Improving the hydrothermal stability of Pd/SSZ-13 for low-temperature NO adsorption: promotional effect of the Mg 2+ co-cation

• Published in: Reaction chemistry & engineering Vol. 8; no. 6; pp. 1312 - 1323
• Main Authors: Cai, Jinhuang, Zhao, Huawang, Li, Xiang, Jing, Guohua, Schwank, Johannes W.
• Format: Journal Article
• Language: English
• Published: 30.05.2023
• ISSN: 2058-9883; 2058-9883
• DOI: 10.1039/D3RE00024A

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 Pd 2+ 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. Mg 2+ 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 Pd 2+ 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 Mg 2+ 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 27 Al NMR and Co 2+ 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 Pd 2+ ion distribution after HTA: Z 2 Pd 2+ 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.