JP2016069211A - Strontium titanate fine particle, photocatalyst, hydrogen and oxygen generating photocatalyst system and manufacturing method of strontium titanate fine particle - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a strontium titanate fine particle excellent in photocatalyst performance and a manufacturing method therefor, a photocatalyst and a hydrogen and oxygen generating photocatalyst system using the strontium titanate fine particle.SOLUTION: There is provided a strontium titanate fine particle containing rhodium of over 1 mol% and 7 mol% or less in a titanium site and having a cube-shape with an exposed (100) surface and lattice constant of over 3.906 Å and 3.910 Å or less.SELECTED DRAWING: None

Description

  The present invention relates to strontium titanate fine particles, a photocatalyst, a hydrogen / oxygen generating photocatalytic system, and a method for producing strontium titanate fine particles.

Strontium titanate (SrTiO ₃ ) is a complex oxide that is expected to be used in various applications as a functional material, such as dielectric properties, thermoelectric properties, photocatalytic properties, and high refractive index properties.
In addition, strontium titanate is highly expected as a photocatalyst that enables hydrogen production using sunlight because of its high stability under light irradiation and strong photoreduction ability.

For example, Patent Document 1 describes “a photocatalyst having visible light activity, characterized by being composed of SrTiO ₃ doped with Rh and / or Ir” ([Claim 1]). Patent Document 1 specifically describes a photocatalyst composed of SrTiO ₃ doped with 0.1 to 3.0 mol% of Rh ([Claim 5]), and describes a preparation method by a solid phase method. ([0013]).

  In Non-Patent Document 1, strontium hydroxide and titanium dioxide are mixed in ethanol in a nitrogen atmosphere, followed by hydrothermal synthesis (high temperature and high pressure) at 170 ° C. for 3 days. Production of cubic-shaped visible light responsiveness (1%) Rh-doped strontium titanate fine particles by performing a time annealing treatment has been reported.

JP 2004008963 A

S. W. Bae et al, Applied Physics Lett. 2008, 92, 104107.

  The present inventor examined the photocatalysts described in Patent Document 1 and Non-Patent Document 1, and depending on the preparation methods specifically disclosed in these documents, the shape of strontium titanate can be micronized or crystallized. It was clarified that the photocatalytic performance of generating hydrogen may be inferior.

  Accordingly, it is an object of the present invention to provide a strontium titanate fine particle having excellent photocatalytic performance, a method for producing the same, a photocatalyst using the strontium titanate fine particle, and a hydrogen / oxygen-generating photocatalyst system.

As a result of intensive studies to achieve the above-mentioned problems, the inventors of the present invention have a cubic shape in which a specific amount of rhodium is contained in the titanium site and the {100} plane is exposed, and the lattice constant A is a predetermined value. A certain strontium titanate fine particle was found to be excellent in photocatalytic performance, and the present invention was completed.
That is, it has been found that the above-described problem can be achieved by the following configuration.

[1] Strontium titanate fine particles having a cubic shape in which rhodium is contained in a titanium site more than 1 mol% and not more than 7 mol%, the {100} plane is exposed, and a lattice constant A is more than 3.906 ３ and less than 3.910 Å .
[2] A photocatalyst comprising the strontium titanate fine particles according to [1].
[3] A hydrogen / oxygen generating photocatalytic system using the strontium titanate fine particles according to [1].

