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Anatase-catalyzed photo-oxidation of cyclohexane was analyzed by in situ ATR-FTIR spectroscopy. A set of seven UV-LEDs (375 nm), with a photon flux of 9 × 10 -9 Einstein‚cm -2 ‚s -1 (at the catalyst surface) was used to initiate the photoreaction. Surface-adsorbed cyclohexanone and water are the primary products of the photocatalytic reaction, formed with a photonic efficiency of 0.5 mmol‚Einstein -1 , through a cyclohexylhydroperoxide intermediate. Desorbed cyclohexanone and surface carboxylates and carbonates become dominant in the subsequent stages of the reaction, leading to deactivation of the catalyst. The carboxylates and carbonates are most likely formed through nonselective peroxide oxidation and consecutive oxidation of adsorbed cyclohexanone by hydroxyl radicals. In the photocatalytic oxidation of D 12 -cyclohexane, D 10cyclohexanone (in the adsorbed state and dissolved in D 12 -cyclohexane) was formed at rates comparable to those of cyclohexanone. The absence of a kinetic isotope effect suggests that the reaction is not limited by the activation of cyclohexane but rather by the activation of oxygen. Desorbed D 10 -cyclohexanone was observed at earlier stages and in higher quantities as compared to desorbed cyclohexanone. This is tentatively explained by a higher water content of the applied D 12 -cyclohexane compared to cyclohexane, inducing cyclohexanone desorption.

A systematic study is presented on the effect of crystallite size of Anatase (Hombikat, Sachtleben), varied by calcination at different temperatures up to 800 degrees C, on photocatalytic activity in cyclohexane selective oxidation. Two different reactors were used to test the materials: a top illumination reactor and an in situ ATR-FTIR cell. Properties such as crystallinity and associated availability of holes and electrons for surface reactions, as well as the amount of surface OH-groups, are shown to have a significant influence on TiO(2) activity, (surface) selectivity, and stability. Upon increasing the crystallite size, productivity (g(-1)(catalyst)) decreases, while (i) the TOF (moles of cyclohexanone formed per minute per OH-site), (ii) the rate of cyclohexanone desorption, (iii) catalytic site stability, and (iv) the cyclohexanol/cyclohexanone ratio increase. The results are discussed on the basis of a reaction scheme, and a simple reaction rate equation.

Cyclohexane photocatalytic oxidation with 18O2 over anatase TiO2 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 TiO2 with increasing concentration of labeled Ti−18OH sites yielded cyclohexanone and surface deactivating species with an isotopic distribution reflecting the concentration of Ti−18OH 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 O2. 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 18O2.