Field of the Invention

[0001 ] The present invention relates to an apparatus and method for producing ammonia.

[0002] The invention has been developed primarily for use in the production of useful precursors for the chemical industry and will be described hereinafter with reference to this application. However, it will be appreciated that the invention is not limited to this particular field of use.

Background of the Invention

[0003] Ammonia (NH3) is one of the world’s most important industrial chemicals, that supports a quadrupling of the world’s food crops, thereby enabling agriculture to sustain an ever-expanding global population. Ammonia has nine times the energy density of Li-ion batteries, and three times the energy density of compressed hydrogen, creating potential as a carbon-free energy carrier [1 ],

[0004] The Haber-Bosch (HB) process (N2+3H2 2NH3) dominants today’s ammonia synthesis, but it requires high temperatures and pressures, the feed of ultra- pure H2, and large and centralized plants to achieve its economic viability. The reactant hydrogen is typically derived from the reforming of fossil hydrocarbons and results in an annual CO2 emission of 300 Mt, accounting for ~1.5% of all greenhouse gas emissions [2,3],

[0005] Green alternative or complementary processes that enable carbon-neutral and decentralized ammonia production directly using water as the hydrogen feedstock and powered by renewable energy sources, are therefore, of immediate demand [3,4],

[0006] Photochemical and electrochemical routes have been devised for ammonia synthesis from N2 and H2O under mild conditions [3], The focus of these studies lies on the suppression of the competing hydrogen evolution reaction to improve selectivity toward N2 reduction and/or the design and selection of desirable catalysts to achieve the goal. However, the electrons transferred to N2 molecules in photocatalysis and electrocatalysis cannot provide sufficient energy to break the strong N=N bonds (bond energy of 945 kJ mol’ ¹ ). As a result, the production rate of ammonia is prohibitively low [3-5],

[0007] Gas discharge plasmas with highly energetic electrons, generated by applying electrical energy to a feeding gas, can activate inert N2 molecules into more reactive, vibrationally or electronically excited states to enable dissociation of nitrogen molecules and thus facilitate the breaking of ultra-stable N = N bonds [6],

[0008] Prior to the HB process, a thermal plasma discharge was used as an industrial process for fertilizer production in the form of nitric acid. This ‘Birkeland- Eyde process’ was however highly energy-demanding due to the thermal plasma heating the bulk gas to a high temperature (equilibrium conditions) and was consequently quickly replaced by the HB process.

[0009] For non-thermal plasma (NTP), however, the energy introduced mainly heats the electrons, creating a thermal nonequilibrium, energy-efficient and highly reactive environment [7], Also, like electrocatalysis, NTP can be powered by renewable electricity [6,7],

[0010] NTP has demonstrated encouraging yields of ammonia; however, the hydrogen species in these reports comes from high-purity H2, which is obtained from energy and carbon-intensive reforming processes [4,8], Substitution of H2O for H2 is, therefore, a key to making NTP-enabled ammonia synthesis sustainable.

[0011 ] The present invention seeks to provide an apparatus and method for producing ammonia, which will overcome or substantially ameliorate at least some of the deficiencies of the prior art, or to at least provide an alternative.

[0012] It is to be understood that, if any prior art information is referred to herein, such reference does not constitute an admission that the information forms part of the common general knowledge in the art, in Australia or any other country.

Summary of the Invention

[0013] According to a first aspect of the present invention there is provided a plasma-bubble reactor, comprising: a vessel configured to hold a liquid; and - a plasma generating means, in association with the vessel, configured to receive an input feed comprising dinitrogen (N2) gas and generate a plasma from the N2 gas to produce an activated N2 gas encapsulated within a plurality of bubbles formed in the liquid, wherein the activated N2 gas reacts with water (H2O) at a plasma-liquid interface formed between the bubbles and the surrounding liquid to produce ammonia (NH3).

[0014] Preferably, the input feed is dinitrogen (N2) gas.

[0015] In one embodiment, the plasma generating means comprises two or more electrodes, wherein at least one of the two or more electrodes is a high voltage (HV) electrode at least partially immersed within the liquid, and configured to generate an electric discharge through the liquid for activating the N2 gas encapsulated within the bubbles when a potential difference is applied across the electrodes.

[0016] Suitably, each of the two or more electrodes is at least partially immersed within the liquid.

[0017] In one embodiment, the other of the two or more electrodes is a ground electrode electrically connected to an external wall of the vessel.

