JP2021138994A - Electrode catalyst for reducing co2, method of producing electrode catalyst for reducing co2, co2 reducing electrode, and co2 reducing system - Google Patents

Abstract

To provide a low-cost and novel electrode catalyst for reducing CO2, having excellent energy efficiency in producing CO and high atom efficiency.SOLUTION: The electrode catalyst for reducing CO2, comprises an electroconductive porous carbon support, a specific nitrogen-containing aromatic heterocyclic compound supported by the carbon support, and a cobalt ion coordinated with four nitrogen atoms present on the nitrogen-containing aromatic heterocycle of the nitrogen-containing aromatic heterocyclic compound, with the molar ratio of the complex comprising a Co-N4C bond being 0.3 or greater relative to the total mount of supported cobalt, or the molar ratio of the complex comprising a Co-N4C bond being 0.2 or greater and the molar ratio of metallic cobalt being 0.5 or lower, relative to the total amount of supported cobalt.SELECTED DRAWING: None

Description

The present invention, CO ₂ reduction electrode catalyst, a method of manufacturing a CO ₂ reduction electrode catalyst, CO ₂ reduction electrode, and CO to _{a two-reducing} system.

As one of the methods to reduce the emission of _{carbon dioxide (CO 2} ), which is the main cause of global climate change, _{CO 2} and water are converted into CO 2 and water using electricity derived from renewable energy, such as carbon monoxide (CO). A method of electrolyzing reduction products and oxygen and recycling them is known.

As _{a reducing electrode of an electrochemical reaction device that reduces CO 2 by electrolysis, a technique using a CO 2} reducing catalyst containing a porous metal layer containing gold and having a dendrite structure has been proposed (see Patent Document 1).

JP-A-2017-57492

The reduction catalyst described in Patent Document 1 is excellent in energy efficiency of CO production, but has low efficiency per atom and is expensive because it uses a precious metal such as gold. _{As a result of diligent studies on the electrode catalyst for CO 2} reduction, the present inventors have a technique capable of providing an electrode catalyst for _{CO 2} reduction which is excellent in energy efficiency of CO generation, has high efficiency per atom, and is inexpensive. I came up with.

The present invention has been made in view of these circumstances, and one of the purposes thereof is to provide _{a new electrode catalyst for CO 2} reduction, which is excellent in energy efficiency of CO generation, has high efficiency per atom, and is inexpensive. To do.

（式中、ＸはＣまたはＮであり、Ｒ_１〜Ｒ_５は、それぞれ独立にＨまたはアルキル基であり、Ｒ_１〜Ｒ_５のうち少なくとも１つがアルキル基である。但し、ＸがＮである場合、Ｒ_４は存在せず、Ｒ_１〜Ｒ_３およびＲ_５のうち少なくとも１つがアルキル基である。）
で表される化合物、および
一般式（１）で表される繰り返し単位を有し、一般式（１）中、Ｒ_１〜Ｒ_５のうち１つのアルキル基が主鎖であり、複素芳香環が側鎖であるポリマー、および
一般式（２）：

（式中、Ｒ_６〜Ｒ_１３は、それぞれ独立にＨまたはアルキル基であり、Ｒ_６〜Ｒ_１３のうち少なくとも１つがアルキル基である。）
で表される化合物からなる群より選択され、カーボン担体に担持されている窒素含有複素芳香環化合物と、
窒素含有複素芳香環化合物の複素芳香環上の窒素原子４つと配位しているコバルトイオンと、
を含み、
全コバルト担持量に対するＣｏ−Ｎ_４Ｃ結合をもつ錯体部のモル比が０．３以上であるか、または
全コバルト担持量に対して、Ｃｏ−Ｎ_４Ｃ結合をもつ錯体部のモル比が０．２以上であり、かつ金属コバルトのモル比が０．５以下である。 One embodiment of the present invention is an electrode catalyst for _{CO 2 reduction.} This _{electrode catalyst for CO 2} reduction is composed of a porous carbon carrier showing conductivity and
The following general formula (1):

(In the formula, X is C or N, R _{1 to} R ₅ are independently H or alkyl groups, and _{at least one of R 1 to} R ₅ is an alkyl group, where X is N. in some cases, _{R 4} is _absent, at least one of alkyl groups of R 1 to R ₃ and _{R 5.)}
It has a compound represented by, and a repeating unit represented by the general formula (1), and in the general formula (1), _{one alkyl group from R 1 to} R ₅ is the main chain, and a complex aromatic ring is formed. Polymers that are side chains, and general formula (2):

(In the formula, R _{6 to} R ₁₃ are independently H or alkyl groups, and _{at least one of R 6 to} R ₁₃ is an alkyl group.)
A nitrogen-containing heteroaromatic ring compound selected from the group consisting of the compounds represented by, and supported on a carbon carrier, and
Cobalt ions coordinated with four nitrogen atoms on the heteroaromatic ring of the nitrogen-containing heteroaromatic ring compound,
Including
Or the molar ratio of the complex portion with Co-N 4 _C bonds to the total cobalt supported amount is 0.3 or more, or the total cobalt loading amount, the molar ratio of the complex portion with Co-N 4 _C bond It is 0.2 or more, and the molar ratio of metallic cobalt is 0.5 or less.

