JP3455779B2 - Apparatus for producing hydrogen comprising semiconductor photocatalyst reactor and electrolyzer - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for producing hydrogen, which is a combination of a semiconductor photocatalytic reaction apparatus and an electrolysis apparatus in the technology of producing hydrogen by electrolysis of water.

[0002]

2. Description of the Related Art Hydrogen energy is a very clean energy source, and various applications such as fuel cells and hydrogen engines can be considered, and it seems that it will be the center of the future energy form replacing fossil energy. However, most of hydrogen today is produced by steam reforming of fossil resources. Considering the future depletion of fossil resources and the problem of global warming due to carbon dioxide, the inexhaustible water will be the only hydrogen source in the end. Electrolysis is easy to produce hydrogen from water, but it does not make sense to use fossil fuels to produce electricity. Therefore, an idea has been proposed in which an inexhaustible, clean and safe solar energy is converted into electric energy by a solar cell, and water is electrolyzed to produce hydrogen. The biggest drawback of this idea is the high cost and low energy balance of the system, in particular the solar cell (energy produced by the system / energy producing the system by its lifetime). Although solar cells such as silicon and electrolysis technology have been vigorously studied,
In order to realize hydrogen production by sunlight, it is necessary to significantly improve the system cost and energy balance with innovative technologies. Also, although the electrolysis technology of water has advanced considerably, the overvoltage that promotes gas generation is very high,
A voltage much higher than the theoretical electrolysis voltage of water, 1.23V, is required, and the energy loss for that is also a big problem.
On the other hand, research on the direct decomposition of water into hydrogen and oxygen with the energy of sunlight using a photocatalyst is also in progress. Although this technology is very low in cost and excellent in recycling and durability, the catalyst which can be directly decomposed at this stage cannot utilize visible light and its efficiency is not high.

[0003]

SUMMARY OF THE INVENTION The present invention relates to a technique for electrolytically producing hydrogen from water using solar energy,
It is an object of the present invention to provide a device that significantly improves the cost and energy balance of the entire system.

[0004]

As a result of intensive studies to solve the above problems, the present inventors have found that when a semiconductor photoreaction device partially incorporating a semiconductor photocatalyst technology with a very low cost is used, hydrogen The inventors have found that the electrolysis voltage of an electrolysis apparatus for manufacturing can be greatly reduced, and have completed the present invention. That is, according to the present invention, the following inventions are provided. (1) In the presence of an aqueous solution containing iron (III) ions, the semiconductor photocatalyst is irradiated with light to generate oxygen, and iron (III) is generated.
A semiconductor photocatalytic reaction device for reducing ions to iron (II) ions, an electrolysis device for generating hydrogen by oxidizing the obtained aqueous solution containing iron (II) ions to iron (III) ions, and generated iron ( III) An apparatus for producing hydrogen, which comprises an apparatus for supplying an aqueous solution containing ions to a semiconductor photoreaction apparatus. (2) The device according to the above 1, wherein RuO ₂ —WO ₃ or WO ₃ —TiO ₂ is used as the semiconductor photocatalyst.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventor has long been conducting research on efficient production of oxygen and iron (II) ions from an aqueous solution containing iron (III) ions using a semiconductor photocatalyst. 4Fe ³⁺ + 2H ₂ O → 4Fe ²⁺ + O ₂ + 4H ⁺ (1) The reaction of the above formula (1) is about 88 kJ / mol of H ₂
This is an energy storage reaction (ΔG> 0, uphill reaction) of O (pH = 0, Fe (III) = 1N). In other words, conversion of solar energy into chemical energy using semiconductor photocatalyst leads to conversion and storage technology of solar energy. Moreover, it is a very simple and inexpensive system. The energy thus stored needs to be extracted in a form that is easy for us to use. There is also a method in which the reverse reaction of this reaction is advanced to take out as heat energy, but its utility value is low. If it can be extracted in the form of high utility value such as hydrogen energy, it will be a very useful technology. The present invention is applicable to WO ₃ and Ti.
Using a semiconductor photocatalyst such as O ₂ or In ₂ O _3, oxygen is generated from water as shown in formula (1), and solar energy is converted to Fe.
^By temporarily accumulating in the form of an aqueous solution of ²⁺ ions and then electrolyzing the aqueous solution containing Fe ²⁺ ions at a slight voltage, it is possible to produce hydrogen at a very low cost. 4Fe ²⁺ + 4H ⁺ → 4Fe ³⁺ + 2H ₂ (2) (1) + (2) 2H ₂ O → O ₂ + 2H ₂ (3)

