Surface heterogeneity in the formation and decomposition of carbon surface oxides

Abstract

The formation of surface oxides on exposure of carbon (Spherocarb) to oxygen (16–6560 Pa) at temperatures at ca. 700 K has been investigated experimentally using controlled atmosphere thermogravimetry and temperature-programmed desorption (TPD) with evolved gas analysis. Under the conditions of the study. Type B oxides are formed, with the gasification of one C atom (as CO or CO 2 ) for every O atom that is bound to the surface. The broad TPD spectrum obtained for the decomposition of Type B oxides formed on an initially clean surface is invariant over a wide range of initial oxide loadings, from 45 to 990 μmol g −1 . It is concluded that lateral interactions between adsorbed species do not affect the kinetics of decomposition of the surface oxides and that the broad TPD spectrum results entirely from the heterogeneous nature of the carbon surface and the oxides formed on it. Activation energy distribution energies for the decomposition of Type B oxides range from ca. 225 to 475 kJ mol −1 . The formation of oxides with a particular activation energy occurs purely statistically, with the fraction of new oxide, having decomposition activation energies in the range E and E+dE , being simply Ψ 0 ( E ) dE , where Ψ 0 ( E ) is a characteristic of the carbon itself and independent of the oxide loading. Therefore, the rate of formation of complexes having a particular activation energy for their thermal decomposition can be expressed simply in terms of the overall rate of formation of Type B complexes and the distribution Ψ 0 ( E ), which is essentially the same as the activation energy distribution function recovered from the TPD spectra.