Electrode catalyst

Abstract

To provide an electrode catalyst with a high Faraday effect even in a high current density of 200 mA / cm2 when a carbon dioxide is reduced to a carbon monoxide.SOLUTION: An electrode catalyst of the present invention comprises Ag-Cu particles containing 96.5-99.0 atomic percent Ag and 1.0-3.5 atomic percent Cu. The Ag(CN) exists on the surface of the Ag-Cu particles to reduce a carbon dioxide to a carbon monoxide. The electrode catalyst may be carried with a gas diffusion electrode.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to an electrode catalyst.

Background Art

[0002] Carbon dioxide reduction technology has attracted attention. As carbon dioxide reduction technologies, there are methods for reducing carbon dioxide biologically and electrochemically. Examples of the method for biologically reducing carbon dioxide include a method of absorbing carbon dioxide by afforestation or the like, or a biochemical reduction and fixation method using microorganisms. Examples of the method for electrochemically reducing carbon dioxide include a method of reducing with hydrogen using a catalyst.

[0003] In the method such as afforestation, a lot of time is required for reducing carbon dioxide. In the biochemical reduction and fixation method using microorganisms, a great deal of energy is required to purify the hydrocarbons produced by the microorganisms. In the method of reducing carbon dioxide with hydrogen using a catalyst, carbon dioxide is generated when synthesizing hydrogen from fossil fuels.

[0004] To solve such problems, for example, Patent Document 1 discloses an electrode catalyst in which silver nanoparticles are supported on a gas diffusion electrode and electrochemically reduce carbon dioxide to carbon monoxide.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] In the electrode catalyst disclosed in Patent Document 1, 200 mA / cm 2Reducing carbon dioxide to carbon monoxide at a high current density such as has been proposed. However, in the electrode catalyst disclosed in Patent Document 1, the Faraday efficiency when reducing carbon dioxide to carbon monoxide remains at 66%.

[0007] The present disclosure has been made to solve the above problems. The present disclosure aims to provide an electrode catalyst with a high Faraday efficiency when reducing carbon dioxide to carbon monoxide even at a high current density such as 200 mA / cm 2 .

Means for Solving the Problems

[0008] The present inventors have intensively studied to achieve the above object and completed the electrode catalyst of the present disclosure. The electrode catalyst of the present disclosure includes the following aspects. 〈Aspect 1〉An electrode catalyst comprising Ag-Cu particles containing 96.5 to 99.0 atomic% of Ag and 1.0 to 3.5 atomic% of Cu, and Ag(CN) is present on the surface of the Ag-Cu particles to reduce carbon dioxide to carbon monoxide. 〈Aspect 2〉The electrode catalyst according to Aspect 1, wherein the electrode catalyst is supported on a gas diffusion electrode.

Advantages of the Invention

[0009] According to the present disclosure, due to the presence of Ag(CN) on the surface of the Ag-Cu particles, it is possible to provide an electrode catalyst that can reduce carbon dioxide to carbon monoxide with a high Faraday efficiency even at a high current density such as 200 mA / cm 2 .

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Figure 3

[0011] Hereinafter, embodiments of the electrode catalyst of the present disclosure will be described. Note that the embodiments shown below do not limit the electrode catalyst of the present disclosure.

[0012] Although not bound by theory, regarding the reason why the electrode catalyst of the present disclosure can obtain a high Faraday efficiency even at a high current density such as 200 mA / cm 2 the findings obtained by the present inventors will be described.

[0013] When reducing carbon dioxide to carbon monoxide at a high current density such as 200 mA / cm 2 hydrogen generation, which is a competing reaction, cannot basically be completely avoided. In the electrode catalyst disclosed in Patent Document 1, it is considered that a large amount of hydrogen generation, which is a competing reaction, occurs, and as a result, the Faraday efficiency decreases.

[0014] When carbon dioxide is reduced, the selectivity of the reduction product is determined by the adsorption energy to the electrode catalyst. When reducing carbon dioxide to produce carbon monoxide, if the adsorption energy of the carbon monoxide intermediate to the electrode catalyst is excessively high, the electrode catalyst is poisoned by carbon monoxide. On the other hand, if the adsorption energy of the carbon monoxide intermediate to the electrode catalyst is excessively low, the reduction reaction does not proceed. In particular, when reducing carbon dioxide to carbon monoxide at a high current density such as 200 mA / cm 2 the overvoltage increases, so the adsorption energy of the carbon monoxide intermediate to the electrode catalyst needs to be more appropriate.

