PHOTOCHEMICAL REACTION DEVICE

The present invention provides: an oxidation reaction electrode that generates oxygen by oxidizing water; and a reduction reaction electrode that synthesizes a carbon compound by reducing carbon dioxide. The two electrodes are electrically connected. Also, the reduction reaction electrode (1) synthesizes a carbon compound by reducing carbon dioxide in a water-containing liquid using radiated light energy.

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Description
TECHNICAL FIELD

The present invention relates to a composite photoelectrode, a photochemical reaction device, and a light energy storage device which reduce carbon dioxide to synthesize a carbon compound using water as an electron source.

BACKGROUND ART

In the related art, Non-Patent Document 1 describes an example of the use of a photoelectrode in a two-electrode system in an oxidation-reduction reaction using water as an electron donor.

Non-Patent Document 1 discloses a technique in which a p-GaInP2 electrode is used as a reduction reaction photoelectrode, a WO3 electrode is used as an oxidation reaction photoelectrode, and light of tungsten-halogen lamp is irradiated in a 0.5 M potassium nitrate solution, to decompose water.

Non-Patent Document 2 discloses a technique in which a semiconductor catalyst powder such as TiO2 is suspended in water as a photocatalyst for reduction reaction which presents a reduction reaction of carbon dioxide, and light is irradiated from an artificial light source such as a xenon lamp and a high-pressure mercury lamp while carbon dioxide is supplied, so that formaldehyde, formic acid, methane, methanol, or the like is produced.

Patent Document 1 discloses a technique related to a method in which light is irradiated to a zirconium oxide semiconductor in water, and hydrogen and oxygen are efficiently produced from water using the light energy, and a method in which hydrogen, oxygen, and carbon monoxide can be simultaneously produced from water and carbon dioxide using light energy under the presence of a catalyst for photodecomposition of water.

Patent Document 2 discloses a technique in which a semiconductor electrode such as TiO2 in a hydroxide salt solution and a gas diffusion electrode which carries a Pd—Ru alloy catalyst in a hydrogen carbonate solution are short-circuited, and light is irradiated on a side of the semiconductor electrode, to reduce carbon dioxide on the side of the gas diffusion electrode and produce formic acid.

Patent Document 3 discloses a technique in which light is irradiated under the presence of a cuprous oxide serving as a visible-light-responsive photocatalyst and triethanolamine serving as an electron donor, to selectively produce methanol with a reaction in water or formic acid with a reaction in acetonitorile.

Non-Patent Document 3 discloses a technique in which light is irradiated onto a p-InP photoelectrode in a methanol solvent in which carbon dioxide is dissolved under a high pressure (40 atm) and a constant current of 50 mA is supplied so that carbon oxide is produced with a current efficiency of 89%.

Non-Patent Document 4 discloses a technique in which light is irradiated onto a p-InP photoelectrode modified with lead in a methanol solvent in which carbon dioxide is dissolved, to produce formic acid with a Faradaic efficiency of 29.9% and light is irradiated onto a p-InP photoelectrode modified with silver in the methanol solvent in which carbon dioxide is dissolved, to produce carbon monoxide with a Faradaic efficiency of 80.4%.

Patent Document 4 discloses a technique in which carbon dioxide is introduced under a high pressure of 0.2-7.5 MPa to an organic solvent in which there are dissolved a photocatalyst selected from among metal complexes having a charge absorption band between a metal and a ligand in a ultraviolet region to the visible light region and an electron donor selected from among organic amines, and light is irradiated under this pressure to selectively reduce carbon dioxide to carbon monoxide.

Non-Patent Document 5 discloses a technique employing a catalyst in which a Ru complex is electrochemically polymerized on a carbon or platinum electrode, in which an electrochemical bias of −0.8 V (vs Ag/AgCl) is applied, so that formic acid is produced with a Faradaic efficiency of 85%.

Non-Patent Document 6 discloses a technique related to production of hydrogen using Cu2ZnSnS4 (CZTS).

RELATED ART REFERENCES [Patent Document]

  • [Patent Document 1] Japanese Patent No. 2526396
  • [Patent Document 2] JP H6-158374 A
  • [Patent Document 3] JP H7-112945 A
  • [Patent Document 4] Japanese Patent No. 3590837

[Non-Patent Document]

  • [Non-Patent Document 1] Turner et al., Journal of the Electrochemical Society 155 (2008), F91
  • [Non-Patent Document 2] Fujishima et al., Nature 277 (1979), 637
  • [Non-Patent Document 3] Fujishima et al., The Journal of Physical Chemistry 102 (1998), 9834
  • [Non-Patent Document 4] Kaneco et al., Applied Catalysis B: Environmental 64 (2006), 139
  • [Non-Patent Document 5] Deronzier et al., The Journal of Electroanalytical Chemistry 444 (1998), 253
  • [Non-Patent Document 6] Domen et al., Applied Physics Express 3 (2010), 101202

DISCLOSURE OF INVENTION Technical Problem

In a reduction reaction of carbon dioxide using water as the electron donor, a photoelectrode material having a high energy level of a conductive band is necessary for the reduction of carbon dioxide. When a visible-light-responsive photoelectrode having a short bandgap is used, the energy level of valance band is also necessarily high, and the use of water as the electron donor becomes difficult. Because of this, a reaction cell of a two-electrode system in which two types of photoelectrodes are combined can be considered effective for efficient use of the solar light over a wide wavelength.

In Non-Patent Document 1, hydrogen and oxygen production reactions by decomposition of water are reported. However, Non-Patent Document 1 does not describe the reduction reaction of carbon dioxide at all. The reduction reaction of carbon dioxide; for example, a reaction in which formic acid is produced from carbon dioxide, has a standard electrode potential of −0.196 V which is higher in energy than a potential (0 V) where hydrogen is produced from a proton. Therefore, on a surface of a single semiconductor electrode such as a p-GalnP2 electrode, a hydrogen production reaction having low energy is highly likely to occur, the reduction reaction of carbon dioxide is difficult to induce, and selectivity of the reduction reaction is low.

Non-Patent Document 2 is an example document which discloses a reduction reaction photoelectrode which presents a reduction reaction of carbon dioxide. Non-Patent Document 2 discloses simultaneous production of formaldehyde, formic acid, methane, methanol, or the like. In addition, Patent Document 1 discloses examples in which only hydrogen is produced or hydrogen and carbon monoxide are simultaneously produced.

Production of hydrogen and simultaneous production of two or more types of carbon dioxide reduction products by radiation of light are characteristics of an inorganic semiconductor photocatalyst, but, in consideration of industrial usage, obtaining the product with a high selectivity is important. In Patent Document 2, 32% of the photocurrent produced by radiation of light on titanium oxide is used for conversion into formic acid, and the selectivity is low. In addition, only titanium oxide, which is an ultraviolet-light-responsive semiconductor, is used, and there is a problem in that the usage efficiency of solar light is low. Moreover, although Patent Document 3 uses a visible-light-responsive photocatalyst, an expensive electron donor made of an organic substance is required for promoting the oxidation reaction pairing with the reduction reaction of the carbon dioxide.

In Non-Patent Document 3, carbon dioxide is reduced using a visible-light-responsive photoelectrode, but carbon dioxide must be dissolved under a high pressure in order to improve reactivity. In Non-Patent Document 4 also, carbon dioxide is reduced using a visible-light-responsive photoelectrode, to produce formic acid and carbon monoxide with high Faradaic efficiencies, but methanol must be used as a reaction solvent. In addition, a high bias voltage of −2.5V (vsAg/AgCl) is applied, and, thus, the advantage of reducing the reaction voltage using the photoelectrode material is not exploited.

In Patent Document 4, only an example using a rhenium complex is shown. It has been reported in academic papers or the like that, when a rhenium complex is used, carbon monoxide tends to be selectively produced. It is also known that, when the rhenium complex is used, in order to realize a photocatalytic reduction reaction of carbon dioxide, only light of relatively short wavelength of 450 nm or less among the visible light is used. In addition, although Patent Document 4 also shows a complex using other metals, this document only lists possible metal elements, and the configuration with the complex using other metals is not realized. In addition, it is known in the research of dye sensitized solar cells or the like that a ruthenium complex can absorb light of a longer wavelength, depending on its structure. However, in this case, the photocatalytic chemical reaction does not occur, and, currently, only electrochemical catalytic reaction through application of electricity is realized.

A reason why the reaction product selectivity on the semiconductor photoelectrode is low can be deduced as follows. The surface of the semiconductor film and the powder are not uniform, and many defects, structural steps in atomic level, etc. exist. Therefore, as a result of the local surface energy variation depending on the sites on the surface, differences exist in adsorption performances of carbon dioxide, proton, solvent, gas, and reaction intermediates which are reactants. Therefore, the processes such as probabilities and rates at which the electron is supplied to these substances are not constant, and various reaction products are produced.

In addition, the reduction reaction of carbon dioxide requires a semiconductor photocatalyst having a high energy level of the conduction band, and, when a visible-light-responsive semiconductor catalyst having a narrow bandgap is used, the energy level of the valance band would also necessarily be high, resulting in difficulties in oxidation reaction of water.

Although the reason is not definite, a reason why there are many limitations in the usage of visible light of a longer wavelength and in photocatalytic limitations in the complex photocatalyst is that, because a lifetime of photoexcited electrons is short, a possibility that the electron moves to the reaction field on the complex is low and the light absorption is not efficiently used for the reaction. Because of this, in Non-Patent Document 5, the material cannot function as a photocatalyst.

In Patent Document 6, CZTS, which does not use rare elements and which is a less expensive electrode material compared to a p-type indium phosphide (p-InP) or the like, is used, but Patent Document 6 does not describe the reduction reaction of carbon dioxide at all. When CZTS is used in an aqueous solution, the hydrogen generation reaction occurs preferentially, and selective reduction of carbon dioxide becomes difficult.

Solution to Problem

According to one aspect of the present invention, there is provided a photochemical reaction device comprising an oxidation reaction electrode which oxidizes water and generates oxygen, and a reduction reaction electrode which reduces carbon dioxide and synthesizes a carbon compound, wherein the oxidation reaction electrode and the reduction reaction electrode are electrically connected, and the reduction reaction electrode reduces carbon dioxide and synthesizes the carbon compound in a solution containing water by means of energy of irradiated light.

According to another aspect of the present invention, preferably, in the photochemical reaction device, an energy level of a conduction band of the oxidation reaction electrode is positioned at a potential on a negative side in relation to an energy level of a valance band of the reduction reaction electrode.

According to another aspect of the present invention, preferably, in the photochemical reaction device, the reduction reaction electrode has a structure in which a semiconductor electrode and a catalyst which presents a reduction action of carbon dioxide are coupled, and the reduction action of carbon dioxide is presented by movement of excited electrons generated by radiation of light on the semiconductor electrode to the catalyst.

According to another aspect of the present invention, preferably, in the photochemical reaction device, the reduction reaction electrode has a structure in which a semiconductor electrode and a catalyst which presents a reduction action of carbon dioxide are coupled by chemical polymerization, and reduces carbon dioxide and synthesizes the carbon compound in the solution containing water by means of the energy of irradiated light.

According to another aspect of the present invention, preferably, in the photochemical reaction device, the oxidation reaction electrode and the reduction reaction electrode are placed in a two-chamber cell separated by a proton exchange membrane, the oxidation reaction electrode and the reduction reaction electrode are electrically connected, and the reduction reaction electrode reduces carbon dioxide and synthesizes the carbon compound in the solution containing water by means of the energy of irradiated light.

