Enhanced nitrogen fixation using DBD plasma with continuous flowing water and TiO2 photocatalytic coupled system

Abstract

In this study, a novel dielectric barrier discharge (DBD) with continuous flowing water is designed while other devices commonly utilize immobile water, which can be easily coupled with UV light and TiO2 to achieve highly efficient nitrogen fixation (NF) from air plasma over water surface. Electrical characterization, optical emission spectroscopy and X-ray photoelectron spectroscopy (XPS) are performed to study the plasma properties and the underlying mechanisms of NF under different conditions. Our results indicate that the maximum nitrate concentration and minimum energy consumption can reach 154 mg/L and 61 MJ/mol, respectively, in the pure plasma case. The total amount of dissolved nitrogen (TN) has a maximum concentration of 199 mg/L, indicating the main nitrogen fixation product is nitrate, and the nitrite concentration is very low. By coupling TiO2 spheres in the water and UV light, the concentration of nitrate and TN increases by 57% and 45%, respectively, compared with the pure plasma condition, at an input power of 200 W and a processing time of 20 min. XPS analysis directly demonstrates the change of the Ti valence state on the TiO2 surface, indicating the formation of more electron-hole pairs. Thus, more highly oxidizing species are formed on the photocatalyst surface to enhance the production of NOx. A mechanism is proposed about the underlying chemistry in gas and liquid phase, as well as for the enhanced NF efficiency upon addition of TiO2 spheres and UV light. Due to the design of the circulating water as ground electrode, which leads to thermal quenching, the novel DBD NF device can realize continuous operation time at a relatively high power, which is expected to have good application prospect.