Nanoconfinement of Pd Ensembles in Ceria's Bridging Hydroxyl Nests for Enhanced C-H Bond Activation.

Precise engineering of the metal-support interaction (MSI) is paramount for manipulating the geometric and electronic structures of supported metal catalysts, yet tailoring the local anchoring microenvironment at the atomic level remains a challenge. This study demonstrates how modulating the hydroxyl chemistry on CeO2 can direct the MSI to stabilize distinct Pd species─from single atoms to subnanometric PdOx ensembles. We show that conventional terminal hydroxyl groups on CeO2 favor the anchoring of Pd single atoms, whereas engineered bridging hydroxyl nests effectively confine undercoordinated PdOx clusters. This PdOx ensemble results in a moderate 4d orbital occupancy of Pd sites, which optimizes the σ-electron-acceptance and back-donation processes critical for C-H bond activation. Consequently, the PdOx ensemble catalyst achieves a dramatic decrease in methane combustion temperatures (T50 and T90 values are 340 and 402 °C, lowered by 175 and 226 °C, respectively) compared to the single-atom counterpart, alongside exceptional stability and water resistance under practical conditions. This work establishes the engineering of the support's hydroxyl microenvironment as a powerful strategy for designing highly efficient metal ensemble catalysts.