Bimetallic electrodes are used in a number of electrochemical processes, but the role of particular arrangements of surface metal atoms (ensembles) has not been studied directly. We have evaluated the electrochemical/catalytic properties of defined atomic ensembles in atomically flat PdAu(111) electrodes with variable surface stoichiometry that were prepared by controlled electrodeposition on Au(111). These properties are derived from infrared spectroscopic and voltammetric data obtained for electrode surfaces for which the concentration and distribution of the respective metal atoms are determined in situ by atomic resolution scanning tunneling microscopy with chemical contrast. Palladium monomers are identified as the smallest ensemble ("critical ensemble") for carbon monoxide adsorption and oxidation, whereas hydrogen adsorption requires at least palladium dimers.

The surface normal structure of the mercury liquid-vapor interface has been investigated by measuring 0 the x-ray reAectivity out to a momentum transfer of q, = 2.5 A '. The results provide direct

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In situ scanning tunnelling microscopy (STM) studies on Au( 11 1) in H2S04 solutions are presented which resolve an ordered structure in the adsorbate layer with a i) unit cell and a coverage of 0.4 monolayers at potentials above cu. 0.75V us. SCE. From the absence of this structure in Na,SO, solutions we conclude that this is a hydrogensulfate adsorbate species. The abrupt disorder-order phase transition at cu. 0.75 V us. SCE coincides with a sharp peak in the voltammogram, underlining the influence of adlayer structural effects on the voltammogram. The phase transition proceeds in a fast and completely reversible way; ordering occurs uiu an island growth mechanism. The absence of strong Coulombic repulsions points to an effective discharging of the adsorbed anions. The ordered adlayer is stable up to the onset of oxidation at 1.2 V us. SCE. Comparative studies on Au( 110) and Au( loo), where no ordered adlayer structures were observed, reveal a strong sensitivity to the substrate geometry. In addition the experimental results provide direct evidence for pronounced electronic effects in anion imaging by STM.