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 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 NOx reduction catalysis.
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films