JP2001129412A - Regenerating method of titanium oxide photocatalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a simple method for regenerating a TiO2 photocatalyst deactivated by a coating film of SiO2 produced when silicon-containing organic gas is decomposed by photocatalyst reaction. SOLUTION: The TiO2 photocatalyst which is used for decomposing a silicon compound and deactivated is washed in a basic solution of pH 12 or higher to regenerate the deactivated TiO2 photocatalyst.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for regenerating a TiO ₂ photocatalyst deactivated by a coating of SiO ₂ generated when a contaminant containing a trace amount of silicon-based organic gas is decomposed by a photocatalytic reaction.

[0002] Titanium oxide photocatalyst has antibacterial, antifouling, deodorizing,
Recently attracting widespread attention by showing environmental purification
[Akira Fujishima, Kazuhito Hashimoto, Toshiya Watanabe, Hikari Clean Revolution, CMC Publishing (1997). Akira Fujishima, Applied Physics, 64,
803 (1995) .Akira Fujishima, Environmental Management, 32 (8), 909 (199)
6). Yoshinobu Kubota et al., Pathophysiology, 12 (11), 878 (199)
3). R. Cai et al, Cancer Research, 52, 2346 (199
2).]. Here, when a titanium oxide photocatalyst is used for environmental purification, the titanium oxide photocatalyst product itself is stable in a normal atmospheric environment, but its photocatalytic activity generally decreases over time. There are many.

[0003] The cause of the deactivation of such a titanium oxide photocatalyst is various, and for example, when used for antifouling of an outdoor facility, the photocatalyst is deactivated by adhesion of dirt and the like. Often like. In contrast, however, by using it in combination with the superhydrophilic properties of titanium oxide, it is possible to allow the adhering dirt to be naturally removed by rainwater. On the other hand, when using a titanium oxide photocatalyst to create a comfortable indoor space,
Although the amount of dirt attached is less than in the outdoors, other factors that deactivate titanium oxide increase.

[0004] One of such elements is the presence of a silicon-based organic substance (particularly, silicone). And, in order to avoid deactivation by this silicon-based organic matter,
Even if we try to remove all silicones from the surrounding environment, silicone products (oil, grease, rubber, etc.) are no longer indispensable in areas such as electrical and electronic, mechanical, chemical, medical, food and civil engineering. [Tetsuya Mayuzumi, Application Development of Silicone, CMC (1998). Kunio Ito, Silicone Handbook (19
90), Nikkan Kogyo Shimbun]] It is virtually impossible to eliminate all silicone products from living space simply because they prevent the deactivation of the titanium oxide photocatalyst.

For example, in civil engineering construction, silicone products are often used as sealing materials and waterproofing materials. Although such high molecular substances themselves are stable, small amounts of low molecular oligomers contained therein are relatively volatile, so they are gradually released into the indoor space. I will. For example, in a clean room of a semiconductor factory, it has been pointed out that such a small amount of organic matter is adsorbed on the silicon wafer surface and adversely affects semiconductor devices [Tadahiro Omi, Takeshi Okawa, Osamu Nakamura, Vacuum, 54 ( 2), 1011 (1998) .KJ Budde, Mat. Res.
Soc. Symp. Proc., Vol, 386, p 165 (1995) .K. Sag
a and T. Hattori, J. Electrochem. Soc., 143 (10),
3279 (1996).].

[0006]

One of the products obtained when such a silicon-based organic material is oxidized and decomposed on a titanium oxide photocatalyst is SiO ₂ . Research [H. Tada, Langmuir, 11
(9), 3281 (1995).]. In this regard, the concentration of the silicon organic substance in the room is usually very low, but if the titanium oxide photocatalyst is left there for a long time, the active sites on the titanium oxide surface will be covered with SiO ₂ , and eventually, Will be deactivated.

Here, the oxygen atom in SiO ₂ is TiO 2
_{(2) Since a} chemical bond is formed between hydroxyl groups on the surface or titanium atoms on the surface, it cannot be removed merely by washing with water. And
When the titanium oxide product is actually applied, it is preferable to regenerate as much as possible because the cost and ease of maintenance must be considered. For this reason, it is important how to regenerate titanium oxide deactivated due to a silicon compound.

The present invention has been made in view of the above problems, and has as its object the purpose of deactivating TiO which has been deactivated by the coating of SiO ₂ generated when a silicon-based organic gas is decomposed by a photocatalytic reaction. An object of the present invention is to provide a simple method for regenerating a _two- photocatalyst.

