Degradation of Benzene Using Dielectric Barrier Discharge Plasma Combined with Transition Metal Oxide Catalyst in Air

Abstract

In this paper, a uniform and stable dielectric barrier discharge plasma is presented for degradation of benzene combined with a transition metal oxide catalyst. The discharge images, waveforms of discharge current, and the optical emission spectra are measured to investigate the plasma characteristics. The effects of catalyst types, applied voltage, driving frequency, and initial VOCs concentration on the degradation efficiency of benzene are studied. It is found that the addition of the packed dielectric materials can effectively improve the uniformity of discharge and enhance the intensity of discharge, thus promoting the benzene degradation efficiency. At 22 kV, the degradation efficiencies of dielectric barrier discharge plasma packed with CuO, ZnO and Fe₃O₄ are 93.6%, 93.2% and 76.2%, respectively. When packing with ZnO, the degradation efficiency of the dielectric barrier discharge plasma is improved from 86.8% to 94.9%, as the applied voltage increases from 16 kV to 24 kV. The catalysts were characterized by XPS, XRD and SEM. The synergistic mechanism and the property of the catalyst are responsible for benzene degradation in the plasma–catalysis system. In addition, the main physiochemical processes and possible degradation mechanism of benzene are discussed.

1. Introduction

Volatile organic compounds (VOCs) lead to vast human diseases and environmental problems [1,2,3]. Humans exposed to VOCs for a long time can suffer from respiratory disorders, cardiovascular diseases, and cancer [4]. Moreover, VOCs reacting with other gaseous pollutants is the main cause of ozone layer depletion and photochemical smog [5]. Tremendous efforts have been devoted to the investigation of VOC degradation technologies, including adsorption, biological decomposition, catalytic oxidation, non-thermal plasma (NTP), etc. [6,7,8].

In the last two decades, the degradation of VOCs by NTP has attracted the attention of researchers for the advantages of high removal efficiency, easy operation, low reaction temperature, etc. [9]. In NTP, strong electric field selectively accelerates electrons to become high-energy electrons, which collide with gas molecules to produce plenty of reactive species [10]. These reactive species, including energetic electrons, ions, radicals and excited molecules and atoms, generated in the NTP can degrade the VOCs through complex physicochemical reactions into small molecule species, such as CO₂ and H₂O [11].

NTP combined with a catalyst can improve the degradation efficiency of VOCs further due to the synergistic effect [12]. Zhang et al. [13] presented a dielectric barrier discharge (DBD) reactor filled with NiO/Pyr composite system, and found it could dramatically improve the removal efficiency of gaseous styrene compared with a bare DBD system. Nguyen et al. [14] compared a plasma combined with Pd/ZSM-5 catalyst system with a plasma alone system. The results show that the former significantly enhanced the styrene degradation efficiency. On the one hand, a packed bed structure is formed by introducing a catalyst into the plasma reactor. The addition of the packing material strongly enhances the intensity of discharge and highly improves the uniformity of discharge, hence improving the VOCs degradation efficiency [15]. On the other hand, the filling materials in the NTP reactor can produce more active species on the surfaces of catalysts [16]. Mao et al. [7] carried out experiments that used the plasma catalyst reactor and the O₃ catalyst reactor. The results show that on the CeO₂ catalyst’s surface, O₃ can be degraded into O₂²⁻ and O²⁻ radicals.

The transition metal oxides, ZnO, MnO_x, NiO, CuO, etc., have attracted great attention owing to their low cost, high catalytic activity, anti-toxicity and wide range of available sources [17,18,19]. In experiments introducing the MnO_x catalyst to the plasma reactor, the xylene removal efficiency was strongly improved [19]. Yan et al. [20] investigated nano-ZnO for bisphenol A degradation in a DBD system, and found that ZnO has a multi-catalysis effects: as a photocatalyst to generate the radical OH, and as a catalyst to react with O₃ and H₂O₂ to form OH. Rostami et al. [21] found that the highest benzene removal efficiency was 52.7 $\pm$ 1.9% in the CuO-coated electrode reactor. However, there have been few studies comparing the synergistic effects of several transition metal oxides in the NTP–catalysis system.

In this study, benzene is chosen as the target VOCs contaminant, and the degradation of benzene by synergy DBD/CuO, DBD/ZnO and DBD/Fe₃O₄ is investigated. The discharge images and the waveforms of voltage and current are discussed. The reactive species are diagnosed using optical emission spectra (OES). The effects of applied voltage, driving frequency and initial concentration of VOCs on the performance of benzene removal are studied. Finally, the main physiochemical processes and the benzene degradation pathway are analyzed in the DBD catalytic system.

