Plasma assisted synthesis of H₂O₂: Effect of gas-liquid mixture

• Nanosecond modulated microwaves pulsations can lead to H2O2 concentrations up to 6 mM or higher using only atmospheric Ar plasma and water.
• The mixture of the gas-liquid phase has an important effect over the reaction pathway.
Introduction
Hydrogen Peroxide (H2O2) is the most used chemical at industrial scale for oxidizing, bleaching, sterilizing/disinfecting, and etching processes. The current challenge about H2O2 is to explore and develop more sustainable ways of synthesis rather than Anthraquinone Auto-oxidation. The aim is the low-cost production at different scales compatible with low-capacities, which is in line with the global goal of green chemistry implementation.
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Plasmas represent a promising alternative platform for the decentralized production of H2O2. Plasma-water interactions have shown influence during plasma-based synthesis of H2O2, 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. The phenomena identified so far include sputtering, OH ion emission due to the induced electric field, and evaporation.1 Furthermore, the mixing of the water-gas phases using a spray water, have been related the enhancement of the H2O2 yield in the aqueous phase.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 to produce H2O2 by means of a microwave plasma reactor. The reactor consists of a coaxial torch based on microwaves modulated with nanosecond pulsations. The system is represented in Figure 1.

A spray mode was also implemented adding argon to the water flow. The concentration of H2O2 was measured by UV-Vis spectroscopy and the the analysis of the chemical species was estimated by Optical Emission Sspectrscopy (OES).

Results and discussion
A previous scanning of the microwave parameters (power and time of pulsations) revealed that the time of pulsations had a slightly higher influence over the formation of H2O2 than the input power, using argon and water. The higher concentrations of H2O2 measured so far are around 6mM. Furthermore, the comparison with the argon+water spray configuration (Figure 2) allowed us to observe a drastic change of the plasma-water behaviour leading to ~3 times lower H2O2 concentrations. This was related to another reaction pathway involving different electron interactions. A qualitative analysis by OES of the plasma-water environment showed the presence of hydrogen phases alpha and beta, atomic oxygen and OH spectra regions, which gave us hints of possible reaction mechanisms. The decrease in water drop size due to the spray modification and increment of contact area between plasma-water is being studied and its influence on the H2O2 yield.
Conclusions
Nanopulsated microwaves have shown high potential for H2O2 synthesis directly from Ar and H2O. The use of a spray mode with argon led to lower H2O2 concentrations related to a different reaction pathway, as interpreted by OES. Deeper analysis towards understanding plasma-water interactions are ongoing.