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

Abstract

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 methane dissociation, was found to play an essential role in methane plasma chemistry. A qualitative estimation of the variation of H atom concentration with operating conditions was done by actinometry, since time-resolved OES provides evidence that atomic hydrogen is mainly formed in the ground state and dissociative excitation can be neglected. In addition to the concentration of atomic hydrogen, the second key parameter is the gas temperature. It was determined from the relative intensity distribution in the rotational structure of the (0,0) C2 Swan band and of the (2,2) H2 Fulcher-α band. Gas temperatures between 1500 and 2500 K were determined for the present discharge conditions. The hydrogen abstraction by hydrogen atoms, favored at high temperature, is responsible for the high methane conversion and low energy requirement achieved (9–10 eV/molecule) and for the distribution of the reaction products.