JP4184451B2 - Method for producing titania-based catalyst - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates to a titania-based catalyst, and more particularly to a method capable of easily producing a titania-based catalyst having excellent photocatalytic activity.
[0002]
[Prior art]
Titania is widely known as a material excellent in catalytic activity, and various photocatalytic reactions have been studied.
[0003]
The photocatalytic reaction consists of (1) a reactive molecule in the proximity of the catalyst surface, (2) a chemical reaction by reducing or oxidizing the reactive molecule on the catalyst surface by the electrons and holes excited by light irradiation inside the catalyst. It is thought to promote.
[0004]
As means for improving the catalyst activity, refinement of the catalyst powder leading to an increase in the specific surface area of the catalyst (increase in the catalyst active point), or a crystal grain size (single crystal) even if it is polycrystalline Therefore, it can be expected that the diameter is reduced, that is, crystal refinement is an effective means. As a method for crystal refinement, a sol-gel method, a gas phase method, and the like are known.
[0005]
Furthermore, in the research paper “Preparation and Photocatalytic Activity of TiO ₂ —SiO ₂ Catalysts Supported on Platinum and Ruthenium” published in “Science and Industry Vol. 66” published in January 1992, the titania-based catalyst powder was prepared by the sol-gel method. It has been reported that the crystal grain size becomes small when silica (SiO ₂ ) is doped (added) in preparing the above.
[0006]
However, in the titania-based catalyst, assuming that platinum and ruthenium are supported as catalyst assistants to increase the photocatalytic activity, TiO ₂ —SiO ₂ plays a role as a carrier that hardly expects catalytic activity. It is estimated that
[0007]
And since these carrying | support operations are performed in the form of chloride aqueous solution in a noble metal, it is troublesome and tends to be expensive.
[0008]
Further, as a result of the examination by the present inventors, it was found that it is difficult to obtain a high catalytic activity only with TiO ₂ —SiO ₂ .
[0009]
Therefore, the present inventors have specially proposed a method for producing a titania-based catalyst that can increase the catalytic activity only with TiO ₂ or TiO ₂ —SiO ₂ without carrying a noble metal promoter or the like. In Japanese Patent Application No. 7-67893 (unpublished at the time of filing this application) and at a conference presentation (Japan Ceramic Society Annual Meeting, April 2, 1995), a method having the following configuration was previously proposed.
[0010]
“After hydrolyzing sol in which titanium alkoxide and silicon alkoxide are mixed so that the molar ratio of (1-x) TiO ₂ · xSiO ₂ (x = 0 to 0.5) is gelled, the gelled product is 350 to A method for producing a titania-based catalyst by firing at 1200 ° C.,
The titania-based catalyst after calcination is subjected to surface treatment (chemical treatment) with acid or alkali. "
[0011]
[Problems to be solved by the invention]
However, there is a demand for further increasing the activity of the titania-based catalyst obtained by the above method.
[0012]
In view of the above, the present invention provides a method for producing a titania-based catalyst that can increase the catalytic activity only with TiO ₂ or TiO ₂ —SiO ₂ without carrying a precious metal promoter or the like. It is an object of the present invention to provide a method for producing a titania-based catalyst capable of increasing the catalyst.
[0013]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the present inventors paid attention to the possibility of some influence on the TiO ₂ crystal if the chemical treatment conditions are changed. The inventors have come up with a method for producing a titania-based catalyst.
[0014]
(1-x) After hydrolyzing sol mixed with titanium alkoxide and silicon alkoxide so as to have a molar ratio of TiO ₂ · xSiO ₂ (x = 0 to 0.5), heat treatment temperature is 350 to 1200 ° C. After baking and further chemical treatment with alkali or acid, re-heat treatment is performed at a temperature of 200 to 1200 ° C.
[0015]
[Detailed description of the means]
Hereinafter, the manufacturing method of the titania catalyst of the present invention will be described with reference to FIG.
[0016]
(1) The method for producing the titania-based catalyst of the present invention is produced by gelling a hydrolyzed sol (colloid solution) in which titanium alkoxide and silicon alkoxide are mixed at a predetermined ratio, and then firing (crystallizing) the gelled product. This is the first premise structure.
[0017]
Here, the molar ratio of titania and silica is x = 0 to 0.5, preferably x = 0.02 to 0.25 in the formula (1-x) TiO ₂ · xSiO ₂ .
