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This paper deals with the ethanol (EtOH) removal in both dry and humid air fed dielectric barrier discharges. The experimental results were compared to the predictions of a zero dimension kinetic model to elucidate the main chemical routes occurring in the plasma phase. This comparison shows that both the dissociative quenching of the nitrogen metastables and the oxidation reactions by the oxygen atom or the hydroxyl radical should be taken into account to explain the EtOH abatement in these kinds of discharges. The CH3CHOH radical seems to be the main product of the nitrogen dissociative collisions, whereas radicals issued from the α- and β-H atom cleavage are the dominant ethanol oxidation by-products. These radicals account for the production of acetaldehyde, the main by-product of the ethanol/air fed discharges investigated here. Apart the complete oxidation products, i.e. carbon oxides and water, aldehydes containing up to six carbon atoms, ketones, carboxylic acids, ozone, nitrogen oxides, nitric acid and organic nitrates were found in the exhaust gas. A kinetic pathway is proposed to explain the formation of the detected by-products. Water vapour addition to the feeding gas slightly improves the EtOH removal and promotes further oxidation of the main by-products, thus enhancing the CO2 selectivity. This behaviour could be ascribed to the higher amount of hydroxyl radicals, which could boost the production of the direct precursors of CO2.

The removal of 500 ppm acetaldehyde in nitrogen at 1 bar is characterized in a pulse dielectric barrier discharge generating a spatial random distribution of plasma filaments. The identification and the quantification of numerous by-products are performed. At 20 °C, CH3CHO is efficiently dissociated, probably owing to quenching of N2 metastable states. The most abundant by-products are CO, H2, and CH4, in consistency with the three important exit channels for the quenching of the N2(A3Σ u + ) state by CH3CHO proposed by Faider et al. (2011). In order of importance, other products are HCN, C2H6, CH3CN, HNCO, CO2, CH3COCH3, C2H4, C2H5CN, NH3, C2H2, and a group of nitriles and of ketones. An increase of the temperature from 20 °C up to 300 °C induces a strong decrease of the removal characteristic energy, but the by-products types remain unchanged. Probably the reaction of H with CH3CHO plays a role in the removal of the molecule at 300 °C.

Electron impact ionization of n -heptane was studied using mass spectrometry. Cross-sections for the formation of molecular ion and ionic fragments are measured between 10 eV and 86 eV with a total cross-section of 1.5 × 10 −16 cm 2 towards 50 eV. The molecular ion is the most abundant below 16 eV. The present results display good agreement between the measured total ionization cross-sections and the calculated one with the BEB model. Five ions C n H 2 n +1 + (2 ≤ n ≤ 6) result from a simple C–C bond split in the molecular ion. C 3 H 7 + , identified as isopropyl cation, is the most abundant of the ionic species above 16 eV. Four ions C n H 2 n + (2 ≤ n ≤ 5) result from a C–C bond split with H-atom rearrangement. C 3 H 6 + , identified as propene cation, is the most abundant of these four cations above 35 eV. Five other ions, C n H 2 n −1 + (2 ≤ n ≤ 4), cyclopropenyl and methyl cations may result from the ionization of C 4 H 9 , the major alkyl issued from the fragmentation of the molecular ion.

The products of 2-heptanone conversion by dielectric barrier discharge plasma are analyzed under different conditions: alternating current (ac) or pulsed mode of excitation, variable energy, variable composition of the carrier gas. The efficiency of the conversion is higher using a pulse excitation mode than an ac mode. With a small oxygen percentage (about 2-3%) added to nitrogen, 2-heptanone is about 30% more efficiently removed than in pure nitrogen, while the 2-heptanone removal decreases with an oxygen percentage higher than 3%. A new analysis method, based on chemical ionization mass spectrometry, is used for volatile organic compound detection along with chromatography. Several products issued from 2-heptanone conversion with ac excitation are identified in nitrogen and in air, and a chemical scheme is proposed to explain their formation and their treatment by the discharge. It appears that byproducts are issued not only from oxidation reactions, but also from C-C bond cleavage by collisions with electrons or nitrogen excited states.