[4] A method for producing strontium titanate fine particles for preparing the strontium titanate fine particles according to [1],
A reaction solution preparation step of preparing a reaction solution in which a strontium raw material, a titanium raw material and a rhodium raw material are added in an alkaline aqueous solution;
A reaction step of heating the reaction solution to 80 ° C. or higher under atmospheric pressure to react the reaction solution to prepare fine particles in an alkaline aqueous solution;
A method of producing strontium titanate fine particles, comprising subjecting the fine particles to a firing treatment at a temperature of 200 ° C. to 1000 ° C. to prepare strontium titanate fine particles.
[5] The method for producing strontium titanate fine particles according to [4], wherein the titanium raw material contains a titanium oxide.
[6] The method for producing strontium titanate fine particles according to [5], wherein the titanium oxide is titanium dioxide.
[7] The method for producing strontium titanate fine particles according to [6], wherein the titanium dioxide is a titanium oxide containing 70% by mass or more of anatase type titanium dioxide.
[8] The strontium raw material includes at least one selected from the group consisting of strontium acetate, nitrate, carbonate, chloride, sulfide, and hydroxide, according to any one of [4] to [7] The manufacturing method of strontium titanate microparticles | fine-particles of description.
[9] The method for producing strontium titanate fine particles according to any one of [4] to [8], wherein the temperature at which the reaction solution is reacted in the reaction step is 90 ° C. or higher and the boiling point or lower of the reaction solution.
[10] The method for producing strontium titanate fine particles according to any one of [4] to [9], wherein the pH of the reaction solution is 13 to 15.
[11] The method for producing strontium titanate fine particles according to any one of [4] to [10], wherein the rhodium raw material is water-soluble.

  ADVANTAGE OF THE INVENTION According to this invention, the strontium titanate microparticles | fine-particles excellent in photocatalytic performance, its manufacturing method, the photocatalyst using a strontium titanate microparticle, and a hydrogen and oxygen production | generation photocatalyst system can be provided.

1A shows a photograph (hereinafter abbreviated as “TEM photograph”) obtained by photographing the strontium titanate fine particles prepared in Example 1 with a transmission electron microscope (TEM). FIG. B) represents a TEM photograph of strontium titanate fine particles containing no rhodium prepared in Comparative Example 1, and FIG. 1C is a TEM photograph of strontium titanate fine particles prepared in Comparative Example 4 (solid phase method). FIG. 1D shows a TEM photograph of the strontium titanate fine particles prepared in Comparative Example 5 and not subjected to the firing treatment. FIG. 2 shows an X-ray diffraction pattern (X−) of a sample subjected to a baking treatment at a temperature condition of 1000 ° C., 750 ° C., 500 ° C., or 250 ° C. and a sample before the baking treatment (Comparative Example 5). Ray Diffraction Patterns (XRDP) shows a high-angle diffraction peak. FIG. 3 shows the XRDP high-angle diffraction peak of a sample immediately after synthesis with a rhodium (Rh) content of 5 mol%, 3 mol% (Example 1), 2 mol%, or 0 mol% (Comparative Example 1). FIG. 4 is a graph showing the correlation between the lattice constant A and the rhodium content. FIG. 5 shows diffuse reflection spectra of strontium titanate fine particles prepared in Example 1, Comparative Example 1 and Comparative Example 5. FIG. 6 (A) is a graph showing the relationship between the rhodium content and the optical band gap before and after the firing treatment, and FIG. 6 (B) shows Comparative Example 5 (before firing: mark ◆) and Example 1 (firing). It is explanatory drawing which shows the change of an optical band gap with (after: ■ mark). FIG. 7 is a graph showing the relationship between the firing temperature and the Rh4 valence by electron spin resonance (ESR). FIG. 8 is a graph showing the relationship between the rhodium content and the hydrogen generation amount.

Hereinafter, the present invention will be described in detail.
The description of the constituent elements described below may be made based on typical embodiments of the present invention, but the present invention is not limited to such embodiments.
In the present specification, a numerical range expressed using “to” means a range including numerical values described before and after “to” as a lower limit value and an upper limit value.

[Strontium titanate fine particles]
The strontium titanate fine particles of the present invention (hereinafter also abbreviated as “fine particles of the present invention”) have a cubic shape containing rhodium at a titanium site of more than 1 mol% and not more than 7 mol%, with the {100} plane exposed, and Strontium titanate fine particles having a lattice constant A of more than 3.906 and less than or equal to 3.910.

Here, “fine particles” refer to particles having an average particle size of less than 1 μm, and are preferably nanoparticles having an average particle size of 200 nm or less.
The average particle diameter can be measured by a method known in the technical field of the present invention, for example, a transmission electron microscope (TEM), an adsorption method, a light scattering method, an X-ray small angle scattering. It can be measured by the method (Small Angle X-ray Scattering: SAXS). In TEM, observation is performed with an electron microscope. When the particle size distribution is wide, it is necessary to pay attention to whether or not the particles entering the field of view represent all particles.
In the present invention, the average particle diameter is an average value obtained by selecting any 10 fine particles having a cubic shape from a TEM photograph at a magnification of 100,000 times and measuring the length (particle diameter) of one side of each fine particle. Say.