2. Results and Discussion

2.1. Visualization and Electrical Characteristics of DBD

Figure 1 shows the discharge images of the DBD without and with packed CuO under various applied voltages. A Cannon 70D digital camera is used for capturing the discharge images with an exposure time of 1 s, as the total synthetic air flow is 500 mL/min and the driving frequency is 8 kHz. The discharge images of DBD without catalysts are shown in Figure 1a, where only a weak surface discharge with purple coloration occurred on the outer dielectric tube at 22 kV, and when the applied voltage was lower than 20 kV, the gas gap did not break down. When the voltage applied was 24 kV, it produced weak plasma between the inner quartz tube and the high-voltage probe, except for the surface discharge on the outer dielectric tube. Figure 1b shows DBD packed with CuO catalyst, wherein a fuse discharge can be generated in the region of the packed catalyst at 16 kV. The discharge was gradually brightened and remained diffused and stable as the applied voltage increased. It is evident that the discharge region of the DBD packed with CuO is much larger than that without catalyst. Obviously, the addition of the catalyst significantly reduces the breakdown voltage and increases the discharge volume into the whole packed region. One of the reasons for this is that the catalyst decreases the discharge gap and increases the reduced electric filed E/N due to the reductions in the gas molecules, thus reducing the breakdown voltage [22]. Furthermore, charge accumulates on the surfaces of the catalysts, and the electric field is stronger at the contact points between catalyst particles and between the catalyst and the DBD reactor tubes, which makes it easier to discharge [23].

In DBD, the gas is broken down at a random location when the applied electric field is larger than the gas breakdown electric field. Discharge channels tend to constrict, and form distinct plasma discharge filaments according to the streamer breakdown mechanism [24]. Figure 2 presents the waveforms of the voltage and current of the DBD reactor without and with packed ZnO catalyst. The driving frequency was kept at 14 kHz. The discharge current filaments of DBD without and with packed ZnO in 24 kV were as shown in Figure 2a,c; the strength of the current pulses apparently increased with the addition of catalyst, while the current peak decreased from about 0.5 A to about 0.15 A. This phenomenon is related to the filling materials significantly reducing the discharge volume, which means the distance of a typical micro-discharge transition reduces [22]. Furthermore, a strong electric field is produced in neighboring regions of contact points between the catalyst particles [25]. The stronger the local electric field, the easier it is for the gas to break down. As such, the microdischarge is enhanced and the current peak decreases. Figure 2b presents the waveforms of the voltage and current in the DBD reactor with ZnO catalyst at 16 kV. Comparing Figure 2b,c, we see that the amount and the intensity of current pulses increased significantly with increase in the applied voltage, while the current peak decreased from about 0.3 A to 0.15 A. This may be because the electric field in the gas gap and on the catalyst’s surface increased with the voltage applied in this condition [26]. Electrons gain more energy from the high electric field, causing more collisions, dissociations and ionizations, and thus more micro-discharges [27]. Since the power supply was constant, when the applied voltage increased to a certain point, the current did not increase, which may be the reason for the reductions in peak current.

2.2. The Power of the Discharge in DBD

The discharge power of different catalysts varying with applied voltage is presented in Figure 3. The discharge power of the DBD packed with three catalysts increased with the increasing of the applied voltage. At 16 kV, the discharge power of DBD packed with ZnO, CuO, and Fe₃O₄ was 4.3 W, 2.6 W, and 2.3 W, respectively. When the applied voltage increased to 24 kV, the discharge power of DBD packed with ZnO, CuO, and Fe₃O₄ can reach 22 W, 18.7 W, and 12.4 W, respectively.

2.3. Emission Spectra of the Discharge in DBD

The OES emitted by the DBD packed with ZnO in synthetic air is shown in Figure 4. The applied voltage was 18 kV, and the driving frequency was 14 kHz in the experiment. It is clearly that the spectra of the DBD are composed of ${\mathrm{N}}_2 \left({\mathrm{C}}^3{\Pi }_{\mathrm{u}} \to { \mathrm{B}}^3{\Pi }_{\mathrm{g}}\right)$, ${\mathrm{N}}_2 \left({\mathrm{B}}^3{\Pi }_{\mathrm{g}} \to { \mathrm{A}}^3{\Pi }_{\mathrm{u}}^{+}\right)$, ${\mathrm{N}}_2^{+} \left({\mathrm{B}}^2{\mathsf{\Sigma }}_{\mathrm{u}}^{+} \to { \mathrm{X}}^2{\mathsf{\Sigma }}_{\mathrm{g}}^{+}\right)$, and the spectral line of oxygen atoms $\mathrm{O} \left({3p}^5\mathrm{P} \to { 3s}^5{\mathrm{S}}_2^{\mathrm{o}}\right)$. Similar emission spectra have also been identified in other atmospheric air discharge studies [28]. The spectra peak of ${\mathrm{N}}_2 \left({\mathrm{C}}^3{\Pi }_{\mathrm{u}} \to { \mathrm{B}}^3{\Pi }_{\mathrm{g}}\right)$ is highest due to the fact that N₂ constitutes three quarters of the synthetic air, and the energetic electrons directly collide with N₂ molecules, the excitation threshold of which is about 11 eV, leading dominantly to ${\mathrm{N}}_2 \left({\mathrm{C}}^3{\Pi }_{\mathrm{u}}\right)$ become excited. The corresponding reactions are as follows [29]:

