JP2000290533A - Titanyl oxalate coating solution and preparation of photocatalytic element using this - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a shelf-stable coating solution which contains titanyl oxalate which forms an unsupported film of anatase-type titanium oxide and does not limit target substrates to acid-resistant materials, and a process for forming in anatase-type titanium oxide photocatalytic film having an excellent hardness, substrate adhesion and photocatalytic activity from this coating solution. SOLUTION: A coating solution has from 1 to 13 wt.% titanyl oxalate, calculated as TiO2, and has a pH adjusted within a range of from 3.8 to 5.0 using aqueous ammonia. A substrate is coated or impregnated with this coating solution and calcined at from 400 to 800 deg.C to form a photocatalytic layer on the surface of the substrate. Preferably, the substrate is made of glass or ceramins.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a titanyl oxalate coating solution for forming a titanium oxide photocatalyst layer on the surface of a support by coating or impregnating on a substrate and firing.

[0002]

2. Description of the Related Art Titanium oxide has a wavelength of 3 or more in the presence of oxygen and water having an energy greater than its band gap.
It is well known to perform a photocatalytic reaction that oxidizes and decomposes substances when irradiated with light of 80 nm or less. In recent years, studies for purifying the environment utilizing this phenomenon have been actively conducted.

Specifically, photodecomposition of harmful or odorous substances such as NO _x , SO _x , ammonia, aldehydes, amines and mercaptans in exhaust gas of automobiles and factories,
Photodecomposition of living pollutants such as oil, tar and tobacco tar, photodecomposition of dyes and pastes contained in industrial wastewater, killing of harmful microorganisms such as bacteria, mold and algae.

[0004] Since titanium oxide itself is a solid powder or a crystal, it must be immobilized and supported on a support or substrate in order to be used in the above-mentioned applications. As one method for this immobilization, there is a method using a coating solution containing a photocatalyst or a component which becomes a photocatalyst by heating, and various coating solutions for that purpose are also known.

Of these, aqueous solutions of silicon tetrachloride and titanyl sulfate and titania sol peptized with hydrochloric acid or nitric acid are inconvenient to handle because the liquid itself is strongly acidic, and the support is also limited to acid-resistant materials. In addition, when these coating liquids are applied and heated to form a coating film, harmful gases are generated.

It has been proposed to use a sol of a hydrolyzate of alkoxytitanium for forming a titanium oxide single film (Japanese Patent Application Laid-Open No. 7-100378). However, this sol is expensive in its raw material, and is unstable in storage. In addition, if a relatively thick titanium oxide film is formed from this sol at one time, cracks occur during firing, and the film easily peels off from the support. Application and baking must be repeated.

A titanium oxalate complex obtained by reacting oxalic acid with titanium hydroxide at a molar ratio of 1: 2 or more is water-soluble and generates titanium oxide by thermal decomposition. For use as a coating liquid for forming a polymer (JP-A-54-98).
716). This liquid is applied to, for example, a glass plate
When heated to 0 ° C. or higher, a titanium oxide film having adhesion to the substrate is formed. However, since the coating solution is strongly acidic, the material of the support is limited to an acid-resistant material.
For example, alumina cannot be used.

Therefore, one of the objects of the present invention is to include titanyl oxalate as a titanium compound which changes to titanium oxide by thermal decomposition, the target substrate is not limited to acid-resistant materials, and is stable in storage. Another object of the present invention is to provide a coating liquid that forms a photocatalytic film having high hardness and high substrate adhesion. Other problems and benefits of the present invention will become apparent as the description proceeds.

[0009]

SUMMARY OF THE INVENTION The above and other problems include:
According to the present invention, a solution is provided by providing a coating solution for forming a photocatalyst layer, which has a titanyl oxalate concentration of 1 to 13% by weight and is adjusted to pH 3.8 to 5.0 with aqueous ammonia. Is done.

The present invention also provides a method for producing a photocatalytic element using the above coating liquid. This method is characterized in that the coating liquid is applied or impregnated on a support, and the titanium oxide oxalate is thermally decomposed by baking at a temperature of 400 to 800 ° C. to form a titanium oxide photocatalyst layer on the surface of the substrate. I have.

