FTIR study of adsorption and photoreactions of acetic acid on TiO₂

Fourier-transformed infrared spectroscopy has been employed to study the adsorption and photoreactions of acetic acid on powdered TiO₂. Molecular acetic acid and acetate groups are present on TiO₂ following acetic acid adsorption at 35°C. Acetic acid is adsorbed via hydrogen bonding or Lewis acid-base interaction and is removed at 170°C, leaving acetate on the surface. The adsorbed acetate groups adopt a bidentate coordination. CO₂(g) is detected as a photoproduct in the UV irradiation of surface acetate. In the presence of water, some surface acetate is converted into acetic acid, and, in this case, both CO₂(g) and CH₄(g) are observed in the photoillumination. However, effect of water on the reaction rate for CO₂(g) and CH₄(g) formation is not significant. O₂ can increase the photodecomposition rate of acetic acid on TiO₂, and in addition to CH₄(g) and CO₂(g), oxygenated compounds such as CH₃OH(g), CH₂O(g) and HCOOCH₃(g) are detected. It is likely that these species are generated due to O₂^-, produced from photoelectrons and O₂, attacking adsorbed acetic acid molecules instead of attacking acetate groups.