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In Plasma Catalytic Oxidation of Toluene Using Monolith CuO Foam as a Catalyst in a Wedged High Voltage Electrode Dielectric Barrier Discharge Reactor: Influence of Reaction Parameters and Byproduct Control

Volatile organic compounds (VOCs) emission from anthropogenic sources has becoming increasingly serious in recent decades owing to the substantial contribution to haze formation and adverse health impact. To tackle this issue, various physical and chemical techniques are applied to eliminate VOC emissions so as to reduce atmospheric pollution. Among these methods, non-thermal plasma (NTP) is receiving increasing attention for the higher removal efficiency, non-selectivity, and moderate operation, whereas the unwanted producing of NO2 and O3 remains important drawback. In this study, a dielectric barrier discharge (DBD) reactor with wedged high voltage electrode coupled CuO foam in an in plasma catalytic (IPC) system was developed to remove toluene as the target VOC. The monolith CuO foam exhibits advantages of easy installation and controllable of IPC length. The influencing factors of IPC reaction were studied. Results showed stronger and more stable plasma discharge in the presence of CuO foam in DBD reactor. Enhanced performance was observed in IPC reaction for both of toluene conversion rate and CO2 selectivity compared to the sole NTP process at the same input energy. The longer the contributed IPC length, the higher the toluene removal efficiency. The toluene degradation mechanism under IPC condition was speculated. The producing of NO2 and O3 under IPC process were effectively removed using Na2SO3 bubble absorption.


Introduction
Volatile organic compounds (VOCs) emission and elimination have become hot issues in recent years. Large quantity of VOC emissions were produced by fugitive anthropogenic emission from industrial processes and chemical products [1]. Industrial processes like thermal power, coking, and iron smelt were often accompanied with NO x and SO 2 emissions [2,3]. In addition to the adverse health effect on the respiratory system and carcinogenesis risks [4][5][6][7][8], VOC emission contributed to regional air pollution [9,10]. VOCs were also major precursors of ozone (O 3 ) and secondary organic aerosols (SOA) [11][12][13]. Taking toluene as an example, smog chamber studies showed SOA production and yields were much higher under urban ambient air against that with purified air when toluene coexisted with NO x and SO 2 [14]. The presence of toluene also promotes aerosol nucleation and The DBD reactor was coaxial type with tubular quartz tube (>99.9% SiO2, dielectric constant: 3.75) as the discharge barrier, and with total length of 300 mm and inner diameter of 20 mm. Stainless steel mesh with a variable length of 5, 10, and 15 cm wrapped outside the quartz tube acted as ground electrode to achieve the discharge volume varying from 8.0, 16.0, and 24.0 cm 3 . A wedged stainless steel rod with a diameter of 14 mm was end-fixed along the axis of the cylinder and acted as a highvoltage electrode. The wedged configuration of the corona electrodes was 1 mm in height. More detailed illustration of the wedged high electrode was shown in Figure S1. As with the decrease of discharge gap showed a better VOCs removal performance [36], the discharge gap in this study was fixed at 3 mm.
The plasma discharge was driven by a high voltage alternating current (AC) power (sine wave, 5-20 kHz, 0-0 kV, Suman Co., Nanjing, China). The electro-parameters of the reactor were monitored by an oscilloscope (DS5062MA, Rigol, Suzhou, China). The consumed energy of the adjacent dielectric barrier was calculated from Lissajous figures, using a 1 μF capacitor inserted between the reactor and the ground. The discharge power added on toluene removal were valued by applied peak voltage and specific input energy (SIE, J/L), with peak voltage varied from 8 to 24 kV corresponding to SIE varying from 70.8 to 856 J/L (as shown in Table S1). The frequency of the AC power was set at a fixed frequency of 7.5 kHz according to the intrinsic character of the high voltage power supply (shown in Figure S2).

