JP5904760B2 - Electrolytic nitrogen fixation method - Google Patents

Description

  The present invention relates generally to nitrogen fixation, and specifically to a method for fixing nitrogen as nitrogen oxides.

  Nitrogen fixation refers to a process of converting nitrogen molecules present in large amounts in the air into nitrogen compounds (ammonia, nitrogen oxides, etc.).

  This nitrogen fixation is performed in nature by, for example, producing ammonia by a fungus having nitrogenase. Alternatively, nitrogen oxides are formed by lightning discharge or ultraviolet rays, and this is dissolved in rainwater to become nitric acid, which is fixed to the soil.

The best example of an artificially synthesized nitrogen compound is ammonia. In the beginning of the 20th century, a method for artificially producing ammonia was developed, and since then, it has brought tremendous benefits to mankind, especially from the viewpoint of increasing food production (nitrogen fertilizer production). The production method is called the Harbor Bosch method and is currently responsible for ammonia production worldwide. In this process, nitrogen and hydrogen are reacted at a high temperature and pressure under a catalyst to synthesize ammonia. However, in the ammonia production by the current Harbor Bosch method, hydrogen is obtained by steam reforming of hydrocarbon, and a large amount of CO ₂ is discharged at this time. In view of the current situation where CO ₂ emission, which is a representative example of global warming gas, is regarded as a problem worldwide, the current ammonia production process is not necessarily an excellent process as an artificial nitrogen fixation method. Can no longer be said.

  On the other hand, nitrogen oxides are also indispensable for producing nitric acid and nitrogen fertilizers (although in recent years, only aspects as air pollutants contained in exhaust gas from internal combustion engines have attracted attention). In order to obtain pure nitrogen oxides, the current production process uses the above ammonia as a raw material. In this process, ammonia is burned under an expensive catalyst such as platinum or a platinum rhodium alloy to obtain nitrogen monoxide, and further reacted with oxygen at room temperature to obtain nitrogen dioxide. Nitric acid is obtained through a reaction between the nitrogen dioxide obtained here and water. This method for producing nitric acid is called the Ostwald method, and a large amount of nitric acid is still produced industrially.

  For example, Japanese Patent Laid-Open No. 58-162276 (Patent Document 1) describes a method of generating nitrogen oxides by converting a mixed gas of nitrogen gas and oxygen gas into plasma.

  Further, for example, Japanese Patent Application Laid-Open No. 11-92120 (Patent Document 2) describes a method for photochemically generating nitrogen oxide from nitrogen gas and oxygen gas using fullerene as a catalyst.

  In addition, for example, Japanese Patent Laid-Open No. 2002-338214 (Patent Document 3) discloses a method and a production method of nitrous oxide that generates nitrous oxide by oxidizing nitrogen with ozone using ozone and nitrogen as raw materials. An apparatus is described.

JP 58-162276 A JP-A-11-92120 JP 2002-338214 A

  However, in the Ostwald method, it is necessary to produce ammonia as a raw material for producing nitrogen oxides.

  In addition, the method described in Patent Document 1 requires an expensive and complicated device for generating plasma, and is poor in mass productivity. Therefore, its use has been limited to sterilization using nitrogen oxides.

  In the method described in Patent Document 2, it was considered that the production rate of nitrogen oxides was slow and not suitable for mass production.

  In the method described in Patent Document 3, it was necessary to produce ozone from oxygen in order to obtain nitrogen oxides.

  Therefore, an object of the present invention is to provide a method that does not use ammonia, ozone, or the like as a raw material, but generates nitrogen oxides directly from nitrogen gas and oxygen gas and can easily cope with mass production. It is.

  As a result of intensive studies, the present inventors have found a new process for nitrogen fixation in which nitrogen gas and oxygen gas are used as raw materials and nitrogen oxides are generated using an electrochemical reaction.

  The present invention provides two methods: a method using nitride ions and oxygen gas (hereinafter referred to as method A) and a method using nitrogen gas and oxide ions (hereinafter referred to as method B).

  The electrolytic nitrogen fixation method (Method A) according to one aspect of the present invention includes the following steps (a1) to (a5). In step (a1), an anode, a cathode, and an electrolyte made of a molten salt in contact with the anode and the cathode are prepared. In step (a2), nitrogen gas is supplied to the cathode. In step (a3), oxygen gas is supplied into the electrolyte. In step (a4), a voltage is applied between the anode and the cathode such that nitrogen is reduced at the cathode to generate nitride ions, and oxide ions are oxidized at the anode to generate oxygen. In step (a5), the nitride ions generated in step (a4) are brought into contact with oxygen supplied in step (a3). The cathode is configured so that nitrogen gas supplied to the cathode can be supplied to the electrolyte.

