JP7045662B2 - Photocatalyst manufacturing method and hydrogen generation method - Google Patents

Description

The present invention relates to a method for producing a photocatalyst and a method for producing hydrogen.

The amount of solar energy that falls on the earth in one year is enormous, which is equivalent to about 10,000 times the amount of energy that we currently consume in one year. Therefore, it is desired to establish a technology for decomposing abundant water using this solar energy to obtain hydrogen, which is a clean resource. In order to achieve this, it is important to develop a new photocatalyst that has the function of absorbing light energy and decomposing water.

Photocatalysts capable of decomposing water into hydrogen and oxygen in a chemical quantitative ratio include transition metal oxides with d-orbitals in the d- ⁰ electron state, such as Ti ⁴⁺ , Zr ⁴⁺ , Ta ⁵⁺ , and Nb ⁵⁺ , or Ga. Photocatalysts carrying a cocatalyst on a typical metal oxide in the d ¹⁰ electron state where the d orbital is filled, such as ³⁺ , In ³⁺ , Ge ⁴⁺ , Sn ⁴⁺ , Sb ⁵⁺ , etc., have been proposed (eg, non-). Patent Document 1).

Yasunobu Inoue, Energy Energy. Sci. , 2009, 2,364-386.

By the way, from the viewpoint of using sunlight, it is important to develop a photocatalyst that works in the visible light region. However, conventional photocatalysts as disclosed in Non-Patent Document 1 act exclusively in the ultraviolet light region where the light absorption wavelength is shorter than 400 nm. In order to deal with this, nitrides, oxynitrides, sulfides, acid sulfides, etc., which have narrow bandwidths of valence bands and conduction bands related to light absorption, have been attracting attention, but long-wavelength visibility has been attracted. At present, it is still difficult to obtain a photocatalyst that works well in the optical region.

Therefore, an object of the present invention is to provide a method for producing a photocatalyst having hydrogen generation activity in a water decomposition reaction in the visible light region, and a method for producing hydrogen using the photocatalyst obtained by the method.

So far, as photocatalysts capable of absorbing visible light, Ta ₃ N ₅ which is a nitride (nitride) compound and TaON, SrTaO ₂ N, BaTaO ₂ N, LaTIO ₂ which are oxynitride compounds have been used. N, LaTaON ₂ and the like are used. These compounds are obtained as powder fine particles by ammonia nitriding and are used as photocatalysts. However, although these conventionally produced nitrides and oxynitrides can be adapted to semi-reactions of water splitting such as hydrogen generation from methanol and oxygen generation from an aqueous solution of silver nitrate, they chemically convert hydrogen and oxygen from water. It has not been adapted to the complete decomposition reaction of water that decomposes by quantity.

Therefore, the present inventors have focused on a method for forming a nitride or an oxynitride on the surface of a metal oxide. That is, oxides such as Ti ⁴⁺ , Zr ⁴⁺ , Nb ⁵⁺ , Ta ⁵⁺ , which are metal ions in the d ⁰ electron state, and Ga ³⁺ , In ³⁺ , Ge ⁴⁺ , Sn ⁴⁺ , Sb, which are metal ions in the d ¹⁰ electron state. It was considered useful to form nitrides and oxynitrides on the surface of oxides such as ⁵⁺ . For that purpose, the metal oxide is nitrided in a short time by using an ammonia gas, an ammonia mixed gas, or the like as a nitrogen source, thereby desorbing a part of the composition component from the metal oxide and forming it on the surface of the metal oxide. It was considered that the method of precipitating only nitride or oxynitride is suitable.

That is, in the present invention, at least one compound selected from the group consisting of nitrides and oxynitrides is deposited on the surface of the metal oxide represented by any of the following general formulas (1) to (3). Provided is a method for producing a photocatalyst used for hydrogen generation from water, comprising a precipitation step of causing the compound to be carried, and a carrying step of carrying at least one selected from the group consisting of a noble metal and a noble metal oxide on the surface of the compound. ..
AXO ₃ ... (1)
A _{n + 1} X _n O _{3n + 1} ... (2)
A _m X _m O _{3m + 2} ... (3)
In the formula, A represents at least one selected from the group consisting of alkali metals, alkaline earth metals and rare earth metals, and X indicates Ti, V, Zr, Ta, Nb, W, Mo, Ga, In, Ge and Indicates at least one selected from the group consisting of Sn, where n indicates 1 or 2 and m indicates 2 or 4.

In the present invention, it is preferable that the precipitation step includes a step of heat-treating the metal oxide in an ammonia-containing gas for 0.05 to 2 hours.

