Catalytic Gold Nanoparticles on an Iron Oxide Surface: A Scanning Tunneling Microscopy/Spectroscopy Study

We present a Scanning Tunneling Microscopy/Spectroscopy study of a model catalyst system consisting of supported gold nanoparticles on a reduced Fe$_{3}$O$_{4}$(111) surface in ultrahigh vacuum. Gold forms two electrically distinct types of nanoparticles on an iron oxide surface upon annealing a multilayer Au/Fe$_{3}$O$_{4}$(111). STS measurements show that large nanoparticles ($\sim $8nm) are metallic while single gold adatoms are bonded to the oxygen sites on the Fe$_{3}$O$_{4}$(111) surface. Site-specific adsorption at oxygen surface atoms and the size sensitive nature of the electronic structure (Coulomb blockade) suggest that Au adatoms are positively charged. When this Au/Fe$_{3}$O$_{4}$(111) catalyst system is dosed with CO at 260K, there is evidence for CO adsorption at gold adatom sites. These observations are consistent with the proposal that nonmetallic, positively charged, ``invisible'' Au particles are the catalytically active species for the water-gas-shift reaction on Au/metal oxide surfaces. http://clippercontrols.com/info/dielectric\_constants.html