Cyclohexane selective photocatalytic oxidation by anatase TiO2: influence of particle size and crystallinity

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.