In the cathode (nitrogen gas cathode) fixed so as to be in contact with the electrolyte, nitrogen gas is electrochemically reduced and nitride ions are supplied into the electrolyte.
1/2 N ₂ + 3 e ⁻ → N ^{3 −}

By supplying the oxygen gas to the electrolyte containing a nitride ion, nitride ion and oxygen gas react directly to form nitrogen oxides NO _n (hereinafter, referred to the reaction and reaction A). Here, NO _n refers to N ₂ O, N ₂ O ₃ , N ₂ O ₄ , N ₂ O _{5 and the} like in addition to NO and NO ₂ .
(Reaction A) N ³ + (2n + 3) / 4 O ₂ → NO _n + 3/2 O ²⁻

Oxide ions generated by this NO _n generation reaction are oxidized on an anode arranged so as to be in contact with the electrolyte, and oxygen gas is generated.
3/2 O ²⁻ → 3/4 O ₂ + 3 e ⁻

The overall reaction formula is as follows, and nitrogen oxides NO _n are generated from nitrogen gas and oxygen gas.
1/2 N ₂ + n / 2 O ₂ → NO _n

The oxygen gas generated at the anode is preferably recovered and used again as a raw material gas for _NOn generation.

  In the electrolytic nitrogen fixation method of the method A according to the present invention, step (a5) is performed by flowing the molten salt from the cathode to the anode through the position where oxygen gas is supplied by step (a3). Are preferred.

  In the electrolytic nitrogen fixation method of Method A according to the present invention, the cathode is preferably formed of a porous body.

  By doing so, it is possible to supply nitrogen gas to the interface between the cathode and the electrolyte, and the cathode can be configured to increase the reaction area. The cathode is not necessarily a porous body.

  The electrolytic nitrogen fixation method (Method B) according to another aspect of the present invention includes the following steps (b1) to (b4). In step (b1), an anode, a cathode, and an electrolyte disposed between the anode and the cathode are prepared. In step (b2), nitrogen gas is brought into contact with the anode. In step (b3), oxygen gas is brought into contact with the cathode. In step (b4), a voltage is applied between the anode and the cathode so that oxygen gas is reduced at the cathode to generate oxide ions, and oxide ions are oxidized at the anode to generate nitrogen oxides.

  The electrolyte is configured such that oxide ions generated by reducing oxygen gas at the cathode can move to the anode. The anode is configured such that nitrogen gas in contact with the anode can be supplied to the interface between the anode and the electrolyte. The cathode is configured such that oxygen gas in contact with the cathode can be supplied to the interface between the cathode and the electrolyte.

Thus, in Method B, it is preferable to use a molten salt or an oxide ion conductive solid electrolyte as the electrolyte. In the oxygen gas cathode fixed in contact with the electrolyte, the oxygen gas is electrochemically reduced and oxide ions are supplied into the electrolyte.
n / 2 O ₂ + 2ne ⁻ → nO ²⁻

The oxide ions generated here are oxidized at the anode supplied with nitrogen gas to generate NO _n (hereinafter, this reaction is referred to as reaction B).
(Reaction B) 1/2 N ₂ + nO ²⁻ → NO _n + 2ne ⁻

The overall reaction formula is the same as method A.
1/2 N ₂ + n / 2 O ₂ → NO _n

  As mentioned above, according to this invention, the method of producing | generating a nitrogen oxide directly from nitrogen gas and oxygen gas can be provided.

  In the electrolytic nitrogen fixation method of Method B according to the present invention, the electrolyte is preferably a molten salt or a solid electrolyte having oxide ion conductivity.

  In the electrolytic nitrogen fixing method of Method B according to the present invention, the anode is preferably formed of a porous body.

  In this way, the anode can be configured so that nitrogen gas can be supplied to the interface between the anode and the electrolyte. Note that the anode is not necessarily a porous body.

  In the electrolytic nitrogen fixation method of Method B according to the present invention, the cathode is preferably formed of a porous body.

  By doing so, oxygen gas can be supplied to the interface between the cathode and the electrolyte, and the cathode can be configured to increase the reaction area. The cathode is not necessarily a porous body.

  As mentioned above, according to this invention, the method of producing | generating a nitrogen oxide directly from nitrogen gas and oxygen gas can be provided.

It is a figure which shows typically the whole structure of the manufacturing apparatus of the nitrogen oxide of 1st Embodiment of this invention. It is a figure which shows typically the whole structure of the manufacturing apparatus of the nitrogen oxide of 2nd Embodiment of this invention. It is a figure which shows typically the whole structure of the manufacturing apparatus of the nitrogen oxide of 3rd Embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(First embodiment)
As shown in FIG. 1, a nitrogen oxide production apparatus 1 for producing nitrogen oxide by method A includes an anode 11, a cathode 12, an electrolyte 13 made of a molten salt, a nitrogen gas supply unit 14, oxygen A gas supply unit 15, an oxygen gas supply pipe 151, a nitrogen oxide gas discharge unit 16, a power supply unit 17, and an oxygen gas discharge unit 18 are provided.

  The anode 11 and the cathode 12 are disposed so as to face each other with the electrolyte 13 in contact with the electrolyte 13. That is, the electrolyte 13 is sandwiched between the anode 11 and the cathode 12. Nitrogen gas is supplied to the surface of the cathode 12 opposite to the surface in contact with the electrolyte 13 by the nitrogen gas supply unit 14. The oxygen gas supply unit 15 in the electrolyte 13 is supplied with oxygen gas from the outside of the electrolyte 13 through the oxygen gas supply pipe 151. Connected to the electrolyte 13 is a nitrogen oxide gas discharge unit 16 for discharging the nitrogen oxide in the electrolyte 13 to the outside of the electrolyte 13. An oxygen gas discharge unit 18 is connected to the surface of the anode 11 opposite to the surface in contact with the electrolyte 13. The power supply unit 17 applies a voltage between the anode 11 and the cathode 12.

