JP2020138188A - Photocatalyst, and method for producing hydrogen and oxygen using said photocatalyst - Google Patents

Abstract

To provide a novel photocatalyst that has an excellent photocatalytic activity, and a method for producing hydrogen and oxygen using said photocatalyst.SOLUTION: Provided is a photocatalyst represented by the following general formula (I), and in which, in XRD measurement with Cu-Kα-ray, the peak top has a diffraction peak at 26.3±0.3 and said peak top, assuming the maximum peak intensity of 100, has an intensity of 20 or more, and the diffraction peak half value width (FWHM) at 30.6 ± 0.5 is in the range of 0.16 to 0.30. A method for producing hydrogen and oxygen, in which, using said photocatalyst, hydrogen and oxygen are generated by the same electrode (photocatalyst). YaTibOcSd -- (I) (Herein, the numbers are a = 1.7 to 2.3, b = 1.7 to 2.3, c = 5, and d = 1.7 to 2.3.).SELECTED DRAWING: Figure 1

Description

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

The development of a high-performance optical energy conversion system that uses solar energy as renewable energy has become extremely important in recent years from the perspective of controlling global warming and breaking away from the depleting dependence on fossil resources. The sex is increasing. Above all, the technology for decomposing water to produce hydrogen using solar energy has become a technology that can be utilized not only as the current 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 widely studied for a long time.
The decomposition reaction of water in an acidic aqueous solution on the photocatalytic particles is estimated as follows.
H ₂ O + 2h ⁺ → 1 / 2O ₂ + 2H ⁺ (1)
_{^{2H + + 2e - → H 2}} (2)

Conventionally, as a photocatalyst, a TiO ₂ doped with a transition metal such as Cr or V has been proposed (for example, Patent Documents 1 to 5), but all of them have low catalyst efficiency and leave a problem 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, not only the photocatalytic activity, specifically, the reaction rate is low, but also for total decomposition that generates hydrogen and oxygen from water. It could not be used as an electrode, but could be used as an electrode that generates either hydrogen or oxygen by coexisting with a sacrificial reagent (Non-Patent Documents 1 and 2).

Japanese Unexamined Patent Publication No. 9-262482 JP-A-11-197512 JP-A-11-255514 JP-A-11-279299 Japanese Unexamined Patent Publication No. 11-33408 Japanese Unexamined Patent Publication No. 2013-62121

Sora Otani, Akio Ishikawa, Tsuyoshi Takada, Kazunari Doen 100th Catalyst Debate Proceedings (2007), p343 Sora Otani, Takahiro Suzuki, Kentaro Teramura, Kazunari Douen 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 having excellent photocatalytic activity and a method for producing hydrogen and oxygen using this photocatalyst.

As a result of diligent studies to solve the above problems, the present inventors have determined that 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 as compared with the case of high-temperature firing, and a new substance having a specific peak shape is generated accordingly, and it is specified on such XRD. The photocatalyst showing the peak pattern of Y ₂ Ti ₂ O ₅ S ₂ shows high water splitting catalytic activity and generates oxygen and hydrogen without using sacrificial reagents, which was the biggest problem with conventional catalysts of Y ₂ Ti ₂ O ₅ S ₂ composition. It has been found that it can be used as a catalyst, that is, a single photocatalyst can realize the generation of oxygen and hydrogen by the total decomposition reaction of water.
That is, the gist of the present invention is as follows.

[1] A diffraction peak having a composition represented by the following general formula (I) and having a peak top of 26.3 ± 0.3 in XRD measurement using Cu-Kα rays according to the following apparatus and measurement conditions. The peak top has an intensity of 20 or more, where the maximum peak intensity on the XRD spectrum is 100, and the diffraction peak half width (FWHM) of 30.6 ± 0.5 is 0.16 to 0. Photocatalyst in the range of 30.
Y _a Ti _b O _c S _d ... (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
Device; SmartLab
Measurement condition;
Powder X-ray diffraction measurement was performed on the photocatalytic powder crushed to a diameter of 100 μm or less by the concentrated method. ・ Measurement range: 5 to 80 °
・ Measurement step: 0.01 °
・ Scan speed: 10 ° / min Analysis without using a monochromator;
FWHM: Calculation is performed directly from the data obtained under the above conditions without undergoing the separation process of Kα1 and Kα2.

[2] A photocatalyst having a composition represented by the following general formula (I), which is obtained by measuring an ultraviolet / visible diffuse reflection spectrum according to the following apparatus and measurement conditions _{. T.} The value (wavelength at which the diffuse reflection 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 whose value (wavelength at which the diffuse reflection spectrum after KM conversion shows an intermediate value) is in the range of 520 nm or more and 570 nm or less.
Y _a Ti _b O _c S _d ... (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 reflection spectrum measurement>
Manufacturer; JASCO
Model number: V-670 Spectrophotometer
Measurement condition;
-Measurement range: 300 nm to 800 nm
・ Data interval: 0.2 nm
-Scanning speed: 200 nm / min-Light source switching: 340.0 nm
-Data analysis software: Spectra Manager version 2
Analysis; vertical axis is Kubelker-Munch (KM) conversion Kubelker-Munch conversion formula f (R _∞ ) = (1-R _∞ ) 2 / 2R _∞ = K / S
Here, f (R _∞ ) is K.I. M. The function, R _∞ is the absolute reflectance, K is the molecular extinction coefficient, and S is the scattering coefficient.
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) (BaSO ₄ is used as the standard sample)
The relative reflectance r _∞ was measured using
f (r _∞ ) = (1-r _∞ ) 2 / 2r _∞ = K / S
Was 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, which generates hydrogen and oxygen using the immobilized product in which the photocatalyst is immobilized or the molded product according to any one of [1] to [3].

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

[6] A method for producing hydrogen and oxygen, which generates hydrogen and / or oxygen by the electrode according to [5].

According to the present invention, a photocatalyst having excellent photocatalytic activity is provided.
Using the photocatalyst of the present invention, hydrogen and oxygen can be produced by efficiently completely decomposing water.

