N2O Formation Characteristics in Dielectric Barrier Discharge Reactor

Nov 2, 2017 - Department of Environmental Engineering, School of Energy and Environmental Engineering, University of Science and Technology Beijing, B...
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N2O Formation Characteristics in Dielectric Barrier Discharge Reactor for Environmental Application: Effect of Operating Parameters Xiaolong Tang, Jiangen Wang, Honghong Yi,* Shunzheng Zhao, Fengyu Gao, Yonghai Huang, Runcao Zhang, and Zhongyu Yang Department of Environmental Engineering, School of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, China ABSTRACT: The present study is devoted to the investigation of N2O formation characteristics in dielectric barrier discharge (DBD) reactor for environmental application. The effect of operating parameters, such as specific energy density (SED), O2 concentration, NO initial concentration, and residence time on N2O concentration was investigated in dry O2/N2 mixtures. N2O formation from bulk gas (O2 and N2) is inevitable in DBD chemical process and was observed in all DBD reactors. N2O concentration shows an exponential increase or decrease with the variation in operating parameter. The experimental results show that N2O concentration first increases and then reaches saturation with the increase in SED and O2 concentration, respectively. N2O concentration shows a monotonic increase with increasing residence time. N2O concentration decreases with NO concentration increasing from 50 ppm to 600 ppm. The N2O formation is enhanced with γ-Al2O3 pellets packing into the DBD reactor compared with that of DBD reactor alone. NO2 + N → N2O + O

1. INTRODUCTION Nonthermal plasma (NTP) generated via dielectric barrier discharge is an excellent source of chemically active species. NTP has been extensively studied in environmental application in the past decades, such as nitrogen oxides (NOx) removal1,2 and volatile organic compounds (VOCs) removal.3−5 However, low energy efficiency, poor product selectivity, and undesirable byproducts (such as O3, CO, and N2O) are serious roadblocks for industrial application. These byproducts can bring new environmental problems. Ozone has be considered as an inevitable product of DBD process in dry air. Because there is no cooling system in most DBD reactors for environmental application, even a relatively small heating and humidity lead to ozone decomposition.6 The CO generated from incomplete oxidation of VOCs may be eliminated by combining with catalysts.7−9 N2O is environmentally important in two quite distinct respects: (i) a potential contributor to the ozone depletion;10 (ii) one kind of greenhouse gases with a high capacity of absorbed infrared radiation that is about 300 times greater than that of CO2.11 However, little attention has been paid to N2O formation characteristics during DBD process. Investigation of NTP chemistry, even for a system as simple as O2/N2, requires a reaction mechanism involving many species (ions, atoms, radicals, and molecules), including excited states and reaction products. Kinetic analysis12−14 indicate that the first excited state of molecular nitrogen N2(A) might play a role in evolution of N2O in NTP chemical process. Herron12 evaluated the chemical kinetics date for gas phase reactions of the first excited state of molecular nitrogen N2(A). N2(A) plays a role in both production and loss of N2O via R1 and R2, respectively, with rate constants in units of 1 × 10−12 cm3 molecule−1 s−1. N2(A) + O2 → N2O + O

(R1)

N2(A) + N2O → 2N2 + O

(R2)

© 2017 American Chemical Society

(R3)

A kinetic scheme for NTP in O2/N2 mixtures was developed to investigated the relevant intermediate species involving a mechanism of ∼450 reactions.14 On the basis of kinetic modeling proposed, R1 and R3 are the dominant reactions to form N 2 O, with the rate coefficients 7.8 × 10 −14 cm3 molecule−1 s−1 and 3 × 10−12 cm3 molecule−1 s−1, respectively. The electronically excited oxygen atom, O(1D), is another species that may be contributed to N2O evolution during NTP process. N2O can be formed via15 N2 + O(1D) → N2O

(R4)

or be oxidized via16 N2O + O(1D) → NO + NO

(R5)

N2O + O(1D) → N2 + N2

(R6)

