JP7462261B2 - CO2 reduction electrode catalyst, method for manufacturing CO2 reduction electrode catalyst, CO2 reduction electrode, and CO2 reduction system - Google Patents

Description

Article 30, paragraph 2 of the Patent Act applies. Abstracts of the 2019 Electrochemical Autumn Meeting, https://confit. Atlas. jp/guide/event/ecsj2019f/proceedings/list (publication address), August 27, 2019 (publication date) [Publications, etc.] 2019 Electrochemical Autumn Meeting (meeting name), September 2019 6th (Event date)

The present invention relates to an electrode catalyst for CO ₂ reduction, a method for producing an electrode catalyst for CO ₂ reduction, a CO ₂ reduction electrode, and a CO ₂ reduction system.

One method of reducing carbon dioxide ( _CO2 ) emissions, which is considered to be the main cause of global climate change, is to use electricity derived from renewable energy sources to electrolyze _CO2 and water into reduction products such as carbon monoxide (CO) and oxygen, and then recycle the resulting products.

A technology has been proposed in which a _CO2 reduction catalyst including a porous metal layer containing gold and having a dendritic structure is used as a reduction electrode of an electrochemical reaction device that reduces _CO2 by electrolysis (see Patent Document 1).

JP 2017-57492 A

The reduction catalyst described in Patent Document 1 has excellent energy efficiency in CO generation, but has low efficiency per atom, and is expensive because it uses precious metals such as gold. As a result of extensive research into _CO2 reduction electrode catalysts, the present inventors have come up with a technology that can provide an inexpensive _CO2 reduction electrode catalyst that has excellent energy efficiency in CO generation, high efficiency per atom, and is inexpensive.

The present invention has been made in consideration of these circumstances, and one of its objectives is to provide a new electrode catalyst for _CO2 reduction that has excellent energy efficiency in CO production, high efficiency per atom, and is inexpensive.

（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 aspect of the present invention is an electrode catalyst for _CO2 reduction. This electrode catalyst for _CO2 reduction comprises a porous carbon support exhibiting electrical conductivity,
The following general formula (1):

(In the formula, X is C or N, R ₁ to R ₅ are each independently H or an alkyl group, 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.)
_A compound represented by the _following general formula (1):

(In the formula, R ₆ to R ₁₃ are each independently H or an alkyl group, and at least one of R ₆ to R ₁₃ is an alkyl group.)
A nitrogen-containing heteroaromatic ring compound supported on a carbon support, the nitrogen-containing heteroaromatic ring compound being selected from the group consisting of compounds represented by the formula:
a cobalt ion coordinated to four nitrogen atoms on a heteroaromatic ring of a nitrogen-containing heteroaromatic ring compound;
Including,
The molar ratio of the complex portion having the Co-N ₄ C bond to the total amount of supported cobalt is 0.3 or more, or the molar ratio of the complex portion having the Co-N ₄ C bond to the total amount of supported cobalt 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 a method for producing the above-mentioned _CO2 reduction electrode catalyst. This method comprises the steps of:

(In the formula, X is C or N, R ₁ to R ₅ are each independently H or an alkyl group, 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.)
_A compound represented by the following general formula ( ₂ ):

(In the formula, R ₆ to R ₁₃ are each independently H or an alkyl group, 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 compounds represented by the following formula (I) is coordinated to a cobalt ion, and the nitrogen-containing heteroaromatic ring compound coordinated to the cobalt ion is adsorbed onto a porous carbon support exhibiting electrical conductivity, followed by calcination.

Yet another embodiment of the present invention is a _CO2 reduction electrode. The _CO2 reduction electrode includes the above-mentioned _CO2 reduction electrode catalyst.

Yet another aspect of the present invention is a _CO2 reduction system. The _CO2 reduction system includes the _CO2 reduction electrode, an oxidation electrode, and a power source connected between the _CO2 reduction electrode and the oxidation electrode.

According to the present invention, it is possible to provide a new electrode catalyst for reducing CO ₂ that has excellent faradaic efficiency for producing CO and is inexpensive.

FIG. 1 is a schematic diagram showing a CO ₂ reduction system according to an embodiment. 1 is a graph showing the relationship between the molar ratio of a complex portion having a Co—N ₄ C bond and the faradaic efficiency of CO production. 1 is a graph showing the relationship between the molar ratio of cobalt oxide and the faradaic efficiency of _H2 production. 1 is a graph showing the relationship between the molar ratio of metallic cobalt and the faradaic efficiency of _H2 production.

