Active Sites and Active Oxygen Species for Photocatalytic Epoxidation of Propene by Molecular Oxygen over TiO2−SiO2 Binary Oxides

TiO2−SiO2 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 TiO2−SiO2 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 [Ti3+−OL - ]* radical pair is formed by UV irradiation. The Ti3+ moiety reacts with O2 to form O2 -, which could not activate propene by itself. The OL - moiety, a hole center on lattice oxygen, reacts with O2 to form O3 -, and the O3 - reacts with propene to yield PO. It is first suggested that the O3 - is the electrophilic oxygen species effective for the epoxidation of propene. When the OL - 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 O3 - and high activity for the consecutive reaction of PO.