Article pubs.acs.org/IECR
Oxidation of Ethanol Vapors in Negative Atmospheric Corona Discharge Mikhail N. Lyulyukin,† Alexey S. Besov,†,‡ and Alexander V. Vorontsov*,†,‡ †
Boreskov Institute of Catalysis, Pr. Ak. Lavrentyeva 5, Novosibirsk 630090, Russia Novosibirsk State University, Pirogova 2, Novosibirsk 630090, Russia
‡
ABSTRACT: Ethanol vapors were oxidized in negative atmospheric corona discharge of double wires-to-planes geometry using three diameters of coronating wires and three power levels. Acetaldehyde, formaldehyde, CH3COOH, and CO were detected as intermediate gaseous products of oxidation, and CO2 was the final product. Comparison was made with the results of the previous work on oxidation of acetone. The lowest energy cost for oxidation was at the thickest electrode ø 0.8 mm, lowest discharge power 15 W, and equaled 1.1 kWh/g for acetone and 0.33 kWh/g for ethanol. In contrast to acetone, the ethanol molecule is oxidized directly by ozone with the rate constant 2.9 × 10−20 cm3/s. Concentrations of intermediate products acetaldehyde and formaldehyde remain at the same levels at all power levels and wire diameters used. Scheme of chemical transformation is proposed for acetone and ethanol vapor oxidation, which suggests that reactions are initiated by interaction with atomic oxygen, ozone, and hydroxyl radicals.
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INTRODUCTION Increased concerns about air pollution and energy efficiency of remediation technologies urged development of novel purification methods. Traditional absorption is a cheap technology for removing pollutants from water and air. However, spent absorbent should be recycled or destroyed to avoid releasing of the pollutants accumulated back to the environment. Other nondestructive purification methods such as adsorption on zeolites or activated carbon, condensation, and membrane separation are also available with the same disadvantage. Destructive technologies have appeared that generally transform pollutants into some products. Creation of nonthermal plasma (NTP) in air is one of the common purification methods for air1−4 and water.5,6 Corona discharge seems to be the easiest way to create nonthermal plasma in the medium.7 Investigation of ethanol oxidation in discharges represents both fundamental and practical interest. The fundamental interest lies in knowing the pathways and kinetics of destructive oxidation. The practical need for the oxidation of ethanol vapors exists due to the fact that they can affect the human health. Ethanol can affect people both itself and through acetaldehyde and formaldehyde which are products of ethanol oxidation in the presence of oxygen.8 A number of works have already been carried out on studying the oxidation of alcohols in nonthermal plasma.9−13 Although studies on the destruction and oxidation of acetaldehyde1,14 and formaldehyde15,16 in plasma discharges at different conditions have been conducted, a complete chain of reactions of ethanol conversion to CO2 and H2O in a closed system under the action of negative atmospheric corona discharge has not been described yet. Previously, we studied degradation of acetone vapors in a corona discharge using the double wires-to-plane discharge gap geometry.17 We have found that the process of oxidation is most energy efficient using the thickest wire and lowest power of negative discharge, while the rate of oxidation increased with © 2013 American Chemical Society
an increase in wires diameter and electrical power. Also, a conclusion was made that the rate of oxidation is directly proportional to the volume of active ionizing corona region around the wires and ozone does not give a significant direct contribution to the oxidation rate of acetone. In the present work, ethanol vapors were used to study the trends of oxidation rate and efficiency as a function of the discharge electrode diameter and electrical power as well as to compare the results with those obtained with acetone vapors. Moreover, it was necessary to elucidate the role of ozone in the oxidation process, detect intermediate products of ethanol oxidation, and study the kinetics of their formation and transformation into the deep oxidation products.
