JP6629598B2 - Photocatalyst, photocatalyst manufacturing method and oxidation-reduction apparatus - Google Patents

Description

  The present invention relates to a highly active photocatalyst used for oxidation and reduction of substances, and more particularly to a highly active photocatalyst for generating oxygen and hydrogen by decomposing water with light.

Photocatalysts can be used for oxidation and reduction of substances.
When the photocatalyst is irradiated with light having energy equal to or greater than the band gap, electrons in the valence band are excited to the conduction band, and holes are generated in the valence band. The holes generated in the valence band can be used for the oxidation reaction, and the electrons excited in the conduction band can be used for the reduction reaction.
If the redox reaction of the photocatalyst is used, it is possible to decompose the substance.
For example, titanium oxide as a photocatalyst is used as a positive electrode, platinum is used as a negative electrode, and the positive electrode and the negative electrode are immersed in water in which an electrolyte is dissolved, and the positive and negative electrodes are connected by a metal conductor to form a closed circuit. By irradiating light to the positive electrode while applying a bias voltage of 0.5 V, water can be decomposed to obtain oxygen from the positive electrode and hydrogen from the negative electrode.
Oxygen and hydrogen are both industrially useful, but in particular, hydrogen does not generate harmful components such as carbon oxides, nitrogen oxides, and sulfur oxides even when burned, and is therefore expected as a clean energy source. .
In order to use the photocatalyst industrially, it is desirable that the photocatalyst be as active as possible.
For the photocatalyst, a solid semiconductor such as an n-type semiconductor, a p-type semiconductor, or a pn-type semiconductor is mainly used.

As an example of increasing the activity of a photocatalyst, an attempt to increase the activity by narrowing the band gap of a semiconductor for a photocatalyst to absorb light having a wavelength as wide as possible has been known.
For example, in the development of a visible light responsive photocatalyst, an example in which nitrogen is added to an oxide semiconductor to reduce the band gap to an oxynitride semiconductor, or an example in which a nitride semiconductor having a narrow band gap is selected and used. Are known (Patent Document 1-2, Non-Patent Document 1-2).
Attempts to achieve high activation by using a cocatalyst are also known.
For example, an example is known in which a metal such as iron, cobalt, nickel, copper, silver, ruthenium, rhodium, palladium, osmium, iridium, platinum, and gold or an oxide thereof is supported as a cocatalyst on a semiconductor for a photocatalyst ( Patent Documents 3-5, Non-Patent Documents 3-6).
Furthermore, an example in which a composite oxide is used as a co-catalyst (Patent Document 6) and an example in which a co-catalyst is coated with a transition metal oxide or the like to form a core-shell structure in order to prevent a decrease in activity due to a reverse reaction Are also known (Patent Documents 7-8 and Non-Patent Document 7).
Furthermore, examples of a two-step photoexcited photocatalyst including a cocatalyst, two types of semiconductors, and an electron transfer system are also known (Non-Patent Documents 8 to 14). Such a two-step photoexcited photocatalyst is also referred to as a Z-scheme photocatalyst.
In addition, examples of using a pn-type semiconductor (Patent Documents 9 to 10) and a stacked semiconductor in which a semiconductor and a metal are alternately stacked as a photocatalyst are also found (Patent Documents 11 to 12).
However, none of the conventional photocatalysts has been sufficiently activated yet, and has not yet been industrially used.

JP 2007-22858 A JP 2007-175659 A Japanese Patent No. 3030328 Japanese Patent No. 3827399 JP 2011-173102 A JP 2007-185605 A JP 2013-230427 A JP-A-2005-71128 Japanese Patent No. 4736267 JP 2005-28225 A JP-A-61-107945 Japanese Patent Publication No. 6-28737

J. Phys. Chem. B, 107 (2003) 1798 Catal. Sci. Technol. , 2 (2012) 818 Chem. Soc. Rev .. , 38 (2009) 253 J. Am. Chem. Soc. , 103 (1981) 4885. J. Phys. Chem. A, 106 (2002) 6750 Chem. Mater. , 20 (2008) 1299 Angew. Chem. Int. Ed. 45 (2006) 7806 J. Chem. Soc. , Faraday Trans. 94 (1998) 3705 Chem. Commun. (2001) 2416 Chem. Lett. 33 (2004) 1348 Chem. Commun. (2005) 3829 J. Catal. 259 (2008) 133 J. Pys. Chem. C. 115 (2011) 3057 J. Am. Chem. Soc. 133 (2011) 11054

The decomposition reaction of a substance using a photocatalyst is attracting attention as an industrially useful technique. If a technology to decompose water with high efficiency using sunlight and a photocatalyst could be completed, it would greatly contribute to the development of industry as a technology for supplying hydrogen as a clean energy source.
However, the photocatalysts of the prior art do not have sufficient activity, and the substance decomposition reaction using the photocatalyst has not yet been industrially used.
Therefore, a highly active photocatalyst capable of decomposing substances with high efficiency is required.
The present invention provides a highly active photocatalyst capable of decomposing a substance by light.

The present invention relates to a highly active photocatalyst.
The photocatalyst of the present invention is a photocatalyst comprising an n-type semiconductor, a p-type semiconductor, and at least one metal selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium, and platinum, wherein n A photocatalyst having a configuration in which a type semiconductor and a p-type semiconductor are connected via the metal.
Further, the photocatalyst of the present invention has at least one kind of metal selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium, and platinum supported on a part of the carrier made of an n-type semiconductor, The photocatalyst is characterized in that it has a configuration in which a p-type semiconductor is supported on a part or all of the photocatalyst.
At least one of the n-type semiconductor and the p-type semiconductor is preferably an oxide semiconductor.
The n-type oxide semiconductor is preferably titanium oxide, and the p-type oxide semiconductor is preferably cobalt oxide or nickel oxide.
In another embodiment of the present invention, a carrier made of an n-type oxide semiconductor carries at least one metal selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium, and platinum, and further comprises p-type metal. Provided is a method for producing a photocatalyst, which supports an oxide semiconductor.
Further, the present invention provides a carrier comprising an n-type oxide semiconductor, on which at least one metal selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium, and platinum or a precursor thereof is further supported. Another object of the present invention is to provide a method for producing a photocatalyst, comprising carrying out a heat treatment at a temperature of 500 ° C. or more and 1200 ° C. or less while supporting a type oxide semiconductor or a precursor thereof.
In the above method for producing a photocatalyst, the n-type oxide semiconductor is preferably titanium oxide, and the p-type oxide semiconductor is preferably cobalt oxide or nickel oxide.
In addition, an oxidation-reduction device of the present invention includes a photocatalyst molded body containing any of the above-described photocatalysts, a substance to be oxidized, a substance to be reduced, and a medium containing a substance to be reduced and redox.

When the photocatalyst of the present invention is used, a substance can be decomposed with high efficiency. For example, when the photocatalyst of the present invention is brought into contact with a substance such as an organic substance, it exhibits a substance decomposition action.
In addition, the photocatalyst of the present invention is particularly useful when industrially producing hydrogen and oxygen by decomposing water with light, and can supply hydrogen as a clean energy source. Is big.
According to the present invention, it is possible to dramatically improve the light conversion efficiency as compared with the related art.

It is a key map of a photocatalyst in one embodiment of the present invention. It is a conceptual diagram of the conventional core-shell type photocatalyst. It is a conceptual diagram showing the water-splitting action of a photocatalyst in one embodiment of the present invention.

  Hereinafter, the photocatalyst of the present invention will be described in detail.

[Configuration of Photocatalyst of the Present Invention]
The configuration of the photocatalyst of the present invention will be described.
The photocatalyst of the present invention is a photocatalyst comprising an n-type semiconductor, a p-type semiconductor, and at least one metal selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium, and platinum, wherein n A photocatalyst having a configuration in which a type semiconductor and a p-type semiconductor are connected via the metal.
That is, the photocatalyst of the present invention is formed by bonding an n-type semiconductor and at least one metal selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium, and platinum, and further comprising the metal and the p-type semiconductor. Are joined.
In the present invention, connection means that two or more things are connected.
Also, in the present invention, the term “joining” means that two or more objects are joined.

