Controlled synthesis of NO in an atmospheric pressure plasma by suppressing NO destruction channels by plasma catalysis

NO synthesis using plasma catalysis is analyzed in a parallel-plate atmospheric pressure RF plasma from N2/O2 admixed to helium exposed to Fe and Pt catalysts on a SiO2 support. The NO x species are measured by Fourier-transform infrared spectroscopy in a multi-pass cell. The trends in species densities can be well explained by air chemistry reactions, where NO’s progressive oxidation occurs with increasing oxygen admixture and ozone generation. The sequence can be controlled by the state of the surface that preferentially quenches O3 and allows for an optimum NO production. The maximum production of NO is found at 70% N2/(N2+O2) mixture ratio at 120 ∘C using sandblasted glass, with a conversion rate of 0.085%.


Introduction
Plasma discharges containing nitrogen and oxygen have been studied for many years, motivated by their broad range of applications in different fields.NO is especially relevant for biological processes such as tissue/wound treatment [1], but is also used as a probe molecule for the determination of gas temperatures [2], in fertilizers [3], can participate in the physiological process in living organisms [4] etc. Different plasma sources have been used to produce NO, such as microwave (MW) discharge [5], gliding arc (GA) discharge [6], glow discharge [7,8], or spark discharge [9].However, the strong triple N ≡ N bond (9.79 eV) makes it hard to transform or fix the di-nitrogen (N 2 ) into nitric oxide (NO).The first plasma-based NO formation was introduced by Birkeland-Eyde, exploiting chemical equilibrium reactions at very high temperatures and a fast quenching to 1000 • C in the exhaust.The thermodynamic limit for the production of this plasmabased NO formation is a few percent only.Therefore, other plasma processes that might provide higher production and operate at lower temperatures are being explored.
The reactive species such as NO are generated using nonthermal plasma (NTP), such as DBDs, or atmospheric pressure plasmas jets (APPJs), where high-energy electrons at a temperature above 10 eV [10] allow for dissociation; nitrogen is oxidized via these electron-induced reactions while the gas temperature is much lower than for conventional thermal N 2 oxidation.The elementary processes of electron-induced nitrogen oxidation, defined as the Zeldovich mechanism, consist of O and N radicals reacting with N 2 and O 2 , respectively, to produce NO [11,12]: or by reaction [13] O + NO 2 k3 ⇌ NO + O 2 , k 3 = 6.5 × 10 −12 exp(120/T) cm 3 s −1 (R3) and reaction [14] N 2 (A) At room temperature, the primary reactions for generating NO 2 are [13] NO or in the downstream region via [13] NO + O 3 k 6 ⇌ NO 2 + O 2 , k 6 = 1.8 × 10 −12 exp(−1370/T) cm 3 s −1 (R6) N 2 O is mainly created by the reaction [15]: By combining NTP with heterogeneous catalysis, synergistic effects may lead to higher production at lower energy costs [16].Patil et al found that the NO production changed with the type of catalyst when using metal oxides as catalysts in a packed-bed DBD [17] with larger surface areas leading to higher N x O y concentrations.Eremin et al used glow discharges with different metal catalysts to generate nitric oxide and found an order of effectiveness of Pt > CuO > Cu > Fe > Ag [18].Ma et al [19] compared the experimental and model results in a radio frequency plasma reactor at low O 2 -to-N 2 mixture ratios (<20%), with and without downstream Pt catalyst and show that low O and high N coverages at high O 2 mole fractions are more effective for NO production.Schmidt et al [20] found that an admixture of small H 2 O concentrations will promote NO production, especially for low O 2 admixtures.In all these experiments, the NO densities remain at the ppm level.
The mechanisms in a plasma catalysis reactor must be investigated to increase NO production.However, this is challenging because of the many chemical reactions involving ground and excited neutrals, radicals, ions, and electrons.Schmidt et al [20] and Van Gaens and Bogaerts et al [21] discussed possible chemical processes during N 2 /O 2 discharges.They show that NO is consumed via several loss channels and can be destroyed by N (N These loss channels may explain the low NO concentrations observed in the experiments [22]. Rapakoulias et al [23] postulated that the vibrationally excited N 2 undergoes dissociative adsorption on the catalytic surface first (R8).Then, atomic nitrogen may react with surface oxygen and produce NO during desorption (R9).The oxygen vacancy can be refilled by gas phase O 2 (R10).These NO species might then oxidize to NO 2 on the catalyst surface [24] N 2 (v) The Zeldovich mechanism, instead, is based on reactive species, such as N, O, and O 3 , created by electron-induced processes in the plasma so that NO is created directly in the gas phase.
This paper aims to devise an optimal source for NO production at room temperature from N 2 and O 2 to be used in, for example, biomedical applications.A 13.56 MHz RF plasma from helium with N 2 and O 2 admixture is used to generate N x O y species in plasma catalysis.The product concentration is lower compared with other plasma sources [25,26], but our setup can generate homogeneous plasma and investigate the role of surface processes in nitrogen oxidation mechanism by changing different dielectric surfaces.The production of nitrogen oxide species is measured by ex situ Fourier-transform infrared spectroscopy (FTIR) combined with a multi-pass cell.