[0018] In one embodiment, the HV electrode is partially enclosed within a column defining a gas passage extending partially along a length of the HV electrode, wherein the column is in fluid communication with the input feed and configured with one or more outlets at a lower portion thereof to allow the activated N2 gas encapsulated within the bubbles to exit therefrom into the liquid in the vessel.

[0019] Suitably, the two or more electrodes are electrically connected to a DC or AC power supply.

[0020] In one embodiment, the reactor comprises a catalyst for catalysing the reaction between the activated N2 gas and H2O.

[0021 ] Suitably, the plasma is generated by applying a potential difference across the two or more electrodes, wherein at least one of the two or more electrodes is a high voltage (HV) electrode at least partially immersed within the liquid, and configured to generate an electric discharge through the liquid for activating the N2 gas encapsulated within the bubbles, - wherein the HV electrode is partially enclosed within a column defining a gas passage extending partially along a length of the HV electrode, and

- wherein the catalyst is supported within the column of the HV electrode.

[0022] In one embodiment, the catalyst is dispersed within the liquid.

[0023] In one embodiment, the catalyst is a metal or a metal oxide.

[0024] In one embodiment, the catalyst comprises a metal oxide and/or a metal.

[0025] In some embodiments, the catalyst comprises a catalytic metal selected from the group comprising palladium, nickel, platinum, rhodium, silver, ruthenium, cobalt, iron, molybdenum, tungsten and vanadium, in combination with a material which is an electron donor or a precursor of an electron donor.

[0026] In some embodiments, the material is a metal or a metal oxide.

[0027] In one embodiment, the metal oxide is magnesium oxide (MgO).

[0028] In one embodiment, the catalyst comprises ruthenium (Ru)Zmagnesium oxide (MgO).

[0029] Preferably, the ratio of ruthenium (Ru) to magnesium oxide (MgO) is about 5%.

[0030] In one embodiment, the catalyst comprises silver nanoparticles.

[0031 ] In some embodiments, the reactor further comprises an oxygen scavenger species dispersed within the liquid to suppress the formation of NOx.

[0032] In one embodiment, the oxygen scavenger species is selected from the group consisting of methanol, ethanol, isopropyl alcohol and mannitol.

[0033] Preferably, the oxygen scavenger species is present within the liquid in a range that falls between about 0.5% and about 2%.

[0034] According to a first aspect of the present invention there is provided a method for producing ammonia (NH3), the method comprising the steps of: generating plasma from an input feed comprising dinitrogen (N2) gas to produce an activated N2 gas encapsulated within a plurality of bubbles formed in liquid; and reacting the activated N2 gas with water (H2O) at a plasma-liquid interface formed between the bubbles and the surrounding liquid to produce ammonia (NH ₃ ).

[0035] Preferably, the plasma is generated by applying a potential difference across two or more electrodes, wherein at least one of the two or more electrodes is a high voltage (HV) electrode at least partially immersed within the liquid, and configured to generate an electric discharge through the liquid for activating the N2 gas encapsulated within the bubbles.

[0036] In one embodiment, the electric discharge is a pulsed discharge.

[0037] Preferably, the potential difference falls within a range of between about 1 kV and about 100kV.

[0038] In some embodiments, the liquid is an aqueous medium.

[0039] Suitably, the aqueous medium comprises an electrolyte.

[0040] In some embodiments, the aqueous medium has a pH that falls within a range of between 5 and 6.

[0041] In one embodiment, the aqueous medium has a pH of 5.6.

[0042] In some embodiments, the reaction is carried out in a vessel substantially under atmospheric pressure and room temperature.

[0043] In some embodiments, the reaction is carried out in a vessel substantially under atmospheric pressure and elevated temperature.

[0044] In one embodiment, the elevated temperature falls within a range of 25°C and 50°C.

[0045] In some embodiments, the input feed comprises a mixture of the dinitrogen (N2) gas and a second gas.

[0046] In one embodiment, the second gas is oxygen (O2) gas.

[0047] In some embodiments, the input feed comprises atmospheric air, comprising the dinitrogen (N2) gas.

[0048] In some embodiments, the input feed comprises a mixture of the dinitrogen (N2) gas and water (H2O) in the form of water-saturated N2 gas. [0049] In one embodiment, the water (H2O) contained in the water-saturated N2 gas is at a concentration of 2.5%.

[0050] In some embodiments, the method further comprises the step of:

- catalysing the reaction between the activated N2 gas and H2O with the addition of a catalyst.

[0051 ] Preferably, the plasma is generated by applying a potential difference across two electrodes, wherein at least one of the two or more electrodes is a high voltage (HV) electrode at least partially immersed within the liquid, and configured to generate an electric discharge through the liquid for activating the N2 gas encapsulated within the bubbles,

- wherein the HV electrode is partially enclosed within a column defining a gas passage extending partially along a length of the HV electrode, and

- wherein the catalyst is supported within the column of the HV electrode.