The present invention will be described below based on preferred embodiments with reference to the drawings. The embodiments are illustrative and do not limit the invention, and all features and combinations 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 to facilitate explanation, and should not be interpreted as being limiting unless otherwise specified. In addition, some of the components that are not important for explaining the embodiments are omitted in each figure.

(Electrode catalyst for _CO2 reduction)
The electrode catalyst for reducing _CO2 according to the embodiment includes a conductive porous carbon support, a compound represented by the above general formula (1), a polymer having a repeating unit represented by the above general formula (1), in which one of R ₁ to R ₅ in general formula (1) is the main chain and a heteroaromatic ring is a side chain, and a compound represented by the above general formula (2), the electrode catalyst includes a nitrogen-containing heteroaromatic ring compound supported on the carbon support, and cobalt ions coordinated to four nitrogen atoms on the heteroaromatic ring of the nitrogen-containing heteroaromatic ring compound, and the molar ratio of the complex part having a Co-N ₄ C bond to the total amount of supported cobalt is 0.3 or more, or the molar ratio of the complex part having a Co-N ₄ C bond to the total amount of supported cobalt is 0.2 or more, and the molar ratio of metallic cobalt is 0.5 or less.

In the _CO2 reduction electrode catalyst according to the embodiment, the molar ratio of the complex part having a Co-N ₄ C bond to the total cobalt loading is 0.3 or more, or the molar ratio of the complex part having a Co-N ₄ C bond to the total cobalt loading is 0.2 or more, and the molar ratio of metallic cobalt is 0.5 or less. In the electrolytic reduction of _CO2 , in addition to the target substance CO, _H2 may be generated as a by-product on the reduction electrode side. The complex part having a Co-N ₄ C bond of the _CO2 reduction electrode catalyst according to the embodiment acts as an active site for the reaction of reducing _CO2 to CO. By setting the molar ratio of the complex part having a Co-N ₄ C bond to 0.3 or more, or the molar ratio of the complex part having a Co-N ₄ C bond to 0.2 or more, and the molar ratio of metallic cobalt to 0.5 or less, the CO ratio in the electrolytic reduction product can be increased, and the energy efficiency of CO generation can be made at a practical level. Furthermore, the CO ₂ reduction electrode catalyst according to the embodiment has a high efficiency per atom and can be manufactured inexpensively compared to conventional catalysts that use precious metals such as gold.

From the viewpoint of further increasing the energy efficiency of CO production, the molar ratio of the complex moiety having a Co-N ₄ C bond to the total amount of supported cobalt is preferably 0.3 or more, and more preferably 0.4 or more. Also, it is preferable that the molar ratio of the complex moiety having a Co-N ₄ C bond is 0.3 or more and the molar ratio of metallic cobalt is 0.4 or less, and it is more preferable that the molar ratio of the complex moiety having a Co-N ₄ C bond is 0.4 or more and the molar ratio of metallic cobalt is 0.2 or less, relative to the total amount of supported cobalt.

In addition, from the viewpoint of further increasing the energy efficiency of CO production, it is preferable that the molar ratio of cobalt oxide contained in the _CO2 reduction electrode catalyst to the total cobalt supported amount is 0.8 or less. Note that cobalt oxide is an inactive substance in the electrolytic reduction of _CO2 .

The carbon support constituting the electrode catalyst for _CO2 reduction is not particularly limited as long as it is conductive and porous and can support a nitrogen-containing heteroaromatic ring compound. Examples of the carbon support include ketjen black, acetylene black, carbon black, carbon nanotubes, graphene, and fullerene.

The compound represented by the above general formula (1) constituting the electrode catalyst for _CO2 reduction has a structure in which the pyridine ring (X = H) or pyrimidine ring (X = N) can freely move molecularly. In formula (1), R ₁ to R ₅ are each independently H or an alkyl group, 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 formula (1) can freely move molecularly, and their length is not limited.

The polymer having a repeating unit represented by the above general formula (1) constituting the electrode catalyst for reducing _CO2 is a polymer in which one of the alkyl groups R ₁ to R ₅ in the general formula (1) is the main chain and a heteroaromatic ring is the side chain. As described above, when X is N in the formula (1), R ₄ does not exist. In this case, the main chain of the polymer is, of course, one of the alkyl groups 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 in the side chain can freely move molecularly. A representative example of the polymer is poly-4-vinylpyridine.

