Photocatalyst having hydrogen-producing activity and method for producing the same

Abstract

To provide an Al-doped LaTiO2N-based photocatalyst having high hydrogen-producing activity, and a method for producing the same.SOLUTION: A photocatalyst having hydrogen-producing activity, comprising main catalyst particles having a composition represented by the formula: LaWTi1-XAlXOYNZ (where X, Y, Z, and W satisfy 1.00≤W≤1.10, 0.03≤X≤0.30, 2.00≤Y≤2.55, and 0.55≤Z≤1.00), and mainly composed of a perovskite-type oxynitride.SELECTED DRAWING: Figure 13

Description

[Technical Field]

[0001] The present invention relates to a photocatalyst active for hydrogen generation and a method for producing the same. [Background technology]

[0002] Due to the problems of global warming and the depletion of fossil fuel resources, technological development related to the water splitting reaction using photocatalysts has attracted attention. By utilizing the water splitting reaction, solar energy can be converted into chemical energy such as hydrogen and put to effective use.

[0003] In photocatalytic water splitting, fine semiconductor particles are dispersed in water and irradiated with light to produce hydrogen and oxygen. More specifically, when the semiconductor particles absorb light with an energy greater than the band gap of the semiconductor particles, electrons in the valence band are excited to the conduction band. As a result, negatively charged free electrons are generated in the conduction band, and positively charged holes are generated in the valence band. These migrate to the surface of the semiconductor particles, where the holes oxidize water to produce oxygen, and the free electrons reduce hydrogen ions to produce hydrogen.

[0004] To initiate a water splitting reaction, semiconductor particles must be irradiated with light having an energy greater than the band gap. In other words, semiconductor particles can only absorb light with an energy greater than their band gap. Meanwhile, the energy ratios of ultraviolet light, visible light, and infrared light that make up sunlight are 5%, 54%, and 41%, respectively. Therefore, to commercialize a photocatalyst for water splitting, it is desirable to use a photocatalyst that can utilize not only ultraviolet light but also visible light.

[0005] Furthermore, in order to generate hydrogen by decomposing water, the potential at the bottom of the conduction band of the photocatalytic material must be more base than the hydrogen generation potential. Furthermore, in order to generate oxygen by decomposing water, the potential at the top of the valence band must be more noble than the oxygen generation potential. Here, the hydrogen generation potential is the potential at which protons in water are reduced to generate hydrogen gas. Furthermore, the oxygen generation potential is the potential at which water is oxidized to generate oxygen.

[0006] Therefore, in order for a photocatalytic material to absorb light and decompose water, thereby generating both hydrogen and oxygen, the photocatalytic material must have a band gap smaller than the light energy, and the conduction band minimum potential must be more base than the hydrogen generation potential and the valence electron maximum potential must be more noble than the oxygen generation potential.

[0007] Recently, oxynitrides have attracted attention as water-splitting photocatalysts, and LaTiO2N, which has a perovskite crystal structure, is considered particularly promising. For example, Patent Document 1 below proposes a photocatalyst composed of an oxynitride containing at least one transition metal (Claim 1 of Patent Document 1). It also discloses that metals and metal compounds bonded with an appropriate amount of nitrogen atoms can function as photocatalysts activated by visible light, and that LaTiO2N is synthesized in the Examples, and that the synthesized LaTiO2N has the ability to reduce protons to hydrogen and oxidize water to oxygen when exposed to visible light with a wavelength of 400 nm or longer (see

[0004] ,

[0022] to

[0024] (Example 6) of Patent Document 1).

[0008] Non-Patent Document 1 below discloses that for Pt / LaTiO2N powder photocatalysts, visible-light-induced changes are investigated by infrared spectroscopy using a CO probe adsorbed on the Pt cocatalyst for water splitting under visible light irradiation (page 23902, Title and Abstract column). It also discloses that the band gap of LaTiO2N is 2.1 eV, enabling it to utilize a wider range of solar energy than GaN:ZnO, and that LaTiO2N has a band position suitable for water splitting (page 23902, 1. INTRODUCTION column of Non-Patent Document 1).

[0009] Non-Patent Document 2 below discloses that in the reaction from the perovskite slab oxide La2Ti2O7 to LaTiO2N, a volume shrinkage of approximately 14% occurs per Ti atom due to the reduction of anions, and pores are formed. Non-Patent Document 3 below discloses that in a conventional solid-phase method in which the workpiece is gradually heated in an atmospheric furnace and then slowly cooled, the perovskite slab oxide La2Ti2O7 is produced, and by reducing this, the perovskite oxide LaTiO3 is synthesized. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-66333 [Non-patent literature]

[0011] [Non-Patent Document 1] Xuwang Lu et al., Infrared Spectroscopic Study of the Potential Change at Cocatalyst Particles on Oxynitride Photocatalysts for Water Splitting by Visible Light Irradiation, The Journal of Physical Chemistry, C201, 115, 48, 23902-23907 [Non-patent document 2] S. Pokrant, MC Cheynet, S. Irsen, AE Maegli, R. Erni, Mesoporosity in Photocatalytically Active Oxynitride Single Crystals. J. Phys. Chem. C 2014, 118, 36, 20940-20947 [Non-patent document 3] G.Herrera, J. Jimenez-Mier, E. Chavira, Layered-structural monoclinic-orthorhombic perovskite La2Ti2O7 to orthorhombic LaTiO3 phase transition and their microstructure characterization. Mater. Characterization. 2014, 89, 13-22 Summary of the Invention [Problem to be solved by the invention]

[0012] Thus, LaTiO2N, which has a perovskite crystal structure, can absorb not only ultraviolet light but also visible light up to wavelengths of 600 nm, and is expected to be a photocatalytic material with excellent potential.

[0013] However, in practice, it has been difficult to obtain a photocatalyst with excellent performance using LaTiO2N. That is, crystal defects are formed inside the crystal of LaTiO2N during the manufacturing process, and these crystal defects become recombination centers. Also, during the nitriding process during manufacturing, some of the titanium (Ti) is reduced from positive tetravalent to positive trivalent, resulting in the formation of Ti reduced species (Ti 3+ ) may occur. 3+ promotes the recombination of carriers (electrons and holes). As a result, carriers do not move to the photocatalyst surface, resulting in low hydrogen generation activity. For these reasons, LaTiO2N synthesized by conventional methods has poor hydrogen generation activity, limiting its use as a photocatalyst for water splitting.

[0014] The inventors conducted extensive research to solve these conventional problems. As a result, they discovered that a highly crystalline photocatalyst can be obtained by doping LaTiO2N with a specific amount of aluminum (Al), and that this photocatalyst has high hydrogen production activity. They also discovered that a highly crystalline photocatalyst can be easily produced by introducing the precursor material into a high-temperature environment of 500°C or higher and then nitriding it.

[0015] The present invention was completed based on these findings, and aims to provide an Al-doped LaTiO2N-based photocatalyst that exhibits high hydrogen generation activity and a method for producing the same. [Means for solving the problem]

[0016] The present invention encompasses the following aspects (1) to (10). In this specification, the expression "to" includes the numerical values ​​on both ends. In other words, "X to Y" is synonymous with "at least X and at most Y."

[0017] (1) Formula: La W Ti 1-X Al X O Y N Z (wherein X, Y, Z, and W satisfy 1.00≦W≦1.10, 0.03≦X≦0.30, 2.00≦Y≦2.55, and 0.55≦Z≦1.00), and includes main catalyst particles whose main component is a perovskite-type oxynitride.

[0018] (2) The perovskite-type oxynitride has an a-axis lattice constant of 5.5343 Å or more and 5.5757 Å or less, a b-axis lattice constant of 5.5129 Å or more and 5.6013 Å or less, and a c-axis lattice constant of 7.7980 Å or more and 7.8992 Å or less, in the hydrogen generation active photocatalyst (1) above.

[0019] (3) A hydrogen generation active photocatalyst according to (1) or (2) above, in which in an X-ray diffraction (XRD) pattern using Cu Kα as a radiation source, the peak full width at half maximum (FWHM) of the (002) diffraction line based on the perovskite-type oxynitride is 0.29° or less.

[0020] (4) The hydrogen-evolving active photocatalyst according to any one of (1) to (3) above, further comprising a co-catalyst supported on the surface of the main catalyst particles.

[0021] (5) A method for producing a hydrogen-evolving photocatalyst containing primary catalyst particles mainly composed of a perovskite-type oxynitride, the method comprising the steps of: preparing a precursor material containing La, Ti and Al; A step of introducing the precursor material into a high-temperature field having a temperature of 500°C or higher to prepare an oxide precursor; nitriding the oxide precursor to produce a nitrided reaction product; The method wherein the La, Ti, and Al contained in the precursor raw material are such that the molar ratio w of the amount of La to the total amount of Ti and Al satisfies 1.00≦w≦1.10, and the molar ratio x of the amount of Al to the total amount of Ti and Al satisfies 0.03≦x≦0.30.

[0022] (6) The method according to (5) above, wherein the oxide precursor comprises a perovskite-type oxide, and the nitridation reaction product comprises a perovskite-type oxynitride formed by nitriding the perovskite-type oxide.

[0023] (7) The method according to (6) above, wherein the volume change rate V when the perovskite oxide is nitrided and transformed into the perovskite oxynitride is 1.82% or more and 2.45% or less.

[0024] (8) In the X-ray diffraction (XRD) pattern of the oxide precursor using Cu Kα as a radiation source, the peak intensity (I A ) and the peak intensity of the diffraction lines originating from perovskite slab-type oxides (I B ) ratio (I A / I B ) is 1.47 or more.

[0025] (9) The method according to any one of (5) to (7) above, further comprising a step of treating the nitriding reaction product with an acid.

