Photocatalytic body and method for making same

Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide

Reexamination Certificate

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C502S349000, C501S134000, C423S608000, C423S610000

Reexamination Certificate

active

06429169

ABSTRACT:

TECHNICAL FIELD
This invention relates to a photocatalytic body having a good photocatalytic function, a method for making the same, and a photocatalytic composition used therefor.
BACKGROUND TECHNOLOGY
When semiconductors are irradiated with light whose wavelength has an energy greater than a band gap thereof, an oxidation-reduction reaction is brought about. Such a semiconductor is called a photocatalytic semiconductor or merely a photocatalyst.
Photocatalysts are in the form of powder and may be used as suspended in a solution, or may be used as supported on a substrate. From the standpoint of photocatalytic activity, the former is more active owing to the greater surface area. From the standpoint of practical applications, it has been frequently experienced to inevitably adopt the latter rather than the former owing to the ease in handing.
In order to support a photocatalyst on a substrate, there has been adopted a method wherein the particles of a photocatalyst are sintered at high temperatures and supported on the substrate. Another method has been proposed wherein a certain type of fluoropolymer is used as a binder, with which a photocatalyst is supported on a substrate. For instance, Japanese Laid-open Patent Application No. 4-284851 sets out a method wherein a mixture of the particles of a photocatalyst and a fluoropolymer is built up as layers and bonded under compression pressure. Japanese Laid-open Patent Application No. 4-334552 sets forth a method wherein the particles of a photocatalyst are thermally bonded to a fluoropolymer. Moreover, Japanese Laid-open Patent Application No. 7-171408 sets out a method wherein the particles of a photocatalyst is bonded on a substrate through a hard-to-decompose binder including an inorganic binder such as water glass or an organic binder such as a silicone copolymer, and also a method for manufacturing a photocatalytic body which includes, on a substrate, a first layer made of a hard-to-decompose binder, and a second layer formed on the first layer and made of a hard-to-decompose binder and the particles of a photocatalyst. In addition, Japanese Laid-open Patent Application No. 5-309267 describes a method wherein the metal oxide obtained from a metal oxide sol is used to support and fix the powder of a photocatalyst therewith. It is stated that the metal oxide sols are obtained from organometallic compounds such as alkoxides, acetylacetonate, carboxylates of metals as used in a sol-gel method, or are obtained by hydrolysis of an alcohol solution of chlorides, such as titanium tetrachloride, in the presence of an acid or alkali catalyst.
DISCLOSURE OF THE INVENTION
In recent years, attempts have been made to decompose, purify and sterilize harmful substances, offensive odor components and oily components ascribed to daily living environments by use of photocatalysts, thus leading to a quick extension of the application range of photocatalysts. This, in turn, requires a method of causing the particles of a photocatalyst to be firmly supported on all types of substrates over a long time without a sacrifice of its photocatalytic function. Especially, where a titanium oxide sol, which exhibits the good photocatalytic function but is poor in the function of bonding to a substrate, is used as a photocatalyst, it is required to improve the bonding property.
However, in these prior art methods, the bonding strength is not satisfactory, so that few methods ensures the support over a long time. If it is intended to make a photocatalytic body which has an improved bonding strength and ensures the support over a long time, there has arisen the problem that the photocatalytic function lowers. In case where the substrate made of an organic polymer resin is employed and rutile titanium oxide, which is weaker in photocatalytic function than anatase titanium oxide, is used, the photocatalytic reaction proceeds. Not only the organic polymer resin per se undergoes a photochemical reaction, but also the use over a long time results in degradation and decomposition.
Moreover, where organic polymer resins are used as a substrate, preliminary coating such as with a silica sol has been attempted, with the attendant problem that during the course of coagulation drying of the silica sol, cracks or voids are formed, thus presenting a problem on their bonding performance.
In order to solve the above problems, studies have been made on how to firmly support the particles of a photocatalyst on all types of substrates over a long time without impeding its photocatalytic function. As a result, it has been unexpectedly found that when using an amorphous titanium peroxide sol as a binder, the particles of a photocatalyst can be firmly supported on all types of substrates over a long time without impeding the photocatalytic function. The invention has been accomplished based on the finding.
More particularly, the invention relates to a method for manufacturing a photocatalytic body by use of a photocatalyst such as of titanium oxide and an amorphous titanium peroxide sol so that the photocatalyst is fixedly supported on a substrate, and also to a method for manufacturing a photocatalytic body which comprises forming, on a substrate, a first layer of an amorphous titanium peroxide sol having no photocatalytic function, and further forming a second layer on the first layer wherein the second layer is made of a photocatalyst and an amorphous titanium peroxide sol. Further, the invention relates to a photocatalytic body obtained by these methods and to a photocatalyst composition used for the manufacture.
The amorphous titanium peroxide sol used in the practice of the invention may be prepared, for example, by the following manner. An alkali hydroxide such as aqueous ammonia or sodium hydroxide is added to an aqueous solution of a titanium salt such as titanium tetrachloride, TiCl
4
. The resultant light bluish white, amorphous titanium hydroxide, Ti(OH)
4
, may be called ortho-titanic acid, H
4
TiO
4
. This titanium hydroxide is washed and separated, after which it is treated with an aqueous hydrogen peroxide solution to obtain an amorphous titanium peroxide solution useful in the present invention. The amorphous titanium peroxide sol has a pH of 6.0~7.0 and a particle size of 8~20 nm, with its appearance being in the form of a yellow transparent liquid. The sol is stable when stored at normal temperatures over a long time. The sol concentration is usually adjusted to a level of 1.40~1.60%. If necessary, the concentration may be optionally controlled. If the sol is used at low concentrations, it is used by dilution such as with distilled water.
The amorphous titanium peroxide sol remains as amorphous and is not crystallized in the form of anatase titanium oxide at normal temperatures. The sol has good adherence, a good film-forming property and is able to form a uniform flat thin film, and a dried film has such a property of being insoluble in water.
It will be noted that when the amorphous titanium peroxide sol is heated to 100° C. or above, it is converted to anatase titanium oxide sol. The amorphous titanium peroxide sol, which has been dried and fixed on a substrate after coating, is converted to anatase titanium oxide when heated to 250° C. or above.
The photocatalysts usable in the present invention include TiO
2
, ZnO, SrTiO
3
, CdS, CdO, CaP, InP, In
2
O
3
, CaAs, BaTiO
3
, K
2
NbO
3
, Fe
2
O
3
, Ta
2
O
5
, WO
3
, SaO
2
, Bi
2
O
3
, NiO, Cu
2
O, SiC, SiO
2
, MoS
2
, MoS
3
, InPb, RuO
2
, CeO
2
and the like. Of these, titanium oxide is preferred. Titanium oxide may be used in the form of particles or powder, or in the form of a sol.
Titanium oxide in the form of a sol, i.e. a titanium oxide sol, can be prepared by heating an amorphous titanium peroxide sol at a temperature of 100° C. or above. The properties of the titanium oxide sol, more or less, change depending on the heating temperature and the heating time. For instance, an anatase titanium oxide sol which is formed by treatment at 100° C. for 6 hours has a pH of 7.5~9.5 and a particle size of 8~20 n

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