(Invited) A Continuous, Atmospheric-Pressure Plasma-Water Droplet Reactor for Nitrogen Fixation

Abstract

Ammonia production via the Haber-Bosch process has a large carbon footprint due to the hydrogen gas feedstock. One possible 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. Toward this end, we investigated a catalyst-free, atmospheric-pressure plasma process to continuously produce ammonia from nitrogen gas and atomized water droplets. Through several control experiments, the production of ammonia and other forms of fixed nitrogen were confirmed. By comparing water droplets versus water vapor as a feed to the reactor, it was discovered that the ammonia forms through gas-phase chemistry and the liquid phase does not appear to significantly contribute. By comparing hydrogen gas to oxygen gas and water as a feedstock, insights into the reaction mechanism were made. When hydrogen gas was used, no nitrates or nitrites were synthesized as expected, and the ammonia production was comparable to water, indicating similar mechanisms involving feedstock dissociation to atomic hydrogen may be occurring. When oxygen gas was used, no ammonia or nitrates/nitrites were detected, which suggests that water provides a unique reaction pathway for nitric and nitrous acid to form in the gas phase. Our study demonstrates that simple 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.