JP2011050895A - Semiconductor photocatalyst having performance heightened by surface modification treatment, method for production thereof, and hydrogen production method using the photocatalyst - Google Patents
Semiconductor photocatalyst having performance heightened by surface modification treatment, method for production thereof, and hydrogen production method using the photocatalyst Download PDFInfo
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- 125000004429 atom Chemical group 0.000 description 8
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- 229910052684 Cerium Inorganic materials 0.000 description 1
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- JPVYNHNXODAKFH-UHFFFAOYSA-N Cu2+ Chemical compound [Cu+2] JPVYNHNXODAKFH-UHFFFAOYSA-N 0.000 description 1
- 229910002651 NO3 Inorganic materials 0.000 description 1
- NHNBFGGVMKEFGY-UHFFFAOYSA-N Nitrate Chemical compound [O-][N+]([O-])=O NHNBFGGVMKEFGY-UHFFFAOYSA-N 0.000 description 1
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/30—Hydrogen technology
- Y02E60/36—Hydrogen production from non-carbon containing sources, e.g. by water electrolysis
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Abstract
Description
本発明は、半導体触媒に関し、特に表面改質処理により高性能化された半導体光触媒及びその製造方法並びに該光触媒を用いた水素製造方法に関する。 The present invention relates to a semiconductor catalyst, and more particularly to a semiconductor photocatalyst improved in performance by surface modification treatment, a method for producing the same, and a method for producing hydrogen using the photocatalyst.
水素エネルギーは非常にクリーンなエネルギー源であり、燃料電池や水素エンジンなど様々な応用が考えられ、まさに化石エネルギーに代わる未来のエネルギー形態の中心になると思われる。
しかし、現在の水素の大部分は化石資源のスチームリフォーミングなどで製造されている。将来の化石資源の枯渇や炭酸ガスによる地球温暖化問題などを考慮すると、最終的には無尽蔵の水を水素源にするしかない。水から水素を製造するには電気分解が簡単であるが、電気を生み出すために化石燃料を用いたのでは意味がない。
Hydrogen energy is a very clean energy source, and various applications such as fuel cells and hydrogen engines can be considered, and it will be the center of future energy forms to replace fossil energy.
However, most of the current hydrogen is produced by steam reforming of fossil resources. Considering the future depletion of fossil resources and global warming due to carbon dioxide, there is no choice but to use inexhaustible water as a hydrogen source. Producing hydrogen from water is easy to electrolyze, but it doesn't make sense to use fossil fuels to produce electricity.
そこで、無尽蔵でクリーンかつ安全な太陽エネルギーを太陽電池で電気エネルギーに変換し、水を電解して水素を製造するアイデアが提案されている。しかしながら、このアイデアの最大の欠点は、システム、特に太陽電池の高いコストおよび低いエネルギー収支（システムがその寿命までに製造するエネルギー／システムを製造するエネルギー)である。シリコンなどの太陽電池や電気分解技術は精力的に研究されてきたが、太陽光による水素製造を実現するためには、革新的な技術でシステムのコストやエネルギー収支を大幅に向上する必要がある。また、水の電気分解技術もかなり進んで来ているが、ガス発生を進行させる過電圧が非常に高く、水の理論電解電圧の１．２３Ｖよりかなり高い電圧が必要とされ、そのためのエネルギーロスも大きな問題である。 Therefore, an idea has been proposed in which inexhaustible, clean and safe solar energy is converted into electric energy by a solar cell, and water is electrolyzed to produce hydrogen. However, the biggest drawback of this idea is the high cost and low energy balance of the system, especially the solar cell (the energy that the system produces by the lifetime / energy that produces the system). Solar cells such as silicon and electrolysis technology have been studied energetically, but in order to realize solar hydrogen production, it is necessary to significantly improve the system cost and energy balance with innovative technology. . Water electrolysis technology has also advanced considerably, but the overvoltage that promotes gas generation is very high, and a voltage much higher than the theoretical electrolysis voltage of water of 1.23V is required, and energy loss for that is also required. It is a big problem.
一方、光触媒を用いて太陽光のエネルギーで水を水素と酸素に直接分解する研究も進んでいる。この技術はコストが非常に低くまたリサイクルや耐久性の面で優れているが、現段階では、その効率の低さが問題となっている。そのため、より革新的な新たなエネルギー変換システムの確立が望まれている。 On the other hand, research on direct decomposition of water into hydrogen and oxygen using solar energy using a photocatalyst is also in progress. This technology is very low cost and excellent in terms of recycling and durability, but at the present stage its low efficiency is a problem. Therefore, establishment of a more innovative new energy conversion system is desired.
本発明者らは、前記課題を解決すべく鋭意研究を重ね、ＷＯ３やＴｉＯ２、Ｉｎ２Ｏ３などの半導体光触媒を用い、Ｆｅ３＋を含有する水溶液に光照射して、酸素とＦｅ２＋を発生させ、次に、Ｆｅ２＋を含む水溶液を電解してＦｅ３＋を再生するとともに水素を発生させることにより、水素発生のための電解電圧を大幅に減少させて、非常に低コストの水素製造を可能にする方法（特許文献１）及びそのための装置（特許文献２）を提案した。
また、本発明者らは、前記Ｆｅ３＋／Ｆｅ２＋以外の新たなレドックスを開発した結果、酸化状態から還元状態に変化させることができるヨウ素化合物を含む水溶液に、光照射を行って酸素と還元状態にあるヨウ素化合物を生成させ、生成した還元状態にあるヨウ素化合物を電解して酸化状態にあるヨウ素化合物を再生すると共に水素を発生させる方法も提案している（特許文献３）。
The inventors of the present invention have made extensive studies to solve the above-mentioned problems. Using semiconductor photocatalysts such as WO 3 , TiO 2 , and In 2 O 3 , light irradiation is performed on an aqueous solution containing Fe 3+ , and oxygen and Fe 2+ are irradiated. Next, by electrolyzing an aqueous solution containing Fe 2+ to regenerate Fe 3+ and generate hydrogen, the electrolysis voltage for hydrogen generation is greatly reduced, thereby producing hydrogen at a very low cost. The method (patent document 1) and the apparatus (patent document 2) for enabling it were proposed.
