Method for the electrochemical synthesis of ammonia and device for performing the method

Abstract

A method for electrochemical synthesis of ammonia comprises (a) forming an atmospheric pressure non-thermal plasma (APNTP) from a gas mixture comprising nitrogen and oxygen in a plasma device, (b) introducing the APNTP into an aqueous electrolyte solution to form a solution of one or more substances of formula NOx-, (c) contacting the solution of (b) with a cathode of an electrochemical cell, the cathode comprises a catalyst capable of catalyzing the electrochemical reduction of the one or more species of formula NOx-to NH3, and (d) applying a potential or current on the electrochemical cell to effect electrochemical synthesis of the ammonia.

Description

Technical Field

[0001] The present invention relates to a method for synthesizing ammonia by electrochemically reducing reactive nitrogen-oxygen species formed from air, and an apparatus (system) and a catalyst for use in the method. Background Art

[0002] Ammonia (NH3) is a widely important chemical, which is currently typically produced on a large scale by the one-step Haber-Bosch (H-B) process. Ammonia production by this process requires a large amount of energy consumption and is also associated with low efficiency. The century-old Haber-Bosch process for ammonia synthesis requires demanding operating conditions, including high temperatures (400 °C to 500 °C) and high pressures (150 atm to 300 atm), uses pure N2 and H2 gases, and a heterogeneous iron-based catalyst. Therefore, the Haber-Bosch process is expensive and requires a large amount of plant infrastructure. In 2019, ammonia (NH3) had a global production volume of 235 million tons, which accounted for 1% to 2% of the world's energy supply and contributed approximately 1% of the total global energy-related CO2 emissions. An emerging alternative to the Haber-Bosch process is the electrochemical synthesis of ammonia by the nitrogen reduction reaction (eNRR). The electrocatalytic reduction process is regarded as an environmentally friendly method for NH3 production; in fact, it can be carried out under mild conditions such as room temperature and atmospheric pressure, and it can also be powered by renewable energy. In addition, the NRR carried out in an aqueous environment eliminates the need for a pure H2 source as a reactant in the reduction reaction. However, the direct electrochemical conversion of H2 to ammonia has disadvantages such as extremely low NH3 yields and low Faradaic efficiency. These disadvantages are due to the highly non-reactive nature of N2, its low solubility and diffusion ability in the electrolyte. In addition, the electrochemical NRR is hindered by competition with the hydrogen evolution reaction (HER), because hydrogen production usually occurs at a lower overpotential than the eNRR.

[0003] It has now been found that the disadvantages associated with the eNRR can be overcome by activating N2 into a more reactive intermediate form. A chargeable nitrogen oxide that is a starting material for electrochemical ammonia synthesis is a worthy intermediate to consider. This is because the charged nitrogen oxide has a relatively low N-O bond dissociation energy and excellent solubility in water. Inspired by the natural lightning fixation of nitrogen and the rapid development of atmospheric pressure non-thermal plasma (APNTP) technology, it has been found that the high-energy electrons of the plasma can achieve the activation of N2 into a reactive nitrogen oxide species of the formula NO x The additional advantage of APNTP is that air can be directly used as a gas source and starting material for converting N2 into a nitrogen oxide species, which can then be absorbed by the electrolyte to form a nitrogen oxide species of the formula NOx - Reactive negatively charged species (mainly nitrites and to a lesser extent nitrates) which in turn can be reduced directly to ammonia at the cathode of the electrochemical cell.

[0004] NO generated in electrochemical cells x - The reduction reaction is:

[0005] NO3 - +9H + +8e - →NH3+3H2O E o =0.88V (relative to RHE)

[0006] NO2 - +7H + +6e - →NH3+2H2O E o =0.86V (relative to RHE)

[0007] Selective electrocatalysis of NO at relatively low potentials x - The key to the conversion of NO to NH3 is the rational design and the ability to favorably catalyze NO x - reduction and provide sufficient H* and e - The electrocatalyst, the H* and e - Instead of undergoing dimerization to form H2, it can participate in intermediate hydrogenation in alkaline electrolytes. Additional tuning of components in, for example, electrochemical nitrate reduction reaction (NitRR) systems (e.g., electrolyte, electrodes, and applied potential) can also contribute to higher ammonia yields and Faradaic efficiencies. Summary of the invention

[0008] The present invention provides a method for the electrochemical synthesis of ammonia.

[0009] The method comprises:

[0010] (a) forming an atmospheric pressure non-thermal plasma (APNTP) from a gas mixture comprising nitrogen and oxygen as starting material in a plasma device,

[0011] (b) introducing the APNTP formed in the plasma device into an alkaline or neutral aqueous electrolyte solution to form a NO x - A solution of one or more substances, wherein x is 1, 2 or 3,

[0012] (c) contacting the solution of (b) with the cathode of an electrochemical cell, the electrochemical cell comprising the cathode, an anode, and an aqueous electrolyte, the cathode comprising a catalyst A, the catalyst A being capable of electrochemically reducing the one or more substances of formula NO x - to NH3 at the cathode, and the anode comprising a catalyst capable of electrochemically oxidizing water to oxygen (O2), and

[0013] (d) applying a potential or current to the electrochemical cell to effect the electrochemical synthesis of ammonia.

[0014] In one embodiment of the method, in step (a), the APNTP is formed by directly applying an electric field across the electrodes and / or a plasma is formed using a compressed gas mixture comprising nitrogen and oxygen (e.g., air or oxygen-enriched air). For example, the compressed gas mixture may be at a pressure from 2 bar to 10 bar and / or the flow rate of the gas mixture in the plasma device may be from 0.1 L / min to 1 L / min and / or the voltage of the plasma device is from 10 kV to 20 kV. For example, the compressed gas mixture may comprise oxygen and nitrogen in a molar ratio from 95:5 to 5:95.