In the present invention, it is preferable that the supporting step includes a step of further supporting an oxide of at least one metal selected from the group consisting of Cr, Co, V, Mo and W on the surface of the compound.

In the present invention, it is preferable that the photocatalyst is excited by irradiating with light having a wavelength selected from the range of 350 to 600 nm.

The present invention also provides a hydrogen generation method for producing hydrogen from water by a photoreaction using visible light in the presence of a photocatalyst obtained by the above production method.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a method for producing a photocatalyst having hydrogen generation activity in a water decomposition reaction in a visible light region, and a method for producing hydrogen using the photocatalyst obtained by the method.

It is a figure which shows the XRD pattern of KTaO ₃ and Ta ₃ N ₅ / KTaO ₃ compound. It is a figure which shows the ultraviolet-visible diffuse reflection spectrum of KTaO ₃ and Ta ₃ N ₅ / KTaO ₃ compound. It is a figure which shows the scanning electron microscope image of KTaO ₃ and Ta ₃ N ₅ / KTaO ₃ compound. It is a figure which shows the relationship between the light irradiation time and the amount of hydrogen and oxygen production using a Cr ₂ O ₃ / Rh-supported Ta ₃ N ₅ / KTaO ₃ photocatalyst. It is a figure which shows the relationship between the nitriding time and the photocatalyst activity in a Cr ₂ O ₃ / Rh-supported Ta ₃ N ₅ / KTaO ₃ photocatalyst. It is a figure which shows the relationship between the kind of the co-catalyst and the photocatalyst activity in the co-catalyst-supported Ta ₃ N ₅ / KTaO ₃ photocatalyst.

<Manufacturing method of photocatalyst>
The method for producing a photocatalyst used for producing hydrogen from water according to the present embodiment is to precipitate at least one compound selected from the group consisting of nitrides and oxynitrides on the surface of a predetermined metal oxide. It comprises a step and a carrying step of supporting at least one selected from the group consisting of noble metals and noble metal oxides on the surface of the compound.

[Precipitation step]
Examples of the metal oxide include metal oxides represented by any of the following general formulas (1) to (3). The layered perovskite structural oxides represented by the general formulas (2) and (3) were laminated so that the _three layers of the perovskite structural oxide AXO represented by the general formula (1) sandwiched the metal ion A. It is a thing. According to the findings of the inventors, both structures have the same mechanism of action when used as a substrate for a photocatalyst used for hydrogen production from water.
Perovskite structure oxide AXO ₃ ... (1)
Layered perovskite structure Oxide _{An + 1} X _n O _{3n + 1} ... (2)
Layered perovskite structure oxide A _m X _m O _{3 m + 2} ... (3)
In the formula, A represents at least one selected from the group consisting of alkali metals, alkaline earth metals and rare earth metals, and alkali metals are preferable from the viewpoint of obtaining better hydrogen generation activity, and Na and K are preferable. Is more preferable. X represents at least one selected from the group consisting of transition metal ions Ti, V, Zr, Ta, Nb, W, Mo, Ga, In, Ge and Sn to obtain better hydrogen production activity. Ti, Ta, Nb and the like are preferable from the viewpoint of being able to. Further, n indicates 1 or 2, and m indicates 2 or 4.

Hereinafter, a method for preparing _KTaO3 as a metal oxide having a perovskite structure will be described, and a representative example of a method for preparing other compounds will be described. The metal oxide can be produced by two methods, a solid phase reaction method or a flux method, as shown below.

Solid phase reaction method: Raw material powder containing the starting materials Ta ₂ O ₅ and K ₂ CO ₃ is mixed at a K / Ta ratio of 0.9 to 1.2, and then placed in an alumina crucible. KTaO ₃ can be obtained by heating this in an electric furnace at 1000 to 1200 ° C. for 10 to 20 hours in the atmosphere.
Flux method: Raw material powder containing the starting materials Ta ₂ O ₅ and K ₂ CO ₃ is mixed at a K / Ta ratio of 0.9 to 1.2, and K ₂ CO ₃ , KCl, NaCl, etc. are further mixed as a flux. Is added 3 to 20 times by weight to the raw material powder. KTaO ₃ can be obtained by heating this in an electric furnace at 800 to 1100 ° C. for 10 to 15 hours in the atmosphere.

