JPH0463113A - Carbon dioxide photoreduction cell - Google Patents

Abstract

PURPOSE:To increase the energy gap (potential difference) between electrodes and also to increase the reduction velocity of carbon dioxide gas by using a photoelectrode using n-type semiconductor as anode and a photoelectrode using p-type semiconductor as cathode and allowing an electrolyte to exist between both. CONSTITUTION:If a photo reduction cell where a photo electrode using n-type semiconductor and a photoelectrode using p-type semiconductor are used as an anode and a cathode, respectively, and an electrolyte is allowed to exist between both electrodes is constituted, the reduction of carbon dioxide gas can be allowed to proceed easily because the potential difference between both electrodes is increased. At this time, a greater effect can be produced when catalytic components by which carbon dioxide gas is absorbed and activated, such as nickel and copper, are carried by the p-type semiconductor. Not only a solution, such as sulfuric acid and phosphoric acid, but also a solid, such as perfluorosulfonic acid film (trade name, 'Nabion 117(R)') prepared by hydrolyzing a copolymer of tetrafluoroethylene and perfluorsulfonyl ethoxyvinylether suffices for the above electrolyte.

Description

DETAILED DESCRIPTION OF THE INVENTION C. Industrial Application Field The present invention relates to a photoelectrode for recycling carbon dioxide gas by reducing it to methanol, methane, carbon oxide, etc. using light energy.

[Conventional technology]

Concerns are beginning to be raised about global warming due to carbon dioxide gas generated from carbon-containing fossil fuels such as LNG, oil, and coal. For this reason, (1) reducing fossil fuel consumption by saving energy and improving thermal efficiency, (2) recycling generated carbon dioxide by reducing it to methanol, methane, carbon oxide, etc., or dumping it into the ocean, etc. Proposals have been made to try to address this.

Methods for reducing carbon dioxide gas to methanol, methane, carbon oxide, etc. include (1) copper, chromium, and zinc;

(2) Copper and mercury are reduced by hydrogen at high temperature and pressure in the presence of a solid catalyst consisting of iron, nickel, etc.

(3) Reduction method using electrical energy using a metal plate such as cadmium, gold, zinc, or tin as an electrode; (3) Reduction method using sunlight using a powdered photocatalyst in which platinum or ruthenium is supported on a semiconductor such as titanium dioxide. Methods are known. However, in method (1), since hydrogen is produced from fossil fuel, carbon dioxide gas is inevitably produced as a by-product.

Furthermore, method (2) does not provide any fundamental solution since electrical energy is obtained using conventional methods. In method (3), when titanium dioxide absorbs light, electrons in the monovalent band are excited to the conduction band, and carbon dioxide gas is reduced by the electrons. At this time, platinum is said to be the site of the reduction reaction. Additionally, after the electrons are removed, positively charged holes are generated, which become sites for oxidation reactions and oxidize water. However, in actual catalysts, the recombination of charges occurs because these two sites are close to each other, so the reduction reaction rate for carbon dioxide gas is extremely slow. To overcome this drawback, by molding a photocatalyst into a flat plate and using it as a photoelectrode, the excited electrons are sent as a current through an external circuit to another electrode, that is, the cathode, where they are used for the reduction reaction of carbon dioxide gas. A method is proposed. The photoelectrode acts as an anode, where the water oxidation reaction takes place. These reactions are (1), (2), (3
).

Tio2←h++e−−・−(t) HzO+2h”←2H”+1/202 =42) CO
2+6H÷+6 e ←CHs○H+H20...(3)
At this time, an electrolyte serving as a proton (H) transfer medium is present between the anode and the cathode, and H+ generated in the reaction (2) is transferred from the anode to the cathode. However, even with this method, the reaction rate is extremely low, making it impractical. The main reason for this is thought to be that when light is absorbed only by the anode, the energy gap (potential difference) between the two poles is small, and the movement of protons and electrons from the anode to the cathode is slow.

[Problem to be solved by the invention]

An object of the invention is to increase the rate of reduction of carbon dioxide gas by increasing the energy gap (potential difference) between electrodes.

[Means to solve the problem]

The above purpose is to use a photoelectrode using an n-type semiconductor as an anode and a p-type semiconductor as an anode.
This is achieved by using a photoelectrode using a type semiconductor as a cathode and placing an electrolyte between the two electrodes. In n-type semiconductors, electrons in the valence band that absorb light are excited and move to the conductor, creating holes.

This hole moves to the surface and participates in the oxidation reaction.

That is, the n-type semiconductor absorbs light and becomes anode polarized. On the contrary, when a p-type semiconductor absorbs light, it becomes cathodically polarized and a reduction reaction occurs on the surface.

Therefore, a photoelectrode using an n-type semiconductor is used as an anode.

When a photoreduction cell is configured in which a photoelectrode using a P-type semiconductor is used as a cathode and an electrolyte is present between the two electrodes, the potential difference between the two electrodes increases, so that reduction of carbon dioxide gas progresses more easily. At this time, the catalyst component 1, for example, nickel, is activated by adsorbing carbon dioxide gas.

It is more effective to support copper or the like on a P-type semiconductor. Furthermore, since the reduction reaction of carbon dioxide gas occurs at a lower potential than the hydrogen generation reaction, it is more effective to suppress hydrogen generation, that is, to add a small amount of lead, mercury, etc. that increase overvoltage. Furthermore, since an oxidation reaction of water occurs at the anode, it is more effective to add platinum, ruthenium, etc., which have a catalytic effect on the reaction. n used for anode
Type semiconductors are silicon carbide and titanium dioxide.

