Direct imaging of catalytically important processes in the oxidation of CO over RuO₂(110)

Ruthenium dioxide (RuO2) reveals unique and promising redox properties, making RuO2 a potential candidate for a versatile oxidation catalyst. Recently Zhang and Kisch1 reported, for instance, that hydrated RuO2 is a robust and efficient catalyst for room temperature oxidation of CO by humid air; recall that typical metal oxides do not tolerate humidity. In this contribution we present scanning tunneling microscopy (STM) data which directly image the catalytically important processes occurring on the RuO2(110) surface after exposing the pristine surface to CO and O2. The STM data are substantiated by density functional theory (DFT) calculations. In the bulk rutile structure of RuO2 the Ru atoms are 6-fold coordinated to oxygen atoms, while the O atoms are coordinated to three Ru atoms in a planar sp2 hybridization. On the stoichiometric RuO2(110) surface two kinds of under-coordinated surface atoms are stabilized (cf. Figure 1a). These are the bridging oxygen atoms (Obr), which are coordinated only to two Ru atoms underneath (Ru-O bond length 1.94 Å), and the so-called 1f-cus-Ru atoms, i.e. 1-fold under-coordinated Ru atoms.2 In Figure 1b we show an experimental 5 nm x 5 nm STM image of this surface taken at room temperature. Clearly, there are rows of protrusions visible along the [001] direction. STM simulations (cf. Figure 1c), using the Tersoff-Hamann model3 within DFT, indicate that the bridging oxygen atoms are imaged as bright regions. This result is quite remarkable as for the TiO2(110) surface the observed protrusions in STM images were ascribed to 1f-cus-Ti atoms.4 It demonstrates also that an interpretation of STM images needs additional information, such as provided by DFT calculations