[0026] (10) The method according to any one of (5) to (7) above, further comprising the step of supporting a promoter on the surface of the main catalyst particles. [Effects of the Invention]

[0027] According to the present invention, an Al-doped LaTiO2N-based photocatalyst exhibiting high hydrogen evolution activity and a method for producing the same are provided. [Brief explanation of the drawings]

[0028] [Figure 1] FIG. 1 is a schematic diagram of a particle manufacturing apparatus. [Figure 2] 1 shows the X-ray diffraction (XRD) patterns of the oxide precursors. [Figure 3] The molar composition ratios calculated from Rietveld analysis of the oxide precursors are shown. [Figure 4] The lattice constant of the LaTiO3 phase calculated from the Rietveld analysis of the oxide precursor is shown. [Figure 5] SEM images of the oxide precursor are shown. [Figure 6] TEM analysis results of the LaTiO3 phase are shown (Comparative Example 1). [Figure 7A] TEM images of the oxide precursor are shown. [Figure 7B] TEM images of the oxide precursor are shown. [Figure 7C] TEM images of the oxide precursor are shown. [Figure 7D] TEM images of the oxide precursor are shown. [Figure 8A] 1 shows an XRD pattern of the nitriding reaction product (after nitriding treatment). [Figure 8B] 1 shows an XRD pattern of the nitriding reaction product (after nitriding treatment). [Figure 8C] 1 shows an XRD pattern of the nitriding reaction product (after nitriding treatment). [Figure 9] The lattice constants calculated from the Rietveld analysis of the nitriding reaction products (after nitriding treatment) are shown. [Figure 10] 1 shows the diffuse reflectance spectrum of the nitriding reaction product (after nitriding treatment). [Figure 11] 1 shows the Ti2p spectrum of the nitriding reaction product (after nitriding treatment) by XPS analysis. [Figure 12] TEM images of the nitriding reaction products (after nitriding treatment) are shown. [Figure 13]The SEM image of the nitriding reaction product (after nitriding treatment) is shown. [Figure 14] 1 shows an XRD pattern of the nitriding reaction product (after acid treatment). [Figure 15] 1 shows the diffuse reflectance spectrum of the nitriding reaction product (after acid treatment). [Figure 16] The apparent quantum yield (AQY) of the promoter-supported nitriding reaction product is shown. [Figure 17] 1 shows the results of evaluating the hydrogen generation activity of the promoter-supported nitriding reaction product. [Figure 18] 1 shows the maximum hydrogen production activity rate of the promoter-supported nitriding reaction product. DETAILED DESCRIPTION OF THE INVENTION

[0029] Specific embodiments of the present invention (hereinafter referred to as "present embodiments") are described below. However, the present invention is not limited to the following embodiments, and various modifications are possible within the scope of the present invention. Furthermore, in this specification, any combination of preferred aspects can be adopted as long as technical consistency can be achieved. For example, one preferred numerical range can be combined with another preferred numerical range.

[0030] <<1. Photocatalyst>> The hydrogen generation active photocatalyst of this embodiment (hereinafter sometimes collectively referred to as "photocatalyst") is a photocatalyst represented by the formula: La W Ti 1-X Al X O Y N Z (wherein X, Y, Z, and W satisfy 1.00≦W≦1.10, 0.03≦X≦0.30, 2.00≦Y≦2.55, and 0.55≦Z≦1.00), and includes main catalyst particles containing a perovskite-type oxynitride as a main component.

[0031] The photocatalyst of this embodiment exhibits hydrogen generation activity. That is, it has the ability to generate hydrogen by decomposing water when irradiated with light. The photocatalyst also contains main catalyst particles whose main component is a perovskite-type oxynitride. Here, the main component refers to the component with the largest mass percentage among the components contained in the main catalyst particles. Typically, the content of the perovskite-type oxynitride in the main catalyst particles is 50 mass% or more.

[0032] The main catalyst particles contain an oxynitride (LaTiAl oxynitride) containing at least lanthanum (La), titanium (Ti), and aluminum (Al) as a main component. This oxynitride is a LaTiON-based compound doped with aluminum (Al) (hereinafter referred to as an "Al-doped LaTiON-based compound") and, like LaTiON undoped with Al, has a perovskite-type crystal structure. Note that LaTiON-based compounds include not only LaTiON with a stoichiometric composition, but also those with compositions that deviate from the stoichiometric composition or those in which some of the constituent elements are substituted with other elements. In other words, as long as the perovskite-type crystal structure is maintained, compounds with compositions that deviate from the stoichiometric composition or compounds in which some of the constituent elements are substituted with other elements may also be used.

[0033] The Al-doped LaTiO2N compounds, which are the main components of the main catalyst particles, correspond to LaTiO2N compounds in which some of the Ti sites have been replaced with Al. Furthermore, the crystal structure of Al-doped LaTiO2N compounds is almost identical to that of LaTiO2N compounds. Because the ionic radius of Al is close to that of Ti, the Al substitution does not cause a significant change in the crystal structure. In fact, when Al-doped LaTiO2N compounds are analyzed using X-ray diffraction (XRD), diffraction lines (diffraction peaks) appear in the XRD pattern at almost the same diffraction angle (2θ) as LaTiO2N compounds.

[0034] However, the ionic radius of Al is close to that of Ti, but slightly smaller. Also, when Al is doped into the Ti site, nitrogen ions are replaced by oxygen ions at the same time as Al doping to maintain charge balance. Therefore, the lattice constants (a, b, c) of the a-axis, b-axis, and c-axis of LaTiO2N-based compounds all decrease with Al doping.

[0035] Al-doped LaTiO2N-based compounds exhibit hydrogen evolution activity similar to LaTiO2N-based compounds. However, Al doping shortens the wavelength of the absorption edge of the diffuse reflectance absorbance (diffuse reflectance spectrum). Therefore, the absorption edge of the diffuse reflectance absorbance is shorter than the absorption edge of LaTiO2N-based compounds (600 nm).

[0036] The Al-doped LaTiO2N compound, which is the main component of this embodiment, is different from the LaTiO2N compound in that it does not have crystal defects or Ti 3+ As mentioned above, LaTiO2N without Al doping has the potential to suppress the recombination of carriers (electrons and holes) and the formation of Ti-reducing species (Ti 3+ ), which results in low hydrogen evolution activity. In contrast, Al-doped LaTiO2N compounds suppress the generation of defects and Ti-reducing species due to the Al doping.

[0037] In fact, the inventors have confirmed that substituting a portion of the Ti in LaTiO2N-based compounds with Al reduces the full width at half maximum (FWHM) of the XRD diffraction peak and increases the crystallite size. They have also synthesized a photocatalyst primarily composed of an Al-doped LaTiO2N compound and confirmed that this photocatalyst exhibits high hydrogen production activity. While the reason for the improved crystallinity due to Al doping should not be interpreted in a restrictive manner, they speculate that this is because, during photocatalyst production, an oxide precursor containing a large amount of perovskite-type crystals is synthesized due to Al doping, and the LaTiO2N-based compound is formed during nitriding while maintaining its crystalline structure.

[0038] The composition formula of the Al-doped LaTiO2N-based compound (perovskite-type oxynitride) that is the main component of this embodiment is (La W Ti 1-X Al X O Y N Z ), the amount of lanthanum (La) W and the amount of aluminum (Al) X satisfy 1.00≦W≦1.10 and 0.03≦X≦0.30. In this way, by doping with a predetermined amount of Al and making the amount of La equal to or slightly in excess of the stoichiometric composition, it is possible to reliably improve the crystallinity and hydrogen evolution activity of the main component compound (Al-doped LaTiO2N-based compound).

[0039] If W is less than 1.00, the composition of the main component compound becomes La-deficient, resulting in reduced crystallinity and reduced hydrogen generation activity. If W exceeds 1.10, the composition becomes La-excessive, resulting in reduced crystallinity and reduced hydrogen generation activity. If X is less than 0.03, the effect of improving crystallinity due to Al substitution becomes insufficient, resulting in reduced hydrogen generation activity. If X exceeds 0.30, the wavelength of the absorption edge in the diffuse reflectance spectrum becomes excessively short. As a result, the amount of light absorbed decreases, resulting in reduced hydrogen generation activity. W preferably satisfies 1.00≦W≦1.05. X preferably satisfies 0.03≦X≦0.30. The composition of the main component particles is almost the same as the raw material blend composition used in producing the photocatalyst.

[0040] In addition, the above-mentioned composition formula (La W Ti 1-X Al X O Y N ZIn ( ), the amount Y of oxygen (O) and the amount Z of nitrogen (N) satisfy 2.00 ≤ Y ≤ 2.55 and 0.55 ≤ Z ≤ 1.00. In the stoichiometric composition LaTiO₂N-based compound, Y = 2.00 and Z = 1.00. Therefore, in the region near Y = 2.00 and Z = 1.00, the Al-doped LaTiO₂N-based compound exists stably. Also, the Al-doping amount tends to affect the amount Y of oxygen (O) and the amount Z of nitrogen (N). As the Al-doping amount increases, Y increases and Z decreases. Y and Z may satisfy 2.00 ≤ Y ≤ 2.30 and 0.75 ≤ Z ≤ 1.00. Or, they may satisfy 2.30 < Y ≤ 2.55 and 0.55 ≤ Z < 0.75.

[0041] The Al-doped LaTiO₂N-based compound (perovskite-type oxynitride), which is the main component of the main catalyst particles, preferably has a lattice constant (a) of the a-axis of 5.5343 Å or more and 5.5757 Å or less, a lattice constant (b) of the b-axis of 5.5129 Å or more and 5.6013 Å or less, and a lattice constant (c) of the c-axis of 7.7980 Å or more and 7.8992 Å or less. More preferably, the lattice constant (a) of the a-axis is 5.5343 Å or more and 5.5720 Å or less, the lattice constant (b) of the b-axis is 5.5129 Å or more and 5.6000 Å or less, and the lattice constant (c) of the c-axis is 7.7980 Å or more and 7.8992 Å or less. In the present embodiment, by controlling the composition and production method of the main catalyst particles, the lattice constant can be controlled. That is, by adjusting W, X, Y in the composition formula (La W Ti 1-X Al X O Y N Z ) within a suitable range and adopting a method of introducing the precursor raw material into a high-temperature field for a short time, the lattice constants (a, b, c) can be controlled to fall within the above-described suitable range.