（式中、ＸはＣまたはＮであり、Ｒ_１〜Ｒ_５は、それぞれ独立にＨまたはアルキル基であり、Ｒ_１〜Ｒ_５のうち少なくとも１つがアルキル基である。但し、ＸがＮである場合、Ｒ_４は存在せず、Ｒ_１〜Ｒ_３およびＲ_５のうち少なくとも１つがアルキル基である。）
で表される化合物、および
一般式（１）で表される繰り返し単位を有し、一般式（１）中、Ｒ_１〜Ｒ_５のうち１つのアルキル基が主鎖であり、複素芳香環が側鎖であるポリマー、および
下記一般式（２）：

（式中、Ｒ_６〜Ｒ_１３は、それぞれ独立にＨまたはアルキル基であり、Ｒ_６〜Ｒ_１３のうち少なくとも１つがアルキル基である。）
で表される化合物からなる群より選択される窒素含有複素芳香環化合物をコバルトイオンに配位させ、コバルトイオンに配位させた窒素含有複素芳香環化合物を、導電性を示す多孔質のカーボン担体に吸着させ、焼成する。 Another aspect of the present invention is the method for producing the electrode catalyst for _{CO 2 reduction.} This manufacturing method is described in the following general formula (1):

(In the formula, X is C or N, R _{1 to} R ₅ are independently H or alkyl groups, and _{at least one of R 1 to} R ₅ is an alkyl group, where X is N. in some cases, _{R 4} is _absent, at least one of alkyl groups of R 1 to R ₃ and _{R 5.)}
It has a compound represented by, and a repeating unit represented by the general formula (1), and in the general formula (1), _{one alkyl group from R 1 to} R ₅ is the main chain, and a complex aromatic ring is formed. The polymer that is the side chain, and the following general formula (2):

(In the formula, R _{6 to} R ₁₃ are independently H or alkyl groups, and _{at least one of R 6 to} R ₁₃ is an alkyl group.)
A nitrogen-containing heteroaromatic ring compound selected from the group consisting of the compounds represented by is coordinated with cobalt ions, and the nitrogen-containing heteroaromatic ring compound coordinated with cobalt ions is a porous carbon carrier exhibiting conductivity. And bake.

Yet another aspect of the present invention is a CO ₂ reducing electrode. This CO ₂ reduction electrode includes the above-mentioned electrode catalyst for _{CO 2 reduction.}

Yet another aspect of the present invention is a CO ₂ reduction system. This CO ₂ reduction system includes the CO ₂ reduction electrode, an oxidation electrode, and _{a power source connected between the CO 2} reduction electrode and the oxidation electrode.

According to the present invention, it is possible to provide _{a new electrode catalyst for CO 2} reduction, which is excellent in Faraday efficiency of CO production and is inexpensive.

It is a schematic diagram which shows the _{CO 2} reduction system which concerns on embodiment. It is a graph showing the relationship between the faraday efficiency of the molar ratio and CO complex formation portion with Co-N 4 _C bond. It is a graph which shows the relationship between the molar ratio of cobalt oxide and _{the Faraday efficiency of H 2 formation.} It is a graph which shows the relationship between the molar ratio of metallic cobalt and _{the Faraday efficiency of H 2 formation.}

Hereinafter, the present invention will be described with reference to the drawings based on preferred embodiments. The embodiments are not limited to the invention, but are exemplary, and all the features and combinations thereof described in the embodiments are not necessarily essential to the invention. The scale and shape of each part shown in each figure are set for convenience of explanation, and are not interpreted in a limited manner unless otherwise specified. In addition, some of the members that are not important for explaining the embodiment in each drawing are omitted and displayed.

(Electrode catalyst for _{CO 2 reduction)
The CO 2} reduction electrode catalyst according to the embodiment contains a porous carbon carrier exhibiting conductivity, a compound represented by the above general formula (1), and a repeating unit represented by the above general formula (1). From _{a polymer having an alkyl group having one of R 1 to} R _{5 as} a main chain and a complex aromatic ring as a side chain in the general formula (1), and a compound represented by the above general formula (2). A nitrogen-containing heteroaromatic ring compound selected from the above group, which is supported on a carbon carrier, and a cobalt ion coordinated with four nitrogen atoms on the heteroaromatic ring of the nitrogen-containing heteroaromatic ring compound. or the molar ratio of the complex portion with Co-N 4 _C bonds to cobalt loading amount is 0.3 or more, or the total cobalt loading amount, the molar ratio of the complex portion with Co-N 4 _C bond 0 .2 or more and the molar ratio of metallic cobalt is 0.5 or less.

