Importance of highly excited vibrational states of molecular nitrogen in ammonia synthesis using low-pressure nitrogen–hydrogen plasma

In a previous work using an atmospheric-pressure plasma, we observed the contribution of vibrationally excited molecular nitrogen in the synthesis of ammonia, while in another previous work using a low-pressure plasma, the synthesis rate was dominated by atomic nitrogen. In this work, we examined the synthesis of ammonia in a low-pressure nitrogen–hydrogen mixture plasma where the vibrational temperature of molecular nitrogen was as high as 4500 K. No specific catalysts were employed, and ammonia was produced on the surface of a stainless-steel vacuum chamber. As a result, we observed an insufficient correlation between the densities of atomic nitrogen and synthesized ammonia, whereas a better correlation was observed between the ammonia density and the vibrational temperature. The experimental result suggests that vibrationally excited molecular nitrogen contributes to ammonia synthesis even in a low-pressure plasma if the vibrational temperature is higher than 3000 K.