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

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 gist of the present invention. In the present specification, when a numerical value or a physical property value is inserted before and after using "~", it is used as including the values before and after that.

[mechanism]
The photocatalyst of the present invention is a photocatalyst having a composition represented by the following formula (I) (hereinafter, a substance represented by the following formula (I) may be abbreviated as "YTOS"), and the following apparatus and measurement In the XRD measurement by Cu-Kα ray according to the conditions, the peak top has a diffraction peak at 26.3 ± 0.3, and the peak top has a maximum peak intensity on the XRD spectrum of 100 and is 20 or more. Intensity and diffraction peak half-value width (FWHM) of 30.6 ± 0.5 is in the range of 0.16 to 0.30, or ultraviolet / visible diffusion reflection spectrum according to the following equipment and measurement conditions. Λ _P. obtained by measurement _{. T.} The value (wavelength at which the diffuse reflection 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.} The value (wavelength at which the diffuse reflection spectrum after KM conversion shows an intermediate value) is in the range of 520 nm or more and 570 nm or less.
Y _a Ti _b O _c S _d ... (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
Device; SmartLab
Measurement condition;
Powder X-ray diffraction measurement was performed on the photocatalytic powder crushed to a diameter of 100 μm or less by the concentrated method. ・ Measurement range: 5 to 80 °
・ Measurement step: 0.01 °
・ Scan speed: 10 ° / min Analysis without using a monochromator;
FWHM: Calculation is performed directly from the data obtained under the above conditions without undergoing the separation process of Kα1 and Kα2.

<Ultraviolet / visible diffuse reflection spectrum (DRS) measurement>
Manufacturer; JASCO
Model number: V-670 Spectrophotometer
Measurement condition;
-Measurement range: 300 nm to 800 nm
・ Data interval: 0.2 nm
-Scanning speed: 200 nm / min-Light source switching: 340.0 nm
-Data analysis software: Spectra Manager version 2
Analysis; vertical axis is Kubelker-Munch (KM) conversion Kubelker-Munch conversion formula f (R _∞ ) = (1-R _∞ ) 2 / 2R _∞ = K / S
Here, f (R _∞ ) is K.I. M. The function, R _∞ is the absolute reflectance, K is the molecular extinction coefficient, and S is the scattering coefficient.
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) (BaSO ₄ is used as the standard sample)
The relative reflectance r _∞ was measured using
f (r _∞ ) = (1-r _∞ ) 2 / 2r _∞ = K / S
Was derived from.

In the photocatalyst of the present invention, as described later, when the photocatalyst is fired at a temperature lower than the temperature described in Patent Document 6, which is 650 ° C. or higher and lower than 1000 ° C., the firing temperature is 1000 ° C. or higher. However, a specific crystal phase develops even if the bulk composition is the same, and YTOS having a specific peak shape is obtained accordingly, and this novel YTOS makes it possible to obtain excellent photocatalytic activity.
Generally, in the production of a metal composite oxide, firing at a high temperature has better crystallinity and excellent physical properties. Therefore, even in Patent Document 6 described above, the firing temperature is preferably 1000 to 1200 ° C. In the examples 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 development of 103 crystal planes, which is important for improving the photocatalytic activity, is developed. Not only is it not sufficient, but as is also clear from the UV-visible diffuse reflection spectrum measurement, λ _P. is also an important parameter for improving photocatalytic activity _{. T.} The value is also λ _{H. S.} The value is also a substance different from YTOS of the present invention, which does not satisfy the requirements of the present invention.

The specific methods of XRD measurement and ultraviolet / visible diffuse reflection spectrum measurement are as shown in the section of Examples described later.

[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, but a = 1.9 is preferable. ~ 2.1, b = 1.9 to 2.1, c = 5, d = 1.9 to 2.1. In this specification, unless otherwise specified, ~ includes the numerical values at the upper end and the lower end.
Here, Y is Ca, Sr, Ag, In, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm as long as the effect of the present invention is not hindered. , Yb, Lu, etc. may be substituted. Similarly, Ti may be substituted with Al, Cr, Mn, Co, Ni, Rh, Ga, Zn, Ta, Nb, and the amount of the substitution is 0 at the value of a or b in the above composition formula. It is preferably 0.3 or less, and more preferably 0.1 or less.

[Characteristics in XRD measurement]
The photocatalyst of the present invention has a diffraction peak whose peak top is in the vicinity of 2θ = 26.3 ± 0.3 (deg.) In XRD (X-ray diffraction) measurement using Cu-Kα rays, and the intensity of the peak top. However, assuming that the maximum peak intensity on the XRD spectrum is 100, it is usually 20 or more, preferably 40 or more, more preferably 60 or more, particularly preferably 100, and around 2θ = 30.6 ± 0.5 (deg.). The lower limit of the half-value width (FWHM) of the diffraction peak of is usually 0.16 or more, preferably 0.18 or more, and the upper limit thereof 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 on the 103rd plane of Y ₂ Ti ₂ O ₅ S ₂ , and the peak near 30.6 ± 0.5 is the peak on the 105th plane. In the following, the intensity of the peak top of 103 planes with respect to the maximum peak intensity of 100 on the XRD spectrum is referred to as “103 plane peak intensity ratio”.
The peak intensity referred to in the present invention means the height of the peak top, and the peak intensity ratio means the ratio of the heights, as is clear from the above description and other descriptions of the present invention.

Peak of 103 surface are the peak observed in XRD diffraction pattern of the normal YTOS, YTOS and impurity phases of interest _(Y 2 _Ti 2 _{O 7 (hereinafter,} YTO) and _{TiS 2, TiO 2, Y 2} S It is effective for grasping the degree of phase transition to ( ₂ O, etc.) (that is, the purity of YTOS pure substance). For example, 103 face the peak intensity ratio is small YTOS are often in a state of forming a eutectic with the YTO and _Y 2 _{S 2} O of YTOS and impurities, such materials, the defects in the crystal Since it has, it often inhibits catalytic activity. 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 crystal planes of the 103-plane of YTOS develop. Therefore, the intensity of the peak top on the YTOS103 plane is usually 20 or more, preferably 40 or more, more preferably 60 or more, and particularly preferably 100, with the maximum peak intensity on the XRD spectrum as 100.

