JPS5844748B2 - How to split water - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for decomposing water, and more particularly to a method for decomposing water by applying light irradiation using a catalyst fixed to a membrane.

Due to the depletion of so-called fossil fuels such as oil and coal, the development of new energy has been required in recent years.

Among the new energies being considered today, hydrogen is ■
When water is used as a raw material, there are no resource constraints and it is not unevenly distributed depending on the region; ■ Since the combustion product is water, it is clean and does not cause pollution; ■ It does not disturb the earth's material circulation cycle. (2) Since it has various advantages such as being relatively easy to store, a method of easily producing hydrogen using water as a raw material is expected to be a powerful means of solving energy problems.

A known method for producing hydrogen from water is one that utilizes the photoexcitation effect on the surface of a semiconductor.

This method is carried out in the apparatus shown conceptually in FIG.

That is, for example, a container divided into two chambers via a diaphragm 5 is filled with an aqueous solution 3, and is filled with, for example, T 102.
A semiconductor electrode 1 and a platinum electrode 2 are immersed to electrically connect the two electrodes, and the surface of the semiconductor electrode 1 is irradiated with light 4.

At this time, hydrogen is generated from the platinum electrode 2 and oxygen is generated from the semiconductor electrode 1.

In this case, sunlight can also be used as the energy source.

Recently, a technology has been developed in which fine semiconductor powder is suspended in an aqueous solution and the suspension is irradiated with light to cause a water splitting reaction on the surface of the fine semiconductor powder to generate oxygen and hydrogen ( JP-A No. 51-83895).

This water splitting mechanism is based on semiconductor fine powder 6 as shown in Figure 2.
This is thought to be based on a photolocal cell mechanism on the surface of the .

That is, the irradiated light 4 generates electron (e-)-hole (h+) pairs on the surface of the semiconductor fine powder 6, and the holes oxidize water to generate oxygen.

On the other hand, the separated electrons reduce hydrogen ions on the other surface of the semiconductor fine powder 6 to generate hydrogen.

If hydrogen could be efficiently produced from solar energy using the method illustrated in Figure 2, the electrical equipment shown in Figure 1 would be unnecessary, and it would be possible to produce hydrogen using extremely simple equipment, even on vast oceans. It is conceivable that hydrogen production will become possible.

However, the method of using semiconductor fine powder (usually particle size 100 μm or less) in its original form for water decomposition on an industrial scale involves great difficulties in uniformly dispersing the fine powder or recovering it. There is a problem.

The present inventor aims to provide a water decomposition method using such a semiconductor fine powder, which allows the reaction to occur without any loss in decomposition efficiency, and which allows homogeneous dispersion or recovery of the water to be carried out extremely easily. As a result of extensive research, the present invention was completed by supporting fine semiconductor powder on a polymer membrane.

That is, the water decomposition method of the present invention involves adding catalyst particles made of a semiconductor excited by light in the visible and/or ultraviolet region or a composite straw of the semiconductor and metal to a porous membrane made of fluororesin in water. This method is characterized by immersing the supported semiconductor and irradiating it with light having an excitation energy higher than the excitation energy of the semiconductor.

The invention will be explained in more detail below.

In the present invention, "excitation" refers to the fact that a semiconductor receives energy due to light irradiation, and electrons in both electronic bands of the semiconductor undergo a transition to a conduction band.

The semiconductors used in the present invention include visible and/or
Alternatively, any substance that can be excited by light in the ultraviolet region can be used, and usually one selected from metal oxides, metal sulfides, metal phosphides, metal arsenides, metal selenides, or metal tellurides. It is composed of two or more types, for example, Tt02 j 5rTx032 Z
n01 Fe2032CdS, CdSe, CdTe, G
aP, GaAs, InP, ZnS.

Examples include Zn5e and the like.

Further, as the metal forming the composite with the semiconductor, noble metals are particularly preferred, but other metals can also be used.

Examples of these are platinum (Pt). Palladium (Pd)
), rhodium (Rh), ruthenium (Ru), nickel (Ni), and the like.

The catalyst, which is a composite of a semiconductor and a metal, is used in various forms, such as supporting a metal on a semiconductor or forming a complex of a metal and a semiconductor.

In addition, the composition ratio of semiconductor and metal is usually as follows:
It uses 0.01 to 10 parts of metal, preferably 0.01 to 1 part.

FIG. 3 is a schematic diagram of a composite semiconductor in which metal is fixed to the semiconductor described above.

It is preferable to use a composite of semiconductor fine powder 6 and metal 7. For example, when using TiO2 particles and several Pt composites, it is better to use water. Decomposition efficiency increases by more than 100 times.

Moreover, in addition to the composite system of a semiconductor and a metal, a composite of different semiconductors can also be used, and in this case, the decomposition efficiency is similarly increased.

The catalyst particles are used by being supported on a fluororesin film.

The fluororesin used in the present invention is made of a polymer of vinyl monomers in which all hydrogen atoms are replaced with fluorine atoms, such as tetrafluoroethylene, hexafluoropropylene, hexafluorobutadiene, etc. Homopolymers or copolymers made of one or more selected from these are used.

