Introducing Ni co-cations to simultaneously maintain a high NOx uptake capacity and improve the hydrothermal stability of Pd/SSZ-13 for low-temperature NO adsorption

Abstract

The hydrothermal stability of Pd/SSZ-13 needs to be improved, particularly when subjected to hydrothermal aging (HTA) at temperatures exceeding 850 °C. Generally, incorporating co-cations in coordination with thermally stable Al pair sites enhances the Pd/SSZ-13 stability. However, this approach significantly reduces the NO storage capacity of fresh samples due to competition between co-cations and Pd2+ ions, leading to the sintering Pd2+ to PdO. In this study, we demonstrate that the “smart” behavior of Ni2+ breaks this activity-hydrothermal stability trade-off. We discovered that in the as-prepared Pd/Ni/SSZ-13 sample, Ni2+ ions predominantly occupied single Al sites as Z[Ni(OH)]+. The abundant ion-exchange sites (with single Al sites accounting for 70 % of the total framework Al) easily accommodated both Pd2+ and Ni2+ ions, mitigating cation competition. The presence of Ni2+ only marginally reduced the NOx storage capacity of Pd/SSZ-13, with the NOx/Pd ratio decreasing from 0.78 to 0.70. During HTA at 850 °C, Z[Ni(OH)]+ gradually transformed into the more thermally stable Z2Ni2+ coordinated to Al pair sites, effectively limiting dealumination. This well-preserved structure maintained a larger number of ion-exchange sites to accommodate Pd2+ ions while simultaneously facilitating the PdO hydrolysis to Pd2+ ions on Brønsted acid sites in Pd/Ni/SSZ-13 during HTA. Consequently, the NOx storage capacity increased (the NOx/Pd ratio rose from 0.70 for the fresh sample to 0.75 for the aged sample). In contrast, undoped Pd/SSZ-13 experienced nearly complete deactivation after HTA (NOx/Pd = 0.2). These findings offer valuable insights for rationally designing Pd/SSZ-13 materials that exhibit both high NO uptake capacity and robust hydrothermal stability.