JP5161555B2 - Method for producing tungsten oxide photocatalyst - Google Patents

Description

  The present invention relates to a method for producing a tungsten oxide photocatalyst that exhibits high photocatalytic activity when irradiated with light from a practical light source such as a fluorescent lamp.

  When a semiconductor is irradiated with light having energy higher than the band gap, electrons in the valence band are excited to the conduction band, and holes are generated in the valence band and electrons are generated in the conduction band. These have strong oxidizing power and reducing power, respectively, and exert a redox action on the molecular species in contact with the semiconductor. Such an action is called a photocatalytic action, and by utilizing this photocatalytic action, organic substances in the atmosphere can be decomposed and removed. Specifically, in the decomposition reaction of organic substances by photocatalysis, when holes generated in the valence band directly oxidize and decompose organic substances, or holes oxidize water and active oxygen species generated therefrom oxidize and decompose organic substances. In addition, it is considered that the electrons generated in the conduction band reduce oxygen, and the active oxygen species generated therefrom oxidatively decompose the organic matter.

  Conventionally, titanium oxide is generally used as a substance exhibiting a photocatalytic action, and photocatalysts in which titanium oxide particles are dispersed or supported on various media or carriers have been put into practical use. However, although titanium oxide shows a good photocatalytic action under the irradiation of light in the ultraviolet region with a relatively short wavelength such as sunlight, in an indoor space illuminated with a light source that occupies most of the visible light, such as a fluorescent lamp. In some cases, sufficient photocatalytic action is hardly exhibited. Therefore, in recent years, a visible light responsive photocatalyst with enhanced photocatalytic activity under visible light irradiation has attracted attention, and as such a photocatalyst, tungsten oxide has been known to be effective so far. .

  For example, a tungsten oxide-titanium oxide mixed photocatalyst obtained by mixing and baking ammonium metatungstate in titanium oxide has been proposed (see Patent Document 1). However, this photocatalyst may not be able to obtain a sufficient photocatalytic action depending on the production conditions or the type of light irradiated during use, and has not yet achieved satisfactory performance.

On the other hand, it is known that tungsten oxide can be produced by a method of neutralizing sodium tungstate (Na ₂ WO ₄ ) with hydrochloric acid, followed by washing and baking, or a method of thermally decomposing ammonium paratungstate ( Non-patent document 1). However, the tungsten oxide obtained by these methods did not necessarily exhibit a sufficiently high photocatalytic activity when used alone as a photocatalyst.
Thus, although tungsten oxide is expected to be used for a visible light responsive photocatalyst, a tungsten oxide photocatalyst exhibiting high photocatalytic activity under irradiation with visible light has not yet been put into practical use.

JP 2006-198464 A New Experimental Chemistry Course 8 Synthesis of Inorganic Compounds I (Maruzen Co., Ltd.)

  Therefore, an object of the present invention is to provide a method for producing a tungsten oxide photocatalyst that can exhibit high photocatalytic activity by visible light contained in a large amount of light from a practical light source such as a fluorescent lamp.

  The present inventors have intensively studied to solve the above problems. As a result, in producing tungsten oxide by firing, it was determined that the combination of the type of raw material used for firing and the firing temperature has a significant effect on the photocatalytic activity of tungsten oxide, and paratungstic acid or its salt was selected as the raw material. It has been found that baking this at a temperature of 480 ° C. or higher is most effective in enhancing the photocatalytic activity under irradiation with visible light. Furthermore, as a reason why the conventional tungsten oxide did not exhibit the photocatalytic activity as expected by visible light, it was determined that some impurities generated by heating during firing were a factor inhibiting photocatalytic activity. It has been found that higher impurities of photocatalytic activity can be obtained by removing impurities that cause inhibition by washing the obtained tungsten oxide. In addition, the effect of improving the photocatalytic activity by this washing produces tungsten oxide. It has also been found that it is important to calcine at a temperature of 650 ° C. or less using paratungstic acid or a salt thereof as a raw material, depending on the raw material and the firing temperature. . The present invention has been completed based on these findings.

