We determined the surface structure of RuO 2 (100) formed on Ru(101 h0) by employing the techniques of low-energy electron diffraction (LEED) and density functional theory (DFT) calculations. The RuO 2 (100) film grows lattice-matched with its [010] direction along the [0001] direction of Ru(101 h0) (4.5% compressively strained), while it grows incommensurately with the [001] direction of RuO 2 (100) along the [1 h21 h0] direction of Ru(101 h0). The RuO 2 (100) surface is terminated by bridging O atoms, which are attached to the coordinatively unsaturated Ru (cus-Ru) atoms with a bond length of 2.01 Å. The other Ru-O bond lengths are in the range of 1.90-2.05 Å, typical for bulk RuO 2 . Due to the presence of cus-Ru atoms on RuO 2 (100), CO molecules adsorb quite strongly as evidenced by a desorption state at 300-400 K. The activity of the RuO 2 (100) surface for the CO oxidation reaction is similar to that of RuO 2 (110). The surface energies of RuO 2 (110) and RuO 2 (100) are 71 and 87 meV/Å 2 , respectively.
The formation of chemisorbed O-phases on Ru (0001) by exposure to O2 at low pressures is apparently limited to coverages Θ ≤ 0.5. Using low-energy electron diffraction and density functional theory we show that this restriction is caused by kinetic hindering and that a dense O overlayer (Θ = 1) can be formed with a (1×1) periodicity. The structural and energetic properties of this new adsorbate phase are analyzed and discussed in view of attempts to bridge the so-called "pressure gap" in heterogeneous catalysis. It is argued that the identified system actuates the unusually high rate of oxidizing reactions at Ru surfaces under high oxygen pressure conditions.
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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