FIELD OF THE INVENTION

The present invention is directed to a paired electrosynthesis process for the (co)production of hydroxylamine (HA) and/or ammonia out of N2 or air and water. It further provides an electrolyzer developed for said purpose comprising both a porous cathode and anode with an effective catalyst layer and based on a continuous flow process. The process and electrolyzer of the present invention are particularly useful to efficiently convert reactants such as NOs', NC>2', NOx and N2 at both, the cathode and anode.

BACKGROUND TO THE INVENTION

The electrosynthesis of nitrogen species such as ammonia and HA are promising in view of renewable energy storage and electrification of the chemical industry. In particular, the current production of ammonia leads to large CO2 emissions due to the fossil fuel based Haber Bosch process. Electrochemical reduction of dinitrogen/NOs' is a sustainable process for ammonia synthesis without CO2 emissions when renewable electricity is used. Moreover, the production of HA is important for the production of caprolactam, a precursor of nylon-6. For the electrochemical production of ammonia and/or HA, also waste water streams can be utilized since they contain nitrates and nitrites.

However, the current technology for an Electrochemical Nitrogen Reduction Reaction (NRR) is facing a poor product selectivity leading to very little amounts of NH3 or HA products to be formed. Moreover, the reliability of many experiments in the field are questioned due to the little amounts of products produced, which cannot be accurately ascribed to the conversion of N2 or NO3; or NO2-. Also the current Electrochemical Nitrate Reduction Reaction (NO3RR) for HA formation is still only realized at lab scale, using batch type of cells.

As such, current key bottlenecks in the electrosynthesis of nitrogen species out of N2 or air and water are (1 ) the poor selectivity of the electrocatalyst, and (2) the electrochemical system (electrode, membranes, reactor) is not optimal for long term (continuous) operation, which is eventually needed for an economic viable process. Both factors limit the desired/achieved currents or productivity of ammonia and HA, hampering applicability, operation and upscaling of this process.

It is an objective of the present invention to address the aforementioned problems and to provide a process and electrolyzer to efficiently convert reactants such as NOs', NO2; NOx and N2 into commercially interesting nitrogen species such as ammonia and HA. As further detailed hereinafter, the electrochemical flow cell as herein provided and the process therein performed, enable a continuous operation to produce ammonia and hydroxylamine from a ^-containing gas. In existing systems, such as for example described in PCT publication WO 2022/060920, and US publication US 2021/0340683 a separate nitrogen oxidation unit is required. Also, a common ^-containing gas can be used both at the cathode and the anode. In the traditional systems for the preparation of nitrogen containing fertilizers such as provided in PCT publication WO2012/051507, the anode system is based on the supply of hydrogen containing gases and as such fail in providing an integrated system for the combined synthesis of ammonia and HA.

SUMMARY OF THE INVENTION

In a first aspect the present invention provides an electrochemical flow reactor configured for paired electrosynthesis of NHg and/or Hydroxylamine (HA), said reactor comprising metal electrocatalyst based porous gas diffusion electrodes (GDEs) as cathode and anode.

In an embodiment, the electrochemical flow cell reactor is characterized in that the metal electrocatalysts are selected from Bismuth/Molybdate, Bismuth/Tin based catalysts, Copper based catalysts, Ruthenium based catalysts, Platinum based catalysts, Cobalt based catalysts, Nickel based catalysts, Palladium based catalysts, Iron based catalysts, Titanium based catalysts, Tungsten based catalysts Cobalt/lron/Zinc/Oxide alloy based catalysts, Iron/Tin alloy based catalysts and combinations or alloys thereof.