According to another aspect of the present invention, preferably, in the photochemical reaction device, the oxidation reaction electrode and the reduction reaction electrode are electrically connected, the oxidation reaction electrode is a semiconductor electrode, and oxidizes water and takes away electrons by means of the energy of irradiated light, and the reduction reaction electrode reduces carbon dioxide and synthesizes the carbon compound in the solution containing water by means of the energy of irradiated light.

According to another aspect of the present invention, preferably, in the photochemical reaction device, the catalyst is a metal complex or a polymer thereof. In particular, the catalyst is preferably a mixture of a first metal complex having an anchor site which is connected to the semiconductor electrode and a second metal complex which is polymerized with the first metal complex and which has a CO2 reduction catalytic function. Further, the second metal complex preferably has a pyrrole site.

According to another aspect of the present invention, preferably, in the photochemical reaction device, a chemical polymerization film of the first metal complex and the second metal complex is formed on a surface of the semiconductor electrode.

According to another aspect of the present invention, preferably, in the photochemical reaction device, the oxidation reaction electrode and the reduction reaction electrode are directly connected in a state where no bias voltage is applied, and light is irradiated on both electrodes so that water functions as an electron donor.

According to another aspect of the present invention, preferably, in the photochemical reaction device, the oxidation reaction electrode and the reduction reaction electrode are connected in a state where a bias power supply is applied, and light is irradiated on both electrodes so that water functions as an electron donor.

According to another aspect of the present invention, preferably, in the photochemical reaction device, the oxidation reaction electrode comprises titanium oxide. In particular, the oxidation reaction electrode preferably comprises anatase-type titanium oxide.

According to another aspect of the present invention, preferably, in the photochemical reaction device, the solution containing water is water or an aqueous solution containing an electrolyte.

According to another aspect of the present invention, preferably, in the photochemical reaction device, the oxidation reaction electrode and the reduction reaction electrode are separated by an ion exchange membrane (a cation exchange membrane or an anion exchange membrane).

According to another aspect of the present invention, preferably, in the photochemical reaction device, there is employed a three-electrode system structure which has a reference electrode in addition to the oxidation reaction electrode and the reduction reaction electrode.

According to another aspect of the present invention, there is provided a composite photoelectrode comprising a catalyst which presents a reduction action of carbon dioxide and a semiconductor electrode coupled with the catalyst, wherein the reduction action of carbon dioxide is presented by movement to the catalyst of excited electrons generated by radiation of light onto the semiconductor electrode.

According to another aspect of the present invention, preferably, in the composite photoelectrode, the catalyst is a metal complex or a polymer thereof. Preferably, the semiconductor electrode is a sulfide semiconductor or a phosphide semiconductor.

According to another aspect of the present invention, there is provided a light energy storage device in which the composite photoelectrode and an oxidation reaction electrode which oxidizes water and generates oxygen are connected.

Advantageous Effects of Invention

According to various aspects of the present invention, carbon dioxide can be reduced and a useful carbon compound can be synthesized by means of light energy.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram schematically showing a structure of a photochemical reaction device according to an embodiment of an electric bias of 0.

FIG. 2 is a diagram schematically showing a structure of a photochemical reaction device according to an embodiment in which an electric bias is applied.

FIG. 3 is a diagram showing a structural formula of a Ru complex and a structural formula of a polymerized Ru complex.

FIG. 4 is a diagram showing a current-voltage characteristic of a device.

FIG. 5 is a partial enlarged view of FIG. 4.

FIG. 6 is a diagram showing an example metal complex according to a preferred embodiment of the present invention.

FIG. 7 is a diagram showing a structure of a photochemical reaction device of a three-electrode system in an example of the present invention.

FIG. 8 is a diagram showing an example metal complex according to a preferred embodiment of the present invention.

FIG. 9 is a diagram showing a result of time-of-flight secondary ion mass spectrometry showing adsorption of a ligand to a semiconductor substrate.

FIG. 10 is a diagram showing a result of time-of-flight secondary ion mass spectroscopy showing adsorption of a ligand to a semiconductor substrate.

FIG. 11 is a diagram showing a structure of a photoelectrochemical measurement device in an example of the present invention.

 BEST MODE FOR CARRYING OUT THE INVENTION

A preferred embodiment of the present invention will now be described with reference to the drawings.

FIG. 1 shows a structure of a photochemical reaction device according to a preferred embodiment of the present invention. A reduction reaction electrode 10 which is a semiconductor electrode and an oxidation reaction electrode 12 which is a counter electrode of the reduction reaction electrode and which is a semiconductor electrode are electrically connected. As shown in FIG. 2, it is preferable to provide a bias power supply 14 between the reduction reaction electrode 10 and the oxidation reaction electrode 12 so that a negative electric bias of a voltage (0-1.4 V) is applied to the reduction reaction electrode 10 with respect to the oxidation reaction electrode 12.

A catalyst of a base material 16 contacts the reduction reaction electrode 10 in a state where electrons e− can be exchanged. In the example structure shown in the figures, a metal complex (ruthenium complex) is used as the base material 16.

In such a system, when light is irradiated onto the reduction reaction electrode 10, photoexcited electrons e− are generated, and are used for the reduction catalytic reaction of the base material 16. In the illustrated example, carbon dioxide (CO2) is reduced to formic acid (HCOOH).

Similarly, when light is irradiated onto the oxidation reaction electrode 12, reactions occur in which water (H2O) is oxidized to oxygen ((1/2)O2) or hydrogen peroxide by photocatalytic reactions, and the generated electrons e− move to the reduction reaction electrode 10 and are combined with holes that are generated as a pair for the photoexcited electrons in the reduction reaction electrode 10.

As described, in the present embodiment, the photoexcited electrons e− generated in the reduction reaction electrode 10 by the radiation of light move to the reaction sites of the base material 16, which presents the reduction action of carbon dioxide so that the reduction reaction of carbon dioxide takes place. In particular, because the photoexcited electrons are used, carbon dioxide can be reduced and useful organic compounds can be synthesized with high efficiency and with a high reaction product selectivity, without application of a bias voltage. In addition, oxidation of water can be realized using holes generated by radiation of light onto the oxidation reaction electrode 12. The electrons generated here efficiently combine with the holes generated at the reduction reaction electrode 10. Because of this, even in the absence of the bias power supply 14, the reduction reaction of carbon dioxide can proceed with the water used as the electron donor. The reaction can be more efficiently promoted by placing the bias power supply 14 and applying a bias voltage between the electrodes.

By employing a two-electrode system in which the reduction reaction electrode 10 and the oxidation reaction electrode 12 are provided separately, it is possible to reduce the carbon dioxide using water as the electron donor, and the energy necessary for the oxidation reaction of water and the reduction reaction of carbon dioxide can be divided into two. Therefore, use of a visible-light-responsive semiconductor material which can absorb only light of a narrow light wavelength region is enabled.

[Reduction Reaction Electrode]

Here, as a semiconductor used for the reduction reaction electrode 10, there is used a material in which a value obtained by subtracting a value of a level having the lowest energy among molecular orbitals, of the base material to be described later, not occupied by the electrons from a value of the lowermost energy level of the conduction band is less than or equal to 0.2 electron volts. For example, the semiconductor may be tantalum oxide, a nitride semiconductor such as tantalum nitride, nitrogen-doped tantalum oxide, etc., tantalum oxynitride, a sulfide semiconductor such as nickel-containing zinc sulfide, copper-containing zinc sulfide, zinc nitride, etc., a selenide semiconductor such as cadmium selenide, a chalcogenide semiconductor including tellurium compounds and other composite compounds, a phosphide semiconductor (phosphorous compound) such as indium phosphide, gallium phosphide, indium gallium phosphide, etc., iron oxide, silicon carbide, an oxide of copper, an arsenide semiconductor such as gallium arsenide, rhodium-doped strontium titanate, or the like.

Alternatively, the semiconductor used for the reduction reaction electrode 10 may be a sulfide semiconductor containing copper, zinc, tin, and sulfur such as Cu2ZnSnS4 (CZTS) or Cu2ZnSn(S,Se)4 (CZTSSe). When a compound semiconductor suitable for the semiconductor to be used for the reduction reaction electrode 10 is represented by AxByCzD4, copper or silver is preferably used as A, zinc or cadmium is preferably used as B, tin, germanium, gallium, or aluminum is preferably used as C, and sulfur, oxygen, selenium, or the like is preferably used as D. The composition conditions are 1.4≦x/y≦2 and 1.4≦x/z≦2, and preferably, 70% or more of A is copper, 90% or more of Bis zinc, 90% of C is tin and germanium, and 70% or more of D is sulfur and selenium.

Here, zinc-doped indium phosphide and sulfide semiconductors containing copper, zinc, tin, and sulfur (for example, CZTS, CZTSSe, or the like) which are used in the Examples of the present invention to be described later are particularly preferable for the reduction reaction electrode 10. For the zinc-doped indium phosphide, there is used, for example, a structure synthesized by the Vapor controlled Czochralski (VCZ) method, the LEC method, or the HB method.

Tantalum nitride and tantalum oxynitride can be generated by thermally processing tantalum oxide in an atmosphere containing ammonia gas. The ammonia is preferably diluted with non-oxidizing gas (such as argon and nitrogen), and, for example, it is preferable to place and heat tantalum oxide in a gas flow in which ammonia and argon are mixed at equal flow rates. The heating temperature is preferably greater than or equal to 500° C. and less than or equal to 900° C., more preferably, greater than or equal to 550° C. and less than or equal to 850° C. The processing period is preferably greater than or equal to 1 hour and less than or equal to 15 hours. For the tantalum oxide before the ammonia processing, there may be used commercially available tantalum oxide having crystallinity or amorphous tantalum oxide obtained by applying a hydrolysis process or the like to a compound solution containing tantalum such as tantalum chloride.

The nickel-containing zinc sulfide can be obtained by dissolving a nickel-containing hydrate and a zinc-containing hydrate, introducing aqueous solution to which a sodium sulfide hydrate is dissolved, stirring, applying centrifugal separation and re-dispersion, removing the supernatant liquor, and drying. The nickel-containing hydrate may be, for example, nickel (II) nitrate hexahydrate. The zinc-containing hydrate may be, for example, zinc (II) nitrate hexahydrate. Here, as a nickel source, in addition to the above-described source, nickel chloride, nickel acetate, nickel perchlorate, nickel sulfate, or the like may be used. Similarly, as a zinc source, zinc chloride, zinc acetate, zinc perchlorate, zinc sulfate, or the like may be used.

Similarly, copper-containing zinc sulfide can be obtained by dissolving a copper-containing hydrate and a zinc nitrate-containing hydrate, introducing a sodium sulfide hydrate, stirring, applying centrifugal separation and re-dispersion, removing the supernatant liquor, and drying. The copper-containing hydrate may be, for example, copper (II) nitrate 2.5-hydrate. The zinc-containing hydrate may be, for example, zinc (II) nitrate hexahydrate. Here, as a copper source, in addition to the above-described source, copper chloride, copper acetate, copper perchlorate, copper sulfate, or the like may be used. Similarly, as a zinc source, zinc chloride, zinc acetate, zinc perchlorate, zinc sulfate, or the like may be used.

