In Situ UV−vis and EPR Study on the Formation of Hydroperoxide Species during Direct Gas Phase Propylene Epoxidation over Au/Ti-SiO2Catalyst

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Abstract
In recent years, there have been great experimental and theoretical advances in the understanding of the epoxidation of propylene by O2 and H2 over Au supported on titanium-containing oxidic supports; however, thus far spectroscopic evidence of reacting species for proposed mechanisms has been lacking. Hydroperoxide species have been postulated as an intermediate responsible for the epoxidation of propylene with O2 and H2. In order to obtain direct evidence for the different type of active oxygen species, in situ UV−vis and EPR measurements were carried out during the epoxidation of propylene with O2 and H2 over a Au/Ti−SiO2 (Ti/Si = 3:100) catalyst. It was determined that the adsorbed species of oxygen (O2-) resided on Au, more likely at a perimeter site, and it led to the formation of titanium hydroperoxo species. These results support the possible mechanism of formation of these hydroperoxo species via H2O2 produced from O2 and H2 adsorbed on the Au surfaces.

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