Plasma-droplet interaction study to assess transport limitations and the role of ^·OH, O^·,H^·,O₂(a¹Δ_g),O₃, He(2³S) and Ar(1s₅) in formate decomposition

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(a¹Δ_g)), 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(a¹Δ_g) 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.