Preparation and Characterization of CuO Foam
CuO foam used in this study was prepared using a simple annealing process. Briefly, 3 mm thickness of copper foam (purity > 99.8%, Kunshan Jiayisheng Electronics Co., Kunshan, China) was manually rolled to a hollow cylindrical shape with different length and fixed external diameter of 20 mm. The tailored copper foam was then pretreated successively with 0.1 M HCl aqueous solution and ethyl alcohol upon ultrasonic vibration for 20 min to remove impurities. The thus-obtained copper foam was washed with deionized (DI) water and ethanol several times before it was dried at 105 °C for 12 h. Finally, the CuO foam was fabricated in situ over the copper foam through a heating process in a tubular furnace at 550 °C under an air atmosphere for 6 h with a heating rate of 5 °C/min from 25 °C. The thus-prepared monolith CuO foam catalyst can be easily filled and removed from the DBD reactor.
The grain morphology of the prepared CuO foam was investigated by field emission scanning electron microscopy (FESEM, Sirion 200, ThermoFisher, Hillsboro, OR, USA) with a resolution of 3.0 nm at 5 kV, 2.0 nm at 10 kV, and 1.5 nm at 15 kV. The phase structural characterization of CuO foam The DBD reactor was coaxial type with tubular quartz tube (>99.9% SiO 2 , dielectric constant: 3.75) as the discharge barrier, and with total length of 300 mm and inner diameter of 20 mm. Stainless steel mesh with a variable length of 5, 10, and 15 cm wrapped outside the quartz tube acted as ground electrode to achieve the discharge volume varying from 8.0, 16.0, and 24.0 cm 3 . A wedged stainless steel rod with a diameter of 14 mm was end-fixed along the axis of the cylinder and acted as a high-voltage electrode. The wedged configuration of the corona electrodes was 1 mm in height. More detailed illustration of the wedged high electrode was shown in Figure S1. As with the decrease of discharge gap showed a better VOCs removal performance [36], the discharge gap in this study was fixed at 3 mm.
The plasma discharge was driven by a high voltage alternating current (AC) power (sine wave, 5-20 kHz, 0-0 kV, Suman Co., Nanjing, China). The electro-parameters of the reactor were monitored by an oscilloscope (DS5062MA, Rigol, Suzhou, China). The consumed energy of the adjacent dielectric barrier was calculated from Lissajous figures, using a 1 µF capacitor inserted between the reactor and the ground. The discharge power added on toluene removal were valued by applied peak voltage and specific input energy (SIE, J/L), with peak voltage varied from 8 to 24 kV corresponding to SIE varying from 70.8 to 856 J/L (as shown in Table S1). The frequency of the AC power was set at a fixed frequency of 7.5 kHz according to the intrinsic character of the high voltage power supply (shown in Figure S2).

Preparation and Characterization of CuO Foam
CuO foam used in this study was prepared using a simple annealing process. Briefly, 3 mm thickness of copper foam (purity > 99.8%, Kunshan Jiayisheng Electronics Co., Kunshan, China) was manually rolled to a hollow cylindrical shape with different length and fixed external diameter of 20 mm. The tailored copper foam was then pretreated successively with 0.1 M HCl aqueous solution and ethyl alcohol upon ultrasonic vibration for 20 min to remove impurities. The thus-obtained copper foam was washed with deionized (DI) water and ethanol several times before it was dried at 105 • C for 12 h. Finally, the CuO foam was fabricated in situ over the copper foam through a heating process in a tubular furnace at 550 • C under an air atmosphere for 6 h with a heating rate of 5 • C/min from 25 • C. The thus-prepared monolith CuO foam catalyst can be easily filled and removed from the DBD reactor.
The grain morphology of the prepared CuO foam was investigated by field emission scanning electron microscopy (FESEM, Sirion 200, ThermoFisher, Hillsboro, OR, USA) with a resolution of 3.0 nm at 5 kV, 2.0 nm at 10 kV, and 1.5 nm at 15 kV. The phase structural characterization of CuO foam was further analyzed by X-ray diffractometer (XRD-6100, Shimadzu, Kyoto, Japan) using Cu Kα radiation, respectively. The XRD patterns were acquired in the diffraction angle range from 20 to 90 • with a scanning rate of 2 o /min.

