JPH1099694A - Photocatalyst and its preparation - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a highly active photocatalyst which consists of a laminar composite compound such as K4 Nb6 O17 having a uniform composition and texture and to provide a method for preparing the photocatalyst. SOLUTION: The compound of group VA elements, a ligand and the compound of alkali metallic elements are mixed with a solvent and the mixture is agitated while heating it at 353-423 K. Esterification reaction is advanced and gelatification is caused. The mixture is heated at <=620 K to remove excess solvent and/or an organic component and heat-treated at 600-800 K. Thereafter, the mixture is pulverized to produce precursor powder. Precursor powder is heat-treated at 773-1200 K to produce powder of a laminar compound consisting of the alkali metallic elements and the group VA elements. At least one kind selected from a group consisting of Ni, Pt, Ir, Ru and oxides thereof is carried on powder of the laminar compound at 0.1-5wt.% (in terms of metallic element).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a photocatalyst suitable for producing hydrogen by decomposing water using light energy, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years,
A method for producing hydrogen by photolyzing water using a semiconductor as an electrode was discovered (NATURE, 238, pp. 37 (l97).
2)), has been proposed as one of the important light energy conversion methods. Among the photocatalysts used in these methods, the layered compound K ₄ Nb ₆ O supporting nickel oxide is used.
₁₇ has the advantage that the product hydrogen and oxygen hardly hinder the progress of the water splitting reaction, and the reaction yield is high (K.
Domen, A. Kudo, A. Shinozuka, A. Tanaka, K. Maruy
a and T. Onishi, J. Chem. Soc. Chem. Commun., pp. 3
59 (1994)).

[0003] However, the yield of conventional photocatalysts is not yet sufficient, and further improvement must be continued for practical use. The present inventors have paid attention to the improvement of a layered composite compound comprising an alkali metal element such as K ₄ Nb ₆ O ₁₇ which is a layered compound and a Group 5A element. As a method for producing K ₄ Nb ₆ O ₁₇ for photocatalyst, there is known a solid phase method in which niobium oxide and potassium carbonate are mixed at a predetermined molar ratio and calcined at a high temperature of 1300 ° C. (JP-A-61-197033). issue). However, the solid phase method has a problem that the reaction is not complete and unreacted particles of niobium oxide and potassium carbonate remain. In order for the reaction of niobium oxide and potassium carbonate to proceed with the synthesis of K ₄ Nb ₆ O ₁₇ , K
The component and the Nb component must diffuse through the solid and contact each other. In order to increase the diffusion rate, it is necessary to repeat the heat treatment at a high temperature. However, heat treatment at a high temperature promotes grain growth and significantly lowers the specific surface area of the obtained K ₄ Nb ₆ O ₁₇ powder, so that sufficient catalytic properties cannot be obtained. Further, the heat treatment at a high temperature causes evaporation of the components, and finally K ₄ Nb ₆ O ₁₇ having a uniform composition and structure.
Is difficult to produce.

In view of such circumstances, there is a need for a method for producing a photocatalyst having a higher activity, particularly a method for producing a layered composite compound such as K ₄ Nb ₆ O ₁₇ having a large surface area and a uniform structure.

Accordingly, an object of the present invention is to solve the drawbacks of the conventional manufacturing method and to obtain a K ₄ N having a uniform composition and structure.
An object of the present invention is to provide a highly active photocatalyst comprising a layered composite compound such as b ₆ O ₁₇ and a method for producing the same.

[0006]

Means for Solving the Problems In view of the above problems, as a result of intensive studies, the present inventor disperses a group 5A element and an alkali metal element as a stable complex in a solution, and obtains a mixture of the group 5A element and the alkali metal element. The surroundings of the complex are polymerized by a polymerization reaction to form a uniform polymer complex, and the layered composite compound is generated by thermal decomposition of the polymer complex.
The obtained layered composite compound was found to have a uniform composition and texture and a high surface area, and when Ni, Pt, Ir, Ru or an oxide thereof was supported on the obtained layered composite compound, high photocatalytic activity was obtained. It has been found that a photocatalyst having the formula (1) is obtained, and the present invention has been completed.