That is, the present invention has the following configuration.
(1) A tungsten oxide photocatalyst comprising calcining paratungstic acid or a salt thereof at 480 to 650 ° C., and then washing the obtained tungsten oxide powder with at least one of water and hydrogen peroxide. Manufacturing method.
(2) the washing, dispersing in the solvent, stirred and the solid-liquid carried by consisting separation operation, the (1) Symbol mounting method for manufacturing the tungsten oxide photocatalyst of.

  ADVANTAGE OF THE INVENTION According to this invention, the tungsten oxide photocatalyst body which can show high photocatalytic activity under the light irradiation by practical light sources, such as a fluorescent lamp, can be provided. This makes it possible to efficiently oxidize and decompose organic substances even in an environment where ultraviolet light is not irradiated.

In the method for producing a tungsten oxide photocatalyst of the present invention, first, paratungstic acid or a salt thereof (hereinafter referred to as “paratungstic acid (salt)”) is fired at a specific firing temperature. Thereby, the tungsten oxide which can express high photocatalytic activity can be produced | generated.
Examples of paratungstic acid (salt) to be used for firing include paratungstic acid, ammonium paratungstate, sodium paratungstate, potassium paratungstate, cesium paratungstate, and the like. However, in the case of an alkali metal salt, there is a possibility that the photocatalytic activity is lowered due to residual alkali metal, and paratungstic acid and ammonium paratungstate without such a concern are particularly preferable. Only one type or two or more types of paratungstic acid (salt) to be used for firing may be used.

  The shape and form of paratungstic acid (salt) to be used for firing are not particularly limited. For example, a paratungstic acid (salt) molded product previously molded into a desired shape (for example, a particle shape, a fiber shape, a tube shape, a thin film shape, etc.) for the photocatalyst body finally obtained may be subjected to firing. Good. Moreover, when using a paratungstic acid (salt) aqueous solution, the solid obtained by the method of removing the water | moisture content in aqueous solution by heating or pressure reduction, the spray-drying method, etc. may be used for baking, or paratungstic acid ( Of course, the salt) aqueous solution can be directly fired in an air-flow firing furnace.

It is important that the firing temperature is 480 to 650 ° C. When the firing temperature is less than 480 ° C., crystal dislocation does not occur in tungsten oxide (WO ₃ ), and thus high photocatalytic activity cannot be obtained. On the other hand, if the calcination temperature exceeds 650 ° C., the effect of improving photocatalytic activity by washing, which will be described later, cannot be obtained sufficiently, and if the calcination temperature is too high, the surface area of the resulting tungsten oxide is reduced. The amount of adsorption decreases and the photocatalytic activity becomes insufficient. Preferably, the lower limit of the firing temperature is 500 ° C. or higher, and the upper limit is 600 ° C. or lower. In addition, although the temperature increase rate at the time of baking is not restrict | limited, 100 degreeC / hour or more is preferable normally and 200 degreeC / hour or more is more preferable.

  The firing of paratungstic acid (salt) can be usually performed using a firing apparatus such as an airflow firing furnace, a tunnel furnace, or a rotary furnace. The firing time may be appropriately set according to the firing temperature, the type of firing apparatus, and the like, but is usually 10 minutes or longer, preferably 30 minutes or longer, and within 30 hours, preferably within 10 hours. Firing is preferably performed in an atmosphere of air, nitrogen, oxygen, argon or the like.

The BET specific surface area of tungsten oxide obtained by firing is not particularly limited, but is preferably 4.5 to 9.0 m ² / g. If the BET specific surface area is less than 4.5 m ² / g, the amount of adsorption of the reaction substrate may be reduced, and high photocatalytic activity may not be obtained. On the other hand, if the BET specific surface area exceeds 9.0 m ² / g, There is a possibility that the crystallinity is lowered and high photocatalytic activity cannot be obtained. In addition, a BET specific surface area can be measured by the nitrogen adsorption method mentioned later in an Example, for example.

