Free-radical pathways in the decomposition of ketones over polycrystalline TiO2:: The role of organoperoxy radicals

The oxidative decomposition of various ketones (including acetone, 2-butane, 4-heptanone, cyclopentanone and cyclohexenone) over dehydrated TiO2 (P25) powder is investigated by electron paramagnetic (EPR) spectroscopy. For the first time, a series of thermally unstable radical intermediates are observed both on the activated and reduced TiO2 surface, depending on the adopted experimental conditions. These radical intermediates are identified as organoperoxy-based species of general formula ROO. and RCO3.. They are formed by reaction of photo-generated charge carriers (either trapped electrons or trapped holes) with the adsorbed ketones in the presence of molecular oxygen. The organoperoxy intermediates are thermally unstable and decompose at temperatures in the region of 180-250 K. This work demonstrates that free-radical pathways involving both organoperoxy and superoxide radicals can be responsible for the thermal- and photodecomposition of ketones over polycrystalline TiO2 (P25).