[0042] The main catalyst particles may contain other components as long as they contain an Al-doped LaTiO2N-based compound (perovskite-type oxynitride) as the main component. Examples of such components include oxides, nitrides, and oxynitrides of La, Ti, and / or Al. However, to maximize the excellent catalytic performance, a high content of the Al-doped LaTiO2N-based compound is desirable. The main catalyst particles preferably contain 70% by mass or more, more preferably 90% by mass or more, and even more preferably 99% by mass or more of the Al-doped LaTiO2N-based compound (perovskite-type oxynitride). The main component particles may contain only the Al-doped LaTiO2N-based compound (perovskite-type oxynitride), i.e., be single-phase.

[0043] The photocatalyst of this embodiment preferably further includes a co-catalyst supported on the surface of the main catalyst particle. The co-catalysts are classified into hydrogen generation co-catalysts, oxygen generation co-catalysts, etc. The hydrogen generation co-catalyst mainly functions to promote the reduction of hydrogen ions by electrons, while the oxygen generation co-catalyst mainly functions to promote the oxidation of water by holes. In particular, by supporting the hydrogen generation co-catalyst on the surface of the photocatalyst made of semiconductor particles, it is possible to dramatically improve water splitting performance.

[0044] The promoter material may be selected from known metals, compounds, and other materials. Examples of such materials include at least one metal selected from the group consisting of platinum (Pt), gold (Au), silver (Ag), iridium (Ir), rhodium (Rh), ruthenium (Ru), cobalt (Co), nickel (Ni), chromium (Cr), and iron (Fe), or one or more metals selected from the group consisting of oxide particles such as ruthenium oxide and nickel oxide, or a mixture of these metal particles or oxide particles, or a composite hydroxide or composite oxide containing rhodium (Rh) and chromium (Cr). More preferably, particles of ruthenium or iridium oxide, or a composite hydroxide or composite oxide containing platinum, iridium, and chromium, can be used.

[0045] Particularly suitable promoters are ruthenium and iridium oxide (Ru / IrOx ) Ru and IrO x When each of these is supported alone, it does not exhibit a very high hydrogen production activity. However, when both are supported together, it exhibits a higher hydrogen production activity.

[0046] The total amount of the promoters supported is preferably 0.01% by mass or more and 7% by mass or less relative to the main catalyst particles. By increasing the supported amount to a certain extent, it becomes possible to fully exert the effect of promoting reduction and / or oxidation by the promoters. Furthermore, by limiting the supported amount to a certain extent, it is possible to prevent a shortage of active sites on the surface of the main catalyst particles.

[0047] The photocatalyst of this embodiment has the advantage that the Al-doped LaTiON-based compound (perovskite-type oxynitride) as its main component has high crystallinity, and therefore has excellent hydrogen generation activity. Although not limited thereto, in the X-ray diffraction (XRD) pattern, the peak full width at half maximum (FWHM) of the (002) diffraction line derived from the perovskite-type oxynitride is preferably 0.29° or less, more preferably 0.27° or less. Here, the XRD pattern is a pattern obtained by XRD analysis using a Cu Kα radiation source. The (002) diffraction line is the most intense of the diffraction lines derived from the perovskite-type oxynitride and appears at a diffraction angle 2θ of approximately 32°. Furthermore, the crystallite diameter of the Al-doped LaTiON-based compound (perovskite-type oxynitride) calculated using the Scherrer equation is preferably 29 nm or more, more preferably 31 nm or more.

[0048] The photocatalyst of this embodiment can be used in the hydrogen production reaction. When light is irradiated onto the photocatalyst in contact with water, protons (H + ) to generate hydrogen. Therefore, the photocatalyst contributes to the creation of clean energy. In addition, the photocatalyst of this embodiment can be used as an environmental purification material. When the photocatalyst is irradiated with light, a strong oxidizing power is generated on its surface, which can decompose and remove harmful substances such as organic compounds and bacteria that come into contact with it.

[0049] <<2. Photocatalyst manufacturing method>> The manufacturing method of this embodiment can produce a hydrogen-generating photocatalyst containing main catalyst particles containing perovskite-type oxynitride. This manufacturing method includes the following steps: preparing a precursor material containing La, Ti, and Al (a raw material preparation step); introducing the precursor material into a high-temperature environment having a temperature of 500°C or higher to produce an oxide precursor (an oxide synthesis step); and nitriding the oxide precursor to produce a nitriding reaction product (a nitriding step). The La, Ti, and Al contained in the precursor material satisfy the following conditions: the molar ratio w of La to the total amount of Ti and Al satisfies 1.00≦w≦1.10, and the molar ratio x of Al to the total amount of Ti and Al satisfies 0.03≦x≦0.30. This manufacturing method may further include, as necessary, a step of acid-treating the nitriding reaction product or a step of supporting a co-catalyst on the surface of the main catalyst particles. Details of each step are described below.

[0050] <Raw material preparation process> In the raw material preparation step, a precursor raw material containing lanthanum (La), titanium (Ti), and aluminum (Al) is prepared. The form of the precursor raw material is not limited as long as it contains La, Ti, and Al. For example, it may be a solution in which the La source, Ti source, and Al source are dissolved in a solvent, or a suspension in which the La source, Ti source, and Al source are dispersed in a solvent. Alternatively, it may be a powdered solid containing La, Ti, and Al. In the case of a solid, La, Ti, and Al may be contained in the form of a complex compound or each may be contained in the form of a mixture. However, a solution in which the La source, Ti source, and Al source are dissolved in a solvent is preferred. This allows for uniform dispersion at the atomic level.

[0051] When the precursor raw material is a solution, examples of the La source used include La compounds such as lanthanum acetate, lanthanum nitrate, and lanthanum isobutyrate. Examples of the Ti source include Ti compounds such as titanium tetraisopropoxide and tetraethoxytitanium. Examples of the Al source include Al compounds such as aluminum nitrate and triisopropoxyaluminum.

[0052] The solvent is not limited as long as it can dissolve the raw materials. However, an organic solvent is desirable because the organic solvent can be used as a combustion source to promote combustion in the subsequent oxide synthesis process. Examples of such solvents include, but are not limited to, ethanol, tetrahydrofuran, benzene, and / or acetone. These may be single solvents or mixed solvents.

[0053] When preparing the precursor raw materials, the raw materials (La source, Ti source, Al source) are blended so that the molar ratio w of the amount of La to the total amount of Ti and Al (w is the La / (Ti+Al) blending molar ratio) satisfies 1.00≦w≦1.10, and the molar ratio x of the amount of Al to the total amount of Ti and Al (x is the Al / (Ti+Al) blending molar ratio) satisfies 0.03≦x≦0.30. This enables the synthesis of a photocatalyst with high crystallinity and hydrogen generation activity.

[0054] As will be described later, in the manufacturing method of this embodiment, an oxide having a perovskite crystal structure (hereinafter referred to as "perovskite oxide") or an oxide having a perovskite slab (layered perovskite) crystal structure (hereinafter referred to as "perovskite slab oxide") is produced in the subsequent oxide synthesis step. In some cases, a different phase may also be produced. However, in order to ultimately obtain a highly crystalline oxynitride (Al-doped LaTiO2N-based compound), it is desirable that the proportion of perovskite oxide contained in the oxide precursor is high. Furthermore, it is desirable that the amount of different phase is small.

[0055] In this regard, by controlling the molar ratio (w, x) of the La content and the Al content in the precursor material within the above-mentioned ranges (1.00≦w≦1.10, 0.03≦x≦0.30), it is possible to increase the proportion of perovskite-type oxide and suppress the formation of heterophases, thereby enabling the synthesis of highly crystalline oxynitride (Al-doped LaTiO2N-based compounds). From the viewpoint of improving the crystallinity of the oxynitride, it is preferable that w satisfies 1.00≦w≦1.05. It is preferable that x satisfies 0.03≦x≦0.30.

[0056] On the other hand, when the molar ratio w of La is less than 1.00, perovskite slab-type oxide is preferentially produced in the oxide synthesis process. The proportion of perovskite-type oxide produced is reduced, resulting in reduced crystallinity of the oxynitride (Al-doped LaTiON-based compound) formed in the nitriding process. When the molar ratio w exceeds 1.10, heterophases such as LaTiO are formed in the oxide synthesis process, inhibiting the production of highly crystalline oxynitride. Furthermore, the incorporation of a large amount of La ions, which have a relatively large ionic radius, can distort the crystals of the final Al-doped LaTiON-based compound, potentially resulting in reduced crystallinity. When the molar ratio x of Al is less than 0.03, the production of perovskite-type oxide in the oxide synthesis process is suppressed, inhibiting the production of highly crystalline oxynitride. When the molar ratio x exceeds 0.30, the absorption edge in the diffuse reflectance spectrum shifts excessively toward shorter wavelengths, resulting in reduced hydrogen evolution activity.

[0057] Furthermore, by controlling the molar ratio (w, x) of the La and Al contents in the precursor materials within the above-mentioned ranges (1.00≦w≦1.10, 0.03≦x≦0.30), the proportion of perovskite-type oxide is increased. This suppresses volumetric change during the nitriding process described below, thereby preventing the formation of pores, as observed in Comparative Example 2 in Figure 13. As a result, a highly crystalline oxynitride can be obtained. Non-Patent Document 2 shows that in the reaction from the perovskite slab oxide La2Ti2O7 to LaTiON, the reduction of anions causes a volumetric shrinkage of approximately 14% per Ti atom, resulting in the formation of pores. However, the inventors' investigations have shown that the volume actually expands. Specifically, the volume change rate V calculated from the change in lattice constant during the nitriding reaction from the perovskite oxide LaTiO3 phase to the LaTiON phase is 1.25% to 2.45%. For this reason, it is necessary to control the molar ratio (w, x) of the La amount and the Al amount within the above-mentioned range.