$${\mathrm{e}+\mathrm{N}}_2 \left({\mathrm{X}}^1{\mathsf{\Sigma }}_{\mathrm{g}}^{+}\right) \to { \mathrm{e}+\mathrm{N}}_2 \left({\mathrm{C}}^3{\Pi }_{\mathrm{u}}\right), 7.9{ \times 10}^{-11}{ \mathrm{cm}}^3 {\mathrm{s}}^{-1}$$

(1)

$${\mathrm{N}}_2 \left({\mathrm{C}}^3{\Pi }_{\mathrm{u}}\right) \to { \mathrm{N}}_2 \left({\mathrm{B}}^3{\Pi }_{\mathrm{g}}\right)+\mathrm{h}\mathsf{\upsilon }, {3 \times 10}^{-7}{ \mathrm{cm}}^3 {\mathrm{s}}^{-1}$$

(2)

$${\mathrm{N}}_2 \left({\mathrm{A}}^3{\mathsf{\Sigma }}_{\mathrm{u}}^{+}\right){+\mathrm{N}}_2 \left({\mathrm{A}}^3{\mathsf{\Sigma }}_{\mathrm{u}}^{+}\right) \to { \mathrm{N}}_2 \left({\mathrm{C}}^3{\mathsf{\Sigma }}_{\mathrm{u}}\right){+\mathrm{N}}_2, 2.1{ \times 10}^{-12}{ \mathrm{cm}}^3 {\mathrm{s}}^{-1}$$

(3)

Energetic electrons (about 18.7 eV), mainly positioned at the streamer head, can directly excite the N₂ molecules to ${\mathrm{N}}_2^{+} \left({\mathrm{B}}^2{\mathsf{\Sigma }}_{\mathrm{u}}^{+}\right)$ [30].

$${\mathrm{e}+\mathrm{N}}_2 \left({\mathrm{X}}^1{\mathsf{\Sigma }}_{\mathrm{g}}^{+}\right) \to { 2\mathrm{e}+\mathrm{N}}_2^{+} \left({\mathrm{B}}^2{\mathsf{\Sigma }}_{\mathrm{u}}^{+}\right)$$

(4)

$${\mathrm{e}+\mathrm{N}}_2 \left({\mathrm{A}}^3{\mathsf{\Sigma }}_{\mathrm{u}}^{+}\right) \to { 2\mathrm{e}+\mathrm{N}}_2^{+} \left({\mathrm{B}}^2{\mathsf{\Sigma }}_{\mathrm{u}}^{+}\right), 2.4{ \times 10}^{-12}{ \mathrm{cm}}^3 {\mathrm{s}}^{-1}$$

(5)

The process of forming $\mathrm{O} \left({3p}^5\mathrm{P}\right)$ is as follows: [23]

$${\mathrm{e}+\mathrm{O}}_2 \to \mathrm{e}+\mathrm{O} \left({}_{ }{}^3\mathrm{P}\right)+\mathrm{O} \left({}_{ }{}^1\mathrm{S}\right), \log k=-8.8–11.9/\mathsf{\Theta }$$

(6)

$${\mathrm{N}}_2 \left({\mathrm{A}}^3{\mathsf{\Sigma }}_{\mathrm{u}}^{+}\right){+\mathrm{O}}_2 \to { \mathrm{N}}_2 \left({\mathrm{X}}^1{\mathsf{\Sigma }}_{\mathrm{g}}^{+}\right)+\mathrm{O}+\mathrm{O}, 2.54{ \times 10}^{-11}{ \mathrm{cm}}^3 { \mathrm{s}}^{-1}$$

(7)

$$\mathrm{e}+\mathrm{O} \to \mathrm{e}+\mathrm{O} \left({3p}^5\mathrm{P}\right)$$

(8)

