STRONG ADSORBATE-ADSORBATE INTERACTIONS IN THE H2O/PD(110) SYSTEM

The chemisoption of water on clean Pd(110) and its coadsorption with CO2 or O was studied using HREELS and TPD. On the clean surface HREELS spectra reveal two distinct arrangements below the onset of multilayer growth while there is only one TPD peak in this coverage range. We propose that at low coverages tetramers are formed whereas at intermediate coverages these tetramers coalesce into islands with the distorted hexagonal bilayer structure. The bilayer model for the saturated chemisorbed layer is carefully scrutinized and found to be in aggreement with all the experimental results for Pd(110). Water coadsorption with CO2 is dominated by a charge transfer from H2O to CO2 which is mediated by the metal surface. This leads to the formation of a bent CO2-delta species. The interaction between water and oxygen is a strong function of oxygen coverage. At low coverage a direct interaction leads either to the dissociation of water or to the formation of an O-H2O complex. When the surface is covered with oxygen, the strength of the water surface bond is significantly reduced compared to the clean surface.