particle aggregate in which multiple glass particles are housed in a container and the glass particles are adjacent to each other. In either case, these are porous bodies in which a large number of holes exist inside (i.e., in the gaps between the glass particles).

[0031] Of these, there are no particular limitations on the method for obtaining the glass particle sintered body, but it can preferably be manufactured as follows. First, prepare a mold, such as a cylindrical tube, and a plate to place underneath it. These materials are not particularly limited as long as they are heat-resistant; for example, alumina, stainless steel, or titanium can be used. Next, place the mold on the plate and load glass particles into the mold. At this time, ensure that the glass particles are packed as densely as possible and make the top surface horizontal. By sintering the glass particles in this state, the glass particles fuse together and are sintered. The sintering temperature and time should be such that the glass particles fuse together. For example, when using soda-lime glass, sintering at around 600-750°C for 5 minutes to 4 hours is sufficient, depending on the particle size. Higher firing temperatures and longer sintering times result in stronger fusion of glass particles, but tend to reduce the void ratio.

[0032] On the other hand, the glass particle aggregate consists of multiple glass particles contained within a container. The multiple glass particles fill the limited space within the container, and the glass particles are in close proximity to each other, forming a porous glass body. The container used here, as will be described later, should be light-transmitting, considering that the photocatalytic reaction will be carried out by irradiating it with light from the outside. It is preferable that it be light-transmitting not only for ultraviolet light but also for visible light, and therefore, it is preferable that the material has a transmittance of 50% or more for light with wavelengths of at least 400 nm to 800 nm. Specifically, the same materials as those used for glass particles can be mentioned.

[0033] Furthermore, the porosity of the glass particle support is preferably 30-60% from the viewpoint of further improving the reaction efficiency in the photocatalytic reaction. Of this, the more preferable lower limit is 40%, and the more preferable upper limit is 50%. In addition, considering the irradiation of light to the photocatalyst present on the surface of the glass particle support, the thickness of the glass particle support is preferably 0.5-30 mm. Within this range, sufficient light can be irradiated to the photocatalyst (BiVO4) present in the gaps between the glass particles forming the glass particle support, thereby contributing to the photocatalytic reaction. This is also advantageous in terms of ensuring the strength of the glass particle support.

[0034] Furthermore, the shape of the glass particle carrier is not particularly limited, as long as the irradiated light can be effectively utilized, for both the glass particle sintered body and the glass particle aggregate. For example, it can be made circular (spherical) to match the shape of a cylindrical container, or it can be made into any shape such as a square (cube or rectangular prism), a triangular pyramid, etc.

[0035] Furthermore, regarding the cylindrical container (cylindrical container) in the present invention having an inner diameter D of 0.8 cm or more, it is preferable that it has light transmittance, considering that the photocatalytic reaction is carried out by irradiating it with light from the outside, similar to the case of glass particles. It is preferable that it has light transmittance not only for ultraviolet light but also for visible light, so it is preferable that the material has a transmittance of 50% or more for light with wavelengths of 400 nm to 800 nm. Specifically, it is preferable that it be made of the same material as in the case of glass particles. In addition, in the present invention, in order to make the ratio of transmitted light amount to irradiated light amount (transmitted light amount / irradiated light amount) to 0.05% or more and 10% or less as described above, it is preferable that the ratio (D / M) of the inner diameter D (cm) of the cylindrical container and the total mass M (g) of the porous glass particle carrier and photocatalyst per 1 cm of length of the cylindrical container contained in the cylindrical container be 0.23 or more and 1.32 or less. For example, if the inner diameter D of the cylindrical container is 0.8 cm or more and 4.5 cm or less, the total mass M (g) is preferably 0.6 g or more and 19.2 g or less. Here, "length of the cylindrical container" refers to the length in the direction 2a relative to the glass tube 1, as shown in Figure 3 described later. That is, it is preferable that the ratio (D / M) of the inner diameter D (cm) of the cylindrical container to the total mass M (g) of the porous glass particle carrier and photocatalyst contained in a width of 1 cm in the longitudinal direction of the cylindrical container is 0.23 or more and 1.32 or less. Using this as a guideline makes it easy to control the ratio of transmitted light to irradiated light (transmitted light amount / irradiated light amount) to be within a predetermined range, allowing light to penetrate deep into the glass particle carrier and sufficiently increasing the photocatalytic activity in response to transmitted light. Note that the cylindrical container may also be an elliptical tube with an elliptical cross-section, in which case the inner diameter D of the elliptical tube corresponds to the major axis of the ellipse. Furthermore, if the inner diameter D of the cylindrical container is less than 0.8 cm, the amount of photocatalyst involved in the photocatalytic reaction is small, resulting in insufficient photocatalytic activity.