Figure 5a presents the effects of discharge time on the emission intensity of ${\mathrm{N}}_2 \left({\mathrm{C}}^3{\Pi }_{\mathrm{u}} \to { \mathrm{B}}^3{\Pi }_{\mathrm{g}}\right)$, ${\mathrm{N}}_2^{+} \left({\mathrm{B}}^2{\mathsf{\Sigma }}_{\mathrm{u}}^{+} \to { \mathrm{X}}^2{\mathsf{\Sigma }}_{\mathrm{g}}^{+}\right)$ and $\mathrm{O} \left({3p}^5\mathrm{P} \to { 3s}^5{\mathrm{S}}_2^{\mathrm{o}}\right)$ emitted from DBD packed with ZnO. It is shown that the intensity of the three active species remains almost unchanged within 12 min of the discharge duration time. This phenomenon indicates the good stability of the packed bed DBD. The emission intensities of ${\mathrm{N}}_2 \left({\mathrm{C}}^3{\Pi }_{\mathrm{u}} \to { \mathrm{B}}^3{\Pi }_{\mathrm{g}}\right)$, ${\mathrm{N}}_2^{+} \left({\mathrm{B}}^2{\mathsf{\Sigma }}_{\mathrm{u}}^{+} \to { \mathrm{X}}^2{\mathsf{\Sigma }}_{\mathrm{g}}^{+}\right)$ and $\mathrm{O} \left({3p}^5\mathrm{P} \to { 3s}^5{\mathrm{S}}_2^{\mathrm{o}}\right)$, varying with the applied voltage and driving frequency, are presented in Figure 5b,c, respectively. The results show that with the increment in both applied voltage and driving frequency, the emission intensities of ${\mathrm{N}}_2 \left({\mathrm{C}}^3{\Pi }_{\mathrm{u}} \to { \mathrm{B}}^3{\Pi }_{\mathrm{g}}\right)$, ${\mathrm{N}}_2^{+} \left({\mathrm{B}}^2{\mathsf{\Sigma }}_{\mathrm{u}}^{+} \to { \mathrm{X}}^2{\mathsf{\Sigma }}_{\mathrm{g}}^{+}\right)$ and $\mathrm{O} \left({3p}^5\mathrm{P} \to { 3s}^5{\mathrm{S}}_2^{\mathrm{o}}\right)$ all rise. When the driving frequency is kept constant, an increasing applied voltage leads to a rise in electron mean energy [31]. When the applied voltage is kept constant, increasing the driving frequency leads to an increases in the amount of micro-discharges in a set period of time, which also leads to an increases in the high-energy electron density [32]. The electrons, ions, molecules, and atoms collide more frequently, which leads to the number of excited particles increasing. Thus, the emission intensities of the active species are increased when increasing both the voltage and the frequency.

2.4. The Influence of Different Catalysts on Benzene Decomposition

Figure 6 shows the effects of DBD combined with different catalysts, ZnO, CuO and Fe₃O₄, on (a) the benzene degradation efficiency, (b) CO₂ selectivity, and (c) energy yield. The initial concentration of benzene is 235 ppm, and the benzene-containing air flow rate is 500 mL/min. It can be noted form Figure 6a that the benzene removal efficiency increases with the increasing of the applied voltage when the DBD is packed with three catalysts, and the degradation performances of the three types of catalysts are different. At 16 kV, the removal efficiency of DBD packed with ZnO, Fe₃O₄ and CuO is 72.7%, 23.4%, and 86.8%, respectively. When the applied voltage increases to 24 kV, the degradation efficiency of DBD packed with ZnO, Fe₃O₄ and CuO can reach 95.3%, 80.4%, and 94.9%, respectively. This suggests that increasing the applied voltage can increase the energy injected into the discharge gap. One part of the energy increases the number of energetic electrons, which means the energy is 1–10 eV. A large amount of active species are obtained, such as free radicals OH and O, through the frequent collision of electrons and background gas molecules [33]. Besides this, the excess energy would transform into heat in the discharge area, and a lot of reactive particles would emit ultraviolet light, which would cause more O₃ to transform into reactive atomic oxygen species [34]. Thus, the probability of benzene molecules colliding with reactive particles in the DBD discharge region is enhanced, causing an increase in the degradation efficiency. At a low applied voltage, the degradation efficiencies of benzene by the three catalysts show great differences, and this difference decreases with the increment in voltage. The apparent catalytic activity can be improved by NTP through inducing the dielectric heating of the catalyst [35]. At 16 kV, the benzene degradation efficiency of DBD/CuO is more than three times that of DBD/Fe₃O₄. This indicates that the synergistic effect of CuO and NTP is better than that of ZnO and Fe₃O₄ at low applied voltage. When a catalyst is added to the reactor, the ozone produced in the NTP can be fully utilized by the catalyst to form more active oxygenated radicals through catalyst surface reactions [36]. CuO has a good effect on ozone degradation, thus has the best degradation efficiency at the applied voltage (ranging from 16 kV to 22 kV) [37]. When the reactor is packed with ZnO, the highest benzene degradation efficiency can reach 95.3%, which is slightly higher than when it is packed with CuO at 24 kV. This may be due to the fact that ZnO is a multi-catalyst, which can be used as a photocatalyst to generate OH, and as the transition metal oxide catalyst to react with O₃ [38]. As the applied voltage increases, the UV light produced in the plasma region increases too, via function (2) discussed in the former section, thus better activating the catalysis of the photocatalyst ZnO. High CO₂ selectivity is expected because the end products are CO₂ and H₂O in VOC degradation. CO₂ selectivity is shown in Figure 6b. That catalysts have totally different levels of CO₂ selectivity may be due to the nature of the catalysts. Although DBD/CuO has no obvious advantage in CO₂ selectivity, its CO₂ concentration is the highest among the three catalysts. The effects of the applied voltage on the EY are shown in Figure 6c. For DBD packed with ZnO and CuO, EY shows a decreasing trend with the increasing of the applied voltage, which decreases from 4.1 to 1 g/kWh and 7.5 to 1.1 g/kWh. This may be due to part of the energy being converted into heat and photons, as well as being used for the formation of byproducts through quenching and recombination reactions [39,40]. It should be noted that unlike ZnO and CuO, the EY of DBD packed with Fe₃O₄ decreases from 2.3 to 1.5 g/kWh as the applied voltage increases, but it has a peak value of 2.6 g/kWh at 20 kV. This is probably because the catalysis of Fe₃O₄ is quickly excited at 20 kV, at which point the degradation efficiency is significantly increased.