[0009]

Means for Solving the Problems As a result of intensive studies conducted by the present inventors in order to achieve the above object, Si produced when a silicon-based organic gas is decomposed by a photocatalytic reaction.
O ₂ has low crystallinity and is more soluble in alkali than ordinary crystalline SiO ₂ , so that it can be removed without affecting the TiO ₂ photocatalyst under predetermined alkaline conditions. And completed the present invention.

More specifically, the present invention includes the following.

(1) A method for regenerating a deactivated TiO ₂ photocatalyst which has been subjected to the decomposition of a silicon compound, wherein the deactivated TiO ₂ photocatalyst is washed with a basic solution having a pH of 12 or more. A method for regenerating a deactivated TiO ₂ photocatalyst.

(2) The method according to (1), wherein the washing is immersion in the basic solution with mechanical stirring.

(3) When regenerating the deactivated TiO ₂ photocatalyst which has been subjected to the decomposition of the silicon compound,
A method of adjusting the regeneration speed and regeneration degree by changing the pH between 12.

(4) The method according to (3), wherein the reproduction speed and the reproduction degree are further adjusted by changing the temperature.

(5) The method according to any one of (1) to (4), wherein the silicon compound is any one of an organic silicon compound, silicone, and a silicon-based organic gas generated from silicone.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, SiO ₂ generated when a silicon-based organic gas is decomposed by a photocatalytic reaction is
The more the NaOH aqueous solution having a pH higher than pH 12 is used, the more the removal of the SiO ₂ can be promoted. However, in order to minimize the adverse effect on the titanium oxide thin film constituting the photocatalyst, It is preferable that the pH is not too high, and it is preferable to use an aqueous solution of sodium hydroxide having a pH of about 12.

In carrying out the present invention, SiO ₂
It is better to perform a certain degree of stirring in order to promote the dissolution of the compound, but it is preferable that stirring by ultrasonic waves is avoided as much as possible and the stirring is performed by macromechanical means. Macromechanical stirring does not mean stirring at a micro level such as ultrasonic stirring, but includes stirring performed by a stirrer or stirrer or stirring performed by bubbling an inert gas such as nitrogen gas. Means agitation performed by adding macro-level effects to the solution. This includes manual agitation that simply rocks.

The present invention, as long as the SiO ₂ is produced on the TiO ₂ surface by the photocatalytic reaction of TiO _2, not limited to the type of those that are degraded by the photocatalytic reaction, to the use of TiO ₂ photocatalyst Not limited.

[0019]

EXAMPLES [Experimental System] First, a TiO ₂ photocatalytic thin film having a size of 5.0 cm × 5.0 cm and a thickness of 0.4 μm was prepared by dip coating to perform a photocatalytic reaction. An experimental system as shown in FIG. 9 was prepared. The reaction vessel uses a 500 mL airtight Pyrex cell, in which octamethyltrisiloxane is used.
(Octamethyltrisiloxane; OMTS) vapor was introduced.

In performing the photocatalytic reaction, light was irradiated from above the reaction vessel as shown in FIG. As the light source, a black light of 2.0 mW / cm ² was used, a fixed amount of OMTS vapor was injected before light irradiation, and light irradiation was started after reaching adsorption equilibrium.

The detection was performed by gas chromatography (FID) for the concentrations of OMTS and CO ₂ . Whether or not SiO ₂ is formed on the substrate surface is determined by X
PS (X-ray Photoelectron Spectroscopy, PHI 5600, P
erkin Elmer). The amount of TiO ₂ and SiO ₂ dissolved when the deactivated titanium oxide thin film was treated with a NaOH solution was determined by ICP (Inductively
Coupled Plasma) was detected by emission spectroscopy.

[Decomposition of Substrate by Photocatalyst and Inactivation of Photocatalyst] FIG. 1 is a diagram showing the adsorption characteristics of OMTS on the surface of a titanium oxide thin film. In FIG. 1, a straight line plotting the reciprocal of the equilibrium concentration in the gas phase and the reciprocal of the amount of adsorption indicates that the adsorption of OMTS on the titanium oxide surface is a chemical adsorption. In addition, Langmuir
As a result of calculation based on the equation, the adsorption equilibrium constant was 0.000036 m ³ /
mg, and the saturated adsorption amount was 0.16 mg / cm ² .