[0036] Next, regarding the method for obtaining a photocatalytic composite consisting of a glass particle carrier and a photocatalyst in a cylindrical photocatalytic cell by placing a photocatalyst on the surface of a glass particle carrier (hereinafter also referred to as the "supporting method"), there are no particular limitations as long as the photocatalyst is present on the surface of the glass particle carrier in a state in which photocatalytic reactions can occur. For example, the following method can be exemplified. Below, an example is given in which the photocatalyst is a bismuth vanadate (BiVO4) semiconductor.

[0037] In other words, for example, one can demonstrate the so-called "impregnation method," in which BiVO4 photocatalyst powder is synthesized in advance, the photocatalyst exists as a photocatalytic dispersion with water, and a glass particle carrier is immersed in the photocatalytic dispersion to allow the BiVO4 photocatalyst to adhere to or be present (supported) on the surface of the glass particles forming the glass particle carrier or in the gaps (i.e., voids) between the glass particles, or the so-called "solution method," in which a raw material solution used for the synthesis of BiVO4 is placed in a cylindrical container that constitutes a cylindrical photocatalyst cell, or in a light-transmitting container that separately contains a glass particle carrier, and the BiVO4 synthesis reaction is carried out by heating the container in that state, thereby precipitating (supporting) BiVO4 on the surface of the glass particles. Furthermore, a so-called "filling method" can also be demonstrated, in which pre-synthesized BiVO4 photocatalyst powder is mixed with liquid paraffin, polyethylene glycol, etc., and this mixture is placed in a cylindrical container that constitutes a cylindrical photocatalyst cell, or in a light-transmitting container that separately contains a glass particle carrier. The photocatalyst is then filled into the voids of the glass particle carrier in a solid or liquid phase, and after filling, the liquid paraffin, polyethylene glycol, etc. are removed with an organic solvent to form a paste of the glass particle carrier and the BiVO4 photocatalyst in a solid or liquid phase, thereby allowing the BiVO4 photocatalyst to be present (supported) on the surface of the glass particles or in the gaps between the glass particles.

[0038] Furthermore, as a glass particle carrier in these methods, a sintered glass particle body can be suitably used in any of the "impregnation method," "solution method," or "filling method." On the other hand, in the case of a glass particle aggregate, it is suitably used in the "impregnation method" and "solution method," except for the "filling method." In particular, when a sintered glass particle body is used, it can stand on its own, so it becomes possible to miniaturize and lighten the photocatalytic reaction apparatus that performs the photocatalytic reaction, for example by forming a self-supporting film.

[0039] In this case, the amount of BiVO4 catalyst supported on the glass particle support is preferably 0.005 to 3% by mass relative to the mass of the glass particle support. Within this range, the BiVO4 catalysts are uniformly supported without aggregation, and high reaction efficiency can be achieved in the photocatalytic reaction described later.

[0040] Here, there are no particular restrictions on the method for obtaining BiVO4 photocatalyst, and it can be synthesized using conventional methods. Specifically, examples include the so-called "suspension synthesis method," in which concentrated nitric acid is added while stirring Bi2O3 and V2O5 in water to precipitate BiVO4, and the so-called "solution synthesis method," in which Bi(NO3)3·5H2O and NH4VO3 are dissolved separately in nitric acid, and the Bi and V salts are mixed, heated, and stirred to precipitate BiVO4.

