The role of atomic ensembles in the reactivity of bimetallic electrocatalysts

Bimetallic electrodes are used in a number of electrochemical processes, bu t the rote of particular arrangements of surface metal atoms (ensembles) ha s not been studied directly. We have evaluated the electrochemical/catalyti c properties of defined atomic ensembles in atomically flat PdAu(111) elect rodes with variable surface stoichiometry that were prepared by controlled electrodeposition on Au(111). These properties are derived from infrared sp ectroscopic and voltammetric data obtained for electrode surfaces for which the concentration and distribution of the respective metal atoms are deter mined in situ by atomic resolution scanning tunneling microscopy with chemi cal contrast. Palladium monomers are identified as the smallest ensemble (" critical ensemble") for carbon monoxide adsorption and oxidation, whereas h ydrogen adsorption requires at least palladium dimers.