JP7206056B2 - photocatalyst - Google Patents

Description

The present invention relates to photocatalysts.

Titania (titanium oxide) is used as a photocatalyst material. Generally, titania powder is dispersed in a solvent and used.

J. Phys. Chem. B 2003, 107, 14336-14341

As for titania for photocatalysts, many of them have high crystallinity, and the higher the crystallinity, the higher the refractive index of titania.

On the other hand, the smaller the particle size, the more likely it is to agglomerate in water or alcohol, which makes it difficult to apply uniformly and ultimately does not provide transparency. In addition, even if applied, the smaller the average particle size, the greater the shrinkage due to drying, so that cracks and peeling are more likely to occur. Moreover, when it is applied to resin, it deteriorates the resin.

On the other hand, when an organic dispersant is used, there is a problem that the activity of the titania surface is impaired. Further, when dispersed using a strong inorganic acid such as nitric acid or hydrochloric acid, the dispersibility is improved, but there is a problem that the volatilization of the acid corrodes the peripheral devices.

There is also a method of performing a hydrothermal synthesis reaction using a titanium source such as titanium alkoxide and an inorganic strong acid such as nitric acid or hydrochloric acid (Non-Patent Document 1, etc.), but industrially there is a problem of drainage and corrosion of the reactor. Nitric acid and hydrochloric acid often cannot be used because they can cause problems.

Accordingly, an object of the present invention is to obtain titania fine particles having coatability, photocatalytic properties, and transparency that could not be achieved with either large titania particles or small titania particles.

In view of the above purpose, as a result of intensive studies, the present inventors have found that the acyl group is bonded to the surface and the weight loss at 200 ° C. or higher when the temperature is raised by a simultaneous differential thermal thermogravimetric measurement device is 5. It has been found that titania nanoparticles with a weight % or more can solve all of the above problems. After further research, the present invention was completed. That is, the present invention includes the following configurations.
Section 1. A photocatalyst containing titania nanoparticles, wherein the titania nanoparticles have an acyl group bonded to the surface, and the weight loss at 200 ° C. or higher when the temperature is raised to 600 ° C. by a simultaneous differential thermogravimetric measurement device is 5% by weight or more, a photocatalyst.
Section 2. The acyl group is bonded to the titanium atom with a group represented by -OCOR (wherein R represents a hydrogen atom, an alkyl group having 1 to 3 carbon atoms, or a hydroxyalkyl group having 1 to 2 carbon atoms) Item 1. The photocatalyst according to item 1.
Item 3. Item 2. The photocatalyst according to item 1, wherein the acyl group is an acyl group derived from at least one organic acid selected from the group consisting of molar carboxylic acids having 1 to 4 carbon atoms and hydroxycarboxylic acids having 2 to 3 carbon atoms.
Section 4. Item 4. The photocatalyst according to Item 3, wherein the organic acid is acetic acid.
Item 5. Item 5. The photocatalyst according to any one of Items 1 to 4, wherein the titania nanoparticles have an average particle size of 1 to 5 nm.
Item 6. Item 6. The photocatalyst according to any one of Items 1 to 5, wherein the titania nanoparticles have a specific surface area of 150 to 500 m ² /g.
Item 7. Item 7. The photocatalyst according to any one of Items 1 to 6, wherein the titania nanoparticles do not contain crystal forms other than anatase.
Item 8. A method for producing a photocatalyst according to any one of Items 1 to 7,
(A) a step of mixing a substance containing titanium, an organic acid and water to obtain a dispersion; and (B) a step of heating the dispersion obtained in step (A) at a temperature higher than 80°C for 1 hour or more. and
In the step (A), the mixing ratio of the titanium-containing substance and the organic acid is 1.5 mol or more of acyl groups in the organic acid per 1 mol of titanium in the titanium-containing substance. Method.
Item 9. Item 9. The production method according to Item 8, wherein the heating conditions in the step (B) are 82° C. or higher for 1.5 hours or longer.
Item 10. Item 10. The production method according to Item 8 or 9, wherein the substance containing titanium is titanium alkoxide, titanium hydroxide or titanium halide.
Item 11. Item 11. The production method according to any one of Items 8 to 10, wherein the concentrations of N, Cl and S elements in the dispersion prepared in step (A) are all 0.01 mol/L or less.
Item 12. Item 12. The production method according to any one of items 8 to 11, wherein the concentration of the inorganic acid in the dispersion prepared in step (A) is 0.01 mol/L or less.
Item 13. Item 13. The production method according to any one of Items 8 to 12, wherein the pH of the dispersion prepared in step (B) is 2 or more and less than 6.
Item 14. Item 8. A photocatalyst dispersion containing 50% by mass or more of water and the photocatalyst according to any one of Items 1 to 7.
Item 15. Item 15. The photocatalyst dispersion according to Item 14, which does not contain an organic dispersant separately from the organic acid.

