Unraveling Nonequilibrium Generation of Atomic Nitrogen and Hydrogen in Plasma-Aided Ammonia Synthesis

Abstract

Nonequilibrium generation of atomic nitrogen and hydrogen governing ammonia production both in the gas phase and on the catalyst surface is critical to plasma-aided ammonia synthesis. Here, this work studies the nonequilibrium generation of atomic nitrogen and hydrogen with a focus on the kinetic role of vibrational energy transfer of hydrogen molecules in plasma-aided ammonia synthesis. By combining two-photon absorption laser-induced fluorescence measurements and plasma kinetic modeling, we found that plasma not only generates ammonia but also produces critical H, N, and NH radicals via both electron impact and vibrational energy transfer excitations. The vibrational energy transfer from the excited hydrogen H 2 (v = 1) to higher vibrational levels H 2 (v = 2−3) via the V−V exchange (H 2 (v)− H 2 (v)) and V−V′ exchange (N 2 (v)−H 2 (v)) significantly enhances the H and NH production and then promotes the coupling between N and NH for ammonia synthesis both in the gas phase and on the catalyst surface.