Oxidation of Ammonia in Water Microdroplets Produces Nitrate and Molecular Hydrogen

Water microdroplets containing dissolved ammonia (30-300 μM) are sprayed through a copper oxide mesh with a 200 μm average pore size, resulting in the formation of nitrate (NO₃^-) and the release of molecular hydrogen (H₂). The products result from a redox process that takes place at the liquid-solid interface through contact electrification, where no external potential is applied. Oxidation is initiated by superoxide radical anions (O₂^-) that originate from the oxygen in the air surrounding the microdroplets and from the hydroxyl radicals (OH^•) originating from the water-air interface. Two spin traps (TEMPO and DMPO) capture these radicals as well as NH₂OH^+•, HNO, NO^•, NO₂^•, and NOOH, which are detected by mass spectrometry. We also directly observed N₂O₂^-• by the same means. We found that the hydrogen atom from the ammonia molecule can be set free not only in the form of H^• but also as H₂, which is detected using a residue gas analyzer. The oxidation process can be significantly enhanced by a factor of 3 when the sprayed microdroplets are irradiated with ultraviolet light (265 nm, 5 W). 35% of 300 μM ammonia can be degraded within 20 μs, and the nitrate conversion rate is estimated to be 15 nmol·mg^-1·h^-1.