Method of manufacturing acetic acid

Abstract

A method for producing acetic acid by reducing carbon dioxide in the atmosphere, comprising a process of immersing a hydrogen-absorbing alloy negative electrode used in a nickel-hydrogen battery in an alkaline electrolyte and then drying it, using an electrolytic cell equipped with an alkaline electrolyte capable of adsorbing carbon dioxide, a positive electrode used in a nickel-hydrogen battery, a hydrogen-absorbing alloy negative electrode after treatment, and a power source connected to the positive electrode and the negative electrode, and a process of reducing carbon dioxide by applying a potential difference to the positive electrode and the negative electrode such that the potential of the positive electrode is higher than the potential of the negative electrode, wherein the alkaline electrolyte equipped in the electrolytic cell is an aqueous K2CO3 solution.

Description

Technology Field

[0001] The present disclosure relates to a method for producing acetic acid. Background Technology

[0002] CO2 (i.e., carbon dioxide) is a greenhouse gas that naturally exists in the atmosphere and is produced by the combustion of fossil fuels. As human activities and energy demand increase, atmospheric CO2 levels are rising, which is believed to cause global warming. The removal of atmospheric CO2 is attracting the attention of many researchers worldwide. Various CO2 conversion methods, including chemical, photocatalytic, and electrochemical methods, are being widely studied. Among these methods, the electrochemical method is one that can be performed at room temperature.

[0003] A method for reducing CO2 using an electrochemical cell and extracting organic carboxylic acids (formic acid, acetic acid, oxalic acid, etc.) in situ, wherein a preferred cathode material for electrochemically reducing CO2 in an alkanolamine that has absorbed CO2 (specifically, monoethanolamine, etc.) to acetic acid is known to include copper, iron, silver, and their oxides or alloys (Patent Document 1).

[0004] It is known that by using a copper single crystal as the cathode, acetic acid and formic acid are produced by the electrolysis of an aqueous KHCO3 solution, and the production ratio of acetic acid to formic acid is 20% to 30% (Non-patent Literature 1).

[0005] It is known that formic acid and acetic acid are synthesized by reducing an aqueous KCl solution containing dissolved CO2 using a boron-doped diamond modified with copper and gold particles as the cathode and platinum as the counter electrode, and the ratio of the amount of acetic acid synthesized to the amount of formic acid synthesized, i.e., the selectivity ratio, was about 3% (Non-patent document 2). Prior art literature

[0006] Japanese Patent Publication No. 2021-516290 Selective Formation of C2 Compounds from Electrochemical Reduction of CO2at a Series of Copper Single Crystal Electrodes, J. Phys. Chem. B, 2002, 106(1), 15-17Electrochemical reduction of carbon dioxide to acetic acid on a Cu-Au modified boron-doped diamond electrode with a flow-cell system, RSC Advances, 2023, 13, 22061-22069 The problem to be solved

[0007] In conventional methods, copper is mainly used as the cathode for electrochemical reactions and expensive platinum is used as the anode, and the selectivity of the generated acetic acid is less than 1.

[0008] One embodiment of the present disclosure aims to provide a method for producing carbon-neutral acetic acid efficiently and with low environmental impact. means of solving the problem

[0009] The means for solving the problem include the following modes.

[0010] <1> A method for producing acetic acid by reducing carbon dioxide in the atmosphere, comprising: a process of immersing a hydrogen-absorbing alloy negative electrode used in a nickel-hydrogen battery in an alkaline electrolyte and then drying it; using an electrolytic cell equipped with an alkaline electrolyte capable of adsorbing carbon dioxide, a positive electrode used in a nickel-hydrogen battery, a hydrogen-absorbing alloy negative electrode after treatment, and a power source connected to the positive electrode and the negative electrode, and a process of reducing carbon dioxide by applying a potential difference to the positive electrode and the negative electrode such that the potential of the positive electrode is higher than the potential of the negative electrode, wherein the alkaline electrolyte equipped in the electrolytic cell is an aqueous K2CO3 solution.