Experimental setup
The plasma channel device has been described previously [27][28][29].Two copper electrodes are covered by glass plates (44 mm × 13 mm × 1 mm) and form a gap distance of 1 mm.The top electrode is connected to a 13.56 MHz RF generator via an impedance-matching network; the bottom electrode is grounded.The plasma volume is 26 mm × 13 mm × 1 mm.The gas is injected at one side and flows through the chamber before being exhausted at the end.The whole chamber is enclosed in a stainless steel body with quartz windows on four sides of the plasma slab.A schematic of the plasma chamber is shown in figures 1(a) and (b).The chamber temperature is controlled by an oil temperature bath in the chamber body.The oil temperature is set by a LAUDA ECO heating thermostat.Due to heat loss along the heating line from the oil bath to the reactor, the temperature of the chamber might be lower by about 20 • -30 • .Such differences may also be deduced from the measured rotational temperatures of the molecules extracted from the IR spectra, although such temperature measurement exhibits a similar error margin [27].The products from the plasma chamber are guided via a tube connected to a multipass cell and are analyzed by FTIR; see figure 1(c).

Catalyst preparation
NO production is analyzed for different surfaces.The top surface always consists of a glass plate; the bottom surface is either (i) also a glass plate, (ii) a sandblasted glass plate, (iii) a sandblasted glass with 0.3 mg cm −2 loading of a Fe catalyst, (iv) or a sandblasted glass with 0.3 mg cm −2 loading of a Pt catalyst, respectively.The particle size of the Pt catalyst is 200 nm, and that of the Fe catalyst is 35-45 nm.The specific surface area of Pt catalyst is 55.9 m 2 g −1 , and that of Fe catalyst is 152.7 m 2 g −1 .The nanoparticle catalysts can be considered as ideal spheres.The micrographs of the different dielectric glass plates we used in the experiment are shown in figure 2. The details about the catalyst preparation procedure are described in [28,30].

Operation parameters
The plasma-forming gas consists of helium with a variable admixture of air or variable amounts of oxygen and nitrogen.The admixture ratio is defined as Φ m /Φ He , Φ m is the flow rate of the admixed gas and Φ He is the flow rate of helium of 250 sccm.In contrast to regular DBD discharges, strong helium dilution is required in RF discharges for stable operation at atmospheric pressure.Mass flow controllers (MFC) feed the working gas to the plasma chamber.The power dissipated in the plasma is measured using VI probes inserted in the electrical circuit of the chamber.The power is determined from the VI signals as [31]: U RMS and I RMS correspond to the effective voltage and current.∆ϕ is the phase shift between voltage and current.At first, a reference phase shift is measured without the plasma to account for the impedance of the setup itself.This is then subtracted from the phase shift measured when the discharge is on to quantify the absorbed power by the plasma.Figure 3 gives an example of the correlation between the rms voltage and the absorbed plasma power in helium plasma with and without an admixture of 0.9% molecular gas at different N 2 :O 2 mixture ratios.All the power curves in our experiment have a similar pattern.