Further, at the peak of the 105th plane, the control of FWHM becomes an important design factor. Generally, it has been proposed that high crystallinity is closely related to high catalytic activity in many photocatalysts, and it is generally desirable that the FWHM be sufficiently small. That is, it is considered that the higher the firing temperature of this material, the better the crystal regularity and the better the catalytic activity. However, surprisingly, in this material, when the firing temperature is raised, it is confirmed that the catalytic activity is improved by improving the crystal regularity up to a certain temperature, but at the same time, as the crystallites grow, the catalytic activity is improved in a specific direction. It has been found that the growth is promoted and the catalytic activity is lowered in the catalyst having the crystal phase grown in these specific directions. This is because when the crystal structure grows in the stacking direction in the layered material, the crystal grows in a direction different from the mixed direction of the 3d orbitals between Ti-Ti bonds, and the electrons and holes excited by light irradiation are on the particle surface. It is presumed that this is because there is a possibility that the reaction mechanism of the photocatalyst, which conducts up to and transmits to the cocatalyst, may be in a very disadvantageous state. From this, the YTOS in which the FWHM value of the present invention is controlled is the YTOS in which the crystallinity of YTOS, which is an important design factor for improving the photocatalytic activity, and the 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 105-plane 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. It is preferably in the range of 30 or less, more preferably 0, 27 or less.

[Characteristics in UV / visible diffuse reflection spectrum measurement]
The photocatalyst of the present invention is obtained by measuring the ultraviolet / visible diffuse reflection spectrum _{. T.} Value (KM conversion (f (r _∞ ) = (1-r _∞ ) 2 / 2r _∞ = K / S, where r _∞ is the KM function, r _∞ is the relative reflectance, and K is the molecular extinction The coefficient (S is the scattering coefficient) and the wavelength at which the diffuse reflection spectrum shows the maximum value) is in the range of 400 nm or more and 495 nm or less, and λ _{H. S.} The value (wavelength at which the diffuse reflection spectrum after KM conversion shows an intermediate value) is in the range of 520 nm or more and 570 nm or less.
Preferably, λ _{P. T.} The value is 430 to 493 nm, and λ _{H. S.} The value is 525-567 nm.

The sample of the present invention is often a powder, and for this reason, the light absorption spectrum cannot be directly measured. Therefore, the λ _{P. T.} Value, λ _{H. S.} The value is determined by measuring the diffuse reflection spectrum and K.I. M. It is a value calculated by the method of converting and converting to the absorbance mode. In the present invention, the observation of a peak in the visible light region is the first condition of the visible light responsive photocatalyst, but in the material of the present invention, in addition to the above conditions, λ _{P. T.} The value is in the range of 400 nm or more and 495 nm or less, and λ _{H. S.} It has been 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, λ _{P. T.} The value is defined as the wavelength at which the diffused reflection spectrum after KM conversion shows the maximum value, and assuming that the absorption spectrum is bilaterally symmetric, this value is the center of the visible light region (generally 360 to 830 nm). It is considered that the closer it is located (about 600 nm), the wider the light in the visible light region can be absorbed, and therefore the catalytic activity is improved. Further, in the present invention, impurities YTO phase and YTO-YTOS intermediate phase are often present in the sample, and in such a structure, the conduction of electrons and holes is often inhibited and the photocatalytic activity is often limited. The impurity YTO phase and YTO-YTOS intermediate phase are λ _{P. T.} The value was confirmed to be 300 to 390 nm, and from the viewpoint of limiting these components, λ _{P. T.} The value is preferably 400 nm or more. This λ _{P. T.} The value becomes higher as the firing temperature is increased, but there is an appropriate range because the catalyst performance is reduced due to the above-mentioned increase in crystallite size.
Further, in the present invention, λ _{H. S.} The value is defined as the wavelength at which the diffuse reflection spectrum after KM conversion shows an intermediate value, and the larger this value is, the more λ _{P. T.} Since light absorption is possible in a wide wavelength range centered on the value, this λ _{H. S.} It is considered that the larger the value, the better the photocatalytic activity. However, in the present invention, λ _{H. S.} The following Ti3 + -containing phase may exist as a factor for increasing the value, and as a result of comparison with the catalytic performance, it was found that there is an optimum range. In the Ti3 + -containing phase, the conduction of electrons and holes is inhibited, and the photocatalytic activity is often limited. This Ti3 + reducing phase is observed as a very gentle slope in the wavelength region of 600 nm or more, and the more this component is, the more λ _{H. S.} Since the value rises, 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, λ _{P. T.} Value and λ _{H. S.} The value-controlled YTOS is a YTOS having a controlled Ti3 + phase content that reduces the visible light absorption characteristics and catalytic performance of YTOS, which is an important design factor for improving photocatalytic activity. For these reasons, λ _{P. T.} The value is in the range of 400 nm or more and 495 nm or less, and λ _{H. S.} Higher activity is exhibited when the value is in the range of 520 nm or more and 570 nm or less, and more preferably, λ _{P. T.} The value is 430 to 493 nm, and λ _{H. S.} The value is preferably in the range of 525-567 nm.

[Manufacturing method of photocatalyst]
The photocatalyst of the present invention can be produced by weighing the raw materials serving as Y source, Ti source, O source, and S source so as to satisfy the above formula (I), mixing them sufficiently, and calcining the obtained mixture. it can.

As the Y source, one or more of Y ₂ O ₃ , Y ₂ O ₂ S, Y ₂ S ₃ , Y, Y Cl _{3, and the} like can be used. Here, Y ₂ O ₃ and Y ₂ O ₂ S are also O sources. In addition, Y ₂ S ₃ and Y ₂ O ₂ S also serve as S sources.