Electron impact ionization of formaldehyde has been studied using mass spectrometry. Cross-sections for the formation of ions are measured between 10 and 85 eV with a total cross-section of 4 × 10 −16 cm 2 at 50 eV. H 2 CO + , HCO + and CO + contribute to about 99% of the total cross-section at 85 eV. The branching ratio for the formation of these cations is 0.32 (H 2 CO + ), 0.49 (HCO + ) and 0.19 (CO + ) between 45 eV and 85 eV. HCO + is the most abundant above 32 eV. Three other minor ions, O + , CH 2 + and CH + , are detected in the range 35–85 eV.

Toluene removal is investigated in filamentary plasmas produced in N2 and in N2/O2 mixtures by a pulse high voltage energised DBD. Influence of the oxygen percentage (lower than 10%) and of the temperature (lower than 350°C) is examined. Toluene is removed in N2 through collisions with electrons and nitrogen excited states. The removal efficiency is a few higher in N2/O2. It increases when the temperature increases for N2 and N2/O2. Both H- and O-atoms play an important role in toluene removal because H can readily recombine with O to form OH, which is much more reactive with toluene than O. H follows from dissociation of toluene and of hydrogenated by-products by electron collisions. Detection of cyanhidric acid, acetylene, formaldehyde, and methyl nitrate strengthens that dissociation processes, to produce H and CH3, must be taken into account in kinetic analysis. Formation and treatment of deposits are also analysed.

The electron impact ionization of acetone and 2-butanone between 10 and 86 eV has been studied using mass spectrometry. The cross-sections are measured for the formation of molecular ions and major fragment ions which are produced. The present results display good agreement between the measured total ionization cross-sections and the calculated with the BEB model. The enthalpies of formation and the ionization energies of several fragments are evaluated using ab initio calculations. For each ketone, the molecular ion and the 43 amu ion contribute to about 80% of the total cross-section at 86 eV. The 43 amu ion, identified as the acetyl cation, is the most abundant above 15 eV. Other ions present in the spectra of acetone are (i) the 42 amu ion, detected in the range 12–86 eV, contributes to about 6% of the total cross-section at the maximum voltage used and is identified at low energy as a ketene cation, (ii) six other minor ions (39, 27, 26, 44, 29 and 15 amu) were detected above 17 eV. Five of them may result from dissociation reactions of the molecular ion while the methyl cation is issued from the 43 amu ion. In the spectra of 2-butanone, other ions are (i) the 57 amu ion detected in the range 11–86 eV and identified as the propionyl cation, contributes to about 6% of the total cross-section over the whole ionization energy range, (ii) four other minor ions (42, 29, 27 and 15 amu) were detected above 18 eV and there formation is similar to that of acetone. Effects of fragment size favour from the molecular ion, the formation of the 57 amu ion near the threshold, and at higher energy, the formation of the 43 amu ion.

A photo-triggered discharge is used to study the influence of three hydrocarbons (HCs), propene (C3H6), n-decane (C10H22), and toluene (C6H5CH3) on NO conversion in N2/O2/NO/HC mixtures, with 18.5% O2 concentration, 700 ppm of NO, and an hydrocarbon concentration ranging between 190 ppm and 2,700 ppm. The electrical system generates a transient homogeneous plasma, working under 400 mbar total pressure, with a 50 ns short current pulse at a repetition frequency up to a few Hz. The NO concentration at the exit of the reactor is quantified using absolute FTIR spectroscopy measurements, as a function of the specific deposited energy in the discharge and the mixture composition. Owing to the plasma homogeneity, the experimental results can be compared to predictions of a self-consistent 0-D discharge and kinetic model based on available data in the literature about reactions and their rate constants. It is shown that the addition of either propene (as for DBD or corona discharges) or n-decane to N2/O2/NO leads to an improvement of the NO removal as compared to the mixture without hydrocarbon molecules. The adopted kinetic schemes explain this effect for the two mixture types. On the other hand, both the experiments and model predictions emphasize that the addition of toluene does not lead to the improvement of NO conversion. Moreover, compounds that are useful for NOx reduction catalysis, such as aldehydes, are less produced in the mixture with toluene.

