JPH09271664A - Production of photocatalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form at least either one of hydrogen and oxygen by photolyzing water by anodically oxidizing a member of which the surface is composed of titanium in an electrolyte soln. containing glycerophosphate and one or more kind of metal acetate respectively in specific concns. SOLUTION: Metal ions of a metal acetate are stably formed under high voltage while taken in an anodically oxidized film by using an electrolyte containing 0.06-0.2mol/l of glycerophosphate and 0.011mol/l of the metal acetate to easily form a relatively thick titanium-containing, oxide film or metal doped titania film with a thickness of several μm. Voltage is pref. within a range of 100-400V and, from a point of time when voltage is ready to exceed 100V, spark discharge is generated on the surface of an anode and the oxide film is locally heated to an extremely high temp. The heating of the film is innumerably repeated as mentioned above and, as a result, the crystallization of the, whole of an anodically oxidized film and the taking in of metal ions are performed at the same time.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a photocatalyst used for photolyzing water to produce at least one of hydrogen and oxygen.

[0002]

2. Description of the Related Art At present, various energy sources are used, but the most consumed fossil fuels (such as petroleum and coal) are burned with carbon dioxide, nitrogen oxides, and sulfur oxides. Objects are discharged. As a result, carbon dioxide causes global warming, and nitrogen oxides and sulfur oxides react with moisture in the air to form nitric acid and sulfuric acid. These nitric acid and sulfuric acid cause acid rain and cause natural destruction. In addition, nuclear power is a next-generation energy source, but it has problems such as safety and waste disposal.

On the other hand, hydrogen only turns into water when burned, and has recently attracted attention as a clean energy source. However, it becomes meaningless if the energy source for generating hydrogen uses fossil fuel or the like. In such a situation, it is very useful to use the inexhaustible sunlight. A photocatalyst is one of the means for obtaining hydrogen by utilizing such sunlight. This is because the catalyst absorbs energy above the band gap to generate holes and electrons, the holes react with water to generate oxygen and hydrogen ions, and the hydrogen ions react with the electrons to generate hydrogen. Is.

As a photocatalyst exhibiting catalytic activity, titania has long been well known. Moreover, it is also known that a titania-containing composite oxide produced by doping this titania with various metal elements or combining it with various metal elements exhibits high catalytic activity.

[0005]

The shape of the photocatalyst used when using the photocatalyst may be powder, plate, fiber, film or the like, but the film shape is the best considering the ease of incorporation into the device and the convenience of handling. Therefore, it is important to make the substance exhibiting catalytic activity into a film shape. The method for producing a film-shaped photocatalyst can be roughly classified into a gas phase method, a liquid phase method, a solid phase method, etc., but the anodic oxidation method among the methods using the liquid phase is considered to be particularly useful for the following reasons. There is.

The anodic oxidation method is a very simple method as compared with other methods for producing a photocatalyst having a film shape, and it is easy to form a film on a surface having a large area. Moreover, since it is possible to form a film on a substrate having a complicated shape, it can be said that this is an industrially useful method for forming a photocatalyst. Further, since the anodizing method oxidizes the surface of the metal that is the substrate, it is characterized in that the substrate metal element itself becomes the constituent element of the oxide film.

An object of the present invention is to produce a titanium-containing composite oxide film or a metal-doped titania film, which is useful as a photocatalyst for photolyzing water to produce at least one of hydrogen and oxygen, by an anodic oxidation method. is there.

[0008]

Means for Solving the Problems In the anodic oxidation method useful for producing a film-shaped photocatalyst, the present inventors have added a metal element such as an alkaline earth metal element to be combined or doped into an electrolyte to be used. It was found to be mixed as ions. Further, based on the experimental results, the member having at least the surface of titanium was anodized in an electrolyte solution containing 0.06 to 0.2 mol / l glycerophosphate and at least one kind of 0.01 to 1 mol / l metal acetate. It has been found that the above (claim 1) is particularly useful for producing a titanium-containing composite oxide film or a metal-doped titania film.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION As described above, according to the present invention, the production of a titania or a composite oxide film containing titania doped with an alkaline earth metal element, which has been difficult in the past, was realized by the anodic oxidation method. Further, in the anodizing method of the present invention, the composition of the composite oxide film changes depending on the concentration of the electrolyte and the voltage. In the anodizing method, a metal of appropriate size (titanium in the present invention) is used as an anode, and an electric field is applied in an appropriate electrolyte to form an oxide film of several μm on the metal surface of the anode. Is.