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EXPERIMENTAL SECTION Chemicals. High purity ethanol and room air having relative humidity around 40% without preliminary cleaning were used for carrying out the experiments. Experimental Setup. An experimental installation used in the present work is very similar to that described previously17 except the analytical part. The scheme of experimental setup is demonstrated in Figure 1. The two wires to the plate discharge unit of 0.91 L active volume is placed inside the 404 L Plexiglas chamber connected to an FTIR analysis system. Concentration of organic vapors, O3, CO, and CO2 in the chamber was measured with a FTIR spectrophotometer Vector 22 (Bruker) equipped with a multipass gas cell (5) of 0.7 L volume. Investigated air was purged through the cell using plastic tubing (8 mm ID) and 0.2 μm air filter (8) under the action of a membrane pump (7). The FTIR spectrophotometer allowed carrying out continuous measurements of concentrations of Received: Revised: Accepted: Published: 5842
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Figure 1. Scheme of experimental setup: (1) test chamber (404 L), (2) corona discharge unit, (3) fan, (4) high voltage DC power supply source, (5) FTIR spectrophotometer Vector 22 (Bruker) with multipass gas cell, (6) computer, (7) membrane pump, (8) 0.2 μm filter.
reagents and products in the chamber (1). Wires with diameters 0.2, 0.4, and 0.8 mm were used in the discharge unit. The FTIR spectrophotometer recorded spectra in the wavenumber range 450−4000 cm−1 with the resolution of 1 cm−1 at 30 s intervals. Each spectrum represents an average of 10 scans. Liquid ethanol (220 μL) was injected through the sampler into the chamber by means of a syringe (Hamilton). Then, ethanol was quickly evaporated and the vapors were evenly distributed in the chamber by a fan. The theoretical initial concentration of ethanol was calculated to be 200 ppm. Within a short time period, the concentration reached a pseudostationary value. The concentration of components in the air was calculated from the areas of absorption bands in the IR spectra. The rates of substance concentration changes were calculated using linear approximations of the corresponding sets of experimental points. Special software was used for numerical processing of numerous spectra obtained. Calculations were based on method of spectral subtraction of gasphase FTIR spectra by minimizing the spectrum length.18
Figure 2. Kinetic curves of concentrations of substances observed in ethanol oxidation. Diameter of the corona wire was 0.2 mm (A) and 0.4 mm (B); power of discharge 60 W.
these compounds. The concentration of ozone continuously increases after the moment of turning on the discharge but gradually decreases after turning off the discharge. Concomitantly, the ethanol and acetaldehyde concentration decreases, CO2 as well as CO concentration increases after turning off the discharge. This shows that some chemical reactions take place with the participation of ozone even in the absence of the discharge. Figure 2 shows kinetic curves of substances in the process of oxidation of ethanol by plasma of corona discharge of the same maximum power 60 W but with different diameters of the corona wire. One can see that the rate of oxidation increases as the wire diameter increases. Figure 3 demonstrates kinetic curves of concentration of ethanol and products of its oxidation for the thickest wire used and three different electrical power levels. One can see that the rate of ethanol oxidation increases with an increase of the power supplied to the discharge. It is also notable that utilization of thicker wire results in a higher rate of ozone production as its concentration is higher for wire of larger diameter. Figure 3C reveals that acetaldehyde, formaldehyde, acetic acid, and carbon monoxide all are intermediate products of ethanol plasmachemical destruction since the concentration of these compounds reaches maxima and decreases then as a result of oxidation in plasma. This shows that upon choosing the proper electrical power, discharge electrode thickness, and residence time, it is possible to obtain complete transformation of ethanol vapors into the deep oxidation products, CO2 and H2O. The initial part of the ethanol and CO2 concentration kinetic curves follow a straight line from which the reaction rate can be obtained. Figure 4 demonstrates the initial rate at different wire diameters and negative discharge electrical power.