Electron impact ionization of toluene is studied using mass spectrometry. Cross-sections for the formation of molecular ions and ionic fragments are measured between 10 and 78 eV with a total cross-section of 1.5 × 10 - 15 cm 2 towards 60 eV. C 7 H 8 + and C 7 H 7 + contribute to 75% of the total cross-section at 78 eV. The molecular ion is the most abundant below 25 eV. Four ionic fragments: C 5 H 5 + , C 4 H 3 + , C 5 H 3 + and C 3 H 3 + , are detected above 20 eV. Enthalpy considerations can lead to think that C 5 H 5 + is issued directly from the molecular ion, whereas the three other species result from two step pathways.

Electron impact ionisation of 2-propanol and ethanal is studied using mass spectrometry. Cross-sections of the formation of molecular ions and ionic fragments are measured between 14 and 86 eV. Free energy changes are evaluated using ab initio calculations. For 2-propanol, two ions, identified as CH 3 CHOH + (45 amu) and CH 3 CHCH 3 + (43 amu), contribute more than 75% to the total cross-section over the whole range of electron energies and are produced by simple bond cleavage in the molecular ion. Both processes occur spontaneously, leaving the molecular ion as a minority species. For ethanal, two ions, identified as HCO + (29 amu) and CH 3 CO + (43 amu), and the molecular ion (44 amu) contribute more than 80% to the total cross-section. The ions of 29 and 43 amu result from a simple bond cleavage in the molecular ion. These sprocesses are not spontaneous and the contribution of the molecular ion becomes predominant at 15 eV and is therefore significant over the whole range of ionisation energies.

The evolution of the OH radical density was measured in the afterglow of a pulsed homogeneous discharge using time-resolved laser induced fluorescence, and the acetaldehyde removal was quantified using chromatography for N2/O2/CH3CHO mixtures at 460 mbar with 5% oxygen and an acetaldehyde concentration between 250 and 5000 ppm. It is shown that the radical density rapidly increases shortly after the discharge for all mixture compositions studied, although it does not contain water molecules. Afterwards this density decreases with a characteristic time much longer than the one estimated from the known reactivity of OH with CH3CHO. This emphasizes that processes other than oxidation of acetaldehyde by O-atoms play a significant role in OH radical production.

2-heptanone is representative of a class of odorous molecules. Recent studies have shown that by adding a catalyst to a dielectric barrier discharge (DBD) plasma, the elimination of 90% of this molecule can be achieved with low consumption of electric energy, at room temperature, for concentrations below 1000 ppm. In the presented work, the removal of the ketone by OBD, both in dry air and within a slice of a honeycomb monolith of cordierite without a catalyst, was studied. In both experiments, the discharge was operated in a plane-to-plane geometry with a discharge volume of 10 cm 3 . A high voltage, bipolar pulse generator (40 kV max, 1-140 Hz freauency range) was used, In dry air, it was found that 2-heptanone is almost totally removed (>95%) for a specific deposited energy of about 500 J . l - 1 , but this elimination is less effective in the porous cordierite reactor (80%) for the same energy. This effect is explained by the very different spatial distribution of the plasma within the discharge volume, as seen using a CCD camera. Moreover, the adsorption-desorption equilibrium of the molecule at the surface of the material is greatly influenced by the discharge.

The electron impact ionisation of 2-heptanone between 13 and 78 eV is studied using mass spectrometry. CH3C(O)CH2CH2CH2CH2CH3+ and fragment ions are produced with a total cross-section of 5 × 10−16 cm2 towards 50 eV. Two ions, identified as CH3CO+ (43 amu) and CH3C(OH)CH2+ (58 amu), contribute to about 60% of the total cross-section for electron energies above the ionisation threshold. The detected ions are identified using ab initio calculations. For E = 14 eV, the ion of 58 amu is the most abundant followed by an ion of 59 amu identified as being CH3C(OH)CH3+; they result from a bond cleavage with one or two H atom rearrangements. For E ≥ 48 eV, the ion of 43 amu is the most abundant; it results from an α-cleavage reaction in the molecular ion.

