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

Paper SS3-TuA6
Interactions of Methanol with Water and Hydrogen in Electrolytic Adlayers on Pt(111)

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

Session: Water/Surface Interactions
Presenter: D.S.W. Lim, University of Washington
Authors: D.S.W. Lim, University of Washington
E.M. Stuve, University of Washington
Correspondent: Click to Email
To model the interactions of methanol with water and hydrogen at electrolye/electrode interface, thermal desorption experiments of the co-adsorbed system were performed on Pt(111). In electrochemical environments, adsorption of methanol on Pt(111) is inhibited by chemisorbed hydrogen, and water competes with methanol for available Pt sites. In ultra-high vacuum, methanol desorbed from Pt(111) in a multilayer state B@sub m@ at 142 and a monolayer state A@sub m1@ at 180 K. However, when the surface was pre-adsorbed with hydrogen, the desorption temperature of the methanol monolayer state, A@sub m1@, was lowered by as much as 20 K. The reduction in desorption temperatures was attribited to weakened interactions between methanol and the surface due to chemisorbed hydrogen. When methanol was co-adsorbed with water, mutual displacement between the methanol monolayer and water bilayer occurred on the surface. Water molecules in direct contact with Pt(111) surface form bilayer structure that desorbs at 173 - 178 K. A realistic simulation of the electrolytic adlayers was to co-adsorb all three species: methanol, water and hydrogen on Pt(111). In a water-lean adlayer system, the methanol monolayer state was destabilized by chemisorbed hydrogen. However, with the introduction of a sufficient amount of water, methanol was re-stabilized and, the effects of hydrogen-induced destabilization were completely erased when excess water was added to the adlayer. Methanol behaved as if it was only interacting with water. The results of these experiments illustrate that there is a delicate balance of hydrogen-induced destabilization and water-promoted stabilization in the electrolytic adlayers on Pt(111), and behavior of methanol interactions with co-adsorbed hydrogen and water followed that observed in electrochemical environments.