As the Ti source, one kind or two or more kinds such as 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 kind or two or more kinds such as S and H ₂ S can be used. At this time, the method of circulating H2S or the like as a gas and reacting it is one of the preferable embodiments in producing the photocatalyst of the present invention. Further, as described above, Y ₂ S ₃ , Y ₂ O ₂ S, and TiS ₂ are also S sources.

It should be noted that the mixing of these raw materials is preferably carried out at −20 to 50 ° C. under an atmosphere of an inert gas such as nitrogen because air and a trace amount of water are mixed and cause impurities such as an oxide phase.

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

The firing atmosphere is not particularly limited, but it is preferable to perform the firing in 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 calcination can be heat-treated by heating 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 as necessary. Good. After this heat treatment, it is preferable to wash with water to remove sulfur oxides and solid-liquid separation of YTOS.

Further, 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 or aqua regia in an amount of about 20 to 80% by mass, if necessary, and the acid treatment is performed on the surface of the photocatalyst particles. Impurities can be removed.

Further, the obtained YTOS may be subjected to sizing treatment such as pulverization and classification, if necessary.
The particle size after pulverization is not particularly limited, but it is preferable to set the particle size to 1 μm or more because it is easy to handle. On the other hand, when the particle size is 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, for example, taking a picture with an SEM, randomly selecting about 50 particles, measuring the diameter, and averaging the particles. If the particles after pulverization deviate significantly from the spherical shape, the particle diameter may be measured and calculated from the photograph with the area equivalent diameter.

Further, the obtained YTOS may be suspended in a cocatalyst-containing solution and subjected to MW (microwave) treatment, if necessary, and the evaluation photocatalyst described later can be prepared in a short time by performing the MW treatment. You may be able to. As the MW treatment, for example, "Microwave synthesis Reactor Monowave 300" manufactured by Antonio Par Co., Ltd. may be used, and recommended conditions may be appropriately selected and carried out.

[Use]
The photocatalyst of the present invention is effective as a photocatalyst for water decomposition, exhibits particularly high photocatalytic activity, and can perform total decomposition of water with a single electrode, that is, as a photocatalyst capable of total decomposition of water without the need for a counter electrode. it can.

[Hydrogen and oxygen production method]
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. Further, 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 base material, or a composite containing the photocatalyst of the present invention is referred to as an electrode.

The photocatalyst of the present invention exhibits sufficient photocatalytic activity by itself, but is preferably used in combination with a cocatalyst.

Examples of the cocatalyst include an oxidation reaction cocatalyst (oxygen evolution side) and a reduction reaction cocatalyst (hydrogen generation side), and it is preferable to support one or both of them on YTOS. Examples of the oxidation reaction co-catalyst include metals of Group 2 to 14 of the periodic table, intermetallic compounds and alloys of the metals, or oxides, composite oxides, nitrides, oxynitrides, sulfides, and acid sulfides of these metals. , Or any of these mixtures is preferred. Here, the "intermetallic compound" is a compound formed from two or more kinds of metal elements, and the component atomic ratio constituting the intermetallic compound is not necessarily a chemical quantitative ratio but has a wide composition range. Say. "These oxides, composite oxides, nitrides, oxynitrides, sulfides, acid sulfides" are metals of Group 2 to 14 of the periodic table, intermetal compounds of the metals, or oxides of alloys. , Composite oxides, nitrides, oxynitrides, sulfides, acid sulfides. The "mixture of these" refers to a mixture of any two or more of the compounds exemplified above.

The oxidation reaction cocatalyst is preferably Mg, Ti, Mn, Fe, Co, Ni, Cu, Ga, Ru, Rh, Pd, Ag, Cd, In, Ce, Ta, W, Ir, Pt or Pb. Metals, oxides or composite oxides thereof, more preferably metals of Mn, Co, Ni, Ru, Rh, Ir, oxides or composite oxides thereof, 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 , IrO ₂ .

Examples of the reduction reaction co-catalyst include metals of groups 3 to 13 of the periodic table, intermetallic compounds and alloys of the metals, or oxides, composite oxides, oxynitrides, sulfides, acid sulfides, and carbides of these metals. It is preferable to use either a nitride or a mixture thereof. Here, the "intermetallic compound" is the same as described above, and the "these oxides, composite oxides, oxynitrides, sulfides, acid sulfides, carbides, and nitrides" are referred to as the periodic tables 3 to 13. Group metals, intermetallic compounds of the metals, alloy oxides, composite oxides, oxynitrides, sulfides, acid sulfides, carbides or nitrides. The "mixture of these" refers to a mixture of any two or more of the compounds exemplified above.

The reduction reaction cocatalyst is preferably Pt, Pd, Rh, Ru, Ni, Au, Fe, NiO, RuO ₂ , IrO ₂ , Rh ₂ O ₃ , and Cr-Rh composite oxide, core-shell type Rh /. Examples thereof include Cr ₂ O ₃ and Pt / Cr ₂ O ₃ .

The supported amount of the cocatalyst described above is not particularly limited, but the metal supported amount of the oxidation reaction cocatalyst is usually 0.01% by mass or more and 1% by mass or less, preferably 0.01% by mass or less, based on YTOS (100% by mass). The upper limit is 0.5% by mass or less, more preferably the upper limit is 0.4% by mass or less, and the lower limit is 0.1% by mass or more. The amount of metal supported by the reduction reaction cocatalyst 30 is not particularly limited, but is usually 0.01% by mass or more and 20% by mass or less, preferably the upper limit is 15% by mass or less, more preferably based on YTOS (100% by mass). Has an upper limit of 10% by mass or less.
The "metal-supported amount" here means the amount occupied by the metal element in the supported cocatalyst.

