Coaxial microwave plasma reactor for continuos production of H₂O₂ using water and argon

Empirical parametric study of a reactor that is mainly a plug-in device for instant H2O2 produtcion.
Nanosecond modulated microwaves pulsations can lead to H2O2 concentrations up to 15 mM or higher using only atmospheric argon plasma and water.
The liquid flow rate highly affects the plasma-water interaction and the product concentration.
Introduction
Hydrogen Peroxide (H2O2) is considered a green oxidizer agent since it decomposes only into water and oxygen, and it is highly demanded for the pulp and paper industry bleaching, water treatment, fuel cells, among others. 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.
Plasmas represent a promising alternative platform for the decentralized production of H2O2. However, the factors and mechanisms involved during the plasma-water reaction are still a matter of investigation. Liu and co-workers proposed that plasma-water interface phenomena controlled the H2O2 synthesis at atmospheric pressure. ... mehrThe phenomena identified so far include sputtering, OH ion emission due to the induced electric field, and evaporation.1 Furthermore, the water flow rate modulation and the quenching conditions, 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 (dropwise) 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.
Different scanning of microwave parameters and quenching conditions have been studied. The concentration of the liquid H2O2 product was measured by UV-Vis spectroscopy and the the analysis of the active chemical species was analyzed by Optical Emission Sspectrscopy (OES).
Results and discussion
A fisrt scanning of the microwave parameters (power and time of pulsations) revealed that the microwave pulsations in the nanosecond scale had great influence enhancing the H2O2 formation in the milimolar range in comparison with continuous microwave plasma, where only micromolar concentrations were obtained. The variation of the time of pulsations had a slightly higher influence over the formation of H2O2 than the input power variation, but the power had a greater influence over the H2O2 concentration in comparison to the ducty clycle variation. Furthermore, lowering the water flow rate had the greatest influence allowing higher concentrations of H2O2 up to 15 mM. Lastly, the comparison with different cooling temperatures revealed that lower temperatures could favour a faster stabilization of the product with a slightly improvement in the final concentration. This could be related to avoiding ion-electron interactions related to the decomposition of H2O2. A qualitative analysis by OES of the plasma-water environment showed the presence of hydrogen phases alpha and beta, atomic oxygen and hydroxyl radical spectra regions, which gave us hints of possible reaction mechanisms. The increase of the residence time of the water drop due to lower flow rate and increment of contact area between plasma-water is being studied and its influence on the H2O2 yield.
Conclusions
The potential of the nanosecond pulsated microwave plasma techonology for H2O2 synthesis directly from Ar and H2O is shown. The use of lower water flow rates led to higher H2O2 concentrations, and the quenching was related to a different reaction pathway, as interpreted by OES. Deeper analysis towards understanding plasma-water interactions are ongoing.