When the photocatalyst of the present invention is irradiated with sunlight, both the n-type semiconductor and the p-type semiconductor excite electrons in the valence band to the conduction band.
The holes generated in the valence band of the n-type semiconductor are subjected to an oxidation reaction, and the electrons excited in the conduction band of the p-type semiconductor are subjected to a reduction reaction.
In the photocatalyst of the present invention, an oxidation reaction occurs in the n-type semiconductor, and a reduction reaction occurs in the p-type semiconductor.
The electrons excited in the conduction band of the n-type semiconductor by light irradiation combine with holes generated in the valence band of the p-type semiconductor via a metal bonded to both the n-type semiconductor and the p-type semiconductor. ,Disappear.
In the photocatalyst of the present invention, since the holes generated in the valence band of the n-type semiconductor are suppressed from being recombined with electrons excited in the conduction band of the n-type semiconductor and disappearing, the generated holes are oxidized. Can be used efficiently.
Similarly, since the electrons excited in the conduction band of the p-type semiconductor are suppressed from being recombined with holes generated in the valence band of the p-type semiconductor and disappearing, the excited electrons are efficiently used for the reduction reaction. it can.

In the photocatalyst of the present invention, the energy level of the valence band of the n-type semiconductor is lower than the energy level of the valence band of the p-type semiconductor. The energy level of the conduction band of the p-type semiconductor is higher than the energy level of the conduction band of the n-type semiconductor. Furthermore, the Fermi level of a metal is lower than the energy level of the conduction band of an n-type semiconductor.
The photocatalyst of the present invention can realize both the oxidation reaction and the reduction reaction with high activity using a single catalyst.
INDUSTRIAL APPLICABILITY The photocatalyst of the present invention can be efficiently used for an oxidation reaction without eliminating holes generated in a valence band of a low energy level of an n-type semiconductor by recombination. At the same time, electrons excited in the conduction band at a high energy level of the p-type semiconductor can be efficiently used for the reduction reaction without disappearing by recombination.
Therefore, the photocatalyst of the present invention has a high activity of the oxidation reaction and the reduction reaction, and its industrial use effect is extremely large.

As an example of the configuration of the photocatalyst of the present invention, n-type semiconductor, ruthenium, rhodium, palladium, osmium, iridium, and at least one metal selected from the group of platinum, and p-type semiconductor, sequentially Supported on the substrate.
For example, the metal is supported on a part of the surface of the n-type semiconductor carrier, and the p-type semiconductor is supported on a part or all of the surface of the metal.
In this case, the shape of the n-type semiconductor carrier may be any shape such as a particle shape, a small piece shape, a film shape, a film shape, and a plate shape.
Further, the shape of the metal carried on the n-type semiconductor may be a particle shape, a small piece shape, a needle shape, a plate shape, a film shape, a film shape, a plate shape, or the like.
In the present invention, when the metal is carried on the n-type semiconductor, the metal is carried on a part of the surface of the n-type semiconductor.
As an example in which the metal is supported on a part of the surface of the n-type semiconductor, a configuration in which the metal is supported in an island shape on particles of the n-type semiconductor is given. Alternatively, the substrate may be covered with an n-type semiconductor, and the metal film may be formed in a desired pattern on a part of the surface of the n-type semiconductor by a method such as photolithography.
In the photocatalyst of the present invention, in a portion where the metal is not supported on the surface of the n-type semiconductor, electrons are excited into the conduction band by light irradiation, holes are generated in the valence band, and the holes are subjected to an oxidation reaction. Provided.
In the present invention, the ratio (s / S) of the area (s) of the portion where the metal is not supported on the surface of the n-type semiconductor to the total surface area (S) of the n-type semiconductor is 0.5 or more. It is more preferably 0.7 or more, and still more preferably 0.9 or more.
The electrons excited in the conduction band move to the metal bonded to a part of the surface of the n-type semiconductor, flow into the metal, and generate holes in the p-type semiconductor and flow into the metal. Combine and disappear.
In the present invention, when a p-type semiconductor is carried on the metal carried on the surface of the n-type semiconductor, a p-type semiconductor is carried on a part or all of the surface of the metal.
The p-type semiconductor supported on the metal may be in the form of particles, small pieces, films, films, plates, or the like.
When the n-type semiconductor carries the metal and the metal carries the p-type semiconductor, the metal and the p-type semiconductor may be separately dispersed and carried on the n-type semiconductor.

A preferred configuration of the photocatalyst of the present invention is a configuration in which the metal particles are supported on an n-type semiconductor, and the p-type semiconductor particles are further supported on the metal particles. In this case, a configuration in which the p-type semiconductor particles are directly supported on the surface of the n-type semiconductor where the metal particles are not supported may be included.
The photocatalyst of the present invention only needs to have a configuration in which the n-type semiconductor and the p-type semiconductor are connected via a metal, and a part of the photocatalyst of the present invention includes an n-type semiconductor and a p-type semiconductor. A portion directly joined may be included.

[N-type semiconductor]
Hereinafter, the n-type semiconductor of the present invention will be described.
When the n-type semiconductor is irradiated with light having energy equal to or greater than the band gap, electrons in the valence band are excited to the conduction band. At that time, holes are generated in the valence band.
When the energy level at the upper end of the valence band of an n-type semiconductor in a vacuum is lower than the energy level of an electron orbit relating to the reaction of a substance such as water or an organic substance, when the substance and the n-type semiconductor come into contact, the electron of the substance becomes Depresses into a valence band at a relatively low energy level and combines with holes generated in the valence band. That is, the holes generated in the valence band have an oxidizing effect on substances such as water and organic substances.
When the energy level at the upper end of the valence band of the n-type semiconductor is sufficiently low, the n-type semiconductor is irradiated with light and is brought into contact with a substance such as water or an organic substance. Is oxidized by
As a result, the substance decomposes or reacts with another substance.

When holes generated in the valence band of the n-type semiconductor are used for oxidation, the energy level at the upper end of the valence band of the n-type semiconductor is preferably as low as possible.
When the standard hydrogen electrode potential at pH = 0 is set as a reference (0 V), the standard electrode potential related to oxygen generation in water electrolysis is 1.23 V. The energy level in vacuum corresponding to the standard electrode potential for the oxygen generation is about -5.7 eV.
On the other hand, for example, the energy level at the upper end of the valence band of titanium oxide in vacuum is said to be about -7.5 eV, which is sufficiently lower than -5.7 eV. Therefore, it is considered that titanium oxide can oxidize water by light to generate oxygen.
In general, it is considered that the absolute value of the energy level at the lower end of the conduction band is equal to the electron affinity of the semiconductor.
Further, the absolute value of the energy level at the upper end of the valence band is considered to be equal to the sum of the electron affinity of the semiconductor and the band gap energy of the semiconductor.
The band gap energy can be determined, for example, by measuring the absorption edge wavelength of a light absorption spectrum.
In the present invention, the band gap refers to the energy width of the forbidden band. Further, the band gap energy refers to an energy value corresponding to the energy width of the forbidden band.
In order to oxidize water by using holes generated in the valence band of the n-type semiconductor to generate oxygen, the energy level at the upper end of the valence band in vacuum needs to be sufficiently lower than the above-5.7 eV. There is.
Therefore, the energy level at the upper end of the valence band of the n-type semiconductor of the present invention is preferably -6.2 eV or less, more preferably -6.5 eV or less, and -7.0 eV or less. Is more preferred.
As described above, when the energy level at the upper end of the valence band of the n-type semiconductor is sufficiently lower than the energy level in vacuum corresponding to the standard electrode potential related to oxygen generation, the valence band of the n-type semiconductor is changed by light irradiation. The generated holes can oxidize substances such as water and organic substances. Therefore, the n-type semiconductor can be used as an oxidation catalyst.

