Process for the Electrochemical Synthesis of Green Urea, an Electrochemical Cell for the Electrochemical Synthesis of Green Urea and the Green Urea Produced Thereby

Abstract

This invention relates to a process for the electrochemical synthesis of green urea, and the urea produced thereby. The electrochemical synthesis of urea involves the reduction of dual purging gases N2 and CO2 via six electron transfer process (N2+CO2+6H++6e−→CO (NH2)2+H2O) & reduction of the NO3− ions and CO2 via sixteen electron transfer process (2NO3−+CO2+18H++16e−→CO(NH2)2+7H2O) under ambient condition using copper phthalocyanine (CuPc) catalyst. The binding of two intermediate products during dual reduction simultaneously, leads to the production of urea in water medium under ambient conditions.

Description

CROSS REFERENCE TO RELATED APPLICATION

The application claims priority to Indian Patent Application No. 202131027290, filed Jun. 23, 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 urea, an electrochemical cell for the electrochemical synthesis of green urea and the green urea produced thereby.

The invention involves a process for electrocatalytic urea synthesis under ambient conditions, using cheap, readily available sources of nitrogen and carbon dioxide and is a sustainable yet economically viable method of urea production.

In the first aspect, this invention relates to a process for the electrochemical synthesis of green urea from the reduction of dual purging gases N₂ and CO₂ via six electron transfer process (N₂+CO₂+6H⁺+6e⁻→CO (NH₂)₂+H₂O) under ambient conditions using copper phthalocyanine (CuPc) catalyst. The binding of two intermediate products during dual gas reduction takes place simultaneously, to produce urea in water medium under ambient conditions.

In a further aspect, this invention relates to the production of green urea utilizing the green house CO₂ gas and nitrogen contaminants (NO₂⁻ and NO₃⁻) present in groundwater, which possess (2NO₃⁻+CO₂+18H⁺+16e⁻→CO(NH₂)₂+7H₂O) a serious threat to mankind, under ambient conditions using copper phthalocyanine (CuPc) catalyst. The process involves the C—N coupling reaction by a sixteen electron transfer process.

Description of Related Art

Nitrogen is considered an essential element for all life, accounting for 78% of the atmosphere but cannot be absorbed directly by plants and animals. Converting N₂ to useful economic fertilizer in mild conditions is a burning demand of the society. On the other hand, excess carbon dioxide (CO₂) emissions have raised serious environmental concerns. Carbon dioxide is a greenhouse gas that creates global warming. Therefore, the conversion of both N₂ and CO₂ into standard urea molecules through the reaction of C—N coupling under preferable conditions is a brilliant pathway to get both the carbon balance technique and the high value use of CO₂. Therefore, great efforts have been made to convert N₂ and CO₂ into valuable chemical compounds and fuels (like NH₃, CH₃OH, N₂H₄ etc.). Urea is the backbone of the fertilizer industry, which is produced on industrial scale by using high temperature (150-200° C.) and high pressure (100-200 atm). In order to produce urea, it is necessary to activate the N₂ and CO₂ so that they can break their bonds to form C—N bond. Breaking the inert N≡N is very difficult and it takes about 941 kJ/mol energy. Under the same condition, carbon dioxide takes about 806 kJ/mol energy. The use of electrochemical methods compared to conventional processes and other hydrogenation processes demonstrate great advantages by way of reduced energy consumption. This prompted several researchers to create an electro-chemical C—N bond connection system using direct CO₂ and amine derivatives (nitrogen sources likes nitrate, NO, nitrite, and also N₂) as feeding reactant. During the feeding of N₂ and CO₂ gas as reactants, the reactions of the electrochemically C—N coupling occurred to make valuable urea [CO(NH₂)₂] components, which are considered to be the most efficient N₂ fertilizers and are significant for chemical industry. Industrial urea synthesis is subjected by two mutual paths, namely N₂+H₂→NH₃ and then NH₃+CO₂→CO (NH₂)₂, which require severe reaction conditions involving elevated temperature and high pressure. For the first step, it takes about 150-350 atm pressure, 350-355° C. temperature and Fe catalyst. For second step, it takes about 150-350 atm pressure and 350-355° C. temperature. Therefore the industrial process requires large amount of energy and involves tremendous financial requirement.

The other sources of nitrogen are present in the environment in the form of notorious contaminants such as nitrogen oxides, which affect the climate, flora, fauna and especially children; they are badly affected by this nitrite contaminant leading to the deadly blue baby syndrome. Nitrogen oxide is gathered in water from engineered compost, chemical reaction, human carelessness to name a few. Nowadays, there is widespread concern about reducing the nitrate contaminants from ground water. Nitrate itself does not affect human beings but when nitrate is reduced to nitrite, it poses a serious threat to humans and animals. So, there is a need to reduce the degree of nitrate contamination.