[0011]

DETAILED DESCRIPTION Titanyl oxalate is a known substance, and its aqueous solution can be obtained, for example, as follows. That is, an aqueous solution of titanyl sulfate or titanium tetrachloride is neutralized with aqueous ammonia, and the precipitate is filtered and washed with water to produce an amorphous water cake of orthotitanic acid. Oxalic acid (dihydrate) at a molar ratio of at least twice the TiO ₂ content is added to the cake and, when stirred, the cake becomes a pale yellow transparent titanyl oxalate solution. Excess oxalic acid can be allowed to settle on standing, filtered or decanted off.

The obtained oxalic acid aqueous solution is so acidic that it cannot be measured by a pH meter, and cannot be applied to a non-acid-resistant substrate such as alumina as it is. According to the present invention, the pH of the aqueous solution is adjusted to 3.8 to 3.8 using aqueous ammonia.
Neutralize to 5.0, preferably pH 4.0-4.5. At this time, it is important for the present invention that ammonia water is used for neutralization and that the pH after neutralization is in the above range. For example, other neutralizing agents such as sodium hydroxide remain in the coating film after calcination and, in some cases, combine with TiO ₂ to adversely affect the photocatalyst such as a decrease in catalytic activity. Is volatilized and does not remain in the coating film. If the pH is within the above range, the handling equipment for the substrate and the coating liquid does not need to be substantially acid-resistant. However, as the pH increases, oxalic acid coordinated to the titanium ion dissociates, and the pH increases.
Above 6.0, such pH range should be avoided because the formation of titanium hydroxide adversely affects the stability of the solution and the performance of the coating film.

The aqueous solution of titanyl oxalate pH-adjusted with ammonia water in this manner has a solid content concentration of 1 to 1 in terms of TiO _2.
It can be adjusted to 3% by weight. In order to form a film as thick as possible in one application, this concentration of the coating liquid is preferably higher, but the solubility at room temperature may be limited to 13% by weight. The concentration can be adjusted by diluting the coating solution with water or, conversely, concentrating the coating solution by evaporation of water.

The substrate must be made of a material that can withstand the calcination temperature described below, such as glass, ceramics containing alumina, cement, slate, gypsum, stone, activated carbon, etc. The shape is selected from arbitrary shapes suitable for the use of the photocatalyst, such as a fibrous shape and a honeycomb shape.

The method of applying or impregnating the substrate with the coating liquid may be any method suitable for the shape and dimensions of the substrate. For example, a brush coating, a spray method, a bar coater method, an applicator method, a spin coating, a dip method, and the like. In the case of spin coating or spraying, it is preferable to set the concentration slightly lower than in other cases.

After applying or impregnating the coating liquid,
The substrate is dried at a temperature of 100 ° C. or more, and 400 to 800
C., preferably at a temperature of 500 to 600.degree. By this baking, oxalic acid and ammonia volatilize, and a layer of anatase-type titanium oxide having photocatalytic ability is formed on the surface of the substrate or the internal cavity. The time required to completely volatilize oxalic acid and ammonia at firing temperatures in the above range is a function of temperature, but generally requires 2-5 hours. However, in a high temperature range exceeding the above range, there is a risk that the form of titanium oxide is transformed into a rutile form having low catalytic activity, so that calcination in such a high temperature range should be avoided.

By using the coating solution of the present invention, it is possible to form an anatase-type titanium oxide photocatalytic layer having a thickness of about 1 to 6 μm by one coating or impregnation. This film is transparent, has high hardness, and thus has excellent adhesion to a substrate.

The substrate on which the anatase-type titanium oxide photocatalyst is immobilized in the form of such a film or layer is useful as a photocatalytic element for decomposing and purifying harmful substances by utilizing the photocatalytic reaction of titanium oxide. For example it is possible to use NO _X decomposition or glazing or outdoor building material having a self-cleaning function, malodorous material decomposition filter to be incorporated into the air purifier, the production of such antimicrobial tiles used in hospitals or bathroom. Elements based on glass beads, alumina beads, activated carbon and the like can be used for light purification of surfactants, dyes, pastes and the like contained in domestic wastewater or industrial wastewater.