The metal oxide may be surface-treated from the viewpoint of precipitating high-quality nitrides and oxynitrides. For the surface treatment, an inorganic acid (aqua regia, hydrofluoric acid) or an organic acid (polystyrene sulfonic acid: PSSA) can be used. For example, when aqua regia is used, the metal oxide may be treated at 80 ° C. for 30 minutes. When hydrofluoric acid is used, a 20% hydrofluoric acid aqueous solution may be prepared and the metal oxide may be treated at room temperature for 10 to 30 minutes. When PSSA is used, the metal oxide may be treated at room temperature for 2 to 20 hours. Regardless of which acid is used, the sample is washed with water, filtered, dried, etc. after the treatment to prepare a sample for nitriding.

The average particle size of the metal oxide is not particularly limited, but from the viewpoint of preparing a suspension (described later) and appropriately performing a photocatalytic reaction, and from the viewpoint of suppressing the formation of nitride / oxynitride to the inside, etc. It is preferably 0.05 to 10 μm, and more preferably 0.1 to 1.0 μm.

The composition of nitrides and oxynitrides depends on the procedure of the precipitation process and the composition of the metal oxide used. When the precipitation method 1 described later is carried out, as the nitride, for example, a nitride such as Ta, Nb, Ga, In, Ge, Sn (Ta ₃ N ₅ , (Ta, Nb) ₃ N ₅ , GaN, InN, Ge ₃ N ₄ etc.).

Further, when the precipitation method 2 described later is carried out, a metal element contained in an inorganic salt or the like is also incorporated. The metal element is as described below, and therefore, examples of the oxynitride include BaTaO ₂ N, CaTaO ₂ N, BaNbO ₂ N, SrTaO ₂ N, LaTaON ₂ , and the like.

Specifically, the precipitation step can be carried out by the following two methods.
Precipitation method 1: The metal oxide is heat-treated in an ammonia-containing gas.
Precipitation method 2: The metal oxide having another metal oxide supported on the surface is heat-treated in an ammonia-containing gas.
Hereinafter, the nitriding method of KTaO ₃ will be described as a representative example.

Precipitation method 1: Add K2 CO ₃ or KCl as a flux to KTaO ₃ so that the weight ratio is 0.5 to ₂ times, and introduce these into an alumina tube. Then, 100% ammonia gas as a reaction gas, or a gas in which nitrogen and ammonia are mixed at a flow rate ratio of NH ₃ / (N ₂ + NH ₃ ) of 0.05 to 0.50 is flown at a total flow rate of 10 to 500 mlmin ^-1 . , Ta ₃ N ₅ / KTaO ₃ having a nitride / metal oxide structure can be obtained by heat treatment at 800 to 950 ° C. for preferably 0.05 to 2 hours using a tubular electric furnace. That is, according to the precipitation method 1, the nitride can be deposited on the surface of the metal oxide.

Precipitation method 2: KTaO ₃ is impregnated into a solution containing a metal salt such as an inorganic salt, an organic salt or a complex salt, and then dried to remove volatile components. Examples of the metal constituting the metal salt include alkaline earth metals, La and the like. Examples of the metal salt include Ba (NO ₃ ) ₂ , Ca (NO ₃ ) ₂ , Sr (NO ₃ ) ₂ , La (NO ₃ ) ₃ , and the like, which are nitrates of these metals. Then, if necessary, the metal oxides are subjected to oxidation treatment in the air or oxygen atmosphere to support the oxides of these metals (other metal oxides) on the surface of the metal oxides. The metal oxide on which the other metal oxide is supported on the surface in this manner is further subjected to nitriding treatment using 100% ammonia gas or the like in the same manner as in the precipitation method 1, for example, BaTaO ₂ N / KTaO ₃ , SrTaO ₂ N / KTaO ₃ , LaTaON ₂ / KTaO ₃ , and the like, compounds having an oxynitride / metal oxide structure can be obtained. That is, according to the precipitation method 2, the oxynitride can be deposited on the surface of the metal oxide.

The heat treatment time in the precipitation step is preferably 0.05 to 2 hours, but may be 0.05 to 1.5 hours, 0.05 to 1 hour, or 0.1 to 0.5 hours. can. In a shorter time than 0.05, it tends to be difficult to obtain sufficient catalytic activity (sufficient amount of nitride / oxynitride is difficult to precipitate), and the time of exposure to ammonia, which is a reducing gas, is long. If it is too much, defects on the surface of the nitride / oxynitride and the reduced species of the above-mentioned element X tend to be generated, which may cause a decrease in catalytic activity.

The obtained KTaO ₃ , Ta ₃ N ₅ / KTaO ₃ , etc. can be identified by comparing the diffraction pattern obtained by XRD with the database of ICDD-PDF (International Center for Diffraction Data-PDF).

