JP2016151064A - Carbon dioxide reduction electrode and carbon dioxide reduction apparatus using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a carbon dioxide reduction electrode and apparatus having high methanol generation efficiency.SOLUTION: A carbon dioxide reduction electrode 1 is used for generating methanol by reduction of carbon dioxide in an electrolyte consisting of an aqueous solution containing a reduction catalyst consisting of a tertiary amine compound. A carbon dioxide reduction apparatus uses the carbon dioxide reduction electrode. The carbon dioxide reduction electrode 1 includes a conductive substrate 11 and a reduction layer 12 formed on the substrate 11. The reduction layer 12 consists of a metal oxide having a hydroxyl group on a surface thereof or either an oxide or alloy at least containing an alkali metal and/or an alkaline earth metal.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a carbon dioxide reduction electrode for reducing carbon dioxide to produce methanol, and a carbon dioxide reduction device using the same.

  In recent years, in order to solve the fossil fuel depletion problem, research for generating hydrogen and organic matter useful as energy sources from water and carbon dioxide is regarded as important. In particular, research on hydrogen generation by water splitting using a photocatalyst or a water splitting electrode is underway. However, it is difficult to say that hydrogen is versatile as an energy source, and there are many problems regarding storage and transportation.

  Then, development which produces | generates alcohol, such as methanol which has versatility from a carbon dioxide, is advanced. For example, a technique for generating methanol by reducing carbon dioxide with a metal electrode made of Au or the like in an electrolytic solution made of an aqueous solution containing a catalyst made of a tertiary amine compound has been developed (see Patent Document 1). .

Special table 2012-516392 gazette

  However, when carbon dioxide is reduced using a metal electrode in an aqueous solution containing a catalyst as described above, the production efficiency of methanol from carbon dioxide decreases. That is, the reactivity between hydrogen ions in the electrolyte and the metal electrode is higher than the reactivity between the reaction intermediate produced from the tertiary amine compound and carbon dioxide and the metal electrode. There is a problem that hydrogen proceeds easily. In particular, when the current density is increased in order to increase the reactivity, the tendency becomes remarkable, and the production efficiency of methanol from carbon dioxide is further reduced.

  The present invention has been made in view of such a background, and intends to provide a carbon dioxide reduction electrode and a carbon dioxide reduction device having high methanol production efficiency.

One aspect of the present invention is a carbon dioxide reduction electrode used for reducing methanol to produce methanol in an electrolyte solution comprising an aqueous solution containing a reduction catalyst comprising a tertiary amine compound,
The carbon dioxide reduction electrode has a conductive substrate and a reduction layer provided on the substrate,
The reduction layer is made of a metal oxide having a hydroxyl group on the surface, or an oxide or alloy containing at least an alkali metal and / or an alkaline earth metal.

  In another aspect of the present invention, the carbon dioxide reduction electrode, the oxidation electrode, the electrolytic solution in which the oxidation electrode and the carbon dioxide reduction electrode are immersed, and the carbon dioxide reduction electrode and the above in the electrolytic solution. The carbon dioxide reduction device includes an ion exchange membrane that separates the oxidation electrode and a supply port that supplies carbon dioxide to the electrolytic solution.

  The carbon dioxide reduction electrode (hereinafter referred to as “reduction electrode” as appropriate) has a reduction layer made of a metal oxide having a hydroxyl group on its surface or an oxide or alloy containing an alkali metal and / or an alkaline earth metal. Yes. Such a reducing layer has high reactivity with a reaction intermediate produced from a tertiary amine compound and carbon dioxide in the electrolytic solution. Therefore, the methanol production efficiency can be improved. Moreover, even if the current density is increased, the production efficiency of methanol can be sufficiently increased. This mechanism is thought to occur as follows.

  That is, when the reduction layer is made of a metal oxide having a hydroxide on the surface, the reaction intermediate polarized to a negative charge and the hydroxyl group are exchanged in the electrolytic solution, and the reaction intermediate and the metal in the metal oxide are exchanged. A coordination bond is formed between the two. Further, when the reducing layer is made of an oxide or alloy containing an alkali metal and / or alkaline earth metal, the alkali metal and / or alkaline earth metal having a high ionization tendency is polarized to a positive charge in the electrolyte, A coordination bond is formed with the reaction intermediate polarized to a negative charge. By forming these coordination bonds, it is considered that the production reaction of methanol from the reaction intermediate easily proceeds at the reduction electrode, and the production efficiency of methanol is improved as described above.

