High-Silica RHO Zeolite-Stabilized Pd Ions Withstanding Extreme High-Temperature Hydrothermal Aging for Passive NO Adsorption

Passive NOx adsorbers (PNAs) have emerged as a promising solution for mitigating cold-start NOx emissions from internal combustion engines. Most PNAs lose their NOx adsorption capacity after stringent hydrothermal aging tests due to either the collapse of the zeolite framework or Pd aggregation. Herein, we report a Pd-based RHO small-pore zeolite (Pd/RHO) that exhibits a nearly 100% increase in low-temperature NO adsorption after rigorous hydrothermal aging at 850 °C, in stark contrast to the severe degradation observed in conventional Pd/CHA. Combined spectroscopic characterizations and density functional theory (DFT) calculations reveal that the unique RHO framework, featuring abundant six-membered rings with aluminum pairs, facilitates the formation of stable [Z2PdII] species. These species exhibit excellent NOx storage capacity, water resistance, and hydrothermal stability, whereas the predominant [ZPdIIOH] sites in CHA zeolites are susceptible to H2O and prone to accumulation during hydrothermal aging. This work provides a general strategy for designing highly stable and efficient zeolite-based adsorption systems by precisely tailoring the local framework geometry.