JP2008006344A - Visible light-responsive photocatalyst - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To respond to such a request that a material having an activity equal to or higher than that of titanium oxide, that of doped titanium oxide or that of an existing visible light-responsive photocatalyst is obtained from oxide materials of easily available materials. <P>SOLUTION: A photocatalyst, which has excellent photocatalytic characteristics even when irradiated with visible light and the photocatalytic activity higher than that of AgNbO<SB>3</SB>being one of visible light-responsive materials, is obtained by forming a solid solution satisfying the composition shown by the general formula: Ag<SB>x</SB>M<SB>1-x</SB>Ti<SB>1-x</SB>Nb<SB>x</SB>O<SB>3</SB>(wherein M is one or more metals selected from bivalent metals consisting of Mg, Ca, Sr, Ba and Zn; and x is a numerical value satisfying 0.05≤x<1) from AgNbO<SB>3</SB>and MTiO<SB>3</SB>. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a visible light responsive photocatalyst that promotes decomposition of a substance by irradiation with visible light.
Specifically, the present invention relates to a perovskite photocatalyst exhibiting high photocatalytic activity for visible light other than ultraviolet rays, which is contained in a lot of sunlight, indoor lighting as well as ultraviolet rays.

In recent years, the daily lives of human beings are being threatened by harmful chemical substances such as formaldehyde that cause dioxins, environmental hormones, sick house syndrome, and the like, which are becoming serious problems.

There is a method for removing harmful chemical substances by using an adsorbent such as activated carbon to adsorb the harmful substances to the adsorbent and removing them, but photocatalytic materials are used to oxidize and decompose the harmful chemical substances. However, the removal method is also an effective method, and various studies have been made.

To date, photocatalytic materials using anatase-type titanium oxide have been frequently used as photocatalytic materials.

However, since the band gap of titanium oxide is large, it exhibits an excellent effect on ultraviolet rays, but does not function at all for visible light. The amount of ultraviolet light contained in sunlight and fluorescent lamps that are the excitation light source is only about 4 to 10% of the visible light amount, and when further increasing efficiency and indoor applications more than ever, There is a need to develop photocatalytic materials that are highly active against visible light. Therefore, in recent years, various photocatalytic materials that are active even at wavelengths in the visible light region have been proposed, and research and development have been conducted extensively.

For example, as one of the catalysts, a catalyst that can exhibit catalytic activity even for visible light by doping titanium oxide with metal ions such as Cr and V has been proposed (Non-patent Document 1). According to this proposal, by doping metal ions such as Cr and V, a new energy level is created between the conduction band and the valence band of titanium oxide. As a result, the band gap becomes narrower and visible light is reduced. Will be able to absorb. However, the energy level introduced by doping metal ions can also be a recombination level of electrons and holes, and in many cases, an increase in activity cannot be expected.

Furthermore, it has been proposed to produce a visible light responsive photocatalytic material by doping titanium oxide with an anion such as nitrogen (Patent Document 1). The titanium oxide photocatalyst produced by this proposal certainly increases the activity under visible light irradiation than the metal ion doped photocatalyst, but doping with nitrogen creates an oxygen defect inside the titanium oxide, reducing the photocatalytic activity. There was a drawback of end. In any case, the visible light responsive photocatalyst material produced by using the dope method is still insufficient at the present stage, and there is a demand for a material that can express a higher level of photocatalytic activity. It has been.

On the other hand, it has been reported that undoped photocatalytic materials are effective materials. Recently, an attempt has been made to produce a visible light responsive photocatalyst using an oxide other than titanium oxide. For example, BiVO ₄ is reported to be a photocatalytic material that generates oxygen from an aqueous silver nitrate solution under irradiation with visible light (Non-patent Document 2). However, this material has insufficient oxidizing power for organic matter, and cannot decompose organic matter such as 4-nonylphenol and completely oxidize and decompose it to carbon dioxide (Non-patent Document 3). In other words, it suggests that the potential of the top of the valence band is too small to completely oxidize and decompose organic matter.

AgNbO ₃ has a band gap of 2.8 eV and absorbs visible light. In fact, AgNbO ₃ is a material that completely oxidizes and decomposes organic substances to carbon dioxide. However, a generally used material manufacturing method such as a solid phase method or a sol-gel method has poor crystallinity and it is difficult to manufacture a single-phase material. For this reason, the specific surface area often becomes relatively small, and it has been reported that the photocatalytic activity in visible light is not so high (Non-patent Document 4).