The average particle size of the nitride or the compound having an oxynitride / metal oxide structure obtained by the precipitation step is not particularly limited, but from the viewpoint of preparing a suspension (described later) and preferably performing a photocatalytic reaction, 0. It is preferably 05 to 10 μm, more preferably 0.1 to 1.0 μm.

[Supporting process]
Precious metals and noble metal oxides function as co-catalysts. Here, the noble metal refers to a metal belonging to the group consisting of Au, Ag, Pt, Pd, Rh, Ir, Ru and Os. From the viewpoint of further improving the catalytic activity, it is preferable that at least one of these precious metals is selected from the group consisting of Rh, Ir, Ru, Pt and Pd. These may be oxides (precious metal oxides).

These noble metals and noble metal oxides may be used together with oxides such as Cr, Co, V, Mo, W (ie, the co-catalyst may be in a "co-supported" mode). Specific examples of such oxides include Cr ₂ O ₃ , CoO _x (0 ≦ x ≦ 2), V ₂ O ₅ , MoO ₃ , WO ₃ , and the like.

From the viewpoint of exhibiting higher hydrogen production activity in the decomposition reaction of water in the visible light region, preferred combinations of metal oxides and nitrides with co-catalysts include Ta 3N ₅ / _{KTaO 3 and} Pt. Examples thereof include any one of Ir, Ru and Rh and a combination of Cr ₂ O ₃ , and among these, a combination of Ta ₃ N ₅ / KTaO ₃ and Rh and Cr ₂ O ₃ is preferable.

Based on the total mass of the metal oxide, the amount of the noble metal or the like (precious metal and noble metal oxide) supported as a co-catalyst is preferably 0.01 to 1.0% by mass, preferably 0.02 to 0.05% by mass. More preferred. Further, the amount of the oxide of the metal that may be supported together with the noble metal or the like is preferably 2 to 5 times the amount of the supported amount of the noble metal or the like. By doing so, the photocatalyst tends to easily develop stable and high hydrogen production activity.

The supporting step can be carried out by a general method such as an impregnation method, a photoelectric adhesion method, and a hydrogen reduction method. Hereinafter, examples of supporting some co-catalysts will be described, and examples of other methods of supporting co-catalysts will be described.

(Example of supporting a co-catalyst by the impregnation method)
When RuO _x (0 ≦ x ≦ 2) is adopted as the co-catalyst, the aqueous solution of ruthenium ruthenium _RuCl3・_H2O is impregnated with the metal oxide that has undergone the precipitation step. This is vacuum-dried and then oxidized in the atmosphere to convert RuCl ₃ into RuO _x (0 ≦ x ≦ 2), whereby RuO _x (0 ≦ x ≦ 2) is added to the metal oxide that has undergone the precipitation step. Can be carried. Alternatively, a THF (tetratetra) solution of Ru ₃ (CO) ₁₂ , which is a carbonyl complex of Ru, is impregnated with a metal oxide that has undergone a precipitation step, dried in vacuum, and further oxidized in an air atmosphere to be Ru. By converting ₃ (CO) ₁₂ to RuO _x (0 ≦ x ≦ 2), RuO _x (0 ≦ x ≦ 2) can be supported on the metal oxide that has undergone the precipitation step.

The oxidation treatment in the impregnation method can be carried out by treating at a temperature condition of preferably 300 to 450 ° C., more preferably 350 to 400 ° C., preferably for 2 to 7 hours, more preferably 3 to 5 hours. .. By performing the oxidation treatment under such conditions, it is possible to more reliably convert RuCl ₃ and Ru ₃ (CO) ₁₂ to RuO _x (0 ≦ x ≦ 2).

(Example of supporting a co-catalyst by the photoelectric adhesion method)
When Rh is used as the co-catalyst and Cr ₂ O ₃ is used as the co-catalyst, first, the metal oxide that has undergone the precipitation step is added to the aqueous methanol solution of RhCl ₃・ H ₂ O. This is transferred to a glass reaction cell, the dissolved oxygen is degassed, and then the Xe lamp light is irradiated under vacuum for 3 hours. Further, Cr (NO ₃ ) ₃ is added to this solution so that the amount of Cr is 3 times the molar ratio of Rh, and the solution is further irradiated with Xe lamp light for 12 hours under vacuum. After the photoadhesion is completed, _Cr2O3 and Rh can be supported _on the metal oxide that has undergone the precipitation step by filtering, washing and drying the solution.

