Patent Application
20240052502
The invention relates to a new, hybrid technology for the production of reduced species, such as ammonia via clean and renewable sources. The technology is based on the coupling of plasma-assisted activation of gas and electrocatalytic conversation of relevant plasma species. The invention also relates to apparatus and catalysts suitable for use in the system.
The transformation of atmospheric nitrogen (N2) to ammonia (NH3) is essential for many eco-systems and industrial processes. Currently, only bacteria and some plants can synthesize ammonia from air and water at ambient conditions via nitrogen fixation processes.
Ammonia is an extremely valuable global commodity at present and seems likely to play a significant role not only in manufacturing but also in energy production and storage in the near future.
Globally, approximately $60 billion worth of ammonia is produced every year for utilisation, mostly in the form of fertilizers. It is estimated that at least half the nitrogen in the human body today comes from a synthetic ammonia plant. Recently, ammonia has been gaining increasing attention as a hydrogen carrier for the hydrogen economy. Ammonia stores almost twice as much energy as liquid hydrogen and is easier to ship and distribute for export purposes. Thus, the global ammonia market has significant potential for expansion in upcoming years.
Commercially, ammonia production has remained essentially unchanged since World War I. The Haber-Bosch process was developed in the early 20th Century and is very well known. The conversion typically occurs at high pressures (150-250 atmospheres) and high temperatures (400-500° C.). Additionally, for this process, relatively high purity hydrogen (from steam reforming of methane) and nitrogen (from air separation) feeds are required. Because of this, the process consumes a significant amount of energy and is fundamentally incompatible with small scale, delocalised ammonia production as well as making it unfeasible to accommodate intermittent and diffusive renewable energy.
In recent years, the production of ammonia through electrocatalytic nitrogen reduction reaction (eNRR) has gained increasing attention. Electrocatalytic NRR has significant benefits over the Haber-Bosch process including operating at mild conditions (ambient temperature and pressures), being fundamentally compatible with renewable energy, well-suited to delocalized production and distribution as well as not requiring any hydrogen feed (with hydrogen coming from water/the electrolyte).
Despite these benefits, electrocatalytic NRR remains significantly hindered with low yields of ammonia and the difficulties in achieving high Faradaic efficiencies at low overpotentials. Specifically, the use of eNRR is intrinsically limited due to the highly unreactive nature of N2 and its low solubility in water. Moreover, the eNRR is hampered by the competition with hydrogen evolution reaction (HER), as hydrogen generation usually occurs at a lower overpotential than the eNRR. Consequently, the eNRR remains significantly hindered by low ammonia production rates (typically 10−9 to 10−10 mol cm−2 s−1) making reliable detection troublesome and, with few exceptions, very low Faradaic efficiencies, below 1%.
To date, some of the highest yields achieved electrocatalytically have been in the order of ˜5 μg/cm2/h, bringing questions about potential scalability in the future.
A promising approach to overcoming the limitations of eNRR is converting N2 into a more reactive intermediary form. In this context, lithium redox intermediary NRR has attracted some recent attention to achieve higher rates and current densities than eNRR. However, the significant overpotential of 3V minimum for the Li-NRR makes this process inherently energy-intensive. Moreover, system stability, the need for ultra-dry and oxygen-free organic solvents and their decomposition at the anode, high pressures (˜50 bar) and hydrogen feed and lithium metal requirements are additional drawbacks of this pathway.
Nitrite and nitrates (NO3− and NO2−, respectively), also referred to collectively when mixed as NOx, are highly soluble and much more easily reduced to ammonia than N2 and benefits from already known chemistry. As such, the generation and exploitation of NOx as an intermediary to overcome the limitations of N2 conversion presents a novel solution to these limitations. However, in industry, nitrites and nitrates are produced from ammonia via the Ostwald process; thus, their direct use as precursors for ammonia production is unfeasible. Additionally, nitrates/nitrites have limited stability in water hence direct production and on-spot utilization is essential. Consequently, the production of NOx for immediate consumption to produce ammonia is of critical industrial importance.
A number of electrocatalytic approaches to the conversion of inert molecules (such as nitrogen) into valuable products have been attempted but these have been significantly hindered from both a conversion and selectivity point of view. In particular, the solubility and activation of these molecules limits conversion and applications in conventional electrocatalysis.
Prior art methods have been attempted using plasma but these invariably the plasmas are generated under oxygen-free conditions, that is the prior art methods have sought to avoid the generation of NOx species by using dry nitrogen or nitrogen in combination with a gas such as helium. Other than eliminating NOx generation possibilities, the use of helium also leads to variation in plasma energetics as high energy electrons can be obtained at lower potentials compared to nitrogen and oxygen mixtures.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.
The invention is a hybrid plasma-electrocatalytic system which activates an input feed gas, in particular nitrogen, which may be in the form of air via plasma formation at the interface of the gas and a liquid (water or electrolyte). The activated NOx species are subsequently dissolved in the liquid, where they are then converted electrocatalytically into ammonia.
In a broad aspect, the invention provides a method of reducing a gaseous compound comprising the steps of:
The gaseous compound may be nitrogen-containing (e.g. air), in which case the reduced compound is ammonia. Alternatively, the gaseous compound is an oxygen containing compound, for example, carbon dioxide (which may produce carbon monoxide and formates etc. as the reduced compound) or a compound mixed with oxygen.
