Yusuf, L. , McHugh, P., Cioncoloni, G., Song, J., Morrison, C., Palmer, E., Franco, F., Gauchotte-Lindsay, C. , Amini Boroujeni, F., Prentice, P. and Symes, M.
(2025)
Towards decentralized nitrogen fixation using pulsed ultrasound.
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The fixation of atmospheric nitrogen is one of the key processes required for life on Earth, and the development of anthropogenic nitrogen fixation methods in the early twentieth century revolutionized agricultural productivity. Non-natural nitrogen fixation is currently dominated by the Haber-Bosch process, which generates ammonia from nitrogen and (usually fossil fuel-derived) hydrogen. However, other potential nitrogen fixation products are equally useful, particularly nitrate, to which a significant proportion of ammonia produced by the Haber-Bosch process is converted via the Ostwald process. Both the Haber Bosch and Ostwald processes require high temperatures, elevated pressures and large, centralized facilities and are major emitters of CO2. Present nitrogen fixation routes therefore leave very little scope for the production of fertilizers in small, decentralized facilities close to the point-of-use, or for the development of truly resilient and sustainable supplies of these vital chemical building blocks. Such decentralized production would require the fixation of nitrogen without the need for high temperature or high pressure reactors, using only air and water and without any additional reagents. Herein, we report on a much under-explored route to nitrogen fixation that is compatible with all the above criteria: fixation using ultrasound. Through the optimization of various acoustic parameters, we demonstrate that just 60 seconds of sonication of an air/water mixture at room temperature, using pulsed ultrasound at 200 kHz with an optimal input protocol (pulse duration = 4 ms, interval between pulses = 80 ms), is sufficient to produce nitrate solutions at concentrations of approximately 15 μM. These results constitute a record rate of production of nitrogen oxides by sonication in aqueous solution and are a step-change in terms of energy requirements per mole of products relative to previous reports. In turn, our results suggest that this simple approach has potential for low capital cost and truly decentralized nitrogen fixation in areas where infrastructure is lacking.
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