JP2002228589A - Method and device for evaluating oxidation- decomposition activity of photocatalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently carry out the development and research of a photocatalyst material by reproducibly obtaining decomposition activity data, in relation to a method for evaluating the oxidation-decomposition activity of a photocatalyst material such as TiO2 and its experiment device. SOLUTION: This method is used for evaluating the oxidation-decomposition activity of the photocatalyst by bringing a titanium oxide (TiO2) photocatalyst test piece into contact with a pigment solution in a cell for spectroscopy and by instantaneously measuring the concentration variation of the solution under light irradiation by light absorption analysis. This device is composed of a transparent cell for spectroscopy provided with an agitating device filled with the pigment solution for agitating the solution and a thermostatic device for keeping it at a certain temperature, the photocatalyst test piece disposed on the cell bottom, an exciting light source installed above the cell, and a light absorption analyzer for entering analysis light into the cell to measure the absorbance of the light transmitted through the cell and the pigment solution, and used for evaluating the oxidation-decomposition activity of the photocatalyst by instantaneously measuring the concentration variation of the solution under light irradiation by light absorption analysis.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention is, with respect to the photocatalytic material, such as TiO _2, it relates to the experimental apparatus and method for evaluating the oxidative decomposition activity. More specifically, the present invention aims to efficiently develop and research photocatalytic materials by obtaining decomposition activity data with good reproducibility.

[0002]

2. Description of the Related Art In order to evaluate the performance of a photocatalytic material, it is desirable to use an experimental method suitable for each purpose of use after sufficiently considering a reaction mechanism. However, in practice, a model substance is often used in place of "dirt" to be decomposed for simplicity. For example, a method of adsorbing a dye as a model substance on the surface of a photocatalyst and measuring the decolorization rate by a change in absorbance is known. That is, in this method, T
The change in absorbance at the absorption peak wavelength due to the irradiation of iO ₂ with ultraviolet rays is measured with a spectrophotometer, and the change (ΔABS) from the initial value of the absorbance is plotted against time. Where Δ
ABS corresponds to the amount of degradation of the dye.

In this method, it is needless to say that ΔA
Since BS must be above the sensitivity of the spectrophotometer,
It is necessary to adjust the amount of excitation light and the amount of dye adsorbed in order to increase ΔABS as much as possible. For the same reason, it is usual that only a sample having a large reaction area (usually corresponding to the surface area of a test piece) is evaluated. As described above, at present, it is relatively difficult to obtain decomposition activity data of a photocatalytic material with good reproducibility, and an evaluation method particularly for a minute test piece has not yet been established.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and aims at overcoming the limitations in the evaluation of photocatalytic activity. An object of the present invention is to provide a method and an experimental apparatus thereof that can easily evaluate the oxidative decomposition activity of a minute photocatalyst test piece with good reproducibility, unlike the conventional one.

[0005]

The present invention solves the above-mentioned problems by contacting a TiO ₂ photocatalyst test piece with a dye solution in a spectroscopic cell, and measuring the change in the concentration of the solution under light irradiation by absorption analysis. The present invention provides a method and an experimental apparatus for evaluating the oxidative decomposition activity of a photocatalyst by performing in-situ measurement in the above.

[0006] The present invention also provides the above-described evaluation method and experimental apparatus, wherein the photocatalyst test pieces to be evaluated include those having a size of 1 cm or less. As shown in FIG. 2, the apparatus for evaluating the oxidative decomposition activity of a photocatalyst according to the present invention includes, as shown in FIG. Specimen 6 is placed on the bottom surface of the device with its TiO ₂ surface facing the light source,
The cell is filled with a dye solution 4 such as a methylene blue aqueous solution. This solution is stirred by the stirrer 5 operated by the magnet stirrer 9 during the measurement process. While the dye solution is being stirred, the incident light 7 is introduced from the spectroscope into the solution in the cell, and the transmitted light 8 is measured by the absorption analyzer.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS TiO ₂ photocatalyst is usually commercially available in the form of a powder. Tent), TiO ₂ film coating material, and the like.
In the method of the present invention, the activity can be evaluated in any of these states, but a thin film in a supported state is more preferable than a powder as an evaluation target in terms of simplicity of measurement.