[0011] <2> The hydrogen-absorbing alloy negative electrode comprises MmNi5, and the positive electrode comprises at least one of Ni(OH)2 and NiOOH. <1> The method for manufacturing acetic acid described in

[0012] <3> The hydrogen-absorbing alloy negative electrode after treatment has a diffraction peak in the range of 2θ = 32° to 33° in X-ray diffraction (XRD) measurements, <1> or <2> The method for manufacturing acetic acid described in

[0013] <4> Acetic acid and formic acid are produced, and the number of moles of acetic acid produced is greater than the number of moles of formic acid produced. <1> ~ <3> A method for manufacturing acetic acid as described in any one of the following. Effects of the invention

[0014] According to one embodiment of the present disclosure, a method is provided for producing carbon-neutral acetic acid efficiently and with low environmental load. Brief explanation of the drawing

[0015] Figure 1 is a graph of the X-ray diffraction results of each MmNi5-based hydrogen-absorbing alloy cathode in the examples and comparative examples. Specific details for implementing the invention

[0016] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0017] In the present disclosure, a numerical range indicated by "~" means a range that includes the values ​​listed before and after "~" as the minimum and maximum values, respectively.

[0018] In the numerical ranges described stepwise in the present disclosure, an upper or lower limit value described in any numerical range may be substituted with an upper or lower limit value of a numerical range described in another stepwise manner. In the numerical ranges described in the present disclosure, an upper or lower limit value described in any numerical range may be substituted with a value shown in the examples.

[0019] In the present disclosure, the term “process” is included not only in independent processes but also in cases where it cannot be clearly distinguished from other processes, provided that the intended purpose of the process is achieved.

[0020] In the present disclosure, a combination of two or more preferred embodiments is a more preferred embodiment.

[0021] In the present disclosure, the “anode” in an electrolytic reaction is also referred to as the “positive electrode,” and the “negative electrode” is also referred to as the “negative electrode.”

[0022] Method for manufacturing acetic acid

[0023] A method for producing acetic acid related to the present disclosure is a method for producing acetic acid by reducing carbon dioxide in the atmosphere, comprising a treatment process and a reduction process. In the treatment process, a hydrogen-absorbing alloy negative electrode used in a nickel-hydrogen battery is immersed in an alkaline electrolyte and then dried. In the reduction process, carbon dioxide is reduced by using an electrolytic cell and applying a potential difference to the positive electrode and the negative electrode by means of a power source such that the potential of the positive electrode becomes higher than the potential of the negative electrode. The electrolytic cell comprises an alkaline electrolyte, a positive electrode, a negative electrode, and a power source. The alkaline electrolyte is an aqueous K2CO3 solution capable of adsorbing carbon dioxide. The positive electrode and the negative electrode are each used in a nickel-hydrogen battery, and the negative electrode is pretreated by a treatment process before being used for electrolysis. The power source is connected to the positive electrode and the negative electrode.

[0024] (Processing process)

[0025] In the processing step, the hydrogen storage alloy negative electrode may be any negative electrode containing a hydrogen storage alloy used in nickel-hydrogen batteries (i.e., nickel-hydrogen secondary batteries (Ni-MH)). As for the hydrogen storage alloy, from the perspective of environmental burden, an AB5 type hydrogen storage alloy widely used in nickel-hydrogen batteries is preferred, a hydrogen storage alloy made from mischmetal is more preferred, and one containing MmNi5 is more preferred. The hydrogen storage alloy negative electrode may be one that is not yet used as a battery, or it may be a hydrogen storage alloy negative electrode included in a nickel-hydrogen battery after use.

[0026] As for the alkaline electrolyte, an alkaline electrolyte used in a nickel-hydrogen battery is preferred. The alkaline electrolyte may be one that has been used before being used in a nickel-hydrogen battery, or one that has been used after being used in a nickel-hydrogen battery. The alkaline electrolyte may be of a single type or a mixture of multiple types. The alkaline electrolyte may be of the same type as the alkaline electrolyte used in the electrolytic cell.

[0027] Examples of alkaline electrolytes include aqueous solutions of potassium carbonate (K2CO3), potassium hydroxide (KOH), potassium bicarbonate (KHCO3), sodium hydroxide (NaOH), sodium carbonate (Na2CO3), and lithium hydroxide (LiOH).

