Abstract. 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…