Mechanism of Photooxidation of Alcohol over Nb₂O₅

Photooxidations of alcohols to carbonyl compounds proceed selectively at a low temperature over niobium oxide (Nb₂O₅) without organic solvents. Although Nb₂O₅ is not able to absorb light at >390 nm wavelengths, the photooixdation of 1-pentanol proceeded under irradiation up to ca. 480 nm. This observation indicates that the photoactivation mechanism of alcohol over Nb₂O₅ is different from the classical electron transfer mechanism found in semiconductor photocatalysis (the formation of an excited electron in the conduction band and the positive hole in the valence band). On the basis of FT-IR and ESR measurements, the following mechanism is proposed: Alcohol is adsorbed onto Nb₂O₅ as an alcoholate species in the dark. Alcoholate adsorbed on Nb₂O₅ is activated by transferring an electron to the conduction band reducing Nb⁵⁺ to Nb⁴⁺, and leaving a hole on the alcoholate. The formed alkenyl radical is converted to a carbonyl compound. The product is desorbed and then the reduced Nb⁴⁺ sites are reoxidized by the reaction with molecular oxygen. The rate-determining step of the photooxidation of alcohol over Nb₂O₅ is the process of desorption of the formed carbonyl compound.