An in-depth investigation of toluene decomposition with a glass beads packed bed DBD reactor

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1 Article Subscriber access provided by Ghent University Library An in-depth investigation of toluene decomposition with a glass beads packed bed DBD reactor Zhiping Ye, Savita Kaliya Perumal Veerapandian, Iuliia Onyshchenko, Anton Nikiforov, Nathalie De Geyter, Jean-Marc Giraudon, Jean-François Lamonier, and Rino Morent Ind. Eng. Chem. Res., Just Accepted Manuscript DOI:./acs.iecr.b00 Publication Date (Web): Aug 0 Downloaded from on August, 0 Just Accepted Just Accepted manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides Just Accepted as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. Just Accepted manuscripts appear in full in PDF format accompanied by an HTML abstract. Just Accepted manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI ). Just Accepted is an optional service offered to authors. Therefore, the Just Accepted Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the Just Accepted Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these Just Accepted manuscripts. Industrial & Engineering Chemistry Research is published by the American Chemical Society. Sixteenth Street N.W., Washington, DC 00 Published by American Chemical Society. Copyright American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

2 Page of Industrial & Engineering Chemistry Research An in-depth investigation of toluene decomposition with a glass beads packed bed DBD reactor Zhiping Ye,, Savita K P Veerapandian, * Iuliia Onyshchenko, Anton Nikiforov, Nathalie De Geyter, Jean-Marc Giraudon, Jean-François Lamonier, and Rino Morent. Univ. Lille, CNRS, Centrale Lille, ENSCL, Univ. Artois, UMR UCCS Unité de Catalyse et Chimie du Solide, F-000 Lille, France Ghent University, Faculty of Engineering and Architecture, Department of Applied Physics, Research Unit Plasma Technology, Sint-Pietersnieuwstraat B, 000 Ghent, Belgium KEYWORDS Packed bed DBD, toluene abatement, OES, by-products identification and quantification ABSTRACT A glass beads packed bed dielectric barrier discharge (DBD) reactor is used for the removal of low concentration toluene (0 ppm) in air. The influence of relative humidity (RH) of the air on the discharge characteristics, toluene removal efficiency and by-product formation is examined. Optical emission spectroscopy (OES) has showed that the amount of N excited states decreases with increasing RH due to the increased quenching of these excited states. A toluene removal efficiency of ±% was obtained at an optimum RH of 0% at a specific input energy of approximately 0 J/L. The main products of the toluene decomposition process were identified as CO, CO, N O, O (both dry and humid air) and HCOOH (dry air only). While

3 Industrial & Engineering Chemistry Research Page of higher RH suppress the formation of formic acid, the highest CO and CO yields, N O and O concentration at an RH of 0% confirm the observed highest removal efficiency at this experimental condition.. INTRODUCTION In recent years, the removal of volatile organic compounds (VOCs) from air streams has become of great interest as these compounds are known to cause adverse environmental effects as well as serious problems to human health. As a result, several VOC abatement techniques such as adsorption, bioprocesses, thermal/catalytic oxidation, and membrane separation, have already been developed at this point, each with their own advantages and limitations. However, for the abatement of dilute VOC streams (< 00 ppm), the above mentioned conventional techniques are often not suitable due to their very high-energy consumption. Over the last three decades, non-thermal plasma (NTP) technology has however been identified as a cost and energy efficient alternative for the elimination of low concentration VOCs. NTPs are very attractive in such a way that the applied energy is mainly used to create an intense electric field in which electrons are selectively accelerated instead of heating up the entire gas flow. These high energy electrons in turn collide with present gaseous molecules such as N, O and H O to induce multiple chemical processes such as excitation, ionization and dissociation. This in turn leads to the creation of active, unstable species like radicals, ions and photons, which can decompose the pollutant molecules. Among various types of NTPs which can be used for VOC abatement, dielectric barrier discharges (DBDs) are very attractive because of their simplicity, their straightforward scalability and their capability to treat atmospheric pressure gas streams with a negligible

4 Page of Industrial & Engineering Chemistry Research pressure drop. When DBDs are applied for VOC abatement, the discharge is typically generated between two concentric cylindrical electrodes enclosing a small gas gap which is often filled with dielectric pellets to create a so-called packed bed DBD reactor., Applying a high voltage to the electrodes leads to the generation of a very strong electric field on each pellet contacting point, resulting in the formation of microdischarges in the void spaces between the pellets and on their surfaces., The dielectric pellets can be either non-catalytic or catalytic, however, the latter system, also referred to as an in-plasma catalysis system lies not within the scope of the present work. Packed bed DBD reactors have received considerable attention in the past for VOC abatement since they are able to achieve a very high energy efficiency,,, due to the use of high dielectric constant pellets, the breakdown voltage can be reduced, which is favorable for industrial applications as higher discharge powers can be obtained at the same applied voltage., In addition, very high electric fields can be obtained in packed bed DBD reactors resulting in discharges with a low electron density, but with high electron energies. The presence of these high energy electrons is a major advantage as they are able to induce electronimpact reactions, which are known to play a major role in plasma-assisted VOC decomposition processes. In this work, a packed bed DBD reactor filled with glass pellets will be used to examine the decomposition of toluene. Toluene is widely used as feedstock in the chemical industry for synthesis of drugs, dyes, explosives, and as a solvent (e.g. thinner, paints, adhesives,0 ). In addition, exposure to toluene is known to affect the central nervous system and may cause tiredness, confusion, weakness, memory loss and nausea.,,0 Moreover, as toluene is water-insoluble, it cannot be scrubbed and should be eliminated from gas streams making use of alternative techniques. Based on its widespread use and its severe health impact, toluene has

