Process for the Electrochemical Synthesis of Ammonia (NH3) and the Ammonia Produced Thereby

Abstract

This invention relates to a process for the electrochemical synthesis of ammonia (NH3) and the ammonia produced thereby. Ammonia is synthesized by the electrochemical reduction of nitrogenous materials such as nitrogen or nitrates (NO3-) using metal phthalocyanine such as iron phthalocyanine (FePc) or β-cobalt phthalocyanine (CoPc) or iron phthalocyanine-molybdenum disulfide (FePc-MoS2) or cobalt phthalocyanine- carbon nitride (CoPc-C3N4) catalyst at very low pressure and room temperature by applying low potential.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Indian Patent Application No. 202131029798 filed Dec. 30, 2021, the disclosure of which is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to a process for the electrochemical synthesis of green ammonia (NH3) and the ammonia produced thereby.

This invention further relates to electrochemical synthesis of green ammonia (NH3) by the electrochemical reduction of nitrogenous materials such as nitrogen (N2) or nitrates (NO3^-) at room temperature and pressure by applying very low potential.

Description of Related Art

Ammonia (NH3) synthesis from dinitrogen (N2) is one of the most interesting and important subjects in chemistry, having won three Nobel Prizes in Chemistry (1918, 1931, 2007). The N2 molecule is exceptionally stable because of the triple bond (941 kJ mol^-1) and enormous energy band gap (10.82 eV) i.e. band gap difference between lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) is high. As a result, biological stabilization of ammonia by the nitrogenase has been the only possible path for few billions of years. It wasn’t until the intelligent invention of the Haber-Bosch procedure in the mid twentieth century that the worldwide nitrogen cycle changed fundamentally. Thus, NH3 has acquired the elevated status of being the world’s second most-used chemical, with more than 80% use in fertilizers, the rest for refrigeration or chemical feed stock. The revolutionary Haber- Bosch process has the greatest limitation owing to the production of 420 million tones of CO2 annually, which is around 1.5% of the total CO2 emissions. Electrochemically ammonia production from nitrate is the new and innovative way for the recent times. The huge scope of electrocatalytic nitrate removal has been limited under both practical conditions, due to the lack of induction by high selectivity and low energy use of nitratereduction and the lack of research on the long adventurous operation of electrode materials in electro-chemical reactions.

Nitrate is one of the probably universal poisonous reagents in the present decade. In fact, nitrate concentration in the earth’s water has increased in recent years as a result ofexcessive usage of natural and synthetic fertilizers. Sources of nitrogenous compound deposition in the environment arises from the discharge of nitrogen composts and modern wastewater, organic manure, septic waste and nitrogen oxides. Some nitrogen compounds contaminate the source of drinking water, as nitrates enter various reservoirs of surface water and groundwater. Since a large portion of drinking water is supplied from groundwater and nitrate is suspected to cause genuine health risks (cancer, blue baby syndrome), lawmakers have stringently fixed the acceptable levels of nitrates in drinking water. In several areas, the waterworks these days need to eliminate nitrate from water so as not to abuse the allowed level. Furthermore, to diminish the contamination of common aquifers, lawmakers have somewhat recently brought down the allowed nitrate levels in wastewater streams, compelling the industries concerned, for example industries involved in creation of composts and explosives, updating of uranium, synthetic industry (e.g. nitration, polyurethane creation), power plants etc., to eliminate their overabundance of nitrates. The most widely recognized innovations for nitrate extraction from water are isolated in biological, physicochemical, and catalytic measures. The actual disadvantages of ion exchange, reverse osmosis, and electro dialysis of physicochemical processes are that the nitrate does not change over into innocuous forms, but is merely eliminated from the water into saline solution which must be treated thereafter or must be discarded. Also, these processes do not specifically target nitrate so the composition of the treated water changes. From an environmental point of view the best way to eliminate nitrate is to convert the harmless gas on it to nitrogen and it can be done very well by biological denitrification. Also, biological denitrification is nitrate-specific, so the water system does not change. However, issues such as the arrival of NO2^-, NOx, and N2O may be manifested by partial denitrification. Also, natural denitrification measures are not simple to deal with broad guidelines of boundaries such as concentration and pH quality are vital and complex, and costly post-treatment procedures are expected to eliminate side-products like organic matter, turbidity, taste loss substances, and others. Another monetary, just as environmental innovation for the expulsion of nitrates from the water was first depicted in 1989 by Vorlop et al [“First steps towards the removal of nitrate and nitrite from drinking water with noble metal catalysts” Chem. Eng. Technol. 1989, 61, 836- 837, DOI: 10.1002/cite.330611023] that an innovation which depends on the reactant hydrogenation of nitrate to nitrogen. Nitrate was discovered to be hydrogenated simply by bimetallic, ideally palladium-copper catalyst, while nitrite and its intermediates can be diminished with a monometallic, ideally palladium catalyst. As a result, nitrates, which reduce with catalyst, are converted to ammonium as basic product. A steep growth in research has been observed on the field of electrochemical reduction of nitrate to ammonia. In this cycle, nitrated wastewater streams are used as a source of nitrogen to produce ammonia, one of the flexible compounds that can be utilized as manure, substance or fuel. This route will strengthen the use of nitrates for the formation of vital molecules, the goal of removing nitrate from untreated waste was to convert nitrate to dinitrogen. As a rule, the transformation or evacuation of nitrates is a significant test that is important for reproducing the nitrogen cycle around the world.For many decades, different human activities contributed to the imbalance of the global nitrogen cycle through the release of reactive nitrogen into the climate. Nitrate emissions have the most detrimental consequences because it can contaminate soil and contaminate ground and surface water, contributing to ecosystem disturbances. A significant portion of the nitrate circulating in the climate begins with the excessive use of fertilizers to produce crops or livestock. Haber-Bosch process produced a wide range of ammonia in favor of the fertilizers and thus had the most influence effect of the nitrogen cycle. Ammonia is formed in a response between H2 and N2 at high pressures and temperatures by Haber-Bosch process. As a result, the Haber-Bosch process relies on the utilization of fossil fuels for H2 creation and is an energy-intensive cycle that leads to higher operational expenses.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to propose a process for the electrochemical synthesis of green ammonia (NH3) and the ammonia produced thereby.