[0041] This section will explain the method for synthesizing BiVO4 photocatalysts using the "solution synthesis method." BiVO4 photocatalysts use bismuth compounds and vanadium compounds as raw materials. The bismuth compound is not particularly limited, but examples include bismuth nitrate (Bi(NO3)3), bismuth chloride (BiCl3), and bismuth trioxide (Bi2O3), with Bi(NO3)3 being preferred. Similarly, the vanadium compound is not particularly limited, but examples include vanadium pentoxide (V2O5), sodium metavanadate (NaVO3), and ammonium metavanadate (NH4VO3), with NH4VO3 being preferred.

[0042] The bismuth compound and vanadium compound are included in the solvent, with concentrations of 15-40 mmol / l preferred for both, and more preferably 20-35 mmol / l. Furthermore, the molar ratio of bismuth in the bismuth compound to vanadium in the vanadium compound is preferably Bi / V = 1 / 1.

[0043] The solvent is not particularly limited as long as it can dissolve the bismuth compound and the vanadium compound, but water is preferred, and it is more preferable to add an acid to the water. The acid is not particularly limited, but nitric acid, hydrochloric acid, sulfuric acid, perchloric acid, acetic acid, etc. are preferred, with nitric acid being more preferred. The concentration of the acid is preferably in the range of 0.25 to 0.8 mol / l, and more preferably 0.3 to 0.5 mol / l.

[0044] It is preferable to react the precursor aqueous solution containing the bismuth salt and the vanadium salt at a maximum temperature of 65 to 90°C for 3 to 24 hours. More preferably, the reaction time is 24 hours at a maximum temperature of 65°C and 6 hours at a maximum temperature of 90°C.

[0045] To the obtained BiVO4, at least one elemental component from the group consisting of Ga, Zn, Ni, Al, and In may be added. When such elemental component is added, the ion of these elemental components, Ga, may be added. 3+ Zn 2+ Ni 2+ , Al 3+ In 3+ It is preferable to dissolve and mix them in this state. Among these, Ga 3+ This is preferable. The amount added is preferably in the range of 0.1 to 10 mol%, and more preferably 0.2 to 1.0 mol%, relative to the amount of substance of the bismuth compound or vanadium compound.

[0046] The BiVO4 prepared under the above conditions consists of a monoclinic crystal system with a cereite structure. The average particle size of the BiVO4 photocatalyst is not particularly limited, but it is preferably 0.5 to 10 μm.

[0047] On the other hand, we will explain the method for synthesizing BiVO4 photocatalysts using the "suspension synthesis method". BiVO4 photocatalysts use bismuth compounds and vanadium compounds as raw materials. The bismuth compound is not particularly limited, but examples include bismuth nitrate (Bi(NO3)3), bismuth chloride (BiCl3), and bismuth trioxide (Bi2O3), with Bi2O3 being preferred. Similarly, the vanadium compound is not particularly limited, but examples include vanadium pentoxide (V2O5), sodium metavanadate (NaVO3), and ammonium metavanadate (NH4VO3), with V2O5 being preferred.

[0048] The solvent contains the above-mentioned bismuth compound and vanadium compound, with concentrations of 0.1 to 0.4 mol / l preferred for both. The molar ratio of bismuth in the bismuth compound to vanadium in the vanadium compound is preferably Bi / V = 1 / 1.

[0049] The solvent is not particularly limited as long as it can dissolve the bismuth compound and the vanadium compound, but water is preferred, and adding an acid to water is more preferred. The acid is not particularly limited, but nitric acid, hydrochloric acid, sulfuric acid, perchloric acid, acetic acid, etc. are preferred, with nitric acid being more preferred. The acid concentration is preferably in the range of 0.5 to 2.0 mol / l, and more preferably in the range of 0.5 to 1.0 mol / l. The liquid temperature is preferably in the range of 60 to 90°C, and more preferably in the range of 75 to 85°C.

