This talk will discuss our most recent findings on the plasma-activated synthesis of ammonia from a nitrogen-hydrogen mixture. The ammonia yield has been measured for the case of a low-pressure RF-driven plasma interacting with various metallic surfaces. Consistent with other reports, we observe a weak dependence of the reactor-level ammonia yield on the catalyst material. We have performed ab initio molecular dynamics simulation to provide a microscopic description of the plasma-surface interaction. In particular, we have focused on atomic nitrogen impinging onto copper and platinum surface. We have found that ammonia forms via an Eley–Rideal mechanism in which atomic nitrogen interacts with the hydrogen-terminated surface, followed by ammonia desorption on a picosecond time scale. While on copper the atomic hydrogen interacts directly with the H-terminated surface, on platinum the saturation of the surface with NH groups is necessary for the reaction to proceed. In either case, the short reaction time and the weak dependence on catalyst material are consistent with a mass transport limited regime.<br/>Moreover, we have explored the use of power pulsing as a route to lowering the ammonia production energy cost. Given the large electron-induced ammonia dissociation cross section (significantly larger than that of nitrogen and hydrogen), it is crucial to remove the ammonia from the reaction volume without exposing it to the plasma. We have found that a significant fraction of the electrical input power is actually utilized to dissociate any ammonia produced at the catalyst surface as it diffuses back into the plasma volume. The energy cost can be significantly lowered by pulsing the RF power so that a sufficient flux of activated nitrogen species is provided to the catalyst surface when the plasma is on, all while preventing ammonia dissociation once it diffuses away from the catalytic surface.

Progress towards a microscopic-level understanding of the plasma synthesis of ammonia

ACS Spring Meeting 2021

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