JP4070516B2 - Visible light-responsive sulfide photocatalyst for hydrogen production from water - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a photocatalyst having activity in light in the visible light region based on NaIn S ₂ , and more particularly to a water-splitting photocatalyst that generates hydrogen under visible light from an aqueous solution containing SO ₃ ²⁻ and S ²⁻ ions.
[0002]
[Prior art]
As a technique for performing a catalytic reaction with light, a method is known in which a solid compound having a photocatalytic activity is irradiated with light, and the reaction product is oxidized or reduced with the generated excited electrons or holes to obtain a target product.
Among them, the photodecomposition reaction of water is of interest from the viewpoint of light energy conversion. In addition, a photocatalyst exhibiting activity in water photolysis reaction can be regarded as an advanced photofunctional material having functions such as light absorption, charge separation, and water oxidation-reduction reaction on the surface.
Kudo, Kato, et al. Have cited many prior literatures that alkali tantalates, alkaline earths, and the like are photocatalysts that exhibit high activity in the complete photolysis of water [see, for example, Catal. Lett. , 58 (1999). 153-155, Chem. Phys. Lett. 331 (5/6) 373-377 (2000), J. MoI. Phys. Chem. B, 105 (19), 4285-4292 (2001), Surface, Vol. 36, no. 12 (1998), 625-645 (referred to as Document A)]. In the above-mentioned documents A, a photocatalytic material useful for proceeding with a reaction for decomposing water into hydrogen and / or oxygen is described. Hydrogen generation reaction by reduction of water-generated electrons, or oxidation of generated holes There are many suggestions for photocatalysts for oxygen production reaction and complete photolysis of water.
It also refers to photocatalysts carrying promoters or promoters such as platinum and NiO.
[0003]
However, what is described here is mainly non-metal containing oxygen. In addition, many solid photocatalysts cannot be operated with visible light having a low energy of less than 3 eV because the width of the forbidden band between the valence band and the conduction band, that is, the band gap energy is larger than 3 eV. On the other hand, most of the conventional solid photocatalysts having a small band gap energy and capable of generating electrons and holes with visible light are unstable under reaction conditions such as water photolysis. For example, although the band gap of CdS, Cu—ZnS, etc. is 2.4 eV, the catalytic reaction is limited because of the oxidative photocorrosion action.
Most of the sunlight that reaches the earth's surface is visible light with low energy, and a stable photocatalyst that operates with visible light and is indispensable in order to allow various catalytic reactions to proceed efficiently with sunlight.
[0004]
Under such circumstances, many researchers who are engaged in the research of photocatalysts are striving to develop photocatalysts that are active in visible light having a longer wavelength, particularly photocatalysts that are active in the decomposition of water. However, a photocatalyst that does not require a sacrificial agent and that can be practically decomposed by visible light has not yet been provided.
In developing a photocatalyst having activity in visible light, it is first important to develop a photo-semiconductor that is active in visible light having a longer wavelength, and in combination with a small amount of an activating element, further to a longer wavelength region. It is intended to improve the activity characteristics and stability of the. If a photocatalyst capable of efficiently decomposing at least one of water is found even if it is not a catalyst for complete decomposition of water (total decomposition), these libraries are constructed and the complete catalyst is selected from many catalysts. It is important in that it offers the potential for the construction of cracking catalyst systems, such as Z-scheme type catalyst systems.
[0005]
As described above, since most of the sunlight that can be used on the ground surface is visible light, there are many proposals for providing a photocatalyst that can generate excited electrons and holes with visible light, and at least the reduction reaction proceeds with high efficiency. Has been made.
Most of the conventional photocatalysts contain metal oxide, that is, oxygen as a nonmetallic element. In the metal oxide, the energy positional relationship between the conduction band and the valence band is largely governed by the valence electrons of oxygen and the energy of the O2p orbital. Therefore, the band gap energy is larger than 3 eV, and the photocatalytic function is expressed with visible light. I can't let you. Therefore, we thought that a catalyst material capable of decomposing water with visible light could be made by comprising N2p with a valence band higher than O2p. [Material Integration Vol. 14, no. 2 (2001), literature B].
