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

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Abstract

The adsorption of O2 from air is an essential step in the use of TiO2 as a photo-oxidation catalyst. This work explores the properties of a weakly bound O2 species which is probably the precursor to “active” O2 needed for the photo-oxidation of molecules by TiO2. Adsorbed molecular oxygen was observed via FTIR for the first time on nanosized TiO2 particles. The observed O−O stretching mode frequency at 1550 cm−1 is identical to the Raman displacement of O2 gas, showing that the interaction between the IR active O2 molecules and TiO2 is relatively weak. It is noteworthy that weak adsorption of O2 on TiO2 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 νO2 observed for increasing coverage indicates that the O2 molecules exhibit only small interactions with each other in the adsorbed layer. The adsorption enthalpy of O2 on TiO2 is measured to be between −16 to −21 kJ mol−1. The O2− species formed by O2 adsorbing on TiO2 surface defect sites (likely to be the active O2 in photochemistry), with a calculated vibrational frequency of 1097 cm−1, is not observed in this work. The influence of coadsorbed CO, CO2, and H2O with the adsorbed O2 species was also investigated. It is found that when the adsorbed surface species, such as CO, CO2, and excess H2O, screen the TiO2 surface electric field and TiO2 surface dispersion forces, the IR active O2 species are no longer observed.

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