A further object of this invention is to propose a process for the electrochemical synthesis of green ammonia (NH3), which is a simple and fast process.

It is a further object of this invention to propose a process for the electrochemical synthesis of green ammonia (NH3), which has much less energy consumption compared to industrial processes.

Another object of this invention is to propose a process for the electrochemical synthesis of green ammonia (NH3), which can be carried out under room temperature and pressure, applying very low reduction potential. Yet another object of this invention is to propose a process for the electrochemical synthesis of green ammonia (NH3), which does not use expensive materials and is therefore a cost-effective process.

A further object of this invention is to propose a process for the electrochemical synthesis of green ammonia (NH3), which leads to ammonia in liquid form which is easy to use.

These and other object and advantages of the invention will be apparent to a person skilled in the art on reading the ensuing description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: XRD pattern of FePc NTs

FIG. 2: FTIR spectrum of FePc NTs

FIG. 3: LSV profile of FePc in nitrate (0.5 M) containing 0.1 M K₂SO₄ solution

FIG. 4: Time dependent current density (j) curves for FePc at different potential

FIG. 5: UV-Vis absorption spectra of varying potential’s electrolytes (after 120 times dilution) with an indophenols blue method after 3600 s electrochemical nitrate reduction

FIG. 6: Bar chart of NH₃ yield rate and Faradaic efficiency of FePc at different potential

FIG. 7: UV-Vis spectra of different concentration ammonium solution with an indophenols blue method after 7200 s incubation in a dark place

FIG. 8: Standard calibration curve used for the determination of NH₄⁺ concentration

FIG. 9: UV-Vis spectra of different concentration hydrazine solution

FIG. 10: Standard calibration curve used for the determination of N₂H₄

FIG. 11: UV-Vis spectra for hydrazine @-1.55V electrolyte solution

FIG. 12: LSV profile of FePc in 0.1 N HCl solution for NRR

FIG. 13: Time dependent current density (j) curves for FePc at varying potential for NRR

FIG. 14: UV-Vis absorption spectra of varying potential’s electrolytes after electrochemical nitrogen reduction reaction using FePc catalyst for NRR

FIG. 15: Bar chart of NH₃ yield rate and Faradaic efficiency of FePc at different potential for NRR

FIG. 16: UV-Vis absorption spectra of varying potential’s electrolytes after electrochemical nitrogen reduction reaction using FePc-MoS₂ catalyst for NRR

FIG. 17: Bar chart of NH₃ yield rate and Faradaic efficiency of FePc-MoS₂ at different potential for NRR

FIG. 18: LSV profile of CoPc_C₃N₄ in 0.1 N HCl solution for NRR

FIG. 19: Time dependent current density (j) curves for CoPc_C₃N₄ at varying potential for NRR

FIG. 20: UV-Vis absorption spectra of varying potential’s electrolytes after electrochemical nitrogen reduction reaction using CoPc_C₃N₄ catalyst for NRR

FIG. 21: Bar chart of NH₃ yield rate and Faradaic efficiency of CoPc_C₃N₄ at different potential for NRR

FIG. 22: Bar chart of NH₃ yield rate and Faradaic efficiency of FePc-MoS₂ at different potential for NRR using an H-type cell and carbon paper as a working electrode

FIG. 23: Bar chart of NH₃ yield rate and Faradaic efficiency of FePc-MoS₂ at different potential for NRR using single cell with GCE as a working electrode

FIG. 24: Isotopic experiment using ¹H-NMR analysis

FIG. 25: Schematic diagram of H-type electrochemical setup (NO₃RR)

FIG. 26: Schematic diagram of Single cell electrochemical setup (NO₃RR)

FIG. 27: Schematic diagram of H-type electrochemical setup (NRR)

FIG. 28: Schematic diagram of Single cell electrochemical setup (NRR)

DESCRIPTION OF THE INVENTION

Thus, according to this invention is provided a process for the electrochemical synthesis of green ammonia (NH3) and the green ammonia produced thereby.

In accordance with this invention, the process for the electrochemical synthesis of green ammonia is conducted in an electrochemical cell, including an electrolyte and a three-electrode system, said three-electrode system comprising:

• a working electrode loaded with an electrocatalyst selected from transition metal phthalocyanine and composites based on transition metal phthalocyanine,
• a reference electrode of silver/silver chloride (Ag/AgCl) saturated with potassium chloride (KCl); and
• an auxiliary or counter electrode being a platinum wire (NRR) and platinum foil (NO₃RR);
• the process comprising the steps of subjecting a nitrogen source to produce green ammonia by electrocatalytic reduction.

In accordance with an embodiment of the invention, the process for the electrochemical synthesis of green ammonia by electroreduction of nitrate (NO₃^-) is conducted in an electrochemical cell, including an electrolyte and a three-electrode system, said three-electrode system comprising:

• a working electrode loaded with an electrocatalyst selected from transition metal phthalocyanine and composites based on transition metal phthalocyanine,
• a reference electrode of silver/silver chloride (Ag/AgCl) saturated with potassium chloride (KCl); and
• an auxiliary or counter electrode being a platinum foil;

the process comprising the steps of subjecting a nitrogen source to produce green ammonia by electrocatalytic reduction.

