JP7230663B2 - Photocatalyst and method for producing hydrogen and oxygen using this photocatalyst - Google Patents

Description

The present invention relates to a photocatalyst and a method for producing hydrogen and oxygen using this photocatalyst.

In recent years, the development of high-performance light energy conversion systems that use solar energy as renewable energy has rapidly increased in importance from the perspective of curbing global warming and breaking away from dependence on fossil resources, which are being depleted. sexuality is increasing. In particular, the technology that uses solar energy to split water and produce hydrogen can be used not only as a raw material supply technology for petroleum refining, ammonia, and methanol, but also as an energy carrier for fuel cells. Social demands are increasing more and more.

Photocatalytic water-splitting reactions have been extensively studied for a long time.
The decomposition reaction of water in the acidic aqueous solution on the photocatalyst particles is presumed as follows.
_H2O +2h ⁺ →1/ _2O2 +2H ⁺ (1)
2H ⁺ +2e ⁻ →H ₂ (2)

As a photocatalyst, TiO ₂ doped with transition metals such as Cr and V has been conventionally proposed (for example, Patent Documents 1 to 5), but all of them have low catalytic efficiency, leaving problems for practical use. Met.

On the other hand, the photocatalyst of the present invention, Y ₂ Ti ₂ O ₅ S ₂ , has been utilized as an active material for sodium ion batteries (Patent Document 6). However, when Y ₂ Ti ₂ O ₅ S ₂ produced by the conventional method is used as a photocatalyst, its photocatalytic activity, specifically, the reaction rate is low, and the total decomposition for generating hydrogen and oxygen from water is low. It could not be used as an electrode, but could be used as an electrode for generating either hydrogen or oxygen by allowing a sacrificial reagent to coexist (Non-Patent Documents 1 and 2).

JP-A-9-262482 JP-A-11-197512 JP-A-11-255514 JP-A-11-279299 JP-A-11-33408 JP 2013-62121 A

Hiroshi Otani, Akio Ishikawa, Tsuyoshi Takada, Kazunari Domen, Proceedings of the 100th Catalysis Symposium (2007), p343 Hiroshi Otani, Takahiro Suzuki, Kentaro Teramura, Kazunari Domen Proceedings of the 87th Annual Meeting of the Chemical Society of Japan (2007), p500

An object of the present invention is to provide a novel photocatalyst with excellent photocatalytic activity and a method for producing hydrogen and oxygen using this photocatalyst.

As a result of intensive studies by the present inventors to solve the above problems, when Y ₂ Ti ₂ O ₅ S ₂ , which has been applied as an active material for conventional sodium ion batteries, is fired at a lower temperature during production, Even if the composition is the same, a specific crystal phase develops in the XRD measurement compared to the case of high-temperature firing, and a new substance with a different specific peak shape is generated accordingly. The photocatalyst exhibiting a peak pattern of , exhibits high catalytic activity for water splitting, and generates oxygen and hydrogen without using a sacrificial reagent, which was the biggest problem with conventional catalysts with the composition Y ₂ Ti ₂ O ₅ S ₂ . It has been found that a single photocatalyst can be used as a catalyst to generate oxygen and hydrogen through the total decomposition reaction of water.
That is, the gist of the present invention is as follows.

[1] A photocatalyst having a composition represented by the following general formula (I), which is a diffraction peak with a peak top at 26.3 ± 0.3 in XRD measurement with Cu-Kα radiation according to the following equipment and measurement conditions: and the peak top has an intensity of 20 or more, with the maximum peak intensity on the XRD spectrum being 100, and the diffraction peak half width (FWHM) of 30.6 ± 0.5 is 0.16 to 0.16. Photocatalyst in the range of 30.
_{YaTibOcSd ( I )}
(However, the numbers are a = 1.7 to 2.3, b = 1.7 to 2.3, c = 5, and d = 1.7 to 2.3.)
<XRD measurement>
Manufacturer: Rigaku
Apparatus; SmartLab
Measurement condition;
Photocatalyst powder pulverized to a diameter of 100 μm or less is subjected to powder X-ray diffraction measurement by the concentration method ・Measurement range: 5 to 80 °
・Measurement step: 0.01°
Scanning speed: 10°/min Analysis without monochromator;
FWHM: Calculated directly from the data obtained under the above conditions without separation of Kα1 and Kα2

[2] A photocatalyst having a composition represented by the following general formula (I), and having a _λP . _T. value (the wavelength at which the diffuse reflectance spectrum after KM conversion shows the maximum value) is in the range of 400 nm or more and 495 nm or less, and λ _{H. S.} A photocatalyst having a value (wavelength at which the diffuse reflectance spectrum after KM conversion shows an intermediate value) is in the range of 520 nm or more and 570 nm or less.
_{YaTibOcSd ( I )}
(However, the numbers are a = 1.7 to 2.3, b = 1.7 to 2.3, c = 5, and d = 1.7 to 2.3.)
<Ultraviolet/visible diffuse reflectance spectrum measurement>
Manufacturer; JASCO
Model number: V-670 Spectrophotometer
Measurement condition;
・Measurement range: 300nm to 800nm
・Data interval: 0.2 nm
・Scanning speed: 200 nm/min ・Light source switching: 340.0 nm
・Data analysis software: Spectra Manager version 2
Analysis; Kubelka-Munk (K.M.) transformation Kubelka-Munk transformation formula f (R _∞ ) = (1-R _∞ ) 2/2R _∞ = K/S on the vertical axis
where f(R _∞ ) is K.K. M. function, R _∞ is the absolute reflectance, K is the molecular extinction coefficient, and S is the scattering coefficient.
It should be noted that it is difficult to measure the absolute reflectance R _∞ of a sample, and in practice it is common to use the relative reflectance r _∞ using a standard sample. Therefore,
r _∞ = r (measurement sample)/r (standard sample) (using BaSO ₄ as standard sample)
to measure the relative reflectance r _∞ using
f(r _∞ )=(1−r _∞ )2/2r _∞ =K/S
derived from

[3] The photocatalyst according to [1] or [2], which is a photocatalyst used for total decomposition of water.

[4] A method for producing hydrogen and oxygen, wherein hydrogen and oxygen are generated using the photocatalyst-immobilized product or molded product according to any one of [1] to [3].

[5] An electrode produced using the photocatalyst according to any one of [1] to [3].

[6] A method for producing hydrogen and oxygen, comprising generating hydrogen and/or oxygen using the electrode according to [5].

ADVANTAGE OF THE INVENTION According to this invention, the photocatalyst excellent in photocatalytic activity is provided.
Using the photocatalyst of the present invention, water can be completely decomposed efficiently to produce hydrogen and oxygen.

