Method for producing acetic acid

Abstract

To provide a method for efficiently producing carbon-neutral acetic acid with low environmental impact. [Solution] A method for producing acetic acid by reducing carbon dioxide in the atmosphere, comprising the steps of: immersing a hydrogen storage alloy negative electrode used in a nickel-metal hydride 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-metal hydride battery, the treated hydrogen storage alloy negative electrode, and a power supply connected to the positive and negative electrodes, and reducing carbon dioxide by applying a potential difference to the positive electrode such that the potential of the positive electrode is higher than the potential of the negative electrode, wherein the alkaline electrolyte in the electrolytic cell is an aqueous K2CO3 solution.

Description

[Technical Field]

[0001] This disclosure relates to a method for producing acetic acid. [Background technology]

[0002] CO2 (i.e., carbon dioxide) is a greenhouse gas that exists naturally in the atmosphere and is produced by the combustion of fossil fuels. Increased human activity and energy demand are thought to be contributing to the increase in atmospheric CO2, leading to global warming. Removing CO2 from the atmosphere has attracted the attention of many researchers worldwide. Various CO2 conversion methods, including chemical, photocatalytic, and electrochemical methods, are being widely studied. Of these methods, electrochemical methods are one that can be performed at room temperature.

[0003] A method for in-situ extraction of organic carboxylic acids (such as formic acid, acetic acid, and oxalic acid) by reducing CO2 using an electrochemical cell is known, in which preferred cathode materials for electrochemically reducing CO2 in an alkanolamine (specifically, monoethanolamine, etc.) that has absorbed CO2 to acetic acid include copper, iron, silver, and their oxides or alloys (Patent Document 1). It is known that by using a copper single crystal as the cathode, acetic acid and formic acid can be produced by electrolysis of an aqueous KHCO3 solution, with the proportion of acetic acid produced being 20% ​​to 30% compared to formic acid (Non-Patent Literature 1). It is known that when boron-doped diamond modified with copper and gold particles is used as the cathode and platinum as the counter electrode, formic acid and acetic acid are synthesized by reduction of an aqueous KCl solution containing dissolved CO2, and the ratio of the amount of acetic acid synthesized to the amount of formic acid synthesized, i.e., the selectivity ratio, is about 3% (Non-Patent Literature 2). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Special Publication No. 2021-516290 [Non-patent literature]

[0005] [Non-Patent Document 1] Selective Formation of C2 Compounds from Electrochemical Reduction of CO2at a Series of Copper Single Crystal Electrodes, J. Phys. Chem. B, 2002, 106(1), 15-17 [Non-Patent Document 2] Electrochemical 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 [Overview of the project] [Problems that the invention aims to solve]

[0006] In conventional methods, copper is mainly used as the cathode and expensive platinum as the anode in the electrochemical reaction, and the selectivity ratio of the resulting acetic acid was less than 1.

[0007] One embodiment of this disclosure aims to provide a method for efficiently producing carbon-neutral acetic acid with low environmental impact. [Means for solving the problem]

[0008] The means for solving the problem include the following: <1> A method for producing acetic acid by reducing carbon dioxide in the atmosphere, comprising the steps of: immersing a hydrogen storage alloy negative electrode used in a nickel-metal hydride 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-metal hydride battery, the treated hydrogen storage alloy negative electrode, and a power supply connected to the positive and negative electrodes, reducing carbon dioxide by applying a potential difference to the positive electrode such that the potential of the positive electrode is higher than the potential of the negative electrode, wherein the alkaline electrolyte in the electrolytic cell is an aqueous K2CO3 solution. <2> The hydrogen storage alloy negative electrode contains MmNi5, and the positive electrode contains at least one of Ni(OH)2 and NiOOH. <1> A method for producing acetic acid as described above. <3> The treated hydrogen storage alloy anode exhibits diffraction peaks in the range of 2θ = 32° to 33° in X-ray diffraction (XRD) measurements. <1> or <2> A method for producing acetic acid as described above. <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 producing acetic acid as described in any one of the following. [Effects of the Invention]

[0009] According to one embodiment of this disclosure, a method for efficiently producing carbon-neutral acetic acid with low environmental impact is provided. [Brief explanation of the drawing]

[0010] [Figure 1] Figure 1 is a graph showing the X-ray diffraction results of each MmNi5-based hydrogen storage alloy cathode in the examples and comparative examples. [Modes for carrying out the invention]