According to the present invention, it is possible to obtain fine titania particles having coatability, photocatalytic properties, and transparency that could not be achieved with either large titania particles or small titania particles.

As used herein, "contain" is a concept that includes both "comprise," "consist essentially of," and "consist of." In this specification, when a numerical range is represented by A to B, it means A or more and B or less.

As used herein, "titanium oxide" or "titania" refers not only to titanium dioxide ( _TiO2 ) _, but also to titanium trioxide ₍ Ti2O3); titanium monoxide ₍ TiO); ₇ , Ti ₅ O ₉ , etc., which are oxygen-deficient compositions of titanium dioxide. In addition, as typified by terminal OH groups, it may contain groups other than Ti--O--Ti partially derived from the synthesis of titanium oxide. Furthermore, those in which an organic acid or the like is bonded to the terminal OH group are also included.

1. Photocatalyst The photocatalyst of the present invention has an acyl group bonded to the surface, and a titania nanotitanium having a weight loss of 5% by weight or more at 200°C or higher when heated to 600°C by a simultaneous differential thermogravimetric measurement device. Contains particles.

Generally, most of water, inorganic acids, free organic acids, etc. volatilize below 200°C. On the other hand, the titania nanoparticles of the present invention, which have acyl groups bonded to their surfaces, gradually desorb in the range of 200 to 600°C. For example, in the case of an acetyl group, it peaks at about 260°C and gradually desorbs in the range of 200 to 600°C. Thus, since the titania nanoparticles of the present invention have acyl groups bonded to their surfaces, the aggregation of the titania nanoparticles during drying or baking can be suppressed, so that cracks, peeling, etc. are difficult to occur, and the coating properties and transparency are particularly improved. In addition to being excellent, cracks, peeling, etc. can be suppressed, resulting in excellent photocatalytic properties. It is common general knowledge that the photocatalytic properties of photocatalysts deteriorate when acyl groups are present. In spite of this, the photocatalytic properties can also be improved.

In addition, the titania nanoparticles preferably have a large amount of acyl groups bonded to their surfaces. If acyl groups are present on the surface, as mentioned above, they will gradually detach in the range of 200 to 600°C. The weight of the raw material in the above is large. Therefore, the weight loss at 200° C. or higher when heated to 600° C. by a simultaneous differential thermogravimetric analyzer (TG-DTA) is 5% by weight or more, preferably 7 to 20% by weight. At this time, the detailed conditions of the simultaneous differential thermogravimetric analyzer (TG-DTA) are atmosphere: air, heating rate: 3° C./min.

As described above, the titania nanoparticles have an acyl group bonded to the surface. The acyl group is -OCOR (wherein R is a hydrogen atom, an alkyl group having 1-2 hydroxyalkyl groups) is preferably bonded to the titanium atom. In other words, this acyl group is preferably an acyl group derived from an organic acid such as a C1-4 molar carboxylic acid or a C2-3 hydroxycarboxylic acid.

