JP5485832B2 - Method for producing photocatalyst dispersion and method for producing photocatalyst - Google Patents

Description

  The present invention relates to a method for producing a photocatalyst dispersion and a method for producing a photocatalyst.

  The photocatalyst dispersion liquid is a dispersion of a photocatalyst that exhibits a catalytic action when irradiated with light. The photocatalyst dispersion liquid may be, for example, irradiated with visible light having lower energy than ultraviolet light. There are known ones that include particulate tungsten oxide capable of exhibiting catalytic action, particulate titanium oxide that exhibits catalytic action when irradiated with ultraviolet light, and a dispersion medium.

  Conventionally, various methods for producing this type of photocatalyst dispersion have been known. Specifically, for example, tungsten oxide particles, titanium oxide particles, and a water-containing dispersion medium are mixed and mixed. A method of preparing a liquid and then heat-treating the mixed liquid at 100 to 160 ° C. is known (Patent Document 1).

  However, in such a method for producing a photocatalyst dispersion liquid, the tungsten oxide particles and the titanium oxide particles and the water-containing dispersion medium can be solid-liquid separated in a relatively short time. There is a problem that is not enough. In addition, there is a problem that the catalyst performance of the photocatalyst deteriorates with time due to aggregation between the tungsten oxide particles and the titanium oxide particles after the production and storage of the dispersion.

JP-A-2005-231935

  In view of the above-mentioned problems, the present invention provides a photocatalyst dispersion manufacturing method capable of manufacturing a photocatalyst dispersion excellent in stability over time and in which deterioration of the catalyst performance of the photocatalyst is suppressed over time, and a photocatalyst from the photocatalyst dispersion. It is an object to provide a method for producing a photocatalyst for obtaining

  In order to solve the above-mentioned problems, a method for producing a photocatalyst dispersion according to the present invention comprises mixing a particulate tungsten oxide, particulate titanium oxide, and a water-containing dispersion medium to prepare a mixed solution, A pH adjusting step for adjusting the mixed solution to a pH not lower than the pH boundary point 0.5 lower than the isoelectric point of titanium oxide, and the pH of the mixed solution is adjusted to the pH while subjecting the mixed solution subjected to the pH adjusting step to pulverization treatment. A photocatalyst dispersion liquid containing particulate tungsten oxide, particulate titanium oxide, and a water-containing dispersion medium is produced by performing a pulverization treatment step that lowers below the boundary point.

  The method for producing a photocatalyst according to the present invention is characterized in that the water-containing dispersion medium is removed from the photocatalyst dispersion produced by the method for producing a photocatalyst dispersion.

  The method for producing a photocatalyst dispersion according to the present invention produces an effect that it is possible to produce a photocatalyst dispersion excellent in temporal stability and in which a decrease in the catalyst performance of the photocatalyst over time is suppressed.

The graph which shows the relationship between the surface charge of a titanium oxide particle and a tungsten oxide particle, and pH value. The graph which shows the result of having evaluated the temporal stability of the photocatalyst dispersion liquid. The graph which shows the time-dependent change of the acetaldehyde density | concentration accompanying the light irradiation to a photocatalyst, and a carbon dioxide level.

  Hereinafter, an embodiment of a method for producing a photocatalyst dispersion according to the present invention will be described.

  The manufacturing method of the photocatalyst dispersion liquid of the present embodiment includes a mixing step of preparing a mixed liquid by mixing particulate tungsten oxide, particulate titanium oxide, and a water-containing dispersion medium, and an isoelectric point of the titanium oxide. 0.5 pH adjustment step for adjusting the mixed solution to a pH not lower than 0.5 pH boundary point, and pulverization for reducing the pH of the mixed solution to less than the pH boundary point while subjecting the mixed solution subjected to the pH adjustment step to pulverization treatment By carrying out the treatment step, a photocatalyst dispersion liquid containing particulate tungsten oxide, particulate titanium oxide and a water-containing dispersion medium is produced.

  In the mixing step, specifically, for example, a mixed liquid can be prepared by mixing tungsten oxide particles, titanium oxide particles, and water as a water-containing dispersion medium at room temperature while stirring.

The tungsten oxide contains crystals represented by the chemical formula WO ₃ . The crystal structure of the tungsten oxide is preferably a monoclinic system.

The tungsten oxide can be obtained in the form of tungsten oxide particles, for example, by directly oxidizing metallic tungsten or by thermally decomposing a tungsten compound such as ammonium paratungstate (APT) in the air.
In addition, as a tungsten oxide particle, what is marketed can be used.

  The average primary particle diameter of the tungsten oxide particles is usually 10 nm to 90 nm, and preferably 20 nm to 60 nm in that the catalytic action of the photocatalyst by light irradiation is further increased. Moreover, the shape of the primary particles of the tungsten oxide particles is not particularly limited, and specific examples include a spherical shape, a plate shape, and a needle shape.

