Adsorption and Photoinduced Decomposition of Acetone and Acetic Acid on Anatase, Brookite, and Rutile TiO2 Nanoparticles

A comparative study of the adsorption and photoinduced degradation (PID) of acetone and acetic acid on thin films of anatase, brookite, and rutile TiO2 nanoparticles is presented. The materials were thoroughly characterized by a wide range of methods, including X-ray diffraction, transmission electron microscopy, and Raman and UV-vis spectroscopies. In situ FTIR transmission spectroscopy was used to follow adsorption and PID reactions. Molecular adsorption of acetone and acetic acid is observed on anatase and brookite, whereas significant dissociation occurs on rutile. It is inferred that adsorbate surface interaction increases in the order anatase < brookite < rutile, favoring formation of bridge-bonded species on rutile (acetate and formate). Illumination with simulated solar light readily dissociates acetic acid and acetone on all TiO2 samples and produces polymorph-specific intermediate surface species, including acetate, formate, carbonate, and water. PID of surface coordinated acetate is rate determining for complete mineralization of acetic acid and prevents further photooxidation on rutile. On anatase and brookite surfaces, acetate formation is suppressed upon photooxidation of acetone, whereas on rutile, acetate readily forms. On anatase, intermediate species form, which are not observed on either brookite or rutile, suggesting different reaction pathways for the different TiO2 polymorphs. Accurate quantum yield measurements were performed. The quantum yield for PID of acetone is larger for brookite than for anatase and much larger than for rutile. In contrast, the quantum yield for PID of acetate is lower for brookite than for anatase, whereas PID of acetate does not occur on rutile under our experimental conditions. The results are discussed in terms of a balance of strong adsorbate surface interactions, moderate bonding of intermediate PID surface species, and efficient surface adsorbate charge transfer of photogenerated electrons and holes.