Furthermore, Ag _0.03 Sr _0.97 Ti _0.97 Nb _0.03 O ₃ forms a miniband with a negative potential compared to a valence band in which a small amount of doped Ag4d orbit is composed of O2p orbitals. Therefore, it has been reported that it is also a visible light responsive photocatalytic material that absorbs visible light (Non-patent Document 5). However, since the amount of Ag is too small in this material, the amount of visible light absorbed and the absorption efficiency are very low. In addition, since the Ag4d orbit corresponding to the top of the valence band is not mixed with the 2p orbit of oxygen and is an isolated discontinuous miniband, the diffusion rate of holes generated by photoexcitation is much slower than that of a normal semiconductor. The improvement of visible light activity is not expected.

E. Borgarello, J.A. Kiwi, M.M. Gratzel, E .; Pelizetti and M.M. Visca: J.M. Am. Chem. Soc. Vol 104 No. 11 2996-3002. American Chemical Society Publications, (1982) A. Kudo, K. et al. Omori, H .; Kato J Am Chem Soc Vol 121 11459-11467. American Chemical Society Publications, (1999) S. Kohtani, S .; Makino, A .; Kudo, K .; Tokumura, Y. et al. Ishigaki, T .; Matsunaga, O .; Nikaido, K .; Hayaka and R.H. Nakagaki Chem. Lett 660-661 The Chemical Society of Japan, (2002) H. Kato, H .; Kobayashi and A.K. Kudo J. et al. Phys. Chem. B Vol 106 12441-12447. American Chemical Society Publications, (2002) Maruyama Yoshihiko, Murase Takashi, Irie Hiroshi, Hashimoto Kazuhito 10th Symposium Recent Developments in Photocatalytic Reaction p64 Optical Functional Materials Study Group (2003) Japanese Patent Laid-Open No. 2004-988

As described above, in recent years, development of a visible light responsive photocatalyst exhibiting high activity with respect to both ultraviolet rays and visible rays is expected. There is a demand for a material having an activity equal to or higher than the catalytic activity of titanium oxide, doped titanium oxide, or an existing visible light responsive photocatalytic material, which is an oxide material that is particularly easily available. The present invention seeks to meet this need. Furthermore, it oxidizes, reduces, and decomposes harmful substances by irradiating light, detoxifies the harmful substances, or cleans the dirt and further detoxifies the harmful substances using this catalyst and photocatalyst material that provides antibacterial properties. The present invention is intended to provide a method, a method for decomposing and cleaning dirt substances, and an antibacterial method.

In order to solve the above problems, the visible light responsive photocatalyst of the present invention has a general formula: Ag _x M _1-x Ti
_1-x Nb _x O ₃ (M is composed of one or more divalent metals of Mg, Ca, Sr, Ba, and Zn. X represents 0.05 ≦ x <1). A configuration characterized by comprising a complex oxide semiconductor having a composition was adopted.

The inventors have conducted extensive research on various materials. As a result, AgNb
A solid solution of O ₃ and MTiO ₃ , Ag _x M _1-x Ti _1-x Nb _x O ₃ (M is Mg, Ca, Sr
It consists of one or more divalent metals of Ba, Zn. X represents 0.05 ≦ x <1. ) Has excellent photocatalytic properties under visible light irradiation and is one of visible light responsive materials.
It has been found that the photocatalytic activity is greatly improved over NbO ₃ . By forming a solid solution, the top position of the valence band can be optimized, that is, sufficient oxidizing power can be maintained while maintaining the ability to absorb visible light, and the reaction can be achieved by solid solution. This is probably because the amount of holes reaching the surface of the field has increased. These phenomena cannot occur in a mixture of AgNbO ₃ and MTiO ₃ simply mixed, and it can be said that solid solution formation is a key point that brings such a larger photocatalytic activity.
In addition, these materials are considered to be photocatalytic materials with fewer defects and less recombination than doping materials. By further solid solution and MTiO ₃ a X = 0.05 or more AgNbO _3, the top position of the valence band and the hybrid is Ag4d trajectory and O2p track, i.e., when considered in potential, is Ag4d orbital and O2p track Ag _0.03 Sr _0.97 Ti _0.97 Nb in which a mini-band is formed while having the advantage of being continuous _{.
In 03} O ₃ , the Ag4d orbit and 02p orbit are discontinuous, and the top position of the valence band is Ag.
Only 4d.
Therefore, there is a big difference in the mobility of generated holes.
The mobility of the generated hole was Ag _0.03 Sr _0.97 Ti _0.97 forming a miniband.
Compared with Nb _0.03 O ₃ , it is considered that the material of the present invention is remarkably improved.
It is known that the diffusion rate of the generated holes to the surface is one of the rate limiting factors of the photocatalytic reaction. The present invention has been made based on this finding.

