Understanding non-equilibrium N₂O/NO_x chemistry in plasma-assisted low-temperature NH₃ oxidation

Ammonia represents one of most promising zero-carbon energy solutions to address the increasingly urgent climate change problem. However, the existing utilization of ammonia faces significant challenges from low chemical reactivity and high NO_x/N₂O emissions, which cause severe inefficiency issues such as cold start and cycle variability of power generation, and detrimental effects on air quality, respectively. A sustainable and energy-efficient approach to tackle the above challenges is low temperature plasma which enables non-equilibrium energy and chemical utilization of ammonia such as oxidation using renewable electricity. As such, this work, for the first time, explores plasma assisted ammonia oxidation at room temperature with a focus on unveiling non-equilibrium NO_x/N₂O reaction pathways by combining in-situ laser diagnostics with plasma modeling. We found that the non-equilibrium plasma controls the NO_x formation by supplying O/H/N atoms via electron-impact dissociation and collisional quenching of excited species. The N₂O formation follows a two-step mechanism, where electron-impact reactions first provide amine radicals which further react with NO_x to generate N₂O. These pathways facilitate a high-efficiency and environment-friendly operation of plasma assisted ammonia oxidation with enhanced reactivity and reduced NO_x/N₂O emissions through manipulating mixture compositions and plasma discharge parameters. © 2023 The Combustion Institute.