In addition, as a result of developing a new redox other than the above Fe 3+ / Fe 2+ , the present inventors performed light irradiation on an aqueous solution containing an iodine compound that can be changed from an oxidized state to a reduced state to reduce oxygen and oxygen. A method has also been proposed in which an iodine compound in a state is generated, the generated iodine compound in a reduced state is electrolyzed to regenerate the iodine compound in an oxidized state and generate hydrogen (Patent Document 3).
しかしながら、前述の半導体光触媒とレドックス媒体を用いた水素製造方法・装置においては、用いている半導体光触媒の光触媒反応が未だ充分とはいえず、そのため更に高性能な新規半導体光触媒もしくは、既存の光触媒を高性能化するための手法の開発が望まれている。
本発明は上記の点に鑑みてなされたものであり、レドックス媒体を還元し、それと同時に酸素を製造する光触媒反応を高効率に進行できる光触媒を提供することを目的とするものである。
However, in the hydrogen production method and apparatus using the above-described semiconductor photocatalyst and redox medium, the photocatalytic reaction of the semiconductor photocatalyst used is still not sufficient, so that a new semiconductor photocatalyst with higher performance or an existing photocatalyst can be used. Development of a technique for improving performance is desired.
The present invention has been made in view of the above points, and an object of the present invention is to provide a photocatalyst capable of reducing the redox medium and simultaneously proceeding the photocatalytic reaction for producing oxygen with high efficiency.
本発明者らは、上記課題を解決するために鋭意検討した結果、タングステン化合物等の可視光応答性光触媒に対し、アルカリ金属、アルカリ土類金属、銀及びニッケルからなる群のうち、少なくとも１つの元素Ｍを含む溶液によって表面改質処理を行うことで、レドックス媒体を還元し、それと同時に酸素を製造する光触媒反応の活性を大きく向上させることができるという知見を得た。 As a result of intensive studies to solve the above problems, the present inventors have determined that at least one of the group consisting of alkali metals, alkaline earth metals, silver and nickel with respect to visible light responsive photocatalysts such as tungsten compounds. It was found that by performing surface modification treatment with a solution containing the element M, the redox medium can be reduced, and at the same time, the activity of the photocatalytic reaction for producing oxygen can be greatly improved.
本発明はこれらの知見に基づいて完成に至ったものであり、本発明によれば、以下の発明が提供される。
［１］可逆的なレドックス媒体の存在下、水から酸素を生成するための半導体光触媒であって、半導体の結晶粒子の表面に、アルカリ金属、アルカリ土類金属、銀、及びニッケルからなる群のうち、少なくとも１つの金属元素が取り込まれた構造を有していることを特徴とする半導体光触媒。
［２］前記半導体が、可視光応答性のタングステン化合物であることを特徴とする上記［１］の半導体光触媒。
［３］前記タングステン化合物が、酸化タングステンである上記［２］の半導体光触媒。
［４］上記［１］〜［３］のいずれかの半導体光触媒を製造する方法であって、アルカリ金属、アルカリ土類金属、銀、及びニッケルからなる群のうち、少なくとも１つの金属元素を含む金属塩溶液を用いて、水熱処理又は含浸処理することを特徴とする半導体光触媒の製造方法。
［５］前記金属塩が、硫酸塩、硝酸塩、炭酸塩、塩化物及び水酸化物のいずれかから選ばれることを特徴とする上記［４］の半導体光触媒の製造方法。
［６］上記［１］〜［３］のいずれかの半導体光触媒の存在下、可逆的なレドックス媒体の酸化体を含有する水溶液に光照射して酸素とレドックス媒体の還元体を生成させる工程、及び生成されたレドックス媒体の還元体を電解して酸化体に再生すると共に水素を発生させる工程を含むことを特徴とする水素の製造方法。
［７］前記可逆的なレドックス媒体が、鉄イオン又はヨウ素化合物イオンであることを特徴とする上記［６］に記載の水素の製造方法。
［８］上記［１］〜［３］のいずれかの半導体光触媒の存在下、可逆的なレドックス媒体の酸化体を含有する水溶液に光照射して酸素と該レドックス媒体の還元体を生成させる半導体光触媒反応装置、生成された該レドックス媒体の還元体を電解して酸化体に再生すると共に水素を発生させる電解装置、及び再生された該レドックス媒体の酸化体を前記半導体光触媒反応装置に供給する装置からなることを特徴とする水素の製造装置。
[９] 上記［１］〜［３］のいずれかの半導体光触媒が導電性基板上で膜状に存在した光電極において、光照射して光電極上で酸素を発生させ、光電極とつながった対極で水素を発生させることを特徴とする水素の製造方法。
The present invention has been completed based on these findings, and according to the present invention, the following inventions are provided.
[1] A semiconductor photocatalyst for generating oxygen from water in the presence of a reversible redox medium, wherein the surface of semiconductor crystal particles is made of an alkali metal, an alkaline earth metal, silver, and nickel. Among them, a semiconductor photocatalyst having a structure in which at least one metal element is incorporated.
[2] The semiconductor photocatalyst according to the above [1], wherein the semiconductor is a visible light responsive tungsten compound.
[3] The semiconductor photocatalyst according to the above [2], wherein the tungsten compound is tungsten oxide.
[4] A method for producing a semiconductor photocatalyst according to any one of [1] to [3], comprising at least one metal element selected from the group consisting of alkali metals, alkaline earth metals, silver, and nickel. A method for producing a semiconductor photocatalyst, characterized by hydrothermal treatment or impregnation treatment using a metal salt solution.
[5] The method for producing a semiconductor photocatalyst according to the above [4], wherein the metal salt is selected from sulfate, nitrate, carbonate, chloride and hydroxide.
