Abstract
Metal-exchanged zeolites are common catalysts and adsorbents, but the relationship between their macroscopic composition (Si:Al and M:Al ratios) and microscopic details of exchange site composition and reactivity are difficult to infer. Here we address this general problem for Fe exchange in an SSZ-13 zeolite. We report periodic supercell density functional theory (DFT) calculations for the structures and energies of candidate Fe-exchange sites, including monomeric and dimeric Fe species with formal oxidation states ranging from 2+ to 5+ and charge-compensated by arbitrary combinations of framework Al, oxo, and hydroxyl ligands plus H2O adsorbates. We show that the chemical identity of an Fe-exchange site depends strongly on the number and proximity of framework Al and, through first-principles thermodynamics models, that these sites evolve in distinct ways as a function of external treatment conditions. By placing the results on a common energy reference and combining with simulated Al distributions, we ...
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