Examples of the alkyl group for R include a methyl group, an ethyl group, and an n-propyl group, and examples of a hydroxyalkyl group include a hydroxymethyl group, a 1-hydroxyethyl group, and a 2-hydroxyethyl group. That is, examples of monocarboxylic acids include formic acid, acetic acid, propionic acid, and butyric acid, and examples of hydroxycarboxylic acids include glycolic acid, lactic acid, and the like.

From the viewpoint of volatility, toxicity and decomposability, R is preferably a hydrogen atom or a methyl group, hydroxymethyl group, 1-hydroxyethyl group, 2-hydroxyethyl group, etc. From the viewpoint of water solubility and odor, methyl groups are preferred. That is, from the viewpoint of volatility, toxicity and decomposability, preferred monocarboxylic acids are formic acid and acetic acid, and preferred hydroxycarboxylic acids are glycolic acid and lactic acid. Acetic acid is particularly preferred from the viewpoint of water solubility and odor. These organic acids can be used alone or in combination of two or more.

The average particle size of the titania nanoparticles is preferably 1 to 5 nm, more preferably 2 to 4 nm. By setting the average particle size of the titania nanoparticles within this range, a film having high photocatalytic properties and high transparency can be formed. In general, when the average particle size is small, the shrinkage during heating is large, so cracks and peeling from the substrate are likely to occur, but the titania nanoparticles of the present invention are materials with excellent coatability despite the small average particle size. be. The average particle size of the titania nanoparticles of the present invention is measured by electron microscope (TEM) observation.

The specific surface area of the titania nanoparticles is preferably 150-500 m ^{2 /g, more preferably 200-400 m 2 /} g. By setting the specific surface area of the titania nanoparticles within this range, the photocatalytic properties can be enhanced. The specific surface area of the titania nanoparticles is measured by the BET method.

In addition, the titania nanoparticles can have concentrations of N, Cl and S elements of 0 to 5000 ppm, particularly 0 to 1000 ppm. By setting the concentrations of the N, Cl and S elements in the titania nanoparticles within this range, corrosion of the substrate can be suppressed. This condition means that impurities derived from acidic titania precursors such as TiCl ₄ and TiOSO ₄ do not exist or are very small. The concentrations of N, Cl and S elements in the titania nanoparticles are measured by WDX (X-ray fluorescence).

Furthermore, the crystal form of the titania nanoparticles is preferably the anatase type. By adopting an anatase type, the photocatalytic properties can be particularly improved. Further, for the same reason, it is preferred that there are no crystal forms other than the anatase form, and that the anatase form is 100%.

Such titania nanoparticles can be adjusted in average particle diameter and specific surface area, and are excellent in transparency and coatability due to their excellent dispersibility. The titania nanoparticles are also excellent in photocatalytic properties. Therefore, the titania nanoparticles are useful as photocatalysts.

2. Method for producing a photocatalyst The photocatalyst of the present invention is
(A) a step of mixing a substance containing titanium, an organic acid and water to obtain a dispersion; and (B) a step of heating the dispersion obtained in step (A) at a temperature higher than 80°C for 1 hour or more. and
In the step (A), the mixing ratio of the titanium-containing substance and the organic acid is 1.5 mol or more of the acyl group in the organic acid per 1 mol of titanium in the titanium-containing substance. can get.

(2-1) Step (A)
In step (A), a specific amount of a substance containing titanium, a specific amount of organic acid and water are mixed to obtain a dispersion.

The substance containing titanium to be used is not particularly limited as long as it is a substance that becomes titanium oxide when heated. That is, the substance containing titanium is preferably titanium oxide and/or a titanium oxide precursor, and specific examples include titanium oxide; titanium hydroxide; titanium alkoxide; titanium halides such as titanium trichloride and titanium tetrachloride (particularly neutralized with a base); metal titanium and the like. These titanium-containing substances can be used alone, or two or more of them can be used in combination. Among these, titanium alkoxide, titanium hydroxide or titanium halide (especially those neutralized with a base) are preferable from the viewpoint of the dispersibility, coatability and photocatalytic properties of the resulting titania, and in particular purity, dispersibility and coatability. Titanium alkoxide is more preferable from the viewpoint of properties and photocatalytic properties.