2.5. The Influence of Initial Concentration on Benzene Decomposition

The effects of the initial concentration on the benzene removal efficiency and CO₂ selectivity, and the EY varying with the applied voltage in the DBD packed with ZnO in synthetic air, are depicted in Figure 7. The driving frequency was kept at 14 kHz. The benzene degradation efficiency increased and the EY decreased in response to increasing the applied voltage, regardless of the initial concentration of benzene. The CO₂ selectivity presents a decreasing trend with the increase in benzene initial concentration in Figure 7b. When the initial concentration was fixed at 235 ppm, benzene degradation efficiency increased from 72.7% to 95.3%, with the applied voltage ranging from 16 kV to 24 kV, while the EY showed the opposite, ranging from 4.1 g/kWh to 1 g/kWh. The removal efficiency of benzene decreased with the increase in initial concentration. At 16 kV, the degradation efficiency of benzene reached 100% when the initial concentration was 60 ppm, while the degradation efficiency was 92% and 79.1% when the initial concentration was 112 ppm and 235 ppm, respectively. Although the benzene degradation efficiency decreased at higher benzene initial concentrations, the amount of converted benzene molecules increased, suggesting more active species are used for the degradation [41]. Under the same applied voltage, when the concentration was increased from 60 ppm to 235 ppm, the EY increased, while when the concentration increased to 300 ppm, the EY decreased. For example, at 16 kV, the EY increased from 1.3 g/kWh to 4.1 g/kWh, and then decreased to 2.8 g/kWh. For a given applied voltage, a certain number of energetic electrons and reactive species would be produced in the DBD reactor discharge region [13]. When the initial concentration of benzene is lower than a certain value, part of the active species cannot be utilized, and the energy stored in the active species is converted into thermal energy; when the initial concentration is too high, the excess benzene molecules cannot be degraded.

2.6. The Influence of Driving Frequency on Benzene Decomposition

The effects of driving frequency on the degradation efficiency of benzene and CO₂ selectivity, and the EY variation with changes in the applied voltage in the DBD packed with ZnO, are depicted in Figure 8. The initial benzene concentration and the flow rate of the synthetic air are fixed, respectively, at 235 ppm and 500 mL/min. It is seen that the benzene removal efficiency increased as the applied voltage increased for all the selected driving frequencies. The removal efficiency of benzene increased as the driving frequency increased at a given applied voltage, whereas the corresponding EY decreased. Figure 8b shows that the variation in driving frequency had no influence on CO₂ selectivity. At 18 kV, the benzene degradation efficiency increased from 63% to 79.1% as the frequency increased from 12 kHz to 14 kHz, while the relevant EY decreased from 4.1 g/kWh to 2.3 g/kWh. This may because more micro-discharges in the DBD reactor would be produced during each period with the increases in the driving frequency, resulting in more reactive species in the discharge region. The probability of collision between the benzene molecules and reactive species is thus increased, contributing to the increases in the benzene degradation efficiency. However, the active species are not fully used, and parts of the active species are quenched and undergo recombination, thus the EY related to benzene degradation decreases [5].

