JP2023142187A - Method for producing photocatalyst and photocatalyst component - Google Patents

Abstract

To provide a method for producing a photocatalyst and a photocatalyst component which uses tin-containing complex oxide such as tin niobate, and shows higher photocatalyst performance.SOLUTION: A photocatalyst according to the present invention contains divalent tin (Sn2+), furthermore includes tin-containing complex oxide containing lanthanoid. Note that the molar ratio of lanthanoid/tin is within the range of 0.003 to 1 mol%. A producing method according to the present invention is a method for producing a photocatalyst component made of tin-containing complex oxide which contains divalent tin (Sn2+) and furthermore contains lanthanoid, where the method includes a process of bringing alkali metal salt of metal acid, tin halide and lanthanoid-containing compound in contact, and the molar ratio of lanthanoid/tin is within the range of 0.003 to 1 mol%.SELECTED DRAWING: Figure 2

Description

The present invention relates to a photocatalyst that can decompose water into hydrogen and oxygen using light, and a method for producing a photocatalyst component. In addition, in this specification, a "photocatalyst component" refers to the component contained in a photocatalyst.

Photosynthesis by plants in nature is known as a method of fixing sunlight energy. In this photosynthesis, it is known that a pigment called chlorophyll has two absorption maxima at 680 nm and 700 nm, but it is currently difficult to industrialize this method.

On the other hand, a system that uses sunlight and inexpensive photocatalysts to split water and produce hydrogen is expected to be put into practical use as a renewable energy technology. There are two types of systems that use sunlight to split water and produce hydrogen. The first is a one-stage excitation type photocatalytic system. In this system, one type of photocatalyst is used, and the band structure of the photocatalyst is such that the lower end of the conduction band has a potential more negative than the reduction potential of water, and the upper end of the valence band has a potential more positive than the oxidation potential of water. It is necessary to have The second is a two-stage excitation type photocatalytic system. This is called a Z-scheme photocatalytic system because its mechanism is similar to that of photosynthesis in plants. This system consists of two types of photocatalysts, namely a hydrogen-producing photocatalyst and an oxygen-producing photocatalyst, and an electron transfer agent. The photocatalyst configured here may have either a hydrogen production potential or an oxygen production potential. By combining it with an appropriate electron transfer agent, water can be decomposed into hydrogen and oxygen. The former is more preferable because it is expected to be simple and low-cost; however, since the photocatalysts that can be selected from the former are limited, the latter, which has a wider selection of photocatalysts, is currently being more actively developed.

Approximately half of the sunlight spectrum that reaches the earth's surface is visible light, and only a few percent is ultraviolet light. On the other hand, many conventional photocatalysts are active in the ultraviolet region and tend to be inactive in visible light. For this reason, even if all the ultraviolet light of sunlight up to 400 nm could be used up, the maximum solar energy conversion efficiency would remain at around 2%, far exceeding the energy conversion efficiency of plants' photosynthesis (1-2%). There isn't. On the other hand, when it becomes possible to use light in the visible light range, the maximum energy conversion efficiency is calculated to be 15% when it can be used up to 600 nm, and 33% when it can be used up to 800 nm. Therefore, it is believed that a photocatalyst capable of converting visible light as described above would dramatically improve the energy conversion efficiency of 1 to 2%.

Metal oxides are often stable compounds in the atmosphere and can also be used as semiconductors, so there are reports that they can be used as photocatalysts. On the other hand, in many ordinary metal oxides that can be used as optical semiconductors, the valence band is composed of oxygen 2p orbitals, so the potential at the top of the valence band is +3V vs. It is near NHE, which is the potential that can oxidize water, 1.23V vs. Since it is located on the much positive side compared to NHE, it satisfies the conditions for oxidizing water, but it becomes a material with a large band gap (potential difference between the lower end of the conduction band and the upper end of the valence band). That is, the potential at the lower end of the conduction band is the potential at which water can be reduced, 0.0V vs. It needs to be on the more negative side than NHE, and the position of the lower end of the valence band is +3.0V vs. If it is NHE or higher, its band gap (potential difference between the lower end of the conduction band and the upper end of the valence band) will be 3.0 eV or more, so the absorption edge will be 410 nm or less, and there is a tendency that only a small amount of visible light can be absorbed.