On the other hand, in an n-type semiconductor used as an oxidation catalyst for substances such as water and organic substances, the energy level at the lower end of the conduction band is not high enough to reduce substances such as water and organic substances. Therefore, it is difficult to industrially use an n-type semiconductor that functions as an oxidation catalyst as a reduction catalyst.
The energy level corresponding to the electrode potential of the standard hydrogen electrode for water electrolysis is about -4.5 eV.
On the other hand, the energy level at the lower end of the conduction band of titanium oxide in vacuum is about -4.2 eV, which is slightly higher than -4.5 eV.
Therefore, it is difficult to reduce water to generate hydrogen even when the titanium oxide is irradiated with light.
In order to generate hydrogen by reducing water using electrons excited in the conduction band of the n-type semiconductor, the energy level at the bottom of the conduction band is higher than the energy level corresponding to the electrode potential of the standard hydrogen electrode. It needs to be high enough. Specifically, the energy level at the lower end of the conduction band is preferably -4.0 eV or more, and more preferably -3.5 eV or more.
On the other hand, when the n-type semiconductor is used for oxidizing a substance such as water or an organic substance, it is preferable that the energy level at the upper end of the valence band is lower than −5.7 eV as described above.
Therefore, if the oxidation and reduction of water are performed simultaneously using only the n-type semiconductor, the band gap must be increased.
However, as the band gap increases, the energy required to excite electrons also increases, so that the energy that can be absorbed by the photocatalyst is limited to high energy.
That is, when sunlight is used as a light energy source, the energy range that can be used is limited to a high energy range, so that the energy of sunlight cannot be used efficiently and high activity cannot be obtained.
In other words, it is difficult for an n-type semiconductor that can oxidatively decompose water by efficiently using sunlight to have energy high enough that electrons excited in the conduction band can reduce substances such as water and organic substances, It is not suitable as a catalyst for reducing substances such as water and organic substances.
However, if electrons excited in the conduction band of the n-type semiconductor remain in the conduction band without being used for reduction, they recombine with holes in the valence band after a certain period of time. Lower.

In order to effectively utilize holes generated in the valence band of the n-type semiconductor for the oxidation reaction, it is necessary to remove electrons excited in the conduction band and avoid recombination with holes.
The electrons excited in the conduction band by light irradiation can move freely along the electric field, so the n-type semiconductor and the counter electrode are connected by a metal conductor to form a closed circuit, and while irradiating the n-type semiconductor with light, If necessary, a voltage can be applied and if the excited electrons can be continuously taken out of the n-type semiconductor before recombination, the excited electrons can be used for the reduction of substances or used as electric energy. it can. Further, holes generated in the conduction band can be effectively used for oxidizing a substance.
In the prior art, electrons excited in the conduction band of an n-type semiconductor are guided to a counter electrode such as platinum and used for a water reduction reaction, and holes generated in a valence band are used for a water oxidation reaction.
The known technique of decomposing water using sunlight and titanium oxide is a typical example using electrons and holes generated by light irradiation.

In the photocatalyst of the present invention, electrons excited in the conduction band of the n-type semiconductor by light irradiation move in the n-type semiconductor, flow into a metal bonded to the n-type semiconductor, and bond with holes generated in the p-type semiconductor. And disappear.
At the junction between the n-type semiconductor and the metal, light is blocked by the metal, so that electrons are hardly excited from the valence band to the conduction band.
Therefore, in the photocatalyst of the present invention, it is not appropriate to use the entire surface of the n-type semiconductor for bonding to the metal.
In order to secure the surface of the n-type semiconductor to which light is directly irradiated, it is preferable to have a structure in which only a part of the surface of the n-type semiconductor is bonded to a metal.
In the photocatalyst of the present invention, electrons are excited to the conduction band by irradiation light on the surface of the n-type semiconductor and on the surface on which no metal is supported. Then, the electrons move to the metal bonded to the n-type semiconductor, recombine with holes generated in the p-type semiconductor, and are continuously removed.
As a result, in the photocatalyst of the present invention, holes generated in the valence band of the n-type semiconductor can be effectively used for the oxidation reaction.
According to the configuration of the photocatalyst of the present invention, electrons excited by light irradiation at the surface portion not bonded to the metal of the n-type semiconductor are converted into the p-type semiconductor via the metal bonded to the n-type semiconductor and the p-type semiconductor. It can be removed by combining with the generated holes.
The photocatalyst of the present invention is a novel one not found in the prior art, and can effectively and continuously utilize holes generated in an n-type semiconductor for an oxidation reaction. Further, electrons excited by the p-type semiconductor can be effectively and continuously used for the reduction reaction.
Therefore, the photocatalyst of the present invention has a remarkable advantage over the prior art.

As the n-type semiconductor of the present invention, for example, an oxide, an oxynitride or a nitride can be used.
Examples of the oxide of the n-type semiconductor of the present invention include, for example, oxides or composite oxides such as titanium, vanadium, zinc, zirconium, niobium, cadmium, indium, tin, tantalum, and tungsten.
For example, titanium oxide (TiO ₂ ), vanadium oxide (V ₂ O ₅ ), zinc oxide (ZnO), zirconium oxide (ZrO ₂ ), niobium oxide (II) (NbO), niobium oxide (V) (Nb ₂ O ₅ ) ), Cadmium oxide (CdO), indium oxide (In ₂ O ₃ ), tin oxide (IV) (SnO ₂ ), tantalum oxide (Ta ₂ O ₅ ), tungsten oxide (IV) (WO ₂ ), tungsten oxide (VI) ) (WO ₃ ), iron titanate (FeTiO ₃ ), strontium titanate (SrTiO ₃ ), barium titanate (BaTiO ₃ ), calcium titanate (CaTiO ₃ ), potassium tantalate (KTaO ₃ ), indium tin oxide ( ITO) and the like.
Examples of the oxynitride of the n-type semiconductor of the present invention include oxynitrides in which the above-described oxides contain nitrogen. Tantalum oxynitride (TaON) in which tantalum oxide (Ta ₂ O ₅ ) contains nitrogen is one example.
Further, the oxide may be a nitride in which oxygen of the above-described oxide is replaced with nitrogen. For example, tantalum nitride (Ta ₃ N ₅ ) in which oxygen of tantalum oxide is replaced with nitrogen may be used.

In the n-type oxide semiconductor of the present invention, the element ratio of the metal element and the oxygen element constituting the n-type oxide semiconductor does not necessarily need to be stoichiometric, and may have a non-stoichiometric composition. I do not care.
For example, in the case of titanium oxide, which is an example of an n-type semiconductor, it is expressed as TiO _{2 in the} chemical formula, but in the present invention, the composition ratio of the titanium element and the oxygen element does not necessarily have to be 1: 2, and is slightly shifted. Includes cases where
The n-type semiconductor of the present invention may have such a non-stoichiometric composition.
The n-type semiconductor of the present invention may contain an impurity.
When the n-type semiconductor has a non-stoichiometric composition or contains impurities, the conductivity of the n-type semiconductor is improved.
In the present invention, in order to improve and control the conductivity of the n-type semiconductor, the n-type semiconductor is intentionally made to have a desired non-stoichiometric composition, or the n-type semiconductor contains a trace amount of impurities. Measures may be taken.
When the n-type semiconductor of the present invention is an oxide, an oxynitride, a nitride, or the like, it may be a solid solution with another metal element oxide, an oxynitride, a nitride, or the like, or a composite oxide, It may be a composite oxynitride, a composite nitride, or the like.
The n-type semiconductor of the present invention is preferably an oxide of an n-type semiconductor, that is, an n-type oxide semiconductor.
As the n-type oxide semiconductor of the present invention, titanium oxide is preferable.

In the present invention, the conductivity of the n-type oxide semiconductor may be controlled as necessary.
As a method for controlling the conductivity of the n-type oxide semiconductor, for example, a method in which an oxide of an element other than the element included in the n-type oxide semiconductor is added to the n-type oxide semiconductor can be employed.
As the element added to the n-type oxide semiconductor other than the element constituting the n-type oxide semiconductor, an element which becomes a cation having a valence higher than that of the cation in the n-type oxide semiconductor is preferable. In particular, an element which becomes a cation having one more valence in an n-type oxide semiconductor is preferable.
For example, when trivalent aluminum is added to zinc oxide composed of divalent zinc, the conductivity of zinc oxide is improved. When aluminum oxide is added to zinc oxide at an element ratio of about 1% of aluminum and zinc, the conductivity of zinc oxide can be improved by 100 times or more.
According to the valence control theory, the reason why the conductivity is improved is understood to be that zinc in zinc oxide is replaced by aluminum. The cation sites in the crystal structure of zinc oxide are occupied by divalent zinc ions, and when one of the zinc ions in the cation site is replaced by a trivalent aluminum ion, the total charge in the crystal is increased. One of the cation sites becomes vacant to keep the zero. At this time, one electron at the anion site becomes excessive, and one free electron is generated. These free electrons improve the conductivity of zinc oxide, which is an n-type oxide semiconductor.
Similarly to the case of zinc oxide, for example, tin oxide is added to indium oxide, niobium oxide is added to tin oxide, indium oxide is added to cadmium oxide, and lanthanum oxide is added to strontium titanate.
In order to increase the effect of improving the conductivity of the n-type oxide semiconductor by adding an element, the added element needs to sufficiently diffuse into the n-type oxide semiconductor and replace as many cation sites as possible. . In general, in an oxide, the cation has a smaller ionic radius than the anion, so that the cation is more easily diffused than the anion. However, the diffusion of the cation is remarkable particularly when the temperature is raised.
In the case where an element other than the element included in the n-type oxide semiconductor is added to the n-type oxide semiconductor, the element to be added is preferably an oxide.
However, the element to be added does not necessarily have to be an oxide from the beginning, and may be any element that becomes an oxide by heating or the like after the addition.
For example, instead of the oxide of the element to be added, a compound such as an acetate, a nitrate, a sulfate, a chloride, or a hydroxide of the element may be added.
This is because these compounds easily become oxides by heating.