[0015] Under the above circumstances, the present inventors have found that in an electrode catalyst in which Ag(CN) is present on the surface of Ag-Cu particles, the adsorption energy of the carbon monoxide intermediate to the electrode catalyst becomes appropriate. And when using such an electrode catalyst, 200 mA / cm 2The inventors have found that even when reducing carbon dioxide to carbon monoxide at a high current density as described above, hydrogen generation, which is a competing reaction, can be suppressed.

[0016] The constituent requirements of the electrode catalyst of the present disclosure, which have been completed based on the findings described so far, will be described below.

[0017] 《Electrode Catalyst》 In the electrode catalyst of the present disclosure, Ag(CN) exists on the surface of Ag-Cu particles. Carbon dioxide (CO2) is brought into contact with this electrode catalyst to reduce carbon dioxide (CO2) to carbon monoxide (CO). Hereinafter, Ag-Cu particles and Ag(CN) will be described. Ag means silver, Cu means copper, C means carbon, N means nitrogen, and O means oxygen.

[0018] 〈Ag-Cu Particles〉 Ag-Cu particles contain Ag in an amount of 96.5 atomic% or more, 97.0 atomic% or more, or 97.5 atomic% or more, and 99.0 atomic% or less, 98.5 atomic% or less, or 98.0 atomic% or less with respect to the Ag-Cu particles. When the Ag-Cu particles contain 96.5 atomic% or more of Ag, it is advantageous for the Ag-Cu particles to function as an electrode catalyst for reducing carbon dioxide to carbon monoxide. When the content of Ag in the Ag-Cu particles is 99.0 atomic% or less, Cu can be contained in the Ag-Cu particles as described below.

[0019] Ag-Cu particles contain Cu in an amount of 1.0 atomic% or more, 1.5 atomic% or more, or 2.0 atomic% or more, and 3.5 atomic% or less, 3.0 atomic% or less, or 2.5 atomic% or less with respect to the Ag-Cu particles. Since the electrode catalyst of the present disclosure electrochemically reduces carbon dioxide, when the Ag-Cu particles contain 1.0 atomic% or more of Cu, it is advantageous for optimizing the conductivity. If the content of Cu in the Ag-Cu particles is 3.0 atomic% or less, in the Ag-Cu particles, Cu is dissolved in Ag, and a decrease in the catalytic function of reducing carbon dioxide to carbon monoxide can be advantageously suppressed. The Ag-Cu particles may contain inevitable impurities in addition to Ag and Cu.

[0020] As described above, in the Ag-Cu particles, Cu is dissolved in Ag in an interstitial and / or substitutional solid solution state, and may have an α-phase. Therefore, the Ag-Cu particles have a crystal structure similar to that of Ag. The α-phase is a phase in the Ag-Cu binary system that can dissolve up to 14.1 atomic% (8.8 mass%) at most, and is also called the α-Ag phase.

[0021] There is no particular limitation on the particle size of the Ag-Cu particles, but it is preferably 50 μm or more, 60 μm or more, 70 μm or more, 80 μm or more, or 90 μm or more, and 200 μm or less, 150 μm or less, or 100 μm or less. If the particle size of the Ag-Cu particles is 50 μm or more, the Ag-Cu particles are less likely to aggregate. On the other hand, if the particle size of the Ag-Cu particles is 200 μm or less, it is advantageous for the Ag-Cu particles to contact carbon dioxide. The particle size is the equivalent circle diameter when observing the Ag-Cu particle size by SEM, and is denoted as D 50 which means.

[0022] 〈Ag(CN)〉 Ag(CN) exists on the surface of the Ag-Cu particles. Ag(CN) is derived from C (carbon) and N (nitrogen) contained in the atmosphere gas or the like when the Ag-Cu particles are generated. Ag(CN) may be a cyanide compound, or may be a state in which Ag (silver), C (carbon), and N (nitrogen) are slightly bonded, and in that case, it is represented by Ag-C-N.

