Production and reactivity of the hydroxyl radical in homogeneous high pressure plasmas of atmospheric gases containing traces of light olefins

Abstract

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 µs) 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.

Propane dissociation in a non-thermal high-pressure nitrogen plasma

“…The photo-triggered reactor used in the present study, called "UV510", has been described in previous publications [21][22][23][24]. It allows to create an homogeneous plasma volume, V D , of 50 cm 3 , with a discharge current pulse of 60 ns for duration.…”

OH kinetics in photo-triggered discharges used for VOCs conversion

“…The photo-triggered discharge reactor used for this work was described previously [5]. Two electrodes, 50 cm long with a spacing d=1 cm and a flat profile over 1 cm width, are directly connected to an energy storage unit of capacitance C=17.44 nF charged up to a voltage V 0 in a few hundred nanoseconds.…”

“…A 0D model has been previously developed [5][6][7][8]26], which couples : i) the solution of the Boltzmann equation for the electrons, ii) the kinetic equations describing the temporal evolution of the various species (molecular excited states, ions, atoms, radicals, molecules) produced during the discharge or during the afterglow, iii) the electric circuit equations given by the Kirchoff laws. Many reaction rates are sensitive functions of the gas temperature so that temperature time evolution is taken into account through the resolution of the energy conservation equation.…”

“…Details about the kinetic scheme adopted for atmospheric gases has been given in [5][6][7][8] As far as a carbonated molecule is added to N 2 /O 2 , numerous other kinetic processes are involved in the discharge. As a first step, we have only considered oxidation and dissociation reactions of acetone and IPA, and the kinetics of by-products containing carbon atoms (ions and radicals) have been neglected.…”

“…However the hydroxyl radical, which is much more reactive with numerous VOCs than the oxygen atom at a temperature close to the ambient one, can be produced through numerous kinetic processes. In particular, it was recently demonstrated by works on homogeneous discharges [5] that the recombination of O-and Hatoms, through three body collisions involving background molecules, is an efficient OH production process in high pressure and low temperature non-thermal plasmas. Therefore the relative importance of O and OH for the removal of VOCs by pulsed discharges deserves to be studied in more details.…”