[0050] To the obtained BiVO4, at least one elemental component from the group consisting of Ga, Zn, Ni, Al, and In may be added. When such elemental component is added, the ion of these elemental components, Ga, may be added. 3+ Zn 2+ Ni 2+ , Al 3+ In 3+ It is preferable to dissolve and mix them in this state. Among these, Ga 3+ This is preferable. The amount added is preferably in the range of 0.1 to 10 mol%, and more preferably 0.2 to 1.0 mol%, relative to the amount of substance of the bismuth compound or vanadium compound.

[0051] The BiVO4 prepared under the above conditions consists of a monoclinic crystal system with a cereite structure. The average particle size of the BiVO4 photocatalyst is not particularly limited, but it is preferably 0.5 to 10 μm.

[0052] In the cylindrical photocatalytic cell of the present invention, when the BiVO4 photocatalyst exists as a photocatalytic dispersion with water, and a glass particle carrier is immersed in this photocatalytic dispersion, a photocatalytic reaction can be performed by including a redox agent in the photocatalytic dispersion and irradiating it with light, thereby reducing the redox agent and oxidizing the water (accompanying the oxidation reaction of water). That is, a photocatalytic reaction apparatus can be constructed by immersing a glass particle carrier in a photocatalytic reaction solution containing a photocatalyst made of bismuth vanadate semiconductor, a redox agent, and water, and irradiating it with light such as an external light source or sunlight to perform the above-described photocatalytic reaction.

[0053] Examples of such redox products include Fe 3+ / Fe 2+ VO2 + / VO 2+ IO3 - / I - These can be used, but among them Fe 3+ / Fe 2+ It is preferable.

[0054] Regarding photocatalytic reactions using these redox compounds, for example, a two-step photoexcitation type photocatalytic reaction known as the Z scheme can be performed using iron ion-based redox compounds (Fe 3+ / Fe 2+ To illustrate with an example using ), it can be shown as shown in equation (3) below. That is, Fe 3+ Fe 2+ It is reduced to and oxidizes water to produce oxygen. At that time, the reduced Fe 2+ The aqueous solution containing the substance can be removed from the photocatalytic reactor and hydrogen can be produced by separately carrying out an electrolytic reaction as shown in formula (4) below. Photocatalytic reaction: 2H2O + 4Fe 3+ →4Fe 2+ +O2+4H + ...(3) Electrolytic reaction: 4Fe2+ +4H + →4Fe 3+ +2H2···(4) If this reaction occurs, the magnitude of the solar energy conversion efficiency is Fe 3+ Fe 2+ The conversion rate at which the reaction is reduced can be expressed, and the photoreduction reaction can be quantified using methods such as colorimetric analysis.

[0055] Considering the need to irradiate a photocatalytic reaction onto a photocatalyst attached to the surface of a glass particle carrier with light, it is desirable to make effective use of the irradiated light. Therefore, as described above, in the present invention, when light is irradiated from the side of the cylindrical container in the diametrical direction of the cylindrical container in a cylindrical photocatalytic cell, the ratio of transmitted light to irradiated light (transmitted light amount / irradiated light amount) is preferably in the range of 0.05% to 10%, and more preferably in the range of 0.1% to 5%. If the ratio of transmitted light to irradiated light amount is less than 0.05%, there is the disadvantage that the light does not penetrate to the depths of the porous glass carrier, and if it exceeds 10%, there is the disadvantage that the photocatalytic activity is not exhibited to the extent of the transmitted light. [Examples]

[0056] The present invention will be described more specifically below based on examples, but the present invention is not limited to the following examples.

