Plasma-Catalyst Reactivity Control of Surface Nitrogen Species through Plasma-Temperature-Programmed Hydrogenation to Ammonia
Patrick Barboun, Hope O. Otor, Hanyu Ma, Anshuman Goswami, William F. Schneider, Jason C. Hicks
Abstract
Nonthermal plasma activation of N2 can facilitate nitrogen adsorption on metal catalysts at low bulk temperatures and atmospheric pressure. We apply a plasma-assisted temperature-programmed reaction (plasma-TPRxn) for ammonia (NH3) synthesis using sequential exposure of a silica-supported metal catalyst to N2 plasma followed by thermal hydrogen treatment while ramping the temperature to decouple the plasma activation of N2 from surface catalyzed hydrogenation steps. This approach eliminates the effects from bulk plasma phase reactions, thereby allowing for direct interrogation of plasma activated nitrogen on the active metal surfaces. We confirm previously reported spectroscopic observations that show plasma-generated surface nitrogen can be converted to NH3 through surface catalyzed pathways. Further, we demonstrate that the ammonia desorption peak temperature is sensitive to metal, with Pt desorbing NH3 at the lowest temperature. Unsteady state microkinetic models of desorption kinetics as a function of initial N coverage and metal recover observed trends in NH3 desorption temperatures and confirm that observed results reflect hydrogenation of plasma-induced N accommodation at each surface. In total, we show that the hydrogenation ability of the catalyst after plasma activation of N2 is responsible for the reactivity trends observed in plasma-assisted NH3 synthesis.