Energy Deposition Timescale Controls Chemistry in Pulsed Methane Plasmas

Nonoxidative methane conversion offers a promising route for the coproduction of hydrogen and value-added hydrocarbons without CO2 emissions. Among emerging approaches, pulsed plasmas can provide potential advantages by enabling high energy deposition, rapid quenching, and generation of active nonthermal species. This work systematically investigates how pulsed energy deposition from nanosecond-to-millisecond time scales (10 to 50 mJ/pulse) influences methane plasma characteristics (electron density and gas temperature), reaction mechanisms, conversion, and energy efficiency. Nanosecond discharges produced the highest overall C2 selectivity (>70%), with up to 20% for C2H4; in this regime, methane dissociation proceeded through both electron-impact and thermal dissociation, whereas longer pulses were predominantly thermal. Pulsed plasma conversion was similar to thermal arcs (∼377 kJ/mol CH4) and specific energy requirements to produce C2H2 and H2 were 15 kWh/kg and 40 kWh/kg, respectively, in 100 μs long discharges. The product distribution was largely determined by thermal processes involving dehydrogenation reactions (Kassel's reaction scheme). High gas temperatures preferentially favored C2H2 formation over C2H4, and C2H4 selectivity decreased with increasing pulse width. Overall, these results demonstrate that pulse width and energy can be used to tailor hydrocarbon selectivity and can help design pulsed plasma systems for efficient production of value-added hydrocarbons and hydrogen.