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Research article

Catalytic Non-Thermal Plasma Reactor for the Decomposition of p-Xylene Over MnOx and CoOx Supported on Anodized Aluminum Oxide

M.S.P. Sudhakaran1, J. Karuppiah1, E. Linga Reddy2, M. Sanjeeva Gandhi1, Quang Hung Trinh1, Young Sun Mok1*

1Department of Chemical and Biological Engineering, Jeju National University, Jeju 690-756, South Korea
2Department of Chemical Engineering, Kyungpook National University, Daegu 702-701, South Korea

*Corresponding author: Dr. Young Sun Mok, Department of Chemical and Biological Engineering, Jeju National University, Jeju 690-756, South Korea, Fax: +82-64-7553670; Tel: +82-64-7543682; Email: smokie@jejunu.ac.kr

Submitted: 02-10-2016 Accepted:  05-04-2016 Published: 06-07-2016

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Article

 

Abstract

This work involved an investigation of the oxidation of p-xylene in a catalytic nonthermal plasma (NTP) reactor using anodized aluminum oxide (AAO) coated with manganese and cobalt (Mn and Co) oxides. The formation of the Mn and Co oxides on the AAO was confirmed by using X-ray diffraction (XRD) and X-ray photoelectron microscopy (XPS), and the surface morphology was analyzed using field emission–scanning electron microscopy (FE-SEM). The experimental results suggested that the impregnation of the AAO with Mn or Co oxide improved the conversion efficiency of p-xylene and the selectivity towards carbon oxides in comparison to bare AAO. The performance of the MnOx catalyst was superior to that of CoOx, which might be due to the formation of atomic oxygen by the in-situ decomposition of reactive ozone and nitrogen species. Overall, the use of AAO impregnated with MnOx and CoOx appears to efficiently catalyze the decomposition of pxylene in the catalytic NTP reactor.

Keywords: Non-thermal plasma; Catalysis; p-xylene; Anodized aluminum oxide

Introduction:

Volatile organic compounds (VOCs) are the most common toxic air pollutants emitted by chemical and petrochemical industries. The release of VOCs into the atmosphere has various harmful effects on the environment and human health [1-6]. Controlling the emission of VOCs by various strategies forms a major part of these industries commitment towards the environment [2]. Among the various VOCs, xylenes are highly toxic organic compounds and harmful to living organisms. Excessive exposure to xylene may affect the eyes, skin, and nose [1]. Yet, xylene is widely used in many industrial processes as a raw material and is the second most frequent organic solvent  detected in ambient air [7]. Sources of xylene include petrol, petroleum refineries, paint manufacturing processes, etc. [8]. The current occupational safety and health act (OSHA) standard for xylene is an average of 100 ppm over an eight-hour work shift in polymer manufacturing and purification industry [8].

Conventional technologies for the abatement of VOCs in air stream include thermal oxidation, catalytic oxidation, and carbon adsorption [9-14]. These techniques are not effective, especially for dilute VOC concentrations for which non-thermal plasma (NTP) generated at atmospheric pressure may be energy saving due to the fast ignition response and the generation of highly energetic electrons that may contribute to the chemical reactions in plasma for the removal of VOCs from the air stream. However, the above-mentioned techniques are limited by drawbacks such as the generation of secondary toxic pollutants [3]. The increased severity of emission limits creates the need for an alternative technology that overcomes the above-mentioned limitations. Of late, the non-thermal plasma technique combined with catalysis has been regarded as an improved alternative method for the treatment of VOCs [4,14]. The catalysts are available on various types of substrates, namely, alumina, silica, zeolites, etc. One of the more recent substrates that are considered attractive is anodic aluminum oxide (AAO) catalyst supports. These supports were prepared by anodic oxidation technology, which offers strong metal catalytic- support interaction [15]. Porous AAO templates are one of the most exciting structures, due to their unexpanded, regular, convenient, and nearly parallel pores [16]. The topology of the catalyst pore structure 10– 100 nm can influence reagent flow, the sequencing of catalytically active sites, and the contact time between the reacting gas and the catalyst. Hence, this offers a highly promising support structure for application in catalytic reactions. The addition of supporting catalysts to the AAO combined with NTP is an effective approach to control VOC elimination.