The rhodium content (mol%) at the titanium site of the fine particles of the present invention is a value expressed by mol% in the composition formula shown in the following formula (1).
That is, “more than 1 mol% and not more than 7 mol%” is synonymous with x in the following formula (1) being “more than 0.01 and not more than 0.07”.
Sr · [Ti _1-x · Rh _x ] · O ₃ (1)
In addition, x represents a value quantified by chemical analysis, and in the present invention, x represents a value quantified using high frequency inductively coupled plasma-optical emission spectrometry (ICP-OES). Specifically, 10 mg of a strontium titanate sample was weighed into a Zr crucible, Na ₂ O ₂ and Na ₂ CO ₃ were added, and the mixture was melted at 900 ° C., and then added to 56 g of water and aqua regia ( ₆ ml of HNO ₃ +18 ml of HCl). It is possible to calculate x by quantifying Sr, Ti and Rh using ICP-OES from a crucible immersed for 60 minutes, diluted to 200 ml with water and diluted 10-fold with aqua regia. In addition, the fine particles prepared by the production method of the present invention described later were almost equal in raw material input amount (molar amount) and quantification by chemical analysis.

In addition, the “cubic shape” includes a substantial cubic shape in which a part of the vertex is chamfered, and in the TEM image, one isolated particle is circumscribed by a rectangle that minimizes the area. When the area S _{0 is} the particle image area Scube, the shape satisfies 0.8 <S _cube / S ₀ .
On the other hand, the evaluation of the crystal plane exposed on the particle surface, that is, whether or not the {100} plane is exposed can be determined by a high-resolution TEM image and an electron diffraction pattern. In the present invention, it is preferable that 85% or more of crystal planes exposed on the particle surface are {100} planes.

“Lattice constant A” refers to a lattice constant precisely determined by powder X-ray diffraction.
Specifically, the angle between the incident X-ray (wavelength = λ) and the plane is θ, and each peak is determined from the diffraction peak position θ in the range of 90 ° <2θ <130 ° and the plane indices h, k, l. The lattice constant a at the position is calculated from the following formula.

Next, the lattice constant a is plotted on the vertical axis, and the Nelson-Riley function 1/2 {(cos θ) ² / sin θ + (cos θ) ² / θ} is plotted on the horizontal axis, and 90 from θ when the least squares fitting is performed with a straight line. The extrapolated value to degrees is defined as “lattice constant A”.
In the case of the fine particles of the present invention, five diffraction peaks appear in the range of 90 ° <2θ <130 °, and each peak has a plane index of {3, 2, 1}, {4, 0, 0}. , {4, 1, 1}, {3, 3, 1}, {4, 2, 0}. The error bar when these five points were fitted to the least square method was about ± 0.001 mm.

As described above, the fine particles of the present invention contain a specific amount of rhodium at the titanium site, have a cubic shape in which the {100} plane is exposed, and the lattice constant A is a predetermined value. It becomes good.
The reason why the photocatalytic performance is good in this way is not clear in detail, but is estimated as follows.
That is, since the fine particles containing a specific amount of rhodium were subjected to a baking treatment described later, and the lattice constant A of the fine particles having a cubic shape was made to be more than 3.906Å and less than 3.910Å, the light absorption of visible light was increased. Can be considered.
As shown in FIG. 5, the comparison between Example 1 and Comparative Example 5 shows that the absorption near 600 nm is increased by performing the baking treatment and reducing the lattice constant, and FIG. As shown, it can be inferred from the fact that the optical band gap is narrowed by performing the baking treatment and reducing the lattice constant from the comparison between Example 1 and Comparative Example 5. That is, in the case where rhodium is not doped, only a small proportion of ultraviolet light can be effectively used even in sunlight, but the fine particles doped with a specific amount of rhodium are subjected to a firing treatment to obtain a band gap. In the case of narrowing, it is considered that light on a longer wavelength side can be absorbed more efficiently.

