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=Dθ exp {–[E1+β(1 –θ)]/RT}, where log10Dθ= 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.