Preferably, the plasma is generated by a combination of glow discharge and spark discharge, albeit either glow or spark discharge could also be viably utilized independently. The plasma electrode embodiments are pin-to-liquid with no enclosure, nozzle enclosure, and bubble column enclosure. The latter is preferred as it provides plasma-water or electrolyte water interface at the interface of a bubble of gas in the water or electrolyte. Preferably, one or both plasma electrodes are covered with a dielectric barrier. In one embodiment the high-voltage electrode is partially covered to offer a combination of glow and spark discharge. Preferably, the ground electrode is of similar design as high voltage electrode and placed inside the liquid. In other configurations dielectric barrier separates the ground and the liquid.
Preferably, in configurations exhibiting underwater plasma bubbles, residence time of bubbles in the liquid is facilitated by Raschig rings.
Preferably, the glow discharge region inside plasma bubble column is packed with metal oxides in form of nanoparticles or monolith with tuneable void fraction and bed height.
Preferably, the gaseous compound is provided at controlled humidity. Also, it is preferred if the water or electrolyte is provided at a controlled temperature. It is also preferred that the water or electrolyte is provided at a controlled pH (both alkaline and acidic). Gas humidity, water temperature and pH can be controlled by conventional method in the art, such as desiccants/bubblers, heaters/coolers and buffers respectively.
The plasma activation and electrolysis can be carried out at any pH range from pH 0 to pH 14 (i.e. for example, in the range of pH's between that of a 1M H+ solution and a 1M OH− solution.
Preferably, the electrocatalytic reduction is facilitated by a transition metal catalyst. Particularly suited transition metals include copper, nickel, tin, bismuth, cobalt, titanium or iron, or the oxides of said transition metals and mixtures thereof.
Preferably the transition metal catalyst is in the form of a foil, a foam, a nanostructured catalyst, a nanoparticulate catalyst or a single atom metal on doped-carbon catalyst. In a preferred form, the catalyst is in the form of a foam with deposits (particularly nanodeposits), such as a metal foam supporting nanowires. Highly preferred are copper catalysts in the form uniformly dispersed thin nanowires on a copper foam. Most preferably, the catalyst is in the form of a metal foam supporting nanowires, such as a copper foam supporting copper nanowires with surface defects.
The term “nanostructured” as herein defined refers to structures that have a feature having at least one dimension on the nanoscale, that is, between 0.1 nm and 1000 nm, preferably between 0.1 nm and 500 nm.
The term “nanowire” as herein defined refers to an elongate wire having a diameter on the nanoscale, that is, between 0.1 nm and 1000 nm, preferably between 0.1 nm and 500 nm. Preferably, the nanowire also has a high length-width ratio of the order of 100 or more or even 1000 or more.
It is preferred that, where present, the catalyst is located in the reaction system in a region adjacent the region of the spark discharge and/or glow discharge.
Preferably the dissolved plasma species is reduced without isolation. In one embodiment, the dissolved plasma species are reduced in the vessel in which plasma is generated. In an alternative embodiment, the dissolved plasma species are stored in a reservoir prior to electrocatalytic reduction.
In a preferred aspect, the invention provides a method of reducing nitrogen gas to produce ammonia, the method comprising the steps of:
The nitrogen containing gas may be pure nitrogen or substantially pure nitrogen, or it may be in admixture with other species. For preference, the nitrogen containing gas further comprises oxygen.
The nitrogen:oxygen ratio may be any ratio between 1:99 and 99:1 wt:wt. In one particular aspect, the nitrogen containing gas is air.
Preferably, the plasma is generated by a combination of glow discharge and spark discharge. The configuration of the plasma electrodes can be a pin-to-liquid with no enclosure, pin-to-liquid with nozzle enclosure, or a pin-to-liquid with a column bubbler enclosure. Most preferably, the plasma is generated by a combination of glow discharge and spark discharge.
Preferably, the plasma-water or electrolyte water interface is at the interface of a bubble of gas in the water or electrolyte.
The gaseous compound may be provided at any relative humidity (i.e. 0 to 100%). In some embodiments, such as carrying out the reduction of nitrogen, a dry gas may be preferred. In some embodiments the gaseous compound is provided at a relative humidity of 20-80%. Preferably, the water or electrolyte temperature is between 20 and 80° C.
In certain preferred embodiments, the electrolyte is aqueous H2SO4 or HCl. Other acidic electrolytes may also be used. In other preferred embodiments, the electrode is a basic species, such as an aqueous solution of hydroxide salt, e.g. KOH or NaOH. In still other preferred embodiments the electrolyte is pure water or an aqueous salt solution (such as, for example KCl or NaCl)
Preferably the dissolved NOx species are NO2
In one embodiment, the dissolved NOx species are moved from a generation vessel to a reservoir prior to electrocatalytic reduction. In another embodiment, the dissolved NOx species are electrocatalytically reduced in the vessel in which they are generated.
In an alternative aspect, the invention provides a method of reducing carbon dioxide gas comprising the steps of:
The gaseous compound contains an oxygen source. In some embodiments that is covalently bound to the reduced species, such as in the case of CO2, where the carbon is bound to the oxygen, or the oxygen source is in the form of gas mixed with the reducible species or co-fed to a plasma forming gas stream, for example, where a nitrogen gas is being reduced, oxygen may be added to the plasma forming feed in a predetermined, controlled amount, or the feed gas to the plasma may be air.
