JP2007222761A - Visible light responsive-type composite oxide photocatalyst - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a photocatalyst which has a catalytic activity to both an ultraviolet radiation of high energy contained in sunlight and a room illumination and light in long wavelength visible light area of energy lower than the former, and is capable of efficiently utilizing optical spectrum. <P>SOLUTION: The visible light responsive-type photocatalyst applied to photochemical reaction such as to decompose hazardous substances into innoxious ones or decompose water to generate hydrogen by a composite oxide semiconductor having a composition expressed by a general formula; PbxMgyNbzOw (0<x≤3, 0<y≤2, 0<z≤3, and 0<w≤10). <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a photocatalytic material, the use of this catalyst, a method for decomposing harmful substances, a method for decomposing and purifying soiled substances, and a method for generating hydrogen. Specifically, composite oxide photocatalyst materials exhibiting high photocatalytic activity for visible light other than ultraviolet rays as well as ultraviolet rays contained in sunlight and indoor lighting, the use of this catalyst, and harmful substances using this catalyst. The present invention relates to a decomposition method, a dirty material decomposition cleaning method, and a hydrogen generation method.

  When the photocatalyst is irradiated with light having energy greater than its band gap, electrons in the valence band are excited to the conduction band, and electrons and holes are generated in the conduction band and the valence band, respectively. In particular, halls have strong oxidizing power, can oxidize and decompose various organic substances, and are applied to various fields such as deodorization and antibacterial.

  As a typical practical material that can function as such a photocatalyst, titanium oxide has been mainly known so far (Non-patent Document 1). However, since the band gap of titanium oxide is as large as 3.2 eV, it is known that it exhibits extremely high photocatalytic activity for ultraviolet light having a wavelength shorter than 400 nm. There was no activity against the light in the region.

  The amount of ultraviolet light contained in sunlight and fluorescent lamps that are light sources is only about 4 to 10% of visible light. In other words, most of the natural light is occupied by visible light, and as long as conventional titanium oxide is used as a photocatalyst, the part of the visible light region that occupies most of the light spectrum of natural light is completely utilized. It will be wasted without anything. For this reason, the light utilization efficiency is extremely low, and the photocatalyst technology / materials are hardly used in the room where the absolute amount of light is particularly small.

  Therefore, the development of a visible light responsive photocatalytic material that functions even in an indoor space with a small absolute amount of light is expected. Under these circumstances, in recent years, various photocatalysts that are active even at wavelengths in the visible light region have been proposed and developed.

  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 2). 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, the band gap is narrowed, and visible light is certainly generated. Be able to absorb. However, the energy level introduced by doping metal ions can also be a recombination site of electrons and holes, and the increase in activity could not be expected so much.

  On the other hand, 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 in activity at this stage, and there is a need for a catalyst that can express a higher level of photocatalytic activity. ing.

Recently, an attempt to design and manufacture a visible light responsive photocatalyst using an oxide other than titanium oxide has been proposed. For example, indium represented by the general formula In _1-x M _x AO ₄ (M represents a transition metal element, A represents an element 5a of the periodic table, and x represents a number 0 <x <1). ) It has been proposed to design a visible light responsive photocatalyst using a composite oxide semiconductor (Patent Document 2).

  Attempts to design photocatalysts with such complex oxides other than titanium oxide have been vigorously and systematically conducted by the present inventors' research group. That is, as a result of intensive studies by the present inventors, the present inventors succeeded in designing a visible light responsive photocatalyst that exhibits activity not only for ultraviolet rays but also for light in the visible light region by using a complex oxide different from the above. A series of patent applications were filed for the results.

These are summarized and listed as described in (1) to (9) below.
That is, the first visible light responsive photocatalyst that has been successfully developed is (1) a general formula; a vanadium (V) -containing composite oxide semiconductor represented by BVO ₄ (where B is a group 3b in the periodic table) (Represents an element or a trivalent transition metal)) (Patent Document 3).