5 Industrial & Engineering Chemistry Research Page of therefore been chosen as model compound in the current study. In an ideal scenario, toluene should be mineralized to CO and H O, however, it is well known that NTP technology often results in incomplete VOC oxidation resulting in the formation of unwanted by-products., Nevertheless, a thorough identification and quantification of these created by-products is often indispensable to elucidate the breakdown mechanism of toluene by NTP. In addition, when the packed bed reactor needs to be filled with catalytic pellets in a next step to perform in-plasma catalytic experiments, it is also of great importance to know all the by-products of the NTP treatment alone to understand the possible synergy between NTP and the used catalyst and to fulfill complete toluene oxidation at a minimal energy cost. As a result, it is thus crucial to first examine toluene abatement in the packed bed DBD reactor alone before combining it with a catalyst, which will therefore be the topic of this paper. Particular attention will be paid to the influence of air humidity on the toluene decomposition process as this parameter can strongly vary in waste streams over time. The influence of humidity on the discharge characteristics will be investigated making use of optical emission spectroscopy (OES), which is only rarely done in the case of plasma reactors for VOC abatement. In addition, the humidity influence on toluene conversion and by-product formation will also be examined with Fourier transform infrared spectroscopy (FTIR), a flue gas analyzer and an ozone monitor. Whenever possible, quantification of the generated by-products will also be conducted. The obtained valuable information will help to unravel the mechanisms occurring during packed bed DBD abatement of toluene and will also be of great value when performing in-plasma catalytic experiments with the same DBD reactor in the near future.

6 Page of Industrial & Engineering Chemistry Research EXPERIMENTAL SET-UP The schematic diagram of the complete experimental plasma set-up for toluene abatement is presented in Figure. As illustrated, the experimental set-up consists of main parts: () the reaction gas supply system, () the packed bed DBD plasma reactor and () the gas analysis instrumentation. Figure. Experimental set-up used for plasma-assisted abatement of toluene.. Dry air cylinder;. Toluene bubbler. Water bubbler;. Humidity meter;. Mixing chamber;. Packed bed DBD reactor;. Resistor (. Ω);. High voltage and current probe;. AC power source (0 khz);. FTIR spectrometer... Gas supply system. The function of the gas supply system is to generate a toluene loaded air stream with different relative humidity (RH) as feed for the plasma reactor. It is essential that this system is able to supply a feed stream with stable conditions such as air flow rate, air humidity and toluene concentration. Therefore, to generate such a feed stream, dry air (Alphagaz

7 Industrial & Engineering Chemistry Research Page of Air Liquide) is fed to three different gas lines and in each of these lines, the air flow rate is controlled by a mass flow controller (MFC, El-Flow, Bronkhorst). In the first line, dry air at a fixed rate of 0.00 L/min goes through a toluene (Toluol.%, Carl Roth) containing bubbler system placed in a water bath at C to obtain a gaseous toluene-loaded air stream. In the second line, dry air is fed at a variable rate (0, 0.0, 0.0 and 0.0 L/min) through a water containing bubbler placed in a water bath at C. By increasing the flow rate of dry air through the water bubbler system, the RH of the gas stream can be varied (0, 0, 0 and 0% RH). Finally, dry air passes through a third line at different gas flow rates (0.0 L/min (0% RH), 0.0 L/min (0% RH), 0.0 L/min (0% RH) and.0 L/min (0% RH)) to dilute the toluene concentration down to 0 ppm. Subsequently, air from the three lines is mixed in a closed vessel where also the RH of the gas stream is monitored using a humidity detector (Testo ). Experiments in this work are thus carried out in air with different humidity at a fixed total air flow rate of.00 L/min and initially containing 0 ppm toluene... Packed bed DBD plasma reactor: set-up and characterization. The packed bed DBD plasma reactor used for this study is schematically presented in Figure. The discharge is generated in a horizontally placed 0 mm long co-axial quartz cylinder with an inner diameter of mm and a wall thickness of mm. The inner high voltage electrode is a stainless steel rod with a diameter of mm which is fixed along the reactor axis while a stainless steel iron mesh with a length of mm placed on the outer surface of the quartz tube acts as ground electrode and determines the discharge volume. The discharge gap between the two parallel electrodes is filled with g of borosilicate glass beads (Sigma Aldrich) with an average diameter of mm,

