Kinetics of NO_x formation from N₂/O₂/C₂H₄/Ar mixtures in repetitively-pulsed dielectric-barrier discharges

The catalytic cycle of NO and NO₂ (NO_x) is known to be one of the mechanisms through which plasma enhances the ignition of hydrocarbons. To better understand the kinetics of NO_x within a non-equilibrium plasma, the formation of NO_x by repetitively pulsed dielectric-barrier discharges was studied experimentally and numerically. The experiments were carried out by applying highvoltage pulses on N₂/O₂/C₂H₄ mixtures highly diluted with Ar, flowing through a flow reactor at isothermal conditions. Numerical simulations were performed by a zero-dimensional plasma kinetics code, which combines CHEMKIN and a Boltzmann equation solver. The present work examines the kinetics at different temperatures and mixture compositions over a wide range of timescales. Even at low temperatures, the major NO_x formation path is identified to be the one found in the Zel'dovich mechanism, namely N+O₂→NO+O, owing to the production of N atoms by the plasma. The enhancement of the hydrocarbon oxidation by the NO_x catalytic cycle is confirmed, and the products of the oxidation process are discussed in detail.