Reactions of Modulated Molecular Beams with Pyrolytic Graphite. II Oxidation of the Prism Plane

Abstract

The reaction of a modulated beam of molecular oxygen with the prism plane of pyrolytic graphite was investigated. Diffusional processes in the bulk dominated the response of the emission rates of CO and CO2. The phase lags of these products relative to the impinging reactant beam indicated that the surface reactions were strongly affected by diffusion of oxygen in the grain boundaries and then into the grains of the pyrolytic graphite structure. This double diffusion process so strongly demodulated the product signals that the apparent reactivity of the prism plane was less than that of the basal plane. This reactivity inversion is peculiar to the ac modulated beam method and would not occur in dc (steady state) experiments. The reactivity of graphite which had been annealed to 3000°C was found to be an order of magnitude larger than that of the as-received material. This increase in reactivity was due to reduction of the demodulation effect which resulted from closing off diffusional paths in the bulk by annealing. Hysteresis of the type found in the basal plane reaction was observed in the prism plane reaction as well. Approximately one-tenth as much CO2 was produced by the reaction as CO. Reaction with a beam enriched in oxygen-18 did not show complete statistical mixing of the isotropes of oxygen in the CO2 product, suggesting that a significant portion of the CO2 product was formed by a direct reaction of surface carbon with molecular oxygen.