JP2000093807A - Photocatalyst body, and heat exchanger and purifying device using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the catalytic activity by using a metal having a chromate film formed on its surface as a substrate and depositing a catalyst film made of a photocatalytic material on the surface. SOLUTION: The photocatalyst consists of an aluminum plate 32 having a smooth surface and subjected to phosphate chromating treatment as its base body, and a titanium oxide film as a catalyst film formed on the base body by sputtering. The titanium oxide film is formed by using an RF magnetron sputtering device. The aluminum plate 32 with the titanium oxide film formed is cut into strips of specified width. A part of the strips is press-formed into way aluminum plates 31. The way aluminum plates 31 thus formed are alternately laminated with a aluminum plates 32 not pressed with an adhesive to form a honeycomb to obtain a purifying functional body 30 made of the photocatalytic medium. Thus, the obtd. photocatalyst body keeps hydrophilicity for a long time and has an improved decomposition effect.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photocatalyst which decomposes organic substances when irradiated with ultraviolet rays, a heat exchanger provided in an air conditioner, etc., and a purifying apparatus which decomposes and removes organic substances in the air or water. It is.

[0002]

2. Description of the Related Art When a photocatalytic substance is irradiated with light having a wavelength having energy equal to or greater than its band gap, electrons are generated in a conduction band by photoexcitation and holes are generated in a valence band. The strong reducing power of electrons and strong oxidizing power of holes cause decomposition and removal of organic substances and nitrogen oxides, decomposition of water, and the like, which is effective for antibacterial and purification. Therefore, conventionally, a photocatalyst in which a catalyst film made of a photocatalytic substance is supported on the surface of a substrate has been used as a purifying function in an air purifier, a water purifier, or the like.

In Japanese Patent Application Laid-Open No. 9-229585, a catalyst film is formed on the surface of a fin of a heat exchanger provided in an air conditioner as a means for hydrophilizing the fin.
That is, when a fin of a heat exchanger is used as a base and a catalyst film composed of a particulate photocatalytic substance and a silicon resin is formed on the surface thereof, the photocatalytic substance decomposes a hydrophobic alkyl group to make the fin surface hydrophilic.

[0004] In a heat exchanger having a hydrophilic surface, even if a large number of fins are provided to increase the heat exchange efficiency and the fin spacing is reduced, dew condensation water is retained between adjacent fins. There is no. Therefore, disadvantages such as obstruction of the airflow by the condensed water and scattering of the condensed water are eliminated.

[0005] In order to carry the photocatalyst substance on the surface of the substrate in the form of a film, the photocatalyst substance in the form of fine particles is mixed with an adhesive (binder) and applied on the substrate, and the mixture is applied at room temperature or by heating. There is a method of curing. As the binder material, silica, fluororesin, and the like have been studied.

[0006] However, the above-mentioned method has the disadvantage that the surface of the photocatalytic substance is covered with the binder, so that the intrinsic activity of the photocatalytic substance is impaired, or the binder itself is decomposed into the photocatalytic substance and the film strength is deteriorated.
Further, by using a porous material as a binder, it is possible to suppress a decrease in catalytic activity, but on the contrary, there arises a problem that adhesion to a substrate is reduced.

There is a sol-gel method for forming a catalyst film without using a binder. In the sol-gel method, after a raw material liquid is applied on a substrate, it is fired at a high temperature of 500 ° C. or higher. However, when the catalyst film is formed using aluminum having a low heat-resistant temperature as a base, the strength of the base is deteriorated by firing at a high temperature.

Further, it is difficult to apply the raw material liquid uniformly over a wide range of the substrate, and the thickness of the catalyst film tends to vary. If there is an extremely thin portion, there arises a problem that the substrate is liable to be corrosion-resistant and the catalytic activity is lowered. As a method for improving the catalytic activity of the photocatalytic substance, for example, a method of supporting metal fine particles on titanium oxide (catalyst film), a method of performing reduction treatment in hydrogen or alcohol gas, and the like are being studied.

A method for forming a catalyst film without using the sol-gel method is disclosed in JP-A-9-192498. Here, the catalyst film is formed on the surface of the substrate by a physical deposition method such as a vacuum deposition method.