In the anode 11, the oxide ions generated at the same time NO _n generated oxygen gas is oxidized to produce. Therefore, an oxygen generating anode usually used in molten salt electrolysis can be used, and examples thereof include conductive ceramics such as nickel ferrite and conductive diamond. However, when the concentration of oxide ions in the electrolyte is low, such as immediately after the start of electrolysis, depending on the magnitude of the electrolysis current, for example, when a molten chloride such as LiCl-KCl is used as the molten salt, the anode may be worn out. There is. Therefore, by using a carbon electrode such as graphite or glassy carbon according to the anodic reaction, or by supplying oxide ions into the electrolyte in advance by adding lithium oxide, calcium oxide, etc., the anodic reaction can be performed even in the initial stage of electrolysis. It is preferable to take measures such as to make it occur.

At the cathode 12, nitrogen gas is reduced to supply nitride ions into the electrolyte. Therefore, the material of the cathode may be any metal, alloy, or compound that can stably reduce nitrogen gas. For example, those containing nickel or iron are preferable, and those containing nickel are particularly preferable. As the shape of the cathode, in order to supply nitrogen to the interface with the electrolyte, a porous, wire mesh, or mesh shape used for an electrode of MCFC (Molten Carbonate Fuel Cell) is preferable. A catalyst such as Pt, Ir, or Ru may be supported on the surface of the cathode. Nitride ions for generating NO _n are supplied from the cathode in this way, but by adding a nitride ion source such as Li ₃ N to the electrolyte in advance, nitride ions can be generated even in the initial stage of electrolysis. It can react smoothly with oxygen gas.

As the electrolyte 13, a molten salt in which nitride ions and oxide ions can exist stably can be used. Examples of the molten salt include at least one of an alkali metal halide and an alkaline earth metal halide. Examples of the alkali metal halide include LiF, NaF, KF, RbF, CsF, LiCl, NaCl, KCl, RbCl, CsCl, LiBr, NaBr, KBr, RbBr, CsBr, LiI, NaI, KI, RbI, CsI, and the like. Can be mentioned. Examples of the alkaline earth metal halides, _{e.g., MgF 2, CaF 2, SrF} 2, BaF 2, MgCl 2, CaCl 2, SrCl 2, BaCl 2, MgBr 2, CaBr 2, SrBr 2, BaBr 2, MgI 2, CaI _{2, SrI 2,} BaI _2, and the like. These compounds can be used alone or in combination of two or more as a molten salt. The combination and mixing ratio of the compounds are not limited, and may be appropriately set according to the operating temperature of the molten salt determined for each purpose.

  In Method A, the electrolyte 13 preferably contains an alkali metal oxide or alkaline earth metal oxide such as lithium oxide or calcium oxide. By doing so, oxide ions can be supplied into the electrolyte 13, so that the oxide ions are oxidized and oxygen gas is generated at the anode 11 immediately after the start of electrolysis. In this case, when a carbon electrode is used for the anode 11, carbon is consumed, and therefore it is preferable to use an insoluble oxygen generating anode such as nickel ferrite from the early stage of electrolysis.

  Operation | movement of the nitrogen oxide manufacturing apparatus 1 comprised as mentioned above is demonstrated.

  In the nitrogen oxide manufacturing apparatus 1, the anode 11 and the cathode 12 are arranged so as to sandwich the electrolyte 13 as described above. First, nitrogen gas is brought into contact with the cathode 12 by supplying nitrogen gas to the nitrogen gas supply unit 14. Next, a voltage is applied between the anode 11 and the cathode 12 so that nitrogen gas is reduced at the cathode 12 to generate nitride ions, and oxide ions are oxidized at the anode 11 to generate oxygen gas. Also, oxygen gas is supplied into the electrolyte 13 by supplying oxygen gas from the oxygen gas supply pipe 151 to the oxygen gas supply unit 15.

  Since the cathode 12 is formed so as to supply nitrogen gas, the nitrogen gas passes through the inside of the cathode 12 and reaches the interface between the cathode 12 and the electrolyte 13. At the interface between the cathode 12 and the electrolyte 13, the following reaction occurs and nitride ions are generated.

(1/2) N ₂ + 3e ⁻ → N ^{3 −}
The generated nitride ions move through the electrolyte 13 and reach the oxygen supply unit 15.

  In the oxygen supply unit 15, the oxygen gas contacts with the nitride ions that have moved from the cathode 12 side, and nitrogen oxides and oxide ions are generated by the following reaction.

(Reaction A) N ³ + (2n + 3) / 4 O ₂ → NO _n +3/2 O ²⁻

  The generated nitrogen oxide is taken out of the electrolyte 13 from the nitrogen oxide gas discharge unit 16. The oxide ions move to the anode 11 side. At the anode 11, oxygen gas is generated by the following reaction.

3/2 O ²⁻ → 3/4 O ₂ + 3e ⁻

The oxygen gas generated at the anode 11 is discharged from the oxygen gas discharge unit 18 to the outside of the electrolyte 13. Oxygen gas generated at the anode 11, as a source gas for the recovery to re NO _n product, preferably utilized to supply the oxygen gas supply tube 151 to the electrolyte 13.

