The Decomposition of Methane in the Negative Glow

Abstract

The decolnposition of methane was studied in the negative glow of a d-c. discharge a t pressures of 0.30 and 0.050 mm. The discharge tube was cooled by liquid or solid nitrogen.The main products were ethane, ethylene, and acetylene in addition to hydrogen and a nonvolatile product which appeared mainly on the cathode as a solid having the formula (CH),. Smaller amounts of propane, propene, propyne, butane, butene, butadiene, and pentene were also found. Lowering the temperature of the discharge tube from -196" t o -210" C greatly increased the amount of ethylene recovered.The solid product is apparently transported t o the cathode in the form of ions and may result from ionic polymerization of the acetylene. Acetylene is the volatile product formed closest t o the cathode which suggests that it may also be fornled by ionic processes. The formati011 of the remaining products is consistent with an excitation mechanism in which the C:! products are formed first and the higher hydrocarbons are formed from them. INTRODUCTIONThe decomposition of methane in the glow discharge has been studied for many years; the early work is summarized in reference 1. More recently, Yeddai~apalli (2) studied the decomposition of methane in a d-c. glow discharge using a discharge tube cooled in liquid air. He worked a t pressures of a few millimeters and found that the products consisted of ethane, ethylene, and acetylene as well as hydrogen and a nonvolatile product having the forinula (CH?),. Wiener and Burton (3), using a d -~. glow discharge a t atmospheric pressure, showed that under conditions of high conversion and relatively high temperature the only significant products were acetylene, hydrogen, and carbon. McCarthy (4) found that, in addition to acetylene, etllyleile and ethane were obtained as products when the gas from a microwave discharge in inethaile was allowed to impinge directly on a wall cooled by liquid nitrogen.The low-pressure d-c. glow discharge is suitable for investigations of this type because it permits studying the decomposition in a number of different discharge regions for which the electrical processes are reasonably well understood ( 5 , 6). In addition, the vapor pressures of methane and its decomposition products are such that most of the products can be removed rapidly from the reaction by freezing thein out on the discharge tube wall. There is thus the possibility of obtaining direct information about the primary products of the decon~position. This approach was used by Yeddanapalli (2), whose work, however, had two main wealcnesses: ( a ) a t the temperature of liquid air the vapor pressure of ethylene is sufficiently high that much of it would have remained in the gas phase, and (b) his method of analysis was such that the products were determined as acetylene and ethylene with the remainder assumed t o be ethane. Any higher hydrocarbons which might have been formed would thus not have been detected.The negative glow was chosen as a suitable region with which t o begin a study of the decomp...

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