K Atom Promotion of O₂ Chemisorption on Au(111) Surface

Alkali atoms are known to promote or poison surface catalytic chemistry. To explore alkali promotion of catalysis and to characterize discharge species in alkali-oxygen batteries, we examine coadsorption of K and O₂ on Au(111) surface at the atomic scale by scanning tunneling microscopy (STM) and density functional theory (DFT). On a clean Au(111) surface, O₂ molecules may weakly physisorb, but when Au(111) is decorated with K⁺ ions, they chemisorb into structures that depend on the adsorbate concentrations and substrate templating. At low K coverages, an ordered quantum lattice of K₂O₂ complexes forms through intramolecular attractive and intermolecule repulsive interactions. For higher K and O₂ coverages, the K₂O₂ complexes condense first into triangular islands, which further coalesce into rhombohedral islands, and ultimately into incommensurate films. No structures display internal contrast possibly because of high structural mutability. DFT calculations explain the alkali-promoted coadsorption in terms of three center, cation-π interactions where pairs of K⁺ coordinate the π-orbitals on each side of O₂ molecules, and in addition O₂ forms a covalent bond to Au(111) surface. The K promoted adsorption of O₂ is catalyzed by charge transfer from K atoms to Au(111) substrate and ultimately to O₂ molecules, forming O₂^-δ in a redox state between the peroxo and superoxo. Tunneling d I/d V spectra of K₂O₂ complexes exhibit inordinately intense inelastic progression involving excitation of the O-O stretching vibration, but absence of a Kondo effect suggests that the magnetic moment of O₂ is quenched.