Adsorption and Formation of BaO Overlayers on gamma-Al2O3 Surfaces

Abstract

First-principles density functional theory slab calculations were used to investigate adsorption, clustering, and overlayer formation of BaO on gamma-Al2O3 surfaces. Multiple stable adsorption configurations were identified for adsorbed BaO monomers and (BaO)(2) dimers on both (100) and (110) surfaces of gamma-Al2O3. Adsorption of BaO and (BaO)(2) induces significant relaxation of these gamma-Al2O3 surfaces. At high BaO coverage, up to a 1:1 ratio of BaO units to surface Al atoms, the adsorbed BaO monomers condensed, organizing to form a buckled monolayer-like overlayer on the surface. An "aggregation energy" was used to characterize this clustering of adsorbed BaO on the surface. Our results show that the initial BaO adsorption configuration has a strong effect on clustering and overlayer formation. A weakly adsorbed BaO monomer will thermodynamically favor clustering over being isolated. On the fully dehydrated gamma-Al2O3(100) surface, the clustering of BaO was thermodynamically unfavorable until 4.26 BaO/nm(2) if the additional BaO was from the most stable monomer adsorption sites; that is, those present at low BaO loadings. In contrast, aggregation became favorable if the additional BaO was from less stable sites occupied by BaO at high loadings. On the fully dehydrated gamma-Al2O3(110) surface, the formation of a BaO dimer was found to have the highest energy cost. On the other hand, the presence of hydroxyls on the surface enhances the stability of the adsorbed BaO monomers. As such, isolated BaO islands, rather than a complete BaO overlayer, are expected on the hydroxylated gamma-Al2O3 surfaces, consistent with recent experimental observations