The reaction rate coefficient of R4 is 3.5 × 10−37 cm6 molecule−2 s−1,15 using gas density of 2.5 × 1019 cm−3 it is 9 × 10−18 cm3 molecule−1 s−1, whereas a competing reaction route resulting in the quenching of O(1D) (N2+O(1D) → O(3P)+N2) has a reaction rate coefficient of 2.5 × 10−11 cm3 molecule−1 s−1. Therefore, reaction R4 is possible but rather not kinetically favored. The reaction rate coefficients of R5 and R6 are 6.71 × 10−11 cm3 molecule−1 s−1 and 4.40 × 10−11 cm3 molecule−1 s−1, respectively.16 It seems that O (1D) play a role on the conversion of N2O. The first two electronically excited nitrogens, N(2D) and N(2P), also play an important role on N2O conversion via R7 and R8, with rate constants in units of 1 Received: August 18, 2017 Revised: November 2, 2017 Published: November 2, 2017 13901

DOI: 10.1021/acs.energyfuels.7b02428 Energy Fuels 2017, 31, 13901−13908

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Energy & Fuels × 10−12 cm3 molecule−1 s−1 and 1 × 10−14 cm3 molecule−1 s−1, respectively.12 N(2 D) + N2O → N2 + NO

(R7)

N(2 P) + N2O → N2 + NO

(R8)

The DBD reactor consists of two coaxial quartz tubes, the outer quartz tube is covered by a copper mesh electrode over a length of 18 mm. The inner quartz tube was inserted with a 6 mm diameter stainless steel rod. The discharge gap is fixed at 2 mm with a total discharge volume of 1.5 mL. Simulated gas mixture and flow rate were controlled by mass flow controllers (MFC). For the plasma-catalytic reactor, 0.85 g of catalyst was used. The discharge zone was filled with catalyst pellets. The voltage Vm across the probe capacitor Cm is proportional to the charge crossing the electrodes. The measured operating voltage and charge values are plotted against each other in a V−Q cyclogram, which is better known as a Lissajous figure. The discharge power (P) of the DBD reactor was calculated from the enclosed area of a V−Q Lissajous figure.29 The SED in joules/liter (J/L) was calculated using the formula

Much research has been performed for NOx removal using NTP-assisted catalysis,1,2,17,18 as well as NTP alone;19,20 however, the N2O formation was not discussed. In several research studies on NOx removal, N2O was detected, but its concentration was negligible.21−23 Zhu et al.,22 Toshiaki et al.,23 and Penetrante et al.24 all claimed that NTP alone converts NO to NO2, and then part of NO2 will be reduced by N to form N2O via R3. The discussion, however, did not include the reaction N2(A) + O2 → N2O + O, which appears to be the dominant N2O production source. The main drawback of NTP for VOCs abatement is the formation of byproducts (NOx, N2O, CO, and intermediates) because of its nonselectivity. Combining NTP with catalyst offers great possibility to improve the mineralization degree and CO2 selectivity. However, the formation of NOx and N2O is still a problem for VOC abatement using NTP.25 Yu et al.26 claimed that the conversion of NOx to N2O decreases with the discharge power increasing, and the N2O formation increases with increase of O2 concentration. In our previous study, N2O is observed as byproduct during NTP-assisted catalysis27 and NTP alone28 for NO removal. The N2O formation increases with the increases of oxygen concentration and discharge power; however, the N2O formation characteristics are not systematically studied. Gaseous pollutant removal is the major applications of DBD reactor regarding environmental issues. In the DBD process, the bulk gas molecules are N2 and O2. The present study aimed to investigate the influence of operating parameters such as specific energy density (SED), oxygen content, NO initial concentration, residence time and catalyst on N2O formation characteristics in the O2/N2 plasma. The DBD reactor was operated at atmospheric pressure and ambient temperature. To the extent of our knowledge, this is the first work about the effect of operating parameters on the formation of N2O.

