Advancing plasma-assisted catalysis for ammonia synthesis through non-perturbative electric field characterization and investigation of surface nickel nitride formation

Abstract

Plasma-assisted catalysis for ammonia synthesis provides a promising alternative to traditional thermal catalysis, which relies on the energy-intensive and heavy CO2 emitting Haber-Bosch process. The present work focuses on the parallel advancement of measuring plasma properties and understanding catalyst behavior. For a N2-H2 plasma, we assert electric field induced second harmonic (E-FISH) generation as a more advanced laser diagnostic to the more widely used Lissajous figures for measuring plasma electric fields. The second focus of this work addresses the structure of nickel nitride formed under plasma-catalyst interactions. We found that E-FISH measured up to 53% and 62% higher electric field strength and electron temperature, respectively, at an applied external voltage of 13 kV compared to Lissajous figures. Consequently, the percentage of the electron population that contributes to the direct N2 dissociation predicted by E-FISH can be higher by a factor of 17. Such deviations suggest that N radical chemistry may have a large impact on NH3 formation in low pressure N2-H2 plasma kinetic models that use E-FISH inputs. For plasma-induced nickel nitride formation on a nickel film, energy-dispersive x-ray spectroscopy (EDS) and high resolution x-ray photoelectron spectroscopy (HR-XPS) indicate the formation of Ni4N and Ni3N after 40 minutes of N2 plasma treatment. Depth profile XPS indicates these nitrides extends beyond the surface atomic layer. Leveraging a previous hollow cathode discharge design, a dielectric barrier discharge (DBD) cell for studying nickel nitride nanoparticles and thin film inside an environmental scanning electron microscope (ESEM) was developed. The ESEM discharge cell will be used for studying catalyst behavior under plasma treatment in a low vacuum with electron microscopy.