The current global energy crisis indicated that increasing our insight into nonfossil fuel nitrogen fixation pathways for synthetic fertilizer production is more crucial than ever. Nonequilibrium plasma is a good candidate because it can use N2 or air as a N source and water directly as a H source, instead of H2 or fossil fuel (CH4). In this work, we investigate NH3 gas phase formation pathways from humid N2 and especially humid air up to 2.4 mol % H2O (100% relative humidity at 20 °C) by optical emission spectroscopy and Fourier-transform infrared spectroscopy. We demonstrate that the nitrogen fixation capacity is increased when water vapor is added, as this enables HNO2 and NH3 production in both N2 and air. However, we identified a significant loss mechanism for NH3 and HNO2 that occurs in systems where these species are synthesized simultaneously; i.e., downstream from the plasma, HNO2 reacts with NH3 to form NH4NO2, which rapidly decomposes into N2 and H2O. We also discuss approaches to prevent this loss mechanism, as it reduces the effective nitrogen fixation when not properly addressed and therefore should be considered in future works aimed at optimizing plasma-based N2 fixation. In-line removal of HNO2 or direct solvation in liquid are two proposed strategies to suppress this loss mechanism. Indeed, using liquid H2O is beneficial for accumulation of the N2 fixation products. Finally, in humid air, we also produce NH4NO3, from the reaction of HNO3 with NH3, which is of direct interest for fertilizer application.

In this work, the kinetics of nitrogen fixation via plasma-induced N 2 oxidation in a 10 ns pulsed atmospheric pressure water-contacting discharge sustained in air is investigated. Two pulse regimes, a single pulse and a three-pulse burst of 100 kHz, are considered. The densities of relevant radicals (NO, O) are studied by time-and space-resolved laser-induced fluorescence spectroscopy. It is concluded that in a single pulse mode, O atoms are mainly generated by O 2 reacting with electronically excited states of N 2 (A 3 Σ + u , B 3 Π g , C 3 Π u ) and are primarily reduced as a result of O 3 formation. The O density shows a maximum at ∼100 ns after the plasma pulse with number density of ∼10 23 m −3 . NO radicals, on the other hand, are primarily formed by reacting with the N 2 A 3 Σ + u state (up to ∼1 μs after the pulse) and with OH radicals (up to ∼10's of μs), peaking at approximately 60 μs with a peak density of ∼10 21 m −3 . The NO loss pathway is represented by the reversed Zeldovich mechanism at short time delays (∼10's μs), whereas at longer delays (>100's of μs) HNO 2 and NO 2 formation causing NO loss start to be dominant. In the burst mode, the energy efficiency of NO formation decreases despite higher N 2 conversion, for which three reasons are suggested:(1) NO removal by the generated O( 1 D) after the discharge pulse through the reverse Zeldovich mechanism, (2) NO oxidation via the accumulated O 3 , (3) pre-ionization induced by high pulse repetition rate (100 kHz) leading to shrinkage of the plasma bulk.

Experimental data on the evolution of a helium atmospheric pressure plasma jet driven by two different voltage waveforms are presented. The characteristics of directed ionization waves (guided streamers) were compared for a sinusoidal voltage waveform with a frequency of 52 kHz and a voltage waveform that was formed via the superposition of 41.6 kHz bipolar square pulses and 300 kHz oscillations. With the sinusoidal voltage, two consecutive ionization waves were observed. With a special tailoring voltage, control of the guided streamer propagation in a stepwise mode was achieved. The observed second streamer and the second step of propagation could be regarded as a secondary ionization wave for both voltages. A change in the voltage waveform led to significant variations in the secondary ionization wave formation and streamer parameters. The voltage waveform enabled the number of ionization waves and their propagation to change, which provided the possibility of controlling the plasma parameters of the jet.