Plasma-droplet interaction study to assess transport limitations and the role of ^⋅OH, O^⋅,H^⋅,O₂(a ¹Δ_g),O₃, He(2³ S) and Ar(1s ₅) in formate decomposition

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, O2 and humidified He) at atmospheric pressure is quantified. The effect of short-lived radicals such as O⋅ and H⋅ atoms, singlet delta oxygen (O2(a 1Δg)), O3 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 H2O 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/O2 plasma was similar, although with an order of magnitude higher O⋅ radical density than the ⋅OH density in the corresponding He/H2O plasma. Using a one-dimensional reaction–diffusion model, we showed that O2(a 1Δg) and O3 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.

Non‐OH‐driven liquid‐phase chemistry in water microdroplets

“…Figure 1a shows the schematic of the plasma-droplet reactor used in this work. A detailed description of this reactor is provided by Oinuma and colleagues [12,19] . In short, an RF-driven glow discharge plasma at atmospheric pressure was generated between the two parallel copper electrodes with an interelectrode gap of 2 mm.…”

“…The absolute calibration of the system is described in detail by Burm [ 45 ] . The spatially averaged electron density ( n e ) and temperature ( T e ), in the absence of droplets, were determined by fitting the absolute intensity of the recorded continuum emission from the plasma $${(\epsilon }_{\text{global}}(\lambda ))$$ in the visible range (350–900 nm) with the theoretical neutral bremsstrahlung radiation as described in previous works [ 19,45‐47 ] . The weighted average of the elastic electron–atom momentum transfer cross‐sections needed for the calculation of the bremsstrahlung cross‐sections in different gas mixtures were obtained from the Phelps database for He and Ar [ 48,49 ] and the Itikawa and Hayashi databases for H 2 and H 2 O [ 50‐53 ] .…”

“…global in the visible range (350-900 nm) with the theoretical neutral bremsstrahlung radiation as described in previous works [19,[45][46][47] . The weighted average of the elastic electron-atom momentum transfer cross-sections needed for the calculation of the bremsstrahlung cross-sections in different gas mixtures were obtained from the Phelps database for He and Ar [48,49] and the Itikawa and Hayashi databases for H 2 and H 2 O [50][51][52][53] .…”

“…The gas temperature was determined by excitation LIF on four different rotational transitions [55,57] . [19] is shown for comparison. The volume-averaged concentrations of formate were calculated using a one-dimensional (1D) reaction-diffusion model (red line) and a simple 1D diffusion model (blue lines) using the diffusion coefficients for formate in water (along with 5 and 10 times larger diffusion coefficients).…”

“…In our previous work, we investigated the decomposition of formate, a model organic compound, in water microdroplets in helium atmospheric pressure radiofrequency (RF) glow plasmas with admixtures of water, Ar, and O 2 [12,19] . In gas mixtures containing O 2 or H 2 O, up to ~70% decomposition of the initial formate concentration was observed for a droplet residence time in the plasma of 12-15 ms. A detailed quantification of the effect of radicals such as hydroxyl (OH), O, and H, along with excited species of He, Ar and O 2 , and O 3 using a one-dimensional (1D) reaction-diffusion model on formate decomposition was made.…”