Infrared absorption spectroscopy
The species densities in the plasma are analyzed by infrared absorption spectroscopy using a Bruker Vertex 70 V FTIR spectrometer with an external MCT detector.A multi-pass cell is used to increase the sensitivity of the system by providing a long absorption path of 7.2 m that is calibrated by measuring a known concentration of CO 2 .The volume of the multipass cell is 2 l.The IR beam passes through two off-axis parabolic (OAP) and one spherical mirror, then goes into the cell.The output beam from the cell is focused with an OAP onto the MCT detector.For each spectrum, 50 scans are taken at a wavenumber resolution of nominal 0.3 cm −1 .
It is well known that atmospheric pressure plasma may cause a strong non-equilibrium in the excitation temperatures.However, we detected the species in the remote multi-pass cell, where the species had enough time to come into equilibrium.The temperature of the multipass cell is fixed at 25 • C. The measured FTIR spectra are fitted by a script described in detail in [32,33].The IR absorption spectra are quantified based on the Einstein coefficients A ul (the spontaneous emission from upper state u to lower state l) taken from the HiTRAN database to calculate line strengths S j [34][35][36].The line profile is a Voight profile that considers pressure broadening, Doppler broadening, and instrumental broadening, where the latter is approximated as a Gaussian in line with literature [32].This yields a cross-section of each line σ j (ε) for different transitions j.The transmittance T ε is calculated based on the Beer-Lambert law: ( n is the density of species and L is the path length.The HITRAN database does not provide the elemental data of N 2 O 5 .Here, we use a variant of equation ( 2) instead given by: And the cross sections as given by Wagner and Birk [37].
The fitting includes small corrections of the baseline.The gas temperature is considered equal to that of the multipass cell, i.e.  red -method is utilized to estimate the uncertainty in the concentration [32].The concentrations of the products are obtained in time from FTIR spectra with (2).The concentrations given in this contribution are obtained after 40 min of continuous plasma operation when the build-up in the multipass cell of the products is stable.The long build-up time is attributed to the residence time in the cell and thermal and chemical equilibrium in the plasma reactor.

Variation of the humid air admixture
Regular humid air is first used because of its commonness and easy availability.We did not control the humidity level and performed this experiment only to see the impact of humidity in general.It is admixed to a constant helium flow as a plasma gas to operate the RF discharge at constant power to identify the optimum air admixture for NO production.The helium flow rate is 250 sccm and the admixture of humid air is at a flow from 0.25 sccm (0.1%) to 4.25 sccm (1.7%) with 6 W plasma power.Figure 6 shows the concentrations of NO, NO 2 , and N 2 O as a function of the humid air admixture.No ozone can be observed.The maximum NO production is found at 0.7%-0.9% of humid air admixture.The production of NO 2 and N 2 O increases with air admixture, with the increase in NO 2 being more pronounced than the increase of N 2 O above an admixture of 1%.
The different trends in N 2 O and NO 2 might be explained by reactions with ozone.Although we do not see any ozone in our data, it might still be produced but quickly consumed in reactions with NO x species, especially at high air admixtures.Reaction (R7) for N 2 O production is slower than reaction (R2) for NO production at small air admixtures but can become very large at high O 2 densities.Consequently, we assume that NO is quickly converted into NO 2 at high air admixtures explaining the decrease in NO production and an increase in NO 2 .At the same time, N 2 O is not affected by reactions with ozone and the concentration of NO 2 becomes larger than N 2 O at high air admixtures.
The decrease of NO production at large air admixtures could also be explained by a depletion of the high energy tail of the electron energy distribution function (EEDF) by inelastic collisions with the admixed molecules.In addition, the electron density may decrease due to attachment to the electronegative oxygen leading to a less efficient NO production for high air admixtures [38,39].However, this deterioration of the plasma generation efficiency should also affect the efficiency of NO 2 and N 2 O production, which is not observed.