In the method for producing a tungsten oxide photocatalyst of the present invention, the obtained tungsten oxide is washed after the firing. Thereby, the impurity which becomes the inhibiting factor of the photocatalytic activity adhering to the surface of a photocatalyst body is removed, and higher photocatalytic activity can be expressed.
The washing is performed using a solvent. Examples of the solvent include water, hydrogen peroxide solution, oxalic acid aqueous solution, and the like. Among these, it is preferable to perform washing using at least one of water and hydrogen peroxide water. In particular, when hydrogen peroxide water is used as a solvent, oxygen defects of tungsten oxide can be compensated and the amount of active oxygen species on the surface of tungsten oxide can be increased. Therefore, even tungsten oxide obtained by firing at a relatively low temperature can be used. It is more preferable because high photocatalytic activity can be imparted. In addition, a solvent may use only 1 type and may use 2 or more types together. Moreover, you may perform washing | cleaning in multiple times as needed, In that case, the solvent to be used can also be changed suitably.

  For example, when the sintered tungsten oxide is in the form of particles, the tungsten oxide is dispersed in a solvent, stirred for a predetermined time (usually about 5 to 60 minutes), and then solidified by solid-liquid separation. This can be done by collecting the minute. Solid-liquid separation may be performed by ordinary means such as centrifugation, decantation, and filtration. When washing is performed by such operations including dispersion in a solvent, stirring and solid-liquid separation, these operations are desirably repeated a plurality of times (usually 3 times or more) as necessary. Further, when the sintered tungsten oxide is a non-particulate molded body, for example, the surface of the tungsten oxide molded body may be washed away by washing, or the tungsten oxide molded body may be immersed in the solvent. A cleaning method such as applying ultrasonic waves can be employed.

  The amount of the solvent used for washing (total amount used in the case of performing washing several times) is not particularly limited, but is 10 times or more, preferably 20 times or more with respect to the weight of tungsten oxide used for washing. And 200 times or less, preferably 100 times or less. If the amount of the solvent is less than 10 times, the removal of impurities becomes insufficient and the photocatalytic activity may be lowered. On the other hand, even if the amount of the solvent exceeds 200 times, the effect of improving the photocatalytic activity corresponding to the increase in the amount of the solvent cannot be obtained, which tends to be disadvantageous in terms of cost.

As described above, when washing is performed by an operation consisting of dispersion in a solvent, stirring, and solid-liquid separation, it is preferable to pulverize tungsten oxide to be used for washing in advance. The pulverization may be dry pulverization that pulverizes in a dry state without adding a liquid such as water, or may be wet pulverization that adds a liquid such as water and pulverizes in a wet state. In the case of pulverization by dry pulverization, for example, a ball mill such as a rolling mill, a vibration ball mill or a planetary mill, a high-speed rotary pulverizer such as a pin mill, a pulverizer such as a medium stirring mill or a jet mill can be used. In the case of pulverization by wet pulverization, for example, a pulverizer similar to the above, such as a ball mill, a high-speed rotary pulverizer, or a medium stirring mill can be used. In addition, you may grind | pulverize after washing | cleaning or after the drying mentioned later as needed.
The tungsten oxide after washing can be dried as necessary. Drying may be performed with an air flow dryer, a medium fluidized dryer, a stationary dryer, or the like.

  The photocatalyst made of tungsten oxide produced as described above exhibits high catalytic activity when irradiated with a fluorescent light, which is a practical light source. Therefore, harmful indoors contained in the reaction substrate (for example, atmospheric air) It is possible to efficiently decompose and remove malodorous substances, organic solvents in water, agricultural chemicals, surfactants, etc.).

  When the tungsten oxide photocatalyst obtained by the production method of the present invention is applied to decompose and remove organic matter, it can be used as it is. However, if necessary, it can be mixed with various additives or dispersed (coating liquid). Or may be used after molding.