In CO ₂ reduction electrode catalyst according to the embodiment, if the molar ratio of the complex portion with Co-N 4 _C bonds to the total cobalt supported amount is 0.3 or more, or the total of cobalt supported amount, the molar ratio of complex parts with _{Co-N 4} C bonds is not less than 0.2, and the molar ratio of the metal cobalt is 0.5 or less. In _{the electrolytic reduction of CO 2 , H 2} may be produced as a by-product in addition to CO, which is the target substance, on the reduction electrode side. The complex portion having a Co-N ₄ C bond of the _{electrode catalyst for CO 2} reduction according to the embodiment acts as an active site for a reaction for reducing _{CO 2 to CO.} The molar ratio of the complex portion having a Co-N _{4 C bond is 0.3 or more, or the molar ratio of the complex portion having a Co-N 4} C bond is 0.2 or more, and the molar ratio of metallic cobalt is set to 0.2 or more. By setting it to 0.5 or less, the ratio of CO in the electrolytic reduction product can be increased, and the energy efficiency of CO production can be set to a level sufficient for practical use. Further, the CO ₂ reduction electrode catalyst according to the embodiment has high efficiency per atom, and can be manufactured at a lower cost than a conventional catalyst using a noble metal such as gold.

Here, from the viewpoint of further increasing the energy efficiency of the CO formation, the molar ratio of the complex portion with Co-N 4 _C bonds to the total of cobalt supported amount is preferably 0.3 or more, more preferably 0. 4 or more. _{Further, it is preferable that the molar ratio of the complex portion having a Co-N 4} C bond is 0.3 or more and the molar ratio of metallic cobalt is 0.4 or less with respect to the total amount of cobalt carried. N 4 molar ratio of the complex part having a _C bond is 0.4 or more, and the molar ratio of the metal cobalt is more preferably 0.2 or less.

Further, from the viewpoint of further increasing the energy efficiency of CO generation, the _{molar ratio of cobalt oxide contained in the electrode catalyst for CO 2} reduction to the total amount of cobalt supported is preferably 0.8 or less. Cobalt oxide is a substance that is inactive in the electrolytic reduction of _{CO 2.}

The _{carbon carrier constituting the electrode catalyst for CO 2} reduction is not particularly limited as long as it is a porous one that exhibits conductivity and can support a nitrogen-containing heteroaromatic ring compound. Examples of carbon carriers include Ketjen black, acetylene black, carbon black, carbon nanotubes, graphene, fullerenes and the like.

The _{compound represented by the above general formula (1) constituting the electrode catalyst for CO 2} reduction has a structure in which the pyridine ring (X = H) or the pyrimidine ring (X = N) can freely move in the molecule. In formula (1), R _{1 to} R ₅ are independently H or alkyl groups, and _{at least one of R 1 to} R ₅ is an alkyl group. However, when X is N, R ₄ does not exist, and _{at least one of R 1 to} R ₃ and R ₅ is an alkyl group. The alkyl groups of R _{1 to} R ₅ may be linear or branched, as long as the heteroaromatic ring (pyridine ring or pyrimidine ring) in the formula (1) can move freely. Is not restricted.

The polymer having a repeating unit represented by the above general formula (1) constituting the electrode catalyst for _{CO 2} reduction has an alkyl group having one of _{R 1 to} R _{5 as a main chain in the general formula (1).} , A polymer having a complex aromatic ring as a side chain. As described above, when X is N in the equation (1), R ₄ does not exist. In this case, of course, the backbone of the polymer _is one alkyl group among R 1 to R ₃ and _{R 5.} The structure of the main chain of the polymer is not particularly limited as long as the heteroaromatic ring of the side chain can move freely. Typical examples of the polymer include poly-4-vinylpyridine and the like.

In the nitrogen-containing heteroaromatic ring compound represented by the above general formula (2) constituting the electrode catalyst for _{CO 2 reduction, R 6 to} R ₁₃ in the general formula (2) are independently H or alkyl groups, respectively. .. The alkyl groups of R ₁ and R ₂ may be linear or branched as long as the bipyridine ring in the formula (2) can move freely, and the length thereof is not limited. _{In the electrode catalyst for CO 2} reduction according to the embodiment, a total of four nitrogen atoms in the bipyridine ring of the two molecules of the compound represented by the formula (2) are coordinated to the cobalt ion.

_{In the electrode catalyst for CO 2} reduction according to the embodiment, the distance between the coordinated nitrogen atom and the cobalt ion is preferably 2.0 Å or more and 3.0 Å or less, and 2.0 Å or more and 2. It is more preferably 5 Å or less, and even more preferably 2.0 Å or more and 2.2 Å or less. By immobilizing the nitrogen-containing heteroaromatic ring compound coordinated to cobalt ions at such a distance on the carbon carrier, the CO ₂ reducing activity of cobalt can be further enhanced.