The porous membrane made of such a fluororesin has a porous structure having a pore size comparable to that of the catalyst particles (usually 0.1 to 100 μm), and supports the catalyst particles.

As a supporting method, for example, a method of uniformly enclosing and fixing it in a porous membrane by applying pressure uniformly with a roller or the like, or a method of fixing it on the membrane by an impregnation method, a deposition method, etc. is used.

The catalyst particles are usually 0.001% of the fluororesin film.
~0.1g mepo is used, preferably 0.0
It is 1 to 0.1 g.

The reason why fluororesin is used as a catalyst fixing film is that it is extremely stable against the strong oxidation effect caused by photoexcitation of the catalyst; other polymer films such as polyethylene, polyvinyl chloride, cellulose, etc. This is not preferable because the membrane itself is decomposed due to the oxidation-reduction action of the catalyst.

A catalyst membrane with catalyst particles encapsulated and fixed is immersed in water,
Next, light with more energy than is necessary to excite the catalyst particles is irradiated.

The light source used in the present invention can be any light source that emits light with a wavelength in the visible and/or ultraviolet region and is sufficient to excite the catalyst particles. For example, an ultra-high pressure mercury lamp, a short arc xenon lamp, etc. can be used. In addition, sunlight can also be used, and from the viewpoint of energy saving, its use is preferable.

Various embodiments can be considered for carrying out the method of the present invention, and a specific example thereof is shown in FIG.

The present invention will be explained below based on this specific example.

In FIG. 4, a photoreaction tank 8 has an upper part covered with a light-transmissive material 9 such as glass, an inlet 10 for water or an aqueous solution containing water as a main component, and an inlet 10 for introducing water or an aqueous solution containing water as a main component and hydrogen as a reaction product gas. An oxygen inlet 11 is provided.

The reaction tank is equipped with a catalyst fixed membrane 12 on its bottom surface, and the water 3 introduced into the tank comes into contact with the catalyst fixed membrane.

An enlarged cross-sectional view of such a catalyst fixed membrane is shown in FIG.
A catalyst 14 whose main component is a semiconductor having a particle size of about the same size as this is fixed therein.

When light 4 is irradiated from the top of the reaction tank through the irradiation window 9, the catalyst particles in the fixed film are excited, and due to this action, water is decomposed to generate hydrogen and oxygen.

These generated decomposed gases are taken out of the system through the outlet 11.

Examples performed using the above apparatus will be described below.

Example platinum 1. A polytetrafluoroethylene film (supporting amount 0.01 g/crit) on which titanium dioxide particles (particle size 11Mn) supported wtφ were fixed was immersed in a reaction tank containing pure water, and then heated with an ultra-high pressure mercury lamp. using room temperature,
When light irradiation was performed at an output of 500 W (light energy of 2.2 eV or more), 0.0.
04 IJTlo 1 generation of hydrogen gas was confirmed.

This result shows that when the catalyst particles are dispersed without being fixed to the membrane,
The amount of hydrogen generated per weight of supported catalyst was approximately the same as that of the sample subjected to light irradiation under the same conditions.

As is clear from the above, water decomposition by the method of the present invention is comparable to conventional decomposition methods in which a catalyst is dispersed, and since the catalyst is fixed to a membrane, When used on an industrial scale, recovery and replacement of the catalyst is easy.

Furthermore, since the catalyst can be uniformly dispersed on the membrane, operations such as stirring are not required, and water can be decomposed extremely easily.

[Brief explanation of the drawing]

Figure 1 is a conceptual diagram of a conventional water splitting device using photoelectrode reaction, Figures 2 and 3 are principle diagrams for explaining water splitting using semiconductor fine powder and composite semiconductor fine powder, respectively, and Figure 4 is a conceptual diagram of a water splitting apparatus according to an embodiment of the present invention, No. 5
The figure is an enlarged sectional view of the catalyst fixed membrane in FIG. 4. 1... Semiconductor electrode, 2... Platinum electrode,
3...Aqueous solution, 4...Light, 5...
...Diaphragm, 6...Semiconductor fine powder, 7...
・Supported metal, 8...Photoreaction vessel, 9...
Light transmission window, 10...Aqueous solution inlet, 11...
...Gas supply port, 12...Catalyst fixed membrane, 13
... Hole or groove, 14 ... Semiconductor fine powder, 15 ... Polytetrafluoroethylene thin film.

Claims (1)

[Claims] 1. Catalyst particles made of a semiconductor excited by light in the visible and/or ultraviolet region, or a composite of the semiconductor and a metal, supported in water on a porous membrane made of a fluororesin. A method for decomposing water, the method comprising immersing the semiconductor in the water and irradiating the semiconductor with light having an excitation energy higher than that of the semiconductor. 2. Claim 1 in which the semiconductor is a metal oxide, a metal sulfide, a metal phosphide, a metal arsenide, a metal selenide, or one or more metal 3. The method for decomposing water according to claim 1, wherein the metal is one or more selected from platinum, palladium, rhodium, ruthenium, or nickel. 4. Water decomposition according to claim 1, wherein the fluororesin is a polymer consisting of one or more selected from tetrafluoroethylene, hexafluoropropylene, or hexafluorobutylene. Method.