  When the tungsten oxide photocatalyst is mixed with various additives, it is preferable to select an additive that can further improve the adsorptivity and photocatalytic activity of the tungsten oxide photocatalyst. Examples of such additives include amorphous silica, silica sol, water glass, silicon compounds such as organopolysiloxane, amorphous alumina, alumina sol, aluminum compounds such as aluminum hydroxide, zeolite, and kaolinite. Alkaline earth metal (hydroxide) oxides such as aluminosilicate, magnesium oxide, calcium oxide, strontium oxide, barium oxide, magnesium hydroxide, calcium hydroxide, strontium hydroxide and barium hydroxide, calcium phosphate, molecular sieve , Activated carbon, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Tc, Re, Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, Ag, Au Zn, Cd, Ga, In, Tl, Ge, Sn, Pb, Bi, La, C Hydroxides of these metal elements, oxides of these metal elements, polycondensates of organic polysiloxane compounds, phosphates, fluoropolymers, silicone polymers, acrylic resins, polyester resins, melamine resins, urethanes Examples thereof include resins and alkyd resins. These additives may be used alone or in combination of two or more.

  When the tungsten oxide photocatalyst is used as a dispersion, the tungsten oxide photocatalyst obtained by the production method of the present invention may be dispersed in an organic solvent such as water or alcohol. At that time, a dispersant may be added as necessary for the purpose of improving the dispersibility of the tungsten oxide photocatalyst. Furthermore, a known inorganic binder or organic binder can be added as necessary for the purpose of improving the adhesion to the substrate when it is formed into a coating film. Such a dispersion (coating liquid) is applied to places that can be irradiated with fluorescent light, halogen lamps, xenon lamps, light-emitting diodes, sunlight, etc., which contain a large amount of visible light, such as walls, ceilings, window glass, and tiles. Is done.

EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited to these.
In addition, about the measurement of each physical property in an Example and a comparative example, and the evaluation of the photocatalytic activity, it performed with the following method.

<Crystal structure>
The X-ray diffraction spectrum of the sample was measured using an X-ray diffractometer (“RINT2000 / PC” manufactured by Rigaku Corporation), and the crystal structure of the main component was determined from the spectrum.

<BET specific surface area>
It measured by the nitrogen adsorption method using the specific surface area measuring apparatus ("Monosorb" by Yuasa Ionics Co., Ltd.).

<Evaluation of photocatalytic activity>
The photocatalytic activity was evaluated by measuring a first-order reaction rate constant in the decomposition reaction of acetaldehyde under irradiation of light from a fluorescent lamp.
First, a sample for measuring photocatalytic activity was prepared. That is, 0.1 g of the photocatalyst was placed in a glass petri dish having an inner diameter of 60 mm, and a small amount of water was added to form a paste, and then the obtained paste was developed so as to be uniform throughout the petri dish. Subsequently, this petri dish was dried with a dryer at 110 ° C. for 1 hour to prepare a sample for photocatalytic activity measurement. The obtained sample was initially irradiated with 16 hours of black light (ultraviolet intensity ² mW / cm ² : measured by attaching a UV receiver “UD-36” manufactured by Topcon UV intensity meter “UVR-2”) for 16 hours. It was converted.

  Next, this initialized sample for photocatalytic activity measurement was placed in a sealed glass container (diameter 8 cm, height 10 cm, capacity about 0.5 L) together with the petri dish, and then the inside of the container was filled with 20% oxygen and nitrogen. The mixture gas was filled with 80% by volume, and 13.4 μmol of acetaldehyde was further sealed therein. And the light of the fluorescent lamp was irradiated with the illumination intensity of 6000 lux (measured with Minolta illuminance meter "T-10") from the outside of the container, and the decomposition reaction of acetaldehyde was performed. At this time, the concentration of acetaldehyde in the container was measured over time from the start of the reaction (start of light irradiation with a fluorescent lamp) with a photoacoustic multi-gas monitor (“1312” manufactured by INNOVA). Then, the concentration decrease of acetaldehyde with respect to the irradiation time was plotted on the logarithmic axis, and the first-order rate constant for the first 10 minutes from the start of the reaction was calculated from the slope of the obtained straight line, and this was evaluated as the acetaldehyde resolution. It can be said that the higher the first-order reaction rate constant, the higher the acetaldehyde resolution (in other words, the photocatalytic activity).