(Example of supporting a co-catalyst by the hydrogen reduction method)
When Ir or Pt is adopted as the co-catalyst, _first , the metal oxide that has undergone the precipitation step is added to the (NH ₄ ) ₂ IrCl ₆ aqueous solution or the H ₂ PtCl _6.6 H2 O aqueous solution and refluxed. Then, this is vacuum-dried, and a mixed gas of hydrogen and nitrogen is circulated, and for example, it is reduced at 300 to 400 ° C. for 3 to 5 hours to support Ir or Pt on the metal oxide that has undergone the precipitation step. be able to. Further, this is further transferred to a reaction cell containing an aqueous solution of K2 CrO ₄ , irradiated with Xe lamp light for ₁₀ to 12 hours to carry out Cr photoelectric adhesion, and the solution is filtered, washed and dried to carry out the precipitation step. Cr ₂ O ₃ and Ir, or Cr ₂ O ₃ and Pt can be supported on the aged metal oxide.

<Photocatalyst>
By carrying out the above steps, a photocatalyst used for hydrogen generation from water of the present embodiment can be obtained. That is, it can be said that the photocatalyst of the present embodiment is as follows.
A metal oxide represented by any of the following general formulas (1) to (3), at least one compound selected from the group consisting of nitrides and oxynitrides on the surface of the metal oxide, and the above-mentioned compound. A photocatalyst used for hydrogen production from water, comprising on the surface of a compound at least one selected from the group consisting of noble metals and noble metal oxides.
AXO ₃ ... (1)
A _{n + 1} X _n O _{3n + 1} ... (2)
A _m X _m O _{3m + 2} ... (3)
In the formula, A represents at least one selected from the group consisting of alkali metals, alkaline earth metals and rare earth metals, and X indicates Ti, V, Zr, Ta, Nb, W, Mo, Ga, In, Ge and Indicates at least one selected from the group consisting of Sn, where n indicates 1 or 2 and m indicates 2 or 4.

The photocatalyst of this embodiment is preferably in the form of particles. The average particle size of the photocatalytic particles is preferably 0.05 to 10 μm, more preferably 0.1 to 1.0 μm from the viewpoint of preparing a suspension (described later) and preferably performing a photocatalytic reaction. ..

<Hydrogen generation method using photocatalyst>
In the hydrogen generation method of the present embodiment, hydrogen is generated from water by a photoreaction using visible light in the presence of the photocatalyst obtained as described above. That is, hydrogen is obtained by the decomposition reaction of water. Specifically, hydrogen can be obtained by preparing a suspension containing a photocatalyst and pure water and irradiating the suspension with light having a wavelength in a specific region from the outside. The photocatalyst of the present embodiment is excited by irradiating with light having a wavelength selected from the range of preferably 350 to 600 nm. Therefore, according to the hydrogen generation method of the present embodiment, hydrogen can be suitably obtained in the decomposition reaction of water in the visible light region (wavelength of about 350 to 600 nm) of sunlight.

For the evaluation of photocatalytic activity, for example, a normal closed circulatory system reactor can be used. This device includes a vacuum exhaust system, a reaction glass cell for light irradiation, a piston pump for gas circulation, a pressure gauge, and the like. The gas (H ₂ , O ₂ ) generated by the photocatalytic reaction can be analyzed at any time by a gas chromatograph directly connected to the reaction system. Since this device is a circulating reaction system, gas products generated over time of the reaction are accumulated in the device. Therefore, when the reaction is repeated, the reaction operation may be repeated again after the gas phase is exhausted.

As a sample for evaluation of the photocatalyst, the photocatalyst obtained as described above is placed in a reaction glass cell for light irradiation, pure water (for example, pure water obtained by further ion-exchanged distilled water) is added thereto, and the suspension is further carried out. Dissolved oxygen and nitrogen in the suspension can be removed by vacuum exhaust. The preferable amount of the photocatalyst for water may be such that the light hits almost all the photocatalyst particles. For example, when Ta ₃ N ₅ / KTaO ₃ is used as the photocatalyst, the content of the photocatalyst is preferably 0.03 to 0.3% by weight, preferably 0.1 to 0.2% based on the total weight of pure water. % By weight is more preferred. In the measurement, the temperature of the evaluation sample is preferably 15 to 40 ° C. The pH of the evaluation sample is preferably 5 to 10, more preferably 6 to 8.