In a particular embodiment the metal electrocatalysts of the cathode and the metal electrocatalysts of the anode are different. Hence in an embodiment the metal electrocatalysts of the cathode are selected from Bismuth/Molybdate based catalysts, Bismuth/Tin based catalysts, Copper based catalysts, Ruthenium based catalysts, Platinum based catalysts, Cobalt based catalysts, Nickel based catalysts, Palladium based catalysts, and combinations thereof; and the metal electrocatalysts of the anode is selected from Ruthenium and Ruthenium/Manganese oxide based catalysts, Platinum based catalysts, Palladium based catalysts, Cobalt/lron/Zinc/Oxide alloy based catalysts, Rhodium based catalysts, Tungsten based, Iron/Tin alloy based catalysts, (supported) boron doped diamond, (supported) nitrogen doped carbon and combinations thereof.

In a preferred embodiment the metal electrocatalysts of the cathode is selected from a Bismuth/Molybdate alloy, a Bismuth/Tin alloy, Copper, a Platinum/Ruthenium alloy, a Platinum/Tin alloy, Cobalt, Nickel, and Palladium; more in particular the metal electrocatalysts of the cathode is selected from a Bismuth/Molybdate alloy, Copper, a Platinum/Ruthenium alloy, a Platinum/Tin alloy, Cobalt, Nickel, and Palladium; even more in particular the metal electrocatalysts of the cathode is selected from Bismuth/Molybdate alloy, Copper, a Platinum/Ruthenium alloy, a Platinum/Tin alloy, Cobalt, and Nickel,

In another preferred embodiment the metal electrocatalysts of the anode is selected from Platinum, a Ruthenium/Titanium alloy, Palladium, a Cobalt/lron/Zinc/Oxide alloy, an Iron/Tin alloy, and a Palladium/Ruthenium alloy; more in particular the metal electrocatalysts of the anode is selected from Platinum, a Ruthenium/Titanium alloy, a Ruthenium/Manganese oxide alloy, Rhodium a Cobalt/lron/Zinc/Oxide alloy, an Iron/Tin alloy, a Palladium/Ruthenium alloy, (supported) boron doped diamond, (supported) nitrogen doped carbon; even more in particular the metal electrocatalysts of the anode is selected from Platinum, a Ruthenium/Titanium alloy, a Cobalt/lron/Zinc/Oxide alloy, an Iron/Tin alloy, and a Palladium/Ruthenium alloy.

The electrochemical flow cell reactor will typically comprise an anode and a cathode compartment separated by an ion exchange membrane or a one-compartment system where the cathode and anode are not separated from each other. The person skilled in the art is aware of the ion exchange membrane separators used in electrochemical flow cell reactors, including cation exchange membranes, anion exchange membranes and bipolar membranes. Making use of metal electrocatalyst based porous GDEs as anode and cathode, said anode and cathode compartment each preferably comprise an electrolyte compartment and a gas compartment separated from one another by means of the metal electrocatalyst based porous GDEs, configured for gas from the gas compartment to reach the metal electrocatalyst from the GDEs, wherein the metal electrocatalyst is configured to be in contact with the electrolyte from the electrolyte compartment.

In an embodiment, the electrolyte compartment of the anode compartment and the electrolyte compartment of the cathode compartment comprise a common electrolyte (either an alkaline or acidic electrolyte; in particular alkaline), wherein the outlet of the electrolyte compartment of the anode compartment is fluidly connected to the inlet of the electrolyte compartment of the cathode compartment. As such a continuous flow of the electrolyte can be realized. In the electrochemical synthesis of NHs and HA, the gas compartment of the anode compartment and the gas compartment of the cathode compartment comprise a gas inlet for a Nitrogen containing gas (such as N2, NO, NO2, N2O and mixtures thereof), in particular dinitrogen (N2) gas.