[0028] From the perspective of the selectivity of acetic acid, it is preferable that the alkaline electrolyte contains KOH, KHCO3, and K2CO3. Additionally, since an aqueous KOH solution becomes an aqueous K2CO3 solution or an aqueous potassium bicarbonate (KHCO3) solution by adsorbing CO2 from the atmosphere, an aqueous KOH solution brought into contact with the atmosphere may be used as the alkaline electrolyte.

[0029] The method of immersing the hydrogen storage alloy negative electrode in the alkaline electrolyte is not limited. It is sufficient if part or all of the hydrogen storage alloy negative electrode is immersed in the alkaline electrolyte. If the hydrogen storage alloy negative electrode is one contained in a used nickel-hydrogen battery, it is already immersed in the alkaline electrolyte, so the hydrogen storage alloy negative electrode extracted from the used nickel-hydrogen battery may be used as is.

[0030] The hydrogen-absorbing alloy anode immersed in the alkaline electrolyte is then dried. The drying does not have to be complete, such as the removal of all moisture, but is sufficient if the alkaline electrolyte attached to the hydrogen-absorbing alloy anode is concentrated.

[0031] The inventors have discovered that by using a dried hydrogen-absorbing alloy negative electrode to which an alkaline electrolyte is attached in an electrolytic cell, the resistance of the hydrogen-absorbing alloy negative electrode in the electrolyte is significantly reduced. Although the reason is not obvious, it is presumed that the surface of the hydrogen-absorbing alloy negative electrode becomes activated due to changes in the surface state, thereby selectively producing acetic acid during electrolysis. As one of the changes in the surface state, it is presumed that at least some of the components of the alkaline electrolyte exist on the surface as crystals. Accordingly, it is presumed that acetate ions are selectively formed during the CO2 reduction reaction by maintaining an environment such as localized strong alkali.

[0032] As for the drying temperature, a relatively low temperature is preferred from the perspective of the selectivity of acetic acid, and as an example, drying is performed in the atmosphere at 80°C for several hours. Drying is performed when the hydrogen-absorbing alloy negative electrode to which the alkaline electrolyte is attached is dried, and although not limited to this condition, it is presumed that crystals that are attached relatively uniformly are produced by drying at such a relatively low temperature.

[0033] Due to the treatment process, it is desirable for the hydrogen-absorbing alloy negative electrode after treatment to have a diffraction peak in the range of 2θ = 32° to 33° in X-ray diffraction (XRD) measurements. The diffraction peak in the range of 2θ = 32° to 33° is a diffraction peak that is newly included in the hydrogen-absorbing alloy negative electrode after treatment and does not exist in the X-ray diffraction (XRD) measurement results for the hydrogen-absorbing alloy negative electrode before treatment. It is presumed that due to the treatment process described above, crystals of KOH, a component of the alkaline electrolyte, exist on a part of the surface of the hydrogen-absorbing alloy negative electrode after treatment, and a new diffraction peak in the range of 2θ = 32° to 33° appears as a diffraction peak originating from these KOH crystals. Furthermore, the hydrogen-absorbing alloy negative electrode after treatment also has a peak caused by the hydrogen-absorbing alloy itself before treatment.

[0034] (Reduction process)

[0035] In the reduction process, an electrolytic cell is used. The alkaline electrolyte provided by the electrolytic cell is an aqueous solution capable of adsorbing carbon dioxide (CO2) and is an aqueous K2CO3 solution in terms of the selectivity of acetic acid. The alkaline electrolyte provided by the electrolytic cell may be an aqueous Na2CO3 solution, a mixture of an aqueous K2CO3 solution and an aqueous Na2CO3 solution, or an aqueous KOH solution. The alkaline electrolyte provided by the electrolytic cell may contain components other than K2CO3.

[0036] Potassium carbonate (K2CO3) reacts with atmospheric CO2 (400 ppm, 40 Pa) in the presence of water to produce potassium bicarbonate (KHCO3). The reaction equation is shown in Equation (2).