Example 1
Ammonium paratungstate (manufactured by Nippon Inorganic Chemical Industry Co., Ltd.) was calcined in air at 500 ° C. for 1 hour. At this time, the temperature rising rate was 200 ° C./hour. The powder after firing had a crystal form of tungsten oxide (WO ₃ ), and its BET specific surface area was 7.0 m ² / g.
Next, 2.0 g of the fired powder (tungsten oxide) was dispersed in 30 g of water, and then the supernatant was removed with a centrifuge. This operation (operation of dispersing in water and removing the supernatant) was repeated a total of 5 times to wash the tungsten oxide. Thereafter, the obtained powder was vacuum-dried at 50 ° C. overnight (18 hours) to obtain a photocatalyst.
When the obtained photocatalyst was used for the decomposition reaction of acetaldehyde under the irradiation of light from a fluorescent lamp and the photocatalytic activity was evaluated, the reaction rate constant was 0.0129 min ⁻¹ .

(Example 2)
Firing was performed in the same manner as in Example 1 except that the firing temperature was 600 ° C. The powder after firing had a crystal form of tungsten oxide (WO ₃ ), and its BET specific surface area was 5.3 m ² / g.
Next, the fired powder (tungsten oxide) was washed with water in the same manner as in Example 1 and then vacuum-dried to obtain a photocatalyst.
When the obtained photocatalyst was used for the decomposition reaction of acetaldehyde under the irradiation of light from a fluorescent lamp and the photocatalytic activity was evaluated, the reaction rate constant was 0.0132 min ⁻¹ .

(Example 3)
After dispersing 2.0 g of the baked powder (tungsten oxide) obtained in the same manner as in Example 1 (that is, baking ammonium paratungstate at a baking temperature of 500 ° C.) in 30 g of 5% aqueous hydrogen peroxide. The supernatant was removed with a centrifuge. This operation (operation of dispersing in hydrogen peroxide solution and removing the supernatant) was repeated a total of 5 times to wash the tungsten oxide. Thereafter, the obtained powder was vacuum-dried at 50 ° C. overnight (18 hours) to obtain a photocatalyst.
When the obtained photocatalyst was used for the decomposition reaction of acetaldehyde under the irradiation of light from a fluorescent lamp and the photocatalytic activity was evaluated, the reaction rate constant was 0.0132 min ⁻¹ .

Example 4
In the same manner as in Example 3, except that the baked powder (tungsten oxide) obtained in the same manner as in Example 2 (that is, by baking ammonium paratungstate at a calcination temperature of 600 ° C.) was used. After washing with water, it was vacuum dried to obtain a photocatalyst.
When the obtained photocatalyst was used for the decomposition reaction of acetaldehyde under the irradiation of light from a fluorescent lamp and the photocatalytic activity was evaluated, the reaction rate constant was 0.0129 min ⁻¹ .

(Comparative Example 1)
Ammonium paratungstate was fired in the same manner as in Example 1 except that the firing temperature was 400 ° C. The powder after firing was presumed to be a decomposition intermediate. The BET specific surface area was 16.0 m ² / g.
Without rinsing the obtained powder after washing with water or hydrogen peroxide water, as a photocatalyst, the decomposition reaction of acetaldehyde under the irradiation of the light of the fluorescent lamp, the photocatalytic activity was evaluated, The reaction rate constant was 0.0010 min ⁻¹ .

(Comparative Example 2)
Ammonium paratungstate was calcined in the same manner as in Example 1 except that the calcining temperature was 450 ° C. The powder after firing was a mixture of a crystal type of tungsten oxide (WO ₃ ) and a presumed decomposition intermediate. The BET specific surface area was 11.0 m ² / g.
Without rinsing the obtained powder after washing with water or hydrogen peroxide water, as a photocatalyst, the decomposition reaction of acetaldehyde under the irradiation of the light of the fluorescent lamp, the photocatalytic activity was evaluated, The reaction rate constant was 0.0043 min ⁻¹ .

