Atmospheric Pressure Non-thermal Plasma for Air Purification: Ions and Ionic Reactions Induced by dc+ Corona Discharges in Air Contaminated with Acetone and Methanol

Atmospheric pressure mass spectrometry (APCI-MS) was used to investigate the positive ions in air containing acetone (A), methanol (M) and mixtures thereof (A + M), subjected to +dc corona discharges. The results of experiments with isotopically labelled analogues, perdeuterated acetone Adeu and methanol Mdeu, and relevant thermochemical data found in the literature allowed us to identify the main ionic reactions occurring in single component systems (A or M) and in binary mixtures (A + M). It is concluded that, thanks to its significantly higher proton affinity, A is very efficient in quenching M-derived ions at atmospheric pressure. These conclusions provide a rationale for interpreting the results of a parallel investigation on the reciprocal effects of M and A when treated together in air at atmospheric pressure with +dc corona in a non-thermal plasma reactor developed previously in our laboratory. Specifically, we observed a marked drop in the degradation efficiency of methanol when it was treated in the presence of an equivalent amount of acetone. This effect is attributed to acetone interfering with ion-initiated degradation processes of methanol, and supports the conclusion\ua0that ions and ionic reactions are important in dc+ corona induced oxidation of volatile organic pollutants in air

Comparison of toluene removal in air at atmospheric conditions by different corona discharges

Different types of corona discharges, produced by DC of either polarity (+/- DC) and positive pulsed (+pulsed) high voltages, were applied to the removal of toluene via oxidation in air at room temperature and atmospheric pressure. Mechanistic insight was obtained through comparison of the three different corona regimes with regard to process efficiency, products, response to the presence of humidity and, for DC coronas, current/voltage characteristics coupled with ion analysis. Process efficiency increases in the order +DC < -DC < +pulsed, with pulsed processing being remarkably efficient compared to recently reported data for related systems. With -DC, high toluene conversion and product selectivity were achieved, CO(2) and CO accounting for about 90% of all reacted carbon. Ion analysis, performed by APCl-MS (Atmospheric Pressure Chemical Ionization-Mass Spectrometry), provides a powerful rationale for interpreting current/voltage characteristics of DC coronas. All experimental findings are consistent with the proposal that in the case of +DC corona toluene oxidation is initiated by reactions with ions (O(2)(+center dot), H(3)O(+) and their hydrates, NO(+)) both in dry as well as in humid air. In contrast, with -DC no evidence is found for any significant reaction of toluene with negative ions. It is also concluded that in humid air OH radicals are involved in the initial stage of toluene oxidation induced both by -DC and +pulsed corona

Products and mechanisms of the oxidation of organic compounds in atmospheric air plasmas

Atmospheric plasma-based technologies are developing as a powerful means for air purification, specifically for the oxidation of organic pollutants. To achieve a better control on the emissions produced by such treatments mechanistic insight is needed in the complex reactions of volatile organic compounds (VOCs) within the plasma. An account is given here of our comparative studies of the behaviour of model VOCs in response to different corona regimes (+dc, -dc and +pulsed) implemented within the same flow reactor. Model VOCs considered include two alkanes (n-hexane and i-octane), one aromatic hydrocarbon (toluene) and two halogenated methanes, dibromomethane (CH(2)Br(2)) and dibromodifluoromethane (CF(2)Br(2), halon 1202). Efficiency and product data are reported and discussed as well as various possible initiation reactions. A powerful diagnostic tool is ion analysis, performed by atmospheric pressure chemical ionization-mass spectrometry: it provides a map of major ions and ion-molecule reactions and a rationale for interpreting current/voltage characteristics of dc coronas. It is shown that, depending on the specific VOC and corona regime adopted, different initiation steps prevail in the VOC-oxidation process and that the presence of a VOC, albeit in small amounts (500 ppm), can greatly affect some important plasma properties (ion population, current/voltage profile, post-discharge products)

Biofiltration combined with non-thermal plasma for air pollution control: A preliminary investigation

Biological technologies have been often employed to remove volatile organic compounds (VOCs) at low concentrations from air streams. However, biodegradation is very sensitive to variations of inlet concentrations and flow rate that usually occurs in real industrial processes; furthermore, an acclimation period is required by microorganisms to adapt to new conditions of flow rate and concentration. A possible solution to overcome these issues is represented by a pre-treatment with non-thermal plasma (NTP). The synergy between an NTP reactor and a biofilter in removing a mixture of VOCs from air is the object of this paper. A mixture of five VOCs (benzene, ethylbenzene, p-xylene, n-heptane and toluene) and humid air was chosen to represent the gaseous effluent stripped from an industrial wastewater treatment plant. A sudden increase in the VOC concentrations was intentionally induced to understand if NTP can manage peaks of the inlet concentration of pollutants and help the biodegradation carried out in the biofilter. NTP revealed to be capable of both pre-treating concentrations peaks and converting the initial VOCs in more soluble compounds; in conclusion, NTP is able to help biodegradation, allows controlling unsteady conditions and prevents stress to bacteria