  The carbon dioxide reduction device (hereinafter referred to as “reduction device” as appropriate) includes a reduction electrode, an oxidation electrode, an electrolytic solution, an ion exchange membrane, and a supply port for supplying carbon dioxide. In the reducing device, a reaction intermediate is generated from the carbon dioxide supplied from the supply port and the catalyst contained in the electrolytic solution. This reaction intermediate is reduced at the reduction electrode to produce methanol. The reduction electrode has a reduction layer that can form a coordinate bond with the reaction intermediate as described above. Therefore, the reducing device has high production efficiency of methanol by the reduction of carbon dioxide.

1 is a cross-sectional view of a reduction electrode in Example 1. FIG. FIG. 3 is an explanatory view showing each step of forming a reduction layer on the substrate from the formation surface side of the FTO film of the substrate in Example 1. FIG. 3 is an explanatory diagram showing each step of forming a reduction layer on a substrate from the cross-sectional side of the substrate in Example 1. FIG. 3 is an explanatory diagram illustrating a cross-sectional configuration of the reduction device according to the first embodiment. Explanatory drawing which shows the structure of the electrochemical cell used at the time of preparation of the reduction electrode of Example 4. FIG. Explanatory drawing (a) which shows the mechanism in which a coordination bond is formed between the reduction | restoration layer of the reduction electrode in Example 1 and Example 2, and a reaction intermediate, Reduction of the reduction electrode in Example 3-7 Explanatory drawing (b) which shows the mechanism in which a coordination bond is formed between a layer and a reaction intermediate body. Explanatory drawing which shows the mechanism of the reaction which progresses between the reduction electrode in Comparative Example 1- Comparative Example 4, and the electrolyte solution containing a reaction intermediate.

Next, preferred embodiments of the reduction electrode and the reduction device will be described.
In the reduction electrode, the entire conductive substrate may have conductivity, or the surface may partially have conductivity. Specifically, for example, a substrate having a conductive layer can be used. The material of the conductive substrate can be selected as appropriate.

  The thickness of the reduction layer in the reduction electrode can be adjusted as appropriate. The thickness of the reducing layer can be, for example, in the range of 0.0005 to 5 mm. The reducing layer may be formed on at least a part of the surface of the substrate. Specifically, for example, the reducing layer may be formed on one side or both sides of the substrate, or may be formed so as to cover the entire surface of the substrate.

The reduction layer made of a metal oxide having a hydroxyl group on the surface is a reduction layer made of a metal oxide and having a hydroxyl group on the surface. It is considered that the hydroxyl group present on the surface is formed from moisture in the atmosphere. Examples of such metal oxides include CuO, Ag ₂ O, AgO, TiO ₂ , SnO, ZnO, Fe ₂ O ₃ , CdO, NiO, In ₂ O ₃ , Au ₂ O ₃ , Ga ₂ O ₃ , Cu. ₂ O, MoO ₂ or the like can be used.

Examples of the reduction electrode made of an oxide or an alloy containing at least an alkali metal and / or an alkaline earth metal include, for example, an alkali metal oxide, a composite oxide containing an alkali metal, an alloy containing an alkali metal, and an oxidation of an alkaline earth metal. Products, composite oxides containing alkaline earth metals, alloys containing alkaline earth metals, and the like can be used. Alternatively, a composite oxide containing an alkali metal and an alkaline earth metal, an alloy containing an alkali metal and an alkaline earth metal, or the like can be used. Specifically, for example, Li ₂ CuO ₂ , Cs ₂ CuO ₂ , CsAgO, LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiFePO ₄ , Li ₂ FePO ₄ F, CaFeO, KAgO, Li ₄ SiO ₂ , Li ₂ Cu Etc. can be used.

  A tertiary amine compound is used as a carbon dioxide reduction catalyst. As the tertiary amine compound, for example, a heterocyclic amine can be used. The heterocyclic amine may be monocyclic or polycyclic. Specifically, pyridine, imidazole, pyrrole, pyrazole, triazole, bipyridyl, and the like can be used. Moreover, these heterocyclic amines may have a substituent such as an alkyl group, a hydroxyl group, a halogen group, an amino group, or a hydrazino group. When the number of carbon atoms of a substituent having a hydrocarbon chain such as an alkyl group is increased, the steric barrier is increased, and the catalytic activity of tertiary amines against carbon dioxide may be reduced. From this viewpoint, the number of carbon atoms of the substituent is preferably 5 or less, and more preferably 2 or less.