As described above, there is a problem in that there are very few metal oxides that can be used as ordinary optical semiconductors that can respond to visible light. There is an approach called valence band-controlled photocatalyst to develop photocatalysts that respond to visible light. A valence electron-controlled photocatalyst is a metal oxide photocatalyst in which a valence band is formed by constituent elements at a shallower position (on the less energetic side) than the valence band that forms the 2p orbital of oxygen. Examples of elements that form such a shallow valence band include Ag(I), Bi(III), Sn(II), and Cu(I).
In addition to the metal oxides mentioned above, photocatalysts made of metal nitrides and metal sulfides have also been reported, but these compounds tend to have lower stability in the atmosphere than metal oxides. SnNb ₂ O ₆ , WO ₃ , BiVO ₄ , TaON, etc. are already known as visible light-responsive oxygen-generating photocatalysts that exhibit relatively high activity, that is, as conventional two-stage excitation-type oxygen-generating photocatalysts. There is.

The metal compound photocatalysts described above may not exhibit sufficient performance as photocatalysts alone. For this reason, the combined use of cocatalysts and surface modification techniques is also an important element, similar to the photocatalyst described above.

When generating oxygen by oxidizing water, if a large overvoltage can be obtained, no co-catalyst may be required, but for photocatalysts that cannot produce large over-voltages, Co, Ir, Ru, Fe, Mn, etc. may be used as co-catalysts for oxygen generation. Containing oxides and hydroxides are required.

The above-mentioned tin niobate (SnNb ₂ O ₆ ) is known to cause hydrogen generation using a sacrificial agent under visible light irradiation, and oxygen generation using a sacrificial agent under visible light irradiation ( For example, Patent Document 1, Non-Patent Documents 1 and 2). Furthermore, it has recently been reported that tin niobate completely decomposes water into hydrogen and oxygen under irradiation with visible light (Patent Document 2). Furthermore, tin niobate is known to be effective as a photocatalytic electrode for oxygen production (Non-Patent Document 3).

Japanese Patent Application Publication No. 2006-88019 Japanese Patent Application Publication No. 2021-13890

Chem. Lett. 2004, 33, 28-29 Chem. Mater. 2008, 20, 1299-1307 Chem. Commun. 2017, 53, 629-632

As mentioned above, there have been reports of technology for decomposing water into hydrogen and oxygen using sunlight and photocatalysts, but there are few reports of highly efficient photocatalysts using metal oxides, which are compounds that are stable in the air. For example, the performance of the aforementioned tin niobate (SnNb ₂ O ₆ ) photocatalyst is also desired to be improved.

Several photocatalysts other than SnNb ₂ O ₆ are already known as visible light-responsive oxygen-generating photocatalysts that exhibit relatively high activity, that is, as conventional two-stage excitation-type oxygen-generating photocatalysts. However, the oxygen-generating activity of these oxygen-generating photocatalysts is not high in practical terms, and oxygen-generating photocatalysts that exhibit higher activity are desired. In particular, further improvement in activity is required in order to bring the conversion efficiency of light energy into hydrogen (STH = Solar to Hydrogen) to the 5% or 10% level.

In view of this situation, an object of the present invention is to provide a method for producing a photocatalyst and a photocatalyst component that exhibit higher photocatalytic performance using a tin-containing composite oxide such as tin niobate.

In order to solve the above problems, the present inventors investigated and found that a tin-containing composite oxide containing divalent tin, further containing lanthanoid, and having a molar ratio of lanthanide and tin within a specific range. It has been discovered that a photocatalyst containing the following is a photocatalyst that efficiently generates oxygen in the presence of water containing a sacrificial agent such as silver nitrate. Furthermore, this photocatalyst is easy to synthesize, and since it is a metal oxide, it can be expected to be stable. Specifically, the present invention provides the following.

[1] A photocatalyst containing a tin-containing composite oxide containing divalent tin (Sn ²⁺ ) and further containing lanthanoid.
(However, the lanthanide/tin molar ratio is in the range of 0.003 to 1 mol%.)

[2] The photocatalyst according to [1], further comprising a promoter containing at least one metal element selected from the group consisting of elements from Group 6 to Group 11 of the Periodic Table.

[3] The photocatalyst according to [1] or [2], wherein the tin-containing composite oxide contains SnNb ₂ O ₆ containing lanthanoid.

[4] The photocatalyst according to [2] or [3], wherein the co-catalyst includes an oxygen-generating co-catalyst.

[5] The photocatalyst according to [4], wherein the oxygen generation promoter contains at least one of Ir and Co.

[6] A method for producing a photocatalyst component made of a tin-containing composite oxide containing divalent tin (Sn ²⁺ ) and further containing lanthanoid, comprising:
contacting an alkali metal salt of a metal acid with a tin halide and a lanthanide-containing compound,
A manufacturing method in which the lanthanide/tin molar ratio is in the range of 0.003 to 1 mol%.

