Adsorption of oxalate on anatase (100) and rutile (110) surfaces in aqueous systems: experimental results vs. theoretical predictions.

A combined experimental and theoretical study of the adsorption of oxalic acid from the aqueous phase at the surface of anatase nanoparticles has been performed. The interfaces were investigated by ATR-FTIR measurements and quantum-chemical calculations using the semiempirical method MSINDO. The vibration spectra of the most stable surface complexes have been calculated and used for the interpretation of experimental results. The theoretical studies have been done using the anatase (100) surface to model the adsorption of oxalic acid and water. The effect of interaction of water and oxalic acid on the adsorption mechanism and the vibration spectra was taken into account in the theoretical models. Inclusion of solvation effects was found crucial to determine the type of denticity and structure of adsorbed complexes. By comparison of experimental data and theoretical calculations the most likely surface species and the effects of hydration in their relative stabilities were determined. The present results are compared to previous studies preformed also by combination of experimental and theoretical calculations of analogous systems using nanoparticulate rutile [C. B. Mendive et al., Phys. Chem. Chem. Phys., 2008, 10, 1960, ref. 1]. Differences between surface complexes on anatase and rutile lie mainly on the denticity type. Whilst in the case of rutile the most stable species consist of two bidentate surface complexes followed in third place by a monodentated form, anatase allows the formation of four species in which the stability order is reversed with respect to the denticity type. In the case of anatase, the main contributors to the surface speciation are two monodentate species differing in the position of the H atom within the molecule (being more stable when it is placed in the O-C-O moiety not bound to the surface); and two bidentate species, one deprotonated and one monoprotonated, in which the C-C bond was parallel or perpendicular to the TiO2 surface, respectively.