Reactions of Modulated Molecular Beams with Pyrolytic Graphite. I. Oxidation of the Basal Plane

Abstract

Reaction of molecular oxygen as a modulated molecular beam with the basal plane of pyrolytic graphite is investigated. The graphite surface participates in the reaction in an unusual fashion. Oxidation tends to create a highly reactive surface while thermal annealing tends to deactivate the surface. The imbalance of these two competing processes results in slow changes in surface reactivity during the course of an experiment, which is manifest as hysteresis in the rate of CO production. The surface chemical reaction occurs on a much smaller time scale than the processes responsible for hysteresis. The data indicate that a two-branch, two-site mechanism explains both the hysteresis and the surface chemistry. Surface migration of adsorbed oxygen is shown to be a step in the production of CO. Carbon dioxide is barely detectible; its production rate is two orders of magnitude smaller than that of carbon monoxide at all temperatures.