Insight into the Role of Unsaturated Coordination O_2c-Ti_5c-O_2c Sites on Selective Glycerol Oxidation over AuPt/TiO₂ Catalysts

In this work, we synthesized a series of TiO₂ species enclosed with different terminated facets and investigated the catalytic performance on glycerol oxidation catalysts over Au₁Pt₃/TiO₂ under base-free conditions. To our surprise, the Au₁Pt₃/TiO₂ catalyst presented a support-facet-dependent catalytic performance. Different TiO₂ crystal planes apparently affect the oxidation product distribution, especially the selectivity of glyceraldehyde and glyceric acid, but have inconspicuous influence on the activity of glycerol oxidation. Primarily, the coordination unsaturated sites O_2c-Ti_5c-O_2c of the (001) facet highly dissociated the CO bond of aldehyde group in a bidentate form, as characterized by in situ CO₂ and formaldehyde Fourier transform infrared spectroscopy. In another aspect, Au^δ+ species were generated on the interface between AuPt nanoparticles and the (001) facet of TiO₂ due to the strong interaction, which facilities the insertion of oxygen species during the process of oxidation to glyceric acid. With the aid of bidentate intermediates and Ti_5c-O_2c-Au^δ+ active sites, the ability of further oxidation of glyceraldehyde is enhanced due to the decreased active energy of glyceraldehyde over Au₁Pt₃/TiO₂-001, as revealed by the kinetic studies of glyceraldehyde oxidation. These findings provide an alternative perspective and feasible approach to manipulate the product distribution in multistep continuous oxidation reactions.