CO Oxidation Reaction over Oxygen-Rich Ru(0001) Surfaces

Abstract

The formation of CO2, by exposing oxygen precovered Ru(0001) surfaces to CO, was investigated as a function of the oxygen coverage for sample temperatures up to 900 K. It turned out that the reaction probability per incident CO molecule is below 5 × 10-4 for O coverages up to 3 monolayers (ML); oxygen in excess of 1 ML is located in the subsurface region. The reaction probability for the (1 × 1)-1O phase is in agreement with the data derived from high-pressure experiments by Peden and Goodman [J. Phys. Chem. 1986, 90, 1360]. Even for CO molecules with a translational energy of 1.2 eV (supersonic molecular beam experiments), the reaction probability is less than 5 × 10-2. This value is consistent with the activation barrier derived from DFT calculations for a reaction by direct collision from the gas phase (Eley−Rideal mechanism). Beyond an oxygen load of 3 ML, however, the reaction probability increases by 2 orders of magnitude. It is suggested that this enhancement is due to a further destabilization of the surface oxygen by the onset of oxide formation.

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

“…S2), suggesting that this sample contains also some silica nanoparticles. The sample was then treated with 16 O 2 , and the TPD spectrum was recorded. The results revealed desorption exclusively of 16 O 2 (Fig.…”

Nanometer and Subnanometer Thin Oxide Films at Surfaces of Late Transition Metals

“…The formation of crystalline, bulk-like RuO 2 (110) during high-pressure CO oxidation catalysis has indeed been observed experimentally at Ru(0001), but it is interesting to note that even after long operation times the film thicknesses never exceeded about 20Å 30,31,52 . Generally, one could interpret this finding as reflecting kinetic limitations to a continued growth, presumably due to slow diffusion of either O or Ru atoms through the formed film.…”

“…Recent experimental studies seem to support this view, i.e., they indicate that thin oxidic structures, now mostly coined surface oxides, start to form roughly at coverages Θ c . 30,31,32,33,34 For the "true" bulk oxide a higher oxygen concentra- tion might be required, but for oxide nucleation and for the formation of oxidic islands such increased oxygen content is only needed locally. In this respect it is remarkable that although for the on-surface adsorption at the late 4d TM basal surface the adatom-adatom interaction is repulsive (see the discussion of Fig.…”

“…2 conform much better with the widespread use of these metals in catalytic car converters, whereas the oxygen bonding at Ru(0001) seems simply too strong. The correspondingly somehow enigmatic high catalytic activity of Ru(0001) at ambient pressures only became comprehensible when experimental studies reported the formation of bulk-like RuO 2 (110) films at the surface in such reactive environments 30,31 . The subsequently established 62 and above described high activity of this oxide surface then not only explained the pressure gap difference (Ru metal in UHV = low activity, RuO 2 at high-pressures = high activity), but also reconciled this system with the Sabatier principle: The RuO 2 (110) surface indeed features oxygen species that are bound with intermediate bond strength comparable to the one of O at Rh(111) and Pd(111).…”

Epitaxial Growth of Ruthenium Dioxides on Ru(0001) Surface

“…It is in analogy to RuO 2 growth on Ru(0001) substrate prepared at temperature 530 o C by insitu sputtering [35], where RuO 2 film exhibits mixed (110) and (100) orientations, but the (100)-orientation is dominant, revealing that RuO 2 grows also in favor of (100) orientation on Ru(0001) in the gas phase conditions [35]. However, a well-ordered (110) RuO 2 film on Ru(0001) has been obtained by gas phase oxidation at a sample temperature of 700 K under UHV conditions [36]. The same RuO 2 film (110) oriented on MgO(100) has been observed by metal-organic vapor deposition at deposition temperature of 350 o C [37], while RuO 2 film (100) oriented on LaAlO 3 has been prepared at deposition temperature of 600 o C [38].…”