Photocatalyst for water splitting

Abstract

To provide a photocatalyst for water decomposition using a catalytic promoter having high water decomposing ability without using rhodium, which is a rare metal.SOLUTION: A photocatalyst for water decomposition comprises an optical semiconductor and a catalytic promoter, wherein the catalytic promoter comprises both Co and Cr, and also comprises Ru and / or Pt. The total content of the Ru and / or Pt is preferably 0.01 mass% or more and 10 mass% or less relative to 100 mass% of the photocatalyst. Each content of Co and Cr is preferably 0.01 mass% or more and 10 mass% or less relative to 100 mass% of the optical semiconductor.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a photocatalyst for water splitting. [Background technology]

[0002] The practical application of high-performance photoenergy conversion systems that utilize renewable energy such as solar energy has become increasingly important in recent years, from the perspective of curbing global warming and moving away from dependence on dwindling fossil fuels. In particular, the technology to produce hydrogen by splitting water using solar energy is essential not only as a raw material supply technology for current petroleum refining, ammonia, and methanol, but also in the coming hydrogen energy society based on fuel cells.

[0003] Water splitting reactions using photocatalysts have been widely studied since the 1970s. Some of these photocatalysts, such as ZrO2, exhibit sufficient activity on their own, but in many cases, a co-catalyst plays a major role. It is known that the reaction rate of various photocatalysts can be significantly improved by supporting a noble metal such as Pt, RuO2, or NiO on the photocatalyst as a co-catalyst for hydrogen generation (reduction reaction) (Non-Patent Document 1).

[0004] The water decomposition reaction in an acidic aqueous solution on photocatalyst particles is presumed to occur as follows. H2O+2h + →1 / 2O2+2H + (1) 2H + +2e - →H2(2) The effect of the co-catalyst is to lower the activation energy of these reactions and promote the reactions. H2+1 / 2O2→H2O (3) If the combustion reaction represented by the following formula is catalyzed, this is undesirable because it is the reverse reaction of the water decomposition reaction. Therefore, the co-catalyst is required to catalyze the above reactions (1) and (2) but not the above reaction (3). It has also been proposed to use not only one type of promoter but also a combination of several types.

[0005] Generally, platinum group metals have a high catalytic effect, but they also catalyze the above reaction (3) in addition to the reactions (1) and (2). Therefore, if one attempts to utilize the high catalytic effect of platinum group metals to obtain a high rate of hydrogen and oxygen production in the water splitting reaction, it becomes necessary to use other promoters in combination (Non-Patent Document 2).

[0006] As an example of a combination of a platinum group metal with another promoter, Patent Document 1 discloses the use of rhodium and chromium oxides in combination as promoters, and states that a high water splitting capacity can be obtained. However, the use of rare metals such as rhodium is undesirable from the viewpoint of securing resources, and there is a demand for promoters using other metals that have equal or greater water splitting capabilities. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] Chem.Soc.Rev.,2009,38,253-278 [Non-patent document 2] Wang, Qian, and Kazunari Domen. “Particulate photocatalysts for light-driven water splitting: mechanisms, challenges, and design strategies.”Chemical reviews 120.2(2019):919-985. [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-185605 Summary of the Invention [Problem to be solved by the invention]

[0009] An object of the present invention is to provide a water splitting photocatalyst that uses a co-catalyst with high water splitting ability. Another object of the present invention is to provide a water splitting photocatalyst that uses a co-catalyst with high water splitting ability without using rhodium, a rare metal. [Means for solving the problem]

[0010] As a result of extensive research aimed at solving the above problems, the present inventors have found that a high rate of hydrogen and oxygen production can be achieved by using a promoter comprising a platinum group metal, Ru and / or Pt, in combination with Co and Cr. The present inventors have also found that by using such a promoter in combination with SrTiO3 containing Al, which is an optical semiconductor, a high rate of hydrogen and oxygen production can be achieved. The present invention was achieved based on these findings and is summarized as follows.

[0011] [1] A photocatalyst for water splitting, comprising an optical semiconductor and a co-catalyst, wherein the co-catalyst contains both Co and Cr, and also Ru and / or Pt.

