Power pulsing in CH₄ microwave plasma for mechanistic insight into ethylene selectivity

Plasma can couple methane to ethylene without CO₂ emissions, providing a unique opportunity for carbon circularity. Efficiency and control over ethylene and acetylene selectivity versus undesired coke formation are the keys to success. The Kassel mechanism predicts a delicate balance between high temperatures (>2000 K) and short gas residence times.

We performed pulsed microwave plasma experiments in mixtures of methane and hydrogen to control the effective residence time and unravel transport and chemistry. Reactor performance measurements, using gas chromatography, are combined with time-resolved in situ reactor temperature profile measurements with Raman spectroscopy to evaluate reaction rates and heating/quenching rates. Scans of plasma pulse duration covered a hydrocarbons selectivity transition from 70% ethane (<50 μs pulses) to ethylene to dominant acetylene formation. Moreover, carbon balances were higher than 97% for these pulsed experiments.

Comparison of the experimental results with a 0D kinetic model highlights the importance of achieving sufficient energy density for H radical production during the pulse. The rapid quenching after the pulse prevents further cracking of ethylene to acetylene. At the same time, temperature control becomes essential to suppress dominant ethane conditions.