Structure and vibrational spectrum of formate and acetate adsorbed from aqueous solution onto the TiO2 rutile (110) surface

The adsorption of formate and acetate from aqueous solutions at pH 3-9 onto the TiO2 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 nu(as)-(COO) - nu(s)(COO) splitting (Deltanu(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-1V, 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.