FIG. 1 is a diagram showing the XRD near 26.3° of the sample prepared by firing at 650° C. in Example 1. FIG. FIG. 2 is a diagram showing the XRD near 30.6° of the sample produced by firing at 650° C. in Example 1. FIG. FIG. 3 is a diagram showing the XRD near 26.3° of the sample produced by firing at 700° C. in Example 2. FIG. FIG. 4 is a diagram showing the XRD near 30.6° of the sample produced by firing at 700° C. in Example 2. FIG. FIG. 5 is a diagram showing the XRD near 26.3° of the sample produced by firing at 800° C. in Example 3. FIG. FIG. 6 is a diagram showing the XRD near 30.6° of the sample produced by firing at 800° C. in Example 3. FIG. FIG. 7 is a diagram showing the XRD near 26.3° of the sample produced by firing at 900° C. in Example 4. FIG. FIG. 8 is a diagram showing the XRD near 30.6° of the sample produced by firing at 900° C. in Example 4. FIG. FIG. 9 is a diagram showing the XRD near 26.3° of the sample produced by firing at 600° C. in Comparative Example 1. FIG. FIG. 10 is a diagram showing the XRD near 30.6° of the sample produced by firing at 600° C. in Comparative Example 1. FIG. FIG. 11 is a diagram showing the XRD near 26.3° of the sample produced by firing at 1000° C. in Comparative Example 2. FIG. FIG. 12 is a diagram showing the XRD near 30.6° of the sample produced by firing at 1000° C. in Comparative Example 2. FIG. FIG. 13 is a diagram showing the results of measurement of capacitance-potential characteristics (Mott-Schottky plot) using the YTOS of the present invention in Example 10. FIG. FIG. 14 is a diagram showing the photoanode current signal corresponding to light irradiation (Light on)-interruption (Light off) at pH 9 in Example 11. FIG. FIG. 15 shows the photoanode current signal corresponding to light irradiation (Light on)-interruption (Light off) at pH 13 in Example 11. FIG.

Embodiments of the present invention will be described in detail below, but the present invention is not limited to the following description, and can be arbitrarily modified and implemented without departing from the scope of the present invention. In this specification, when a numerical value or a physical property value is sandwiched before and after the "~", it is used to include the values before and after it.

[mechanism]
The photocatalyst of the present invention is a photocatalyst having a composition represented by the following formula (I) (hereinafter, the substance represented by the following formula (I) may be abbreviated as "YTOS"), and the following apparatus and measurement In XRD measurement with Cu-Kα radiation according to the conditions, it has a diffraction peak with a peak top at 26.3 ± 0.3, and the peak top has a maximum peak intensity on the XRD spectrum of 20 or more. intensity and a diffraction peak half width (FWHM) of 30.6 ± 0.5 in the range of 0.16 to 0.30, or an ultraviolet/visible diffuse reflectance spectrum according to the following equipment and measurement conditions The _λP obtained from the measurement. _T. value (the wavelength at which the diffuse reflectance spectrum after KM conversion shows the maximum value) is in the range of 400 nm or more and 495 nm or less, and λ _{H. S.} A value (wavelength at which the diffuse reflectance spectrum after KM conversion exhibits an intermediate value) is in the range of 520 nm or more and 570 nm or less.
_{YaTibOcSd ( I )}
(However, the numbers are a = 1.7 to 2.3, b = 1.7 to 2.3, c = 5, and d = 1.7 to 2.3.)

<XRD measurement>
Manufacturer: Rigaku
Apparatus; SmartLab
Measurement condition;
Photocatalyst powder pulverized to a diameter of 100 μm or less is subjected to powder X-ray diffraction measurement by the concentration method ・Measurement range: 5 to 80 °
・Measurement step: 0.01°
Scanning speed: 10°/min Analysis without monochromator;
FWHM: Calculated directly from the data obtained under the above conditions without separation of Kα1 and Kα2

<Ultraviolet/visible diffuse reflectance spectrum (DRS) measurement>
Manufacturer; JASCO
Model number: V-670 Spectrophotometer
Measurement condition;
・Measurement range: 300nm to 800nm
・Data interval: 0.2 nm
・Scanning speed: 200 nm/min ・Light source switching: 340.0 nm
・Data analysis software: Spectra Manager version 2
Analysis; Kubelka-Munk (K.M.) transformation Kubelka-Munk transformation formula f (R _∞ ) = (1-R _∞ ) 2/2R _∞ = K/S on the vertical axis
where f(R _∞ ) is K.K. M. function, R _∞ is the absolute reflectance, K is the molecular extinction coefficient, and S is the scattering coefficient.
It should be noted that it is difficult to measure the absolute reflectance R∞ of a sample, and in practice it is common to use the relative reflectance _r∞ using a standard sample. Therefore,
r _∞ = r (measurement sample)/r (standard sample) (using BaSO ₄ as standard sample)
to measure the relative reflectance r _∞ using
f(r _∞ )=(1−r _∞ )2/2r _∞ =K/S
derived from

In the photocatalyst of the present invention, as described later, the firing temperature at the time of photocatalyst production is 650 ° C. or more and less than 1000 ° C., which is lower than the temperature described in Patent Document 6 above. However, even if the bulk composition is the same, a specific crystal phase develops, resulting in YTOS with a different specific peak shape, and this new YTOS can provide excellent photocatalytic activity.
Generally, in the production of metal composite oxides, sintering at a high temperature generally results in better crystallinity and excellent physical properties. In the example of Patent Document 6, firing is performed at 1100°C.
However, as shown in Comparative Example 2 below, the conventional YTOS has the same bulk composition as the YTOS of the present invention, but the 103 crystal face, which is important for improving the photocatalytic activity, does not develop. Not only is it not sufficient, but also λ _P. , which is also an important parameter for improving photocatalytic activity, as is clear from UV/visible diffuse reflectance spectroscopy. _T. The value is also _{λH. S.} It is a substance different from the YTOS of the present invention, whose value also does not meet the requirements of the present invention.

Specific methods for the XRD measurement and the ultraviolet/visible diffuse reflectance spectrum measurement are as shown in Examples below.

[YTOS composition]
The photocatalyst of the present invention is represented by the above formula (I).
In formula (I), a = 1.7 to 2.3, b = 1.7 to 2.3, c = 5, d = 1.7 to 2.3, preferably a = 1.9 ~2.1, b = 1.9-2.1, c = 5, d = 1.9-2.1. In the present specification, unless otherwise specified, the numbers ∼ include upper and lower numbers.
Here, Y is Ca, Sr, Ag, In, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm within a range that does not interfere with the effects of the present invention. , Yb, Lu, or the like. Similarly, Ti may be substituted with Al, Cr, Mn, Co, Ni, Rh, Ga, Zn, Ta, and Nb, and the amount of substitution is 0 as the value of a or b in the above composition formula. 0.3 or less, more preferably 0.1 or less.

[Characteristics in XRD measurement]
The photocatalyst of the present invention has a diffraction peak with a peak top near 2θ = 26.3 ± 0.3 (deg.) in XRD (X-ray diffraction) measurement using Cu-Kα rays, and the intensity of the peak top is usually 20 or more, preferably 40 or more, more preferably 60 or more, particularly preferably 100, with the maximum peak intensity on the XRD spectrum being 100, and 2θ = around 30.6 ± 0.5 (deg.) The lower limit of the half-value width (FWHM) of the diffraction peak is usually 0.16 or more, preferably 0.18 or more, and the upper limit is usually 0.50 or less, preferably 0.30 or less, more preferably 0 , 27 or less.
Here, the peak near 26.3±0.3 is the peak of the 103 plane of Y ₂ Ti ₂ O ₅ S ₂ , and the peak near 30.6±0.5 is the peak of the same 105 plane. In addition, below, the intensity|strength with respect to the maximum peak intensity 100 on the XRD spectrum of the peak top of 103 planes is called "103 plane peak intensity ratio."
As is clear from the above description and other descriptions of the present invention, the peak intensity in the present invention means the height of the peak top, and the peak intensity ratio means the ratio of heights.