Chemistry of organic pollutants in atmospheric plasmas

A large corona reactor of wire/cylinder configuration was used to compare the performance of different regimes (\u2013DC, +DC and +pulsed corona) in the removal of VOCs via oxidation in air at room temperature and atmospheric pressure. Mechanistic insight was gained through comparison of the different corona regimes with regard to process efficiency, response to the presence of humidity and, for DC coronas, current/voltage characteristics coupled with the analysis of the ionized component of the plasma. The results of ion analysis, performed by APCI-MS (Atmospheric Pressure Chemical Ionization \u2013 Mass Spectrometry), provide a powerful rationale for interpreting current/voltage characteristics of DC coronas. All experimental findings are consistent with the proposal that in the case of +DC corona the oxidation of hydrocarbons is initiated by reactions with ions (O2+\u2022, H3O+ and their hydrates, NO+) both in dry as well as in humid air. In contrast, with \u2013DC no evidence is found for any significant reaction of hydrocarbons with negatively charged ions. Thus, electron induced bond dissociation and reactions with O atoms and OH radicals must prevail in this case. Evidence is presented for the involvement of OH radicals in the initial stage of hydrocarbon oxidation induced both by \u2013DC and +pulsed corona. Finally, the major role of VOC chemical composition is highlighted by the strikingly different effects on \u2013DC corona produced by ppm concentrations of any of the hydrocarbons studied and of a chlorocarbon, dichloromethane: no effect is seen on the \u2013DC corona current/voltage profiles for any of the former, as opposed to a significant increase in current for the latter. These observations are consistent with and rationalized by the detection of many Cl-containing anions in the APCI mass spectra of dichloromethane

A new system for plasma-driven catalytic dry reforming of methane

Among the possibilities for syngas production, CO2 reforming of methane is very attractive (CH4 + CO2 => 2 CO + 2 H2). The environmental importance of this process consists in the possibility of recycling CH4 and CO2, which are the two major greenhouse gases, in a process useful for industry. Syngas can be stocked, transported and/or converted in hydrocarbons or in valuable oxygenated compounds. Unfortunately, dry reforming is a highly endothermic reaction, which requires a high operating temperature and the use of a catalyst [1]. Moreover, the occurrence of the Boudouard reaction (2 CO => CO2 + C) and methane cracking (CH4 => C + 2 H2) as side reactions induces carbon deposition which deactivates the catalyst. To overcome these problems, activation of dry reforming by the combination of an heterogeneous catalyst with non-thermal plasma has been recently proposed in the literature. When the catalyst is placed in the discharge zone, the results in most cases demonstrate the occurrence of a synergy between plasma and heterogeneous catalysis which allows to work at lower temperatures and decrease coke deposition. [2,3]. We report here the development of a prototype reactor for CO2 reforming of methane based on the application of corona discharges above the surface of a catalyst. The system has been designed to perform activity trials with and without plasma and with and without an heterogeneous catalyst. The catalyst can be introduced as a powder or in grains and is placed directly into the discharge zone. It is also possible to vary the distance between the active electrode and the catalyst to optimize the synergy between plasma and catalyst. The reactor was designed in such a way that it can be operated at temperatures up to 800\ub0C, necessary for a purely catalytic process, without suffering any damage to the electrodes due to thermal expansion. The first tests will be carried out without the heterogeneous catalyst, so as to study the process induced by plasma alone. Plasma will be produced by different types of discharges in order to find and optimize the best power supply to be used in the combined experiments. Analogously, the process induced by innovative Ni- and noble metal-based catalysts supported on nanostructured ceria and zirconia or perovskites will be studied in the absence of plasma. The preliminary results obtained will be used to optimize the experimental conditions for the study of the effects of the combination of catalyst and plasma. Particular attention will be devoted to the study of the mechanism of the synergy between plasma and catalyst

Advanced oxidation process for degradation of aqueous phenol in a dielectric barrier discharge reactor

A novel dielectric barrier discharge reactor was developed for the oxidative degradation of organic pollutants in water. Phenol, chosen as a model compound, is efficiently removed from the aqueous solution according to an exponential decay as a function of treatment time at constant voltage. The effect of different experimental variables was investigated, including the active electrodes material and size and the flow rate of air above the solution. A few intermediates and CO2, the final product of phenol decomposition, were detected and identified by LC/ESI-MS and FT/IR analysis. The major reactive species formed upon application of the discharge in air, the OH radical and ozone, were determined by means of specific chemical probes

Development of a new hybrid reactor for plasma-driven catalytic dry reforming of methane

We report here the development of a prototype reactor for CO2 reforming of methane based on the application of corona discharges above the surface of a catalyst. The system has been designed to perform activity trials with and without plasma and with and without a heterogeneous catalyst. The catalyst can be introduced as a powder or in grains and is placed directly into the discharge zone. It is also possible to vary the distance between the active electrode and the catalyst to optimize the synergy between plasma and catalyst. The reactor was designed in such a way that it can be operated at temperatures up to 800\ub0C, necessary for a purely catalytic process, without suffering any damage to the electrodes due to thermal expansion. The first tests will be carried out without the heterogeneous catalyst, so as to study the process induced by plasma alone. Plasma will be produced by different types of discharges in order to find and optimize the best power supply to be used in the combined experiments. Diagnostics includes spectroscopy techniques like FT-IR and OES (optical emission spectroscopy) as well as GC analysis coupled to different detectors (MS, FID and TCD)

Characterization of plasma-induced phenol advanced oxidation process in a DBD reactor

Using phenol as a model organic pollutant we studied and characterized an innovative advanced oxidation process in water using a prototype dielectric barrier discharge (DBD) reactor in which electrical discharges are produced in the air above the water surface. Phenol is decomposed quite efficiently in this reactor operated at room temperature and atmospheric pressure. The process selectivity to form CO2 is, however, to be improved since a large fraction of the treated organic carbon is unaccounted for. The rate of phenol conversion increases linearly with the reciprocal of the pollutant initial concentration, suggesting the operation of a mechanism of inhibition by products as found earlier for oxidation of volatile organic pollutants in air plasmas