The form of the photocatalyst when the photocatalyst of the present invention is actually used for decomposing water is not particularly limited, and the form in which the photocatalyst particles are dispersed in water and the form in which the photocatalyst particles are solidified to form a molded body are used in water. The photocatalyst layer is provided on the base material to form a laminate, and the laminate is installed in water. The photocatalyst is immobilized on the current collector to form an electrode for photowater decomposition reaction, which is installed in water together with the counter electrode. The form and the like can be mentioned. In particular, when the photo-water decomposition reaction is carried out on a large scale, it is preferable to use an electrode for the photo-water decomposition reaction from the viewpoint of imparting a bias to promote the water-water decomposition reaction. Further, as another aspect, the photocatalyst layer containing the photocatalyst of the present invention is formed on the base material without imparting a bias by utilizing the fact that the photocatalyst of the present invention can completely decompose water by itself. The provided laminate or the composite containing the photocatalyst of the present invention can also be installed in water. With this aspect, it is possible to reduce the ease of processing and handling, the ease of maintenance, and the cost when used as an artificial photosynthesis device that uses a large area, and it is an industrially superior water splitting device and oxygen generator. , A hydrogen generator, or an artificial photosynthesis system can be obtained.

The electrode for photo-water decomposition reaction can be produced by a known method. For example, it can be easily prepared by the 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 a photo-water decomposition reaction by the following procedure. That is, the photocatalyst particles are placed on the first base material such as glass to obtain a laminate of the photocatalyst layer and the 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 surface layer on the conductive layer side of the photocatalyst layer are immobilized on the conductive layer. After that, the 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 a part of the photocatalyst particles is immobilized on the surface of the conductive layer, it is peeled off together with the conductive layer, and as a result, an electrode for photowater decomposition reaction having a photocatalyst layer, a conductive layer, and a second base material layer is obtained. be able to.
Alternatively, as another method, an electrode for photowater decomposition reaction may be obtained by applying a slurry in which the photocatalyst particles are dispersed to the surface of the current collector and drying it, or the photocatalyst particles and the current collector may be separated from each other. An electrode for photowater decomposition reaction may be obtained by integrating by pressure molding or the like. Alternatively, the current collector may be immersed in a slurry in which the photocatalyst particles are dispersed, a voltage may be applied, and the photocatalyst particles may be accumulated on the current collector by electrophoresis.
Alternatively, the co-catalyst may be supported in a subsequent step. For example, in the above-mentioned particle transfer method, using optical semiconductor particles instead of photocatalytic particles, a laminate having an optical semiconductor layer, a conductive layer, and a second base material layer is obtained by the same method, and then an optical semiconductor is obtained. An electrode for photowater decomposition reaction may be obtained by supporting oxide particles as a cocatalyst on the surface of the layer.

By immersing the photocatalyst of the present invention or the electrode for photowater decomposition reaction described above in water or an aqueous electrolyte solution and irradiating the photocatalyst or the electrode for photowater decomposition reaction with light to perform photowater decomposition, hydrogen and 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 decomposition reaction, while a conductor carrying a hydrogen generation catalyst is used as a counter electrode, and a liquid or gas is used. Irradiate light while supplying water in the form of water to promote the water splitting reaction. The water splitting reaction can be promoted by providing a potential difference between the electrodes as needed. Alternatively, a photosemiconductor carrying a hydrogen generation catalyst may be used as a counter electrode. In this case, as the optical semiconductor, a known optical semiconductor that catalyzes the hydrogen production reaction can be used.

On the other hand, the water splitting reaction may proceed by irradiating the immobilized product in which the photocatalyst particles are immobilized on the insulating base material or the molded body in which the photocatalyst particles are pressure-molded with water while supplying water. .. Alternatively, the photocatalyst particles may be dispersed in water or an aqueous electrolyte solution and irradiated with light to allow the water decomposition reaction to proceed. 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 thereto. Hydrogen and oxygen can be produced if there is a means and a means for extracting hydrogen and / or oxygen.
This simplifies the structure, and at the same time, it is possible to operate the oxygen generating electrode and the hydrogen generating electrode in half the area as compared with arranging them in parallel. The generated hydrogen and oxygen can be separated into hydrogen and oxygen by using, for example, a zeolite membrane.

The reaction conditions during the production of hydrogen and / or oxygen are not particularly limited, but for example, the reaction temperature is 0 ° C. or higher and 200 ° C. or lower, and the reaction pressure is 2 MPa (G) or lower.
The irradiation light is visible light or ultraviolet light having a wavelength of 650 nm or less. Examples of the light source of the irradiation light include the sun, a lamp capable of irradiating sunlight-approximate 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 the photowater decomposition reaction.

Hereinafter, the present invention will be described in more detail based on Examples, but the present invention is not limited to the following Examples. The values of various production conditions and evaluation results in the following examples have meanings as preferable values of the upper limit or the lower limit in the embodiment of the present invention, and the preferable range is the above-mentioned upper limit or lower limit value. , The range specified by the combination of the values of the following examples or the values of the examples may be used.

[XRD measurement / DRS measurement]
The conditions, apparatus, etc. of the XRD measurement and the ultraviolet / visible diffuse reflection spectrum (DRS) measurement of the photocatalyst powder synthesized in the following Examples and Comparative Examples are as follows.

<XRD measurement>
Manufacturer; Rigaku
Device; SmartLab
Measurement condition;
Powder X-ray diffraction measurement was performed on the photocatalytic powder crushed to a diameter of 100 μm or less by the concentrated method. ・ Measurement range: 5 to 80 °
・ Measurement step: 0.01 °
・ Scan speed: 10 ° / min Analysis without using a monochromator;
FWHM: Calculation is performed directly from the data obtained under the above conditions without undergoing the separation process of Kα1 and Kα2.

<DRS measurement>
Manufacturer; JASCO
Model number: V-670 Spectrophotometer
Measurement condition;
-Measurement range: 300 nm to 800 nm
・ Data interval: 0.2 nm
-Scanning speed: 200 nm / min-Light source switching: 340.0 nm
-Data analysis software: Spectra Manager version 2
Analysis; vertical axis is Kubelker-Munch (KM) conversion Kubelker-Munch conversion formula f (R _∞ ) = (1-R _∞ ) 2 / 2R _∞ = K / S
Here, f (R _∞ ) is K.I. M. The function, R _∞ is the absolute reflectance, K is the molecular extinction coefficient, and S is the scattering coefficient.
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) (BaSO ₄ is used as the standard sample)
The relative reflectance r _∞ was measured using
f (r _∞ ) = (1-r _∞ ) 2 / 2r _∞ = K / S
Was derived from.