In another aspect, the invention provides apparatus for reducing a gas comprising
In yet another aspect, the invention provides apparatus for reducing a gas comprising
The invention also provides a catalyst comprising a transition metal catalyst in the form of a nanowire. Preferably the nanowire is supported on a transition metal foam. Most preferably the catalyst is a copper nanowire supported by a copper foam.
The present invention relates to a new, hybrid technology for the production of a reduced gaseous species (such as ammonia) via clean and renewable sources. The technology is based on the coupling between two fundamental aspects: plasma-assisted activation of gas; and electrocatalytic conversation of relevant plasma species to the reduced gaseous species.
Gaseous species, for example, ground-state nitrogen molecules, exhibit high ionization potential. This is intrinsically non-reactive for thermodynamic standpoint, but plasma activation can provide avenues for the conversion of highly stable nitrogen molecules into easier-to-breakdown species. These species can then be more efficiently converted into ammonia electrochemically. The hybrid system of the present invention can operate under ambient conditions, with water and air being reactants.
Further, the ammonia produced is in aqueous phase, thus requiring no further pre-treatment stages for application areas such as direct use as fertilizer and in the textile and explosives industries. The present invention is a hybrid plasma-electrocatalytic system which activates an input feed gas to form a plasma at the liquid/gas interface of the reactant gas within the liquid (typically water/electrolyte). The resulting activated species are dissolved in the liquid, and subsequently converted into valuable chemicals by means of electrocatalysis. The invention relates to the method, apparatus and also to specific features of the system, in particular features such as the catalyst design.
This present invention can be used to convert a variety of reducible gases into reduced species but in general, it will be discussed herein with reference to the conversion of nitrogen (either as supplied nitrogen or air) and water to ammonia. In this process nitrogen is bubbled into the liquid (water or electrolyte) while being subjected to an atmospheric pressure plasma discharge, enabling the transport of the activated species within the liquid. These species (particularly nitrates and nitrites) can be then efficiently converted into ammonia by using a designed electrocatalyst.
In addition to the reduction of nitrogen, it should be noted that the process can be advantageously used in electrocatalytic carbon dioxide reduction reaction, which requires transformation of stable carbon dioxide molecules into comparatively more energetic and reactive states, which this invention can provide thus delivering enhanced performance as well as controllable selectivity.
A particular advantage of the present invention may be found where in reactions where the gas phase activation is the rate determining step.
As aforementioned, two of the most significant inhibitors to electrocatalytic NRR (eNRR) are the high stability and low solubility of the N2 molecule in liquids. The present invention seeks to overcome these limitations by converting N2 into a more reactive and soluble form.
Nitrate and nitrates are highly soluble and much more easily reduced to ammonia than N2. While this approach may seem promising, it needs to be kept in mind that the industrial process for producing nitrates and nitrates are is from ammonia via the Ostwald process, thus their direct use as precursors for ammonia production is highly circuitous and impractical. Additionally, nitrates/nitrites have limited stability in water hence direct production and on-spot utilisation is desirable. Consequently, the production of NOx via a plasma-driven process for the direct consumption to produce ammonia would be a desirable industrial process, if practicable.
The first step in the process of the present invention is the plasma-activation of air, at the water/electrolyte interface, to produce NOx (i.e. a mixture of NO2
Plasma can be categorized into thermal and non-thermal plasmas (NTP). Thermal plasmas exhibit equilibrium between electrons and bulk gas temperatures (typically higher than 5×103 K). Meanwhile, in NTP such equilibrium is not established thus the temperature of the electrons can be several orders of magnitude higher than ambient. NTP is less energy intensive than thermal plasmas, and still possess electrons with high translational energies required to overcome the stability of the N2 molecule via electronic structure transitions, which makes NTP a suitable choice for aforementioned process.
Nitrogen activation, and oxidation, is difficult due to the thermodynamic and kinetic stability of nitrogen in the energy required to break the N2 triple bond. Plasma is able to provide sufficient energy to activate N2. This reaction occurs commonly in nature as a consequence of lightning to produce NO.
Three key approaches have been studied in order to drive the plasma-driven generation of NOx species.
There are a range of variables which in can be used to control the (i) amount of species, (ii) the overall energy efficiencies of the system (NOx produced/power input) and (iii) ratio of nitrate to nitrite. These variables include plasma input voltage (amplitude, pulse width and repetition frequency), time, gas flow rate, and liquid flow rate.
The impact of these parameters of the performance of the different plasma systems are interrelated. For example, the impact of changing voltage and pulse/discharge frequency on the pin-in-nozzle design on the total quantity of NO produced is relatively minor, as shown in
Table 1 demonstrates a sample results comparison between the differing designs for plasma NOx generation, specifically comparing the pin-in-nozzle and column bubbler. It is clear that the production rate is significantly higher in the case of the column bubbler, however, in this case the ratio of nitrates/nitrates is notably different.
Ultimately, the design of the plasma-system, along with the input voltage, frequency, time, gas type and flow rate, humidity and temperature liquid type (i.e. electrolyte/water) and flow rate all have a significant impact on the quantity of NOx, the energy efficiencies of the species produced (NOx produced/power input) as well as the ratio of nitrate to nitrite. Given the teachings in the present specification, it would be expected that variations in design to optimise NOx production would be within the capacity of a person of ordinary skill in the art.
