First-Principles Molecular Dynamics Simulations of H2O on α-Al2O3(0001)

We present a more detailed account of our recently reported [Hass, K. C.; Schneider, W. F.; Curioni, A.; Andreoni, W. Science 1998, 282, 265] first-principles molecular dynamics investigation of the static and dynamical behavior of adsorbed H 2 O on α-Al 2 O 3 (0001). Al-terminated surfaces with varying degrees of H 2 O coverage are modeled using large periodic supercells. A predicted large relaxation of the clean surface agrees well with previous density functional theory calculations. Both molecular and dissociative H 2 O adsorption modes are identified, with the latter favored by ∼10 kcal mol -1 . Complementary Al 8 O 12 cluster results are shown to be unreliable because of their finite lateral extent. Constrained dynamical calculations of free-energy barriers indicate that the dissociation rate is very high, even in the absence of defects, but differs by 3 orders of magnitude for two equally exothermic pathways (proton transfer being more favorable across a six-membered ring than to the nearest O site). Unconstrained simulations at intermediate H 2 O coverages exhibit (1) spontaneous unimolecular and (2) H 2 O-mediated dissociation events, as well as (3) the diffusion and hydrogen bonding of physisorbed H 2 O and (4) an additional proton transfer reaction between adsorbed H 2 O and OH species. An experimentally observed decrease in H 2 O binding energies with coverage is explained in terms of a separation into defect-dominated, intrinsic (0001) terrace, and “hydrogen-bonding” regimes, with reasonable quantitative agreement throughout. Calculated O−H vibrational frequencies are consistent with known trends on aluminas but indicate a discrepancy between experimental observations for α-Al 2 O 3 (0001) and models based on simple hydroxylation. Simulations for high H 2 O coverages suggest the possibility of more complicated behavior, including the interchange of adsorbed and lattice oxygen and the etching of surface Al. A “fully”-hydroxylated α-Al 2 O 3 (0001) surface in which each surface Al is replaced by three protons to give uniform OH-termination, as in aluminum hydroxides, is the most likely result of prolonged exposure. Results for this surface confirm its anticipated stability, provide a reasonable explanation of observed vibrational spectra, and reveal a complex, dynamical structure with extensive intraplanar hydrogen bonding.