JP3735711B2 - Visible light-responsive rare earth compound 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 zircon-type crystal structure that contains Y or a lanthanoid element in the periodic table and vanadium (V) and belongs to the I4 ₁ / amd space group. The present invention relates to a highly active photocatalyst for hydrogen production that absorbs efficiently, a photocatalyst for water decomposition, and a photocatalyst for decomposition of harmful chemical substances.
[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 decomposition of organic substances such as agricultural chemicals and malodorous substances in the water and air and self-cleaning of solid surfaces coated with catalysts have been researched and proposed, most of which are using 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 photocatalyst that efficiently absorbs ultraviolet rays contained in sunlight and the like. By using this catalyst, the harmful substance or hydrogen-containing compound is irradiated with light, and the harmful substance or hydrogen It is intended to provide a method for detoxifying harmful substances or producing and producing hydrogen by decomposing contained compounds.
[0009]
[Means for Solving the Problems]
Therefore, as a result of intensive studies, the inventors have succeeded in solving and achieving the above object by the following means (1) to (10).
[0010]
(1) A photocatalyst comprising a vanadium (V) -containing composite oxide semiconductor represented by the general formula (I): RVO ₄ ;
In the formula, R is a Y element or a lanthanoid element. A rare earth complex oxide semiconductor in which the number of moles of R is equal to the number of moles of V is used, and this component is included, thereby solving the problem.
(2) R is at least one element selected from the group consisting of Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. The photocatalyst according to (1), characterized in that it exists.
[0011]
(3) The rare earth complex oxide semiconductor represented by the general formula (I) is a complex oxide semiconductor having a zircon-type crystal structure belonging to the I 4 ₁ / amd space group, according to (1) or (2) photocatalyst.
(4) Pt, NiO _x (x represents a value of more than 0 and 1 or less) comprising a promoter composed of one or more components selected from RuO ₂ , The photocatalyst according to any one of (1) to (3).
(5) A photocatalyst for hydrogen production comprising the composite oxide semiconductor according to any one of (1) to (4).
(6) 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 hydrogen production according to (5).
[0012]
(7) A photocatalyst for water splitting comprising the composite oxide semiconductor according to any one of (1) to (4).
(8) A method for producing hydrogen, wherein the water is irradiated with light containing ultraviolet rays and visible light in the presence of the photocatalyst for water splitting according to (7).
(9) A photocatalyst for decomposing a harmful chemical substance comprising the composite oxide semiconductor according to any one of (1) to (4).
(10) 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 a photocatalyst for decomposing the hazardous chemical substance according to (9).
[0013]
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.
[0014]
In the oxide RVO _{4 according} to claim 1 of the present invention, R is a Y element or a lanthanoid element. A photocatalyst using a complex oxide semiconductor in which the number of moles of R is equal to the number of moles of V. Although oxygen in the chemical formula is represented by four in terms of form, the number of oxygen obtained as a result of actual preparation does not need to be exactly 4 because there are oxygen defects and the like. The basic skeleton is a zircon type crystal belonging to the I4 ₁ / amd space group represented by the general formula R ³⁺ V ⁵⁺ O ₄ (“International Tables for Crystallography”, vol. A, p. 472, Kluwer Academic Publishers, 1995). It is a compound having a structure, and its crystal structure may be maintained.
[0015]
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.
[0016]
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 50 μm or less. Further, fine particles can be molded and used as a plate. Alternatively, other materials can be coated in a thin film.
[0017]
Further, for the semiconductor of the present invention, a cocatalyst such as a noble metal such as Pt, a transition metal such as Ni, NiO _x (x represents a value of more than 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%.
[0018]
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.
[0019]
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.
[0020]
(Example)
Hereinafter, the present invention will be described in detail. In the following examples, YVO ₄ was synthesized using Y element and V element. The synthesis was performed by a solid phase method by preparing oxides of respective components in a stoichiometric ratio.
[0021]
Example 1
YVO ₄ was synthesized using Y element and V element. In the synthesis, 1.0 mol Y ₂ O ₃ and 1.0 mol V ₂ O ₅ were prepared in a stoichiometric ratio per metal. (Figure 1)
For example, when synthesizing 10 g YVO ₄ , 5.539 g of Y ₂ O ₃ and 4.461 g of V ₂ O ₅ were weighed. This was put into an alumina crucible and sintered at 700 ° C. for 12 hours in an electric furnace under normal pressure in air. After firing, the fired product was pulverized to a size of 10 mm or less with 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 tetragonal system and has a space group I4 ₁ / amd, lattice constant a = 0.7123, c = 0.6292 nm, and a zircon type crystal structure. From the ultraviolet-visible absorption spectrum measurement, it was found that the present photocatalyst showed absorption from the ultraviolet region to the visible light region with an upper limit of 600 nm, the band cap was estimated to be 2.1 eV or less, and had visible light responsiveness.
