Nitrogen Fixation with Water Vapor by Nonequilibrium Plasma: toward Sustainable Ammonia Production

Ammonia is a crucial nutrient used for plant growth and as a building block in the pharmaceutical and chemical industry, produced via nitrogen fixation of the ubiquitous atmospheric N₂. Current industrial ammonia production relies heavily on fossil resources, but a lot of work is put into developing nonfossil-based pathways. Among these is the use of nonequilibrium plasma. In this work, we investigated water vapor as a H source for nitrogen fixation into NH₃ by nonequilibrium plasma. The highest selectivity toward NH₃ was observed with low amounts of added H₂O vapor, but the highest production rate was reached at high H₂O vapor contents. We also studied the role of H₂O vapor and of the plasma-exposed liquid H₂O in nitrogen fixation by using isotopically labeled water to distinguish between these two sources of H₂O. We show that added H₂O vapor, and not liquid H₂O, is the main source of H for NH₃ generation. The studied catalyst- and H₂-free method offers excellent selectivity toward NH₃ (up to 96%), with energy consumption (ca. 95–118 MJ/mol) in the range of many plasma-catalytic H₂-utilizing processes.