Oxidation of a pyrophoric iron. Part 1.—Kinetics of chemisorption of oxygen on the finely divided iron–carbon substrate

Abstract

A kinetic study of the reaction of oxygen with a finely divided, pyrophoric, iron–carbon mixture is reported. At low pressures ( < 0.2 Torr) of oxygen, chemisorption proceeded to about two monolayers. Above 0.8 Torr oxygen uptake corresponded to the formation of about nine monolayers of oxide. From kinetic data, and electron micrographs of the reactant we conclude that the rate limiting processes were: (i) in the low pressure range, intercrystalline diffusion of gas to the active surfaces of the metal, and (ii) in the high pressure range, by diffusion across an oxide barrier layer which covered the metal surfaces. Reactions > 0.8 Torr were accompanied by self-heating. The activation energy for both oxygen chemisorption reactions was low ( < 0.5 kJ mol^–1, 77–373 K). The surface oxide layer could be reduced through reaction with constituent carbon by heating at 800 K in vacuum. This ability to regenerate the active surface enabled several successive oxidation reactions to be studied on a particular sample of reactant.