Urea is used as a significant nitrogen source in composts and is consumed worldwide at the rate of 100 million tons per year and is probably the most important usable product. Nitrogen and hydrogen are important components for making ammonia in the fertilizer industry, which is utilized as a crude material of urea production. During the time spent in making ammonia and urea, hydrogen is created through the steam methane improving measure, in which methane and steam respond to deliver carbon monoxide and hydrogen. This kind of response is profoundly endothermic, requiring huge energy while responding. Because of limitations and costs of natural gas, it is important to discover alternative wellsprings of hydrogen. Hydrogen can fill in as a mediator product of urea and ammonia plants in the biomass and electrolysis of the water. Testing of green science centers on synthetic items and cycle progress typically includes waste reduction, supplanting of existing items with other less harmful alternatives and progress to sustainable feedstock. The synthetic business is as of now looking for imaginative methods of diminishing greenhouse gas emission (GHG) related with the creation of ammonia, to supplant the century old Haber-Bosch interaction to produce alkali ammonia from H₂ and N₂. Nitrogen fertilizers were made utilizing wind energy. Air based ammonia can essentially diminish fossil energy and GHG as compared to ordinary production. Different investigators have observed that the production of alkali ammonia through wind-produced power on remote islands, using the abundant wind energy was not expected to fulfill the need for alkali ammonia synthesis and production of green ammonia, and have utilized a sun based alkali ammonia treatment facility, which uses solar energy for the cycle of ammonia salts production. There is a built-in strategy for evaluating the energy of various sustainable power sources, coordinated in ammonia production plants which can be derived from the gasification of biomass, biogas transforming, or electrical analysis of water using energy sun-based or wind-based energy. Electrochemical ammonia generation has been created by reduced the energy consumption by over 20% and reduced the cost and complexity compared to the conventional route for ammonia production.

Since practically all synthetic fertilizers are derived from fossil fuels, the demand for continuous fertilizers is further involved. Processing of these fuel releases greenhouse gases (GHG) such as carbon dioxide (CO₂), nitrous oxide (N₂O) and methane (CH₄). The increasing environmental concentration of GHG has been a matter of serious concern for a long time. Their outflow is a significant reason for anthropogenic environmental change (e.g. global warming), prompting environmental disasters such as glacier melting, sea level rise, ocean acidity etc. Therefore, there has been significant activity towards the far and wide execution of reduction systems, including Carbon Capture and Storage (CCS) and Carbon Capture and Utilization (CCU). For CCS, CO₂ is caught and put away in geological designs (e.g. drained oil wells, gas fields, saline springs), conceivably permitting practical evacuation of a lot of CO₂ from the climate (Leung et al., 2014 [Leung, Dennis Y. C.; Caramanna, Giorgio; Maroto-Valer, M. Mercedes (2014). An overview of current status of carbon dioxide capture and storage technologies. Renewable and Sustainable Energy Reviews, 39, 426-443. doi:10.1016/j.rser.2014.07.093]). Conversely, CO₂ captured for CCU is handled into the variety of business items (e.g., methanol, methane, polyurethanes, formaldehyde etc.) that offer an option in contrast to their fossil-determined equivalents. CN_108977841B to Changchun Inst Applied Chemistry, discloses a method for synthesizing urea through synchronous electrochemical reduction of nitrogen and carbon dioxide gas. The method comprises mixing nitrogen and carbon dioxide gas and are subjecting the mixture to electrochemical reaction to obtain urea. According to this method, carbon dioxide and the nitrogen are adopted as raw materials and are subjected to electro-catalysis synchronously to synthesize urea.

U.S. Pat. No. 8,524,066B2 to Avantium Knowledge Centre BV, discloses methods and systems for electrochemical production of urea. The method includes the steps of introducing carbon dioxide and NOx to a solution of an electrolyte and a heterocyclic catalyst in an electrochemical cell. The divided electrochemical cell may include an anode in a first cell compartment and a cathode in a second cell compartment. The cathode may reduce the carbon dioxide and the NOx into a first sub-product and a second sub-product, respectively. The sub-products may be combined in the next step, to produce urea.

U.S. Pat. No. 9,890,114B2 to Toyo Engineering Corporation, discloses a urea synthesis method in which the amount of oxygen used as a corrosion-resistant agent is minimized without using special duplex stainless steel. The urea synthesis is carried out in a urea synthesis apparatus having a synthesis tower, a stripper, and a condenser, general-purpose austenitic-ferritic duplex stainless steel is used as a urea synthesis apparatus material in at least some of parts where the urea synthesis apparatus comes into contact with a fluid having corrosiveness, and oxygen feed concentration with respect to carbon dioxide is 100 to 2,000 ppm.