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RESULTS Figures 2 and 3 show kinetic curves of concentrations of ethanol vapors, organic intermediate products of oxidation, CO, CO2, and ozone during evaporation of ethanol, stabilization of its concentration, and the subsequent oxidation of ethanol after switching on and off the corona discharge. The corona discharge unit was turned on after reaching of the pseudostationary level of ethanol concentration, and this moment is marked with a vertical line. After 150 min of discharge operation, it was turned off which is marked by the second vertical line in these figures. For clarity of presentation, Figures 2 and 3 show every tenth experimental point. It is easy to see that after turning on the discharge, the ethanol vapor concentration starts to decrease rapidly, CO2, CO, ozone, and such organic gaseous intermediates as acetaldehyde, formaldehyde, and acetic acid appear in the chamber’s air. One can see in these figures that concentrations of acetaldehyde and formaldehyde reach their maxima at some moments confirming thereby the intermediate character of 5843
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The highest rate of ethanol consumption and CO 2 generation is observed at the highest used electrical power and thickest electrode wires. It is interesting to compare the ethanol consumption rate at fixed power levels. Every time the wire thickness increases from 0.2 to 0.4 and from 0.4 to 0.8 mm, the rate increases at every fixed discharge power level. One can see in Figure 4 that the CO2 generation rate is always less than the doubled rate of ethanol consumption. This is obviously due to the formation of oxidation byproducts shown in Figures 2 and 3. Some CO2 generation occurs even in the absence of ethanol or any other organic compound in the chamber which is associated with the destruction of the walls and other organic material in the chamber by ozone. A control experiment without purging the test substance into the chamber has shown a constant carbon dioxide generation rate of 0.15 ppm/min. This value did not change for ozone concentrations up to 50 ppm. So this CO2 generation gives the systematic addition to the overall generation of carbon dioxide. This value is subtracted from the corresponding data points in Figure 4. Energy efficiency is an important parameter for characterization of a process which uses electrical power. It is noted often that there is no universal parameter to determine the energy efficiency of NTP processes.19 One of the most widely used parameters for this purpose is the energy cost (EC). We calculated this value in correspondence with the following expressions: ⎡ eV ⎤ P [W] energy cost ⎢ = 34.7 ppm ⎣ molecule ⎥⎦ rate ⎡⎣ min ⎤⎦
(1)
and ⎡ kW h ⎤ P [W] energy cost ⎢ ⎥ = 0.926 g ppm ⎤ ⎡ g ⎣ ⎦ rate ⎣ min ⎦ Ma ⎡⎣ mol ⎤⎦
(2)
The factors 34.7 and 0.926 in the equations are the conversion factors at 20 °C and atmospheric pressure. Ma is the molecular weight of the test substance. Figure 5 shows the corresponding
Figure 3. Kinetic curves of concentration of substances observed during ethanol plasmachemical destruction. Diameter of corona wire was 0.8 mm, electrical power consumed by the negative corona discharge was 15 (A), 30 (B), and 60 W (C).
Figure 5. Specific value of ethanol molecule decomposition energy (energy cost).
specific values of the required energy to break a single molecule of ethanol at different electrical power consumed and wire electrode thickness. The lowest energy cost is observed at the thickest electrode and lowest electrical power. Frequently used in the literature, the energy yield value, also called energy efficiency of pollutant removal, is expressed in grams per kilowatt hour,2,4 the reverse value to energy cost (kW h/g). For example, values obtained for acetone in air using a
Figure 4. Linear approximation results of ethanol consumption and CO2 generation rates.
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This value corresponds to the following possible mechanism of ethanol with ozone reaction.1
silent discharge pilot scale reactor are in the range of 1.2−3.2 g/ (kW h).20 So the corresponding energy cost values may be calculated as 0.31−0.83 kW h/g. These values are of about the same order as those presented for acetone in Figure 8B. For ethanol oxidation (Figure 5), at the thickest electrode and lowest power input, the energy cost is 0.32 kW h/g that agrees well with the literature on VOCs destruction.4
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DISCUSSION Ozone and Ethanol. The fall of ethanol vapor concentration after turning off the corona discharge was observed in the experiments on the oxidation of ethanol vapor. That was a consequence of the interaction of ethanol vapor and ozone, formed by the corona discharge processing. We managed to calculate the rate of ethanol consumption, and it is much higher than the rate of acetone consumption in reaction with ozone.17 Figure 6 shows concentrations of ethanol vapors, concentration of ozone, and the initial rate of the ethanol consumption
CH3CH 2OH + O3 → products
(3)
O3 + M → O2 + O + M
(4)
CH3CH 2OH + O → products
(5)