[0011] The embodiments of this disclosure will be described below with reference to the drawings. In this disclosure, a numerical range indicated using "~" means a range that includes the numbers written before and after "~" as the minimum and maximum values, respectively. In the numerical ranges set forth stepwise in the present disclosure, an upper limit or a lower limit described for a certain numerical range may be replaced with an upper limit or a lower limit of another numerical range set forth stepwise. In the numerical ranges described in the present disclosure, an upper limit or a lower limit described for a certain numerical range may be replaced with the values shown in the working examples. In the present disclosure, the term "step" includes not only an independent step, but also a case that cannot be clearly distinguished from other steps, as long as the intended object of the step is achieved. In the present disclosure, a combination of two or more preferred embodiments is a more preferred embodiment. In the present disclosure, the "anode" in an electrolytic reaction is also referred to as the "positive electrode", and the "cathode" is also referred to as the "negative electrode".

[0012] <Method for Producing Acetic Acid> The method for producing acetic acid according to the present disclosure is a method for producing acetic acid by reducing carbon dioxide in the atmosphere, and comprises a treatment step and a reduction step. In the treatment step, a hydrogen storage alloy negative electrode used for a nickel-metal hydride battery is immersed in an alkaline electrolyte and then dried. In the reduction step, an electrolytic cell is used, and a power supply applies a potential difference between the positive electrode and the negative electrode such that the potential of the positive electrode is higher than the potential of the negative electrode, thereby reducing carbon dioxide. The electrolytic cell comprises an alkaline electrolyte, a positive electrode, a negative electrode, and the power supply. The alkaline electrolyte is a K2CO3 aqueous solution capable of adsorbing carbon dioxide. The positive electrode and the negative electrode are each used in a nickel-metal hydride battery, and the negative electrode is pretreated by the treatment step before being used for electrolysis. The power supply is connected to the positive electrode and the negative electrode.

[0013] (Treatment Step) In the treatment process, the hydrogen storage alloy negative electrode only needs to be a negative electrode containing a hydrogen storage alloy used in nickel-metal hydride batteries (that is, nickel-metal hydride secondary batteries (Ni-MH)). From the perspective of environmental impact, AB5-type hydrogen storage alloys widely used in nickel-metal hydride batteries are preferable as the hydrogen storage alloy; hydrogen storage alloys using mischmetal as a raw material are more preferable; and it is even more preferable that the hydrogen storage alloy contains MmNi5. The hydrogen storage alloy negative electrode may be one that has not been used as a battery, or may be a hydrogen storage alloy negative electrode contained in a used nickel-metal hydride battery.

[0014] As the alkaline electrolyte, an alkaline electrolyte used in nickel-metal hydride batteries is preferable. The alkaline electrolyte may be one that has not been used in a nickel-metal hydride battery, or may be an alkaline electrolyte that has been used in a nickel-metal hydride battery. The alkaline electrolyte may be of a single type, or may be a mixture of a plurality of types. The alkaline electrolyte may be the same type of electrolyte as the alkaline electrolyte used in the electrolytic cell.

[0015] Examples of the alkaline electrolyte include aqueous potassium carbonate (K2CO3) solution, aqueous potassium hydroxide (KOH) solution, aqueous potassium bicarbonate (KHCO3) solution, aqueous sodium hydroxide (NaOH) solution, aqueous sodium carbonate (Na2CO3) solution, and aqueous lithium hydroxide (LiOH) solution.

[0016] From the viewpoint of the selectivity of acetic acid, the alkaline electrolyte preferably contains KOH, KHCO3, and K2CO3. Note that an aqueous KOH solution becomes an aqueous K2CO3 solution or an aqueous potassium bicarbonate (KHCO3) solution by adsorbing CO2 in the atmosphere, so an aqueous KOH solution that has been brought into contact with the atmosphere may be used as the alkaline electrolyte.

[0017] The method for immersing the hydrogen storage alloy anode in the alkaline electrolyte is not limited. It is sufficient for part or all of the hydrogen storage alloy anode to be immersed in the alkaline electrolyte. If the hydrogen storage alloy anode is one contained in a used nickel-metal hydride battery, it has already been immersed in the alkaline electrolyte, so the hydrogen storage alloy anode removed from the used nickel-metal hydride battery can be used as is.

