Coadsorption systems are of high importance in the understanding of heterogeneous catalysis since the various reaction fragments adsorb on the surface prior to reaction, and the presence and nature of neighboring species can affect the physical and chemical properties, and hence reaction, markedly.

Studies for the (mCO+nO)/Ru(0001) system show that for the systems that form in nature where CO occupies the top site, with increasing oxygen coverage, the adsorption energy of CO can remain practically unchanged or even exhibit a slight increase. We attribute the increase to an O-induced lateral weakening of Ru-Ru bonds of non-O-bonded surface Ru atoms. Thus, these non-O-bonded Ru atoms can form stronger bonds to an on-top CO adsorbate. In contrast, a more expected behavior of a notable decrease in CO adsorption energy with increasing O coverage is observed only if the O atoms bond to the same Ru atoms as CO as, for example, is the case when CO occupies hollow sites. Furthermore, for some of the structures, we find that there is a manifestation of small activation energy barriers for CO adsorption well above  the surface. [C. Stampfl and M. Scheffler, Phys. Rev. B 65, 155417 (2002).]