[0052] In one embodiment, the catalyst is dispersed within the liquid.

[0053] In some embodiments, the method further comprises the step of:

- dispersing an oxygen scavenger species within the liquid to suppress the formation of NOx.

[0054] Other aspects of the invention are also disclosed.

Brief Description of the Drawings

[0055] Notwithstanding any other forms which may fall within the scope of the present invention, preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

[0056] FIG. 1 shows (a) a schematic representation of a plasma-catalytic bubble (PCB) reactor and (b) an actual prototype of the PCB reactor (without catalyst) in operation;

[0057] FIG. 2 shows schematic representations of three (3) different configurations of the plasma-catalytic bubble (PCB) reactor of FIG. 1 , including (a) plasma bubble column (PBC) generating both dielectric barrier discharge (DBD) and spark, (b) a PBC with catalysts packed in column, and (c) a PBC with catalysts in water; [0058] FIG. 3 shows a plot of the normalized relative intensity (a.u.) from discharge optical emission spectra (OES) obtained from a comparison of the N2 ⁺ and NH species and ammonia (NH3) production rate (N2 flow rate of 1 L/min with 2.5% of H2O vapour) obtained using the plasma-bubble reactor 110 shown in FIG. 2(b), when the column 135 is packed with (a) plasma only (i.e., zero catalyst), (b) MgO, or (c) Ru/MgO;

[0059] FIG. 4 shows a plot showing the amount (mg/hour) of ammonia (NH3) produced using the PBC reactor of Fig. 2(a) when performed at (a) pH 5.60, T 298 K, (b) pH 2.40, T 298 K, (c) pH 2.40, T 318 K, and (d) pH 2.40, T 318 K, in the presence of an oxygen (O2) scavenger; and

[0060] FIG. 5 shows schematic representations of a plasma bubble column reactor according to another preferred embodiment of the present invention, comprising (a) 2 columns, and (b) 3 columns.

Detailed Description of Specific Embodiments

[0061 ] It should be noted in the following description that like or the same reference numerals in different embodiments denote the same or similar features.

[0062] The present invention is predicated on the finding of a one-step, plasma- driven process for producing ammonia (NH3) gas via clean and renewable sources.

[0063] Here, the inventors have found that by employing the use of non-thermal plasma (NTP), it is possible to overcome the activation energy gap associated with the highly stable N=N bond (bond energy of 945 kJ mol ^-1 ) of N2to generate active species that can be used to react with the hydrogen (H) from water (H2O) molecules to form NH ₃ .

[0064] The two key steps in this process mainly include plasma pre-activation and interactions between H2O and the plasma-activated N2 gas. Various species (including electrons, ions, radicals, molecular fragments) with different energy levels are present in the plasma ionised gas.

[0065] Different from thermal plasma (equilibrium plasma) with high bulk gas temperature (typically higher than 5 x 10 ³ K), NTP operates in a more ambient temperature condition, but it gives enough energies to activate stable molecules and drive the reaction across the energy gap, with excellent selectivity of products and high energy efficiency. [0066] This process is thus in stark contrast to the conventional Haber-Bosch (HB) process, which is energy-intensive and is highly eco-destructive and is not compatible with renewable-energy. In addition to the environmental costs of producing ammonia, for efficiency, current production plants need to be large, which then means a significant infrastructure is required to transport fertilizer to rural farms and locations.

[0067] Moreover, as will be described in more detail below, the presently disclosed apparatus is capable of generating NH3 with a rate enhanced by ~100 times and an energy yield reduced by -4 times, when compared to the only other one-step nonthermal plasma (NTP) production of NHsfrom N2 and H2O, which is still challenged by slow production rate and high energy efficiency, with the best performing system which reports an ammonia production of 0.44 mg/h and energy yield of 40.82 kWh/mol (-2400 kWh/kg-NHs).

[0068] The technique is based on the reactions in gas-phase plasma and at the plasma-liquid interface approaching by a reactor termed “plasma bubble column (PBC)”.

[0069] FIG. 1 (a) shows a plasma bubble reactor 5a comprising a catalyst packed within a reaction column of the reactor 5a, in which the reactor 5a is configured to activate dinitrogen (N2) gas using non-thermal plasma (NTP), generated by a High Voltage (HV) electrode immersed in a liquid medium comprising water (H2O), wherein the plasma-activated N2 reacts with H2O to produce ammonia (NH3) gas.

