Titanium dioxide (TiO2) is widely used for applications in heterogeneous photocatalysis. We prepared nanocrystalline powders of the anatase as well as the rutile modification by high-energy ball milling of the coarse grained source materials for up to 4 h. The resulting average grain size was about 20 nm. The morphology of the powders was investigated with transmission electron microscopy, X-ray powder diffraction, and BET surface area determination. Measurements of the catalytic activity reveal a maximum as a function of the milling time at about 40 min. This maximum could be explained by a superposition of two counteracting effects. The first one is the increase of the specific surface area resulting in an increase of the catalytic activity, and the second one is a change of the electronic structure at the surface of the TiO2 particles corresponding to a reduction of the surface. The latter one was confirmed by light absorption experiments, X-ray photoelectron spectroscopy, and electron paramagnetic resonance spectroscopy.
In order to obtain a useful expression for the kinetics of the photocatalyzed total oxidation reaction of alkanes with oxygen and TiO 2 as the catalyst, the dependence of the reaction rate on the concentration of the starting material species, the absorbed irradiation intensity, the wavelength of the light and the temperature was investigated. The experiments were performed in a CSTR system. The rate law found for alkane oxidation differs from that obtained for olefin oxidation due to a different reaction mechanism. While a reversible catalyst deactivation process takes place in the latter case, there is no hint of such a process during alkane oxidation. The kinetic data lead to the conclusion that the formation of an alkane and a hydrogen-radical at a defect site of the catalyst is the first step in the total oxidation reaction of these species followed by the formation of a stronger adsorbed hydroperoxide species. The subsequent transformation of the hydroperoxide into further oxidized species mediated by the electron-hole pairs generated by the absorbed light on the semiconductor's surface was recognized to be the rate determining step. The reaction behavior for the investigated alkanes could be modeled using this concept with the exception of methane. Its rate law differs strongly from that found for ethane, propane as well as iso-butane.
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