Direct Conversion of N2 and Air to Nitric Acid in Gas–Water Microbubbles

We report a simple, direct, and green conversion of air/N2 to nitric acid by bubbling the gas through an aqueous solution containing 50 μM Fe2+ ions. Air stone, along with ultrasonication, was employed to generate gas microbubbles. H2O2 produced at the water–gas interface undergoes Fenton’s reaction with Fe2+ ions to produce OH• that efficiently activates N2, yielding nitric acid as the final product. Nitrate (NO3–) formation occurs without the use of any external electric potential or radiation. The concentration of NO3– increased linearly with time over a period of 132 h. The average NO3– production rate is found to be 12.9 ± 0.05 μM h–1. We envision that this nitrogen fixation strategy that produces nitric acid in an eco-friendly way might open the possibility for the energy-efficient and green production of nitric acid.