Titanium alkoxides include titanium tetraisopropoxide, titanium tetra-n-butoxide, titanium tetra-n-propoxide, titanium tetraethoxide and the like, and from the viewpoints of cost, water solubility of by-products, coating properties and photocatalytic properties, , and titanium tetraisopropoxide are preferred.

Depending on the combination of titanium alkoxide and organic acid, the resulting titania may be used as a catalyst to liberate an ester compound that is not soluble in water. However, from the viewpoint of obtaining a uniform dispersion liquid, a combination of an organic acid and a titanium alkoxide that yields an organic acid alkoxide with excellent water solubility is adopted. is preferred.

Titanium halides (titanium tetrachloride, titanium trichloride, etc.) should be neutralized with a base and precipitated from the viewpoint of impurities (halogen), corrosion of reactors during mass production, crystallinity control, coatability and photocatalytic properties. It is preferable to use it after washing. In that case, from the viewpoint of the dispersibility of the resulting titania, it is preferable to use it without drying.

When a solid such as titanium oxide or metallic titanium is used, the average particle size is preferably 100 nm or less, more preferably 50 nm or less. Although the lower limit is not particularly set, it is usually about 1 nm. In addition, when the particle size is large, it can be pulverized in a dry or wet manner using a planetary ball mill, a paint shaker, or the like. The average particle size of solids such as titanium oxide and metallic titanium is measured by electron microscope (SEM or TEM) observation or the like.

The concentration of the titanium-containing substance in the dispersion is preferably 0.01 to 5 mol/L, more preferably 0.05 to 3 mol/L, from the viewpoints of productivity, reaction liquid viscosity, coatability and photocatalytic properties.

The acid used in the reaction is an organic acid, preferably a _{volatile acid} . Carboxylic acid etc. are mentioned.

From the viewpoint of volatility, toxicity and decomposability, preferred monocarboxylic acids are formic acid with n = 0 and acetic acid with n = 1, and preferred hydroxycarboxylic acids are glycolic acid, lactic acid, etc., and from the viewpoint of water solubility and odor. acetic acid is particularly preferred. These organic acids can be used alone or in combination of two or more.

From the viewpoint of dispersibility, coating properties, photocatalytic properties, and cost, the amount of organic acid used is adjusted so that 1.5 mol or more, preferably 2 mol, of COOH groups are contained per 1 mol of titanium in the substance containing titanium. is preferred. The more the organic acid is used, the more the stability over time and the applicability are improved. Although the upper limit is not particularly limited, it is usually 10 mol per 1 mol of titanium in the substance containing titanium.

The concentration of the organic acid in the dispersion is preferably 0.02 to 10 mol/L, more preferably 0.1 to 7 mol/L, from the viewpoints of dispersibility, coatability, photocatalytic properties and cost.

As the reaction solvent, it is preferable to use an aqueous solvent such as water as a main component (specifically, for example, 50% by mass or more), but alcohol or ester may be included during the reaction.

For example, when titanium tetraisopropoxide is used as a raw material, isopropanol is produced by reaction with an organic acid. Heating may also produce isopropyl esters of organic acids. In other words, the alcohol or ester may be introduced into the dispersion liquid obtained in step (A), or may be generated in the system. This alcohol or ester may be removed by heating in an open system at 100° C. or less, may be removed by reduced pressure, or may remain in the reaction solution.

When alcohol is contained in the dispersion liquid, the average particle size of the obtained titania nanoparticles tends to be small, and alcohol may be intentionally added in order to control the average particle size.