[0018] The hydrogen storage alloy anode, immersed in an alkaline electrolyte, is then dried. The drying does not need to be complete, such as the removal of all moisture; it may suffice to dry it to the extent that the alkaline electrolyte adhering to the hydrogen storage alloy anode is concentrated.

[0019] The inventors have found that by using a hydrogen storage alloy anode with an alkaline electrolyte attached and then dried, the resistance of the hydrogen storage alloy anode in the electrolyte is significantly reduced. Although the reason for this is not clear, it is presumed that a change in the surface state activates the surface of the hydrogen storage alloy anode, leading to the selective production of acetic acid during electrolysis. One possible change in the surface state is that at least some of the components of the alkaline electrolyte are present on the surface as crystals. This is presumed to maintain a localized environment of strong alkalinity, thereby selectively forming acetate ions in the CO2 reduction reaction.

[0020] From the viewpoint of acetic acid selectivity, a relatively low drying temperature is preferred. For example, drying is performed in air at 80°C for several hours. While drying is not limited to these conditions, it is presumed that drying at such a relatively low temperature will result in the formation of relatively uniformly adhering crystals.

[0021] The processing step preferably results in the treated hydrogen storage alloy anode exhibiting diffraction peaks in the range of 2θ = 32° to 33° in X-ray diffraction (XRD) measurements. These diffraction peaks in the range of 2θ = 32° to 33° are not present in the X-ray diffraction (XRD) measurements of the hydrogen storage alloy anode before processing, and are newly present in the treated hydrogen storage alloy anode. It is presumed that, due to the processing step described above, crystals of KOH, a component of the alkaline electrolyte, are present on a portion of the surface of the treated hydrogen storage alloy anode, and that these KOH crystals are the origin of the new diffraction peaks in the range of 2θ = 32° to 33°. The treated hydrogen storage alloy anode also exhibits peaks from the hydrogen storage alloy itself before processing.

[0022] (Reduction process) In the reduction process, an electrolytic cell is used. The alkaline electrolyte in the electrolytic cell is an aqueous solution capable of adsorbing carbon dioxide (CO2), and from the viewpoint of the selectivity of acetic acid, it is an aqueous K2CO3 solution. The alkaline electrolyte in the electrolytic cell may also be an aqueous Na2CO3 solution, a mixture of aqueous K2CO3 solution and aqueous Na2CO3 solution, or an aqueous KOH solution. The alkaline electrolyte in the electrolytic cell may also contain components other than K2CO3.

[0023] 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).

[0024] K2CO3 + CO2 + H2O ←→ 2KHCO3(2)

[0025] In the van't Hoff equation shown in equation (3) below, ΔH 0 This represents the standard enthalpy change (-96.1 kJ / molCO2), ΔS 0 This represents the standard entropy change (-208 J / KmolCO2). P0 is atmospheric pressure (0.1 MPa), R is the gas constant, and 8.314 JK. -1 mol -1 、T represents temperature (298K) substituted. As a result, the pressure P is 0.112 Pa, and the equilibrium concentration of CO2 absorption is 1.12 ppm. Therefore, it can absorb carbon dioxide (400 ppm) in the atmosphere, and potassium bicarbonate (KHCO3) is electrochemically used as a CO2 source. Thereafter, potassium bicarbonate (KHCO3) is regenerated into potassium carbonate (K2CO3).

[0026] [Mathematical Formula]

[0027] A KOH aqueous solution becomes a K2CO3 aqueous solution or a potassium bicarbonate (KHCO3) aqueous solution by adsorbing CO2 in the atmosphere, so the KOH aqueous solution may be used in contact with the atmosphere. KOH absorbs CO2 (400 ppm) in the atmosphere to partially generate potassium carbonate (K2CO3) and water. The reaction formula is shown in Formula (4).

[0028] 2KOH+CO2←→K2CO3+H2O (4)

[0029] In the van't Hoff equation shown in the above Formula (3), ΔH 0 is the standard enthalpy change (-194kJ / molCO2), ΔS 0 is the standard entropy change (-151J / K·molCO2), P0 is atmospheric pressure (0.1MPa), R is the gas constant, 8.314 J·K -1 ·mol -1 、 T is the temperature (298K) substituted. As a result, the pressure P is 0.112 Pa, and the equilibrium concentration of CO2 absorption is 7.92×10 -21 ppm, so KOH can absorb carbon dioxide in the atmosphere. Since this K2CO3 is thermodynamically stable, it is considered that K2CO3 can be more effectively used as a CO2 source from the viewpoint of the selectivity of acetic acid in the above-mentioned K2CO3 aqueous solution than in the KOH aqueous solution, and it is therefore preferable as an alkaline electrolyte. When a KOH aqueous solution is used as the alkaline electrolyte in the electrolytic cell, electrolysis may be performed by, for example, increasing the potential difference.