Next, the visible light responsive photocatalyst that has been successfully developed next is (2) a general formula; a vanadium (V) -containing composite oxide semiconductor represented by RVO ₄ (where R represents a Y element or a lanthanoid element). ) Is a catalyst designed by (Patent Document 4).

Furthermore, the visible light responsive photocatalyst that has been developed third is a composite oxide semiconductor represented by (3) general formula: (BaO) _n (In ₂ O ₃ ) _m (where n = 1 to 8). , M = 1 to 3) (Patent Document 5).

Subsequently, the fourth visible light responsive photocatalyst that has been successfully developed is (4) general formula: ABO ₃ (where A is the Ca, Sr, Ba element, and B is the two elements B1 and B2). Perovskite, wherein B1 is an In, Co, Ni, Cu, Zn element, and B2 is a V, Nb, Ta, Cr, Mo, W element). Designed by a complex oxide semiconductor having a type crystal structure (Patent Document 6), the fifth one is (5) an indium complex oxide semiconductor represented by the general formula: MIn ₂ O ₄ (In the formula, M = Ca, Sr, Ba represents at least one element) Visible light responsive photocatalyst (Patent Document 7), and the sixth is (6) general formula : Composite oxide semiconductor represented by MBi ₂ O ₄ However, it is a visible light responsive photocatalyst composed of M = Ca, Sr, Ba) (Patent Document 8).

Furthermore, the seventh developed visible light responsive photocatalyst is (7) General formula: MBiO ₃
Complex oxide semiconductor represented by nH ₂ O (where M = Li, Na, K, Ag, 0 ≦ n ≦
2) (Patent Document 9), and the eighth developed one is (8) General formula: Ag _x Bi _y M _z O _w (where M is V, One or more elements selected from the group 5A metal elements of Nb and Ta, represented by 0 <x, y ≦ 3, 0 <z ≦ 9, 0 <w ≦ 24, any numerical value) that composite oxides are those composed designed by semiconductor (Patent Document 10), followed by a visible-light-responsive photocatalyst developed ninth is (9) in the formula BaBi _x O _y (wherein, 0. It is designed by a composite oxide semiconductor represented by 5 <x <2, 2.5 <y <4) (Patent Document 11).

A. Fujishima, K .; Hashimoto, T .; Watanabe, TiO2 photocatalysis: Fundamentals and Applications, BKC Inc, (1999. 5) 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) Japanese Patent Laid-Open No. 2004-988 JP 2003-19437 A JP 2003-33661 A JP 2003-251197 A JP 2004-66028 A JP 2004-275946 A JP 2004-275947 A JP 2004-358332 A JP 2005-34716 A JP 2005-199134 A JP 2005-254154 A

  As described above, in addition to the high-energy ultraviolet light contained in sunlight and indoor lighting in recent years, light that has catalytic activity for light in the visible light region with a lower energy and longer wavelength than this. Research has been actively conducted to find a photocatalyst that can efficiently use the spectrum, that is, a visible light responsive photocatalyst exhibiting high activity against both ultraviolet rays and visible light. As a result, various photocatalysts can be obtained, the degree of freedom of selection is ensured, and a temporary result has been obtained, but it has a sufficiently high activity for reactions using light. However, there is a need for a photocatalyst that is adaptable to various environments and exhibits high activity.

  In particular, it is prepared by oxides that are readily available in the design of catalyst materials, and is composed of a composition different from titanium oxide, doped titanium oxide, or the composite oxide proposed above, and its performance is compared with these catalytic activities. Therefore, a visible light responsive photocatalyst having a catalytic activity that is inferior or superior to that is required.