(Manufacturing method of electrode catalyst for _{CO 2 reduction)
The method for producing an electrode catalyst for CO 2} reduction according to the present embodiment is a nitrogen-containing heteroaromatic ring selected from the group consisting of a vinyl polymer having a pyridyl group in the side chain and a compound represented by the above general formula (1). The compound is coordinated with cobalt ions, and the nitrogen-containing heteroaromatic ring compound coordinated with cobalt ions is adsorbed on a porous carbon carrier exhibiting conductivity and fired. Here, the nitrogen-containing heteroaromatic ring compound and the carbon carrier are the same as those in _{the above-described electrode catalyst for CO 2 reduction.}

The method for coordinating the nitrogen-containing heteroaromatic ring compound to the cobalt ion is not particularly limited, and for example, it can be carried out by dispersing the nitrogen-containing heteroaromatic ring compound and the cobalt salt in a solvent. Further, a cobalt complex solution of the nitrogen-containing heteroaromatic ring compound may be formed by dispersing each of the nitrogen-containing heteroaromatic ring compound and the cobalt salt in a solvent and mixing the two dispersions.

The method for adsorbing the nitrogen-containing heteroaromatic ring compound coordinated with cobalt ions to the carbon carrier is not particularly limited, and for example, the carbon carrier can be added to the obtained cobalt complex solution and mixed. After adsorption, the solution can be removed, for example, by evaporation to dryness to obtain a carbon carrier carrying a cobalt complex.

In the production method according to the present embodiment, the carbon carrier on which the obtained cobalt complex is supported is calcined. The firing temperature is preferably 600K to 873K, more preferably 600K to 800K, and even more preferably 650K to 750K. By setting the calcination temperature within such a range, the CO ₂ reduction activity of cobalt can be further enhanced. The calcination may be performed once, but may be performed twice or more by changing the temperature from the viewpoint of appropriately fixing the cobalt complex with the carbon carrier. The firing time is not particularly limited, and can be appropriately set, for example, between 1 hour and 10 hours.

The catalyst formed by calcination is preferably washed with an acidic aqueous solution. During the formation of the catalyst, cobalt oxide and metallic cobalt may be produced on the carbon carrier as by-products. In order to improve the CO production rate per cobalt amount, it is desirable to remove cobalt oxide among these by-products, and to set the molar ratio of cobalt oxide to the total amount of cobalt supported to 0.8 or less. Therefore, the acidic aqueous solution is not particularly limited as long as it can remove cobalt oxide without affecting the activity of the catalyst. Examples of such an acidic aqueous solution include dilute nitric acid, dilute sulfuric acid, and dilute hydrochloric acid. Further, the formation of the by-product metallic cobalt can be suppressed by controlling the firing temperature within the above-mentioned temperature range.

(CO ₂ reduction electrode)
The CO ₂ reduction electrode according to the embodiment includes the CO ₂ reduction electrode catalyst according to the above embodiment. The CO ₂ reduction electrode, from the fact that by using the CO ₂ reduction electrode catalyst, when used as a reduction electrode for CO ₂ reduction systems, high energy efficiency of CO generation, high efficiency per atom. Moreover, this CO ₂ reduction electrode can be manufactured at low cost.

The shape of the CO ₂ reduction electrode is not particularly limited, and examples thereof include various shapes such as a plate shape, a mesh shape, a wire shape, a particle shape, a porous shape, a thin film shape, and an island shape. For example, the CO ₂ reduction electrode _{can be formed by arranging the CO 2} reduction electrode catalyst on the surface of the substrate. As _{an example of the base material on which the electrode catalyst for CO 2} reduction is arranged, carbon paper or the like can be mentioned. Further, for example, the CO ₂ reduction electrode can be formed by molding the _{CO 2 reduction electrode catalyst itself.}

(CO ₂ reduction system)
FIG. 1 is a schematic diagram showing a _{CO 2 reduction system according to an embodiment.} The CO ₂ reduction system 10 accommodates a reduction electrode 12, an oxidation electrode 14, a power source 16 that connects the reduction electrode 12 and the oxidation electrode 14, and the reduction electrode 12, _{and performs a CO 2} reduction reaction. including 18 houses the oxide electrode 14, and the oxidation reaction unit 20 that performs of H 2 _O oxidation, with an ion exchange membrane 22 disposed between the reduction electrode 12 and the oxide electrode 14, the. As will be described later, in the CO ₂ reduction system 10, since the _{electrode catalyst for CO 2} reduction according to the above embodiment is used for the reduction electrode 12, the energy efficiency of CO generation is high and the efficiency per atom is high. , Can be manufactured at low cost.

The reduction electrode 12 is an electrode _{that reduces CO 2} to produce a carbon compound. The oxidation electrode 14 is an electrode that oxidizes water to generate oxygen and hydrogen ions. In order to cause a reduction reaction at the reduction electrode 12, the reduction electrode 12 is connected to the negative electrode terminal of the power supply 16. The oxide electrode 14 is connected to the positive electrode terminal of the power supply 16 in order to cause an oxidation reaction at the oxide electrode 14.