Since the peak of the 103 plane is a peak normally observed in the XRD diffraction pattern of YTOS, the target YTOS and the impurity phase (Y ₂ Ti ₂ O ₇ (hereinafter referred to as YTO) and TiS ₂ , TiO ₂ , Y ₂ S ₂ O, etc.) (ie, the purity of the YTOS pure substance). For example, YTOS with a small 103 plane peak intensity ratio often forms a eutectic with YTO or Y ₂ S ₂ O as an impurity, and such a substance has defects in the crystal. It often inhibits the catalytic activity due to the presence of Further, in the process of completing the present invention, it was found that the photocatalytic activity of YTOS increases as the 103 plane peak intensity ratio increases, that is, as the 103 plane crystal plane of YTOS develops. Therefore, the intensity of the YTOS 103-plane peak top is usually 20 or more, preferably 40 or more, more preferably 60 or more, and particularly preferably 100, where 100 is the maximum peak intensity on the XRD spectrum.

Also, in the peak of the 105 plane, control of FWHM is an important design factor. In many photocatalysts, it is generally proposed that the degree of crystallinity is closely related to the degree of catalytic activity, and it is generally considered desirable that the FWHM be sufficiently small. That is, it is considered that the crystal regularity is improved and the catalytic activity is improved as the calcination temperature is raised to a higher temperature in this material as well. However, with this material, surprisingly, when the sintering temperature is increased, the crystal regularity is improved up to a certain temperature, and the catalytic activity is improved. It has been found that the growth is accelerated, and the catalytic activity of catalysts having crystal phases grown in these specific directions is lowered. This is because when the crystal structure growth progresses in the stacking direction in the layered material, the crystal grows in an orientation different from the hybrid orientation of the 3d orbital between Ti-Ti bonds, and electrons and holes excited by light irradiation are generated on the particle surface. It is presumed that this is because there is a possibility that the reaction mechanism of the photocatalyst, in which the light is conducted to the co-catalyst, will be in a very disadvantageous state. From this, the YTOS with controlled FWHM value of the present invention is YTOS in which the crystallinity of YTOS, which is an important design factor in improving the photocatalytic activity, and excessive growth in a specific specific orientation are controlled. Become. For these reasons, the lower limit of the full width at half maximum (FWHM) of the diffraction peak on the 105 plane is usually 0.16 or more, preferably 0.18 or more, and the upper limit is usually 0.50 or less, preferably 0.50. It is preferably in the range of 30 or less, more preferably 0.27 or less.

[Characteristics of UV/visible diffuse reflectance spectrum measurement]
The photocatalyst of the present invention has a _λP obtained by ultraviolet/visible diffuse reflectance spectroscopy. _T. value (KM transform (f(r _∞ )=(1−r _∞ )2/2r _∞ =K/S, where r _∞ is the KM function, r _∞ is the relative reflectance, K is the molecular absorption coefficient, S is the scattering coefficient) and the wavelength at which the diffuse reflectance spectrum shows the maximum value) is in the range of 400 nm or more and 495 nm or less, and _{λH. S.} A value (wavelength at which the diffuse reflectance spectrum after KM conversion exhibits an intermediate value) is in the range of 520 nm or more and 570 nm or less.
Preferably, the _{λP. T.} values are between 430 and 493 nm and the λ _{H. S.} Values are between 525 and 567 nm.

The sample of the present invention is often powder, and for this reason, the absorption spectrum of light cannot be measured directly _{. T.} value, λ _{H. S.} Values are obtained by measuring the diffuse reflectance spectrum, K.K. M. It is a value calculated by a method of converting into absorbance mode. In the present invention, the first condition for a visible light responsive photocatalyst is that a peak be observed in the visible light region _{. T.} value is in the range of 400 nm or more and 495 nm or less, and λ _{H. S.} It was found that excellent catalytic activity is exhibited when the value is in the range of 520 nm or more and 570 nm or less.

In the present invention, λP _{. T.} The value is defined as the wavelength at which the diffuse reflectance spectrum after KM conversion shows the maximum value, and assuming that the absorption spectrum is symmetrical, this value is the center of the visible light region (generally 360 to 830 nm) (About 600 nm), the closer it is, the more light in the visible light region can be absorbed, so it is thought that the catalytic activity is improved. Furthermore, in the present invention, the sample often contains an impurity YTO phase or a YTO-YTOS intermediate phase, and in such structures, conduction of electrons and holes is inhibited, and photocatalytic activity is often limited. This impurity YTO phase and YTO-YTOS intermediate phase have a wavelength _{of λP. T.} value = 300-390 nm, and from the point of view of limiting these components, λ _{P. T.} The value is preferably 400 nm or greater. This _{λP. T.} The value becomes higher as the calcination temperature is raised, but there is an appropriate range because the catalytic performance is reduced due to the aforementioned increase in crystallite size.
Also, in the present invention, λ _{H. S.} The value is defined as the wavelength at which the diffuse reflectance spectrum after KM conversion exhibits _an intermediate value. _T. This _{λH. S.} It is considered that the higher the value, the higher the photocatalytic activity. However, in the present invention, λH _{. S.} As a factor for increasing the value, the following Ti3+-containing phase may be present, and as a result of comparison with the catalyst performance, it was found that there is an optimum range. Ti3+-containing phases often inhibit electron and hole conduction, limiting photocatalytic activity. This Ti3+ reduced phase is observed as a very gentle slope in the wavelength region of 600 nm or more, and the more this component, the more the _{λH. S.} value increases, the λ _{H. S.} The values are useful for understanding the distribution of light absorption wavelengths and the Ti3+ phase content that reduces catalytic activity.

From the above, the _{λP. T.} value and λ _{H. S.} YTOS with a controlled value is a YTOS with a controlled Ti3+ phase content that reduces the visible light absorption characteristics and catalytic performance of YTOS, which are important design factors in improving photocatalytic activity. For these reasons λ _{P. T.} value is in the range of 400 nm or more and 495 nm or less, and λ _{H. S.} A value in the range of 520 nm or more and 570 nm or less indicates higher activity, and more preferably λ _{P. T.} values are between 430 and 493 nm and the λ _{H. S.} Preferably the value lies between 525 and 567 nm.

[Method for producing photocatalyst]
The photocatalyst of the present invention can be produced by weighing and sufficiently mixing the raw materials for the Y source, Ti source, O source, and S source so as to satisfy the above formula (I), and calcining the resulting mixture. can.

As the Y source, one or more of Y ₂ O ₃ , Y ₂ O ₂ S, Y ₂ S ₃ , Y, YCl ₃ and the like can be used. Here, Y ₂ O ₃ and Y ₂ O ₂ S also serve as O sources. Y ₂ S ₃ and Y ₂ O ₂ S also serve as S sources.

As a Ti source, one or more of TiO ₂ , TiS ₂ , Ti and the like can be used. Here, TiO ₂ also serves as an O source. TiS ₂ also serves as an S source.

As the O source, as described above, it is preferable to use both the Y source and the Ti source.

As the S source, one or more of S, H ₂ S and the like can be used. At this time, the technique of allowing H2S or the like to flow as a gas for reaction is one of the preferred modes for producing the photocatalyst of the present invention. Moreover, as described above, Y ₂ S ₃ , Y ₂ O ₂ S, and TiS ₂ also serve as S sources.

The mixing of these raw materials is preferably carried out at −20 to 50° C. in an atmosphere of an inert gas such as nitrogen, since air and a small amount of moisture are mixed in and cause the formation of impurities such as oxide phases.