Attention was then focussed enhancing energy efficiency with respect to NOx formation. An AC sinusoidal waveform with periodic gaps between discharges was employed as compared to DC plasmas, AC waveform is considered more efficient, inexpensive, and reliable for longer operation. This is because of higher excited state active species at similar powers (in some cases 5 times higher) due to less energy dissipation into heat at the electrodes in aqueous environments. Compared to thermal plasmas, where gas is heated to temperatures typically in order of 20,000 K, the present invention utilized non-thermal (cold) plasma which is generated at ambient temperatures and pressures but still exhibit elevated electron temperatures.
The use of underwater plasma bubbles intensifies gas-to-liquid mass transfer, which is assisted by interfacial areas, residence time and internal pressures. Thus, the present inventors developed a non-thermal AC plasma, exploiting a bubble column with varying discharge regimes (including spark and glow discharges). Five different design configurations (
It is clearly shown in
The key features of the reactor design (
The incorporation of Raschig rings further increased the energy efficiency. This enhancement can be attributed changes in mass transfer and residence times, allowing for an intensification of mass transfer from the gas phase NOx species into solution. Ultimately, these key design approaches resulted in an energy efficient, scalable approach to aqueous NOx production.
Once the NOx (nitrate/nitrite mixture) is produced from the plasma, it needs to be reduced and the method chosen in the present case is electrocatalytic reduction. In order to better understand the system, and, the ability of the NOx intermediaries to be electrocatalytically reduced to ammonia, the present inventors initially focused on H-cell experiments. In these experiments, the electrocatalytic conversion of NOx to ammonia was performed using an integrated system that incorporates a custom-design plasma-bubbler to the electrochemical H-cell, as well as with NOx salts, to understand the electrocatalytic conversion pathways.
The conversion of NOx species can produce ammonia at higher rates and faradaic efficiencies than N2 directly. In acidic media, the reaction proceeds as shown in Equations 1-3 below. The reaction competes with the hydrogen evolution reaction (HER), Equation 4. Whilst HER occurs at more negative potential than the nitrate/nitrite reduction, slow kinetics for nitrate/nitrite reduction may lead to HER occurrence and it has been found that unwanted HER may be addressed by electocatalytic optimization.
Thus, the different pathways for the conversion of NOx, the competing HER as well as different reactants (NO3
The present invention has established that a range of transition metals can be used to facilitate the electrocatalytic conversion of nitrogen to ammonia. Of these copper and nickel were the most preferred. The description of the electrocatalyst will be provided with reference to copper but it will be appreciated that it can apply to other transition metals.
In the present invention, as found in
A range of Cu-based catalysts was prepared and evaluated for their performance for the electrocatalytic conversion of NOx to ammonia (Cu foil, foam and nanowires (NWs) grown on foam). Representative scanning electron microscopy (SEM) images of the Cu NWs (
It was found that the Cu NWs sample was able to attain the highest current density (j) for the reduction of plasma-activated electrolyte whilst achieving an ammonia production rate of 45 nmol·s−1·cm−2 and Faradaic efficiency (FE) of ˜100%.
Comparatively, the Cu foil and foam facilitated somewhat lower FEs of ˜80 and 71%, respectively, with ammonia yields of 6.0 nmol·s−1·cm−2 and 8.9 nmol·s−1·cm−2 (at −0.5 V). This variation in catalytic activity can be ascribed to the variation in electrochemical active surface area (ECSA) between the electrodes.
The ECSA for the Cu NWs catalyst was significantly larger for the foil and foam samples, indicating an increase in active sites for the NWs sample, ultimately improving the overall yield of ammonia. The high FE is due to the presence of Cu1+/Cu0 is well-known for the suppression of the competing hydrogen evolution reaction HER, which is the breakdown of water into oxygen and hydrogen.
To understand the origin of the NH3, in particular whether it arises from NOx or dissolved N2 reduction, control experiments were performed showing that the electrolyte alone (with no plasma activation) resulted in no NH3 production. Furthermore, the polarisation curves (
It was observed a portion of the *NO2 desorbed into the solution as NO2
To further understand the reaction pathway, and the consumption of both nitrate and nitrite as a function of electrolysis duration, a batch experiment was undertaken with a successive sampling of nitrite, nitrate and ammonia (
It is highly desirable to integrate the plasma-driven production of NOx with the electrocatalytic system for the production of ammonia. The stability of the produced nitrate/nitrites is low and thus the direct conversion of the activated species to ammonia is highly desirable.
Two possible approaches to integrating the plasma/electrocatalytic systems as shown in
Alternative embodiments are envisaged in which a reservoir is provided intermediate the NOx generation vessel and the NOx reduction vessel. A reservoir can provide benefits in terms of feeding the NOx for reduction at a predetermined rate, which can avoid build-up of NOx or NOx starvation at the site of electrocatalytic reduction.
The present system was tested and it was established that an increase in cell voltage from 1 V to 1.4 V resulted in an increase in j from 27 mA cm−2 to 52 mA cm−2 and ammonia rate from 15 mg h−1 to up to 24 mg h−1. Furthermore, the stability of the flow system at a current density of 30 mA cm−2 was investigated, where plasma-activated electrolyte was fed continuously while ammonia was collected from the outlet. The hybrid system maintained a stable applied cell voltage of 1.5±0.04 V and an average Faradaic efficiency of —58% for 8 h continuously (
The electrochemical conversion of the resultant NOx intermediaries, using a scalable electrolyser, resulted in current densities of over 50 mA/cm2, Faradaic efficiencies of ˜60%, an ammonia production rate of 23.2 mg/h (42.1 nmol/scm2) at a very low cell voltage of 1.4 V.