[0070] The plasma bubble reactor 5a generates tuneable discharge regions (glow or DBD, and spark) and can be paired with catalysts either in the reaction column or directly in the bulk liquid (both termed plasma-catalytic bubbles, PCBs), further enabling thermodynamically unfavourable reactions to proceed under ambient conditions. The water vapours carried by the inducer gas (N2) and from the gas-liquid interface and bulk solution sustain the hydrogen sources for ammonia (NH3) production.

[0071 ] FIG. 1 (b) shows a prototype of a different plasma bubble reactor 5b in operation. In this arrangement, the plasma bubble reactor 5b is configured without a catalyst packed into the reaction column. [0072] By controlling the reactor design, the plasma conditions and, where appropriate, coupling with a catalyst, it is possible to produce NH3 gas as a chemical output.

[0073] What follows is a detailed description of three (3) different configurations of a plasma bubble reactor, and a method for the production of ammonia (NH3) using said reactors.

[0074] It should be noted that the following description outlines the use of pure N2 as the inlet gas and H2O as the hydrogen source, however, it will be appreciated by a person of ordinary skill in the relevant art that it is possible to use an alternative source of nitrogen and tune the plasma/inlet parameters to obtain the desired products.

[0075] For instance, in the examples given below, the results presented are for the use of dinitrogen (N2) gas, however, air and N2/ oxygen (O2) mixtures, along with H2O, also show promise for ammonia (NH3) production and artificial nitrogen fixation (generating NOx).

Plasma-Bubble Reactors

[0076] FIG. 2 shows schematic representations of three (3) different configurations of a plasma-bubble reactor according to embodiments of the present invention, including (a) a reactor 10 equipped with a single plasma bubble column (PBC), (b) a reactor 110 equipped with a single plasma bubble column (PBC) comprising a catalyst supported within the column, and (c) a reactor 210 equipped with a single plasma bubble column (PBC) with a catalyst dispersed within the liquid medium.

[0077] In both instances (FIG. 2(b) and 1 (c)), the catalyst is provided for the purpose of catalysing the reaction between the activated N2 gas and H2O.

[0078] The following description outlines the structural details of each of the three (3) different configurations in turn.

[0079] In the simplest configuration, as represented in FIG. 2(a), the plasma-bubble reactor 10 comprises a vessel 15 that comprises a base 15a and a wall 15b upstanding from the base 15a to define a cavity 20 configured to hold a liquid medium 25 substantially within the cavity 20, and an opening 15c at an upper portion of the vessel 15. [0080] The plasma-bubble reactor 10 further comprises a plasma generating means in the form of two electrodes 30, 40 that are located within the cavity 20 of the vessel 15, via the opening 15c, and partially immersed in the liquid medium 25.

[0081 ] The two electrodes 30, 40 are electrically connected to an AC power supply 50. Although it will be appreciated by persons of ordinary skill in the relevant art that in an alternative embodiment, the two electrodes 30, 40 may be electrically connected to a DC power supply (not shown).

[0082] The first electrode 30 is a High Voltage (HV) electrode (or cathode), while the second electrode 40 is a ground or counter electrode (or anode) electrically connected to the external wall 15b of the vessel 15.

[0083] The HV electrode 30 is partially enclosed within a quartz column 35 defining a gas passage extending partially along a length of the HV electrode 30. The column 35 comprises a gas inlet (not shown) at an upper portion thereof that is configured to fluidly receive an input feed comprising dinitrogen (N2) gas from a N2 gas supply (not shown), and one or more gas outlets 35a, 35b at a lower portion thereof, wherein the lower portion of the column 35 is fully immersed within the liquid medium 25.

[0084] As will be discussed below, the gas outlets 35a, 35b allow the activated N2 gas encapsulated within the bubbles to exit from the lower portion of the column 35 into the liquid medium 25.

[0085] Combining conventional dielectric barrier discharge (DBD) plasma reactors with plasma-bubbles enhances the generation of species, as the DBD discharge acts as an excitation pre-treatment prior to the spark discharge in the forming bubbles. This combination or the ‘ladder’ excitation approach offers differing excitation pathways between the glow and spark discharges for NOx production from air with outstanding yields [1 ],

[0086] Since oxygen exists in the water (H2O) molecules, they generally favour the formation of reactive oxygen species (ROS), leading to NOx generation and directly competing with NH3 production. Therefore, in some embodiments, an oxygen scavenger species (not shown) may be dispersed within the liquid medium 25 to suppress the formation of NOx during the plasma driven process. [0087] Suitable oxygen scavengers may be selected from the group consisting of methanol, ethanol, isopropyl alcohol and mannitol.