In a further aspect the present invention provides an electrochemical flow cell reactor for paired electrosynthesis of NH3 and/or Hydroxylamine (HA) from a Nitrogen containing gas, wherein the dinitrogen (N2) from the Nitrogen containing gas from a gas compartment of the anode compartment is oxidized at the anode with the formation of NOx, Nitrate (NO3 ) and/or Nitrite (NO2 ) in the electrolyte of the anode compartment, wherein the thus obtained Nitrate (NO3 ) containing electrolyte is fed into to electrolyte compartment of the cathode compartment, to be reduced at the cathode together with the Nitrogen (N2) from the Nitrogen containing gas from the gas compartment of the cathode compartment with the formation of Ammonia (NH3) and/or hydroxylamine (HA) in the electrolyte of the cathode compartment, and removing the thus obtained Ammonia (NH3) and/or hydroxylamine (HA) containing electrolyte from the electrolyte compartment of the cathode compartment.

In an embodiment the electrochemical flow cell reactor, further comprising a separator to retrieve the Ammonia (NH3) and/or hydroxylamine (HA) from the Ammonia (NH3) and/or hydroxylamine (HA) containing electrolyte, and recycling the electrolyte into the electrolyte compartment of the anode compartment.

In a second aspect the present invention provides a method of paired electrosynthesis of NHg and/or Hydroxylamine (HA) in an electrochemical flow cell reactor comprising an anode and a cathode compartment separated by an ion exchange membrane, said method comprising;

• oxidizing diNitrogen (N2) from a Nitrogen containing gas using a metal electrocatalyst based porous gas diffusion electrodes (GDEs) as anode with the formation of Nitrate (NO3-) and/or Nitrite (NO2 ) in an electrolyte present in an electrolyte compartment of the anode compartment or NOx in the gas compartment of the anode compartment,

• feeding the thus obtained Nitrate (NOs-J/Nitrite (NO2')/NOx containing electrolyte into an electrolyte compartment of the cathode compartment,

• reducing the Nitrate (NOs-J/Nitrite (NO2 ) from said Nitrate (NO3-) containing electrolyte together with Nitrogen (N2) from a Nitrogen containing gas using a metal electrocatalyst based porous gas diffusion electrodes (GDEs) as cathode, with the formation of Ammonia (NH3) and hydroxylamine (HA) in the alkaline/acidic electrolyte of the cathode compartment, and

• removing the thus obtained Ammonia (NH3) and hydroxylamine (HA) containing electrolyte from the electrolyte compartment of the cathode compartment.

In an embodiment the method of paired electrosynthesis of NH3 and/or Hydroxylamine (HA), further comprising;

• separating the Ammonia (NH3) and hydroxylamine (HA) from the Ammonia (NH3) and hydroxylamine (HA) containing electrolyte, and

• recycling the electrolyte into the electrolyte compartment of the anode compartment.

In an embodiment the method of paired electrosynthesis of NH3 and/or Hydroxylamine (HA), is characterized in that the metal electrocatalysts of the cathode is selected from Bismuth/Molybdate based catalysts, Copper based catalysts, Ruthenium based catalysts, Platinum based catalysts, Cobalt based catalysts, Nickel based catalysts, Palladium based catalysts, and combinations thereof; and wherein the metal electrocatalysts of the anode is selected from Ruthenium based catalysts, Platinum based catalysts, Palladium based catalysts, Cobalt/lron/Zinc/Oxide alloy based catalysts, Iron/Tin alloy based catalysts and combinations thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 , herein also referred to as Fig. 1 , provides a schematic diagram of the paired electrosynthesis process for NH3 / HA production. DETAILED DESCRIPTION OF THE INVENTION

As mentioned herein before, the present invention is in particular directed to a paired electrosynthesis process for the (co)production of hydroxylamine (HA) and/or ammonia out of N2 or air and water. It provides thereto a continuous flow process and a continuous electrochemical flow reactor with porous electrodes at both the anode and the cathode, wherein each of said porous electrodes comprise a metal electrocatalyst layer.

Per reference to Fig.1 , and further detailed hereinafter, such continuous electrochemical flow reactor consists of a membrane or diaphragm, separating the anode and the cathode compartment and at either side a metal electrocatalyst layer incorporated in a gas diffusion electrode (GDE) to enhance mass transport and increase the electrochemical active surface area.