  The present invention is intended to meet such a demand. Furthermore, it is possible to oxidize, reduce, and decompose harmful substances by irradiating light, detoxify harmful substances, or to clean dirt, and to detoxify harmful substances using this catalyst and photocatalyst materials used for hydrogen generation. The present invention intends to provide a treatment method, a method for decomposing and cleaning dirt substances, and a method for generating hydrogen. In particular, recently, a self-cleaning technique in which a photocatalyst is coated on a surface on which dirt easily adheres and the attached dirt is decomposed by the action of light has attracted attention. As a photocatalyst material used in such a self-cleaning technique, good film formability and durability that does not change even when exposed to the environment are required. This includes such a self-cleaning technique and is intended to meet this demand.

For this reason, the present inventors have made extensive studies focusing on ternary oxides containing lead, magnesium, and niobium as complex oxides having a composition different from those of the conventional photocatalysts based on the various photocatalysts described above. Repeated. As a result, it has been found that when these ternary oxides have a specific ratio, photocatalytic ability with excellent visible light response can be expressed. Its activity is higher than that of titanium oxide, and the recombination site increases dramatically by doping, as seen in the visible light responsive material using the doped type, and this does not increase the activity sufficiently. In addition, the present inventors have found that the material is excellent in film formability and can stably exhibit an excellent catalytic effect without being altered even when applied to the surface and exposed to the environment.
The present invention has been made based on this finding. The configuration is as described in (1) to (7) below.

(1) A composite oxide semiconductor having a composition represented by the general formula: Pb _x Mg _y Nb _z O _w (0 <x ≦ 3, 0 <y ≦ 2, 0 <z ≦ 3, 0 <w ≦ 10) A visible light responsive photocatalyst characterized by comprising:
(2) The visible light responsive photocatalyst described in (1) above, wherein the visible light responsive photocatalyst is used for decomposing harmful substances.
(3) The visible light responsive photocatalyst described in (1) above, wherein the visible light responsive photocatalyst is used for decomposing and cleaning dirt.
(4) The visible light responsive photocatalyst described in (1) above, wherein the visible light responsive photocatalyst is used to produce water by decomposing water or a hydrogen-containing substance.
(5) A composite oxide semiconductor having a composition represented by the general formula: Pb _x Mg _y Nb _z O _w (0 <x ≦ 3, 0 <y ≦ 2, 0 <z ≦ 3, 0 <w ≦ 10) A method for decomposing and removing harmful substances, comprising using a visible light responsive photocatalyst comprising: irradiating a harmful substance with light containing ultraviolet rays and visible light in the presence of the photocatalyst to decompose the harmful substance.
(6) A composite oxide semiconductor having a composition represented by the general formula: Pb _x Mg _y Nb _z O _w (0 <x ≦ 3, 0 <y ≦ 2, 0 <z ≦ 3, 0 <w ≦ 10) A method for decomposing and cleaning a dirt substance, comprising using a visible light responsive photocatalyst comprising: irradiating a dirt substance with light containing ultraviolet rays and visible light in the presence of the photocatalyst to decompose the dirt substance.
(7) A composite oxide semiconductor having a composition represented by the general formula: Pb _x Mg _y Nb _z O _w (0 <x ≦ 3, 0 <y ≦ 2, 0 <z ≦ 3, 0 <w ≦ 10) It is characterized by using a visible light responsive photocatalyst comprising, irradiating water or a hydrogen-containing substance with light containing ultraviolet rays and visible light in the presence of the photocatalyst, and decomposing the water or hydrogen-containing substance to generate hydrogen. Hydrogen generation method.

