Hydrophobic coadsorption

On Pt(lll) the HREELS features due to water are unchanged by the presence of CO. These observations indicate that water and CO adsorb onto separate patches on the surface, in a form of hydrophobic coadsorption. Water condenses into hydrogen-bonded islands, as indicated by the low 0-H stretching frequency. CO spreads to cover the rest of surface, giving a phase similar to that for CO alone, but with a coverage normalized to the water-free, not total, surface area. COCO repulsions, which have been well documented on Pt(lll) (10), produce a surface pressure within the CO patches which bears upon the edges of the water islands. It is this lateral pressure which causes water to desorb from Pt(lll) at lower temperatures in the presence of coadsorbed CO. 

HydrophiIic versus hydrophobic coadsorption. The contrast between the hydrophilic and hydrophobic coadsorption seen on Rh(111) and Pt(lll), if confirmed under normal electrochemical conditions, might be of electrocatalytic importance. On Rh(lll), where net attractive CO-HgO interactions produce a mixed phase in which CO is displaced to a three-fold binding site which is not occupied in the absence of water, CO and water appear to occupy adjacent binding sites. Such thorough mixing of the oxygen source (water) and the intermediate [or poison] (CO) should improve electrooxidation rates for C 0 H fuels (11). On Pt(lll), where net repulsions cause condensation of CO and water into separate patches, reaction between the adsorbed species could occur only at the boundaries between patches, and one would expect slower kinetics. 

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