[7] The manufacturing method according to [6], wherein the tin halide is tin chloride.

As described above, the photocatalyst of the present invention can efficiently generate oxygen in the presence of water containing a sacrificial agent. Furthermore, since the photocatalyst of the present invention contains a metal oxide, it can be expected to be a stable catalyst.

FIG. 1 is a graph showing the results of a water splitting reaction in a comparative example. FIG. 2 is a graph showing the results of water splitting reactions in Examples. FIG. 3 is a graph showing variations in initial activity when the doping amount is changed in Example 1 or 2.

The photocatalyst of the present invention contains a tin-containing composite oxide that contains divalent tin (Sn ²⁺ ), further contains lanthanoid, and has a molar ratio of lanthanide and tin within a specific range. Such a photocatalyst of the present invention can efficiently generate oxygen under visible light irradiation and in the presence of water containing a sacrificial agent such as silver nitrate. Note that in this specification, visible light is light with a wavelength of substantially 380 nm to 800 nm. The photocatalyst of the present invention is a photocatalyst whose main component is a metal oxide that is said to be highly stable, and the photocatalyst uses a metal oxide that contains tin and also contains other trace components. . Each component constituting the photocatalyst of the present invention will be described below.

(1) Tin-containing composite oxide containing divalent tin (Sn ²⁺ ) and further containing lanthanoid The photocatalyst of the present invention contains tin-containing composite oxide containing divalent tin (Sn ²⁺ ) and further containing lanthanide. Contains complex oxide as one component. Examples of the tin-containing composite oxide include tin-containing composite oxides that contain divalent tin (Sn ²⁺ ) and are doped with lanthanoids. The tin-containing composite oxide preferably does not contain Group 15 elements of the periodic table, such as nitrogen and sulfur, or Group 16 elements other than oxygen, but it may contain them as long as it does not violate the purpose of the present invention. You can stay there. The tin-containing composite oxide may contain nitrogen, sulfur, etc., preferably at most 5% by weight, more preferably at most 3% by weight, even more preferably at most 1% by weight, particularly preferably at most 0.3% by weight. good.

The tin-containing composite oxide contains metal elements other than tin and lanthanoids. The metal elements other than tin and lanthanoids are not particularly limited as long as they are elements used in known tin-containing composite oxides, but elements from Groups 4 to 6 of the periodic table are preferably used. Preferred examples include niobium (Nb), titanium (Ti), tantalum (Ta), and tungsten (W). Among these, niobium is preferred. That is, as the tin-containing composite oxide, tin niobate containing lanthanoid is preferable. Examples of the lanthanoid-containing tin niobate include lanthanoid-doped tin niobate.

Further, specific chemical formulas of the tin-containing composite oxides include, for example, SnNb ₂ O ₆ , Sn ₂ Nb ₂ O ₇ , Sn ₂ TiO ₄ , SnTa ₂ O ₆ , Sn ₂ Ta ₂ O ₇ , SnWO ₄ , _{Sn2Sb2O7 etc.} are mentioned, _{SnNb2O6 , Sn2Nb2O7 , Sn2TiO4 are preferable , SnNb2O6 and Sn2TiO4} are more preferable, _{and SnNb2O6} is most _preferable .

Of course, metals other than the metals constituting the present invention and their derivatives (oxides, etc.) may be included as long as they do not contradict the purpose of the present invention. The content of such components is such that the metal atoms are preferably 5 mol % or less, more preferably 3 mol % or less, still more preferably 1 mol % or less, particularly preferably 0. The content is 3 mol% or less.

The SnNb ₂ O ₆ mentioned above is an n-type semiconductor and has a layered fordite structure consisting of two layers: an octahedral phase layer of Nb ₂ O ₆ and a Sn layer, and has a band gap of 2.3 eV and an absorption edge. is 540 nm.

As mentioned above, it is known that SnNb ₂ O ₆ generates oxygen when silver nitrate is used as a sacrificial agent under visible light irradiation. This is considered to suggest that SnNb ₂ O ₆ , as a band structure, potentially has the ability to oxidize water and generate oxygen under visible light irradiation. The present invention was made based on the discovery that the excellent performance of this tin-containing composite oxide can be further enhanced by the use of trace components described below.

The tin-containing composite oxide is usually in powder form, and from the viewpoint of activity and ease of handling, its size is preferably about 100 nm to 5 μm, more preferably about 300 nm to 2 μm.