That is, the method of the present invention comprises Ni, Pt,
A method for producing a photocatalyst in which at least one selected from the group consisting of Ir, Ru and their oxides is supported on a layered compound comprising an alkali metal element and a Group 5A element, comprising: One or more solvents selected from the group consisting of hydrogen, monohydric alcohols and polyhydric alcohols,
Alkoxides of group elements, one or more compounds selected from the group consisting of organic acid salts and inorganic acid salts, and (b) a carboxylic acid,
Acetylacetone, one or more ligands selected from the group consisting of diamines and pyridines, and (c) an alkoxide of an alkali metal element, one or more compounds selected from the group consisting of organic acid salts and inorganic acid salts And (2) stirring the obtained solution while heating it to 353 to 423 K, causing the esterification reaction to proceed, and gelling. (3) Heating the obtained gel at 620 K or less to excess (4) heat treatment at 600-800K, then pulverize to produce a precursor powder, and (5) heat treat the precursor powder at 773-1200K to remove alkali metal. A powder of a layered compound comprising an element and a Group 5A element is generated. (6) The layered compound powder contains at least one selected from the group consisting of Ni, Pt, Ir, Ru and oxides thereof.
It is characterized by being supported by 1 to 5% by weight (in terms of metal element).

[0008] The photocatalyst of the present invention comprises a layered compound comprising an alkali metal element and a group 5A element, which is composed of Ni, Pt, Ir,
0.1 to 5% by weight of at least one selected from the group consisting of Ru and their oxides is supported, and the layered compound comprising the alkali metal element and the group 5A element includes (1) water, hydrogen peroxide, One or more solvents selected from the group consisting of monohydric alcohols and polyhydric alcohols include (a) 5A
Alkoxides of group elements, one or more compounds selected from the group consisting of organic acid salts and inorganic acid salts, and (b) a carboxylic acid,
Acetylacetone, one or more ligands selected from the group consisting of diamines and pyridines, and (c) an alkoxide of an alkali metal element, one or more compounds selected from the group consisting of organic acid salts and inorganic acid salts (2) The obtained solution is stirred while heating to 353 to 423 K, and the esterification reaction proceeds to gel. (3) The obtained gel is heated at 620 K or lower to (4) heat treatment at 600-800K, then pulverize to produce precursor powder, and (5) heat-treat the precursor powder at 773-1200K. It is characterized in that it is.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. [1] Raw material (a) Compound of group 5A element The group 5A element is selected from V, Nb, and Ta.
Among them, Nb is particularly preferred. As the compound of the group 5A element, at least one selected from the group consisting of alkoxides and inorganic acid salts is used. As the alkoxide of the 5A group element,
Methoxide, ethoxide, propoxide, isopropoxide and n-butoxide, and are selected from vanadyl triethoxide (VO (OC
₂ H _{5) 3),} niobium triethoxide, to use tantalum penta propoxide or the like. Examples of the inorganic acid salts of Group 5A elements include chlorides and sulfates, and examples of the organic acid salts include oxalates and acetates. Among them, it is preferable to use chlorides such as vanadium chloride, niobium chloride and tantalum chloride and / or organic acid salts such as niobium oxalate, vanadium oxalate and niobium acetate.

(B) Ligand As the ligand, at least one selected from the group consisting of carboxylic acid, acetylacetone, diamines, pyridines and bipyridine is used. By adding a ligand,
A stable complex of titanium and barium can be formed. As the carboxylic acid, α-hydroxycarboxylic acids such as citric acid, malic acid, tartaric acid, lactic acid, and glycolic acid, and carboxylic acids such as tricarballylic acid, succinic acid, oxalic acid, and acetic acid are preferably used. Diamines include ethylenediamine, ethylenediaminetetraacetic acid,
It is preferable to use 1,2-propanediamine, 1,3-propanediamine and the like. As pyridines, it is preferable to use pyridine, bipyridine and the like. Among these ligands, citric acid is particularly preferred.

(C) Compound of alkali metal element The alkali metal element is selected from the group consisting of Li, Na, K, Rb and Cs. As the alkali metal element compound, at least one selected from the group consisting of alkoxides, organic acid salts and inorganic acid salts is used. Alkoxides of alkali metal elements include methoxide, ethoxide,
It is selected from the group consisting of propoxide, isopropoxide and n-butoxide, and it is preferable to use lithium methoxide, sodium methoxide and the like.
Examples of the inorganic acid salt of an alkali metal element include carbonates, chlorides, and nitrates. Particularly, carbonates such as lithium carbonate, sodium carbonate, potassium carbonate, rubidium carbonate, and cesium carbonate are particularly preferable. Examples of the organic acid salt include acetate, oxalate and the like.