In the present invention, the titanium base material is anodized in a special electrolyte to form on its surface a titania or a composite oxide film containing titania doped with an alkaline earth metal element having high crystallinity. To provide a photocatalytic effect. Then, in some cases, the photocatalytic effect can be improved by performing hydrothermal treatment in high-temperature and high-pressure steam to further enhance the crystallinity of the film and make the surface porous to increase the surface area. Note that the titanium base material referred to in the present invention also includes a thin titanium layer formed on the surface. That is, the base material is made of any material such as metal, ceramics, and plastic, and the present method can be applied to the case where a thin titanium layer is formed only on the surface by a method such as sputtering, plating, or thermal spraying. The present invention will be described in detail below. As an electrolyte for anodic oxidation, phosphoric acid, sulfuric acid or a mixed acid thereof, and a mixed solution of glycerophosphate and metal acetate or a solution of glycerophosphate alone is used.

In the present invention, it is necessary to use a salt in order to introduce metal ions to be incorporated into the anodic oxide film into these electrolytes. That is, it is necessary to use a mixed solution of glycerophosphate as a salt and metal acetate or a solution of glycerophosphate alone as the electrolyte. However, since there are few types of good water-soluble glycerophosphate,
It is difficult to incorporate the metal ion of glycerophosphate into the film as a metal ion. Therefore, it is not appropriate to use a solution of glycerophosphate alone as the electrolyte, and a mixed solution of glycerophosphate and metal acetate is desirable and practical.

Further, in order to introduce a large amount of metal ions into the film, it is necessary to increase the salt concentration. Therefore, it is better to introduce the target metal ion as a metal ion with a good water-soluble metal acetate. Further, although the reason is not clear, a high-concentration salt solution can be prepared by using a mixed solution of a combination of glycerophosphate and metal acetate as an electrolyte. Further, since a high voltage anodizing treatment can be performed, a thick film can be formed. Moreover, when a mixed solution of a combination of glycerophosphate and metal acetate is used as an electrolyte, it is safer than an electrolyte of a strong acid such as phosphoric acid or sulfuric acid, and post-treatment such as waste liquid treatment is easy.

Examples of glycerophosphate include sodium glycerophosphate and calcium glycerophosphate.
Sodium glycerophosphate is most preferred because it is very soluble in water. As the metal acetate, a metal acetate of a metal ion to be incorporated in the film may be used. As mentioned above,
Since metal acetates are generally easily soluble in water, various metal ions can be introduced. For example, alkali metal acetates (lithium, sodium, potassium, rubidium,
Cesium salts), alkaline earth metal acetates (magnesium, calcium, strontium, barium salts), lanthanum acetate, aluminum acetate, lead acetate, chromium acetate, manganese acetate and the like. Among them, although the reason is not clear, there is an alkaline earth metal as a metal that is easily incorporated into the film. Further, when titanium is used as the substrate metal, it is known that the composite oxide of alkaline earth metal and titanium has high activity as a photocatalyst. Therefore, it is preferable to use a metal acetate of an alkaline earth metal (magnesium, calcium, strontium, barium, etc.).

In order to perform stable anodic oxidation in an electrolyte composed of glycerophosphate and metal acetate, the optimum concentration of these salts must be determined. The concentration is related to the voltage for anodic oxidation and the type of metal acetate. The concentration of glycerophosphate is preferably 0.06 mol / l to 0.2 mol / l. Because if it is less than 0.06mol / l,
Even if anodization is performed using any concentration of metal acetate, anatase phase titania is formed, and a titanium-containing oxide film or a metal-doped titania film is not formed. Also, 0.2 mol
When it is at least 1 / l, it is not preferable because it reacts with the metal acetate to dissolve it in water and precipitates.

The concentration of metal acetate should be 0.01 mol / l or more. This is because when the amount is less than 0.01 mol / l, a sufficient amount of metal is not taken into the anodic oxide film. If it is more than 1 mol / l, it is not preferable because it reacts with glycerophosphate to cause precipitation.

Using an electrolyte of 0.06 mol / l to 0.2 mol / l of glycerophosphate and 0.01 mol / l or more of metal acetate, metal ions of metal acetate are stably contained in the anodic oxide film at high voltage. It is possible to easily form a relatively thick titanium-containing oxide film or metal-doped titania film having a thickness of several μm.
On the other hand, the voltage is an important factor in forming the anodic oxide film.

Although the film thickness increases in proportion to the voltage,
The voltage should not be lower than 100V as it requires a thickness of a few μm to obtain sufficient photocatalytic activity. On the other hand, if it is too thick, stable anodic oxidation will not be possible, so it should not be higher than 500V. Therefore, the voltage is preferably in the range of 100 to 400V. When the voltage exceeds 100V, spark discharge occurs on the surface of the anode, and the oxide film is locally heated to a very high temperature. In this way, as a result of innumerable heating of the film, crystallization of the entire anodized film and incorporation of metal ions are performed simultaneously.

When the voltage is higher than 400 V, anodic oxidation cannot be performed in a high-concentration electrolyte, and a sufficient amount of metal ions cannot be taken in. As described above, the present invention can form a composite oxide film having a uniform thickness even with a large area and a complicated shape, as in the conventional anodic oxidation method. Further, the reaction time for one time is about a few minutes, which is relatively short. Furthermore, since it requires no special equipment, is relatively easy to operate, and can be prepared in an aqueous solution at room temperature, it consumes very little energy.