Although the firing temperature of the mixture is not particularly limited because it depends on the firing time, it is preferably 650°C or more and less than 1000°C. If the calcination temperature is 1000° C. or higher, it is often impossible to obtain YTOS exhibiting the above-mentioned XRD and ultraviolet/visible diffuse reflectance spectrum measurement results characteristic of the photocatalyst of the present invention and having excellent photocatalytic activity. Also, if the firing temperature is too low, as shown in Comparative Example 1. In many cases, the solid-phase reaction does not proceed sufficiently and high-purity YTOS cannot be obtained. Therefore, the firing temperature is usually 650°C or higher, preferably 670°C or higher, and usually lower than 1000°C, preferably 930°C. It is below.

Although there are no particular restrictions on the firing atmosphere, it is preferable to carry out the firing in a vacuum from the viewpoint of preventing side reactions.
The firing time varies depending on the firing temperature, but is usually 12 to 240 hours, preferably 48 to 120 hours.

YTOS obtained by firing may be subjected to heat treatment in air at a temperature of 100 to 300° C. for about 0.1 to 3 hours in order to oxidize and remove excess sulfur content as necessary. good. After this heat treatment, it is preferable to remove sulfur oxides by washing with water and separate YTOS into solid and liquid.

In addition, the obtained YTOS may be subjected to an acid treatment in which it is brought into contact with an acid such as sulfuric acid, nitric acid, aqua regia, etc. of about 20 to 80% by mass, if necessary. Impurities can be removed.

Moreover, the obtained YTOS may be subjected to granule regulating treatment such as pulverization and classification, if necessary.
The particle size after pulverization is not particularly limited, but is preferably 1 μm or more because it facilitates handling. On the other hand, by setting the particle size to 20 μm or less, the surface area of the catalyst is increased and the catalytic activity is improved, which is preferable. This particle size is calculated from the average value of the diameters of about 50 particles selected at random, for example, by taking a photograph with an SEM. When the particles after pulverization are largely out of a spherical shape, the particle diameter may be calculated by measuring the area equivalent diameter from a photograph.

Furthermore, the obtained YTOS may be suspended in a cocatalyst-containing solution and subjected to MW (microwave) treatment as necessary. By performing MW treatment, the photocatalyst for evaluation described later can be prepared in a short time. sometimes it is possible. As the MW treatment, for example, "Microwave synthesis Reactor Monowave 300" manufactured by Anton Paar may be used, and recommended conditions may be appropriately selected.

[Use]
The photocatalyst of the present invention is effective as a water-splitting photocatalyst, exhibits particularly high photocatalytic activity, and can completely decompose water with a single electrode, that is, as a photocatalyst capable of completely decomposing water without the need for a counter electrode. can.

[Method for producing hydrogen and oxygen]
The method for producing hydrogen and oxygen of the present invention is characterized by using the photocatalyst of the present invention to generate hydrogen and oxygen without using a sacrificial reagent. Moreover, by using the photocatalyst of the present invention, hydrogen and oxygen can be generated on the same electrode. In the present invention, a laminate in which a photocatalyst layer containing the photocatalyst of the present invention is provided on a substrate, or a composite containing the photocatalyst of the present invention is referred to as an electrode.

Although the photocatalyst of the present invention exhibits sufficient photocatalytic activity by itself, it is preferably used together with a co-catalyst.

As the co-catalyst, there are an oxidation reaction co-catalyst (oxygen generation side) and a reduction reaction co-catalyst (hydrogen generation side), and it is preferable to use one or both of these by supporting YTOS. As the oxidation reaction co-catalyst, metals of groups 2 to 14 of the periodic table, intermetallic compounds and alloys of these metals, or oxides, composite oxides, nitrides, oxynitrides, sulfides, and oxysulfides thereof , or mixtures thereof. Here, the "intermetallic compound" is a compound formed from two or more kinds of metal elements, and the atomic ratio of the components constituting the intermetallic compound is not necessarily a stoichiometric ratio, and it may have a wide composition range. say. "These oxides, composite oxides, nitrides, oxynitrides, sulfides, and oxysulfides" are metals of groups 2 to 14 of the periodic table, intermetallic compounds of these metals, or oxides of alloys , composite oxides, nitrides, oxynitrides, sulfides, and oxysulfides. "These mixtures" means a mixture of any two or more of the compounds exemplified above.

The oxidation reaction promoter is preferably Mg, Ti, Mn, Fe, Co, Ni, Cu, Ga, Ru, Rh, Pd, Ag, Cd, In, Ce, Ta, W, Ir, Pt or Pb. Metals, their oxides or composite oxides, more preferably Mn, Co, Ni, Ru, Rh, Ir metals, their oxides or composite oxides, still more preferably Ir, MnO, MnO ₂ , Mn ₂ O ₃ , Mn ₃ O ₄ , CoO, Co ₃ O ₄ , NiCo ₂ O ₄ , RuO ₂ , Rh ₂ O ₃ and IrO ₂ .

As the reduction reaction co-catalyst, metals of Groups 3 to 13 of the periodic table, intermetallic compounds and alloys of these metals, or oxides, composite oxides, oxynitrides, sulfides, oxysulfides, carbides thereof, It is preferred to use either nitrides or mixtures thereof. Here, the “intermetallic compound” is the same as above, and “these oxides, composite oxides, oxynitrides, sulfides, oxysulfides, carbides, and nitrides” refer to Periodic Table Nos. 3 to 13. group metals, intermetallic compounds of said metals, alloy oxides, composite oxides, oxynitrides, sulfides, oxysulfides, carbides or nitrides. "These mixtures" means a mixture of any two or more of the compounds exemplified above.

Pt, Pd, Rh, Ru, Ni, Au, Fe, NiO, RuO ₂ , IrO ₂ , Rh ₂ O ₃ and Cr—Rh composite oxides, core-shell type Rh/ Cr ₂ O ₃ , Pt/Cr ₂ O ₃ and the like can be mentioned.

The amount of the co-catalyst supported is not particularly limited, but is usually 0.01% by mass or more and 1% by mass or less, preferably 1% by mass or less, based on YTOS (100% by mass). The upper limit is 0.5 mass % or less, more preferably the upper limit is 0.4 mass % or less, and the lower limit is 0.1 mass % or more. The amount of metal supported by the reduction reaction cocatalyst 30 is not particularly limited, but based on YTOS (100% by mass), it is usually 0.01% by mass or more and 20% by mass or less, preferably the upper limit is 15% by mass or less, and more preferably. has an upper limit of 10% by mass or less.
As used herein, the term "amount of metal supported" refers to the amount of the metal element in the supported cocatalyst.

When the photocatalyst of the present invention is actually used for water decomposition, the form of the photocatalyst is not particularly limited. A mode in which a photocatalyst layer is provided on a base material to form a laminate and the laminate is placed in water. morphology and the like. In particular, when the photo-water splitting reaction is performed on a large scale, it is preferable to use the electrode for the photo-water splitting reaction from the viewpoint of promoting the water splitting reaction by applying a bias. In another aspect, a photocatalyst layer containing the photocatalyst of the present invention is formed on a substrate without applying a bias, utilizing the fact that the photocatalyst of the present invention can completely decompose water with the present catalyst alone. The provided laminate or composite containing the photocatalyst of the present invention can also be placed in water. Due to this aspect, it is possible to reduce the cost when used as an artificial photosynthesis device that uses a large area, such as ease of processing and handling, ease of maintenance, and industrially superior water decomposition device and oxygen generator. , a hydrogen generator, or an artificial photosynthesis system.