(D) Solvent As the solvent to which the components (a) to (c) are added, at least one selected from the group consisting of water, hydrogen peroxide, monohydric alcohol and polyhydric alcohol is used. Monohydric alcohols include methanol, ethanol, isopropanol, n-butanol, i-butanol, sec-butanol, tert-
Butanol and the like are preferable, and as the polyhydric alcohol, ethylene glycol, propylene glycol, butylene glycol and the like are preferable. Among them, ethylene glycol and propylene glycol are particularly preferred.

[2] Method for producing layered composite compound A method for producing the layered composite compound K ₄ Nb ₆ O ₁₇ will be described by citing niobium as an example of a Group 5A element and potassium as an example of an alkali metal element. Elements can be manufactured in a similar manner.

(1) Mixing / stirring step First, (a) a niobium compound, (b) a ligand, and (c) a potassium compound are added to a solvent and mixed with stirring. The order of addition of each component is preferably (a) → (b) → (c).

The amount of the solvent is 10 to 100 mol, preferably 20 to 50 mol, based on 1 mol of the niobium compound.
If the amount of the solvent is less than 10 mol, the resulting K ₄ Nb ₆ O ₁₇
Is difficult to homogenize. Conversely, if the amount of the solvent exceeds 100 mol, the mixed solution is too dilute.

The amount of the ligand to be added is 2 to 10 mol, preferably 4 to 8 mol, based on 1 mol of the niobium compound. If the amount of the ligand is less than 2 mol, a stable niobium complex cannot be obtained. Further, if the amount of the ligand exceeds 10 mol, no further promoting effect can be obtained.

The amount of the potassium compound added is K ₄ Nb ₆ O ₁₇
Is set so as to obtain the composition of

Each component is mixed at the above mixing ratio, and 300 to 373
When stirred at the temperature of K (preferably room temperature) for 2 to 5 hours,
A clear solution is obtained. During this process, niobium and potassium form a stable complex with the ligand.

(2) Esterification Step Next, the transparent solution is stirred while being heated to 353 to 423K, and the esterification reaction between the ligand and the solvent proceeds to gel. By this esterification reaction, a polymer complex is formed. The reaction time is preferably 3 to 20 hours.

(3) Heating step The obtained gel is heated at a temperature of 620 K or lower, particularly 500 to 600 K, to remove excess solvent and / or organic components. The preferred heating time is 2 to 5 hours.

(4) Pre-heat treatment step Subsequently, heat treatment is carried out at a temperature of 600 to 800 K, preferably 600 to 700 K for 2 to 5 hours to obtain a black powder as a precursor. This precursor is lightly ground with an agate mortar or the like.

(5) Post heat treatment step The obtained precursor powder is subjected to 773-1200K, especially 800-1100K.
To obtain K ₄ Nb ₆ O ₁₇ powder. The preferred heat treatment time is 2 to 5 hours. The thus-obtained K ₄ Nb ₆ O ₁₇ for photocatalyst has a small unreacted component and a uniform composition and structure.

[3] Production method of photocatalyst The obtained layered composite compound such as K ₄ Nb ₆ O ₁₇ is impregnated with
Ni, Pt, Ir, R by a known method such as a solid phase reaction.
and one or more photocatalytically active species selected from the group consisting of u and their oxides. The preferred loading method is the impregnation method. In the impregnation method, K ₄ Nb ₆ O ₁₇ is impregnated with an aqueous solution of a compound such as a chloride or nitrate of a photocatalytically active species, and then dried at 323 to 423 K for about 2 to 5 hours. The firing is preferably performed at a temperature of 600 to 750 K or less and in a reducing atmosphere and / or an oxidizing atmosphere for 2 to 5 hours. If the firing temperature exceeds 750 K, the grain growth proceeds, which is not preferable.

The content of the catalytically active species in the photocatalyst of the present invention is preferably 0.1 to 5% by weight (in terms of a metal element), particularly preferably 0.8 to 1.5% by weight (in terms of a metal element).

Although the method for producing a layered compound has been described above by taking K ₄ Nb ₆ O ₁₇ as an example, other layered compounds can be produced by the same method. Examples of the layered compound include KTiNbO ₅ , KLaNb ₂ O ₇ , and KCa ₂
Nb ₃ O _{10 and the} like.

The photocatalyst of the present invention obtained by the above method has higher activity than the conventional one. The cause is greater than the conventional specific surface area of the layered composite compound K ₄ Nb ₆ O _17, but unreacted components which inhibit the electrification separated therein are considered to be the substantial absence, K ₄ Nb ₆
Is the charge generation rate of O ₁₇ increased or K ₄ Nb
Details such as whether the charge transfer rate from ₆ O ₁₇ to a catalytically active species such as Ni has increased have not yet been elucidated.

