ADSORPTION OF H2O ON ALKALI-METAL-COVERED PT(111) AND RU(001) - A SYSTEMATIC COMPARISON

The adsorption of H2O on Na-covered Pt(111) was studied at 110 and 320 K by recording changes in work function, DELTA-phi. At a Na coverage below 0.10 the adsorption was molecular and fully reversible. A comparison of work function changes due to H2O on Na/Pt(111) and K/Pt(111) shows general similarities, with large dipole moments of H2O, opposite in sign to H2O adsorbed on clean Pt(111). A further comparison of DELTA-phi versus H2O coverage for alkali-covered Pt(111) and Ru(001) surfaces illustrates the alkali-metal and substrate specificity of H2O/alkali coadsorption. H2O dipole moments derived from DELTA-phi versus coverage are interpreted in terms of H2O reorientation compared to the adsorbed H2O state on the clean surface. These dipole moments indicate increasing tilt of the H2O molecule from Na to K to Cs for Ru(001). Also, the amount of tilt is stronger, the weaker the H2O substrate interaction (for a particular alkali). A higher degree of alkali-induced H2O reorientation generally correlates with a more difficult dissociation of H2O.