As the tungsten oxide, tungsten oxide in which a compound is not supported other than tungsten oxide, metal-supported tungsten oxide in which a metal element other than tungsten is supported, and the like can be used.
As the metal-supported tungsten oxide, copper-supported tungsten oxide on which copper is supported, platinum-supported tungsten oxide on which platinum is supported, gold-supported tungsten oxide on which gold is supported, palladium-supported oxide on which palladium is supported Tungsten or the like can be used. Among these, platinum-supported tungsten oxide is preferable in that the photocatalyst is superior in catalytic action by light irradiation, and copper-supported tungsten oxide is preferable in that the photocatalyst is excellent in catalytic action by light irradiation while being relatively inexpensive. preferable.

  The copper content in the copper-supported tungsten oxide is not particularly limited, but is usually 0.0001 to 3% by weight.

The copper-supported tungsten oxide is, for example, added to a tungsten oxide particle by adding an aqueous solution or an ethanol solution of a copper compound such as copper sulfate or copper nitrate, volatilizing water or ethanol, and then firing the copper-supported oxide. It can be obtained in the form of tungsten particles.
In addition, what is marketed can be used as tungsten oxide particles, such as this copper carrying | support tungsten oxide particle.

The titanium oxide contains crystals represented by the chemical formula TiO ₂ .
Examples of the crystal form of the titanium oxide include an anatase type, a rutile type, and a brookite type. Among these, as the crystal type of titanium oxide, anatase type is preferable because it is excellent in catalytic action by light irradiation.

The titanium oxide is obtained by, for example, baking a product obtained by reacting an aqueous solution of a titanium compound such as titanium oxysulfate, titanium sulfate, titanium oxychloride, or titanium chloride with a base such as ammonia or amine. It can be obtained in the form of particles.
In addition, as a titanium oxide particle, what is marketed can be used.

  Examples of the titanium oxide include doped titanium oxide doped with oxygen or an element other than titanium, undoped titanium oxide that has not been doped. The titanium oxide may be one in which a compound other than titanium oxide is supported.

Doped titanium oxide doped with an element other than oxygen or titanium can exhibit a catalytic action even when visible light having a wavelength of 400 nm or more is doped with a compound other than oxygen or titanium. Examples thereof include nitrogen-doped titanium oxide and sulfur-doped titanium oxide.
Undoped titanium dioxide that has not been doped does not exhibit a catalytic action with visible light having a wavelength of 400 nm or more, but is inexpensive and decomposes organic environmental pollutants into carbon dioxide by irradiation with ultraviolet light. It is preferable in that it can impart the performance to the photocatalyst. That is, as the titanium oxide, even undoped titanium oxide that has not been doped can be suitably used.

  The nitrogen-doped titanium oxide is, for example, at least one of titanium nitride oxide, titanium oxide, and metal titanium as a target material. After vapor deposition or ion plating of this material in an atmosphere containing nitrogen gas, ammonia gas is used. Heat treatment at a temperature of 400 ° C. or higher and 700 ° C. or lower in an atmosphere including the atmosphere, or an atmosphere containing ammonia gas, an atmosphere containing nitrogen gas, or a mixed atmosphere of nitrogen gas and hydrogen gas containing titanium oxide or hydrous titanium oxide. It can be prepared by heat treatment or the like.

  The average primary particle diameter of the titanium oxide particles is usually 5 nm to 50 nm, and is preferably 5 nm to 40 nm in that the catalytic action by light irradiation is further enhanced. Moreover, the shape of the primary particles of the titanium oxide particles is not particularly limited, and specific examples include a spherical shape, a plate shape, and a needle shape.

  The weight ratio of the tungsten oxide to the titanium oxide is preferably a weight ratio of tungsten oxide: titanium oxide = 3: 7 to 9: 1, more preferably a weight ratio of 6: 4 to 9: 1. Preferably, the weight ratio is 6: 4 to 8: 2. By such a weight ratio, the performance of the photocatalyst for decomposing organic environmental pollutants to carbon dioxide is enhanced even under the condition of being irradiated with visible light, and the stability over time of the photocatalyst dispersion is improved. There is an advantage that it can be increased.

The water-containing dispersion medium is a liquid dispersion medium containing water. The water-containing dispersion medium is not particularly limited as long as it contains water. For example, an aqueous solution containing water and an organic solvent, water, or the like can be used. As the organic solvent, alcohols such as methanol, ethanol and 2-propanol, ketones such as dimethyl ketone, and the like can be used. Especially, as a water containing dispersion medium, water is preferable at the point that it is easy to handle.
In addition, an appropriate amount of a dispersant can be appropriately added to the water-containing dispersion medium as necessary in order to increase the dispersion stability of the particles. However, the performance of the photocatalyst is improved by the dispersant remaining on the surface of the photocatalyst. It is preferable that the water-containing dispersion medium does not contain a dispersant in that it can be reduced.

