Plasma nitrogen fixation in the presence of a liquid interface: role of OH radicals

Physical backgrounds of a nitrogen fixation process in a nanosecond pulsed discharge in the presence of a plasma/liquid interface are reported. The role of OH radicals and O atoms in NO formation is experimentally revealed for the plasma operating in a single pulse mode and high-frequency burst. The kinetic curves demonstrate NO radical formation up to 10's of mu s that well agrees with the observed OH radical behavior. The measurements in the single mode illustrate the sequence of the gas-phase chemistry that can be simplified as follows: electrons -> N-2(electronically excited) -> O(D) -> OH -> NO. The increase of the operating frequency up to 100 kHz does not affect the pathway of NO formation in high electrical field plasma (such as dielectric barrier discharges or ns-pulsed plasma studied here). Independently of the frequency, the NO generation takes place through the extended Zeldovich mechanism dominantly driven by OH radicals because of the presence of the plasma/liquid interface whereas the O atoms' contribution in direct NO formation is low.