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

Area of Science:

• Materials Science
• Catalysis
• Environmental Science

Background:

• Passive NOx adsorbers (PNAs) are crucial for mitigating cold-start NOx emissions from internal combustion engines.
• Conventional PNAs often degrade under hydrothermal aging due to zeolite framework collapse or palladium aggregation, reducing NOx adsorption capacity.
• Developing stable and efficient PNAs is essential for effective emission control.

Purpose of the Study:

• To develop a highly stable and efficient passive NOx adsorber for internal combustion engines.
• To investigate the structural and chemical factors influencing PNA performance under hydrothermal aging.
• To provide a general strategy for designing advanced zeolite-based adsorption systems.

Main Methods:

• Synthesis and characterization of a palladium-based RHO small-pore zeolite (Pd/RHO).
• Rigorous hydrothermal aging tests at 850 °C to assess material stability.
• Spectroscopic characterizations and density functional theory (DFT) calculations to elucidate active site formation and stability.
• Evaluation of low-temperature NOx adsorption capacity before and after aging.

Main Results:

• Pd/RHO demonstrated a nearly 100% increase in low-temperature NOx adsorption after hydrothermal aging, outperforming conventional Pd/CHA.
• The RHO zeolite framework, with its unique six-membered rings and aluminum pairs, promotes the formation of stable [Z2PdII] species.
• These stable species exhibit superior NOx storage capacity, water resistance, and hydrothermal stability compared to [ZPdIIOH] sites in CHA zeolites.
• DFT calculations confirmed the stability of [Z2PdII] species and the susceptibility of [ZPdIIOH] sites to water.

Conclusions:

• The unique RHO zeolite framework enables the formation of highly stable palladium species, significantly enhancing NOx adsorption and hydrothermal stability.
• Tailoring local framework geometry in zeolites is a viable strategy for designing robust and efficient PNA materials.
• This research offers a promising pathway for developing next-generation catalysts for automotive emission control.