  The operation temperature is not particularly limited as long as the operation is performed in a temperature range where the molten salt as the electrolyte 13 can be stably used.

The voltage applied between the anode 11 and the cathode 12 may be appropriately adjusted in accordance with conditions such as the bath composition and operating temperature. For example, when molten LiCl-KCl at 450 ° C. is used as the electrolyte, the cathode The nitrogen gas reduction reaction proceeds at about 0.5 V (vs. Li ⁺ / Li: based on the potential indicated by metal Li in the same bath), and the oxygen gas generation reaction at the anode Proceeds at a potential nobler than about 2.5 V (vs. Li ⁺ / Li), so a voltage higher than about 2.0 V may be applied.

  As described above, the electrolytic nitrogen fixing method performed in the nitrogen oxide manufacturing apparatus 1 includes the following steps (a1) to (a5). In step (a1), an anode 11 and a cathode 12 and an electrolyte 13 made of a molten salt in contact with the anode 11 and the cathode 12 are prepared. In step (a2), nitrogen gas is supplied to the cathode 12. In step (a3), oxygen gas is supplied into the electrolyte 13. In step (a4), a voltage is applied between the anode 11 and the cathode 12 such that nitrogen gas is reduced at the cathode 12 to generate nitride ions, and oxide ions are oxidized at the anode 11 to generate oxygen gas. To do. In step (a5), the nitride ions generated in step (a4) are brought into contact with oxygen supplied in step (a3). The cathode 12 is configured so that the nitrogen gas supplied to the cathode 12 can be supplied to the electrolyte 13.

  Note that step (a2), step (a3), and step (a4) may be performed simultaneously, or any one of them may be performed first.

  By doing in this way, the method of producing | generating a nitrogen oxide directly from nitrogen gas and oxygen gas can be provided.

  Moreover, in the nitrogen oxide manufacturing apparatus 1, the cathode 12 is formed of a porous body. The cathode 12 does not necessarily need to be porous as long as gas can be supplied.

  By doing in this way, it is possible to supply nitrogen gas to the interface of the cathode 12 and the electrolyte 13, and the cathode 12 can be comprised so that a reaction area may be enlarged.

Here, when the nitrogen oxide is synthesized by the method A, for example, the nitride ions generated at the cathode 12 reach the anode 11 while remaining unreacted without being consumed by the NO _n generation unit (oxygen gas supply unit 15). In this case, as shown in the following formula, the gas is oxidized at the anode 11 to become nitrogen gas, and the current share of oxygen generation at the anode 11 is reduced.
N ³⁻ → 1/2 N ₂ + 3e ⁻

Alternatively, even when the oxygen gas supplied into the electrolyte 13 reaches the cathode 12 without being reacted, for example, as shown below, it is reduced at the cathode 12 to generate oxide ions, and nitrogen reduction at the cathode 12 is performed. Reduce the current share of the reaction.
3/2 O ₂ + 2 e ⁻ → O ²⁻

Therefore, in the method A, nitride ions generated at the cathode 12 are all NO _n generator is supplied to the (oxygen gas supply unit 15), also oxygen ions generated in the NO _n generator (oxygen gas supply unit 15) All must be supplied to the anode 11 and consumed reliably. Therefore, the molten salt that is the electrolyte 13 is circulated in a loop shape in the electrolytic cell, and the cathode 12, the NO _n generation unit (oxygen gas supply unit 15), and the anode 11 are disposed along the circulation of the molten salt. Is preferred. In this way, all the nitride ions generated at the cathode 12 to supply NO _n generating unit (oxygen gas supply unit 15), and oxides generated by the NO _n generator (oxygen gas supply unit 15) All ions can be supplied to the anode 11.

  Therefore, an embodiment in which nitrogen oxide is produced by Method A using a nitrogen oxide production apparatus configured to circulate and flow an electrolyte in a loop will be described below.

(Second Embodiment)
As shown in FIG. 2, as a more preferable embodiment of the nitrogen oxide production apparatus 1 of the first embodiment, the nitrogen oxide production apparatus 2 includes an anode 21, a cathode 22, an electrolytic cell 20, and an electrolytic cell 20. An electrolyte 23 made of molten salt, a nitrogen gas supply pipe 24, an oxygen gas supply section 25, an oxygen gas supply pipe 251, a nitrogen oxide gas discharge section 26, a power supply section 27, and an oxygen discharge section. 28 and an inert gas supply unit 29. The cathode 22 is formed of a porous body. The cathode 22 is not necessarily porous as long as the gas can be supplied. For example, the cathode 22 may be formed in a wire mesh shape or a mesh shape as described in the first embodiment.

  In this embodiment, the electrolytic cell 20 includes a first flow path 201 and a second flow path 202 that extend in the vertical direction, and a third flow path 203 and a fourth flow path 204 that extend in the horizontal direction. The first flow path 201 and the second flow path 202 are connected by the third flow path 203 in the upper part and are connected by the fourth flow path 204 in the lower part.

  The flow path inside the electrolytic cell 20 may be in any form as long as it has a loop shape, such as one flow path formed in a circular shape in the vertical plane. The road is not necessary.

