Abstract
According to a first aspect of the present invention, there is provided an apparatus for forming NOx from nitrogen and oxygen, the apparatus comprising: a gliding arc discharge, GAD, device arranged to generate a plasma; a passageway including an inlet for a feed gas comprising nitrogen and oxygen and an outlet for the NO.sub.x, wherein the passageway extends, at least in part, through the GAD device wherein, in use, the nitrogen and oxygen are reacted in the generated plasma, thereby forming the NO.sub.x from at least some of the nitrogen and oxygen; and a post-discharge container for adjusting the NO.sub.2/NO ratio in the formed NO.sub.x to from 1:2 to 2:1.
Claims
1. According to a first aspect of the present invention, there is provided an apparatus for forming NO.sub.x from nitrogen and oxygen, the apparatus comprising: a gliding arc discharge, GAD, device arranged to generate a plasma; a passageway including an inlet for a feed gas comprising nitrogen and oxygen and an outlet for the NO.sub.x, wherein the passageway extends, at least in part, through the GAD device wherein, in use, the nitrogen and oxygen are reacted in the generated plasma, thereby forming the NO.sub.x from at least some of the nitrogen and oxygen; and a post-discharge container for adjusting the NO2/NO ratio in the formed NO.sub.x to from 1:2 to 2:1.
2. The apparatus according to claim 1, wherein the post-discharge container comprises a micro-porous membrane dividing the container into two parts.
3. The apparatus according to claim 1, wherein the apparatus further comprises a means for converting the NO.sub.x to ammonia.
4. The apparatus according to claim 3, wherein the means for converting the NO.sub.x to ammonia is an electrochemical means, suitably comprising a H-type cell arranged as a divided electrochemical cell.
5. The apparatus according to claim 4, wherein the divided electrochemical cell comprises a working electrode comprising an electrocatalyst comprising cobalt or nickel.
6. The apparatus according to claim 1, wherein the nitrogen and oxygen are reacted in the generated plasma at temperatures of at most 300 C.
7. The apparatus according to claim 1, wherein the feed gas is air.
8. The apparatus according to claim 1, wherein the GAD device comprises at least a pair of diverging electrodes.
9. A method of forming NO.sub.x from nitrogen and oxygen, the method comprising: generating a plasma using a gliding arc discharge, GAD, device; and reacting the nitrogen and oxygen in the generated plasma, thereby forming the NO.sub.x from at least some of the nitrogen and oxygen adjusting the NO2/NO ratio in the formed NO.sub.x to from 1:2 to 2:1.
10. The method according to claim 9, wherein the method comprises reacting air comprising nitrogen and oxygen in the generated plasma.
11. A method of synthesising ammonia, the method comprising: (a) reacting the NO.sub.x obtained from claim 9 with an aqueous solution to form nitrate/nitrite from at least some of the NO.sub.x; and (b) electrochemically reducing the nitrate/nitrite obtained in step (a) to ammonia.
12. The method according to claim 9, wherein reacting the methane in the generated plasma at temperatures less than 300 C.
13. The method according to claim 11 wherein step (b) involves applying a potential on the working electrode in a H-type cell.
14. The method according to claim 13 wherein step (b) occurs on a catalyst comprising cobalt and/or nickel, suitably consisting essentially of cobalt, supported on a conductive substrate.
15. Use of a catalyst comprising Co metal film in the electrochemical reduction of nitrate and/or nitrite to ammonia.
16. The apparatus according to claim 1, wherein the nitrogen and oxygen are reacted in the generated plasma at temperatures of at most 250 C.
17. The apparatus according to claim 8, wherein the at least a pair of diverging electrodes, comprises diverging steel electrodes.
18. The method according to claim 9, wherein reacting the methane in the generated plasma at temperatures of less than 250 C.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0128] For a better understanding of the invention, and to show how exemplary embodiments of the same may be brought into effect, reference will be made, by way of example only, to the accompanying diagrammatic Figures, in which:
[0129] FIG. 1A schematically depicts the experimental setup;
[0130] FIG. 2A shows the NO.sub.x concentration at different N.sub.2/O.sub.2 ratios; FIG. 2B shows the energy consumption of NO.sub.x production and NO selectivity at different N.sub.2/O.sub.2 ratios, for a gas flow rate of 1 SLM; a power of 17 W and an applied voltage frequency of 6 kHz;
[0131] FIG. 3 shows the NO.sub.x concentration and energy consumption of NO.sub.x production at different applied voltage frequencies, for an air gas flow rate of 1 SLM, a power of 15 W;
[0132] FIG. 4A shows digital photos of gliding arc discharge with increasing power under an applied voltage frequency of 6 KHz; FIG. 4B shows the digital photos of gliding arc discharge with increasing power under an applied voltage frequency of 11 kHz; FIG. 4C shows the digital photos of gliding arc discharge with increasing power under an applied voltage frequency of 40 kHz;
[0133] FIG. 5A shows the NO.sub.x concentration at different discharge powers and frequencies; FIG. 5B shows the NO and NO.sub.2 selectivity at different discharge powers and frequencies; FIG. 5C shows the energy consumption of NO.sub.x production at different discharge powers and frequencies, for a gas flow rate of 1 SLM.