Since a proper Nitrate (NOs ) source is hard to get (waste water can be used, however, the concentration of nitrates and nitrites herein are typically too low to produce large amounts of ammonia and/or HA) the technical realization of an electrochemical process for HA and/or ammonia synthesis has thus far been hindered by;

• very low current densities, in particular at the cathode (thus low productivity),

• poor product selectivity of the electrocatalyst for NRR towards HA or ammonia,

• large overpotentials due to the anodic reaction, typically oxygen evolution reaction (hence low energetic efficiency),

• process durability which is a result of the interplay of simultaneous processes occurring on both electrodes, and crossover of species through the membrane, and

• stability of the electrode structure.

This problem has been solved by the paired electrosynthesis process and the continuous electrochemical flow reactor according to the invention. Using a paired electrosynthesis process, wherein the anodic reaction produces nitrates, nitrites and/or NOx which are subsequently used as reactant for the cathodic reaction to produce NH3 and/or HA, the aforementioned problem of a proper Nitrate (NOs ) source has been resolved. With the reactor of the present invention a N2- containing gas, such as air can be used instead, or even a gaseous reactant, such as NOx can be utilized, optionally together with an electrolyte based on liquid reactants or dissolved species such as NOs', NOs' and N2H4. Using a paired electrosynthesis process, wherein the anodic reaction produces nitrates, nitrites and/or NOx which are subsequently used as reactant for the cathodic reaction to produce NHs or HA, also results in a decreased cell potential, and thus the energy efficiency is increased. The invention also benefits from a synergistic effect by co- reduction of N2 and NOs' or NC>2' at the cathode which can be controlled by the potential window. Compared to the existing technologies, this results in high product yields of HA and/or ammonia.

Expressed differently, the present invention utilizes the anode reaction products in a paired electrosynthesis concept for the production of HA and ammonia. Although there have been electrodes reported for NRR to ammonia, there are no indications in the literature to co-produce with 100% selectivity only ammonia or HA. Nitrogen oxidation has not been carried out electrochemically in a continuous flow cell which is the key of our anode reaction. It is accordingly an object of the present invention to provide the use of Nitrogen oxidation, i.e. the Electrochemical Nitrogen Oxidation Reaction (NOR), in the electrochemical synthesis of ammonia and/or hydroxylamine, more in particular the use of NOR in a continuous flow cell configuration, in the electrochemical synthesis of ammonia and/or hydroxylamine. The present invention accordingly provides for the use of the products obtained from the anodic oxidation of Nitrogen as reactant for a cathodic reaction to produce NH3 and/or HA. In an embodiment the present invention accordingly provides for the use of the products obtained from the anodic oxidation of Nitrogen in a continuous flow cell, as reactant for a cathodic reaction to produce NH3 and/or HA, more in particular as reactant for a cathodic reaction to produce NH3 and/or HA in a continuous flow cell.

