(569ea) Identification of Catalytic Nickel Nitride Structures for Plasma-Assisted Ammonia Synthesis

The synergy between heterogeneous catalysts and excited gas-phase species generated by nonthermal plasma (NTP) has been studied and has shown promising performance for multiple catalytic applications. Plasma-assisted catalytic ammonia synthesis from N₂ and H₂ is one of the most studied processes due to the high demand for ammonia as a feedstock for fertilizer production and as an energy carrier. To optimize catalysts for plasma-assisted catalytic ammonia synthesis, a wide range of metals catalysts have been evaluated, and non-noble Ni catalysts were reported to be more active than the benchmark Ru catalyst. Plasma-induced formation of N-containing species on Ni surfaces (likely Ni nitride, Ni_xN_y) has been observed and may serve as a contributor to high catalytic activity. However, due to the lack of in situ spectroscopic studies, the composition and structure of Ni nitride under plasma-assisted reaction conditions is not well understood. Additionally, the catalytic performance of Ni nitride has not been studied systematically. Herein, we reported on in situ/ex situ spectroscopic studies and reaction kinetic testing for the structures and catalytic activity of Ni nitride catalysts for plasma-assisted ammonia synthesis.

Ni catalysts were tested in a coaxial dielectric barrier discharge (DBD) flow reactor in a programmable furnace, and the products were analyzed with online sampling using a mass spectrometer (MS). Formation of nitride on the surface of Ni catalysts was probed using high-resolution X-ray photoelectron spectroscopy (HR-XPS), transmission electron microscopy/energy dispersive X-ray spectroscopy (TEM/EDS), and in situ/operando Raman spectroscopy. Our experiments demonstrated that Ni nitride structures formed under N₂- and N₂/H₂-plasma. The catalysts with Ni nitride were more active than metallic Ni. The identification of Ni nitride will be useful in the development of catalysts for a wide range of reactions that require a transformation of nitrogen-containing molecules, including ammonia synthesis and nitrogen-rich biomass processing