Exposure of a Ru(0001) surface to large doses of O2 at elevated temperatures leads to the growth of an epitaxial layer of RuO2 with (110) surface orientation whose structure was analyzed quantitatively by combination of low energy electron diffraction, scanning tunneling microscopy and density functional calculations. The surface exposes essentially bridging O atoms and Ru atoms not capped by oxygen. The latter play the role of coordinatively unsaturated sites (cus) - a hypothesis introduced long ago to account for the catalytic activity of oxide surfaces - onto which CO may become chemisorbed and from where it may react with neighboring lattice-O to CO2. The distortion of the surface lattice thereby caused is restored by uptake of oxygen from the gas phase, i.e., the oxide surface itself is actively participating in the catalytic reaction. In this way a general mechanism originally proposed by Mars and van Krevelen could be verified.
Corresponding author: H. Over (FHI Berlin). Reprints also available from M. Schmid (schmid).
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Part of this work is on display in the IAP/TU Wien STM Gallery (see the non-metals page).