The anode 21 is disposed in the third flow path 203 of the electrolytic cell 20. At the anode 21, oxide ions generated at the same time as NO _n generation are oxidized to generate oxygen gas. The cathode 22 is disposed on the upper part of the first flow path 201 of the electrolytic cell 20. At the cathode 22, nitrogen gas is reduced to supply nitride ions into the electrolyte. The nitrogen gas supply pipe 24 supplies nitrogen gas to the cathode 22 from the outside of the electrolytic cell 20.

  The configuration and operating temperature of the anode 21, the cathode 22, and the electrolyte 23 are as described with reference to FIG.

  Oxygen gas is supplied from the outside of the electrolytic cell 20 through the oxygen gas supply pipe 251 to the lower oxygen gas supply unit 25 in the second flow path 202 of the electrolyte 23. Since the reaction between nitride ions and oxygen gas in the electrolyte is a gas-liquid reaction, when supplying using a gas supply pipe including a porous gas supply unit such as quartz glass or high-purity alumina. It is preferable to reduce the bubble size of the supplied oxygen gas because the reaction proceeds smoothly. Also, holding a filler carrying a catalyst in a metal supply pipe such as Ni or the glass or alumina supply pipe described above may cause the nitride ions and oxygen gas in the electrolyte due to the catalytic effect. It is more preferable because the reaction with can be promoted.

  From the nitrogen oxide gas discharge unit 26 at the upper part of the second flow path 202, the nitrogen oxide gas generated by the nitrogen oxide manufacturing apparatus 2 is discharged outside the nitrogen oxide manufacturing apparatus 2. The power supply unit 27 applies a voltage between the anode 21 and the cathode 22. The power supply unit 27 is configured such that the voltage applied by the power supply unit 27 can be controlled by the user. Oxygen generated in the electrolytic cell 20 is discharged to the outside of the electrolytic cell 20 from the oxygen discharge unit 28 in the upper part of the third flow path 203 of the electrolytic cell 20. The inert gas supply unit 29 at the bottom of the second flow path 202 supplies an inert gas such as argon from the bottom of the electrolytic cell 20 into the electrolyte 23 as indicated by an arrow P in the drawing.

  Operation | movement of the nitrogen oxide manufacturing apparatus 2 comprised as mentioned above is demonstrated.

  In the nitrogen oxide manufacturing apparatus 2, the anode 21 and the cathode 22 are disposed so as to contact the electrolyte 23 as described above. First, oxygen gas is supplied into the electrolyte 23 by supplying oxygen gas from the oxygen gas supply pipe 251 to the oxygen gas supply unit 25. Nitrogen gas is brought into contact with the cathode 22 by supplying nitrogen gas to the nitrogen gas supply pipe 24. Next, a voltage is applied between the anode 21 and the cathode 22 so that nitrogen gas is reduced at the cathode 12 to generate nitride ions, and oxide ions are oxidized at the anode 11 to generate oxygen gas.

  Since the cathode 22 is formed so that nitrogen gas can be supplied, the nitrogen gas passes through the inside of the cathode 22 and reaches the interface between the cathode 22 and the electrolyte 23. At the interface between the cathode 22 and the electrolyte 23, the following reaction occurs and nitride ions are generated.

(1/2) N ₂ + 3e ⁻ → N ^{3 −}

  An inert gas is supplied to the electrolyte 23 from an inert gas supply unit 29 at the bottom. By doing so, an upward flow is generated in the second flow path 202 of the electrolyte 23 by the gas lift, and the molten salt is circulated and fluidized in the loop of the electrolytic cell 20. For the flow of the molten salt, a mechanical method using a propeller or the like may be used, but if a gas lift using supply of oxygen gas or inert gas is used, a mechanical drive unit is unnecessary, This is preferable because the device configuration is simplified. The generated nitride ions move in the direction indicated by the arrow Q in the figure, and reach the oxygen gas supply unit 25 through the first flow path 201 and the fourth flow path 204.

  In the oxygen gas supply unit 25, the oxygen gas and the nitride ions that have moved from the cathode 22 come into contact with each other, and nitrogen oxides and oxide ions are generated by the following reaction.

(Reaction A) N ³ + (2n + 3) / 4 O ₂ → NO _n +3/2 O ²⁻

  The generated nitrogen oxides and oxide ions move from the bottom to the top in the second flow path 202 by gas lift. Nitrogen oxide is taken out of the electrolytic cell 20 from the nitrogen oxide gas exhaust 26 at the top of the second flow path 202. The oxide ions move from the second flow path 202 to the third flow path 203 as shown by an arrow R in the figure, and reach the anode 21. At the anode 21, oxygen gas is generated by the following reaction.

3/2 O ²⁻ → 3/4 O ₂ + 3e ⁻

Oxygen gas generated at the anode 21 is discharged from the oxygen discharge section 28 at the top of the third flow path 203 to the outside of the electrolytic cell 20. Oxygen gas generated at the anode 21, as a source gas for the recovery to re NO _n product, preferably utilized to supply the oxygen gas supply tube 251 to the electrolyte 23.

  As described above, the electrolyte 23 can move the nitride ions generated by reducing the nitrogen gas at the cathode 22 to the oxygen gas supply unit 25, and at the time of generating the nitrogen oxide in the oxygen gas supply unit 25. The oxide ions generated at the same time are configured to move to the anode 21, and the anode 21 is configured to discharge the generated oxygen gas to the outside of the anode 21. 22 is configured such that nitrogen gas can be supplied to the interface with the electrolyte 23 in contact with the cathode 22 (cathode-nitrogen gas-electrolyte three-phase interface).