[0019]

The present invention will be described in more detail with reference to the following non-limiting examples.

Example 1 An aqueous solution of titanyl sulfate (an aqueous solution obtained by dissolving a TM crystal (trade name) manufactured by Teika Co., Ltd. in pure water at a concentration of 5% by weight as TiO ₂ ) was mixed with reagent grade ammonia water (Katayama Chemical Industry Co., Ltd.) Made,
(NH ₃ concentration 25% by weight) to neutralize to pH 7.5, and precipitate was filtered and washed with twice the amount of pure water to obtain amorphous orthotitanic acid. The content of titanium in terms of TiO _{2 in} this cake was 14% by weight.

57.1 g of this cake (8 g in terms of TiO ₂ ) was placed in a 100 ml glass beaker, and 27.9 g of oxalic acid dihydrate (reagent grade, manufactured by Katayama Chemical Industry Co., Ltd.) was used.
(A molar ratio of 2.2 times the amount of TiO ₂ ) was added, and the temperature was raised to 70 ° C. while stirring to completely dissolve the cake. The solution was cooled to room temperature and allowed to stand, and the precipitated excess oxalic acid was removed by filtration. 82.4 g of this filtrate
A mixture of 14 g of aqueous ammonia (same as above) and 3.6 g of pure water was gradually added with stirring, and the pH of the solution was adjusted to 4.0 to obtain a coating solution having a concentration of 8% by weight in terms of TiO ₂ . .

This coating solution was applied to a slide glass plate (Matsunami Glass Co., Ltd., size 8 × 2.5 cm, thickness 1.3).
mm) using a bar coater # 22 (manufactured by Daisuke Kikai Co., Ltd.), dried at 110 ° C. for 30 minutes, and then dried for 60 minutes.
It was baked at 0 ° C. for 2 hours to prepare a coated plate.

Example 2 Except that the pH of an aqueous solution of titanyl oxalate was adjusted to 5.0 with a mixture of 14.5 g of aqueous ammonia and 3.1 g of pure water, the concentration in terms of TiO _{2 was} 8 wt. % Of the coating liquid was obtained.

Example 3 A mixture of 7 g of ammonia water and 51.8 g of pure water was added to 41.2 g of the titanyl oxalate solution of Example 1 to obtain a pH 4.0 solution.
It was adjusted to 0 to obtain a transparent coating solution having a concentration of 4% by weight in terms of TiO ₂ .

Example 4 85.7 g of the orthotitanate cake in Example 1
(Equivalent to 12 g of TiO ₂ ) and 41.8 of oxalic acid dihydrate
g (molar ratio to TiO ₂ of 2.2), and removing excess oxalic acid to obtain an aqueous solution of titanyl oxalate 123.7.
g was obtained. 21 g of aqueous ammonia was added to this solution (T
iO ₂ concentration in terms 8.3 wt%), evaporating the water in a 110 ° C. oven and concentrated, pH 4.0, to give the terms of TiO ₂ concentration of 12% by weight of the coating solution.

Example 5 The firing temperature of a glass plate coated with a coating liquid was 450
Same as Example 1 except that the temperature was changed to ° C.

Example 6 The firing temperature of a glass plate coated with a coating liquid was 750.
Same as Example 1 except that the temperature was changed to ° C.

Comparative Example 1 In Example 1, the amount of aqueous ammonia added to the aqueous solution of titanyl oxalate was changed to 15.0 g and the amount of pure water to 2.6 g to obtain a coating liquid having a concentration of 8% by weight in terms of TiO _2. Was. The pH of this solution was 6.0, and titanium oxide was precipitated and became cloudy.

Comparative Example 2 In Example 1, the amount of ammonia water added to the aqueous solution of titanyl oxalate was changed to 13.6 g and the amount of pure water was changed to 4.0 g to obtain a coating solution having a concentration of 8% by weight in terms of TiO _2. Was. This solution had a pH of 3.5 and was transparent.

