Analysis of reactive species during plasma-water interaction for H2O2 formation using nanosecond pulsated microwaves

Hydrogen peroxide is the preferred green oxidant reagent since it decomposes into water and oxygen, and therefore it is widely used in green chemistry applications. It has been demonstrated that plasma-based synthesis of H2O2 is influenced by plasma-water interactions, but the factors involved in the reaction mechanisms and synthesis are still a matter of investigation. Previous studies proposed that plasma-water interface phenomena as sputtering, OH ion emission, and evaporation controlled the H2O2 synthesis at atmospheric pressure.1 Furthermore, the direct cooling of the plasma-liquid zone, and the use of spray water, has been related to the enhancement of the H2O2 yield in the aqueous phase because of quenching of radicals and side undesired reactions.2,3

Among plasma configurations, microwave plasmas have been less studied for H2O2 production given the higher temperatures of operation leading to H2O2 decomposition. Nevertheless, they are interesting for handling higher gas flow rates and do not require complex electrodes. In this work, we directly use water in combination with argon/oxygen mixtures to produce H2O2 by means of a microwave plasma reactor. ... mehrThe reactor consists of a coaxial torch based on microwaves modulated with nanosecond pulsations, which would allow a better control of the temperature4. We compared the behaviour of the system using a water spray adaption and argon/oxygen mixtures as carrier gases obtaining H2O2 concentrations up to 6 mM. Through OES we could identify atomic oxygen lines, OH region and the typical Balmer lines for H. The differences of reactive species observed by OES gave a hint of reaction pathways for the H2O2 formation. Decrease in water drop size and increment of contact area between plasma-water is studied and its influence on the H2O2 yield.