Non-thermal plasma (NTP) assisted ammonia synthesis has received significant interest for its potential as an atmospheric pressure, low temperature, and “green” alternative to the Haber-Bosch process. Plasma-phase ammonia synthesis is a well-known process in which production rates can be enhanced by the addition of a solid catalyst to the discharge. While the mechanism of this enhancement is still largely speculative, kinetic studies provide evidence that nitrogen adsorption enabled by plasma-activation of gas phase nitrogen is one of the primary modes of plasma-assisted enhancement. Fourier transform infrared spectroscopy (FTIR) studies have observed NHx species on catalyst surfaces from plasma treatment; however, the low visibility of nitride and hydride species in FTIR makes it less suited for studying ammonia synthesis. In this presentation, we provide recent inelastic neutron scattering (INS) studies of a plasma treated Ni/Al2O3 catalyst. INS is a spectroscopic technique that is highly sensitive to hydrogen containing species and is not limited by the selection rules governing infrared spectroscopy techniques. INS studies were conducted with sequential nitrogen and hydrogen treatments of the catalyst in order to isolate NHx species generated by surface reaction from those generated in the plasma phase. We show that activating nitrogen with a plasma facilitates the adsorption of nitrogen on the Ni surface, and that following this adsorption step with either hydrogen plasma treatments or thermal hydrogen treatments, adsorbed nitrogen can be hydrogenated to NHx species. Density functional theory calculations were used to model the INS spectra of NHx on the surface, which allowed for the identification of NH2 and NH. Under these conditions, ammonia is generated via surface reactions that follow the initial adsorption of plasma-activated N2. Finally, we show that these results are translatable to laboratory scale reactors following similar sequential experiments for ammonia synthesis. Overall, this study provides direct evidence of plasma-catalyst interactions via INS experiments and can aid in our fundamental understanding of surface reactions occurring under plasma stimulation.

Inelastic neutron scattering study of the formation of NHx species on Ni/Al2O3 from plasma stimulation

ACS Spring Meeting 2021

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