Dielectric barrier discharge (DBD) reactors are widely studied for ammonia synthesis due to their ability to operate at mild conditions and their compatibility with catalytic materials. However, the underlying plasma–surface chemistry responsible for ammonia (NH3) formation remain unclear.
This work focuses on understanding the role of surface in NH3 formation under DBD plasma conditions by correlating ammonia production with the corresponding surface chemistry for nickel (Ni) and silver (Ag) electrode surfaces at 50 mbar pressure.
Concentration of ammonia formed was measured using mass spectrometry (MS), while surface NHx species were quantified by transferring in-vacuo the plasma-exposed electrode for X-Ray Photoelectron Spectroscopy (XPS) analysis. The XPS system has been calibrated for quantitative determination of the concentrations of elements and compounds on the surface.
The XPS results after plasma exposure show that that nitrogen-containing species (i.e NHx) from the plasma interact more strongly with Ni than with Ag, with indication of nickel nitride formation. The presence of surface NHx compounds is evidence for the formation of intermediate species participating in ammonia formation reactions.
Despite the different surface chemistry between Ni and Ag, MS results show similar NH3 concentrations in the gas phase across different N2/H2 gas mixtures. These finding are tentatively explained by different ammonia formation mechanisms for these two surfaces. On surfaces with weak nitrogen interaction, NH3 likely forms through an Eley–Rideal mechanism involving direct abstraction of surface hydrogen by impinging nitrogen radicals. In contrast, on surfaces that favor nitrogen adsorption, saturation of the surface with NH species is required to enable subsequent abstraction reactions leading to NH3 formation[1].
This work demonstrates the capability of the newly developed in-vacuo DBD-XPS experiment for analyzing the surface chemistry of plasma exposed DBD electrodes and provides new insights into the role of the surface in NH3 synthesis.
[1] J. Phys. Chem. Lett. 2020, 11, 10469−10475