[0027]

The present invention will be described in more detail with reference to the following specific examples. Example 1 Niobium chloride (NbCl ₅ ), citric acid (anhydrous), potassium carbonate, and ethylene glycol were successively added to ethanol in 0.1 parts.
By mixing at a molar ratio of 2: 1: 0.04: 4, mixing and stirring, a citrate complex of potassium and niobium was formed, and a clear solution was obtained. This solution was heated and stirred at 385 K to allow the esterification reaction between citric acid and ethylene glycol to proceed. After about 5 hours, a yellow foamed polymeric gel was obtained. The gel was heated at 550 K (600 K or less) to remove excess solvent and organic components, and then heat-treated at 630 K for 2 hours. Then, the precursor was pulverized with a Teflon rod for 10 minutes to obtain a black powdery precursor. This precursor is 873
Was heat treated 2 hours at K (773 or _{K), K 4 Nb 6 O} 17
Crystals were produced. The product was subjected to X-ray diffraction analysis (X-ray diffraction analyzer MXP3VA system, manufactured by Mac Science Co., Ltd.). As a result, K ₄ Nb ₆ O ₁₇ crystals were confirmed, and the crystal structure of unreacted particles such as niobium oxide was observed. Was not seen.

Nickel chloride in distilled water, Ni is K ₄ Nb ₆
It was weighed and added to 1% by weight of O ₁₇ and dissolved. The above K ₄ Nb ₆ O ₁₇ powder was added to this solution and mixed and stirred. After uniformly dispersing, heat and stir at 50-80 ° C,
The water was evaporated and the dried sample was dried at 373 ° C. for 3 hours. Then, heat treatment was performed at 673 K in air for 3 hours to obtain a photocatalyst. When this catalyst was measured by the BET method, the surface area was 35 m ² / g.

1 g of this catalyst was added to 1.4 liters of pure water, and irradiated with a high-pressure mercury lamp (400 W, hν ≧ 290 nm), and oxygen and hydrogen generated by the water splitting reaction were quantified by gas chromatography. The molar ratio of the amounts of generated oxygen and hydrogen was approximately 1: 2, and complete decomposition of water was observed. Table 1 shows the amounts of hydrogen and oxygen generated.

COMPARATIVE EXAMPLE 1 Niobium oxide powder (average particle size: 2 μm) and potassium carbonate powder (average particle size: 10 μm) were mixed at a molar ratio of 3: 1.
Baking at 1300 K for 5 hours by solid phase method, K ₄ Nb
₆ O ₁₇ was obtained. X-ray diffraction analysis was performed in the same manner as in Example 1. As a result, formation of K ₄ Nb ₆ O ₁₇ crystals was observed, but remaining niobium oxide and potassium carbonate were confirmed.

Nickel nitrate in distilled water, Ni is K ₄ Nb ₆
It was weighed and added to 1% by weight of O ₁₇ and dissolved. The above K ₄ Nb ₆ O ₁₇ powder was added to this solution and mixed and stirred. After uniformly dispersing, heat and stir at 50-80 ° C,
The water was evaporated and the dried sample was dried at 100 ° C. for 24 hours. Then, while flowing a mixed gas of hydrogen and nitrogen (hydrogen gas / nitrogen gas = 2/98) (flow rate 250 ml / min), 50
After reduction treatment at 0 ° C. for 2 hours, heat treatment was performed in air at 475 ° C. for 7 hours to obtain a photocatalyst. When this catalyst was measured by the BET method, the surface area was 4.7 m ² / g.

In the same manner as in Example 1, 1 g of this catalyst was added to 1.4 liter of pure water, and a high-pressure mercury lamp (400 W, h
(ν ≧ 290 nm), and oxygen and hydrogen generated by the water splitting reaction were quantified by gas chromatography. Table 1 shows the amounts of hydrogen and oxygen generated.

Table 1 Amount of hydrogen and oxygen produced in photolysis of water (unit: μmol / h) Example No. Amount of hydrogen produced Amount of oxygen produced Example 1 202 102 Comparative Example 4 63 31

As can be seen from Table 1, in Example 1 and Comparative Example 1 in which Ni was used as the catalytically active species, the activity of the photocatalyst of Example 1 supported on K ₄ Nb ₆ O ₁₇ of the present invention was The activity of the catalyst of Comparative Example 1 supported on K ₄ Nb ₆ O ₁₇ produced by the solid phase method was three times or more.

