DFT investigation of intermediate steps in the hydrolysis of α-Al2O3(000I)

The α-Al2O3(0001) surface is well-known to become hydroxylated in the presence of water, and this hydroxylation is important to subsequent alumina surface chemistry. Here, we use plane-wave, supercell density functional theory to examine the progression of multiple water dissociation steps from the hydrogen-free stoichimetric surface to the fully hydroxylated, gibbsite-like surface. Consistent with earlier reports, we find that water molecules adsorb and dissociate exothermically and with a small activation barrier at unhydroxylated and coordinatively unsaturated surface Als sites and that the formation energy of these hydroxylated Als is coverage-independent. Subsequent water dissociations at a singly hydroxylated Als site, steps necessary to liberate Als and reach the fully hydroxylated surface, are approximately thermoneutral at any surface hydroxyl coverage. Further, within the pathways we are able to identify, these subsequent dissociation steps proceed along more complex reaction coordinates and have higher activation energies than the first water dissociation step. Although the fully hydroxylated surface is the thermodynamic ground state in the presence of water, the actual α-Al2O3(0001) surface composition under any particular conditions may exhibit strong dependence on sample history.