[0015] In one embodiment, the aqueous electrolyte solution employed in step (b) of the method of the present invention comprises an alkali metal and / or an alkaline earth metal hydroxide. For example, the alkali metal and / or alkaline earth metal hydroxide may consist of or comprise KOH. For example, the concentration of the alkali metal and / or alkaline earth metal hydroxide in the solution may be from 0.1 M to 3 M.

[0016] In one embodiment, the electrochemical cell further comprises a separator, such as a separator comprising an anion exchange membrane sheet.

[0017] In one embodiment, the aqueous electrolyte in the electrochemical cell is an aqueous solution of an alkali metal and / or an alkaline earth metal hydroxide. For example, the alkali metal or alkaline earth metal hydroxide may consist of or comprise KOH, and / or the concentration of the alkali metal and / or alkaline earth metal hydroxide in the alkaline aqueous electrolyte may be from 0.1 M to 6.6 M, such as from 1 M to 3 M. The aqueous electrolyte in the electrochemical cell may be the same as or substantially the same as the alkaline aqueous solution employed in step (b) of the method of the present invention. However, the aqueous electrolyte may also be substantially neutral and may, for example, comprise Na2SO4 or any other water-soluble sulfate. For example, the electrolyte may be an aqueous salt solution having a pH from 6.5 to 7.5 using 0.1 M to 3 M Na2SO4.

[0018] In one embodiment of the method, formula NOx - The one or more substances described above at least include nitrite (NO2 - ) substances.

[0019] In one embodiment, the concentration of the one or more substances in the solution used in step (b) of the formula NO x - is from 0.1 mM to 50 mM, for example from 1 mM to 30 mM.

[0020] In one embodiment of the method of the present invention, a catalyst A capable of electrochemically reducing the one or more substances of the formula NO x - to NH3 at the cathode of an electrochemical cell includes one or more of Ni, Co, Ru, Pt, Cu, Fe, La, Y, Ce, Ti, Pd, B, Sr, Ba, W, Rh, Au, Cr, Re, Os, In, Pb, Sb. For example, catalyst A at least includes Ru and Cu. For example, the atomic ratio of Ru:Cu is from 90:10 to 10:90. In addition, catalyst A may be present on a mesh (e.g., a Ni mesh). Catalyst A may also be present in and / or on a carbon-based or metal foam, felt, cloth, sponge, mesh. Catalyst A may further be present in the form of particles, for example particles in the range from 1 nm to 100 μm in size. Additionally, catalyst A may be present in the form of dispersed single atoms or atomic clusters.

[0021] The present invention further provides a device (also referred to herein as a "system") for performing the method of the present invention as described above (including its various embodiments). The device at least includes (i) a plasma device capable of forming APNTP from a gas mixture including nitrogen and oxygen, and (ii) an electrochemical cell including a cathode, an anode, and an aqueous electrolyte, the cathode including catalyst A, the catalyst A being capable of electrochemically reducing the one or more substances of the formula NO x - to NH3 at the cathode of the electrochemical cell.

[0022] In one embodiment of the device, the plasma device is configured to form the APNTP by directly applying an electric field across electrodes. For example, the voltage of the plasma device may be from 10 kV to 20 kV.

[0023] In one embodiment, the aqueous electrolyte includes an alkali metal and / or an alkaline earth metal hydroxide. For example, the alkali metal and / or alkaline earth metal hydroxide may consist of or include KOH, and / or the concentration of the alkali metal and / or alkaline earth metal hydroxide in the electrolyte may be from 0.1 M to 6.6 M. However, the aqueous electrolyte may also be substantially neutral and may include, for example, Na2SO4 or any other water-soluble sulfate. For example, the electrolyte may be an aqueous salt solution having a pH from 6.5 to 7.5 using 0.1 M to 3 M Na2SO4.

[0024] In one embodiment, the electrochemical cell further includes a separator, such as a separator including an anion exchange membrane sheet.

[0025] In one embodiment of the device, a catalyst A capable of catalytically electrochemically reducing NO x - at the cathode of the electrochemical cell to NH3 includes one or more of Ni, Co, Ru, Pt, Cu, Fe, La, Y, Ce, Ti, Pd, B, Sr, Ba, W, Rh, Au, Cr, Re, Os, In, Pb, Sb. For example, catalyst A at least includes Ru and Cu, and for example, the atomic ratio of Ru:Cu is from 90:10 to 10:90. In addition, catalyst A may be present on a mesh (e.g., a Ni mesh).

[0026] In one embodiment of the device, the device further includes a container / reservoir for receiving APNTP in the plasma device and containing an aqueous electrolyte solution. For example, the aqueous electrolyte solution may include an alkali metal and / or an alkaline earth metal hydroxide. The alkali metal and / or alkaline earth metal hydroxide may consist of or include KOH, and / or the concentration of the alkali metal and / or alkaline earth metal hydroxide in the aqueous solution may be from 0.1 M to 6.6 M. However, the aqueous electrolyte solution may also be substantially neutral and may include, for example, Na2SO4 or any other water-soluble sulfate. For example, the solution may be an aqueous salt solution having a pH from 6.5 to 7.5 using 0.1 M to 3 M Na2SO4.

[0027] In one embodiment, the device further includes a potentiostat and / or a galvanostat.

[0028] The present invention further provides a method for converting a gas mixture including nitrogen and oxygen into NO of the formula x -A method of one or more substances, where x is 1, 2, or 3. The method includes forming APNTP as a starting material from a gas mixture and introducing the formed APNTP into an alkaline aqueous solution to form a solution of the one or more substances of formula NO x - in the alkaline aqueous solution.

[0029] In one embodiment, the one or more substances of formula NO x - at least include nitrite (NO2 - ).

[0030] In one embodiment of the method, the APNTP is formed by directly applying an electric field across electrodes and / or using a compressed gas mixture including nitrogen and oxygen (e.g., air or oxygen-enriched air). For example, the pressure of the gas mixture can be from 2 bar to 10 bar and / or the flow rate of air in the plasma device can be from 0.1 L / min to 1 L / min and / or the voltage of the plasma device can be from 10 kV to 20 kV.

