Photocatalyst and method for producing the same, as well as method for producing oxygen and hydrogen using the photocatalyst.

Abstract

To provide a photocatalyst capable of exhibiting photocatalytic activity even under acidic conditions.SOLUTION: There is provided a photocatalyst which has a carrier containing BiVO4 and WO3 which is supported on the carrier and has a hexagonal phase. There is provided a method for producing a photocatalyst which comprises: a mixing step of agitating a bismuth salt and a vanadium salt in a liquid to obtain a solution or suspension; and a dispersion step of adding the tungsten salt into the solution or suspension, followed by further agitating to obtain a dispersion of a photocatalyst having a carrier containing BiVO4 and WO3 supported on the carrier.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This application relates to a photocatalyst that can be used for water electrolysis in a photocatalyst-electrolytic hybrid system. [Background technology]

[0002] In recent years, research has been progressing on using photocatalysts to decompose water into hydrogen and oxygen using solar energy. This technology is very low-cost and has excellent recyclability and durability. However, the low efficiency of this technology is a problem at this stage. For this reason, the establishment of a more innovative energy conversion system is desired. The inventors of this application have developed a method to decompose water into hydrogen and oxygen using photocatalysts such as TiO2 or In2O3. 3+ When light is shone on an aqueous solution containing oxygen and Fe 2+ It generates, and then Fe 2+ By electrolyzing water in an aqueous solution containing Fe, 3+ We proposed a method (Patent Document 1) and a hydrogen production apparatus (Patent Document 2) for producing hydrogen at low cost by significantly reducing the electrolytic voltage required for hydrogen generation through regeneration and hydrogen generation.

[0003] Bismuth vanadate (BiVO4), known for its visible light responsiveness and high oxygen-evolving capacity from water, is a promising photocatalyst for hydrogen production using redox media. However, the photocatalytic activity of BiVO4 is highly pH-dependent, decreasing significantly at low pH levels. One possible reason for this decrease in photocatalytic activity at low pH is that BiVO4 dissolves in strongly acidic solutions. In photocatalytic-electrolytic hybrid systems that combine photocatalytic reactions with water electrolysis, BiVO4, which can utilize long-wavelength light, is suitable as a photocatalyst. Therefore, improving the acid resistance of BiVO4 so that it can exhibit photocatalytic activity even at low pH levels is required. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 11-157801 [Patent Document 2] Japanese Patent Publication No. 2001-233602 [Overview of the project] [Problems that the invention aims to solve]

[0005] This application was made in view of these circumstances, and aims to provide a photocatalyst that can exhibit photocatalytic activity even under acidic conditions. [Means for solving the problem]

[0006] The inventors of this application have discovered that by adding metatungstate salt during the process of preparing BiVO4 from Bi2O3 and V2O5 using a liquid-solid phase method, WO3 with high resistance to acidic solutions can be supported on BiVO4 particles, and a photocatalyst with high photocatalytic activity even under acidic conditions can be obtained.

[0007] The photocatalyst of the present invention comprises a carrier containing BiVO4 and WO3 supported on the carrier and having a hexagonal phase. The method for producing the photocatalyst of the present invention is as follows: Bismuth nitrate, bismuth chloride, and one or more types of bismuth trioxide and Vanadium chloride, ammonium metavanadate, and one or more of the following: vanadium chloride, ammonium metavanadate, and vanadium pentoxide. The process includes a mixing step of stirring in a liquid to obtain a solution or suspension, and a dispersion step of adding a tungsten salt to the solution or suspension and further stirring to obtain a dispersion of a photocatalyst comprising a carrier containing BiVO4 and WO3 supported on the carrier.

[0008] The present invention provides an oxygen production method which involves irradiating a reaction solution containing an aqueous solution with iron(III) ions and the present invention's photocatalyst with light to decompose the water in the reaction solution and produce oxygen. The present invention provides a hydrogen production method which involves irradiating a reaction solution containing an aqueous solution with iron(III) ions and the present invention's photocatalyst with light to decompose the water in the reaction solution to generate oxygen and reduce iron(III) ions to iron(II) ions, and an electrolysis step which involves electrolyzing an aqueous solution containing iron(II) ions to regenerate iron(III) ions from iron(II) ions and generate hydrogen. [Effects of the Invention]