The present invention will be described in detail below. Since the reaction of the formula (1) is a large energy storage reaction, energy must be supplied from the outside. A semiconductor photocatalyst with a bandgap of 3.3 eV or less is used to carry out this reaction using solar energy. Oxygen generation is a multi-electron photoreaction, so it is difficult to use a simple dye, but a semiconductor photocatalyst can pool a large number of electrons and holes, so it has sufficient oxygen generation capability. As conditions for the semiconductor for the catalyst, in addition to (i) the size of the band gap, (ii) the conduction band potential is Fe (III) / Fe.
Higher (negatively higher) than the Redox level of (II), (ii
It must satisfy the conditions that i) the potential of the valence band is lower (positively higher) than the Redox rank of O ₂ / H ₂ O, and (iv) that it is stable under the reaction conditions.
Specifically, WO ₃ , TiO ₂ , SrTiO ₃ , Ta ₂ O ₅ ,
There are also simple oxides such as In ₂ O ₃ , complex oxides such as FeTiOx, and oxide semiconductors such as WO ₃ that can be limitedly used under certain reaction conditions (low pH). Other than oxides, SiC is a semiconductor having a high conduction band potential and is useful.

As a method for preparing a semiconductor, a commercially available product may be used as it is, or a semiconductor may be synthesized from a metal precursor. It is desirable that the purity is as high as possible. Various preparation methods such as precipitation of hydroxide from precursor and baking, thermal decomposition of ammonium salt, sol-gel method, etc. can be used, but the prepared semiconductor has as good crystallinity as possible and has a relatively large surface area. Is good. The range of crystal diameter is 1 to 200
0 nm, preferably 2 to 200 nm, is medium. However, depending on the semiconductor, the preparation method is limited, and thus it is difficult to control the crystal diameter. In that case, a large crystal diameter may be used. Further, the particle size and surface area may be controlled, or a semiconductor may be supported on a simple substance such as silica so that light can efficiently hit the catalyst. Furthermore, when a plurality of semiconductors are used at the same time, light of various wavelengths can be used efficiently.

The semiconductor powder has a sufficient photocatalytic activity by itself, but an additive having a function of accelerating charge separation such as RuO 2 or reducing the overvoltage of the reaction is added in an amount of 0.01 to 0.01%.
It is desirable to support the content in the range of 20 wt%, preferably 0.1 to 5 wt%, and the performance is greatly improved.

The redox potential of Fe (III) / Fe (II) depends on the total iron ion concentration and Fe (III) / Fe (I
It is affected by the ratio of I), pH, anionic species, etc. Naturally, the reaction of the formula (1) has a high Fe (III) ion concentration and p
The higher H is, the easier the process is. On the other hand, the formula (2) is Fe (II)
The higher the ion concentration and the lower the pH, the better. Since both reactions are in equilibrium with the reverse reaction, a highly active catalyst and an efficient light irradiation method are important for increasing the reaction rate and transferring the equilibrium over a wide range. The influence of anions is also great. For example, in the presence of CN ⁻ ions, iron ions form complex ions such as [Fe (CN) ₆ ] ³⁻ and [Fe (CN) ₆ ] ⁴⁻ , and the redox potential becomes high (becomes negatively large). It is disadvantageous for hydrogen generation but is advantageous for hydrogen generation. Fe (III) / Fe (II) ions can be used only in an acidic region because they cause hydroxide precipitation in alkaline conditions, but when they form a complex ion with CN ^−, they have the advantage of being stable even at high pH. .