The present invention is a composite oxide semiconductor photocatalyst composed of silver and a divalent metal element, niobium, titanium, and oxygen. When irradiated with light, the photocatalytic reaction is caused by sufficiently absorbing the wavelength spectrum in the visible light region. Can do. In this respect, it is a material having an extremely high advantage compared with the ultraviolet light-responsive photocatalyst based on titanium oxide that has been put into practical use so far.
Also, compared to materials doped with Cr or N, the amount of defects is small, recombination of electrons and holes hardly occurs, and photocatalytic activity is high. According to the present invention, not only ultraviolet light but also visible light, one of the VOCs most often used in factories, 2-propyl alcohol (IPA)
) Can be decomposed efficiently.
This photocatalyst is not limited to this, but it also decomposes and removes other harmful gases such as aldehyde gas and environmental hormones that are one of the cause gases of sick house syndrome by irradiating light. Has the ability to
Antiviral and antibacterial effects can also be expected against viruses and bacteria.
As described above, the visible light responsive photocatalyst of the present invention has activity in the visible light and ultraviolet light regions, and because of its characteristics, it can be used for various applications other than the use examples shown above. Expected and expected to play a very important role in the future.

In order to obtain a composite oxide semiconductor as a photocatalyst of the present invention, an ordinary solid phase reaction method, that is, an oxide, metal carbonate, metal nitrate, metal sulfate, or metal chloride of each metal component as a raw material is used. It can synthesize | combine by mixing by the ratio of the target composition and baking in air under normal pressure.
Also, depending on the raw material components during firing, the material design of the catalyst may deviate from the planned design, but in this case, add an amount appropriate for sublimation from the beginning. Can be dealt with by.
In addition to the above raw materials, a metal alkoxide or a metal salt can be used as a raw material, which can be prepared by various methods such as so-called sol-gel method, coprecipitation method, complex polymerization method, sputtering method, chemical vapor deposition method, hydrothermal synthesis method and the like. It can be carried out by any preparation process.
The firing temperature when firing the adjusted blended raw material is converted into an oxide by decomposition of the raw material,
Any temperature may be used as long as a sintered body made of an oxide is obtained.

The shape and particle size of the composite oxide semiconductor used for the photocatalyst of the present invention are desirably designed so that the surface area becomes as large as possible in order to effectively use light.
Since the complex oxide semiconductor manufactured by the solid phase reaction method is obtained as a large molded product or lump, the surface area can be further increased by pulverizing it with a ball mill or etching with an acid or the like.
Further, the surface area may be increased by synthesizing so as to have a mesoporous structure, or may be supported on a carrier having a high surface area.
Furthermore, it is possible to use the powder particles by appropriately shaping them into a shape and shape. The composite oxide semiconductor used for the photocatalyst of the present invention can use the various adjusting means described above in addition to the sintering method. For example, a reaction raw material solution such as an aqueous solution containing a catalyst component is prepared, It can also be produced by co-precipitation and co-precipitation of a substance containing a catalyst component by precipitation reaction or co-precipitation reaction, and further drying and dehydrating or baking them.

In order to accelerate the charge separation of the photocatalytic material of the present invention and accelerate the photocatalytic reaction, Pt, Pd, A
A noble metal such as g may be supported on the composite oxide semiconductor to a few percent or less.

Hazardous substances that can be decomposed or removed by oxidation or reduction reaction by the photocatalytic reaction of the photocatalyst of the present invention include environmental hormones, agricultural chemicals, insecticides, molds, bacteria, viruses, algae, environmental pollutants, chlorofluorocarbons, hydrocarbons, alcohols, aldehydes , Ketone, carboxylic acid, carbon monoxide, amine, oil, aromatic compound, organic halogen compound, nitrogen compound, sulfur compound,
Examples include organophosphorus compounds and proteins. Furthermore, examples include soap scum and oil, dirt, tea astringency, and kitchen sinks that cause personal contamination.