[0037] K2CO3+CO2+H2O←→2KHCO3(2)

[0038] In the van't Hoff equation shown in the following equation (3), ΔH 0 ε is the standard enthalpy change (-96.1 kJ / mol CO2), ΔS 0 ε is the standard entropy change (-208 J / Kmol CO2). P0 is atmospheric pressure (0.1 MPa), R is the gas constant, 8.314 JK -1 mol -1 , T is set to the temperature (298K). As a result, the pressure P is 0.112 Pa, and the equilibrium concentration of CO2 absorption is 1.12 ppm. Because of this, carbon dioxide (400 ppm) in the atmosphere can be absorbed, and potassium bicarbonate (KHCO3) is electrochemically used as a CO2 source. Afterwards, potassium bicarbonate (KHCO3) is regenerated into potassium carbonate (K2CO3).

[0039]

[0040] Since the KOH aqueous solution becomes an aqueous K2CO3 solution or a potassium bicarbonate (KHCO3) solution by adsorbing CO2 from the atmosphere, the KOH aqueous solution may be used in contact with the atmosphere. KOH absorbs CO2 (400 ppm) from the atmosphere and produces some potassium carbonate (K2CO3) and water. The reaction equation is shown in Equation (4).

[0041] 2KOH+CO2←→K2CO3+H2O (4)

[0042] In the van't Hoff equation shown in the above equation (3), ΔH 0 ΔS is the standard enthalpy change (-194 kJ / mol CO2). 0 ε is the standard entropy change (-151 J / Kmol CO2). P0 is atmospheric pressure (0.1 MPa), R is the gas constant, 8.314 JK -1 mol -1 , substitute the temperature (298K) for T. As a result, the pressure P is 0.112 Pa, and the equilibrium concentration of CO2 absorption is 7.92 × 10⁻⁶ -21 When the concentration becomes ppm, KOH can absorb carbon dioxide from the atmosphere. Since this K2CO3 is thermodynamically stable, the aforementioned aqueous K2CO3 solution is considered to be more effective in utilizing K2CO3 as a CO2 source compared to the aqueous KOH solution in terms of the selectivity of acetic acid, and is therefore preferred as an alkaline electrolyte. Furthermore, when using an aqueous KOH solution as the alkaline electrolyte in an electrolytic cell, electrolysis may be performed by increasing the potential difference, etc.

[0043] The alkaline electrolyte may be an electrolyte contained in a nickel-hydrogen battery (i.e., a nickel-hydrogen secondary battery (Ni-MH)). The electrolyte may be an electrolyte contained in a nickel-hydrogen battery before use or an electrolyte contained in a nickel-hydrogen battery after use. The alkaline electrolyte may contain trace amounts of elements or compounds other than those mentioned above.

[0044] As for the electrodes provided in the electrolytic cell, a positive electrode used in a nickel-hydrogen battery is utilized. It is preferable that the positive electrode comprises at least one of Ni(OH)2 and NiOOH. Elements other than Ni(OH)2 or NiOOH, such as Co and Fe, may be added to the positive electrode. The positive electrode may be the positive electrode included in the nickel-hydrogen battery before use or the positive electrode included in the nickel-hydrogen battery after use.

[0045] For the positive electrode, it is desirable to perform the same treatment process as the negative electrode in terms of the selectivity of acetic acid. The treatment for the positive electrode is the same as the treatment for the negative electrode.

[0046] As for the power source, it is sufficient if it is capable of performing electrolysis using an electrolytic cell, and is not limited thereto. Carbon dioxide is reduced by applying a potential difference to the positive electrode and the negative electrode using the power source, such that the potential of the positive electrode becomes higher than the potential of the negative electrode. As for the power source, it is preferable to use a power source that does not use fossil fuels, etc. This is because acetic acid, which serves as a raw material for fuel, can be produced from carbon dioxide in the atmosphere with less environmental burden.

[0047] The electrolytic cell performs electrolysis in a state where the alkaline electrolyte can come into contact with the atmosphere. Accordingly, CO2 adsorbed by the alkaline electrolyte can be electrochemically reduced. In the method for producing acetic acid disclosed herein, acetate ions are generated by the reduction of CO2, and it is believed that most of them exist as potassium acetate in the alkaline electrolyte. Acetic acid can be recovered by recovering the alkaline electrolyte containing potassium acetate and neutralizing the electrolyte.