As the reducing electrode 12, the CO ₂ reducing electrode according to the above embodiment is used. The oxidation electrode 14 is not particularly limited as long as it is a material capable of oxidizing water to generate oxygen and hydrogen ions, and can be made of a known material. Examples of such materials include metals such as iridium, platinum, palladium and nickel, alloys and intermetal compounds containing those metals, iridium oxide, manganese oxide, nickel oxide, cobalt oxide, iron oxide, tin oxide and oxidation. Dual metal oxides such as indium and ruthenium oxide, and ternary metal oxides such as Ni-Co-O, Ni-Fe-O, La-Co-O, Ni-La-O and Sr-Fe-O. , Pb-Ru-Ir-O, La-Sr-Co-O and other quaternary metal oxides, and metal complexes such as Ru complex and Fe complex. Various shapes such as a plate shape, a mesh shape, a wire shape, a particle shape, a porous shape, a thin film shape, and an island shape can be applied to the oxide electrode 14. The oxide electrode 14 may be a composite electrode in which these materials are laminated on a base material.

_{CO 2} reduced by the reduction electrode 12 is supplied to the reduction reaction unit 18. The CO _{2 to be} reduced may be a gas or may be in the form of a solution containing _{CO 2.} In the form of a solution, it is preferable to use a solution having a high _{CO 2 absorption rate.} Examples of such solutions include aqueous solutions such as LiHCO ₃ , NaHCO ₃ , KHCO ₃ , CsHCO ₃ , Li ₂ CO ₃ , Na ₂ CO ₃ , K ₂ CO ₃ , and Cs ₂ CO ₃ . Further, a solution containing _{CO 2} may be prepared by using a solvent of alcohols such as methanol, ethanol and acetone. The solution containing the CO ₂ raises the reduction potential of the CO _2, high ion conductivity, it is desirable that a solution containing a CO ₂ absorbent that absorbs CO _2. Examples of such solutions, and cations such as imidazolium ions, pyridinium ions, BF ₄ ^- or PF ₆ ^- is composed of a salt with an anion such as, ionic liquids or a liquid state in a wide temperature range An aqueous solution can be mentioned. Examples of other solutions include amine solutions such as ethanolamine, imidazole, and pyridine, or aqueous solutions thereof. The amine may be any of a primary amine, a secondary amine, and a tertiary amine.

_{H 2} O that is oxidized by the oxidation electrode 14 is supplied to the oxidation reaction unit 20. Pure water can be used as the H _{2 O to be oxidized.} Instead of pure water, may be used an aqueous solution containing any electrolyte, it is preferable to use an aqueous solution to promote the oxidation reaction of H 2 _O. Examples of such aqueous solutions include sulfuric acid, nitric acid, perchloric acid and hydrochloric acid.

The ion exchange membrane 22 may be made of a material capable of transferring ions between the reducing electrode 12 and the oxidizing electrode 14, and the type of the material is not particularly limited. Examples of ion exchange membranes include cation exchange membranes such as Nafion (registered trademark) and Flemion (registered trademark), and anion exchange membranes such as Neocepta (registered trademark) and Selemion (registered trademark).

Next, _{the operation of the CO 2} reduction system 10 will be described. When an electric potential is applied from the power source 16 to the oxide electrode 14, _{an oxidation reaction of H 2} O occurs in the vicinity of the oxide electrode 14, and oxygen (O ₂ ) and hydrogen ions (H ⁺ ) are generated. ^{The H +} generated on the oxidation electrode 14 side reaches the reduction electrode 12 via the ion exchange membrane 22. The CO ₂ reduction reaction occurs due ^{to the electrons (e −} ) supplied from the power source 26 to the reduction electrode 12 and ^{the H +} transferred to the reduction electrode 12. CO is produced by the reduction reaction of _{CO 2.} In the CO ₂ reduction system 10, the energy efficiency of CO production is preferably 63% or more at a ^{CO production rate of 1 mmol / h / cm 2.} Here, the energy efficiency of CO generation is _{obtained by dividing 1.33 V, which is the theoretical potential of CO generation and O 2} generation, by the voltage applied between the reduction electrode and the oxidation electrode.

_{The configuration of the CO 2} reduction system shown in FIG. 1 is only an example, and various modifications are possible as long as the system can carry out _{a CO 2 reduction reaction using the CO 2} reduction electrode according to the above embodiment. Is. For example, in the CO ₂ reduction system shown in FIG. 1, the ion exchange film 22 is arranged between the reduction electrode 12 and the oxidation electrode 14 so that the reduction electrode 12 and the oxidation electrode 14 are in contact with each other. And the oxide electrode 14 may be separated from the ion exchange film. Further, for example, an electrolytic solution flow path for circulating the electrolytic solution may be provided so that the reducing electrode 12 and the oxidizing electrode 14 are in contact with the electrolytic solution. In addition to these, various modifications are possible.

Hereinafter, examples of the present invention will be described, but the examples are merely examples for preferably explaining the present invention, and do not limit the present invention in any way.

(Example 1)
The CO ₂ reducing side electrode of Example 1 was prepared by the following procedure.