In the present invention, the rhodium content (dope amount) is preferably 2 to 6 mol%, more preferably 2 to 5 mol%, and more preferably 3 to 4 mol%, because the photocatalytic performance becomes better. More preferably.
Here, as the rhodium content increases, the high-angle diffraction peak of XRDP shifts to the low-angle side from the results shown in FIG. 3, and the constituent constant A increases from the results shown in FIG. I understand.

In addition, in the present invention, electrons are easy to move from the band structure, and as a result, the photocatalytic ability is better, and as described above, 85% or more of the crystal faces exposed on the particle surface. The {100} plane is preferable, and 90% or more is more preferably the {100} plane.
Similarly, moisture and the like are removed by heat treatment from the lattice expanded by moisture and the like, resulting in higher crystallization, resulting in better photocatalytic performance. Therefore, the lattice constant A is more than 3.9060 Å 3.9100 Å It is preferable that it is 3.9065 to 3.9090 and more preferably 3.9065 to 3.9085.

  Furthermore, in the present invention, it is possible to increase the irradiation area of sunlight by increasing the specific surface area by reducing the size, increasing the catalytic reaction active point, and maintaining high crystallinity, As a result, the average particle diameter of the fine particles of the present invention is preferably 10 nm or more and 500 nm or less, more preferably 20 nm or more and 100 nm or less, for the reason that the photocatalytic performance becomes better.

[photocatalyst]
The photocatalyst of the present invention is a photocatalyst comprising the strontium titanate fine particles of the present invention described above.
As described above, the photocatalyst of the present invention has strontium titanate fine particles containing a specific amount of rhodium at a titanium site, having a cubic shape with an exposed {100} plane, and having a lattice constant A of a predetermined value. Therefore, the photocatalytic performance is improved.

<Metal promoter>
The photocatalyst of the present invention may be a catalyst composed only of the fine particles of the present invention, but is preferably a catalyst further supporting a metal promoter on the surface of the fine particles of the present invention.
The strontium titanate fine particles carrying a metal promoter on the surface recombine with holes because electrons excited in the strontium titanate by light irradiation move to the surface quickly due to the presence of the metal promoter on the surface. Therefore, hydrogen can be generated more efficiently.

When the metal promoter is supported, the method for supporting the metal promoter is not particularly limited, and examples thereof include a method using a photo-deposition method, an impregnation method, electroless plating, and the like.
Among these, the method using the photo-deposition method is preferable from the viewpoint that the photocatalytic performance becomes better and the photocatalytic performance can be evaluated simultaneously with the loading of the metal catalyst.

As a light source in the photo-deposition method, a xenon lamp, a mercury lamp or sunlight having a distribution with an energy higher than the absorption wavelength of the oxide particles can be mentioned, but a viewpoint of uniformly irradiating a large area with inexpensive equipment To xenon lamps are preferred.
In the photo-deposition method, it is preferable to irradiate the surface of the reaction vessel with visible light having a wavelength of 420 nm or more with illuminance.
Moreover, the atmosphere in the photo-deposition method is not particularly limited, and it may be under atmospheric pressure or in an arbitrary gas, but it is preferably performed in an argon atmosphere from the viewpoint of simultaneously evaluating the photocatalytic performance.
Moreover, in order to suppress the evaporation of the solvent, the maximum temperature reached by the reaction solution in the photodeposition method is preferably 40 ° C. or less, and more preferably 10 ° C. to 25 ° C.

Examples of the metal promoter include platinum, silver, gold, rhodium, chromium, etc. Among them, platinum and rhodium exhibiting high photocatalytic activity are preferable.
For example, as the platinum raw material, it is preferable to use either hexachloroplatinic acid hexahydrate or tetrachloroplatinic acid hexahydrate from the viewpoint of adhering to the strontium titanate fine particles by a reduction reaction in a solvent. The platinum concentration is preferably 0.1% by mass to 20.0% by mass with respect to the weight of strontium titanate.