In the light irradiation method, the catalyst and the solution must be irradiated with light as efficiently as possible. When an artificial light source is used, a cell of a type in which diffusely reflected light returns to the reaction solution again, such as an internal irradiation type reaction tube, is desirable. In the case of external irradiation type such as reaction of sunlight, mirrors and aluminum foil are used properly so that light does not escape. The catalyst and the solution may be in a stationary system, but it is preferable to circulate the solution or to vigorously disperse the solution with a magnetic stirrer or a shaker. The gas phase is preferably depressurized, but may be replaced with an inert gas such as argon. The types of glass for window plates are
Inexpensive materials such as Pyrex (registered trademark) and plastic can be used.

The reaction of formula (2) is carried out in a two-chamber electrolysis cell using partition walls. An ion exchange membrane, a salt bridge, a ceramic membrane or the like can be used as the diaphragm, but a cation exchange membrane is preferable because the mobility of protons needs to be sufficiently higher than the mobility of iron ions. A material with a small hydrogen overvoltage is desirable for the cathode electrode on the hydrogen generation side.
A carbon electrode carrying a small amount of platinum or the like can be used. On the other hand, as an electrode for oxidizing Fe2 ⁺ ions, in addition to the above electrodes, a carbon electrode not supporting platinum can be used. In order to lower the electrolysis voltage, shorten the distance between electrodes, raise the reaction temperature, lower the electrode current density,
It is important to devise such as using current collectors. As the electrolysis proceeds, the Fe ²⁺ ion concentration around the anode decreases, so the aqueous solution around the electrode is vigorously stirred, or is directly connected to the reaction device of reaction (1), and a solution with a high Fe ²⁺ concentration is constantly circulated. System is preferred.

[0012]

EXAMPLES Examples of the present invention will be described below.

Example 1 1 g of tungsten oxide (Kanto Kagaku) and iron (III) sulfate
(2 mmol as Fe ³⁺ ) and 400 ml of water were mixed, charged into an internal irradiation type reaction vessel, and set in a closed circulation system. After degassing the air in the gas phase and the liquid phase, argon was introduced into the system to bring the total pressure in the system to about 35 torr. A 400 W high pressure mercury lamp (Science and Technology) was used as a light source, and light was irradiated while the catalyst was dispersed by a stirrer. Pyrex was used as the lamp cooling tube. The produced hydrogen and oxygen were qualitatively quantified by gas chromatography and a pressure gauge. Oxygen generation rate is 40 μmol / h, oxygen generation amount after 4 hours is 270 μm
reached ol. After reacting for 20 hours, this solution was quickly filtered, the filtrate was put in one of the two-chamber cells using an ion exchange membrane, and the same amount of pure water was put in the other chamber. Furthermore, S in both rooms
It was prepared by adding sulfuric acid so that the O ₄ ²⁻ concentration was 0.5 mol / l. A platinum wire was used for the working electrode and the counter electrode, and Ag / AgCl (NaCl) was used for the reference electrode. The solution was vigorously stirred with a magnetic stirrer. When the current was measured with a potentiostat at a delivery speed of 0.02 V / min, the current started to flow at about 1.0 V (Ag / AgCl).
Generation of hydrogen was confirmed. Fe as shown in Comparative Example 1
Electrolysis without ²⁺ ion is 1.5V (Ag
/ AgCl) or more, the current does not flow, which indicates that the electrolysis efficiency is very good.

Example 2 In Example 1, 0.5 mol /
The reaction was carried out by adding 1 L of H ₂ SO ₄ , but there was no great influence on oxygen generation and iron ion reduction reaction.

Example 3 In Example 1, chloride was used instead of sulfate.
The initial oxygen generation rate was reduced to about half that of Example 1, but there was no significant difference in the amount of Fe2 ⁺ produced after 4 hours. Also for the electrolytic reaction, hydrochloric acid was used instead of sulfuric acid.
The anodic potential was slightly lower than that of sulfate. As shown in Comparative Example 2, the electrolysis without Fe2 ⁺ ions shows that the electrolysis efficiency is very good in comparison with the fact that no current flows unless electrolysis is 1.3 V (Ag / AgCl) or more.

