Initial stages in the oxidation and reduction of the4×4surface oxide phase on Ag{111}: A combined density-functional theory and STM simulation study (original) (raw)

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.

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.

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.

In Situ Observation of CO Oxidation on Ag(110)(2×1)-O by Scanning Tunneling Microscopy: Structural Fluctuation and Catalytic Activity

The Journal of Physical Chemistry B, 2005

On the added-row reconstructed Ag(110)(n×1)-O surfaces where one-dimensional -Ag-O-Ag-Ochains arrange periodically, the clean-off reaction of O adatoms by CO was investigated using variable temperature scanning tunneling microscopy (VT-STM). Based on the in situ STM observations of the surface structure variation in the course of the reaction at various temperatures, we found that the reaction kinetics are significantly affected by the structural transition of AgO chains from a solid straight line configuration to dynamically fluctuating configurations. Below 230 K where the chains are straight, the reaction takes place only at the end of the chains, so that the reaction progresses in the zero-order kinetics with the reaction front propagating along the chain. The temperature dependence of the reaction rates yields the activation barrier of 41 kJ/mol and the preexponential factor of 1.7 × 10 3 cm -2 s -1 . At room temperature, the reaction rate is drastically accelerated when almost half of the O adatoms are eliminated and the chains start fluctuating. The dynamic formation of active sites equivalent to the end of chains upon the chain fluctuation results in the nonlinear increase of the reaction rate. † The Graduate University for Advanced Studies (Sokendai).

Combined STM, LEED and DFT study of Ag(1 0 0) exposed to oxygen near atmospheric pressures

Surface Science, 2006

We have investigated the interaction of molecular oxygen with the Ag(1 0 0) surface in a temperature range from 130 K to 470 K and an oxygen partial pressure ranging up to 10 mbar by scanning tunneling microscopy, low electron energy diffraction, Auger electron spectroscopy and ab initio density functional calculations. We find that at 130 K, following oxygen exposures of 6000 Langmuirs O 2 , the individual oxygen atoms are randomly distributed on the surface. When the sample is exposed to 10 mbar O 2 at room temperature, small, p(2 · 2) reconstructed patches are formed on the surface. After oxidation at %470 K and 10 mbar O 2 pressure the surface undergoes a c(4 · 6) reconstruction coexisting with a (6 · 6) superstructure. By ab initio thermodynamic calculations it is shown that the c(4 · 6) reconstruction is an oxygen adsorption induced superstructure which is thermodynamically stable for an intermediate range of oxygen chemical potential.

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).

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...

Imaging the Surface and the Interface Atoms of an Oxide Film on Ag{111} by Scanning Tunneling Microscopy: Experiment and Theory

Physical Review Letters, 2000

High-resolution images of the vacuum oxide surface atomic structure of Ag͕111͖-p͑4 3 4͒-O reveal large terraces of a perfect ͑4 3 4͒ reconstruction. Under certain conditions, the surface Ag atoms at the interface between the reconstructed oxide layer and the underlying Ag͕111͖ lattice are also imaged, providing the structural registry. Scanning tunneling microscopy simulations reveal a strong sensitivity to structure and comparison with the experimental images, therefore providing an atom-by-atom model for the entire metal-oxide-vacuum structure.

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.