[0009] The photocatalyst of this invention is supported with WO3 having a hexagonal phase. Therefore, the photocatalyst of this invention has excellent acid resistance. According to the method for producing the photocatalyst of this invention, a photocatalyst with high activity even in acidic conditions can be obtained. The oxygen production method of this invention uses the photocatalyst of this invention, which has excellent acid resistance. Therefore, according to the oxygen production method of this invention, oxygen production proceeds by the decomposition of water under acidic conditions using a redox medium. The hydrogen production method of this invention uses the photocatalyst of this invention, which has excellent acid resistance, in a photocatalyst-electrolysis hybrid system. Therefore, according to the hydrogen production method of this invention, the reduction of the redox medium proceeds even under acidic conditions, and hydrogen can be efficiently produced while re-oxidizing the reduced redox medium. [Brief explanation of the drawing]

[0010] [Figure 1] Conceptual diagram of water electrolysis using a photocatalytic-electrolytic hybrid system. [Figure 2] Powder X-ray diffraction pattern of the photocatalyst from Example 4. [Figure 3] Powder X-ray diffraction pattern of the photocatalyst in Comparative Example 2. [Modes for carrying out the invention]

[0011] First, let's explain the photocatalyst-electrolytic hybrid system. Figure 1 conceptually illustrates the electrolysis of water in a photocatalyst-electrolytic hybrid system. In this system, light is irradiated onto the photocatalyst, generating excited electrons in the conduction band and holes in the valence band. Redox reactions occur when the standard redox potential is positive than +1.23V. That is, the holes generated in the valence band produce oxygen, the oxidation product, from water, the oxidizer in the reaction solution.

[0012] On the other hand, excited electrons generated in the conduction band reduce the redox medium. For example, if the redox medium is iron ions, Fe 3+ Fe 2+is reduced. In this system, by electrolyzing this reaction solution at a voltage lower than 1.23V, the reduced redox medium is oxidized at the anode. For example, when the redox medium is iron ions, at the anode, Fe 2+ becomes Fe 3+ and is oxidized. At the cathode of this system, water is reduced to produce hydrogen.

[0013] The photocatalyst of the embodiment of the present application includes a carrier and WO3 supported on the carrier. The carrier contains BiVO4. In order to fully function the photocatalytic activity of BiVO4, the carrier is preferably composed of BiVO4 excluding inevitable impurities. However, other components may be included in the carrier as long as the photocatalytic activity of BiVO4 is not significantly hindered. WO3 has a hexagonal phase. The fact that WO3 having a hexagonal phase is supported on the carrier can be confirmed by the presence of peaks at 2θ = 22.8°, 28.2°, and 36.6° in the powder X-ray diffraction pattern.

[0014] The shape of the photocatalyst of the present embodiment is not particularly limited, and may be particulate, powdery, or film-like, etc. When the shape of the photocatalyst of the present embodiment is particulate or powdery, the average particle size is not particularly limited, but is usually about 1μm to 10μm. Since the photocatalytic activity is high, in the photocatalyst of the present embodiment, the ratio of the amount of substance of WO3 to the amount of substance of BiVO4 (so-called molar amount) is preferably 0.5 mol% or more and 30 mol% or less. The photocatalyst of the present embodiment is extremely effective as a catalyst for promoting an energy storage type photocatalytic reaction that reduces a redox medium and simultaneously produces oxygen. In particular, it is effective when the redox medium is iron ions. That is, the photocatalyst of the present embodiment can be used in a reaction that decomposes water to generate oxygen using iron ions as a reversible redox medium.

[0015] The method for producing the photocatalyst of the embodiment of the present application includes a mixing step and a dispersion step. In the mixing step, Bismuth nitrate, bismuth chloride, and one or more types of bismuth trioxide and Vanadium chloride, ammonium metavanadate, and one or more of the following: vanadium chloride, ammonium metavanadate, and vanadium pentoxide. are stirred in a liquid to obtain a solution or a suspension. Bismuth nitrate, bismuth chloride, and one or more types of bismuth trioxide , Vanadium chloride, ammonium metavanadate, and one or more of the following: vanadium chloride, ammonium metavanadate, and vanadium pentoxide. , FurthermoreDepending on the type and concentration of the liquid, etc., a solution or suspension is obtained in the mixing step. . glass Bismuth acid, bismuth chloride, and bismuth trioxide S Among So , bismuth trioxide (Bi2O3 )but is preferred.

