Cation movements during dehydration and NO₂ desorption in a Ba-Y,FAU zeolite: An in situ time-resolved X-ray diffraction study

Synchrotron-based in situ time-resolved X-ray diffraction and Rietveld analysis were used to probe the interactions between Ba-Y,FAU zeolite frameworks and H₂O or NO₂ molecules. These results provide information about the migration of the Ba²⁺ cations in the zeolite framework during dehydration and during NO₂ adsorption/desorption processes in a water-free zeolite. In the hydrated structure, water molecules form four double rings of hexagonal icelike clusters [(H₂O) ₆] in the 12 ring openings of the supercage. These water rings interact with the cations and the zeolite framework through four cation/water clusters centered over the four six-membered rings of the supercage (site II). Interpenetrating tetrahedral water clusters [(H₂O)₄] and tetrahedral Ba cation clusters are observed in the sodalite cage. Consistent with the reported FT-IR results, three different ionic NO_x species (NO⁺, NO⁺-NO₂, and NO₃^-) are observed following NO₂ adsorption by the dehydrated Ba-Y,FAU zeolite. The structure of the water and the NO_x species are correlated with the interactions between the adsorbates, the cations, and the framework. The population of Ba²⁺ ions at different cationic positions strongly depends on the amount of bound water or NO_x species. Both dehydration and NO₂ adsorption/desorption result in facile migration of Ba²⁺ ions among the different cationic positions. Data obtained in this work have provided direct evidence for the Ba ²⁺ cation migration to accommodate the binding of gas molecules. This important feature may play a pivotal role in the strong binding of NO ₂ to Ba-Y,FAU zeolite, a prerequisite for high catalytic activity in lean NO_x reduction catalysis.