(Invited) Plasma-Activated Synthesis of Ammonia Using Nitrogen Gas and Water As Feedstocks

Abstract

The reduction of nitrogen to ammonia by the Haber-Bosch (H-B) process entails large energy and environmental costs that motivate research on alternative strategies. A major issue with the H-B process is the need for hydrogen gas that currently comes from fossil fuels (steam methane reforming). Plasma activation is a potential alternative as it was historically the first industrial method employed to fix nitrogen, but by oxidation. More recently, plasmas have been combined with solid catalyst materials to reduce nitrogen to ammonia at lower pressures and temperatures than the H-B process, but also use hydrogen gas as a feedstock.Here, we present our efforts to develop plasma-activated processes for the reductive synthesis of ammonia using nitrogen and water as feedstocks. One configuration is a hybrid electrochemical system in which a microplasma jet impinging on an aqueous electrolyte surface replaces a metal electrode and reactions are carried out at the interface of the plasma and liquid bath. A second configuration is a hybrid aerosol system in which water droplets are introduced into a larger-volume plasma and reactions are carried out at the interface of the plasma and droplet surface. In both cases, the ammonia is collected in the liquid phase and measured by chemical assay methods. We will discuss process optimization, product selectivity, and energy consumption for both systems. Insights into the reaction mechanisms will also be addressed and in particular, the possible role of water vapor.