Plasma-droplet interaction study to assess transport limitations and the role of OH, O,H,O2(a 1Δg),O3, He(23 S) and Ar(1s 5) 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, 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)Delta(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 similar to 70% decomposition occurring within similar to 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/O-2 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 O-2(a (1)Delta(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.

Automated summary

The study quantified the role of different gas mixtures in initiating reactions in water microdroplets through plasma interaction, aiming to effectively decompose organic pollutants in water. By analyzing the impact of various short-lived radicals on formate decomposition in droplets, the study revealed the importance of gas phase .OH flux and electron/ion-induced reactions in the process.