Probing Catalytic Solid‐Liquid Interfaces by Attenuated Total Reflection Infrared Spectroscopy: Adsorption of Carboxylic Acids on Alumina and Titania

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 ν_AS(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.