Cyclohexane photocatalytic oxidation with 18 O 2 over anatase TiO 2 was analyzed by in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, to gain insight in the mechanistic steps of formation of cyclohexanone and deactivating carboxylate species, respectively. Cyclohexane photo-oxidation on TiO 2 with increasing concentration of labeled Ti− 18 OH sites yielded cyclohexanone and surface deactivating species with an isotopic distribution reflecting the concentration of Ti− 18 OH sites on the surface. This finding suggests that oxygen incorporated in cyclohexanone, as well as in deactivating carbonates and carboxylates, originates from the catalyst surface, rather than from dissolved O 2 . Cyclohexyl hydroperoxide, observed spectroscopically, is a spectator species, rather than a commonly assumed reaction intermediate. The proposed Mars−van Krevelen cycle is completed by regeneration of surface sites by reaction with 18 O 2 .
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Almeida et al. (2011) studied this question.