SED (J/L) =

P Q

where the P is the discharge power calculated from V−Q Lissajous figure (J/s) and Q is the flow rate of gas in L/s. 2.2. Catalyst Preparation. Single metal oxide catalyst of CoOx/ Al2O3, CuOx/Al2O3, and MnOx/Al2O3 were synthesized by incipient wetness impregnation method. Commercial γ-Al2O3 powder was impregnated with an aqueous solution of manganese(II) nitrate or cerium(III) nitrate or copper(II) nitrate with a metal loading of 5 wt %. The impregnation was carried out under ultrasound for 2 h. The impregnation was then dried at 383 K and calcined at 823 K under air for 5 h. Finally, all catalyst samples were crushed and sieved to a 20− 40 mesh size. 2.3. Chemical Analysis. The N2O concentration was analyzed online by Nicolet iS50 Fourier transform infrared (FTIR) spectrometer (Nicolet iS50, Thermo fisher scientific, America, path length is 2.4 m) operation in the adsorption mode, with a resolution of 4 cm−1. Characteristic features were obtained for NO2 in the spectral range 1660−1540 cm−1, for NO in the range 1980−1860 cm−1, for N2O the range in 2270−2140 cm−1, and for O3 the range was 1080− 940 cm−1. The oxygen content is detected by flue gas analyzer (Kane, KM9106).

3. RESULTS AND DISCUSSION 3.1. N2O Formation of DBD Reactor Alone. NTP generated by DBD is a multicomponent system, containing a large number of excited atoms and molecules, active atoms and molecules, and electrons. Changing parameters of DBD system (SED, gas component and residence time) permit control and change plasma-chemical process. The effects of SED, oxygen content, NO initial concentration, and residence time on the N2O formation has been investigated in this section. 3.1.1. Effect of SED. The effect of specific energy density (SED) on N2O formation characteristic was investigated by varying SED from 750 to 3400 J/L with the absence of NO at different oxygen content. The flow rate was fixed at 600 mL/ min. The specific energy density (SED), in units of joules per liter (J/L), is used to estimate the energy dissipated into the gas during DBD process, which is the most important parameter in plasma. The N2O concentration was detected at the outlet of DBD reactor after it operating for 20 min. N2O concentration as a function of SED is shown in Figure 2. It is clear that N2O outlet concentration increases first with SED increasing from 750 to 1500 J/L, and then the fluctuation of N2O concentration is negligible with further increase of SED more than 1500 J/L. It seems that there is a saturation for N2O formation for the SED in the range of 1500 to 3500 J/L. The reason for this may be that the increase of SED not only improves N2O formation via the reaction R1 but also N2O loss by R2 and R5−R8. N2O concentration and the amount of

2. EXPERIMENTAL SETUP 2.1. DBD System. As shown in Figure 1, NTP was obtained in a coaxial cylinder-type DBD reactor energized by a high-voltage power supply (CTP-2000k, Nanjing Suman Electronics Co., Ltd., China).

Figure 1. Schematic of the experimental system. 13902

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Figure 2. N2O concentration as a function of SED.

reactive species (O, N, and N2(A)) both increase with the SED increasing from 750 to 1500 J/L. The increase of N2O concentration will raise the probability of reaction between N2O and reactive species which are contributed to N2O loss. The proportion of different species are also changed with the increase of SED, which will further influence the N2O formation. The increase in SED indicates that more energy may be used to dissociate N2 and O2 molecules and more reactive species would be generated, such as N2(A), O(1D), N(2D), and N(2P). N2O can be formed by reactions of metastable nitrogen N2(A) with O2;14 on the contrary, this reactive species also play an important role in N2O loss.30 For a fixed ratio of O2/N2, the density of N2(A) and O radical increase obviously with the increase of SED. With the increase in N2O concentration and O(1D) density, the N2O can be eliminated via R5 and R6 by O(1D). The evolution of N2O in DBD process depends on the combined action of R1−R8. Equilibrium may be established between N2(A), O2, N, O(1D), and N2O with SED increasing more than 1500 J/L. NOx and O3 concentration as a function of SED are shown in Figure 3. In the absence of initial NO, the NOx is formed when the SED is over 1500 J/L, and the O3 is disappeared with the SED more than 1500 J/L. For a fixed SED, the NOx and O3 concentration increase with the increase of oxygen content. It is generally known that the dissociation energy of oxygen is lower than that of nitrogen. It appears that O2 dissociation occurs first, and then the O3 is formed. The NO synthesis in NTP is controlled by an endothermic step involving vibrationally excited nitrogen molecules (N2*(vib)): O + N2*(vib) → NO + N, following by a secondary exothermic process: N + O2 → NO + N, known as the Zeldovich mechanism.6 Because of the endothermic nature of this reaction and high dissociation energy of N2 (9.77 eV), it is necessary to realize that this plasma process requires highly energy input. Therefore, the NOx can be only observed when the SED is more than 1500 J/L. The reason why O3 concentration decreases with the increase in SED is because of the increase in temperature of DBD reactor, which will increase the decomposition of O3. With the presence of NO, O3 formation can also be deteriorated because of the catalyzed decomposition reaction with NO. 3.1.2. Effect of Oxygen Content. For the combustion exhaust, the oxygen content is typically 5−15%. The oxygen content can reach to 21% in some DBD applications for VOCs

