CN113737205B - Method for directly preparing ammonia gas by electrochemical reduction of nitrite - Google Patents

Method for directly preparing ammonia gas by electrochemical reduction of nitrite Download PDF

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CN113737205B
CN113737205B CN202111138428.9A CN202111138428A CN113737205B CN 113737205 B CN113737205 B CN 113737205B CN 202111138428 A CN202111138428 A CN 202111138428A CN 113737205 B CN113737205 B CN 113737205B
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nitrate
nitrite
electrolyte
electrochemical reduction
ammonia
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CN113737205A (en
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向开松
刘恢
易慧敏
沈锋华
王珠江
刘旭东
付迎雪
伍琳
陈昊
柴立元
李青竹
王庆伟
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Central South University
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Abstract

The invention discloses a method for preparing ammonia by electrochemical direct reduction of nitrite (nitrite) nitrate, which comprises the steps of electrochemically reducing electrolyte containing nitrite and/or nitrate through a porous membrane electrode to generate ammonia; the method can realize the efficient and high-selectivity conversion of nitrate (nitrite) ions into ammonia gas, and the method can convert ammonium ions in the solution into ammonia gas and realize separation without adding alkali, thereby greatly reducing the cost for separating ammonia gas, being capable of being carried out under the conditions of normal temperature and normal pressure, having simple operation and being convenient for large-scale application.

Description

Method for directly preparing ammonia gas by electrochemical reduction of nitrite
Technical Field
The invention relates to a method for directly preparing ammonia by electrochemical reduction of nitrite (nitrite) and particularly relates to a method for realizing resource utilization of ammonia by converting nitrite (nitrite) in denitration waste liquid or nitrate sewage by a wet process into high-economic-value ammonia with high selectivity through electrochemical reduction, belonging to the technical field of resource treatment of nitrate sewage.
Background
The large amount of nitrate nitrogen wastewater generated in industrial activities seriously threatens the water ecology. With increasingly stringent environmental requirements, the harmless treatment of nitrate-nitrogen wastewater becomes the key point of environmental supervision, and the traditional treatment method mainly converts nitrate-nitrogen into stable and harmless nitrogen through denitrification by a microbiological method. On the other hand, if the nitrate nitrogen is used as an important form of nitrogen resource and can be converted into an ammonia resource which is needed urgently at present, the win-win situation of environmental improvement and resource recovery can be realized.
In recent years, methods for converting nitrate nitrogen into ammonium ions have been reported in succession, but subsequent reports for extracting nitrate nitrogen into ammonia gas have not been reported yet. The reason for this is that the electrolysis of (nitrite) -containing waste water to ammonium nitrogen, the main method for separating ammonium nitrogen from water, is to consume large amounts of alkali to adjust the pH to alkaline and then to let ammonia escape by heating or large amounts of aeration, the escaped ammonia still needs to be further concentrated, which is not cost-effective.
Disclosure of Invention
Aiming at the defects in the prior art, the invention aims to provide a method for directly obtaining ammonia by electrochemically reducing nitrate (nitrite) by using a porous membrane electrode, the method has high efficiency and high selectivity for reducing nitrate into ammonia, can be carried out under the conditions of normal temperature and normal pressure, has mild conditions, low energy consumption, simple operation and green and environment-friendly process, has almost zero ammonia separation cost, is beneficial to large-scale popularization and application, and provides a new solution for recycling industrial nitrate (nitrite) wastewater.
In order to achieve the technical purpose, the invention providesThe electrochemical reduction process of preparing ammonia directly with nitrite and/or nitrate radical includes electrochemical reduction of nitrite and/or nitrate radical containing electrolyte via porous membrane electrode to produce ammonia; the porous membrane electrode has a catalytic effect on electrochemical reduction of nitrite and nitrate; when the pH value of the electrolyte containing nitrite and/or nitrate is greater than or equal to 7 and less than 13, the current density of the electrode in the electrochemical reduction process is greater than 10mA/cm 2 When the pH value of the electrolyte containing nitrite and/or nitrate is more than 1 and less than 7, the current density of the electrode in the electrochemical reduction process is more than 50mA/cm 2 。
The technical scheme of the invention is that nitrite and/or nitrate-containing electrolyte is electrochemically reduced to generate ammonium ions, and the key is that protons on the surface of the porous membrane electrode can be consumed under appropriate current density by controlling the current density of the porous membrane electrode and utilizing the hydrophobic and porous structures of the porous membrane electrode, so that local high-concentration hydroxide radicals are generated on the surface of the porous membrane electrode, and the ammonium ions are promoted to be converted into free ammonia on the interface of the porous membrane electrode.
