JPH07188961A - Carbon dioxide reducing electrode and carbon dioxide converter - Google Patents

Abstract

PURPOSE:To reduce CO2 with high current efficiency at the time of reducing CO2 electrochemically and photoelectrochemically to reduce it into hydrocarbons and alcohols by using an electrode obtained by coating the surface of a copper sheet with a specified metal oxide as the reducing electrode. CONSTITUTION:A KHCO3 soln. 2 is filled in an electrolytic cell 1 provided with a light transmitting window, a photoelectrode 4 as a thin film of Pt or an n-type optical semiconductor is used as an anode, an electrode 3 obtained by coating the surface of metallic copper with a conductive oxide consisting of at least one kind of metallic oxide among ZnO, Fe2O3, TiO, SnO2, IrO2, RuO2, ReO2, PbO2, etc., and having <=3.5eC of band gap or <=100OMEGAm electric resistance is used as the cathode, both electrodes 3 and 4 are separated by an ion-exchange membrane 6, a current is applied while blowing CO2 8 into the electrolyte 2, and the electrolyte is irradiated with the light of an Xe lamp 10. The CO2 blown into the electrolyte 2 is reduced to hydrocarbons and alcohols with high current efficiency.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrode for electrochemically or photoelectrochemically reducing carbon dioxide into hydrocarbons and alcohols, and a carbon dioxide converter using the same.

[0002]

2. Description of the Related Art It is the type of electrode material and the shape of the surface that have a great influence on the rate and selectivity of the reaction for electrochemically or photoelectrochemically reducing carbon dioxide. So far, many studies have been conducted on plain metals, formic acid for indium, tin and lead, carbon monoxide for gold, silver and zinc, methane for copper,
It is said that ethylene and the like are selectively produced. For example, Electrochemistry 58, No. 11, 1990, pages 984-989, and pages 996-100.
According to page 2, metallic copper shows a high reaction selectivity (current efficiency) to hydrocarbons and alcohols for the reduction reaction of carbon dioxide and bicarbonate ions, and the total current efficiency of methane, ethylene and ethanol is about 70. It is said that it will reach%. Further, in JP-A-1-28462, an electrode containing at least one of platinum, palladium, iridium, rhodium, iron, nickel and cobalt as a first metal and at least one of lead, copper, zinc and cadmium as a second metal. Is said to be excellent in reducing carbon dioxide.

[0006] Among conventional simple metals, metal copper, which has the highest reaction selectivity in conversion to hydrocarbons and alcohols, has a limited reaction selectivity of a single component and has not yet reached a practical level. Further, in the above-mentioned Japanese Patent Application Laid-Open No. 1-28462, hydrogen atoms are adsorbed and activated, and the atoms of the first metal are separated from each other by the second metal, whereby hydrogen atoms are bonded to each other to generate hydrogen gas. The use of hydrogen atoms for the reduction of carbon dioxide gas is suppressed. However, in any of these known techniques, the reaction rate is low, and the selectivity of the product is low, and its improvement is desired.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of these prior arts, and an object thereof is an electrode capable of reducing carbon dioxide gas to hydrocarbons and alcohols with high current efficiency and carbon dioxide utilizing the electrode. It is to provide a gas conversion device.

[0005]

The carbon dioxide gas reduction electrode of the present invention has a band gap of zinc, iron, titanium, tin, iridium, ruthenium, rhenium, and lead of 3.5 eV on a part or all of the surface of metallic copper. Below or 100 Ω electric resistance
It is characterized by being coated with at least one metal oxide having a conductivity of m or less. Of these, particularly preferable metal oxides are ZnO, Fe ₂ O ₃ , TiO ₂ , SnO ₂ , and IrO.
₂ , RuO ₂ , ReO ₂ and PbO ₂ . It is effective that these metal oxides are dispersed or coated in the form of islands on the surface of metallic copper in a thickness of 200 Å or less.
The second component, that is, at least one of zinc, iron, titanium, tin, iridium, ruthenium, rhenium, and lead, can be dispersed or entirely coated on the surface of metallic copper by vapor deposition, coating, sintering, thermal spraying, or the like. It is possible. For example, the metal oxide can be formed by depositing the metal of the second component on the copper surface by a sputtering method or an ion plating method in an oxygen atmosphere.

The shape of the base material copper may be any of plate, net, foam, thin film and fine particles. If the surface of the second component metal oxide and the surface of copper coexist, or if the copper surface is coated with a metal oxide, the adsorption state of carbon dioxide and protons changes and the reduction reaction is advantageous for the production of hydrocarbons and alcohols. Proceed in the direction. This reaction is a surface electrochemical reaction including the transfer of electrons, and the conductive metal oxide promotes the reduction reaction.

The thickness of the metal oxide on the metallic copper surface is 200
If it is less than Å, the synergistic effect of metallic copper and metallic oxide acts more strongly.