All reagents and solvents were purchased from Sigma-Aldrich or from Chem-Supply Pty Ltd. Cu foam was purchased from Xiamen TMAX Machine Limited. Oakton pH/Ion 700 Ion 700 Benchtop Meter and Cole-Parmer Combination Ion Selective Electrodes (nitrate) were purchased from John Morris Group. Milli-Q water with a resistivity of 18.2 MΩ·cm was obtained from an inline Millipore RiOs/Origin H2O purification system, was used throughout the experiments for sample preparation and reaction.
The commercial Cu foam and foil was cut into desired sizes and ultrasonically cleaned with acetone, ethanol, and finally Milli-Q water for 15 min intervals, and then washed with dilute H2SO4 solution to remove any surface impurities and oxide layers. Cu(OH)2 nanowires were first synthesized on Cu foam by immersion into a solution containing 0.133M (NH4)2S2O8 (ammonium persulfate) and 2.667m NaOH for 0.5 h at room temperature. Subsequently, the Cu foam was removed out from solution, rinsed with Milli-Q water and absolute ethanol, and air-dried. CuO NWs were then fabricated by annealing the prepared Cu(OH)2 NW arrays at 180° C. for 1 h in air. The resulting CuO NW sample was electrochemically reduced to Cu/Cu2O NW arrays in 0.5M Na2SO4 under −1V vs RHE.
All electrochemical evaluations were conducted using Autolab Potentiostat (Autolab M204) in a custom-designed H-type electrochemical cell and electrolyser. The cathodic chamber was separated from the anodic chamber by Nafion© 117 membrane. For the H-type cell, a three-electrode set-up using the Cu catalyst (foil, foam and Cu NWs) as the working electrode (WE), platinum wire as the counter electrode (CE) and Ag/AgCl (sat. KCl) reference electrode (RE) was used. 10 mM H2SO4 was used as the background electrolyte in this study, and the optimization of acid concentration was performed. Typically, for the H-type cell studies, 50 mL of electrolyte was used in the cathodic chamber to allow for electrolyte sampling. The electrode size for the H-cell was 1 cm−2 and Cu foil was used for optimization studies. The reaction was facilitated with magnetic stirrer at the speed of 650 rpm. All potentials for H-type cell were described versus the reversible hydrogen electrode (RHE) via the following equation:
E
RHE
=E
(Ag/AgCl)+0.197+0.059×pH(pH=1.68 in this study)
To further translate this concept for large scale application, the plasma-activated water (PAW) from the scaled-up reactor (vide infra) was fed into a high throughput electrolyser to understand the potential for ammonia production rate and yield. A membrane electrode assembly (MEA) was prepared by sandwiching the Cu NWs cathode (electrode size 9 cm2) and Ru/TiO2 anode between a commercial Nafion membrane. The MEA was loaded within the electrolyser with PAW being used as the catholyte and 0.1 M H2SO4 as the anolyte (using a peristaltic pump with a flow rate of 1.5 mL/min). For the electrolyser optimization, 250 mL of the PAW was circulated in the cathodic chamber. For the stability test, a continuous flow was used for 8 h with 30 mA·cm−2 being applied.
From the cathodic chamber electrolyte solution, 0.5 mL of electrolyte was taken and transferred into a 2 mL sample tube. Into the tube, 0.4 mL of 1 M NaOH solution (with 5 wt. % salicylic acid and 5 wt. % sodium citrate), 0.1 mL of 0.05 M NaClO and 30 μL of 1 wt. % C5FeN6Na2O (sodium nitroferricyanide) in water was added. The mixture was then incubated in the dark at room temperature for 2 h prior to UV-Vis testing. The concentration of ammonia was determined via a calibration curve. The calibration curve was prepared using a set of standard solutions with a known amount of (NH4)2SO4 (concentrations were based on NH4+) in 10 mM H2SO4. Into these solutions, the above-mentioned indophenol blue reagents were added, and the indophenol blue absorbance at 655 nm was determined after 2 h. The limit of detection (LOD) of UV-Vis used in this study refers to the absorbance at 655 nm obtained from blank 10 mM H2SO4 for the lower limit and from 200 μM NH4+ for the upper limit.
50 μL of the sample was taken and transferred into a cuvette and combined with 50 μL of Griess Reagent and 0.9 mL of Milli-Q water. The resulting sample was mixed thoroughly. The mixture was incubated at room temperature in the dark for 0.5 h prior to UV-Vis testing. Solutions of NaNO2 with known concentrations (in 10 mM H2SO4) were used as calibration standards, with the absorbance at 525 nm used to plot the calibration curves. Upper LOD of UV-Vis used in this study refers to the absorbance at 525 nm obtained from 200 μM NaNO2. A dilution factor was applied to measure nitrite concentration in plasma-activated water (PAW).