U.S. Pat. No. 9,005,422B2 to Energy and Environmental Research Center Foundation (EERC Foundation), discloses electrochemical process for the preparation of nitrogen fertilizers. Methods and apparatus for the preparation of nitrogen fertilizers including ammonium nitrate, urea, urea-ammonium nitrate, and/or ammonia, at low temperature and pressure, preferably at ambient temperature and pressure, utilizing a source of carbon, a source of nitrogen, and/or a source of hydrogen or hydrogen equivalent. Implementing an electrolyte serving as ionic charge carrier, ammonium nitrate, urea or its isomers, ammonia and urea-ammonium nitrate is produced via the simultaneous cathodic reduction of a carbon source and a nitrogen source, and anodic oxidation of a nitrogen source.

Therefore, the need exists to propose an alternative process for the production of urea using simpler, most cost-effective modes of production, which will at the same time improve the yields, use materials which are a threat to the environment and also those materials which are abundantly available from nature.

SUMMARY OF THE INVENTION

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

A further object of this invention is to propose a process for the electrochemical synthesis of green urea, 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 urea, 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 urea, 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 urea, which uses natural eco-friendly, easily available raw materials and the process is environment friendly.

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

A still further object of this invention is to propose a process for the electrochemical synthesis of green urea, which does not use expensive noble metals and is therefore a cost-effective process.

Another object of this invention is to propose a process for the electrochemical synthesis of green urea, which uses greenhouse gases and is a long-term sustainable process.

Yet another object of this invention is to propose a process for the electrochemical synthesis of green urea, where the process involves the utilization of nitrogen in two different forms, first in the form gaseous N₂ molecule and another includes the ionic forms of nitrogen oxides (NO₂ and NO₃⁻).

A further object of this invention is to propose a process for the electrochemical synthesis of green urea, where the process involves the conversion of the greenhouse CO₂ gas into value added product in the form of urea.

These and other objects 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 copper phthalocyanine (CuPc)

FIG. 2: FTIR spectrum of CuPc

FIG. 3: LSV profile of CuPc 0.1 M KHCO₃ solution [For N₂+CO₂ to Urea]

FIG. 4: Current density (j) vs time (s) curves of CuPc at different potentials [For N₂+CO₂ to Urea]

FIG. 5: UV-VIS absorption spectra of different potential's electrolytes after 7200 s electrochemically N₂ and CO₂ reduction [For N₂+CO₂ to Urea]

FIG. 6: Bar chart of urea yield rate and Faradaic efficiency of CuPc at different potential [For N₂+CO₂ to Urea]

FIG. 7: UV-VIS spectra of different concentration urea solution [For N₂+CO₂ to Urea]

FIG. 8: Standard calibration curve used for the determination of urea concentration [For N₂+CO₂ to Urea]

FIG. 9: LSV profile of CuPc in 1000 ppm NO₃⁻electrolyte solution [For NO₃+CO₂ to Urea]

FIG. 10: Current density (j) vs time (s) curves of CuPc at different potentials [For NO₃+CO₂ to Urea]

FIG. 11: UV-VIS absorption spectra of different potential's electrolytes after 7200 s electrochemically NO₃ and CO₂ reduction [For NO₃+CO₂ to Urea]

FIG. 12: Bar chart of urea yield rate and Faradaic efficiency of CuPc at different potentials [For NO₃+CO₂ to Urea]

FIG. 13: Control experiment study [For NO₃+CO₂ to Urea]

FIG. 14: Schematic diagram of H-type electrochemical setup

FIG. 15: UV-VIS spectra of different concentration urea solution [For NO₃^-+CO₂ to Urea]

FIG. 16: Standard calibration curve used for the determination of urea concentration [For NO₃+CO₂ to Urea]

FIG. 17: ¹H NMR spectra of electrolyte saturated withand Ar after 25 h of electrolysis

DESCRIPTION OF THE INVENTION

Thus, according to this invention is provided a process for the electrochemical synthesis of green urea, an electrochemical cell for the electrochemical synthesis of green urea and the green urea produced thereby.

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

• • a working electrode comprising carbon paper loaded with copper phthalocyanine (CuPc) catalyst ink,
  • 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 and carbon dioxide gas to electro-reduction followed by combination of the electro-reduced products to produce urea.

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

• • a working electrode comprising carbon paper loaded with copper phthalocyanine (CuPc) catalyst ink,
  • 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 purging nitrogen (N₂) and carbon dioxide (CO₂) gases into the electrolyte, followed by electro-reduction of the gases and combination of the electro-reduced products to produce green urea.

