Oxidation of carbons and graphites by atomic oxygen kinetic studies

Abstract

The oxidation of three amorphous carbons and an artificial graphite by atomic oxygen has been studied in the temperature range 14 to 350'C. The plasma of the radio-frequency discharge used to produce oxygen atoms was not in contact with the carbon sample. The rate of oxidation, at a fixed temperature, is dependent upon the duration of the experiment. This rate is also temperature dependent and activation energies of 9.9 to 10-8 kcal mole-1 are reported. The reaction is approximately first order with respect to atomic oxygen concentration. The rate of oxidation of the carbons, at constant temperature and concentration of atomic oxygen, is not influenced appreciably by the wide variations shown by the carbons in their chemical composition, internal surface area and poresizes, and degree of crystallinity. At temperatures above 200°C, the measured activation energy decreases and approaches almost zero at 350°C. The several factors, to which this is attributable, are discussed and evidence produced to show that surface-oxide can retard the rate of oxidation. For one carbon, surface-oxide is formed more readily from atomic oxygen than from molecular oxygen. The activation energy of gasification by atomic oxygen, and its relevance to the understanding of the mechanism of the reaction of molecular oxygen with carbon, is discussed.

The formation of olefins and alkynes from the reaction of hydroxyl radical and carbonaceous material

“…oxygen atoms and hydrogen atoms. [33][34][35] Several mechanistic interpretations have been suggested. 19,20,22,33,34 Because of the higher energy of vacancies and defects relative to the bulk surface, when the surface of graphite is exposed to reacting gases at high temperature, gasification reactions are initiated at vacancies or reactive defects by removing the atoms surrounding these sites.…”

“…[33][34][35] Several mechanistic interpretations have been suggested. 19,20,22,33,34 Because of the higher energy of vacancies and defects relative to the bulk surface, when the surface of graphite is exposed to reacting gases at high temperature, gasification reactions are initiated at vacancies or reactive defects by removing the atoms surrounding these sites. 23 Various experiments have shown that atoms and radicals probably have a degree of mobility on the surface 22 and can recombine to give observed products.…”

“…Kinetic studies of the oxidation of three amorphous carbons and graphites by atomic oxygen clearly established that the gasification rate by oxygen atoms is much higher than by oxygen molecules. 33 If the rate controlling step in the gasification of carbon by molecular oxygen is dissociative chemisorption, then the rate controlling step in the gasification by atomic oxygen is the formation of the carbon-oxygen activated-complex.…”

Carbons

“…It was found necessary in an extensive formulation of hypothetical elementary steps to invoke the existence of stationary and mobile initial oxidation products (step 4) in order to explain the varying selectivity between CO and CO 2 . The experimental evidence showed different TDS peaks for carbon-oxygen compounds and not for di-oxygen [91,83,93,85] which led the authors to consider only the possibility that reaction products are mobile and not activated oxygen. The process of oxygen activation was assumed to only be possible at the reaction sites themselves ("C" in the reaction scheme).…”

“…The process of oxygen activation was assumed to only be possible at the reaction sites themselves ("C" in the reaction scheme). If the activation of molecular oxygen occurs by non-chemical means (glow discharge [83,93] or radiolysis [82]) the sensitivity of the overall reaction kinetics to the presence of graphene layers for oxygen activation is lost and a fully structure-insensitive process occurs. In the other extreme of a di-oxygen reaction on a perfect graphene layer reaction of excessive activated oxygen will always occur at the unavoidable external prism surfaces and very efficiently at the few intralayer defects.…”