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

Passive NO_<i>x</i> adsorbers (PNAs) have emerged as a promising solution for mitigating cold-start NO_<i>x</i> emissions from internal combustion engines. Most PNAs lose their NO_<i>x</i> 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 [Z₂Pd^II] species. These species exhibit excellent NO_<i>x</i> storage capacity, water resistance, and hydrothermal stability, whereas the predominant [ZPd^IIOH] 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.