8 Page of Industrial & Engineering Chemistry Research which are kept in place using quartz wool on the sides. The volume of the discharge is. cm and the corresponding residence time of the gas in the plasma zone is 0. s. Figure. Schematic representation of the packed bed DBD reactor.. Gas inlet;. Gas outlet;. Quartz glass tube;. Connector for grounded stainless steel mesh electrode;. Stainless steel HV electrode;. Quartz wool;. Stainless steel wire mesh grounded electrode;. Borosilicate glass beads;. Cross section view of the DBD reactor An AC high voltage with amplitudes ranging between. and.0 kv with a fixed frequency of 0 khz is applied to the inner electrode, while the outer electrode is connected to ground through a resistor R (. Ω). The applied voltage is measured using a 00: high voltage probe (Tektronix P0A) while the discharge current is monitored by measuring the voltage across the resistor R with a Picotech 0 MHz voltage probe. The instantaneous voltage (V(t)) and current (I(t)) waveforms of which examples are shown in Figure are then recorded using a digital oscilloscope (PicoScope 0A). As can be seen in Figure, the discharge current consists of numerous short current peaks, which are an indication of the filamentary character of the used packed bed DBD reactor in addition to the surface discharge on the surface of the pellets near the contact points. Using these voltage-current waveforms, the average power P of the discharge can be calculated according to the following equation (T = period of the discharge):,

9 Industrial & Engineering Chemistry Research Page of P = t+t T I(t)V(t)dt t In this work, the discharge power will be varied between. and.0 W to examine the plasmaassisted decomposition of toluene. Figure. Voltage V(t) and current I(t) waveforms of the packed bed DBD reactor at an applied voltage of. kv. The influence of the relative humidity of the air stream on the excited radiative species generated in the discharge is also studied using optical emission spectroscopy (OES) in this work. Optical emission spectra are collected through an optical fiber placed perpendicular to the axis of the quartz tube and located a few millimeters away from the middle of the discharge region. Overview spectra of the discharge are recorded in the spectral range 00 to 00 nm using an Ocean Optics spectrometer S000 with a resolution of. nm. High resolution spectra for gas ()

10 Page of Industrial & Engineering Chemistry Research temperature determination are also obtained using another spectrometer (Avantes AvaSpec - 0) with a resolution of 0.0 nm... Gas analysis instrumentation. The conversion efficiency of toluene and the formation of gaseous by-products after plasma treatment are on-line investigated with a Fourier-transform infrared (FTIR) spectrometer (Bruker, Tensor ) equipped with a DTGS (deuterated tri-glycine sulfate) detector and a gas cell with a 0 cm optical path length. After reaching steady state conditions, FTIR spectra averaged over different samples are obtained with a resolution of cm - and an aperture of mm. OPUS (Bruker) software is used to collect and analyze the obtained spectra. The background spectra for the FTIR measurement is obtained after introducing dry air and the required relative humidity. Quantification of toluene and CO is performed based on the obtained FTIR spectra by measuring the corresponding FTIR peak areas after calibration with a standard toluene/co gas mixture (Air Liquide). A straightforward quantification of CO based on FTIR is not easy in this work as the characteristic CO peaks at and cm - are overlapping with the peaks of N O and O respectively. In this case, although the absorption peaks of O and CO can be deconvoluted, there is a possibility that the amount of CO will be underestimated. As a result, it was preferred in this work to determine the CO amount making use of a Testo 0 flue gas analyser with a resolution of 0. ppm for CO in the measurement range of 0-00 ppm. Other gaseous by-products of the plasma assisted toluene oxidation process, which can be identified from the FTIR spectra, are not quantified due to the unavailability of a calibration mixture, however, for these by-products, FTIR peak integration areas will be used to make a comparison between different experimental conditions.

11 Industrial & Engineering Chemistry Research Page of The amount of O which is one of the by-products detected in the gas stream after plasma treatment is also obtained by using a UV absorption based ozone detector (Teledyne, Model M). A mass spectrometer is used for the identification of different by-products. The mass spectrometer used in these experiments is a Quadrupole MS (HPR0 QIC R&D, Hiden Analytical) equipped with dual Faraday and electron multiplier detector. MASoft PC software is used for collecting and displaying data. The peaks are represented as bar lines over the corresponding mass (amu).. RESULTS AND DISCUSSION.. Discharge characteristics of the reactor. In a first stage of the research, optical emission spectroscopy is used to study the effect of RH of the feeding gas on the generation of excited, radiative species in the discharge. For this purpose, air with different RH containing 0 ppm toluene has been sent through the packed bed DBD reactor operating at a discharge power ranging between. and.0 W. OES spectra were subsequently obtained a few minutes after plasma ignition. As an example, OES spectra of the discharge operating at.0 W in gas streams containing toluene for different RH are shown in Figure.