[0088] In a preferred embodiment, the oxygen scavenger species, ethanol, is present within the liquid medium 25 in a range that falls between about 0.5% and about 2%.

Alternative Arrangements

[0089] The components of the two other plasma-bubble reactor configurations 110, 210 of FIG. 2(b), (c) are labelled in a similar manner to those in Fig. 2(a), where a prefix of “1” or “2” is employed before the reference numeral of each component to signify that the component relates to the corresponding plasma-bubble reactor 1.10 and 210 of FIG. 2(b) and FIG. 2(c), respectively.

[0090] Catalyst

In FIG. 2(b), the column 135 of the HV electrode 130 further comprises a catalyst supported therein for catalysing the reaction between the plasma-activated N2gas and H2O. Catalysts can assist nitrogen activation and conversion by decreasing the required energy for N2 dissociative adsorption. It is established that catalysts packed within a plasma discharge can increase dinitrogen (N2) gas conversion and ammonia (NH3) production either through enhancing the plasma discharge and/or augmenting the activated dinitrogen (N2) gas and hydrogen (H) species over the catalytic surface

[0091 ] In some embodiments, the catalyst comprises a catalytic metal selected from the group comprising palladium, nickel, platinum, rhodium, silver, ruthenium, cobalt, iron, molybdenum, tungsten and vanadium, in combination with a material which is an electron donor or a precursor of an electron donor. The material in this case is either a metal or a metal oxide.

[0092] For instance, in one embodiment, the catalyst comprises ruthenium (Ru) metal in combination with magnesium oxide (MgO) as a carrier. The inventors have observed good results when the ratio of ruthenium (Ru) to magnesium oxide (MgO) is about 5%.

[0093] In some embodiments, the catalyst may simply comprise of a catalytic metal oxide. For instance, the catalytic metal oxide may comprise magnesium oxide (MgO). [0094] In other embodiments, the catalyst may comprise a plurality of silver nanoparticles.

[0095] In FIG. 2(c), the catalytically active material for catalysing the reaction between the plasma-activated N2 gas and H2O is simply dispersed within the liquid medium 25 within the vessel 15.

Method

[0096] A method for producing ammonia (NH3) gas using the single reactor of FIG. 2(a) as a general guide will now be described.

[0097] According to a first step of the method, a potential difference is applied across the two electrodes 30, 40 causing the HV electrode 30 to generate an electric discharge within the column 35. The electric discharge generates a plasma from the N2 gas that has been fed into the column 35 via the input feed to produce an activated N2 gas. The activated N2 gas exits the column 35 via the gas outlets 35a, 35b and forms a plurality of bubbles in the liquid medium 25, which for the purpose of this embodiment is an aqueous liquid medium comprising an electrolyte.

[0098] In some embodiments, the aqueous medium has a pH that falls within a range of between 5 and 6.

[0099] For instance, in one embodiment, the aqueous medium has a pH of 5.6.

[00100] In one embodiment, the electric discharge is a pulsed discharge, that is repeatedly applied at a frequency that falls with a range of about 50 Hz and about 10 MHz. Under such conditions, the potential difference that is to be applied across the two electrodes 30, 40 typically falls within a range of between about 1 kV and about 100 kV.

[00101 ] According to a second step of the method, the activated N2 gas encapsulated within the bubbles produces a plurality of excited molecules selected from the group consisting of N2*, N2 ⁺ , N, H and NH _X (as well as NOx when supplying humid N2). These excited molecules then react with the water (H2O) in the aqueous liquid medium 25 at a plasma-liquid interface formed between the bubbles and the surrounding liquid medium 25 to produce ammonia (NH3) gas. [00102] In one embodiment, the reaction is carried out substantially under atmospheric pressure and at room temperature, although it will be appreciated by persons of ordinary skill in the relevant art that altering one or both of these parameters can be used as a means by which to increase or decrease the rate of conversion of N2 gas to ammonia (NH3) gas.

[00103] For instance, the method may be performed under alternative conditions, whereby the reaction is carried out in the vessel 15 substantially under atmospheric pressure and an elevated temperature that falls within a range of 25°C and 50°C.

Results

[00104] Without wishing to be bound by any one particular theory, the inventors believe that the reaction at the plasma-liquid interface involves energetic electrons, as well as plasma-generated and activated gaseous species in thus-forming bubbles, that undergo further collisions, charge transfer, quenching and other reactions during the plasma propagation stage until reaching the plasma-liquid interface. In essence, the inventors believe that the closer these active species are to the plasma-liquid interface, the closer they are to the higher H2O content, which enables the formation of more H, sustained by water dissociation.