Further, a magnet stirrer or the like provided in the reaction device may be used for stirring the suspension, and an Xe lamp (for example, 300W Xe lamp device R300-3J manufactured by Eagle Engineering Co., Ltd.) or the like may be used for light irradiation. can. At this time, the wavelength of the irradiated light can be 350 to 800 nm, and 420 to 800 nm, from the viewpoint that the photocatalyst of the present embodiment exhibits high hydrogen generation activity in the decomposition reaction of water in the visible light region. can do. Since the photocatalyst of the present embodiment is excited by irradiating light having a wavelength selected from the range of 350 to 600 nm as described above, the wavelength of the irradiated light may be 350 to 600 nm. ..

The photocatalyst particles can be used by suspending them in water, but can also be used as a photocatalyst panel by fixing the photocatalyst particles on a substrate such as alumina, glass, or plastic in a film form.

Hereinafter, the present invention will be specifically described with reference to Examples, but this is intended to make the present invention easier to understand, and it is natural that the present invention is not construed in a limited manner.

(Example 1: Ta ₃ N ₅ / KTaO ₃ compound)
1) Preparation of photocatalyst 1-1) Preparation of KTaO ₃ Raw material powders of Ta ₂ O ₅ and K ₂ CO ₃ which are starting materials are mixed at a molar ratio of K: Ta at 1.05: 1.0, and then. It was placed in an alumina pentoxide pot and heated in an electric furnace at 1150 ° C. for 10 hours in an electric furnace to obtain a white KTaO ₃ powder.

1-2) Nitride of KTaO ₃ The KTaO ₃ sample prepared in 1-1) above was transferred to a boat-type alumina talc, introduced into an alumina tube, and then 100% ammonia gas was used as the reaction gas at a total flow rate of 100 mlmin. The flow was carried out at ^-1 , and the alumina tube was heated in a tubular electric furnace at 900 ° C. for 0.05 to 10 hours to obtain a powder.

The obtained powder was subjected to XRD (CuKα ray) measurement using a powder X-ray diffractometer (RINT2000HF) manufactured by Rigaku Corporation. The X-ray diffraction pattern of the powder before and after nitriding is shown in FIG. In FIG. 1, (A) shows the entire region of the X-ray diffraction pattern, and (B) shows a part of the X-ray diffraction pattern enlarged. Further, (a) is a pattern before nitriding KTaO ₃ , and (b) to (g) are after nitriding KTaO ₃ with nitriding times of 0.25h, 0.5h, 1h, 2h, 4h, and 10h, respectively. Show the pattern. Comparing the obtained X-ray diffraction pattern with the data of the database ICDD-PDF for powder X-ray diffraction reported so far, the powder obtained in 1-1) above is the structure of _KTaO3 . It was confirmed that the target starting material could be synthesized in almost a single phase. Further, when the nitriding temperature was set to 900 ° C. and the nitriding time was lengthened, an XRD peak attributed to Ta ₃ N ₅ was generated in the vicinity of 2θ = 36 ° in addition to the diffraction pattern based on KTaO ₃ . Its peak intensity has been shown to increase with nitriding time, and nitriding KTaO ₃ produces a Ta ₃ N ₅ / KTaO ₃ compound (a compound having Ta ₃ N ₅ on the surface of KTaO ₃ ). It was confirmed that Although not shown in FIG. 1, the Ta 3N ₅ / _{KTaO 3 compound} was also produced in the case of nitriding KTaO ₃ with a nitriding time of 0.05 h and 0.125 h, respectively.

The ultraviolet-visible diffuse reflection spectra of the obtained KTaO ₃ powder and the Ta ₃ N ₅ / KTaO ₃ powder obtained after nitriding were measured. Specifically, the obtained powder was fixed to a predetermined glass plate and measured using an ultraviolet-visible near-infrared spectrophotometer V-670 (reference powder: alumina) manufactured by JASCO Corporation. The results are shown in FIG. (A) to (g) in the figure correspond to (a) to (g) in FIG. According to the figure, the light absorption wavelength of KTaO ₃ is about 350 nm, but the light absorption wavelength of Ta ₃ N ₅ / KTaO ₃ after nitriding is shifted to about 600 nm in the visible light region.

The obtained KTaO ₃ powder and the Ta ₃ N ₅ / KTaO ₃ powder obtained after nitriding were observed with a high-resolution field emission scanning electron microscope (HITACHI, 24TK004300), respectively. The scanning electron microscope image is shown in FIG. In FIG. 3, (A) shows the scanning electron microscope images of KTaO ₃ , and (B) and (C) show the scanning electron microscope images of KTaO ₃ after nitriding with nitriding times of 0.25 h and 0.5 h, respectively. As shown in (A), KTaO ₃ showed a cubic structure having a surface on the ridgeline. The particle size of these particles was 0.2 to 0.8 μm. On the other hand, as shown in (B) and (C), although the cubic structure of the _KTaO3 particles was almost maintained after nitriding, columnar crystals were precipitated mainly on the ridgeline. The appearance of columnar crystals coincided with the appearance of X-ray diffraction peaks, and the light absorption characteristics showed good agreement with those of the Ta _{3N 5 compound} . From this, it is considered that the columnar crystals belong to Ta ₃ N ₅ .

