  Using density functional theory and an exchange-correlationfunctional that
includes the van der Waals interaction, we study the coadsorption of CO on
Ru(0001) saturated with 0.5 ML of oxygen. Different coexisting CO coverages are
considered that are experimentally motivated, the room temperaturecoverage
consisting of 0.5 ML-O + 0.25 ML-CO (low coverage), the saturation coverage
achieved at low temperatures (0.5 ML-O + 0.375 ML-CO, intermediate coverage),
and the equally mixed monolayer that is stable according to our calculations
but not experimentally observed yet (0.5 ML-O + 0.5 ML-CO, high coverage). For
each coverage, we study the competition between the desorption and oxidation of
CO on the corresponding optimized structure by analyzing their reaction
energies and minimum energy reaction paths. The desorption process is
endothermic at all coverages, although the desorption energy decreases as the
CO coverage increases. The process itself (and also the reverted adsorption)
becomes more involved at the intermediate and high coverages because of the
appearance of a physisorption well and concomitant energy barrier separating it
from the chemisorbed state. Remarkably, the oxidation of CO, which is
endothermic at low coverages, turns exothermic at the intermediate and high
coverages. In all cases, the minimum reaction path for oxidation, which
involves the chemisorbed and physisorbed CO2, is ruled by one of the large
energy barriers that protect these molecular states. Altogether, the larger
activation energies for oxidation as compared to those for desorption and the
extreme complexity of the oxidation against the desorption paths explain that
CO desorption dominates over the oxidation in experiments.