Power variation with humid/synthetic air mixture
Figure 7 shows the concentrations of products for varied plasma power when admixing 0.7% humid air (dashed lines) or synthetic air admixture (O 2 :N 2 = 20:80, solid lines) to 250 sccm helium.The concentrations of products are affected by the gas composition.In a synthetic mixture, the NO production is smaller than in humid air.O 3 is generated at small input powers, and the densities of NO and NO 2 are small.At increasing plasma power, the O 3 production decreases and vanishes, whereas the NO production gradually increases, while NO 2 first increases faster than NO and then slightly decreases.In humid air, the NO production is higher than in synthetic admixtures and gradually increases with plasma power.The NO 2 production remains constant.In both gases, the N 2 O production only slightly increases, with higher production in the synthetic air admixture.
One can see that the N 2 O production varies slightly with plasma power and only differs when comparing humid vs. synthetic air.However, the NO 2 and the NO production vary strongly with absorbed plasma power following opposite trends.When humid air is admixed, the NO production increases by a factor of two, and the concentration of NO 2 decreases correspondingly.The main NO loss channel can explain these trends: depending on the O 3 concentration, NO may decrease at the expense of the NO 2 (R6).It is well known that air humidity strongly affects ozone formation, with dry air being typically three times more efficient [21].Consequently, we expect a much lower ozone production in humid air, leading to more efficient NO formation, as observed in the experiment.To assess the plasma catalytic formation of NO, we use only synthetic air further on.

Power dependencies of product formation for different surfaces at room temperature
Figure 8 shows the impact of different surfaces (glass, sandblasted glass, Fe catalyst, Pt catalyst) at different plasma power for an admixture of 2.25 sccm synthetic air (O 2 :N 2 = 20:80) to 250 sccm helium.O 3 is produced under low plasma power and decreases with the increase of plasma power.NO is observed only for the glass, sandblasted, and Pt surfaces but not for Fe surfaces.The concentration of NO increases with plasma power.The production of N 2 O is only slightly affected by the different surfaces and exhibits only a small increase with plasma power.NO 2 shows an increase with plasma power before it saturates at high plasma powers.The impact of the surfaces on the plasma chemistry is discussed as follows: • glass: O 3 is produced at low plasma power, and any NO is efficiently oxidized to NO 2 and N 2 O 5 .With increasing plasma power, O 3 is destroyed by the reactions in the plasma but also in reactions with oxygen vacancies on the glass surface.The yields of NO and NO 2 gradually increase while the latter gradually saturates at high plasma power.We assume that NO formation occurs via the Zeldovich mechanism.In the absence of ozone, any further oxidation by ozone cannot occur.
• sandblasted glass: The trends are almost identical to those observed for glass.But NO has a higher concentration than normal glass, indicating that the rough surfaces might facilitate NO production.• Pt catalyst: The production of N x O y and O 3 is similar to that with normal glass and sandblasted glass.Meanwhile, the use of Pt catalysts may increase the adsorption of NO on the surface and thus be oxidized.So the yield of NO 2 is higher while the NO is lower compared to the case of using the former two glass plates.• Fe catalyst: The NO formation using a Fe catalyst shows different behavior, NO is not detected even at high plasma powers.The O 3 concentration is rather large, so any produced NO is quickly oxidized into NO 2 or N 2 O 5 .Only at higher plasma power, when the ozone production decreases, the formation of NO 2 does become visible.
These data may indicate that ozone chemistry significantly impacts the selectivity of NO formation.We use a variation of the N 2 :O 2 mixture ratios in the following to evaluate this.