(Comparative Example 3)
After washing with water in the same manner as in Example 1 except that the fired powder obtained in the same manner as in Comparative Example 2 (ie, calcining ammonium paratungstate at a firing temperature of 450 ° C.) was used, vacuum was applied. It dried and the photocatalyst body was obtained.
The obtained photocatalyst was subjected to a decomposition reaction of acetaldehyde under irradiation of light from a fluorescent lamp, and its photocatalytic activity was evaluated. The reaction rate constant was 0.0034 min ⁻¹ .

(Comparative Example 4)
Washed with hydrogen peroxide solution in the same manner as in Example 3 except that the fired powder obtained in the same manner as in Comparative Example 2 (ie, calcining ammonium paratungstate at a firing temperature of 450 ° C.) was used. Then, it vacuum-dried and obtained the photocatalyst body.
The obtained photocatalyst was subjected to a decomposition reaction of acetaldehyde under irradiation of light from a fluorescent lamp, and its photocatalytic activity was evaluated. The reaction rate constant was 0.0046 min ⁻¹ .

(Comparative Example 5)
The photocatalyst without washing the fired powder (tungsten oxide) in the same manner as in Example 1 (that is, calcining ammonium paratungstate at a calcining temperature of 500 ° C.) without washing with water or aqueous hydrogen peroxide. As a result, a decomposition reaction of acetaldehyde was performed under the irradiation of light from a fluorescent lamp, and its photocatalytic activity was evaluated. As a result, the reaction rate constant was 0.0106 min ⁻¹ .

(Comparative Example 6)
A photocatalyst without washing the powder (tungsten oxide) after firing obtained in the same manner as in Example 2 (that is, by calcining ammonium paratungstate at a firing temperature of 600 ° C.) with water or aqueous hydrogen peroxide. As a result, a decomposition reaction of acetaldehyde was performed under the irradiation of light from a fluorescent lamp, and its photocatalytic activity was evaluated. As a result, the reaction rate constant was 0.0104 min ⁻¹ .

(Comparative Example 7)
Ammonium paratungstate was calcined in the same manner as in Example 1 except that the calcining temperature was 700 ° C. The powder after firing had a crystal form of tungsten oxide (WO ₃ ), and its BET specific surface area was 3.7 m ² / g.
The obtained powder after baking (tungsten oxide) is used as a photocatalyst without washing with water or hydrogen peroxide, and acetaldehyde is decomposed under the irradiation of light from a fluorescent lamp. When evaluated, the reaction rate constant was 0.0119 min ⁻¹ .

(Comparative Example 8)
Washing with water in the same manner as in Example 1 except that the baked powder (tungsten oxide) obtained in the same manner as in Comparative Example 7 (that is, by baking ammonium paratungstate at a baking temperature of 700 ° C.) was used. Then, it was vacuum-dried to obtain a photocatalyst.
The obtained photocatalyst was subjected to a decomposition reaction of acetaldehyde under light irradiation of a fluorescent lamp, and the photocatalytic activity was evaluated. The reaction rate constant was 0.0121 min ⁻¹ .

(Comparative Example 9)
In the same manner as in Example 3, except that the baked powder (tungsten oxide) obtained in the same manner as in Comparative Example 7 (that is, by baking ammonium paratungstate at a baking temperature of 700 ° C.) was used. After washing with water, it was vacuum dried to obtain a photocatalyst.
The resulting photocatalyst was used to undergo a decomposition reaction of acetaldehyde under the irradiation of light from a fluorescent lamp, and when the photocatalytic activity was evaluated, the reaction rate constant was 0.0109 min ⁻¹ .

(Comparative Example 10)
Ammonium paratungstate was fired in the same manner as in Example 1 except that the firing temperature was 800 ° C. The powder after firing had a crystal form of tungsten oxide (WO ₃ ), and its BET specific surface area was 1.0 m ² / g.
The obtained powder after baking (tungsten oxide) is used as a photocatalyst without washing with water or hydrogen peroxide, and acetaldehyde is decomposed under the irradiation of light from a fluorescent lamp. When evaluated, the reaction rate constant was 0.0080 min ⁻¹ .