An ion-selective electrode (ISE), also known as a specific ion electrode (SIE), is a transducer (or sensor) that converts the activity of a specific ion dissolved in a solution into an electrical potential. The voltage is theoretically dependent on the logarithm of the ionic activity, according to the Nernst equation. Cole Palmer Nitrate selective probe has a concentration range of 7 μM to 1M (0.5 to 62,000 ppm). The ionic strength of ion solutions varies with the concentration of the ion to be measured. To maintain a constant ionic strength, an Ionic Strength Adjuster (ISA) is added. This ensures the total ionic strength is independent of the analyte concentration. In this study, 2M ammonium sulfate (NH4)2SO4 was added, as the ISA, at 400 μL to each 20 mL of standard or sample to adjust the ionic strength to about 0.12 M.
H2 detection was tested by GC (Shimidzu, Model 2010 Plus) equipped with both thermal conductivity detector (TCD) and flame ionization detector (FID) detectors.
X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Scientific K-Alpha X-ray spectrometer. The morphology and structure of Cu NWs were imaged by scanning electron microscopy (SEM) using a JEOL JSM-IT-500 HR. UV-Vis absorption spectra were recorded on a Shimadzu UV-3600 UV-VIS-NIR spectrophotometer.
The two crucial descriptors, revealing ammonia synthesis performance, are the Faradaic efficiency and the ammonia production rate. The Faradaic efficiency indicates the selectivity of the electrocatalysis for ammonia synthesis, which refers to the ratio of the electrical energy consumed for the synthesis of ammonia to the overall energy through the electrochemical system. The Faradaic efficiency (η) of ammonia synthesis was determined by Eq. (51), where n is the figure of the desired electrons for synthesizing one ammonia molecule (n=6 when ammonia is from nitrite and n=8 when ammonia is from nitrate), F is the Faraday constant (F=96485.33), C is the detected ammonia molar concentration, V is the electrolyte volume, and Q is the overall electrical energy travelled over the electrodes. To calculate the number of the exchanged electron, both nitrite and nitrate concentrations were measured before and after each reaction; the average found to be 7.6. For the reactions that both nitrite and nitrate are fully exhausted, n was calculated based on their initial ratio.
Ammonia production rate (R) is the ammonia production over unit time and over unit electrode surface area. It can be determined by Eq. (S2), where C is the detected ammonia molar concentration, V is the electrolyte volume, t is the reaction time, and S is the catalytically active surface area of the electrode.
Plasma activation of water in the H-cell.
Ground-state nitrogen molecules exhibit high ionization potential making it intrinsically unreactive from a thermodynamic standpoint. Still, plasma activation provides avenues for the conversion of highly stable nitrogen molecules into easier to breakdown intermediaries (NOx). Plasma can be categorized into thermal and non-thermal plasmas (NTP). Thermal plasmas exhibit equilibrium between electrons and bulk gas temperatures (typically higher than 5×103 K). Meanwhile, in NTP such equilibrium is not established; thus the temperature of the electrons can be several orders of magnitude higher than ambient. NTP is less energy-intensive than thermal plasmas, and still possess electrons with high translational energies required to overcome the stability of the N2 molecule via electronic structure transitions, which makes NTP a suitable choice for the aforementioned process.
The design of the plasma-system, along with the input voltage, frequency, time, gas type and flow rate, liquid type (i.e. electrolyte/water) and flow rate all have a significant impact on the quantity of NOx, the energy efficiencies of the species produced (NOx produced/power input) as well as the ratio of nitrate to nitrite.
For the batch electrochemical tests, custom plasma bubbler was used in an H-cell and connected to the plasma generator (‘Leap 100’ from PlasmaLeap Technologies). The optimized plasma generator parameters were using a voltage of 100V, duty of 83 μs, discharge frequency of 600 Hz and resonance frequency of 60 kHz. Dry air (Coregas, dry air) was introduced from the top of the custom plasma bubbler at 20 mL/min to generate PAW. The plasma activation was performed for 0.5 h to achieve NOx concentration of ˜4 mM in 100 mL water.
Five reactor design configurations using underwater plasma bubbles were tested. Photographs of plasma bubble column reactors design configurations are shown in
Raschig rings. Plasma bubble column reactors were capable of dual-discharge mode operation. i.e. glow and spark discharge. To achieve the former, the high voltage electrode was sheathed with borosilicate. The latter incorporated a sharpened high voltage electrode with a 1 cm protrusion which induced a spark extending longitudinally towards the bubbles. Meanwhile, combinative discharge reactors coupled both these concepts in a single unit. In configurations involving single reactor, water was used as ground, meanwhile double-reactor configurations utilized secondary plasma reactor as ground. Plasma reactors were fabricated using a quartz tube with one end sealed and 12 laser-drilled holes with a diameter of 200 μm located radially 5 mm above the sealed base. Stainless steel rod was used as high voltage electrode inserted concentrically into the quartz tube. A tee fitting was connected to the quartz tube to position the electrodes. Instrument grade air was injected as the feed gas at a flow rate of 1 L/min in each reactor via a mass flow controller. Reactors were powered by plasma generator (‘Leap100’, PlasmaLeap Technologies) capable of yielding voltage output of 0-80 kV (peak-to-peak), discharge power of up to 700 W, and a discharge frequency range of 100 Hz-3000 Hz. For all experiments, power was provided in form of batches of sinusoidal pulses with a lag time between each batch. Resonance frequency of pulse was set at 60 kHz while discharge frequency of each batch of pulses was 300 Hz (duty cycle of 103 μs).