[0018]
Although titania alone may be used, it is desirable to dope (add) a small amount of silica to reduce the crystal grain size as described above. The range is x = 0.02 to 0.25. When x is less than 0.02, it is difficult to obtain the effect of silica addition (crystal refinement: increase in specific surface area). When the amount of silica added is excessive, crystal refinement is not promoted so much and relatively titania. On the other hand, the catalytic activity decreases. (See FIG. 1, which is a graph showing the relationship between the amount of silica added and the specific surface area.)
Moreover, although baking conditions change with ratios of a silica, it is 350-1200 degreeC x 0.3-2h normally, It is desirable that it is 450-600 degreeC x 0.5-1.5h normally. It is desirable that the temperature is relatively low because crystal refinement (increase in specific surface area) can be achieved (see FIG. 2 which is a graph showing the relationship between the calcination temperature and catalyst activity). However, if it is too low, firing becomes difficult and the firing time becomes relatively long, and productivity is lowered.
[0019]
The titania-based catalyst (titania-silica composite fine particle crystal) used in the present invention is prepared, for example, by the method described in JP-A-5-58649.
[0020]
It is not simply a mixture of silica fine particles and titania fine particles, but is prepared by gelling a hydrolyzed sol solution of silicon alkoxide and titanium alkoxide so as to have a predetermined ratio, followed by firing and, if necessary, grinding.
[0021]
When preparing the hydrolyzed sol solution, after mixing titanium alkoxide and silicon alkoxide, hydrolyzing at the same time, or partially hydrolyzing one (hereinafter referred to as prehydrolysis), then adding the other, There is a method of hydrolysis (hereinafter referred to as final hydrolysis). The latter method is particularly used when the hydrolysis rate of the silicon alkoxide and titanium alkoxide to be used is greatly different and precipitation is likely to occur during hydrolysis.
[0022]
As the titanium alkoxide, a compound represented by the general formula Ti (OR) ₄ (where R is an alkyl group or an alkoxyalkyl group), or one or two alkoxide groups (OR) in the general formula is a carboxyl group. Alternatively, a compound substituted with a β-dicarbonyl group or a mixture thereof is preferable.
[0023]
Specific examples of the titanium alkoxide include Ti (O-isoC ₃ H ₇ ) ₄ , Ti (O-nC ₄ H ₉ ) ₄ , Ti (O—CH ₂ CH (C ₂ H ₅ ) C ₄ H ₉ ). ₄ , Ti (OC ₁₇ H ₃₅ ) ₄ , Ti (O-isoC ₃ H ₇ ) ₂ [CO (CH ₃ ) CHCOCH ₃ ] ₂ , Ti (O-nC ₄ H ₉ ) ₂ [OC ₂ H ₄ N (C _{2 H 4 OH) 2] 2} , Ti (OH) 2 [OCH (CH 3) COOH] 2, Ti (OCH 2 CH (C 2 H 5) CH (OH) C 3 H 7) 4, Ti (O- nC ₄ H ₉ ) ₂ (OCOC ₁₇ H ₃₅ ) and the like.
[0024]
There are various types of silicon alkoxides, but examples of those which are easily available industrially include, for example, compounds represented by the general formula Si (OR ¹ ) ₄ (where R ¹ is an alkyl group or an alkoxyalkyl group), or the above general formula A compound in which one or two alkoxide groups (OR ¹ ) are substituted with a carboxyl group or a β-dicarbonyl group, or a low condensate obtained by partially hydrolyzing them or a mixture thereof. Used without limitation. Examples of the alkyl group include lower alkyl groups such as a methyl group, an ethyl group, an isopropyl group, and a butyl group. Examples of the alkoxyalkyl group include a methoxymethyl group, an ethoxymethyl group, a methoxyethyl group, an ethoxyethyl group, and a methoxypropyl group. Groups and the like can be preferably mentioned. These silicon alkoxides may be used as they are or after being purified by distillation.
[0025]
The hydrolysis is performed with stirring in the presence of a solvent such as alcohol, an acid or a basic catalyst, if necessary, in addition to water. At this time, it is desirable to perform hydrolysis in a water bath or a hot water bath. A catalyst and a solvent such as alcohol are not necessarily required, but the catalyst is effective in increasing the speed of hydrolysis and polycondensation, and the solvent such as alcohol is effective in suppressing the generation of precipitates and preparing a more uniform sol solution. There is.