[0031] In one embodiment of the method, the alkaline aqueous solution includes alkali metal and / or alkaline earth metal hydroxides. For example, the alkali metal and / or alkaline earth metal hydroxides can consist of or include KOH, and / or the concentration of the alkali metal and / or alkaline earth metal hydroxides in the solution can be from 0.1 M to 3 M.

[0032] The present invention also provides a catalyst for catalytic electrochemical synthesis of ammonia. The catalyst is capable of catalytically reducing substances of formula NO x - to ammonia at the cathode of an electrochemical cell, where x is 1, 2, or 3. The catalyst at least includes, for example, Cu and Ru with an atomic ratio from 90:10 to 10:90.

[0033] In one embodiment, the catalyst is present on a Ni mesh, for example, in the form of a nano sponge (coral-like).

[0034] In one embodiment, the catalyst can be obtained by reducing an aqueous solution of Cu salts and Ru salts. The reducing agent can be, for example, NaBH4 or include NaBH4. However, other reducing agents such as ethylene glycol, hydrogen, hydrazine, Na4O6P2, and C2H6O can also be suitable. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The present invention is further described in the following detailed description by way of non-limiting examples of exemplary embodiments of the present invention. In the drawings:

[0036] Figure 1 Schematically shows the design of a plasma-assisted device prototype for electrochemical synthesis used in the experiments described below;

[0037] Figure 2 Graphically represents the results of linear sweep voltammetry obtained using nickel mesh samples treated differently in the presence and absence of nitrate ions in the electrolyte in the experiments described below;

[0038] Figure 3 Graphically represents the results of linear sweep voltammetry obtained using acid-etched Ni mesh at different nitrate ion concentrations in the electrolyte in the experiments described below;

[0039] Figure 4 Graphically represents the ammonia production rate and Faraday efficiency obtained using acid-etched Ni mesh as a function of the applied potential in the experiments described below; and

[0040] Figure 5 Graphically represents the ammonia production rate and Faraday efficiency obtained using acid-etched Ni mesh as a function of the applied potential in the experiments described below under conditions different from those Figure 4 employed. Detailed Description

[0041] The details presented herein are by way of example and are for purposes of illustrative discussion of embodiments of the invention only and are presented to provide a description believed to be the most useful and readily understood of the principles and conceptual aspects of the invention. In this regard, the level of detail shown in the structure of the invention is only that necessary for a basic understanding of the invention, and the description using the drawings enables those skilled in the art to understand how to embody several forms of the invention in practice.

[0042] As used herein, the singular forms "a," "an," and "the" include plural references unless the context clearly dictates otherwise. For example, a reference to "a catalyst" will also mean a mixture of two or more catalysts may be present, unless specifically excluded.

[0043] Unless otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, etc. used in this specification and the appended claims are to be understood as being modified in all instances by the term "about." Accordingly, unless indicated to the contrary, the numerical parameters set forth in this specification and the appended claims are approximations that may vary depending upon the desired properties sought to be obtained by the invention. At the very least, each numerical parameter should be construed in light of the number of significant digits and the ordinary rounding convention.

[0044] In addition, the disclosure of numerical ranges in this specification is considered to be a disclosure of all the numerical values and ranges within said ranges. For example, if the range is from 0.1 to 50, then it is considered to include, for example, 0.3, 1, 7, 34, 46.1, 23.7 or any other value or range within said range.

[0045] As described above, the method of the present invention for the electrochemical synthesis of ammonia comprises:

[0046] (a) forming an atmospheric pressure non-thermal plasma (APNTP) as a starting material from a gas mixture comprising nitrogen and oxygen in a plasma device,

[0047] (b) introducing the APNTP formed in the plasma device into an alkaline or neutral aqueous electrolyte solution to form a solution of one or more substances of the formula NO x - wherein x is 1, 2 or 3,

[0048] (c) contacting the solution of step (b) with the cathode of an electrochemical cell, said electrochemical cell comprising said cathode, an anode (and preferably a separator) and an aqueous electrolyte, said cathode comprising a catalyst A which is capable of catalytically electrochemically reducing one or more substances of the formula NO x - to NH3 at the cathode, and

[0049] (d) applying a potential or current to the electrochemical cell to effect the electrochemical synthesis of ammonia.

[0050] If air is used as a component of the gas mixture, then the air can be admixed with oxygen to increase the amount of active N-O species and, after introducing the plasma into the alkaline aqueous solution, is conducive to the formation of nitrite rather than nitrate. The molar ratio of nitrogen (N2) to oxygen (O2) in the gas mixture is not particularly limited but will generally be from 95:5 to 5:95, such as from 90:10 to 10:90, from 85:15 to 15:85, from 80:20 to 20:80, from 75:25 to 25:75, from 70:30 to 30:70 or from 60:40 to 40:60. Only the ratio of nitrogen to oxygen in air is approximately 81:19.

[0051] In step (a), an APNTP can be formed by directly applying an electric field across the electrodes. Additionally, the gas mixture used to form the plasma will typically be compressed. For example, the gas mixture can be at a pressure ranging from 2 bar to 10 bar (e.g., from 3 bar to 8 bar or from 4 bar to 7 bar). The flow rate of the gas mixture in the plasma device can be, for example, from 0.1 L / min to 1 L / min, but higher or lower flow rates are also suitable. The voltage of the plasma device can be from 10 kV to 20 kV, but higher or lower voltages are also suitable.

[0052] The alkaline aqueous solution used in step (b) of the method of the present invention includes alkali metal and / or alkaline earth metal hydroxides. For example, the alkali metal and / or alkaline earth metal hydroxides can consist of or include hydroxides of NaOH and / or KOH and / or Mg and / or Ca, but other hydroxides are also suitable. The concentration of the hydroxide can, for example, range from about 0.1 M to about 9 M (e.g., from about 0.1 M to about 5 M, or from about 0.1 M to about 3 M), and the pH of the solution will typically be at least about 8 (e.g., at least about 9, at least about 10, or at least about 11).