(Preparation of Co-N ₄ C coordination complex)
Cobalt nitrate hexahydrate (special grade reagent, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was dissolved in ethanol (special grade reagent, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) to prepare a cobalt solution so that the cobalt content was 40 mM. .. Dissolve 80 mg of poly (4-vinylpyridine) (weight average molecular weight 60,000, manufactured by Sigma-Aldrich) in ethanol (special grade reagent, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) to make the total volume 50 mL, and stir for 20 minutes to sufficiently mix the polymer. Distributed. Then, 1.7 mL of the cobalt solution was mixed and stirred for 15 minutes. The complex obtained here is referred to as a cobalt complex.

(Support of cobalt complex on conductive porous material)
After mixing 107.6 mg of Ketjen black carrier (ECP, manufactured by LION) with the above solution, the mixture was heated at 343 K with stirring on a hot plate and evaporated to dryness. The obtained powder was dried on a hot plate at 343 K for 16 hours. This is referred to as a cobalt catalyst precursor.

(Partial firing of cobalt catalyst precursor)
The cobalt catalyst precursor was placed in a flat-bottomed quartz reactor and heat-treated at a heating rate of 25 K / min at 423 K for 1 hour and then at 673 K for 3 hours under helium flow. This is referred to as a cobalt catalyst (before purification).

(Cleaning of cobalt catalyst (before purification))
A cobalt catalyst (before purification) was added to 0.1 M dilute nitrate, stirred at 300 rpm for 1 hour, suction filtered using a hydrophilic membrane filter with a pore size of 0.1 μm, and the pH was 6 to 6 to 200 mL of ion-exchanged water. It was washed until it became 7. It was then washed 3 times with 20 mL 2-propanol. The filtrate was dried under reduced pressure at 353 K for 1 hour. This is referred to as a cobalt catalyst (after purification).

(Example 2)
A cobalt catalyst (after purification) of Example 2 was prepared in the same manner as in Example 1 except that the temperature of partial firing of the cobalt catalyst precursor was set to 723 K.

(Example 3)
A cobalt catalyst (after purification) of Example 3 was prepared in the same manner as in Example 1 except that the temperature of partial firing of the cobalt catalyst precursor was 773 K.

(Example 4)
The cobalt catalyst (after purification) of Example 4 was prepared in the same manner as in Example 1 except that the temperature of partial firing of the cobalt catalyst precursor was set to 673 K and the cobalt catalyst (before purification) was not washed.

(Example 5)
The cobalt catalyst (after purification) of Example 5 was prepared in the same manner as in Example 1 except that the temperature of partial firing of the cobalt catalyst precursor was 773 K and the cobalt catalyst (before purification) was not washed.

(Example 6)
Same as Example 1 except that BP2000 (manufactured by Cabot Japan Co., Ltd.) was used instead of the Ketjen black carrier, the temperature of partial firing of the cobalt catalyst precursor was set to 673 K, and the cobalt catalyst (before purification) was not washed. The cobalt catalyst of Example 6 (after purification) was prepared.

(Example 7)
Example 1 and Example 1 except that an acetylene black carrier (manufactured by Denka Co., Ltd.) was used instead of the Ketjen black carrier, the temperature of partial firing of the cobalt catalyst precursor was set to 673 K, and the cobalt catalyst (before purification) was not washed. Similarly, the cobalt catalyst of Example 7 (after purification) was prepared.

(Example 8)
An active carbon carrier (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used instead of Ketjen Black, the temperature of partial firing of the cobalt catalyst precursor was set to 673 K, and the cobalt catalyst (before purification) was not washed. The cobalt catalyst of Example 8 (after purification) was prepared in the same manner as in Example 1.

(Example 9)
Instead of poly (4-vinylpyridine) having a weight average molecular weight of 60,000, poly (4-vinylpyridine) (manufactured by Sigma Aldrich) having a weight average molecular weight of 81500 was used to control the partial firing temperature of the cobalt catalyst precursor. The cobalt catalyst of Example 9 (after purification) was prepared in the same manner as in Example 1 except that the cobalt catalyst (before purification) was not washed at 773K.

(Reference example 1)
The cobalt catalyst (after purification) of Reference Example 1 was prepared in the same manner as in Example 1 except that the temperature of partial firing of the cobalt catalyst precursor was set to 973 K.

(Reference example 2)
The cobalt catalyst (after purification) of Reference Example 2 was prepared in the same manner as in Example 1 except that the temperature of partial firing of the cobalt catalyst precursor was set to 973 K and the cobalt catalyst (before purification) was not washed.

(Reference example 3)
The cobalt catalyst (before purification) is washed using poly (4-vinylpyridine) (manufactured by Sigma-Aldrich) having a weight average molecular weight of 15,000 instead of poly (4-vinylpyridine) having a weight average molecular weight of 60,000. A cobalt catalyst (after purification) of Reference Example 3 was prepared in the same manner as in Example 1 except that it was not present.

(Comparative Example 1)
When cleaning the cobalt catalyst (before purification), the cobalt catalyst of Comparative Example 1 (after purification) was prepared in the same manner as in Example 1 except that 0.5 M nitric acid was used instead of 0.1 M nitric acid. bottom.

