Invited Paper PS+SS-ThA8
Plasma-Assisted Ammonia Synthesis in Hybrid Plasma-Catalysis DBD Reactors

Thursday, October 24, 2019, 4:40 pm, Room B131

Solar and wind power are creating increasingly large amounts of electricity, and an important question is how can we take advantage of the expanding increase of renewable electricity for catalysis? One approach is plasma-assisted catalysis, which utilizes excited gaseous molecules or new reactive species formed in a (non-equilibrium, low temperature) gas discharge plasma, along with a catalyst to enable increases in the activity and selectivity for carrying out desirable chemical reactions. A significant challenge in plasma-catalysis hybrid systems is to achieve the strongest synergistic interactions between the plasma and catalyst to increase performance and overall energy efficiency. We report on two types of dielectric barrier discharge (DBD) reactors (with a coaxial tube and parallel plates) that have been used to screen catalytic effects of different metal surfaces and supported catalysts in plasma-catalysis hybrid systems at near atmospheric pressure, utilizing both AC and nanosecond pulsed discharges. We observed strong synergistic effects between non-equilibrium plasma and catalysts for both NH₃ synthesis and methane coupling reactions. We compared the performance for ammonia synthesis of catalysts using active metals (Pd, Pt, and Fe) or less active metals (Au, Ag, and Cu) or their alloys. We found that the metal-nitrogen (M-N) bond energy was not the only parameter governing the catalytic activity for NH₃ synthesis in plasma. Better catalytic activity could be achieved by bimetallic catalysts that contained catalytic sites for both N₂* dissociation and hydrogenation of M-N bonds, leading to our observations of a highly active PdFe catalyst for NH₃ synthesis in plasma. In addition, we will also report briefly about results in CO₂ reforming of methane in the coaxial reactor, where we found that under thermal only conditions, PtFe/Al₂O₃ catalyzed mainly the formation of CO and H₂, but with the plasma on, the selectivity shifted to methane coupling reactions. Interestingly, an Ag/Al₂O₃ catalyst with an AC discharge demonstrated 100% selectivity to CH₄ coupling reactions at 350 ^oC. Methane coupling using the plasma-catalysis reactor at low temperatures and pressures mainly produced higher hydrocarbons, suggesting a potential route for converting cheap and abundant methane gas into high value hydrocarbons and fuels.