Toluene Removal and Evaluation
Before the plasma catalytic reaction, the toluene stream was fed through the DBD reactor for 10 min to achieve saturated absorption (as shown in Figure S3). The gaseous products after toluene IPC reaction mainly included toluene, carbon oxides (CO and CO 2 ) and byproducts (NO x , O 3 , and organic intermediates). The inlet and outlet concentration of toluene were analyzed by a gas chromatograph (GC-2010, Shimadzu, Kyoto, Japan) equipped with a flame ionization detector (FID) and with an Rtx-1 column. The gas chromatograph column oven temperature was held at 100 • C, with injector and detector temperatures of 200 • C. CO 2 was measured using a gas chromatograph flame ionization detector with a methanizer (GC-950, Haixin Co., Shanghai, China) and with TDX-01 packed molecular sieve column. The methanizer temperature was held at 350 • C, the column oven temperature was 80 • C, and the injector and detector temperature was 200 • C. The quantification of gas phase toluene and CO 2 were carried out by using external standard method. The organic intermediates were further analyzed using gas chromatography with time-of-flight mass (GC-TOF-MS, with Agilent GC 7890A and LECO PEGASUS high throughput TOF-MS, Saint Joseph, MI, USA). The GC column was DM-FAMEWAX with diameter of 0.25 µm and length of 30 m. NO x and O 3 generated by the DBD reaction were monitored by a flue gas analyzer (Testo 340, Testo, Lenzkirch, Germany) and an ozone monitor (UV 300B, Limei Co., Guangzhou, China), respectively. All experiments were conducted at least three times (n ≥ 3) and data reported were mean values.
Toluene conversion rate (R T , %), CO 2 selectivity (SCO 2 , %), and the specific input energy (SIE) were used to evaluate different processes and calculated as follows: SIE (J/L) = Discharge power Gas flow rate × 60 where [toluene] in and [toluene] out are the toluene inlet and outlet concentration, and [CO 2 ] out is the CO 2 outlet concentration, respectively. Discharge power unit: W, gas flow rate unit: L/min.

XRD of CuO Foam
As shown in Figure 2, the prepared CuO foam exhibited sharp diffraction peaks which were well matched with Joint Committee on Powder Diffraction Standards (JCPDS) card no. 48-1548 [37]. The diffraction patterns indicated that CuO nanoparticle was single crystalline and there were no other peaks related to Cu 2 O, confirming the complete oxidation of Cu foam during preparation and good phase purity. Additionally, the CuO crystalline structure exhibited no phase change and slight decrease of diffraction peaks after 50 cycles of IPC reaction, indicating the stability of CuO foam under plasma reaction and slight surface corrosion of CuO particle caused by toluene decomposition byproducts.  Figure 3 shows the morphology of the CuO foam before and after IPC toluene reaction characterized by FESEM. It is observed that the foam framework shown no obvious change after annealing preparation previously described. CuO nanoparticles (with average size of 200-500 nm, Figure 3d) uniformly grew on the framework without any holes. Notably, the CuO grain surface was partly covered by oil-like substance after the IPC reaction (Figure 3h), which was the surface corrosion caused by toluene degradation intermediates. This phenomenon was also proved by the XRD results, in accordance with the slight decrease of CuO diffraction peaks. The granularity of the CuO grain after the IPC reaction becomes smaller and slightly obscure, leading to more uniform distribution and higher dispersion. Meanwhile, it was apparent that the oil-like substance on the inner tube wall was much less than that of the NTP reaction (as shown in Figure S4), confirming that the toluene degradation byproduct was mainly adsorbed by the CuO framework in the IPC reaction [38].