  The present invention is a photocatalyst composed of a complex oxide semiconductor composed of lead, niobium and magnesium, and the ratio of these is very wide, and sufficiently absorbs the spectrum of the wavelength in the visible light region when irradiated with light. Therefore, it is a material that has a significant advantage over the titanium oxide-based ultraviolet light-responsive photocatalyst that has been put into practical use. In addition, the amount of defects is small, recombination of electrons and holes hardly occurs, and the photocatalytic activity is high as compared with materials doped with Cr or N. According to the present invention, not only ultraviolet light but also visible light can be used to effectively decompose one type of VOC, 2-propyl alcohol (IPA), which is most often used in factories and the like. It will be. This photocatalyst is not limited to this, but it also decomposes and removes other harmful gases, such as aldehyde gas and environmental hormones, which are one of the cause gases of sick house syndrome, by irradiating light. Has the ability to As described above, the composite oxide semiconductor photocatalyst of the present invention has activity in the visible light and ultraviolet light regions, and because of its characteristics, it can be used in various applications other than the use examples shown above. Expected and expected to play a very important role in the future. Furthermore, the catalyst performance of the present invention is not inferior to the photocatalytic ability of the present invention as compared with the visible light responsive photocatalyst using the composite oxide described above.

  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 a normal pressure.

  The ratio x, y, z of each component of Pb, Mg, and Nb constituting this catalyst may be x: y: z = 0 to 3 or less: 0 to 2 or less: 0 to 3 or less. The molar ratio of each component is effective in a very wide range. Depending on the raw material components during firing, the material design of the catalyst may deviate from the planned design. In this case, add a suitable amount for sublimation from the beginning. can do.

  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 at which the adjusted blended raw material is fired may be any temperature at which the raw material is decomposed and converted into an oxide to obtain a sintered body made of the oxide. Specifically, the temperature range may be 700 ° C. or more and 1500 ° C. or less, and preferably 850 ° C. or more and 1050 ° C. or less.

The raw material adjustment in preparing the catalyst is determined based on the general formula: Pb _x Mg _y Nb _z O _w (0 <x ≦ 3, 0 <y ≦ 2, 0 <z ≦ 3, 0 <w ≦ 10) Is done. It is prepared by blending the raw materials so as to satisfy this general formula and firing. The meaning and meaning of this general formula are those that can function as a photocatalyst as long as lead, magnesium, and niobium satisfy the specified amounts, respectively, but some of them may be substituted with other elements. Visible light responsiveness can be shown, and the present invention can include such an embodiment and is not excluded.

  Specifically, a part of Pb or Mg may be replaced with another divalent metal element, or even if a part of Nb is replaced with another pentavalent metal element, it can be used as a visible light responsive photocatalyst. it can. The mixing / substitution of these other components cannot be uniquely defined. That is, when the photocatalyst is improved by mixing / substitution of these other components, it can be said that mixing / substitution of these other components is rather preferable and does not need to be excluded. However, when the photocatalytic effect is diluted by mixing and replacing the component, it is preferable to consider not mixing as much as possible in order to prevent a decrease in the catalytic effect.

  The shape and particle size of 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 composite oxide photocatalyst produced by the solid phase reaction method is obtained as a large molded product or a 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. Furthermore, it is possible to use the powder particles by appropriately shaping them into a shape and shape. 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, and a eutectoid reaction or coprecipitation is performed from the reaction solution. It can also be produced by co-depositing and co-precipitating a substance containing a catalyst component by reaction and further drying or dehydrating or calcining them.

  Hazardous substances that can be decomposed or removed by oxidation or reduction reaction of the photocatalytic material 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, organic phosphorus compound, protein and the like. In addition, soap and oil, hand stains, tea astringents, and slimes such as kitchen sinks that cause personal contamination can be decomposed by the photocatalytic reaction of this photocatalytic material.

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 examples described below, the general formula: Pb _x Mg _y Nb _z O _w
As the composite oxide semiconductor photocatalyst represented by (0 <x ≦ 3, 0 <y ≦ 2, 0 <z ≦ 3, 0 <w ≦ 10), the values of x, y, z, and w are specific values. , Pb _1.83 Mg _0.29 Nb _1.71 O _6.39 is disclosed as an example, and this is an example when synthesized by a solid phase reaction method. However, although it repeats, this Example is one Example for demonstrating this invention to the last, Comprising: This invention is not limited by this Example.

