Unusual water-assisted NO adsorption over Pd/FER calcined at high temperatures: The effect of cation migration

Moisture contained in vehicle exhaust gas normally degrades the capacity and efficiency of Pd ion-exchanged zeolites as NOx adsorbents by competitive adsorption on active sites. Here, we report a counterexample to this general proposition, in which moisture facilitates the storage of NO as a nitrosyl complex on hydrated Pd ions in high temperature calcined FER-type zeolites. The divalent Pd2+ cations upon elevated temperature (>800 °C) calcination occupy sterically constrained cationic position that render them inactive for the adsorption of probe molecules such as NO. These ‘hidden’ Pd ions, however, are accessible by NO when the zeolite is hydrated, but readily release NO at around 200°C as dehydration proceeds. By combining systematic in situ infra-red data with X-ray diffraction Rietveld analyses, we revealed that the high temperature-induced relocation of Pd ions to more stable cationic positions located near 6-membered ring of the ferrierite cage is responsible for this anomalous behavior. This finding constitutes a notable advance in understanding coordination chemistry of cations in zeolites.