The photo-water splitting reaction electrode can be produced by a known method. For example, it can be easily produced by a so-called particle transfer method (Chem. Sci., 2013, 4, 1120-1124). Here, in the particle transfer method, it is common to manufacture an electrode for photo-water splitting reaction by the following procedure. That is, photocatalyst particles are placed on a first base material such as glass to obtain a laminate of a photocatalyst layer and a first base material layer. A conductive layer (current collector) is provided on the surface of the photocatalyst layer of the obtained laminate by vapor deposition or the like. Here, the photocatalyst particles on the conductive layer side surface layer of the photocatalyst layer are immobilized on the conductive layer. After that, a second base material is adhered to the surface of the conductive layer, and the conductive layer and the photocatalyst layer are peeled off from the first base material layer. Since some of the photocatalyst particles are immobilized on the surface of the conductive layer, they are peeled off together with the conductive layer, resulting in a photowater-splitting reaction electrode having a photocatalyst layer, a conductive layer, and a second substrate layer. be able to.
Alternatively, as another method, a slurry in which photocatalyst particles are dispersed may be applied to the surface of a current collector and dried to obtain an electrode for a photohydrolysis reaction, or photocatalyst particles and a current collector may be combined. The electrode for photo-water splitting reaction may be obtained by integrating by pressure molding or the like. Alternatively, a current collector may be immersed in a slurry in which photocatalyst particles are dispersed, and a voltage may be applied to collect the photocatalyst particles on the current collector by electrophoresis.
Alternatively, the supporting of the co-catalyst may be carried out in a post-process. For example, in the above-described particle transfer method, using photo-semiconductor particles instead of photocatalyst particles, a laminate having a photo-semiconductor layer, a conductive layer, and a second base layer is obtained in a similar manner, and then the photo-semiconductor An electrode for a photo-water splitting reaction may be obtained by supporting oxide particles as a co-catalyst on the surface of the layer.

The photocatalyst of the present invention or the above-described electrode for photowater decomposition reaction is immersed in water or an aqueous electrolyte solution, and the photocatalyst or electrode for photowater decomposition reaction is irradiated with light to perform photowater decomposition, thereby hydrogen and /or oxygen can be produced.

For example, as described above, a photocatalyst is immobilized on a current collector composed of a conductor to obtain an electrode for a photowater splitting reaction, while a conductor supporting a hydrogen generation catalyst is used as a counter electrode, and a liquid or gaseous conductor is used. Light is irradiated while supplying water in the form of water, and the water-splitting reaction proceeds. The water-splitting reaction can be promoted by providing a potential difference between the electrodes as necessary. Alternatively, an optical semiconductor supporting a hydrogen generation catalyst may be used as the counter electrode. In this case, as the optical semiconductor, a known optical semiconductor that catalyzes the hydrogen generation reaction can be used.

On the other hand, an immobilized product in which photocatalyst particles are immobilized on an insulating base material, or a molded body in which photocatalyst particles are pressure-molded, may be irradiated with light while supplying water to allow the water-splitting reaction to proceed. . Alternatively, the photocatalyst particles may be dispersed in water or an aqueous electrolyte solution, and then irradiated with light to promote the water-splitting reaction. In this case, the reaction can be promoted by stirring as necessary.
Since the photocatalyst of the present invention can completely decompose water by itself, it is not necessary to connect the oxygen generating electrode and the hydrogen generating electrode, and the photocatalyst is placed in water and water is supplied there. Hydrogen and oxygen can be produced if there are means and means for extracting hydrogen and/or oxygen.
This simplifies the structure, and at the same time, it is possible to operate in half the area compared to arranging the oxygen generating electrode and the hydrogen generating electrode in parallel. The generated hydrogen and oxygen can be separated into hydrogen and oxygen using, for example, a zeolite membrane.

Although the reaction conditions for producing hydrogen and/or oxygen are not particularly limited, for example, the reaction temperature is set at 0° C. or higher and 200° C. or lower, and the reaction pressure is set at 2 MPa (G) or lower.
The irradiation light is visible light having a wavelength of 650 nm or less, or ultraviolet light. Examples of the light source for the irradiation light include the sun, a lamp capable of irradiating near-sunlight light such as a xenon lamp and a metal halide lamp, a mercury lamp, and an LED.

As described above, according to the present invention, by using the photocatalyst of the present invention, hydrogen and/or oxygen can be efficiently produced by a photo-water splitting reaction.

EXAMPLES Hereinafter, the present invention will be described in more detail based on examples, but the present invention is not limited by the following examples. It should be noted that various production conditions and values of evaluation results in the following examples have the meaning of preferred values for the upper limit or lower limit in the embodiments of the present invention, and the preferred range is the above-described upper limit or lower limit value. , the range defined by the values in the following examples or a combination of the values in the examples.

[XRD measurement/DRS measurement]
Conditions, devices, etc. for XRD measurement and ultraviolet/visible diffuse reflectance spectrum (DRS) measurement of the photocatalyst powders synthesized in the following Examples and Comparative Examples are as follows.

<XRD measurement>
Manufacturer: Rigaku
Apparatus; SmartLab
Measurement condition;
Photocatalyst powder pulverized to a diameter of 100 μm or less is subjected to powder X-ray diffraction measurement by the concentration method ・Measurement range: 5 to 80 °
・Measurement step: 0.01°
Scanning speed: 10°/min Analysis without monochromator;
FWHM: Calculated directly from the data obtained under the above conditions without separation of Kα1 and Kα2

<DRS measurement>
Manufacturer; JASCO
Model number: V-670 Spectrophotometer
Measurement condition;
・Measurement range: 300nm to 800nm
・Data interval: 0.2 nm
・Scanning speed: 200 nm/min ・Light source switching: 340.0 nm
・Data analysis software: Spectra Manager version 2
Analysis; Kubelka-Munk (K.M.) transformation Kubelka-Munk transformation formula f (R _∞ ) = (1-R _∞ ) 2/2R _∞ = K/S on the vertical axis
where f(R _∞ ) is K.K. M. function, R _∞ is the absolute reflectance, K is the molecular extinction coefficient, and S is the scattering coefficient.
It should be noted that it is difficult to measure the absolute reflectance R _∞ of a sample, and in practice it is common to use the relative reflectance r _∞ using a standard sample. Therefore,
r _∞ = r (measurement sample)/r (standard sample) (using BaSO ₄ as standard sample)
to measure the relative reflectance r _∞ using
f(r _∞ )=(1−r _∞ )2/2r _∞ =K/S
derived from

[Preparation of photocatalyst for evaluation and evaluation of photocatalytic activity]
Photocatalyst powders synthesized in the following examples and comparative examples were evaluated for photocatalytic activity by preparing photocatalysts for evaluation by the following method.

<Preparation of IrO ₂ colloidal solution>
After dissolving Na ₃ IrCl _6.n hydrate in pure water so that the Ir metal concentration becomes 0.0548 mg/mL, adjust the pH to 12 with 1N aqueous sodium hydroxide solution and stir at 80° C. for 30 minutes. bottom. Then, it was cooled to 25° C. in an ice bath and adjusted to pH=9.0 with 0.5N and 0.05N nitric acid. After the adjustment, the mixture was stirred again at 80° C. for 30 minutes and cooled again to room temperature to prepare an IrO ₂ colloidal solution.