Product formation for varying N 2 :O 2 mixture ratios at room temperature
The correlation between the power absorbed by plasma and the applied rms voltage is shown in figure 3. The voltage at which plasma ignition occurs shifts to higher values with increasing N 2 :O 2 mixture ratio due to the higher ionization energy of nitrogen molecules (15.58 eV) versus oxygen molecules (12.13 eV).The near linear scaling between the generator and absorbed plasma power indicates that the plasma always remains homogeneous.Any transition into a constricted mode would lead to a strong non-linear scaling, as it is known from APPJs [25], especially at very high plasma powers.Figure 9 shows the concentration of products for different surfaces (glass, sandblasted glass, Fe, Pt) and feed gas mixtures at 6 W plasma power.The O 2 percentage (Φ O2 /Φ O2+N2 ) is changed from 10% to 90%, and the total mixture gas flow is 2.25 sccm, which is injected into 250 sccm helium.In general, NO concentrations decrease with decreasing N 2 :O 2 mixture ratios.O 3 appears after the NO concentration becomes zero and increases further with higher O 2 fractions.N 2 O and NO 2 increase for small O 2 fractions but then decrease at very low nitrogen concentrations.According to Jõgi et al [40], O 3 plays an essential role in the N 2 /O 2 chemistry, since NO is oxidized by O 3 to NO 2 and further to N 2 O 5 through reactions (R11) and (R12) [13]: and by [41]: It shows that O 3 production is higher for Fe catalysts.The catalyst's presence does apparently not affect NO formation at room temperature.This could be explained by the fact that the Zeldovich mechanism, according to (R1) and (R2) does not depend on the state of the surface.It is interesting to note that the N 2 O production is not affected by the different surfaces.This could be explained if reaction (R7) for N 2 O formation is dominating and the concentration of oxygen atoms is not affected by the surfaces.Only the NO 2 production varies with N 2 :O 2 mixture ratios being much higher for Pt than for Fe, which should be associated with the loss channels involving O 3 .These might vary for the different surfaces: • glass: We use normal glass plates as the reference to compare the effect of different surfaces on the reactions.The concentration of NO 2 is the highest at O 2 :N 2 = 30:70 mixture ratio, where most of the NO is oxidized by ozone.• sandblasted glass: N x O y exhibits higher concentrations when using sandblasted instead of regular glass as a surface.
One may argue that more O 3 loss occurs on sandblasted surface sites due to the higher roughness compared to regular glass.The loss of O 3 implies also less oxidation of N x O y species by O 3 [42].The NO 2 has a higher concentration, and the corresponding gas mixture ratio shifts to O 2 :N 2 = 40:60.• Pt catalyst: The concentration of NO 2 is lower for the case of Pt catalyst surface than normal glass.This could be explained either due to an enhanced loss of NO 2 in a surface process because NO 2 exhibits a high sticking coefficient and preferentially dissociatively adsorbs on Pt [43].It is known, i.e. that Pt/Ba/Al 2 O 3 catalysts have a higher N x O y storage capacity at 200 • C and 300 • C than Fe/Ba/Al 2 O 3 [44].However, the enhanced loss of NO 2 could also occur due to reactions with O 3 at high oxygen concentrations.• Fe catalyst: NO is oxidized by O 3 when using a Fe catalyst, and it only exists under the N 2 :O 2 = 90:10 gas mixture.This Fe catalyst is not able to destroy O 3 upon surface impact, and thus the concentration of O 3 is higher than when using the other three surfaces.
The oxidation of nitrogen is an endothermic reaction; increasing the temperature might facilitate the conversion of N 2 to N x O y .To look into the oxidation process at a higher temperature, the temperature of the plasma chamber is set to 120 • C. Figure 10 shows an increase in NO production with plasma power for all surface materials, and the NO densities are much higher than the room temperature measurements.No O 3 is observed, and thus, the ozone-induced NO loss channels are suppressed.Whereas the densities of NO 2 and N 2 O remain almost constant.Only Fe is different, with lower NO densities and higher NO 2 densities.This could be explained by the very inefficient ozone destruction by the Fe catalyst.• glass: The normal glass plate is used as a reference.No ozone is produced because it dissociates easily at high temperatures.The production of NO is much higher than that at room temperature.The trend in NO production with O 2 :N 2 mixture ratio is similar to that observed at room temperature.However, NO is still efficiently produced even at high O 2 concentration in the feed gas.The production of NO 2 increases with the O 2 :N 2 mixture ratio due to NO oxidation.• sandblasted glass: In the case of sandblasted glass, the NO production is very high, and no O 3 is produced even at a high O 2 admixtures.As discussed by Meyer et al [42], the sandblasted glass surface provides more reactive sites such as oxygen vacancies where O 3 readily adsorbs dissociatively.This may lead to the so-called zero ozone effect [45], where commercial DBD ozonizers show an extreme drop in ozone generation in the absence of N 2 .Little admixtures of N 2 can suppress the zero ozone effect because N atoms from the dissociation in the plasma can block these active sites in typical DBD plasmas.Without O 3 , the dominant loss channel for NO reacting with O 3 is suppressed, and NO formation is maximized.In our experiment, we observe the disappearance of O 3 even for N 2 containing discharges.Therefore, we assume that the significant helium dilution and the associated activated species, such as helium metastables, might cause a plasma-induced species desorption from the surface so that a finite surface density of active sites is always present.• Pt catalyst: It is known that the decomposition of O 3 is enhanced in the presence of a Pt catalyst at high temperatures [46].Thereby, the loss channel of NO with O 3 is suppressed, and NO generation is maximal [47].The removal of O 3 in surface processes with the glass substrate seems as efficient as with the Pt catalyst.A slight difference can be observed since the NO densities are a bit smaller than for the sandblasted surfaces, whereas the NO 2 production is higher.This difference might be caused by (i) a slightly • Fe catalyst: NO generation using a Fe catalyst is inefficient.O 3 is observed at very high oxygen admixtures, although much less than the room temperature experiments.The Fe catalyst is much less efficient in decomposing O 3 , consistent with the literature [49].Consequently, NO is quickly converted into NO 2 and N 2 O 5 .The NO 2 production is at its maximum at 50% oxygen percentage.
The temperature we use is still not high enough to activate the catalyst.However, the different results of Fe and Pt indicate that the catalyst has an impact on the decomposition of ozone and not only the enhanced temperature.It should be mentioned that the analysis of the N 2 /O 2 chemistry in our setup is based on data acquired in the multi-pass cell.During the transport from the plasma channel to the long-path cell, NO x species might further react with long-living species such as ozone in the plasma effluent.Therefore, the presented NO densities can be regarded as a lower bound.On the other hand, the applications of these plasmas are often based on treatment in an effluent, where the same reactions of ozone consumption via NO oxidation occur.Therefore, the presented data can be used to assess the plasma performance and chemistry.

