Inelastic Neutron Scattering Observation of Plasma-Promoted Nitrogen Reduction Intermediates on Ni/γ-Al2O3

Plasma-assisted catalysis is an emerging technology for the atmospheric pressure and low bulk gas temperature synthesis of ammonia from molecular nitrogen and hydrogen. Direct evidence for plasma-induced surface reaction intermediates relevant to ammonia production, including surface hydrides and NHx (x = 1, 2, 3) species, has remained elusive. Here we report inelastic neutron scattering (INS) observations of alumina-supported Ni particles after treatment with N2 and H2 plasmas. INS experiments reveal the presence of NHx species and hydrides on Ni sites after exposure to sequential N2 and H2 plasma treatments. By separating exposure, we exclude the presence of plasma-phase reactions and demonstrate that these species are generated through plasma-facilitated surface reactions. INS spectra obtained from first-principles of NH3, NH2, and NH adsorbates on Ni support the experimental assignments of surface intermediates. The results directly implicate plasma stimulation of dinitrogen in generation of surface-bound nitrogen that participates in further hydrogenation reactions driven either thermally or with H2 plasma.