(Comparative Example 11)
Washing with water in the same manner as in Example 1 except that the fired powder (tungsten oxide) obtained in the same manner as in Comparative Example 10 (that is, calcining ammonium paratungstate at a firing temperature of 800 ° C.) was used. Then, it was vacuum-dried to obtain a photocatalyst.
The obtained photocatalyst was subjected to a decomposition reaction of acetaldehyde under light irradiation of a fluorescent lamp, and the photocatalytic activity was evaluated. As a result, the reaction rate constant was 0.0091 min ⁻¹ .

(Comparative Example 12)
In the same manner as in Example 3, except that the baked powder (tungsten oxide) obtained in the same manner as in Comparative Example 10 (that is, by baking ammonium paratungstate at a baking temperature of 800 ° C.) was used. After washing with water, it was vacuum dried to obtain a photocatalyst.
The obtained photocatalyst was subjected to a decomposition reaction of acetaldehyde under irradiation of light from a fluorescent lamp, and its photocatalytic activity was evaluated. The reaction rate constant was 0.0081 min ⁻¹ .

(Comparative Example 13)
Ammonium paratungstate was fired in the same manner as in Example 1 except that the firing temperature was 900 ° C. The powder after firing had a crystal form of tungsten oxide (WO ₃ ), and its BET specific surface area was 0.3 m ² / g.
The obtained powder after baking (tungsten oxide) is used as a photocatalyst without washing with water or hydrogen peroxide, and acetaldehyde is decomposed under the irradiation of light from a fluorescent lamp. When evaluated, the reaction rate constant was 0.0043 min ⁻¹ .

(Comparative Example 14)
Washing with water in the same manner as in Example 1 except that the fired powder (tungsten oxide) obtained in the same manner as in Comparative Example 13 (ie, calcining ammonium paratungstate at a firing temperature of 900 ° C.) was used. Then, it was vacuum-dried to obtain a photocatalyst.
When the obtained photocatalyst was used for the decomposition reaction of acetaldehyde under the irradiation of light from a fluorescent lamp and the photocatalytic activity was evaluated, the reaction rate constant was 0.0049 min ⁻¹ .

(Comparative Example 15)
In the same manner as in Example 3, except that the fired powder (tungsten oxide) obtained in the same manner as in Comparative Example 13 (that is, calcined ammonium paratungstate at a firing temperature of 900 ° C.) was used. After washing with water, it was vacuum dried to obtain a photocatalyst.
The obtained photocatalyst was subjected to a decomposition reaction of acetaldehyde under irradiation of light from a fluorescent lamp, and its photocatalytic activity was evaluated. The reaction rate constant was 0.0046 min ⁻¹ .

As in Examples 1 to 4, after baking at 480 to 650 ° C. and washing, high photocatalytic activity is obtained. Specifically, from the comparison between Example 1 or 3 and Comparative Example 5 or the comparison between Example 2 or 4 and Comparative Example 6, when calcined at the specific temperature, photocatalytic activity (reaction rate constant) is obtained by washing. It is clear that this is improved by 1.3 times.
On the other hand, when calcined at a temperature lower than 480 ° C. as in Comparative Examples 1 to 4, since the paratungstate does not sufficiently become tungsten oxide, the photocatalytic activity (reaction rate constant) becomes very low, As in Comparative Examples 7 to 15, the photocatalytic activity (reaction rate constant) also decreases when calcined at a temperature exceeding 650 ° C. Specifically, in the case of firing at a temperature lower than 480 ° C. or higher than 650 ° C., the improvement effect of the photocatalytic activity (reaction rate constant) by washing is 1.1 times at most (Comparative Example 4 with respect to Comparative Example 2, It is clear that Comparative Example 9 for Comparative Example 8 and Comparative Example 12 for Comparative Example 11) do not provide sufficient photocatalytic activity.

Claims (2)

A method for producing a tungsten oxide photocatalyst comprising calcining paratungstic acid or a salt thereof at 480 to 650 ° C. and then washing the obtained tungsten oxide powder with at least one of water and hydrogen peroxide water . The washing, dispersing in the solvent, stirred and the solid-liquid carried by consisting separation method of the tungsten oxide photocatalyst according to claim 1 Symbol placement.