[0030] The alkaline electrolyte may be the electrolyte contained in a nickel-metal hydride battery (i.e., a nickel-metal hydride secondary battery (Ni-MH)). The electrolyte may be the electrolyte contained in a nickel-metal hydride battery before use, or the electrolyte contained in a nickel-metal hydride battery after use. The alkaline electrolyte may contain trace amounts of elements or compounds other than those mentioned above.

[0031] The electrodes in the electrolytic cell use the same positive electrode as those used in nickel-metal hydride batteries. The positive electrode preferably contains at least one of Ni(OH)2 and NiOOH. Other elements besides 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 contained in a nickel-metal hydride battery before use, or the positive electrode contained in a nickel-metal hydride battery after use.

[0032] From the viewpoint of the selectivity ratio of acetic acid, it is preferable to perform the same treatment process on the positive electrode as on the negative electrode. The treatment of the positive electrode is the same as the treatment of the negative electrode.

[0033] The power source can be any power source capable of performing electrolysis using an electrolytic cell, and is not limited to that. 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 is higher than the potential of the negative electrode, carbon dioxide is reduced. It is preferable to use a power source that does not use fossil fuels, etc., because it is possible to produce acetic acid, which is a raw material for fuel, from carbon dioxide in the atmosphere with less environmental impact.

[0034] Electrolytic cells perform electrolysis in a state where the alkaline electrolyte can come into contact with the atmosphere. This allows for the electrochemical reduction of CO2 adsorbed by the alkaline electrolyte. In the method for producing acetic acid according to this disclosure, acetate ions are generated by the reduction of CO2, and it is believed that much of this exists as potassium acetate in the alkaline electrolyte. Acetic acid can be recovered by recovering the alkaline electrolyte containing potassium acetate and neutralizing the electrolyte.

[0035] In the method for producing acetic acid according to this disclosure, other ions such as formate ions may be produced in addition to acetate ions. In the method for producing acetic acid according to this 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 five times or more than the number of moles of formic acid produced. Oxygen is generated from the positive electrode by electrolysis. The generated oxygen may be released into the atmosphere or recovered and used.

[0036] The selectivity ratio S, which is the ratio of acetic acid production to formic acid production, can be calculated from the measured amounts of acetic acid (Ma) and formic acid (Mf) using the following formula (1). Note that the production amounts may be measured by mass.

[0037] S = Ma / Mf (1)

[0038] According to the method for producing acetic acid according to this disclosure, acetic acid, which can be used as a raw material for products such as vinegar, polymers, and ethanol (a fuel), 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-metal hydride battery. The method for producing acetic acid according to this disclosure can be carried out, for example, by opening a used nickel-metal hydride battery, removing the negative electrode, subjecting it to a processing step, returning it, and applying a potential difference to both electrodes. Furthermore, even when using electrolyte, positive electrode, or negative electrode that have not been used in a nickel-metal hydride battery, existing components can be used as they are. Therefore, according to the method for producing acetic acid according to this disclosure, carbon-neutral acetic acid can be produced efficiently and with a low environmental impact. [Examples]

[0039] The present disclosure will be further described below with reference to examples. The examples herein are illustrative and not intended to limit the scope. A person with ordinary skill in the art relating to this disclosure may modify, transform, or substitute materials, compositions, manufacturing methods, and applications as appropriate without departing from the spirit and scope of the invention. Note that "Ni(OH)2 / NiOOH cathode" means that the cathode contains at least one of Ni(OH)2 and NiOOH, and may contain both.

[0040] (Measurement method) In the following examples and comparative examples, electrolysis was performed using an electrolytic cell under predetermined conditions, followed by ion chromatography analysis to measure the amount of acetic acid produced (Ma) and the amount of formic acid produced (Mf). From these production amounts, the selectivity ratio S, which is the amount of acetic acid produced relative to the amount of formic acid produced, was calculated using formula (1) above. Production amounts were measured by mass. The measurement results and calculation results are shown in Table 1.