The presence of divalent tin contained in the tin-containing composite oxide can be identified by XPS analysis. Moreover, the composite oxide of the present invention may contain tin other than divalent tin. Examples of tin other than divalent tin include tetravalent tin (Sn ⁴⁺ ). The proportion of tetravalent tin present in the tin-containing composite oxide can be estimated by Mössbauer spectroscopy or XPS analysis. In XPS analysis, it can be estimated from the peak derived from the 3d orbit of tin. The proportion of tetravalent tin (Sn ⁴⁺ ) in the tin-containing composite oxide is 20% or less of the proportion of divalent tin (Sn ²⁺ ) in the tin-containing composite oxide. The content is preferably 10% or less, more preferably 5 mol% or less, and even more preferably 5 mol% or less.

The above XPS analysis can be performed, for example, under the following conditions.
・Device: AXIS-NOVA (manufactured by Kratos)
・X-ray source: Monochromatic AI Ka (1486.6ev)
・Analysis area: 700μm x 300μm (analysis of the outermost surface)
・A” ion etching rate: Raster 15mm square 0.48 A sec (sio. conversion)
Furthermore, by using this method, it is also possible to determine the valence of other metal group elements, such as the co-catalyst (2) described below.

(lanthanoid)
The composite metal oxide containing divalent tin of the present invention is characterized by containing lanthanoid in a specific proportion. Specifically, it is characterized in that the lanthanide/tin molar ratio is in the range of 0.003 to 1 mol%. The preferable lower limit is 0.005 mol%, more preferably 0.007 mol%, even more preferably 0.01 mol%. On the other hand, the preferable upper limit is 1.0 mol%, more preferably 0.5 mol%, still more preferably 0.3 mol%. If the molar ratio is smaller than the lower limit, the effect of the present invention may not be sufficient.

The above lanthanoids refer to a total of 15 elements with atomic numbers 57 to 71. Among these, lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), gadolinium (Gd), terbium (Tb), dysprosium (Dy), and holmium (Ho) are preferred. , erbium (Er), and lutetium (Lu). More preferred are lanthanum (La), neodymium (Nd), samarium (Sm), gadolinium (Gd), terbium (Tb), and erbium (Er), and even more preferred are lanthanum (La), samarium (Sm), and erbium. (Er), particularly preferably lanthanum (La) or samarium (Sm).

Although the function of lanthanoids in the present invention is not clearly known, the inventors believe that lanthanides may function as dopants in semiconductors. It is likely that lanthanoids are isomorphically substituted for tin in the tin-containing complex oxide, unexpectedly improving its function as a semiconductor. At this time, for example, there is an upper limit to the amount of tin that can be isomorphically substituted by lanthanide atoms, and since lanthanide atoms cannot be substituted above a certain lanthanide content, it can be assumed that there is an upper limit to the content of lanthanide atoms. Furthermore, when the content of lanthanoid atoms is high, it is possible that the lanthanoid cannot form a structure that allows isomorphic substitution with the main metal such as tin. It is likely that one or more of these factors are involved, and that photocatalysts can exhibit effective oxygen generation capabilities within a specific range.

(2) Co-catalyst In the present invention, (1) a tin-containing composite oxide is used to decompose water into hydrogen and oxygen under visible light irradiation to generate hydrogen and oxygen from water. A promoter containing at least one metal element selected from the group consisting of elements from Groups 1 to 11 may be used. The co-catalyst is preferably supported on, for example, a tin-containing composite oxide. For example, SnNb ₂ O ₆ preferably supports a co-catalyst in order to generate hydrogen and oxygen from water under visible light irradiation. A catalytic embodiment is disclosed.

The co-catalyst may be either a hydrogen-producing co-catalyst or an oxygen-producing co-catalyst, or both a hydrogen-producing co-catalyst and an oxygen-producing co-catalyst may be used.

Patent Document 1 reports that iridium is introduced into a composite metal oxide such as SnNb ₂ O ₆ and oxygen is generated under visible light in the presence of a sacrificial agent. From this point of view, the promoter of the present invention is preferably an oxide or hydroxide containing Co, Ir, Ru, Fe, Mn, etc., and more preferably an oxide or hydroxide containing Ir or Co.