[0010]

However, in the technique disclosed in Japanese Patent Application Laid-Open No. 9-192498, the catalyst film is directly formed on the substrate, so that the catalyst film is easily affected by the material of the substrate. For example, when the substrate is a substance containing a metal or an alkali metal such as sodium (such as glass), there is a problem that catalytic activity is reduced. In addition, when such a photocatalyst is used in a heat exchanger or a purifying device, a sufficient hydrophilizing effect and a decomposing effect cannot be obtained, and the photocatalyst does not have good performance.

The present invention has been made in view of the above problems, and provides a photocatalyst having a catalyst film supported on a substrate without using a binder, the photocatalyst having good catalytic activity. With the goal. Another object of the present invention is to provide a heat exchanger having a surface having good hydrophilicity or a purifying apparatus provided with a purifying function body having a good purifying ability.

[0012]

According to a first aspect of the present invention, there is provided a photocatalyst comprising a metal having a chromate film formed on a surface thereof as a base, and a catalyst film made of a photocatalytic substance carried on the surface. Features. A photocatalyst according to a second aspect is the photocatalyst according to the first aspect, wherein the metal is aluminum. The photocatalysts of the first and second aspects have a chromate film as an intermediate layer between the substrate and the catalyst film. This chromate film is formed by a general metal chromate treatment, and may be a phosphoric acid chromate film or a chromic chromate film.

In the second aspect, aluminum is used for the substrate. However, aluminum is inexpensive as a material, and by making it thin, processing such as punching and bending becomes easier as compared with other metals such as iron and stainless steel. For this reason, it is suitable for continuously forming a photocatalyst while being wound up on a roll-to-roll basis. In particular, aluminum subjected to chromate treatment is mass-produced as a fin material for a heat exchanger and is inexpensive.

According to a third aspect of the present invention, there is provided a photocatalyst wherein aluminum having a surface on which an alumite film is formed is used as a base and a catalyst film made of a photocatalytic substance is supported on the surface. Therefore, this photocatalyst has an alumite film as an intermediate layer between the substrate and the catalyst film. This alumite film is formed by a general aluminum alumite treatment.

According to a fourth aspect of the present invention, there is provided a photocatalyst wherein a catalyst film made of a photocatalytic substance is carried on a silica film formed on the surface of a metal or a metal having a chromate film on the surface. A photocatalyst according to a fifth aspect is the photocatalyst according to the fourth aspect, wherein the metal is aluminum.

A photocatalyst according to a sixth aspect of the present invention is characterized in that a catalyst film made of a photocatalytic substance is carried on a silica film formed on the surface of an aluminum substrate on which an alumite film is formed. Each of the photocatalysts according to the fourth, fifth and sixth aspects has a silica film as an intermediate layer between the substrate and the catalyst film.

According to a seventh aspect of the present invention, in the photocatalyst according to any one of the fourth to sixth aspects, the silica film is formed of colloidal silica. Since this silica film has a large surface roughness, the catalyst film formed thereon has a large surface area.

The photocatalyst of claim 8 is characterized in that, in the photocatalyst according to any one of claims 4 to 6, the silica film is formed by a sputtering method, a vapor deposition method, or a CVD method.

The photocatalyst according to claim 9 is the photocatalyst according to any one of claims 1 to 8, wherein the photocatalytic substance is titanium oxide and the catalyst film is formed by a sputtering method, a vapor deposition method, or a CVD method. It is characterized by being formed by a method.

As described above, when a sputtering method, a vapor deposition method, or a CVD method is used for forming a silica film or a catalyst film, the film can be formed over a large area with a uniform thickness. In addition, a film can be formed at 300 ° C. or lower by a sputtering method or an evaporation method.

A heat exchanger according to a tenth aspect is characterized in that the photocatalyst according to any one of the first to ninth aspects is used for a fin. In this heat exchanger, organic groups such as alkyl groups exhibiting hydrophobicity are decomposed by the catalyst film, and the fins become hydrophilic.

According to a eleventh aspect of the present invention, there is provided a purifying apparatus using the photocatalyst according to any one of the first to ninth aspects as a purifying functional body. This purifying functional body can decompose odors and harmful gases in the air, odors and organic substances in water by the action of a photocatalyst.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, an embodiment of the photocatalyst according to the present invention will be described, and its features will be described using a conventional photocatalyst as a comparative example.

Example 1 The photocatalyst of Example 1 was obtained by forming a titanium oxide film (catalyst film) by a sputtering method using an aluminum plate having a smooth surface and a phosphoric acid chromate treatment as a base. .