In the nitrogen-containing heteroaromatic ring compound represented by the general formula (2) constituting the _CO2 reduction electrode catalyst, _R6 to _R13 in the general formula (2) are each independently H or an alkyl group. The alkyl groups of _R1 and _R2 may be linear or branched, and their length is not limited, as long as the bipyridine ring in the formula (2) can freely move. In the _CO2 reduction electrode catalyst according to the embodiment, a total of four nitrogen atoms in the bipyridine ring of two molecules of the compound represented by the formula (2) are coordinated to the cobalt ion.

In the _CO2 reduction electrode catalyst according to the embodiment, the distance between the coordinated nitrogen atom and the cobalt ion is preferably 2.0 Å to 3.0 Å, more preferably 2.0 Å to 2.5 Å, and even more preferably 2.0 Å to 2.2 Å. The nitrogen-containing heteroaromatic ring compound coordinated to the cobalt ion at such a distance is fixed on the carbon support, thereby further increasing the _CO2 reduction activity of cobalt.

(Method for producing electrode catalyst for _CO2 reduction)
In the method for producing the electrode catalyst for _CO2 reduction according to the present embodiment, a nitrogen-containing heteroaromatic ring compound selected from the group consisting of vinyl polymers having a pyridyl group in the side chain and compounds represented by the above general formula (1) is coordinated to a cobalt ion, and the nitrogen-containing heteroaromatic ring compound coordinated to the cobalt ion is adsorbed on a conductive porous carbon support and then calcined. Here, the nitrogen-containing heteroaromatic ring compound and the carbon support are the same as those in the embodiment of the electrode catalyst for _CO2 reduction described above.

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

The method of adsorbing the nitrogen-containing heteroaromatic ring compound coordinated to the cobalt ion onto the carbon support is not particularly limited, but can be carried out, for example, by adding the carbon support to the obtained cobalt complex solution and mixing. After adsorption, the solution can be removed, for example, by evaporation to dryness, to obtain the carbon support carrying the cobalt complex.

In the manufacturing method according to the present embodiment, the obtained carbon support carrying the cobalt complex is calcined. The calcination 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 _CO2 reduction activity of cobalt can be further increased. Calcination may be performed once, but from the viewpoint of more appropriately fixing the cobalt complex to the carbon support, the temperature may be changed and the calcination may be performed two or more times. The calcination 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 generated as by-products on the carbon support. In order to improve the CO generation rate per amount of cobalt, it is desirable to remove cobalt oxide from among these by-products and 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 acidic aqueous solutions include dilute nitric acid, dilute sulfuric acid, and dilute hydrochloric acid. Furthermore, the generation of metallic cobalt as a by-product can be suppressed by controlling the calcination temperature to the above-mentioned temperature range.

( _CO2 reduction electrode)
The _CO2 reduction electrode according to the embodiment includes the _CO2 reduction electrode catalyst according to the above embodiment. Since this _CO2 reduction electrode uses the above _CO2 reduction electrode catalyst, when used as a reduction electrode in a _CO2 reduction system, the energy efficiency of CO generation is high and the efficiency per atom is high. In addition, this _CO2 reduction electrode can be manufactured inexpensively.

The shape of the _CO2 reduction electrode is not particularly limited, and may be, for example, a plate, a mesh, a wire, a particle, a porous, a thin film, an island, or other various shapes. For example, the _CO2 reduction electrode can be formed by disposing the _CO2 reduction electrode catalyst on the surface of a substrate. An example of the substrate on which the _CO2 reduction electrode catalyst is disposed is carbon paper. In addition, for example, the _CO2 reduction electrode can be formed by molding the _CO2 reduction electrode catalyst itself.

( _CO2 reduction system)
1 is a schematic diagram showing a CO ₂ reduction system according to an embodiment. The CO ₂ reduction system 10 includes a reduction electrode 12, an oxidation electrode 14, a power source 16 connecting the reduction electrode 12 and the oxidation electrode 14, a reduction reaction unit 18 that houses the reduction electrode 12 and performs a CO ₂ reduction reaction, an oxidation reaction unit 20 that houses the oxidation electrode 14 and performs an H ₂ O oxidation reaction, and an ion exchange membrane 22 disposed between the reduction electrode 12 and the oxidation electrode 14. As described later, the CO ₂ reduction system 10 uses the CO ₂ reduction electrode catalyst according to the above embodiment in the reduction electrode 12, so that the energy efficiency of CO generation is high, the efficiency per atom is high, and it can be manufactured inexpensively.