As described above, the photocatalyst of the present invention includes (1) a tin-containing composite oxide, and may optionally include (2) a co-catalyst. The ratio of these components is not particularly limited, but preferably (2) co-catalyst is 0.01 to 20 parts by weight, more preferably 0.1 to 10 parts by weight per 100 parts by weight of photocatalyst. be. The proportions of (1) tin-containing composite oxide and (2) co-catalyst can be determined, for example, by ICP optical emission spectrometry (high-frequency inductively coupled plasma optical emission spectrometry, ICP-OES/ICP- It can be determined by quantifying each metal using AES). Of course, it can also be specified from the charging ratio of each raw material component during photocatalyst synthesis. In the present invention, an ICP emission spectrometer ICPS-8100 manufactured by Shimadzu Corporation is used for the above-mentioned ICP analysis, and a value obtained by measurement according to a conventional method is used as the above-mentioned ratio.

(Sacrificial agent (sacrificial reagent))
When evaluating oxygen production activity, a generally well-known method is to oxidize water in the presence of a sacrificial agent and evaluate the activity. As the sacrificial agent, for example, a silver Arrhenius salt compound such as silver nitrate can be used. As the sacrificial agent used for oxygen generation, a compound that undergoes reduction by photoexcited electrons when a tin-containing composite oxide, such as SnNb ₂ O ₆ , is irradiated with light can be used. In that case, a compound exhibiting a redox potential more noble than the redox potential at which hydrogen molecules are produced by reduction of water is preferred. For example, in addition to compounds containing monovalent Ag, compounds containing monovalent Au or trivalent Au, compounds containing tetravalent Pt, compounds containing trivalent Co, etc. may be mentioned.

The amount of the sacrificial agent is preferably 0.001 to 0.5 mol, more preferably 0.005 to 0.4 mol, per 1 mol of water. If the amount of sacrificial agent used is within the above range, oxygen can be efficiently generated.

(Method of generating oxygen from water)
If such a photocatalyst of the present invention is used, oxygen can be generated from water using a sacrificial reagent such as silver nitrate even under visible light conditions, as shown in the examples described below. The reaction conditions are not particularly limited, and the reaction can be carried out under known reaction conditions. For example, by putting a photocatalyst in water and irradiating it with visible light, oxygen can be generated from water and a sacrificial agent. Examples of the light source of the visible light to be irradiated include, in addition to the sun, lamps that can irradiate light similar to sunlight or simulated sunlight, such as xenon lamps and metal halide lamps; mercury lamps; LEDs, and the like.

The photocatalyst of the present invention is an embodiment containing a composite metal oxide containing tin. Although the photocatalyst of the present invention contains metal oxides, it is possible to react with water using visible light. Furthermore, the photocatalyst of the present invention can react with water using visible light without sacrificial agents such as methanol or silver nitrate, but its characteristics are more likely to be exhibited when a co-catalyst is used in combination.

The photocatalyst of the present invention has a metal complex oxide (1) tin-containing complex oxide as its main component, and thus tends to be stable and easy to manufacture. Furthermore, because it has a special configuration, oxygen can be efficiently generated in the presence of water and a sacrificial reagent even under visible light. Such photocatalysts are considered to be a technology that has the potential to respond to the depletion of fossil fuels and become the basis of a hydrogen society expected in the near future. In addition, it is expected that in the future it may become a basic component of photocatalysts that can completely decompose water.

(Method for producing photocatalyst)
The method for producing the photocatalyst of the present invention is not particularly limited.
In the present invention, (1) the method for producing a tin-containing composite metal oxide includes (1) reacting an alkali metal salt of a metal acid with a tin halide, and then firing at a temperature of 800°C or higher. A method for preparing a tin-containing composite oxide can be mentioned. There is no particular restriction on the upper limit of the firing temperature, but in view of the material of the reactor, etc., it is preferably 1500°C, more preferably 1200°C.
A more specific and preferred method will be described below.

A tin-containing composite oxide containing divalent tin (Sn ²⁺ ) is usually obtained as a powder, but each particle constituting the powder is preferably produced as a single crystal or an aggregate of crystals, Moreover, it is more preferable that the crystal be a high-quality crystal or a collection thereof with few impurities and/or defects and grain boundaries. Further, as described above, the size of each particle is preferably about 100 nm to 5 μm, more preferably about 300 nm to 2 μm, from the viewpoint of activity and ease of handling.

Conventional solid phase methods can also _be applied to produce powders of high quality and _{specific sizes} . ₂ Nb ₂ O ₆ etc.) is once produced. The method for producing this alkali niobate salt is not particularly limited, but it is usually carried out by so-called hydrothermal synthesis in which niobic acid and an alkali metal hydroxide are reacted in an aqueous solvent in the range of 100°C to 250°C. Acid-alkali salts can be produced. The reaction time at that time is 1 to 40 hours. The reaction operation is preferably carried out using a closed autoclave or the like. More preferred is a method (flux method) in which a tin-containing composite oxide is produced by reacting this alkali metal-containing metal oxide with a tin halide.