In the mixing step, for example, a mixed liquid can be prepared by stirring tungsten oxide particles and titanium oxide particles in water. Further, for example, a mixture obtained by mixing tungsten oxide particles in water and stirring the titanium oxide particles in water and mixing can be prepared. In addition, for example, a mixed liquid can be prepared by adding one particle of tungsten oxide particles or titanium oxide particles in water and stirring the mixture, and then stirring the other particle.
In the mixing step, specifically, for example, stirring can be performed at room temperature using a homogenizer.
Moreover, in the said mixing process, the grinding | pulverization process (after-mentioned) which makes the secondary particle of each particle contained in a liquid mixture smaller can be performed with respect to a liquid mixture as needed.

  Next, in the pH adjustment step, a pH boundary point lower than the isoelectric point of the titanium oxide is set, and the pH of the mixed solution is adjusted to a pH equal to or higher than the pH boundary point.

The isoelectric point of the titanium oxide is a value determined as follows.
That is, in order to obtain the isoelectric point of titanium oxide, first, 900 g of pure water (JIS K0557 “Water used for water and wastewater test” A2 class) so that the particulate titanium oxide becomes 10% by weight. While adding 100 g of titanium oxide and dispersing with a homogenizer (rotation speed: 5000 rpm), the pH is adjusted to 2.5 by adding a 60 wt% nitric acid aqueous solution to prepare a titanium oxide slurry. Then, using the titanium oxide slurry, the zeta potential of titanium oxide at pH 6 and pH 8 is determined at 25 ° C., and the zeta potential is defined as the surface potential.
Specifically, the zeta potential is measured by using a 1% by weight or 0.1% by weight hydrochloric acid aqueous solution or a 1% by weight or 0.1% by weight sodium hydroxide aqueous solution in the above-mentioned pure water. The solution is adjusted to pH 6 or pH 8 by adding to the above solution using a dilute solution in which the titanium oxide slurry is added until the following titanium oxide concentration enables electrophoretic light scattering measurement. The diluted solution was subjected to electrophoretic light scattering measurement using a zeta potential measuring device (equipment name “ELS6000” manufactured by Otsuka Electronics Co., Ltd.). From the electromobility determined by the electrophoretic light scattering measurement, Smoluchowski's The zeta potential is calculated using the formula.
When calculating the electric mobility, 1.3328 is used as the refractive index of the solvent, and when calculating the zeta potential, 0.8878 (cP) is used as the viscosity of the solvent, and 78.3 is used as the dielectric constant of the solvent. Is used.
In the measurement of the surface charge at pH 6, one point in the range of pH 6 ± 0.3 is selected. In the measurement of the surface charge at pH 8, one point in the range of pH 8 ± 0.3 is selected, and the decimal point is selected. Record up to the second value. A glass electrode pH meter is used for pH measurement. Further, in titanium oxide, the surface charge is usually positive at pH 6 and negative at pH 8.
Next, a graph in which the horizontal axis represents the pH value and the vertical axis represents the zeta potential value is drawn, a straight line connecting the two measured values is drawn, and the pH value at which the surface charge (zeta potential) becomes zero (decimal point) The second point is obtained as the isoelectric point. If the surface charges are both negative at pH 6 and pH 8, the isoelectric points are obtained in the same manner by substituting the surface charges at pH 5 and pH 7, respectively.

The pH boundary point is a value determined as follows.
That is, the second decimal place of the obtained isoelectric point is rounded off to obtain an isoelectric point up to the first decimal place, and a value 0.5 lower than this isoelectric point is set as the pH boundary point.
In the case of producing a photocatalyst dispersion using two or more kinds of titanium oxide, the isoelectric point and pH in the same manner as above in the mixed titanium oxide having the same mixing ratio as the mixing ratio of titanium oxide in the photocatalyst dispersion. Find the boundary point.

In the pH adjusting step, the pH of the mixed solution can be adjusted with an alkali agent or an acid.
Specific examples of the alkali agent include ammonia, sodium hydroxide, potassium hydroxide, and the like. Among these, ammonia is preferable as the alkali agent in that it hardly remains in the produced photocatalyst dispersion.
Moreover, as the alkali agent, alkalized inorganic powder capable of increasing the pH of the mixed solution can be used. As the alkalinized inorganic powder, zeolite, apatite or the like can be used.
On the other hand, hydrochloric acid, sulfuric acid, nitric acid and the like can be used as the acid.
In the pH adjusting step, it is not always necessary to use an alkaline agent or the like as long as the pH of the mixed solution is a pH that is equal to or higher than the pH boundary point.

  In the pH adjusting step, the pH can be adjusted while stirring the mixed solution. Moreover, temperature is not specifically limited, Specifically, pH can be adjusted at normal temperature, for example, stirring a liquid mixture with the homogenizer of rotation speed 1000-10000rpm.

