Pulsated microwave plasma water activation towards hydrogen peroxide production

Background
Hydrogen Peroxide (H2O2) is a highly demanded chemical for oxidation, bleaching, sterilizing/disinfecting, and etching processes in the industry. The current challenge for H2O2 synthesis is to explore and develop more sustainable ways of synthesis that replace the Anthraquinone Auto-oxidation process, considered as not environmentally friendly.[1] Plasmas represent a promising alternative platform for the decentralized production of chemicals. Plasma-water interactions have shown influence during plasma-based synthesis of H2O2, [2] but the factors involved in the reaction mechanisms and synthesis are still a matter of investigation. Liu and co-workers proposed that plasma-water interface phenomena controlled the H2O2 synthesis at atmospheric pressure.[3] Furthermore, the mixing of the water in spray plus gas phases have been related to the enhancement of the H2O2 yield in the aqueous phase.[4]
Introduction
Among plasma configurations, microwave plasmas are the less studied for H2O2 production, due to the higher operation temperatures that lead to H2O2 decomposition. Nevertheless, they are interesting for handling higher gas flow rates and do not require complex electrodes. ... mehrKIT has developed the first nanosecond pulsed microwave plasmas, which allows the precise modulation of the energy supplied and control of temperatures.[5] In this work, we directly use water in combination with argon to produce H2O2 in a reactor by means of a nanosecond pulsated microwave plasma. Some configurations were studied as follows: the water is added directly to the plasma zone dropwise (1) or by spray adding argon to the water flow (2). Additionally, the influence of quenching the plasma-water system was investigated (3). The concentration of H2O2 was measured by UV-Vis spectroscopy and the the analysis of the chemical species was estimated by Optical Emission Sspectroscopy (OES).
Results and discussion
A previous scanning of the microwave parameters revealed that the pulse time had a slightly higher influence over the formation of H2O2 than the average input power, using argon and water. The maximum concentration of H2O2 obtained so far is 6mM (1). Furthermore, the comparison with the water spray (2) showed a drastic change of the plasma-water behaviour leading to ~3 times lower H2O2 concentrations possibly related to H2O2 decomposition. On the other hand, the plasma quenching variation (3) showed the most significant effect towards higher H2O2 concentrations; the concentrations were more than double than the ones obtained in the initial configuration, reaching up to 14 mM. A qualitative analysis by OES of the plasma-water environment gave us hints about the possible reaction mechanisms.
Conclusions
• Nanopulsated microwaves show potential for rapid H2O2 synthesis directly from Ar and H2O.
• The use of spray water promoted reaction pahtways leading to poor H2O2 concentrations.
• Direct quenching of the plasma-water zone led to higher H2O2 production. Deeper analysis towards understanding plasma-water interactions and improvement of the plasma quenching process are ongoing with the support of OES.