COMPARATIVE EXAMPLE 3 In Example 4, an aqueous solution of titanyl oxalate having a concentration of 8.3% by weight in terms of TiO ₂ after addition of aqueous ammonia was replaced with TiO 2.
_It was concentrated to a concentration of ₂ % by weight of 14% by weight. Although the pH of this coating solution was 4.0, many oxalate crystals were precipitated.

Comparative Example 4 The firing temperature of the glass plate to which the coating liquid was applied was 900
Same as Example 1 except that the temperature was changed to ° C.

Comparative Example 5 31.5 g of reagent grade titanyl ammonium oxalate (manufactured by Katayama Chemical Co., Ltd., 25.4% by weight in terms of TiO ₂ ) was stirred and dissolved in 68.5 g of pure water, and the concentration in terms of TiO _{2 was} 8% by weight.
Thus, 100 g of an aqueous solution of titanyl ammonium oxalate was obtained. This solution had a pH of 2.0 and was transparent.

Storage stability of coating solution and substrate decay
Food habits 1. Storage stability 100 g of the coating liquid is placed in a glass container with a lid,
It was allowed to stand at 25 ° C. for 2 months, and the presence or absence of sediment was visually evaluated. 2. Substrate Corrosion 100 g of the coating solution and 1 g of granular aluminum oxide (manufactured by Katayama Chemical Industry Co., Ltd.) were mixed, placed in a glass container with a lid, and allowed to stand at 50 ° C. for 12 hours. Thereafter, the particulate aluminum oxide was recovered by filtration, washed with water, dried and weighed. The substrate corrosion was evaluated based on the weight loss rate at the beginning.

Performance Evaluation of Photocatalytic Film Method of forming coating film A coating solution was applied to a slide glass plate as in Example 1, dried, and dried at 450 ° C. in Example 5, 750 ° C. in Example 6, and 900 ° C. in Comparative Example 4.
° C, and 600 ° C in other Examples and Comparative Examples. 2. Adhesion of Coating Film The adhesiveness of the coating film was evaluated according to the Gobang tape method according to JIS K5400. When the peeled portion of the coating film after peeling off the tape was less than 10% of the total gross pattern, it was regarded as good, and when it was 10% or more, it was regarded as bad. 3. Film hardness was evaluated by pencil hardness according to JIS K5400. 4. Photocatalytic activity The coated plate was placed in a glass container having an inner volume of 400 ml, and 10 ppm of NO gas was sealed therein. This container is placed in a high-pressure mercury lamp (H-400- mercury lamp activator for physics and chemistry manufactured by Wako Electric Co., Ltd.).
A / B, Lamp Toshiba H-400F, Lamp power consumption 40
0 W), and finely adjusted so that the irradiation intensity of the mercury lamp was 3.7 mW / cm ² . Thereafter, the coated plate was irradiated with a high-pressure mercury lamp for 10 minutes while rotating the glass container. The residual NO gas concentration after the irradiation was measured using a gas detector (GV-100S detector tube No. 11 manufactured by GASTEC).
L), and the photodegradation rate was determined by the following equation. Photolysis rate (%) = (initial gas concentration−residual gas concentration) / initial gas concentration × 100 The results are shown in Tables 1 and 2.

[0035]

[Table 1]

[0036]

[Table 2]

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Ryoji Masuno 1-47, Funamachi, Taisho-ku, Osaka City, Osaka Prefecture F-term (in reference) 4J038 AA012 EA011 HA212 HA216 JA43 KA04 PA19 PC03 PC03

Claims (3)

[Claims] 1. It has a titanyl oxalate concentration of 1 to 13% by weight in terms of TiO ₂ and a pH of 3.8 to 3.8 with ammonia water.
A coating solution for forming a photocatalyst layer, which is adjusted to a range of 5.0. 2. A photocatalytic element, wherein a titanium oxide photocatalyst layer is formed on a surface of a substrate by applying or impregnating the coating solution of claim 1 on a substrate and firing at a temperature of 400 to 800 ° C. Production method. 3. The method according to claim 2, wherein the substrate is made of glass or ceramic.