[0053] The concentration of one or more substances of the formula NO in the solution used in step (b) will typically be from 0.1 mM to 50 mM (e.g., at least about 1 mM), but higher or lower concentrations can also be suitable. x -

[0054] The aqueous electrolyte of the electrochemical cell can be a liquid and / or gel electrolyte and will typically include hydroxides of alkali metals such as Na and / or K (specifically, KOH) and / or hydroxides of alkaline earth metals such as Mg and / or Ca. The concentration of the hydroxide can, for example, range from about 0.1 M to about 9 M (e.g., from about 0.1 M to about 5 M, or from about 0.1 M to about 3 M), and the pH of the electrolyte will typically be at least about 8 (e.g., at least about 9, at least about 10, or at least about 11).

[0055] The device for performing the method of the present invention includes an electrochemical cell, which includes a cathode, an anode, and an aqueous electrolyte. The cathode includes one or more catalysts capable of catalyzing the reduction of NO substances to ammonia (and preferably minimizing the formation of H2). The anode is made of a conductive material that is inert with respect to the electrolyte and antioxidant. For example, the anode can include a Ni mesh coated with a catalyst capable of catalyzing the electrochemical oxidation of water to oxygen. x -

[0056] ​​An electrochemical cell will typically further include a separator (e.g., an anion exchange membrane or a polymer film) that permits the passage of ions and separates the cathode side of the electrolyte from the anode side of the electrolyte.

[0057] The method of the present invention and specifically NO x - The electrochemical reduction of NO substances to form ammonia can be carried out at a temperature from about 20 °C to about 200 °C and / or at a pressure from about atmospheric pressure to about 10 atm. For example, it can (and preferably) be carried out at atmospheric pressure and ambient temperature (room temperature) (e.g., from about 20 °C to about 30 °C).

[0058] The method can be carried out continuously or batchwise, with continuous or semi - continuous operation being preferred.

[0059] Catalysts capable of catalyzing the x - electrochemical reduction of NO substances to ammonia can be deposited on metal or carbon - based foams, felts, cloths, sponges, meshes, etc., and can include, for example, one or more of Ni, Co, Ru, Pt, Cu, Fe, La, Y, Ce, Ti, Pd, B, Sr, Ba, W, Rh, Au, Cr, Re, Os, In, Pb, Sb, in their as - received form or in the form of their physical mixtures, alloys, and compounds. The catalyst can also be a ceramic material, such as oxides, nitrides, carbides, etc. In addition, the catalyst can exist in various shapes, such as nanoparticles, cubes, dendrites, rods, flowers, spikes, etc.

[0060] For example, known catalysts suitable for NitRR are disclosed in the following: "The origin of selective nitrate-to-ammonia electroreduction on metal-free nitrogen-doped carbon aerogel catalysts" by Li et al., Applied Catalysis B: Environmental, Vol. 331, August 15, 2023, 122677; "Electrocatalytic nitrate reduction to ammonia via amorphous cobalt boride" by Shi et al., Chemical Communications, Issue 62, 2022; "Efficient Electrochemical Nitrate Reduction to Ammonia with Copper-Supported Rhodium Cluster and Single-Atom Catalyst" by Liu et al., Angewandte Chemie, published on March 17, 2022; "A multifunctional copper single-atom electrocatalyst aerogel for smart sensing and producing ammonia from nitrate" by Li et al., June 20, 2023, doi.org / 10.1073 / pnas.2305489120; "A two-dimensional MXene-supported CuRu catalyst for efficient electrochemical nitrate reduction to ammonia" by Zhao et al., Catalysis Science & Technology, 13(19), 5543 to 5548 (2023); "Alloying of Cu with Ru Enabling the Relay Catalysis for Reduction of Nitrate to Ammonia" by Gao et al., Advanced Materials, 35(19), 2202952 (2023); "Roles of Copper in Nitrate Reduction at Copper-Modified Ru / C Catalysts" by Chen et al., The Journal of Physical Chemistry C, 127(6), 2918 to 2928 (2023); "Efficient conversion of low-concentration nitrate sources into ammonia on a Ru-dispersed Cu nanowire electrocatalyst" by Chen et al., Nature Nanotechnology, 17(7), 759 to 767 (2022). The entire disclosures of these documents are incorporated herein by reference.

[0061] Capable of catalytically NO at the cathode of an electrochemical cell x -The currently preferred catalyst A for electrochemically reducing one or more substances to NH3 may include one or more of Ni, Co, Ru, Pt, Cu, Fe, La, Y, Ce, Ti, Pd, B, Sr, Ba, W, Rh, Au, Cr, Re, Os, In, Pb, Sb. Other elements may also be present. For example, catalyst A may at least include Ru and Cu, such as an atomic ratio of Ru:Cu from 9:1 to 1:9, such as from 8:2 to 2:8, from 7:3 to 3:7, from 6:4 to 4:6, or about 1:1. In addition, catalyst A may be present on a mesh such as a Ni mesh. Catalyst A may also be present in and / or on carbon-based or metal foams, felts, cloths, sponges, meshes, etc. Catalyst A may further be present in particulate form, such as particles in the range from 1 nm to 100 μm in size.

[0062] To prepare a suitable cathode, several methods can be used, including electrochemical and / or chemical deposition and / or impregnation and / or other coatings on metal or carbon-based foams, felts, cloths, sponges, meshes, etc.

[0063] The apparatus for carrying out the method of the present invention includes an electrochemical cell, which includes a cathode, an anode (preferably a separator), and an aqueous electrolyte. The cathode includes one or more catalyst A, which is capable of catalyzing the reduction of NO x - substances (and preferably minimizing the formation of H2). The anode is made of a conductive material that is inert with respect to the electrolyte and oxidation and includes one or more catalysts capable of catalyzing the oxidation of water to oxygen. For example, the anode may be in the form of a metal mesh such as a Ni mesh, which is preferably coated with a suitable catalyst for the oxidation reaction.

