Selective Nitrate Synthesis by Plasma/Liquid Water Reaction: An Enhanced Oxidative Strength Mechanism in Nitrogen Discharge

Plasma/liquid (P/L) reaction is an emerging green chemical process for nitrogen fixation into ammonia or nitrogen oxides directly from air and water. On the other hand, the P/L synthesis is often limited by product nonselectivity. Mechanistic understanding is essential to achieve selective nitrate synthesis in P/L reactions, which would also be profitable in practice. In this study, selective NO₃^– synthesis from nitrogen gas and liquid water was achieved via a cold microwave plasma (CMP) at ambient temperature and pressure. The product yield and selectivity were found to be controlled by tuning the types of discharge gases as well as the treatment time. The results demonstrate that NO₃^– was exclusively produced in nitrogen discharge at an extended treatment time of 20 min. Various spectroscopy characterizations and reactive nitrogen or oxygen species measurements were conducted to understand the underlying mechanism. In the nitrogen plasma, the abundant metastable N₂(A) species were suggested to be responsible for the ample hydroxyl radicals generated from water, which in turn plays a key role for the high NO₃^– selectivity. The results in this work not only give insight into the nitrogen fixation process in plasma and liquid water systems but also have a more general relevance to the understanding of P/L based chemical transformation.