Au Core-Pd Shell Bimetallic Nanoparticles Immobilized within Hyper-Cross-Linked Polystyrene for Mechanistic Study of Suzuki Cross-Coupling: Homogeneous or Heterogeneous Catalysis?

The synthesis of catalytically active bimetallic Au-Pd nanoparticles stabilized in hyper-cross-linked polystyrene (HPS) for Suzuki cross-coupling of 4-bromoanisole (4-BrAn) and phenylboronic acid is presented. The core-shell structure with a thin (less than 1 nm) Pd shell and a Au core was proven by X-ray diffraction and high-angle annular dark-field scanning transmission electron microscopy combined with energy-dispersive X-ray spectroscopy. More than a 2-fold increase in 4-BrAn conversion was found in comparison with monometallic Pd/HPS. Moreover, when the Suzuki reaction was carried out under visible-light irradiation, the product yield further increased by about 1.3 times (from 56.1% up to 73.7%). This effect was assigned to a local surface plasmon resonance arising in the Au core that allowed electron transfer to the extremely thin Pd layer due to intimate contact with gold. The results suggest that the rate-limiting step of the catalytic cycle takes place on the surface of the Pd shell, serving as evidence of the heterogeneous catalysis nature.