The present work involved the design of a DBD reactor based on an AAO catalytic support for the complete removal of p-xylene
from gas streams. Different operational parameters, such as specific input energy (SIE), were studied by varying the applied voltage, selection of a suitable catalyst, influence of the initial concentration, effect of water vapor, and the formation of ozone and organic byproducts. Finally, the possible decomposition mechanism of p-xylene with DBDs is proposed as well.

Experimental

Preparation of Catalysts


The porous AAO support was prepared by an anodization process using freshly prepared oxalic acid solution. The preparation of the AAO support followed the work of Reddy et al. [17]. Briefly, high-purity aluminum foil was anodized in 0.3 M oxalic acid at 40 V for 24 hours at 10 °C to form a porous γ-alumina supported layer, after which porewidening treatment (PWT) was carried out with the same oxalic acid solution used for anodization at 30 °C for 4 hours. After completion of the PWT, the plate was washed in a sufficient amount of deionised water to allow for the removal of residual oxalic acid. Finally, an AAO support with a high surface area was obtained by calcination in air at 450 °C.

The catalysts were prepared by using a wetness impregnation method previously developed by us [18]. AAO was used as catalyst support. The incorporation of the transition metal oxides of 3 wt.% MnOx and CoOx into the AAO catalyst, was achieved by impregnation with aqueous nitrate solutions of Mn(NO3)2.6H2O and Co(NO3)2.6H2O, respectively, followed by drying at 110 °C overnight. The dried AAO template was calcined in air at 450 °C for 5 hours to obtain the MnOx/AAO and CoOx /AAO catalysts. The MnOx and CoOx/AAO was limited to 3 wt.% of the metal oxide, respectively. Finally, the AAO supports were rolled into cylindrical form with a diameter of 22 mm to obtain the desired discharge gap of 9.5 mm.

Methods

The schematic diagram of the NTP plasma catalytic reactor for the decomposition of p-xylene is shown in Figure 1. The dielectric barrier discharge was generated in a cylindrical  eactor made of a quartz tube, with an outer diameter and wall thickness of 31.5 and 3 mm, respectively. The inner electrode consisted of a metallic rod with a diameter of 8 mm. The outer electrode was made of aluminum foil and was positioned around the quartz tube, of which the discharge gap was 9.5 mm, and the length was 10 cm, resulting in a volume for the plasma of 77 cm3. Thus, the modified AAO cylindrical rolled shape (22 mm diameter) acts as the inner electrode as well as the catalyst.

Figure 1. Schematic diagram of the experimental setup

envsci fig 15.1

The SIE was changed in the range of 480 __1180 J/L by varying the high-voltage AC (15.5 __ 23.5 kV/400 Hz). The voltage (V)-charge (Q) Lissajous technique is widely used to measure the discharge power (W) in a plasma regime. The output V-Q waveforms were monitored by a digital oscilloscope (Tektronix, TDS 3062) and plotted to obtain a typical VQ Lissajous diagram [19]. The SIE of the discharge was determined by the following equation (1) [18,19]. Increasing the applied voltage of the plasma reactor increases the power and specific input energy.

The gas contaminated by p-xylene was prepared by mixing pure p-xylene with air (O2: 21%; N2: 79%) using mass flow controllers (MFCs). The effect of water vapor was studied by preparing the feed gas to contain 2.5 vol% of water vapor by bubbling water saturated with N2 through the feed gas in a temperature-controlled water bath. The experimental setup consisted of a continuous flow gas operation and the experiments were conducted at room temperature. The initial concentration range of p-xylene was 250 ppm to 1000 ppm and the total flow rate of the feed gas was fixed at 1000 mL min-1.

The conversion efficiency of p-xylene at each voltage was measured after 30 min. The concentration of pxylene at the outlet of the reactor was monitored by a gas chromatograph (Bruker 450) equipped with a flame ionization detector (FID) and a BR-624 capillary column (60 m length, 0.32 mm thickness). In addition, the formation of CO2, CO, and the concentration of residual O3, N2O, and NOx were simultaneously monitored with a Fourier-transform infrared (FTIR) spectrometer (FTIR 7600, Lambda). The yellow organic byproducts deposited on the catalyst surface were analyzed using FTIR spectroscopy (NICOLET 6700) and a gas chromatographmass spectrometer (GC-MS Agilent GC 7890) coupled with a mass spectrometer (Agilent 5975 MSD).The conversion of p-xylene as well as CO2 and CO selectivity is defined as follows [20]:

where [COx] is the sum of the outlet concentration of CO2 and CO.