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

• • a working electrode comprising carbon paper loaded with copper phthalocyanine (CuPc) catalyst ink,
  • 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 providing the NO₂⁻ and NO₃⁻ ion containing nitrogen source as the electrolyte, purging carbon dioxide (CO₂) gas into the electrolyte solution, followed by electro-reduction of the ions and CO₂ gas, and combination of the electro-reduced products to produce green urea.

The processes are carried out in an electrochemical cell using a three-electrode system where the platinum (Pt) wire is an auxiliary or counter electrode and Ag/AgCl (saturated in 3.5 M KCl) is the reference electrode and carbon paper loaded with CuPc catalyst ink is the working electrode. All potential values are changed to the reversible hydrogen electrode (RHE).

In accordance with a further embodiment, the invention provides an electrochemical cell for the electrochemical synthesis of green urea, 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 copper phthalocyanine (CuPc) catalyst ink,
  • a reference electrode of silver/silver chloride (Ag/AgCl) saturated with potassium chloride (KCl); and
  • an auxiliary or counter electrode being a platinum wire; said cathodic chamber including at least two inlets for the entry of gases and at least one outlet for the exit of gases therefrom.

The nitrogen source is selected from N₂ gas and ionic forms of nitrogen oxides (NO₂ and NO₃⁻). These ions are to be found as contaminants in sewage and groundwater.

As used herein, the term “cell” is used to refer to an electrochemical cell, more particularly, a H-type cell.

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 (Sigma-Aldrich).

The power can be supplied as DC current.

The voltage applied across the electrodes is typically in the range of −0.1 to −1 volts with respect to the reversible hydrogen electrode. As the applied potential is very low, hence it can be provided by the solar cell/photo-voltaic cell, therefore the process is green in nature

In accordance with an embodiment, when the nitrogen source is N₂ gas, the gas is diffused into the electrolyte along with CO₂, for electro-reduction. The electrolyte used is ordinarily an aqueous bicarbonate solution such as sodium bicarbonate or potassium bicarbonate.

In accordance with an embodiment, when the nitrogen source comprises the ionic forms of nitrogen oxides (NO₂ and NO₃), they are used as the electrolyte during the electrochemical process.

The reference electrode of silver/silver chloride (Ag/AgCl) is saturated with 3.5 M Potassium chloride (KCl).

The working electrode is prepared by loading a catalyst ink on a substrate such as carbon paper. While the present invention is not intended to be limited by the catalyst it will normally be selected from transition metal complex catalysts such as copper phthalocyanine, iron phthalocyanine, zinc phthalocyanine, cobalt phthalocyanine, preferably copper phthalocyanine.

While the invention is not intended to be limited by the method of deposition of the catalyst, it is envisaged that the catalyst is dispersed in an alcohol and the solution mixture is sonicated. A solution of an ionic polymer is added to the previous solution and the solution is placed in vortex to produce useable ink. By way of a preferred embodiment and without implying any limitation on the scope of the invention, copper phthalocyanine (CuPc) crystal is dispersed in an alcohol such as 2-propanol and the solution mixture is sonicated. A solution of an ionic polymer is added to the solution. Finally, the solution is placed in vortex for a minute and useable ink is prepared for further use. The CuPc ink thus produced is loaded on carbon paper.

The ionic polymer used in the catalyst ink is a proton conductor, which may be any synthetic polymer with ionic properties, such as a sulfonated tetrafluoroethylene based fluoropolymer-copolymer, such as for instance Nafion 117 (Sigma-Aldrich).

For the process wherein N₂ and CO₂ are converted to urea, nitrogen and carbon dioxide gases are continuously purged into the 0.1 M KHCO₃ electrolyte solution for 30 min. During the dual gases reduction process, the rate of entry of nitrogen gas was 4 ml/min and carbon dioxide gas was 2 ml/min.

For the process wherein NO₃ and CO₂ are converted to urea, carbon dioxide gas is continuously purged into the 1000 ppm KNO₃ electrolyte solution for 30 min. The rate of entry of carbon dioxide gas was 2 ml/min.

Electrochemical dual reduction of N₂ and CO₂ gases to Urea:

In accordance with an embodiment of this invention, urea is synthesized electrochemically from the reduction of dual purging gases N₂ and CO₂ via six electron transfer process at ambient condition using copper phthalocyanine (CuPc) catalyst. Carbon dioxide is electrochemically reduced and captured. During electro-reduction an intermediate product CO is produced from CO₂.

The synthesis of urea from the dual reduction of N₂ and CO₂ is an advantageous improvement over the Haber-Bosch process (ammonia synthesis, one step during urea synthesis) where high temperature and pressure were required to complete the reaction and also high energy consumption for the CO₂ reduction to CO. However, in accordance with the instant invention, CuPc catalyst is used to produce urea from the electro-reduction of N₂ and CO₂ at normal pressure and room temperature. Urea yield and Faradaic efficiency are measured after electrochemical dual gas reduction by UV-VIS spectrophotometer. The calculated urea yield and the Faradaic efficiency are 1.13 mmol h⁻¹ g_cat⁻¹ and 18.67% (at −0.60V vs RHE).