[Hydrogen / Oxygen generation photocatalyst system]
The hydrogen / oxygen generating photocatalytic system of the present invention is a hydrogen / oxygen generating photocatalytic system using the above-described strontium titanate fine particles of the present invention, that is, the above-described photocatalyst of the present invention.
As such a system, for example, a thin film for hydrogen production (specifically, an artificial photosynthesis film or the like) obtained by applying the photocatalyst of the present invention to a substrate; a photocatalyst of the present invention and an oxygen generation photocatalyst (for example, tungsten oxide, And a photocatalytic device in combination with bismuth vanadate).

[Production method of strontium titanate fine particles]
The method for producing strontium titanate fine particles of the present invention (hereinafter also referred to as “the method of production of the present invention”) is a method for producing strontium titanate fine particles for preparing the strontium titanate fine particles of the present invention described above, and is alkaline. A reaction liquid preparation step for preparing a reaction liquid in which an strontium raw material, a titanium raw material, and a rhodium raw material are added in an aqueous solution; and the reaction liquid is reacted at 80 ° C. or higher under atmospheric pressure to react the reaction liquid in an alkaline aqueous solution. A method for producing strontium titanate fine particles comprising: a reaction step of preparing fine particles; and a calcination step of preparing fine particles of strontium titanate by subjecting the fine particles to a firing treatment at a temperature of 200 ° C. to 1000 ° C.
Hereinafter, materials and conditions in each processing step will be described in detail.

[Reaction solution preparation step]
The reaction solution preparation step is a step of preparing a reaction solution in which a strontium raw material, a titanium raw material, and a rhodium raw material are added to an alkaline aqueous solution.

<Titanium raw material>
The titanium raw material preferably contains titanium oxide.
The titanium oxide is preferably titanium dioxide.
Titanium dioxide has an anatase type and a rutile type, and since it has a skeleton relatively close to a perovskite skeleton, the anatase type is preferable.
Therefore, the titanium dioxide is preferably a titanium oxide containing 70% by mass or more of anatase type titanium dioxide.
As a titanium raw material, it is preferable to use a Ti-containing aqueous solution in which titanium oxide is dissolved in water.

<Strontium raw material>
The strontium raw material preferably contains at least one selected from the group consisting of strontium acetate, nitrate, carbonate, chloride, sulfide and hydroxide.
Among these, from the viewpoint of incorporating strontium ions into the structure of a titanium raw material (for example, titanium dioxide), water-soluble is preferable, and strontium nitrate, strontium hydroxide, and strontium chloride are more preferable.

<Rhodium raw material>
The rhodium raw material is preferably water-soluble, and specifically, rhodium (III) chloride, rhodium oxide (III), and rhodium oxide (IV) are more preferable.
Here, the content of the rhodium raw material in the reaction solution is preferably more than 1 mol% and 7 mol% or less with respect to the total mol amount of the titanium raw material and the rhodium raw material.
Since the rhodium raw material is a small amount relative to the strontium raw material, it is preferable to prepare an aqueous solution of the rhodium raw material in advance.

<Alkaline aqueous solution>
The alkaline aqueous solution to which the strontium raw material, titanium raw material and rhodium raw material are added is not particularly limited as long as it is an alkaline aqueous solution, but is preferably a strong alkaline aqueous solution having a pH of 13 to 15.
The pH can be adjusted with a basic compound (alkali source) such as potassium hydroxide or sodium hydroxide.

The reaction solution is prepared by adding the strontium raw material, the titanium raw material and the rhodium raw material as solutes to the alkaline aqueous solution described above, and stirring and dissolving them. It is preferable to prepare it by adding to the alkaline aqueous solution.
The reaction solution prepared in this manner preferably has a pH of 13 to 15 like the alkaline aqueous solution described above.
The concentration of the titanium raw material in the reaction solution can be arbitrarily selected, but is preferably 1 mmol / L or more and 50 mmol / L or less from the viewpoint of the uniformity and productivity of the fine particles.

[Reaction process]
The reaction step is a step of preparing fine particles in an alkaline aqueous solution by heating the reaction solution to 80 ° C. or higher under atmospheric pressure to react the reaction solution.
Here, the atmospheric pressure generally refers to 1 atmosphere (about 0.1 MPa), but in the present invention, “under atmospheric pressure” means a range of 0.09 to 0.12 MPa including the vicinity of atmospheric pressure. is there.
Further, “reacting the reaction solution” means reacting one or more components contained in the reaction solution.

