Ammonia Synthesis by Nanosecond Pulsed Atmospheric Pressure Plasma Jets Impinging on Water

Developing energy-efficient technologies for carbon-neutral ammonia (NH 3 ) synthesis is critical for decentralized fertilizer production and global decarbonization. This study investigates generating NH 3 from water using a nanosecond pulsed atmospheric pressure plasma jet (ns‑APPJ) operating in either N₂ or dry air. The plasma jet reactor employed approximately 250 ns, up-to-22 kV pulses at 500 Hz to sustain a nonequilibrium discharge impinging directly on static liquid water. The kinetics, energy efficiency, and product selectivity of NH 3 formation were quantified as functions of the pulse voltage, repetition frequency (PRF), and gas flow rate. NH 3 production increased linearly with treatment time and scaled strongly with pulse voltage, reaching a maximum of 0.0345 mg h⁻¹ at 22 kV. Even at higher voltages, the production of NH 3 was more favorable than oxidative nitrogen species (i.e., NO 2 - and NO 3 - ), with the NH 3 selectivity increasing from 25% at 17 kV to 85% at 22 kV. Reducing the PRF improved energy efficiency but decreased production rates, while lower gas flow rates enhanced NH 3 formation, suggesting increased residence time and improved plasma–liquid mass transport. Optical emission spectroscopy revealed dominant N 2 (C–B) emissions, confirming that electron impact excitation is important for NH 3 formation. Despite modest production rates relative to other plasma–liquid systems, the reactor operated on a low average power (< 3 W) and maintained liquid temperatures below 41 °C, demonstrating a strongly nonthermal process. This work established a baseline for ns‑APPJ‑driven NH 3 synthesis and identified the key operating parameters for optimizing the reaction efficiency. The findings highlight the potential of nanosecond pulsed plasma strategies for localized, low‑carbon NH 3 production and encourage designing reactors that incorporate flowing liquid architectures to enhance the yield.