[0058] <Oxide synthesis process> In the oxide synthesis process, precursor materials are introduced into a high-temperature field having a temperature of 500°C or higher to produce an oxide precursor (LaTiAl oxide). The manufacturing method of this embodiment is characterized in that the oxide precursor is produced by introducing the precursor materials into a high-temperature field in a short time, rapidly heating, and then rapidly cooling. By introducing the precursor materials into a high-temperature field, the reaction between La, Ti, and Al in the precursor materials progresses. At this time, a metastable structure is formed by rapid heating and rapid cooling. Specifically, perovskite-type oxides and perovskite slab-type oxides are produced. Examples of perovskite-type oxides include LaTiO3, La 0.66 TiO 2.993 Examples of perovskite slab oxides include La2Ti2O7 and compounds in which Al is dissolved. In some cases, a different phase, such as La2TiO5, may be produced. Therefore, the oxide precursor obtained in the oxide synthesis process is a perovskite oxide (LaTiO3 in which Al is dissolved, or La 0.66 TiO 2.993 etc.), perovskite slab-type oxides (such as La2Ti2O7 with Al as a solid solution), and other heterophases (such as La2TiO5).

[0059] When the oxide precursor is nitrided in the subsequent nitriding step, any compound contained in the perovskite oxide and the perovskite slab oxide (LaTiO3, La 0.66 TiO 2.993 , La2Ti2O7, etc.) also undergo a topotactic reaction and are transformed into Al-doped LaTiON-based compounds (perovskite-type oxynitrides). Here, the topotactic reaction is a reaction in which elements are substituted while maintaining the structure, and in this embodiment, it refers to a reaction in which some of the oxygen atoms contained in the oxide precursor are replaced with nitrogen atoms.

[0060] Thus, although both perovskite-type oxides and perovskite slab-type oxides are transformed into Al-doped LaTiO2N-based compounds by topotactic reactions, the effects on their crystallinity differ. That is, compounds derived from perovskite-type oxides have relatively high crystallinity, while compounds derived from perovskite slab-type oxides have relatively low crystallinity. Therefore, a high proportion of perovskite-type oxide is desirable for obtaining highly crystalline Al-doped LaTiO2N-based compounds.

[0061] For example, in the X-ray diffraction (XRD) pattern of the oxide precursor, the peak intensity (I A ) and the peak intensity of the diffraction lines originating from perovskite slab-type oxides (I B ) ratio (I A / I B ) is preferably 1.47 or more, more preferably 2.53 or more, and even more preferably 5.24 or more. B is zero (0), that is, the ratio I A / I B can be infinite (∞). A / I B The higher the value, the more crystalline the Al-doped LaTiO2N-based compound can be synthesized. A is the peak intensity of the strongest peak derived from perovskite oxide that appears around 2θ = 32° in the XRD pattern, and I B is the peak intensity of the strongest peak originating from the perovskite slab-type oxide, which appears around 2θ = 30°. The 2θ value is the diffraction angle in XRD analysis using Cu Kα as the radiation source.

[0062] On the other hand, heterophases other than perovskite oxides and perovskite slab oxides (e.g., La2Ti5) inhibit the topotactic reaction during the nitriding process, making it difficult to synthesize highly crystalline Al-doped LaTiO2N compounds if the oxide precursor contains a large amount of heterophases.

[0063] Thus, to synthesize highly crystalline Al-doped LaTiON-based compounds (perovskite-type oxynitrides), a high proportion of perovskite-type oxide in the oxide precursor is desirable. Furthermore, a low amount of heterophase is desirable. In this regard, the desired oxide precursor can be obtained by controlling the composition (w, x) of the precursor raw material in the raw material preparation process and employing a technique of rapid heating and rapid cooling of the precursor raw material in the oxide synthesis process. This is believed to be because rapid heating and rapid cooling maintain the metastable structure of the synthesized oxide precursor and effectively promote Al solid solution (doping). In contrast, the conventional solid-phase method, in which the workpiece is gradually heated and slowly cooled in an atmospheric furnace, preferentially produces perovskite slab-type oxide (Non-Patent Document 3), making it difficult to obtain the desired oxide precursor raw material and highly crystalline Al-doped LaTiON-based compounds.

[0064] The method for synthesizing an oxide precursor, i.e., the method for introducing a precursor material into a high-temperature field having a temperature of 500°C or higher, is not particularly limited as long as the oxide precursor is rapidly heated and rapidly cooled. Examples include a method of spraying the precursor material into a flame (flame spraying), a method of rapidly introducing the precursor material into a furnace maintained at a high temperature and then rapidly removing it, or a method of introducing the precursor material into high-temperature plasma. The temperature of the high-temperature field is preferably 550°C or higher, more preferably 1000°C or higher. There is no particular upper limit to the temperature, but from the viewpoint of reducing energy consumption, a temperature of, for example, 4000°C or lower is preferred. Furthermore, the high-temperature field is preferably an oxidizing atmosphere.

[0065] An example of a method for spraying precursor materials into a flame (flame spraying) is described below. When synthesizing oxide precursors using the flame spraying method, the precursor materials are supplied into the flame in the form of an aerosol. Specifically, the precursor material aerosol (fine droplets) is transported using a carrier gas such as oxygen and introduced into the flame.

[0066] The aerosol-forming means is not particularly limited and may be adjusted depending on the state of the raw material. For example, when the precursor raw material is a liquid (such as a solution), the aerosol can be formed by spraying the liquid precursor raw material toward a carrier gas using various atomizers such as a centrifugal atomizer or a two-fluid nozzle. Microdroplets can also be formed by irradiating the liquid precursor raw material with ultrasonic waves. On the other hand, when the precursor raw material is in powder form, the aerosol is formed by dispersing the powder precursor raw material in an airflow. Whether the precursor raw material is in liquid or powder form, the size of the formed aerosol is, for example, 1 μm or more and 100 μm or less.

[0067] When using a liquid precursor material, the liquid material is sprayed into a carrier gas such as oxygen to form droplets (aerosol), and the resulting droplets are sprayed into the flame. The organic solvent contained in the sprayed droplets is burned and decomposed in the flame. The generated combustion heat contributes to the decomposition of the La source, Ti source, and Al source in the precursor material. The decomposed La source, Ti source, and Al source are cooled in the tail flame of the flame, causing the oxide precursor to precipitate.

[0068] The conditions for forming the flame are not particularly limited. For example, a flame can be formed using a gas mixture containing a flammable liquid such as hydrocarbon and oxygen. In this case, the heat power can be controlled by adjusting the flow rates of the flammable liquid and oxygen and their ratio (flow rate ratio). By controlling the heat power, the particle size of the resulting oxide precursor can be adjusted. For example, when a flame is formed using a gas mixture containing oxygen and propane, the volume ratio of the flow rates of propane and oxygen (burner) in the gas mixture is preferably 5 to 8 oxygen parts per 1 part of propane, and the flow rate of propane is preferably in the range of 0.5 to 2 L / min.

[0069] In this way, the oxide precursor is synthesized by the flame spray method. The synthesized oxide precursor may be recovered using a recovery means such as a filter.

[0070] An example of a particle production apparatus suitable for synthesis by the flame spray method is shown schematically in Figure 1. Particle production apparatus 10 has a storage section 11 for containing the raw material solution (liquid precursor raw material), a two-fluid nozzle 12 for forming droplets of the raw material and forming a flame at the same time, and a reaction tube 13 connected to a filter 14 for collecting the formed oxide precursor.

[0071] A raw material solution and a carrier gas are supplied to the two-fluid nozzle 12 to form an aerosol (aerosol formation step). For example, oxygen and a hydrocarbon are supplied to the two-fluid nozzle 12, which together form a flame reaction field. The formed aerosol can be supplied into the flame and subjected to heat treatment (heat treatment step). A cooling water pipe 131 is arranged around the reaction tube 13, and cooling water is circulated. The LaTiAl oxide particles introduced into the reaction tube 13 are collected by a filter 14 such as a bag filter. An ejector 15 may be provided at the most downstream side to adjust the amount of carrier gas supplied.

[0072] <Nitriding process> In the nitriding step, the oxide precursor is nitrided to produce a nitriding reaction product containing an Al-doped LaTiO2N-based compound. That is, the oxide precursor contains a perovskite-type oxide, and the nitriding reaction product contains a perovskite-type oxynitride formed by nitriding this perovskite-type oxide. Specifically, for example, the oxide precursor may be heat-treated at a temperature of 700°C or higher in a flow of ammonia. In this case, the ammonia gas passing through becomes radical species such as NH2 and NH2 and is supplied to the workpiece, where the reaction proceeds. For example, La w Ti 1-x Al x O y When an oxide precursor having the composition is subjected to nitriding treatment at 700° C. or higher, the reaction of the following formula (A) proceeds to the right.

[0073] [ka]

[0074] The heat treatment holding time is not limited as long as the desired photocatalyst is obtained. However, from the viewpoint of promoting the production of the nitriding reaction product and improving the crystallinity, a relatively long holding time is desirable. On the other hand, an excessively long holding time increases the cycle time, leading to increased production costs. The heat treatment holding time is preferably 6 hours or more, and more preferably 8 hours to 15 hours. Note that lanthanum oxide, a heterophase, may be produced in the nitriding reaction product. The produced lanthanum oxide can be removed in the subsequent acid treatment step.

[0075] Incidentally, when an oxide precursor is nitrided and converted into a nitriding reaction product, a volume change occurs. Specifically, when the perovskite oxide (Al-doped LaTiO) contained in the oxide precursor is converted into a perovskite oxynitride (Al-doped LaTiON-based compound), the lattice elongates. As a result, the unit lattice expands and the volume increases. The volume change rate during nitriding is, for example, 1.25% to 2.45%, and preferably 1.82% to 2.45%.

[0076] The volume change rate is calculated from the unit lattice volume (V1) of the perovskite oxide and the unit lattice volume (V2) of the perovskite oxynitride according to the following formula (1). Furthermore, V1 and V2 are calculated according to the following formula (2) or (3) using the lattice constants (a1, b1, c1) of the perovskite oxide and the lattice constants (a2, b2, c2) of the perovskite oxynitride, respectively.

[0077]

number

[0078] <Acid treatment process> If necessary, an acid treatment step may be provided in which the nitriding reaction product is acid-treated (acid washed). During the nitriding step, heterogeneous phases such as lanthanum oxide may be generated and remain in the nitriding reaction product. If such heterogeneous phases (lanthanum oxide, etc.) remain, there is a risk that the bonding strength of the co-catalyst may be reduced if it is supported. Therefore, it is desirable to acid treat the nitriding reaction product to remove the heterogeneous phases. However, the acid treatment step is not essential. If the amount of heterogeneous phases is zero or negligible, the acid treatment step may not be provided.