[0023] The amount of Ag(CN) present on the surface of the Ag-Cu particles is trace. The amount of its existence can be specified by the peak height when performing XRD analysis on the Ag-Cu particles having Ag(CN) present on the surface. Specifically, the maximum peak height of Ag(CN) is 3.50 to 4.50% with respect to the maximum peak height of the α-phase.

[0024] If the maximum peak height of Ag(CN) is 3.50% or more with respect to the maximum peak height of the α-phase, the function of reducing carbon dioxide to carbon monoxide can be practically recognized. If the maximum peak height of Ag(CN) is 4.50% or less, it is possible to suppress the reduction of the function of reducing carbon dioxide to carbon monoxide by affecting the crystal structure of the α-phase.

[0025] 〈Gas Diffusion Electrode〉 The electrode catalyst of the present disclosure is not particularly limited in its form as long as it can contact carbon dioxide, but typically it is supported on a gas diffusion electrode. A gas diffusion electrode means an electrode in which three states of solid, liquid, and gas are in contact with each other. The electrode catalyst supported on the gas diffusion electrode has a catalytic function for the electrochemical reaction between the liquid phase and the gas phase.

[0026] The gas diffusion electrode includes a conductive support. The conductive support is typically carbon paper, but is not limited thereto, and may be a metal porous body, an oxide porous body, a semiconductor porous body, or the like. The gas diffusion electrode is obtained by mixing Ag-Cu particles having Ag(CN) on the surface in an organic solvent, dropping the mixed solution onto the aforementioned conductive support (carrier), and drying it. This is used as the working electrode.

[0027] The loading amount of the electrode catalyst may be appropriately determined in consideration of the reduction conditions of carbon dioxide and the like. The loading amount is, for example, 0.1 mg / cm 2 or more, 0.3 mg / cm 2 or more, 0.5 mg / cm 2 or more, 1.0 mg / cm 2 or more, or 1.5 mg / cm 2 or more, and may be 5.0 mg / cm 2 or less, 4.0 mg / cm 2 or less, 3.0 mg / cm 2 or less, 2.5 mg / cm 2 or less, or 2.0 mg / cm 2 or less.

[0028] Regarding the electrode catalyst of the present disclosure, when carbon dioxide is brought into contact with the electrode catalyst of the present disclosure, carbon dioxide can be reduced to carbon monoxide. Typically, the electrode catalyst supported on the gas diffusion electrode is brought into contact with carbon dioxide to reduce carbon dioxide to carbon monoxide. This will be described together with the apparatus, but is not limited thereto.

[0029] FIG. 1 is an explanatory diagram schematically showing an overview of an example of an electrochemical reduction apparatus. Hereinafter, this electrochemical reduction apparatus will be described.

[0030] The electrochemical reduction apparatus 100 includes an anode chamber 10, a cathode chamber 20, and a gas chamber 30. The anode chamber 10 and the cathode chamber 20 are connected via a separator 50. The cathode chamber 20 and the gas chamber 30 are connected via a working electrode 60. The working electrode 60 is a gas diffusion electrode on which an electrode catalyst is supported, and the electrode catalyst is Ag—Cu particles on which Ag(CN) is present on the surface.

[0031] The inside of the anode chamber 10 and the inside of the cathode chamber 20 are filled with an electrolyte solution 70. In the anode chamber 10, a counter electrode 80 is installed so that a part thereof is immersed in the electrolyte solution 70. In the cathode chamber 20, a reference electrode 90 is installed so that a part thereof is immersed in the electrolyte solution 70. By the reference electrode 90, a desired potential can be accurately applied to the working electrode 60.

[0032] In the gas chamber 30, an inlet pipe 40 and an outlet pipe 42 are installed. Carbon dioxide gas is introduced into the inside of the gas chamber 30 from the inlet pipe 40. The carbon dioxide gas introduced into the inside of the gas chamber 30 comes into contact with the working electrode 60, and the working electrode 60 functions as an electrode catalyst to promote the reduction of the carbon dioxide gas. Then, the carbon monoxide gas obtained by the reduction of the carbon dioxide gas is discharged from the outlet pipe 42.

[0033] When reducing carbon dioxide with the apparatus shown in FIG. 1, a gas diffusion electrode on which an electrode catalyst is supported is installed as the working electrode 60. Hereinafter, the working electrode 60, the counter electrode 80, the reference electrode 90, the separator 50, and the electrolyte solution 70 will be described.

