Properties of Adsorbed Oxygen Forms on a Defective Ag(111) Surface. DFT Analysis (original) (raw)
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To help provide insight into the remarkable catalytic behavior of the oxygen/silver system for heterogeneous oxidation reactions, purely sub-surface oxygen, and structures involving both on-surface and sub-surface oxygen, as well as oxide-like structures at the Ag(111) surface have been studied for a wide range of coverages and adsorption sites using density-functional theory. Adsorption on the surface in fcc sites is energetically favorable for low coverages, while for higher coverage a thin surface-oxide structure is energetically favorable. This structure has been proposed to correspond to the experimentally observed (4 × 4) phase. With increasing O concentrations, thicker oxide-like structures resembling compressed Ag2O(111) surfaces are energetically favored. Due to the relatively low thermal stability of these structures, and the very low sticking probability of O2 at Ag , their formation and observation may require the use of atomic oxygen (or ozone, O3) and low temperatures. We also investigate diffusion of O into the sub-surface region at low coverage (0.11 ML), and the effect of surface Ag vacancies in the adsorption of atomic oxygen and ozone-like species. The present studies, together with our earlier investigations of on-surface and surface-substitutional adsorption, provide a comprehensive picture of the behavior and chemical nature of the interaction of oxygen and Ag , as well as of the initial stages of oxide formation.
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We present a theoretical study--based on first principles calculations--aimed at characterizing the surface reconstructions which occur at the Ag(0 0 1) surface when oxygen is dosed on it. We first model this system at different coverages using (1  1), c(2  2), and p(2  2) structures of oxygen atoms adsorbed on the hollow sites of the Ag(0 0 1) surface. The corresponding equilibrium geometries are obtained by accurate energy minimizations performed within density-functional theory in the local density or in the generalized gradient approximations. We then compare the energies of these structures with that of oxygen adsorbed on a (2 ffiffi ffi 2 p  ffiffi ffi 2 p ) missing-row reconstructed substrate, recently proposed to be the stable phase at low temperature on the basis of X-ray photo-electron diffraction experiments. We do find evidence that the surface structure might be stabilized by a missing-row reconstruction, though our predicted geometry differs from that previously proposed. Ó
Ab initio density functional study of O on the Ag(001) surface
Surface Science, 2003
The adsorption of oxygen on the Ag(100) is investigated by means of density functional techniques. Starting from a characterization of the clean silver surfaces oxygen adsorption in several modifications (molecularly, on-surface, sub-surface, Ag 2 O) for varying coverage was studied. Besides structural parameters and adsorption energies also work-function changes, vibrational frequencies and core level energies were calculated for a better characterization of the adsorption structures and an easier comparison to the rich experimental data.
Oxygen adsorption on Ag(111): A density-functional theory investigation
Physical Review B, 2002
The oxygen/silver system exhibits unique catalytic behavior for several large-scale oxidation ͑and partial oxidation͒ industrial processes. In spite of its importance, very little is known on the microscopic level concerning the atomic geometry and chemical nature of the various O species that form. Using densityfunctional theory within the generalized gradient approximation, the interaction between atomic oxygen and the Ag͑111͒ surface is investigated. We consider, for a wide range of coverages, on-surface adsorption as well as surface-substitutional adsorption. The on-surface fcc-hollow site is energetically preferred for the whole coverage range considered. A significant repulsive interaction between adatoms is identified, and on-surface adsorption becomes energetically unstable for coverages greater than about 0.5 monolayer ͑ML͒ with respect to gas-phase O 2 . The notable repulsion even at these lower coverages causes O to adsorb in subsurface sites for coverages greater than about 0.25 ML. The O-Ag interaction results in the formation of bonding and antibonding states between Ag 4d and O 2p orbitals where the antibonding states are largely occupied, explaining the found relatively weak adsorption energy. Surface-substitutional adsorption initially exhibits a repulsive interaction between O atoms, but for higher coverages switches to attractive, towards a (ͱ3ϫͱ3)R30°structure. Scanning tunneling microscopy simulations for this latter structure show good agreement with those obtained from experiment after high-temperature and high-O 2 -gas-pressure treatments. We also discuss the effect of strain and the found marked dependence of the adsorption energy on it, which is different for different kinds of sites.
Adsorption ofO2on Ag(111): Evidence of Local Oxide Formation
Physical Review Letters, 2016
The atomic structure of the disordered phase formed by oxygen on Ag(111) at low coverage is determined by a combination of low-temperature scanning tunneling microscopy and density functional theory. We demonstrate that the previous assignment of the dark objects in STM to chemisorbed oxygen atoms is incorrect and incompatible with trefoil-like structures observed in atomic-resolution images in current work. In our model, each object is an oxidelike ring formed by six oxygen atoms around the vacancy in Ag(111).
Structure and dynamics of oxygen adsorbed on Ag(100) vicinal surfaces
Physical Review B, 2004
The structure and dynamics of atomic oxygen adsorbed on Ag(410) and Ag(210) surfaces has been investigated using density functional theory. Our results show that the adsorption configuration in which O adatoms decorate the upper side of the (110) steps forming O-Ag-O rows is particularly stable for both surfaces. On Ag , this arrangement is more stable than other configurations at all the investigated coverages. On Ag(410), adsorption on the terrace and at the step edge are almost degenerate, the former being slightly preferred at low coverage while the latter is stabilized by increasing the coverage. These findings are substantiated by a comparison between the vibrational modes, calculated within density-functional perturbation theory, and the HREEL spectrum which has been recently measured in these systems.
Oxygen diffusion through the Ag(111) surface: A quantum chemical study by the NDDO/MC method
Journal of Structural Chemistry, 2000
A tomicfomts of oaygen on the (111)face of metallic silver are studied by the NDDO/MC semiempilical method. The surface (above the octahedral void) and subsulface (within the void) positions of oaygen between the fitxt and second layers ~>/" the (111) .face are investigated. The potential surface cross section is calculated for the subsurface position of oaygen. A new approach is used to take into account surface relaxation due to reaction with adsorbate. The barrier o/" atomic o.~3'gen diffusion through the sulface is much lower than the banier of its desolption from the surface. The con'elation conection to the d~ff'usion and desotption banier energies is estimated by the double CI (DCIP) method. The greatest con'elation effects are obtained in the desolption banier calculation.
Structures and thermodynamic phase transitions for oxygen and silver oxide phases on Ag{111}
Chemical Physics Letters, 2003
With density functional theory, we have examined oxygen adsorption at surface and subsurface sites of Ag{1 1 1}. The microscopic structure of Ag oxide epitaxed to Ag{1 1 1} has also been determined. In agreement with a recent scanning tunneling microscopy study, non-stoichiometric oxide growth is favoured over the previously assumed stoichiometric growth. An ab initio phase diagram for O on Ag{1 1 1} has been constructed from the adsorption free energy of the various O and Ag oxide phases. The key finding is that under real conditions for ethylene epoxidation the active catalyst is likely to be non-stoichiometric Ag oxide.
Surface Science, 1995
We have investigated the interaction of oxygen with Ag(lll) by using a supersonic molecular beam in the impact energy range 93-800 meV. At 105 K, contrary to the results of Carley et al. [Surf. Sci. 238 (1990) L467], we find no evidence for O 2 adsorption even after very high O 2 exposures (~ 25000 L) indicating that for a clean surface the sticking probability S is lower than 6 X 10 -7 for the whole impact energy range. At room temperature dissociative oxygen adsorption occurs at E i = 0.80 eV, with S = 9 x 10 -7. The data show however evidence that the adsorption process is mediated also in this case by adsorbed OH so that S is even smaller for the clean surface.