AVS 47th International Symposium
    Surface Science Tuesday Sessions
       Session SS3-TuA

Paper SS3-TuA9
Can Clustering Lower the Barrier to Dissociation of Water on Nickel Surfaces?

Tuesday, October 3, 2000, 4:40 pm, Room 210

Session: Water/Surface Interactions
Presenter: K. Griffiths, University of Western Ontario, Canada
Authors: L. Mao, University of Western Ontario, Canada
K. Griffiths, University of Western Ontario, Canada
P.R. Norton, University of Western Ontario, Canada
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The interaction of water with Ni(110) surface has been extensively explored in this group due to its fundamental and practical importance in the fields of heterogeneous catalysis, electrochemistry as well as corrosion. At temperatures  400K, water molecules react rapidly with Ni(110) producing adsorbed O and H2(g). Reaction probabilities can reach as high as 0.24 and our studies indicate that the reaction mechanism involves single water molecules. Dissociation of water can also occur at low temperatures (220K) provided that we begin with an adsorbed chemisorbed layer. The activation energy for the dissociation reaction is believed to be lowered by some means which involves a cluster of at least two water molecules. The exact mechanism is still speculative at this stage. Between these two temperatures, the Ni surface appears to be inert towards the dissociation of water. Under normal UHV dosing conditions, the surface is sufficiently hot to prevent an appreciable dynamic coverage of dimers. At the same time, the activation energy barrier for the unimolecular , high temperature mechanism cannot be overcome. The aim of this study is to investigate the reaction between water and Ni(110) in this >inert= temperature regime. In this study, we have produced locally relatively high pressures at the surface using a capillary doser to demonstrate that there is a non-linear dependence of reaction rate on dosing pressure. The absolute number of impinged water molecules as well as the instantaneous coverage of water (governed by the dosing pressure) determines the extent of reaction.