Removal of Volatile Organic Compounds (VOCs) at Room Temperature Using Dielectric Barrier Discharge and Plasma-Catalysis

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1 Plasma Chem Plasma Process (214) 34:81 81 DOI 1.17/s ORIGINAL PAPER Removal of Volatile Organic Compounds (VOCs) at Room Temperature Using Dielectric Barrier Discharge and Plasma-Catalysis Yizhuo Li Zeyun Fan Jianwei Shi Zhenyan Liu Jiwen Zhou Wenfeng Shangguan Received: 8 January 214 / Accepted: 17 February 214 / Published online: 28 February 214 Ó Springer Science+Business Media New York 214 Abstract Non-thermal plasma (NTP) was produced in a dielectric barrier discharge reactor for degradation of acetaldehyde and benzene, respectively. The effect of volatile organic compounds (VOCs) chemical structure on the reaction was investigated. In addition, acetaldehyde was removed in different background gas. The results showed that, no matter in nitrogen, air or oxygen, NTP technology always exhibited high acetaldehyde removal efficiency at ambient temperature. However, it also caused some toxicity by-product such as NOx and ozone. Meanwhile, some intermediates such as acetic acid, amine and nitromethane were formed and resulted in low carbon dioxide selectivity. To solve above problems, Co OMS-2 catalysts were synthesized and combined with plasma. It was found that, the introduction of catalysts improved VOCs removal efficiency and inhibited by-product formation of plasma significantly. The plasma-catalysis system was operated in a recycling experiment to investigate its stability. The acetaldehyde removal efficiency can be kept at 1 % in the whole process. However, slight deactivation in ozone control was observed at the later stage of the experiment, which may be ascribed to deposition of VOCs on the catalysts surface and reduction of catalysts surface area. Keywords Plasma Catalysis VOCs degradation Co OMS-2 catalyst Y. Li Z. Fan J. Shi Z. Liu J. Zhou W. Shangguan (&) Research Center for Combustion and Environment Technology, Shanghai Jiao Tong University, 8 Dong Chuan Road, Shanghai 224, People s Republic of China shangguan@sjtu.edu.cn J. Shi W. Shangguan Key Laboratory for Power Machinery and Engineering of Ministry of Education, Shanghai Jiao Tong University, Shanghai 224, People s Republic of China

2 82 Plasma Chem Plasma Process (214) 34:81 81 Introduction Volatile organic compounds (VOCs), which are emitted from various industrial process and transportation activities, are considered to be major air pollutants. They are not only hazardous to human health but also harmful to the environment. With the increases in emissions, control of VOCs has become an important issue [1]. Some conventional methods were applied for VOCs degradation such as adsorption [2], catalytic oxidation [3], bioreaction [4] and photocatalysis [5]. However, those methods became cost-inefficient and difficult to operate when low concentrations of VOCs need to be treated [6, 7]. In last two decades, non-thermal plasma technology attracted growing interest of scientists because of its high efficiency and quick response at ambient temperature [8 1]. In the plasma discharge zone, background gas molecules (e.g. nitrogen, oxygen) are excited, ionized and dissociated to form some unstable reactive species such as excited nitrogen molecule (N 2 (A 3 P u? )), hydroxyl radicals (OH) and reactive oxygen atoms (O). Those free radicals are regarded as ideal species for VOCs degradation [11 13]. However, we still know little about those reaction mechanisms. Therefore, the involved reaction processes should be investigated. On the other hand, NTP technology also causes some problems of toxicity by-product and low carbon dioxide selectivity in VOCs removal [14 16]. One way to overcome these disadvantages is combining catalysts with plasma. Previous studies [8, 17] have revealed that manganese oxides are suitable for the plasmacatalysis reaction. However, complete removal of VOCs and by-product for these catalysts is still difficult [17]. Therefore, a more effective catalyst should be developed in a plasmacatalysis system. In this work, a dielectric barrier discharge (DBD) reactor was applied to produce plasma for degradation of acetaldehyde and benzene, respectively. The effect of VOCs chemical structure and background gas on the plasma reaction were investigated. Co OMS-2 catalyst was synthesized and placed downstream of NTP to form a post plasma-catalysis system. The influence of catalysts introduction was discussed. In addition, the stability of plasma-catalysis system was also evaluated. Experimental Set-Up Experimental Set-Up of VOCs Degradation The experimental set-up of VOCs degradation by NTP or plasma-catalysis is described in Fig. 1. It includes a VOCs generator, a plasma-catalysis reactor and an exhaust gas analysis section. VOCs was evaporated by bubbling with the background gas (nitrogen, air or oxygen) and diluted before introduced into the plasma reactor. The flow rate and VOCs concentration could be adjusted by mass flow controllers (MFC), and fixed at 4 ml/min and 1 ppmv, respectively. The space velocity for catalytic reaction was kept at 12 h -1. As shown in Fig. 2, the length of plasma discharge zone was 2 mm and the gap spacing was 3 mm. Plasma reactor was driven by a high voltage AC power supply ( 1 kv, 5 2, Hz, sine wave). The input specific energy density (SED) of plasma reactor, in J/L, could be varied by adjusting the input voltage of the AC power supply. In the experiments, the frequency was fixed at 1, Hz. Areas of the voltage-charge Lissajous figures were used to measure the input electric discharge power [18]. A quartz tube filled with.9 g Co OMS-2/Al 2 O 3 catalyst was located downstream of the DBD reactor. The tube has an inner diameter of 12 mm and an outer diameter of 14 mm. Acetaldehyde

