Mechanism of C2 hydrocarbon formation from methane in a pulsed microwave plasma

Methane dissociation, followed by the formation of C2 hydrocarbons, in a pulsed microwave discharge in methane was investigated by mass spectrometry and optical emission spectroscopy (OES). Long microwave pulses (>200 μs) are characterized by a pronounced dehydrogenation, but have a disadvantage in the saturation of the methane conversion at relatively low values, due to methane depletion toward the end of the pulse. For shorter pulses, the conversion degree increases approximately linearly as a function of energy input, and a maximum conversion of 90% with 80% selectivity toward acetylene was obtained for 60 μs pulses at 1 kHz repetition frequency. A further decrease of the pulse duration (20 μs) at higher frequency, in order to ensure a similar energy input, resulted in a decrease in conversion and dehydrogenation. The explanation of the effect of the pulse duration is based on information provided by optical emission spectroscopy of active species generated in the discharge. Atomic hydrogen, formed by ...