NH<sub>3</sub>and HNO<sub><i>x</i></sub>Formation and Loss in Nitrogen Fixation from Air with Water Vapor by Nonequilibrium Plasma
Elise Vervloessem, Mikhail Gromov, Nathalie De Geyter, Annemie Bogaerts, Yury Gorbanev, Anton Nikiforov
Abstract
The current global energy crisis indicated that increasing our insight into nonfossil fuel nitrogen fixation pathways for synthetic fertilizer production is more crucial than ever. Nonequilibrium plasma is a good candidate because it can use N 2 or air as a N source and water directly as a H source, instead of H 2 or fossil fuel (CH 4 ). In this work, we investigate NH 3 gas phase formation pathways from humid N 2 and especially humid air up to 2.4 mol % H 2 O (100% relative humidity at 20 °C) by optical emission spectroscopy and Fourier-transform infrared spectroscopy. We demonstrate that the nitrogen fixation capacity is increased when water vapor is added, as this enables HNO 2 and NH 3 production in both N 2 and air. However, we identified a significant loss mechanism for NH 3 and HNO 2 that occurs in systems where these species are synthesized simultaneously; i.e., downstream from the plasma, HNO 2 reacts with NH 3 to form NH 4 NO 2, which rapidly decomposes into N 2 and H 2 O. We also discuss approaches to prevent this loss mechanism, as it reduces the effective nitrogen fixation when not properly addressed and therefore should be considered in future works aimed at optimizing plasma-based N 2 fixation. In-line removal of HNO 2 or direct solvation in liquid are two proposed strategies to suppress this loss mechanism. Indeed, using liquid H 2 O is beneficial for accumulation of the N 2 fixation products. Finally, in humid air, we also produce NH 4 NO 3, from the reaction of HNO 3 with NH 3, which is of direct interest for fertilizer application.