Reaction Mechanism of Selective Photooxidation of Hydrocarbons over Nb2O5

Selective aerobic photooxidation of hydrocarbons to oxidized products, mainly ketones, was conducted over Nb2O5 in the absence of solvent. For the oxidation of cyclohexane and ethylbenzene, Nb2O5 shows significantly higher selectivity to partial oxidation products than does TiO2. In the oxidation of cyclohexane, Nb2O5 gave a much higher ketone/alcohol ratio than TiO2. On the basis of spectroscopic and kinetic studies, the following mechanism is proposed. Both hydrocarbon and dioxygen adsorb on Nb2O5. The C–H bond of the hydrocarbon is activated to produce an alkyl radical by a photogenerated positive hole, and adsorbed oxygen is reduced by a photogenerated electron. The alkyl radical reacts with superoxide to produce a hydroperoxide. Then, the hydroperoxide is converted to a ketone, which subsequently desorbs from Nb2O5. This proposed mechanism is a typical Langmuir–Hinshelwood mechanism, including the formation of the hydroperoxide species as an intermediate. A linear correlation between logarithms of the reaction rates and bond dissociation enthalpies of several hydrocarbons indicates that the rate-determining step is C–H bond activation. An electron spin resonance (ESR) study suggests that the higher selectivity of Nb2O5 for partial oxidation products than TiO2 is due to the absence of O3–, which is highly active for the complete oxidation of hydrocarbons.