2.7. Characteristics of the Catalysts

Figure 9 shows the scanning spectra of Cu2p, Zn2p and FeLM2. It can be seen that the spectral lines at 934.6 eV and 954.4 eV correspond to the Cu2p3/2 and Cu2p1/2 of the copper element in CuO, and the shake-up peak at 944 eV indicates the existence of a paramagnetic state in Cu²⁺. The Zn2p spectrum’s peaks at 1022.6 eV and 1045.8 eV correspond to the Zn2p3/2 and Zn2p1/2 states, which are the normal states of Zn²⁺ in the ZnO catalyst [42]. The binding energy of Fe2p3/2, 710.5 eV, is quite insignificant compared to the apparent kinetic energy of electrons.

Figure 10 shows the XRD patterns of CuO, ZnO, and Fe₃O₄ catalysts. The diffraction peaks can be observed at a 2θ of 35.46°, 38.60°, 48.78°, 52.54°, 58.42°, 61.63°, 66.42°, 67.93°, 72.38°, and 75.28°, corresponding to the (002), (111), (−202), (202), (−113), (022), (113), (311), and (004) facets of the monoclinic CuO [43]. As for the ZnO catalyst, there are 11 diffraction peaks at a 2θ of 31.8°, 34.4°, 36.3°, 47.5°, 56.6°, 62.9°, 66.4°, 68.0°, 69.1°, 72.6°, and 77.0° [44]. The peaks of the Fe₃O₄ catalyst at 35.5°, 43.1°, 53.4°, 57.0° and 62.6° correspond to (311), (400), (422), (511), and (440) in the cubic spinel crystal-structured Fe₃O₄ [45]. The peaks at 24.1°, 35.6°, 57.4°, 64.0°, and 66.0° correspond to the (012), (110), (122), (300), and (125) crystal planes of hexa-Fe₂O₃ [46].

SEM micrographs of the three catalysts CuO, ZnO, and Fe₃O₄ are presented in Figure 11. It can be clearly seen from the low magnification scanning that these materials all take the form of very fine particles. Among these catalysts, CuO has the smallest particle size, and the sizes of CuO, ZnO, and Fe₃O₄ are about 5 μm, 10 μm, and 20 μm, respectively.

It can be seen from the above that the three catalysts exist in crystal states. The metal elements of ZnO and CuO have higher banding energies, which are relatively more conducive to discharge performance than that of Fe₃O₄. The binding energy of Cu²⁺ is higher than that of Zn²⁺, and CuO is more likely to play a photocatalytic role than ZnO. Therefore, CuO has a better degradation effect on benzene than ZnO at a lower voltage.

2.8. Mechanism of Benzene Decomposition

The benzene degradation efficiency using DBD combined with a catalyst is much higher than that without a catalyst, and ZnO/CuO/Fe₃O₄ cannot decompose benzene without the correct discharge conditions, implying that DBD and the ZnO/CuO/Fe₃O₄ catalyst have synergistic effects on benzene degradation. On the one hand, the catalyst can be activated effectively under plasma conditions [47]. The benzene degradation of the catalyst alone in the DBD-packed catalyst reactor has been studied at room temperature. For the three catalysts, no degradation of the benzene, nor any byproduct formation, is observed. On the other hand, the discharge from the DBD can be more uniform, and can generate more active particles, which largely improves the benzene degradation efficiency. The degradation efficiency of bare DBD only is 22.6% when the applied voltage is 24 kV, while benzene does not degrade at all at lower voltages. The interaction of the plasma and catalyst is shown in Figure 12a; the transition metal oxides can strongly promote the decomposition of O₃ on the surface of the catalyst to reactive oxygen particles, such as O⁻ and O₂⁻. ZnO is a photocatalyst, which can absorb UV to generate electrons and holes (e⁻ and h⁺), as well as being a surface that can generate OH through a sequence of complex chemical reactions [48].

$$\mathrm{ZnO}+\mathrm{h}\mathsf{\nu } \to { \mathrm{ZnO}+\mathrm{e}}^{-}{+\mathrm{h}}^{+},$$

(9)

$${\mathrm{e}}^{-}{+\mathrm{O}}_3 \to { •\mathrm{O}}_3^{-},$$

(10)

$${•\mathrm{O}}_3^{-} \to { •\mathrm{O}}^{-}{+\mathrm{O}}_2,$$

(11)

$${\mathrm{h}}^{+}{+\mathrm{H}}_2\mathrm{O} \to { •\mathrm{OH}+\mathrm{H}}^{+},$$

(12)

$${\mathrm{h}}^{+}{+\mathrm{OH}}^{-} \to •\mathrm{OH},$$

(13)

$${\mathrm{H}}^{+}{+•\mathrm{O}}^{-} \to •\mathrm{OH},$$

(14)

However, the bands of $\mathrm{OH} \left({\mathrm{A}}^2\mathsf{\Sigma }\to {\mathrm{X}}^2\Pi \right)$ cannot be distinguished, maybe because only small amounts are produced and quickly react with other particles. Furthermore, the discharge is strongly enhanced near the catalyst’s surface due to the accumulation of charge on the surface of the dielectric material, which results in more uniform plasma.