The method and conditions for forming the titanium oxide film are as follows. That is, an RF magnetron sputtering apparatus was used as a film forming apparatus, and the size of an aluminum plate as a substrate was 6 cm × 3 cm.
(18 cm ² ), the target is TiO _{2 with} a purity of 99.9% or more,
The introduced gases are argon and oxygen.

FIG. 1 is a schematic structural view of an RF magnetron sputtering apparatus. A substrate 2 is placed in a vacuum chamber 1 of the apparatus main body so as to face a target 3. First, the inside of the vacuum chamber 1 is evacuated from the exhaust valve 5 by the vacuum pump 4 to a predetermined degree of vacuum. Then, the base 2 is heated to a predetermined temperature, and argon and oxygen are introduced through the valves 6 and 7 to form a mixed gas atmosphere. At this time, the flow rate of the introduced gas and the amount of exhaust gas are adjusted so that argon and oxygen have a predetermined partial pressure ratio. And
High frequency power is applied to the target 3 by the power source 8 to form a titanium oxide film on the surface of the base 2. The respective forming conditions are shown in Table 1 below.

[0027]

[Table 1]

The film growth rate under this film formation condition is 0.3.
× 10 ⁻⁶ cm / min. Here, the film was formed until the film thickness became 0.6 μm.

Example 2 The photocatalyst of Example 2 was obtained by forming a titanium oxide film (catalyst film) by a sputtering method using an aluminum plate having a smooth surface and anodized aluminum as a base material. The method and conditions for forming the titanium oxide film are the same as those in the first embodiment.

Example 3 The photocatalyst of Example 3 was based on an aluminum plate having a smooth surface and a phosphoric acid chromate treatment as a base, a colloidal silica film was formed on the surface, and a titanium oxide film was formed by sputtering. (Catalyst film).

The method of forming the colloidal silica film is as follows. That is, 10% of SiO ₂ particles having a particle size of 10 to 20 mm
After the substrate was immersed in an acidic aqueous solution (2 times diluted aqueous solution of Snowtex O manufactured by Nissan Chemical Industries, Ltd.), the substrate was pulled out of the solution and cured at 200 ° C. for 30 minutes.

The method and conditions for forming the titanium oxide film are the same as in the first embodiment. Here, the thickness of the titanium oxide is 0.3 μm.

Example 4 The photocatalyst of Example 4 was formed by using an aluminum plate having a smooth surface as a base, forming a silica film on the surface by a sputtering method, and further forming a titanium oxide film (catalyst) by a sputtering method. Film).

The method and conditions for forming the silica film are as follows. That is, an RF magnetron sputtering apparatus is used as the film forming apparatus (see FIG. 1), the target is SiO _{2 having} a purity of 99.9% or more, and the introduced gases are argon and oxygen. Other forming conditions are the same as in Table 1. As a result, the growth rate of the thickness of the silica film became 0.3 × 10 ⁻⁶ cm / min. In this case, the film was formed until the film thickness became 0.6 μm.

The method and conditions for forming the titanium oxide film are the same as in the first embodiment.

Comparative Example 1 The photocatalyst of Comparative Example 1 was obtained by forming a titanium oxide film (catalyst film) by a sputtering method using an aluminum plate having a smooth surface as a base. The method and conditions for forming the titanium oxide film are the same as those in the first embodiment.

[Comparative Example 2] The photocatalyst of Comparative Example 2 has a glass plate as a substrate and a titanium oxide film (catalyst film) formed on the surface thereof by a sol-gel method.

That is, a titanium alkoxide is applied on a barium borosilicate-based alkali-free glass having a smooth surface,
Baking at 0 ° C. for 1 hour to form a titanium oxide film. Then, application and baking are repeated until the film thickness becomes 0.6 μm.

Comparative Example 3 In Comparative Example 3, an aluminum plate having a smooth surface and subjected to a phosphoric acid chromate treatment was used as a base, and a colloidal silica film was formed on the surface thereof. The method of forming the colloidal silica film is the same as that of the third embodiment.

An experiment was conducted to compare the catalytic activities of the photocatalysts of Examples 1 to 4 and Comparative Examples 1 and 2 described above. In the experimental method, each photocatalyst is first placed in a 5 liter container, and acetaldehyde, one of the malodorous substances, is injected into the container so as to be 100 ppm. next,
Each photocatalyst is irradiated with a 6 W black light, and the time required for the acetaldehyde concentration to decrease to 1 ppm is measured.
The results are shown in Table 2 below.