Strontium titanate, tin oxide, zinc oxide.

The material may be cadmium sulfide, cadmium selenium, gallium phosphide, gallium arsenide, silicon, or germanium, with titanium dioxide being particularly preferred. The n-type semiconductor used for the cathode is silicon carbide, gallium, or gallium arsenide, with silicon carbide being particularly preferred.

An electrolyte must exist between the anode and the cathode to transfer protons generated at the anode to the cathode. Electrolytes include liquids such as sulfuric acid and phosphoric acid, as well as
Perfluorosulfonic acid membrane (trade name: Nafion 11) made by hydrolyzing a copolymer of tetrafluoroethylene and perfluorosulfonyl ethoxy vinyl ether
It may be a solid such as 7).

A photoreduction cell can be produced, for example, by a membrane manufacturing apparatus that can laminate anode and cathode materials on the surface of a solid electrolyte membrane. Particularly suitable is an apparatus that can coat or spray and laminate anode and cathode materials and solid electrolyte fine particles while adjusting the concentration on the solid electrolyte membrane.

Furthermore, if the solid electrolyte has ion exchange ability, it may be used to support the catalyst component.

When used as a carbon dioxide recycling system, a photoreduction cell is equipped with a solar concentrating irradiation device and a device for absorbing or concentrating carbon dioxide from flue gas or flue gas. Contribute to countermeasures against global warming issues.

〔Example〕

<Example 1> A photocatalyst was prepared by using anatase type titanium dioxide as an n-type semiconductor and supporting 1 wt % of platinum thereon. This catalyst and perfluorosulfonic acid (trade name Nafion 1)
The dispersion of Example 17) was mixed to form a paste and applied to porous carbon paper to prepare an anode. Similarly, a cathode was prepared by applying a photocatalyst in which 5 wt % of platinum and copper were supported on strontium titanate, which is a P-type semiconductor. A solid electrolyte membrane of Nafion 117 was sandwiched between these electrodes and hot pressed to produce a photoreduction cell.

Both electrodes are made to sufficiently absorb a sulfuric acid aqueous solution of Q, 1 moQ/Q, the anode is brought into contact with a sulfuric acid aqueous solution of 0.1 moQ/Q, and the cathode is brought into contact with carbon dioxide gas.

When the reaction was carried out at 55°C, 1.1% of carbon dioxide gas was converted into methanol, formaldehyde, formic acid, methane, etc.

<Example 2> Carbon dioxide was reacted in the same manner as in Example 1 except that a photocatalyst in which 1 wt% of platinum, nickel, and copper were supported on silicon carbide was used as the cathode.
9% was reduced and methane production was significant.

<Example 3> On one surface of the Nafion 117 solid electrolyte.

A cathode was formed by spraying a methanol dispersion of platinum ruthenium black and silicon carbide. An anode was formed by spraying only titanium dioxide on one side. After hot-pressing this, carbon dioxide gas was reduced by the method of Example 1, and the reduction rate was about 0.5%.

Comparative Example 1> When carbon dioxide gas was reduced under the same conditions as in Example 3 except that silicon carbide was not included in the cathode, the reduction rate was about 0.1%.

<Example 4> A photoreduction cell was prepared by sandwiching a paste of phosphoric acid and zirconium phosphate powder between the electrodes prepared in Example 1, and nitrogen gas containing water vapor saturated at 50°C was brought into contact with the anode. When the cathode was brought into contact with carbon dioxide gas containing saturated water vapor at 50°C and reacted with carbon dioxide at 60°C, the reduction rate was 1.5%, mainly due to gas components such as methane and carbon monoxide. was generated.

〔Effect of the invention〕

Claims (1)

[Claims] 1. Carbon dioxide light characterized by comprising a photoelectrode using an n-type semiconductor as an anode, a photoelectrode using a p-type semiconductor as a cathode, and an electrolyte existing between the two. reduction cell. 2. In claim 1, the n-type semiconductor is any one of silicon carbide, titanium dioxide, strontium titanate, tin oxide, zinc oxide, cadmium sulfide, cadmium selenium, gallium phosphide, gallium arsenide, silicon, and germanium, A photoreduction cell in which the p-type semiconductor is silicon carbide, gallium phosphide, or gallium arsenide. 3. In claim 1 or 2, the semiconductor includes Ib group,
A photoreduction cell supported or doped with any element of Group IIb, Group IIIb, Group IVb, Group Vb, or Group VIII. 4. In claim 3, the Ib group is copper, silver, gold, the IIb group is mercury, the IIIb group is gallium, indium, the IVb group is germanium, tin, lead, the Vb group is phosphorus, arsenic, The group VIII is a photoreduction cell in which any one of nickel, ruthenium, platinum, and palladium is supported or doped. 5. The photoreduction cell according to claim 1, wherein the electrolyte is a liquid or solid proton conductor. 6. The photoreduction cell according to claim 1, wherein the n-type semiconductor or the p-type semiconductor is laminated on the surface of a solid electrolyte membrane. 7. A photoreduction cell manufacturing device, which is capable of laminating a solid electrolyte membrane while controlling the ratio of a semiconductor and a solid electrolyte. 8. A carbon dioxide gas recycling system according to any one of claims 1 to 6, comprising the photoreduction cell, a solar irradiation device for the photoreduction cell, and a device for absorbing or concentrating carbon dioxide from flue gas or exhaust gas.