Overcoming thermal degradation in continuous conversion of water into hydrogen peroxide in a flexible microwave plasma reactor

Hydrogen peroxide is a key oxidant for environmentally benign oxidations and
water treatment, but today it is produced almost exclusively by the anthraquinone process in
large, centralized plants that are dependent on the constant supply of fossil resources. This
makes on-site, flexible use difficult. Plasma-based routes can in principle generate H₂O₂
directly from water and electricity, yet current concepts struggle to combine continuous
operation, commercially relevant concentrations, and acceptable energy yields.
We developed a nanosecond-pulsed coaxial microwave plasma reactor that converts water
directly into H₂O₂ using argon as plasma gas. A 2.45 GHz microwave torch is driven with
pulses of 300–700 ns, while water is fed as a fine spray (0.2–2.5 mL·min⁻¹) into the afterglow.
A compact cooling coil provides strong quenching of the post-discharge. H₂O₂ is quantified by
titration/UV–vis. Gas-phase H₂ and O₂ are monitored online with a high-sensitivity gas
analyzer, and time-resolved optical emission spectroscopy provides qualitative trends in
excitation and electron density. A simple plug-flow model is used to separate effective H₂O₂
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formation and decomposition terms as a function of residence time and peak power.
Results and discussion. Under optimized pulsed conditions and with quenching, the reactor
produces H₂O₂ continuously at concentrations up to 50–54 mM (0.17 wt%), i.e. in the range
of commercial formulations, with energy yields up to 1.2 g·kWh⁻¹ based on absorbed power.
A systematic parametric study shows that duty cycle and pulse time control the balance
between formation and thermal degradation, whereas the water flow (residence time) governs
the achievable concentration. The PFR analysis indicates that the apparent formation rate is
nearly independent of power, while the decomposition rate increases sharply at high peak
power, consistent with thermally activated pathways. Long-term experiments demonstrate
stable operation over several hours. We will discuss the resulting design rules for nanosecond-
pulsed plasma–liquid reactors, implications for scale-up, and opportunities to integrate such
electrified reactors as modular, on-site H₂O₂ sources in chemical and environmental
processes.