[6] A step of generating oxygen and a reduced form of redox medium by irradiating an aqueous solution containing an oxidant of a reversible redox medium in the presence of the semiconductor photocatalyst according to any one of [1] to [3] above, And a method for producing hydrogen, comprising the step of electrolyzing the reductant of the produced redox medium to regenerate it into an oxidant and generating hydrogen.
[7] The method for producing hydrogen according to the above [6], wherein the reversible redox medium is an iron ion or an iodine compound ion.
[8] A semiconductor that generates an oxygen and a reduced form of the redox medium by irradiating an aqueous solution containing an oxidant of a reversible redox medium in the presence of the semiconductor photocatalyst according to any one of the above [1] to [3]. Photocatalyst reaction apparatus, electrolysis apparatus for electrolyzing the reductant of the generated redox medium to regenerate it into an oxidant and generating hydrogen, and an apparatus for supplying the regenerated oxidant of the redox medium to the semiconductor photocatalyst reaction apparatus An apparatus for producing hydrogen, comprising:
[9] In the photoelectrode in which the semiconductor photocatalyst of any one of the above [1] to [3] is present in the form of a film on the conductive substrate, light is irradiated to generate oxygen on the photoelectrode, and the counter electrode connected to the photoelectrode A method for producing hydrogen, characterized in that hydrogen is generated in the process.
本発明の表面改質処理を施したタングステン系の可視光応答性光触媒によれば、レドックス媒体を還元し、同時に酸素を生成する光触媒反応に対する活性を、未処理のものと比べ飛躍的に向上させることができる。 According to the tungsten-based visible light responsive photocatalyst subjected to the surface modification treatment of the present invention, the redox medium is reduced, and at the same time, the activity for the photocatalytic reaction that generates oxygen is drastically improved compared to the untreated one. be able to.
本発明の半導体光触媒は、可逆的なレドックス媒体の存在下、水から酸素を生成するための半導体光触媒であって、半導体の結晶粒子の表面に、アルカリ金属、アルカリ土類金属、銀、及びニッケルからなる群のうち、少なくとも１つの金属元素Ｍが取り込まれた構造を有していることを特徴とするものである。
図１は、本発明の半導体光触媒に係る半導体結晶粒子の構造を示す模式図であり、図中、１は、半導体結晶粒子、２は、表面処理で半導体結晶粒子の表面近傍内部に生成した金属Ｍと半導体との複合酸化物の薄膜層を表している。
The semiconductor photocatalyst of the present invention is a semiconductor photocatalyst for generating oxygen from water in the presence of a reversible redox medium, and an alkali metal, alkaline earth metal, silver, and nickel are formed on the surface of semiconductor crystal particles. It has a structure in which at least one metal element M is taken in the group consisting of:
FIG. 1 is a schematic view showing the structure of a semiconductor crystal particle according to the semiconductor photocatalyst of the present invention. In the figure, 1 is a semiconductor crystal particle, and 2 is a metal produced near the surface of the semiconductor crystal particle by surface treatment. The thin film layer of the complex oxide of M and a semiconductor is represented.
本発明の半導体光触媒は、アルカリ金属、アルカリ土類金属、銀及びびニッケルからなる群のうち、少なくとも１つの元素Ｍを含む金属塩溶液を用いて水熱処理法もしくは含浸法により、半導体光触媒に表面改質処理を施すことにより製造される。 The semiconductor photocatalyst of the present invention is formed on the surface of the semiconductor photocatalyst by hydrothermal treatment or impregnation using a metal salt solution containing at least one element M from the group consisting of alkali metals, alkaline earth metals, silver and nickel. Manufactured by applying a modification treatment.
半導体光触媒に対して行った上記の表面改質処理は、表面で進行する光触媒作用を促進させ、その光触媒活性を大幅に向上させることができる。
したがって、半導体光触媒に対して施されるこの表面改質処理方法は、可視光照射によって、たとえば、Ｆｅ３＋やＩＯ３ −などといったレドックス媒体を還元して、同時に酸素を製造するエネルギー蓄積型の光触媒反応に対する活性を大幅に向上させることができる。
The surface modification treatment performed on the semiconductor photocatalyst can promote the photocatalytic action that proceeds on the surface, and can greatly improve the photocatalytic activity.
Therefore, this surface modification method applied to a semiconductor photocatalyst is an energy storage type photocatalyst that reduces redox media such as Fe 3+ and IO 3 − by visible light irradiation and simultaneously produces oxygen. The activity against the reaction can be greatly improved.
表面改質処理に用いられる金属塩溶液は、アルカリ金属、アルカリ土類金属及び銀、ニッケルからなる群のうち、少なくとも１つの元素Mを含む金属塩溶液であれば特に制限はないが、特にセシウムが好ましい。 The metal salt solution used for the surface modification treatment is not particularly limited as long as it is a metal salt solution containing at least one element M from the group consisting of alkali metals, alkaline earth metals, silver, and nickel, but in particular cesium. Is preferred.
本発明における水熱処理法又は含浸法としては、図１のように、金属元素Ｍが半導体の表面近傍の結晶に取り込まれている構造を形成することのできる手法であればどのような方法であっても良いが、特に、金属を溶解した溶液を半導体粉末に含浸して適切な温度で短時間加熱する方法が好ましい。表面処理で半導体結晶粒子の表面内部に金属Mと半導体との複合酸化物の薄膜層が形成されることが好ましい。高温または長時間処理すると半導体の改質が表面だけでなく半導体結晶粒子内部にまで及ぶことになり、好ましくない。
ＸＲＤ測定を表面処理前後で行い、半導体結晶粒子のＸＲＤピークが変化していないことが好ましい。ＸＲＤ測定は、バルク全体の結晶情報を反映しているので、処理が半導体結晶粒子内部に影響を与えていないことが分かる。
また、過酷な条件（長時間処理や高温処理、高濃度処理など）で表面処理を行うと、半導体内部まで金属Ｍと半導体との複合酸化物が形成されることがＸＲＤ測定で観測できるが、その場合は活性が大きく低下するので、処理条件を緩和（短時間処理や低温処理、低濃度処理など）する必要がある。
As the hydrothermal treatment method or the impregnation method in the present invention, any method can be used as long as it can form a structure in which the metal element M is incorporated in the crystal near the surface of the semiconductor as shown in FIG. However, a method in which a semiconductor powder is impregnated in a semiconductor powder and heated at an appropriate temperature for a short time is particularly preferable. It is preferable that a thin film layer of a composite oxide of metal M and semiconductor is formed inside the surface of the semiconductor crystal particles by the surface treatment. Treatment at a high temperature or for a long time is not preferable because the modification of the semiconductor extends not only to the surface but also to the inside of the semiconductor crystal grains.