[Base Material]

As the base material 16, there is used a material in which a value obtained by subtracting a value of a level of the lowest energy among the molecular orbitals, of the base material 16, not occupied by the electrons from a value of the lowermost energy level of the conduction band of the semiconductor of the reduction reaction electrode 10 is less than or equal to 0.2 electron volts. The base material 16 may be a metal complex or a polymer thereof, and, for example, there are used a rhenium complex having a carboxybipyridine ligand ((Re(dcbpy)(CO)3P(OEt)3)), ((Re(dcbpy)(CO)3Cl)), Re(dcbpy)(CO)3MeCN, or Re(dcbqi)(CO)3MeCN, and a ruthenium (Ru) complex [Ru(dcbpy)(bpy)(CO)2]2+ (bpy=2,2′-bipyridine, dcbpy=4,4′-dicarboxy-2,2′-bipyridine).

In particular, a ruthenium (Ru) complex or a polymer thereof used in the Example of the present invention is preferable. In particular, [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)2]n as shown in FIG. 3(b) obtained by polymerizing [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)2Cl2] shown in FIG. 3(a) is preferably used for formation of the reduction reaction electrode 10. In addition, the use of [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)(CH3CN)Cl2] is also preferable.

In the base material 16, no particular limitation is imposed on the method of synthesizing the complex polymer, so long as the polymer contains a metal complex which shows a reduction activity of carbon dioxide. For example, the methods may be (1) chemical polymerization in which the polymerization is realized through a chemical reaction, (2) electrolytic polymerization in which the polymerization is realized through an electrochemical reaction, or (3) photochemical polymerization and photoelectrochemical polymerization which use light for the above-described reactions. No particular limitation is imposed on the method of modifying the complex polymer on the reduction reaction electrode, and, for example, (1) spin coating, (2) dip coating, (3) spraying, (4) dropping, or the like may be employed.

The reduction reaction electrode 10 and the base material 16 are coexistent such that electrons can be exchanged. For example, the base material 16 may float in an electrolytic solution or may be coupled. When the base material 16 is coupled, for example, the base material 16 is mixed in the solvent. The solvent is dropped on the reduction reaction electrode 10, to adhere the base material 16 on the surface of the electrode. The photoelectrode can be obtained by a method, for example, in which the base material 16 is coupled on the surface of the reduction reaction electrode 10 by drying the adhered material. For the solvent, an organic solvent may be employed, and, for example, acetonitorile, methanol, ethanol, acetone, or the like may be employed.

The base material 16 may be any compound which shows a carbon dioxide reduction activity using electrons, and no particular limitation is imposed thereon. In the case of a metal complex, a metal complex of at least one metal selected from the VII family metals and the VIII family metals on the periodic table may be used. For example, there may be used a complex of a metal such as ruthenium, rhenium, manganese, iron, copper, osmium, cobalt, rhodium, iridium, nickel, palladium, platinum, or the like and a ligand. Examples of the metal complex include [Re(N—N)(CO)3L] (N—N=a complex compound containing nitrogen) (L=unidentate ligand such as PR3, SCN, Cl, MeCN, and DMF), [Ru(N—N) (N′—N′)(CO)2], [Ru(tpy)(N—N)(CO)], [Cu(N—N)2], [Co(N—N)3], [Fe(N—N)3], [Ni(N—N)3], etc.

No particular limitation is imposed on the ligand, and examples of typical primary ligands include a heterocyclic compound containing nitrogen, a heterocyclic compound containing oxygen, and a heterocyclic compound containing sulfur. Preferable examples of secondary ligands include CO, halogens, phosphines, or the like. In addition, solvent molecules such as acetonitrile, DMF, water, or the like may also be used. These secondary ligands may be converted and generated during the reaction. One or a combination of two or more of these ligands may be used.

As the heterocyclic compound containing nitrogen, for example, pyridine, bipyridine, phenanthroline, terpyridine, pyrrole, indole, carbazole, imidazole, pyrazole, quinoline, isoquinoline, acridine, pyridazine, pyrimidine, pyrazine, phthalazine, quinazoline, quinoxaline, or the like can be exemplified. As the heterocyclic compound containing oxygen, for example, furan, benzofuran, oxazole, pyran, pyrone, coumarin, benzopyrone, or the like can be exemplified. As the heterocyclic compound containing sulfur, for example, thiophene, thionaphthene, thiazole, or the like can be exemplified. Such a ligand may be used as a single entity or in a combination of two or more ligands.

The reduction reaction electrode 10 and the base material 16 are preferably chemically bonded by a linking group. No particular limitation is imposed on the linking group, and it may be any group which enables chemical bonding. Examples include a carboxyl group, a phosphate group, a sulfonic acid group, a silanol group, a thiol group, and derivatives thereof. Here, in the state where the linking group is linked to the reduction reaction electrode 10, the linking group may have a structure in which a proton is detached or a structure in which a metal and an oxygen atom are coordinate bonded. These linking groups may be used as a single entity or in a combination of two or more linking groups. Alternatively, a plurality of linking groups may be used.

The linking method of the reduction reaction electrode 10 and the base material 16 may be any method which chemically bonds the semiconductor of the reduction reaction electrode 10 and the base material, and no particular limitation is imposed thereon. For example, the method may be (1) adhering a metal complex in which a linking group is introduced into the ligand which is adsorbed on the semiconductor, (2) directly forming the complex after the ligand to which the linking group is introduced is adsorbed on the semiconductor, or (3) bonding the metal complex to the semiconductor to which the linking group is introduced.

A coverage of the base material 16 is preferably greater than or equal to 1% and less than or equal to 100% with respect to a surface area of the reduction reaction electrode 10. When the coverage of the base material 16 is less than 1%, the amount of the base material is too low, and sufficient carbon dioxide reduction activity cannot be expressed.

A particularly preferable combination is, in a case of the semiconductor being a metal oxide, a phosphate group serving as the linking group, and, in a case of the semiconductor being a compound semiconductor such as GaP and InP, ester phosphate serving as the linking group.

As will be described below in the Examples of the present invention, it is preferable to electrochemically deposit a Ru complex on the reduction reaction electrode 10. The reduction reaction electrode 10 and the counter electrode can be immersed, and the Ru complex can be bonded on the reduction reaction electrode 10 by electrodeposition.

Alternatively, the use of a metal complex having an anchor ligand is also preferable. Examples of the metal complex having an anchor ligand include [Ru({4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)2Cl2)] and [Ru({4,4′-dicarboxylic acid-2,2′-bipyridine}(CO)2Cl2)], or the like. In addition to these, a metal complex including a pyrrole ligand may be polymerized and linked to the reduction reaction electrode 10, to improve the catalytic performance of the reduction reaction of carbon dioxide. In addition, selectivity of the reduction reaction can also be improved.

[Oxidation Reaction Electrode]

For the oxidation reaction electrode 12, there is used a material which exhibits a photocatalytic function and causes the oxidation reaction of water with radiation of light. For example, titanium oxide (TiO2), nitrogen-doped titanium oxide (N—TiO2), rutile-type titanium oxide, tungsten oxide (WO3), strontium titanate, tantalum oxynitride (TaON), a bismuth vanadate compound, or the like is used. These materials are formed by direct synthesis such as sputtering, hydrolysis, and polymerization, or by a method of fixing powders with a binder, or the like. These materials are used as a single entity or in a configuration of being formed on a conductive substrate. Commercially available titanium oxide particles (TiO2(P25)) and TiO2, obtained by reducing titanium oxide by hydrogen, which are used in the Examples of the present invention, are particularly preferable.

In such a photochemical reaction device, for example, the reduction reaction electrode 10 and the oxidation reaction electrode 12 are immersed in water in which carbon dioxide is dissolved, and light is irradiated on both electrodes 10 and 12. With this process, as described above, the reduction catalytic reaction at the base material 16 causes production of formic acid from carbon dioxide in the water, and water is oxidized to oxygen gas at the oxidation reaction electrode 12 by means of a photocatalytic reaction. The product is not limited to formic acid, and, by selecting the base material 16 and causing the catalytic reaction at a suitable environment, it is also possible to synthesize useful organic substances such as alcohol from carbon dioxide.

When the bias power supply 14 is provided and the bias voltage is applied as shown in FIG. 2, the operation can employ a lower bias voltage (0-1.4 V) than in the case of a two-electrode system using non-photoelectrode material. This is because photoexcitation by light energy is used in the reduction reaction electrode 10 and the oxidation reaction electrode 12.

As described, according to the present embodiment, by means of light energy, carbon dioxide can be converted into useful carbon compounds and the light energy can be stored in the carbon compound. In particular, because carbon dioxide can be reduced using water as the electron donor, there is obtained an advantage in that the cost of the overall system can be reduced. In addition, with the use of a complex catalyst which presents a reduction action of carbon dioxide, the carbon compound can be synthesized with a high reaction product selectivity.

By using the light energy, carbon dioxide can be reduced with a lower voltage than can the non-photoelectrode material. By using the two-electrode system of the reduction reaction electrode 10 and the oxidation reaction electrode 12, the reaction fields of reduction and oxidation can be separated, and products can be easily separated. In addition, by using photoelectrodes for both electrodes, light energy necessary for the oxidation reaction of water and the reduction reaction of carbon dioxide can be absorbed and used over a wide wavelength range.

Example 1

With a device shown in FIG. 2 (including a bias voltage of 0 V), experiments were performed. First, for photoelectrochemical measurement, an electrochemical analyzer (BAS) was used and measurement was conducted with the two-electrode system. A Pyrex (registered trademark) glass cell was used for the container. A xenon lamp (MAX-302 manufactured by Asahi Spectra) was used as the light source.

For evaluation of the products involved with the photoelectrochemical measurement, ion chromatograph (DIONEX with ICS-2000 auto-sampler AS) was used. For the column of the ion chromatograph, “IonPac AS15” was used, a KOH eluate was used for the eluate, and an electric conductivity detector was used for the detector.

Example 1

In an acetonitrile solution containing about 1 mg of a ruthenium complex [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)2Cl2] (FIG. 3(a)), zinc-doped indium phosphide (p-InP) synthesized through a VCZ method (having a carrier concentration of 4×1018-6×1018/cm3) was used for a working electrode, platinum was used for a counter electrode, an I−/I3− electrode was used for a reference electrode, and argon gas was passed for 10 minutes. A potential of −1.2 V with respect to the reference electrode was applied, and electrodeposition was performed for one hour under irradiation with a fluorescent lamp, to deposit a ruthenium complex polymer [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)2]n (FIG. 3(b)) on the surface of the working electrode.

A carbon dioxide reduction reaction was performed using the above-described zinc-doped indium phosphide on which the ruthenium complex polymer was deposited (p-InP—Zn (Ru-polymer)) as the working electrode (reduction reaction electrode 10). For the counter electrode (oxygen reaction electrode 12), there was used a rutile monocrystalline titanium oxide electrode (TiO2-x), to which a reduction process was applied with hydrogen. As the electrolyte, 8 ml of distilled water was used. After argon gas was bubbled in the solution for about 20 minutes to remove the dissolved gas, carbon dioxide gas was bubbled in the solution for about 10 minutes, and then, a current-voltage measurement was conducted under a carbon dioxide gas atmosphere. Finally, under a carbon dioxide gas atmosphere, a current-time measurement was conducted while a bias voltage of −0.8 V was applied with respect to the reduction reaction electrode 10 and the oxidation reaction electrode 12.

