The kinetics of chemical vapor deposited diamond-oxygen reaction

The kinetics of reaction between chemical vapor deposited diamond films (prepared by the hot filament method) and oxygen gas was studied by thermogravimetry. The reactions were carried out at atmospheric pressure in gas mixtures containing between 25 and 100 vol. % oxygen (balance argon), and in the temperature range of 973–1073 K. The apparent order of the reaction is close to 0.6, and the apparent activation energy is 232 kJ/mole. The kinetic data are explained by assuming no mass transfer limitations, direct reaction between CVD diamond and oxygen to form CO and CO₂, and thermodynamic equilibrium between CVD diamond, CO, and CO₂. The dominant chemical reaction involves the formation of CO, while the formation of CO₂ is not significant. Three stage mechanistic schemes are developed involving adsorption of oxygen on CVD diamond surface, surface chemical reaction, and desorption of adsorbed species to CO or CO₂. The experimental rate data conform to the reaction rate expressions developed for the mechanistic schemes leading to the formation of CO and CO₂, assuming adsorption as the rate-controlling step. The adsorption rate constants for the formation of CO and CO₂ are determined. The activation energy of the adsorption step leading to the formation of CO is 213 kJ/mole.