Controlling Energy Transfer in Plasma-Driven Ammonia Synthesis by Adding Helium Gas
Rusen Zhou, Dejiang Zhou, Baowang Liu, Lanlan Nie, Yubin Xian, Tianqi Zhang, Renwu Zhou, Xinpei Lu, Kostya Ostrikov, Patrick J. Cullen
Abstract
Plasma-driven ammonia (NH 3 ) 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 N 2 and H 2, has been developed for enhancing NH 3 synthesis. Optical emission spectra (OES) show an increased electron density with a significantly enhanced N 2 + 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 He 2 + species, which collide with N 2 molecules to generate N 2 + through Penning ionization and charge transfer, contributing to more efficient N 2 activation and NH 3 production.