  In the reduction device, the oxidation electrode is the counter electrode of the reduction electrode. As the oxidation electrode, for example, an electrode made of a noble metal or the like can be used. As the electrolytic solution, for example, an aqueous solution containing the above-described reduction catalyst can be used. Various commercially available products can be used as the ion exchange membrane.

Example 1
Next, embodiments of the carbon dioxide reduction electrode and the carbon dioxide reduction device will be described with reference to the drawings. As shown in FIG. 1, the reduction electrode 1 of this example includes a conductive substrate 11 and a reduction layer 12 provided on the substrate 11. The substrate 11 includes a quartz glass substrate 111 and a fluorine-doped tin oxide (FTO) film 112 formed on the surface thereof. The reduction layer 12 is formed on the conductive FTO film 112. The reducing layer 12 in this example is made of CuO and has a hydroxyl group on the surface.

  A method for manufacturing the reduction electrode 1 of this example will be described. First, the reduction layer 12 is formed on the substrate 11 by a squeegee method. Specifically, first, a quartz glass substrate 111 having an FTO film 112 formed on the surface thereof was prepared as the conductive substrate 11 (see FIGS. 2A and 3A). As the substrate 11, a product of Meijo Kagaku Kogyo Co., Ltd. was used. In the substrate 11, the FTO film 112 is laminated on one side of the quartz glass substrate 111. The dimensions of the substrate 11 are 30 mm long × 30 mm wide × 1.8 mm thick.

  Next, as shown in FIGS. 2 (b) and 3 (b), a plate-like spacer 120 having a hollow interior was prepared. The external dimensions of the spacer 120 are 30 mm long × 30 mm wide like the substrate 11, and the dimensions of the internal space are 25 mm long × 25 mm wide. The thickness of the spacer 120 is 1 mm. This spacer 120 was placed on the surface of the substrate 11 where the FTO film 111 is formed. Next, as shown in FIGS. 2C and 3C, a CuO paste layer 121 is formed on the FTO film 111 of the substrate 11 by applying a CuO paste in the internal space of the spacer 120 by a squeegee method. did. As the CuO paste, a paste was used in which the amount of CuO powder contained in 500 μl of ethanol (reagent manufactured by Sigma-Aldrich) was 0.500 mg.

  Next, after the CuO paste layer 121 was dried at a temperature of 50 ° C. for 1 hour, the spacer 120 was removed. Next, the substrate 11 on which the CuO paste layer 121 was laminated was baked at a temperature of 550 ° C. for 1 hour. Thereby, as shown in FIG. 2D and FIG. 3D, the reducing layer 12 having a thickness of 0.02 mm was formed on the substrate 11. The reduction layer 12 is made of CuO and has a hydroxyl group on the surface.

  In the above example, the reducing layer 12 is produced by the squeegee method, but the reducing layer 12 can also be produced by, for example, a spin coater method, an electrophoresis method, an electrodeposition method or the like.

Next, a reduction device was assembled using the reduction electrode 1. As shown in FIG. 4, the reduction device 6 includes a reduction electrode 1, an oxidation electrode 2, an electrolytic solution 3 containing a catalyst (pyridine), an ion exchange membrane 4 (hereinafter referred to as “membrane 4” as appropriate), CO 2 ₂ and a supply port 5 for supplying ₂ . The electrolytic solution 3 is a pyridine aqueous solution having a concentration of 0.01 mol / L. In the reducing device 6, 50 ml of the electrolytic solution 3 is injected into the case 60. Case 60 is an H-type cell VB-9 manufactured by EC Frontier. That is, the case 60 is H-shaped and includes two cases 601 and 602 and a connecting portion 603 that connects the two cases.

