Tunable Conversion of Ammonia to Hydrazine or Ammonium Nitrite Induced by Acoustic Cavitation Bubbles

In this work, we explore the possibility to harness the energy released during the implosion of acoustic cavitation bubbles to activate and selectively convert NH₃ in water without the assistance of any catalyst. Two primary products, hydrazine and ammonium nitrite, were identified in the liquid phase. Hydrazine was formed as a result of the cleavage of the N─H bond of NH₃ at the cavitation bubble collapse time, while ammonium nitrite was formed through oxidation of NH₃ with HO^● radicals stemming from the sonolysis of water. By adjusting the gas atmosphere, the generation of HO^● radicals can be suppressed or enhanced, thereby enabling selectivity control over the sonochemical conversion pathway of NH₃ toward either hydrazine or ammonium nitrite. For instance, we show that under H₂ atmosphere, NH₃ is predominantly converted to hydrazine, while under oxygen atmosphere ammonium nitrite becomes the major product formed. Finally, we discuss the unexpected beneficial role of salts (mono-, di-, and trivalent), which influence the efficiency of this sonochemical activation of NH₃.