< _{Preparation of} photocatalyst _CrOx /Rh/ _IrO2 / _Y2Ti2O5S2 for total _{decomposition} evaluation>
The photocatalyst powder was suspended in an aqueous solution of _IrO2 colloid (0.3 mass % Ir with respect to the photocatalyst powder) and stirred for 40 minutes to allow the _IrO2 colloid to be adsorbed on the photocatalyst powder. After the adsorption of _IrO2 , it was suspended in an aqueous _RhCl3 solution (2.0 wt% Rh relative to the photocatalyst powder) and irradiated with visible light (λ>420 nm) for 3 hours in the absence of air to transform Rh(III) into metal. Photoreduced to Rh. After precipitation of Rh, the obtained sample was suspended in an aqueous K ₂ CrO ₄ solution (1.5% by mass of Cr with respect to the photocatalyst powder), and irradiated with UV-visible light (λ>300 nm) for 15 hours to form K ₂ CrO ₄ . was reduced to chromium oxide _CrOx . A 300 W xenon lamp equipped with a cut-off filter was used for light irradiation. Cooling water was used during light irradiation to keep the temperature of the solution at room temperature. The product was washed thoroughly with distilled water and dried under reduced pressure at 40° C. for 2 hours.

< _{Preparation of} photocatalyst _IrO2 / _Y2Ti2O5S2 for evaluation _of hydrogen/oxygen generation>
The photocatalyst powder was suspended in an aqueous solution of _IrO2 colloid (0.3 mass % Ir with respect to the photocatalyst powder) and stirred for 40 minutes to allow the _IrO2 colloid to be adsorbed on the photocatalyst powder. The product was washed thoroughly with distilled water and dried under reduced pressure at 40° C. for 2 hours.

< _{Preparation of} photocatalyst CoOx/ _Y2Ti2O5S2 for evaluation of _hydrogen /oxygen generation>
The photocatalyst powder was suspended in a cobalt nitrate solution in which cobalt nitrate hexahydrate was dissolved, and after removing the solvent, the suspension was calcined at 300° C. for 1 hour.

<Total decomposition evaluation test and hydrogen/oxygen generation evaluation test>
Using the prepared photocatalyst for evaluation of total decomposition or the photocatalyst for evaluation of hydrogen/oxygen generation, the photowater decomposition reaction performance was evaluated. The photowater splitting reaction was carried out in a closed-system reaction apparatus equipped with an evacuation pump, a circulation pump, a cell containing a photocatalyst immobilized substance, a gas sampling valve, and a gas chromatograph analyzer (GC). A 300 W xenon lamp (λ>420 nm) was used as the light source, a water filter was provided between the lamp and the cell to avoid temperature rise, and the cell was cooled from the outside using cooling water. During the evaluation, it was confirmed that no air remained after degassing the inside of the reactor several times in advance. The degree of vacuum was about 4×10 ⁴ Pa. After that, light irradiation was started and the amount of gas produced was measured. Analysis conditions were column (molecular sieve 5A), carrier gas (argon), and temperature (50 to 70°C).

In the total decomposition evaluation test, 150 mL of water and 100 mg of La ₂ O ₃ (pH=9) as a pH adjuster were added to 200 mg of the photocatalyst for total decomposition evaluation, and the mixture was sealed in a cell and tested.
In the hydrogen generation evaluation test, 150 mL of 0.02M Na ₂ S—Na ₂ SO ₃ aqueous solution was sealed in a cell with respect to 200 mg of the photocatalyst for hydrogen generation evaluation.
In the oxygen generation evaluation test, 150 mL of 0.02 M AgNO ₃ aqueous solution was sealed in a cell with respect to 200 mg of the photocatalyst for oxygen generation evaluation, and the test was conducted.
Hereinafter, the generation rate of hydrogen and oxygen (the amount of moles generated per unit time) in the total decomposition evaluation test is referred to as the total decomposition rate, and the hydrogen generation rate is referred to as "V _H2 " and the oxygen generation rate as "V _O2 ".
In addition, the hydrogen generation rate in the hydrogen generation evaluation test is defined as a half reaction rate and is indicated as "V _H2 ", and the oxygen generation rate in the oxygen generation evaluation test is referred to as a half reaction rate and is indicated as "V _O2 ".

[Synthesis, analysis and evaluation of photocatalyst powder]
<Example 1>
Y ₂ O ₃ , Y ₂ S ₃ , TiO ₂ and S were weighed at a molar ratio of 1:2:6:2.05 and mixed in a N ₂ glove box (with a dew point of minus 70° C. or less) for about 40 minutes, After being sealed in a quartz tube in vacuum, it was fired at 650° C. for 96 hours. After that, the excess sulfur content was oxidized by heat treatment at 200° C. in the air for 1 hour, washed with water, and filtered to obtain a photocatalyst powder A.
As a result of XRD measurement of the obtained photocatalyst powder A, almost all peaks were attributed to Y ₂ Ti ₂ O ₅ S ₂ . Table 1 shows the XRD measurement results and the ultraviolet/visible diffuse reflectance spectrum measurement results of the obtained photocatalyst powder A.
Table 1 shows the evaluation results of the photocatalytic activity of the photocatalyst powder A.
Further, FIG. 1 shows the XRD peak shape around 26.3°, and FIG. 2 shows the peak shape around 30.6°.

<Example 2>
Photocatalyst powder B was obtained in the same manner as in Example 1, except that the firing temperature was 700°C.
As a result of XRD measurement of the obtained photocatalyst powder B, almost all peaks were attributed to Y ₂ Ti ₂ O ₅ S ₂ . Table 1 shows the XRD measurement results and the ultraviolet/visible diffuse reflectance spectrum measurement results of the obtained photocatalyst powder B.
Table 1 shows the evaluation results of the photocatalytic activity of the photocatalyst powder A.
Further, FIG. 3 shows the XRD peak shape around 26.3°, and FIG. 4 shows the peak shape around 30.6°.

<Example 3>
A photocatalyst powder C was obtained in the same manner as in Example 1, except that the firing temperature was 800°C.
As a result of XRD measurement of the obtained photocatalyst powder C, almost all peaks were attributed to Y ₂ Ti ₂ O ₅ S ₂ . Table 1 shows the XRD measurement results and the ultraviolet/visible diffuse reflectance spectrum measurement results of the obtained photocatalyst powder C.
Table 1 shows the evaluation results of the photocatalytic activity of the photocatalyst powder C.
Further, FIG. 5 shows the XRD peak shape near 26.3°, and FIG. 6 shows the peak shape near 30.6°.

<Example 4>
A photocatalyst powder D was obtained in the same manner as in Example 1, except that the firing temperature was 900°C.
As a result of XRD measurement of the obtained photocatalyst powder D, almost all peaks were attributed to Y ₂ Ti ₂ O ₅ S ₂ . Table 1 shows the XRD measurement results and the ultraviolet/visible diffuse reflectance spectrum measurement results of the obtained photocatalyst powder D.
Table 1 shows the evaluation results of the photocatalytic activity of the photocatalyst powder D.
FIG. 7 shows the XRD peak shape near 26.3°, and FIG. 8 shows the peak shape near 30.6°.

<Example 5>
The photocatalyst powder C synthesized in Example 3 was immersed in 49% by mass sulfuric acid and stirred at room temperature for 2 hours. After the acid treatment, the photocatalyst powder E was obtained by washing with water and filtering.
As a result of XRD measurement of the obtained photocatalyst powder E, almost all peaks were attributed to Y ₂ Ti ₂ O ₅ S ₂ . Table 1 shows the XRD measurement results and the ultraviolet/visible diffuse reflectance spectrum measurement results of the obtained photocatalyst powder E.
Table 1 shows the evaluation results of the photocatalytic activity of the photocatalyst powder E.