[0041]

[Table 2]

As shown in Table 2, in Comparative Example 1 in which a titanium oxide film was formed directly on an aluminum plate, it took 6.2 hours to decompose acetaldehyde, whereas in Comparative Example 1, an intermediate layer was formed between the aluminum plate and the titanium oxide film. Example 1 having a chromate film, Example 2 having an alumite film, Example 3 having a colloidal silica film on a chromate film, and Example 4 having a sputtered silica film were all capable of decomposing acetaldehyde in a shorter time. are doing.

In particular, in Example 3, since the surface area of the titanium oxide film was increased by the colloidal silica film, acetaldehyde was decomposed in a shorter time than in Examples 1, 2, and 4. Further, Comparative Example 2 in which the titanium oxide film was formed by the sol-gel method required more time for decomposition than Examples 1 to 4 in which the titanium oxide film was formed by the sputtering method.

Next, the above-described Examples 1 to 4 and Comparative Example 1
For each of the photocatalysts of Comparative Example 3 and the sample of Comparative Example 3, an experiment was conducted to compare the change over time in wettability. The experimental method irradiates each photocatalyst and the sample with a 6W black light for 2 hours,
Pure water is dropped within 10 minutes after the irradiation is completed, and the contact angle of water is measured. After that, it is kept in a dark box, and the contact angle of water is measured by the same method every predetermined time. Table 3 shows the results.

[0045]

[Table 3]

As shown in Table 3, Examples 1 to 4 are more hydrophilic than Comparative Example 1, and they are maintained for a long time. In particular, in Example 3, since the surface area of the titanium oxide film was increased by the colloidal silica film, the hydrophilicity which was larger than that of the other Examples 1, 2, and 4 was maintained. Comparative Example 3
Is a prior art in which a silica film is formed on the surface to impart hydrophilicity, but the hydrophilicity is inferior to Examples 1-4.

As described above, the embodiment according to the present invention has a chromate film, an alumite film, and a silica film as an intermediate layer between the substrate and the catalyst film, and the catalyst film is formed by a sputtering method. As a result, the catalytic activity of the photocatalytic substance does not decrease, and good hydrophilizing action and decomposition action can be obtained. In particular, hydrophilicity is maintained for a long period of time. Further, when a colloidal silica film is provided in the intermediate layer, the catalyst film formed thereon has a large surface area, so that the decomposition rate of organic substances and the like is faster, and the hydrophilicity is maintained for a longer period.

Next, an embodiment of the heat exchanger according to the present invention will be described. The method for producing the fins of the heat exchanger is as follows. That is, a mixture of colloidal silica liquid (Nissan Chemical Snowtex O) and pure water at a weight ratio of 1: 1 on both surfaces of an aluminum plate subjected to a phosphoric acid chromate treatment was applied in an amount of 1 g on one side of the aluminum plate. / m ² and cured at 200 ° C. for 30 minutes.

Next, a titanium oxide film is formed on the colloidal silica film one by one by a sputtering method. The formation method and conditions at this time are the same as those in the first embodiment, and the thickness of the titanium oxide film is 0.3 μm. Then, the aluminum plate on which the colloidal silica film and the titanium oxide film are formed is cut and pressed into a predetermined fin shape.
FIG. 2 is a perspective view of the heat exchanger of the present embodiment, and FIG. 3 is a cross-sectional view showing a part of the heat exchanger in an enlarged manner. Here, the fins 11 are formed in a rectangular shape, and the refrigerant pipes 12 are attached to a large number of the fins 11 so as to pass therethrough.

FIG. 4 shows an example of an indoor unit of an air conditioner provided with the above heat exchanger, and shows a cross section thereof. An intake port 21a is formed in an upper portion of the indoor unit main body 21, and air taken in from the intake port 21a is first subjected to dust removal by an electrostatic filter 22. In the heat exchanger 10, the refrigerant pipe 1
The refrigerant passing through 2 absorbs heat from the air and cools it. Then, the cooled air is sent into the room by the sirocco fan 23 from the outlet 21b at the lower part of the main body 21.

The heat exchanger 10 is provided with a black light 24 opposed thereto, and is irradiated with ultraviolet rays. Since the heat exchanger 10 has a fin 11 on which a titanium oxide film is formed using a chromate film and a colloidal silica film as an intermediate layer, the heat exchanger 10 has good hydrophilicity when irradiated with ultraviolet rays.