[0048] In the method for producing acetic acid of the present disclosure, other ions such as formate ions may be generated in addition to acetate ions. In the method for producing acetic acid of the present disclosure, acetic acid and formic acid are produced, and it is preferable that the number of moles of acetic acid produced is greater than the number of moles of formic acid produced. It is more preferable that the number of moles of acetic acid produced is at least five times the number of moles of formic acid produced. In addition, oxygen is generated from the positive electrode by electrolysis. The generated oxygen may be released into the atmosphere or recovered and utilized.

[0049] The selectivity ratio S, which is the amount of acetic acid produced relative to the amount of formic acid produced, can be calculated from the measured amount of acetic acid produced Ma and the amount of formic acid produced Mf by the following equation (1). Additionally, the amount produced may be measured by mass.

[0050] S=Ma / Mf (1)

[0051] According to the method for producing acetic acid related to the present disclosure, acetic acid, which serves as a raw material for, for example, vinegar, polymers, and fuel ethanol, can be selectively produced from carbon dioxide in the atmosphere by electrolysis. The electrolytic cell used for electrolysis may utilize the electrodes and electrolyte of a used nickel-hydrogen battery. The method for producing acetic acid related to the present disclosure can be implemented, for example, by opening a used nickel-hydrogen battery, removing the negative electrode, applying it to a processing process, returning it, and applying a potential difference to both electrodes. Furthermore, even when using an electrolyte, positive electrode, or negative electrode that is not currently in use as a nickel-hydrogen battery, existing components can be utilized as they are. Accordingly, according to the method for producing acetic acid related to the present disclosure, carbon-neutral acetic acid can be produced efficiently and with low environmental impact.

[0052] [Example]

[0053] The present disclosure is described in more detail below by way of examples. The examples in this specification are illustrative and are not intended to be limiting. Those skilled in the art can appropriately modify, alter, or substitute materials, compositions, manufacturing methods, and objects of application without departing from the spirit and scope of the present invention. Furthermore, "Ni(OH)2 / NiOOH positive electrode" means that the positive electrode comprises at least one of Ni(OH)2 and NiOOH, and may comprise both.

[0054] (measurement method)

[0055] In the following examples and comparative examples, electrolysis was performed using an electrolytic cell under predetermined conditions, and then ion chromatography was performed to measure the amount of acetic acid produced Ma and the amount of formic acid produced Mf. From these production quantities, the selectivity ratio S, which is the amount of acetic acid produced relative to the amount of formic acid produced, was calculated by the above Equation (1). In addition, the production quantity was measured in mass. The measurement results and calculation results are shown in Table 1.

[0056] (Example 1)

[0057] A nickel-hydrogen battery was disassembled to extract a Ni(OH)2 / NiOOH positive electrode and an MmNi5-based hydrogen storage alloy negative electrode. As a processing step, these positive and negative electrodes were heat-treated in air at 80°C for 3 hours. The heat-treated positive electrode was used as the positive electrode and the heat-treated negative electrode as the negative electrode, and an electrolysis experiment with an aqueous K2CO3 solution was conducted under standby at 2V for 400 hours. The current ranged from 15mA to 6mA. Subsequently, ion chromatography analysis was performed on the electrolyte.

[0058] (Example 2)

[0059] A nickel-hydrogen battery was disassembled to extract a Ni(OH)2 / NiOOH positive electrode and an MmNi5-based hydrogen storage alloy negative electrode. As a processing step, these positive and negative electrodes were heat-treated in air at 80°C for 3 hours. The heat-treated positive electrode was used as the positive electrode and the heat-treated negative electrode as the negative electrode, and an electrolysis experiment with an aqueous K2CO3 solution was conducted under standby at 2V for 250 hours. The current ranged from 15mA to 7mA. Subsequently, ion chromatography analysis was performed on the electrolyte.

[0060] (Example 3)

[0061] A nickel-hydrogen battery was disassembled to extract a Ni(OH)2 / NiOOH positive electrode and an MmNi5-based hydrogen storage alloy negative electrode. As a processing step, these positive and negative electrodes were heat-treated in air at 80°C for 3 hours. The heat-treated positive electrode was used as the positive electrode and the heat-treated negative electrode as the negative electrode, and an electrolysis experiment with an aqueous K2CO3 solution was conducted under standby at 2V for 100 hours. The current ranged from 15mA to 8mA. Subsequently, ion chromatography analysis was performed on the electrolyte.