[0079] In the acid treatment, the nitriding reaction product obtained in the nitriding step is brought into contact with an acidic aqueous solution. Specific examples include immersing the nitriding reaction product in the acidic aqueous solution or spraying the acidic aqueous solution onto the nitriding reaction product. The acidic aqueous solution is not particularly limited as long as it can dissolve and remove heterogeneous phases (such as lanthanum oxide). Examples include aqueous solutions containing hydrochloric acid (HCl), aqua regia, and / or sulfonic acid. The concentration of the aqueous solution is also not limited as long as it can dissolve and remove heterogeneous phases. For example, when the aqueous solution is dilute hydrochloric acid, the dilute hydrochloric acid concentration is preferably 0.1 M or more. The acidic aqueous solution may be an aqueous solution at room temperature or a heated aqueous solution.

[0080] After contact with the acidic aqueous solution, the nitriding reaction product is preferably washed with water to remove the acid component, and the nitriding reaction product after washing with water is preferably dried.

[0081] In this way, the main catalyst particles of the photocatalyst can be produced. That is, if the acid treatment is not performed, the nitriding reaction product after the nitriding treatment can be used as the main catalyst particles. If the acid treatment is performed, the nitriding reaction product after the acid treatment can be used as the main catalyst particles. The nitriding reaction product contains a highly crystalline Al-doped LaTiO2N-based compound as its main component.

[0082] <Co-catalyst supporting step> If necessary, a step of supporting a co-catalyst on the surface of the main catalyst particles (co-catalyst supporting step) may be provided. The co-catalyst supporting step may be performed at any timing after the nitriding step. The co-catalyst may be supported on the nitriding reaction product (main catalyst particles) after the nitriding treatment, or the co-catalyst may be supported on the nitriding reaction product (main catalyst particles) after the acid treatment.

[0083] The co-catalyst may be supported by a known method such as impregnation, microwave heating, or photoelectrodeposition. The impregnation method is a method in which a solution containing a co-catalyst raw material such as a metal salt is mixed with main catalyst particles, followed by drying and heat treatment to support the co-catalyst. The microwave heating method is a method in which a solid-liquid mixture containing a solvent, a co-catalyst source, and main catalyst particles is irradiated with microwaves to heat and support the co-catalyst. The photoelectrodeposition method is a method in which a solution containing a co-catalyst raw material is stirred with main catalyst particles, and then irradiated with light to support the co-catalyst.

[0084] Ruthenium and iridium oxide (Ru / IrO x In the case of supporting a co-catalyst consisting of iridium oxide, for example, an Ir raw material and main catalyst particles are dispersed in water, microwave-heated, and an aqueous solution containing a Ru raw material and an iridium oxide-supported catalyst is mixed, heated, and calcined in a hydrogen stream. Examples of Ir raw materials include, but are not limited to, iridium trichloride, iridium tetrachloride, and hydrates thereof. Examples of Ru raw materials include, but are not limited to, ruthenium chloride and hydrates thereof. Calcination can be carried out in a reducing atmosphere such as hydrogen at a temperature of 200°C to 300°C.

[0085] In this way, the photocatalyst of this embodiment can be obtained. The photocatalyst of this embodiment is characterized by high crystallinity and high hydrogen generation activity. Therefore, it is particularly useful for producing hydrogen and decomposing and removing harmful substances.

[0086] Photocatalysts with such characteristics have not been known up to now. Conventionally known LaTiO2N-based compounds have crystal defects and Ti reduction species (Ti 3+), there is a problem that the hydrogen generation activity is poor. Furthermore, there is no known method for effectively doping an oxynitride with a different element to improve its activity. That is, although a flux method for doping an oxide with a different element is known, when this method is applied to an oxynitride, the doping element forms an oxide solid solution, which reduces the catalytic activity. Therefore, it is difficult to obtain the photocatalyst of this embodiment using conventional methods.

[0087] <<3. Hydrogen production methods>> In the hydrogen production method of this embodiment, a suspension containing the above-described photocatalyst, water, and methanol is irradiated with light, thereby decomposing water and generating hydrogen. Specifically, the photocatalyst and an aqueous methanol solution are mixed to prepare a suspension. The resulting suspension is then irradiated with light.

[0088] The light to be irradiated preferably has sufficient intensity in the wavelength range of 400 nm to 600 nm. Light irradiation causes a water decomposition reaction on the surface of the photocatalyst in the suspension, generating hydrogen. Light irradiation may be performed by placing the suspension in a closed circulation system. The suspension may also be stirred during irradiation. [Example]

[0089] The present invention will be described in more detail with reference to the following examples and comparative examples, but the present invention is not limited to these examples.

[0090] (1) Preparation of photocatalyst [Example 1] In Example 1, primary catalyst particles containing an Al-doped LaTiON compound (perovskite-type oxynitride) were synthesized, and a co-catalyst was supported on the primary catalyst particles to prepare a photocatalyst. The specific preparation procedure was as follows.

[0091] <Raw material preparation process> Lanthanum nitrate hexahydrate (Kanto Chemical Co., Ltd., 4N), titanium tetraisopropoxide (Fujifilm Wako Pure Chemical Industries, Ltd., purity 95% or higher), and aluminum nitrate nonahydrate (Kanto Chemical Co., Ltd., special grade, purity 98% or higher) were used as raw materials, and these raw materials were added to and dissolved in ethanol as a solvent to prepare a mixed raw material solution. The amounts of raw materials added were adjusted so that the La concentration in the mixed raw material solution was 0.100M, the Ti concentration was 0.095M, and the Al concentration was 0.005M.

[0092] <Oxide synthesis process> Next, the obtained mixed raw material solution was sprayed into a flame to synthesize an oxide precursor (LaTiAl oxide). The particle manufacturing device shown in Figure 1 was used for the synthesis. The mixed raw material solution was sprayed under the following conditions: LP gas flow rate: 1.2 NL / min, O2 spray gas flow rate: 6 NL / min, O2 carrier gas flow rate: 15 NL / min, and raw material supply rate: 3 g / min. The oxide precursor composition calculated from the raw material blend amounts was LaTi 0.95 Al 0.05 O y (where y is a positive real number that makes the oxide stable).

[0093] <Nitriding process> The resulting oxide precursor (LaTiAl oxide) was nitrided to produce a nitriding reaction product. Specifically, the oxide precursor was transferred to a boat and placed in a horizontal tubular furnace. Ammonia was passed through at a flow rate of 200 mL / min, and the temperature of the placed oxide precursor was increased to 1000°C at a rate of 10°C / min. The temperature was maintained at 1000°C for 15 hours, after which the precursor was cooled (furnace-cooled). The cooled sample was removed and collected as the nitriding reaction product.

[0094] <Acid treatment process> The resulting nitridation product was acid-treated to produce primary catalyst particles composed of perovskite-type oxynitride. Specifically, the nitridation product was added to and suspended in a 0.1 M HCl aqueous solution, stirred for 30 minutes, and then collected by filtration. The collected material was then added to and suspended in 300 mL of water, stirred for an additional 15 minutes, and then collected by filtration. This water-washing procedure was repeated twice. The collected sample was then dried at 40°C under a vacuum atmosphere.

[0095] <Co-catalyst supporting step> The obtained main catalyst particles were loaded with Ru / IrOx as a promoter. Specifically, the main catalyst particles and iridium (III) chloride n-hydrate were added to 15 mL of water to obtain an Ir-containing mixed solution. The amount of iridium (Ir) added was adjusted so that the amount of Ir relative to the main catalyst particles was 1 mass%. The obtained Ir-containing mixed solution was then heated to 150°C and irradiated with microwaves for 10 minutes. The mixed solution after microwave irradiation was filtered to recover the product, and the recovered product was dried at 40°C under a vacuum atmosphere.

[0096] Next, the recovered material and ruthenium chloride trihydrate were added to water to obtain a Ru-containing mixed solution. At this time, the amount of ruthenium (Ru) added was adjusted so that the amount of Ru relative to the main catalyst particles was 1 mass %. The Ru-containing mixed solution was then heated to evaporate the water, and a co-catalyst-supported precursor was obtained.

[0097] The obtained co-catalyst-supported precursor was transferred to a boat and placed in a horizontal tubular furnace. A mixed gas of hydrogen (flow rate: 20 mL / min) and nitrogen (flow rate: 200 mL / min) was then passed through the boat, and the temperature was raised to 250°C at a rate of 10°C / min. The temperature was maintained for 1 hour, and then the boat was cooled (furnace-cooled). After cooling, the sample was removed and recovered. In this way, the photocatalyst sample of Example 1 was prepared.

[0098] [Example 2] In Example 2, during the raw material preparation step, the amounts of raw materials added were adjusted so that the La concentration in the mixed raw material solution was 0.105M, the Ti concentration was 0.095M, and the Al concentration was 0.005M. Except for this, the mixed raw material solution (precursor raw material) was prepared in the same manner as in Example 1. In addition, the oxide synthesis step, nitriding step, acid treatment step, and co-catalyst supporting step were carried out in the same manner as in Example 1. The oxide precursor composition calculated from the raw material blend amounts was La 1.05 Ti 0.95 Al 0.05 O y It was.

[0099] [Example 3] In Example 3, during the raw material preparation step, the amounts of raw materials added were adjusted so that the La concentration in the mixed raw material solution was 0.110 M, the Ti concentration was 0.095 M, and the Al concentration was 0.005 M. Except for this, the mixed raw material solution (precursor raw material) was prepared in the same manner as in Example 1. In addition, the oxide synthesis step, nitriding step, acid treatment step, and co-catalyst supporting step were carried out in the same manner as in Example 1. The oxide precursor composition calculated from the raw material blend amounts was La 1.10 Ti 0.95 Al 0.05 O y It was.