[P-type semiconductor]
Hereinafter, the p-type semiconductor of the present invention will be described.
In a p-type semiconductor, similarly to the case of an n-type semiconductor, when light having energy equal to or greater than the band gap is irradiated, electrons in a valence band are excited to a conduction electron, and holes are generated in the valence band. .
When the energy level at the lower end of the conduction band of the p-type semiconductor in vacuum is higher than the energy level of the electron orbit associated with the reaction of a substance such as water or an organic substance, the electrons excited in the conduction band contact the p-type semiconductor. The substance falls into an empty orbit at the relatively low energy level of the substance and is taken into the substance. That is, the electrons excited in the conduction band have a reducing effect on substances such as water and organic substances.
When the energy level at the lower end of the conduction band of the p-type semiconductor is sufficiently high, the substance is contacted with a substance such as water or an organic substance while irradiating the p-type semiconductor with light, and the substance is excited by excited electrons of the p-type semiconductor. Be reduced.
As a result, the substance decomposes or reacts with another substance.
As described above, the energy level corresponding to the electrode potential of the standard hydrogen electrode is about -4.5 eV.
On the other hand, for example, the energy level at the lower end of the conduction band of cobalt oxide (CoO) in vacuum is about -2.8 eV, which is sufficiently higher than -4.5 eV. Therefore, cobalt oxide can reduce water by light to generate hydrogen.
In order to generate hydrogen by reducing water using electrons excited in the conduction band of the p-type semiconductor, the energy level at the lower end of the conduction band in vacuum needs to be sufficiently higher than -4.5 eV.
The energy level at the lower end of the conduction band of the p-type semiconductor of the present invention is preferably at least -4.0 eV, more preferably at least -3.5 eV.
As described above, when the energy level at the lower end of the conduction band of the p-type semiconductor is sufficiently higher than the energy level corresponding to the electrode potential of the standard hydrogen electrode of -4.5 eV, the conduction band of the p-type semiconductor is irradiated with light. The excited electrons can reduce substances such as water and organic substances. Therefore, the p-type semiconductor can be used as a reduction catalyst.

As described above, the energy level in vacuum corresponding to the standard electrode potential of the oxygen electrode related to the electrolysis of water is set to about -5.7 eV.
On the other hand, for example, the energy level at the upper end of the valence band of cobalt oxide in vacuum is about -5.0 eV, which is higher than -5.7 eV. Therefore, even if light is applied to the cobalt oxide, water cannot be oxidized to generate oxygen.
In order to generate oxygen by oxidizing water using holes generated in the valence band of the p-type semiconductor by light irradiation, the energy level at the upper end of the valence band needs to be sufficiently lower than -5.7 eV. is there. Specifically, the energy level at the upper end of the valence band is preferably -6.2 eV or less, more preferably -6.5 eV or less, and still more preferably -7.0 eV or less. .
In the present invention, it is recommended that the energy level at the lower end of the conduction band be higher than -4.0 eV as a p-type semiconductor used for reducing substances such as water and organic substances. Therefore, assuming a band gap useful as a photocatalyst, it is not practically preferable to lower the energy level at the upper end of the valence band to -6.2 eV or less. Lowering to -6.2 eV or lower means increasing the band gap to 2.2 eV or higher, and requires high energy to excite valence band electrons, and consequently can be utilized. This is because the energy range of sunlight is limited, the efficiency becomes low as a photocatalyst, and industrial utility is lost.
The p-type semiconductor, which can reduce water with high efficiency by light irradiation, has the energy of holes generated in the valence band is not low enough to oxidize substances such as water and organic substances, and oxidizes substances such as water and organic substances. Therefore, it is not suitable as an oxidation catalyst.
However, if the holes generated in the valence band of the p-type semiconductor remain in the valence band without being used for oxidation, they recombine with the excited electrons in the conduction band after a certain period of time and disappear. It reduces the oxidizing action of the type semiconductor.
In order to effectively use the electrons excited in the conduction band of the p-type semiconductor for the reduction reaction, it is necessary to remove the holes generated in the valence band to avoid recombination with the excited electrons.
In the photocatalyst of the present invention, holes generated in the valence band of the p-type semiconductor by light irradiation flow into the metal bonded to the p-type semiconductor, and are combined with electrons excited in the conduction band of the n-type semiconductor and disappear. I do.
As described above, by continuously removing holes generated in the valence band of the p-type semiconductor, electrons excited in the conduction band of the p-type semiconductor can be effectively subjected to an oxidation reaction without disappearing by recombination. Can be used.

Examples of the p-type semiconductor of the present invention include oxides or composite oxides such as bismuth, chromium, manganese, iron, cobalt, nickel, yttrium, and lanthanum.
For example, bismuth oxide (Bi ₂ O ₃ ), chromium (II) oxide (CrO), chromium (III) oxide (Cr ₂ O ₃ ), manganese oxide (II) (MnO), manganese oxide (II, III) (Mn) ₃ O ₄ ), manganese (III) oxide (Mn ₂ O ₃ ), manganese (IV) (MnO ₂ ), iron oxide (II) (FeO), iron oxide (II, III) (Fe ₃ O ₄ ), Cobalt oxide (II) (CoO), cobalt oxide (II, III) (Co ₃ O ₄ ), cobalt oxide (III) (Co ₂ O ₃ ), nickel oxide (II) (NiO), copper oxide (I) ( Cu ₂ O), copper (II) oxide (CuO), yttrium oxide (Y ₂ O ₃ ), lanthanum manganate (LaMnO ₃ ), lanthanum chromate (LaCrO ₃ ), and the like.

In the p-type oxide semiconductor of the present invention, the element ratio of the metal and oxygen constituting the p-type oxide semiconductor does not necessarily need to be stoichiometric, and may have a non-stoichiometric composition. .
For example, in the case of nickel oxide, even though it is expressed as CoO in the chemical formula, the composition ratio of the cobalt element and the oxygen element is not always 1: 1, but may be slightly shifted.
The p-type semiconductor of the present invention may have such a non-stoichiometric composition.
Further, the p-type semiconductor of the present invention may contain impurities.
When the p-type semiconductor has a non-stoichiometric composition or contains impurities, the conductivity of the p-type semiconductor is improved.
In the present invention, in order to improve and control the conductivity of the p-type semiconductor, the p-type semiconductor is intentionally made to have a desired non-stoichiometric composition, or the p-type semiconductor contains a trace amount of impurities. Measures may be taken.

Another example of the p-type semiconductor of the present invention is a p-type IIIV compound semiconductor. Examples of the p-type group IIIV compound semiconductor include a group IIIV compound semiconductor doped with an element such as beryllium, magnesium, zinc, cadmium, iron, silicon, sulfur, zinc or an oxide thereof as an acceptor. Can be
Here, the group IIIV compound refers to a compound of a group III element such as aluminum, gallium, and indium and a group V element such as nitrogen, phosphorus, arsenic, and antimony. Specifically, gallium phosphide (GaP), gallium arsenide (GaAs), indium phosphide (InP), indium gallium phosphide (InGaP), indium gallium arsenide (InGaAs) and the like are exemplified.
For example, gallium nitride (GaN) to which zinc oxide is added can be used as a p-type semiconductor.