[0026]
As the catalyst, an acid or a basic compound is used as it is or in a state in which it is dissolved in a solvent such as water or alcohol (hereinafter referred to as an acidic catalyst and a basic catalyst, respectively). The concentration at that time is not particularly limited, but when the concentration is high, the hydrolysis and polycondensation rates tend to increase. However, when a basic catalyst having a high concentration is used, a precipitate may be generated in the sol solution. Therefore, the concentration of the basic catalyst is preferably 1 N (concentration in aqueous solution) or less.
[0027]
The type of acidic catalyst or basic catalyst is not particularly limited, but when a catalyst having a high concentration needs to be used, a catalyst composed of an element that hardly remains in the catalyst crystal grains after sintering is preferable. Specifically, examples of the acidic catalyst include hydrogen halides such as hydrochloric acid, nitric acid, sulfuric acid, sulfurous acid, hydrogen sulfide, perchloric acid, hydrogen peroxide, carbonic acid, carbonic acid such as formic acid and acetic acid, and R of the structural formula RCOOH. Examples of basic catalysts such as substituted carboxylic acids substituted with other elements or substituents, sulfonic acids such as benzenesulfonic acid, and the like include ammoniacal bases such as aqueous ammonia and amines such as ethylamine and aniline.
[0028]
(2) In the present invention, the titania catalyst obtained by the above method is chemically treated with an alkali or an acid. The conditions at that time may be conditions under normal temperature and normal pressure (atmospheric pressure), but it is desirable from the standpoint that the reaction is promoted, etc., under conditions of high temperature and pressure at which a hydrothermal synthesis reaction occurs. .
[0029]
Here, sodium hydroxide / potassium hydroxide aqueous solution or the like can be suitably used as the alkali, and hydrofluoric acid or the like can be suitably used as the acid.
[0030]
Here, the high temperature and pressure under which hydrothermal synthesis reaction occurs, for example, in the case of chemical treatment (surface treatment) with sodium hydroxide, in a pressure-resistant sealed container, the temperature is 70 to 150 ° C. × 1 to 100 h, Desirably, it is performed under conditions of 100 to 120 ° C. × 2 to 120 hours.
[0031]
By this chemical treatment, the amorphous silica remaining on the surface of the titania-based catalyst powder (which is thought to adversely affect the catalytic activity) is removed. When the chemical treatment is performed at a high temperature and a high concentration , the specific surface area is remarkably increased (see FIG. 3 which is a graph showing the relationship between the alkali treatment temperature and the specific surface area at various alkali concentrations).
[0032]
In addition, it wash | cleans with water after each chemical treatment. And when chemically treating with an alkali, it is desirable to neutralize with an inorganic acid such as dilute hydrochloric acid after washing with water. This neutralization treatment method is usually performed by a method such as immersion or spraying.
[0033]
(3) The titania-based catalyst after the chemical treatment prepared as described above is further subjected to a reheating treatment at a temperature of 200 to 1200 ° C. for 10 to 400 minutes, preferably 300 to 600 ° C. for 60 to 160 minutes. Do. This reheating treatment improves the crystallinity of TiO ₂ and improves the catalytic activity. That is, when chemical treatment (surface treatment) is performed, it is considered that a bond of Ti—OH or Ti—O is formed on the surface layer of TiO _2. When this is heat-treated, a bond of Ti—O—Ti is formed. Therefore, it is presumed that the catalytic activity is improved because the amount of TiO ₂ that contributes relatively to the catalytic reaction increases.
[0034]
When the heat treatment temperature is less than 200 ° C., Ti—O—Ti bonds are not formed, and when it exceeds 1200 ° C., crystals grow and the specific surface area decreases, so that the catalytic activity may be lowered.
[0035]
(4) The reheat-treated product may be used as it is as a catalyst (photoactive), but is usually used after being pulverized. As a pulverization method, a fine pulverizer / ultra-fine pulverizer such as a ball / rod mill or a micronizer is usually used.
[0036]
[Operation and effect of the invention]
The method for producing a titania-based catalyst of the present invention has the following operations and effects with the above-described configuration.
[0037]
(1) The silica-doped or dopeless titania-based fired body is supported by the below-described examples by subjecting it to a condition chemical treatment that induces a hydrothermal synthetic reaction with an alkali or an acid, and a reheating treatment. As shown, the photocatalytic properties are greatly increased. This is because the titania-silica amorphous remaining in the titania-based fired body is removed by chemical treatment, and modified by reheating treatment so that the catalytic activity of each crystal on the TiO ₂ surface is increased. Presumed.
[0038]
(2) When a predetermined amount or more of silica is doped, the photocatalytic properties are also increased by lowering the firing temperature as much as possible. It is presumed that the particle size of the single crystal is reduced (relatively increased specific surface area), and the catalytic activity surface is increased.
