Water desorption from an oxygen covered Pt(111) surface:: Multichannel desorption

Mixed OH/H2O structures, formed by the reaction of O and water on Pt(111), decompose near 200 K as water desorbs. With an apparent activation barrier that varies between 0.42 and 0.86 eV depending on the composition, coverage, and heating rate of the film, water desorption does not follow a simple kinetic form. The adsorbate is stabilized by the formation of a complete hydrogen bonding network between equivalent amounts of OH and H2O, island edges, and defects in the structure enhancing the decomposition rate. Monte Carlo simulations of water desorption were made using a model potential fitted to first-principles calculations. We find that desorption occurs via several distinct pathways, including direct or proton-transfer mediated desorption and OH recombination. Hence, no single rate determining step has been found. Desorption occurs preferentially from low coordination defect or edge sites, leading to complex kinetics which are sensitive to both the temperature, composition, and history of the sample. (c) 2006 American Institute of Physics.