  As described above, the electrolytic nitrogen fixing method performed in the nitrogen oxide manufacturing apparatus 2 includes the following steps (a1) to (a5). In step (a1), an anode 21, a cathode 22, and an electrolyte 23 made of a molten salt in contact with the anode 21 and the cathode 22 are prepared. In step (a2), nitrogen gas is supplied to the cathode 22. In step (a3), oxygen gas is supplied into the electrolyte 23. In step (a4), a voltage is applied between the anode 21 and the cathode 22 such that nitrogen gas is reduced at the cathode 22 to generate nitride ions, and oxide ions are oxidized at the anode 21 to generate oxygen. . In step (a5), the nitride ions generated in step (a4) are brought into contact with oxygen supplied in step (a3). The cathode 22 is configured so that the nitrogen gas supplied to the cathode 22 can be supplied to the electrolyte 23.

  Further, in the electrolytic nitrogen fixing method performed in the nitrogen oxide production apparatus 2, step (a5) is performed by causing the electrolyte 23 to flow to the anode 21 through the position where oxygen gas is supplied in step (a3). Done.

  Note that step (a2), step (a3), and step (a4) may be performed simultaneously, or any one of them may be performed first.

  By doing in this way, the method of producing | generating a nitrogen oxide directly from nitrogen gas and oxygen gas can be provided.

  Other configurations and effects of the nitrogen oxide production apparatus 2 are the same as those of the nitrogen oxide production apparatus 1 (FIG. 1).

(Third embodiment)
As shown in FIG. 3, the nitrogen oxide production apparatus 3 for producing nitrogen oxide by the method B includes an anode (nitrogen electrode) 31, a cathode (oxygen electrode) 32, an electrolyte 33, and a nitrogen gas supply unit. 34, an oxygen gas supply unit 35, a nitrogen oxide gas discharge unit 36, and a power supply unit 37.

  The anode 31 and the cathode 32 are disposed so as to face each other with the electrolyte 33 in contact with the electrolyte 33. That is, the electrolyte 33 is sandwiched between the anode 31 and the cathode 32. Both the anode 31 and the cathode 32 are formed of a porous body. The anode 31 and the cathode 32 are not necessarily porous as long as the gas can be supplied.

The anode 31 needs to be in a form that can supply oxygen to the interface with the electrolyte 33. Any material may be used as long as the components contained in the anode 31 are eluted by electrolysis, or the reaction is not progressed due to alteration by the generated nitrogen oxides. When molten salt is included as the electrolyte, conductive ceramics such as nickel ferrite and conductive diamond can be exemplified. In this case, as in method A, for example, oxide ions are supplied to the electrolyte in advance by adding lithium oxide, calcium oxide, or the like, so that the anode 31 reaction generates oxygen even in the initial stage of electrolysis. It is preferable to take action. When an oxide ion conductive solid electrolyte is used as the electrolyte, for example, Ni can be used as long as it is a metal. Alternatively, as described above, a La—Sr—MnO system, a La—Sr—CoO ₃ system, a Sm—Sr—CoO ₃ system such as those used in SOFC (Solid Oxide Fuel Cell), MIEC (Mixed Ionic-Electronic Conductors, mixed ion / electron conductors) such as (La, Sr) (Co, Fe) O ₃ system can also be used. In addition, for the purpose of promoting the formation reaction of nitrogen oxides, a catalyst of a metal or a compound such as Pt, Ir, Ru, Rh, or Fe may be supported.

At the cathode 32, oxygen gas is reduced and oxide ions are supplied to the electrolyte 33. Accordingly, the material of the cathode 32 may be any metal, alloy, or compound that can stably reduce oxygen gas. For example, a La—Sr—MnO system or a La—Sr—CoO ₃ system used in SOFC is used. MEC such as Sm—Sr—CoO ₃ and (La, Sr) (Co, Fe) O ₃ can be used. Further, in order to supply oxygen to the interface with the electrolyte 33, it is necessary to be in a form capable of supplying gas such as porous or wire mesh. In addition to pure oxygen gas, it can be used as long as it is a gas containing oxygen and the electrochemical reduction reaction of gas components other than oxygen does not proceed. For example, dry air can be exemplified.

  As the electrolyte 33, a molten salt in which oxide ions can exist stably or an oxide ion conductive solid electrolyte can be used. As the molten salt, the same molten salt as in Method A can be used. Specific examples of the oxide ion conductive solid electrolyte include zirconia-based ceramics such as yttria-stabilized zirconia and scandia-stabilized zirconia, ceria-based ceramics, lanthanum gallate-based ceramics, and the like used in SOFC, for example. Is available.

  Nitrogen gas is supplied to the nitrogen gas supply unit 34.

  Oxygen gas is supplied to the oxygen gas supply unit 35. The oxygen gas supply unit 35 may be supplied with a gas containing oxygen, such as dry air, instead of pure oxygen.

  From the nitrogen oxide gas discharge unit 36, the nitrogen oxide gas generated by the nitrogen oxide manufacturing apparatus 3 is discharged to the outside of the nitrogen oxide manufacturing apparatus 3.