Electrical and optical measurements. A digital oscilloscope (DS6104, Rigol) was employed to record both the sinusoidal voltage and current waveform via a high voltage probe (PVM-6, North Star) and a current probe (4100, Pearson), respectively. The time-averaged discharge power (P) was calculated from the measured discharge voltage and current with the following formula:
P=f∫
t
t
+T
u(t)i(t)dt
The electrical parameters across various reactor configurations are presented in Table 2. Optical Emission Spectra (OES) were recorded using a spectrometer (SR-500i-A-R, Andor Shamrock), with a grating groove of 300 lines mm−1 and exposure time of 20 ms.
Using an AC system was desirable as polarity inversion in AC systems prompts current passing through zero at half cycle which enhances the lifetime of electrodes.
Cold plasma is particularly useful to selectively transfer incident electric power to the electrons rather than volumetric heating of the entire gas as it is an energy-efficient route to formation of active species via collisions.
Without wishing to be bound by theory, it is believed that higher production of reactive radicals in the aqueous phase takes place because of mechanical agitation and local heating caused by bursting of bubbles.
Single reactor glow discharge (SRGD) was operated at glow-only discharge scheme by applying 7.38 W power and using a dielectric barrier around the high voltage electrode. In principle, the use of dielectric barrier limits the flow of charge enabling higher voltages at the same power. In such discharge scheme, production of NO3 predominated over NO2, which corroborates with literature. Meanwhile, spark-only discharge scheme was dominated by NO2 over NO3, and higher current to voltage ratios than the glow-only scheme and comparatively higher power (9.22 W). High intensity electric fields in glow discharge scheme favour ozone production, which maintain oxidation environment the entire volume in the tube facilitating conversion of NO2 to NO3. However, spark streamers are confined in the concentrated volume prompting formation of high energy species and back-reactions of NO3 to NO2.
Thus, the underwater plasma bubbler reactors of the present invention , combine both glow and spark discharges to generate the NOx intermediaries at the unprecedented energy efficiency of 263 mmol/kWh.
It should be noted that the above sections outline the use of air as the inlet gas, however, to date a range of different gases have been examined. It is possible to use any gas and tune the plasma/inlet parameters to obtain the desired product. In the present specification, the results presented are predominantly for the use of air, however mixtures of N2/O2, along with H2O also show promise. It should be further noted that the system has also been adapted for use for CO2 conversion showing favourable results.
The incorporation of a catalyst into the glow and/or spark discharge region for the plasma-driven NOx generation.
As mentioned above, transition metal catalysts, an specifically copper, nickel, tin, iron, bismuth, cobalt, titanium and oxides thereof are particularly useful in the present invention.
Any suitable catalyst binder, such as silica, alumina, clays, polymers or carbon based supports can be used.
To probe the active sites responsible for eNRR, XPS analysis was carried out on the post-reaction Cu NW electrode to investigate any variation in the surface chemical state of the electrode owing from the negative bias applied herein. From
A number of calibration and control experiments were conducted using UV-visible spectroscopy to investigate the background of NH3 and NOx in the used electrolytes, PAW, solutions and electrodes,
It was observed that a in some bubbler configurations a barely detectable amount of ammonia is generated at the rate of 0.21 nmol s−1 during the plasma activation, however, with the plasma column bubbler, no ammonia could be detected in the electrolyte, showing the specificity of the preferred embodiments of the invention to be specific toward the production of NOx. In the case of the electrolysis of PAW, the significant production rate of 45 nmol cm−2 s−1 was obtained.
Moreover, the measured concentration of ammonia in the background electrolyte, as well as all of the other controls, was more than four orders of magnitudes lower than the measured ammonia in the electrolysis tests. This result indicates that the environmental contaminations are not contributing to the ammonia production rate reported in this work.
The concentration of NOx was controlled by the plasma activation time under the optimized parameters (voltage of 100V, duty of 83 μs, discharge frequency of 600 Hz and resonance frequency of 60 Hz). It is shown from
The first step toward the optimization of the electrocatalytic conversion of NOx to ammonia was performed using nitrate (KNO3) and nitrite (NaNO2) salts as the NOx source, and Cu foil (1 cm×1 cm) as the cathode, Pt wire as the anode and Ag/AgCl (sat. KCl) as the reference electrode in a custom-designed H-cell (
In acidic media, the reaction proceeds, as shown in Equations S1-S2 below. The reaction competes with the hydrogen evolution reaction (HER), Equation S3[1].
Whilst H+ is required to facilitate the reaction, a high concentration of H+ may result in occurrence of HER. In the present experiment, when H+ was not available in the electrolyte, both FE and ammonia yield rate was very low (>30% and 1 nm cm−2s−1, respectively). The addition of acid (10 mM H2SO4) in the electrolyte resulted in a significant increase in the ammonia production rate and FE, from 0.81 to 8.94 nmolcm−2 s−1 and from 31% to 73%, respectively. However, a further increase in the acid concentration did not positively impact the ammonia production while the FE dropped significantly as
HER became more competitive. See
Therefore, in this study, 10 mM H2SO4 was used as the background electrolyte which (a) increases the conductivity of the electrochemical system to minimize energy losses caused by the resistance; (b) provides proton for ammonia synthesis; (c) supports synthesis of ammonium sulphate (NH4)2SO4 which is soluble in water and can be used directly as a fertilizer. See
When nitrite was used as the reactant, ammonia production rate increased with more negative potentials, and the rate reaches its maximum at −0.5V with FE of about 73%. While for nitrate, its maximum production rate (˜3.8 nmol cm−2 s−1) and FE (˜60%) occurred at −0.4V. Beyond the optimal potential, both rate and FE start to decrease due to a possible occurrence of HER.
