Abstract
The adsorption of carboxylic acids (formic, acetic, and pyruvic acid) from corresponding solutions in CH2Cl2 solvent on Al2O3 and TiO2 thin films has been studied by attenuated total reflection infrared spectroscopy. The metal-oxide films were vapor-deposited on a Ge internal reflection element, which was mounted into a specially designed flow cell. The system allowed in situ monitoring of the processes occurring at the solid-liquid interface. The metal-oxide films were characterized by X-ray photoelectron spectroscopy, ellipsometry, and atomic force microscopy. Formic acid and acetic acid adsorbed predominantly as bridging species on alumina surfaces. Adsorbed free acids were not observed under a flow of neat solvent. Based on the position of the νAS(COO) and of the keto-group stretching vibration of the pyruvate ion, pyruvic acid is proposed to coordinate to the Al2O3 surface in a monodentate fashion, whereas, on TiO2, a bidentate species is preferred. Comparison of the adsorption behavior on the vapor-deposited alumina film and on an α-Al2O3 layer deposited from a water suspension of the corresponding metal-oxide powder indicated that pyruvic acid adsorbs in a similar mode, irrespective of the metal-oxide deposition technique.
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