Example 1;
Pb _1.83 Mg _0.29 Nb _1.71 O _6.39 , which is one of complex oxide semiconductors composed of lead, niobium and magnesium, was synthesized by a solid phase reaction method as described below.
First, 3.0 g of lead oxide, 0.21 g of magnesium hydroxide carbonate, and 1.8 g of niobium oxide were weighed. These were sufficiently pulverized and mixed using a pulverizer such as a ball mill or a mortar, then placed in an alumina crucible and sintered at 950 ° C. for 5 hours in an atmospheric air atmosphere to obtain a powder. When this powder was measured using an X-ray diffractometer, it was found that a substance having a single-phase pyrochlore structure of the target Pb _1.83 Mg _0.29 Nb _1.71 O _6.39 was obtained (FIG. 1).
As a result of ultraviolet-visible absorption spectrum measurement, the photocatalyst of this example showed absorption from the ultraviolet region to the visible light region of about 450 nm, and the band cap could be estimated to be about 2.8 eV (FIG. 2).

Example 2;
A decomposition test of about 250 ppm of 2-propyl alcohol was conducted with 0.4 g of Pb _1.83 Mg _0.29 Nb _1.71 O _6.39 obtained in Example 1. A 300 W Xe lamp was used as the light source, and visible light (light quantity: 0.9 mWcm ⁻² ) of 420 nm to 520 nm was irradiated to the reaction vessel using a cutoff filter. Detection and quantification of 2-propyl alcohol and its decomposition substances acetone and carbon dioxide are performed by gas chromatography with a methanizer (detector is FID), and the amount of intermediate acetone generated when 2-propyl alcohol is decomposed is measured. The change with time was examined (FIG. 3). As a result, about 80 ppm of acetone was produced in one hour, and it was revealed that this material is a visible light responsive photocatalyst. It was also revealed that when light was irradiated for a longer time, it was completely oxidatively decomposed to carbon dioxide.

Example 3;
Using 0.5 g of Pb _1.83 Mg _0.29 Nb _1.71 O _6.39 obtained in Example 1, a hydrogen generation experiment from an aqueous methanol solution was performed. Using a cut-off filter and a xenon lamp, visible light of 400 nm or more was used as a light source. As a result, it was confirmed that about 40 μmol of hydrogen was produced in about 5 hours.

Comparative Example 1;
The visible light decomposition activity of 2-propyl alcohol decomposition was investigated using TiO ₂ which is a typical photocatalyst. The equipment used for the measurement was the same as in Example 2. as a result,
Even if 1 hour passed, there was no production amount of acetone and carbon dioxide, and since there was no change in the amount of 2-propyl alcohol in the gas phase, it was confirmed that 2-propyl alcohol was not decomposed at all (FIG. 3). . TiO ₂ exhibiting excellent activity in ultraviolet light also showed no activity in visible light irradiation, and its photocatalytic activity in the visible light region was significantly inferior to a composite oxide semiconductor composed of lead, niobium and magnesium. From the above, it was reconfirmed that this TiO ₂ photocatalyst has no ability to decompose organic substances such as 2-propyl alcohol under irradiation with visible light.

Comparative Example 2;
The visible light decomposition activity of 2-propyl alcohol was examined using nitrogen-doped TiO ₂ which is a typical visible light responsive photocatalyst. The sample preparation method was synthesized based on the procedure described in Non-Patent Document 3 described later. The equipment used for the measurement was the same as in Example 2. As a result, it was confirmed that about 20 ppm of acetone was produced in 1 hour, and it was found that the photocatalytic activity was about 4 times inferior when compared with Example 2 (FIG. 3). In addition, when comparing the specific surface area, Comparative Example 2, respectively the specific surface area of Example 2, about 43m ² g ^-1, about 1.7 m ² g ^-1 and about 20 times or more, was greater towards the Comparative Example 2 . If other properties are the same, it is generally known that a material having a larger specific surface area has higher activity. If Example 2 and Comparative Example 2 have the same specific surface area, the difference in activity is several. Ten times Example 2 is expected to be higher. Thus, it can be seen that the complex oxide semiconductor composed of lead, niobium, and magnesium has a higher activity than existing visible light responsive materials, and is a very effective visible light responsive photocatalytic material.