The X-ray diffraction (XRD) patterns of the prepared catalysts were recorded by an X-ray diffractometer (Rigaku D/max- 2200H) with a Cu Kα radiation source. Field emissionscanning electron microscopy (FE-SEM) (JEM-1200EX II, JEOL) at an accelerating voltage of 200 kV was used to examine the surface structure and the particle distribution over the AAO support. In order to verify the formation and adsorption of the transition metal oxides on the AAO and also to analyze the carbon deposit, an XPS analysis was carried out using XPS (Theta Probe AR-XPS System, Thermo Fisher Scientific with monochromatic Al Kα radiation (1486.6eV) operated at 15 kV).

Results and Discussion

Catalyst Characterization

The XRD patterns of the bare AAO, MnOx/AAO and CoOx/AAO catalysts are shown in Figure 2. Figure 2a shows the XRD pattern
of the AAO-support, where the strong intensities of the three main peaks at 45°, 66.2°, and 78.2° correspond to the (400), (440), and (311) planes, which indicates the amorphous phase of γ-Al2O3 and Al [21,22]. The MnOx/AAO and CoOx/ AAO catalysts show strong peaks for Mn and Co, thereby indicating the presence of bulk species on the surface.

envsci fig 15.2

 

Figure 2. XRD pattern of the AAO and MnOx, CoOx-modified AAO.

The Al2O3 and Al peaks are diminished due to shielding of the AAO surface with metal particles. As can be seen from Figure 2b the characteristic peak of the MnOx phase (2θ = 37.9°, 43°, and 65°) corresponds to the (101), (111), and (002) planes, which correspond well with JCPDS PDF No. 03-065-2821 [23,24]. In Figure 2c the CoOx/AAO diffraction peaks at 32°, 36.8°, 38°, 44°, 55°, 59°, and 65° correspond to the (220), (311), (222), (400), (422), (511), and (400) planes of CoOx, which corresponds well with JCPDS PDF No.042-1467 [25]. Figure 3 shows the Mn 2p and Co 2p XPS spectra of the 3% MnOx, CoOx/AAO catalyst samples. Figure 3a shows the typical deconvoluted XPS spectra of the 3 wt. % MnOx/AAO, from which it is evident that there are two kinds of Mn species present, the first, with a Mn 2p binding energy in the range of 641.5 – 641.9 eV is assigned to Mn3+, whereas that in the range 642.6 – 643.1 eV is ascribed to Mn4+ [26-28]. In a similar manner, Figure 3b shows the XP spectra of 3 wt. % CoOx/AAO in the XP spectra of CoOx/SMF in the Co 2p region and also shows two peaks in the 776 – 784 eV region (Figure 3b). The first peak, centered on 779.9 eV, was assigned to Co in the mixed-valence state between +2 and +3 (Co3O4), whereas the second peak centered at 781.7 eV, was assigned to Co in the +2 oxidation state (Co(OH)2) [27,29,30].

envsci_fig_15.3

Figure 3. XPS spectra of fresh catalyst (a) MnOx/AAO (b) CoOx/AAO

The surface morphologies of bare AAO and AAO modified by the MnOx, CoOx catalysts were analyzed using FE-SEM at a 100 nm magnification as shown in Figure 4a. It showed a well-defined porous homogeneous structure. Figure 4b and 4c show the typical morphology of (3 wt. %) MnOx and CoOx samples. It can be seen that for both formulations, the shape of the particles was spherical with a nano-size distribution with a relatively homogenous size distributed throughout the AAO pores. As can be seen in Figure 4b-4c, the formation of a monolayer of nanoparticles on the AAO surface was observed, which is possibly due to interactions between the metal NPs and AAO support.