Here, as shown in FIG. 2, since the concentration of OMTS gradually decreases while the concentration of CO ₂ increases while being irradiated with ultraviolet light, the decomposition of siloxane proceeds by the photocatalytic reaction. It was confirmed that it went.
However, as shown in FIG. 3, when the photocatalytic reaction proceeds for a long time, the catalytic activity of titanium oxide is lost. (Note that “One cycle” in FIG.
This is the period from injecting TS until it disappears).

That is, as shown in FIG. 3, in this experiment, the injection of OMTS was repeated five times until it disappeared (one cycle). However, in the fourth cycle, OMTS was completely decomposed. The time required to perform the process was prolonged significantly, and all OMTS could not be decomposed in the fourth cycle. From this, when OMTS is decomposed by the titanium oxide photocatalyst, O
It can be seen that the titanium oxide photocatalyst is deactivated when the MTS is completely decomposed five times.

Here, in order to investigate the cause of the deactivation, the surface state of TiO ₂ before and after light irradiation was measured by XPS. FIG. 4A shows a change in the peak of Si2p before and after light irradiation. Before irradiation, there was no Si2p peak, but after irradiation, a strong peak appeared. This peak increases with light irradiation until the titanium oxide is deactivated. FIG. 4B is a diagram showing a change in the O1s peak before and after light irradiation. Before irradiation, O1s of SiO ₂
Only peaks appeared, but irradiation resulted in new peaks at higher binding energies. This is SiO
₂ corresponds to the O1s peak. From the above experimental data, it can be inferred that the surface of the TiO ₂ is covered with the decomposition product, SiO _2, which is the cause of the deactivation.

[Regeneration of Photocatalyst] FIG. 5 shows that the deactivated titanium oxide thin film was treated in pure water and a NaOH solution having a different pH.
(a)) and XPS spectra of O1s (FIG. 5 (b)). As can be seen from FIG. 5, TiO ₂
The SiO ₂ coated on the surface could not be removed by treating with a pH 11 NaOH aqueous solution or pure water. On the other hand, when treated with a pH 12 NaOH aqueous solution, the 2p peak of Si (FIG. 5 (a))
₂ O1s (FIG. 5 (b)) disappeared. At the same time, IC
By the P measured, by treating about 20 minutes NaOH solution of pH 12, it was found that the SiO ₂ coated on the surface of the TiO ₂ has been completely removed.

Then, the TiO ₂ thin film from which the SiO ₂ coated on the surface is completely removed is placed in a reaction vessel of an experimental system shown in FIG. 9, and OMTS is flowed into the reaction vessel and light is emitted. Upon irradiation, OMTS photodegradation began to occur again, as shown in FIG.

[0028] From the results of FIG. 6, once the coating of SiO ₂ has a photocatalytic activity of the TiO ₂ thin films deactivated, pH 1
It can be seen that recovery (regeneration) was achieved by treating with the NaOH aqueous solution of No. 2 for 20 minutes.

Recovery (regeneration) of photocatalytic activity of TiO ₂ thin film
FIG. 7 shows the results of an experiment carried out with respect to, in which the substrate was changed from OMTS to methylene blue. As shown in Figure 7, once the photocatalytic activity of the TiO ₂ thin films deactivated, it recovered (reproduced) by treating for 20 minutes aqueous NaOH pH 12, TiO ₂ thin film photocatalyst activity was reproduced Methylene blue was decomposed in the same manner as a new product.

In this connection, AFM observation showed that pH 1
It was found that the state of the surface of the titanium oxide thin film hardly changed even when treated with the NaOH aqueous solution of No. 2 for 40 minutes (compare FIG. 8A and FIG. 8B.
(A) is before the NaOH aqueous solution treatment, and FIG. 8 (b) is after the treatment. There is a change in the image shown in FIG. 8 (a) and the image shown in FIG. 8 (b). Absent). Also,
According to the UV-Vis measurement, no change in the absorption spectrum of the titanium oxide thin film was observed before and after the treatment with the NaOH aqueous solution.

[0031] [cleaning operation at the time of the SiO ₂ removal] SiO ₂
It is better to perform a certain amount of agitation in order to promote the dissolution of. In this regard, our experiments show that, for example,
When the deactivated titanium oxide thin film was treated in a sodium hydroxide aqueous solution having a pH of 13 by manual stirring for 60 minutes, no apparent dissolution of the film was observed (Table 1 below).
On the other hand, a result was obtained in which the titanium oxide thin film was melted when continuously sonicated for 60 minutes in the same aqueous solution of sodium hydroxide at pH 13 (Table 2 below).

