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.