Scanning Tunneling Microscopy of the RuO2(110) Surface at Ambient Oxygen Pressure

To test predictions about the activity of Ru catalysts the RuO2(110) surface was investigated in an oxygen atmosphere at ambient pressure using scanning tunneling microscopy (STM). Epitaxial RuO2(110) films were grown on a Ru(0001) sample following an established preparation technique from ultrahigh vacuum (UHV) investigations. The sample was then exposed to 200 mbar of O2 at 300 K, and STM images were taken during exposure. The mesoscopic morphology of the film and the row structure of the RuO2(110) surface known from UHV were preserved. However, a 2-fold periodicity was observed along the [001] rows which is inconsistent with the expected surface termination by O atoms bonded to the coordinatively unsaturated sites of the RuO2(110) surface. In addition, a second type of features that partially form clusters within the ordered surface was observed. In a pure CO atmosphere at pressures of up to 21 mbar no atomic changes of this structure were observed, meaning that it does not contain O species that can react with CO. The new surface phase was stable after removal of the O2 atmosphere, so that it could be further characterized in UHV. Thermodesorption spectra showed strong desorption of CO2 with peaks at 520 and 570 K but not the expected recombinative desorption of O atoms from the coordinatively unsaturated sites. Photoelectron spectroscopy showed an O 1s state at 531.0 eV in addition to the bulk oxygen state of the RuO2 film at 529.5 eV. The most likely interpretation of the surface species in the oxygen atmosphere is a strongly bound carbonate formed by reaction of the surface with traces of CO or CO2 in the O2 atmosphere. The carbonate passivates the surface, leading to complete catalytic deactivation at 300 K. It is concluded that the established model for the unusual activity of Ru catalysts, which is based on the unique chemical properties of the RuO2(110) surface, cannot be extrapolated to ambient conditions for temperatures below the decomposition temperature of the carbonate species.