Active Sites and Active Oxygen Species for Photocatalytic Epoxidation of Propene by Molecular Oxygen over TiO2−SiO2Binary Oxides

TiO 2 −SiO 2 binary oxides of low Ti content promoted photocatalytic epoxidation of propene by molecular oxygen. By catalytic runs and UV spectra, it was revealed that the isolated tetrahedral Ti species on the TiO 2 −SiO 2 samples are active for the photoepoxidation of propene, while the aggregated titanium oxide species are active mainly for the side reactions. By ESR and stoichiometric reaction tests of radical species, the following mechanism was proposed. Over the isolated tetrahedral Ti species, a [Ti 3+ −O L - ]* radical pair is formed by UV irradiation. The Ti 3+ moiety reacts with O 2 to form O 2 -, which could not activate propene by itself. The O L - moiety, a hole center on lattice oxygen, reacts with O 2 to form O 3 -, and the O 3 - reacts with propene to yield PO. It is first suggested that the O 3 - is the electrophilic oxygen species effective for the epoxidation of propene. When the O L - moiety reacts with propene, acrolein or ethanal is produced through H abstraction or CC bond fission. The lower selectivity of the aggregated titanium oxide species was attributed to the lower stability of the O 3 - and high activity for the consecutive reaction of PO.