[0016] salt Vanadium chloride, ammonium metavanadate, and vanadium pentoxide Mu Among So , vanadium pentoxide (V2O5 )but is preferred. The liquid is Bismuth nitrate, bismuth chloride, and one or more types of bismuth trioxide and Vanadium chloride, ammonium metavanadate, and one or more of the following: vanadium chloride, ammonium metavanadate, and vanadium pentoxide. are suspended, and a tungsten salt used in the dispersion step after the mixing step is dissolved. For example, water, which is a hydrophilic liquid, is preferred. The liquid may be only water or may contain water as the main component. The Vanadium chloride, ammonium metavanadate, and one or more of the following: vanadium chloride, ammonium metavanadate, and vanadium pentoxide. substance amount of the Bismuth nitrate, bismuth chloride, and one or more types of bismuth trioxide substance amount ratio of the Bismuth nitrate, bismuth chloride, and one or more types of bismuth trioxide substance amount of / Vanadium chloride, ammonium metavanadate, and one or more of the following: vanadium chloride, ammonium metavanadate, and vanadium pentoxide. substance amount) is preferably 1.

[0017] To grow BiVO4 crystals at an appropriate speed, the temperature during stirring in the mixing step is preferably 40 °C or higher and 130 °C or lower, and more preferably 70 °C or higher and 120 °C or lower. Also, Bismuth nitrate, bismuth chloride, and one or more types of bismuth trioxide and Vanadium chloride, ammonium metavanadate, and one or more of the following: vanadium chloride, ammonium metavanadate, and vanadium pentoxide. To promote the reaction between, in the mixing step, it is preferable to add an acid to the liquid before or during stirring. There is no particular limitation on the type of acid added to the liquid, and examples of the acid include nitric acid, hydrochloric acid, sulfuric acid, and perchloric acid. The concentration of the acid added is usually 0.5 mol / L -1 ~2.0 mol / L -1 or so.

[0018] In the dispersion step, a tungsten salt is added to the solution or suspension obtained in the mixing step, and further stirred to obtain a dispersion of a carrier containing BiVO4 and WO3 supported on this carrier. By going through the mixing step and the dispersion step, more specifically, Bismuth nitrate, bismuth chloride, and one or more types of bismuth trioxide and Vanadium chloride, ammonium metavanadate, and one or more of the following: vanadium chloride, ammonium metavanadate, and vanadium pentoxide.A dispersion containing a photocatalyst with excellent acid resistance is obtained by adding a tungsten salt to a stirred solution or suspension and stirring further. The reason why a photocatalyst with excellent acid resistance is obtained is that WO3, which has a hexagonal phase with excellent acid resistance, is supported on a BiVO4-containing carrier such as a ceallite monoclinic phase with high photocatalytic activity.

[0019] On the other hand, for example, in a liquid Bismuth nitrate, bismuth chloride, and one or more types of bismuth trioxide , Vanadium chloride, ammonium metavanadate, and one or more of the following: vanadium chloride, ammonium metavanadate, and vanadium pentoxide. , Furthermore The photocatalyst contained in the dispersion obtained by stirring together with a tungsten salt has poor acid resistance and photocatalytic activity because it contains BiVO4 with a crystalline structure such as a zircon structure, which differs from the cereite monoclinic phase, and because WO3 with a hexagonal phase is hardly supported on the support. Examples of tungsten salts include tungstates, metatungstates, and peroxotungstates. Among these, ammonium metatungstate, which is a metatungstate, is preferred as the tungsten salt.

[0020] The temperature at which the tungsten salt is added to the solution or suspension obtained in the mixing step and further stirred is preferably 60°C to 120°C, and more preferably 80°C to 110°C. Furthermore, in order to obtain a photocatalyst in which the ratio of the amount of WO3 to the amount of BiVO4 in the support is 0.5 mol% to 30 mol%, the amount of tungsten in the tungsten salt added to the solution or suspension is Vanadium chloride, ammonium metavanadate, and one or more types of vanadium pentoxide The amount of vanadium in substance and Bismuth nitrate, bismuth chloride, and one or more types of bismuth trioxide It is preferable that the amount of bismuth contained is between 0.5 mol% and 30 mol%.

[0021] The oxygen production method of the embodiment of the present invention involves irradiating a reaction solution containing an aqueous solution with iron(III) ions and the photocatalyst of the present invention with light, thereby decomposing the water in the reaction solution and producing oxygen. As described above, when light is irradiated onto the photocatalyst, excited electrons are generated in the conduction band and holes are generated in the valence band. Then, the holes generated in the valence band produce oxygen, which is the oxidation product, from the water, which is the oxide in the reaction solution. On the other hand, the excited electrons generated in the conduction band are Fe 3+ Fe 2+ The reaction solution is reduced to [a certain value]. When oxygen is produced from water, the hydrogen ion concentration increases, meaning the acidity of the reaction solution increases. Since the photocatalyst of this invention has excellent acid resistance, oxygen can be produced efficiently according to the oxygen production method of this embodiment using the photocatalyst of this invention. Thus, according to the oxygen production method of this embodiment, the photocatalytic function can be exhibited even in reaction solutions that are acidic, especially those with a pH of 0 or higher and 2 or lower.