In accordance with a further embodiment of the invention, the process for the electrochemical synthesis of green ammonia by electroreduction of nitrogen gas is conducted in an electrochemical cell, including an electrolyte and a three-electrode system, said three-electrode system comprising:

• a working electrode loaded with an electrocatalyst selected from transition metal phthalocyanine and composites based on transition metal phthalocyanine,,
• a reference electrode of silver/silver chloride (Ag/AgCl) saturated with potassium chloride (KCl); and
• an auxiliary or counter electrode being a platinum wire;

the process comprising the steps of subjecting a nitrogen source to produce green ammonia by electrocatalytic reduction

In accordance with a still further embodiment, the invention provides an electrochemical cell for the electrochemical synthesis of green ammonia, comprising an anodic chamber and a cathodic chamber in fluid connectivity with each other through a tubular structure configured to hold a membrane separating said anodic chamber and cathodic chamber,

said anodic chamber and cathodic chamber being configured to include an electrolyte and a three-electrode system, said three-electrode system comprising,

• -a working electrode comprising carbon paper loaded with iron phthalocyanine, iron phthalocyanine-molybdenum disulfide and cobalt phthalocyanine-carbon nitride
• a reference electrode of silver/silver chloride (Ag/AgCl) saturated with potassium chloride (KCl); and
• an auxiliary or counter electrode being a platinum wire or foil.

The nitrogen source is selected from nitrates (NO₃^-) and Nitrogen gas (N₂). The nitrates are found as contaminants in sewage and groundwater. When the nitrogen source is NO₃^-, the NO₃^- is diffused into the catalyst surface for electrocatalytic reduction and when the nitrogen source is N₂, N₂ gas is diffused into the electrolyte for electrocatalytic reduction.

The electrolyte used (for NO₃RR) is ordinarily a 0.1 to 0.5 M aqueous nitrate solution such as sodium or potassium nitrate with 0.1 M K₂SO₄.

The electrolyte used (for NRR) is ordinarily a 0.1 M aqueous solution HCl.

The process is carried out in an electrochemical cell, including an electrolyte and a three-electrode system, where the three-electrode system comprises a working electrode loaded with an electrocatalyst selected from transition metal phthalocyanine and composites based on transition metal phthalocyanine, a reference electrode of silver/silver chloride (Ag/AgCl) saturated with potassium chloride (KCl); and an auxiliary or counter electrode being a platinum wire. The working electrode is a glassy carbon electrode or a carbon paper. The reference electrode is saturated with 3.5 M KCl.

All potential values are changed to the Reversible Hydrogen Electrode (RHE). The voltage applied across the electrodes is typically in the range of -0.1 V to -0.7 V with respect to the RHE (for NRR) & -1.25 V to -1.65 V (for NO₃RR). As the applied potential is very low, it can be provided by solar cell/ photo-voltaic cell. Hence the process is green in nature.

In the process according to the invention, the electrocatalyst is selected from transition metal phthalocyanines and composites of transition metal phthalocyanines. When used alone, the transition metal phthalocyanine is selected from nano-tubes and nano-rods.

The composites comprise transition metal phthalocyanines with a compound selected from the disulfide and selenide of Molybdenum (Mo) and Tungsten (W), carbon nitride (C₃N₄), boron nitride, graphene and Borophene.

In accordance with a preferred embodiment, the compounds used in the composite are selected from molybdenum disulfide (MoS₂), molybdenum diselenide (MoSe₂), tungsten disulfide (WS₂), tungsten didiselenide (WSe₂),), carbon nitride, boron nitride, reduced graphene oxide (RGO) and Borophene.

The transition metal phthalocyanine is selected from iron phthalocyanine (FePc), cobalt phthalocyanine (CoPc), nickel phthalocyanine (NiPc), copper phthalocyanine (CuPc), chromium phthalocyanine (CrPc), manganese phthalocyanine (MnPc).

In accordance with a preferred embodiment according to the invention, the composite is selected from FePc-MoS₂, FePc-MoSe₂, FePc-WS₂, FePc-WSe₂, CoPc-MoS₂, CoPc-MoSe₂, CoPc-WS₂, CoPc-WSe₂, NiPc-MoS₂, NiPc-MoSe₂, NiPc-WS₂, NiPc-WSe₂, CuPc-MoS₂, CuPc- MoSe₂, CuPc-WS₂, CuPc-WSe₂, CrPc-MoS₂, CrPc-MoSe₂, CrPc-WS₂, CrPc-WSe₂, MnPc-MoS₂, MnPc-MoSe₂, MnPc-WS₂, MnPc-WSe₂, FePc- C₃N₄, FePc-B₃N₄, FePc-RGO, FePc-Borophene, CoPc-C₃N₄, CoPc-B₃N₄, CoPc-RGO, CoPc-Borophene,, NiPc-C₃N₄, NiPc-B₃N₄, NiPc-RGO, NiPc-Borophene, CuPc- C₃N₄, CuPc-B₃N₄, CuPc-RGO, CuPc-Borophene, CrPc-C₃N₄, CrPc- B₃N₄, CrPc-RGO, CrPc-Borophene, MnPc-C₃N₄, MnPc-B₃N₄, MnPc-RGO and MnPc- Borophene.

The ammonia is green ammonia which is obtained from the electrolyte in liquid form. In the embodiment according to the invention where nitrate is used as the nitrogenous source for the electrocatalytic reduction, the concentration of nitrate ion is 0.1-0.5 M. The electro-reduction is carried out for a period ranging from about 1-2 hours.