Preferably, when the electrolyte containing nitrite and/or nitrate is neutral or alkaline (pH is greater than or equal to 7 and less than 12), the electrode current density during electrochemical reduction is preferably 10-200 mA/cm 2 When the electrolyte containing nitrite and/or nitrate is acidic (pH is more than 1 and less than 7), the current density of the electrode in the electrochemical reduction process is more than 50-300 mA/cm 2 . Too high a pH is detrimental to the reduction of nitrite and/or nitrate to ammonia, e.g. pH =13, current density 10mA/cm 2 In the process, the ammonia process has the first efficiency of 11 percent and the separation rate of 90 percent; too low a pH leads to severe hydrogen evolution during electrolysis, and it is difficult to achieve a high pH at the gas-liquid interface under low current conditions, e.g., pH =0.5, current density 10mA/cm 2 The ammonia process had 54% of efficiency, and almost no ammonia was separated. In the preferred current density range, the higher the current density, the more favorable the evolution of ammonia gas.
Preferably, the concentration of the nitrite and/or nitrate containing electrolyte is 10. Mu. Mol/L to 1.0mol/L.
Preferably, the porous membrane electrode is made of a hydrophobic material and a catalytic material supported on the surface thereof or made of a hydrophobic catalytic material.
As a preferable scheme, the hydrophobic material is selected from PTFE, PEEK, PP, PE, carbon cloth or porous carbon paper, or is formed by performing surface hydrophobic treatment on a porous material matrix; the porous material matrix is selected from a metal material, an inorganic non-metal material or an organic polymer material. The metal material is specifically such as copper foam, nickel foam and the like, the inorganic nonmetal material is specifically such as porous carbon material, carbon cloth and the like, the organic polymer material is specifically such as reticular PTFE, PEEK, PP, PE and the like, and the hydrophobic property can be further improved by surface hydrophobic treatment to be used as the electrode base material of the porous membrane regardless of the hydrophobic material or the hydrophilic material.
As a preferable scheme, the surface hydrophobic treatment is surface modification treatment by hydrophobic macromolecules or hydrophobic micromolecules or surface micro-nano scale processing treatment. The surface modification treatment is performed by hydrophobic macromolecules or hydrophobic small molecules, specifically, PTFE, biological wax or octadecanethiol and the like are used for surface modification of a porous material matrix, for example: and (3) soaking the porous material with the gap size of 0.1mm in an ethyl acetate solution in which 1% octadecanethiol is dissolved for 1-5 minutes, and naturally drying to obtain the porous material. The method comprises the following specific steps of surface micro-nano scale processing treatment, for example: a porous material with the gap size of 0.1mm is subjected to anodic oxidation in a 3mol/L potassium hydroxide solution to build a nano array with the needle-shaped length of about 2 micrometers in situ, so that the surface of the nano array is hydrophobic.
As a preferable scheme, the catalytic material is at least one of a metal simple substance, a metal sulfide, a metal selenide, a metal phosphide, a metal nitride and boron-doped diamond; the preferred elemental metal is selected from at least one of copper, cobalt, iron, nickel, gold, silver, platinum or palladium. Preferred metal sulfides are selected from at least one of the sulfides of copper, cobalt, iron, nickel, gold, silver, platinum or palladium. Preferred metal selenides are selected from at least one of copper, cobalt, iron, nickel, gold, silver, platinum or palladium selenides; preferred metal phosphides are selected from at least one of copper, cobalt, iron, nickel, gold, silver, platinum or palladium phosphides. Preferred metal nitrides are selected from at least one of the nitrides of copper, cobalt, iron, nickel, gold, silver, platinum or palladium. These catalytic materials are catalytic materials which are common in the art and have a catalytic effect on the electrochemical reduction of nitrite and nitrate.
In a preferred embodiment, the hydrophobic catalytic material is at least one selected from copper, cobalt, iron, nickel, gold, silver, platinum and palladium, or is at least one of copper, cobalt, iron, nickel, gold, silver, platinum and palladium subjected to surface hydrophobic treatment. The surface hydrophobic treatment is, for example, surface modification treatment by hydrophobic macromolecules or hydrophobic small molecules or surface micro-nano scale processing treatment.
In a preferable scheme, in the electrochemical reduction process, an electrolyte containing nitrite and/or nitrate is used as a cathode chamber electrolyte, a porous membrane electrode is used as a working electrode, one side of the porous membrane electrode faces the electrolyte, and the other side faces a gas collection chamber.