1-3) Support of co-catalyst (Cr ₂ O ₃ / Rh) on Ta ₃ N ₅ / KTaO ₃ Ta ₃ N ₅ / KTaO ₃ was added to a 10% by volume methanol aqueous solution of RhCl ₃・ H ₂ O. At this time, the amount of Rh supported was adjusted to be 0.01 to 0.5% by weight in terms of Rh with respect to Ta ₃ N ₅ / KTaO ₃ . This was transferred to a glass reaction cell for light irradiation, and after degassing the dissolved oxygen in the liquid, it was irradiated with 300 W Xe lamp light for 3 hours under vacuum. Further, K2 CrO ₄ was added to this solution so that the amount of Cr was ₃ times the molar ratio of Rh, and the solution was further irradiated with Xe lamp light for 12 hours under vacuum. After the photoelectric adhesion of Rh and Cr ₂ O ₃ was completed, the solution was filtered, washed and dried to obtain a Cr ₂ O ₃ / Rh-supported Ta ₃ N ₅ / KTaO ₃ photocatalyst.

2) Evaluation of hydrogen production activity A measurement sample containing the obtained photocatalyst was prepared, and the activity of the hydrogen production reaction from water was evaluated using the above-mentioned closed circulatory system reactor. At this time, a photocatalyst containing KTaO ₃ nitrided with a nitriding time of 0.25 h was used. The amount of the photocatalyst was 0.1 to 0.3% by weight based on the total weight of pure water, and the evaluation was performed with the temperature of the evaluation sample at 15 ° C. and the pH at 7. Further, a Xe lamp (300W Xe lamp device R300-3J manufactured by Eagle Engineering Co., Ltd.) was used for light irradiation, and light having a wavelength of 420 to 800 nm was irradiated by an external irradiation method.

FIG. 4 is a graph showing the relationship between the irradiation time and the amount of hydrogen produced and the amount of oxygen produced. First, it was confirmed that hydrogen and oxygen were constantly generated with a stoichiometric ratio maintained by irradiation with light having a wavelength of 420 to 800 nm. Even when the reaction was repeated under the same light irradiation conditions after exhausting hydrogen and oxygen in the gas phase (Evac.), Hydrogen and oxygen were constantly generated while maintaining a stoichiometric ratio. Even when the exhaust gas and the reaction under the same conditions were performed again, hydrogen and oxygen were generated with good reproducibility. From these facts, it was demonstrated that this photocatalyst is active in the complete decomposition of water in a visible wide range with a wavelength longer than 420 nm. The photocatalyst had a hydrogen production activity of about 10 μmolh ^-1 and an oxygen production activity of about 5 μmolh ^-1 when irradiated with visible light having a wavelength of 420 to 800 nm.

3) Examination of photocatalytic activity by nitriding time FIG. 5 is a graph showing the relationship between the nitriding time and the amount of hydrogen produced and the amount of oxygen produced. As shown in FIG. 5, sufficient hydrogen and oxygen generation activity was also produced in KTaO ₃ after nitriding in a short time with a nitriding time of 0.05 h. In this example, the activity peaked at the nitriding time of 0.25 h and the activity gradually decreased until the nitriding time of 10 h, but the production of hydrogen and oxygen was confirmed.

4) Examination of photocatalyst activity depending on the type of co-catalyst A co-catalyst-supported Ta ₃ N ₅ / KTaO ₃ photocatalyst was prepared in the same manner as above except that Pt, Ir, or Ru was used instead of Rh. For photoelectric adhesion of Pt, Ir and Ru, H2 PtCl ₆ , K2 _{IrCl 6 and} RuCl _{3 were} used as starting materials, respectively. Further, a photocatalyst containing KTaO ₃ nitrided with a nitriding time of 0.25 h was used. Then, the activity of hydrogen and oxygen production in the decomposition reaction of water was measured for each. The results are shown in FIG. Both co-catalysts showed remarkable hydrogen and oxygen producing ability, but their activity sequence was Rh>Pt>Ir> Ru.