It is preferable that the XRD measurement is performed before and after the surface treatment, and the XRD peak of the semiconductor crystal particles is not changed. Since the XRD measurement reflects the crystal information of the entire bulk, it can be seen that the treatment does not affect the inside of the semiconductor crystal particles.
In addition, when surface treatment is performed under harsh conditions (long-time treatment, high-temperature treatment, high-concentration treatment, etc.), it can be observed by XRD measurement that a complex oxide of metal M and semiconductor is formed inside the semiconductor. In that case, since the activity is greatly reduced, it is necessary to relax the processing conditions (short-time processing, low-temperature processing, low-concentration processing, etc.).
また、金属元素Ｍが半導体の表面近傍の結晶に取り込まれていることは、金属元素Ｍが溶解する条件の溶液で良く洗浄した後の半導体をＸＰＳで観測できる。ＸＰＳ測定は、表面１ｎｍ前後の深さの情報が得られるので、金属元素Ｍが安定に表面に存在していることを確認することで検証できる。 In addition, the fact that the metal element M is taken into the crystal in the vicinity of the surface of the semiconductor can be observed by XPS after the semiconductor is thoroughly washed with a solution under a condition that the metal element M is dissolved. The XPS measurement can be verified by confirming that the metal element M is stably present on the surface because information on the depth of about 1 nm on the surface can be obtained.
レドックス媒体を還元し、同時に酸素を製造する光触媒反応に対して、その反応効率を促進するための手法としては、半導体の表面の上にＰｔやＲｕＯ２などの助触媒担持することが報告されている（J.Phys.Chem.1984,88,4001.）。助触媒は反応中に溶解しない安定な元素材料を半導体に担持して用いる。アルカリ金属など元素Ｍが半導体表面上に担持や吸着しているだけでは、酸性溶液中に簡単にほとんど脱離してしまうので活性向上効果は無い。 For photocatalytic reactions that reduce redox media and at the same time produce oxygen, it has been reported that promoters such as Pt and RuO 2 are supported on the surface of the semiconductor as a method for promoting the reaction efficiency. (J. Phys. Chem. 1984, 88, 4001.). The cocatalyst is used by supporting a stable element material that does not dissolve during the reaction on a semiconductor. If the element M, such as an alkali metal, is merely supported or adsorbed on the semiconductor surface, the activity is not improved because the element M is easily desorbed in the acidic solution.
一方、本発明では、半導体結晶粒子自体を改質し、元素Ｍが半導体表面近傍の内部に存在している。アルカリ金属など反応中に溶解する不安定な元素材料を用いることができる。また、例えば銀の場合、半導体結晶粒子の表面に光電着という手法で銀粒子を担持することができるが、これでは活性は向上しない。銀イオンを含む水溶液と半導体をオートクレーブに入れて高温で水熱処理することで銀が半導体の一部と複合し、半導体表面近傍の内部にまで存在することで、活性が向上できる。本発明の半導体光触媒を水溶液に入れると中に外表面に露出している元素Ｍが一部外れることもあるが、半導体表面近傍の内部は構造が保持されるので、活性は向上したまま持続できる。外表面に露出している元素Ｍが他の元素とイオン交換しても良い。イオン交換するイオンとしては、酸素発生の中間体である過酸化水素の自己酸化分解を促進するイオンが好ましい。つまり、＋０．６８２〜＋１．７７V（SHE）の間の標準酸化還元準位であるイオンが好ましく、例えば、鉄イオンや銅イオン、マンガンイオン、クロムイオン、セリウムイオンなどがあるが、特にＦｅ２＋とイオン交換するのは好ましい。 On the other hand, in the present invention, the semiconductor crystal particles themselves are modified, and the element M exists in the vicinity of the semiconductor surface. An unstable element material that dissolves during the reaction, such as an alkali metal, can be used. Further, for example, in the case of silver, silver particles can be supported on the surface of the semiconductor crystal particles by a technique called photo-deposition, but this does not improve the activity. An aqueous solution containing silver ions and a semiconductor are placed in an autoclave and subjected to hydrothermal treatment at a high temperature, so that silver is combined with a part of the semiconductor and exists in the vicinity of the semiconductor surface, whereby the activity can be improved. When the semiconductor photocatalyst of the present invention is put in an aqueous solution, the element M exposed on the outer surface may be partially removed, but the structure is maintained inside the vicinity of the semiconductor surface, so that the activity can be maintained while being improved. . The element M exposed on the outer surface may be ion exchanged with other elements. As the ion to be ion-exchanged, an ion that promotes auto-oxidative decomposition of hydrogen peroxide, which is an intermediate for generating oxygen, is preferable. In other words, ions are preferably standard redox level between + 0.682~ + 1.77V (SHE), for example, iron ions, copper ions, manganese ions, chromium ions, and the like cerium ions, in particular Fe 2+ It is preferred to ion exchange with.
これらの金属塩溶液は、一般的には半導体光触媒に対して０．０１atom％〜１０atom％の範囲内において使用することが考慮される。この使用量については、金属塩溶液と可視光応答性光触媒の種類、そして、光触媒粉末の表面積などによって具体的に定めることができる。 These metal salt solutions are generally considered to be used in the range of 0.01 atom% to 10 atom% with respect to the semiconductor photocatalyst. About this usage-amount, it can determine concretely with the kind of metal salt solution and visible light responsive photocatalyst, the surface area of photocatalyst powder, etc.
