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Abstract Plasmas interacting with liquid microdroplets are gaining momentum due to their ability to significantly enhance the reactivity transfer from the gas phase plasma to the liquid. This is, for example, critically important for efficiently decomposing organic pollutants in water. In this contribution, the role of ⋅ OH as well as non- ⋅ OH-driven chemistry initiated by the activation of small water microdroplets in a controlled environment by diffuse RF glow discharge in He with different gas admixtures (Ar, O 2 and humidified He) at atmospheric pressure is quantified. The effect of short-lived radicals such as O ⋅ and H ⋅ atoms, singlet delta oxygen (O 2 ( a 1 Δ g )), O 3 and metastable atoms of He and Ar, besides ⋅ OH radicals, on the decomposition of formate dissolved in droplets was analyzed using detailed plasma diagnostics, droplet characterization and ex situ chemical analysis of the treated droplets. The formate decomposition increased with increasing droplet residence time in the plasma, with ∼70% decomposition occurring within ∼15 ms of the plasma treatment time. The formate oxidation in the droplets is shown to be limited by the gas phase ⋅ OH flux at lower H 2 O concentrations with a significant enhancement in the formate decomposition at the lowest water concentration, attributed to e − /ion-induced reactions. However, the oxidation is diffusion limited in the liquid phase at higher gaseous ⋅ OH concentrations. The formate decomposition in He/O 2 plasma was similar, although with an order of magnitude higher O ⋅ radical density than the ⋅ OH density in the corresponding He/H 2 O plasma. Using a one-dimensional reaction–diffusion model, we showed that O 2 ( a 1 Δ g ) and O 3 did not play a significant role and the decomposition was due to O ⋅ , and possibly ⋅ OH generated in the vapor containing droplet-plasma boundary layer.
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Nitric oxide decomposition in a helium radio frequency atmospheric pressure plasma: kinetic and transport processes
Abstract Plasma can remediate nitrogen oxides (NOx) emission from combustion processes. Nitric oxide (NO) oxidation reactions have been studied extensively. However, NO decomposition by plasma in a non-oxidizing environment, the timescales for NO dissociation reactions and their coupling with transport processes, the focus of this work, remains relatively unexplored. We report on axially resolved laser-induced fluorescence (LIF) measurements of NO densities in a plasma in helium (He) with small admixtures of NO generated in a capillary tube by two outer ring electrodes, where one ring is powered by radio frequency (rf) high voltage. A limited number of chemical reactions describe the He/NO model system, and this description of the plasma chemistry allows for the quantification of the kinetic and transport mechanisms associated with the plasma-mediated NO decomposition. A 1D plug flow model shows the dominant role of electrons in addition to helium metastable species, and atomic nitrogen for NO decomposition, while the effect of metastables is largely counteracted by the NO+ recombination at the capillary wall, yielding a significant source of NO. A 2D reaction transport model assuming a given distribution of short-lived species in the plasma zone is also able to describe the NO recovery experimentally found in the plasma effluent. The observed NO recovery as a result of inhomogeneous NO decomposition by short-lived species and the resulting radial transport underlines the limitations of the widely used plug flow approximation for such systems.
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- Award ID(s):
- 2234270
- PAR ID:
- 10648653
- Publisher / Repository:
- IOP Publishing
- Date Published:
- Journal Name:
- Journal of Physics D: Applied Physics
- ISSN:
- 0022-3727
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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