JPH09276707A - Thin film semiconductor photocatalyst element and reaction device using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thin film semiconductor photocatalyst element which enables rapid decomposition of a harmful material and to provide a reaction device using this element. SOLUTION: This thin film semiconductor photocatalyst element accelerates decomposition of charges and oxygen reduction reaction by the effect of a metal catalyst and addition of PTFE particles, so that rapid decomposition of a harmful material can be performed even when the element is used in a single form. Moreover, the photocatalyst element is deposited on a base body and plural sheets of photocatalyst elements thus prepared are arranged parallel to each other at intervals so as to perform a three-dimensional laminating method of irradiating the element surface with light beams at a small incident angle. Thereby, the decomposition rate and treating ability per unit illuminated area can be largely improved.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a semiconductor photocatalyst, and is a film semiconductor for oxidatively decomposing environmental pollutants such as halogen compounds, sulfur, and nitrogen compounds existing in water or gas phase by photocatalysis. The present invention relates to a photocatalyst element and a reaction device using the same.

[0002]

2. Description of the Related Art In recent years, pollution of environmental water by pesticides and insecticides sprayed on a large scale, trihalomethane contained in tap water, sulfur oxides, nitrogen oxides and halogen compounds released into the atmosphere, The treatment of various pollutants such as unburned hydrocarbons has become a problem.

As one of the techniques for oxidatively decomposing these environmental pollutants, studies utilizing the photocatalytic action of semiconductor substances have been conducted. The present inventors made a report on the decomposition of trichlorobenzene by a titanium oxide photocatalyst added with a noble metal and its mechanism on page 164 of the abstracts of the 62nd convention of the Electrochemical Society of Japan. As shown in FIG.
When a semiconductor such as titanium oxide or tungsten oxide is irradiated with light having an energy higher than its forbidden band width (in most cases, near ultraviolet light and ultraviolet light), electrons are excited from the valence band to the conduction band. The holes generated in the valence band react with hydroxyl groups and water on the semiconductor surface to generate OH radicals having high oxidation activity, which oxidatively decompose harmful substances. On the other hand, the electrons excited in the conduction band reduce oxygen in the air or dissolved oxygen in water to produce ionic oxygen. This oxygen also goes through some routes to become hydrogen peroxide partially and contributes to the decomposition of harmful substances. That is, since harmful substances can be decomposed into harmless substances simply by irradiating the semiconductor substance with sunlight or artificial light, it has attracted attention as one of environmental purification methods. It has been proposed so far that photocatalyst fine particles such as titanium oxide are dispersed in a liquid or directly coated on the surface of a substrate, and then the substrate is irradiated with sunlight or artificial light to decompose it. This catalyst system is expected to be very economical with stable performance over a long period of time. However, in the former case, it is difficult to collect the catalyst powder and it has not been put to practical use. The latter is being applied to tiles such as toilets, and attempts have been made to apply it to deodorization. However, its reaction rate is small and it is not suitable for rapid removal of a large amount of pollutants.

[0004]

In order to rapidly decompose harmful substances, it is necessary to sufficiently irradiate the semiconductor material with light to efficiently cause charge separation of holes and electrons.
As described above, holes generated in the valence band by light irradiation are consumed to oxidize water to generate OH radicals, and conduction band electrons are used to reduce oxygen molecules to generate oxygen ions. Consumed. However, in the photocatalyst, the oxidation reaction and the reduction reaction are paired reactions and proceed at the same speed.
If one of the consumption reactions of holes or electrons is slow,
The subsequent charge separation cannot proceed efficiently.
Usually, a reduction reaction is unlikely to occur because the oxygen reduction catalytic activity of the plain oxide semiconductor is low, and the reduction reaction of oxygen is apt to be a rate-determining reaction including a charge separation reaction and thus a decomposition reaction of pollutants. Therefore, it is indispensable to develop a catalyst for promoting charge separation between electrons and holes and imparting high oxygen reduction catalytic ability.