When the composite oxide film produced by the present invention is heat-treated (hydrothermal treatment) in high-pressure steam or water in a closed container, crystallization of the film or reaction between TiO ₂ and metal ions occurs. As a result, the membrane becomes porous and the surface area increases, and the formation of a composite oxide of alkaline earth metal and titanium is further promoted, and the catalytic activity for water decomposition increases. Therefore, the catalytic activity is improved by hydrothermally treating the composite oxide film produced in the present invention.

Since the hydrothermal treatment method applies high pressure, the crystallinity of the anodic oxide film can be increased at a relatively low temperature.
That is, there is an advantage that the same effect as that obtained by the heat treatment at a high temperature can be obtained. In addition, when the photocatalyst of the present invention is used in a water-splitting reaction by light, it does not matter even if the photocatalyst of a normal powder is modified with a catalyst such as carrying a noble metal.

[0021]

[Example 1] (Method for producing anodic oxide film) Sodium β-glycerophosphate and strontium acetate were added at 0.08 mol / l and 0.5 m, respectively.
It was dissolved in distilled water at a concentration of ol / l to prepare an electrolyte solution. In the solution, a metal titanium plate bent in a cylindrical shape (diameter 5c
m, height 15 cm, thickness 0.5 mm) was anodized at a voltage of 350 V (FIG. 3). The current density was 50 mA / cm ² , and the electrolyte temperature was 40 ° C.

The prepared anodic oxide film had TiO ₂ as a matrix phase, but when elementally analyzed by an energy dispersive X-ray analyzer, it contained Sr and P incorporated from the electrolyte. Furthermore, when the crystal phase of the anodic oxide film was identified by a powder X-ray diffractometer, it was found to be a composite oxide film composed of TiO ₂ (anatase and rutile) and strontium titanate (FIG. 1). This strontium titanate composite oxide film was formed by the reaction of TiO ₂ and Sr due to the high heat of the spark discharge generated during the anodic oxidation. (Evaluation of catalytic activity) The catalytic activity was evaluated using the anodic oxide film (TiO ₂ (containing Sr)) produced by the above method. An aqueous solution of methanol (50 ml of methanol, 300 ml of water) was placed in a reaction tube in the closed circulation system, and light was irradiated while stirring with a magnetic stirrer to quantify the generated hydrogen gas. A 450 W high-pressure mercury lamp was used as a light source, and all light was irradiated. The reaction tubes used were all made by Pyrex. Gas chromatography was used to detect and quantify the produced hydrogen.

Table 1 shows the measurement results of the steady-state activity of hydrogen production reaction from aqueous methanol solution. Example 2 a (hydrothermal treatment) the anodic oxide film in Example 1 (TiO ₂ (Sr content)) titanium metal plate produced a capacity of 1.3 l O - Tokure - placed hanging in the center of the probe Hydrothermal treatment was carried out by adding 0.2 liter of water and heating at 300 ° C. for 2 hours.

An X-ray diffraction chart of the anodized film after hydrothermal treatment is shown in FIG. By the hydrothermal treatment, crystallization of each crystal phase proceeds, and the peak becomes higher than that before the hydrothermal treatment. (Evaluation of catalytic activity) Anodized film (Ti
The catalytic activity of O ₂ (containing Sr)) was evaluated in the same manner as in Example 1 by using a closed circulation system catalytic reactor to generate hydrogen from an aqueous methanol solution to which methanol was added as a sacrificial reagent. Table 1 shows the measurement results.

[0025]

[Table 1]

As described above, all the anodic oxide films prepared in Examples 1 and 2 were found to have catalytic activity for the water splitting reaction.

[0027]

As described above, the metal oxide film formed on the metal surface by the anodic oxidation method in the electrolyte containing a high concentration of metal ions is useful as a highly efficient photocatalyst for water splitting.

[Brief description of drawings]

FIG. 1 is an X-ray diffraction pattern of the anodic oxide film produced in Example 1.

FIG. 2 is an X-ray diffraction pattern of the anodized film produced in Example 2.

FIG. 3 is a conceptual diagram of an apparatus (one example) used when manufacturing an anodized film according to the present invention.

[Explanation of symbols]

 1 Anode (metal titanium substrate) 2 Cathode 3 Electrolyte 4 DC power supply

Claims (2)

[Claims] 1. A member having at least a surface made of titanium
From a titanium-containing composite oxide film or a metal-doped titania film obtained by anodizing in an electrolyte solution containing 0.06 to 0.2 mol / l glycerophosphate and at least one kind of 0.01 to 1 mol / l metal acetate. Method for producing a photocatalyst. 2. The method for producing a photocatalyst according to claim 1, further comprising hydrothermal treatment at high temperature and high pressure.