Performance of NTP Reactor for the Decomposition Of P-Xylene

In the present work, the concentration of p-xylene was varied between 250 and 1000 ppm to study the performance of the reactor. Figure 5a represents the activity of various catalytic AAO modified electrodes during the conversion of 1000 ppm of p-xylene in the SIE range of 480 to 1180 J/L. It shows that the conversion of p-xylene increases when the overall SIE of the catalyst increases. Conversion of all the catalysts increased from ~ 85 % at 480 J/L to more than 95 % at 1180 J/L;

 

envsci_fig_15.4

Figure 4. Representative FE-SEM images of the monolith catalyst (a) bare AAO (b) MnOx/AAO (c) CoOx/AAO

however, bare Al only showed an increase of 80% at 480 J/L, whereas 90% conversion was observed at 1180 J/L. In addition, the metal oxide (MnOx, CoOx) modified AAO did not show any significant influence, indicating that high concentrations of p-xylene lead to oxidation to form yellow polymeric products on the catalyst, thereby covering the active sites of the catalyst. This might be the cause of catalyst deactivation during the reaction. In the oxidation of p-xylene, the desired products are CO2 and H2O; i.e., total oxidation. However, as the NTP removal of VOCs may lead to undesired toxic byproducts such as CO, selective conversion to CO2 is generally not 100 %.

envsci_fig_15.5a

envsci_fig_15.5b

Figure 5. Effect of catalyst as a function of SIE for 1000 ppm of p-xylene (a) conversion (b) selectivity of COx and CO2

During this study, the formation of organic byproducts was detected along with unreacted p-xylene at the outlet, which enabled us to determine the total COx selectivity and also the  carbon balance. Figure 5b represents the selectivity toward the formation of COx (CO + CO2) and the carbon balance. The selectivity toward COx was close to 50% with all catalysts and never reached 100 % at an SIE of 480 J/L. However, when the SIE is increased, the COx selectivity also increases and reaches 95 % at 1180 J/L. Hence, with 1000 ppm of p-xylene, in the SIE range of the present study, the bare aluminum electrodes showed poor carbon balance as a significant amount of organic compounds was deposited on the reactor walls. However, AAO modified with MnOx and CoOx results in higher COx and CO2 selectivity due to the effect of ozone decomposition by metal oxides. The selectivity towards CO2 followed a trend similar to that of COx. The modification of AAO with transition metal oxides
did not show any significant influence even at higher SIE; for example, catalysts modified with MnOx and CoOx showed a maximum CO2 selectivity of 45 % even at 1180 J/L [31]. During the destruction of 1000 ppm of p-xylene, the COx selectivity was not 100 %, which may be due to the high concentration of p-xylene. For this concentration of p-xylene the activity of the examined catalysts followed the trend MnOx/AAO>CoOx/ AAO>AAO>bare Al. The effect of the inlet concentration of p-xylene was examined by varying the concentration between 1000 and 250 ppm.

Figure 6 presents the performance of the catalytic NTP reactor for the destruction of 250 ppm p-xylene. As shown in Figure 6a,decreasing the p-xylene concentration from 1000 ppm to 250 ppm has a significant effect on the conversion rate. However, AAO modified with MnOx and CoOx did not show any significant conversion for 250 ppm p-xylene, with all the electrodes showing nearly the same activity. Most of the industrial oxidation processes produce flue gases containing water vapor; thus, the effect of water vapor on the VOC oxidation process always needs careful investigation. The effect of water vapor on the oxidation of p-xylene has been examined by employing feed gas containing 250 ppm of pxylene in 2.5% of water vapor (25,000 ppmv) with the MnOx/AAO catalyst. The conversion of 250 ppm p-xylene in both dry and humid conditions as a function of SIE was varied between 480 and 1180 J/L over the various catalytic electrodes. The MnOx and CoOx modified AAO electrodes showed improved conversion under both humid and dry conditions, and this enhanced activity may be due to the formation of strongly oxidant atomic oxygen [32, 33]. Figure 6b represents the selectivity to COx during the destruction of 250ppm of p-xylene, showing an increase in the selectivity toward COx reaching 100% (no carbon deposits) on MnOx and CoOx, when the SIE increased to 480 J/L and 630 J/L, whereas bare Al required it to be higher than 990 J/L. An interesting observation is that, under humid conditions at an SIE of 480 J/L, the selectivity of MnOx/AAO to COx was shown to be only 75%. Hence, during the destruction of 250 ppm of p-xylene, an SIE of 630 J/L is required in order to avoid the deposition of partially oxidized organic compounds under humid air conditions with a MnOx/AAO catalyst. However, the selectivity of MnOx and CoOx/AAO in terms of the formation of COx was superior to that of unmodified AAO and bare aluminum in both dry and humid environments.

