Controlling Energy Transfer in Plasma-Driven Ammonia Synthesis by Adding Helium Gas

Abstract

Plasma-driven ammonia (NH3) synthesis is regarded as a green complementary to the conventional Haber–Bosch process, especially for decentralized and on-demand production. However, a major challenge remains in reducing its energy cost because a huge amount of energy is consumed but not specifically for exciting, ionizing, and dissociating the feedstock. Therefore, controlling the energy transfer to predominantly generate more reactive species instead of gas heating is crucial to enhancing energy yields in the plasma synthesis of ammonia. Helium (He) is commonly used in atmospheric-pressure gas discharges to assist the generation of stable nonthermal plasmas with reduced breakdown voltages and gas temperatures while enhancing thermal conductivity and diffusion rate. In this work, a simple process based on adding He, which can be recycled and returned to the reactor along with unreacted N2 and H2, has been developed for enhancing NH3 synthesis. Optical emission spectra (OES) show an increased electron density with a significantly enhanced N2+ emission centered at 391.5 nm. With the addition of He, the energy transfer in the plasma is modified, with electron impact leading to the formation of metastable He and He2+ species, which collide with N2 molecules to generate N2+ through Penning ionization and charge transfer, contributing to more efficient N2 activation and NH3 production.