Elucidating the Structural Origin of Photocatalytic Activity in High-Ti-Content TS-1 Zeolite

Titanium silicate zeolite TS-1 integrates the photocatalytic activity of TiO2 with the shape selectivity of MFI zeolite and is widely used in shape-selective catalytic oxidation processes with excellent hydrothermal stability. However, insufficient knowledge on the microscopic structure and photocatalytic active sites hinders the further application of TS-1. In this work, we determined a reliable composite structure of high-Ti-content TS-1, consisting of isomorphous Ti substitution and extra-framework Ti species (EFTi), and systematically investigated the photocatalytic activity in terms of photohole trapping, water adsorption and activation, and ·OH radical production, via extensive ab-initio molecular dynamics (AIMD) simulations and density functional theory calculations with on-site Coulomb interaction correction (DFT+U). Density of states analysis revealed that, although framework Ti substitution can reduce the TS-1 bandgap to a minimum of ∼3.5 eV (∼5.0 eV from HSE06), EFTi clusters significantly introduce a narrow bandgap of ∼2.4 eV (∼3.6 eV from HSE06) with more prominent band edge levels. Accordingly, lattice oxygen atoms in EFTi clusters show a stronger hole trapping capacity than those in the TS-1 framework, presenting a linear relationship with the center position of the occupied O 2p band. Moreover, EFTi clusters also demonstrate enhanced activities in water adsorption and dissociation as well as ·OH generation compared to framework Ti sites, manifesting them to be the dominate photocatalytic active component in high-Ti-content TS-1. Further inspired by the superior photoactivity of the overcoordinated Si5c-O-Ti sites at the EFTi@framework interface, we theoretically demonstrated that a simple Si substitution strategy can significantly boost the photocatalytic activity of TiO2, potentially providing a general guideline for designing improved photocatalysts.