Photocatalytic oxidation of cyclohexane over TiO₂ nanoparticles by molecular oxygen under mild conditions

The structure, physical characteristics and photocatalytic selective oxidation properties of nanometer-size TiO₂ particles produced by a sol-gel method were studied by X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR) and photocatalytic selective oxidation measurements. Analysis of the XRD results shows that sol-gel-produced TiO₂ nanoparticles have the anatase structure at annealing temperatures ≤973 K, that the rutile structure begins to emerge at annealing temperatures ≥973 K and the particles have the pure rutile structure at 1023 K. DRS indicates that the obtained TiO₂ nanoparticles exhibit a blue shift with decreasing crystallite size. Analysis of the XPS results shows that the TiO₂ nanoparticles have a lot of oxygen vacancies. The EPR spectrum of TiO₂ at 77 K is composed of a strong isotropic EPR Surface-Ti³⁺ signal(I) at g = 1.926 and a weak broad Bulk-Ti³⁺ signal (II) at g = 1.987. Quantitative EPR indicates that both signals show a size and temperature dependence. Photocatalytic oxidation of cyclohexane into cyclohexanol with high selectivity and activity has been obtained by activation of molecular oxygen over sol-gel-produced TiO₂ nanoparticles under mild conditions in dry solvent, which reveals that the quantum size effect and surface state effect of nanoparticles are key points for governing the selective photocatalytic reaction. The photocatalytic oxidation mechanism under dry solvent is different from that in aqueous solutions. © 2003 Society of Chemical Industry.