AVS 49th International Symposium
    Electrochemistry and Fluid-Solid Interfaces Monday Sessions
       Session EC+SS-MoM

Paper EC+SS-MoM2
A Specular He Scattering Study of Water Adsorption, Desorption, and Clustering on Pt(111)

Monday, November 4, 2002, 8:40 am, Room C-104

Session: Fuel Cells and Surface Electrochemical Reactions
Presenter: J.L. Daschbach, Pacific Northwest National Laboratory
Authors: J.L. Daschbach, Pacific Northwest National Laboratory
B.M. Peden, Pacific Northwest National Laboratory
R.S. Smith, Pacific Northwest National Laboratory
B.D. Kay, Pacific Northwest National Laboratory
Correspondent: Click to Email
Specular He atom scattering is used to probe the adsorption, desorption, and clustering kinetics of sub-monolayer H@sub 2@O on Pt(111) over the temperature range 22 K to 185 K. Water deposited on clean Pt at low temperatures is shown to be arranged random ly on the Pt substrate. Over a narrow temperature range, as the clean substrate temperature is raised, the deposited water transforms to a 2-D condensed phase. Rearrangement of randomly adsorbed H@sub 2@O is studied as a function of coverage and temperat ure. At low initial H@sub 2@O coverage the specular He waveform is dominated by the clustering of the isolated H@sub 2@O molecules. At higher H@sub 2@O coverage and temperature a second feature is manifest, which we interpret as the Oswald ripening of the 2-D islands. Adsorption and desorption kinetics are examined isothermally. Water, when fully clustered in two dimensions, gives rise to a He specular intensity that decreases linearly with coverage. Over essentially the entire sub-monolayer coverage regi m e the sample coverage changes linearly in time during both adsorption and desorption of H@sub 2@O. This requires that the desorption rate be independent of coverage and thus the desorption kinetics are zero-order. The zero-order kinetics are a consequen ce of the coexistence of a 2-D H@sub 2@O gas with a 2-D condensed H@sub 2@O phase on the Pt surface. At higher temperatures, depending on flux, non-zero order kinetics are observed which are indicitive of a transition to a single H@sub 2@O 2-D phase. Details of the experimental techniques and results will be presented. Pacific Northwest National Laboratory is a multiprogram National Laboratory operated for the Department of Energy by Battelle under Contract DE-AC067-76RLO 1830.