A Yolk-Shell Fe3O4/Pd@ZrO2 Catalyst with Space-Confined Effect for Enhancing 4-Nitrophenol Reduction.

Submicron catalysts with tailored 3D architectures offer unique opportunities to manipulate nanoscale mass transport and active site distribution, thereby enhancing catalytic performance. Herein, we constructed a yolk-shell Fe₃O₄@H-ZrO₂ carrier to provide a semi-open interstitial cavity. Subsequently, Pd nanoparticles were introduced into this cavity by a vacuum-assisted strategy, which resulted in precise confinement and uniform dispersion of Pd nanoparticles. This structural confinement improved the accessibility of the active site while inhibiting aggregation and surface migration. To systematically evaluate the confinement effect, we synthesized and compared three structurally distinct carriers: hollow (H-ZrO₂), core-shell (Fe₃O₄@ZrO₂), and yolk-shell (Fe₃O₄@H-ZrO₂). Among them, Fe₃O₄/Pd@H-ZrO₂ completely reduced 4-nitrophenol to 4-aminophenol within 7 min under ambient conditions. Post-catalytic characterizations (HRTEM, PXRD, FTIR) confirmed the excellent structural robustness and palladium retention. Magnetic separation-driven cycling further demonstrated the high reusability of the catalyst, with 96.2% of the activity retained after eight cycles. In addition, pH effects, common environmental anions, and humic acid interference were investigated to fully assess catalytic stability and environmental suitability. This work can generally construct space-confined catalysts and provide new perspectives on structure-performance relationships in environmental catalysis and beyond.