  In the pH adjustment step, preferably, the pH of the mixed solution is adjusted to be not less than the pH boundary point and not more than the isoelectric point. By adjusting the pH of the mixed solution to be not less than the pH boundary point and not more than the isoelectric point, there is an advantage that the manufactured photocatalyst dispersion liquid can have more excellent temporal stability.

  Next, in the pulverization step, the pH of the mixed solution is lowered below the pH boundary point while pulverizing the mixed solution that has undergone the pH adjustment step.

  The pulverization process in the pulverization step is performed to make the tungsten oxide particles and titanium oxide particles smaller particles, that is, to make the tungsten oxide and titanium oxide secondary particles smaller secondary particles. To do.

The isoelectric point of the titanium oxide can take various values depending on the difference in the crystal type of the titanium oxide, but is usually about 5 to 7. Further, the isoelectric point of the tungsten oxide can take various values as well, but is usually about 1-2.
When the pH of the mixed solution is higher than the isoelectric point of titanium oxide, the surface charges of both the tungsten oxide particles and the titanium oxide particles become negative, and the titanium oxide and tungsten oxide particles are electrically connected to each other. It is thought to repel.
On the other hand, when the pH of the mixed solution is lower than the isoelectric point of titanium oxide and higher than the isoelectric point of tungsten oxide, the surface charge of the tungsten oxide particles becomes negative and the surface charge of the titanium oxide particles becomes positive. Accordingly, it is considered that the particles of titanium oxide and tungsten oxide are electrically attracted to each other and aggregate.
Further, when the pH of the mixed solution is near the isoelectric point of titanium oxide, the absolute value of the surface charge of the titanium oxide particles is relatively small, and the electric repulsive force acting between the titanium oxide particles is relatively small. It is considered that the particles of titanium oxide aggregate together.

However, in the pulverization step, the pH of the mixed solution is made less than the pH boundary point while pulverizing the mixed solution having a pH higher than the pH boundary point 0.5 lower than the isoelectric point of titanium oxide. As a result, the photocatalyst dispersion liquid that has undergone the pulverization process is less likely to cause solid-liquid separation, and the stability over time is excellent.
The principle that the photocatalyst dispersion has excellent stability over time is not completely clear, but at a pH boundary point where the electrical repulsion between the titanium oxide particles is relatively small, When both the secondary particles and the secondary particles of tungsten oxide are subjected to a pulverization treatment, relatively stable secondary particles including both titanium oxide particles and tungsten oxide particles are newly formed after the pulverization. Conceivable. In the photocatalyst dispersion liquid having a pH lower than the pH boundary point, even if the surface charge of the titanium oxide particles is positive and the surface charge of the tungsten oxide particles is negative, the above relatively stable secondary particles are formed. Therefore, it is considered that the phenomenon of agglomeration due to electrical attraction between the titanium oxide particles and the tungsten oxide particles is less likely to occur.

In the pulverization process, the pH of the mixed solution usually decreases with the pulverization process. This is considered to be due to the following reasons.
That is, usually, the surface of the metal oxide particles is covered with OH groups generated by reacting with water vapor in the atmosphere. In water, the following reaction (M represents a metal element) can occur.
Mainly on the low pH side: M-OH + H ⁺ → MOH ⁺ ... Formula (1)
Mainly high pH ^{side: M-OH + OH - →} MO - + H 2 O ··· formula (2)
Here, regarding tungsten oxide particles, since the electronegativity of tungsten ions in tungsten oxide is relatively large, the tungsten ions relatively strongly attract electrons, and H ⁺ tends to dissociate from hydroxyl groups on the particle surface. Therefore, the reaction of the above formula (2) is relatively easy to occur. Further, as the new surface of the tungsten oxide particles increases with the pulverization treatment, the reaction of the above formula (2) can occur one after another. For these reasons, it is considered that the pH of the mixed solution is lowered with the pulverization treatment.
In addition, the electronegativity χ _i of the metal ion is obtained by the following formula (3) from the Pauling electronegativity χ _p of the metal atom and the valence Z of the metal ion)
χ _i = (1 + 2Z) χ _p Equation (3)

  In the pulverization treatment step, an acid or the like that lowers the pH can be added to the mixed solution in order to lower the pH more rapidly, and conversely, ammonia water or the like can be added to the mixed solution to make the pH lower more slowly. Can be added to.

  In the pulverization step, the width of the pH drop is not particularly limited as long as it is less than the pH boundary point, but preferably, the pH of the mixed solution that has undergone the pH adjustment step is less than the isoelectric point. The pH is lowered to 1.0 or more. By lowering the pH to 1.0 or less, there is an advantage that the temporal stability of the photocatalyst dispersion can be improved. In the pulverization process, the temperature is not limited, but the pulverization process is usually performed at room temperature.

