A number of recent studies indicate that, under the oxygen rich conditions of oxidation catalysis, some transition metal catalysts may be covered by thin oxide overlayers. Moreover, it has been suggested that such “surface-oxide” layers are catalytically active, possibly more active than the pure metal surfaces as was traditionally assumed. This contemporary picture can be traced back to Ag catalysis, where over 30years ago it was suggested that the top layer of Ag(111) reconstructed to an epitaxial Ag2O like overlayer upon exposure to oxygen [Rovida et al., Surf. Sci.43, 230 (1974)]. Extensive experimental work, including scanning tunneling microscopy studies in which the oxide was apparently imaged with atomic resolution, as well as density-functional theory calculations, largely confirmed this interpretation. However, a review of published experimental data and new density-functional theory results presented here indicate that previous conclusions are significantly incomplete and that the structure of this original surface oxide must be reconsidered.

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39.

This estimate was made through a somewhat circuitous route, namely, by comparing the ratio of O to Ag Auger peak heights for O on Ag(111) to similar data for a phase of known coverage of O on Ag(100).

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46.

The isolated depressions on the Ag(111) triangular terraces are believed to be O atoms. See Refs. 20 and 21 for more details.

47.
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48.

These Ag adatoms correspond to Ag atoms in bulk Ag2O that would be bonded to O atoms in adjoining trilayers.

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The latter differs from the former merely by the surface energy of the clean reference surface [here this is Ag(111)].

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59.
All new DFT calculations reported here have been performed with the CASTEP code [
M. C.
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63.

These calculations were done in unit cells similar to the (4×4) cell used for the adsorption calculations. The structure of a free-standing trilayer film was optimized as a function of unit cell area by varying together both unit cell vectors from 20% to +20% of their value in the (4×4) adsorption cell.

64.

The O–Ag distances within the triangular building blocks of each overlayer are all 2.07±0.05Å. The Ag–Ag distances differ by as much as 0.6Å, specifically the Ag–Ag distances representing the base of each pyramid are 3.30±0.30Å. This reflects, however, large variations in Ag–Ag distances within individual overlayers, in particular within the Ag2O like models, rather than an inherent difference between the various overlayer structures themselves.

65.

We notice that the coordination of the basal oxygen atoms to Ag atoms in the overlayer also affects the registry of the oxide-like overlayer with the Ag(111) substrate. Stable structures with onefold coordinated O atoms invariably have their Ag–O bonds of these “undercoordinated” basal O atoms directed toward atop sites of the underlying substrate. Structures with twofold coordinated O atoms tend to have these oxygens located at bridge sites on the underlying Ag(111) substrate. For the structures in which all oxygen atoms are threefold coordinated, i.e., the Ag2O like overlayers, a strong preference for the location of the basal oxygen atoms has not been observed.

66.
With the current computational setup we calculate the heat of formation of bulk Ag2O (T=0K, P=0) to be 0.30eV. The experimental value for the standard enthalpy of formation of Ag2O is 0.32eV [
Handbook of Chemistry and Physics
, 76th ed., edited by
D. R.
Lide
(
CRC Press
, Boca Raton, FL,
1995
)].
67.

We add a (somewhat obvious) word of caution concerning the interpretation of Fig. 5 and indeed plots like this in general. Only phases which we have calculated will appear on it. Thus there may always exist a lower energy structure which we have not yet identified. Of course, however, nature does not always find the lowest energy structures either, diamonds and nanotubes are metastable yet long lived forms of carbon!

68.

Some valence band photoemission experiments have also been reported for the (4×4) overlayer (Refs. 18 and 30). However, they do not agree on the location of the O adsorbate induced resonances. Moreover, when we calculate the total and partial densities of states for the various low energy (4×4) structures under consideration here we find them to be essentially indistinguishable.

69.
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70.
Following the procedure introduced in Ref. 69, the core level shifts for each atom were computed by comparing the total energy of the ground-state to calculations with an “impurity” atom. The impurity atom was an atom with a core hole in its pseudopotential. For the Ag pseudopotential, where a valuable comparison to experiment can be made, we calculate a difference of 0.5eV between the Ag3d binding energies in bulk Ag and bulk Ag2O in agreement with experiment. A detailed discussion on this approach for calculating core level shifts and its successful application to the adsorption of CO on Rh(111) can be found in:
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Birgersson
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74.
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 et al. (to be published).
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