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

Abstract

To test predictions about the activity of Ru catalysts the RuO 2 (110) surface was investigated in an oxygen atmosphere at ambient pressure using scanning tunneling microscopy (STM). Epitaxial RuO 2 (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 O 2 at 300 K, and STM images were taken during exposure. The mesoscopic morphology of the film and the row structure of the RuO 2 (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 RuO 2 (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 O 2 atmosphere, so that it could be further characterized in UHV. Thermodesorption spectra showed strong desorption of CO 2 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 RuO 2 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 CO 2 in the O 2 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 RuO 2 (110) surface, cannot be extrapolated to ambient conditions for temperatures below the decomposition temperature of the carbonate species.

Low Temperature CO Oxidation on Ruthenium Oxide Thin Films at Near-Atmospheric Pressures

“…1), as a function of surface oxygen coverage measured prior to the reaction. The reaction was performed in the circulating mixture of 10 mbar CO and 50 mbar O 2 (He balance) at 450 K sharp LEED patterns of RuO 2 (110)/Ru(0001) and showed STM images very similar to the previously reported by Over et al [12] and Rössler et al [27], with rectangularshaped terraces dominating the large scale morphology (Fig. 4a).…”

“…6(a,b), clearly show the catalysts deactivation with time. Several mechanisms for the deactivation have been discussed in the literature: a surface reconstruction into a less active phase [30] and a carbonaceous (e.g., a carbonate) contamination mechanism [23,27]. Which of these two is operative in our experiments is difficult to ascertain on the basis of solely AES and LEED characterizations of the post-reacted surfaces.…”

Reply to comment on “CO oxidation on ruthenium: The nature of the active catalytic surface” by H. Over, M. Muhler, A.P. Seitsonen

“…In point #3, the authors take issue with the findings of Roessler et al [18]. Although we believe these results are quite relevant to the discussion, we believe that our argument is sufficient without invoking the supporting evidence presented by Roessler et al Whether there is deactivation of RuO 2 and whether this is by carbonate formation has not been a subject addressed by us and is a question that we leave to others.…”

Oxidation of the Ru(0001) surface covered by weakly bound, ultrathin silicate films

“…Commonly, thin RuO 2 (110) films are prepared by exposing Ru(0001) to high doses of molecular oxygen at 10 -6 -10 -4 mbar and elevated temperatures [22,23,29,30]. In this study, we oxidized Ru(0001) in 10 -5 mbar of O 2 at 700 K for 45 min, and sample cooling to room temperature was performed in the same oxygen ambient.…”