[0100] [Example 4] In Example 4, during the raw material preparation step, the amounts of raw materials added were adjusted so that the La concentration in the mixed raw material solution was 0.105M, the Ti concentration was 0.090M, and the Al concentration was 0.010M. Except for this, the mixed raw material solution (precursor raw material) was prepared in the same manner as in Example 1. The oxide synthesis step, nitriding step, acid treatment step, and co-catalyst supporting step were carried out in the same manner as in Example 1. The oxide precursor composition calculated from the raw material blend amounts was La 1.05 Ti 0.90 Al 0.10 O y It was.

[0101] [Example 5] In Example 5, during the raw material preparation step, the amounts of raw materials added were adjusted so that the La concentration in the mixed raw material solution was 0.105M, the Ti concentration was 0.080M, and the Al concentration was 0.020M. Except for this, the mixed raw material solution (precursor raw material) was prepared in the same manner as in Example 1. The oxide synthesis step, nitriding step, acid treatment step, and co-catalyst supporting step were carried out in the same manner as in Example 1. The oxide precursor composition calculated from the raw material blending amounts was La. 1.05 Ti 0.80 Al 0.20 O y It was.

[0102] [Example 6] In Example 6, during the raw material preparation step, the amounts of raw materials added were adjusted so that the La concentration in the mixed raw material solution was 0.105M, the Ti concentration was 0.070M, and the Al concentration was 0.030M. Except for this, the mixed raw material solution (precursor raw material) was prepared in the same manner as in Example 1. The oxide synthesis step, nitriding step, acid treatment step, and co-catalyst supporting step were carried out in the same manner as in Example 1. The oxide precursor composition calculated from the raw material blending amounts was La. 1.05 Ti 0.70 Al 0.30 It was Oy.

[0103] [Comparative Example 1] In Comparative Example 1, no Al raw material was added during the raw material preparation step. The amounts of raw materials added were adjusted so that the La concentration in the mixed raw material solution was 0.100M and the Ti concentration was 0.100M. Except for this, the mixed raw material solution (precursor raw material) was prepared in the same manner as in Example 1. The oxide synthesis step, nitriding step, acid treatment step, and co-catalyst loading step were performed in the same manner as in Example 1. The oxide precursor composition calculated from the raw material blend amounts was LaTiO y It was.

[0104] Comparative Example 2 In Comparative Example 2, no Al raw material was added during the raw material preparation step. Also, the raw material addition amounts were adjusted so that the La concentration in the mixed raw material solution was 0.105 M and the Ti concentration was 0.100 M. Other than that, the mixed raw material solution (precursor raw material) was prepared in the same procedure as in Example 1. Also, the same operations as in Example 1 were performed in the oxide synthesis step, nitridation step, acid treatment step, and co-catalyst loading step. The oxide precursor composition determined from the raw material formulation amounts was La 1.05 TiO y Thereof.

[0105] [Comparative Example 3] In Comparative Example 3, no Al raw material was added during the raw material preparation step. Also, the raw material addition amounts were adjusted so that the La concentration in the mixed raw material solution was 0.110 M and the Ti concentration was 0.100 M. Other than that, the mixed raw material solution (precursor raw material) was prepared in the same procedure as in Example 1. Also, the same operations as in Example 1 were performed in the oxide synthesis step, nitridation step, acid treatment step, and co-catalyst loading step. The oxide precursor composition determined from the raw material formulation amounts was La 1.10 TiO y Thereof.

[0106] [Comparative Example 4] In Comparative Example 4, during the raw material preparation step, the raw material addition amounts were adjusted so that the La concentration in the mixed raw material solution was 0.105 M, the Ti concentration was 0.050 M, and the Al concentration was 0.050 M. Other than that, the mixed raw material solution (precursor raw material) was prepared in the same procedure as in Example 1. Also, the same operations as in Example 1 were performed in the oxide synthesis step, nitridation step, acid treatment step, and co-catalyst loading step. The oxide precursor composition determined from the raw material formulation amounts was La 1.05 Ti 0.50 Al 0.50 Oy Thereof.

[0107] (2) Evaluation of photocatalyst Using the photocatalysts obtained in Examples 1 to 6 and Comparative Examples 1 to 4 and the intermediate products (oxide precursors, nitridation reaction products) during their production as samples, evaluations of various properties were conducted as follows.

[0108] [XRD Analysis] The samples were analyzed by powder X-ray diffraction (XRD) to identify the crystalline phase. XRD analysis was performed under the following conditions:

[0109] - X-ray diffractometer: Rigaku Corporation, Mini Flex - Source: CuKα -Tube voltage: 30kV -Tube current: 10mA - Scan speed: 1.25° / min - Scan range (2θ): 15~60°

[0110] <Rietveld analysis> The XRD patterns of the samples were analyzed by Rietveld analysis to examine the details of the crystalline phase. The XRD patterns of the samples were obtained under the following conditions:

[0111] -X-ray diffraction device: Bruker AXS,D2 PHASER - Source: CuKα -Tube voltage: 40kV -Tube current: 30mA - Scan range (2θ): 10~100°

[0112] Next, Rietveld analysis of the XRD pattern was performed using analysis software (Total Pattern Analysis Solutions (TOPAS) by Bruker AXS) to determine the proportion of crystalline phases contained in the sample and the lattice constant.

[0113] <Diffuse reflectance spectrum> The diffuse reflectance characteristics of the samples were evaluated. Specifically, the reflectance of the samples was measured using an ultraviolet-visible-near-infrared spectrophotometer (Jasco, V-670), and the obtained data was subjected to Kubleka-Munk transformation to obtain the diffuse reflectance spectrum, which was then evaluated.

[0114] <Elemental analysis> The amounts of lanthanum (La), titanium (Ti), and aluminum (Al) in the sample were analyzed by ICP emission spectrometry. The analysis was performed using an inductively coupled plasma optical emission spectrometer (Agilent Technologies, ICP-OES, 5900 SVDV). Also, the amounts of oxygen (O) and nitrogen (N) in the sample were analyzed by inert gas fusion method. The analysis was performed using an oxygen, nitrogen, and hydrogen analyzer (LECO, ON836).

[0115] <SEM Observation> The sample was observed using a scanning electron microscope (SEM; Hitachi High-Tech Corporation, SU8000).

[0116] <TEM Observation> The sample was observed using a transmission electron microscope (TEM; JEOL Ltd., JEM-ARM200F) to obtain an electron diffraction pattern. Also, in STEM mode, a high-angle annular dark-field (HAADF) image was observed. Furthermore, an electron beam diffraction image was obtained, and for a predetermined location of the obtained TEM image, semi-quantitative composition analysis was performed using an EDS device (JEOL Ltd., JED-2300T) attached to the TEM.

[0117] <XPS Analysis> The sample was analyzed by X-ray photoelectron spectroscopy (XPS). The analysis was performed using a scanning X-ray photoelectron spectrometer (ULVAC-PHI, Versa Probe II). Also, during the analysis, peak separation of Ti2p spectrum for Ti 3+ and Ti 4+ was performed.

[0118] <Photocatalytic Performance for Water Splitting> A hydrogen generation activity performance test was conducted by irradiating the sample with visible light in a gas closed circulation system. Specifically, after introducing 0.1 g of the sample and 10 ml of a 15 vol% methanol aqueous solution into the reaction vessel, ultrasonic dispersion was performed for 30 seconds to prepare a suspension. The reaction vessel was placed in the closed circulation system. After sufficiently removing the dissolved gas in the suspension and degassing the closed circulation system, argon (Ar) gas was enclosed in the closed circulation system, and the pressure inside the system was maintained at 10 kPa.

[0119] Next, the photocatalytic performance was evaluated using a 300 W xenon lamp equipped with a filter that transmits only light with a wavelength of 420 nm or more. At that time, in order to prevent heating of the suspension by light irradiation, the periphery of the reaction vessel was cooled with cooling water set at 12°C, and while continuously stirring the suspension, the gas generated by the photocatalytic reaction was analyzed and the amount of generation was measured. The analysis and measurement of the generated gas were performed using a gas chromatograph (Shimadzu Corporation, GC-8A) equipped with a molecular sieve 5A column.

[0120] <AQY measurement> The apparent quantum yield (AQY) of the hydrogen generation reaction was evaluated. Specifically, except for using a 300 W xenon lamp equipped with a 420 nm band-pass filter as the light source, a hydrogen generation activity performance test was conducted under the above-described conditions, and based on the obtained data, AQY was calculated according to the following formula (4). In the following formula (4), n(H2) represents the amount of generated H2 molecules. Also, n(photons) is the amount of incident photons, which was measured using a grating spectroscopic radiometer (Eihong Seiki Co., Ltd., LS-100).

[0121] [Number]

[0122] (3) Evaluation results [[ID=2,2]]Oxide precursor <XRD analysis> The XRD patterns of the oxide precursors (after flame spraying) synthesized in Examples 1 to 6 and Comparative Examples 1 to 4 are shown in FIG. 2. In FIG. 2, the standard diffraction peaks of cubic LaTiO3 (JCPDS card 01-075-0267; LaTiO3, Pm-3m(221)), and the standard diffraction peaks of La2Ti2O7 (JCPDS card 01-081-1066; La2Ti2O7, P21(4)), La5Ti5O 17 (JCPDS card 00-048-0480) are also shown.

[0123] As shown in Figure 2, for all samples, the diffraction peaks in the XRD patterns were those of cubic LaTiO, LaTiO, and LaTiO. 17 This indicates that the oxide precursors include perovskite-type oxide (LaTiO3) and perovskite slab-type oxide (La2Ti2O7). In fact, LaTiO3, La2Ti2O7, and La5Ti5O 17 The Rietveld analysis of the crystalline phase has a convergence factor R wp The value was able to converge to 10 or less.

[0124] The intensity of peak A (I A ), the intensity of peak B (I B ), and the ratio of these (I A / I B ) together with the diffraction angle (2θ) are shown in Table 1 below. Peak A is an XRD peak appearing near 2θ = 32° and derived from perovskite oxide, and Peak B is an XRD peak appearing near 2θ = 30° and derived from perovskite slab oxide.