In the present invention, inorganic acids (especially strong inorganic acids) such as nitric acid, hydrochloric acid, and sulfuric acid, which are often used in the hydrothermal synthesis reaction of titania nanoparticles, are used to obtain titania nanoparticles in the crystal form of anatase and brookite. Not only is it mixed, but the storage stability of the obtained dispersion is inferior, and it is not used in principle from the viewpoint of corrosion of equipment, impurities, waste water, and the like. However, in the case of improving the dispersibility, uniformity, etc. of the raw material and facilitating handling, it can be used supplementarily within a range that does not impair the effect, for example, in the range of 0.01 mol/L or less. In this case, the concentrations of N, Cl and S elements in the dispersion are all 0.01 mol/L or less.

The pH of the dispersion obtained in the step (A) is preferably 2 or more and less than 6, more preferably 2.1 to 5, from the viewpoints of equipment corrosion, handling safety, and dispersibility.

In step (A), the method for preparing the dispersion is not particularly limited, and the substance containing titanium, the organic acid and water (solvent) may be mixed simultaneously or sequentially. In particular, it is preferable to mix the organic acid and water (solvent) and then add the substance containing titanium while stirring, from the viewpoint of preventing the formation of large lumps due to agglomeration and allowing the stirring to be continued.

(2-2) Process (B)
In step (B), the dispersion obtained in step (A) is heated at a temperature higher than 80°C for 1 hour or more.

Step (B) may be carried out under normal pressure, or may be carried out under pressure in a closed vessel. From the viewpoint of reducing the average particle size of titania nanoparticles, it is preferable to carry out under normal pressure, specifically 0.09 to 0.11 MPa. In addition, when performing under pressure, from the viewpoint of forming a film with high photocatalytic properties and high transparency, the reaction is performed for a short time (for example, about 5 to 30 minutes) at 0.2 MPa or less (0.11 to 0.2 MPa). preferably.

During heating, stirring is preferred from the viewpoint of sufficiently reacting the substance containing titanium, the organic acid, and water. The stirring method is not particularly limited, and a conventional method can be used. The stirring time is preferably 1 hour or longer, more preferably 1.5 hours or longer, from the viewpoint of sufficiently reacting the substance containing titanium, the organic acid and water. Although the upper limit of the stirring time is not particularly limited, it is usually 240 hours.

The heating temperature is higher than 80°C, preferably 82°C or higher. If the heating temperature is 80° C. or lower, cracks are likely to occur, and the coatability is poor and the coating is easily peeled off, making it difficult to form a coating film. Although the upper limit of the heating temperature is not particularly limited, it is usually 120°C when the reaction is carried out under normal pressure.

The pH of the dispersion obtained in the step (B) is preferably 2 or more and less than 6, more preferably 2.1 to 5, from the viewpoints of equipment corrosion, handling safety, and dispersibility.

Thereafter, the photocatalyst of the present invention can be recovered by precipitating and centrifuging the titania nanoparticles by a conventional method. That is, it is possible to obtain a photocatalyst containing titania nanoparticles with a large amount of acyl groups attached to the surface.

3. Photocatalyst Dispersion The photocatalyst dispersion of the present invention can be made more uniform by using the reaction solution that has undergone the above steps (A) and (B) and adding a dispersion step such as ultrasonic dispersion. At this time, in the conventional titania dispersion, a uniform dispersion could not be obtained without the use of a dispersant. Even with this, a dispersion with much better dispersibility than ordinary titania nanoparticles can be obtained. Good dispersibility results in excellent crack resistance of the coating. In addition, as a result of not having to add a dispersant, a dense coating of titania becomes possible.

At this time, in the photocatalyst dispersion of the present invention, the total amount of the photocatalyst dispersion of the present invention is 100% by mass, and the content of water as a solvent is 50% by mass from the viewpoint of ease of coating and film properties of coating. % or more, particularly preferably 60 mass % or more.

It is also possible to remove the photocatalyst of the present invention from the reaction solution and change the solvent. Moisture may be removed from the reaction solution by centrifugation, a filtration membrane, or the like, and replaced with an organic solvent. In that case, it is preferable not to dry the photocatalyst of the present invention from the viewpoint of dispersibility, transparency, and the like.