Comparison of Ethanol and Acetone Oxidation. Previously, we performed plasmachemical oxidation of acetone vapors using the same experimental setup. Ethanol can be expected to be oxidized more easily than acetone and to require less specific power input. Figure 8 shows acetone decomposition rate and acetone molecule decomposition energy cost plotted at different power consumed in negative corona discharge and different wires diameter according to ref 17. If we compare the rates of acetone oxidation in Figure 8A and ethanol oxidation in Figure 4, we can see that both molecules demonstrate the same tendency of increasing the rate at greater wire thickness and higher electrical power. However, the rate of acetone oxidation is much smaller. The highest rate of acetone consumption is about 3 times lower than the highest rate of ethanol consumption. The specific values of acetone and ethanol decomposition energy should be compared by considering Figures 8B and 5. The general tendencies of the decrease in energy cost with an increase in the discharge electrode diameter and decrease in electrical power are preserved for both molecules with only one point deviation of acetone oxidation at the lowest electrode thickness at the lowest power. Speaking quantitatively, the gain in energy cost is generally higher for ethanol oxidation when we decrease the electrical power. The lowest energy cost for ethanol plasmachemical oxidation is around 0.6 keV/molecule for the thickest electrode and lowest power, while it is about 1.9 keV/molecule for acetone at the same conditions. Thus, ethanol needs triple less energy for removal from air than acetone. As stated earlier, ozone directly reacts with ethanol in gas phase. But the contribution of this reaction to the overall decomposition rate depends on discharge parameters and concentration of ozone and ethanol. The process of the destruction of ethanol in plasma region of the discharge dominates at the start of the experiments due to a low concentration of ozone in the experimental chamber. Direct destruction of ethanol vapors in the plasma region remains dominant when the ethanol concentration decreases and the ozone concentration increases. But the contribution of ozone interaction with ethanol to the overall decomposition rate reaches up to about 30%. In this case, the molecules of ozone became the carriers of discharge energy, which form by combining an oxygen atom with an oxygen molecule. So the overall ethanol decomposition rate that includes both decomposition of ethanol molecule directly in plasma region and decomposition in reaction with ozone rise and the effective decomposition energy cost decreases. This is noticeable for oxidation of ethanol at low power levels especially with a small diameter of wire. Conversion to Carbon Oxides and Selectivity. Selectivity is often used for the analysis of VOC removal. Selectivity values were calculated by the well-known21 formulas
Figure 6. Ethanol and ozone concentrations at the moment of switching off the corona discharge and the ethanol concentration fall rate after switching off the discharge.
after turning off the discharge. From considering data in Figure 6, it became obvious that the rate of ethanol consumption is proportional to the concentration of ozone and ethanol. Indeed, if we plot the rate of ethanol consumption as a function of the product of concentrations of ozone and ethanol, we obtain a linear dependence that is shown in Figure 7. The coefficient of proportionality between the rate of ethanol consumption, and the product of ozone and ethanol concentration is the reaction rate constant. Using a linear fit to data in Figure 7, the bimolecular rate coefficient of reaction of ethanol with ozone was calculated to be 2.9 × 10−20 cm3/s.
Figure 7. Rate of ethanol consumption as a function of the product of ozone and ethanol concentrations and linear approximation for this function. 5845
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Figure 8. Acetone oxidation by negative corona discharge at different discharge electrode diameter and power:17 (A) acetone consumption rate; (B) energy cost of the acetone molecule consumption.
SCO2(%) =
Generally, the selectivity obtained for CO2 is at the level of 30−50% which is a typical value for conventional NTP reactors;19 the selectivity for CO is about 30%. The dependence of the selectivity for CO and CO2 on the power of the discharge is stronger with the smallest diameter of the wire. Selectivity for CO2 and CO is below 30% and 10%, respectively, for the discharge power 15 W. In other cases selectivity remains slightly dependent on the powerit slowly rises with the growth of power. A single data point drops out of the common regularity: total selectivity for carbon oxides is 100% and carbon dioxide selectivity reaches 80% for 0.8 mm wire diameter and 60 W of electrical power. The difference of results of this experiment from others becomes noticeable when looking at Figure 3the concentration of CO reaches its maximum, and then descends. Application of high power and wires of large diameter results in decrease of CO selectivity and high CO 2 selectivity, a result important for practical applications for air purification. Intermediate Products. The maximum concentration of acetaldehyde and formaldehyde which are intermediates of oxidation was found to be a constant value in all the experiments on oxidation of ethanol vapors. The values are 30.3 ± 1 ppm for acetaldehyde and 24.5 ± 2 ppm for formaldehyde (Table 1). The time needed to reach these values depended on the power and the diameter of the corona discharge wire. Acetic acid is formed in ethanol oxidation from acetaldehyde and was found in the gas mixture. The role of different active species in reactions of volatile organic compounds was considered in the literature.4,19 In atmospheric pressure nonthermal plasma in air, atomic oxygen O(3P) + O(1D) yield was suggested to exceed yield of atomic nitrogen and excited nitrogen molecules by an order of magnitude, the yield of OH• radicals was exceed by at least two times. The following reactions could lead to the production of acetic acid.