[0034] As the working electrode 60, a gas diffusion electrode (substrate) through which the electrolytic solution 70 does not pass and through which carbon dioxide and reduction products pass, and on which Ag-Cu particles having Ag(CN) on the surface are supported, is used. The gas diffusion electrode (substrate) used for the working electrode 60 is as described above.

[0035] The counter electrode 80 is not particularly limited as long as it does not prevent the efficient progress of the reduction of carbon dioxide. Typically, a platinum mesh is used.

[0036] The reference electrode 90 only needs to be able to accurately measure the potential of the working electrode 60. Examples of the material contained in the reference electrode 90 include Hg / Hg2Cl2, Ag / AgCl, and / or reversible hydrogen, etc. Ag / AgCl is particularly preferable.

[0037] As the separator 50, well-known materials can be used. Examples of the material contained in the separator 50 include Nafion, glass filter, ceramion, and / or Zirfon, etc. Nafion is particularly preferable. Typically, a Nafion membrane is used as the separator 50.

[0038] As the electrolytic solution 70, well-known electrolytic solutions can be used. Examples include KHCO3, HClO4, H2SO4, HCl, and / or HNO3, etc. KHCO3 is preferable.

[0039] The reduction conditions of carbon dioxide may be appropriately determined according to the combination of the working electrode 60, the counter electrode 80, the reference electrode 90, the separator 50, and the electrolytic solution 70, etc. The potential of the working electrode 60 may be, for example, in the range of 0V vs. RHE to -3.5V vs. RHE. The current density may be, for example, with respect to the working electrode, -800 mA / cm 2 or more, -700 mA / cm 2 or more, or -600 mA / cm 2 or more, and may be -500 mA / cm 2 or less, -400 mA / cm 2 or less, -300 mA / cm2 or less, or -200 mA / cm 2 may be. The supply rate of carbon dioxide gas is, for example, 5 cm 3 / min or more, 10 cm 3 / min or more, or 50 cm 3 / min or more, and may be 200 cm 3 / min or less, 150 cm 3 / min or less, or 100 cm 3 / min or less.

[0040] <Modification> The electrode catalyst of the present disclosure can be appropriately modified within the scope of the content described in the claims. For example, the electrode catalyst of the present disclosure only needs to include Ag-Cu particles having Ag(CN) on the surface, and does not exclude containing substances other than Ag-Cu particles having Ag(CN) on the surface as long as the effects of the electrode catalyst of the present disclosure are not impaired.

Examples

[0041] Hereinafter, the electrode catalyst of the present disclosure will be described more specifically with reference to Examples and Comparative Examples. Note that the electrode catalyst of the present disclosure is not limited to the conditions used in the following examples.

[0042] 《Preparation of Samples》 Each sample was prepared as follows.

[0043] 〈Example 1〉 Powder (particle size (D 50 ): 100 μm) manufactured by Sigma-Aldrich was prepared. This powder has Ag(CN) on the particle surface of Ag-Cu powder containing 97.5 atomic% of Ag and 2.5 atomic% of Cu. This can be confirmed from the fact that, in the XRD pattern (K α line) of this powder shown in FIG. 2, in addition to the α-phase peak, there is a peak of Ag(CN). The peak height of Al(CN) is 4.07% of the maximum peak height of the α-phase.

[0044] The above powder was mixed with isopropanol and Nafion 117 solution (manufactured by Sigma-Aldrich) and dispersed using an ultrasonic cleaner. Then, this dispersion solution was dropped onto carbon paper so that the total loading amount of the above particles became 0.5 mg / cm 2 and dried to obtain a working electrode. This working electrode was used as the sample of Example 1. The area of the carbon paper was 0.5 cm 2 .

[0045] <Example 2> A sample of Example 2 was prepared in the same manner as in Example 1, except that the dispersion solution of Example 1 was dropped onto carbon paper so that the total loading amount of the above particles became 1.0 mg / cm 2 .

[0046] <Example 3> A sample of Example 3 was prepared in the same manner as in Example 1, except that the dispersion solution of Example 1 was dropped onto carbon paper so that the total loading amount of the above particles became 1.5 mg / cm 2 .

[0047] <Example 4> A sample of Example 4 was prepared in the same manner as in Example 1, except that the dispersion solution of Example 1 was dropped onto carbon paper so that the total loading amount became 2.0 mg / cm 2 .