Even with the above two improvements, the structure of the conventional semiconductor photocatalyst element still has a low solubility of oxygen in the solution and a slow diffusion rate in the solution. Therefore, development of a new semiconductor photocatalyst element that promotes these is indispensable. The present invention has been made in view of such circumstances, and an object of the present invention is to provide a thin film semiconductor photocatalyst element capable of rapidly decomposing harmful substances and a reaction apparatus using the same.

[0006]

In order to achieve the above-mentioned object, the present invention has a particle size of 0.005 to 5 μm in a porous fluorine compound film having an affinity for oxygen and a gas flow passage.
The semiconductor photocatalyst particles of m are dispersed. Further, the present invention, in order to achieve the above object, platinum,
A highly dispersed support of at least one catalyst selected from gold, silver, palladium, rhodium, ruthenium, iron, cobalt, nickel, copper or alloys thereof.
Wherein the semiconductor photocatalyst particles are held on a polymer base material coated with a light-transmitting inorganic thin film, or a light-transmitting inorganic base material, or an inorganic or metal base material having a high light reflectance. To do.

Further, in order to achieve the above object, the present invention provides that the substrate holding the film-shaped semiconductor photocatalytic element according to claim 1 is a plate-like or fibrous substrate, and
A plurality of film-shaped semiconductor photocatalyst elements held on this base material are arranged in parallel at intervals, and a light source for irradiating the surface of this thin-film semiconductor photocatalyst element with light having a low incident angle is provided. It is characterized by being

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION In the thin film semiconductor photocatalyst element of the present invention, as shown in FIG.
~ 10% by weight Highly dispersed particle size 0.005 ~ 5μ
The m semiconductor photocatalyst particles are dispersed in a low-density porous film made of a fine fibrous or particulate fluorine compound having an affinity for oxygen in order to facilitate the flow of gas or liquid. . Since the element is not easy to handle as it is, it is held on a light-transmissive base material or a metal plate having a high light reflectance to be incorporated in the reaction apparatus.

As the semiconductor material, fine powder of titanium oxide or tungsten oxide is used. Particle size is 0.005-5 μm
Within the range, a smaller one has a larger specific surface area and is more likely to react. The oxide powder having a small particle size and high activity is obtained by hydrolyzing an alkoxide or chloride in an aqueous solution or an alcohol solution, heating and drying. As the metal catalyst, platinum, gold, silver, palladium, rhodium, ruthenium, iron,
At least one selected from cobalt, nickel, copper and alloys thereof is added in an amount of 10% by weight or less based on the semiconductor photocatalyst. The method of supporting a metal catalyst is to put a semiconductor photocatalyst in a solution of water or alcohol in which metal ions are dissolved, to chemically reduce and precipitate with hydrazine, or to irradiate light with conduction band electrons generated in the semiconductor. It can be reduced and precipitated. When the platinum fine particles are photo-deposited on titanium oxide in a 2-propanol solution containing chloroplatinic acid, the maximum activity can be achieved with an amount in the range of 2 to 3 wt% with respect to titanium oxide. The smaller the particle size of the metal catalyst, the more the contact interface between the metal and the semiconductor increases, so that the addition amount can be reduced.

The semiconductor photocatalyst particles are fixed with a fluororesin binder such as polytetrafluoroethylene (PTFE). The fluororesin has a high affinity with oxygen, and can adsorb and concentrate many organic substances, and rapidly supply these reactants onto the semiconductor catalyst, thereby greatly improving the photocatalytic reaction rate. When the fluororesin completely covers the semiconductor surface, the light absorption efficiency and the water supply speed decrease. However, when a suspension of PTFE particles is vigorously stirred with an ultrasonic homogenizer and a shearing force is applied to the particles, fine fibers are formed, and a cotton-like porous fluorine compound aggregate film can be formed (see FIG. 2). ). Each fiber of this assembly has a diameter smaller than that of the photocatalyst particles, and is entangled with each other to form a cotton shape, which allows light to pass through to the inside. Therefore, when the suspension of PTFE particles and the semiconductor photocatalyst particles are stirred together, the photocatalyst particles are confined between the fibers, and the exposed portion of the photocatalyst particles can be directly irradiated with light.