Example 4 Instead of WO ₃ in Example 1, TiO ₂ (Ishihara Sangyo, Rutile) was used. The initial oxygen generation rate was lower than that of WO ₃ , but continued oxygen generation was observed, and Fe 2 after 4 hours
^{+ There} was no big difference in the production amount.

Example 5 In Example 1 WO₃On RuO₂Of carrying its activity
I examined the changes. RuO ₂Is RuCl₃Aqueous solution to WO_ThreeIncluded in
It was dipped and air-baked at 500 degrees for 2 hours. RuO _TwoA trace amount
Oxygen generation rate and Fe2 after 4 hours just by adding⁺
It can be seen that the production amount greatly increases. RuO₂Carrying amount
It was found that 1 to 3 wt% is preferable. RuO₂(3w
t%)-TiO₂Electrolysis of the solution after reacting for 20 hours at
Has an anode potential of 0.85 V (Ag
/ AgCl (NaCl)) (see Figure 1).

Example 6 In Example 5, the dependence of RuO ₂ (3 wt%)-WO _{3 on} the air calcination temperature was investigated. As a result, it was found that the catalyst calcined at 500 degrees had the highest activity.

Example 7 In Example 1, WO ₃ was replaced by TiO ₂ (Ishihara Sangyo, Rutile)
The catalyst supported on was used. After impregnating TiO2 with ammonium tungstate as a precursor, it was air-baked at 500 degrees. It was found that the oxygen generation rate and the amount of Fe2 ⁺ produced after 4 hours were greatly improved as compared with Examples 1 and 4.

Comparative Example 1 When photocatalyst was added and electrolysis was performed without photocatalytic reaction in Example 1, the potential at which current started to flow was 1.5 V (Ag / AgCl (NaCl)) (see FIG. 1). ).

Comparative Example 2 When the photocatalyst was added and electrolysis was performed without the photocatalytic reaction in Example 3, the potential at which the current started to flow was 1.3 V (Ag / AgCl (NaCl)).

[0022]

[Table 1]

[0023]

EFFECTS OF THE INVENTION According to the present invention, hydrogen can be obtained by decomposing water with very little electric power as compared with ordinary electrolysis. Normally, electrolysis of water decomposes water at a voltage of 1.5 V or higher, which is close to 2 V, but if a photocatalytic reaction can produce an aqueous solution containing high-concentration Fe2 ⁺ ions, the electrolysis voltage can reach a maximum of 0.7 V. Because it can be lowered
As a result, significant power saving is achieved. If the photocatalytic reaction is performed by sunlight and the electric power is generated by the solar cell, a highly efficient solar energy conversion system can be constructed.

[Brief description of drawings]

FIG. 1 shows the relationship between voltage and electrolytic current. a: Example 5 b: Comparative example 1

─────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-51-88492 (JP, A) JP-A-10-87303 (JP, A) Int. J. Hydrogen. Energy. , 1983, 8 [1], p. 23-31 Adv. Hydrogen. Energy 2 (Hydrogen Energy Prog. 2), 1981, p. 631-640 (58) Fields investigated (Int. Cl. ⁷ , DB name) C01B 3/04 B01J 23/652 B01J 35/02 C01B 13 / 02 C25B 1/04

Claims (2)

(57) [Claims] 1. A semiconductor photocatalytic reaction device for reducing iron (III) ions to iron (II) ions by irradiating the semiconductor photocatalyst with light in the presence of an aqueous solution containing iron (III) ions to generate oxygen. The obtained aqueous solution containing iron (II) ions is oxidized into iron (III) ions to generate hydrogen, and an aqueous solution containing the generated iron (III) ions is supplied to the semiconductor photoreaction device. An apparatus for producing hydrogen, which comprises a device. 2. RuO ₂ -W as a semiconductor photocatalyst
The device according to claim 1, wherein O ₃ or WO ₃ —TiO ₂ is used.