In the embodiment according to the invention where nitrogen gas is used as the nitrogenous source for the electrocatalytic reduction, during the reduction process, the rate of entry of nitrogen gas is 2-5 mL/min. During the reduction process, nitrogen gas is purged in the cathode chamber and the electro-reduction is affected for a period ranging from about 1-2 hours.

In the electrochemical cell, the cathodic chamber and anodic chamber are separated by a membrane. The membrane used is a proton-conductive polymer membrane which allows only protons to cross over. Any membrane which has the desired properties of high ionic conductivity, low gas permeability and high mechanical strength may be used. Particularly preferred are synthetic polymers with ionic properties such as a sulfonated tetrafluoroethylene based fluoropolymer-copolymer, such as for instance, Nafion 117 membrane (Sigma-Aldrich) for H-type cell.

The cathodic chamber includes at least one inlet for the entry of gases and at least one outlet for the exit of gases for nitrogen reduction to green ammonia

The cathodic chamber includes no inlets for the entry of gases and at least one outlet for the exit of gases for nitrate reduction to green ammonia.

This invention relates to the production of green ammonia (NH3) by the electrochemical reduction of nitrogenous material such as nitrogen (N2) or nitrates (NO3^-) at room temperature and pressure with applying very low potential. This may be considered to be an alternative to Haber-Bosch process. In Haber-Bosch process hightemperature and pressure are required to complete the reaction, whereas in the instant process, metal phthalocyanines or composites based on metal phthalocyanines are used as electrocatalyst to synthesize ammonia by the reduction of starting materials selected from N2 or nitrates at very low pressure and room temperature.

In accordance with an embodiment of the invention, ammonia is produced under normal temperature and pressure by applying low potential with FePc. Crystalline structure and phase purity of FePc is confirmed by X-ray diffraction (XRD) study where the main peaks (100), (102), (102), (105), (401), (114) and (314) were observed at the diffraction angle of 6.977, 10.07, 15.675, 24.184, 25.35, 26.756 and 27.83 respectively (FIG. 1).

The different chemical bonds present in FePc are confirmed by FTIR spectra as shown in FIG. 2. The main peak at 1165 cm^-1 assigned for the Fe-N bond in the FePc molecule. The peaks of 908, 1072, 1089, and 1119 cm^-1 are assigned to pyrrole in the plane mode of FePc, and the peaks of 865 cm^-1 indicate the pyrrole-out-of-plane mode of iron phthalocyanine. The peaks of 1288 and 1331 cm^-1 are assigned for C=N-C bridge bond in FePc.

Case I Electrochemical Nitrate Reduction to Ammonia

All electrochemical measurements are carried out of 0.5 M nitrate concentration in 0.1 M K2SO4 electrolyte solution. During electro-reduction of nitrate first, nitrate ions come to the surface of the catalyst and are then adsorbed (ad) on the catalyst surface. The reaction mechanism below shows how nitrate ion is converted to ammonium ion.

So, the overall nitrate ion is converted to ammonia via 8 electron transfer process. The onset potential started at near -1.2V (vs RHE) (FIG. 3). The time dependent chronoamperometry curve shows (FIG. 4) a negligible decay in current densities which indicates the stability of the catalyst after 3600 s electrochemical nitrate reduction. UV-Vis absorption spectra were carried out at different potential’s electrolytes sample using indophenols blue method after 3600 s electrochemical nitrate reduction in 0.1 M K2SO4 electrolyte. The maximum absorption peaks appeared at 655 nm (FIG. 5) which confirmed the production of ammonia. At the different potential window -1.25V to -1.65V (vs RHE), the strongest absorption came at -1.55V (vs RHE). So, the ammonia yield is highest at -1.55V (vs RHE). After the electrochemical nitrate reduction at all possible different potential, the ammonia yield and Faradaic efficiency (FE) were calculated (FIG. 6). As the Faradaic efficiency is dependent on current density (current per unit area), the maximum FE is ~100% at -1.55V (vs RHE) and ammonia yield is 35247 µg h^-1mg cat^-1 (at -1.55V vs RHE). The unknown ammonia concentration was determined using the known ammonia concentration series (FIG. 7). Standard ammonium concentration absorption spectra were created and a linear correlation between concentration and absorbance was observed (FIG. 8). During nitrate reduction reaction to ammonia, there is a possible chance to form hydrazine as a byproduct. For that case, a series of hydrazine concentration was prepared (FIGS. 9&10). But, there is no absorption at 455 nm in the UV-Vis spectrum (FIG. 11) and this indicates that hydrazine (N2H4) is not formed during NO₃RR, making it a good choice of catalyst to convert nitrate to NH3 and gives the evidence and also establishes the choice of ammonia formation with negligible hydrogen formation.

Case II Electrochemical Nitrogen Reduction to Ammonia

In accordance with a further embodiment of the invention, ammonia is synthesized by the reduction of N2 under room temperature and pressure by applying low potential with the synthesized β-CoPc nano-catalyst. Stable β-cobalt phthalocyanine (CoPc) nanotubes (NTs) have been combined by an adaptable solvothermal technique for electrocatalytic NRR [Ghorai et al. / https://doi.org/10.1021/acsnano.0c10596, ACS Nano 2021, 15, 3, 5230-5239]. The chemically integrated CoPc NTs show magnificent electrochemical NRR because of its high reactant activity. Subsequently, CoPc NTs deliver a higher NH3 yield of 107.9 µg h^-1 mg^{- 1} and FE of 27.7% in 0.1 M HCl at -0.3 V versus RHE. The utilitarian hypothesis affirms that theCo place in CoPc is the primary dynamic site answerable for electrochemical NRR.