[0039]
As described above, the production method of the titania-based catalyst of the present invention can further increase the photocatalytic activity in the prior method in which the photocatalytic activity is easily increased without supporting the noble metal promoter.
[0040]
[Test example]
Hereinafter, test examples conducted for confirming the effects of the present invention will be described.
[0041]
(1) Relationship between silica addition amount and specific surface area: Commercially available tetraisopropoxy titanium so that the composition is (1-x) TiO ₂ · xSiO ₂ (x = 0, 0.05, 0.1) In addition, dilute hydrochloric acid was added as a hydrolysis catalyst to a sol solution in which tetraethoxysilane was dissolved in ethanol and water, and the mixture was allowed to stand and gelled after hydrolysis. (See the process diagram in FIG. 5)
Each gelled product is calcined by heating for 2 hours at temperatures of 500, 600, and 700 ° C., and the calcined product is pulverized in an agate mortar to form fine powder (particle size <320 mesh) titania. The specific surface area of each titania catalyst from which the system catalyst was prepared was measured based on the BET method. The results are shown in FIG. 1, and it can be seen that the specific surface area increases as the amount of silica added increases. Moreover, it turns out that a specific surface area becomes small as a calcination temperature (crystallization temperature) becomes high.
[0042]
(2) Relationship between calcination temperature and catalyst activity:
The catalytic activity of each catalyst obtained above with a silica addition amount of 0.05 mol% and a crystallization temperature of 500, 600, and 700 ° C. was measured by the optical Kolbe method. The results are shown in FIG. 2, and it can be seen that the lower the crystallization temperature (the higher the specific surface area) is.
[0043]
(3) Relationship between alkali treatment temperature and specific surface area at various alkali concentrations: Silica added amount of 20 mol%, and prepared at calcination temperature of 600 ° C. at each sodium hydroxide concentration of 20%, 40% and 68% Each titania-based catalyst was prepared by performing alkali treatment at 20 ° C., 60 ° C., and 110 ° C. for 20 hours (without reheating treatment).
[0044]
For each titania-based catalyst, the specific surface area was measured based on the BET method. The result is shown in FIG. 3, and it can be seen that the specific surface area increases as the concentration and temperature increase, which are alkali treatment conditions.
[0045]
The chemical treatment was completed by washing with water after acid treatment and acid washing with HClaq (0.1N). The same applies hereinafter.
[0046]
(4) Relationship between alkali treatment / reheat treatment and catalytic activity: Silica added at 20 mol% and prepared at a calcination temperature of 600 ° C., 1) untreated product, 2) normal temperature / normal pressure alkali treated product, 3) High-temperature / pressurized alkali-treated products (40% NaOH × 110 ° C. × 20 h, 4) Reacted high-temperature / pressurized-alkali-treated products were measured for catalytic activity by the optical Kolbe method. The results are shown in FIG. 4, but the product 3) not subjected to reheating treatment has almost the same catalytic activity as that of the normal temperature / normal pressure alkali treated product 2) (but the catalytic activity is increased as compared with the non-treated product). ), Reheat-treated 4) shows a marked increase in catalytic activity .
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between silica addition amount and specific surface area. FIG. 2 is a graph showing the relationship between calcination temperature and catalytic activity. FIG. 3 is a graph showing the relationship between alkali treatment temperature / concentration and specific surface area. Graph [Figure 4] Graph showing the relationship between the presence or absence of alkali treatment / reheating treatment and catalyst activity [Figure 5] Process diagram of sol-gel method in the above test example

Claims (4)

(1-x) A hydrolytic sol in which a titanium alkoxide and a silicon alkoxide are mixed so as to have a molar ratio of TiO ₂ .xSiO ₂ (x = 0.02 to 0.25 ) is gelled, and a heat treatment temperature is 350 to 1200. and fired at ° C., further, after the chemical treatment with alkali or acid, the production method of the titania-based catalysts, characterized in that the reheating treatment at a temperature of 200 to 1200 ° C.. The method for producing a titania-based catalyst according to claim 1 , wherein the heat treatment temperature is 450 to 600 ° C. The method for producing a titania-based catalyst according to claim 1 or 2 , wherein the chemical treatment is an alkali treatment, and is performed under a condition of a sodium hydroxide concentration of 20% or more and a temperature of 20 to 150 ° C. The method for producing a titania-based catalyst according to any one of claims 1 to 3 , wherein the reheating treatment temperature is 300 to 600 ° C.

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