In addition, focusing on the fact that the valence band S3p is also at a higher level than O2p, studies on photocatalysts composed of oxysulfide compounds have already been made by Domen and Hara et al. . 79th, No. 1, pp366; examination of visible light decomposition of water with oxysulfide, literature C]. In addition, Kudo et al. Have proposed that Bi ³⁺ and Ag ⁺ are also candidates as valence band forming elements other than O2p. BiVO ₄ and AgNbO ₃ are photocatalysts that exhibit activity in oxygen generation from aqueous solutions under visible light irradiation [J. Am. Chem. Soc. 121 (49), 11459-11467 (1999), Material Stage, No. 5, 21-26 (2002), Documents D.] The oxygen generation catalyst is a catalyst that forms one of the Z scheme catalyst systems. It has the potential as
[0006]
On the other hand, studies are being made to improve the photoactive characteristics and the stability in combination with a small amount of activating element or compound. For example, for SrTiO ₃ , Lehn et al. Have demonstrated photoactivity for complete photolysis of water, eg, for Rh / SrTiO ₃ , in combination with a noble metal promoter. Japanese Patent Application Laid-Open No. 2000-189806 discloses that a metal or metal oxide such as Pt, Ru, Rh, Ir, or Ni is supported in order to improve the visible light activity of the photocatalyst. However, these precious metals are supported on a photocatalyst, and an effect of expanding the energy band in the visible light region cannot be expected.
[0007]
In contrast to the above technique, SrTiO ₃ and TiO ₂ co-doped with Cr ³⁺ and Sb ⁵⁺ or Ta ⁵⁺ show catalytic activity to generate hydrogen from aqueous methanol and oxygen from aqueous silver nitrate under visible light irradiation, respectively. It is known [J. Phys. Chem. 106 (19), 5029-5034 (2002), Material Stage, No. 5, 21-26 (2002), literature E)], showing that the doping of the element not only broadens the energy band in the visible light region but also imparts the activity of generating H ₂ or O _2. Yes. Furthermore, it has also been reported that a compound having a layer structure composed of In or In and Zn oxide has an activity of generating hydrogen from an aqueous methanol solution under visible light [A. KudoandI. Mikami, Chem. Lett. 1027 (1998), Literature F]. Furthermore, many attempts have been made to improve the activity in visible light by doping ZnS with various metal elements [Cataly. Lett. 58 [4], 241-243 (1999), Chem. Commun. , 1371- 1372 (2000); Document G]
[0008]
[Problems to be solved by the invention]
An object of the present invention is to propose a novel catalyst that is efficient at least in the production of H ₂ by photolysis of water in order to realize enrichment of the photocatalyst having visible light activity. Another object of the present invention is to provide a substance useful as the photocatalyst that is environmentally friendly and has no toxicity. Therefore, while examining whether the photocatalyst combining the characteristics of the valence band S3p and the compound having the layer structure with visible light activity can be designed, NaInS ₂ and AgInZn ₇ S ₉ compounds which are sulfides are used. Generation of hydrogen under visible light, and further support of Pt significantly improve the activity, and a quantum yield under visible light irradiation of 6 and 15%, respectively, As a result, it was found that a compound obtained by co-doping BiS, Pb, and Cl into ZnS was active in visible light.
[0009]
[Means for Solving the Problems]
This onset Ming is a photocatalyst having a visible light activity, characterized in that it consists NaInS _2. 2. The photocatalyst having visible light activity according to claim 1, wherein a noble metal such as a platinum catalyst is supported. The second aspect of the present invention is a visible light active water-splitting photocatalyst composed of NaInS ₂ that generates hydrogen under visible light from an aqueous solution containing SO ₃ ²⁻ and S ²⁻ ions, preferably a noble metal such as a platinum catalyst. A visible light active water-splitting photocatalyst that generates hydrogen under visible light from an aqueous solution containing the SO ₃ ²⁻ and S ²⁻ ions.
[0010]
[Embodiments of the present invention]
The present invention will be described in more detail.
A. The first NaInS ₂ of the present invention has a layer structure schematically shown in FIG. 1a, and has a layer structure in which Na is present between sulfide layers connected to the octahedron shown in b.
Compared to oxides, sulfides are inferior in stability to oxidation like the optical semiconductor CdS, but are attractive compounds as photocatalysts because they have absorptivity in the visible light region. Therefore, we studied the synthesis method of In sulfide, and gradually added the aqueous sodium sulfide solution to the mixed aqueous solution of indium nitrate and sodium nitrate, and then reacted at room temperature to obtain an amorphous precursor in the XRD measurement of NaInS ₂ ( In ₂ S ₃ was not produced in the precipitation reaction in the presence of Na ⁺ .), This precursor was dried at 150 ° C. (432 K) in a nitrogen stream, and then heat treated at 300 ° C. (573 K) for crystallinity. NaInS ₂ was obtained.
According to the XRD measurement, it was a single phase and the BET surface area was 14 m ² / g. The activity as a photocatalyst was obtained by treating the crystalline NaInS ₂ in water at room temperature.
The crystalline NaInS ₂ can significantly improve the catalytic activity by supporting a noble metal such as Pt.
[0011]
B. The sulfide photocatalyst of the present invention is expected to be effective when combined with the use of a sulfur-based reducing agent by-produced in the petrochemical industry or present in large quantities on the earth.