Figure 3. (a) NOx concentration and (b) O3 concentration as a function of SED at different oxygen content.

removal. The flow rate of gas mixtures was fixed at 600 mL/ min. The effect of O2 concentration on N2O formation was investigated by varying the O2 concentration from 0 to 21%, the results are shown in Figure 4. The N2O concentration increases with oxygen content increasing form 0 to 21%.

Figure 4. N2O yield as a function of oxygen content at different SED with and without initial NO. 13903

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Energy & Fuels Remarkable increase of the N2O concentration occurred with O2 concentration increasing from 0 to 10%. The increase of N2O concentration is not obvious with O2 concentration increasing from 10 to 21%. In the 0 ppm of NO/1000 J/L system, the N2O concentration for different O2 concentrations are 27.4 ppm for 3%, 43 ppm for 6%, 50.3 ppm for 10%, and 53.6 ppm for 21%, respectively. It seems that there is a plateau for N2O formation with O2 concentration increasing form 10 to 21%. O2 and the reactive oxygen species play an important role in both production and loss of N2O in NTP. A higher oxygen content in the gas streams result in a higher N2O productivity via reaction R1. However, oxygen also has an adverse effect on N2O formation due to its electronegative characteristics, physical quenching of N2(A), and abatement of N2O. Therefore, with the increase of O2 concentration, more O would be formed, and R4 and R5 become gradually dominant,31,14 which would result in the decrease in N2O formation rate. In general, the N2O is considered as a byproduct of NO removal using DBD reactor. NTP convert NO to NO2, then part of NO2 will be reduced by N to form N2O via reaction NO2 + N → N2O + O. However, as shown in Figure 4, remarkable drop in the N2O concentration was observed as a fraction of initial NO was added into the mixture gas. It confirms that the N2O was formed from bulk N2 and O2 molecule via R1. It is obvious that R1 is the dominant N2O production source for two reason: (i) the concentration of O2 molecules is much higher than that of O atoms; (ii) the probability of reaction R1 is higher than R4. In conclusion, the evolution of N2O in NTP is determined by the reaction involving N2(A) and O(1D) at the SED lower than 1500 J/L. With the increase of O2 content, the probability of O(1D) formation is also increased, on the contrary, N2(A) concentration is decreased due to the physical quenching by O2.32 In addition, the quenching rate of N2(A) by O2 increase with the increase in O2 content. Therefore, the production rate of N2O become slow with the increase of O2 concentration more than 10%. 3.1.3. Effect of Initial NO Concentration. N2O generally considered as a byproducts of NO reduction33 and conversion.24 The effect of initial NO concentration on the N2O formation was investigated with oxygen content fixed at 6%. The N2O concentrations as a function of NO concentration are shown in Figure 5 for different SED. The NO concentration has somewhat effect on the N2O formation. At a fixed SED, the N2O concentration decreases with increase of NO initial concentration. The reason for this may be that the reactive species contributed to N2O formation decrease with the NO molecule feeding into DBD reactor. N2(A) and O(1D) play an important role on the evolution of N2O in the NTP chemistry via R1−R6. N2(A), the lowest electronically excited state of N2, has been invoked as a possible energy reservoir in plasma systems. The role of electronically excited species N2(A) in plasma chemical systems has long been of interest, however, the importance of such intermediates within the overall mechanism is still poorly understood. Young et al. indicated that the N2(A) plays an important role in the formation of the excited state of NO.34 N2(A) + NO(X2Π) → N2(X1Σ+g ) + NO(A2Σ+)

