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.

In the search for precious-metal free electrode materials for electrochemical water splitting, transition metal oxides have been receiving much recent interest as active and stable electrocatalysts for the anodic oxygen evolution reaction (OER). We present operando surface X-ray diffraction studies of two structurally well-defined epitaxial cobalt oxide thin films -Co3O4(111) and CoOOH(001) electrodeposited on Au(111). The potential-dependent structural changes during cyclic voltammograms were monitored with high time resolution up to OER current densities as high as 150 mA cm -2 . The CoOOH(001) film is found to be smooth and perfectly stable over a wide potential range. In the case of Co3O4(111), fast and fully reversible structural changes are observed. Specifically, the surface region of Co3O4( 111) starts restructuring at potentials 300 mV negative of the onset of the OER, indicating that the process is related to the thermodynamically predicted Co3O4 / CoOOH(001) transition rather than to the catalytic reaction. The formed skin layer is of defined thickness, which changes linearly with applied potential, and is the OER active phase. Surprisingly, the catalytic activity of the skin layer covered Co3O4 film and that of the smooth CoOOH(001) are almost identical, if the true microscopic surface area is taken into account.This indicates that the number of OER active sites on the two oxides is similar, despite the very different defect density, and is at variance with previous suggestions that di-µ-oxo bridged Co cations are exclusively responsible for the OER activity of Co oxides. For the smooth CoOOH(001) a turnover frequency of 4.2 s -1 per surface atom is determined at an overpotential of 400 mV. Furthermore, our studies demonstrate that the pseudo-capacitive charging current in the pre-OER potential range must be assigned to a bulk process that is accompanied by potential-dependent changes of the unit cell volume in the Co3O4 bulk.

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