12 Page of Industrial & Engineering Chemistry Research Figure. OES spectra of the discharge (.0 W) generated in toluene/air with different relative humidity From Figure, it can be concluded that the main component of the spectrum is emission of excited molecular nitrogen under all experimental conditions despite the presence of molecular oxygen and (in some cases) water vapor in the gas mixture. The main contributions to the spectra presented in Figure are emission of the second positive system of the nitrogen molecule (N (C Π u B Π g )) and the first negative system of the nitrogen molecular ion (N + (B Σ + u X Σ + g )). The intensive N molecular bands in the spectra indicate that the discharge generates abundant metastable electronic excited nitrogen molecules and ionized nitrogen species. In air at atmospheric pressure, stepwise dissociation and ionization of excited nitrogen is however hardly possible because of the very effective collisional quenching of excited states by molecular oxygen. As a result, N + species are mainly generated by direct electron impact, as shown in the following reaction: e + N (X Σ + g ) ν=0 N + (B Σ + u ) ν =0 + e (ΔE=. ev) ()

13 Industrial & Engineering Chemistry Research Page of Furthermore, in a dielectric barrier discharge, the mean electron energy is typically ranging between and ev. Hence, only a very small fraction of electrons possesses energies that exceed the threshold value of reaction (), which can explain the relatively low intensity of the peak of the first negative system of N +. Emission of atomic oxygen ( S 0 - P at and S 0 - P at nm) is hardly detected in the spectra, although the energy required for these transitions (. and.0 ev respectively) is well below the threshold for nitrogen emission at nm. Similar results have also been reported by other researchers, and can be explained by the fact that the quenching processes of excited oxygen atoms with oxygen and nitrogen molecules are two orders of magnitude faster than the radiative processes, making it very difficult to observe atomic oxygen in the spectra, although it is certainly present in the different air plasma discharges. The discharge in humid air also contains OH radicals, however, emission of excited OH radicals at 0 nm cannot be detected in the OES spectra. This observation could also be explained by the fact that quenching of excited OH radicals by oxygen and nitrogen molecules is much faster than the de-excitation processes to the ground state. In the inset of Figure, it can be seen that the intensity of the band at. nm gradually decreases with increasing humidity and this behavior could be observed for all bands present in the OES spectra. The observed decrease in emission can be attributed to the increased quenching of N (C Π u ) excited states due to collisions with water molecules before they undergo spontaneous emission to the lower N (B Π g ) excited state. Unfortunately, since no emission peaks from excited oxygen atoms or OH radicals could be observed in the emission spectra, it is not possible to draw any conclusions on the effect of humidity on the amount of these excited species.

14 Page of Industrial & Engineering Chemistry Research The obtained OES spectra with strong emission of the nitrogen second positive system can however be used to determine the N rotational temperature. 0 This temperature is determined by analysis of the rotational structure of a nitrogen molecular band in the second positive system, namely the 0-0 vibrational band of N (C Π u B Π g ) emission at a wavelength of. nm. The experimental spectra are fitted with simulated ones and a typical fitting procedure of the spectrum in a toluene/dry air mixture is shown in Figure. The experimental spectrum presented in Figure has been simulated with different rotational temperatures ranging from 00 to 00 K in steps of 0 K and the best fit was observed for a rotational temperature of 0±0 K. This rather high temperature is in good agreement with literature, where DBD reactors have also shown to result in the generation of high rotational temperatures. It should however be highlighted that this rotational temperature is an overestimation of the bulk gas temperature as it only applies to the central part of the microdischarges and not to the whole gas volume., As a result, the overall gas temperature in the plasma discharge region is much lower than the rotational temperature as was confirmed by measuring the temperature of the DBD reactor wall making use of a thermocouple, which was found to be equal to ± K.

15 Industrial & Engineering Chemistry Research Page of Figure. Determination of the N rotational temperature by fitting procedure of an experimental spectrum of N (C Π u B Π g, 0 0) with a calculated spectrum. The plasma is generated in a toluene/dry air mixture at.0 W. Simulations of the experimental spectra have also been performed for discharges generated in toluene containing humid air with different RH at.0 W and it was observed that the addition of water vapor to the air feed gas does not result in significant changes in rotational N temperature as well as DBD reactor wall temperature in the range of the applied power from. to W and sensitivity of the applied spectroscopy method... Toluene abatement in the packed bed DBD reactor.... Toluene removal efficiency. After characterizing the DBD itself, plasma-assisted toluene abatement experiments are carried out to study the effect of air humidity. For this purpose, an air gas stream with different RH containing 0 ppm toluene is sent through the packed bed DBD reactor. A constant flow rate of.00 L/min is maintained during all experiments while discharge power was varied between. and.0 W enabling a variation in specific input energy (SIE) (= power [J/s] / gas flow rate [L/s]) between 0 and 00 J/L. As previously mentioned, a calibrated FTIR spectrometer is used to analyze the toluene content in the gas stream before and after plasma exposure enabling the determination of the toluene removal efficiency (η toluene ) according to the following formula: η toluene (%) = 0 [toluene] in [toluene] out [toluene] in () where [toluene] in is the toluene concentration introduced into the plasma reactor and [toluene] out the toluene concentration at the exit of the DBD reactor. The toluene removal efficiency can be seen in Figure as a function of SIE for air streams with different RH. For all cases of humidity, the toluene conversion significantly increases with