Catalysis

[00105] By employing catalyst matching, the opportunity exists to exploit the underused energy of the vibrationally excited plasma species, further reducing the activation barrier.

[00106] In the plasma bubble column (PBC) reactor 110 shown in FIG. 2(b), the column 135 of the HV electrode 130 is packed with a catalyst (labelled “X”) supported within the column 135 by a mesh plate and glass wool (not shown). The inventors have found that packing the DBD discharge zones within the column 135 with catalytic pellets causes the discharge behaviour to shift from volumetric micro discharges to a combination of surface discharge on the catalyst surface and weak micro-discharges in the space between the catalysts, leading to an enhancement of reactive specie generation and conversion efficiency.

[00107] For example, FIG. 3 shows a plot of the normalized relative intensity (a.u.) from discharge optical emission spectra (OES) obtained from a comparison of the N2 ⁺ and NH species and ammonia (NH3) concentration in the water (N2 flow rate of 1 L/min with 2.5% of H2O vapour) obtained using the plasma-bubble reactor 110 shown in FIG. 2(b), when the column 135 is packed with (a) plasma only (i.e. zero catalyst), (b) MgO, or (c) Ru/MgO.

[00108] As shown in FIG. 3, the OES of the DBD indicate a higher intensity (or formation) of the reactive N2* and key intermediate NH species when the column 135 is packed with (b) MgO and (c) Ru/MgO under the same discharge conditions.

[00109] This then leads to the ammonia (NH3) concentration being enhanced by up to 2 times.

[00110] Besides being packed in the column 135, catalysts can also be simply dispersed within the liquid medium 25 in the vessel. For instance, in the plasma-bubble reactor 210 shown in FIG. 2(c), the catalyst is dispersed within the liquid medium 25. No extra stirring systems are required since the gas inlets 235a, 235b and forming bubbles are able to disperse these catalysts. By this means, the catalysts do not interface with the discharges (no visible changes in the optical emission spectra (OES) and other discharge properties, data not shown), however, their presence in the gasliquid interface may extend plasma effects.

[00111 ] Also, some demonstrated nitrogen reducing catalysts are expected to transform thus-generated NOx in the liquid medium 25 to ammonia (NH3), enhancing ammonia production and selectivity.

[00112] Temperature/pH

[00113] FIG. 4 shows a plot showing the amount (mg/hour) of ammonia (NH3) produced using the PBC reactor of FIG. 2(a) when performed at different pH values and temperatures (K), with or without the presence of an oxygen (O2) scavenger.

[00114] The experimental results in FIG. 4(a) clearly show that ammonia (NH3) can be generated directly with the PBC reactor 10 when using N2 as the feed gas, and the water (H2O) in the aqueous liquid medium 25 as the H source for ammonia (NH3) formation, since no extra hydrogen sources are added (pH 5.60, D.l. water).

[00115] When the same reaction is performed in an acidic environment (pH 2.40), the results in FIG. 4(b) clearly show that this environment contributes to better ammonia (NH3) adsorption. [00116] In addition, when the temperature of the aqueous liquid medium 25 was increased from 293 K (20°C) to 318 K (45°C), as shown in FIG. 4(c), this resulted in an almost 50% enhancement of the ammonia (NH3) production rate. The inventors believe that this occurs since the increase in the temperature of the aqueous liquid medium 25 enhances the H2O content at the gas-liquid interface, thereby allowing more H2O to be involved in the ammonia (NH3) forming reactions.

[00117] In addition, replacing the dry N2 with water saturated N2 (containing 2.5% of H2O), as in the case of the results shown in FIG. 4(b), also enhanced ammonia (NH3) production, while the solution was kept at 293 K (20°C).

[00118] In a further experiment, 0.5g/L of Ag-nanoparticles (< 150 nm) were dispersed within the aqueous liquid medium 25 of the PBC reactor 210 shown in FIG. 2(c) as a catalyst. The results obtained showed an enhanced ammonia production by around 40% (FIG. 4(d)).

[00119] Since oxygen exists in the water (H2O) molecules, there is a high propensity for the formation of reactive oxygen species (ROS) to occur as a result of plasma treatment. The formation of ROS often leads to the unfavourable generation of NOx, which can directly compete with NH3 production. However, by adding a small amount (about 1 %) of an oxygen species scavenger to the aqueous liquid medium 25, it becomes possible to reduce the amount of ROS produced.

