Investigation of Plasma-Induced Nickel Nitride Formation during Ammonia Synthesis

Plasma-assisted catalysis for ammonia synthesis provides a potentially more sustainable alternative to traditional thermal catalysis. Advancing the use of plasma with catalysts in tandem requires a more thorough understanding of plasma-surface interactions and reaction kinetics. In the present work, we investigate the formation of nickel nitride on a nickel film deposited on a stainless steel substrate in a low pressure N₂ dielectric barrier plasma discharge (capacitively coupled; 12 kV, 20 kHz). Surface morphology of the nickel film after 40 minutes of plasma treatment was examined by scanning electron microscopy (SEM). Energy-dispersive x-ray spectroscopy (EDS) and high-resolution x-ray photoelectron spectroscopy (HR-XPS) determined the formation of nickel nitride on the plasma-treated nickel surface. Depth profiling using XPS shows that the formation of a nitride extends beyond a surface atomic layer. To further understand the formation of nickel nitride in the presence of a N₂ plasma, a microdischarge cell capable of generating a N₂ plasma at 5-10 Torr inside an environmental scanning electron microscope (ESEM) while providing optical access for direct SEM imaging and EDS analysis was constructed. This ESEM cell enables surface analysis after plasma treatment without any distortion of elemental and structural information by air contamination. These initial results and additional studies will be helpful in understanding the role of nickel nitride in plasma-assisted catalysis of ammonia synthesis using nickel catalysts.