Electrochemical reduction of N₂ and CO₂ to urea is the alternative to Haber-Bosch process where ammonia is produced which further reacts with CO (reduction of CO₂) to produce urea. Herein, the inventors have produced urea under normal temperature and pressure by applying low potential with CuPc crystal.

Crystalline structure of commercial CuPc and phase purity was confirmed by X-ray diffraction (XRD) technique (FIG. 1) where the strong peaks (101), (101), (002), (200), (202), (103) and (103) were seen at the 20 values of 7.319, 9.492, 10.815, 12.811, 14.36, 15.45, and 18.89 respectively.

The various chemical bonds present in CuPc system are confirmed by FTIR spectra as shown in FIG. 2. The main peak at 1165 cm⁻¹ is attributed to the Cu—N bond in the CuPc molecule. The peaks of 900, 1068, 1091, and 1120 cm⁻¹ are assigned to pyrrole in the plane mode of CuPc, and the peaks of 876 cm⁻¹ indicate the pyrrole-out-of-plane mode of copper phthalocyanine. The peaks of 1289 and 1335 cm⁻¹ are assigned for C=N—C bridge bond in CuPc.

During electro-reduction, the gases are diffused into the electrolyte, and then they are adsorbed onto the catalytic surface followed by the reorientation of the catalyst to facilitate the electron transfer process. Thereafter, a reorientation of the molecular catalyst occurs for facile release of the product (urea) formed and then desorption of the product from the surface of the catalyst occurs and finally the urea is diffused out into the electrolyte. Electrochemically N₂ and CO₂ gases undergo simultaneous reduction to form urea. In the first stage, nitrogen gas reduces to form ammonia and carbon dioxide gas to carbon mono-oxide. In the second stage, both the reduced products combine with each other to produce the final component urea. The on-set potential (from LSV curve, FIG. 3) is found to be near at −0.4 V (vs RHE) and after that the current density suddenly increases. The time relevant chronoamperometry profile (j vs t) was shown in FIG. 4 with applying the different potential window. A potential window of −0.45V to −0.9V (vs RHE) is applied for the process and the strongest absorption peak appears at −0.6V (vs RHE), indicating the highest urea yield at −0.6V (vs RHE).

There is a negligible decay in current densities which indicates a good stability of catalyst after two hours of electrochemical dual gas reduction. The UV-VIS absorption spectroscopy was conducted on the electrolyte samples after two hours of electrochemical N₂ and CO₂ reduction and the maximum absorption peaks were shown (FIG. 5) at 520 nm which confirmed the production of urea after the electro reduction of dual gas. At the different potential window −0.4V to −0.9V (vs RHE), the strongest absorption peak appeared at −0.6V (vs RHE). So, the urea yield is highest at −0.6V (vs RHE). After the electrochemical N₂ and CO₂ reduction at all the possible different potentials, the urea yield and Faradaic efficiency (FE) was calculated. As the Faradaic efficiency is dependent on current density, the maximum FE is 18.67% and urea yield is 1.13 mmol h⁻¹g_cat⁻¹ (at −0.6V vs. RHE) (FIG. 6). The unknown urea concentration was determined using the known urea concentration series. Standard urea concentration absorption spectra were observed in FIG. 7 details mention in experimental section and from a linear correlation between concentration and absorbance was observed which is shown in FIG. 8.

Electrochemical reduction of NO₃⁻ ions and CO₂ gas to Urea: All electrochemical measurements were performed in 1000 ppm NO₃ electrolyte solution with the continuous purging of CO₂ gas. During dual-reduction, carbon dioxide is reduced to carbon monoxide and nitrate ions are reduced to —NH2 intermediate, which combine together to form the final product urea. From the Linear Sweep Voltammetry (LSV) profile (FIG. 9), it is clearly shown that when Argon (Ar) gas and CO₂ gas are purged into the nitrate electrolyte solution, the current density is increased in the case of when CO₂ gas is purged which signifies the reduction of CO₂ gas instead of reduction of inert Ar gas. LSV profile was taken at the potential window of −0.45 to −0.85 V vs. RHE. The time dependent chronoamperometry (j, current density vs t, time) was shown in FIG. 10 with applying the different potential range for 2 h of dual reduction process. In FIG. 10, j vs t, shows a very negligible change in current densities which indicate the stability of catalyst after 2h dual reduction process at different applied potential range. After 2 h of electrochemical dual reduction of nitrate ions and carbon dioxide gas, produced urea was quantified by UV-VIS spectrophotometers utilizing the well-established diacetyl monoxime method. UV-VIS curves (FIG. 11) show that the maximum absorption peak is observed at −525 nm confirming the formation of urea in the electrolyte solution. After getting the absorption data from individual potential range, urea yield rate and Faradaic efficiency (FE) was calculated utilizing a standard urea calibration curve. FIG. 12 shows the maximum urea yield rate or production rate is 1.85 mmol h⁻¹g_cat⁻¹ and corresponding FE is 34.42% at −0.75V vs. RHE. Several control experiments were carried out to confirm the urea is actually coming by the dual reduction of nitrate ions and carbon dioxide gas. Firstly, Ar gas was purged into the NO₃⁻ solution and after about 2 h of electroreduction process there was negligible amount of urea produced as shown in FIG. 13. However, maximum amount of urea was produced at −0.75V when carbon dioxide gas was continuously purged into electrolyte solution.

