Isolated Metal Atom Geometries as a Strategy for Selective Heterogeneous Hydrogenations

Tuning Hydrogen Adsorption Heterogeneous metal catalysts for hydrogenating unsaturated organic compounds need to bind molecular hydrogen strongly enough that it dissociates and forms adsorbed hydrogen atoms, but must not bind these atoms too strongly, or the transfer to the organic molecule will be impeded. Kyriakou et al. (p. 1209) examined surface alloy catalysts created when palladium (Pd) atoms are adsorbed on a copper (Cu) surface using scanning tunneling microscopy and desorption techniques under ultrahigh vacuum conditions. The Pd atoms could bind hydrogen dissociatively—which, under these conditions, the Cu surfaces could not—allowing the Cu surface to take up adsorbed hydrogen atoms. These weakly bound hydrogen atoms were able to selectively hydrogenate styrene and acetylene. Palladium atoms adsorbed on a copper surface activate hydrogen adsorption for subsequent hydrogenation reactions. Facile dissociation of reactants and weak binding of intermediates are key requirements for efficient and selective catalysis. However, these two variables are intimately linked in a way that does not generally allow the optimization of both properties simultaneously. By using desorption measurements in combination with high-resolution scanning tunneling microscopy, we show that individual, isolated Pd atoms in a Cu surface substantially lower the energy barrier to both hydrogen uptake on and subsequent desorption from the Cu metal surface. This facile hydrogen dissociation at Pd atom sites and weak binding to Cu allow for very selective hydrogenation of styrene and acetylene as compared with pure Cu or Pd metal alone.