[0041] (Example 1) A nickel-metal hydride battery was disassembled to extract the Ni(OH)2 / NiOOH positive electrode and the MmNi5-based hydrogen storage alloy negative electrode. As a processing step, these positive and negative electrodes were heat-treated at 80°C for 3 hours in air. Using the heat-treated positive electrode as the anode and the heat-treated negative electrode as the cathode, an electrolysis experiment with a K2CO3 aqueous solution was performed under air at 2V for 400 hours. The current ranged from 15A to 6A. Subsequently, ion chromatography analysis was performed on the electrolyte.

[0042] (Example 2) A nickel-metal hydride battery was disassembled to extract the Ni(OH)2 / NiOOH positive electrode and the 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. Using the heat-treated positive electrode as the anode and the heat-treated negative electrode as the cathode, an electrolysis experiment with a K2CO3 aqueous solution was performed in air at 2V for 250 hours. The current ranged from 15A to 7A. Subsequently, ion chromatography analysis was performed on the electrolyte.

[0043] (Example 3) A nickel-metal hydride battery was disassembled to extract the Ni(OH)2 / NiOOH positive electrode and the 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. Using the heat-treated positive electrode as the anode and the heat-treated negative electrode as the cathode, an electrolysis experiment with a K2CO3 aqueous solution was performed in air at 2V for 100 hours. The current ranged from 15A to 8A. Subsequently, ion chromatography analysis was performed on the electrolyte.

[0044] (Example 4) A nickel-metal hydride battery was disassembled, and the Ni(OH)2 / NiOOH positive electrode and the MmNi5-based hydrogen storage alloy negative electrode were extracted. The negative electrode was heat-treated in air at 80°C for 3 hours. Using the positive electrode as the anode and the heat-treated negative electrode as the cathode, an electrolysis experiment of a K2CO3 aqueous solution was performed in air at 2V for 100 hours. The current ranged from 9A to 4A. Subsequently, ion chromatography analysis was performed on the electrolyte.

[0045] (Comparative Example 1) A nickel-metal hydride battery was disassembled, and the Ni(OH)2 / NiOOH positive electrode and the MmNi5-based hydrogen storage alloy negative electrode were extracted. The positive electrode was heat-treated at 80°C for 3 hours in air. Using the heat-treated positive electrode as the anode and the negative electrode as the cathode, an electrolysis experiment with a K2CO3 aqueous solution was performed in air at 2V for 100 hours. The current ranged from 7A to 3A. Subsequently, ion chromatography analysis was performed on the electrolyte.

[0046] (Comparative Example 2) A nickel-metal hydride battery was disassembled to obtain the Ni(OH)2 / NiOOH positive electrode and the MmNi5-based hydrogen storage alloy negative electrode. Using the Ni(OH)2 / NiOOH positive electrode as the anode and the MmNi5-based hydrogen storage alloy negative electrode as the cathode, an electrolysis experiment with a K2CO3 aqueous solution was performed under air at 2V for 100 hours. The current ranged from 5A to 2A. Subsequently, ion chromatography analysis was performed on the electrolyte.

[0047] (Comparative Example 3) A nickel-metal hydride battery was disassembled, and the Ni(OH)2 / NiOOH positive electrode and the MmNi5-based hydrogen storage alloy negative electrode were extracted. These positive and negative electrodes were heat-treated at 80°C for 3 hours in air. Using the heat-treated positive electrode as the anode and the heat-treated negative electrode as the cathode, an electrolysis experiment with a K2CO3 aqueous solution was performed under nitrogen at 2V for 100 hours. The current ranged from 15A to 8A. Subsequently, ion chromatography analysis was performed on the electrolyte.

[0048] (Comparative Example 4) A nickel-metal hydride battery was disassembled, and the Ni(OH)2 / NiOOH positive electrode and the MmNi5-based hydrogen storage alloy negative electrode were extracted. These positive and negative electrodes were heat-treated in air at 80°C for 3 hours. Using the heat-treated positive electrode as the anode and the heat-treated negative electrode as the cathode, an electrolysis experiment with a KOH aqueous solution (6 mol / L) was performed in air at 2V for 100 hours. The current ranged from 15A to 8A. Subsequently, ion chromatography analysis was performed on the electrolyte.