The selectivity toward CO2 followed the same trend, where MnOx/AAO modified electrodes under humid conditions produced superior results compared to CoOx/AAO and MnOx/ SMF/dry air.

envsci_fig_15.6a

envsci_fig_15.6b

Figure 6. Effect of catalyst as a function of SIE for 250 ppm of p-xylene (a) conversion (b) selectivity of COx and CO2.

This difference between dry and wet air conditions may be explained by the additional production of OH radicals generated in the NTP reactor with the addition of water vapor. These radicals are considered to be more active than oxygen radicals and other active species such that the oxidation of VOCs would be accelerated considerably under humid conditions [34,35]. Hence, it might be concluded that lower pxylene concentrations and humid conditions improve the oxidation of p-xylene by NTP.

Effect of Initial p-xylene Concentration on the Performance of the Reactor

To understand the influence of the concentration of p-xylene on the performance of the reactor, the initial concentration was varied between 250 and 1000 ppm at an SIE of 810 J/L, and the results are presented in Figure 7a.

envsci_fig_15.7a

envsci_fig_15.7b

Figure 7. Influence of initial concentration of p-xylene on catalyst at SIE of 810 J/L (a) conversion (b) selectivity of COx and CO2.

The conversion decreased with an increasing concentration of p-xylene from 250 to 1000 ppm. With the aluminum electrode at an SIE of 810 J/L, the removal efficiency for 250 ppm of p-xylene was ~99%, which decreased to ~90% for 1000 ppm. A similar trend was observed in all cases for p-xylene concentrations >500 ppm. In the case of higher concentrations of 750 ppm and 1000 ppm, the MnOx/AAO, CoOx/AAO shows a slightly higher conversion than the aluminum electrode. For concentrations of 750 ppm and 1000 ppm, the catalytic activity followed the trend MnOx/AAO>CoOx/AAO>AAO>Al. The desired outcome of the destruction of VOCs is total oxidation, i.e., complete conversion to CO2 and H2O. However, the NTP destruction of VOCs is non-selective and may lead to undesired yellow polymeric deposits in addition to CO and CO2; hence, the elimination of the carbonaceous deposit is warranted. As these are the only gaseous products observed, SCOx may also represent the percentage of carbon, as shown in Figure 7b, which depicts the selectivity to COx as a function of the concentration of p-xylene.

It is shown that, for 250 ppm of p-xylene with the Al electrode, the selectivity toward forming COx was 80% at an SIE of 810 J/L, which decreased to 56% for 1000 ppm. AAO modification with metal oxides (MnOx, CoOx) also increased the selectivity toward COx. The unmodified AAO electrode showed a slightly higher selectivity than Al, whereas CoOx and MnOx/AAO showed still higher activity. For 1000 ppm of p-xylene, MnOx/ AAO showed around 80% SCOx against 56% on Al electrode. CO2 selectivity followed the same trend where the MnOx/AAO catalyst showed improved activity as presented in Figure 7b. For 250 ppm of p-xylene, the Al electrode showed 39% selectivity to CO2 against 60% with the MnOx/AAO catalyst. For all cases, the SCO2 decreased significantly with increasing p-xylene concentration; for example, during the oxidation of 1000 ppm p-xylene, MnOx/AAO showed only 35% selectivity.

envsci_fig_15.8

Figure 8. Stability of catalysts for 500 ppm of p-xylene at constant SIE 810 J/L for 11.6 h.

Among the catalysts explored, the MnOx-coated AAO was exposed to higher amounts of CO2 than the other catalyst and bare AAO or Al, suggesting that the MnOx effectively oxidized the intermediate byproducts to carbon dioxide. The enhanced decomposition efficiency could be related to the adsorption of p-xylene on the surface of alumina, with the adsorbed p-xylene slowly migrating to the metal oxide by surface diffusion and then interacting with the plasma discharge decomposition  process. Subsequently, the carbon oxides produced in this way migrate from the metal oxide into the gas phase, regenerating the adsorption capacity of the alumina. Meanwhile, in the presence of the plasma, the metal oxide can produce reactive surface oxygen species (such as O2 - and O-) via ozone decomposition on the surface, following which subsequent decomposition reactions would take place, leading to the efficient decomposition of p-xylene [30,36,37]. The metal oxide was found to play a significant role in the oxidative decomposition of p-xylene when combined with the plasma [20]. The selectivity towards carbon oxides and the carbon balance obtained with the metal oxide-coated alumina also exceeded those obtained with the bare alumina.