[0134] FIG. 6A shows digital photos of gliding arc discharge upon increasing power using a 100 k resistance in the circuit; FIG. 6B shows the digital photos of gliding arc discharge upon increasing power without using a 100 k resistance in the circuit, at an applied voltage frequency of 6 kHz, for a gas flow rate of 1 SLM
[0135] FIG. 7A shows the NO.sub.x concentration, energy consumption of NO.sub.x production using or without using a 100 k resistance; FIG. 7B shows NO.sub.x selectivity using or without using a 100 k resistance, for a gas flow rate of 1 SLM and an applied voltage frequency of 6 kHz;
[0136] FIG. 8A shows the digital photos of gliding arc discharge upon increasing the flow rate at an applied frequency of 40 kHz at a fixed power of 18 W; FIG. 8B shows digital photos of gliding arc discharge upon increasing the flow rate at an applied frequency of 6 kHz at a fixed power of 21 W;
[0137] FIG. 9A shows the NO.sub.x concentration, energy consumption of NO.sub.x production at different flow rates; FIG. 9B shows the NO selectivity at different flow rates and, for frequencies of 6 kHz and 40 kHz, at fixed powers of 21 W and 18 W;
[0138] FIG. 10A shows pH and conductivity of aqueous solution in the acrylic cylindrical container at different discharge times using water as the absorbing solution; FIG. 10B shows concentrations of nitrate and nitrite in the aqueous solution at different discharge times using water as the absorbing solution; FIG. 10C shows concentrations of nitrate and nitrite in the aqueous solution at different discharge times using 1 M KOH as the absorbing solution; at a discharge power of 18 W and a flow rate of 1.5 SLM;
[0139] FIG. 11 shows LSV curves of Co(OH) 2 on different conductive substrate (carbon cloth, carbon paper and Ni foam) in the electrolysis producing ammonia from nitrate;
[0140] FIG. 12 shows LSV curves of Co(OH).sub.2/Ni foam, Co/Ni foam, and Co.sub.3O.sub.4/Ni foam in the electrolysis producing ammonia from nitrate;
[0141] FIG. 13A shows LSV curves of Co/Ni foam at different concentrations of nitrate (concentration of KOH=1 M); FIG. 13 B shows faradaic efficiency toward ammonia production from 0.2 to 1.0 V in 1 M KOH containing different concentrations of nitrate; FIG. 13C shows LSV curves of Co/Ni foam at different concentrations of nitrite (concentration of KOH=1 M); FIG. 13 D shows faradaic efficiency toward ammonia production from 0.2 to 1.0 V in 1 M KOH containing different concentrations of nitrite;
[0142] FIG. 14A shows LSV curves of Co/Ni foam at different concentrations of KOH using nitrate solution (concentration of nitrate=0.1 M); FIG. 14B shows faradaic efficiency toward ammonia production from 0.2 to 1.0 V in different concentrations of KOH containing 0.1 M nitrate; FIG. 14C shows LSV curves of Co/Ni foam at different concentrations of nitrite using nitrite solution (concentration of nitrite=0.1 M); FIG. 14D shows faradaic efficiency toward ammonia production from 0.2 to 1.0 V in different concentrations of KOH containing 0.1 M nitrite;
[0143] FIG. 15A shows LSV curve of Co/Ni foam using plasma-activated solution in the acrylic cylindrical container (discharge time=30 min, 1 M KOH as absorbing solution, discharge power=18 W, flow rate=1.5 SLM);
[0144] FIG. 16A shows the effect of discharge power on the NO.sub.x concentration; FIG. 16B shows the effect of discharge power on the energy consumption for the production of NO.sub.x (air GAD, 50 Hz, total gas flow rate 2.8 SLM);
[0145] FIG. 17A shows the effect of total gas flow rate on the NO.sub.x concentration; FIG. 17B shows the effect of total gas flow rate on the energy consumption for the production of NO.sub.x (air GAD, 50 Hz, discharge power 28 W).
EXAMPLES
Experimental
[0146] The following procedures were used in the examples which follow.
[0147] The arc voltage was measured by a high voltage probe (Tektronix P6015A), while the current was measured by a current monitor (Pearson 2877). The electrical signals (arc voltage, current) were recorded by a four-channel digital oscilloscope (Tektronix MDO 3054, 500 MHZ, 2.5 GS/s) at a sampling rate of 5 Mpts per record to ensure precise measurement.
[0148] The gaseous reaction products were analyzed online using a Fourier transform infrared (FTIR) spectrometer (Bruker Tensor II) at a wavenumber resolution of 2 cm.sup.1, and each spectrum was obtained by averaging 16 scans. The absorption spectra were recorded 10 minutes after the discharge ignition to ensure a stable discharge, and each measurement was repeated at least three times. For quantitatively analyzing the concentrations of NO.sub.x, precise calibration gas mixtures (NO or NO.sub.2 in Argon) with a wide range of concentrations were introduced to the gas cell by mass flow controllers.