[0012] [2] The photocatalyst for water splitting according to [1], wherein the content of Ru and / or Pt is 0.01% by mass or more and 10% by mass or less in total, with the photocatalyst being 100% by mass.

[0013] [3] The photocatalyst for water splitting according to [1] or [2], wherein the Co and Cr contents are each 0.01% by mass or more and 10% by mass or less, with the optical semiconductor being 100% by mass.

[0014] [4] The photocatalyst for water splitting according to any one of [1] to [3], wherein the optical semiconductor is SrTiO3 containing Al.

[0015] [5] The photocatalyst for water splitting according to any one of [1] to [4], wherein the Al content in the Al-containing SrTiO3 is 0.01 mol % or more and 20 mol % or less, with SrTiO3 being 100 mol %. [Effects of the Invention]

[0016] According to the photocatalyst for water splitting of the present invention, by using a promoter having high water splitting ability, it is possible to realize a high rate of production of hydrogen and oxygen without using rhodium, which is a rare metal. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not limited to the following description and can be implemented by any modifications within the scope of the gist of the present invention. In this specification, when an expression using "~" with a numerical value or physical property value before and after it is used, it is used as including the values ​​before and after it.

[0018] The water splitting photocatalyst of the present invention is a water splitting photocatalyst comprising an optical semiconductor and a co-catalyst, characterized in that the co-catalyst contains both Co and Cr, and also Ru and / or Pt.

[0019] The cocatalyst for the photocatalyst for water splitting of the present invention (hereinafter, sometimes referred to as "the cocatalyst of the present invention") will be described. The promoter of the present invention contains both Co and Cr, and also Ru and / or Pt. When Ru and / or Pt alone is used as the promoter, a high rate of hydrogen and oxygen production is not realized, but when Co and Cr are used as the promoter in addition to Ru and / or Pt, a high rate of hydrogen and oxygen production is realized.

[0020] The method for supporting the co-catalyst in the photocatalyst for water splitting of the present invention is not particularly limited, and for example, photoelectrodeposition or impregnation can be used. In particular, photoelectrodeposition allows the co-catalyst to be selectively supported in the vicinity of the active site of the optical semiconductor, thereby enhancing the effect of the co-catalyst.

[0021] A common method of photoelectrodeposition is to disperse a substance containing an optical semiconductor and a metal used as a co-catalyst in a medium, and then irradiate the medium with light to electrodeposit the desired metal onto the surface of the optical semiconductor. In this case, light irradiation may be performed after dispersing all of the desired metals, but it is more preferable to electrodeposit the elements constituting the co-catalyst one by one in sequence. As the medium, it is generally preferable to use water, particularly pure water including deionized water, in order to prevent the inclusion of impurities.

[0022] Although there are no particular limitations on the metal-containing substances used, in terms of availability, electrodeposition efficiency, etc., it is preferable to use a Co(NO3)2 aqueous solution for Co, a K2CrO4 aqueous solution for Cr, a HPtCl6 aqueous solution for Pt, and a RuCl3 aqueous solution for Ru. However, other acid salts, chlorides, etc. may also be used as long as they can be electrodeposited by light. There are no particular limitations on the concentration of each promoter metal aqueous solution, but from the viewpoint of the stability of the aqueous solution, it is preferable to use an aqueous solution with a promoter metal compound concentration of about 0.05 to 1 mass %.

[0023] These promoter metal aqueous solutions are used for photoelectrodeposition in an amount that contains the amount of metal to be electrodeposited onto the optical semiconductor by light. The order of electrodeposition is not particularly limited, but the order of Pt, Co, Cr or Ru, Cr, Co is preferred. It is also possible to use Pt and Ru in combination, in which case either Pt or Ru may be electrodeposited first.

[0024] Furthermore, metals other than Pt, Ru, Co, and Cr may be added as long as they do not impede the effects of the present invention, but the amount is preferably 10 mass % or less of the total amount of Pt, Ru, Co, and Cr.