Example 2

In the configuration of Example 1, the current-time measurement was conducted while a bias voltage of −0.4 V was applied with respect to the reduction reaction electrode 10 and the oxidation reaction electrode 12.

Example 3

In the configuration of Example 1, a tungsten oxide electrode (WO3) was used for the oxidation reaction electrode 12, and a cut-off filter of λ>422 nm was used for the light source so that only visible light was irradiated, and the current-time measurement was conducted while a bias voltage of −0.8 V was applied with respect to the reduction reaction electrode 10 and the oxidation reaction electrode 12.

Comparative Example 1

In the configuration of Example 1, carbon dioxide gas was not bubbled and the current-time measurement was conducted under an argon gas atmosphere.

Comparative Example 2

In the configuration of Example 2, carbon dioxide gas was not bubbled and the current-time measurement was conducted under the argon gas atmosphere.

Comparative Example 3

In the configuration of Example 1, there was used a wafer (8 mm×20 mm) of zinc-doped indium phosphide (p-InP—Zn) on which the Ru complex polymer was not electrodeposited, and the current-time measurement was conducted under the carbon dioxide gas atmosphere.

[Result]

TABLE 1 collectively shows conditions and results of Examples 1-3 and Comparative Examples 1-3.

TABLE 1 WORKING COUNTER LIGHT ELECTRODE ELECTRODE ELECTROLYTE GAS SOURCE EXAMPLE 1 p-InP TiO2 H2O CO2 ALL LIGHTS Ru polymer EXAMPLE 2 p-InP TiO2 H2O CO2 ALL LIGHTS Ru polymer EXAMPLE 3 p-InP WO3 H2O CO2 VISIBLE LIGHT Ru polymer (λ > 422 nm) COMPARATIVE p-InP TiO2 H2O Ar ALL LIGHTS EXAMPLE 1 Ru polymer COMPARATIVE p-InP TiO2 H2O Ar ALL LIGHTS EXAMPLE 2 Ru polymer COMPARATIVE p-InP TiO2 H2O CO2 ALL LIGHTS EXAMPLE 3 POTENTIAL FORMIC ACID FARADAIC DIFFERENCE TIME CONCENTRATION CHARGE EFFICIENCY (V vs CE) (hour) (μM) (C) (%) EXAMPLE 1 −0.8 20 31 1.26 3.8 EXAMPLE 2 −0.4 20 4 0.85 0.7 EXAMPLE 3 −0.8 20 19 1.18 2.5 COMPARATIVE −0.8 20 5 0.24 3.3 EXAMPLE 1 COMPARATIVE −0.4 20 1 0.16 0.9 EXAMPLE 2 COMPARATIVE −0.8 20 0 0.73 0.1 EXAMPLE 3

While 31 μM of formic acid was detected in 20 hours under the carbon dioxide gas atmosphere as a result of the current-time measurement with application of the bias voltage of −0.8 V in Example 1, only 5 μM of the formic acid was detected in 20 hours under the argon gas atmosphere of Comparative Example 1.

While 4 μM of formic aced was detected in 20 hours under the carbon dioxide atmosphere in Example 2 in which the time-current measurement was conducted with application of the bias voltage of −0.4 V, only 1 μM of formic acid was detected in 20 hours under the argon gas atmosphere in Comparative Example 2.

In addition, only 0.3 μM of formic acid was detected in 20 hours also in the case of Comparative Example 3 where only the indium phosphide electrode was used under the carbon dioxide gas atmosphere. Based on this, it was suggested that carbon dioxide is reduced to formic acid on the indium phosphide electrode on which the Ru complex polymer is adhered in the distilled water which does not use the electrolytes.

In Example 3 in which the counter electrode was changed from the TiO2, photoelectrode to the WO3 photoelectrode and the current-time measurement was conducted under irradiation with visible light of λ>422 nm and the bias voltage of −0.8 V, 19 μM of formic acid was detected in 20 hours under the carbon dioxide gas atmosphere. Thus, it was suggested that carbon dioxide was reduced to formic acid even when only the visible light was irradiated.

FIGS. 4 and 5 show results of measurements of current for cases where bias voltage was changed in the configurations of Example 1 and Comparative Example 1, and in configurations where no light was irradiated in the configurations of Example 1 and Comparative Example 1. When no light was irradiated, no current flowed. On the other hand, when light was irradiated, a photocurrent flows, but, in a condition with the presence of carbon dioxide as in Example 1, the photocurrent is larger than that in the case of the argon gas atmosphere. In particular, the photocurrent is increased even in the case of the bias voltage of 0 V, due to the presence of carbon dioxide. Based on this, it was found that the reduction reaction of carbon dioxide took place even with the bias voltage of 0 V. Thus, it was suggested that the p-InP—Zn (Ru-polymer)/TiO2-x based devices can be operated with a zero bias.

In addition, as a result of the current-voltage measurement under the carbon dioxide gas atmosphere in Example 1, it was confirmed that a value of the photocurrent was increased as the absolute value of the negative bias voltage applied to the reduction reaction electrode 10 with respect to the oxidation reaction electrode 12 was increased.

Example 4

0.25 mg of a ruthenium complex [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine](CO)(CH3CN)Cl2] was dissolved in 0.25 ml of an acetonitrile solution, 5 μl of pyrrole solution (having a molar ratio of pyrrole with respect to the ruthenium complex of 1.1%) was mixed, and then, 5 μl of 0.2 M iron (III) chloride solution (having a molar ratio of iron chloride with respect to the ruthenium complex of a factor of 3.1) was added. The pyrrole solution was prepared by diluting 50 μl of pyrrole by 1 ml of acetonitrile. The iron (III) chloride solution was prepared by dissolving 1.08 g of iron (III) chloride hexahydrate into 20 ml of ethanol. 50 μl of the above-described mixture solution was applied on a p-InP—Zn photoelectrode, and dried in an oven at 45° C. Such application and drying were repeated for 5 times, to create a Ru-polymer (CP)/p-InP—Zn photoelectrode. The Ru-polymer (CP)/p-InP—Zn photoelectrode thus created was used as the working electrode (reduction reaction electrode 10).

In addition, 30 μl of acetyl acetone, 400 μl of water, and one drop of surfactant (Triton X-100) were well-kneaded with 0.2 g of particles of commercially available titanium oxide particle (TiO2(P25)), to prepare a paste, and the paste was dropped on a glass substrate on which a transparent conductive layer was provided, applied and rolled with a glass rod, dried, and then sintered at a temperature of 550° C., to create a TiO2(P25) photoelectrode. The TiO2(P25) photoelectrode thus created was used as the counter electrode (oxidation reaction electrode 12).

The above-described Ru-polymer (CP)/p-InP—Zn photoelectrode (reduction reaction electrode 10) and the TiO2(P25) photoelectrode (oxidation reaction electrode 12) were placed in chambers of a two-chamber cell separated by a proton exchange membrane (Nafion 117). Pure water was used for the electrolyte. Such a photochemical reaction device was placed under a carbon dioxide gas atmosphere, and the current-time measurement was conducted while radiating light in a state where no bias voltage was applied to the reduction reaction electrode 10 and the oxidation reaction electrode 12.

[Result]

In Example 4, 0.26 C of charges were observed and 115 μM of formic acid was detected when light corresponding to 1.4 SUN was irradiated for 20 hours. A ratio of the produced formic acid with respect to the amount of observed charges (Faradaic efficiency) was calculated and found to be 35.8%. Even in comparison to Example 1, characteristics of both the amount of production of formic acid and the Faradaic efficiency were significantly improved even though no bias voltage was applied.

The following three reasons can be considered as reasons why the characteristics were improved: (1) because the reaction fields were separated using the two-chamber cell, formic acid produced on the side of the reduction reaction could be efficiently accumulated and recovered without the formic acid being decomposed on the side of the oxidation reaction; (2) because the TiO2, photoelectrode was changed to the TiO2(P25) photoelectrode, the current value at the zero-bias condition was improved by a factor of approximately 4; and (3) because the method of modifying the ruthenium complex polymer was changed, the amount of production of formic acid was improved by a factor of approximately 4.

Example 5

A MeCN solution containing [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)(MeCN)Cl2] (refer to FIG. 6(a)).FeCl3.pyrrol was applied to a wafer (8 mm×20 mm) of zinc-doped indium phosphide (p-InP—Zn manufactured by Sumitomo Electric Industries) which is a p-type semiconductor, dried, and then washed with water. This electrode was used as the working electrode. In the present example, a three-electrode system as shown in FIG. 7 was employed, a glassy carbon electrode (GC) was used for the counter electrode, and a silver/silver chloride electrode (Ag/AgCl) was used for the reference electrode. For the electrolyte, 5 ml of distilled water was used. After argon gas was bubbled in the solution for about 20 minutes to remove the dissolved gas, carbon dioxide gas was bubbled in the solution for about 10 minutes, and then, light was irradiated under a carbon dioxide gas atmosphere, and reduction and oxidation reactions were measured. For the potential, −0.4 V was applied with respect to the reference electrode.

Example 6

An MeCN solution including FeCl3.pyrrol in which [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)(MeCN) Cl2] (refer to FIG. 6(a)) and [Ru{4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)2Cl2] (refer to FIG. 6(b)) were mixed in a 1:1 ratio was applied on a wafer (8 mm×20 mm) of zinc-doped indium phosphide (p-InP—Zn manufactured by Sumitomo Electric Industries), which is a p-type semiconductor, dried, and then washed with water. This electrode was used as the working electrode. In the present example, a three-electrode system as shown in FIG. 7 was employed, the glassy carbon electrode was used for the counter electrode, and the silver/silver chloride electrode (Ag/AgCl) was used for the reference electrode. For the electrolyte, 5 ml of distilled water was used. After argon gas was bubbled in the solution for about 20 minutes to remove the dissolved gas, carbon dioxide gas was bubbled in the solution for about 10 minutes, and then, light was irradiated under a carbon dioxide gas atmosphere and reduction and oxidation reactions were measured. For the potential, −0.4 V was applied with respect to the reference electrode.

Example 7

An MeCN solution containing [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO) (MeCN)Cl2] (refer to FIG. 6(a)).FeCl3.pyrrol was applied on a wafer (8 mm×20 mm) of zinc-doped gallium phosphide (p-GaP—Zn manufactured by Sumitomo Electric Industries), which is a p-type semiconductor, dried, and then washed with water. This electrode was used as the working electrode. In the present example, a three-electrode system as shown in FIG. 7 was employed, the glassy carbon electrode was used for the counter electrode, and the silver/silver chloride electrode (Ag/AgCl) was used for the reference electrode. For the electrolyte, 5 ml of distilled water was used. After argon gas was bubbled in the solution for about 20 minutes to remove the dissolved gas, carbon dioxide gas was bubbled for about 10 minutes, and then, light was irradiated under the carbon dioxide gas atmosphere and the reduction and oxidation reactions were measured. For the potential, −0.4 V was applied with respect to the reference electrode.