[0149] The concentrations of aqueous nitrate, nitrite and ammonia were measured by spectrophotometric method using a microplate reader (Thermo Scientific Varioskan Flash Reader). For the detection of nitrite, 100 L Griess reagent was added into 100 L sample, and the absorbance was measured at 540 nm. For the detection of nitrate, 100 L saturated VCl3 was added into the sample, after which 100 L Griess reagent was added into the above solution, and the absorbance was measured at 540 nm after the solution was incubated at 37 C. for 12h to insure fully reduction of nitrate by VCl.sub.3. Finally the nitrate concentration was obtained by subtracting the nitrite concentration from the total concentration of nitrate and nitrite. For detection of ammonia, 100 L potassium sodium tartrate was added into 100 L sample and then 100 L Nessler reagent was added into the above solution, and the absorbance was measured at 420 nm. All the measurements were calibrated by using the standard curves.
Examples: Ammonia Production Via Plasma-Electrolysis Process
Example 1: NO.SUB.x .Production at Different N.SUB.2./O.SUB.2 .Ratios
[0150] FIG. 1 schematically depicts the experimental setup. The experiments were conducted in a flat gliding arc reactor that used either a mixture of nitrogen and oxygen or dry air as feed gas under atmospheric pressure.
[0151] The gliding arc reactor consists of two thin diverging stainless steel electrodes (thickness of 3 mm) fixed symmetrically in a transparent flat (thickness of 10 mm) quartz container with a rectangular cross-section (10060 mm) to achieve a uniform drag and high processing fraction of the arc column by the surrounding gas flow. The feed gas is introduced through a cylindrical nozzle with a diameter of 1 mm, and the nozzle is 5 mm above the tip of the electrodes, where has the narrowest gap distance of 2 mm.
[0152] Feed gas: mixture of N.sub.2 and O.sub.2; gas flow rate: 1 SLM; discharge power: 17 W; applied voltage frequency: 6 KHz.
[0153] FIG. 2A shows the measured NO.sub.x (NO and NO.sub.2) concentrations as different N.sub.2/O.sub.2 ratios. The NO and NO.sub.2 concentrations both follow parabolic trends with increasing N.sub.2 fraction. The NO concentration increases upon increasing N.sub.2 fraction until a maximum value of 10900 ppm is reached at a N.sub.2/O.sub.2 ratio of 4, while the NO.sub.2 concentration reaches its maximum (10850 ppm) at a N.sub.2/O.sub.2 ratio of 1.5.
[0154] FIG. 2B shows the selectivity of NO and energy consumption of NO.sub.x production as a function of N.sub.2/O.sub.2 ratio. At N.sub.2/O.sub.2 ratios of lower than 0.11, increasing the N.sub.2 fraction leads to a slightly lower NO selectivity. After this point, the NO selectivity increases upon varying the N.sub.2/O.sub.2 ratio as NO.sub.2 production by NO oxidation is less favoured at low O.sub.2 fractions and the highest N.sub.2/O.sub.2 ratio gives the highest NO selectivity of 81.6%. Energy consumption for NO.sub.x production drops sharply upon increasing N.sub.2 fraction when the N.sub.2/O.sub.2 ratio is lower than 0.43 and it continues to decrease to a minimum value of 1.26 MJ/mol when it reaches the optimum N.sub.2/O.sub.2 ratio (1.5), after which energy consumption begins to increase. Clearly, too much or too little N.sub.2 is unfavorable for efficient NO.sub.x generation because both N.sub.2 and O.sub.2 are precursors for NO and NO.sub.2 formation.
[0155] Interestingly, at a N.sub.2/O.sub.2 ratio of 4, similar to the composition of air, the energy consumption (1.35 MJ/mol) is only 7% higher than the optimised N.sub.2/O.sub.2 feed ratio, making air a suitable feed gas for NO.sub.x production as no additional energy is needed to prepare pure O.sub.2 and N.sub.2. Consequently, in the following experiments, we focus on NO.sub.x production only using air as the feed gas.
Example 2: NO.SUB.x .Production at Different Frequencies
[0156] FIG. 3 compares NO.sub.x production performances under different applied voltage frequencies at a fixed discharge power (feed gas: air; gas flow rate: 1 SLM; discharge power: 15 W). Clearly, higher or lower applied voltage frequencies do not necessarily lead to a higher NO.sub.x production. 40 KHz frequency give the best performance, where the NO.sub.x concentration and energy consumptions of NO.sub.x production can reach 15500 ppm and 1.41 MJ/mol, respectively. Similar results can be seen at 20 kHz. While 6 kHz, 25 kHz, 30 kHz, and 43 kHz frequencies show slightly worse performances and the worst NO.sub.x production performances is observed at 11 kHz.
Example 3: NO.SUB.x .Production at Different Discharge Powers and Frequencies
[0157] FIG. 4 shows that the gliding arc showed different phenomena at different frequencies with increasing discharge power (feed gas: air; gas flow rate: 1 SLM).
