JP3890414B2 - Perovskite complex oxide visible light responsive photocatalyst, hydrogen production method using the same, and hazardous chemical decomposition method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention is a composite oxide semiconductor having a perovskite type crystal structure, a photocatalyst excellent in photoresponsiveness that efficiently absorbs ultraviolet rays and visible light contained in sunlight and the like, and in particular, highly active in hydrogen production ability. The present invention relates to a photocatalyst for hydrogen production, a photocatalyst for water splitting, a photocatalyst for decomposing toxic chemicals, a method for producing hydrogen using these catalysts, and a method for decomposing toxic chemicals.
[0002]
[Prior art and problems to be solved by the invention]
Global warming has become a global problem. If carbon dioxide in the atmosphere continues to increase at the current pace, it is expected that in 2030, its concentration will double that of the pre-industrial level. Due to the greenhouse effect, the temperature in the polar region rises by about 14 degrees, and if the sea level rises by 60 cm, it is said that it will have a serious adverse effect on the earth's ecosystem and climate change. Specific carbon dioxide emission reduction figures for each country were examined, and Japan set a target of reducing average emissions from 2008 to 2012 by at least 6% from the 1990 level. In order for mankind to continue to develop after the 21st century, it is essential to develop clean energy that does not emit carbon dioxide or environmental pollutants. It is also essential to purify the environment that is already being destroyed.
[0003]
Hydrogen is considered to be the ultimate fuel that has three times the thermal efficiency of gasoline and burns back to water, producing no harmful substances. In fact, hydrogen-fueled fuel cells are being researched and put to practical use in the near future. In addition, hydrogen automobiles and hydrogen turbines are urgently being developed by companies and other companies as clean systems that do not generate toxic substances. And the synthesis method of hydrogen is an urgent problem because it is the source of all of them. Currently, most of the hydrogen is generated from reforming reactions from oil and natural gas, or from electrolysis of water, which simultaneously generates carbon dioxide, which causes global warming, or uses valuable energy sources. Will end up.
[0004]
On the other hand, the amount of solar energy that reaches the ground in one year is enormous, equivalent to 10,000 times the annual energy consumption of mankind. As solar energy utilization methods, solar cells and solar heat utilization systems have been developed, but their utilization rate is still insufficient, and large-scale ones are difficult and cost is high.
In order to realize the effective use of sunlight, an artificial photosynthesis technology that can directly produce hydrogen and oxygen as clean fuels from inexhaustible sunlight and water using a visible light semiconductor photocatalyst can be considered.
[0005]
When the photocatalyst absorbs energy greater than its band gap, it generates holes and electrons, which respectively undergo oxidation and reduction reactions with water to generate oxygen and hydrogen. Considering the practical application of this photocatalyst, it is essential to use sunlight as a light source. Sunlight falling on the surface of the earth has a maximum intensity of radiation in the vicinity of a wavelength of 500 nm that is visible light, and the amount of energy in the visible light region having a wavelength of 400 to 750 nm is about 43% of the total sunlight. On the other hand, it is less than 5% in the ultraviolet region with a wavelength of 400 nm or less. Therefore, in order to efficiently use the sunlight spectrum, a photocatalyst having catalytic activity for visible light is desired.
[0006]
However, many conventional semiconductor photocatalysts can produce hydrogen when irradiated with high-energy ultraviolet light, but the examples of hydrogen production using visible light-responsive semiconductor photocatalysts are very limited and their activity is low. . In order to use sunlight, it is essential to develop a new photocatalyst that can effectively use visible light.
[0007]
In recent years, as an applied study of photocatalysts, the use of photocatalysts for the decomposition of harmful chemical substances has been widely studied in the field. Application examples such as the decomposition of organic substances such as pesticides and malodorous substances in the water and the atmosphere, and self-cleaning of solid surfaces coated with catalysts have been researched and proposed, most of which use titanium dioxide, Moreover, it hardly functioned with visible light.