An alcohol etc. are mentioned as an organic solvent used for a dispersion liquid. Alcohols include aliphatic alcohols having 1 to 6 carbon atoms such as methanol, ethanol and isopropanol, non-aliphatic alcohols such as α-terpineol; butyl carbitol (diethylene glycol monobutyl ether), hexylene glycol (2-methyl -2,4-pentanediol), ethylene glycol-2-ethylhexyl ether, ethylene glycol monomethyl ether, propylene glycol monomethyl ether, and other glycol-based solvents; diols such as hexanediol;

In addition, even if it does not have an OH group, it is sufficient if it has affinity with titania and other solvents (water, alcohol, etc.). triethylene glycol butyl methyl ether, tetraethylene glycol dimethyl ether, diethylene glycol monobutyl ether acetate, diethylene glycol monoethyl ether acetate, diethylene glycol diacetate, triethylene glycol diacetate, tetraethylene glycol diacetate and the like. Among them, diethylene glycol monobutyl ether acetate, tetraethylene glycol dimethyl ether, and the like are preferable from the viewpoint of boiling point and the like.

The viscosity of the photocatalyst dispersion of the present invention is adjusted according to the application. For example, when used for spin coating, dip coating, spraying, etc., the viscosity is low. However, when used for screen printing, it is preferable to further increase the viscosity and suppress the fluidity. The coating film of the present invention thus obtained is a dense coating as described above.

The present invention will be specifically described based on Examples, but the present invention is not limited to these.

[Example 1]
120 g (2 mol) of acetic acid was added to 142.1 g (0.5 mol) of titanium tetraisopropoxide, stirred for 60 minutes, and 538 g of water was added. This dispersion had a titanium tetraisopropoxide concentration of 0.625 mol/L, an acetic acid concentration of 2.5 mol/L, and a pH of 2.2. A large amount of translucent precipitate was generated, but when the mixture was stirred for 60 minutes and then heated, the precipitate was completely dissolved at 70°C. In this dispersion liquid, the concentration of the inorganic acid and the concentrations of the N, Cl and S elements are all 0 mol/L.

After that, when the mixture was stirred at 85°C under normal pressure (0.1 MPa) for 3 hours, a translucent uniform titania dispersion was obtained without using an organic dispersant. Adding ultrasonic dispersion to this dispersion reduced the viscosity and increased the transparency. This dispersion had a water content of 67 mass % and a pH of 2.3. When this dispersion was applied to glass by spin coating and dried, a transparent coating film was obtained.

This dispersion was dried to obtain titania nanoparticles. The BET specific surface area of the titania nanoparticles was measured to be 250 m ² /g. Further, TEM observation revealed that the average particle size was about 3 nm. Further, when the crystallinity of the obtained titania nanoparticles was analyzed by X-ray diffraction, it was found to be 100% anatase type (no other crystal forms were present).

This dispersion was held at 200°C using a moisture meter, and the TG-DTA of the titania nanoparticles was dried until the weight loss disappeared. As a result, the weight loss at 200°C or higher was 10%. This weight loss at 200° C. or higher corresponds to weight loss due to elimination of acetic acid, which is an organic acid. Since most of the liberated acetic acid evaporates below 200°C, the fact that the weight loss above 200°C is 10% indicates that a large amount of acetyl groups, which are acyl groups, are present on the surface of the titania nanoparticles of the present invention in the form of —OCOCH ₃ . suggesting that they are bonded to atoms.

