First-Principles Molecular Dynamics Simulations of H(2)O on alpha-Al(2)O(3) (0001)

We present a more detailed account of our recently reported [Hass, K.C.; Schneider, W.F.; Curioni, A.; Andreoni, A.: Science, 1998, Vol. 282, p. 265] first-principles molecular dynamics investigation of the static and dynamical behavior of adsorbed H(2)O on alpha-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 approx. 10 kcal/mol. 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 three orders of magnitute for two equally-exothermic pathways ("1-4" >> "1-2"). Unconstrained simulations at intermediate H(2)O coverages exhibit (1) spontaneous unimoleculelar 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 an aluminas, but indicate a discrepancy between experimental observations for alpha-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 alpha-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 intra-planar hydrogen bonding.