Effect of Catalyst Loadings and Applied Power
The oxidation capacity of IPC system comparing with that of NTP only process for toluene decomposition was investigated at steady-state conditions and results were listed in Figure 4. It was  Figure 3 shows the morphology of the CuO foam before and after IPC toluene reaction characterized by FESEM. It is observed that the foam framework shown no obvious change after annealing preparation previously described. CuO nanoparticles (with average size of 200-500 nm, Figure 3d) uniformly grew on the framework without any holes. Notably, the CuO grain surface was partly covered by oil-like substance after the IPC reaction (Figure 3h), which was the surface corrosion caused by toluene degradation intermediates. This phenomenon was also proved by the XRD results, in accordance with the slight decrease of CuO diffraction peaks. The granularity of the CuO grain after the IPC reaction becomes smaller and slightly obscure, leading to more uniform distribution and higher dispersion. Meanwhile, it was apparent that the oil-like substance on the inner tube wall was much less than that of the NTP reaction (as shown in Figure S4), confirming that the toluene degradation byproduct was mainly adsorbed by the CuO framework in the IPC reaction [38].  Figure 3 shows the morphology of the CuO foam before and after IPC toluene reaction characterized by FESEM. It is observed that the foam framework shown no obvious change after annealing preparation previously described. CuO nanoparticles (with average size of 200-500 nm, Figure 3d) uniformly grew on the framework without any holes. Notably, the CuO grain surface was partly covered by oil-like substance after the IPC reaction (Figure 3h), which was the surface corrosion caused by toluene degradation intermediates. This phenomenon was also proved by the XRD results, in accordance with the slight decrease of CuO diffraction peaks. The granularity of the CuO grain after the IPC reaction becomes smaller and slightly obscure, leading to more uniform distribution and higher dispersion. Meanwhile, it was apparent that the oil-like substance on the inner tube wall was much less than that of the NTP reaction (as shown in Figure S4), confirming that the toluene degradation byproduct was mainly adsorbed by the CuO framework in the IPC reaction [38].

Effect of Catalyst Loadings and Applied Power
The oxidation capacity of IPC system comparing with that of NTP only process for toluene decomposition was investigated at steady-state conditions and results were listed in Figure 4. It was

Effect of Catalyst Loadings and Applied Power
The oxidation capacity of IPC system comparing with that of NTP only process for toluene decomposition was investigated at steady-state conditions and results were listed in Figure 4. It was obvious that the plasma discharge became more intense and uniform under IPC condition comparing with that of NTP process (shown in Figure S5). It was also noticed that breakdown voltage was 18 kV for the NTP process, and was only 12 kV for the IPC process. Only 65.6% of toluene conversion (R T ) and 28.9% of CO 2 selectivity (SCO 2 ) were obtained at maximum peak voltage of 24 kV under the sole NTP condition. Enhanced R T and SCO 2 at the same input peak voltage were obtained after CuO foam catalyst filling into plasma region (IPC) compared with that of sole NTP process. With the IPC region length increasing from 5 cm to 15 cm, both R T and SCO 2 increased simultaneously. The longer of the IPC length leaded to a larger discharge volume thus enhancing the toluene oxidation capacity. The maximum R T and SCO 2 were achieved at 15 cm of IPC length and 24 kV of peak voltage with R T = 99.7% and SCO 2 = 72.9%, respectively. Notably, SCO 2 showed no obvious increase at 5 cm of IPC length when peak voltage was higher than 15 kV (40.8% to 46.2% at 24 kV), whereas the R T maintained an increasing trend. obvious that the plasma discharge became more intense and uniform under IPC condition comparing with that of NTP process (shown in Figure S5). It was also noticed that breakdown voltage was 18 kV for the NTP process, and was only 12 kV for the IPC process. Only 65.6% of toluene conversion (RT) and 28.9% of CO2 selectivity (SCO2) were obtained at maximum peak voltage of 24 kV under the sole NTP condition. Enhanced RT and SCO2 at the same input peak voltage were obtained after CuO foam catalyst filling into plasma region (IPC) compared with that of sole NTP process. With the IPC region length increasing from 5 cm to 15 cm, both RT and SCO2 increased simultaneously. The longer of the IPC length leaded to a larger discharge volume thus enhancing the toluene oxidation capacity. The maximum RT and SCO2 were achieved at 15 cm of IPC length and 24 kV of peak voltage with RT = 99.7% and SCO2 = 72.9%, respectively. Notably, SCO2 showed no obvious increase at 5 cm of IPC length when peak voltage was higher than 15 kV (40.8% to 46.2% at 24kV), whereas the RT maintained an increasing trend. It is well-known that high-energy electrons produced by plasma discharge can cleave the bond between between the methyl group and the aromatic ring (5.0-5.3 eV) [39,40], whereas the dissociation energy of C＝C in the aromatic ring is beyond 5.4 eV [41]. At 20 kV of peak voltage, the toluene conversion reached 98.9 % at IPC length of 15 cm, and 90.0% at 10 cm of IPC length, 68.5% at 5 cm of IPC length compared with that of 49.6% under the sole NTP process. The mechanism of toluene degradation under IPC reaction will be discussed after further analysis.

