Mechanistic insights into the non-equilibrium thermodynamics of nitrogen fixation via acoustic cavitation

Abstract Non-equilibrium reaction environments offer a route to bypass the thermodynamic constraints that limit conventional nitrogen fixation, yet such conditions remain inaccessible in traditional thermal systems. Here, we show that rapid activation-quenching chemistry inside cavitation bubbles provides a viable non‑equilibrium pathway for nitrogen fixation. The violent collapse of ultrasound-driven bubbles generates an intense temperature pulse that enables direct nitrogen activation and subsequent redox chemistry within a transient gas phase microreactor. Nitrogen-containing products are produced with tuneable rates and selectivity controlled by feed gas composition, cavitation dynamics, and solution properties. Introduced cavitation nuclei lower the cavitation threshold and improve collapse reproducibility, while noble‑gas doping modulates collapse temperatures and shifts nitrate-nitrite distributions through enhancing the involvement of water‑derived species. Isotopic labelling and single‑bubble modelling indicate that nitrogen reaction proceeds predominantly through gas‑phase pathways during collapse, which can be described by a dynamic thermodynamic model within a temperature pulse. These findings establish cavitation‑driven non-equilibrium thermal cycling as a distinct mechanism for nitrogen fixation and underscore the broader potential of transient thermal microenvironments for chemical synthesis.