[0062] (Example 4)

[0063] A nickel-hydrogen battery was disassembled to extract a Ni(OH)2 / NiOOH positive electrode and an MmNi5-based hydrogen storage alloy negative electrode. The negative electrode was heat-treated in air at 80°C for 3 hours. The positive electrode was used as the anode and the heat-treated negative electrode as the cathode, and an electrolysis experiment with an aqueous K2CO3 solution was conducted under standby at 2V for 100 hours. The current ranged from 9mA to 4mA. Subsequently, ion chromatography analysis was performed on the electrolyte.

[0064] (Comparative Example 1)

[0065] A nickel-hydrogen battery was disassembled to extract a Ni(OH)2 / NiOOH positive electrode and an MmNi5-based hydrogen storage alloy negative electrode. The positive electrode was heat-treated in air at 80°C for 3 hours. The heat-treated positive electrode was used as the positive electrode and the negative electrode as the negative electrode, and an electrolysis experiment with an aqueous K2CO3 solution was conducted under standby at 2V for 100 hours. The current ranged from 7mA to 3mA. Subsequently, ion chromatography analysis was performed on the electrolyte.

[0066] (Comparative Example 2)

[0067] A nickel-hydrogen battery was disassembled to extract the Ni(OH)2 / NiOOH positive electrode and the MmNi5-based hydrogen storage alloy negative electrode. The Ni(OH)2 / NiOOH positive electrode was used as the positive electrode and the MmNi5-based hydrogen storage alloy negative electrode as the negative electrode, and an electrolysis experiment of an aqueous K2CO3 solution was conducted at 2V for 100 hours under standby. The current was 5mA to 2mA. Afterward, ion chromatography analysis was performed on the electrolyte.

[0068] (Comparative Example 3)

[0069] A nickel-hydrogen battery was disassembled to extract a Ni(OH)2 / NiOOH positive electrode and an MmNi5-based hydrogen storage alloy negative electrode. These positive and negative electrodes were heat-treated in air at 80°C for 3 hours. The heat-treated positive electrode was used as the positive electrode and the heat-treated negative electrode as the negative electrode, and an electrolysis experiment with an aqueous K2CO3 solution was conducted under nitrogen at 2V for 100 hours. The current ranged from 15mA to 8mA. Subsequently, ion chromatography analysis was performed on the electrolyte.

[0070] (Comparative Example 4)

[0071] A nickel-hydrogen battery was disassembled to extract a Ni(OH)2 / NiOOH positive electrode and an MmNi5-based hydrogen storage alloy negative electrode. These positive and negative electrodes were heat-treated in air at 80°C for 3 hours. The heat-treated positive electrode was used as the positive electrode and the heat-treated negative electrode as the negative electrode, and an electrolysis experiment with an aqueous KOH solution (6 mol / L) was conducted at 2V for 100 hours while waiting. The current was 15 mA to 8 mA. Afterward, ion chromatography analysis was performed on the electrolyte.

[0072] (Comparative Example 5)

[0073] A nickel-hydrogen battery was disassembled to extract a Ni(OH)2 / NiOOH positive electrode and an MmNi5-based hydrogen storage alloy negative electrode. These positive and negative electrodes were heat-treated in air at 80°C for 3 hours. The heat-treated positive electrode was used as the positive electrode and the heat-treated negative electrode as the negative electrode, and an electrolysis experiment with an aqueous KOH solution (6 mol / L) was conducted under nitrogen at 2V for 100 hours. The current was 15 mA to 8 mA. Afterward, ion chromatography analysis was performed on the electrolyte.

[0074]

[0075] As shown in Table 1, in Examples 1 to 3, the amount of acetic acid produced by electrolyzing an aqueous K2CO3 solution under atmospheric conditions using a heat-treated Ni(OH)2 / NiOOH as the anode and a heat-treated MmNi5-based hydrogen-absorbing alloy as the cathode was 11 mg / L to 120 mg / L, and the selectivity ratio (amount of acetic acid / amount of formic acid) was 5 to 40. In Example 4, even when an untreated Ni(OH)2 / NiOOH was used as the anode and a heat-treated MmNi5-based hydrogen-absorbing alloy as the cathode, acetic acid was produced by electrolyzing an aqueous K2CO3 solution under atmospheric conditions, and the selectivity ratio (amount of acetic acid / amount of formic acid) was 5 or higher. On the other hand, as shown in Comparative Example 1 or Comparative Example 2, when the MmNi5-based hydrogen-absorbing alloy was not heat-treated, almost no acetic acid was produced.