[0158] FIG. 4A shows the digital photos of gliding arc discharge with increasing power at a frequency of 6 kHz. Under the lowest discharge power, the arc can only propagate to a short distance; with increasing power from 8 W to 11 W, the propagation distance and the arc length increase significantly and further increasing the power only slightly increases the propagation distance but lead to a larger plasma volume and more diffuse appearance. The discharge contains a large number of bright filamentous arcs at low powers in the upstream of the gliding arc. The evolution process from a mode containing numberless short bright arcs (short arc mode) to a mode that contains long propagated arcs (diffuse mode) upon increasing power was also observed at frequencies of 8 kHz, 11 kHz, 25 kHz, 30 kHz, and 43 kHz. This short arc mode is most remarkable at 11 kHz, where the arcs in the upstream are even brighter and hard to drag down as shown in FIG. 4B. Interesting but differently, the discharge cannot sustain itself in the short arc mode at frequencies of 20 KHz and 40 KHz. As shown in FIG. 4C, the gliding arc discharge directly appears in diffuse mode at the lowest discharge power and the arc propagates slightly downwards upon increasing power.
[0159] FIG. 5 shows the NO.sub.x concentrations, NO.sub.x selectivity and energy consumption of NO.sub.x production at different frequencies as a function of discharge power. In FIG. 5A, it can be seen that the NO.sub.x concentrations increase, more quickly at low discharge powers, upon higher discharge powers for all the frequencies. The highest NO.sub.x concentrations are always observed at 20 KHz and 40 kHz at a fixed discharge power. While 11 kHz sees the lowest NO.sub.x concentrations, consistent with the above results. Clearly, frequency does affect NO.sub.x production, but the NO.sub.x production of the best frequency is only 5% higher than that of the worst frequency at high powers. Note that for frequencies of 6 kHz and 11 kHz, the highest discharge power can be attained are 35 W and 21 W, respectively.
[0160] FIG. 5B depicts the effects of frequency and discharge power on NO.sub.x selectivity. For all the frequencies, increasing the power leads to a remarkably NO.sub.2 selectivity (lower NO selectivity) under low powers (<24 W), and a further power increase only slightly increases NO selectivity. This can be partly explained by the fact that increase the power leads to a longer propagation distance and a larger volume of the plasma region as stated before. As a result, the previous formed NO molecules are more likely oxidized by excited O.sub.2 into NO.sub.2 with a longer residence time in the plasma region. As shown in FIG. 4, the propagation distance increases significantly under low powers, consistent with the increasing NO.sub.2 selectivity upon increasing power. The highest and lowest NO.sub.2 selectivities are 36.1% and 47.8%, which are achieved at 6 kHz with the lowest power and 40 kHz with the highest power.
[0161] In FIG. 5C, the calculated energy consumption for NO.sub.x production drops sharply upon increasing power (<15 W) for frequencies of 6 kHz, 11 kHz. The rapid decrease in the energy consumption upon increasing power under low powers at frequencies of 6 kHz and 11 kHz is well in accordance with the discharge behaviours at these frequencies. As illustrated in FIG. 4A and FIG. 4B, the discharge evolves from short arc mode to diffuse mode upon increasing power at low powers, accompanied by a longer propagation distance and a larger plasma volume. In contrast, at frequencies of 20 kHz and 40 kHz, the energy consumptions do not change much upon increasing power because the discharge directly operates at diffuse mode as seen in FIG. 4C. For all the frequencies, the optimum power for NO.sub.x production lies between 15 W and 30 W, and a further power increase yields a slightly higher energy consumption. Under the optimum power, the energy consumption can reach approximately 1.29 MJ/mol at 20 kHz and 40 kHz, the numbers are 1.34 MJ/mol and 1.39 MJ/mol for 6 KHz and 11 kHz, respectively.
Example 4: NO.SUB.x .Production with Restricting Discharge Current
[0162] The effect of discharge current on NO.sub.x production was investigated by restricting the discharge current with a 100 k resistance. The experiment is performed at a frequency of 6 kHz and an air flow rate of 1 SLM.
[0163] In FIG. 6, the discharge can sustain itself at a much lower power of about 5 W with a 100 k resistance. For comparison, the lowest power to maintain a gliding arc without a resistance is about 8 W. Note the arc propagation distances are close at the lowest discharge power with or without restricting the current, indicating restricting current does not change the discharge mode at this frequency but lower the discharge power. Visually, the arc plasma in the upstream of the discharge region is much brighter without restricting the current when the discharge operates at low powers, whereas the discharge is more uniform with restricting current.
