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 NO storage over gamma-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/gamma-Al2O3(1 0 0), (BaO)(2)/gamma-Al2O3(1 00), and (BaO)(S)/gamma-Al2O3(1 0 0) surfaces were investigated using first-principles density functional theory calculations. A single NO2 molecule prefers to adsorb at basic O-Ba 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 gamma-Al2O3(1 0 0) support enhance the stability of adsorbed NO2. The interaction between NO2 and the (BaO)(n)/gamma-Al2O3(1 0 0) catalysts was found to be markedly affected by the sizes and morphologies of the supported (BaO),, clusters. The adsorption energy of NO2 increases from -0.98 eV on the BaO/gamma-Al2O3(1 0 0) surface to -3.01 eV on (BaO)(5)/-Al2O3(1 00). NO2 adsorption on (BaO)(2) clusters in a parallel configuration on the gamma-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)/gamma-Al2O3(1 0 0) surfaces. Following the widely accepted NO2 storage mechanism of BaO + 3NO(2)(g) -> Ba(NO3)(2) + NO(g), our thermodynamic analysis indicates that the largest energy gain for this overall process of NO uptake is obtained on the amorphous monolayer-like (BaO)(5)/gamma-Al2O3(1 0 0) surface. This suggests that gamma-Al2O3-supported BaO materials with similar to 6-12 wt% loadings may provide optimum structures for NOx storage. (C) 2010 Elsevier B.V. All rights reserved