<Example 6>
0.5 g of the photocatalyst powder C synthesized in Example 3 was sealed in a vial along with 12 g of zirconia beads having a diameter of 1 mm and 3 mL of water, and the entire container was pulverized by a tumbling mill (200 rpm, 30 minutes). After the pulverization treatment, the photocatalyst powder F was obtained by removing the beads and performing filtration treatment.
As a result of XRD measurement of the obtained photocatalyst powder F, almost all peaks were attributed to Y ₂ Ti ₂ O ₅ S ₂ . Table 1 shows the XRD measurement results and the ultraviolet/visible diffuse reflectance spectrum measurement results of the obtained photocatalyst powder F.
Table 1 shows the evaluation results of the photocatalytic activity of the photocatalyst powder F.

<Example 7>
The photocatalyst powder C synthesized in Example 3 was sealed in a sample bottle together with distilled water and an _IrO colloidal solution, and subjected to "Microwave synthesis" manufactured by Anton Paar.
MW (microwave) treatment was performed at 200° C. for 10 minutes in a Reactor Monowave 300. After removing the solvent, it was suspended in ethylene glycol, Na ₃ (RhCl ₆ ).nH ₂ O and K ₂ CrO ₄ were added, and MW treatment was performed again at 200° C. for 10 minutes. At this time, the concentrations of Ir metal, Rh metal, and Cr metal were set to 0.15, 0.1, and 0.1% by mass, respectively, with respect to the photocatalyst powder. A photocatalyst powder G was obtained by distilling off the solvent from the resulting suspension.
As a result of XRD measurement of the obtained photocatalyst powder G, almost all peaks were attributed to Y ₂ Ti ₂ O ₅ S ₂ . Table 1 shows the XRD measurement results and the ultraviolet/visible diffuse reflectance spectrum measurement results of the obtained photocatalyst powder G.
Table 1 shows the evaluation results of the photocatalytic activity of the photocatalyst powder G.

<Example 8>
The photocatalyst powder C synthesized in Example 3 was dispersed in a cobalt nitrate solution in which cobalt nitrate hexahydrate was dissolved, the solvent was removed, and the dispersion was calcined at 300° C. for 1 hour in the air. At this time, the Co metal concentration was set to 0.015% by mass with respect to the photocatalyst powder. A photocatalyst powder J was obtained by distilling off the solvent from the resulting suspension.
As a result of XRD measurement of the obtained photocatalyst powder J, almost all peaks were attributed to Y ₂ Ti ₂ O ₅ S ₂ . Table 1 shows the XRD measurement results and the ultraviolet/visible diffuse reflectance spectrum measurement results of the obtained photocatalyst powder J.
Table 1 shows the evaluation results of the photocatalytic activity of the photocatalyst powder J.

<Example 9>
The photocatalyst powder C synthesized in Example 3 was dispersed in a cobalt nitrate solution in which cobalt nitrate hexahydrate was dissolved, the solvent was removed, and the dispersion was calcined at 300° C. for 1 hour in the air. At this time, the Co metal concentration was set to 0.15% by mass with respect to the photocatalyst powder. A photocatalyst powder K was obtained by distilling off the solvent from the resulting suspension.
As a result of XRD measurement of the obtained photocatalyst powder K, almost all peaks were attributed to Y ₂ Ti ₂ O ₅ S ₂ . Table 1 shows the XRD measurement results and the ultraviolet/visible diffuse reflectance spectrum measurement results of the obtained photocatalyst powder K.
Table 1 shows the evaluation results of the photocatalytic activity of the photocatalyst powder K.

<Comparative Example 1>
Photocatalyst powder H was obtained in the same manner as in Example 1, except that the firing temperature was 600°C.
As a result of XRD measurement, the obtained photocatalyst powder H has a very weak peak near 26.3°, and has an intensity of only 6 when the maximum peak is taken as 100, and the main component is Y ₂ Ti ₂ O ₅ S ₂ It turned out not to belong. Table 1 shows the XRD measurement results and the ultraviolet/visible diffuse reflectance spectrum measurement results of the obtained photocatalyst powder H.
Table 1 shows the evaluation results of the photocatalytic activity of the photocatalyst powder H.
Further, FIG. 9 shows the XRD shape near 26.3°, and FIG. 10 shows the peak shape near 30.6°.

<Comparative Example 2>
A photocatalyst powder I was obtained in the same manner as in Example 1, except that the sintering temperature was 1000°C.
As a result of XRD measurement of the obtained photocatalyst powder I, almost all peaks were attributed to Y ₂ Ti ₂ O ₅ S ₂ . Table 1 shows the XRD measurement results and the ultraviolet/visible diffuse reflectance spectrum measurement results of the obtained photocatalyst powder I. The peak intensity near 26.3° is low, and it can be seen that even with YTOS, the growth direction is different from that of YTOS of the example. In addition, the measurement results of the ultraviolet/visible diffuse reflectance spectrum are different from those of the examples, and it is presumed that the surface state of the photocatalyst powder is also different.
Table 1 shows the evaluation results of the photocatalytic activity of the photocatalyst powder I.
Further, FIG. 11 shows the XRD peak shape near 26.3°, and FIG. 12 shows the peak shape near 30.6°.

From Table 1, the photocatalyst powders A to G, J, and K of Examples 1 to 9 obtained by firing at a temperature of 650 to 900 ° C. are photocatalyst powder H with a firing temperature of 600 ° C. and photocatalyst powder with a firing temperature of 1000 ° C. I shows different measured values in the XRD measurement and the ultraviolet/visible diffuse reflectance spectrum measurement, and it can be seen that they are different substances even if the composition is the same.
The photocatalyst powders H and I showed almost no photocatalytic activity against water decomposition, but the photocatalyst powders A to G, J and K showed good photocatalytic activity.

As is clear from the above results, the photocatalyst of the present invention, which is YTOS having specific characteristics, has high photocatalytic activity and can completely decompose water with only this photocatalyst. As a result, without connecting the photocatalyst for decomposing water and generating oxygen and the photocatalyst for generating hydrogen by wiring, this photocatalyst can be placed in water and light like sunlight can be emitted. It can be seen that water can be decomposed into hydrogen and oxygen with high efficiency only by irradiation.

<Example 10>
Experiments have been conducted to show that the YTOS of the present invention is environmentally insensitive and operates over a wide range of pH when used as an electrode.

<Fabrication of electrodes>
A YTOS electrode was produced by a well-known particle transfer method using the YTOS powder produced by the method described in Example 3 and having XRD data equivalent to the XRD data in Example 3. That is, 60 mg of YTOS powder was suspended in 0.5 ml of isopropyl alcohol and applied to a thin film by drop casting onto a 3 cm square glass sacrificial substrate. After drying this at room temperature, a 3 μm-thick titanium thin film was deposited by sputtering (ULVAC, MPS series). By peeling off this titanium thin film from the glass sacrificial substrate, a very thin YTOS electrode thin film formed on the titanium thin film was obtained.