本発明に係る可視光応答性光触媒の触媒活性促進効果が、当該光触媒の機能を補填し、レドックス媒体の還元作用を促進する理由の詳細は現時点では明らかではないが、以下のように推定している。
半導体結晶粒子表面の金属Ｍと半導体との複合酸化物の薄膜層が形成された部分では、光励起で生成した正孔と水との反応がすみやかに進行し、酸素発生が効率よく進行すると考えられる。または、半導体結晶粒子表面の金属Ｍと半導体との複合酸化物の薄膜層が形成された部分では、レドックス酸化体の吸着がすみやかに進み、光励起で生成した電子とレドックス酸化体との反応がすみやかに進行し、レドックス還元が効率よく進行すると考えられる。
The details of the reason why the catalytic activity promoting effect of the visible light responsive photocatalyst according to the present invention supplements the function of the photocatalyst and promotes the reduction action of the redox medium is not clear at present, but is estimated as follows. Yes.
It is considered that the reaction between the holes generated by photoexcitation and water proceeds promptly and oxygen generation proceeds efficiently in the portion where the thin film layer of the composite oxide of metal M and semiconductor on the semiconductor crystal particle surface is formed. . Alternatively, in the portion where the thin film layer of the composite oxide of the metal M and the semiconductor on the surface of the semiconductor crystal particle is formed, the adsorption of the redox oxidant proceeds promptly, and the reaction between the electron generated by photoexcitation and the redox oxidant is rapid. It is considered that redox reduction proceeds efficiently.
このように、可視光応答性光触媒を用いたレドックス媒体を還元して同時に酸素を製造するエネルギー蓄積型の光触媒反応において、反応効率を低下させる要因をうまく抑制することで、劇的に反応活性を向上させることができる。
以上の理由から、本発明の表面処理法と併用できる可視光応答性光触媒としては、可視光を吸収することにより、レドックス媒体を還元し、同時に酸素を製造できるものであれば、その光触媒活性の大小に拘わらず、従来公知の可視光応答性半導体化合物、例えば、ＢｉＶＯ４などの複合酸化物、ＴａＯＮなどの含窒素化合物、Ｎ−ＴｉＯ２などのドープ化合物などの何れも使用できるが、特にタングステン化合物半導体に対して特に有効である。
In this way, in the energy storage type photocatalytic reaction in which redox medium using visible light responsive photocatalyst is reduced to produce oxygen at the same time, the reaction activity is dramatically reduced by successfully suppressing the factors that lower the reaction efficiency. Can be improved.
For the above reasons, as a visible light responsive photocatalyst that can be used in combination with the surface treatment method of the present invention, any photocatalytic activity can be used as long as it can reduce redox medium and absorb oxygen at the same time by absorbing visible light. Regardless of the size, any conventionally known visible light responsive semiconductor compound such as a composite oxide such as BiVO 4 , a nitrogen-containing compound such as TaON, and a doped compound such as N—TiO 2 can be used. This is particularly effective for compound semiconductors.
タングステン系可視光応答性光触媒の半導体化合物としては、タングステン酸化物、タングステンを含む複合化合物が用いられる。酸素欠陥のあるＷＯｘや異種金属やアニオン（Ｎ,Ｃ,Ｓ）をドーピングや置換した化合物でも良い。タングステンと同族であり似た特性を持つモリブデン化合物との固溶体半導体も用いることができる。
半導体光触媒の具体例は、ＷＯｘ（Ｘ≦３）、ＷｙＭｏ１−ｙＯｘ（x≦３、y≦１）、Ｂｉ２ＷＯ６、Ｂｉ２ＭｏＯ６が例示される。その中でも、酸化タングステンが特に好ましい。
As the semiconductor compound of the tungsten-based visible light responsive photocatalyst, a composite compound containing tungsten oxide and tungsten is used. A compound in which WOx having an oxygen defect, a different metal, or an anion (N, C, S) is doped or substituted may be used. A solid solution semiconductor with a molybdenum compound which is similar to tungsten and has similar characteristics can also be used.
Specific examples of the semiconductor photocatalyst include WO x (X ≦ 3), W y Mo 1-y O x (x ≦ 3, y ≦ 1), Bi 2 WO 6 , and Bi 2 MoO 6 . Among these, tungsten oxide is particularly preferable.
次に、本発明に係る可視光応答性光触媒に対する代表的な表面改質処理方法について説明する。
その一つはタングステン化合物に対して炭酸セシウム溶液を含浸するものである。この場合、典型的には、炭酸セシウム水溶液を磁性るつぼにとり、そこにタングステン化合物をケン濁させる。そして、１００℃に加温したホットプレート上で蒸発乾固させ、その後電気炉を用い５００℃で１０分間焼成する。最後に表面のセシウムを取り除くために、純水で洗浄する。光触媒を導電性基板上に成膜した半導体光電極に対しては、作成した光電極に炭酸セシウム水溶液を滴下して常温で乾燥させ、その後５００℃で１０分間焼成する。
Next, a representative surface modification method for the visible light responsive photocatalyst according to the present invention will be described.
One is to impregnate a tungsten compound with a cesium carbonate solution. In this case, typically, an aqueous cesium carbonate solution is placed in a magnetic crucible, and a tungsten compound is suspended therein. Then, it is evaporated to dryness on a hot plate heated to 100 ° C., and then baked at 500 ° C. for 10 minutes using an electric furnace. Finally, in order to remove the surface cesium, it is washed with pure water. For a semiconductor photoelectrode in which a photocatalyst is formed on a conductive substrate, an aqueous cesium carbonate solution is dropped onto the prepared photoelectrode, dried at room temperature, and then baked at 500 ° C. for 10 minutes.
