Catalyst-Free Nitrogen Fixation by Microdroplets through a Radical-Mediated Disproportionation Mechanism under Ambient Conditions

Abstract

Nitrogen fixation is essential for the sustainable development of both human society and the environment. Due to the chemical inertness of the N≡N bond, the traditional Haber–Bosch process operates under extreme conditions, making nitrogen fixation under ambient conditions highly desirable but challenging. In this study, we present an ultrasonic atomizing microdroplet method that achieves nitrogen fixation using water and air under ambient conditions in a rationally designed sealed device, without the need for any catalyst. The total nitrogen fixation rate achieved is 6.99 μmol/h, yielding ammonium as the reduction product and nitrite and nitrate as the oxidation products, with hydrogen peroxide produced as a byproduct at a rate of 4.29 μmol/h. Using electron paramagnetic resonance (EPR) spectroscopy, we captured reactive species, including hydrogen, hydroxyl, singlet oxygen, superoxide anion, and NO radicals. In conjunction with in situ mass spectrometry (MS) and isotope labeling, we confirmed the presence of nitrogen-containing intermediates, such as HN═NOH^+•, H₂N–N(OH)₂^+•, HNO⁺, and NH₂OH^+•. Supported by these findings and theoretical calculations, we propose a radical-mediated nitrogen disproportionation mechanism. Simulations of naturally occurring condensed microdroplets also demonstrated nitrogen redox fixation. This microdroplet-based method not only offers a potential pathway for nitrogen fixation in practical applications and sustainable development but also deepens our understanding of the natural nitrogen cycle.