[0164] FIG. 7 shows the effects of restricting discharge current on NO.sub.x concentrations, NO.sub.x selectivity, and energy consumption of NO.sub.x production plotted as a function of discharge power. The NO.sub.x concentration at the lowest discharge power with restricting current is about 1400 ppm, lower that for 8 W without restricting current (1800 ppm), but the selectivity of NO is higher in the former case. Notably, at the same power of about 8 W, the NO.sub.x concentration can reach 6000 ppm with restricting the current, and therefore the energy consumption is merely one third of that without restricting the current. This is reasonable because at the same power of 8 W, the discharge clearly shows a different mode, short arc mode without restricting the current and diffuse glow-type mode with restricting the current, and as shown above the short arc mode is more energy consuming for NO.sub.x production. Actually, at discharge powers lower than 12 W, the energy consumptions of NO.sub.x production with restricting the current are significantly lower than these of the case without restricting the current, mainly due to the discharges are in different modes as clearly shown in FIG. 6.
[0165] However, the superiority of restricting discharge current begins to disappear upon further increasing the power above 12 W, because the discharge without restricting the current has transitioned into a diffuse glow-like mode. From the perspective of the overall energy input, restricting the current by employing a high value resistance is not encouraged because much energy is consumed by the resistor in the form of heat. For example, the resistor power and the temperature of the resistor can reach about 35 W and 200 C. at the optimum power of 15 W, and these values are even higher at higher powers. If the energy spent on the resistor is not used appropriately and efficiently, the overall energy efficiency of the system will be considerably low.
Example 5: NO.SUB.x .Production at Different Flow Rates
[0166] FIG. 8A and FIG. 8B shows the digital photos of gliding arc discharge upon increasing the flow rate at an applied frequency of 6 kHz and 40 KHz, at fixed powers of 21 W and 18 W, respectively, except for the first case at 40 kHz, where the power is 22 W in glow mode (feed gas: air).
[0167] FIG. 8 shows the discharge phenomena show different patterns upon increasing the flow rate at frequencies of 6 kHz and 40 kHz. The gliding arc is observed at a flow rate range of 0.5-2.5 SLM, and the power supply fails to sustain the gliding arc at 21 W when further increasing the flow rate at a frequency of 6 kHz. Differently, at 40 kHz, we observed static glow-type discharges at flow rates of 0.5 SLM and 0.75 SLM, and the gliding arc could operate at a higher maximum flow rate (4 SLM). Notably, the propagation distance decreases and the arcs become brighter and non-uniform upon increasing the flow rate.
[0168] In FIG. 9, we see the effects of flow rate on the NO.sub.x concentrations, NO.sub.x selectivity, and energy consumption of NO.sub.x production at two distinct frequencies. When the discharge operates at a static glow-type mode as shown in FIG. 8, the energy consumption is considerably higher than these in gliding arc mode. The NO.sub.x concentrations decrease steadily upon increasing the flow rate at both frequencies, which is also the case for NO.sub.2 selectivity. For example, at 6 kHz, the lowest flow rate gives the highest NO.sub.2 selectivity (83%) and the NO.sub.2 selectivity decreases almost linearly upon increasing the flow rate. Lower flow rate indicates longer residence time and therefore previously formed NO molecules are more likely oxidized into NO.sub.2. However, too high or too low flow rate is not benefit for energy efficient NO.sub.x production.
[0169] As shown in FIG. 9B, the optimum gas flow rates at 6 kHz and 40 KHz are 1.25 SLM and 1.5 SLM, respectively. The corresponding energy consumptions are 1.28 MJ/mol and 1.23 MJ/mol at the optimum conditions. Low flow rate benefits from long propagation distance but suffers from high concentrations of products, whereas high flow rates could overcome the influence of high concentrations of products but it results in a short propagation distance and non-uniform discharge.
Example 6: Nitrate/Nitrite Production by Absorbing NO.SUB.x .in Aqueous Solution
[0170] The produced NO.sub.x in the GAD device was firstly introduced to a post-discharge container to achieve suitable NO.sub.2/NO ratio, and then the NO.sub.x was introduced to a vessel containing 100 mL solution to produce nitrate/nitrite from the reaction of NO.sub.x and solution.
[0171] FIG. 10A shows pH and conductivity of aqueous solution in the acrylic cylindrical container at different discharge times using water as the absorbing solution at the optimum discharge condition for NO.sub.x production in terms of energy consumption (discharge power: 18 W, applied frequency: 40 kHz, flow rate: 1.5 SLM). The post-container used in this experiment has a length of 15 m, and the NO.sub.2 selectivity increased from 34.5% to 68%. The pH drops sharply from 5.9 to 2.1 with a discharge time of 2.5 min, and then it further decreases with increasing discharge time. At a discharge time of 30 min, the pH reaches about 1. The conductivity of the solution increases almost linearly with increasing discharge time and it reaches approximately 25000 s/cm at a discharge time of 30 min.
[0172] FIG. 10B shows concentrations of nitrate and nitrite in the solution at different discharge times. The concentrations of nitrate and nitrite both increase upon increasing discharge time. The concentration of nitrate increases almost linearly as the discharge time increases, and it reaches a value of about 100 mM at a discharge time of 30 min, while the value for nitrite at the same discharge time is only 5 mM. The concentration of nitrate in the solution is more than then times higher than that for nitrite, especially with a long discharge time, which is as a result of the high NO.sub.2 selectivity in the NO.sub.x.
