109.    ---------------. [AUDIBERT, E] [Mechanism of the Oxidation of Methane.]  Compt. rend., vol. 216, 1943, pp. 348-350, 449-451; Chem. Abs., vol. 38, 1944, p. 2257.

                 Mechanism and the kinetics of the reaction taking place between CH₄ and O₂ at temperatures 300°-650° were studied by use of quartz or Pyrex tubes.  Reaction products are H₂O, CO, CO₂, H₂, and HCHO (1); the amounts of (1) and H₂ formed are small, sometimes none of the latter.  Even under equal conditions the results sometimes vary considerably and depend upon the diameter of the tube and the state of its inner surface.  With a quartz tube of 10 mm. inside diameter, the inner surface of which is covered with a NaCl or KCl layer, the reaction isotherm remains constantly slow at 650° with a mixture of 1 molecule CH₄ and 2, O₂.  With NaCl-lined quartz tubes of greater diameter the reaction may be accelerated to such an extent that the gas mixture ignites at 550° or higher.  With unlined quartz tubes, the reaction may become violent at 550°, even if the diameter of the tube is 10 mm.  To study the transformation isotherm further, the oxidation of CH₄ at 300°-700° was accomplished by passing the mixture at atmospheric pressure through a ring-like tube formed by inserting 1 quartz tube within another 4 mm. wider.  The latter was coated on its inner surface, the former on its outer, with NaCl.  With such an apparatus CH₄ and O₂ react at all temperatures between 300° and 700° with formation of H₂O, CO, CO₂, H₂, and (1).  The amounts of H₂ and (1) are very small; the amount of CO₂ increases with the duration of heating, whereas the amount of CO increases to a maximum and then decreases.  The ratio of consumption of CH₄:O₂ is about 1:2 as long as the heating time is short and the temperature low.  Based upon the CH₄ consumption, the reaction follows the law of Arrhenius, and the heat of activation is about 40,000 cal.; based upon the O₂ consumption, it does not follow this law, and the activation heat is 40,000 cal. at 650° or higher and 50,000 cal. at 625°.  The results indicate that under the conditions used, the reaction is heterogeneous and that its course is influenced by the state of the surface of the reaction tube.