3 Plasma Chem Plasma Process (214) 34: Fig. 1 Experimental set-up of VOCs degradation by plasma or plasma-catalysis Fig. 2 Schematic overview of NTP reactor was analyzed by gas chromatography (GC956, Shanghai Huaai), equipped with a flame ionization detector (FID), and a packed column (Parapak-Q) with a column length of 2 m. Benzene was also monitored by GC956, with a SE-3 column. To determine the carbon dioxide concentration, a reformer furnace was used in the GC instrument. The production of NOx and ozone were defined by a chemiluminescent NO/NO 2 analyzer (Thermo Environmental Instruments Inc., 42i LS) and an ozone analyzer (Thermo Environmental Instruments Inc., Model 49i). The other intermediates in the exhaust were detected by an on-line fast FTIR infrared spectrometer (Nicolet-67). All the experiments were repeated for three times to keep an accurate result. Catalyst Preparation In a typical synthesis of Co OMS-2 catalyst, cryptomelane (K OMS-2) was prepared as precursor at first. In this step, KMnO 4 solution (13.3 g in 225 ml of distilled water) was added to a mixture of MnSO 4 H 2 O solution (19.8 g in 67.5 ml distilled water) and concentrated HNO 3 (6.8 ml). The suspension was stirred vigorously and refluxed at 373 K for 24 h. After a filtration, the precipitate was washed with distilled water until neutral ph and dried at 393 K. In this way, cryptomelane (K OMS-2) was synthesized [19]. After that, 2 g prepared cryptomelane was added to a Co(NO 3 ) 2 solution (.375 mol in 5 ml distilled water). The mixture was stirred for 24 h and collected by filtration. The precipitate was washed with water for several times, followed by drying at 353 K for 12 h and calcining at 573 K for 3 h. In order to decrease pressure resistance of catalysts and increase the adsorption of VOCs, Co OMS-2 was supported on Al 2 O 3 pellets (with a diameter of 2 mm) in the post plasma-catalysis reaction. To obtain the Co OMS-2/Al 2 O 3 catalysts, 3 g commercial c-al 2 O 3 pellets and.5 g Co OMS-2 were mixed together into a 5 ml sealed

