Oxygen adsorption and oxidation of metal surfaces: corrosion
- Oxygen on Ruthenium
- Oxygen on Silver
- Oxidation of Transition Metal Surfaces
Oxygen on Ruthenium
It
is known from experiments that under ultra high vacuum (UHV)
conditions, at room temperature, dissociative adsorption of O2
results in an (apparent) saturation coverage of half a monolayer of
oxygen atoms on the surface. On the basis of DFT calculations we
predicted that even higher coverages should be attainable and that
their formation under UHV conditions is only kinetically hindered,
namely, we predicted formation of a full monolayer structure. This
phase was subsequently verified experimentally. In order to achieve
this, use of an oxygen carrying molecule that readily dissociates in
the presence of O on the surface is necessary e.g. NO2 , or high gas pressures of O2
needs to be employed. Identification of this phase illustrates
the different behavior of a system under UHV and high pressure
conditions (the pressure-gap) and also immediately raises the
question of the possible existence of other high coverage surface
structures
likewise achievable by bypassing the "pressure gap"
which may be of importance in the understanding of heterogeneous
catalysis.[C.Stampfl, S. Schwegmann, H. Over, M. Scheffler, and G.
Ertl, Phys. Rev. Lett. 77 3371 (1996)]
Oxygen on Silver
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 (4x4) phase. With increasing O concentrations, thicker oxide-like structures resembling compressed Ag2O(111) surfaces are energetically favored. The present studies, provide a comprehensive picture of the behavior and chemical nature of the interaction of oxygen and Ag(111), as well as of the initial stages of oxide formation. [W.-X. Li, C. Stampfl, and M. Scheffler, Phys. Rev. B 67, 064108 (2003)]
Oxidation of Transition Metal Surfaces
Using density-functional theory, we performed a trend study addressing the incorporation of oxygen into the basal plane of the late 4d transition metals (TM) from Ru to Ag. Occupation of sub-surface sites is always connected with a significant distortion of the host lattice, rendering it initially less favorable than on-surface chemisorption. Penetration only starts after a critical coverage, which is lower for the softer metals towards the right of the TM series. The computed critical coverages are found to be very similar to the ones, above which the bulk oxide phase becomes thermodynamically more stable, thus suggesting that the initial incorporation of oxygen is the key step in oxide formation at transition metal surfaces.[M. Todorova, W.X. Li, M.V. Ganduglia-Pirovano, C. Stampfl, K. Reuter, and M. Scheffler, Phys. Rev. Lett. 89, 096103 (2002)]
Calculated binding energies for on-surface O chemisorption at the basal surface of the late 4d transition metals. The energies are given with respect to molecular oxygen. The bonding is strongest for Ru and weakest for Ag, and in all cases the O-metal bond becomes weaker with increasing coverage, indicating a repulsive interaction between the O atoms.
Two-dimensional contour plots of the average binding energy as a function of the total oxygen coverage of which a certain fraction is located below the surface. It can be seen that for Ru(0001) and Rh(111) for any given total oxygen coverage, it is energetically favorable that there be no sub-surface O. For Pd(111) and Ag(111) on the other hand, after total coverages of about 0.5ML and 0.25 ML on the surface, respectively, it becomes energetically favorable for O to occupy sub-surface sites.