  In the case 601, the reduction electrode 1 and the reference electrode 19 made of Ag / AgCl are inserted. The reduction electrode 1 and the reference electrode 19 are electrically connected. On the other hand, an oxidation electrode 2, which is a counter electrode of the reduction electrode 1, is inserted in the case 602. The oxidation electrode 2 is made of a Pt wire. The reduction electrode 1 is electrically connected to the positive electrode of the potentiostat 69, and the oxidation electrode 2 is electrically connected to the negative electrode of the potentiostat 69. A membrane 4 (Nafion 117 manufactured by Sigma-Aldrich) is disposed at the connecting portion 603. The membrane 4 exists between the reduction electrode 1 and the oxidation electrode 2 and separates the electrolytic solution 3. In the case 60, the reduction electrode 1, the reference electrode 19, the oxidation electrode 2, and the membrane 4 are immersed in the electrolytic solution 3. In the reduction device 6, the case 601 side constitutes a reduction tank, and the case 602 side constitutes an oxidation tank.

The openings of the cases 601 and 602 are sealed with plugs 603 and 604. A tube 50 for supplying carbon dioxide is inserted into the plug 603 on the case 601 side. Carbon dioxide (CO ₂ ) is supplied into the electrolytic solution 3 from the supply port 5 of the pipe 50.

  Further, the reduction device 6 is electrically connected to an electrochemical analyzer 7 including an ammeter 71 and a voltmeter 72 as shown in FIG. The electrochemical analyzer 7 is an ALS model 660E manufactured by BAS. The ammeter 71 is connected between the reduction electrode 1 and the oxidation electrode 2, and the voltmeter 72 is connected between the reduction electrode 1 and the reference electrode 19.

Next, CO ₂ was reduced using the reducing device 6 and the amount of methanol produced was analyzed. Specifically, as shown in FIG. 5, carbon dioxide gas (CO ₂ ) having a G1 level, that is, a purity of 99.99995 vol%, from the supply port 5 into the electrolytic solution 3 in the case 601 (reduction tank side) of the reducing device 6. Was blown in for 1 hour. The flow rate of carbon dioxide is 10 ml / min. Next, a carbon dioxide reduction reaction was carried out by applying a voltage of 2 V to the reduction electrode 1 by means of a potentiostat 69. During the reduction reaction, carbon dioxide gas was continuously blown into the electrolytic solution 3 at a flow rate of 10 ml / min. This reduction reaction was carried out for 2 hours, and the reduction products methanol (MeOH), carbon monoxide (CO), hydrogen (H ₂ ), methane (CH ₄ ), and formic acid (HCOOH) were quantified.

Each reduction product was quantified by the following apparatus and conditions.
(1) Methanol apparatus: gas chromatography (FID) (GC-2014 manufactured by Shimadzu Corporation)
Condition: Constant temperature measurement at 70 ° C. Operation: The electrolytic solution after the reduction reaction was injected into the apparatus by a syringe.

(2) Carbon monoxide, hydrogen, methane Apparatus: Gas chromatography (TCD) (Agilent 7890A manufactured by Agilent Technologies)
Conditions: The initial temperature was increased from 60 ° C. to a temperature of 180 ° C. at a temperature increase rate of 20 ° C./min. Operation: The gas in the case after the reduction reaction was injected into the apparatus by a syringe.

(3) Formic acid apparatus: ion chromatography (Dionex IC20 manufactured by Thermo Fisher Scientific)
Condition: Separation / extraction with sodium carbonate / sodium bicarbonate ionization liquid Operation: The electrolytic solution after the reaction was injected into the apparatus with a syringe.

The amount of methanol produced measured by the gas chromatography shown in (1) above is shown in Table 1 described later. Moreover, the selectivity S of methanol production was calculated from the production amount of each reduction product measured by the methods (1) to (3) based on the following formula (α). The results are shown in Table 1 below. In formula (α), A is the amount of methanol produced (mol), B is the amount of carbon monoxide produced (mol), C is the amount of hydrogen produced (mol), and D is the amount of methane. A production amount (mol), and E is a production amount (mol) of formic acid.
S (%) = 100 × A / (A + B + C + D + E) (α)

(Examples 2 to 7)
Next, a plurality of reduction electrodes 1 having a composition of the reduction layer 12 different from that of Example 1 were produced (see FIG. 1). In Examples 2 to 7 to be described later, the same reference numerals as those in Example 1 indicate the same components as in Example 1 and refer to the preceding description.

In the reduction electrode 1 of Example 2, the reduction layer 12 is made of Ag ₂ O, and the other configuration is the same as that of Example 1. The reduction electrode 1 of Example 2 was produced in the same manner as in Example 1 except that Ag ₂ O powder (reagent manufactured by Sigma-Aldrich) was used instead of the CuO powder in Example 1 described above.