[0064] Experimental section

[0065] A prototype of a plasma-assisted electrochemical device for the electrochemical synthesis of ammonia was constructed. In Figure 1 its overall design is depicted, where the reference symbols are as follows:

[0066] 1 Power supply (DC 10V; 20A)

[0067] 2 Air gas plasma (APNTR)

[0068] 3 NO x Generation chamber

[0069] 4 Cathode electrolyte tank

[0070] 5 Cathode

[0071] 6a / b Cathode KOH chamber

[0072] 7 Anode electrolyte tank

[0073] 8 Anode

[0074] 9 Anode KOH Chamber

[0075] 10 Separator

[0076] 11 NH3 Trap

[0077] 12 KOH + NO x - Pump

[0078] 13 Cathode Electrolyte Pump

[0079] 14 Anode Electrolyte Pump

[0080] 15 Gas Inlet

[0081] 16 Excess N2 Outlet

[0082] 17 NH3 / H2 Outlet

[0083] 18 O2 Outlet

[0084] 19 H2 Outlet

[0085] EH Electric Heater

[0086] TS Temperature Sensor

[0087] PS Pressure Sensor

[0088] The electrochemical ammonia synthesis using the prototype of the electrochemical device includes the following features / components:

[0089] · A non-thermal plasma device operated using a compressed air stream (about 6 bar) and a high-voltage plasma generator. The plasma device gas outlet is directly introduced into the electrolyte in the NO x - production reservoir.

[0090] · A NO x - production reservoir for collecting the reactive nitrogen oxides produced by the plasma device. The reservoir contains KOH with a concentration ranging from 0.1 M to 3 M.

[0091] · NO x - The production reservoir is connected to the cathode electrolyte reservoir through a peristaltic pump / centrifugal pump and a feedback loop for introducing NO x - into the electrochemical cell.

[0092] · Including NO embedded in the metal (foam Ni mesh) working electrode x -The cathode of the reduced catalyst particles (see below).

[0093] · An anode composed of a nickel mesh and a catalyst for water oxidation.

[0094] · A peristaltic / centrifugal pump with variable flow rate for flowing the liquid electrolyte from the reservoir tank into the electrochemical cell in a closed loop.

[0095] · A KOH alkaline electrolyte with a concentration ranging from 0.1 M to 6.6 M.

[0096] · An electrolyte reservoir tank equipped with a heating rod to achieve temperature control of the electrolyte.

[0097] · A mercury / mercuric oxide - Hg / HgO (MMO) reference electrode connected to the cell through a salt bridge.

[0098] · The cathode electrolyte chamber is separated from the anode electrolyte chamber by an anion exchange membrane.

[0099] · The outlet of the cathode electrolyte and gas chamber is connected to a trap containing a dilute solution of sulfuric acid (H2SO4) to retain the synthesized ammonia as ammonium ions.

[0100] · An electronic device (potentiostat / galvanostat) for applying a constant potential or current between the cathode and the anode.

[0101] The procedure for operating the cell is as follows:

[0102] 1. Once the cell is assembled and all electrodes are in place, load the cell with fresh electrolyte.

[0103] 2. Introduce fresh electrolyte into the NO x - production reservoir.

[0104] 3. Place fresh sulfuric acid solution in the trap and connect it to the cathode gas outlet.

[0105] 4. Bubble N2 gas through the cathode electrolyte and gas chamber to purge the dissolved oxygen.

[0106] 5. Connect the electrodes to the potentiostat / galvanostat and measure the open - circuit voltage (OCV) of the system.

[0107] 6. Introduce compressed air at a pressure of about 6 bar and a flow rate of 0.1 L / min to 1 L / min into the plasma device, which operates at a voltage of 10 kV to 20 kV to produce reactive nitrogen oxides, and introduce the reactive nitrogen oxides into the container / reservoir.

[0108] 7. The generated and dissolved NO in the reservoirx - The substance is transferred to the cathode electrolyte cell.

[0109] 8. Apply a constant potential technique between -0.8 V and -1.3 V relative to MMO (mercury / mercuric oxide) to the cell for a set time period while the electrolyte continues to flow into the chamber and the ammonia produced is collected in an acid trap. It is also possible to operate at a constant current density of 0.001 A cm -2 to 0.1 A cm -2 .

[0110] 9. Periodically sample the solution in the acid trap and test for the presence of ammonia.

[0111] 10. Record and observe the current and electrode potentials (cathode and anode) throughout the operation of the cell.

[0112] Preferably, use feedback loops from temperature sensors before and after the electrochemical cell to control the temperature of the electrolyte cell (EH), and use a feedback loop from a pressure sensor to control the flow rate by the speed of a liquid pump (centrifugal pump).

[0113] Determine the concentration of the ammonia produced by a colorimetric method using a UV VIS spectrometer. Two identification and quantification methods are used: one using Nessler's reagent - potassium tetraiodomercurate (II) and the other using the Berthelot reaction that uses salicylic acid as an indophenol derivative. The NO x - (i.e., nitrite) concentration in the reservoir is determined by a colorimetric Greiss method and verified by ion chromatography techniques.

[0114] Preliminary results:

[0115] Use a 100 ml glass electrochemical H-cell loaded with 0.1 M KOH as the electrolyte at both compartments to perform an initial check of the NO x - reduction reaction. Adding nitrate ions to the cathode electrolyte at a fixed concentration of 50 mM is used as a simulation of the product of the plasma system. As an initial cathode for the nitrate reduction reaction, a nickel mesh is used to catalyze the reduction reaction. The mesh is subjected to various activation treatments such as acid etching, sodium borohydride reduction, and acetone exposure. After each treatment, place the Ni mesh in the electrochemical cell as the cathode and record linear sweep voltammograms using an MMO electrode as the reference in the presence of 50 mM potassium nitrate in the electrolyte and an MMO electrode as the control without adding nitrate.

