JPS6327428B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a novel method for producing hydrogen, and more particularly to a method for producing hydrogen from water by passing a reducing fluid through one side of a zirconia-based solid electrolyte membrane having a metal electrode attached thereto. In recent years, hydrogen has attracted attention as a new energy source due to its non-polluting nature and many other advantages, but it has not been put into practical use due to the high cost of hydrogen itself. Various methods are being researched to produce hydrogen at low cost, and in particular, methods for producing hydrogen from inexhaustible water are being researched in Japan and other countries. These include the photoelectrolysis method that uses solar energy, the electrolysis method that uses surplus electricity, the thermochemical method that uses multi-stage chemical reactions, and the steam iron method that uses iron as a reaction medium, but all of them are highly efficient. However, there are still major problems in terms of cost, etc., and it is currently far from being possible to produce high-purity hydrogen at low cost. In view of this situation, the present inventors investigated a new method for producing high-purity hydrogen, and as a result, they used a zirconia-based solid electrolyte membrane with oxygen ion conductivity as a hydrogen separation wall to separate reducing compounds from water. We discovered a new method for producing high-purity hydrogen and arrived at the present invention. That is, in the present invention, metal electrodes of the same type or different types are attached to both sides of a zirconia solid electrolyte membrane to which a divalent metal oxide is added, and the attached material is used as a hydrogen separation wall, and conductivity is established between the metal electrodes on both sides. Electrically connected by a wire, one side (anode side) is brought into fluid contact with a reducing substance and the other side (cathode side) is brought into fluid contact with water vapor at a temperature range of 600℃ to 1000℃. This is a method for producing hydrogen. To explain the hydrogen production method of the present invention using diagrams, first, reducing substances (H 2 , H ₂ ,
When CO, CH ₄ ) are flowed to one side of the hydrogen separation wall (the right side in the figure, the anode side), they react with oxygen ions (O ^2- ) in the zirconia solid electrolyte 1 and release electrons. The chemical formula is H ₂ +O ^2- →H ₂ O+2e ^- CO+O ^2- →CO ₂ +2e ^- CH ₄ +4O ^2- →CO ₂ +2H ₂ O+8e ^- C+2O ^2- →CO ₂ +4e ^- , etc. The emitted electrons are transferred to the electrodes 2 and 3 on both sides.
It is supplied to the opposite side of the solid electrolyte (the left side in the figure, the cathode side) through a conductive wire 4 that electrically connects the solid electrolyte. On the other hand, water vapor is flowing to the cathode side, and when the water vapor is decomposed, the oxygen atoms receive the electrons supplied from the anode side and become oxygen ions, and the hydrogen becomes hydrogen gas. This can be expressed as a chemical formula: H ₂ O + 2e ^- → H ₂ + O ^{2 -} The generated oxygen ions diffuse through the zirconia solid electrolyte, reach the anode side, and are used again for reaction with the reducing compound. Through the mechanism described above, hydrogen gas with high purity can be produced in one step using a reducing substance and water as raw materials. If CO ₂ is used instead of water in the above explanation, pure CO gas can also be produced as shown in the following equation. CO ₂ + 2e ^- →CO + O ^2- Various types of zirconia-based solid electrolytes are already known to be used in the method of the present invention, but all of them have the property that oxygen ions move easily. Specifically, it is necessary to add oxides of divalent metals such as CaO, MgO, and SrO to zirconia, or oxides of trivalent metals such as rare earths (Y ² O ₃ , S _C2 O _3, etc.). Those obtained by adding one type or several types of can be used. Furthermore, the metal materials used as electrodes in the present invention include Au, Pt,
Pd, Rh, Ru, Ag, Fe, Co, Ni, Sn, In, Cu
In principle, any electrode that is normally used as an electrode can be used, but the electrode used in this method is not only an electrical conductor, but can also be expected to have a catalytic effect that promotes oxidation and reduction reactions. is preferable, and the effect is particularly great on the anode side.
Furthermore, thermal stability, adhesion to zirconia, etc. are also issues. Ag and Fe are good in terms of adhesion, but in terms of reaction rate on the anode side, Ni, Pd,
Pt, Ag, and Fe are superior in this order (Examples 8 to 12). The metal electrode surface has micropores and requires a three-phase interface where the metal electrode, the base zirconia, and the reaction raw material come into contact, and this is the method for creating many such three-phase interfaces in the case of Au, Pt, and Ag. A metal electrode surface made of fine particles having the above performance can be obtained by coating fine particles of metals or metal oxides such as zirconia on the base zirconia and firing them.In the case of metals other than the above, after firing, it is possible to obtain a metal electrode surface made of fine particles having the above performance by applying fine particles of metals or metal oxides such as Alternatively, a similar metal electrode surface can be obtained by electrochemical reduction. Further, it is also possible to use a metal electrode made porous by adhering a thin metal foil to it physically or by chemical treatment such as etching. The reaction temperature in the method of the present invention may be within a range where the zirconia-based solid electrolyte has oxygen ion conductivity, but in most cases, it is practically 600°C to
It is in the range of 1000℃. If it is too low, the reaction rate will be slow, and if it is too high, the energy efficiency will be poor. In any case, in this method, under steady state conditions in which the reaction proceeds rapidly, the reaction on the anode side is an extremely exothermic reaction, so there is little need to supplement the amount of heat from the outside. Furthermore, if waste heat from coke ovens, boilers, etc. is used directly or indirectly, economic efficiency will be further increased. Furthermore, although the reaction in this method proceeds even under normal pressure, the reaction rate is naturally faster under increased pressure. In principle, the reducing substance used as one of the raw materials in the method of the present invention can be any substance that reacts with oxygen and burns and has fluidity.
Any gaseous, liquid, or powdered gas may be used, but a gaseous one is most desirable in terms of contact with the electrode, reaction temperature, etc., and various mixed gases can be used industrially. Specifically, various waste gases containing low concentrations of hydrogen gas, methane gas, and other saturated and unsaturated hydrocarbons, incompletely combusted waste gases containing carbon monoxide, coke oven gas, and blast furnace gas. Examples include converter gas, and gases containing SO ₂ , NH ₃ , etc. can also be used. In addition, there is a possibility that various waste oils can be used as liquids, and powdered coal can be used as powders. In the method of the present invention, it is necessary to electrically connect the electrodes attached to both sides of the zirconia solid electrolyte with a conductive wire. The reaction rate and yield can be significantly increased by applying a voltage. In this case, it is most economically effective to combine two types of metals having a Seebeck effect as a power source and to utilize thermoelectromotive force generated by utilizing waste heat. As explained above, in the method of the present invention, since the reducing substance, which is one of the raw materials, and the water vapor, which is the direct raw material for hydrogen, are separated by the partition wall, high-purity hydrogen gas containing no impurities is produced. It can be easily obtained directly or by simple separation with water. Hereinafter, the method of the present invention will be explained in more detail with reference to Examples. Examples 1 to 7 Commercially available cylindrical zirconia-based solid electrolyte membranes with a thickness of 2 mm, a diameter of 21 mm, and a length of 30 cm;
type was used. One of them is stabilized by adding 15 mol% of CaO and is hereinafter abbreviated as CSZ. The other one is stabilized by adding 9 mol% of Y ₂ O ₃ and is abbreviated as YSZ. The method for attaching metal electrodes to the solid electrolyte is to thoroughly grind 1 g of AgO powder in a mortar, suspend it in acetone, apply it to both the inner and outer surfaces of the zirconia tube (applied area: 48 cm ² ), and then Acetone was completely removed by firing in oxygen at 700°C for 2 hours, and AgO was thermally decomposed to produce an Ag electrode consisting of fine particles. The reaction was carried out in a normal flow system at normal pressure. A reducing substance was passed through the tube at a rate of 14 ml/min with the inside of the tube serving as the anode side, and water vapor saturated with helium as a carrier gas at room temperature was passed at a rate of 7 ml/min with the outside of the tube serving as the cathode side. and reacted. The carrier gas helium was used for convenience of the experiment; it does not participate in the chemical reaction at all and is essentially unnecessary. The temperature was maintained at the specified temperature of 700-800°C as shown in the table. The hydrogen produced was measured by the value of the current flowing through the external conductor, and the correspondence between this current value and the amount of hydrogen produced was confirmed in advance by gas chromatography. Further, the applied voltage in Examples 4 to 7 utilized thermoelectromotive force generated by a Chromer-Constantan thermocouple. The experimental results obtained are shown in Table 1.