Therefore, even if the interval between the fins 11 becomes narrow, dew condensation water is not retained between the fins 11, so that a large number of fins 11 can be provided. Become. In addition, since the hydrophilicity is maintained for a long period of time, the irradiation of ultraviolet rays may be performed only intermittently.

[0053] Furthermore, the use of heat exchangers carrying the catalyst film made of a photocatalytic material in the air conditioner, since smell and NO _x in the air is decomposed by the photocatalytic material is added air cleaning function and tobacco It is known that even if dirt such as tar is attached to the heat exchanger, it is decomposed and maintenance is not required. Particularly, in the heat exchanger of the present embodiment, these effects are greatly obtained because the catalyst activity is good. Further, even if dirt adheres to the surface, the hydrophilicity does not decrease due to the self-cleaning action.

Next, an embodiment of a water purifier as a purification device according to the present invention will be described. The method for producing the purifying function body provided in this water purifier is as follows. That is, a titanium oxide film is formed on both surfaces of an aluminum plate subjected to a phosphoric acid chromate treatment by a sputtering method. The formation method and conditions at this time are the same as those in the first embodiment, and the thickness of the titanium oxide film is 0.3 μm.

FIG. 5 is an external perspective view of the purifying function body. The aluminum plate on which the titanium oxide film was formed was cut into a band having a predetermined width, and a part of the band was pressed to form a wave (in the figure, a wave-shaped aluminum plate 31). And
The corrugated aluminum plate 31 and the remaining unpressed aluminum plate 32 are alternately adhered to each other with an adhesive to form a honeycomb.

FIG. 6 is a schematic structural view of a water purifier provided with the purifying function body 30. In the main body 41, the purifying function body 30 is held by a holder 42, and the water supply pipe 4
Water is sent from 3. The purifying function body 30 is irradiated with ultraviolet rays by the black light 44, and organic matter and odor in the water are decomposed and removed by the photocatalytic substance. In particular, since the purifying function body 30 is formed by forming a titanium oxide film using a chromate film as an intermediate layer, the catalytic activity is good and the effect of the decomposition action is greatly obtained. Then, the purified water is sent from the water pipe 45 to the outside of the main body 41.

In the above-described embodiments 1 to 4 of the present invention and the embodiments of the heat exchanger and the purifier, the silica film may be formed by a vapor deposition method or a CVD method in addition to the sputtering method. Good. In the vapor deposition method, for example, SiO ₂ is heated by resistance heating or electron beam irradiation to be vaporized,
Forming a silica film on a substrate installed in SiO ₂ and a position facing. In the CVD method, for example, a monosilane gas (SiF ₄ ) is introduced into oxygen plasma, and a silica film is formed on a substrate provided in the vicinity.

The titanium oxide film may be formed by a vapor deposition method or a CVD method other than the sputtering method. In the vapor deposition method, for example, TiO ₂ is heated and vaporized by resistance heating or electron beam irradiation, and a titanium oxide film is formed on a substrate provided at a position opposed to TiO ₂ . In the CVD method, for example, a titanium alkoxide (TiO (C
H ₃ ) ₄ ) gas is introduced to form a titanium oxide film.

The chromate film may be a chromate chromate film in addition to phosphoric acid chromate.

Although the water purifier has been described as an example in the embodiment of the purifying apparatus, the present invention can be applied to an air purifier, a toxic gas removing apparatus, and the like.

[0061]

As described above, in the photocatalyst according to any one of claims 1 to 9, any one of a chromate film, an alumite film, and a silica film is provided as an intermediate layer between the metal substrate and the catalyst film. Several of them are interposed. As a result, the catalyst film is not affected by the metal of the substrate, so that its catalytic activity is not impaired, and a good organic substance decomposing and hydrophilizing action can be obtained. In addition, the corrosion resistance of the photocatalyst can be improved by these intermediate layers, and the photocatalyst has high reliability for long-term use.

The photocatalysts according to the second, third, fifth, and sixth aspects use aluminum as the base. Therefore, this photocatalyst is inexpensive, can be mass-produced, and can be easily processed by punching and bending.

Further, in the photocatalyst according to the seventh aspect, since the catalyst film is formed on the colloidal silica film having a large surface roughness, the surface area of the catalyst film becomes large. Therefore, the reaction speed by the photocatalytic substance is increased. Also, the hydrophilicity is improved, and it is maintained for a long time.