The electrolyte containing nitrite and/or nitrate radical can adopt wet denitration waste liquid or nitrate nitrogen sewage to realize resource utilization.
The invention directly prepares ammonia by electrochemically reducing nitrate (nitrite) through a porous membrane electrode by the following implementation process: a three-electrode system is adopted for carrying out nitrate radical electrochemical reduction, sulfur dioxide absorption liquid is taken as electrolyte of a cathode chamber, a membrane electrode is taken as a working electrode, and two surfaces of a catalyst membrane respectively face an electrolyte and a gas collecting chamber. A saturated calomel electrode is used as a reference electrode to construct a three-electrode system taking nitrate as electrolyte. The pH value of the electrolyte is preferably controlled to be more than 7, and the current density is preferably controlled to be more than 10mA/cm 2 . At this time, ammonia gas can be obtained in the gas collection chamber.
Compared with the prior art, the technical scheme of the invention has the following beneficial technical effects:
1) The technical scheme of the invention realizes the high-selectivity electro-catalytic reduction of nitrite into ammonia gas by adopting the porous membrane electrode for the first time, and compared with the prior art, the technical scheme of the invention has incomparable advantages in the separation cost of ammonia gas.
2) The technical scheme of the invention adopts the membrane electrode with porous structure and high catalytic activity to realize the electrochemical reduction of nitrate (nitrite) electrolyte, and can quickly and selectively separate the ammonia generated in the reduction process from the electrolyte, thereby promoting the chemical reaction balance of the whole electrochemical reduction reaction to move towards the direction favorable for generating the ammonia, and being favorable for improving the Faraday efficiency and the gas purity of the ammonia.
3) According to the technical scheme, the nitrate (nitrite) radical can be efficiently converted only by controlling proper current density at room temperature and normal pressure, the Faraday efficiency of ammonia gas can reach more than 90%, the long-time catalytic stability can be kept, the reaction condition is mild, the energy consumption is low, and the industrial application is facilitated; the reaction process can separate and generate high-concentration ammonia gas without adding other chemical agents, no waste salt is generated, no energy is consumed in the separation process, and the advantages of environmental protection and energy consumption are obvious.
Detailed Description
The following examples are intended to further illustrate the present invention, but not to limit the scope of the claims.
Nitrate (nitrite) electrolyte in the following examples is electrochemically reduced by using a three-electrode system, a cathode chamber and an anode chamber of the three-electrode system are separated by using a dupont N117 proton membrane or a cation exchange membrane, the electrolyte in the cathode chamber is nitrate (nitrite) electrolyte, a membrane electrode is used as a working electrode, pt is used as a counter electrode, saturated mercurous sulfate is used as a reference electrode, and a proper current density is controlled by a constant current or constant potential mode according to the pH of the electrolyte.
The membrane electrode preparation process in the following examples takes Cu/PTFE and Ag-Cu/stainless steel wire mesh as examples:
(1) Cu/PTFE: cleaning a commercial PTFE breathable film by using 0.1M hydrofluoric acid, and airing; treating the film with an oxygen plasma at a power of 5W for 5 minutes; the membrane was immersed in a copper sulfate (10 g/L)/sodium tartrate (50 g/L) solution, the pH was adjusted to 12 using sodium hydroxide, a formaldehyde solution (10 g/L) was added, and the mixture was allowed to stand at room temperature for 1 hour to control the thickness of the plating layer to about 20 μm.
(2) Ag-Cu/stainless Steel wire gauze: cleaning a commercial 1000-mesh stainless steel wire net with 0.1M dilute hydrochloric acid and acetone, and airing; covering one side of the stainless steel wire mesh with a raw material belt, contacting the uncovered side with 1% v/v octadecanethiol/ethyl acetate solution page for 5min, taking out, and placing in an oven for drying at 50 ℃; tearing off the raw material belt, immersing the stainless steel screen membrane in a copper sulfate (10 g/L)/sodium tartrate (50 g/L) solution, adjusting the pH to 12 by using sodium hydroxide, adding a formaldehyde solution (10 g/L), standing for 1 hour at room temperature, and controlling the thickness of a plating layer to be about 20 mu m to obtain a Cu/stainless steel screen; placing the Cu/stainless steel wire net in 1mM AgNO 3 Standing the solution for 5min, taking out, cleaning and drying the solution to obtain the Ag-Cu/stainless steel wire mesh with one hydrophobic surface.