Figure 5. N2O yield as a function of NO initial concentration

N2O formation from R1 decrease because of the competition between R7 and R1 for the N2(A). Reaction NO + O → NO2 are rapid reaction which efficiently consume O. O(1D) is the main reactive species for N2O abatement. It seems that the decrease in N2(A3Σu) has more effect on the evolution of N2O compared with the decrease in O. 3.1.4. Effect of Residence Time. The residence time will affect the extent of the reaction in chemistry process. Thus, the N2O formation of DBD reactor with varying residence time was investigated, the results are shown in Figure 6. The residence

Figure 6. N2O yield as a function of residence time.

time is calculated by (volume of the reactor)/(flow rate). The residence time of 0.1, 0.15, 0.3, 0.6, and 0.12 s were achieved by changing the gas feed flow rate to 900, 600, 300, 150, and 75 mL/min, respectively. Two different concentrations of O2 with and without initial NO were investigated. The results in Figure 6 show that the N2O concentration increases with increasing residence time and increasing oxygen content, and decreases with NO feeding into DBD reactor. Longer residence time results in a higher N2O concentration because of longer reaction time and the higher SED per unit volume of the feed gases. Supplying a higher SED per unit volume gives high mean

(R9)

According to the mechanism presented in R9,35 the N2(A) concentration decreases with increasing NO concentration. 13904

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The Al2O3 exposed to the discharge is a favorable surface for oxygen atom recombination.39 U. Roland et al.40 proposed that Al−O−O* aluminum peroxyl structure is formed during plasma catalysis process in the presence of O2 using electron paramagnetic resonance (EPR). The enhancement of N2O formation of PBR is mainly due to the formation of Al−O−O*. 3.2.2. Effect of Oxygen Content and NO Initial Concentration. the effect of oxygen content and NO concentration on the N2O formation in NTP alone has been investigated in the section above. In this section, the effect of the two factor on the N2O formation will be investigated in the PBR reactor. As shown in Figure 8, N2O concentration shows a

energy to electrons to produce more reactive species, which leads to an increase in the formation of N2O. The long residence time not only increase the extent of R1 but also the extent of R2, R5, and R6. The formation rate of N2O decreases with the increase of residence time. The reason for this maybe that the increase of N2O concentration will bring more O(1D) into the R4 and R5, which will cause the decrease in the N2O formation rate. 3.2. N2O Formation of NTP-Assisted Catalysis Process. NTP assisted catalysis reactors, constructed by packing catalytic dielectric pellets inside NTP reactors, have been considered as an effective method to alleviate the major bottleneck encountered by NTP, i.e., low energy efficiency, poor products selectivity and undesirable byproducts. NTP-assisted catalysis was regarded as an emerging technology by combing the high reactivity of the plasma with the high selectivity of the catalyst. The chemical processes NTP-assisted catalysis are manifold, such as bulk plasma chemistry, surface interaction and heterogeneous catalytic chemistry. Catalysts are regarded as one of the most important factors to determine the reaction performance of a plasma catalysis process. Different catalysts including γ-Al2O3,21 TiO2,36 MnOx,37 and zeolite38 have been shown to enhance the performance of plasma catalysis process. The γ-Al2O3 with particles size of 0.45−0.90 mm were used in this study. 3.2.1. N2O Formation in the DBD Reactor Packed with γAl2O3. DBD reactor packed with γ-Al2O3 was used to investigate the difference in N2O formation compared with NTP alone. Figure 7 shows the N2O concentration as a

Figure 8. N2O yield of PBR as a function of oxygen content.

