CO adsorption on the reduced RuO₂(110) surface: Energetics and structure

The adsorption sites of CO on the reduced RuO₂(110) surface were determined by employing the techniques of density-functional theory calculations and quantitative low-energy electron diffraction. On the mildly reduced RuO₂(110) surface, where the bridging oxygens are removed, twofold (2f) undercoordinated Ru atoms [2f- coordinatively unsaturated site (cus) Ru] are exposed. On this surface, the CO molecules initially adsorb above these 2f-cus Ru atoms in symmetric bridge positions. The CO coordinated 2f-cus Ru atoms are drawn towards the CO molecule by 0.2 Å. With increasing CO coverage, the CO adsorption site changes from the symmetric to the asymmetric bridge position. When all 2f-cus Ru atoms have been capped by bridging CO molecules, the 1f-cus Ru atoms are also progressively occupied by on-top CO. For comparison, on the stoichiometric RuO₂(110) surface, CO molecules adsorb exclusively on top of the onefold coordinatively unsaturated Ru atoms (1f-cus Ru) at temperatures below 200 K. The energetics of the CO adsorption on the reduced RuO₂(110) surface compares favorably with the corresponding thermal-desorption spectrum.