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Methods for reduction of volatile organic compounds (VOCs) content in air depend on the application considered. For low concentration and low flux, nonthermal plasma methods are often considered as efficient. However, the complex chemistry involved is still not well understood because there is a lack of data sets of byproducts formation. To overcome this issue, rapid analytical methods are needed. We present the coupling of a rapid chemical ionization mass spectrometer (CIMS) for the real-time analysis of the VOCs formed during a degradation experiment. The high-resolution instrument used allows for chemical ionization and direct quantification of nontargeted compounds. This method is successfully applied to degradation experiments of acetone in a phototriggered nitrogen plasma discharge. Two regimes were highlighted: efficient conversion at low concentrations (100 ppm) and moderate efficiency conversion at higher concentrations (100 ppm). Those two regimes were clearly delimited as the sum of two exponential curves occurring at respectively low and high concentrations. Many byproducts were detected; in particular, HCN presented a significantly high yield. Nitrile compounds (acetonitrile, propionitrile, ...) are formed as well. To a lower extent, ketene, acetaldehyde, and formaldehyde are observed. The association of the high-resolution mass spectrometer to the plasma reactor will allow further insights into the plasma chemistry and comparison to modelization.

International audience; We compare the predictions of our recently developed statistical molecular fragmentation (SMF) model with experimental results from plasma induced hydrocarbon decay. The SMF model is an exactly solvable statistical model, capable to calculate the probabilities for all possible fragmentation channels as a function of the deposited excitation energy. The weights of the channels are calculated from the corresponding volume of the accessible phase space of the system, taking into account all relevant degeneracies, symmetries and density functions. An experiment designed to study the abatement of propene in N 2 using a photo-triggered discharge producing a homogeneous plasma at sub-atmospheric pressure was also performed. Using a 0D model that simulates the complex chemical kinetics in the plasma, it was possible to assess the percentages of the original parent hydrocarbon's fragmentation channels based on the detected species. These results were compared to those obtained from the SMF model. Previous plasma induced hydrocarbon fragmentation experiments for ethene, ethane and propane, where also compared to the predictions of the SMF model. For energies below that of metastable dinitrogen (i.e. below 6.17 eV and 8.4 eV), the SMF model and the experimental fragmentation channels coincide. This study allows one to draw conclusions both on the range of excitation energies transferred to the parent hydrocarbon molecules during plasma discharge and on the probability of the dynamical coupling of two H atoms from neighbouring carbon atoms to form H 2 molecules.

Gaseous naphthalene has been removed by air plasma generated by pulsed corona discharges at 100 °C (LSPM) and dielectric barrier discharges (DBD) up to 250 °C (LPGP) in different reactors geometries. Naphthalene has been chosen as one of unburned hydrocarbon present in exhaust gas engine during the cold start of vehicles. The comparison between the different discharge geometries has been possible using the specific input energy (SIE) as relevant parameter for pollutant removal process control considering the differences in the electrical characteristics and the differences of gas flow. The best naphthalene degradation is obtained in the wire-to cylinder (WTC) corona discharge and the stem-to-cylinder DBD with an energy cost β respectively of 10 and 20 J L−1. The main by-products issues of the naphthalene oxidation are CO2 and CO reaching 45% in Multi-Pin-to-Plan corona discharge. We detected polyaromatic hydrocarbons in the gas phase (few ppm) and in the solid phase deposited in the reactors. The introduction of water in the discharges promotes the naphthalene degradation by OH-atom, which has better oxidising power than O-atom in dry air.

The proton transfer reaction mass spectrometry (PTR-MS) is used to follow the toluene concentration at the exit of a dielectric barrier discharge, for an inlet gas mixture of dry air with organic molecules at a concentration in the range 10–100 ppm, at atmospheric pressure and room temperature. The precursor ion is H3O+, and toluene is detected at mass 93 u, corresponding to the (C6H5CH3)–H+ ion. The PTR-MS is used together with optical absorption spectroscopies to follow ozone and carbon dioxide molecules. Two different discharge regimes are studied: without or with significant self-warming effects. The PTR-MS diagnostic appears as a very useful tool for precise monitoring of the concentration of this organic molecule, and to get real-time information about the energy efficiency for the removal of the pollutant during the discharge operation, as function of the discharge regime.

