Effect of noble gas addition on plasma-assisted ammonia synthesis*

Ammonia synthesis assisted by non-thermal plasmas has gained increasing interest because non-thermal plasmas allow ammonia synthesis to occur at atmospheric pressure and temperatures lower than what is required by the Haber-Bosch process [1]. We report here the effect of noble gas addition on plasma-assisted ammonia synthesis. We performed kinetics experiments and optical emission spectroscopy (OES) measurements using a coaxial dielectric barrier discharge (DBD) reactor at several applied voltages with different percentages of Ar and He added to the N₂-H₂ reaction mixture. Our kinetics experiments showed that the NH₃ mole fraction produced reached a maximum for 10% Ar or He, and that He addition gave a higher NH₃ mole fraction than Ar. OES data showed the existence of reactive gas phase species (N₂⁺, N, H_α, H_β, and N₂^*) in both cases, with a larger amount of H observed for He addition. Atomic hydrogen lines H_α and H_β were also utilized to estimate the plasma parameters. [1] J. G. Chen et al., Science 360, eaar6611 (2018).