16 Page of Industrial & Engineering Chemistry Research increasing energy density. This can be attributed to the fact that a higher amount of reactive plasma species, which can initiate and/or promote the decomposition processes of toluene, are available at higher energy density. In addition, the toluene conversions obtained at high SIE values in dry air are comparable with previously reported conversion efficiencies,,, and the comparison is shown in Table. For a particular SIE value, the removal efficiency significantly increases when the relative humidity is increased from 0 to 0%, while at higher levels of humidity, a trend inversion occurs with the efficiency at 0% RH becoming lower than the one at 0% RH. The result obtained is in accordance with the results shown by Schiorlin et al for abatement of toluene using negative corona discharge. With increasing humidity levels, an increasing amount of OH radicals are present in the discharge, which are known to be much stronger oxidants than for example atomic oxygen or peroxyl radicals.,0 This in turn results in an increase in removal efficiency with increasing humidity up to 0%. The trend inversion observed for higher humidity levels can be attributed to the saturation and inhibition of OH radical forming reactions such as for example the dissociation of H O molecules by reactions with electrons or atomic oxygen at high humidity levels.

17 Industrial & Engineering Chemistry Research Page of Figure. Toluene removal efficiency as a function of SIE for different relative humidities (RH=0%, 0%, 0% and 0%) Table. Comparison of toluene removal efficiency in glass beads packed bed DBD reactor using dry air non thermal plasma Initial toluene conc. (ppm) Flow rate (L/min) Energy density (J/L) Present work Chang et al Delagrange et al Toluene removal efficiency (%)... Analysis of by-product formation. As previously mentioned in the introduction, incomplete oxidation of VOCs by non-thermal plasma often occurs resulting in the formation of unwanted by-products, such as NO x, CO, O, other VOCs, 0, A thorough analysis of the identification of the by-products is thus indispensable to elucidate the plasma-assisted breakdown mechanism of toluene. Therefore, in the following paragraphs, FTIR spectra will be

18 Page of Industrial & Engineering Chemistry Research closely examined to qualitatively determine the by-products of toluene abatement with a glass beads packed bed DBD reactor. Figure shows the FTIR spectra of (a) the inlet and (b and c) the outlet stream when the plasma reactor is operated in a dry air/toluene mixture at two different SIE values ( and J/L). The inlet stream only shows the characteristic IR bands of toluene, which are listed in Table. On the other hand, several other IR bands appear after plasma abatement thereby suggesting incomplete oxidation of toluene. To examine the composition of the plasma exhaust stream at both SIE values in detail, wavenumbers values are being used to allocate specific IR bands to the appropriate by-product. An overview of the wavenumbers and the corresponding by-product are presented in Table. By carefully comparing Figure (b and c) with NIST reference spectra, several gaseous by-products in the exhaust stream can be identified after plasma abatement at both SIE values. First of all, CO is clearly present in the outlet stream as confirmed by the double bands at cm - and cm -. Moreover, toluene is also decomposed into CO which has characteristic FTIR peaks at and cm -. The formation of N O is also suggested by the presence of the absorption doublet at and 0 cm -. Ozone is also detectable in the outlet stream since strong absorption bands at, and cm - can be observed in the FTIR spectra. On the other hand, NO and NO cannot be detected in the plasma-treated gas stream as suggested by the absence of strong absorption doublets in the region 0-00 cm - and 0-0 cm -. Additionally, formic acid can also be identified in the outlet stream. This is confirmed by the presence of the peaks, allocated with number, at 0,, and 00 cm -.

19 Industrial & Engineering Chemistry Research Page of Table. Characteristic FTIR bands of toluene., Wavenumber range (cm - ) Vibration 0- aromatic ring =C-H stretching -0 methyl group C-H stretching -0 overtones 0-0 aromatic ring C-C stretching 0- aromatic in-plane C-H bending 0-0 aromatic out-of-plane C-H bending Table. (By)products of plasma-assisted toluene abatement in dry air detected with FTIR spectroscopy Number Compound Structure Peak locations (cm - ) Toluene C H 0, 00,, 0, 0,,, Carbon dioxide CO, Carbon monoxide CO, Nitrous oxide N O, 0 Ozone O,, Formic acid HCOOH 00,, 0,, Figure also clearly shows that the intensities of the by-product IR bands increase with increasing SIE value suggesting that these by-products are generated in higher amounts at higher

20 Page of Industrial & Engineering Chemistry Research SIE values, which is consistent with the increased toluene conversion observed at higher SIE values. After a close investigation of the outlet FTIR spectra in humid air (not shown here), a great resemblance with the spectra obtained in dry air could be observed. However, two major differences could be clearly observed: () a broad absorption band in the region cm - due to the presence of water appears in the humid air gas streams and () the peaks attributed to formic acid disappear with the introduction of water vapor. Figure. FTIR spectra of a dry air/toluene mixture before (a) and after plasma at an energy density of J/L (b) and J/L (c)