A plasma-catalyst reactor was used to eliminate a low concentration (180 ppm) of 2-heptanone in air by total oxidation. This VOC is representative of a class of odorous ketones. A DBD-type plasma was generated through a platinum-based catalyst supported on an alumina wash-coated honeycomb monolith by means of a high voltage bi-polar pulsed excitation. The 2-heptanone elimination efficiency increases with the energy density. More than 97% of 2-heptanone conversion is observed, in dry air condition for an energy density of 200 J/L. An important synergy effect has been observed between cold plasma and catalyst, the 2-heptanone conversion remaining less than 50% on an uncoated monolith, even at an energy density value higher than 500 J/L. The presence of water (3 mol%) in the gas phase, slightly decreases the efficiency of the 2-heptanone elimination, but strongly decreases the ozone formation in the 100–300 J/L energy density range.

Electron impact ionisation of iso-octane is studied using mass spectrometry. Cross-sections for the formation of fragment ions are measured between 17 and 79 eV. C4H9+ is the most abundant ion, C8H18+ is completely absent. A total ionisation cross-section of 2×10−15 cm2 is found at 79 eV. Ab initio calculations show that the principal ionic species observed, C4H9+, C4H8+, C7H15+, C3H6+ and C3H11+, may be issued directly from iso-octane by simple C–C bond split processes or with H atom rearrangements. Ionisation of the two major neutral species issued from bond split, C4H9 and C4H10, can lead to the principal ions observed.

Experimental investigations and kinetic interpretation indicate the presence of several peroxidic species during the oxidation of n-dodecane. The oxidation was performed either in the gaseous phase in a flow system, or with a liquid/gas interaction in a bulb, because in many combustion processes liquid and gas coexist during preignition. The different peroxides were identified by mass spectrometry. Simple hydroperoxides with two oxygen atoms per molecule were observed at relatively low temperatures above 370 K with liquid and gas present. Ketohydroperoxides with three oxygen atoms per molecules are formed at ∼500 K. Di-, and even tri-, ketohydroperoxides with four or five oxygen atoms per molecule are also present under these experimental conditions at 518 K. Their formation is explained by isomerization reactions of alkoxy radicals OR −2H O bearing a carbonyl group; these stem from the decomposition of carbonyl-hydroperoxides. Depending on the experimental conditions, the nature of the hydroperoxides is different and the respective reactions of these species should be introduced in low temperature oxidation mechanisms. These reaction schemes should include the isomerization of peroxy radicals RO 2 and of ketoalkoxy radicals OR −2H O, leading to the formation of peroxides with several carbonyl groups.

Engine and laboratory experiments are performed to understand the formation of deposits during ignition delay. An experimental Diesel engine allows to observe deposit formation as a global phenomenon including homogeneous and heterogeneous reactions as well as interaction between liquid and gaseous phases. A CFR engine is used to work under the same global conditions but without the liquid phase. A flow reactor enables to investigate the effects of defined and controlled temperatures and fuel/air ratios. Results show that deposit formation is facilitated by temperatures of the oxidized gases within the 290 - 350°C range and by the presence of the liquid phase. In this temperature range, carbo-nyl-hydroperoxides ORO2H, RO and RO2 radicals are known to be present and to play an important role. Various species are identified (carboxylic acids, aldehydes, ketones, dione, furanone) by GC/MS. Homogeneous and heterogeneous reactions of RO and RO2 radicals yield analogous types of compounds. All these results all...