[0025] Next, the optical semiconductor for use as a photocatalyst for water splitting of the present invention (hereinafter sometimes referred to as "the optical semiconductor of the present invention") will be described. Optical semiconductors generate electrons and holes by absorbing ultraviolet or visible light, and examples include metal oxides, composite oxides, nitrides, oxynitrides, sulfides, and oxysulfides. Optical semiconductors with different crystal planes exposed are particularly suitable for photocatalysis because electrons and holes are less likely to recombine. Examples of such catalysts include TiO2, WO3, BiVO4, BiOBr, BiOCl, PbTiO3, Ag3PO4, SrTiO3, and Al-containing SrTiO3, with Al-containing SrTiO3 (hereinafter sometimes referred to as "Al-containing SrTiO3") being particularly preferred. In this optical semiconductor, the {100} and {110} planes are exposed.

[0026] The Al content in Al-containing SrTiO3 is preferably 0.01 mol % to 20 mol %, more preferably 0.05 mol % to 15 mol %, and even more preferably 0.1 mol % to 10 mol %, based on 100 mol % of SrTiO3. If the Al content is equal to or greater than the lower limit, the {110} plane growth and the effect of suppressing photocarrier recombination can be significantly achieved, and if it is equal to or less than the upper limit, the SrTiO3 structure is not significantly disturbed, which is preferable. Al-containing SrTiO3 can be synthesized according to a conventionally known method.

[0027] As described above, the co-catalyst of the present invention essentially contains Co and Cr. The contents of Co and Cr in the photocatalyst for water splitting of the present invention are preferably from 0.01 to 10% by mass, more preferably from 0.02 to 5% by mass, and even more preferably from 0.03 to 1% by mass, based on 100% by mass of the optical semiconductor of the present invention. If the contents of Co and Cr are each equal to or greater than the respective lower limits, the promoter effect can be sufficiently obtained, and if they are equal to or less than the respective upper limits, reduction in the surface area of ​​the promoter due to aggregation is unlikely to occur, which is preferable.

[0028] The co-catalyst of the present invention essentially contains Ru and / or Pt. The total content of Ru and / or Pt in the photocatalyst for water splitting of the present invention is preferably 0.01% by mass or more and 10% by mass or less, more preferably 0.02% by mass or more and 5% by mass or less, and even more preferably 0.03% by mass or more and 1% by mass or less, based on 100% by mass of the optical semiconductor. If the total content of Ru and / or Pt is equal to or greater than the lower limit, the promoter effect can be sufficiently obtained, and if it is equal to or less than the upper limit, reduction in the surface area of ​​the promoter due to aggregation is unlikely to occur, which is preferable.

[0029] The total content of Co, Cr, Ru and / or Pt in the photocatalyst for water splitting of the present invention is not particularly limited as long as the content of each metal is within the above-mentioned range, but is preferably 0.01 to 10 mass%, particularly 0.03 to 3 mass%, based on 100 mass% of the optical semiconductor. Furthermore, the ratio of the total content of Co and Cr to the total content of Ru and / or Pt is preferably (total amount of Co and Cr):(total amount of Ru and / or Pt)=1:0.1-10, particularly preferably 1:0.2-3, in mass ratio.

[0030] The water splitting photocatalyst of the present invention can be operated as a photocatalyst by placing the powder in water and irradiating it with light, but if necessary, it can also be used by coating it on a substrate to form a photocatalyst layer.

[0031] The wavelength of light absorbed varies depending on the optical semiconductor used, but as described in the prior art section above, the co-catalyst is required to have the property of catalyzing reactions (1) and (2) while not catalyzing reaction (3), and therefore the co-catalyst of the present invention is not limited to Al-containing SrTiO3, and can basically be considered to work with different optical semiconductors as well. However, the co-catalyst of the present invention is particularly preferably used as a co-catalyst for the above-mentioned Al-containing SrTiO3. The hydrogen and oxygen generated by splitting water using the photocatalyst for water splitting of the present invention can be separated as needed and used as raw materials or energy sources. [Example]

[0032] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. Note that the values ​​of various production conditions and evaluation results in the following examples represent preferred upper or lower limit values ​​in the embodiments of the present invention, and a preferred range may be defined by a combination of the above-mentioned upper or lower limit values ​​and the values ​​in the following examples or values ​​between the examples.

