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.

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The oxidation states formed during low-temperature oxidation (T < 500 K) of a Ru(0001) surface are identified with photoelectron spectromicroscopy and thermal desorption (TD) spectroscopy. Adsorption and consecutive incorporation of oxygen are studied following the distinct chemical shifts of the Ru 3d(5/2) core levels of the two topmost Ru layers. The evolution of the Ru 3d(5/2) spectra with oxygen exposure at 475 K and the corresponding O2 desorption spectra reveal that about 2 ML of oxygen incorporate into the subsurface region, residing between the first and second Ru layer. Our results suggest that the subsurface oxygen binds to the first and second layer Ru atoms, yielding a metastable surface "oxide", which represents the oxidation state of an atomically well ordered Ru(0001) surface under low-temperature oxidation conditions. Accumulation of more than 3 ML of oxygen is possible via defect-promoted penetration below the second layer when the initial Ru(0001) surface is disordered. Despite its higher capacity for oxygen accumulation, also the disordered Ru surface does not show features characteristic for the crystalline RuO2 islands. Development of lateral heterogeneity in the oxygen concentration is evidenced by the Ru 3d(5/2) images and microspot spectra after the onset of oxygen incorporation, which becomes very pronounced when the oxidation is carried out at T > 550 K. This is attributed to facilitated O incorporation and oxide nucleation in microregions with a high density of defects.

Scanning photoelectron spectromicroscopy has been used to study the onset and the initial stages of oxidation of Ru(0001) at three oxidation temperatures, 625, 675, and 775 K, and oxygen exposures of about 105 Langmuir. The lateral heterogeneity developed during oxide nucleation and growth and the local composition of the coexisting phases have been determined using as fingerprints the O 1s and Ru 3d spectra, thus combining chemical mapping with spectroscopy from selected features from the maps. The onset of oxide formation is characterized by the appearance of randomly distributed small islands (⩾0.5 μm) identified as germinal patches exhibiting some spectral features of bulk RuO2. The following anisotropic growth of the RuO2 phase and in particular the shape of the oxide islands shows a strong dependence on the oxidation temperature. The spectroscopic information obtained for the areas surrounding the oxide islands reveals an intermediate oxygen state characterized by distinct O 1s and Ru 3d features different from both the chemisorbed and the oxide form. It is assigned to an intermediate state acting as precursor of the oxide. The experimental data are discussed in the framework of the oxidation pathway and of core level fingerprints for the various oxygen–Ru phases suggested in recent theoretical models.

The conditions for formation of subsurface oxygen on the Ru(0001) surface have been studied using thermal desorption spectroscopy, low energy electron diffraction (LEED) and specular helium scattering. The incorporation of oxygen has been performed via dissociative chemisorption of three molecular carriers of atomic oxygen: NO2, O2 and N2O. The rates for oxygen dissolution can be related to the initial dissociative sticking probability of the molecules on the bare Ru surface. For sample temperatures below 800 K, oxygen penetration into the subsurface region starts only when oxygen molecules impinge on the saturated adsorbed layer characterized by the 1×1 O LEED pattern, indicative of 1 monolayer. A thermally induced transformation of this chemisorbed 1×1 O phase into subsurface oxygen could not be caused even at temperatures close to the onset of oxygen desorption. Oxygen incorporation into the subsurface region by passing through the 1×1 O adsorbed layer, however, shows Arrhenius-type behavior. For impinging O2 molecules, the onset of subsurface oxygen formation appears at 550 K and the entire process is characterized by a rather low activation energy of about 0.5 eV. Deposition of alkali metals on the Ru(0001) surface does not enhance the probability for oxygen dissolution. The resulting oxygen content is substantially reduced and this effect strongly depends on the coverage of alkali–metal atoms. In contrast structural defects on the Ru surface, as generated by Ar+ sputtering and characterized by specular He scattering, act as promoters for oxygen accommodation. The onset for oxygen penetration on a rough surface already begins at about 350 K and the resulting oxygen content can be directly related to the surface roughness.