The following examples illustrate the effectiveness of the invention using Cu/PTFE and Ag-Cu/stainless steel wire mesh as examples. The chemical reagents used are all conventional commercial products, and are analytically pure reagents.
Example 1
10mL of nitrate electrolyte (0.1 mol/L) was used as a catholyte, 10mL of an aqueous solution of anhydrous sodium sulfate (1.0 mol/L) was used as an anolyte, and 2mol/L of sodium hydroxide solution was used to adjust the pH of the catholyte to 10. Nitrate electrochemical reduction was performed using a three-electrode system with Cu/PTFE as the working electrode, the catalyst side and the non-catalyst side of the catalyst membrane facing the electrolyte and gas collection chamber, respectively. The reduction voltage is set to-0.8V (vs Hg/Hg) 2 SO 4 ) Constant potential electrolysis with current density of about 25mA/cm 2 And ammonia gas is generated in the gas collecting chamber, the Faraday efficiency is 92%, and the separation efficiency is 99%.
Example 2
10mL of nitrate electrolyte (0.1 mol/L) was used as catholyte, and 10mL of anhydrous sodium sulfate aqueous solution (1.0 mol/L) was used as anolyte. Nitrate radical electrochemical reduction is carried out by adopting a three-electrode system and Cu/PTFE is used as a working mediumAs electrodes, the catalyst side and the non-catalyst side of the catalyst membrane face the electrolyte and gas collection chambers, respectively. Constant current electrolysis with current density set at 50mA/cm 2 And ammonia gas is generated in the gas collecting chamber, the Faraday efficiency is 93 percent, and the separation efficiency is 98 percent. The ammonia gas can be obtained without adding alkali liquor.
Example 3
10mL of nitrate electrolyte (0.1 mol/L) was used as catholyte, 10mL of anhydrous sodium sulfate aqueous solution (1.0 mol/L) was used as anolyte, and 0.1mol/L sulfuric acid solution was used to adjust the pH of the catholyte to 5. Nitrate electrochemical reduction was performed using a three-electrode system with Cu/PTFE as the working electrode, the catalyst side and the non-catalyst side of the catalyst membrane facing the electrolyte and gas collection chamber, respectively. Constant current electrolysis with a current density of 100mA/cm 2 And ammonia gas is generated in the gas collecting chamber, the Faraday efficiency is 89%, and the separation efficiency is 90%.
Example 4 (comparative example)
10mL of nitrate electrolyte (0.1 mol/L) was used as catholyte, 10mL of an aqueous solution of anhydrous sodium sulfate (1.0 mol/L) was used as anolyte, and the pH of the catholyte was adjusted to 0.5 with 0.1mol/L sulfuric acid solution. A three-electrode system is adopted for electrochemical reduction of nitrate, cu/PTFE is used as a working electrode, and the catalyst side and the non-catalyst side of the catalyst membrane respectively face an electrolyte and a gas collecting chamber. Constant current electrolysis with current density set at 10mA/cm 2 The Faraday efficiency was 54%, but the gas collection chamber was ammonia-free and no ammonia was separated by the membrane.
Example 5 (comparative example)
10mL of nitrate electrolyte (0.1 mol/L) was used as catholyte, 10mL of an aqueous solution of anhydrous sodium sulfate (1.0 mol/L) was used as anolyte, and the pH of the catholyte was adjusted to 13 with 2mol/L sodium hydroxide solution. A three-electrode system is adopted for electrochemical reduction of nitrate, cu/PTFE is used as a working electrode, and the catalyst side and the non-catalyst side of the catalyst membrane respectively face an electrolyte and a gas collecting chamber. Constant current electrolysis with current density set at 10mA/cm 2 Faraday efficiency is 11%, a small amount of ammonia gas exists in the gas collecting chamber, and the ammonia separation efficiency is 90%.
Example 6
10mL of nitrate electrolyte (0.1 mol/L) was used as a catholyte, and 10mL of an aqueous solution of anhydrous sodium sulfate (1.0 mol/L) was used as an anolyte. Nitrate electrochemical reduction was performed using a three-electrode system with Cu/PTFE as the working electrode, the catalyst side and the non-catalyst side of the catalyst membrane facing the electrolyte and gas collection chamber, respectively. Constant current electrolysis with current density set at 100mA/cm 2 And ammonia gas is generated in the gas collecting chamber, the Faraday efficiency is 90%, and the separation efficiency is 97%.