The fluorine resin is added in an amount of 1 to 30% by weight with respect to the semiconductor photocatalyst, but a range of 3 to 10% by weight which facilitates the supply of oxygen and water and does not hinder the light irradiation effect. preferable. The thin-film semiconductor photocatalyst element described above has an effective thickness of several tens of milo-meters or less in consideration of light transmission, and thus a suitable base material for holding the element is required.
An element film is formed on a transparent substrate such as thin glass or polyethylene terephthalate (see FIG. 3). In such a case, three-dimensional lamination can be performed as shown in FIG. That is, a plurality of photocatalyst films fixed on both surfaces of the transparent substrate are arranged in parallel at intervals, and light is irradiated onto the surface of the semiconductor photocatalyst at a low incident angle. When irradiated at an appropriate incident angle, light diffusely reflects on the surface of the photocatalyst and the base material and penetrates deeply, and in addition, multiple reflections and scattering occur inside the transparent base material, and the entire photocatalyst on both sides of the base material effectively acts. Therefore, the reaction rate per unit area irradiated with light is significantly increased.

Further, glass fiber having excellent light transmittance is used as P
A thin film photocatalytic element can be supported by mixing it in a mixed solution of a suspension of TFE particles and semiconductor photocatalyst particles (see FIG. 5). In this case, since the glass fiber serves as a path for light, the entire photocatalyst can be made to work more effectively. In addition, as the translucent base material, light with a wavelength having an energy equal to or more than the forbidden band width of the semiconductor (about 40
Visible light and ultraviolet rays of 0 nm or less) should be transmitted,
The material is not necessarily limited to glass materials. For example, a polymer material such as polyethylene terephthalate, which has high light transmittance and good workability, is used. In this case, in order to prevent the photocatalytic decomposition of the polymer material itself, a thin film protective coat of an inorganic compound such as tin oxide having a wide band gap is applied to the polymer surface.

Further, it is effective to process the substrate into a shape that allows more effective use of light depending on the type of light source used. For example, in the case of a straight tube type ultraviolet lamp, a substrate having a hole for receiving a light source at its center may be laminated.
Optimizing the geometrical arrangement is important because the distance over which light can be scattered within the substrate varies depending on the type of light source (wavelength, parallelism, intensity, etc.).

When the element film is made thick, the film is formed on a metal plate or a ceramic plate having a high light reflectance, a plurality of films are arranged in parallel at intervals, and the surface of the semiconductor catalyst is irradiated at a low incident angle. . By this method, the light can be reflected on the surface of the photocatalyst or the substrate and the catalytic effect can be exhibited in a wide range. For supporting the thin film semiconductor photocatalytic element on various substrates, a screen printing method, a blade coating method, a dip coating method, a spray method or the like can be applied. However, a gaseous or liquid reactant of the thin film semiconductor photocatalytic element can be applied. In order to make the supply and the removal of products proceed smoothly, it is necessary to adjust the filling degree appropriately. When titanium oxide powder having an average particle diameter of 0.021 μm and PTFE particles having an average particle diameter of 0.3 μm are used, 0.7 to 2 mg of titanium oxide per square centimeter is used for a filling factor of 30 to
It is preferable to support in the range of 70%.

As the light source, a black light (ultraviolet lamp), a low pressure mercury lamp, a high pressure mercury lamp, an ultrahigh pressure mercury lamp or sunlight can be used.

[0016]