All the electrochemical processes are carried out in an electrochemical workstation (CHI 760E) using a three-electrode system. The schematic diagram of the H-type electrochemical setup is illustrated in FIG. 14. A pair of containers acts as the cathodic chamber (A) and anodic chamber (B) and which are in fluid connectivity with each other through a tubular structure there between. The two chambers (A, B) configure a three-electrode system (1, 3, 7), of which the first electrode of carbon paper loaded with CuPc catalyst ink (6) embedded within the first chamber (A) acts as a working electrode (1), the second electrode of Ag/AgCl (saturated in 3.5 M KCl) embedded within the cathodic chamber (A) acts a reference electrode (3) and the third electrode of platinum (Pt) wire embedded within the anodic chamber (B) acts as an auxiliary or counter electrode (7). The first chamber (A) configures a pair of gas inlets, a first inlet (4) for N₂ gas and a second inlet (5) for CO₂ gas to enter into the chamber (A) and a gas outlet (2) for excess gases to exit therefrom. The tubular structure configures a Nafion membrane (8) (Sigma-Aldrich) to allow only protons to pass through it.

The invention will now be explained in greater detail with the help of the following non-limiting examples.

EXAMPLES

Preparation of Catalyst Ink:

For N₂+CO₂ to Urea: 2 mg CuPc crystal was taken in a vial and dispersed with 200 μL 2-propanol (Merck) solution. The solution mixture was sonicated for few seconds. Further, 20 μL Nafion 117 (5 wt %) (Sigma-Aldrich) solution was added to the previous solution. Finally, the solution was placed in vortex for a minute and useable ink was prepared for further use. 25 μL prepared ink was loaded on 1×1 cm² carbon paper.

For NO₃⁻+CO₂ to Urea: In a vial, 2.5 mg of CuPc was taken and dispersed with 250 μL 2-propanol (Mark) solution. The solution was sonicated for 2 min. In addition, 20 μL, Nafion 117 (5 wt %) (Sigma-Aldrich) solution was mixed to the previous solution. Finally, the solution was stirred for 1 minute and a usable ink was prepared for further use. Finally, 40 μL of prepared ink was loaded on 1×1 cm² carbon paper.

Experimental Procedure:

Electrochemical dual reduction of N₂ and CO₂ gases to Urea:

The set-up as described hereinbefore was used for the process. Before starting the experiment, nitrogen and carbon dioxide (N₂+CO₂), gases were continuously purged into the 0.1 M KHCO₃ electrolyte solution for 30 min. During the dual gases reduction process, the rate of entry of nitrogen gas was 4 ml/min and carbon dioxide gas was 2 ml/min.

All the electrochemical processes were carried out in an electrochemical workstation (CHI 760E) using a three-electrode system where the platinum (Pt) wire is an auxiliary or counter electrode (7), Ag/AgCl (saturated in 3.5 M KCl) is the reference electrode (3) and carbon paper loaded with CuPc catalyst ink (6) is the working electrode (1). All potential values were changed to the reversible hydrogen electrode (RHE). A H-type cell (FIG. 14) was used for all electrochemical reduction processes. FIG. 8 was used for process of conversion using N₂+CO₂ to urea & FIG. 16 was used for process for conversion of NO₃+CO₂ to urea. In an H-type cell, two cell chambers (anodic & cathodic chambers) were separated by a Nafion 117 membrane (Sigma-Aldrich) (where only proton can pass through it). Before using the Nafion 117 membrane, it was first treated in 5 wt % H202 at 80° C. for 1 hour and 0.1 M H₂SO₄ at 80° C. for 1 hour and then in distilled water for 3 hours at 80° C.

A measurable amount (70 ml) of electrolyte solution was taken in both anode as well as cathode chambers. Then, the working electrode and reference electrode were kept in cathode chamber and counter electrode was kept in anode chamber.

