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

Abstract

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 α-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 H2O adsorption modes are identified, with the latter favored by ∼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). Unconstrain...