The adsorption of formate and acetate from aqueous solutions at pH 3-9 onto the TiO 2 rutile (110) surface was studied by ATR-FTIR spectroscopy. The spectra indicated that there was only one type of adsorbed species, and that formate and acetate were adsorbed in a similar manner. On the basis of the measured ν as -(COO)ν s (COO) splitting (∆ν as-s ) of 191 and 87 cm -1 , for formate and acetate, respectively, the monodentate (ester type) binding mode could be excluded. Ab initio calculations at the Hartree-Fock level showed that for pentacoordinated Ti IV , present in the (110) surface, the chelating bidentate binding mode is unstable with respect to the rearrangement to the monodentate or the bridging bidentate mode. The computed vibrational frequencies of formate and acetate adsorbed in a bridging bidentate mode onto Ti clusters with 2-5 Ti centers, representing the (110) surface, agreed with experiment and thus showed that this methodology can be used for the determination of the structures of adsorbates on, for example, metal oxide surfaces in contact with aqueous solutions.

Photodegradation on soil and mineral surfaces is an important pathway for the transformation of recalcitrant pollutants in the atmosphere and on terrestrial surfaces. Monitoring degradation on surfaces under controlled conditions without intrusive methods, however, remains a challenge. Here, we report in situ measurements of the photodegradation of 4,4‘-bis(2-sulfostyryl)biphenyl (DSBP), a fluorescent whitening agent used in laundry detergents, on the surfaces of metal oxides under conditions of controlled humidity using attenuated total reflectance infrared spectroscopy (ATR-FTIR). A ZnSe ATR crystal was coated with 1−5 μm thick layers of high specific surface area hematite, δ-alumina, lepidocrocite, rutile, and anatase. The 0.4 mg oxide layers were spiked with 20 nmol of (E,E)-DSBP and illuminated with 365 nm light at 25 °C and 88% relative humidity. Extensive degradation was observed on rutile and anatase due to the photocatalytic action of the TiO2 semiconductor particles. Degradation on alumina, hematite, and lepidocrocite, on the other hand, was slower and occurred primarily through the direct photodegradation of adsorbed DSBP. For hematite and lepidocrocite, light absorption by the oxide layer reduced the degradation rate by reducing the amount of light available to DSBP. Implications for the photodegradation reactions of environmental contaminants on soils are discussed.