In the reduction electrode 1 of Example 3, the reduction layer 12 is made of Li ₂ CuO ₂ , and the other configuration is the same as that of Example 1. The reduction electrode 1 of Example 3 was produced in the same manner as in Example 1 except that Li ₂ CuO ₂ powder was used instead of the CuO powder in Example 1 described above. The Li ₂ CuO ₂ powder was prepared as follows. Specifically, first, using a mortar, Li ₂ CO ₃ powder and CuO powder were mixed in equimolar amounts. The mixed powder was transferred to an alumina container and baked at a temperature of 685 ° C. for 24 hours. Next, the above-mentioned Li ₂ CuO ₂ powder was obtained by pulverizing the sintered body obtained after firing in a mortar.

In the reduction electrode 1 of Example 4, the reduction layer 12 is made of Li ₂ Cu, and the other configuration is the same as that of Example 1. The reduction electrode of Example 4 was produced by reducing the reduction layer 12 made of Li ₂ CuO ₂ in the reduction electrode 1 of Example 3 described above using the electrochemical cell 8 shown in FIG. First, an electrochemical cell 8 was constructed as follows. As shown in FIG. 5, first, the reduction electrode 1 of Example 3 and the conductive substrate 11 were immersed in the electrolytic solution 800 in the case 80. The inside of the case 80 is sealed with a plug 801. The conductive substrate 11 is a quartz glass substrate 111 on the surface of which an FTO film 112 similar to that of the first embodiment is formed. The electrolytic solution 800 is a Na ₂ SO ₄ aqueous solution having a concentration of 0.1 mol / L. The reduction electrode 1 is electrically connected to the positive electrode of the potentiostat 81, and the substrate 11 is electrically connected to the negative electrode of the potentiostat 81. A reference electrode 19 made of Ag / AgCl is inserted in the case 80, and a voltmeter 72 is connected between the reduction electrode 1 and the reference electrode 19. In addition, an ammeter 71 for measuring a current value flowing between the reduction electrode 1 and the substrate 11 is provided. In addition, the electrochemical cell 8 is provided with a light source 85 for irradiating the reducing layer 12 of the reducing electrode 1 with light 851.

While applying a voltage of 2 V to the reducing electrode 1 of the electrochemical cell 8 having such a configuration, the reducing layer 12 was irradiated with light 851. The wavelength of the light 851 is 365 nm, and the intensity is 10 mW / cm ² . By applying this voltage and irradiating with light, the reduced layer 12 made of Li ₂ CuO ₂ was reduced to form a reduced layer made of Li ₂ Cu. In this way, a reduction electrode of Example 4 was produced.

In the reduction electrode 1 of Example 5, the reduction layer 12 is made of Cs ₂ CuO ₂ , and the other configuration is the same as that of Example 1. The reduction electrode 1 of Example 5 was produced in the same manner as Example 1 except that Cs ₂ CuO ₂ powder was used instead of the CuO powder in Example 1 described above. The Cs ₂ CuO ₂ powder was prepared as follows. Specifically, first, Cs ₂ CO ₃ powder and CuO powder were mixed in equimolar amounts using a mortar. The mixed powder was transferred to an alumina container and baked at a temperature of 500 ° C. for 10 hours. Next, the above-mentioned Cs ₂ CuO ₂ powder was obtained by pulverizing the sintered body obtained after firing in a mortar.

In the reduction electrode 1 of Example 6, the reduction layer 12 is made of CsAgO, and other configurations are the same as those of Example 1. The reduction electrode 1 of Example 6 was produced in the same manner as in Example 1 except that CsAgO powder was used instead of the CuO powder in Example 1 described above. CsAgO powder was prepared as follows. Specifically, first, Cs ₂ CO ₃ powder and Ag ₂ O powder were mixed in equimolar amounts using a mortar. The mixed powder was transferred to an alumina container and baked at a temperature of 500 ° C. for 10 hours. Subsequently, the above-mentioned CsAgO powder was obtained by pulverizing the sintered body obtained after firing in a mortar.

