Formation and decomposition of surface oxide in carbon combustion

Abstract

The formation of surface oxide on graphitized carbon black by reaction with oxygen at 500°C and the subsequent desorption of the oxide as CO at 600–750°C have been studied gravimetrically. The surface oxide is formed exclusively during the initial transient period of fast combustion observed by previous workers, and for each O atom chemisorbed one labile C atom is removed from the surface. The kinetics of desorption follow the relation: –dθ/dt=D_θ exp {–[E₁+β(1 –θ)]/RT}, where log₁₀D_θ= log₁₀D₁+α(1 –θ), and θ is the fraction of O on the surface at time t related to that present during steady combustion. D₁, E₁, α and β are constants. Values of 70 and 57 kcal mole^–1 were found for E₁ with two specimens at lower and higher burn-off respectively; the corresponding values of β were 43 and 47 kcal mole^–1.

Apart from the effect of catalytic impurities, two distinct processes are required to account for the kinetics of combustion. Only one of these produces stable oxide. A hypothesis is proposed which identifies this with reaction at “arm-chair”{112l} edges of the carbon lattice.