Therefore, in the above applied research, if a photocatalyst that can use visible light is developed and used, it can be expected that the efficiency will be improved. What is important at that time is the level of the conduction band. Holes in the valence band of an oxide semiconductor have a very strong oxidizing ability and can oxidize electron donors such as water and many organic substances. At that time, electrons generated in the conduction band are consumed by reducing oxygen in the air. That is, the conduction band level must be more negative than the oxygen reduction level. A photocatalyst capable of generating hydrogen is a novel homogeneous photocatalyst having a potential to reduce oxygen, and can be expected to be applied to the above-mentioned fields.
[0008]
[Problems to be solved by the invention]
The present invention is intended to provide a novel photocatalyst that efficiently absorbs not only ultraviolet rays contained in sunlight and the like but also visible light. By using this catalyst, harmful substances and hydrogen-containing compounds can be obtained. It is intended to provide a method for detoxifying a harmful substance or a method for producing and producing hydrogen by irradiating light to decompose the harmful substance or the hydrogen-containing compound. In addition, the research group including the present inventor has so far developed and proposed a number of photocatalysts with the aim of being shown. The present invention has also been made as part of the research, and proposes a novel photocatalyst that differs in composition from what has been proposed so far, and has been successfully developed. The photocatalyst proposed by the present inventors as the prior art of the present invention is in a patent application and is still in a state before publication, so the document name cannot be disclosed. Described below.
[0009]
[Patent Document 1]
Patent application 2001-221148
[Patent Document 2]
Patent Application 2002-59804
[Patent Document 3]
Patent application 2002-225296
[0010]
[Means for Solving the Problems]
As described above, as a result of intensive studies on the photocatalyst having a specific function, the present inventors have succeeded in solving and achieving the above object by the following means (1) to (8).
[0011]
(1) A visible light responsive photocatalyst comprising a composite oxide semiconductor having a perovskite type crystal structure represented by the general formula (I): ABO ₃ .
In the formula, A represents any of Ca, Sr, and Ba elements. B is the one in which two elements B1 and B2 are substituted while maintaining a charge balance so as to be tetravalent. B1 is Co, represents one of Ni element, B2 represents V, Nb, one of Ta elements.
[0012]
(2) noble metal elements, transition metal elements, _NiO X (x is greater than 0, 1 or less represents a _value.) From IrO 2, 1, two or more components selected from the group consisting of RuO ₂ The visible light responsive photocatalyst described in the item (1), which further comprises a cocatalyst.
(3) A photocatalyst for visible light responsive hydrogen production, comprising the composite oxide semiconductor according to any one of (1) to (2).
(4) A method for producing hydrogen, comprising irradiating a hydrogen-containing compound with light containing ultraviolet rays and visible light in the presence of the photocatalyst for visible light responsive hydrogen production according to (3).
[0013]
(5) A visible light responsive water splitting photocatalyst comprising the composite oxide semiconductor according to any one of (1) to (2).
(6) A method for producing hydrogen, comprising irradiating water containing ultraviolet light and visible light in the presence of the visible light responsive water-decomposing photocatalyst described in (5).
(7) A photocatalyst for decomposing visible light responsive harmful chemical substances, comprising the composite oxide semiconductor according to any one of (1) to (2).
(8) A method for decomposing a hazardous chemical substance, which comprises irradiating the hazardous chemical substance with light containing ultraviolet rays and visible light in the presence of the photocatalyst for decomposing a visible light responsive harmful chemical substance according to (7) .
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail, but these all disclose one specific embodiment of the present invention, and the present invention is not limited to this.
[0015]
In the oxide ABO _{3 according} to claim 1 of the present invention, A represents any one of Ca, Sr, and Ba elements. B is one in which the two types of elements B1 and B2 are replaced with 1/2 to 1/2 or 1/3 to 2/3 while maintaining a charge balance so that the total is tetravalent. B1 is Co, represents one of Ni element, B2 represents V, Nb, one of Ta elements. The basic skeleton is a compound having a perovskite type crystal structure represented by the general formula A ²⁺ B ⁴⁺ O ₃ , and the crystal structure may be maintained. By allowing two types of metal elements to coexist at the B site of the crystal structure, a new electronic level can be introduced, the semiconductor band gap can be reduced, and the photoresponse range can be expanded.
