| The aromatization of methane under oxygen-free conditions can produce COx-free hydrogen for clean energy and aromatics for chemical feedstocks. The two-stage plasma-followed-by-catalyst (PFC) process and one-stage plasma-over-catalyst (POC) process were used for oxygen-free conversion of methane to aromatics and hydrogen at lower temperatures in this paper.Oxygen-free conversions of methane in atmospheric non-thermal plasma generated by using different discharge techniques, pulsed streamer discharge, pulsed spark discharge, unipolar-pulsed dielectric barrier discharge (DBD) and AC DBD were investigated. Of the four electric discharge techniques, C2 hydrocarbons and hydrogen were the major products, with only extremely small amount of aromatic product.Energy costs for methane conversion, acetylene (for pulsed streamer discharge and pulsed spark discharge) or ethane (for DBD processes) formation, and H2 formation increase with methane conversion percentage, and were found to be: in pulsed spark discharge (methane conversion 18-69%), 14-25, 35-65 and 10-17 eV/molecule; in pulsed streamer discharge (methane conversion 19-41%), 17-21, 38-59, and 12-19 eV/molecule; in unipolar-pulsed DBD (methane conversion 6-13%), 38-57, 137-227 and 47-75 eV/molecule; in AC DBD (methane conversion 5-8%), 116-175, 446-637, and 151-205 eV/molecule, respectively. The results of optical emission spectroscopy in pulsed streamer discharge and pulsed spark discharge showed that high selectivity of acetylene could be related to C2 radicals. Supports immersed in pulsed streamer discharge and DBD enhanced the methane conversion and C2 hydrocarbons production.This study investigated the oxygen-free conversion of methane to aromatics and hydrogen for the first time in a two-stage plasma-followed-by-catalyst (PFC) reactor. By using pulsed spark discharge plasma at the first stage and Ni/HZSM-5 catalyst bed at the second stage, conversion of methane to aromatics and hydrogen has been achieved at lower temperatures of 300～400℃. The effect ofNi loadings, reaction temperatures and on-stream time on aromatics selectivity was investigated. The methane conversion percentage and yield of aromatics and hydrogen on the average during 300 rain were 72.1%, 32.7% and 41.2% over Ni(1 wt.%)/HZSM-5 at 400℃with 68.6 kJ/L of specific input energy. Furthermore, the results of temperature-programmed reaction showed that the Bronsted acid sites played a key role in conversion of acetylene to. aromatics and that Ni promoted acetylene conversion and aromatics production.The conversion of methane to aromatics in a plasma-over-catalyst (POC) reactor was explored in this paper. The results of the one-stage combination of DBD with Mo/HZSM-5 catalyst showed that DBD had a slightly positive effect on methanearomatization. By using pulsed streamer discharge plasma over Ni/HZSM-5 catalysts process, conversion of methane to aromatics has been achieved at lower temperatures of 250～350℃. The effects of Ni loadings, reaction temperatures and on-stream time on aromatics selectivity were investigated. The average methane conversion percentage and aromatics yield during 300 min were 41.4% and 15.0% over Ni(0.5 wt.%)/HZSM-5 at 300℃with 50 kJ/L of specific input energy.
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