The temperature at which the reaction solution is reacted is 80 ° C. or higher, preferably 90 ° C. or higher and lower than the boiling point of the reaction solution, and has a boiling point of 100 ° C. or higher at normal pressure of water and lower than the boiling point of the reaction solution. Is more preferable.
Moreover, the heating method of the said reaction liquid is not specifically limited, For example, it can select from a hot plate heating, a mantle heater, an aluminum block heating, etc.
Moreover, it is preferable that a temperature increase rate is 1 degree-C / min-100 degree-C / min from a viewpoint of productivity.

During the reaction step, the reaction solution is preferably stirred. The stirring speed should just be a solute disperse | distributing uniformly and about 100 rpm-1000 rpm are preferable.
The reaction time is preferably shorter as long as the reaction proceeds sufficiently, but is preferably 2 hours or longer and 12 hours or shorter.

[Optional process]
The production method of the present invention preferably has an optional step of purifying and drying the fine particles produced in the alkaline aqueous solution after the reaction step.

<Purification process>
In order to purify the fine particles prepared in the alkaline aqueous solution, it is preferable to perform a centrifugation operation for separating the solvent and the fine particles.
The conditions for the centrifugal separation operation are not particularly limited, but are preferably 5000 to 20000 rpm, 5 to 15 minutes, and 3 to 30 ° C.
The centrifugation operation is preferably repeated twice or more, and it is preferable to include a step of discarding the supernatant and suspending with 20 ml to 100 ml of pure water between the centrifugation operations. In this case, from the viewpoint of removing alkali components and the like in the suspension, it is more preferable to repeat the suspension so that the pH value of the suspension is at least less than 10.

<Drying process>
In order to completely evaporate the alkaline aqueous solution remaining in the fine particles, it is preferably dried at 100 to 120 ° C. for 1 to 3 hours.
The heating method is not particularly limited, and can be selected from hot plate heating, electric furnace heating, infrared heating, microwave heating, and the like.
Moreover, although it does not specifically limit to the atmosphere in drying, it is preferable to carry out under atmospheric pressure and air | atmosphere from viewpoints, such as manufacturing cost.

[Baking process]
The firing step is a firing step of preparing strontium titanate fine particles by subjecting the fine particles obtained by the reaction step (optional step if any of the above steps) to a firing treatment at a temperature of 200 ° C. or higher and 1000 ° C. or lower. is there.
Here, the fine particles before being subjected to the firing treatment are considered to have a hydroxyl group inside the fine particles, and it is considered that the hydroxyl constant is eliminated and the lattice constant A is reduced by performing the firing treatment.
This can also be inferred from the fact that, as shown in FIG. 2, the XRDP high-angle diffraction peak shifts to the high-angle side as the temperature of the baking treatment increases.

The method for the baking treatment is not particularly limited, and can be selected from, for example, a muffle furnace, infrared heating and the like.
Further, in order to sufficiently desorb the hydroxyl group, it is preferable that the baking is performed at 500 ° C. or higher for 7 hours or longer, and the baking temperature is 750 ° C. or higher and 850 ° C. or lower from the viewpoint of suppressing the rhodium oxidation reaction. More preferably.
This is also due to the fact that, as shown in FIG. 7, the Rh4 valence having an intrinsic absorption band in the visible light region and an absorption band caused by excitation of electrons not used in the photocatalyst is reduced around 800 ° C. Can be guessed.

  Hereinafter, the present invention will be described in more detail based on examples. The materials, amounts used, ratios, processing details, processing procedures, and the like shown in the following examples can be changed as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be construed as being limited by the following examples.

