Understanding the nature of surface nitrates in BaO/γ-Al2O3 NOx storage materials: A combined experimental and theoretical study

A combined experimental-theory approach was applied to unambiguously determine the nature of “surface nitrates” in BaO/γ-Al2O3 NOx storage materials. High resolution scanning transmission electron microscopy images reveal that at a low BaO coverage of 2 wt% on γ-Al2O3 monomeric BaO units are present almost exclusively. These molecularly dispersed BaO units are concentrated on the (100) facets of the alumina crystallites, while other facets remain practically BaO-free The results of the density functional theory calculations predicted adsorption geometries for the (BaO)x (image and 2) units. The energetically most favorable BaO monomer and dimer units anchor to pentacoordinate Al3+ sites on the (100) facets of γ-Al2O3 in such geometries that maximize their interactions with the support surface. The calculated vibrational frequencies of the energetically most favorable nitrate species formed upon the interaction of NO2 with the monomeric and dimeric BaO units agree remarkably well with those observed experimentally by infrared spectroscopy and identified as “surface nitrates.”