Bonding geometry and reactivity of methoxy and ethoxy groups adsorbed on powdered TiO2

Thermo- and photoreactivities of methoxy (CH3O(a)) and ethoxy groups (C2H5O(a)) bonded, via the oxygen atom, to one Ti ion (monodentate adsorption form) or to two Ti ions (bidentate adsorption form) on TiO2 have been studied by Fourier transformed infrared spectroscopy. Regardless of the similar thermal stability for the two adsorption geometries of the adsorbed alkoxy groups, difference in photoreactivity is observed by monitoring the change of their integrated IR absorptions as a function of UV irradiation time. The monodentate photooxidation rate is similar to 1.5 times that of bidentate for both methoxy and ethoxy groups. CH3O(a) on TiO2 is photooxidized to H2O(a) and HCOO(a) in the presence of O-2 On the other hand, C2H5O(a) is photooxidized to H2O(a), HCOO(a), and CH3COO(a). A Russell-like mechanism is invoked to explain the formation of the reaction products. Possible reaction steps that control the photoreactivity of the monodentate and bidentate adsorption forms are discussed in terms of this mechanism.