Figure 8 presents the performance of the DBD reactor over a period of time at an SIE of 810 J/L during the destruction of 500 ppm of p-xylene for ~12 hours.

As seen from Figure 8, all the catalytic electrodes maintain nearly the same activity throughout the course of the reaction. Meanwhile, a very small extent of catalyst deactivation was observed for bare aluminum after 5 hours. The catalytic efficiency was slightly reduced due to the yellow polymeric deposition on the surface of the catalyst after some time. This might cover the catalytically active sites of the catalyst.

Effect of Ozone Concentration on the Performance of the Reactor

Ozone is one of the important oxidation byproducts in the NTP reactor and is produced from air by the ionization of oxygen molecules. In catalytic plasma reactions, the role of the catalyst is to facilitate in situ decomposition of ozone to produce a highly active atomic oxygen oxidant. Figure 9a shows the influence of the catalysts on in situ ozone decomposition under both dry and humid conditions, at an SIE of 810 J/L. In dry air the ozone formation was 200 ppm with the Al electrode, and this decreased to 90 and 80 ppm with the CoOx/AAO and MnOx/AAO catalytic electrodes, respectively. The interaction of ozone with the metal oxide catalyst probably increased the formation of reactive atomic oxygen, which resulted in improved performance. Under humid conditions, the ozone formation was further decreased to 40 ppm. This decrease in the ozone concentration in humid air suggests the formation of atomic oxygen.

envsci_fig_15.9

Figure 9. (a) Influence of catalyst in the plasma reactor as a function of ozone under dry and humid conditions (b) influence of ozone concentration on p-xylene decomposition for 250 ppm of p-xylene and SIE 810 J/L.

It is known that ozone acts as an electron acceptor to produce O3•−, in turn generating hydroxyl radicals, which have a strong capability to oxidize VOCs. The hydroxyl radicals would react with O3 to generate HO2 radicals and O2, and then the HO2 radicals would react with O3 to form hydroxyl radicals and O2 [2, 32, 34, 38, 39]. These findings revealed that the MnOx/AAO catalyst showed improved catalytic ozonation from the standpoint of catalytic activities and efficiency for ozone utilization.

Considering the results, improved catalytic performance of the reactor was achieved with the MnOx/AAO electrode, which changed the product distribution towards the total oxidation of p-xylene. Figure 9b presents the xylene conversion and selectivity as a function of ozone concentration at a fixed SIE of 810 J/L. When the ozone concentration decreased from 200 to 40 ppm, this did not significantly influence the conversion for any of the candidates, but in the case of CO2 the selectivity was ~65% with MnOx-modified AAO in humid air and the corresponding ozone concentration was 40 ppm. However, in the case of dry air, the selectivity toward CO2 was 60% and the concentration of ozone was 80 ppm. This observation suggested that the molecular oxygen was involved in an autoxidation reaction in which radical intermediates (R*) were oxidized by O2 to form the final products of CO2 and CO [33,34,40]. Ozone concentration is in Figure 9a.

R*+ O2 → RO2 * → CO2, CO      (6)

The p-xylene oxidation produced R* radicals as intermediate products. When the amount of O2 involved in the autoxidation processes increased, the ratio of ozone decomposition to pxylene oxidation increased. This leads to the selective formation of CO2 due to the reaction of various organic species and acceleration of the oxidative processes.

Formation of N2O during the p-xylene Decomposition

A critical drawback of the NTP reactor operated in air, is that it may produce secondary pollutants such as nitric oxides species (NO, NO2, N2O) and acids such as HNO3 and HNO2 [41]. In this study, a negligible amount of NOx (NO or NO2) was detected, but the formation of N2O was significant. Figure 10 shows the formation of N2O during the decomposition of p-xylene as a function of the SIE of 480-1180 J/L. The formation of N2O linearly increases with increasing SIE for all the catalysts. The amount of N2O produced slightly decreased with the use of MnOx/AAO.