  As the pulverization process, for example, a pulverization process using a pulverization medium that performs pulverization by the shearing force accompanying the collision of the pulverization medium in the presence of a pulverization medium that collides with each other, and pulverization is performed by the collision force between particles. An impact pulverization process, an ultrasonic pulverization process in which pulverization is performed by irradiating ultrasonic waves using an ultrasonic cleaner, or the like may be employed.

As the grinding treatment with the grinding medium, grinding with a bead mill, grinding with a wet ball mill, or the like can be employed.
Specifically, for example, the grinding medium corresponds to beads in grinding by a bead mill, and corresponds to balls in grinding by a wet ball mill. In the pulverization process performed in the presence of the pulverization medium, when the pulverization media such as beads collide with each other, a shear force acts on the secondary particles between the pulverization media, the secondary particles are pulverized, and the particle size is further increased. Secondary particles.

  As the collision force pulverization process, for example, a pulverization process using a wet jet mill can be employed. In the collision force pulverization treatment, by causing the mixed liquid obtained by mixing the secondary particles and the dispersion medium to collide with each other, the secondary particles are subjected to shearing force and the like, and the secondary particles are pulverized, and the particle diameter is further increased. Secondary particles.

In particular, as the pulverization treatment, by reducing the secondary particle diameter of tungsten oxide particles or titanium oxide particles, organic environmental pollutants are decomposed into carbon dioxide even under conditions where the amount of ultraviolet light irradiation is small. The performance of the photocatalyst is improved, the crystal structure of the tungsten oxide particles or the titanium oxide particles is less affected, and the stability over time of the photocatalyst dispersion can be improved. It is preferable to employ a pulverization treatment using a bead mill, and it is more preferable to employ a pulverization treatment using a bead mill.
The pulverization treatment by the bead mill can be performed by a general method using a conventionally known bead mill apparatus. Specifically, for example, the material may be ceramic, glass, or metal, and a plurality of spherical beads having a diameter of 0.03 to 3 mm may be used to perform a pass method or a circulation method.

  In the method for producing the photocatalyst dispersion, when a pulverization process using a pulverization medium is employed in the pulverization process, the photocatalyst dispersion obtained through the pulverization process and the pulverization medium can be separated. Specifically, for example, the photocatalyst dispersion liquid is allowed to pass through a mesh fabric that cannot pass the pulverization medium and the photocatalyst dispersion liquid can pass through, and the dispersion medium stays on the mesh, thereby separating the photocatalyst dispersion liquid and the pulverization medium. Can do.

  In addition, pH of the photocatalyst dispersion liquid produced by the method for producing the photocatalyst dispersion liquid is adjusted as necessary, and can be appropriately adjusted to a predetermined pH.

  In addition, the photocatalyst dispersion liquid produced by the method for producing a photocatalyst dispersion liquid can be used as it is, for example, as a coating agent for application to a coating object.

Then, one Embodiment of the manufacturing method of the photocatalyst based on this invention is described.
In the photocatalyst production method, the water-containing dispersion medium is removed from the photocatalyst dispersion produced by the photocatalyst dispersion production method.

  As a method for removing the water-containing dispersion medium from the photocatalyst dispersion liquid, for example, a method of placing the photocatalyst dispersion liquid at a temperature at which the water-containing dispersion medium volatilizes can be employed. Specifically, the photocatalyst can be produced, for example, by placing the photocatalyst dispersion under a temperature condition of 100 ° C. or more and evaporating the water-containing dispersion medium.

  The photocatalyst produced by the method for producing a photocatalyst does not necessarily have to be calcined, and can exhibit a catalytic action by light irradiation without being calcined. Preferably, the photocatalyst is produced without being fired in that the cost during production can be suppressed. Note that the firing means heating to a temperature at which the organic matter can be decomposed to sinter the subject to be fired. Specifically, for example, the subject to be fired in the presence of oxygen or in the absence of oxygen. Heating to 200 ° C. or higher.

  The photocatalyst is used, for example, under conditions where light from various lamps such as sunlight and fluorescent lamps is irradiated, and exhibits a catalytic action by light irradiation. More specifically, the photocatalyst is excellent in performance of decomposing an organic environmental pollutant to carbon dioxide even by irradiation with visible light. The principle of superior performance in decomposing organic environmental pollutants to carbon dioxide is not always fully understood, but the difference in the band structure of tungsten oxide and the band structure of titanium oxide This is considered to be due to the fact that electrons are efficiently exchanged between the two compounds during irradiation and the oxidation-reduction reaction is promoted.

  In addition to the tungsten oxide and the titanium oxide, the photocatalyst may include an oxide of a metal such as silicon or zirconium, zeolite, apatite, or the like.

  The photocatalyst can be used as a coating agent by mixing with a paint such as a liquid paint or a powder paint. Examples of the liquid paint include those in which a base resin is dissolved in an organic solvent, and aqueous emulsions in which the base resin is dispersed in water.