21 Industrial & Engineering Chemistry Research Page 0 of Besides identifying the by-products in the outlet, it is also very important to quantify these products and to examine the influence of applied SIE values and the RH of the air stream on their concentration. These results will therefore be given in the following paragraphs for the identified by-products (CO, CO, ozone, N O and HCOOH). First of all, the amount of generated CO will be investigated as this is the targeted degradation product. Figures and show the evolution of the CO yield and CO selectivity respectively as a function of SIE for toluene loaded air streams with different RH. The CO yield and selectivity are defined as follows: Y CO (%) = S CO (%) = [CO ] [toluene] in 0 () [CO ] [toluene] conv 0 () where [CO ] is the amount of CO detected in the outlet gas stream and [toluene] conv the amount of toluene converted by the plasma. Figure. CO yield as a function of SIE for different relative humidities (RH = 0%, 0%, 0% and 0%) 0

22 Page of Industrial & Engineering Chemistry Research Figure clearly shows similar trends as the ones depicted in Figure. The CO yield increases with increasing SIE values and with increasing humidity until an RH value of 0%. At higher humidity levels, the CO yield again decreases. These results thus suggest that the more toluene is destroyed due to the addition of extra energy or the increase in humidity, the more CO is produced, which is a rather straightforward conclusion. However, when looking at the evolution of the CO selectivity as presented in Figure, a completely different behavior can be observed: the CO selectivity first slightly decreases and then remains more or less constant in the range -0% with increasing SIE values. In addition, the presence of water vapor also seems to have no significant influence on CO selectivity although it was already observed that it can influence the decomposition efficiency. This result thus means that although more CO is produced in the effluent gas with increasing SIE values and increasing RH up to 0%, these CO molecules will not be more preferentially generated compared to the other by-products. Air humidity can thus not affect CO selectivity, but might have an influence on the formation of other by-products which will be examined in the next few paragraphs.

23 Industrial & Engineering Chemistry Research Page of Figure. CO selectivity as a function of SIE for different relative humidities (RH = 0%, 0%, 0% and 0%) The second by-product that will be investigated in this work is CO and the results will be presented hereafter. Figures and show the evolution of the CO yield and CO selectivity respectively as a function of SIE for toluene loaded air streams with different RH. The CO yield and selectivity are defined as follows: Y CO (%) = S CO (%) = [CO] [toluene] in 0 () [CO] [toluene] conv 0 () where [CO] is the amount of CO detected in the outlet gas stream and [toluene] conv the amount of toluene converted by the plasma.

24 Page of Industrial & Engineering Chemistry Research Figure. CO yield as a function of the SIE for different relative humidities (RH = 0%, 0%, 0% and 0%) From Figure, it can be clearly observed that the CO yield slightly increases with increasing SIE values for dry air and a RH of 0% while it strongly increases for an RH of 0% and 0%. The evolution of the CO selectivity as presented in Figure shows that the CO selectivity is very low (<%) for dry air and a RH of 0% and that it remains more or less constant with increasing SIE values. In contrast, a higher CO selectivity (-0%) can be observed at a RH of 0 and 0% and this CO selectivity slightly increases with increasing SIE values.

25 Industrial & Engineering Chemistry Research Page of Figure. CO selectivity as a function of the SIE for different relative humidities (RH = 0%, 0%, 0% and 0%) Figure. Formation of N O as a function of SIE for different relative humidities (RH=0%, 0%, 0% and 0%)

26 Page of Industrial & Engineering Chemistry Research Figure. Ozone productions as a function of SIE for different relative humidities (RH=0%, 0%, 0% and 0%) During plasma-assisted VOC decomposition, formation of nitrogen oxides is an undesirable process because of their adverse effects on human health. Interestingly, NO and NO have not been detected in the plasma outlet stream under the experimental conditions used in this work. However, the formation of NO was found to occur at very high energy densities (>00 J/L) in both dry as well as humid air (more info will be further given). In addition, the formation of N O was also found to be considerable as could already be observed from the FTIR spectra presented in Figure (b). As quantification of N O is not possible in this work, the N O FTIR peak integration area will be used as a measure for the amount of N O and the results as a function of SIE are presented in Figure. As shown in this figure, the formation of N O strongly increases with increasing SIE values for all examined RH

