Vibrational Spectroscopic Observation of Weakly Bound Adsorbed Molecular Oxygen on Powdered Titanium Dioxide

The adsorption of O 2 from air is an essential step in the use of TiO 2 as a photo-oxidation catalyst. This work explores the properties of a weakly bound O 2 species which is probably the precursor to “active” O 2 needed for the photo-oxidation of molecules by TiO 2 . Adsorbed molecular oxygen was observed via FTIR for the first time on nanosized TiO 2 particles. The observed O−O stretching mode frequency at 1550 cm −1 is identical to the Raman displacement of O 2 gas, showing that the interaction between the IR active O 2 molecules and TiO 2 is relatively weak. It is noteworthy that weak adsorption of O 2 on TiO 2 is accompanied by the development of a weak IR absorption band due to the production of a small dipole in the molecule. In addition, the constant ν O 2 observed for increasing coverage indicates that the O 2 molecules exhibit only small interactions with each other in the adsorbed layer. The adsorption enthalpy of O 2 on TiO 2 is measured to be between −16 to −21 kJ mol −1 . The O 2 − species formed by O 2 adsorbing on TiO 2 surface defect sites (likely to be the active O 2 in photochemistry), with a calculated vibrational frequency of 1097 cm −1, is not observed in this work. The influence of coadsorbed CO, CO 2, and H 2 O with the adsorbed O 2 species was also investigated. It is found that when the adsorbed surface species, such as CO, CO 2, and excess H 2 O, screen the TiO 2 surface electric field and TiO 2 surface dispersion forces, the IR active O 2 species are no longer observed.