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 H2O on alpha-Al2O3 (0001). Al-terminated surfaces with varying degrees of H2O 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 (HO)-O-2 adsorption modes are identified, with the latter favored by similar to 10 kcal mol(-1). Complementary Al8O12 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 H2O coverages exhibit (1) spontaneous unimolecular and (2) H2O-mediated dissociation events, as well as (3) the diffusion and hydrogen bonding of physisorbed H2O and (4) an additional proton transfer reaction between adsorbed H2O and OH species, An experimentally observed decrease in H2O 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 alpha-Al2O3 (0001) and models based on simple hydroxylation. Simulations for high H2O 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 alpha-Al2O3 (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.