Example 8

An MeCN solution containing FeCl3.pyrrol in which [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO) (MeCN)Cl2] (refer to FIG. 6(a)) and [Ru{4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)2Cl2] (refer to FIG. 6(b)) were mixed in a 1:1 ratio was applied on a wafer (8 mm×20 mm) of zinc-doped gallium phosphide (p-GaP—Zn manufactured by Sumitomo Electric Industries), which is a p-type semiconductor, dried, and then washed with water. This electrode was used as the working electrode. In the present embodiment, the three-electrode system as shown in FIG. 7 was employed, the glassy carbon electrode was used for the counter electrode, and the silver/silver chloride electrode (Ag/AgCl) was used for the reference electrode. For the electrolyte, 5 ml of distilled water was used. After argon gas was bubbled in the solution for about 20 minutes to remove the dissolved gas, carbon dioxide gas was bubbled in the solution for about 10 minutes, and then, light was irradiated under the carbon dioxide gas atmosphere and the reduction and oxidation reactions were measured. For the potential, −0.4 V was applied with respect to the reference electrode.

Example 9

An MeCN solution containing [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)(MeCN)Cl2] (refer to FIG. 6(a)).FeCl3.pyrrol was applied on a wafer (8 mm×20 mm) of silicon (p-Si), which is a p-type semiconductor, dried, and then washed with water. This electrode was used as the working electrode. In the present example, the three-electrode system as shown in FIG. 7 was employed, the glassy carbon electrode was used for the counter electrode, and the silver/silver chloride electrode (Ag/AgCl) was used for the reference electrode. For the electrolyte, 5 ml of distilled water was used. After argon gas was bubbled in the solution for about 20 minutes to remove the dissolved gas, carbon dioxide gas was bubbled in the solution for about 10 minutes, and then, light was irradiated under the carbon dioxide gas atmosphere, and the reduction and oxidation reactions were measured. For the potential, −0.4 V was applied with respect to the reference electrode.

Example 10

An MeCN solution containing FeCl3.pyrrol in which [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)(MeCN)Cl2] (refer to FIG. 6(a)) and [Ru{4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)2Cl2] (refer to FIG. 6(b)) were mixed in a 1:1 ratio was applied on a wafer (8 mm×20 mm) of silicon (p-Si), which is a p-type semiconductor, dried, and then, washed with water. This electrode was used as the working electrode. In the present example, the three-electrode system as shown in FIG. 7 was employed, the glassy carbon electrode was used for the counter electrode, and the silver/silver chloride electrode (Ag/AgCl) was used for the reference electrode. For the electrolyte, 5 ml of distilled water was used. After argon gas was bubbled in the solution for about 20 minutes to remove the dissolved gas, carbon dioxide gas was bubbled in the solution for about 10 minutes, and then, light was irradiated under the carbon dioxide gas atmosphere and reduction and oxidation reactions were measured. For the potential, −0.4 V was applied with respect to the reference electrode.

Example 11

An MeCN solution containing [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)(MeCN)Cl2] (refer to FIG. 6(a)).FeCl3.pyrrol was applied on a sputtered film (20 mm×20 mm) of nitrogen-doped tantalum oxide (N—Ta2O5), which is a p-type semiconductor, dried, and then washed with water. This electrode was used as the working electrode. In the present example, the three-electrode system as shown in FIG. 7 was employed, the glassy carbon electrode was used for the counter electrode, and the silver/silver chloride electrode (Ag/AgCl) was used for the reference electrode. For the electrolyte, 5 ml of distilled water was used. After argon gas was bubbled for about 20 minutes to remove the dissolved gas, carbon dioxide gas was bubbled in the solution for about 10 minutes, and then, light was irradiated under the carbon dioxide gas atmosphere and the reduction and oxidation reactions were measured. For the potential, −0.4 V was applied with respect to the reference electrode.

Example 12

An MeCN solution containing FeCl3.pyrrol in which [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)(MeCN)Cl2] (refer to FIG. 6(a)) and [Ru{4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)2Cl2] (refer to FIG. 6(b)) were mixed in a 1:1 ratio was applied on a sputtered film (20 mm×20 mm) of nitrogen-doped tantalum oxide (N—Ta2O5), which is a p-type semiconductor, dried, and then washed with water. This electrode was used as the working electrode. In the present example, the three-electrode system as shown in FIG. 7 was employed, the glassy carbon electrode was used for the counter electrode, and the silver/silver chloride electrode (Ag/AgCl) was used for the reference electrode. For the electrolyte, 5 ml of distilled water was used. After argon gas was bubbled in the solution for about 20 minutes to remove the dissolved gas, carbon dioxide gas was bubbled in the solution for about 10 minutes, and then, light was irradiated under the carbon dioxide gas atmosphere and the reduction and oxidation reactions were measured. For the potential, −0.4 V was applied with respect to the reference electrode.

Example 13

An MeCN solution containing [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)(MeCN)Cl2] (refer to FIG. 6(a)).FeCl3.pyrrol was applied on a wafer (8 mm×20 mm) of zinc-doped indium phosphide (p-InP—Zn manufactured by Sumitomo Electric Industries), which is a p-type semiconductor, dried, and then washed with water. This electrode was used as the working electrode. In the present example, the three-electrode system as shown in FIG. 7 was employed, the glassy carbon electrode was used for the counter electrode, and the silver/silver chloride electrode (Ag/AgCl) was used for the reference electrode. For the electrolyte, 5 ml of 10 mM aqueous solution of NaHCO3 was used. After argon gas was bubbled in the solution for about 20 minutes to remove the dissolved gas, carbon dioxide gas was bubbled in the solution for about 10 minutes, and then, light was irradiated under the carbon dioxide gas atmosphere, and the reduction and oxidation reactions were measured. For the potential, −0.4 V was applied with respect to the reference electrode.

Example 14

An MeCN solution containing [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)(MeCN)Cl2] (refer to FIG. 6(a)).FeCl3.pyrrol was applied on a wafer (8 mm×20 mm) of zinc-doped indium phosphide (p-InP—Zn manufactured by Sumitomo Electric Industries), which is a p-type semiconductor, dried, and then washed with water. This electrode was used as the working electrode. In the present example, the three-electrode system as shown in FIG. 7 was employed, the glassy carbon electrode was used for the counter electrode, and the silver/silver chloride electrode (Ag/AgCl) was used for the reference electrode. For the electrolyte, 5 ml of 10 mM aqueous solution of Na3PO4 was used. After argon gas was bubbled in the solution for about 20 minutes to remove the dissolved gas, carbon dioxide gas was bubbled in the solution for about 10 minutes, and then, light was irradiated under the carbon dioxide gas atmosphere and the reduction and oxidation reactions were measured. For the potential, −0.4 V was applied with respect to the reference electrode.

Example 15

An MeCN solution containing [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)(MeCN)Cl2] (refer to FIG. 6(a)).FeCl3.pyrrol was applied on a wafer (8 mm×20 mm) of zinc-doped indium phosphide (p-InP—Zn manufactured by Sumitomo Electric Industries), which is a p-type semiconductor, dried, and then washed with water. This electrode was used as the working electrode. In the present example, the three-electrode system as shown in FIG. 7 was employed, the glassy carbon electrode was used for the counter electrode, and the silver/silver chloride electrode (Ag/AgCl) was used for the reference electrode. For the electrolyte, 5 ml of a 10 mM aqueous solution of Na2SO4 was used. After argon gas was bubbled in the solution for about 20 minutes to remove the dissolved gas, carbon dioxide gas was bubbled in the solution for about 10 minutes, and then, light was irradiated under the carbon dioxide gas atmosphere and the reduction and oxidation reactions were measured. For the potential, −0.4 V was applied with respect to the reference electrode.

Example 16

In the configuration of Example 6, [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)(MeCN)Cl2] (refer to FIG. 6(a)) and [Ru{4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)2Cl2] (refer to FIG. 6(b)) were mixed in a ratio of 1:4 and applied, and a catalytic activity was measured.

Example 17

In the configuration of Example 6, [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)(MeCN) Cl2] (refer to FIG. 6(a)) and [Ru{4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)2Cl2] (refer to FIG. 6(b)) were mixed in a ratio of 4:1 and applied, and a catalytic activity was measured.

Example 18

In the configuration of Example 6, [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)(MeCN)Cl2] (refer to FIG. 6(a)) and [Ru{4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)2Cl2] (refer to FIG. 6(b)) were mixed in a ratio of 9:1 and applied, and a catalytic activity was measured.

Comparative Example 4

In the configuration of Example 5, the complex catalyst was not applied and the catalytic activity was measured with only the semiconductor.

Comparative Example 5

In the configuration of Example 7, the complex catalyst was not applied and the catalytic activity was measured with only the semiconductor.

Comparative Example 6

In the configuration of Example 9, the complex catalyst was not applied and the catalytic activity was measured with only the semiconductor.

Comparative Example 7

In the configuration of Example 11, the complex catalyst was not applied, and the catalytic activity was measured with only the semiconductor.

Comparative Example 8

In the configuration of Example 5, the working electrode was changed to the glassy carbon electrode, and the catalytic activity was measured.

Comparative Example 9

In the configuration of Example 6, [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)(MeCN)Cl2] (refer to FIG. 6(a)) was not used, and only [Ru{4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)2Cl2] (refer to FIG. 6(b)) was applied, and the catalytic activity was measured.

Comparative Example 10

In the configuration of Example 13, the catalytic activity was measured not under the carbon dioxide gas atmosphere, but under the argon gas atmosphere.

[Result]

TABLES 2 and 3 show the results of the catalytic activity measurements of the above-described Examples 5-18 and Comparative Examples 4-10.

TABLE 2 APPLIED IRRADIATION HCOO− SATURATED POTENTIAL PERIOD CONCEN- EFF CATHODE ANODE GAS SOLVENT (V; VS Ag/AgCl) (HOURS) TRATION (mM) (%) EXAMPLE 5 InP/Ru COMPLEX GC CO2 DISTILLED −0.4 1 0.155 81.1 WATER EXAMPLE 6 InP/Ru COMPLEX GC CO2 DISTILLED −0.4 1 0.197 78.2 WATER COMPARATIVE InP GC CO2 DISTILLED −0.6 3 0 0 EXAMPLE 4 WATER EXAMPLE 7 GaP/Ru COMPLEX GC CO2 DISTILLED −0.4 1 0.001 44.7 WATER EXAMPLE 8 GaP/Ru COMPLEX GC CO2 DISTILLED −0.4 1 0.109 56.7 WATER COMPARATIVE GaP GC CO2 DISTILLED −0.4 1 0 0 EXAMPLE 5 WATER EXAMPLE 9 p-Si/Ru COMPLEX GC CO2 DISTILLED −0.4 1 0.006 52.6 WATER EXAMPLE 10 p-Si/Ru COMPLEX GC CO2 DISTILLED −0.4 1 0.018 75.5 WATER COMPARATIVE p-Si GC CO2 DISTILLED −0.4 1 0 0 EXAMPLE 6 WATER EXAMPLE 11 N—Ta2O5/ GC CO2 DISTILLED −0.4 1 0.022 48.2 Ru COMPLEX WATER EXAMPLE 12 N—Ta2O5/ GC CO2 DISTILLED −0.4 1 0.029 63.3 Ru COMPLEX WATER COMPARATIVE N—Ta2O5/ GC CO2 DISTILLED −0.4 1 0 0 EXAMPLE 7 Ru COMPLEX WATER EXAMPLE 13 InP/Ru COMPLEX GC CO2 NaHCO3 −0.4 1 0.311 70.6 EXAMPLE 14 InP/Ru COMPLEX GC CO2 Na3PO4 −0.4 1 0.201 58.6 EXAMPLE 15 InP/Ru COMPLEX GC CO2 Na2SO4 −0.4 1 0.243 58.1 COMPARATIVE InP GC CO2 NaHCO3 −0.4 1 0.001 5.7 EXAMPLE 10

