Performance of high-frequency spark discharge for efficient NO production with tunable selectivity

Plasma-based NOx synthesis from ambient air is a promising sustainable technology for producing value-added chemicals, such as nitrogen fertilizer and medication. The relatively low efficiency and product selectivity are major concerns that hinder the commercialization of this technology. Despite advancements in reducing energy costs, limited attention has been paid to improving and controlling selectivity, which significantly contributes to the overall cost. Thus, the present study aimed to gain insights into the efficient pathways and key factors for highly selective NOx synthesis in a recently developed high-frequency spark discharge (HFSD) reactor. The thorough investigations conducted proved that the higher energy efficiency of HFSD is due to higher vibrational and rotational excitations and their efficient utilization. However, the significant impact of higher temperatures on reactions led to poor selectivity of NO and NO2. Therefore, the role of influential factors, such as O2 and H2O contents, as well as residence time, on reactions, were analyzed and optimized. It is found that controlling the oxygen content remarkably improved product selectivity but negatively affected the energy cost. Moreover, water vapor triggered the so-called extended Zeldovich mechanism, which was evident in N2/H2O, affecting NOx formation and selectivity. Interestingly, tuning the gas flow rate effectively controlled selectivity without sacrificing the energy cost. A high NO selectivity of 95% was achieved at an energy cost of 2.1 MJ/mol when a relatively high flow rate of 4 L/min was used.