[0116] The acid etching of the Ni mesh was carried out as follows: the Ni mesh was immersed in 6M or 3M HCl solution in a glass beaker at 25 °C, and then the beaker was placed in an ultrasonic bath and sonicated for 10 minutes (6M) or 30 minutes (3M). Then the mesh was removed from the acid, thoroughly rinsed with double-distilled H2O and then thoroughly rinsed with ethanol, and then dried overnight in a vacuum oven at 60 °C.

[0117] The results of these experiments are presented in Figure 2 (the Ni mesh “0.3ap” means a mesh with 0.3 mm mesh size). The highest response was obtained with the acid etching of the Ni mesh, where the potential for nitrate reduction shifted towards lower potentials, indicating higher reduction activity of the cathode.

[0118] Based on this preliminary result, the experiments with acid-etched Ni mesh were extended and linear sweep voltammetry (LSV) experiments were carried out at different nitrate concentrations (i.e., 0 mM, 1 mM, 5 mM, 10 mM and 50 mM). The results are presented in Figure 3 , which demonstrates that the onset potential as well as the current density increase with increasing nitrate concentration. The standard potential of NO3 - / NH3 is higher than that of H+ / H2, as can be seen from the following reactions:

[0119]

[0120] Therefore, by using a selective catalyst for NitRR, the electrochemical process can be separated from the hydrogen evolution reaction.

[0121] In addition, the limiting current for nitrate reduction can be observed at a high nitrate concentration of 50 mM (and slightly at 10 mM) at approximately -1.1 V, indicating that the working potential recommended for chronoamperometry can be close to -1.1 V, where mass transfer limitation is dominant. The separation between HER and NitRR can also be distinguished, as they start at approximately -1.1 V and approximately -0.9 V, respectively. The table summarizes the onset potential and limiting current for NitRR.

[0122] Table 1. Onset potential and limiting current for NitRR derived from LSV in Figure 3 (the onset potential and limiting current for NitRR derived from LSV in

[0123] <![CDATA[NO3 - Concentration [mM]]]> Initial potential [V, vs. MMO] <![CDATA[Limiting current [mA cm -2 > 0 -1.01 - 1 -0.93 - 5 -0.92 - 10 -0.92 ~ -1.5 @ -1.1 V 50 -0.89 ~ -2.8 @ -1.1 V

[0124] Next, to quantify the NOx reduction activity of the acid-etched Ni mesh, the electrochemical cell was operated at a constant nitrate concentration of 50 mM, thereby changing the applied potential at the cathode and monitoring the amount of nitrate converted to ammonia in the gas trap and electrolyte compartment as well as the Faradaic yield of the system. The results are presented in Figure 4 .

[0125] The figure is well correlated with Figure 2 the LSV results shown in, where the FE (Faradaic efficiency) of NitRR decreases from about -1.1 V because HER starts (starting at about -1.1 V). It can be seen that the FE and ammonia productivity are 2 to 3 orders of magnitude higher than those of the conventional nitrogen reduction reaction (NRR), where in the absence of a NO X selective catalyst, the peak FE reaches 96.8% at -1.0 V and the peak ammonia productivity reaches about 240 μg h -1 cm -2 . The background of the system is negligible, so the signal is related to NitRR and any contamination in the electrochemical system is not taken into account.

[0126] The ammonia production experiment described above was repeated using a lower nitrate concentration of 5 mM, which is one order of magnitude lower. The results are presented in Figure 5 .

[0127] Completely different characteristic distribution curves for both FE and ammonia productivity were obtained in this experiment. At -1.1 V, the FE decreased from about 65% to about 7%, which highlights the need to select a catalyst with higher selectivity in the lower concentration range (1 mM to 10 mM). Therefore, the productivity is characterized by an optimum value of 3.0 μg h at -1.1 V -1 cm -2 , which represents a 6-fold decrease compared to the experiment using 50 mM NO3 - . These results emphasize the importance of a selective catalyst for reducing NOx species.

[0128] Further experiments were conducted as follows:

[0129] Plasma: Operate the PVA TePla plasma pen device according to its manual. Flow 6 bar of compressed air into the device. Fill 500 ml of 1 M KOH into a 1000 ml glass beaker. Place the pen in a glass tube cone, where the tip of the cone is immersed in the KOH solution by about 1 cm. Place the plasma torch 5 cm to 6 cm above the liquid surface. Stir the KOH solution using a magnetic stirrer. Then ignite the plasma for 30 min and collect samples of the KOH solution every 10 min. Determine the NO x - concentration by a Grace test.

[0130] In the above experiment, NO3 could not be detected -The presence of ions in the electrolyte, thus indicating the preferred activation of nitrogen in this plasma leads to the formation of nitrite. The following nitrite concentrations [ppm] were found after 10 min, 20 min, and 30 min: 35.85, 59.96, 72.32.

[0131] Electrochemical cell: First, rinse with distilled water and then assemble the electrochemical cell. Each chamber consists of an electrode holder, a sintered glass gas purge tube, and a gas outlet stream. The cathode electrolyte has a salt bridge for the reference electrode. The cathode electrolyte and the anode electrolyte are separated by an ionomer AF3 membrane.

[0132] All experiments were conducted at room temperature with a 1 M KOH solution and an Ar gas flow rate of 0.1 L / min. The outlet streams of the cathode electrolyte and the anode electrolyte were connected to an acid trap (5 mM H2SO4). The working electrode is a Ni mesh with a 0.3 mm pore size that has been etched with HCl (as described above), and a CuRu catalyst (see below) has been deposited onto the Ni mesh. The reference electrode is MMO (1 M KOH) and the anode consists of a nickel plate.