[0016]
The composite oxide semiconductor of the present invention can also be synthesized by a normal solid phase reaction method, that is, by mixing oxides of respective metal components as raw materials in a ratio of a target composition and firing in air under normal pressure. It is necessary to add a little more in the raw material which is easy to sublimate. Various methods such as various sol-gel methods, coprecipitation methods, and complex polymerization methods using metal alkoxides and metal salts as raw materials are also used. Among them, an oxide precursor is prepared and synthesized by firing.
[0017]
The shape of the photocatalyst of the present invention is preferably fine particles and has a large surface area in order to effectively use light. Although the oxide prepared by the solid phase reaction method has large particles and a small surface area, the particle diameter can be reduced by grinding with a ball mill or the like. In general, the size of the particles is 10 nm to 200 μm, preferably 1 μm or less. Further, fine particles can be molded and used as a plate. Alternatively, other materials can be coated in a thin film.
[0018]
Further, for the semiconductor of the present invention, noble metals such as Pt which are co-catalysts, transition metals such as Ni, NiO _X (x represents a value exceeding 0 and 1 or less), IrO ₂ , RuO _2, etc. Modifications commonly used in photocatalyst production such as oxide loading can be performed. The supporting method can be performed by an impregnation method or a photo-deposition method. In the impregnation method, a semiconductor is impregnated with an aqueous solution of a photocatalytically active species such as chloride and nitrate, and then dried at 100 to 200 ° C. for about 2 to 5 hours to be 800 ° C. or less, preferably 200 to 200 ° C. Firing is performed at 500 ° C. in a reducing atmosphere and / or an oxidizing atmosphere for 2 to 5 hours. The amount of cocatalyst is 0.01 to 10 wt%, preferably 0.1 to 5 wt%.
[0019]
In addition, the reaction solution used for the water decomposition reaction is not limited to pure water. Usually, salts such as carbonates, hydrogen carbonates, iodine salts and bromine salts are used so that they are often used for water decomposition reactions. Mixed water may be used. And the photocatalyst of this invention is added to the said aqueous solution. The amount of catalyst added is basically selected so that incident light can be efficiently absorbed. 0.05~10g the irradiation area 25 cm ^2, preferably 0.2 to 3 g. Thus, by irradiating light to the aqueous solution to which the photolysis catalyst is added, water is decomposed and hydrogen is generated. The wavelength of the light to be irradiated needs to include light having a wavelength in a region where the semiconductor is absorbed. In the present invention, sunlight may be irradiated.
[0020]
The photocatalyst of the present invention can be applied not only to water decomposition but also to many photocatalytic reactions.
For example, in the case of decomposition of organic substances, alcohol, agricultural chemicals, malodorous substances and the like generally act as electron donors, and are oxidatively decomposed by holes, and hydrogen is generated by electrons or oxygen is reduced. As a reaction form, the catalyst may be suspended in an aqueous solution containing an organic substance and irradiated with light, or the catalyst may be fixed to a substrate. A gas phase reaction may be used, such as decomposition of malodorous substances.
[0021]
(Example)
Hereinafter, the present invention will be described in detail. In the following examples, BaCo _1/3 Nb _2/3 O ₃ and BaNi _1/3 Nb _2/3 O ₃ were synthesized. The synthesis was performed by a solid phase method by preparing oxides of respective components in a stoichiometric ratio.
[0022]
Example 1;
BaCo _1/3 Nb _2/3 O ₃ was prepared by a solid phase reaction method.
For example, in the case of synthesizing 10 g BaCo _1/3 Nb _2/3 O ₃ , 7.4 g BaCO ₃ , 0.94 g CoO, and 3.32 g Nb ₂ O ₅ were weighed. This was put in an alumina crucible and sintered at 950 ° C. for 8 hours in an electric furnace under atmospheric pressure in air, and then sintered at 1150 ° C. for 32 hours. After the completion of firing, the fired product was pulverized to a size of 10 μm or less in a mortar. The chemical composition and crystal structure of the catalyst were investigated using XRD and SEM-EDS. Rietveld structure analysis revealed that this system belongs to the cubic system and has a space group Pm3m, a lattice constant a = 0.409 nm, and a perovskite crystal structure. From the ultraviolet-visible absorption spectrum measurement, it was found that the present photocatalyst absorbs from the ultraviolet region to the visible light region with an upper limit of 800 nm and has a response in a wide visible light region.
