Reaction Mechanism and the Role of Copper in the Photooxidation of Alcohol over Cu/Nb2O5

• Published in: Chemphyschem 2011-10, Vol.12 (15), p.2823-2830
• Main Authors: Furukawa, Shinya, Ohno, Yasuhiro, Shishido, Tetsuya, Teramura, Kentaro, Tanaka, Tsunehiro
• Format: Article
• Language: English
• Subjects: alcohol oxidation; Catalysis; Chemistry; copper; Exact sciences and technology; General and physical chemistry; niobium oxide; photocatalysis; Photochemistry; Physical chemistry of induced reactions (with radiations, particles and ultrasonics); reaction mechanisms; Surface physical chemistry; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.201100388

Loading of a small amount of copper on Nb2O5 significantly enhances the activity of alcohol photooxidation without organic solvents. Alcohol is adsorbed on the Lewis acid site (NbV) to form an alkoxide species. Photogenerated holes and electrons on Cu/Nb2O5 are trapped by the adsorbed alkoxide and CuII species to form the alkoxide carbon radical and CuI species. The formed alkoxide carbon radical is converted to a carbonyl compound and then desorbed. Finally, the reduced CuI sites are reoxidized by reaction with O2. The alcohol photooxidation over Nb2O5 takes place under not only UV irradiation but also under visible light irradiation up to 450 nm, although the band gap of Nb2O5 is 390 nm (3.2 eV). DFT calculations reveal that 1) the surface donor level derived from the adsorbed alkoxide species is located in the forbidden band, 2) direct electron transition from the surface donor level to the conduction band takes place by absorbing a photon, 3) the excitation energy from surface donor level to the Nb 4d conduction band is lower than that from the O 2p valence band to Nb 4d. The kinetic study and FT/IR spectra suggest that CuI acts as an effective desorption site for the products. Based on these results, we conclude that copper functions as an effective redox promoter and desorption site for the product. Role playing: The photocatalytic activity of Nb2O5 in the oxidation of alcohol (see picture) is significantly increased by the addition of a small amount of Cu. A mechanistic study, carried out using several spectroscopic techniques, kinetic analysis and DFT calculations, reveals that copper is both a redox promoter and an effective desorption site for the product.