(269g) Determining Siting Preference of Exchanged Fe Ions in Fe-SSZ-13 Zeolite through Density Functional Theory and Ab Initio Molecular Dynamics
Here we present a theoretical approach to determine Fe siting preference in Fe-SSZ-13 zeolite using density functional theory (DFT) and ab Initio molecular dynamics (AIMD). Relative stabilities of exchanged Fe monomers in isolation and dimers ligated by arbitrary combinations of O and OH ligands at isolated Al and different paired Al sites are determined by construction of reactions that interconvert these species. AIMD and time-dependent DFT are combined to compute ensemble-averaged UV-visible spectra of different Fe species. We show that both Fe2+ and Fe3+ have strong tendency to form hydroxo-bridged dimers at paired Al sites, and OH-ligated Fe3+ monomer and dimers have qualitatively similar computed UV-visible spectra, suggesting previous UV-vis based assignments of isolated Fe3+ cation preference may need to be revisited. Through first-priciples thermodynamic analysis, we identify potential active sites by determining abundant Fe species at different Al sites (either isolated or paired) under conditions of catalyst activation for partial methane oxidation and other reactions. These findings help in better understanding of relevant active sites as well as redox mechanism in Fe-SSZ-13 catalysis.
 Gao, F., Zheng, Y., Kukkadapu, R.K., Wang, Y., Walter, E. D., Schwenzer, B., Szanyi, J. and Peden, C. H. G. ACS. Catal. 6, 5 (2016)