[0033] Example 1: Ru / Cr / Co / SrTiO:Al Al-doped SrTiO3 (hereafter referred to as SrTiO3:Al) was synthesized by placing commercially available SrTiO3 and Al2O3, along with SrCl2 as a flux, in an alumina crucible and firing it in air at 1150°C for 10 hours. The molar ratio was SrTiO3:Al2O3:SrCl2 = 100:2:1000. After firing, the mixture was washed in distilled water to remove excess SrCl2. Stirring and filtration were repeated in distilled water until the pH of the filtrate reached a neutral range.

[0034] Next, the cocatalyst was loaded onto the synthesized SrTiO3:Al. The loading was performed using a sequential photoelectrodeposition method. First, 0.1 g of SrTiO3:Al was suspended in 100 mL of distilled water, and a RuCl3 aqueous solution (RuCl3 concentration: 0.41 mass%) containing 0.0001 g of Ru was added. The suspension was then irradiated with light from a Xe lamp for approximately 10 minutes. The light irradiation was then stopped, and a K2CrO4 aqueous solution (K2CrO4 concentration: 0.149 mass%) containing 0.00005 g of Cr was added, followed by irradiating with light for approximately 5 minutes. Finally, a Co(NO3)2 aqueous solution (Co(NO3)2 concentration: 0.124 mass%) containing 0.00005 g of Co was added, followed by irradiating with light for approximately 5 minutes to complete the loading procedure. The sample obtained in this manner will be referred to as "Ru / Cr / Co / SrTiO3:Al" below.

[0035] The photocatalytic activity (water splitting activity) of Ru / Cr / Co / SrTiO3:Al was evaluated in a Pyrex glass reaction vessel directly connected to a closed circulation system as follows. After the cocatalyst loading was completed, the reaction system was evacuated to remove dissolved air, and then Ar was introduced to over 2 kPa and irradiated with light from a Xe lamp. The qualitative and quantitative analysis of the produced gas was carried out using gas chromatography (TCD-GC, Ar carrier, molecular sieve 5A column). The water splitting activity (H2 production rate and O2 production rate) of Ru / Cr / Co / SrTiO3:Al is shown in Table 1. As shown in Table 1, high water splitting activity was demonstrated.

[0036] Example 2: Pt / Cr / Co / SrTiO:Al 0.1 g of SrTiO3:Al synthesized in the same manner as in Example 1 was suspended in 100 mL of distilled water, to which an HPtCl6 aqueous solution (HPtCl6 concentration: 0.42 mass%) containing 0.0001 g of Pt was added, followed by approximately 10 minutes of light irradiation using a Xe lamp. The light irradiation was then stopped, and an KCrO4 aqueous solution (KCrO4 concentration: 0.149 mass%) containing 0.00005 g of Cr was added, followed by approximately 5 minutes of light irradiation. The loading procedure was then completed by adding an aqueous Co(NO3)2 solution (Co(NO3)2 concentration: 0.124 mass%) containing 0.00005 g of Co, followed by approximately 5 minutes of light irradiation. The sample obtained in this manner is hereinafter referred to as "Pt / Cr / Co / SrTiO3:Al." In the same manner as in Example 1, the water catalytic activity (water dispersion activity) of Pt / Cr / Co / SrTiO3:Al was evaluated. The water splitting activity (H2 production rate and O2 production rate) of Pt / Cr / Co / SrTiO3:Al is shown in Table 1. As shown in Table 1, high water splitting activity was demonstrated.

[0037] [Comparative example 1: Ru / Cr / SrTiO3:Al] 0.1 g of SrTiO3:Al synthesized in the same manner as in Example 1 was suspended in 100 mL of distilled water, to which an aqueous RuCl3 solution containing 0.0001 g of Ru (RuCl3 concentration: 0.41 mass%) was added, followed by approximately 10 minutes of light irradiation using a Xe lamp. The light irradiation was then stopped, and an aqueous K2CrO4 solution containing 0.00005 g of Cr (K2CrO4 concentration: 0.149 mass%) was added, followed by approximately 5 minutes of light irradiation to complete the loading procedure. The sample obtained in this manner will be referred to as "Ru / Cr / SrTiO3:Al" below. As in Example 1, the water catalytic activity (water dispersion activity) of Ru / Cr / SrTiO3:Al was evaluated. The water splitting activity (H2 production rate and O2 production rate) of Ru / Cr / SrTiO3:Al is shown in Table 1. As shown in Table 1, the water splitting activity was significantly low.