(Comparative Example 2)
A cobalt catalyst (after purification) of Comparative Example 2 was prepared in the same manner as in Example 1 except that iron (II) nitrate was used instead of cobalt nitrate hexahydrate.

Examples 1-9, Reference Examples 1-3, the molar ratio of the complex portion with _{Co-N 4} C bonds to the total of cobalt loading amount of cobalt catalyst of Comparative Example 1 and 2 (after generation), the molar ratio of cobalt oxide, The molar ratio of metallic cobalt was determined by the following procedure. The spectrum near the cobalt K absorption edge X-ray absorption edge of the powder sample was measured, and the abundance ratio of the cobalt species was identified by linear combination fitting of the spectrum. The respective molar ratios are shown in Table 1 below.

An electrochemical reaction evaluation cell using the cobalt catalysts (after purification) of Examples 1 to 9, Reference Examples 1 to 3, and Comparative Examples 1 and 2 was prepared by the following procedure.

( _{Preparation of CO 2} reduction side electrode)
A catalyst ink was obtained by mixing 8 mg of a cobalt catalyst (after purification), 50 μL of a 10 wt% Nafion dispersion (manufactured by Sigma-Aldrich), and 400 μL of acetone, and stirring the mixture by ultrasonic irradiation. This was uniformly applied to the water-repellent layer side of carbon paper (GDL-25-BC, SGL GROUP) cut out to a diameter of 1.6 cm and a geometric surface area of 2 cm ^{2 in several portions.} The electrode was dried under reduced pressure for 15 minutes.

(Oxidation reaction side electrode fabrication)
8 g of Ketjen black powder carrying 20% by weight of Ir was mixed with 40 μL of 10% by weight Nafion solution and 400 μL of acetone, and supported on carbon paper in the same manner as in the previous section.

(Pretreatment of ion exchange membrane)
Cut Nafion (registered trademark) 117 (manufactured by Du pont Co., Ltd.) to a size required in the electrolysis cell, 1 hour with _3% H 2 _{O 2} in water, 1 hour with deionized water, in the order of 1 hour in 2N sulfuric acid Heat reflux was performed. Finally, it was heated to reflux with ion-exchanged water for 1 hour, and the obtained membrane was stored in ion-exchanged water.

(Making a membrane / electrode assembly)
The Nafion membrane treated in the above procedure was sandwiched between the catalysts on the reducing side and the oxidizing side, and hot-pressed at 413K and 50 MPa for 10 minutes to obtain a membrane-electrode assembly (MEA). After hot pressing, MEA was immersed in ion-exchanged water for 5 minutes.

(On the electrochemical reaction evaluation cell set)
The MEA produced as described above is contacted from both sides with a current collector in the shape of a perforated Teflon (registered trademark) plate (thickness 1 mm) wrapped in a gold mesh (Kubaa Co., Ltd., 99.9%). The anode chamber and the cathode chamber were separated by sealing with another Teflon frame.

Using the obtained electrochemical reaction evaluation cell, an electrochemical reaction was carried out according to the following procedure.

(Electrochemical reaction)
Pure water was immersed in the anode chamber, and the foot portion on which the chloride film was stretched was _{immersed in 0.5 MH 2} SO ₄ , and the Ag / AgCl reference electrode was connected via a saturated KCl solution as a liquid junction. _{CO 2} gas was supplied to the cathode chamber at 10 ml / min. Potentiostat (HZ-5000, manufactured by Hokuto Denko Co., Ltd.) was used for the electrochemical measurement.

The electrochemical reaction was carried out under −0.6 V vs SHE. The generated CO and H ₂ were evaluated according to the following procedure. The total Faraday efficiency FE (FE (CO) + FE (H ₂ )) was 100%. Table 1 shows the CO production rate and the CO production Faraday efficiency in each of Examples, Reference Examples, and Comparative Examples.

(Evaluation of product (CO))
The outlet gas from the cathode chamber was injected into an online gas chromatograph (GC-8APT, manufactured by Shimadzu Corporation, detector: TCD, column: MS-5A, detector temperature 210 ° C., analysis temperature 90 ° C.) for analysis. The CO production rate r (CO) [μmol / h / cm ² ] was calculated by _{adding the CO 2} gas supply rate and the electrode area to the obtained concentration. Considering that the reduction from CO ₂ to CO is a two-electron reaction, the CO production rate was converted to the CO production current density j (CO) according to the following equation.
j (CO) [mA / cm ² ] = r (CO) [μmol / h / cm ² ] x 96485 [C / mol] x 2 (number of reaction electrons) x 1/3600 [s / h]
The CO-generated Faraday efficiency was calculated as follows using the total current density j.
FE (CO) = j (CO) / j x 100

(Evaluation of product (H ₂₎₎
The outlet gas from the cathode chamber is separated by a gas tight syringe and injected into a gas chromatograph (GC-8APT, manufactured by Shimadzu Corporation, detector: TCD, column: activated carbon, detector temperature 150 ° C, analysis temperature 100 ° C). And analyzed. _{The CO production rate r (H 2} ) [μmol / h / cm ² ] was calculated by _{adding the CO 2} gas supply rate and the electrode area to the obtained concentration. _{The H 2} generation current density j (H ₂ ) and the H ₂ generation Faraday efficiency FE (H ₂ ) were calculated in the same manner as in the case of CO.