Hereinafter, although the present invention is explained in detail based on a concrete example and a drawing, these do not limit the present invention. In the following examples, the general formula: Ag _x M _1-x Ti _1-x Nb _x
O ₃ (M is made of one or more divalent metals such as Mg, Ca, Sr, Ba, and Zn. In addition, x
Indicates 0.05 ≦ x <1. As an example, a photocatalyst composed of a complex oxide semiconductor represented by the following formula) is synthesized by a solid phase reaction method.

Example 1;
First, silver oxide, niobium oxide, strontium carbonate, and titanium oxide were weighed according to the composition ratio and sufficiently pulverized and mixed using a pulverizing and mixing device such as a ball mill or a mortar.
Thereafter, the mixture was put in an alumina crucible and sintered at 1000 ° C. for 24 hours in an atmospheric air atmosphere to obtain a powder.
Further, if necessary, firing may be performed in an oxygen atmosphere instead of an air atmosphere.
When this powder was measured using an X-ray diffractometer (FIG. 1), it was found that the diffraction peak shifted in one direction with the change of x, and the crystal structure was systematically changed.
Further, no impurity phase other than silver was confirmed, and Ag _x Sr _1-x Ti _1-x Nb _x O ₃ (x = 0
. 05, 0.5, 0.75) was found to be obtained.
Moreover, it turned out that said solid solution has absorption with respect to visible light from the measurement of an absorption spectrum (FIG. 2). In particular, in the materials of x = 0.5 and x = 0.75, the absorption edge is clearly red-shifted compared to SrTiO ₃ which does not absorb visible light at x = 0, and the absorption of visible light in these solid solution materials. Is not absorption between the isolated discrete Ag4d miniband and the bottom of the conduction band, but absorption derived from the Ag4d + O2p hybrid orbit and the bottom of the conduction band (FIG. 3).

The result of ultraviolet-visible absorption spectrum measurement of Ag _0.5 Sr _0.5 Ti _0.5 Nb _0.5 O ₃ showed absorption from the ultraviolet region to the visible light region from about 450 nm (FIG. 2).
This semiconductor was made into a powder having a specific surface area of about 1-2 m ² g ⁻¹ , mixed and suspended in 0.27 liter of a silver nitrate aqueous solution having a concentration of 3 g L ⁻¹ , and a decomposition test was conducted.
A 300 W Xe lamp was used as a light source, and a visible light of 400 nm or more was irradiated to the reaction vessel using a cut-off filter. The produced oxygen was quantified by gas chromatography (detector was TCD).
As a result, it was confirmed that about 120 μmol of oxygen was generated in 1 hour. From the above, it was found that this material is a visible light responsive photocatalyst.

Example 2;
_{Ag x M 1-x Ti 1} -x Nb x O 3 is one of the complex oxide semiconductor _{Ag-Ag 0.75} S
r _0.25 Ti _0.25 Nb _0.75 O ₃ was synthesized by the solid phase reaction method as described below.
First, 0.582 g of silver oxide, 0.400 g of niobium oxide, and 0.16 of strontium carbonate.
2 g and 0.0877 g of titanium oxide were weighed.
In the same manner as in Example 1, these were sufficiently pulverized and mixed, and then sintered at 1000 ° C. for 24 hours in an atmospheric air atmosphere to obtain a powder. When this powder was measured using an X-ray diffractometer, no impurity phase other than silver was confirmed, and Ag _0.75 Sr _0.25 Ti _0.25 Nb _0.75 O _3.
It was found that a solid solution was obtained.

0.5 liters of 0.4 g of the powder obtained in this example in which the specific surface area of Ag—Ag _0.75 Sr _0.25 Ti _0.25 Nb _0.75 O ₃ is about 1 to ² m ² g ⁻¹ The decomposition test of 2-propyl alcohol (about 160 ppm) gas was conducted.
A 300 W Xe lamp was used as the light source, and the reaction vessel was irradiated with visible light (light amount: 1.6 mWcm ⁻² ) from 400 nm to 520 nm using a cutoff filter.
The detection and quantification of 2-propyl alcohol and carbon dioxide were performed by gas chromatography with a methanizer (detector was FID), and the final product and the amount of carbon dioxide generated were also investigated (FIG. 4).
As a result, about 130 ppm of 2-propyl alcohol decomposes in one hour after the light irradiation, and then the light irradiation is continued. The carbon dioxide concentration is about 240 p by light irradiation for about one day.
Reached pm. This means that almost half the amount of 2-propyl alcohol charged first was completely decomposed to carbon dioxide.
Since the specific surface area of this material is 1 to ² m ² g ^−1, it takes time to decompose, but the decomposition time can be greatly shortened by expanding the specific surface area by using a carrier or making fine particles. Easy to predict. From the above, it is clear that this material exhibits photocatalytic properties in visible light, and it can be said that this material is a new and promising visible light responsive photocatalytic material.