[Examples 1-3, Comparative Examples 1-3]
<Reaction solution preparation process>
To a Teflon (registered trademark) three-necked flask, 160 mL of pure water and 1 mol of sodium hydroxide NaOH were added to prepare an alkaline aqueous solution.
Next, strontium nitrate (II) anhydride (made by Wako Pure Chemical Industries, 98%) 2.04 mmol and titanium oxide TiO ₂ (P25) (Degussa, 99.5%) 2.04 mmol, which are fine particle raw materials, After mixing with pure water, the mixture was mixed with the sodium hydroxide solution to prepare a mixed solution.
Subsequently, rhodium chloride RhCl ₂ (manufactured by Wako Pure Chemicals, 95%) was mixed with the mixed solution so as to have a content described in Table 1 below, thereby preparing a reaction solution having a pH of 13.5 or more. The amount of titanium oxide was appropriately adjusted so that the total of the titanium raw material and the rhodium raw material was 2.04 mmol. In Comparative Example 1, rhodium chloride was not added.

<Reaction process>
Next, the three-necked flask was placed in an aluminum block thermostatic bath with a stirrer (Synflex, BBS-108RB), stirred at a stirring speed of 500 rpm, set temperature was 150 ° C., and refluxed for 6 hours after boiling. The temperature of the reaction liquid at the time of boiling was 100 ° C.

<Purification / Drying Process>
After completion of the reaction, the reaction solution containing the reaction product was divided into 4 centrifuge tubes each in 40 mL, and the following purification steps were performed 3 times.
Specifically, after the centrifugation, the supernatant of the centrifuge tube was discarded, and a purification step of sonicating the suspension obtained by adding 20 mL of pure water was performed three times. The centrifugation conditions in the purification step were all 10 minutes at 10,000 rpm.
After the third purification step, 5 mL of pure water is added to each centrifuge tube containing the precipitate after discarding the supernatant to prepare a suspension in which the precipitate is dispersed, and then the suspension is placed in a petri dish. It was transferred to a hot plate and dried at 100 ° C. for 1 hour.

<Baking process>
Next, the dried powder was placed in a crucible and subjected to a baking treatment in a muffle furnace at a heating rate of 7 ° C./min and 800 ° C. for 7 hours.
After the firing, the rate of temperature decrease was about 5 ° C./min. After confirming that the temperature of the muffle furnace had returned to room temperature, the powder sample was collected and then pulverized and mixed using a pestle and mortar to obtain strontium titanate fine particles. Prepared.

[Comparative Example 4]
Using a pestle and mortar, 0.408 mmol of strontium carbonate, rutile, 0.404 mmol and 4 μmol of rhodium oxide (Wako Pure Chemicals, 95%), which are the raw materials for the fine particles, were used. After pulverization and mixing, the mixture was baked in a crucible at 900 ° C. for 1 hour.
Next, after collecting the powder sample after firing, it was pulverized and mixed, and fired in a crucible at 1000 ° C. for 7 hours.
Subsequently, the powder sample after firing was collected and then pulverized and mixed to prepare strontium titanate fine particles.

[Comparative Example 5]
A strontium titanate fine particle, ie, a dried suspension in which a precipitate was dispersed, was prepared in the same manner as in Example 1 except that the firing treatment was not performed.

<TEM photo>
A TEM photograph of the strontium titanate fine particles prepared in Example 1, Comparative Example 1, Comparative Example 4, and Comparative Example 5 is shown in FIG.
As shown in FIG. 1, it was found that the strontium titanate fine particles prepared in Comparative Example 4 were not in a cubic shape.

<Photocatalytic performance>
0.01 g of each synthesized strontium titanate fine particle was suspended in 120 ml of a 10% methanol aqueous solution in a photocatalytic ability evaluation apparatus (glass reaction vessel) and ultrasonically dispersed.
Subsequently, in order to attach platinum Pt as a metal promoter to the surface of the strontium titanate fine particles, a hexachloroplatinic acid hexahydrate solution (1 g / 100 ml) was mixed to 0.3 mass%.
Next, the reaction vessel was connected to a glass gas pipe. The gas pipe is connected to a gas chromatography device (Shimadzu Corporation, GC2014), and can perform gas component analysis, pressure measurement with a digital manometer, evacuation, air filling, and the like.
Next, the reaction vessel was evacuated by a vacuum pump while stirring with a stirrer at a stirring speed of 300 rpm, and continued until the display on the digital manometer became 3 kPa or less. Thereafter, argon filling and evacuation were repeated three times, and pretreatment was performed until the amount of mixed oxygen became 1000 ppm or less.
Thereafter, while continuing stirring, a xenon lamp (Eagle Engineering) was irradiated with an intensity indicating current display 2A, and a photocatalytic reaction was started through a 420 nm cut filter. After 15 minutes, 30 minutes, 60 minutes, 90 minutes, and 120 minutes after the reaction, 2.5 ml of gas was sampled with a gas chromatography apparatus, and the amount of hydrogen was measured.
As a result, a hydrogen generation amount of 70 μmol / h or more was evaluated as “A”, a hydrogen generation amount of 50 μmol / h or more and less than 70 μmol / h was evaluated as “B”, and a hydrogen generation amount of 35 μmol / h or more and less than 50 μmol / h was evaluated. Evaluation was made as “C”, and those less than 35 μmol / h were evaluated as “D”.
FIG. 8 shows the hydrogen generation amount of the fine particles produced in Examples 1 to 3 and Comparative Examples 1 to 3.