  The said coating agent can hold | maintain the said photocatalyst in the surface vicinity of an application target object, for example by apply | coating to an application target object. The photocatalyst member formed by applying the coating agent to the application object can decompose organic harmful substances and organic pollutants in the air to carbon dioxide even when irradiated with visible light. .

The method for producing a photocatalyst dispersion and the method for producing a photocatalyst according to this embodiment are as described above, but the present invention is not limited to the method for producing a photocatalyst dispersion and the method for producing a photocatalyst exemplified above. .
In addition, various modes used in a general photocatalyst dispersion manufacturing method and a photocatalyst manufacturing method can be employed as long as the effects of the present invention are not impaired.

  EXAMPLES Next, although an Example is given and this invention is demonstrated in more detail, this invention is not limited to these.

(Measurement of isoelectric point of titanium oxide)
The isoelectric point of titanium oxide was determined as shown below.
・ Titanium oxide particles: Trade name “ST-01” (Ishihara Sangyo Co., Ltd.)
No supported element, no doping treatment Anatase type crystal Secondary particle size D10 = 1.6μm D50 = 7.1μm D90 = 22.7μm
The secondary particle diameter was measured by a light scattering method using a device name “MT3000” (manufactured by Nikkiso Co., Ltd.).
900 g of pure water (JIS K0557 “Water used for water and wastewater testing” A2 class pure water production device “ElixUV5” manufactured by Nihon Millipore, Inc., so that the solid content concentration is 10% by weight. In addition, a titanium oxide slurry was prepared by stirring at a rotational speed of 5000 rpm using a homogenizer (trade name “Homogenizer” manufactured by ASONE). While stirring the slurry with a homogenizer, a 60% by weight nitric acid aqueous solution was added, and the pH was measured at 20 ° C. with a glass electrode pH meter (trade name “pH meter D-50” manufactured by Horiba, Ltd.). A titanium oxide slurry was prepared.
On the other hand, a 0.1 wt% hydrochloric acid aqueous solution is added to the pure water, and a part of the titanium oxide slurry is further added until the titanium oxide concentration is measurable with a zeta potential measuring device, and finally the pH is 5.92. A diluted titanium oxide solution was prepared. For this diluted solution, electrophoretic light scattering measurement was performed at 25 ° C. using a zeta potential measurement device (trade name “ELS6000” manufactured by Otsuka Chemical Co., Ltd.), and the zeta potential was calculated from the obtained electric mobility using the Smoluchowski equation. did. The calculated zeta potential value was defined as the surface charge. As a result, the surface charge of the titanium oxide particles at pH 5.92 was +18.
Further, the pH of the diluted solution was adjusted to 8.27 by adding a 0.1 wt% aqueous sodium hydroxide solution, and the surface charge of titanium oxide was measured in the same manner. As a result, the surface charge of the titanium oxide particles at pH 8.27 was −14.
On the other hand, the following operation was performed for reference. That is, by adding 0.1% by weight or 1.0% by weight hydrochloric acid aqueous solution or 0.1% by weight or 1.0% by weight sodium hydroxide aqueous solution, the pH of the diluted solution is adjusted to 2 in the same manner. The surface charge when adjusted to 0.08, 4.08, and 9.95 was measured.

(Measurement of isoelectric point of tungsten oxide)
A tungsten oxide slurry containing 10% by weight of copper-supported tungsten oxide was prepared by the same method as that described above, except that 60% by weight of nitric acid aqueous solution was not added. The surface charge was measured at 03, 4.10, 6.08, 8.83, 10.08. Copper-supported tungsten oxide particles: product number “HP-CW091”
(Showa Titanium)
Secondary particle size D10 = 0.5μm D50 = 5.3μm D90 = 22.0μm
The secondary particle diameter was measured by a light scattering method using a device name “MT3000” (manufactured by Nikkiso Co., Ltd.).

  Table 1 shows the results of surface charge at each pH of each slurry. A graph of the results is shown in FIG.

(Calculation of pH boundary point)
Based on the result of the surface charge of titanium oxide at pH 5.92 and pH 8.27, the isoelectric point at which the surface charge was 0 was determined by the method described above, and the isoelectric point was 7.2. Therefore, the pH boundary point was set to 6.7, which is 0.5 lower than this value.

Example 1
A photocatalyst dispersion was produced as shown below.
That is, the copper-supported tungsten oxide and the titanium oxide were mixed with pure water by a homogenizer at a rotational speed of 5000 rpm so that the total concentration of copper-supported tungsten oxide: titanium oxide = 7: 3 was 10% by weight. Thus, a mixed solution was prepared and a mixing step was performed.
While stirring the mixed solution with a homogenizer with a rotation speed of 5000 rpm, 25% by weight ammonia water was added to the mixed solution until the pH reached 6.90, and a pH adjustment step was performed.
Next, the mixed solution that has undergone the pH adjustment step is subjected to a grinding process with a bead mill for 20 minutes using a bead mill device (device name “Ultra Apex Mill UAM-015” manufactured by Kotobuki Kogyo Co., Ltd.), and pulverized until the pH reaches 5.21. Processing steps were performed.
And the obtained photocatalyst dispersion liquid and beads were separated to produce a photocatalyst dispersion liquid.