Next, the dispersion was applied (spin-coated) onto a 1 mm-thick glass substrate, dried at 120°C, and the transmittance at 587.6 nm (d-line: a wavelength close to the absorption peak of methylene blue) was measured by ultraviolet/visible spectroscopy. It was 90.9 when measured by a device (Shimadzu Corporation UV3400). A 1 mmol/L solution of methylene blue was dropped onto the substrate, and excess liquid was removed after 10 minutes. The transmittance at 587.6 nm after immersion was measured with an ultraviolet/visible spectrophotometer and found to be 74.9%. After that, when UV light with a peak wavelength of 352 nm was irradiated with a black light, the color disappeared in 5 hours, and the transmittance recovered to 91.2% before the dye immersion.

[Comparative Example 1]
30 g (0.5 mol) of acetic acid was added to 142.1 g (0.5 mol) of titanium tetraisopropoxide, stirred for 15 minutes, and 625 g of water was added. This dispersion had a titanium tetraisopropoxide concentration of 0.625 mol/L, an acetic acid concentration of 0.625 mol/L, and a pH of 2.9. A large amount of translucent precipitate was generated, but when 4 ml of 65% nitric acid was added and heated with stirring for 60 minutes, all the precipitate was dissolved at 50°C. In this dispersion, the inorganic acid concentration is 0.05 mol/L, the N concentration is 0.05 mol/L, and the Cl and S element concentrations are both 0 mol/L.

After that, water was added to the liquid which was stirred at 80° C. for 5 hours under normal pressure (0.1 MPa) to prepare a total of 800 g, and then ultrasonic waves were applied to obtain a translucent uniform titania dispersion liquid. This dispersion had a water content of 62.5% by mass or more and a pH of 0.7.

This dispersion was dried to obtain titania nanoparticles. The BET specific surface area of the titania nanoparticles was measured to be 280 m ² /g. Further, TEM observation revealed that the average particle size was about 3 nm. Further, when the crystallinity of the obtained titania nanoparticles was analyzed by X-ray diffraction, they were mostly anatase type, but a small amount of brookite type was mixed. When this dispersion was applied to a glass substrate and dried, the coating film cracked and fell off from the glass, although the specific surface area and particle size were the same as in Example 1.

[Comparative Example 2]
142.1 g (0.5 mol) of titanium tetraisopropoxide was added while stirring 650 g of an aqueous nitric acid solution having a pH of 0.7. This dispersion had a titanium tetraisopropoxide concentration of 0.625 mol/L and a pH of 1.0. In this dispersion liquid, the inorganic acid concentration is 0.25 mol/L, and the Cl and S element concentrations are both 0 mol/L.

After stirring this dispersion for 1 hour, the temperature was raised to 80° C. under normal pressure (0.1 MPa) and maintained for 8 hours to synthesize a translucent titania dispersion. The final weight was adjusted to 800g. This dispersion had a water content of 80% by mass and a pH of 1.0.

This dispersion was dried to obtain titania nanoparticles. The BET specific surface area of the titania nanoparticles was measured to be 220 m ² /g. Further, TEM observation revealed that the average particle size was about 3 nm. Further, when the crystallinity of the obtained titania nanoparticles was analyzed by X-ray diffraction, they were mostly anatase type, but a small amount of brookite type was mixed. When this dispersion was applied to a glass substrate and dried, the coating film cracked and peeled off from the glass.

In addition, when the raw material titania dispersion was observed after 7 days, precipitation occurred, gelation progressed, and the viscosity increased, and storage stability was also difficult.

[Comparative Example 3]
30 g of acetic acid and 160 g of water were added to 10 g of titania nanoparticles ST-01 (manufactured by Ishihara Sangyo Co., Ltd., specific surface area: 300 m ² /g, average particle size calculated from the specific surface area: 5 nm), and ultrasonic dispersion was performed. No solution was obtained.

When this dispersion was applied onto a glass substrate, the titania film was completely opaque.

[Comparative Example 4]
30 g of acetic acid and 160 g of water were added to 10 g of titania nanoparticles ST-01 (manufactured by Ishihara Sangyo Co., Ltd., specific surface area: 300 m ² /g, average particle size calculated from the specific surface area: 5 nm). Sonication was performed, but a homogeneous solution was not obtained.

