Structures and thermodynamic phase transitions for oxygen and silver oxide phases on Ag{111} (original) (raw)
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Subsurface oxygen and surface oxide formation at Ag(111): A density-functional theory investigation
Physical Review B, 2003
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.
Physical review, 2003
With density-functional theory, we have examined the initial stages in the oxidation and reduction of the high coverage Ag 1.83 O oxide phase that forms on Ag͕111͖. Various oxidized and reduced structures have been identified and their relative energies assessed at 0 K and at finite temperatures and pressures. We find that in the temperature and pressure regime characteristic of industrial epoxidation conditions, the Ag 1.83 O oxide is easily oxidized and reduced demonstrating a high chemical flexibility of this oxide for redox reactions. In addition scanning tunneling microscopy ͑STM͒ image simulations have been performed. These reveal that as well as the Ag 1.83 O oxide structure previously proposed a second oxide overlayer is consistent with reported STM images of this system, making this different oxide overlayer another likely candidate for the surface structure obtained in the experiment.
Structural transformations on an oxidized Ag(111) surface
JETP Letters, 2017
The structure of the Ag(111) surface after the adsorption of molecular oxygen at a temperature of 300 K is studied by low-temperature scanning tunneling microscopy. It is established that local surface oxide is formed at the first stage of adsorption. The subsequent adsorption of O results in the appearance of new objects with a size of 3-8 Å and a height of 1.0-1.5 Å on the Ag(111) surface, which form quasi-ordered structures with increasing degree of coating. The heating of the system obtained up to 500 K leads to a structural transition resulting in the formation of single islands of the (4 4) phase on the surface. Surface structures are identified by a simulation based on the density functional theory.
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.
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).
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.
Properties of Adsorbed Oxygen Forms on a Defective Ag(111) Surface. DFT Analysis
Journal of Structural Chemistry - J STRUCT CHEM-ENGL TR, 2002
A cluster model of an Ag12–3O (ASV) adsorption center using layered silver oxide as a prototype is proposed. The model includes a cation vacancy V on the Ag(111) surface and oxide type subsurface oxygen atoms Oox. Density functional theory (DFT) (B3LYP/LANL1MB approximation) is used to analyze the electronic structure of ASV and oxygen adsorption on this center, ASV+O ? AS–O. As shown by the calculations, the adsorbed oxygen is associated with the subsurface oxygen atoms Oss to form structures similar to metal ozonides — Ag–Oss–Oep–Oss–Ag–Oox–Ag, containing electrophilic oxygen Oep along with the oxide oxygen Oox. The optical spectra of the ASV and AS–O centers were calculated by the configuration interaction method with single excitations (CIS). For ASV, the most intense absorption bands were obtained in the region 500-700 nm. Oxygen association is accompanied by a sharp decrease in spectrum intensity in the range 600-700 nm and an increase in the intensity of the peak at 500 nm. V...
Adsorption of atomic oxygen on Ag(): a study based on density-functional theory
Surface Science, 2002
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
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.