As the tin halide, tin fluoride (SnF ₂ ), tin chloride (SnCl ₂ ), tin bromide (SnBr ₂ ), and tin iodide (SnI ₂ ) can be used, with tin chloride being particularly preferred. Furthermore, tin halides have a low melting point; for example, tin chloride has a melting point of 247° C., and is thought to function not only as a Sn source but also as a fluxing agent. In this method (flux method) of producing a tin-containing composite oxide by reacting an alkali metal-containing metal oxide with a tin halide, alkali metal chlorides such as LiCl, NaCl, KCl, CsCl, MgCl ₂ , CaCl ₂ , SrCl ₂ , BaCl ₂ and other alkaline earth metal chlorides, which can be used as a fluxing agent, may also be present.

The tin halide is preferably used in an excess amount of Sn/Nb atomic ratio of 1 to 10 times, more preferably 1.1 to 5 times, with respect to the alkaline niobate salt, and the temperature is preferably 300 to 900°C. The reaction is preferably carried out at a temperature of 350 to 700°C, more preferably 400 to 600°C, under a nitrogen stream, in a vacuum sealed tube, or in an autoclave.

After the reaction, excess tin chloride and by-produced alkali chloride salts are removed using an aqueous hydrochloric acid solution, alcohol, water, etc., and the reaction product is washed and further dried to obtain SnNb ₂ O ₆ powder. Since tin chloride (SnCl ₂ ) has a low melting point of 247° C., it is believed that under the conditions described above, reaction and crystal growth occur conveniently in the presence of molten tin chloride.

Furthermore, it is desirable to subject the tin-containing composite oxide thus obtained to a treatment such as firing (annealing). More specifically, in the case of SnNb ₂ O ₆ , for example, tin niobate is obtained by the reaction of the above-mentioned alkali niobate and tin chloride, and then heated (calcined (annealed)) at a temperature of 600°C or higher. A preferable SnNb ₂ O ₆ powder is obtained by performing the following steps. A preferable lower limit of the firing (annealing) treatment temperature is 800°C. As mentioned above, the upper limit of the firing (annealing) treatment temperature is not particularly limited, but is preferably 1500°C, more preferably 1200°C, still more preferably 1100°C, and particularly preferably 1000°C. Further, this firing (annealing) treatment is preferably performed in an inert gas atmosphere such as nitrogen or argon or in a vacuum sealed tube. This is to prevent divalent tin from being oxidized to tetravalent tin. Through this firing (annealing) treatment, it is possible to obtain SnNb ₂ O ₆ powder consisting of high-quality crystals or aggregates of crystals suitable for oxygen generation.

Of course, other manufacturing methods capable of obtaining high-quality crystals, such as various thin film manufacturing methods, can also be used. Also, other manufacturing methods can be used in combination with the above method.

In the steps of the above production method, the lanthanide-containing compound can be brought into contact with the metal oxide or tin-containing compound at any step. The step includes, for example, a step of contacting an alkali metal salt of a metal acid, a tin halide, and a lanthanoid-containing compound, and more specifically, a step of bringing the metal oxide, the alkali metal hydroxide, and a lanthanide-containing compound into contact with each other. A preferred example is to use a lanthanide-containing compound in the step of contacting with tin. In addition, lanthanoids can also be introduced by bringing an alkali metal salt of a metal acid, a tin halide, and a lanthanide-containing compound into contact with each other by a method similar to the above-described calcination (annealing) treatment.

Examples of the lanthanoid-containing compound that can be used include lanthanide oxides, lanthanide hydroxides, lanthanide carbonates, and mixtures thereof. Lanthanide oxides are particularly preferred.

Methods for confirming the structure of complex oxides include elemental analysis methods such as ICP, X-ray crystallography (XRD), which is a crystal structure analysis method, and ultraviolet-visible-near-infrared light analysis. Any known method can be used.