[0173] To achieve a high nitrite selectivity in the aqueous solution, the NO.sub.2 selectivity in the NO.sub.x was adjusted to around 50% using a post-discharge container with a length of 5 m, and 1 M KOH solution was used as trapping solution.
[0174] FIG. 10C shows concentrations of nitrate and nitrite in the solution at different discharge times using 1 M KOH as trapping solution. Similar as the case using water as trapping solution, the concentrations of nitrate and nitrite both increase with increasing discharge time. Differently, the concentration of nitrite is much higher than the concentration of nitrate at all discharge time; the former reaches about 130 mM at a discharge time of 30 min and the number for the latter is about 7.5 mM. Therefore in this case, solution with high nitrite selectivity is obtained
Example 7: Ammonia Production from Electrolysis Using Co(OH).SUB.2 .Catalyst Supporting on Different Conductive Substrates
[0175] Concentration of nitrate solution: 0.1 M; concentration of KOH: 1 M
[0176] FIG. 11 shows LSV curves of Co(OH) 2 on carbon cloth, carbon paper and Ni foam in the electrolysis producing ammonia from standard electrolyte containing 0.1 M nitrate and 1 M KOH. The current densities of both curves stay around 0 mA/cm.sup.2 as the potential is higher than 0.1 V versus RHE. The current density of LSV curve of Co(OH).sub.2 on Ni foam begins to drop at a potential of around 0.12 V versus RHE, while the potential for Co(OH).sub.2 on carbon cloth and carbon paper is around 0.2 V versus RHE. After the dropping point, at which the current density begin to drop sharply with decreasing potential, the current densities decrease at a similar speed with decreasing potential for Co(OH).sub.2 on all types of conductive substrate.
[0177] The current density of LSV curve of Co(OH) 2 on Ni foam reaches 650 mA/cm.sup.2 at a potential of 0.56 V versus RHE, and at the same potential, the current densities of LSV curves of Co(OH).sub.2 on carbon cloth and carbon paper are 391 mA/cm.sup.2 and 331 mA/cm.sup.2. Clearly, Co(OH).sub.2 on Ni foam has a higher activity than it on other substrates, possibly because of the 3 D porous structure of Ni foam. In the following example, Ni foam was used as the conductive substrate.
Example 8: Ammonia Production from Electrolysis Using Different Catalysts Comprising Cobalt Supporting on a Ni Foam
[0178] Concentration of nitrate solution: 0.1 M; concentration of KOH: 1 M
[0179] FIG. 12 shows LSV curves of Co(OH) 2/Ni foam, Co/Ni foam, and Co.sub.3O.sub.4/Ni foam in the electrolysis producing ammonia from standard electrolyte containing 0.1 M nitrate and 1 M KOH. The current densities of LSV curves of Co(OH).sub.2 on Ni foam and Co.sub.3O.sub.4 on Ni foam both stay around 0 mA/cm.sup.2 as the potential is higher than 0.0 V versus RHE. The current density of LSV curve of Co on Ni foam begins to drop at a potential of around 0.17 V versus RHE, while the potential for Co(OH).sub.2 on Ni foam and CO.sub.3O.sub.4 on Ni foam is around 0.17 V versus RHE. After the dropping point, at which the current density begin to drop sharply with decreasing potential, the current densities decrease at a roughly similar speed with decreasing potential for all catalysts comprising cobalt on Ni foam.
[0180] The absolute value of LSV curve of Co on Ni foam is higher than that of Co(OH).sub.2 and Co.sub.3O.sub.4 at a potential range from 0.1 V versus RHE to 1.0 V versus RHE, especially at a potential range from 0.2 V versus RHE to 0.5 V versus. For example, the current density of LSV of Co on Ni foam reaches 648 mA/cm.sup.2 at a potential of 0.4 V versus RHE, and at the same potential, the current densities of LSV curves of Co(OH).sub.2 on Ni foam and Co.sub.3O.sub.4 on Ni foam are 388 mA/cm.sup.2 and 212 mA/cm.sup.2. From the above example, it can be seen that Co on Ni foam has a higher activity than Co(OH).sub.2 and CO.sub.3O.sub.4. In the following example, Co on Ni foam was used as the catalyst.
Example 9: Ammonia Production from Electrolysis Using Co/Ni Foam at Different Concentrations of KOH
[0181] Concentration of nitrate solution: 0.1 M; catalyst: Co/Ni foam
[0182] FIG. 14A shows LSV curves of Co/Ni foam at different concentrations of KOH (0.0 M, 0.1 M, 0.5 M, and 1 M) using nitrate solution as nitrogen source. The absolute value of LSV curves is higher at higher concentrations of KOH at a potential range from 0.2 V versus RHE to 0.2 V versus RHE, indicating the catalyst possesses higher activity at more alkaline environment. For example, at a potential of 0.8 V versus RHE, the current density reaches 1270 mA/cm.sup.2 at 1 M KOH solution, while it only reaches 658 mA/cm.sup.2, 390 mA/cm.sup.2, and 154 mA/cm.sup.2 when the electrolysis was conducted in 0.5 M, 0.1 M, and 0 M KOH solution.
