Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Silicon containing or process of making
Reexamination Certificate
2001-07-27
2003-10-14
Silverman, Stanley S. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Silicon containing or process of making
C502S242000
Reexamination Certificate
active
06632771
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to silica gel carrying a titanium oxide photocatalyst in a high concentration that is capable of deodorizing automobiles, rooms, and stockbreeding pens or detoxifying toxic pollutants, of removing NOx in the atmosphere, decolorizing waste water caused by dyeing, preventing the emergence of algae in water tanks, and sterilizing water and air, in which the titanium oxide is contained in a high concentration in the pores of the silica gel, which has an adsorption performance, and relates to a method for preparation thereof.
BACKGROUND OF THE INVENTION
It has been well known that for a photocatalytic reaction of a semiconductor, when powders of a semiconductor are dispersed in a water solution to which a light having the same amount or more of energy as that of the band gap of the semiconductor (light having a wavelength of 400 nm or less) is made to irradiate, electrons and electron holes generated by optical pumping are transferred onto the surface of semiconductor particles and act on an ionic species and a molecular species in the water solution to cause various reactions, such as the decomposition of water. Titanium oxide is exemplified as a representative photocatalyst.
By irradiating a semiconductor by light such as sunlight, or the light of a fluorescent tube, incandescent lamp, black light, lamp, metal-halide lamp, or a cold cathode fluorescent tube, the decomposition and removal of environmental pollutants can be conducted, such as the decomposition and removal of smelly and toxic substances in the air, the treatment of waste water, a water-purifying treatment, or the sterilization of microorganisms in water.
Due to the difficulty of handling fine powders for those uses, suggested in Japanese Patent Early-publication No. 6-65012 is a titanium oxide film photocatalyst that is excellent in water-resistant and heat-resistant properties and in durability. It is obtained by producing a sol of titanium oxide from an alkoxide of titanium, coating the sol on a glass substrate by dip coating, drying and calcining the sol-coated glass substrate to form a transparent titanium oxide film photocatalyst, or by further coating a metal on the photocatalyst by photo-electrodeposition, etc. However, it has in its use big problems, such as that this catalyst can be applied only to an inorganic substance, the shape of which is limited, such as glass, ceramics, etc., and also that since a decomposition reaction by a photocatalyst is caused only on the surface of the photocatalyst, to continuously decompose and remove environmental pollutants a very large area must be provided for the photocatalyst.
DISCLOSURE OF THE INVENTION
Considering these problems, the present invention has been achieved. This invention provides silica gel carrying a photocatalyst in a high concentration and a method for preparation thereof. The silica gel of this invention has an improved performance in decomposing environmental pollutants such as smelly or toxic substances in the air, or organic solvents, agricultural chemicals, etc., contained in water, by providing a concentration gradient of titanium oxide such that the concentration near the surface of the silica gel is high and the concentration of the central part is low, said silica gel also having excellent properties of safety, economic efficiency, stability, and water-resistance (even if it is put into water it does not break apart).
This invention provides silica gel carrying a titanium oxide photocatalyst in a high concentration characterized in that, to achieve the above objects, the amount of the titanium oxide contained in the pores near the surface of the silica gel is 7-70% by weight, provided that a concentration gradient is provided such that the amount is 1.5 times or more than the amount of the titanium oxide contained in the pores near the center of the silica gel, said silica gel having an average pore diameter of 6-100 nm. This invention also provides a method for the preparation of silica gel carrying a titanium oxide photocatalyst in a high concentration characterized by mixing silica gel with a titanium-containing solution in the same volume as or less than the total pore volume of the silica gel, to have the silica gel contain the titanium-containing solution, followed by heating and calcining, said silica gel having an average pore diameter of 6-100 nm.
The silica gels used in the present invention are amorphous silicon dioxides having an average pore diameter of 6-100 nm, said silica gel not breaking apart even if it is immersed in water or an organic solvent. They can be obtained by a known method such as is described in Japanese Patent Publication No. 7-64543, etc. In this method a silica hydrogel obtained by neutralizing an alkaline silicate water-solution is dried at 100-1,000° C. by superheated steam to result in a silica xerogel, to adjust the sizes of the colloidal particles that constitute the silica gel, or by obtaining Q-6, Q-10, Q-15, Q-30, or Q-50, which are commercially available as series of CARiACT Q (produced by Fuji-Silysia Chemical Ltd.). Those silica gels greatly differ from silica gel generally used as a desiccant for packaging (an A-type or B-type standardized product under JIS Z 0701, desiccant for packaging). Since they have good mechanical strength, adsorption performance, and the property of not breaking apart even if they are immersed in water or an organic solvent, they are also used for chromatography or catalytic supports. They are applicable in various forms, from fine powders to granules. The desiccant as described above that is used for packaging has a critical defect in that when it is immersed in water or an organic solvent it crumbles. Thus it cannot be used for the present invention.
In the present invention, when the average pore diameter of the silica gel used is less than 6 nm the titanium-containing solution clogs the pores near the surface of the silica gel and thus the solution does not sufficiently impregnate the internal parts. Also, when silica gel has an average pore diameter of more than 100 nm, it is difficult to produce it and it is very expensive, and thus it is undesirable.
The crystalline structure of the titanium oxide to be contained in the pores of the silica gel of the present invention is desirably anatase, which has a high photocatalytic activity.
The silica gel carrying a titanium oxide photocatalyst in a high concentration of the present invention is one in which the amount of titanium oxide contained in pores near the surface of silica gel is 7-70% by weight, and a concentration gradient is provided such that the amount is 1.5 times or more than the amount of the titanium oxide contained in the pores near the center of the silica gel. The photocatalytic reaction of the titanium oxide occurs with an ultaviolet light having a wavelength of 400 nm or less. For an ultraviolet light having a wavelength of 380 nm, about 60% of the light is absorbed in a titanium oxide film having a thickness of 1 &mgr;m, and the titanium oxide that is involved with the photocatalytic reaction is the only one present near the surface of the silica gel. Since a titanium-containing solution is very expensive, the silica gel carrying a titanium oxide photocatalyst in a high concentration of the present invention is economically very excellent. The silica gel of this invention contains titanium oxide in a high concentration in the pores near its surface, while the titanium oxide is contained in a lower concentration or none at all in the pores of the inner part, where ultraviolet light does not reach and thus no photocatalytic reaction occurs. The titanium oxide content contained in the pores near the surface of the silica gel can be determined by EPMA, which can measure to a depth of 1-2 &mgr;m from the surface of the silica gel. The titanium oxide content contained in the pores near the center of the silica gel can be determined by EPMA by measuring the cross section obtained by cracking the silica gel.
The titanium oxide-containing solution used in the pres
Maekawa Masaaki
Shirai Chisako
Takeuchi Satoshi
Yamada Zenichi
Finnegan Henderson Farabow Garrett & Dunner LLP
Silverman Stanley S.
Sintokogio Ltd.
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