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.