As described above, in the present invention, the oxidative decomposition activity of a photocatalyst is evaluated by using a dye solution as an object to be decomposed and measuring the concentration change of the solution under light irradiation in situ by absorption spectroscopy. Such photocatalytic decomposition of the liquid phase involves diffusion and adsorption of molecules in the liquid phase to the TiO ₂ surface,
Three processes of the decomposition reaction are involved. Although it is possible to experimentally determine the concentration in the liquid phase, it is the surface concentration that directly affects the reaction rate, which makes the activity evaluation complicated.

However, in a solution system capable of supplying a sufficient amount of dye molecules to the photocatalyst surface, the adsorption equilibrium is maintained, so that the reaction speed is determined only by the amount of light and the charge separation efficiency of the photocatalyst. Therefore, the rate of the decomposition reaction determined at this time is as close as possible to the true oxidative decomposition activity. In the conventional method of decomposing a dye previously adsorbed on the surface of the photocatalyst, it is necessary to consider the possibility that the result depends on the amount of the dye adsorbed, but the method of the present invention does not need to do so.

When a dye solution is used, the actual amount of decomposition can be estimated from ΔABS. In general, the relationship between the absorbance ABS of the sample solution and the concentration c can be expressed by the Lambert-Beer rule, ABS = εcι [Equation 1]. Here, the extinction coefficient ε is a constant specific to the dye, and ι is the thickness of the liquid phase. Therefore, [Equation 1] is a simple proportional function, and the concentration, that is, the dye molecular weight per unit volume can be calculated from the absorbance. . In the conventional method, it is difficult to determine the absolute value of the photoactivity, and only the relative ratio to the standard sample can be obtained. However, in the method of the present invention, the activity can be converted to the absolute value of the amount of dye decomposition.

The decomposition rate is proportional to the surface area of the test piece irradiated with light. Therefore, in the case of a small test piece with a small amount of decomposition, the change in concentration (ie, ΔABS) is apparently increased by minimizing the volume of the dye solution.
In the method of the present invention, using a spectroscopic cell as a reaction vessel,
An embodiment of the present invention is a method for in-situ measurement of a change in absorbance of a solution under light irradiation. For example, even for a test piece having a size of 1 cm or less, a sufficient ΔABS can be observed by making the solution filling the cell 3 ml. At this time, a preferable initial concentration of the solution is 1 × 10 ⁻⁶ mol dm.
⁻³ to 1 × 10 ⁻⁴ mol dm ⁻³ .

The dye used in the evaluation of the decomposition activity of the present invention is: (i) that the dye itself is resistant to ultraviolet light;
It is in (ii) dark do not decompose even in a state adsorbed on TiO _2, the wavelength range used to excite the (iii) TiO ₂ (3
(At around 30 nm to 370 nm), and (iv) easily decompose by photocatalytic reaction. Such dyes include, for example, methylene blue (FIG. 1).

The material for the spectroscopic cell that fills the dye solution is not particularly limited as long as it is transparent over almost the entire visible light region where the dye is strongly absorbed, but quartz glass is frequently used in terms of ease of handling. Used. The dimensions of the cell are also not limited, but are appropriately determined in consideration of the volume of the dye solution to be filled and the size of the test piece (which largely depends on the bottom area of the cell as described later). For example, when evaluating a small test piece having a dimension of 1 cm or less, the bottom dimension is 1 cm × 1c.
m cells are used.

Next, an outline of an experiment using the method and apparatus of the present invention will be described. A photocatalyst specimen is placed face-up on the bottom of a clean spectroscopic cell, and the dye solution is filled from above with a defined volume. Always keep the specimen parallel to the bottom of the cell during the measurement. For this reason,
The shape and dimensions of the test piece that can be measured by this method are naturally determined by the bottom surface of the cell.