The oxide semiconductor was supported by 1.0 wt% NiO _X by impregnation with an aqueous Ni (NO ₃ ) ₂ solution, dried at 200 ° C. for 5 hours, reduced by hydrogen at 500 ° C., and reoxidized at 200 ° C.
1.0 g of NiO _X / BaCo _1/3 Nb _2/3 O ₃ was suspended in 250 ml of pure water to cause water photolysis reaction. Visible light was irradiated from the outside using a closed circulation system catalytic reactor while stirring with a magnetic stirrer. A 300 W xenon lamp was used as the light source, and a reaction cell made of Pyrex glass (registered trademark of Corning) was used. The light from the light source was transmitted through a cut-off filter (wavelength> 420 nm) for cutting light on the short wavelength side, and then irradiated to the sample (photocatalyst). The generated hydrogen was detected and quantified by gas chromatography.
As a result, hydrogen was constantly generated at a rate of 60 μmol / h. It was found that water was decomposed with visible light to generate hydrogen. The results are shown in Table 1.
[0023]
Example 2;
In order to confirm whether the decomposition of the organic matter proceeded efficiently with visible light, the methanol in the aqueous solution was decomposed. As the catalyst, the above oxide semiconductor carrying Pt (0.1 wt%) was used. 1.0 g of the catalyst was suspended in a mixed solution of 240 ml of pure water and 10 ml of methanol and subjected to a photolysis reaction. Visible light was irradiated from the outside using a closed circulation system catalytic reactor while stirring with a magnetic stirrer. A 300 W xenon lamp was used as the light source, and a reaction cell made of Pyrex glass (registered trademark of Corning) was used. The light from the light source was transmitted through a cut-off filter (wavelength> 420 nm) for cutting light on the short wavelength side, and then irradiated to the sample (photocatalyst).
The generated hydrogen and oxygen were detected and quantified by gas chromatography.
As a result, hydrogen was constantly generated at 100 μmol / h. Oxygen was not generated. This indicates that while the methanol is oxidatively decomposed by holes, the reaction in which electrons reduce water and generate hydrogen proceeds under irradiation with visible light. These results are shown in Table 1.
[0024]
Example 3;
BaNi _1/3 Nb _2/3 O ₃ was prepared by a solid phase reaction method.
For example, when synthesizing 10 g BaNi _1/3 Nb _2/3 O ₃ , 7.41 g of BaCO ₃ , 0.93 g of NiO, and 3.32 g of Nb ₂ O ₅ were weighed. This was put in an alumina crucible and sintered at 950 ° C. for 8 hours in an electric furnace under atmospheric pressure in air, and then sintered at 1150 ° C. for 32 hours. After the completion of firing, the fired product was pulverized to a size of 10 μm or less in a mortar. The chemical composition and crystal structure of the catalyst were investigated using XRD and SEM-EDS. Rietveld structure analysis revealed that this system belongs to the cubic system and has a space group Pm3m, a lattice constant a = 0.4057 nm, and a perovskite crystal structure. The UV-visible absorption spectrum measurement showed that the present photocatalyst showed absorption from the ultraviolet region to the visible light region with an upper limit of 500 nm, and had visible light responsiveness. In order to confirm whether decomposition of the organic substance of the oxide photocatalyst proceeds efficiently with visible light, methanol in the aqueous solution was decomposed. As the catalyst, the above oxide semiconductor carrying Pt (0.1 wt%) was used. 1.0 g of the catalyst was suspended in a mixed solution of 240 ml of pure water and 10 ml of methanol and subjected to a photolysis reaction. Visible light was irradiated from the outside using a closed circulation system catalytic reactor while stirring with a magnetic stirrer. A 300 W xenon lamp was used as the light source, and a reaction cell made of Pyrex glass (registered trademark of Corning) was used. The light from the light source was transmitted through a cut-off filter (wavelength> 420 nm) for cutting light on the short wavelength side, and then irradiated to the sample (photocatalyst). The generated hydrogen and oxygen were detected and quantified by gas chromatography.
