Investigating Plasma Catalyst Interactions for N₂ Reactions

The potential for non-thermal plasma to activate strong chemical bonds has provided alternative routes for chemical transformations at milder reaction conditions. The choice of catalytic material is pivotal in harnessing the synergistic relationship observed when stimulated by plasma. However, material design that can effectively harness the plasma-catalyst synergy requires a fundamental understanding of relevant plasma-activated surface reaction pathways. Moreover, in these plasma-catalytic systems, plasma phase reactions can dominate, which often complicates the analysis and contribution of the catalyst reactivity to the overall product yields.

This thesis focuses on the investigation of surface catalyst pathways for plasma N₂ transformation. A sequential exposure of the catalyst surface to the different reactant gases in which the plasma-activated nitrogen is subjected to temperature-programmed reactions with another reactant is employed to 1) highlight the opportunities for catalytic coupling and 2) decouple plasma-catalytic interactions for nitrogen reduction and oxidation chemistry. Initial studies focus on the evaluation of the surface reactivity of supported metal catalysts (Pt, Co, Ni, Fe) in plasma-assisted NH₃ synthesis. Under these hydrogenation conditions, the formation of NH₃ over different silica-supported metal catalysts is observed, showing metal surface hydrogenation of plasma-activated nitrogen species. Additionally, microkinetic modeling calculations show that the temperature of NH₃ formation scales with the activation barrier for surface hydrogenation. Under oxidative conditions over Pt and Au catalysts, the formation of NO and N₂O was observed. Interestingly, the metal affinity towards oxygen activation was observed to be critical in directing product selectivity, with Pt being more selective to NO and Au more selective to N₂O. Additionally, temperature regimes where product selectivity can be controlled on Pt catalyst are explored. Also, the influence of catalyst particle size on produce selectivity for N₂O synthesis was evaluated. Lastly, the modification of oxide material under plasma stimulation was studied as an avenue for chemical looping synthesis.