The electrode according to the present invention can be used as a carbon dioxide conversion device in combination with an anode electrode. As the anode, not only ordinary metals such as platinum, gold, rhodium, and iridium but also a photoelectrode obtained by thinning an n-type optical semiconductor can be used. Carbon dioxide is chemically very stable and its reduction requires a lot of energy.
The electrode according to the present invention causes a reduction reaction of carbon dioxide gas, and the anode causes an oxidation reaction of water. For example, the reaction when methane is produced can be expressed by the following equation.

[0009]

[Chemical 1]

These two reaction equations are summarized as follows.

CO ₂ + 2H ₂ O …………> CH ₄ + 2O ₂ The standard free energy of this reaction formula is +818 kJ / mol.
Therefore, the reaction does not proceed unless some kind of energy is applied from the outside. If fossil fuel is used as an energy source, it will lead to the emission of new carbon dioxide gas, which is contrary to carbon dioxide reduction. The most ideal energy source for this is sunlight. A photoelectrode that absorbs solar energy and acts as an anode is the preferred electrode to combine. An n-type optical semiconductor such as TiO ₂ may be used for the photoelectrode in order to absorb the light energy and cause the water decomposition reaction.

As another method using sunlight, it is also possible to adopt a hybrid system in which sunlight is once converted into electricity and the electricity is supplied to the carbon dioxide converter to carry out a reduction reaction.

[0013]

[Function] A part or all of the surface of metallic copper has conductivity such that the band gap selected from zinc, iron, titanium, tin, iridium, ruthenium, rhenium and lead is 3.5 eV or less or the electric resistance is 100 Ωm or less. By coating with at least one of the metal oxides, carbon dioxide gas can be efficiently converted into hydrocarbon and alcohol.

[0014]

【Example】

Example 1 A platinum net was placed on a metal copper plate having a purity of 99.999% and an area of 8 cm ² , and ZnO was dispersed and coated on both surfaces by sputtering in an oxygen atmosphere to prepare an electrode. Figure 1
0.1 mol / l potassium bicarbonate aqueous solution (electrolyte) 2 was put into the H-type cell 1 as shown in Fig. 3, and the metallic copper electrode modified with ZnO was the cathode 3, the platinum plate was the anode 4, and Ag / AgCl was the reference electrode. 5 and partition between both electrodes with an ion exchange membrane 6,
A reduction experiment was conducted under the condition that the cathode potential was controlled to −1.8 V _VS. Ag / AgCl by the power supply 7 while blowing carbon dioxide gas 8. The current efficiency of the product is compared with Comparative Example 1 and Table 1
Shown in. Comparative Example 1 is the same as Example 1 except that metallic copper is used as the cathode. The ZnO-modified electrode increased the current efficiency of methane as compared with the electrode of Comparative Example 1.

(Example 2) Instead of ZnO in Example 1, F
An electrode was prepared in the same manner as in Example 1 except that e ₂ O ₃ was used, and the same experiment as in Example 1 was performed. The results are shown in Table 1. The electrode modified with Fe ₂ O ₃ also increased the current efficiency of methane as compared with the electrode of Comparative Example 1.

[0016]

[Table 1]

(Example 3) Purity 99.999%, area 8c
Both sides of a metal copper plate of m ² were dispersedly coated with TiO ₂ by an ion cluster beam in an oxygen atmosphere to prepare an electrode. An H-type cell was charged with 0.1 mol / l potassium bicarbonate aqueous solution, a metal copper electrode modified with TiO ₂ was used as a cathode, a platinum plate was used as an anode, Ag / AgCl was used as a reference electrode, and both electrodes were partitioned by an ion exchange membrane, Room temperature, cathode potential -1.
When a reduction experiment was carried out while blowing carbon dioxide gas under the condition of 8 V _VS. Ag / AgCl, as shown in Table 2, the current efficiency of methane was increased and that of H ₂ was decreased as compared with Comparative Example 1.

Example 4 An electrode was prepared in the same manner as in Example 3 except that SnO ₂ was used instead of TiO ₂ in Example 3, and the same experiment as in Example 3 was conducted. The results are shown in Table 2.
Shown in. When the electrode modified with SnO ₂ was used, the current efficiency of methane was increased and that of H ₂ was decreased as compared with the electrode of Comparative Example 1.

[0019]

[Table 2]

(Example 5) Purity 99.999%, area 8c
IrO ₂ coating films were formed on both surfaces of a metal copper plate of m ^{2 by} a thermal spraying method to prepare electrodes. A 0.1 mol / l potassium bicarbonate aqueous solution was placed in an H-type cell, a metallic copper electrode coated with IrO ₂ was used as a cathode, a platinum plate was used as an anode, and Ag / AgCl was used as a reference electrode, and both electrodes were partitioned by an ion exchange membrane. At room temperature and cathode potential of −1.8 V _VS. Ag / AgCl, a reduction experiment was conducted while blowing carbon dioxide gas. As shown in Table 3, the current efficiency of methane was increased as compared with Comparative Example 1.