As the p-type semiconductor of the present invention, an oxide of a p-type semiconductor, that is, a p-type oxide semiconductor is more preferable.
The p-type oxide semiconductor of the present invention may be a solid solution with an oxide composed of an element other than the element constituting the p-type oxide semiconductor, or may be composed of an element other than the element constituting the p-type oxide semiconductor. It may be a composite oxide with an oxide.
As the p-type oxide semiconductor of the present invention, cobalt (II) oxide (CoO) or nickel oxide (II) (NiO) is preferable, and cobalt (II) oxide (CoO) is particularly preferable.
In the present invention, when a p-type oxide semiconductor is used as the p-type semiconductor, the conductivity of the p-type oxide semiconductor may be controlled as necessary.
As a method for controlling the conductivity of the p-type oxide semiconductor, for example, a method in which an oxide of an element other than the element included in the p-type oxide semiconductor is added to the p-type oxide semiconductor can be employed.
As the element added to the p-type oxide semiconductor other than the element constituting the p-type oxide semiconductor, an element which becomes a cation having a valence lower than that of a cation in the p-type oxide semiconductor is preferable. In particular, an element which becomes a cation having one less valence in a p-type oxide semiconductor is preferable.
For example, when the valence of a nickel ion in nickel oxide is 2, a lithium oxide having a monovalent valence is preferable as the oxide to be dissolved in the nickel oxide. In fact, when lithium oxide is solid-dissolved in nickel oxide, the effect of improving electrical conductivity is obtained.
According to the valence control theory, this is such that when the nickel ions in the nickel oxide are replaced by lithium ions, the lithium ions are monovalent, so that the total charge of the nickel oxide becomes zero. This is because the valence of some nickel ions increases from divalent to trivalent, and holes are generated.
Examples of the addition of an element other than the elements constituting the p-type oxide semiconductor to the p-type oxide semiconductor to improve the conductivity include an example in which lithium oxide is added to the above nickel oxide, and a case in which lithium oxide is added to manganese oxide. , An example of adding lithium oxide to cobalt oxide, an example of adding magnesium oxide to chromium oxide, and the like.
In the case where an element other than the element included in the p-type oxide semiconductor is added to the p-type oxide semiconductor, the element to be added is preferably an oxide.
However, the element to be added does not necessarily have to be an oxide from the beginning, and may be any element that becomes an oxide by heating or the like after the addition.
For example, instead of the oxide of the element to be added, a compound such as an acetate, a nitrate, a sulfate, a chloride, or a hydroxide of the element may be added.
This is because these compounds easily become oxides by heating.

[Metal interposed between n-type semiconductor and p-type semiconductor]
The photocatalyst of the present invention contains at least one metal selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium, and platinum.
The photocatalyst of the present invention has a configuration in which an n-type semiconductor and a p-type semiconductor are connected via the above-mentioned metal (hereinafter, referred to as “intermediate metal”).
In the present invention, the purpose of interposing an intermediate metal between the n-type semiconductor and the p-type semiconductor is to generate electrons excited in the conduction band of the n-type semiconductor by light irradiation, and to generate electrons in the valence band of the p-type semiconductor by light irradiation. And annihilated holes to make them disappear.
As described above, by making the configuration of the photocatalyst a configuration in which an intermediate metal is interposed between the n-type semiconductor and the p-type semiconductor, the electrons excited in the conduction band of the n-type semiconductor are converted to the valence band of the n-type semiconductor. It is possible to avoid recombination with the holes generated in the n-type semiconductor to consume the holes, and to effectively provide the holes generated in the valence band of the n-type semiconductor to the oxidation reaction. Furthermore, it is possible to prevent the holes generated in the valence band of the p-type semiconductor from recombining with the electrons excited in the conduction band of the p-type semiconductor and consuming the excited electrons. The excited electrons can be effectively used for the reduction reaction.
By combining and eliminating holes generated in the valence band of the p-type semiconductor and electrons excited in the conduction band of the n-type semiconductor, the n-type semiconductor is continuously irradiated during light irradiation. Oxidization and reduction with a p-type semiconductor.
Therefore, it is necessary to select, as the intermediate metal, a metal in which excited electrons of the n-type semiconductor can easily flow and holes generated in the p-type semiconductor can easily flow.
For that purpose, it is necessary to select a metal having an appropriate work function as an intermediate metal in relation to the n-type semiconductor and the p-type semiconductor.
However, it is difficult to predict and select such metals.

Under such circumstances, the present inventors have conducted intensive studies and have found that at least one metal selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium, and platinum (hereinafter, referred to as “platinum group metal or the like”) It has been surprisingly found that the use of as an intermediate metal makes it possible to realize a photocatalyst having high activity, and has finally accomplished the present invention.
In order to sufficiently obtain the effects of the present invention, the platinum group metal or the like of the present invention is preferably in the form of particles.
The average particle size of the particles of the platinum group metal or the like of the present invention is preferably from 0.1 nm to 1 μm, more preferably from 0.1 nm to 100 nm, more preferably from 1 nm to 30 nm, and further preferably from 1 nm to 10 nm.
Within the range of the average particle size, a photocatalyst having higher activity that can be used industrially can be obtained.
The platinum group metal and the like of the present invention may contain a small amount of impurities.
Further, an alloy may be formed with a metal other than the platinum group metal or the like. However, in the case of an alloy, the platinum group metal or the like of the present invention must be a main component. Specifically, the content of the platinum group metal or the like must be 50% by weight or more.
The photocatalyst of the present invention is such that the n-type semiconductor and the p-type semiconductor are connected via at least one metal selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium, and platinum. High photocatalytic activity, which cannot be obtained by the prior art, can be achieved.

Hereinafter, the present invention will be described through the method for producing a photocatalyst of the present invention.
The method for producing a photocatalyst described below shows one embodiment of the present invention, and does not limit the invention according to the claims. In addition, not all combinations of features described in the following photocatalyst manufacturing methods are necessarily essential for solving the invention.
The photocatalyst produced by the following production method uses an n-type oxide semiconductor as a carrier, and on this carrier, ruthenium, rhodium, palladium, osmium, iridium, and at least one metal selected from the group of platinum and , And a p-type oxide semiconductor. However, even when an n-type semiconductor other than the n-type oxide semiconductor or a p-type semiconductor other than the p-type oxide semiconductor is used, the photocatalyst of the present invention can be produced by a known method by appropriately setting the atmosphere and the like. it is obvious.

The following method for producing a photocatalyst includes a first step, a second step, and a third step.
As a first step, a water slurry containing an n-type oxide semiconductor is manufactured.
As a method for producing a water slurry containing an n-type oxide semiconductor, for example, a method of dispersing particles of the n-type oxide semiconductor in water can be mentioned.
Further, in the present invention, instead of the particles of the n-type oxide semiconductor, a precursor of the n-type oxide semiconductor which becomes an n-type oxide semiconductor by chemical treatment, thermal treatment, or the like may be used.
Examples of the precursor of the n-type oxide semiconductor include alkoxides, hydroxides, chlorides, sulfates, nitrates, acetates, and the like of metals constituting the n-type oxide semiconductor.
For example, examples of the precursor of titanium oxide include titanium alkoxide, titanium chloride, titanium nitrate, titanium sulfate, and titanium acetate.
When such a precursor of an n-type oxide semiconductor is hydrolyzed or neutralized in the presence of water, it becomes a precursor of an n-type oxide semiconductor in a sol state.
For example, when a titanium alkoxide is hydrolyzed in the presence of water, a hydroxide of titanium is generated, which becomes a precursor of titanium oxide in a sol state.
When producing a precursor of an n-type oxide semiconductor in a sol state by neutralization, a precursor of the n-type oxide semiconductor and an alkaline neutralizing agent such as ammonia are simultaneously added to water while controlling the pH. May be selected. In this case, the pH is preferably controlled in the range of 5 to 9, more preferably in the range of 6 to 8.
For example, 3 L of water is placed in a 5 L beaker, and titanium tetrachloride and ammonia are gradually dropped while controlling the pH to 7 while stirring, to obtain a sol. The obtained sol is filtered, dispersed in water, washed with stirring, further filtered, washed repeatedly with water to remove ammonium chloride, and then mixed with water to form a sol-state titanium oxide precursor. obtain.
In the present invention, the precursor of the n-type oxide semiconductor in the sol state may be used as it is as a water slurry containing the n-type oxide semiconductor.
For example, a precursor of titanium oxide in a sol state may be used as a water slurry containing titanium oxide.

