Plasma-assisted catalysis is one possibility for synthesis of fuels and chemicals that takes advantage of the expanding increase of renewable electricity from solar and wind power. This approach 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 at lower temperature and pressure conditions than commonly utilized in commercial thermal heterogeneous catalysis. Our recent research has focused on fundamental aspects of one of the simplest, and also most important, industrial catalytic reactions, i.e., ammonia (NH₃) synthesis. These experiments use a dielectric barrier discharge (DBD) reactor packed with γ-Al₂O₃ support particles or Ru/γ-Al₂O₃ supported metal catalysts in the discharge zone with AC or ns-pulsed discharges. We will discuss the effects of the support and a Ru/γ-Al₂O₃ catalyst on both the forward and the reverse reactions of plasma-assisted NH₃ synthesis. Experiments at different plasma power levels found that N₂ conversion was only slightly increased by Ru/γ-Al₂O₃ compared to γ-Al₂O₃ particles. Utilizing zero-dimensional plasma kinetic modeling with ZDPlasKin, we found that the direct adsorption of N and H, and two Eley-Rideal (ER) reactions, i.e., H + NH(s) → NH₂(s) and H + NH₂(s) → NH₃ + surf, make the largest contributions to plasma-assisted NH₃ synthesis in the presence of γ-Al₂O₃. Modeling also indicates that gas-phase radicals can diffuse into the pores of the support to some extent. Measurements using an electron-ionization molecular beam mass spectrometer (MBMS) that was connected directly to a plasma reactor detected NNH radicals in the gas phase, indicating that this reactive species should be considered further in the reaction kinetics. Overall, these results increase our understanding of plasma-catalysis hybrid systems and aid efforts to achieve the strongest synergistic interactions between the plasma and catalyst to increase performance and overall energy efficiency.

Hao Zhao

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