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 g of NiO _x / YVO ₄ was suspended in 370 ml of pure water and subjected to water photolysis reaction. Using a closed circulation system catalytic reactor, light was irradiated from the outside while stirring with a magnetic stirrer. A 400 W high-pressure mercury lamp was used as the light source, and a reaction cell made of quartz glass was used. The generated hydrogen and oxygen were detected and quantified by gas chromatography. The result is shown in FIG.
As a result, it was found that hydrogen was constantly generated at a rate of 300 μmol / h and oxygen at a rate of 150 μmol / h.
[0022]
Example 2
In Example 1, RuO ₂ was used as the supporting metal instead of NiO _x . The YVO ₄ semiconductor carrying 1.0 wt% RuO ₂ was impregnated with an aqueous RuCl ₄ solution, dried at 200 ° C. for 5 hours, and baked at 500 ° C. in an oxidizing atmosphere for 2 hours. 1 g of RuO ₂ / YVO ₄ was suspended in 370 ml of pure water, and water was subjected to photolysis reaction. Using a closed circulation system catalytic reactor, light was irradiated from the outside while stirring with a magnetic stirrer.
A 400 W high-pressure mercury lamp was used as the light source, and a reaction cell made of quartz glass was used. The generated hydrogen and oxygen were detected and quantified by gas chromatography. The result is shown in FIG. Also in this case, it was found that hydrogen was constantly generated at a rate of 150 μmol / h and oxygen at a rate of 75 μmol / h.
[0023]
Example 3
In Example 1, Pt was used instead of NiO _{x as} the supported metal. Platinum equivalent to 0.1 wt% with respect to the oxide was added with an aqueous chloroplatinic acid solution and supported on the oxide by photo-deposition. 1 g of RuO ₂ / YVO ₄ was suspended in 370 ml of pure water, and water was subjected to photolysis reaction. Using a closed circulation system catalytic reactor, light was irradiated from the outside while stirring with a magnetic stirrer. A 400 W high-pressure mercury lamp was used as the light source, and a reaction cell made of quartz glass was used. The generated hydrogen and oxygen were detected and quantified by gas chromatography. The result is shown in FIG.
[0024]
Example 4
In Example 1, a catalyst without a supported metal was used. 1 g of YVO ₄ was suspended in 370 ml of pure water and subjected to photolysis reaction of water. Using a closed circulation system catalytic reactor, light was irradiated from the outside while stirring with a magnetic stirrer. A 400 W high-pressure mercury lamp was used as the light source, and a reaction cell made of quartz glass was used. The generated hydrogen was detected and quantified by gas chromatography. The result is shown in FIG. Even in this case, steady hydrogen generation was observed.
As described above, when the results from Example 1 to Example 4 are seen, for the YVO ₄ -based semiconductor photocatalyst, the catalyst having a supported metal (Example 1, Example 2, Example 3) is a catalyst having no supported metal ( It was found that the photocatalytic performance was superior to that of Example 4). It was also found that there was a great difference in the photocatalytic performance depending on the supported metal component. This YVO ₄ -based semiconductor photocatalyst had the highest activity among the photocatalysts of the formulation designed to support 1.0 wt% NiO _x .
[0025]
Example 5
In order to confirm whether decomposition of organic matter proceeded efficiently by light irradiation, methanol in an aqueous solution was decomposed. As the catalyst, the above oxide semiconductor carrying Pt (0.1 wt%) was used. 1 g of the catalyst was suspended in a mixed solution of 320 ml of pure water and 50 ml of methanol and subjected to a photolysis reaction. Using a closed circulation system catalytic reactor, light was irradiated from the outside while stirring with a magnetic stirrer. A 400 W high-pressure mercury lamp was used as the light source, and a reaction cell made of quartz glass was used. The generated hydrogen was detected and quantified by gas chromatography. As a result, hydrogen was constantly generated at 1.5 mmol / 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 light irradiation.
[0026]
Example 6
GdVO ₄ was synthesized using Gd element and V element. In the synthesis, 1.0 mol Gd ₂ O ₃ and 1.0 mol V ₂ O ₅ were prepared in a stoichiometric ratio per metal (FIG. 1).
For example, when synthesizing 10 g GdVO ₄ , 6.659 g of Gd ₂ O ₃ and 3.341 g of V ₂ O ₅ were weighed. Sample preparation is the same as in Example 1. This 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 tetragonal system and has a space group of I 4 ₁ / amd and a zircon type crystal structure. From the ultraviolet-visible absorption spectrum measurement, it was found that the present photocatalyst showed absorption from the ultraviolet region to the visible light region with an upper limit of 600 nm, the band cap was estimated to be 2.1 eV or less, and had visible light responsiveness.