remarkable increase with the oxygen content increasing from 0 to 6%; however, fluctuation of N2O concentration between 6, 10, and 21% are negligible. The trend is similar to that in NTP alone. For a fixed SED at 950 J/L, the N2O concentration of PBR (O2/N2) for different O2 content are 89 ppm for 6%, 93 ppm for 10% and 92 ppm for 21%. The amounts of plasma species are changed with increase of oxygen content: (i) the amount of N2(A) decrease due to the quenching by O2, (ii) the production of O(1D) and Al2O3−O* increased with the increase in oxygen content. Al2O3−O* and N2(A) are the dominate N2O formation source via R10. N2O can be eliminated via R5 and R6 by O(1D). It seems there is chemical equilibrium between the plasma species which are estimated to contribute to the formation and reduction of N2O. The N2O concentration of NO/O2/N2 system is smaller than that of O2/ N2 system at fixed oxygen content and SED. The reason for this may be the effective quenching of N2(A) by NO. 3.2.3. Effect of Different Catalyst (MOx/Al2O3, M = Mn, Cu, and Co). A number of efforts have been made to improve the energy efficiency and products selectivity of plasma catalysis process for environmental application. Porous materials like γAl2O3 coated or impregnated with transition metal oxides such as MnOx, CuOx and CoOx have been used as the catalyst. MnOx/Al2O3 as a catalyst has been used in plasma catalysis process for both NO oxidation41 and VOC decomposition.7,9,4 In this section, a series of γ-Al2O3 supported transition metal oxides (MnOx, CuOx, and CoOx) catalysts were prepared, and their catalytic performances of N2O formation were compared.

Figure 7. N2O yield of PBR as a function of applied power.

function of SED for packed bed reactor (PBR). There is a marked increase of N2O concentration of PBR compared to NTP alone. The reasons could be the changes in discharge behavior and presence of surface reaction on the surface of γAl2O3. As shown in Figure 7, the N2O was even detected in the PBR system feeding with pure nitrogen (99.99%). The reason for this is that the presence of γ-Al2O3 in the discharge zone will bring the surface oxygen species (Al2O3−O*) into the plasma chemical process. N2(A) + Al 2O3‐O* → N2O + Al 2O3

(R10)

where * represents an active site on the catalyst and O* represents atomic oxygen bound to the site. 13905

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Energy & Fuels XRD measurements were obtained to confirm the crystal structure of three support transition metal oxide catalysts, as show in Figure 9. After MnOx or CoOx doping, although the

Figure 11. N2O yield of PBR packed with different catalyst.

catalyst MnOx/Al2O3 is obviously higher than that of the other catalysts. In the plasma catalysis system, active oxygen species, including O2, O atom, and O*, are main oxygen source for the formation of N2O. The O2 concentration in this section was fixed at 6%. The amount of atomic O generated form collisions between excited species and O2 molecules is mainly dependent on SED, whereas the amount of O* is mainly dependent on the nature of the catalysts. Because the fluctuation of SED between different catalysts is not obvious, the increase in N2O concentration for different catalysts may be due to the increase in O* formation. There are two possible pathways to produce O*. On the one hand, O* can be generated from collisions between excited species and catalyst. One the other hand, O* can be also generated from “in situ” decomposition of ozone on catalyst surface via the following reaction:42,43

Figure 9. XRD patterns of the supported metal oxides catalyst.

structure of γ-Al2O3 remained intact, the intensities of all these peaks decreased dramatically, implying interaction between γAl2O3 and MnOx or CoOx. The sample MnOx/Al2O3 may be assigned to MnO2 (JCPDS 24−0735) with the peaks at 28.9, 37.6, 42.0, 56.4, and 72.7°. The sample CoOx/Al2O3 may be assigned to Co3O4 (JCPDS 42−1467) with the peaks at 17.2, 31.3, 36.8, 59.4, and 65.2°. The diffraction peaks of CuOx/ Al2O3 are very weak and do not show intense or sharp peaks of copper oxides. For a fixed applied power, the SED and discharge power for different supported metal oxides was nearly the same (Figure 10). It seems that supported metal oxides did not change the

O3 + Catalyst → O2 + Catalyst−O*

(R11)