The reduction electrode 12 is an electrode that reduces _CO2 to produce carbon compounds. The oxidation electrode 14 is an electrode that oxidizes water to produce oxygen and hydrogen ions. To cause a reduction reaction at the reduction electrode 12, the reduction electrode 12 is connected to the negative terminal of a power source 16. To cause an oxidation reaction at the oxidation electrode 14, the oxidation electrode 14 is connected to the positive terminal of the power source 16.

The reduction electrode 12 uses the _CO2 reduction electrode according to the above embodiment. 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 intermetallic compounds containing these metals, binary metal oxides such as iridium oxide, manganese oxide, nickel oxide, cobalt oxide, iron oxide, tin oxide, indium oxide, and ruthenium oxide, ternary metal oxides such as Ni-Co-O, Ni-Fe-O, La-Co-O, Ni-La-O, and Sr-Fe-O, quaternary metal oxides such as Pb-Ru-Ir-O and La-Sr-Co-O, and metal complexes such as Ru complexes and Fe complexes. The oxidation electrode 14 can be applied in various shapes such as a plate, mesh, wire, particle, porous, thin film, and island shape. The oxidation electrode 14 may be a composite electrode in which these materials are laminated on a substrate.

The reduction reaction section 18 is supplied with CO ₂ to be reduced by the reduction electrode 12. The CO ₂ to be reduced may be in the form of a gas or a solution containing CO _2. In the case of a solution, it is preferable to use a solution with a high absorption rate of CO _2. Examples of such solutions include aqueous solutions of 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. In addition, a solution containing CO ₂ may be prepared using an alcohol solvent such as methanol, ethanol, or acetone. The solution containing CO ₂ is preferably a solution containing a CO ₂ absorbent that increases the reduction potential of CO ₂ , has high ionic conductivity, and absorbs CO _2. Examples of such solutions include ionic liquids or aqueous solutions thereof that are composed of a salt of a cation such as an imidazolium ion or a pyridinium ion and an anion such as BF ₄ ^- or PF ₆ ^- and are in a liquid state over a wide temperature range. Other solutions include solutions of amines such as ethanolamine, imidazole, pyridine, etc., or aqueous solutions thereof. The amine may be any of primary amines, secondary amines, and tertiary amines.

The oxidation reaction section 20 is supplied with H ₂ O to be oxidized by the oxidation electrode 14. Pure water can be used as the H ₂ O to be oxidized. Instead of pure water, an aqueous solution containing any electrolyte can be used, and it is preferable to use an aqueous solution that promotes 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 any material that can move ions between the reduction electrode 12 and the oxidation electrode 14, and the type of 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 _CO2 reduction system 10 will be described. When a potential is applied from the power source 16 to the oxidation electrode 14, an oxidation reaction of _H2O occurs near the oxidation electrode 14, and oxygen ( _O2 ) and hydrogen ions (H ⁺ ) are generated. The H ⁺ generated on the oxidation electrode 14 side reaches the reduction electrode 12 through the ion exchange membrane 22. A reduction reaction of _CO2 occurs due to electrons ( ^e- ) supplied from the power source 26 to the reduction electrode 12 and H ⁺ that has moved to the reduction electrode 12. CO is generated by the reduction reaction of _CO2 . In the _CO2 reduction system 10, the energy efficiency of CO generation is preferably 63% or more at a CO generation rate of 1 mmol/h/ ^cm2 . Here, the energy efficiency of CO generation is calculated by dividing 1.33V, which is the theoretical potential of CO generation and _O2 generation, by the voltage applied between the reduction electrode and the oxidation electrode.