[0049] (Comparative Example 5) A nickel-metal hydride battery was disassembled, and the Ni(OH)2 / NiOOH positive electrode and the MmNi5-based hydrogen storage alloy negative electrode were extracted. These positive and negative electrodes were heat-treated at 80°C for 3 hours in air. Using the heat-treated positive electrode as the anode and the heat-treated negative electrode as the cathode, an electrolysis experiment with a KOH aqueous solution (6 mol / L) was performed under nitrogen at 2V for 100 hours. The current ranged from 15A to 8A. Subsequently, ion chromatography analysis was performed on the electrolyte.

[0050] [Table 1]

[0051] As shown in Table 1, in Examples 1-3, when heat-treated Ni(OH)2 / NiOOH was used as the anode and heat-treated MmNi5-based hydrogen storage alloy was used as the cathode, the amount of acetic acid produced by electrolysis of a K2CO3 aqueous solution under atmospheric conditions was 5 mg / L to 120 mg / L, and the selectivity ratio (amount of acetic acid / amount of formic acid) was 5 mg / L to 40 mg / L. In Example 4, even when untreated Ni(OH)2 / NiOOH was used as the anode and a heat-treated MmNi5-based hydrogen storage alloy cathode was used, acetic acid was produced by electrolysis of a K2CO3 aqueous 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 storage alloy was not heat-treated, almost no acetic acid was produced.

[0052] In Comparative Example 3, when a K2CO3 aqueous 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 storage 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 results, it was observed that, as shown in formula (2) above, potassium carbonate absorbs carbon dioxide from the atmosphere and then converts it into acetic acid.

[0053] As shown in Comparative Example 4, when an aqueous KOH solution was used as the electrolyte, the amount of acetic acid produced was greatly reduced. In Comparative Example 5, when a heat-treated Ni(OH)2 / NiOOH was used as the anode and a heat-treated MmNi5-based hydrogen storage alloy was used as the cathode, and an aqueous KOH solution was electrolyzed under a nitrogen atmosphere, no acetic acid was produced because CO2 was not present.

[0054] (Example 5) A MmNi5-based hydrogen storage alloy cathode extracted from a nickel-metal hydride 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 Figure 1.

[0055] (Comparative Example 6) Figure 1 shows the X-ray diffraction intensity curve of an MmNi5-based hydrogen storage alloy cathode that was removed from a nickel-metal hydride battery, washed, and then left untreated.

[0056] In Example 5, heat treatment of the MmNi5 alloy produced a novel peak at 2θ = 32°~33° that was not present in Comparative Example 6. This peak is thought to be due to the crystallization of KOH, the electrolyte used in nickel-metal hydride batteries. Novel peaks were also present at 12.9° and 25.7°, and these are also thought to be due to the crystallization of KOH, the electrolyte used in nickel-metal hydride batteries. [Explanation of symbols]

[0057] X-ray diffraction results of a 10 mm mNi5-based hydrogen storage alloy cathode. 11. X-ray diffraction intensity curve of heat-treated MmNi5-based hydrogen storage alloy cathode. 12. X-ray diffraction intensity curve of MmNi5-based hydrogen storage alloy cathode without processing steps.

Claims

1. A method for producing acetic acid by reducing carbon dioxide in the atmosphere, A process of immersing a hydrogen storage alloy negative electrode used in nickel-metal hydride batteries in an alkaline electrolyte and then drying it. The method includes a step of reducing carbon dioxide by using an electrolytic cell comprising an alkaline electrolyte capable of adsorbing carbon dioxide, a positive electrode used in a nickel-metal hydride battery, a hydrogen storage alloy negative electrode after the treatment, and a power supply connected to the positive and negative electrodes, and applying a potential difference to the positive and negative electrodes such that the potential of the positive electrode is higher than the potential of the negative electrode. The alkaline electrolyte provided in the electrolytic cell is K ２ CO ３ It is an aqueous solution. A method for producing acetic acid.

2. The hydrogen storage alloy anode is MmNi ５ Includes, The positive electrode is Ni(OH) ２ A method for producing acetic acid according to claim 1, comprising at least one of and NiOOH.

3. The method for producing acetic acid according to claim 1, wherein the hydrogen storage alloy anode after processing has a diffraction peak in the range of 2θ = 32° to 33° in X-ray diffraction (XRD) measurements.

4. Acetic acid and formic acid are produced, The method for producing acetic acid according to claim 1, wherein the number of moles of acetic acid produced is greater than the number of moles of formic acid produced.

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