NOx and O3 Production
The main drawbacks of non-thermal plasma were the unavoidable production of ozone (O3) and nitric oxides (NOx) once discharge using air as carrier gas (the generation processes were illustrated as Equation [4][5][6][7][8]. Briefly, the atomic oxygen produced by O2 collision recombined with O2 molecule, which leaded to the O3 formation. The massive active nitrogen free radicals produced by N2 collision recombined with O2, thus causing the production of NOx. It is well-known that high-energy electrons produced by plasma discharge can cleave the bond between between the methyl group and the aromatic ring (5.0-5.3 eV) [39,40], whereas the dissociation energy of C=C in the aromatic ring is beyond 5.4 eV [41]. At 20 kV of peak voltage, the toluene conversion reached 98.9 % at IPC length of 15 cm, and 90.0% at 10 cm of IPC length, 68.5% at 5 cm of IPC length compared with that of 49.6% under the sole NTP process. The mechanism of toluene degradation under IPC reaction will be discussed after further analysis.

NO x and O 3 Production
The main drawbacks of non-thermal plasma were the unavoidable production of ozone (O 3 ) and nitric oxides (NO x ) once discharge using air as carrier gas (the generation processes were illustrated as Equation (4)-(8)). Briefly, the atomic oxygen produced by O 2 collision recombined with O 2 molecule, which leaded to the O 3 formation. The massive active nitrogen free radicals produced by N 2 collision recombined with O 2 , thus causing the production of NO x .
The influence of the input energy on O 3 and NO x formation was presented in Figure 5. As NO is readily oxidized to NO 2 by abundant ozone after discharge, the major NO x species detected in this study was NO 2 for both IPC and sole NTP process. In general, higher input peak voltage led to increasing production of NO 2 and O 3 when peak voltage increased from 12 kV to 20 kV. After the applied peak voltage was higher than 20 kV, NO 2 concentration in IPC process started to decrease. The reason can be attibuted to NO 2 reduction by active nitrogen free radical under stronger plasma discharge condition (Equations (9) and (10)). The NO 2 concentration for the NTP process maintained an increasing tendency with the input voltage ranging from 12 to 24 kV, which was attributed to the lower energy density comparing with that of IPC process. The O 3 concentration also showed deceasing trend after the maximum concentration when the input peak voltage was higher at all conditions. The ozone concentration of both NTP and IPC reactions decreased to nearly zero when the peak voltage reached 24 kV, which means that the ozone dissociation reactions become dominant at higher energy input. With the increase of energy density, the plasma-forming gas temperature was slightly increased, which also contributed to O 3 decomposition to a certain extent [42,43].  The influence of the input energy on O3 and NOx formation was presented in Figure 5. As NO is readily oxidized to NO2 by abundant ozone after discharge, the major NOx species detected in this study was NO2 for both IPC and sole NTP process. In general, higher input peak voltage led to increasing production of NO2 and O3 when peak voltage increased from 12 kV to 20 kV. After the applied peak voltage was higher than 20 kV, NO2 concentration in IPC process started to decrease. The reason can be attibuted to NO2 reduction by active nitrogen free radical under stronger plasma discharge condition (Equations (9) and (10)). The NO2 concentration for the NTP process maintained an increasing tendency with the input voltage ranging from 12 to 24 kV, which was attributed to the lower energy density comparing with that of IPC process. The O3 concentration also showed