β-CoPc nanotubes (NTs) have been synthesized by the ethylene glycol helped with versatile solvothermal strategy for NRR to NH3. The hollow CoPc NTs showed magnificent NRR study with a high NH3 yield of 107.9 µg h^-1 mg^-1cat, and it remains practically the same until five back to back cycles. The catalyst showed stability for up to 20 h with no crumbling in execution, with the structural integrity staying unblemished. The hollow morphology, stable beta phase of CoPc makes them hearty electrocatalysts toward NRR to NH3 under encompassing conditions. Crystal structure and phase confirmation of β-CoPc NTs is confirmed by X-ray diffraction (XRD) study where the main peaks (100), (102), (002), (202), (104), (112) and (311) were observed at the different diffraction angle. The main two peaks 6.99° and 9.184° confirm a β phase formation of CoPc NTs with space group of ^P21/c. All electrochemical measurements were carried out by the purging of ultra high pure nitrogen gas in 0.1 M HCl electrolyte solution in the H-type cell using the Nafion 117 membrane. During electro-reduction of nitrogen, first N2 comes to the surface of the catalyst and is then adsorbed (ad) on the catalyst surface. After that, electro-reduction of nitrogen triple bond starts via the preferable alternative pathway compare to the distal pathway. The linear sweep Voltammetry (LSV) curve of CoPc NTs showed higher current density in the potential window of -0.2 to -0.6 V (vs. RHE) (in N2 saturated solution) compared to Ar saturated solution. This indicates that CoPc NTs are reasonable for NRR. It is seen from LSV curve that the onset potential begins at near -0.2V (vs RHE). Time-dependent chronoamperometry tests performed using CoPc NTs at various applied potentials of the N2 saturated electrolyte show a negligible loss of stability at current densities up to 7200 seconds which indicates the stability of the catalyst. The electrochemical NRR process with distinct absorption at UV-Visible spectrum 655 nm in various possible ranges from -0.2V to -0.6V, and the CoPc catalyst exhibits the strongest absorption at -0.3V (vs. RHE) potential. After analysis in each possible case, the indophenols blue method is used to quantify the amount of NH3 produced and FE by given formula. NH3 yields and FE are calculated at different potential window. The highest NH3 production rate and FE of CoPc NTs obtained at -0.3V are 107.9 µg h^-1 mg^-1cat and 27.7%, respectively. The unknown ammonia (NH3) solution’s concentration was calculated utilizing the known ammonia concentration series making.

In case of iron phthalocyanine (FePc) electrocatalyst, it showed an excellent result during nitrogen reduction reaction. The linear sweep Voltammetry (LSV) curve (with scan rate of 100 mV/s) of FePc showed (FIG. 12) higher current density in the potential window of -0.2 to -0.8 V (vs. RHE) (in N2 saturated solution) compared to Ar saturated solution. This indicates that FePc are reasonable for NRR. It is seen from LSV curve that the onset potential begins at near -0.2V (vs RHE). Time-dependent chronoamperometry tests (FIG. 13) performed using FePc at various applied potentials of the N2 saturated electrolyte show a negligible loss of stability at current densities up to 7200 seconds which indicates the stability of the catalyst. The electrochemical nitrogen reduction process in various possible potential from -0.2V to -0.7V was examined for 2 hours. An indophenols blue method was used to quantify the amount of ammonia and FE by the given formula. After electroreduction process, a distinct UV-Visible absorption spectrum at 655 nm was observed, and the maximum absorption peak for FePc catalyst exhibits at -0.3V (vs. RHE) potential (FIG. 14). NH3 yields and FE were calculated at different potential window. The highest NH3 production rate and FE of FePc obtained at -0.3V are 120.5 µg h^-1 mg^-1cat and 41.2%, respectively (FIG. 15).

Further, the inventors investigated NRR study using iron phthalocyanine-molybdenum disulfide (FePc-MoS₂) electrocatalyst; it showed an excellent data during nitrogen reduction reaction. The electrochemical nitrogen reduction process in various possible potential from -0.2V to -0.7V was examined for 2 hours. An indophenols blue method was used to quantify the amount of ammonia and FE by the given formula. After electroreduction process, a distinct UV-Visible absorption spectrum at 655 nm was observed, and the maximum absorption peak for FePc-MoS₂ catalyst exhibits at -0.3V (vs. RHE) potential (FIG. 16). NH3 yields and FE were calculated at different potential window. The highest NH3 production rate and FE of FePc-MoS₂ obtained at -0.3V are 218.6 µg h^-1 mg^-1cat and 44.2%, respectively (FIG. 17).

Further, the inventors also conducted NRR study using cobalt phthalocyanine-carbon nitride (CoPc-C₃N₄, weight ratio basis CoPc:C₃N₄ = 1:2) electrocatalyst; it also showed an excellent ammonia yield during nitrogen reduction reaction. The linear sweep voltammetry (LSV) curve of CoPc-C₃N₄ showed (FIG. 18) higher current density in the potential window of -0.1 to -0.5 V (vs. RHE) (in N2 saturated solution) compared to Ar saturated solution. This indicates that CoPc-C₃N₄ is reasonable for NRR. It is seen from LSV curve that the onset potential begins at near -0.1 V (vs RHE). Time-dependent chronoamperometry tests (FIG. 19) performed using CoPc-C₃N₄ at various applied potentials of the N2 saturated electrolyte show a negligible loss of stability at current densities up to 7200 seconds which indicates the stability of the catalyst. The electrochemical nitrogen reduction process in various possible potential from -0.1 V to -0.5V was examined for 2 hours. An indophenols blue method was used to quantify the amount of ammonia and FE by the given formula. After electroreduction process, a distinct UV-Visible absorption spectrum at 655 nm was observed, and the maximum absorption peak for CoPc-C₃N₄ catalyst exhibits at -0.2V (vs. RHE) potential (FIG. 20). NH3 yields and FE were calculated at different potential window. The highest NH3 production rate and FE of CoPc-C₃N₄ obtained at -0.2V are 412.2 µg h^-1 mg^-1 cat and 32%, respectively (FIG. 21).