In the reduction electrode 1 of Example 7, the reduction layer 12 is made of CaFeO, and the other configuration is the same as that of Example 1. The reduction electrode 1 of Example 7 was produced in the same manner as Example 1 except that CaFeO powder was used instead of CuO powder in Example 1 described above. The CaFeO powder was prepared as follows. Specifically, first, anhydrous calcium acetate ((CH ₃ CO ₂ ) ₂ Ca · H ₂ O) and iron nitrate (Fe (NO ₃ ) ₃ ) were dissolved in water in equimolar amounts. Next, after putting polyethylene glycol (PEG) having a molecular weight of 600 into the solution, the solution was dried while stirring at a temperature of 120 ° C. to remove moisture. The obtained solid content was baked at a temperature of 450 ° C. for 2 hours, and further baked at a temperature of 1050 ° C. for 10 hours. Next, the sintered body obtained after firing was pulverized in a mortar to obtain the CaFeO powder described above.

Using the reduction electrodes 1 of Examples 1 to 7 obtained as described above, a reduction device 6 having the same configuration as that of Example 1 was produced (see FIG. 4). Then, CO ₂ was reduced using each reducing device in the same manner as in Example 1, and the amount of methanol produced and the selectivity of methanol production were examined. The results are shown in Table 1 below.

(Comparative Examples 1-4)
Next, a reduction electrode composed of four types of metal electrodes was prepared for comparison with Examples 1 to 7. The reduction electrode of Comparative Example 1 is made of Pt. The reduction electrode of Comparative Example 2 is made of Cu. The reduction electrode of Comparative Example 3 is made of Ag. The reduction electrode of Comparative Example 4 is made of Ti. Moreover, the reduction apparatus of the structure similar to Example 1 was produced using the reduction electrode of each comparative example. Then, CO ₂ was reduced using each reducing device in the same manner as in Example 1, and the amount of methanol produced and the selectivity of methanol production were examined. The results are shown in Table 1 below.

(Comparison between Examples 1-7 and Comparative Examples 1-4)
The production amounts of methanol and the selectivity of methanol production in each of Examples 1 to 7 and Comparative Examples 1 to 4 are as shown in the following table.

As can be seen from Table 1, in the electrode device 6 using the reduction electrode 1 of the example, the amount of methanol that is a reduction product of CO ₂ is high, and the selectivity of methanol is also high. On the other hand, in the reduction apparatus using the reduction electrode of the comparative example, the amount of methanol produced was low or the selectivity was low. That is, by using the reduction electrode 1 having the reduction layer 12 made of a metal oxide having a hydroxyl group on the surface or an oxide or alloy containing at least an alkali metal and / or an alkaline earth metal as in the embodiment, methanol is obtained. It can be seen that the production efficiency of can be sufficiently increased. This mechanism will be considered with reference to FIGS.

  That is, as in Example 1 and Example 2, when the reducing layer 12 is made of a metal oxide having a hydroxide on the surface, as shown in FIG. The reaction intermediate 31 polarized to charge and the hydroxyl group present on the surface of the reduction layer 12 are interchanged. As a result, a coordinate bond is formed between the reaction intermediate 31 and the metal M polarized to a positive charge in the metal oxide in the reduction layer 12. As a result, since a bond is formed between the reaction intermediate 31 and the reduction layer 12, it is considered that the reaction intermediate 31 is easily reduced and the production efficiency of methanol is increased.

  Further, when the reducing layer 12 is made of an oxide or an alloy containing an alkali metal and / or an alkaline earth metal as in Example 3 to Example 7, as shown in FIG. Alkali metals such as Li and Cs and / or alkaline earth metals such as Ca, which have a high tendency, are polarized to a positive charge in the electrolyte, and a coordination bond is formed with the reaction intermediate 31 polarized to a negative charge. In FIG. 6 (b), the state in which this coordination bond is formed is illustrated by taking the alkali metal Li as an example, but other alkali metals such as Cs and other alkaline earths such as Ca are illustrated. The same applies to metals. By forming these coordination bonds, it is considered that in the reduction layer 12, the methanol formation reaction from the reaction intermediate 31 proceeds easily, and the production efficiency of methanol is increased as described above.

  On the other hand, in the case of the reduction electrode 9 made of a metal such as Pt as in Comparative Examples 1 to 4, it is difficult for the metal M constituting the reduction electrode 9 and the reaction intermediate 31 to attract each other as shown in FIG. Therefore, the reactivity between the hydrogen ions in the electrolytic solution and the reduction electrode 9 is higher than the reactivity between the reaction intermediate 31 and the reduction electrode 9, and the electricity of water is higher than the production of methanol from the reaction intermediate 31. Decomposition proceeds easily and hydrogen is easily generated.