Influence of VOC Initial Concentration and Stream Flux
The VOC stream flux and concentration usually fluctuated during emission. Therefore, the influences of VOC stream flux and concentration should be taken into consideration because it directly reflected the capability of plasma screening VOCs efficiency. As shown in Figure 6a, the toluene initial concentration was fixed at 1000 ppm, with the increase of toluene flux from 100 mL/min to 2000 mL/min, both toluene conversion efficiency and CO 2 selectivity showed a decreasing tendency. At the maximum flux of 2000 mL/min, R T and SCO 2 decreased to 76.4% and 52.9%, respectively, comparing with that of 99.8% and 86.9% at a minimum flux of 100 mL/min. With the increase of the VOC stream flux, the gas retention time decreased, then leading to a decreasing number of active radicals reaction with toluene molecules and reducing collision probability with electrons. The influence of toluene initial concentration was shown in Figure 6b with a fixed flux of 400 mL/min, the R T kept relative steady and achieved 98% at maximum toluene initial concentration of 2000 ppm, while the SCO 2 decreased significantly when toluene concentration increased from 200 ppm to 600 ppm. SCO 2 kept relatively steady when toluene concentration was higher than 600 ppm, and reached 66% at 2000 ppm. The influences of VOC stream flux and inlet concentration indicated the high removal capacity of the CuO foam IPC system for both endurance of variable toluene concentrations and flux.

Influence of VOC Initial Concentration and Stream Flux
The VOC stream flux and concentration usually fluctuated during emission. Therefore, the influences of VOC stream flux and concentration should be taken into consideration because it directly reflected the capability of plasma screening VOCs efficiency. As shown in Figure 6a, the toluene initial concentration was fixed at 1000 ppm, with the increase of toluene flux from 100 mL/min to 2000 mL/min, both toluene conversion efficiency and CO2 selectivity showed a decreasing tendency. At the maximum flux of 2000 mL/min, RT and SCO2 decreased to 76.4% and 52.9%, respectively, comparing with that of 99.8% and 86.9% at a minimum flux of 100 mL/min. With the increase of the VOC stream flux, the gas retention time decreased, then leading to a decreasing number of active radicals reaction with toluene molecules and reducing collision probability with electrons. The influence of toluene initial concentration was shown in Figure 6b with a fixed flux of 400 mL/min, the RT kept relative steady and achieved 98% at maximum toluene initial concentration of 2000 ppm, while the SCO2 decreased significantly when toluene concentration increased from 200 ppm to 600 ppm. SCO2 kept relatively steady when toluene concentration was higher than 600 ppm, and reached 66% at 2000 ppm. The influences of VOC stream flux and inlet concentration indicated the high removal capacity of the CuO foam IPC system for both endurance of variable toluene concentrations and flux.

Influence of Relative Humidity (RH)
Flue gas from industry process usually contains variable water content. Herein, the influence of water vapor was investigated by bubbling air through water thus giving variable relative humidity (RH). As shown in Figure 7, enhanced toluene conversion and CO 2 selectivity were achieved at higher relative humidity. Notably, obvious toluene conversion enhancement was obtained from 12 to 15 kV under saturated water vapor (RH = 100%), comparing with that of unsaturated water vapor conditions (RH = 0%, 25%, 50%, and 75%). Some relevant studies also showed the same phenomenon as for the humidity influence [44,45].

Influence of Relative Humidity (RH)
Flue gas from industry process usually contains variable water content. Herein, the influence of water vapor was investigated by bubbling air through water thus giving variable relative humidity (RH). As shown in Figure 7, enhanced toluene conversion and CO2 selectivity were achieved at higher relative humidity. Notably, obvious toluene conversion enhancement was obtained from 12 to 15 kV under saturated water vapor (RH = 100%), comparing with that of unsaturated water vapor conditions (RH = 0%, 25%, 50%, and 75%). Some relevant studies also showed the same phenomenon as for the humidity influence [44,45]. It is well known that direct electron impact and radical attack are the dominant degradation mechanisms for toluene molecules decomposition by NTP technology. Under higher humidity, increasing hydroxyl radicals were produced in the presence of water vapor because of the electron reaction with H2O molecule (Equation 11), thus leading to the enhancing oxidation ability and higher decomposition rate of toluene molecules. In plasma discharge process, oxygen molecules react with high-energy electron to generate oxygen radicals. The free oxygen radical (·O) also attack H2O thus leading to the generation of •OH radicles (Equation (12)):

