A Plasma-Water Droplet Reactor for Process-Intensified, Continuous Nitrogen Fixation at Atmospheric Pressure

Current industrial production of ammonia via the Haber–Bosch process has a massive carbon footprint because the hydrogen gas feedstock comes from the reformation of fossil fuel which releases large amounts of carbon dioxide. One possible solution is to provide hydrogen gas by water electrolysis that is powered by renewable sources. A more radical solution is to use water as the hydrogen source in a process-intensified scheme whereby ammonia is produced by directly reacting nitrogen gas and water. Here, we investigated an atmospheric-pressure plasma process to continuously produce ammonia from nitrogen gas and atomized water droplets. The successful production of ammonia was carefully confirmed by several control experiments. We find that other forms of fixed nitrogen are synthesized as well, predominantly nitrate and some nitrite. To understand the role of water droplets, we characterized their lifetimes in the reactor and compared the ammonia production with a water vapor feed. The results show that the ammonia forms through gas-phase chemistry and the liquid phase does not appear to substantially contribute. Additional insight into the reaction mechanism was provided by replacing the water with hydrogen gas or oxygen gas as the feedstock. As expected, no nitrates or nitrites were synthesized with hydrogen gas, and the ammonia production was comparable, indicating similar mechanisms may be occurring that involve feedstock dissociation to atomic hydrogen. No ammonia or nitrates/nitrites were detected with oxygen gas, which suggests that water provides a unique reaction pathway for nitric and nitrous acid to be formed in the gas phase. Our study demonstrates that nitrogen gas and water droplets can serve as feeds in a plasma process for the sustainable and distributed production of ammonia or other fixed nitrogen compounds.