FTIR and GC-MS Study of the Byproducts

The FTIR spectra of the yellow organic byproducts deposited on the catalyst surface are presented in Figure 11, which shows the presence of C-C (2991 cm-1 and 2943 cm-1), C=C (1640 cm- 1), O-H (3439 cm-1), CH3 (1385 cm-1 , 1216 cm-1), C-O( 1095 cm- 1) and C=O (1734 cm-1) stretching vibrations, indicating that some of the p-xylene molecules are oxidized to form alcohol, ether, aldehydes, and carboxylic acid. [40-44].

envsci_fig_15.10

Figure 10. Nitrous oxide (N2O) concentration in the plasma reactor as a function of SIE for 250 ppm of p-xylene.

envsci_fig_15.11


Figure 11. FTIR spectrum of organic byproduct formation during decompositionof p-xylene in NTP.


Different kinds of organic byproducts and some partially oxidized organic intermediates evolved during p-xylene decomposition under non-thermal plasma oxidative conditions. Moreover, some brownish yellow polymer-like deposition was also observed on the reactor wall and surface of the catalyst. These deposits were dissolved in acetone and analyzed by GC-MS. The detected byproducts are listed in Table S1. Among these byproducts, benzene and phenol are ring-retaining products, whereas the spectral analysis indicated that the aromatic ring of other products, such as higher cyclical hydrocarbon organic compounds, had been opened during the destruction of VOCs by the plasma. (9b)

Van Durme detected both ring-retaining and ring-opening products during the degradation of toluene by positive corona discharge, and the ring-opening byproducts were not in agreement with those in this study [45]. The resulting GC-MS spectra for the identification of intermediate products are provided as supplementary information (S1). The possible decomposition mechanism of p-xylene is given in the chart. The oxidation of p-xylene with NTP plasma was primarily accomplished through attack with electrons, ions (O2 +, O+, N2 + or N+), O radicals, and OH radicals as follows[20, 44],

C6H4 (CH3)2 + e + → CO2, CO + intermediate (7)


C6H4 (CH3)2 + (O2 +, O+, N2 + or N+ )→ CO2 + H2O (8)


C6H4(CH3)2 + OH → C6H4CH2CH3 + H2O (9a)


→ C6H4 (CH3)2OH (9b)

Figure 12 shows the possible decomposition pathways of p-xylene, with the peroxyl radicals formed by the O2 via H abstraction
or reversible addition with hydroxilated p-xylene adducts. In addition to radical reactions, O3 provides another possible route leading to ring-cleavage [47-49].

envsci fig 15.12

Figure 12. Proposed plasma-catalytic decomposition pathway of p-xylene.

Further, the secondary adducts are easily oxidized due to their unstable chemical structure. The remaining compounds are further oxidized to form CO and CO2 via various oxidation reactions.

Conclusions

The combination of transition metal oxides on AAO plate-type catalysts with NTP significantly increased the p-xylene removal efficiency and COx selectivity and obviously also reduced the formation of toxic organic intermediate compounds such as  alcohols, carbonic acids, and esters. The decomposition efficiency of the MnOx-modified AAO was found to be higher than that
of CoOx/AAO followed by that of AAO. Treatment of the AAO electrode with oxides of Mn and Co improved the total oxidation towards the formation of CO2. The complete decomposition of 250 ppm of p-xylene to CO2 was achieved with a specific input energy (SIE) of 480 J/L. Optimal p-xylene oxidation was achieved with 3 wt.% MnOx, CoOx loading. The highest CO2 selectivity was achieved at an SIE of 1180 J/L and an H2O content of 2.5% with the MnOx/AAO. This was closely related to the in situ decomposition of ozone on MnOx/AAO to induce the formation of reactive oxygen species. In this study, 3 wt.% MnOx/ AAO showed the best performance, with complete oxidation achieved at an SIE of 610J/L.

Acknowledgements

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (Grant No. 2013R1A2A2A01067961).

References

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Cite this article:Young Sun MokD. Catalytic Non-Thermal Plasma Reactor for the Decomposition of p-Xylene Over MnOx and CoOx Supported on Anodized Aluminum Oxide. J J Environ Sci. 2016, 2(2): 015.

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