TABLE 3 APPLIED HCOO− MIXTURE SATURATED POTENTIAL CONCENTRATION EFF CATHODE RATIO ANODE GAS SOLVENT (V; VS Ag/AgCl) (mM) (%) EXAMPLE 6 InP/Ru COMPLEX 1:1 GC CO2 DISTILLED −0.4 0.197 78.2 WATER EXAMPLE 16 InP/Ru COMPLEX 1:4 GC CO2 DISTILLED −0.4 0.128 68.4 WATER EXAMPLE 17 InP/Ru COMPLEX 4:1 GC CO2 DISTILLED −0.4 0.163 80.8 WATER EXAMPLE 18 InP/Ru COMPLEX 9:1 GC CO2 DISTILLED −0.4 0.198 81.2 WATER COMPARATIVE InP/Ru COMPLEX 0:1 GC CO2 DISTILLED −0.4 0.040 23.9 EXAMPLE 9 WATER

In the case of Comparative Example 4 in which the measurement was conducted with only the semiconductor, as opposed to Example 5 in which 0.155 mM of formic acid was detected in one hour under the carbon dioxide gas atmosphere, only 0.01 mM of formic acid was detected in three hours. In other words, reduction of carbon dioxide to the formic acid in the aqueous solution was suggested merely by applying a complex catalyst on the indium phosphide electrode. In Example 6, a complex catalyst having an anchor ligand and a complex catalyst having a pyrrole ligand were combined, and, as a result, 0.197 mM of formic acid was detected in one hour, and the amount of production was increased as compared to Example 5. This can be considered to have been caused by the improvement of electron movement velocity between the semiconductor and the complex catalyst and consequent improvement in the reactivity, due to the use of the anchor ligand.

In Example 7, only 0.001 mM of formic acid was detected in one hour under the carbon dioxide gas atmosphere, and, in Comparative Example 5 in which the measurement was conducted only with the semiconductor, only 0.001 mM of formic acid was detected in one hour. In Example 8, a complex catalyst having an anchor ligand and a complex catalyst having a pyrrole ligand were combined, and, as a result, 0.109 mM of formic acid was detected in one hour, and the amount of production was increased as compared to Example 7 and Comparative Example 5. This can also be considered to have been caused by the smoothening of the electron movement between the semiconductor and the complex catalyst and consequent improvement of reactivity, due to the use of the anchor ligand. In addition, it was suggested that the reduction reaction from carbon dioxide to the formic acid in the aqueous solution was caused by merely applying the complex catalyst on the gallium phosphide electrode.

In Example 9, 0.006 mM of formic acid was detected in one hour under the carbon dioxide gas atmosphere, whereas in Comparative Example 6 in which the measurement was conducted only with the semiconductor, only 0.002 mM of formic acid was detected in one hour. In other words, reduction of carbon dioxide to the formic acid in the aqueous solution was suggested merely by applying the complex catalyst on the p-type silicon electrode. In Example 10, a complex catalyst having an anchor ligand and a complex catalyst having a pyrrole ligand were combined, and, as a result, 0.018 mM of formic acid was detected in one hour, and the amount of production was increased compared to Example 9. This can also be considered to have been caused by the improvement of the electron movement velocity between the semiconductor and the complex catalyst and consequent improvement of reactivity, due to the use of the anchor ligand.

In Example 11, 0.022 mM of formic acid was detected in one hour under the carbon dioxide gas atmosphere, whereas in Comparative Example 7 in which the measurement was conducted only with the semiconductor, only 0.01 mM of formic acid was detected in one hour. In other words, reduction of carbon dioxide to the formic acid in the aqueous solution was suggested by merely applying the complex catalyst on the N—Ta2O5 electrode. In Example 12, a complex catalyst having an anchor ligand and a complex catalyst having a pyrrole ligand were combined, and, as a result, 0.029 mM of formic acid was detected in one hour, and the amount of production was increased as compared to Example 11. This can also be considered to have been caused by the smoothening of the electron movement between the semiconductor and the complex catalyst and consequent improvement in reactivity, due to the use of the anchor ligand.

In Comparative Example 8, no formic acid was detected even when a voltage of −0.6 V (vs silver/silver chloride electrode) was applied for 20 hours on the glassy carbon electrode under the carbon dioxide gas atmosphere. Based on this, it can be deduced that in Examples 5-12, various semiconductor electrodes use the light energy to enable production of formic acid at a low voltage.

From Comparative Example 9 and Examples 5, 6, and 16-18, it was found that the catalyst activity was low when single structure of [Ru({4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)2Cl2)] was used. In addition, it was found that the catalyst activity was improved by mixing [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)(MeCN)Cl2] and [Ru{4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)2Cl2], and the mixture ratio of 1:1 to 9:1 results in a high effect on the activity improvement.

Upon comparison of Examples 13-15 and Comparative Example 10, it was found that the reduction reaction of carbon dioxide is not blocked even when salt was added. In particular, it was found that the catalyst activity was improved when NaHCO3 was added. In Comparative Example 10, it was found that formic acid was produced from NaHCO3, and the catalyst activity is not improved.

A ruthenium complex [Ru(dpebpy)(bpy)(CO)2]2+ (FIGS. 7 and 8) having a 4,4′-diphosphate ethyl-2,2′-bipyridine ligand (dpebpy) was adsorbed on zinc-doped gallium phosphide (p-GaP—Zn manufactured by Sumitomo Electric Industries) in the following manner, and presence or absence of the adsorption of the ligand to the semiconductor substrate was analyzed. Specifically, 1 ml of dichloromethane/methanol mixture solution of 2 mM ruthenium complex [Ru(dpebpy) (bpy) (CO)2]2+ and a wafer (5 mm×5 mm) of p-GaP—Zn were inserted into a container made of Teflon (registered trademark), left for one night at the room temperature and taken out, washed with a solvent (dichloromethane/methanol) twice, and vacuum dried at a temperature of 40° C. FIG. 9 shows a result of analysis of this sample by a time-of-flight secondary ion mass spectrometry (TOF-SIMS).

In addition, a ruthenium complex [Ru(dpebpy) (bpy) (CO)2]2+ (FIGS. 7 and 8) having a 4,4′-diphosphate ethyl-2,2′-bipyridine ligand (dpebpy) was adsorbed on zinc-doped indium phosphide (p-InP—Zn manufactured by Sumitomo Electric Industries) in the following manner, and the presence or absence of adsorption of the ligand on the semiconductor substrate was analyzed. Specifically, 1 ml of dichloromethane/methane mixture solution of 2 mM ruthenium complex [Ru(dpebpy) (bpy) (CO)2]2+ and a wafer (5 mm×5 mm) of p-InP—Zn were inserted in a container made of Teflon (registered trademark), left for one night at room temperature and taken out, washed with a solvent (dichloromethane/methanol) twice, and vacuum dried at a temperature of 40° C. FIG. 10 shows a result of analysis of this sample by the time-of-flight secondary ion mass spectrometry (TOF-SIMS).

From FIGS. 9 and 10, it was found that a spectrum corresponding to the ruthenium ion was observed and that the ruthenium complex was adsorbed on gallium phosphide and indium phosphide.

Next, in Example 16, the reduction reaction electrode 10 for reducing carbon dioxide and the oxidation reaction electrode 12 for oxidizing water and generating oxygen were combined, and a value of photocurrent when the bias voltage applied between the two electrodes was set to 0 V was checked.

Example 19

For the reduction reaction electrode 10, a wafer (8 mm×20 mm) of zinc-doped indium phosphide (p-InP—Zn manufactured by Sumitomo Electric Industries), which is a p-type semiconductor was used, and, for the oxidation reaction electrode 12, TiO2(P25) electrode formed on a conductive glass (FTO manufactured by Asahi Glass) by a squeegee method using commercially available titanium oxide (TiO2) particles (P25 manufactured by Degussa) was used. The TiO2(P25) electrode includes approximately 80% of anatase-type titanium oxide.

For the photoelectrochemical measurement, an electrochemical analyzer (BAS) was used, and the measurement was conducted in the two-electrode system which uses the working electrode and the counter electrode. The reduction reaction electrode 10 was used for the working electrode, the oxidation reaction electrode 12 was used for the counter electrode, and the two electrodes were placed in parallel and in an overlapping manner. A rectangular quartz glass cell was used for the cell, and 2.5 ml of 0.2 M K2SO4 was used for the electrolyte. A xenon lamp of 300 W (MAX-302 manufactured by Asahi Spectra) was used for the light source, applied voltage was set to 0 V, and light of all wavelengths was irradiated from the side of the oxidation reaction photoelectrode.

Comparative Example 11

A wafer (8 mm×20 mm) of zinc-doped indium phosphide (p-InP—Zn manufactured by Sumitomo Electric Industries), which is a p-type semiconductor, was used for the reduction reaction electrode 10, and a TiO2-x electrode in which monocrystal of rutile-type titanium oxide was reduced by hydrogen was used for the oxidation reaction electrode 12. The other conditions were set to be the same as those of Example 19.

[Result]

TABLE 4 shows a result of photoelectrochemical measurement on Example 19 and Comparative Example 11. In Example 19, with the use of the TiO2(P25) electrode primarily including anatase-type titanium oxide, a photocurrent of 21 μA was observed under the condition of the applied voltage of 0 V. On the other hand, in Example 11 in which TiO2-x electrode which is a rutile-type titanium oxide was used, the photocurrent at the applied voltage of 0 V was 4.4 μA. Therefore, the photocurrent was increased by factor of four or greater with the use of the anatase-type titanium oxide.

TABLE 4 APPLIED POTENTIAL PHOTOCURRENT CATHODE ANODE SOLVENT (V; VS Ag/AgCl) (μA) EXAMPLE 19 InP TiO2 (P25) K2SO4 0 21 COMPARATIVE InP TiO2−x K2SO4 0 4.4 EXAMPLE 11

An energy level of the conduction band of the anatase-type titanium oxide is positioned on a negative side with respect to an energy level of the conduction band of the rutile-type titanium oxide, and, with the use of the anatase-type titanium oxide, a difference with respect to an energy level of the valance band of the indium phosphide is larger as compared to the structure where the rutile-type titanium oxide is used. In other words, it can be deduced that, with the use of the anatase-type titanium oxide, a potential gradient created between the two electrodes is increased, movement of electrons between the electrodes is promoted, and the photocurrent was increased even with the applied voltage of 0 V.

Next, in Examples 20 and 21, the reduction reaction of carbon oxide when the reduction reaction electrode 10 and the oxidation reaction electrode 12 were combined and water was used as the electron donor was checked. For the photoelectrochemical measurement, as shown in FIG. 11, an electrochemical analyzer (BAS) was used, and the measurement was conducted in the two-electrode system which uses the working electrode and the counter electrode. For the cell, a two-chamber cell separated by a proton exchange membrane (Nafion 117 manufactured by Du Pont) was used. For the light source, a xenon lamp of 300 W (MAX-302 manufactured by Asahi Spectra) or a solar simulator (HAL-320 manufactured by Asahi Spectra) was used. For evaluation of the product involved with the photoelectrochemical measurement, ion chromatograph (DIONEX with ICS-2000 auto-sampler AS) was used. For the column, IonPac AS15 was used. For the eluate, a KOH eluate was used, and, for the detector, an electric conductivity detector was used.