[0057] (Examples 1-6) <Fabrication of BiVO4 photocatalyst composites using packing method> To obtain the BiVO4 photocatalyst composites described in Examples 1 to 6, the BiVO4 photocatalysts were first prepared using the suspension synthesis method. Specifically, the process was as follows: 4.66 g (10 mmol) of Bi2O3, 1.82 g (10 mmol) of V2O5, 0.04 g of Ga(NO3)3·nH2O, and 48 ml of water were stirred while 2.64 ml of concentrated nitric acid (1.42 g / ml, 69% by mass) was added. This was heated and stirred at 80°C for 24 hours. After cooling to room temperature and filtering, BiVO4:Ga (6.48 g, yellow powder) was obtained.

[0058] The BiVO4 photocatalyst (BiVO4:Ga) obtained above, and glass particles with the particle size shown in Table 1 below (material: soda-lime glass, density: 2.5 g / cm³) 3 The mixture was kneaded with liquid paraffin as a dispersion solvent and packed into a cylindrical glass tube (material: borosilicate glass) with an inner diameter D as shown in Table 1 and a length of 3 cm, fitted with a filter. Next, the liquid paraffin was removed from the glass tube by hexane washing to form glass particle aggregates in which glass particles were placed adjacent to each other within the glass tube, and a BiVO4 photocatalyst composite was prepared by supporting the BiVO4 photocatalyst on the surface of the glass particles in the glass particle aggregates. The BiVO4 photocatalyst composite contained in the glass tube (cylindrical container) described above is called a cylindrical photocatalyst cell.

[0059] As described above, BiVO4 photocatalytic composites were prepared with the values ​​in Table 1 for each condition. In doing so, the amount of BiVO4 photocatalyst supported was made equivalent among Examples 1 to 6 when converted to the amount supported per unit volume of the glass tube. Furthermore, the glass tubes in which each BiVO4 photocatalytic composite was housed were all cylindrical glass tubes with filters, similar to those described above. The material (borosilicate glass), length, and glass thickness were the same, but the inner diameter D differed as shown in Table 1, thus forming the cylindrical photocatalytic cells of Examples 1 to 6.

[0060] (Example 7) <Fabrication of BiVO4 photocatalyst composites using solution method> In order to obtain the BiVO4 photocatalytic composite according to Example 7, first, a glass particle sintered body was prepared as a glass particle support as follows. A 50 μm thick titanium foil was cut into strips, and the ends were spot-welded to create a mold with a diameter of 1.6 cm and a height of 6 cm. This mold was placed on an alumina plate, and a 50 μm thick titanium foil strip was wrapped around the inside of the mold. 16 g of glass particles (material: soda-lime glass) with the particle size shown in Table 1 were then placed inside the mold. The mold filled with glass particles was placed in a firing furnace together with the alumina plate and sintered. The sintering conditions were 725°C for 1 hour, with the temperature being raised to 725°C over 30 minutes and then held at 725°C for 1 hour. After that, the mold was removed, the strip inside the mold was peeled off, and a sintered glass particle body (16 g) with a diameter of 1.6 cm, a length of 6 cm, and a porosity of 40-45% was prepared.

[0061] Next, BiVO4 photocatalysts were prepared using a solution synthesis method. Specifically, the process was as follows: 100 ml of 0.4 mol / l nitric acid was prepared. 1.5522 g (3.2 mmol) of Bi(NO3)3·5H2O was weighed into a container. 0.3743 g (3.2 mmol) of NH4VO3 was weighed into another container. 100 ml of the prepared nitric acid was distributed to each container, and they were stirred and dissolved to obtain Bi solution and V solution. 0.0064 g of Ga(NO3)3·nH2O was added to the dissolved Bi solution. The V solution was added to the Bi solution and mixed. In this way, a mixed solution was prepared.