[0048] <Example 5> A sample of Example 5 was prepared in the same manner as in Example 1, except that the dispersion solution of Example 1 was dropped onto carbon paper so that the total loading amount of the above particles became 3.0 mg / cm 2 .

[0049] <Comparative Example 1> Powder (particle size (D 50 ): 90 μm) manufactured by MKnano was prepared. This powder is Ag-Cu particles containing 97.0 atomic% Ag and 3.0 atomic% Cu. There is no Ag(CN) on the surface of the powder particles. This is shown in Fig. 3, the XRD pattern (K αIn the (line), it can be confirmed that in addition to the peak of the α phase, there is no peak of Ag(CN).

[0050] The Au-Cu particles without Al(CN) on the surface of the powder particles described above were mixed with isopropanol and Nafion 117 solution (manufactured by Sigma-Aldrich) and dispersed using an ultrasonic cleaner. Then, this dispersion solution was dropped onto carbon paper so that the total loading amount of the Au-Cu particles without Al(CN) on the surface of the powder particles became 0.5 mg / cm 2 , and it was dried to obtain a working electrode. This working electrode was used as the sample of Comparative Example 1. The area of the carbon paper was 0.5 cm 2 .

[0051] 《Reduction of Carbon Dioxide》 Carbon dioxide was reduced using the electroreduction device shown in FIG. 1. A platinum mesh was used as the counter electrode, an Ag / AgCl electrode (saturated KCl) was used as the reference electrode, and 1 M KHCO3 was used as the electrolyte. The working electrodes of each of the above samples were installed in such an electroreduction device, and carbon dioxide gas was flowed through the gas compartment at 10 cm 3 / min. The current density was a constant current of -200 mA / cm 2 with respect to the working electrode. To obtain such a constant current, a potentiostat (HZ-7000 manufactured by Hokuto Denko Corporation) was used. The reduction time (electrolysis time) was 30 minutes.

[0052] 《Evaluation》 Qualitative and quantitative analyses were performed on the products obtained when carbon dioxide was reduced using the working electrodes of each sample using a gas chromatograph and an ion chromatograph. The selectivity of the reduction products was evaluated by the Faraday efficiency. The electrode catalysts of each sample were observed by TEM, and the particle size distribution was measured based on the TEM images.

[0053] The Faraday efficiency (FE) was calculated by the following formula (A). The Faraday efficiency also indicates how advantageous a specific reaction process is kinetically. FE = αnF / Q ···(A) Here, α is the number of electrons required for the formation of one molecule of the target product, n is the amount of the target product, F is the Faraday constant, and Q is the total charge.

[0054] The results are shown in Table 1.

[0055]

Table 1

[0056] When the samples of Examples 1 to 5 were used, it was confirmed that high Faradaic efficiency was obtained. As shown in the XRD pattern of Figure 2, the powders used in Examples 1 to 5 had Ag(CN) present on the surface of the powder particles, and it is considered that the presence of this Ag(CN) enabled the achievement of high Faradaic efficiency. When EDS analysis was performed on Ag(CN), it was confirmed that, on average of three measurement values, 10.9 atomic% of N was present and an equivalent amount of C was present. The presence of N and C was also confirmed by XPS analysis.

[0057] On the other hand, when the sample of Comparative Example 1 was used, since Ag(CN) was not present on the surface of the powder particles, it is considered that a large amount of hydrogen generation, which is a competing reaction, occurred.

[0058] From the above results, the effect of the electrode catalyst of the present disclosure was confirmed.

Explanation of Reference Numerals

[0059] 10 Anode chamber 20 Cathode chamber 30 Gas chamber 40 Inlet pipe 42 Outlet pipe 50 Separator 60 Working electrode 70 Electrolyte 80 Counter electrode 90 Reference electrode 100 Electrochemical reduction apparatus

Claims

1. An electrode catalyst comprising Ag-Cu particles containing 96.5 to 99.0 atomic% of Ag and 1.0 to 3.5 atomic% of Cu, and Ag(CN) is present on the surface of the Ag-Cu particles to reduce carbon dioxide to carbon monoxide. Electrode catalyst.

2. The electrode catalyst according to claim 1, wherein the electrode catalyst is supported on a gas diffusion electrode.

Images