DFT Investigation of Intermediate Steps in the Hydrolysis of α-Al₂O₃(0001)

Abstract

The R-Al 2 O 3 (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 hydrogenfree 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 Al s sites and that the formation energy of these hydroxylated Al s is coverage-independent. Subsequent water dissociations at a singly hydroxylated Al s site, steps necessary to liberate Al s 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 R-Al 2 O 3 (0001) surface composition under any particular conditions may exhibit strong dependence on sample history. † Part of the special section "Physical Chemistry of Environmental Interfaces".

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