Further, (2) the co-catalyst can be formed in a form that is bonded onto the surface of the composite oxide using a so-called supported formulation. This supporting method is not particularly limited, and ordinary supporting methods can be applied. Preferred supporting methods include an impregnation method, a coprecipitation method, a CVD (chemical vapor deposition) method, a pore filling method, a photoelectrodeposition method, and the impregnation method and photoelectrodeposition method are preferred. For example, in the impregnation method, an impregnating solution is prepared by uniformly dissolving a metal salt or metal complex of Groups 6 to 11 of the periodic table in water or an organic solvent, and then a tin-containing composite oxide powder is prepared. is added, water or organic solvent is distilled off while heating and stirring, and further heat treatment (calcination) in a hydrogen or hydrogen-containing gas atmosphere (nitrogen or argon can also be used together), or in an air or oxygen-containing gas atmosphere. However, depending on the case, it can be supported by further heat treatment in an atmosphere of an inert gas such as nitrogen or argon. The heat treatment (calcination) temperature is preferably in the range of 100 to 500°C, more preferably 150 to 450°C. The firing time is preferably 1 minute to 50 hours, more preferably 1 to 10 hours. Again, since divalent tin is easily oxidized, when heat-treated in an air or oxygen atmosphere, the heat treatment (firing) temperature is preferably in the range of 100 to 300°C, more preferably in the range of 100 to 250°C.

When applying the photocatalyst according to the present invention, which has excellent oxygen production activity, to actual water splitting, various methods are possible. For example, there is a method in which the photocatalyst for oxygen generation of the present invention and a photocatalyst for hydrogen generation prepared separately are suspended in water and irradiated with light in the presence of a compound for electron transfer called a mediator. A method of irradiating a photocatalyst sheet in water with a photocatalyst sheet coated with a photocatalyst for oxygen generation of the present invention and a separately prepared photocatalyst for hydrogen generation on a conductive metal substrate such as Examples include a method in which a photocatalyst and a separately prepared photocatalyst for hydrogen generation are each made into electrodes, connected with a conductor wire, and irradiated with light in water.

Hereinafter, the photocatalyst of the present invention will be specifically explained based on Examples, but the present invention is not limited to these Examples.

[Comparative example 1]
<Synthesis of SnNb ₂ O ₆ powder (lanthanoid-free mode)>
First, 9.30 g of niobic acid (Sigma-Aldrich, 99.99%), 4.0 g of NaOH granules (Fujifilm Wako Pure Chemical, first grade, 93+%), and 100 ml of water were placed in an autoclave with a Teflon (registered trademark) inner cylinder. , the reaction was carried out at 160°C for 20 hours. After the contents were filtered and washed with water, they were vacuum dried at 60°C overnight. Na ₂ Nb ₂ O _{6.H 2} O thus obtained was heat treated at 250° C. for 1 hour under vacuum to obtain Na ₂ Nb ₂ O ₆ anhydride.

After thoroughly mixing 2.0 g of this Na ₂ Nb ₂ O ₆ anhydride and 3.41 g of SnCl ₂ (Fuji Film Wako Pure Chemical, 99.9%) in an agate mortar, the mixture was placed in a quartz glass tube and placed under vacuum. It was sealed and heated in an electric furnace at 500° C. for 5 hours (flux treatment). Thereafter, the contents were washed three times with 3N hydrochloric acid aqueous solution and three times with water, and then vacuum dried at 60° C. overnight. The SnNb ₂ O ₆ thus obtained was further placed in a quartz glass tube, sealed under vacuum, and annealed at 950° C. for 10 hours in an electric furnace.

Regarding the obtained crystalline compound, in addition to ICP analysis, crystal structure analysis was performed using XRD, and structural analysis was also attempted by diffuse reflection spectrum measurement using a UV-VIS-NIR spectrometer, and the crystalline SnNb ₂ O ₆ structure was determined. I confirmed that it is.

<Supported promoter>
A solution of 0.0458 g of tris(2,4-pentanedionato)iridium (Tokyo Kasei) dissolved in 6 mL of methanol was impregnated into 0.6 g of the SnNb ₂ O ₆ powder. Thereafter, treatment was performed at 350° C. for 2 hours under gas flow at a nitrogen/hydrogen ratio of 95/5 (volume ratio). As a result, SnNb ₂ O ₆ powder supporting Ir was obtained. ICP analysis showed that the amount of Ir supported was 0.7% by weight.

<Method for evaluating oxygen production activity by water decomposition using visible light>
Using the Ir-supported SnNb ₂ O ₆ compound as a photocatalyst, the activity was evaluated using an upward irradiation type reaction cell connected to a closed circulation type reaction device. A xenon lamp (a filter was used to cut light with a wavelength of 420 nm or less) was used as a light source. A reaction cell was charged with 150 mL of an aqueous solution containing 0.1 g of a photocatalyst and 20 mM of silver nitrate as a sacrificial reagent, and the atmosphere in the system was replaced with Ar. The reaction conditions are a reaction temperature of 15 to 20°C and a reaction pressure of 40 to 50 torr. Light was irradiated from above the Pyrex (registered trademark) glass window using the above xenon lamp. The generated oxygen was quantified every hour using an online gas chromatograph (manufactured by Shimadzu Corporation; GC-8A, MS-5A, TCD, Ar carrier). The amount of oxygen generated 1 hour after the start of the reaction was defined as the initial activity (unit: μmol/h). The results are shown in Figure 1.