以下に酸化タングステン粉末を、炭酸セシウム水溶液を用いて表面改質処理する場合を例として説明する。酸化タングステン粉末を、炭酸セシウム水溶液を用いて表面処理する場合には、含浸するセシウムは酸化タングステン粉末に対して０．１atom％〜１０atom％が好ましく、より好ましくは１atom％である。水熱処理法でも同様の量を用いるのが好ましい。 Hereinafter, a case where the surface modification treatment is performed on the tungsten oxide powder using a cesium carbonate aqueous solution will be described. When the surface treatment of the tungsten oxide powder is performed using an aqueous cesium carbonate solution, the impregnated cesium is preferably 0.1 atom% to 10 atom%, more preferably 1 atom% with respect to the tungsten oxide powder. It is preferable to use the same amount in the hydrothermal treatment method.
本発明に係る半導体光触媒に対する表面改質処理方法は、レドックス媒体を還元して、同時に酸素を製造するエネルギー蓄積型の光触媒反応を進行するための触媒として極めて有効である。
レドックス媒体としては、いくつかのレドックスが使用できるが、特に鉄イオンの他にＩＯ３ −等のヨウ素化合物に有効である。
The surface modification method for a semiconductor photocatalyst according to the present invention is extremely effective as a catalyst for proceeding an energy storage type photocatalytic reaction in which a redox medium is reduced and oxygen is produced at the same time.
Several redox media can be used as the redox medium, but it is particularly effective for iodine compounds such as IO 3 − in addition to iron ions.
以下、本発明を実施例によりさらに具体的に説明するが、本発明はこの実施例によって何ら限定されるものではない。
（実施例１〜８）
様々な金属塩水溶液にて表面処理したＷＯ３を以下のように調製した。
高純度化学製のＷＯ３粉末１ｇと表１に示した様々な金属塩水溶液１０ｍＬを、タングステンと金属イオンの化学両論比１：０．０１になるようにＰＡＲＲ社製の酸分解用反応容器（４７４９）に仕込んだ。そして、２４０度に加温したオーブンの中で１２時間水熱処理した。その後、純水で十分に洗浄し、７０度に加温されたオーブンで乾燥した。Ｘ線回折計（ＸＲＤ；ＭＸ Ｌａｂｏマックサイエンス社製）によりＷＯ３が主生成物であることを同定した。
これらの表面処理されたＷＯ３粉末（０．４ｇ）とＦｅ３＋を含んだ反応溶液（４．２ｍＭ／３００ｍＬ）を側面照射型の反応管に入れ、ガスクロと真空ポンプを備えた閉鎖循環系に接続した。反応溶液はマグネチックスターラーで攪拌した。そして、閉鎖循環系内を真空脱気したのち、光源にＬ４２のカットオフフィルターで照射光を可視光に制御した３００ＷのＸｅランプを用いて光照射を行った。その後、生成した気体をガスクロにて定性定量した。その結果を表１に示す。
Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the examples.
(Examples 1-8)
WO 3 surface-treated with various aqueous metal salt solutions was prepared as follows.
A reaction vessel for acid decomposition manufactured by PARR was prepared by adding 1 g of WO 3 powder made of high purity chemical and 10 mL of various metal salt aqueous solutions shown in Table 1 so that the stoichiometric ratio of tungsten and metal ions was 1: 0.01. 4749). Then, hydrothermal treatment was performed for 12 hours in an oven heated to 240 degrees. Thereafter, it was thoroughly washed with pure water and dried in an oven heated to 70 degrees. It was identified by the X-ray diffractometer (XRD; manufactured by MX Labo Mac Science) that WO 3 was the main product.
These surface-treated WO 3 powder (0.4 g) and a reaction solution (4.2 mM / 300 mL) containing Fe 3+ were placed in a side-irradiation type reaction tube, and a closed circulation system equipped with a gas chromatograph and a vacuum pump was placed. Connected. The reaction solution was stirred with a magnetic stirrer. Then, after the inside of the closed circulation system was vacuum degassed, light irradiation was performed using a 300 W Xe lamp in which the irradiation light was controlled to be visible light with a cut-off filter of L42 as a light source. Thereafter, the generated gas was qualitatively determined by gas chromatography. The results are shown in Table 1.
ＷＯ３は、水熱処理を行うだけでも、性能が向上することがわかった。それに対して、Ｒｈ、Ｉｒ、Ｃｒ、Ｃｏ、Ｌａ、Ｂ、そしてＰｔなどの様々な種類の金属イオン存在下で水熱処理したものは、活性が変わらないかまたは減少した。一方、アルカリ金属、アルカリ土類金属、そして銀、ニッケルで処理したものは、特異的に酸素生成活性が向上することがわかった。特に、セシウムで処理したもので性能が最も高かった。水熱処理された粒子の表面状態を光電子スペクトル計(Ulvac-Phi; XPS-1800)および電子顕微鏡（ＳＥＭ；Hitachi S-800）により確認した。その結果、粒径や見た目には大きな変化はなかった。ＸＲＤ測定でも変化はなかった。ＸＰＳ測定では表面に表面改質処理した元素が存在していることが確かめられた。タングステン化合物は一般的にアルカリ性に弱いが、このように条件を最適化すれば表面付近のみ改質することができる。アルカリ金属、アルカリ土類金属、銀、ニッケルを加熱せずに半導体表面上に吸着や光電着させただけでは活性は向上しなかった。つまり、ある程度の温度で水熱加熱処理することで、半導体表面自体を改質する必要があることが言える。 WO 3 has been found to improve performance simply by performing hydrothermal treatment. In contrast, the hydrothermal treatment in the presence of various types of metal ions such as Rh, Ir, Cr, Co, La, B, and Pt did not change or decreased in activity. On the other hand, it was found that the oxygen generation activity was specifically improved in the case of treatment with alkali metal, alkaline earth metal, silver or nickel. In particular, those treated with cesium had the highest performance. The surface state of the hydrothermally treated particles was confirmed by a photoelectron spectrometer (Ulvac-Phi; XPS-1800) and an electron microscope (SEM; Hitachi S-800). As a result, there was no significant change in particle size or appearance. There was no change in the XRD measurement. The XPS measurement confirmed that the surface-modified element was present on the surface. Tungsten compounds are generally weak in alkalinity, but if the conditions are optimized in this way, only the vicinity of the surface can be modified. The activity was not improved by simply adsorbing or photo-depositing the alkali metal, alkaline earth metal, silver or nickel on the semiconductor surface without heating. That is, it can be said that it is necessary to modify the semiconductor surface itself by performing a hydrothermal treatment at a certain temperature.
