CO adsorption on the reducedRuO2(110)surface: Energetics and structure

The adsorption sites of CO on the reduced ${\mathrm{RuO}}_{2}(110)$ surface were determined by employing the techniques of density-functional theory calculations and quantitative low-energy electron diffraction. On the mildly reduced ${\mathrm{RuO}}_{2}(110)$ 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 \AA{}. 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 ${\mathrm{RuO}}_{2}(110)$ 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 ${\mathrm{RuO}}_{2}(110)$ surface compares favorably with the corresponding thermal-desorption spectrum.