Optimum design for selective low temperature NO production
The dominant reaction scheme is summarized in a sketch as shown in figure 12. NO is predominantly created from N 2 and O 2 via the Zeldovich mechanism.Similarly, ozone is generated from O 2 and O recombination.We regard the initial dissociation reactions to be dominated by electron impact in the plasma phase, so a direct impact of temperature or the surface is of minor importance.The consecutive steps, however, depend sensitively on the temperature and surface state: (i) At room temperature (figure 12(a)), the surfaces are rather nonreactive and most of the O 3 react with NO to create NO 2 or even further to N 2 O 5 species; (ii) at elevated temperature of 120 • C, the surfaces become active by decomposing O 3 into O 2 at active sites.These could be oxygen vacancies at the glass substrates or directly at the surface of the metal catalyst.The efficiency of Pt is much larger than that of Fe, as known from the literature.The ability of the glass surface to inactivate O 3 is unusual, especially at finite nitrogen concentrations.Here, we assume that excited He species from the plasma continually created oxygen vacancies, which allows the incident O 3 to decompose.In our experiment, NO is most efficiently produced at an N 2 :O 2 ratio of 70:30 under 120 • C when using sandblasted glass.The conversion rate of NO (α NO ) is 0.085%, which is calculated by [NO] and [N 2 ] are the concentrations of NO and N 2 , respectively.Consequently, an optimum NO source based on He diluted RF plasma jets should be operated at high N 2 :O 2 ratios.A higher power will enhance NO formation and suppress O 3 formation inside the plasma at the expense of atomic oxygen.However, O 3 will then form in the effluent.Instead, the surface temperature should be at least 120 • C to provide a reactive surface for O 3 decomposition.Thereby further oxidation of NO to NO 2 or N 2 O 5 is suppressed, and the NO concentration is at its optimum.The efficiency for NO conversion is at 0.1%.The surface could be either glass providing oxygen vacancies or a catalyst for O 3 decomposition such as Pt.Following this guideline, an optimal and selective source for NO is realized.