（実施例９〜１４）
炭酸セシウム水溶液にて表面処理したＷＯ３を以下のように調製した。
高純度化学製のＷＯ３粉末と表２に示した様々な濃度の炭酸セシウム水溶液１０ｍＬを、タングステンと金属イオンの化学両論比１：０．０１〜０．１になるようにＰＡＲＲ社製の酸分解用反応容器（４７４９）に仕込んだ。そして、２４０℃に加温したオーブンの中で１２時間水熱処理した。その後、純水で十分に洗浄し、７０℃に加温されたオーブンで乾燥した。Ｘ線回折計（ＸＲＤ；MX Laboマックサイエンス社製）によりＷＯ３が主生成物であることを同定した。紫外可視分光光度計（ＤＲＳ；Jasco;Ubest-570）により、吸収スペクトルに変化が見られないことを確認した。
セシウムの仕込み量を変化させて水熱処理により表面処理を行ったＷＯ３を用いて、実施例１と同様の実験条件において、光触媒活性を比較した結果を表２に示した。この条件では０．０１atom％では効果が無く、それを超えるセシウムイオン濃度が必要であった。１atom％の量のセシウムイオンを共存させた溶液下で水熱処理することで最も高い活性を示すことがわかった。
(Examples 9 to 14)
WO 3 surface-treated with an aqueous cesium carbonate solution was prepared as follows.
A high-purity chemical WO 3 powder and 10 mL of various concentrations of cesium carbonate aqueous solutions shown in Table 2 were prepared by using an acid manufactured by PARR so that the stoichiometric ratio of tungsten to metal ions was 1: 0.01 to 0.1. A decomposition reaction vessel (4749) was charged. And it hydrothermally treated for 12 hours in the oven heated at 240 degreeC. Thereafter, it was thoroughly washed with pure water and dried in an oven heated to 70 ° C. It was identified by the X-ray diffractometer (XRD; manufactured by MX Labo Mac Science) that WO 3 was the main product. It was confirmed by an ultraviolet-visible spectrophotometer (DRS; Jasco; Ubest-570) that no change was observed in the absorption spectrum.
Table 2 shows the results of comparison of the photocatalytic activity under the same experimental conditions as in Example 1 using WO 3 which was surface-treated by hydrothermal treatment while changing the amount of cesium charged. Under these conditions, there was no effect at 0.01 atom%, and a cesium ion concentration exceeding that was required. It was found that the highest activity was exhibited by hydrothermal treatment in a solution coexisting with an amount of 1 atom% of cesium ions.
（実施例１５〜１９）
炭酸セシウム水溶液にて表面処理したＷＯ３を実施例１〜１４とは別法にて調製した。高純度化学製のＷＯ３粉末１ｇを前処理として７００℃〜９００℃で焼成し、その後、純水１０ｍLを入れたＰＡＲＲ社製の酸分解用反応容器（４７４９）に仕込んで、１００℃〜２４０℃に加温したオーブンの中で１２時間水熱処理した。次に、表面改質処理として、任意の量のセシウム溶液を入れた磁性るつぼにＷＯ３（０．５ｇ）をケン濁させ、１００℃に加温したホットプレート上で蒸発乾固した。そして５００℃で１０分焼成した後に、純水で十分に洗浄し、７０℃に加温されたオーブンで乾燥した。Ｘ線回折計（ＸＲＤ；ＭＸ Ｌａｂｏマックサイエンス社製）によりＷＯ３が主生成物であることを同定した。種々の条件で表面処理を行ったＷＯ３を用いて、実施例１および２と同様の実験条件において、光触媒活性を比較した結果を表３に示した。ＷＯ３光触媒は、セシウムを添加せず、焼成と水熱処理のみでも、条件を最適化すれば、４倍程度まで活性が向上することがわかった。さらに、その最適条件で調製したＷＯ３に対し、セシウムを含浸し、表面処理を行うことでさらに２倍以上活性が向上した。最適化された条件で改めて水熱処理時にセシウムを共存させ、表面処理したＷＯ３は、含浸法で行ったものよりも活性が低かったことから、含浸法で処理することがもっとも好ましいことがわかった。実施例１７のセシウム表面処理した触媒を光触媒反応実験後に濾過回収して、もう一度Ｆｅ３＋を含んだ反応溶液に入れて光触媒反応を行うと酸素発生活性は約２倍に向上した。一回目の光触媒反応実験中に鉄イオンが触媒上のセシウムと一部イオン交換したためと推察される。
(Examples 15 to 19)
WO 3 surface-treated with an aqueous cesium carbonate solution was prepared by a method different from Examples 1-14. 1 g of WO 3 powder made by high-purity chemical was calcined at 700 ° C. to 900 ° C. as a pretreatment, and then charged into a reaction vessel for acid decomposition (4749) made by PARR containing 10 mL of pure water, and 100 ° C. to 240 ° C. Hydrothermal treatment was carried out for 12 hours in an oven heated to ° C. Next, as a surface modification treatment, WO 3 (0.5 g) was suspended in a magnetic crucible containing an arbitrary amount of cesium solution and evaporated to dryness on a hot plate heated to 100 ° C. And after baking at 500 degreeC for 10 minutes, it fully wash | cleaned with the pure water and dried in the oven heated at 70 degreeC. It was identified by the X-ray diffractometer (XRD; manufactured by MX Labo Mac Science) that WO 3 was the main product. Table 3 shows the results of comparison of photocatalytic activity under the same experimental conditions as in Examples 1 and 2 using WO 3 subjected to surface treatment under various conditions. It was found that the WO 3 photocatalyst improves the activity up to about 4 times if the conditions are optimized without adding cesium and only firing and hydrothermal treatment. Furthermore, the activity was further improved twice or more by impregnating cesium and performing surface treatment on WO 3 prepared under the optimum conditions. It was found that WO 3 treated with cesium at the time of hydrothermal treatment under the optimized conditions and surface-treated was less active than that obtained by the impregnation method, so that the treatment by the impregnation method was most preferable. . When the catalyst treated with the cesium surface in Example 17 was collected by filtration after the photocatalytic reaction experiment, and once again placed in a reaction solution containing Fe 3+ and subjected to the photocatalytic reaction, the oxygen generation activity was improved about twice. This is probably because iron ions partially exchanged with cesium on the catalyst during the first photocatalytic reaction experiment.