[0183] FIG. 14B shows the Faradic efficiency of nitrate toward ammonia using Co/Ni foam as catalyst at different concentrations of KOH (0.0 M, 0.1 M, 0.5 M, and 1 M). The Faradic efficiencies of nitrate toward ammonia are similar at different concentrations of KOH; they are all above 90% when the potential is higher than 0.9 V versus RHE. Combing the current density and Faradic efficiency at different concentrations of KOH, high concentrations of KOH is favoured as it possesses higher ammonia production rates.
[0184] FIG. 14C shows LSV curves of Co/Ni foam at different concentrations of KOH (0.0 M, 0.1 M, 0.5 M, and 1 M) using nitrite solution as nitrogen source. The absolute value of LSV curves is higher at higher concentrations of KOH at a potential range from 0.2 V versus RHE to 0.2 V versus RHE, indicating the catalyst possesses higher activity at more alkaline environment, the same as the trend when nitrate is used as nitrogen source.
[0185] FIG. 14D shows the Faradic efficiency of nitrate toward ammonia using Co/Ni foam as catalyst at different concentrations of KOH (0.0 M, 0.1 M, 0.5 M, and 1 M). The Faradic efficiencies of nitrate toward ammonia are similar at different concentrations of KOH; they are all above 90% when the potential is higher than 0.9 V versus RHE, the same as the trend when nitrate is used as nitrogen source. Combing the current density and Faradic efficiency at different concentrations of KOH, high concentrations of KOH favours higher ammonia production rates.
Example 11: Ammonia Production from Electrolysis Using Co/Ni Foam as a Catalyst at Different Concentrations of Nitrate/Nitrite
[0186] Concentration of KOH: 1 M
[0187] FIG. 13A shows LSV curves of Co/Ni foam at different concentrations of nitrate (0.005 M, 0.02 M, 0.05 M, 0.1 M, and 0.2 M). The absolute value of LSV curves is higher at higher concentrations of nitrate at a potential range from 0.2 V versus RHE to 0.2 V versus RHE, indicating the catalyst possesses higher activity at higher concentrations of nitrate. For example, at a potential of 0.8 V versus RHE, the current density reaches 1410 mA/cm.sup.2 at 0.2 M nitrate solution, while it attains 1269 mA/cm.sup.2, 1103 mA/cm.sup.2, and 636 mA/cm.sup.2 when the electrolysis was conducted in 0.1 M, 0.05 M, 0.02 M and 0.005 M KOH solution.
[0188] FIG. 14B shows the Faradic efficiency of nitrate toward ammonia using Co/Ni foam as catalyst at different concentrations of nitrate (0.005 M, 0.02 M, 0.05 M, 0.1 M, and 0.2 M). At a nitrate concentration of 0.2 M, the Faradic efficiency stays above 90% at all potentials ranging from 0.2 V versus RHE to 1.0 V versus RHE. Similar trend of the Faradic efficiency can be observed at a nitrate concentration of 0.1 M, except for the potential of 1.0 V versus RHE, which has a Faradic efficiency of 88%. At a nitrate concentration of 0.05 M, the Faradic efficiency stays above 90% at potentials higher than 0.5 V versus RHE, and it begins to decrease almost linearly with decreasing potential. At lower nitrate concentrations, the Faradic efficiency begins to drop at a higher potential. Combing the current density and Faradic efficiency at different concentrations of nitrate, high concentrations of nitrate favours higher ammonia production rates as a result of higher activity of catalyst and higher Faradic efficiency toward ammonia.
[0189] FIG. 13A shows LSV curves of Co/Ni foam at different concentrations of nitrite (0.005 M, 0.02 M, 0.05 M, 0.1 M, and 0.2 M). The absolute value of LSV curves is higher at higher concentrations of nitrite at a potential range from 0.2 V versus RHE to 0.2 V versus RHE, indicating the catalyst possesses higher activity at higher concentrations of nitrite, the same as the trend when nitrate is used as nitrogen source.
[0190] FIG. 14B shows the Faradic efficiency of nitrite toward ammonia using Co/Ni foam as catalyst at different concentrations of nitrite (0.005 M, 0.02 M, 0.05 M, 0.1 M, and 0.2 M). At a nitrite concentration of 0.2 M, the Faradic efficiency stays above 90% at all potentials ranging from 0.2 V versus RHE to 1.0 V versus RHE. Similar trend of the Faradic efficiency can be observed at a nitrite concentration of 0.1 M, except for the potential of 1.0 V versus RHE. At lower nitrite concentrations, the Faradic efficiency begins to drop at a higher potential, the same as the trend when nitrate is used as nitrogen source. Combing the current density and Faradic efficiency at different concentrations of nitrite, high concentrations of nitrite favour higher ammonia production rates as a result of higher activity of catalyst and higher Faradic efficiency toward ammonia.
