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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.

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 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.

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.

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 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 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.