[Example 1]
In Comparative Example 1, during the flux treatment in which 2.0 g of Na ₂ Nb ₂ O ₆ anhydride and 3.41 g of SnCl ₂ were reacted, 2.0 mg of La ₂ O ₃ (2.0 mg of Na ₂ Nb ₂ O ₆ anhydride was The same operation was performed except that La (corresponding to 0.2 mol %) was added. XRD and DRS analysis were performed for characterization of the La-doped SnNb ₂ O ₆ thus obtained, and no major changes were found compared to Comparative Example 1. When the amount of La was measured by ICP analysis, it was found to be 0.09 mol%. Further, an Ir co-catalyst was supported by the same operation as for supporting the above-mentioned co-catalyst. Using the thus obtained Ir-supported La-doped SnNb ₂ O ₆ , the oxygen production activity by water decomposition using visible light was evaluated in the same manner as above. The initial activity (unit: μmol/h) is shown in FIG.

Furthermore, as shown in FIG. 3, the same operation as above was performed except that the amount of La doped with respect to Na ₂ Nb ₂ O ₆ anhydride was changed to obtain La-doped SnNb ₂ O ₆ . The oxygen production activity due to decomposition was evaluated. The initial activity (unit: μmol/h) at each doping amount is shown in FIG.

[Example 2]
Sm was doped by the same operation as in Example 1, except that 1.1 mg of Sm ₂ O ₃ (equivalent to 0.2 mol % of Sm with respect to Na ₂ Nb ₂ O ₆ anhydride) was used instead of La ₂ O ₃ . synthesized SnNb ₂ O ₆ . When the obtained Sm-doped SnNb ₂ O ₆ was subjected to XRD and DRS analysis for characterization, similar to Example 1, there was no significant change from Comparative Example 1. When the amount of Sm was measured by ICP analysis, it was found to be 0.10 mol%. Furthermore, Ir-supported Sm-doped SnNb ₂ O ₆ was synthesized, and the oxygen production activity by water decomposition using visible light was evaluated in the same manner as above. The initial activity (unit: μmol/h) is shown in FIG.

Furthermore, as shown in FIG. 3, the same operation as above was performed except that the amount of Sm doped with respect to Na ₂ Nb ₂ O ₆ anhydride was changed to obtain Sm-doped SnNb ₂ O ₆ , and the water was analyzed using visible light. The oxygen production activity due to decomposition was evaluated. The initial activity (unit: μmol/h) at each doping amount is shown in FIG.

[Example 3 to Example 6]
Neodymium oxide (Example 3), gadolinium oxide (Example 4), terbium oxide (Example 5), or erbium oxide (Example 6) was used instead of lanthanum oxide (Na ₂ Nb ₂ O ₆ anhydride). Ir-supported lanthanide-doped SnNb ₂ O ₆ was obtained in the same manner as in Example 1, except that each lanthanoid (equivalent to 0.2 mol %) was evaluated for oxygen production activity. The initial activity (unit: μmol/h) is shown in FIG.

Claims (7)

A photocatalyst containing a tin-containing composite oxide containing divalent tin (Sn ²⁺ ) and further containing lanthanoid.
(However, the lanthanide/tin molar ratio is in the range of 0.003 to 1 mol%.) The photocatalyst according to claim 1, further comprising a promoter containing at least one metal element selected from the group consisting of elements from Group 6 to Group 11 of the periodic table. The photocatalyst according to claim 1 or 2, wherein the tin-containing composite oxide contains SnNb ₂ O ₆ containing lanthanoid. The photocatalyst according to claim 2 or 3, wherein the co-catalyst includes an oxygen-generating co-catalyst. The photocatalyst according to claim 4, wherein the oxygen generation promoter contains at least one of Ir and Co. A method for producing a photocatalyst component made of a tin-containing composite oxide containing divalent tin (Sn ²⁺ ) and further containing lanthanoid, the method comprising:
contacting an alkali metal salt of a metal acid with a tin halide and a lanthanide-containing compound,
A manufacturing method in which the lanthanide/tin molar ratio is in the range of 0.003 to 1 mol%. 7. The manufacturing method according to claim 6, wherein the tin halide is tin chloride.

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