[0022] The hydrogen production method of the embodiment of the present invention applies the photocatalyst of the present invention to a photocatalyst-electrolytic hybrid system. The hydrogen production method of this embodiment comprises a photocatalytic reaction step and an electrolysis step. In the photocatalytic reaction step, as described above, a reaction solution having an aqueous solution containing iron(III) ions and the photocatalyst of the present invention is irradiated with light to decompose the water in the reaction solution to generate oxygen and reduce the iron(III) ions to iron(II) ions. In the electrolysis step, as described above, the aqueous solution containing iron(II) ions is electrolyzed to regenerate iron(III) ions from iron(II) ions and generate hydrogen. [Examples]

[0023] Examples (Photocatalyst fabrication) 0.8 mol L -1 70 mL of nitric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. (hereinafter the same)) was used to suspend 10 mol each of Bi2O3 (manufactured by Kojun Chemical Laboratories, Ltd. (hereinafter the same)) and V2O5 (manufactured by Kojun Chemical Laboratories, Ltd. (hereinafter the same)), and this suspension was stirred at 80°C for 24 hours. Subsequently, the tungsten salt ammonium metatungstate hydrate (NH4)6H2W was used. 12 O40 xH2O (manufactured by Strem Chemicals) was added, and stirring was continued to obtain a dispersion of the photocatalyst. The amount of tungsten salt added (expressed as the amount of W added / the amount of Bi used × 100, shown as "W added (mol%)" in the table), the temperature during stirring after the addition of the tungsten salt (shown as "Stirring temperature after W addition (°C)" in the table), and the stirring time after the addition of the tungsten salt (shown as "Stirring time after W addition (h)" in the table) were varied as shown in Table 1.

[0024] Then, the dispersion was cooled to 40°C, filtered while washing with pure water, and vacuum dried to obtain the WO3-supported BiVO4 photocatalysts of Examples 1 to 7, in which WO3 is supported on BiVO4. The powder X-ray diffraction pattern of the photocatalyst of Example 4 (powder X-ray diffractometer: PANalytical, EMPYREAN (hereinafter the same)) is shown in Figure 2. As shown in Figure 2, in the photocatalyst of Example 4, peaks were present at 2θ = 22.8°, 28.2°, and 36.6° in the powder X-ray diffraction pattern, indicating that WO3 with a hexagonal phase was supported on the BiVO4 support. In other words, the photocatalysts of each example are photocatalysts in which WO3 with a hexagonal phase is supported on the BiVO4 support (hereinafter sometimes referred to as "hexagonal phase WO3-supported BiVO4 photocatalysts").

[0025] (Photocatalytic activity evaluation) Dissolve Fe(ClO4)3·nH2O (manufactured by Fujifilm Wako Pure Chemical Industries) in water, and add 0.4g of the WO3-supported BiVO4 photocatalyst from Examples 1 to 7 to form 4 mmol L -1 A reaction solution containing Fe(ClO4)3 was prepared in a volume of 300 mL. This reaction solution was placed in a side-irradiation type reaction tube, and the tube was connected to a closed-circulation glass reactor (PCAT, Makuhari Chemical Glass Works) equipped with a vacuum pump and a thermal conductivity detector gas chromatograph.

[0026] Using HClO4 (manufactured by Fujifilm Wako Pure Chemical Industries), the pH of the reaction solution was varied within the range of 0 to 2.0 as shown in Table 1, and the mixture was stirred with a magnetic stirrer. After degassing the closed-circulation reactor, the reaction tube was irradiated with light from a 300W xenon lamp, whose irradiation light was controlled to visible light using a cutoff filter, to generate oxygen. The amount of oxygen generated (indicated as "oxygen generation activity (μmolh)" in the table) was measured using a thermal conductivity detector gas chromatograph (Shimadzu Corporation, GC-8A). -1 The result was calculated. The result is shown in Table 1.

[0027] Comparative Example (Photocatalyst fabrication) 0.8 mol L -1 10 moles each of Bi2O3 and V2O5 were suspended in 70 mL of an aqueous nitric acid solution. This suspension was stirred at 80°C for 48 hours to obtain a photocatalyst dispersion. The dispersion was then cooled to 40°C, filtered while washing with pure water, and vacuum dried to obtain the BiVO4 photocatalyst of Comparative Example 1.