International audience; The present work is devoted to the precise spatiotemporal mapping of the absolute density of Ar(1s(5)) in a ns-pulsed argon plasma jet. The plasma impinges on glass and alumina targets at floating potential placed 5mm away from the reactor's nozzle. Under these conditions, diffuse discharges are established in the small gas gap. As so, the line-of-sight absolute density of Ar(1s(5)) is effectively evaluated via laser absorption spectroscopy. The application of the Abel-inversion is also demonstrated for different operating conditions leading to the precise radial mapping of the Ar(1s(5)) absolute density. The influence of each target is studied for two gas flow rates, 0.3 and 0.4lmin(-1). The temporal density profiles over a voltage pulse period reveal two maxima related with the Ar(1s(5)) production in the streamer head and in the residual diffuse plasma channel. Furthermore, the maximum Ar(1s(5)) axial/radial density (approximate to 10(13)-3.5x10(14)cm(-3)) depends on the target material and gas flow rate. Finally, the plasma is proved to be very effective for the fast desorption of organic molecules (bibenzyl) deposited on both targets. The results obtained suggest that the desorption of bibenzyl is due to the production of high Ar(1s(5)) densities at the close vicinity of the targets. [GRAPHICS] .

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.

6th Central European Symposium on Plasma Chemistry (CESPC), Bressanone, ITALY, SEP 06-10, 2015; International audience; The propagation of a DC-pulsed argon plasma jet through the surrounding ambient air, and its interaction with an ungrounded glass plate placed on the jet trajectory, was studied by means of fast imaging. The surface plays an important role in the spatio-temporal characteristics of the plasma. Indeed, for an argon jet propagating perpendicularly to the surface, the plasma jet structure changes from filamentary to diffuse when the distance between the nozzle of the capillary tube and the surface is short (

6th Central European Symposium on Plasma Chemistry (CESPC), Bressanone, ITALY, SEP 06-10, 2015; The kinetics of four volatile organic compounds (VOCs) (propene, propane, acetaldehyde, acetone) were studied in plasmas of atmospheric gases using a photo-triggered discharge (homogeneous plasma) or a dielectric barrier discharge (filamentary plasma). It was shown for the homogeneous plasma that quenchings of nitrogen metastable states, A(3)Sigma(+)(u) and the group of singlets a `(1)Sigma(-)(u), a (1)Pi(g) and w(1)Delta(u), are important processes for the decomposition of such molecules. Recent measurements of the H-2 concentration produced in the N-2/C3H6 mixture emphasize that the hydrogen molecule can be an exit route for propene dissociation. It is also found that H-2 and CO molecules are efficiently produced following the dissociation of CH3COCH3 and the subsequent chemical reactivity induced by radicals coming from acetone. Addition of oxygen to a N-2/VOC mixture can change drastically the kinetics. However, the quenching processes of N-2 metastables by the VOC are always present and compete with oxidation reactions for the conversion of the pollutant. At low temperature, oxidations by O or by OH are not always sufficiently effective to induce an increase of the molecule decomposition when oxygen is added to the mixture. In particular, the presence of O-2 has a detrimental effect on the acetone removal. Also, as evidenced for acetaldehyde and propane, some kinetic analogies appear between filamentary and homogeneous plasmas.

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.

A photo-triggered discharge has been used to study the production kinetic mechanisms and the reactivity of the hydroxyl radical in a N2/O2 mixture (5% oxygen) containing ethane or ethene for hydrocarbon concentration values in the range 1000?5000?ppm, at 460?mbar total pressure. The discharge (current pulse duration of 60?ns) has allowed the generation of a transient homogeneous non-equilibrium plasma, and the time evolution of the OH density has been measured (relative value) in the afterglow (up to 200??s) by laser induced fluorescence (LIF). Experimental results have been explained using predictions of a self-consistent 0D discharge and plasma reactivity modelling, and reduced kinetic schemes for OH have been validated. It has been shown that recombination of H- and O-atoms, as well as reaction of O with the hydroperoxy radical HO2, plays a very important role in the production of OH radicals in the mixture with ethane. H is a key species for production of OH and HO2 radicals. As for ethane, O, H and HO2 are key species for the production of OH in the case of ethene, but carbonated radicals, following the partial oxidation of the hydrocarbon molecule by O, also play a non-negligible role. The rate constant for O- and H-atom recombination has been estimated to be 3 ? 10?30?cm6?s?1 at near ambient temperature, consistent with LIF measurements on OH for both mixtures with ethane and ethene.

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.

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.

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 influence of the gas mixture temperature, from 20 °C up to 300 °C, on the removal of formaldehyde, diluted at low concentration (less than 800 ppm) in dry air at atmospheric pressure, by a pulsed dielectric barrier discharge (DBD) is studied by means of Fourier transform infrared spectroscopy and micro gas chromatography. Efficient removal of CH2O is obtained and it is found that the characteristic energy, less than 200 J l−1, is a decreasing function of the temperature over the whole range of concentration values under consideration. Byproducts issued from the removal are identified and quantified (CO, CO2, HCOOH, HNO3). Experimental results are analysed using a zero-dimensional simplified DBD-reactor model in order to gain insights on the chemical processes involved. It is shown that the dissociation of the molecule competes with oxidation reactions at low temperature, whereas at high temperature oxidation processes dominate.

