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. Divalent Pd2+ cations upon elevated temperature (>800 ◦C)

calcination occupy sterically constrained cationic position in the zeolite framework, and become 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 as dehydration proceeds. Combined systematic in situ infra-red data

with X-ray diffraction Rietveld analyses reveal 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 behavior.