27 Industrial & Engineering Chemistry Research Page of values. The formation of N O does not change when the RH is increased from 0 to 0%, but does increases when the RH is increased to 0%. A higher RH value of 0% however results in a lower N O formation. Figure also shows that increasing the humidity of the toluene-loaded air flow results in a noticeable decrease in ozone concentration at the plasma reactor outlet. It is well know from literature, that oxygen atoms play a crucial role in the main O formation mechanism, which is shown hereafter: O + O + M O + M () However, in humid air, oxygen atoms tend to be rapidly consumed by water related species and the following reaction is believed to be responsible for the largest removal of oxygen atoms: O + H O OH + OH () As a result, less atomic oxygen atoms are present in humid air resulting into a reduction in O production. In addition, it can also be observed in Figure that the ozone concentration first increases with increasing SIE values to reach a maximal value at approximately J/L after which the ozone concentration starts to decrease. This behavior is in good agreement with former studies performed by Yamamoto et al. and Holzer et al.. The phenomenon that the ozone concentration decreases at high SIE values is peculiar to ozone generation in air DBD discharges as it never appears in pure oxygen DBDs. The reason for this can be twofold: () the concentration of NO x species increases at high SIE values and/or () the gas temperature increases with input energy resulting into thermal dissociation of ozone. However, when looking at the FTIR spectra of the gas outlet stream at an SIE value of 0 J/L, no NO x species can be observed suggesting that in this case most likely the occurring high gas temperature at

28 Page of Industrial & Engineering Chemistry Research J/L is responsible for the decreasing ozone generation due to thermal O dissociation. Indeed, the temperature of the DBD reactor wall was found to be equal to ± K under these experimental conditions which is an underestimation of the actual gas temperature in the reactor. As a result, it is expected that the gas temperature in the reactor can be high enough to induce thermal dissociation of ozone which is also in agreement with emission spectroscopy results. The concentration of ozone measured using the ozone monitor is in good agreement with the surface area obtained from the peak corresponding to ozone (at cm - ) in FTIR spectra for different relative humidities. It is observed that when the used DBD reactor is operated in dry air at very high energy densities, like for example J/L, the generation of NO x species does occur rapidly as can be seen in Figure, where the evolution of the concentration of NO and ozone is shown as a function of plasma operating time. This figure clearly shows that at the time point NO species are being created, the ozone concentration strongly decreases and reaches a concentration close to zero when high amounts of NO species are present in the discharge. Similar trends have also been observed in the examined humid air discharges, but are not shown in this paper. A high concentration of NO is known to completely stop the generation of ozone since it consumes oxygen atoms at a very fast rate according to the following reactions: O + NO NO + O () O + NO + M NO + M () As a result, at high energy densities, the atomic oxygen reactions with NO become faster than their reaction with molecular oxygen thereby resulting in an ozone generation stop (so-called discharge poisoning). This observation is thus in good agreement with literature stating that ozone and nitrogen oxides do not coexist well in a non-thermal plasma. Indeed, the used

29 Industrial & Engineering Chemistry Research Page of packed bed DBD reactor effectively produces ozone and no NO at low energy densities, while at high energy densities, high amounts of NO are generated without any ozone formation. To avoid the phenomenon of discharge poisoning, the specific input energy of the experiments conducted in this paper has therefore been restricted to 00 J/L. Figure. Formation of NO and disappearance of ozone as a function of time in a toluene/dry air mixture for a specific input energy of J/L

30 Page of Industrial & Engineering Chemistry Research Figure. Formic acid production as a function of SIE for different relative humidities (RH=0%, 0%, 0% and 0%) Figure. Mass spectrum of inlet toluene/dry air before plasma treatment and after plasma treatment at energy density of J/L.

31 Industrial & Engineering Chemistry Research Page 0 of The final by-product which will be examined in this paper is formic acid, which has been detected by FTIR (and also confirmed by mass spectrometry, not presented here) in the effluent gas and which has been often reported as an important gaseous byproduct of plasma-assisted toluene removal. Figure shows the integrated FTIR peak area of the formic acid ester band (0-0 cm - ) as a function of specific input energy. This figure clearly shows that in dry air the formation of formic acid is considerable while it also increases with increasing SIE value, whereas in the presence of water vapor, the formation of formic acid becomes almost negligible. This is due to the presence of OH radicals in the humid air discharges which are effectively quenched by formic acid leading into complete oxidation of formic acid to CO and H O. The mass spectra of the inlet toluene/dry air before plasma treatment and after abatement at the SIE of J/L is shown in Figure. Before plasma treatment, the mass spectrum shows only the characteristics fragments of toluene. The mass spectrum of the outlet gas after the plasma treatment at the energy density of J/L shows the decrease in the abundance of toluene. From the mass spectrum obtained after the plasma treatment, it is observed that the by-products such as benzene (mass = amu), benzaldehyde (mass =,, amu) and benzoic acid (mass =,, amu) are absent.. CONCLUSIONS In this work, the decomposition of toluene using a packed bed DBD reactor has been systematically studied in air streams with different RH. Optical emission spectroscopy has showed that with increasing humidity, less N excited states are present in the discharge, which can be attributed to the increased quenching of these states due to collisions with water 0