In order to see the accurate role of low temperature reactions in autoignition nd nock phenomena, oxidation of stoichiometric hydrocarbons/air mixtures has been performed in a CFR engine and in a flow system. The results show that reactions of isomerization of R02 radicals areimponant for hydrocarbons containing five or more carbon atoms. The pernxidic compounds formed through these isomerization reactions are alkylketohydroperoxides. A correlation is made between fuel structure, octane number. isomerization reactions and autoignition. A chemical kinetic model. created to interpret autoignition and knock. reproduces the results obtained with n-heptane and the influence of engine parameters as speed, intake temperature and pressure.

International audience; The kinetic of the hydroxyl radical is studied in N/O/HO mixtures with small amounts of acetone or isopropyl alcohol (0.5%). The radical density is measured in absolute value in the afterglow of a photo-triggered discharge, which generates an homogeneous transient non-equilibrium plasma, using a time resolved absorption measurement method. For dry mixtures, experimental results are compared to predictions of a self-consistent 0D discharge and kinetic model. It is shown that dissociation of the VOCs through quenching collisions of nitrogen metastable states plays an important role in the production of OH. Measurements can not be explained looking only at the oxidation of acetone or IPA by the oxygen atom. This result is reinforced by experimental results about the OH density in wet mixtures, with or without VOCs, compared to dry ones.

A photo-triggered discharge producing a homogeneous plasma was used to investigate, experimentally and with the help of a self-consistent 0D model, the decomposition processes of acetaldehyde (concentration up to 0.5%) in N2/O2/CH3CHO mixtures containing up to 20% oxygen, at a total pressure of 460 mbar. This work follows a previous one about N2/CH3CHO, having provided the necessary data about the quenching of the N2 metastable states by the acetaldehyde molecule. For the condition of the experiment, it was shown that oxygen has a weak influence on the acetaldehyde removal. Nevertheless, the kinetic reactions involved drastically change when the oxygen percentage is increased. Quenching reactions gradually give way to oxidation reactions by O(3P) and OH. Oxidation by OH dominates for a high acetaldehyde concentration or a high oxygen percentage. Moreover, CH3 is an important primary compound for the formation of CH4 and C2H6. Ethane is less populated than methane in the whole range of oxygen percentage values studied, and there are still hydrocarbon molecules in the gas mixture at 20% oxygen. This is well explained by the adopted kinetic scheme.

In plasmas of atmospheric gases, the kinetics of some aliphatic organic molecules belonging to the hydrocarbons (propene, propane), aldehydes (acetaldehyde) and ketones (acetone) families were studied using a photo-triggered discharge (homogeneous plasma). It was shown that quenchings of N2 metastable states, and the group of singlets , a?1?g and w?1?u, are important processes for the decomposition of such molecules. It plays a fundamental role in the nitrogen plasma, but it is also present in air. At low temperature, the oxidation reactions by the oxygen atom or by the hydroxyl radical are not always sufficiently effective to induce an increase of the molecule decomposition when oxygen is added to the nitrogen/organic mixture. For most cases, quenching processes appear purely dissociative. However, recent results obtained for propene lead to the conclusion that a non-dissociative exit route could exist. The quenching of the singlet states induces a break of the double bound C?=?O for the acetaldehyde and acetone molecules. Some kinetic analogies appear between filamentary and homogeneous plasmas, which could be very useful to get a comprehensive understanding of the physico-chemical processes in dielectric barriers or corona discharges used for various applications.

A photo-triggered discharge is used to study the decomposition processes of acetaldehyde in a high-pressure (460 mbar) nitrogen plasma, for a concentration of CH3CHO ranging from 500 up to 5000 ppm. Results of chromatographic measurements are compared with predictions of a self-consistent discharge and plasma kinetic model, for the primary molecule and for a number of detected by-products: H2, CH4, C2H2, C2H4, C2H6, CO and CH3COCH3. The main by-products are H2, CH4 and CO. It is proposed that CH3CHO mainly decomposes owing to quenching collisions of metastable states of the nitrogen molecule. The estimated coefficients for the quenching of is 4.2 × 10−11 cm3 s−1, assuming that the coefficient for the singlet states equals the one previously known for the quenching of N2(a') by ethene, i.e. 4.0 × 10−10 cm3 s−1. A value of 6.5 × 10−11 cm3 s−1 constitutes a maximum for and a minimum for N2(a'). The most probable exit routes (and the branching ratios) for the dissociation process of CH3CHO are CH3 + HCO (45%), CH4 + CO (30%), CH2CO + H2 (17%) and CH3CO + H (8%), as regards . For singlet states, a break of the double C = O bond occurs and the branching ratios are 15% for both exit channels producing C2H2 and C2H4 together with the oxygen atom. The model predictions for concentration values of C2H6 and CH3COCH3 are in good accordance with measurements, supporting the proposed dissociation pathways that lead to the production of methyl and acetyl radicals.

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.