Temperature-programmed desorption of CO2, formed by CO photooxidation on TiO2 surface

Temperature-programmed desorption (TPD) spectra of CO2, remaining on the dehydroxylated powder TiO2 (anatase and rutile) surface after photooxidation of CO at T = 293 K, were analysed. It was shown that the CO2 TPD peak appears at T > 400 K, has a symmetric shape, and its maximum position (T-m) depends on the CO2 coverage: the lower is the CO2 amount, the higher is the T-m value. The CO2 TPD peak is well fitted with the second-order desorption kinetics with E-des = 1.15 +/- 0.05 eV and nu -= 0.32 +/- 0.02 cm(2) s(-1) for anatase and E-des = 1.24 +/- 0.05 eV and nu = 0.16 +/- 0.01 cm(2) s(-1) for rutile. The desorption process can be interpreted as monomolecular desorption of carbonate species with two types of precursor states: 1) "over an empty site" and 2) "over a filled site". It was supposed that CO2 thermal desorption resulted from migration of strongly bonded carbonate species (CO3) along the surface (via the precursor state "over an empty site") and interaction of them (desorption from precursor state "over a filled site").