4 84 Plasma Chem Plasma Process (214) 34:81 81 centrifuge tube with strong mechanical vibrations for 1 min. In this manner, c-al 2 O 3 pellets would be coated with the Co OMS-2 powder. Catalyst Characterization X-ray diffraction (Rigaku D/max-22/PC Japan) with Cu Ka radiation (4 kv, 2 ma) was applied to analyze the structure and phase purity of the catalyst. The chemical composition of catalysts bulk was measured by a coupled plasma atomic emission spectroscopy (ICP AES) on an ICAP-6-Radial instrument. Fourier-transform infrared (FT-IR) spectra were recorded in the range of 4 4, cm -1 with a resolution of 2 cm -1 on a Nicolet-67 spectrometer. The samples were mixed with KBr and well grounded before the FT-IR measurement. In addition, surface areas of Co OMS-2/Al 2 O 3 catalyst were determined by BET measurement (Micromeritics, USA, ASAP 21 M? C). Results and Discussion The Effect of VOCs Chemical Structure for NTP The degradation of acetaldehyde and benzene by NTP were carried out in an air background, respectively. The DBD reactor was worked at a low SED (defined as input energy divided by reaction flow rate) of 9.9 J/L in this section. VOCs removal efficiencies are shown in Fig. 3.It can be found that, NTP are effective to degrade acetaldehyde and benzene. The removal efficiency could achieve 62. and 39.1 %, respectively. However, some toxicity by-product are also detected in this process. In particular, the formation of ozone could reach more than 18 ppmv, which is even higher than the initial concentration of VOCs. On the other hand, compared to benzene, higher removal efficiency and lower production of ozone could be obtained in the reaction of acetaldehyde degradation. The reason for this may be that acetaldehyde has a less stable chemical structure than benzene, and it is easier to be oxidized by ozone. The reaction between ozone and acetaldehyde results in reduction of both them. Meanwhile, it is also observed that, more NOx is formed in acetaldehyde degradation. It has been reported that NOx is generated by the reaction between nitrogen and oxygen in the discharge zone of NTP [2]. But in our experiment, NTP is worked at the same condition just for different VOCs, and the production of NOx is Fig. 3 VOCs removal efficiencies and by-product formation of NTP at the SED of 9.9 J/L VOC removal efficiency (%) degradation of acetaldehyde degradation of benzene VOCs removal efficiency NOx concentration Ozone concentration Byproduct concentration (ppm)

5 Plasma Chem Plasma Process (214) 34: still different. Therefore, it can be inferred that NOx is not only resulted from the reaction between nitrogen and oxygen directly, but also attributed to the VOCs chemical structure. The Effect of Background Gas for NTP To investigate the effect of background gas on NTP, acetaldehyde was removed in nitrogen, air and oxygen respectively. As a function of input specific energy density (SED), the acetaldehyde removal efficiencies are plotted in Fig. 4a. Obviously, the enhancement of plasma input energy could improve acetaldehyde degradation for each background gas. At the same SED, more acetaldehyde can be degraded in oxygen plasma because of oxidation. It is noted that, nitrogen plasma without any oxygen species is also active for acetaldehyde degradation. Liu et al. [21] reported that, nitrate radical can play an important role in butyl-mercaptan treatment by plasma. According to their study, VOCs could react with excited nitrogen molecule to form some nitrogen-containing products. Carbon dioxide selectivity of NTP is shown in Fig. 4b. In oxygen plasma, higher input energy increases carbon dioxide selectivity, meanwhile, acetaldehyde can be completely converted into carbon dioxide at the SED of 13.1 J/L. The same trend is observed in air plasma. However, its carbon dioxide selectivity is much lower than oxygen plasma, which may be ascribed to the formation of intermediary product. When acetaldehyde is removed in nitrogen plasma at a high SED, a small amount of carbon dioxide is also detected. It can be concluded, acetaldehyde may contribute oxygen to the formation of carbon dioxide in nitrogen discharge. Acetaldehyde removal efficiency (%) plasma in air plasma in nitrogen plasma in oxygen A Ozone concentration (ppm) plasma in air plasma in nitrogen plasma in oxygen 4 6 C Carbon dioxide selectivity (%) plasma in air plasma in nitrogen plasma in oxygen B NO x concentration (ppm) plasma in air plasma in nitrogen plasma in oxygen D Fig. 4 The effect of background gas on acetaldehyde degradation by NTP: a acetaldehyde removal efficiency, b CO 2 selectivity, c O 3 concentration, d NOx concentration. Experimental condition: a flow rate of 4 ml/min, 1 ppmv acetaldehyde initial concentration, at 3 C and 6 % RH