In order to investigate the effect produced by adding ketohydroperoxide to hydrocarbons, experiments were performed in a a motored CFR engine. First, n-heptane, then n-heptane containing added heptyl hydroperoxide, and finally n-heptane containing added ketohydroperoxide were successively used as fuels

Two experiments have been performed under conditions as close as possible to those existing in a diesel engine. The first is oxidation of n -dodecane in a motored diesel engine running under conditions close to ignition but avoiding it. The progress of chemical reactions is followed by measurements of the global temperature increase ΔT of the exhaust gases, and by continuous sampling of the combustion chamber gases, to measure the concentrations of hydroperoxides and molecular hydrogen; about 4.2% of the energy introduced as hydrocarbon is consumed, thus showing significant transformations during the ignition delay of n -dodecane. The location of the maximum concentration of hydroperoxides coincides with the fuel jet's edge. Tarlike compounds are present in the unburnt dodecane at the engine exhaust. The second experiment is the study of ignition delay of an n -dodecane spray in an oxidation chamber filled with air, between 715 and 760 K and 15 and 25 bar. A reduced mechanism of 32 reactions, with three types of branching due to the species (RO 2 , RO 2 H), (HO 2 , H 2 O 2 ), and H, enable one to predict the ignition delay. Computer simulations are made with the KIVA II code. They show good agreement between the experimental and the calculated ignition delays. They also indicate that, during the ignition delay, reactions occur first at the boundary of the fuel spray. A temperature increase of about 100 K takes place at the hottest points, which correspond to concentration maxima of the three branching species. Time-dependent evolutions of average concentrations show that RO 2 H reaches a maximum first, then H 2 O 2 , and lastly the H atom.

The space and time development of a single nanosecond pulsed corona discharge is studied in atmospheric air-propane mixtures with different concentrations of propane up to 8%. Time-resolved imaging of the discharge shows a complete diffuse pattern in pure dry air, which becomes more and more filamentary when propane is added. Filaments are thinner with higher concentrations of propane, and the whole discharge energy increases and gets saturated.

The use of pulsed nanosecond scale discharges is promising for automotive engine ignition because air-hydrocarbon mixtures can be chemically activated with a nonthermal plasma. Very short high voltage pulses are a good way to control the energy which is transferred into the gas at pressures above atmospheric. The development of such a discharge, in a point-to-plane configuration, under a short and high overvoltage, is investigated in air for different pressures through fast imaging and electrical records.

In the context of expanding plasma-catalytic cleaning techniques where discharges develop inside heterogeneous catalytic supports like packed beds or honeycomb monoliths, the understanding of streamers propagation physics in such nonhomogeneous media is of great interest. For that purpose, have been compared pulsed discharges inside two identical plane-to-plane dielectric barrier devices, one of them being coupled with a honeycomb monolith of cordierite. Electrical records and corresponding images of plasma properties in both these reactors are presented.

The decomposition of propane in non-thermal plasmas of N2/C3H8 and N2/O2/C3H8 mixtures (oxygen percentage up to 20%) at low temperature is studied in a photo-triggered discharge. Quenching of nitrogen metastable states dissociate C3H8 to produce propene and hydrogen. Oxidation reactions are growing in importance when the O2 concentration increases, but the dissociation quenching reactions still occurs for the air-based mixture. Even for a low concentration of oxygen, OH is an important specie involved in the conversion of the hydrocarbon. A kinetic analysis emphasises that OH comes in great part from the production of H, in which the methyl radical plays a role, strengthening the role of the dissociation processes of propane and propene in the medium reactivity. Results of PLIF measurements performed on OH during the diffuse afterglow of a nanosecond corona discharge correlate with results obtained on the photo-triggered discharge.