[0125] Comparing the results of Comparative Example 1 and Example 1, the intensity ratio I A / I B It was found that the amount of perovskite oxide produced increased, that is, a larger amount of perovskite oxide was produced. A similar tendency was also observed when the results of Comparative Example 2 and Example 2, or the results of Comparative Example 3 and Example 3 were compared.

[0126] From the results of Examples 1 to 3, by increasing the amount of La to more than the stoichiometric composition, the intensity ratio I A / I B It was found from the results of Examples 2, 4 and 5 that the intensity ratio I A / I B In Example 6 and Comparative Example 4, the peak of the perovskite slab-type oxide (La2Ti2O7) disappeared, and only the peak of the perovskite-type oxide was observed. Therefore, the intensity ratio I A / IB became infinity (∞).

[0127] [Table 1]

[0128] <Rietveld analysis> The crystalline phases (LaTiO, LaTiO, LaTiO) in several samples (Comparative Example 2, Example 4, Example 6, and Comparative Example 4) with different Al doping amounts were 17 The molar composition ratio of each crystalline phase is shown in FIG. 3. Here, the molar composition ratio of each crystalline phase was calculated by Rietveld analysis of the XRD data of the sample. 1.05 TiO y ), Example 4 (La 1.05 Ti 0.9 Al 0.1 O y ), Example 6 (La 1.05 Ti 0.7 Al 0.3 O y ), and Comparative Example 4 (La 1.05 Ti 0.5 Al 0.5 O y The Al doping amounts of the above SiO2 and SiO2 were 0 mol %, 10 mol %, 30 mol %, and 50 mol %, respectively.

[0129] In the range of Al doping amount from 0 to 30 mol%, the proportion of LaTiO3 increased with increasing Al doping amount, and at the same time, the proportion of La2Ti2O7 decreased (Comparative Example 2, Example 4, Example 6). Therefore, it was found that Al doping contributes to the formation of LaTiO3. However, the composition ratio of the crystalline phase in the sample with 50 mol% Al doping amount (Comparative Example 4) was almost unchanged compared to the sample with 30 mol% Al doping amount (Example 6).

[0130] The lattice constants of the perovskite-type oxide precursors (LaTiO3 phase) of Comparative Example 2 (Al doping amount: 0 mol%), Example 4 (Al doping amount: 10 mol%), Example 6 (Al doping amount: 30 mol%), and Comparative Example 4 (Al doping amount: 50 mol%) are shown in Fig. 4. Here, the lattice constants are values calculated from the Rietveld analysis. Also, the filled marks are sample data, and the open marks are data of reference LaTiO3.

[0131] With the increase in the Al doping amount, all lattice constants of the a-axis, b-axis, and c-axis decreased. Looking at the effective ionic radii by Shannon, the ionic radius of Ti (tetravalent, 6-coordination) is 0.605 Å, while the ionic radius of Al (trivalent, 6-coordination) is 0.535 Å. It is considered that the lattice constants decreased due to the doping of Al with a small ionic radius into the Ti sites.

[0132] From the above results, it was found that by making the La amount equal to or more than the stoichiometric composition and doping with Al, the formation of perovskite slab-type oxide (La2Ti2O7) was suppressed, and the ratio of the perovskite-type oxide increased. By increasing the ratio of the perovskite-type oxide, it is expected that the ratio of the perovskite-type oxynitride (Al-doped LaTiO2N) generated by topotactic conversion in the subsequent nitridation process will increase, and the crystallinity will be enhanced.

[0133] <SEM Observation> SEM images of the oxide precursors obtained in Comparative Example 2, Example 4, Example 6, and Comparative Example 4 are shown in Fig. 5. The oxide precursors are particles with a rounded shape, and their particle size was on the order of several tens of nm to submicron. Also, their surface was smooth. From the rounded submicron-sized particle shape, it is considered that the surface tension of the droplets acted more dominantly than the combustion reaction of the solvent during flame spray synthesis.

[0134] <Elemental Analysis> Table 2 below shows the elemental analysis results of the oxide precursors obtained in Comparative Example 2, Example 4, Example 6, and Comparative Example 4. In Table 2 below, the content of each element (La, Ti, Al, O) in the oxide precursor is shown together with the atomic ratios (La / (Ti + Al) ratio, Ti / (Ti + Al) ratio, O / (Ti + Al) ratio). It was confirmed that the bulk composition of the oxide precursor synthesized by the flame spray method was almost as charged.

[0135] The valence of Ti calculated from La(III), Al(III), and O(III) was 3 or more in all samples (Comparative Example 2, Example 4, Example 6, Comparative Example 4). Considering that the valence of Ti in LaTiO3 is trivalent and La2Ti2O7 (Ti is tetravalent) is included, this is contradictory. On the other hand, from the results of Rietveld analysis (Figure 4), it is known that the lattice constant of the sample of Comparative Example 2 is smaller than that of the reference LaTiO3. From these results, it is suggested that cations of La and Ti are deficient.

[0136]

Table 2

[0137] <TEM Observation> The TEM analysis results (TEM image, electron diffraction pattern, HAADF image, EDS analysis result) obtained for the oxide precursor of Comparative Example 1 are shown in Fig. 6. From the EDS analysis, it was found that the oxide precursor was composed of a perovskite-type LaTiO3 phase with 25% deficiency of Ti and La2Ti2O7 with a slight deficiency of La. Therefore, the perovskite-type LaTiO3 phase synthesized by the flame spray method is considered to be La(Ti 3 / 4 □ 1 / 4 □)O3 with a Ti-site deficiency perovskite structure composed of tetravalent Ti.

[0138] TEM images obtained for the oxide precursors of Comparative Example 2, Example 4, Example 6, and Comparative Example 4 are shown in FIGS. 7A to 7D. Also, the atomic ratios (La / (Ti + Al) ratio, Al / (Ti + Al) ratio) determined by EDS analysis are shown in the lower middle part of the figures. Note that the numbers (such as 1) of the positions where EDS analysis was performed correspond to the numbers of the positions indicated by square frames in the TEM images. It was confirmed from the EDS analysis that Al is present in the particles.

[0139] Nitridation reaction product (after nitridation step) <XRD analysis> XRD patterns of the nitridation reaction compositions (immediately after the nitridation step) obtained in Examples 1 to 6 and Comparative Examples 1 to 4 are shown in FIGS. 8A to 8C. FIG. 8A is a pattern in the range of 2θ = 20 to 60°, and FIGS. 8B and 8C are patterns in the range of 2θ = 31.5 to 33.0°.

[0140] As shown in FIG. 8A, the diffraction peaks in the XRD pattern almost coincided with the standard diffraction peaks of LaTiO2N (JCPDS card 01-079-6427; LaTiO2N, Imma(74)). No other by-products were confirmed, and it was found that a single-phase LaTiO2N-based compound was formed.

[0141] Comparing the XRD patterns of Comparative Example 1 and Example 1, it was found that the peak shifted to the high-angle side due to Al doping (FIG. 8B). Similar trends were also observed when comparing the results of Comparative Example 2 and Example 2, or the results of Comparative Example 3 and Example 3.

[0142] As shown in FIG. 8C, as the amount of Al doping increased, the peak shifted more to the high-angle side. In addition to Al being doped into the Ti site, in order to maintain charge balance, nitrogen ions were substituted for oxygen ions simultaneously with Al doping, and as a result, it is considered that the lattice constant decreased. And it is considered that the peak shifted to the high-angle side due to the decrease in the lattice constant.

[0143] The fact that Al doping reduces the lattice constant is also supported by the results of Rietveld analysis. Specifically, as shown in Figure 9, with increasing Al doping, the a-axis lattice constant of the perovskite oxynitride decreased from 5.5757 Å to 5.5343 Å, the b-axis lattice constant decreased from 5.6013 Å to 5.5129 Å, and the c-axis lattice constant decreased from 7.8760 Å to 7.7980 Å. Note that the points in the figure with Al doping levels of 0 mol%, 10 mol%, 30 mol%, and 50 mol% correspond to the data for Comparative Example 2, Example 4, Example 6, and Comparative Example 4, respectively. The solid symbols represent sample data, and the open symbols represent data for the reference LaTiO2N.

[0144] The XRD peak intensities and XRD full widths at half maximum (FWHM) of the nitriding reaction products are summarized in Table 3 below. Comparing the results of Comparative Example 1 and Example 1, it was found that Al doping reduced the XRD full width at half maximum, i.e., improved crystallinity. A similar tendency was also observed when comparing the results of Comparative Example 2 and Example 2, or the results of Comparative Example 3 and Example 3.

[0145] From the results of Examples 1 to 3, it was found that the XRD half-width decreased when the La content was increased to 1.05, while the XRD half-width increased when the La content was increased to 1.10. This is thought to be due to distortion in the crystal. On the other hand, from the results of Examples 2, 4, 5, and 6, it was found that the XRD half-width decreased when the Al doping amount was large. This suggests an improvement in crystallinity.

[0146] [Table 3]

[0147] <Elemental analysis> The elemental analysis results of the nitridation reaction products obtained in Comparative Example 2, Example 4, Example 6, and Comparative Example 4 are shown in Table 4 below. Table 4 below also shows the molar ratios of La, Ti, Al, O, and N to (Ti + Al). The molar ratios of the cations (La, Ti, Al) to (Ti + Al) were almost the same as the results obtained for the oxide precursors. On the other hand, the molar ratio of N / (Ti + Al) was less than the theoretical amount, and O was more. Also, the molar ratio of O / (O + N) increased with an increase in the Al doping amount. This is considered to be because LaTiO2N and LaAlO3 form a solid solution.

[0148]

Table 4

[0149] <Diffuse reflection spectrum> The diffuse reflection spectra of the nitridation reaction products (after the nitridation step) obtained in Examples 1 to 6 and Comparative Examples 1 to 4 are shown in Fig. 10. The short wavelength shift of the absorption edge was observed due to Al doping. This is attributed to the decrease in the N content and the widening of the band gap. Also, the higher the La content, the lower the background absorption in the wavelength range of 600 nm or more. When Ti reduction species (Ti 3+ ) or crystal defects that act as recombination centers of electrons and holes are formed, the background absorption becomes high. Therefore, the decrease in the background absorption means the suppression of the generation of recombination centers (Ti 3+ , crystal defects). It is considered that the generation of recombination centers was suppressed by the excessive blending of La, and from this, an improvement in the catalytic activity was expected.