An embodiment of the present invention will be described. Example 1 FIG. 6 shows an example of a method for producing a thin film semiconductor photocatalytic element film constructed according to the present invention. FIG.
TiO2-PTFE is semiconductor particles TiO2 and PTFE particles supported on a substrate in a thin film form. In this example, commercially available titanium oxide powder (anatase type, average particle size 0.021μ
m), but PTFE particles are commercially available Teflon dispersion (average particle size 0.3 μm, particle content about 60 wt%)
Is used. After adding 10 g of water to 4 g of titanium oxide powder and adding a predetermined amount of Teflon dispersion, the paste was prepared by stirring with an ultrasonic homogenizer for 10 minutes and applied onto a substrate. In this example, a method of screen-printing on one side of a slide glass substrate (thickness 1 mm) is shown. Both sides of the slide glass were previously sandblasted (polished glass) in order to increase the surface area and the supporting strength. The amount of PTFE added is in the range of 1 to 30 wt% and the heat treatment temperature is from room temperature to 350 ° C.
Was changed in the range. The optimum amount of PTFE added and the heat treatment temperature after drying differ depending on the type of carrier and the supporting method. In the method of screen-printing the paste on the slide glass shown in FIG. 6, 5 wt% PTFE was added to titanium oxide. Those dried at room temperature showed the highest photocatalytic activity. [Example 2] After TiO2-PTFE was prepared in the same manner as in Example 1, a metal catalyst was highly dispersed and supported on photocatalyst particles. In this example, a method is shown in which the photocatalytic film is immersed in a 2-propanol solution containing a predetermined amount of chloroplatinic acid and irradiated with light to photoprecipitate platinum fine particles on titanium oxide. When the method of FIG. 6 was followed, it showed the highest activity in amounts ranging from 2-3% by weight with respect to titanium oxide. The thin film semiconductor photocatalyst element Pt-TiO2-PTFE manufactured by the method of FIG. 6 has a film thickness of 3 μm,
Titanium oxide loading is 0.9 mg / cm2, PTFE loading is 0.045
The amount of platinum carried was 0.025 mg / cm2. [Comparative Example 1] As a comparative control sample, a plain TiO2 film on which neither platinum nor PTFE was carried was prepared by the method shown in FIG. [Comparative Example 2] After the TiO2 film of Comparative Example 1 was prepared, a platinum-supported Pt-TiO2 film was prepared by the method shown in FIG.

An example of measuring the decomposition performance of harmful substances by these photocatalysts is shown below. Example 1, Example 2,
Effective area produced as shown in Comparative Example 1 and Comparative Example 2
Pyrex test tube (inner diameter = 14
mm, length = 165 mm). As a test for removing harmful substances in water, 8 ml of a 9.1 ppm trichlorobenzene (TCB) aqueous solution that had been air-saturated in advance was injected, and the container was sealed with a rubber septum. This was irradiated with light from a 500 W xenon lamp while keeping it at 25 ° C in a constant temperature bath. Since TCB is decomposed into carbon dioxide and chloride ions by the photocatalytic action of titanium oxide, the solutions were sampled at regular intervals and their concentrations were measured by ion chromatography. In addition, when performing a harmful substance removal test in the gas phase, in order to keep the amount of water adsorbed on titanium oxide constant, 4.6 To
Air containing water vapor of rr was allowed to flow for 1 hour and then sealed with a septum. Trichlorethylene (TCE) vapor was injected into this test tube so that the concentration was 0.1 vol%, and light was irradiated with an ultraviolet lamp (black light) with a power consumption of 15 W. Since TCE is decomposed into carbon dioxide and hydrochloric acid by a photocatalytic reaction, the gas in the test tube was sampled at regular intervals and the carbon dioxide concentration was quantified by gas chromatography. Table 1 shows the measurement results.

[Table 1] In either case, blank experiments, i.e., light irradiation in the state of containing no harmful substances, light irradiation in the state of containing harmful substances but no photocatalyst, and reaction in the dark in the presence of harmful substances and photocatalyst, Neither carbon dioxide nor hydrochloric acid was produced, and it was confirmed that the harmful substances were decomposed only by the photocatalytic action. Table 1
As is clear from the above, by adding the metal catalyst and PTFE to the photocatalyst layer, the decomposition rate of the harmful substances in the liquid phase and the gas phase can be significantly improved. Especially in water where the reduction reaction of dissolved oxygen is slow, the effect of PTFE is great,
-It can be seen that the TCB decomposition activity of TiO2-PTFE in water is more than twice as high as that of plain TiO2. Example 3 Next, an experimental example of a three-dimensional layered photocatalytic reaction will be described. The Pt-TiO2-PTFE photocatalyst of Example 2 supported on both sides of a Pyrex slide glass base material (26 mm x 38 mm) (effective surface area on one side: 10 cm2) was 10
The sheets were laminated and set in a Pyrex test container as shown in FIG. The harmful substance removal experiment was performed in the same manner as the above experiment, but the amount of TCB solution was 70 m
The amount of air containing 0.1% by volume of TCE was 120 ml. The light was applied to the cross section of the substrate at an incident angle of 30 degrees. Figure 8 shows the decomposition characteristics of TCB in water. By stacking photocatalysts and irradiating light obliquely, only 1
The second reaction rate constant is about 4 when a single film is vertically irradiated.
The CO2 can be rapidly decomposed to complete decomposition in about 1/4 of the time (40 minutes) of a single membrane. Comparing the decomposition amount per unit light irradiation area after 30 minutes of light irradiation, by layering,
It was possible to increase the amount of decomposition into chloride ions by 12 times, and the amount of decomposition into CO2 by about 5 times.