[0012]
【Example】
EXAMPLES Hereinafter, although an Example demonstrates this invention concretely, this invention is not interpreted limitedly by this illustration.
Description of the device for measuring the characteristics of the obtained photocatalyst;
For XRD, MiniFlex manufactured by Rigaku Corporation was used.
As BET, Coulter SA3100B was used.
The diffuse reflection spectrum was measured using Ubest V570 manufactured by JASCO Corporation.
[0013]
Example 1
Preparation of a photocatalyst comprising NaInS _{2 In} 80 mL of a mixed aqueous solution of 99.99% high-purity indium nitrate and 99.0% sodium nitrate each having a concentration of 0.25 mol / L, a concentration of 1.25 mol / L After gradually adding 120 mL of a sodium sulfide solution, the mixture was reacted at room temperature for 20 hours to obtain a NaInS ₂ precursor. The precursor was amorphous from XRD measurement. In ₂ S ₃ was not generated in the precipitation reaction in the presence of Na ⁺ . The obtained precursor powder was dried at 150 ° C. for 0.5 hours in a nitrogen stream, and then heat-treated at 300 ° C. (573 K) for 2 hours. As a result, crystalline NaInS ₂ was obtained. Light yellow. It was a single phase by XDR measurement, and the BET surface area was 14 m ² / g. This crystalline NaInS ₂ was further treated in room temperature water for about 10 hours. 0.7 g of the obtained NaInS ₂ powder was suspended in potassium sulfite (K ₂ SO ₃ ) having a concentration of 0.5 mol / L and a volume of 320 mL, and irradiated with visible light using a 300 W xenon lamp and an ultraviolet light cut filter. did. The produced H ₂ was quantified by gas chromatography directly connected to the closed circulation system. The diffuse reflection spectrum was measured with an ultraviolet-visible near-infrared spectrophotometer. FIG. 1 shows the structure of NaFeO ₂ type NaInS ₂ . This is composed of an anion layer shared by edges of an InS ₆ octahedron and a sodium ion layer between the anion layers. This structure is different from a layered semiconductor such as MoS ₂ . Absorption of NaInS ₂ occurs in the indium sulfide anion layer, and this conductor and valence band are considered to be In5s and S3p, respectively.
[0014]
The diffusion spectrum of NaInS ₂ is shown in FIG. The amorphous precursor was white, but the heat-treated crystal was light yellow. Further, NaInS ₂ treated with water had an orange-yellow color, and had a large absorption in the visible light region. The band gap of NaInS ₂ can be estimated as 2.3 eV from the absorption edge wavelength. There was no change in the XDR pattern before and after the water treatment.
FIG. 3 shows 28 μmol / 1 hour H ₂ production activity from aqueous sodium sulfite aqueous solution of NaInS ₂ treated with water under visible light irradiation. Even when no Pt promoter was supported, H ₂ production activity was exhibited. As can be seen from FIG. 3, it can be seen that the activity is dramatically increased by loading the Pt promoter. This indicates that the conduction band level of NaInS ₂ is slightly higher than the reduction potential of water. Initially, H ₂ was produced at a rate of 470 μmol / hour. The quantum yield at this time was about 6% at a wavelength of 440 nm. Even with the heat-treated catalyst, the color changed during the reaction in the same manner as the water treatment, and photocatalytic activity was obtained. The activity spectrum of the H ₂ production reaction using crystalline NaInS ₂ treated with water is in good agreement with the diffuse reflectance spectrum, demonstrating that hydrogen generation proceeds by band gap excitation.
【The invention's effect】
As described above, the sulfide photocatalyst of the present invention has an excellent effect that a bandgap of 2.5 eV to 2.3 eV was able to be provided.
[Brief description of the drawings]
FIG. 1 is a sulfide layer b in which a NaInS ₂ wurtzite layer structure a and an octahedron are connected.
[2] 300 ° C. (573K) heat treatment and diffuse reflectance spectra of the heat treatment after the hydrothermal treatment was NaInS ₂ [3] 300 ° C. (573K) heat treatment NaInS ₂ photocatalyst H from _K 2 SO ₃ aqueous solution under visible light irradiation by ₂ production reaction

Claims (4)

Photocatalyst with visible light activity, characterized in that it consists NaInS _2. The photocatalyst having visible light activity according to claim 1, wherein a platinum catalyst is supported. A visible light active water splitting photocatalyst composed of NaInS ₂ that generates hydrogen under visible light from an aqueous solution containing SO ₃ ²⁻ and S ²⁻ ions. Visible light activated water decomposition catalysts according to claim 3, characterized in that platinum is supported catalysts.

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