  The power supply unit 37 applies a voltage between the cathode 32 and the anode 31. The power supply unit 37 is configured such that the voltage applied by the power supply unit 37 can be controlled by the user.

  Operation | movement of the nitrogen oxide manufacturing apparatus 3 comprised as mentioned above is demonstrated.

  In the nitrogen oxide manufacturing apparatus 3, as described above, the anode 31 and the cathode 32 are arranged so as to sandwich the electrolyte 33. First, nitrogen gas is brought into contact with the anode 31 by supplying nitrogen gas to the nitrogen gas supply unit 34. By supplying oxygen gas to the oxygen gas supply unit 35, oxygen gas is brought into contact with the cathode 32. Next, a voltage is applied between the anode 31 and the cathode 32 such that oxygen is reduced at the cathode 32 to generate oxide ions, and oxide ions are oxidized at the anode 31 to generate nitrogen oxides.

  Since the cathode 32 is formed so as to supply oxygen gas, the oxygen gas passes through the inside of the cathode 32 and reaches the interface between the cathode 32 and the electrolyte 33. At the interface between the cathode 32 and the electrolyte 33, the following reaction occurs to generate oxide ions.

(1/2) n O ₂ + 2ne ⁻ → n
O ^2-

  The generated oxide ions move through the electrolyte 33 and reach the anode 31.

  On the other hand, since the anode 31 is formed so as to supply nitrogen gas, the nitrogen gas passes through the inside of the anode 31 and reaches the interface between the anode 31 and the electrolyte 33. At the interface between the anode 31 and the electrolyte 33, the following reaction proceeds between the nitrogen gas and the oxide ions that have moved from the cathode 32 side.

^{_{(B) 1/2 N 2 + n}} O 2- → NO n + 2n e -

  In this way, nitrogen oxides are generated. The generated nitrogen oxide passes through the anode 31 and is discharged from the nitrogen oxide gas discharge unit 36.

  The operating temperature is not particularly limited as long as it is operated in a temperature range where the molten salt can be stably used or in a temperature range where oxide ion conductivity is exhibited. When using the above solid oxide electrolyte, it is preferable to operate at 400 to 1000 ° C, preferably 500 to 800 ° C.

What is necessary is just to adjust suitably the voltage applied between the anode 31 and the cathode 32 according to conditions, such as bath composition operating temperature. For example, in the case of using 450 ° C. molten LiCl-KCl, referring to thermodynamic calculation, the anode 31 and the cathode are arranged so that the anode potential is a potential region that is nobler than about 3 V (vs. Li ⁺ / Li). A voltage to be applied between the two terminals may be set.

  As described above, the electrolytic nitrogen fixing method performed in the nitrogen oxide production apparatus 3 includes the following steps (b1) to (b4). In step (b1), an anode 31, a cathode 32, and an electrolyte 33 arranged so as to be sandwiched between the anode 31 and the cathode 32 are prepared. In step (b2), nitrogen gas is brought into contact with the anode 31. In step (b3), oxygen gas is brought into contact with the cathode 32. In step (b4), a voltage is applied between the anode 31 and the cathode 32 such that oxygen is reduced at the cathode 32 to generate oxide ions, and oxide ions are oxidized at the anode 31 to generate nitrogen oxides. To do.

  The electrolyte 33 is a molten salt or oxide ion conductive solid electrolyte, and is configured such that oxide ions generated by reducing oxygen gas at the cathode 32 can move to the anode. The anode 31 is configured so that nitrogen gas in contact with the anode 31 can be supplied to the electrolyte 33. The cathode 32 is configured so that oxygen gas in contact with the cathode 32 can be supplied to the electrolyte 33.

  By doing in this way, the method of producing | generating a nitrogen oxide directly from nitrogen gas and oxygen gas can be provided.

  Note that step (b2), step (b3), and step (b4) may be performed simultaneously, or any one of them may be performed first.

  In the nitrogen oxide production apparatus 3, the anode 31 is formed of a porous body. The anode 31 does not necessarily need to be porous as long as the gas can be supplied.

  In this way, the anode 31 can be configured so that the nitrogen gas in contact with the anode 31 can be supplied to the electrolyte 33.

  In the nitrogen oxide manufacturing apparatus 3, the cathode 32 is formed of a porous body. The anode 32 is not necessarily porous as long as the gas can be supplied.

  In this way, the cathode 32 can be configured so that the oxygen gas in contact with the cathode 32 can be supplied to the electrolyte 33.

(Example 1)
In steps (a2) to (a4) of Method A of the present invention, nitrogen is reduced at the cathode to produce nitride ions, and oxide ions are oxidized at the anode at the anode to generate oxygen. Has been clarified by the results of research on the electrode reaction of nitrogen gas and oxygen gas in the molten salt that the inventors have conducted so far.

Therefore, the formation of nitrogen oxides by the method A of the present invention is confirmed by showing the NO _n generation reaction A in which the nitride ions and oxygen react to form nitrogen oxides in step (a5). can do.

