Catalyst size and morphological effects on the interaction of NO2 with BaO/γ-Al2O3 materials

The capability of NOx storage on the supported BaO catalyst largely depends on the Ba loading. With different Ba loadings, the supported BaO component exposes various phases ranging from well-dispersed nanoclusters to large crystalline particles on the oxide support materials. In order to better understand size and morphological effects on NOx storage over γ-Al2O3-supported BaO materials, the adsorption structures and energetics of single NO2 molecule, as well as NOx + NOy (NO2 + NO2, NO + NO3 and NO2 + NO3) pairs on the BaO/γ-Al2O3(1 0 0), (BaO)2/γ-Al2O3(1 0 0), and (BaO)5/γ-Al2O3(1 0 0) surfaces were investigated using first-principles density functional theory calculations. A single NO2 molecule prefers to adsorb at basic OBa site forming anionic nitrate species. Upon adsorption, a charge redistribution in the supported (BaO)n clusters occurs. Synergistic effects due to the interaction of NO2 with both the (BaO)n clusters and the γ-Al2O3(1 0 0) support enhance the stability of adsorbed NO2. The interaction between NO2 and the (BaO)n/γ-Al2O3(1 0 0) catalysts was found to be markedly affected by the sizes and morphologies of the supported (BaO)n clusters. The adsorption energy of NO2 increases from −0.98 eV on the BaO/γ-Al2O3(1 0 0) surface to −3.01 eV on (BaO)5/γ–Al2O3(1 0 0). NO2 adsorption on (BaO)2 clusters in a parallel configuration on the γ-Al2O3(1 0 0) surface is more stable than on dimers oriented in a perpendicular fashion. Similar to the bulk BaO(1 0 0) surface, a supported (BaO)n cluster-mediated electron transfer induces cooperative effects that dramatically increase the total adsorption energy of NOx + NOy pairs on the (BaO)n/γ-Al2O3(1 0 0) surfaces. Following the widely accepted NO2 storage mechanism of BaO + 3NO2(g) → Ba(NO3)2 + NO(g), our thermodynamic analysis indicates that the largest energy gain for this overall process of NOx uptake is obtained on the amorphous monolayer-like (BaO)5/γ-Al2O3(1 0 0) surface. This suggests that γ-Al2O3-supported BaO materials with ∼6–12 wt% loadings may provide optimum structures for NOx storage.