[0150] Similarly, a remarkable decrease in the background absorption was also observed in Example 6 with a large amount of Al blended. By increasing the Al blending amount to more than 0.3, it is considered that the generation of recombination centers (Ti 3+ , crystal defects) was suppressed.

[0151] <XPS analysis> The XPS analysis results of Ti2p of the nitridation reaction product are shown in Fig. 11. The Ti2p spectrum shows Ti 3+ species and Ti4+ can be attributed to two spin - orbit species of the kind. With the increase of the Al doping amount, Ti 3+ species peak area ratio decreases, while Ti 4+ species peak area ratio increases. From this result, it was found that with the increase of the Al doping amount, the Ti of the recombination center 3+ decreases.

[0152] <TEM image> The TEM image of the nitridation reaction product obtained in Example 4 is shown in Fig. 12. The nitridation reaction product was in the form of particles having a particle size of several tens of nm to sub - micron level. Also, similar to the oxide precursor (LaTiAl oxide) shown in Fig. 5, it maintained a rounded shape. Also, at the stage of the oxide precursor (LaTiAl oxide), there were connected particles with a particle size of about several nm, while such particles were not observed in the nitridation reaction product. It is considered that during the nitridation treatment, it combined with large - sized spherical particles and the Al amount in the particles was homogenized.

[0153] The component amounts and atomic ratios determined by EDS analysis are shown in the lower part of the figure. The numbers (No. 1, etc.) of the locations where EDS analysis was performed correspond to the numbers of the locations indicated by square frames in the TEM image. The presence of Al was confirmed in all particles.

[0154] <SEM observation> The SEM image of the nitridation reaction product is shown in Fig. 13. In Comparative Example 2, it was confirmed that pores were formed on the particle surface. Non - Patent Document 2 shows that in the reaction from La2Ti2O7 to LaTiO2N, due to the decrease of anions, about 14% volume shrinkage occurs and pores are formed. On the other hand, from the SEM images of Comparative Example 2, Example 4, Example 6, and Comparative Example 4 shown in Fig. 13, it was observed that the formation of pores was suppressed by increasing the Al doping amount.

[0155] The lattice constants and lattice volumes of the LaTiO3 phase and the LaTiO2N phase obtained by the Rietveld analysis are shown in Table 5 below. Since the reaction from the LaTiO3 phase to the LaTiO2N phase involves only the conversion of O and N, the volume expands, and the change rate is from 1.25% to 2.45%. Compared with the volume change due to the reaction from La2Ti2O7 to LaTiO2N, it was found that the volume change of the reaction from the LaTiO3 phase to the LaTiO2N phase is small. From these results, as the amount of Al doping increases, the formation of the LaTiO3 phase increases, so it is considered that the formation of pores is suppressed. Such a minimal structural change is considered to result in little structural change during the nitridation reaction and to suppress the formation of structural defects that serve as recombination centers.

[0156]

Table 5

[0157] Nitridation reaction product (after the acid treatment step) <XRD analysis> The XRD patterns of the nitridation reaction compositions (immediately after the acid treatment step) obtained in Examples 1 to 6 and Comparative Examples 1 to 3 are shown in FIG. 14. In any of the samples, the diffraction peaks in the XRD pattern almost coincided with the standard diffraction peaks of LaTiO2N (JCPDS card 01-079-6427; LaTiO2N, Imma (74)). No other peaks were confirmed, and it was confirmed that a single-phase LaTiO2N crystal was maintained even after the acid treatment.

[0158] The XRD peak intensities and XRD full width at half maximum (FWHM) of the nitridation reaction product (after the acid treatment step) are summarized in Table 6 below. When compared with the XRD full width at half maximum of the nitridation reaction product immediately after the nitridation step (Table 3), no significant difference was found in the magnitude of the XRD full width at half maximum between before and after the nitridation treatment for any of the samples. Also, no change was observed in the order of the XRD full width at half maximum among the samples. From this, it was confirmed that the acid treatment has almost no effect on the crystallinity.

[0159]

Table 6

[0160] <Diffuse reflection spectrum> The diffuse reflection spectra of the nitridation reaction products (after the acid treatment step) obtained in Examples 1 to 6 and Comparative Examples 1 to 4 are shown in Fig. 15. When compared with the diffuse reflection spectrum of the sample after nitridation treatment (Fig. 10), no change was observed in the position of the absorption edge in the samples with the La amount above the stoichiometric composition and Al-doped. Also, looking at the results of Example 2, Example 5, Example 6, and Comparative Example 4, the background absorption in the wavelength range of 600 nm or more was reduced. This suggests that defects on the catalyst surface and interface defects, etc. were removed by the acid treatment, improving the catalyst activity.

[0161] <AQY measurement> The external quantum efficiency (AQY) of the hydrogen generation reaction determined for Comparative Example 2, Example 4, Example 6, and Comparative Example 4 is shown in Fig. 16. When comparing the results of Comparative Example 2 and Example 6, it was found that the AQY was improved 1.9 times by doping with 30 mol% of Al. This is considered to be because the crystallinity was improved and recombination centers (Ti 3+ , structural defects) could be suppressed by Al doping. However, in Comparative Example 4, the AQY was lower than that in Example 6. This is considered to be because the amount of Al doping at the Ti site in LaTiO2N was excessive, reducing the mobility of photoexcited carriers.

[0162] <Water splitting photocatalytic performance> The evaluation results of the hydrogen generation activity and the maximum hydrogen generation rate of the photocatalysts (promoter-supported nitridation reaction products) in Examples 1 to 6 and Comparative Examples 1 to 3 are shown in Figs. 17 and 18, respectively.

[0163] Comparing the results obtained for Examples 1 to 6 and Comparative Examples 1 to 3, it was found that the hydrogen evolution activity was improved by increasing the La content to or greater than the stoichiometric composition and by doping with Al (FIGS. 17 and 18). Furthermore, the results for Examples 2 and 4 showed that when the Al content x was 0.10 or less, the order of the hydrogen evolution rate correlated with the order of the XRD half-width and the crystallite diameter (Tables 3 and 6) (FIG. 18). That is, the smaller the XRD half-width and the larger the crystallite diameter, the higher the catalytic activity. This showed that the sample with higher crystallinity had higher catalytic activity.

[0164] On the other hand, the maximum hydrogen evolution rates of Examples 5 and 6, in which the Al blending amount x was 0.2 or 0.3, were higher than those of Example 1, but lower than those of Example 4, in which the Al blending amount x was 0.1 (FIG. 18). As shown in the diffuse reflectance spectra (FIG. 15) for Example 1 (x=0.1), Example 5 (x=0.2), and Example 6 (x=0.3), it is believed that the shift of the absorption edge to a shorter wavelength reduced the amount of light absorption, resulting in a decrease in activity.

[0165] From the above results, it can be understood that by setting the amount of La to be equal to or greater than the stoichiometric composition and doping with a predetermined amount of Al, the crystallinity of the photocatalyst containing the LaTiO2N-based compound is improved, thereby improving the catalytic activity. [Explanation of symbols]

[0166] 10 Particle production equipment 11 Storage area 12 Two-fluid nozzle 13 Reaction tube 131 Cooling water piping 14 filters 15 Ejector

Claims

1. Formula: La Ｗ Ti １－Ｘ Al Ｘ O Ｙ N Ｚ (wherein X, Y, Z, and W satisfy 1.00≦W≦1.10, 0.03≦X≦0.30, 2.00≦Y≦2.55, and 0.55≦Z≦1.00), and includes main catalyst particles containing a perovskite-type oxynitride as a main component.

2. 2. The hydrogen generation active photocatalyst according to claim 1, wherein the perovskite oxynitride has an a-axis lattice constant of 5.5343 Å or more and 5.5757 Å or less, a b-axis lattice constant of 5.5129 Å or more and 5.6013 Å or less, and a c-axis lattice constant of 7.7980 Å or more and 7.8992 Å or less.

3. 3. The hydrogen generation active photocatalyst according to claim 1, wherein in an X-ray diffraction (XRD) pattern using Cu Kα as a radiation source, the peak full width at half maximum (FWHM) of the (002) diffraction line based on the perovskite-type oxynitride is 0.29° or less.

4. 3. The hydrogen generation active photocatalyst according to claim 1, further comprising a promoter supported on the surface of the main catalyst particles.

5. A method for producing a hydrogen-evolving photocatalyst containing primary catalyst particles mainly composed of a perovskite-type oxynitride, the method comprising the steps of: preparing a precursor material containing La, Ti, and Al; A step of introducing the precursor material into a high-temperature field having a temperature of 500°C or higher to prepare an oxide precursor; nitriding the oxide precursor to produce a nitrided reaction product; With regard to La, Ti, and Al contained in the precursor raw material, a molar ratio w of the amount of La to the total amount of Ti and Al satisfies 1.00≦w≦1.10, and a molar ratio x of the amount of Al to the total amount of Ti and Al satisfies 0.03≦x≦0.

30.

6. The method of claim 5 , wherein the oxide precursor comprises a perovskite oxide, and the nitridation reaction product comprises a perovskite oxynitride formed by nitriding the perovskite oxide.

7. 7. The method according to claim 6, wherein a volume change rate V when the perovskite oxide is nitrided and transformed into the perovskite oxynitride is 1.82% or more and 2.45% or less.

8. In the X-ray diffraction (XRD) pattern of the oxide precursor using Cu Kα as a radiation source, the peak intensity (I Ａ ) and the peak intensity of the diffraction line originating from the perovskite slab-type oxide (I Ｂ ) ratio (I Ａ / I Ｂ 8. The method according to claim 5, wherein the value of (a) is 1.47 or more.

9. The method of any one of claims 5 to 7, further comprising the step of treating the nitriding reaction product with an acid.

10. The method according to any one of claims 5 to 7, further comprising the step of supporting a promoter on the surface of the main catalyst particles.

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