Example 20

For the reduction reaction electrode 10, an MeCN solution containing [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)(MeCN)Cl2].FeCl3.pyrrol was applied on a wafer (8 mm×20 mm) of zinc-doped indium phosphide (p-InP—Zn manufactured by Sumitomo Electric Industries), which is a p-type semiconductor, dried, and then washed with water and used. For the oxidation reaction electrode 12, a TiO2 (P25) electrode created on a conductive glass (FTO manufactured by Asahi Glass) by a squeegee method using commercially available titanium oxide (TiO2) particles (P25 manufactured by Degussa) was used. For the electrolyte, 5 ml of distilled water was used. After argon gas was bubbled in the solution for about 20 minutes to remove the dissolved gas, carbon dioxide gas was bubbled in the solution for about 10 minutes, and then, the measurement was conducted under carbon dioxide gas atmosphere. A bias voltage between the two electrodes was set to 0 V, and xenon light corresponding to 1.4 SUN was irradiated.

Example 21

For the reduction reaction electrode 10, an MeCN solution containing FeCl3.pyrrol in which [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)(MeCN)Cl2] and [Ru({4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)2Cl2)] were mixed in a 1:1 ratio were applied on a wafer (8 mm×20 mm) of zinc-doped indium phosphide (p-InP—Zn manufactured by Sumitomo Electric Industries), which is a p-type semiconductor, dried, and then washed with water and used. For the oxidation reaction electrode 12, there was used an electrode in which platinum was carried on a TiO2(P25) electrode created on a conductive glass (FTO manufactured by Asahi Glass) by a squeegee method using commercially available titanium oxide (TiO2) particles (P25 manufactured by Degussa). For the electrolyte, 5 ml of an aqueous solution of 10 mM NaHCO3 was used. After argon gas was bubbled in the solution for about 20 minutes to remove the dissolved gas, carbon dioxide gas was bubbled in the solution for about 10 minutes, and the measurement was conducted under the carbon dioxide gas atmosphere. A bias voltage between the two electrodes was set to 0 V, and light of a solar simulator corresponding to 1 SUN was irradiated.

[Result]

TABLE 5 shows results of photoelectrochemical measurements with respect to Examples 20 and 21. In Example 20, 0.115 mM of formic acid was detected in 20 hours under the carbon dioxide gas atmosphere, and, when a ratio of charges (EFF) consumed for production of formic acid compared to the total amount of observed charges was calculated, the ratio was found to be 35.8%. In Example 21, 0.190 mM of formic acid was detected in 3 hours under the carbon dioxide gas atmosphere, and, when a ratio of charges (EFF) consumed for production of formic acid compared to the total amount of observed charges was calculated, the ratio was found to be 67.2%.

TABLE 5 APPLIED IRRADIATION HCOO− SATURATED POTENTIAL PERIOD CONCENTRATION EFF CATHODE ANODE GAS SOLVENT (V; VS Ag/AgCl) (HOURS) (mM) (%) EXAMPLE 20 InP/Ru TiO2 (P25) CO2 DISTILLED 0 20 0.115 35.8 COMPLEX WATER EXAMPLE 21 InP/Ru Pt/TiO2 (P25) CO2 NaHCO3 0 3 0.190 67.2 COMPLEX

The TiO2 (P25) electrode contains the anatase-type titanium oxide. An energy level of the conduction band of the anatase-type titanium oxide is high, and an energy difference with the valance band of indium phosphide (p-InP—Zn) is large. Therefore, it can be deduced that electrons efficiently move between the two electrodes even under the zero-bias condition, and a large photocurrent is generated. In addition, it can be considered that, with the use of the two-chamber cell, the reaction fields of the oxidation reaction and the reduction reaction are separated, resulting in inhibition of re-oxidation of the formic acid, produced by the reduction reaction, into carbon dioxide by an oxidation reaction.

As described above, by combining the reduction reaction electrode 10 for reducing carbon dioxide and the photoelectrode for oxidizing water and generating oxygen, a reduction reaction of carbon dioxide using water as the electron donor can be realized.

Next, in Examples 22-24, a sulfide semiconductor and a metal complex were combined for the reduction reaction electrode 10, and the reduction reaction of carbon dioxide using water as the electron donor was checked.

For photoelectrochemical measurement for Examples 22-24 and Comparative Examples 12-14, the electrochemical analyzer (BAS) was used, and measurement was conducted in the three-electrode system which uses the working electrode, the counter electrode, and the reference electrode in a structure as shown in FIG. 7. For the cell, a cylindrical Pyrex (registered trademark) glass cell was used. For the light source, a xenon lamp of 300 W (MAX-302 manufactured by Asahi Spectra) was used, and, using a cutoff filter of a wavelength of 422 nm, only visible light was irradiated. For the evaluation of product involved with the photoelectrochemical measurement, an ion chromatograph (DIONEX with ICS-2000 auto-sampler AS) was used. For the column, IonPac AS15 was used, for the eluate, a KOH eluate was used, and for the detector, an electric conductivity detector was used.

Example 22

For the working electrode, there was used an electrode in which gallium-doped Cu2ZnSnS4(Ga-CZTS), which is a p-type semiconductor, was modified with a ruthenium complex polymer. An MeCN solution containing FeCl3.pyrrol in which ruthenium complex polymers [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)(MeCN)Cl2] (refer to FIG. 6(a)) and [Ru({4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)2Cl2)] (refer to FIG. 6(b)) were mixed in a 1:1 ratio was applied on a CZTS substrate, dried, and then washed with water and used. For the counter electrode, the glassy carbon electrode was used, and, for the reference electrode, the silver/silver chloride electrode (Ag/AgCl) was used. For the electrolyte, 5 ml of purified water was used. After argon gas was bubbled in the solution for about 20 minutes to remove the dissolved gas, carbon dioxide gas was bubbled in the solution for about 10 minutes, and then, light of 70 SUN was irradiated under the carbon dioxide gas atmosphere, and the reduction and oxidation reactions were measured. For the potential, −0.4 V was applied with respect to the reference electrode.

Example 23

For the working electrode, there was used an electrode in which Cu2ZnSn (S, Se)4 (CZTSSe) which is a p-type semiconductor was modified by a ruthenium complex polymer. An MeCN solution containing FeCl3.pyrrol in which ruthenium complex polymers [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO) (MeCN) Cl2] (refer to FIG. 6(a)) and [Ru{4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)2Cl2] (refer to FIG. 6(b)) were mixed in a 1:1 ratio was applied on a CZTSSe substrate, dried, and then washed with water and used. For the counter electrode, the glassy carbon electrode was used and, for the reference electrode, the silver/silver chloride electrode (Ag/AgCl) was used. For the electrolyte, 5 ml of purified water was used. After argon gas was bubbled in the solution for about 20 minutes to remove the dissolved gas, carbon dioxide gas was bubbled in the solution for about 10 minutes, and then, light of 70 SUN was irradiated under the carbon dioxide gas atmosphere and the reduction and oxidation reactions were measured. For the potential, −0.4 V was applied with respect to the reference electrode.

Example 24

For the working electrode, there was used an electrode in which Cu2ZnSnS4 (CZTS), which is a p-type semiconductor, was modified with a ruthenium complex polymer. An MeCN solution containing FeCl3.pyrrol in which ruthenium complex polymers [Ru {4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)(MeCN)Cl2] (refer to FIG. 6(a)) and [Ru{4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)2Cl2] (refer to FIG. 6(b)) were mixed in a 1:1 ratio was applied on a CZTS substrate, dried, and then washed with water and used. For the counter electrode, the glassy carbon electrode was used and, for the reference electrode, the silver/silver chloride electrode (Ag/AgCl) was used. For the electrolyte, 5 ml of purified water was used. After argon gas was bubbled in the solution for about 20 minutes to remove the dissolved gas, carbon dioxide gas was bubbled in the solution for about 10 minutes, and then, light of 70 SUN was irradiated under the carbon dioxide gas atmosphere and the reduction and oxidation reactions were measured. For the potential, −0.4 V was applied with respect to the reference electrode.

Comparative Example 12

For the working electrode, gallium-doped Cu2ZnSnS4 (Ga-CZTS), which is a p-type semiconductor, was used. For the counter electrode, the glassy carbon electrode was used and, for the reference electrode, the silver/silver chloride electrode (Ag/AgCl) was used. For the electrolyte, 5 ml of purified water was used. After argon gas was bubbled in the solution for about 20 minutes to remove the dissolved gas, carbon dioxide gas was bubbled in the solution for about 10 minutes, and then, light of 70 SUN was irradiated under the carbon dioxide gas atmosphere and the reduction and oxidation reactions were measured. For the potential, −0.4 V was applied with respect to the reference electrode.

Comparative Example 13

For the working electrode, there was used an electrode in which gallium-doped Cu2ZnSnS4 (Ga-CZTS), which is a p-type semiconductor, was modified with a ruthenium complex polymer. An MeCN solution containing FeCl3.pyrrol in which ruthenium complex polymers [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)(MeCN)Cl2] (refer to FIG. 6(a)) and [Ru{4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)2Cl2] (refer to FIG. 6(b)) were mixed in a 1:1 ratio was applied on a CZTS substrate, dried, and then washed with water and used. For the counter electrode, the glassy carbon electrode was used and, for the reference electrode, the silver/silver chloride electrode (Ag/AgCl) was used. For the electrolyte, 5 ml of purified water was used. After argon gas was bubbled in the solution for about 20 minutes to remove the dissolved gas, light of 70 SUN was irradiated under an argon gas atmosphere, and the reduction and oxidation reactions were measured. For the potential, −0.4 V was applied with respect to the reference electrode.

Comparative Example 14

For the working electrode, there was used an electrode in which gallium-doped Cu2ZnSnS4 (Ga-CZTS), which is a p-type semiconductor, was modified with a ruthenium complex polymer. An MeCN solution containing FeCl3.pyrrol in which ruthenium complex polymers [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)(MeCN) Cl2] (refer to FIG. 6(a)) and [Ru{4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)2Cl2] (refer to FIG. 6(b)) were mixed in a 1:1 ratio was applied on a CZTS substrate, dried, and then washed with water and used. For the counter electrode, the glassy carbon electrode was used and, for the reference electrode, the silver/silver chloride electrode (Ag/AgCl) was used. For the electrolyte, 5 ml of purified water was used. After argon gas was bubbled in the solution for about 20 minutes to remove the dissolved gas, carbon dioxide gas was bubbled in the solution for about 10 minutes, and then, the reduction and oxidation reactions were measured under the carbon dioxide gas atmosphere and with radiation of no light. For the potential, −0.4 V was applied with respect to the reference electrode.

[Result]

TABLE 6 shows results of the photoelectrochemical measurements for Examples 22-24 and Comparative Examples 12-14.

