Investigation of Ammonia Synthesis Assisted by Non-thermal Plasmas

Abstract

Ammonia (NH3) is produced from N2 and H2 in the industry via the Haber–Bosch (H–B) process, which uses a thermal catalytic reactor operating at a high temperature (700 K) and high pressure (100 atm), and consumes 1–2% of the energy production of the world annually. NH3 synthesis assisted by non-thermal plasmas has drawn increasing attention because non-thermal plasmas allow NH3 synthesis to carry out at atmospheric pressure and temperatures lower than what is required by the H–B process. Despite tremendous efforts spent on plasma-assisted NH3 synthesis, the NH3 energy yield of this process, defined as grams of NH3 produced per kWh of energy consumed, is still much lower than that of the H–B process. This thesis focuses on providing a better understanding of the reaction mechanism of NH3 synthesis assisted by non-thermal plasmas and identifying the potential causes of low energy yield of this process. We performed NH3 synthesis experiments in a coaxial dielectric barrier discharge (DBD) reactor and also conducted a complementary zero-dimensional plasma kinetic model analysis. Our analysis demonstrates that NH3 synthesis in the plasma reactor proceeds principally via the formation of reactive gas-phase radicals and the subsequent radical adsorption and Eley–Rideal (E–R) reactions on both the metal and support material surfaces. Such a reaction pathway is energy inefficient since it involves the direct breaking of chemical bonds by plasma. We proceeded to perform NH3 decomposition experiments to study the significance of product decomposition in plasma-assisted NH3 synthesis. The experimental results show that product decomposition alone cannot fully explain the low energy yield of the plasma reactor. We then studied the performances of porous and nonporous catalyst supports in plasma-assisted NH3 synthesis through experiments and simulations. We argue that surface porosity provides more surface sites for reactive gas-phase species to participate in radical adsorption and E–R reactions, leading to a higher concentration of NH3 in the presence of porous catalyst support beads. The last part of this thesis presents the preliminary experimental results of NH3 synthesis assisted by nanosecond-pulsed discharge at different pulse widths and pulse repetition frequencies.