The ozone decomposition activities of these catalysts decreased in the following order: MnO2/Al2O3 > CuO/Al2O3 > Al2O3.9 This is consistent with the observation in Figure 11 that the MnO2/Al2O3 had highest N2O concentration among the catalysts studied in this work. Therefore, combining the high oxidation capacity of supported metal oxides catalyst with NTP may not only improve the performance of plasma catalysis process but also increase the production of byproduct N2O. 3.3. Calculation Analysis and Mechanism Discussion. The effects of various parameters such as SED, O 2 concentration, NO initial concentration, and residence time on the N2O concentration of DBD reactor have been experimentally investigated in section 3.1. N2O concentration as a function of these parameters were calculated using exponential function: [N2O] = αexp(X/τ) + β, where [N2O] is the outlet concentration of N2O; α, β, and τ are coefficient depended on parameters X. N2O concentration shows an exponential increase with the increase in SED, O2 content, or residence time. The increasing rate of N2O concentration decreases with the increase in these three parameters. The main reason for this may be that the increase in N2O concentration will conversely enhance the probability of N2O loss reaction route. There is an exponential decline in N2O concentration with increasing NO initial concentration due to the competition of consumption of N2(A), which is the most

Figure 10. Discharge power of PBR packed with different catalyst. (MOx/Al2O3, M = Mn, Cu, and Co).

physical properties of catalyst supports.21 The different performance of the catalysts may be attributed to the difference in the surface property of catalysts. Figure 11 exhibits the effect of packing supported metal oxides on the formation of N2O as a function of SED. N2O concentration increase linearly with SED increasing from 900 to 1800 J/L. The N2O concentration of different catalysts are ordered as MnOx/Al2O3 > CuOx/ Al2O3 > Al2O3 > CoOx/Al2O3. The N2O concentration of the 13906

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supported by National Natural Science Foundation of China (21677010, U1660109).

important species for N2O formation in the NTP process. N2O concentration shows a monotonic increase with the increasing of residence time from 0.1 to 1.2 s. The increase in residence time is able to improve both the reaction extent of N2O formation and N2O loss. There is a saturation for N2O formation with the X increasing to a point of 1500 J/L for SED and 10% for oxygen content. The evolutions of plasma species during DBD processes are complex and nonselective. The amounts of plasma species such as N2(A), O(1D), N(2D), and N(2P) increase with the increase of SED. Different plasma species play a distinct role on the evolution of N2O: (i) N2(A) and O2 for N2O formation; (ii) O(1D), N(2D), N(2P), and N2(A) for N2O loss. The analysis showed that the increase of these two kinds of plasma species is responsible for the saturation of N2O formation in the DBD reactor at high SED values and high O2 concentration. There is a complex interaction between these different plasma species. The effects of SED, O2 concentration, and NO initial concentration on the N2O concentration of PBR have been experimentally investigated in section 3.2. N2O concentration as a function of SED and O2 concentration were calculated using exponential function [N 2 O] = αexp(X/τ) + β, respectively. The N2O concentration increases monotonically with SED increasing from 900 to 3500 J/L, which is different with that of DBD reactor alone. The reason for this may be that the O* species adsorbed on the catalyst may increase with the increase in SED, which will further enhance N2O formation. However, how catalysts enhance the N2O formation and the inhibition of N2O formation in plasma catalysis need to be further studied.

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4. CONCLUSIONS N2O formation from bulk gas (O2 and N2) is inevitable in DBD chemistry. The reaction of N2(A) with O2 (N2(A) + O2 → N2O + O) are the dominant process for N2O formation during DBD process. The coexistence of plasma species both for N2O formation and N2O loss leads to a saturation of N2O formation. N2O concentration increase with the increase in SED, O2 concentration or residence time, respectively. NO molecule is an excellent source for physical quenching of N2(A), which would decrease the N2O formation in DBD reactor. N2O formation of DBD reactor packed with γ-Al2O3 pellets has obviously been enhanced comparing with that of DBD reactor alone. The reason for this may be the formation of O* which can react with N2(A) to form N2O. Many other studies should be done to optimize plasma-assisted catalysis process and eliminate the N2O formation of this technology.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.Y.). ORCID

Xiaolong Tang: 0000-0003-3130-3883 Honghong Yi: 0000-0002-0097-9007 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was primarily supported by National Key R Program of China (2017YFC0210303-01). This work was also partly 13907

DOI: 10.1021/acs.energyfuels.7b02428 Energy Fuels 2017, 31, 13901−13908

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DOI: 10.1021/acs.energyfuels.7b02428 Energy Fuels 2017, 31, 13901−13908