As an example for confirming the NO _n generation reaction A in the method A, the following examination was performed. The eutectic composition LiCl—KCl was held at 450 ° C. in a high-purity alumina crucible under an Ar atmosphere, and 0.2 mol% Li ₃ N added as an N ³ source was used as a molten salt. Ar gas containing 10 vol% O ₂ in the molten salt was blown through an alumina tube at 100 cc · min ⁻¹ to react with N ³⁻ in the molten salt. After the elapse of a predetermined time, the generated gas was recovered, and NO and NO ₂ were evaluated for the generated gas using a Kitagawa gas detector tube. Table 1 shows the relationship between the sampling time (elapsed time after the start of O ₂ blowing) and the product gas.

When only Ar was blown into the molten salt (indicated as blank in Table 1), the production of NO and NO ₂ was not confirmed. Generation of NO was confirmed immediately after the start of injecting O ₂ into the molten salt, and NO and NO ₂ exceeding the measurable upper limit of the concentration of the detector tube were confirmed after about 1000 seconds. From this result, it was confirmed that NO _{n was} generated by the reaction A, and therefore, combined with the known nitrogen gas cathode reduction reaction in the molten salt and the oxygen gas generation reaction at the anode, the electrochemical NO _n by the method A was used. Generation was confirmed.

(Example 2)
In step (b3) of the method B of the present invention, oxygen is reduced at the cathode to generate oxide ions. This reaction is also performed by the inventors of the oxygen gas in the molten salt. The results of research on

Therefore, the generation of nitrogen oxides by the method B of the present invention indicates the NO _n generation reaction B in which nitrogen oxides are generated from oxide ions and nitrogen by the anodic reaction in steps (b2) and (b4). This can be confirmed.

As an example for confirming the NO _n generation reaction B in the method B, the following examination was performed. The eutectic composition LiCl—KCl was held at 450 ° C. in a high-purity alumina crucible under an Ar atmosphere, and 0.2 mol% Li ₂ O added as an O ²⁻ source was used as the molten salt. Square anode nickel ferrite (Ni _0.75 Fe _2.25 O ₄ ) is used for the anode, which is placed inside a quartz cylinder and immersed in the molten salt while being isolated from the atmosphere inside the cell. N ₂ gas was supplied at 100 cc · min ⁻¹ . An Al plate was used as the cathode, and Li generated by the cathode reduction reaction of Li ⁺ during the electrolysis was held in the cathode by using an Al—Li alloy. An Ag ⁺ / Ag electrode was used as a reference electrode, and the electrode potential was calibrated with an open circuit potential of metal Li deposited on a Ni wire electrode (Li ⁺ / Li potential, hereinafter referred to as the same potential reference). After starting the supply of N ₂ to the anode in the cylinder, constant potential electrolysis was performed at an anode potential of 3.4V. The gas generated from the anode during electrolysis was taken out from the inside of the cylinder and collected, and as in Example 1, the generated gas was evaluated for the NO and NO _{2 using} the Kitagawa gas detector tube. Although the generation of NO _n If you immediately sampled after the start of electrolysis could not be confirmed, where sampling was carried out after the lapse of about 2,000 seconds, the formation of NO of about 5ppm has been confirmed. From this result, it was confirmed that NO _{n was} produced by reaction B, so that electrochemical NO _n production by method B could be confirmed together with the oxygen gas cathode reduction reaction in the already known molten salt.

  The embodiment disclosed above should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and includes all modifications within the scope and meaning equivalent to the terms of the claims.

  11, 21, 31: anode, 12, 22, 32: cathode, 13, 23, 33: electrolyte.

Claims (6)

Providing an anode, a cathode, and an electrolyte comprising a molten salt in contact with the anode and the cathode (a1);
Supplying nitrogen gas to the cathode (a2);
Supplying oxygen gas into the electrolyte (a3);
Applying a voltage between the anode and the cathode to generate nitrogen ions by reducing nitrogen at the cathode and oxidizing the oxide ions at the anode to generate oxygen (a4);
Contacting (a5) the nitride ions produced in step (a4) with the oxygen supplied in step (a3),
The electrolytic nitrogen fixation method, wherein the cathode is configured to be able to supply the nitrogen gas supplied to the cathode to the electrolyte.   2. The electrolytic nitrogen according to claim 1, wherein the step (a5) is performed by flowing the molten salt from the cathode to the anode through a position where oxygen gas is supplied by the step (a3). Fixed method. Providing an anode, a cathode, and an electrolyte disposed between the anode and the cathode (b1);
Contacting the anode with nitrogen gas (b2);
Contacting the cathode with oxygen gas (b3);
A step of applying a voltage between the anode and the cathode to reduce the oxygen gas at the cathode to generate oxide ions and to oxidize the oxide ions at the anode to generate nitrogen oxides (b4). ) And
The electrolyte is configured such that oxide ions generated by reducing the oxygen gas at the cathode can move to the anode,
The anode is configured to be able to supply the nitrogen gas in contact with the anode to the interface with the electrolyte,
The electrolytic nitrogen fixation method, wherein the cathode is configured to be able to supply the oxygen gas in contact with the cathode to the interface with the electrolyte.   The electrolytic nitrogen fixing method according to claim 3, wherein the electrolyte is a molten salt or an oxide ion conductive solid electrolyte.   The electrolytic nitrogen fixing method according to claim 3 or 4, wherein the anode is formed of a porous body.   The electrolytic nitrogen fixing method according to any one of claims 1 to 5, wherein the cathode is formed of a porous body.

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