We also compared the NRR data by changing the working electrode. FIG. 22 shows that the maximum ammonia yield and FE was obtained 88.7 µg h^-1 mg^-1 cat and 22.3% respectively at -0.3V for FePc-MoS₂ when we change the working electrode GCE to carbon paper. Again, we compared the NRR data by changing the cell chamber. FIG. 23 shows that the maximum ammonia yield and FE was obtained 187.5 µg h^-1 mg^-1cat and 39.2% respectively at -0.3V for FePc-MoS₂ loaded on GCE when we change the H-type cell to a single cell.

Further, to validate the actual N source in ammonia, we performed an isotopic labeling experiment (FIG. 24). A triplet NMR peak at ~7 ppm occurred when we purged ¹⁴N₂ gas for the electroreduction process. But when we purged ¹⁵N₂ gas a doublet peak came. This can definitely say that ammonia is formed by purged nitrogen gas and not from any other contaminates.

We also obtained good results using other metal phthalocyanine-based complexes (NiPc, MnPc, CuPc, etc.) in both cases (NRR and NO₃RR).

The invention will now be explained in greater details with the help of the following nonlimiting examples.

EXAMPLES

Catalytic Ink Preparation for Case I

0.75 mg FePc was dispersed in 130 µL 2-propanol (Merck) and ultra-sonication was performed for 1 min. Then, 20 µL of Nafion 117 (5 wt %) solution (Sigma Aldrich)was added to the previous solution. The solution mixture was placed in vortex for 2 min to ensure the homogeneous mixing. Finally, the ink is ready to load on 1 × 2 cm² carbon paper for electrochemical nitrate reduction process.

Catalytic Ink Preparation for Case II

1 mg synthesized β-CoPc was dispersed in 100 µL isopropanol (Merck) and ultra-sonication was done for 1 minute. Then, 3 µL of Nafion 117 solution (5 wt %, Sigma Aldrich) was mixed to the previous solution. Then prepared mixture solution was placed in vortex for 2 minutes to become the homogeneous mixing solution. Lastly, the prepared ink (10 µL) is loaded on the glassy carbon working electrode (area with 0.07 cm²) for electrochemical nitrogen reduction process.

Similarly, FePc, FePc-MoS₂ and CoPc-C₃N₄ ink was prepared following Case II.

Determination of Ammonia (NH3)

Ammonia was determined by UV-Vis spectrophotometer using the indophenol blue method. For this purpose, three chemical precursor solutions were prepared. Solution A was prepared by mixing of 5 g trisodium citrate dihydrate (Merck), 5 g salicylic acid (Merck), and 4 g sodium hydroxide (Merck) in 100 mL triple distilled water (Millipore). Solution B was prepared by mixing 5 mL sodium hypochlorite solution (4% w/v available chlorine) (Merck) in 45 mL triple distilled water. Solution C was prepared by mixing 0.5 g of coloring agent sodium nitroprusside dihydrate (Loba Chemie) in 50 mL triple distilled water. Finally, 2 mL solution A, 1 mL solution B, and 0.2 mL solution C were mixed with 2 mL of electrolyte solution and incubated for 2h in a dark place. Maximum absorption peaks in UV-Vis spectra were shown nearly at 655 nm.

Standard calibration curve was made using ammonium sulfate (Merck) with varying the concentration of 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0 ppm (FIG. 7). After three repeated experiment the straight-line curve from the standard calibration curve was obtained as y = 0.0012 + 0.27829 x, R² = 0.999 (FIG. 8). This revealed that the linear co-relation between concentration and the absorbance.

Determination of Hydrazine (N2H4)

The hydrazine produced by the ENRR was quantitatively determined using the Watt and Chrisp method. In detail, the pigment solution was making by 5.99 g of para- dimethylamino benzaldehyde (Merck) containing 30 mL concentrate HCl (Merck) and 300 mL of ethanol (Merck). After the ENRR test, 5 mL of electrolyte is taken and added5 mL of color solution and then stirred for 10 minutes and then kept at the room temperature in a dark place for another 5 minutes, after which the absorption spectrum of solution is taken utilizing UV-Vis spectrophotometer. As measured, the maximum absorption peak was observed at 455 nm. The calibration series were prepared in the following pathway: A series of different concentrations of hydrazine was prepared as standard solution (0.0, 0.4, 0.8, 1.2, 1.6, 2.0 ppm) (FIG. 9) in an electrolyte solution. An absorption and concentration plot, obtained a fitting line (y = 0.82811x + 0.06981; R²=0.999) (FIG. 10) which gave a nice linear relationship between hydrazine concentration and the absorption, then after three distinct calibrations, it was reported. FIG. 11 showed UV-Vis spectra of hydrazine @-1.55V of the electrolyte solution. This figure confirmed that there is no hydrazine produce during the electrochemical nitrate reduction process (using FePc) as there is no significant signature peak present at 455 nm range.

Characterizations

Crystal structure and phase isolation of FePc or β-CoPc was confirmed by the powder X-ray diffraction (XRD) study. A monochromatic Cu-K_α radiation with wavelength (λ) 0.15404 nm was used and the environmental conditions were maintained 25 mA current and 40 kV voltages respectively, and quantification of ammonia was determined.