[00120] For instance, as shown in FIG. 4(e), when the oxygen species scavenger, ethanol, is added to the aqueous liquid medium 25, the amount of ammonia (NH3) produced was notably double (8.5 mg/hour) that when compared to the results where no oxygen species scavenger was present (see FIG. 4(a), (b), (c) and (d)). Overall, this approach favours an energy efficiency (~20 kWh/mol-NHs without catalyst) among decentralized or plasma-enabled ammonia (NH3) production studies.

[00121 ] Reactor System

[00122] The inventors have also demonstrated that it is possible to realise a plasma bubble column (PBC) reactor system, whereby multiple columns may be used to expand the capability of the PBC reactor system to further enhance conversion and production of ammonia (NH3), without requiring an extra power source. [00123] FIG. 5 shows schematic representations of a plasma bubble column (PBC) reactor system according to another preferred embodiment of the present invention, whereby the system takes the form of (a) a PBC reactor 310 that is configured with 2 columns, and (b) a PBC reactor 410 that is configured with 3 columns.

[00124] The components of the two PBC reactor configurations 310, 410 of FIG. 3(a), (b) are labelled in a similar manner to those in Fig. 2(a), where a prefix of “3” or “4” is employed before the reference numeral of each component to signify that the component relates to the corresponding PBC reactor 310 and 410 of FIG. 3(a) and FIG. 3(b), respectively.

[00125] In the 2-column arrangement shown in FIG. 5(a), rather than a ground electrode (40, 140, 240), as in the case of the single column PBC reactors 10, 110, 210 in FIG. 2(a), (b) and (c), a quartz column 345 comprising a low voltage (LV) electrode 340 partially enclosed there within is employed instead.

[00126] As described above for the single column PBC reactors 10, 110, 210, the column 345 in FIG. 5(a) (hereinafter referred to as the LV column) also defines a gas passage extending along its length. The HV column 335 and the LV column 345 each comprise a corresponding gas inlet 338, 348 at an upper portion thereof that is configured to fluidly receive an input feed comprising dinitrogen (N2) gas from a N2 gas supply (not shown). The HV electrode 330 and the LV electrode 340 are each configured to generate an electric discharge through the liquid medium 25 for activating the N2 gas encapsulated within the bubbles formed when a potential difference is applied across the two electrodes 330, 340.

[00127] Like the HV column 335, the LV column 345 also comprises one or more gas outlets 345a, 345b at a lower portion thereof, wherein the lower portion of each of the HV column 335 and the LV column 345 is fully immersed within the liquid medium 25 within the vessel 315 of the PBC reactor 310. The gas outlets 345a, 345b allow the activated N2 gas encapsulated within the bubbles to exit from the lower portion of the corresponding HV column 335 and LV column 345 into the liquid medium 25.

[00128] In the 3-column arrangement shown in FIG. 5(b), a third quartz column 455 can be introduced into the vessel 415 of the PBC reactor 410. The third column 455 also comprises a high voltage (HV) electrode 450 that can be connected to the same power source (not shown) as the other HV column 435 and the LV column 445. The two HV electrodes 430, 450 and the LV electrode 440 are all configured to generate an electric discharge through the liquid medium 25 for activating the N2 gas encapsulated within the bubbles formed when a potential difference is applied across the three electrodes 430, 440, 450.

[00129] Table 1 provides experimental results obtained for different PBC reactor systems 10, 310, 410, equipped with either 1 , 2 or 3 columns (discharge power 25 W, D.l. water only (0.2 L), discharge duration 10 min, catalyst-free) at T 293 K (20 °C).

[00130] It is apparent from the results in Table 1 that the ammonia (NH3) gas production and total nitrogen fixation in the water increased almost linearly with an increase in the number of columns used within the PBC reactor system.

[00131 ] For instance, the 3-column PBC reactor system 410 shown in FIG. 5(b) generated ammonia (NH3) at a rate of 20.4 mg/h with an energy yield of 20.83 kWh/mol. Again, the addition of an oxygen species scavenger (about 1 %) to the vessel 415 of the 3-column PBC reactor system 410 further doubled the ammonia (NH3) production, with the selectivity being increased by ~190%, and the energy yield being enhanced to ~10 kWh/mol.

[00132] Table 1: NH3, NO _X (NO & NC>3~) generation results in different PBC reactor systems equipped with either 1, 2 or 3 columns (discharge power 25 W, D.l. water only (0.2 L), discharge duration 10 min, catalyst-free).

