Construction of Highly Active Pd–Ti3+ Sites in Defective Pd/TiO2 Catalysts for Efficient Hydrogenation of Styrene–Butadiene–Styrene

Defect-rich Pd/TiO2 catalysts are intensively adopted in heterogeneous hydrogenation reactions; however, the complexity of the defect structure makes it difficult to precisely identify which Pd-defect combination dominates the catalytic activity. Herein, defective TiO2 nanoflakes with tunable ratios of Vo to Ti3+ defects were synthesized and used to construct Pd–Vo and Pd–Ti3+ active sites after loading Pd to investigate the role of defects in regulating the structural and catalytic properties of defective Pd/TiO2 catalysts. Combining the experimental results and theoretical calculations, we reveal that both Vo and Ti3+ defects act as the electron donors for Pd and induce the strong metal–support interaction. When compared to the Vo defect, the Ti3+ defect behaves more significantly and donates more electrons, causing the Pd species on the catalysts to be better dispersed and more rich in electrons. These unique features endow the Pd–Ti3+ active centers with enhanced adsorption–activation ability toward C═C and H2 as well as reduced energy barrier of the rate-limiting step, thus improving the intrinsic activity. The Pd–Ti3+ site manifests a high turnover frequency of 348 h–1 and hydrogenation degree of 97% for hydrogenation of C═C in styrene–butadiene–styrene, which significantly outperforms the Pd–Vo site (254 h–1 and 78%) and Pd nanoparticle (217 h–1 and 53%). This work provides deep insight into the role of defects in regulating the properties of metal active sites, which can be used to guide the development of high-performance Pd/TiO2 catalysts for versatile applications.