Conclusion
In this work, the concentration of nitrogen oxide (NO, NO 2 , N 2 O and N 2 O 5 ) and ozone generated in an atmospheric pressure RF He/N 2 /O 2 discharge are measured by FTIR spectroscopy combined with a multi-pass cell.Gas mixture composition, plasma power, surface material (catalyst), and temperature are varied to find an optimum design for NO generation.
Our experiments reveal that NO itself is not generated by any plasma catalysis synergisms, but instead a dominant NO destruction channel by O 3 is suppressed by a catalytic process for ozone destruction either on the metal catalyst or at oxygen vacancies on glass, which are continuously created by active species from the plasma.When using an admixture of synthetic air (N 2 :O 2 = 80:20) to the helium plasma and gradually increasing the plasma power, O 3 is first produced, and oxides NO to NO 2 or N 2 O 5 .With increasing plasma power, the concentration of O 3 goes to zero, and NO is produced.When fixing the plasma power and changing the ratio of N 2 :O 2 from 90:10 to 10:90, the concentration of O 3 increases, NO is converted to NO 2 , and further to N 2 O 5 .A Fe catalyst is the most ineffective in NO production.The production of N 2 O remains constant.
When increasing the temperature to 120 • C, the concentration of O 3 and N 2 O 5 significantly decreases, and the production of NO is more efficient than that at room temperature.The NO concentration monotonically increases with plasma power while NO 2 and N 2 O do not show a significant change.The most abundant product is NO even at O 2 rich conditions.
The production of NO is highly related to the reaction with ozone.At room temperature, the catalysts are non-reactive and the product NO is oxidized to NO 2 and N 2 O 5 by O 3 .With a higher temperature of 120 • C, O 3 is quickly decomposed on the surface or dissociated in reactions with H 2 O. Thus, NO x has a higher yield.
The advantage of the source is the controlled production of NO (in N 2 and O 2 ) with no other reactive byproducts such as O 3 .Such sources may be beneficial for biomedical applications, where on-demand NO production is required.In our experiment, the products generated by the plasma source travel through a long tube to the FTIR, which has similarities to the use of NO as a selective pulmonary vasodilator in medicine [50].To optimize the production, the essential part is the enhancement of ozone decomposition, thus inhibiting NO oxidation.However, the helium-diluted plasma may not be a preferred method to produce NO only since methods to separate the produced NO from N 2 and O 2 would be required.

Figure 1 .
Figure 1.Cross-section of the experimental setup (a) along the gas flow, (b) perpendicular to the gas flow, and (c) long multi-pass cell from sampling small concentrations via FTIR absorption.

Figure 2 .
Figure 2. Optical microscopy images of different dielectric surfaces.
25 • C. The spectra of NO (dominant peak at 1876 cm −1 ), NO 2 (at 1618 cm −1 ), N 2 O (at 2224 cm −1 ), O 3 (at 1042 cm −1 ) and N 2 O 5 (at 743 cm −1 ) are fitted by the script, and the concentrations in ppm are calculated.The production and oxidation processes of NO are investigated by using gas admixtures to a flow of 250 sccm helium at different N 2 :O 2 mixture ratios.The typical survey and background FTIR spectra at a mixture ratio of O 2 :N 2 = 40:60 (black) and of O 2 :N 2 = 60:40 (orange) are shown in figure 4. The background spectrum (grey curve in figure 4) is taken after purging the multi-pass cell with helium for more than 40 min.The appearance of CO 2 and H 2 O in the spectra during discharge originates either from absorption in the external path of the IR beam or from impurities in the admixed gases.Due to the long path length in the multipass cell, a slight change in the background gas will show on the spectrum.The 'negative absorption' is related to the decrease of the CO 2 content in the external beam path.The absorption peaks of the products, i.e.N 2 O 5 , N 2 O, NO 2 , NO, O 3 and N 2 O 5 , are observed.

Figure 4 .
Figure 4. Survey spectrum as the ratio between the IR spectrum before and after plasma ignition of N 2 :O 2 plasma discharges for different N 2 :O 2 mixture ratios, as indicated (black and orange curves).Background spectrum for pure helium (grey curve).

Figure 6 .
Figure 6.NO, NO 2 , and N 2 O concentration as a function of the humid air admixture with Φ He = 250 sccm and 6 W plasma power at room temperature and using glass surfaces.

Figure 7 .
Figure 7. Product concentration as a function of the plasma power measured at room temperature using glass surfaces and a flow of Φ He = 250 sccm and with an admixture of 0.7% synthetic or humid air.

Figure 9 .
Figure 9. Product concentration as a function of O 2 :N 2 mixture ratios at 6 W plasma power at room temperature using different surfaces (normal glass, sandblasted glass, Fe and Pt), Φ He = 250 sccm and Φ O2 + Φ N2 = 2.25 sccm.

3. 6 .
Figure11shows the N x O y concentration at 120 • C using different N 2 :O 2 mixture ratios in the feed gas.Again, the N 2 O production is not affected by the different surfaces at 120 • C, indicating that the concentration of oxygen atoms is not affected by the surfaces and reaction (R7) dominates N 2 O production.However, the densities of NO and NO 2 are strongly affected by the catalyst, as discussed in the following: