Adsorption-competition driven volcano catalysis in TS-1: the critical role of Ti3+-VO defect concentration tailored by hydrogen reduction

A novel hydrogen-mediated thermal reduction strategy is employed to introduce Ti³⁺-V_O defects into TS-1 without foreign element contamination. Defect concentrations are precisely tuned by adjusting reduction conditions, and their impact on propylene epoxidation with hydrogen and oxygen (HOPO) was investigated. The formation of Ti³⁺ species and oxygen vacancies is confirmed by XPS and EPR. Meanwhile, XRD, FT-IR, and N₂-BET indicated no significant alterations to the MFI framework and porosity of TS-1. In-situ DRIFTS and kinetics experiments revealed that Ti³⁺-V_O defects simultaneously promoted the adsorption of both propylene and PO, with propylene adsorption dominating at low defect concentrations and PO adsorption dominating at high concentrations, causing a volcano-shaped relationship between catalytic activity and Ti³⁺-V_O defect concentration. Consequently, TS-1 with optimal defect concentration can significantly enhance propylene conversion, PO selectivity, and PO formation rate in HOPO reaction. The findings highlight the importance of precise defect engineering for optimizing the catalytic activity of TS-1.