[CO2 ]final − [CO2 ]initial × 100% 2([ethanol]initial − [ethanol]final ) (6)
and SCO(%) =
[CO]final − [CO]initial × 100% 2([ethanol]initial − [ethanol]final ) (7)
where the factor 2 means that two molecules of carbon oxides may be released from a single molecule of ethanol. The calculations were made for the points at the moment of turning off the corona discharge unit. Figure 9 demonstrates selectivity of ethanol oxidation to carbon oxides for the three power levels and wire diameters
Figure 9. Selectivity of ethanol to carbon oxides at the time of switching off the discharge.
used. It is seen that the CO2 selectivity increases when power and wire diameter increase.
CH3CHO + O3 → CH3COOH + O2
(8)
Table 1. Highest Concentration of Aldehydes Measured during Oxidation of Ethanol in Corona Discharge and the Time of Maxima diameter, mm power level, W acetaldehyde, ppm (time, min) formaldehyde, ppm (time, min)
0.2 mm
0.4 mm
0.8 mm
15 31 (84)
30 31.2 (70)
60 29.2 (60.5)
15 31.2 (79.5)
30 30.8 (51)
60 31.2 (45.5)
15 30 (80)
30 29.3 (47)
60 28.9 (22.5)
25 (150)
23.6 (126)
25.1 (104.5)
22.1 (140)
23.6 (92.5)
25.1 (81.5)
29 (130)
23.1 (77.5)
24.1 (51)
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(9)
The first reaction proceeds outside the corona discharge while the second one can dominate inside the discharge region where molecular oxygen decomposes into atoms which then can form ozone molecules. Acetic acid is further transformed into CO, CO2, and H2O possibly by reactions with atomic oxygen and OH• radicals.22 Methyl radicals CH3• are proposed often as intermediate in ethanol oxidation,23 and acetic acid can also be formed in a reaction of CH3• with O2 followed by hydrogen atom abstraction from an organic molecule.24 The time period of the corona discharge operation was too short for acetic acid to reach its maximum in all the experiments except the single one. It is not surprised that formic acid which is a product of formaldehyde oxidation was also registered in the gas phase. Its maximum concentration was about 5 ppm. Since the detection error is about 2 ppm, the kinetic curves of its concentration are not shown in Figures 2 and 3. The close values of the maximum concentration of aldehydes observed possibly mean that both the rates of their formation and consumption change simultaneously. The decrease of the time of maximum concentration at higher discharge power and wire diameter is again indicative of the increase in rates of both formation and consumption of these intermediate products. This follows from the well-known example of simple consecutive reaction. Schemes of Transformations. Acetone oxidation can be represented as a reaction of acetone with ozone molecules or atomic oxygen25 as follows. CH3COCH3 + 8O3 → 3CO2 + 3H 2O + 8O2
(10)
CH3COCH3 + 8O → 3CO2 + 3H 2O
(11)
Figure 11. Scheme of ethanol conversion in corona discharge. All compounds shown have been detected.
Kinetic curves in Figures 2 and 3 suggest that ethanol is oxidized into acetaldehyde and directly into CO2. Analogously acetaldehyde is converted via two pathways into acetic acid and formaldehyde. Formaldehyde is oxidized into formic acid, and the acids are transformed into CO and CO2. All these pathways may proceed with participation of ozone outside the discharge region or inside the plasma of discharge by reactions with atomic oxygen. Figure 11 contains only one way of formaldehyde oxidation with participation of ozone and OH• radical. An alternative formaldehyde oxidation pathway to CO and ultimate products CO2 and H2O exists inside discharge15,16 and proceed via formation of formyl radical CHO•. These reactions proceed via hydrogen atom abstraction by atomic oxygen or by OH• radical:
Formaldehyde and CO were detected as intermediate products of acetone oxidation in our experiments. Figure 10 shows the suggested scheme of acetone transformation under the action of corona discharge that includes all detected gaseous products.
corona NTP
O2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 2O•• ••
HCHO + O
■ Figure 10. Scheme of acetone conversion in corona discharge.
