Plasma-Assisted Synthesis of Mg3n2 in Gas-Powder Rotating Gliding Arc Discharge Plasma

Abstract

Magnesium nitride (Mg3N2) synthesis by nonthermal (NTP) plasma is a promising approach that overcomes the limitations of thermodynamics by conventional thermal synthesis methods. In this study, a gas-powder rotating gliding arc (RGA) plasma excited by a high-frequency AC power supply is proposed for Mg3N2 synthesis in N2 at ambient temperature. The arc evolution behaviors of RGA are explored macroscopically and microscopically, and the reactive species generated are diagnosed to analyze the Mg3N2 synthesis mechanisms in N2/MgO RGA system. The experimental result shows that the arc formation and development processes were hindered when introducing MgO powder into the RGA system, and the single arc column becomes narrower in width and weaker in brightness, which results in enhancing the transition time from B-G to A-G mode and decreasing the discharge power. It is worth noting that the arc color changes from purple to green after MgO powder is introduced, and simultaneously two new characteristic peaks corresponding to Mg atom emissions appear, indicating that the dissociation of Mg atoms from MgO may occur via interactions with energetic electrons or reactive nitrogen species. The Mg3N2 synthesis yield is closely related to the gliding arc mode, and the gliding arc mode depends both on the applied voltage and gas flow rate. The optical emission spectra analysis and Mg3N2 synthesis performance demonstrate that the non-equilibrium RGA plasma in A-G mode is beneficial to enhance the excitation and dissociation of MgO, promoting nitridation reaction and improving Mg3N2 synthesis. The optimal nitridation performance can be achieved when the arc glides over the maximum range with a consistently slow and steady motion in the A-G mode, reaching a maximum Mg3N2 yield of 0.51% with a maximum Mg3N2 generation efficiency of 0.15 g/kWh. This work is expected to offer an environmentally-friendly and effective approach to indirect ammonia synthesis.

Keywords: Non-thermal plasma, Rotating gliding arc (RGA), Nitridation, Nitrogen carrier, Ammonia synthesis