[Table] Examples 8 to 12 Using the same CSZ as Examples 4 to 7, Pt, Pd,
1 g each of NiO and Fe ₂ O ₃ powders were applied to the anode side in exactly the same manner as in Examples 1 to 7 (applied area: 48 cm ² ).
It was fired in oxygen at 700°C for 2 hours. Pd, NiO, and Fe ₂ O ₃ were heated to create metal electrodes of Pd, Ni, and Fe through H ₂ gas. Note that Pt was used in the reaction as it was without being treated with H ₂ like Ag in Examples 1 to 7. These were used as anode electrodes, Ag was used as the cathode electrode, and H ₂ gas was used as the reducing substance. The comparison below shows the following.

【table】 [Brief explanation of the drawing]

FIG. 1 is a drawing for explaining the principle of the method of the present invention, and is a schematic cross-sectional view of a zirconia-based solid electrolyte membrane to which electrodes are attached. In the drawings: 1...solid electrolyte, 2...metal electrode (anode side), 3...metal electrode (cathode side), 4...conductor, 5...ammeter.

Claims (1)

[Claims] 1. A hydrogen separation wall consisting of a zirconia solid electrolyte membrane to which a divalent or trivalent metal oxide is added and metal electrodes of the same or different types attached to both sides, and a hydrogen separation wall between the metal electrodes on both sides. electrically connected by conductive wire,
Its one side (anode) in the temperature range of 600℃~1000℃
A method for producing hydrogen from water, which comprises bringing one side (cathode) into fluid contact with a reducing substance and the other side (cathode) with water vapor. 2 The divalent or trivalent metal oxide is CaO or Y ₂ O ₃
A method for producing hydrogen according to claim 1. 3. Claims 1 or 2, in which a part of the conductive wire connecting the electrodes is made of a combination of different metals that generate thermoelectromotive force, and is connected so that a positive voltage is applied to the anode side. The method for producing hydrogen as described.