[0028] Furthermore, the photocatalyst of Example 4 was heated in an electric furnace (KDF Corporation, Electric Furnace 300-Plus) at 600°C for 0.5 hours to obtain the WO3-supported BiVO4 photocatalyst of Comparative Example 2. The powder X-ray diffraction pattern of the photocatalyst of Comparative Example 2 is shown in Figure 3. As shown in Figure 3, in this photocatalyst, the peaks at 2θ = 22.8°, 28.2°, and 36.6°, which indicate WO3 with a hexagonal phase, were not observed, and the peaks at 2θ = 23.1°, 23.6°, and 24.4°, which indicate WO3 with a monoclinic phase, were observed. In other words, the photocatalyst of Comparative Example 2 is a photocatalyst in which WO3 with a monoclinic phase, rather than a hexagonal phase, is supported on the BiVO4 support (hereinafter sometimes referred to as "monoclinic phase WO3-supported BiVO4 photocatalyst").

[0029] (Photocatalytic activity evaluation) The amount of oxygen produced was calculated using the BiVO4 photocatalyst of Comparative Example 1 and the WO3-supported BiVO4 photocatalyst of Comparative Example 2, in the same manner as in the examples. The results are shown in Table 1.

[0030] [Table 1]

[0031] As shown in Table 1, the hexagonal phase WO3-supported BiVO4 photocatalysts of Examples 1 to 7, produced under conditions where the amount of tungsten salt added was between 0.5 mol% and 30 mol%, exhibited higher oxygen generation activity compared to the BiVO4 photocatalyst of Comparative Example 1, which did not have WO3 support. In particular, in reaction solutions with a pH between 0 and 1, the oxygen generation activity of the WO3-supported BiVO4 photocatalysts of Examples 3 to 5, with an added amount of 5 mol% tungsten salt, was more than 10 times that of the BiVO4 photocatalyst of Comparative Example 1. Furthermore, the monoclinic phase WO3-supported BiVO4 photocatalyst of Comparative Example 2 exhibited lower oxygen generation activity than the hexagonal phase WO3-supported BiVO4 photocatalyst of Example 4 in a reaction solution with a pH of 1.5.

Claims

1. BiVO ４ A carrier containing and a WO supported on the carrier having a hexagonal phase ３ A photocatalyst having the following characteristics.

2. In claim 1, BiVO ４ WO ３ A photocatalyst in which the ratio of the amount of substance is 0.5 mol% or more and 30 mol% or less.

3. In claim 1 or 2, A photocatalyst that uses iron ions as a reversible redox medium in a reaction that decomposes water to produce oxygen.

4. A mixing step of stirring one or more of bismuth nitrate, bismuth chloride, and bismuth trioxide with one or more of vanadium chloride, ammonium metavanadate, and vanadium pentoxide in a liquid to obtain a solution or suspension, Add a tungsten salt to the above solution or suspension and stir further to obtain BiVO ４ A carrier containing and a WO supported on the carrier having a hexagonal phase ３ A dispersion step to obtain a dispersion of a photocatalyst having the following: A method for producing a photocatalyst.

5. In claim 4, A method for producing a photocatalyst, comprising the mixing step of stirring one or more of the bismuth nitrate, bismuth chloride, and bismuth trioxide with one or more of the vanadium chloride, ammonium metavanadate, and vanadium pentoxide under acidic conditions.

6. In claim 4 or 5, One or more of the above bismuth nitrate, bismuth chloride, and bismuth trioxide is Bi ２ O ３ wherein one or more of the above vanadium chloride, ammonium metavanadate, and vanadium pentoxide is V ２ O ５ A method for producing a photocatalyst in which the tungsten salt is a metatungstate.

7. A method for producing oxygen, comprising irradiating a reaction solution having an aqueous solution containing iron(III) ions and a photocatalyst according to claim 1 or 2 with light to decompose the water in the reaction solution and produce oxygen.

8. In claim 7, A method for producing oxygen in which the reaction solution is acidic.

9. In claim 8, A method for producing oxygen, wherein the pH of the reaction solution is 0 or higher and 2 or lower.

10. A photocatalytic reaction step comprising irradiating a reaction solution having an aqueous solution containing iron(III) ions and a photocatalyst of claim 1 or 2 with light, thereby decomposing the water in the reaction solution to generate oxygen and reducing the iron(III) ions to iron(II) ions, An electrolysis step is performed by electrolyzing the aqueous solution containing iron(II) ions to regenerate iron(III) ions from the iron(II) ions and generate hydrogen. A method for producing hydrogen.

Images