[Preparation of photocatalyst for evaluation and evaluation of photocatalytic activity]
For the photocatalyst powders synthesized in the following examples and comparative examples, a photocatalyst for evaluation was prepared by the following method and the photocatalytic activity was evaluated.

<Preparation of IrO ₂ colloidal solution>
After the Na ₃ IrCl ₆ · n hydrate Ir metal concentration was dissolved in pure water so as to 0.0548mg / mL, adjusted to pH = 12 with 1N aqueous sodium hydroxide solution, stirred at 80 ° C. 30 minutes did. Then, the mixture 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 IrO ₂ colloidal solution was prepared by stirring again at 80 ° C. for 30 minutes and recooling to room temperature.

<Preparation of photocatalyst CrO _x / Rh / IrO ₂ / Y ₂ Ti ₂ O ₅ S ₂ for total decomposition evaluation>
The photocatalyst powder was suspended in an aqueous IrO ₂ colloid solution (0.3 mass% Ir with respect to the photocatalyst powder), and the mixture was stirred for 40 minutes to adsorb the IrO ₂ colloid on the photocatalyst powder. After adsorption of IrO ₂ , it is suspended in an aqueous solution of RhCl ₃ (2.0% by mass Rh with respect to the photocatalytic powder) and irradiated with visible light (λ> 420 nm) for 3 hours in the absence of air to irradiate Rh (III) with metal. It was photoreduced to Rh. After precipitation of Rh, the obtained sample was suspended in an aqueous solution of K ₂ CrO ₄ (1.5 mass% Cr with respect to the photocatalytic powder), irradiated with ultraviolet visible light (λ> 300 nm) for 15 hours, and K ₂ CrO _{4 was} irradiated. Was reduced to chromium oxide CrO _x . For light irradiation, a 300 W xenon lamp equipped with a cutoff filter was used. Cooling water was used during light irradiation to keep the solution temperature at room temperature. The product was washed well with distilled water and dried under reduced pressure at 40 ° C. for 2 hours.

<Preparation of photocatalyst IrO ₂ / Y ₂ Ti ₂ O ₅ S ₂ for evaluation of hydrogen / oxygen production>
The photocatalyst powder was suspended in an aqueous IrO ₂ colloid solution (0.3 mass% Ir with respect to the photocatalyst powder), and the mixture was stirred for 40 minutes to adsorb the IrO ₂ colloid on the photocatalyst powder. The product was washed well with distilled water and dried under reduced pressure at 40 ° C. for 2 hours.

<Preparation of photocatalyst CoOx / Y ₂ Ti ₂ O ₅ S ₂ for hydrogen / oxygen production evaluation>
The photocatalyst powder was suspended in a cobalt nitrate solution in which cobalt nitrate hexahydrate was dissolved, the solvent was removed, and the mixture was calcined at 300 ° C. for 1 hour.

<Total decomposition evaluation test and hydrogen / oxygen production evaluation test>
The photowater decomposition reaction performance was evaluated using the prepared photocatalyst for total decomposition evaluation or photocatalyst for hydrogen / oxygen production evaluation. The photowater decomposition reaction was carried out in a closed reactor equipped with a vacuum exhaust pump, a circulation pump, a cell containing a photocatalytic 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 a temperature rise, and the cell was cooled from the outside using cooling water. At the time of 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. The analysis conditions were a column (molecular sieve 5A), a carrier gas (argon), and a temperature (50 to 70 ° C.).

In the total decomposition evaluation test, 150 mL of water and 100 mg of the pH adjuster La ₂ O ₃ (pH = 9) were added to 200 mg of the photocatalyst for total decomposition evaluation and sealed in a cell to carry out the test.
In the hydrogen production evaluation test, 150 mL of a 0.02 M Na ₂ S-Na ₂ SO ₃ aqueous solution was sealed in a cell with respect to 200 mg of a photocatalyst for hydrogen production evaluation, and the test was carried out.
In the oxygen production evaluation test, 150 mL of a 0.02M AgNO ₃ aqueous solution was sealed in a cell with respect to 200 mg of a photocatalyst for oxygen production evaluation, and the test was carried out.
Hereinafter, the hydrogen and oxygen production rate (molar production per unit time) in the total decomposition evaluation test will be referred to as the total decomposition rate, the hydrogen production rate will be referred to as “V _H2 ”, and the oxygen production rate will be referred to as “ _VO2 ”.
In addition, the hydrogen production rate in the hydrogen production evaluation test is referred to as "V _H2 " as the half reaction rate, and the oxygen production rate in the oxygen production evaluation test is referred to as " _VO2 ".

[Synthesis, analysis and evaluation of photocatalytic powder]
<Example 1>
Weigh Y ₂ O ₃ , Y ₂ S ₃ , TIO ₂ , and S at a molar ratio of 1: 2: 6: 2.05, mix in an N ₂ glove box (dew point minus 70 ° C or less) for about 40 minutes, and mix. After sealing in a quartz tube in vacuum, it was fired at 650 ° C. for 96 hours. Then, the excess sulfur content was oxidized by heat treatment in air at 200 ° C. for 1 hour, washed with water, and then filtered to obtain photocatalyst powder A.
As a result of XRD measurement, almost all peaks of the obtained photocatalyst powder A were assigned to Y ₂ Ti ₂ O ₅ S ₂ . Table 1 shows the XRD measurement results and the ultraviolet / visible diffuse reflection 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 peak shape of XRD near 26.3 °, and FIG. 2 shows the peak shape near 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, almost all peaks of the obtained photocatalyst powder B were assigned to Y ₂ Ti ₂ O ₅ S ₂ . Table 1 shows the XRD measurement results and the ultraviolet / visible diffuse reflection 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 peak shape of XRD near 26.3 °, and FIG. 4 shows the peak shape near 30.6 °.