As shown above, the Ta ₃ N ₅ / KTaO ₃ photocatalyst exhibits extremely good hydrogen production activity. The inventors infer the reason for this as follows. That is, the growth of Ta ₃ N ₅ crystals from KT a O ₃ in the NH ₃ atmosphere involves the process of K desorption. Therefore, it is considered that the formation of Ta ₃ N ₅ proceeds slowly, so that columnar crystals having high crystallinity are deposited on the surface of KTaO ₃ . Such high crystalline Ta ₃ N ₅ has a low defect concentration, and excited electrons and holes generated by photoexcitation can move without recombination, so that it is considered to bring high activity to the decomposition of water.

(Example 2: BaTaO ₂ N / KTaO ₃ compound)
The KTaO ₃ prepared in Example 1 was impregnated into a Ba (NO ₃ ) ₂ aqueous solution, and Ba (NO ₃ ) ₂ was supported on the surface of KTaO ₃ in an amount of 1 to 16 mol%. This was oxidized in the air at 900 ° C. Then, in the same manner as in Example 1, a nitriding treatment (0.5 to 4 hours) was performed to obtain BaTaO ₂ N / KTaO ₃ powder, and a co-catalyst was further supported to support Cr _{2O 3} / Rh-supported BaTaO ₂ N / KTaO. ₃ Photocatalysts were obtained. The obtained photocatalyst showed hydrogen and oxygen production activity in the decomposition reaction of water when irradiated with the above visible light.

(Comparative Example 1)
G. Hitoki, A. Ishikawa, T.M. Takata, J. et al. Kondo, M.D. Hara, K.K. Domen, Catal. Let. , 2002, 736-737. After firing Ta ₂ O ₅ at 1150 ° C. for 10 hours at 900 ° C., nitriding until Ta ₂ O ₅ is completely converted to Ta ₃ N ₅ at 900 ° C. Was done. A co-catalyst was supported on the obtained Ta ₃ N ₅ in the same manner as in Example 1, and a Cr ₂ O ₃ / Rh-supported Ta ₃ N ₅ photocatalyst was obtained. The obtained photocatalyst showed almost no hydrogen production activity with respect to the decomposition reaction of water when irradiated with the above visible light.

(Comparative Example 2)
Similar to Comparative Example 1, the Ba ₅ Ta ₄ O ₁₅ oxide was calcined at 1150 ° C. for 15 hours based on the conventional method, and then Ba ₅ Ta ₄ O ₁₅ was completely calcinated at 900 ° C. under ammonia gas. Nitriding was performed until it was converted to _BaTaO 2N. A co-catalyst was supported on the obtained BaTaO ₂ N in the same manner as in Example 1, and a Cr _{2O 3} / Rh-supported BaTaO ₂ N photocatalyst was obtained. The obtained photocatalyst showed almost no hydrogen production activity with respect to the decomposition reaction of water when irradiated with the above visible light.

As described above, the discovery that hydrogen can be produced by complete decomposition of water in the photocatalyst obtained by the present invention, which can absorb long wavelength light up to about 600 nm, is found in the visible light region for hydrogen generation from water. It opens a breakthrough in the development of photocatalysts and contributes sufficiently to the establishment of renewable energy technology. The photocatalyst obtained by the present invention, which exhibits high hydrogen generation activity in the decomposition reaction of water in the visible light region, has extremely high industrial utility because it can effectively utilize the inexhaustibly supplied solar energy. be.

Claims (5)

In the step of precipitating at least one compound selected from the group consisting of nitrides and oxynitrides containing the X element of the metal oxide on the surface of the metal oxide represented by the following general formula (1 ) . A precipitation step comprising a step of heat-treating the metal oxide or the metal oxide having an oxide of an alkaline earth metal on the surface in an ammonia-containing gas .
A method for producing a photocatalyst used for hydrogen generation from water, comprising a supporting step of supporting at least one selected from the group consisting of a noble metal and a noble metal oxide on the surface of the compound.
AXO ₃ ... (1 )
( In the formula, A represents an alkali metal and X represents Ta .) The production method according to claim 1, wherein the heat treatment time in the precipitation step is 0.05 to 2 hours. The carrying step according to claim 1 or 2, wherein the supporting step further comprises a step of supporting an oxide of at least one metal selected from the group consisting of Cr, Co, V, Mo and W on the surface of the compound. Manufacturing method. The production method according to any one of claims 1 to 3, wherein the photocatalyst is excited by irradiating with light having a wavelength selected from the range of 350 to 600 nm. A hydrogen production method for producing hydrogen from water by a photoreaction using visible light in the presence of a photocatalyst obtained by the production method according to any one of claims 1 to 4.

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