（実施例２０）
炭酸セシウム水溶液にて表面処理したＷＯ３を実施例１５〜１９と同様の手法にて調製した。高純度化学製のＷＯ３粉末１ｇをタングステンに対して１atom%にあたる量の炭酸セシウム溶液を入れた磁性るつぼにケン濁させ、１００℃に加温したホットプレート上で蒸発乾固した。そして５００℃で１０分焼成した後に、純水で十分に洗浄し、７０℃に加温されたオーブンで乾燥した。その後乾燥した触媒を用いて、０．５ｗｔ％にあたるＰｔを、Ｈ２ＰｔＣｌ６溶液を用いて炭酸セシウムと同じ手法で含浸し、５００℃で３０分焼成した。表面処理を行ったＷＯ３を用いて、Ｆｅ３＋イオンの代わりに、ＩＯ３ −からの酸素生成反応の光触媒活性を比較した結果を表４に示した。このように、セシウムによる表面処理を施したＷＯ３はＦｅ３＋を還元しながら酸素を生成する反応だけでなく、ＩＯ３ −を還元しながら酸素を生成する反応に対しても非常に高い性能を発揮することがわかった。
(Example 20)
WO 3 surface-treated with an aqueous cesium carbonate solution was prepared in the same manner as in Examples 15-19. 1 g of WO 3 powder made by high purity chemical was suspended in a magnetic crucible containing an amount of cesium carbonate solution equivalent to 1 atom% with respect to tungsten, and evaporated to dryness on a hot plate heated to 100 ° C. And after baking at 500 degreeC for 10 minutes, it fully wash | cleaned with the pure water and dried in the oven heated at 70 degreeC. Thereafter, using a dried catalyst, Pt corresponding to 0.5 wt% was impregnated with H 2 PtCl 6 solution in the same manner as cesium carbonate, and calcined at 500 ° C. for 30 minutes. Table 4 shows the result of comparison of the photocatalytic activity of the oxygen generation reaction from IO 3 − instead of Fe 3+ ions using WO 3 subjected to surface treatment. Thus, WO 3 subjected to the surface treatment with cesium has a very high performance not only for the reaction that generates oxygen while reducing Fe 3+ but also for the reaction that generates oxygen while reducing IO 3 −. I found it to work.
（実施例２１）
炭酸セシウム水溶液にて表面処理したＷＯ３光電極を実施例１５〜１９と同様の手法にて調製した。日本板ガラス製の導電性ガラスに厚さ１０００nｍの酸化タングステン粒子膜のスパッタ法で成膜し、５００℃で３０分間焼成した。その後、炭酸セシウム水溶液を滴下してさらに５００℃で１０分間焼成し、純水で十分に洗浄した。
この表面処理を行ったＷＯ３光電極を用いて、１．２Ｖ vs Ａｇ／ＡｇＣｌ(ｐＨ２．３)のバイアスを印加した場合の光電流を比較した結果を表５に示した。このように、セシウムによる表面処理を施したＷＯ３光電極は、ＷＯ３粉末同様性能が向上し、高い光電流が得られることがわかった。白金対極ではこの時光電流に比例した水素発生が起こる。故に、セシウムの表面処理は半導体光触媒粉末だけでなく、光触媒を導電性基板上に成膜した半導体光電極にも向上効果があることがわかった。
(Example 21)
WO3 photoelectrodes surface-treated with an aqueous cesium carbonate solution were prepared in the same manner as in Examples 15-19. A 1000 nm thick tungsten oxide particle film was formed on a conductive glass made of Japanese plate glass by sputtering, and baked at 500 ° C. for 30 minutes. Thereafter, an aqueous cesium carbonate solution was added dropwise, further baked at 500 ° C. for 10 minutes, and sufficiently washed with pure water.
Table 5 shows the results of comparison of photocurrents when a bias of 1.2 V vs. Ag / AgCl (pH 2.3) was applied using the WO 3 photoelectrode subjected to this surface treatment. As described above, it was found that the WO 3 photoelectrode subjected to the surface treatment with cesium has improved performance like the WO 3 powder, and a high photocurrent can be obtained. At the platinum counter electrode, hydrogen is generated in proportion to the photocurrent. Therefore, it was found that the surface treatment of cesium is effective not only for the semiconductor photocatalyst powder but also for the semiconductor photoelectrode in which the photocatalyst is formed on a conductive substrate.
１：半導体結晶粒子
２：表面処理で半導体結晶粒子の表面近傍内部に生成した金属Ｍと半導体との複合酸化物の薄膜層
1: Semiconductor crystal particle 2: Thin film layer of composite oxide of metal M and semiconductor produced in the vicinity of the surface of the semiconductor crystal particle by surface treatment
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| JPH0889800A (en) * | 1994-09-19 | 1996-04-09 | Nikon Corp | Photocatalyst |
| JP2001002403A (en) * | 1999-06-16 | 2001-01-09 | Sharp Corp | Oxygen producing device |
| JP2001233602A (en) * | 2000-12-22 | 2001-08-28 | Natl Inst Of Advanced Industrial Science & Technology Meti | System for producing hydrogen comprising semiconductor photocatalyst reacting device and electrolytic device |
| JP2004256378A (en) * | 2003-02-27 | 2004-09-16 | National Institute Of Advanced Industrial & Technology | Method and apparatus for manufacturing hydrogen and oxygen |
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