As a result, hydrogen was constantly generated at 50 μmol / h. Oxygen was not generated. This indicates that while methanol is oxidatively decomposed by holes, a reaction in which electrons reduce water and generate hydrogen proceeds under visible light irradiation. These results are shown in Table 1.
[0025]
The examples shown above are very specific about the relationship between the photocatalyst component used, the supported cocatalyst component and its presence, the type of reaction (reaction purpose), the light source used, the amount of hydrogen gas generated per unit time, etc. In particular, this embodiment is shown, and this makes it possible to easily implement the present invention. Table 1 summarizing these matters briefly presents certain suggestions regarding the importance of the above items.
[0026]
[Table 1]
[0027]
【The invention's effect】
As described above, general formula (I): ABO ₃ (wherein A represents any one of Ca, Sr, and Ba elements. B represents a charge balance such that two kinds of elements B1 and B2 are tetravalent. is obtained by replacing while taking .B1 represents Co, one of the Ni element, B2 is V, Nb, a composite oxide having a perovskite type crystal structure of the present invention represented by indicating one of Ta element) Considering that the wavelength range where photoresponse is possible extends to visible light with an upper limit of 800 nm, and that the conventional photocatalyst functioned only in the ultraviolet light range, the wavelength range that can be used effectively is greatly expanded, and the solar energy is large. Since it can function with respect to the light occupying the main area of the part, its significance is extremely large. In addition, holes and electrons generated by photoexcitation can quickly move to the surface of the catalyst, reducing the probability of recombination of holes and electrons, and exhibiting high catalytic activity for light. According to the present invention, hydrogen can be generated by decomposing water using visible light energy. In the future, if a photocatalyst is laid in an artificial pond, hydrogen can be efficiently produced in large quantities with inexhaustible sunlight, which can overcome the energy problem. Further, these photocatalysts may be used for other chemical reactions instead of water decomposition reactions. For example, it can be applied to organic substance decomposition reactions and metal ion reduction reactions. It can greatly contribute to environmental purification. As described above, the compound photocatalyst of the present invention is active over a wide area of light, and because of its characteristics, it is expected to be used in various applications other than the above-mentioned use examples. The role played is considered to be very large.

Claims (8)

A visible light responsive photocatalyst comprising a composite oxide semiconductor having a perovskite crystal structure represented by the general formula (I): ABO ₃ .
In the formula, A represents any of Ca, Sr, and Ba elements. B is obtained by substituting two kinds of elements B1 and B2 while maintaining a charge balance. B1 is any one of Co and Ni elements, and B2 is any one of V, Nb and Ta elements. Noble metal elements, transition metal elements, _NiO X (exceed x is 0,. Representing a value of 1 or _less), aid consisting of one or more components selected from the group consisting of IrO 2, RuO ₂ The visible light responsive photocatalyst according to claim 1, further comprising a catalyst.   A photocatalyst for visible light responsive hydrogen production, comprising the composite oxide semiconductor according to claim 1.   A method for producing hydrogen, comprising irradiating a hydrogen-containing compound with light containing ultraviolet rays and visible light in the presence of the photocatalyst for visible light responsive hydrogen production according to claim 3.   A visible light responsive water-decomposing photocatalyst comprising the composite oxide semiconductor according to claim 1.   A method for producing hydrogen, comprising irradiating water containing ultraviolet light and visible light in the presence of the photocatalyst for visible light responsive water splitting according to claim 5.   A photocatalyst for decomposing a visible light responsive harmful chemical substance, comprising the composite oxide semiconductor according to claim 1.   A method for decomposing a harmful chemical substance, comprising irradiating the hazardous chemical substance with light containing ultraviolet rays and visible light in the presence of the photocatalyst for decomposing a visible light responsive harmful chemical substance according to claim 7.