Periodic DFT Characterization of NOx Adsorption in Cu-Exchanged SSZ-13 Zeolite Catalysts

We report a density functional theory (generalized gradient approximation (GGA)) and hybrid-exchange (HSE06 and B3LYP) characterization of NOx adsorption on Cu-exchanged SSZ-13, in particular to compare predictions of computational models, to understand the relative stability of adsorbates under reaction conditions, and to aid in the interpretation of experiments. We consider Cu exchanged near a single Al tetrahedral site without and with a proximal Brønsted acid. Computed structures, vibrational spectra, and oxidation states are consistent with prior observations. The GGA generally overpredicts adsorbate binding energies to extents that are species specific. The tendency for a Cu site to coadsorb two species is in particular exaggerated within the GGA. Proximal acid sites generally have a small effect on adsorbate binding. Computed vibrational spectra support the assignment of various experimentally observed features, including the assignment of an experimentally observed feature near 2100 cm–1 to an Al site charge-compensated by NO. First-principles thermodynamic analysis shows that Cu-bound nitrite and nitrate are thermodynamically preferred under oxidizing conditions in the presence of NO or NO2.