The configuration of the _CO2 reduction system shown in FIG. 1 is merely an example, and various modifications are possible as long as the system can perform a _CO2 reduction reaction using the _CO2 reduction electrode according to the above embodiment. For example, the _CO2 reduction system shown in FIG. 1 is arranged between the reduction electrode 12 and the oxidation electrode 14 so that the ion exchange membrane 22 is in contact with the reduction electrode 12 and the oxidation electrode 14, but the reduction electrode 12 and the oxidation electrode 14 may be separated from the ion exchange membrane. In addition, for example, an electrolyte flow path through which the electrolyte flows may be provided so that the reduction electrode 12 and the oxidation electrode 14 are in contact with the electrolyte. Various modifications other than these are possible.

The following describes examples of the present invention, but these examples are merely illustrative examples for the purpose of explaining the present invention in a suitable manner and do not limit the present invention in any way.

Example 1
The _CO2 reduction 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 with a cobalt concentration of 40 mM. 80 mg of poly(4-vinylpyridine) (weight average molecular weight 60,000, manufactured by Sigma-Aldrich) was dissolved in ethanol (special grade reagent, manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) to make the total volume 50 mL, and the solution was stirred for 20 minutes to thoroughly disperse the polymer. Next, 1.7 mL of cobalt solution was mixed and stirred for 15 minutes. The complex obtained here is referred to as a cobalt complex.

(Supporting cobalt complex on conductive porous material)
107.6 mg of Ketjen Black carrier (ECP, Lion) was mixed into the above solution, and then heated to 343 K while stirring on a hot plate to evaporate and dry. 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 Calcination of Cobalt Catalyst Precursor)
The cobalt catalyst precursor was placed in a flat-bottom quartz reactor and heat-treated under a helium flow at a heating rate of 25 K/min at 423 K for 1 hour and then at 673 K for 3 hours. This is referred to as cobalt catalyst (before purification).

(Washing of cobalt catalyst (before purification))
The cobalt catalyst (before purification) was added to 0.1 M diluted nitric acid and stirred at 300 rpm for 1 hour, then suction filtered using a hydrophilic membrane filter with a pore size of 0.1 μm and washed with 200 mL of ion-exchanged water until the pH reached 6 to 7. Then, it was washed three times with 20 mL of 2-propanol. The filtered product was dried under reduced pressure at 353 K for 1 hour. This is referred to as the 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 for partial calcination of the cobalt catalyst precursor was 723K.

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

Example 4
A cobalt catalyst (purified) of Example 4 was prepared in the same manner as in Example 1, except that the temperature for partial calcination of the cobalt catalyst precursor was 673 K and washing of the cobalt catalyst (before purification) was not performed.

Example 5
A cobalt catalyst (purified) of Example 5 was prepared in the same manner as in Example 1, except that the temperature for partial calcination of the cobalt catalyst precursor was 773 K and washing of the cobalt catalyst (before purification) was not performed.

Example 6
The cobalt catalyst (after purification) of Example 6 was prepared in the same manner as in Example 1, except that BP2000 (manufactured by Cabot Japan Co., Ltd.) was used instead of the Ketjen Black carrier, the temperature for partial calcination of the cobalt catalyst precursor was 673 K, and washing of the cobalt catalyst (before purification) was not performed.

(Example 7)
A cobalt catalyst (after purification) of Example 7 was prepared in the same manner as in Example 1, except that an acetylene black carrier (manufactured by Denka Co., Ltd.) was used instead of the Ketjen black carrier, the temperature for partial calcination of the cobalt catalyst precursor was set to 673 K, and washing of the cobalt catalyst (before purification) was not performed.

(Example 8)
The cobalt catalyst (purified) of Example 8 was prepared in the same manner as in Example 1, except that an active carbon support (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) was used instead of Ketjen black, the temperature for partial calcination of the cobalt catalyst precursor was set to 673 K, and washing of the cobalt catalyst (before purification) was not performed.

Example 9
A cobalt catalyst (purified) of Example 9 was prepared in the same manner as in Example 1, except that poly(4-vinylpyridine) having a weight average molecular weight of 81,500 (manufactured by Sigma-Aldrich) was used instead of poly(4-vinylpyridine) having a weight average molecular weight of 60,000, the temperature for partial calcination of the cobalt catalyst precursor was set to 773 K, and washing of the cobalt catalyst (before purification) was not performed.

(Reference Example 1)
A cobalt catalyst (after purification) of Reference Example 1 was prepared in the same manner as in Example 1, except that the temperature for partial calcination of the cobalt catalyst precursor was 973K.

