Pulsed microwave plasma reactor for H2O2 production from H2O and O2

Hydrogen Peroxide (H2O2) is a green chemical mostly used at industrial scale for oxidizing, bleaching, sterilizing/disinfecting, and etching processes. It´s marketing value was more than 5 million of Tons in 2021 with a considerable growing demand in the last two years due to the global impact of Covid-19, for disinfecting applications. The current industrial production of H2O2 by means of the Anthraquinone Auto-oxidation process is not consider an environmentally friendly process and requires significant amounts of energy. Searching for a more sustainable way to produce H2O2 at low cost in different scales compatible with low-capacities is in line with the global goal of green chemistry implementation.
Various approaches made so far include the direct synthesis of H2O2 from hydrogen and oxygen via electrocatalysis,1 and photochemical synthesis from water and oxygen.2 These approaches would allow for decentralized production in regions disconnected of big, industrial clusters.
Plasmas represent a promising alternative platform for the decentralized production as well with the advantages of having instant on/off times, and the potential to activate molecules such as oxygen and water without the assistance of a catalyst. ... mehrAmong different plasma configurations studied for H2O2 synthesis, dielectric barrier discharges (DBDs) and gliding arc (GA) discharges are the most common. The efficiency between the different configurations is usually compared by the energy yield and H2O2 production rate. Figure 1 presents a comparison of energy yield and production rate between different plasma configurations using H2O, O2 or H2 mixtures as reactants. 81 g/kWh of energy yield was achieved using water spray in a Gliding Arc configuration; authors also observed that the energy yields increased with the flow rate of the water sprayed into the plasma zone.3 In turn, Liu and co-workers concluded that plasma-water interface phenomena controlled the H2O2 synthesis at atmospheric pressure. These phenomena included sputtering; OH ion emission due to the induced electric field, and evaporation.4
From the previous literature comparison, a wide window of research remains open since the general aim is to improve the energy yield without sacrificing the production of H2O2 from its main component molecules. Microwave plasmas haven´t been much studied for H2O2 production, even if they are interesting since they can handle higher gas flow rates, do not require electrodes and have shown potential for efficient use of energy in comparison with the other plasmas. Nevertheless, the direct combination of H2 and O2 at high temperatures usually present in microwave plasmas (up to 3000K) are a concern for safety reasons and the thermal stability of hydrogen peroxide.
In this sense, in line with a decentralized production concept, we aim to directly use water in combination with oxygen to produce H2O2 by means of a microwave plasma reactor. A general sketch of the reaction system configuration is shown in Figure 2a. The H2O2 production and gas phase products would be analysed by UV-Vis and Gas Chromatography, respectively. The interface phenomena between water and plasma are the main subject of study (Figure 2b), along with the mixing regime, spray conditions and flow pulses which have shown to affect both, the efficiency and yield of the reaction. Thus, we aim to balancing the increased contact of plasmas and water together with the reaction control inside the aqueous phase.