Infrared Spectroscopic Analysis of the Adsorption of Pyridine Carboxylic Acids on Colloidal Ceria.

Surface adsorption of a homologous series of pyridine carboxylic acids on a hydrated colloidal cerium dioxide (ceria) film is characterized using the combination of experimental and computationally determined infrared (IR) spectra. Experimental analyses employ attenuated total reflectance (ATR) IR spectroscopy of deposited colloidal ceria thin films equilibrated with three pyridine carboxylic acids at pH 3.0, 5.5, and 8.5. The corresponding computational IR spectra for the energy-minimized intermediate and base forms of the pyridine carboxylic acids use density functional theory calculations at the B3LYP/6-311++G** level of theory. Solvent effects are modeled using both the COSMO implicit solvation model and the inclusion of explicit water molecules. Experimental IR spectra show that the adsorptive interactions between the pyridine carboxylic acids and ceria surface are due to the outer-sphere coordination of cerium ions in the films. Vibrational assignments based on combined experimental and computational results indicate that both pyridyl ring nitrogen and carboxylate functional groups account for the interaction of pyridine carboxylic acids at ceria surfaces. Experimentally determined Langmuir constants point to the intermediate form of picolinic acid (pyridine-2-carboxylic acid) as having the strongest adsorption to ceria compared to the other pyridine carboxylic acids investigated. The enhanced adsorption of picolinic acid is attributed to the adjacency of the protonated pyridyl nitrogen and the carboxylate group relative to nicotinic acid (pyridine-3-carboxylic acid) and isonicotinic acid (pyridine-4-carboxylic acid).