As another method for obtaining a water slurry containing the n-type oxide semiconductor of the present invention, a gel of a sol-state n-type oxide semiconductor precursor is dried by drying at a temperature of 20 ° C to 100 ° C for 1 to 48 hours. Obtained and baked in air in a temperature range of 200 ° C. to 800 ° C. for 1 to 48 hours to form an oxide, and then pulverized as necessary to obtain particles having an average particle size of 0.01 μm to 100 μm. After obtaining, a method of dispersing the particles in water can be adopted.
In the present invention, a known method such as a ball mill, a jet mill, and a hammer mill can be used for pulverization.

Further, as still another method for obtaining a water slurry containing an n-type oxide semiconductor, a known method is known in which a sol-state precursor of an n-type oxide semiconductor and water are mixed and the viscosity is adjusted in the range of 5 to 1000 cP. Spray drying by the method, firing in air in the range of 200 ° C. to 800 ° C. for 1 to 48 hours, pulverizing as necessary to obtain particles having an average particle size of 0.01 μm to 5 mm, A method of dispersing in water can be adopted.
Here, for the spray drying, a known spraying device such as a rotating disk system, a two-fluid nozzle system, and a pressurized nozzle system can be used. In addition, in order to obtain droplets having a uniform shape, composition, size, or the like, a mixed solution of a sol-state precursor of an n-type oxide semiconductor and water is added by using a method such as adding an acid or an alkali. The pH may be adjusted.
The n-type oxide semiconductor contained in the water slurry prepared by the above method can be used as a carrier.

In the present invention, in order to improve the chemical stability and mechanical stability of an n-type oxide semiconductor, an element other than an element constituting the n-type oxide semiconductor (hereinafter referred to as “stabilization”) is added to the n-type oxide semiconductor. Element ”) may be added.
Examples of such a stabilizing element include aluminum and silicon.
As a method for adding such a stabilizing element, for example, when producing a water slurry containing an n-type oxide semiconductor in the first step, an organic acid salt, an inorganic acid salt, or a hydroxide of the stabilizing element may be used. An n-type oxide semiconductor to which a stabilizing element is added can be obtained by adding a compound selected from a substance, an oxide, and the like.
For example, when adding aluminum as a stabilizing element to titanium oxide, a precursor of titanium oxide in a sol state and an aluminum compound are mixed, and a hydrothermal reaction is performed at 20 ° C to 100 ° C for 1 to 48 hours, After drying to obtain a gel, the gel may be fired in the air at a temperature of 200 ° C to 800 ° C for 1 to 48 hours. Alternatively, after mixing the titanium sol and the aluminum compound, an alkaline aqueous solution may be added to co-precipitate titanium and aluminum, dried, and then fired under the above conditions. Drying can be performed using a vacuum dryer, a hot-air circulation dryer, an atmosphere control dryer, a spray dryer, or the like, in addition to a general electric furnace dryer.
As a compound of aluminum to be added as a stabilizing element, a water-insoluble compound such as aluminum oxide and aluminum hydroxide can be used. In addition, inorganic acid salts such as sodium aluminate, aluminum chloride hexahydrate, aluminum perchlorate hexahydrate, aluminum sulfate hexadecahydrate, aluminum nitrate nonahydrate, aluminum acetate, aluminum formate, etc. A water-soluble compound such as an organic acid salt can be used.
In the case where a stabilizing element is added to an n-type oxide semiconductor, the composition ratio of the stabilizing element and the n-type oxide semiconductor is such that the amount of the stabilizing element is such that It is preferably in the range of 0.001 mol% to 10 mol%, more preferably in the range of 0.01 mol% to 1 mol%, based on the total amount.

In the present invention, for the purpose of improving the stability over time of photocatalytic activity, at least one metal selected from alkali metals, alkaline earth metals, and rare earth metals (hereinafter, referred to as “base”) is added to an n-type oxide semiconductor. Metal ").
Examples of such an alkali metal include Li, Na, K, Rb, and Cs. Examples of the alkaline earth metal include Be, Mg, Ca, Sr, and Ba. Further, examples of the rare earth metal include La, Ce, Pr and the like.
In the present invention, the composition ratio of the basic metal to the metal constituting the n-type oxide semiconductor (moles of the basic metal / moles of the metal constituting the n-type oxide semiconductor) is from 0.001 to 0.1. It is preferably in the range of 5.
For example, when magnesium is contained in titanium oxide, the element ratio of magnesium to titanium is preferably in the range of 0.001 to 0.5. When strontium titanate contains magnesium, the element ratio between magnesium and (titanium + strontium) is preferably in the range of 0.001 to 0.5.
As a method of including a basic metal in the n-type oxide semiconductor, for example, the following method can be selected.
The water slurry containing the n-type oxide semiconductor obtained in the first step is mixed with a compound of a basic metal to obtain a mixture, and the mixture is optionally heated at 20 ° C to 100 ° C for 1 hour to After performing the hydrothermal treatment for 48 hours, the resultant is dried by a known method such as a spray drying method, and further calcined at 200 ° C. to 800 ° C. for 1 hour to 48 hours to include an n-type containing a component of a basic metal. An oxide semiconductor can be obtained.

The n-type oxide semiconductor obtained by the above method is often in the form of particles, and is at least one metal or p-type selected from the group of ruthenium, rhodium, palladium, osmium, iridium, and platinum of the present invention. It is suitable for supporting an oxide semiconductor.
The average particle size of the n-type oxide semiconductor of the present invention is preferably 0.01 μm or more, more preferably 0.1 μm or more, and further preferably 1 μm or more. If the particle size is too small, it becomes difficult to support a metal or a p-type oxide semiconductor. Therefore, in order to efficiently realize the configuration of the photocatalyst of the present invention in which an n-type oxide semiconductor and a p-type oxide semiconductor are connected via a metal, the average particle size of the n-type oxide semiconductor must be It is preferable to be within the above range.
In addition, the average particle size of the n-type oxide semiconductor of the present invention is, for example, 5 mm or less, preferably 1 mm or less, more preferably 0.1 mm or less, further more preferably 0.05 mm or less, More preferably, it is 0.01 mm or less. If the particle size is too large, it becomes difficult to carry out a photocatalytic reaction even if a metal or a p-type oxide semiconductor is carried inside the particles of the n-type oxide semiconductor, and the weight per unit weight of the n-type oxide semiconductor Photocatalytic efficiency deteriorates.

In the present invention, as the compound of the basic metal, oxides, hydroxides, acetates, nitrates, chlorides, sulfates and the like of the basic metal can be used.
In the present invention, the stabilizing element and the basic metal added to the n-type oxide semiconductor go through a sintering step in the production of the photocatalyst, and are considered to eventually become an oxide.

When an n-type oxide semiconductor is used as a carrier, the shape is preferably granular having fine pores, but is polyhedral, columnar, cylindrical, polygonal cylindrical, polygonal columnar, plate-like, polygonal. The shape may be plate-like, scale-like, small-piece-like, needle-like, fibrous, honeycomb-like, irregular, or the like. By having fine pores, the specific surface area of the carrier surface can be increased, and at least one kind of metal or p-type oxide semiconductor selected from the group of ruthenium, rhodium, palladium, osmium, iridium, and platinum Can be increased. As a result, an effect of increasing the photocatalytic activity per unit weight of the carrier can be obtained.
The specific surface area can be measured, for example, by a BET nitrogen adsorption method. In the present invention, the range of the specific surface area of the n-type oxide semiconductor used for the carrier is preferably 10 m ² / g or more, more preferably 20 m ² / g or more, and still more preferably, in order to obtain a sufficient carrying amount. Is 50 m ² / g or more. In order to obtain sufficient mechanical strength and durability, the thickness is preferably 700 m ² / g or less, more preferably 500 m ² / g or less, further preferably 300 m ² / g or less, and most preferably 250 m ² / g or less. ² / g or less.