Example 6 An electrode was prepared in the same manner as in Example 5 except that RuO ₂ was used instead of IrO ₂ in Example 5, and the same experiment as in Example 5 was conducted. The results are shown in Table 3.
Shown in. Also when the electrode modified with RuO ₂ was used, the current efficiency of methane was increased as compared with the electrode of Comparative Example 1.

Example 7 An electrode was prepared in the same manner as in Example 5 except that ReO ₂ was used instead of IrO ₂ in Example 5, and the same experiment as in Example 5 was conducted. The results are shown in Table 3.
Shown in. Even when the electrode modified with ReO ₂ was used, the current efficiency of methane was increased as compared with the electrode of Comparative Example 1.

[0023]

[Table 3]

(Example 8) Purity 99.999%, area 8c
An electrode was prepared by coating both surfaces of a metal copper plate of m ² with ZnO by a Zn oxygen plasma sputtering method. 0.1 mol in H type cell
/ L of potassium bicarbonate aqueous solution was added, the prepared electrode was used as a cathode and the platinum plate was used as an anode, and both electrodes were partitioned by an ion exchange membrane. A solar cell applied a voltage of 4 V between both electrodes to reduce carbon dioxide gas. When an experiment was conducted, as shown in Table 4, the current efficiency of methane was increased as compared with Comparative Example 2. Comparative Example 2 is Example 8 except that metallic copper is used as the cathode.
Is the same as.

[0025]

[Table 4]

(Embodiment 9) This embodiment will be described with reference to FIG. A 0.1 mol / l potassium bicarbonate aqueous solution 2 was placed in an H-shaped cell 1 provided with a light transmission window 9, and a titanium oxide thin film was formed on a tin oxide coating glass by a sol-gel method to a thickness of 0.5 μm.
The formed photoelectrode is anode 4, purity 99.999%, area 8 cm
^With the metal net of masking not placed on the metal copper plate of ² ,
The electrode of Example 9 prepared by vapor-depositing ZnO for 1 minute by the sputtering method was used as the cathode 3. The two electrodes are partitioned by an ion exchange membrane 6, and at room temperature, the anode 4 side is irradiated with light of a 500 W xenon lamp 10 similar to sunlight,
When the output terminal of the solar cell 11 of 2 W was connected between both electrodes and the reduction experiment of carbon dioxide gas 8 was performed, as shown in Table 5, the current efficiency of methane increased as compared with Comparative Example 3. Comparative Example 3 is the same as Example 9 except that metallic copper is used as the cathode.

[0027]

[Table 5]

[0028]

INDUSTRIAL APPLICABILITY According to the present invention, carbon dioxide can be efficiently converted into hydrocarbon and alcohol by an electrochemical and photoelectrochemical reduction method.

[Brief description of drawings]

FIG. 1 is a schematic view of an electrochemical reduction device according to an embodiment of the present invention.

FIG. 2 is a schematic view of a photoelectrochemical reduction device according to an embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... H-type cell, 2 ... Electrolyte, 3 ... Cathode, 4 ... Anode, 5 ... Reference electrode, 6 ... Ion exchange membrane, 7 ... Potentiostat, 8 ... Carbon dioxide, 9 ... Light transmission window, 10 ... Xenon lamp , 11 ... Solar cells.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Hiroshi Tobita, 7-1, 1-1 Omika-cho, Hitachi-shi, Ibaraki Hitachi, Ltd. Hitachi Research Laboratory (72) Inventor Hiroshi Miyadera 1-chome, Omika-cho, Hitachi-shi, Ibaraki No. 1 Hitachi Ltd. Hitachi Research Laboratory

Claims (8)

[Claims] 1. A part or all of the surface of metallic copper is zinc, iron,
Carbon dioxide gas characterized by being coated with at least one conductive metal oxide having a band gap selected from titanium, tin, iridium, ruthenium, rhenium, and lead of 3.5 eV or less or an electric resistance of 100 Ωm or less. Reduction electrode. 2. The metal oxide according to claim 1, wherein the metal oxide is Zn.
O, Fe ₂ O ₃ , TiO ₂ , SnO ₂ , IrO ₂ , RuO ₂ ,
A carbon dioxide gas reduction electrode comprising at least one of ReO ₂ and PbO ₂ . 3. The carbon dioxide gas reduction electrode according to claim 1, wherein the metal oxide is dispersed or coated on the surface of the metal copper in an island state of 200 Å or less. 4. The metallic copper according to claim 1, wherein the metallic copper is a plate, a net,
A carbon dioxide gas reduction electrode characterized by comprising either a foam or a thin film. 5. A carbon dioxide gas converter comprising the electrode according to any one of claims 1 to 4 as a cathode and the photoelectrode as an anode. 6. A carbon dioxide converter according to claim 5, wherein the photoelectrode is composed of an n-type semiconductor. 7. The carbon dioxide gas conversion device according to claim 5, wherein the photoelectrode is made of titanium oxide. 8. The energy required for conversion of carbon dioxide gas is supplied by sunlight and a solar cell.
The carbon dioxide gas conversion device according to any one of 1 to 7.