Chemistry of inorganic compounds – Oxygen or compound thereof – Metal containing
Reexamination Certificate
2001-08-03
2004-08-03
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Oxygen or compound thereof
Metal containing
Reexamination Certificate
active
06770257
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a process for producing anatase titanium oxide particularly having high photocatalytic activity as one of photocatalysts used, for example, as environment cleaning materials, for example, for the removal of harmful materials, the deodorisation/decomposition of offensive odor substances, antifouling, and sterilization, and more particularly to a process for producing anatase titanium oxide powder having high photocatalytic activity and large specific surface area, and a process for producing anatase titanium oxide slurry, containing the anatase titanium oxide powder, with excellent stability and dispersibility.
The present invention also relates to a process for producing a coating material of titanium oxide as one of photocatalysts used, for example, as environment cleaning materials, for example, for the removal of harmful materials, the deodorisation/decomposition of offensive odor substances, antifouling, and sterilization. More particularly, the present invention relates to a process for producing a titanium oxide coating material which, particularly when anatase titanium oxide having high photocatalytic activity is coated at a low temperature, the photocatalytic activity can persist for a long period of time, and, at the same time, can maintain high adhesion strength between the coating and the member as the substrate for a long period of time, does not deteriorate the appearance of the member by virtue of excellent transparency of the coating, and enables the production of a binder for improving adhesion strength.
BACKGROUND ART
Photocatalysts are materials that, through radicals (hydroxy radicals, superoxide anions, etc.) generated upon the application of ultraviolet light onto the surface thereto, can function to perform, for example, the removal of harmful materials (for example, benzene, dioxin, and volatile organic compounds), th deodorisation/decomposition of offensive odor substances (substanc s regulated by the Offensive Odor Control Law), antifouling, and sterilization.
In recent years, an attempt to utilize the above functions by coating the photocatalyst onto the surface of objects has been made. A number of oxides can be utilized as photocatalysts. Among them, titanium oxide is in many cases used as one of the photocatalysts, and, among others, anatase titanium oxide is superior from the viewpoints of both the function and safety.
Specifically, titanium oxide is classified into three crystal forms, i.e., anatase, rutile and hrookite forms, and amorphous form. Among them, anatase titanium oxide has the highest photocatalytic activity.
Anatase titanium oxide powder can be produced by a gas phase process and a liquid phase process. For each process, conventional techniques will be described.
Degussa P-25 manufactured by Nippon Aerosil Co., Ltd. is representative anatase titanium oxide produced by the gas phase process. In this process, titanium oxide powder having a specific surface area of 40 m
2
/g (BET method) can be produced by hydrolysis of titanium chloride in an oxygen atmosphere at a high temperature of 1000° C.
Further, there is also a report such that anatase titanium oxide is produced by a CVD (chemical vapor deposition) process at a controlled furnace temperature of 600 to 800° C. (Kagaku Kogaku Ronbunshu (J. Chem. Eng. Japan), Vol. 16, No. 3, 584-587, May 1990).
Sol-gel process, HyCOM (hydrothermal crystallization in Organic Media), and sulfuric acid process have been proposed for the production of anatase titanium oxide by the liquid phase process.
According to the sol-gel process, titanium oxide is produced from an alkoxide in the same manner as in the production of silica, and the sol-gel process should involve two steps, i.e., the step of preparing titanium hydroxide by hydrolysis and the step of sintering wherein titanium hydroxide is polycondensed by heating to give titanium oxide. Further, both the steps are carried out under the atmospheric pressure (for th sol-gel process, see, for example, “The Science of Sol-Gel Method,” 8-15, published in July 1988 by AGNE SHOFU PUBLISHING INC.)
When anatase titanium oxide is prepared by the sol-gel process, the above step of sintering is indispensable, and the heating temperature for sintering should be in the range of 300 to 700° C. The reason why the heat treatment in the specific temperature range is necessary is as follows. When the heat treatment is carried out at a temperature below 300° C., titanium oxide remains unchanged from the amorphous form. On the other hand, when the heat treatment is carried out at a temperature above 700° C., anatase titanium oxide is converted to a crystal form having lower photocatalytic activity, i.e., rutile form.
In HyCOM, moisture contained in gas or water vapor produced from a separate water tank is fed, as water necessary for hydrolysis of an alkoxide, into a solvent with the titanium alkoxide being dissolved therein by the application of pressure (10 kg/cm
2
G), thereby producing titanium oxide. In this case, the solvent with the alkoxide being dissolved therein and water are placed separately form each other in the apparatus. That is, water is absent in the starting material.
Titanium oxide produced by HyCOM is highly heat-resistant anatase titanium oxide which, for example, even after baking at 900° C., maintains the anatase form and has a specific surface area of 40 m
2
/g (J. Mater. Sci. Lett., 15, 197 (1996)).
As described in Japanese Patent Laid-Open No. 171408/1995, in the sulfuric acid process, an acidic titanium sol, prepared by heating and hydrolyzing titanium sulfate, is adjusted to pH 7 by the addition of sodium hydroxide, and filtration and washing are then carried out to prepare a crystal. Subsequently, water is added to the resultant titanium oxide wet cake to prepare titanium oxide slurry. The titanium oxide slurry is adjusted to pH 7 by the addition of sodium hydroxide, followed by hydrothermal treatment in an autoclave at 150° C. for 3 hr. Thereafter, the hydrothermally treated slurry is adjusted to pH 7 by the addition of nitric acid, and is then filtered. The cake is then washed with water, and is dried (110° C., 3 hr) to prepare titanium oxide.
Next, conv ntional production processes of titanium oxide-containing liquid and slurry will be d scribed.
Japanese Patent Laid-Open No. 99041/1996 proposes a production process which comprises the steps of: adding polyethylene glycol or ethylene oxide to a titania sol, prepared, for example, from an alkoxide of titanium and an alcohol amine; coating the mixture onto a substrate; and then heating the coated substrate gradually from room temperature to a temperature of 600 to 700° C. to prepare a thin film of an anatase titanium oxide. In this publication, there is a description to the effect that the baking temperature is preferably 600 to 700° C. This indicates that the step of sintering is necessary for the production of anatase titanium oxide.
Japanese Patent Laid-Open No. 277147/1996 also proposes a coating material produced by the sol-gel process. In this case, the step of baking at 350° C. is provided. Japanese Patent Laid-Open No. 21557/1996 also proposes the use, as a coating material, of a titanium oxide sol, prepared by hydrolysis of a titanyl sulfate, after dilution with water. Also in this case, baking at 300° C. in the air is carried out.
Japanese Patent Laid-Open No. 257360/1996 proposes a production process which comprises the steps of: dispersing previously prepared anatase titanium oxide powder (F-25, manufactured by Nippon Aerosil Co., Ltd.) together with finely divided cellulose in water; and adding polyaluminum chloride as a coagulant to prepare a slurry material.
Regarding the dispersion of powdery anatase titanium oxide into water, for example, there is a report such that metatitanic acid is prepared from ilmenite as a starting material by the sulfuric acid process, and nitric acid is added to the metatitanic acid, followed by the dispersion of titanium oxide in the mixture to improve the dispersion and storage stability of the co
Imura Tatsuya
Terada Seiji
Bos Steven
Kawasaki Jukogyo Kabushiki Kaisha
Oliff & Berridg,e PLC
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