The mean energy of electrons accelerated by plasma usually ranges from 1 to 10 eV [49], and energetic electrons can impact molecules in the air, which makes H₂O, O₂, and N₂ molecules ionize to form reactive particles, including O₂⁻, OH, N, O₂⁺, O, O(¹D), O(³P), O⁻, etc. [9]. As mentioned above, it is by greatly increasing the number of high-energy electrons and other reactive species that the synergy between plasma and catalyst improves the degradation efficiency of benzene. For the benzene molecule, the dissociation energies of the C–C and C–H bonds are 5.0–5.5 eV and 4.4 eV [50]. As such, the C–H bonds of the benzene molecule are primarily attacked by high-energy electrons and other reactive particles to form phenyl radical, phenol, and o-benzoquinone, as well as other organic byproducts [7]. These derivatives of benzene can be further decomposed into unsaturated linear hydrocarbons and corresponding radicals, as well as organic acids, etc., such as cyclopentadienyl radical, vinylacetylene and 1,3-butadiene. In addition, these benzene derivatives may also polymerize to form aromatic hydrocarbon complex compounds [39]. Finally, these carbon-containing fragments degrade into CO, CO₂ and H₂O through a series of complex reactions, including hydrogenation and substitution [21,51]. The corresponding main degradation reactions are presented in Figure 12b.

3. Experimental

3.1. Experimental Setup

The setup diagram of DBD combined with a catalyst degrading benzene is shown in Figure 13. It consists of an AC power supply, a double DBD reactor, a benzene-containing gas generator system, an electric signal acquisition system, an optical diagnosis system, and an exhaust gas detection system. The DBD reactor is composed of two concentric cylindrical quartz glass tubes with different diameters, which are used as inner (outer diameter: 12 mm) and outer (outer diameter: 22 mm) dielectric barriers (1 mm thickness). A stainless steel screw is connected to a high voltage (HV) power supply as an HV electrode with a 10 mm diameter. A sheet of stainless steel mash 50 mm long is wrapped around the DBD reactor’s outer quartz glass wall serving as a ground electrode. ZnO, CuO, and Fe₃O₄ are chosen as catalysts with 1–3 mm particle sizes, which are packed in the DBD discharge region. The benzene-containing air is simulated by evaporating benzene through bubbling N₂ in a water bath, which is then diluted with synthetic air in a mixing chamber. The initial concentration of benzene is fixed by controlling the temperature of the temperature-controlled bath and the N₂ flow rate through a mass flow controller (D07-series, Sevenstar, Beijing, China), which also controls the total synthetic air (N₂:O₂ = 4:1) with a 500 mL·min⁻¹ flow rate. The discharge parameter of the DBD is obtained by an oscilloscope (TDS 3054B, Tektronix, Beaverton, OR, USA) coupled with a 1000:1 high-voltage probe (P6015A, Tektronix, Beaverton, OR, USA) and a 1:1 current monitor (A622, Tektronix, Beaverton, OR, USA). The OES are recorded by a grating spectrometer (DS-300-122B, Acton, MA, USA). The concentrations of benzene, CO₂ and CO are analyzed through gas chromatography (GC-7900, Techcomp, Shanghai, China).

3.2. Materials

In this work, CuO, ZnO and Fe₃O₄ were purchased from Beijing Zhong Nuo Xin Cai Scientific Co., Ltd., China. Benzene (99.5%) was purchased from Shanghai Titan Scientific Co., Ltd., China.

3.3. Catalyst Preparation

Firstly, 17.5 g of anhydrous calcium chloride was dispersed in 232.5 mL deionized water, and then 40 g CuO/ZnO/Fe₃O₄ powder was added to the solution; 2 g of alginic acid was dissolved in 98 mL deionized water and slowly added to the anhydrous calcium chloride mixture solution. After 2 h of stirring, the mixed solution was dropped onto 2–3 mm pellets via a peristaltic pump. Finally, the pellets were dried at 80 °C overnight, followed by calcining at 800 °C for 10 h using a muffle furnace.