NO3−30 2H++2e−→NO2−+H2O (Eq. S4)
This result indicates that at lower potentials, nitrate is more favourable to be converted to nitrite rather than ammonia (which is in agreement with the literature [2]). To investigate, a 15 min electrolysis experiment was conducted in 25 mL of 1 mM KNO3 solution at −0.3V. It was found that 0.7 μmol of ammonia was produced, while 1.17 μmol of nitrite was produced in this potential. On the other hand, when the experiment was performed at −0.5V, 1.64 μmol ammonia and 0.66 μmol nitrite were produced.
During NOx reduction, it established that NO3
To investigate the effect of NOx concentration on the ammonia production rate and FE, a range of concentrations of nitrite and nitrate salts was tested (
Effect of pH Upon Electrolysis
A comparative study was carried out in which a series of electrocatalytic reduction reactions were carried out under identical conditions, save for varying the starting pH. Table 3 displays the corresponding experimental conditions.
Analysis was undertaken with a range of pH conditions. For example, plasma activated water (PAW), salt (KCl) with a concentration of 50 mM was added to PAW, allowing the electrolysis to occur in a neutral media. From this and other experiments, the present inventors have concluded that the reduction method of the present invention may be carried out at any pH.
To compare copper-based catalyst performance as a function of available surface area, control experiments were performed with Cu foil and Cu foam. Cu NWs electrode facilitated a very high current density (j), −45 mA cm−2 at −1V, compared to −22 mA cm−2 for Cu foil and −26 mA cm−2 for Cu foam. Cu NWs also facilitated a much higher catalytic activity for ammonia synthesis with a production rate of 40±3.3 nmol cm−2 s−1 and FE of 100±7%. At the same time, Cu foil only provided ammonia production rate of 6.1±0.6 nmol cm−2 s−1 with FE of 80.6±0.3% and Cu foam had a rate of 8.8±1.3 nmol cm−2 s−1 with FE of 71.1±1.7%. See
In the case of Cu foil and Cu foam, non-Faradaic charging currents are measured in the potential range of 0.5V and 0.55V vs RHE and for Cu NWs, the potential range is 0.25V to 0.30V vs RHE. The scan rate is varied between 5, 10, 15, 20 and 25 mV/s and the anodic (positive) and cathodic (negative) current densities are obtained from the double layer charge/discharge curves at 0.525V vs RHE for Cu foil and Cu foam and 0.275V vs RHE for Cu NWs. See
The double-layer capacitance was then calculated by averaging the absolute values of cathodic and anodic current densities and take the slopes of the linear fits. The slopes obtained with Cu foil, Cu foam and Cu NWs are 0.13 mF/cm2, 3.03 mF/cm2 and 15.24 mF/cm2 respectively, indicating that the fabricated catalyst Cu NWs has much larger electrochemical active surface area compared to the commercial Cu foil and Cu foam.
The performance of a nickel based and single-atom copper catalysts in the electrocatalytic reduction of nitrate to ammonia were investigated and the results are shown in
With an extended reaction time of 2.5 h (See
NMR analysis (see
A similar series of experiments was conducted to those described above, with the input gas changed from air to carbon dioxide reduction, in order to investigate carbon dioxide reduction.
Plasma runs were conducted for 10 minutes by continuously bubbling CO2 gas through Milli-Q water at 0.1-0.5 L min−1. For these tests, Plasma Leap was operated at voltage of 200 V, duty cycle of 83 μs, discharge frequency of 2 kHz and resonance frequency of 60 kHz. Subsequently, activated species in liquid phase were electrochemically converted to hydrocarbon products. Two different catalysts, i.e. Cu foam and Ni, were selected as cathodes. While CO2 to CO conversion was prevalent, some higher hydrocarbons produced. The tests evidenced that the invention can be successfully utilized for CO2 conversion to high value chemicals.
The global $60 billion ammonia fertiliser market is supplied by ammonia generated using the conventional Haber-Bosch process at an average price of $0.23-25 per kg. Locally, the price ranges from $0.2-$0.5 per kg of ammonia. Due to the advantages experienced by large plants due to economies of scale, almost all fertilizer plants are large-scale (˜100,000 MT per year), located strategically near ports for water requirement as well as for shipping and as a result, a significant infrastructure is required to transport fertilizer to rural farms and locations. Hence, local farmers are required to pay a significantly higher price, i.e. 5 kg of ammonia fertilizer costs $10.58. (AUD as at September 2020)
As such, considerable efforts are in place to generate ammonia in small-scale delocalized units at a competitive cost. While electrochemical nitrogen reduction reactions to ammonia are proposed as a promising technology, the best performing NRR catalyst can generate ammonia with a low yield of merely 0.23 μmol h−1 cm−2 at RTP, with a high energy input of 1410 kWh/kgNH3. It must be stated that in addition to the high cost, the overall yield within these electrochemical NRR are very low, making these systems unfavourable for scalability.
In contrast, the hybrid NRR system of the present invention is capable of generating ammonia with a yield which is ˜3,000 times greater than the NRR counterpart. As indicated in
| Number | Date | Country | Kind |
|---|---|---|---|
| 2020903637 | Oct 2020 | AU | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/AU2021/051172 | 10/7/2021 | WO |