(Example 2)
A photocatalyst dispersion liquid was produced in the same manner as in Example 1 except that the mixed solution that had undergone the pH adjustment step was pulverized by a bead mill for 40 minutes and the pulverization step was performed until the pH reached 5.07.

(Example 3)
A photocatalyst dispersion liquid was produced in the same manner as in Example 1 except that the mixed liquid that had undergone the pH adjustment process was pulverized by a bead mill for 60 minutes and the pulverization process was performed until the pH reached 4.84.

(Example 4)
The pH adjustment step was performed by adding 25% by weight ammonia water to the mixture until the pH reached 8.24. The mixture after the pH adjustment step was pulverized with a bead mill for 20 minutes to reach pH 5.72. A photocatalyst dispersion was produced in the same manner as in Example 1 except that the pulverization treatment step was performed.

(Example 5)
The pH adjustment step was performed by adding 25% by weight ammonia water to the mixture until the pH reached 8.24, and the mixture after the pH adjustment step was crushed by a bead mill for 40 minutes to reach pH 5.08. A photocatalyst dispersion was produced in the same manner as in Example 1 except that the pulverization treatment step was performed.

(Example 6)
The pH adjustment step was performed by adding 25% by weight ammonia water to the mixture until the pH reached 8.24. The mixture after the pH adjustment step was pulverized by a bead mill for 60 minutes to reach pH 4.65. A photocatalyst dispersion was produced in the same manner as in Example 1 except that the pulverization treatment step was performed.

(Example 7)
24 g of the above titanium oxide particles and 20 g of zeolite particles (trade name “Hisiv-3000” manufactured by Showa Union Co., Ltd.) were placed in pure water (600 g) and stirred with a homogenizer to obtain a suspension (pH 8.87). This suspension and a suspension (pH 3.98) obtained by adding 56 g of the above tungsten oxide particles to 300 g of pure water and stirring with a homogenizer, titanium oxide, tungsten oxide particles (3: 7 weight ratio), zeolite particles, A mixed solution was prepared by mixing so that the total amount of was 10% by weight, and the mixing step was performed.
The pH of the mixed solution after the mixing step was 6.88. That is, the pH adjustment step was performed by adding zeolite particles.
Then, the pulverization treatment step was carried out in the same manner as in Example 1 except that the pulverization treatment with a bead mill was performed for 20 minutes on the mixed solution that had undergone the pH adjustment step, and the pulverization treatment step was carried out until the pH reached 6.41. Moreover, the obtained photocatalyst dispersion liquid and beads were separated to produce a photocatalyst dispersion liquid.

(Example 8)
A photocatalyst dispersion liquid was produced in the same manner as in Example 7 except that the mixed liquid that had undergone the pH adjustment process was pulverized by a bead mill for 40 minutes, and the pulverization process was performed until the pH reached 6.17.

Example 9
A photocatalyst dispersion liquid was produced in the same manner as in Example 7 except that the mixed liquid that had undergone the pH adjustment process was pulverized by a bead mill for 60 minutes and the pulverization process was performed until the pH reached 5.92.

(Comparative Example 1)
The pH of the mixed solution that passed through the mixing step in the same manner as in Example 1 was 6.01. The pH adjustment step was not performed, and the mixture was pulverized in the same manner as in Example 1.
When the pulverization was performed, solid-liquid separation occurred, and thus the photocatalyst dispersion liquid could not be produced.

(Comparative Example 2)
Only the above titanium oxide was mixed with pure water so as to be 10% by weight to prepare a titanium oxide dispersion. The pH of this titanium oxide dispersion was 6.68.
The pH of the titanium oxide dispersion was adjusted to 8.47 with 25% by weight ammonia water in the same manner as in Example 1. Then, the grinding | pulverization process by a bead mill was performed like Example 1. FIG.
When the pulverization was performed, solid-liquid separation occurred, and thus the photocatalyst dispersion liquid could not be produced.

(Comparative Example 3)
Only the copper-supported tungsten oxide was mixed with pure water so as to be 10% by weight to prepare a titanium oxide dispersion. The copper-supported tungsten oxide dispersion had a pH of 4.08.
The pH of the copper-supported tungsten oxide dispersion was adjusted to 7.98 with 25 wt% ammonia water in the same manner as in Example 1. Then, the grinding | pulverization process by a bead mill was performed like Example 1. FIG.
The pulverization process was carried out for 20 minutes, and when the pH reached 3.93, the pulverization process step was terminated to produce a photocatalyst dispersion.