From the results shown in Table 1 and FIG. 1 and FIG. 8, it is understood that the fine particles having a rhodium content of 1 mol% or less are inferior in photocatalytic performance even if they have a cubic shape with exposed {100} faces ( Comparative Examples 1 and 2) It was found that the fine particles having a rhodium content exceeding 7 mol% were inferior in photocatalytic performance even if they had a cubic shape with exposed {100} faces (Comparative Example 3).
Moreover, it turned out that the microparticles | fine-particles which are prepared by a solid-phase system and do not have a cube shape are inferior in photocatalytic performance (comparative example 4).
In addition, even if the rhodium content is 3 mol% and the cubic shape has an exposed {100} plane, fine particles having a lattice constant A exceeding 3.910 mm are inferior in photocatalytic performance. It was.

On the other hand, strontium titanate fine particles containing a specific amount of rhodium, having a cubic shape with exposed {100} faces, and having a lattice constant A of a predetermined value all have good photocatalytic performance. (Examples 1-3).
In particular, from the comparison results of Examples 1 to 3 and the result of FIG. 8, it was found that the photocatalytic performance is extremely good when the rhodium content is 3 to 4 mol%.

Claims (11)

  Strontium titanate fine particles having a cubic shape in which rhodium is contained in a titanium site in an amount of more than 1 mol% and not more than 7 mol%, the {100} plane is exposed, and a lattice constant A is more than 3.906 ３ and not more than 3.910 Å.   A photocatalyst comprising the strontium titanate fine particles according to claim 1.   A hydrogen / oxygen generating photocatalytic system using the strontium titanate fine particles according to claim 1. A method for producing strontium titanate fine particles for preparing the strontium titanate fine particles according to claim 1,
A reaction solution preparation step of preparing a reaction solution in which a strontium raw material, a titanium raw material and a rhodium raw material are added in an alkaline aqueous solution;
A reaction step of heating the reaction solution to 80 ° C. or higher under atmospheric pressure to react the reaction solution to prepare fine particles in the alkaline aqueous solution;
A method for producing strontium titanate fine particles, comprising: subjecting the fine particles to a firing treatment at a temperature of 200 ° C. to 1000 ° C. to prepare strontium titanate fine particles.   The manufacturing method of the strontium titanate microparticles | fine-particles of Claim 4 with which the said titanium raw material contains a titanium oxide.   The method for producing strontium titanate fine particles according to claim 5, wherein the titanium oxide is titanium dioxide.   The manufacturing method of the strontium titanate microparticles | fine-particles of Claim 6 whose said titanium dioxide is a titanium oxide containing 70 mass% or more of anatase type titanium dioxide.   The strontium raw material includes at least one selected from the group consisting of strontium acetate, nitrate, carbonate, chloride, sulfide, and hydroxide, according to any one of claims 4 to 7. Method for producing strontium titanate fine particles.   The method for producing strontium titanate fine particles according to any one of claims 4 to 8, wherein a temperature at which the reaction solution is reacted in the reaction step is 90 ° C or more and not more than a boiling point of the reaction solution.   The method for producing strontium titanate fine particles according to any one of claims 4 to 9, wherein the pH of the reaction solution is 13 to 15.   The method for producing strontium titanate fine particles according to any one of claims 4 to 10, wherein the rhodium raw material is water-soluble.