Isophorone on Au/MgO/Ag(001) : Physisorption with Electrostatic Site Selection

Abstract

We report a computational study of isophorone C9H14O adsorption on a Ag(001)-supported ultrathin MgO film with Au adatoms and clusters employing density functional theory calculations. The calculations show that the keto form of isophorone is more stable than the enol tautomers both in gas phase and on the MgO/Ag(001) surface. The interaction between the keto isophorone and step and terrace sites of MgO/Ag(001) displays long interaction distances, relatively weakly exothermic adsorption energies, lack of charge transfer, and minor changes in the density of states, all of which indicate that the molecule merely physisorbs on the surface. The step sites are energetically preferred adsorption sites due to attractive electrostatic interaction between more exposed Mg2+ cations and the polar O=C bond in isophorone. The adsorption on the step sites is 0.6 to 0.8 eV more exothermic compared with terrace sites. The calculations suggest that isophorone can chemisorb on MgO/Ag(001) surface but requires the presence of O vacancy. The interaction between a Au atom and isophorone was studied both in vacuum and on the MgO/Ag(001) terrace. In vacuum calculations, we did not find chemical bonding between an anionic, cationic, or neutral Au atom and the keto species. On the MgO/Ag(001) surface, a negatively charged Au adatom repels the keto isophorone, while electrostatic attraction is observed with the enol form. The calculations employing a Au stripe supported by MgO/Ag(001) to mimic the edge of the 2D Au nanoisland show that keto isophorone physisorbs at the edge and on-top of a Au stripe. The enol isophorone demonstrates adsorption site selectivity to the edge of the stripe due to electrostatic attraction between the negatively charged cluster edge and the positive end of the polar H–O bond, which makes physisorption on the Au edge energetically preferred.