[0133] At the applied potential selected according to the LSV results, use the plasma-treated solution or a 5 mM KNO3 solution or a 50 mM KNO3 solution to perform chronoamperometry (CA) for 1 h under the same conditions. Samples were taken from each part of the system at t = 0 and t = 1 h. At the end of the experiment, samples from the acid trap were analyzed by the Nessler's reagent and samples from the electrolyte were analyzed by the salicylate method.

[0134] The FE of the CuRu catalyst (see below, Cu:Ru = 1:1) was found to be 100% + / - 2% and 89.07% at -0.85 V and -0.9 V (versus MMO), respectively.

[0135] The ammonia productivity of the catalyst was found to be 603.9 μg h -1 cm -2 and 1240.9 μg h -1 cm -2 .

[0136] Decided to focus on the concentration of 5 mM NO3 - because it is believed that the plasma will produce this concentration of NO per hour x - .

[0137] Based on LSV, the E-onset and ΔE window for NitRR can be determined. ΔE is the difference between the E-onset for NitRR and the E-onset for HER.

[0138] The following results were obtained:

[0139]

[0140] E onset indicates which catalyst has the best intrinsic properties for the NitRR reaction (Ru), and the ΔE window indicates which catalyst is the most selective (Cu 75 Ru 25 ).

[0141] At E = -0.9 vs. the reference electrode, the FE for NitRR was approximately 96%.

[0142] NitRR experiments using solutions containing plasma-derived NO2 - :

[0143] In the NitRR experiments performed in a glass H-cell, a KOH solution activated by the torch plasma described above was used as the nitrite-containing matrix. The cathode used for this experiment was a 2.5 cm nickel mesh coated with a Cu-Ru catalyst (Cu:Ru = 1:1), and the applied potential was -0.9 V for one hour. 2 nickel mesh, and the applied potential was -0.9 V for one hour.

[0144] In the first experiment, the electrolyte contained approximately 40 ppm nitrite, corresponding to approximately 1 mM nitrite. Ammonia was detected in the electrolyte, and its production rate was 1485.3 μg / h (corresponding to 675 μg / h / cm 2 ), and the FE was 75%.

[0145] A second batch of electrolyte containing 72 ppm nitrite (corresponding to approximately 3 mM nitrite) was tested in the same system under the same conditions. Ammonia was detected in the electrolyte, and its production rate was 1152.5 μg / h (corresponding to 524 μg / h / cm 2 ), and the FE was 99%, indicating that almost all of the current was converted, with an experimental error of approximately ±5% due to the deviation caused by the colorimetric method used.

[0146] Preparation of RuCu catalyst

[0147] Materials:

[0148] · Nickel mesh (electrode size 5 cm 2 )

[0149] · Ethanol

[0150] · 3M HCl

[0151] · Solution A: 2.8180 g of RuCl3 and 2.1311 g of CuCl2 in 50 ml of distilled water.

[0152] · Solution B: 1.8915 g of NaBH4 and 0.2000 g of NaOH in 50 ml of distilled water.

[0153] Steps for Solution A:

[0154] · Place the salt in a beaker.

[0155] · Add distilled water to the beaker.

[0156] · Stir the mixture with a glass rod for a few seconds until the solution becomes homogeneous.

[0157] Steps for Solution B:

[0158] · Measure NaOH in a conical flask.

[0159] · Add distilled water and swirl the mixture until the NaOH is completely dissolved.

[0160] · Add NaBH4 and swirl the mixture until the NaBH4 is completely dissolved.

[0161] Reaction steps:

[0162] 1. Immerse the Ni mesh sample in 3M HCl for 20 min.

[0163] 2. Rinse the sample three times with distilled water and ethanol.

[0164] 3. Immerse each sample in Solution A for about 3 seconds and then in Solution B for 10 seconds.

[0165] 4. Rinse the sample with distilled water to remove loose particles and NaBH4.

[0166] 5. Repeat steps 3 and 4 three times.

[0167] 6. Dry the sample in vacuum overnight.

[0168] The resulting CuRu / Ni mesh has a nanosponge morphology (coral-like) of Cu and Ru present on the Ni mesh. Cu is easily reducible (compared to Ru), so most of the Ru is deposited on Cu. The CuRu does not cover the entire Ni mesh surface. The stoichiometry (atomic %) measured by electron diffraction spectroscopy is an average of Cu5Ru / Ni mesh.

[0169] It was found that the FE of the CuRu catalyst produced using 5 mM nitrate added to an aqueous alkaline solution was 45.29% and 96.46% at -0.85 V and -0.9 V, respectively. Under these conditions, the ammonia production rate was found to be 23.02 μg / h / cm 2 and 90.72 μg / h / cm 2 .

Claims

1. A method for the electrochemical synthesis of ammonia (NH3), wherein the method comprises: (a) forming APNTP as a starting material from a gas mixture comprising nitrogen and oxygen in a plasma device, (b) Introduce the APNTP formed in the plasma device into an alkaline or neutral aqueous electrolyte solution to form a solution of one or more substances of the formula NO x - where x is 1, 2 or 3, (c) contacting the solution of (b) with the cathode of an electrochemical cell comprising the cathode, an anode, and an aqueous electrolyte, the cathode comprising a catalyst A capable of electrochemically reducing the one or more substances of formula NO x - to NH3 at the cathode, and (d) applying a potential or current to the electrochemical cell to achieve the electrochemical synthesis of the ammonia.

2. The method according to claim 1, wherein in (a), the APNTP is formed by directly applying an electric field across the electrodes.

3. The method according to claim 1 or claim 2, wherein in (a), a compressed gas mixture is used.

4. The method according to claim 3, wherein the flow rate of the gas mixture in the plasma device is from 0.1 L / min to 1 L / min.

5. The method according to any one of the preceding claims, wherein the voltage of the plasma device is from 10 kV to 20 kV.

6. The method according to any one of the preceding claims, wherein the aqueous electrolyte solution in (b) comprises an alkali metal and / or an alkaline earth metal hydroxide.

