Effect of BaO Morphology on NOx Abatement: NO2 Interaction with Unsupported and γ-Al2O3-Supported BaO

Abstract

First-principles density functional theory calculations have been applied to study NO2 interaction with BaO in various morphologies, including extended BaO surfaces as well as unsupported and γ-Al2O3-supported BaO clusters. Adsorption of NO2 over BaO of different morphologies studied with the same method allowed a direct comparison of adsorption structures and energetics. In comparison with the flat BaO(100) surface, the presence of the low-coordinated sites on the small BaO clusters and at the edge of the stepped BaO(310) surface not only strengthens the binding of NO2 at these sites but also creates new NO2 binding modes. The adsorption of NO2 at the accessible edge oxygen sites on the BaO clusters and stepped BaO(310) surface led to the formation of a nitrate species with the new N−O (surface or cluster) bond length of 1.3−1.5 Å. Furthermore, BaO(310) created an adsorption configuration with two oxygen atoms of the adsorbed NO2 bridging the step-edge and terrace Ba sites. This adsorption configuration has an adsorption energy of 1.39 eV and cannot be formed on BaO(100). The nonstoichiometric clusters and defective surfaces with either O or Ba vacancies exhibit much stronger affinity toward NO2, up to 4.09 eV for NO2 adsorption at the edge oxygen site of Ba14O13. NO2 prefers interacting with the nucleophilic substrate (O deficient) through its O atoms in an O-down configuration. Consequently, the adsorbed NO2 molecule gains partial electronic charge from the substrate, forming a NO2δ− species. On the other hand, the NO2 interacts with the electrophilic substrate (Ba deficient) by sharing its lone pair electrons on the N atom with a surface O site which resulted in a NO3δ− species. Furthermore, the synergistic effect of γ-Al2O3 and BaO toward NO2 adsorption reported previously (Cheng, L.; Ge, Q. Surf. Sci. 2007, 601, L65) was confirmed with a larger γ-Al2O3 surface unit cell that isolates the supported BaO cluster from its periodic images. The synergistic effect was attributed to the enhanced basicity and acidity of the supported BaO cluster and the γ-Al2O3 surface, respectively.