[0038] [Comparative example 2: Pt / Cr / SrTiO3:Al] 0.1 g of SrTiO3:Al synthesized in the same manner as in Example 1 was suspended in 100 mL of distilled water, to which an H2PtCl6 aqueous solution (H2PtCl6 concentration 0.42 mass%) containing 0.0001 g of Pt was added, followed by approximately 10 minutes of light irradiation using a Xe lamp. The light irradiation was then stopped, and an aqueous K2CrO4 solution (K2CrO4 concentration 0.149 mass%) containing 0.00005 g of Cr was added, followed by approximately 5 minutes of light irradiation to complete the loading procedure. The sample obtained in this manner will be referred to as "Pt / Cr / SrTiO3:Al" below. As in Example 1, the water catalytic activity (water dispersion activity) of Pt / Cr / SrTiO3:Al was evaluated. The water splitting activity (H2 production rate and O2 production rate) of Pt / Cr / SrTiO3:Al is shown in Table 1. As shown in Table 1, the water splitting activity was significantly low.

[0039] [Comparative example 3: Ru / Co / SrTiO3:Al] 0.1 g of SrTiO3:Al synthesized in the same manner as in Example 1 was suspended in 100 mL of distilled water, to which an aqueous RuCl3 solution containing 0.0001 g of Ru (RuCl3 concentration: 0.41 mass%) was added, followed by approximately 10 minutes of light irradiation using a Xe lamp. The light irradiation was then stopped, and an aqueous Co(NO3)2 solution containing 0.00005 g of Co (Co(NO3)2 concentration: 0.142 mass%) was added, followed by approximately 5 minutes of light irradiation to complete the loading procedure. The sample obtained in this manner will be referred to as "Ru / Co / SrTiO3:Al" below. As in Example 1, the water catalytic activity (water dispersion activity) of Ru / Co / SrTiO3:Al was evaluated. The water splitting activity (H2 production rate and O2 production rate) of Ru / Co / SrTiO3:Al is shown in Table 1. As shown in Table 1, the water splitting activity was significantly low.

[0040] [Comparative Example 4: Pt / Co / SrTiO3:Al] 0.1 g of SrTiO3:Al synthesized in the same manner as in Example 1 was suspended in 100 mL of distilled water, to which was added an HPtCl6 aqueous solution (HPtCl6 concentration 0.42 mass%) containing 0.0001 g of Pt, and the suspension was irradiated with light using a Xe lamp for approximately 10 minutes. The light irradiation was then stopped, and an aqueous Co(NO3)2 solution (Co(NO3)2 concentration 0.149 mass%) containing 0.00005 g of Co was added, followed by irradiating with light for approximately 5 minutes to complete the loading procedure. The sample obtained in this manner will be referred to as "Pt / Co / SrTiO3:Al" below. As in Example 1, the water catalytic activity (water dispersion activity) of Pt / Co / SrTiO3:Al was evaluated. The water splitting activity (H2 production rate and O2 production rate) of Pt / Co / SrTiO3:Al is shown in Table 1. As shown in Table 1, the water splitting activity was significantly low.

[0041] [Table 1]

[0042] Table 1 shows that the water splitting photocatalyst of the present invention can achieve very high water splitting activity by using relatively easily available Co and Cr and Pt and / or Ru, without using rhodium, a rare metal.

Claims

1. A photocatalyst for water splitting, comprising a photosemiconductor and a co-catalyst, wherein the co-catalyst contains both Co and Cr, and also contains Ru and / or Pt; A photocatalyst for water splitting, characterized in that the content of Ru and / or Pt is 0.01 mass % or more and 10 mass % or less in total, with the photocatalyst being 100 mass %.

2. 2. The photocatalyst for water splitting according to claim 1, wherein the content of Co and the content of Cr are each 0.01% by mass or more and 10% by mass or less, with the optical semiconductor being 100% by mass.

3. The optical semiconductor is SrTiO containing Al ３ 3. The photocatalyst for water splitting according to claim 1 or 2, wherein

4. The Al-containing SrTiO ３ The content of Al in SrTiO ３ The photocatalyst for water splitting according to claim 3 , wherein the content of the cations in ...

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