Cation Movements during Dehydration and NO2 Desorption in a Ba-Y,FAU Zeolite: An in Situ Time-Resolved X-ray Diffraction Study

Abstract

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 NOx 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 NOx 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 NOx 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 Ba2+ cation migration to accommodate the binding of gas molecules. This important feature may play a pivotal role in the strong binding of NO2 to Ba-Y,FAU zeolite, a prerequisite for high catalytic activity in lean NOx reduction catalysis.