The acetone molecule oxidizes and forms three main carboncontaining products, CO2, CO, and formaldehyde. Formaldehyde and CO being the intermediate products oxidized further to carbon dioxide. Ethanol oxidation under the action of the negative corona discharge is more complex than that for acetone and additionally includes acetaldehyde, acetic and formic acids. Figure 11 demonstrates scheme of ethanol transformation pathways. 5847
(12) •
→ CHO + OH
•
(13)
HCHO + OH• → CHO• + H 2O
(14)
CHO• + O2 → CO + HO2•
(15)
CO + O•• → CO2
(16)
CONCLUDING REMARKS • For both acetone and ethanol, the rate of oxidation increases with an increase in corona discharge wire diameter and in electrical power supplied to the discharge. • Energy cost for destruction of ethanol was calculated to be not less than 0.33 kW h/g (570 eV/molecule). This value is 1.1 kW h/g (1.9 keV/molecule) for acetone under the same conditions. • It was shown experimentally that the rate and energy efficiency of ethanol oxidation in corona discharge plasma are strongly dependent on the concentration of accumulated ozone. The rate constant of the bimolecular gas-phase reaction of ozone and ethanol was calculated as 2.9 × 10−20 cm3/s. • Concentrations of intermediate products (acetaldehyde and formaldehyde) remain at the same levels despite the 4-fold rise of power level and analogous change of corona dx.doi.org/10.1021/ie400476p | Ind. Eng. Chem. Res. 2013, 52, 5842−5848
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(15) Chang, M. B.; Lee, C. C. Destruction of Formaldehyde with Dielectric Barrier Discharge Plasmas. Environ. Sci. Technol. 1995, 29, 181. (16) Starch, D. G.; Kushner, M. J. Destruction Mechanisms for Formaldehyde in Atmospheric Temperature Plasmas. J. App. Phys. 1993, 73, 51. (17) Lyulyukin, M. N.; Besov, A. S.; Vorontsov, A. V. The Influence of Corona Electrodes Thickness on the Efficiency of Plasmachemical Oxidation of Acetone. Plasma Chem. Plasma Process. 2011, 31, 23. (18) Kozlov, D.; Besov, A. Method of Spectral Subtraction of GasPhase Fourier Transform Infrared (FT-IR) Spectra by Minimizing the Spectrum Length. Appl. Spectrosc. 2011, 65, 918. (19) Kim, H. H. Nonthermal Plasma Processing for Air-Pollution Control: A Historical Review, Current Issues, and Future Prospects. Plasma Process. Polym. 2004, 1, 91. (20) Wang, H.; Li, D.; Wu, Y.; Li, J.; Li, G. Removal of four kinds of volatile organic compounds mixture in air using silent discharge reactor driven by bipolar pulsed power. J. Electrost. 2009, 67, 547. (21) Subrahmanyam, C. Catalytic Non-Thermal Plasma Reactor for Total Oxidation of Volatile Organic Compounds. Ind. J. Chem. 2009, 48A, 1062. (22) Sano, N.; Yamamoto, D. Simulation Model of the Decomposition Process of Phenol in Water by Direct Contact of Gas Corona Discharge in a Cylindrical Reactor. Ind. Eng. Chem. Res. 2005, 44, 2982. (23) Frassoldati, A.; Cuoci, A.; Faravelli, T.; Ranzi, E. Kinetic Modeling of the Oxidation of Ethanol and Gasoline Surrogate Mixtures. Combust. Sci. Technol. 2010, 182, 653. (24) Liu, C. J.; Wang, J. G.; Wang, Y.; Eliasson, B. On the Mechanism of Synthesis of Acetic Acid Directly from CH4 and CO2 Using Dielectric-Barrier Discharges. Fuel Chem. Div. Prepr. 2003, 48, 268. (25) Xi, Y.; Reed, C.; Lee, Y. K.; Oyama, S. T. Acetone Oxidation Using Ozone on Manganese Oxide Catalysts. J. Phys. Chem. B 2005, 109, 17587.
discharge wire diameter. This is due to unchanged chemical transformation pathways. • The proposed scheme of chemical transformation of ethanol molecule in the process of decomposition by corona discharge takes into account all detected gasphase products.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge the financial support for this work by the Russian Federal Department of Science and Education via Federal Target Program “Scientific and Educational Personnel” contract 8440 as well as President Grant for the Leading Scientific Schools NSh 524.2012.3.
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