Example 12: Ammonia Production from Electrolysis Using Co/Ni Foam as a Catalyst and Plasma-Activated Solution as Working Electrolyte
[0191] FIG. 15A shows LSV curve of Co/Ni foam using plasma-activated solution in the acrylic cylindrical container. (Discharge time: 30 min, 1 M KOH as absorbing solution, discharge power: 18 W, flow rate: 1.5 SLM, concentration of nitrate: 7.5 mM, concentration of nitrite: 130 mM). The LSV curve of Co/Ni foam using plasma-activated solution is similar to that of using 0.1 M nitrite solution in 1 M KOH.
[0192] Table 1 below provides a summary of the results.
TABLE-US-00001 TABLE 1 Ammonia production rate, Faradic efficiency, energy consumption of ammonia production in the electrolysis, and energy consumption from air with full NO.sub.x absorption.sup.a at different potentials Energy Energy consumption Poten- consumption from air with tial (V Ammonia production Faradic in full NO.sub.x versus rate efficiency electrolysis absorption RHE) mmol/h/cm.sup.2 mg/h (%) (MJ/mol) .sup.b (MJ/mol) 0.9 7.14 30.35 85 0.61 1.84 0.7 6.29 26.73 91 0.45 1.68 0.5 4.58 19.47 92 0.32 1.55 0.3 2.98 12.67 96 0.18 1.41 0.1 1.46 6.21 94 0.06 1.29 .sup.aOptimized energy consumption of NO.sub.x production + energy consumption of ammonia production in electrolysis
Example 13: Plasma Synthesis of NO.SUB.2 .Rich NO.SUB.x .from Air Using a Gliding Arc Reactor
[0193] A further experiment was carried out as follows:
[0194] A flat GAD reactor used in this study contains two diverging stainless-steel electrodes (60 mm in length, 18 mm in width) installed 3 mm downstream of the nozzle exit with the narrowest gap of 2 mm. The GAD reactor was connected to an AC high voltage neon transformer with a maximum peak-to-peak voltage of 10 kV and a fixed frequency of 50 Hz. The applied voltage was measured by a high voltage probe (Testec, TT-HVP 15 HF), while arc current was recorded by a current monitor (Magnelab CT-E0.5). A four-channel digital oscilloscope (Tektronix, MDO3024) was used to sample the electrical signals. The discharge power was determined via the integral of applied voltage multiplied by arc current. The gas temperature in the GAD reactor was measured by a fibre optic thermometer (Omega, FOB102) with the fibre positioned at 70 mm downstream of the nozzle exit.
[0195] Air (Zero grade, BOC) was used as the reactant and introduced into the DBD reactor by a mass flow controller (Omega, FMA-2404). The gas products were analysed using an online Fourier-transform infrared (FTIR) spectrometer (FTIR-4200, Jasco) with a resolution of 0.5 cm.sup.1. The effluent gases passed a standard gas cell with a 10 cm path length placed in the FTIR. The products from the air GAD were determined as NO and NO.sub.2 in all working conditions and the sum of their concentrations was defined as NO.sub.x concentration. Calibration gases with known concentrations of NO and NO.sub.2 diluted in N.sub.2 were used to quantify the concentration of these products. Each experiment was repeated 3 times, and the margin of error in this work was within 3%. The total gas flow rate was 2.8-4.0 L/min and the discharge power was 24-38 W, resulted in the specific energy input (SEI) ranged between 450 and 814 J/L.
[0196] NO.sub.2 and NO were found as the dominant products in this process. FIG. 16A demonstrates the overall NO.sub.x concentration increased from 16374 to 22555 with the increase of discharge power from 24 to 38 W. FIG. 16B demonstrates that the energy consumption of NO.sub.x synthesis increased from 0.66 to 0.76 MJ/mol NO.sub.x when increasing the discharge power from 24 to 38 W, suggesting a low discharge power leads to lower energy consumption of NO.sub.x production. Within the test range, the lowest energy consumption was 0.66 MJ/mol NO.sub.x at a discharge power of 24 W.
[0197] The concentration of individual NO.sub.2 and NO and overall NO.sub.x showed identical trend with the increase of total flow rate, as shown in FIG. 17A. The concentration of NO.sub.x increased from 2.8 to 3.0 SLM and reached the maximum value of 20271 ppm at 3.0 SLM, after which it declined linearly to 15846 ppm at a total flow rate of 4.0 SLM. The concentrations of NO.sub.2 and NO also reached the peak values of 16563 and 3709 ppm at a gas flow rate of 3.0 SLM, respectively. FIG. 17B demonstrates the influence of total gas flow rate on the energy consumption of NO.sub.x synthesis. The energy consumption of NO.sub.x synthesis slightly dropped when increasing the total flow rate from 2.8 SLM to 3.2 SLM, after which it remained stable at 0.65 MJ/mol NO.sub.x with the increase of total flow rate until 4.0 SLM. Notably, the minimum energy consumption of NO.sub.x synthesis (0.65 MJ/mol NO.sub.x) was achieved at the highest total flow rate of 4.0 SLM.