[0062] As previously described, a sintered glass particle body made using glass particles with an average number diameter of 2.00 mm was placed in a cylindrical glass tube with a cap, having an inner diameter of 1.6 cm and a length of 6 cm. 12 ml of the mixture obtained above was placed in this reaction tube and heated at 65°C for 24 hours to precipitate BiVO4:Ga. After that, it was cooled to room temperature, filtered, washed with water, and dried to prepare a BiVO4 photocatalyst composite. At this time, the amount of BiVO4 photocatalyst loaded was adjusted so that it was equivalent to that in Examples 1 to 6 when converted to the amount loaded per unit volume of the glass tube. The obtained BiVO4 photocatalyst composite was then loaded into a cylindrical glass tube with a filter (material: borosilicate glass) having an inner diameter D as shown in Table 1, as in Examples 1 to 6, to form a cylindrical photocatalyst cell according to Example 7.

[0063] Here, in order to determine the particle size of the glass particles forming the glass particle aggregate, SEM images (50 to 200x magnification) were taken of the glass particles used in each example, and 200 arbitrary glass particle images were selected. The circumscribed circle of the glass particle was considered as the diameter, and the average particle size D was calculated as the number-average diameter by dividing the sum of the diameters of each particle by 200. Furthermore, for the specific surface area of ​​the BiVO4 photocatalytic composite in each example from Examples 1 to 7, the volume V of one glass particle was calculated as V = (4 / 3)π(D / 2), where ρ is the density of the glass particle material. 3 It is expressed as follows, and the surface area S of one glass particle is S = 4π(D / 2) 2 Since it can be expressed as, the specific surface area (surface area per unit mass) S m The formula was calculated from =S / (ρV). These results are summarized in Table 1. Note that the amount of catalyst supported in Table 1 was obtained by subtracting the mass of the glass particle assembly before supporting the BiVO4 photocatalyst from the mass of the BiVO4 photocatalyst composite obtained as described above.

[0064] <Evaluation of cylindrical photocatalytic cells> Each of the cylindrical photocatalytic cells obtained above was evaluated using the following experimental setup and photocatalytic reaction. As shown in Figure 1, first, 900 ml of 20°C water was placed in tank 4 with a stopcock, and 150 μl of 60% HClO4 aqueous solution was added to adjust the pH. Next, 7.2 ml of 0.25 mol / l Fe(ClO4)3 aqueous solution was added, and Fe 3+ A solution (photocatalytic reaction solution) 3 containing the redox product was prepared with a concentration of 2 mmol / l.

[0065] The tank 4 containing the photocatalytic reaction solution 3 prepared in this manner and the inlet and outlet of the cylindrical photocatalytic cell, in which the BiVO4 photocatalytic composite 2 is housed in each cylindrical glass tube 1, were connected by tubes 5. The glass tube 1 was positioned horizontally in its longitudinal direction, and the photocatalytic reaction solution 3 was circulated so that the residence time inside the glass tube 1 was 1 minute. Then, in the area where the BiVO4 photocatalytic composite 2 was housed, light 6 was irradiated from directly above the glass tube 1 in the diametrical direction 1a of the glass tube 1 to carry out the photocatalytic reaction. At that time, as shown in Figure 2, the surface in contact with the uppermost point of the circumference of the glass tube 1 was defined as the irradiated surface 8a, and the amount of light irradiated onto this irradiated surface 8a was defined as the irradiated light intensity (i). In addition, the irradiated light intensity when light was irradiated using a solar simulator (Yamashita Densou YSS-100A) was measured with a diffraction grating spectroradiometer (Eiko Seiki LS-100), and 1 SUN (100 mW / cm²) was defined as 1 SUN (100 mW / cm²). 2 The position of the glass tube 1 was adjusted so that ). Light irradiation was started, and the light-receiving part 7 of the diffraction grating spectroradiometer was placed on the outlet surface 8b so as to be in contact with the lowest point of the circumference of the glass tube, and the transmitted light amount (ii) was measured. After that, the outflowed aqueous solution was collected, colored using the phenanthroline method, and the absorbance at 510 nm was measured to determine Fe 2+ The quantity was quantified.

[0066] (Comparative Examples 1-2) Following the same procedure as in Examples 1-6, BiVO4 photocatalytic composites with the values ​​in Table 1 were prepared to obtain cylindrical photocatalytic cells corresponding to Comparative Examples 1-2. The amount of BiVO4 photocatalyst loaded was adjusted so that it was equivalent to that of Examples 1-7 when converted to the amount loaded per unit volume of the glass tube.