As shown in Table 1, in _{the cells using the catalysts of Examples 1 to 9 in which the molar ratio of the complex portion having the Co-N 4} C bond was 0.3 or more as the reducing electrode, the Faraday efficiency of CO production was high. rice field. Further, as in Examples 3 and 6, when compared with Reference Examples 1-3, the molar ratio of the complex portion with _{Co-N 4} C bond is 0.2 or more, and the molar ratio of the metal cobalt 0.5 If the following, it was suggested that the Faraday efficiency of CO production would be at a practical level.

FIG. 2 is a graph showing the relationship between the molar ratio of the complex portion having _{a Co-N 4 C bond and the Faraday efficiency of CO production.} Figure 3 is a graph showing the relationship between the faraday efficiency of the molar ratio and H ₂ generated in the cobalt oxide. FIG. 4 is a graph showing the relationship between the molar ratio of metallic cobalt and _{the Faraday efficiency of H 2 formation.} As shown in FIG. 2, a large change in the Faraday efficiency of CO production was observed near the molar ratio of 0.3 in the complex portion _{having a Co-N 4 C bond.} Further, as shown in FIG. 4, a large change in the Faraday efficiency of H ₂ generated in the vicinity of molar ratio 0.5 of the metal cobalt was observed.

10 CO ₂ reduction system 12 reduction electrode 14 oxidation electrode 16 power supply

Claims (7)

Porous carbon carrier showing conductivity and
The following general formula (1):

(In the formula, X is C or N, R _{1 to} R ₅ are independently H or alkyl groups, and _{at least one of R 1 to} R ₅ is an alkyl group, where X is N. in some cases, _{R 4} is _absent, at least one of alkyl groups of R 1 to R ₃ and _{R 5.)}
It has a compound represented by the above formula (1) and a repeating unit represented by the general formula (1), and in the general formula (1), _{one alkyl group from R 1 to} R ₅ is the main chain and has a complex aromaticity. Polymers whose ring is a side chain, and the following general formula (2):

(In the formula, R _{6 to} R ₁₃ are independently H or alkyl groups, and _{at least one of R 6 to} R ₁₃ is an alkyl group.)
A nitrogen-containing heteroaromatic ring compound selected from the group consisting of the compounds represented by the above carbon carriers and supported on the carbon carrier.
Cobalt ions coordinated with four nitrogen atoms on the heteroaromatic ring of the nitrogen-containing heteroaromatic ring compound,
Including
Or the molar ratio of the complex portion with Co-N 4 _C bonds to the total cobalt supported amount is 0.3 or more, or the total cobalt loading amount, the molar ratio of the complex portion with Co-N 4 _C bond An electrode catalyst for _{CO 2} reduction, which is 0.2 or more and has a molar ratio of metallic cobalt of 0.5 or less. The electrode catalyst for _{CO 2} reduction according to claim 1, wherein the molar ratio of cobalt oxide to the total supported amount of cobalt is 0.8 or less. The following general formula (1):

(In the formula, X is C or N, R _{1 to} R ₅ are independently H or alkyl groups, and _{at least one of R 1 to} R ₅ is an alkyl group, where X is N. in some cases, _{R 4} is _absent, at least one of alkyl groups of R 1 to R ₃ and _{R 5.)}
It has a compound represented by the above formula (1) and a repeating unit represented by the general formula (1), and in the general formula (1), _{one alkyl group from R 1 to} R ₅ is the main chain and has a complex aromaticity. Polymers whose ring is a side chain, and the following general formula (2):

(In the formula, R _{6 to} R ₁₃ are independently H or alkyl groups, and _{at least one of R 6 to} R ₁₃ is an alkyl group.)
A nitrogen-containing heteroaromatic ring compound selected from the group consisting of the compounds represented by is coordinated with cobalt ions, and the nitrogen-containing heteroaromatic ring compound coordinated with the cobalt ions is made of a porous material exhibiting conductivity. The method for producing an electrode catalyst for _{CO 2} reduction according to claim 1 or 2, wherein the compound is adsorbed on a carbon carrier and fired. The method for producing an electrode catalyst for _{CO 2} reduction according to claim 3, wherein the carbon carrier on which the nitrogen-containing heteroaromatic ring compound is adsorbed is calcined at a temperature of 600 K to 873 K. CO ₂ reduction electrode, characterized in that it comprises a CO ₂ reduction electrode catalyst according to claim 1 or 2. The CO ₂ reducing electrode according to claim 5 and
Oxidation electrode and
A power supply connected between the CO ₂ reduction electrode and the oxidation electrode,
_{A CO 2} reduction system characterized by containing. _{The CO 2} reduction system according to claim 6, wherein the energy efficiency of CO production is 63% or more at a CO production rate of 1 mmol / h / cm ^2.

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