Degradation Mechanism
The reaction mechanisms of VOCs decomposition by NTP are complicated because NTP is unique in that it induced various non-equilibrium chemical reactions besides direct dissociation of VOC molecules by energetic electrons [25]. For further understanding of the mechanism of the toluene degradation in IPC reaction, the gaseous toluene degradation intermediates were analyzed using GC-TOF-MS at different toluene conversions. Figure 8 showed the total ion chromatogram (TIC) of the gaseous organic intermediates with the toluene conversion of 20%, 50%, 70%, and 90%, respectively. It is clear to see that, except for toluene, the main following organic by-products were detected: formic acid, acetic acid, benzaldehyde, maleic anhydride, and phenol, while the concentrations of those by-products were much lower than that of toluene. With the promotion of toluene conversion, increases of benzaldehyde, maleic anhydride, acetic acid, and formic acid were It is well known that direct electron impact and radical attack are the dominant degradation mechanisms for toluene molecules decomposition by NTP technology. Under higher humidity, increasing hydroxyl radicals were produced in the presence of water vapor because of the electron reaction with H 2 O molecule (Equation (11)), thus leading to the enhancing oxidation ability and higher decomposition rate of toluene molecules. In plasma discharge process, oxygen molecules react with high-energy electron to generate oxygen radicals. The free oxygen radical (·O) also attack H 2 O thus leading to the generation of •OH radicles (Equation (12)): e + H 2 O → ·OH + ·H (11) ·O

Degradation Mechanism
The reaction mechanisms of VOCs decomposition by NTP are complicated because NTP is unique in that it induced various non-equilibrium chemical reactions besides direct dissociation of VOC molecules by energetic electrons [25]. For further understanding of the mechanism of the toluene degradation in IPC reaction, the gaseous toluene degradation intermediates were analyzed using GC-TOF-MS at different toluene conversions. Figure 8 showed the total ion chromatogram (TIC) of the gaseous organic intermediates with the toluene conversion of 20%, 50%, 70%, and 90%, respectively. It is clear to see that, except for toluene, the main following organic by-products were detected: formic acid, acetic acid, benzaldehyde, maleic anhydride, and phenol, while the concentrations of those by-products were much lower than that of toluene. With the promotion of toluene conversion, increases of benzaldehyde, maleic anhydride, acetic acid, and formic acid were observed. When the R T was higher than 90%, the organic byproducts decreased significantly, and CO 2 accounted for the majority.
observed. When the RT was higher than 90%, the organic byproducts decreased significantly, and CO2 accounted for the majority. It is generally accepted that the energetic electrons and a large quantity of radicals (•O, •N, and •OH, et al.) produced by discharge dominated the toluene destruction, whereas the contributions of O3 directly reacted with toluene and ion collisions were negligible [33]. During the toluene oxidation reaction, the activated sites belonged to the C-H bond on the methyl group of toluene. The C-H bond dissociation energy of the phenyl group and methyl group are 110 and 75 kcal/mol, respectively, and benzene is relatively difficult to be oxidized compared with toluene [46,47]. Combining with the above intermediates analysis, we speculated the toluene degradation mechanism under IPC reaction: toluene was decomposed and underwent a series of consequent reactions: toluene → benzaldehyde → benzoic acid → chain carboxylic acids (maleic anhydride) → formic acid and acetic acid → CO2.

Byproducts Control
As shown in Figure 5, the unavoidable production of O3 and NOx after the discharge process is a major drawback of the non-thermal plasma reaction under air atmosphere. Herein, Na2SO3 bubble absorption was preliminarily utilized to remove NO2 and O3 produced in the plasma reaction ( Figure  9). The initial NO2 and O3 levels after the plasma discharge process were fixed at 480 ppm and 330 ppm, respectively. It can be concluded that NO2 and O3 were completely absorbed by Na2SO3 in the initial 40 min, and the promotion of Na2SO3 absorption concentration and the increase of pH value will prolong NO2 and O3 removal effectiveness [48]. The post sodium-based alkali solution absorption is an effective application of the NTP technique in industrial flue gas treatment.  [33]. During the toluene oxidation reaction, the activated sites belonged to the C-H bond on the methyl group of toluene. The C-H bond dissociation energy of the phenyl group and methyl group are 110 and 75 kcal/mol, respectively, and benzene is relatively difficult to be oxidized compared with toluene [46,47]. Combining with the above intermediates analysis, we speculated the toluene degradation mechanism under IPC reaction: toluene was decomposed and underwent a series of consequent reactions: toluene → benzaldehyde → benzoic acid → chain carboxylic acids (maleic anhydride) → formic acid and acetic acid → CO 2 .

