Evidence for two temperature régimes in the slow oxidation of methane

Abstract

The kinetics of the slow oxidation of methane have been studied using a technique such that the partial pressures of methane and oxygen—and in many experiments, carbon monoxide as well—were followed throughout the course of the reaction. The dependences of the rate of the disappearance of methane upon the sum of the pressures with or without diluents, upon the injection of formaldehyde into a reacting methaneoxygen mixture, and upon the removal of carbon monoxide from the reaction mixture have been determined directly for the first time. The results of these experiments indicate that: 1. (1) When carbon monoxide accumulates in the slow oxidation of methane at the temperatures used here, it retards the rate of disappearance of methane. When it is removed, the rate of disappearance of methane remains constant throughout 70 to 80 per cent of the reaction indicating that the rate is an apparent zero order reaction. 2. (2) Below the range of about 416° to 430°C, the rate depends only upon the initial pressures of methane and oxygen and is not affected by the total pressure, by diluents, or by the injection of nearly equi-molar amounts of formaldehyde. The rate can be expressed by the relation: − d (CH 4) dt =Ae − 30200 RT (CH 4 0) 2(O 2 0) 3. (3) Above about 430°C, diluents and formaldehyde accelerate the rate of the reaction. The rate is higher than predicted from the above expression and it appears that an additional branched chain(s) may be developed at these higher temperatures. 4. (4) The present evidence suggests that formaldehyde is not the rate-controlling intermediate at low temperatures but that it may serve as an additional chain branching agent at the higher temperatures.