32 Page of Industrial & Engineering Chemistry Research molecules before they undergo spontaneous emission to a lower excited state. Unfortunately, it was not able to derive any conclusions on the effect of RH on the amount of excited oxygen atoms as well as excited OH radicals since no corresponding emission peaks have been observed. OES was however able to determine the nitrogen rotational temperature which was found to be equal to 0±0 K for the highest discharge power used in this work (.0 W). At the same time, under these conditions, the DBD reactor wall temperature was found to be ± K, which is significantly lower than the nitrogen rotational temperature due to the fact that this rotational temperature is an overestimation of the bulk gas temperature as it only gives an indication of the temperature in the core of the microdischarges. It was also observed that neither the nitrogen rotational temperature nor the reactor wall temperature was influenced by the RH of the gas stream. After characterizing the discharge itself, toluene decomposition experiments were also performed thereby limiting the discharge power to.0 W as higher discharge powers were found to result in the formation of the unwanted by-product NO. It was observed that the highest toluene removal efficiency was observed at a RH of 0% most likely due to the fact that the OH radical amount in the plasma is the highest under these experimental conditions. In addition, all gaseous products in the plasma reactor outlet have been identified and the main products of the toluene decomposition process were found to be CO, CO, N O, O (both dry and humid air) and HCOOH (dry air only). Increasing the RH of the air stream was found to completely suppress the formation of formic acid while the formation of ozone was also found to decrease. In contrast, the CO selectivity was found to increase with RH, while the CO selectivity was unaffected by the addition of water to the air stream. The highest N O concentration was observed at a RH of 0%: this observation in combination with the observed relatively high amounts of CO, CO and O molecules in the gas outlet at a RH of 0% confirm the observed highest removal efficiency

33 Industrial & Engineering Chemistry Research Page of at this experimental condition. It can thus be concluded that toluene decomposition in the examined packed bed DBD reactor is the most efficient at a RH of 0%. AUTHOR INFORMATION Corresponding Author * Savita.KaliyaPerumalVeerapandian@ugent.be Notes The author declares no competing financial interest ACKNOWLEDGMENT This study is a part of the Special Research Fund of Ghent University and of Region Hautsde-France for a joint PhD scholarship. The joint research program PICS n (Preparation of catalysts and catalytic depollution assisted by plasma) from CNRS, Chevreul institute (FR ), Ministère de l Enseignement Supérieur et de la Recherche, Région Hauts-de- France and FEDER are acknowledged for supporting and funding this work. The authors also acknowledge the support of the European Union for funding this work though an Interreg V France-Wallonie-Vlaanderen project Depollutair. ABBREVIATIONS C H, Toluene; CO, Carbon monoxide; CO, Carbon dioxide; DBD, Dielectric Barrier Discharge; DTGS, Deuterated tri-glycine sulfate; FTIR, Fourier Transform Infrared Spectroscopy; H O, Water; HCOOH, Formic acid; IR bands, Infrared bands; I(t), Instantaneous current; MFC, Mass Flow Controller; NIST, National Institute of Standards and Technology; N, Nitrogen; N O, Nitrous oxide; NO, Nitrogen dioxide; NO x, Nitrogen oxides; NTP, Non-thermal Plasma; O, Oxygen; O, Ozone; ppm, OES, Optical Emission Spectroscopy; OH, Hydroxyl; ppm, parts per million; R, Resistor; RH, Relative Humidity; S CO, Carbon monoxide

34 Page of Industrial & Engineering Chemistry Research selectivity; S CO, Carbon dioxide selectivity; SIE, Specific Input Energy; VOC, Volatile Organic Compound; V(t), Instantaneous voltage; T, Period of discharge; Y CO, Carbon monoxide yield; Y CO, Carbon dioxide yield; η toluene, Toluene removal efficiency; REFERENCES () Wang, Y.; Zhang, Y.; Su, L.; Li, X.; Duan, L.; Wang, C.; Huang, T. Hazardous Air Pollutant Formation from Pyrolysis of Typical Chinese Casting Materials. Environ. Sci. Technol. 0, (),. () Yi, F.-Y.; Lin, X.-D.; Chen, S.-X.; Wei, X.-Q. Adsorption of VOC on Modified Activated Carbon. J. porous Mater. 00, (),. () Kennes, C.; Rene, E. R.; Veiga, M. C. Bioprocesses for Air Pollution Control. J. Chem. Technol. Biotechnol. 00, (),. () Everaert, K.; Baeyens, J. Catalytic Combustion of Volatile Organic Compounds. J. Hazard. Mater. B 00, ( ),. () Li, W.; Gong, H. Recent Progress in the Removal of Volatile Organic Compounds by Catalytic Combustion. Acta Physico-Chimica Sin. 0, (),. () Reij, M. W.; Keurentjes, J. T. F.; Hartmans, S. Membrane Bioreactors for Waste Gas Treatment. J. Biotechnol.,,. () Degreve, J.; Everaert, K.; Baeyens, J. The Use of Gas Membranes for VOC-Air Separations. Filtr. Sep. 00, (),. () Subrahmanyam, C.; Renken, A.; Kiwi-Minsker, L. Catalytic Non-Thermal Plasma

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