<Evaluation of stability over time of photocatalyst dispersion>
The produced photocatalyst dispersion was allowed to stand at room temperature, and the particle size of the photocatalyst dispersion after standing for a predetermined number of days was measured over time to evaluate the temporal stability of the photocatalyst dispersion. The particle size was measured by a dynamic light scattering method using a particle size distribution measuring device (device name “FPER-1000” manufactured by Otsuka Electronics Co., Ltd.).
FIG. 2 shows the results of measuring the particle diameters (D10, D50, D90) of the photocatalyst dispersion liquids in Examples 1 to 3 over time. In Examples 1 to 3, solid-liquid separation was not observed even 52 days after production.

<Evaluation of photocatalysis by light irradiation>
The photocatalyst dispersion liquid produced in each Example and each Comparative Example was placed at 100 ° C. immediately after production or after standing for 52 days, and water was removed by volatilization to produce a photocatalyst.
Each photocatalyst was pulverized with a mortar and pestle to obtain a photocatalyst powder. 0.1 g of the photocatalyst powder was placed in a petri dish having an inner diameter of φ58 mm, and an appropriate amount of pure water was added to uniformly disperse the powder. Thereafter, the petri dish was placed under a condition of 100 ° C., and a photocatalytic film was produced in the petri dish.
The petri dish on which the photocatalyst film was produced was placed in a 2 L capacity Tedlar bag, sealed, and 1 L of pure air was injected therein. In addition, gaseous acetaldehyde was added to a concentration of approximately 100 ppm.
In order to wait for the acetaldehyde to adsorb to the surface of the photocatalyst powder and reach an equilibrium state, after leaving the Tedlar bag in the dark for 16 to 18 hours, use a daylight color fluorescent lamp ("FL20SS-ENW / 18X" manufactured by Panasonic). Light irradiation (measured with 4000 Lux Konica Minolta "T-10" JIS AA class), and the amount of acetaldehyde and carbon dioxide was measured by gas chromatography ("GC-9A" manufactured by Shimadzu Corporation). Measurements with the course were obtained.
In addition, light irradiation was performed on the conditions which cut the shorter wavelength side than 410 nm in the irradiation light of a daylight color fluorescent lamp using "UGP20WL10" (made by Fuji Film).
And the density | concentration of acetaldehyde was measured with time, and the acetaldehyde reduction constant was computed from the measurement result. Further, the concentration of carbon dioxide was measured over time, and the carbon dioxide production rate was determined from the measurement results. Details of the calculation method of the acetaldehyde reduction constant and the carbon dioxide production rate are as follows.
[Calculation method of acetaldehyde reduction constant]
Since the acetaldehyde reduction rate depends on the initial concentration, in order to cancel the initial concentration, the logarithm of the concentration with respect to time was plotted, an approximate line was drawn in the linear region, and the slope of the approximate line was defined as the acetaldehyde reduction constant. The unit is the reciprocal of time.
[Calculation method of carbon dioxide production rate]
In the linear region of the plot of the carbon dioxide concentration generated in the initial stage, an approximate straight line passing through the origin was drawn, and the slope thereof was defined as the carbon dioxide production rate.
When acetaldehyde is completely decomposed into carbon dioxide, the carbon dioxide concentration is theoretically about 200 ppm.

About Examples 1-3 and the comparative example 3, according to the above-mentioned method, evaluation of the photocatalysis by light irradiation was performed simultaneously.
A graph showing changes with time of the acetaldehyde concentration and the carbon dioxide concentration is shown in FIG. Specifically, FIG. 3 (a) shows the results of measuring the time-dependent changes in the acetaldehyde concentration and the carbon dioxide concentration for Example 1 and Comparative Example 3, and FIG. 3 (b) is similarly carried out. The results measured for Example 2 and Comparative Example 3 are shown, and FIG. 3C shows the results measured for Example 3 and Comparative Example 3 in the same manner. In each example, the photocatalyst obtained by removing water immediately after producing the photocatalyst dispersion (described as “immediately after production”) and the photocatalyst dispersion produced were allowed to stand for 52 days. Each result of the photocatalyst (described as “52 days after production”) is shown.

Claims (2)

  A mixing step of mixing a particulate tungsten oxide, a particulate titanium oxide and a water-containing dispersion medium to prepare a mixed solution; and a pH not lower than a pH boundary point 0.5 lower than the isoelectric point of the titanium oxide. By carrying out a pH adjusting step for adjusting the mixed solution, and a pulverizing treatment step for reducing the pH of the mixed solution to less than the pH boundary point while subjecting the mixed solution that has undergone the pH adjusting step to a pulverizing treatment, A method for producing a photocatalyst dispersion, comprising producing a photocatalyst dispersion containing tungsten oxide, particulate titanium oxide, and a water-containing dispersion medium.   A method for producing a photocatalyst comprising removing a water-containing dispersion medium from a photocatalyst dispersion produced by the production method according to claim 1.

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