Characterization of surface and bulk nitrates of γ-Al2O3–supported alkaline earth oxides using density functional theory

“Surface” and “bulk” nitrates formed on a series of alkaline earth oxides (AEOs), AE(NO3)2, were investigated using first-principles density functional theory calculations. The formation of these surface and bulk nitrates was modeled by the adsorption of NO2 + NO3 pairs on γ-Al2O3-supported monomeric AEOs (MgO, CaO, SrO, and BaO) and on the extended AEO(001) surfaces, respectively. The calculated vibrational frequencies of the surface and bulk nitrates based on our proposed models are in good agreement with experimental measurements of AEO/γ-Al2O3 materials after prolonged NO2 exposure. This indicates that experimentally observed “surface” nitrates are most likely formed with isolated two-dimensional (including monomeric) AEO clusters on the γ-Al2O3 substrate, while the “bulk” nitrates are formed on exposed (including (001)) surfaces (and likely in the bulk as well) of large three-dimensional AEO particles supported on the γ-Al2O3 substrate. Also in line with the experiments, our calculations show that the low and high frequency components of the vibrations for both surface and bulk nitrates are systematically red-shifted with the increasing basicity and cationic size of the AEOs. The adsorption strengths of NO2 + NO3 pairs are nearly the same for the series of alumina-supported monomeric AEOs, while the adsorption strengths of NO2 + NO3 pairs on the AEO surfaces increase in the order of MgO < CaO < SrO ∼ BaO. Compared to the NO2 + NO3 pair that only interacts with monomeric AEOs, the stability of NO2 + NO3 pairs which interact with both the monomeric AEO and the γ-Al2O3 substrate is enhanced by about 0.5 eV.