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 g of NiO _x / GdVO ₄ was suspended in 370 ml of pure water and subjected to water photolysis reaction. Using a closed circulation system catalytic reactor, light was irradiated from the outside while stirring with a magnetic stirrer. A 400 W high-pressure mercury lamp was used as the light source, and a reaction cell made of quartz glass was used. The generated hydrogen and oxygen were detected and quantified by gas chromatography. The result is shown in FIG.
As a result, it was found that hydrogen was constantly generated at a rate of 300 μmol / h and oxygen at a rate of 150 μmol / h.
[0027]
Example 7
In Example 6, RuO ₂ was used as the supported metal instead of NiO _x .
The GdVO ₄ semiconductor loaded with 1.0 wt% RuO ₂ was impregnated with an aqueous RuCl ₄ solution, dried at 200 ° C. for 5 hours, and fired at 500 ° C. in an oxidizing atmosphere for 2 hours. 1 g of RuO ₂ / GdVO ₄ was suspended in 370 ml of pure water and subjected to water photolysis reaction. Using a closed circulation system catalytic reactor, light was irradiated from the outside while stirring with a magnetic stirrer. A 400 W high-pressure mercury lamp was used as the light source, and a reaction cell made of quartz glass was used. The generated hydrogen and oxygen were detected and quantified by gas chromatography. The result is shown in FIG. Also in this case, it was found that hydrogen is constantly generated at a rate of 250 μmol / h and oxygen is 125 μmol / h.
[0028]
Example 8
In order to confirm whether decomposition of organic matter proceeded efficiently by light irradiation, methanol in an aqueous solution was decomposed. As the catalyst, the above oxide semiconductor carrying Pt (0.1 wt%) was used. 1 g of the catalyst was suspended in a mixed solution of 320 ml of pure water and 50 ml of methanol and subjected to a photolysis reaction. Using a closed circulation system catalytic reactor, light was irradiated from the outside while stirring with a magnetic stirrer. A 400 W high-pressure mercury lamp was used as a light source, and a reaction cell made of quartz glass was used. The generated hydrogen was detected and quantified by gas chromatography. As a result, hydrogen was constantly generated at 2 mmol / 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 light irradiation.
[0029]
The above results are summarized in FIG. That is, FIG. 1 shows the relationship between the used photocatalyst component, the component of the supported promoter and its presence, the type of reaction (reaction purpose), the light source used, the amount of hydrogen gas generated per unit time, and the like. Is.
[0030]
【The invention's effect】
As described above, RVO ₄ oxidation having a zircon type crystal structure belonging to the I4 ₁ / amd space group of the present invention represented by the general formula (I); RVO ₄ (wherein R represents a Y element or a lanthanoid element) Considering that the wavelength range in which photoresponse is possible extends to visible light with an upper limit of 600 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 Its significance is extremely great because it can function for the light that occupies most of the main areas. 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, there is a possibility that hydrogen can be efficiently produced in large quantities with inexhaustible sunlight, which can overcome the energy problem. In addition, 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 rare earth compound photocatalyst of the present invention has an activity for 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, In the future, it will play a very important role.
[Brief description of the drawings]
FIG. 1 is a graph showing the amount of hydrogen generated by the decomposition reaction of water and methanol using the visible light-responsive rare earth compound photocatalyst of the present invention.

Claims (10)

General formula (I): A photocatalyst comprising a vanadium (V) -containing composite oxide semiconductor represented by RVO ₄ .
In the formula, R represents a Y element or a lanthanoid element.   R is at least one element selected from the group consisting of Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. The photocatalyst according to claim 1, wherein 3. The photocatalyst according to claim 1, wherein the rare earth composite oxide semiconductor represented by the general formula (I) is a composite oxide semiconductor having a zircon-type crystal structure belonging to the I 4 ₁ / amd space group. . 2. A cocatalyst composed of one or more components selected from Pt, NiO _x (x represents a value greater than 0 and less than 1) and RuO ₂ is included. 4. The photocatalyst according to any one of 3 to 3.   A photocatalyst for hydrogen production comprising the composite oxide semiconductor according to any one of claims 1 to 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 hydrogen production according to claim 5.   A photocatalyst for water splitting comprising the composite oxide semiconductor according to any one of claims 1 to 4.   A method for producing hydrogen, comprising irradiating water containing ultraviolet light and visible light in the presence of the photocatalyst for water splitting according to claim 7. A photocatalyst for decomposing harmful chemical substances, comprising the composite oxide semiconductor according to any one of claims 1 to 4. A method for decomposing a hazardous chemical substance, comprising irradiating the hazardous chemical substance with light containing ultraviolet rays and visible light in the presence of the photocatalyst for decomposing the hazardous chemical substance according to claim 9.

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