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.