<Measurement of capacitance-potential characteristics>
Impedance was measured by a three-probe method using the above YTOS electrode as a working electrode, an Ag/AgCl reference electrode containing a saturated KCl aqueous solution as an internal solution as a reference electrode, and a platinum wire as a counter electrode. A 0.1 M Na ₂ SO ₄ aqueous solution was used as the electrolyte, the solution was stirred during the measurement, and the headspace of the container was purged with argon gas. Experiments to adjust the pH were carried out by adding an aqueous H ₂ SO ₄ solution and an aqueous NaOH solution. Cyclic voltammetry measurements were performed at a sweep rate of 10 mV/s before impedance measurements. The amplitude of the AC signal applied during impedance measurement was 10 mV, and the measurement was performed in a dark place.

<Mott-Schottky Plot>
1/ ^C2 was obtained from the capacitance (C) per unit area of the YTOS electrode surface obtained by impedance measurement, and plotted against the potential (V) applied to the YTOS electrode (Mott-Schottky plot).
The results are shown in FIG. In FIG. 13, ● is the data of pH 13.0, ▪ is the data of pH 6.8, and ▲ is the data of pH 5.1.
In general, when a depletion layer exists on the surface of a semiconductor, the width of this depletion layer changes depending on the potential applied to the semiconductor, so the value of 1/ ^C2 also changes. Since the energy band is tilted in the depletion layer on the surface of the semiconductor, when light corresponding to the band gap is absorbed and electron-hole pairs are generated, the electrons and holes move in opposite directions to the front and back sides of the thin film, respectively. will move to The semiconductor electrode from which electrons migrate to the thin film surface side is the photocathode, and the semiconductor electrode from which holes migrate to the thin film surface side is the photoanode.

The Mott-Schottky plot of the YTOS electrode becomes a straight line with a positive slope as shown in FIG. This positive slope indicates that the YTOS electrode surface has a depletion layer formed by depletion of the n-type semiconductor on the surface and operates as a photoanode. A feature of this result is that the linear plot is measured up to a fairly negative potential side of -1.0 V vs Ag/AgCl. From this result, it can be seen that the YTOS electrode surface operates as a photoanode independent of the pH, that is, regardless of the operating environment, and the potential of the operating range is extremely wide. This means that when the YTOS electrode and another cathode electrode are combined to perform water splitting in a two-electrode system, there are many choices for the cathode electrode, and therefore it is easy to combine with a highly active cathode, which leads to highly efficient water splitting. Advantageous.

<Example 11>
<Fabrication of electrodes>
A YTOS electrode was produced by a well-known particle transfer method using the YTOS powder produced by the method described in Example 3 and having XRD data equivalent to the XRD data in Example 3. That is, on a YTOS particle thin film coated on a glass sacrificial substrate, titanium was first deposited to a thickness of 1 μm, and gold was deposited thereon to a thickness of 3 μm by vapor deposition. A thin film was produced.

<Measurement of photocurrent-potential characteristics>
Photocurrent-potential characteristics were measured by a three-probe method using the above YTOS electrode as a working electrode, an Ag/AgCl reference electrode containing a saturated KCl aqueous solution as an internal solution as a reference electrode, and a platinum wire as a counter electrode. The aqueous solution used was a phosphoric acid aqueous solution with a pH of 9 or 13 (prepared so that the phosphate concentration was 0.5M in either case). Stirring of the solution and bubbling with argon gas were carried out during the measurement. The anode current was measured by sweeping the potential of the YTOS electrode from the positive potential to the negative potential side, and the photoanode current was observed by repeating irradiation and interruption of simulated sunlight during this sweep.

The measurement results are shown in FIG. 14 (pH 9) and FIG. 15 (pH 13). A sawtooth-like signal corresponding to light irradiation (Light on)-interruption (Light off) is visible up to about 0.1 V vs RHE (RHE: reversible hydrogen potential). If the photoresponse exists even at 0 V vs RHE, if this YTOS electrode is connected to an ideal hydrogen-producing electrode, water splitting will proceed simply by irradiating the YTOS electrode with sunlight, which is extremely significant. Further, even when the onset potential of the photoresponse does not reach 0 V vs RHE, if a potential exceeding the onset potential is applied from the outside, water decomposition proceeds when the YTOS electrode is irradiated with sunlight. Therefore, the low onset potential of the YTOS electrode is useful in applications.

In Ta ₃ N ₅ having a bandgap close to that of YTOS of the present invention, it is known that photoresponse disappears at about 0.5 V in a similar experiment. It can be seen that the usefulness is excellent.

Claims (5)

A photocatalyst having a composition represented by the following general formula (I) and having a diffraction peak with a peak top at 26.3 ± 0.3 in XRD measurement with Cu-Kα radiation according to the following equipment and measurement conditions , The peak top has an intensity of 20 or more, with the maximum peak intensity on the XRD spectrum being 100, and a diffraction peak half width (FWHM) of 30.6 ± 0.5 in the range of 0.16 to 0.30 A photocatalyst used for the total decomposition of water in
_{YaTibOcSd ( I )}
(However, the numbers are a = 1.7 to 2.3, b = 1.7 to 2.3, c = 5, and d = 1.7 to 2.3.)
<XRD measurement>
Manufacturer: Rigaku
Apparatus; SmartLab
Measurement condition;
Photocatalyst powder pulverized to a diameter of 100 μm or less is subjected to powder X-ray diffraction measurement by the concentration method ・Measurement range: 5 to 80 °
・Measurement step: 0.01°
Scanning speed: 10°/min Analysis without monochromator;
FWHM: Calculated directly from the data obtained under the above conditions without separation of Kα1 and Kα2 A photocatalyst having a composition represented by the following general formula (I) and having a _{λP.P. T.} value (the wavelength at which the diffuse reflectance spectrum after KM conversion shows the maximum value) is in the range of 400 nm or more and 495 nm or less, and λ _{H. S.} A photocatalyst used for total decomposition of water having a value (wavelength at which the diffuse reflectance spectrum after KM conversion exhibits an intermediate value) is in the range of 520 nm or more and 570 nm or less.
_{YaTibOcSd ( I )}
(However, the numbers are a = 1.7 to 2.3, b = 1.7 to 2.3, c = 5, and d = 1.7 to 2.3.)
<Ultraviolet/visible diffuse reflectance spectrum measurement>
Manufacturer; JASCO
Model number: V-670 Spectrophotometer
Measurement condition;
・Measurement range: 300nm to 800nm
・Data interval: 0.2 nm
・Scanning speed: 200 nm/min ・Light source switching: 340.0 nm
・Data analysis software: Spectra Manager version 2
Analysis; Kubelka-Munk (K.M.) transformation Kubelka-Munk transformation formula f (R _∞ ) = (1-R _∞ ) 2/2R _∞ = K/S on the vertical axis
where f(R _∞ ) is K.K. M. function, R _∞ is the absolute reflectance, K is the molecular extinction coefficient, and S is the scattering coefficient.
It should be noted that it is difficult to measure the absolute reflectance R _∞ of a sample, and in practice it is common to use the relative reflectance r _∞ using a standard sample. Therefore,
r _∞ = r (measurement sample)/r (standard sample) (using BaSO ₄ as standard sample)
to measure the relative reflectance r _∞ using
It was derived from f(r _∞ )=(1−r _∞ )2/2r _∞ =K/S. 3. A method for producing hydrogen and oxygen, wherein hydrogen and oxygen are generated using the photocatalyst-immobilized immobilized article or molded article according to claim 1 or 2 . An electrode produced using the photocatalyst according to claim 1 or 2 . A method for producing hydrogen and oxygen, wherein the electrode according to claim 4 generates hydrogen and/or oxygen.

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