6 86 Plasma Chem Plasma Process (214) 34:81 81 Fig. 5 FTIR analysis of intermediates product formed in acetaldehyde degradation by NTP at the SED of 9.9 J/L: a Acetaldehyde in air without NTP discharge; b Degradation of acetaldehyde by NTP in air; c Degradation of acetaldehyde by NTP in pure oxygen; d Degradation of acetaldehyde by NTP in pure nitrogen Absorbance (a.u.) -NO 2 C-N (amine) (nitromethane) C-N (nitromethane) } C-O (acetic acid) NO 2 CO CO 2 C=O (acetaldehyde or acetic acid) CH 3 - (d) H 2 O/OH Wavenumber (cm -1 ) (c) (b) (a) Figure 4c shows the ozone concentration of NTP in different background gas. As byproduct, ozone is formed in the discharge zone, but it is inefficient to react with VOCs directly [22]. Therefore, both ozone and acetaldehyde exist in the NTP exhaust. Oxygen plasma can produce much more ozone than air, while no ozone is observed in the nitrogen plasma. Figure 4d presents NOx production. It can be found that, the NOx formation in different background gas decreases in the order of air [ nitrogen [ oxygen =. As previously mentioned, NOx generated in air plasma is mainly ascribed to the reaction between nitrogen and oxygen. However, when acetaldehyde is removed in nitrogen plasma without oxygen, NOx is still formed. Consequently, it can be speculated that, some NOx originate from the reaction between acetaldehyde and excited nitrogen molecule. The on-line fast FTIR also proves the above conclusions. As shown in Fig. 5d, in the exhaust of acetaldehyde degradation by nitrogen plasma, a small amount of carbon dioxide, NOx, as well as residual acetaldehyde are detected. As nitrogen-containing intermediates, amine (band at 1,115 cm -1 ) and nitromethane (band at 915, 1,373 and 1,545 cm -1 ) are observed. When acetaldehyde is removed in air, less amine and nitromethane are produced, while more NO 2 is generated (Fig. 5b). In addition, carbon monoxide and acetic acid are also formed in air plasma, which may result in low carbon dioxide selectivity. In oxygen plasma, most intermediates can be oxidized, therefore, high acetaldehyde removal efficiency and carbon dioxide selectivity can be achieved. The Effect of Catalysts Introduction on NTP Reaction Although acetaldehyde can be completely oxidized into carbon dioxide and water in oxygen plasma, most NTP reactor is expected to work in an atmosphere environment for practical application. To improve the carbon dioxide selectivity and reduce the byproduct of air plasma, Co OMS-2/Al 2 O 3 catalysts are synthesized and combined with NTP in this work. Figure 6a d shows the acetaldehyde degradation of NTP with/without catalysts as function of SED. For instance, at the SED of 9.9 J/L, when Co OMS-2/Al 2 O 3 catalyst is introduced, the acetaldehyde removal efficiency of NTP could be improved from 62. to 1 %, and carbon dioxide selectivity is increased by 24.3 %. At the same time, the ozone formation is suppressed from to 3.8 ppmv, moreover, NOx production is also limited from 25.8 to 7.6 ppmv. In our experiment, Al 2 O 3 pellets without supporting Co OMS-2

7 Plasma Chem Plasma Process (214) 34: Acetaldehyde removal efficiency (%) A Plasma PPC:Co-OMS-2/γ-Al 2 O Ozone concentration (ppm) Plasma PPC:Co-OMS-2/γ-Al 2 O 3 C Carbon dioxide selectivity (%) B Plasma PPC:Co-OMS-2/γ-Al 2 O 3 NO x concentration (ppm) Plasma PPC:Co-OMS-2/ γ-al 2 O 3 D Fig. 6 The effect of Co OMS-2 catalysts introduction on NTP reaction: a acetaldehyde removal efficiency, b CO 2 selectivity, c O 3 concentration, d NOx concentration. Experimental condition: a flow rate of 4 ml/ min, 1 ppmv acetaldehyde initial concentration, at 3 C and 6 % RH are also applied in plasma-catalysis. The acetaldehyde removal efficiency, carbon dioxide selectivity and by-product formation are similar to those of plasma-alone. Therefore, Al 2 O 3 pellets just act as adsorbents and the effects of Co OMS-2/Al 2 O 3 catalysts on plasma are mainly contributed by Co OMS-2. Chen et al. [1] suggested that the suitable catalyst for combining with plasma must possess two characteristics: the effectiveness in decomposing ozone, and the generation of reactive oxygen species toward VOCs. Therefore, it can be inferred, in Co OMS-2 plasmacatalysis, ozone could be firstly decomposed by the cobalt ions and manganese ions. In this step, electrons are transferred from those cations to absorbed ozone [23], while the reactive oxygen species are formed and adsorbed on the surface of catalysts. And then, oxidation of acetaldehyde and intermediates by those oxygen species are taken place. In a word, oxygen species released from the decomposing of ozone participate in the oxidation of metal cations in Co OMS-2, afterwards re-reduction by acetaldehyde and intermediates are achieved. Finally, ozone is catalytically dissociated into oxygen, and acetaldehyde is further oxidized into carbon dioxide and water. The Stability of Plasma-Catalysis System To evaluate the stability, the plasma-catalysis system was operated in a recycling experiment at the SED of 7.9 J/L. The experiment was repeated for 3 cycles. In each cycle, the