[0067] [Table 1]

[0068] Table 1 shows the ratio of transmitted light (ii) to irradiated light (i) measured before injecting photocatalytic reaction solution 3, an aqueous solution containing the redox product, into the cylindrical photocatalytic cell [transmitted light (i) / irradiated light (ii)], and the same ratio after injecting photocatalytic reaction solution 3 into the cylindrical photocatalytic cell [transmitted light (i) / irradiated light (ii)]. Also, in Table 1, "Fe per unit area of ​​composite" 2+ "Production rate" is the amount of Fe per unit area of ​​the photocatalytic composite irradiated with light in the experimental apparatus. 2+ This represents the generation rate. More specifically, as shown in Figure 3, when viewed from directly above when light 6 is irradiated onto the glass tube 1 in the experimental apparatus described above, a rectangle 1a × 2a, having the inner diameter 1a of the glass tube 1 and the length 2a of the BiVO4 photocatalytic complex 2 contained within the glass tube 1, is considered as the light irradiation area of ​​the BiVO4 photocatalytic complex 2. 2+ The generation rate was calculated by dividing it by the light irradiation area (1a × 2a).

[0069] As a result, as shown in Table 1, when a cylindrical photocatalytic cell is used in which the ratio of transmitted light amount (ii) to irradiated light amount (i) is within the range of the present invention, the Fe per unit area of ​​the BiVO4 photocatalytic composite is higher compared to when a cylindrical photocatalytic cell of the comparative example that does not satisfy this condition is used. 2+ It can be seen that the generation rate is excellent. This is because, below the lower limit of the ratio in the present invention, light does not reach the deeper parts of the photocatalytic composite, and conversely, above the upper limit of the ratio, it is thought that light passes straight through the photocatalytic composite. However, with the cylindrical photocatalytic cell according to the example, it can be said that the irradiated light was able to be fully utilized in the photocatalytic reaction. [Explanation of Symbols]

[0070] 1: Cylindrical container, 2: Photocatalytic composite, 3: Photocatalytic reaction solution, 4: Tank, 5: Tube, 6: Light (irradiation light), 7: Light receiving unit, 8a: Irradiated surface, 8b: Light irradiation outlet surface.

Claims

1. A light-transmitting cylindrical container with an inner diameter D of 0.8 cm or more contains a porous glass particle carrier formed of multiple glass particles with an average particle size of 0.1 to 2 mm, and a photocatalyst is supported on the porous glass particle carrier. The ratio (D / M) of the inner diameter D (cm) of the cylindrical container to the total mass M (g) of the porous glass particle carrier and the photocatalyst per 1 cm of the length of the cylindrical container is 0.23 or more and 1.32 or less. A cylindrical photocatalytic cell characterized in that, when light is irradiated from the side of the cylindrical container in the diametrical direction of the cylindrical container, the ratio of transmitted light to irradiated light (transmitted light amount / irradiated light amount) is 0.05% or more and 10% or less.

2. The cylindrical photocatalytic cell according to claim 1, wherein the glass particle carrier is a glass particle sintered body in which glass particles are sintered together and integrated.

3. The cylindrical photocatalytic cell according to claim 1, wherein the glass particle carrier is a glass particle aggregate in which a plurality of glass particles are housed in the cylindrical container and the glass particles are adjacent to each other.

4. The cylindrical photocatalytic cell according to any one of claims 1 to 3, wherein the photocatalyst is a bismuth vanadate semiconductor.

5. The cylindrical photocatalytic cell according to any one of claims 1 to 4, further comprising an oxidation-reduction agent in the cylindrical container.

6. A cylindrical photocatalytic cell according to any one of claims 1 to 5, for use in artificial photosynthesis.

7. The cylindrical photocatalytic cell according to any one of claims 1 to 6, wherein the light is sunlight.

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