Electron Properties and Reaction Mechanisms in Plasma-Assisted Catalysis of Ammonia Synthesis

Ammonia synthesis by the Haber-Bosch process contributes 1-3% of the world’s total energy demand per year. Plasma-assisted catalysis is being investigated as an alternative method for ammonia production, with attractive features of distributed production, ease of on-off operation suitable for intermittent electrical energy supply, and possibly even increasing the energy yield of ammonia. Most studies of plasma-assisted catalysis have either characterized the ammonia production rate, showing the benefits of coupling a catalyst with a plasma environment, or characterized the bulk plasma and particle dynamics. These separate approaches have limited our understanding of important plasma-catalyst interactions such as sheath effects, reaction pathways, and catalyst active sites. Here, we report results from our investigations into the particle dynamics and plasma properties observed in a dielectric barrier discharge (DBD) reactor for ammonia synthesis via plasma-assisted catalysis. The DBD was produced by a nanosecond pulser (NSP), which enables selective heating of electrons and a plasma with tunable properties. To understand the particle dynamics in the plasma adjacent to the catalyst, we used the non-perturbative methods of Thompson scattering for electron density and energy and Raman scattering for molecular vibrational energies, taken concurrently at a given location. This enables us to make correlations between electron dynamics, N­₂ energy states, and ammonia production rates. Insights from these results for plasma-assisted catalysis of ammonia synthesis will be discussed.