Electrochemical reduction of NO₃⁻ ions and CO₂ gas to Urea: The set-up as described hereinbefore was used for the process, keeping the first inlet (4) for N₂ gas closed during the entire procedure. Before starting the experiment, carbon dioxide gas was continuously purged into the 1000 ppm KNO₃ electrolyte solution for 30 min. The rate of entry of carbon dioxide gas was 2 ml/min.

The electrochemical experiments involved Linear Sweep Voltammetry (LSV) study, involves two cases, first in which argon gas was purged and in the second case carbon dioxide (CO₂) gas into the nitrate electrolyte solution, the current density was found to increase in the case of CO₂ gas rich electrochemical environment which signify that the reduction of CO₂ gas occurred instead of reduction of inert Ar gas. LSV profile was taken at the potential window of −0.45 to −0.85 V vs. RHE. The time dependent chronoamperometry study (j, current density vs t, time) was done at different potential for the duration of 2 h for the dual reduction process. The j vs t revealed that very negligible fluctuation in current densities was observed which indicates the stability of catalyst after 2h dual reduction process at different applied potential range. After 2 h of electrochemical dual reduction of nitrate ions and carbon dioxide gas, the electrochemically produced urea was quantified by UV-VIS spectrophotometers utilizing the well establish diacetyl monoxime method. UV-VIS curves show that the maximum absorption peak occurs at −525 nm confirming the formation of urea into the electrolyte solution.

After 2 hours of the electro-reduction process, the urea produced in the electrolyte was isolated in a container for further detection and quantification.

Determination of Urea:

Urea concentration was quantified by UV-VIS spectrophotometer using diacetyl monoxime method. For this process color reagent was prepared as follows:

Solution A: Acid ferric solution: 30 ml concentrated H₂SO₄ and 10 ml concentrated phosphoric acid (Sigma-Aldrich) were mixed with 60 ml triple distil water (Millipore). Then 10 mg ferric chloride was added into the solution.

Solution B: DAMO-TSC solution: 0.25 g diacetyl monoxime (DAMO) (Sigma-Aldrich) and 5 mg thiosemicarbazide (TSC) (Sigma-Aldrich) were dissolved in 50 ml triple distilled water.

Previously urea solutions of different concentrations (0.0, 0.1, 0.3, 0.6, 0.9, and 1.2 ppm) were prepared for standard calibration purpose. 2 ml solution of A, 1 ml solution of B were mixed with each of the urea containing solutions and shaken vigorously. Then the solutions were heated at 100° C. for 30 min to generate pink color solution. The solutions were cooled to room temperature and UV-VIS spectra were shown that the maximum absorbance peaks at 520 nm. From these curves a linear relation (y=0.072×+0.113, R²=0.982) was obtained after plotting concentration vs absorbance of all solutions.

For nitrate and carbon dioxide to urea, a urea concentration series (0.0, 0.4, 0.8, 1.2, 1.6, and 2.0 ppm) (FIG. 15) were made where y=0.0811×+0.0417, R²=0.997 (FIG. 16).

Calculation of the yield and Faradaic efficiency:

After electrochemically NO₃⁻/N₂ and CO₂ reduction to urea, the yield rate of urea and Faradaic efficiency were calculated by the following equation:

$\mathrm{Urea}\mathrm{yield}\mathrm{rate}=\frac{\left(C_{\mathrm{urea}}\times V\right)}{\left(m_{\mathrm{cat}}\times t\times 60.06\right)}$ $\begin{array}{cc}\mathrm{FE}\left(\%\right)=\frac{\left(6\times F\times C_{\mathrm{urea}}\times V\right)}{\left(60.06\times Q\right)}\times 100\%& \left[\mathrm{For}{N}_2+{\mathrm{CO}}_2\mathrm{to}\mathrm{Urea}\right]\end{array}$ $\begin{array}{cc}\mathrm{FE}\left(\%\right)=\frac{\left(16\times F\times C_{\mathrm{urea}}\times V\right)}{\left(60.06\times Q\right)}\times 100\%& \left[\mathrm{For}{\mathrm{NO}}_{3^{-}}+{\mathrm{CO}}_2\mathrm{to}\mathrm{Urea}\right]\end{array}$ $\begin{array}{cc}\mathrm{FE}\left(\%\right)=\frac{\left(16\times F\times C_{\mathrm{urea}}\times V\right)}{\left(60.06\times Q\right)}\times 100\%& \left[\mathrm{For}{\mathrm{NO}}_{3^{-}}+{\mathrm{CO}}_2\mathrm{to}\mathrm{Urea}\right]\end{array}$

where, C_urea is the concentration of urea produced during electrochemical reduction process, V is the volume of electrolyte, t is the time duration for reduction, 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.

