Structure and bonding of the water-hydroxyl mixed phase on Pt(111)

We combine low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and Auger electron spectroscopy (AES) with density functional theory (DFT) to reveal the structure and bonding of water-hydroxyl mixed layers adsorbed on Pt(111). We find that the stable water-hydroxyl adlayer forms a mixed phase of nearly coplanar hexamer structures resulting in (root 3 x root 3)R30 degrees and (3 x 3) unit cells, respectively. In the asymmetric (3 x 3) structure the lateral O-O distances alternate between long and short bond lengths similar to the chemical bonding network for OH- ions in solution. The chemical driving force behind this similarity is discussed in a molecular orbital picture.