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• W179087689 abstract "Plasma chemistry is a rapidly growing field to develop and exploit the great potential of plasma to perform chemical reactions. This thesis deals with the oxidation of organic pollutants in air and water promoted by the action of air non-thermal plasma (NTP). Such plasmas, which are conveniently produced by electric non-thermalizing discharges like corona and dielectric barrier discharges in air at atmospheric pressure and ambient temperature, provide highly reactive oxidizing environments comprising electrons, excited molecules, atomic and radical species (O, OH), ions (O2+, N2+, NO+, O–, O2–, O3–), O3 and NO. Despite the numerous successful applications of NTP in processes of environmental and commercial relevance, the chemistry of organic compounds within air non-thermal plasmas is still not well characterized, both as far as products and mechanisms are concerned. This thesis developed as a contribution to this field of research along three lines dealing, respectively, with plasma processing of: volatile organic compounds (VOCs) in air with plasma alone; VOCs in air with plasma plus an heterogeneous catalyst; organic pollutants in aqueous solutions above which an air plasma is generated. All three projects had a common focus in the mechanistic characterization of the oxidation processes within these very complex and highly reactive systems.The study of VOC oxidation in air NTP was carried out with a large prototype corona reactor developed at the Department of Chemical Sciences in Padova, which can be energized with DC or pulsed high voltage of either polarity.Comparative studies were carried out to evaluate the response of selected model VOCs to different corona regimes (+DC, −DC and +pulsed). The VOCs considered include two alkanes (n-hexane and i-octane), toluene and two halogenated methanes, dibromomethane (CH2Br2) and dibromodifluoromethane (CF2Br2, halon 1202). Remarkably, all these different VOCs, including the highly inert halon, can be oxidized to CO2 at room temperature with efficiencies which depend on VOC type (despite their high reactivity NTPs display some selectivity), on VOC concentration (the efficiency increases linearly with the reciprocal of VOC concentration) but also on the way energy is given to the plasma. Thus, for all VOCs examined the efficiency decreases in the order:+pulsed > -DC > +DC corona. This means that any given amount of energy produces an extent of VOC conversion ([VOC]/[VOC]0) which depends on the corona regime and decreases in the order +pulsed > –DC > +DC. The greater efficiency of +pulsed corona is due to the higher mean electron energy achieved, for any given input of energy, with this type of power supply with respect to DC high voltage. The mean electron energy of the plasma under the different corona regimes was experimentally determined in our reactor by emission spectroscopy measurements following a published procedure. Another important variable tested was the humidity in the air, which is known to produce the strongly oxidizing OH radicals via electron induced dissociation or via reaction with O2+ and N2+ ions. Thus, the greater efficiency in humid with respect to dry air observed for the oxidation of hydrocarbons and of CH2Br2 with –DC and +pulsed corona was attributed to reaction with OH radicals.Surprisingly, in experiments with + DC the same VOCs undergo less efficient oxidation in humid air than in dry air, despite the presence of OH radicals.Analysis of the plasma ionized species, performed by APCI-MS (Atmospheric Pressure Chemical Ionization – Mass Spectrometry), coupled to the determination of current/voltage characteristics of DC coronas, led to the proposal that in the case of +DC corona the oxidation of hydrocarbons and of CH2Br2 is initiated by reactions with ions (O2+, H3O+ and their hydrates, NO+)both in dry as well as in humid air. In contrast, with –DC and +pulsed corona in humid air, OH radicals are involved in the initial stage of hydrocarbons and of CH2Br2 oxidation. Consistent with its very low reactivity with the OH radical, the oxidation of CF2Br2 in humid air proceeds less efficiently with both +DC and –DC corona. It was thus proposed that the oxidation of CF2Br2 occurs via a common mechanism under all corona regimes tested, the initial step being electron induced C-Br dissociation. The process efficiency is lower in humid air because the mean electron energy is lowered due to reaction of the electrons with water molecules. The two halomethanes also form different products: FTIR analysis of post-discharge gas has shown that CH2Br2 produces both CO2 and CO, whereas CF2Br2 forms CO2 and F2C=O. The latter product is a longlived oxidation intermediate due to its low reactivity with atmospheric radicals. It is however very rapidly hydrolized to CO2 and HF as shown by combined ion chromatography and FT-IR analysis of the solution and of the exhaust gas obtained after a water scrubbing step. Other non-carbon containing products of the discharge were analyzed by FT-IR analysis, including ozone, HNO3 and N2O. In experiments with both halomethanes evidence was found for brominesustained catalytic ozone destruction cycles, responsible also for increased conversion of NOx into HNO3.Efficiency and, especially, product selectivity can be improved by the combined action of plasma and heterogeneous catalysts which usually result in synergic effects. The nature and origin of this synergy was the focus of the research I carried out in my second year in graduate school during a stage at the Advanced Industrial Science and Technology Institute (AIST) in Tsukuba (Japan), in the group of dr. Hyun-Ha Kim. As molecular probe to compare the effects of plasma alone and plasma plus catalyst we selected the O-scrambling reaction to form 16O18O starting from a mixture of 16O2 and 18O2. Four different reactors were used and various catalysts, including TiO2, MS-13X and gAl2O3 also containing a few % of Ag. It was possible to conclude that, in the absence of a catalyst the O-exchange reaction occurs in the gas phase and not on the electrodes surface. It was also possible to use the results of these experiments to estimate, for any given energy input the average concentration of O atoms within the plasma. This is a most interesting outcome of these studies since the traditional method for obtaining O atom density involves sophisticated laser spectroscopy instrumentation and procedures. As for the catalyst/plasma interaction, using again a labelled molecular probe, 18O2, it was possible to conclude that the plasma induces oxygen fission on the catalyst surface and that this oxygen is then used in the oxidation of VOCs.The third project dealt with the plasma induced oxidation of phenol in aqueous solution. For these studies two prototype reactors were developed and tested, both characterized by application of electric discharges in the air above the solution. The first is a dielectric barrier discharge reactor which afforded the efficient removal of phenol from the aqueous solution according to an exponential decay as a function of treatment time at constant power. A qualitative analysis of the intermediate and final products of phenol oxidation was performed and the major reactive species formed upon the application of the discharge in air were identified and determined. It is concluded that the decomposition of phenol is initiated by reactions with ozone, taking place on the surface of the solution, and with hydroxyl radicals, both at the surface and within the bulk of the solution. An interesting and most convenient result is the better efficiency of phenol removal in tap water than in milliQ water. After ruling out possible effects due to conductivity, to Fenton’s reaction due to Fe2+ and toactive chlorine, it was concluded that the efficiency increase is due to the higher pH of the solution mantained by the presence of bicarbonate salts. The second developed reactor allowed us to perform some interesting comparisons since it can be powered by different types of high voltage for the generation of plasma.The possibility to apply this technology to the treatment of waste water depends on many factors: the process efficiency, the final composition of the treated solution, the general applicability of the system to the organic pollutants. The data obtained so far are very promising due to the efficient oxidation all the way to CO2 and to the absence of any hazardous organic byproduct after a proper treatment time.The results of this thesis confirm that plasma processing is a promising highly efficient means for the advanced oxidation of organic pollutants both in air and in aqueous solution." @default.
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• W179087689 date "2009-12-31" @default.
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• W179087689 title "Non-thermal plasma processing for the decomposition of organic pollutants" @default.
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