The Decomposition of Methane by Low-Energy Electrons

Abstract

The decomposition of methane by a beam of electrons having energies between 15 and 100 ev has been studied using methane pressures between and mm of mercury. The products were frozen out on a surface cooled to about -220" C and situated approximately 5 millimeters from the electron beam.Ethane, ethylene, and acetylene were found to be the main products along with smaller amounts of saturated and unsaturated higher hydrocarbons. The results provide some evidence that under these experimental conditions ions do not play a major part in the decomposition and a free radical mechanism has been proposed to explain the formation of the main products. INTRODUCTIONRecently the authors have become interested in the study of chemical reactions in the electric discharge. For many years, the interpretation of the results of such studies was based largely on the reactions of free radicals, ionic reactions not being considered of importance. During the last few years, however, it has been observed, initially by Stevenson (I), that ions frequently have enhanced cross sections for reaction. This has led t o a reappraisal of the role played by ions in radiation and discharge chemistry.As an aid to understanding the decomposition of methane in a discharge, the experimental conditions have been simplified in such a way that many of the experimental parameters could be separately controlled. In addition, an effort has been made to reduce the extent t o which secondary reactions occur by providing for the rapid removal of the products froin the reaction zone. In this way, it was hoped t o obtain some information concerning the primary products of the discharge reaction. An electrically heated (a-c.) tungsten ribbon filament was the source of electrons, the filament being enclosed in a housing with its own pumping for the removal of pyrolysis products. The beam of electrons was defined by apertures 1.5 mm in diameter in a series of Chrome1 A plates. Further alignment and collimation of the beam were obtained by the use of a permanent magnet which provided a uniform field strength of 870 gauss over an area about 4 centimeters in diameter a t the center of the gap. Onto the final collimator (collimator 2) was soldered a hollow brass cone, A, which fitted into the glass tube surrounding the reaction zone. This arrangement caused the major pressure drop to be across the defining aperture in the center of the collimator. The collector served to monitor the electron beam. The collector grid, about 1 mm in front of it, consisted of 90% transparent tungsten mesh and was maintained a t the same potential as the final collimator, thus creating a field free reaction zone 4.5 centimeters in length.The various potentials were obtained from a regulated d-c. supply and were chosen t o give the maximum beam current consistent with stability and reasonable filament 'Manuscript

Corona Chemistry in Titan.

“…These results were in agreement with previous corona discharge studies of gaseous methane in which basically saturated hydrocarbons were detected (Ponnamperuma and Pering, 1966;Ponnamperuma and Woeller, 1964;Ponnamperuma et al, 1969). Manton and Ticker (1960) have shown that when methane is irradiated with lowenergy electrons (<20 eV) and the products are protected from secondary degradation, the C 2 -hydrocarbon distribution is ethane > ethene > ethyne; as the energy of the electrons is raised to about 20 eV, the distribution is changed to ethane > ethene = ethyne, and at higher energies to ethane > ethyne > ethene. Based on these results, we inferred that the electron energy in our corona discharge experiments was very low (<20 eV).…”

The Decomposition of Methane in the Negative Glow

“…The products found in these experiments are very similar to those found by Manton and Tickner (9) in their study of the decoinposition of methane by a beam of electrons. Since it is ltnown that the electrons in the negative regions of a d-c. glow discharge possess beam properties and have energies similar to those used in the electron beam experiments, this suggests that the products of the negative glow are formed by a inechanism siinilar to the one which they proposed.…”

The role of unsaturated products in the radiolysis of methane

“…Previous papers 2-5 on the radiolysis of pure methane show that sinall amounts of ethylene have been detected (G values from 0.05 to 0*12), as well as traces of iso-butene4 and isopentene.3~ 4 Acetylene and propylene have not been reported, althougb the former is a major product in gas discharge experiments,6, 7 and has recently been found 8 in methane passed at low pressures (10-2mmHg) through a beam of low-energy electrons.…”