Comparative Example 1;
The visible light decomposition activity of 2-propyl alcohol decomposition was investigated using anatase TiO ₂ which is a typical photocatalyst. The specific surface area of this powder is about 8 m ² g ⁻¹ . The equipment used for the measurement was the same as in Example 2. As a result, it was confirmed that 2-propyl alcohol was not decomposed at all because there was no production of intermediate acetone and carbon dioxide even after 1 hour, and there was no change in the amount of 2-propyl alcohol in the gas phase. It was done. TiO ₂ which shows excellent activity in ultraviolet light also shows no activity in visible light irradiation, and the photocatalytic activity in the visible light region is Ag.
_{x M 1-x Ti 1-} x Nb x O 3 (M is Mg, Ca, Sr, Ba, consisting of one or more divalent metals Zn. Further, x is shows a 0.05 ≦ x <1 )). From the above, it was reconfirmed that this TiO ₂ photocatalyst has no ability to decompose organic substances such as 2-propyl alcohol under visible light irradiation (FIG. 4).

Comparative Example 2;
The visible light decomposition activity of 2-propyl alcohol was examined using powder of AgNbO ₃ which is a typical visible light responsive photocatalyst. AgNbO ₃ was synthesized by the solid phase reaction method in the same manner as in Examples 1 and 2, and had the same specific surface area. The equipment used for the measurement was the same as in Example 2. As a result, it was confirmed that 2-propanol decomposes about 20 ppm in one hour. When compared with Example 2, it was found that the photocatalytic activity of AgNbO ₃ was clearly inferior. Thus, the composite oxide solid solution semiconductor composed of silver, divalent metal, niobium, and titanium has much higher activity than existing visible light responsive materials, and is a very effective visible light responsive photocatalyst material. (FIG. 4).

The above results are shown in FIGS. 1-4 as described above.
That is, a composite oxide semiconductor composed of silver, a divalent metal, and niobium is a highly active visible light responsive photocatalytic material, indicating that the material that meets the above-mentioned purpose has been successfully developed. As a result, the use efficiency is increased with respect to the wavelength of the irradiated light, and it is expected to be more effectively utilized and contribute to the photocatalytic reaction.

As described above, the present invention has the general formula: Ag _x M _1-x Ti _1-x Nb _x O ₃ (M is one or more divalent metals of Mg, Ca, Sr, Ba, Zn). X is 0.05.
<= X <1 is shown. ) A photocatalyst made of a complex oxide semiconductor can absorb not only ultraviolet light but also visible light.
Considering that the present practical photocatalyst, TiO ₂ , functioned only in the ultraviolet light region according to the present invention, it is extremely significant that the wavelength region that can be effectively used can be greatly expanded. In the visible light region, the activity is much higher than that of existing AgNbO ₃ . According to the present invention, environmental countermeasure technology used to make various harmful compounds using visible light, such as environmental hormones and bacteria, so-called harmful substances, sterilize, decompose, remove, etc. It is expected to contribute to the development of industry by being used for various chemical reactions.

X-ray diffraction pattern of the photocatalyst of Example 1 The figure which shows the absorption spectrum of the photocatalyst of Example 1 Schematic diagram of the band structure near the top position of the valence band The figure which shows each photocatalytic activity of Example 2 and Comparative Examples 1 and 2

Claims (1)

A visible light responsive photocatalyst that promotes oxidative decomposition of a substance by irradiation with visible light, and has a general formula: Ag _x M _1-x Ti _1-x Nb _x O ₃ (M is Mg, Ca, Sr, Ba, Zn) And a composite oxide semiconductor having a composition represented by the following formula: x is 0.05 ≦ x <1. photocatalyst.

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