[0035]

As described above in detail, since the method of the present invention produces K ₄ Nb ₆ O ₁₇ by complex polymerization, no unreacted components remain in the obtained K ₄ Nb ₆ O _17. , Composition and texture are very uniform. In addition, K for photocatalyst at low temperature
It is possible to produce a ₄ Nb ₆ O _17, advantages of low manufacturing cost can be obtained. Further, the above K ₄ Nb ₆ O ₁₇
The photocatalyst according to the present invention using the compound has high catalytic activity and is suitable for photolysis of water.

Continued on the front page (72) Inventor Kiyohide Yoshida 4-1-1, Suehiro, Kumagaya-shi, Saitama Pref.Rikken Kumagaya Plant (72) Inventor Masato Kakihana 1-6-12 Teraonaka, Ayase-shi, Kanagawa (72) Inventor Kazunari Domen 4259 Nagatsuta-cho, Midori-ku, Yokohama-shi, Kanagawa Pref.

Claims (7)

[Claims] 1. A method for producing a photocatalyst comprising supporting at least one selected from the group consisting of Ni, Pt, Ir, Ru and oxides thereof on a layered compound comprising an alkali metal element and a Group 5A element, (1) One or more solvents selected from the group consisting of water, hydrogen peroxide, monohydric alcohols and polyhydric alcohols,
Alkoxides of group elements, one or more compounds selected from the group consisting of organic acid salts and inorganic acid salts, and (b) a carboxylic acid,
Acetylacetone, one or more ligands selected from the group consisting of diamines and pyridines, and (c) an alkoxide of an alkali metal element, one or more compounds selected from the group consisting of organic acid salts and inorganic acid salts (2) The obtained solution is stirred while heating to 353 to 423 K, and the esterification reaction proceeds to gel. (3) The obtained gel is heated at 620 K or lower to (4) heat treatment at 600-800K, pulverize to form precursor powder, and (5) heat treat the precursor powder at 773-1200K to remove alkali metal. A powder of a layered compound composed of an element and a Group 5A element is generated. (6) The layered compound powder contains at least one selected from the group consisting of Ni, Pt, Ir, Ru and oxides thereof.
A method comprising loading 1 to 5% by weight (in terms of a metal element). 2. The method according to claim 1, wherein the alkali metal element is selected from the group consisting of Li, Na, K, Rb, and Cs, and the group 5A element is V,
A method selected from the group consisting of Nb and Ta. 3. The method according to claim 1, wherein the monohydric alcohol is selected from the group consisting of methanol, ethanol, isopropanol and butanol, and the polyhydric alcohol is ethylene glycol or propylene glycol. And butylene glycol. 4. The method according to claim 1, wherein the alkoxide of the Group 5A element is selected from the group consisting of methoxide, ethoxide, propoxide, isopropoxide and n-butoxide. Wherein the inorganic acid salt of a Group 5A element is chloride and / or sulfate. 5. The method according to claim 1, wherein the carboxylic acid comprises citric acid, malic acid, tartaric acid, lactic acid, glycolic acid, tricarballylic acid, succinic acid, oxalic acid and acetic acid. A method selected from the group consisting of ethylenediamine, ethylenediaminetetraacetic acid, 1,2-propanediamine and 1,3-propanediamine. 6. The method according to claim 1, wherein the alkoxide of the alkali metal element is selected from the group consisting of methoxide, ethoxide, propoxide, isopropoxide and n-butoxide. Wherein the inorganic acid salt of the alkali metal element is selected from the group consisting of carbonate, chloride and nitrate. 7. A layered compound comprising an alkali metal element and a Group 5A element, wherein at least one selected from the group consisting of Ni, Pt, Ir, Ru and oxides thereof is 0.1 to 5% by weight.
In the supported photocatalyst, the layered compound comprising the alkali metal element and the group 5A element is (1) water, hydrogen peroxide, one or more solvents selected from the group consisting of monohydric alcohols and polyhydric alcohols, a) 5A
Alkoxides of group elements, one or more compounds selected from the group consisting of organic acid salts and inorganic acid salts, and (b) a carboxylic acid,
Acetylacetone, one or more ligands selected from the group consisting of diamines and pyridines, and (c) an alkoxide of an alkali metal element, one or more compounds selected from the group consisting of organic acid salts and inorganic acid salts (2) The obtained solution is stirred while heating to 353 to 423 K, and the esterification reaction proceeds to gel. (3) The obtained gel is heated at 620 K or lower to (4) heat treatment at 600-800K, then pulverize to produce precursor powder, and (5) heat-treat the precursor powder at 773-1200K. A photocatalyst characterized by the fact that