All the electrocatalytic nitrate reduction measurements were done using an electrochemical workstation with a three-electrode system where the platinum wire acts as an auxiliary electrode and Ag/AgCl (saturated in 3.5 M KCl solution) as a reference electrode and carbon paper or glassy carbon electrode with loaded FePc or β-CoPc or FePc-MoS₂ or CoPc-C₃N₄ catalyst in as a working electrode. All potentials were converted to the reversible hydrogen electrode (RHE). The electro-reduction is carried out in H-type cell as well as single cell. Schematic diagrams of the H-type and single cells used are shown in FIGS. 25- 28. FIG. 25 depicts a schematic design of the H-type electrochemical system for NO₃RR. A pair of containers serve as the cathodic chamber (left) and anodic chamber (right) and are connected to each other through a tubular framework. The two chambers (left, right) form a three-electrode system (1, 2, 6), with the first electrode of carbon paper/GCE loaded with catalyst (3) embedded within the cathodic chamber (left) acting as a working electrode (1), the second electrode of Ag/AgCl (saturated in 3.5 M KCl) embedded within the cathodic chamber (left) acting as a reference electrode (2), and the third electrode of (6) embedded in anodic chamber (right). The tubular construction is configured to enable only protons to flow through a Nafion membrane (4) (Sigma-Aldrich).

FIG. 26 shows the general layout of a single-type, three-electrode electrochemical system for NO₃RR. The working electrode (2) comprises of carbon paper or glassy carbon electrode, and it is loaded with catalyst (5). The counter electrode (1) is composed of a platinum rod, while the reference electrode (4) is composed of Ag/AgCl (Sat. with KCl). There is a gas outlet chamber (3) for producing any kind of gases during electrolysis from counter electrode as well as working electrode.

The H-type electrochemical system for NRR is shown schematically in FIG. 27. The cathodic chamber (left) and anodic chamber (right) are two containers that are connected to one another by a tubular structure. The two chambers (left, right) come together to form a three-electrode system (1, 2, 6), with the first electrode of carbon paper/GCE loaded with catalyst (3) acting as a working electrode (1), the second electrode of Ag/AgCl (saturated in 3.5 M KCl) acting as a reference electrode (2), and the third electrode of (6) platinum rod as a counter electrode embedded within the anodic chamber (right). Only protons can pass through a Nafion membrane to the tubular design (4). (Sigma-Aldrich). A gas is leaving the system, (5) (right chamber). Additionally, there is a source of nitrogen gas that enters and leaves the system (left chamber).

FIG. 28 depicts the overall design of a single-type (NRR), three-electrode electrochemical system. The working electrode (3) is made of glassy carbon electrode or carbon paper and is filled with catalyst (6). The reference electrode (4) is made of Ag/AgCl, whereas the counter electrode (1) is made of a platinum rod or foil (Sat. with KCl). The cell chamber has a provision for allowing gas to enter (1) and for releasing gas (4).

Calculation of the Yield and Faradaic Efficiency

After electrochemical nitrogen or nitrate reduction to ammonia, the yield of ammonia and Faradaic efficiency were calculated by the following equation:

${\text{Yield of NH}}_3=\frac{\left({\text{C}}_{{\text{NH}}_3}\times \text{V}\right)}{\left(\text{t}\times \text{m}\right)}\left[\text{For both cases}\right]$

$\begin{array}{l}\text{Faradaic efficiency}\left(\%\right)=\frac{\left(3\text{F}\times {\text{C}}_{\text{NH3}}\times \text{V}\right)}{\left({\text{M}}_{{\text{NH}}_3}\times \text{Q}\right)}\times 100\%\\ \left[{\text{For N}}_2\text{to}{\text{NH}}_3\right]\end{array}$

$\begin{array}{l}\text{Faradaic efficiency}\left(\%\right)=\frac{\left(\text{8F}\times {\text{C}}_{\text{NH3}}\times \text{V}\right)}{\left({\text{M}}_{{\text{NH}}_3}\times \text{Q}\right)}\times 100\%\\ \left[{\text{For NO}}_3{}^{-}\text{to}{\text{NH}}_3\right]\end{array}$

Where, C_NH3 is the concentration of ammonia produced during electroreduction, V is the volume of electrolyte, M_NH3 is the molecular mass of ammonia, t is the time duration for electroreduction, m is the catalyst mass, F is the Faradaic constant (96,485 C mol^-1), and Q is the total charge passing through the electrode.

The present invention relates to a process for the electrochemical synthesis of ammonia (NH₃) and the ammonia produced thereby. The process involves the electrochemical synthesis of ammonia (NH₃) by the reduction of nitrogenous materials such as nitrogen or nitrate (NO₃^-) under room temperature and pressure with applying very low potential. The electrochemical transformation of nitrate to ammonia can be coupled to powerthat preferably comes from economical sources, similar to wind and solar energy. Moreover, it will facilitate the production of decentralized ammonia at room temperature. Thus, converting nitrogen or nitrate to ammonia would give a solution to restore the imbalance of the worldwide nitrogen cycle and at the same time give a maintainable option in contrast to the Haber-Bosch process. The inventors acknowledge SERB, Govt. of India for providing the partial financial support for filing the Pat. Application (CRG/2022/009427).

Claims

1. A process for the electrochemical synthesis of green ammonia in an electrochemical cell, comprising an electrolyte and a three-electrode system, said three-electrode system comprising:

a working electrode loaded with an electrocatalyst selected from transition metal phthalocyanine and composites based on transition metal phthalocyanine,

a reference electrode of silver/silver chloride (Ag/AgCl) saturated with potassium chloride (KCl); and

an auxiliary or counter electrode being a platinum wire or foil;

the process comprising the steps of subjecting a nitrogen source to produce green ammonia by electrocatalytic reduction.

2. The process as claimed in claim 1, wherein said nitrogen source is selected from nitrates (NO3- ) and Nitrogen gas (N2).