In the second step of the present invention, a photocatalyst precursor is produced.
As a method for producing the catalyst precursor of the present invention, for example, a water slurry containing the n-type oxide semiconductor obtained in the first step, and ruthenium, rhodium, palladium, osmium, iridium, and platinum A method of mixing a water slurry containing at least one selected metal and a water slurry containing a p-type oxide semiconductor is exemplified.
Ruthenium, rhodium, palladium, osmium, iridium, and a method for producing a water slurry containing at least one metal selected from the group of platinum, for example, ruthenium, rhodium, palladium, osmium, iridium, and platinum A method of dispersing at least one kind of metal particles selected from the group in water is exemplified.
Further, ruthenium, rhodium, palladium, osmium, iridium, and particles of at least one metal selected from the group of platinum, instead of chemical treatment, thermal treatment, etc., ruthenium, rhodium, palladium, osmium, iridium. And at least one metal selected from the group of platinum, ruthenium, rhodium, palladium, osmium, iridium, and a precursor of at least one metal selected from the group of platinum may be used. .

  Ruthenium, rhodium, palladium, osmium, iridium, and a precursor of at least one metal selected from the group of platinum, for example, ruthenium, rhodium, palladium, osmium, iridium, and platinum group Oxides, chlorides, metal chlorides, metal chlorides, sulfates, nitrates, acetates, hydroxides, metal complexes, and the like of at least one metal selected from among them can be used. Specifically, ruthenium oxide, ruthenium chloride, sodium ruthenate, ruthenium sulfate, ruthenium nitrate, ruthenium acetate, rhodium oxide, rhodium chloride, ammonium chloride rhodate, rhodium sulfate, rhodium nitrate, rhodium acetate, palladium oxide, palladium chloride, Palladium chloride, sodium chloride, palladium sulfate, palladium nitrate, palladium acetate, dinitrodiamminepalladium, dichlorotetraamminepalladium, tetraamminepalladium dichloride, palladium acetate, amminepalladium hydroxide, iridium oxide, iridium chloride, ammonium iridium chloride, chloride Potassium iridate, iridium sulfate, iridium nitrate, iridium acetate, platinum oxide, platinum chloride, chloroplatinic acid, chloroplatinic acid Chloride, potassium chloroplatinic acid, sodium chloroplatinic acid, platinum sulfate, platinum nitrate, dinitrodiammine platinum, tetraamine platinum dichloride, tetraamine platinum hydroxide, hexahydroxoplatinum acid, hexahydroxoplatinum acid ethanolamine, and the like.

The valence of the metal contained in the precursor of at least one metal selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium, and platinum according to the present invention is any of the possible valences. You may. For example, potassium chloroplatinate is known to have two compounds of potassium divalent potassium chloroplatinate (II) and potassium tetravalent chloroplatinate (IV). It may be a compound having a valence.
The precursor of at least one metal selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium and platinum of the present invention may be used in the form of an aqueous solution.
As a method for preparing such an aqueous solution, a known method may be used.
In this case, ruthenium, rhodium, palladium, osmium, iridium, and the total elemental amount of the metal in the water slurry containing at least one metal selected from the group of platinum is an element of the metal constituting the n-type oxide semiconductor The amount is at least 0.001 mol%, preferably at least 0.005 mol%, more preferably at least 0.01 mol%, even more preferably at least 0.05 mol%, most preferably at least 0.1 mol%, based on the amount. % Or less, and preferably 10 mol% or less, preferably 5 mol% or less, more preferably 3 mol% or less, and still more preferably 1 mol% or less.

As a method of producing a water slurry containing a p-type oxide semiconductor, for example, a method of dispersing particles of a p-type oxide semiconductor in water can be mentioned.
Instead of the particles of the p-type oxide semiconductor, a precursor of a p-type oxide semiconductor which becomes a p-type oxide semiconductor by chemical treatment, thermal treatment, or the like may be used.
Examples of such a precursor of the p-type oxide semiconductor include alkoxides, hydroxides, chlorides, sulfates, nitrates, and acetates of metals constituting the p-type oxide semiconductor.
Specifically, examples of the cobalt oxide precursor include divalent or trivalent cobalt alkoxides, hydroxides, chlorides, sulfates, nitrates, and acetates. Specifically, examples thereof include cobalt 2-methoxyethoxide, cobalt hydroxide, cobalt chloride, cobalt nitrate, cobalt sulfate, cobalt carbonate, and cobalt acetate.

In the present invention, the amount of the metal element constituting the p-type oxide semiconductor is at least 0.01 mol%, preferably at least 0.05 mol%, more preferably at least 0.05 mol%, based on the metal element constituting the n-type oxide semiconductor. Is preferably 0.1 mol% or more and 10 mol% or less, preferably 5 mol% or less, more preferably 3 mol% or less.
Further, the amount of the metal element constituting the p-type oxide semiconductor and the total amount of the metal element in the water slurry containing at least one metal selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium, and platinum The ratio is preferably 0.1 or more, preferably 0.5 or more, more preferably 1 or more, and 10 or less, preferably 5 or less, more preferably 3 or less.

In the present invention, it is preferable that the temperature of the mixture is adjusted to be 60 ° C. or more and 100 ° C. or less after or during the mixing in the second step.
In the n-type oxide semiconductor, ruthenium, rhodium, palladium, osmium, iridium, and at least one metal selected from the group of platinum or a precursor of the metal, and a p-type oxide semiconductor or a precursor of the p-type semiconductor A precursor of the photocatalyst, in which the body is precipitated, is obtained.

In the third step of the present invention, the n-type oxide semiconductor carries at least one metal selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium, and platinum, and the p-type oxide semiconductor The obtained photocatalyst is obtained.
As a method for obtaining the photocatalyst in the third step, for example, a method of filtering, washing with water, drying the precursor of the photocatalyst obtained in the second step, and then performing a heat treatment can be mentioned.
The purpose of the heat treatment is to oxidize the precursor of the p-type oxide semiconductor, further sinter it, and turn it into a p-type oxide semiconductor.
Only by such heat treatment can the highly active photocatalyst of the present invention be obtained.
The p-type oxide semiconductor of the present invention is preferably polycrystalline by firing.
Note that the n-type oxide semiconductor of the present invention may be a polycrystalline body.
The temperature of the heat treatment is preferably 500 ° C. or higher, more preferably 600 ° C. or higher, and further preferably 700 ° C. or higher. If the heat treatment temperature is lower than 500 ° C., it is not preferable because the precursor of the p-type oxide semiconductor cannot be sufficiently converted into a p-type oxide semiconductor and the activity of the photocatalyst cannot be increased.
Further, the temperature of the heat treatment is preferably 1300 ° C. or lower, more preferably 1200 ° C. or lower, further preferably 1100 ° C. or lower, and most preferably 1000 ° C. or lower. If the heat treatment temperature is higher than 1300 ° C., the photocatalyst is sintered, the pores of the n-type oxide semiconductor are closed, and the catalytic activity is undesirably reduced.
The heat treatment time is not particularly limited, but is preferably 0.5 hours or more and 72 hours or less, more preferably 1 hour or more and 48 hours or less. If the heat treatment time is shorter than 0.5 hour, the precursor of the p-type oxide semiconductor may not be sufficiently converted into a p-type oxide semiconductor, which is not preferable. On the other hand, if the firing is performed for more than 72 hours, the sintering of the catalyst is promoted, and the activity may be reduced.
In the heat treatment, it is preferable to avoid a rapid temperature rise and to gradually raise the temperature according to the condition of the heat treatment target. If the temperature rises sharply, gasification and combustion become intense, and it is not preferable because it is exposed to a higher temperature than set or causes pulverization.
The range of the preferable average particle diameter of the photocatalyst obtained in the third step is 0.1 μm or more, more preferably 1 μm or more, still more preferably 10 μm or more, and 1 mm or less, more preferably 300 μm or less, More preferably, it is 100 μm or less.

