EVALUATION OF THERMAL-DESORPTION SPECTRA FOR HETEROGENEOUS SURFACES - APPLICATION TO CARBON SURFACE OXIDES

The existence of a distribution of intrinsically heterogeneous surface sites gives rise to broad spectra in the temperature-programmed decomposition of surface species. A simple method is developed to allow the determination of activation-energy distributions of population, PSI(E), and frequency factor, A(E), for the decomposition of surface complexes by using a combination of isothermal and linear TPD experiments. The main requirement in applying the method is that of very precise measurement of sample temperature during TPD. The decomposition of carbon surface oxide complexes occurs with an intrinsic distribution of activation energies. For complexes formed from oxygen at 520 K, the decomposition activation energies range from 150 kJ mol-1 to 420 kJ mol-1, with a mean of 295 kJ mol-1. The frequency factor for decomposition is approximately constant, with an average value of 10(14+/-0.4) s-1. These values cannot be compared directly with the kinetic parameters for CO desorption from metals because the decomposition of carbon surface oxides is irreversible and does not occur via a simple bond scission. The formation of CO2 from carbon oxide decomposition occurs with kinetic parameters distinctly different from those observed for CO formation.