(Reference Example 2)
A cobalt catalyst (purified) of Reference Example 2 was prepared in the same manner as in Example 1, except that the temperature for partial calcination of the cobalt catalyst precursor was 973 K and washing of the cobalt catalyst (before purification) was not performed.

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

(Comparative Example 1)
A cobalt catalyst (after purification) of Comparative Example 1 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 when washing the cobalt catalyst (before purification).

(Comparative Example 2)
A cobalt catalyst (purified) 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.

The molar ratios of the complex portion having a Co-N ₄ C bond, the molar ratio of cobalt oxide, and the molar ratio of metallic cobalt relative to the total amount of cobalt supported in the cobalt catalysts (after production) of Examples 1 to 9, Reference Examples 1 to 3, and Comparative Examples 1 and 2 were determined by the following procedure. The cobalt K-edge X-ray absorption edge vicinity spectrum 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.

Electrochemical reaction evaluation cells were produced using the cobalt catalysts (after purification) of Examples 1 to 9, Reference Examples 1 to 3, and Comparative Examples 1 and 2 by the following procedure.

(Preparation of _CO2 reduction side electrode)
8 mg of cobalt catalyst (purified), 50 μL of 10 wt% Nafion dispersion (Sigma-Aldrich), and 400 μL of acetone were mixed and stirred by ultrasonic irradiation to obtain a catalyst ink. This was uniformly applied in several portions to the water-repellent layer side of carbon paper (GDL-25-BC, SGL GROUP) cut into a size of 1.6 cm in diameter and ² cm2 in geometric surface area. This electrode was dried under reduced pressure for 15 minutes.

(Preparation of the oxidation reaction side electrode)
8 g of Ketjenblack powder carrying 20 wt % Ir was mixed with 40 μL of 10 wt % Nafion solution and 400 μL of acetone, and the mixture was carried on carbon paper in the same manner as in the previous section.

(Pretreatment of ion exchange membrane)
Nafion (registered trademark) 117 (manufactured by Du Pont) was cut to the size required for the electrolysis cell, and _heated and refluxed in 3% _H2O2 water for 1 hour, ion-exchanged water for 1 hour, and 2N sulfuric acid for 1 hour in that order. Finally, it was heated and refluxed in ion-exchanged water for 1 hour, and the obtained membrane was stored in ion-exchanged water.

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

(Electrochemical reaction evaluation cell assembly)
The MEA prepared as described above was brought into contact with a current collector on both sides, which was a perforated Teflon (registered trademark) plate (thickness 1 mm) wrapped in gold mesh (manufactured by Kureha Corporation, 99.9%), and the anode chamber and cathode chamber were isolated by sealing with another Teflon frame.

The electrochemical reaction was carried out using the obtained electrochemical reaction evaluation cell according to the following procedure.

(Electrochemical reaction)
The anode chamber was filled with pure water, the legs of the Nafion membrane were immersed in _{0.5M H2SO4} , and the Ag/AgCl reference electrode was connected via a saturated KCl solution as a liquid junction. _CO2 gas was supplied to the cathode chamber at 10 ml/min. A potentiostat (HZ-5000, Hokuto Denko Corporation) was used for electrochemical measurements.

An electrochemical reaction was carried out under -0.6 V vs. SHE. The generated CO and _H2 were evaluated according to the following procedure. The total faradaic efficiency FE (FE(CO)+FE( _H2 )) was 100%. The CO generation rate and the CO generation faradaic efficiency in each of the examples, reference examples, and comparative examples are shown in Table 1.

(Evaluation of Product (CO))
The outlet gas from the cathode chamber was injected into an online gas chromatograph (GC-8APT, Shimadzu Corporation, detector: TCD, column: MS-5A, detector temperature 210°C, analysis temperature 90°C) and analyzed. The CO production rate r(CO) [μmol/h/ ^cm2 ] was calculated from the obtained concentration, taking into account the _CO2 gas supply rate and electrode area. Considering that the reduction of _CO2 to CO is a two-electron reaction, the CO production rate was converted into the CO production current density j(CO) according to the following formula.
j(CO) [mA/ ^cm2 ] = r(CO) [μmol/h/ ^cm2 ] x 96485 [C/mol] x 2 (number of reaction electrons) x 1/3600 [s/h]
The faradaic efficiency of CO production was calculated using the total current density j as follows:
FE(CO)=j(CO)/j×100

(Evaluation of Product (H ₂ ))
The outlet gas from the cathode chamber was collected using a gas-tight syringe and injected into a gas chromatograph (GC-8APT, Shimadzu Corporation, detector: TCD, column: Active carbon, detector temperature 150°C, analysis temperature 100°C) for analysis. The CO production rate r( _H2 ) [μmol/h/ ^cm2 ] was calculated from the obtained concentration, taking into account the _CO2 gas supply rate and electrode area. The _H2 production current density j( _H2 ) and _H2 production Faraday efficiency FE( _H2 ) were calculated in the same manner as in the case of CO.