TABLE 6 APPLIED IRRADI- HCOO− SATUR- POTENTIAL ATION CONCEN- ATED (V; VS IRRADIATED PERIOD TRATION EFF CATHODE ANODE GAS SOLVENT Ag/AgCl) LIGHT (HOURS) (mM) (%) EXAMPLE 22 Ga-CZTS/ GC CO2 DISTILLED −0.4 70SUN (λ > 3 0.246 74 Ru COMPLEX WATER 422 nm) EXAMPLE 23 CZTSSe/ GC CO2 DISTILLED −0.4 70SUN (λ > 3 0.382 71 Ru COMPLEX WATER 422 nm) EXAMPLE 24 CZTS/ GC CO2 DISTILLED −0.6 70SUN (λ > 3 0.285 82 Ru COMPLEX WATER 422 nm) COMPARATIVE Ga-CZTS GC CO2 DISTILLED −0.4 70SUN (λ > 3 0.006 — EXAMPLE 12 WATER 422 nm) COMPARATIVE Ga-CZTS/ GC Ar DISTILLED −0.4 70SUN (λ > 3 0.008 — EXAMPLE 13 Ru COMPLEX WATER 422 nm) COMPARATIVE Ga-CZTS/ GC CO2 DISTILLED −0.4 DARK CONDITION 3 0 — EXAMPLE 14 Ru COMPLEX WATER

In Example 22, 0.246 mM of formic acid was detected when light was irradiated for three hours under the carbon dioxide gas atmosphere, and a ratio of charges consumed for production of formic acid with respect to a total amount of measured charges was 74.1%. On the other hand, in Comparative Example 12 in which light was irradiated only on the semiconductor under the carbon dioxide gas atmosphere, only 0.006 mM of formic acid was detected in three hours. In Comparative Example 13 in which light was irradiated under the argon gas atmosphere, only 0.008 mM of formic acid was detected in three hours. In Comparative Example 14 in which light was not irradiated under the carbon dioxide gas atmosphere, no formic acid was detected. From the above-described results, it was suggested that the Ga-CZTS electrode modified with the ruthenium complex polymer has selectively reduced with light carbon dioxide into formic acid in an aqueous solution.

In Example 23 in which light was irradiated under the carbon dioxide gas atmosphere for three hours, 0.382 mM of formic acid was detected, and a ratio of charges consumed for production of formic acid with respect to a total amount of measured charges was 71.2%. With the use of the CZTSSe electrode also, similar to the CZTS electrode, carbon dioxide can be selectively reduced with light into formic acid.

In Example 24 in which light was irradiated under the carbon dioxide gas atmosphere for three hours, 0.285 mM of formic acid was detected, and a ratio of charges consumed for production of the formic acid with respect to a total amount of measured charges was 81.8%. Based on the measurement results of the current-voltage characteristic, the uppermost energy level of the valance band of CZTS can be estimated to be at a potential of about 0.2 V with respect to the reference electrode (Ag/AgCl). In consideration of the bandgap of 1.5 eV, the lowermost energy level of the conduction and is at a potential more negative than the potential of −0.6 V necessary for reducing carbon dioxide on the ruthenium complex polymer. Because of this, it can be deduced that the electrons excited to the conduction band can move to the complex, and the reduction reaction of carbon dioxide has progressed on the complex. In CZTSSe also, it can be deduced that the reaction has occurred with a similar mechanism.

The above-described reduction reaction electrode (composite photoelectrode) for reducing carbon dioxide and the oxidation reaction electrode (photoelectrode) for oxidizing water and generating oxygen were combined and a reduction reaction of carbon dioxide using water as the electron donor was measured. For the photoelectrochemical measurement, the electrochemical analyzer (BAS) was used, and the measurement was conducted in the two-electrode system which uses the working electrode and the counter electrode. For the cell, a two-chamber cell separated by a proton exchange membrane (Nafion 117 manufactured by Du Pont) was employed. For the light source, a solar simulator (HAL-320 manufactured by Asahi Spectra) was used. For evaluation of products involved with the photoelectrochemical measurement, ion chromatograph (DIONEX with ICS-2000 auto-sampler AS) was used. For the column, IonPac As15 was used, for the eluent, KOH eluent was used, and for the detector, an electric conductivity detector was used.

Example 25

For the working electrode, there was used an electrode in which Cu2ZnSn(S,Se)4 (CZTSSe), which is a p-type semiconductor, was modified with a ruthenium complex polymer. An MeCN solution containing FeCl3.pyrrol in which ruthenium complex polymers [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)(MeCN)Cl2] (refer to FIG. 6(a)) and [Ru{4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)2Cl2] (refer to FIG. 6(b)) were mixed in a 1:1 ratio was applied on a CZTSSe substrate, dried, and then washed with water and used. For the counter electrode, there was used an electrode in which titanium oxide (TiO2) particles (P25 manufactured by Degussa) were applied on a conductive glass (FTO manufactured by Asahi Glass) through a squeegee method and platinum was carried on the titanium oxide (TiO2) electrode. For the electrolyte, 4 ml of an aqueous solution of 10 mM NaHCO3 was used in each cell. After argon gas was bubbled for about 20 minutes in the solution to remove the dissolved gas, carbon dioxide gas was bubbled in the solution for about 10 minutes, and then, the reduction and oxidation reactions were measured under the carbon dioxide gas atmosphere. No bias voltage was applied (0 V), and light of solar simulator corresponding to 1 SUN was irradiated.

[Result]

TABLE 7 shows a result of the photoelectrochemical measurements for Example 25 and Comparative Examples 12-14.

TABLE 7 APPLIED IRRADI- IRRADIATION HCOO− SATURATED POTENTIAL ATED PERIOD CONCEN- EFF CATHODE ANODE GAS SOLVENT (V) LIGHT (HOURS) TRATION (mM) (%) EXAMPLE 25 CZTSSe/Ru Pt/TiO2(P25) CO2 NaHCO3 0 1SUN 3 0.1 52 COMPLEX

In Example 25 in which light was irradiated under the carbon dioxide gas atmosphere for three hours, 0.1 mM of formic acid was detected, and a ratio of charges consumed for production of formic acid with respect to a total amount of measured charges was 51.7%. It can be considered that, because titanium oxide (TiO2) electrode (P25) contains anatase-type titanium oxide, the energy level of the conduction band of the anatase-type titanium oxide is high, and an energy difference with the valance band of the CZTSSe electrode is large, the electrons efficiently move between the two electrodes and a photocurrent was generated even under the condition that no bias voltage was applied (0 V).

EXPLANATION OF REFERENCE NUMERALS

  • 10 REDUCTION REACTION ELECTRODE; 12 OXIDATION REACTION ELECTRODE; 14 BIAS POWER SUPPLY; 16 BASE MATERIAL

Claims

1. A photochemical reaction device comprising:

an oxidation reaction electrode which oxidizes water and generates oxygen; and
a reduction reaction electrode which reduces carbon dioxide and synthesizes a carbon compound, wherein
the oxidation reaction electrode and the reduction reaction electrode are electrically connected, and
the reduction reaction electrode reduces carbon dioxide and synthesizes the carbon compound in a solution containing water, by means of energy of irradiated light.

2. The photochemical reaction device according to claim 1, wherein

an energy level of a conduction band minimum of the oxidation reaction electrode is positioned at a potential on a negative side with respect to an energy level of a valance band minimum of the reduction reaction electrode.

3. The photochemical reaction device according to claim 1, wherein

the reduction reaction electrode has a structure in which a semiconductor electrode and a catalyst which presents a reduction action of carbon dioxide are coupled, and
the reduction reaction of carbon dioxide is presented by movement of excited electrons generated by radiation of light on the semiconductor electrode to the catalyst.

4. The photochemical reaction device according to claim 1, wherein

the reduction reaction electrode has a structure in which a semiconductor electrode and a catalyst which presents a reduction action of carbon dioxide are coupled by chemical polymerization, and
the reduction reaction electrode reduces carbon dioxide and synthesizes the carbon compound in the solution containing water by means of the energy of irradiated light.

5. The photochemical reaction device according to claim 1, wherein

the oxidation reaction electrode and the reduction reaction electrode are placed in two chambers separated by a proton exchange membrane, and the oxidation reaction electrode and the reduction reaction electrode are electrically connected, and
the reduction reaction electrode reduces carbon dioxide and synthesizes the carbon compound in the solution containing water by means of the energy of irradiated light.

6. The photochemical reaction device according to claim 1, wherein

the oxidation reaction electrode and the reduction reaction electrode are electrically connected,
the oxidation reaction electrode is a semiconductor electrode, and oxidizes water and extracts electrons by means of the energy of irradiated light, and
the reduction reaction electrode reduces carbon dioxide and synthesizes the carbon compound in the solution containing water by means of the energy of irradiated light.

7. The photochemical reaction device according to claim 3, wherein

the catalyst is a metal complex or a polymer thereof.

8. The photochemical reaction device according to claim 7, wherein

the catalyst is a mixture of a first metal complex having an anchor site which is connected to the semiconductor electrode and a second metal complex which is polymerized with the first metal complex and which has a catalytic function.

9. The photochemical reaction device according to claim 8, wherein

the second metal complex has a pyrrole site.

10. The photochemical reaction device according to claim 8, wherein

a chemical polymerization film of the first metal complex and the second metal complex is formed on a surface of the semiconductor electrode.

11. The photochemical reaction device according to claim 1, wherein

the oxidation reaction electrode and the reduction reaction electrode are directly connected in a state where no external bias voltage is applied, and light is irradiated on both electrodes so that water functions as an electron donor for CO2 reduction.

12. The photochemical reaction device according to claim 1, wherein

the oxidation reaction electrode and the reduction reaction electrode are connected in a state where a bias power supply is applied, and light is irradiated on both electrodes so that water functions as an electron donor.

13. The photochemical reaction device according to claim 1, wherein

the oxidation reaction electrode comprises titanium oxide.

14. The photochemical reaction device according to claim 13, wherein

the oxidation reaction electrode comprises anatase-type titanium oxide.

15. The photochemical reaction device according to claim 1, wherein

the solution containing water is water or an aqueous solution containing an electrolyte.

16. The photochemical reaction device according to claim 1, wherein

the oxidation reaction electrode and the reduction reaction electrode are separated by an ion exchange membrane.

17. The photochemical reaction device according to claim 1, wherein

a three-electrode system structure is employed which has a reference electrode in addition to the oxidation reaction electrode and the reduction reaction electrode.

18. A composite photoelectrode comprising:

a catalyst which presents a reduction reaction of carbon dioxide, and
a semiconductor electrode coupled with the catalyst, wherein
the reduction reaction of carbon dioxide is presented by transfer to the catalyst of excited electrons generated by radiation of light on the semiconductor electrode.

19. The composite photoelectrode according to claim 18, wherein

the catalyst is a metal complex or a polymer thereof.

20. The composite photoelectrode according to claim 18, wherein

the semiconductor electrode is a sulfide semiconductor or a phosphide semiconductor.

21. A light energy storage device in which the composite photoelectrode according to claim 18 and an oxidation reaction electrode which oxidizes water and generates oxygen are connected.

Patent History
Publication number: 20130277209
Type: Application
Filed: Dec 27, 2011
Publication Date: Oct 24, 2013
Applicant: KABUSHIKI KAISHA TOYOTA CHUO KENKYUSHO (Nagakute-shi)
Inventors: Shunsuke Sato (Aichi-gun), Takeo Arai (Nisshin-shi), Takeshi Morikawa (Seto-shi), Keiko Uemura (Seto-shi), Tsutomu Kajino (Toyoake-shi), Tomiko Mori (Seto-shi)
Application Number: 13/977,374
Classifications
Current U.S. Class: Diaphragm Type (204/252); Cells (204/242); Electrodes (204/280); Metallic (204/292); Composition (204/291)
International Classification: C25B 1/00 (20060101);