[0076] In Comparative Example 3, when a K2CO3 aqueous solution was electrolyzed under a nitrogen atmosphere using heat-treated Ni(OH)2 / NiOOH as the anode and a heat-treated MmNi5-based hydrogen-absorbing alloy as the cathode, the amount of acetic acid produced decreased significantly. On the other hand, as shown in Example 3, the amount of acetic acid produced increased in the atmosphere. From these, it was confirmed that potassium carbonate absorbs carbon dioxide from the atmosphere and converts it into acetic acid, as shown in Equation (2) above.

[0077] As shown in Comparative Example 4, when an aqueous KOH solution was used as the electrolyte, the amount of acetic acid produced was significantly reduced. In Comparative Example 5, when an aqueous KOH solution was electrolyzed under a nitrogen atmosphere using a heat-treated Ni(OH)2 / NiOOH as the anode and a heat-treated MmNi5-based hydrogen-absorbing alloy as the cathode, acetic acid was not produced because CO2 was not present.

[0078] (Example 5)

[0079] An MmNi5-based hydrogen storage alloy cathode extracted from a nickel-hydrogen battery was heat-treated in air at 80°C for 3 hours. The X-ray diffraction intensity curve of the heat-treated MmNi5-based hydrogen storage alloy cathode is shown in Fig. 1.

[0080] (Comparative Example 6)

[0081] Figure 1 shows the X-ray diffraction intensity curve of an MmNi5-based hydrogen storage alloy cathode that was removed from a nickel-hydrogen battery, cleaned, and not subjected to a treatment process.

[0082] In Example 5, by heat-treating the MmNi5-based alloy, a new peak occurred at 2θ=32°~33°, which was not present in Comparative Example 6. This peak is thought to be the crystallization of KOH in the electrolyte used in nickel-hydrogen batteries. Additionally, new peaks also exist at 12.9° and 25.7°, and these are also thought to be the crystallization of KOH in the electrolyte used in nickel-hydrogen batteries. Explanation of the symbols

[0083] X-ray diffraction results of 10 MmNi5-based hydrogen absorption alloy cathodes 11 X-ray diffraction intensity curve of heat-treated MmNi5-based hydrogen-absorbing alloy cathode 12 X-ray diffraction intensity curve of an MmNi5-based hydrogen-absorbing alloy cathode that has not undergone the treatment process

Claims

Claim 1 A method for producing acetic acid by reducing carbon dioxide in the atmosphere, comprising: a process of immersing a hydrogen-absorbing alloy negative electrode used in a nickel-hydrogen battery in an alkaline electrolyte and then drying it; using an electrolytic cell having an alkaline electrolyte capable of adsorbing carbon dioxide, a positive electrode used in a nickel-hydrogen battery, a hydrogen-absorbing alloy negative electrode after the above treatment, and a power source connected to the positive electrode and the negative electrode; and a process of reducing carbon dioxide by applying a potential difference to the positive electrode and the negative electrode such that the potential of the positive electrode becomes higher than the potential of the negative electrode, wherein the alkaline electrolyte provided in the electrolytic cell is an aqueous K2CO3 solution. Claim 2 A method for producing acetic acid according to claim 1, wherein the hydrogen absorption alloy negative electrode comprises MmNi5 and the positive electrode comprises at least one of Ni(OH)2 and NiOOH. Claim 3 A method for producing acetic acid according to claim 1, wherein the hydrogen-absorbing alloy negative electrode after treatment has a diffraction peak in the range of 2θ=32°~33° in X-ray diffraction (XRD) measurement. Claim 4 A method for producing acetic acid according to claim 1, wherein acetic acid and formic acid are produced, and the number of moles of acetic acid produced is greater than the number of moles of formic acid produced.

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