The role of the surface complex in the kinetics of the reaction of oxygen with carbon

Abstract

Using thin films of pyrolytic carbon the kinetics of the reaction with oxygen were studied in a static system over the temperature range 748–1173 K at pressures in the neighborhood of l Torr (0.13 kPa). The formation of the stable surface complex, measured by pressure change and by temperature-programmed desorption, and products carbon monoxide and carbon dioxide were monitored as a function of reaction time during the complete course of the reaction. At lower temperatures an induction period was observed for formation of carbon monoxide and carbon dioxide but not for formation of the complex. The presence of the complex on the surface before the start of a reaction shortened the induction period and increased slightly the over-all rate. It was concluded that the surface complex is an intermediate in the production of carbon monoxide and carbon dioxide. A mechanism for the reaction is proposed which involves secondary reaction of oxygen with the complex and which accounts qualitatively for the kinetics of the reaction. It is suggested that a turn-over number may be estimated based on a number of active carbon atoms on the surface.