As shown in Table 1, the faradaic efficiency of CO production was high in the cells using as the reduction electrode the catalysts of Examples 1 to 9 in which the molar ratio of the complex portion having a Co-N ₄ C bond was 0.3 or more. Furthermore, a comparison of Examples 3 and 6 with Reference Examples 1 to 3 suggests that the faradaic efficiency of CO production will be at a practical level if 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 0.5 or less.

Fig. 2 is a graph showing the relationship between the molar ratio of the complex part having a Co-N ₄ C bond and the faradaic efficiency of CO generation. Fig. 3 is a graph showing the relationship between the molar ratio of cobalt oxide and the faradaic efficiency of H ₂ generation. Fig. 4 is a graph showing the relationship between the molar ratio of metallic cobalt and the faradaic efficiency of H ₂ generation. As shown in Fig. 2, a large change in the faradaic efficiency of CO generation was observed when the molar ratio of the complex part having a Co-N ₄ C bond was around 0.3. Also, as shown in Fig. 4, a large change in the faradaic efficiency of H ₂ generation was observed when the molar ratio of metallic cobalt was around 0.5.

10 _CO2 reduction system 12 Reduction electrode 14 Oxidation electrode 16 Power source

Claims (7)

A porous carbon support exhibiting electrical conductivity;
The following general formula (1):

(In the formula, X is C or N, R ₁ to R ₅ are each independently H or an alkyl group, 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.)
_A compound represented by the following general formula ( ₂ ):

(In the formula, R ₆ to R ₁₃ are each independently H or an alkyl group, and at least one of R ₆ to R ₁₃ is an alkyl group.)
A nitrogen-containing heteroaromatic ring compound supported on the carbon support, the nitrogen-containing heteroaromatic ring compound being selected from the group consisting of compounds represented by the formula:
a cobalt ion coordinated to four nitrogen atoms on a heteroaromatic ring of the nitrogen-containing heteroaromatic ring compound;
Including,
An electrode catalyst for reducing CO2, characterized in that the molar ratio of the complex portion having a Co-N ₄ C bond to the total amount of supported cobalt is 0.3 or more, or the molar ratio of the complex portion having a Co-N ₄ C bond to the total amount of supported cobalt is _0.2 or more and the molar ratio of metallic cobalt is 0.5 or less. 2. The electrode catalyst for reducing CO ₂ according to claim 1, characterized in that the molar ratio of cobalt oxide to the total amount of supported 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 each independently H or an alkyl group, 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.)
_A compound represented by the following general formula ( ₂ ):

(In the formula, R ₆ to R ₁₃ are each independently H or an alkyl group, and at least one of R ₆ to R ₁₃ is an alkyl group.)
The method for producing an electrode catalyst for CO2 reduction according to claim 1 or 2, characterized in that a nitrogen-containing heteroaromatic ring compound selected from the group consisting of compounds represented by the following formula (I) is coordinated to a cobalt ion, and the nitrogen-containing heteroaromatic ring compound coordinated to the _cobalt ion is adsorbed onto a porous carbon support exhibiting electrical conductivity, and then calcined. The method for producing an electrode catalyst for reducing CO ₂ according to claim 3, characterized in that the carbon support having the nitrogen-containing heteroaromatic ring compound adsorbed thereon is calcined at a temperature of 600K to 873K. A _CO2 reduction electrode comprising the electrode catalyst for _CO2 reduction according to claim 1 or 2. The _CO2 reduction electrode according to claim 5,
An oxidation electrode;
A power source connected between the _CO2 reduction electrode and the oxidation electrode;
A _CO2 reduction system comprising: The _CO2 reduction system according to claim 6, characterized in that the energy efficiency of CO production is 63% or more at a CO production rate of 1 mmol/h/ ^cm2 .

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