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

<Example 4>
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, almost all peaks of the obtained photocatalyst powder D were assigned to Y ₂ Ti ₂ O ₅ S ₂ . Table 1 shows the XRD measurement results of the obtained photocatalyst powder D and the ultraviolet / visible diffuse reflection spectrum measurement results.
Table 1 shows the evaluation results of the photocatalytic activity of the photocatalyst powder D.
Further, FIG. 7 shows the peak shape of XRD 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 sulfuric acid having a concentration of 49% by mass 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, almost all peaks of the obtained photocatalyst powder E were assigned to Y ₂ Ti ₂ O ₅ S ₂ . Table 1 shows the XRD measurement results and the ultraviolet / visible diffuse reflection 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 together with 12 g of 1 mm diameter beads made of zirconia and 3 mL of water, and the whole container was pulverized by a rolling mill (200 rpm, 30 minutes). After the pulverization treatment, the beads were removed and filtered to obtain a photocatalyst powder F.
As a result of XRD measurement, almost all peaks of the obtained photocatalyst powder F were assigned to Y ₂ Ti ₂ O ₅ S ₂ . Table 1 shows the XRD measurement results of the obtained photocatalyst powder F and the ultraviolet / visible diffuse reflection spectrum measurement results.
Table 1 shows the evaluation results of the photocatalytic activity of the photocatalyst powder F.

<Example 7>
The photocatalytic powder C synthesized in Example 3 was sealed in a sample bottle together with distilled water and an IrO ₂ colloidal solution, and "Microwave synthesis" manufactured by Antonio Par Co., Ltd. was sealed.
A MW (microwave) treatment was carried out at 200 ° C. for 10 minutes in "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 Ir metal, Rh metal, and Cr metal concentrations were 0.15, 0.1, and 0.1% by mass, respectively, with respect to the photocatalyst powder. The solvent was distilled off from the obtained suspension to obtain a photocatalyst powder G.
As a result of XRD measurement, almost all peaks of the obtained photocatalyst powder G were assigned to Y ₂ Ti ₂ O ₅ S ₂ . Table 1 shows the XRD measurement results of the obtained photocatalyst powder G and the ultraviolet / visible diffuse reflection spectrum measurement results.
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 then the photocatalyst powder C was calcined in air at 300 ° C. for 1 hour. At this time, the Co metal concentration was set to 0.015% by mass with respect to the photocatalyst powder. The solvent was distilled off from the obtained suspension to obtain a photocatalyst powder J.
As a result of XRD measurement, almost all peaks of the obtained photocatalyst powder J were assigned to Y ₂ Ti ₂ O ₅ S ₂ . Table 1 shows the XRD measurement results and the ultraviolet / visible diffuse reflection 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 then the photocatalyst powder C was calcined in air at 300 ° C. for 1 hour. At this time, the Co metal concentration was 0.15% by mass with respect to the photocatalyst powder. The solvent was distilled off from the obtained suspension to obtain a photocatalyst powder K.
As a result of XRD measurement, almost all peaks of the obtained photocatalyst powder K were assigned to Y ₂ Ti ₂ O ₅ S ₂ . Table 1 shows the XRD measurement results and the ultraviolet / visible diffuse reflection 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 set to 600 ° C.
As a result of XRD measurement, the obtained photocatalyst powder H had a very weak peak near 26.3 ° and had an intensity of only 6 when the maximum peak was 100, and the main component was 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 reflection 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 shape of the XRD near 26.3 °, and FIG. 10 shows the peak shape near 30.6 °.

<Comparative example 2>
Photocatalyst powder I was obtained in the same manner as in Example 1 except that the firing temperature was 1000 ° C.
As a result of XRD measurement, almost all peaks of the obtained photocatalyst powder I were assigned to Y ₂ Ti ₂ O ₅ S ₂ . Table 1 shows the XRD measurement results of the obtained photocatalyst powder I and the ultraviolet / visible diffuse reflection spectrum measurement results. The peak intensity around 26.3 ° is low, and it can be seen that the growth direction of YTOS is different from that of YTOS of the example. In addition, the UV / visible diffuse reflection spectrum measurement results are also different from those in 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 peak shape of XRD around 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 a photocatalyst powder H having a firing temperature of 600 ° C. and a photocatalyst powder having a firing temperature of 1000 ° C. It shows different measured values from I in the XRD measurement and the ultraviolet / visible diffusion reflection spectrum measurement, and it can be seen that they are different substances even if they have the same composition.
Further, the photocatalytic powders H and I hardly obtained the photocatalytic activity against water decomposition, but the photocatalytic powders A to G, J and K obtained good photocatalytic activity.

As is clear from the above results, since the photocatalyst of the present invention is YTOS having specific characteristics, it has high photocatalytic activity, and it can be seen that water can be completely decomposed only by this photocatalyst. As a result, even if the photocatalyst for decomposing water to generate oxygen and the photocatalyst for generating hydrogen are not connected by wiring, this photocatalyst is placed in water to emit light like sunlight. It can be seen that water can be decomposed into hydrogen and oxygen with high efficiency just by irradiating.

<Example 10>
Experiments have been conducted to show that the YTOS of the present invention is not easily affected by the environment and operates at a wide range of pH when used as an electrode.

<Preparation of electrodes>
A YTOS electrode was prepared by a particle transfer method, which is a known method, using YTOS powder prepared by the method described in Example 3 and obtained 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 dropped-cast onto a sacrificial substrate of 3 cm □ glass to apply it in a thin film form. After drying this at room temperature, a titanium thin film having a thickness of 3 μm was deposited by sputtering (ULVAC, MPS series). By peeling off this titanium thin film from the sacrificial substrate of glass, a YTOS electrode thin film formed very thinly on the titanium thin film was obtained.

