Density functional theory is used to investigate CO oxidation over an ultrathin MgO film supported on Ag(100). O(2) is found to be activated on MgO/Ag(100) whereas CO is only weakly bonded to the surface. These adsorption properties together with a low activation barrier render the MgO/Ag system an efficient catalyst for CO oxidation at low temperatures. As the predicted mechanism is general in nature, the result is suggested to have implications for a wide range of oxidation reactions.

Surface structures formed by exposing Ag(111) to atomic oxygen have been studied by X-ray photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory calculations. From the combination of the experimental and theoretical results, a model is proposed for the 0.5 ML oxygen coverage with a c(4 × 8) periodicity. Moreover, we find that a bulk-like Ag 2 O phase starts to form at coverages above 0.5 ML.

The oxidation of small silver clusters (Ag n , n e 9) was investigated through electronic structure calculations based on density functional theory. The adsorption energies of molecular and dissociated adsorption show a pronounced odd/even alternation, with lower energies calculated for even-sized clusters. Molecular adsorption is favored for n e 5, whereas dissociation is preferred for the larger sizes. Molecular oxygen is adsorbed in atop (Ag, Ag 2 , Ag 6 , Ag 8 ) or bridge (Ag 3 , Ag 4 , Ag 5 , Ag 7 , Ag 9 ) configurations, and atomic oxygen is preferably adsorbed in 3-fold hollow positions. Results for stoichiometric (Ag 2n O n ) clusters were compared to O 2 adsorption on Ag(111), and ab initio thermodynamics was used to estimate the temperature for the oxide-to-metal phase transition. The barrier for O 2 dissociation on Ag 8 was calculated to be higher than the corresponding barrier on Ag(111), which indicates a slower oxidation process. Adsorption of NO x onto the oxidized clusters was found to proceed through a formal reduction of the clusters; that is, NO x is adsorbed as NO x+1 with x ) 1, 2.

Supporting Information. Particle size distributions of MgO, Ba 0.02 Mg 0.98 O, Ba 0.04 Mg 0.96 O, and Ba 0.06 Mg 0.94 O, X-ray diffraction patterns of MgO and Ba 0.06 Mg 0.94 O, phase diagram of MgOÀBaO. This material is available free of charge via the Internet at http://pubs.acs.org.

Density functional theory calculations and high-resolution core-level spectroscopy are used to explore the remarkable observation of decreased Ag 3d binding energy upon silver oxidation. The shift in Ag 3d binding energy is investigated at different degrees of oxidation and compared to results for Pd 3d, which exhibits a normal shift. Analysis of initial-state effects and valence electronic structure shows that the onsite Ag core potential is insensitive to oxidation despite a clear metal-to-oxygen charge transfer. The substantial negative shift for oxidized Ag is instead attributed to final-state effects as screening of the core-hole occurs in metal s states of bonding character.