(269g) Determining Siting Preference of Exchanged Fe Ions in Fe-SSZ-13 Zeolite through Density Functional Theory and Ab Initio Molecular Dynamics

Fe-exchanged zeolites are known to be catalytically active for various reactions, for example, N2O decomposition, paritial methane oxidation, selective catalytic reduction of NOx, etc. In spite of great efforts spent on understanding catalytic mechanisms of these reactions over Fe-zeolites, identification of available Fe species and their siting preference, even under ex situ conditions, is still under debate. Several factors complicate the site identification, including: (1) potential coexistence of Fe with different nuclearities, ranging from monomeric cations to oligomeric Fe complexes and Fe oxides, (2) coexistence of Fe with oxidation states (Fe2+ and Fe3+) and (3) uncertainties in interpretations of experimental characterization results. The available literature [1] suggests that in low-loaded SSZ-13 samples, each Fe cation charge-compensate single framework Al, and that these cations are present primarily as isolated and dimeric species.

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.

Reference

[1] 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)