About the above result, it has shown to FIGS. 1-3 as above-mentioned. In addition to the above examples, Pb ₃ MgNb ₂ O ₉ having a perovskite structure absorbs visible light and efficiently decomposes organic substances under irradiation with visible light.
That is, a complex oxide semiconductor composed of lead, niobium, and magnesium 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.

R. Asahi, T .; Morikawa, T .; Ohwaki, K .; Aoki and Y.A. Taga: Science Vol 293, No 5528, pp 269-271 AMER ASSOC ADVANCEMENT SCIENCE, (2001.7.13).

As described above, the present invention provides a Pb _x Mg _y Nb _z O _w (0 <x ≦ 3, 0 <y ≦ 2, 0 <z ≦ 3, 0 <w ≦ 10) composite oxide semiconductor photocatalyst. It can absorb not only ultraviolet light but also visible light. Considering that the present practical photocatalyst, TiO _2, has 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 the existing nitrogen-doped titanium oxide. According to the present invention, environmental countermeasure technology used to act on various harmful compounds using visible light, for example, so-called harmful substances such as environmental hormones and bacteria, and to 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 fabricated Pb _1.83 Mg _0.29 Nb _1.71 O _6.39 Diagram showing absorption spectrum of the prepared photocatalyst The figure which shows each photocatalytic activity of Example 2 and Comparative Examples 1 and 2

Claims (7)

It is made of a complex oxide semiconductor having a composition represented by the general formula: Pb _x Mg _y Nb _z O _w (0 <x ≦ 3, 0 <y ≦ 2, 0 <z ≦ 3, 0 <w ≦ 10) A visible light responsive photocatalyst characterized by   The visible light responsive photocatalyst according to claim 1, wherein the visible light responsive photocatalyst is used for decomposing a harmful substance.   The visible light responsive photocatalyst according to claim 1, wherein the visible light responsive photocatalyst is used for decomposing and cleaning dirt.   The visible light responsive photocatalyst according to claim 1, wherein the visible light responsive photocatalyst is used to produce hydrogen by decomposing water or a hydrogen-containing substance. Visible compound oxide semiconductor having a composition represented by the general formula: Pb _x Mg _y Nb _z O _w (0 <x ≦ 3, 0 <y ≦ 2, 0 <z ≦ 3, 0 <w ≦ 10) A method for decomposing and removing harmful substances, comprising using a photoresponsive photocatalyst and irradiating the harmful substance with light containing ultraviolet rays and visible light in the presence of the photocatalyst to decompose the harmful substance. Visible compound oxide semiconductor having a composition represented by the general formula: Pb _x Mg _y Nb _z O _w (0 <x ≦ 3, 0 <y ≦ 2, 0 <z ≦ 3, 0 <w ≦ 10) A method for decomposing and cleaning a dirt substance, comprising using a photoresponsive photocatalyst and irradiating the dirt substance with light containing ultraviolet rays and visible light in the presence of the photocatalyst to decompose the dirt substance. Visible compound oxide semiconductor having a composition represented by the general formula: Pb _x Mg _y Nb _z O _w (0 <x ≦ 3, 0 <y ≦ 2, 0 <z ≦ 3, 0 <w ≦ 10) Hydrogen, characterized by using a photoresponsive photocatalyst and irradiating water or a hydrogen-containing substance with light containing ultraviolet rays and visible light in the presence of the photocatalyst to decompose water or the hydrogen-containing substance to generate hydrogen. How it occurs.