<Measurement of electrical capacity-potential characteristics>
Impedance was measured by a three-terminal method in which the above YTOS electrode was the working electrode, the internal solution was a saturated KCl aqueous solution, the Ag / AgCl reference electrode was the reference electrode, and the platinum wire was the counter electrode. A 0.1 M aqueous solution of Na ₂ SO ₄ was used as the electrolytic solution, the solution was stirred during the measurement, and the head space of the container was purged with argon gas. In the experiment to adjust the pH, the pH was adjusted by adding an aqueous solution of H ₂ SO _{4 and an} aqueous solution of NaOH. Prior to the impedance measurement, cyclic voltammetry measurement was performed at a sweep rate of 10 mV / s. The amplitude of the AC signal applied during the impedance measurement was 10 mV, and the measurement was performed in a dark place.

<Mott-Schottky plot>
1 / C ² 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 result is 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.
Generally, when a depletion layer is present on the surface of a semiconductor, the width of the depletion layer changes depending on the potential applied to the semiconductor, so that the value of 1 / C ² also changes. Since the energy band is tilted in the depletion layer on the semiconductor surface, when the light corresponding to the band gap is absorbed and electron-hole pairs are generated, the electrons and holes flow in opposite directions to the front surface side and the back surface side of the thin film, respectively. Will move to. The semiconductor electrode in which electrons move to the thin film surface side is the optical cathode, and the semiconductor electrode in which holes move to the thin film surface side is the optical anode.

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

<Example 11>
<Preparation of electrodes>
A YTOS electrode was prepared by a particle transfer method, which is a known method, using YTOS powder prepared by the method described in Example 3 and obtained XRD data equivalent to the XRD data in Example 3. That is, a YTOS electrode formed on a titanium / gold laminated film by first depositing 1 μm of titanium and 3 μm of gold on the YTOS particle thin film coated on the glass sacrificial substrate and peeling it off from the sacrificial substrate. A thin film was prepared.

<Measurement of photocurrent-potential characteristics>
The photocurrent-potential characteristics were measured by a three-terminal method in which the above YTOS electrode was the working electrode, the internal solution was a saturated KCl aqueous solution, the Ag / AgCl reference electrode was the reference electrode, and the platinum wire was the counter electrode. As the aqueous solution, a phosphoric acid aqueous solution having a pH of 9 or 13 (in each case, the phosphate concentration was adjusted to 0.5 M) was used. During the measurement, the solution was stirred and bubbling with argon gas was performed. The potential of the YTOS electrode was swept from the positive potential to the negative potential side to measure the anode current, and the photoanode current was observed by repeating irradiation and interruption of pseudo-sunlight during this sweep.

The measurement results are shown in FIGS. 14 (pH 9) and 15 (pH 13). The sawtooth signal corresponding to light on-interruption (Light off) is visible up to about 0.1 V vs RHE (RHE: lossless hydrogen potential). When a photoresponse exists even at 0 V vs RHE, if this YTOS electrode is connected to an ideal hydrogen generation electrode, water decomposition will proceed only by irradiating the YTOS electrode with sunlight, which is extremely significant. Even when the onset potential of the optical response does not reach 0 V vs RHE, if a potential exceeding the onset potential is applied from the outside, water decomposition will proceed when the YTOS electrode is irradiated with sunlight. Therefore, the low onset potential of the YTOS electrode is useful in application.

In Ta ₃ N ₅ having a band gap close to that of YTOS of the present invention, it is known that the optical response disappears at about 0.5 V when the same experiment is performed. From this point as well, the YTOS of the present invention It turns out that the usefulness is excellent.

Claims (6)

A photocatalyst having a composition represented by the following general formula (I), which has a diffraction peak with a peak top of 26.3 ± 0.3 in XRD measurement with Cu—Kα rays according to the following apparatus and measurement conditions. The peak top has an intensity of 20 or more, where the maximum peak intensity on the XRD spectrum is 100, and the diffraction peak half width (FWHM) of 30.6 ± 0.5 is in the range of 0.16 to 0.30. Photocatalyst in.
Y _a Ti _b O _c S _d ... (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
Device; SmartLab
Measurement condition;
Powder X-ray diffraction measurement was performed on the photocatalytic powder crushed to a diameter of 100 μm or less by the concentrated method. ・ Measurement range: 5 to 80 °
・ Measurement step: 0.01 °
・ Scan speed: 10 ° / min Analysis without using a monochromator;
FWHM: Calculation is performed directly from the data obtained under the above conditions without undergoing the separation process of Kα1 and Kα2. A photocatalyst having a composition represented by the following general formula (I), which is obtained by measuring an ultraviolet / visible diffuse reflection spectrum according to the following apparatus and measurement conditions _{. T.} The value (wavelength at which the diffuse reflection 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 whose value (wavelength at which the diffuse reflection spectrum after KM conversion shows an intermediate value) is in the range of 520 nm or more and 570 nm or less.
Y _a Ti _b O _c S _d ... (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 reflection spectrum measurement>
Manufacturer; JASCO
Model number: V-670 Spectrophotometer
Measurement condition;
-Measurement range: 300 nm to 800 nm
・ Data interval: 0.2 nm
-Scanning speed: 200 nm / min-Light source switching: 340.0 nm
-Data analysis software: Spectra Manager version 2
Analysis; vertical axis is Kubelker-Munch (KM) conversion Kubelker-Munch conversion formula f (R _∞ ) = (1-R _∞ ) 2 / 2R _∞ = K / S
Here, f (R _∞ ) is K.I. M. The function, R _∞ is the absolute reflectance, K is the molecular extinction coefficient, and S is the scattering coefficient.
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) (BaSO ₄ is used as the standard sample)
The relative reflectance r _∞ was measured using
It was derived from f (r _∞ ) = (1-r _∞ ) 2 / 2r _∞ = K / S. The photocatalyst according to claim 1 or 2, which is a photocatalyst used for total decomposition of water. A method for producing hydrogen and oxygen that generates hydrogen and oxygen using the immobilized product in which the photocatalyst is immobilized or the molded product according to any one of claims 1 to 3. An electrode produced by using the photocatalyst according to any one of claims 1 to 3. A method for producing hydrogen and oxygen, which generates hydrogen and / or oxygen by the electrode according to claim 5.

Images