Active Sites and Active Oxygen Species for Photocatalytic Epoxidation of Propene by Molecular Oxygen over TiO_2-SiO_2 Binary Oxides

摘要

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 active 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.