FIG. 9 shows the decomposition characteristics of TCE in air. The decomposition rate of TCE in the stack type device was increased to about twice that of a single film, and it was possible to completely decompose in 60 minutes. Per unit of light irradiation area
The amount of TCE decomposition treatment was also more than doubled. If the thickness of the base material is reduced and the degree of stacking is increased, even higher characteristics can be expected.

[0020]

As is apparent from the above examples, in the thin film semiconductor photocatalyst element of the present invention, the charge separation and the oxygen reduction reaction can be accelerated by the addition of the metal catalyst and the PTFE particles. It is possible to decompose and remove harmful substances. Further, the photocatalytic element supported on the base material is arranged in parallel at a plurality of intervals to irradiate light at a low incident angle to the surface of the semiconductor photocatalyst by the three-dimensional stacking method, and the decomposition rate and unit light irradiation. The processing capacity per area can be greatly improved.

[Brief description of drawings]

FIG. 1 is a principle of decomposition reaction of harmful substances by a semiconductor photocatalyst supporting a metal catalyst.

FIG. 2 is a working principle of the thin film semiconductor photocatalytic element of the present invention.

FIG. 3 is an example of holding the thin film semiconductor photocatalyst element of the present invention on a light transmissive substrate.

FIG. 4 is a three-dimensional layered photocatalytic reaction device according to an embodiment of the present invention and a light irradiation method thereof.

FIG. 5 is an example of holding the thin film semiconductor photocatalyst element of the present invention on a glass fiber.

FIG. 6 is a method for producing a photocatalyst film of the present invention (Examples 1 and 2 and Comparative Examples 1 and 2).

FIG. 7 is a three-dimensional layered photocatalytic reaction experimental apparatus which is an embodiment of the present invention.

FIG. 8 is a decomposition amount of TCB into carbon dioxide and chloride ions per unit irradiation area. Comparison between the case where the three-dimensional laminated photocatalytic reaction experimental device according to the present invention is obliquely irradiated with light (light irradiation area: 16.1 cm 2) and the case where the single film (5 cm 2) is vertically irradiated.

FIG. 9 is the decomposition amount of TCE into carbon dioxide per unit of light irradiation area. Comparison between the case where the three-dimensional laminated photocatalytic reaction experimental apparatus according to the present invention is obliquely irradiated with light (light irradiation area: 12.2 cm 2) and the case where the single film (10 cm 2) is vertically irradiated.

[Explanation of symbols]

Claims (3)

[Claims] 1. A particle size of 0.005 to 5 μm in a porous fluorine compound film having an affinity for oxygen and having a gas flow passage.
A film-like semiconductor photocatalytic element, wherein m semiconductor photocatalytic particles are dispersed. 2. The semiconductor according to claim 1, wherein at least one catalyst selected from the group consisting of platinum, gold, silver, palladium, rhodium, ruthenium, iron, cobalt, nickel, copper or alloys thereof is highly dispersed and supported. A film characterized in that the photocatalyst particles are held on a polymer base material coated with a light-transmitting inorganic thin film, a light-transmitting inorganic base material, or an inorganic or metal base material having a high light reflectance. Semiconductor photocatalyst element. 3. A film-shaped semiconductor photocatalyst element, wherein the base material holding the film-shaped semiconductor photocatalyst element according to claim 1 or 2 is a plate-like or fibrous base material and is held by this base material. Are arranged in parallel at a plurality of intervals and are equipped with a light source for irradiating the surface of the thin film semiconductor photocatalytic element with light having a low incident angle.