3. The process as claimed in claim 1, wherein said working electrode is a glassy carbon electrode or a carbon paper.

4. The process as claimed in claim 1, wherein the reference electrode is saturated with 3.5 M KCl.

5. The process as claimed in claim 1, wherein when the nitrogen source is NO3-, the NO3- is diffused into the catalyst surface for electrocatalytic reduction.

6. The process as claimed in claim 1, wherein when the nitrogen source is N2, N2 gas is diffused into the electrolyte for electrocatalytic reduction.

7. The process as claimed in claim 1, wherein said transition metal catalyst is selected from nano-tubes and nano-rods.

8. The process as claimed in claim 1, wherein said composite based on transition metal phthalocyanine comprises a composite of transition metal phthalocyanine and a compound selected from the disulfide and selenide of Molybdenum (Mo) and Tungsten (W), carbon nitride (C3N4), boron nitride, graphene and Borophene.

9. The process as claimed in claim 8, wherein said compound used in the composite is selected from molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2), tungsten disulfide (WS2), tungsten diselenide (WSe2),), carbon nitride, boron nitride, reduced graphene oxide (RGO) and Borophene.

10. The process as claimed in claim 1, wherein said transition metal phthalocyanine is selected from iron phthalocyanine (FePc), cobalt phthalocyanine (CoPc), nickel phthalocyanine (NiPc), copper phthalocyanine (CuPc), chromium phthalocyanine (CrPc), manganese phthalocyanine (MnPc).

11. The process as claimed in claim 1, wherein said composite is selected from FePc-MoS2, FePc-MoSe2, FePc-WS2, FePc-WSe2, CoPc-MoS2, CoPc-MoSe2, CoPc-WS2, CoPc-WSe2, NiPc-MoS2, NiPc-MoSe2, NiPc-WS2, NiPc-WSe2, CuPc-MoS2, CuPc-MoSe2, CuPc-WS2, CuPc-WSe2, CrPc-MoS2, CrPc-MoSe2, CrPc-WS2, CrPc-WSe2, MnPc-MoS2, MnPc-MoSe2, MnPc-WS2, MnPc-WSe2, FePc- C3N4, FePc-B3N4, FePc-RGO, FePc-Borophene, CoPc- C3N4, CoPc-B3N4, CoPc-RGO, CoPc-Borophene„ NiPc-C3N4, NiPc-B3N4, NiPc-RGO, NiPc-Borophene, CuPc- C3N4, CuPc-B3N4, CuPc-RGO, CuPc-Borophene, CrPc-C3N4, CrPc-B3N4, CrPc-RGO, CrPc-Borophene, MnPc-C3N4, MnPc-B3N4, MnPc-RGO and MnPc-Borophene.

12. The process as claimed in claim 1, wherein the ammonia is green ammonia which is obtained from the electrolyte in liquid form.

13. A process for the electrochemical synthesis of green ammonia by electroreduction of nitrate (NO3- ) in an electrochemical cell, comprising an electrolyte and a three-electrode system, said three-electrode system comprising:

a working electrode loaded with an electrocatalyst selected from transition metal phthalocyanine and composites based on transition metal phthalocyanine,

a reference electrode of silver/silver chloride (Ag/AgCl) saturated with potassium chloride (KCl); and

an auxiliary or counter electrode being a platinum foil;

the process comprising the steps of subjecting a nitrogen source to produce green ammonia by electrocatalytic reduction.

14. The process as claimed in claim 13, wherein during the reduction process, the concentration of nitrate ion is 0.1-0.5 M.

15. The process as claimed in claim 13, wherein the electro-reduction is effected for a period ranging from about 1-2 hours.

16. A process for the electrochemical synthesis of green ammonia by electroreduction of nitrogen gas in an electrochemical cell, comprising an electrolyte and a three-electrode system, said three-electrode system comprising:

a working electrode loaded with an electrocatalyst selected from transition metal phthalocyanine and composites based on transition metal phthalocyanine,,

a reference electrode of silver/silver chloride (Ag/AgCl) saturated with potassium chloride (KCl); and

an auxiliary or counter electrode being a platinum wire;

the process comprising the steps of subjecting a nitrogen source to produce green ammonia by electrocatalytic reduction.

17. The process as claimed in claim 16, wherein during the reduction process, the rate of entry of nitrogen gas is 2-5 mL/min.

18. The process as claimed in claim 16, wherein during the reduction process, nitrogen gas is purged in the cathode chamber.

19. The process as claimed in claim 16, wherein the electro-reduction is effected for a period ranging from about 1-2 hours.

20. The process as claimed in claim 16, wherein the electro-reduction is occurred in H-type cell as well as single cell.

21. An electrochemical cell for the electrochemical synthesis of green ammonia, comprising an anodic chamber and a cathodic chamber in fluid connectivity with each other through a tubular structure configured to hold a membrane separating said anodic chamber and cathodic chamber,

said anodic chamber and cathodic chamber being configured to comprise an electrolyte and a three-electrode system, said three-electrode system comprising, a working electrode comprising carbon paper loaded with iron phthalocyanine, iron phthalocyanine-molybdenum disulfide and cobalt phthalocyanine-carbon nitride a reference electrode of silver/silver chloride (Ag/AgCl) saturated with potassium chloride (KCl); and an auxiliary or counter electrode being a platinum wire.

22. The system as claimed in claim 20, wherein the cathodic chamber and anodic chamber are separated by Nafion 117 membrane for H-type cell.

23. The system as claimed in claim 20, wherein the cathodic chamber comprise at least one inlet for the entry of gases and at least one outlet for the exit of gases for nitrogen reduction to green ammonia.

24. The process as claimed in claim 20, wherein the cathodic chamber comprises no inlets for the entry of gases and at least one outlet for the exit of gases for nitrate reduction to green ammonia.