8 88 Plasma Chem Plasma Process (214) 34:81 81 Fig. 7 Recycling experiment of plasma-catalysis reaction for acetaldehyde degradation at the SED of 7.9 J/L Acetaldehyde removal efficiency (%) acetaldehyde removal efficiency ozone concentration Ozone concentration (ppm) Number of cycles system was performed for 1.5 h. As shown in Fig. 7, acetaldehyde removal efficiency can be kept at 1 % in the whole process without any deactivation. Moreover, ozone formed in NTP can be maintained at about 1.5 ppmv in the first 2 cycles. However, a slight increase of ozone concentration is observed afterwards. Obviously, this deactivation in ozone control is caused by the Co OMS-2/Al 2 O 3 catalysts. The catalysts were characterized before and after the reaction. The XRD patterns of samples are shown in Fig. 8. For the fresh sample, the peaks at 2h = 37.5, 45.9, 6.3 and 66.8 can be indexed to c-al 2 O 3. The other peaks are assigned to cryptomelane-type crystalline phase (JCPDS 29-12) [24]. Cobalt oxides are not observed in Fig. 8a. However, cobalt content of the samples could achieve 3.2 % according to the ICP analysis. Therefore, it can be inferred that, cobalt ions substitute some manganese in cryptomelane to form the Co OMS-2 catalysts. Hu et al. [25] also reported that cobalt would readily replace surface manganese in the cryptomelane materials, due to their almost equivalent ionic radii to those of the manganese ions. The XRD patterns of used catalysts are similar to those of fresh catalysts (shown in Fig. 8b). It indicates that, phase structure of catalysts is not changed in the plasma-catalysis reaction. The specific surface area, pore size and pore volume of catalysts from the BET characterization are summarized in Table 1. Obvious decrease of catalysts surface area is observed after the recycling experiment. FTIR spectra of the samples are shown in Fig. 9. The bands of fresh Co OMS-2 observed at 532, 581 and 79 cm -1, are ascribed to vibrations of the MnO 6 octahedral framework [26], which presents a clear signature of cryptomelane structure. The broad band at 3,441 and 1,627 cm -1 are assigned to stretching of OH groups and tunnel water species in cryptomelane [27]. Compare to the fresh samples, some residual acetaldehyde are detected on the surface of the used samples. It can be inferred that, the deposition of acetaldehyde reduced the surface area of catalysts, and resulted in a deactivation of catalysts. Finally, ozone control got worse. However, abundant reactive oxygen species can be still supplied in this catalytic process, therefore, high acetaldehyde removal efficiency can be kept. Conclusions Acetaldehyde and benzene could be effectively removed by non-thermal plasma. Compared with benzene, acetaldehyde is less stable and easier to be oxidized by ozone.

9 Plasma Chem Plasma Process (214) 34: Fig. 8 XRD patterns of Co OMS-2 catalyst before and after the reaction cryptomelane γ-al 2 O 3 Intensity (a.u.) Used material B Fresh material A θ (degree) Table 1 Surface Area, Pore Volume and Pore Size of catalysts before and after the reaction Sample S BET (m 2 g -1 ) V pore (cm 3 g -1 ) D pore (nm) c-al 2 O Co OMS 2/c-Al 2 O 3 (fresh) Co OMS 2/c-Al 2 O 3 (used) Fig. 9 FTIR spectra of Co OMS-2 catalyst before and after the reaction Transmittance (%) Fresh material Used material adsorped acetaldehyde Wavenumber (cm -1 ) Therefore, higher removal efficiency and lower production of ozone could be obtained. However, more NOx is observed in acetaldehyde degradation. It can be inferred that, VOCs chemical structure will affect the NOx formation in NTP. For acetaldehyde degradation, oxygen plasma exhibits higher removal efficiency and carbon dioxide selectivity than air plasma or nitrogen plasma. Air plasma shows low carbon dioxide selectivity, which may be ascribed to the formation of intermediary product such as amine, nitromethane, carbon monoxide and acetic acid. In nitrogen plasma, carbon

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