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

Abstract

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 NH x (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 NH x 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.

Shielding Protection by Mesoporous Catalysts for Improving Plasma-Catalytic Ambient Ammonia Synthesis

“…Figure b shows that H 2 plasma was able to break the NN triple bond of the adsorbed N 2 on the Ni surface, forming coordinated *NH 3 (ν as (NH 3 ): 3393 cm –1 ; ν s (NH 3 ): 3222 cm –1 ; δ as (NH 3 ): 1639 cm –1 ; δ as (NH 3 ) or δ as (NH 4 + ): 1550, 1525 cm –1 ). A small IR peak at 1403 cm –1 represents *NH species, which is one of the critical intermediates in the surface reactions of ammonia synthesis . Compared to the bare MCM-41, the metallic Ni sites, especially on the external surface of MCM-41, promoted the formation of surface *NH x ( x = 1 and 2) amide groups significantly, as indicated by the increased intensity of the characteristic IR peaks at 3120 cm –1 (for ν as (NH x )), 3019 cm –1 (for ν s (NH x )), and 1403 cm –1 (Ni–*NH x ).…”

“…A small IR peak at 1403 cm −1 represents *NH species, which is one of the critical intermediates in the surface reactions of ammonia synthesis. 55 Compared to the bare MCM-41, the metallic Ni sites, especially on the external surface of MCM-41, promoted the formation of surface *NH x (x = 1 and 2) amide groups significantly, as indicated by the increased intensity of the characteristic IR peaks at 3120 cm −1 (for ν as (NH x )), 3019 cm −1 (for ν s (NH x )), and 1403 cm −1 (Ni−*NH x ). Note that the IR peaks of NN stretching between 1900 and 2000 cm −1 are absent, which reveals that in the presence of H and H 2 radicals, *N 2 species could be rapidly dissociated and hydrogenated to form *NH x species on the catalyst surface, instead of forming adsorbed *N 2 H x species (which are the major intermediates in the electrochemical nitrogen reduction reactions 56,57 ).…”

In Situ Identification of NNH and N2H2 by Using Molecular-Beam Mass Spectrometry in Plasma-Assisted Catalysis for NH3 Synthesis

“…In the context of ammonia synthesis, the formation of N, H, NH, NH 2 , and NH 3 in the gas phase ,, and their interactions with solid surfaces ,,, have been examined in plasma kinetic model analysis. The presence of NH x species in the gas-phase plasma or on catalyst surfaces in plasma has been demonstrated in experiments by using optical emission spectroscopy, X-ray photoelectron spectroscopy, and inelastic neutron scattering . Hwang and Mebel calculated rate constants for thermal NH 3 synthesis in the gas phase including intermediates such as NH, NH 2 , NNH, and N 2 H 2 ; however, according to their rate models, the concentrations of these intermediates in the gas phase are very low at the temperature used in our study, and therefore, they play an insignificant role in thermal reactions.…”

“…The presence of NH x species in the gas-phase plasma or on catalyst surfaces in plasma has been demonstrated in experiments by using optical emission spectroscopy, 28 X-ray photoelectron spectroscopy, 14 and inelastic neutron scattering. 29 Hwang and Mebel 30 calculated rate constants for thermal NH 3 synthesis in the gas phase including intermediates such as NH, NH 2 , NNH, and N 2 H 2 ; however, according to their rate models, the concentrations of these intermediates in the gas phase are very low at the temperature used in our study, and therefore, they play an insignificant role in thermal reactions. Winter et al 31 identified surface reaction intermediates such as N 2 H y in plasma catalytic ammonia synthesis, although these species were not separately identified and quantified.…”

Direct Observation of Plasma-Stimulated Activation of Surface Species Using Multimodal In Situ/Operando Spectroscopy Combining Polarization-Modulation Infrared Reflection-Absorption Spectroscopy, Optical Emission Spectroscopy, and Mass Spectrometry

“…Stere et al developed an in situ DRIFTS coupled with mass spectrometry (MS) instrument to simultaneously investigate the surface species and formation of products in hydrocarbon selective catalytic reduction of nitrogen oxides (NO x ) over a silverbased catalyst with a helium plasma jet and identified the importance of isocyanate species being the active intermediates. 26 X-ray absorption spectroscopy 30 and inelastic neutron scattering 31 have also been used to reveal the role of plasmas in the hybrid NTP catalytic oxidation of methane and plasmapromoted nitrogen reduction, respectively.…”