7. The method according to claim 6, wherein the alkali metal and / or alkaline earth metal hydroxide comprises KOH.

8. The method according to any one of claims 6 and 7, wherein the concentration of the alkali metal and / or alkaline earth metal hydroxide in the solution in (b) is from 0.1 M to 3 M.

9. The method according to any one of the preceding claims, wherein the electrochemical cell further comprises a separator.

10. The method according to claim 9, wherein the separator comprises an anion exchange membrane sheet.

11. The method according to any one of the preceding claims, wherein the aqueous electrolyte in the electrochemical cell is an aqueous solution of an alkali metal and / or an alkaline earth metal hydroxide.

12. The method according to claim 11, wherein the alkali metal and / or alkaline earth metal hydroxide comprises KOH.

13. The method according to any one of claims 11 and 12, wherein the concentration of the alkali metal and / or alkaline earth metal hydroxide in the aqueous electrolyte is from 0.1 M to 3 M.

14. The method according to any one of the preceding claims, wherein the one or more substances of formula NO x - comprise at least nitrite (NO2 - ) substances.

15. The method according to any one of the preceding claims, wherein the concentration of said one or more substances of formula NO x - in the solution of (b) is from 1 mM to 50 mM.

16. The method according to any one of the preceding claims, wherein the catalyst A comprises one or more of Ni, Co, Ru, Pt, Cu, Fe, La, Y, Ce, Ti, Pd, B, Sr, Ba, W, Rh, Au, Cr, Re, Os, In, Pb, Sb.

17. The method according to any one of the preceding claims, wherein the catalyst A comprises at least Ru and Cu.

18. The method according to claim 17, wherein the atomic ratio Ru:Cu in the catalyst is from 90:10 to 10:

90.

19. The method according to any one of claims 18 and 19, wherein the catalyst A is present on a Ni mesh.

20. The method according to any one of the preceding claims, wherein the catalyst A is present in or on a carbon-based or metal foam, felt, cloth, sponge, mesh.

21. The method according to any one of the preceding claims, wherein the catalyst A is present in the form of particles sized from 1 nm to 100 μm or in the form of a nanosponge.

22. An apparatus for performing the method according to any one of claims 1 to 21, wherein the apparatus comprises (i) a plasma device capable of forming APNTP from a gas mixture comprising nitrogen and oxygen, and (ii) an electrochemical cell comprising a cathode, an anode and an aqueous electrolyte, the cathode comprising a catalyst A capable of catalytically electrochemically reducing one or more substances of the formula NO x - to NH3.

23. The apparatus according to claim 22, wherein the plasma device is configured to form the APNTP by directly applying an electric field across the electrodes.

24. The apparatus according to any one of claims 22 and 23, wherein the voltage of the plasma device is from 10 kV to 20 kV.

25. The apparatus according to any one of claims 22 to 24, wherein the aqueous electrolyte comprises an alkali metal and / or an alkaline earth metal hydroxide.

26. The apparatus according to claim 25, wherein the alkali metal and / or alkaline earth metal hydroxide comprises KOH.

27. The apparatus according to any one of claims 25 and 26, wherein the concentration of the alkali metal and / or alkaline earth metal hydroxide in the electrolyte is from 0.1 M to 6.6 M.

28. The apparatus according to any one of the preceding claims, wherein the electrochemical cell further comprises a separator.

29. The apparatus according to claim 28, wherein the separator comprises an anion exchange membrane sheet.

30. The apparatus according to any one of claims 22 to 29, wherein the catalyst A comprises one or more of Ni, Co, Ru, Pt, Cu, Fe, La, Y, Ce, Ti, Pd, B, Sr, Ba, W, Rh, Au, Cr, Re, Os, In, Pb, Sb.

31. The apparatus according to any one of claims 22 to 30, wherein the catalyst A comprises at least Ru and Cu.

32. The apparatus according to claim 31, wherein the atomic ratio Ru:Cu is from 90:10 to 10:

90.

33. The apparatus according to any one of claims 31 and 32, wherein the catalyst A is present on a Ni mesh.

34. The apparatus according to any one of claims 22 to 33, wherein the equipment further comprises a container for receiving the alkaline aqueous solution for receiving the APNTP from the plasma device.

35. The apparatus according to claim 34, wherein the alkaline aqueous solution comprises an alkali metal and / or an alkaline earth metal hydroxide.

36. The apparatus according to claim 34, wherein the alkali metal and / or alkaline earth metal hydroxide comprises KOH.

37. The apparatus according to any one of claims 35 and 36, wherein the concentration of the alkali metal and / or alkaline earth metal hydroxide in the aqueous solution is from 0.1 M to 3 M.

38. The apparatus according to any one of claims 22 to 37, wherein the equipment further comprises a potentiostat and / or a galvanostat.

39. A method for converting a gas mixture comprising nitrogen and oxygen into one or more substances of the formula NO x - where x is 1, 2 or 3, the method comprising forming APNTP as a starting material from the gas mixture and introducing the formed APNTP into an aqueous electrolyte solution to form a solution of the one or more substances of the formula NO x - in the aqueous electrolyte solution.

40. The method according to claim 39, wherein the one or more substances of formula NO x - comprise at least nitrite (NO2 - ) substances.

41. A catalyst for the electrochemical synthesis of ammonia, wherein the catalyst is capable of catalyzing the reduction of a substance of formula NO x - to ammonia at the cathode of an electrochemical cell and comprises at least Cu and Ru in an atomic ratio from 9:1 to 1:9, where x is 1, 2 or 3.

42. The catalyst according to claim 41, wherein the catalyst is present on a Ni mesh and / or present as a nanosponge.

43. The catalyst according to any one of claims 41 and 42, wherein the catalyst can be obtained by contacting an aqueous solution of a Cu salt and a Ru salt with a reducing agent.

44. The catalyst according to claim 43, wherein the Cu salt and the Ru salt are reduced by NaBH4.

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