The methane decomposition and the formation of C-2 hydrocarbons, in particular acetylene, in a microwave plasma were studied. It was found that pulsing the discharge presents major advantages over the cw operation. The effect of the operating parameters, including pressure (15-65 mbar), flow rate (33-190 sccm), and discharge power (16-81 W) was investigated, with the aim to improve the efficiency for methane conversion and to reduce the energy requirement for the formation of acetylene. Maximum values of the methane conversion degree over 90% were obtained. As a function of the discharge conditions, acetylene can become the main reaction product, with 80% selectivity. The minimum energy requirement for methane conversion was approximately 7 eV/molecule and for acetylene formation 20 eV/molecule. The results show that active species generated in the plasma contribute to the methane dissociation and influence the product distribution. The correlation between the dehydrogenation and the gas temperature supports the view of thermally activated neutral-neutral reactions. (C) 2002 American Institute of Physics.
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