Confirmation of the Electrochemical Urea by Isotopic Labelling Experiment

To confirm the origin of urea production, an isotope labelling experiment was carried out using the dual gases ¹⁵N₂ (98 atom percent ¹⁵N Sigma-Aldrich) and ¹²CO₂ (99.999 percent ultra-high purity grade). To saturate the gases in the electrolyte solution, ultra-high purity gases such as Argon, nitrogen (N₂), carbon dioxide (CO₂), and nitrogen (¹⁵N₂) isotope gas were constantly purged into the cathode chamber for half an hour before starting the experiment. The final electrolyte solution was concentrated in a distillation setup after 25 hours of co-reduction of nitrogen and carbon dioxide gases at a potential of −0.6V (versus RHE). The production of urea was then detected using the ¹H-NMR technique (Bruker 600 MHz, USA) using an internal standard of d⁶-DMSO (Cambridge Isotope Laboratories) as an internal standard. After 2 hours and 2,000 scans, the NMR spectrum was seen. When ¹⁴N₂ and ¹²CO₂ gases were purged in the electrolyte, a singlet peak was observed at a chemical shift of 5.7 ppm for the formation of urea, and a doublet peak was observed when ¹⁵N₂ and ¹²CO₂ gases were purged in the electrolyte, but no such peaks were observed when Ar was purged, indicating that the origin of urea comes from the dual reduction of gases (N₂ and CO₂) and not from (FIG. 17).

The invention relates to a process for the electrochemical synthesis of green urea, and the urea produced thereby. The urea is green urea which is produced in liquid form. The process does not use expensive noble metal catalysts, but easily available copper catalyst and is therefore cost-effective. The catalyst is a single molecule which can adsorb the reactant gases and at an ultra-low applied potential (-0.6 V vs. RHE) the catalyst can simultaneously reduce the two gases (N₂ and CO₂). The catalyst can not only reduce (N₂ and CO₂) gases but also it can reduce (NO_x ions and CO₂ gases) to produce green urea. The process uses heterogeneous catalytic system which operates under mild conditions; hence the product formed is eco-friendly green urea. As opposed to prior art methods where two different catalysts, heterostructure or alloys are required for the two reductions of NO₃ /N₂ and CO₂, the instant process uses a single catalyst for reduction. These steps reduce the cost still further.

Claims

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

a working electrode comprising carbon paper loaded with copper phthalocyanine (CuPc) catalyst ink,

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 subjecting a nitrogen source and carbon dioxide gas to produce green urea by electrocatalytic reduction.

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

3. The process as claimed in claim 1, wherein said nitrogen source is selected from N2 gas and ionic forms of nitrogen oxides (NO2− and NO3−).

4. The process as claimed in claim 3, wherein when the nitrogen source is N2, N2 gas is diffused into the electrolyte for electrocatalytic reduction.

5. The process as claimed in claim 2, wherein when the nitrogen source is ionic forms of nitrogen oxides, the ionic forms of nitrogen oxides (NO2− and NO−3) are present as the electrolyte.

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

7. The process as claimed in claim 1, wherein during the reduction process, the rate of entry of nitrogen gas is 2-5 ml/min and carbon dioxide gas is 1-3 ml/min.

8. The process as claimed in claim 1, wherein the electro-reduction is effected for a period ranging from about 1-3.5 hours.

9. A process for the electrochemical synthesis of green urea in an electrochemical cell, comprising an electrolyte and a three-electrode system said three-electrode system comprising: the process comprising purging nitrogen (N2) and carbon dioxide (CO2) gases into the electrolyte, followed by electro-reduction of the gases and combination of the electrocatalytic reduced products to produce green urea.

a working electrode comprising carbon paper loaded with copper phthalocyanine (CuPc) catalyst ink,

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

an auxiliary or counter electrode being a platinum wire;

10. A process for the electrochemical synthesis of urea in an electrochemical cell, comprising an electrolyte and a three-electrode system said three-electrode system comprising: the process comprising providing the NO2 and NO3 ion containing nitrogen source as the electrolyte, purging carbon dioxide (CO2) gas into the electrolyte solution, followed by electrocatalytic reduction of the ions and CO2 gas, and combination of the electrocatalytic reduced products to produce green urea.

a working electrode comprising carbon paper loaded with copper phthalocyanine (CuPc) catalyst ink,

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

an auxiliary or counter electrode being a platinum wire;

11. An electrochemical cell for the electrochemical synthesis of green urea, 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 copper phthalocyanine (CuPc) catalyst ink a reference electrode of silver/silver chloride (Ag/AgCl) saturated with potassium chloride (KCl); and an auxiliary or counter electrode being a platinum wire;

said cathodic chamber comprising at least two inlets for the entry of gases and at least one outlet for the exit of gases therefrom.