[0032]

[Table 1]

[0033]

[Table 2]

From the above, in carrying out the present invention, a situation arises in which the TiO ₂ is easily dissolved by the heat generated during the ultrasonic treatment, so that the stirring by the ultrasonic waves should be avoided as much as possible. It is preferable to do so.

According to experiments conducted by the present inventors, it is found that SiO ₂ can be completely removed by merely shaking by hand or mechanically stirring with a motor without adversely affecting the titanium oxide thin film. If the above two stirring methods cannot be used in some cases, bubbling may be performed with nitrogen gas or the like.

[0036]

According to the method for regenerating a titanium oxide photocatalyst of the present invention as described above, the TiO ₂ deactivated by the coating of SiO ₂ generated when the silicon-based organic gas is decomposed by the photocatalytic reaction. a photocatalyst, without adversely affecting the TiO _2, can be reproduced by a simple operation.

[Brief description of the drawings]

FIG. 1 OMTS (Octam) on the surface of titanium oxide thin film
FIG. 3 is a graph showing the adsorption characteristics of ethyltrisiloxane).

[Fig. 2] O accompanying the progress of photocatalytic reaction by ultraviolet irradiation
Changes in MTS concentration and CO _Two5 is a graph showing a change in concentration.
You.

FIG. 3 shows the OM when the cycle (1 cycle) from the injection of OMTS to its disappearance (1 cycle) is repeated 5 times
It is a graph which shows the change of TS concentration.

FIG. 4 is a graph showing the result of XPS measurement of the surface state of TiO ₂ before and after light irradiation, and in particular, FIG. 4 (a) is a graph showing a change in the peak of Si2p before and after light irradiation;
FIG. 4B is a graph showing a change in the O1s peak before and after light irradiation.

FIG. 5 shows a deactivated titanium oxide thin film treated with pure water and a NaOH solution having a different pH, before and after the treatment with Si2p (FIG. 5 (a)) and O1s (FIG. 5).
It is the graph which showed the XPS spectrum of (b)).

FIG. 6 shows a TiO ₂ thin film treated for about 20 minutes in a NaOH solution having a pH of 12 and irradiated with light to obtain an OMTS.
5 is a graph showing a change in OMTS concentration and a change in CO ₂ concentration when the compound is photolyzed.

FIG. 7 is a graph showing the results of an experiment performed on the recovery (regeneration) of the photocatalytic activity of a TiO ₂ thin film by changing the substrate from OMTS to methylene blue. In the graph, 1 (black square) is a plot showing the change in methylene blue concentration with respect to light irradiation time when a new titanium oxide thin film was used, and 2 (black circle) used a titanium oxide thin film inactivated by SiO ₂ . 3 is a plot showing a change in methylene blue concentration with respect to light irradiation time in the case where 3 (open square) indicates a change in methylene blue concentration with respect to light irradiation time when a titanium oxide thin film regenerated by treating with an aqueous NaOH solution at pH 12 is used. It is a plot.

FIG. 8 is an AFM image showing the state of the surface of the titanium oxide thin film before and after the treatment with the NaOH aqueous solution having a pH of 12 (FIG. 8A shows the state before the NaOH aqueous solution treatment;
FIG. 8B shows the state after the processing.

FIG. 9 is a block diagram showing an experimental system for performing a photocatalytic reaction.

Continued on front page F-term (reference) 4D048 BA07X BA13X BA41X BD05 EA01 4G069 AA10 BA04A BA04B BA04C BA48A BA48C BB04A BB04B BB04C FA08 FB27 FC07 FC09 GA12

Claims (5)

[Claims] 1. A method for regenerating a TiO ₂ photocatalyst deactivated by being subjected to decomposition of a silicon compound, wherein the deactivated TiO ₂ photocatalyst is washed with a basic solution having a pH of 12 or more. A method for regenerating activated TiO ₂ photocatalyst. 2. The method according to claim 1, wherein the washing is immersion in the basic solution with mechanical stirring.
The described method. 3. A method of adjusting a regeneration speed and a regeneration degree by changing a pH between pH 12 when regenerating a TiO ₂ photocatalyst deactivated by being subjected to decomposition of a silicon compound. 4. The method according to claim 3, further comprising adjusting a reproduction speed and a reproduction degree by changing a temperature. 5. The method according to claim 1, wherein the silicon compound is one of an organic silicon compound, silicone, and a silicon-based organic gas generated from silicone.
5. The method according to any one of to 4 above.