  As described above, by using the reduction electrode 1 having the reduction layer 12 made of a metal oxide having a hydroxyl group on the surface or an oxide or alloy containing at least an alkali metal and / or an alkaline earth metal as in the embodiment. The production efficiency of methanol can be sufficiently increased.

Moreover, as in Example 1 and Example 2, the metal oxide constituting the reduction layer 12 is preferably CuO or Ag ₂ O. In this case, as shown in Table 1 above, the production efficiency of methanol can be reliably increased.

Further, as in Examples 3 to 7, the alkali metal is preferably Li and / or Cs, and the alkaline earth metal is preferably Ca. Furthermore, the reduction layer 12 is more preferably made of Li ₂ CuO ₂ , Li ₂ Cu, Cs ₂ CuO ₂ , CsAgO, or CaFeO. In these cases, as shown in Table 1 above, the production efficiency of methanol can be reliably increased. Further, from the viewpoint of further increasing the production efficiency of methanol, the reducing layer 12 is more preferably made of Li ₂ CuO ₂ or Li ₂ Cu (see Table 1).

  In the present example, an example in which pyridine is used as a reduction catalyst for reducing carbon dioxide has been described. However, based on the above-mentioned mechanism, not only pyridine but also another tertiary amine compound is used as a catalyst, the same reaction intermediate 31 as in the case of pyridine is produced. It can be seen that the reactivity of is improved (see FIG. 6). Therefore, various tertiary amine compounds can be used as the reduction catalyst.

  The tertiary amine compound is preferably a heterocyclic amine, and more preferably pyridine or a pyridine derivative. In this case, as shown in Table 1 above, the production efficiency of methanol can be reliably increased.

  As mentioned above, although the Example of this invention was described in detail, this invention is not limited to the said Example, A various change is possible within the range which does not impair the meaning of this invention.

DESCRIPTION OF SYMBOLS 1 Carbon dioxide reduction electrode 11 Conductive substrate 12 Reduction layer 2 Oxidation electrode 3 Electrolyte 6 Carbon dioxide reduction apparatus

Claims (8)

A carbon dioxide reduction electrode (1) used for producing methanol by reducing carbon dioxide in an electrolyte solution (3) comprising an aqueous solution containing a reduction catalyst comprising a tertiary amine compound,
The carbon dioxide reduction electrode (1) has a conductive substrate (11) and a reduction layer (12) provided on the substrate (11),
The carbon dioxide reduction electrode (1), wherein the reduction layer (12) comprises a metal oxide having a hydroxyl group on the surface, or an oxide or alloy containing at least an alkali metal and / or an alkaline earth metal. The carbon dioxide reduction electrode (1) according to claim 1, wherein the metal oxide is CuO or Ag ₂ O.   The carbon dioxide reducing electrode (1) according to claim 1, wherein the alkali metal is Li and / or Cs, and the alkaline earth metal is Ca. The carbon dioxide reduction electrode (1) according to claim 3, wherein the reduction layer (12) is made of Li ₂ CuO ₂ , Li ₂ Cu, Cs ₂ CuO ₂ , CsAgO, or CaFeO. The carbon dioxide reducing electrode (1) according to claim 4, wherein the reducing layer (12) is made of Li ₂ CuO ₂ or Li ₂ Cu.   The carbon dioxide reducing electrode (1) according to any one of claims 1 to 5, wherein the tertiary amine compound is a heterocyclic amine.   The carbon dioxide reducing electrode (1) according to any one of claims 1 to 6, wherein the tertiary amine compound is pyridine or a pyridine derivative.   The carbon dioxide reduction electrode (1) according to any one of claims 1 to 7, the oxidation electrode (2), the oxidation electrode (2), and the carbon dioxide reduction electrode (1) are immersed in the above An electrolyte solution (3), an ion exchange membrane (4) separating the carbon dioxide reduction electrode (1) and the oxidation electrode (2) in the electrolyte solution (3), and carbon dioxide in the electrolyte solution (3). A carbon dioxide reduction device (6), characterized in that it has a supply port (5) for supplying water.

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