In the photocatalyst according to the eighth aspect, since the silica film is formed by a sputtering method, a vapor deposition method or a CVD method, the film is formed over a wide range with a uniform thickness.
Therefore, the substrate is protected by the silica film and has high corrosion resistance. Further, there is no disadvantage that the catalyst film is directly affected by the substrate in the portion where the silica film is thin, and the catalytic activity is reduced.

Further, in the photocatalyst of the ninth aspect, since the catalyst film is formed by the sputtering method, the vapor deposition method or the CVD method, the film is formed with a uniform thickness over a wide range. Therefore, there is no inconvenience that the catalytic activity is reduced in the thin portion. Further, since the substrate does not need to be heated to a high temperature, a metal having a low heat-resistant temperature such as aluminum can be used for the substrate. Further, since no binder is used, a catalyst film having excellent water resistance is obtained.

In the heat exchanger according to the tenth aspect, since the fins have good hydrophilicity for a long period of time, even if the fin spacing becomes narrow, dew condensation water is not retained between the fins. Fins can be provided. Therefore, the heat exchange efficiency of the heat exchanger is improved, and the energy saving effect is large.

In the purifying apparatus according to the eleventh aspect, since the purifying functional body has good catalytic activity, it is possible to quickly decompose odors and organic substances in the air and water. In addition, the photocatalyst body having excellent corrosion resistance and water resistance has a long life.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an RF magnetron sputtering apparatus.

FIG. 2 is a perspective view showing an embodiment of the heat exchanger of the present invention.

FIG. 3 is an enlarged sectional view showing a part of the heat exchanger.

FIG. 4 is a cross-sectional view of an indoor unit of an air conditioner including the heat exchanger.

FIG. 5 is an external perspective view of a purification function body provided in the purification device of the present invention.

FIG. 6 is a schematic configuration diagram of a water purifier provided with the purification function body.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Vacuum chamber 2 Substrate 3 Target 4 Vacuum pump 5 Exhaust valve 6 Valve 7 Valve 8 Power supply 10 Heat exchanger 11 Fin 12 Refrigerant pipe 21 Indoor unit main body 21a Inlet 21b Outlet 22 Electrostatic filter 23 Sirocco fan 24 Black light 30 Purification Functional body 31 Corrugated aluminum plate 32 Aluminum plate 41 Water purifier main body 42 Holder 43 Water supply pipe 44 Black light 45 Water supply pipe

 ──────────────────────────────────────────────────の Continued on the front page F-term (reference) 4F100 AA19A AA20C AA21C AA21D AA22A AB01B AB10B BA03 BA04 BA07 BA10B BA10C EH66C EH66D EJ69A GB51 JB02 JB05 JB07 JC00 JL01 JL02 JL08C JL08MBA JA02JA02A BB02A BB02B BC16A BC16B BC58A BC58B BD02A BD02B BD05A BD05B CA06 CA10 FA02 FB02 FB03

Claims (11)

[Claims] 1. A photocatalyst comprising a metal having a chromate film formed on its surface as a substrate and a catalyst film made of a photocatalyst substance carried on its surface. 2. The photocatalyst according to claim 1, wherein the metal is aluminum. 3. A photocatalyst comprising an aluminum substrate on which an alumite film is formed on its surface and a catalyst film made of a photocatalytic substance carried on its surface. 4. A photocatalyst wherein a catalyst film made of a photocatalytic substance is carried on a silica film formed on a surface of a metal or a metal having a chromate film on its surface as a substrate. 5. The photocatalyst according to claim 4, wherein the metal is aluminum. 6. A photocatalyst, comprising a catalyst film made of a photocatalytic substance supported on a silica film formed on an aluminum substrate having an alumite film formed on a surface thereof. 7. The photocatalyst according to claim 4, wherein the silica film is formed of colloidal silica. 8. The photocatalyst according to claim 4, wherein the silica film is formed by a sputtering method, an evaporation method, or a CVD method. 9. The photocatalytic substance is titanium oxide, and the catalyst film is formed by a sputtering method, a vapor deposition method, or a CVD method.
The photocatalyst according to any one of the above. 10. A heat exchanger, wherein the photocatalyst according to claim 1 is used for a fin. 11. A purifying apparatus using the photocatalyst according to claim 1 as a purifying function.