Fig. 1 provides a schematic diagram for the paired electrosynthesis of ammonia and/or HA in a continuous electrochemical flow cell reactor (1 ) using the anodic reaction products as reactant for the cathodic reduction. The electrochemical flow cell reactor comprises an anode compartment (2) and a cathode compartment (3) separated from one another by an ion exchange membrane (4). Both the anode compartment and the cathode compartment are configured as a continuous flow cell, and accordingly comprise an electrolyte compartment (5, 6) comprising a common electrolyte (20) (in the present example an alkaline electrolyte) and a gas compartment (7, 8) separated from one another by means of a metal electrocatalyst based porous GDE (9, 10). The GDE of the anode compartment (9) is configured for N2 containing gas (13) from the gas compartment (7) to reach the metal electrocatalyst layer (11) of the anode which is in contact with the alkaline electrolyte (20) of the anode electrolyte compartment (5). At the interface between said liquid alkaline electrolyte and the metal electrocatalyst layer (11) of the anode, Nitrogen is oxidized with the formation of Nitrate (NOs j/N itrite (NO2 ) in the alkaline electrolyte. The thus obtained Nitrate (NO3 ) containing alkaline electrolyte (21) is fed by means of a fluid connection (19) into the electrolyte compartment of the cathode cell (6). In the cathode compartment (3) another GDE (10) is present and configured for N2 containing gas (14) from the gas compartment (8) the reach the metal electrocatalyst layer (12) of the cathode which is in contact with the Nitrate (NO3 ) containing alkaline electrolyte (21 ) of the cathode electrolyte compartment (6). At the interface between said Nitrate (NOs ) containing alkaline electrolyte (21) and the metal electrocatalyst layer (12) of the cathode, Dinitrogen (N2) and/or Nitrate/Nitrite is reduced with the formation of Ammonia (NHg) and/or Hydroxylamine (HA) in the alkaline electrolyte of the cathode compartment. The thus obtained Ammonia (NHg) and/or Hydroxylamine (HA) containing alkaline electrolyte (22) being removed from the cathode electrolyte compartment (6) through an outlet (18). The double arrow symbolises a separator to separate Ammonia (NHg) and/or Hydroxylamine (HA) from the Ammonia (NHg) and/or Hydroxylamine (HA) containing alkaline electrolyte (22), with possible recycling of the alkaline electrolyte (20) into the electrolyte compartment of the anode (5). The recycling of the alkaline electrolyte (20) is an optional step in the method as disclosed. Also the separator as used, is simply to indicate any possible means and procedures to obtain the reaction products Ammonia (NHg) and/or Hydroxylamine (HA), from the Ammonia (NHg) and/or Hydroxylamine (HA) containing alkaline electrolyte (22).

In this schematic representation the alkaline electrolyte is an aqueous alkaline electrolyte, hence the electrochemical flow cell reactor may also act as an electrolyzer with the formation of Oxygen (O2) gas at the anode and Hydrogen (H2) gas at the cathode. However, via selective electrocatalysts and selecting the optimal reaction conditions (potential, electrolyte composition, etc.) these side reactions are minimized. Also at either side the formation or consumption of NOx gasses cannot be excluded.

Abbreviations

CL - Catalyst layer

GDE - Gas Diffusion Electrode

GDL - Gas Diffusion Layer

HA - Hydroxylamine

HER - Hydrogen Evolution Reaction

NRR - Electrochemical Nitrogen Reduction Reaction NOR - Electrochemical Nitrogen Oxidation Reaction NO3RR - Electrochemical Nitrate Reduction Reaction NO2RR - Electrochemical Nitrite Reduction Reaction OER - Oxygen Evolution Reaction ORR - Oxygen Reduction Reaction

Reference Numbers

1 - electrochemical flow cell reactor

2 - anode compartment, also known as anode cell

3 - cathode compartment, also known as cathode cell

4 - membrane 5 - electrolyte compartment of the anode cell

6 - electrolyte compartment of the cathode cell

7 - gas compartment of the anode cell

8 - gas compartment of the cathode cell

9 - GDE of the anode cell or the anode GDE

10 - GDE of the cathode cell or the cathode GDE

11 - the metal electrocatalyst layer of the anode GDE

12 - the metal electrocatalyst layer of the cathode GDE

13 - gas input to the gas compartment of the anode cell

14 - gas input to the gas compartment of the cathode cell

15 - electrolyte inlet to the electrolyte compartment of the anode cell

16 - electrolyte outlet of the electrolyte compartment of the anode cell

17 - electrolyte inlet to the electrolyte compartment of the cathode cell

18 - electrolyte outlet of the electrolyte compartment of the cathode cell

19 - fluid connection between the electrolyte compartments

20 - alkaline electrolyte

21 - Nitrate (NO3')/nitrite (NO2 ) containing alkaline electrolyte

22 - Ammonia (NH3) and/or Hydroxylamine (HA) containing alkaline electrolyte