Example 7
10mL of nitrate electrolyte (0.1 mol/L) was used as a catholyte, and 10mL of an aqueous solution of anhydrous sodium sulfate (1.0 mol/L) was used as an anolyte. A three-electrode system is adopted for electrochemical reduction of nitrate, cu/PTFE is used as a working electrode, and the catalyst side and the non-catalyst side of the catalyst membrane respectively face an electrolyte and a gas collecting chamber. Constant current electrolysis with a current density of 150mA/cm 2 And ammonia gas is generated in the gas collecting chamber, the Faraday efficiency is 85 percent, and the separation efficiency is 97 percent.
Example 8
10mL of nitrate electrolyte (0.1 mol/L) was used as catholyte, and 10mL of anhydrous sodium sulfate aqueous solution (1.0 mol/L) was used as anolyte. A three-electrode system is adopted for electrochemical reduction of nitrate, cu/PTFE is used as a working electrode, and the catalyst side and the non-catalyst side of the catalyst membrane respectively face an electrolyte and a gas collecting chamber. Constant current electrolysis with current density set at 200mA/cm 2 And ammonia gas is generated in the gas collecting chamber, the Faraday efficiency is 80 percent, and the separation efficiency is 97 percent.
Example 9
10mL of nitrate electrolyte (0.1 mol/L) was used as a catholyte, and 10mL of an aqueous solution of anhydrous sodium sulfate (1.0 mol/L) was used as an anolyte. A three-electrode system is adopted for carrying out nitrate electrochemical reduction, an Ag-Cu/stainless steel wire net is used as a working electrode, the Ag-Cu side of a catalyst film faces to an electrolyte, and the hydrophobic treatment side faces to a gas collection chamber. Constant current electrolysis with a current density of 100mA/cm 2 And ammonia gas is generated in the gas collection chamber, the Faraday efficiency is 83 percent, and the separation efficiency is 91 percent.
Example 10
10mL of nitrate electrolyte (1.0 mol/L) was used as catholyte, and 10mL of anhydrous sodium sulfate aqueous solution (1.0 mol/L) was used as anolyte. A three-electrode system is adopted for electrochemical reduction of nitrate, cu/PTFE is used as a working electrode, and the catalyst side and the non-catalyst side of the catalyst membrane respectively face an electrolyte and a gas collecting chamber. Constant current electrolysis with a current density of 100mA/cm 2 And ammonia gas is generated in the gas collecting chamber, the Faraday efficiency is 91 percent, and the separation efficiency is 97 percent. The invention is also suitable for the conversion of high-concentration nitrate wastewater and the separation of ammonia.
Example 11
10mL of nitrite electrolyte (0.1 mol/L) was used as a catholyte, and 10mL of an aqueous solution of anhydrous sodium sulfate (1.0 mol/L) was used as an anolyte. Nitrate electrochemical reduction was performed using a three-electrode system with Cu/PTFE as the working electrode, the catalyst side and the non-catalyst side of the catalyst membrane facing the electrolyte and gas collection chamber, respectively. Constant current electrolysis with a current density of 100mA/cm 2 And ammonia gas is generated in the gas collection chamber, the Faraday efficiency is 88 percent, and the separation efficiency is 93 percent. The invention is equally applicable to the conversion of nitrite and the separation of ammonia.

Claims (2)

1. A method for directly preparing ammonia by electrochemical reduction of nitrite and/or nitrate is characterized in that: electrochemical reduction of nitrite and/or nitrate containing electrolyte via porous membrane electrode to produce ammonia gas; the porous membrane electrode has a catalytic effect on the electrochemical reduction of nitrite and nitrate; when the pH value of the electrolyte containing nitrite and/or nitrate is greater than or equal to 7 and less than 13, the current density of the electrode in the electrochemical reduction process is greater than 10mA/cm 2 When the pH value of the electrolyte containing nitrite and/or nitrate is more than 1 and less than 7, the current density of the electrode in the electrochemical reduction process is more than 50mA/cm 2 ；
In the electrochemical reduction process, electrolyte containing nitrite and/or nitrate is taken as electrolyte of a cathode chamber, a porous membrane electrode is taken as a working electrode, one side of the porous membrane electrode faces the electrolyte of the cathode chamber, and the other side faces a gas collecting chamber;
the porous membrane electrode is Cu/PTFE or Ag-Cu/stainless steel wire mesh.
2. The method for directly preparing ammonia gas by electrochemical reduction of nitrite and/or nitrate according to claim 1, is characterized in that: the concentration of the electrolyte containing nitrite and/or nitrate is 10 mu mol/L-1.0 mol/L.
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