Density Functional/All-Electron Basis Set Slab Model Calculations of the Adsorption/Dissociation Mechanisms of Water on α-Al2O3(0001) Surface

The microscopic reaction mechanism for the water adsorption/dissociation processes on the alpha-Al2O3(0001) surface was calculated using density functional theory with the all-electron triple numerical polarized basis sets. Both unit-cell and 2 x 2 supercell slab models were employed to investigate the coverage-dependent hydroxylation of the surface. Geometries of the molecular adsorbed intermediates, transition states, and the hydroxylated products were fully optimized, and the energetic reaction routes were clarified. The hydroxylation occurs predominantly via the low-barrier 1,4-hydrogen migration path, and the 1,2-dissociation path is competitive. The 1,2-hydroxylated surface is more preferable thermodynamically in the consideration of reaction exothermicity. It was found that the in-plane hydrogen atoms can roam between the surface oxygen atoms, resulting in isomerization between the 1,2- and 1,4-hydroxylated products. Calculations for the multiple layer adsorption confirm that the hydroxylated surface is relatively inert to further hydroxylation by water. Further added water molecules prefer to form multilayered hexagonal ice-like arrangements through a hydrogen-bonding network. The electric field might not play a significant role in either surface reconstruction or the hydroxylation process until it exceeds 10(8) V/m. The present theoretical work is useful to gain some new insights on the ice accumulation of high-voltage power lines under high humidity and supercooled environment.