The removal and the conversion processes of propane in N2/C3H8 mixtures (concentration of hydrocarbon molecules up to 5500 ppm) energized by a photo-triggered discharge (homogeneous plasma) are studied at 460 mbar total pressure, both experimentally and theoretically. A self-consistent 0D discharge and kinetic model is used to interpret chromatographic measurements of propane and some by-products' concentrations (hydrogen and hydrocarbons with two or three carbon atoms). It is suggested, from the comparison between measurements and model predictions, that quenching processes of nitrogen metastable states by C3H8 lead to the dissociation of the hydrocarbon molecule, and are the most important processes for the removal of propane. Such a result is obtained using the quenching coefficient value previously determined by Callear and Wood (1971 Trans. Faraday Soc. 67 272) for the state, whereas the coefficient for collisions of the singlet states with C3H8 is estimated to be 3.0 × 10−10 cm3 s−1 in order to explain the measured propane disappearance in the N2/C3H8 mixture excited by the photo-triggered discharge. The hydrogen molecule is the measured most populated by-product and, also from the comparison between experimental results and model predictions, the most probable dissociation products of propane appear to be H2 and C3H6. The propene molecule is also efficiently dissociated by the quenching processes of N2 states, and probably leads to the production of hydrogen atoms and methyl radicals with equivalent probabilities. The kinetic model predicts that the carbon atom is distributed amongst numerous molecules, including HCN, CH4, C2H2, C2H4, C2H6 and C3H6.

The absolute value of the hydroxyl radical was measured in the afterglow of an homogeneous photo-triggered discharge generated in N2/O2/H2O/C2H6 mixtures, using a UV absorption diagnostic synchronized with the discharge current pulse. Measurements show that OH is efficiently produced even in the absence of water vapour in the mixture, and that the radical production is closely linked to the degradation kinetic of the hydrocarbon. Experimental results for dry mixtures, both for OH and for the removal of ethane in the discharge volume, are compared with predictions of a self-consistent 0D discharge and the kinetic model. It appears that the oxidation reaction of the ethane molecule by O(3P) atoms plays a minor role. Dissociation of the hydrocarbon through quenching collisions of the nitrogen metastable states are of great importance for a low oxygen concentration value. Also, the oxidation of ethane by O(1D) cannot be neglected at high oxygen concentration. The most probable exit channel for N2 states quenching collisions by ethane is the production of ethene and hydrogen molecules. Afterwards C2H4 should be dissociated to produce H and H2. As previously suggested from the study of the OH density time evolution in relative value, the recombination of H and O atoms appears as a main process for the production of OH in transient low temperature plasmas generated in atmospheric gases at high pressure. Another important reaction is the reduction of the HO2 radical by O, this radical coming from the addition of H on the oxygen molecule. H atoms come from numerous kinetic processes, amongst which is the dissociation of ethene.

The removal of traces of formaldehyde (150?ppm) in nitrogen and in dry air, at atmospheric pressure, by the filamentary plasma of a dielectric barrier discharge (in a cylindrical geometry) energized by a HV pulse is experimentally studied, at ambient temperature (20??C) and at 300??C. It is found that the pollutant molecule is more efficiently removed in nitrogen than in air at 20??C, whereas it is the opposite at 300??C. In air, the removal of CH2O strongly increases when the temperature increases. This effect also occurs in nitrogen, but it is less important. A qualitative explanation for these results can be found in the competitive influence of quenching collisions of the nitrogen metastable states by formaldehyde and oxidation reactions of this molecule.

Le dosage de SO2 en presence de composes intermediaires d’oxydation d’hydrocarbures (peroxydes) etant particulierement delicat par suite des reactions secondaires en phase liquide, nous proposons une methode experimentale permettant de separer les differents produits.Dans ce but, les gaz de combustion traversent des solutions appropriees.Grâce a cette methode, il est montre que la concentration de SO2 n’est pratiquement pas modifiee au cours de l’oxydation lente du pentane. Il en resulte que la reaction de SO2 avec les radicaux hydroperoxyles en phase gazeuse n’est pas importante dans nos conditions experimentales.

Presentation d'une nouvelle methode d'etude semi-statique; premiers resultats obtenus sur le mecanisme de reaction

L'oxydation lente de l'hydrogene donne naissance au radical hydroperoxyle HO2 susceptible de s'additionner sur SO 2 pour former un nouveau radical dont la structure a ete etudiee par R.P.E. L'invariance du spectre par substitution de l'hydrogene par le deuterium ainsi que les valeurs principales (g 1 = 2,0331, g 2 = 2,0077 et g 3 = 2,0026) du tenseur g indiquent qu'il s'agit d'un radical peroxyle de structure probable :[math]