Improving the hydrothermal stability of Pd/SSZ-13 for low-temperature NO adsorption: promotional effect of the Mg2+ co-cation

Abstract

Insufficient hydrothermal stability is an issue that restricts application of Pd/SSZ-13 for low-temperature NO adsorption from vehicle emissions. Pd/SSZ-13 suffers from severe dealumination upon hydrothermal aging (HTA) at 850 °C, resulting in the aggregation of NO trapping active Pd²⁺ ions into inactive PdO particles, as well as a significant decline in NO_x storage capacity. Mg was introduced to improve the stability of Pd/SSZ-13. Mg²⁺ ions, anchoring preferentially to the Al pair sites, show strong affinity to the framework of SSZ-13 even upon HTA at 850 °C, which reduces the dealumination. The preserved structure retains Pd²⁺ and contributes to the high NO storage capacity of HTA Mg/Pd/SSZ-13. Specifically, the PdO species, formed due to detachment of Al pair sites that are unoccupied by Mg²⁺ ions, disintegrate into PdO monomers via the Ostwald ripening process, which are subsequently captured and reacted with anionic exchange sites (framework Al) in the zeolite support. The ²⁷Al NMR and Co²⁺ ion-exchange results indicate that a large amount of single Al sites remains in Mg/Pd/SSZ-13 after HTA, so PdO monomers are dispersed on these sites to generate Z[Pd(OH)]⁺ (ZH⁺ + PdO = Z[Pd(OH)]⁺). This results in significant changes of Pd²⁺ ion distribution after HTA: Z₂Pd²⁺ decreases and Z[Pd(OH)]⁺ increases. Significantly, experimental and DFT results reveal the advantage of divalent Mg co-cations that occupy the Al pair sites on improving the stability of Pd/SSZ-13.