Byproducts Control
As shown in Figure 5, the unavoidable production of O 3 and NO x after the discharge process is a major drawback of the non-thermal plasma reaction under air atmosphere. Herein, Na 2 SO 3 bubble absorption was preliminarily utilized to remove NO 2 and O 3 produced in the plasma reaction ( Figure 9). The initial NO 2 and O 3 levels after the plasma discharge process were fixed at 480 ppm and 330 ppm, respectively. It can be concluded that NO 2 and O 3 were completely absorbed by Na 2 SO 3 in the initial 40 min, and the promotion of Na 2 SO 3 absorption concentration and the increase of pH value will prolong NO 2 and O 3 removal effectiveness [48]. The post sodium-based alkali solution absorption is an effective application of the NTP technique in industrial flue gas treatment. Figure 9. Elimination of NO2 and O3 using Na2SO3 absorption (0.5%, wt%).

Conclusions
In this study, CuO foam was prepared and was taken as IPC reaction catalyst for the oxidation of gaseous toluene. The influencing factors of IPC reaction were studied. The monolith CuO foam catalyst featured easy installization, controllable plasma catalytic region length, and good gas permeability compared with powder catalysts. Results showed that, in the presence of CuO foam, obvious promotion of plasma energy and a synergistic effect were obtained under IPC reaction, then led to enhanced toluene conversion and CO2 selectivity compared with the sole NTP process. Both of the CuO foam framework and phase structure maintained good stability after cycles of IPC reaction. The maximum toluene conversion and CO2 selectivity were achieved at 15 cm of IPC length and peak voltage of 24 kV with RT = 99.7% and SCO2 = 72.9%, respectively. The increase of the IPC region and relative humidity led to enhanced toluene conversion. The possible reaction pathway for toluene decomposition was also speculated based on the analysis of gaseous organic byproducts. In view of future application of NTP techniques in flue gas treatment, the post sodium-based alkali solution absorption can be combined with NTP for its effective control of unwanted NO2 and O3.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Table S1: Specific input energy (SIE) with different input power and peak voltage. Figure S1: Enlarged illustration of the wedged high electrode and DBD reactor. Figure S2: Waveforms of applied voltage and V-Q Lissajous diagrams of IPC process at 15 kV peak voltage. Figure S3: Toluene adsorption balance of different CuO foam loading in IPC reactor.

Conclusions
In this study, CuO foam was prepared and was taken as IPC reaction catalyst for the oxidation of gaseous toluene. The influencing factors of IPC reaction were studied. The monolith CuO foam catalyst featured easy installization, controllable plasma catalytic region length, and good gas permeability compared with powder catalysts. Results showed that, in the presence of CuO foam, obvious promotion of plasma energy and a synergistic effect were obtained under IPC reaction, then led to enhanced toluene conversion and CO 2 selectivity compared with the sole NTP process. Both of the CuO foam framework and phase structure maintained good stability after cycles of IPC reaction. The maximum toluene conversion and CO 2 selectivity were achieved at 15 cm of IPC length and peak voltage of 24 kV with R T = 99.7% and SCO 2 = 72.9%, respectively. The increase of the IPC region and relative humidity led to enhanced toluene conversion. The possible reaction pathway for toluene decomposition was also speculated based on the analysis of gaseous organic byproducts. In view of future application of NTP techniques in flue gas treatment, the post sodium-based alkali solution absorption can be combined with NTP for its effective control of unwanted NO 2 and O 3 .

Supplementary Materials:
The following are available online at http://www.mdpi.com/1660-4601/16/5/711/s1, Table S1: Specific input energy (SIE) with different input power and peak voltage. Figure S1: Enlarged illustration of the wedged high electrode and DBD reactor. Figure S2: Waveforms of applied voltage and V-Q Lissajous diagrams of IPC process at 15 kV peak voltage. Figure S3: Toluene adsorption balance of different CuO foam loading in IPC reactor. Figure S4: Toluene decomposition byproduct on inner barrier tube comparison of NTP and IPC process. Figure