Plasma-catalytic ammonia generation and plasma-chemical nitrogen oxidation in a ‘hybrid’ ns pulse/RF discharge

Abstract

Plasma-catalytic ammonia synthesis in a ‘hybrid’ ns pulse/RF discharge, operated in the repetitive burst mode, is studied by Fourier transform infrared absorption spectroscopy. The data are taken in preheated H2-N2 mixtures, with and without Ni/γ-Al2O3, Rh/γ-Al2O3, and Ru/γ-Al2O3 catalyst placed in the plasma flow reactor. The sub-breakdown RF waveform added to the ns pulse bursts is used to isolate the effect of the enhanced vibrational excitation of N2 on ammonia and nitric oxide yields. The enhancement of the N2 vibrational temperature in the hybrid N2-H2 plasma has been demonstrated in our previous work. Adding the RF waveform results in a reproducible increase of the ammonia yield, measured over the ‘blank’ alumna, Ni, Rh, and Ru catalyst samples, by up to 25% compared to the ns pulse discharge operating alone. The yield enhancement in the empty reactor (without the alumina or catalyst samples) is significantly lower, about 10%. This indicates that the surface exposed to the plasma is an essential factor for the RF enhancement. The effect scales with the number of RF-augmented pulses per burst. In a closely related experiment, nitric oxide yield in preheated O2-N2 mixtures excited by the hybrid ns/RF discharge in the empty reactor was compared to that in a ns pulse discharge operating alone, at the same conditions. Significant enhancement of NO yield in the hybrid discharge has been detected, up to 50%. Measurements of N2 (A3Σu+) populations in ns/RF and ns pulse discharges did not show evidence of additional generation of N2 ( A ) , N, H, or O atoms by the sub-breakdown RF waveform. We conclude that the NH3 yield enhancement is likely caused by the surface reactions of vibrationally excited nitrogen, leading to its dissociation on the surface, while the NO yield is enhanced by the reactions of vibrationally excited nitrogen in the plasma volume.