[0015] The excitation light is introduced from above the cell. TiO
_In the case of ₂ , since the light is ultraviolet light in the vicinity of 330 nm to 370 nm, for example, a black light is preferable as the light source. Since methylene blue has no strong absorption in this wavelength range, the light from the light source is
Reach the O ₂ surface. On the other hand, light from the spectrometer for measuring the absorbance is transmitted perpendicularly to the cell and guided to the detection system. At this time, it is absolutely necessary to prevent ultraviolet light for excitation from entering the detection side.

Since the decomposition of the dye occurs only on the TiO ₂ surface, a concentration distribution occurs in the cell over time in the above experimental arrangement. As described above, the evaluation method of the present invention does not consider the diffusion effect of the molecules in the liquid phase, so that the conditions for sufficiently supplying the dye molecules to the photocatalyst surface are maintained and the concentration in the cell is made uniform. The solution is constantly stirred during the reaction.

Further, in order to obtain data with higher reproducibility, the initial state of the test piece is kept constant. Therefore, before immersion in the dye solution, the TiO ₂ surface is sufficiently irradiated with ultraviolet rays to clean the surface. Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0018]

Embodiment 1 FIG. 2 is a schematic view of an experimental apparatus, which is a schematic view of an evaluation experiment of oxidative decomposition activity by the method and apparatus of the present invention, in the case of a TiO ₂ photocatalyst test piece having a size of 1 cm or less in a thin film state. Is shown. (A) shows details of a quartz glass spectroscopic cell, and (b) shows a sample arrangement and an optical arrangement of an experiment.

As shown here, a spectroscopic cell made of quartz glass and having a bottom dimension of 1 cm × 1 cm (FIG. 2A) was used as it was as a reaction vessel. That is, a test piece was placed on the bottom surface of the cell with the surface facing up, and 1 × 10 ⁻⁵ mol was placed from above.
3 ml of the dm ^-3 dye solution was filled. A micro-stirrer (dimensions: 6.4 mm × 3 mmφ) coated with Teflon (registered trademark) was placed on the test piece, and the solution was constantly stirred with a magnetic stirrer during the reaction. Further, the temperature of the dye solution was kept at 20 ° C. by circulating water from the thermostatic water bath around the cell.

To excite TiO ₂ , ultraviolet light (maximum wavelength 352 nm, output 3.0 W) from a black light was irradiated from above. The distance from the light source to the bottom of the cell is about 10c
m. On the other hand, the analysis light (intensity I ₀ ) monochromatized by the monochromator in the spectroscope is perpendicularly incident on the cell, and the light (intensity I) transmitted through the cell and the dye solution (liquid phase thickness 1 cm) is detected by the detection system. Led to. Absorbance at that time ABS = 1
og ₁₀ (I ₀ / I) was plotted as a function of wavelength and an absorption spectrum was obtained.

[0021]

Embodiment 2 Under the arrangement and conditions of the experiment shown in Embodiment 1, the activity of a TiO ₂ thin film deposited on a substrate by a laser deposition method was evaluated. That is, this test piece has a Kr
F excimer laser (λ = 248 mm; Energ
y: 50 mJ / pulse) and ablation of the Ti target in an oxygen atmosphere (35 mTorr),
It is a crystalline thin film (both rutile and anatase structures are mixed) formed on a (0001) plane α-Al ₂ O ₃ single crystal substrate heated to 400 ° C.

FIG. 3 shows the results obtained for this test piece.
5 shows the change over time in the absorption spectrum of a dye solution. That is, the time-dependent changes in the absorption spectrum of the aqueous solution of methylene blue, which is a dye solution, are shown in (a) 0 minutes, (b) 20 minutes, (c) 4
It is a spectrum after 0 minute and (d) 117 minutes. With the reaction time, the absorbance at the absorption peak wavelength near 660 nm decreased significantly.