^# as an oxygen scavenger. Advantages:

[00133] The present invention provides a number of advantages, including, but not limited to:

[00134] [1] The realisation of a means by which to utilise dinitrogen (N2) gas, or a combination of oxygen (O2) gas and dinitrogen (N2) gas (as in, for example, atmospheric air), to generate a major industrially important product, ammonia (NH3).

[00135] [2] The realisation of a means that could help to reduce the amount of NOx present in the environment.

[00136] [3] The reactor design could be used for other gas conversion processes such as CO2 and methane.

[00137] [4] The reactor design facilities ease of scaling with horizontal scaling approaches being feasible.

[00138] [5] The reactor design overcomes heating build-up issues due to the use of the bubble interface as part of the reactor and the convection currents of the surrounding bulk water.

Embodiments:

[00139] Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

[00140] Similarly, it should be appreciated that in the above description of example embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the Detailed Description of Specific Embodiments are hereby expressly incorporated into this Detailed Description of Specific Embodiments, with each claim standing on its own as a separate embodiment of this invention.

[00141 ] Furthermore, while some embodiments described herein include some, but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

Other Embodiments:

[00142] While the preferred embodiments of the invention described above relate to the use of pure dinitrogen (N2) gas as the input feed gas, it will be appreciated by persons of ordinary skill in the relevant art that the plasma-driven process may also employ N2 gas in combination with a second gas.

[00143] For instance, the input feed may comprise atmospheric air comprising the dinitrogen (N2) gas as the inlet gas.

[00144] Alternatively, the input feed may comprise a mixture of dinitrogen (N2) gas and oxygen (O2) gas as the inlet gas.

[00145] In yet another embodiment, the input feed may comprise a mixture of the dinitrogen (N2) gas and water (H2O) in the form of water-saturated N2 gas.

[00146] For instance, in one embodiment, the water (H2O) contained in the water- saturated N2 gas is present at a concentration of 2.5%.

References:

1 . J. Sun et al. A hybrid plasma electrocatalytic process for sustainable ammonia production. Energy & Environmental Science. 14(2), 865-8722021 (2021 ).

2. F. Jiao, B. Xu, Electrochemical ammonia synthesis and ammonia fuel cells. Adv. Mater. 31 , e1805173 (2018).

3. J. G. Chen et al., Beyond fossil fuel-driven nitrogen transformations. Science 360, eaar6611 (2018). 4. L. Wang et al., Greening ammonia toward the solar ammonia refinery. Joule 2, 1055-1074 (2018).

5. S Wang, et al. Nitrogen fixation reaction derived from nanostructured catalytic materials. Advanced Functional Materials, 28(50), 1803309 (2018).

6. A. Bogaerts, E. C. Neyts, Plasma technology: an emerging technology for energy storage. ACS Energy Letters 3, 1013-1027 (2018).

7. X. Lu, et al. Reactive species in non-equilibrium atmospheric-pressure plasmas: Generation, transport, and biological effects. Physics Reports 630, 1 - 84 (2016).

8. K. H. Rouwenhorst, et al. Plasma-driven catalysis: green ammonia synthesis with intermittent electricity. Green Chemistry 22, 6258-6287 (2020).

Different Instances of Objects

[00147] As used herein, unless otherwise specified the use of the ordinal adjectives "first", "second", "third", etc., to describe a common object, merely indicate that different instances of like objects are being referred to, and are not intended to imply that the objects so described must be in a given sequence, either temporally, spatially, in ranking, or in any other manner.

Specific Details

[00148] In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

Terminology

[00149] In describing the preferred embodiment of the invention illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, the invention is not intended to be limited to the specific terms so selected, and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar technical purpose. Terms such as "forward", "rearward", "radially", "peripherally", "upwardly", "downwardly", and the like are used as words of convenience to provide reference points and are not to be construed as limiting terms.

Comprising and Including

[00150] In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" are used in an inclusive sense, i.e. , to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

[00151 ] Any one of the terms: including or which includes or that includes as used herein is also an open term that also means including at least the elements/features that follow the term, but not excluding others. Thus, including is synonymous with and means comprising.

Scope of Invention

[00152] Thus, while there has been described what are believed to be the preferred embodiments of the invention, those skilled in the art will recognize that other and further modifications may be made thereto without departing from the spirit of the invention, and it is intended to claim all such changes and modifications as fall within the scope of the invention. For example, any formulas given above are merely representative of procedures that may be used. Functionality may be added or deleted from the block diagrams and operations may be interchanged among functional blocks. Steps may be added or deleted to methods described within the scope of the present invention.

[00153] Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.

Industrial Applicability

[00154] It is apparent from the above, that the arrangements described are applicable to the chemical industry.