3.4. Chemical Analysis

The performance of the benzene decomposition was evaluated by the factors of benzene removal efficiency (${\eta }_{\mathrm{benzene}}$), CO₂ selectivity (${\mathrm{S}}_{{\mathrm{CO}}_2}$), specific input energy (SIE) and energy yield (EY), defined as:

$${\eta }_{\mathrm{benzene}} (\%)=\left(\frac{C_{\mathrm{in}}{-C}_{\mathrm{out}}}{C_{\mathrm{in}}}\right) \times 100,$$

(15)

$${\mathrm{S}}_{{\mathrm{CO}}_2} (\%)=\frac{C_{{\mathrm{CO}}_2}}{\left(C_{{\mathrm{CO}}_2}{+C}_{\mathrm{CO}}\right)} \times 100,$$

(16)

$$\mathrm{SIE} (\mathrm{J}/\mathrm{L})=\frac{P}{Q} \times 60,$$

(17)

$$\mathrm{EY} (\mathrm{g}/\mathrm{kWh})=\frac{3.6 \times M \times \left(C_{\mathrm{in}}-{ C}_{\mathrm{out}}\right)}{24.4 \times \mathrm{SIE}},$$

(18)

where $C_{\mathrm{in}}$ and $C_{\mathrm{out}}$ present the inlet and outlet benzene concentrations (ppm). $C_{{\mathrm{CO}}_2}$ and $C_{\mathrm{CO}}$ represent the concentrations of CO₂ (ppm) and CO (ppm) at the outlet. Q is the total flow rate (L/min) of the gas, and P is the input power (W).

The classical Lissajous method is as follows [52]:

$$\mathrm{I} \left(\mathrm{t}\right)=\frac{\mathrm{d}q}{\mathrm{d}t}=C\frac{{\mathrm{d}U}_{\mathrm{c}}}{\mathrm{d}t},$$

(19)

where C is the capacitance with 0.33 μF, $U_{\mathrm{c}}$ is the voltage of the capacitor, and $U_{\mathrm{HV}}$ is the voltage of the whole device.

$$U_{\mathrm{t}} {= U}_{\mathrm{HV}}-{ U}_{\mathrm{C}} \approx { U}_{\mathrm{HV}},$$

(20)

since $U_{\mathrm{c}}\ll U_{\mathrm{HV}}$. So, the instantaneous power P (W) is:

$${P \left(\mathrm{W}\right)=U}_{\mathrm{t}}{ \times I}_{\mathrm{t}} \approx { U}_{\mathrm{HV}}\left(\mathrm{t}\right) \times C\frac{{\mathrm{d}U}_{\mathrm{c}}}{\mathrm{d}t},$$

(21)

Figure 14 shows the Lissajous figures of DBD packed with ZnO/CuO/Fe₃O₄. The applied voltage was 20 kV and the driving frequency was 14 kHz. The Lissajous figures of DBD packed with three catalysts all resemble a parallelogram. The shapes of the Lissajous figures change with the types of filling materials, because of the different dielectric strengths of different catalysts [53]. It is indicated that ZnO and CuO transfers more charge than Fe₃O₄, judging from the slopes of the graph [54].

4. Conclusions

In this paper, a DBD reactor packed with transition metal oxide was employed to generate uniform plasma and degrade benzene. As the applied voltage increased, the discharge intensity was strongly enhanced. With the applied voltage and driving frequency increasing, the emission intensities of the three active species, ${\mathrm{N}}_2 \left({\mathrm{C}}^3{\Pi }_{\mathrm{u}}\to {\mathrm{B}}^3{\Pi }_{\mathrm{g}}\right)$, ${\mathrm{N}}_2^{+} \left({\mathrm{B}}^2{\mathsf{\Sigma }}_{\mathrm{u}}^{+}\to {\mathrm{X}}^2{\mathsf{\Sigma }}_{\mathrm{g}}^{+}\right)$, and $\mathrm{O} \left({3p}^5\mathrm{P}\to {3s}^5{\mathrm{S}}_2^{\mathrm{o}}\right)$, were enhanced. The benzene degradation efficiency was promoted as the applied voltage and driving frequency increased, while EY decreased because part of the energy was lost as heat and light. With the initial benzene concentration increasing from 60 ppm to 300 ppm, the degradation efficiency decreased from 100% to 52.3% under the conditions of 18 kV applied voltage and a 14 kHz driving frequency over the ZnO catalyst. Catalyst type can greatly influence the degradation efficiency of benzene; CuO has the best performance in terms of benzene conversion below 22 kV, while benzene’s synergistic degradation with ZnO is better at higher applied voltages. When the initial concentration is 235 ppm, the applied voltage is 22 kV and the driving frequency is 14 kHz, the degradation efficiencies of CuO, ZnO and Fe₃O₄ are 93.6%, 93.2% and 76.2%, respectively.

This paper also establishes the synergetic mechanisms of hybrid plasma/ZnO, plasma/CuO and plasma/Fe₃O₄, as well as the differences in the synergistic performance of transition metal oxides, conforming that the combination of plasma and transition metal oxides can effectively improve the removal of benzene.