The amount of change ΔABS from the initial value is 20
FIG. 4A shows the results measured and plotted at minutes, 40 minutes and 117 minutes. That is, it shows the time change of the absorbance at the absorption peak wavelength (around 660 nm). (A) shows the result of the TiO ₂ thin film deposited on the (0001) α-Al ₂ O ₃ single crystal substrate by the laser vapor deposition method, and is compared with (b) where no test piece was inserted. As apparent from this figure, ΔABS was obtained by photocatalytic decomposition of the dye for 117 minutes.
Reached -0.211. [Equation 1] and ε = 6.3 × 10
^{By using 4} (liter / mol · cm),
This value could be calculated to correspond to a decomposition amount of 1.0 × 10 ⁻⁸ mol (6.0 × 10 ¹⁵ ).

[0024]

[Comparative Example 1] Under the same conditions as in Example 2, the change in absorbance was measured when there was no TiO ₂ test piece, that is, when only the dye solution and the micro stirrer were placed in the cell. The result is shown in FIG. Final ΔABS after 117 minutes
Is -0.004, which is negligibly smaller than the value of Example 1, and it was confirmed that methylene blue has resistance to ultraviolet light from black light. Further, by taking the difference (−0.207) from the first embodiment, a correction value closer to the true decomposition amount could be obtained.

[0025]

According to the method and the experimental apparatus of the present invention, the oxidative decomposition activity of the TiO ₂ photocatalyst test piece can be evaluated very easily. In addition, the obtained oxidative decomposition activity data has high reproducibility and enables quantitative comparison between samples. Furthermore, since the measurement is performed in-situ in the spectroscopic cell, it cannot be overlooked that it is relatively easy to apply to various analysis techniques such as evaluation of temperature dependency and simultaneous measurement of multiple samples.

The efficiency of the research and development is expected by constantly feeding back the performance evaluation data according to the present invention as described above to the production process of the photocatalytic material.

[Brief description of the drawings]

FIG. 1 shows the chemical structure of methylene blue.

FIG. 2 is a schematic view of an evaluation experiment of oxidative decomposition activity by the method and the apparatus of the present invention.

FIG. 3 is a diagram showing a change over time in an absorption spectrum of an aqueous methylene blue solution.

FIG. 4 is a diagram showing a temporal change in absorbance at an absorption peak wavelength (around 660 nm).

[Explanation of symbols]

(1) Black light (excitation light source) (2) Ultraviolet light for TiO ₂ excitation (3) Quartz glass spectroscopic cell (4) Methylene blue aqueous solution (5) Micro stirrer (6) TiO ₂ test piece (7) Light incident from the spectroscope (intensity I ₀ ) (8) Transmitted light entering the detection system (intensity I) (9) Magnetic stirrer

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2G054 EA04 FA06 GA03 GB01 JA08 JA09 4G069 BA04A BA04B BA48A CA01 CA07 CA10 DA06 EA08 FB02

Claims (3)

[Claims] 1. Titanium oxide (TiO ₂ ) in a spectroscopic cell
A method for evaluating the oxidative decomposition activity of a photocatalyst by bringing a dye solution into contact with a photocatalyst test piece and measuring the concentration change of the solution under light irradiation in situ by absorption spectroscopy. 2. A photocatalyst test piece to be evaluated has a size of 1
The method according to claim 1, wherein the method includes a minute thing of less than cm. 3. A transparent spectroscopic cell filled with a dye solution and equipped with a stirrer and a thermostat for stirring and maintaining the solution at a constant temperature, a photocatalyst test piece disposed on the bottom of the cell, and above the cell. It consists of an excitation light source installed in the cell, and an absorption spectrometer that measures the absorbance of light transmitted through the cell and the dye solution by injecting the analysis light into the cell and measuring the concentration change of the solution under light irradiation. A device that evaluates the oxidative decomposition activity of a photocatalyst by measuring it in situ by absorption spectrometry.