When this dispersion was applied onto a glass substrate, the titania film was completely opaque.

[Comparative Example 5]
30 g of acetic acid and 160 g of water were added to 10 g of titania nanoparticles = 25 (manufactured by Nippon Aerosil Co., Ltd., specific surface area: 50 m ² /g, average particle size calculated from the specific surface area: 5 nm), and ultrasonic dispersion was performed. was not obtained.

Next, the dispersion was applied (spin-coated) onto a 1 mm-thick glass substrate, dried at 120°C, and the transmittance at 587.6 nm (d-line: a wavelength close to the absorption peak of methylene blue) was measured by ultraviolet/visible spectroscopy. It was 91.3% when measured by a device (Shimadzu Corporation UV3400). When this dispersion liquid was spin-coated on glass with a thickness of 1 mm, a uniform and translucent coating film was obtained although it was cloudy. Next, this coating film was dried at 120° C., a solution of 1 mmol/L of methylene blue was dropped onto the substrate, and excess liquid was removed after 10 minutes. The transmittance of visible light after immersion was measured by an ultraviolet/visible spectrophotometer and found to be 82.7%. After that, when UV light with a peak wavelength of 352 nm was irradiated with a black light, the transmittance of 587.6 nm after 5 hours was 87.0%, and the transmittance of 587.6 nm after 20 hours was 91.1%, indicating that the color disappeared. It can be understood that the photocatalytic properties are inferior because it took 20 hours to complete.

Claims (12)

A photocatalyst containing titania nanoparticles, wherein the titania nanoparticles have -OCOR (wherein R is a hydrogen atom, an alkyl group having 1 to 3 carbon atoms, or a hydroxyalkyl group having 1 to 2 carbon atoms ) is bonded to the acyl group bonded to the titanium atom, and the titania nanoparticles to which the acyl group is bonded have an average particle size of 1 to 5 nm, and differential thermal thermogravimetric simultaneous A photocatalyst having a weight loss of 5% by weight or more at 200°C or higher when heated to 600°C by a measuring device. 2. The photocatalyst according to claim 1, wherein the acyl group is an acyl group derived from at least one organic acid selected from the group consisting of monocarboxylic acids having 1 to 4 carbon atoms and hydroxycarboxylic acids having 2 to 3 carbon atoms. . 3. The photocatalyst according to claim 2, wherein said organic acid is acetic acid. The photocatalyst according to any one of claims 1 to 3, wherein the titania nanoparticles have a specific surface area of 150 to 500 m ² /g. The photocatalyst according to any one of claims 1 to 4, wherein 100% of the titania nanoparticles are anatase titanium oxide. A method for producing a photocatalyst according to any one of claims 1 to 5,
(A) a step of mixing a titanium alkoxide, titanium hydroxide or titanium halide, an organic acid and water to obtain a dispersion; heating for 1 hour or more (however, in the case of 0.11 to 0.2 MPa, heating for 5 to 30 minutes) , and
In the step (A), the mixing ratio of the titanium-containing substance and the organic acid is 1.5 mol or more of acyl groups in the organic acid per 1 mol of titanium in the titanium-containing substance. Method. 7. The production method according to claim 6, wherein the heating conditions in step (B) are 0.09 to 0.11 MPa for 1.5 hours or longer. 8. The production method according to claim 6 or 7, wherein the concentrations of N, Cl and S elements in the dispersion prepared in step (A) are all 0.01 mol/L or less. 9. The production method according to any one of claims 6 to 8, wherein the concentration of inorganic acid in the dispersion prepared in step (A) is 0.01 mol/L or less. 10. The production method according to any one of claims 6 to 9, wherein the dispersion prepared in step (B) has a pH of 2 or more and less than 6. A photocatalyst dispersion containing 50% by mass or more of water and the photocatalyst according to any one of claims 1 to 5. 12. The photocatalyst dispersion according to claim 11, which does not contain an organic dispersant separately from the organic acid.