[Redox device]
Hereinafter, the oxidation-reduction device of the present invention will be described.
The photocatalyst of the present invention can be used after being formed into a plate by a known method such as cold pressing, hot pressing, or isostatic pressing. Further, a layer containing the photocatalyst of the present invention can be formed on a conductive substrate by a known method such as loading, coating, and baking.
A molded article containing the photocatalyst of the present invention (hereinafter, referred to as “photocatalyst molded article”) is subjected to oxidation (hereinafter, referred to as “oxidized substance”) and reduction substance (hereinafter, referred to as “substance to be reduced”). (Hereinafter, referred to as a “medium containing an oxidation-reduction substance”) to constitute an apparatus.
Such an apparatus composed of the photocatalyst molded body, the substance to be oxidized, the substance to be reduced, and the medium containing the substance to be redox is the oxidation-reduction apparatus of the present invention.
In the oxidation-reduction device of the present invention, when the photocatalyst molded body is irradiated with light, the substance to be oxidized is oxidized on the photocatalyst molded body and the substance to be reduced is reduced.
For example, when the photocatalyst molded article of the present invention is immersed in water and irradiated with sunlight, water is oxidized to generate oxygen, and water is reduced to generate hydrogen. In this case, water is the substance to be oxidized, the substance to be reduced, and the medium containing the substance to be oxidized and reduced.
When water is decomposed using the photocatalyst of the present invention, a substance that ionizes in water, such as sodium hydroxide or potassium hydroxide, may be added.
The redox apparatus of the present invention is for performing such a redox reaction.

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

(Example 1)
2 L of distilled water is placed in a 5 L beaker, and 1 mol of titanium tetrachloride and 1 L of an aqueous sodium hydroxide solution (1 mol / L) are stirred into the distilled water so that the pH falls within the range of 6 to 8 while stirring. Then, titanium tetrachloride and an aqueous solution of sodium hydroxide were gradually added dropwise to obtain a product. After the obtained product was filtered, it was added to 2 L of hot water at about 80 ° C., stirred, and washed. This filtration and hot water washing were repeated three times to obtain 1 mol of gel-form orthotitanic acid having a water content of 93.
The obtained orthotitanic acid was dried at 80 ° C. for 3 hours and calcined at 300 ° C. for 1 hour to obtain a powder, which was dispersed in 0.5 L of water to obtain a water slurry containing titanium oxide.
On the other hand, 15 mL of an aqueous solution of cobalt nitrate (1 mol / L) and 0.1 mL of an aqueous solution of lithium nitrate (1 mol / L) were mixed at room temperature to prepare a mixed solution of cobalt nitrate and lithium nitrate.
While stirring the aqueous slurry containing titanium oxide, 10 mL of an aqueous ruthenium nitrate solution (0.1 mol / L) was added, and then the above-mentioned mixture of cobalt nitrate and lithium nitrate was added. The temperature was adjusted from 8 to 8.5 by heating with a heater while further stirring, and the temperature was increased from room temperature to 80 ° C. After stirring at 80 ° C for 6 hours, the mixture was allowed to cool to room temperature, and stopped stirring for 12 hours. I left it. Thereafter, filtration and washing were repeated three times to obtain a photocatalyst precursor.
The obtained photocatalyst precursor was dried in air at 110 ° C. for 3 hours, calcined in air at 800 ° C. for 1 hour to convert the cobalt oxide into a p-type semiconductor, and then cooled to room temperature to obtain a photocatalyst powder.
0.2 g of the obtained photocatalyst was dispersed in 400 mL of water, put into an internal irradiation reaction cell, and irradiated with light from a 450 W mercury lamp for 5 hours. As a result, 2.2 mmol of hydrogen and 1.1 mmol of oxygen were obtained.

(Example 2)
A photocatalyst was obtained in the same manner as in Example 1, except that a rhodium nitrate aqueous solution was used instead of the ruthenium nitrate aqueous solution.
When this photocatalyst was subjected to water splitting in the same manner as in Example 1, about 2.4 mmol of hydrogen and 1.2 mmol of oxygen were obtained.

(Example 3)
A photocatalyst was obtained in the same manner as in Example 1 except that lithium nitrate was not used, and firing was performed at 800 ° C. for 1 hour in nitrogen instead of firing at 800 ° C. for 1 hour in air.
When subjected to water splitting in the same manner as in Example 1, 3 mmol of hydrogen and 1.5 mmol of oxygen were obtained.

(Example 4)
A photocatalyst was obtained in the same manner as in Example 1, except that an aqueous solution of palladium nitrate was used instead of the aqueous solution of ruthenium nitrate.
When subjected to water splitting in the same manner as in Example 1, 2 mmol of hydrogen and 1 mmol of oxygen were obtained.

(Example 5)
A photocatalyst was obtained in the same manner as in Example 1, except that nickel nitrate was used instead of cobalt nitrate.
When subjected to water splitting in the same manner as in Example 1, 1.8 mmol of hydrogen and 0.6 mmol of oxygen were obtained.

(Example 6)
The photocatalyst obtained by the method of Example 1 was formed into pellets having a diameter of 8 mm and a thickness of 1 mm using a tableting machine.
This molded body was fired at 1100 ° C. for 3 hours to obtain a disc-shaped sintered body of the photocatalyst.
This photocatalyst was placed in a petri dish having a diameter of 60 mmφ and a height of 15 mm, and 2.5 cc of water and 0.3 mL of sodium hydroxide (1 mol / L) were added to prepare a redox apparatus for water.
When this oxidation-reduction device was placed on a hot plate and heated to maintain the temperature at 60 ° C. and exposed to sunlight, a gas was generated from the disc-shaped sintered body of the photocatalyst. As a result, it was found that the mixed gas was a mixture of oxygen and hydrogen at a mixing ratio of 1: 2.

(Comparative Example 1)
A photocatalyst was obtained in the same manner as in Example 1, except that firing at 800 ° C. in air for converting cobalt oxide into a p-type semiconductor was not performed.
When this photocatalyst was subjected to water splitting in the same manner as in Example 1, 1.0 mmol of hydrogen and 0.5 mmol of oxygen were obtained.

(Comparative Example 2)
A photocatalyst was obtained in the same manner as in Example 1, except that an aqueous solution of chloroauric acid was used instead of the aqueous solution of ruthenium nitrate.
When this photocatalyst was subjected to water splitting in the same manner as in Example 1, 0.8 mmol of hydrogen and 0.4 mmol of oxygen were obtained.

By using the photocatalyst of the present invention, the substance can be oxidized and reduced with high activity. In addition, the use of the oxidation-reduction apparatus of the present invention makes it possible to decompose substances industrially.
Therefore, for example, industrial use can be expected as a photocatalyst for obtaining hydrogen and oxygen by decomposing water with sunlight.

DESCRIPTION OF SYMBOLS 11 n-type semiconductor 12 platinum group metal etc. 13 p-type semiconductor 21 semiconductor particle 22 metal (oxide) core 23 oxide shell 31 n-type semiconductor 32 platinum group metal etc. 33 p-type semiconductor

Claims (9)

an n-type semiconductor;
a p-type semiconductor;
Palladium, a photocatalyst comprising at least one metal, a member selected from the group consisting of 及 beauty platinum,
A photocatalyst having a configuration in which an n-type semiconductor and a p-type semiconductor are connected via the metal.



  Part of the surface of the carrier made of the n-type semiconductor, ruthenium, rhodium, palladium, osmium, iridium, and at least one metal selected from the group of platinum is supported, part or all of the surface of the metal A photocatalyst comprising a p-type semiconductor supported thereon.   The photocatalyst according to claim 1, wherein at least one of the n-type semiconductor and the p-type semiconductor is an oxide semiconductor.   The photocatalyst according to claim 3, wherein the n-type semiconductor is titanium oxide.   The photocatalyst according to claim 3, wherein the p-type semiconductor is cobalt oxide or nickel oxide.   Supporting at least one metal selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium, and platinum on a carrier made of an n-type oxide semiconductor, and further supporting a p-type oxide semiconductor. A method for producing a photocatalyst.   On a carrier made of an n-type oxide semiconductor, ruthenium, rhodium, palladium, osmium, iridium, and at least one metal selected from the group of platinum or a precursor thereof, and further a p-type oxide semiconductor or A method for producing a photocatalyst, comprising supporting a precursor and heat-treating the precursor at a temperature of 500 ° C to 1200 ° C.   The method according to claim 6, wherein the n-type oxide semiconductor is titanium oxide, and the p-type oxide semiconductor is cobalt oxide or nickel oxide. An oxidation-reduction device comprising a photocatalyst molded article containing the photocatalyst according to any one of claims 1 to 5, an oxidized substance, a reduced substance, and a medium containing a redox substance.